A molecule targeting VHL-deficient renal cell carcinoma that induces autophagy

A HIF-regulated VHL-PTP1B-Src signaling axis identifies a therapeutic target in renal cell carcinoma

Metastatic renal cell carcinoma (RCC) is a molecularly heterogeneous disease that is intrinsically resistant to chemotherapy and radiotherapy. Although therapies targeted to the molecules vascular endothelial growth factor and mammalian target of rapamycin have shown clinical effectiveness, their effects are variable and short-lived, underscoring the need for improved treatment strategies for RCC. Here, we used quantitative phosphoproteomics and immunohistochemical profiling of 346 RCC specimens and determined that Src kinase signaling is elevated in RCC cells that retain wild-type von Hippel-Lindau (VHL) protein expression. RCC cell lines and xenografts with wild-type VHL exhibited sensitivity to the Src inhibitor dasatinib, in contrast to cell lines that lacked the VHL protein, which were resistant. Forced expression of hypoxia-inducible factor (HIF) in RCC cells with wild-type VHL diminished Src signaling output by repressing transcription of the Src activator protein tyrosine phosphatase 1B (PTP1B), conferring resistance to dasatinib. Our results suggest that a HIF-regulated VHL-PTP1B-Src signaling pathway determines the sensitivity of RCC to Src inhibitors and that stratification of RCC patients with antibody-based profiling may identify patients likely to respond to Src inhibitors in RCC clinical trials.

Autophagy as a mediator of chemotherapy-induced cell death in cancer

Since the 1940s, chemotherapy has been the treatment of choice for metastatic disease. Chemotherapeutic agents target proliferating cells, inducing cell death. For most of the history of chemotherapy, apoptosis was thought to be the only mechanism of drug-induced cell death. More recently, a second type of cell death pathway has emerged: autophagy, also called type II programmed cell death. Autophagy is a tightly regulated process by which selected components of a cell are degraded. It primarily functions as a cell survival adaptive mechanism during stress conditions. However, persistent stress can also promote extensive autophagy, leading to cell death, hence its name. Alterations in the autophagy pathway have been described in cancer cells that suggest a tumor-suppressive function in early tumorigenesis, but a tumor-promoting function in established tumors. Moreover, accumulating data indicate a role for autophagy in chemotherapy-induced cancer cell death. Here, we discuss some of the evidence showing autophagy-dependent cell death induced by anti-neoplastic agents in different cancer models. On the other hand, in some other examples, autophagy dampens treatment efficacy, hence providing a therapeutic target to enhance cancer cell killing. In this paper, we propose a putative mechanism that could reconcile these two opposite observations.

The role of autophagy in cancer: therapeutic implications

Autophagy is a homeostatic, catabolic degradation process whereby cellular proteins and organelles are engulfed by autophagosomes, digested in lysosomes, and recycled to sustain cellular metabolism. Autophagy has dual roles in cancer, acting as both a tumor suppressor by preventing the accumulation of damaged proteins and organelles and as a mechanism of cell survival that can promote the growth of established tumors. Tumor cells activate autophagy in response to cellular stress and/or increased metabolic demands related to rapid cell proliferation. Autophagy-related stress tolerance can enable cell survival by maintaining energy production that can lead to tumor growth and therapeutic resistance. As shown in preclinical models, inhibition of autophagy restored chemosensitivity and enhanced tumor cell death. These results established autophagy as a therapeutic target and led to multiple early phase clinical trials in humans to evaluate autophagy inhibition using hydroxychloroquine in combination with chemotherapy or targeted agents. Targeting autophagy in cancer will provide new opportunities for drug development, because more potent and specific inhibitors of autophagy are needed. The role of autophagy and its regulation in cancer cells continues to emerge, and studies aim to define optimal strategies to modulate autophagy for therapeutic advantage.

Notch in the kidney: development and disease

Notch signalling is a highly conserved cell-cell communication mechanism that regulates development, tissue homeostasis, and repair. Within the kidney, Notch has an important function in orchestrating kidney development. Recent studies indicate that Notch plays a key role in establishing proximal epithelial fate during nephron segmentation as well as the differentiation of principal cells in the renal collecting system. Notch signalling is markedly reduced in the adult kidney; however, increased Notch signalling has been noted in both acute and chronic kidney injury. Increased glomerular epithelial Notch signalling has been associated with albuminuria and glomerulosclerosis, while tubular epithelial Notch activation caused fibrosis development most likely inducing an improper epithelial repair pathway. Recent studies thereby indicate that Notch is a key regulator of kidney development, repair, and injury.

The autophagic paradox in cancer therapy

Autophagy, hallmarked by the formation of double-membrane bound organelles known as autophagosomes, is a lysosome-dependent pathway for protein degradation. The role of autophagy in carcinogenesis is context dependent. As a tumor-suppressing mechanism in early-stage carcinogenesis, autophagy inhibits inflammation and promotes genomic stability. Moreover, disruption of autophagy-related genes accelerates tumorigenesis in animals. However, autophagy may also act as a pro-survival mechanism to protect cancer cells from various forms of cellular stress. In cancer therapy, adaptive autophagy in cancer cells sustains tumor growth and survival in face of the toxicity of cancer therapy. To this end, inhibition of autophagy may sensitize cancer cells to chemotherapeutic agents and ionizing radiation. Nevertheless, in certain circumstances, autophagy mediates the therapeutic effects of some anticancer agents. Data from recent studies are beginning to unveil the apparently paradoxical nature of autophagy as a cell-fate decision machinery. Taken together, modulation of autophagy is a novel approach for enhancing the efficacy of existing cancer therapy, but its Janus-faced nature may complicate the clinical development of autophagy modulators as anticancer therapeutics.

