Nicotinamide N-Methyltransferase Interacts with Enzymes of the Methionine Cycle and Regulates Methyl Donor Metabolism

Methyl donor balance is critical for epigenetic regulation in cells and is maintained by the so-called methionine cycle proteins that regenerate S-adenosylmethionine (SAM), the universal methyl donor, from homocysteine formed by the activity of methyltransferases. Nnmt is a liver enzyme that methylates nicotinamide, but its role in regulating methyl donor balance in the liver is unclear. In this study, we assessed the effect of altered Nnmt expression on various aspects of methyl donor metabolism in the liver. We found that Nnmt overexpression decreased SAM levels and the SAM/ S-adenosylhomocysteine (SAH) ratio both in vivo and in vitro. Nnmt knockdown did not change methyl donor balance in mouse primary hepatocytes but increased SAM levels and the SAM/SAH ratio when Gnmt, the dominantly expressed methyltransferase in liver, was simultaneously knocked down. Paradoxically, expression of enzymatically deficient Nnmt increased the SAM/SAH ratio, suggesting that Nnmt can regulate methyl donor balance independent of its methyltransferase activity. Proteomics analysis of Nnmt-interacting proteins in the liver identified Bhmt, Mat1a, and Ahcy, all components of the methionine cycle, and functional experiments showed that mutant Nnmt increased the level of remethylation of homocysteine to SAM. In summary, we show that the function of Nnmt in hepatic methyl donor balance is multifactorial. On one hand, Nnmt decreases methyl donor balance, consistent with its activity as a methyltransferase consuming methyl donors. On the other hand, by co-opting the enzymes of the methionine cycle, Nnmt aids the recycling of homocysteine to SAM for another round of methylation.