Progress 12/01/00 to 11/30/04
Outputs
We have shown that all-trans-retinoic acid (RA) induced the hepatic enzyme glycine N-methyltransferase (GNMT), a key regulatory protein that controls both folate and methionine metabolism to optimize the availability of methyl groups for transmethylation reactions. Retinoid-mediated alterations in GNMT were liver-specific and resulted in a diminished ability to methylate DNA, an epigenetic process that is critical in a number of pathological states. Moreover, we have shown that the minimum dose of RA required to perturb methyl group metabolism was well within the level used therapeutically by humans. RA also enhances the catabolism of methionine and the folate-dependent remethylation of homocysteine. We have extended our RA findings to explore a gluconeogenic state, namely treatment with the glucocorticoid dexamethasone or a diabetic condition. Treatment of rats or hepatic cells in culture with dexamethasone perturbed GNMT similar to, but independent of RA. GNMT was
also induced using a streptozotocin-induced diabetic rat model, a condition that promoted folate-independent remethylation of homocysteine. It appeared that the catabolism of homocysteine was also enhanced, and that treatment of diabetic rats with RA prevented many of the changes observed in homocysteine metabolism in diabetic-only animals. Recently, we have found that the induction of GNMT protein in diabetic rats was associated with increased abundance of GNMT mRNA, indicating that the regulation of this protein was at the transcriptional/translational level. Moreover, the changes in methyl and homocysteine metabolism were exacerbated under dietary folate restriction, whereas inclusion of folate in the diet attenuated many of the observed changes. This data with respect to diabetes and perturbation of methyl group metabolism is in contrast to our recent report examining these pathways, with and without RA administration, using a hyperthyroid rat model, as alterations in thyroid
status have been reported to disrupt the normal metabolism of folate and methyl groups. Treatment with triiodothyronine prevented the increase in GNMT activity, but only partially attenuated the accumulation of GNMT protein, indicating that its ability to restore normal metabolism of methyl groups was at a posttranslational level, possibly by altering the levels of the known folate coenzymes that allosterically regulate GNMT activity. Taken together, these results clearly indicate that physiologic/hormonal conditions (i.e., gluconeogenic; hyperthyroid) as well as the administration of RA have profound adverse effects on hepatic methyl group metabolism and exhibit a distinct interaction when manipulated together. Because these pathways are vital in maintenance of health, understanding how nutritional and hormonal factors is important in preventing potential pathologies associated with their disruption. The results of this research indicates that the dietary recommendations with respect
to ensuring optimal folate function and methyl group metabolism may be significantly influenced by potential changes in hormonal profiles, such as diabetes or altered thyroid function.
Impacts
The proper metabolism of folate and methyl groups is critical to health, as perturbation of these pathways is associated with a number of pathologies, including cardiovascular disease, cancer development, and birth defects. Understanding how hormonal and nutritional factors, such as retinoid compounds and/or a diabetic state, perturb these metabolic pathways is important for future dietary recommendations directed at optimizing human health and preventing disease. Moreover, this research will begin to identify individuals that are more sensitive to moderate nutritional deficiencies, hormonal imbalances, or retinoid toxicity.
Publications
- Tanghe, K.A., Garrow, T.A. & Schalinske, K.L. (2004) Triiodothyronine treatment attenuates the induction of hepatic glycine N-methyltransferase by retinoic acid and elevates plasma homocysteine concentrations in rats. J. Nutr. 134: 2913-2918.
- Nieman, K.M., Rowling, M.J., Garrow, T.A. & Schalinske, K.L. (2004) Modulation of methyl group metabolism by streptozotocin-induced diabetes and all-trans-retinoic acid. J. Biol. Chem. 279: 45708-45712.
