Association between circulating CTRP9 levels and coronary artery disease: a systematic review and meta-analysis

Introduction

Coronary artery disease (CAD) is a prevalent condition among elderly individuals in China. It is a chronic inflammatory response disease of arterial intima initiated by lipid entry. During the early stages of atherosclerosis, monocytes aggregate, adhere, and migrate to the subendothelial layer of the intima where they differentiate into macrophages and finally foam cells. Subsequently, atherosclerotic plaques are formed (Libby, 2021). Biomarkers play a critical role in definition, prognostication and decision-making regarding cardiovascular events management (Wang et al., 2017). Currently, the commonly used biomarkers for CAD include cardiac troponin (cTn), creatine kinase MB (CK-MB), brain natriuretic peptide (BNP), C-reactive protein (CRP), and etc. (Keffer, 1996). However, their prognostic value is limited when assessing future development of cardiovascular disease (Lobbes et al., 2010). Therefore, there is a need for new biomarkers that are more sensitive and accurate for early diagnosis of CAD.

C1q tumor necrosis factor (TNF) related proteins (CTRPs), a highly conserved family of adiponectin paralogs, is a well-known homeostatic factor that regulate glucose levels, lipid metabolism, and insulin sensitivity through its anti-inflammatory, anti-fibrotic, and antioxidant effects. Researches indicated that CTRP1 and CTRP5 may serve as potential risk factors for CAD, whereas CTRP3, CTRP9, CTRP12, and CTRP13 function as protective factors (Si, Fan & Sun, 2020). Among all the CTRP paralogs, CTRP9 exhibits the highest degree of amino acid identity to adiponectin, which possesses anti-atherogenic properties, in its globular C1q domain (Liu et al., 2022a; Shanaki et al., 2020; Wong et al., 2009). The expression of CTRP9 in adipocytes is significantly lower than adiponectin, but far exceeds in cardiac tissue (Peterson, Wei & Wong, 2009). This discovery has heightened awareness regarding the pivotal role of CTRP9 in cardiovascular disease.

CTRP9 is mainly expressed in adipose tissue and interstitial vascular cells (Schäffler & Buechler, 2012). Sun et al. (2013) have found a significant reduction in the average expression of CTRP9 in both adipocytes and plasma of mice after myocardial infarction. In vivo CTRP9 administration to mice improved their condition by enhancing survival rate and systolic function recovery. Moreover, overexpression of CTRP9 has been shown to alleviate myocardial ischemia-reperfusion injury (IRI) and improve cardiac function (Kambara et al., 2012). It has also been demonstrated that CTRP9 can inhibit superoxide production in diabetes mice (Su et al., 2013).

Numerous reports have indicated that low CTRP9 levels can serve as an independent risk factor for CAD. However, the existing studies primarily concentrate on animal models, with a dearth of studies examining the role of CTRP9 in CAD occurrence (Li et al., 2013). Recently, conflicting results regarding the association between circulating levels of CTRP9 and CAD have emerged. Therefore, our study aims to investigate association between circulating levels of CTRP9 and CAD.

Materials and Methods

Results

Search

A total of 193 articles were obtained through initial screening. After reviewing the titles and abstracts, 118 articles were excluded. Among the 118 excluded articles, there were 17 that lacked a non CAD control group, five unrelated to CAD, 28 on other molecules in CTRPs family and 65 on the genes or pathogenesis of CTRP9. Additionally, three articles were withdrawn. After removing duplicates, twenty-nine articles were left. We carefully examined the remaining 29 articles while eliminating those with repeated data sources or unavailable full texts. Ultimately, we included a total of 25 studies that met our inclusion and exclusion criteria (Fig. 1).

Circulating levels of CTRP9

The results of the meta-analysis revealed a significant decrease in circulating CTRP9 levels among patients with CAD compared to the control group (Z = 3.26, P = 0.001) (Fig. 2). Considering the high heterogeneity, statistical analysis was conducted using a random effects model. Leave-1-out sensitivity analysis was used to assess the impact of each individual study on the overall results. Notably, consistent findings were observed before and after excluding each study one by one.

