Heme Oxygenase-1-derived Carbon Monoxide Requires the Activation of Transcription Factor NF-κB to Protect Endothelial Cells from Tumor Necrosis Factor-α-mediated Apoptosis*

We have shown that carbon monoxide (CO) generated by heme oxygenase-1 (HO-1) protects endothelial cells (EC) from tumor necrosis α (TNF-α)-mediated apoptosis. This effect relies on the activation of p38 MAPK. We now demonstrate that HO-1/CO requires the activation of the transcription factor NF-κB to exert this anti-apoptotic effect. Our data suggest that EC have basal levels of NF-κB activity that sustain the expression of NF-κB-dependent anti-apoptotic genes required to support the anti-apoptotic effect of HO-1/CO. Over-expression of the inhibitor of NF-κB α (IκBα) suppresses the anti-apoptotic action of HO-1/CO. Reconstitution of NF-κB activity, by co-expression of IκBα with different members of the NF-κB family, i.e. p65/RelA or p65/RelA plus c-Rel, restores the anti-apoptotic effect of HO-1/CO. Expression of the NF-κB family members p65/RelA or p65/RelA with p50 or c-Rel up-regulates the expression of the anti-apoptotic genes A1, A20, c-IAP2, and manganese superoxide dismutase (MnSOD). Inhibition of NF-κB activity by over-expression of IκBα suppresses the expression of some of these anti-apoptotic genes, i.e. c-IAP2. Under inhibition of NF-κB, co-expression of some of these anti-apoptotic genes, i.e. c-IAP2 and A1, restores the anti-apoptotic action of HO-1/CO, whereas expression of A20 or MnSOD cannot. The ability of c-IAP2 and/or A1 to restore the anti-apoptotic action of HO-1/CO is abolished when p38 MAPK activation is blocked by over-expression of a p38 MAPK dominant negative mutant. In conclusion, we demonstrate that HO-1/CO cooperates with NF-κB-dependent anti-apoptotic genes, i.e. c-IAP2 and A1, to protect EC from TNF-α-mediated apoptosis. This effect is dependent on the ability of HO-1/CO to activate the p38 MAPK signal transduction pathway.