Role for PKCδ on Apoptosis in the DNA Damage Response

Genotoxic stress induces cell cycle arrest, DNA repair, and apoptotic cell death. The decision by cells either to repair DNA lesions and continue through the cell cycle or to undergo apoptosis is relevant to the incidence of mutagenesis and, subsequently, carcinogenesis. In this regard, incomplete repair of DNA damage prior to replication or mitosis can result in the accumulation of heritable genetic changes. Therapeutic anti-cancer treatments that use genotoxic agents must strike a balance between induction of repair and apoptosis in order to maximize the therapeutic effect. However, the nature of the cellular signaling response that determines cell fate such as survival or death is far from being understood. Certain insights have been derived from the finding that diverse isozymes of the protein kinase C (PKC) family are activated in response to DNA damage. PKC-mediated signaling pathway modulates destiny of cells following genotoxic insults (Yoshida 2007a, Yoshida 2008a). In particular, recent studies have shown that certain isozyme of PKC controls function of the p53 tumor suppressor in induction of cell cycle arrest, DNA repair, and apoptosis. In the past 10 years, understanding the molecular mechanisms of apoptosis mediated by PKC has advanced considerably, and the primary focus of this review is to provide an overview of PKC and p53, its mode of action and its physiological role in DNA damage-induced apoptosis.


Introduction
Genotoxic stress induces cell cycle arrest, DNA repair, and apoptotic cell death.The decision by cells either to repair DNA lesions and continue through the cell cycle or to undergo apoptosis is relevant to the incidence of mutagenesis and, subsequently, carcinogenesis.In this regard, incomplete repair of DNA damage prior to replication or mitosis can result in the accumulation of heritable genetic changes.Therapeutic anti-cancer treatments that use genotoxic agents must strike a balance between induction of repair and apoptosis in order to maximize the therapeutic effect.However, the nature of the cellular signaling response that determines cell fate such as survival or death is far from being understood.Certain insights have been derived from the finding that diverse isozymes of the protein kinase C (PKC) family are activated in response to DNA damage.PKC-mediated signaling pathway modulates destiny of cells following genotoxic insults (Yoshida 2007a, Yoshida 2008a).In particular, recent studies have shown that certain isozyme of PKC controls function of the p53 tumor suppressor in induction of cell cycle arrest, DNA repair, and apoptosis.In the past 10 years, understanding the molecular mechanisms of apoptosis mediated by PKC has advanced considerably, and the primary focus of this review is to provide an overview of PKC and p53, its mode of action and its physiological role in DNA damage-induced apoptosis.

Protein kinase C
The protein kinase C (PKC) family of serine-threonine kinases was first described as a calcium-activated, phospholipid-dependent serine/threonine protein kinase (Takai et al. 1977).PKC is activated diacylglycerol (DAG) hydrolyzed from phosphatidylinositol (PI) by phospholipase C (PLC) under a different cell-signaling system (Nishizuka 1984, Nishizuka 1988, Nishizuka 1992, Nishizuka 1995).It has attracted attention as an intracellular receptor for tumor-promotor phorbol esters, such as 12-O-tetradecanoyl-13-phorbol acetate (TPA) (Niedel et al. 1983).Although PKC had been recognized as a protein kinase, subsequent studies have revealed that it belongs to a family of serine/threonine-specific protein kinases and is activated by diverse stimuli and participates in various cellular processes, such as growth, differentiation, apoptosis, and cellular senescence (Casabona 1997, Clemens et al. 1992, Goodnight et al. 1994, Hofmann 1997, Hug and Sarre 1993, Nishizuka 1984, Nishizuka 1988, Nishizuka 1992, Nishizuka 1995).PKC consists of at least 11 isozymes (, I, II, , , , , , , / and ) with selective tissue distribution, activators, and substrates.PKC isozymes have been categorized into three groups: i) the classical/conventional PKCs (cPKCs: , , II ), which are calcium dependent and activated by DAG; ii) the novel PKCs (nPKCs: , , , ), which are calcium-independent and activated by DAG; and iii) the atypical PKCs (aPKCs: , ), which are calcium-independent and not activated by DAG (Casabona 1997, Goodnight et al. 1994, Hug and Sarre 1993, Nishizuka 1988, Nishizuka 1992, Nishizuka 1995).The cell-specific expression and subcellular localization of individual PKC isozymes indicate important isozyme-specific functions.To elucidate these functions, it should be necessary to study the individual features of each isozyme, such as expression, posttranslational modification, substrate specificity, subcellular localization and signaling crosstalk with other proteins.Moreover, the involvement of a PKC isozyme in a signaling pathway resulting in a specific cellular response can be investigated by diverse distinct methods such as overexpression or inhibition of enzyme.