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Progress 01/01/03 to 12/31/03
Outputs
To date, we have shown that all-trans-retinoic acid (ATRA) induced the hepatic enzyme glycine N-methyltransferase (GNMT), a key regulatory protein that controls both folate and methionine metabolism to optimize the availability of methyl groups for transmethylation reactions. Retinoid-mediated alterations in GNMT were liver-specific and resulted in a diminished ability to methylate DNA, an epigenetic process that is critical in a number of pathological states. During the past year, we have extended these findings on several fronts, in particular linking perturbation of methyl group metabolism with a diabetic condition. First, the induction of GNMT by ATRA was demonstrated at levels equivalent to those utilized therapeutically by humans. Second, treatment with ATRA resulted in a decrease in both plasma homocysteine and methionine concentrations, indicating that the catabolism of homocysteine was enhanced by retinoids and perturbation of hepatic methyl group metabolism
may compromise the availability of methyl groups for other cells and tissues. Moreover, the reduction of homocysteine appears to be largely due to an increase in folate-dependent remethylation of homocysteine, whereas folate-independent remethylation was not affected. Third, the induction of GNMT by ATRA was exacerbated in both diabetic rats and rats treated with the glucocorticoid, dexamethasone. We have confirmed this finding in a cell culture model, indicating that the ability of ATRA or diabetes/ glucocorticoids to perturb methyl group metabolism was direct and mechanistically distinct from each other. In support of this finding, we have found that a diabetic condition enhances homocysteine metabolism by increasing catabolism through the transsulfuration pathway and enhancing folate-independent remethylation. Fourth, we have found that supplementation of folate-deficient rats with dietary folate attenuated the increase in GNMT activity, but ATRA did not prevent the
hyperhomocysteinemia owing to folate deficiency. Fifth, we have found that treatment of ATRA-treated rats with thyroid hormone prevented the increase in GNMT activity, but not induction of GNMT protein, indicating that triiodothyronine was likely altering the folate coenzyme levels known to inhibit GNMT activity at a posttranslational level. Taken together, these results clearly indicate that the administration of ATRA and a diabetic condition have profound adverse effects on hepatic methyl group metabolism, thereby having significant implications for millions of individuals that are diabetic and/or receive therapeutic retinoids.
Impacts
The proper metabolism of folate and methyl groups is critical to health, as perturbation of these pathways is associated with a number of pathologies, including cardiovascular disease, cancer development, and birth defects. Understanding how nutrients and/or physiological conditions, such as retinoid compounds and a diabetic state, perturb these metabolic pathways is important for future dietary recommendations directed at optimizing human health and preventing disease. Moreover, this research will begin to identify individuals that are more sensitive to retinoid toxicity due to unsupervised or therapeutic use of retinoid compounds or hormonal imbalances.
Publications
- Rowling, M.J. & Schalinske, K.L. (2003) Retinoic acid and glucocorticoid treatment induces hepatic glycine N-methyltransferase and lowers plasma homocysteine concentrations in the rat and rat hepatoma cells. J. Nutr. 133: 3392-3398.
- Schalinske, K.L. (2003) Interrelationship between diabetes and homocysteine metabolism: hormonal regulation of cystathionine b-synthase. Nutr. Rev. 61: 136-138.
- Ozias, M.K. & Schalinske, K.L. (2003) All-trans-retinoic acid rapidly induces glycine N-methyltransferase in a dose-dependent manner and reduces circulating methionine and homocysteine levels in rats. J. Nutr. 133: 4090-4094.
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Progress 01/01/02 to 12/31/02
Outputs
During year 1, we demonstrated that the administration of retinoid compounds (30 mmol/kg body weight) to rats profoundly altered the metabolism of methionine in the liver. In particular, all-trans-retinoic acid (ATRA) induced the enzyme glycine N-methyltransferase (GNMT), a key regulatory protein that controls both folate and methionine metabolism to optimize the availability of methyl groups for transmethylation reactions. Retinoid-mediated alterations in GNMT were liver-specific and resulted in a diminished ability to methylate DNA, an epigenetic process that is critical in a number of pathological states. We have extended these findings on several fronts. First, the induction of GNMT by ATRA was dose-dependent as more physiological levels of retinoid compounds (5 mmol/kg body weight) were also effective. This is a key finding, as it demonstrates that retinoid levels that are achievable by supplementation and/or therapeutic uses are sufficient to perturb methyl
group metabolism. Second, treatment with ATRA resulted in a decrease in the plasma homocysteine concentrations, indicating that the catabolism of homocysteine was enhanced by retinoids. The enhanced catabolism of homocysteine was also evident by the increase in taurine synthesis, at the expense of plasma glutathione and urinary inorganic sulfate. Third, the induction of GNMT by ATRA was exacerbated in diabetic rats, a condition known to perturb homocysteine metabolism. Fourth, increasing the protein (i.e., casein) content of the diet exacerbated the induction of GNMT by ATRA; however, substituting soy protein for casein attenuated this effect. This indicates that the level of dietary cysteine may serve as a viable intervention strategy to abrogate the adverse effects of retinoid administration on methyl group and homocysteine metabolism. Taken together, these results clearly indicate that the administration of ATRA has profound adverse effects on hepatic methyl group metabolism,
thereby having significant implications for liver toxicity, cancer development, cardiovascular disease, and birth defects.
