Mechanism of inhibition defines CETP activity: a mathematical model for CETP in vitro

Because cholesteryl ester transfer protein (CETP) inhibition is a potential HDL-raising therapy, interest has been raised in the mechanisms and consequences of CETP activity. To explore these mechanisms and the dynamics of CETP in vitro, a mechanistic mathematical model was developed based upon the shuttle mechanism for lipid transfer. Model parameters were estimated from eight published experimental datasets, and the resulting model captures observed dynamics of CETP in vitro. Simulations suggest the shuttle mechanism yields behaviors consistent with experimental observations. Three key findings predicted from model simulations are: 1) net CE transfer activity from HDL to VLDL and LDL can be significantly altered by changing the balance of homoexchange versus heteroexchange of neutral lipids via CETP; 2) lipemia-induced increases in CETP activity are more likely caused by increases in lipoprotein particle size than particle number; and 3) the inhibition mechanisms of the CETP inhibitors torcetrapib and JTT-705 are significantly more potent than a classic competitive inhibition mechanism with the irreversible binding mechanism having the most robust response. In summary, the model provides a plausible representation of CETP activity in vitro, corroborates strong evidence for the shuttle hypothesis, and provides new insights into the consequences of CETP activity and inhibition on lipoproteins.     

Evidence for a role of CETP in HDL remodeling and cholesterol efflux: Role of cysteine 13 of CETP

Cholesteryl ester transfer protein (CETP), a key regulator of high-density lipoprotein (HDL) metabolism, induces HDL remodeling by transferring lipids between apolipoprotein B-containing lipoproteins and HDL, and/or by promoting lipid transfer between HDL subparticles. In this study, we investigated the mechanism as to how CETP induces the generation of lipid-poor particles (pre-β-HDL) from HDL, which increases ATP-binding cassette transporter 1-mediated cholesterol efflux. This CETP-dependent HDL remodeling is enhanced by the CETP modulator dalcetrapib both in plasma and isolated HDL. The interaction of dalcetrapib with cysteine 13 of CETP is required, since this effect was abolished when using mutant CETP in which cysteine 13 was substituted for a serine residue. Other thiol-containing compounds were identified as CETP modulators interacting with cysteine 13 of CETP. In order to mimic dalcetrapib-bound CETP, mutant CETP proteins were prepared by replacing cysteine 13 with the bulky amino acid tyrosine or tryptophan. The resultant mutants showed virtually no CETP-dependent lipid transfer activity but demonstrated preserved CETP-dependent pre-β-HDL generation. Overall, these data demonstrate that the two functions of CETP i.e., cholesteryl ester transfer and HDL remodeling can be uncoupled by interaction of thiol-containing compounds with cysteine 13 of CETP or by introducing large amino acid residues in place of cysteine 13.     

Association of cholesteryl ester transfer protein with HDL particles reduces its proteolytic inactivation by mast cell chymase

Human atherosclerotic intima contains mast cells that secrete the neutral protease chymase into the intimal fluid, which also contains HDL-modifying proteins, such as cholesteryl ester transfer protein (CETP), in addition to abundant amounts of nascent discoidal HDL particles. Here, we studied chymase-dependent degradation of a) CETP isolated from human plasma and b) CETP-HDL complexes as well as the functional consequences of such degradations. Incubation with chymase caused a rapid cleavage of CETP, yielding a specific proteolytic pattern with a concomitant reduction in its cholesteryl ester transfer activity. These chymase-dependent effects were attenuated after CETP was complexed with HDL. This attenuation was more effective when CETP was complexed with HDL(3) and HDL(2) than with discoidal reconstituted high density lipoprotein (rHDL). Conversely, rHDL, but not spherical HDLs, was protected in such CETP complexes against functional inactivation by chymase. Thus, in contrast to the complexes of CETP with spherical HDLs, the ability of the CETP-rHDL complexes to promote cholesterol efflux from macrophage foam cells remained unchanged, despite treatment with chymase. In summary, complexation of CETP and HDL modifies their resistance to proteolytic inactivation: spherical HDLs protect CETP, and CETP protects discoidal HDL. These results suggest that in inflamed atherosclerotic intima, CETP, via its complexation with HDL, has a novel protective role in early steps of reverse cholesterol transport.     

