Remnant cholesterol

Remnant cholesterol is the cholesterol content of triglyceride-rich lipoproteins (TGRL), which is composed of chylomicron remnants, very low-density lipoprotein (VLDL), and intermediate density lipoprotein (IDL) ¹. Clinically, plasma triglyceride concentrations serve as a surrogate measure of triglyceride-rich lipoproteins (TGRL) and remnant cholesterol ².

Experimental studies have shown that remnant cholesterol is involved in the formation and progression of atherosclerosis by multiple mechanisms, like direct accumulation in the arterial wall and enhanced inflammatory response ³. Mendelian randomization studies and recent guidelines also reported a causal association between genetically elevated remnant cholesterol and coronary artery disease ⁴. Remnant cholesterol has been proposed to promote atherosclerotic cardiovascular disease independent of low-density lipoprotein cholesterol, yet the underlying mechanisms are not well understood ⁵. In statin-treated patients with atherosclerotic cardiovascular disease, remnant cholesterol was associated with coronary atheroma progression regardless of conventional lipid parameters, C-reactive protein or clinical risk factors ⁵. Higher remnant cholesterol levels also correlated with higher major adverse cardiovascular events ⁵. These data support further investigations into remnant cholesterol-lowering interventions in statin-treated patients harboring residual atherosclerotic cardiovascular disease risk.

Several studies have also examined the relationship between remnant cholesterol (both calculated and measured) and atherosclerotic cardiovascular disease ⁶. Varbo et al. ⁷ demonstrated that each 1 mmol/L (39 mg/dL) increase in nonfasting remnant cholesterol (calculated as total cholesterol minus HDL-C minus LDL-C) associated with a 2.8-fold increase in risk of ischemic heart disease independent of low HDL-C (high-density lipoprotein cholesterol). In another study of 97,962 participants from the Copenhagen City Heart Study and the Copenhagen General Population Study ⁸, nonfasting calculated remnant cholesterol was associated with a stepwise increase in risk of ischemic heart disease, myocardial infarction, and all-cause mortality. In the study by Jepsen et al. ⁶, both elevated levels of measured and calculated remnant cholesterol were associated with all-cause mortality in patients with ischemic heart disease. More recently, a nested-case control study of 4,662 individuals from the China Kadoorie Biobank demonstrated that remnant cholesterol concentrations (measured by NMR spectroscopy) were associated with a 1.27-fold increased risk of myocardial infarction and 1.20-fold increased risk of ischemic stroke ⁹. It is important to note that most of these observational epidemiologic studies were done in the pre-statin era.

Results from genetic studies also show that elevated levels of remnant cholesterol are causally associated with both low-grade inflammation and ischemic heart disease ¹⁰. However, elevated levels of LDL cholesterol (LDL-C) are associated with ischemic heart disease, but not with low-grade inflammation. Such results indicate that elevated LDL cholesterol (LDL-C) levels cause atherosclerosis without a major inflammatory component, whereas an inflammatory component of atherosclerosis is driven by elevated remnant cholesterol levels. Post-hoc subgroup analyses of randomized trials using fibrates in individuals with elevated triglyceride levels, elevated remnant cholesterol levels, show a benefit of lowering triglycerides or remnant cholesterol levels; however, large randomized trials with the primary target of lowering remnant cholesterol levels are still missing ¹⁰.

In this multi-center prospective study ¹¹, the researchers found that the pre-diabetes mellitus patients who had high remnant cholesterol tended to present worse prognosis when presented as calculated or directly measured forms. Moreover, they also demonstrated that high levels of remnant cholesterol were significant predictors of major adverse cardiovascular events in patients having both coronary artery disease and diabetes mellitus independent of traditional risk factors, suggesting that assessing remnant cholesterol levels in coronary artery disease patients with diabetes mellitus or pre-diabetes mellitus might be likely to have clinical utility in terms of cardiovascular risk stratification and future intervention.

