CETP and Atherosclerosis
Ever since cholesteryl ester transfer protein (CETP) was first identified, there have been conflicting opinions regarding its role (if any) in the development of atherosclerosis. According to 1 view, CETP is antiatherogenic by virtue of its ability to increase the rate of reverse cholesterol transport (RCT), the pathway in which cholesterol in peripheral tissues is transported to the liver for elimination in bile. This pathway involves an initial uptake of cell cholesterol by HDL, where it is esterified before being transferred by CETP to LDL and VLDL. The cholesteryl esters in the VLDL/LDL pool are subsequently delivered to the liver and ultimately eliminated from the body as a component of bile. Thus, to the extent that CETP enhances the rate of RCT, it may be an antiatherogenic factor. However, the fact that CETP redistributes cholesteryl esters from the nonatherogenic HDL to the potentially atherogenic VLDL/LDL implies that it may also be proatherogenic.
The situation is even further complicated by mounting evidence that concentration-dependent, anti-inflammatory functions of HDL may also contribute to its antiatherogenicity.1 Because CETP decreases the concentration of HDL,2 it may decrease the anti-inflammatory impact of this lipoprotein fraction in a process that is ultimately proatherogenic. Thus, on theoretical grounds, CETP may be either proatherogenic or antiatherogenic, depending on which of the HDL functions is dominant. As outlined below, the situation has not been clarified by more direct studies in vivo in animals in which evidence supporting both a proatherogenic3 4 5 and an antiatherogenic6 7 role of CETP has been reported.
What Is CETP and What Does It Do?
CETP is a hydrophobic glycoprotein that is secreted from the liver and that circulates in plasma, bound mainly to HDL.8 It promotes the redistribution of cholesteryl esters, triglyceride, and, to a lessor extent, phospholipids between plasma lipoproteins. CETP picks up lipids from lipoprotein particles and deposits them in other lipoproteins in a process that results in an equilibration of lipids between lipoprotein fractions.9 Because most of the cholesteryl esters in plasma originate in HDL in the reaction catalyzed by lecithin:cholesterol acyltransferase (LCAT) and most of the triglyceride enters plasma as a component of chylomicrons and VLDL (triglyceride-rich lipoproteins), the overall effect of CETP is a net mass transfer of cholesteryl esters from HDL to triglyceride-rich lipoproteins and LDL and of triglyceride from triglyceride-rich lipoproteins to LDL and HDL.
One consequence of these CETP-mediated transfers of cholesteryl esters from HDL is a reduction in the cholesterol content, the apoA-I content, and the size of HDL particles.10 This “remodeling” of HDL is accompanied by the dissociation from the HDL of lipid-poor, pre-β-migrating apoA-I,10 the fraction that has been reported to be the preferred plasma acceptor of cell cholesterol.
Is CETP Rate Limiting In Vivo?
Under normal conditions, the rate of CETP-mediated cholesteryl ester transfer is rapid relative to the rate of HDL and LDL catabolism.9 As a consequence, the pools of cholesteryl esters in HDL and LDL are close to equilibrium in vivo, with transfers between the 2 fractions being approximately equal in each direction.9 An increase in the activity of CETP may increase the rate of bidirectional transfers but will have little effect on the distribution of cholesteryl esters between the 2 lipoprotein fractions. Furthermore, a reduction in CETP activity will become rate limiting only when the rate of transfer becomes slow relative to the rate of LDL and HDL catabolism. Thus, under normal conditions, the amount of CETP is probably not rate limiting in the exchange of lipids between HDL and LDL unless its activity is markedly reduced. However, in the case of transfers between HDL and the much more rapidly catabolized VLDL, the amount of CETP in plasma is almost certainly rate limiting. Indeed, when the concentration of VLDL is increased, the amount of CETP is the limiting factor in the rate at which cholesteryl esters are transferred from HDL.11
In Vivo Evidence That CETP May Be Antiatherogenic
Human Studies: The Honolulu Heart Study
Analysis of subjects in the Honolulu Heart Study indicated that when a deficiency of CETP was associated with an HDL cholesterol level between 1.06 and 1.55 mmol/L, the risk of coronary heart disease was increased relative to that in normal-CETP subjects with the same level of HDL cholesterol.12 However, for subjects with levels of HDL cholesterol >1.55 mmol/L, the coronary risk was low, whether or not there was a deficiency of CETP. This finding provides strong circumstantial evidence that a deficiency of CETP can be proatherogenic (possibly because of a compromised RCT) if the associated increase in HDL is modest. If, however, the CETP deficiency is accompanied by a more substantial increase in HDL, the effects of the resulting increase in (for example) the anti-inflammatory potential of this lipoprotein fraction may override the proatherogenic consequences of the compromised RCT.
Expression of CETP in Hypertriglyceridemic Mice
Mice engineered to overexpress human apoC-III have high levels of triglyceride-rich lipoproteins remnants and develop atherosclerosis. Introduction and expression of the CETP gene into these animals reduce the extent of atherosclerosis.6 The development of atherosclerosis in apoC-III–transgenic mice indicates that the proatherogenic potential of these remnants exceeds the antiatherogenic potential of HDL. When CETP is expressed in these animals, the resulting increase in RCT may provide an antiatherogenic balance to the remnant particles and hence, reduce atherosclerosis.
