The Utilization of Extracellular Proteins as Nutrients Is Suppressed by mTORC1

Macropinocytosis and autophagy crosstalk in nutrient scavenging

Adaptive strategies used by cells to scavenge and recycle essential nutrients are important for survival in nutrient-depleted environments such as cancer tissues. Autophagy and macropinocytosis are two major mechanisms that promote nutrient recycling and scavenging, which share considerable, yet poorly understood, cross-regulation. Here we review recent findings that connect these starvation response mechanisms and discuss the implications of their crosstalk. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

High-Throughput Screening Identified Compounds Sensitizing Tumor Cells to Glucose Starvation in Culture and VEGF Inhibitors In Vivo

Tumor cells utilize glucose to fuel their anabolic needs, including rapid proliferation. However, due to defective vasculature and increased glucose uptake, tumor cells must overcome glucose deprivation. Accordingly, tumor cells depend on cellular pathways promoting survival under such conditions. Targeting these survival mechanisms can thus serve as a new therapeutic strategy in oncology. As such, we sought to identify small-molecule inhibitors which sensitize tumor cells to glucose starvation by high-throughput drug screening in vitro. Specifically, we searched for inhibitors that selectively killed tumor cells growing in glucose-free but not in normal medium. This phenotypic drug screen of 7000 agents with MCF7 cells led to the identification of 67 potential candidates, 31 of which were validated individually. Among the identified compounds, we found a high number of compounds known to target mitochondria. The efficacies of two of the identified compounds, QNZ (EVP4593) and papaverine, were validated in four different tumor cell lines. We found that these agents inhibited the mTOR(Mechamistic\Mammilian Target of Rapamycin) pathway in tumor cells growing under glucose starvation, but not under normal conditions. The results were validated and confirmed in vivo, with QNZ and papaverine exhibiting superior antitumor activity in a tumor xenograft model when combined with the VEGF inhibitor bevacizumab (avastin). Administering these drug combinations (i.e., avastin and papaverine, and avastin and QNZ) led to significant reductions in proliferation and mTOR activity of the aggressive DLD1 colon cell line in mice. Given our findings, we propose that compounds targeting metabolically challenged tumors, such as inhibitors of mitochondrial activity, be considered as a therapeutic strategy in cancer.

mTOR as a central hub of nutrient signalling and cell growth

The highly conserved protein kinase mechanistic target of rapamycin (mTOR; originally known as mammalian target of rapamycin) is a central cell growth regulator connecting cellular metabolism and growth with a wide range of environmental inputs as part of mTOR complex 1 (mTORC1) and mTORC2. In this Review, we introduce the landmark discoveries in the mTOR field, starting from the isolation of rapamycin to the molecular characterizations of key components of the mTORC signalling network with an emphasis on amino acid sensing, and discuss the perspectives of mTORC inhibitors in therapeutic applications.

Lysosomal protease deficiency or substrate overload induces an oxidative-stress mediated STAT3-dependent pathway of lysosomal homeostasis

Diverse cellular processes depend on the lysosomal protease system but how cells regulate lysosomal proteolytic capacity is only partly understood. We show here that cells can respond to protease/substrate imbalance in this compartment by de novo expression of multiple lysosomal hydrolases. This response, exemplified here either by loss of asparagine endopeptidase (AEP) or other lysosomal cysteine proteases, or by increased endocytic substrate load, is not dependent on the transcription factor EB (TFEB) but rather is triggered by STAT3 activation downstream of lysosomal oxidative stress. Similar lysosomal adaptations are seen in mice and cells expressing a constitutively active form of STAT3. Our results reveal how cells can increase lysosomal protease capacity under ‘fed’ rather than ‘starved’ conditions that activate the TFEB system. In addition, STAT3 activation due to lysosomal stress likely explains the hyperproliferative kidney disease and splenomegaly observed in AEP-deficient mice.

How cells regulate their lysosomal proteolytic capacity is only partly understood. Here, the authors show that lysosomal protease deficiency or substrate overload induces lysosomal stress leading to activation of a STAT3-dependent, TFEB-independent pathway of lysosomal hydrolase expression.

Higher plane of nutrition pre-weaning enhances Holstein calf mammary gland development through alterations in the parenchyma and fat pad transcriptome

Background

To reduce costs of rearing replacement heifers, researchers have focused on decreasing age at breeding and first calving. To increase returns upon initiation of lactation the focus has been on increasing mammary development prior to onset of first lactation. Enhanced plane of nutrition pre-weaning may benefit the entire replacement heifer operation by promoting mammary gland development and greater future production.

Methods

Twelve Holstein heifer calves (< 1 week old) were reared on 1 of 2 dietary treatments (n = 6/group) for 8 weeks: a control group fed a restricted milk replacer at 0.45 kg/d (R, 20% crude protein, 20% fat), or an accelerated group fed an enhanced milk replacer at 1.13 kg/d (EH, 28% crude protein, 25% fat). At weaning (8 weeks), calves were euthanized and sub-samples of mammary parenchyma (PAR) and mammary fat pad (MFP) were harvested upon removal from the body. Total RNA from both tissues was extracted and sequenced using the Illumina HiSeq2500 platform. The Dynamic Impact Approach (DIA) and Ingenuity Pathway Analysis (IPA) were used for pathway analysis and functions, gene networks, and cross-talk analyses of the two tissues.

Results

When comparing EH vs R 1561 genes (895 upregulated, 666 downregulated) and 970 genes (506 upregulated, 464 downregulated) were differentially expressed in PAR and MFP, respectively. DIA and IPA results highlight a greater proliferation and differentiation activity in both PAR and MFP, supported by an increased metabolic activity. When calves were fed EH, the PAR displayed transcriptional signs of greater overall organ development, with higher ductal growth and branching, together with a supportive blood vessel and nerve network. These activities were mediated by intracellular cascades, such as AKT, SHH, MAPK, and Wnt, probably activated by hormones, growth factors, and endogenous molecules. The analysis also revealed strong communication between MFP and PAR.

Conclusion

The transcriptomics and bioinformatics approach highlighted key mechanisms that mediate the mammary gland response to a higher plane of nutrition in the pre-weaning period.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5303-8) contains supplementary material, which is available to authorized users.

