ANGIOTENSIN-II STIMULATES PROTEIN-TYROSINE-PHOSPHATASE ACTIVITY THROUGH A G-PROTEIN INDEPENDENT MECHANISM

Most of angiotensin II's (Ang II) documented effects have been attributed to the interaction of this peptide with a G-protein coupled receptor termed AT1. The role and the signalling mechanisms of the more recently characterized AT2 receptor, which does not appear to interact with G-proteins, are however still unclear. We report here that this receptor mediates the rapid dephosphorylation of tyrosine residues of specific proteins in the 60 to 150 KDa range in PC12W cells which express only AT2 receptors. We further characterized this phosphatase activity using the synthetic substrate para-nitrophenyl phosphate. Dephosphorylation of this substrate in response to Ang II is not affected by Ser/Thr phosphatase inhibitors, but is completely prevented by the protein tyrosine phosphatase (PTPase) inhibitor sodium orthovanadate. This effect is mimicked by the AT2 selective agonist CGP 42112 and is not affected by the AT1 antagonist losartan, In contrast to the recently reported PTPase stimulation by somatostatin and dopamine, PTPase stimulation by Ang II is not affected by the guanyl nucleotides GTP(gamma)S and GDP(beta)S. Moreover, depletion of solubilized membrane preparations from G-proteins by lectin affinity chromatography does not alter Ang II stimulation of the measured PTPase activity. These findings indicate that Ang II stimulates a PTPase activity through AT2 receptors via G-protein independent pathways. This signalling mechanism may be involved in AT2 receptor mediated actions of Ang II such as particulate guanylate cyclase inhibition, modulation of T-type Ca++ channels and regulation of cell proliferation and differentiation.