Modification of CETP function by changing its substrate preference: a new paradigm for CETP drug design

We previously determined that hamster cholesteryl ester transfer protein (CETP), unlike human CETP, promotes a novel one-way transfer of TG from VLDL to HDL, causing HDL to gain lipid. We hypothesize that this nonreciprocal lipid transfer activity arises from the usually high TG/cholesteryl ester (CE) substrate preference of hamster CETP. Consistent with this, we report here that ∼25% of the total lipid transfer promoted by the human Q199A CETP mutant, which prefers TG as substrate, is nonreciprocal transfer. Other human CETP mutants with TG/CE substrate preferences higher or lower than wild-type also possess nonreciprocal lipid transfer activity. Mutants with high TG/CE substrate preference promote the nonreciprocal lipid transfer of TG from VLDL to HDL, but mutants with low TG/CE substrate preference promote the nonreciprocal lipid transfer of CE, not TG, and this lipid flow is in the reverse direction (from HDL to VLDL). Anti-CETP TP2 antibody alters the TG/CE substrate preference of CETP and also changes the extent of nonreciprocal lipid transfer, showing the potential for externally acting agents to modify the transfer properties of CETP. Overall, these data show that the lipid transfer properties of CETP can be manipulated. Function-altering pharmaceuticals may offer a novel approach to modify CETP activity and achieve specific modifications in lipoprotein metabolism.     

Human apoA-I expression in CETP transgenic rats leads to lower levels of apoC-I in HDL and to magnification of CETP-mediated lipoprotein changes

Plasma cholesteryl ester transfer protein (CETP) has a profound effect on neutral lipid transfers between HDLs and apolipoprotein B (apoB)-containing lipoproteins when it is expressed in combination with human apoA-I in HuAI/CETP transgenic (Tg) rodents. In the present study, human apoA-I-mediated lipoprotein changes in HuAI/CETPTg rats are characterized by 3- to 5-fold increments in the apoB-containing lipoprotein-to-HDL cholesterol ratio, and in the cholesteryl ester-to-triglyceride ratio in apoB-containing lipoproteins. These changes occur despite no change in plasma CETP concentration in HuAI/CETPTg rats, as compared with CETPTg rats. A number of HDL apolipoproteins, including rat apoA-I and rat apoC-I are removed from the HDL surface as a result of human apoA-I overexpression. Rat apoC-I, which is known to constitute a potent inhibitor of CETP, accounts for approximately two-thirds of CETP inhibitory activity in HDL from wild-type rats, and the remainder is carried by other HDL-bound apolipoprotein inhibitors. It is concluded that human apoA-I overexpression modifies HDL particles in a way that suppresses their ability to inhibit CETP. An apoC-I decrease in HDL of HuAI/CETPTg rats contributes chiefly to the loss of the CETP-inhibitory potential that is normally associated with wild-type HDL.     

Remodeling of HDL by CETP in vivo and by CETP and hepatic lipase in vitro results in enhanced uptake of HDL CE by cells expressing scavenger receptor B-I.

The transport of HDL cholesteryl esters (CE) from plasma to the liver involves a direct uptake pathway, mediated by hepatic scavenger receptor B-I (SR-BI), and an indirect pathway, involving the exchange of HDL CE for triglycerides (TG) of TG-rich lipoproteins by cholesteryl ester transfer protein (CETP). We carried out HDL CE turnover studies in mice expressing human CETP and/or human lecithin:cholesterol acyltransferase (LCAT) transgenes on a background of human apoA-I expression. The fractional clearance of HDL CE by the liver was delayed by LCAT transgene, while the CETP transgene increased it. However, there was no incremental transfer of HDL CE radioactivity to the TG-rich lipoprotein fraction in mice expressing CETP, suggesting increased direct removal of HDL CE in the liver. To evaluate the possibility that this might be mediated by SR-BI, HDL isolated from plasma of the different groups of transgenic mice was incubated with SR-BI transfected or control CHO cells. HDL isolated from mice expressing CETP showed a 2- to 4-fold increase in SR-BI-mediated HDL CE uptake, compared to HDL from mice lacking CETP. The addition of pure CETP to HDL in cell culture did not lead to increased selective uptake of HDL CE by cells. However, when human HDL was enriched with TG by incubation with TG-rich lipoproteins in the presence of CETP, then treated with hepatic lipase, there was a significant enhancement of HDL CE uptake. Thus, the remodeling of human HDL by CETP, involving CE;-TG interchange, followed by the action of hepatic lipase (HL), leads to the enhanced uptake of HDL CE by cellular SR-BI.These observations suggest that in animals such as humans in which both the selective uptake and CETP pathways are active, the two pathways could operate in a synergistic fashion to enhance reverse cholesterol transport.     

