Open access peer-reviewed chapter

Drug Resistance and Molecular Cancer Therapy: Apoptosis Versus Autophagy

Written By

Rebecca T. Marquez, Bryan W. Tsao, Nicholas F. Faust and Liang Xu

Submitted: January 11th, 2012Reviewed: December 5th, 2012Published: May 15th, 2013

DOI: 10.5772/55415

Chapter metrics overview

3,855 Chapter Downloads

View Full Metrics

1. Introduction

The majority of chemo/radiotherapies inhibit cancer cell growth by activating cell death pathways, such as apoptosis, necrosis, and autophagy-associated cell death. However, as the disease progresses, cancer cells can acquire a variety of genetic and epigenetic alterations, which leads to dysregulation of cell death-associated signaling pathways and chemo/radioresistance. Designing novel drugs and enhancing therapeutic strategies to improve survival and quality of life for cancer patients must specifically target pathways responsible for drug resistance. Two cellular mechanisms can contribute to chemo/radioresistance: inhibition of apoptotic cell death pathways and induction of autophagy, a cell survival response. The development of novel drugs and extensive research studies has provided significant insight into the aberrant regulation of apoptosis and key apoptosis inhibitor proteins during tumorigenesis. However, the extensive dysregulation of cell growth pathways in cancer cells makes it necessary to target multiple pathways in order to elicit a lasting death response. Autophagy, classically designated as a cell “survival” mechanism, appears to play a greater role in cell death than previously conceived. This contradiction between autophagy-associated cell survival versus cell death has intensified the interest in this field of research in cancer therapeutics. Understanding how autophagic cells cross the threshold from cell survival to cell death during drug treatments is imperative for identifying more potent therapies. Utilizing novel treatments that will re-activate apoptotic cell death pathways, while driving autophagy-associated cell death will lead to more effective chemotherapies, thereby enhancing overall patient survival.

Keywords:apoptosis; autophagy; programmed cell death; molecular therapy; personalized medicine; signaling pathways


2. Apoptosis pathways

Cancer cells can acquire apoptosis-resistance during treatment by up-regulating multiple pro-survival factors, such as inhibitors of apoptosis proteins (IAPs), nuclear factor-κB (NF-κB), and the B cell CLL/lymphoma-2 (BCL-2) family proteins. There are two major apoptosis signaling pathways, the extrinsic and intrinsic apoptosis signaling pathways (Figure 1). The extrinsic (death receptor) apoptosis pathway is induced by the binding of cell death ligands, such as FAS ligand or TNF to cell death receptors, FAS receptor or tumor necrosis factor receptor,TNFR1, respectively. Activation of these death receptors results in caspase 8 activated cell death [1]. The intrinsic (mitochondrial or BCL-2 regulated) apoptosis pathway can be activated by cellular stresses or chemo/radiotherapies that lead to functional activation of the pro-apoptotic BCL-2 family proteins, which induce mitochondrial outer membrane permeabilitization (MOMP) and cytochrome crelease into the cytosol. Once in the cytosol, cytochrome cinduces formation of the apoptosome complex, which contains cytochrome c, caspase 9 and apoptotic protease-activating factor-1 (APAF-1), followed by activation of downstream caspases 3, and 7[2]. While the intrinsic apoptosis pathway is considered to be regulated by BCL-2, the extrinsic pathway can also be regulated by BCL-2 family members via crosstalk with the intrinsic pathway. This crosstalk occurs through caspase 8 cleavage and activation of the BH3-interacting domain death agonist (BID). The cleavage product, truncated BID (tBID), is required for death receptor-induced apoptosis in some cell types. During tumorigenesis, both the extrinsic and intrinsic apoptosis signaling pathways become dramatically dysregulated thereby leading to increased cell survival upon chemo/radiotherapy. This chapter will discuss exploitation of factors regulating apoptosis, such as second mitochondria-derived activator of caspases (SMAC) and BH3-only proteins, as molecular targets utilized to overcome apoptosis resistance in cancer cells.


3. IAP Family proteins promote apoptosis-resistance

IAPs are a pivotal class of pro-survival factors that suppress apoptosis against a large variety of apoptotic stimuli, including chemotherapeutic agents, radiation, and immunotherapy in cancer cells[3-5]. Elevated expression of IAPs is a common occurrence in multiple cancer types, while eliciting a wide range of biological responses that promote cancer cell survival and proliferation[6]. Therefore, IAPs are attractive molecular targets for anti-cancer therapies in order to decrease apoptosis-resistance, thereby enhancing cancer therapeutics and increasing patient survival.

IAPs are characterized by baculoviral IAP repeat (BIR) domains, which are required for the majority of IAP-mediated protein-protein interactions and inhibition of apoptosis[7]. Eight IAPs have currently been identified in humans, but the most studied IAP members include the X chromosome-linked IAP protein (XIAP), cellular IAP1 (cIAP-1), and cellular IAP2 (cIAP-2)[8]. IAPs inhibit both the intrinsic and extrinsic apoptotic pathways (Figure 1). XIAP binds to and inhibit caspases 3, 7, and 9, while cIAPs negatively regulate caspase 8 activation through TNFR1 signaling[9]. IAPs also contain a carboxyl-terminal RING domain, which enables them to function as E3 ubiquitin ligases[6]. XIAP and cIAPs can promote cancer cell survival and proliferation by inhibiting caspase activation, IAP-antagonist binding, or by acting as critical mediators of the NF-κB pathway.

Figure 1.

Extrinsic and intrinsic apoptosis signaling pathways.The extrinsic (death receptor) apoptosis pathway is induced by the binding of cell death ligands, TNF, FASL or TRAIL, to cell death receptors TNFR, FAS, or DR5, respectively. Activation of the death receptors results in caspase 8 activated cell death. The intrinsic (mitochondrial or BCL-2 regulated) apoptosis pathway can be activated by cellular stresses or chemo/radiotherapies. This leads to functional activation of the pro-apoptotic BCL-2 family proteins which induces cytochromecor SMAC release into the cytosol. Cytochromecinduces formation of the apoptosome complex, which contains cytochromec, caspase 9, and APAF-1, followed by activation of downstream caspase 3 and 7. SMAC can promote apoptosis by binding to XIAP, which results in the subsequent release of caspase 9 and downstream activation of apoptosis. cIAPs are capable of inhibiting SMAC by blocking this interaction. The crosstalk between the extrinsic and intrinsic pathways occurs through caspase 8 cleavage and activation of the BID. The cleavage product, tBID, is required for death receptor-induced apoptosis in some cell types. During tumorigenesis, both the extrinsic and intrinsic apoptosis signaling pathways become dramatically dysregulated thereby leading to increased cell survival during chemo/radiotherapy. IAP antagonists can inhibit the anti-apoptotic actions of XIAP and cIAPs in both the intrinsic and extrinsic apoptosis pathways.


4. XIAP is a potent caspase inhibitor

XIAP protein is the first well-characterized IAP family member[4, 10, 11]. XIAP is overexpressed in approximately 25% of the 60 NCI human cancer cell lines and can predict response to chemotherapy[12-16]. Although it was initially believed that all IAP proteins blocked apoptosis by directly binding caspases, it was later found that only XIAP directly binds to and inhibits caspases 3, 7, and 9 (Figure 1)[10, 17, 18]. As caspase 9 is an initiator caspase, it is considered the most critical target for XIAP’s anti-apoptotic function[19]. Structural studies have outlined the protein interactions utilized by XIAP to inhibit caspase function. The BIR3 domain of XIAP binds to the catalytic domain of caspase 9 while the linker region between XIAP BIR1 and BIR2 binds to caspase 3 or 7[20, 21],[22]. In addition to binding and blocking caspase catalytic sites, XIAP also utilizes its E3 ubiquitin ligase function for targeting and ubiquitylating caspase 3 for proteasome degradation[23]. Therefore, due to XIAP’s ability to inhibit multiple caspases, either directly or via ubiquitylation, XIAP has become a premiere molecular target for current chemotherapies (Figure 1).


5. cIAP regulation of the NF-κB signaling pathway

Although IAPs are typically known to bind and inhibit caspases, cIAPs also modulate ubiquitin-dependent signaling events of the extrinsic apoptosis pathway and regulate activation of NF-κB[24]. cIAPs are required for stimulus-dependent activation of the canonical pathway and for constitutive suppression of the non-canonical NF-κB pathway (Figure 2)[8]. NF-κB is a transcription factor involved in angiogenesis, metastasis, and cell proliferation[8]. Upon activation, NF-κB regulates transcription of pro-survival genes such as TNFα, cIAPs, BCL-2 and other apoptosis-related proteins. Furthermore, blocking NF-κB pathway can sensitize cancer cells to chemotherapeutic agents and radiation[25-27].

In the canonical NF-κB pathway, the inhibitor of NF-κB (IκBα) binds to NF-κB, thereby preventing NF-κB nuclear translocation from the cytoplasm into the nucleus in unstimulated cells[28]. TNF-mediated activation of NF-κB requires the assembly of an ubiquitin-dependent signaling complex[29]. TNF ligand binding to TNFR1 induces the formation of a signaling complex by initially recruiting TNFR1-associated death domain protein (TRADD) and TNFR-associated factor 2 (TRAF2), followed by recruitment of receptor-interacting protein 1 (RIP1) and c-IAP proteins (Figure 2)[30, 31]. Within this complex, cIAPs promote nondegradative polyubiquitylation of RIP1, in addition to themselves, to generate a binding platform for assembly of the IκB kinase (IKK) complex[32-34]. This leads to the activation of IKKβ, which results in phosphorylation of IκBα, prompting IκBα polyubiquitylation and subsequent degradation. This allows NF-κB to translocate to the nucleus and activate target genes[28]. Therefore, cIAPs positively regulate the canonical NF-κB pathway.

Alternatively, in the non-canonical pathway, cIAPs negatively regulate NF-κB transcription by ubiquitylating and targeting NF-κB-inducing kinase (NIK) for proteasomal degradation[35]. In unstimulated cells, a cytoplasmic complex composed of cIAPs, TRAF2, TRAF3 and NIK, maintains constitutive ubiquitin-dependent proteasomal degradation of NIK (Figure 2) [35-41]. Accumulation of NIK is acquired by dissociation of this cytoplasmic complex. Upon ligand binding, receptors of the TNFR family, such as CD40, recruit TRAF2, TRAF3 and the cIAP proteins into their respective signaling complexes. This results in cIAP ubiquitylation and degradation of the cIAPs, TRAF2, and TRAF3, which leads to stabilization and accumulation of NIK and downstream activation of NF-κB anti-apoptotic target genes[9, 42]. The conflicting roles that cIAPs play in inducing or inhibiting NF-κB signaling pathway display an additional layer of complexity when developing therapeutic drugs targeting cIAPS.


6. SMAC: IAP-antagonist

SMAC is a regulator of the intrinsic apoptosis pathway and becomes released from the mitochondria upon mitochondrial outer membrane permeabilitization (MOMP) (Figure 1). Structural studies show that SMAC induces apoptosis by binding to and sequestering IAPs from binding to caspases[43-45]. As previously mentioned, the BIR3 domain of XIAP binds to the N-terminus of small subunit p12 of processed caspase 9. SMAC protein contains a region homologous to the caspase 9 p12 subunit, therefore, it can also bind to XIAP BIR3 domain[20]. SMAC binding of XIAP allows the subsequent release of caspase 9 and activation of downstream signaling leading to apoptosis[46]. While cIAPs are not potent inhibitors of caspases, cIAPs are able to bind to SMAC with high affinity, thereby preventing SMAC from disrupting XIAP-mediated inhibition of caspases[6].

6.1. The role of IAP and SMAC and clinical outcome

Due to the importance of apoptosis resistance during chemo/radiotherapy, the expression of IAP proteins and IAP inhibiting proteins, such as SMAC, have demonstrated significant correlation with clinicopathological data[6, 47]. Altered expression of cIAPs in cancer cells is typically due to chromosomal aberrations, such as genomic ampifications, translocations and deletions. Genomic amplification at the 11q21-q23 genomic loci of both cIAP1 and cIAP2 has been detected in many cancers, including esophageal squamous cell carcinomas, liver cancer, lung cancer, and cervical cancer[48-51]. Furthermore, immunohistochemical analysis of cervical cancers from patients treated only with radiotherapy had high levels of nuclear cIAP1 staining and demonstrated that both overall survival and local recurrence-free survival was significantly poorer compared to patients with little or no nuclear cIAP1[50]. Genomic translocations, such as t(11;18)(q21;q21), results in the fusion of the BIR domains of cIAP2 with paracaspase mucosa-associated lymphoid tissue lymphoma translocation protein 1(MALT1) and occurs frequently in mucosa-associated lymphoid tissues[52-54]. The resulting cIAP2-MALT1 fusion protein constitutively activates the NF-κB signaling pathway[53, 55].

As previously discussed, cIAPs act as oncogenes in most cancers, however, cIAPs in multiple myeloma has demonstrated tumor suppressive properties. In multiple myeloma, chromosomal deletions of cIAP-1/2 resulted in stabilization of NIK, which induced constitutive aberrant activation of the non-canonical survival NF-κB pathway[37, 39]. This further delineates the important balancing act of cIAPs in regulating the NF- κB pathways and cell survival.

XIAP expression is also dysregulated in many cancers and correlates with clinical outcome[6]. XIAP is upregulated in clear-cell renal cell carcinoma and correlates with increasing tumor stage, dedifferentiation, and aggressive growth[56]. XIAP was also shown to be an independent prognostic marker for non-muscular invasive bladder cancer, colon cancer, and liver cancer[57-59]. In invasive breast ductal carcinoma, nuclear expression of XIAP correlated with shortened overall survival[60]. Interestingly, a prostate cancer study showed patients with high XIAP levels had a much lower probability of tumor recurrence than those with lower XIAP expression. Furthermore, patients with high-grade prostate tumors who had high XIAP levels had a lower risk of recurrence compared with patients whose tumors express low XIAP[61]. This demonstrates that while many cancers have a correlation with high XIAP expression levels and poor prognosis, some cancers have additional altered mechanisms associated with poor clinical outcome. This further supports the need for tumor expression profiling in order to determine whether an individual’s tumor is apoptosis-resistant. Pre-treatment screening will allow physicians to identify the proper treatment regimen in order to avoid unnecessary toxicity and relapse.

The down-regulation of IAP inhibitor, SMAC, has also been shown to play a significant role in inhibiting IAPs in cancer and correlates with clinical outcome[6]. In rectal cancer, high expression levels of SMAC correlated with 5-year recurrence free survival rate and 5-year local relapse-free survival rate[62]. Down-regulation of SMAC has been shown to be associated with disease progression in many cancer types, such as lung, hepatocellular carcinoma, testicular cancer[63-65]. In renal cell carcinoma, low levels of SMAC correlated with advanced tumor stage, poor prognosis, and a reduced probability of recurrence-free survival[56, 66]. Furthermore, XIAP expression increased with stage and grade, while mRNA and protein expression levels of SMAC did not significantly change. This results in a relative increase of anti-apoptotic XIAP over pro-apoptotic SMAC, thereby contributing to apoptosis resistance in renal cell carcinoma[66].

