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

2009; Elsevier BV; Volume: 50; Issue: 11 Linguagem: Inglês

10.1194/jlr.m900015-jlr200

1539-7262

Laura K. Potter, Dennis L. Sprecher, Max Walker, Frank Tobin,

Drug Transport and Resistance Mechanisms

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. 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. Cholesteryl ester transfer protein (CETP) plays a critical role in reverse cholesterol transport (1Barter P.J. Hugh Sinclair lecture: the regulation and remodelling of HDL by plasma factors.Atheroscler. Suppl. 2002; 3: 39-47Abstract Full Text Full Text PDF PubMed Scopus (103) Google Scholar, 2Yamashita S. Matsuzawa Y. Cholesteryl ester transfer protein.in: Betteridge D.J. Illingworth D.R. Shepherd J. Lipoproteins in Health and Disease. 1999: 277-299Google Scholar, 3de Grooth G.J. Klerkx A.H. Stroes E.S. Stalenhoef A.F. Kastelein J.J. Kuivenhoven J.A. A review of CETP and its relation to atherosclerosis.J. Lipid Res. 2004; 45: 1967-1974Abstract Full Text Full Text PDF PubMed Scopus (216) Google Scholar). CETP facilitates the net transfer of cholesterol ester (CE) from HDL to VLDL and LDL, which is followed by hepatic uptake of LDL via the LDL receptor (4Soutar A.K. Low-density lipoprotein receptors.in: Betteridge D.J. Illingworth D.R. Shepherd J. Lipoproteins in Health and Disease. 1999: 303-322Google Scholar). The CETP-mediated transfer of CE is part of a bidirectional exchange that also includes the transfer of triglyceride (TG) from VLDL and LDL to HDL (1Barter P.J. Hugh Sinclair lecture: the regulation and remodelling of HDL by plasma factors.Atheroscler. Suppl. 2002; 3: 39-47Abstract Full Text Full Text PDF PubMed Scopus (103) Google Scholar, 2Yamashita S. Matsuzawa Y. Cholesteryl ester transfer protein.in: Betteridge D.J. Illingworth D.R. Shepherd J. Lipoproteins in Health and Disease. 1999: 277-299Google Scholar, 3de Grooth G.J. Klerkx A.H. Stroes E.S. Stalenhoef A.F. Kastelein J.J. Kuivenhoven J.A. A review of CETP and its relation to atherosclerosis.J. Lipid Res. 2004; 45: 1967-1974Abstract Full Text Full Text PDF PubMed Scopus (216) Google Scholar).The relationship between CETP and atherosclerosis/coronary heart disease (CHD) is unclear, as is the importance of the net transfer of CE from HDL to apolipoprotein (apo) B-containing lipoproteins. CETP deficiency and inhibition studies in animals and humans have produced conflicting results. Pharmacologic CETP inhibition has increased HDL cholesterol and reduced atherosclerosis in rabbit models (5Rader D.J. Regulation of reverse cholesterol transport and clinical implications.Am. J. Cardiol. 2003; 92: 42J-49JAbstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar). In humans, CETP deficiency has been associated with both increased and decreased CHD risk (3de Grooth G.J. Klerkx A.H. Stroes E.S. Stalenhoef A.F. Kastelein J.J. Kuivenhoven J.A. A review of CETP and its relation to atherosclerosis.J. Lipid Res. 2004; 45: 1967-1974Abstract Full Text Full Text PDF PubMed Scopus (216) Google Scholar, 5Rader D.J. Regulation of reverse cholesterol transport and clinical implications.Am. J. Cardiol. 2003; 92: 42J-49JAbstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar). The CETP inhibitors JTT-705 and torcetrapib have been shown to effectively reduce CETP activity in humans and raise HDL cholesterol, although the effect of this class of compounds on atherosclerosis and CHD risk is as yet unclear (6Barter P.J. Kastelein J.J. Targeting cholesteryl ester transfer protein for the prevention and management of cardiovascular disease.J. Am. Coll. Cardiol. 2006; 47: 492-499Crossref PubMed Scopus (142) Google Scholar, 7Athyros V.G. Mikhailidis D.P. Kakafika A.I. Karagiannis A. Hatzitolios A. Tziomalos K. Ganotakis E.S. Liberopoulos E.N. Elisaf M. Identifying and attaining LDL-C goals: mission accomplished? Next target: new therapeutic options to raise HDL-C levels.Curr. Drug Targets. 