Overexpression of VIRMA confers vulnerability to breast cancers via the m6A-dependent regulation of unfolded protein response

Virilizer-like m⁶A methyltransferase-associated protein (VIRMA) maintains the stability of the m⁶A writer complex. Although VIRMA is critical for RNA m⁶A deposition, the impact of aberrant VIRMA expression in human diseases remains unclear. We show that VIRMA is amplified and overexpressed in 15-20% of breast cancers. Of the two known VIRMA isoforms, the nuclear-enriched full-length but not the cytoplasmic-localised N-terminal VIRMA promotes m⁶A-dependent breast tumourigenesis in vitro and in vivo. Mechanistically, we reveal that VIRMA overexpression upregulates the m⁶A-modified long non-coding RNA, NEAT1, which contributes to breast cancer cell growth. We also show that VIRMA overexpression enriches m⁶A on transcripts that regulate the unfolded protein response (UPR) pathway but does not promote their translation to activate the UPR under optimal growth conditions. Under stressful conditions that are often present in tumour microenvironments, VIRMA-overexpressing cells display enhanced UPR and increased susceptibility to death. Our study identifies oncogenic VIRMA overexpression as a vulnerability that may be exploited for cancer therapy.

Glutamine addiction promotes glucose oxidation in triple-negative breast cancer

Glutamine is a conditionally essential nutrient for many cancer cells, but it remains unclear how consuming glutamine in excess of growth requirements confers greater fitness to glutamine-addicted cancers. By contrasting two breast cancer subtypes with distinct glutamine dependencies, we show that glutamine-indispensable triple-negative breast cancer (TNBC) cells rely on a non-canonical glutamine-to-glutamate overflow, with glutamine carbon routed once through the TCA cycle. Importantly, this single-pass glutaminolysis increases TCA cycle fluxes and replenishes TCA cycle intermediates in TNBC cells, a process that achieves net oxidation of glucose but not glutamine. The coupling of glucose and glutamine catabolism appears hard-wired via a distinct TNBC gene expression profile biased to strip and then sequester glutamine nitrogen, but hampers the ability of TNBC cells to oxidise glucose when glutamine is limiting. Our results provide a new understanding of how metabolically rigid TNBC cells are sensitive to glutamine deprivation and a way to select vulnerable TNBC subtypes that may be responsive to metabolic-targeted therapies.

Ablation of the ASCT2 (SLC1A5) gene encoding a neutral amino acid transporter reveals transporter plasticity and redundancy in cancer cells

The neutral amino acid transporter solute carrier family 1 member 5 (SLC1A5 or ASCT2) is overexpressed in many cancers. To identify its roles in tumors, we employed 143B osteosarcoma cells and HCC1806 triple-negative breast cancer cells with or without ASCT2 deletion. ASCT2ko 143B cells grew well in standard culture media, but ASCT2 was required for optimal growth at <0.5 mm glutamine, with tumor spheroid growth and monolayer migration of 143B ASCT2ko cells being strongly impaired at lower glutamine concentrations. However, the ASCT2 deletion did not affect matrix-dependent invasion. ASCT2ko 143B xenografts in nude mice exhibited a slower onset of growth and a higher number of small tumors than ASCT2wt 143B xenografts, but did not differ in average tumor size 25 days after xenotransplantation. ASCT2 deficiency was compensated by increased levels of sodium neutral amino acid transporter 1 (SNAT1 or SLC38A1) and SNAT2 (SLC38A2) in ASCT2ko 143B cells, mediated by a GCN2 EIF2α kinase (GCN2)-dependent pathway, but this compensation was not observed in ASCT2ko HCC1806 cells. Combined SNAT1 silencing and GCN2 inhibition significantly inhibited growth of ASCT2ko HCC1806 cells, but not of ASCT2ko 143B cells. Similarly, pharmacological inhibition of l-type amino acid transporter 1 (LAT1) and GCN2 significantly inhibited growth of ASCT2ko HCC1806 cells, but not of ASCT2ko 143B cells. We conclude that cancer cells with reduced transporter plasticity are more vulnerable to disruption of amino acid homeostasis than cells with a full capacity to up-regulate redundant transporters by an integrated stress response.

