Artherogenesis: Predictor of CVD – the Smaller and Denser LDL Particles
Reporter: Aviva Lev-Ari, PhD, RN
Genetic Associations with Valvular Calcification and Aortic Stenosis
February 7, 2013DOI: 10.1056/NEJMoa1109034
We determined genomewide associations with the presence of aortic-valve calcification (among 6942 participants) and mitral annular calcification (among 3795 participants), as detected by computed tomographic (CT) scanning; the study population for this analysis included persons of white European ancestry from three cohorts participating in the Cohorts for Heart and Aging Research in Genomic Epidemiology consortium (discovery population). Findings were replicated in independent cohorts of persons with either CT-detected valvular calcification or clinical aortic stenosis.
Genetic variation in the LPA locus, mediated by Lp(a) levels, is associated with aortic-valve calcification across multiple ethnic groups and with incident clinical aortic stenosis. (Funded by the National Heart, Lung, and Blood Institute and others.)
HDL is more than an eNOS Agonist
In addition to the modulation of NO production by signaling events that rapidly dictate the level of enzymatic activity, important control of eNOS involves changes in the abundance of the enzyme. In a clinical trial by the Karas laboratory of niacin therapy in patients with low HDL levels (nine males and two females), flow-mediated dilation of the brachial artery was improved in association with a rise in HDL of 33% over 3 months (Kuvin et al., 2002).
They also demonstrated that eNOS expression in cultured human endothelial cells is increased by HDL exposure for 24 hours. They further showed that the increase in eNOS is related to an increase in the half-life of the protein, and that this is mediated by PI3K–Akt kinase and MAPK (Ramet et al., 2003).
Thus, the same mechanisms that underlie the acute activation of eNOS by HDL appear to be operative in upregulating the expression of the enzyme.
The current understanding of the mechanism by which HDL enhances endothelial NO production is summarized in Shaul & Mineo (2004), Figure 1.
It describes the mechanism of action for HDL enhancement of NO production by eNOS in vascular endothelium.
(a) HDL causes membrane-initiated signaling, which stimulates eNOS activity. The eNOS protein is localized in cholesterol-enriched (orange circles) plasma membrane caveolae as a result of the myristoylation and palmitoylation of the protein. Binding of HDL to SR-BI via apoAI causes rapid activation of the nonreceptor tyrosine kinase src, leading to PI3K activation and downstream activation of Akt kinase and MAPK. Akt enhances eNOS activity by phosphorylation, and independent MAPK-mediated processes are additionally required (Duarte, et al., 1997). Eur J Pharmacol, 338:25–33.
HDL also causes an increase in intracellular Ca2+ concentration (intracellular Ca2+ store shown in blue; Ca2+ channel shown in pink), which enhances binding of calmodulin (CM) to eNOS. HDL-induced signaling is mediated at least partially by the HDL-associated lysophospholipids SPC, S1P, and LSF acting through the G protein–coupled lysophospholipid receptor S1P3. HDL-associated estradiol (E2) may also activate signaling by binding to plasma membrane–associated estrogen receptors (ERs), which are also G protein coupled. It remains to be determined if signaling events are also directly mediated by SR-BI (Yuhanna et al., 2001), (Nofer et al., 2004), (Gong et al., 2003), (Mineo et al., 2003).
(b) HDL regulates eNOS abundance and subcellular distribution. In addition to modulating the acute response, the activation of the PI3K–Akt kinase pathway and MAPK by HDL upregulates eNOS expression (open arrows). HDL also regulates the lipid environment in caveolae (dashed arrows). Oxidized LDL (OxLDL) can serve as a cholesterol acceptor (orange circles), thereby disrupting caveolae and eNOS function. However, in the presence of OxLDL, HDL maintains the total cholesterol content of caveolae by the provision of cholesterol ester (blue circles), resulting in preservation of the eNOS signaling module (Ramet et al., 2003), (Blair et al., 1999), (Uittenbogaard et al., 2000).
Shaul, PW and Mineo, C, (2004). HDL action on the vascular wall: is the answer NO? J Clin Invest., 15; 113(4): 509–513.
Are Additional Lipid Measures Useful?
Ryan D. Bradley, ND; and Erica B. Oberg, ND, MPH
Total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) are the well-established standards by which clinicians identify individuals at risk for coronary artery disease (CAD), yet nearly 50% of people who have a myocardial infarction have normal cholesterol levels. Measurement of additional biomarkers may be useful to more fully stratify patients according to disease risk. The typical lipid panel includes TC, LDL-C, high-density lipoprotein cholesterol (HDL-C), and triglycerides (TGs). Emerging biomarkers for cardiovascular risk include measures of LDL-C pattern, size, and density; LDL particle number; lipoprotein(a); apolipoproteins (apoA1 and apoB100 being the most useful); C-reactive protein; and lipoprotein-associated phospholipase
Some of these emerging biomarkers have been proven to add to, or be more accurate than, traditional risk factors in predicting coronary artery disease and, thus, may be useful for clinical decision-making in high-risk patients and in patients with borderline traditional risk factors. However, we still believe that until treatment strategies can uniquely address these added risk factors—ie, until protocols to rectify unhealthy findings are shown to improve cardiovascular outcomes—healthcare providers should continue to focus primarily on helping patients reach optimal LDL-C, HDL-C, and TG levels
Table 1. Traditional Lipid Panel and Recommended Treatment
Goals for Cardiovascular Disease Prevention34
- Total Cholesterol Desirable (low) < 200 mg/dL
- Borderline high 200-239 mg/dL
- High 240 mg/dL or greater
- HDL Cholesterol Desirable (high) > 60 mg/dL
- Acceptable 40-60 mg/dL
- Low < 40 mg/dL
- LDL Cholesterol Desirable (low) < 100 mg/dL
- Acceptable 100-129 mg/dL
- Borderline high 130-159 mg/dL
- High 160-189 mg/dL
- Very high 190 mg/dL or greater
- Triglycerides Desirable (low) < 150 mg/dL
- Borderline high 150-199 mg/dL
- High 200-499 mg/dL
- Very high 500 mg/dL or greater
LDL-C and HDL-C: Pattern, Size, and Density
Two patterns predominate and are used to describe the average size of LDL particles. Pattern A refers to a preponderance of large LDL particles, while Pattern B refers to a preponderance of small LDL particles; a minority of individuals displays an intermediate or mixed pattern. Some commercially available assays further subdivide LDL-C into 7 distinct designations based on particle size.9,10
LDL Lipoprotein Particle Number
LDL particle number (LDL-P) is a measure of the number of lipoprotein particles independent of the quantity of lipid within the cholesterol particle; ie, LDL-P measures the number of individual particles, not a concentration like LDL-C. It is measured using nuclear magnetic resonance technology and is unaffected by fasting status.21 Higher LDL-P measures have been associated with a higher risk of CAD. This might simply be because there are more particles susceptible to oxidation in circulation.
