Tyrosine kinase activation by the angiotensin II receptor in the absence of calcium signaling

The angiotensin II type 1 (AT(1)) receptor signals via heterotrimeric G-pro teins and intracellular tyrosine kinases, Here, we investigate a modified A T(1) receptor, termed M5, where the last five tyrosines (residues 292, 302, 312, 319, and 339) within the intracellular carboxyl tail have been mutate d to phenylalanine. This receptor did not elevate cytosolic free calcium or inositol phosphate production in response to angiotensin II, suggesting an uncoupling of the receptor from G-protein activation. Despite this, the M5 receptor still activated tyrosine kinases, induced STAT1 tyrosine phosphor ylation, and stimulated cell proliferation. We also studied another AT(1) m utant receptor, D74E, stably expressed in Chinese hamster ovarian cells and a fibroblast cell line from mice with a genetic inactivation of G alpha (q /11). Both cell lines have a deficit in calcium signaling and in G-protein activation, and yet in both cell lines, angiotensin II induced the time-dep endent tyrosine phosphorylation of STAT1, These studies are the first to sh ow the ability of a seven-transmembrane receptor to activate intracellular tyrosine kinase pathways in the absence of a G-protein-coupled rise in intr acellular calcium.