Summary
Numerous epidemiological studies have demonstrated the atheroprotective roles of high density lipoproteins (HDL), so that HDL is established as an independent negative risk factor. The protective effect of HDL against atherosclerosis is mainly attributed to their capacity to bring peripheral excess cholesterol back to the liver for further elimination into the bile. In addition, HDL can exert other protective functions on the vascular wall, through their anti-inflammatory, antioxidant, antithrombotic and cytoprotective properties. HDL-targeted therapy is thus an innovative approach against cardiovascular risk and atherosclerosis. These pleiotropic atheroprotective properties of HDL have led experts to believe that “HDL-related therapies” represent the most promising next step in fighting against atherosclerosis. However, because of the heterogeneity of HDL functions, targeting HDL is not a simple task and HDL therapies that lower cardiovascular risk are NOT yet available. In this paper, an overview is presented about the therapeutic strategies currently under consideration to raise HDL levels and/or functions. Recently, clinical trials of drugs targeting HDL-C levels have disappointingly failed, suggesting that HDL functions through specific mechanisms should be targeted rather than increasing per se HDL levels.
Résumé
Le rôle athéroprotecteur des lipoprotéines de haute densité (HDL) est maintenant largement démontré par les études épidémiologiques. Cet effet bénéfique est principalement attribué à la fonction des HDL dans le transporteur retour du cholestérol, des cellules périphériques, notamment les macrophages de la paroi artérielle, vers le foie, où il est ensuite éliminé via les voies biliaires. En outre, a été démontré que les HDL possèdent de nombreux effets pléïotropes telles que des activités anti-inflammatoires, anti-oxydantes, anti-thrombotiques ou cyto-protectrices. Cibler spécifiquement les HDL pourrait constituer une approche innovante dans la lutte contre l’athérosclérose et le risque cardiovasculaire. Cependant, en raison de l’hétérogénéité de leurs fonctions, cibler les HDL n’est pas un objectif simple. Cet article vise à présenter les stratégies actuellement à l’étude, celles-ci ayant pour objectif, soit d’augmenter les concentrations circulantes de HDL-cholestérol (HDL-C), soit de moduler les fonctions de ces lipoprotéines. Des essais cliniques récents, portant sur des molécules augmentant les taux de HDL-C, n’ont pas permis de conclure de manière claire à une efficacité dans la réduction du risque cardiovasculaire. Cela semble donc suggérer que ce sont les fonctions des HDL qui devraient être ciblées plutôt que simplement l’élévation de leur concentration.
Background
Cardiovascular pathologies are now considered as the biggest scourge of our modern society; during 2008, nearly 30% of deaths worldwide were due to cardiovascular diseases (CVDs) . At the origin of most cardiovascular pathologies, atherosclerosis is a deleterious phenomenon responsible for coronary artery diseases, peripheral vascular diseases and strokes. This inflammatory process is characterized by alteration of the arterial wall, followed by lipid infiltration leading to thickening of the atheroma plaque and then, ultimately, to its rupture with the formation of a thrombus. Cholesterol deposit is crucial and two main lipoproteins are able to transport cholesterol in the plasma: while increased low-density lipoprotein cholesterol (LDL-C) concentration is among the main risk factors for CVDs, high-density lipoprotein cholesterol (HDL-C) concentration is inversely related to atherosclerosis severity. Therefore, these two variables are included in systematic routine measurements to screen dyslipidaemias in the general population, but also to set therapeutic objectives. Lowering LDL-C, known to be responsible for cholesterol deposition in the vessel wall, has always been one of the most attractive targets of lipid-lowering drugs. Indeed, statin therapy has a beneficial effect on the atherosclerotic process and a 12% reduction in overall mortality has been observed for each mmol/L decrease in LDL-C concentration (40 mg/dL) . Unfortunately, the residual risk remains important, which emphasizes the need to find new targets to achieve further benefits. Among all possibilities, raising HDL-C concentration has appeared as a most promising strategy. Indeed, epidemiological studies have shown that a 0.03 mmol/L (1 mg/dL) reduction in HDL-cholesterol concentration is associated with a 2–3% increase in cardiovascular risk . And, while a concentration < 1 mmol/L (40 mg/dL) is considered as an independent risk factor for CVD, it has recently been shown that 33% of men and 40% of women treated for dyslipidaemia in Europe display low concentrations of HDL-C . A low HDL-C concentration is therefore very common and strategies aiming at raising the plasma concentration could be promising. In the first part of this review, we will focus on the different protective effects of HDL before addressing treatments that are currently used or under development.
