Structural Plasticity of Cholesteryl Ester Transfer Protein Assists the Lipid Transfer Activity

Abstract

Coronary heart disease (CHD)2 is one of the prime causes of an increase in the mortality rate worldwide. Over the past few years, various epidemiological studies have provided ample evidence that increased levels of HDL, by transporting cholesterol from atherosclerotic plaques to liver for excretion, can arrest the progression of CHD (1, 2). Subsequent preclinical studies on overexpression of apolipoprotein A-I (apoA-I), the major protein constituent of HDL, have reported significant reduction in CHD risk (3, 4). Although raising HDL levels is a longstanding strategy for mitigating CHD risk, the current lipidtargeted therapies, including the usage of statins, fibrates, and niacin, have shown reduced therapeutic potential (5-7). Hence, there is a continuous requirement for new strategies that can elevate HDL levels to halt the progression of CHD.Cholesteryl ester transfer protein (CETP), a plasma glycoprotein with 476 residues, has been found to mediate the transfer of cholesteryl esters (CEs) from HDL to LDL and VLDL as well as the reciprocal transfer of triglycerides (TGs) from LDL and VLDL to HDL (8, 9). The identification of deficient CETP gene levels and correlated high HDL levels in a Japanese population (10 -13) brought CETP into focus as a therapeutic target for controlling HDL levels. Preclinical animal model studies have further substantiated these findings by showing an elevated level of HDL in mice with non-expressing CETP (14 -16). Since then, the inhibition of CETP activity by small molecule inhibitors has been pursued as an active approach to prevent atherosclerosis with varying levels of success at clinical trial phases (17)(18)(19)(20). Given the complexity of lipid metabolism and the pivotal role of CETP in the process, understanding CETP structure, CETP inhibition, and the mechanism by which CETP transfers neutral lipids is of prime importance. The last decade, therefore, has seen the employment of sophisticated techniques, like cryo-EM (21, 22), x-ray (23, 24), and MD simulations (25-28) on this front. Nevertheless, the story of CETP remains far from complete.The crystal structure of CETP (Protein Data Bank entry 2OBD) has been solved with four bound lipid molecules (23). The structure shows two ␤-barrel domains at the N and C termini, each with a twisted ␤-sheet and a long ␣-helix; a central ␤-sheet between the two ␤-barrels; a C-terminal ␣-helix; and a 60-Å-long hydrophobic tunnel occupied by two CE molecules. Additionally, there were two plug-in dioleoylphosphatidylcholine molecules with polar headgroups exposed to plasma and acyl chains buried in the hydrophobic tunnel of CETP. Interestingly, one of the CEs in the hydrophobic tunnel was in the bent conformation, whereas the second CE molecule was in the linear conformation. Recent cryopositive staining EM and optimized negative-staining electron microscopy (OpNS-EM) protocols combined with CETP C terminus-specific polyclonal antibody studies have suggested that CETP penetrates its C-terminal ␤-barrel domain into LDL or VLDL and its N-termi...

How cholesteryl ester transfer protein can also be a potential triglyceride transporter

“…The subsequent visual inspection of the trajectory demonstrates that the frequent inter-switching of TG C between the chair and trident/random conformations is the possible reason for the variation in CETP tunnel length. The extended tunnel observed in the current study is consistent with our recent report on the structural plasticity of CE-bound CETP, where we have shown that CE-bound CETP frequently switches to an extended conformation of about 100 Å in length 14 . Thus all these analyses suggest that TG-bound CETP maintains a similar stable conformation as that of the CE bound CETP.

Figure 6Comparison of TG- and CE-bound CETP structures.

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Cholesteryl ester transfer protein and its inhibitors

“…That study identified the presence of a continuous, central tunnel within the CETP molecule, which is unique among members of the LTP/LBP gene family. Qiu et al further identified two lipid binding pockets in the N-and C-terminal domains of CETP (18,19), and an amphipathic helix, helix X, that is located in the C-terminal domain of the protein. The central CETP tunnel can accommodate two CE molecules, one CE and one TG molecule, or two TG molecules (18).…”

“…The results of these studies support the penetration of the N-terminal domain of CETP into the surface of an HDL particle together with a concomitant interaction of the C-terminal domain of CETP with an LDL or VLDL particle. Additional analyses have indicated that the transfer of cholesteryl esters between HDLs and LDLs, or HDLs and VLDLs, by this mechanism is dependent on conformational changes in the N-and the C-terminal domains of CETP that increase tunnel continuity and improve neutral lipid accessibility (19,21). …”

Role of glycans in cholesteryl ester transfer protein revealed by molecular dynamics simulation

“…More recent electron microscopy (EM) images revealed that the N‐terminal of CETP‐C penetrates the HDL surface and core (∼50 Å), whereas the C‐terminal penetrates the LDL or VLDL only ∼25 Å . Furthermore, molecular dynamics (MD) simulations showed that a hydrophobic tunneling inside CETP‐C is sufficient to allow a CE molecule to completely transfer through the entire CETP . These results strongly suggest a tunneling mechanism, in which the cavities of CETP form a continuous tunneling that mediates the CE‐TG transfer .…”

“…While plasma CETP has 4 potential N‐linked glycosylation sites (CETP‐G), which play an important role in the heteroexchange of CEs and TGs between lipoproteins . Up to now, most studies on CETP have only focused on the nascent structure of CETP (CETP‐N, mutated residues were reversed) and CETP‐C . Thus, little is known about the detailed structural information of CETP‐G and effect of glycans on the structure and function of CETP, as well as the accompanying modulation of CEs transfer.…”