Specific killing of Rb mutant cancer cells by inactivating TSC2

Aurora Kinase A Is Overexpressed in Human Retinoblastoma and Correlates with Histopathologic High-Risk Factors: Implications for Targeted Therapy

Retinoblastoma (RB) is an intraocular malignancy initiated by loss of RB1 function and/or dysregulation of MYCN oncogene. RB is primarily treated with chemotherapy; however, systemic toxicity and long-term adverse effects remain a significant challenge necessitating the identification of specific molecular targets. Aurora kinase A (AURKA), a critical cell cycle regulator, contributes to cancer pathogenesis, especially in RB1-deficient and MYCN-dysregulated tumors. Our immunohistochemistry study in patient specimens (n = 67) discovered that AURKA is overexpressed in RB, and elevated expression correlates with one or more histopathologic high-risk factors, such as tumor involvement of the optic nerve, choroid, sclera, and/or anterior segment. More specifically, AURKA is ubiquitously expressed in most advanced-stage RB tumors that show a suboptimal response to chemotherapy. shRNA-mediated depletion/pharmacologic inhibition studies in cell lines, patient-derived cells, in vivo xenografts, and enucleated patient specimens confirm that RB cells are highly sensitive to a lack of functional AURKA. In addition, we deciphered that AURKA and MYCN associate with each other to regulate their levels in RB cells. Overall, our results demonstrate a previously unknown up-regulation of AURKA in RB, facilitated by its crosstalk with MYCN, and elevated levels of this kinase may indicate unfavorable prognosis in tumors refractory to chemotherapy. This study provides a rationale and confirms that therapeutic targeting of elevated AURKA in RB could be a potential treatment approach.

Therapeutic Strategies for RB1-Deficient Cancers: Intersecting Gene Regulation and Targeted Therapy

The retinoblastoma (RB) transcriptional corepressor 1 (RB1) is a critical tumor suppressor gene, governing diverse cellular processes implicated in cancer biology. Dysregulation or deletion in RB1 contributes to the development and progression of various cancers, making it a prime target for therapeutic intervention. RB1's canonical function in cell cycle control and DNA repair mechanisms underscores its significance in restraining aberrant cell growth and maintaining genomic stability. Understanding the complex interplay between RB1 and cellular pathways is beneficial to fully elucidate its tumor-suppressive role across different cancer types and for therapeutic development. As a result, investigating vulnerabilities arising from RB1 deletion-associated mechanisms offers promising avenues for targeted therapy. Recently, several findings highlighted multiple methods as a promising strategy for combating tumor growth driven by RB1 loss, offering potential clinical benefits in various cancer types. This review summarizes the multifaceted role of RB1 in cancer biology and its implications for targeted therapy.

Stearate-derived very long-chain fatty acids are indispensable to tumor growth

Reprogramming of lipid metabolism is emerging as a hallmark of cancer, yet involvement of specific fatty acids (FA) species and related enzymes in tumorigenesis remains unclear. While previous studies have focused on involvement of long-chain fatty acids (LCFAs) including palmitate in cancer, little attention has been paid to the role of very long-chain fatty acids (VLCFAs). Here, we show that depletion of acetyl-CoA carboxylase (ACC1), a critical enzyme involved in the biosynthesis of fatty acids, inhibits both de novo synthesis and elongation of VLCFAs in human cancer cells. ACC1 depletion markedly reduces cellular VLCFA but only marginally influences LCFA levels, including palmitate that can be nutritionally available. Therefore, tumor growth is specifically susceptible to regulation of VLCFAs. We further demonstrate that VLCFA deficiency results in a significant decrease in ceramides as well as downstream glucosylceramides and sphingomyelins, which impairs mitochondrial morphology and renders cancer cells sensitive to oxidative stress and cell death. Taken together, our study highlights that VLCFAs are selectively required for cancer cell survival and reveals a potential strategy to suppress tumor growth.

Exploiting Cancer Synthetic Lethality in Cancer-Lessons Learnt from PARP Inhibitors

PARP inhibitors now have proven utility in the treatment of homologous recombination (HR) defective cancers. These drugs, and the synthetic lethality effect they exploit, have not only taught us how to approach the treatment of HR defective cancers but have also illuminated how resistance to a synthetic lethal approach can occur, how cancer-associated synthetic lethal effects are perhaps more complex than we imagine, how the better use of biomarkers could improve the success of treatment and even how drug resistance might be targeted. Here, we discuss some of the lessons learnt from the study of PARP inhibitor synthetic lethality and how these lessons might have wider application. Specifically, we discuss the concept of synthetic lethal penetrance, phenocopy effects in cancer such as BRCAness, synthetic lethal resistance, the polygenic and complex nature of synthetic lethal interactions, how evolutionary double binds could be exploited in treatment as well as future horizons for the field.

Glucose-6-phosphate dehydrogenase exerts antistress effects independently of its enzymatic activity

G6PD (glucose-6-phosphate dehydrogenase) is the rate-limiting enzyme in the oxidative pentose phosphate pathway that can generate cytosolic NADPH for biosynthesis and oxidative defense. Since cytosolic NADPH can be compensatively produced by other sources, the enzymatic activity deficiency alleles of G6PD are well tolerated in somatic cells but the effect of null mutations is unclear. Herein, we show that G6PD KO sensitizes cells to the stresses induced by hydrogen peroxide, superoxide, hypoxia, and the inhibition of the electron transport chain. This effect can be completely reversed by the expressions of natural mutants associated with G6PD deficiency, even without dehydrogenase activity, exactly like the WT G6PD. Furthermore, we demonstrate that G6PD can physically interact with AMPK (AMPK-activated protein kinase) to facilitate its activity and directly bind to NAMPT (nicotinamide phosphoribosyltransferase) to promote its activity and maintain the NAD(P)H/NAD(P)⁺ homeostasis. These functions are necessary to the antistress ability of cells but independent of the dehydrogenase activity of G6PD. In addition, the WT G6PD and naturally inactive mutant also can similarly regulate the metabolism of glucose, glutamine, fatty acid synthesis, and GSH and interact with the involved enzymes. Therefore, our findings reveal the previously unidentified functions of G6PD that can act as the important physiological neutralizer of stresses independently of its enzymatic activity.

Targeting RB1 Loss in Cancers

Simple Summary

Irreversible defects in RB1 tumor suppressor functions often predict poor outcomes in cancer patients. However, the RB1-defecient status can be a benefit as well for them, as it generates a variety of vulnerabilities induced through the upregulation of RB1 targets, relief from functional restrictions due to RB1 binding, presence of genes whose inactivation cause synthetic lethality with RB1 loss, or collateral synthetic lethality owing to simultaneous loss of neighboring genes.