Amino acid starvation accelerates replication of Ibaraki virus

Ibaraki virus (IBAV) is a strain of epizootic hemorrhagic disease virus 2 that belongs to the genus Orbivirus of the family Reoviridae. IBAV replication is suppressed by the inhibition of autophagy, and since mechanistic target of rapamycin complex 1 (mTORC1) is a key regulator of autophagy, we examined if mTORC1 inhibition by amino acid starvation or mTOR inhibitors (Torin 1 and rapamycin) affects IBAV replication. We found that IBAV replication is significantly enhanced after amino acid starvation of host cells, but not after treatment with mTOR inhibitors, during early stages of viral infection (0-1 hpi). Notably, inhibition of mTORC1 by amino acid starvation was reversible and thus restricted to 0-1 hpi, whereas mTOR inhibitors sustainably suppressed mTORC1 even after the 1-h treatment, suggesting that mTORC1 suppression itself does not affect IBAV replication. To investigate the mechanism of enhanced IBAV replication by amino acid starvation, we examined the endocytic pathway, since IBAV utilizes acidification of endosomes as a trigger for viral replication. Accordingly, we found that amino acid starvation, but not mTOR inhibitors, strongly induced acidification of endosomes/lysosomes and that inhibition of endosomal acidification by bafilomycin A1 effectively blocked enhancement of IBAV replication. Altogether, the inactivation of mTORC1 by amino acid starvation during early stages of infection enhances acidification of endosomes, which in turn enhances IBAV replication.

The role of metabolic adaptation to nutrient stress in pancreatic cancer

Pancreatic cancer is the fourth most common cause of cancer-related mortality, with a dismal prognosis that has changed little over the past few decades. Despite extensive efforts in understanding the oncogenetics of this pathology, pancreatic cancer remained largely elusive. One of the main characteristics of pancreatic cancer is the reduced level of oxygen and nutrient perfusion, caused by the new matrix formation, through the activation of stromal cells (desmoplasia). This stromal reaction leads to metabolic adaptations in surviving tumor cells in order to cope with these challenging conditions. The oncogenic signaling driven by KRAS mutation is necessary to fuel pancreatic tumors by activating key metabolic processes, including enhanced glycolysis and glutamine consumption. Here we review our current understanding of the pancreatic cancer metabolism as well as discuss recent work pointing to the importance of various metabolic strategies as well as autophagy and macropinocytosis as critical nutrient supply pathways. The elucidation of these metabolic networks may highlight new opportunities to further develop novel therapeutic strategies.

Transcription Factor Eb Is Required for Macropinocytosis-Mediated Growth Recovery of Nutrient-Deprived Kras-Mutant Cells

Macropinocytosis is a regulated form of endocytosis that mediates the nonselective uptake of nutrients to support growth under nutrient-deprived conditions. KRAS-mutant cancer cells upregulate macropinocytosis to import extracellular proteins, which subsequently undergo proteolytic degradation in the lysosome. Although transcription factor EB (TFEB) is a master regulator of lysosomal biogenesis and function, its role in the degradation of extracellular protein from macropinocytosis in KRAS-mutant cells has not previously been elucidated. In this study, we investigated the role of TFEB in the recovery of macropinocytosis-mediated mTORC1 activity and cell growth under nutrient depletion. Mouse embryonic fibroblasts (MEFs) expressing Kras^G12D and KRAS-mutant human cancer cells took up markedly higher levels of tetramethylrhodamine (TMR)-dextran than the corresponding wild-type cells. siRNA-mediated inhibition of TFEB did not influence extracellular TMR-dextran uptake, but significantly attenuated lysosomal degradation of extracellular protein. Bovine serum albumin (BSA) treatment restored p-S6K levels and cell proliferation suppressed by leucine deprivation, and these effects were blocked by siTFEB. Collectively, our results show that TFEB plays a role in macropinocytosis-mediated KRAS-mutant cell growth under nutrient deprivation by promoting lysosomal degradation of extracellular proteins.

Arginine starvation kills tumor cells through aspartate exhaustion and mitochondrial dysfunction

Defective arginine synthesis, due to the silencing of argininosuccinate synthase 1 (ASS1), is a common metabolic vulnerability in cancer, known as arginine auxotrophy. Understanding how arginine depletion kills arginine-auxotrophic cancer cells will facilitate the development of anti-cancer therapeutic strategies. Here we show that depletion of extracellular arginine in arginine-auxotrophic cancer cells causes mitochondrial distress and transcriptional reprogramming. Mechanistically, arginine starvation induces asparagine synthetase (ASNS), depleting these cancer cells of aspartate, and disrupting their malate-aspartate shuttle. Supplementation of aspartate, depletion of mitochondria, and knockdown of ASNS all protect the arginine-starved cells, establishing the causal effects of aspartate depletion and mitochondrial dysfunction on the arginine starvation-induced cell death. Furthermore, dietary arginine restriction reduced tumor growth in a xenograft model of ASS1-deficient breast cancer. Our data challenge the view that ASNS promotes homeostasis, arguing instead that ASNS-induced aspartate depletion promotes cytotoxicity, which can be exploited for anti-cancer therapies.

Induction of non-apoptotic programmed cell death by oncogenic RAS in human epithelial cells and its suppression by MYC overexpression

Oncogenic mutations of RAS genes, found in about 30% of human cancers, are considered to play important roles in cancer development. However, oncogenic RAS can also induce senescence in mouse and human normal fibroblasts. In some cell lines, oncogenic RAS has been reported to induce non-apoptotic programed cell death (PCD). Here, we investigated effects of oncogenic RAS expression in several types of normal human epithelial cells. Oncogenic RAS but not wild-type RAS stimulated macropinocytosis with accumulation of large-phase lucent vacuoles in the cytoplasm, subsequently leading to cell death which was indistinguishable from a recently proposed new type of PCD, methuosis. A RAC1 inhibitor suppressed accumulation of macropinosomes and overexpression of MYC attenuated oncogenic RAS-induced such accumulation, cell cycle arrest and cell death. MYC suppression or rapamycin treatment in some cancer cell lines harbouring oncogenic mutations in RAS genes induced cell death with accumulation of macropinosomes. These results suggest that this type of non-apoptotic PCD is a tumour-suppressing mechanism acting against oncogenic RAS mutations in normal human epithelial cells, which can be overcome by MYC overexpression, raising the possibility that its induction might be a novel approach to treatment of RAS-mutated human cancers.

Hypoalbuminemia: Pathogenesis and Clinical Significance

Hypoalbuminemia is associated with inflammation. Despite being addressed repeatedly in the literature, there is still confusion regarding its pathogenesis and clinical significance. Inflammation increases capillary permeability and escape of serum albumin, leading to expansion of interstitial space and increasing the distribution volume of albumin. The half‐life of albumin has been shown to shorten, decreasing total albumin mass. These 2 factors lead to hypoalbuminemia despite increased fractional synthesis rates in plasma. Hypoalbuminemia, therefore, results from and reflects the inflammatory state, which interferes with adequate responses to events like surgery or chemotherapy, and is associated with poor quality of life and reduced longevity. Increasing or decreasing serum albumin levels are adequate indicators, respectively, of improvement or deterioration of the clinical state. In the interstitium, albumin acts as the main extracellular scavenger, antioxidative agent, and as supplier of amino acids for cell and matrix synthesis. Albumin infusion has not been shown to diminish fluid requirements, infection rates, and mortality in the intensive care unit, which may imply that there is no body deficit or that the quality of albumin “from the shelf” is unsuitable to play scavenging and antioxidative roles. Management of hypoalbuminaemia should be based on correcting the causes of ongoing inflammation rather than infusion of albumin. After the age of 30 years, muscle mass and function slowly decrease, but this loss is accelerated by comorbidity and associated with decreasing serum albumin levels. Nutrition support cannot fully prevent, but slows down, this chain of events, especially when combined with physical exercise.