PKC and apoptotic cell death upon genotoxic insults
Novel PKC, , and  are substrates for the effector caspase-3, and proteolytic activation of these novel PKCs has been associated with cell death (Datta et al. 1997, Emoto et al. 1995, Endo et al. 2000).However, recent studies have shown that PKC acts upstream of caspases to regulate cell death.For example, PKC activators enhanced caspase activation, whereas an inhibitor of PKC prevented caspase activation in response to DNA damage (Basu et al. 2001).In particular, studies with PKC -/-mice suggest that PKC plays pivotal roles in the regulation of cell proliferation and apoptosis (Humphries et al. 2006, Leitges et al. 2001).PKC is activated by a variety of stimuli including ionizing radiation, anti-cancer agents, reactive oxygen species (ROS), ultraviolet radiation, growth factors and cytokines (Carpenter et al. 2002, Chen et al. 1999, Denning et al. 1996, Konishi et al. 2001, Reyland et al. 1999, Yoshida and Kufe 2001, Yoshida et al. 2002).Molecular mechanisms such as tyrosine phosphorylation and proteolytic cleavage by caspase-3 are of importance to understand the pro-apoptotic role for PKC activation.PKC isozymes have been implicated in the growth factor signal transduction pathway (Nishizuka 1992).By contrast, activation of PKC inhibits cell cycle progression and down-regulation of PKC is linked to tumor promotion, suggesting that PKC may have a negative effect on cell survival (Lu et al. 1997, Watanabe et al. 1992).In many cases, the growth-inhibitory effects of PKC have been linked to changes in the expression of factors that influence cell cycle progression.Furthermore, PKC plays a pivotal role in the genotoxic stress response leading to apoptosis in various cell types (Brodie and Blumberg 2003, Reyland 2007, Yoshida 2007a).In addition, cells derived from PKC -/-mice were shown to be defective in mitochondria-dependent apoptosis (Humphries et al. 2006, Leitges et al. 2001).These findings thus support our proposition of a pro-genotoxic role for PKC.PKC is activated in response to diverse cellular stimuli by various processes, including membrane translocation (Joseloff et al. 2002, Wang et al. 1999), protein-protein interaction (Benes et al. 2005), tyrosine phosphorylation (Denning et al. 1996, Kaul et al. 2005), and proteolytic cleavage (Emoto et al. 1995, Ghayur et al. 1996, Yoshida 2007a, Yoshida et al. 2003).The translocation of PKC to discrete subcellular compartments and/or proteolytic cleavage can be induced by numerous stimuli, such as ceramide, TNF, UV irradiation, ionizing radiation, oxidative stress, and etoposide (DeVries et al. 2002, Majumder et al. 2000, Matassa et al. 2001, Reyland et al. 1999, Yamaguchi et al. 2007b, Yoshida 2007a, Yoshida et al. 2006a, Yoshida et al. 2002, Yoshida et al. 2003, Yoshida et al. 2006b).Importantly, recent studies have shown that genotoxin-induced PKC activation is in part dependent upon Ataxia telangiectasia mutated (ATM) (Yoshida et al. 2003).Whereas ATM activates c-Abl, and c-Abl activates PKC, a potential explanation is that DNA damage induces an ATMc-AblPKC pathway (Yoshida 2007b, Yoshida and Miki 2005, Yoshida et al. 2005).Alternatively, ATM may directly activate PKC in the DNA damage response.In either case, nuclear targeting of PKC is pre-requisite for ATM-mediated full activation of PKC.

