Activation of immediate‐early gene expression by peroxisome proliferators in vitro

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Abstract
The hepatocarcinogenicity of peroxisome proliferators (PPs) in rodents has been attributed both to oxidative DNA damage resulting from excessive leakage of peroxisomal H2O2 and to increased hepatocellular replication that may be independent of peroxisome proliferation. Because of the growing association between tumor promotion and alterations in growth‐regulatory signal transduction pathways, we investigated whether PPs can modulate these pathways in a mouse liver epithelial cell line, BNL‐CL.2. We tested two PPs that differ markedly in rodent tumorigenicity for their ability to activate immediate‐early proto‐oncogene expression. 4‐Chloro‐6‐(2,3‐xylidino)‐2‐pyrimidinylthioacetic acid (Wy‐14643), a highly tumorigenic PP, was an exceptionally strong inducer of c‐fos expression. In contrast, diethylhexyl phthalate (DEHP), a weakly tumorigenic PP, was a very poor inducer of c‐fos expression. Wy‐14643 was also stronger than DEHP in stimulating c‐jun expression, whereas both PPs were fairly strong inducers of jun‐B and jun‐D. The induction of fos and jun expression by Wy‐14643 was specifically inhibited by the protein kinase C inhibitor 1‐(5‐isoquinolinesulfonyl)‐2‐methylpiperizine dihydrochloride (H‐7). DEHP‐induced gene expression was strongly inhibited by H‐7, but was also partially inhibited by an inhibitor of protein kinase A. The activation of fos and jun gene expression by PPs was independent of peroxisome proliferation since it was an immediately‐early response not requiring protein synthesis and since the cell lines used in this study do not undergo peroxisome proliferation. Our results raise the possibility that the carcinogenicity of PPs may be due, in part, to epigenetic modulation of growth‐regulatory signal transduction pathways.