Open access peer-reviewed chapter

The Role of Astrocytes in Tumor Growth and Progression

Written By

Emily Gronseth, Ling Wang, David R. Harder and Ramani Ramchandran

Submitted: May 17th, 2017 Reviewed: November 24th, 2017 Published: March 21st, 2018

DOI: 10.5772/intechopen.72720

From the Edited Volume

Astrocyte

Edited by Maria Teresa Gentile and Luca Colucci D’Amato

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Abstract

Current research is continually implicating the importance of astrocytes as active participants in neurological injury, disease, and tumor progression. This chapter will discuss some of these emerging concepts, especially as they relate to tumor biology. Astrocytes themselves can become tumorigenic, such as the case in gliomas, which often have aberrant signaling in key regulating genes of astrocyte development. Astrocytes secrete factors that maintain the tight junctions of the blood brain barrier (BBB), which in turn regulates the success or failure of metastatic cells extravasating into the brain. This astrocytic association with the brain vasculature also promotes brain tumor stem cell characteristics, which are known to be necessary for tumor initiation. Tumor cells within the brain make direct contacts with astrocytes through gap junctions, which subsequently lead to increased chemoresistance of the tumor cells. Astrocytes have also been shown to effect tumors cells via secretion of degradative enzymes, cytokines, chemokines, and growth factors, all of which have been shown to promote tumor cell proliferation, survival, and invasion. Thus, research in astrocyte biology and the role of astrocytes in the tumor microenvironment has and will likely continue to reveal novel targets for cancer intervention.

Keywords

  • astrocytes
  • metastasis
  • blood-brain barrier
  • reactive astrogliosis
  • cancer

1. Introduction

The tumor microenvironment plays a critical role in tumor progression. Tumors within the central nervous system (CNS) include primary brain tumors originating from a CNS resident cell, or secondary tumors that came from extraneural origins. The brain microenvironment consists of multiple cell types including the most abundant glial cell, astrocytes. Astrocytes have very diverse and microenvironment-dependent morphologies; for a long time, this structural contribution was considered their main purpose. Present in gray matter, protoplasmic astrocytes are the most common types of astrocytes and are stellate in nature with branching processes or “endfeet” [1]. These endfeet make important contacts with neurons and other cells within the brain microenvironment. Importantly for this chapter, one of these interfaces, which we will discuss further, is the astrocyte endfeet connections made with endothelial cells and pericytes, commonly referred to as the blood brain barrier (BBB). This barrier allows for select metabolites to enter and toxic waste to exit the brain.

Homeostasis in the brain is of utmost importance to maintain neural function and prevent potentially detrimental immune responses from occurring. An invading tumor cell normally encounters enormous barriers before it can colonize the brain. On entering the brain, it will need to overcome brain defense mechanisms, which are partly mediated by astrocytes and brain macrophage cells called microglia. These and other mechanisms are in place to thwart tumor cell entrance, however, in some cases these mechanisms are either not adequate to prevent tumor cell invasion, or exploited to aid in tumor cell extravasation into the brain. In addition to regulating brain metastases, astrocytes, which develop from neural stem cells (NSCs), can become transformed and undergo developmental dysregulation due to aberrant gene activation, resulting in various types of brain tumors, including gliomas.

In this chapter, we will further discuss autocrine, paracrine, and juxtacrine mechanisms in which astrocytes influence surrounding cells in the brain microenvironment and tumor progression within the CNS. We will discuss the underlying mechanisms that regulate these processes, and provide examples of possible interventions that could eventually be translated into successful clinical treatment for patients.

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2. Primary tumors of astrocytic origin

We begin this chapter by understanding how astrocytes themselves may become transformed and discuss the key features of these types of tumors. The cellular origin of many brain tumors can be traced back to multipotent NSCs, which are able to self-renew and differentiate into all subtypes of mature neurons and glial cells. However, many tumors with more distinct cellular origins exist along the glial cell differentiation axis, and are traced back to more restricted and differentiated astrocyte progeny [2]. During development of mature astrocytes, NSCs first partially differentiate into neuronal precursor cells where, in the presence of specific growth factors and their cognate receptors, they differentiate into various cellular lineages [3]. Early astrocyte precursors are characterized by their expression of fibroblast growth factor receptor (FGFR), nestin, and epidermal growth factor receptor (EGFR), while mature astrocytes express markers such as glutamate aspartate transporter (GLAST), FGFR3, S100β and glial fibrillary acidic protein (GFAP) [4, 5, 6, 7]. These astrocyte precursor cells are perhaps most vulnerable to transformation, and depending on the stage of these cells, fatal adult primary brain tumors (gliomas) may arise. Because mature astrocytes maintain their ability to proliferate throughout adulthood (an uncommon characteristic of many CNS cells), it is hypothesized that this is a contributing reason for why astrocytic tumors are so common overall and most common in adults [8, 9].

The term “glial cells” describes a broader group of cells including astrocytes, ependymal cells and oligodendrocytes, not all gliomas are specifically astrocytic in nature. Gliomas which are thought to originate or histologically resemble astrocytes include astrocytomas, mixed gliomas or oligoastrocytomas, diffuse intrinsic pontine gliomas [10], and high grade astrocytomas called glioblastoma multiforme (GBM). There are also several types of mixed neuronal-glial tumors. These tumors are extremely heterogeneous, differing in histology, location in the brain, molecular biology, karyotype, age of onset, and survival prognosis of the patient. Gliomas share many characteristics with astrocytes, particularly activated astrocytes, which will be discussed in a later section of this chapter. Some of these include migration capabilities, growth factor expression pattern, stem cell-like characteristics, and the ability for anchorage-independent growth which is correlated with invasiveness of a tumor [11, 12, 13].

In general, cellular origins of the previously mentioned gliomas are astrocyte precursor cells. However, the diversity in the distinct molecular/genetic alterations of the tumors suggests that different stages or types of precursor cells have different sensitivities to specific genetic mutations. One of the most notable genetic signatures of GBM is EGFR amplification and overexpression which, as previously mentioned, is also involved in regulating astrocyte differentiation [14, 15, 16, 17]. There have been several mechanisms associating EGFR overexpression with astrocyte tumor malignancy. Several known ligands of EGFR, including EGF and transforming growth factor-α (TGF-α), promote proliferation of astrocytes and astrocyte precursor cells, thus contributing to the malignancy of the tumors [14, 18, 19]. Additionally, cell cycle regulators such as Rb, p53 and CDKN2A are commonly mutated and inactivated in low grade gliomas and GBM, [15, 20, 21, 22, 23]. Mutations in isocitrate dehydrogenase (IDH)-1 and -2 are also extremely common, but only in certain gliomas; they are present in 70% of grade II and III astrocytomas and oligodendrogliomas, as well as secondary GBMs, but are rare in primary GBMs [15, 24].

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3. The role of astrocytes in tumor growth and progression

As will be discussed in more detail throughout this chapter, astrocytes are very heterogeneous in regards to function and influence on tumors within the CNS. This fact, combined with the heterogeneity that encompasses the transformation of cells results in unique tumor genotypes and phenotypes, plus many other contextual factors, equates to interactions that are both tumor promoting and tumor suppressive (Figure 1). Arguably, there is more evidence suggesting how astrocytes can be tumor promoting, which will be covered in this section. Some functions of astrocytes that are known within the literature to be tumor promoting to both primary and metastatic brain tumors are summarized and illustrated (Figure 2).

Figure 1.

Research shows astrocytes have functions that result in tumor supporting and tumor suppressing mechanisms, and sometimes both. Like many physiological responses, this demonstrates the context dependent balancing act that occurs when homeostasis is breached, and the many factors that play a role in tipping the balance one way or another.

Figure 2.

Many signaling mechanisms have been identified that implicate astrocytes in tumor progression. Some of these functions include secretion of factors that have been shown to increase invasiveness and malignancy of established brain tumors, in addition to factors that enable brain metastasizing cells to enter the brain. Astrocytes have also been shown to maintain the vascular niche in the brain which can promote stem-like characteristics in BTSCs. Astrocytes directly interact with tumor cells and communicate via gap junctions, leading to increased intracellular calcium and resistance to treatments. Indirect or paracrine communication with surrounding cells often occurs via secretion of exosomes known to carry miRNAs to target key tumor suppressor genes in tumor cells and surrounding microenvironment. Lastly, astrocytes have been shown to regulate CNS immune suppression, weakening the innate tumor killing response of the body.

3.1. Metastatic tumors: Interactions with astrocytes at the blood-brain barrier

One of the critical steps in the life time of tumor progression is tumor metastasis, especially brain metastasis. This step results in catastrophic consequences from a patient perspective. The metastases from extraneural tumors in the brain are actually the most common sources of tumors in the CNS, as shown in Table 1 [25, 26].

CNS tumor type Incidence rate (per 100,000 persons) (all ages) References
All brain metastases 8.3/11.1/14.3 [195, 196, 197]
Lung cancer brain metastases ~ 3.2–8(estimation based on 39–56% of all brain metastases) [32]
Breast cancer brain metastases ~ 1.1–4.3 (estimation based on 13–30% of all brain metastases) [32]
Melanoma brain metastases ~ 0.5–1.6 (estimation based on 6–11% of all brain metastases) [32]
Primary malignant CNS tumors 7.2 [198]
GBM 3.2 [198]
Nerve sheath tumors 1.82 [198]
Other astrocytomas 1.2 [198]
CNS lymphoma 0.43 [198]
Embryonal tumors (medulloblastoma, ATRT, and PNET) 0.62 (ages 0–19 only) [198]

Table 1.

Primary and metastatic CNS tumors with their respective incidence rates per 100,000 persons. All tumors are accounting for all ages, except embryonal tumors which only includes persons’ age 0–19 in the population study.

The process of metastasis, in brief, involves invasion of a tumor cell away from the tumor to a blood vessel, entry into and survival in the blood circulation, extravasation from the blood vessel into the secondary organ, and survival, engraftment, and proliferation into a secondary tumor. Extravasating into the brain provides an added challenge: that of getting past the BBB. The most functionally important component of the BBB are the tight junctions held between brain microvascular endothelial cells. Thus, substances that get into the brain parenchyma are tightly controlled. Both para-cellular and trans-cellular diffusion are low; most solutes that get in and out through the BBB, such as glucose and other nutrients, do so through transporters expressed on endothelial cells [27, 28, 29]. Despite this added barrier, many extraneural tumors have a strong tendency to metastasize to the brain. To note, there are regions within the brain that lack BBB, and could also be a potential avenue of metastasis [30]. The recent discovery of brain lymphatics is also suggestive of an alternative route given the already known function of lymphatics to carry tumor cells [31].

Breast cancer, melanoma, and lung cancer are three tumor types that show proclivity to go to the brain. The most common type of brain metastases originate from lung cancer, accounting for up to 56% of brain metastases, followed by breast cancer metastases at 13–30% [32, 33]. Interestingly, specific subtypes of these tumors have a much higher frequency of brain metastases, including non-small cell lung cancer (NSLC), triple negative breast cancer cells that are estrogenic receptor (ER), progesterone receptor (PR) and epidermal growth factor receptor-2 (HER2) (ER, PR, HER2), and HER2-enriched (HER2+) breast cancer cells [32, 33, 34, 35]. One theory behind a specific tumors’ proclivity to the brain is explained by Paget’s seed and soil hypothesis, which suggests that for a seed (tumor cell) to take up in a soil (brain), it must adapt itself and make changes that will favor the soil [26, 36]. In support of this idea, genes associated with breast cancer metastasis to brain have been discovered and efforts continue to identify new targets for lung cancer cell metastasis to the brain and for other cancers as well [37, 38, 39, 40]. However, the question raised by the seed-soil hypothesis is whether the soil influences the seed, and if so, how it is accomplished. One could argue that in the first place, the soil is influenced by the seed. Therefore, in this “circular logic,” both seed and soil appear to contribute together, ultimately, for the growth of the tumor cell.

