Inhibition of cytokine-induced matrix metalloproteinase 9 expression by peroxisome proliferator-activated receptor alpha agonists is indirect and due to a NO-mediated reduction of mRNA stability.

Rat renal mesangial cells express high levels of matrix metalloproteinase 9 (MMP-9) in response to inflammatory cytokines such as interleukin 1beta (IL-1beta). We tested whether ligands of the peroxisome proliferator-activated receptor (PPARalpha) could influence the cytokine-induced expression of MMP-9. Different PPARalpha agonists dose-dependently inhibited the IL-1beta-triggered increase in gelatinolytic activity mainly by decreasing the MMP-9 steady-state mRNA levels. PPARalpha agonists on their own had no effects on MMP-9 mRNA levels and gelatinolytic activity. Surprisingly, the reduction of MMP-9 mRNA levels by PPARalpha activators contrasted with an amplification of cytokine-mediated MMP-9 gene promoter activity and mRNA expression. The potentiation of MMP-9 promoter activity functionally depends on an upstream peroxisome proliferator-responsive element-like binding site, which displayed an increased DNA binding of a PPARalpha immunopositive complex. In contrast, the IL-1beta-induced DNA-binding of nuclear factor kappaB was significantly impaired by PPARalpha agonists. Most interestingly, in the presence of an inducible nitric-oxide synthase (iNOS) inhibitor, the PPARalpha-mediated suppression switched to a strong amplification of IL-1beta-triggered MMP-9 mRNA expression. Concomitantly, activators of PPARalpha potentiated the cytokine-induced iNOS expression. Using actinomycin D, we found that NO, but not PPARalpha activators, strongly reduced the stability of MMP-9 mRNA. In contrast, the stability of MMP-9 protein was not affected by PPARalpha activators. In summary, our data suggest that the inhibitory effects of PPARalpha agonists on cytokine-induced MMP-9 expression are indirect and primarily due to a superinduction of iNOS with high levels of NO reducing the half-life of MMP-9 mRNA.