Nutrient scavenging in cancer

While cancer cell proliferation depends on access to extracellular nutrients, inadequate tumour perfusion means that glucose, amino acids and lipids are often in short supply. To overcome this obstacle to growth, cancer cells utilize multiple scavenging strategies, obtaining macromolecules from the microenvironment and breaking them down in the lysosome to produce substrates for ATP generation and anabolism. Recent studies have revealed four scavenging pathways that support cancer cell proliferation in low-nutrient environments: scavenging of extracellular matrix proteins via integrins, receptor-mediated albumin uptake and catabolism, macropinocytic consumption of multiple components of the tumour microenvironment and the engulfment and degradation of entire live cells via entosis. New evidence suggests that blocking these pathways alone or in combination could provide substantial benefits to patients with incurable solid tumours. Both US Food and Drug Administration (FDA)-approved drugs and several agents in preclinical or clinical development shut down individual or multiple scavenging pathways. These therapies may increase the extent and durability of tumour growth inhibition and/or prevent the development of resistance when used in combination with existing treatments. This Review summarizes the evidence suggesting that scavenging pathways drive tumour growth, highlights recent advances that define the oncogenic signal transduction pathways that regulate scavenging and considers the benefits and detriments of therapeutic strategies targeting scavenging that are currently under development.

PKCδ stimulates macropinocytosis via activation of SSH1-cofilin pathway

Macropinocytosis is an actin-dependent endocytic mechanism mediating internalization of extracellular fluid and associated solutes into cells. The present study was designed to identify the specific protein kinase C (PKC) isoform(s) and downstream effectors regulating actin dynamics during macropinocytosis. We utilized various cellular and molecular biology techniques, pharmacological inhibitors and genetically modified mice to study the signaling mechanisms mediating macropinocytosis in macrophages. The qRT-PCR experiments identified PKCδ as the predominant PKC isoform in macrophages. Scanning electron microscopy and flow cytometry analysis of FITC-dextran internalization demonstrated the functional role of PKCδ in phorbol ester- and hepatocyte growth factor (HGF)-induced macropinocytosis. Western blot analysis demonstrated that phorbol ester and HGF stimulate activation of slingshot phosphatase homolog 1 (SSH1) and induce cofilin Ser-3 dephosphorylation via PKCδ in macrophages. Silencing of SSH1 inhibited cofilin dephosphorylation and macropinocytosis stimulation. Interestingly, we also found that incubation of macrophages with BMS-5, a potent inhibitor of LIM kinase, does not stimulate macropinocytosis. In conclusion, the findings of the present study demonstrate a previously unidentified mechanism by which PKCδ via activation of SSH1 and cofilin dephosphorylation stimulates membrane ruffle formation and macropinocytosis. The results of the present study may contribute to a better understanding of the regulatory mechanisms during macrophage macropinocytosis.

Vps34-mediated macropinocytosis in Tuberous Sclerosis Complex 2-deficient cells supports tumorigenesis

Tuberous Sclerosis Complex (TSC), a rare genetic disorder with mechanistic target of rapamycin complex 1 (mTORC1) hyperactivation, is characterized by multi-organ hamartomatous benign tumors including brain, skin, kidney, and lung (Lymphangioleiomyomatosis). mTORC1 hyperactivation drives metabolic reprogramming including glucose and glutamine utilization, protein, nucleic acid and lipid synthesis. To investigate the mechanisms of exogenous nutrients uptake in Tsc2-deficient cells, we measured dextran uptake, a polysaccharide internalized via macropinocytosis. Tsc2-deficient cells showed a striking increase in dextran uptake (3-fold, p < 0.0001) relative to Tsc2-expressing cells, which was decreased (3-fold, p < 0.0001) with mTOR inhibitor, Torin1. Pharmacologic and genetic inhibition of the lipid kinase Vps34 markedly abrogated uptake of Dextran in Tsc2-deficient cells. Macropinocytosis was further increased in Tsc2-deficient cells that lack autophagic mechanisms, suggesting that autophagy inhibition leads to dependence on exogenous nutrient uptake in Tsc2-deficient cells. Treatment with a macropinocytosis inhibitor, ethylisopropylamiloride (EIPA), resulted in selective growth inhibition of Atg5-deficient, Tsc2-deficient cells (50%, p < 0.0001). Genetic inhibition of autophagy (Atg5^−/− MEFs) sensitized cells with Tsc2 downregulation to the Vps34 inhibitor, SAR405, resulting in growth inhibition (75%, p < 0.0001). Finally, genetic downregulation of Vps34 inhibited tumor growth and increased tumor latency in an in vivo xenograft model of TSC. Our findings show that macropinocytosis is upregulated with Tsc2-deficiency via a Vps34-dependent mechanism to support their anabolic state. The dependence of Tsc2-deficient cells on exogenous nutrients may provide novel approaches for the treatment of TSC.

Targeting the Warburg effect via LDHA inhibition engages ATF4 signaling for cancer cell survival

Nutrient restriction reprograms cellular signaling and metabolic network to shape cancer phenotype. Lactate dehydrogenase A (LDHA) has a key role in aerobic glycolysis (the Warburg effect) through regeneration of the electron acceptor NAD⁺ and is widely regarded as a desirable target for cancer therapeutics. However, the mechanisms of cellular response and adaptation to LDHA inhibition remain largely unknown. Here, we show that LDHA activity supports serine and aspartate biosynthesis. Surprisingly, however, LDHA inhibition fails to impact human melanoma cell proliferation, survival, or tumor growth. Reduced intracellular serine and aspartate following LDHA inhibition engage GCN2-ATF4 signaling to initiate an expansive pro-survival response. This includes the upregulation of glutamine transporter SLC1A5 and glutamine uptake, with concomitant build-up of essential amino acids, and mTORC1 activation, to ameliorate the effects of LDHA inhibition. Tumors with low LDHA expression and melanoma patients acquiring resistance to MAPK signaling inhibitors, which target the Warburg effect, exhibit altered metabolic gene expression reminiscent of the ATF4-mediated survival signaling. ATF4-controlled survival mechanisms conferring synthetic vulnerability to the approaches targeting the Warburg effect offer efficacious therapeutic strategies.