RAB27A promotes melanoma cell invasion and metastasis via regulation of pro-invasive exosomes

Despite recent advances in targeted and immune-based therapies, advanced stage melanoma remains a clinical challenge with a poor prognosis. Understanding the genes and cellular processes that drive progression and metastasis is critical for identifying new therapeutic strategies. Here, we found that the GTPase RAB27A was overexpressed in a subset of melanomas, which correlated with poor patient survival. Loss of RAB27A expression in melanoma cell lines inhibited 3D spheroid invasion and cell motility in vitro, and spontaneous metastasis in vivo. The reduced invasion phenotype was rescued by RAB27A-replete exosomes, but not RAB27A-knockdown exosomes, indicating that RAB27A is responsible for the generation of pro-invasive exosomes. Furthermore, while RAB27A loss did not alter the number of exosomes secreted, it did change exosome size and altered the composition and abundance of exosomal proteins, some of which are known to regulate cancer cell movement. Our data suggest that RAB27A promotes the biogenesis of a distinct pro-invasive exosome population. These findings support RAB27A as a key cancer regulator, as well as a potential prognostic marker and therapeutic target in melanoma.

Abstract A036: Blocking DNA and RNA synthesis by targeting glutamine metabolism in prostate cancer

Cancer cells greatly increase their uptake of nutrients (glucose, amino acids, and lipids), and metabolize them to provide the necessary building blocks for new cancer cells. It was recently shown that extracellular amino acids make up by far the majority of the carbon sources used by cancer cells for cell division, highlighting amino acid uptake as a viable therapeutic target. In addition, amino acids such as glutamine are critical in providing nitrogen for purine and pyrimidine metabolism. We have previously shown that glutamine uptake is mediated predominantly by ASCT2 in prostate cancer, with ASCT2 knockdown blocking cell growth in vitro and in vivo. Using the TCGA dataset, we have discovered increased expression of downstream glutamine metabolism enzymes in approximately 25% of prostate cancer patients. One of these enzymes, guanine monophosphate synthase (GMPS), is a glutamine amidotransferase involved in de novo purine biosynthesis and is responsible for the last step in the synthesis of the guanine nucleotide. Expression of GMPS correlates with increasing Gleason score in prostate cancer patient samples. Furthermore, patients with high GMPS expression had significantly reduced disease-free survival in the TCGA dataset. Immunofluorescent staining shows that GMPS is localized in both the cytoplasm and nucleus of LNCaP and PC-3 cells—consistent with a secondary role for GMPS in p53 stabilization. Inhibition of GMPS using decoyinine significantly decreased cell growth of both LNCaP and PC-3 cells. Knockdown of GMPS by shRNA significantly decreased cell growth, which could be rescued by addition of extracellular guanosine to the media, suggesting a direct effect on nucleotide synthesis. These results show the importance of downstream glutamine metabolism in prostate cancer, and suggest that GMPS is a potential therapeutic target in “glutamine-addicted” prostate cancers.

Citation Format: Qian (Kevin) Wang, Michelle van Geldermalsen, Angel Pang, Blake Zhang, Jeff Holst. Blocking DNA and RNA synthesis by targeting glutamine metabolism in prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A036.

Abstract B20: Glutamine metabolic vulnerabilities define triple-negative from luminal A breast cancer subsets

Although a nonessential amino acid in normal cells, the demand for glutamine is dramatically increased throughout malignant transformation to support increased metabolic demands; namely, provision of catabolic substrates for ATP production and anabolic substrates for the citric acid cycle and subsequent macromolecule biosynthesis, as well as potentiating the uptake of other critical amino acids by acting as an obligate exchange substrate. Elevated expression of glutamine metabolism-related genes, MYC-driven transcriptional events, and increased consumption and reliance on glutamine are all associated with aggressive breast cancers, including the high-risk triple-negative (TN) subtype. We recently showed that in breast cancer cells, glutamine uptake by the small neutral amino acid transporter, ASCT2, is required to sustain TN cell growth in vitro and in vivo. We therefore hypothesized that highly proliferative TN breast cancers that are sensitive to ASCT2 inhibition may have unique metabolic signatures that could be additionally exploited for therapeutic purposes.

