Date Published: August 2, 2017
Publisher: Public Library of Science
Author(s): Ziyun Wang, Manabu Niimi, Qianzhi Ding, Zhenming Liu, Ling Wang, Jifeng Zhang, Jun Xu, Jianglin Fan, Ivan Tancevski.
Cholesteryl ester transfer protein (CETP) is a plasma protein that mediates bidirectional transfers of cholesteryl esters and triglycerides between low-density lipoproteins and high-density lipoproteins (HDL). Because low levels of plasma CETP are associated with increased plasma HDL-cholesterol, therapeutic inhibition of CETP activity is considered an attractive strategy for elevating plasma HDL-cholesterol, thereby hoping to reduce the risk of cardiovascular disease. Interestingly, only a few laboratory animals, such as rabbits, guinea pigs, and hamsters, have plasma CETP activity, whereas mice and rats do not. It is not known whether all CETPs in these laboratory animals are functionally similar to human CETP. In the current study, we compared plasma CETP activity and characterized the plasma lipoprotein profiles of these animals. Furthermore, we studied the three CETP molecular structures, physicochemical characteristics, and binding properties with known CETP inhibitors in silico. Our results showed that rabbits exhibited higher CETP activity than guinea pigs and hamsters, while these animals had different lipoprotein profiles. CETP inhibitors can inhibit rabbit and hamster CETP activity in a similar manner to human CETP. Analysis of CETP molecules in silico revealed that rabbit and hamster CETP showed many features that are similar to human CETP. These results provide novel insights into understanding CETP functions and molecular properties.
Cholesteryl ester transfer protein (CETP) is a hydrophobic glycoprotein synthesized mainly in the liver and circulates in plasma in association with HDL. CETP transports cholesteryl esters from HDLs to apolipoprotein (apo)-B containing particles, therefore playing an important role in the metabolism of lipoproteins and the reverse cholesterol transport from the peripheral tissues to the liver. Patients genetically deficient in the CETP gene showed low or no CETP activity along with hyper-HDL-cholesterolemia. Furthermore, it has been known that high levels of plasma HDL-C are inversely associated with low risk of coronary heart disease (CHD); thus, elevation of plasma HDL-C levels through inhibition of CETP was also considered an alternative therapy to treat CHD. This notion was initially supported by the finding that therapeutic inhibition of CETP (such as CETP antisense, vaccine, or inhibitors) in experimental animals led to the elevation of plasma HDL-C and the reduction of atherosclerosis[5–9]. However, in human clinical trials, three CETP inhibitors either failed due to excess death (torcetrapib) or were terminated due to insufficient efficacy (dalcetrapib and evacetrapib)[10–12]. Currently, only anacetrapib is still under testing in a Phase III clinical trial. Because it is still controversial regarding whether CETP inhibition is beneficial for the treatment of CHD, there is a need to examine the pathophysiological functions of CETP using experimental animals. Human CETP and its interactions with CETP inhibitors have been extensively investigated[16–18]. Interestingly, in addition to humans and other primates, only a few laboratory animals, such as rabbits, guinea pigs, and hamsters, exhibit detectable plasma CETP activity, whereas rodents (mice and rats) do not have endogenous CETP genes. To study pathophysiological roles of CETP in lipid metabolism and atherosclerosis, it is essential to use appropriate animal models with plasma CETP activity. In fact, it is not known whether CETP-possessing mammals have CETP functions similar to those of human CETP. To examine this question, we performed the current study in an attempt to (1) construct three CETP 3-D molecule structures by homology in silico and examine possible pockets of these CETP models; (2) compare their CETP activity along with characterization of the plasma lipoprotein profiles; and (3) examine CETP interactions with known inhibitors. Our results indicate that rabbit and hamster CETP but not guinea pig CETP is similar to human CETP in terms of activity and inhibitor interactions.
We first constructed an evolutionary tree of eight species that have CETP genes based on a search of GenBank (Fig 1). Among five non-primates, rabbit CETP is the closest to that of primates. We focused on three commonly-used laboratory animals (rabbits, guinea pigs, and hamsters) regarding the CETP gene and protein sequence and compared their similarities with human CETP. CETP proteins in all species are 53 kDa in size, but the rabbit CETP sequence is slightly more identical to human CETP compared with guinea pig and hamster CETP, as summarized in Table 1.
In the current study, we characterized three CETP-possessing laboratory animals regarding their CETP activity, lipoprotein profiles, and CETP interactions with four known inhibitors. Although all of these animals are considered useful for the study of lipoprotein metabolism and atherosclerosis, it has not been defined whether their CETP is similar in terms of the molecular structures and interactions with the inhibitors. Biochemical analysis of plasma lipoproteins along with molecular analysis of the CETP structure and interactions with CETP inhibitors suggest that rabbits and hamsters are appropriate models for investigating CETP functions since they show similar lipoprotein profiles and CETP functions.