Abstract 5148: Amino acid transporters ASCT2 and LAT1 ubiquitously function in N-myc (+) epithelial and mesenchymal human HCC cells exhibiting a wide array of mTOR and glycolytic reliance for growth
Authors: Kron Clare , Wittke Evan , Mottar Ashley , Bothwell Paige , Finger Richard , Bode Barrie P. 
Published: August 2012

Amino acid transporters ASCT2 and LAT1 are coordinately enhanced in a broad array of human cancers where they associate in the plasma membrane and are proposed to work cooperatively to activate mTOR signaling through a tertiary-active transport mechanism involving glutamine (ASCT2) and essential branch-chain amino acids (LAT1). Silencing of ASCT2 expression in liver cancer cells has been shown to induce apoptosis and inhibit mTOR signaling while Myc expression has been implicated in promoting ASCT2 and LAT1 expression as well as a glutamine-intensive metabolic profile typical of many cancers, which helps support the switch to aerobic glycolysis (Warburg effect). In particular, ASCT2 and LAT1 are nearly undetectable in normal liver but are enhanced in hepatocellular carcinoma (HCC). To test the relationship between amino acid transporters, myc expression, metabolism and mTOR, we examined the expression and function of ASCT2 and LAT1 as a function of N-myc and C-myc status, reliance on glycolysis for growth and sensitivity to rapamycin (mTOR) in a broad panel of 14 human HCC lines (7 Group I (epithelial) and 7 Group II (mesenchymal)). We hypothesized that transporter expression is proportional to myc expression, and that glycolytic reliance and both transporters are enhanced in the more aggressive Group II HCC lines. Our results show that all human HCC lines are N-myc-, ASCT2- and LAT1-positive, and variably c-myc positive, with no clear correlation between specific myc isoform and transporter expression levels. In general, LAT1 protein expression was uniformly high in all mesenchymal HCC lines, but more variable in epithelial HCC, while BCH-inibitable leucine uptake rates displayed a wide array of values in both HCC groups; the same was true for ASCT2 protein and glutamine uptake activities. Treatment of HCC cells for 24 hours with ASCT2 and LAT1 substrate inhibitors GPNA and BCH, respectively, failed to inhibit mTOR signaling as indexed by 4EBP1 phosphorylation. Inhibition of glycolysis and growth with 3-bromopyruvate was generally more pronounced in epithelial HCC, but variably evident in both HCC groups. Rapamycin-dependent growth inhibition was variable between HCC lines and showed no preferential epithelial or mesenchymal HCC efficacy. Collectively, the results suggest that N-myc, ASCT2 and LAT1 are ubiquitously expressed in both epithelial and mesenchymal human HCC cells, where they help drive growth in cells variably reliant on mTOR signaling and glycolysis. Thus, both transporters remain the subject of ongoing investigation as targeted therapies for HCC.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5148. doi:1538-7445.AM2012-5148