Harnessing synthetic lethal interactions in anticancer drug discovery

Unique features of tumours that can be exploited by targeted therapies are a key focus of current cancer research. One such approach is known as synthetic lethality screening, which involves searching for genetic interactions of two mutations whereby the presence of either mutation alone has no effect on cell viability but the combination of the two mutations results in cell death. The presence of one of these mutations in cancer cells but not in normal cells can therefore create opportunities to selectively kill cancer cells by mimicking the effect of the second genetic mutation with targeted therapy. Here, we summarize strategies that can be used to identify synthetic lethal interactions for anticancer drug discovery, describe examples of such interactions that are currently being investigated in preclinical and clinical studies of targeted anticancer therapies, and discuss the challenges of realizing the full potential of such therapies.

The autophagy conundrum in cancer: influence of tumorigenic metabolic reprogramming

Tumorigenesis is often accompanied by metabolic changes that favor rapid energy production and increased biosynthetic capabilities. These metabolic adaptations promote the survival and proliferation of tumor cells, and in conjunction with the hypoxic and metabolically challenged tumor microenvironment, influence autophagic activity. Autophagy is a catabolic process that allows cellular macromolecules to be broken down and re-utilized as metabolic precursors. Stimulation of autophagy promotes the survival of tumor cells under stressful metabolic and environmental conditions, and counters the potentially deleterious effects of mitochondrial dysfunction and the ROS that these organelles generate. However, inhibition of autophagy has also been reported to fuel tumorigenesis. In spite of the advances in our understanding of the relationship between autophagy and tumorigenesis, it remains unclear whether the therapeutic approaches targeting autophagy should aim to increase or decrease autophagic flux in tumor tissues in human patients. Here, we review how metabolic reprogramming influences autophagic activity in tumors, and discuss how inhibition of autophagy might be exploited to target tumor cells that show altered metabolism.

SAR studies of 4-pyridyl heterocyclic anilines that selectively induce autophagic cell death in von Hippel-Lindau-deficient renal cell carcinoma cells

We recently identified a class of pyridyl aniline thiazoles (PAT) that displayed selective cytotoxicity for von Hippel-Lindau (VHL) deficient renal cell carcinoma (RCC) cells in vitro and in vivo. Structure-activity relationship (SAR) studies were used to develop a comparative molecular field analysis (CoMFA) model that related VHL-selective potency to the three-dimensional arrangement of chemical features of the chemotype. We now report the further molecular alignment-guided exploration of the chemotype to discover potent and selective PAT analogues. The contribution of the central thiazole ring was explored using a series of five- and six-membered ring heterocyclic replacements to vary the electronic and steric interactions in the central unit. We also explored a positive steric CoMFA contour adjacent to the pyridyl ring using Pd-catalysed cross-coupling Suzuki-Miyaura, Sonogashira and nucleophilic displacement reactions to prepare of a series of aryl-, alkynyl-, alkoxy- and alkylamino-substituted pyridines, respectively. In vitro potency and selectivity were determined using paired RCC cell lines: the VHL-null cell line RCC4 and the VHL-positive cell line RCC4-VHL. Active analogues selectively induced autophagy in RCC4 cells. We have used the new SAR data to further develop the CoMFA model, and compared this to a 2D-QSAR method. Our progress towards realising the therapeutic potential of this chemotype as a targeted cytotoxic therapy for the treatment of RCC by exploiting the absence of the VHL tumour suppressor gene is reported.

Proteases in autophagy

Autophagy is a process involved in the proteolytic degradation of cellular macromolecules in lysosomes, which requires the activity of proteases, enzymes that hydrolyse peptide bonds and play a critical role in the initiation and execution of autophagy. Importantly, proteases also inhibit autophagy in certain cases. The initial steps of macroautophagy depend on the proteolytic processing of a particular protein, Atg8, by a cysteine protease, Atg4. This processing step is essential for conjugation of Atg8 with phosphatidylethanolamine and, subsequently, autophagosome formation. Lysosomal hydrolases, known as cathepsins, can be divided into several groups based on the catalitic residue in the active site, namely, cysteine, serine and aspartic cathepsins, which catalyse the cleavage of peptide bonds of autophagy substrates and, together with other factors, dispose of the autophagic flux. Whilst most cathepsins degrade autophagosomal content, some, such as cathepsin L, also degrade lysosomal membrane components, GABARAP-II and LC3-II. In contrast, cathepsin A, a serine protease, is involved in inhibition of chaperon-mediated autophagy through proteolytic processing of LAMP-2A. In addition, other families of calcium-dependent non-lysosomal cysteine proteases, such as calpains, and cysteine aspartate-specific proteases, such as caspases, may cleave autophagy-related proteins, negatively influencing the execution of autophagic processes. Here we discuss the current state of knowledge concerning protein degradation by autophagy and outline the role of proteases in autophagic processes. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

5-Methoxyflavanone induces cell cycle arrest at the G2/M phase, apoptosis and autophagy in HCT116 human colon cancer cells

Natural flavonoids have diverse pharmacological activities, including anti-oxidative, anti-inflammatory, and anti-cancer activities. In this study, we investigated the molecular mechanism underlying the action of 5-methoxyflavanone (5-MF) which has a strong bioavailability and metabolic stability. Our results show that 5-MF inhibited the growth and clonogenicity of HCT116 human colon cancer cells, and that it activated DNA damage responses, as revealed by the accumulation of p53 and the phosphorylation of DNA damage-sensitive proteins, including ataxia-telangiectasia mutated (ATM) at Ser1981, checkpoint kinase 2 (Chk2) at Thr68, and histone H2AX at Ser139. 5-MF-induced DNA damage was confirmed in a comet tail assay. We also found that 5-MF increased the cleavage of caspase-2 and -7, leading to the induction of apoptosis. Pretreatment with the ATM inhibitor KU55933 enhanced 5-MF-induced γ-H2AX formation and caspase-7 cleavage. HCT116 cells lacking p53 (p53(-/-)) or p21 (p21(-/-)) exhibited increased sensitivity to 5-MF compared to wild-type cells. 5-MF further induced autophagy via an ERK signaling pathway. Blockage of autophagy with the MEK inhibitor U0126 potentiated 5-MF-induced γ-H2AX formation and caspase-2 activation. These results suggest that a caspase-2 cascade mediates 5-MF-induced anti-tumor activity, while an ATM/Chk2/p53/p21 checkpoint pathway and ERK-mediated autophagy act as a survival program to block caspase-2-mediated apoptosis induced by 5-MF.