Expression of CETP in LCAT-Transgenic Mice
Mice made transgenic with human LCAT have an increase in concentration of HDL but paradoxically, also an increased susceptibility to atherosclerosis.13 Expression of simian CETP in these animals eliminates the atherosclerosis.7 It has been argued that the HDLs that accumulate in the LCAT-transgenic animals are overly enriched in cholesteryl esters and, in the absence of CETP, become dysfunctional in terms of their ability to promote cell cholesterol efflux. A possible increase in anti-inflammatory potential resulting from the increased concentration of HDL may be insufficient to counterbalance the reduced cholesterol efflux. Introduction of CETP enables the excess cholesteryl esters to be transferred from the HDL, thereby restoring their efficiency as acceptors of cell cholesterol and inhibiting the development of atherosclerosis.
In Vivo Evidence That CETP May Be Proatherogenic
Expression of CETP in Atherosclerosis-Prone Mice
In studies of cholesterol-fed, atherosclerosis-prone mice, the introduction and overexpression of the simian CETP gene results in worse atherosclerosis than in nonexpressing controls3 . It was concluded that the enhancement of atherosclerosis by CETP was secondary to a redistribution of cholesterol from HDL to the VLDL/LDL fraction. In another study,4 the human CETP gene was introduced into apoE-knockout mice and LDL receptor–knockout mice, both of which develop spontaneous atherosclerosis. CETP expression in these animal models redistributed cholesterol from HDL to the VLDL/LDL pool and again increased the development of atherosclerosis.4 However, when the CETP was overexpressed in apoE-knockout mice that also overexpressed human apoA-I, the effect of CETP on the development of atherosclerosis was nonsignificant, despite being associated with a major reduction in the concentration of HDL cholesterol.4 Expression of CETP in the animals also expressing human apoA-I increased the rate of clearance of cholesteryl esters from HDL. On this basis, it was suggested that a CETP-mediated increase in the rate of RCT may have overcome the consequences of the reduced HDL concentration in these animals. These results are again consistent with CETP’s being proatherogenic by virtue of reducing the concentration of HDL and redistributing cholesteryl esters from HDL to the VLDL/LDL pool and antiatherogenic as a consequence of increasing the rate of RCT.
Inhibition of CETP in Rabbits
Rabbits have a naturally high level of CETP.14 In a recent study of cholesterol-fed rabbits, inhibition of CETP by injection of antisense oligodeoxynucleotides against CETP resulted in a reduction in CETP mRNA and mass, a reduction in plasma total cholesterol, and an increased concentration of HDL cholesterol.5 There was also an increase in LDL receptor mRNA associated with the antisense oligodeoxynucleotides. These changes were accompanied by a marked reduction in aortic cholesterol content. The results of this study are again consistent with the multiple effects of CETP, the sum of which (at least in the rabbit) is proatherogenic.
Another approach to inhibiting CETP in vivo has involved infusion of anti-CETP antibodies into rabbits.15 An effect of CETP on the distribution of cholesteryl esters between HDL and VLDL/LDL was confirmed, but the study did not address the effects on the development of atherosclerosis. In a report in the current issue of Arteriosclerosis, Thrombosis, and Vascular Biology,16 a vaccine approach has been used to generate antibodies against CETP in vivo in rabbits. The effect of this vaccination on the development of atherosclerosis in cholesterol-fed animals was assessed. The immunized animals had a reduced activity of CETP that was accompanied by a substantial increase in the concentration of HDL cholesterol, a modest decrease in LDL cholesterol concentration, and a significant reduction in aortic atherosclerotic lesions. This study demonstrates that long-term inhibition of CETP is not only possible but, in rabbits at least, reduces the susceptibility to atherosclerosis. Whether or not such long-term inhibition of CETP is also beneficial in humans is currently unknown.
On the basis of the animal studies reported to date, it must be concluded that CETP can be both proatherogenic and antiatherogenic, depending on the metabolic setting and perhaps on the species being investigated. The results of the Honolulu Heart Study suggest that the consequences of inhibiting CETP in vivo in human subjects may also depend on other genetic and environmental factors that influence the concentration of HDL. If the CETP deficiency is not associated with an increase in the HDL level, then the consequences appear to be proatherogenic. If, however, the CETP deficiency is accompanied by a substantial elevation in the level of HDL, the consequences appear to be antiatherogenic.
It is clearly premature to contemplate the long-term inhibition of CETP as a means for preventing coronary disease in human subjects. At the very least, we need short-term studies utilizing either anti-CETP vaccines or small-molecule inhibitors to understand how the acute inhibition of CETP affects the concentration, composition, metabolism, and function of HDL and other lipoproteins in humans in various dyslipidemic states. Armed with such information, it may become possible to explore the therapeutic potential of long-term CETP inhibition from a base that depends on more than guesswork.
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