Phosphorylation of serine-504 of tNOX (ENOX2) modulates cell proliferation and migration in cancer cells

Tumor-associated NADH oxidase (tNOX; ENOX2) is a growth-related protein expressed in transformed cells. Consistent with this function, tNOX knockdown by RNA interference leads to a significant reduction in cell proliferation and migration in HeLa cells, whereas tNOX overexpression confers an aggressive phenotype. Here, for the first time, we report that tNOX is phosphorylated by protein kinase Cδ (PKCδ) both in vitro and in vivo. Replacement of serine-504 with alanine significantly reduces phosphorylation by PKCδ. Co-immunoprecipitation experiments reveal an interaction between tNOX and PKCδ. Moreover, whereas overexpression of wild-type tNOX in NIH3T3 cells increases cell proliferation and migration, overexpression of the S504A tNOX mutant leads to diminished cell proliferation and migration, reflecting reduced stability of the unphosphorylatable tNOX mutant protein. Collectively, these results suggest that phosphorylation of serine-504 by PKCδ modulates the biological function of tNOX.