A NUMB-EFA6B-ARF6 recycling route controls apically restricted cell protrusions and mesenchymal motility

The endocytic protein NUMB has been implicated in the control of various polarized cellular processes, including the acquisition of mesenchymal migratory traits through molecular mechanisms that have only been partially defined. Here, we report that NUMB is a negative regulator of a specialized set of understudied, apically restricted, actin-based protrusions, the circular dorsal ruffles (CDRs), induced by either PDGF or HGF stimulation. Through its PTB domain, NUMB binds directly to an N-terminal NPLF motif of the ARF6 guanine nucleotide exchange factor, EFA6B, and promotes its exchange activity in vitro. In cells, a NUMB-EFA6B-ARF6 axis regulates the recycling of the actin regulatory cargo RAC1 and is critical for the formation of CDRs that mark the acquisition of a mesenchymal mode of motility. Consistently, loss of NUMB promotes HGF-induced cell migration and invasion. Thus, NUMB negatively controls membrane protrusions and the acquisition of mesenchymal migratory traits by modulating EFA6B-ARF6 activity.

Identification of novel macropinocytosis inhibitors using a rational screen of Food and Drug Administration-approved drugs

BACKGROUND AND PURPOSE

Macropinocytosis is involved in many pathologies, including cardiovascular disorders, cancer, allergic diseases, viral and bacterial infections. Unfortunately, the currently available pharmacological inhibitors of macropinocytosis interrupt other endocytic processes and have non-specific endocytosis-independent effects. Here we have sought to identify new, clinically relevant inhibitors of macropinocytosis, using an FDA-approved drug library.

EXPERIMENTAL APPROACH

In the present study, 640 FDA-approved compounds were tested for their ability to inhibit macropinocytosis. A series of secondary assays were performed to confirm inhibitory activity, determine IC50 values and investigate cell toxicity. The ability of identified hits to inhibit phagocytosis and clathrin-mediated and caveolin-mediated endocytosis was also investigated. Scanning electron microscopy and molecular biology techniques were utilized to examine the mechanisms by which selected compounds inhibit macropinocytosis.

KEY RESULTS

The primary screen identified 14 compounds that at ~10 μM concentration inhibit >95% of macropinocytotic solute internalization. Three compounds - imipramine, phenoxybenzamine and vinblastine - potently inhibited (IC50  ≤ 131 nM) macropinocytosis without exerting cytotoxic effects or inhibiting other endocytic pathways. Scanning electron microscopy imaging indicated that imipramine inhibits membrane ruffle formation, a critical early step leading to initiation of macropinocytosis. Finally, imipramine has been shown to inhibit macropinocytosis in several cell types, including cancer cells, dendritic cells and macrophages.

CONCLUSIONS AND IMPLICATIONS

Our results identify imipramine as a new pharmacological tool to study macropinocytosis in cellular and biological systems. This study also suggests that imipramine could be a good candidate for repurposing as a therapeutic agent in pathological processes involving macropinocytosis.

The Canonical Wnt Pathway Drives Macropinocytosis in Cancer

Macropinocytosis has emerged as an important pathway of protein acquisition in cancer cells, particularly in tumors with activated Ras such as pancreatic and colon cancer. Macropinocytosis is also the route of entry of Bacillus Calmette-Guerin (BCG) and other microbial therapies of cancer. Despite this important role in tumor biology and therapy, the full mechanisms by which cancer cells can activate macropinocytosis remain incompletely defined. Using BCG uptake to assay macropinocytosis, we executed a genome-wide shRNA screen for macropinocytosis activators and identified Wnt pathway activation as a strong driver of macropinocytosis. Wnt-driven macropinocytosis was downstream of the β-catenin-dependent canonical Wnt pathway, was PAK1 dependent, and supported albumin-dependent growth in Ras-WT cells. In cells with activated Ras-dependent macropinocytosis, pharmacologic or genetic inhibition of Wnt signaling suppressed macropinocytosis. In a mouse model of Wnt-driven colonic hyperplasia via APC silencing, Wnt-activated macropinocytosis stimulated uptake of luminal microbiota, a process reversed by topical pharmacologic inhibition of macropinocytosis. Our findings indicate that Wnt pathway activation drives macropinocytosis in cancer, and its inhibition could provide a therapeutic vulnerability in Wnt-driven intestinal polyposis and cancers with Wnt activation.Significance: The Wnt pathway drives macropinocytosis in cancer cells, thereby contributing to cancer growth in nutrient-deficient conditions and, in the context of colon cancer, to the early phases of oncogenesis. Cancer Res; 78(16); 4658-70. ©2018 AACR.

Microenvironmental regulation of cancer cell metabolism: implications for experimental design and translational studies

Cancers have an altered metabolism, and there is interest in understanding precisely how oncogenic transformation alters cellular metabolism and how these metabolic alterations can translate into therapeutic opportunities. Researchers are developing increasingly powerful experimental techniques to study cellular metabolism, and these techniques have allowed for the analysis of cancer cell metabolism, both in tumors and in ex vivo cancer models. These analyses show that, while factors intrinsic to cancer cells such as oncogenic mutations, alter cellular metabolism, cell-extrinsic microenvironmental factors also substantially contribute to the metabolic phenotype of cancer cells. These findings highlight that microenvironmental factors within the tumor, such as nutrient availability, physical properties of the extracellular matrix, and interactions with stromal cells, can influence the metabolic phenotype of cancer cells and might ultimately dictate the response to metabolically targeted therapies. In an effort to better understand and target cancer metabolism, this Review focuses on the experimental evidence that microenvironmental factors regulate tumor metabolism, and on the implications of these findings for choosing appropriate model systems and experimental approaches.

Imaging Cancer Metabolism: Underlying Biology and Emerging Strategies

Dysregulated cellular metabolism is a characteristic feature of malignancy that has been exploited for both imaging and targeted therapy. With regard to imaging, deranged glucose metabolism has been leveraged using ¹⁸F-FDG PET. Metabolic imaging with ¹⁸F-FDG, however, probes only the early steps of glycolysis; the complexities of metabolism beyond these early steps in this single pathway are not directly captured. New imaging technologies-both PET with novel radiotracers and MR-based methods-provide unique opportunities to investigate other aspects of cellular metabolism and expand the metabolic imaging armamentarium. This review will discuss the underlying biology of metabolic dysregulation in cancer, focusing on glucose, glutamine, and acetate metabolism. Novel imaging strategies will be discussed within this biologic framework, highlighting particular strengths and limitations of each technique. Emphasis is placed on the role that combining modalities will play in enabling multiparametric imaging to fully characterize tumor biology to better inform treatment.

When fats commit crimes: fatty acid metabolism, cancer stemness and therapeutic resistance

The role of fatty acid metabolism, including both anabolic and catabolic reactions in cancer has gained increasing attention in recent years. Many studies have shown that aberrant expression of the genes involved in fatty acid synthesis or fatty acid oxidation correlate with malignant phenotypes including metastasis, therapeutic resistance and relapse. Such phenotypes are also strongly associated with the presence of a small percentage of unique cells among the total tumor cell population. This distinct group of cells may have the ability to self-renew and propagate or may be able to develop resistance to cancer therapies independent of genetic alterations. Therefore, these cells are referred to as cancer stem cells/tumor-initiating cells/drug-tolerant persisters, which are often refractory to cancer treatment and difficult to target. Moreover, interconversion between cancer cells and cancer stem cells/tumor-initiating cells/drug-tolerant persisters may occur and makes treatment even more challenging. This review highlights recent findings on the relationship between fatty acid metabolism, cancer stemness and therapeutic resistance and prompts discussion about the potential mechanisms by which fatty acid metabolism regulates the fate of cancer cells and therapeutic resistance.

