Abstract 274: Functional coupling between the amino acid transporters SLC38A5 and SLC7A11 in TNBC via selenomethionine
Authors: Mathew Marilyn , Dharmalingam Gunadharini Nandagopal , Sivaprakasam Sathish , Ramachandran Sabarish , Sennoune Souad R. , Ganapathy Vadivel 
Published: June 2023

Abstract
Breast cancer is the most common cancer in women, affecting 1 in every 8 women, with a combination of radiation, surgery, and chemotherapy as treatment. SLC38A5 is a glutamine/serine/glycine/methionine transporter, which mediates the influx of Na+/amino acid into cells coupled to the efflux of H+. SLC7A11 mediates the cellular uptake of cystine in exchange for glutamate. Both transporters are upregulated in triple-negative breast cancer (TNBC). SLC38A5 supports “glutamine addiction” and the increased need for one-carbon metabolism in cancer cells. SLC7A11 supports cellular synthesis of glutathione by providing cysteine, and thus protects the cancer cells against oxidative stress and iron-induced cell death (ferroptosis). To date, no functional crosstalk has been identified between the two transporters. We hypothesized that (i) SLC38A5 might mediate the cellular uptake of the micronutrient selenium in the form of selenomethionine (Se-Met) like it does Metionine (Met) and (ii) since Se-Met is known to induce the anti-oxidative transcription factor Nrf2 that promotes SLC7A11 expression, there might be a crosstalk between the two transporters via Se-Met. We tested these hypotheses using two TNBC cell lines, MB231 and MB453. The activity of SLC38A5 was monitored by measuring the uptake of serine in the presence of an uptake buffer (pH 8.5) containing LiCl in place of NaCl. Li+ tolerance and higher uptake at alkaline pH are unique features of SLC38A5. The interaction of Met and Se-Met with SLC38A5 was investigated by studying the dose-response effects for the two amino acids in inhibiting serine uptake. Met and Se-Met competed with serine for SLC38A5-mediated uptake in both cell lines with comparable IC50 values (~500 μM). We then investigated the effect of Se-Met on the expression and activity of SLC7A11. Pretreatment of cell lines with 1 mM Se-Met for 16 h increased SLC7A11 mRNA levels and increased the transport activity of SLC7A11. In these studies, monomethylfumarate was used as a positive inducer of Nrf2. Previously work showed shRNA-mediated downregulation of SLC38A5 in TNBC cell lines suppresses growth and proliferation of cancer in cell culture and in mouse xenografts.RNAseq analysis of the control tumors and shRNA-tumors show transcriptomic variance in the profile of the tumors in response to SLC38A5 deficiency. We found upregulation of genes involved in oxidative phosphorylation, TGF-β signaling, and hypoxia signaling and downregulation of KRAS and epithelial-mesenchymal transition pathways in SLC38A5-deficient tumors. We conclude that though both SLC38A5 and SLC7A11 promote TNBC via independent mechanisms, the two transporters are functionally coupled by Se-Met. This study provides valuable insight into how these two amino acid transporters work together to support TNBC growth, thus setting the stage for pharmacologic targeting of these transporters as a novel therapeutic strategy.
Citation Format: Marilyn Mathew, Gunadharini Nandagopal Dharmalingam, Sathish Sivaprakasam, Sabarish Ramachandran, Souad R. Sennoune, Vadivel Ganapathy. Functional coupling between the amino acid transporters SLC38A5 and SLC7A11 in TNBC via selenomethionine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 274.