Heme oxygenase–carbon monoxide signalling pathway in atherosclerosis: anti-atherogenic actions of bilirubin and carbon monoxide?

Atherosclerosis is a major contributor to cardiovascular disease, and genetic disorders of lipoprotein metabolism are recognized risk factors in atherogenesis. The gaseous monoxides nitric oxide (NO) and carbon monoxide (CO), generated within the blood vessel wall, have been identified as important cellular messengers involved in the regulation of vascular smooth muscle tone. Microsomal heme oxygenases degrade heme to biliverdin and CO, and the cytosolic enzyme biliverdin reductase then catalyzes reduction of biliverdin to bilirubin, both powerful chain-breaking antioxidants. Two principal isozymes of heme oxygenase have been identified, a constitutive isoform HO-2 (M(r) approximately 34,000) and an inducible isoform HO-1 (M(r) approximately 32,000), which is expressed at a low basal level in vascular endothelial and smooth muscle cells and is induced by heavy metals, oxidative stress, inflammatory mediators and oxidized low density lipoproteins. Although NO and CO modulate intracellular cGMP levels, platelet aggregation and smooth muscle relaxation, CO has a much lower affinity for soluble guanylyl cyclase than NO. Decreased production or sensitivity to NO in atherosclerosis may be compensated for by an induction of HO-1, with bilirubin acting as a cellular antioxidant and CO as a vasodilator. This review examines the evidence that oxidized low density lipoproteins (LDL), hypoxia and pro-inflammatory cytokines induce HO-1 expression and activity in vascular endothelial and smooth muscle cells, and evaluates the anti-atherogenic potential of the heme oxygenase signalling pathway.