Overexpression of wild-type and nuclear-targeted catalase modulates resistance to oxidative stress but does not alter spontaneous mutant frequencies at APRT.

Animal cells generate hydrogen peroxide as a byproduct of energy metabolism. In the presence of reduced metals H(2)O(2) can decompose to a highly reactive hydroxyl radical that attacks essentially all organic molecules, including DNA. We wished to determine if overexpression of catalase and/or the targeting of the enzyme to the nucleus could protect cells from oxidative stress and reduce the frequency of mutation. Wild-type human catalase, which localizes to peroxisomes, and a modified construct, which targets catalase to the nucleus, were overexpressed in a murine line of embryonic carcinoma cells (P19). Both constructs enhanced the resistance of the cells to hydrogen peroxide, but sensitized them to bleomycin. Overexpression of wild-type catalase protected cells against paraquat, while nuclear targeting sensitized them to this agent. Expression of neither construct significantly altered spontaneous mutant frequencies at the endogenous murine adenosine phosphoribosyl transferase (APRT) locus; however, nuclear-targeted catalase prevented an increase in mutant frequency after H(2)O(2) treatment. These results suggest that endogenous levels of hydrogen peroxide may not generate DNA damage in vivo, or that such damage may be efficiently repaired in murine embryonic carcinoma cells.