BACKGROUND: -Numerous molecular and biochemical changes have been linked with the cardioprotective effects of exercise, including increases in anti-oxidant enzymes, heat shock proteins (Hsps), and regulators of cardiac myocyte proliferation. However, a master regulator of exercise-induced protection has yet to be identified. Here, we assess whether phosphoinositide 3-kinase [PI3K(p110alpha)] is essential for mediating exercise-induced cardioprotection, and if so, whether its activation independent of exercise can restore function of the failing heart. METHODS AND RESULTS: -Cardiac-specific transgenic mice with elevated or reduced PI3K(p110alpha) activity (constitutively active-caPI3K and dominant negative-dnPI3K, respectively) and non-transgenic controls (Ntg) were subjected to 4 weeks of exercise training followed by 1 week of pressure overload (aortic-banding) to induce pathological remodeling. Aortic-banding in untrained Ntg led to pathological cardiac hypertrophy, depressed systolic function and lung congestion. This phenotype was attenuated in Ntg that had undergone exercise prior to aortic-banding. Banded caPI3K mice were protected from pathological remodeling independent of exercise status, whereas exercise provided no protection in banded dnPI3K mice, suggesting that PI3K is necessary for exercise-induced cardioprotection. Transgenic overexpression of Hsp70 could not rescue the phenotype of banded dnPI3K mice, and deletion of Hsp70 from banded caPI3K mice had no effect. Next, we used a gene therapy approach (recombinant adeno-associated viral vector 6, rAAV6) to deliver caPI3K expression cassettes to hearts of mice with established cardiac dysfunction due to aortic-banding. Mice treated with rAAV6-caPI3K had improved heart function after 10 weeks. CONCLUSIONS: -PI3K(p110alpha) is essential for exercise-induced cardioprotection and delivery of caPI3K vector can improve function of the failing heart.