The Oncologist, Vol. 12, No. 12, 1395-1403, December 2007; doi:10.1634/theoncologist.12-12-1395
© 2007 AlphaMed Press
Proautophagic Drugs: A Novel Means to Combat Apoptosis-Resistant Cancers, with a Special Emphasis on Glioblastomas
Florence Lefranca,b,
Vincenzo Facchinic,
Robert Kissb
aDepartment of Neurosurgery, Erasme University Hospital, Brussels, Belgium;
bLaboratory of Toxicology, Institute of Pharmacy, Université Libre de Bruxelles, Brussels, Belgium;
cUnibioscreen SA, Brussels, Belgium
Key Words. Apoptosis • Autophagy • Glioblastoma • Melanoma • Metastatic cancer • Temozolomide
Correspondence: Florence Lefranc, M.D., Ph.D., Department of Neurosurgery, Erasmus Academic Hospital, Université Libre de Bruxelles, 808 Route de Lennik, 1070 Brussels, Belgium. Telephone: 32-474-477-192; Fax: 32-2-332-5335; e-mail: fllefran{at}ulb.ac.be
Received April 24, 2007;
accepted for publication October 9, 2007.
Disclosure: No potential conflicts of interest were reported by the authors, planners, reviewers, or staff managers of this article.
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Learning Objectives
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After completing this course, the reader will be able to:- Describe the pathways involved in the natural resistance of cancer cells to cytotoxic insults including radio-/chemotherapy.
- Explain autophagic cell death as a potent alternative tumor-suppressing mechanism.
- Identify the common targets in apoptosis and autophagy resistance pathways and the surrogate markers that could be used in clinical practice for proautophagic therapy.
- Discuss the rationale for incorporating endoplasmic reticulum stress inhibitors as adjuvant chemotherapies against apoptosis-resistant cancers.
Access and take the CME test online and receive 1 AMA PRA Category 1 CreditTM at CME.TheOncologist.com
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ABSTRACT
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The therapeutic goal of cancer treatment has been to trigger tumor-selective cell death. Although cell death can be achieved not only by apoptosis (type I programmed cell death) but also by necrosis, mitotic catastrophe, and autophagy, drugs inducing apoptosis remain the main chemotherapeutic agents in medical oncology. However, cancer cells in their relentless drive to survive, hijack cell processes, resulting in apoptosis resistance, which underlies not only tumorigenesis but also the inherent resistance of certain cancers to radiotherapy and chemotherapy. Unlike apoptosis, which is a caspase-dependent process characterized by nuclear condensation and fragmentation, autophagic cell death is a caspase-independent process characterized by the accumulation of autophagic vacuoles in the cytoplasm accompanied by extensive degradation of the Golgi apparatus, the polyribosomes, and the endoplasmic reticulum, which precedes the destruction of the nucleus. The most striking evidence for proautophagic chemotherapy to overcome apoptosis resistance in cancer cells comes from the use of temozolomide, a proautophagic cytotoxic drug, which has demonstrated real therapeutic benefits in glioblastoma patients and is in clinical trials for several types of apoptosis-resistant cancers. A number of potential common targets in autophagy and apoptosis resistance pathways, that is, mammalian target of rapamycin (mTOR), phosphatidylinositol 3' kinase (PI3K), and Akt have been identified. Thus, further success in certain devastating cancers might be achieved by the combination of proautophagic drugs such as temozolomide with mTOR, PI3K, or Akt inhibitors, or with endoplasmic reticulum stress inhibitors as adjuvant chemotherapies.
