Nuclear Translocation of Mismatch Repair Proteins MSH2 and MSH6 as a Response of Cells to Alkylating Agents*

Mammalian mismatch repair has been implicated in mismatch correction, the prevention of mutagenesis and cancer, and the induction of genotoxicity and apoptosis. Here, we show that treatment of cells specifically with agents inducing O6-methylguanine in DNA, such asN-methyl-N′-nitro-N-nitrosoguanidine and N-methyl-N-nitrosourea, elevates the level of MSH2 and MSH6 and increases GT mismatch binding activity in the nucleus. This inducible response occurs immediately after alkylation, is long-lasting and dose-dependent, and results from translocation of the preformed MutSα complex (composed of MSH2 and MSH6) from the cytoplasm into the nucleus. It is not caused by an increase in MSH2 gene activity. Cells expressing the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT), thus having the ability to repair O6-methylguanine, showed no translocation of MutSα, whereas inhibition of MGMT by O6-benzylguanine provoked the translocation. The results demonstrate that O6-methylguanine lesions are involved in triggering nuclear accumulation of MSH2 and MSH6. The finding that treatment of cells with O6-methylguanine-generating mutagens results in an increase of MutSα and GT binding activity in the nucleus indicates a novel type of genotoxic stress response.

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