Abstract

Retinoblastoma protein 1 (RB1) is encoded by a tumor suppressor gene that was discovered more than 30 years ago. Almost all mitogenic signals promote cell cycle progression by braking on the function of RB1 protein through mono- and subsequent hyper-phosphorylation mediated by cyclin-CDK complexes. The loss of RB1 function drives tumorigenesis in limited types of malignancies including retinoblastoma and small cell lung cancer. In a majority of human cancers, RB1 function is suppressed during tumor progression through various mechanisms. The latter gives rise to the acquisition of various phenotypes that confer malignant progression. The RB1-targeted molecules involved in such phenotypic changes are good quarries for cancer therapy. Indeed, a variety of novel therapies have been proposed to target RB1 loss. In particular, the inhibition of a number of mitotic kinases appeared to be synthetic lethal with RB1 deficiency. A recent study focusing on a neighboring gene that is often collaterally deleted together with RB1 revealed a pharmacologically targetable vulnerability in RB1-deficient cancers. Here we summarize current understanding on possible therapeutic approaches targeting functional or genomic aberration of RB1 in cancers.

Cross-species identification of PIP5K1-, splicing- and ubiquitin-related pathways as potential targets for RB1-deficient cells

The RB1 tumor suppressor is recurrently mutated in a variety of cancers including retinoblastomas, small cell lung cancers, triple-negative breast cancers, prostate cancers, and osteosarcomas. Finding new synthetic lethal (SL) interactions with RB1 could lead to new approaches to treating cancers with inactivated RB1. We identified 95 SL partners of RB1 based on a Drosophila screen for genetic modifiers of the eye phenotype caused by defects in the RB1 ortholog, Rbf1. We validated 38 mammalian orthologs of Rbf1 modifiers as RB1 SL partners in human cancer cell lines with defective RB1 alleles. We further show that for many of the RB1 SL genes validated in human cancer cell lines, low activity of the SL gene in human tumors, when concurrent with low levels of RB1 was associated with improved patient survival. We investigated higher order combinatorial gene interactions by creating a novel Drosophila cancer model with co-occurring Rbf1, Pten and Ras mutations, and found that targeting RB1 SL genes in this background suppressed the dramatic tumor growth and rescued fly survival whilst having minimal effects on wild-type cells. Finally, we found that drugs targeting the identified RB1 interacting genes/pathways, such as UNC3230, PYR-41, TAK-243, isoginkgetin, madrasin, and celastrol also elicit SL in human cancer cell lines. In summary, we identified several high confidence, evolutionarily conserved, novel targets for RB1-deficient cells that may be further adapted for the treatment of human cancer.

Inhibiting both proline biosynthesis and lipogenesis synergistically suppresses tumor growth

Cancer cells often proliferate under hypoxia and reprogram their metabolism. However, how to find targets to effectively block the hypoxia-associated metabolic pathways remains unclear. Here, we developed a tool to conveniently calculate electrons dissipated in metabolic transformations. Based on the law of conservation of electrons in chemical reactions, we further built up an electron balance model for central carbon metabolism, and it can accurately outline metabolic plasticity under hypoxia. Our model specifies that glutamine metabolism reprogrammed for biosynthesis of lipid and/or proline actually acts as the alternative electron bin to enable electron transfer in proliferating cells under hypoxia. Inhibition of both proline biosynthesis and lipogenesis can synergistically suppress cancer cell growth under hypoxia and in vivo tumor onset. Therefore, our model helps to reveal combinations of potential targets to inhibit tumor growth by blocking hypoxia-rewired metabolism and provides a useful tool for future studies on cancer metabolism.

An autophagy-related model of 4 key genes for predicting prognosis of patients with laryngeal cancer

Autophagy, a major cause of cancer-related death, is correlated with the pathogenesis of various diseases including cancers. Our study aimed to develop an autophagy-related model for predicting prognosis of patients with laryngeal cancer.We analyzed the correlation between expression profiles of autophagy-related genes (ARGs) and clinical outcomes in 111 laryngeal cancer patients from The Cancer Genome Atlas (TCGA). Afterward, gene functional enrichment analyses of gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) were performed to find the major biological attributes. Univariate Cox regression analyses and multivariate Cox regression analyses were performed to screen ARGs whose expression profiles were significantly associated with laryngeal cancer patients overall survival (OS). Furthermore, to provide the doctors and patients with a quantitative method to perform an individualized survival prediction, we constructed a prognostic nomogram.Thirty eight differentially expressed ARGs were screened out in laryngeal cancer patients through the TCGA database. Related functional enrichments may act as tumor-suppressive roles in the tumorigenesis of laryngeal cancer. Subsequently, 4 key prognostic ARGs (IKBKB, ST13, TSC2, and MAP2K7) were identified from all ARGs by the Cox regression model, which significantly correlated with OS in laryngeal cancer. Furthermore, the risk score was constructed, which significantly divided laryngeal cancer patients into high- and low-risk groups. Integrated with clinical characteristics, gender, N and the risk score are very likely associated with patients OS. A prognostic nomogram of ARGs was constructed using the Cox regression model.Our study could provide a valuable prognostic model for predicting the prognosis of laryngeal cancer patients and a new understanding of autophagy in laryngeal cancer.

NatB regulates Rb mutant cell death and tumor growth by modulating EGFR/MAPK signaling through the N-end rule pathways

Inactivation of the Rb tumor suppressor causes context-dependent increases in cell proliferation or cell death. In a genetic screen for factors that promoted Rb mutant cell death in Drosophila, we identified Psid, a regulatory subunit of N-terminal acetyltransferase B (NatB). We showed that NatB subunits were required for elevated EGFR/MAPK signaling and Rb mutant cell survival. We showed that NatB regulates the posttranscriptional levels of the highly conserved pathway components Grb2/Drk, MAPK, and PP2AC but not that of the less conserved Sprouty. Interestingly, NatB increased the levels of positive pathway components Grb2/Drk and MAPK while decreased the levels of negative pathway component PP2AC, which were mediated by the distinct N-end rule branch E3 ubiquitin ligases Ubr4 and Cnot4, respectively. These results suggest a novel mechanism by which NatB and N-end rule pathways modulate EGFR/MAPK signaling by inversely regulating the levels of multiple conserved positive and negative pathway components. As inactivation of Psid blocked EGFR signaling-dependent tumor growth, this study raises the possibility that NatB is potentially a novel therapeutic target for cancers dependent on deregulated EGFR/Ras signaling.

Mitochondrial dysfunction and chronic lung disease

The functions of body gradually decrease as the age increases, leading to a higher frequency of incidence of age-related diseases. Diseases associated with aging in the respiratory system include chronic obstructive pulmonary disease (COPD), IPF (idiopathic pulmonary fibrosis), asthma, lung cancer, and so on. The mitochondrial dysfunction is not only a sign of aging, but also is a disease trigger. This article aims to explain mitochondrial dysfunction as an aging marker, and its role in aging diseases of lung. We also discuss whether the mitochondria can be used as a target for the treatment of aging lung disease.

