Differential mechanisms of retinoid transfer from cellular retinol binding proteins types I and II to phospholipid membranes

Cellular retinol-binding proteins types I and II (CRBP-I and CRBP-II) are known to differentially facilitate retinoid metabolism by several membrane-associated enzymes. The mechanism of ligand transfer to phospholipid small unilamellar vesicles was compared in order to determine whether differences in ligand trafficking properties could underlie these functional differences. Unidirectional transfer of retinol from the CRBPs to membranes was monitored by following the increase in intrinsic protein fluorescence that occurs upon ligand dissociation. The results showed that ligand transfer of retinol from CRBP-I was >5-fold faster than transfer from CRBP-II. For both proteins, transfer of the other naturally occurring retinoid, retinaldehyde, was 4-5-fold faster than transfer of retinol. Rates of ligand transfer from CRBP-I to small unilamellar vesicles increased with increasing concentration of acceptor membrane and with the incorporation of the anionic lipids cardiolipin or phosphatidylserine into membranes. In contrast, transfer from CRBP-II was unaffected by either membrane concentration or composition. Preincubation of anionic vesicles with CRBP-I was able to prevent cytochrome c, a peripheral membrane protein, from binding, whereas CRBP-II was ineffective. In addition, monolayer exclusion experiments demonstrated differences in the rate and magnitude of the CRBP interactions with phospholipid membranes. These results suggest that the mechanisms of ligand transfer from CRBP-I and CRBP-II to membranes are markedly different as follows: transfer from CRBP-I may involve and require effective collisional interactions with membranes, whereas a diffusional process primarily mediates transfer from CRBP-II. These differences may help account for their distinct functional roles in the modulation of intracellular retinoid metabolism.

Expression and conservation of apolipoprotein AIV in an avian species

In birds, intestinally derived lipoproteins are thought to be secreted directly into the portal vein rather than to enter the circulation via the lymphatic system as in mammals. Hepatic clearance of these so-called portomicrons must be rapid, but the protein(s) mediating their catabolism, presumably analogues of the 36-kDa mammalian apolipoprotein E, have not been identified. In searching for such a mediator(s), we have isolated a hitherto unknown 38-kDa protein from chicken serum, which we identified by microsequencing and molecular cloning as a counterpart to mammalian apolipoprotein AIV (apoAIV). Mature chicken apoAIV consists of 347 amino acids, lacks cysteine residues, and displays 57% sequence identity with human apoAIV and, to a significantly lesser extent, with apoAIVs of rodents. This first nonmammalian apoAIV characterized is the smallest homologue reported so far, because of the lack of repeated motifs at the carboxyl terminus with the consensus sequence Glu-Gln-Glu/Ala-Gln, a hallmark of mammalian apoAIVs. Chicken apoAIV (isoelectric point, 4.65) is also considerably more acidic than its human counterpart. Agarose gel electrophoresis revealed that unlike human apoAIV, which migrates to a pre-alpha-position, chicken apoAIV shows fast alpha migration. Functional characterization demonstrated that the avian protein is able to activate the enzyme lecithin:cholesterol acyltransferase. Roosters and hens express apoAIV predominantly in the gut, one-fifth as much in the liver, and no other sites of expression are identifiable by Northern blot analysis. Although pronounced intestinal synthesis is common to apoAIVs, the features of the avian protein support the notion that it represents a prototype of an apoprotein that evolved to acquire possibly distinct functions in mammals and birds.