Evacetrapib is a novel, potent, and selective inhibitor of cholesteryl ester transfer protein that elevates HDL cholesterol without inducing aldosterone or increasing blood pressure

Cholesteryl ester transfer protein (CETP) catalyses the exchange of cholesteryl ester and triglyceride between HDL and apoB containing lipoprotein particles. The role of CETP in modulating plasma HDL cholesterol levels in humans is well established and there have been significant efforts to develop CETP inhibitors to increase HDL cholesterol for the treatment of coronary artery disease. These efforts, however, have been hampered by the fact that most CETP inhibitors either have low potency or have undesirable side effects. In this study, we describe a novel benzazepine compound evacetrapib (LY2484595), which is a potent and selective inhibitor of CETP both in vitro and in vivo. Evacetrapib inhibited human recombinant CETP protein (5.5 nM IC(50)) and CETP activity in human plasma (36 nM IC(50)) in vitro. In double transgenic mice expressing human CETP and apoAI, evacetrapib exhibited an ex vivo CETP inhibition ED(50) of less than 5 mg/kg at 8 h post oral dose and significantly elevated HDL cholesterol. Importantly, no blood pressure elevation was observed in rats dosed with evacetrapib at high exposure multiples compared with the positive control, torcetrapib. In addition, in a human adrenal cortical carcinoma cell line (H295R cells), evacetrapib did not induce aldosterone or cortisol biosynthesis whereas torcetrapib dramatically induced aldosterone and cortisol biosynthesis. Our data indicate that evacetrapib is a potent and selective CETP inhibitor without torcetrapib-like off-target liabilities. Evacetrapib is currently in phase II clinical development.     

Adipose tissue-specific CETP expression in mice: impact on plasma lipoprotein metabolism

Adipose tissue appears to be a highly conserved site of cholesteryl ester transfer protein (CETP) expression across species. To investigate the impact of adipose CETP expression on lipid metabolism, we created adipose tissue-specific CETP transgenic (CETPTg) mice. CETP mRNA is predominantly expressed in adipose tissue. Plasma CETP mass and activity are readily detectable in CETPTg mice but not in controls. Plasma lipoprotein analysis shows marked reductions in HDL cholesterol and phospholipids, increases non-HDL lipids, decreases apolipoprotein A-I (apoA-I), and increases apoB. Unexpectedly, CETPTg adipocytes are significantly smaller than those in control mice (44%), triglyceride and cholesterol in adipose tissue were significantly decreased compared with controls (50% and 37%, respectively), and phospholipids showed no significant changes. To study the mechanism, we measured peroxisome proliferator-activated receptor gamma, sterol-regulatory element binding protein-1c, LPL, and hormone-sensitive lipase (HSL) in aP2-CETPTg adipose tissue and controls and found that, except for HSL, all mRNA levels are significantly decreased in the transgenic mice compared with controls (26, 33, and 22%). In conclusion, adipose tissue CETP makes a major contribution to CETP in the circulation, reduces HDL, and increases non-HDL cholesterol levels. Moreover, adipose tissue CETP expression changes triglyceride and cholesterol content and the size of adipocytes.     

