β‐adrenergic receptor‐mediated DNA synthesis in neonatal rat cardiac fibroblasts proceeds via a phosphatidylinositol 3‐kinase dependent pathway refractory to the antiproliferative action of cyclic AMP

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Abstract
The following study was undertaken to elucidate the cytoskeletal phenotype of neonatal rat cardiac fibroblasts (NNCF) and the signaling pathways coupled to β‐adrenergic receptor stimulated DNA synthesis. The cytoskeletal proteins vimentin, and smooth muscle α‐actin were detected in NNCF, suggestive of a myofibroblast phenotype. Isoproterenol (ISO) treatment stimulated 3H‐thymidine uptake, and concomitantly increased intracellular cyclic AMP levels. However, cyclic AMP‐elevating agents markedly decreased DNA synthesis. Coincident with growth, ISO‐stimulated phosphatidylinositol 3‐kinase (PI3‐K) activity, and the PI3‐K inhibitor LY294002 abrogated enzyme activity, and DNA synthesis. Unexpectedly, the serine/threonine kinase protein kinase Bα (PKBα), a putative downstream target of PI3‐K, was dephosphorylated following ISO treatment. Despite PKBα inactivation, the phosphorylation of its putative downstream target, the pro‐apoptotic enzyme glycogen synthase kinase‐3α was significantly increased in response to ISO. These latter effects of ISO were mimicked by the cyclic AMP‐elevating agent forskolin. Lastly, ISO treatment increased p70 ribosomal S6 kinase (p70S6K) phosphorylation, as reflected by an upward electrophoretic mobility shift. The pretreatment with rapamycin abrogated the ISO‐mediated mobility shift of p70S6K, and DNA synthesis. Collectively, these data demonstrate that NNCF express a myofibroblast phenotype, and β‐adrenergic agonists promote DNA synthesis via a PI3‐K‐dependent pathway involving p70S6K. Although unable to suppress ISO‐stimulated DNA synthesis, cyclic AMP can influence specific downstream targets of PI3‐K highlighting a novel crosstalk between these signaling pathways.