There are suggestions, but not definitive proof, that reducing LDL-P increases intra-LDL antioxidant capacity. The European Prospective Investigation of Cancer (EPIC)-Norfolk cohort, a study that has followed 25 663 participants (men and women aged 45-79 years) over 6 years, evaluated associations between LDL-P and risk of CAD. Compared to controls, cases of CAD had a higher number of LDL particles (LDL-P P<.0001), smaller average LDL-particle size (P=.002), and higher concentrations of small LDL particles (P<.0001).22
Once again, small, dense LDL-C were positively associated with TG and negatively associated with HDL. In another study investigating incident angina and MI with LDL-P, females, but not males, had a significantly increased odds ratio for incident MI and angina for higher LDL-P—but not for LDL size—after adjustment for LDL, age, and race. Males had increased (but not significant) point estimates showing the same relationship.23 Of note, LDL-P and non-HDL-C (ie, TC minus HDL-C, or, specifically, LDL-C plus VLDLs), added equivalently to Framingham-predicted CAD risk stratification, thus reducing our enthusiasm for this additional measurement when TC and HDL-C are routinely available.22 Based on these results, LDL-P is becoming recognized as a more-precise measure of LDL-related risk and, as it becomes more available, is likely to replace LDL-C in risk-stratification tools. Clinical availability is currently limited; however, Medicare recently began reimbursing for regular testing of LDL-P in highrisk patients, so we should see availability increase soon. There are no novel treatments based on LDL-P at this time, and data shows therapies that lower LDL-C lower LDL-P as well.
Apolipoproteins are the protein components of plasma lipoproteins. Several different apolipoproteins have been identified and numbered; however, apoB48, apoB100, and apoA are the most commonly referenced. ApoB48 is associated with LDL particles that transport dietary cholesterol to the liver for processing. ApoB100 is found in lipoproteins originating from the liver (eg, LDL and VLDL); it transports these lipoproteins and, also, TGs to the periphery. In addition, ApoB100 is involved with the binding of LDL particles to the vascular wall, implicating itself as a key player in the development of atherogenic plaques. Importantly, there is one apoB100 molecule per hepatic-derived lipoprotein. Hence, it is possible to quantify the number of LDL/VLDL particles by noting the total apoB100 concentration.
Lipoprotein(a)—Lp(a)—is attached to apoB. The association of Lp(a) with CAD and its ability to act as a biomarker of risk appears to be strongest in patients with hypercholesterolemia and, in particular, in young patients with premature atherosclerosis (males younger than 55 and females younger than 65). Part of the reason for this is the observation that there seem to be important threshold effects such that only very high Lp(a) levels (> 30 mg/dL) are associated with elevated vascular risk; in this regard, these increased plasma levels of Lp(a) independently predict the presence of CAD, particularly in patients with elevated LDL-C levels.28
In the Cardiovascular Health Study, a relative risk of approximately 3-fold for death from vascular events and stroke was seen in the highest quintile compared to the lowest quintile of Lp(a) but for males only, whereas no such relation existed for women.29 Lp(a) is commonly considered a marker for familial hypercholesterolemia. Lp(a) may best be used in assessing the risk of younger males with strong family histories of CVD but should not be used more generally.
Risk Factors for Cardiovascular Disease
(Exclusive of LDL Cholesterol)34
- Cigarette smoking
- Hypertension (BP > 140/90 mmHg or on antihypertensive medication)
- Low HDL cholesterol (< 40 mg/dL)
- Family history of premature CHD (CHD in first-degree male relative <
- 55 years; CHD in first-degree female relative < 65 years)
- Age (men > 44 years; women > 54 years
- Clinical coronary heart disease,
- symptomatic carotid artery disease,
- peripheral arterial disease, or
- abdominal aortic aneurysm
In the United States, treatment guidelines for high CVD risk factors are set by the National Cholesterol Education Program (NCEP) Expert Panel, which developed the third report of the Adult Treatment Panel (ATPIII).34 Treatment goals are determined according to risk stratification by LDL-C and by known additional risk factors such as smoking, low HDL, hypertension, family history, and age. Yet, clinically, decision-making is always more complex than this. Additional risk stratification can be accomplished by measuring the biomarkers discussed above, and this may potentially provide additive benefit beyond NCEP guidelines. However, we always encourage clinicians to treat known risks to goal levels before adding additional goals for treatment. In a future article we will provide further detail on treatment options for novel biomarkers.
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