Abstract 18652: Phopshoinositide-Dependent Protein Kinase-1 Mediates Fasting Induced Autophagy in the Heart
Fasting is a powerful inducer of cardiac autophagy, but the mechanisms are incompletely understood. The goal of this study was to determine if impaired insulin signaling via PI3K mediates fasting-induced autophagy. We characterized insulin signaling and markers of autophagy in control (WT) mice and mice with cardiomyocyte-restricted deletion of 3-phosphoinositide-dependent protein kinase-1 (PDK1) (PDKO). Consistent with loss of PDK1 signaling, basal Akt phosphorylation at Thr 308 was reduced by 67% and did not increase following in vivo insulin stimulation, however basal Akt phosphorylation at Ser 473 was preserved. In WT mice, fasting for 24 hr increased autophagy as evidenced by a 3.5-fold increase in LC3II/LC3I ratio (P<0.01 vs. Fed), a 60% increase in Cathepsin D level (P<0.01 vs. Fed), reduced p62 levels (P<0.05 vs. Fed), and increased AMPK phosphorylation (P<0.01 vs. Fed) that was associated with reduced mTOR, S6 and Foxo1/3 phosphorylation. In fed 6-week-old PDKO mice, similar changes in mTOR, S6 and FOXO1/3 phosphorylation were observed. The ratio of LC3II/LC3I was increased by 2-fold (P<0.01 vs. WT) and Cathepsin D level by 50% (P<0.05 vs. WT). p62 levels were reduced (P<0.05 vs. WT) and AMPK phosphorylation increased (P<0.05 vs. WT). In response to fasting, there was no further reduction of S6 or FOXO phosphorylation in PDKO mice but a further 25% increase in LC3II/LC3I ratios was observed. 6-week-old PDKO mice had significantly decreased heart weight (P<0.05 vs. WT), but LV contractile function was preserved. However, between 8–12 weeks of age, PDKO developed progressive heart failure characterized by LV dilatation, decreased wall thickness, increased fibrosis, and decreased fractional shortening resulting in death of all mice by 17-weeks of age. These data indicate that fasting-induced autophagy is largely regulated via PI3K-mediated signaling pathways. However, additional PI3K and mTOR independent pathways exist that contribute in part to fasting-induced autophagy. Second, chronic deficiency of PDK-1 signaling leads to progressive heart failure that may be due in part to unrestrained autophagy.
- © 2010 by American Heart Association, Inc.