HDL as Targets for Type 2 Diabetes and Cardiovascular Disease: What Have We Learned?

The longitudinal Framingham Heart Study was the first to report, in the early 1950s, that risk of coronary heart disease (CHD) decreased with increasing high density lipoprotein (HDL) levels. Interest in HDL waned, however, then re-emerged in the 1970s when it was again demonstrated that HDL cholesterol levels were inversely related to heart disease.¹

The epidemiological data relating HDL to heart disease have been extensively validated, and more recently, results from clinical trials have produced the same conclusions. Coronary event data from several statin trials showed a relationship between baseline HDL levels and cardiovascular event incidence. In 5 major trials, the risk of coronary events in patients with low HDL was reduced on statin therapy to rates similar to those of placebo-treated patients who had higher HDL concentrations (Figure 1).^2-7 In each study, patients in statin and placebo groups with lower HDL at baseline experienced a higher incidence of coronary events compared with patients with higher HDL levels.

Figure 1. Statin Trials: Effects on Cardiovascular Risk by Baseline HDL

In 5 major trials, the risk of coronary events in patients with low HDL was reduced with statin therapy to rates similar to those of placebo treated patients who had higher HDL concentrations.
Sources: Ballantyne CM, et al. Circulation. 1999;99:736-743; Scandinavian Simvastatin Survival Study Group. Lancet. 1995;345:1274-1275; LIPID Study Group. N Engl J Med. 1998;339:1349-1357; Pfeffer MA, et al. J Am Coll Cardiol. 1999;33:125-130; Shepherd J, et al. N Engl J Med. 1995;333:1301-1307; Downs JR, et al. JAMA. 1998;279:1615-1622.

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The Treating to New Targets (TNT) study compared atorvastatin 80 mg with atorvastatin 10 mg in patients with CHD. Post hoc analyses explored the ability of on-treatment HDL to predict outcomes.⁸ Patients with on-treatment LDL levels < 70 mg/dL were divided into quintiles based on HDL levels during the third treatment month, with the lowest having < 37 mg/dL and the highest > 55 mg/dL. Event rates were higher in the lower 2 quintiles when compared with those occurring in patients with on-treatment HDL levels > 43 mg/dL, and patients in the highest HDL quintile were at significantly less risk for a major cardiovascular event compared with those in the lowest quintile (P = .03). Therefore, data from several statin trials show that both baseline and on-treatment HDL levels are associated with cardiovascular risk.

HDL Structure and Physiology

HDL has a cholesterol-rich spherical structure with a phospholipid surface, typical of other lipoproteins. The primary difference between HDL and the atherogenic lipoproteins is the presence of apoA-I as the predominant surface protein on HDL. ApoA-I is produced primarily in the liver, although some production occurs in the intestine. ApoA-I is secreted in combination with the phospholipid that makes up the coating of lipoproteins.

Low HDL is related to hypertriglyceridemia, obesity, and insulin resistance, which are all genetic and environmentally induced metabolic disorders characterized by an excess of energy that is not being stored efficiently. Persons with obesity and insulin resistance have a range of HDL levels from the 20s to low 40s.

Atheroprotective effect of HDL

The mechanism by which HDL exerts its protective effect is believed to be related to its mediation of reverse cholesterol transport. Foam cells — macrophages filled with cholesterol — in fatty streaks and plaque contain several surface proteins that can facilitate the removal of cholesterol from the foam cells. When it is transferred to the apoA-I phospholipid combination that was secreted from the liver and intestine, a nascent HDL particle is created. This cholesterol is then enzymatically esterified in the process of producing a mature HDL particle. The particle is then believed to be taken up by the liver, where the cholesterol is removed and subsequently excreted in bile as either biliary cholesterol or bile acid. This linear pathway from the macrophage foam cell to the feces comprises reverse cholesterol transport.

However, if very low density lipoprotein (VLDL) is elevated and cholesteryl ester transfer protein (CETP) is active, cholesterol can be diverted from this linear pathway into an atherogenic particle. Some of those atherogenic particles are transported to the liver via LDL receptors. Alternatively, they may enter the vessel wall, returning the cholesterol to the plaque. This futile recycling interrupts reverse cholesterol transport.

Insulin resistance and dyslipidemia

The multi-organ mechanism by which insulin resistance is linked to dyslipidemia originates with fat cells that have become insulin-resistant. Being less responsive to insulin, these fat cells release free fatty acids inappropriately, rather than maintaining them as stored triglycerides (Figure 2). The liver removes the fatty acids from the circulation, uses them to synthesize new triglyceride, and releases VLDL that carry those triglycerides to the periphery. In this setting of increased circulatory VLDL and triglycerides, CETP initiates the process of transferring the HDL cholesteryl ester into the VLDL. The net effect is reduction of cholesterol being transported in a protective HDL particle and an enrichment of cholesterol in an atherogenic VLDL particle.