Ginseng metabolite Protopanaxadiol induces Sestrin2 expression and AMPK activation through GCN2 and PERK

Ginseng is one of the most commonly used herbs that is believed to have a variety of biological activities, including reducing blood sugar and cholesterol levels, anti-cancer, and anti-diabetes activities. However, little is known about the molecular mechanisms involved. In this study, we showed that protopanaxadiol (PPD), a metabolite of the protopanaxadiol group ginsenosides that are the major pharmacological constituents of ginsengs, significantly altered the expression of genes involved in metabolism, elevated Sestrin2 (Sesn2) expression, activated AMPK, and induced autophagy. Using CRISPR/CAS9-mediated gene editing and shRNA-mediated gene silencing, we demonstrated that Sesn2 is required for PPD-induced AMPK activation and autophagy. Interestingly, we showed that PPD-induced Sesn2 expression is mediated redundantly by the GCN2/ATF4 amino acid-sensing pathway and the PERK/ATF4 endoplasmic reticulum (ER) stress pathway. Our results suggest that ginseng metabolite PPD modulates the metabolism of amino acids and lipids, leading to the activation of the stress-sensing kinases GCN2 and PERK to induce Sesn2 expression, which promotes AMPK activation, autophagy, and metabolic health.

Cells Lacking the RB1 Tumor Suppressor Gene Are Hyperdependent on Aurora B Kinase for Survival

Small cell lung cancer (SCLC) accounts for 15% of lung cancers and is almost always linked to inactivating RB1 and TP53 mutations. SCLC frequently responds, albeit briefly, to chemotherapy. The canonical function of the RB1 gene product RB1 is to repress the E2F transcription factor family. RB1 also plays both E2F-dependent and E2F-independent mitotic roles. We performed a synthetic lethal CRISPR/Cas9 screen in an RB1 -/- SCLC cell line that conditionally expresses RB1 to identify dependencies that are caused by RB1 loss and discovered that RB1 -/- SCLC cell lines are hyperdependent on multiple proteins linked to chromosomal segregation, including Aurora B kinase. Moreover, we show that an Aurora B kinase inhibitor is efficacious in multiple preclinical SCLC models at concentrations that are well tolerated in mice. These results suggest that RB1 loss is a predictive biomarker for sensitivity to Aurora B kinase inhibitors in SCLC and perhaps other RB1 -/- cancers. SIGNIFICANCE: SCLC is rarely associated with actionable protooncogene mutations. We did a CRISPR/Cas9-based screen that showed that RB1 -/- SCLC are hyperdependent on AURKB, likely because both genes control mitotic fidelity, and confirmed that Aurora B kinase inhibitors are efficacious against RB1 -/- SCLC tumors in mice at nontoxic doses.See related commentary by Dick and Li, p. 169.This article is highlighted in the In This Issue feature, p. 151.

Ginseng metabolite protopanaxadiol interferes with lipid metabolism and induces endoplasmic reticulum stress and p53 activation to promote cancer cell death

Protopanaxadiol (PPD), a ginseng metabolite generated by the gut bacteria, was shown to induce colorectal cancer cell death and enhance the anticancer effect of chemotherapeutic agent 5-FU. However, the mechanism by which PPD promotes cancer cell death is not clear. In this manuscript, we showed that PPD activated p53 and endoplasmic reticulum (ER) stress and induced expression of BH3-only proteins Puma and Noxa to promote cell death. Induction of Puma by PPD was p53-dependent, whereas induction of Noxa was p53-independent. On the other hand, PPD also induced prosurvival mechanisms including autophagy and expression of Bcl2 family apoptosis regulator Mcl-1. Inhibition of autophagy or knockdown of Mcl-1 significantly enhanced PPD-induced cell death. Interestingly, PPD inhibited expression of genes involved in fatty acid and cholesterol biosynthesis and induced synergistic cancer cell death with fatty acid synthase inhibitor cerulenin. As PPD-induced ER stress was not significantly affected by inhibition of new protein synthesis, we suggest PPD may induce ER stress directly through causing lipid disequilibrium.

Synthetic lethal therapies for cancer: what's next after PARP inhibitors?

The genetic concept of synthetic lethality has now been validated clinically through the demonstrated efficacy of poly(ADP-ribose) polymerase (PARP) inhibitors for the treatment of cancers in individuals with germline loss-of-function mutations in either BRCA1 or BRCA2. Three different PARP inhibitors have now been approved for the treatment of patients with BRCA-mutant ovarian cancer and one for those with BRCA-mutant breast cancer; these agents have also shown promising results in patients with BRCA-mutant prostate cancer. Here, we describe a number of other synthetic lethal interactions that have been discovered in cancer. We discuss some of the underlying principles that might increase the likelihood of clinical efficacy and how new computational and experimental approaches are now facilitating the discovery and validation of synthetic lethal interactions. Finally, we make suggestions on possible future directions and challenges facing researchers in this field.

An Rb family-independent E2F3 transcription factor variant impairs STAT5 signaling and mammary gland remodeling during pregnancy in mice

E2F transcription factors are regulated by binding to the retinoblastoma (Rb) tumor suppressor family of proteins. Previously, we reported an E2F^LQ mutation that disrupts the binding with Rb proteins without affecting the transcriptional activity of E2F. We also showed that mouse embryonic fibroblasts with an E2F3^LQ mutation exhibit increased E2F activity and more rapid cell proliferation. In this report, we analyzed E2F3^LQ mice to further characterize the in vivo consequences of Rb family-independent E2F3 activity. We found that homozygous E2F3^LQ mice were viable and had no obvious developmental defects or tumor growth. Our results also indicated that E2F3^LQ cells largely retain normal control of cell proliferation in vivo However, female E2F3^LQ mice had partial nursing defects. Examination of the E2F3^LQ mammary glands revealed increased caveolin-1 (CAV1) expression, reduced prolactin receptor/Stat5 signaling, and impaired pregnancy-induced cell proliferation and differentiation. Of note, ChIP experiments disclosed that E2F3 binds the CAV1 promoter. Furthermore, E2F3 overexpression induced CAV1 expression, and CRISPR/CAS9-mediated E2F3 knockout reduced CAV1 levels and also increased prolactin receptor-induced Stat5 signaling in mammary epithelial cells. Our results suggest that the Rb family-independent E2F3 ^LQ variant inhibits pregnancy-induced mammary gland cell proliferation and differentiation by up-regulating CAV1 expression and inhibiting Stat5 signaling.

PKM2 promotes reductive glutamine metabolism

Objective:

Pyruvate kinases M (PKM), including the PKM1 and PKM2 isoforms, are critical factors in glucose metabolism. PKM2 promotes aerobic glycolysis, a phenomenon known as " the Warburg effect”. The purpose of this study was to identify the roles of PKM2 in regulating cellular metabolism.

Methods:

The CRISPR/Cas9 system was used to generate the PKM-knockout cell model to evaluate the role of PKM in cellular metabolism. Lactate levels were measured by the Vitros LAC slide method on an autoanalyzer and glucose levels were measured by the autoanalyzer AU5800. The metabolism of ¹³C₆-glucose or ¹³C₅-glutamine was evaluated by liquid chromatography/mass spectrometry analyses. The effects of PKM on tumor growth were detected in vivo in a tumor-bearing mouse model.

Results:

We found that both PKM1 and PKM2 enabled aerobic glycolysis, but PKM2 converted glucose to lactate much more efficiently than PKM1. As a result, PKM2 reduced glucose levels reserved for intracellular utilization, particularly for the production of citrate, and thus increased the α-ketoglutarate/citrate ratio to promote the generation of glutamine-derived acetyl-coenzyme A through the reductive pathway. Furthermore, reductive glutamine metabolism facilitated cell proliferation under hypoxia conditions, which supports in vivo tumor growth. In addition, PKM-deletion induced a reverse Warburg effect in tumor-associated stromal cells.

