RATIONALE: Mechanotransduction and the response to biomechanical stress is a fundamental response in heart disease. Loss of phosphoinositide 3-kinase (PI3K)γ, the isoform linked to G protein-coupled receptor signaling, results in increased myocardial contractility, but the response to pressure overload is controversial. OBJECTIVE: To characterize molecular and cellular responses of the PI3Kγ knockout (KO) mice to biomechanical stress. METHODS AND RESULTS: In response to pressure overload, PI3KγKO mice deteriorated at an accelerated rate compared with wild-type mice despite increased basal myocardial contractility. These functional responses were associated with compromised phosphorylation of Akt and GSK-3α. In contrast, isolated single cardiomyocytes from banded PI3KγKO mice maintained their hypercontractility, suggesting compromised interaction with the extracellular matrix as the primary defect in the banded PI3KγKO mice. β-Adrenergic stimulation increased cAMP levels with increased phosphorylation of CREB, leading to increased expression of cAMP-responsive matrix metalloproteinases (MMPs), MMP2, MT1-MMP, and MMP13 in cardiomyocytes and cardiofibroblasts. Loss of PI3Kγ resulted in increased cAMP levels with increased expression of MMP2, MT1-MMP, and MMP13 and increased MMP2 activation and collagenase activity in response to biomechanical stress. Selective loss of N-cadherin from the adhesion complexes in the PI3KγKO mice resulted in reduced cell adhesion. The β-blocker propranolol prevented the upregulation of MMPs, whereas MMP inhibition prevented the adverse remodeling with both therapies, preventing the functional deterioration in banded PI3KγKO mice. In banded wild-type mice, long-term propranolol prevented the adverse remodeling and systolic dysfunction with preservation of the N-cadherin levels. CONCLUSIONS: The enhanced propensity to develop heart failure in the PI3KγKO mice is attributable to a cAMP-dependent upregulation of MMP expression and activity and disorganization of the N-cadherin/β-catenin cell adhesion complex. β-Blocker therapy prevents these changes thereby providing a novel mechanism of action for these drugs.
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