Effects of the Neutral Lipid Content of High Density Lipoprotein on Apolipoprotein A-I Structure and Particle Stability

Daniel L. Sparks, W. Sean Davidson, Sissel Lund-Katz, Michael C. Phillips
1995 Journal of Biological Chemistry  
Alterations in high density lipoprotein (HDL) composition that occur in dyslipidemic states may modulate a number of events involved in cholesterol homeostasis. To elucidate the details of how HDL-core composition can affect the molecular structure of different kinds of HDL particles, the conformation and stability of apoA-I have been investigated in homogeneous recombinant HDL particles (LpA-I) containing palmitoyloleoyl phosphatidylcholine (POPC), triolein (TG), and/or cholesteryl linoleate
more » ... E). In a discoidal particle containing two molecules of apoA-I and 85 molecules of POPC, apoA-I exhibits an ␣-helix content of 70% and a free energy of stability of its ␣-helical segments (⌬G D 0 ) of 2.2 kcal/mol. Inclusion of eight molecules of TG into the complex significantly reduces the ␣-helix content and stability of apoA-I, whereas inclusion of four molecules of CE into the complex has an opposite effect in that the ␣-helix content is significantly reduced and the stability of the remaining ␣-helical structure of apoA-I is increased. Neutral lipids have a different effect on apoA-I conformation in spherical LpA-I particles. In a sonicated-spherical LpA-I particle containing two molecules of apoA-I and 70 molecules of POPC, apoA-I exhibits an ␣-helix content of about 60% and a ⌬G D 0 of 1.2 kcal/mol apoA-I. Inclusion of either 10 molecules of TG or six molecules of CE into such a particle increases both the ␣-helix content and stability of apoA-I. Increasing the CE/TG ratio in LpA-I particles that contain both neutral lipids enhances the stability of the ␣-helical segments. ApoA-I molecules tend to dissociate and cause particle instability when ⌬G D 0 for the lipid-bound ␣-helices is less than that for helices in the lipid-free state. The stabilities of both discoidal and spherical LpA-I particles are relatively low when the only neutral lipid present is TG but the particle stability is enhanced by the presence of CE molecules. Such dissociation of apoA-I molecules from LpA-I particles that have a low CE/TG ratio would be promoted in the hypertriglyceridemic state in vivo. High density lipoproteins (HDL) 1 comprise a heterogeneous class of particles that contain apolipoprotein A-I (apoA-I) (LpA-I) or apoA-I and A-II (LpA-I,A-II) as their primary protein constituents (1). The central role that HDL plays in cholesterol metabolism is thought to involve the transport of cholesterol from peripheral tissues to the liver (2). Several studies have suggested that the efficiency of HDL in mediating this flux may be impaired in hypertriglyceridemic patients and that this effect may be related to modifications in HDL composition and size (for review, see Ref. 3). Investigations in a variety of laboratories have shown that changes in HDL size and composition can lead to altered interactions between HDL and lecithin:cholesterol acyltransferase (4 -6), cholesteryl ester transfer protein (7-9), and cell surfaces (10, 11). There is evidence that an increased number of small, neutral lipid-poor, HDL particles in hypertriglyceridemic patients may directly affect cholesterol transport by stimulating the production of cholesteryl ester within the HDL pool (12) and by promoting an enhanced transfer and potentially atherogenic accumulation of these lipids in apoB-containing lipoproteins (3, 13). Alterations in HDL composition also give rise to specific changes in the conformation and charge of the primary protein of HDL, apoA-I (14 -17), and it appears that these changes in the molecular properties of HDL closely correlate with the altered function of these lipoprotein particles (6, 14, 18, 19) . While a substantial amount of information about the organization of apoA-I molecules in discoidal particles is now available (for review, see Refs. 20 and 21), very little information exists that describes the structure of apoA-I in spherical HDL particles. In recent studies with reconstituted HDL particles (LpA-I), we showed that the overall conformation of human apoA-I is significantly different in spherical particles containing a cholesteryl ester core than in discoidal complexes that do not contain a neutral lipid core (16, 17) . These studies also showed that the surface charge and secondary structure of apoA-I are significantly different on spherical particles and appear to be modulated by changes in composition in a different manner than for apoA-I on discoidal LpA-I particles (17). In this study, we further show that the type and amount of neutral lipid in reconstituted discoidal and spherical LpA-I directly affects the surface charge and structural characteristics of the lipoprotein particles. The results indicate that cholesteryl ester and triglyceride have distinct effects on the physical properties of apoA-I that are specific to the kind, spherical or discoidal, of LpA-I particle. A decrease in the cholesteryl ester content in the LpA-I particles is associated with a reduction in the structural integrity of these lipoprotein structures. Such particles are very similar to the abnormal HDL particles found in the plasma of hypertriglyceridemic subjects (1, 3, 13), and it is probable that the unusual charge and structural characteristics of small, neutral lipid-poor, HDL give rise to differences in the functional properties of these lipoproteins.
doi:10.1074/jbc.270.45.26910 pmid:7592936 fatcat:vxuqa23uurfvzfvjox3ttubwny