—In cardiac fibroblasts, angiotensin II (Ang II) induced a rapid increase in extracellular signal–regulated kinase (ERK) activity in a pertussis toxin–insensitive manner. This ERK activation was abolished by the G_q-associated phospholipase C inhibitor U73122 but was insensitive to protein kinase C (PKC) inhibitors or PKC downregulation by phorbol ester. Intracellular Ca²⁺chelation by BAPTA-AM or TMB-8 abolished Ang II–induced ERK activation, whereas treatment with EGTA or nifedipine did not affect it. Ca²⁺ionophore A23187 also induced a rapid increase in ERK activity to an extent similar to that of Ang II stimulation. Calmodulin inhibitors (W7 and calmidazolium) and tyrosine kinase inhibitors (genistein and ST638) completely blocked ERK activation by Ang II and A23187. Both Ang II and A23187 caused a rapid increase in the binding of GTP to p21^Ras, which was nearly abolished by genistein and calmidazolium. Transfection with the dominant negative mutant of Ras and the Ras inhibitor manumycin completely inhibited Ang II–induced ERK activation. It was also found for the first time that cardiac fibroblasts abundantly expressed Ca²⁺-sensitive tyrosine kinase Pyk2/CAKβ/RAFTK and that Ang II markedly induced its activation in a Ca²⁺/calmodulin-sensitive manner. Overexpression of the dominant negative mutant of Pyk2 significantly attenuated Ang II– or A23187-induced ERK activities (36% and 38% inhibition compared with that in mock-transfected cells, respectively) and ERK tyrosine phosphorylation levels, as well as an increase in the binding of GTP to p21^Ras. These findings demonstrate that in cardiac fibroblasts, Ang II–induced Ras/ERK activation is dominantly regulated by G_q-coupled Ca²⁺/calmodulin signaling and that Pyk2 plays an important role in the signal transmission for efficient activation of the Ang II–induced Ras/ERK pathway.

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