Cholesteryl ester transfer protein (CETP), also called plasma lipid transfer protein, is a plasma protein that facilitates the transport of cholesteryl esters and triglycerides between the lipoproteins. It collects triglycerides from very-low-density (VLDL) or low-density lipoproteins (LDL) and exchanges them for cholesteryl esters from high-density lipoproteins (HDL), and vice versa. Most of the time, however, CETP does a heteroexchange, trading a triglyceride for a cholesteryl ester or a cholesteryl ester for a triglyceride.
The CETP gene is located on the sixteenth chromosome (16q21).
The crystal structure of CETP is that of dimer of two TUbular LIPid (TULIP) binding domains. [3] [4] Each domain consists of a core of 6 elements: 4 beta-sheets forming an extended superhelix; 2 flanking elements that tend to include some alpha helix. The sheets wrap around the helices to produce a cylinder 6 x 2.5 x 2.5 nm. CETP contains two of these domains that interact head-to-head via an interface made of 6 beta-sheets, 3 from each protomer. The same fold is shared by Bacterial Permeability Inducing proteins (examples: BPIFP1 BPIFP2 BPIFA3 and BPIFB4), phospholipid transfer protein (PLTP), and long-Palate Lung, and Nasal Epithelium protein (L-PLUNC). The fold is similar to intracellular SMP domains, [5] and originated in bacteria. [6] [7] [8] The crystal structure of CETP has been obtained with bound CETP inhibitors. [9] However, this has not resolved the doubt over whether CETP function as a lipid tube or shuttle. [10]
Rare mutations leading to reduced function of CETP have been linked to accelerated atherosclerosis. [11] In contrast, a polymorphism (I405V) of the CETP gene leading to lower serum levels has also been linked to exceptional longevity [12] and to metabolic response to nutritional intervention. [13] However, this mutation also increases the prevalence of coronary heart disease in patients with hypertriglyceridemia. [14] The D442G mutation, which lowers CETP levels and increases HDL levels also increases coronary heart disease. [11]
Elaidic acid, a major component of trans fat, increases CETP activity. [15]
As HDL can alleviate atherosclerosis and other cardiovascular diseases, and certain disease states such as the metabolic syndrome feature low HDL, pharmacological inhibition of CETP is being studied as a method of improving HDL levels. [16] To be specific, in a 2004 study, the small molecular agent torcetrapib was shown to increase HDL levels, alone and with a statin, and lower LDL when co-administered with a statin. [17] Studies into cardiovascular endpoints, however, were largely disappointing. While they confirmed the change in lipid levels, most reported an increase in blood pressure, no change in atherosclerosis, [18] [19] and, in a trial of a combination of torcetrapib and atorvastatin, an increase in cardiovascular events and mortality. [20]
A compound related to torcetrapib, Dalcetrapib (investigative name JTT-705/R1658), was also studied, but trials have ceased. [21] It increases HDL levels by 30%, as compared to 60% by torcetrapib. [22] Two CETP inhibitors were previously under development. One was Merck's MK-0859 anacetrapib, which in initial studies did not increase blood pressure. [23] In 2017, its development was abandoned by Merck. [24] The other was Eli Lilly's evacetrapib, which failed in Phase 3 trials.
Click on genes, proteins and metabolites below to link to respective articles. [§ 1]
Cholesterol is an organic molecule. It is a sterol, a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell membranes.
High-density lipoprotein (HDL) is one of the five major groups of lipoproteins. Lipoproteins are complex particles composed of multiple proteins which transport all fat molecules (lipids) around the body within the water outside cells. They are typically composed of 80–100 proteins per particle and transporting up to hundreds of fat molecules per particle.
Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein which transport all fat molecules around the body in the extracellular water. These groups, from least dense to most dense, are chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein and high-density lipoprotein (HDL). LDL delivers fat molecules to cells. LDL is involved in atherosclerosis, a process in which it is oxidized within the walls of arteries.
A lipoprotein is a biochemical assembly whose primary function is to transport hydrophobic lipid molecules in water, as in blood plasma or other extracellular fluids. They consist of a Triglyceride and Cholesterol center, surrounded by a phospholipid outer shell, with the hydrophilic portions oriented outward toward the surrounding water and lipophilic portions oriented inward toward the lipid center. A special kind of protein, called apolipoprotein, is embedded in the outer shell, both stabilising the complex and giving it a functional identity that determines its fate.
Hypolipidemic agents, cholesterol-lowering drugs or antihyperlipidemic agents, are a diverse group of pharmaceuticals that are used in the treatment of high levels of fats (lipids), such as cholesterol, in the blood (hyperlipidemia). They are called lipid-lowering drugs. These are drugs which lower the level of lipids and lipoproteins in blood.
Very-low-density lipoprotein (VLDL), density relative to extracellular water, is a type of lipoprotein made by the liver. VLDL is one of the five major groups of lipoproteins that enable fats and cholesterol to move within the water-based solution of the bloodstream. VLDL is assembled in the liver from triglycerides, cholesterol, and apolipoproteins. VLDL is converted in the bloodstream to low-density lipoprotein (LDL) and intermediate-density lipoprotein (IDL). VLDL particles have a diameter of 30–80 nm. VLDL transports endogenous products, whereas chylomicrons transport exogenous (dietary) products. In the early 2010s both the lipid composition and protein composition of this lipoprotein were characterised in great detail.
Chylomicrons, also known as ultra low-density lipoproteins (ULDL), are lipoprotein particles that consist of triglycerides (85–92%), phospholipids (6–12%), cholesterol (1–3%), and proteins (1–2%). They transport dietary lipids from the intestines to other locations in the body. ULDLs are one of the five major groups of lipoproteins that enable fats and cholesterol to move within the water-based solution of the bloodstream. A protein specific to chylomicrons is ApoB48.