6.2. IAP antagonists as therapy to overcome apoptosis-resistance

Due to the dysregulation and contribution of IAPs towards chemo/radioresistance, researchers have developed several targeting strategies, such as small-molecule IAP antagonists, including SMAC mimetics, and antisense oligonucleotides. Table 1 shows a subset of IAP antagonists currently used in clinical trials.

6.3. SMAC-mimetics

Several studies have shown that overexpression of SMAC sensitizes neoplastic cells to apoptotic cell death[67, 68]. Therefore, SMAC mimetics have been developed in order to sensitize cancer cells to apoptotic stimuli, such as chemo/radiotherapy. Synthetic SMAC N-terminal peptides fused to cell-permeabilizing peptides were initially used as SMAC mimetics for treating cancer cells. These peptides were found to bypass mitochondrial regulation and sensitize both human cancer cells in culture and tumor xenographs in mice to apoptosis when combined with TNF-related apoptosis-inducing ligand (TRAIL) or chemotherapeutic drug treatments[69, 70]. While appearing effective, SMAC peptides did not possess good pharmacological properties and, therefore, could not be used as therapeutic agents. Researchers then utilized 3D structure analysis of SMAC bound to XIAP BIR3 domain to design and synthesize small molecule SMAC mimetics[71-73]. These compounds show at least 20-fold enhanced binding to XIAP BIR3 domain over the natural SMAC peptide in a cell-free system[72-74]. Small molecule SMAC mimetics also bind and inhibit cIAP-1 and cIAP-2 activities and promote apoptosis synergistically with proapoptotic stimuli, such as TRAIL or TNFα, in cancer cells that were previously determined to be resistant to TRAIL or TNFα[71].

DrugCancer type(s)Clinical TrialCo-therapyOutcome
AT-406Solid tumors, lymphomaPhase 1NoneOngoing.[225]
AMLPhase 1Daunorubicin and CytarabineOngoing.[225]
AEG35156AMLPhase 1/2High-dose Cytarabine and IdarubicinAEG35156 treatment led to dose-dependent decreases of XIAP mRNA and protein levels. Apoptosis induction was detected.[195]
AMLPhase 1/2Cytarabine and IdarubicinVery effective when combined with chemotherapy in patients with AML refractory to a single induction regimen.[87]
YM155Advanced refractory solid tumorsPhase 1NoneThe safety profile, plasma concentrations achieved, and antitumor activity.[209]
NSCLCPhase 2NoneModest single-agent activity in patients with refractory, advanced NSCLC. A favorable safety/tolerability profile was reported.[210]

Table 1.

Selective list of IAP antagonists undergoing clinical trials with and without combination therapy.

Pre-clinical and clinical data has demonstrated that SMAC mimetics may show more therapeutic promise in combination with conventional chemotherapeutic drugs, death receptor agonist or radiation therapy (Table 1). Research from our lab demonstrated that the SMAC mimetic, SH130, disrupts the binding between XIAP/cIAP and SMAC. Upon combination treatment, SH130 enhances ionizing radiation-induced apoptosis in vitroand induces 80% tumor regression in hormone-refractory prostate cancer models[75]. We also demonstrated that SMAC mimetic, SH122, can induce cell death via both the extrinsic and intrinsic apoptosis pathways. In combined treatment with death receptor ligand, TRAIL, SH122 induces TRAIL-mediated cell death in prostate cancer cell lines by blocking IAPs and NF-κB[76].

SMAC mimetics have proven tremendous efficacy when used in combination with treatments to induce apoptosis in apoptosis-resistant cells[77]. Interestingly, it was also shown that SMAC mimetic treatment alone could induce apoptosis in a subset of non-small-cell lung cancer cell lines[78]. It was later determined that autocrine-secreted TNFα-mediated apoptosis signals that were inhibited by IAP proteins. Treatment with the SMAC mimetic promoted formation of RIP1-dependent caspase 8-activating complex leading to apoptosis in these cells[78]. It has also been demonstrated that SMAC mimetic binding of cIAPs leads to rapid ubiquitination and proteasomal degradation of cIAPs[35]. Therefore, in addition to targeting XIAP to relieve caspase 9 inhibition in the intrinsic cell death pathway, SMAC mimetics can induce cIAPs auto-ubiquitination and degradation, which leads to NF-κB activation and TNFα secretion. The autocrine TNFα signaling in turn induces caspase 8 activation and cancer cell death (Figure 2).

6.4. cIAP- and XIAP-selective antagonists

SMAC mimetics have broad specificity by inhibiting both XIAP and cIAPs. Currently, the individual roles of IAPs in apoptosis resistance, as well as BIR domain structure, are unexplored. Therefore, more selective antagonists are designed in order to provide greater specificity for the diverse IAPs. CS3 is a cIAP1/2 selective antagonist and has been shown to induce degradation of cIAP1/2, activate canonical, non-canonical NF-κB signaling pathways, and induce cell death[79]. Although CS3 is capable of inducing cell death, cIAP-selective antagonists are significantly less potent in promoting apoptosis than pan-selective compounds[79].

Embelin, the active ingredient of traditional herbal medicine, is a potent IAP antagonist that binds to the XIAP BIR3 domain. We have shown that embelin inhibits cell growth, induces apoptosis, and activates caspase 9 in prostate cancer cells with high levels of XIAP, but has a minimal effect on normal prostate epithelial cells with low levels of XIAP[80]. Furthermore, embelin combined with radiation potently suppressed prostate cancer cell proliferation that was associated with S and G2/M cell cycle arrest[81]. Moreover, the combination treatment promoted caspase-independent apoptosis. In vivo, embelin significantly improved tumor response to x-ray radiation in PC-3 xenograft model. Combination therapy resulted in tumor growth delay and prolonged time to tumor progression, with minimal systemic toxicity. These findings demonstrate the potential to utilize embelin as a novel adjuvant therapeutic candidate for the treatment of hormone-refractory prostate cancer that is resistant to radiation therapy[81].

Figure 2.

Canonical and non-canonical prosurvival NF-κB pathways.cIAPs are required for stimulus-dependent activation of NF-κB canonical pathway and alternatively for constitutive suppression of the non-canonical NF-κB pathway. TNF-mediated activation of the canonical NF-κB pathway requires the assembly of an ubiquitin-dependent signaling complex comprised of TRADD, TRAF2, RIP1, and cIAPs. cIAPs induce non-degradative ubiquitylation of both RIP1 as well as themselves which leads to activation of downstream pro-survival NF-κB signaling. IAP antagonists can inhibit NF-κB canonical pathway by preventing cIAP ubiquitylation of RIP1 which leads to recruitment of pro-caspase 8, thereby inducing apoptosis. Alternatively, in the non-canonical pathway, cIAPs negatively regulate NF-κB transcription by ubiquitylating and targeting NIK for proteasomal degradation. In unstimulated cells, a cytoplasmic complex composed of cIAPs; TRAF2, TRAF3 and NIK, maintains constitutive ubiquitin-dependent proteasomal degradation of NIK, thereby preventing activation of NF-κB pathway. Upon ligand binding, receptors of the TNFR family, such as CD40, recruit TRAF2, TRAF3 and the cIAP proteins into their respective signaling complexes. This results in cIAP ubiquitylation and subsequent degradation of cIAPs, TRAF2, and TRAF3. Degradation of this complex leads to stabilization and accumulation of NIK and downstream activation of NF-κB anti-apoptotic target genes. Interestingly, IAP antagonists can switch the non-canonical NF-κB signaling pathway from pro-survival to pro-apoptotic pathway (dashed arrow). IAP antagonists induce activation of this pathway by blocking cIAP inhibition, which leads to TNFα secretion. The autocrine TNFα signaling in turn induces caspase 8 activation and cancer cell death.

IAP antagonists have proven to be effective in overcoming apoptosis resistance in cancer cells. Clinical trials are currently underway to test the applicability of small-molecule IAP antagonists in single and combined anti-cancer therapies. Preliminary results suggest that IAP antagonists are well tolerated and effective in inhibiting IAPs (Table I)[6]. In addition, these molecules are providing insight into additional regulatory networks that exist in cancer cells, thereby providing new understanding of apoptosis resistance.

6.5. Inhibition of IAPs through RNA interference

Inhibition of IAPs using RNAi has further demonstrated the role of IAPs in drug resistance. Esophageal cancer cell lines transfected with XIAP siRNA demonstrated increased cell apoptosis[82]. Another study demonstrated that RNAi targeting of XIAP increased breast and pancreatic cancer cell susceptibility to functionally diverse chemotherapeutic agents, including TRAIL and taxanes and therefore increasing the effectiveness of chemotherapeutic agents[83]. Furthermore, in vivostudies also demonstrated that inhibition of XIAP by RNAi radiosensitized lung cancer cells by up-regulating apoptotic signaling and down-regulating cell survival[84]. We have also shown that combination treatment using RNAi silencing of IAPs and SH122 SMAC mimetic shows a greater sensitization of cells to apoptosis, than SMAC mimetic alone[76].

Clinical trials using anti-sense oligonucleotide AEG35156 is proving to be successful. The first-in-human study with AEG35156 in patients with advanced refractory cancers demonstrated that the compound was well tolerated and showed some anti-tumor activity[83]. However, AEG35156 was less effective in Phase I clinical trials with pancreatic cancer patients[85, 86]. Phase II trials treating primary refractory AML patients with both chemotherapy and AEG35156 demonstrated a 91% rate of complete remission[87]. Therefore, RNAi therapy shows significant promise in treating apoptosis-resistant cancers. While AEG35156 demonstrates promise in treating primary refractive disease, it is important to identify the patients that express high levels of IAPs in order to gain the most therapeutic benefit. Again, this demonstrates the need for molecular marker screening in order to develop personalized therapies.


7. BCL-2 family proteins regulate the intrinsic apoptotic pathway

In addition to XIAPs, the BCL-2 protein family members are essential regulators of the intrinsic apoptotic pathway, also known as the BCL-2-regulated pathway, and significant contributors to apoptosis-resistance during chemo/radiotherapies[88]. BCL-2 family members are characterized by their BCL-2 homology (BH) domain and can be categorized into three classes: the anti-apoptotic multi-domain proteins, such as BCL-2, BCL-xL, and MCL-1, are essential for cell survival, the pro-apoptotic BH3-only proteins, such as BID, BIM, BAD, and PUMA, initiate apoptosis signaling; and the pro-apoptotic multi-domain effector proteins, such as BAX and BAK, are required for MOMP and activation of caspases that leads to cell death[89, 90]. Both the anti-apoptotic and pro-apoptotic functions of BCL-2 family members are regulated through their BH domains [91, 92]. Furthermore, the BH1-BH3 domains of anti-apoptotic proteins form a hydrophobic binding pocket that binds the α-helix of the BH3-only pro-apoptotic proteins [93, 94]. BCL-2 proteins are typically found at the outer mitochondrial membrane (OMM), however, they can also be localized to the endoplasmic reticulum (ER) and in the cytosol[95].

Death signals induced by DNA damage, growth factor deprivation, or chemotherapies induce apoptosis via the mitochondrial pathway (Figure 1) by transcriptional or post-translational activation of BH3-only proteins[96-98]. After activation, the pro-apoptotic BH3-only proteins prompt a conformational change of monomeric BAX and BAK resulting in homo-oligomerization and activation[99-101]. Activated BAX and BAK cause MOMP, followed by release of cytochrome cand other pro-apoptotic factors, such as SMAC, from the mitochondria. BAX and BAK are essential for the pro-apoptotic function of BH3-only proteins, therefore, loss of BAX and BAK prevents apoptotic cell death[101, 102]. Cancer cells that display overexpression of anti-apoptotic proteins and/or down-regulation of pro-apoptotic proteins, have the potential to evade chemotherapeutic cell death resulting in drug resistance.

While it is generally accepted that activation of BAX and BAK is required to induce permeabilization of the mitochondria, there are multiple models that describe the mechanisms used in the activation/inhibition of BAK/BAX. One model suggests that BH3-only activating proteins, such as Bid or Bim, directly bind to BAX/BAK to induce oligomerization and subsequent activation [65, 103-106]. Another model describes an indirect mechanism. Anti-apoptotic proteins, such as BCL-2 and BCL-xL, inhibit cell death by binding to and sequestering activating BH3-only proteins thereby preventing their activation of BAX/BAK[107-110]. The indirect mechanism involves a subset of BH3-only proteins, called sensitizers, which induce BAX/BAK oligomerization indirectly, by binding anti-apoptotic proteins, thereby displacing the activating BH3-only proteins allowing them to bind to BAX/BAK[111, 112].

Anti-apoptotic proteins, BCL-2 and BCL-xL, are also capable of heterodimerizing with BAX or BAK, thereby inhibiting BAX or BAK[113-115]. It has been shown that BCL-2 undergoes a conformational change to bind to and inhibit oligomerization of mitochondrial membrane bound Bax. However, if BAX is in excess, apoptosis resumes due to the availability of free BAX able to activate the apoptotic pathway [115].

The activation models, as described in the previous paragraphs, are simplified examples of the complex interactions required to carry out the intrinsic apoptosis pathway. Dysregulation of intrinsic apoptosis pathways, due to altered ratios of antiapoptotic members to proapoptotic members, leads to apoptotic blocks. Identifying the proteins involved in these blocks is essential for designing more effective rational therapies. Studies called “BH3 profiling” used BH3 peptides that selectively antagonize BCL-2 family members to identify apoptotic blocks in cancer cells[107, 116]. It was demonstrated that BH3-only proteins show distinct binding preferences to anti-apoptotic BCL-2 family members[107, 116]. Identifying differential BH3-only protein binding affinities for anti-apoptotic BCL-2 protein family members has led to the development of specific small molecule inhibitors of anti-apoptotic BCL-2 proteins which are designed to overcome apoptosis resistance in cancer cells and induce cell death.

7.1. Dysregulation of the BCL-2 family of proteins augments chemo/radioresistance

The dysregulation of BCL-2 family members, such as overexpression of anti-apoptotic genes or silencing of pro-apoptotic genes, is a key determinant for apoptosis-resistance during tumorigenesis and chemotherapy. BCL-2 was initially discovered to be overexpressed in human B-cell lymphomas and is located near chromosomal translocation break points frequently found in B-cell lymphomas[118]. Additional studies have demonstrated that BCL-2 protein levels in cancers are enhanced due to promoter hypomethylation, loss of inhibitory microRNA expression, and gene amplifications, signifying that up-regulation of BCL-2 expression is often found in a variety of cancers[119, 120].

