Improvement of cardiac functions by chronic metformin treatment is associated with enhanced cardiac autophagy in diabetic OVE26 mice

OBJECTIVE

Autophagy is a critical cellular system for removal of aggregated proteins and damaged organelles. Although dysregulated autophagy is implicated in the development of heart failure, the role of autophagy in the development of diabetic cardiomyopathy has not been studied. We investigated whether chronic activation of the AMP-activated protein kinase (AMPK) by metformin restores cardiac function and cardiomyocyte autophagy in OVE26 diabetic mice.

RESEARCH DESIGN AND METHODS

OVE26 mice and cardiac-specific AMPK dominant negative transgenic (DN)-AMPK diabetic mice were treated with metformin or vehicle for 4 months, and cardiac autophagy, cardiac functions, and cardiomyocyte apoptosis were monitored.

RESULTS

Compared with control mice, diabetic OVE26 mice exhibited a significant reduction of AMPK activity in parallel with reduced cardiomyocyte autophagy and cardiac dysfunction in vivo and in isolated hearts. Furthermore, diabetic OVE26 mouse hearts exhibited aggregation of chaotically distributed mitochondria between poorly organized myofibrils and increased polyubiquitinated protein and apoptosis. Inhibition of AMPK by overexpression of a cardiac-specific DN-AMPK gene reduced cardiomyocyte autophagy, exacerbated cardiac dysfunctions, and increased mortality in diabetic mice. Finally, chronic metformin therapy significantly enhanced autophagic activity and preserved cardiac functions in diabetic OVE26 mice but not in DN-AMPK diabetic mice.

CONCLUSIONS

Decreased AMPK activity and subsequent reduction in cardiac autophagy are important events in the development of diabetic cardiomyopathy. Chronic AMPK activation by metformin prevents cardiomyopathy by upregulating autophagy activity in diabetic OVE26 mice. Thus, stimulation of AMPK may represent a novel approach to treat diabetic cardiomyopathy.Autophagy is a physiologic process whereby cytoplasmic components, including long-lived proteins and organelles, are engulfed by a double-membrane structure and targeted for destruction in lysosomes (1). It selectively removes damaged mitochondria as a cytoprotective mechanism for limiting mitochondria-derived oxidative stress and preventing apoptosis (2,3). A low level of constitutive autophagy is important in the heart for maintaining normal cellular function and the quality of proteins and organelles. Defects in this process cause cardiac dysfunction and heart failure, particularly when cellular stress is increased (4). Although autophagy is implicated in various pathologic conditions, including cardiac hypertrophy, cardiomyopathy, and heart failure, there is little information on the pathophysiologic roles of autophagy in the pathogenesis of diabetic cardiomyopathy.Metformin, one of the most commonly prescribed antidiabetic drugs, improves cardiac function and reduces the incidence of myocardial infarction in type 2 diabetic patients (5,6). The UK Prospective Diabetes Study reported that metformin was more effective than sulfonylureas or insulin in reducing all-cause mortality and diabetes-related end points in diabetic patients, even though these agents decreased HbA_1c by comparable magnitudes. These findings suggest that metformin provides cardiovascular protection independent of its hypoglycemic effects (7).Indeed, metformin ameliorates cardiac dysfunctions induced by global ischemia, without affecting blood glucose in nondiabetic animals (8,9), by activating the AMP-activated protein kinase (AMPK) (10,11). AMPK acts as a sensor of cellular energy status and controls several cellular functions in the cardiovascular system, including protein synthesis (12,13), apoptosis (14–16), and autophagy (17,18) in physiologic and pathologic conditions, such as hemodynamic stress (12,13), myocardial ischemia, and reperfusion injury (16,19,20). However, the roles and molecular mechanisms by which AMPK regulates diabetic cardiomyopathy remain to be established.Diabetic cardiomyopathy, which develops in diabetic patients in the absence of coronary artery disease or hypertension (21–24), is a major cause of heart failure in diabetic patients. It is characterized by reduced cardiomyocyte contractility, cardiac apoptosis, mitochondrial pathology, and dysfunction (25,26). Despite the importance of this complication, the underlying mechanisms of diabetic cardiomyopathy are still poorly understood. Thus, this study was designed to test whether decreased autophagy is associated with the development of cardiomyopathy in diabetic OVE26 mice and to evaluate whether metformin improves cardiac function by modulating autophagic activity in diabetes.