Conclusions:

PKM2 plays a critical role in promoting reductive glutamine metabolism and maintaining proton homeostasis. This study is helpful to increase the understanding of the physiological role of PKM2 in cancer cells.

Intersection of retinoblastoma tumor suppressor function, stem cells, metabolism, and inflammation

The Retinoblastoma (RB) tumor suppressor regulates G₁/S transition during cell cycle progression by modulating the activity of E2F transcription factors. The RB pathway plays a central role in the suppression of most cancers, and RB mutation was initially discovered by virtue of its role in tumor initiation. However, as cancer genome sequencing has evolved to profile more advanced and treatment‐resistant cancers, it has become increasingly clear that, in the majority of cancers, somatic RB inactivation occurs during tumor progression. Furthermore, despite the presence of deregulation of cell cycle control due to an INK4A deletion, additional CCND amplification and/or other mutations in the RB pathway, mutation or deletion of the RB gene is often observed during cancer progression. Of note, RB inactivation during cancer progression not only facilitates G₁/S transition but also enhances some characteristics of malignancy, including altered drug sensitivity and a return to the undifferentiated state. Recently, we reported that RB inactivation enhances pro‐inflammatory signaling through stimulation of the interleukin‐6/STAT3 pathway, which directly promotes various malignant features of cancer cells. In this review, we highlight the consequences of RB inactivation during cancer progression, and discuss the biological and pathological significance of the interaction between RB and pro‐inflammatory signaling.

Activated E2F activity induces cell death in papillary thyroid carcinoma K1 cells with enhanced Wnt signaling

Disruption of Wnt signaling often happens in tumorigenesis, but whether Wnt signaling affects the early stages of thyroid tumor, such as papillary thyroid carcinoma, is still a question, especially in the papillary thyroid carcinoma without genomic RET/PTC mutation. In this study, we demonstrated the important function of Wnt signaling in papillary thyroid carcinoma K1 cells, which have no RET/PTC mutation. We found that K1 cells have enhanced Wnt signaling in comparison to normal thyroid cells. We further demonstrated that K1 cells require the enhanced Wnt signaling for growth and survival. Interestingly, we identified that enhancing E2F activity by either knockdown of Rb or overexpression of Cyclin D1 induces cell death in K1 cells. And we further revealed that the cell death is caused by enhanced oxidative stress. Our studies present a novel cell model to support the key roles of Wnt signaling in early stage of thyroid tumor, and also provide an alternative way to limit thyroid cancer.

RB1: a prototype tumor suppressor and an enigma

In this review, Dyson summarizes some recent developments in pRB research and focuses on progress toward answers for the three fundamental questions that sit at the heart of the pRB literature: What does pRB do? How does the inactivation of RB change the cell? How can our knowledge of RB function be exploited to provide better treatment for cancer patients?

The retinoblastoma susceptibility gene (RB1) was the first tumor suppressor gene to be molecularly defined. RB1 mutations occur in almost all familial and sporadic forms of retinoblastoma, and this gene is mutated at variable frequencies in a variety of other human cancers. Because of its early discovery, the recessive nature of RB1 mutations, and its frequency of inactivation, RB1 is often described as a prototype for the class of tumor suppressor genes. Its gene product (pRB) regulates transcription and is a negative regulator of cell proliferation. Although these general features are well established, a precise description of pRB's mechanism of action has remained elusive. Indeed, in many regards, pRB remains an enigma. This review summarizes some recent developments in pRB research and focuses on progress toward answers for the three fundamental questions that sit at the heart of the pRB literature: What does pRB do? How does the inactivation of RB change the cell? How can our knowledge of RB function be exploited to provide better treatment for cancer patients?

Synthetic Lethality in Cancer Therapeutics

Treatment with targeted drugs has primarily focused on the genes and pathways that are mutated in cancer, which severely limits the repertoire of drug targets. Synthetic lethality exploits the notion that the presence of a mutation in a cancer gene is often associated with a new vulnerability that can be targeted therapeutically, thus greatly expanding the arsenal of potential drug targets. Here we discuss both the experimental and the computational biology tools that can be used to identify synthetic lethal interactions. We also discuss strategies for using synthetic lethality to discover new drug targets and in the rational design of more potent drug combinations. We review the progress made and future opportunities offered by synthetic lethal approaches to treating cancer more effectively.

Enhanced Rb/E2F and TSC/mTOR Pathways Induce Synergistic Inhibition in PDGF-Induced Proliferation in Vascular Smooth Muscle Cells

Platelet-derived growth factor (PDGF) plays an essential role in proliferation of vascular smooth muscle cells (VSMCs). The Rb/E2F and TSC/mTOR pathways contribute to the proliferation of VSMCs, but its exact roles in PDGF-induced proliferation are unclear. In this study, we demonstrated the roles of Rb/E2F and TSC/mTOR pathways in PDGF-induced proliferation in VSMCs. We found that PDGF stimulates the activity of E2F and mTOR pathways, and knockdown of either Rb or TSC2 increases PDGF-induced proliferation in VSMCs. More interestingly, we revealed that enhancing both E2F and mTOR activity leads to synergistic inhibition of PDGF-induced proliferation in VSMCs. We further identified that the synergistic inhibition effect is caused by the induced oxidative stress. Summarily, these data suggest the important regulations of Rb/E2F and TSC/mTOR pathways in PDGF-induced proliferation in VSMCs, and also present a promising way to limit deregulated proliferation by PDGF induction in VSMCs.

Drugging the addict: non-oncogene addiction as a target for cancer therapy

Historically, cancers have been treated with chemotherapeutics aimed to have profound effects on tumor cells with only limited effects on normal tissue. This approach was followed by the development of small‐molecule inhibitors that can target oncogenic pathways critical for the survival of tumor cells. The clinical targeting of these so‐called oncogene addictions, however, is in many instances hampered by the outgrowth of resistant clones. More recently, the proper functioning of non‐mutated genes has been shown to enhance the survival of many cancers, a phenomenon called non‐oncogene addiction. In the current review, we will focus on the distinct non‐oncogenic addictions found in cancer cells, including synthetic lethal interactions, the underlying stress phenotypes, and arising therapeutic opportunities.

RB1 dual role in proliferation and apoptosis: cell fate control and implications for cancer therapy

Inactivation of the retinoblastoma (RB1) tumor suppressor is one of the most frequent and early recognized molecular hallmarks of cancer. RB1, although mainly studied for its role in the regulation of cell cycle, emerged as a key regulator of many biological processes. Among these, RB1 has been implicated in the regulation of apoptosis, the alteration of which underlies both cancer development and resistance to therapy. RB1 role in apoptosis, however, is still controversial because, depending on the context, the apoptotic cues, and its own status, RB1 can act either by inhibiting or promoting apoptosis. Moreover, the mechanisms whereby RB1 controls both proliferation and apoptosis in a coordinated manner are only now beginning to be unraveled. Here, by reviewing the main studies assessing the effect of RB1 status and modulation on these processes, we provide an overview of the possible underlying molecular mechanisms whereby RB1, and its family members, dictate cell fate in various contexts. We also describe the current antitumoral strategies aimed at the use of RB1 as predictive, prognostic and therapeutic target in cancer. A thorough understanding of RB1 function in controlling cell fate determination is crucial for a successful translation of RB1 status assessment in the clinical setting.