Autophagy modulation for cancer therapy

Autophagy is a homeostatic and catabolic process that enables the sequestration and lysosomal degradation of cytoplasmic organelles and proteins that is important for the maintenance of genomic stability and cell survival. Beclin 1 (+/- ) gene knockout mice are tumor prone, indicating a tumor suppressor role for autophagy. Autophagy is also mechanism of stress tolerance that maintains cell viability and can lead to tumor dormancy, progression, and therapeutic resistance. Many anticancer drugs induce cytotoxic stress that can activate pro-survival autophagy. In some contexts, excessive or prolonged autophagy can lead to tumor cell death. Inhibition of cytoprotective autophagy by genetic or pharmacological means has been shown to enhance anticancer drug-induced cell death, suggesting a novel therapeutic strategy. Studies are ongoing to define optimal strategies to modulate autophagy for cancer prevention and therapy, and to exploit it as a target for anticancer drug discovery.

Targeting autophagy during cancer therapy to improve clinical outcomes

Autophagy is a catabolic process that turns over long-lived proteins and organelles and contributes to cell and organism survival in times of stress. Current cancer therapies including chemotherapy and radiation are known to induce autophagy within tumor cells. This is therefore an attractive process to target during cancer therapy as there are safe, clinically available drugs known to both inhibit and stimulate autophagy. However, there are conflicting positive and negative effects of autophagy and no current consensus on how to manipulate autophagy to improve clinical outcomes. Careful and rigorous evaluation of autophagy with a focus on how to translate laboratory findings into relevant clinical therapies remains an important aspect of improving clinical outcomes in patients with malignant disease.

Autophagy as a therapeutic target in cancer

Autophagy is a self-catabolic process that maintains intracellular homeostasis and prolongs cell survival under stress via lysosomal degradation of cytoplasmic constituents and recycling of amino acids and energy. Autophagy is intricately involved in many aspects of human health and disease, including cancer. Autophagy is a double-edged sword in tumorigenesis, acting both as a tumor suppressor and a protector of cancer cell survival, and elucidation of its exact role at different stages of cancer progression and in treatment responsiveness is a complex and challenging task. Better understanding of autophagy regulation and its impact on treatment outcome will potentially allow us to identify novel therapeutic targets in cancer. In this review, we summarize current knowledge on the regulation and dual function of autophagy in tumorigenesis, as well as ongoing efforts in modulating autophagy for cancer treatment and prevention. This is a very exciting and highly promising area of cancer research, as pharmacologic modulation of autophagy appears to augment the efficacy of currently available anticancer regimens and opens the way to the development of new combinatorial therapeutic strategies that will hopefully contribute to cancer eradication.

Hypoxia-inducible factors and the response to hypoxic stress

Oxygen (O(2)) is an essential nutrient that serves as a key substrate in cellular metabolism and bioenergetics. In a variety of physiological and pathological states, organisms encounter insufficient O(2) availability, or hypoxia. In order to cope with this stress, evolutionarily conserved responses are engaged. In mammals, the primary transcriptional response to hypoxic stress is mediated by the hypoxia-inducible factors (HIFs). While canonically regulated by prolyl hydroxylase domain-containing enzymes (PHDs), the HIFα subunits are intricately responsive to numerous other factors, including factor-inhibiting HIF1α (FIH1), sirtuins, and metabolites. These transcription factors function in normal tissue homeostasis and impinge on critical aspects of disease progression and recovery. Insights from basic HIF biology are being translated into pharmaceuticals targeting the HIF pathway.

A serotype 5/3 adenovirus expressing MDA-7/IL-24 infects renal carcinoma cells and promotes toxicity of agents that increase ROS and ceramide levels

Agents that generate reactive oxygen species (ROS) are recognized to enhance MDA-7/IL-24 lethality. The present studies focused on clarifying how such agents enhanced MDA-7/IL-24 toxicity in renal cell carcinoma cells (RCCs). Infection of RCCs with a tropism-modified serotype 5/3 adenovirus expressing MDA-7/IL-24 (Ad.5/3-mda-7) caused plasma membrane clustering of CD95 and CD95 association with pro-caspase 8, effects that were enhanced by combined exposure to 17-N-allylamino-17-demethoxygeldanamycin (17AAG), As(2)O(3), or fenretinide and that correlated with enhanced cell killing. Knockdown of CD95 or expression of cellular FADD (Fas-associated protein with death domain)-like interleukin-1β-converting enzyme inhibitory protein, short form (c-FLIP-s) blocked enhanced killing. Inhibition of ROS generation, elevated cytosolic Ca(2+), or de novo ceramide synthesis blocked Ad.5/3-mda-7 ± agent-induced CD95 activation and the enhancement of apoptosis. Ad.5/3-mda-7 increased ceramide levels in a PERK-dependent fashion that were responsible for elevated cytosolic Ca(2+) levels that promoted ROS generation; 17AAG did not further enhance cytokine-induced ceramide generation. In vivo, infection of RCC tumors with Ad.5/3-mda-7 suppressed the growth of infected tumors that was enhanced by exposure to 17AAG. Our data indicate that in RCCs, Ad.5/3-mda-7-induced ceramide generation plays a central role in tumor cell killing and inhibition of multiple signaling pathways may have utility in promoting MDA-7/IL-24 lethality in renal cancer.

