The results of the present study suggested that FGF21 has the ability to attenuate myocardial I/R injury by inhibiting NET formation. Furthermore, the cardioprotective effect of FGF21 was mediated through the involvement of AMPK.
A large number of previous studies have found that FGF21 attenuates myocardial I/R injury, increases cardiomyocyte activity, and decreases the rate of cardiomyocyte apoptosis, which is consistent with our study [19–21]. In a rat model of myocardial I/R injury, a notable upregulation in the expression of NETs was observed within the ischemic zone. Interestingly, facilitating the degradation of NETs through deoxy ribonuclease I led to a reduction in myocardial infarct size and a significant improvement in left ventricular remodeling induced by I/R injury in rats [22]. In a mice model of chronic pancreatitis, mice treated with FGF21 significantly reduced levels of MPO, a biomarker of NETs, and improved the inflammatory state of serum, pancreatic, and peritoneal macrophages [23]. Significantly, our current investigation, within a myocardial I/R injury model, has unveiled for the first time that FGF21 exhibits the capacity to inhibit the formation of NETs induced by I/R injury, thereby attenuating myocardial I/R injury.
Previous studies have shown that FGF21 is an endocrine factor that can regulate energy metabolism and that AMPK, as an energy sensor, is a target for FGF21 signaling [24]. FGF21 activates AMPK both directly through the FGFR1/klotho-β complex and indirectly by inducing the expression of lipofuscin and corticosterone [25]. Once activated, AMPK plays a crucial role in regulating mitochondrial activity by stimulating SIRT1 and PGC-1α [26]. Furthermore, AMPK contributes to the maintenance of lipid homeostasis, protein synthesis regulation, and promotion of glucose metabolism, all aimed at sustaining optimal intracellular adenosine triphosphate levels [27].
Beyond its role in stimulating AMPK to regulate energy metabolism, FGF21 also exhibits anti-inflammatory properties. Studies have shown that a lack of FGF21 increases susceptibility to inflammation, leading to local and systemic inflammatory responses, and supplementation with exogenous FGF21 has been shown to reverse elevated levels of these inflammatory markers [28–30]. In hypertensive and diabetic mice models, the absence of FGF21 resulted in a pronounced exacerbation of inflammatory response and oxidative stress. Conversely, administration of FGF21 significantly alleviated these conditions by promoting the phosphorylation of AMPK, highlighting the crucial role of FGF21 in mitigating inflammatory and oxidative stress pathways [31, 32]. Subsequent investigations have unveiled that FGF21 exerts its anti-inflammatory, antioxidant stress, and inhibitory effects on myocardial remodeling and apoptosis through two distinct pathways. The first pathway involves the AMPK-AKT2-Nrf2 antioxidant pathway, while the second pathway operates through the AMPK-ACC-CPT-1 lipid-lowering pathway, both of which are mediated by AMPK [33].
Studies have shown that imbalances in the mechanisms that produce or eliminate NETs can lead to tissue damage, which is an important mechanism in autoimmune and inflammatory diseases that are closely linked to cardiovascular disease, and that inhibiting NETs formation has been shown to be beneficial [34, 35]. Several studies have consistently demonstrated that AMPK, known for its anti-inflammatory effects, plays a pivotal role in the formation of NETs [36–39]. Activation of AMPK both inhibits NETs formation and promotes NETs clearance to reduce NETs levels[40, 41]. However, inhibition of AMPK significantly increased the levels of NETs in the peripheral circulation and in ischemic tissues [42].
The present study has some limitations. Due to the limitation of funds and experimental equipment, cardiac ultrasonography was not performed in I/R-injured mice in this study, so it was not possible to judge the improvement of cardiac function in mice after the application of FGF21. Although the present study suggests a possible mechanism by which FGF21 inhibits the formation of NETs through activation of AMPK, the actual biological mechanism is likely to be more complex, and other unconsidered signaling pathways or cell types may also be involved.