Using a targeted metabolomics approach, we combined labeled substrate tracing, liquid chromatography coupled tandem-mass spectrometry (LC-MS/MS), and gas chromatography mass spectrometry (GC-MS) to analyze intracellular levels of key tricarboxylic acid (TCA) cycle intermediates, glycolytic metabolites, fatty acid precursors, and amino acids in human breast cancer cell lines. These analyses revealed distinct metabolic effects when glutamine uptake was blocked in vitro by L-γ-glutamyl-p-nitroanilide (GPNA), a pharmacologic inhibitor of ASCT2. These data confirm a broad reliance on glutamine availability in TN breast cancers, reinforced by TCGA gene expression data showing a specific upregulation of multiple glutamine metabolism enzymes that is completely absent in the luminal A subtype. These data emphasize the link between increased glutamine metabolism and clinically aggressive breast cancers, thus highlighting the therapeutic potential of targeting glutamine metabolism pathways in these patients.

Citation Format: Michelle van Geldermalsen, Lake-Ee Quek, Nigel Turner, Seher Balaban, Andrew Hoy, Qian Wang, Jeff Holst. Glutamine metabolic vulnerabilities define triple-negative from luminal A breast cancer subsets [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr B20.

Abstract 5167: Rab27a promotes melanoma cell invasion and metastasis via the regulation of exosome secretion

Introduction: The Rab GTPase family has been increasingly related to carcinogenesis and cancer biology during the past decade. In melanoma, Rab27a has been identified as a tumor dependency gene. Rab27a is thought to regulate exosome secretion in several cell types, however the exact role of Rab27a in melanoma biology and the underlying mechanisms are not well investigated.

Experimental Procedures: To investigate the influence of Rab27a on melanoma biology and exosome secretion, Rab27a shRNA knockdown (KD) or CRISPR knockout (KO) was performed in human and murine melanoma cell lines. The effect of Rab27a loss on melanoma cell invasion and motility was analyzed by in vitro 3D spheroid invasion and live cell imaging. Rab27a function on melanoma metastasis was investigated using a mouse melanoma spontaneous metastasis model. Exosomes secreted by melanoma cell lines with/without Rab27a KD/KO were purified and characterized by electron microscopy, NanoSight analysis, BCA assay, western blotting and mass spectrometry.

Results: Our results indicate that Rab27a loss in Rab27a high metastatic melanoma cell lines reduced 3D spheroid invasion and cell motility and Rab27a loss also reduced spontaneous melanoma metastasis in vivo. Rab27a KD invasion phenotype can be partially rescued by addition of exosomes from Rab27a replete cell conditioned media, but not exosomes derived from Rab27a KD cells. Loss of Rab27a does not alter the number of exosomes secreted from melanoma cells, but does alter exosome protein composition and morphology. Mass spectrometry analysis has shown that Rab27a replete cell secreted exosomes are more involved in regulating cell invasion and motility than the exosomes from Rab27a KD cells.

Conclusion: In summary, Rab27a promotes the invasion and metastasis of a subset of melanoma cells via the regulation of pro-invasive exosomes, which indicates Rab27a as a potential therapeutic target for preventing melanoma progression.

Citation Format: DAJIANG GUO, Goldie Y. Lui, Danae M. Sharp, Siew Li Lai, Shweta Tikoo, Rain Kwan, Michelle van Geldermalsen, Jeff Holst, Nikolas K. Haass, Wolfgang Weninger, Kimberley A. Beaumont. Rab27a promotes melanoma cell invasion and metastasis via the regulation of exosome secretion [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5167.

Benzylserine inhibits breast cancer cell growth by disrupting intracellular amino acid homeostasis and triggering amino acid response pathways

Background

Cancer cells require increased levels of nutrients such as amino acids to sustain their rapid growth. In particular, leucine and glutamine have been shown to be important for growth and proliferation of some breast cancers, and therefore targeting the primary cell-surface transporters that mediate their uptake, L-type amino acid transporter 1 (LAT1) and alanine, serine, cysteine-preferring transporter 2 (ASCT2), is a potential therapeutic strategy.

Methods

The ASCT2 inhibitor, benzylserine (BenSer), is also able to block LAT1 activity, thus inhibiting both leucine and glutamine uptake. We therefore aimed to investigate the effects of BenSer in breast cancer cell lines to determine whether combined LAT1 and ASCT2 inhibition could inhibit cell growth and proliferation.

