Angiotensin II activates pp60^c-src in vascular smooth muscle cells

The angiotensin II type-1 (AT₁) receptor, a G protein-coupled receptor, lacks intrinsic kinase activity. However, recent data show that angiotensin II (Ang II) stimulates tyrosine phosphorylation of phospholipase C-γ1 (PLC-γ1), Stat91 (one of the signal transducers and activators of transcription), and paxillin in vascular smooth muscle cells. The tyrosine kinases responsible for these phosphorylation events are unknown. Src family kinases have been shown to phosphorylate PLC-γ1 and to be activated by G protein-coupled receptors. We hypothesized that pp60^c-src associates with the AT₁ receptor and is activated after Ang II stimulation of smooth muscle cells. We immunoprecipitated pp60^c-src from Ang II-stimulated vascular smooth muscle cells and measured pp60^c-src activity by autophosphorylation and by phosphorylation of enolase. Both assays demonstrated an approximately threefold increase in pp60^c-src activity within 1 minute. A similar increase in Ang II-stimulated pp60^c-src activity was observed in Chinese hamster ovary cells transfected with the AT₁ receptor but not in untransfected cells. These data are the first to show that pp60^c-src is activated by Ang II. To determine if pp60^s-src associated with the AT₁ receptor, the AT₁ receptor was immunoprecipitated (with two different antibodies), and Western blots were performed with two different anti-pp60^c-src antibodies. No pp60^c-src was detected. In addition, direct interaction between the AT₁ receptor and pp60^c-src could not be demonstrated by using a glutathione S-transferase (GST)-AT₁ fusion protein to bind proteins from cell lysates stimulated by Ang II. In combination with recent findings that anti-pp60^c-src antibodies inhibit Ang II-mediated PLC-γ1 phosphorylation, our data suggest an important role for pp60^c-src in Ang II signal transduction.