Roles for the Histone Modifying and Exchange Complex NuA4 in Cell Cycle Progression in Drosophila melanogaster

Robust and synchronous repression of E2F-dependent gene expression is critical to the proper timing of cell cycle exit when cells transition to a postmitotic state. Previously NuA4 was suggested to act as a barrier to proliferation in Drosophila by repressing E2F-dependent gene expression. Here we show that NuA4 activity is required for proper cell cycle exit and the repression of cell cycle genes during the transition to a postmitotic state in vivo However, the delay of cell cycle exit caused by compromising NuA4 is not due to additional proliferation or effects on E2F activity. Instead NuA4 inhibition results in slowed cell cycle progression through late S and G2 phases due to aberrant activation of an intrinsic p53-independent DNA damage response. A reduction in NuA4 function ultimately produces a paradoxical cell cycle gene expression program, where certain cell cycle genes become derepressed in cells that are delayed during the G2 phase of the final cell cycle. Bypassing the G2 delay when NuA4 is inhibited leads to abnormal mitoses and results in severe tissue defects. NuA4 physically and genetically interacts with components of the E2F complex termed D: rosophila, R: bf, E: 2F A: nd M: yb/ M: ulti-vulva class B: (DREAM/MMB), and modulates a DREAM/MMB-dependent ectopic neuron phenotype in the posterior wing margin. However, this effect is also likely due to the cell cycle delay, as simply reducing Cdk1 is sufficient to generate a similar phenotype. Our work reveals that the major requirement for NuA4 in the cell cycle in vivo is to suppress an endogenous DNA damage response, which is required to coordinate proper S and G2 cell cycle progression with differentiation and cell cycle gene expression.

ESCRT-0 complex modulates Rbf-mutant cell survival by regulating Rhomboid endosomal trafficking and EGFR signaling

The Rb tumor suppressor is conserved in Drosophila, and its inactivation can lead to cell proliferation or death depending on the specific cellular context. Therefore, identifying genes that affect the survival of Rb-mutant cells can potentially identify novel targets for therapeutic intervention in cancer. From a genetic screen in Drosophila, we identified synthetic lethal interactions between mutations of fly Rb (rbf) and the ESCRT-0 components stam and hrs We show that inactivation of ESCRT-0 sensitizes rbf-mutant cells to undergo apoptosis through inhibition of EGFR signaling and accumulation of Hid protein. Mutation of stam inhibits EGFR signaling upstream of secreted Spi and downstream of Rhomboid expression, and causes Rhomboid protein to accumulate in the abnormal endosomes labeled with both the early and late endosomal markers Rab5 and Rab7. These results reveal that ESCRT-0 mutants inhibit EGFR signaling by disrupting Rhomboid endosomal trafficking in the ligand-producing cells. Because ESCRT-0 also plays crucial roles in EGFR downregulation after ligand binding, this study provides new insights into how loss of ESCRT-0 function can either increase or decrease EGFR signaling.

Proteomic analysis of pRb loss highlights a signature of decreased mitochondrial oxidative phosphorylation

Nicolay et al. ablated Rb in adult mice and conducted a quantitative analysis of RNA and proteomic changes in the colon and lungs. The proteomic changes in common between Rb^KO tissues showed a striking decrease in proteins with mitochondrial functions, highlighting the importance of pRb for mitochondrial function.

The retinoblastoma tumor suppressor (pRb) protein associates with chromatin and regulates gene expression. Numerous studies have identified Rb-dependent RNA signatures, but the proteomic effects of Rb loss are largely unexplored. We acutely ablated Rb in adult mice and conducted a quantitative analysis of RNA and proteomic changes in the colon and lungs, where Rb^KO was sufficient or insufficient to induce ectopic proliferation, respectively. As expected, Rb^KO caused similar increases in classic pRb/E2F-regulated transcripts in both tissues, but, unexpectedly, their protein products increased only in the colon, consistent with its increased proliferative index. Thus, these protein changes induced by Rb loss are coupled with proliferation but uncoupled from transcription. The proteomic changes in common between Rb^KO tissues showed a striking decrease in proteins with mitochondrial functions. Accordingly, RB1 inactivation in human cells decreased both mitochondrial mass and oxidative phosphorylation (OXPHOS) function. RB^KO cells showed decreased mitochondrial respiratory capacity and the accumulation of hypopolarized mitochondria. Additionally, RB/Rb loss altered mitochondrial pyruvate oxidation from ¹³C-glucose through the TCA cycle in mouse tissues and cultured cells. Consequently, RB^KO cells have an enhanced sensitivity to mitochondrial stress conditions. In summary, proteomic analyses provide a new perspective on Rb/RB1 mutation, highlighting the importance of pRb for mitochondrial function and suggesting vulnerabilities for treatment.

Drosophila as a Potential Model for Ocular Tumors

Drosophila has made many contributions to our understanding of cancer genes and mechanisms that have subsequently been validated in mammals. Despite anatomical differences between fly and human eyes, flies offer a tractable genetic model in which to dissect the functional importance of genetic lesions found to be affected in human ocular tumors. Here, we discuss different approaches for using Drosophila as a model for ocular cancer and how studies on ocular cancer genes in flies have begun to reveal potential strategies for therapeutic intervention. We also discuss recent developments in the use of Drosophila for drug discovery, which is coming to the fore as Drosophila models are becoming tailored to study tumor types found in the clinic.

β-Catenin is required for the tumorigenic behavior of triple-negative breast cancer cells

Our previous data illustrated that activation of the canonical Wnt signaling pathway was enriched in triple-negative breast cancer and associated with reduced overall survival in all patients. To determine whether Wnt signaling may be a promising therapeutic target for triple-negative breast cancer, we investigated whether β-catenin was necessary for tumorigenic behaviors in vivo and in vitro. β-catenin expression level was significantly reduced in two human triple-negative breast cancer cell lines, MDA-MB-231 and HCC38, using lentiviral delivery of β-catenin-specific small hairpin RNAs (shRNAs). Upon implantation of the cells in the mammary fat pad of immunocompromised mice, we found that β-catenin shRNA HCC38 cells formed markedly smaller tumors than control cells and grew much more slowly. In in vitro assays, β-catenin silencing significantly reduced the percentage of Aldefluor-positive cells, a read-out of the stem-like cell population, as well as the expression of stem cell-related target genes including Bmi-1 and c-Myc. β-catenin-knockdown cells were also significantly impaired in their ability to migrate in wound-filling assays and form anchorage-independent colonies in soft agar. β-catenin-knockdown cells were more sensitive to chemotherapeutic agents doxorubicin and cisplatin. Collectively, these data suggest that β-catenin is required for triple-negative breast cancer development by controlling numerous tumor-associated properties, such as migration, stemness, anchorage-independent growth and chemosensitivity.