Astrocytes are vital to the development and maintenance of the BBB, therefore understanding their role in this process is necessary to understand how they also may influence brain metastatic tumor cells attempting to breach the BBB. The tight junctions between the endothelial cells of the BBB are comprised of many junctional proteins, notably claudin-5 and occludin [41, 42]. Vascular endothelial (VE)-cadherin is also of importance within adherens junctions which associate with tight junctions, as well as cytoplasmic scaffolding proteins such as zonula occludens (ZO)-1 and -2 [27, 43, 44, 45]. In normal conditions, the BBB homeostasis and junctional complexes are partially supported at the structural and physiological functional levels by astrocytes. Astrocytes contact brain endothelial cells via their end feet processes. This contact was shown to maintain BBB permeability characteristics. However, it was later found that secreted factors alone in astrocyte-conditioned media also upheld the tight junction characteristics in endothelial cells, demonstrating the importance of both astrocyte contact and paracrine actions in BBB function [46, 47, 48]. For example, Sonic hedgehog (Shh), an important developmental signaling protein, is known to be secreted by astrocytes and bind its cognate receptor, Patched-1, expressed at the cell membrane of brain endothelial cells. This induces a signaling cascade mediated by β-catenin that bolsters tight junctions by increasing expression of occludin [49, 50]. Other proteins secreted by astrocytes that regulate and maintain tight junctions of brain ECs (most often by increased gene expression of junctional proteins) include angiotensin-1, FGF, TGF-β, glia derived neurotropic factor (GDNF), and retinoic acid (RA) [51, 52, 53, 54, 55].

Just as astrocytes are important for maintaining the BBB in homeostatic conditions, astrocytes also play key roles when the BBB is disrupted, which can occur during extravasation of tumor cells metastasizing to the brain. Several groups utilized mouse models of melanoma, lung, and breast cancer combined with histological and fluorescent imaging modalities to visualize very early interactions with tumor cells and the BBB. This work demonstrates that tumor cells first arrest in the brain capillaries, often at branch points [56, 57]. Lorger et al. (2010) show that very early on, astrocytes become activated and associate around vasculature in the brain where breast tumor cells are present, but in some cases have not yet extravasated or visibly altered the BBB [56]. This suggests that signals secreted by tumor cells are reaching astrocytes either directly or indirectly through the endothelial cells; a topic that requires further investigation. This association of tumor cells with reactive astrocytes persists throughout metastases formation, characterized by increased astrocyte expression of GFAP, nestin, and matrix metalloproteinase 9 (MMP-9), all of which aid in tumor extravasation mechanisms that will be discussed in later sections.

3.2. Astrocytes’ direct cell-cell interactions with tumor cells

Astrocytes have multiple primary and branching endfeet which expand and contract, allowing them to dynamically contact both synapses and the microvasculature. Also, astrocytes regulate communication between neuronal networks and glial-vascular coupling by forming independent contact network [58, 59, 60, 61]. Therefore, it has been widely accepted that astrocytes directly contact and communicate with neurons to regulate neuronal function at the synaptic and network levels, which provides a significant impact on physiological and pathological state of the CNS. Subsequently, direct interactions with astrocytes and tumor cells, often in the form of gap junctions, has also been discovered to be significant for tumor progression and resistance to therapy.

As discussed, gliomas are the most lethal primary intracranial tumors. The proliferative dysfunction and invasion of gliomas are associated with changes in gap junction communication [62, 63]. In metastatic brain tumors, reactive astrocytes protect melanoma cells from chemotherapy induced cell death by sequestering intracellular calcium through gap junctions [64]. In the brain, metastases from breast and lung cancer show upregulation of many survival genes which is dependent on the direct contact through gap junctions between the astrocytes and tumor cells, which was found to be causal for developing resistance [65]. These data suggest that reactive astrocytes participate in tumor progression and chemo-resistance by their direct physical contacts and gap junctional communication with tumor cells in the brain.

Gap junctions are efficient tools for intercellular communication. In astrocytes, they are composed of connexins 30 and 43 (Cx30, Cx43) [66]. Cx43 is widely expressed in adult astrocytes and exhibits increased expression in reactive astrocytes induced by various brain pathologies and intercellular calcium signaling [67, 68, 69, 70, 71, 72, 73]. Also, Cx43-mediated intercellular communication between astrocytes plays an important role in the invasion of glioma cells in the brain [63]. A recent study has also revealed that breast and lung cancer cells express proto-cadherin 7 (PCDH7) to promote tumor-astrocyte gap junction formation by recruiting Cx43, which allows the transfer of cGAMP from tumor cells to astrocytes to trigger the secretion of inflammatory cytokines, which further promote tumor growth and chemo-resistance [74].

3.3. Astrocytes’ secretome and paracrine signaling mechanisms that influence tumor cells

3.3.1. Cytokines and growth factors

Astrocytes can synthesize a host of biologically interesting growth factors and cytokines. Previous studies have shown that sphingosine-1-phosphate (S1P), which shows the highest expression in the brain and is only expressed by astrocytes, induces cell motility in GBM cell lines that express S1P receptor-1 and S1P receptor-3 [75, 76]. Other neurotrophic factors secreted by astrocytes, such as TGF-α, C-X-C motif chemokine 12 (CXCL12), and GDNF, have also revealed the potential to increase the invasive capacity of GBM cells [77, 78]. In brain metastatic tumors, an early study found that metastatic MDA-MB-435 breast cancer cells, when cultured with astrocyte conditioned media, exhibit better growth in response to the conditioned medium. However, the growth-stimulatory effect was partially reversed by anti-IL-6, anti-TGF-β, and anti-insulin like growth factor-1 (IGF-I) antibodies [79]. Another study showed that reactive astrocytes expressed phosphorylated platelet-derived growth factor receptor β at tyrosine 751 (p751-PDGFRβ). Pazopanib, an inhibitor of PDGFRs, inhibited the activation of p-PDGFR expressing astrocytes, and thus prevented brain metastasis formation in the HER2-transfected MDA-MB-231 breast cancer cells [80]. Taken together, this work demonstrates that paracrine signaling by astrocyte secreted cytokines and growth factors facilitates tumor metastasis formation in the brain.

3.3.2. Extracellular matrix (ECM) proteins and degradative enzymes

ECM proteins are important participants in the tumorigenic process, as they are involved in not only the physical adhesion and migration of tumors cells, but also the regulation of intracellular signaling. The brain parenchyma is high in proteoglycans, glycoproteins, and matricellular proteins, all of which astrocytes express and secrete [4]. Specifically, some astrocyte secreted matricellular proteins have been studied in regulation of various brain tumors, including secreted protein acidic and rich in cysteine (SPARC) and CYR61/CTGF/NOV (CCN). Both SPARC and CCN2 have been shown to be secreted by activated astrocytes proximal to brain tumors or injuries [81, 82]. While increases in CCN2 secretion have been correlated with negative glioblastoma outcomes, expression of SPARC and its effect on tumor cells is tumor dependent. In gliomas and astrocytomas, tumor secretion of SPARC promotes invasion, angiogenesis, and a negative prognosis [83, 84]; however medulloblastoma tumor cells have increased loss of SPARC, which when rescued induces cell cycle arrest, neuronal differentiation, and limits radioresistant DNA damage response [85, 86, 87].

Previously, we discussed MMP-related mechanisms in which astrocytes assist tumor cells in extravasating the BBB. The secretion of these matrix degrading enzymes also supports brain tumor progression by breaking down the barriers induced by the ECM. Heparanase degrades the glycosaminoglycan side chains of heparan sulfate proteoglycans, which are essential and ubiquitous macromolecules associated with the cell surface [88, 89, 90]. Reactive astrocytes have been frequently found in areas surrounding melanoma-related lesions and produce nerve growth factor (NGF), the prototypic neurotrophin [91]. Neurotrophins can stimulate heparanase production in astrocytes and thus contribute to the brain colonization of melanoma cells [88].

MMP-2 and -9 have been observed in secretory vesicles in astrocytes [92]. Stimulation of astrocytes with lipopolysaccharide, IL1-α, IL1-β, or TNF-α induces MMP-2 and -9 secretion [93]. MMP-9 also promotes the growth of primary brain tumors by releasing vascular endothelial growth factor (VEGF) sequestered in the surrounding matrix [94]. The expression of MMP-9 was up-regulated in reactive astrocytes, which was involved in the brain metastases of MDA-MB-435 cells [56]. Moreover, both MMP-2 and -9 secreted by astrocytes contribute to breast cancer MDA-MB-231 cell invasion and brain metastases [95]. In addition to secreting MMPs themselves, astrocytes can also induce tumor cells to secrete MMPs, as shown by Mendes et al. (2007), where they found breast cancer cells to secrete significantly more MMP-2 in the presence of astrocyte conditioned media, aiding metastasis to the brain [96].

3.3.3. Exosomes

The topic of exosomes, which are endosome-derived microvesicles between 50 and 100 nm in size that carry specific protein and RNA cargo, has become a subject of intense interest in tumor biology. Exosomes are released from cells by fusion of multi-vesicular structures with the plasma membrane through the process of exocytosis [97]. Exosomes are a general mode of intercellular communication and can interact with neighboring cells, thus mediating signals between astrocytes and other cells in the brain microenvironment [98, 99].

Among RNA cargo, microRNA (miRNA) transcripts, specifically miR-26a, is highly expressed in astrocytes and is present in astrocyte-derived exosomes [100, 101]. MiR-26a targets mRNAs that impact neuronal function and morphology, and was first implicated in many neuronal disorders [102, 103, 104]. Moreover, miR-26a can be sorted to exosomes and transported by these vesicles in the plasma, serum, whole blood, urine, or secreted in vitro by human umbilical vein endothelial cells [105, 106, 107, 108, 109]. In addition, miR-26a autonomously regulates primary gliomas by increasing de novo tumor formation and radiosensitivity through targeting of suppressor phosphatase and tensin homolog (PTEN) and ataxia-telangiectasia mutated (ATM), respectively [110, 111]. Therefore, it is plausible that miR-26a in astrocyte-derived exosomes may function to regulate the surrounding tumor environment. A recent study found that primary breast tumor cells express tumor suppressor PTEN, however this expression of PTEN was lost reversibly after tumor cells metastasized into the brain [112]. Astrocyte-derived exosomal miR-19a reversibly mediated the downregulation of PTEN expression in cancer cells, thus providing one mechanism for loss of PTEN in tumor cells that enter the brain. Further, miR-19a also increased C-C motif chemokine ligand 2 (CCL2) secretion and recruitment of myeloid cells, thus facilitating changes in the brain microenvironment to promote metastasis [112].

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4. The role of astrocytes in brain tumor stem cell biology

An important attribute of brain tumor biology regarding tumor initiation and propagation is the existence of brain tumor stem cells (BTSCs). These cells have been found to, in many ways, resemble adult NSCs that exist in distinct regions of the brain, including the subventricular zone (SVZ) and the subgranular zone (SGZ) [113, 114]. Many groups identified CD133, Nestin, and sex determining region Y-box 2 (SOX2) as markers to isolate NSCs which maintain the essential properties of stem cells (self-renewal and ability to differentiate into multiple progeny) [115, 116, 117, 118]. Using the NSC neurosphere culturing method, CD133 and/or CD15 have also been found to be expressed on BTSCs from GBM, medulloblastoma, ependymoma, and astrocytoma tumors [118, 119, 120]. Interestingly, Singh et al. (2003) found CD133+ cells to be tumor initiating, whereas CD133 cells could not initiate a tumor or self-renew in a mouse xenograft model [118, 121]. The levels of CD133+ BTSCs has since been correlated to negative prognoses in gliomas, and have been found to be particularly enriched in recurrent tumors after radiation and chemotherapy [122, 123, 124]. These findings highlight the importance of stem cells in the overall initiation, malignancy, and recurrence of brain tumors.

