ANGIOTENSIN-II STIMULATES RAPID SERINE PHOSPHORYLATION OF TRANSCRIPTION FACTOR STAT3

In rat neonatal cardiac fibroblasts and CHO-K1 cells expressing angiot ensin type 1 receptors, angiotensin II (AII) rapidly caused a time dep endent reduction in the SDS-polyacrylamide gel electrophoretic mobilit y of Stat3 (Signal Transducer and Activator of Transcription). This wa s concentration dependent and detected at a low/physiological concentr ation of AII (1 nM), with initial effect observed as early as 2 min; a nd maximal at 5 min. The rapid stimulation of Stat3 mobility retardati on by AII, paralleled the rapid activation of MAP kinases (mitogen-act ivated protein kinases), and both were sensitive to the MAP kinase kin ase I inhibitor, PD98059. Immunoprecipitation of Stat3 from [P-32] lab eled cells demonstrated a 4-fold increase in Stat3 phosphorylation in response to AII, and phosphoamino acid analysis indicated that phospho rylation occurred on serine residues. Angiotensin II-induced rapid pho sphorylation of Stat3 was also sensitive to the MAP kinase kinase 1 in hibitor, PD98059. Treatment of immunoprecipitated Stat3 from AII-treat ed cells with protein phosphatase-PP-2A, reversed the AII-induced reta rdation of Stat3 mobility. These results demonstrate that AII rapidly induces Stat3 serine phosphorylation through a MAP kinase kinase 1 dep endent pathway. Rapid stimulation of Stat3 serine phosphorylation by A II may have implications in the modulation of its transcriptional acti vity and gene expression.