Hepatic folate, methyl group, and homocysteine metabolism are interrelated pathways that when disrupted are associated with numerous pathologies. Maintenance of normal methyl group and homocysteine homeostasis is dependent on the balance between: S-adenosylmethionine (SAM)-dependent transmethylation, which utilizes methyl groups and produces homocysteine; remethylation of homocysteine back to methionine by folate-dependent and -independent mechanisms; and homocysteine catabolism via the transsulfuration pathway. Recent studies have demonstrated that hormonal imbalance is a factor in the control of key proteins that regulate these pathways. A diabetic state is characterized by increased expression of specific methyltransferases that utilize SAM-derived methyl groups and produce homocysteine. Although the supply of methyl groups from the folate-dependent 1-carbon pool appears to be diminished under diabetic conditions, the increased production of homocysteine is compensated for by stimulation of folate-independent remethylation and catabolism by transsulfuration, resulting in hypohomocysteinemia. Similar changes have been observed with glucocorticoid administration and in a growth hormone-deficient model, which can be prevented by insulin and growth hormone treatment, respectively. Taken together, these reports clearly indicate that hormonal regulation is a major factor in the metabolic control of folate, methyl groups, and homocysteine, thereby providing a potential link between the pathologies associated with these pathways and hormonal imbalance.

Homocysteine is a metabolic intermediate in methyl group metabolism that is dependent on a number of nutritional B-vitamin cofactors. An emerging aspect of homocysteine metabolism is its relation to health and disease. Perturbations of homocysteine metabolism, particularly intracellular and subsequently circulating accumulation of homocysteine (i.e., hyperhomocysteinemia), are associated with vascular disease risk, as well as other pathologies. However, intervention with B-vitamin supplementation has been shown to successfully restore normal homocysteine concentrations, but without concomitant reductions in disease risk. Thus, the mechanistic relation between homocysteine balance and disease states, as well as the value of homocysteine management, remains an area of intense investigation.

Numerous perturbations of methyl group and homocysteine metabolism have been documented as an outcome of diabetes. It has also been observed that there is a transition from hypo- to hyperhomocysteinemia in diabetes, often concurrent with the development of nephropathy. The objective of this study was to characterize the temporal changes in methyl group and homocysteine metabolism in the liver and kidney and to determine the impact these alterations have on DNA methylation in type 1 diabetic rats. Male Sprague-Dawley rats were injected with streptozotocin (60 mg/kg body weight) to induce diabetes and samples were collected at 2, 4, and 8 wk. At 8 wk, hepatic and renal betaine-homocysteine S-methyltransferase activities were greater in diabetic rats, whereas methionine synthase activity was lower in diabetic rat liver and kidney did not differ. Cystathionine beta-synthase abundance was greater in the liver but less in the kidney of diabetic rats. Both hepatic and renal glycine N-methyltransferase (GNMT) activity and abundance were greater in diabetic rats; however, changes in renal activity and/or abundance were present only at 2 and 4 wk, whereas hepatic GNMT was induced at all time points. Most importantly, we have shown that genomic DNA was hypomethylated in the liver, but not the kidney, in diabetic rats. These results suggest that diabetes-induced perturbations of methyl group and homocysteine metabolism lead to functional methyl deficiency, resulting in the hypomethylation of DNA in a tissue-specific fashion.

BackgroundAltered methyl group and homocysteine metabolism were tissue-specific, persistent, and preceded hepatic DNA hypomethylation in type 1 diabetic rats. Similar metabolic perturbations have been shown in the Zucker (type 2) diabetic fatty (ZDF) rat in the pre-diabetic and early diabetic stages, but tissue specificity and potential impact on epigenetic marks are unknown, particularly during pathogenesis.

Betaine homocysteine S-methyltransferase (BHMT) catalyzes the transfer of a methyl group from betaine to homocysteine forming dimethylglycine and methionine. We previously showed that inhibiting BHMT in mice by intraperitoneal injection of S-(α-carboxybutyl)-DL-homocysteine (CBHcy) results in hyperhomocysteinemia. In the present study, CBHcy was fed to rats to determine whether it could be absorbed and cause hyperhomocysteinemia as observed for the intraperitoneal administration of the compound in mice. We hypothesized that dietary administered CBHcy will be absorbed and will result in the inhibition of BHMT and cause hyperhomocysteinemia. Rats were meal-fed every 8 hours an L-amino acid-defined diet either containing or devoid of CBHcy (5 mg/meal) for 3 days. The treatment decreased liver BHMT activity by 90% and had no effect on methionine synthase, methylenetetrahydrofolate reductase, phosphatidylethanolamine N-methyltransferase and CTP:phosphocholine cytidylyltransferase activities. In contrast, cystathionine β-synthase activity and immunodetectable protein decreased (56 and 26%, respectively) and glycine N-methyltransferase activity increased (52%) in CBHcy-treated rats. Liver S-adenosylmethionine levels decreased by 25% in CBHcy-treated rats and S-adenosylhomocysteine levels did not change. Further, plasma choline decreased (22%) and plasma betaine increased (15-fold) in CBHcy-treated rats. The treatment had no effect on global DNA and CpG island methylation, liver histology and plasma markers of liver damage. We conclude that CBHcy mediated BHMT inhibition causes an elevation in total plasma homocysteine that is not normalized by the folate-dependent conversion of homocysteine to methionine. Further, metabolic changes caused by BHMT inhibition affect cystathionine β-synthase and glycine N-methyltransferase activities, which further deteriorate plasma homocysteine levels.