Glycine-N methyltransferase (GNMT) is a potential
tumor suppressor that is commonly inactivated in human
hepatoma. We systematically investigated how GNMT regulates methyl group kinetics and global DNA methylation. HepG2 cells (GNMT inactive, GNMT-) and cells transfected with GNMT expressed vector (GNMT+) were cultured in low (10 μmol/L), adequate (100 μmol/L), or high (500 μmol/L)
l-methionine, each with 2.27 μmol/L
folate. Transmethylation kinetics were studied using stable isotopic tracers and GC-MS. Methylation status was determined by
S-adenosylmethionine (SAM) and
S-adenosylhomocysteine (SAH) levels, SAM:SAH ratio,
DNA methyltransferase (DNMT) activity, and methylated
cytidine levels in
DNA. Compared with GNMT- cells, GNMT+ cells had lower
homocysteine and greater
cysteine concentrations. GNMT expression increased
methionine clearance by inducing
homocysteine transsulfuration and remethylation metabolic fluxes when cells were cultured in high or adequate
l-methionine. In contrast,
homocysteine remethylation flux was lower in GNMT+ cells than in GNMT- cells and
homocysteine transsulfuration fluxes did not differ when cells were cultured in low
methionine, suggesting that normal GNMT function helps to conserve methyl groups. Furthermore, GNMT expression decreased SAM and increased SAH levels and reduced DNMT activity in high or adequate, but not low,
methionine cultures. In low
methionine cultures, restoring GNMT in HepG2 cells did not lead to
sarcosine synthesis, which would waste methyl groups. Methylated
cytidine levels were significantly lower in GNMT- cells than in GNMT+ cells. In conclusion, we have shown that GNMT affects transmethylation kinetics and SAM synthesis and facilitates the conservation of methyl groups by limiting
homocysteine remethylation fluxes.