Heme Oxygenase-1 Modulates the Expression of Adhesion Molecules Associated with Endothelial Cell Activation

Abstract

Heme oxygenase-1 (HO-1) cleaves the porphyrin ring of heme into carbon monoxide, Fe²⁺, and biliverdin, which is then converted into bilirubin. Heme-derived Fe²⁺ induces the expression of the iron-sequestering protein ferritin and activates the ATPase Fe²⁺-secreting pump, which decrease intracellular free Fe²⁺ content. Based on the antioxidant effect of bilirubin and that of decreased free cellular Fe²⁺, we questioned whether HO-1 would modulate the expression of proinflammatory genes associated with endothelial cell (EC) activation. We tested this hypothesis specifically for the genes E-selectin (CD62), ICAM-1 (CD54), and VCAM-1 (CD106). We found that HO-1 overexpression in EC inhibited TNF-α-mediated E-selectin and VCAM-1, but not ICAM-1 expression, as tested at the RNA and protein level. Heme-driven HO-1 expression had similar effects to those of overexpressed HO-1. In addition, HO-1 inhibited the activation of NF-κB, a transcription factor required for TNF-α-mediated up-regulation of these genes in EC. Bilirubin and/or Fe²⁺ chelation mimicked the effects of HO-1, whereas biliverdin or carbon monoxide did not. In conclusion, HO-1 inhibits the expression of proinflammatory genes associated with EC activation via a mechanism that is associated with the inhibition of NF-κB activation. This effect of HO-1 is mediated by bilirubin and/or by a decrease of free intracellular Fe²⁺ but probably not by biliverdin or carbon monoxide.