2007; 8: 483-488Crossref PubMed Scopus (27) Google Scholar, 8Dullaart R.P. Dallinga-Thie G.M. Wolffenbuttel B.H. van Tol A. CETP inhibition in cardiovascular risk management: a critical appraisal.Eur. J. Clin. Invest. 2007; 37: 90-98Crossref PubMed Scopus (48) Google Scholar).Adding to this uncertainty is the termination of a Phase III torcetrapib study following an unexpected increase in deaths when dosed in combination with statin therapy versus statin therapy alone. "Off-target" activities of torcetrapib have been suggested as the reason for increased mortality and morbidity (9Barter P.J. Caulfield M. Eriksson M. Grundy S.M. Kastelein J.J. Komajda M. Lopez-Sendon J. Mosca L. Tardif J.C. Waters D.D. et al.Effects of torcetrapib in patients at high risk for coronary events.N Engl J Med. 2007; 357: 2109-2122Crossref PubMed Scopus (2578) Google Scholar) and atherosclerotic plaque volume decreases were seen in the subset of patients having the greatest HDL changes (10Nicholls S.J. Brennan D.M. Wolski K. Kalidindi S.R. Moon K.-W. Tuzcu E.M. Nissen S.E. Abstract 684: Changes in levels of high density lipoprotein cholesterol predict the impact of torcetrapib on progression of coronary atherosclerosis: insights from ILLUSTRATE.Circulation. 2007; 116 (II_127-b-.)Google Scholar). It is not yet known, however, if the increases in mortality and morbidity were caused entirely by the proposed mechanism or were in part due to changes in HDL subfractions or in functionality induced by CETP inhibition itself (11Tall A.R. Yvan-Charvet L. Wang N. The failure of torcetrapib: was it the molecule or the mechanism?.Arterioscler. Thromb. Vasc. Biol. 2007; 27: 257-260Crossref PubMed Scopus (268) Google Scholar). Studies with other CETP inhibitors in development may shed light on whether this is a generalized phenomenon or specific to certain compounds, patient groups, or lipid phenotypes.The inhibitors JTT-705 and torcetrapib block CETP activity via different mechanisms. JTT-705 irreversibly binds to CETP and prevents CETP-lipoprotein binding (7Athyros V.G. Mikhailidis D.P. Kakafika A.I. Karagiannis A. Hatzitolios A. Tziomalos K. Ganotakis E.S. Liberopoulos E.N. Elisaf M. Identifying and attaining LDL-C goals: mission accomplished? Next target: new therapeutic options to raise HDL-C levels.Curr. Drug Targets. 2007; 8: 483-488Crossref PubMed Scopus (27) Google Scholar). Torcetrapib is more potent (8Dullaart R.P. Dallinga-Thie G.M. Wolffenbuttel B.H. van Tol A. CETP inhibition in cardiovascular risk management: a critical appraisal.Eur. J. Clin. Invest. 2007; 37: 90-98Crossref PubMed Scopus (48) Google Scholar) and acts as a noncompetitive inhibitor by binding reversibly to CETP where the resulting complex can bind to lipoproteins and form an inactive complex that is unable to complete transfer of lipids (7Athyros V.G. Mikhailidis D.P. Kakafika A.I. Karagiannis A. Hatzitolios A. Tziomalos K. Ganotakis E.S. Liberopoulos E.N. Elisaf M. Identifying and attaining LDL-C goals: mission accomplished? Next target: new therapeutic options to raise HDL-C levels.Curr. Drug Targets. 2007; 8: 483-488Crossref PubMed Scopus (27) Google Scholar). It is unclear, however, what the consequences of different inhibition mechanisms may be on CETP transfer activity and lipoprotein metabolism in general.Evidence suggests that both the lipid composition and the relative particle numbers of lipoproteins influence CETP activity and the net transfer of CE from HDL to apoB-containing lipoproteins (2Yamashita S. Matsuzawa Y. Cholesteryl ester transfer protein.in: Betteridge D.J. Illingworth D.R. Shepherd J. Lipoproteins in Health and Disease. 1999: 277-299Google Scholar, 12Guerin M. Dolphin P.J. Chapman M.J. A new in vitro method for the simultaneous evaluation of cholesteryl ester exchange and mass transfer between HDL and apoB-containing lipoprotein subspecies. Identification of preferential cholesteryl ester acceptors in human plasma.Arterioscler. Thromb. 