Synthetic lethality: general principles, utility and detection using genetic screens in human cells

Synthetic lethality occurs when the simultaneous perturbation of two genes results in cellular or organismal death. Synthetic lethality also occurs between genes and small molecules, and can be used to elucidate the mechanism of action of drugs. This area has recently attracted attention because of the prospect of a new generation of anti-cancer drugs. Based on studies ranging from yeast to human cells, this review provides an overview of the general principles that underlie synthetic lethality and relates them to its utility for identifying gene function, drug action and cancer therapy. It also identifies the latest strategies for the large-scale mapping of synthetic lethalities in human cells which bring us closer to the generation of comprehensive human genetic interaction maps.

New cancer targets emerging from studies of the Von Hippel-Lindau tumor suppressor protein

Inactivation of the von Hippel-Lindau tumor suppressor protein (pVHL) causes the most common form of kidney cancer. pVHL is part of a complex that polyubiquitinates the alpha subunit of the heterodimeric transcription factor HIF. In the presence of oxygen, HIF1α is prolyl hydroxylated by EglN1 (also called PHD2); this modification recruits pVHL, which then targets HIF1α for proteasomal degradation. In hypoxic or pVHL-defective cells, HIF1α accumulates, binds to HIF1β, and transcriptionally activates genes such as VEGF. VEGF inhibitors and mTOR inhibitors, which indirectly affect HIF, are now approved for the treatment of kidney cancer. EglN1 is a 2-oxoglutarate-dependent dioxygenase; such enzymes can be inhibited with drug-like small molecules and EglN1 inhibitors are currently being tested for the treatment of anemia. EglN2 (PHD1) and EglN3 (PHD3), which are EglN1 paralogs, appear to play HIF-independent roles in cell proliferation and apoptosis, respectively, and are garnering interest as potential cancer targets. A number of JmjC-containing proteins, including RBP2 and PLU-1, are 2-oxoglutarate-dependent dioxygenases that demethylate histones. Preclinical data suggest that inhibition of RBP2 or PLU-1 would suppress tumor growth.

The genetic basis of kidney cancer: a metabolic disease

Kidney cancer is not a single disease but comprises a number of different types of cancer that occur in the kidney, each caused by a different gene with a different histology and clinical course that responds differently to therapy. Each of the seven known kidney cancer genes, VHL, MET, FLCN, TSC1, TSC2, FH and SDH, is involved in pathways that respond to metabolic stress or nutrient stimulation. The VHL protein is a component of the oxygen and iron sensing pathway that regulates hypoxia-inducible factor (HIF) levels in the cell. HGF-MET signaling affects the LKB1-AMPK energy sensing cascade. The FLCN-FNIP1-FNIP2 complex binds AMPK and, therefore, might interact with the cellular energy and nutrient sensing pathways AMPK-TSC1/2-mTOR and PI3K-Akt-mTOR. TSC1-TSC2 is downstream of AMPK and negatively regulates mTOR in response to cellular energy deficit. FH and SDH have a central role in the mitochondrial tricarboxylic acid cycle, which is coupled to energy production through oxidative phosphorylation. Mutations in each of these kidney cancer genes result in dysregulation of metabolic pathways involved in oxygen, iron, energy or nutrient sensing, suggesting that kidney cancer is a disease of cell metabolism. Targeting the fundamental metabolic abnormalities in kidney cancer provides a unique opportunity for the development of more-effective forms of therapy for this disease.

Autophagy and tumorigenesis

Autophagy, a catabolic process involved in the sequestration and lysosomal degradation of cytoplasmic contents, is crucial for cellular homeostasis. The current literature supports that autophagy plays diverse roles in the development, maintenance, and progression of tumors. While genetic evidence indicates autophagy functions as a tumor suppressor mechanism, it is also apparent that autophagy can promote the survival of established tumors under stress conditions and in response to chemotherapy. In this review, we discuss the mechanisms and the evidence underlying these multifaceted roles of autophagy in tumorigenesis, the prospects for targeting autophagy in cancer therapy, and overview the potential markers that may be utilized to reliably detect autophagy in clinical settings.

4-Pyridylanilinothiazoles that selectively target von Hippel-Lindau deficient renal cell carcinoma cells by inducing autophagic cell death

Renal cell carcinomas (RCC) are refractory to standard therapy with advanced RCC having a poor prognosis; consequently treatment of advanced RCC represents an unmet clinical need. The von Hippel-Lindau (VHL) tumor suppressor gene is mutated or inactivated in a majority of RCCs. We recently identified a 4-pyridyl-2-anilinothiazole (PAT) with selective cytotoxicity against VHL-deficient renal cells mediated by induction of autophagy and increased acidification of autolysosomes. We report exploration of structure-activity relationships (SAR) around this PAT lead. Analogues with substituents on each of the three rings, and various linkers between rings, were synthesized and tested in vitro using paired RCC4 cell lines. A contour map describing the relative spatial contributions of different chemical features to potency illustrates a region, adjacent to the pyridyl ring, with potential for further development. Examples probing this domain validated this approach and may provide the opportunity to develop this novel chemotype as a targeted approach to the treatment of RCC.

