title: The methyl donor S-Adenosylmethionine inhibits active demethylation of DNA: a candidate novel mechanism for the pharmacological effects of S-Adenosylmethionine.
abstract: S-Adenosylmethionine (AdoMet) is the methyl donor of numerous methylation reactions. The current model is that an increased concentration of AdoMet stimulates DNA methyltransferase reactions, triggering hypermethylation and protecting the genome against global hypomethylation, a hallmark of cancer. Using an assay of active demethylation in HEK 293 cells, we show that AdoMet inhibits active demethylation and expression of an ectopically methylated CMV-GFP (green fluorescent protein) plasmid in a dose-dependent manner. The inhibition of GFP expression is specific to methylated GFP; AdoMet does not inhibit an identical but unmethylated CMV-GFP plasmid. S-Adenosylhomocysteine (AdoHcy), the product of methyltransferase reactions utilizing AdoMet does not inhibit demethylation or expression of CMV-GFP. In vitro, AdoMet but not AdoHcy inhibits methylated DNA-binding protein 2/DNA demethylase as well as endogenous demethylase activity extracted from HEK 293, suggesting that AdoMet directly inhibits demethylase activity, and that the methyl residue on AdoMet is required for its interaction with demethylase. Taken together, our data support an alternative mechanism of action for AdoMet as an inhibitor of intracellular demethylase activity, which results in hypermethylation of DNA.
authors.affiliation: Department of Pharmacology and Therapeutics, McGill University, Montreal, Quebec H3G 1YG, Canada.
authors.forename: Nancy
authors.initials: N
authors.lastname: Detich
authors.forename: Stefan
authors.initials: S
authors.lastname: Hamm
authors.forename: George
authors.initials: G
authors.lastname: Just
authors.forename: J David
authors.initials: JD
authors.lastname: Knox
authors.forename: Moshe
authors.initials: M
authors.lastname: Szyf
mesh_terms: D002478:Cells, Cultured; D019175:DNA Methylation / Q000187:drug effects*; D015254:DNA Modification Methylases / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015870:Gene Expression / Q000187:drug effects; D049452:Green Fluorescent Proteins; D006801:Humans; D006877:Hydroxamic Acids / Q000494:pharmacology; D066298:In Vitro Techniques; D007202:Indicators and Reagents / Q000378:metabolism; D007668:Kidney / Q000166:cytology; D008164:Luminescent Proteins / Q000235:genetics; D012435:S-Adenosylhomocysteine / Q000494:pharmacology; D012436:S-Adenosylmethionine / Q000494:pharmacology*; D014162:Transfection
chemical_list: D004791:Enzyme Inhibitors; D006877:Hydroxamic Acids; D007202:Indicators and Reagents; D008164:Luminescent Proteins; D049452:Green Fluorescent Proteins; C012589:trichostatin A; D012436:S-Adenosylmethionine; D012435:S-Adenosylhomocysteine; D015254:DNA Modification Methylases