Among 321 unstable angina patients in our study, there are 16% of the patients has wide QRS without BBB and near one third of them have a prolonged QTc by definition. We found the plasma FGF21 level was increased in patients with a wide QRS complex and prolonged QTc interval with and without adjusted model. From our analysis, the plasma FGF21 level was positively associated with fasting glucose, HbA1c, creatinine and adiponectin, but negatively associated with albumin and eGFR. To the best of our knowledge, this is the first study to show an association between increased plasma FGF21 level and wide QRS complex and prolonged QTc interval in human subjects.
The biological effect of FGF21 on CAD are not well understood. Literatures review showed a higher FGF21 in patient with diastolic heart failure or CAD with greater risk of MACEs [21,22] but have a cardio protecting and antiarrhythmic effect in animal studies [16-19]. This conflict observation is interesting. In previous animal study, FGF21 treatment reduced susceptibility to arrhythmia in infarcted mice hearts by suppressing miR-143 expression and regulate the EGR1-SCN5A /KCNJ2 pathway in MI [17]. However, some vivo experiments, FGF21 levels was found to be correlated with the dysregulated metabolic status. For examples, FGF21 resistance has been observed in in vivo experiments with DIO mice livers and white adipose tissue [38], and another report in ex vivo experiments with obese rat hearts [39]. Furthermore, FGF21 resistance has also been observed in clinical reports where the serum FGF21 level was significantly increased in patients with nonalcoholic fatty liver disease (NAFLD) [40], coronary heart disease [40,41], metabolic syndrome [42], and T2DM [43]. While FGF21 mainly secreted by the liver which regulates insulin sensitivity and glucose homeostasis [44]. FGF21 has also been identified in the heart as a paracrine signal protein [45]. In one clinical study, FGF21 is found abundantly secreted into the plasma in response to cardiac stress stimuli in patients with cardiovascular diseases [46].
QRS complex is modified by various mechanisms and clinical conditions. A wide QRS complex may be related to obesity [47], cigarette smoking [48], hypertension [47], NAFLD [49], diabetes mellitus [50], metabolic syndrome [51], heart failure [52], CKD [53], and inflammation [54]. As patient with heart diseases always has metabolic dysregulation problems, high serum FGF21 levels in these patients are reasonable. Therefore, we suggest serum FGF21 levels could be considered an indicator of adverse metabolic dysregulation but not the explanation of the cause of these abnormal cardiac function parameters findings.
Previous studies reported there is an increase of FGF21 levels in cardio-metabolic conditions such as CAD, heart failure, atrial fibrillation, MI, obesity, and diabetes mellitus [55,56]. In recent one study regarding the influence of lifestyle to the FGF21 serum level. It showed the age, aminotransferase, gamma-glutamyl transpeptidase, smoking status, and breakfast and alcohol consumption frequency were independently associated with FGF21 levels. More interestingly, the FGF21 levels were found more profound correlated with waist circumference, SBP, and total cholesterol in the relative healthy the non-obese group [57]. These findings are consistent with our study result. With increasing FGF21 tertiles, we found our patients had a higher waist-to-hip ratio and HbA1c as well as higher prevalence of LVH and incrementally lower LVEF.
In our study, we found that the QT interval is prolonged with the increase of FGF21 levels. QT interval prolongation has been associated with diabetes, obesity, and adiposity, cardiovascular diseases which are all related to an increased FGF21 level [40]. Yılmaz et al. further suggested regional adipose tissue deposition especially epicardial fat volume may play an important role in QT interval prolongation pathogenesis [58]. As FGF21 was identified as a hepatic endocrine factor which modulates lipid metabolism, and the serum FGF21 level progressively increased with visceral fat, and associated with NAFLD [59], it is reasonable to proposed there is a possible link between FGF21 and prolonged QTc interval via the lipid metabolism happened at epicardial fatty tissue. However, like the phenomenon we found in QRS interval. FGF21 might not directly induced the prolonged QTc interval but reflect the complex underlying metabolic dysregulation and comorbidity of the patients.
In our study, we found that a high FGF21 level was significantly associated with all-cause mortality in stable angina patients which similar to previous observations. FGF21 levels were known significantly correlated with left ventricular systolic dysfunction at baseline and has a greater risk to develop MACEs and cardiac death [21,60]. Furthermore, elevated circulating FGF21 levels also found in patients with carotid atherosclerosis [61], subclinical atherosclerosis [62], CHD [41], and acute MI [63]. All these data logically conduct the idea that high levels of serum FGF21 may be indicative for the adverse cardiovascular events following CAD. However, because of the relative short duration of follow-up and small sample size, further studies with a longer follow-up period and larger patient population are still required to confirm this finding.
Interestingly, we found that T2DM patients had significantly higher FGF21 levels compared to patients without T2DM in our study. The T2DM patients with a wide QRS complex had higher levels of FGF21 than those with normal QRS complex with and without T2DM. Moreover, the patients with a prolonged QTc interval and T2DM also had higher levels of FGF21 than those with normal QTc interval with or without T2DM. Recently, few studies have shown the early compensatory high serum level of FGF21 levels might be responsive to the occurrence and development of DM-induced cardiovascular complications [62,64]. In addition, the deletion of FGF21 has also been proved to have the relationship with the aggravation of DM-induced cardiovascular injury in some reports [18,65].
In our study, a higher FGF21 level was positively associated with plasma adiponectin level. Adiponectin and FGF21 both control the metabolism of lipids and carbohydrates, which is essential for the maintenance of energy homeostasis in the body and hence for survival. In one of our previous studies, plasma adiponectin levels have been proved to have the correlation with QTc interval prolongation [67]. A previous study further demonstrated adiponectin could mediates the metabolic effects of FGF21 [66]. In addition, insulin resistance, which is associated with adiponectin resistance, also found could predict a future increase in Tpeak-Tend interval which will influence QT interval in the general population [68].
Therefore, on the basis with all our findings and other reports, we hypotheses the mechanisms that may be involved the effects of an elevated FGF21 level to the electrocardiographic abnormalities we observed. First, the increase of serum FGF21 may be associated with the corresponding metabolic dysregulation. Second, in individuals with metabolic abnormalities, the FGF21 signaling pathway may be impaired, leading to FGF21 resistance. As the more severe of the underlying metabolic abnormalities, the secretion of FGF21 increased and the associated electrocardiographic abnormalities become obvious.
There were some limitations to the current study. First, the cross-sectional design of our study limits the ability to infer any causal relationships between FGF21 level and wide QRS complex and QTc prolongation and limited the robust comparisons. Further case controlled and cohort studies are required. Second, we did not determine the more complex cardiac electroactivity parameters such as heart rate variability, PR and QT interval variability in this report, further calculation should be performed in the future to evaluate the sympathetic and parasympathetic effect of the FGF21. Third, if the study population had different severity and phases of heart diseases (such as stable angina versus acute coronary syndrome or chronic ischemic heart diseases), differences in clinical scenery may have different impact on the results. To avoid selection bias, we had chosen individuals with stable angina in this study, and thus our results may not be generalizable to other populations. Forth, we did not evaluate the genetic factor in our study, possible confounding effects cannot be excluded. Finally, the underlying mechanism how FGF21 is associated with cardiomyocyte ion channel expression and electrophysiology remains unclear, further investigations are warranted to elucidate these issues.