BACKGROUND

Gene therapy with inducible nitric oxide synthase (iNOS) markedly reduces myocardial infarct size; this effect is associated with cyclooxygenase-2 (COX-2) upregulation and is ablated by COX-2 inhibitors. However, pharmacological inhibitors are limited by relative lack of specificity; furthermore, the mechanism whereby iNOS gene therapy upregulates COX-2 remains unknown. Accordingly, we used genetically engineered mice to test the hypothesis that the cardioprotection afforded by iNOS gene transfer is mediated by COX-2 upregulation via a nuclear factor (NF)-kappaB-dependent pathway.

METHODS AND RESULTS

Mice received an intramyocardial injection of Av3/LacZ (LacZ group) or Av3/iNOS (iNOS group); 3 days later, myocardial infarction was produced by a 30-minute coronary occlusion followed by 4 hours of reperfusion. Among Av3/LacZ-treated mice, infarct size was similar in COX-2(-/-) and wild-type groups. iNOS gene transfer (confirmed by iNOS immunoblotting and activity assays) markedly reduced infarct size in wild-type mice but failed to do so in COX-2(-/-) mice. In transgenic mice with cardiac-specific expression of a dominant-negative mutant of IkappaB alpha (IkappaB alpha(S32A,S36A)), the upregulation of phosphorylated IkappaB alpha, activation of NF-kappaB, and cardiac COX-2 protein expression 3 days after iNOS gene therapy were abrogated, which was associated with the abolishment of the cardioprotective effects afforded by iNOS gene therapy.

CONCLUSIONS

These data provide strong genetic evidence that COX-2 is an obligatory downstream effector of iNOS-dependent cardioprotection and that NF-kappaB is a critical link between iNOS and COX-2. Thus, iNOS imparts its protective effects, at least in part, by recruiting NF-kappaB, leading to COX-2 upregulation. However, COX-2 does not play an important cardioprotective role under basal conditions (when iNOS is not upregulated).