PTEN Deficiency and AMPK Activation Promote Nutrient Scavenging and Anabolism in Prostate Cancer Cells

We report that PTEN-deficient prostate cancer cells use macropinocytosis to survive and proliferate under nutrient stress. PTEN loss increased macropinocytosis only in the context of AMPK activation, revealing a general requirement for AMPK in macropinocytosis and a novel mechanism by which AMPK promotes survival under stress. In prostate cancer cells, albumin uptake did not require macropinocytosis, but necrotic cell debris proved a specific macropinocytic cargo. Isotopic labeling confirmed that macropinocytosed necrotic cell proteins fueled new protein synthesis in prostate cancer cells. Supplementation with necrotic debris, but not albumin, also maintained lipid stores, suggesting that macropinocytosis can supply nutrients other than amino acids. Nontransformed prostatic epithelial cells were not macropinocytic, but patient-derived prostate cancer organoids and xenografts and autochthonous prostate tumors all exhibited constitutive macropinocytosis, and blocking macropinocytosis limited prostate tumor growth. Macropinocytosis of extracellular material by prostate cancer cells is a previously unappreciated tumor-microenvironment interaction that could be targeted therapeutically.Significance: As PTEN-deficient prostate cancer cells proliferate in low-nutrient environments by scavenging necrotic debris and extracellular protein via macropinocytosis, blocking macropinocytosis by inhibiting AMPK, RAC1, or PI3K may have therapeutic value, particularly in necrotic tumors and in combination with therapies that cause nutrient stress. Cancer Discov; 8(7); 866-83. ©2018 AACR.See related commentary by Commisso and Debnath, p. 800This article is highlighted in the In This Issue feature, p. 781.

Metabolic reprogramming for cancer cells and their microenvironment: Beyond the Warburg Effect

While metabolic reprogramming of cancer cells has long been considered from the standpoint of how and why cancer cells preferentially utilize glucose via aerobic glycolysis, the so-called Warburg Effect, the progress in the following areas during the past several years has substantially advanced our understanding of the rewired metabolic network in cancer cells that is intertwined with oncogenic signaling. First, in addition to the major nutrient substrates glucose and glutamine, cancer cells have been discovered to utilize a variety of unconventional nutrient sources for survival. Second, the deregulated biomass synthesis is intertwined with cell cycle progression to coordinate the accelerated progression of cancer cells. Third, the reciprocal regulation of cancer cell's metabolic alterations and the microenvironment, involving extensive host immune cells and microbiota, have come into view as critical mechanisms to regulate cancer progression. These and other advances are shaping the current and future paradigm of cancer metabolism.

mTORC1 pathway in DNA damage response

Living organisms have evolved various mechanisms to control their metabolism and response to various stresses, allowing them to survive and grow in different environments. In eukaryotes, the highly conserved mechanistic target of rapamycin (mTOR) signaling pathway integrates both intracellular and extracellular signals and serves as a central regulator of cellular metabolism, proliferation and survival. A growing body of evidence indicates that mTOR signaling is closely related to another cellular protection mechanism, the DNA damage response (DDR). Many factors important for the DDR are also involved in the mTOR pathway. In this review, we discuss how these two pathways communicate to ensure an efficient protection of the cell against metabolic and genotoxic stresses. We also describe how anticancer therapies benefit from simultaneous targeting of the DDR and mTOR pathways.

High glucose reduces megalin-mediated albumin endocytosis in renal proximal tubule cells through protein kinase B O-GlcNAcylation

The role of albumin reabsorption in proximal tubule (PT) cells has emerged as an important factor in the genesis of albuminuria observed in the early stages of diabetes. Evidence has shown that a decrease in megalin expression could be the key mechanism in this process. In the present work, we investigated the molecular mechanism underlying the modulation of albumin endocytosis in LLC-PK1 cells, a model of PT cells. High glucose concentrations (HG) inhibited megalin expression and albumin endocytosis after 48 h of incubation. This inhibitory effect involves the entrance of glucose into PT cells through SGLT located at the luminal membrane. Once inside PT cells, glucose is diverted to the hexosamine biosynthetic pathway (HBP) increasing O-GlcNAcylation of several intracellular proteins, including PKB. This process promotes the inhibition of PKB activity measured by its phosphorylation at Thr-308 and Ser-473 and phosphorylation of specific substrates, glycogen synthase kinase 3β (GSK3β) and tuberous sclerosis complex 2. The decrease in PKB activity led to a decrease in megalin expression and, consequently, reducing albumin endocytosis in LLC-PK1 cells. HG did not change mammalian target of rapamycin (mTOR) C2 activity, responsible for phosphorylated PKB at Ser-473. In addition, HG activated the mTORC1/S6K pathway, but this effect was not correlated to the decrease in megalin expression or albumin endocytosis. Taken together, our data help to clarify the current understanding underlying the genesis of tubular albuminuria induced by hyperglycemia in the early stage of diabetes pathogenesis.

mTOR signalling: jack-of-all-trades 1

The mechanistic target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase that senses and integrates environmental information into cellular regulation and homeostasis. Accumulating evidence has suggested a master role of mTOR signalling in many fundamental aspects of cell biology and organismal development. mTOR deregulation is implicated in a broad range of pathological conditions, including diabetes, cancer, neurodegenerative diseases, myopathies, inflammatory, infectious, and autoimmune conditions. Here, we review recent advances in our knowledge of mTOR signalling in mammalian physiology. We also discuss the impact of mTOR alteration in human diseases and how targeting mTOR function can treat human diseases.

A RAB35-p85/PI3K axis controls oscillatory apical protrusions required for efficient chemotactic migration

How cells move chemotactically remains a major unmet challenge in cell biology. Emerging evidence indicates that for interpreting noisy, shallow gradients of soluble cues a system must behave as an excitable process. Here, through an RNAi-based, high-content screening approach, we identify RAB35 as necessary for the formation of growth factors (GFs)-induced waves of circular dorsal ruffles (CDRs), apically restricted actin-rich migratory protrusions. RAB35 is sufficient to induce recurrent and polarized CDRs that travel as propagating waves, thus behaving as an excitable system that can be biased to control cell steering. Consistently, RAB35 is essential for promoting directed chemotactic migration and chemoinvasion of various cells in response to gradients of motogenic GFs. Molecularly, RAB35 does so by directly regulating the activity of p85/PI3K polarity axis. We propose that RAB35 is a molecular determinant for the control of an excitable, oscillatory system that acts as a steering wheel for GF-mediated chemotaxis and chemoinvasion.