New molecular insights into CETP structure and function: a review

Cholesteryl ester transfer protein (CETP) is important clinically and is the current target for new drug development. Its structure and mechanism of action has not been well understood. We have combined current new structural and functional methods to compare with relevant prior data. These analyses have led us to propose several steps in CETP's function at the molecular level, in the context of its interactions with lipoproteins, e.g., sensing, penetration, docking, selectivity, ternary complex formation, lipid transfer, and HDL dissociation. These new molecular insights improve our understanding of CETP's mechanisms of action.     

eNOS Activation by HDL Is Impaired in Genetic CETP Deficiency

Mutations in the CETP gene resulting in defective CETP activity have been shown to cause remarkable elevations of plasma HDL-C levels, with the accumulation in plasma of large, buoyant HDL particles enriched in apolipoprotein E. Genetic CETP deficiency thus represents a unique tool to evaluate how structural alterations of HDL impact on HDL atheroprotective functions. Aim of the present study was to assess the ability of HDL obtained from CETP-deficient subjects to protect endothelial cells from the development of endothelial dysfunction. HDL isolated from one homozygous and seven heterozygous carriers of CETP null mutations were evaluated for their ability to down-regulate cytokine-induced cell adhesion molecule expression and to promote NO production in cultured endothelial cells. When compared at the same protein concentration, HDL and HDL3 from carriers proved to be as effective as control HDL and HDL3 in down-regulating cytokine-induced VCAM-1, while carrier HDL2 were more effective than control HDL2 in inhibiting VCAM-1 expression. On the other hand, HDL and HDL fractions from carriers of CETP deficiency were significantly less effective than control HDL and HDL fractions in stimulating NO production, due to a reduced eNOS activating capacity, likely because of a reduced S1P content. In conclusion, the present findings support the notion that genetic CETP deficiency, by affecting HDL particle structure, impacts on HDL vasculoprotective functions. Understanding of these effects might be important for predicting the outcomes of pharmacological CETP inhibition.     

Association Between Lipids, Lipoproteins Composition of HDL Particles and Triglyceride-Rich Lipoproteins, and LCAT and CETP Activity in Post-renal Transplant Patients

High-density lipoprotein (HDL) remodeling within the plasma compartment and the association between lecithin-cholesterol acyltransferase (LCAT) and cholesterol ester transfer protein (CETP) activity, and lipid, lipoprotein concentrations and composition were investigated. The aim was to examine the high sensitivity of C-reactive protein (hsCRP), lipid, apolipoprotein B (apoB), apoAI, total apoAII, apoAIInonB, apoB-containing apoAII (apoB:AII), total apoCIII, apoCIIInonB, apoB-containing apoCIII (apoB:CIII) concentration and LCAT and CETP activity to gain an insight into the association between them and LCAT and CETP, 57 post-renal transplant (Tx) patients with and without statin therapy and in 15 healthy subjects. Tx patients had moderate hypertriglyceridemia, hypercholesterolemia, and dyslipoproteinemia, disturbed triglyceride-rich lipoproteins (TRLs) and HDL composition, decreased LCAT, and slightly increased hsCRP but no CETP activity. Spearman’s correlation test showed the association between lipids and lipoproteins and LCAT or CETP, and multiple ridge stepwise forward regression showed that immunosuppressive therapy in Tx patients can disturb HDL and TRLs composition. The results suggest that inhibition or activation of LCAT is due, in part, to HDL-associated lipoprotein. Lipoprotein composition of apoAI, apoAIInonB, and apoCIIInonB in HDL particle and apoB:AII TRLs can contribute to decrease LCAT mass in Tx patients. Tx patients without statin and with lower triglycerides but higher HDL cholesterol concentration and disturbed lipoprotein composition of ApoAI and apoAII in HDL particle can decrease LCAT, increase LDL cholesterol, aggravate renal graft, and accelerate atherosclerosis and chronic heart diseases.     