1994; 14: 199-206Crossref PubMed Google Scholar, 13Murakami T. Michelagnoli S. Longhi R. Gianfranceschi G. Pazzucconi F. Calabresi L. Sirtori C.R. Franceschini G. Triglycerides are major determinants of cholesterol esterification/transfer and HDL remodeling in human plasma.Arterioscler. Thromb. Vasc. Biol. 1995; 15: 1819-1828Crossref PubMed Scopus (166) Google Scholar). Several human studies have shown increased CE transfer and CETP activity in individuals with higher TG levels including temporary postprandial increases as well as in hyperlipidemia (13Murakami T. Michelagnoli S. Longhi R. Gianfranceschi G. Pazzucconi F. Calabresi L. Sirtori C.R. Franceschini G. Triglycerides are major determinants of cholesterol esterification/transfer and HDL remodeling in human plasma.Arterioscler. Thromb. Vasc. Biol. 1995; 15: 1819-1828Crossref PubMed Scopus (166) Google Scholar, 14Le N.A. Innis-Whitehouse W. Li X. Bakker-Arkema R. Black D. Brown W.V. Lipid and apolipoprotein levels and distribution in patients with hypertriglyceridemia: effect of triglyceride reductions with atorvastatin.Metabolism. 2000; 49: 167-177Abstract Full Text PDF PubMed Scopus (67) Google Scholar, 15Guerin M. Egger P. Soudant C. Le Goff W. van Tol A. Dupuis R. Chapman M.J. Cholesteryl ester flux from HDL to VLDL-1 is preferentially enhanced in type IIB hyperlipidemia in the postprandial state.J. Lipid Res. 2002; 43: 1652-1660Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar, 16Guerin M. Le Goff W. Lassel T.S. Van Tol A. Steiner G. Chapman M.J. Atherogenic role of elevated CE transfer from HDL to VLDL(1) and dense LDL in type 2 diabetes: impact of the degree of triglyceridemia.Arterioscler. Thromb. Vasc. Biol. 2001; 21: 282-288Crossref PubMed Scopus (203) Google Scholar, 17Tall A. Sammett D. Granot E. Mechanisms of enhanced cholesteryl ester transfer from high density lipoproteins to apolipoprotein B-containing lipoproteins during alimentary lipemia.J. Clin. Invest. 1986; 77: 1163-1172Crossref PubMed Scopus (154) Google Scholar, 18Lahdenpera S. Syvanne M. Kahri J. Taskinen M.R. Regulation of low-density lipoprotein particle size distribution in NIDDM and coronary disease: importance of serum triglycerides.Diabetologia. 1996; 39: 453-461Crossref PubMed Google Scholar). When high TG levels are sustained, the increased CE transfer mediated by CETP yields higher levels of small, dense, proatherogenic LDL particles (8Dullaart R.P. Dallinga-Thie G.M. Wolffenbuttel B.H. van Tol A. CETP inhibition in cardiovascular risk management: a critical appraisal.Eur. J. Clin. Invest. 2007; 37: 90-98Crossref PubMed Scopus (48) Google Scholar). This increase in CETP activity may be due to the higher levels of TG rather than increases in CETP protein (13Murakami T. Michelagnoli S. Longhi R. Gianfranceschi G. Pazzucconi F. Calabresi L. Sirtori C.R. Franceschini G. Triglycerides are major determinants of cholesterol esterification/transfer and HDL remodeling in human plasma.Arterioscler. Thromb. Vasc. Biol. 1995; 15: 1819-1828Crossref PubMed Scopus (166) Google Scholar). CE transfer activity is also augmented in hypercholesterolemia, playing a role in shifting the LDL profile toward smaller, more dense particles (19Guerin M. Dolphin P.J. Chapman M.J. Preferential cholesteryl ester acceptors among the LDL subspecies of subjects with familial hypercholesterolemia.Arterioscler. Thromb. 