A recent study provides evidence to explain how cholesteryl ester transfer protein (CETP) mediates the conversation of cholesterol from 'good' HDLs to 'bad' LDLs.
These findings point the way to the design of safer, more effective next generation CETP inhibitors that could help prevent the development of heart disease.
In a study, researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) have recorded the first structural images of CETP interacting with HDLs and LDLs.
The images and structural analyses support the hypothesis that cholesterol is transferred from high density lipoproteins (HDLs) to low density lipoproteins (LDLs) via a tunnel running through the center of the CETP molecule.
"Our images show that CETP is a small (53 kilodaltons) banana-shaped asymmetric molecule with a tapered N-terminal domain and a globular C-terminal domain," Gang Ren, a materials physicist and electron microscopy expert with Berkeley Lab's Molecular Foundry, a DOE nanoscience research center, who led the study.
"We discovered that the CETP's N-terminal penetrates HDL and its C-terminal interacts with LDL forming a ternary complex. Structure analyses lead us to hypothesize that the interaction may generate molecular forces that twist the terminals, creating pores at both ends of the CETP. These pores connect with central cavities in the CETP to form a tunnel that serves as a conduit for the movement of cholesterol from the HDL," Ren explained.
Cardiovascular or heart disease, mainly atherosclerosis, remains the leading cause of death in the United States and throughout the world. Elevated levels of LDL cholesterol and/or reduced levels of HDL cholesterol in human plasma are major risk factors for heart disease.
Since CETP activity can reduce HDL-cholesterol concentrations and CETP deficiency is associated with elevated HDL-cholesterol levels, CETP inhibitors have become a highly sought-after pharmacological target for the treatment of heart disease.
However, despite this intense clinical interest in CETP, little is known concerning the molecular mechanisms of CETP-mediated cholesterol transfers among lipoproteins, or even how CETP interacts with and binds to lipoproteins.
"It has been very difficult to investigate CETP mechanisms using conventional structural imaging methods because interaction with CETP can alter the size, shape and composition of lipoproteins, especially HDL," said Ren.
"We were successful because we used our optimized negative-staining electron microscopy protocol that allows us to flash-fix the structure and efficiently screen more than 300 samples prepared under different conditions," he stated.
Ren and his colleagues used their optimized negative-staining electron microscopy protocol to image CETP as it interacted with spherical HDL and LDL particles.
Image processing techniques yielded three-dimensional reconstructions of CETP and CETP-bound HDL. Molecular dynamic simulations were used to assess CETP molecular mobility and predict the changes that would be associated with cholesterol transfer. CETP antibodies were used to identify the CEPT interaction domains and validate the cholesterol transfer model by inhibiting CETP. This model presents inviting new targets for future CETP inhibitors.
"Our model identifies new interfaces of CETP that interact with HDL and LDL and delineates the mechanism by which the transfer of cholesterol takes place. This is an important step toward the rational design of next generation CETP inhibitors for treating cardiovascular disease," Ren added.
The results of this study have been reported in the journal Nature Chemical Biology.