In this report, we found that TERT was significantly upregulated after myocardial ischemia-reperfusion injury (MIRI). TERT can promote proliferation, attenuate apoptosis, and enhance autophagy in myocardial cells after MIRI.
The protective role of TERT in promoting cell proliferation and reducing apoptosis has also been reported in many other fields6. Our previous study showed that TERT can protect neurons from apoptosis induced by hypoxic-ischemic injury10. Oh reported that TERT overexpression can increase telomerase activity in cardiomyocytes and maintain telomere length, thus delaying irreversible cell cycle exit and promoting cell survival7. TERT can also promote proliferation, hypertrophy, and survival of cardiomyocytes, and both in vivo and in vitro experiments have shown that TERT can reduce apoptosis induced by ischemia injury in cardiac myocytes7. In a mouse model of myocardial infarction, TERT promoted the proliferation and survival of cardiomyocytes after myocardial infarction in mice, reduced the infarction area, increased the metabolic activity in the infarcted area, and improved cardiac function. TERT also reduced scar formation by reducing fibre formation and increasing tissue remodelling and regeneration potential, thus ultimately reducing the mortality of heart failure after myocardial infarction9. Moreover, TERT did not enhance cardiac hypertrophy after myocardial infarction and thus, it did not alter heart morphology. The study also suggested that this mechanism may be related to the effect of TERT in maintaining telomere length and activating some signalling pathways associated with cardiac protection and regeneration. Thus, TERT may play a significant role in preventing heart failure after myocardial infarction9. However, whether TERT can play a role in promoting cell proliferation and anti-apoptosis in MIRI remains unclear. In this report we found that TERT expression was increased after MIRI, and that TERT overexpression promoted cardiomyocyte proliferation and reduced apoptosis after MIRI.
So far, the underlying mechanisms of TERT in cell protection upon myocardial ischemia-reperfusion injury remain to be explored. Acute myocardial reperfusion injury can occur due to complex mechanisms including reactive oxygen species(ROS) generation, Ca2+ overload, and mitochondrial permeability transition pore (mPTP) opening11. Yellon and Hausenloy concluded that the potential mediators during myocardial reperfusion injury include oxidative stresses, calcium paradox, inflammation, and mPTP1. Mitochondrial permeability transition pores may be a significant target for cardioprotection as they plays a critical role in injury1. The mitochondrial pathway mediating cell death (including apoptosis and necrosis) is critical in ischemic/reperfusion injury12. The key event in mitochondrial pathway-mediated apoptosis is mitochondrial outer membrane permeabilization resulting in the release of apoptogenic factors such as cytochrome C from the mitochondrial membrane, and activation of caspase, finally triggering apoptosis. Opening of the mPTP is a critical event in mitochondrial pathway-mediated necrosis. Ischemia results in intracellular acidosis, which leads to an increase in intracellular Ca2+; Ca2+ overload promote mPTP opening during reperfusion and stimulates ROS production. These changes lead to the cessation of ATP synthesis and mitochondrial swelling, which may finally cause necrosis12. Nevertheless, there is an interconnection between the mitochondrial pathways of apoptosis and necrosis; it is also reported that mPTP opening may cause cytochrome C release and lead to apoptosis, and that apoptosis may lead to necrosis12. These changes significantly weaken cardiac function and increase heart infarction during reperfusion3. Therefore, these researches suggest that mitochondrial function plays an important role in the mechanisms underlying MIRI.
The effect of TERT on regulating mitochondrial function has been reported recently. TERT was believed to be located in the nucleus and cytoplasm and has active forms in the nucleus. However, studies have revealed that TERT can translocate into the mitochondria after injury, suggesting that TERT may also play a role in the mitochondrial injury pathways3, 6. Emerging evidences have shown the protective effect of TERT in mitochondrial function during stress; TERT can protect mitochondrial DNA (mDNA) damage by binding to mDNA, increasing the mitochondrial membrane potential, decreasing ROS production, and increasing respiratory chain activity13, 14. However, whether this function of TERT is also present in myocytes during MIRI remains unclear. In a rat model of TERT deficiency, lack of TERT was found to increase mDNA damage and decreased the mitochondrial respiratory capacity during cardiac stress. These results confirmed a critical role of TERT in regulating mitochondrial functions in cardiac injury during stress, and suggested that TERT may regulate cardiac function during stress by regulating the mitochondrial Ca2+, ROS production, and ATP production, which finally affect cell death3. Our previous research has shown that TERT may act as a neuroprotective agent via anti-apoptosis in neurons after hypoxic-ischemic injury. The underlying mechanisms may be associated with regulating the Bcl-2/Bax expression ratio, attenuating ROS generation, and increasing mitochondrial membrane potential10. Therefore, the effect of TERT on mitochondrial pathway-mediated cell death including apoptosis or necrosis during myocardial ischemic-reperfusion injury is the next focus of our research.
Autophagy is a process that delivers and degrades cytoplasmic materials in the lysosome. It is a recycling system that produces materials and energy for cell renovation15. However, the role of autophagy in cell injury is still controversial; generally, autophagy can promote cell survival, but some evidence suggests that autophagy can also induce cell death12, 15. In the heart, autophagy can promote adaption to hemodynamic stress15. It is already reported that ischemia-reperfusion injury can induce cardiac autophagy, but the role of autophagy during ischemia-reperfusion injury is still unclear. Increased autophagic capacity is suggested to protect myocytes against ischemia-reperfusion injury16, whereas other reports have proposed that autophagy may be protective during ischemia, but may be harmful during reperfusion17. Microtubule-associated protein 1 light chain 3(LC3) is reported to be a reliable marker of autophagic activity18. In this study, the expression ratio of LC3-II/LC3-I was used to reflect autophagic activity. We found that the autophagic activity of myocardial cells was enhanced after myocardial ischemia-reperfusion injury; however, the exact role of autophagy during MIRI needs further research. Recently, a few studies have reported the role of TERT in regulating autophagy. In a model of nutrient deprivation, TERT was reported to activate autophagy activity by inhibiting mTORC1 kinase activity19. In mice with a specific genetic deletion of TERT, the autophagy activity was delayed after renal ischemia-reperfusion injury. The underlying mechanism may be mediated in part by increased mTOR signalling20. A recent study reported that TERT can promote autophagy in cancer cells under glucose deprivation through an HK2-mTOR pathway21. Our study also revealed that TERT can increase the autophagy activity of myocardial cells after myocardial ischemia-reperfusion injury. However, further research is needed to confirm the underlying mechanisms.
Further, telomere-independent effects of TERT, including regulation of gene expression, cell differentiation, and proliferation, are also reported. These effects of TERT might also be involved in the protection of myocardial ischemic injury6. TERT may participate in regulating the proliferation and differentiation of myocardial stem cells and endothelial progenitor cells, thus affecting myocardial regeneration and vascular regeneration after myocardial ischemic injury6. However, whether these mechanisms of TERT exist in MIRI and the underlying protection mechanisms of TERT in MIRI will be the focus of our next study.