NADPH oxidase 5 promotes proliferation and fibrosis in human hepatic stellate cells

NADPH oxidase (Nox) variants Nox1, Nox2 and Nox4 are implicated in the progression of liver fibrosis. However, the role of Nox5 is not yet known, mainly due to the lack of this enzyme in rat and mouse genomes. Here we describe the expression and functional relevance of Nox5 in the human cell line of hepatic stellate cells (HSC) LX-2. Under basal conditions, three long (Nox5-L: Nox5 alpha, -beta, and -delta) and a short (Nox5-S or Nox5 epsilon) splice variants were detected, which were silenced with specific siRNAs for Nox5. The most abundant isoform was Nox5-S, accounting for more than 90% of Nox5 protein. Overexpression of Nox5 beta generated reactive oxygen species (ROS) in the presence of calcium, as judged by the production of hydrogen peroxide, L-012 luminescence and cytochrome c reduction. Nox5 epsilon did not generated ROS under these conditions, and a reduced ROS production was observed when co-expressed with Nox5 beta. In contrast, dihydroethidium oxidation was increased by Nox5 beta or Nox5 epsilon, suggesting that Nox5 epsilon induced intracellular oxidative stress by an unknown mechanism. Functional studies showed that both Nox5 beta and Nox5 epsilon stimulated the proliferation of LX-2 cells and the collagen type I levels, while Nox5 siRNAs inhibited these effects. Interestingly, TGF-beta and angiotensin II upregulated Nox5 expression, which was reduced in cells pre-incubated with catalase. Further studies silencing Nox5 in TGF-beta-treated cells resulted in a reduction of collagen levels via p38 MAPK. Collectively, these results show for the first time that Nox5 can play a relevant role in the proliferation and fibrosis on human HSC.