SLC6A14 (ATB0,+) Protein, a Highly Concentrative and Broad Specific Amino Acid Transporter, Is a Novel and Effective Drug Target for Treatment of Estrogen Receptor-positive Breast Cancer

SLC6A14, also known as ATB(0,+), is an amino acid transporter with unique characteristics. It transports 18 of the 20 proteinogenic amino acids. However, this transporter is expressed only at low levels in normal tissues. Here, we show that the transporter is up-regulated specifically in estrogen receptor (ER)-positive breast cancer, demonstrable with primary human breast cancer tissues and human breast cancer cell lines. SLC6A14 is an estrogen/ER target. The transport features of SLC6A14 include concentrative transport of leucine (an activator of mTOR), glutamine (an essential amino acid for nucleotide biosynthesis and substrate for glutaminolysis), and arginine (an essential amino acid for tumor cells), suggesting that ER-positive breast cancer cells up-regulate SLC6A14 to meet their increased demand for these amino acids. Consequently, treatment of ER-positive breast cancer cells in vitro with alpha-methyl-DL-tryptophan (alpha-MT), a selective blocker of SLC6A14, induces amino acid deprivation, inhibits mTOR, and activates autophagy. Prolongation of the treatment with alpha-MT causes apoptosis. Addition of an autophagy inhibitor (3-methyladenine) during alpha-MT treatment also induces apoptosis. These effects of alpha-MT are specific to ER-positive breast cancer cells, which express the transporter. The ability of alpha-MT to cause amino acid deprivation is significantly attenuated in MCF-7 cells, an ER-positive breast cancer cell line, when SLC6A14 is silenced with shRNA. In mouse xenograft studies, alpha-MT by itself is able to reduce the growth of the ER-positive ZR-75-1 breast cancer cells. These studies identify SLC6A14 as a novel and effective drug target for the treatment of ER-positive breast cancer.