Metabolism of ApoA-I as Lipid-Free Protein or as Component of Discoidal and Spherical Reconstituted HDLs

Abstract

Objective-Apolipoprotein (apo)A-I exists in 3 forms in plasma: as lipid-free apoA-I, as a component of pre-␤-migrating discoidal high density lipoproteins (HDLs), and as a component of ␣-migrating spherical HDLs. This study investigates (1) the in vivo metabolism of apoA-I in each of these forms and (2) the effects of hepatic lipase (HL) on apoA-I metabolism. Methods and Results-Wild-type and HL transgenic rabbits were studied. When lipid-free 125 I-apoA-I and 125 I-apoA-I in pre-␤-migrating discoidal reconstituted HDLs (rHDLs) were injected into wild-type rabbits, the label rapidly appeared in ␣-migrating particles and decayed with the same fractional catabolic rate (FCR) as when they were injected as a component of spherical rHDLs. Spherical rHDLs did not change in size when they were injected into wild-type rabbits but were reduced in size in HL transgenic rabbits. The FCR of apoA-I in HL transgenic rabbits was double that in wild-type rabbits. Conclusions-In vivo, (1) lipid-free apoA-I rapidly incorporates into preexisting ␣-migrating particles, (2) pre-␤-migrating discoidal HDLs are rapidly converted into ␣-migrating HDLs, (3) the FCR of apoA-I is independent of the form in which it is introduced into plasma, and (4) Key Words: apolipoprotein A-I Ⅲ hepatic lipase Ⅲ high density lipoproteins Ⅲ metabolism Ⅲ rabbits T he discovery that HDLs protect against the development of atherosclerosis 1,2 has stimulated a major interest in the factors that regulate these lipoproteins. This interest has applied particularly to the regulation of apoA-I, the main protein constituent of HDL.Most of the HDLs in normal plasma are spherical particles that exhibit ␣-migration when subjected to agarose gel electrophoresis. They consist of a surface monolayer of apolipoproteins, phospholipids, and unesterified cholesterol (UC) surrounding a core of cholesteryl esters (CEs) and triglycerides. However, there is the potential for apoA-I also to circulate in a lipid-free (or lipid-poor) form and as a component of pre-␤-migrating discoidal complexes containing apoA-I, phospholipid, and UC. 3 In vitro studies have shown that apoA-I cycles between the lipid-free and lipid-associated forms in processes mediated by lecithin:cholesterol acyltransferase (LCAT), CE transfer protein (CETP), phospholipid transfer protein, and hepatic lipase (HL). 4 It has also been shown in vitro that pre-␤-migrating discoidal HDLs are converted into ␣-migrating spherical particles by the action of LCAT. 5,6 In vitro studies have also shown that ␣-migrating HDLs are increased in size by LCAT and that they decrease in size when they interact with CETP, HL, and triglyceride-rich lipoproteins. 4 The reduction in size is accompanied by a dissociation of lipid-free apoA-I from the HDL. [7][8][9] Despite the abundant evidence that HDL remodeling occurs in vitro, its contribution to HDL metabolism in vivo remains unclear. Indeed, the fact that apoA-I exists predominantly in ␣-migrating spherical HDLs in the plasma of most species, including humans, raises questions a...

Preβ high density lipoprotein has two metabolic fates in human apolipoprotein A-I transgenic mice

“…3 and 4). Our results are consistent with those of other investigators who have observed rapid assimilation of synthetic pre␤ HDL particles into the medium HDL size range (53,58). However, in the previous studies, only homogeneous medium-sized HDL particles were present in mouse plasma, whereas in hA-I Tg mice, there are abundant large and small HDL particles (29,59).…”

Reconstituted High-Density Lipoproteins Inhibit the Acute Pro-Oxidant and Proinflammatory Vascular Changes Induced by a Periarterial Collar in Normocholesterolemic Rabbits

“…This amount, when given to a 3-kg rabbit (plasma volume approximately 120 mL), would have increased the plasma apoA-I concentration in the immediate postinfusion period by 0.2 mg/mL, an increase of only 40% over the preinfusion concentration of 0.5 mg/mL. Furthermore, with a fractional catabolic rate for rabbit plasma apoA-I of 0.8 pools/d, 44 it would be predicted (as observed) that the apoA-I concentration would have returned to preinfusion levels by the time of euthanasia 24 hours after the last infusion. Yet despite what was no more than a minor and transient increase in the concentration of plasma HDL, the infusions resulted in an almost complete inhibition of the collar-induced arterial inflammation.…”

Initial interaction of apoA-I with ABCA1 impacts in vivo metabolic fate of nascent HDL

“…There is also mounting evidence that assembly of most HDL particles occurs at the cell surface (45,(58)(59)(60)(61)(62)(63)(64) or in a recycling endosomal compartment by ABCA1 (65)(66)(67)(68)(69)(70) and not in the secretory pathway of the endoplasmic reticulum and Golgi apparatus (71). Because we have shown that poorly lipidated apoA-I (i.e., pre-b1 HDL) is rapidly catabolized by the kidney, whereas lipid-free apoA-I rapidly and quantitatively transfers to plasma HDLs (17,51), newly synthesized apoA-I from hepatocytes or intestinal epithelial cells has several metabolic fates. If apoA-I escapes lipidation in the secretory pathway and at the cell surface by ABCA1, it is likely to associate with mature HDL particles after entering the circulation (17,51).…”

“…Because we have shown that poorly lipidated apoA-I (i.e., pre-b1 HDL) is rapidly catabolized by the kidney, whereas lipid-free apoA-I rapidly and quantitatively transfers to plasma HDLs (17,51), newly synthesized apoA-I from hepatocytes or intestinal epithelial cells has several metabolic fates. If apoA-I escapes lipidation in the secretory pathway and at the cell surface by ABCA1, it is likely to associate with mature HDL particles after entering the circulation (17,51). If lipidation in the secretory pathway or at the cell surface by ABCA1 is sufficient to form pre-b2, -3, or -4 HDLs, these particles will undergo further lipidation by non-ABCA1-mediated pathways and ultimately be converted to mature plasma HDL particles by LCAT in plasma.…”