Apo CIII Proteoforms, Plasma Lipids, and Cardiovascular Risk in MESA

Abstract

Background: Apo CIII (apolipoprotein CIII) is an important regulator of triglyceride metabolism and was associated with cardiovascular risk in several cohorts. It is present in 4 major proteoforms, a native peptide (CIII_0a), and glycosylated proteoforms with zero (CIII_0b), 1 (CIII₁, most abundant), or 2 (CIII₂) sialic acids, which may differentially modify lipoprotein metabolism. We studied the relationships of these proteoforms with plasma lipids and cardiovascular risk.

Methods: Apo CIII proteoforms were measured by mass spectrometry immunoassay in baseline plasma samples of 5791 participants of Multi-Ethnic Study of Atherosclerosis, an observational community-based cohort. Standard plasma lipids were collected for up to 16 years and cardiovascular events (myocardial infarction, resuscitated cardiac arrest, or stroke) were adjudicated for up to 17 years.

Results: Apo CIII proteoform composition differed by age, sex, race and ethnicity, body mass index, and fasting glucose. Notably, CIII₁ was lower in older participants, men and Black and Chinese (versus White) participants, and higher in obesity and diabetes. In contrast, CIII₂ was higher in older participants, men, Black, and Chinese persons, and lower in Hispanic individuals and obesity. Higher CIII₂ to CIII₁ ratio (CIII₂/III₁) was associated with lower triglycerides and higher HDL (high-density lipoprotein) in cross-sectional and longitudinal models, independently of clinical and demographic risk factors and total apo CIII. The associations of CIII_0a/III₁ and CIII_0b/III₁ with plasma lipids were weaker and varied through cross-sectional and longitudinal analyses. Total apo CIII and CIII₂/III₁ were positively associated with cardiovascular disease risk (n=669 events, hazard ratios, 1.14 [95% CI, 1.04-1.25] and 1.21 [1.11-1.31], respectively); however, the associations were attenuated after adjustment for clinical and demographic characteristics (1.07 [0.98-1.16]; 1.07 [0.97-1.17]). In contrast, CIII_0b/III₁ was inversely associated with cardiovascular disease risk even after full adjustment including plasma lipids (0.86 [0.79-0.93]).

Conclusions: Our data indicate differences in clinical and demographic relationships of apo CIII proteoforms, and highlight the importance of apo CIII proteoform composition in predicting future lipid patterns and cardiovascular disease risk.

Keywords: apolipoprotein; cardiovascular disease; lipids; risk.

Figure 1.

Cross-sectional (baseline; left panels) and longitudinal (follow-up adjusted for baseline and time of follow-up; right panels) relationships of baseline total apoC-III concentrations, and log-ratios of apoC-III proteoforms to C-III₁ (all included in the same additive model) with plasma lipids. Multiple linear regression models were run unadjusted (total apoC-III and proteoforms ratios in separate models) and adjusted (total apoC-III and proteoforms ratios in the same model) for baseline age, gender, race/ethnicity, BMI, diabetes status, fasting glucose, tobacco use, lipid-lowering therapy and eGFR. Longitudinal mixed regression models for repeated measures were further adjusted for BMI, diabetes status, fasting glucose, tobacco use, lipid-lowering therapy and eGFR at each follow-up exam. Symbols and labels are β-estimates. All 95% CI not crossing the zero-x-axis value are consistent with p<0.05. All apoC-III and lipid measures were natural log-transformed and scaled to 1 SD, e.g., an increase of 1 SD in C-III₂/C-III₁ (adjusted model) was associated with reductions of 28% and 4.5% of 1 SD in baseline and follow-up plasma triglycerides, respectively.

Figure 2.. Association between baseline total apoC-III concentrations and log-ratios of apoC-III proteoforms to apoC-III₁ (most abundant), and CVD events.

Panel A: Kaplan-Meier curves of top (High) and bottom (Low) quartile of apoC-III measures for entire follow-up period. Unadjusted Cox proportional hazard ratios (HR) and 95% confidence intervals (95% CI) per 1 SD of apoC-III measures are listed in each figure. Panel B: Cox proportional risk models of CVD risk. Model 1 included total apoC-III and apoC-III proteoforms ratios; Model 2 also included baseline age, gender, race/ethnicity, BMI, diabetes, systolic blood pressure, use of antihypertensive and lipid lowering medications, smoking status and eGFR, and Model 3 was further adjusted for plasma triglycerides and HDL cholesterol. All apoC-III measures were natural log-transformed and expressed per 1 SD unit.