Expression of anti-apoptotic BCL-2 family members has a significant effect on chemoresistance and prognosis[120, 121]. BCL-2, BCL-xL, and MCL-1 expression increases during prostate cancer progression[122]. Furthermore, BCL-2/BCL-xL expression levels correlate with resistance to a wide spectrum of chemotherapeutic agents[123, 124]. Alternatively, the pro-apoptotic BCL-2 family members can be down-regulated resulting in suppressed apoptosis. Spontaneous deletions or mutations of BAX have been observed in colorectal tumors, which results in significant reduction of apoptosis in response to anticancer agents[125, 126]. The BH3-only protein PUMA is also down-regulated in melanoma and Burkitt lymphomas[127, 128].

Dysregulation of BCL-2 family of proteins also occurs in cancer cells due to a loss of p53 tumor suppressor expression or function. p53 expression is lost in a majority of cancers. p53 can activate transcription of BAX, BID, PUMA and NOXA (Figure 1)[97, 129-132]. Cytosolic accumulation of p53 results in activation of BAX similarly to the BH3-only activating BCL-2 proteins, thereby inducing apoptosis[133]. Interestingly, p53 has also been shown to inhibit anti-apoptotic BCL-2 family members as well. DNA damage induces p53-Bcl-2 binding, thereby sequestering BCL-2 from inhibiting BAX/BAK oligomerization resulting in apoptotic cell death in cancer cells[134]. Inhibiting apoptosis via p53-associated regulation of the BCL-2 family displays another level of complexity in inducing cell death of cancer cells.

7.2. BH3-mimetics as a therapeutic strategy to overcome apoptosis resistance

Due to the dysregulation and importance of BCL-2 family members for inhibiting apoptosis in cancer cells, attempts aimed at developing novel drugs that can inhibit anti-apoptotic BCL-2 proteins. Crystal structure analysis of BCL-xL revealed that the BH1-BH3 domains formed a hydrophobic groove[93]. Further studies demonstrated that this BCL-xL hydrophobic groove could bind to a BAK BH3 peptide indicating the ability to design small molecules that could bind to BCL-xL and inhibit its anti-apoptotic function[94]. Indeed, numerous small molecule BH3-mimetics have been identified or designed to bind to this BH3 binding pocket with the potential to block BCL-2/xL binding to pro-apoptotic BCL-2 proteins. The BH3 mimetics have demonstrated diverse binding specificity and efficacy in inducing apoptosis (Figure 3)[135-137].

Figure 3.

BH3 mimetics inhibit anti-apoptotic BCL-2 proteins therefore inducing both apoptosis and autophagy.BH3 mimetics are designed to bind to anti-apoptotic BCL-2 proteins and induce apoptosis. BH3 mimetics also induce autophagy-associated cell death by preventing BCL-2 proteins from binding to the autophagy activating protein, Beclin1.

One of the first small molecules developed via in silicoscreens was HA14-1[136]. HA14-1 was initially demonstrated to induce the activation of Apaf-1 and caspases in human acute myeloid leukemia cells. HA14-1 was subsequently found to prevent BCL-2 binding to BAK[138]. In addition, treatment with HA14-1 caused cytosolic Ca(2+) increase, change in mitochondrial membrane potential, BAX translocation, and reactive oxygen species (ROS) generation prior to cytochrome c release[139]. Obatoclax (GX15-070MS) was one of the first pan anti-apoptotic BCL-2 protein inhibitors capable of inhibiting BCL-2, BCL-XL, and MCL-1[140]. Clinical trials using obatoclax treatment have demonstrated success across many cancer types both independently as well as in combined therapies. Representative clinical trials are listed in Table 2.

Clinical TrialCo-therapyOutcome
AT-101/ GossypolSCLCPhase 2NoneNot active in patients with recurrent chemosensitive SCLC.[226]
NSCLCPhase 2DocetaxelAT-101 plus docetaxel was well tolerated with an adverse event profile indistinguishable from the base docetaxel regimen.[227]
SCLCPhase 1/2TopotecanRelapsed progression - 17.4 weeks, refractory progression - 11.7 weeks.[228]
CRPCPhase 1/2NoneEvidence of single-agent clinical activity was observed with prostate-specific antigen declines in some patients.[229]
Metastatic Breast CancerPhase 1/2NoneGossypol appears to affect the expression of Rb protein and cyclin D1; negligible antitumor activity against anthracycline and taxane refractory metastatic breast cancer.[230]
ABT-263/ NavitoclaxCLLPhase 1NoneLow MCL1 expression and high BIM:MCL1 or BIM:BCL-2 ratios in leukemic cells correlated with response.[143]
SCLCPhase 1NoneChanges in a surrogate marker of BCL-2 amplification (pro-gastrin releasing peptide) correlated with changes in tumor volume.[144]
LymphomaPhase 1NoneNavitoclax has a novel mechanism of peripheral thrombocytopenia and T-cell lymphopenia, attributable to high-affinity inhibition of BCL-XL and BCL-2, respectively.[231]
GX15-070MS/ Obatoclax mesylateLeukemiaPhase INone
Well tolerated and these results support its further investigation in patients with leukemia and myelodysplasia.[232]
Solid tumorsPhase ITopotecanSafe and well tolerated when given in combination with topotecan.[233]
CLLPhase INoneActivation of Bax and Bak was demonstrated in peripheral blood mononuclear cells, and apoptosis induction was related to obatoclax exposure, as monitored by the plasma concentration of oligonucleosomal DNA/histone complexes.[234]
Oblimersen (Genasense)Breast CancerPhase ITACTwo of 13 patients showed a decrease of BCL-2 transcripts after 4 days of treatment with oblimersen.[235]
CRPCPhase IIDocetaxelThe primary end points of the study were not met: PSA response rate "/>30% and a major toxic event rate <45% were not observed with docetaxel-oblimersen.[236]
Breast CancerPhase IITACOblimersen up to a dose of 7 mg/kg/day administered as a 24-h infusion on days 1-7 can be safely administered in combination with standard TAC on day 5.[196]
HRPCPhase IIDocetaxelOblimersen combined with docetaxel is an active combination demonstrating both an encouraging response rate and an overall median survival. [237[

Table 2.

Selective list of published BH3 mimetics clinical trials with and without combination therapy.

Using nuclear magnetic resonance-based screening and structure-based design, the BH3 mimetic, ABT-737, was developed and shown to possess greater affinity and ability to inhibit BCL-2, BCL-xL and BCL-w, than MCL-1[141]. ABT-737 was initially developed by screening a library of BH-3 like analogues with high binding efficiency to the hydrophobic groove of BCL-xL. ABT-737 has been shown to synergistically enhance cell death in combined treatments with chemotherapeutics and radiation[141]. An oral form of ABT-737, called ABT-263 (Navitoclax), has also been developed and is also undergoing clinical trials for lymphoma, leukemia, and small cell lung cancer[142-144].

The BH3 mimetic (-)-gossypol is a natural polyphenol purified from the cottonseed. We previously demonstrated that the (-)-gossypol significantly enhances the antitumor activity of docetaxel chemotherapy in hormone-refractory prostate cancer patients with BCL-2/BCL-xL/MCL-1 overexpression[145]. Mechanistically, we demonstrated that (-)-gossypol blocked the interactions of BCL-2/Bcl-xL with Bax or Bad in cancer cells. (-)-Gossypol (AT-101) is the first BCL-2/BCL-xL inhibitor entered clinical trial and is now in Phase IIb clinical trials for hormone-refractory prostate cancer and many other types of cancer at multiple centers in the United States. In addition, more potent and less toxic gossypol derivatives, such as Apogossypolone and TW-37, are being developed[146-148].

BH3 mimetics are designed to inhibit anti-apoptotic BCL-2 proteins and demonstrate significant therapeutic potential in clinical trials. Interestingly, BH3 mimetics induced toxicity independent of Bax/Bak suggesting the existence of an alternative route of cell death induction[149]. BCL-2 has also been linked to a non-apoptotic cell death mechanism associated with autophagy, usually known as a cell survival mechanism[150]. It was later determined that BCL-2 and BCL-xL can bind the BH3 domain of tumor suppressor Beclin1 (BECN1) and inhibit autophagy (Figure 3)[151, 152]. This discovery revealed a new role for anti-apoptotic BCL-2 protein family as anti-autophagic proteins. The following sections will discuss autophagy and the role played by the BCL-2:Beclin 1 interaction for inducing/inhibiting autophagy, and the mechanism of novel therapies, such as BH3 mimetics, aimed at disrupting the interaction in order to induce autophagy-associated cell death.


8. Autophagy and autophagic cell death: background

Autophagy is a highly regulated catabolic process that functions as a cell survival mechanism activated upon cellular stresses such as nutrient deprivation, starvation, hypoxia and chemo/radiotherapy[153]. There are three primary types of autophagy, chaperone-mediate autophagy, microautophagy and macroautophagy[154]. This chapter will focus on macroautophagy, referred as autophagy further in the text. Activation of autophagy induces the formation of autophagosomes that engulf damaged organelles or particles. Eventually, the autophagsome fuses with the lysosome and degrades its interiors to provide cells the nutrients such as amino acids or fatty acids necessary for cell metabolism[155]. Defective autophagy machinery can lead to diseases such as neurodegenerative, liver, cardiac, and muscle diseases, as well as a variety of cancers. Recent studies have reported that apoptosis-resistant cancer cells can avoid chemo/radiotherapeutic-induced cell death by activating autophagy [156-159]. Furthermore, apoptosis-associated proteins, such as NF-κB, p53, UVRAG and the above-discussed BCL-2, have been shown to play dual regulatory roles in both apoptosis and autophagy [160-162]. Paradoxically, activation of autophagy upon drug treatments can induce cell death independent of or in parallel with apoptosis and necrosis.[163]. Therefore, researchers are actively developing novel cancer therapies that aim to promote cell death by modulating autophagy pathways.

8.1. Autophagy pathways

Autophagy can be activated by a variety of stimuli and signaling pathways. The classical induction of autophagy occurs upon nutrient deprivation; however, autophagy can also be induced by other factors, such as hypoxia, cytokines, hormones, genotoxic stress, p53 activation, and chemo/radiotherapy. Autophagy has also been attributed to tumor suppression. This was first demonstrated in mice with allelic loss of Beclin1, a key protein involved in inducing autophagy. Complete loss of the Beclin1 resulted in death during early embryogenesis whereas heterozygous loss of Beclin1 resulted in formations of spontaneous tumors [164, 165]. Autophagy involves a conserved family of proteins known as the autophagy-related gene families (ATGs). The canonical autophagy pathway in mammals occurs in a series of stages: initiation, nucleation, elongation, and degradation. All stages are regulated by a core molecular machinery (Figure 4).

Figure 4.

Cross-talk between apoptosis and autophagy.Autophagy takes place in a series of stages; initiation, nucleation, elongation, and degradation. Autophagy can be activated by a variety of stimuli and signaling pathways, including nutrient deprivation, hypoxia, p53 genotoxic stress, suppression of mTOR, or chemo/radiotherapy, followed by activation of AMPK. ULK1, ATG13, ATG101, FIP200 protein complex forms and mediates autophagy initiation. ATG13 mediates ULK1 phosphorylation of FIP200 and activates the ULK complex. Subsequently, the ULK complex localizes to the ER and initiates pre-autophagosome formation. The vesicle nucleation involves the core complex consisting of PI3KIII, p150, ATG14L, Beclin1 and AMBRA1. ATG14L induces a translocation of the PI3KIII complex to the site of autophagosome formation and initiates the formation of the phagophore. Phagophore elongation into an autophagosome requires ATG12, LC3-I, and two ubiquitin-like protein conjugation systems. The first system involves ATG7 and ATG10 conjugation of ATG12 into the ATG16L-ATG12-ATG5 complex. The second conjugation system involves LC3-I modification by ATG7 and ATG3 into LC3-II and inserts into the autophagosome membrane. Finally, the autophagosome fuses with the lysosome and contents within the autophagosome are degraded. Beclin1 can interact with autophagy machinery at the ER and induce autophagy. In addition, Beclin1 can bind to anti-apoptotic BCL-2 family of proteins, preventing BCL-2 binding to BAX or BAK monomers, therefore inducing apoptosis. Beclin1 can also be cleaved by caspase 3 to form Beclin1-C which inhibits autophagy. However, Beclin1-C can induce apoptosis by localizing to the mitochondria facilitiating the release of apoptotic factors.

A protein complex consisting of unc-51-like kinase 1(ULK1, homolog of yeast ATG1), ATG13, ATG101 and a scaffolding protein FIP200 (ortholog of yeast ATG16) mediates autophagy initiation (Figure 4)[166]. In nutrient-rich environment, an upstream regulator called mammalian target of rapamycin (mTOR) phosphorylates ATG13 and ULK1 to inhibit the initiation of autophagy. Following starvation or cellular stress, mTOR is inhibited and dissociates from the ULK1 complex. Then, ATG13 mediates ULK1 to phosphorylate FIP200 and activates the ULK complex[167]. Subsequently, the ULK complex localizes to the endoplasmic reticulum (ER) and initiates pre-autophagosome formation [168]. The vesicle nucleation involves the core complex consisting of Class III phosphatidylinositol 3-kinase (PI3KIII/ homolog of yeast Vps34), p150 (Vps15), ATG14L, Beclin1 (ATG6), and activating molecule in Beclin 1-regulated autophagy (AMBRA1)[165, 169]. ATG14L induces a translocation of the PI3KIII complex to the site of autophagosome formation and initiates the formation of an isolated membrane, also known as the phagophore[170]. A recent study revealed that PI3KIII lipid kinase activity produces and accumulates phosphatidylinositol 3-phosphate (PI3P) at the ER to induce a high membrane curvature that attracts ATG14L binding. Bound to the ER, ATG14L produces more PI3Ps and recruits other parts of the core complex. Recruitment of these proteins induces phagophore elongation [171, 172]. Phagophore elongation into an autophagosome requires ATG12, ATG8/LC3-I, and two ubiquitin-like protein conjugation systems. The first system involves an E1-like ATG7 and an E2-like ATG10 conjugation of ATG12 to ATG12-ATG5 that interacts with ATG16L to form the ATG16L-ATG12-ATG5 complex[173-176]. The second conjugation system involves the cytosolic protein isoform known as the LC3-I (ATG8) to undergo modification by ATG7 and E2-like ATG3 into LC3-phosphatidylethanolamine (LC3-II), an important biomarker for autophagy[177, 178]. The ATG16L complex acts as an E3-like enzyme to promote lipidation of cytosolic LC3-I into LC3-II and correctly localizes LC3-II onto the autophagosome formation site to help form the membrane [179]. Finally, the autophagosome fuses with the lysosome and contents within the autophagosome are degraded. This final step requires the endosome marker, RAB7, and a lysosomal membrane protein, LAMP2, however, the exact mechanism involved in the fusion of autophagosome and lysosomes is still unclear [180, 181].

