Abstract P185: Cardiac Characterization of Phlpp1 Knockout Mice: A Novel Phosphatase to Terminate Akt Signaling

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The serine/threonine kinase Akt, also known as protein kinase B (PKB), regulates a wide variety of cellular processes including metabolism, cell growth, survival, protein synthesis, and gene transcription. Akt is activated by upstream kinases through sequential phosphorylation on its activation loop (Thr308) followed by phosphorylation on the hydrophobic motif (Ser473). Studies using overexpression of Akt in vitro and in vivo have implicated the kinase in either attenuating or increasing cardiac hypertrophy, depending on signal intensity and localization of Akt in the cell. Recently the protein phosphatase PHLPP (PH domain leucine-rich repeat protein phosphatase) was shown to dephosphorylate Akt on its hydrophobic motif (Ser473), thereby decreasing kinase activity. We hypothesized that PHLPP could serve as an important regulator of Akt signaling in the heart and that loss of PHLPP would accentuate Akt activation under physiological conditions. We generated phlpp1 null mice to investigate the cardiac phenotype induced by modulating the kinetics of Akt activation in vivo . Myocytes were isolated from PHLPP1 knock-out and wild-type mice to demonstrate loss of PHLPP1 protein and increased Akt phosphorylation following stimulation with the cytokine LIF. Histological examination of hearts revealed that PHLPP1 KO mice have an increased capillary to myocyte ratio compared to age matched wild-type mice at baseline. Increased capillary networks were also detected in the PHLPP1 KO mice relative to wild-type mice using corrosion casting. To determine the role of PHLPP removal on cardiac hypertrophy, PHLPP1 null mice and wild-type mice were subjected to two weeks pressure overload by transverse aortic constriction (TAC). Cardiac hypertrophy was unexpectedly attenuated in PHLPP1 null mice (26% increase in HW/BW ratio vs. 48% in wild-type mice). In contrast, PHLPP1 null mice display an accentuated response to physiological hypertrophy induced by swimming compared to wild-type controls (46% vs. 38% respectively). Our data suggests that there is enhanced Akt activation following PHLPP removal and this attenuates pathological hypertrophy due to changes in capillary density.