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

Lipid-lowering drug therapy for reducing cardiovascular risk in diabetes. A clinical view of the Cardiovascular Disease Working Group of the Spanish Diabetes Society

Patients with type 2 diabetes mellitus (T2DM) managed in both hospital and out-ofhospital settings usually have a high/very high cardiovascular risk, with a high burden of cardiovascular disease. All this justifies that the reduction of low-density lipoprotein cholesterol is the main therapeutic goal in T2DM. However, residual cardiovascular risk is very prevalent in T2DM, and is usually associated with atherogenic dyslipidemia and hyperlipoproteinemia(a); therefore, it is also necessary to reverse these lipoprotein abnormalities to achieve effective cardiovascular prevention. Given the considerable armamentarium of lipid-lowering drugs currently available, the Cardiovascular Disease Working Group of the Spanish Diabetes Society has considered it appropriate to carry out a narrative review and update of the effectiveness of these lipid-lowering drugs in the population with T2DM taking into account their effect on the lipoprotein profile and their potential impact on glycemic control.

ApoC-III proteoforms are associated with better lipid, inflammatory, and glucose profiles independent of total apoC-III

BACKGROUND

Apolipoprotein (apo) C-III is involved in several processes that increase triglyceride levels, inflammation, and insulin resistance. Four of its proteoforms have been the focus of several studies and have shown differential associations with cardiovascular risk biomarkers, mostly lipids. However, there are other proteoforms of apoC-III that have not yet been investigated in detail. The aim of this study was to evaluate the associations of seven apoC-III proteoforms with a comprehensive set of biomarkers, including lipid metabolism, inflammation, and glucose homeostasis.

METHODS

Seven apoC-III proteoforms (apoC-III0a, apoC-III0b, apoC-III1, apoC-III1d, apoC-III2, apoC-III2d, and apoC-III0f) were measured using a mass spectrometry immunoassay in 875 participants from the cross-sectional study of the Di@bet.es cohort. The complete lipoprotein profile was obtained via the Liposcale test, and the proton nuclear magnetic resonance (1H-NMR)-assessed glycoprotein signals were also obtained as biomarkers of inflammation.

RESULTS

Three proteoform ratios (apoC-III2d, apoC-III2, and apoC-III0f normalized to apoC-III1) showed protective associations with most of the cardiovascular risk biomarkers in comparison with total apoC-III in linear regression models and were negatively associated with triglycerides (β=-0.173, p < 0.001; β=-0.297, p < 0.001; β=-0.223, p = 0.002), very low-density (VLDL) particle concentration (β=-0.133, p < 0.001; β=-0.265, p < 0.001; β=-0.203, p < 0.001), GlycA (β=-0.148, p < 0.001; β=-0.263, p < 0.001; β=-0.211, p < 0.001) and homeostatic model assessment of insulin resistance (HOMA-IR) (β=-0.096, p = 0.003; β=-0.199, p < 0.001; β=-0.114, p = 0.002). These associations were partly independent of total apoC-III concentrations. Participants with high levels of these proteoforms had a lower prevalence of cardiometabolic disorders, such as type 2 diabetes (p = 0.022), obesity (p = 0.001), and metabolic syndrome (p = 0.013).

CONCLUSIONS

While apoC-III is positively associated with biomarkers of cardiometabolic risk, the proportions of three apoC-III proteoforms show opposite associations, independent of total apoC-III concentrations. Measuring not only apoC-III but also the proportions of apoC-III proteoforms can provide valuable information since individuals with similar levels of total apoC-III could display opposite lipid profiles depending on the proportion of apoC-III proteoforms.

Relationship of apolipoprotein C-III proteoform composition with ankle-brachial index and peripheral artery disease in the Multi-Ethnic Study of Atherosclerosis (MESA)

BACKGROUND AND AIMS

Apolipoprotein C-III (apoC-III) proteoform composition shows distinct relationships with plasma lipids and cardiovascular risk. The present study tested whether apoC-III proteoforms are associated with risk of peripheral artery disease (PAD).

METHODS

ApoC-III proteoforms, i.e., native (C-III0a), and glycosylated with zero (C-III0b), one (C-III1) or two (C-III2) sialic acids, were measured by mass spectrometry immunoassay on 5,734 Multi-Ethnic Study of Atherosclerosis participants who were subsequently followed for clinical PAD over 17 years. Ankle-brachial index (ABI) was also assessed at baseline and then 3 and 10 years later in 4,830 participants.

RESULTS

Higher baseline C-III0b/C-III1 and lower baseline C-III2/C-III1 were associated with slower decline in ABI (follow-up adjusted for baseline) over time, independently of cardiometabolic risk factors, and plasma triglycerides and HDL cholesterol levels (estimated difference per 1 SD was 0.31 % for both, p < 0.01). The associations between C-III2/C-III1 and changes in ABI were stronger in men (-1.21 % vs. -0.27 % in women), and in Black and Chinese participants (-0.83 % and -0.86 % vs. 0.12 % in White). Higher C-III0b/C-III1 was associated with a trend for lower risk of PAD (HR = 0.84 [95%CI: 0.67-1.04]) that became stronger after excluding participants on lipid-lowering medications (0.73 [95%CI: 0.57-0.94]). Neither change in ABI nor clinical PAD was related to total apoC-III levels.

