Mitochondrial Sirt3 Is Upregulated By Glucagon-like Peptide-1 Receptor Activation And Contributes To Reversal Of Cardiac Mitochondrial Remodeling Induced By Type 2 Diabetes

Rationale: Sirtuin 3 (SIRT3) is a mitochondrial protein deacetylase that regulates mitochondrial function and maintains basal ATP yield, whereas the role in myocardium remain uncertain. We reported that the GLP-1 receptor agonist exendin-4 (Ex-4) ameliorated cardiac mitochondrial remodeling in diabetic cardiomyopathy via cyclic AMP axis (ESC 2012, AHA 2012). Changes in intracellular ATP levels are associated with cyclic AMP, which is generated by adenylyl cyclase utilizing ATP, we thus hypothesized that SIRT3 may involve in Ex-4-mediated myocardial reverse remodeling in diabetes. Methods: Type 2 diabetic mice were allocated into Ex-4 (24 nmole/kg/day, DIO/Ex-4) and vehicle (DIO/CON). Heart samples and cultured cardiomyocytes were subjected to mitochondrial fractionation followed by mRNA and protein analyses. Results: Cardiac cyclic AMP concentration and phosphorylation of CREB were elevated in DIO/Ex-4, suggesting successful administration of Ex-4. Electron microscopy revealed that DIO/CON exhibited mitochondria with destroyed cristae structure and defragmented, which was reversed by Ex-4 (Fig. A). The ratio of Mitofusin-1 (Mfn1) and mitofusin-2 (Mfn2), which modulates mitochondria morphology, was consistently suppressed by Ex-4 (Fig. B), suggesting normalization of mitochondrial morphology. DIO-CON exhibited decrease in cardiac SIRT3 level compared to nondiabetic counterpart, which was reversed by Ex-4 (Fig. C). Increased cAMP levels (Isoprotelenol and 8-bromo-cyclic AMP) enhanced SIRT3 mRNA and protein levels in cultured cardiomyocytesm which was blocked by AMP-activated protein kinase (AMPK) inhibitor (compound C). Conclusions: Our results indicate that Ex-4 reversed abnormal suppression of SIRT3 in mitochondria via activating the PKA/AMPK-dependent pathway.