Figure 2. Mechanisms Relating Insulin Resistance and Dyslipidemia

The multi-organ mechanism by which insulin resistance is linked to dyslipidemia originates with fat cells that have become insulin-resistant.
KEY: CE — cholesteryl ester; CETP — cholesteryl ester transfer protein; FFA — free fatty acid; HDL — high density lipoprotein; IR — insulin resistance; TG — triglyceride; VLDL — very low density lipoprotein
Source: Ginsberg HN. J Clin Invest. 2000;106:453-458.

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A triglyceride is transferred from VLDL to HDL concomitant with the CETP-mediated cholesterol transfer from HDL to VLDL, creating an unstable, triglyceride-enriched HDL particle. The transferred triglyceride is removed from the HDL by endothelial-bound lipases, including hepatic lipase. The transformed and smaller HDL is unstable and loses some of the surface apoA-I, which is cleared rapidly from the circulation. A significant site for apoA-I removal is the kidney. Therefore, as a result of the insulin resistant hypertriglyceridemic state, there are fewer HDL particles and less apoA-I.

Cholesterol efflux capacity

Factors affecting cholesterol efflux capacity, a metric of HDL function, was investigated in a case control study in an ex vivo system.⁹ Participants included healthy volunteers and patients with coronary artery disease (CAD). The assay involves incubating macrophages with apoB-depleted serum and quantifying cholesterol efflux. The association between cholesterol efflux capacity and CAD was adjusted for numerous factors including participant age, sex, smoking status, diabetes, hypertension, and the levels of LDL cholesterol, HDL cholesterol, and apoA-I. Although levels of HDL cholesterol and apoA-I were significant determinants of cholesterol efflux capacity, they accounted for < 40% of the variation among samples. The presence and extent of atherosclerosis were inversely related to efflux, and efflux was a significant predictor of coronary disease status that remained after adjusting for HDL cholesterol and apoA-I levels.

Other proposed anti-atherogenic mechanisms

Many mechanisms other than reverse cholesterol transport have been proposed to explain the anti-atherogenic properties of HDL; most are primarily based on in vitro studies.^10-12 Several studies have shown that HDL transports antioxidants including vitamin E and peroxidase. In vitro studies have also demonstrated HDL inhibition of the production of endothelial cell adhesion molecules, proteins that facilitate movement of monocytes from the circulation to atherosclerotic plaques. HDL also modulates monocyte production in bone marrow, suggesting that an earlier anti-atherosclerotic effect may occur that precedes those provided by anti-inflammatory and antioxidant effects in the vessel walls.¹³ Nitric oxide (NO) production is promoted by HDL, as is prostacyclin stabilization. Prostacyclins work with NO to inhibit platelet activation and induce vascular smooth muscle relaxation. Finally, insulin secretion from β-cells is regulated in part by HDL. HDL levels are commonly reduced in diabetes, and high HDL is known to be associated with greater insulin sensitivity. Results of a recent in vitro study showed that apoA-I and apoA-II increased β-cell secretion of insulin up to 5-fold.¹⁴

The complexity of HDL and its structural and functional heterogeneity pose challenges to specifically defining its biochemical interactions and confirming its benefits. This complexity also hinders full understanding of the mechanisms by which the benefits of HDL are achieved. Numerous HDL subspecies differ by size, charge, density, and composition.¹⁵ HDL particles are associated with approximately 100 different proteins, with an even greater number of lipid species that may contribute to variability in HDL structure. These structural differences are related to metabolic and functional differences, and contribute to the abundance of targets through which HDL may provide its protective effects.

Discussion

What are the prospects of a cholesterol efflux capacity assay reaching the stage of clinical applicability?

Henry Ginsberg, MD: I do not know if there are plans to commercialize that assay. The technique adds to our understanding, but I do not think it will be generally helpful because, although it was independently predictive on a statistical basis, the resulting receiver operating characteristic curve revealed a minimally increased ability to predict coronary disease. In a patient with coronary disease and very high HDL, it may be interesting to see if a defect is shown.

Patients with acute coronary syndrome (ACS) are not the only group for whom HDL may become acutely dysfunctional; anyone who has atherosclerosis or who has had a cardiovascular event may have HDL that is bearing the burden of constantly removing atherogenic compounds from the vessel wall. This is an important research area that requires considerable additional investigation.

