Effects of Insulin and Glucose on Cellular Metabolic Fluxes in Homocysteine Transsulfuration, Remethylation, S-Adenosylmethionine Synthesis, and Global Deoxyribonucleic Acid Methylation

Background: The mechanisms underlying the impact of pathophysiological elevations in insulin or glucose on hepatic cellular homocysteine kinetics is not fully understood. Objective: The objective of the study was to investigate the impact of elevated insulin/glucose on hepatic homocysteine kinetics at the cellular level. Design and Methods: Effects of insulin and glucose on homocysteine remethylation and transsulfuration metabolic fluxes were investigated in a cell model using stable isotopic tracers and gas chromatography/mass spectrometry. The methylation status was assessed by S-adenosylmethionine (adoMet), the adoMet to S-adenosylhomocysteine ratio, DNA methyltransferase activity, and methylated cytidine content of DNA. The expression profile of homocysteine remethylation, transmethylation, and transsulfuration-associated genes was determined. Results: Insulin increased cellular homocysteine production primarily by its inhibition of transsulfuration. When cells were exposed to elevated insulin and glucose, homocysteine remethylation was enhanced, which consequently increased intracellular adoMet concentrations by inducing adoMet synthase activity. Elevated glucose further enhanced DNA methyltransferase activity that subsequently led to increased global DNA methylation. Conclusions: We demonstrated the novel finding of a direct promoting effect of high cellular insulin or glucose exposure on homocysteine remethylation, adoMet synthase activity, and adoMet synthesis. We also provided new evidence indicating that when hepatic tissue is exposed to elevated insulin or glucose, the cellular methylation balance can be altered, which may have potential epigenetic impacts gene regulation in diabetic individuals. These findings in a cell line may or may not reflect what happens in humans. In vivo studies on the homocysteine transmethylation fluxes and DNA methylation in diabetic state are underway. (J Clin Endocrinol Metab 94: 1017-1025, 2009)