Fibrates down-regulate IL-1–stimulated C-reactive protein gene expression in hepatocytes by reducing nuclear p50-NFκB–C/EBP-β complex formation

Abstract

C-reactive protein (CRP) is a major acute-phase protein in humans. Elevated plasma CRP levels are a risk factor for cardiovascular disease. CRP is predominantly expressed in hepatocytes and is induced by interleukin-1 (IL-1) and IL-6 under inflammatory situations, such as the acute phase. Fibrates are hypolipidemic drugs that act through the nuclear receptor peroxisome proliferator-activated receptor-α (PPAR-α). Fibrates have been shown to reduce elevated CRP levels in humans, but the molecular mechanism is unknown. In this study, we demonstrate that different PPAR-α activators suppress IL-1–induced, but not IL-6–induced, expression of CRP in primary human hepatocytes and HuH7 hepatoma cells. Induction of CRP expression by IL-1 occurs at the transcriptional level. Site-directed mutagenesis experiments show that IL-1 induces CRP expression through 2 overlapping response elements, the binding sites for CCAAT-box/enhancer–binding protein-β (C/EBP-β) and p50-nuclear factor-κB (p50-NFκB). Cotransfection of C/EBP-β and p50-NFκB enhances CRP promoter activity, and coimmunoprecipitation experiments indicate that the increase in CRP promoter activity by IL-1 is related to the generation and nuclear accumulation of C/EBP-β–p50-NFκB complexes. Interestingly, PPAR-α activators reduce the formation of nuclear C/EBP-β–p50-NFκB complexes, and thereby CRP promoter activity, by 2 mechanisms. First, PPAR-α increases IκB-α expression and thus prevents p50-NFκB translocation to the nucleus. Second, fibrates decrease hepatic C/EBP-β and p50-NFκB protein levels in mice in a PPAR-α–dependent way. Our findings identify C/EBP-β and p50-NFκB as novel targets for PPAR-α and provide a molecular explanation for the reduction of plasma CRP levels by fibrates.