1994; 14: 679-685Crossref PubMed Scopus (45) Google Scholar). Whether lipemia-induced increases in CETP activity result directly from heightened apoB-containing lipoprotein particle number, size, or both is unclear.To gain better insight into the overall effect of CETP on net CE transfer and reverse cholesterol transport, it is important to understand the consequences of CETP inhibition and the relationship between CETP and lipoprotein particle number and composition. Learning more about the mechanisms and implications of CETP-mediated lipid transfer can help shed light on the complex interrelationships between HDL and the apoB-containing lipoproteins and how therapeutic modifications to the system could impact HDL and LDL mass, composition, and clearance.Mathematical models can help dissect the complex dynamics of lipoprotein metabolism and provide a greater understanding of the role CETP plays. By comprehensively detailing the kinetics and interactions of CETP activity, a kinetic model could predict the effects of CETP inhibition and the differences in inhibitory mechanisms. A model also could explore issues related to lipoprotein particle size and composition with a view toward predicting and explaining the relative atherogenicity of a given lipoprotein particle profile. The first step in building such a model is to capture the dynamics of CETP activity in vitro, including the binding and the details of the lipid exchange processes. Such a model could be a building block in a larger model of in vivo lipoprotein metabolism to explore CETP biology and investigate inhibition of different components of lipoprotein metabolism.In this article, a mathematical model for CETP activity in vitro is presented. The model tracks the binding and lipid transfer kinetics of CETP and lipoprotein species by class including the cholesterol and triglyceride contents of the various lipoprotein classes. Model parameters were carefully estimated via mathematical optimization by simultaneously comparing the model to multiple experimental datasets. The resulting model is able to predict the dynamics of CETP activity and inhibition on lipoprotein particles in vitro.METHODSConceptual model for CETP activityThere are two prevailing hypotheses for the mechanism of CETP activity: the shuttle model and the ternary complex model (20Barter P. Rye K.A. Cholesteryl ester transfer protein: its role in plasma lipid transport.Clin. Exp. Pharmacol. Physiol. 1994; 21: 663-672Crossref PubMed Scopus (45) Google Scholar, 21Tall A.R. Plasma cholesteryl ester transfer protein.J. Lipid Res. 1993; 34: 1255-1274Abstract Full Text PDF PubMed Google Scholar). In the shuttle model, CETP can bind to CE and TG, and accumulates "stores" of CE and TG that can be subsequently exchanged with lipoproteins. Such an exchange begins with CETP binding to a lipoprotein (Fig. 1, Step 1) (22Swenson T.L. Brocia R.W. Tall A.R. Plasma cholesteryl ester transfer protein has binding sites for neutral lipids and phospholipids.J. Biol. Chem. 1988; 263: 5150-5157Abstract Full Text PDF PubMed Google Scholar) followed by the bidirectional transfer of lipids between the lipoprotein and CETP (Fig. 1, Step 2). The exchange is followed by CETP dissociation (Fig. 1, Step 3) and the CETP molecule is then free to continue binding and exchanging with additional lipoproteins.The ternary complex model (23Ihm J. Quinn D.M. Busch S.J. Chataing B. Harmony J.A. Kinetics of plasma protein-catalyzed exchange of phosphatidylcholine and cholesteryl ester between plasma lipoproteins.J. Lipid Res. 1982; 23: 1328-1341Abstract Full Text PDF PubMed Google Scholar) postulates that CETP binds to a lipoprotein particle, which, in turn, leads to binding with another lipoprotein to form a complex of CETP bound between two lipoproteins. This is followed by the transfer of neutral lipids between the two lipoproteins, and, finally, the dissociation of the lipoprotein particles.Experimental evidence from several studies provides strong support for the shuttle model. CETP can bind to neutral lipids (22Swenson T.L. Brocia R.W. Tall A.R. Plasma cholesteryl ester transfer protein has binding sites for neutral lipids and phospholipids.J. Biol. Chem. 1988; 263: 5150-5157Abstract Full Text PDF PubMed Google Scholar, 24Qiu X. Mistry A. Ammirati M.J. Chrunyk B.A. Clark R.W. Cong Y. Culp J.S. Danley D.E. Freeman