Diametrically opposed actions of the heme oxygenase-1 and endothelial nitric oxide synthase pathways in angiogenesis via p21^WAF1

INTRODUCTION: Vascular endothelial growth factor (VEGF)-induced angiogenesis requires endothelial nitric oxide synthase (eNOS) activation, however, the mechanism is largely unknown. As nitric oxide(NO) inhibits endothelial proliferation to promote capillary formation (Am J Path,159:993-1008,2001) and p21WAF1 is an important cell cycle inhibitor, we hypothesised that eNOS-induced angiogenesis requires up regulation of p21^WAF1.
METHODS: Human and porcine endothelial cells were cultured on growth factor reduced Materigel for in vitro tube formation and in vivo angiogenesis was assessed by hind limb ligation ischemia model.Conversely, we propose that the cytoprotective enzyme, heme oxygenase-1(HO-1), may suppress p21^WAF1 to limit angiogenesis.
RESULTS: The expression of p21^WAF1 was up regulated in porcine aorticenothelial cells stablely transfected with a constitutively activated form of eNOS (eNOS^S1177D) as well as in HUVEC infected by adenovirus encoding eNOS^S1177D. When these cells were plated on growth-factor reduced Matrigel (compaired to empty vector), they enhanced in vitro angiogenesis, which was inhibited following knockdown of p21^WAF1. Furthermore, over expression of p21^WAF1 led to increased tube formation while p21WAF1 knockdown abrogated vascular endothelial growth factor(VEGF) and fibroblast growth factor (FGF-2) mediated angiogenesis.Conversely, the cytoprotective enzyme, heme oxygenase-1 (HO-1) when over expressed decreased p21^WAF1 expression and reduced VEGF, FGF-2 and eNOSS1177D-induced angiogenesis.
CONCLUSIONS: These results demonstrate that eNOS-induced angiogenesis requires up regulation of p21^WAF1/CIP1 wherease, induction of HO-1 will decrease the expression of p21^WAF1/CIP1 to limit angiogenesisindicating that eNOS and HO-1 regulate angiogenesis via p21^WAF1/CIP1 in adiametrically opposed manner and that p21^WAF1/CIP1 appears to be a central regulator of angiogenesis that offers a new therapeutic target.