Abbreviations: I/R: ischemia-reperfusion; H/R, hypoxia/reoxygenation; PFKFB3, 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase isoform 3; AAV; adeno-associated virus
What is already known:
Pinocembrin protects myocardial from ischemic injury in animals.
What this study adds:
Pinocembrin postconditioning significantly reduced the infarct size and improved cardiac contractile function after acute myocardial I/R.
Acute cardioprotective benefits of pinocembrin are mediated in part via glycolytic stimulation through PFKFB3.
What is the clinical significance:
Pinocembrin may be an effective agent for improving tissue perfusion in I/R-related diseases.
Introduction
Cardiovascular disease remains the leading cause of death worldwide (Bice & Baxter, 2015). Currently, there are few effective drugs to protect the heart after ischemia/reperfusion (I/R) injury. Therefore, it has become a hot research field to find the molecular mechanism of coronary artery disease progression and development that can protect myocardium from I/R injury. With this in mind, more and more attention is focused on pharmacological interventions because it can induce cardioprotection and easy to implement (Zheng et al., 2017). However, for patients with ischemic heart disease, there are still many restrictive drugs available clinically.
Traditional Chinese medicine has a history of thousands of years and provides a large amount of medicinal materials. Pinomline (5,7-dihydroxyflavone) is an abundant flavonoid isolated from propolis and some plants. It has various biological functions such as anti-inflammatory, anti-oxidation, anti-bacterial, etc. (Hanieh et al., 2017). Previous studies have confirmed that pinocembrin confer neuroprotective effects during cerebral ischemic I/R (Saad, Abdel Salam, Kenawy, & Attia, 2015; Shi et al., 2011; Tao, Shen, Sun, Chen, & Yan, 2018). Recent research has also tried to determine whether pinocembrin is beneficial for cardiac injury. For example, several studies suggest that pinocembrin can improve the cardiac function of myocardial I/R rats, reduce ventricular arrhythmia, and reduce the area of myocardial infarction (Lungkaphin et al., 2015; Zhang, Xu, Hu, Yu, & Bai, 2018). However, it is still unclear whether pinocembrin given at the onset of reperfusion has cardioprotective effects, which is a more clinically effective method. In addition, the underlying mechanism by which pinocembrin can provide cardioprotection is largely unknown.
Recent studies have confirmed that there are changes in energy metabolism in many human diseases, and targeted energy metabolism may have therapeutic potential on these diseases (Gohil et al., 2010). Due to the mechanical function of the heart, it is an organ with high energy requirements. In general, most of the energy (about 70%) of a healthy heart comes from the β-oxidation of fatty acids, and the rest of the energy comes from glucose oxidation (Lopaschuk, Ussher, Folmes, Jaswal, & Stanley, 2010). Nevertheless, in a pathological environment, substrate utilization may change (Lionetti, Stanley, & Recchia, 2011). In patients with diabetes, circulating blood glucose levels at admission are related to the clinical outcome after acute myocardial infarction (AMI), suggesting that it may be related to myocardial metabolism (Malmberg, Norhammar, Wedel, & Ryden, 1999). Metabolic shift from β-oxidation to glycolysis metabolism will reduce the cell’s need for oxygen by 11-13%, and NAD+ precursors have been shown to activate cellular glycolysis to protect the heart from ischemic injury (Nadtochiy, Wang, Nehrke, Munger, & Brookes, 2018). It is worth noting that redirect energy metabolism to glycolysis can reduce oxidative damage and inhibit apoptosis (Hunter, Hendrikse, & Renan, 2007; Jeong, Kim, Cho, & Kim, 2004; Vaughn & Deshmukh, 2008). However, few agents that target energy metabolism are clinically safe and useful for patients. At the same time, it is unclear whether and how pinocembrin regulates acute myocardial I/R glycolysis.
To address these issues, this study was designed to (i) the current study was designed to determine the role of pinocembrin in rat and mouse cardiac I/R injury ex vivo and in vivo respectively. (ii) clarify the effects of pinocembrin on the glycolytic metabolism during I/R; (iii) explore the underlying molecular basis that contributes to the pinocembrin-induced cardioprotection. Our results provide new insight into the mechanism of pinocembrin-induced cardioprotection and suggest a potential application value of pinocembrin in the protection of hearts against I/R injury.
Methods
2.1 Chemicals and reagents
Pinocembrin were obtained from Sigma-Aldrich (St. Louis, MO, USA). 3-(3-Pyridinyl)-1-(4-pyridinyl)-2- propen-1-one (3PO) was from EMD Millipore (MA, USA). All other reagents were from Sigma-Aldrich (St. Louis, MO, USA).