Circular dorsal ruffles (CDRs) are apical actin enriched structures involved in the interpretation of growth factor gradients during cell migration. Here, the authors find that a RAB35/PI3K axis is necessary and sufficient for the formation and stabilization of polarized CDRs and persistent directional migration.

Differential glucose requirement in skin homeostasis and injury identifies a therapeutic target for psoriasis

Proliferating cells depend on glucose uptake more than quiescent cells for their growth. While glucose transport in keratinocytes is mediated largely by the Glut1 facilitative transporter, the keratinocyte-specific ablation of Glut1 did not compromise mouse skin development and homeostasis. Ex vivo metabolic profiling revealed altered sphingolipid, hexose, amino acid, and nucleotide metabolism in Glut1-deficient keratinocytes, suggesting metabolic adaptation. On the other hand, Glut1-deficient keratinocytes in culture displayed metabolic and oxidative stress and impaired proliferation. Similarly, Glut1 deficiency impaired in vivo keratinocyte proliferation and migration within wounded or UV-damaged mouse skin. Notably, both genetic and pharmacological Glut1 inactivation reduced hyperplasia in mouse models of psoriasis-like disease. Topical application of a Glut1 inhibitor also reduced inflammation in these models. Glut1 inhibition decreased expression of pathology-associated genes in human psoriatic skin organoids. Thus, Glut1 is selectively required for injury- and inflammation-associated keratinocyte proliferation, and its inhibition offers a novel treatment strategy for psoriasis.

Lysosomal regulation of cholesterol homeostasis in tuberous sclerosis complex is mediated via NPC1 and LDL-R

Tuberous sclerosis complex (TSC) is a multisystem disease associated with hyperactive mTORC1. The impact of TSC1/2 deficiency on lysosome-mediated processes is not fully understood. We report here that inhibition of lysosomal function using chloroquine (CQ) upregulates cholesterol homeostasis genes in TSC2-deficient cells. This TSC2-dependent transcriptional signature is associated with increased accumulation and intracellular levels of both total cholesterol and cholesterol esters. Unexpectedly, engaging this CQ-induced cholesterol uptake pathway together with inhibition of de novo cholesterol synthesis allows survival of TSC2-deficient, but not TSC2-expressing cells. The underlying mechanism of TSC2-deficient cell survival is dependent on exogenous cholesterol uptake via LDL-R, and endosomal trafficking mediated by Vps34. Simultaneous inhibition of lysosomal and endosomal trafficking inhibits uptake of esterified cholesterol and cell growth in TSC2-deficient, but not TSC2-expressing cells, highlighting the TSC-dependent lysosome-mediated regulation of cholesterol homeostasis and pointing toward the translational potential of these pathways for the therapy of TSC.

Eating the Beast: Dietary Protein and Anticancer Immunity

Studies on the impact of diet on cancer progression have predominantly focused on the metabolic demands of the tumor. In this issue of Cell Metabolism, Rubio-Patiño et al. (2018) examine how a small reduction in dietary protein effectively commandeers the adaptive immune response to attack the tumor, highlighting a role for endoplasmic reticulum stress.

Macropinocytosis, mTORC1 and cellular growth control

The growth and proliferation of metazoan cells are driven by cellular nutrient status and by extracellular growth factors. Growth factor receptors on cell surfaces initiate biochemical signals that increase anabolic metabolism and macropinocytosis, an actin-dependent endocytic process in which relatively large volumes of extracellular solutes and nutrients are internalized and delivered efficiently into lysosomes. Macropinocytosis is prominent in many kinds of cancer cells, and supports the growth of cells transformed by oncogenic K-Ras. Growth factor receptor signaling and the overall metabolic status of the cell are coordinated in the cytoplasm by the mechanistic target-of-rapamycin complex-1 (mTORC1), which positively regulates protein synthesis and negatively regulates molecular salvage pathways such as autophagy. mTORC1 is activated by two distinct Ras-related small GTPases, Rag and Rheb, which associate with lysosomal membranes inside the cell. Rag recruits mTORC1 to the lysosomal surface where Rheb directly binds to and activates mTORC1. Rag is activated by both lysosomal luminal and cytosolic amino acids; Rheb activation requires phosphoinositide 3-kinase, Akt, and the tuberous sclerosis complex-1/2. Signals for activation of Rag and Rheb converge at the lysosomal membrane, and several lines of evidence support the idea that growth factor-dependent endocytosis facilitates amino acid transfer into the lysosome leading to the activation of Rag. This review summarizes evidence that growth factor-stimulated macropinocytosis is essential for amino acid-dependent activation of mTORC1, and that increased solute accumulation by macropinocytosis in transformed cells supports unchecked cell growth.

Entosis Is Induced by Glucose Starvation

Entosis is a mechanism of cell death that involves neighbor cell ingestion. This process occurs in cancers and promotes a form of cell competition, where winner cells engulf and kill losers. Entosis is driven by a mechanical differential that allows softer cells to eliminate stiffer cells. While this process can be induced by matrix detachment, whether other stressors can activate entosis is unknown. Here, we find that entosis is induced in adherent cells by glucose withdrawal. Glucose withdrawal leads to a bimodal distribution of cells based on their deformability, where stiffer cells appear in a manner requiring the energy-sensing AMP-activated protein kinase (AMPK). We show that loser cells with high levels of AMPK activity are eliminated by winners through entosis, which supports winner cell proliferation under nutrient-deprived conditions. Our findings demonstrate that entosis serves as a cellular response to metabolic stress that enables nutrient recovery through neighbor cell ingestion.

Extracellular matrix internalization links nutrient signalling to invasive migration

Integrins are the key mediators of cell-extracellular matrix (ECM) interaction, linking the ECM to the actin cytoskeleton. Besides localizing at the cell surface, they can be internalized and transported back to the plasma membrane (recycled) or delivered to the late endosomes/lysosomes for degradation. We and others have shown that integrin can be endocytosed together with their ECM ligands. In this short review, I will highlight how extracellular protein (including ECM) endocytosis impinges on the activation of the mechanistic target of rapamycin (mTOR) pathway, a master regulator of cell metabolism and growth. This supports the intriguing hypothesis that ECM components may be considered as nutrient sources, primarily under soluble nutrient-depleted conditions.

The Role of the Mammalian Target of Rapamycin (mTOR) in Pulmonary Fibrosis

The phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR)-dependent pathway is one of the most integral pathways linked to cell metabolism, proliferation, differentiation, and survival. This pathway is dysregulated in a variety of diseases, including neoplasia, immune-mediated diseases, and fibroproliferative diseases such as pulmonary fibrosis. The mTOR kinase is frequently referred to as the master regulator of this pathway. Alterations in mTOR signaling are closely associated with dysregulation of autophagy, inflammation, and cell growth and survival, leading to the development of lung fibrosis. Inhibitors of mTOR have been widely studied in cancer therapy, as they may sensitize cancer cells to radiation therapy. Studies also suggest that mTOR inhibitors are promising modulators of fibroproliferative diseases such as idiopathic pulmonary fibrosis (IPF) and radiation-induced pulmonary fibrosis (RIPF). Therefore, mTOR represents an attractive and unique therapeutic target in pulmonary fibrosis. In this review, we discuss the pathological role of mTOR kinase in pulmonary fibrosis and examine how mTOR inhibitors may mitigate fibrotic progression.