Antitumor mechanisms when pRb and p53 are genetically inactivated

pRb and p53 are the two major tumor suppressors. Their inactivation is frequent when cancers develop and their reactivation is rationale of most cancer therapeutics. When pRb and p53 are genetically inactivated, cells irreparably lose the antitumor mechanisms afforded by them. Cancer genome studies document recurrent genetic inactivation of RB1 and TP53, and the inactivation becomes more frequent in more advanced cancers. These findings may explain why more advanced cancers are more likely to resist current therapies. Finding successful treatments for more advanced and multi-therapy resistant cancers will depend on finding antitumor mechanisms that remain effective when pRb and p53 are genetically inactivated. Here, we review studies that have begun to make progress in this direction.

Stress-mediated translational control in cancer cells

Tumor cells are continually subjected to diverse stress conditions of the tumor microenvironment, including hypoxia, nutrient deprivation, and oxidative or genotoxic stress. Tumor cells must evolve adaptive mechanisms to survive these conditions to ultimately drive tumor progression. Tight control of mRNA translation is critical for this response and the adaptation of tumor cells to such stress forms. This proceeds though a translational reprogramming process which restrains overall translation activity to preserve energy and nutrients, but which also stimulates the selective synthesis of major stress adaptor proteins. Here we present the different regulatory signaling pathways which coordinate mRNA translation in the response to different stress forms, including those regulating eIF2α, mTORC1 and eEF2K, and we explain how tumor cells hijack these pathways for survival under stress. Finally, mechanisms for selective mRNA translation under stress, including the utilization of upstream open reading frames (uORFs) and internal ribosome entry sites (IRESes) are discussed in the context of cell stress. This article is part of a Special Issue entitled: Translation and Cancer.

Regulation of cell transformation by Rb-controlled redox homeostasis

Rb is a tumor suppressor, and regulates various biological progresses, such as cell proliferation, development, metabolism and cell death. In the current study, we show that Rb knockout in 3T3 cells leads to oxidative redox state and low mitochondrial membrane potential by regulating mitochondrial activity. Our results indicate that Rb plays an important role in controlling redox homeostasis. More importantly, the functions of Rb in modulating cell proliferation, death and transformation are, at least in part, mediated by its controlling cellular redox state. In addition, our results also suggest that the cellular redox state possibly determines various biological activities, including cell survival, death and transformation, where Rb is functioning as a regulator of redox homeostasis.

Hyperactivated Wnt signaling induces synthetic lethal interaction with Rb inactivation by elevating TORC1 activities

Inactivation of the Rb tumor suppressor can lead to increased cell proliferation or cell death depending on specific cellular context. Therefore, identification of the interacting pathways that modulate the effect of Rb loss will provide novel insights into the roles of Rb in cancer development and promote new therapeutic strategies. Here, we identify a novel synthetic lethal interaction between Rb inactivation and deregulated Wg/Wnt signaling through unbiased genetic screens. We show that a weak allele of axin, which deregulates Wg signaling and increases cell proliferation without obvious effects on cell fate specification, significantly alters metabolic gene expression, causes hypersensitivity to metabolic stress induced by fasting, and induces synergistic apoptosis with mutation of fly Rb ortholog, rbf. Furthermore, hyperactivation of Wg signaling by other components of the Wg pathway also induces synergistic apoptosis with rbf. We show that hyperactivated Wg signaling significantly increases TORC1 activity and induces excessive energy stress with rbf mutation. Inhibition of TORC1 activity significantly suppressed synergistic cell death induced by hyperactivated Wg signaling and rbf inactivation, which is correlated with decreased energy stress and decreased induction of apoptotic regulator expression. Finally the synthetic lethality between Rb and deregulated Wnt signaling is conserved in mammalian cells and that inactivation of Rb and APC induces synergistic cell death through a similar mechanism. These results suggest that elevated TORC1 activity and metabolic stress underpin the evolutionarily conserved synthetic lethal interaction between hyperactivated Wnt signaling and inactivated Rb tumor suppressor.

Inactivation of Rb tumor suppressor is common in cancers. Therefore, identification of genes and pathways that are synthetic lethal with Rb will provide new insights into the role of Rb in cancer development and promote the development of novel therapeutic approaches. Here we identified a novel synthetic lethal interaction between Rb inactivation and hyperactivated Wnt signaling and showed that this synthetic lethal interaction is conserved in mammalian systems. We demonstrate that hyperactivated Wnt signaling activate TORC1 activity and induce excessive energy stress with inactivated Rb tumor suppressor, which underpins the evolutionarily conserved synthetic lethal interaction. This study provides novel insights into the interactions between the Rb, Wnt, and mTOR pathways in regulating cellular energy balance, cell growth, and survival.

Retinoblastoma tumor suppressor pathway in breast cancer: prognosis, precision medicine, and therapeutic interventions

A series of recent studies have demonstrated that the retinoblastoma tumor suppressor (RB) pathway plays a critical role in multiple clinically relevant aspects of breast cancer biology, spanning early stage lesions to targeted treatment of metastatic disease. In ductal carcinoma in situ, multiple groups have shown that dysregulation of the RB pathway is critically associated with recurrence and disease progression. Functional models have similarly illustrated key roles for RB in regulating epithelial–mesenchymal transition and other features contributing to aggressive disease. Invasive breast cancers are treated in distinct fashions, and heterogeneity within the RB pathway relates to prognosis and response to commonly used therapeutics. Luminal B breast cancers that have a poor prognosis amongst estrogen receptor-positive disease are defined based on the expression of RB-regulated genes. Such findings have led to clinical interventions that directly target the RB pathway through CDK4/6 inhibition which have promise in both estrogen receptor-positive and Her2-positive disease. In contrast, RB loss results in improved response to chemotherapy in triple-negative breast cancer, where ongoing research is attempting to define intrinsic vulnerabilities for targeted intervention. These findings support a wide-reaching impact of the RB pathway on disease that could be harnessed for improved clinical interventions.

Anticancer compound Oplopantriol A kills cancer cells through inducing ER stress and BH3 proteins Bim and Noxa

Oplopantriol-A (OPT) is a natural polyyne from Oplopanax horridus. We show here that OPT preferentially kills cancer cells and inhibits tumor growth. We demonstrate that OPT-induced cancer cell death is mediated by excessive endoplasmic reticulum (ER) stress. Decreasing the level of ER stress either by inactivating components of the unfolded protein response (UPR) pathway or by expression of ER chaperone protein glucose-regulated protein 78 (GRP78) decreases OPT-induced cell death. We show that OPT induces the accumulation of ubiquitinated proteins and the stabilization of unstable proteins, suggesting that OPT functions, at least in part, through interfering with the ubiquitin/proteasome pathway. In support of this, inhibition of protein synthesis significantly decreased the accumulation of ubiquitinated proteins, which is correlated with significantly decreased OPT-induced ER stress and cell death. Finally, we show that OPT treatment significantly induced the expression of BH3-only proteins, Noxa and Bim. Knockdown of both Noxa and Bim significantly blocked OPT-induced cell death. Taken together, our results suggest that OPT is a potential new anticancer agent that induces cancer cell death through inducing ER stress and BH3 proteins Noxa and Bim.