4.1. Astrocytes’ direct influence on cancer stem cells

It is clear that BTSCs play an important role in the progression of all brain tumors. Therefore, cells in the microenvironment that influence BTSCs are of interest from a clinical therapy perspective. Interestingly, astrocytes seem to affect normal NSCs and BTSCs quite differently. While astrocyte secreted factors have been shown to promote neurogenesis of normal adult NSCs, astrocytes within the microenvironment of brain tumors have also been shown to promote stem-like characteristics in BTSCs and enrich the stem cell population, thus worsening the malignancy of such brain tumors [125, 126, 127, 128]. GBM CD133+ stem cells co-cultured both directly and indirectly with astrocytes show gene expression signatures known to be involved in GBM invasion and metastasis, such as a disintegrin and metalloproteinase domain-containing protein 10 (ADAM10), hyaluronan synthase 2 (HAS2), and vascular cell adhesion molecule-1 (VCAM1). Interestingly, although there were many overlapping genes in conditions where astrocytes and tumor cells were and were not in direct contact, even the distinct gene expression changes in each condition were still related to tumor cell invasion. This emphasizes the role of astrocytes in GBM invasion, which is one of the most challenging traits of GBM [127]. Indeed, CD133+ GBM cells were found to be more invasive, whereas CD133 GBM cells did not have the same gene expression and invasion changes [127]. Later, it was shown that indirect co-culture with CD133+ GBM cells and astrocytes resulted in cytokine release from astrocytes that reduced radiosensitivity of the GBM cells; again, this same phenotype and crosstalk with astrocytes was absent in CD133 GBM cells [128]. Some of the astrocyte secreted cytokines that induced radioresistance include CXCL1, IL-4, IL-6, and CCL7 [128]. These differences suggest that cancer stem (or stem-like) cells signal differently with astrocytes compared to tumor cells lacking stem characteristics. The reverse effect, which is tumor stem cells influencing astrocytes has also been observed in GBM. GBM stem cells provide signals that block the expression of p53 in surrounding astrocytes [129]. P53 is a tumor suppressor often found mutated in many tumors [130], and is classically known for its function in DNA damage response. However, recently p53 has been shown to have non-autonomous cellular functions, particularly in the tumor microenvironment, by influencing secretion of proteins, including ECM proteins [129, 131, 132]. Thus, the interaction between astrocytes and BTSCs are bi-directional and influence each other’s development.

In addition to primary brain tumors, cancer stem cells of brain metastatic tumors are also influenced by astrocytes. It has been shown that cyclooxygenase 2 (COX2) is highly expressed in breast cancer brain metastatic cells, which autonomously induces expression of MMP-1 and prostaglandins [133]. While MMP-1 allows for BBB tight junction and basement membrane degradation to aid brain metastasis, prostaglandins are able to activate astrocytes and subsequently increase astrocyte expression of CCL7, which was shown to significantly increase self-renewal and survival of breast cancer stem cells through increased expression of Nanog, a key stem cell regulator [133, 134]. This study provides evidence that astrocytes enrich breast cancer stem cells in brain metastases and aid in their ability to extravasate the BBB.

4.2. Astrocytes as a part of the perivascular niche

Normal NSCs in the SVZ and SGZ are maintained by specialized vascular regions called the perivascular niche (PVN) [135]. The PVN consists of the endothelial cells lining the vasculature, as well as astrocytes, pericytes, macrophages, microglia, fibroblasts, and vascular smooth muscle cells. These cells function and signal together to maintain structure and provide signals to NSCs. Evidence exists which demonstrates the vital role endothelial cells play in maintaining and regulating NSC/BTSCs’ survival and differentiation status [136, 137, 138, 139]. Astrocytes also play a vital role within the PVN. First and foremost, they play an indirect but obvious role in the structural and chemical maintenance of endothelial cell-BBB phenotypes, as discussed earlier. Studies have shown that BTSCs maintain close proximity to angiogenic regions of the tumor microenvironment, providing evidence that these regions phenocopy the PVN within the SVZ/SGZ to provide enrichment signals to BTSCs [140].

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5. Astrocytes as an immune regulator in the tumor microenvironment

The immune system within the CNS is tightly controlled. In addition to the BBB, there are other barriers that maintain the CNS as an immune-privileged system, including the blood-meningeal barriers and the blood-cerebrospinal fluid (CSF) barriers [141, 142]. During homeostasis, these barriers do not allow entry of pathogens or blood-borne immune cells. Only upon CNS injury do some of these cellular barriers become fenestrated to allow for immune cell entrance. Although microglia are thought to be the main regulator of immune responses within the brain, astrocytes (and other cells) also play key roles in this function [143]. A vast amount of work investigating the astrocyte function in either normal or activated state is often related to the regulation of the immune environment in the CNS, as shown in functional studies and astrocyte secretome studies, summarized well by Sofroniew et al. [144, 145, 146, 147]. As suggested by Yang et al. (2013), the presence of classical immunological surface molecules, such as major histocompatibility (MHC) antigen and intercellular adhesion molecule-1 (ICAM-1) on astrocytes underlines their importance in CNS immune function [11, 148, 149]. We will discuss next how astrocytes control immune responses to invading tumor cells, and the immune-related concepts associated with this process.

5.1. Immune responses to tumor cell presence

As mentioned in the discussion of astrocytes and tumor cell interactions at the BBB, there have been a few key studies observing the cellular events that take place when metastatic cells extravasate into the brain parenchyma [56, 57]. From these studies, it is known that astrocytes are the first to respond to extravasating metastatic tumor cells entering the brain, followed by microglia [56]. Regardless of whether a CNS tumor is primary or metastatic, microglia and astrocytes control the immune response; therefore, it is upon their activation that other immune cells, such as macrophages or lymphocytes, may infiltrate [144, 145, 146, 150]. Activated astrocytes secrete pro-inflammatory molecules such as CXCL12, CCI2, Il15, CCL8, and CXCL1, all of which are known to regulate recruitment, activation and proliferation of T-cells, B-cells, or natural killer (NK) cells [144, 145, 146].

As it is often seen with any local or systemic inflammation, CNS immune responses can often persist or be dysregulated by tumor cells to become pathogenic. Many of these signaling responses are mediated by astrocytes. For example, Valiente et al. [151] reported that astrocytes produce FasL and plasmin ligands as defense mechanisms to kill brain-invading tumor cells. In response, tumor cells secrete serpins, which thwart the lethal action of plasmin [151]. Thus, Fas-mediated tumor cell apoptosis is blocked, leading to tumor survival. Other cells, such as endothelial cells, in the brain microenvironment are also co-opted, which is facilitated by up-regulation of L1 cell adhesion molecules (L1CAM). All these mechanisms work together to initiate brain metastasis [151].

Astrocytes are also known to function in immunosuppression. This is accomplished by downregulating the pro-inflammatory cytokine TNF-α in surrounding microglia, and suppressing the antigen presenting abilities of various immune cells by downregulating their expression of MHCII and CD80 [152, 153]. Additionally, activated astrocytes can co-localize and induce apoptosis in T-cells attempting to infiltrate the brain parenchyma by expression of the “death ligand,” CD95L, which binds the receptor on T-cells [152, 154].

5.2. Reactive Astrogliosis

Arguably, the most important feature of astrocytes in relation to their immune function is their ability to activate, a process called reactive astrogliosis. What determines whether an astrocyte is “activated” or not has not been clearly defined, however Sofroniew summarized the existing research into four key features. First, reactive astrogliosis is a spectrum of molecular, cellular, and functional changes among astrocytes in response to CNS injury of many kinds [147]. Second, the changes can vary in severity and the response can be sequential and/or progressive. Third, the changes are regulated by intra- and inter-cellular signals and lastly, signaling events can be both gain and loss of function in nature, resulting in both beneficial and detrimental outcomes [147, 155]. In other words, reactive astrogliosis is spectral in nature; the triggers can vary and therefore the “activation” or response can vary and is context dependent, which is also true in regards to how reactive astrocytes affect tumor progression and/or tumor death.

The activation responses can be as small as a transient upregulation of GFAP, to permanent structural changes in the brain from a process called glial scar formation. Scar formation occurs when astrocytes proliferate and overlap to a point that causes dense, compact barriers around necrotic tissue [147, 156]. In between these two extremes, other phenotypic changes that occur include hypertrophy of the cell body and processes, a vast array of gene expression changes, and varying degrees of proliferation up to the point of scar formation. Some of the chemical activators of astrocytes known to be secreted by or induced by tumor cells include EGF (glioblastoma and medulloblastoma), TGF-α (medulloblastoma), receptor activator of nuclear factor kappa-B (NFκB) ligand (RANKL) (glioma), macrophage migration inhibitory factor (MIF), interleukin-8 (IL-8), and plasminogen activator inhibitor-1 (PAI-1) (lung cancer metastases) [11, 157, 158, 159, 160].

In addition to chemical activation, astrocytes can also be activated by tumor cells mechanically. Although extremely abundant, astrocytes hold a highly regulated, non-overlapping distribution that plays an important role in morphology and contact-dependent inhibition of proliferation [11, 61]. This distribution and homeostasis is mediated by contact inhibition and adherens junctions. Therefore, mechanical disruption occurs when processes such as migration and/or proliferation of surrounding cells is initiated. Such mechanical signals could come potentially emerge from tumor cells, subsequently triggering astrocyte activation via disruption of these cell surface complexes such as cadherins and β-catenin [161, 162]. The genes activated by β-catenin signaling are regulatory and often lead to proliferation and migration [11, 163]. Interestingly, Yang et al. (2012) found this contact initiated activation of astrocytes to parallel what occurs in the transformation of astrocytomas, further coupling the process of astrocyte activation and tumor progression [162].

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6. Therapeutic opportunities for cancer emerging from astrocyte-tumor cross talk

As stated previously, homeostasis in brain environment is key for the functionality of the brain, and therefore key checkpoints, such as the BBB, are responsible for maintaining homeostasis [30]. The BBB also prevents access of key drugs into the brain for targeting tumor cells. Any surgical intervention in the brain clearly has quality of life considerations, and does not offer complete disease-free state. Therefore, it is of importance to prevent tumor cells from entering the brain or block the target routes and underlying mechanisms used by tumors to circumvent checkpoints. It is worth noting that the regions of the brain which are free from the protections of the junctional characteristics of the BBB, such as the stroma of the choroid plexus and area postrema have increased vascular permeability which can be problematic, and therefore must be considered when trying to block tumor cell entrance into the brain [30].

As we know, astrocytes are capable of signaling to trigger tumor cell (breast, lung, skin, and brain) migration, invasion and metastasis in vivo [88, 95, 127, 160]. There are many targets in the brain microenvironment that provide effective intervention strategies for metastasis, and is reviewed elsewhere [164]. Here, we will discuss targets and mechanisms at the signaling interface of tumor cells and astrocytes that offer fresh perspective on intervention strategies.