Can HDL particles be pro-inflammatory?

Ginsberg: This concept originated from observation of patients with an HDL of 60 mg/dL or 70 mg/dL who still have CAD. One group that has been studying this showed that some HDLs did not display the protective activities typically observed in laboratory models.¹⁶ However, other groups have not been able to validate this assay and there may be a variety of other explanations for these patients having coronary disease. Nevertheless, as the complexity of HDL is better understood, it is possible that a total HDL cholesterol level might not always be as revealing as population epidemiology suggests it is.

Alan R. Tall, MD: I believe it is likely that HDL becomes dysfunctional in certain circumstances. For example, it was recently shown that HDL loses its ability to stimulate NO release from endothelial cells in the setting of ACS.¹⁷ An important question is whether the HDL dysfunction is the primary defect or whether it is a secondary defect. I believe that under certain conditions, for example when people have ACS, HDL may be overwhelmed, becoming filled with oxidized lipids that exert pro-inflammatory functions. Therefore, I believe these are likely secondary events. The primary abnormality is not that dysfunctional HDL is causing disease, but it is simply an issue of the HDL becoming overwhelmed.

References

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Ballantyne CM, Herd JA, Ferlic LL, et al. Influence of low HDL on progression of coronary artery disease and response to fluvastatin therapy. Circulation. 1999;99:736-743.
Scandinavian Simvastatin Survival Study Group. Baseline serum cholesterol and treatment effect in the Scandinavian Simvastatin Survival Study (4S). Lancet. 1995;345:1274-1275.
The Long-Term Intervention With Pravastatin in Ischaemic Disease (LIPID) Study Group. Prevention of cardiovascular events and death with pravastatin in patients with coronary heart disease and a broad range of initial cholesterol levels. N Engl J Med. 1998;339:1349-1357.
Pfeffer MA, Sacks FM, Moyé LA, et al. Influence of baseline lipids on effectiveness of pravastatin in the CARE trial. Cholesterol and recurrent events. J Am Coll Cardiol. 1999;33:125-130.
Shepherd J, Cobbe SM, Ford I, et al. Prevention of coronary heart disease with pravastatin in men with hypercholesterolemia. West of Scotland Coronary Prevention Study Group. N Engl J Med. 1995;333:1301-1307.
Downs JR, Clearfield M, Weis S, et al. Primary prevention of acute coronary events with lovastatin in men and women with average cholesterol levels: Results of AFCAPS/TexCAPS. Air Force/Texas Coronary Atherosclerosis Prevention Study. JAMA. 1998;279:1615-1622.
Barter P, Gotto AM, LaRosa JC, et al; Treating to New Targets Investigators. HDL cholesterol, very low levels of LDL cholesterol, and cardiovascular events. N Engl J Med. 2007;357:1301-1310.
Khera AV, Cuchel M, de la Llera-Moya, et al. Cholesterol efflux capacity, high-density lipoprotein function, and atherosclerosis. N Engl J Med. 2011;364:127-135.
Tall AR. Cholesterol efflux pathways and other potential mechanisms involved in the athero-protective effect of high density lipoproteins. J Int Med. 2008;263:256-273.
Mineo C, Deguchi H, Griffin JH, Shaul PW. Endothelial and antithrombotic actions of HDL. Circ Res. 2006;98:1352-1364.
Chapman MJ, Ginsberg HN, Amarenco P, et al; European Atherosclerosis Society Consensus Panel. Triglyceride-rich lipoproteins and high-density lipoprotein cholesterol in patients at high risk of cardiovascular disease: Evidence and guidance for management. Eur Heart J. 2011;32:1345-1361.
Yvan-Charvet L, Pagler T, Gautier EL, et al. ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation. Science. 2010;328:1689-1693.
Fryirs MA, Barter PJ, Appavoo M, et al. Effects of high-density lipoproteins on pancreatic beta-cell insulin secretion. Arterioscler Thromb Vasc Biol. 2010;30:1642-1648.
Asztalos BF, Tani M, Schaefer EJ. Metabolic and functional relevance of HDL subspecies. Curr Opin Lipidol. 2011;22:176-185.
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Cannon CP, Braunwald E, McCabe CH, et al; Pravastatin or Atorvastatin Evaluation and Infection Therapy — Thrombolysis in Myocardial Infarction 22 Investigators. Intensive versus moderate lipid lowering with statins after acute coronary syndromes. N Engl J Med. 2004;350:1495-1504.