T.B. Geoghegan K.F. et al.Crystal structure of cholesteryl ester transfer protein reveals a long tunnel and four bound lipid molecules.Nat. Struct. Mol. Biol. 2007; 14: 106-113Crossref PubMed Scopus (210) Google Scholar) and those lipids can be transferred back and forth between CETP and lipoproteins (25Connolly D.T. McIntyre J. Heuvelman D. Remsen E.E. McKinnie R.E. Vu L. Melton M. Monsell R. Krul E.S. Glenn K. Physical and kinetic characterization of recombinant human cholesteryl ester transfer protein.Biochem. J. 1996; 320: 39-47Crossref PubMed Scopus (30) Google Scholar). Moreover, a recent study detailing the crystal structure of CETP suggests that CETP binds to only a single lipoprotein at a time, forming a "tunnel" with both ends bound to the lipoprotein (24Qiu X. Mistry A. Ammirati M.J. Chrunyk B.A. Clark R.W. Cong Y. Culp J.S. Danley D.E. Freeman T.B. Geoghegan K.F. et al.Crystal structure of cholesteryl ester transfer protein reveals a long tunnel and four bound lipid molecules.Nat. Struct. Mol. Biol. 2007; 14: 106-113Crossref PubMed Scopus (210) Google Scholar). Both CE and TG can be exchanged between the CETP molecule and the bound lipoprotein by the flow of neutral lipids through the tunnel. These findings are consistent with the shuttle theory for CETP-mediated lipid transfer.Limited kinetic models have been developed based on both the shuttle (25Connolly D.T. McIntyre J. Heuvelman D. Remsen E.E. McKinnie R.E. Vu L. Melton M. Monsell R. Krul E.S. Glenn K. Physical and kinetic characterization of recombinant human cholesteryl ester transfer protein.Biochem. J. 1996; 320: 39-47Crossref PubMed Scopus (30) Google Scholar, 26Barter P.J. Hopkins G.J. Gorjatschko L. Jones M.E. A unified model of esterified cholesterol exchanges between human plasma lipoproteins.Atherosclerosis. 1982; 44: 27-40Abstract Full Text PDF PubMed Scopus (30) Google Scholar, 27Barter P.J. Jones M.E. Kinetic studies of the transfer of esterified cholesterol between human plasma low and high density lipoproteins.J. Lipid Res. 1980; 21: 238-249Abstract Full Text PDF PubMed Google Scholar) and ternary complex (23Ihm J. Quinn D.M. Busch S.J. Chataing B. Harmony J.A. Kinetics of plasma protein-catalyzed exchange of phosphatidylcholine and cholesteryl ester between plasma lipoproteins.J. Lipid Res. 1982; 23: 1328-1341Abstract Full Text PDF PubMed Google Scholar) hypotheses. They provided reasonable approximations to CE transfer data, although none of these models were designed to describe TG transfer activity directly, and hence they are not able to capture the key exchange dynamics of CE and TG together. The detailed crystal structure of CETP (24Qiu X. Mistry A. Ammirati M.J. Chrunyk B.A. Clark R.W. Cong Y. Culp J.S. Danley D.E. Freeman T.B. Geoghegan K.F. et al.Crystal structure of cholesteryl ester transfer protein reveals a long tunnel and four bound lipid molecules.Nat. Struct. Mol. Biol. 2007; 14: 106-113Crossref PubMed Scopus (210) Google Scholar) provides compelling evidence for the shuttle model over the ternary complex model because it appears that the CETP molecule can bind to only one lipoprotein particle at a time. Therefore, in this article, we will consider only the shuttle mechanism for CETP activity. The resulting model, presented here, captures all the relevant behaviors of CETP activity, including binding, bidirectional lipid exchange of CE and TG, and dissociation.The major assumptions used in building the shuttle-based model follow.Equimolar exchange of lipids.Equimolar exchange occurs when the number of molecules transferred from a CETP-bound lipoprotein to CETP is equal to the number of molecules transferred back from CETP to the lipoprotein, yielding no net gain or loss of core lipids. Several experimental studies have reported equimolar CETP-mediated exchange of neutral lipids (28Ko K.W. Ohnishi T. Yokoyama S. Triglyceride transfer is required for net cholesteryl