A role for cAMP in angiotensin II mediated inhibition of cell growth in AT1A receptor-transfected CHO-K1 cells

G-protein coupled Angiotensin II receptors (AT1A), mediate cellular responses through multiple signal transduction pathways. In AT1A receptor-transfected CHO-K1 cells (T3CHO/AT1A), angiotensin II (AII) stimulated a dose-dependent EC50 = 3.3 nM) increase in cAMP accumulation, which was inhibited by the selective AT1, nonpeptide receptor antagonist EXP3174. Activation of protein kinase C, or increasing intracellular Ca2+ with ATP, the calcium ionophore A23187 or ionomycin failed to stimulate cAMP accumulation. Thus, AII-induced cAMP accumulation was not secondary to activation of a protein kinase C- or ca2+/calmodulin-dependent pathway. Since cAMP has an established role in cellular growth responses, we investigated the effect of the AII-mediated increase in cAMP on cell number and [3H]thymidine incorporation in T3CHOA/AT1A cells. AII (1 microM) significantly inhibited cell number (51% at 96 h) and [3H]thymidine incorporation of 68% at 24 h) compared to vehicle controls. These effects were blocked by EXP3174, confirming that these responses were mediated through the AT1 receptor. Forskolin (10 microM) and the cAMP analog dibutyryl-cAMP (1 mM) also inhibited [3H]thymidine incorporation by 55 and 25% respectively. We extended our investigation on the effect of AII-stimulated increases in cAMP, to determine the role for established growth related signaling events, i.e., mitogen-activated protein kinase activity an tyrosine phosphorylation of cellular proteins. AII-stimulated mitogen-activated protein kinase activity and phosphorylation of the 42 and 44 kD forms. These events were unaffected by forskolin stimulated increases in cAMP, thus the AII-stimulated mitogen-activated protein kinase activity was independent of cAMP in these cells. AII also stimulated tyrosine phosphorylation of a number of cellular proteins in T3CHO/AT1A cells, in particular at 127 kD protein. The phosphorylation of the 127 kD protein was transient, reaching a maximum at 1 min, and returning to basal levels within 10 min. The dephosphorylation of this protein was blocked by a selective inhibitor of cAMP dependent protein kinase A, H89-dihydrochloride and preexposure to forskolin prevented the AII-induced transient tyrosine phosphorylation of the 127 kD protein. These data suggest that cAMP, and therefore protein kinase A can contribute to AII-mediated growth inhibition by stimulating the dephosphorylation of substrates that are tyrosine phosphorylated in response to AII.