Negative regulation of human fibrinogen gene expression by peroxisome proliferator-activated receptor alpha agonists via inhibition of CCAAT box/enhancer-binding protein beta

Fibrinogen is a coagulation factor and an acute phase reactant up-regulated by inflammatory cytokines, such as interleukin 6 (IL-6). Elevated plasma f ibrinogen levels are associated with coronary heart diseases. Fibrates are clinically used hypolipidemic drugs that act via the nuclear receptor perox isome proliferator-activated receptor a (PPA-R alpha). In addition, most fi brates also reduce plasma fibrinogen levels, but the molecular mechanism is unknown. In this study, we demonstrate that fibrates decrease basal and IL -6-stimulated expression of the human fibrinogen-beta gene in human primary hepatocytes and hepatoma HepG2 cells. Fibrates diminish basal and IL-6-ind uced fibrinogen-beta promoter activity, and this effect is enhanced in the presence of co-transfected PPAR alpha. Site-directed mutagenesis experiment s demonstrate that PPAR alpha activators decrease human fibrinogen-beta pro moter activity via the CCAAT box/enhancer-binding protein (C/EBP) response element. Co-transfection of the transcriptional intermediary factor glucoco rticoid receptor-interacting protein 1/transc riptional intermediary factor 2 (GRIP1/TIF2) enhances fibrinogen-beta gene transcription and alleviates the repressive effect of PPAR alpha. Co-immunoprecipitation experiments dem onstrate that PPAR alpha and GRIP1/TIF2 physically interact in vivo in huma n liver. These data demonstrate that PPARa agonists repress human fibrinoge n gene expression by interference with the C/EBP beta pathway through titra tion of the coactivator GRIP1/TIF2. We observed that the anti-inflammatory action of PPAR alpha is not restricted to fibrinogen but also applies to ot her acute phase genes containing a C/EBP response element; it also occurs u nder conditions in which the stimulating action of IL-6 is potentiated by d examethasone. These findings identify a novel molecular mechanism of negati ve gene regulation by PPAR alpha and reveal the direct implication of PPAR alpha in the modulation of the inflammatory gene response in the liver.