Discussion
In the present studies, we address the functional roles of glycolysis in mediating cardioprotective responses of pinocembrin. We demonstrated that (i) pinocembrin delivered at the onset of reperfusion (postconditioning) significantly improved post-ischemic myocardial function and reduced infarct size after I/R ex vivo ; (ii) pinocembrin pretreatment significantly protected mouse hearts from acute myocardial I/R injury in vivo ; (iii) those protection is at least partially related to enhanced glycolysis by pinocembrin in the I/R cardiomyocytes; and (iv) the cardioprotective effects of pinocembrin are mediated by the activation of PFKFB3. These results extend previous findings indicating the cardioprotection of pinocembrin against I/R injury and reveal the new mechanisms of pinocembrin in the cardioprotection.
It would be an attractive treatment principle to reduce the infarct size through pharmaceutical intervention to assist classic reperfusion intervention (Morel et al., 2012). Pinocembrin is a potential cardiovascular drug with potential neuroprotective effects on transient and long-term ischemic stroke in rats (Wu et al., 2013). Also, previous study indicated that administration of pinocembrin before myocardial ischemia improved LV function (Lungkaphin et al., 2015). Pharmacological postconditioning is easier to implement and has therapeutically potential in both clinical and experimental setting, which avoids the potential injury induced by ischemic conditioning, therefore has good clinical application prospects (Heusch, 2015). Therefore, our study aims to explore the cardioprotective effects of pinocembrin postconditioning. The ex vivo results showed that pinocembrin from 10 to 100 µM delivered at the first 5 minutes of reperfusion remarkably improved post-ischemic myocardial function and attenuates cell death in a concentration-dependent manner. Furthermore, in vivo mouse myocardial I/R injury model was prepared and pinocembrin postconditioning was fulfilled by an intraperitoneal injection of pinocembrin (5 mg/kg and 10 mg/kg body weight) 5 min before reperfusion. Cardiac function, serum LDH activity and cTnT content, and infarct size were significantly improved with pinocembrin treatment. These data are consistent with the observation of cardioprotective effects of pinocembrin preconditioning others reported.
Various metabolic abnormalities occur during myocardial I/R, such as increased fatty acid oxidation and decreased glucose oxidation (Fillmore, Mori, & Lopaschuk, 2014). This phenomenon is related to uncoupling of mitochondrial respiration, increased proton leakage, ROS formation and, more importantly, increased myocardial oxygen consumption (Boudina & Abel, 2006). During myocardial I/R, the metabolic shift aimed at increasing glucose oxidation have proved to be beneficial. Although some therapeutic strategies have tried to reverse this metabolic imbalance, there is still no approved treatment regimen so far (Jaswal, Keung, Wang, Ussher, & Lopaschuk, 2011; Kantor, Lucien, Kozak, & Lopaschuk, 2000; Taniguchi et al., 2001). In addition, strategies aimed at increasing clinical glucose consumption have different results and have not yet reached routine clinical practice. Looking for drugs that can safely induce the transfer of cellular energy metabolism and a better understanding of the protective mechanisms of increased glucose oxidation may facilitate transfer to the clinic. Our results show that increasing glycolysis is responsible for pinocembrin induced protective effects. This is consistent with previous studies that metabolic shift towards increased glycolysis protects the heart from I/R injury (Nadtochiy et al., 2018). Moreover, PFKFB3 expression is notably upregulated during I/R, which directs cellular glucose metabolism from PPP to aerobic glycolysis (Li et al., 2019). Our data shown that PFKFB3 was significantly upregulated with pinocembrin treatment and specific inhibitor of PFKFB3 abolished pinocembrin-afforded protective effects in cardiomyocytes. Most importantly, knock down of PFKFB3 in myocardium using AAV9 reversed the cardioprotection of pinocembrin postconditioning. Previous studies have shown that the main reason responsible for the protection offered by the metabolic shift is the fact that glycolysis is able to produce two molecules of ATP without need for oxygen, so that uncoupling between mitochondrial full glucose oxidation and glycolysis leads to increased cardiac efficiency (Lopaschuk, 2017). However, potential protective mechanisms of the metabolic shift are largely unknown. Further studies are needed to investigate how pinocembrin regulate the glycolysis and search for its putative downstream targets.
In summary, our findings demonstrate that compound pinocembrin exhibits significant protective effects on cardiac I/R injury when delivered at the beginning of or before reperfusion in ex vivo rat andin vivo mouse models through enhancing glycolysis. Pinocembrin may be considered as an effective lead compound for large animal experiments, and is expected to be used in clinical research of acute myocardial infarction
Acknowledgements:
This work was supported by the Project of National Natural Science Foundation of China [grant numbers 81700354, 81970229].
Author contributions:
ZYJ and GXF performed and analysed the experiments. ZYJ wrote the first draft of the manuscript. WGQ participated in the analysis of the experiments. LJR revised the manuscript. ZYJ and LJR contributed to the experimental design and revised the manuscript.
Conflict of interest: No conflicts of interest.
Declaration of transparency and scientific rigour
This Declaration acknowledges that this paper adheres to the principles for transparent reporting and scientific rigour of preclinical research as stated in the BJP guidelines for Design & Analysis, Immunoblotting and Immunochemistry, and Animal Experimentation, and as recommended by funding agencies, publishers and other organisations engaged with supporting research.