Publication bias

The funnel plot depicting the included articles was used to assess the publication bias (Fig. 3), and the results of Egger’s tests (t = −0.30, P = 0.769) and Begger’s tests (z = −2.00, P = 0.0483) were insignificant. No article was imputed after trim-and-fill correction, indicating that the results of the meta-analysis were robust and there was no publication bias.

Discussion

CTRP9, the closest paralog of adiponectin, plays a cardioprotective role in the CAD process due to its anti-inflammatory and anti-atherosclerosis features. Previous studies have investigated CTRP9 levels in patients with and without CAD, but their results were contradictory. Therefore, our research analyzed the circulating CTRP9 levels in CAD patients, and result showed that circulating CTRP9 levels in patients with CAD were significantly lower than those without CAD. A study has demonstrated that CTRP9 reduces cell apoptosis in myocardial tissue by activating the AMPK pathway, thereby reducing myocardial IRI and playing a protective role in the cardiovascular system (Kambara et al., 2012). Additionally, AMI inhibits both mRNA and protein levels of CTRP9 generation. CTRP9 supplementation ameliorates myocardial infarction cardiac remodeling through PKA-dependent pathway, improving survival rates of AMI mice (Sun et al., 2013). All these studies confirm that CTRP9 levels are lower in patients with CAD compared to those without CAD.

Due to the high heterogeneity, subgroup analysis and meta-regression were conducted to investigated the impact of different factors on CTRP9 levels. The results indicated that most characteristics of articles (year of publication, patients’ age, and BMI) were not sources of heterogeneity, but the population distribution may contribute to the observed heterogeneity. As an adipocytokine, CTRP9 levels may be associated with obesity status. Studies have shown that obesity disrupts adipocytokine production and promotes the progression of diseases related to lipid metabolism disorders. However, there is no independent correlation between circulating CTRP9 levels and age, gender, BMI, TC, visceral fat, and etc. (Hwang et al., 2014). As for the population distribution, our results proposed the possibility of regional impact on serum CTRP9 of CAD patients. This impact may be due to ethnic and regional differences in genetic, environmental, and lifestyle. But it’s worth noting that the majority of the included studies were conducted in China (92%, 23/25), which may introduce regional bias into our data. Clinical studies need to be conducted in more regions around the world.

Numerous studies have highlighted a significant correlation between diabetes and cardiovascular disease (CVD), which stands as the leading cause of morbidity and mortality associated with CVD (Stamler et al., 1993). Overexpression of CTRP9 has been shown to decrease insulin and blood glucose levels, indicating that CTRP9 may offer improvements in diabetes (Wong et al., 2009). Thus, diabetes or T2DM may be the cause of heterogeneity. However, results indicated that diabetes was not the source of heterogeneity, and there’s no relation between diabetes or T2DM and serum CTRP9 levels. CTRP9 seems to play a role in the pathogenesis of diabetes (Peterson et al., 2013), but the problem becomes more complicated when vascular complications exist. Circulating CTRP9 levels decreases in patients with T2DM (Song et al., 2023), and its protective effect on atherosclerosis may be weakened.

Characterized as an adipocytokine, CTRP9 exhibits the highest expression within adipose tissue and plays a role in lipid metabolism (Su et al., 2013). The expression of CD36 protein in macrophages, which is a receptor of oxidized low density lipoprotein (ox-LDL), can be diminished by CTRP9, indicating a protective effect of CTRP9 on the accumulation of ox-LDL (Zeng et al., 2023). These studies suggest a possible connection between blood lipids and CTRP9 level. Standard lipid blood tests include measurements of TG, LDL, and HDL. In our study, patients with CAD were stratified based on varying levels of these indicators, followed by subgroup analysis. The results reflected that TG, LDL, and HDL did not account for the observed heterogeneity. Some emerging evidence suggested that the low-density lipoprotein receptor-related protein 1 (LRP1)/calreticulin co-receptor system act on CTRP9-induced effects, and proprotein convertase subtilisin/kexin-9 (PCSK9) mediated reduction of LRP1 can diminish the effect of CTRP9 (Potere et al., 2019; Rohrbach et al., 2021). Nevertheless, the lipid-related signaling pathways by which CTRP9 protect against CVD are not yet comprehensive, so direct evidence is needed to clarify the mechanism (Yang et al., 2016).

