Interaction between dietary methionine and methyl donor intake on rat liver betaine-homocysteine methyltransferase gene expression and organization of the human gene.

We previously showed that rat liver betaine-homocysteine methyltransferase (BHMT) mRNA content and activity increased 4-fold when rats were fed a methionine-deficient diet containing adequate choline, compared with rats fed the same diet with control levels of methionine (Park, E. I., Renduchintala, M. S., and Garrow, T. A. (1997) J. Nutr. Biochem. 8, 541-545). A further 2-fold increase was observed in rats fed the methionine-deficient diet with supplemental betaine. The nutrition studies reported here were designed to determine whether other methyl donors would induce rat liver BHMT gene expression when added to a methionine-deficient diet and to define the relationship between the degree of methionine restriction and level of methyl donor intake on BHMT expression. Therefore, rats were fed amino acid-defined diets varying in methionine and methyl donor composition. The effect of diet on BHMT expression was evaluated using Northern, Western, and enzyme activity analyses. Similar to when betaine was added to a methionine-deficient diet, choline or sulfonium analogs of betaine induced BHMT expression. The diet-induced induction of hepatic BHMT activity was mediated by increases in the steady-state level of its mRNA and immunodetectable protein. Using methyl donor-free diets, we found that methionine restriction was required but alone not sufficient for the high induction of BHMT expression. Concomitant with methionine restriction, dietary methyl groups were required for high levels of BHMT induction, and a dose-dependent relationship was observed between methyl donor intake and BHMT induction. Furthermore, the severity of methionine restriction influenced the magnitude of BHMT induction. To study the molecular mechanisms that regulate the expression of BHMT, we have cloned the human BHMT gene. This gene spans about 20 kilobases of DNA and contains 8 exons and 7 introns. Using RNA isolated from human liver and hepatoma cells, a major transcriptional start site has been mapped using the 5' rapid amplification of cDNA ends technique, and this start site is 26 nucleotides downstream from a putative TATA box.