Results

BenSer treatment significantly inhibited both leucine and glutamine uptake in MCF-7, HCC1806 and MDA-MB-231 breast cancer cells, causing decreased cell viability and cell cycle progression. These effects were not primarily leucine-mediated, as BenSer was more cytostatic than the LAT family inhibitor, BCH. Oocyte uptake assays with ectopically expressed amino acid transporters identified four additional targets of BenSer, and gas chromatography-mass spectrometry (GCMS) analysis of intracellular amino acid concentrations revealed that this BenSer-mediated inhibition of amino acid uptake was sufficient to disrupt multiple pathways of amino acid metabolism, causing reduced lactate production and activation of an amino acid response (AAR) through activating transcription factor 4 (ATF4).

Conclusions

Together these data showed that BenSer blockade inhibited breast cancer cell growth and viability through disruption of intracellular amino acid homeostasis and inhibition of downstream metabolic and growth pathways.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4599-8) contains supplementary material, which is available to authorized users.

Adipocyte lipolysis links obesity to breast cancer growth: adipocyte-derived fatty acids drive breast cancer cell proliferation and migration

Background

Obesity is associated with increased recurrence and reduced survival of breast cancer. Adipocytes constitute a significant component of breast tissue, yet their role in provisioning metabolic substrates to support breast cancer progression is poorly understood.

Results

Here, we show that co-culture of breast cancer cells with adipocytes revealed cancer cell-stimulated depletion of adipocyte triacylglycerol. Adipocyte-derived free fatty acids were transferred to breast cancer cells, driving fatty acid metabolism via increased CPT1A and electron transport chain complex protein levels, resulting in increased proliferation and migration. Notably, fatty acid transfer to breast cancer cells was enhanced from “obese” adipocytes, concomitant with increased stimulation of cancer cell proliferation and migration. This adipocyte-stimulated breast cancer cell proliferation was dependent on lipolytic processes since HSL/ATGL knockdown attenuated cancer cell responses.

Conclusions

These findings highlight a novel and potentially important role for adipocyte lipolysis in the provision of metabolic substrates to breast cancer cells, thereby supporting cancer progression.

Electronic supplementary material

The online version of this article (doi:10.1186/s40170-016-0163-7) contains supplementary material, which is available to authorized users.

LAT1 is a putative therapeutic target in endometrioid endometrial carcinoma

l-type amino acid transporters (LAT1-4) are expressed in various cancer types and are involved in the uptake of essential amino acids such as leucine. Here we investigated the expression of LAT1-4 in endometrial adenocarcinoma and evaluated the contribution of LATs to endometrial cancer cell growth. Analysis of human gene expression data showed that all four LAT family members are expressed in endometrial adenocarcinomas. LAT1 was the most highly expressed, and showed a significant increase in both serous and endometrioid subtypes compared to normal endometrium. Endometrioid patients with the highest LAT1 levels exhibited the lowest disease-free survival. The pan-LAT inhibitor BCH led to a significant decrease in cell growth and spheroid area in four endometrial cancer cell lines tested in vitro. Knockdown of LAT1 by shRNA inhibited cell growth in HEC1A and Ishikawa cells, as well as inhibiting spheroid area in HEC1A cells. These data show that LAT1 plays an important role in regulating the uptake of essential amino acids such as leucine into endometrial cancer cells. Increased ability of BCH compared to LAT1 shRNA at inhibiting Ishikawa spheroid area suggests that other LAT family members may also contribute to cell growth. LAT1 inhibition may offer an effective therapeutic strategy in endometrial cancer patients whose tumours exhibit high LAT1 expression.

Abstract 1043: Targeting ASCT2-mediated glutamine uptake and metabolism in breast cancer

Although a nonessential amino acid in normal cell growth, the demand for glutamine is dramatically increased throughout malignant transformation to provide catabolic substrates for ATP production and anabolic substrates for macromolecule biosynthesis. To maintain glutamine availability for these metabolic processes, cancer cells overexpress cell surface transporters that function to exchange amino acids across the plasma membrane. One such transporter is ASCT2 (alanine, serine, cysteine-preferring transporter 2; SLC1A5), a sodium-dependent symporter that mediates uptake of small, neutral amino acids, including glutamine.

