Abstract 4459: a-Methyltryptophan (a-MT) inhibits estrogen receptor (ER)-positive breast cancer growth through blockade of SLC6A14

Abstract

Introduction: SLC6A14 is an amino acid transporter that transports all essential amino acids as well as glutamine and arginine, and is energized by Na+ gradient, Cl- gradient, and membrane potential. The expression of SLC6A14 is upregulated in ER-positive breast cancer cells but not in ER-negative breast cancer cells. Since cancer cells have an increased need for essential amino acids (protein synthesis), Gln (nucleotide synthesis/glutaminolysis), and Arg (essential amino acid in cancer), we hypothesize that cancer cells satisfy their need for these amino acids by upregulating SLC6A14, and that blockade of SLC6A14 offers a rational strategy for treatment of ER-positive breast cancer. Methods: The transport of Leu, Gln, and Arg via SLC6A14 was studied in X. laevis oocyte expression system. SLC6A14 expression was monitored by RT-PCR and immunohistochemistry in normal and tumors (ER-positive and ER-negative) from primary breast cancer specimens. The influence of estrogen on SLC6A14 expression was investigated in ER-positive BT474 breast cancer cells. α-MT was used as the blocker of SLC6A14. The differential effects of α-MT on MCF7 (ER-positive), MB231 (ER-negative) and MCF10A (non-malignant) cells were investigated in terms of mTOR activity, autophagy, and apoptosis. The ability of α-MT to inhibit the growth of breast cancer cells was evaluated by xenograft in nude mice. α-MT was administered to mice in drinking water (2 mg/ml). Plasma levels of α-MT were measured by HPLC. Results: The transport of Leu, Gln, and Arg via SLC6A14 was dependent on Na+ and Cl- with a Na+:Cl-:amino acid stoichiometry of 2:1:1, and was blocked by α-MT. SLC6A14 expression was upregulated in ER-positive breast cancer. SLC6A14 was not expressed in ER-negative breast cancer. Estrogen induced SLC6A14 expression in ER-positive breast cancer cells. Exposure of ER-positive MCF7 breast cancer cells to α-MT induced the expression of asparagine synthetase and CHOP, inhibited mTOR activity, and induced autophagy and apoptosis. When combined with an autophagy inhibitor, α-MT induced apoptosis in MCF7 cells more potently. These effects were specific for ER-positive breast cancer cells, and were not seen in ER-negative cells or in non-malignant cells. In mouse xenografts, α-MT reduced the growth of ER-positive ZR-75-1 breast cancer cells, but did not have any effect on the growth of ER-negative MB231 cells. MCF7 cells did not grow in nude mice unless estrogen was administered, and under these conditions, α-MT did not block the growth of the cells. Plasma concentration of α-MT in treated mice was 8.5 ± 1.5 μM. α-MT was not detectable in the plasma of untreated mice. Conclusions: SLC6A14 is an ideal and effective drug target for treatment of ER-positive breast cancer. α-MT has potential for use as an anticancer drug, and may also serve as a lead compound in the design/development of newer blockers of SLC6A14 for use in humans. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4459. doi:10.1158/1538-7445.AM2011-4459