This study demonstrates that metformin therapy significantly retards the progression of ischemic cardiomyopathy and heart failure in mice after myocardial infarction. The key findings in this study are: (1) metformin therapy improved survival by 47% after permanent occlusion of the left coronary artery compared to treatment with saline, (2) metformin therapy led to significant improvements in cardiac remodeling and function during heart failure, (3) metformin-mediated cardioprotection was associated with an increase in phosphorylation of AMPK and eNOS and an increase in expression of PGC-1α and (4) metformin therapy attenuated mitochondrial dysfunction during heart failure. These data provide additional insight into the pleiotropic effects of metformin in cardiovascular disease and its therapeutic role in ischemic induced heart failure.
The pleiotrophic actions of metformin are thought to be mediated by the activation of AMPK18,19
, an important regulator of diverse cellular pathways20
. Chronic activation of AMPK phosphorylates transcription factors altering gene expression21
and modulates mitochondrial biogenesis22
. In vitro
studies have shown that AMPK activation is a key mediator of the changes in substrate utilization during cardiac ischemia and functions to maintain energy homeostasis, cardiac function and myocardial viability23
. Our data demonstrate that metformin increases AMPK phosphorylation and that the cardioprotective actions of metformin were ablated in AMPKα2 dominant negative mice. These results suggest that the chronic activation of AMPK during the development of ischemia-induced heart failure is a critical mechanism mediating the beneficial actions of metformin.
Heart failure is associated with abnormalities of mitochondrial biogenesis24
and mitochondrial injury correlates strongly with its severity25
. In heart failure, there is a decrease in activity of complexes of the respiratory chain and Krebs cycle enzymes. The reduced expression of mitochondrial proteins results in decreased mitochondrial respiration efficiency and limited ATP synthesis capacity and myocardial energy production26
. The decreased oxidative capacity of the failing myocardium therefore limits the ability of the heart to meet hemodynamic demands and leads to symptoms of heart failure. Both, eNOS and PGC-1α are important regulators of mitochondrial biogenesis and function and play important roles in the pathophysiology of HF27,28
. For instance, targeted overexpression of the eNOS gene within the vascular endothelium has been shown to attenuate cardiac dysfunction and improve survival in ischemic cardiomyopathy29
. AMPK has been shown to increase the phosphorylation of eNOS leading to an increase in eNOS activity and NO bioavailability30
. Additionally, we15
have demonstrated that metformin increases the phosphorylation of eNOS in an AMPK-dependent manner, as evidenced by the finding that metformin fails to increase eNOS phosphorylation in the hearts of AMPKα2 dominant negative mice. The results of the current study support these previous findings, as we found that metformin therapy promotes the phosphorylation of eNOS during heart failure and that the metformin-mediated improvements in LV function were abolished in the absence of eNOS. AMPK is also an upstream activator of PGC-1α and may exert its actions by increasing the expression of PGC-1α9
. PGC-1α is a member of a family of transcription coactivators that plays a central role in the regulation of cellular energy metabolism31
. PGC-1α is induced in response to conditions that demand increased myocardial ATP synthesis32,33
and has been shown to drive mitochondrial biogenesis and improve mitochondrial function in cardiac myocytes and hearts of transgenic mice28
. PGC-1α deficient mice have decreased expression of genes involved in mitochondrial oxidative phosphorylation and have decreased state 3 mitochondrial respiration rates27,33
. In our study metformin treatment increases the expression of PGC-1α during heart failure. Furthermore, we have demonstrated that metformin improves mitochondrial oxygen consumption and ATP synthesis. These beneficial actions may be mediated by an increase in PGC1-α expression and/or eNOS phosphorylation.
The findings of the current study highlight a metformin-mediated cardioprotective signaling pathway involving AMPK, eNOS, and PGC-1α Previously, we evaluated the cardioprotective effects of a single administration of metformin in the setting of acute myocardial ischemia-reperfusion injury and found that metformin reduced infarct size when it was administered at the time of reperfusion in an AMPK-eNOS dependent fashion15
. While the current study also demonstrates that metformin provides protection in a similar manner there are some important differences. First of all, the findings of the current study are significant because they demonstrate that chronic metformin therapy initiated after myocardial ischemia is beneficial for the treatment of heart failure. Importantly, we found that although a single administration of metformin at the time of reperfusion is beneficial in attenuating infarct size, this alone is not sufficient to cause a significant improvement in cardiac function after 4 wk. On the other hand, daily metformin therapy initiated at the time of reperfusion provided significant improvements in cardiac function and LV dimensions. This suggests that metformin treatment could potentially be initiated at the time of coronary artery reperfusion and then continued daily in patients undergoing myocardial ischemia to achieve a long-term improvement in cardiac function and to decrease the morbidity and mortality resulting from heart failure. These findings support several experimental and clinical studies reporting that metformin possesses significant cardioprotective actions and is safe in the setting of diabetes and HF34,35
. Although previously contraindicated in heart failure due to the potential risk for development of lactic acidosis, the Food and Drug Administration (FDA), in response to the findings of several recent studies, has now updated the prescribing information for metformin to eliminate this contraindication36
. A meta-analysis of controlled studies evaluating anti-diabetic agents and outcomes in patients with heart failure and diabetes found that metformin when compared to other anti-hyperglycemic therapies significantly reduced mortality and hospital admissions in treated patients despite a similar decrease in hemoglobin A1C
values, suggesting that metformin may have additional cytoprotective actions beyond blood glucose lowering actions34
. This observation is further supported by experimental studies demonstrating that metformin does not affect glucose values in non-diabetic rodents37
, yet improves cardiac function following in vitro
. Finally, the findings of the current study expand on our initial findings and provide data demonstrating that metformin can attenuate mitochondrial dysfunction through the activation of AMPK and the downstream signaling pathway involving eNOS and/or PGC-1α. As such, this current study is timely and provides important insights into the use of metformin treatment for cardiovascular disease in all patient populations.
The current study mainly focused on the ability of metformin to improve mitochondrial function during heart failure, however, there are certainly a number of other effects mediated by AMPK, eNOS, and PGC-1α that could account for the observed cardioprotection. In particular, the ability of metformin therapy to promote the phosphorylation of eNOS and increase NO bioavailability provides numerous potential cardioprotective actions in the setting of HF, such as vasodilation and the inhibition of oxidative stress and apoptosis39
. All of these actions in addition to the effects of NO on the mitochondria could account for the improvements in LV function following metformin treatment. In particular, the effects of NO on hemodynamics could play an important role in providing prolonged changes in afterload and coronary blood flow regulation, which could then promote LV function and improve LV ejection fraction. However, in a previous study, we found that a single administration of metformin (125 μg/kg) did not alter hemodynamics in the period immediately following its administration. Nonetheless, since we have not evaluated if chronic metformin therapy could alter hemodynamics, we cannot rule out the possibility that the activation of eNOS over the period of 4 weeks can improve outcome through changes in afterload and/or coronary blood flow. Therefore, additional studies are warranted to fully understand the cardioprotective signaling mechanisms of metformin in the treatment of heart failure.
In summary, our findings demonstrate that low dose metformin administered at the time of reperfusion and daily improves survival and affords significant cardioprotection against ischemia-induced heart failure by improving mitochondrial function via activation of AMPK and the downstream signaling pathway involving eNOS and PGC-1α. These data suggest that metformin therapy should not be limited to the treatment of hyperglycemia but may rather have practical clinical use following myocardial infarction in all patient populations to reduce the morbidity and mortality from ischemia induced heart failure.