A novel tumor suppressor function of glycine N-methyltransferase is independent of its catalytic activity but requires nuclear localization.
Glycine N-methyltransferase (GNMT), an abundant cytosolic enzyme, catalyzes the transfer of a methyl group from S-adenosylmethionine (SAM) to glycine generating S-adenosylhomocysteine and sarcosine (N-methylglycine). This reaction is regulated by 5-methyltetrahydrofolate, which inhibits the enzyme catalysis. In the present study, we observed that GNMT is strongly down regulated in human cancers and is undetectable in cancer cell lines while the transient expression of the protein in cancer cells induces apoptosis and results in the activation of ERK1/2 as an early pro-survival response. The antiproliferative effect of GNMT can be partially reversed by treatment with the pan-caspase inhibitor zVAD-fmk but not by supplementation with high folate or SAM. GNMT exerts the suppressor effect primarily in cells originated from malignant tumors: transformed cell line of non-cancer origin, HEK293, was insensitive to GNMT. Of note, high levels of GNMT, detected in regenerating liver and in NIH3T3 mouse fibroblasts, do not produce cytotoxic effects. Importantly, GNMT, a predominantly cytoplasmic protein, was translocated into nuclei upon transfection of cancer cells. The presence of GNMT in the nuclei was also observed in normal human tissues by immunohistochemical staining. We further demonstrated that the induction of apoptosis is associated with the GNMT nuclear localization but is independent of its catalytic activity or folate binding. GNMT targeted to nuclei, through the fusion with nuclear localization signal, still exerts strong antiproliferative effects while its restriction to cytoplasm, through the fusion with nuclear export signal, prevents these effects (in each case the protein was excluded from cytosol or nuclei, respectively). Overall, our study indicates that GNMT has a secondary function, as a regulator of cellular proliferation, which is independent of its catalytic role.
Department of Biochemistry and Molecular Biology, Medical University of South Carolina, Charleston, South Carolina, USA.
Suchandra
S
DebRoy
Inga I
II
Kramarenko
Sampa
S
Ghose
Natalia V
NV
Oleinik
Sergey A
SA
Krupenko
Natalia I
NI
Krupenko
D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000235:genetics; D002384:Catalysis; D002453:Cell Cycle / Q000187:drug effects / Q000235:genetics; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D049109:Cell Proliferation / Q000187:drug effects; D004249:DNA Damage; D004789:Enzyme Activation; D005492:Folic Acid / Q000737:chemistry / Q000378:metabolism; D015870:Gene Expression; D050938:Glycine N-Methyltransferase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D051379:Mice; D008958:Models, Molecular; D008968:Molecular Conformation; D009154:Mutation; D011485:Protein Binding; D021381:Protein Transport; D011994:Recombinant Proteins / Q000494:pharmacology; D025521:Tumor Suppressor Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology
D011994:Recombinant Proteins; D025521:Tumor Suppressor Proteins; D005492:Folic Acid; D050938:Glycine N-Methyltransferase