High-density lipoprotein protective mechanisms
Reverse cholesterol transport
Classically, the main HDL atheroprotective function is ‘reverse cholesterol transport’ (RCT), a process whereby excess cell cholesterol is taken up from peripheral (and vascular foam) cells and is delivered to the liver for further elimination into the bile. To have a better understanding of this process, it is important to be reminded of the structure of HDLs: these are nanoparticles composed of a lipid moiety (free and esterified cholesterol, triglycerides, phospholipids, lysosphingolipids) and a protein part, including the major apolipoprotein A-I (apoA-I) and various enzymes, each one being responsible for numerous beneficial effects. Briefly, after synthesis by the liver, apoA-I acquires phospholipids and free cholesterol through efflux of cellular cholesterol by active (adenosine triphosphate [ATP] binding cassette A1 and G1 [ABCA1, ABCG1]) and passive (scavenger receptor class B type I [SR-BI]) transporters, to form pre-β HDL. Then, several enzymes carried by HDL itself lead to the formation of mature HDL. Lecithin cholesterol acyl transferase (LCAT) esterifies free cholesterol, leading to migration of esterified cholesterol into the particle core, generating a continuous gradient of free cholesterol from cells towards HDL. After plasma remodelling, HDL-C is taken up by the liver through SR-BI and is eliminated into the bile as free cholesterol or as biliary acids after metabolism.
Another putative pathway involved in RCT has also been evoked. Demonstrated in mice, it is responsible for the uptake of the whole HDL particle and involves activation of the membrane ecto-F 1 -ATPase by apoA-I . The generated adenosine diphosphate further activates the purinergic receptor P2Y 13 , which in turn stimulates endocytosis of the entire HDL particle . Besides this general process, an alternate route exists in humans for the delivery of HDL-C back to the liver, involving cholesteryl ester transfer protein (CETP); this pathway is quantitatively important in normolipaemic conditions. CETP is combined with HDL lipoprotein and mediates transfer of esterified cholesterol towards very-low-density lipoprotein (VLDL) and LDL in exchange for triglycerides. This transfer protein is therefore responsible for a decrease in HDL-C concentration but also accounts for an enrichment of LDL particles in cholesterol, allowing cholesterol elimination by the liver through the LDL receptor. This indirect route that is quantitatively important in humans is absent in rodents, which questions the relevance of murine models for studies on lipoprotein metabolism.
The whole RCT process is therefore physiologically important as it allows removal of excess cholesterol from the artery wall and from atherosclerotic plaques.
Other atheroprotective effects of high-density lipoprotein
Beyond this main atheroprotective mechanism, HDL exerts pleiotropic functions that protect against atherosclerosis ( Fig. 1 ). Indeed, as detailed in different reviews , HDL can protect endothelium by different mechanisms: by stimulating endothelial cell nitrite oxide and prostacyclin production, HDLs promote better regulation of vascular structure and tone, and thus display antithrombotic and antiaggregating properties . Sphingosine-1-phosphate, a major lysosphingolipid associated with HDL particles, also promotes endothelial survival via activation of its specific receptor. HDLs are also able to decrease endothelial apoptosis induced by tumour necrosis factor-α and growth factor deprivation by several mechanisms and, particularly, through pathways triggered by SR-BI activation, involving proapoptotic factors Bcl-2-associated death promoter and Bcl-2-associated X protein or by apoA-I-mediated ecto-F 1 -ATPase activation . Furthermore, HDLs present potent antioxidative properties due to numerous enzymes carried by these lipoproteins, such as paraoxonase, platelet-activating factor-acetyl hydrolase, LCAT or glutathione selenoperoxidase, which degrade oxidized lipids and therefore prevent LDL oxidation, which is a key determinant of atherogenesis. Apolipoproteins (A-I, A-II, A-IV, E or J) also display antioxidative properties and also have an anti-inflammatory impact. Moreover, HDLs exhibit an anti-infectious role against bacteria and parasites. They protect against endotoxaemia by accelerating bile clearance of gram-negative bacteria due to their binding to membrane lipopolysaccharides, but they also show specific lytic activity against Trypanosoma brucei brucei , the sleeping sickness parasite . In addition, HDLs seem to be important in cellular immunity, through macrophage expression of inflammatory chemokines, such as monocyte chemoattractant protein-1. HDLs also promote humoral immunity by modulating activation of the complement system .
