The downregulation of NADPH oxidase Nox4 during hypoxia in hemangioendothelioma cells: a possible role of p22(phox) on Nox4 protein stability

NADPH oxidase (Nox) 4 produces H2O2 by forming a heterodimer with p22(phox) and is involved in hemangioendothelioma development through monocyte chemoattractant protein-1 (MCP-1) upregulation. Here, we show that Nox4 protein levels were maintained by p22(phox) in hemangioendothelioma cells and Nox4 protein stability was dependent on p22(phox) coexpression. Conversely, the degradation of Nox4 monomer was enhanced by p22(phox) knockdown. Under hypoxic conditions in hemangioendothelioma cells, p22(phox) was downregulated at the mRNA and protein levels. Downregulation of p22(phox) protein resulted in the enhanced degradation of Nox4 protein in hypoxia-treated hemangioendothelioma cells. In contrast, Nox2, a Nox isoform, was not altered at the protein level under hypoxic conditions. Nox2 exhibited a higher affinity for p22(phox) compared with Nox4, suggesting that when coexpressed with Nox4 in the same cells, Nox2 acts as a competitor. Nox2 knockdown restored Nox4 protein levels partially reduced by hypoxic treatment. Thus, Nox4 protein levels were attenuated in hypoxia-treated cells resulting from p22(phox) depletion. MCP-1 secretion was decreased concurrently with hypoxia-induced Nox4 downregulation compared with that under normoxia.