Clofibric acid (CA), a potent peroxisome proliferator (PP), has been shown to cause
tumor formation in rat liver. The precise mechanism of action of PPs remains largely unknown. However, it has been proposed that they act by increasing
reactive oxygen species (ROS), leading to a cellular oxidative stress. In the present study, we have investigated the effect of CA on the
activator protein-1 (AP-1) expression in PP-responsive H4IIEC3 rat
hepatoma cells. Electrophoretic mobility shift assays demonstrated that
AP-1 activation was induced in cells treated with CA for 24 h at all concentrations of the
fibrate. This activation was prolonged up to 48 h. Using transfection experiments with H4IIEC3 cells, we found that CA induced the expression of a reporter gene driven by
AP-1 and that of the
glutathione S-transferase P target gene. By supershift experiments, jun and fos
proteins were identified as components of the CA-activated
AP-1 complexes. Western blot analyses revealed that the induction of the
AP-1 activity was not dependent to an increase in the levels of jun and fos
proteins. Cotreatment of H4IIEC3 cells with CA and the
antioxidant N-acetylcysteine or
calphostin C, a specific inhibitor of
protein kinase C (PKC), blocked the
AP-1 activation and the expression of the AP-1-driven
luciferase reporter gene. These results demonstrate that CA activates
AP-1 in H4IIEC3 cells and that this induction is mediated via ROS and PKC.