6.1. Enzyme targets

As discussed earlier, we and others have identified astrocyte secreted MMP-2, MMP-9, and MMP-1 to promote tumor progression, and blocking them with broad spectrum MMP inhibitors does influence tumor metastasis in pre-clinical models [95, 96, 165, 166, 167]. Interestingly, MMP-1 was one of 21 MMPs that showed clinical significance in regards to breast cancer brain metastasis, and expression analysis of brain-seeking triple negative breast cancer clonal cells confirm MMP-1 and MMP-9 as potential targets [133, 168]. Therefore, these studies suggest either MMPs or the underlying pathways that regulate their expression as pharmaceutical targets. Given that targeting MMPs in the past using first generation MMP inhibitors resulted in disappointing results in the clinic, we also suggest that next generation, highly-specific MMP inhibitors, applied locally, could be effective new strategies to consider in preventing further growth and movement of tumor cells to a second location in the brain [169].

6.2. Gap junction protein targets

Astrocytes are co-opted to up-regulate survival genes in tumor cells and induce protection from chemotherapy [65]. Downregulation of the astrocyte-initiated survival gene expression in tumor cells will render tumor cells sensitive to chemotherapy [65]. This chemoprevention role, however, appears to be contact dependent, utilizing gap junctions to mediate the changes in tumor cells. Previously, gap junction proteins Cx43 and Cx26 were utilized by breast cancer and melanoma cells to initiate brain metastatic lesion formation in cohort with the vasculature [170]. Indeed, patient data analysis revealed increased cancer recurrence and metastasis with increased expression of Cx26 and Cx43 in primary melanoma and breast tumor cells. The recent work done by Chen et al. shows that brain metastatic breast and lung cancer cells initiate contact with astrocytes through gap junctions, which produces a signaling response (discussed in detail earlier in the chapter) resulting in chemoresistance [74]. Bioavailable modulators of gap junctions, meclofenamate and tonabersat, could influence this paracrine signaling loop, and thus could be proposed for treatment of established brain metastases [74].

6.3. PTEN, exosomes and miRNA targeting

Breast cancer metastases often show common alterations in the EGFR and HER2 driven pathways, both of which are regulated by PTEN gene [171]. PTEN is mutated in human brain, breast and prostate cancer, and loss of PTEN was found in a substantial portion of breast cancer brain metastases samples significantly associated with triple negative breast cancer [172, 173]. Interestingly, PTEN loss promotes a feedback loop between tumor cells and glial cells, which contributes to disease progression. We already know one mechanism in which PTEN expression is lost; through the targeting and degradation of transcript by miR-26a and miR-19a from astrocyte secreted exosomes [110, 111, 112].

Blocking the astrocytes from secreting the PTEN-targeting microRNA rescues the PTEN loss and importantly suppresses brain metastasis in vivo [112, 174]. Similarly, miR-200 containing extracellular vesicles, which regulates the mesenchymal to epithelial transition, can be transferred from metastatic cells to non-metastatic cells leading to promotion of metastasis [175]. Therefore, collectively, approaches that promote PTEN expression or prevent loss of PTEN expression has the potential to influence metastatic outcome in the clinic for select cancers such as breast, brain and prostate cancer.

6.4. Adaptations (environment)

The finding that breast cancer cells take up a neuronal phenotype when they are in the brain suggests co-evolution adaptive mechanisms associated with metastatic cells and their microenvironment. The variable PTEN expression in metastatic tumor cells in response to different organ environments suggests a genetic component that drives co-evolution adaptive behavior between metastatic cells and their microenvironment [176]. Brain homing MDA-MB-231 cells secrete bone morphogenic protein-2 (BMP-2), which mediates the differentiation of NSCs into astrocytes; subsequently, downregulation of BMP-2 in the brain homing tumor cells diminished their engraftment and colonization abilities [176]. Further, when co-cultured with NSCs, primary (non-brain homing) MDA-MB-231 cells fail to proliferate over 15 days, but brain homing MDA-MB-231 cells escaped this growth inhibition, and proliferation occurred in parallel with NSCs' differentiation into astrocytes [176]. This suggests that both the brain homing MDA-MB-231 cells? adaptive phenotype and the NSCs' differentiation into astrocytes are codependent, meaning the brain homing MDA-MB-231 cells require astrocytic signals to survive. This group extended these observations further and demonstrated that human breast cancer cells found in the brain and not in the primary tumor, upregulated γ-aminobutyric acid (GABA) pathway genes, and displayed GABAergic phenotypes that are similar to neuronal cells [177]. This phenotype offers a proliferative advantage to tumor cells because GABA is catabolized into succinate which generates NADH, a critical metabolite necessary for tumor cell sustenance. It is noteworthy that GABA is abundant in the brain, and perhaps tumor cells have adapted to this environment that gives them a proliferative advantage. Of the different cells in the brain, neurons, because of their function, require the majority of ATP [178]. As such, astrocytes expend less energy, and secrete lactate that is generated by glycolysis [179]. This evolutionary adaptation feature of tumors and their reliance on astrocytic signals open up avenues for targeting; several inhibitors of metabolites such as GABA are available and could be repurposed for brain metastases treatment. Of course, more research and context-specific treatment of such modalities will be needed.

6.5. CNS tumor immunotherapies and astrocytes

The emergence and success of immunotherapy techniques in many blood, lymph, and some solid tumors is bringing groundbreaking and exciting work in the cancer research field [180]. Currently, researchers are now looking for ways to modulate this therapy so it can be applied to more tumors, including tumors in the CNS [141]. Importantly, the effectiveness of strategies such as vaccine and immune checkpoint therapies rely on a strong response and presence of tumor infiltrating immune cells for antigen presentation, which is low in most brain tumors due to the limited presence of resident immune cells within the brain [141, 181]. Some strategies to stimulate the immune response, such as adjuvants or tetanus and diphtheria boosters with vaccine administration have increased effectiveness [141, 182, 183]. As discussed earlier, astrocytes play an important role in immunosuppression in the brain tumor microenvironment, however this function has not yet been targeted. Therefore, one could postulate investigation of a combination therapy targeting an immunosuppressive factor(s) produced by astrocytes as an additional option worthy of research.

Much excitement in the immune therapy world surrounds the programmed death ligand-1 (PD-L1), an immune checkpoint signal that is immunosuppressive by binding its cognate receptor, programmed death-1 (PD-1) receptor, expressed on T-cells to induce apoptosis [184]. Targeting and blocking PD-L1 or PD-1 with antibody therapies has been an effective treatment for several cancers [185, 186]. It has been shown that GBM tumors highly express PD-L1, in addition to infiltrating microglia [187, 188]. Normal astrocytes have also been found to highly express PD-L1, however astrocyte expression of PD-L1 in a tumor setting has not yet been investigated [189]. Future work investigating astrocyte (normal and reactive) expression of PD-L1 is needed and will provide mechanistic hypothesis to current clinical trial that utilizes nivolumab, a PD-1 antibody, in combination with temozolomide for treatment GBM. PD-L1 has also been investigated in metastatic brain tumors, however expression and correlation to outcomes appear to be tumor dependent, leading to conflicting reports on whether PD-L1 expression correlates to a positive or negative prognosis, therefore more research is needed [190, 191, 192].

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7. Conclusions

Until recently, the complexities of astrocyte signaling and influence on human pathologies were not fully appreciated in the literature, especially in regards to astrocytes’ influence on tumor biology. Cancer researchers began recognizing that tumor cells themselves may not be the sole perpetrators in tumor initiation and progression, and the resulting research has made it increasingly clear that tumor cells and the host environment they reside in are constantly communicating to facilitate growth, sustenance and metastasis [193, 194]. To facilitate tumor progression, the host environment is either co-opted by tumor cells or defense mechanisms of host cells are overcome by the tumor cells. Whether astrocytes are “friends” or “foes” of tumor cells is a matter of context, as evidence exists for both scenarios. Figure 1 depicts the balancing act of these functions, in addition to the outside factors which dictate them, eventually determining the fate of the respective tumor cells and tumor as a whole. There are many known (and unknown) factors that must be considered in understanding CNS tumors and their relation to astrocytes. We summarize some of the tumor promoting mechanisms of astrocytes, which have been highlighted in this chapter (Figure 2), effecting both primary brain tumors and secondary brain metastases. The diversity of astrocyte mechanism modalities will hopefully bring about unique and novel intervention strategies, some of which were also discussed in this chapter. In conclusion, astrocytes are a critical cell type that participate in various physiological and pathological conditions, and their role in the history of tumor progression is beginning to be appreciated.

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Acknowledgments

We thank lab members of the Developmental Vascular Biology Program for their input during numerous presentations of this subject. EG is supported by Department of Pediatrics, and Children’s Research Institute. LW is partly supported by Department of Obstetrics and Gynecology funds, and Department of Defense grant # BC161839 and HL033833. DH is supported by funds from VA hospital and NIH grant HL033833. RR is partly supported by NIH grant HL123338, HL112639, HL033833, endowment funds from OBGYN and start-up funds from Department of Pediatrics and Children’s Research Institute.

