Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 22, 2012
Publication Date: October 19, 2012
Citation: Jackson, M.I., Cao, J.J., Zeng, H., Uthus, E.O., Combs, G.F. 2012. S-Adenosylmethionine-dependent protein methylation Is required for expression of selenoprotein P and gluconeogenic enzymes in HepG2 human hepatocytes. Journal of Biological Chemistry. 287(43):36455-36464. Interpretive Summary: Hepatic methylation is a process critical to several physiological functions, including control of expression of gluconeogenic enzymes important to glucose production. Selenoprotein P (SEPP1) is a protein known to be involved in the progression of Type-II Diabetes, and to be regulated on a cellular level similar to gluconeogenic enzymes. This work has determined that methylation serves as a nexus of control for the unified coordination of SEPP1 and gluconeogenic enzyme expression. Further it shows that disrupting methionine amino acid transmethylation has the potential to be a leverage point for therapeutic intervention in diseases such a Type-II diabetes which present hyperglycemia due to disregulated gluconeogenesis.
Technical Abstract: Cellular methylation processes enable expression of gluconeogenic enzymes and metabolism of the nutrient selenium (Se). Se status may relate to type-II diabetes and plasma levels of selenoprotein P (SEPP1) are positively correlated with insulin resistance. Increased expression of gluconeogenic enzymes glucose-6-phosphatase (G6PC) and phosphoenolpyruvate carboxykinase 1 (PCK1) has negative consequences for blood-sugar management in type-II diabetics. Transcriptional regulation of SEPP1 is directed by the same factors as those controlling the expression of G6PC and PCK1, and those transcription factors are methylated on arginine residues to increase activity. Here we sought to determine whether expression of SEPP1 and the aforementioned glucoconeogenic enzymes is regulated by relative concentrations of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH). A human hepatocyte cell line, HepG2, was treated with periodate-oxidized adenosine (POA) and other mechanistically distinct inhibitors of SAH hydrolase (AHCY) known to reduce methylation capacity, or was induced to form SAH prior to analysis of G6PC, PCK1 and SEPP1 mRNA. Inhibiting AHCY or increasing SAH decreased SAM/SAH ratio, protein-arginine methylation and the expression of SEPP1, G6PC and PCK1 transcripts. Further, hormonal induction of gluconeogenic enzymes was reduced by inhibition of methylation capacity. When protein-arginine methyltransferase 1 expression was reduced by siRNA-treatment, G6PC expression was inhibited. This work demonstrates that hepatocellular SAM-dependent methylation regulates both SEPP1 and gluconeogenic enzyme expression, and that inhibition of protein arginine methylation might provide a route to therapeutic interventions in Type-II diabetes.