Signal transduction mechanism for potentiation by α1- and β2-adrenoceptor agonists of l-ascorbic acid-induced DNA synthesis and proliferation in primary cultures of adult rat hepatocytes

We investigated the effects of α- and β-adrenoceptor agonists on l-ascorbic acid-induced hepatocyte DNA synthesis and proliferation in primary cultures of adult rat hepatocytes. The results showed that phenylephrine (10−6 M) and metaproterenol (10−6 M) alone did not induce hepatocyte DNA synthesis and proliferation. However, when combined with l-ascorbic acid (10−6 M), these adrenoceptor agonists potentiated the hepatocyte DNA synthesis and proliferation induced by l-ascorbic acid. Then intracellular signal transduction mechanisms for the effects of phenylephrine and metaproterenol on l-ascorbic acid-induced hepatocyte mitogenesis were examined. Western blot analysis showed that phenylephrine and metaproterenol did not potentiate l-ascorbic acid-induced insulin-like growth factor I receptor tyrosine kinase phosphorylation. In contrast, they both significantly potentiated l-ascorbic acid-induced extracellular-signal regulated kinase-2 (ERK2) phosphorylation within 5 min. Moreover, cell-permeable second messenger analogs phorbol ester (10−7 M) and 8-bromo cAMP (10−7 M) mimicked the effects of phenylephrine and metaproterenol on l-ascorbic acid-induced ERK2 phosphorylation. The effects of these adrenoceptor agents were specifically antagonized by GF109203X and H-89, respectively. These results indicate that activation of ERK2 via protein kinas C and protein kinase A represents a mechanism for potentiation of l-ascorbic acid-induced hepatocyte DNA synthesis and proliferation in primary cultures of adult rat hepatocytes.