Bioavailability and efficacy of omeprazole given orally and by nasogastric tube

We compared the bioavailability and the efficacy of omeprazole provided either as encapsulated enteric-coated granules or as enteric-coated granules delivered via a nasogastric tube in 10 healthy subjects. Omeprazole reduced mean pentagastrin-stimulated peak gastric acid secretion by 85.5% +/- 23.7% when delivered orally and by 79.6% +/- 32.1% when delivered by nasogastric tube; the mean plasma omeprazole concentration area under the curve (AUC) was 2.02 +/- 0.79 after oral delivery and 1.74 +/- 1.89 after nasogastric tube delivery. There was no significant difference in these parameters between the two routes of administration, and there was excellent intrasubject correlating between oral and nasogastric percent acid suppression and AUC. There was a close correlation between AUC and percent acid suppression at AUC values below 0.6, and complete acid suppression at AUC values above 0.6, regardless of the delivery route. We conclude that omeprazole delivered as enteric-coated granules via nasogastric tube provides equal bioavailability and gastric acid suppression as omeprazole given orally in its proprietary formulation.

Effect of interfacial pressure on the binding and phospholipase A2 activity of recombinant human lecithin-cholesterol acyltransferase

We examined the effect of surface pressure on the interfacial binding and phospholipase A2 activity of lecithin-cholesterol acyltransferase. The enzyme bound to phosphatidylcholine monolayers with an apparent dissociation constant of 1.5 nM was excluded from the interface at pressures > 29 mN/m and exhibited maximal phospholipase activity at pressures between 29-28 mN/m. These data suggest that lipoprotein surface pressure may regulate lecithin-cholesterol acyltransferase activity in vivo.

Identification of functional domains in the plasma apolipoproteins by analysis of inter-species sequence variability

Molecular evolution theory posits that sequence motifs essential for protein function are constrained by selective pressure from changing over long stretches of evolutionary time. Thus, analysis of inter-species amino acid sequence variability, by identifying highly conserved intervals, should predict the location of domains critical for protein function. We have analyzed the amino acid sequences of the mammalian apolipoproteins A-I, A-IV, C-I, C-II, C-III, D, and E with a computer algorithm that calculates numerical residue variability scores. The application of a median sieve filter to the data facilitated identification of the exact boundaries of highly conserved domains, which coincided with the location of known structural features and functional domains in this family of proteins. The analysis also identified highly conserved intervals in every apolipoprotein whose function is unknown at present, but which are candidates for regions with specific functional roles.

Interfacial properties of recombinant human cholesterol ester transfer protein

We investigated the interfacial behavior of recombinant human cholesterol ester transfer protein (rCETP) using monolayer and surface balance techniques. rCETP bound to egg phosphatidylcholine monolayers spread at the air/water interface with a maximum surface pressure of 23 millinewtons (mN)/m at subphase concentrations between 3 and 5 x 10(-5) g/dl; the estimated dissociation constant was 7.5 x 10(-6) g/dl or 1 nM. The binding of rCETP to the lipid interface decreased linearly with increasing initial surface pressure; rCETP was excluded at pressures greater than 31 mN/m. rCETP catalyzed the desorption of [14C]cholesterol oleate from mixed lipid monolayers in a concentration dependent fashion. Similar studies with apolipoproteins A-I and A-IV established that cholesterol ester desorption was not caused by changes in surface pressure or cholesterol ester solubility. The desorption rate was proportional to subphase rCETP concentration, but at all concentrations surface radioactivity remained constant until surface pressure reached a plateau. The calculated binding stochiometry was one molecule of cholesterol ester desorbed for every 1000 molecules of rCETP in the subphase. We conclude that rCETP is surface active, binds to phospholipid monolayers with an affinity equivalent to that of the plasma apolipoproteins, and effects the desorption of cholesterol ester molecules from phospholipid monolayers by a carrier mechanism. Moreover, the relatively low equilibrium surface pressure of rCETP suggests that when bound to lipid the entire rCETP molecule may not penetrate the interface.

Attenuated hypercholesterolemic response to a high-cholesterol diet in subjects heterozygous for the apolipoprotein A-IV-2 allele

BACKGROUND

Previous studies have suggested that the variant apolipoprotein (apo) allele apo A-IV-2 may influence the response of the plasma cholesterol concentration to dietary cholesterol.

