We report for the first time that NRG-1 improves the density of capillaries and arterioles in the different ischemic regions post AMI. In addition, NRG-1 also improved left ventricular function after infarction and inhibited myocardial fibrosis in a rat model of AMI. Mechanistically, the protective effects of NRG-1 were mediated via activation of the proangiogenic VEGF/VEGFR2 and PI3K/AKT/eNOS pathways in the cardiac tissues.
The rat model of AMI has been established previously [18]. AMI results in myocardial cell apoptosis, cardiac inflammation and myocardial fibrosis that adversely affect cardiac systolic and diastolic functions [22]. It eventually leads to ventricular wall thinning, left ventricular cavity expansion and cardiac ventricular remodeling, all of which increase the risk of heart failure [23]. We found that NRG-1 significantly inhibited myocardial collagen deposition post-AMI and improved cardiac function. The NRG-1/ErbB system is crucial for adapting to cardiac demands, and its disruption reduces tolerance to myocardial ischemia [24]. NRG-1 administration also improves cardiac function in animal models of ischemic heart disease, dilated cardiomyopathy and viral cardiomyopathy [8]. At the cellular and molecular level, it improves cardiomyocyte survival [25], ameliorates myofilament injury [26], prevents mitochondrial dysfunction [27], enhances reparative inflammatory response [28], and increases calcium intake in the sarcoplasmic reticulum [29].
Our study showed a pro-angiogenic effect of NRG-1 in an animal model of AMI, which was the likely basis of improved cardiac function. This is consistent with Hedhli et al. who reported that exogenous injection of NRG-1 improved blood flow in an ischemia model [14], as well as our previous study wherein we found that NRG-1 increased the levels of angiogenic factors in vitro and promoted myocardial angiogenesis in vivo [15]. In the current study, NRG-1 treatment increased capillary density in peri-infarct region and arteriole density in the infarct region. Angiogenesis, or the formation of new blood vessels from pre-existing vascular beds, is activated during ischemia [30]. The ischemic and hypoxic conditions in the core infarct regions urgently require vascular compensation, resulting in increased collateral arterioles following NRG-1 treatment. However, since the injury in the peripheral region is relatively minor, NRG-1 promotes the formation of new capillary cavities that act as reserves of mature blood vessels.
Angiogenesis is a complex process that requires the participation of various angiogenic factors, pathways and cells [31, 32]. a rat model of diabetic cardiomyopathy along with other angiogenic factors [15]. PI3K-AKT-eNOS signaling cascade is closely related to angiogenesis and VEGF is the key and strongest angiogenic factor to induce angiogenesis. We found that NRG-1 significantly activated this pro-angiogenic pathway in both the infarct and peri-infarct regions, which is consistent with previous studies showing that NRG-1 induces angiogenic factors in various tissues and cells. For instance, NRG-1β promotes glucose uptake in neonatal rat cardiomyocytes via the PI3K/Akt pathway [33], and induces VEGF secretion by endothelial cells [34]. The PI3K-Akt-eNOS pathway lies downstream of VEGF/VEGFR [35, 36], although the relationship is more complex in vivo. Nevertheless, our results clearly show that they are critical to myocardial angiogenesis post AMI.
Our study has several limitations that should be acknowledged. First, although the hemodynamic parameters improved significantly following NRG-1 treatment, the actual infarct size was not measured. Second, the use of α-SMA as a marker of angiogenesis is controversial despite recent studies showing that activated fibroblasts express higher levels of α-SMA following myocardial infarction [5, 36]. Third, the effect of NRG-1 on the angiogenic pathways was not validated by genetic or pharmacological inhibition of the said factors. Finally, it remains to be clarified whether NRG-1 regulates other angiogenic pathways in AMI.
In conclusion, NRG-1 improved cardiac function and promoted myocardial angiogenesis in a rat model of AMI by up-regulating VEGF and activating the PI3K-Akt-eNOS pathway.