beta-adrenergic stimulation of rat cardiac fibroblasts promotes protein synthesis via the activation of phosphatidylinositol 3-kinase

beta -adrenergic agonists stimulate neonatal rat cardiac fibroblast growth, albeit the identity of the signaling event(s) remains equivocal. Isoproter enol (ISO) treatment increased intracellular cyclic AMP levels: however, cy clic AMP-elevating agents had no effect on protein synthesis. The tyrosine kinase inhibitor tyrphostin A25, and the inhibition of ras processing by th e farnesyltransferase inhibitor BMS-191563 attenuated ISO-stimulated protei n synthesis. Concomitant with increased protein synthesis, ISO stimulated e xtracellular signal-regulated protein kinase (ERK) and phosphatidylinositol . 3-kinase (PI3-K) activity. The MEK1/2 inhibitor PD098059 abrogated ISO-st imulated ERK activity, albeit the increase in protein synthesis was unaffec ted. By contrast, LY294002 inhibited both ISO-stimulated PI3-K activity, an d protein synthesis. ISO treatment did not increase the expression of trans forming growth factor-beta (1) (TGF-beta (1)) mRNA, whereas a significant d ecrease in the steady-state mRNA level of TGF-beta (3) was observed. This l atter effect was mimicked by cyclic AMP-elevating agents. Angiotensin II (A II) activation of the AT(1) receptor increased protein synthesis, but in co ntrast to ISO, the growth response was not inhibited by either tyrphostin A 25 or BMS-191563, and was associated with the concomitant expression of bot h TGF-beta (1) and TGF-beta (3) mRNAs, Analogous to ISO, AII treatment incr eased ERK and PI3-K activity, and PI3-K was required for protein synthesis. These findings are the first to highlight the activation of PI3-K by a Gs( alpha)-coupled receptor, and its essential role in beta -adrenergic as well as AT(1) receptor-mediated protein synthesis in neonatal rat cardiac fibro blasts. However, despite the conserved role of PI3-K, additional disparate signaling pathways are recruited by ISO and AE, which may differentially in fluence fibroblast phenotype. (C) 2001 Academic Press.