Glutamine deprivation alters TGF-β signaling in hepatocellular carcinoma

Metabolic reprogramming is one of the hallmarks of cancer. Glutamine is one of the most important nutrients that fuels the TCA cycle and therefore takes part in the production of energy. Glutamine is used as starting metabolite for the synthesis of nucleotides, fatty acids and non-essential amino acids. Since nutrients are uptaken from the blood stream, and considering the 3-dimensional state of solid tumors, access of nutrients is highly dependent on the location of individual cells within a tumor, which results in affecting their metabolic activity. This gives rise to two disctincts cell population: the ones that have access to nutrient and the ones that are nutrient-deprived. We studied the effect of the lack of glutamine by creating glutamine-resistent hepatocellular carcinoma cell lines chosen based on their epithelial (Hep3B) or mesenchymal phenotype (SNU-499 and HLF). We found that glutamine deprivation decreased the proliferation rate, clonogenicity and stemness frequency of the three cell lines but in a greater extent of the mesenchymal cells. Transcriptomic analysis performed in HLF cells showed that glutamine deprivation decreased the activation of signaling pathways involved in cell-cell junction, cell-extracellular matrix interactions and decreased the expression of the hallmarks of epithelial-to-mesenchymal transition. We therefore investigated the role of TGFβ, a master regulator of these three processes, by transcriptomic and functional analyses in epithelial (Hep3B) and mesenchymal cells (HLF). We found that the lack of glutamine strongly impared the activation of TGFβ signaling which correlated with an altered regulation of TGFβ target genes: the expression of mesenchymal genes was no longer induced by TGFβ while the epithelial genes were more strongly induced. Functional analyses showed that glutamine deprivation abolished the invasive capacities of HCCs and decreased cell adhesion. Altogehter, our results show that glutamine metabolism is necessary to maintain a mesenchymal phenotype and to maintain an efficient TGFβ signaling in hepatocellularcarcinoma.