Abstract

Physiologically, angiogenesis is tightly regulated, or otherwise it leads to pathological processes, such as tumors, inflammatory diseases, gynecological diseases and diabetic retinopathy. The vascular endothelial growth factor (VEGF) is a potent and critical inducer of angiogenesis. The VEGF gene expression is regulated by a variety of stimuli. Hypoxia is one of the most potent inducers of the VEGF expression. The hypoxia inducible factor 1 (HIF-1) plays as a key transcription factor in hypoxia-mediated VEGF gene upregulation. Nitric oxide (NO) as well as hypoxia is reported to upregulate the VEGF gene by enhancing HIF-1 activity. The Akt/protein kinase B (PKB) pathway may be involved in NO-mediated HIF-1 activation in limited cell lines. There are some reports of negative effects of NO on HIF-1 and VEGF activity. These conflicting data of NO effects may be attributed mainly to the amount of released NO. Indeed, NO can be a positive or negative modulator of the VEGF gene under the same conditions simply by changing its amounts. The VEGF-mediated angiogenesis requires NO production from activated endothelial NO synthase (eNOS). Activation of eNOS by VEGF involves several pathways including Akt/PKB, Ca(2+)/calmodulin, and protein kinase C. The NO-mediated VEGF expression can be regulated by HIF-1 and heme oxygenase 1 (HO-1) activity, and the VEGF-mediated NO production by eNOS can be also modulated by HIF-1 and HO-1 activity, depending upon the amount of produced NO. These reciprocal relations between NO and VEGF may contribute to regulated angiogenesis in normal tissues.