Graphical abstract: Triglyceride-rich lipoprotein (TRL) remnant particles and their role in atherosclerosis. ApoB48 and apoB100-containing lipoproteins with a diameter of 70 nm or smaller enter the artery wall by transcytosis, and become retained via interactions with artery wall proteoglycans. The retained lipoproteins become modified by oxidative and non-oxidative enzymes, and release lipolytic products. The modified lipoproteins are taken up bymacrophages converting them to foam-cells.
The critical role of cholesterol-rich apolipoprotein B (apoB)-containing lipoproteins in atherogenesis cannot be overstated.[1,2] LDL is considered the archetypal atherogenic lipoprotein. As the main carrier of cholesterol in the blood, it is for most people the principal vehicle delivering cholesterol into the artery wall, and thereby initiating and progressing lesion formation. However, mounting evidence has established that triglyceride-rich lipoproteins (TRLs) and their remnants are also causal factors in atherosclerotic cardiovascular disease (ASCVD), and their contribution to pathological processes appears statistically independent of, and additional to, that of LDL.[3,4] Remnants are partially lipolysed lipoproteins derived from TRLs of liver and intestinal origin. While remnant particles contain significant amounts of triglycerides, they also contain large amounts of cholesterol; the cholesterol/apoB molar ratio is approximately two-fold higher than for LDL particles. Conceptually, all lipoproteins that carry a cholesterol ‘payload’ and have surface proteins—particularly apoB100 and apoE—that bind to arterial wall proteoglycans (and hence are retained in the intima) are potential contributors to focal cholesterol deposition and macrophage foam cell generation. So, it is not surprising that other members of the apoB-containing lipoprotein spectrum are now emerging as possible targets for intervention.[1,4]
Studies demonstrating the importance of TRL remnants in ASCVD are based mainly on genetics and epidemiology. However, there are significant hurdles in investigating and quantifying the contribution of remnant particles to atherogenesis. The spectrum of TRL remnants in the circulation is heterogenous, and to date there is a lack of a definitive biomarker(s)—’signature’—that permits the unequivocal quantitation of remnant levels. This is partly explained by the fact that TRL remnants in the circulation are modified by metabolic processes (i.e. hydrolysis, cholesteryl-ester transfer protein-mediated lipid exchange, and uptake by hepatic receptors) that constantly alter both their lipid and protein composition. Several different approaches have been used to date including sequential ultracentrifugation, immunoaffinity assays, nuclear magnetic resonance (NMR) spectroscopy, calculations based on cholesterol measurements in various lipoprotein classes, or simply dividing plasma triglyceride concentration by a factor. Assessment of TRL cholesterol (TRL-C) provides an approximation of the abundance of remnants present in the circulation since the level of this lipid is believed to vary in proportion to remnant concentration. However, it is not a precise or specific biomarker and it must be used with a degree of caution, keeping in mind the pathophysiology of TRL metabolism and the molecular basis of remnant atherogenicity.
The study by Quispe et al. in the present issue of the European Heart Journal furthers our understanding of the importance of TRL remnants in ASCVD risk in asymptomatic people. By pooling data from 17 532 ASCVD-free individuals in three well-known, large US epidemiological studies [Atherosclerosis Risk in Communities study (ARIC; n = 9748), Multi-Ethnic Study of Atherosclerosis (MESA; n = 3049), and Coronary Artery Risk Development in Young Adults (CARDIA; n = 4735)], remnant-C was shown to associate with incident ASCVD independent (statistically) of both LDL cholesterol (LDL-C) and apoB levels over a median observation period of 18.7 years.
In light of the comments above, it is important in studies such as that of Quispe et al. to understand how TRL-/remnant-C was determined. The authors calculated remnant-C as [non-HDL-C minus calculated LDL-C]. The LDL-C itself was calculated as [total cholesterol minus HDL-C minus very low-density lipoprotein (VLDL)-C] where VLDL-C was derived from plasma triglyceride using the Martin/Hopkins equation. Thus, the calculated ‘remnant-C’ corresponds arithmetically to the cholesterol in VLDL (i.e. TRL-C). While this derived estimate of remnant-C is likely to be the best that can be achieved in these large population datasets given the lipid profile results available, it is a proxy measure. Varbo and Nordestgaard recently showed that the correlation between directly measured and calculated TRL-/remnant-C shows significant discordance especially at high concentrations (Figure 1).