Nuclear translocation of PKC in the apoptotic responses
Translocation of PKC into the nucleus has been demonstrated in various cells (Blass et al. 2002, DeVries et al. 2002, DeVries-Seimon et al. 2007, Eitel et al. 2003, Scheel-Toellner et al. 1999, Yoshida et al. 2003, Yuan et al. 1998).Recent study showed that PKC translocates to nucleus after exposure of cells with 1--D-arabinofuranosylcytosine (ara-C) (Yoshida et al. 2003).Moreover, pretreatment with PKC inhibitor, rottlerin, attenuates nuclear targeting of PKC (Yoshida et al. 2003), suggesting that its kinase activity is required for nuclear translocation.A putative nuclear localization signal has been identified at the C-terminus of the catalytic domain of PKC (DeVries et al. 2002).Numerous PKC targets and substrates, including the p53 tumor suppressor, are nuclear proteins that function in induction of apoptosis.

Role for p53 in response to DNA damage
The tumor suppressor protein p53 plays a central role in mediating stress and DNA damageinduced cell cycle arrest and apoptosis (Vogelstein et al. 2000).The p53 protein controls normal responses to DNA damage and other forms of genotoxic stress and is an indispensable element in maintaining genomic stability (Vogelstein et al. 2000).In fact, p53 is the most frequently mutated gene in human cancers (Nigro et al. 1989).The level of p53 protein is mostly undetectable in normal cells but rapidly increases in response to a variety of stress stimuli.The mechanism by which the p53 protein is stabilized is not completely understood, but post-translational modification plays a crucial role (Shieh et al. 1997).Mutations in the p53 gene are frequently correlated with generation of human cancers; however, the p53 pathway can be also derailed by diverse oncogenic molecules (Oren et al. 2002).The p53 gene knockedout mice develop tumors with an increased rate (Donehower et al. 1992).It is reasonable that many agents may inhibit the p53 pathway as part of the road toward tumor promotion.However, mechanisms for action of many chemical agents that promote tumor development have not been elucidated.With the central role of p53 in mind, agents that promote tumor formation might block the p53 pathway.Importantly, p53 is regulated primarily via posttranslational modifications, especially phosphorylation, and the accumulation of p53 is the first step following cellular stress (Oren 1999).The mdm2 gene is a transcriptional target of p53, and once synthesized, the MDM2 protein can bind to p53 at its NH2 terminus leading to its rapid degradation through the ubiquitin proteasome-mediated pathway (Kubbutat and Vousden 1998, Oren 1999, Ryan et al. 2001).Upon DNA damage, p53 is phosphorylated at multiple sites at the NH2 terminus, thereby inhibiting MDM2 binding (Burns and El-Deiry 1999, Canman et al. 1998, Kubbutat and Vousden 1998, Oren 1999, Ryan et al. 2001, Siliciano et al. 1997).As a result, p53 degradation stops and p53 accumulates.p53 can also be phosphorylated at its COOH-terminal regulatory domain, which influences its DNA binding (Meek 1998).In this context, constitutive phosphorylation of p53 by PKC at its COOH-terminal domain can lead to its degradation through ubiquitin proteasome-mediated pathway (Chernov et al. 2001).Moreover, treatment with PKC inhibitors, such as H7 or bisindolylmaleimide I, prohibited COOH-terminal phosphorylation of p53 and increased accumulation of p53 without any effect on the formation of the p53-MDM2 complex (Chernov et al. 2001).However, PKC inhibitors were incapable of p53 accumulation in human papilloma virus-positive HeLa cells (Chernov et al. 2001, Chernov et al. 1998).

