HDL Complexity and the Effect of Disease on HDLs

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Summary

HDL Complexity and the Effect of Disease on HDLs

The potential for disease status to change the structure and, hence, function of HDLs is of paramount importance to our understanding of the subsequent effects of raising HDL-C concentration. Interpretation and prediction of the impact of CETP inhibition is complicated by the growing awareness that the effects of HDL-C may be modified in different clinical settings. Human plasma high-density lipoprotein (HDL) particles constitute a spectrum of pseudo-micellar protein/ lipid complexes, with hydrated densities within the range of 1.063 to 1.210 g/ml⁴⁰. This spectrum may be further defined by five physicochemically-defined particle subpopulations determined by their buoyant density: HDL2b, 2a, 3a, 3b, and 3c. Compared with other lipoproteins, HDLs are protein-rich, with an average ratio protein:lipid of 1:1. Approximately 70 % of HDL protein mass is Apo AI, with Apo AII accounting for a further 15–20 %. Major components of the remaining 15–20 % of proteins are other amphipathic apolipoproteins (eg., ApoCI, ApoCIV, E, D, M and AIV), enzymes and lipid transfer proteins (eg paraoxonase (PON), PAF-acetlyhydrolase (PAFAH), LCAT, and CETP).

Proteomic analysis of HDLs, on the other hand, has been identifying putative mechanistic information for functional observations reported in previous decades. Recent proteomic studies have identified up to 49 proteins associated with centrifugally-isolated HDL^41–44. Studies by Vaisar and colleagues⁴⁴, investigating both total HDL and HDL3 from normocholesterolaemic control subjects and age-matched patients with CAD, identified a role for HDLs in protease inhibition in addition to an effect on complement activation, supporting findings from ten years earlier⁴⁵. Data from a meticulous study, in which the proteome of HDL2b, 2a, 3a, 3b, and 3c, also from normocholesterolaemic subjects, were analysed, defined 28 distinct HDL-associated proteins which associated in clusters in subpopulations⁴⁶. In this study the investigators were able to show that ApoL-I, PON1 and PON3 correlated with the capacity of HDL3 to protect against oxidation, confirming their results through measurement of the ability of HDL3 to reduce the rate of accumulation of conjugated dienes in an LDL oxidation assay. These observations confirm earlier functional studies in which PON was shown to be one of the major proteins responsible for the anti- oxidative function of HDLs^47–49. In a recent report of a 10-year follow up of 88 type 2 diabetic patients, the incidence of cardiovascular events increased in proportion to reduced PON1 levels and activity, suggesting that PON1 may be an independent predictor of cardiovascular events in people with diabetes⁴⁹. Modulation of the structure and function of HDL-C has been reported, initially with regard to the anti-oxidative function of the particle50, but also with respect to the anti-inflammatory, cholesterol transfer, and anti-thrombotic function of HDL-C^51–53.

Clinical Outcomes, so Far

The first Phase III randomised controlled trial for a member of the CETP inhibitor family (Torcetrapib: ILLUMINATE; NCT00134264) began in July 2004 comparing 15,067 patients randomised to high-intensity statin alone or high-intensity statin plus torcetrapib⁵⁴. This study was terminated early when it became clear that the drug increased the incidence of the primary CVD endpoint, despite raising HDL-C by 70 % and lowering LDL-C by 20 %. Following analysis of the trial samples collected, the authors concluded that the effect was due to an off- target rise in the concentration of an aldosterone-like factor, which resulted in an unanticipated increase in blood pressure. However, this is not entirely consistent with the fact that CHD mortality was inversely related to a raised blood pressure and the incidence of stroke was not greater in the treatment group. A second member of the CETP inhibitors, dalcetrapib, presented an opportunity to test the HDL hypothesis as this compound raised plasma HDL-C by 30–40 % but had little or no effect on LDL-C. A Phase III randomised controlled trial (Dalcetrapib: Dal-OUTCOMES; NCT00658515), involving 15,600 patients with recent acute coronary syndrome, began in 2008 but was halted in 2012, because of a perceived lack of efficacy⁵⁵. Anacetrapib is the most potent CETP inhibitor to date and in the first clinical trial (Anacetrapib: DEFINE; NCT00685776)⁵⁶ was shown to lower LDL-C by approximately 50 % and increase HDL-C by 140 %. Although, this gross effect may reflect an extreme disturbance of HDL-C metabolism and its consequences,the DEFINE trial did not report any significant increase in CVD in the test arm. However, this study was too small to provide robust information on the clinical events⁵⁶.