Role of osteoprotegerin (OPG) in angiogenesis

Abstract

Background: Angiogenesis, the development of blood vessels from an already existing microvascular network, is not only of fundamental importance to many physiological processes, but has also been associated with various pathologies, ranging from cancer and chronic inflammatory diseases to cardiovascular pathologies, including atherosclerosis and ischemic heart disease. Heme oxygenase-1 (HO-1) is the cytoprotective enzyme which catalyzes the rate-limiting step in the oxidative degradation of heme into its products carbon monoxide (CO), free iron (Fe), and biliverdin, which is rapidly converted to bilirubin. Complex anti-inflammatory, antiapoptotic, and anti-oxidanteffectshavebeen identified in thevasculature forHO-1 and its products. In addition, increasing evidence suggests a role for HO-1 in angiogenesis. However, the precise mechanisms through which HO-1 exerts its effects on angiogenesis remain elusive. Results: Herein, we show that inhibition of HO-1 either by synthetic inhibitor (ZnPP) or specific siRNA alters the angiogenic process at various levels.HO-1 depletion significantly reduced vascular endothelial growth factor A (VEGF)-mediated human endothelial cell (EC) proliferation and significantly inhibited capillary-like formation on a 2D-Matrigel in vitro. Further, we demonstrate that VEGF-induced EC cell cycle progression is inhibited by HO-1 siRNA; an observation associated with decreased expression of the cell cycle regulators cyclin A1 and cyclin E1. In contrast, HO-1 depletion did not alter susceptibility of endothelial cells to serum-starvation induced cell death and HO-1deficient cells were still protected from apoptosis by VEGF, most likely through induction of the anti-apoptotic genes Bcl-2 and A1. Interestingly,HO-1depletion alsonegatively affecteddirectionalmigrationof EC towards a VEGF gradient; a phenotype reversed by HO-1 overexpression using an adenoviral vector. Conclusion: Importantly, on-going experiments will utilise a proteomics approach and targeted microarrays to identify novel downstream targets of HO-1 involved in the regulation of angiogenesis. This may in turn reveal potential therapeutic targets for the manipulation of angiogenesis at sites of ischemia or wound healing, or conversely to inhibit angiogenesis associatedwith atherosclerosis or tumourogenesis.