METHODS

We measured plasma lipids and lipoproteins in 11 subjects who were heterozygous for the apo A-IV-2 allele (apo A-IV-1/2 heterozygotes) and a control group of 12 subjects who were homozygous for the common apo A-IV allele (apo A-IV-1/1 homozygotes) in an outpatient dietary-modification study. (Approximately one in seven persons in the United States is a heterozygote.) The subjects consumed a low-cholesterol diet (about 200 mg [0.5 mmol] of cholesterol per day) during a two-week run-in period; daily cholesterol intake was then increased to approximately 1100 mg (2.8 mmol) by the addition of four egg yolks per day.

RESULTS

The fat intake and the ratio of polyunsaturated to saturated fat were similar in the two groups throughout the study. After three weeks of egg intake, the mean plasma total cholesterol increased by 22 mg per deciliter (0.57 mmol per liter) in the apo A-IV-1/1 group, but by only 6 mg per deciliter (0.15 mmol per liter) in the apo A-IV-1/2 group (P = 0.05). The mean plasma low-density lipoprotein cholesterol increased by 19 mg per deciliter (0.49 mmol per liter) in the apo A-IV-1/1 group, but by only 1 mg per deciliter (0.03 mmol per liter) in the apo A-IV-1/2 group (P = 0.03). There were no changes in the plasma triglyceride or high-density lipoprotein cholesterol concentrations in either group.

CONCLUSIONS

The apo A-IV-2 allele attenuates the hypercholesterolemic response to the short-term ingestion of a very-high-cholesterol diet and may partially account for the heterogeneous response to dietary cholesterol. However, cholesterol intake in this study was more than twice that of the general population; whether the apo A-IV-2 allele alters responsiveness at lower levels of cholesterol intake remains to be determined.

Efflux of lipid from fibroblasts to apolipoproteins: dependence on elevated levels of cellular unesterified cholesterol

Earlier work from this laboratory showed that enrichment of cells with free cholesterol enhanced the efflux of phospholipid to lipoprotein acceptors, suggesting that cellular phospholipid may contribute to high density lipoprotein (HDL) structure and the removal of sterol from cells. To test this hypothesis, we examined the efflux of [3H]cholesterol (FC) and [32P]phospholipid (PL) from control and cholesterol-enriched fibroblasts to delipidated apolipoproteins. The percentages of [3H]cholesterol and [32P]phospholipid released from control cells to human apolipoprotein A-I were 2.2 +/- 0.5%/24 h and 0.8 +/- 0.1%/24 h, respectively. When the cellular cholesterol content was doubled, efflux of both lipids increased substantially ([3H]FC efflux = 14.6 +/- 3.6%/24 h and [32P]PL efflux = 4.1 +/- 0.3%/24 h). Phosphatidylcholine accounted for 70% of the radiolabeled phospholipid released from cholesterol-enriched cells. The cholesterol to phospholipid molar ratio of the lipid released from cholesterol-enriched cells was approximately 1. This ratio remained constant throughout an incubation time of 3 to 48 h, suggesting that there was a coordinate release of both lipids. The concentrations of apoA-I, A-II, A-IV, E, and Cs that promoted half-maximal efflux of phospholipid from cholesterol-enriched fibroblasts were 53, 30, 68, 137, and 594 nM, respectively. With apoA-I and A-IV, these values for half-maximal efflux of phospholipid were identical to the concentrations that resulted in half-maximal efflux of cholesterol. Agarose gel electrophoresis of medium containing apoA-I that had been incubated with cholesterol-enriched fibroblasts revealed a particle with alpha to pre-beta mobility. We conclude that the cholesterol content of cellular membranes is an important determinant in the ability of apolipoproteins to promote lipid removal from cells. We speculate that apolipoproteins access cholesterol-phosphatidylcholine domains within the plasma membrane of cholesterol-enriched cells, whereupon HDL is generated in the extracellular compartment. The release of cellular lipid to apolipoproteins may serve as a protective mechanism against the potentially damaging effects of excess membrane cholesterol.

