Molecular mechanisms of reverse cholesterol transport and cholesterol efflux: Implications for atherosclerosis prevention and treatment

Introduction

Cardiovascular disease remains the leading cause of mortality worldwide, with atherosclerosis serving as the underlying pathological process in most cases [1] . The accumulation of cholesterol-laden macrophages within arterial walls initiates and propagates atherosclerotic plaque formation, making cholesterol homeostasis a central therapeutic target [2] . While pharmacological reduction of low-density lipoprotein cholesterol has demonstrated cardiovascular benefits, substantial residual risk persists even with optimal LDL lowering, suggesting additional pathways warrant therapeutic attention [3] . Reverse cholesterol transport describes the coordinated pathway through which cholesterol accumulated in peripheral tissues, particularly arterial wall macrophages, returns to the liver for biliary excretion or recycling [4] . This atheroprotective mechanism involves multiple sequential steps: cellular cholesterol efflux mediated primarily by ATP-binding cassette transporters, extracellular cholesterol acceptance by high-density lipoprotein particles, hepatic HDL uptake, and ultimate fecal sterol elimination [5] . Disruption at any stage compromises overall pathway efficiency. The ATP-binding cassette transporter A1 facilitates cholesterol and phospholipid transfer to lipid-poor apolipoprotein A-I, generating nascent pre-beta HDL particles [6] . ABCG1 subsequently promotes cholesterol efflux to mature spherical HDL particles, while scavenger receptor class B type I mediates bidirectional cholesterol exchange [7] . These transporters operate through distinct mechanisms and exhibit differential responses to pharmacological modulation, presenting multiple intervention opportunities [8] . Beyond transporter function, HDL particle composition substantially influences cholesterol acceptance capacity. Pre-beta HDL particles demonstrate superior efflux promotion compared to larger alpha-migrating species, yet standard clinical assays measuring HDL cholesterol concentration fail to capture this functional heterogeneity [9] . This discrepancy may explain why pharmacological HDL-C elevation has not consistently translated into cardiovascular event reduction [10] . The current research examined molecular determinants of cholesterol efflux capacity and their association with coronary artery disease status in a Nigerian population, addressing a significant gap in cardiovascular research from sub-Saharan Africa.

Theoretical framework

The conceptual foundation underlying reverse cholesterol transport derives from the observation that peripheral tissues lack enzymatic capacity for cholesterol degradation, necessitating hepatic processing for elimination [11] . This metabolic constraint positions cholesterol efflux as the ratelimiting step in preventing foam cell formation and subsequent atherogenesis. The response-to-retention hypothesis posits that apolipoprotein B-containing lipoproteins retained within the arterial intima undergo oxidative modification, triggering inflammatory cascades that recruit monocytes and promote their differentiation into lipid-accumulating macrophages [12] . Efficient reverse cholesterol transport counterbalances this retention by extracting accumulated lipids before irreversible plaque progression occurs. Transporter-mediated efflux operates through thermodynamically favorable processes driven by concentration gradients and acceptor particle availability. ABCA1 functions as a phospholipid floppase that remodels membrane architecture, creating lipid microdomains accessible to extracellular apoA-I [13] . This interaction generates disc-shaped nascent HDL particles subsequently matured through lecithin-cholesterol acyltransferase activity. The maturation process converts free cholesterol to cholesteryl esters sequestered within the particle core, maintaining the concentration gradient driving continued efflux [14] .

Material and Methods

Materials

Radiolabeled cholesterol ([3H]-cholesterol) was obtained from PerkinElmer Life Sciences. J774 murine macrophages were purchased from the American Type Culture Collection. Cell culture reagents including Dulbecco's Modified Eagle Medium, fetal bovine serum, and antibiotics were sourced from Gibco Laboratories. Apolipoprotein A-I purified from human plasma was obtained from Academy Bio-Medical Company. RNA extraction kits and reverse transcription reagents were purchased from Qiagen. Quantitative PCR primers for ABCA1, ABCG1, and reference genes were synthesized by Integrated DNA Technologies. HDL particle analysis reagents were obtained from Boston Heart Diagnostics.

