The role of tyrosine phosphorylation in angiotensin II signaling

The cellular effects of angiotensin II are mediated by a seven transmembrane receptor called AT₁. We have found that AT₁ receptor signaling stimulates three important signalling pathways via tyrosine (Y) phosphorylation. In rat vascular smooth muscle and renal mesangial cells, ang II stimulated the rapid Y phosphorylation of PLC-yl. The time course of phosphorylation was coincident with the time course of 1,4,5-IP₃ generation. Inhibitors of Y kinases blocked both ang II mediated production of 1,4,5-IP₃ and the release of intracellular Ca²⁺, This was not a toxic effect as shown by thapsigargin release of Ca²⁺. When smooth muscle cells were electroporated with anti-Src antibody, the ang H-mediated Y phosphorylation of PLC-γ1 was blocked and 1,4,5-IP₃ production was markedly diminished. Ang II directly stimulated Src as measured by autophosphorylation. In smooth muscle cells ang II stimulates Ras-GTP production, the formation of a coprecipitable Ras-Rafl complex, and Y phosphorylation of GAPs. The electroporation of anti-Src antibody blocked these manifestations of Ras pathway activation. Ang II also stimulates Jak2 Y phosphorylation and activation. This leads to Stall phosphorylation and translocation to the nucleus. Our studies underline the importance of Y phosphorylation in the signaling of a seven transmembrane receptor. Src appears to play an important role in these signaling pathways.