PKC regulation of p53
The p53 tumor suppressor is activated following genotoxic stress.Transactivation of p53 target genes dictates cell cycle arrest and DNA repair or apoptosis.Accumulating studies have demonstrated that PKC regulates p53 expression at the transcriptional and posttranslational levels.

Control at the transcription
Recent reports document that PKC transactivates expression of p53 at the transcriptional level (Abbas et al. 2004, Liu et al. 2007, Yoshida 2008a).The tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) hinders DNA damage-induced up-regulation of p53 by down-regulating PKC.TPA initiates tumor formation in a variety of mice and tissue culture models, and this has been correlated with the down-regulation of PKC (Hansen et al. 1990).TPA initially induces and then diminishes the activity of the diacylglycerol-dependent PKC isoforms (Fournier andMurray 1987, Hansen et al. 1990).Previous studies showed that the tumor-promoting activities of TPA are mediated at least in part by down-regulating PKC (Lu et al. 1997).Moreover, transgenic mice over-expressing PKC were resistant to tumor promotion by TPA (Reddig et al. 1999).In this regard, previous studies implied that TPA can inhibit the DNA damage-mediated induction of p53 (Magnelli et al. 1995).Moreover, other studies with protein kinase inhibitors suggested that PKC regulates the p53 signalsome pathway (Ghosh et al. 1999).Regulation of p53 upon stress most commonly occurs by inhibiting ubiquitination and degradation of the p53 protein.In contrast, repression of p53 by inhibiting PKC is caused by the prevention of p53 synthesis, not augmented degradation of p53 protein.Inhibiting PKC blocks both basal transcription of the human p53 gene and initiation of transcription from the human p53 promoter.The DNA damage-elicited increase in p53 accumulation is drastically inhibited by pre-treatment with TPA.In addition, the PKC inhibitor, rottlerin, is also able to block the DNA damage-mediated induction of p53.More importantly, pre-treatment of cells with TPA or treatment with rottlerin results in the inhibition of basal p53 transcription.In this regard, accumulation of p53 could not be achieved by any means, including proteasome inhibition, after TPA or rottlerin treatment, since p53 transcription is hindered.Thus, the tumorsuppressing effects for PKC are mediated at least in part through activating p53 transcription.Suppression of the p53 promoter has been implied as a mechanism for tumor promotion (Raman et al. 2000, Stuart et al. 1995).Damaged genes in tumor cells are generally the mechanistic drivers toward oncogenesis.However, abrogation of endogenous genes, specifically tumor suppressors, may be also a crucial regulatory mechanism for tumor promotion.In this context, agents that interfere with the activity of PKC may inhibit p53 responses.
Recent study also demonstrated that PKC induces the promoter activity of p53 via the p53 core promoter element (CPE-p53) and that such induction is enhanced after DNA damage.Upon genotoxic insults, PKC is activated and interacts with the death-promoting transcription factor Btf (Bcl-2-associated transcription factor) to co-occupy CPE-p53.Inhibition of PKC decreases the affinity of Btf to CPE-p53, thereby reducing p53 expression.Concomitant with these results, abrogation of Btf-mediated p53 transcription by RNA interference leads to repression of p53-mediated apoptosis in response to genotoxic stress.These findings demonstrate that activation of p53 transcription by PKC induces p53dependent apoptosis following DNA damage (Liu et al. 2007).