References

  1. 1. Sun D, Jakobs TC. Structural remodeling of astrocytes in the injured CNS. The Neuroscientist. 2012;18(6):567-588
  2. 2. Liu C, Zong H. Developmental origins of brain tumors. Current Opinion in Neurobiology. 2012;22(5):844-849
  3. 3. Lee JC, Mayer-Proschel M, Rao MS. Gliogenesis in the central nervous system. Glia. 2000;30(2):105-121
  4. 4. Wiese S, Karus M, Faissner A. Astrocytes as a source for extracellular matrix molecules and cytokines. Frontiers in Pharmacology. 2012;3:120
  5. 5. Lillien L. Changes in retinal cell fate induced by overexpression of EGF receptor. Nature. 1995;377(6545):158-162
  6. 6. Kornblum HI, Hussain R, Wiesen J, Miettinen P, Zurcher SD, Chow K, Derynck R, Werb Z. Abnormal astrocyte development and neuronal death in mice lacking the epidermal growth factor receptor. Journal of Neuroscience Research. 1998;53(6):697-717
  7. 7. Sibilia M, Steinbach JP, Stingl L, Aguzzi A, Wagner EF. A strain-independent postnatal neurodegeneration in mice lacking the EGF receptor. The EMBO Journal. 1998;17(3):719-731
  8. 8. Collins VP. Gliomas. Cancer Surveys. 1998;32:37-51
  9. 9. Rasheed BK, Wiltshire RN, Bigner SH, Bigner DD. Molecular pathogenesis of malignant gliomas. Current Opinion in Oncology. 1999;11(3):162-167
  10. 10. Warren KE. Diffuse intrinsic pontine glioma: poised for progress. Frontiers in Oncology. 2012;2:205
  11. 11. Yang C, Rahimpour S, AC Y, Lonser RR, Zhuang Z. Regulation and dysregulation of astrocyte activation and implications in tumor formation. Cellular and Molecular Life Sciences. 2013;70(22):4201-4211
  12. 12. Tezel G, Hernandez MR, Wax MB. Vitro evaluation of reactive astrocyte migration, a component of tissue remodeling in glaucomatous optic nerve head. Glia. 2001;34(3):178-189
  13. 13. Yang H, Cheng XP, Li JW, Yao Q, Ju G. De-differentiation response of cultured astrocytes to injury induced by scratch or conditioned culture medium of scratch-insulted astrocytes. Cellular and Molecular Neurobiology. 2009;29(4):455-473
  14. 14. Wechsler-Reya R, Scott MP. The developmental biology of brain tumors. Annual Review of Neuroscience. 2001;24:385-428
  15. 15. Altieri R, Agnoletti A, Quattrucci F, Garbossa D, Calamo Specchia FM, Bozzaro M, Fornaro R, Mencarani C, Lanotte M, Spaziante R, et al. Molecular biology of gliomas: Present and future challenges. Translational medicine @ UniSa. 2014;10:29-37
  16. 16. Libermann TA, Nusbaum HR, Razon N, Kris R, Lax I, Soreq H, Whittle N, Waterfield MD, Ullrich A, Schlessinger J. Amplification and overexpression of the EGF receptor gene in primary human glioblastomas. Journal of Cell Science. Supplement. 1985;3:161-172
  17. 17. Wong AJ, Bigner SH, Bigner DD, Kinzler KW, Hamilton SR, Vogelstein B. Increased expression of the epidermal growth factor receptor gene in malignant gliomas is invariably associated with gene amplification. Proceedings of the National Academy of Sciences of the United States of America. 1987;84(19):6899-6903
  18. 18. Prigent SA, Lemoine NR. The type 1 (EGFR-related) family of growth factor receptors and their ligands. Progress in Growth Factor Research. 1992;4(1):1-24
  19. 19. Garrett TP, McKern NM, Lou M, Elleman TC, Adams TE, Lovrecz GO, Zhu HJ, Walker F, Frenkel MJ, Hoyne PA, et al. Crystal structure of a truncated epidermal growth factor receptor extracellular domain bound to transforming growth factor alpha. Cell. 2002;110(6):763-773
  20. 20. Biernat W, Debiec-Rychter M, Liberski PP. Mutations of TP53, amplification of EGFR, MDM2 and CDK4, and deletions of CDKN2A in malignant astrocytomas. Polish Journal of Pathology: Official Journal of the Polish Society of Pathologists. 1998;49(4):267-271
  21. 21. Henson JW, Schnitker BL, Correa KM, von Deimling A, Fassbender F, HJ X, Benedict WF, Yandell DW, Louis DN. The retinoblastoma gene is involved in malignant progression of astrocytomas. Annals of Neurology. 1994;36(5):714-721
  22. 22. Ichimura K, Schmidt EE, Goike HM, Collins VP. Human glioblastomas with no alterations of the CDKN2A (p16INK4A, MTS1) and CDK4 genes have frequent mutations of the retinoblastoma gene. Oncogene. 1996;13(5):1065-1072
  23. 23. Simon M, Koster G, Menon AG, Schramm J. Functional evidence for a role of combined CDKN2A (p16-p14(ARF))/CDKN2B (p15) gene inactivation in malignant gliomas. Acta Neuropathologica. 1999;98(5):444-452
  24. 24. Yan H, Parsons DW, Jin G, McLendon R, Rasheed BA, Yuan W, Kos I, Batinic-Haberle I, Jones S, Riggins GJ, et al. IDH1 and IDH2 mutations in gliomas. The New England Journal of Medicine. 2009;360(8):765-773
  25. 25. Landis SH, Murray T, Bolden S, Wingo PA. Cancer statistics, 1999. CA: a Cancer Journal for Clinicians. 1999;49(1):8-31 31
  26. 26. Langley RR, Fidler IJ. The seed and soil hypothesis revisited--the role of tumor-stroma interactions in metastasis to different organs. International Journal of Cancer. 2011;128(11):2527-2535
  27. 27. Daneman R, Prat A. The blood-brain barrier. Cold Spring Harbor Perspectives in Biology. 2015;7(1):a020412
  28. 28. Coomber BL, Stewart PA. Morphometric analysis of CNS microvascular endothelium. Microvascular Research. 1985;30(1):99-115
  29. 29. Mittapalli RK, Manda VK, Adkins CE, Geldenhuys WJ, Lockman PR. Exploiting nutrient transporters at the blood-brain barrier to improve brain distribution of small molecules. Therapeutic Delivery. 2010;1(6):775-784
  30. 30. Moody DM. The blood-brain barrier and blood-cerebral spinal fluid barrier. Seminars in Cardiothoracic and Vascular Anesthesia. 2006;10(2):128-131
  31. 31. Louveau A, Smirnov I, Keyes TJ, Eccles JD, Rouhani SJ, Peske JD, Derecki NC, Castle D, Mandell JW, Lee KS, et al. Structural and functional features of central nervous system lymphatic vessels. Nature. 2015;523(7560):337-341
  32. 32. Nayak L, Lee EQ, Wen PY. Epidemiology of brain metastases. Current Oncology Reports. 2012;14(1):48-54
  33. 33. Wilhelm I, Fazakas C, Molnar K, Vegh AG, Hasko J, Krizbai IA. Foe or friend? Janus-faces of the neurovascular unit in the formation of brain metastases. Journal of Cerebral Blood Flow and Metabolism. 2017. 271678X17732025
  34. 34. Cheng X, Hung MC. Breast cancer brain metastases. Cancer Metastasis Reviews. 2007;26(3–4):635-643
  35. 35. Bravo Marques JM. Treatment of brain metastases in patients with HER2+ breast cancer. Advances in Therapy. 2009;26(Suppl 1):S18-S26
  36. 36. Paget S. The distribution of secondary growths in cancer of the breast. Cancer Metastasis Reviews 1989. 1889;8(2):98-101
  37. 37. Bos PD, Zhang XH, Nadal C, Shu W, Gomis RR, Nguyen DX, Minn AJ, van de Vijver MJ, Gerald WL, Foekens JA, et al. Genes that mediate breast cancer metastasis to the brain. Nature. 2009;459(7249):1005-1009
  38. 38. Kikuchi T, Daigo Y, Ishikawa N, Katagiri T, Tsunoda T, Yoshida S, Nakamura Y. Expression profiles of metastatic brain tumor from lung adenocarcinomas on cDNA microarray. International Journal of Oncology. 2006;28(4):799-805
  39. 39. Grinberg-Rashi H, Ofek E, Perelman M, Skarda J, Yaron P, Hajduch M, Jacob-Hirsch J, Amariglio N, Krupsky M, Simansky DA, et al. The expression of three genes in primary non-small cell lung cancer is associated with metastatic spread to the brain. Clinical Cancer Research: An Official Journal of the American Association for Cancer Research. 2009;15(5):1755-1761
  40. 40. Hamilton R, Krauze M, Romkes M, Omolo B, Konstantinopoulos P, Reinhart T, Harasymczuk M, Wang Y, Lin Y, Ferrone S, et al. Pathologic and gene expression features of metastatic melanomas to the brain. Cancer. 2013;119(15):2737-2746
  41. 41. Morita K, Furuse M, Fujimoto K, Tsukita S. Claudin multigene family encoding four-transmembrane domain protein components of tight junction strands. Proceedings of the National Academy of Sciences of the United States of America. 1999;96(2):511-516
  42. 42. McCarthy KM, Skare IB, Stankewich MC, Furuse M, Tsukita S, Rogers RA, Lynch RD, Schneeberger EE. Occludin is a functional component of the tight junction. Journal of Cell Science. 1996;109(Pt 9):2287-2298
  43. 43. Taddei A, Giampietro C, Conti A, Orsenigo F, Breviario F, Pirazzoli V, Potente M, Daly C, Dimmeler S, Dejana E. Endothelial adherens junctions control tight junctions by VE-cadherin-mediated upregulation of claudin-5. Nature Cell Biology. 2008;10(8):923-934
  44. 44. Stevenson BR, Siliciano JD, Mooseker MS, Goodenough DA. Identification of ZO-1: A high molecular weight polypeptide associated with the tight junction (zonula occludens) in a variety of epithelia. The Journal of Cell Biology. 1986;103(3):755-766
  45. 45. Gumbiner B, Lowenkopf T, Apatira D. Identification of a 160-kDa polypeptide that binds to the tight junction protein ZO-1. Proceedings of the National Academy of Sciences of the United States of America. 1991;88(8):3460-3464
  46. 46. Neuhaus J, Risau W, Wolburg H. Induction of blood-brain barrier characteristics in bovine brain endothelial cells by rat astroglial cells in transfilter coculture. Annals of the New York Academy of Sciences. 1991;633:578-580
  47. 47. Prat A, Biernacki K, Wosik K, Antel JP. Glial cell influence on the human blood-brain barrier. Glia. 2001;36(2):145-155
  48. 48. Tao-Cheng JH, Nagy Z, Brightman MW. Tight junctions of brain endothelium in vitro are enhanced by astroglia. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 1987;7(10):3293-3299
  49. 49. Alvarez JI, Dodelet-Devillers A, Kebir H, Ifergan I, Fabre PJ, Terouz S, Sabbagh M, Wosik K, Bourbonniere L, Bernard M, et al. The hedgehog pathway promotes blood-brain barrier integrity and CNS immune quiescence. Science. 2011;334(6063):1727-1731
  50. 50. Runkle EA, Rice SJ, Qi J, Masser D, Antonetti DA, Winslow MM, Mu D. Occludin is a direct target of thyroid transcription factor-1 (TTF-1/NKX2-)1. The Journal of Biological Chemistry. 2012;287(34):28790-28801
  51. 51. Lavoie JL, Sigmund CD. Minireview: Overview of the renin-angiotensin system--an endocrine and paracrine system. Endocrinology. 2003;144(6):2179-2183
  52. 52. Reuss B, Dono R, Unsicker K. Functions of fibroblast growth factor (FGF)-2 and FGF-5 in astroglial differentiation and blood-brain barrier permeability: Evidence from mouse mutants. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 2003;23(16):6404-6412
  53. 53. Igarashi Y, Utsumi H, Chiba H, Yamada-Sasamori Y, Tobioka H, Kamimura Y, Furuuchi K, Kokai Y, Nakagawa T, Mori M, et al. Glial cell line-derived neurotrophic factor induces barrier function of endothelial cells forming the blood-brain barrier. Biochemical and Biophysical Research Communications. 1999;261(1):108-112
