Involvement of net and Hif1alpha in distinct yet intricately linked hypoxia-induced signaling pathways.

The present study compares negative Ets transcription factor (Net) and hypoxia-inducible factor 1alpha (HIF1alpha) regulation by hypoxia. Their protein stabilities are differently regulated by hypoxia, defining three periods in the kinetics: normoxia (high Net levels and low HIF1alpha levels), early hypoxia (high levels of Net and HIF1alpha), and late hypoxia (degradation of Net and HIF1alpha). Modulators of prolyl hydroxylase domain protein (PHD) activity induce a mobility shift of Net, similar to HIF1alpha, suggesting that post-translational modifications of both factors depend on PHD activity. The three PHDs have different roles in the regulation of Net protein levels; PHD1 and PHD3 are involved in the stabilization of Net, whereas PHD2 controls its degradation in late hypoxia. Net physically interacts with PHD2 in hypoxia, whereas PHD1 and PHD3 bind to Net in normoxia and hypoxia. Under the same conditions, PHD2 and PHD3 regulate both HIF1alpha stabilization in early hypoxia and its degradation at late hypoxia, whereas PHD1 is involved in HIF1alpha degradation in late hypoxia. We describe interconnections between the regulation of both Net and HIF1alpha at the protein level. Evidence is provided for a direct physical interaction between Net and HIF1alpha and indirect transcriptional regulation loops that involve the PHDs. Taken together our results indicate that Net and HIF1alpha are components of distinct signaling pathways that are intricately linked.