Blocking ASCT2 to prevent glutamine uptake and glutaminolysis has been shown to successfully prevent tumor cell proliferation in melanoma, non-small cell lung cancer, prostate cancer and acute myeloid leukaemia. We recently showed that in breast cancer, although ASCT2 is highly expressed in most tumor subtypes, only aggressive triple-negative (TN) breast cancer cells require ASCT2-mediated uptake of glutamine to sustain cell growth in vitro and in vivo. Gene expression analysis of xenograft-derived tumor tissue and TN patient samples suggested coordinate regulation of ASCT2 and other glutamine metabolism-related genes, such as glutaminase (GLS) and glutamate-ammonia ligase (GLUL), with global activation of glutaminolytic energy production pathways in these tumors. The metabolism-regulating transcription factors, MYC and ATF4, were significantly correlated with these genes, suggesting a dynamic MYC and ATF4-driven transcriptional program in TN breast cancer. We therefore hypothesized that highly proliferative TN breast cancers that are sensitive to ASCT2 inhibition may have unique metabolic signatures that could be additionally exploited for therapeutic purposes.

We developed a targeted metabolomics approach that combined labelled substrate tracing, liquid chromatography coupled tandem-mass spectrometry (LC-MS/MS) and gas chromatography mass spectrometry (GC-MS) to analyze intracellular levels of key tricarboxylic acid (TCA) cycle intermediates, glycolytic metabolites, fatty acid precursors, and amino acids in human breast cancer cell lines. These analyses revealed distinct metabolic effects when ASCT2 transporter function was blocked in vitro by pharmacological inhibitors or inducible shRNA knockdown, in combination with CB-839, a GLS inhibitor in Phase I clinical trials. To confirm the clinical utility of these findings, we also determined mRNA and protein expression of glutamine metabolism-related genes in tissue microarrays of TN patient samples. These data suggest a reliance on glutamine availability in TN breast cancers and reinforce the link between increased glutamine metabolism and clinically aggressive breast cancers, thus highlighting the therapeutic potential of targeting the ASCT2 glutamine uptake and metabolism pathway in these patients.

Citation Format: Michelle van Geldermalsen, Qian Wang, Jeff Holst. Targeting ASCT2-mediated glutamine uptake and metabolism in breast cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1043.

Abstract 1007: The amino acid transporter SNAT4: Potential role as a tumor suppressor in melanoma

Cancer cells utilize amino acids to satisfy their need for nutrients and fuel their accelerated growth. The amino acid glutamine has been identified as one of the key building blocks in cancer cells, utilized for macromolecular synthesis and energy production. Due to the increased requirement for glutamine and other amino acids, cancer cells commonly increase expression of amino acid transporters, such as SLC1A5 (ASCT2). Amino acid transporters are membrane transport proteins that are used by cells to move important amino acids in and out of the cell. Interestingly, most amino acid transporters are upregulated in cancer to ensure continued access to nutrients.

We have recently shown that ASCT2 plays a critical role in regulating glutamine uptake in melanoma, prostate and breast cancer. Many other glutamine transporters are upregulated in different cancers, including SLC38 family members SNAT1, SNAT2 and SNAT3. The role of SLC38A4 (SNAT4) in amino acid uptake and cancer growth, however, has not been examined. Like the other SNATs, SNAT4 is a sodium-dependent amino acid transporter that transports neutral amino acids, such as alanine, across the plasma membrane, although it has low affinity for glutamine. Interestingly, unlike the other SNAT family members, our analysis showed that SNAT4 expression is downregulated in 879/917 cancer cell lines in Oncomine. Analysis of SNAT4 mutations in patient samples using cBioPortal showed infrequent mutations in the majority of cancers. However, the TCGA melanoma data (cBioPortal) showed that 32 of 278 melanoma cases (11.5%) have a point mutation in SNAT4 suggesting an important role in melanoma. These mutations included 9 SNAT4 hotspot mutations, present in 2-4 patients each. Using a SNAT4 homology model to predict loss-of-function mutants, we selected five of these mutations to assess in a SNAT4 transport assay in Xenopus oocytes. Two of the five hotspot mutations (G31E, S76F, G78E, G150E and S371F) were able to completely inactivate SNAT4 alanine transport in oocytes. Further analysis of 52 melanoma patient samples in Oncomine showed SNAT4 downregulation in all patients, suggesting that, in contrast to most amino acid transporters, SNAT4 plays a tumor suppressor role in melanoma.

We are currently examining these mutations in melanoma cell lines to determine their effects on cell growth. We are also examining how SNAT4 is involved in either the import or export of amino acids and the downstream metabolic consequences that may directly inhibit melanoma cell growth. Through this research we will gain a better understanding of the role of SNAT4- mediated amino acid metabolism in preventing melanoma cell growth.

