Venous thromboembolism (VTE) is a chronic cardiovascular disease that mainly consists of deep vein thrombosis (DVT) and pulmonary embolism (PE). Each year, VTE affects roughly 10 million individuals of all races globally and significantly contributes to the burden of illness worldwide (ISTH Steering Committee for World Thrombosis Day, 2014). Medical expenditure related to venous thromboembolism is approximately worth €1.5 to €3 billion every year in Europe and $7 to $10 billion in the United States (Barco et al., 2016; Grosse et al., 2016). After a heart attack or stroke, acute venous thromboembolism is the third most prevalent acute cardiovascular illness. It has an annual incidence of roughly 1–2/1,000, rising exponentially with age in both men and women. Meanwhile, VTE is four times more prevalent in high-income nations than in low-income nations (Tagalakis et al., 2013; ISTH Steering Committee for World Thrombosis Day, 2014; Heit, 2015).

High body mass index (BMI) has been identified as a risk factor of VTE in several major observational studies (Borch et al., 2010; Cushman et al., 2016; Klarin et al., 2017). According to the 2016 ISTH SSC guideline and its 2021 update, oral anticoagulants are recommended for VTE prophylaxis in patients with a BMI greater than 40 kg/m² (Martin et al., 2016; Martin et al., 2021). Recent Mendelian randomization and observational research have further indicated the causal association between obesity and an increased incidence of VTE (Lindström et al., 2017; Kim et al., 2021; Yuan et al., 2021).

The molecular processes associating adiposity and the development of VTE have not yet been elucidated, which may be relevant to insulin resistance, adipocyte hypertrophy, and increased production of inflammatory adipocytokines and free fatty acids (Yang et al., 2012). As a highly active metabolic and endocrine organ, adipose tissue is now recognized to express and secrete a variety of adipokines regulating numerous metabolic processes and inflammatory responses inside the body (Halberg et al., 2008; Ouchi et al., 2011; Lehr et al., 2012; Raucci et al., 2013). Previous research has connected adipokines to various obesity-related diseases, with greater adiponectin levels deemed cardioprotective and higher leptin and resistin levels usually seen as detrimental to cardiovascular health. However, the causal association between adipokines and VTE is not well-established, and previous observational studies produced inconsistent results.

Conventional observational epidemiological studies are susceptible to residual confounding and reverse causality and are consequently incapable of drawing causal inferences. Mendelian randomization (MR) is an epidemiological methodology based on germline genetic data that increases causal inference in association by employing genetic variations as the exposed instrumental variables (Burgess and Thompson, 2015). As a result of the random assignment of alleles from parents to children during inheritance, Mendelian randomization prevents interference from potential confounding variables and reverse causal effects (Burgess and Thompson, 2015).

We used a two-sample MR framework to examine the association of six adipokines (adiponectin, leptin, PAI-1, MCP-1, leptin receptor, and RETN) with the risk of VTE and its two subtypes (DVT and PE) using genetic variants associated with adipokine concentrations from published genome-wide association studies (GWAS). In addition, we used the mediation analysis method based on multivariable Mendelian randomization (MVMR) to explore the proportion mediated by adipokines for the effect between BMI and VTE, DVT, and PE.

With publicly accessible datasets that yielded genome-wide association results for BMI, adipokines, and outcomes (VTE, DVT, and PE), we conducted two-step two-sample MR. The two-sample MR refers to the use of various datasets (samples) to determine the relationship between genes and risk factors (like BMI and adipokines) and outcomes (like VTE). We used two-step MR to first examine the effects of BMI and adipokines on outcomes. The first step was to determine the causal effect of BMI and adipokines on outcomes. In the second step, we figured out the causal effect between BMI and the significant adipokines (q < 0.05) found in the first step.