High-density lipoprotein under inflammatory conditions
HDLs are complex particles, which are continuously remodelled. Systemic inflammation associated with oxidative stress induces structural and compositional modifications. These abnormal HDLs are considered dysfunctional, with loss of their normal properties. Indeed, it has been shown that acute phase proteins, such as serum amyloid A, can displace apoA-I from HDL, causing a negative impact on cholesterol efflux capacity. Modification of HDL composition is also deleterious because of enrichment in triglycerides at the expense of cholesterol esters. Finally, a decrease in antioxidative properties is related to decreased activities of paraoxonase, platelet-activating factor-acetyl hydrolase and LCAT . Furthermore, phospholipase A2, either lipoprotein associated or secreted, has been involved in the inflammatory reaction occurring during atherogenesis. HDLs, due to their anti-inflammatory effects, might counteract actions of phospholipase A2. However, clinical trials using specific phospholipase A2 inhibitors did not show significant effects on HDL or LDL concentrations, as shown by Mohler et al. . Hence, in HDL particles, pro- and anti-inflammatory properties are in subtle equilibrium and some authors have proposed an ‘inflammatory index’ to quantify HDL properties. This approach assesses either LDL-induced monocyte chemotaxis or dichlorofluorescein oxidation , with and without HDL. A ratio between these two conditions allows separation into two groups: a ratio below 1 indicates that HDLs are anti-inflammatory and, conversely, a ratio greater than 1 indicates a proinflammatory profile for HDL. In 2003, a study reported that this index could be more useful than a single measurement of HDL-C for evaluating coronary artery patients: among 26 patients, 77% presented an inflammatory index above 1, while only 11% had a low HDL-C concentration . Unfortunately, these techniques have not been introduced in routine measurements so far, probably because of a lack of standardization.
Modification of HDL composition during acute or chronic inflammation leads to functional alterations; this emphasizes the need to evaluate HDL composition and functions, rather than simple measurement of HDL-C concentration. Therefore, therapies improving HDL functionality could be a more promising bet compared with therapies that only increase concentration.
Pharmacological therapies
Niacin (nicotinic acid)
Niacin, also known as nicotinic acid or vitamin B3, is a physiological precursor of nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate, two coenzymes involved in oxidoreductive reactions and energy metabolism. An inadequate nutritional intake leads to pellagra, an old disease characterized by dermatitis, dementia and diarrhoea. Vitaminic potential is demonstrated with milligram doses, but at a pharmacological dose of approximately 1.5–2 g per day, niacin is one of the most potent agents available for increasing HDL-C concentration. Niacin also reduces all proatherogenic lipids and lipoproteins, including total cholesterol, triglycerides, VLDL, LDL and lipoprotein(a). Different potential mechanisms underlying the antidyslipidaemic effects of niacin have been recently extensively reviewed and are summarized in Fig. 2 and Table 1 . Beyond its lipid-modifying activity, niacin has also been shown to exert other potential antiatherosclerotic effects, in part through mechanisms involving its receptor (hydroxycarboxylic acid receptor 2 [also called GPR109A]) on immune cells as well as through direct and indirect effects on the vascular endothelium . In accordance with these pleiotropic potentially beneficial actions of niacin in CVDs, its therapeutic use has been considered for decades in the prevention and treatment of atherosclerosis, but negative outcomes of recent clinical trials – discussed below – have led to questions about its efficacy.