ester transfer between lipoproteins in plasma by lipid transfer protein. Evidence for a hetero-exchange transfer mechanism demonstrated by using novel monoclonal antibodies.J. Biol. Chem. 1994; 269: 28206-28213Abstract Full Text PDF PubMed Google Scholar, 29Morton R.E. Greene D.J. The surface cholesteryl ester content of donor and acceptor particles regulates CETP: a liposome-based approach to assess the substrate properties of lipoproteins.J. Lipid Res. 2003; 44: 1364-1372Abstract Full Text Full Text PDF PubMed Scopus (14) Google Scholar, 30Morton R.E. Zilversmit D.B. Inter-relationship of lipids transferred by the lipid-transfer protein isolated from human lipoprotein-deficient plasma.J. Biol. Chem. 1983; 258: 11751-11757Abstract Full Text PDF PubMed Google Scholar, 31Serdyuk A.P. Morton R.E. Lipid transfer inhibitor protein activity deficiency in normolipidemic uremic patients on continuous ambulatory peritoneal dialysis.Arterioscler. Thromb. Vasc. Biol. 1997; 17: 1716-1724Crossref PubMed Scopus (17) Google Scholar), whereas others have shown a depletion of core lipids in HDL species, particularly in the presence of unesterified fatty acids (32Barter P.J. Chang L.B. Rajaram O.V. Sodium oleate dissociates the heteroexchange of cholesteryl esters and triacylglycerol between HDL and triacylglycerol-rich lipoproteins.Biochim. Biophys. Acta. 1990; 1047: 294-297Crossref PubMed Scopus (26) Google Scholar, 33Van Tol A. Scheek L.M. Groener J.E. Net mass transfer of cholesteryl esters from low density lipoproteins to high density lipoproteins in plasma from normolipidemic subjects.Arterioscler. Thromb. 1991; 11: 55-63Crossref PubMed Google Scholar, 34Barter P.J. Chang L.B. Rajaram O.V. Sodium oleate promotes a redistribution of cholesteryl esters from high to low density lipoproteins.Atherosclerosis. 1990; 84: 13-24Abstract Full Text PDF PubMed Scopus (37) Google Scholar, 35Liang H.Q. Rye K.A. Barter P.J. Dissociation of lipid-free apolipoprotein A-I from high density lipoproteins.J. Lipid Res. 1994; 35: 1187-1199Abstract Full Text PDF PubMed Google Scholar). Given the data showing equimolar exchange can occur and that monoclonal antibody evidence (28Ko K.W. Ohnishi T. Yokoyama S. Triglyceride transfer is required for net cholesteryl ester transfer between lipoproteins in plasma by lipid transfer protein. Evidence for a hetero-exchange transfer mechanism demonstrated by using novel monoclonal antibodies.J. Biol. Chem. 1994; 269: 28206-28213Abstract Full Text PDF PubMed Google Scholar) suggests a tight coupling between CE and TG exchange, we will assume that the exchanges are equimolar, at least in the case of the in vitro environment with isolated lipoproteins and low levels of unesterified fatty acids. If necessary, this assumption can easily be relaxed to allow nonequimolar exchange.Homoexchange and heteroexchange of lipids.Assuming equimolar exchange, there are four possible scenarios for the exchange of lipid between CETP and a bound lipoprotein: a heteroexchange of 1) CE for TG or 2) TG for CE, or a homoexchange of 3) CE for CE or 4) TG for TG. Evidence suggests a relative preference for homoexchange over heteroexchange (36Serdyuk A.P. Morton R.E. Lipid transfer inhibitor protein defines the participation of lipoproteins in lipid transfer reactions: CETP has no preference for cholesteryl esters in HDL versus LDL.Arterioscler. Thromb. Vasc. Biol. 1999; 19: 718-726Crossref PubMed Scopus (33) Google Scholar). Homoexchange effectively competes with heteroexchange by tying up CETP in a nonproductive manner, blocking the net CE transfer from HDL to the apoB-containing lipoproteins.Effects of core lipid composition on transfer activity.The relative preference for the donation of CE versus TG from a lipoprotein to CETP is a function of the relative concentrations of CE and TG in the lipoprotein's core (29Morton R.E. Greene D.J. The surface cholesteryl ester content of donor and acceptor particles regulates CETP: a liposome-based approach to assess the substrate properties of lipoproteins.J. Lipid Res. 2003; 44: 1364-1372Abstract Full Text Full Text PDF PubMed Scopus (14) Google Scholar). In an in vitro incubation, CETP activity will attain equilibrium when all lipoproteins in the incubation have the same CE:TG ratio (20Barter P. Rye K.A. Cholesteryl ester transfer protein: its role in plasma lipid transport.Clin. Exp. Pharmacol. Physiol. 