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Pharmacological modulation of cholesteryl ester transfer protein, a new therapeutic target in atherogenic dyslipidemia

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Abstract

In mediating the transfer of cholesteryl esters (CE) from antiatherogenic high density lipoprotein (HDL) to proatherogenic apolipoprotein (apo)-B-containing lipoprotein particles (including very low density lipoprotein [VLDL], VLDL remnants, intermediate density lipoprotein [IDL], and low density lipoprotein [LDL]), the CE transfer protein (CETP) plays a critical role not only in the reverse cholesterol transport (RCT) pathway but also in the intravascular remodeling and recycling of HDL particles. Dyslipidemic states associated with premature atherosclerotic disease and high cardiovascular risk are characterized by a disequilibrium due to an excess of circulating concentrations of atherogenic lipoproteins relative to those of atheroprotective HDL, thereby favoring arterial cholesterol deposition and enhanced atherogenesis. In such states, CETP activity is elevated and contributes significantly to the cholesterol burden in atherogenic apoB-containing lipoproteins. In reducing the numbers of acceptor particles for HDL-derived CE, both statins (VLDL, VLDL remnants, IDL, and LDL) and fibrates (primarily VLDL and VLDL remnants) act to attenuate potentially proatherogenic CETP activity in dyslipidemic states; simultaneously, CE are preferentially retained in HDL and thereby contribute to elevation in HDL-cholesterol content.

Mutations in the CETP gene associated with CETP deficiency are characterized by high HDL-cholesterol levels (>60 mg/dL) and reduced cardiovascular risk. Such findings are consistent with studies of pharmacologically mediated inhibition of CETP in the rabbit, which argue strongly in favor of CETP inhibition as a valid therapeutic approach to delay atherogenesis. Consequently, new organic inhibitors of CETP are under development and present a potent tool for elevation of HDL in dyslipidemias involving low HDL levels and premature coronary artery disease, such as the dyslipidemia of type II diabetes and the metabolic syndrome. The results of clinical trials to evaluate the impact of CETP inhibition on premature atherosclerosis are eagerly awaited.

Introduction

Cholesterol homeostasis is maintained through a specific multistep pathway termed reverse cholesterol transport (RCT), whose action results in the net movement of cholesterol from peripheral tissues via the plasma compartment to the liver for excretion (Fig. 1). Among the key actors involved in this potentially antiatherogenic pathway is the cholesteryl ester transfer protein (CETP), which plays a fundamental role in mediating the transfer of CE from cardioprotective high density lipoprotein (HDL) particles to atherogenic apolipoprotein (apo)-B-containing lipoproteins (very low density lipoprotein [VLDL], VLDL remnants, intermediate density lipoprotein [IDL], and low density lipoprotein [LDL]). Indeed, the CETP-mediated transfer of CE to these particles, which may subsequently be taken up by both receptor-dependent and receptor-independent pathways in the liver, may represent up to 70% of total cholesterol flux to the liver in some animal species, such as the rabbit (Goldberg et al., 1991).

The action of CETP is however equally implicated in the intravascular remodeling of lipoproteins, which favors HDL particle recycling and thus the formation of preβ-HDL, thereby leading to enhanced cellular free cholesterol (FC) efflux and plasma lecithin:cholesterol acyltransferase (LCAT) activity (Fig. 1). Such actions are synonymous with an antiatherogenic role for CETP. Nevertheless, by depleting plasma HDL-cholesterol levels and by increasing CE content in all apoB-containing lipoproteins in both fasting and postprandial states, CETP can potentially contribute to the establishment of an atherogenic lipoprotein profile. Indeed, when circulating concentrations of apoB-containing particles are elevated, as occurs typically in the major forms of hyperlipidemia associated with premature atherosclerosis (hypercholesterolemia, mixed hyperlipidemia, and hypertriglyceridemia), then both CETP activity and mass are typically elevated significantly (up to 3-fold) as compared with the normolipidemic state McPherson et al., 1991b, Guérin et al., 1994a, Guérin et al., 1996a. Paradoxically then, CETP possesses the potential to simultaneously exert both proatherogenic and antiatherogenic effects, the balance between them clearly depending primarily on the metabolic context. Plasma CETP activity and mass should clearly be sufficient to facilitate generation of optimal amounts of small HDL particles to ensure the initial step of RCT (Fig. 1) but should not exceed such an optimal level so as to limit the overall cholesterol load in atherogenic apoB-containing lipoproteins.

From the foregoing discussion, it is evident that pharmacological inhibition of CETP in hyperlipidemic subjects at high cardiovascular risk can be envisaged with the goal of attenuating CETP-mediated CE enrichment of apoB-containing lipoproteins. In this way, HDL-cholesterol levels may be raised at the expense of reduction in the CE content of atherogenic VLDL, IDL, and LDL. The goal of this review is therefore to discuss the potential clinical relevance of CETP inhibition in hyperlipidemic subjects at elevated cardiovascular risk and more specifically (1) the action of CETP in both normolipidemic and hyperlipidemic states and its potential role in the establishment of a proatherogenic or antiatherogenic lipid phenotype, (2) the effect of CETP deficiency on lipid metabolism and phenotype and its relationship to cardiovascular risk, (3) the impact of lipid-lowering drugs currently used in the treatment of atherogenic hyperlipidemias on CETP-mediated CET, (4) the effects of pharmacological inhibition of CETP activity on lipid metabolism and atherosclerosis in animal models and human subjects, and (5) the potential approaches to the attenuation of CETP gene expression.

