Coronary artery disease (CAD) remains the major cause of mortality globally, with its pathogenesis intricately linked to atherosclerosis [1]. A critical hallmark of advanced atherosclerosis is the development of coronary artery calcification (CAC), which is recognized as a notable independent predictor associated with future cardiovascular events and all-cause mortality [2]. CAC is therefore a key target in the prevention and treatment of CAD, especially in CAD patients with multivessel disease [3, 4]. The progression of CAC is accelerated in individuals with metabolic disorders, particularly those with insulin resistance (IR) [5, 6]. IR fosters a pro-atherogenic milieu through multiple pathways, including chronic inflammation, endothelial dysfunction, dyslipidemia, and oxidative stress [7]. Collectively, these factors collectively promote the transition of vascular smooth muscle cells (VSMCs) toward an osteoblast-like phenotype, driving the deposition of hydroxyapatite crystals in the coronary arteries, which is a typical pathophysiological feature of CAC [8]. The triglyceride-glucose (TyG) index is a practical and reliable surrogate marker of IR that can independently predict both the presence and progression of CAC [9, 10]. Despite its established clinical significance, the precise molecular mechanisms linking IR to the accelerated calcification process have not been fully elucidated, highlighting the need for novel biomarkers and pathophysiological insights. Fibroblast growth factor 21 (FGF21), a member of the fibroblast growth factor family, is recognized as a pivotal metabolic regulator with multifaceted roles in glucose and lipid homeostasis [11]. FGF21 is primarily secreted by the liver and acts by enhancing insulin sensitivity, promoting glucose uptake, and improving lipid profiles [12]. These properties make FGF21 a critical defender against metabolic disorders such as IR, type 2 diabetes, and obesity [13, 14]. Our previous studies showed that FGF21 attenuated vascular calcification both in vivo and in vitro by inhibiting endoplasmic reticulum stress and reducing oxidative stress [15, 16, 17]. Hence, the exogenous administration of FGF21 could be a promising therapeutic intervention for vascular calcification [18].

The aim of this study was therefore to evaluate the association between serum FGF21 levels and the severity of CAC in a well-characterized cohort of patients with varying degrees of IR. By elucidating this relationship, we seek to identify a potential biomarker for risk stratification in metabolically susceptible patients with CAC.

The present study found that the association between serum FGF21 level and severe CAC in a cohort of patients with CAD was significantly modified by the degree of IR, as assessed by the TyG index. Specifically, among patients with a higher degree of IR (TyG index $>$8.62), the serum FGF21 level was significantly lower in those with severe CAC, was negatively correlated with the CAC score, and was identified as an independent protective factor against severe CAC. In contrast, no significant association was observed between FGF21 and CAC in patients with a lower degree of IR (TyG index $\le$8.62). Importantly, a significant interaction was found between the TyG index and FGF21 levels in predicting severe CAC. To our knowledge, this is the first study to elucidate the impact of the degree of IR on the association between serum FGF21 and severe CAC in CAD patients. IR, characterized by reduced insulin sensitivity and responsiveness, is a hallmark of type 2 diabetes and is strongly linked to cardiovascular diseases [24, 25]. The TyG index is a convenient and reliable alternative indicator for IR that is significantly correlated with the hyperinsulinemic-euglycemic clamp and is widely used in clinical practice and research [20, 26, 27]. In the current study, CAD patients with a higher TyG index had a higher prevalence of diabetes and elevated FBG levels, further supporting its clinical utility as a marker reflecting the severity of IR. Meanwhile, given the close relationship between IR and glucose-lipid metabolism, FGF21 has garnered attention as a key metabolic regulator involved in this process [13, 28, 29]. In addition to improving glucolipid metabolism and reducing inflammation, FGF21 was also shown in our earlier studies to alleviate vascular calcification by reducing endoplasmic reticulum stress and oxidative stress. However, the association between serum FGF21 levels and CAC, as well as the specific pathological mechanisms involved, remains unclear. In this study, we found that FGF21 levels in CAD patients were significantly lower in the severe CAC group. In addition, they were negatively correlated with the degree of calcification, suggesting that higher FGF21 levels may reflect a compensatory metabolic response associated with lower calcification severity.