Induction of autophagy and inhibition of melanoma growth in vitro and in vivo by hyperactivation of oncogenic BRAF

Activating mutations in NRAS and BRAF are found frequently in cutaneous melanomas. Because concurrent mutations of both BRAF and RAS are extremely rare, it is thought that transformation by RAS and BRAF occurs through a common mechanism. Also, there is evidence for a relationship of synthetic lethality between NRAS and BRAF oncogenes that leads to selection against cells with a hyperactive mitogen-activated protein kinase (MAPK) pathway. However, it is not known whether the hyperactivation of the MAPK pathway by overexpression of either oncogene alone could also inhibit melanoma tumorigenesis. Here, we show that in melanoma cells with oncogenic BRAF (mBRAF), high levels of mBRAF induce hyperactivation of ERK and senescence-like phenotype and trigger autophagy by inhibiting the mammalian target of rapamycin complex signaling. Growth inhibition and cell death caused by high mBRAF levels are partially rescued by downregulation of BRAF protein or inhibition of autophagy, but not by inhibition of the MAPK or apoptotic pathways. In nude mice, growth of mBRAF-overexpressing tumors is inhibited. Quantitative immunohistochemical analysis of human melanomas and cell lines showed a significant positive correlation between the levels of BRAF protein and autophagy marker light chain 3. Our data suggest that high oncogenic BRAF levels trigger autophagy, which may have a role in melanoma tumor progression.

A novel sphingosine kinase inhibitor induces autophagy in tumor cells

The sphingolipids ceramide, sphingosine, and sphingosine 1-phosphate (S1P) regulate cell signaling, proliferation, apoptosis, and autophagy. Sphingosine kinase-1 and -2 (SK1 and SK2) phosphorylate sphingosine to form S1P, shifting the balanced activity of these lipids toward cell proliferation. We have previously reported that pharmacological inhibition of SK activity delays tumor growth in vivo. The present studies demonstrate that the SK2-selective inhibitor 3-(4-chlorophenyl)-adamantane-1-carboxylic acid (pyridin-4-ylmethyl)amide (ABC294640) induces nonapoptotic cell death that is preceded by microtubule-associated protein light chain 3 cleavage, morphological changes in lysosomes, formation of autophagosomes, and increases in acidic vesicles in A-498 kidney carcinoma cells. ABC294640 caused similar autophagic responses in PC-3 prostate and MDA-MB-231 breast adenocarcinoma cells. Simultaneous exposure of A-498 cells to ABC294640 and 3-methyladenine, an inhibitor of autophagy, switched the mechanism of toxicity to apoptosis, but decreased the potency of the SK2 inhibitor, indicating that autophagy is a major mechanism for tumor cell killing by this compound. Induction of the unfolded protein response by the proteasome inhibitor N-(benzyloxycarbonyl)leucinylleucinylleucinal Z-Leu-Leu-Leu-al (MG-132) or the heat shock protein 90 inhibitor geldanamycin synergistically increased the cytotoxicity of ABC294640 in vitro. In severe combined immunodeficient mice bearing A-498 xenografts, daily administration of ABC294640 delayed tumor growth and elevated autophagy markers, but did not increase terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling-positive cells in the tumors. These data suggest that ABC294640 promotes tumor cell autophagy, which ultimately results in nonapoptotic cell death and a delay of tumor growth in vivo. Consequently, ABC294640 may effectively complement anticancer drugs that induce tumor cell apoptosis.

Regulation of the histone demethylase JMJD1A by hypoxia-inducible factor 1 alpha enhances hypoxic gene expression and tumor growth

The hypoxia-inducible transcription factors (HIFs) directly and indirectly mediate cellular adaptation to reduced oxygen tensions. Recent studies have shown that the histone demethylase genes JMJD1A, JMJD2B, and JARID1B are HIF targets, suggesting that HIFs indirectly influence gene expression at the level of histone methylation under hypoxia. In this study, we identify a subset of hypoxia-inducible genes that are dependent on JMJD1A in both renal cell and colon carcinoma cell lines. JMJD1A regulates the expression of adrenomedullin (ADM) and growth and differentiation factor 15 (GDF15) under hypoxia by decreasing promoter histone methylation. In addition, we demonstrate that loss of JMJD1A is sufficient to reduce tumor growth in vivo, demonstrating that histone demethylation plays a significant role in modulating growth within the tumor microenvironment. Thus, hypoxic regulation of JMJD1A acts as a signal amplifier to facilitate hypoxic gene expression, ultimately enhancing tumor growth.

Targeting cancer cells through autophagy for anticancer therapy

Autophagy is a cellular degradation process in which portions of the cell's cytoplasm and organelles are sequestered in a double-membrane bound vesicle called an autophagosome. Fusion of autophagosomes with lysosomes results in the formation of autolysosomes, where the proteins and organelles are degraded. This degradation pathway is induced under nutrient deprivation, metabolic stress or microenvironmental conditions to ensure energy balance, clearance of damaged proteins and adaptation to stress. Disruption of autophagy is involved in diverse human diseases including cancer. In particular, the regulation of autophagy in cancer cells is complex since it can enhance tumor cell survival in response to certain stresses, yet it can also act to suppress the initiation of tumor growth. Understanding the signaling pathways involved in the regulation of autophagy as well as the autophagy process itself represents new directions in the development of anticancer therapies. In this review, we discuss recent advances in our understanding the complexity of the autophagy process and the development of targeted therapies that modulate autophagy in cancer cells in the clinic.

Oxidative stress and autophagy in cardiac disease, neurological disorders, aging and cancer

Autophagy is a catalytic process of the bulk degradation of long-lived cellular components, ultimately resulting in lysosomal digestion within mature cytoplasmic compartments known as autophagolysosomes. Autophagy serves many functions in the cell, including maintaining cellular homeostasis, a means of cell survival during stress (e.g., nutrient deprivation or starvation) or conversely as a mechanism for cell death. Increased reactive oxygen species (ROS) production and the resulting oxidative cell stress that occurs in many disease states has been shown to induce autophagy. The following review focuses on the roles that autophagy plays in response to the ROS generated in several diseases.