  54. 54. Dohgu S, Yamauchi A, Takata F, Naito M, Tsuruo T, Higuchi S, Sawada Y, Kataoka Y. Transforming growth factor-beta1 upregulates the tight junction and P-glycoprotein of brain microvascular endothelial cells. Cellular and Molecular Neurobiology. 2004;24(3):491-497
  55. 55. Mizee MR, Wooldrik D, Lakeman KA, van het Hof B, Drexhage JA, Geerts D, Bugiani M, Aronica E, Mebius RE, Prat A, et al. Retinoic acid induces blood-brain barrier development. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 2013;33(4):1660-1671
  56. 56. Lorger M, Felding-Habermann B. Capturing changes in the brain microenvironment during initial steps of breast cancer brain metastasis. The American Journal of Pathology. 2010;176(6):2958-2971
  57. 57. Kienast Y, von Baumgarten L, Fuhrmann M, Klinkert WE, Goldbrunner R, Herms J, Winkler F. Real-time imaging reveals the single steps of brain metastasis formation. Nature Medicine. 2010;16(1):116-122
  58. 58. Garcia-Segura LM, Chowen JA, Duenas M, Torres-Aleman I, Naftolin F. Gonadal steroids as promoters of neuro-glial plasticity. Psychoneuroendocrinology. 1994;19(5–7):445-453
  59. 59. Blutstein T, Baab PJ, Zielke HR, Mong JA. Hormonal modulation of amino acid neurotransmitter metabolism in the arcuate nucleus of the adult female rat: A novel action of estradiol. Endocrinology. 2009;150(7):3237-3244
  60. 60. Haydon PG, Blendy J, Moss SJ, Rob Jackson F. Astrocytic control of synaptic transmission and plasticity: A target for drugs of abuse? Neuropharmacology. 2009;56(Suppl 1):83-90
  61. 61. Bushong EA, Martone ME, Jones YZ, Ellisman MH. Protoplasmic astrocytes in CA1 stratum radiatum occupy separate anatomical domains. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 2002;22(1):183-192
  62. 62. Tabernero A, Medina JM, Giaume C. Glucose metabolism and proliferation in glia: Role of astrocytic gap junctions. Journal of Neurochemistry. 2006;99(4):1049-1061
  63. 63. Sin WC, Aftab Q, Bechberger JF, Leung JH, Chen H, Naus CC. Astrocytes promote glioma invasion via the gap junction protein connexin43. Oncogene. 2016;35(12):1504-1516
  64. 64. Lin Q, Balasubramanian K, Fan D, Kim SJ, Guo L, Wang H, Bar-Eli M, Aldape KD, Fidler IJ. Reactive astrocytes protect melanoma cells from chemotherapy by sequestering intracellular calcium through gap junction communication channels. Neoplasia. 2010;12(9):748-754
  65. 65. Kim SJ, Kim JS, Park ES, Lee JS, Lin Q, Langley RR, Maya M, He J, Kim SW, Weihua Z, et al. Astrocytes upregulate survival genes in tumor cells and induce protection from chemotherapy. Neoplasia. 2011;13(3):286-298
  66. 66. Giaume C, McCarthy KD. Control of gap-junctional communication in astrocytic networks. Trends in Neurosciences. 1996;19(8):319-325
  67. 67. Rozental R, Giaume C, Spray DC. Gap junctions in the nervous system. Brain Research. Brain Research Reviews. 2000;32(1):11-15
  68. 68. Theodoric N, Bechberger JF, Naus CC, Sin WC. Role of gap junction protein connexin43 in astrogliosis induced by brain injury. PLoS One. 2012;7(10):e47311
  69. 69. Hossain MZ, Peeling J, Sutherland GR, Hertzberg EL, Nagy JI. Ischemia-induced cellular redistribution of the astrocytic gap junctional protein connexin43 in rat brain. Brain Research. 1994;652(2):311-322
  70. 70. Goldberg GS, Lampe PD, Nicholson BJ. Selective transfer of endogenous metabolites through gap junctions composed of different connexins. Nature Cell Biology. 1999;1(7):457-459
  71. 71. De Bock M, Decrock E, Wang N, Bol M, Vinken M, Bultynck G, Leybaert L. The dual face of connexin-based astroglial Ca(2+) communication: A key player in brain physiology and a prime target in pathology. Biochimica et Biophysica Acta. 2014;1843(10):2211-2232
  72. 72. Scemes E, Suadicani SO, Spray DC. Intercellular communication in spinal cord astrocytes: Fine tuning between gap junctions and P2 nucleotide receptors in calcium wave propagation. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 2000;20(4):1435-1445
  73. 73. Toyofuku T, Yabuki M, Otsu K, Kuzuya T, Hori M, Tada M. Intercellular calcium signaling via gap junction in connexin-43-transfected cells. The Journal of Biological Chemistry. 1998;273(3):1519-1528
  74. 74. Chen Q, Boire A, Jin X, Valiente M, Er EE, Lopez-Soto A, Jacob L, Patwa R, Shah H, Xu K, et al. Carcinoma-astrocyte gap junctions promote brain metastasis by cGAMP transfer. Nature. 2016;533(7604):493-498
  75. 75. Edsall LC, Spiegel S. Enzymatic measurement of sphingosine 1-phosphate. Analytical Biochemistry. 1999;272(1):80-86
  76. 76. Van Brocklyn JR, Jackson CA, Pearl DK, Kotur MS, Snyder PJ, Prior TW. Sphingosine kinase-1 expression correlates with poor survival of patients with glioblastoma multiforme: Roles of sphingosine kinase isoforms in growth of glioblastoma cell lines. Journal of Neuropathology and Experimental Neurology. 2005;64(8):695-705
  77. 77. Kaur B, Khwaja FW, Severson EA, Matheny SL, Brat DJ, Van Meir EG. Hypoxia and the hypoxia-inducible-factor pathway in glioma growth and angiogenesis. Neuro-Oncology. 2005;7(2):134-153
  78. 78. Banisadr G, Skrzydelski D, Kitabgi P, Rostene W, Parsadaniantz SM. Highly regionalized distribution of stromal cell-derived factor-1/CXCL12 in adult rat brain: Constitutive expression in cholinergic, dopaminergic and vasopressinergic neurons. The European Journal of Neuroscience. 2003;18(6):1593-1606
  79. 79. Sierra A, Price JE, Garcia-Ramirez M, Mendez O, Lopez L, Fabra A. Astrocyte-derived cytokines contribute to the metastatic brain specificity of breast cancer cells. Laboratory Investigation; A Journal of Technical Methods and Pathology. 1997;77(4):357-368
  80. 80. Gril B, Palmieri D, Qian Y, Anwar T, Liewehr DJ, Steinberg SM, Andreu Z, Masana D, Fernandez P, Steeg PS, et al. Pazopanib inhibits the activation of PDGFRbeta-expressing astrocytes in the brain metastatic microenvironment of breast cancer cells. The American Journal of Pathology. 2013;182(6):2368-2379
  81. 81. Huang H, Colella S, Kurrer M, Yonekawa Y, Kleihues P, Ohgaki H. Gene expression profiling of low-grade diffuse astrocytomas by cDNA arrays. Cancer Research. 2000;60(24):6868-6874
  82. 82. Schwab JM, Postler E, Nguyen TD, Mittelbronn M, Meyermann R, Schluesener HJ. Connective tissue growth factor is expressed by a subset of reactive astrocytes in human cerebral infarction. Neuropathology and Applied Neurobiology. 2000;26(5):434-440
  83. 83. Halliday JJ, Holland EC. Connective tissue growth factor and the parallels between brain injury and brain tumors. Journal of the National Cancer Institute. 2011;103(15):1141-1143
  84. 84. Schultz C, Lemke N, Ge S, Golembieski WA, Rempel SA. Secreted protein acidic and rich in cysteine promotes glioma invasion and delays tumor growth in vivo. Cancer Research. 2002;62(21):6270-6277
  85. 85. Chetty C, Dontula R, Ganji PN, Gujrati M, Lakka SS. SPARC expression induces cell cycle arrest via STAT3 signaling pathway in medulloblastoma cells. Biochemical and Biophysical Research Communications. 2012;417(2):874-879
  86. 86. Bhoopathi P, Chetty C, Dontula R, Gujrati M, Dinh DH, Rao JS, Lakka SS. SPARC stimulates neuronal differentiation of medulloblastoma cells via the Notch1/STAT3 pathway. Cancer Research. 2011;71(14):4908-4919
  87. 87. Chetty C, Dontula R, Gujrati M, Dinh DH, Lakka SS. Blockade of SOX4 mediated DNA repair by SPARC enhances radioresponse in medulloblastoma. Cancer Letters. 2012;323(2):188-198
  88. 88. Marchetti D, Li J, Shen R. Astrocytes contribute to the brain-metastatic specificity of melanoma cells by producing heparanase. Cancer Research. 2000;60(17):4767-4770
  89. 89. Iozzo RV, Murdoch AD. Proteoglycans of the extracellular environment: Clues from the gene and protein side offer novel perspectives in molecular diversity and function. FASEB Journal: Official Publication of the Federation of American Societies for Experimental Biology. 1996;10(5):598-614
  90. 90. Irimura T, Nakajima M, Nicolson GL. Chemically modified heparins as inhibitors of heparan sulfate specific endo-beta-glucuronidase (heparanase) of metastatic melanoma cells. Biochemistry. 1986;25(18):5322-5328
  91. 91. Condorelli DF, Dell'Albani P, Mudo G, Timmusk T, Belluardo N. Expression of neurotrophins and their receptors in primary astroglial cultures: Induction by cyclic AMP-elevating agents. Journal of Neurochemistry. 1994;63(2):509-516
  92. 92. Sbai O, Ould-Yahoui A, Ferhat L, Gueye Y, Bernard A, Charrat E, Mehanna A, Risso JJ, Chauvin JP, Fenouillet E, et al. Differential vesicular distribution and trafficking of MMP-2, MMP-9, and their inhibitors in astrocytes. Glia. 2010;58(3):344-366
  93. 93. Gottschall PE, Yu X, Bing B. Increased production of gelatinase B (matrix metalloproteinase-9) and interleukin-6 by activated rat microglia in culture. Journal of Neuroscience Research. 1995;42(3):335-342
  94. 94. Wu H, Du J, Zheng Q. Expression of MMP-1 in cartilage and synovium of experimentally induced rabbit ACLT traumatic osteoarthritis: Immunohistochemical study. Rheumatology International. 2008;29(1):31-36
  95. 95. Wang L, Cossette SM, Rarick KR, Gershan J, Dwinell MB, Harder DR, Ramchandran R. Astrocytes directly influence tumor cell invasion and metastasis in vivo. PLoS One. 2013;8(12):e80933
  96. 96. Mendes O, Kim HT, Lungu G, Stoica G. MMP2 role in breast cancer brain metastasis development and its regulation by TIMP2 and ERK1/2. Clinical & Experimental Metastasis. 2007;24(5):341-351
  97. 97. Colombo M, Raposo G, Thery C. Biogenesis, secretion, and intercellular interactions of exosomes and other extracellular vesicles. Annual Review of Cell and Developmental Biology. 2014;30:255-289
  98. 98. Simons M, Raposo G. Exosomes–Vesicular carriers for intercellular communication. Current Opinion in Cell Biology. 2009;21(4):575-581
  99. 99. Camussi G, Deregibus MC, Bruno S, Cantaluppi V, Biancone L. Exosomes/microvesicles as a mechanism of cell-to-cell communication. Kidney International. 2010;78(9):838-848
  100. 100. Smirnova L, Grafe A, Seiler A, Schumacher S, Nitsch R, Wulczyn FG. Regulation of miRNA expression during neural cell specification. The European Journal of Neuroscience. 2005;21(6):1469-1477
  101. 101. Lafourcade C, Ramirez JP, Luarte A, Fernandez A, Wyneken U. MiRNAs in astrocyte-derived exosomes as possible mediators of neuronal plasticity. Journal of Experimental Neuroscience. 2016;10(Suppl 1):1-9