Control at the post-translation
Recent study demonstrated that both PKC and IKK, but not IKK, are targeted to the nucleus after oxidative stress (Yamaguchi et al. 2007a, Yamaguchi et al. 2007b).PKC interacts with and activates IKK.Significantly, upon exposure to oxidative stress, PKCmediated IKK activation does not contribute to NF-B activation; rather, nuclear IKK controls transcription activity of p53 by phosphorylation on Ser20.These findings indicate a novel mechanism in which the PKCIKK signaling pathway contributes to ROS-induced p53 activation.Recent studies have also demonstrated that phosphorylation of p53 at Ser46 induces p53AIP1 expression, resulting in the commitment to the apoptotic cell death (Matsuda et al. 2002, Oda et al. 2000, Taira et al. 2007, Yoshida 2008b).Furthermore, upon genotoxic stress, p53DINP1 is induced and then recruits a kinase(s) to p53, which specifically phosphorylate Ser46 (Okamura et al. 2001).We initially found that PKC is associated with Ser46 phosphorylation (Yoshida et al. 2006a).This phosphorylation was required for the interaction of PKC to p53.Importantly, p53DINP1 associated with PKC in response to anti-cancer agents.In concert with these findings, PKC potentiates p53dependent apoptotic cell death by Ser46 phosphorylation.Taken together, PKC controls p53 to induce apoptosis in the cellular response to DNA damage (Yoshida et al. 2006a).Of note, our subsequent studies have demonstrated that another kinase DYRK2 plays a major and direct role on apoptosis induction by phosphorylating p53 at Ser46 in response to DNA damage (Taira et al. 2007, Taira et al. 2010).We also recently found that PKC regulates MDM2 expression independently of p53.Given that Mdm2 mRNA change was detected in p53-proficient, but not deficient cells, PKCδ affected Mdm2 at the post-translational level.In this context, treatment of proteasome inhibitor MG132 restored Mdm2 expression to the steady-state level.Moreover, PKCδ inhibited Mdm2 ubiquitination in p53-deficient cells and loss of PKCδ resulted in an increase in Mdm2 proteasomal degradation.P300/CBPassociated factor (PCAF), an ubiquitin ligase 3 for Mdm2, was observed to participate in Mdm2 ubiquitination by PKCδ inhibition and PCAF silencing rescued Mdm2 diminution.We thus conclude that PKCδ regulates Mdm2 expression distinctively of p53 pathway by affecting Mdm2 ubiquitination and maintenance of Mdm2 expression by PKCδ is important to ensure normal genotoxic cell death response in human cancer cells (Hew et al. 2011).

Future perspective
PKC plays a pivotal role in the control of apoptotic cell death in response to a diverse array of stress stimuli.Thus PKC is a pro-apoptotic kinase activated by multiple mechanisms, including subcellular translocation and proteolysis.The proteolytic activation of PKC is also important not only in activating the downstream apoptotic cascade including p53, but also in amplifying upstream caspase signaling.Most of the studies mentioned above suggest that the role of PKC in the induction of apoptosis is tightly associated with its caspasedependent cleavage and the regulation of p53.However, functional regulation of p53 by PKC remains largely unclear.In this regard, thorough investigation coupled with PKC and p53 should be enhanced from multiple views.In the encounter with genotoxic insults, ATM controls various cellular responses, such as cell cycle arrest, transcription, DNA repair, and apoptosis.In this context, DNA damage-induced PKC is modulated under ATM, suggesting the notion that establishment of the ATMPKCp53 signaling cascade provides new mechanistic light on how PKC functions as the pro-apoptotic kinase in the nucleus (Figure 1) (Yoshida 2007a, Yoshida 2008a).While dysregulation of the PKC signalsome confers resistance to anticancer drugs (Meinhardt et al. 1999), there is little Fig. 1.A hypothetical schema for nuclear targeting of PKC in response to DNA damage.Following DNA damage, PKC translocates from the cytoplasm into the nucleus.In addition, some genotoxic stress also exerts cytoplasmic oxidative stress to activate PKC.In the nucleus, PKC is activated by ATM, then induces apoptosis (or DNA repair) in a p53dependent manner.
understanding of how the PKC signaling pathway is influenced when cancer cells acquire resistance to chemotherapeutic drugs.Considering the importance of PKC in genotoxic stress-induced apoptosis, a thorough understanding of how it controls apoptosis should benefit cancer therapeutic potentials.Finally, novel PKC-based therapy may be used in combination with other agents to confer synergism and prevent the development for drug resistance.