8.2. Autophagy induction

Activation of autophagy is regulated by multiple molecular pathways depending upon the stimuli (Figure 4). As mentioned above, mTOR is activated under nutrient-rich environment thereby suppressing autophagy. Starvation of growth factors and certain amino acids represses class I PI3K signaling to promote cell survival via autophagy induction [182, 183]. PI3KI forms the substrate PI3P which leads to activation of the PKB/AKT protein that inhibits a heterodimer complex involving the tuberous sclerosis complexes 1 and 2 (TSC1 and TSC2). The TSC2 protein suppresses mTOR activity via activation of a Ras family small GTPase called Ras homolog enriched in brain (Rheb) [184]. Tumor suppressor phosphatase

and tensin homolog (PTEN) dephosphorylates the PI3K product PI3P, thereby suppressing AKT signaling. Loss of PTEN occurs in multiple cancers including brain, breast, and prostate cancer [185]. Additional aberrant signaling of PI3KI can result in cancers that exhibit mutated amplification of upstream receptor tyrosine kinase, such as HER2 in gastric cancer or PDGFR and EGFR in glioblastoma [186, 187]. Under metabolic stress, such as high AMP level, hypoxia and cytosolic calcium level increase, AMP-activated protein kinase (AMPK) can mediate autophagy by negatively regulating mTOR and inducing the dephosphorylation of ATG13 and ULK1 [188-190]. Alternatively, AMPK has been found to activate autophagy by direction phosphorylation of ULK1 [191].

The tumor suppressor protein p53 plays a more complicated role and can induce as well as inhibit autophagy, based upon subcellular location and cellular context. Upon exposure to DNA-damaging agents, nuclear p53 can induce autophagy by transcriptionally activating damage-regulated autophagy modulator (DRAM)[192]. DRAM activates target proteins Sestrin1 and Sestrin2, which subsequently activate AMPK thereby inhibiting mTOR and inducing autophagy [193]. In addition, nuclear p53 can up-regulate ULK1 transcriptionally and directly activate autophagy[194]. Cytoplasmic p53 has the opposite effect and can actually inhibit autophagy[195]. High mobility group box 1 (HMGB1) is a Beclin1-interacting accessory protein that assists in autophagy activation. p53 has been discovered to form a complex with HMGB1 in the cytoplasm resulting in the inhibition of autophagy and induction of cell death (22345153)[87]. Loss or knockdown of p53 increases the binding of HMGB1 to Beclin1 and mediates cytosolic localization of the complex to the ER [87]. Subsequently, HMGB1 mediates the Beclin1-PI3KIII complex formation and initiates autophagosome production.

8.3. Beclin1:BCL2 interaction regulates autophagy/apoptosis switch

As discussed above, Beclin1 is a critical inducer of autophagy. Interestingly, Beclin1 is also a BH3-only protein and therefore interacts with anti-apoptotic BCL-2 family members via its BH3 domain [151, 196]. BCL-2 binding of Beclin1 at the ER prevents Beclin1 from assembling the pre-autophagosomal structure mediated by the Beclin1/PI3KIII complex (Figure 3 and 4)[196, 197]. Therefore, BCL-2 anti-apoptotic proteins have dual pro-survival roles by preventing both apoptosis and autophagy-associated cell death that makes these proteins ideal chemotherapeutic targets.

The expression of BCL-2 and/or Beclin1 is critical for regulating the switch between autophagy and apoptosis. Down-regulation of Beclin1 also contributes to tumorigenesis, evident in hepatocellular carcinoma, brain, colorectal, and gastric cancer [198-200]. Low expression of Beclin1 results in insufficient removal of damaged organelles. Deficient Beclin1 causes cell transformation through the accumulation of reactive oxygen species and genotoxic stress, [165]. Furthermore, it was shown that inhibiting BCL-2 in breast cancer cells via siRNA knockdown did not induce apoptosis as expected but observed a form of autophagic cell death [201]. The autophagic cell death was the result of combinatorial treatment with doxorubicin that lead to increased expression of Beclin1[201]. Therefore, maintaining adequate levels of Beclin1 is important to override BCL-2 inhibition of autophagy-related cell death. Evidently, BCL-2 and Beclin1 expressions are important determinants for identifying the proper chemotherapy or combination treatments that would provide the greatest therapeutic benefit.

Autophagy regulatory proteins can promote or inhibit the BCL-2:Beclin1 interactions. As previously mentioned, AMBRA1 is a key regulator in initiating autophagy by binding to Beclin1. In presence of autophagic stimulus, ULK1 phosphorylates AMBRA1, which results in AMBRA1 dissociation from the Dynein motor complex [202]. After dissociation, AMBRA1 translocates to the ER, binds to Beclin1 in the autophagy initiation complex and results in the induction of autophagy [202]. Moreover, a recent study demonstrated that BCL-2 localized to the mitochondria can also bind AMBRA1whereas ER-localized BCL-2 does not [203]. The BCL-2:AMBRA1 interaction at the mitochondria is down-regulated during autophagy and apoptosis. Therefore, BCL-2 can regulate Beclin1-induced autophagy by directly binding to Beclin1, as well as by sequestering AMBRA1, the activator of Beclin1 at the mitochondrion [203].

BCL-2/Beclin1 complex can be disrupted by otherBCL-2 and Beclin1 binding partners. As discussed above, HMGB1 can bind to Beclin1 and initiate autophagy. Inhibition of HMGB1 decreases autophagy and increases apoptosis[204]. For example, a study has shown that deletion or deactivation of HMGB1 in mouse embryonic fibroblasts reduces LC3-I expression. In response to starvation, cells lacking HMGB1 cannot initiate autophagy and undergo apoptotic cell death[205]. Additionally, HMGB1 bound to Beclin1 has also been found to induce the phosphorylation of BCL-2 which disrupts the BCL-2:Beclin1 complex.

Autophagy can also be inhibited by Beclin1 cleavage. Chemotherapy-induced and mitochondria-mediated apoptosis was shown to induce Beclin1 cleavage by caspase 8 to form Beclin1-C. This event renders defective Beclin1 activity and autophagy pathway [206]. Furthermore, the C-terminus of cleaved Beclin1 can acquire pro-apoptotic ability by translocation to the mitochondria and inducing release of apoptotic factors [207]. This demonstrates a novel therapeutic approach to induce apoptosis by inhibiting autophagy.

8.4. Inducing autophagy-associated cell death using BH3 mimetics

This chapter has previously discussed the therapeutic benefits of using BH3 mimetics to induce apoptosis by preventing anti-apoptotic BCL-2 proteins from binding to pro-apoptotic proteins, BAX/BAK. Upon the discovery that Beclin1 was a novel autophagic BH3-only protein, BH3 mimetics have been utilized to induce autophagy[208]. ABT737 was the first BH3 mimetic reported to induce apoptosis and autophagy by inhibiting anti-apoptotic action of BCL-2 or BCL-xL[208]. At first the findings were counterintuitive; how could a drug induce both apoptotic cell death and autophagic cell survival? As discussed above, BH3 mimetics appeared to kill cells in a BAK/BAX-independent manner suggesting that apoptotic cell death was not the only mechanism for BH3 mimetic-induced cell death[149]. It was later determined that BH3 mimetics could induce autophagy-associated cell death, especially in apoptosis-resistant cells [209].

We recently investigated the effect of the natural BH3-mimetic (-)-gossypol in apoptosis-resistant prostate cancer cells with high levels of BCL-2 versus prostate cancer cells with low BCL-2 expression[210]. (-)-Gossypol induced similar levels of total cell death in both prostate cancer cell lines. However, the dominant mode of cell death depended upon the expression of the anti-apoptotic BCL-2 family of proteins[210]. BH3 mimetics induced apoptotic cell death in prostate cancer cells with low BCL-2 expression. Conversely, prostate cancer cells with high BCL-2 expression died via modulation of the autophagy pathway [210]. Furthermore, overexpressing BCL-2 decreased the level of (-)-gossypol-induced autophagy, possibly due to the stoichiometric abundance of BCL-2 sequestering Beclin1 and inhibiting autophagy induction. The data demonstrate that BH3 mimetics can be utilized to kill cells with both high and low BCL-2, therefore, enhancing the ability to overcome chemo/radioresistance.

BH3 mimetics induce autophagy by disrupting the BCL-2:Beclin1 inhibitory complex as well as additional autophagy pathways. BH3 mimetics, ABT-737 and HA14-1, also stimulate other pro-autophagic pathways and hence activate the nutrient sensors Sirtuin1 and AMPK, inhibit mTOR, deplete cytoplasmic p53, and trigger the IKK Kinase[211]. Activation of autophagy was independent of reduced oxidative phosphorylation or reduced cellular ATP concentrations. Furthermore, induction of autophagy by ABT-737 and HA14-1 was completely inhibited by knockdown of Beclin1 or PI3KIII. This suggests that BH3 mimetics can interfere with multiple pathways, eliciting a coordinated effort to induce autophagy-associated cell death.


9. The role of autophagy in therapy resistance

A number of therapeutic strategies have been developed to target autophagy in cancer cells. Similarly with apoptosis-resistance, autophagy-associated resistance to chemotherapy has become a challenging variable in the successful treatment of patients. For example, in human lung cancer cells treated with EGFR tyrosine kinase inhibitors (TKI), gefitinib and erlotinib, autophagy contributed to cell survival[212]. Inhibition of EGFR suppresses PI3KI activity and results in downstream activation of the ULK complex[212]. Other studies have shown that autophagy contributes to chemotherapy resistance through its cytoprotective mechanism. For example, chronic myeloid leukemia treated with imatinib, glioblastoma multiforme treated with temozolomide, colorectal cancer treated with 5-FU, and breast cancer treated with both tamoxifen and trastuzumab have all shown resistance that is associated with increased autophagy[213]. Recent studies have shown that cytotoxic agents and starvation may play a role in activating autophagy via HMGB1[214]. Increased expression of HMGB1 during treatment with doxorubicin, cisplatin, and methotrexate in osteosarcoma patients has been found to facilitate chemotherapy resistance by promoting the formation Beclin1/PI3KIII complex. In addition, HMGB1 also antagonizes drug-induced cell death in leukemia, colon cancer, and prostate cancer by up-regulating autophagy but the exact mechanism remains unclear[214].

Not only does autophagy contribute to chemotherapy resistance, it also plays a role in radiotherapy resistance. Investigators exposed radioresistant MDA-MB-231 cells to ionizing radiation at different doses and found increasing levels of LC3-II, a hallmark of autophagy activation. This indicates that activation of autophagy may protect these cells from radiation-induced cell death[215]. In addition, researchers found upregulation of autophagy in radiosensitive HBL-100 cells after inhibition of mTOR by rapamycin. In further experiments, inhibition of autophagy by 3-methyladenine (3-MA) resulted in reduced cell survival and displayed a radiosensitizing effect[215]. From these experiments, researchers deduced that cancer cells use autophagy as an escape mechanism from apoptosis to overcome radiotherapeutic stress via degradation of IR-induced cellular damage.


10. Re-sensitization of cancer cells to treatment by autophagy inhibition

To counter autophagy in cancer resistance, novel cancer therapies uses target inhibition of autophagy for re-sensitizing cancer cells to drug treatments. Researchers have used autophagy inhibitors such as 3-MA, LY294002, wortmannin to inhibit the PI3K [158, 216]. 3-MA contributes to autophagy suppression by down-regulating the PI3KI/Akt/mTOR signaling pathway. Surprisingly, the autophagy inhibitor 3-MA has been found to induce autophagy and contribute to cell survival when used for a prolonged period[217]. This controversial phenomenon is most likely due to the dual effect of 3-MA on PI3KI and PI3KIII. 3-MA blocks Class I permanently, but only temporarily Class III PI3K. Thus, treatment with 3-MA should only be considered under specific conditions such as limited treatment periods. Other types of autophagy inhibitors include LC3 knockdown by siRNA, which decreased breast cancer resistance to trastuzumab and increased cell death in CML in combination with imatinib[156, 218]. Chloroquine (CQ) and Hydroxychloroquine (HCQ) are the most successful autophagy inhibitors that suppress the autophagic lysosomal protease activity to promote the accumulation of autophagic vacuoles that often leads to apoptotic and necrotic cell death[219, 220]. Phase I and II clinical trials are ongoing using HCQ or CQ in combination with treatment such as docetaxel in prostate cancer, tamoxifen in breast cancer, and gemcitabine in pancreatic cancer[221].

Inhibiting autophagy poses another potential problem since anti-autophagic therapeutic drugs reduce tumor-specific immune response thereby limiting the therapeutic success[222]. Activated autophagy in glioblastoma cells treated with EGF toxin has been found to release HMGB1 that binds to and activates Toll-like receptor 4 (TLR4). Activated TLR4 increases T-cell mediated anti-tumor response to eliminate the malignant cells[223]. Deactivating autophagy decreases the release of HMGB1, leaves tumor cells unattended by the host immune system, and results in increased resistance[224]. Although inhibiting autophagy is effective, researchers must take its adverse side effects into consideration.

11. Concluding remarks

As this chapter has outlined, chemo/radioresistance is a key contributor to decreased patient survival. In order to develop more effective cancer therapies and improve treatment outcome, more research is required to delineate this complicated biological mechanism. Furthermore, the ability of cancer cells to acquire heterogeneous genetic and epigenetic alterations across tumors elicits deregulation of cell death-associated signaling pathways in a variety of ways. Cancer cells are smart to quickly figure out ways to overcome a treatment that targets any particular cellular signaling pathway. Therefore, designing novel drugs and enhancing therapeutic strategies must simultaneously target multiple pathways and mechanisms. Using IAP antagonists that target multiple cell survival pathways, as well as BH3 mimetics that can overcome anti-apoptotic BCL-2 proteins to induce both apoptosis and autophagy-related cell death can improve survival and quality of life for cancer patients. The complexity of tumor biology and drug resistance suggests that we need to design treatment strategy based on the genetic/signaling profiles of the patient in order to provide the safest and most effective cancer therapies tailored to a particular patient, the ultimate goal of the personalized medicine.


This study was supported in part by NIH grants R01 CA121830(S1) and R01 CA134655 (LX) a pilot grant from NIH COBRE CCET grant (8P30GM103495), Bridging Grant from the Kansas IDeA Network of Biomedical Research Excellence (K-INBRE) (P20 GM103418), and by the Kansas Bioscience Authority Rising Star Award (LX). RTM is supported in part by NIH K-INBRE Post-Doctoral Award. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of NIH.


