Involvement of insulin receptor substrates in epidermal growth factor induced activation of phosphatidylinositol 3‐kinase in rat hepatocyte primary culture

Short‐term incubation of adult rat hepatocytes with epidermal growth factor (EGF) caused tyrosine phosphorylation of insulin receptor substrate (IRS)‐1 and IRS‐2 when the cells had been submitted to primary culture from 1–18 h. Tyrosine‐phosphorylated IRS‐1 and IRS‐2 bound to the regulatory subunit (p85) of phosphatidylinositol (PtdIns) 3‐kinase, thereby activating the enzymic activity. Tyrosine phosphorylation of the IRSs and activation of PtdIns 3‐kinase in 3 h cultured hepatocytes both proceeded similarly to the same actions of insulin; the activation was rapid and transient, with peak values at 15–30 s and with similar EC 50 s in the n m range in both cases. A possible involvement of insulin receptors in these insulin‐like actions of EGF was excluded by the following three lines of evidence. Insulin caused tyrosine phosphorylation of the insulin receptor β‐subunit but EGF did not. In contrast, the EGF receptor was phosphorylated by EGF, but the insulin receptor was not. The actions of EGF, but not those of insulin, were inhibited by AG1478, a selective inhibitor of EGF receptor tyrosine kinase. Cultured hepatocytes exposed to insulin or insulin‐like growth factor‐I (IGF‐I) for a short period responded to the subsequent addition of EGF, whereas EGF‐treated cells responded to insulin. The cells, however, displayed receptor desensitization under the same conditions, that is, no response was observed upon repeated addition of the same agonist, EGF, insulin or IGF‐I. Thus, the EGF receptor‐initiated signalling was mediated by PtdIns 3‐kinase associated with tyrosine‐phosphorylated IRSs in short‐term cultured rat hepatocytes.