Role of amino acid transporters ASCT2 and LAT1 and their substrates in stimulating mTORC1-mediated growth signaling in a broad panel of human liver cancer cells

Cancers exhibit enhanced uptake and consumption of nutrients including glucose and amino acids compared to normal cells and tissues. Two amino acid transporters – ASCT2 and LAT1 – have been shown to be augmented coordinately in a number of human cancers, where they are implicated in driving growth via stimulation of an amino acid-stimulated protein kinase termed mammalian target-of-rapamycin complex 1 (mTORC1). Hepatocellular carcinoma (HCC) is a recalcitrant and deadly cancer for which therapies are desperately needed, and in which mTOR signaling is enhanced in at least 50% of the cases. Here, the role of ASCT2 and LAT1 and their amino acid substrates in driving mTOR-dependent growth signaling in HCC was investigated. The relative importance of these transporters and substrates for mTOR signaling was investigated in15 cell lines representing primary (epithelial) HCC and mesenchymal (metastatic) HCC through the use of amino acid depletion/repletion, specific biochemical inhibitors and western blot-base cell signaling analysis. The hypothesis tested is that metastatic HCC cells rely more heavily on ASCT2 and LAT1 for mTORC1 growth signaling than epithelial cells due to their more aggressive nature and limited glutamine transporter repertoire vs. epithelial HCC cells. The results indicated that mTORC1 signaling (4EBP1 phosphorylation) in specific HCC cell lines exhibits a wide variety of responses to amino acids and transporter inhibition independent of epithelial vs. mesenchymal classification, although mesenchymal HCC tended to respond more robustly to essential amino acids (LAT1 substrates), which were the dominant mTORC1 stimulatory factor in nearly all HCC.