277 - Hydrogen Peroxide Formation by Nox4 Limits Malignant Transformation

Abstract

Through the constitutive production of H2O2 the NAPDH Oxidase Nox4 promotes differentiation of cells and contributes to cellular quiescence. Our previous observations in mesenchymal tissue suggested that the absence of Nox4 promotes inflammation and de-differentiation. Chronic inflammation and lack of differentiation are pre-carcinogenic states. We therefore hypothesized that lack of Nox4 promotes tumor development. The process of inflammation-dependent tumor formation was studied in two carcinogen-induced murine tumor models, leading to the formation of fibro sarcomas and colon carcinomas. In both models tumor burden was massively enhanced in Nox4-deficient mice. Genetic deletion of Nox4 resulted in increased DNA damage as detected by Nicoletti-staining and Comet assay. In contrast, the DNA damage response marker γH2AX was significantly reduced in Nox4-/- mice. As underlying mechanism we identified that Nox4 oxidizes Akt, which subsequently retains the phosphatase PP2A in the cytosol. In the absence of Nox4 PP2A accumulates in the nucleus. As a consequence, H2AX is dephosphorylated before DNA repair can take place resulting in genomic instability. In line with this, phosphorylated Akt accumulates in the cytosol promoting proliferation and survival of tumor cells as observed. Overexpression of all three Akt isoforms could rescue the described phenotype whereas overexpression of Akt mutants lacking the redox-sensitive cysteines, had no effect on PP2A translocation, percentage of DNA damage and γH2AX phosphorylation. Taken together these results suggest a protective role of endogenous Nox4 for malignant transformation. By the oxidation of Akt the enzyme prevents nuclear translocation of PP2A and thereby enables adequate DNA damage repair.