The NADPH oxidase Nox4 produces H₂O₂ constitutively and thereby promotes differentiation of a variety of cells. Nox4 knockout mice exhibit a hyper-inflammatory phenotype. Thus we hypothesize that Nox4 limits chronic inflammatory processes and thereby prevents inflammation-induced carcinogenesis. Macrophages play an important role in inflammation in which they can polarize to a pro-inflammatory (M1) or a pro-angiogenic (M2) phenotype. We assume that Nox4 controls inflammatory activity by affecting macrophage polarization.

MCA (3-methylcholanthrene) was utilized to induce fibrosarcoma in mice. In this model the tumor induction is driven by inflammation, whereas tumor growth depends on angiogenesis. MCA (0.1 mg) was injected and tumor development was analyzed subsequently. Nox4-deficient mice displayed an enhanced initial tumor formation and an increased amount of F4/80 positive cells within the tumor. Moreover, the pro-inflammatory cytokines TNF-α and Il-1β and the pro-inflammatory NADPH oxidase Nox2 were elevated in tumors of Nox4-/- mice, indicating a greater proportion of M1 macrophages compared to wildtype. Accordingly, the expression of VEGF, an M2 macrophage marker was reduced in tumors of Nox4-/- mice and so was angiogenesis. Indeed, in vitro the polarization into the M2 but not M1 phenotype was blocked in Nox4-deficient macrophages.

These results suggest that endogenous Nox4 exerts anti-inflammatory properties due to M2 polarization of macrophages. By maintaining VEGF production, Nox4 however contributes to angiogenesis and thus tumor growth.