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.