Characterization of the GNMT-HectH9-PREX2 tripartite relationship in the pathogenesis of hepatocellular carcinoma

The pathogenesis of hepatocellular carcinoma (HCC) involves many molecular pathways. Glycine N-methyltransferase (GNMT) is downregulated in almost all HCC and its gene knockout mice developed HCC with high penetrance. We identified PREX2, a novel PTEN inhibitor, as a GNMT-interacting protein. Such interaction enhanced degradation of PREX2 through an E3 ligase HectH9-mediated proteasomal ubiquitination pathway. Depletion of GNMT or HectH9 resulted in AKT activation in a PREX2 dependent manner and enhanced cell proliferation. An elevated PREX2 protein expression accompanied by activation of AKT was observed in the liver of Gnmt knockout mice. PREX2 protein expression was upregulated in 54.9% of human HCC samples, while its mRNA level was comparable in tumor and tumor-adjacent tissue, suggesting a post-translational alteration of PREX2 expression. Higher level of PREX2 in the tumor tissues was associated with poorer survival. These results reveal a novel mechanism in which GNMT participates in AKT signaling and HCC tumorigenesis by promoting HectH9-mediated PREX2 degradation. What's new? Hepatocellular carcinoma (HCC) is associated with various molecular anomalies, including downregulation of glycine N-methyltransferase (GNMT), an enzyme with metabolic functions in the liver. Our study sheds light on the enigmatic role of GNMT deficiency in liver tumorigenesis, showing that GNMT interacts with PREX2, an oncogenic regulator of PTEN/PI3K/AKT signaling. In cell and animal models, GNMT interaction enhanced HectH9-mediated PREX2 degradation, while its depletion was associated with elevated post-translational PREX2 expression. Increased PREX2 was further linked to poor survival in HCC patients. Hence, GNMT downregulation appears to contribute to HCC via reduced PREX2 ubiquitination, consequent AKT signaling and dysregulated cell proliferation.