CONCLUSIONS

We found associations of apoC-III proteoform composition with changes in ABI that were independent of other risk factors, including plasma lipids. Our data further support unique properties of apoC-III proteoforms in modulating vascular health that go beyond total apoC-III levels.

Emerging Roles of UDP-GalNAc Polypeptide N-Acetylgalactosaminyltransferases in Cardiovascular Disease

UDP-GalNAc polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts) catalyze mucin-type O-glycosylation by transferring α-N-acetylgalactosamine (GalNAc) from UDP-GalNAc to Ser or Thr residues of target proteins. This post-translational modification is common in eukaryotes, yet its biological functions remain unclear. Recent studies have identified specific receptors in the heart and vascular wall cells that can be mucin-type O-glycosylated, and there is now substantial evidence confirming that patients with various cardiovascular diseases (CVDs), such as heart failure, coronary artery disease, myocardial hypertrophy, and vascular calcification, exhibit abnormal changes in GalNAc-Ts. This review aims to highlight recent advances in GalNAc-Ts and their roles in the cardiovascular system, intending to provide evidence for clinical treatment and prevention of CVDs.

Exploring apolipoprotein C-III: pathophysiological and pharmacological relevance

The availability of pharmacological approaches able to effectively reduce circulating LDL cholesterol (LDL-C) has led to a substantial reduction in the risk of atherosclerosis-related cardiovascular disease (CVD). However, a residual cardiovascular (CV) risk persists in treated individuals with optimal levels of LDL-C. Additional risk factors beyond LDL-C are involved, and among these, elevated levels of triglycerides (TGs) and TG-rich lipoproteins are causally associated with an increased CV risk. Apolipoprotein C-III (apoC-III) is a key regulator of TG metabolism and hence circulating levels through several mechanisms including the inhibition of lipoprotein lipase activity and alterations in the affinity of apoC-III-containing lipoproteins for both the hepatic receptors involved in their removal and extracellular matrix in the arterial wall. Genetic studies have clarified the role of apoC-III in humans, establishing a causal link with CVD and showing that loss-of-function mutations in the APOC3 gene are associated with reduced TG levels and reduced risk of coronary heart disease. Currently available hypolipidaemic drugs can reduce TG levels, although to a limited extent. Substantial reductions in TG levels can be obtained with new drugs that target specifically apoC-III; these include two antisense oligonucleotides, one small interfering RNA and an antibody.

Apolipoprotein C3: form begets function

Increased circulating levels of apolipoprotein C3 (APOC3) predict cardiovascular disease (CVD) risk in humans, and APOC3 promotes atherosclerosis in mouse models. APOC3's mechanism of action is due in large part to its ability to slow the clearance of triglyceride-rich lipoproteins (TRLs) and their remnants when APOC3 is carried by these lipoproteins. However, different pools and forms of APOC3 exert distinct biological effects or associations with atherogenic processes. Thus, lipid-free APOC3 induces inflammasome activation in monocytes whereas lipid particle-bound APOC3 does not. APOC3-enriched LDL binds better to the vascular glycosaminoglycan biglycan than does LDL depleted of APOC3. Patterns of APOC3 glycoforms predict CVD risk differently. The function of APOC3 bound to HDL is largely unknown. There is still much to learn about the mechanisms of action of different forms and pools of APOC3 in atherosclerosis and CVD, and whether APOC3 inhibition would prevent CVD risk in patients on LDL-cholesterol lowering medications.

Triglyceride-Rich Lipoprotein Metabolism: Key Regulators of Their Flux

The residual risk for arteriosclerotic cardiovascular disease after optimal statin treatment may amount to 50% and is the consequence of both immunological and lipid disturbances. Regarding the lipid disturbances, the role of triglyceride-rich lipoproteins (TRLs) and their remnants has come to the forefront in the past decade. Triglycerides (TGs) stand as markers of the remnants of the catabolism of TRLs that tend to contain twice as much cholesterol as compared to LDL. The accumulation of circulating TRLs and their partially lipolyzed derivatives, known as "remnants", is caused mainly by ineffective triglyceride catabolism. These cholesterol-enriched remnant particles are hypothesized to contribute to atherogenesis. The aim of the present narrative review is to briefly summarize the main pathways of TRL metabolism, bringing to the forefront the newly discovered role of apolipoproteins, the key physiological function of lipoprotein lipase and its main regulators, the importance of the fluxes of these particles in the post-prandial period, their catabolic rates and the role of apo CIII and angiopoietin-like proteins in the partition of TRLs during the fast-fed cycle. Finally, we provide a succinct summary of the new and old therapeutic armamentarium and the outcomes of key current trials with a final outlook on the different methodological approaches to measuring TRL remnants, still in search of the gold standard.