ANGIOTENSIN-II TYPE-1 RECEPTOR-INDUCED EXTRACELLULAR SIGNAL-REGULATEDPROTEIN-KINASE ACTIVATION IS MEDIATED BY CA2+ CALMODULIN-DEPENDENT TRANSACTIVATION OF EPIDERMAL GROWTH-FACTOR RECEPTOR/

The signaling cascade elicited by angiotensin II (Ang II) resembles th at characteristic of growth factor stimulation, and recent evidence su ggests that G protein-coupled receptors transactivate growth factor re ceptors to transmit mitogenic effects. In the present study, we report the involvement of epidermal growth factor receptor (EGF-R) in Ang II -induced extracellular signal-regulated kinase (ERK) activation, c-Sos gene expression, and DNA synthesis in cardiac fibroblasts, Ang II ind uced a rapid tyrosine phosphorylation of EGF-R in association with pho sphorylation of Shc protein and ERK activation, Specific inhibition of EGF-R function by either a dominant-negative EGF-R mutant or selectiv e tyrphostin AG1478 completely abolished Ang II-induced ERK activation , Induction of c-fos gene expression and DNA synthesis were also aboli shed by the inhibition of EGF-R function. Calmodulin or tyrosine kinas e inhibitors, but not protein kinase C (PKC) inhibitors or downregulat ion of PKC, completely abolished transactivation of EGF-R by An II or the Ca2+ ionophore A23187, Epidermal growth factor (EGF) activity in c oncentrated supernatant from Ang II-treated cells was not detected, an d saturation of culture media with anti-EGF antibody did not affect th e Ang II-induced transactivation of EGF-R, Conditioned media in which cells were incubated with Ang II could not induce phosphorylation of E GF-R on recipient cells. Platelet-derived growth facror-beta receptor was not phosphorylated on Ang II stimulation, and Ang II-induced c-jun gene expression was not affected by tyrphostin AG1478, Our resuIts de monstrated that in cardiac fibroblasts Ang II-induced ERK activation a nd its mitogenic signals are dominantly mediated by EGF-R transactivat ed in a Ca2+/calmodulin-dependent manner and suggested that the effect s of Ang II on cardiac fibroblasts should be interpreted in associatio n with the signaling pathways regulating cellular proliferation and/or differentiation by growth factors.