  102. 102. Beveridge NJ, Gardiner E, Carroll AP, Tooney PA, Cairns MJ. Schizophrenia is associated with an increase in cortical microRNA biogenesis. Molecular Psychiatry. 2010;15(12):1176-1189
  103. 103. Perkins DO, Jeffries CD, Jarskog LF, Thomson JM, Woods K, Newman MA, Parker JS, Jin J, Hammond SM. microRNA expression in the prefrontal cortex of individuals with schizophrenia and schizoaffective disorder. Genome Biology. 2007;8(2):R27
  104. 104. Cogswell JP, Ward J, Taylor IA, Waters M, Shi Y, Cannon B, Kelnar K, Kemppainen J, Brown D, Chen C, et al. Identification of miRNA changes in Alzheimer's disease brain and CSF yields putative biomarkers and insights into disease pathways. Journal of Alzheimer's Disease. 2008;14(1):27-41
  105. 105. Li Y, Zhang L, Liu F, Xiang G, Jiang D, Pu X. Identification of endogenous controls for analyzing serum exosomal miRNA in patients with hepatitis B or hepatocellular carcinoma. Disease Markers. 2015;2015:893594
  106. 106. Lin X, He Y, Hou X, Zhang Z, Wang R, Wu Q. Endothelial cells can regulate smooth muscle cells in contractile phenotype through the miR-206/ARF6&NCX1/exosome axis. PLoS One. 2016;11(3):e0152959
  107. 107. Wu SC, Yang JC, Rau CS, Chen YC, Lu TH, Lin MW, Tzeng SL, Wu YC, Wu CJ, Hsieh CH. Profiling circulating microRNA expression in experimental sepsis using cecal ligation and puncture. PLoS One. 2013;8(10):e77936
  108. 108. Ichii O, Otsuka-Kanazawa S, Horino T, Kimura J, Nakamura T, Matsumoto M, Toi M, Kon Y. Decreased miR-26a expression correlates with the progression of podocyte injury in autoimmune glomerulonephritis. PLoS One. 2014;9(10):e110383
  109. 109. Cheng L, Sharples RA, Scicluna BJ, Hill AF. Exosomes provide a protective and enriched source of miRNA for biomarker profiling compared to intracellular and cell-free blood. Journal of Extracellular Vesicles. 2014;3:23743
  110. 110. Huse JT, Brennan C, Hambardzumyan D, Wee B, Pena J, Rouhanifard SH, Sohn-Lee C, le Sage C, Agami R, Tuschl T, et al. The PTEN-regulating microRNA miR-26a is amplified in high-grade glioma and facilitates gliomagenesis in vivo. Genes & Development. 2009;23(11):1327-1337
  111. 111. Guo P, Lan J, Ge J, Nie Q, Guo L, Qiu Y, Mao Q. MiR-26a enhances the radiosensitivity of glioblastoma multiforme cells through targeting of ataxia-telangiectasia mutated. Experimental Cell Research. 2014;320(2):200-208
  112. 112. Zhang L, Zhang S, Yao J, Lowery FJ, Zhang Q, Huang WC, Li P, Li M, Wang X, Zhang C, et al. Microenvironment-induced PTEN loss by exosomal microRNA primes brain metastasis outgrowth. Nature. 2015;527(7576):100-104
  113. 113. Alvarez-Buylla A, Garcia-Verdugo JM. Neurogenesis in adult subventricular zone. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 2002;22(3):629-634
  114. 114. Kempermann G. Why new neurons? Possible functions for adult hippocampal neurogenesis. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 2002;22(3):635-638
  115. 115. Uchida N, Buck DW, He D, Reitsma MJ, Masek M, Phan TV, Tsukamoto AS, Gage FH, Weissman IL. Direct isolation of human central nervous system stem cells. Proceedings of the National Academy of Sciences of the United States of America. 2000;97(26):14720-14725
  116. 116. Lendahl U, Zimmerman LB, McKay RD. CNS stem cells express a new class of intermediate filament protein. Cell. 1990;60(4):585-595
  117. 117. Wegner M. SOX after SOX: SOXession regulates neurogenesis. Genes & Development. 2011;25(23):2423-2428
  118. 118. Singh SK, Clarke ID, Terasaki M, Bonn VE, Hawkins C, Squire J, Dirks PB. Identification of a cancer stem cell in human brain tumors. Cancer Research. 2003;63(18):5821-5828
  119. 119. Read TA, Fogarty MP, Markant SL, McLendon RE, Wei Z, Ellison DW, Febbo PG, Wechsler-Reya RJ. Identification of CD15 as a marker for tumor-propagating cells in a mouse model of medulloblastoma. Cancer Cell. 2009;15(2):135-147
  120. 120. Mao XG, Zhang X, Xue XY, Guo G, Wang P, Zhang W, Fei Z, Zhen HN, You SW, Yang H. Brain tumor stem-like cells identified by neural stem cell marker CD15. Translational Oncology. 2009;2(4):247-257
  121. 121. Reynolds BA, Weiss S. Generation of neurons and astrocytes from isolated cells of the adult mammalian central nervous system. Science. 1992;255(5052):1707-1710
  122. 122. Yi Y, Hsieh IY, Huang X, Li J, Zhao W. Glioblastoma stem-like cells: Characteristics, microenvironment, and therapy. Frontiers in Pharmacology. 2016;7:477
  123. 123. Zeppernick F, Ahmadi R, Campos B, Dictus C, Helmke BM, Becker N, Lichter P, Unterberg A, Radlwimmer B, Herold-Mende CC. Stem cell marker CD133 affects clinical outcome in glioma patients. Clinical Cancer Research: An Official Journal of the American Association for Cancer Research. 2008;14(1):123-129
  124. 124. Tamura K, Aoyagi M, Ando N, Ogishima T, Wakimoto H, Yamamoto M, Ohno K. Expansion of CD133-positive glioma cells in recurrent de novo glioblastomas after radiotherapy and chemotherapy. Journal of Neurosurgery. 2013;119(5):1145-1155
  125. 125. Wilhelmsson U, Faiz M, de Pablo Y, Sjoqvist M, Andersson D, Widestrand A, Potokar M, Stenovec M, Smith PL, Shinjyo N, et al. Astrocytes negatively regulate neurogenesis through the Jagged1-mediated notch pathway. Stem Cells. 2012;30(10):2320-2329
  126. 126. Seri B, Garcia-Verdugo JM, McEwen BS, Alvarez-Buylla A. Astrocytes give rise to new neurons in the adult mammalian hippocampus. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 2001;21(18):7153-7160
  127. 127. Rath BH, Fair JM, Jamal M, Camphausen K, Tofilon PJ. Astrocytes enhance the invasion potential of glioblastoma stem-like cells. PLoS One. 2013;8(1):e54752
  128. 128. Rath BH, Wahba A, Camphausen K, Tofilon PJ. Coculture with astrocytes reduces the radiosensitivity of glioblastoma stem-like cells and identifies additional targets for radiosensitization. Cancer Medicine. 2015;4(11):1705-1716
  129. 129. Biasoli D, Sobrinho MF, da Fonseca AC, de Matos DG, Romao L, de Moraes MR, Rehen SK, Moura-Neto V, Borges HL, Lima FR. Glioblastoma cells inhibit astrocytic p53-expression favoring cancer malignancy. Oncogene. 2014;3:e123
  130. 130. Muller PA, Vousden KH. p53 mutations in cancer. Nature Cell Biology. 2013;15(1):2-8
  131. 131. Kiaris H, Chatzistamou I, Trimis G, Frangou-Plemmenou M, Pafiti-Kondi A, Kalofoutis A. Evidence for nonautonomous effect of p53 tumor suppressor in carcinogenesis. Cancer Research. 2005;65(5):1627-1630
  132. 132. Alexandrova A, Ivanov A, Chumakov P, Kopnin B, Vasiliev J. Changes in p53 expression in mouse fibroblasts can modify motility and extracellular matrix organization. Oncogene. 2000;19(50):5826-5830
  133. 133. Wu K, Fukuda K, Xing F, Zhang Y, Sharma S, Liu Y, Chan MD, Zhou X, Qasem SA, Pochampally R, et al. Roles of the cyclooxygenase 2 matrix metalloproteinase 1 pathway in brain metastasis of breast cancer. The Journal of Biological Chemistry. 2015;290(15):9842-9854
  134. 134. Boyer LA, Lee TI, Cole MF, Johnstone SE, Levine SS, Zucker JP, Guenther MG, Kumar RM, Murray HL, Jenner RG, et al. Core transcriptional regulatory circuitry in human embryonic stem cells. Cell. 2005;122(6):947-956
  135. 135. Tavazoie M, Van der Veken L, Silva-Vargas V, Louissaint M, Colonna L, Zaidi B, Garcia-Verdugo JM, Doetsch F. A specialized vascular niche for adult neural stem cells. Cell Stem Cell. 2008;3(3):279-288
  136. 136. Goldman SA, Chen Z. Perivascular instruction of cell genesis and fate in the adult brain. Nature Neuroscience. 2011;14(11):1382-1389
  137. 137. Dawson DW, Volpert OV, Gillis P, Crawford SE, Xu H, Benedict W, Bouck NP. Pigment epithelium-derived factor: A potent inhibitor of angiogenesis. Science. 1999;285(5425):245-248
  138. 138. Cao L, Jiao X, Zuzga DS, Liu Y, Fong DM, Young D, During MJ. VEGF links hippocampal activity with neurogenesis, learning and memory. Nature Genetics. 2004;36(8):827-835
  139. 139. Leventhal C, Rafii S, Rafii D, Shahar A, Goldman SA. Endothelial trophic support of neuronal production and recruitment from the adult mammalian subependyma. Molecular and Cellular Neurosciences. 1999;13(6):450-464
  140. 140. Folkins C, Man S, Xu P, Shaked Y, Hicklin DJ, Kerbel RS. Anticancer therapies combining antiangiogenic and tumor cell cytotoxic effects reduce the tumor stem-like cell fraction in glioma xenograft tumors. Cancer Research. 2007;67(8):3560-3564
  141. 141. Lyon JG, Mokarram N, Saxena T, Carroll SL, Bellamkonda RV. Engineering challenges for brain tumor immunotherapy. Advanced Drug Delivery Reviews. 2017
  142. 142. Abbott NJ. Dynamics of CNS barriers: Evolution, differentiation, and modulation. Cellular and Molecular Neurobiology. 2005;25(1):5-23
  143. 143. Gieryng A, Pszczolkowska D, Walentynowicz KA, Rajan WD, Kaminska B. Immune microenvironment of gliomas. Laboratory Investigation; A Journal of Technical Methods and Pathology. 2017;97(5):498-518
  144. 144. Sofroniew MV. Multiple roles for astrocytes as effectors of cytokines and inflammatory mediators. The Neuroscientist. 2014;20(2):160-172
  145. 145. Hamby ME, Coppola G, Ao Y, Geschwind DH, Khakh BS, Sofroniew MV. Inflammatory mediators alter the astrocyte transcriptome and calcium signaling elicited by multiple G-protein-coupled receptors. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 2012;32(42):14489-14510
  146. 146. Zamanian JL, Xu L, Foo LC, Nouri N, Zhou L, Giffard RG, Barres BA. Genomic analysis of reactive astrogliosis. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 2012;32(18):6391-6410
  147. 147. Sofroniew MV. Molecular dissection of reactive astrogliosis and glial scar formation. Trends in Neurosciences. 2009;32(12):638-647
  148. 148. Sobel RA, Mitchell ME, Fondren G. Intercellular adhesion molecule-1 (ICAM-1) in cellular immune reactions in the human central nervous system. The American Journal of Pathology. 1990;136(6):1309-1316
  149. 149. Massa PT, Ozato K, McFarlin DE. Cell type-specific regulation of major histocompatibility complex (MHC) class I gene expression in astrocytes, oligodendrocytes, and neurons. Glia. 1993;8(3):201-207
  150. 150. Aloisi F, Ria F, Columba-Cabezas S, Hess H, Penna G, Adorini L. Relative efficiency of microglia, astrocytes, dendritic cells and B cells in naive CD4+ T cell priming and Th1/Th2 cell restimulation. European Journal of Immunology. 1999;29(9):2705-2714
  151. 151. Valiente M, Obenauf AC, Jin X, Chen Q, Zhang XH, Lee DJ, Chaft JE, Kris MG, Huse JT, Brogi E, et al. Serpins promote cancer cell survival and vascular co-option in brain metastasis. Cell. 2014;156(5):1002-1016