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PROAPOPTOTIC DRUGS: CURRENTLY THE MAIN CHEMOTHERAPEUTIC AGENTS IN MEDICAL ONCOLOGY
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The therapeutic goal of cancer treatment has been to trigger tumor-selective cell death [1, 2]. The response of tumors to chemotherapy, radiotherapy, and hormone therapy or to treatment with biologically active agents depends at least in part on their propensity to undergo cell death [1]. Some cancers, for example, leukemia, small cell lung cancer, and seminomas, respond quickly to first-line therapy. This fast response is thought to result from induction of apoptosis (type I programmed cell death [PCD]). Solid tumors, on the other hand, usually respond slowly and less effectively, with cell death characterized not only by apoptosis but also by necrosis or mitotic catastrophe [1–3]. However, proapoptotic drugs have been and remain the main chemotherapeutic agents in medical oncology. Over the past two decades, the role of apoptosis in the cytotoxicity of anticancer drugs has become clear [1]. Apoptosis may occur via death receptor–dependent (extrinsic) or independent (intrinsic or mitochondrial) pathways [3, 4]. The mitochondrial pathway of cell death is mediated by Bcl-2 family proteins, a group of antiapoptotic and proapoptotic proteins that regulate the passage of small molecules, such as cytochrome c, second mitochondria-derived activator of caspase (Smac)/direct IAP binding protein with low pI and apoptosis-inducing factor, which activate caspase cascades through the mitochondrial transition pore [4]. In cells ready to die, proapoptotic Bcl-2 family members like Bax, disrupt mitochondria, causing the release of other proteins that lead to caspase release and cell death [3]. Activation of this so-called intrinsic apoptotic pathway is the goal of many of the new cancer drugs [2, 4, 5]. Thus, many small-molecule Bcl-2 family inhibitors are currently in the pipeline [2, 5–7]. The extrinsic cell death pathway is also an important target [8]. Around 1996, the first so-called death receptor, DR4, was discovered. Bound by an endogenous ligand dubbed tumor necrosis factor–related apoptosis-inducing ligand (TRAIL), DR4 and its sister receptor, DR5, activate a pathway that ultimately also leads to cell death, with remarkable and still largely mysterious specificity for cancer cells [8]. Human Genome Sciences Inc., under license from Cambridge Antibody Technology Ltd, has mounted a major effort to find drugs activating the TRAIL death receptors (agonist antibodies and recombinant TRAIL ligands) [3, 9, 10]. Three products are currently in clinical trials, with the most advanced candidate in phase II [10].
Another apoptosis target generating great excitement is X-linked inhibitor of apoptosis (XIAP) [3], which binds to three key caspases, preventing them from activating and killing cancer cells. However, like Bcl-2 inhibitors, XIAP inhibitors must block a protein–protein interaction. Smac, an endogenous protein that is released from mitochondria, binds with XIAP and inactivates it, triggering apoptosis [11]. This discovery raised the possibility of Smac mimetics to treat cancer [11]. Most companies are designing drugs targeting one of the two XIAP-caspase binding domains in order to mimic Smac's full caspase activation function and ensure that apoptosis takes place. Small-molecule XIAP inhibitors are in preclinical evaluation. All three approaches, Bcl-2 inhibitors, TRAIL modulators, and Smac mimetics, have thus yet to be clinically validated.
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NATURAL RESISTANCE OF CANCER CELLS TO APOPTOSIS
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Resistance to apoptosis is considered to be a characteristic of many diverse cancer cells [3]. Defects in apoptosis underlie not only tumorigenesis but also resistance to cancer treatments [3]. Furthermore, the inherent resistance of cancer cells to radiotherapy and chemotherapy is contributed to by changes at genomic, transcriptional, and post-transcriptional levels of proteins and protein kinases and their transcriptional factor effectors [3] (Fig. 1). The phosphatase and tensin homologue deleted on chromosome ten (PTEN)/phosphatidylinositol 3' kinase (PI3K)/Akt /mammalian target of rapamycin (mTOR)/nuclear factor kappa B (NF-
B) and the Ras/Raf/mitogen-activated protein kinase–extracellular signal-related kinase (ERK) kinase (MEK)/ERK signaling cascades play critical roles in the transmission of signals from growth factor receptors to regulate gene expression and prevent apoptosis [12] (Fig. 1). Components of these pathways are mutated or aberrantly expressed in human cancer (e.g., Ras, B-Raf, PI3K, PTEN, and Akt) [12]. The activity of the PI3K/Akt and the mTOR pathway is often constitutively upregulated in tumors as a result of excessive stimulation by growth factor receptors, as well as mutations in the PTEN tumor suppressor gene, whose tumor suppressor properties are closely related to its inhibitory effect on the PI3K-dependent activation of Akt signaling [12] (Fig. 1). In some cells, PTEN mutation may contribute to suppression of the Raf/MEK/ERK cascade because of the ability of elevated activated Akt levels to phosphorylate and inactivate Raf-1 [3, 12] (Fig. 1). An aberrantly activated PI3K/Akt pathway renders tumor cells resistant to cytotoxic insults, including those related to