2.2 Animals
250-300 g Adult Male Sprague-Dawley (SD) rats and C57BL/6 mice (age, 8-10 weeks) from Shanghai Slac Laboratory Animal Co. Ltd. were cared in accordance with the Guidelines for Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication, 8th Edition, 2011). Animal procedures were approved by the Institutional Review Board of Shanghai University of Health and Medicine (Shanghai, China).
2.3 I/R injury model in Langendorff-perfused rat hearts
As mentioned before, the heart was rapidly excised at 37 °C and perfused with Krebs-Henseleit buffer (KHB) at a constant pressure of 80 mmHg using Langendorff technique (Xie et al., 2005). Left ventricular (LV) pressure was monitored with a water-filled latex balloon connected to a pressure sensor (AD instrument, Bella Vista, New South Wales, Australia) and inserted into the left ventricle cavity to achieve left ventricular end-diastolic pressure (LVEDP) at 0 to 10 mm Hg. PowerLab system (AD instrument, Australia) was used to monitor left ventricular imaging pressure (LVDP), left ventricular imaging pressure (LVEDP), maximum rate of pressure changes over time (+ dP/dtmax) and pressure decay over time (-dP/dtmax). Pinocembrin’s final concentration of 10, 30 and 100 µM perfusate was added with 5 minutes of reperfusion. In order to assess the infarct size, the isolated rat heart was re-perfused for 2 hours after 30 minutes of ischemia. The slices were incubated in 1% w/v triphenyltetrazolium chloride (TTC, pH 7.4) for 15 min, and then fixed in 10% formaldehyde. Image-Pro-Plus software (media cybernetics) was used to calculate the infarct area. The infarct area was expressed as a percentage of the LV area at risk.
2.4 Myocardial I/R Model
Surgical ligation of the left coronary artery (LCA) was performed as described previously (Qin et al., 2017). Briefly, mice were anesthetized with ketamine (50 mg/kg) and pentobarbital sodium (50 mg/kg) by intraperitoneal injection, followed by orally intubated and ventilated. The core body temperature is always maintained at 37 °C. An internal sternotomy was then performed with electrocautery, and then the proximal LCA was displayed and ligated. After 30 minutes of coronary artery occlusion, the suture was cut and the blood vessels were allowed to reperfusion. After 24 hours of reperfusion, the mice were anesthetized with isoflurane. Transthoracic echocardiography was used to determine the left ventricular ejection fraction. After reperfusion, blood samples were collected and centrifuged at 3000 rpm for 10 minutes for cardiac troponin-T (cTnT) and lactate dehydrogenase (LDH) measurements. Measurement of area at risk and infarct size was performed as reported previously. Each of the myocardial slices were weighed and the areas of infarction, risk, and nonischemic left ventricle were assessed by a blinded observer using computer-assisted planimetry (NIH ImageJ 1.37).
2.5 Isolation and culture of primary cardiomyocytes
Continuous enzymatic digestion and isolation were used to obtain neonatal rat and mouse cardiomyocytes (Irvine et al., 2013; Irvine et al., 2012). Neonatal rats were decapitated and hearts were immediately placed in HBSS. The ventricle was taken and digested with trypsin overnight at 4 °C and collagenase four times at 37 °C for 10 min. Cardiomyocytes were suspended in sterile DMEM, supplemented with penicillin 100Uml-1, streptomycin 100 mgml-1 and 10% fetal bovine serum. The cells were pre-plated twice (37 °C for 45 minutes) to reduce fibroblast contamination. Hypoxia/reoxygenation (H/R, which simulated MI/R in vivo ) was employed as previously described (Hou et al., 2019).
2.6 Measurement of LDH release and cTnT release
Necrotic cell death was evaluated by supernatant LDH activity, as in previous studies (Kishi et al., 2015). A spectrophotometric kit (Nanjing jiancheng, Jiangsu, China) was used according to the manufacturer’s instruction. In short, 20μl supernatants were collected in a 96-well, then 25μl matrix buffer and 5μl coenzyme I was added to the 96-well. The mixture was incubated at 37°C for 15 min. After 25μl 2,4-dinitrophenylhydrazine was added to each well and incubated at 37°C for 15 min, 250μl 0.4M NaOH was added to each well and the mixture was incubated at room temperature for 5 min. The absorbance value was measured at 450nm with the spectrophotometer (BioTek, VT, USA) and the LDH activity was calculated. Plasma cTnT levels as an indicator of cardiomyocyte damage was measured using a mouse cTnT ELISA kit (Elabscience Biotechnology Co., Ltd, Wuhan, China) according to the manufacturer’s instruction.