Hyperlipidemia is abnormally elevated levels of any or all lipids or lipoproteins in the blood. Hyperlipidemia is an umbrella term that refers to acquired or genetic disorders that result in high levels of lipids circulating in the blood. This disease is usually chronic and requires ongoing medication to control blood lipid levels.
Torcetrapib was a drug being developed to treat hypercholesterolemia and prevent cardiovascular disease. Its development was halted in 2006 when phase III studies showed excessive all-cause mortality in the treatment group receiving a combination of atorvastatin (Lipitor) and torcetrapib.
A CETP inhibitor is a member of a class of drugs that inhibit cholesterylester transfer protein (CETP). They are intended to reduce the risk of atherosclerosis by improving blood lipid levels. At least three medications within this class have failed to result in benefits however.
Familial hypercholesterolemia (FH) is a genetic disorder characterized by high cholesterol levels, specifically very high levels of low-density lipoprotein, in the blood and early cardiovascular disease.The most common mutations diminish the number of functional LDL receptors in the liver. Since the underlying body biochemistry is slightly different in individuals with FH, their high cholesterol levels are less responsive to the kinds of cholesterol control methods which are usually more effective in people without FH. Nevertheless, treatment is usually effective.
Acid lipase disease or deficiency is a name used to describe two related disorders of fatty acid metabolism. Acid lipase disease occurs when the enzyme lysosomal acid lipase that is needed to break down certain fats that are normally digested by the body is lacking or missing. This results in the toxic buildup of these fats in the body's cells and tissues. These fatty substances, called lipids, include waxes, oils, and cholesterol.
Apolipoprotein A1 is a protein that in humans is encoded by the APOA1 gene. It has a specific role in lipid metabolism. The text in a 2014 report suggested that APOA1 mRNA is regulated by endogenously expressed antisense RNA.
Apolipoprotein C-III also known as apo-CIII is a protein that in humans is encoded by the APOC3 gene. Apo-CIII is secreted by the liver as well as the small intestine, and is found on triglyceride-rich lipoproteins such as chylomicrons, very low density lipoprotein (VLDL), and remnant cholesterol.,
Hepatic lipase (HL), also called hepatic triglyceride lipase (HTGL) or LIPC, is a form of lipase, catalyzing the hydrolysis of triacylglyceride. Hepatic lipase is coded by chromosome 15 and its gene is also often referred to as HTGL or LIPC. Hepatic lipase is expressed mainly in liver cells, known as hepatocytes, and endothelial cells of the liver. The hepatic lipase can either remain attached to the liver or can unbind from the liver endothelial cells and is free to enter the body's circulation system. When bound on the endothelial cells of the liver, it is often found bound to HSPG, heparan sulfate proteoglycans (HSPG), keeping HL inactive and unable to bind to HDL or IDL. When it is free in the bloodstream, however, it is found associated with HDL to maintain it inactive. This is because the triacylglycerides in HDL serve as a substrate, but the lipoprotein contains proteins around the triacylglycerides that can prevent the triacylglycerides from being broken down by HL.
Phospholipid transfer protein is a protein that in humans is encoded by the PLTP gene.
Reverse cholesterol transport is a multi-step process resulting in the net movement of cholesterol from peripheral tissues back to the liver first via entering the lymphatic system, then the bloodstream.
In molecular biology, the lipid-binding serum glycoproteins family, also known as the BPI/LBP/Plunc family or LBP/BPI/CETP family represents a family which includes mammalian lipid-binding serum glycoproteins. Members of this family include:
Evacetrapib was a drug under development by Eli Lilly & Company that inhibits cholesterylester transfer protein. CETP collects triglycerides from very low-density lipoproteins (VLDL) or low-density lipoproteins (LDL) and exchanges them for cholesteryl esters from high-density lipoproteins (HDL), and vice versa, but primarily increasing high-density lipoprotein and lowering low-density lipoprotein. It is thought that modifying lipoprotein levels modifies the risk of cardiovascular disease. The first CETP inhibitor, torcetrapib, was unsuccessful because it increased levels of the hormone aldosterone and increased blood pressure, which led to excess cardiac events when it was studied. Evacetrapib does not have the same effect. When studied in a small clinical trial in people with elevated LDL and low HDL, significant improvements were noted in their lipid profile.
Apabetalone is an orally available small molecule created by Resverlogix Corp. that is being evaluated in clinical trials for the treatment of atherosclerosis and associated cardiovascular disease (CVD). In the phase II clinical trial ASSURE in patients with angiographic coronary disease and low high-density lipoprotein cholesterol (HDL-C) levels, apabetalone showed no greater increase in HDL-cholesterol (HDL-c) and apolipoprotein A-I (ApoA-I) levels or incremental regression of atherosclerosis than administration of placebo, while causing a statistically significant greater incidence of elevated liver enzymes. However, pooled analysis of the effect of apabetalone in three phase II clinical trials ASSERT, ASSURE, and SUSTAIN demonstrated increases in HDL-cholesterol (HDL-c) and apolipoprotein A-I (ApoA-I) levels, as well as decreases in the incidence of major adverse cardiac events (MACE). Reduction of MACE was more profound in patients with diabetes mellitus. In a short term study in prediabetics, favorable changes in glucose metabolism were observed in patients receiving apabetalone. An international, multicenter phase III trial, “Effect of RVX000222 on Time to Major Adverse Cardiovascular Events in High-Risk Type 2 Diabetes Mellitus Subjects with Coronary Artery Disease” (BETonMACE) commenced in October 2015. The trial is designed to determine whether apabetalone in combination with statins can decrease cardiac events compared to treatment with statins alone.