Critical role of cholesterol ester transfer protein in nicotinic acid-mediated HDL elevation in mice

Nicotinic acid is a commonly used anti-dyslipidemic agent that increases plasma levels of HDL-cholesterol and decrease triglycerides (TG), and VLDL- and LDL-cholesterol. The most well-studied effect of nicotinic acid is its ability to lower plasma free fatty acids, which has been observed in humans and many animal models. However, its ability to raise HDL in humans has not been replicated in animal models, which precludes studying the mechanism of HDL elevation. Here we studied lipid-modulating effects of nicotinic acid in mice carrying genomic DNA fragments that drive expression of various human genes in the mouse liver. Treatment with nicotinic acid reduced serum levels of HDL cholesterol in wild-type and human apolipoprotein B100 (apoB100)-transgenic mice. In contrast, nicotinic acid treatment of mice that express human cholesteryl ester transfer protein (CETP), with or without concomitant apoB100 expression, resulted in a significant increase of HDL cholesterol and reduction of TG, VLDL- and LDL-cholesterol. These data demonstrate a critical role of CETP in nicotinic acid-mediated HDL elevation, and suggest that mice carrying the human CETP gene may be useful animal models for studying the HDL-elevating effect of nicotinic acid.     

Cholesteryl Ester Transfer Protein (CETP) Deficiency and CETP Inhibitors

Epidemiologic studies have shown that low-density lipoprotein cholesterol (LDL-C) is a strong risk factor, whilst high-density lipoprotein cholesterol (HDL-C) reduces the risk of coronary heart disease (CHD). Therefore, strategies to manage dyslipidemia in an effort to prevent or treat CHD have primarily attempted at decreasing LDL-C and raising HDL-C levels. Cholesteryl ester transfer protein (CETP) mediates the exchange of cholesteryl ester for triglycerides between HDL and VLDL and LDL. We have published the first report indicating that a group of Japanese patients who were lacking CETP had extremely high HDL-C levels, low LDL-C levels and a low incidence of CHD. Animal studies, as well as clinical and epidemiologic evidences, have suggested that inhibition of CETP provides an effective strategy to raise HDL-C and reduce LDL-C levels. Four CETP inhibitors have substantially increased HDL-C levels in dyslipidemic patients. This review will discuss the current status and future prospects of CETP inhibitors in the treatment of CHD. At present anacetrapib by Merck and evacetrapib by Eli Lilly are under development. By 100mg of anacetrapib HDL-C increased by 138%, and LDL-C decreased by 40%. Evacetrapib 500 mg also showed dramatic 132% increase of HDL-C, while LDL-C decreased by 40%. If larger, long-term, randomized, clinical end point trials could corroborate other findings in reducing atherosclerosis, CETP inhibitors could have a significant impact in the management of dyslipidemic CHD patients. Inhibition of CETP synthesis by antisense oligonucleotide or small molecules will produce more similar conditions to human CETP deficiency and may be effective in reducing atherosclerosis and cardiovascular events. We are expecting the final data of prospective clinical trials by CETP inhibitors in 2015.     

Antisense oligonucleotide inhibition of cholesteryl ester transfer protein enhances RCT in hyperlipidemic,CETP transgenic,LDLr-/- mice

Due to their ability to promote positive effects across all of the lipoprotein classes, cholesteryl ester transfer protein (CETP) inhibitors are currently being developed as therapeutic agents for cardiovascular disease. In these studies, we compared an antisense oligonucleotide (ASO) inhibitor of CETP to the CETP small molecule inhibitor anacetrapib. In hyperlipidemic CETP transgenic (tg) mice, both drugs provided comparable reductions in total plasma cholesterol, decreases in CETP activity, and increases in HDL cholesterol. However, only mice treated with the antisense inhibitor showed an enhanced effect on macrophage reverse cholesterol transport, presumably due to differences in HDL apolipoprotein composition and decreases in plasma triglyceride. Additionally, the ASO-mediated reductions in CETP mRNA were associated with less accumulation of aortic cholesterol. These preliminary findings suggest that CETP ASOs may represent an alternative means to inhibit that target and to support their continued development as a treatment for cardiovascular disease in man.     