Remnant cholesterol calculation

An estimate of remnant cholesterol can be calculated from a standard lipid profile:

• Remnant cholesterol = Total cholesterol (TC) minus LDL cholesterol (LDL-C) minus HDL-C (high-density lipoprotein cholesterol)

In addition to calculation methods, remnant cholesterol can be directly measured using a variety of analytical methods including ultracentrifugation, nuclear magnetic resonance (NMR) spectroscopy, and by a direct automated assay. The automated assay by Denka Seiken measures the cholesterol content in chylomicrons and VLDL remnants utilizing enzymes and surfactants ⁶. Remnant cholesterol can also be measured by immunoseparation assays using antibodies to apoA1 and apoB100 ¹². This immunoaffinity mixed gel containing apoA1 and B100 antibodies absorb almost all lipoproteins with the exception of certain subpopulations of triglyceride-rich lipoproteins (d<1.006 g/dl), specifically chylomicrons and VLDL remnants, which can then be quantified ¹³. The automated assay only measures a fraction (approximately 13%) of the calculated remnant cholesterol compared to the immunoseparation assay, which correlates better with the calculated levels ⁶. The vertical auto profile (VAP) method measures remnant cholesterol (IDL-C and VLDL-C) after separation of lipoproteins by density-gradient ultracentrifugation ¹⁴. The quantification of remnant cholesterol from the exogenous pathway (i.e. chylomicron remnants) can be done via ultracentrifugation or, indirectly, by measuring apoB48 ¹⁵. ApoB48 is the truncated gene product created by post-translational modification of the larger apoB100 and is found exclusively on chylomicrons and their remnants ¹⁶. ApoB48 is primarily synthesized in the small intestine and plays a key role in chylomicron assembly. Each chylomicron and chylomicron remnant particle contains a single apoB48 molecule which can be used as a measure of intestinally-derived remnant cholesterol ¹⁶. At this time, these direct assays to measure remnant cholesterol have limited clinical applicability as they do not measure all remnant cholesterol in the plasma and are labor intensive and costly. Currently, there is an unmet need for the development of automated assays that accurately and reproducibly measure all remnant cholesterol, which may take on increased urgency if remnant cholesterol is ultimately vindicated as an important causal and targetable atherosclerotic cardiovascular disease risk factor ¹⁷.