1994; 21: 663-672Crossref PubMed Scopus (45) Google Scholar).A complete list of assumptions used in developing the conceptual model are detailed in Appendix I.Mathematical modelThe corresponding mathematical model is a system of ordinary differential equations that describes the rate kinetics of each process: changes in CETP binding, lipid transfer activities, and the composition of the lipoproteins. Depending on the type of in vitro experiment, the lipoproteins involved in the exchange may include two or more separate classes or subclasses (e.g., LDL, VLDL, HDL3, etc.). For each lipoprotein species, the model tracks the "average" lipoprotein particles of each class, as well as their corresponding TG and CE compositions. These concentrations are further distinguished by the lipoproteins that are bound and unbound to CETP. Finally, the model also includes the concentrations of CETP and the CE and TG associated with the CETP particles.The model includes the simultaneous binding and exchange between CETP and the different lipoprotein species as individual lipoproteins bind to CETP molecules, exchange lipids, and dissociate. For example, consider the in vitro incubation of LDL, VLDL, and lipoprotein-free plasma (a source of CETP) for 18 h, as described in (37Deckelbaum R.J. Eisenberg S. Oschry Y. Butbul E. Sharon I. Olivecrona T. Reversible modification of human plasma low density lipoproteins toward triglyceride-rich precursors. A mechanism for losing excess cholesterol esters.J. Biol. Chem. 1982; 257: 6509-6517Abstract Full Text PDF PubMed Google Scholar) and Experiment 1 of Table 1. In this case, the following simultaneous activities are modeled:TABLE 1Experimental data used in model calibrationExper.IncubationMeasurementReference1VLDL, LDL, LFP for 18 hLDL TG:CE ratio over time(37Deckelbaum R.J. Eisenberg S. Oschry Y. Butbul E. Sharon I. Olivecrona T. Reversible modification of human plasma low density lipoproteins toward triglyceride-rich precursors. A mechanism for losing excess cholesterol esters.J. Biol. Chem. 1982; 257: 6509-6517Abstract Full Text PDF PubMed Google Scholar)2VLDL, LDL, LFP for 18 hFinal weight ratios of CE and TG in VLDL and LDL(37Deckelbaum R.J. Eisenberg S. Oschry Y. Butbul E. Sharon I. Olivecrona T. Reversible modification of human plasma low density lipoproteins toward triglyceride-rich precursors. A mechanism for losing excess cholesterol esters.J. Biol. Chem. 1982; 257: 6509-6517Abstract Full Text PDF PubMed Google Scholar)3VLDL, LDL, LFP for 18 hFinal LDL TG:CE ratio with respect to different initial VLDL:LDL protein ratios(37Deckelbaum R.J. Eisenberg S. Oschry Y. Butbul E. Sharon I. Olivecrona T. Reversible modification of human plasma low density lipoproteins toward triglyceride-rich precursors. A mechanism for losing excess cholesterol esters.J. Biol. Chem. 1982; 257: 6509-6517Abstract Full Text PDF PubMed Google Scholar)4VLDL, HDL2, LFP for 18 hFinal HDL2 TG:CE ratio with respect to different initial VLDL-TG:HDL2-CE ratios(40Deckelbaum R.J. Eisenberg S. Oschry Y. Granot E. Sharon I. Bengtsson-Olivecrona G. Conversion of human plasma high density lipoprotein-2 to high density lipoprotein-3. Roles of neutral lipid exchange and triglyceride lipases.J. Biol. Chem. 1986; 261: 5201-5208Abstract Full Text PDF PubMed Google Scholar)5VLDL, HDL3, LFP for 18 hFinal HDL3 TG:CE ratio with respect to different initial VLDL-TG:HDL3-CE ratios(40Deckelbaum R.J. Eisenberg S