Stimulation of the DNA binding activity of AP-1 by the peroxisome proliferator ciprofibrate and eicosanoids in cultured rat hepatocytes

Peroxisome proliferators induce hepatic peroxisome proliferation and hepati c tumors in rodents. These chemicals increase the expression of the peroxis omal beta-oxidation pathway and the cytochrome P-450 4A family, which metab olize lipids, including eicosanoids. Peroxisome proliferators also induce i ncreased cell proliferation in vivo. However, peroxisome proliferators are only weakly mitogenic and are not comitogenic with epidermal growth factor (EGF) in cultured hepatocytes. Our earlier studies found that the peroxisom e proliferator ciprofibrate is comitogenic with eicosanoids. We therefore h ypothesized that the comitogenicity of the peroxisome proliferator ciprofib rate and eicosanoids may result from a synergistic increase of the DNA bind ing activity of AP-1. Primary rat hepatocytes were cultured on collagen gel s in serum-free L-15 medium with ciprofibrate, eicosanoids, and/or growth f actors. The DNA binding activity of AP-1 was determined in nuclear protein extracts by electrophoretic mobility shift assay. The DNA binding activity of AP-1 was not induced by ciprofibrate or eicosanoids alone, but the addit ion of eicosanoids along with ciprofibrate increased the induction of DNA b inding activity of AP-1 at 30 min and 2 h after exposure. The combination o f ciprofibrate and PGF(2 alpha) blocked the inhibitory effect of transformi ng growth factor (TGF)-beta on the DNA binding activity of AP-1 induced by EGF. These results show that the peroxisome proliferator ciprofibrate and e icosanoids co-stimulate the DNA binding activity of AP-1 and suggest that c hanges in eicosanoid concentrations may modulate mitogenic signal transduct ion pathways by the peroxisome proliferator ciprofibrate. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.