The Cardioprotective Effect of Metformin in Doxorubicin-Induced Cardiotoxicity: The Role of Autophagy
The molecular mechanisms underlying doxorubicin-induced cardiotoxicity are still being investigated, but are known to involve oxidative stress, mitochondrial dysfunction, and the dysregulation of autophagy. The objective of the current study was to examine the protective role of metformin and its effect on autophagy in doxorubicin-induced cardiotoxicity. Sprague-Dawley rats were divided into four groups at random. The doxorubicin-treated group received doxorubicin (3 mg/kg every second day)
... aperitoneally. The metformin-treated group received 250 mg/kg/day metformin via gavage. The doxorubicin + metformin-treated group received both at the above-mentioned doses. The control group received vehicle only. Following the two-week treatment, the hearts were isolated, and cardiac functions were registered. Serum levels of lactate dehydrogenase (LDH), creatine kinase iso-enzyme MB (CK-MB) enzyme, Troponin T, and cardiac malondialdehyde (MDA) were also measured. Heart tissue samples were histopathologically examined by using Masson's trichrome staining and Western blot analysis was conducted for evaluating the expression level of AMP-activated protein kinase (AMPK) and autophagy-associated proteins beclin-1, LC3B-II, and p62, respectively. The results revealed that treatment with metformin conferred increased cardiac protection against the development of cardiotoxicity manifested by a significant decrease in serum Troponin T and cardiac MDA levels, and remarkable improvement in heart function in connection with histopathological features. Furthermore, by focusing on the contribution of autophagic proteins, it was found that metformin normalised autophagy, which may help cardiomyocytes survive doxorubicin-induced toxicity. These results promote the use of metformin, which would be a preferable drug for patients receiving doxorubicin. DOX-induced cardiotoxicity are multifactorial and are still unclear, but mitochondrial dysfunction, oxidative stress, apoptosis, and dysregulation of autophagy are involved [6, 7] . Furthermore, the heart is very susceptible to DOX-induced lipid peroxidation and toxicity because of its high energy requirement and mitochondrial density  . Autophagy is a highly conserved process which is aimed to maintain cell and tissue homeostasis, and involves the elimination of damaged and long-lived organelles under both physiological and pathological conditions , including energy and oxygen status, nutrient starvation, and modification in metabolism  . Three types of autophagy can be described including microautophagy, chaperone-mediated autophagy, and macroautophagy. The autophagy term hereafter refers to macroautophagy in this study. The role of autophagy in cardiac tissue is apparently dual from the view of survival or death; it depends on the type and the duration of the stress  . Several studies have found that DOX treatment affects autophagy, however, it is still not clearly elucidated how DOX alters this process. Previous studies on this matter have shown many controversial results [7, 12] . Recently reported studies have demonstrated that DOX induces autophagy; however, it causes dysregulation in the autophagic flux and the autophagic process cannot be completed  . These findings are also supported by Hill and co-workers, showing that the administration of DOX inhibits the lysosomal acidification, thus causing disruption in the autophagy flux  . In addition, Tokarska et al. reported that DOX can cause dysregulation in most processes of myocardial energy metabolism, such as the AMP-activated protein kinase (AMPK) signaling pathway  . AMPK is a major sensor of cell energetic homeostasis. Low cellular energy levels and increased reactive oxygen species (ROS) result in the phosphorylation and activation of AMPK, which is able to induce autophagic processes    . Metformin (MET) is an orally used first-line anti-diabetic drug for the treatment of type 2 diabetes. Several studies reported that application of MET decreases mortality and cardiovascular end-points of type 2 diabetes and has protective effects in cardiac function [18, 19] . Recently, several studies have found that MET activates AMPK, and through the AMPK signaling pathway it induces cardiac autophagy and improves cardiac functions. Indeed, Kobashigawa and colleagues demonstrated that cardioprotective effect of MET against DOX-induced toxicity is mediated via upregulation of AMPK and its downstream target molecules  . However, high doses of MET treatment induce the same alteration in the AMPK pathway, but its protective effect is lost. The authors suggested that this could be due to the downregulation of platelet-derived growth factor receptor. Furthermore, silencing of adiponectin receptors supressed AMPK activation and cell viability in . Nowadays, several extensive studies have investigated the role of autophagy in DOX-induced cardiotoxicity, but we are one of the first research groups to investigate the effect of MET in the autophagic process in DOX-treated animals under in vivo followed by ex vivo conditions. Various studies have revealed that boosting or restoring autophagy could help the cardiomyocytes to survive during DOX therapy. In the present study, we co-administered DOX and MET in order to investigate the role of MET in the autophagic process and its cardioprotective properties in DOX-induced cardiotoxicity. Thus, our investigation may offer further understanding of the role of cardiac autophagy in DOX-treated animal subjects. In addition, our hypothesis was that MET could activate AMPK, restore autophagy, and improve cardiac function, which may consequently mean that DOX co-administered with MET help the cardiomyocytes to survive. This could be a promising new strategy for patients suffering from cancer and receiving the DOX regimen. Moreover, it remains a challenge to find an effective agent which might be combined with DOX that is able to reduce its cardiotoxicity, whilst maintaining its efficacy and safety in tumor therapies.