  152. 152. Lorger M. Tumor microenvironment in the brain. Cancers (Basel). 2012;4(1):218-243
  153. 153. Kostianovsky AM, Maier LM, Anderson RC, Bruce JN, Anderson DE. Astrocytic regulation of human monocytic/microglial activation. Journal of Immunology. 2008;181(8):5425-5432
  154. 154. Bechmann I, Steiner B, Gimsa U, Mor G, Wolf S, Beyer M, Nitsch R, Zipp F, Astrocyte-induced T. Cell elimination is CD95 ligand dependent. Journal of Neuroimmunology. 2002;132(1–2):60-65
  155. 155. Sofroniew MV, Vinters HV. Astrocytes: Biology and pathology. Acta Neuropathologica. 2010;119(1):7-35
  156. 156. Voskuhl RR, Peterson RS, Song B, Ao Y, Morales LB, Tiwari-Woodruff S, Sofroniew MV. Reactive astrocytes form scar-like perivascular barriers to leukocytes during adaptive immune inflammation of the CNS. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 2009;29(37):11511-11522
  157. 157. Ekstrand AJ, James CD, Cavenee WK, Seliger B, Pettersson RF, Collins VP. Genes for epidermal growth factor receptor, transforming growth factor alpha, and epidermal growth factor and their expression in human gliomas in vivo. Cancer Research. 1991;51(8):2164-2172
  158. 158. Meco D, Servidei T, Riccardi A, Ferlini C, Cusano G, Zannoni GF, Giangaspero F, Riccardi R. Antitumor effect in medulloblastoma cells by gefitinib: Ectopic HER2 overexpression enhances gefitinib effects in vivo. Neuro-Oncology. 2009;11(3):250-259
  159. 159. Kim JK, Jin X, Sohn YW, Jin X, Jeon HY, Kim EJ, Ham SW, Jeon HM, Chang SY, SY O, et al. Tumoral RANKL activates astrocytes that promote glioma cell invasion through cytokine signaling. Cancer Letters. 2014;353(2):194-200
  160. 160. Seike T, Fujita K, Yamakawa Y, Kido MA, Takiguchi S, Teramoto N, Iguchi H, Noda M. Interaction between lung cancer cells and astrocytes via specific inflammatory cytokines in the microenvironment of brain metastasis. Clinical & Experimental Metastasis. 2011;28(1):13-25
  161. 161. Huttenlocher A, Lakonishok M, Kinder M, Wu S, Truong T, Knudsen KA, Horwitz AF. Integrin and cadherin synergy regulates contact inhibition of migration and motile activity. The Journal of Cell Biology. 1998;141(2):515-526
  162. 162. Yang C, Iyer RR, AC Y, Yong RL, Park DM, Weil RJ, Ikejiri B, Brady RO, Lonser RR, Zhuang Z. Beta-catenin signaling initiates the activation of astrocytes and its dysregulation contributes to the pathogenesis of astrocytomas. Proceedings of the National Academy of Sciences of the United States of America. 2012;109(18):6963-6968
  163. 163. Behrens J, Jerchow BA, Wurtele M, Grimm J, Asbrand C, Wirtz R, Kuhl M, Wedlich D, Birchmeier W. Functional interaction of an axin homolog, conductin, with beta-catenin, APC, and GSK3beta. Science. 1998;280(5363):596-599
  164. 164. Weidle UH, Birzele F, Kollmorgen G, Ruger R. Dissection of the process of brain metastasis reveals targets and mechanisms for molecular-based intervention. Cancer Genomics Proteomics. 2016;13(4):245-258
  165. 165. Shumakovich MA, Mencio CP, Siglin JS, Moriarty RA, Geller HM, Stroka KM. Astrocytes from the brain microenvironment alter migration and morphology of metastatic breast cancer cells. FASEB Journal: Official Publication of the Federation of American Societies for Experimental Biology. 2017
  166. 166. Cathcart J, Pulkoski-Gross A, Cao J. Targeting matrix metalloproteinases in cancer: Bringing new life to old ideas. Genes and Diseases. 2015;2(1):26-34
  167. 167. Radisky ES, Raeeszadeh-Sarmazdeh M, Radisky DC. Therapeutic potential of matrix metalloproteinase inhibition in breast cancer. Journal of Cellular Biochemistry. 2017;118(11):3531-3548
  168. 168. Stark AM, Anuszkiewicz B, Mentlein R, Yoneda T, Mehdorn HM, Held-Feindt J. Differential expression of matrix metalloproteinases in brain- and bone-seeking clones of metastatic MDA-MB-231 breast cancer cells. Journal of Neuro-Oncology. 2007;81(1):39-48
  169. 169. Zucker S, Cao J, Chen WT. Critical appraisal of the use of matrix metalloproteinase inhibitors in cancer treatment. Oncogene. 2000;19(56):6642-6650
  170. 170. Stoletov K, Strnadel J, Zardouzian E, Momiyama M, Park FD, Kelber JA, Pizzo DP, Hoffman R, VandenBerg SR, Klemke RL. Role of connexins in metastatic breast cancer and melanoma brain colonization. Journal of Cell Science. 2013;126(Pt 4):904-913
  171. 171. Hohensee I, Lamszus K, Riethdorf S, Meyer-Staeckling S, Glatzel M, Matschke J, Witzel I, Westphal M, Brandt B, Muller V, et al. Frequent genetic alterations in EGFR- and HER2-driven pathways in breast cancer brain metastases. The American Journal of Pathology. 2013;183(1):83-95
  172. 172. Li J, Yen C, Liaw D, Podsypanina K, Bose S, Wang SI, Puc J, Miliaresis C, Rodgers L, McCombie R, et al. PTEN, a putative protein tyrosine phosphatase gene mutated in human brain, breast, and prostate cancer. Science. 1997;275(5308):1943-1947
  173. 173. Hohensee I, Chuang HN, Grottke A, Werner S, Schulte A, Horn S, Lamszus K, Bartkowiak K, Witzel I, Westphal M, et al. PTEN mediates the cross talk between breast and glial cells in brain metastases leading to rapid disease progression. Oncotarget. 2017;8(4):6155-6168
  174. 174. Shipman L. Microenvironment: Astrocytes silence PTEN to promote brain metastasis. Nature Reviews Cancer. 2015;15(12):695
  175. 175. Le MT, Hamar P, Guo C, Basar E, Perdigao-Henriques R, Balaj L, Lieberman J. miR-200-containing extracellular vesicles promote breast cancer cell metastasis. The Journal of Clinical Investigation. 2014;124(12):5109-5128
  176. 176. Neman J, Choy C, Kowolik CM, Anderson A, Duenas VJ, Waliany S, Chen BT, Chen MY, Jandial R. Co-evolution of breast-to-brain metastasis and neural progenitor cells. Clinical & Experimental Metastasis. 2013;30(6):753-768
  177. 177. Neman J, Termini J, Wilczynski S, Vaidehi N, Choy C, Kowolik CM, Li H, Hambrecht AC, Roberts E, Jandial R. Human breast cancer metastases to the brain display GABAergic properties in the neural niche. Proceedings of the National Academy of Sciences of the United States of America. 2014;111(3):984-989
  178. 178. Attwell D, Laughlin SB. An energy budget for signaling in the grey matter of the brain. Journal of Cerebral Blood Flow and Metabolism. 2001;21(10):1133-1145
  179. 179. Belanger M, Allaman I, Magistretti PJ. Brain energy metabolism: Focus on astrocyte-neuron metabolic cooperation. Cell Metabolism. 2011;14(6):724-738
  180. 180. Menon S, Shin S, Dy G. Advances in cancer immunotherapy in solid Tumors. Cancers (Basel). 2016;8(12)
  181. 181. Papaioannou NE, Beniata OV, Vitsos P, Tsitsilonis O, Samara P. Harnessing the immune system to improve cancer therapy. Annals of Translational Medicine. 2016;4(14):261
  182. 182. Khong H, Overwijk WW. Adjuvants for peptide-based cancer vaccines. Journal for ImmunoTherapy of Cancer. 2016;4:56
  183. 183. Mitchell DA, Batich KA, Gunn MD, Huang MN, Sanchez-Perez L, Nair SK, Congdon KL, Reap EA, Archer GE, Desjardins A, et al. Tetanus toxoid and CCL3 improve dendritic cell vaccines in mice and glioblastoma patients. Nature. 2015;519(7543):366-369
  184. 184. Ishida Y, Agata Y, Shibahara K, Honjo T. Induced expression of PD-1, a novel member of the immunoglobulin gene superfamily, upon programmed cell death. The EMBO Journal. 1992;11(11):3887-3895
  185. 185. Topalian SL, Hodi FS, Brahmer JR, Gettinger SN, Smith DC, McDermott DF, Powderly JD, Carvajal RD, Sosman JA, Atkins MB, et al. Safety, activity, and immune correlates of anti-PD-1 antibody in cancer. The New England Journal of Medicine. 2012;366(26):2443-2454
  186. 186. Iwai Y, Hamanishi J, Chamoto K, Honjo T. Cancer immunotherapies targeting the PD-1 signaling pathway. Journal of Biomedical Science. 2017;24(1):26
  187. 187. Berghoff AS, Kiesel B, Widhalm G, Rajky O, Ricken G, Wohrer A, Dieckmann K, Filipits M, Brandstetter A, Weller M, et al. Programmed death ligand 1 expression and tumor-infiltrating lymphocytes in glioblastoma. Neuro-Oncology. 2015;17(8):1064-1075
  188. 188. Bloch O, Crane CA, Kaur R, Safaee M, Rutkowski MJ, Parsa AT. Gliomas promote immunosuppression through induction of B7-H1 expression in tumor-associated macrophages. Clinical Cancer Research: An Official Journal of the American Association for Cancer Research. 2013;19(12):3165-3175
  189. 189. Mishra V, Schuetz H, Haorah J. Differential induction of PD-1/PD-L1 in Neuroimmune cells by drug of abuse. International Journal of Physiology, Pathophysiology and Pharmacology. 2015;7(2):87-97
  190. 190. Duchnowska R, Peksa R, Radecka B, Mandat T, Trojanowski T, Jarosz B, Czartoryska-Arlukowicz B, Olszewski WP, Och W, Kalinka-Warzocha E, et al. Immune response in breast cancer brain metastases and their microenvironment: The role of the PD-1/PD-L axis. Breast Cancer Research : BCR. 2016;18(1):43
  191. 191. Berghoff AS, Ricken G, Widhalm G, Rajky O, Dieckmann K, Birner P, Bartsch R, Holler C, Preusser M. Tumour-infiltrating lymphocytes and expression of programmed death ligand 1 (PD-L1) in melanoma brain metastases. Histopathology. 2015;66(2):289-299
  192. 192. Harter PN, Bernatz S, Scholz A, Zeiner PS, Zinke J, Kiyose M, Blasel S, Beschorner R, Senft C, Bender B, et al. Distribution and prognostic relevance of tumor-infiltrating lymphocytes (TILs) and PD-1/PD-L1 immune checkpoints in human brain metastases. Oncotarget. 2015;6(38):40836-40849
  193. 193. Langley RR, Fidler IJ. Tumor cell-organ microenvironment interactions in the pathogenesis of cancer metastasis. Endocrine Reviews. 2007;28(3):297-321
  194. 194. Gupta GP, Massague J. Cancer Metastasis: Building a framework. Cell. 2006;127(4):679-695
  195. 195. Walker AE, Robins M, Weinfeld FD. Epidemiology of brain tumors: the national survey of intracranial neoplasms. Neurology. 1985;35(2):219-226
  196. 196. Percy AK, Elveback LR, Okazaki H, Kurland LT. Neoplasms of the central nervous system. Epidemiologic considerations. Neurology. 1972;22(1):40-48
  197. 197. Counsell CE, Collie DA, Grant R. Incidence of intracranial tumours in the Lothian region of Scotland, 1989-90. Journal of Neurology, Neurosurgery, and Psychiatry. 1996;61(2):143-150
  198. 198. Ostrom QT, Gittleman H, Xu J, Kromer C, Wolinsky Y, Kruchko C, Barnholtz-Sloan JS. CBTRUS statistical report: Primary brain and other central nervous system tumors diagnosed in the United States in 2009–2013. Neuro-Oncology. 2016;18(suppl_5):v1-v75

Written By

Emily Gronseth, Ling Wang, David R. Harder and Ramani Ramchandran

Submitted: May 17th, 2017 Reviewed: November 24th, 2017 Published: March 21st, 2018