Notably, this relationship was significantly modified by the degree of IR, our findings revealed a significant interaction between the TyG index and FGF21 levels in predicting severe CAC, with the protective association of FGF21 primarily observed in patients with a higher degree of IR (TyG index $>$8.62). Regarding the likely pathological mechanism, IR leads to impaired glycemic stability and exposes patients to a persistent state of metabolic stress characterized by dysregulated glucose and lipid metabolism, chronic inflammation, and oxidative stress [24, 30]. These pathological processes not only exacerbate vascular injury and endothelial dysfunction, but also promote the osteogenic trans-differentiation of VSMCs, thereby driving the development and progression of CAC [7, 31, 32]. In this abnormal metabolic environment, the liver and other tissues increase their secretion of FGF21 as a compensatory protective response to restore metabolic homeostasis and mitigate metabolic damage [33, 34]. Therefore, in individuals with a higher degree of IR, elevated serum FGF21 levels may reflect a more effective adaptive response. Patients capable of mounting a stronger FGF21 response can attenuate atherosclerosis and vascular calcification through multiple mechanisms, such as improving insulin sensitivity, reducing cardiac lipotoxicity, suppressing inflammatory pathways, and alleviating oxidative stress [35, 36]. This may partially explain the significant negative correlation we observed between the FGF21 level and the severity of CAC, as well as its independent predictive value for severe CAC. On the other hand, in individuals with relatively healthy metabolism (i.e., those with a lower TyG index), baseline FGF21 levels may be sufficient to maintain vascular homeostasis, meaning that any fluctuations in the level are less critical for regulating the calcification process. Furthermore, given that the low TyG index subgroup typically exhibits a lower CAC burden, the compensatory upregulation of FGF21 tends to be weaker, resulting in further attenuation of the association between serum FGF21 levels and CAC. Taken together, the observed negative correlation in the overall population may reflect the average effect of FGF21 on severe CAC under the heterogeneous metabolic states of CAD patients. The significant interaction between TyG and FGF21 indicates that the observed protective association of FGF21 is primarily driven by individuals with a higher TyG index.

Although IR is known to be closely related to vascular calcification and FGF21 secretion, our study did not find significant differences in CAC scores or serum FGF21 levels between the high and low TyG index groups. There may be several reasons for this. First, CAC is influenced by multiple factors, including age, gender, ethnicity, and cumulative exposure to other traditional cardiovascular risk factors, and is not entirely attributable to IR [37]. Therefore, the degree of IR defined by the TyG index may not directly translate into group-level differences in the calcification burden within this relatively small CAD cohort. Second, FGF21 is a stress-induced cytokine produced primarily by the liver, adipose tissue, and skeletal muscle. Consequently, circulating FGF21 levels are regulated by various factors beyond IR, including liver function, inflammation, and drug treatments [38, 39]. These factors may partially attenuate any significant differences between groups. Third, the relatively small sample size may have limited the ability to identify modest group-level differences.

In summary, our findings indicate the association of FGF21 with CAC is not static, but is modulated by the underlying metabolic environment. The interaction between the degree of IR and FGF21 is crucial for predicting CAC risk, and the inverse association of FGF21 with CAC is more pronounced in CAD patients with impaired metabolism. Therefore, in individuals with IR, FGF21 may not only serve as a valuable biomarker for CAC risk, but also as a complementary indicator for risk stratification in metabolically susceptible CAD patients. Given the growing evidence supporting FGF21 as a potential therapeutic target for various metabolic disorders [40, 41, 42], regular monitoring of FGF21 levels and the implementation of early preventive measures in high-risk populations may hold significant clinical importance.

This study found that the association between FGF21 and CAC was significantly modified by the degree of IR. In CAD patients with a higher degree of IR (TyG index $>$8.62), the serum FGF21 level was negatively correlated with CAC severity and served as an independent predictor associated with a lower risk of severe CAC. These findings suggest that FGF21 may be used as a novel biomarker to identify metabolically susceptible individuals at increased risk of CAC, with potential value for risk stratification and early prevention.

AUC, area under the curve; BMI, body mass index; CAC, coronary artery calcification; CAD, coronary artery disease; CAG, coronary angiography; CCTA, coronary computed tomography angiography; CI, confidence interval; eGFR, estimated glomerular filtration rate; FBG, fasting blood glucose; FGF21, fibroblast growth factor 21; HDL-C, high-density lipoprotein cholesterol; IQR, interquartile range; IR, insulin resistance; LAD, left anterior descending artery; LCX, left circumflex artery; LDL-C, low-density lipoprotein cholesterol; MI, myocardial infarction; OR, odds ratio; PCI, percutaneous coronary intervention; RCA, right coronary artery; ROC, receiver operating characteristic; SD, standard deviation; TC, total cholesterol; TG, triglyceride; TyG, triglyceride-glucose; VSMCs, vascular smooth muscle cells.

The data regarding this article will be shared by the corresponding author upon reasonable request.

CW: Conceptualization, Data curation, Formal analysis, Methodology, Writing—original draft. YL: Investigation, Writing—review & editing. HP: Conceptualization, Writing—review & editing. JL: Project administration, Supervision, Writing—review & editing. All authors contributed to the conception and editorial changes in the manuscript. All authors have read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The study was carried out in accordance with the guidelines of the Declaration of Helsinki and approved by the Ethics Committee of Beijing Anzhen Hospital (Approval no. 2025174x). The participants provided their written informed consent to participate in this study.

Not applicable.

The study was supported by the National Natural Science Foundations of China (No. 82170344).

The authors declare no conflict of interest.

Supplementary material associated with this article can be found, in the online version, at https://doi.org/10.31083/RCM46781.