Presence of potential nickel-responsive element(s) in the mouse MTH1 promoter

The murine MTH1 gene codes for MTH1, an 8-oxo-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolase (8-oxo-dGTPase) which hydrolyzes 8-oxo-dGTP, a promutagenic product of reactive oxygen species' attack on the nucleotide pool. This gene is regulated by oxidative stress. Therefore, we hypothesized that MTH1 expression can be affected by carcinogenic nickel(II), known to induce such stress. Three plasmid constructs, carrying different upstream regions of the mouse MTH1 and the chloramphenicol acetyltransferase (CAT) reporter gene, were transiently transfected into NIH 3T3 cells and the CAT protein was measured in nickel(II) acetate-treated and untreated cells. Nickel concentration-dependent increase of CAT protein level was observed for low Ni(II) concentrations, up to 400 microM Ni(II), in cells transfected with pHI103 plasmid (-5969 to +530 of the MTH1 sequence) only. Cells transfected with the pHI104 (-1331 to +530) or pHI108 (-151 to +530) plasmids did not respond to nickel(II) whatsoever. This finding demonstrated that the MTH1 sequence between -5969 and -1331 contained element(s) responsive to nickel(II) treatment. DNA sequencing revealed the presence of AP-1, NF-kappaB, and ATF-1 binding sites in both the -5969 to -1331 and -1331 to +530 regions. In contrast, two (CA)n repeats (-5642 to -5582 and -2078 to -2031), a family of B2 (-5428 to -5247) and B1 (-4559 to -4420) short interspersed repeated elements, and an (AT)n repeat (-5243 to -5230) were identified only in the -5969 to -1331 sequence. The results suggest that up-regulation of murine MTH1 expression by nickel(II) is controlled by the repeat sequences, potential candidates for nickel-responsive elements.