Modulation of Methyl Group Metabolism by Streptozotocin-induced Diabetes and All-trans-retinoic Acid

Abstract

The hepatic enzyme glycine N-methyltransferase (GNMT) plays a major role in the control of methyl group and homocysteine metabolism. Because disruption of these vital pathways is associated with numerous pathologies, understanding GNMT control is important for evaluating methyl group regulation. Recently, gluconeogenic conditions have been shown to modulate homocysteine metabolism and treatment with glucocorticoids and/or all-trans-retinoic acid (RA)-induced active GNMT protein, thereby leading to methyl group loss. This study was conducted to determine the effect of diabetes, alone and in combination with RA, on GNMT regulation. Diabetes and RA increased GNMT activity 87 and 148%, respectively. Moreover, the induction of GNMT activity by diabetes and RA was reflected in its abundance. Cell culture studies demonstrated that pretreatment with insulin prevented GNMT induction by both RA and dexamethasone. There was a significant decline in homocysteine concentrations in diabetic rats, owing in part to a 38% increase in the abundance of the transsulfuration enzyme cystathionine ␤-synthase; treatment of diabetic rats with RA prevented cystathionine ␤-synthase induction. A diabetic state also increased the activity of the folate-independent homocysteine remethylation enzyme betaine-homocysteine S-methyltransferase, whereas the activity of the folatedependent enzyme methionine synthase was diminished 52%. In contrast, RA treatment attenuated the streptozotocin-mediated increase in betaine-homocysteine Smethyltransferase, whereas methionine synthase activity remained diminished. These results indicate that both a diabetic condition and RA treatment have marked effects on the metabolism of methyl groups and homocysteine, a finding that may have significant implications for diabetics and their potential sensitivity to retinoids.

Exercise prevents hyperhomocysteinemia in a dietary folate-restricted mouse model

“…This shift is also reflected in other 270 gluconeogenic states, such as diabetes. We and others have demonstrated that a diabetic state or 271 administration of synthetic glucocorticoid compounds has a major impact on methyl group and 272 homocysteine metabolism [23][24][25][26][49][50][51][52]. A consistent finding from these reports is a reduction 273 in circulating homocysteine concentrations owing to an increase in folate-independent 274 remethylation (i.e., BHMT) and catabolism of homocysteine through the transsulfuration 275 pathway.…”

“…This was based on our previous 95 research demonstrating that a gluconeogenic state and related hormonal alterations, similar to 96 what is exhibited as a function of exercise, results in reduced homocysteine concentrations via 97 enhanced folate-independent remethylation of homocysteine, as well as increased catabolism 98 [23][24][25][26] initially housed in groups of 2 or 3 in a 12-h light: dark cycle and provided an AIN-93G semi-120 purified diet (Table 1) and water ad libitum [28]. No antibiotics were added to the diets or 121 drinking water, resulting in a moderate degree of folate deficiency as we have previously 122…”

“…reported [24]. After an acclimation period of 3 d, mice were randomly assigned to one of 2 123 groups: sedentary or free-access wheel exercised.…”

“…Euthanasia consisted of exsanguination and removal of major organs for their subsequent 131 processing as described previously [23][24][25][26]. Heparinized whole blood was collected via cardiac 132 puncture, centrifuged at 4,000 g for 6 min, and plasma was stored at -20°C for subsequent 133 homocysteine analysis.…”

Homocysteine metabolism and its relation to health and disease

“…A GNMT knockout mouse model was characterized by extremely high levels of methionine and SAM in the circulation (15); however, plasma homocysteine concentrations were not reported. Although overexpression of GNMT might be expected to result in excessive production of homocysteine, various rodent models exhibiting a marked increase in GNMT abundance and activity did not translate into similar changes in the circulating concentrations of homocysteine (16)(17)(18)(19). In fact, homocysteine concentrations were actually diminished in these studies, owing to the increased activity of other enzymes involved in homocysteine remethylation and catabolism.…”