Abstract 4838: Alpha-methyltryptophan, a blocker of the amino acid transporter SLC6A14, induces autophagy and apoptosis in SLC6A14-positive breast cancer and colon cancer cell lines

Abstract

Abstract SLC6A14 (also known as ATB0,+) is a Na+/Cl− -coupled amino acid transporter that transports 18 of the 20 proteinogenic amino acids, including all essential amino acids. Normal tissues express this transporter at low levels, whereas tumor tissues (colon, breast, and cervix) upregulate this transporter several-fold. The unique functional features of SLC6A14 (broad substrate specificity and multiple driving forces) make it an ideal transporter for tumor cells to maintain amino acid nutrition necessary for enhanced cell proliferation. We speculate that SLC6A14 could not only be an ideal tumor-specific delivery system for amino acid-based anti-cancer drugs but also be a direct drug target for cancer chemotherapy. Recently we identified alpha-methyltryptophan (ALT) as a blocker of SLC6A14. Here we examine the potential of this blocker as an anti-cancer agent in SLC6A14-positive breast and colon cancer cells. Treatment of MCF7 (a breast cancer cell line), HCT116 and LS174T (colon cancer cell lines) with ALT causes amino acid starvation as evidenced by the upregulation of asparagine synthetase and CHOP. These three cell lines are SLC6A14-positive. In contrast, ALT treatment of breast cancer cell line MDA-MB231, which is SLC6A14-negative, does not show any evidence of amino acid starvation. Similarly, ALT treatment has no effect on normal breast and colon epithelial cell lines (MCF10A and CCD841, respectively). These cell lines also do not express SLC6A14. These data show that ALT-induced amino acid starvation is mediated specifically through the blockade of SLC6A14. Since nutrient signaling is also associated with changes in mTOR pathway, we examined the influence of ALT treatment on phosphorylation of S6 and S6kinase. Blockade of SLC6A14 with ALT in SLC6A14-positive cancer cell lines leads to decreased mTOR signaling as evidenced by decreased phosphorylation of S6 and S6kinase. Again, such changes are not seen in SLC6A14-negative cancer or normal cell lines. The initial response of the SLC6A14-positive cancer cell lines to ALT treatment is induction of autophagy. ALT-induced autophagy is documented in these cells by light and electron microscopic examination as well as by biochemical means. Inhibition of autophagy with 3-methyladenine under these conditions leads to apoptosis. We conclude that SLC6A14 is a potential drug target for cancer chemotherapy and that ALT could serve as a lead compound for the future design of high-affinity blockers of SLC6A14. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4838.