Nutrient Sensing in Cancer

Cell-intrinsic mechanisms of nutrient sensing are intimately linked to adaptive metabolic responses, and these pathways play critical roles in the complex and dynamic nutrient environment of a growing tumor. Nutrient-responsive transcription factors (e.g., HIF, SREBP, ATF4) and signaling pathways (e.g., mTORC1, AMPK) allow tumor cells to tune their metabolic output and strategies to fluctuations in nutrient availability, thus balancing tumor cell proliferation and survival with a combination of anabolic and adaptive responses. Coupling these nutrient-sensing mechanisms to the control of recycling and scavenging processes, such as autophagy and macropinocytosis, further enhances the adaptability to nutrients within tumors. Here, we discuss the key nutrient-sensing pathways active in cancer cells, how oncogenic events influence these pathways, and their likely contributions to tumor growth and survival. A better understanding of nutrient-sensing strategies and metabolic adaptations within the tumor microenvironment is critical to defining and targeting metabolic vulnerabilities in cancer.

IRE1α RNase-dependent lipid homeostasis promotes survival in Myc-transformed cancers

Myc activation is a primary oncogenic event in many human cancers; however, these transcription factors are difficult to inhibit pharmacologically, suggesting that Myc-dependent downstream effectors may be more tractable therapeutic targets. Here, we show that Myc overexpression induces endoplasmic reticulum (ER) stress and engages the inositol-requiring enzyme 1α (IRE1α)/X-box binding protein 1 (XBP1) pathway through multiple molecular mechanisms in a variety of c-Myc- and N-Myc-dependent cancers. In particular, Myc-overexpressing cells require IRE1α/XBP1 signaling for sustained growth and survival in vitro and in vivo, dependent on elevated stearoyl-CoA-desaturase 1 (SCD1) activity. Pharmacological and genetic XBP1 inhibition induces Myc-dependent apoptosis, which is alleviated by exogenous unsaturated fatty acids. Of note, SCD1 inhibition phenocopies IRE1α RNase activity suppression in vivo. Furthermore, IRE1α inhibition enhances the cytotoxic effects of standard chemotherapy drugs used to treat c-Myc-overexpressing Burkitt's lymphoma, suggesting that inhibiting the IRE1α/XBP1 pathway is a useful general strategy for treatment of Myc-driven cancers.

Unique metabolic features of pancreatic cancer stroma: relevance to the tumor compartment, prognosis, and invasive potential

Pancreatic ductal adenocarcinoma (PDAC) has a dismal prognosis. The aggressiveness and therapeutic recalcitrance of this malignancy has been attributed to multiple factors including the influence of an active desmoplastic stroma. How the stromal microenvironment of PDAC contributes to the fatal nature of this disease is not well defined. In the analysis of clinical specimens, we observed diverse expression of the hypoxic marker carbonic anhydrase IX and the lactate transporter MCT4 in the stromal compartment. These stromal features were associated with the epithelial to mesenchymal phenotype in PDAC tumor cells, and with shorter patient survival. Cultured cancer-associated fibroblasts (CAFs) derived from primary PDAC exhibited a high basal level of hypoxia inducible factor 1a (HIF1α) that was both required and sufficient to modulate the expression of MCT4. This event was associated with increased transcription and protein synthesis of HIF1α in CAFs relative to PDAC cell lines, while surprisingly the protein turnover rate was equivalent. CAFs utilized glucose predominantly for glycolytic intermediates, whereas glutamine was the preferred metabolite for the TCA cycle. Unlike PDAC cell lines, CAFs were resistant to glucose withdrawal but sensitive to glutamine depletion. Consistent with the lack of reliance on glucose, CAFs could survive the acute depletion of MCT4. In co-culture and xenograft studies CAFs stimulated the invasive potential and metastatic spread of PDAC cell lines through a mechanism dependent on HIF1α and MCT4. Together, these data indicate that stromal metabolic features influence PDAC tumor cells to promote invasiveness and metastatic potential and associate with poor outcome in patients with PDAC.

Selective mTORC2 Inhibitor Therapeutically Blocks Breast Cancer Cell Growth and Survival

Small-molecule inhibitors of the mTORC2 kinase (torkinibs) have shown efficacy in early clinical trials. However, the torkinibs under study also inhibit the other mTOR-containing complex mTORC1. While mTORC1/mTORC2 combined inhibition may be beneficial in cancer cells, recent reports describe compensatory cell survival upon mTORC1 inhibition due to loss of negative feedback on PI3K, increased autophagy, and increased macropinocytosis. Genetic models suggest that selective mTORC2 inhibition would be effective in breast cancers, but the lack of selective small-molecule inhibitors of mTORC2 have precluded testing of this hypothesis to date. Here we report the engineering of a nanoparticle-based RNAi therapeutic that can effectively silence the mTORC2 obligate cofactor Rictor. Nanoparticle-based Rictor ablation in HER2-amplified breast tumors was achieved following intratumoral and intravenous delivery, decreasing Akt phosphorylation and increasing tumor cell killing. Selective mTORC2 inhibition in vivo, combined with the HER2 inhibitor lapatinib, decreased the growth of HER2-amplified breast cancers to a greater extent than either agent alone, suggesting that mTORC2 promotes lapatinib resistance, but is overcome by mTORC2 inhibition. Importantly, selective mTORC2 inhibition was effective in a triple-negative breast cancer (TNBC) model, decreasing Akt phosphorylation and tumor growth, consistent with our findings that RICTOR mRNA correlates with worse outcome in patients with basal-like TNBC. Together, our results offer preclinical validation of a novel RNAi delivery platform for therapeutic gene ablation in breast cancer, and they show that mTORC2-selective targeting is feasible and efficacious in this disease setting.Significance: This study describes a nanomedicine to effectively inhibit the growth regulatory kinase mTORC2 in a preclinical model of breast cancer, targeting an important pathogenic enzyme in that setting that has been undruggable to date. Cancer Res; 78(7); 1845-58. ©2018 AACR.