proapoptotic anticancer drugs [12–18]. Deregulation of the PTEN/PI3K/Akt pathway has been associated with resistance to chemotherapeutic drugs used in breast cancer therapy, and also prostate cancer, ovarian cancer, and malignant gliomas [12–18]. Shingu et al. [19] have shown, in the context of glioma cells, that the inhibition of this pathway restores or even augments the effectiveness of chemotherapy. In the same manner, the identification of activating mutations in the Ras signaling pathway opens up new therapeutic options for Ras and Raf inhibitors [12, 20]. In breast cancer cells, activated Raf conferred resistance to the chemotherapeutic drugs doxorubicin and paclitaxel [12]. Ras/Raf pathway activation is also involved in head and neck cancer resistance to apoptosis [20]. The complexity of cell survival through Akt signaling was recently reviewed by McCormick [21], while the TOR pathway as a target for cancer therapy has been reviewed by Bjornsti and Houghton [22]. Preclinical studies suggested that sensitivity to mTOR inhibitors may correlate with activation of the PI3K pathway and/or with aberrant expression of cell-cycle regulatory or antiapoptotic proteins. mTOR inhibitors are currently under evaluation in clinical trials and include rapamycin (sirolimus) and the related derivatives temsirolimus (CCI-779), everolimus (RAD001), and AP23573 [23].
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Figure 1. Cancer cell resistance to apoptosis. Pathways involved in the natural resistance of cancer cells to radio/chemotherapy. Pathways involved in cytotoxic insult resistance are presented in orange and the inhibitors that could be used to restore chemosensitivity to pro-apoptotic drugs are presented in blue.
Abbreviations: ERK, extracellular signal–related kinase; MAPK, mitogen-activated protein kinase; mTOR, mammalian target of rapamycin; NFB, nuclear factor kappa B; P, phosphorylated; PI3K, phosphatidylinositol 3' kinase; PIP2, phosphatidylinositol-4,5-bisphosphate; PTEN, phosphatase and tensin homologue deleted on chromosome ten; SOS, guanine nucleotide exchange factor SOS (Son of Sevenless).
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These trials have shown that mTOR inhibitors are well tolerated and may induce prolonged stable disease and tumor regressions in cancer patients [23].
Constitutive activation of the NF-B pathway also enables cancer cells (including glioblastoma cells) to resist cytotoxic insults [16, 17, 24]. These changes also affect tumor necrosis factor, Fas, and TRAIL receptors, which play important roles in tumor resistance to apoptosis during cancer progression, and principally melanoma development [25, 26].
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FROM AUTOPHAGY TO AUTOPHAGIC CELL DEATH
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Autophagy (Greek for "the eating of oneself"), more correctly referred to as macroautophagy, is a major intracellular process for the degradation and recycling of long-lived superfluous proteins and cytoplasmic damaged organelles, contributing to the maintenance of cellular homeostasis [27]. The autophagy story began in 1966 when De Duve and Wattiaux [28] discovered lysosomes, with the significance that autophagy is a lysosome-mediated degradation of proteins and cellular organelles [27]. The term autophagic cell death or type II PCD was coined by Schweichel and Merker in 1973 [29].
Autophagy is essential for maintaining cell survival following a variety of extracellular and intracellular stimuli including growth factor deprivation. This mechanism of promoting cell autonomous survival is necessarily self-limited and ultimately results in autophagic cell death if the stress imposed on the cell is sustained (Fig. 2) [30]. In fact, autophagy has been reported to initiate cell death in response to intracellular damage caused by hypoxia, chemotherapeutic agents, virus infection, or toxins (Fig. 2) [31–33]. Unlike apoptosis, which is a caspase-dependent process characterized by nuclear condensation and fragmentation, but without major ultrastructural changes in cytoplasmic organelles, autophagic cell death is a caspase-independent process characterized by the accumulation of autophagic vacuoles in the cytoplasm accompanied by extensive degradation of the Golgi apparatus, the polyribosomes, and the endoplasmic reticulum, which precedes the destruction of the nucleus [3, 32, 34, 35]. Autophagy involves the sequestration of cytosol or cytoplasmic organelles within double membranes, thus creating autophagosomes (also called autophagic vacuoles) (Fig. 3) [35]. Autophagosomes subsequently fuse with endosomes and eventually with lysosomes, thereby creating autophagolysosomes or autolysosomes. In the lumen of these latter structures, lysosomal enzymes operating at low pH then catabolize the trapped material (Fig. 3) [33]. The point at which autophagy becomes autophagic cell death or even apoptosis remains unclear. In some cases, apoptotic and autophagic cell death coincide in vivo in certain tissues, and in rare cases both processes may coincide within the same cells [32]. Apoptosis can begin with autophagy, and autophagy can end with apoptosis (Figs. 2 and 3). More interestingly, the blockage of caspase activity can cause a cell to default to type II cell death from type I [36]. Autophagy can therefore be seen as a backup mechanism that comes into action when apoptosis is inhibited.