2.7 Quantitative real-time PCR
We isolated total RNA was from the heart tissue with Trizol Reagent (Invitrogen, Carlsbad, CA, USA). Relative quantitation by real-time PCR involved SYBR Green detection of PCR products in real time with the ABI PRISM 7700 Sequence Detection System (Applied Biosystems). The primers were list in Supplemental Table 1. The reactions were conducted in triplicate by heating the reactant to 95 °C for 5 min, followed by 40 cycles of 94 °C for 30 s, 58 °C for 30 s and 72 °C for 30 s.
2.8 Western blot analysis
Left ventricles were homogenized and the cells were lysed as described previously (Matsushima et al., 2013). The samples were analyzed by SDS-PAGE. Transfer the protein to a polyvinylidene fluoride microporous membrane (Bio Rad) with primary antibody PFKFB3 (Abcam, USA; 1: 1000) and anti-GAPDH (internal control; Kangcheng Co., Ltd., Shanghai, China; 1: 8000). The secondary antibody was coupled to horseradish peroxidase (Cell Signaling Technology, 1: 6000). Enhanced ECL detection kit (Amersham Pharmacia Biotech) was employed to visualize the immunoreaction, followed by exposing to film and quantified with a video documentation system (Gel Doc 2000, Bio-Rad).
2.9 Generation and administration of adeno-associated virus (AAV)
Serotype 9 AAV vectors (AAV9) encoding shNC or shPFKFB3 (AAV9-shNC and AAV9–shPFKFB3) were prepared as previously described (Xie et al., 2016). 3 x 1011 vg of AAV9-shNC or AAV9–shPFKFB3 were injected intravenously into tail veins as previously described (Somanathan et al., 2014) of 4-5 weeks old male C57 mice. Sham or myocardial ischemia surgeries were conducted 4 weeks after AAV9 injection.
2.10 Seahorse extracellular flux analyzer assays
Cellular bioenergetics was measured using a Seahorse XFe24 extracellular flux analyzer in intact cardiomyocytes. We conducted glycolysis stress testing according to the manufacturer’s instructions previously reported (He et al., 2017). Glycolysis stress test: Cells were incubated in glucose-free Seahorse assay media supplemented with 1mM pyruvate at 37oC in incubator without CO2 for 1-h prior to the assay. Injectors were loaded to add 20mM Glucose, 1μM Oligomycin and 100mM 2 Deoxy-Glucose (2-DG) and glycolysis, glycolytic capacity and glycolytic reserves were calculated as extracellular acidification rate (ECAR).
2.11 Statistical analysis
Data were presented as means ± SEM. The unpaired, 2-tailed t test was used for comparisons between 2 groups. For multiple comparisons, ANOVA or repeated ANOVA followed by the LSD post hoc test was used with GraphPad Prism® version 7.0 software. A P value < 0.05 was considered as statistically significant.
Results
3.1 Compound pinocembrin significantly improved the cardiac function and reduced infarct size after I/R ex vivo
We perfused isolated rat hearts to explore the cardioprotective effects of pinocembrin against I/R injury, 10 to 100 µM pinocembrin were delivered during the first 5 min of reperfusion (fig 1A). During 45 min reperfusion following 30min ischemia, the contractile function of left ventricular (LV), including LV developed pressure (LVDP), LV end-diastolic pressure (LVEDP), maximal rates of pressure development over time (+dP/dtmax) and pressure decay over time (−dP/dtmax) were significantly suppressed (fig 2 A-D). Pinocembrin itself does not affect the heart rate during reperfusion (Supplementary fig 1), while it dose-dependently improved the postischemic myocardial performance from 10 to 100 µM (fig 2 A-D).
Next, we explore whether pinocembrin improves cell survival during I/R via examining lactate dehydrogenase (LDH) release, an indicator of myocardial injury. Little LDH release was detected in the coronary efflux before ischemia, while LDH release was obviously induced at the end of reperfusion, while pinocembrin significantly inhibited the release of LDH from 10 to 100 µM (fig 2E). Consistently, the I/R-induced infarction after 2 h of reperfusion was significantly attenuated by pinocembrin at the concentration of 30 µM (fig 2F). These data demonstrated that pinocembrin exhibits beneficial effects on cardiac I/R injury ex vivo .
