Effects of ethanol on mitogen-activated protein kinase and stress-activated protein kinase cascades in normal and regenerating liver.

To understand the mechanisms by which ethanol inhibits hepatocyte proliferation, we studied the effects of ethanol on p42/44 mitogen-activated protein kinase (MAPK), p38 mitogen-activated protein kinase (p38 MAPK) and c-Jun N-terminal kinase (JNK) in normal and regenerating rat liver. Treatment of rat hepatocytes with 100 mM ethanol in vitro for 16 h prolonged the activation of p42/44 MAPK and p38 MAPK induced by various agonists. Such treatment also increased basal JNK activity, but did not potentiate or prolong agonist-induced JNK activation. Ethanol potentiation of the activation of p42/44 MAPK was abolished by pertussis toxin. In contrast, chronic ethanol consumption in vivo inhibited the activation of p42/44 MAPK, p38 MAPK and JNK induced either by partial hepatectomy or by various agonists. However, both acute and chronic ethanol inhibited hepatocyte proliferation induced by insulin and epidermal growth factor. A selective inhibitor of p42/44 MAPK partially prevented the inhibition of hepatocyte proliferation caused by acute, but not by chronic, ethanol exposure, whereas a selective inhibitor of p38 MAPK further inhibited hepatocyte proliferation under both conditions. These data suggest that acute and chronic ethanol inhibit hepatocyte proliferation by different mechanisms. The effect of acute ethanol may be related to the prolongation of p42/44 MAPK activation, whereas inhibition of hepatocyte proliferation by chronic ethanol may be due to inhibition of p38 MAPK activation.