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Figure 2. Autophagy and endoplasmic reticulum stress (ERS) pathways: survival or cell death? Conditions of starvation in the central area of a tumor mass and subsequent ATP depletion or sublethal damage induce autophagy and/or ERS/unfolded protein response (UPR) in the tumor cells to survive. These processes can ultimately result in programmed cell death if the stress imposed is sustained.
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Figure 3. Simplified view of the pathways of autophagy.
Abbreviations: LC3, light chain 3; mTOR, mammalian target of rapamycin; PDK-1, phosphoinositide-dependent protein kinase-1; PI3K, phosphatidylinositol 3' kinase.
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SIMPLIFIED VIEW OF THE PATHWAYS OF AUTOPHAGY
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The autophagic process in mammalian cells is regulated by homologues of the Apg and Aut autophagy-relevant family of yeast genes, now renamed Atg genes [27]. Several Atg proteins have been implicated in autophagosome formation. Atg5, Atg7, Atg10, and Atg12 are required to form the autophagic vacuole (Fig. 3). PI3K, the enzyme synthesizing phosphatidylinositol-3-phosphate (PtdIns 3P) from PtdIns, is a major player in mammalian autophagic pathways. While class III PI3K is required in the early stages of autophagosome generation, class I PI3K activity has an inhibitory effect, mediated at least partially through mTOR (Fig. 3) [32, 34]. Thus class I PI3K, Akt, and mTOR are components of the pathways that are involved in both apoptosis and autophagy resistance of cancer cells. Beclin 1, a homologue of the yeast autophagy protein Atg6, which belongs to the class III PI3K complex, is required for vacuolar formation and transport [32]. In addition to interacting with class III PI3K, Beclin 1 is able to bind Bcl-2 proapoptotic family members (Fig. 3) [37]. While the interaction of Beclin 1 with class III PI3K stimulates autophagy and inhibits oncogenesis, its interaction with Bcl-2 inhibits autophagy and stimulates oncogenesis (Fig. 3) [37]. Autophagy is also induced by cell death–associated protein kinase and death-associated related protein kinase 1 [32, 34]. Recent research has also revealed that autophagy is activated by p53, the guardian of the genome, a critical tumor suppressor that is involved and mutated in >50% of cancers of all tissue origins [38, 39]. Crighton et al. [40] have described a damage-regulated autophagy modulator gene (DRAM), a p53 target gene encoding a lysosomal protein that induces autophagy. Like Beclin 1, analysis of DRAM in a subset of epithelial tumors revealed frequent decreased expression [40]. Finally, microtubule-associated protein 1 light chain 3 (MAP1LC3) exists in two forms: MAP1LC3-I, localized in the cytosol, and its proteolytic derivative, MAP1LC3-II, localized in autophagosomal membranes [32]. During autophagy, MAP1LC3-I is cleaved and conjugated to phosphatidylethanolamine to form MAP1LC3-II, which is essential for the formation of the autophagosome [32]. MAP1LC3-II thus can be used to estimate the abundance of autophagosomes before they are destroyed through fusion with lysosomes (Fig. 3) [32].