  1. 1.GalluzziLVitaleIAbramsJ. MAlnemriE. SBaehreckeE. HBlagosklonnyM. Vet alMolecular definitions of cell death subroutines: recommendations of the Nomenclature Committee on Cell Death2012Cell death and differentiation. 2012;19110720Epub 2011/07/16.
  2. 2.FuldaSGalluzziLKroemerGTargeting mitochondria for cancer therapy. Nature reviews Drug discovery.20109644764Epub 2010/05/15.
  3. 3.SchimmerA. DInhibitor of apoptosis proteins: translating basic knowledge into clinical practice. Cancer research.20046420718390Epub 2004/10/20.
  4. 4.SrinivasulaS. MAshwellJ. DIAPs: what’s in a name? Molecular cell.200830212335Epub 2008/04/29.
  5. 5.DeverauxQ. LStennickeH. RSalvesenG. SReedJ. CEndogenous inhibitors of caspases. J Clin Immunol.199919638898Epub 2000/01/14.
  6. 6.FuldaSVucicDTargeting IAP proteins for therapeutic intervention in cancer. Nature reviews Drug discovery.201211210924Epub 2012/02/02.
  7. 7.TakahashiRDeverauxQTammIWelshKAssa-muntNSalvesenG. Set alA single BIR domain of XIAP sufficient for inhibiting caspases. The Journal of biological chemistry.199827314778790Epub 1998/05/09.
  8. 8.DaiYLawrenceTXuLOvercoming cancer therapy resistance by targeting inhibitors of apoptosis proteins and nuclear factor-kappa B. American journal of translational research.2009eb15eeff-35e4-8951-208a-fb085ebac5df):116
  9. 9.VucicDDixitV. MWertzI. EUbiquitylation in apoptosis: a post-translational modification at the edge of life and death. Nature reviews Molecular cell biology.201112743952Epub 2011/06/24.
  10. 10.DeverauxQ. LTakahashiRSalvesenG. SReed JC. X-linked IAP is a direct inhibitor of cell-death proteases. Nature.199738866393004Epub 1997/07/17.
  11. 11.WilkinsonJ. CCeperoEBoiseL. HDuckettC. SUpstream regulatory role for XIAP in receptor-mediated apoptosis. Mol Cell Biol.20042416700314Epub 2004/07/30.
  12. 12.FongW. GListonPRajcan-separovicESt Jean M, Craig C, Korneluk RG. Expression and genetic analysis of XIAP-associated factor 1 (XAF1) in cancer cell lines. Genomics.200070111322Epub 2000/11/23.
  13. 13.MurisJ. JCillessenS. AVosWVan HoudtI. SKummerJ. AVan KriekenJ. Het alImmunohistochemical profiling of caspase signaling pathways predicts clinical response to chemotherapy in primary nodal diffuse large B-cell lymphomas. Blood.20051057291623Epub 2004/12/04.
  14. 14.YangX. HFengZ. EYanMHanadaSZuoHYangC. Zet alXIAP is a predictor of cisplatin-based chemotherapy response and prognosis for patients with advanced head and neck cancer. PloS one.2012e31601. Epub 2012/03/10.
  15. 15.HectorSRehmMSchmidJKehoeJMccawleyNDickerPet alClinical application of a systems model of apoptosis execution for the prediction of colorectal cancer therapy responses and personalisation of therapy. Gut.201261572533Epub 2011/11/16.
  16. 16.TammIKornblauS. MSegallHKrajewskiSWelshKKitadaSet alExpression and prognostic significance of IAP-family genes in human cancers and myeloid leukemias. Clinical cancer research : an official journal of the American Association for Cancer Research.2000651796803Epub 2000/05/18.
  17. 17.DeverauxQ. LLeoEStennickeH. RWelshKSalvesenG. SReedJ. CCleavage of human inhibitor of apoptosis protein XIAP results in fragments with distinct specificities for caspases. The EMBO journal.19991819524251Epub 1999/10/03.
  18. 18.DattaROkiEEndoKBiedermannVRenJKufeDXIAP regulates DNA damage-induced apoptosis downstream of caspase-9 cleavage. The Journal of biological chemistry.200027541317338Epub 2000/08/10.
  19. 19.DeverauxQ. LRoyNStennickeH. RVan ArsdaleTZhouQSrinivasulaS. Met alIAPs block apoptotic events induced by caspase-8 and cytochrome c by direct inhibition of distinct caspases. The EMBO journal.1998178221523Epub 1998/05/26.
  20. 20.SrinivasulaS. MHegdeRSalehADattaPShiozakiEChaiJet alA conserved XIAP-interaction motif in caspase-9 and Smac/DIABLO regulates caspase activity and apoptosis. Nature.200141068241126Epub 2001/03/10.
  21. 21.ShiozakiE. NChaiJRigottiD. JRiedlS. JLiPSrinivasulaS. Met alMechanism of XIAP-mediated inhibition of caspase-9. Molecular cell.200311251927Epub 2003/03/07.
  22. 22.HuangYParkY. CRichR. LSegalDMyszkaD. GWuHStructural basis of caspase inhibition by XIAP: differential roles of the linker versus the BIR domain. Cell.2001104578190Epub 2001/03/21.
  23. 23.SuzukiYNakabayashiYTakahashiRUbiquitin-protein ligase activity of X-linked inhibitor of apoptosis protein promotes proteasomal degradation of caspase-3 and enhances its anti-apoptotic effect in Fas-induced cell death. Proceedings of the National Academy of Sciences of the United States of America.2001981586627Epub 2001/07/12.
  24. 24.PopCSalvesenG. SHuman caspases: activation, specificity, and regulation. The Journal of biological chemistry.2009284332177781Epub 2009/05/29.
  25. 25.MagneNToillonR. ABotteroVDidelotCHoutteP. VGerardJ. Pet alNF-kappaB modulation and ionizing radiation: mechanisms and future directions for cancer treatment. Cancer Lett.2006231215868Epub 2006/01/10.
  26. 26.VoborilRWeberova-voborilovaJConstitutiveN. F-k. a. p. p. aB activity in colorectal cancer cells: impact on radiation-induced NF-kappaB activity, radiosensitivity, and apoptosis. Neoplasma.200653651823Epub 2006/12/15.
  27. 27.RhoH. SKimS. HLeeC. EMechanism of NF-kappaB activation induced by gamma-irradiation in B lymphoma cells : role of Ras. J Toxicol Environ Health A.2005Epub 2005/12/06.
  28. 28.ScheidereitCIkappaB kinase complexes: gateways to NF-kappaB activation and transcription. Oncogene.200625516685705Epub 2006/10/31.
  29. 29.DardingMMeierPIAPs: guardians of RIPK1. Cell death and differentiation.20121915866Epub 2011/11/19.
  30. 30.MicheauOTschoppJInduction of TNF receptor I-mediated apoptosis via two sequential signaling complexes. Cell.2003114218190Epub 2003/07/31.
  31. 31.RotheMPanM. GHenzelW. JAyresT. MGoeddelD. VThe TNFR2-TRAF signaling complex contains two novel proteins related to baculoviral inhibitor of apoptosis proteins. Cell.1995837124352Epub 1995/12/29.
  32. 32.VarfolomeevEGoncharovTFedorovaA. VDynekJ. NZobelKDeshayesKet alc-IAP1 and c-IAP2 are critical mediators of tumor necrosis factor alpha (TNFalpha)-induced NF-kappaB activation. J Biol Chem.200828336242959Epub 2008/07/16.
  33. 33.BertrandM. JMilutinovicSDicksonK. MHoW. CBoudreaultADurkinJet alcIAP1 and cIAP2 facilitate cancer cell survival by functioning as E3 ligases that promote RIP1 ubiquitination. Molecular cell.2008306689700Epub 2008/06/24.
  34. 34.MahoneyD. JCheungH. HMradR. LPlenchetteSSimardCEnwereEet alBoth cIAP1 and cIAP2 regulate TNFalpha-mediated NF-kappaB activation. Proceedings of the National Academy of Sciences of the United States of America.2008105331177883Epub 2008/08/14.
  35. 35.VarfolomeevEBlankenshipJ. WWaysonS. MFedorovaA. VKayagakiNGargPet alIAP antagonists induce autoubiquitination of c-IAPs, NF-kappaB activation, and TNFalpha-dependent apoptosis. Cell.2007131466981Epub 2007/11/21.
  36. 36.DemchenkoY. NGlebovO. KZingoneAKeatsJ. JBergsagelP. LKuehlW. MClassical and/or alternative NF-kappaB pathway activation in multiple myeloma. Blood.201011517354152Epub 2010/01/08.
  37. 37.AnnunziataC. MDavisR. EDemchenkoYBellamyWGabreaAZhanFet alFrequent engagement of the classical and alternative NF-kappaB pathways by diverse genetic abnormalities in multiple myeloma. Cancer cell.200712211530Epub 2007/08/19.
  38. 38.VinceJ. EWongW. WKhanNFelthamRChauDAhmedA. Uet alIAP antagonists target cIAP1 to induce TNFalpha-dependent apoptosis. Cell.2007131468293Epub 2007/11/21.
  39. 39.KeatsJ. JFonsecaRChesiMSchopRBakerAChngW. Jet alPromiscuous mutations activate the noncanonical NF-kappaB pathway in multiple myeloma. Cancer cell.200712213144Epub 2007/08/19.
  40. 40.HeJ. QZarnegarBOganesyanGSahaS. KYamazakiSDoyleS. Eet alRescue of TRAF3-null mice by100NF-kappa B deficiency. The Journal of experimental medicine.2006Epub 2006/10/04.
  41. 41.GrechA. PAmesburyMChanTGardamSBastenABrinkRTRAF2 differentially regulates the canonical and noncanonical pathways of NF-kappaB activation in mature B cells. Immunity.200421562942Epub 2004/11/13.
  42. 42.VarfolomeevEGoncharovTMaeckerHZobelKKomuvesL. GDeshayesKet alCellular Inhibitors of Apoptosis Are Global Regulators of NF-kappaB and MAPK Activation by Members of the TNF Family of Receptors. Sci Signal.2012ra22. Epub 2012/03/22.
  43. 43.WuGChaiJSuberT. LWuJ. WDuCWangXet alStructural basis of IAP recognition by Smac/DIABLO. Nature.20004086815100812Epub 2001/01/05.
  44. 44.ChaiJDuCWuJ. WKyinSWangXShiYStructural and biochemical basis of apoptotic activation by Smac/DIABLO. Nature.2000406679885562Epub 2000/09/06.
  45. 45.LiuZSunCOlejniczakE. TMeadowsR. PBetzS. FOostTet alStructural basis for binding of Smac/DIABLO to the XIAP BIR3 domain. Nature.2000408681510048Epub 2001/01/05.
  46. 46.SalvesenG. SDuckettC. SIAP proteins: blocking the road to death’s door. Nature reviews Molecular cell biology.20023640110Epub 2002/06/04.
  47. 47.VucicDFairbrotherW. JThe inhibitor of apoptosis proteins as therapeutic targets in cancer. Clinical cancer research : an official journal of the American Association for Cancer Research.2007132059956000Epub 2007/10/20.
  48. 48.ImotoIYangZ. QPimkhaokhamATsudaHShimadaYImamuraMet alIdentification of cIAP1 as a candidate target gene within an amplicon at 11q22 in esophageal squamous cell carcinomas. Cancer research.20016118662934Epub 2001/09/18.
  49. 49.ZenderLSpectorM. SXueWFlemmingPCordon-cardoCSilkeJet alIdentification and validation of oncogenes in liver cancer using an integrative oncogenomic approach. Cell.20061257125367Epub 2006/07/04.
  50. 50.ImotoITsudaHHirasawaAMiuraMSakamotoMHirohashiSet alExpression of cIAP1, a target for 11q22 amplification, correlates with resistance of cervical cancers to radiotherapy. Cancer research.2002621748606Epub 2002/09/05.
  51. 51.DaiZZhuW. GMorrisonC. DBrenaR. MSmiragliaD. JRavalAet alA comprehensive search for DNA amplification in lung cancer identifies inhibitors of apoptosis cIAP1 and cIAP2 as candidate oncogenes. Human molecular genetics.2003127791801Epub 2003/03/26.
  52. 52.AkagiTMotegiMTamuraASuzukiRHosokawaYSuzukiHet alA novel gene, MALT1 at 18q21, is involved in t(11;18) (q21;q21) found in low-grade B-cell lymphoma of mucosa-associated lymphoid tissue. Oncogene.19991842578594Epub 1999/10/19.
  53. 53.ZhouHDuM. QDixitV. MConstitutiveN. F-k. a. p. p. aB activation by the t(11;18)(q21;q21) product in MALT lymphoma is linked to deregulated ubiquitin ligase activity. Cancer cell.20057542531Epub 2005/05/17.
  54. 54.MorganJ. AYinYBorowskyA. DKuoFNourmandNKoontzJ. Iet alBreakpoints of the t(11;18)(q21;q21) in mucosa-associated lymphoid tissue (MALT) lymphoma lie within or near the previously undescribed gene MALT1 in chromosome 18. Cancer research.19995924620513Epub 2000/01/08.
  55. 55.VarfolomeevEWaysonS. MDixitV. MFairbrotherW. JVucicDThe inhibitor of apoptosis protein fusion c-IAP2.MALT1 stimulates NF-kappaB activation independently of TRAF1 AND TRAF2. The Journal of biological chemistry.200628139290229Epub 2006/08/08.
  56. 56.RampUKriegTCaliskanEMahotkaCEbertTWillersRet alXIAP expression is an independent prognostic marker in clear-cell renal carcinomas. Hum Pathol.200435810228Epub 2004/08/07.
  57. 57.LiMSongTYinZ. FNaY. QXIAP as a prognostic marker of early recurrence of nonmuscular invasive bladder cancer. Chinese medical journal.2007120646973Epub 2007/04/19.
  58. 58.XiangGWenXWangHChenKLiuHExpression of X-linked inhibitor of apoptosis protein in human colorectal cancer and its correlation with prognosis. Journal of surgical oncology.2009100870812Epub 2009/09/25.
  59. 59.ShiY. HDingW. XZhouJHeJ. YXuYGambottoA. Aet alExpression of X-linked inhibitor-of-apoptosis protein in hepatocellular carcinoma promotes metastasis and tumor recurrence. Hepatology.2008482497507Epub 2008/07/31.
  60. 60.ZhangYZhuJTangYLiFZhouHPengBet alX-linked inhibitor of apoptosis positive nuclear labeling: a new independent prognostic biomarker of breast invasive ductal carcinoma. Diagnostic pathology.2011Epub 2011/06/08.
  61. 61.SeligsonD. BHongoFHuerta-yepezSMizutaniYMikiTYuHet alExpression of X-linked inhibitor of apoptosis protein is a strong predictor of human prostate cancer recurrence. Clinical cancer research : an official journal of the American Association for Cancer Research.20071320605663Epub 2007/10/20.
  62. 62.YanHYuJWangRJiangSZhuKMuDet alPrognostic value of Smac expression in rectal cancer patients treated with neoadjuvant therapy. Med Oncol.201229116873Epub 2011/01/26.
