Differential regulation of cyclooxygenase‐2 and inducible nitric oxide synthase by 4‐hydroxynonenal in human osteoarthritic chondrocytes through ATF‐2/CREB‐1 transactivation and concomitant inhibition of NF‐κB signaling cascade

Abstract

4-hydroxynonenal (HNE), a lipid peroxidation end product, is produced abundantly in osteoarthritic (OA) articular tissues and was recently identified as a potent catabolic factor in OA cartilage. In this study, we provide additional evidence that HNE acts as an inflammatory mediator by elucidating the signaling cascades targeted in OA chondrocytes leading to cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) gene expression. HNE induced COX-2 protein and mRNA levels with accompanying increases in prostaglandin E2 (PGE(2)) production. In contrast, HNE had no effect on basal iNOS expression or nitric oxide (NO) release. However, HNE strongly inhibited IL-1beta-induced iNOS or NO production. Transient transfection experiments revealed that the ATF/CRE site (-58/-53) is essential for HNE-induced COX-2 promoter activation and indeed HNE induced ATF-2 and CREB-1 phosphorylation as well as ATF/CRE binding activity. Overexpression of p38 MAPK enhanced the HNE-induced ATF/CRE luciferase reporter plasmid activation, COX-2 synthesis and promoter activity. HNE abrogated IL-1beta-induced iNOS expression and promoter activity mainly through NF-kappaB site (-5,817/-5,808) possibly via suppression of IKKalpha-induced IkappaBalpha phosphorylation and NF-kappaB/p65 nuclear translocation. Upon examination of upstream signaling components, we found that IKKalpha was inactivated through HNE/IKKalpha adduct formation. Taken together, these findings illustrate the central role played by HNE in the regulation of COX-2 and iNOS in OA. The aldehyde induced selectively COX-2 expression via ATF/CRE activation and inhibited iNOS via IKKalpha inactivation.