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THE CANCER CONNECTION TO AUTOPHAGY
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The first clues linking the autophagic process and cancer appeared in the late 1970s. Several studies revealed lower rates of proteolysis in transformed cells than in their normal counterparts [41, 42]. In the early stages of tumor development, autophagy functions as a tumor suppressor [32]. A decrease in protein degradation by autophagy would shift the balance between protein synthesis and degradation toward synthesis, increasing intracellular protein content and thus favoring cellular growth [32]. By contrast, a proautophagic drug could block tumor growth and function as a tumor suppressor. Beclin 1, which is necessary to induce autophagy, is monoallelically deleted in a large number of sporadic breast, ovarian, and prostate cancers [43, 44]. Miracco et al. [45] recently examined Beclin 1 protein expression in a series of 212 primary human brain tumors, including gliomas, meningiomas, and medulloblastomas. The authors demonstrated that the expression levels of Beclin 1 were inversely proportional to primary brain tumor grade [45]. Furthermore, the introduction of Beclin 1 into MCF7 breast cancer cells induced autophagy and inhibited tumorigenesis when these transfected MCF7 cells were implanted into nude mice [43]. Several other autophagy genes (i.e., Atg7, MAP1LC3, see previous paragraph) are also localized in chromosomal regions that are frequently lost in human cancers, including breast, prostate, ovarian, and lung cancers [46–49]. Although autophagy is suppressed during the early stages of tumor progression, it seems to be upregulated during later stages as a protective mechanism against stress conditions such as low oxygen and/or nutrient levels [50, 51]. Given that autophagy is decreased or suppressed during the early stages of tumor progression and cancer cells carry mutations that inactivate apoptotic pathways [3, 12, 17], therapeutically increasing autophagy could represent an alternative means to destroy cancer cells.
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TARGETING AUTOPHAGY: THE POTENTIAL FOR CANCER THERAPY
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A variety of chemical or physical treatments, including rapamycin (mTOR inhibitor) [22, 52], radiation [53], arsenic trioxide [54], ceramide [55], temozolomide [31], dopamine [56], endostatin [57], the histone deacetylase (HDAC) inhibitors butyrate and suberoylanilide hydroxamic acid [58], neodymium oxide [59], the chemotherapeutic vitamin D analogue EB1089 [60], saponins [61], and resveratrol [62], have been reported to induce autophagy in vitro and in vivo in certain cancer cells. Notably, rapamycin analogues, arsenic trioxide, and temozolomide bring great hope of combating apoptosis-resistant cancers [23].
Rapamycin and its analogues (such as CCI-779, RAD001, and AP23573) inhibit mTOR, the kinase that normally suppresses both apoptosis and autophagy and is active when nutrients are abundant [22]. Rapamycin activates the autophagic process [32, 34], and the inhibition of autophagy by small interfering RNA (siRNA) directed against the autophagy-related gene beclin 1 attenuates the cytotoxicity of rapamycin in rapamycin-sensitive tumor cells, indicating that autophagy is a primary mediator of rapamycin-mediated antitumor effects rather than a protective response [63]. Exogenous expression of an mTOR mutant interfering with its kinase activity markedly enhances the incidence of rapamycin-induced autophagy [63]. Moreover, silencing of mTOR with siRNA increases the inhibitory effect of rapamycin on tumor cell viability by stimulating autophagy [63]. Importantly, not only rapamycin-sensitive malignant glioma cells but also rapamycin-resistant malignant glioma cells with wild-type PTEN are sensitized to rapamycin by mTOR siRNA [63]. In addition, mTOR inhibitors sensitize tumor cells to DNA-damaging agents in vitro. Phase I and II clinical trials with mTOR inhibitors are ongoing in patients with recurrent malignant gliomas, advanced solid tumors, advanced non-small cell lung carcinoma, endometrial cancer, metastatic breast cancer, prostate cancer, and metastatic renal carcinoma [22, 23].
It appears that five phase I and II clinical trials with arsenic trioxide are currently recruiting patients with gliomas, advanced solid tumors, metastatic liver cancer, and pediatric solid tumors. One trial with resveratrol is recruiting patients with colon cancer (http://www.clinicaltrials.gov website).
Temozolomide contributes significant therapeutic benefits in glioblastoma patients [64, 65]. Indeed, the addition of temozolomide to radiotherapy resulted in a longer median survival time in newly diagnosed glioblastoma patients, 14.6 versus 12.1 months, and a higher 2-year survival rate, 26.5% versus 10.4% [64].
