GNMT Expression Increases Hepatic Folate Contents and Folate-Dependent Methionine Synthase-Mediated Homocysteine Remethylation

Abstract

Glycine N-methyltransferase (GNMT) is a major hepatic enzyme that converts S-adenosylmethionine to S-adenosylhomocysteine while generating sarcosine from glycine, hence it can regulate mediating methyl group availability in mammalian cells. GNMT is also a major hepatic folate binding protein that binds to, and, subsequently, may be inhibited by 5-methyltetrafolate. GNMT is commonly diminished in human hepatoma; yet its role in cellular folate metabolism, in tumorigenesis and antifolate therapies, is not understood completely. In the present study, we investigated the impacts of GNMT expression on cell growth, folate status, methylfolate-dependent reactions and antifolate cytotoxicity. GNMT-diminished hepatoma cell lines transfected with GNMT were cultured under folate abundance or restriction. Folate-dependent homocysteine remethylation fluxes were investigated using stable isotopic tracers and gas chromatography/mass spectrometry. Folate status was compared between wild-type (WT), GNMT transgenic (GNMT tg ) and GNMT knockout (GNMT ko ) mice. In the cell model, GNMT expression increased folate concentration, induced folate-dependent homocysteine remethylation, and reduced antifolate methotrexate cytotoxicity. In the mouse models, GNMT tg had increased hepatic folate significantly, whereas GNMT ko had reduced folate. Liver folate levels correlated well with GNMT expressions (r = 0.53, P = 0.002); and methionine synthase expression was reduced significantly in GNMT ko , demonstrating impaired methylfolate-dependent metabolism by GNMT deletion. In conclusion, we demonstrated novel findings that restoring GNMT assists methylfolate-dependent reactions and ameliorates the consequences of folate depletion. GNMT expression in vivo improves folate retention and bioavailability in the liver. Studies on how GNMT expression impacts the distribution of different folate cofactors and the regulation of specific folate dependent reactions are underway.

Sarcosine as a Potential Prostate Cancer Biomarker—A Review

“…Glycine N-methyltransferase (GNMT) acts as an essential component that influences synthesis of sarcosine [29–32]. Synthesis of GNMT is controlled by the same gene named GNMT .…”

A novel role of the tumor suppressor GNMT in cellular defense against DNA damage

“…Among all forms of folates, the 5‐methyl tetrahydrofolate (5‐methyl‐THF) is the most abundant . Although GNMT binds to 5‐methyl‐THF and can subsequently be inhibited by 5‐methyl‐THF, we have demonstrated that GNMT‐bound‐5‐methyl‐THF is bioavailable for methylfolate‐dependent homocysteine remethylation . Mice with Gnmt deletion have poor folate status; and cells without GNMT expression exhibited more cytotoxicity in response to antifolate drugs.…”

“…The Gnmt knockout mouse model was established by gene‐targeting disruption as described previously . Nullizygotes (Gnmt −/− ) had undetectable GNMT protein expression, and the heterozygotes (Gnmt +/− ) had ∼50% of GNMT expression compared to their wild‐type) littermates (Gnmt +/+ ).…”

“…Mice with Gnmt deletion have poor folate status; and cells without GNMT expression exhibited more cytotoxicity in response to antifolate drugs. Restoring GNMT not only assists methylfolate‐dependent reactions but also ameliorates the consequences of folate depletion in GNMT‐expressing tissues …”

Long-Term Prednisolone Treatments Increase Bioactive Vitamin B6Synthesis In Vivo

“…In study II, we used an amino acid-defined diet to carefully control dietary supplies of amino acid and vitamins. This approach is also helpful when we need to compare results among different studies on folate and vitamin metabolism (Wang et al, 2011). In study II, mice were fed a modified Clifford/Koury amino acid-defined rodent diet containing the recommended daily allowance for vitamin B 6 for rodents (7 mg/Kg pyriodoxine-HCl) (Dyets).…”