Abstract 1376: The Nadph Oxidase Nox4 Controls Mkp1 Expression and Thereby the Development of White Adipose Tissue

Abstract

Obesity is a cardiovascular risk factor. Insulin promotes the formation of adipocytes by promoting fibroblast into adipocyte differentiation. Insulin however also acts as a mitogen for fibroblasts and the balance of insulin-induced proliferation and differentiation is incompletely understood. We hypothesize that the NADPH oxidase Nox4 acts as a central switch between these two processes. In 3T3-fibroblasts Nox4 siRNA reduced the cellular radical formation and attenuated the insulin-stimulated differentiation from fibroblasts into adipocytes. Importantly, insulin-induced proliferation was enhanced by Nox4 siRNA. Accordingly, Nox4 overexpression enhanced differentiation and reduced insulin-stimulated proliferation. We generated Nox4 knockout mice to study the in vivo relevance of this observation: The amount of adipose tissue was reduced in the Nox4−/− animals as compared to their wildtype littermates suggesting indeed an attenuated adipocyte differentiation, whereas fibroblasts obtained from Nox4−/− mice presented with enhanced proliferation. To uncover the underlying mechanism, we focused on the Erk1/2 pathway. Erk1/2 signaling is prevented by dephosphorylation through the dualspecific phosphatase MKP1. Luciferase reportergene assays using the full-length MKP1 promotor revealed that Nox4 siRNA reduced, and overexpression of Nox4 as well as treatment of the cells with H 2 O 2 induced MKP1 promotor activity. Indeed, Nox4 siRNA reduced MKP1 protein expression and thus enhanced basal and insulin-induced activation of Erk1/2 in fibroblasts. Nox4 overexpression had the opposite effect. Importantly, MKP1 expression was also reduced in adipose tissue from Nox4−/− mice. Overexpression of MKP1 in fibroblastes increased insulin-induced differentiation and attenuated proliferation, whereas MKP1 siRNA had the opposite effect. We conclude that Nox4, by regulating MKP1, is essentially involved in adipocyte differentiation and development of obesity.