In this large-scale MR analysis, we found a causal relationship between genetically determined circulating leptin levels and the risk of VTE, DVT, and PE. In addition, it was acknowledged that adiponectin may be a protective factor for VTE and PE. Simultaneously, MCP-1 may be a protective factor for VTE. However, we found no evidence supporting the causal associations between genetically determined levels of the other three adipokines (RETN, leptin receptor, and PAI-1) and the risk of VTE, DVT, and PE. These findings implied that adiponectin, PAI-1, MCP-1, leptin receptor, and RETN might have no significance in the pathogenesis of VTE, DVT, and PE. Although the difference between univariate Mendelian randomization and multivariate Mendelian randomization results suggests that leptin may mediate the effect of BMI on PE, the results of the mediation analysis suggest that this mediating effect of leptin did not reach statistical differences (p > 0.05).

Leptin is a 16-kDa hormone released by adipocytes, the secretion of which is dependent on the fat depot. Additionally, leptin production is greater in the subcutaneous fat depot than that in the visceral depot. The blood levels of leptin show a proportional distribution pattern in adipose tissue mass, which also manifests the instantaneous changes in nutritional status, as reflected by a sharp decrease after the onset of fasting (Raucci et al., 2013). Previous observational studies on the relationship between circulating leptin levels and VTE risk have shown inconclusive findings to some extent. Observational studies and in vitro tests have demonstrated that the circulating leptin levels are related to an extensive array of hemostatic factors, including platelet activation parameters, procoagulant factors, and fibrinolytic factors. Leptin has been demonstrated to stimulate TF activity, antigen and mRNA expression, and the production of microparticles containing TF in human peripheral mononuclear cells (Napoleone et al., 2007; Rafail et al., 2008; Petrini et al., 2016). It is worth mentioning that Ayer et al. found that the elevation of the tissue factor (TF) activity in mononuclear cells was only accomplished at supra-physiological quantities of leptin but not at the lower amounts often encountered in obese individuals (Ayer et al., 2010). A prospective study of 264 patients with acute pulmonary embolism revealed that a low plasma leptin concentration was an independent risk factor for high mortality among patients with acute pulmonary embolism (Dellas et al., 2013). A prospective study of 203 OA patients reveals that a high preoperative leptin level may be an independent risk factor for postoperative DVT (OR = 2.17, 95% CI, 1.01–4.64) (Lu et al., 2018). However, a recent nested case–control analysis (416 VTE cases and 848 controls) based on the trauma cohort revealed that blood leptin levels were related to the incidence of VTE, before adjusting for BMI, which indicates that the association between circulating leptin levels and VTE risk might be confounded by BMI (Frischmuth et al., 2022). This is consistent with the results of the current MR study. In the mediation analysis, the mediating effect of circulating leptin levels failed Sobel’s test (p = 0.10), suggesting leptin may not be a strong mediator of BMI on the risk of PE.

Adiponectin is primarily released by the visceral adipose tissue and is the most abundant adipokine, with quantities in the bloodstream inversely associated with obesity, which specifically exerts anti-apoptotic, anti-inflammatory/anti-fibrotic, and insulin-sensitizing effects (Straub and Scherer, 2019). In a rat model of hyperlipidemia, adiponectin inhibits platelet aggregation by enhancing eNOS activation and reducing oxidative/nitrosative stress, such as by inhibiting iNOS expression and superoxide production (Wang et al., 2011). In an observational study, Restituto et al. (2010) found that adiponectin decreased platelet aggregation and activation and demonstrated antithrombotic characteristics in metabolic syndrome patients. A retrospective study involving 95 patients with pulmonary embolism revealed that patients with pulmonary embolism had lower adiponectin levels than normal controls (Gul et al., 2016). Recent prospective cohort research also demonstrated that low adiponectin levels are associated with post-thrombotic syndrome, regardless of BMI (Mrozinska et al., 2018). In the current MR study, we found that adiponectin was associated with a decreased risk of VTE (OR = 0.85; p = 0.013; q = 0.053) and PE (OR = 0.81; p = 0.032; q = 0.083). Although the FDR-corrected significance threshold was not met, adiponectin could be considered a potential protective factor for VTE and PE when combined with previous findings.

