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Richard E. Morton, Ph.D.Staff
Department of Cell Biology |
Lipid and lipoprotein metabolism, membrane structure-function relationships, lipid-protein interactions.
It is increasingly appreciated that the lipid constituents of human lipoproteins are not passive components, but are rapidly transferred between lipoproteins and other membrane structures. Whether this transfer occurs by simple diffusion through the aqueous space or whether it is protein-mediated depends on the nature of the lipid itself.
The transfer of the apolar components of lipoproteins, cholesteryl ester and triglyceride, is dependent on the action of a specific plasma protein called cholesteryl ester transfer protein (CETP). CETP can facilitate the net transfer of both triglyceride and cholesteryl ester between lipoproteins. Thus, this protein can play an important role in defining the lipid composition of plasma lipoproteins.
The major focus of our lab is to investigate the role of CETP in the intravascular and extravascular metabolism of plasma lipoproteins and lipids. Our current interests take several directions. First, we are investigating the mechanism of the transfer process itself. We have made significant progress in the past in defining the kinetics of binding between CETP and plasma lipoproteins. We have shown that lipid transfer requires the formation of a CETP-lipoprotein complex and that all plasma lipoproteins bind CETP with similar affinities but display markedly different binding capacities. By reconstitution techniques we are investigating how lipoprotein composition affects the binding event and how this may affect the association of CETP with different lipoprotein classes in vivo. Our ultimate goals are to define the "binding site" of CETP and to delineate the mechanism of lipid transfer.
A second major direction involves the regulation of CETP activity. Two approaches are being taken: regulation of CETP activity by the physicochemical properties of its lipoprotein substrates, and the regulation of the circulating CETP by other plasma proteins. The initial approach focuses on the ability of modified lipoprotein composition, induced by dietary factors or metabolic aberrations, to alter the rate and directionality of lipid transfer.. We have shown that the capacity of CETP to facilitate the mass transfer of cholesteryl ester is positively correlated with the unesterified cholesterol content of plasma lipoproteins, demonstrating that unesterified cholesterol stimulates the metabolic pathways that deliver tissue cholesterol to the liver where it can be excreted.
Recently, we have focused on the characterization of a novel protein in human plasma, designated LTIP, which suppresses CETP activity in vitro. We have demonstrated that LTIP is not simply a general suppressor of CETP activity, but that it preferentially inhibits CETP-mediated lipid transfers involving low-density lipoprotein. This results in a reduced capacity of the apolar lipids within the low-density lipoprotein pool to equilibrate with those in other lipoproteins. Therefore, LTIP plays a key role in defining the lipid transfer events that CETP can mediate in plasma. In general, LTIP appears to promote a pattern of lipid transfers that are considered to be anti-atherogenic. We have purified LTIP and cloned its cDNA. We are now staged to perform detailed kinetic and functional studies in order to define the role of LTIP in determining the composition and concentration of individual lipoprotein classes and subclasses.
We are also interested in defining the capacity of CETP to alter the accumulation or deposition of lipids within cells. These studies are an extension of our observations that CETP can promote the net removal of cholesteryl esters from lipid-loaded macrophages in culture, and that CETP expression is essential for normal cellular cholesterol homeostasis, suggesting novel roles for CETP in extravascular lipid metabolism.
Collectively, these studies should not only yield important information concerning the function and regulation of CETP, but should provide useful, additional insights into the mechanisms underlying the formation of putatively atherogenic lipoproteins and the cellular deposition of lipids leading to foam cell formation.
Wang, X, Driscoll, DM, Morton, RE: Molecular cloning and expression of lipid transfer inhibitor protein reveals its identity with apolipoprotein F. J. Biol. Chem. 274: 1814-1820, 1999.
Greene, DG, Skeggs, JW, Morton, RE: Elevated triglyceride content diminishes the capacity of HDL to deliver cholesteryl esters via the scavenger receptor, class B, type I (SR-BI). J. Biol. Chem. 276: 4804-4811, 2001.
Morton, RE, Nunes, V, Izem, L, Quintão, ECR: Markedly elevated lipid transfer inhibitor protein in hypercholesterolemic subjects is mitigated by plasma triglyceride levels. Arterioscler. Thromb. Vasc. Biol. 21: 1642-1649, 2001.
Izem, L, Morton, RE: Cholesteryl ester transfer protein biosynthesis and cellular cholesterol homeostasis are tightly interconnected. J. Biol. Chem. 276: 26534-26541, 2001.
Skeggs, JW, Morton, RE: LDL and HDL enriched in triglyceride promote abnormal cholesterol transport. J. Lipid Res. 43: 1264-1274, 2002.
Paromov, VM, Morton, RE: Lipid transfer inhibitor protein defines the participation of HDL subfractions in lipid transfer reactions mediated by CETP. J. Biol. Chem 278: 40859-40866, 2003.
Morton, RE, Greene, DJ: CETP and lipid transfer inhibitor protein are uniquely affected by the negative charge density of the lipid and protein domains of lipoproteins. J. Lipid Res. 44: 2287-2296, 2003.
Paromov VM, Morton RE. Lipid transfer inhibitor protein defines the participation of high density lipoprotein subfractions in lipid transfer reactions mediated by cholesterol ester transfer protein (CETP). J Biol Chem 2003;278:40859-66.
Izem L, Morton RE. Possible role for intracellular cholesteryl ester transfer protein in adipocyte lipid metabolism and storage. J Biol Chem 2007;282:21856-65.
Morton RE, Greene DJ. Partial suppression of CETP activity beneficially modifies the lipid transfer profile of plasma. Atherosclerosis 2007;192:100-7.