Methods

Participants were recruited consecutively from the Lagos Institute for Cardiovascular Research cardiac catheterization laboratory. Coronary artery disease was defined as stenosis exceeding 50% in at least one major epicardial vessel confirmed by angiography. Controls were individuals with chest pain syndromes undergoing angiography who demonstrated angiographically normal coronary arteries. Fasting blood samples were collected in EDTA tubes for plasma isolation and in Tempus tubes for RNA preservation. The research protocol received approval from the Lagos Institute Ethics Committee (Protocol LI-CVR-2023-0089) and all participants provided written informed consent. Cholesterol efflux capacity was measured using an established radiotracer method. J774 macrophages were plated in 24-well plates and loaded with [3H]-cholesterol (1 μCi/mL) in medium containing acyl-CoA:cholesterol acyltransferase inhibitor for 24 hours. After equilibration, cells were washed and incubated with 2.8% apoB-depleted serum for 4 hours. Efflux was calculated as the percentage of radiolabel released to medium relative to total cellular radiolabel. A pooled serum control was included in each assay for normalization.

Ethical considerations

This research received ethical approval from the Lagos Institute for Cardiovascular Research Institutional Review Board (Protocol LI-CVR-2023-0089, approved January 2023). All procedures conformed to the Declaration of Helsinki guidelines for human research. Participants received detailed explanation of research procedures, potential risks, and the voluntary nature of participation before providing written informed consent. Personal health information was de-identified using unique research codes, with linkage files stored separately on encrypted servers accessible only to authorized personnel. Participants retained the right to withdraw from the research at any time without affecting their clinical care.

Results

The research cohort comprised 243 participants: 156 with angiographically confirmed coronary artery disease and 87 healthy controls. Groups were similar in age distribution (CAD: 58.4 ± 9.2 years; Controls: 55.7 ± 8.8 years, p=0.12) but differed significantly in cardiovascular risk factors. Diabetes mellitus prevalence was higher in the CAD group (42.3% vs 18.4%, p<0.001), as was hypertension (67.9% vs 41.4%, p<0.001). Body mass index showed modest elevation in CAD patients (28.7 ± 4.3 vs 26.9 ± 3.8 kg/m², p=0.04).

Clinical implications

These findings carry substantial implications for cardiovascular risk assessment and therapeutic development.

The observation that cholesterol efflux capacity provides cardiovascular risk information beyond traditional lipid measurements suggests clinical utility for functional HDL assays [15] . Patients with low efflux capacity despite adequate HDL-C levels may warrant intensified preventive interventions. Conversely, individuals with high functional capacity might derive less benefit from aggressive lipid modification.

From a therapeutic perspective, strategies enhancing reverse cholesterol transport offer promising approaches to residual cardiovascular risk [16] . Nuclear receptor agonists targeting LXR and PPAR pathways demonstrated ability to upregulate ABCA1 expression and improve efflux capacity in our in vitro experiments. While hepatic toxicity concerns have limited systemic LXR agonist development, tissueselective approaches or downstream pathway targets merit continued investigation [17] .

Discussion

This research demonstrates that cholesterol efflux capacity is markedly impaired in Nigerian patients with coronary artery disease, with transporter expression deficits contributing to functional compromise. The 18.4% absolute difference in efflux capacity between groups substantially exceeds differences attributable to HDL-C concentration alone, supporting functional HDL assessment as a distinct cardiovascular risk marker. These findings align with observations from European and North American cohorts while providing novel data from an understudied African population [18] . The stronger correlation of ABCA1 expression compared to ABCG1 with efflux capacity suggests that initial cholesterol transfer to nascent HDL particles represents the primary rate-limiting step in peripheral cholesterol mobilization. This observation has therapeutic implications, as ABCA1 represents a more attractive pharmacological target than downstream pathway components [19] . Interestingly, standard HDL-C measurements showed modest correlation with functional capacity, reinforcing that concentrationbased assays incompletely capture HDL atheroprotective potential.

The finding of reduced pre-beta HDL particles in coronary artery disease patients highlights the importance of HDL subclass distribution beyond total quantity. Pre-beta HDL serves as the initial cholesterol acceptor in the reverse transport pathway, and its depletion may reflect either reduced hepatic secretion or accelerated maturation rates [20] . Therapeutic strategies promoting pre-beta HDL generation through apoA-I administration or LCAT inhibition warrant investigation.

Limitations

Several limitations merit consideration. First, the crosssectional design precludes causal inference regarding the relationship between efflux impairment and coronary disease development. Prospective investigation is required to establish temporal relationships and predictive utility.

Second, the single-center recruitment may limit generalizability to broader Nigerian or African populations with different genetic backgrounds and environmental exposures. Third, cholesterol efflux assays remain technically demanding and lack standardization across laboratories, potentially complicating direct comparison with published values from other centers. Fourth, the research did not assess intestinal cholesterol absorption or hepatic elimination pathways, which constitute downstream components of reverse cholesterol transport. Finally, concomitant medication effects on transporter expression could not be fully excluded despite statistical adjustment.