Autophagy and tumorigenesis

Autophagy, or cellular self-digestion, is activated in cancer cells in response to multiple stresses and has been demonstrated to promote tumor cell survival and drug resistance. Nonetheless, genetic evidence supports that autophagy functions as a tumor suppressor mechanism. Hence, the precise role of autophagy during cancer progression and treatment is both tissue and context dependent. Here, we discuss our current understanding of the biological functions of autophagy during cancer development, overview how autophagy is regulated by cancer-associated signaling pathways, and review how autophagy inhibition is being exploited to improve clinical outcomes.

Hypoxia-induced autophagy: cell death or cell survival?

Hypoxia (approximately 3-0.1% oxygen) is capable of rapidly inducing, via the hypoxia-inducible factor (HIF-1), a cell survival response engaging autophagy. This process is mediated by the atypical BH3-only proteins the Bcl-2/E1B 19kDa-interacting protein 3 (BNIP3/BNIP3L (NIX)) that are induced by HIF-1. These mitochondrial associated BNIP proteins also mediate mitophagy, a metabolic adaptation for survival that is able to control reactive oxygen species (ROS) production and DNA damage. In contrast, severe hypoxic conditions or anoxia (<0.1% oxygen), where the latter is often confused with physiological hypoxia, are capable of inducing a HIF-independent autophagic response, generated via an extreme nutritional stress response implicating the AMPK-mTOR and unfolded protein response (UPR) pathways. The autophagic cell death that is often observed in these extreme stress conditions should be seen as the outcome of failed adaptation.

Microglia are mediators of Borrelia burgdorferi-induced apoptosis in SH-SY5Y neuronal cells

Inflammation has long been implicated as a contributor to pathogenesis in many CNS illnesses, including Lyme neuroborreliosis. Borrelia burgdorferi is the spirochete that causes Lyme disease and it is known to potently induce the production of inflammatory mediators in a variety of cells. In experiments where B. burgdorferi was co-cultured in vitro with primary microglia, we observed robust expression and release of IL-6 and IL-8, CCL2 (MCP-1), CCL3 (MIP-1α), CCL4 (MIP-1β) and CCL5 (RANTES), but we detected no induction of microglial apoptosis. In contrast, SH-SY5Y (SY) neuroblastoma cells co-cultured with B. burgdorferi expressed negligible amounts of inflammatory mediators and also remained resistant to apoptosis. When SY cells were co-cultured with microglia and B. burgdorferi, significant neuronal apoptosis consistently occurred. Confocal microscopy imaging of these cell cultures stained for apoptosis and with cell type-specific markers confirmed that it was predominantly the SY cells that were dying. Microarray analysis demonstrated an intense microglia-mediated inflammatory response to B. burgdorferi including up-regulation in gene transcripts for TLR-2 and NFκβ. Surprisingly, a pathway that exhibited profound changes in regard to inflammatory signaling was triggering receptor expressed on myeloid cells-1 (TREM1). Significant transcript alterations in essential p53 pathway genes also occurred in SY cells cultured in the presence of microglia and B. burgdorferi, which indicated a shift from cell survival to preparation for apoptosis when compared to SY cells cultured in the presence of B. burgdorferi alone. Taken together, these findings indicate that B. burgdorferi is not directly toxic to SY cells; rather, these cells become distressed and die in the inflammatory surroundings generated by microglia through a bystander effect. If, as we hypothesized, neuronal apoptosis is the key pathogenic event in Lyme neuroborreliosis, then targeting microglial responses may be a significant therapeutic approach for the treatment of this form of Lyme disease.

Lyme disease, which is transmitted to humans through the bite of a tick, is currently the most frequently reported vector-borne illness in the northern hemisphere. Borrelia burgdorferi is the bacterium that causes Lyme disease and it is known to readily induce inflammation within a variety of infected tissues. Many of the neurological signs and symptoms that may affect patients with Lyme disease have been associated with B. burgdorferi-induced inflammation in the central nervous system (CNS). In this report we investigated which of the primary cell types residing in the CNS might be functioning to create the inflammatory environment that, in addition to helping clear the pathogen, could simultaneously be harming nearby neurons. We report findings that implicate microglia, a macrophage cell type in the CNS, as the key responders to infection with B. burgdorferi. We also present evidence indicating that this organism is not directly toxic to neurons; rather, a bystander effect is generated whereby the inflammatory surroundings created by microglia in response to B. burgdorferi may themselves be toxic to neuronal cells.

Synthetic lethality: a framework for the development of wiser cancer therapeutics

The challenge in medical oncology has always been to identify compounds that will kill, or at least tame, cancer cells while leaving normal cells unscathed. Most chemotherapeutic agents in use today were selected primarily for their ability to kill rapidly dividing cancer cells grown in cell culture and in mice, with their selectivity determined empirically during subsequent animal and human testing. Unfortunately, most of the drugs developed in this way have relatively low therapeutic indices (low toxic dose relative to the therapeutic dose). Recent advances in genomics are leading to a more complete picture of the range of mutations, both driver and passenger, present in human cancers. Synthetic lethality provides a conceptual framework for using this information to arrive at drugs that will preferentially kill cancer cells relative to normal cells. It also provides a possible way to tackle 'undruggable' targets. Two genes are synthetically lethal if mutation of either gene alone is compatible with viability but simultaneous mutation of both genes leads to death. If one is a cancer-relevant gene, the task is to discover its synthetic lethal interactors, because targeting these would theoretically kill cancer cells mutant in the cancer-relevant gene while sparing cells with a normal copy of that gene. All cancer drugs in use today, including conventional cytotoxic agents and newer 'targeted' agents, target molecules that are present in both normal cells and cancer cells. Their therapeutic indices almost certainly relate to synthetic lethal interactions, even if those interactions are often poorly understood. Recent technical advances enable unbiased screens for synthetic lethal interactors to be undertaken in human cancer cells. These approaches will hopefully facilitate the discovery of safer, more efficacious anticancer drugs that exploit vulnerabilities that are unique to cancer cells by virtue of the mutations they have accrued during tumor progression.