Visualization of caspase-3-like activity in cells using a genetically encoded fluorescent biosensor activated by protein cleavage

Cytosolic caspase-3-like proteases, such as caspase-3 and caspase-7, have a central role in mediating the progress of apoptosis. Here to conveniently monitor caspase-3-like activity in the multicellular environment, we have developed genetically encoded switch-on fluorescence-base indicators that are cyclized chimeras containing a caspase-3 cleavage site as a switch. When cleaved by caspase-3-like proteases, the non-fluorescent indicator rapidly becomes fluorescent, and thus detects in real-time the activation of such caspases. We generate cultured cells constitutively expressing these chimeras, and all the healthy cells are non-fluorescent. When these cells are exposed to apoptotic stimuli, dead cells show strong fluorescence depending on caspase activation. With these tools, we monitor in real-time caspase-3-like activity in each cell under various conditions, and show for the first time that the environment of cancer cells affects their sensitivity to chemotherapeutic drugs in a modified soft agar assay. These biosensors should enable better understanding of the biological relevance of caspase-3-like proteases.

Modulation of oxidative stress as an anticancer strategy

The regulation of oxidative stress is an important factor in both tumour development and responses to anticancer therapies. Many signalling pathways that are linked to tumorigenesis can also regulate the metabolism of reactive oxygen species (ROS) through direct or indirect mechanisms. High ROS levels are generally detrimental to cells, and the redox status of cancer cells usually differs from that of normal cells. Because of metabolic and signalling aberrations, cancer cells exhibit elevated ROS levels. The observation that this is balanced by an increased antioxidant capacity suggests that high ROS levels may constitute a barrier to tumorigenesis. However, ROS can also promote tumour formation by inducing DNA mutations and pro-oncogenic signalling pathways. These contradictory effects have important implications for potential anticancer strategies that aim to modulate levels of ROS. In this Review, we address the controversial role of ROS in tumour development and in responses to anticancer therapies, and elaborate on the idea that targeting the antioxidant capacity of tumour cells can have a positive therapeutic impact.

Mutation of the retinoblastoma tumor suppressor gene sensitizes cancers to mitotic inhibitor induced cell death

The retinoblastoma gene Rb is a prototype tumor suppressor, which encodes a protein that is inactivated in a broad range of human cancers through different mechanisms. Rb functions to regulate cell proliferation, differentiation, as well as cell death. Therefore, even though Rb inactivation promotes cancer development, this may also open up certain vulnerabilities of cancers that can potentially be targeted with drug intervention. Based on the assumption that cancers that have mutation, deletion, or rearrangement in the Rb locus represent strong loss of Rb function while cancers with WT Rb on average retain some Rb function, we searched Genomics of Drug Sensitivity in Cancer database to identify cancer drugs that are particularly effective to cancers with Rb genomic alterations. Three mitotic inhibitors were identified from this analysis. We further tested the effects of two mitotic inhibitors, Taxol and STLC, on prostate and breast cancer cells. We demonstrate that the Rb status affects cancer cell sensitivity to these mitotic drugs and that the sensitizing effects of Rb are mediated in part by its regulation of the cell cycle checkpoint protein Mad2. Since the mitotic inhibitors identified in our analysis inhibit mitosis through distinct targets, it is possible that the Rb functional status may serve as a general biomarker for cancer sensitivity to mitotic inhibitors. Because the Rb pathway is inactivated in a large number of human cancers, identification of agents that are particularly effective or ineffective based on the Rb status in cancers can potentially be used generally to matching patients with appropriate treatments to achieve better therapeutic outcome.

Autophagy-dependent metabolic reprogramming sensitizes TSC2-deficient cells to the antimetabolite 6-aminonicotinamide

The mammalian target of rapamycin complex 1 (mTORC1) is hyperactive in many human cancers and in tuberous sclerosis complex (TSC). Autophagy, a key mTORC1-targeted process, is a critical determinant of metabolic homeostasis. Metabolomic profiling was performed to elucidate the cellular consequences of autophagy dysregulation under conditions of hyperactive mTORC1. It was discovered that TSC2-null cells have distinctive autophagy-dependent pentose phosphate pathway (PPP) alterations. This was accompanied by enhanced glucose uptake and utilization, decreased mitochondrial oxygen consumption, and increased mitochondrial reactive oxygen species (ROS) production. Importantly, these findings revealed that the PPP is a key autophagy-dependent compensatory metabolic mechanism. Furthermore, PPP inhibition with 6-aminonicotinamide (6-AN) in combination with autophagy inhibition suppressed proliferation and prompted the activation of NF-κB and CASP1 in TSC2-deficient, but not TSC2-proficient cells. These data demonstrate that TSC2-deficient cells can be therapeutically targeted, without mTORC1 inhibitors, by focusing on their metabolic vulnerabilities.

IMPLICATIONS

This study provides proof-of-concept that therapeutic targeting of diseases with hyperactive mTORC1 can be achieved without the application of mTORC1 inhibitors.

Effect of atmospheric gas plasmas on cancer cell signaling

Cancer is one of the most life-threatening diseases with many forms still regarded as incurable. The conventional cancer treatments have unwanted side effects such as the death of normal cells. A therapy that can accurately target and effectively kill tumor cells could address the inadequacies of the available therapies. Atmospheric gas plasmas (AGP) that are able to specifically kill cancerous cells offer a promising alternative approach compared to conventional therapies. AGP have been shown to exploit tumor-specific genetic defects and a recent trial in mice has confirmed its antitumor effects. The mechanism by which the AGP act on tumor cells but not normal cells is not fully understood. A review of the current literature suggests that reactive oxygen species (ROS) generated by AGP induce death of cancer cells by impairing the function of intracellular regulatory factors. The majority of cancer cells are defective in tumor suppressors that interfere normal cell growth pathways. It appears that pro-oncogene or tumor suppressor-dependent regulation of antioxidant/or ROS signaling pathways may be involved in AGP-induced cancer cell death. The toxic effects of ROS are mitigated by normal cells by adjustment of their metabolic pathways. On the other hand, tumor cells are mostly defective in several regulatory signaling pathways which lead to the loss of metabolic balance within the cells and consequently, the regulation of cell growth. This review article evaluates the impact of AGP on the activation of cellular signaling and its importance for exploring mechanisms for safe and efficient anticancer therapies.

Rapamycin extends life span of Rb1+/- mice by inhibiting neuroendocrine tumors

Chronic treatment of mice with an enterically released formulation of rapamycin (eRapa) extends median and maximum life span, partly by attenuating cancer. The mechanistic basis of this response is not known. To gain a better understanding of these in vivo effects, we used a defined preclinical model of neuroendocrine cancer, Rb1^+/− mice. Previous results showed that diet restriction (DR) had minimal or no effect on the lifespan of Rb1^+/− mice, suggesting that the beneficial response to DR is dependent on pRb1. Since long-term eRapa treatment may at least partially mimic chronic DR in lifespan extension, we predicted that it would have a minimal effect in Rb1^+/− mice. Beginning at 9 weeks of age until death, we fed Rb1^+/− mice a diet without or with eRapa at 14 mg/kg food, which results in an approximate dose of 2.24 mg/kg body weight per day, and yielded rapamycin blood levels of about 4 ng/ml. Surprisingly, we found that eRapa dramatically extended life span of both female and male Rb1^+/− mice, and slowed the appearance and growth of pituitary and decreased the incidence of thyroid tumors commonly observed in these mice. In this model, eRapa appears to act differently than DR, suggesting diverse mechanisms of action on survival and anti-tumor effects. In particular the beneficial effects of rapamycin did not depend on the dose of Rb1.

