Amino Acid Transporter SLC6A14: A Novel Drug Target for Colorectal Cancer & Colitis and Its Transcriptional Regulation by TCF4/β-catenin Pathway

Mohd Omar Faruk Sikder, Sathish Sivaprakasam, Vadivel Ganapathy

Texas Tech University Health Sciences Center

SLC6A14 is a Na+/Cl -dependent amino acid transporter capable of transporting 18 of the 20 proteinogenic amino acids, including leucine (mTOR activator), glutamine (necessary for nucleotide biosynthesis), and arginine (substrate for iNOS). This transporter is expressed at basal levels in normal colon but significantly upregulated in colorectal cancer (CRC) and colitis. However, the relevance of this upregulation to disease progression and the mechanisms involved in the upregulation remain unknown. We postulated that deletion of SLC6A14 or pharmacological blockade of its function may suppress CRC by depleting amino acids and interfering with mTOR signaling selectively in tumor cells. It may also ameliorate the severity of colitis by diminishing NO synthesis by iNOS in colonic epithelial cells. Since TCF4/β-catenin/Wnt signaling is activated in CRC and colitis, we postulated that TCF4/β-catenin might control the expression of SLC6A14. We tested these hypotheses in the present study. CRC cells treated in vitro with α-methyl tryptophan, a selective blocker of SLC6A14, showed evidence of amino acid deprivation, decreased mTOR activity, and increased autophagy and apoptosis. In nude mouse xenografts with LS174T cells (a CRC cell line), silencing of SLC6A14 by shRNA markedly reduced tumor growth. Accordingly, Slc6a14-/- mice showed reduced tumor incident and tumor size compared to the wild type mice in AOM/DSS-induced colitis-associated colon cancer mouse model. Furthermore, Slc6a14-null mice showed decreased disease progression in an experimental colitis model (Dextran sulfate sodium-induced colitis) compared to wild type mice. In vitro treatment of LS174T cells with calphostin-C, a potent inhibitor of TCF4/β-catenin, dramatically reduced the expression of SLC6A14 mRNA and protein levels, whereas treatment of KM12L4 and KM12C cells with Wnt agonist AMBMP showed the opposite trend. Finally, ChIP assay demonstrated that TCF4/β-catenin complex directly regulates the expression of SLC6A14 in human CRC cells by binding with its promoter. We conclude that the increased β-catenin/Wnt signaling in CRC and colitis is responsible for the upregulation of SLC6A14 under these pathological conditions and that deletion of the transporter or its pharmacological blockade

protects against colitis and CRC. These studies identify SLC6A14 as a novel drug target for the treatment of colonic inflammation and CRC.

Citation Format: Mohd Omar Sikder, Sathish Sivaprakasam, Ganapathy Vadivel. Amino acid transporter SLC6A14: A novel drug target for colorectal cancer and colitis and its transcriptional regulation by TCF4/beta-catenin pathway [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 5735.