Leukocyte-derived hepatic lipase increases HDL and decreases en face aortic atherosclerosis in LDLr-/- mice expressing CETP

In addition to hepatic expression, cholesteryl ester transfer protein (CETP) and hepatic lipase (HL) are expressed by human macrophages. The combined actions of these proteins have profound effects on HDL structure and function. It is not known how these HDL changes influence atherosclerosis. To elucidate the role of leukocyte-derived HL on atherosclerosis in a background of CETP expression, we studied low density lipoprotein receptor-deficient mice expressing human CETP (CETPtgLDLr(-/-)) with a leukocyte-derived HL deficiency (HL(-/-) BM). HL(-/-) bone marrow (BM), CETPtgLDLr(-/-) mice were generated via bone marrow transplantation. Wild-type bone marrow was transplanted into CETPtgLDLr(-/-) mice to generate HL(+/+) BM, CETPtgLDLr(-/-) controls. The chimeras were fed a high-fat, high-cholesterol diet for 14 weeks to promote atherosclerosis. In female HL(-/-) BM, CETPtgLDLr(-/-) mice plasma HDL-cholesterol concentration during high-fat feeding was decreased 27% when compared with HL(+/+) BM, CETPtgLDLr(-/-) mice (P < 0.05), and this was associated with a 96% increase in en face aortic atherosclerosis (P < 0.05). In male CETPtgLDLr(-/-) mice, leukocyte-derived HL deficiency was associated with a 16% decrease in plasma HDL-cholesterol concentration and a 25% increase in aortic atherosclerosis. Thus, leukocyte-derived HL in CETPtgLDLr(-/-) mice has an atheroprotective role that may involve increased HDL levels.     

Structural basis of the lipid transfer mechanism of phospholipid transfer protein (PLTP)

Human phospholipid transfer protein (PLTP) mediates the transfer of phospholipids among atheroprotective high-density lipoproteins (HDL) and atherogenic low-density lipoproteins (LDL) by an unknown mechanism. Delineating this mechanism would represent the first step towards understanding PLTP-mediated lipid transfers, which may be important for treating lipoprotein abnormalities and cardiovascular disease. Here, using various electron microscopy techniques, PLTP is revealed to have a banana-shaped structure similar to cholesteryl ester transfer protein (CETP). We provide evidence that PLTP penetrates into the HDL and LDL surfaces, respectively, and then forms a ternary complex with HDL and LDL. Insights into the interaction of PLTP with lipoproteins at the molecular level provide a basis to understand the PLTP-dependent lipid transfer mechanisms for dyslipidemia treatment.     

CETP does not affect triglyceride production or clearance in APOE*3-Leiden mice

The cholesteryl ester transfer protein (CETP) facilitates the bidirectional transfer of cholesteryl esters and triglycerides (TG) between HDL and (V)LDL. By shifting cholesterol in plasma from HDL to (V)LDL in exchange for VLDL-TG, CETP aggravates atherosclerosis in hyperlipidemic APOE*3-Leiden (E3L) mice. The aim of this study was to investigate the role of CETP in TG metabolism and high-fat diet-induced obesity by using E3L mice with and without the expression of the human CETP gene. On chow, plasma lipid levels were comparable between both male and female E3L and E3L.CETP mice. Further mechanistic studies were performed using male mice. CETP expression increased the level of TG in HDL. CETP did not affect the postprandial plasma TG response or the hepatic VLDL-TG and VLDL-apolipoprotein B production rate. Moreover, CETP did not affect the plasma TG clearance rate or organ-specific TG uptake after infusion of VLDL-like emulsion particles. In line with the absence of an effect of CETP on tissue-specific TG uptake, CETP also did not affect weight gain in response to a high-fat diet. In conclusion, the CETP-induced increase of TG in the HDL fraction of E3L mice is not associated with changes in the production of TG or with tissue-specific clearance of TG from the plasma.     

