Reactive oxygen species (ROS) can cause cellular damage and promote cancer development. Besides such harmful consequences of overproduction of ROS, all cells utilize ROS for signaling purposes and stabilization of cell homeostasis. In particular, the latter is supported by the NADPH oxidase 4 (Nox4) that constitutively produces low amounts of H2O2. By that mechanism, Nox4 forces differentiation of cells and prevents inflammation. We hypothesize a constitutive low level of H2O2 maintains basal activity of cellular surveillance systems and is unlikely to be cancerogenic. Utilizing two different murine models of cancerogen-induced solid tumors, we found that deletion of Nox4 promotes tumor formation and lowers recognition of DNA damage. Nox4 supports phosphorylation of H2AX (γH2AX), a prerequisite of DNA damage recognition, by retaining a sufficiently low abundance of the phosphatase PP2A in the nucleus. The underlying mechanism is continuous oxidation of AKT by Nox4. Interaction of oxidized AKT and PP2A captures the phosphatase in the cytosol. Absence of Nox4 facilitates nuclear PP2A translocation and dephosphorylation of γH2AX. Simultaneously AKT is left phosphorylated. Thus, in the absence of Nox4, DNA damage is not recognized and the increased activity of AKT supports proliferation. The combination of both events results in genomic instability and promotes tumor formation. By identifying Nox4 as a protective source of ROS in cancerogen-induced cancer, we provide a piece of knowledge for understanding the role of moderate production of ROS in preventing the initiation of malignancies.

PrecisBy oxidizing AKT and keeping PP2A in the cytosol, the NADPH oxidase Nox4 allows proper DNA damage repair and averts cancer development.not peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was . http://dx.doi.org/10.1101/177055 doi: bioRxiv preprint first posted online Aug. 16, 2017; AbstractReactive oxygen species (ROS) can cause cellular damage and are thought to promote cancer-development. Nevertheless, under physiological conditions, all cells constantly produce ROS, either as chemical by-products or for signaling purpose. During differentiation cells induce the NADPH oxidase Nox4, which constitutively produces low amounts of H 2 O 2 . We infer that this constitutive H 2 O 2 is unlikely to be carcinogenic and may rather maintain basal activity of cellular surveillance systems.Utilizing two different murine tumor models we demonstrate that Nox4 prevents malignant transformation and facilitated the recognition of DNA-damage. Upon DNA-damage repair is initiated as consequence of phosphorylation of H2AX (γH2AX). Repair only occurs if nuclear activity of the γH2AX-dephosphorylating phosphatase PP2A is kept sufficiently low, a task fulfilled by Nox4: Nox4 continuously oxidizes AKT, and once oxidized AKT captures PP2A in the cytosol. Absence of Nox4 facilitates nuclear PP2A translocation and dephosphorylation of γH2AX. Simultaneously the proportion of active, phosphorylated AKT is increased. Thus, DNA-damage is not recognized and the increase in AKT activity promotes proliferation. The combination of both events resulted in genomic instability and tumor initiation.With the identification of the first cancer-protective source of reactive oxygen species, Nox4, the paradigm of reactive-oxygen species-induced initiation of malignancies should be revised. (200 words) SignificanceThe stereotype of ROS produced by NADPH oxidases as cause of malignant diseases persists generalized since decades. We demonstrate that the NADPH oxidase Nox4, as constitutive source of ROS, prevents malignant transformation and that its pharmacological not peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was . http://dx.doi.org/10.1101/177055 doi: bioRxiv preprint first posted online Aug. 16, 2017; inhibition as currently aspired by several companies will potentially increase the risk of malignant cell transformation and eventually tumor formation.