Association between directly measured and calculated remnant cholesterol. Each grey dot indicates concentrations for an individual, red dashed lines indicate 80th percentiles, and the dark blue line is from a local polynomial regression (unadjusted) between directly measured and calculated remnant cholesterol (both capturing cholesterol content in triglyceride-rich lipoproteins). The black dashed line is the identity line Y = X. From Eur Heart J. ehab293, https://doi.org/10.1093/eurheartj/ehab293.
Quispe et al. adopt two approaches in evaluating the contribution of remnant-C to ASCVD risk in these combined cohorts: multivariate models in which remnant-C, LDL-C, and apoB are included as explanatory variables; and a ‘discordancy’ analysis relating population percentile for remnant-C to that for LDL-C. The discordant high remnant-C (percentile)/low LDL-C (percentile) group, but not the low remnant-C/high LDL-C group, was associated with increased ASCVD risk compared with the concordant group (where the percentiles for remnant-C and LDL-C were similar) with a hazard ratio (HR) of 1.21 [95% confidence interval (CI) 1.08–1.35] for ASCVD. This observation supports the view that TRL-C/remnant-C represents an additional risk factor beyond LDL-C. The finding is in line with the recent publication from Johannesen et al. that explored in a similar discordancy analysis the impact of TRL-C on ASCVD risk in statin-treated individuals in the Copenhagen population. In the study of Quispe et al. it was concluded further, based on multivariate statistical models, that the risk associated with remnant-C was independent of traditional risk factors, LDL-C and apoB, and remained a statistically significant risk predictor even after adjusting for non-HDL-C.
There are a number of issues worthy of comment in the interpretation of these findings. First, the Cox model results can give the impression that LDL-C is not associated with increased ASCVD risk. However, the model adjusts for apoB which is present as the major protein in LDL particles (LDL-C and apoB are highly correlated for this reason), and it is no surprise that LDL-C becomes non-significant if apoB is included in the model. Second, apoB is also an integral structural component of all TRL remnant particles (apoB100 in VLDL remnants, apoB48 in chylomicron remnants). It may be that the remnant-C level as determined by the authors is a statistically independent risk predictor when total plasma apoB is inserted into the regression model, but the cholesterol in remnants does not cause atherosclerosis outside of its constituent association with apoB-containing lipoprotein particles. How total apoB is distributed across the VLDL-LDL spectrum may differ in individuals with concordant and discordant remnant-C/LDL-C profiles.
The authors further report that elevated remnant-C levels remained associated with ASCVD even after adjusting for non-HDL-C. The physiological relevance of this adjustment is unclear so it is hard to discern the implications of this result. However, the authors offer the following suggestion ‘this observation suggests that elevated levels of remnant-C, regardless of total non-HDL-C level, may indirectly reflect risk information related to other atherogenic mechanisms such as increased apoC3 or ANGPTL3 activity rather than the risk captured in the cholesterol content of remnant particles’. However, since remnants carry around twice more cholesterol per particle than LDL (not 40 times more as stated) it seems feasible that the increased cholesterol/apoB molar ratio could explain, at least partly, the difference.
In addition, the baseline characteristics differed between the high remnant-C/low LDL-C group and the low remnant-C/high LDL-C group; diabetes was 2.4-fold more common and plasma triglycerides 2-fold higher in the high remnant-C/low LDL-C group. It is well known that the expression of apoC-III is associated with elevation of TRLs in subjects with type 2 diabetes, potentially through the influence of glucose homeostasis on the production of apoC-III, and plasma levels of apoC-III have been shown to associate with ASCVD events in several studies, including the ARIC study. Thus, the characteristics of the remnant particles may differ between the high remnant-C/low LDL-C group and the low remnant-C/high LDL-C groups. This is important since it is unclear if all remnant particles display equal atherogenicity. Thus, it is possible that remnants in the high remnant-C/low LDL-C group have characteristics (lipid and/or protein composition) that makes them more atherogenic, than in the low remnant-C/high LDL-C group.
The role of TRL and their remnants in ASCVD is increasingly appreciated not only as a component of residual risk in statin-treated patients with well-controlled LDL levels, but also as an unaddressed causal factor in asymptomatic individuals at increased risk for disease. In focusing on evaluating the contribution of remnant particles to atherosclerosis and as a potential target for intervention, we must not lose sight of the fact that LDL for many is the major driver of atherogenesis. Individual measures of remnant abundance are at present surrogates, and findings need to be interpreted in light of the known pathophysiology of lipoprotein particles.