Transfer of cholesteryl ester into high density lipoprotein by cholesteryl ester transfer protein: effect of HDL lipid and apoprotein content

Recombinant high density lipoprotein (rHDL) particles were prepared by cosonication of purified lipids and human apoproteins and incubated with partly purified cholesteryl ester transfer protein (CETP) and low density lipoprotein (LDL) containing [3H]cholesteryl ester. Increasing the triglyceride content relative to cholesteryl ester in rHDL significantly decreased the ability of the particles to accept cholesteryl esters transferred by CETP. Kinetic analysis of the data was performed to numerically define the maximum velocity of lipid transfer, Tmax, and the HDL concentration required for half maximal velocity, KH. Increases in rHDL-triglyceride content were shown to result in a significant reduction in the Tmax without a major change in KH. When the free cholesterol content was increased relative to phospholipid, the ability of the particles to accept cholesteryl esters was also decreased in a similar manner. Conversely, rHDL prepared from purified apoprotein A-I, A-II, or mixtures of both, had significantly elevated Tmax and KH values for their interaction with CETP. The results suggest that increases in triglyceride or free cholesterol content of an rHDL particle decrease the catalytic ability of CETP by noncompetitive inhibition. In addition, some component(s) of HDL apoproteins, other than A-I or A-II, were shown to uncompetitively inhibit the activity of CETP, by modifying both Tmax and the KH for the reaction. This study has shown that altered HDL composition may have marked effects on the transfer and equilibration of cholesteryl esters within the HDL pool.     

Cholesteryl ester transfer protein (CETP) expression enhances HDL cholesteryl ester liver delivery, which is independent of scavenger receptor BI, LDL receptor related protein and possibly LDL receptor

Cholesteryl ester transfer protein (CETP) is a hydrophobic plasma glycoprotein that mediates the transfer and exchange of cholesteryl ester (CE) and triglyceride (TG) between plasma lipoproteins, and also plays an important role in HDL metabolism. Previous studies have indicated that, compared to wild type mice, human CETP transgenic mice had significantly lower plasma HDL CE levels, which was associated with enhancement of HDL CE uptake by the liver. However, the mechanism of this process is still unknown. To evaluate the possibility that this might be directly mediated by CETP, we utilized CETP transgenic (CETPTg) mice with liver scavenger receptor BI (SR-BI) deficiency [i.e., PDZK1 gene knockout (PDZK1O)], and with receptor associated protein (RAP) overexpression, to block LDL receptor-related protein (LRP) and LDL receptor (LDLR). We found that (1) CETPTg/PDZK1O mice have significantly lower HDL-C than that of PDZK1 KO mice (36%, p<0.01); (2) CETPTg and CETPTg/PDZK1O mice have same HDL-C levels; (3) CETPTg/PDZK1O/RAP mice had significant lower plasma HDL-C levels than that of PDZK1O/RAP ones (50%, p<0.001); (4) there is no incremental transfer of HDL CE radioactivity to the apoB-containing lipoprotein fraction in mice expressing CETP; and (5) CETPTg/PDZK1O/RAP mice had significant higher plasma and liver [(3)H]CEt-HDL turnover rates than that of PDZK1O/RAP ones (50% and 53%, p<0.01, respectively). These results suggest that CETP expression in mouse increases direct removal of HDL CE in the liver and this process is independent of SR-BI, LRP, and possibly LDLR.     

Association of Two CETP Polymorphisms With HDL Levels in the Chinese Obese Population

The association of two cholesterol ester transfer protein (CETP) polymorphisms, D442G and TAQIB (B1→B2), with high‐density lipoprotein (HDL) levels in 932 Chinese obese individuals (BMI ≥ 27) was investigated in comparison with normal controls (BMI ≤ 24). Independent association was demonstrated for TAQIB minor allele B2 and CETP442 minor allele G with elevated HDL levels. The CETP D442G polymorphism was associated with a much greater increase in HDL levels in subjects with BMI exceeding 27 kg/m² (+5.42 mg/dl, P = 0.0007) compared to normal controls (+1.97 mg/dl, P = 0.275), and the increase in HDL reached the highest level among subjects with BMI exceeding 30 kg/m² (+6.80 mg/dl, P = 0.016). TAQIB showed significant association with HDL levels only in normal BMI subgroup (P = 0.0017). TAQIB significantly interacted with serum triglyceride (TG) on modulating HDL levels (P = 0.027). The TAQIB–TG interaction effect remained marginally significant after controlling for BMI (P = 0.057). We conclude that D442G polymorphism is associated with more HDL elevation in obesity. TAQIB interacts with serum TG on modulating HDL levels, and the interaction is partly independent of BMI.