Identifying druggable disease-modifying gene products

Many disease genes encode proteins that are difficult to target directly using small molecule drugs. Improvements in libraries based on synthetic compounds, natural products, and other types of molecules may ultimately allow some challenging proteins to be successfully targeted; however, these developments alone are unlikely to be sufficient. A complementary strategy exploits the functional interconnectivity of intracellular networks to find druggable targets lying upstream, downstream, or in parallel to a disease-causing gene, where modulation can influence the disease process indirectly. These targets can be selected using prior knowledge of disease-associated pathways or identified using phenotypic chemical and genetic screens in model organisms and cells. These approaches should facilitate the identification of effective drug targets for many genetic disorders.

Targeting the hypoxia-inducible factor (HIF) pathway in cancer

The central component of hypoxia sensing in the cell is the hypoxia-inducible factor (HIF) transcriptional complex. HIF activity is deregulated in many human cancers, especially those that are highly hypoxic. Hypoxic tumour cells are usually resistant to radiotherapy and most conventional chemotherapeutic agents, rendering them highly aggressive and metastatic. Overexpression of HIF-alpha, the regulatory subunit of HIF, is associated with increased vascular density, severity of tumour grade, treatment failure and a poor prognostic outcome with conventional therapies. Therefore HIF is an attractive, although challenging, therapeutic target, and several different strategies have been developed to target HIF directly or indirectly in recent years. This review outlines the preclinical and clinical advances in this arena and discusses which cancers may benefit from HIF-targeted therapy.

The double-edged sword of autophagy modulation in cancer

Macroautophagy (autophagy) is a lysosomal degradation pathway for the breakdown of intracellular proteins and organelles. Although constitutive autophagy is a homeostatic mechanism for intracellular recycling and metabolic regulation, autophagy is also stress responsive, in which it is important for the removal of damaged proteins and organelles. Autophagy thereby confers stress tolerance, limits damage, and sustains viability under adverse conditions. Autophagy is a tumor-suppression mechanism, yet it enables tumor cell survival in stress. Reconciling how loss of a prosurvival function can promote tumorigenesis, emerging evidence suggests that preservation of cellular fitness by autophagy may be key to tumor suppression. As autophagy is such a fundamental process, establishing how the functional status of autophagy influences tumorigenesis and treatment response is important. This is especially critical as many current cancer therapeutics activate autophagy. Therefore, efforts to understand and modulate the autophagy pathway will provide new approaches to cancer therapy and prevention.

Tricking melanoma to self-digest: a deal of a meal!

Melanoma cells acquire multiple genetic and epigenetic alterations that promote their metastasis and resistance to available therapies. In this issue of Cancer Cell, Soengas and colleagues reveal that the induction of endosome-mediated autophagy results in efficient melanoma cell death, thereby offering new potential means for treatment of this devastating cancer.

Differences in regulation of tight junctions and cell morphology between VHL mutations from disease subtypes

Background

In von Hippel-Lindau (VHL) disease, germline mutations in the VHL tumor suppressor gene cause clear cell renal carcinomas, hemangioblastomas, and pheochromocytomas. The VHL gene product is part of an ubiquitin E3 ligase complex and hypoxia-inducible factor alpha (HIF-α) is a key substrate, although additional VHL functions have been described. A genotype-phenotype relationship exists in VHL disease such that specific VHL mutations elicit certain subsets of these tumors. Here, we examine VHL genotype-phenotype correlations at the cellular level, focusing on the regulation of tight junctions and cell morphology.

Methods

Wild-type and various mutant VHL proteins representing VHL disease subtypes were stably expressed in 3 VHL-negative renal carcinoma cell lines. Using these cell lines, the roles of various VHL-associated cellular functions in regulation of cell morphology were investigated.

Results

As a whole, type 1 mutants varied greatly from type 2 mutants, demonstrating high levels of HIF-2α, cyclin D1 and α5 integrin, lower p27 levels, and a spindly, fibroblastic cellular appearance. Type 2 mutations demonstrated an epithelial morphology similar to wild-type VHL in the majority of the renal cell lines used. Knockdown of p27 in cells with wild-type VHL led to perturbations of both epithelial morphology and ZO-1 localization to tight junctions. ZO-1 localization correlated well with VHL disease subtypes, with greater mislocalization observed for genotypes associated with a higher risk of renal carcinoma. HIF-2α knockdown in 786-O partially restored ZO-1 localization, but did not restore an epithelial morphology.

Conclusion

VHL has both HIF-α dependent and HIF-α independent functions in regulating tight junctions and cell morphology that likely impact the clinical phenotypes seen in VHL disease.

Identification and characterization of a novel anticancer agent with selectivity against deleted in pancreatic cancer locus 4 (DPC4)-deficient pancreatic and colon cancer cells

OBJECTIVES

The deleted in pancreatic cancer locus 4 (DPC4)/SMAD4 tumor suppressor gene is frequently inactivated in pancreatic (approximately 55%) and colorectal cancers (approximately 30%). Like other tumor suppressor genes, the loss-of-function mutations found in the DPC4 gene are specific to cancer cells. This provides an attractive and unique opportunity for therapeutic intervention. The aim of this study was to identify and characterize small molecules that selectively kill DPC4-deficient cancer cells.

