OBJECTIVE: Carbon monoxide (CO) is generated from vascular smooth muscle cells via the degradation of heme by the enzyme heme oxygenase-1. Since smooth muscle cell apoptosis is associated with numerous vascular disorders, we investigated whether CO regulates apoptosis in vascular smooth muscle. METHODS AND RESULTS: Treatment of cultured rat aortic smooth muscle cells with a combination of cytokines (interleukin-1beta, 5 ng/ml; tumor necrosis factor-alpha, 20 ng/ml; interferon-gamma, 200 U/ml) for 48 h stimulated apoptosis, as demonstrated by DNA laddering, annexin V binding, and caspase-3 activation. However, the exogenous administration of CO inhibited cytokine-mediated apoptosis. The antiapoptotic action of CO was partially dependent on the activation of soluble guanylate cyclase and was associated with the inhibition of mitochondrial cytochrome c release and with the suppression of p53 expression. Incubation of smooth muscle cells with the cytokines also resulted in a pronounced increase in heme oxygenase-1 protein after 24 h of stimulation. The addition of the heme oxygenase inhibitor, zinc protoporphyrin-IX, or the CO scavenger, hemoglobin, stimulated apoptosis following 24 h of cytokine exposure. CONCLUSIONS: These results demonstrate that CO, either administered exogenously or endogenously derived from heme oxygenase-1 activity, inhibits vascular smooth muscle cell apoptosis. The ability of CO to block smooth muscle cell apoptosis may play an important role in blocking lesion formation at sites of vascular injury.
