Heme oxygenase-1 inhibition prevents intimal hyperplasia enhancing nitric oxide-dependent apoptosis of vascular smooth muscle cells.

Heme oxygenase-1 (HO-1, encoded by the HMOX1 gene) and inducible nitric oxide synthase (iNOS) have been implicated in vascular disease; however the role of these genes remains unclear. Therefore, we studied the mechanism by which iNOS-derived nitric oxide (NO) affects the intimal hyperplasia (IH) formation in relation to HO-1. We show, in a model of balloon injury in rats, that the suppression of vascular smooth muscle cells (VSMC) proliferation by NO required HO-1, while induction of apoptosis of the VSMC by NO does not involve HO-1. To better clarify the molecular mechanism of this finding, we used Hmox1(+/+) and Hmox1(-/-) VSMC exposed to NO. In Hmox1(+/+) VSMC, NO is antiproliferative (up to 34% inhibition) and it is associated to an increase of apoptosis (up to 35%) due to a decrease of X-linked inhibitor of apoptosis protein (XIAP) expression level and to the activation of caspase-3. In the absence of HO-1 (Hmox1(-/-) VSMC) apoptosis was significantly greater (69% p<0.01 vs. Hmox1(+/+) VSMC) demonstrating that HO-1 attenuated the pro-apoptotic effect of NO on VSMC. In the context of IH, the pro-apoptotic effect of NO on VSMC is increased in the absence of HO-1 and exerts therapeutic effects with a significant reduction in IH.

Bilirubin: a natural inhibitor of vascular smooth muscle cell proliferation.

BACKGROUND: Bilirubin, a natural product of heme catabolism by heme oxygenases, was considered a toxic waste product until 1987, when its antioxidant potential was recognized. On the basis of observations that oxidative stress is a potent trigger in vascular proliferative responses, that heme oxygenase-1 is antiatherogenic, and that several studies now show that individuals with high-normal or supranormal levels of plasma bilirubin have a lesser incidence of atherosclerosis-related diseases, we hypothesized that bilirubin would have salutary effects on preventing intimal hyperplasia after balloon injury. METHODS AND RESULTS: We found less balloon injury-induced neointima formation in hyperbilirubinemic Gunn rats and in wild-type rats treated with biliverdin, the precursor of bilirubin, than in controls. In vitro, bilirubin and biliverdin inhibited serum-driven smooth muscle cell cycle progression at the G1 phase via inhibition of the mitogen-activated protein kinase signal transduction pathways and inhibition of phosphorylation of the retinoblastoma tumor suppressor protein. CONCLUSIONS: Bilirubin and biliverdin might be potential therapeutics in vascular proliferative disorders.