Part of temozolomide cytotoxic activity is exerted through proautophagic processes, at least in glioblastoma cells, as a result of the formation of O6-methylguanine in DNA, which mispairs with thymine during the following cycle of DNA replication [31, 66]. Glioma cells thus respond to temozolomide by undergoing G2/M arrest, but ultimately die from autophagy [31, 66]. Knowing that O6-alkylguanine-DNA alkyltransferase (AGT) is a DNA repair enzyme that limits the efficacy of temozolomide in glioblastoma cells, Kanzawa et al. [66] first showed that inhibition of AGT by O6-benzylguanine can render previously resistant glioblastoma cells sensitive to temozolomide. Hegi and colleagues [67] subsequently offered hope of even greater improvements in the survival of glioblastoma patients in the near future, through identification of a molecular marker in the tumor that allows the prediction of benefit from the new treatment. Indeed, the data obtained by Hegi et al. [67] show that patients who had glioblastomas that contained a methylated O6-methylguanine-DNA methyltransferase (MGMT) promoter benefited from temozolomide, while those who did not were less responsive.
Part of temozolomide cytotoxic activity is also a result of the induction of late apoptosis [68]. Indeed, Roos et al. [68] showed that malignant glioma cells undergo apoptosis following treatment with the methylating agents N-methyl-N'- nitro-N-nitrosoguanidine and temozolomide. Transfection experiments with MGMT and depletion of MGMT by O6-benzylguanine showed that, in gliomas, the apoptotic signal originates from O6-methylguanine (O6MeG) and that repair of O6MeG by MGMT prevents apoptosis [68]. Roos et al. [68] further demonstrated that O6MeG-triggered apoptosis requires Fas/CD95/Apo-1 receptor activation in p53 nonmutated glioma cells, whereas in p53 mutated gliomas the same DNA lesion triggers the mitochondrial apoptotic pathway. This occurs less effectively via Bcl-2 degradation and caspase-9, -2, -7 and -3 activation [68]. O6MeG-triggered apoptosis in gliomas is a late response (occurring >120 hours after treatment) that requires extensive cell proliferation. Overall, the data reported by Roos et al. [68] thus demonstrate that cell death induced by temozolomide in gliomas is a result of apoptosis and that determinants of sensitivity of gliomas to temozolomide are MGMT, p53, proliferation rate, and double-strand break repair.
The data reported by Kanzawa et al. [31] and Roos et al. [68] are not contradictory, because autophagy and apoptosis may be triggered by common upstream signals, and sometimes this results in combined autophagy and apoptosis [69]. In other instances, the cell switches between the two responses in a mutually exclusive manner [69]. On a molecular level, this means that the apoptotic and autophagic response machineries share common pathways that either link or polarize the cellular responses [69].
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POTENTIAL SURROGATE MARKERS FOR PROAUTOPHAGIC THERAPY
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As traditional clinical endpoints prove more difficult to apply in the evaluation of molecularly targeted therapies, a great need exists to define and validate surrogate markers of effect and benefit [70]. Given that the response to temozolomide is at least partly associated with low MGMT protein expression [67], MGMT methylation analysis by means of reverse transcription-polymerase chain reaction techniques or MGMT immunostaining could be used to predict tumor sensitivity to the drug. However, this surrogate marker of response is not really related directly to the proautophagic phenomenon. In the same manner, mTOR expression could be evaluated and high tumor mTOR protein levels might indicate suitability for a proautophagic inhibitor strategy. Most trials using mTOR inhibitors do not measure mTOR levels. Instead, the commonly studied biomarkers usually involve downstream effectors, such as the phosphorylation of ribosomal p70 S6 kinase, which is considered to be a good indicator of the activated Akt/mTOR pathway as well as rapamycin sensitivity [71].
Analysis of DRAM expression, a damage-regulated autophagy modulator gene encoding a lysosomal protein that induces autophagy, could potentially also be used as a surrogate marker before the administration of a proautophagic drug. In addition, the analysis of MAP1LC3-II that estimates the abundance of autophagosomes before they are destroyed could be used to follow the proautophagic effects of a new compound.