The multiple connections between pRB and cell metabolism

The pRB tumor suppressor is traditionally seen as an important regulator of the cell cycle. pRB represses the transcriptional activation of a diverse set of genes by the E2F transcription factors and prevents inappropriate S-phase entry. Advances in our understanding of pRB have documented roles that extend beyond the cell cycle and this review summarizes recent studies that link pRB to the control of cell metabolism. pRB has been shown to regulate glucose tolerance, mitogenesis, glutathione synthesis, and the expression of genes involved in central carbon metabolism. Several studies have demonstrated that pRB directly targets a set of genes that are crucial for nucleotide metabolism, and this seems likely to represent one of the ways by which pRB influences the G1/S-phase transition and S-phase progression.

Cancer metabolism: feeding a worm to starve a tumor

The tumor suppressor Rb is known to have its hand in many pots. New findings have added another pot to the mix - cell metabolism. This may lead to a better understanding of Rb mutant phenotypes and Rb's roles in oncogenesis.

Loss of TSC2 confers resistance to ceramide and nutrient deprivation

Nutrient stress that produces quiescence and catabolism in normal cells is lethal to cancer cells, because oncogenic mutations constitutively drive anabolism. One driver of biosynthesis in cancer cells is the mammalian target of rapamycin complex 1 (mTORC1) signaling complex. Activating mTORC1 by deleting its negative regulator tuberous sclerosis complex 2 (TSC2) leads to hypersensitivity to glucose deprivation. We have previously shown that ceramide kills cells in part by triggering nutrient transporter loss and restricting access to extracellular amino acids and glucose, suggesting that TSC2-deficient cells would be hypersensitive to ceramide. However, murine embryonic fibroblasts (MEFs) lacking TSC2 were highly resistant to ceramide-induced death. Consistent with the observation that ceramide limits access to both amino acids and glucose, TSC2(-/-) MEFs also had a survival advantage when extracellular amino acids and glucose were both reduced. As TSC2(-/-) MEFs were resistant to nutrient stress despite sustained mTORC1 activity, we assessed whether mTORC1 signaling might be beneficial under these conditions. In low amino acid and glucose medium, and following ceramide-induced nutrient transporter loss, elevated mTORC1 activity significantly enhanced the adaptive upregulation of new transporter proteins for amino acids and glucose. Strikingly, the introduction of oncogenic Ras abrogated the survival advantage of TSC2(-/-) MEFs upon ceramide treatment most likely by increasing nutrient demand. These results suggest that, in the absence of oncogene-driven biosynthetic demand, mTORC1-dependent translation facilitates the adaptive cellular response to nutrient stress.

Deregulated G1-S control and energy stress contribute to the synthetic-lethal interactions between inactivation of RB and TSC1 or TSC2

Synthetic lethality is a potential strategy for cancer treatment by specifically promoting the death of cancer cells with particular defects such as the loss of the RB (RB1) tumor suppressor. We previously showed that inactivation of both RB and TSC2 induces synergistic apoptosis during the development of Drosophila melanogaster and in cancer cells. However, the in vivo mechanism of this synthetic-lethal interaction is not clear. Here, we show that synergistic cell death in tissues that have lost the RB and TSC orthologs rbf and dtsc1/gig, respectively, or overexpress Rheb and dE2F1, are correlated with synergistic defects in G1-S control, which causes cells to accumulate DNA damage. Coexpression of the G1-S inhibitor Dap, but not the G2-M inhibitor dWee1, decreases DNA damage and reduces cell death. In addition, we show that rbf and dtsc1 mutant cells are under energy stress, are sensitive to decreased energy levels and depend on the cellular energy stress-response pathway for survival. Decreasing mitochondrial ATP synthesis by inactivating cova or abrogating the energy-stress response by removing the metabolic regulator LKB1 both enhance the elimination of cells lacking either rbf or dtsc1. These observations, in conjunction with the finding that deregulation of TORC1 induces activation of JNK, indicate that multiple cellular stresses are induced and contribute to the synthetic-lethal interactions between RB and TSC1/TSC2 inactivation. The insights gained from this study suggest new approaches for targeting RB-deficient cancers.

Capicua regulates proliferation and survival of RB-deficient cells in Drosophila

Mutations in rbf1, the Drosophila homologue of the RB tumour suppressor gene, generate defects in cell cycle control, cell death, and differentiation during development. Previous studies have established that EGFR/Ras activity is an important determinant of proliferation and survival in rbf1 mutant cells. Here, we report that Capicua (Cic), an HMG box transcription factor whose activity is regulated by the EGFR/Ras pathway, regulates both proliferation and survival of RB-deficient cells in Drosophila. We demonstrate that cic mutations allow rbf1 mutant cells to bypass developmentally controlled cell cycle arrest and apoptotic pressure. The cooperative effect between Cic and RBF1 in promoting G1 arrest is mediated, at least in part, by limiting Cyclin E expression. Surprisingly, we also found evidence to suggest that cic mutant cells have decreased levels of reactive oxygen species (ROS), and that the survival of rbf1 mutant cells is affected by changes in ROS levels. Collectively, our results elucidate the importance of the crosstalk between EGFR/Ras and RBF1 in coordinating cell cycle progression and survival.

Functional inactivation of Rb sensitizes cancer cells to TSC2 inactivation induced cell death

We showed previously that inactivation of TSC2 induces death in cancer cells lacking the Retinoblastoma (Rb) tumor suppressor under stress conditions, suggesting that inactivation of TSC2 can potentially be used as an approach to specifically kill cancers that have lost WT Rb. As Rb is often inactivated in cancers by overexpression of cyclin D1, loss of p16(ink4a) cdk inhibitor, or expression of viral oncoproteins, it will be interesting to determine if such functional inactivation of Rb would similarly sensitize cancer cells to TSC2 inactivation induced cell death. In addition, many cancers lack functional Pten, resulting in increased PI3K/Akt signaling that has been shown to modulate E2F-induced cell death. Therefore it will be interesting to test whether loss of Pten will affect TSC2 inactivation induced killing of Rb mutant cancer cells. Here, we show that overexpression of Cyclin D1 or the viral oncogene E1a sensitizes cancer cells to TSC2 knockdown induced cell death and growth inhibition. On the other hand, knockdown of p16(ink4a) sensitizes cancer cells to TSC2 knockdown induced cell death in a manner that is likely dependant on serum induction of Cyclin D1 to inactivate the Rb function. Additionally, we demonstrate that loss of Pten does not interfere with TSC2 knockdown induced cell death in Rb mutant cancer cells. Together, these results suggest that TSC2 is potentially a useful target for a large spectrum of cancer types with an inactivated Rb pathway.