To the best of our knowledge, the is the first comprehensive evaluation of the association between HDL-mediated CEC and cardiovascular risk performed by meta-analysis. Three important results were found in this study. First, there was a strong inverse association between HDL-meditated CEC and the incidence of cardiovascular events, especially in healthy individuals and patients with CVD at baseline. Dose-response analysis demonstrated a linear, dose-dependent association, while the cardiovascular risk decreased as the increased CEC. Second, CEC was also inversely associated with the prevalence of cardiovascular events. Third, although there was a trend toward an inverse association between HDL-CEC and all-cause mortality, this was not found to be statistically significant based on the current data.

HDLs comprise various subspecies and vary widely in particle size, as well as the lipid and protein composition [18]. Indeed, it is now widely known that specific HDL subspecies exert highly specific functions [9, 19]. Static concentration measurements are poorly reflecting the biologic activities of HDL [20]. This may account for the failure of pharmacologic interventions (niacin and CETP inhibitors) designed to raise plasma HDL-cholesterol levels to improve cardiovascular outcomes. HDL-mediated cholesterol efflux from macrophages, which is the key functional property of HDL in protecting against atherosclerosis, is a critical process that is evaluated by RCT [7, 21, 22]. In recent years, a growing number of studies have focused on HDL function, especially CEC, instead of simply assessing HDL concentrations, with the aim of gaining an improved understanding of the clinical benefits of HDL in protecting against CVD. However, these clinical studies have yielded inconsistent results, and the link between HDL-mediated CEC and cardiovascular risk remains to be fully clarified.

Importantly, analysis of the pooled results in this study showed that the capacity of HDL to promote cholesterol efflux was inversely associated with the future incidence of cardiovascular events. Furthermore, the dose-response analysis demonstrated that increased CEC was related to decreased cardiovascular risk, in a relationship that is apparently independent of HDL levels. In the subgroup analysis stratified by ethnicity, significant inverse associations between CEC and cardiovascular risk were observed both in European and Asian populations. The pooled effect size (RR) was much smaller in Asian populations than that in European populations. It is well recognized that there are differences in cardiovascular risk between European and Asian populations involving many factors, including genetic factors, lifestyle, and environment. Studies have demonstrated that lifestyle changes also influence CEC [23]. Nevertheless, we were unable to identify ethnicity as a determinant factor that may influence the predictive value of CEC for cardiovascular risk, because of limited data. This is an interesting issue that warrants further investigation. Further subgroup analysis stratified according to the baseline clinical characteristics of participants revealed that the results are influenced by disease. First, we found that HDL-cholesterol efflux capacity was a strong predictor of cardiovascular risk both in healthy individuals and in patients with CVD. As an integrated measure of HDL quantity and quality, CEC accurately reflects the role of HDL in atheroprotection. Studies have demonstrated that HDL-cholesterol efflux exerts multiple cardioprotective properties, the most important being that the cholesterol efflux from macrophages inhibits foam cell formation and protects macrophages from LDL-induced apoptosis [24]. In addition, CEC has been shown to be an important signaling pathway required for nitric oxide activation and protein transport, and enhances endothelial function by mediating anti-inflammatory and anti-oxidant activities [25, 26]. Accordingly, increased CEC enhances the cardioprotective activities of HDL, and vice versa. Although limited data were pooled in this meta-analysis and statistical heterogeneity was present, these results provide important information showing that HDL-mediated macrophage-specific CEC is potentially a stronger predictor of cardiovascular risk among patients with various CVDs and healthy individual. However, CEC was not correlated with cardiovascular risk in the patients with kidney disease. The potential underlying reason for this could be the dysfunction of HDL in kidney disease. HDL particles are characterized by an altered molecular composition under conditions of kidney disease, which is not restricted to the anti-inflammatory and anti-oxidative effects, but also result in decreased CEC [27]. Moreover, these patients usually lose lipoproteins via the urine and as a consequence, it is likely that not only the quality, but also the quantity of HDL is decreased. In fact, cardiovascular risk is notably increased in individuals with chronic kidney disease, mainly due to the high prevalence of traditional risk factors, such as hypertension, diabetes, dyslipidemia and high levels of inflammation [28]. The findings of this study showed that disease characteristics affect the predictive value of CEC for cardiovascular risk. HDL-mediated CEC may be a good predictive biomarker of cardiovascular risk in healthy individuals and in patients with CVD, but not in patients with impaired kidney function.

A previous cross-sectional study reported a strong inverse association between CEC and the prevalence of coronary artery disease [29]. A similar association was found between CEC and the prevalence of cardiovascular risk in this study. Because of the obvious heterogeneity and potential publication bias, the association was weaker and remains to be confirmed by more clinical studies. However, our results did not show a statistical association between CEC and all-cause mortality. This indicates that HDL-mediated CEC is predominantly correlated with cardiovascular risk, but not with the other risks. The small sample (n = 6824) included in this meta-analysis and the existence of heterogeneity may reduce the credibility of this result.

The findings of our study may have important clinical implications. Although cholesterol efflux from macrophages represents only a small component of the overall turnover of cholesterol in the body as a whole, it is probably the property that is most relevant to atheroprotection and accurately reflects HDL function. The results of this study suggest a potential role of HDL-mediated CEC in cardiovascular risk prediction and support the use of CEC assays in guiding the development of new HDL-targeted therapies. However, before the measurement of CEC is widely recommended in clinical practice, it is necessary to standardize CEC assays and develop high-throughput platforms that are suitable for routine clinical use.

Several limitations of this study should be recognized. First, combined cardiovascular events were identified as the outcome to estimate the cardiovascular risk in the included studies, thus, making it difficult to identify the risk of specific cardiovascular events, such as myocardial infarctions and ischemic strokes. Larger clinical trials dealing the association of CEC with the incidence of specific cardiovascular events are urgently needed to validate this concept. Second, the pooled results were based on observational studies, and although multiple-adjusted effect size was selected, the adjusted factors were not consistent. Hence, we cannot exclude the impact of the other confounders, which may account for some between-study variation. Finally, the most important limitation is that there is no established gold-standard for ex vivo CEC assays. For example, CEC measurement using different forms of labeled cholesterol (radiolabel or fluorescence label) and differences in donor cell lines (J774 or THP-1) can yield between-study variation. At present, the cholesterol efflux assay is not readily applicable in routine clinical practice. Our findings support the implementation of efforts to establish a standardized CEC assay that is suitable for clinical use.