Epidermal Growth Factor Mediated Signal Transduction in Rat Hepatocytes
Treatment of rat hepatocytes with epidermal growth factor (EGF) produced an enhanced tyrosine phosphorylation of the EGF receptor and phospholipase C-γ (PLC-γ) in conjunction with the mobilization of Ca2+. Approximately 30% of the total PLC-γ was tyrosine phosphorylated with a maximum being reached after 30s of incubation with EGF. Pretreatment of the rats with pertussis toxin prior to isolation of the hepatocytes blocked EGF-induced tyrosine phosphorylation of PLC-γ and Ca2+ mobilization but had no effect on autophosphorylation of the EGF receptor or Ca2+ responses elicited by angiotensin II or phenylephrine. Under these conditions Gi-protein α subunits were fully ADP-ribosylated. Antibodies specific to PLC-γ coimmunoprecipitated phosphorylated proteins with molecular masses of 190kDa, 96kDa, 47kDa and 25kDa but not the EGF receptor. A 41kDa Gi-protein α subunit was found to be present in the anti-PLC-γ immune complex as shown by in vitro ADP-ribosylation using [32P]NAD+ and activated pertussis toxin. Antibodies specific for the EGF receptor also coimmunoprecipitated a Gi-protein α subunit. Treatment of hepatocytes with EGF caused a decrease in the amount of Gi-protein α subunit associated with the EGF receptor and an increase in its association with PLC-γ. These results indicate that a pertussis toxin-sensitive Gi-protein is intimately involved in the signal transduction pathway mediating EGF-induced activation of PLC-γ and Ca2+ mobilization in hepatocytes.
KeywordsCorn Tyrosine Serine Angiotensin Hepes
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