Betaine attenuates alcoholic steatosis by restoring phosphatidylcholine generation via the phosphatidylethanolamine methyltransferase pathway.

BACKGROUND/AIMS: Previous studies in our laboratory implicated ethanol-induced decreases in hepatocellular S-adenosylmethionine to S-adenosylhomocysteine (SAM:SAH) ratios in lowering the activity of phosphatidylethanolamine methyltransferase (PEMT), which is associated with the generation of steatosis. Further in vitro studies showed that betaine supplementation could correct these alterations in the ratio as well as attenuate alcoholic steatosis. Therefore, we sought to determine whether the protective effect of betaine is via its effect on PEMT activity. METHODS: Male Wistar rats were fed the Lieber DeCarli control or ethanol diet with or without 1% betaine supplementation for 4 weeks. RESULTS: We observed that ethanol feeding resulted in decreased phosphatidylcholine (PC) production by a PEMT-catalyzed reaction. Betaine supplementation corrected the ethanol-induced decrease in hepatic SAM:SAH ratios and by normalizing PC production via the PEMT-mediated pathway, significantly reduced fatty infiltration associated with ethanol consumption. This restoration of hepatocellular SAM:SAH ratio by betaine supplementation was associated with increases in the activity, enzyme mass and gene expression of the enzyme, betaine homocysteine methyltransferase (BHMT), that remethylates homocysteine. CONCLUSIONS: Betaine, by virtue of promoting an alternate remethylation pathway, restores SAM:SAH ratios that, in turn, correct the defective cellular methylation reaction catalyzed by PEMT resulting in protection against the generation of alcoholic steatosis.

Role of elevated S-adenosylhomocysteine in rat hepatocyte apoptosis: protection by betaine.

Previous studies from our laboratory have shown that ethanol consumption results in an increase in hepatocellular S-adenosylhomocysteine levels. Because S-adenosylhomocysteine is a potent inhibitor of methylation reactions, we propose that increased intracellular S-adenosylhomocysteine levels could be a major contributor to ethanol-induced pathologies. To test this hypothesis, hepatocytes isolated from rat livers were grown on collagen-coated plates in Williams' medium E containing 5% FCS and exposed to varying concentrations of adenosine in order to increase intracellular S-adenosylhomocysteine levels. We observed increases in caspase-3 activity following exposure to adenosine. This increase in caspase activity correlated with increases in intracellular S-adenosylhomocysteine levels and DNA hypoploidy. The adenosine-induced changes could be significantly attenuated by betaine administration. The mechanism of betaine action appeared to be via the methylation reaction catalyzed by betaine-homocysteine-methyltransferase. To conclude, our results indicate that the elevation of S-adenosylhomocysteine levels in the liver by ethanol is a major factor in altering methylation reactions and in increasing apoptosis in the liver. We conclude that ethanol-induced alteration in methionine metabolic pathways may play a crucial role in the pathologies associated with alcoholic liver injury and that betaine administration may have beneficial therapeutic effects.

A comparison of the effects of betaine and S-adenosylmethionine on ethanol-induced changes in methionine metabolism and steatosis in rat hepatocytes.

Previous studies showed that chronic ethanol administration alters methionine metabolism in the liver, resulting in increased intracellular S-adenosylhomocysteine (SAH) levels and increased homocysteine release into the plasma. We showed further that these changes appear to be reversed by betaine administration. This study compared the effects of betaine and S-adenosylmethionine (SAM), another methylating agent, on ethanol-induced changes of methionine metabolism and hepatic steatosis. Wistar rats were fed ethanol or control Lieber-Decarli liquid diet for 4 wk and metabolites of the methionine cycle were measured in isolated hepatocytes. Hepatocytes from ethanol-fed rats had a 50% lower intracellular SAM:SAH ratio and almost 2-fold greater homocysteine release into the media compared with controls. Supplementation of betaine or SAM in the incubation media increased this ratio in hepatocytes from both control and ethanol-fed rats and attenuated the ethanol-induced increased hepatocellular triglyceride levels by approximately 20%. On the other hand, only betaine prevented the increase in generation of homocysteine in the incubation media under basal and methionine-loaded conditions. SAM can correct only the ratio and the methylation defects and may in fact be detrimental after prolonged use because of its propensity to increase homocysteine release. Both SAM and betaine are effective in increasing the SAM:SAH ratio in hepatocytes and in attenuating hepatic steatosis; however, only betaine can effectively methylate homocysteine and prevent increased homocysteine release by the liver.

Betaine lowers elevated s-adenosylhomocysteine levels in hepatocytes from ethanol-fed rats.

Previous studies showed that chronic ethanol administration inhibits methionine synthase activity, resulting in impaired homocysteine remethylation to form methionine. This defect in homocysteine remethylation was shown to increase plasma homocysteine and to interfere with the production of hepatic S-adenosylmethionine (SAM) in ethanol-fed rats. These changes were shown to be reversed by the administration of betaine, an alternative methylating agent. This study was undertaken to determine additional effects of ethanol on methionine metabolism and their functional consequences. The influences of methionine loading and betaine supplementation were also evaluated. Adult Wistar rats were fed ethanol or a control Lieber-DeCarli liquid diet for 4 wk, and metabolites of the methionine cycle were measured in vitro in isolated hepatocytes under basal and methionine-supplemented conditions. S-Adenosylhomocysteine (SAH) concentrations were elevated in hepatocytes isolated from ethanol-fed rats compared with controls and in hepatocytes from both groups when supplemented with methionine. The addition of betaine to the methionine-supplemented incubation media reduced the elevated SAH levels. The decrease in the intracellular SAH:SAM ratio due to ethanol consumption inhibited the activity of the liver-specific SAM-dependent methyltransferase, phosphatidylethanolamine methyltransferase. Our data indicate that betaine, by remethylating homocysteine and removing SAH, overcomes the detrimental effects of ethanol consumption on methionine metabolism and may be effective in correcting methylation defects and treating liver diseases.

Methionine synthase. a possible prime site of the ethanolic lesion in liver.

Among the most important pathways for liver integrity in the body are the two that synthesize methionine and S-adenosylmethionine (SAM) through methylation of homocysteine. Results of studies in this laboratory have demonstrated ethanolic inhibition of one of these pathways catalyzed by methionine synthetase. It has been shown elsewhere that alcohol per se does not inhibit the enzyme, but that the metabolite of ethanol, acetaldehyde, is responsible through the formation of an inhibiting covalent adduct. Because hepatic SAM has been shown to be essential in the transport of fat from the liver, avoiding steatosis and further liver damage, it is entirely feasible that this repression of methionine synthase is an important site of the injurious action of alcohol metabolism in the liver. This loss of activity is particularly important in human beings who cannot produce methionine and SAM by means of the alternate pathway mediated by betaine:homocysteine:transmethylase, because of the lack of production of the betaine substrate for this enzyme.