CTRPs are widely expressed in various tissues, especially in mouse and human adipose tissue and plasma. Various studies have confirmed effect of excess adipose tissue on increasing coronary atherosclerosis. Recently, Asada et al. (2016) also found that plasma CTRP9 levels were associated with atherosclerosis in patients with type 2 diabetes without renal dysfunction. CTRP9 can enhance atherosclerotic plaque stability by attenuating vascular smooth muscle cells (VCMCs) proliferation and neointimal formation (Uemura et al., 2013), increasing the expression of adiponectin receptor (AdipoR) 1 (Li et al., 2015) and reducing the level of pro-inflammatory cytokines (Zhang et al., 2016). However, the results suggested that plaque stability and the number of coronary artery lesions were not heterogeneous sources. More research is needed in the future.

CAD can be clinically classified into three categories: UAP, SAP, and AMI. CAD classification was used as subgroups for analysis, and the between-subgroup heterogeneity manifested no significant change. There is no worldwide consensus on whether CAD should be classified or how many categories it should be divided into. Additionally, the included articles did not provide an explanation for the criteria used in classifying CAD. Therefore, classifying CAD into subgroups for analysis may lack accuracy. More mechanistic research is needed to elucidate the relationship between CAD classification and CTRP9 levels.

In addition to the above factors, there may be other factors affecting the observed heterogeneity. Li et al. (2020) reported that patients with CAD who experienced moderate to severe obstructive sleep apnea (OSA) exhibited lower CTRP9 levels compared to those without OSA or with mild OSA. Fadaei et al. (2023) discovered a increase in CTRP9 levels among OSA patients. Besides, studies showed a negative correlation between CTRP9 levels and the severity of peripheral arterial disease in T2DM (Jiang et al., 2018). Studies demonstrated a positive correlation between adiponectin and CTRP9 (Wang et al., 2015). In addition, evidence showed that serum CTRP9 may also be correlated to adiponectin, systolic pressure, CRP, leptin, and etc. (Bai et al., 2017; Hwang et al., 2014; Liu et al., 2014). Regretfully, data from the included studies were insufficient to conduct further subgroup analysis, and more factors affecting heterogeneity need to be explored in the future.

Our research still has the following limitations. First, all studies included in this meta-analysis were retrospective, making it challenging to establish a causal relationship between CTRP9 levels and CAD. Second, two articles we incorporated presented non-normally distributed data, necessitating the use of median and quartiles for data description (Hu & Lin, 2021; Liu et al., 2022b). Nonetheless, we proceeded with conversions to describe CTRP9 levels using mean and standard deviation, which could introduce errors. Third, SMDs were pooled in this meta-analysis due to their uniform unit across studies. Fourth, most included studies were conducted in China (92%, 23/25), which may introduce regional bias. Besides, it is difficult to draw rigorous conclusions with only 25 articles reviewed, and more accurate studies with richer data may be conducted in the future.

Conclusions

In conclusion, our meta-analysis has substantiated the significant reduction of circulating CTRP9 levels in patients with CAD compared to those without CAD, and the association may be modified by population distribution. Our findings align with existing fundamental research on the association between CTRP9 and CAD. However, given that all studies included in this meta-analysis were observational in nature, larger population-based studies and studies in more regions are warranted to further strengthen the credibility of this conclusion.

Supplemental Information