METHODS

An unbiased chemical screening using isogenic cell lines that only differ in the DPC4 gene was carried out to identify positive hits. Selected hits were further verified in additional isogenic cell lines and characterized in cancer cells using several different cellular assays.

RESULTS

A lead molecule, UA62784, was identified to be selectively cytotoxic against cancer cells with deficient DPC4. UA62784 preferentially induces cell cycle arrest and apoptosis in cells with deficient DPC4. It also selectively reduces the clonogenicity of DPC4-deficient cells on soft agar when compared with cells with wild type DPC4. We further demonstrate that UA62784 induces CDC2 kinase activity preferentially in DPC4-negative cells.

CONCLUSIONS

UA62784 is a small molecule that selectively kills DPC4-deficient cancer cells. Its potent activity and relatively low molecular weight make it a decent candidate for further lead optimization.

MDA-7/IL-24-induced cell killing in malignant renal carcinoma cells occurs by a ceramide/CD95/PERK-dependent mechanism

Melanoma differentiation associated gene-7/interleukin-24 (mda-7/IL-24) is a novel cytokine displaying selective apoptosis-inducing activity in transformed cells without harming normal cells. The present studies focused on clarifying the mechanism(s) by which glutathione S-transferase (GST)-MDA-7 altered cell survival of human renal carcinoma cells in vitro. GST-MDA-7 caused plasma membrane clustering of CD95 and the association of CD95 with procaspase-8. GST-MDA-7 lethality was suppressed by inhibition of caspase-8 or by overexpression of short-form cellular FLICE inhibitory protein, but only weakly by inhibition of cathepsin proteases. GST-MDA-7-induced CD95 clustering (and apoptosis) was blocked by knockdown of acidic sphingomyelinase or, to a greater extent, ceramide synthase-6 expression. GST-MDA-7 killing was, in parallel, dependent on inactivation of extracellular signal-regulated kinase 1/2 and on CD95-induced p38 mitogen-activated protein kinase and c-jun NH(2)-terminal kinase-1/2 signaling. Knockdown of CD95 expression abolished GST-MDA-7-induced phosphorylation of protein kinase R-like endoplasmic reticulum kinase. GST-MDA-7 lethality was suppressed by knockout or expression of a dominant negative protein kinase R-like endoplasmic reticulum kinase that correlated with reduced c-jun NH(2)-terminal kinase-1/2 and p38 mitogen-activated protein kinase signaling and maintained extracellular signal-regulated kinase-1/2 phosphorylation. GST-MDA-7 caused vacuolization of LC3 through a mechanism that was largely CD95 dependent and whose formation was suppressed by knockdown of ATG5 expression. Knockdown of ATG5 suppressed GST-MDA-7 toxicity. Our data show that in kidney cancer cells GST-MDA-7 induces ceramide-dependent activation of CD95, which is causal in promoting an endoplasmic reticulum stress response that activates multiple proapoptotic pathways to decrease survival.

Methodological approaches in application of synthetic lethality screening towards anticancer therapy

A promising direction in the development of selective less toxic cancer drugs is the usage of synthetic lethality concept. The availability of large-scale synthetic low-molecular-weight chemical libraries has allowed HTS for compounds synergistic lethal with defined human cancer aberrations in activated oncogenes or tumour suppressor genes. The search for synthetic lethal chemicals in human/mouse tumour cells is greatly aided by a prior knowledge of relevant signalling and DNA repair pathways, allowing for educated guesses on the preferred potential therapeutic targets. The recent generation of human/rodents genome-wide siRNAs, and shRNA-expressing libraries, should further advance this more focused approach to cancer drug discovery.

Autophagic cell death: the story of a misnomer

Dying cells often display a large-scale accumulation of autophagosomes and hence adopt a morphology called autophagic cell death. In many cases, it is agreed that this autophagic cell death is cell death with autophagy rather than cell death by autophagy. Here, we evaluate the accumulating body of literature that argues that cell death occurs by autophagy. We also list the caveats that must be considered when deciding whether or not autophagy is an important effector mechanism of cell death.

Autophagic cell death: the story of a misnomer

Dying cells often display a large-scale accumulation of autophagosomes and hence adopt a morphology called autophagic cell death. In many cases, it is agreed that this autophagic cell death is cell death with autophagy rather than cell death by autophagy. Here, we evaluate the accumulating body of literature that argues that cell death occurs by autophagy. We also list the caveats that must be considered when deciding whether or not autophagy is an important effector mechanism of cell death.

Targeted therapy for the loss of von Hippel-Lindau in renal cell carcinoma: a novel molecule that induces autophagic cell death

Radiation and conventional cytotoxic chemotherapies are ineffective in treating renal cancer. Approximately 75 percent of renal cell carcinoma (RCC) is associated with an inactivation of the tumor suppressor gene von Hippel-Lindau (VHL). We exploited the possibility of targeting VHL-deficient RCC through synthetic lethality using a high-throughput screening approach. In this screen, STF-62247 was identified to be selectively toxic and growth inhibitory to renal cells lacking VHL. We recently demonstrated that the cytotoxicity of STF-62247 is due to dysregulated autophagy. Furthermore, the reduction of protein levels of essential autophagy pathway components such as Atg5, Atg7 and Atg9 reduces sensitivity of VHL-deficient cells to killing by STF-62247. Loss of proteins involved in Golgi trafficking sensitized RCC with wild-type VHL to killing by STF-62247, indicating a potential role for these proteins as a target of the compound. Our study supports the concept of using synthetic lethality to selectively kill VHL-deficient cells that represents a new type of targeted therapy for the treatment of RCC.