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Effect of phorbol ester (PMA) on antioxidant enzyme expression in TGF‐β1‐induced apoptosis in primary cultures of hepatocytes

Apoptosis has been documented as a fundamental component of the life cycle of many cell types. One of the characteristics of this process is the cleavage of genomic DNA into oligonucleosomal fragments. The multifunctional cytokine TGF‐β 1 has been described to induce apoptosis in cultured hepatocytes although in this condition DNA fragmentation has not yet been detected. We investigated whether TGF‐β 1‐induced apoptosis was associated with DNA fragmentation and was affected by PMA. Agarose gel electrophoresis of TGF‐β 1‐treated hepatocytes shows a typical ladder‐like pattern of DNA fragments and PMA, a selective stimulator of protein kinase C, diminishes the DNA fragmentation and cell death. It has been described that the antioxidant enzyme systems play an important role in the control of apoptosis and that the apoptogenic ability of TGF‐β1 is through the inhibition of antioxidant enzyme expression in cultured hepatocytes [Y. Kayanoki, J. Fujii, K. Suzuki, S. Kawata, Y. Matsuzawa and N. Taniguchi, Suppression of antioxidative enzyme expression by transforming growth factor‐β 1 in rat hepatocytes, J. Biol. Chem. 269 (1994), 15 488–15 492]. However, PMA does not induce significant changes levels of manganese superoxide dismutase, copper—zinc superoxide dismutase and catalase mRNAs. Our data reveal that the attenuation of TGF‐β 1‐induced DNA fragmentation by PMA is not associated with changes in the expression of antioxidant systems and is probably due selectively to the stimulation of protein kinase C.