  63. 63.SekimuraAKonishiAMizunoKKobayashiYSasakiHYanoMet alExpression of Smac/DIABLO is a novel prognostic marker in lung cancer. Oncol Rep.2004114797802Epub 2004/03/11.
  64. 64.KempkensteffenCJagerTBubJWeikertSHinzSChristophFet alThe equilibrium of XIAP and Smac/DIABLO expression is gradually deranged during the development and progression of testicular germ cell tumours. Int J Androl.200730547683Epub 2007/02/15.
  65. 65.BaoS. TGuiS. QLinM. SRelationship between expression of Smac and Survivin and apoptosis of primary hepatocellular carcinoma. Hepatobiliary Pancreat Dis Int.2006545803Epub 2006/11/07.
  66. 66.YanYMahotkaCHeikausSShibataTWethkampNLiebmannJet alDisturbed balance of expression between XIAP and Smac/DIABLO during tumour progression in renal cell carcinomas. Br J Cancer.2004917134957Epub 2004/08/26.
  67. 67.KashkarHHaefsCShinHHamilton-dutoitS. JSalvesenG. SKronkeMet alXIAP-mediated caspase inhibition in Hodgkin’s lymphoma-derived B cells. The Journal of experimental medicine.200319823417Epub 2003/07/23.
  68. 68.KashkarHSeegerJ. MHombachADeggerichAYazdanpanahBUtermohlenOet alXIAP targeting sensitizes Hodgkin lymphoma cells for cytolytic T-cell attack. Blood.200610810343440Epub 2006/07/27.
  69. 69.FuldaSWickWWellerMDebatinK. MSmac agonists sensitize for Apo2L/TRAIL- or anticancer drug-induced apoptosis and induce regression of malignant glioma in vivo. Nat Med.20028880815Epub 2002/07/16.
  70. 70.PardoO. ELesayAArcaroALopesRNgB. LWarneP. Het alFibroblast growth factor 2-mediated translational control of IAPs blocks mitochondrial release of Smac/DIABLO and apoptosis in small cell lung cancer cells. Mol Cell Biol.20032321760010Epub 2003/10/16.
  71. 71.LiLThomasR. MSuzukiHDe BrabanderJ. KWangXHarranP. GA small molecule Smac mimic potentiates TRAIL- and TNFalpha-mediated cell death. Science.2004305568914714Epub 2004/09/09.
  72. 72.SunHNikolovska-coleskaZYangC. YXuLTomitaYKrajewskiKet alStructure-based design, synthesis, and evaluation of conformationally constrained mimetics of the second mitochondria-derived activator of caspase that target the X-linked inhibitor of apoptosis protein/caspase-9 interaction site. Journal of medicinal chemistry.20044717414750Epub 2004/08/06.
  73. 73.SunHNikolovska-coleskaZYangC. YXuLLiuMTomitaYet alStructure-based design of potent, conformationally constrained Smac mimetics. J Am Chem Soc.200412651166867Epub 2004/12/23.
  74. 74.SunHNikolovska-coleskaZLuJQiuSYangC. YGaoWet alDesign, synthesis, and evaluation of a potent, cell-permeable, conformationally constrained second mitochondria derived activator of caspase (Smac) mimetic. Journal of medicinal chemistry.20064926791620Epub 2006/12/22.
  75. 75.DaiYLiuMTangWDesanoJBursteinEDavisMet alMolecularly targeted radiosensitization of human prostate cancer by modulating inhibitor of apoptosis. Clinical cancer research : an official journal of the American Association for Cancer Research.20081423770110Epub 2008/12/03.
  76. 76.DaiYLiuMTangWLiYLianJLawrenceT. Set alA Smac-mimetic sensitizes prostate cancer cells to TRAIL-induced apoptosis via modulating both IAPs and NF-kappaB. BMC Cancer.2009Epub 2009/11/10.
  77. 77.SchimmerA. DWelshKPinillaCWangZKrajewskaMBonneauM. Jet alSmall-molecule antagonists of apoptosis suppressor XIAP exhibit broad antitumor activity. Cancer cell.2004512535Epub 2004/01/30.
  78. 78.PetersenS. LWangLYalcin-chinALiLPeytonMMinnaJet alAutocrine TNFalpha signaling renders human cancer cells susceptible to Smac-mimetic-induced apoptosis. Cancer cell.200712544556Epub 2007/11/13.
  79. 79.NdubakuCVarfolomeevEWangLZobelKLauKElliottL. Oet alAntagonism of c-IAP and XIAP proteins is required for efficient induction of cell death by small-molecule IAP antagonists. ACS chemical biology.20094755766Epub 2009/06/06.
  80. 80.Nikolovska-coleskaZXuLHuZTomitaYLiPRollerP. Pet alDiscovery of embelin as a cell-permeable, small-molecular weight inhibitor of XIAP through structure-based computational screening of a traditional herbal medicine three-dimensional structure database. Journal of medicinal chemistry.20044710243040Epub 2004/04/30.
  81. 81.DaiYDesanoJQuYTangWMengYLawrenceT. Set alNatural IAP inhibitor Embelin enhances therapeutic efficacy of ionizing radiation in prostate cancer. American journal of cancer research.20111212843Epub 2011/08/02.
  82. 82.ZhangSDingFLuoAChenAYuZRenSet alXIAP is highly expressed in esophageal cancer and its downregulation by RNAi sensitizes esophageal carcinoma cell lines to chemotherapeutics. Cancer Biol Ther.20076697380Epub 2007/07/06.
  83. 83.McmanusD. CLefebvreC. ACherton-horvatGSt-jeanMKandimallaE. RAgrawalSet alLoss of XIAP protein expression by RNAi and antisense approaches sensitizes cancer cells to functionally diverse chemotherapeutics. Oncogene.20042349810517Epub 2004/09/21.
  84. 84.CaoCMuYHallahanD. ELuBXIAP and survivin as therapeutic targets for radiation sensitization in preclinical models of lung cancer. Oncogene.20042342704752Epub 2004/07/20.
  85. 85.DeanEJodrellDConnollyKDansonSJolivetJDurkinJet alPhase I trial of AEG35156 administered as a 7-day and 3-day continuous intravenous infusion in patients with advanced refractory cancer. J Clin Oncol.2009271016606Epub 2009/02/25.
  86. 86.MahadevanDChalasaniPRensvoldDKurtinSPretzingerCJolivetJet alPhase I Trial of AEG35156 an Antisense Oligonucleotide to XIAP Plus Gemcitabine in Patients With Metastatic Pancreatic Ductal Adenocarcinoma. Am J Clin Oncol.2012Epub 2012/03/24.
  87. 87.SchimmerA. DEsteyE. HBorthakurGCarterB. ZSchillerG. JTallmanM. Set alPhase I/II trial of AEG35156 X-linked inhibitor of apoptosis protein antisense oligonucleotide combined with idarubicin and cytarabine in patients with relapsed or primary refractory acute myeloid leukemia. J Clin Oncol.2009272847416Epub 2009/08/05.
  88. 88.KarnakDXuLChemosensitization of prostate cancer by modulating Bcl-2 family proteins. Current drug targets.2010c06aabad-8fbd-e7ce-4422fb085eb07a5c):699-1406.
  89. 89.AdamsJ. MCorySThe Bcl-2 apoptotic switch in cancer development and therapy. Oncogene.2007269132437Epub 2007/02/27.
  90. 90.KroemerGGalluzziLBrennerCMitochondrial membrane permeabilization in cell death. Physiol Rev.200787199163Epub 2007/01/24.
  91. 91.YouleR. JStrasserAThe BCL-2 protein family: opposing activities that mediate cell death. Nature reviews Molecular cell biology.2008914759Epub 2007/12/22.
  92. 92.ChanS. LYuV. CProteins of the bcl-2 family in apoptosis signalling: from mechanistic insights to therapeutic opportunities. Clinical and experimental pharmacology & physiology.200431311928Epub 2004/03/11.
  93. 93.MuchmoreS. WSattlerMLiangHMeadowsR. PHarlanJ. EYoonH. Set alX-ray and NMR structure of human Bcl-xL, an inhibitor of programmed cell death. Nature.1996381658033541Epub 1996/05/23.
  94. 94.SattlerMLiangHNettesheimDMeadowsR. PHarlanJ. EEberstadtMet alStructure of Bcl-xL-Bak peptide complex: recognition between regulators of apoptosis. Science.199727553029836Epub 1997/02/14.
  95. 95.ZhuWCowieAWasfyG. WPennL. ZLeberBAndrewsD. WBcl-2 mutants with restricted subcellular location reveal spatially distinct pathways for apoptosis in different cell types. The EMBO journal.19961516413041Epub 1996/08/15.
  96. 96.EvanG. IWyllieA. HGilbertC. SLittlewoodT. DLandHBrooksMet alInduction of apoptosis in fibroblasts by c-myc protein. Cell.199269111928Epub 1992/04/03.
  97. 97.OdaEOhkiRMurasawaHNemotoJShibueTYamashitaTet alNoxa, a BH3-only member of the Bcl-2 family and candidate mediator of53apoptosis. Science.2000Epub 2000/05/12.
  98. 98.NakanoKVousdenK. HPUMA, a novel proapoptotic gene, is induced by53Molecular cell.2001Epub 2001/07/21.
  99. 99.EskesRDesagherSAntonssonBMartinouJ. CBid induces the oligomerization and insertion of Bax into the outer mitochondrial membrane. Mol Cell Biol.200020392935Epub 2000/01/11.
  100. 100.KorsmeyerS. JWeiM. CSaitoMWeilerSOhK. JSchlesingerP. HPro-apoptotic cascade activates BID, which oligomerizes BAK or BAX into pores that result in the release of cytochrome c. Cell death and differentiation.2000712116673Epub 2001/02/15.
  101. 101.WeiM. CZongW. XChengE. HLindstenTPanoutsakopoulouVRossA. Jet alProapoptotic BAX and BAK: a requisite gateway to mitochondrial dysfunction and death. Science.2001292551772730Epub 2001/04/28.
  102. 102.ZongW. XLindstenTRossA. JMacGregor GR, Thompson CB. BH3-only proteins that bind pro-survival Bcl-2 family members fail to induce apoptosis in the absence of Bax and Bak. Genes & development.2001151214816Epub 2001/06/19.
  103. 103.WeiM. CLindstenTMoothaV. KWeilerSGrossAAshiyaMet altBID, a membrane-targeted death ligand, oligomerizes BAK to release cytochrome c. Genes & development.20001416206071Epub 2000/08/19.
  104. 104.KuwanaTMackeyM. RPerkinsGEllismanM. HLatterichMSchneiterRet alBid, Bax, and lipids cooperate to form supramolecular openings in the outer mitochondrial membrane. Cell.2002111333142Epub 2002/11/07.
  105. 105.KimHRafiuddin-shahMTuH. CJeffersJ. RZambettiG. PHsiehJ. Jet alHierarchical regulation of mitochondrion-dependent apoptosis by BCL-2 subfamilies. Nature cell biology.2006812134858Epub 2006/11/23.
  106. 106.SharpeJ. CArnoultDYouleR. JControl of mitochondrial permeability by Bcl-2 family members. Biochim Biophys Acta.2004Epub 2004/03/05.
  107. 107.CertoMDel Gaizo Moore V, Nishino M, Wei G, Korsmeyer S, Armstrong SA, et al. Mitochondria primed by death signals determine cellular addiction to antiapoptotic BCL-2 family members. Cancer cell.20069535165Epub 2006/05/16.
  108. 108.ChengE. HWeiM. CWeilerSFlavellR. AMakT. WLindstenTet alBCL-2, BCL-X(L) sequester BH3 domain-only molecules preventing BAX- and BAK-mediated mitochondrial apoptosis. Molecular cell.20018370511Epub 2001/10/05.
  109. 109.KuwanaTBouchier-hayesLChipukJ. EBonzonCSullivanB. AGreenD. Ret alBH3 domains of BH3-only proteins differentially regulate Bax-mediated mitochondrial membrane permeabilization both directly and indirectly. Molecular cell.200517452535Epub 2005/02/22.
  110. 110.LetaiASorcinelliM. DBeardCKorsmeyerS. JAntiapoptotic BCL-2 is required for maintenance of a model leukemia. Cancer cell.2004632419Epub 2004/09/24.
  111. 111.WillisS. NChenLDewsonGWeiANaikEFletcherJ. Iet alProapoptotic Bak is sequestered by Mcl-1 and Bcl-xL, but not Bcl-2, until displaced by BH3-only proteins. Genes & development.200519111294305Epub 2005/05/20.
  112. 112.WillisS. NFletcherJ. IKaufmannTVan DelftM. FChenLCzabotarP. Eet alApoptosis initiated when BH3 ligands engage multiple Bcl-2 homologs, not Bax or Bak. Science.200731558138569Epub 2007/02/10.
  113. 113.HsuY. TWolterK. GYouleR. JCytosol-to-membrane redistribution of Bax and Bcl-X(L) during apoptosis. Proceedings of the National Academy of Sciences of the United States of America.1997948366872Epub 1997/04/15.
  114. 114.HsuY. TYouleR. JNonionic detergents induce dimerization among members of the Bcl-2 family. The Journal of biological chemistry.1997272211382934Epub 1997/05/23.
  115. 115.DlugoszP. JBillenL. PAnnisM. GZhuWZhangZLinJet alBcl-2 changes conformation to inhibit Bax oligomerization. The EMBO journal.20062511228796Epub 2006/04/28.
  116. 116.LetaiABassikM. CWalenskyL. DSorcinelliM. DWeilerSKorsmeyerS. JDistinctB. Hdomains either sensitize or activate mitochondrial apoptosis, serving as prototype cancer therapeutics. Cancer cell.20022318392Epub 2002/09/21.
  117. 117.DengJCarlsonNTakeyamaKDal Cin P, Shipp M, Letai A. BH3 profiling identifies three distinct classes of apoptotic blocks to predict response to ABT-737 and conventional chemotherapeutic agents. Cancer cell.200712217185Epub 2007/08/19.
  118. 118.TsujimotoYCossmanJJaffeECroceC. MInvolvement of the bcl-2 gene in human follicular lymphoma. Science.1985228470614403Epub 1985/06/21.
  119. 119.HanadaMDeliaDAielloAStadtmauerEReedJ. Cbcl-2 gene hypomethylation and high-level expression in B-cell chronic lymphocytic leukemia. Blood.199382618208Epub 1993/09/15.
  120. 120.YipK. WReedJ. CBcl-2 family proteins and cancer. Oncogene.200827506398406Epub 2008/10/29.
  121. 121.KellyP. NStrasserAThe role of Bcl-2 and its pro-survival relatives in tumourigenesis and cancer therapy. Cell death and differentiation.2011189141424Epub 2011/03/19.
  122. 122.KrajewskaMKrajewskiSEpsteinJ. IShabaikASauvageotJSongKet alImmunohistochemical analysis of bcl-2, bax, bcl-X, and mcl-1 expression in prostate cancers. The American journal of pathology.19961485156776Epub 1996/05/01.