Impacts
The proper metabolism of folate and methyl groups is critical to health, as perturbation of these pathways is associated with a number of pathologies, including hepatotoxicity and cancer development. Understanding how nutrients such as retinoid compounds perturb these metabolic pathways is important for future dietary recommendations directed at optimizing human health and preventing disease. Moreover, this research will begin to identify individuals that are more sensitive to retinoid toxicity due to unsupervised or therapeutic use of retinoid compounds.
Publications
- Rowling MJ, MH McMullen and KL Schalinske. 2002. Vitamin A and its derivatives induce hepatic glycine N-methyltransferase and hypomethylation of DNA in rats. J. Nutr. 132:365-369.
- McMullen MH, MJ Rowling, MK Ozias and KL Schalinske. 2002. Activation and induction of glycine N-methyltransferase by retinoids are tissue- and gender-specific. Arch. Biochem. Biophys. 401:73-80.
- Rowling MJ, MH McMullen, DC Chipman and KL Schalinske. 2002. Hepatic glycine N-methyltransferase is up-regulated by methionine in rats. J. Nutr. 132:2545-2549.
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Progress 01/01/01 to 12/31/01
Outputs
The oral administration of retinoid compounds to rats had profound effects on the hepatic metabolism of methyl groups. All-trans-retinoic acid and 13-cis-retinoic acid elevated the hepatic activity of glycine N-methyltransferase more than 2-fold following a 10-day treatment at 30 mmol retinoid/kg body weight. Moreover, changes in GNMT activity were reflected in the abundance of the protein, suggesting that retinoids mediate GNMT modulation at a transcriptional/translational level. GNMT is a key protein that regulates folate and methyl group metabolism, two interrelated metabolic pathways that are important in preventing liver disease and cancer development. The administration of vitamin A (i.e., retinyl palmitate) was also effective in inducing modulation of GNMT, but to a lesser degree. The ability of retinoids to modulate changes in methyl group metabolism appears to be tissue- and gender-specific. The activity and abundance of GNMT was not altered in renal or
pancreatic tissue, the only other known localizations of the protein. Hepatic GNMT activity and abundance was elevated by retinoids in female as well as male rats, however, the degree to which it was increased was significantly less in female rats. Furthermore, the ability of retinoid compounds to alter methyl group metabolism does not appear to be dependent on adrenal function, as has been shown for retinoid-mediated alterations in lipid metabolism. The retinoid-mediated increase in GNMT function and subsequent loss of methyl groups was evident for a number of transmethylation-dependent processes. Rats receiving all-trans-retinoic acid exhibited a significant decrease in DNA methylation and creatinine synthesis. In contrast, glutathione synthesis, a product of transmethylation and transsulfuration, was not compromised. Taken together, these results clearly indicate that retinoid compounds, including vitamin A, has a profound affect on hepatic methyl group metabolism. These results
are significant, because alterations in this pathway are known to be associated with hepatic steatosis, liver disease, and neoplastic development.
Impacts
Abnormal functioning of methyl group metabolism is associated with liver disease and cancer development, as well as cardiovascular disease and birth defects. Understanding the interaction between retinoid compounds and methyl group metabolism will impact future dietary recommendations directed towards optimal health. Moreover, it will provide a basis for the evaluation of current and new therapeutic retinoid compounds.
Publications
- Rowling MJ and KL Schalinske. 2001. Retinoid compounds activate and induce hepatic glycine N-methyltransferase in rats. J. Nutr. 131:1914-1917.
- Rowling MJ, MH McMullen and KL Schalinske. 2002 Vitamin A and its derivatives induce hepatic glycine N-methyltransferase and hypomethylation of DNA in rats. J. Nutr. 132:(in press).
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