References

1. Twickler TB, Dallinga-Thie GM, Cohn JS, Chapman MJ. Elevated remnant-like particle cholesterol concentration: a characteristic feature of the atherogenic lipoprotein phenotype. Circulation. 2004;109(16):1918-1925. doi:10.1161/01.CIR.0000125278.58527.F3 https://doi.org/10.1161/01.CIR.0000125278.58527.F3
2. Sandesara, P.B., Virani, S.S., Fazio, S., & Shapiro, M.D. (2019). The Forgotten Lipids: Triglycerides, Remnant Cholesterol, and Atherosclerotic Cardiovascular Disease Risk. Endocrine reviews, 40 2, 537-557. https://pdfs.semanticscholar.org/00d0/4d7a7fff434cb63106fb0f9eb44f92e70bfb.pdf
3. Varbo A, Benn M, Tybjaerg-Hansen A, et al. Elevated remnant cholesterol causes both low-grade inflammation and ischemic heart disease, whereas elevated low-density lipoprotein cholesterol causes ischemic heart disease without inflammation. Circulation. 2013;128:1298–1309. doi: 10.1161/CIRCULATIONAHA.113.003008
4. Mach F, Baigent C, Catapano AL, et al. 2019 ESC/EAS Guidelines for the management of dyslipidaemias: lipid modification to reduce cardiovascular risk. Eur Heart J. 2020;41:111–188. doi: 10.1093/eurheartj/ehz455
5. Elshazly, M. B., Mani, P., Nissen, S., Brennan, D. M., Clark, D., Martin, S., … Puri, R. (2020). Remnant cholesterol, coronary atheroma progression and clinical events in statin-treated patients with coronary artery disease. European Journal of Preventive Cardiology, 27(10), 1091–1100. https://doi.org/10.1177/2047487319887578
6. Jepsen AM, Langsted A, Varbo A, Bang LE, Kamstrup PR, Nordestgaard BG. Increased Remnant Cholesterol Explains Part of Residual Risk of All-Cause Mortality in 5414 Patients with Ischemic Heart Disease. Clin Chem2016; 62:593-604
7. Varbo A, Benn M, Tybjaerg-Hansen A, Jorgensen AB, Frikke-Schmidt R, Nordestgaard BG. Remnant cholesterol as a causal risk factor for ischemic heart disease. J Am Coll Cardiol2013; 61:427-436
8. Varbo A, Freiberg JJ, Nordestgaard BG. Extreme nonfasting remnant cholesterol vs extreme LDL cholesterol as contributors to cardiovascular disease and all-cause mortality in 90000 individuals from the general population. Clin Chem2015; 61:533-543
9. Holmes MV, Millwood IY, Kartsonaki C, Hill MR, Bennett DA, Boxall R, Guo Y, Xu X, Bian Z, Hu R, Walters RG, Chen J, Ala-Korpela M, Parish S, Clarke RJ, Peto R, Collins R, Li L, Chen Z, China Kadoorie Biobank Collaborative G. Lipids, Lipoproteins, and Metabolites and Risk of Myocardial Infarction and Stroke. J Am Coll Cardiol2018; 71:620-632
10. Varbo A, Benn M, Nordestgaard BG. Remnant cholesterol as a cause of ischemic heart disease: evidence, definition, measurement, atherogenicity, high risk patients, and present and future treatment. Pharmacol Ther. 2014;141(3):358-367. https://doi.org/10.1016/j.pharmthera.2013.11.008
11. Cao YX, Zhang HW, Jin JL, et al. The longitudinal association of remnant cholesterol with cardiovascular outcomes in patients with diabetes and pre-diabetes. Cardiovasc Diabetol. 2020;19(1):104. Published 2020 Jul 6. doi:10.1186/s12933-020-01076-7
12. Nguyen SV, Nakamura T, Uematsu M, Fujioka D, Watanabe K, Watanabe Y, Obata JE, Nakamura K, Kugiyama K. Remnant lipoproteinemia predicts cardiovascular events in patients with type 2 diabetes and chronic kidney disease. J Cardiol2017; 69:529-535
13. Nakajima K, Saito T, Tamura A, Suzuki M, Nakano T, Adachi M, Tanaka A, Tada N, Nakamura H, Campos E, et al. Cholesterol in remnant-like lipoproteins in human serum using monoclonal anti apo B-100 and anti apo A-I immunoaffinity mixed gels. Clin Chim Acta1993; 223:53-71
14. Joshi PH, Khokhar AA, Massaro JM, Lirette ST, Griswold ME, Martin SS, Blaha MJ, Kulkarni KR, Correa A, D’Agostino RB, Sr., Jones SR, Toth PP, Lipoprotein Investigators Collaborative Study G. Remnant Lipoprotein Cholesterol and Incident Coronary Heart Disease: The Jackson Heart and Framingham Offspring Cohort Studies. J Am Heart Assoc2016; 5
15. Masuda D, Yamashita S. Postprandial Hyperlipidemia and Remnant Lipoproteins. J Atheroscler Thromb2017; 24:95-109
16. Nakajima K, Nagamine T, Fujita MQ, Ai M, Tanaka A, Schaefer E. Apolipoprotein B-48: a unique marker of chylomicron metabolism. Adv Clin Chem2014; 64:117-177
17. Nordestgaard BG. Triglyceride-Rich Lipoproteins and Atherosclerotic Cardiovascular Disease: New Insights From Epidemiology, Genetics, and Biology. Circ Res2016; 118:547-563