MCP-1 is from the family of CC chemokines, which can bind to several receptors but mainly enforces its biological effect by attaching to the extracellular region of CCL2 (chemokine ligand 2). The primary sources of monocyte chemoattractant protein-1 are epithelial cells, endothelial cells, smooth muscle cells, monocytes/macrophages, fibroblasts, astrocytes, and microglial cells (Singh et al., 2021). Obesity-related insulin resistance promotes the inflammation of the adipose tissue through MCP-1 from adipocytes, resulting in the recruitment of monocytes and activation of macrophages (Shimobayashi et al., 2018). As reported, monocyte recruitment plays a pivotal role in thrombus lysis; endogenous MCP-1 levels increased in spontaneously dissolving venous thrombi, and the injection of MCP-1 into the thrombosed tissue enhances thrombus lysis, according to previous research studies (Humphries et al., 1999; Ali et al., 2006; Gutmann et al., 2020). Zhang et al. reported the subsequent elevation experiment on rabbits, where they discovered that increased MCP-1 protein expression caused by bone marrow MSC transplantation promoted the regression and recanalization of DVT (Zhang et al., 2019). Georgakis et al. discovered that higher MCP-1 levels were related to an increased risk of cardioembolic arterial thrombosis in stroke, as well as mortality (Georgakis et al., 2019a; Georgakis et al., 2019b; Georgakis et al., 2021). MCP-1 was associated with a decreased risk of VTE in our study (OR = 0.88, p = 0.048). However, this association was no longer significant after FDR correction (q = 0.14), suggesting the association observed in the initial analysis may have been false positive, and further research would be needed to determine if there is a true association between MCP-1 and VTE. Although there are some distinctions in the processes of formation and pathological alterations for arterial and venous thrombosis, there are many similarities in the pathogenesis (Lijfering et al., 2011; Prandoni, 2020). Given that the conclusion regarding the positive contribution of MCP-1 in the process of venous thrombus dissolution is derived from in vitro experiments and the association of MCP-1 with an elevated risk of cardiovascular embolic events is supported by higher quality studies, further research with robust evidence is warranted to establish the precise role played by MCP-1 in venous thromboembolism.

Here, we found no association between the genetically determined PAI-1 concentration and the risk of VTE, DVT, and PE. Zhang et al. (2020) and Wang et al. (2022) revealed the PAI-1 4G/5G polymorphism associated with the risk of the onset of VTE in East Asian ancestry, but the causal relationship between serum PAI-1 levels and VTE risk was not recognized in our study, probably because the GWAS data we assessed were generated from European ancestry. Because of variances in gene linkage disequilibrium among races, SNPs indicating PAI-1 levels in GWAS studies of European populations do not reflect PAI-1 levels in other races.

Several aspects of our research study make it superior to others. 1) Statistical data summary of exposure and outcomes was from the largest and latest GWAS in related fields, and there was no overlapping sample in our study. 2) Strict criteria were established to select IVs which may strengthen the statistical power. 3) Since genetic variants were scattered over multiple chromosomes, it is possible that potential gene–gene interactions will not have much effect on the results. 4) To enhance the accuracy of the estimation, we utilized different methods for sensitivity analysis. 5) A mediation analysis based on MVMR was used to investigate the mediating role of adipokines in the risk of obesity and VTE. This study does have some limitations. 1) GWAS used included only the European ancestry population. Thus, additional studies should be performed to explore the intermediated effect in the non-European population. 2) There were some heterogeneities in the analysis. Because meta-GWAS data were used, it was impossible to investigate the possibility of non-linear connections or stratification effects that varied according to age, health status, or gender. 3) Since the GWAS research employed as an endpoint was not stratified by gender, it could not elucidate if adipokines’ influence on VTE was subjected to the gender differences. 4) Limited adipokines were included in the current study. 5) Further research studies were required to determine if the adipokines that showed negative results in current investigation can trigger the formation of DVT, VTE, or PE on their own or contribute to the pathogenesis of DVT, VTE, or PE in collaboration with other adipokines. 6)

Our MR investigation revealed that the circulating leptin level was a risk factor for VTE, DVT, and PE, but it may not be a mediator of BMI on the risk of VTE, DVT, and PE and that adiponectin was a potential protective factor for both VTE and PE.