Abstract 5361: PDK1 Plays Crucial Roles in β-Adrenergic Response and Cell Survival in the Heart
Background: The 3-phosphoinositide-dependent protein kinase-1 (PDK1) plays a homeostatic role in the regulation of cellular function in multiple organs, by working downstream of phosphatidylinositol-3 kinase (PI3-K) through activating several kinases including Akt and p70 S6 kinase. We found that myocardial expression of PDK1 was decreased in murine models of heart failure. Although previous studies have reported that PDK1 is important for the regulation of cardiomyocyte size, the precise role of PDK1 in the heart remains to be fully characterized.
Methods and Results: To elucidate the roles of PDK1 in the postnatal heart, we generated tamoxifen-inducible heart specific PDK1 knockout (PDK1-KO) mice. Deletion of PDK1 at the age of 10 weeks caused severe heart failure. At 1 week after Pdk1 gene disruption, left ventricular systolic function was already deteriorated without reduction in cardiomyocyte size. Langendorff-perfused hearts from PDK1-KO mice exhibited impaired responsiveness to isoproterenol in spite of preserved responsiveness to forskolin. In PDK1-KO hearts, PI3-K γ activity was enhanced and the expression levels of β1-adrenergic receptor (β1AR) in membrane fraction were decreased. Overexpression of PIK-domain of PI3-K γ blocked β1AR internalization by competitively inhibiting the association between PI3-K γ and G-protein coupled receptor kinase 2, and improved cardiac function in PDK1-KO hearts. In addition, we found that cardiomyocyte apoptosis was significantly increased 1 week after Pdk1 gene disruption. PDK1-KO hearts showed reduction of Akt and SGK activities, upregulation of Bax protein and endoplasmic reticulum stress signals, which were the potential causes of cardiomyocyte apoptosis. Overexpression of Bcl-2 in PDK1-KO hearts prevented cardiomyocyte apoptosis and partially improved cardiac function in PDK1-KO mice.
Conclusions: These results suggest that PDK1 is essential for normal cardiac function by not only preserving responsiveness to β-adrenergic stimulation but also preventing cardiomyocyte apoptosis. Since PDK1-KO mice reproduces many aspects of human heart failure, we propose that PDK1 may be a promising molecule targeted for the treatment of heart failure.