Citation Format: Nicholas J. Otte, Angelika Broer, Patrick O’Young, Michelle van Geldermalsen, Qian Wang, Charles G. Bailey, Stefan Broer, Jeff Holst. The amino acid transporter SNAT4: Potential role as a tumor suppressor in melanoma. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1007.

Targeting ASCT2-mediated glutamine uptake blocks prostate cancer growth and tumour development

Glutamine is conditionally essential in cancer cells, being utilized as a carbon and nitrogen source for macromolecule production, as well as for anaplerotic reactions fuelling the tricarboxylic acid (TCA) cycle. In this study, we demonstrated that the glutamine transporter ASCT2 (SLC1A5) is highly expressed in prostate cancer patient samples. Using LNCaP and PC‐3 prostate cancer cell lines, we showed that chemical or shRNA‐mediated inhibition of ASCT2 function in vitro decreases glutamine uptake, cell cycle progression through E2F transcription factors, mTORC1 pathway activation and cell growth. Chemical inhibition also reduces basal oxygen consumption and fatty acid synthesis, showing that downstream metabolic function is reliant on ASCT2‐mediated glutamine uptake. Furthermore, shRNA knockdown of ASCT2 in PC‐3 cell xenografts significantly inhibits tumour growth and metastasis in vivo, associated with the down‐regulation of E2F cell cycle pathway proteins. In conclusion, ASCT2‐mediated glutamine uptake is essential for multiple pathways regulating the cell cycle and cell growth, and is therefore a putative therapeutic target in prostate cancer. © 2015 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Targeting ASCT2-mediated glutamine uptake blocks prostate cancer growth and tumour development

Glutamine is conditionally essential in cancer cells, being utilized as a carbon and nitrogen source for macromolecule production, as well as for anaplerotic reactions fuelling the tricarboxylic acid (TCA) cycle. In this study, we demonstrated that the glutamine transporter ASCT2 (SLC1A5) is highly expressed in prostate cancer patient samples. Using LNCaP and PC-3 prostate cancer cell lines, we showed that chemical or shRNA-mediated inhibition of ASCT2 function in vitro decreases glutamine uptake, cell cycle progression through E2F transcription factors, mTORC1 pathway activation and cell growth. Chemical inhibition also reduces basal oxygen consumption and fatty acid synthesis, showing that downstream metabolic function is reliant on ASCT2-mediated glutamine uptake. Furthermore, shRNA knockdown of ASCT2 in PC-3 cell xenografts significantly inhibits tumour growth and metastasis in vivo, associated with the down-regulation of E2F cell cycle pathway proteins. In conclusion, ASCT2-mediated glutamine uptake is essential for multiple pathways regulating the cell cycle and cell growth, and is therefore a putative therapeutic target in prostate cancer.

Targeting glutamine transport to suppress melanoma cell growth

Amino acids, especially leucine and glutamine, are important for tumor cell growth, survival and metabolism. A range of different transporters deliver each specific amino acid into cells, some of which are increased in cancer. These amino acids consequently activate the mTORC1 pathway and drive cell cycle progression. The leucine transporter LAT1/4F2hc heterodimer assembles as part of a large complex with the glutamine transporter ASCT2 to transport amino acids. In this study, we show that the expression of LAT1 and ASCT2 is significantly increased in human melanoma samples and is present in both BRAF(WT) (C8161 and WM852) and BRAF(V600E) mutant (1205Lu and 451Lu) melanoma cell lines. While inhibition of LAT1 by BCH did not suppress melanoma cell growth, the ASCT2 inhibitor BenSer significantly reduced both leucine and glutamine transport in melanoma cells, leading to inhibition of mTORC1 signaling. Cell proliferation and cell cycle progression were significantly reduced in the presence of BenSer in melanoma cells in 2D and 3D cell culture. This included reduced expression of the cell cycle regulators CDK1 and UBE2C. The importance of ASCT2 expression in melanoma was confirmed by shRNA knockdown, which inhibited glutamine uptake, mTORC1 signaling and cell proliferation. Taken together, our study demonstrates that ASCT2-mediated glutamine transport is a potential therapeutic target for both BRAF(WT) and BRAF(V600E) melanoma.