Compound pinocembrin protects hearts from myocardial I/R injuryin vivo
To explore the cardioprotective effects of pinocembrin on myocardial I/R injury in vivo , pinocembrin was intravenously injected into wild-type (WT) mice 5 minutes before the end of sustained ischemia (i.v., 5 mg/kg and 10 mg/kg), followed by reperfusion for 24 hours (fig 1B). Evans-blue/TTC dye method was used to determine the infarct size. After reperfusion for 24 hours, there was no difference of area at risk (AAR) between each group (Figure 3A and 3B). Nevertheless, compared with I/R group, pinocembrin i.v. treatment significantly reduced the infarct size by 20% (fig 3A and 3C). Besides, plasma levels of cTnI and LDH activity were markedly elevated during myocardial I/R, which were both suppressed with pinocembrin i.v. treatment (fig 3 D-E). Furthermore, the echocardiographic results showed that pinocembrin can significantly improve I/R-suppressed the ejection fraction (EF%) and fractional shortening (FS%) (fig 3F).
3.3 Protective actions of pinocembrin against cardiomyocyte injury responses in vitro
Next, we examined the direct effects of pinocembrin on cardiomyocytes. To determine whether pinocembrin confers cardioprotective effects through its direct action on the cardiomyocytes, we subjected the isolated neonatal rat and mouse cardiomyocytes to H/R and applied 30 µM pinocembrin to cells during the onset of reperfusion. In line with the effects of pinocembrin on the myocardial I/R injury, simulated I/R-reduced cell viability was significantly improved by pinocembrin (data not shown). Moreover, our data demonstrated that pinocembrin rescued cardiac troponin I (cTnI) release and LDH release post-simulated I/R in vitro , in both rat and mouse cardiomyocytes (fig 4 A-D).
Pinocembrin increases glycolysis in cardiomyocyte
During myocardial ischemia, enhanced glycolytic metabolism is essential for maintaining homeostasis of cardiomyocytes. In addition, previously studies reported that pinocembrin is involved in regulating glucose uptake in cancer cells (Liu et al., 2018). Subsequently, we explored the effects of pinocembrin on cellular bioenergetics with the Seahorse extracellular flux analyzer and performed glycolysis stress tests to measure glycolysis and glycolytic capacity both in intact rat and mouse cardiomyocytes. We observed that brief pretreatment with pinocembrin versus the control increased glycolysis by 21.4% (extracellular acidification rate [ECAR]). Pinocembrin also increased glycolytic capacity in cardiomyocyte by 23.7% (fig 5 A-B).
To further explore the underlying mechanism of pinocembrin promoting myocardial glycolysis, mRNA expressions of a panel of glycolysis-related genes was determined with qRT-PCR. As shown in Fig 5C, pinocembrin significantly increased the expression of glycolysis-related genes, especially PFKFB3 gene.
3.5 PFKFB3 inhibition alters glycolysis and abolished pinocembrin-induced cardioprotection in cardiomyocytes
Next, we explored whether blockade of PFKFB3 inhibits cardiomyocyte glycolysis and impairs pinocembrin-afforded cardioprotective effects. As shown in Fig 6 A-D, exposure of cardiomyocyte to 10 µM PFKFB3 inhibitor, 3PO remarkably reversed pinocembrin-enhanced glycolysis. What’s more, inhibition of PFKFB3 resulted in a significant increase of cTnI release and LDH release post-H/R in vitro (fig 6 E-H), suggesting that disruption of glucose metabolism leads to impaired cardioprotection of pinocembrin during H/R.
3.6 PFKFB3 deficiency in normal mice using AAV9 abolished pinocembrin improved cardiac injury and dysfunction after MI/R.
Since that pinocembrin alleviated H/R-induced cardiomyocytes death by upregulating glycolysis via PFKFB3, we further evaluated the roles of PFKFB3 on myocardial I/R injury in vivo . To determine whether PFKFB3 regulation is directly involved in pinocembrin-related MI/R injury improvement, the WT mice were injected with AAV9 encoding PFKFB3 shRNA to knockdown endogenous PFKFB3. Intravenous injection of AAV9-shPFKFB3 on mice has successfully reduced protein level of PFKFB3 in heart (Supplementary fig 2). Subsequently, we exposed the mice to in situ myocardial ischemia–reperfusion to assess the functional role of myocardial-specific PFKFB3 in pinocembrin-afforded cardioprotection. We measured myocardial injury by infarct size area, serum cTnT levels and LDH activity. Exposure of mice infected with AAV9-shPFKFB3 to myocardial ischemia followed by reperfusion revealed larger infarct sizes and elevated levels of the myocardial necrosis marker troponin I and LDH release (fig 7 A–C). More importantly, delete myocardial PFKFB3 abolished pinocembrin-conferred protective effects on those indexes. Furthermore, the pinocembrin-improved EF and FS were also abolished by PFKFB3 deficiency with AAV9-shPFKFB3 (fig 7D). Collectively, these data support that pinocembrin confers cardioprotective effects by enhanced cardiomyocytes glycolysis through activation of PFKFB3.