Section snippets

Physiological function of cholesteryl ester transfer protein

The analysis of the CETP primary sequence (476 amino acids) indicates that the mature protein is composed of 45% of nonpolar residues, suggesting it to be highly hydrophobic in nature Drayna et al., 1987, Hesler et al., 1987. However, the fact that CETP is readily soluble in water implies that such hydrophobic residues are mainly inaccessible to the aqueous phase; indeed, they form a hydrophobic pocket that permits the binding of neutral lipids (Hesler et al., 1987). Study of structure-function

Role of cholesteryl ester transfer protein in the intravascular remodeling of lipoproteins

In vitro studies indicate that CETP influences LDL particle size by favoring a shift in LDL distribution toward a profile composed mainly of CE-rich, buoyant LDL particles of large size Gambert et al., 1990, Lagrost et al., 1993. Conversely, inhibition of CETP in the hamster is associated with a shift in LDL profile toward dense particles of small size (Evans et al., 1994). Indeed, the presence of heterogeneous small dense LDL is a key feature in patients presenting CETP deficiency Yamashita et

Primary dyslipidemias

CETP activity is sensitive to variation in the relative concentrations and composition of acceptor and donor lipoproteins that determine the specificity of neutral lipid transfer. Lipid abnormalities characteristic of dyslipoproteinemic states are associated with marked changes not only in the metabolism but also equally in the quantitative and qualitative distribution of plasma lipoproteins. In addition, postprandial lipemia induces major modifications in plasma lipid and lipoprotein profile

Human cholesteryl ester transfer protein deficiency

CETP deficiency is the most frequent cause of hyperalphalipoproteinemia in Asian populations, and mutations of the CETP gene, which underlie CETP deficiency, have been mainly described in Japanese patients. Nevertheless, some cases have been more recently reported in Chinese and Caucasian populations (Table 1). It has been estimated that CETP deficiency is responsible for some 61.7% of severe hyperalphalipoproteinemia and for 31.4% of moderate hyperalphalipoproteinemia in the Japanese

Potential atherogenicity of human cholesteryl ester transfer protein deficiency

Elevated levels of plasma HDL in CETP-deficient subjects strongly suggest that absence of CETP activity may exert a protective effect against atherosclerosis (Inazu et al., 1990). Nonetheless, both homozygous and heterozygous CETP-deficient individuals appear to be susceptible to cardiovascular disease Hirano et al., 1995, Hirano et al., 1997. The relationship between CETP deficiencies and atherosclerosis therefore remains controversial, possibly as a result of the small number of homozygous

Inhibition of cholesteryl ester transfer protein

Studies of the inhibition of CETP activity have been mainly conducted in the rabbit, an animal species that displays high CETP activity and is highly susceptible to atherosclerosis Ha & Barter, 1985, Speijer et al., 1991. Several approaches to CETP inhibition have been conducted in this model, including use of monoclonal antibodies Abbey & Calvert, 1989, Whitlock et al., 1989, antisense oligodeoxynucleotides (ODN; Sugano & Makino, 1996, Sugano et al., 1998), vaccination (Rittershaus et al.,

Pharmacological modulation of cholesteryl ester transfer protein activity in humans

HMG-CoA reductase inhibitors act primarily by inhibiting the cellular biosynthesis of cholesterol in the liver (Fig. 2A). The reduction of intracellular cholesterol content is associated with an increased expression of hepatic LDL receptors, which in turn leads to marked reduction in plasma LDL-cholesterol levels. Recent studies have shown that statin therapy markedly decreases the plasma concentration and proportion not only of small dense LDL particles but also of all LDL subfractions, with a

Regulation of cholesteryl ester transfer protein gene expression

The human CETP gene contains 16 exons and 15 introns and is located on chromosome 16 (16q12–16q21; Lusis et al., 1987, Agellon et al., 1990). The cDNA of the human CETP gene was originally sequenced by Drayna et al. (1987) and several mutations have been described (Table 1). These mutations are common in the Japanese population (see above) and are associated with reduction or lack of CETP activity in plasma. In addition, numerous polymorphisms have been identified in the human CETP gene,

Conclusion

The impact of CETP-mediated lipid transfer reactions on atherogenesis remains controversial, probably reflecting the complexity of CETP activity in vivo. Animal studies have confirmed the dual—and indeed chameleon-like—action of this protein. On the one hand, expression of CETP in animals lacking CETP can exert marked atherogenic activity; on the other hand, inhibition of CETP can reduce the progression of atherosclerosis in proatherogenic metabolic contexts (for review, see Barter et al., 2003

Acknowledgements

Dr. W. Le Goff was a recipient of a research fellowship from the French Ministry of Research and Technology. Studies undertaken at INSERM Unit 551 (W.L.G., M.G., and M.J.C.) were supported by INSERM, University Pierre and Marie Curie (Paris VI), ARLA, Fondation pour la Recherche Médicale, Bristol-Myers Squibb, Laboratoires Fournier, Pfizer, and Sanofi-Synthelabo. It is a pleasure to acknowledge stimulating discussions of CETP inhibition with Dr. C.L. Shear.

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