Dynamin inhibitors block activation of mTORC1 by amino acids independently of dynamin

mTORC1 plays a crucial role in protein synthesis and cell proliferation and growth. It is activated by growth factors and amino acids, including essential amino acids (EAAs), such as leucine; Leu enters cells via the Leu transporter LAT1-4F2hc (also known as SLC7A5-SLC3A2) and potentially via endocytosis. Here, we investigated the contribution of the different routes of Leu entry into cells to mTORC1 activation using pharmacological inhibitors and cells that lack LAT1 or dynamin-1, -2 and -3. Our results show that LAT1 is the major route of Leu entry into cells and mTORC1 activation (∼70%), whereas dynamin-dependent endocytosis and macropinocytosis contribute minimally to both (5-15%). However, macropinocytosis contributes significantly (∼40%) to activation of mTORC1 by other EAAs. Surprisingly, the dynamin inhibitors dynasore and Dyngo 4A, which minimally inhibited Leu uptake, abolished mTORC1 activation independently of dynamin. Instead, dynasore inhibited RagA binding to Raptor, reduced mTORC1 recruitment to the lysosome, and inhibited Akt activation and TSC2-S939 phosphorylation; this resulted in inhibition of Rheb and mTORC1 activity. Our results suggest that these commonly used inhibitors of dynamin and endocytosis are potent suppressors of mTORC1 activation via off-target effects and not via dynamin inhibition.This article has an associated First Person interview with the first author of the paper.

Lymphangioleiomyomatosis: A Monogenic Model of Malignancy

Lymphangioleiomyomatosis (LAM) is a rare, low-grade, metastasizing neoplasm that arises from an unknown source, spreads via the lymphatics, and targets the lungs. All pulmonary structures become infiltrated with benign-appearing spindle and epithelioid cells (LAM cells) that express smooth-muscle and melanocyte-lineage markers, harbor mTOR-activating mutations in tuberous sclerosis complex (TSC) genes, and recruit abundant stromal cells. Elaboration of lymphangiogenic growth factors and matrix remodeling enzymes by LAM cells enables their access to lymphatic channels and likely drives the cystic lung remodeling that often culminates in respiratory failure. Dysregulated cellular signaling results in a shift from oxidative phosphorylation to glycolysis as the preferred mode of energy generation, to allow for the accumulation of biomass required for cell growth and tolerance of nutrient-poor, anaerobic environments. Symptomatic LAM occurs almost exclusively in females after menarche, highlighting the central but as yet poorly understood role for sex-restricted anatomical structures and/or hormones in disease pathogenesis. LAM is an elegant model of malignancy because biallelic mutations at a single genetic locus confer all features that define cancer upon the LAM cell-metabolic reprogramming and proliferative signals that drive uncontrolled growth and inappropriate migration and invasion, the capacity to exploit the lymphatic circulation as a vehicle for metastasis and access to the lungs, and destruction of remote tissues. The direct benefit of the study of this rare disease has been the rapid identification of an effective FDA-approved therapy, and the collateral benefits have included elucidation of the pivotal roles of mTOR signaling in the regulation of cellular metabolism and the pathogenesis of cancer.

Targeting carbonic anhydrase IX improves the anti-cancer efficacy of mTOR inhibitors

The inhibition of the mechanistic target of rapamycin complex 1 (mTORC1) by chemical inhibitors, such as rapamycin, has demonstrated anti-cancer activity in preclinical and clinical trials. Their efficacy is, however, limited and tumors eventually relapse through resistance formation. In this study, using two different cancer mouse models, we identify tumor hypoxia as a novel mechanism of resistance of cancer cells against mTORC1 inhibitors. Indeed, we show that the activity of mTORC1 is mainly restricted to the non-hypoxic tumor compartment, as evidenced by a mutually exclusive staining pattern of the mTORC1 activity marker pS6 and the hypoxia marker pimonidazole. Consequently, whereas rapamycin reduces cancer cell proliferation in non-hypoxic regions, it has no effect in hypoxic areas, suggesting that cancer cells proliferate independently of mTORC1 under hypoxia. Targeting the hypoxic tumor compartment by knockdown of carbonic anhydrase IX (CAIX) using short hairpin RNA or by chemical inhibition of CAIX with acetazolamide potentiates the anti-cancer activity of rapamycin. Taken together, these data emphasize that hypoxia impairs the anti-cancer efficacy of rapalogs. Therapeutic strategies targeting the hypoxic tumor compartment, such as the inhibition of CAIX, potentiate the efficacy of rapamycin and warrant further clinical evaluation.

Eukaryotic Elongation Factor 2 Kinase (eEF2K) in Cancer

Eukaryotic elongation factor 2 kinase (eEF2K) is a highly unusual protein kinase that negatively regulates the elongation step of protein synthesis. This step uses the vast majority of the large amount of energy and amino acids required for protein synthesis. eEF2K activity is controlled by an array of regulatory inputs, including inhibition by signalling through mammalian target of rapamycin complex 1 (mTORC1). eEF2K is activated under conditions of stress, such as energy depletion or nutrient deprivation, which can arise in poorly-vascularised tumours. In many such stress conditions, eEF2K exerts cytoprotective effects. A growing body of data indicates eEF2K aids the growth of solid tumours in vivo. Since eEF2K is not essential (in mice) under ‘normal’ conditions, eEF2K may be a useful target in the treatment of solid tumours. However, some reports suggest that eEF2K may actually impair tumorigenesis in some situations. Such a dual role of eEF2K in cancer would be analogous to the situation for other pathways involved in cell metabolism, such as autophagy and mTORC1. Further studies are needed to define the role of eEF2K in different tumour types and at differing stages in tumorigenesis, and to assess its utility as a therapeutic target in oncology.

A Unified Approach to Targeting the Lysosome's Degradative and Growth Signaling Roles

Lysosomes serve dual roles in cancer metabolism, executing catabolic programs (i.e., autophagy and macropinocytosis) while promoting mTORC1-dependent anabolism. Antimalarial compounds such as chloroquine or quinacrine have been used as lysosomal inhibitors, but fail to inhibit mTOR signaling. Further, the molecular target of these agents has not been identified. We report a screen of novel dimeric antimalarials that identifies dimeric quinacrines (DQ) as potent anticancer compounds, which concurrently inhibit mTOR and autophagy. Central nitrogen methylation of the DQ linker enhances lysosomal localization and potency. An in situ photoaffinity pulldown identified palmitoyl-protein thioesterase 1 (PPT1) as the molecular target of DQ661. PPT1 inhibition concurrently impairs mTOR and lysosomal catabolism through the rapid accumulation of palmitoylated proteins. DQ661 inhibits the in vivo tumor growth of melanoma, pancreatic cancer, and colorectal cancer mouse models and can be safely combined with chemotherapy. Thus, lysosome-directed PPT1 inhibitors represent a new approach to concurrently targeting mTORC1 and lysosomal catabolism in cancer.Significance: This study identifies chemical features of dimeric compounds that increase their lysosomal specificity, and a new molecular target for these compounds, reclassifying these compounds as targeted therapies. Targeting PPT1 blocks mTOR signaling in a manner distinct from catalytic inhibitors, while concurrently inhibiting autophagy, thereby providing a new strategy for cancer therapy. Cancer Discov; 7(11); 1266-83. ©2017 AACR.See related commentary by Towers and Thorburn, p. 1218This article is highlighted in the In This Issue feature, p. 1201.