  123. 123.KonoplevaMZhaoSHuWJiangSSnellVWeidnerDet alThe anti-apoptotic genes Bcl-X(L) and Bcl-2 are over-expressed and contribute to chemoresistance of non-proliferating leukaemic CD34+ cells. British journal of haematology.2002118252134Epub 2002/07/26.
  124. 124.ReedJ. CBcl-2 family proteins: strategies for overcoming chemoresistance in cancer. Adv Pharmacol.19974150132Epub 1997/01/01.
  125. 125.RampinoNYamamotoHIonovYLiYSawaiHReedJ. Cet alSomatic frameshift mutations in the BAX gene in colon cancers of the microsatellite mutator phenotype. Science.199727553029679Epub 1997/02/14.
  126. 126.ZhangLYuJParkB. HKinzlerK. WVogelsteinBRole of BAX in the apoptotic response to anticancer agents. Science.2000290549398992Epub 2000/11/04.
  127. 127.GarrisonS. PJeffersJ. RYangCNilssonJ. AHallM. ARehgJ. Eet alSelection against PUMA gene expression in Myc-driven B-cell lymphomagenesis. Mol Cell Biol.200828175391402Epub 2008/06/25.
  128. 128.KarstA. MDaiD. LMartinkaMLiGPUMA expression is significantly reduced in human cutaneous melanomas. Oncogene.200524611116Epub 2005/02/04.
  129. 129.MiyashitaTKrajewskiSKrajewskaMWangH. GLinH. KLiebermannD. Aet alTumor suppressor53is a regulator of bcl-2 and bax gene expression in vitro and in vivo. Oncogene.1994Epub 1994/06/01.
  130. 130.MiyashitaTReedJ. CTumor suppressor53is a direct transcriptional activator of the human bax gene. Cell.1995Epub 1995/01/27.
  131. 131.SaxJ. KFeiPMurphyM. EBernhardEKorsmeyerS. JEl-DeiryW. SBID regulation by53contributes to chemosensitivity. Nature cell biology.2002Epub 2002/10/29.
  132. 132.YuJWangZKinzlerK. WVogelsteinBZhangLPUMA mediates the apoptotic response to53in colorectal cancer cells. Proceedings of the National Academy of Sciences of the United States of America.2003Epub 2003/02/08.
  133. 133.ChipukJ. EKuwanaTBouchier-hayesLDroinN. MNewmeyerD. DSchulerMet alDirect activation of Bax by53mediates mitochondrial membrane permeabilization and apoptosis. Science.2004Epub 2004/02/14.
  134. 134.DengXGaoFFlaggTAndersonJMayW. SBcl2’s flexible loop domain regulates53binding and survival. Mol Cell Biol.2006Epub 2006/06/02.
  135. 135.ShangarySJohnsonD. ERecent advances in the development of anticancer agents targeting cell death inhibitors in the Bcl-2 protein family. Leukemia : official journal of the Leukemia Society of America, Leukemia Research Fund, UK.2003178147081Epub 2003/07/30.
  136. 136.WangJ. LLiuDZhangZ. JShanSHanXSrinivasulaS. Met alStructure-based discovery of an organic compound that binds Bcl-2 protein and induces apoptosis of tumor cells. Proceedings of the National Academy of Sciences of the United States of America.2000971371249Epub 2000/06/22.
  137. 137.WangSYangDLippmanM. ETargeting Bcl-2 and Bcl-XL with nonpeptidic small-molecule antagonists. Seminars in oncology.2003Suppl 16):133-42. Epub 2003/11/13.
  138. 138.ManeroFGautierFGallenneTCauquilNGreeDCartronP. Fet alThe small organic compound HA14-1 prevents Bcl-2 interaction with Bax to sensitize malignant glioma cells to induction of cell death. Cancer research.2006665275764Epub 2006/03/03.
  139. 139.AnJChenYHuangZCritical upstream signals of cytochrome C release induced by a novel Bcl-2 inhibitor. The Journal of biological chemistry.2004279181913340Epub 2004/02/18.
  140. 140.NguyenMMarcellusR. CRoulstonAWatsonMSerfassLMurthy Madiraju SR, et al. Small molecule obatoclax (GX15-070) antagonizes MCL-1 and overcomes MCL-1-mediated resistance to apoptosis. Proceedings of the National Academy of Sciences of the United States of America.200710449195127Epub 2007/11/28.
  141. 141.OltersdorfTElmoreS. WShoemakerA. RArmstrongR. CAugeriD. JBelliB. Aet alAn inhibitor of Bcl-2 family proteins induces regression of solid tumours. Nature.2005435704267781Epub 2005/05/20.
  142. 142.WalenskyL. DFrom mitochondrial biology to magic bullet: navitoclax disarms BCL-2 in chronic lymphocytic leukemia. J Clin Oncol.20123055547Epub 2011/12/21.
  143. 143.RobertsA. WSeymourJ. FBrownJ. RWierdaW. GKippsT. JKhawS. Let alSubstantial susceptibility of chronic lymphocytic leukemia to BCL2 inhibition: results of a phase I study of navitoclax in patients with relapsed or refractory disease. J Clin Oncol.201230548896Epub 2011/12/21.
  144. 144.GandhiLCamidgeD. RRibeiro de Oliveira M, Bonomi P, Gandara D, Khaira D, et al. Phase I study of Navitoclax (ABT-263), a novel Bcl-2 family inhibitor, in patients with small-cell lung cancer and other solid tumors. J Clin Oncol.201129790916Epub 2011/02/02.
  145. 145.MengYTangWDaiYWuXLiuMJiQet alNatural BH3 mimetic (-)-gossypol chemosensitizes human prostate cancer via Bcl-xL inhibition accompanied by increase of Puma and Noxa. Molecular cancer therapeutics.2008772192202Epub 2008/07/23.
  146. 146.KitadaSKressC. LKrajewskaMJiaLPellecchiaMReedJ. CBcl-2 antagonist apogossypol (NSC736630) displays single-agent activity in Bcl-2-transgenic mice and has superior efficacy with less toxicity compared with gossypol (NSC19048). Blood.2008111632119Epub 2008/01/19.
  147. 147.SunYWuJAboukameelABanerjeeSArnoldA. AChenJet alApogossypolone, a nonpeptidic small molecule inhibitor targeting Bcl-2 family proteins, effectively inhibits growth of diffuse large cell lymphoma cells in vitro and in vivo. Cancer Biol Ther.200879141826Epub 2008/09/05.
  148. 148.Al-katibA. MSunYGoustinA. SAzmiA. SChenBAboukameelAet alSMI of Bcl-2 TW-37 is active across a spectrum of B-cell tumors irrespective of their proliferative and differentiation status. Journal of hematology & oncology.2009Epub 2009/02/18.
  149. 149.TammIAEG-35156, an antisense oligonucleotide against X-linked inhibitor of apoptosis for the potential treatment of cancer. Current opinion in investigational drugs.20089663846Epub 2008/06/03.
  150. 150.ShimizuSKanasekiTMizushimaNMizutaTArakawa-kobayashiSThompsonC. Bet alRole of Bcl-2 family proteins in a non-apoptotic programmed cell death dependent on autophagy genes. Nature cell biology.200461212218Epub 2004/11/24.
  151. 151.ObersteinAJeffreyP. DShiYCrystal structure of the Bcl-XL-Beclin 1 peptide complex: Beclin 1 is a novel BH3-only protein. The Journal of biological chemistry.2007282171312332Epub 2007/03/06.
  152. 152.PattingreSTassaAQuXGarutiRLiangX. HMizushimaNet alBcl-2 antiapoptotic proteins inhibit Beclin 1-dependent autophagy. Cell.2005122692739Epub 2005/09/24.
  153. 153.ChenSRehmanSZhangWWenAYaoLZhangJAutophagy is a therapeutic target in anticancer drug resistance. Biochimica et biophysica acta.2010c2ea68e465ce-52d4-86d3-fb085eb006dc):220-9.
  154. 154.MizushimaNLevineBCuervoA. MKlionskyD. JAutophagy fights disease through cellular self-digestion. Nature.20084517182106975Epub 2008/02/29.
  155. 155.LucocqJWalkerDEvidence for fusion between multilamellar endosomes and autophagosomes in HeLa cells. European journal of cell biology.199772430713Epub 1997/04/01.
  156. 156.Vazquez-martinAOliveras-ferrarosCMenendezJAutophagy facilitates the development of breast cancer resistance to the anti-HER2 monoclonal antibody trastuzumab. PloS one.2009fdb94e203b9-d18b-5b7d-9bf165abe7df).
  157. 157.AbedinM. JWangDMcdonnellM. ALehmannUKelekarAAutophagy delays apoptotic death in breast cancer cells following DNA damage. Cell death and differentiation.200714350010Epub 2006/09/23.
  158. 158.LiuDYangYLiuQWangJInhibition of autophagy by 3-MA potentiates cisplatin-induced apoptosis in esophageal squamous cell carcinoma cells. Med Oncol.201128110511Epub 2009/12/31.
  159. 159.RenJ. HHeW. SNongLZhuQ. YHuKZhangR. Get alAcquired cisplatin resistance in human lung adenocarcinoma cells is associated with enhanced autophagy. Cancer biotherapy & radiopharmaceuticals.20102517580Epub 2010/03/02.
  160. 160.Djavaheri-mergnyMAmelottiMMathieuJBesanconFBauvyCCodognoPRegulation of autophagy by NFkappaB transcription factor and reactives oxygen species. Autophagy.2007343902Epub 2007/05/02.
  161. 161.ItakuraEMizushimaNAtg14 and UVRAG: mutually exclusive subunits of mammalian Beclin 1-PI3K complexes. Autophagy.2009545346Epub 2009/02/19.
  162. 162.TrocoliADjavaheri-mergnyMThe complex interplay between autophagy and NF-kappaB signaling pathways in cancer cells. Am J Cancer Res.20111562949Epub 2011/10/14.
  163. 163.ShenSKeppOKroemerGThe end of autophagic cell death? Autophagy.20128113Epub 2011/11/16.
  164. 164.YueZJinSYangCLevineA. JHeintzNBeclin 1, an autophagy gene essential for early embryonic development, is a haploinsufficient tumor suppressor. Proceedings of the National Academy of Sciences of the United States of America.2003100251507782Epub 2003/12/06.
  165. 165.QuXYuJBhagatGFuruyaNHibshooshHTroxelAet alPromotion of tumorigenesis by heterozygous disruption of the beclin 1 autophagy gene. The Journal of clinical investigation.200311212180920Epub 2003/11/26.
  166. 166.JungC. HJunC. BRoS. HKimY. MOttoN. MCaoJet alULK-Atg13-FIP200 complexes mediate mTOR signaling to the autophagy machinery. Mol Biol Cell.200920719922003Epub 2009/02/20.
  167. 167.HosokawaNHaraTKaizukaTKishiCTakamuraAMiuraYet alNutrient-dependent mTORC1 association with the ULK1Atg13-FIP200 complex required for autophagy. Molecular biology of the cell.2009ccf76-f86a-2e4b-8ca9-af545e417259):1981-2072.
  168. 168.HaraTTakamuraAKishiCIemuraSNatsumeTGuanJ. Let alFIP200, a ULK-interacting protein, is required for autophagosome formation in mammalian cells. J Cell Biol.20081813497510Epub 2008/04/30.
  169. 169.HeCLevineBThe Beclin 1 interactome. Current opinion in cell biology.20102221409Epub 2010/01/26.
  170. 170.ObaraKOhsumiYAtg14: a key player in orchestrating autophagy. Int J Cell Biol.2011Epub 2011/10/21.
  171. 171.AxeE. LWalkerS. AManifavaMChandraPRoderickH. LHabermannAet alAutophagosome formation from membrane compartments enriched in phosphatidylinositol 3-phosphate and dynamically connected to the endoplasmic reticulum. J Cell Biol.20081824685701Epub 2008/08/30.
  172. 172.FanWNassiriAZhongQAutophagosome targeting and membrane curvature sensing by Barkor/Atg14(L). Proceedings of the National Academy of Sciences of the United States of America.201110819776974Epub 2011/04/27.
  173. 173.MizushimaNSugitaHYoshimoriTOhsumiYA new protein conjugation system in human. The counterpart of the yeast Apg12p conjugation system essential for autophagy. J Biol Chem.1998273513388992Epub 1998/12/16.
  174. 174.MizushimaNYoshimoriTOhsumiYMouse Apg10 as an Apg12-conjugating enzyme: analysis by the conjugation-mediated yeast two-hybrid method. FEBS Lett.200253234504Epub 2002/12/17.
  175. 175.TanidaITanida-miyakeEUenoTKominamiEThe human homolog of Saccharomyces cerevisiae Apg7p is a Protein-activating enzyme for multiple substrates including human Apg12p, GATE-16, GABARAP, and MAP-LC3. J Biol Chem.2001276317016Epub 2000/11/30.
  176. 176.MizushimaNKumaAKobayashiYYamamotoAMatsubaeMTakaoTet alMouse Apg16L, a novel WD-repeat protein, targets to the autophagic isolation membrane with the Apg12Apg5 conjugate. J Cell Sci.2003Pt 9):1679-88. Epub 2003/04/01.
  177. 177.IchimuraYKirisakoTTakaoTSatomiYShimonishiYIshiharaNet alA ubiquitin-like system mediates protein lipidation. Nature.2000408681148892Epub 2000/12/02.
  178. 178.KirisakoTIchimuraYOkadaHKabeyaYMizushimaNYoshimoriTet alThe reversible modification regulates the membrane-binding state of Apg8/Aut7 essential for autophagy and the cytoplasm to vacuole targeting pathway. The Journal of cell biology.2000151226376Epub 2000/10/19.
  179. 179.FujitaNItohTOmoriHFukudaMNodaTYoshimoriTThe Atg16L complex specifies the site of LC3 lipidation for membrane biogenesis in autophagy. Mol Biol Cell.20081952092100Epub 2008/03/07.
  180. 180.JagerSBucciCTanidaIUenoTKominamiESaftigPet alRole for Rab7 in maturation of late autophagic vacuoles. J Cell Sci.2004Pt 20):4837-48. Epub 2004/09/02.
  181. 181.KimuraSNodaTYoshimoriTDissection of the autophagosome maturation process by a novel reporter protein, tandem fluorescent-tagged LC3. Autophagy.20073545260Epub 2007/05/31.
  182. 182.NobukuniTJoaquinMRoccioMDannS. GKimS. YGulatiPet alAmino acids mediate mTOR/raptor signaling through activation of class 3 phosphatidylinositol 3OH-kinase. Proceedings of the National Academy of Sciences of the United States of America.2005102401423843Epub 2005/09/24.
  183. 183.LumJ. JBauerD. EKongMHarrisM. HLiCLindstenTet alGrowth factor regulation of autophagy and cell survival in the absence of apoptosis. Cell.2005120223748Epub 2005/02/01.
  184. 184.LiYCorradettiM. NInokiKGuanK. LTSC2: filling the GAP in the mTOR signaling pathway. Trends in biochemical sciences.2004291328Epub 2004/01/20.
