Chronic nitric oxide deprivation induces hypoxia inducible factor-1α stabilization in human endothelial cells

The endothelium explicates its physiological function by producing active molecules among which nitric oxide (NO) is endowed with very important properties. Endothelial-produced NO possesses vasorelaxant and antiaggregant activity that protects the cardiovascular system from thrombosis and acute events. Indeed, NO loss is associated with endothelial dysfunction typical of atherosclerosis, diabetes and senescence. To reproduce the pathological situation of NO deficiency we chronically treated human umbilical vein endothelial cells (HUVECs) with NG-Nitro-L-arginine methyl ester (L-NAME), an analogue of L-arginine that competitively block the active site of endothelial nitric oxide synthase (eNOS), the enzyme responsible for endothelial NO production. The chronic NO deprivation induced profound modification in HUVEC behaviour, consisting in increased cell motility, enhanced expression of Vascular Endothelial Growth Factor (VEGF) and of its receptor KDR, and very interestingly, stabilization of hypoxia-inducible factor-1 α (HIF-1 α). To investigate whether the observed effects of L-NAME treatment on HIF-1α accumulation were due to the specific inhibitory effect on the eNOS activity, expression of the enzyme was silenced in HUVECs by using RNA interference methodology. In agreement with the results obtained after long-term exposure to L-NAME, HIF-1 α clearly accumulated in the nucleus of eNOS knock-down cells. Moreover, the NO donor DETA/NO reverted the HIF-1α accumulation induced by L-NAME treatment, confirming the dependence of HIF-1α stabilization from NO deficiency. Based on our results, we suggest that basal release of NO may act as a negative controller of HIF-1α levels and cell motility with important consequences for the endothelial cell physiology. Finally, long term L-NAME treatment induced in HUVECs a significant decrease in mitochondrial mass, confirming also in endothelial cells the central role of NO in promoting mitochondrial biogenesis.