A novel signaling pathway for β‐adrenergic receptor‐mediated activation of PI3K in H9c2 cardiomyocytes

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Stimulation of cardiac β-adrenergic receptors (βAR) activates both Gs-and Gi-coupled signaling cascades, including the PI3K pathway, that have important physiological implications. Multiple isoforms of PI3K are found in the heart. The goals of the current study were to examine the intracellular signaling pathways linking βAR to PI3K and to identify the PI3K isoform mediating this transactivation in a cardiac context. Acute βAR stimulation with isoproterenol (ISO) resulted in increased tyrosine kinase-associated PI3K activity and phosphorylation of Akt and p70S6K in H9c2 cardiomyocytes. Co-treatment with ICI 118,551, but not CGP 20712, abolished ISO-induced increase in PI3K activity, suggesting a β2AR-mediated event. The increase in PI3K activity was also abrogated by co-treatment with PP2, a selective inhibitor of the Src-family tyrosine kinases, and by AG 1296, a selective inhibitor of PDGFR, but not by inhibitors of Gαi, PKA, PKC, Ras, adenylyl cyclase, EGFR or IGF-1R. Consistent with these results, βAR-induced increases in tyrosine phosphorylation of PDGFR and Src, which can be abolished by siRNA specific for Src. Moreover, H9c2 cardiomyocytes stably transfected with a vector expressing a Gβγ sequestrant peptide derived from the C-terminus of βAR kinase-1 failed to activate PI3K after βAR stimulation, suggesting Gβγ is required for the transactivation. More important, acute βAR stimulation in vivo resulted in increases in tyrosine kinase-sensitive PI3K and PI3Kα activities but not the activities of other isoforms (PI3Kβ, -δ, -γ) in adult mouse heart. Taken together, these data provide in vitro and in vivo evidence for a novel mechanism of βAR-mediated transactivation of cardiac PI3Kα via sequential involvement of Gβγ, Src and PDGFR. (Supported by NIH 1 P20 RR018728)