Increased expression of heme oxygenase-1 and bilirubin accumulation in foam cells of rabbit atherosclerotic lesions

Heme oxygenase-1 (HO-1) catalyzes the regiospecific oxidative degradation o f heme to biliverdin IX alpha, iron, and carbon monoxide. Biliverdin IX alp ha is subsequently reduced to bilirubin IX alpha by biliverdin reductase. H O-1 expression is induced under various disease conditions, including ather osclerosis, but it is unknown whether HO-1 catalyzes heme breakdown in the regions at risk. Using hypercholesterolemic rabbits fed a cholesterol-enric hed diet, we attempted to demonstrate the involvement of HO-1 induction and bilirubin IX alpha production in atherosclerotic regions. Expression level s of HO-1 mRNA were elevated in the aortas of hypercholesterolemic rabbits. In situ hybridization and immunohistochemistry revealed that mRNA and prot ein of HO-1 are induced in endothelial cells and foam cells (lipid-filled m acrophages) in atherosclerotic lesions. Furthermore, immunohistochemistry w ith the use of an anti-bilirubin-IX alpha monoclonal antibody, 24G7, demons trated accumulation of bilirubin IX alpha in foam cells, indicating that he me is actually degraded in atherosclerotic lesions. Remarkably, bilirubin I X alpha, like HO-1 protein, is predominantly accumulated in the perinuclear regions of foam cells. These results provide the first in vivo evidence of the colocalization of HO-1 and bilirubin IX alpha in foam cells, suggestin g a role of HO-1 induction in the modulation of macrophage activation in at herosclerosis.