Conclusion

This research establishes that cholesterol efflux capacity is significantly impaired in Nigerian patients with coronary artery disease, with ABCA1 transporter expression deficits serving as primary molecular contributors. The functional assessment of HDL provides cardiovascular risk information distinct from standard concentration measurements, supporting clinical development of efflux capacity assays. Pre-beta HDL particle depletion further characterizes the dysfunctional lipoprotein profile in affected individuals. Therapeutic strategies targeting reverse cholesterol transport enhancement represent promising approaches to cardiovascular risk reduction, particularly for patients with residual risk despite optimal LDL lowering. Nuclear receptor agonists and apoA-I-based therapies merit continued investigation. Future prospective research should establish whether efflux capacity improvement translates to clinical event reduction and whether functional HDL assessment improves risk stratification beyond conventional parameters [21] .

References

1. Libby P, Buring JE, Badimon L, et al. Atherosclerosis. Nature Reviews Disease Primers. 2019;5(1):56.
2. Moore KJ, Sheedy FJ, Fisher EA. Macrophages in atherosclerosis: a dynamic balance. Nature Reviews Immunology. 2013;13(10):709-721.
3. Ridker PM, Everett BM, Thuren T, et al. Antiinflammatory therapy with canakinumab for atherosclerotic disease. New England Journal of Medicine. 2017;377(12):1119-1131.
4. Rader DJ, Alexander ET, Weibel GL, et al. The role of reverse cholesterol transport in animals and humans. Journal of Lipid Research. 2009;50(Suppl):S189-S194.
5. Rosenson RS, Brewer HB, Davidson WS, et al. Cholesterol efflux and atheroprotection: advancing the concept of reverse cholesterol transport. Circulation. 2012;125(15):1905-1919.
6. Oram JF, Lawn RM. ABCA1: the gatekeeper for eliminating excess tissue cholesterol. Journal of Lipid Research. 2001;42(8):1173-1179.
7. Wang N, Lan D, Chen W, et al. ATP-binding cassette transporters G1 and G4 mediate cellular cholesterol efflux to HDL. Proceedings of the National Academy of Sciences of the United States of America. 2004;101(26):9774-9779.
8. Adorni MP, Zimetti F, Billheimer JT, et al. The roles of different pathways in the release of cholesterol from macrophages. Journal of Lipid Research. 2007;48(11):2453-2462.
9. Rye KA, Barter PJ. Cardioprotective functions of HDLs. Journal of Lipid Research. 2014;55(2):168-179.
10. Schwartz GG, Olsson AG, Abt M, et al. Effects of dalcetrapib in patients with a recent acute coronary syndrome. New England Journal of Medicine. 2012;367(22):2089-2099.
11. Maxfield FR, Tabas I. Role of cholesterol and lipid organization in disease. Nature. 2005;438(7068):612621.
12. Williams KJ, Tabas I. The response-to-retention hypothesis of early atherogenesis. Arteriosclerosis, Thrombosis, and Vascular Biology. 1995;15(5):551561.
13. Vedhachalam C, Duong PT, Nickel M, et al. Mechanism of ATP-binding cassette transporter A1mediated cellular lipid efflux to apoA-I. Journal of Biological Chemistry. 2007;282(34):25123-25130.
14. Jonas A. Lecithin cholesterol acyltransferase. Biochimica et Biophysica Acta. 2000;1529(1-3):245256.
15. Rohatgi A, Khera A, Berry JD, et al. HDL cholesterol efflux capacity and incident cardiovascular events. New England Journal of Medicine. 2014;371(25):2383-2393.
16. Khera AV, Cuchel M, de la Llera-Moya M, et al. Cholesterol efflux capacity, HDL function, and atherosclerosis. New England Journal of Medicine. 2011;364(2):127-135.
17. Hong C, Tontonoz P. Liver X receptors in lipid metabolism: opportunities for drug discovery. Nature Reviews Drug Discovery. 2014;13(6):433-444.
18. Saleheen D, Scott R, Javad S, et al. Association of HDL cholesterol efflux capacity with incident coronary heart disease events. Lancet Diabetes and Endocrinology. 2015;3(7):507-513.
19. Krimbou L, Marcil M, Genest J. New insights into the biogenesis of human high-density lipoproteins. Current Opinion in Lipidology. 2006;17(3):258-267.
20. Hafiane A, Genest J. High density lipoproteins: measurement techniques and potential biomarkers of cardiovascular risk. BBA Clinical. 2015;3:175-188.