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WHAT HAPPENS IF PROAUTOPHAGIC TREATMENT FAILS? THE ENDOPLASMIC RETICULUM STRESS RESPONSE CONNECTION
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Endoplasmic reticulum stress (ERS) is related to the fact that cancer cells in poorly vascularized areas of rapidly growing solid tumors (including glioblastomas) are constantly or intermittently exposed to nutrient deprivation, notably glucose deprivation, hypoxia, and redox and glycosylation reactions, as well as disturbances to their calcium mobilization [72–74] (Fig. 2). This activates a cytoprotective signal known as the unfolded protein response (UPR), which leads to translational attenuation and selective upregulation of a number of transcription factors [72, 73, 75]. The UPR has multiple functions, including assisting protein folding via the upregulated ER protein chaperones and enhancing the degradation of misfolded proteins via the upregulation of molecules involved in the ER-associated degradation pathway. However, excessive or long-term stress in the ER could lead to cell death because the compensatory mechanisms may not be able to fully sustain ER function [72, 73] (Fig. 2).
Several studies point to UPR activation in a variety of tumors from patients and in animal models for various cancer types [76, 77]. A lower level of susceptibility to cell death upon activation of the UPR may contribute to tumor progression and drug resistance in solid tumors [72, 76, 78, 79]. Thus, inhibition of the UPR might improve the effectiveness of chemotherapy [72]. Several recent studies have demonstrated that interfering with the activation of different arms of the UPR or altering the levels of the ER molecular chaperone Grp78/BiP, a master regulator of ER function and the UPR, can enhance chemosensitivity and inhibit tumor growth in vivo [76, 77, 80]. Inhibitors of the cell stress response are also being examined as potential cancer therapeutic agents [78, 80, 81–84]. Heat shock protein 90 (Hsp90) is another regulator of ER function and Hsp90 inhibitors cause inactivation, destabilization, and eventual degradation of Hsp90 client proteins, and they have shown promising antitumor activity in various preclinical tumor models [81–83]. The first-in-class Hsp90 inhibitor in clinical trials is the geldanamycin analogue 17-allylamino 17-demethoxygeldanamycin (17-AAG) [83]. The results that have emerged from these trials have been encouraging, with stable disease observed in two melanoma patients [83]. However, 17-AAG has shown only very limited activity to date in trials. There is also interest in targeting Hsp70 interactions as a potential cancer therapy [82].
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CONCLUSIONS
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Resistance to apoptosis, a characteristic of many different cancers, underlies not only tumorigenesis but also the inherent resistance of cancer cells to radiotherapy and chemotherapy. This results from changes at the genomic, transcriptional, and post-transcriptional level of proteins, protein kinases, and their transcriptional factor effectors. Thus, the PTEN/PI3K/Akt /mTOR/NF-B and Ras/Raf/MEK/ERK signaling cascades play critical roles in the regulation of gene expression and prevention of apoptosis. Components of these pathways are mutated or aberrantly expressed in human cancer. Notably, therefore, glioblastomas and metastatic cancers escape the most sophisticated surgical approaches and are resistant to apoptosis and thus to radiotherapy and most chemotherapeutic agents that are associated with proapoptotic effects.
Autophagy represents an alternative tumor-suppressing mechanism to overcome, at least partly, the dramatic resistance of many cancers to radiotherapy and proapoptotic-related chemotherapy. The most striking evidence for proautophagic chemotherapy comes from the use of temozolomide, a proautophagic cytotoxic drug that has demonstrated real therapeutic benefits in glioblastoma patients. Further clinical trials are underway with temozolomide targeting metastatic melanoma, metastatic breast cancer, lung cancer, and advanced solid tumors in adult and pediatric patients.
Several potential common targets in apoptosis and autophagy resistance pathways, that is, mTOR, class I PI3K, and Akt, have been identified. Inhibitors of such targets might be able to increase the level of sensitivity of migrating apoptosis-resistant cancer cells to both proapoptotic and proautophagic drugs. Thus, novel successes in the fight against certain devastating cancers might be achieved by the combination of proautophagic drugs such as temozolomide with mTOR, class I PI3K, or Akt inhibitors or with ERS inhibitors as adjuvant chemotherapies.
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ACKNOWLEDGMENTS
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R.K. is a Director of Research with the Fonds National de la Recherche Scientifique (FNRS, Belgium) and F.L. is a Clinical Research Fellow with the FNRS.
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Learning Objectives
Abstract
