Characterization of the 4 S polycyclic aromatic hydrocarbon-binding protein in human liver and cells

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.

Rat liver 4S-benzo[a]pyrene-binding protein is distinct from glycine N-methyltransferase

Rat liver 4S-benzo[a]pyrene-binding protein (BAP) was reported to be identical to the subunit (Mr 32,500) of the tetrameric glycine N-methyltransferase (GNMT). We have reevaluated this study. When a liver cytosol was subjected to Sephacryl S-200 gel permeation chromatography, enzyme activity as well as cross-reactivity with anti-GNMT antibody were found only at the elution position of the purified GNMT. Chromatograph of the cytosol pretreated with [3H]benzo[a]pyrene showed two peaks in the void volume and at the position of an approximate Mr of 40,000. The latter, corresponding to the 4 S BAP, did not cross-react with the anti-GNMT antibody. An extract of nuclei in which GNMT was proposed to act as a mediator of cytochrome P4501A1 gene expression contained the tetrameric GNMT but no binding activity. The lung, in which no GNMT mRNA occurred, had the 4 S BAP. Extracts of Escherichia coli and COS-7 cells expressing large amounts of GNMT showed no 4 S BAP. These findings suggest that the 4 S BAP is distinct from the subunit of GNMT.