  185. 185.LiJYenCLiawDPodsypaninaKBoseSWangS. Iet alPTEN, a putative protein tyrosine phosphatase gene mutated in human brain, breast, and prostate cancer. Science.1997275530819437Epub 1997/03/28.
  186. 186.StommelJ. MKimmelmanA. CYingHNabioullinRPonugotiA. HWiedemeyerRet alCoactivation of receptor tyrosine kinases affects the response of tumor cells to targeted therapies. Science.2007318584828790Epub 2007/09/18.
  187. 187.ChenC. TKimHLiskaDGaoSChristensenJ. GWeiserM. RMET activation mediates resistance to lapatinib inhibition of HER2-amplified gastric cancer cells. Molecular cancer therapeutics.20121136609Epub 2012/01/13.
  188. 188.GwinnD. MShackelfordD. BEganD. FMihaylovaM. MMeryAVasquezD. Set alAMPK phosphorylation of raptor mediates a metabolic checkpoint. Molecular cell.200830221426Epub 2008/04/29.
  189. 189.PapandreouILimA. LLaderouteKDenkoN. CHypoxia signals autophagy in tumor cells via AMPK activity, independent of HIF-1, BNIP3, and BNIP3L. Cell death and differentiation.20081510157281Epub 2008/06/14.
  190. 190.Hoyer-hansenMJaattelaMAMP-activated protein kinase: a universal regulator of autophagy? Autophagy.2007343813Epub 2007/04/26.
  191. 191.KimJKunduMViolletBGuanK. LAMPK and mTOR regulate autophagy through direct phosphorylation of Ulk1. Nature cell biology.201113213241Epub 2011/01/25.
  192. 192.CrightonDWilkinsonSOPreyJSyedNSmithPHarrisonPR, et al. DRAM, a53modulator of autophagy, is critical for apoptosis. Cell.2006Epub 2006/07/15.
  193. 193.BudanovA. VKarinM. ptarget genes sestrin1 and sestrin2 connect genotoxic stress and mTOR signaling. Cell.2008134345160Epub 2008/08/12.
  194. 194.GaoWShenZShangLWangXUpregulation of human autophagy-initiation kinase ULK1 by tumor suppressor53contributes to DNA-damage-induced cell death. Cell death and differentiation.2011Epub 2011/04/09.
  195. 195.CarterB. ZMakD. HMorrisS. JBorthakurGEsteyEByrdA. Let alXIAP antisense oligonucleotide (AEG35156) achieves target knockdown and induces apoptosis preferentially in CD34+38- cells in a phase 1/2 study of patients with relapsed/refractory AML. Apoptosis : an international journal on programmed cell death.20111616774Epub 2010/10/13.
  196. 196.RomJVon MinckwitzGEiermannWSievertMSchleheBMarmeFet alOblimersen combined with docetaxel, adriamycin and cyclophosphamide as neo-adjuvant systemic treatment in primary breast cancer: final results of a multicentric phase I study. Annals of oncology : official journal of the European Society for Medical Oncology / ESMO.200819101698705Epub 2008/05/15.
  197. 197.LiangX. HJacksonSSeamanMBrownKKempkesBHibshooshHet alInduction of autophagy and inhibition of tumorigenesis by beclin 1. Nature.199940267626726Epub 1999/12/22.
  198. 198.MiraccoCCosciEOliveriGLuziPPacentiLMonciattiIet alProtein and mRNA expression of autophagy gene Beclin 1 in human brain tumours. Int J Oncol.200730242936Epub 2007/01/05.
  199. 199.ShiY. HDingZ. BZhouJQiuS. JFanJPrognostic significance of Beclin 1-dependent apoptotic activity in hepatocellular carcinoma. Autophagy.2009533802Epub 2009/01/16.
  200. 200.AhnC. HJeongE. GLeeJ. WKimM. SKimS. HKimS. Set alExpression of beclin-1, an autophagy-related protein, in gastric and colorectal cancers. APMIS : acta pathologica, microbiologica, et immunologica Scandinavica.20071151213449Epub 2008/01/11.
  201. 201.AkarUChaves-reyezABarriaMTariASanguinoAKondoYet alSilencing of Bcl-2 expression by small interfering RNA induces autophagic cell death in MCF-7 breast cancer cells. Autophagy.20084566979Epub 2008/04/22.
  202. 202.Di Bartolomeo SCorazzari M, Nazio F, Oliverio S, Lisi G, Antonioli M, et al. The dynamic interaction of AMBRA1 with the dynein motor complex regulates mammalian autophagy. The Journal of cell biology.2010191115568Epub 2010/10/06.
  203. 203.StrappazzonFVietri-rudanMCampelloSNazioFFlorenzanoFFimiaG. Met alMitochondrial BCL-2 inhibits AMBRA1-induced autophagy. The EMBO journal.20113071195208Epub 2011/03/02.
  204. 204.TangDKangRChehC. WLiveseyK. MLiangXSchapiroN. Eet alHMGB1 release and redox regulates autophagy and apoptosis in cancer cells. Oncogene.201029385299310Epub 2010/07/14.
  205. 205.KangRLiveseyKZehHLozeMTangDHMGB1: a novel Beclin1binding protein active in autophagy. Autophagy.2010a93e1e-c67d-83e4-87ca-9aea803ddcd1):1209-20.
  206. 206.JankuFMcconkeyDHongDKurzrockRAutophagy as a target for anticancer therapy. Nature reviews Clinical oncology.2011ee0c5394e2da-32d4-597e-9b23fbfbab39):528-67.
  207. 207.WirawanEVande Walle L, Kersse K, Cornelis S, Claerhout S, Vanoverberghe I, et al. Caspase-mediated cleavage of Beclin-1 inactivates Beclin-1induced autophagy and enhances apoptosis by promoting the release of proapoptotic factors from mitochondria. Cell death & disease.2010e18. Epub 2010/01/01.
  208. 208.MaiuriMCriolloATasdemirEVicencioJTajeddineNHickmanJet alBH3only proteins and BH3 mimetics induce autophagy by competitively disrupting the interaction between Beclin 1 and Bcl-2/Bcl-X(L). Autophagy.3(f1536ce7-df32-56eb-f51e-7b9f6f70c571):374-80.
  209. 209.SatohTOkamotoIMiyazakiMMorinagaRTsuyaAHasegawaYet alPhase I study of YM155, a novel survivin suppressant, in patients with advanced solid tumors. Clinical cancer research : an official journal of the American Association for Cancer Research.20091511387280Epub 2009/05/28.
  210. 210.GiacconeGZatloukalPRoubecJFloorKMusilJKutaMet alMulticenter phase II trial of YM155, a small-molecule suppressor of survivin, in patients with advanced, refractory, non-small-cell lung cancer. J Clin Oncol.2009272744816Epub 2009/08/19.
  211. 211.MalikS. AOrhonIMorselliECriolloAShenSMarinoGet alBH3 mimetics activate multiple pro-autophagic pathways. Oncogene.20113037391829Epub 2011/04/05.
  212. 212.HanWPanHChenYSunJWangYLiJet alEGFR tyrosine kinase inhibitors activate autophagy as a cytoprotective response in human lung cancer cells. PloS one.2011e9b519257f-e019-7009-9b903ae72bc4).
  213. 213.SwampillaiA. LSalomoniPShortS. CThe Role of Autophagy in Clinical Practice. Clin Oncol (R Coll Radiol).2011Epub 2011/10/29.
  214. 214.HuangJNiJLiuKYuYXieMKangRet alHMGB1 promotes drug resistance in osteosarcoma. Cancer research.2012f06c5ead-ce77b4c8-b289-95ddcffe93ca):230-8.
  215. 215.ChaachouayHOhneseitPToulanyMKehlbachRMulthoffGRodemannHAutophagy contributes to resistance of tumor cells to ionizing radiation. Radiotherapy and oncology : journal of the European Society for Therapeutic Radiology and Oncology.2011f586ccb512b0-be73-4db2-fb085eb012a7):287-379.
  216. 216.BlommaartE. FKrauseUSchellensJ. PVreeling-sindelarovaHMeijerA. JThe phosphatidylinositol3kinase inhibitors wortmannin and LY294002 inhibit autophagy in isolated rat hepatocytes. European journal of biochemistry / FEBS.1997Epub 1997/01/15.
  217. 217.WuY-TTanH-LShuiGBauvyCHuangQWenkMet alDual role of3methyladenine in modulation of autophagy via different temporal patterns of inhibition on class I and III phosphoinositide 3-kinase. The Journal of biological chemistry.2010c2c6-9a36-95f349333c77):10850-911.
  218. 218.BellodiCLidonniciM. RHamiltonAHelgasonG. VSolieraA. RRonchettiMet alTargeting autophagy potentiates tyrosine kinase inhibitor-induced cell death in Philadelphia chromosome-positive cells, including primary CML stem cells. The Journal of clinical investigation.20091195110923Epub 2009/04/14.
  219. 219.ShenH. MCodognoPAutophagic cell death: Loch Ness monster or endangered species? Autophagy.20117545765Epub 2010/12/15.
  220. 220.SolomonV. RLeeHChloroquine and its analogs: a new promise of an old drug for effective and safe cancer therapies. European journal of pharmacology.2009Epub 2009/10/20.
  221. 221.SwampillaiASalomoniPShortSThe Role of Autophagy in Clinical Practice. Clinical oncology (Royal College of Radiologists (Great Britain)).2011d048bf9-caed-2448-fb085eba55e5).
  222. 222.ThorburnJFrankelAThorburnARegulation of HMGB1 release by autophagy. Autophagy.2009c807524d29e-a4fe-3f06-9a0cae660f50):247-56.
  223. 223.ApetohLGhiringhelliFTesniereACriolloAOrtizCLidereauRet alThe interaction between HMGB1 and TLR4 dictates the outcome of anticancer chemotherapy and radiotherapy. Immunological reviews.20072204759Epub 2007/11/06.
  224. 224.ThorburnJHoritaHRedzicJHansenKFrankelA. EThorburnAAutophagy regulates selective HMGB1 release in tumor cells that are destined to die. Cell death and differentiation.200916117583Epub 2008/10/11.
  225. 225.CaiQSunHPengYLuJNikolovska-coleskaZMceachernDet alA potent and orally active antagonist (SM-406/AT-406) of multiple inhibitor of apoptosis proteins (IAPs) in clinical development for cancer treatment. Journal of medicinal chemistry.2011548271426Epub 2011/03/30.
  226. 226.BaggstromM. QQiYKoczywasMArgirisAJohnsonE. AMillwardM. Jet alA phase II study of AT-101 (Gossypol) in chemotherapy-sensitive recurrent extensive-stage small cell lung cancer. Journal of thoracic oncology : official publication of the International Association for the Study of Lung Cancer.2011610175760Epub 2011/09/16.
  227. 227.ReadyNKarasevaN. AOrlovS. VLuftA. VPopovychOHolmlundJ. Tet alDouble-blind, placebo-controlled, randomized phase 2 study of the proapoptotic agent AT-101 plus docetaxel, in second-line non-small cell lung cancer. Journal of thoracic oncology : official publication of the International Association for the Study of Lung Cancer.2011647815Epub 2011/02/04.
  228. 228.HeistR. SFainJChinnasamiBKhanWMolinaJ. RSequistL. Vet alPhase I/II study of AT-101 with topotecan in relapsed and refractory small cell lung cancer. Journal of thoracic oncology : official publication of the International Association for the Study of Lung Cancer.2010510163743Epub 2010/09/03.
  229. 229.LiuGKellyW. KWildingGLeopoldLBrillKSomerBAn open-label, multicenter, phase I/II study of single-agent AT-101 in men with castrate-resistant prostate cancer. Clinical cancer research : an official journal of the American Association for Cancer Research.200915931726Epub 2009/04/16.
  230. 230.Van PoznakCSeidmanA. DReidenbergM. MMoasserM. MSklarinNVan ZeeKet alOral gossypol in the treatment of patients with refractory metastatic breast cancer: a phase I/II clinical trial. Breast cancer research and treatment.200166323948Epub 2001/08/21.
  231. 231.WilsonWH,OConnorOA,CzuczmanMS,LaCasce AS, Gerecitano JF, Leonard JP, et al. Navitoclax, a targeted high-affinity inhibitor of BCL-2, in lymphoid malignancies: a phase 1 dose-escalation study of safety, pharmacokinetics, pharmacodynamics, and antitumour activity. The lancet oncology. 2010;11(12):1149-59. Epub 2010/11/26.
  232. 232.SchimmerA. DOBrienSKantarjianHBrandweinJChesonB. DMindenMD, et al. A phase I study of the pan bcl-2 family inhibitor obatoclax mesylate in patients with advanced hematologic malignancies. Clinical cancer research : an official journal of the American Association for Cancer Research.200814248295301Epub 2008/12/18.
  233. 233.PaikP. KRudinC. MBrownARizviN. ATakebeNTravisWet alA phase I study of obatoclax mesylate, a Bcl-2 antagonist, plus topotecan in solid tumor malignancies. Cancer chemotherapy and pharmacology.2010666107985Epub 2010/02/19.
  234. 234.OBrienS. MClaxtonD. FCrumpMFaderlSKippsTKeatingMJ, et al. Phase I study of obatoclax mesylate (GX15-070), a small molecule pan-Bcl-2 family antagonist, in patients with advanced chronic lymphocytic leukemia. Blood.20091132299305Epub 2008/10/22.
  235. 235.RomJVon MinckwitzGMarmeFAtasevenBKozianDSievertMet alPhase I study of apoptosis gene modulation with oblimersen within preoperative chemotherapy in patients with primary breast cancer. Annals of oncology : official journal of the European Society for Medical Oncology / ESMO.20092011182935Epub 2009/07/17.
  236. 236.SternbergC. NDumezHVan PoppelHSkonecznaISellaADaugaardGet alDocetaxel plus oblimersen sodium (Bcl-2 antisense oligonucleotide): an EORTC multicenter, randomized phase II study in patients with castration-resistant prostate cancer. Annals of oncology : official journal of the European Society for Medical Oncology / ESMO.200920712649Epub 2009/03/20.
  237. 237.TolcherA. WChiKKuhnJGleaveMPatnaikATakimotoCet alA phase II, pharmacokinetic, and biological correlative study of oblimersen sodium and docetaxel in patients with hormone-refractory prostate cancer. Clinical cancer research : an official journal of the American Association for Cancer Research.20051110385461Epub 2005/05/18.

Written By

Rebecca T. Marquez, Bryan W. Tsao, Nicholas F. Faust and Liang Xu

Submitted: January 11th, 2012Reviewed: December 5th, 2012Published: May 15th, 2013