Decreased low-density lipoprotein receptor function and mRNA levels in lymphocytes from uremic patients

The mechanisms by which renal failure causes hyperlipoproteinemia remain unclear. To investigate the potential role of the low-density lipoprotein (LDL) receptor in lipoprotein metabolism in uremia we measured LDL receptor function in peripheral blood mononuclear cells (PBMC) from uremic patients and control subjects using a functional assay in which proliferation of lectin-stimulated PBMC in the presence of lovastatin was dependent upon internalization of exogenous cholesterol via a functional LDL receptor. The amount of LDL required to reverse 50% of lovastatin-induced inhibition of proliferation in PBMC from uremic patients was significantly greater (3.6 +/- 1.8 micrograms/ml, N = 33, P < 0.05) than controls, (1.99 +/- 0.6 micrograms/ml, N = 37). Abnormal LDL receptor function in four uremic patients normalized following renal transplantation. To investigate the molecular basis for LDL receptor dysfunction, we directly quantitated LDL receptor messenger RNA (mRNA) in PBMC from uremic patients and control subjects using a ribonuclease protection assay. LDL receptor mRNA expression in uremic patients was 0.42 +/- 0.08 (N = 10), significantly lower (P < 0.015) than in normal subjects, 0.71 +/- 0.08 (N = 14). These data suggest that an acquired defect in LDL receptor function in PBMC from uremic patients exists which may be due to decreased LDL receptor expression. These abnormalities, if present in other tissues, could contribute to the aberrant lipoprotein metabolism which is a consistent feature of uremia.

Adsorption of apolipoprotein A-IV to phospholipid monolayers spread at the air/water interface. A model for its labile binding to high density lipoproteins

The mechanisms that mediate the labile binding of apolipoprotein A-IV (apoA-IV) to high density lipoproteins (HDL) are not known. We therefore used a surface balance and surface radioactivity detector to investigate the adsorption of apoA-IV to egg phosphatidylcholine monolayers spread at the air/water interface. ApoA-IV bound rapidly and reversibly to phospholipid monolayers and generated a maximum increase in surface pressure of 19 millinewtons (mN)/m at a subphase concentration of 2 x 10(-5) g/dl. Binding decreased linearly with increasing initial surface pressure; at pressures greater than 28-29 mN/m, apoA-IV could no longer penetrate the lipid monolayer. The area occupied by the amino acid residues in apoA-IV reached an unusually low limiting molecular area of 10-12 A2/residue at surface saturation. The surface pressure of native HDL3 was calculated to be 33 mN/m, and it rapidly decreased with the action of lecithin:cholesterol acyltransferase on the particle surface. We conclude that the surface activity of apoA-IV is lower than that of any other human apolipoprotein; its binding and surface conformation are particularly sensitive to pressure; and at saturation, a significant portion of the molecule is excluded from the interface. The exclusion pressure of apoA-IV may be only slightly lower than the surface pressure of HDL; in vivo, the action of lecithin:cholesterol acyltransferase and lipid transfer proteins may cause the HDL3 surface pressure to oscillate about a narrow range that spans the exclusion pressure of apoA-IV. The resultant labile association of apoA-IV and HDL may be of central importance to its role in lipoprotein metabolism.

Distinctive structure and function of human apolipoprotein variant ApoA-IV-2

We have investigated the molecular structure, phospholipid binding, and lecithin-cholesterol acyltransferase catalytic activity of pure apoA-IV-2, a basic variant isoform of apoA-IV which is inherited as a classical Mendelian allele with a gene frequency of 0.09. Circular dichroism spectroscopy established that the alpha-helical content of apoA-IV-2 was 75% in the native state (versus 56% for apoA-IV-1), and increased to 88% in the presence of phospholipid. Fluorescence titration established that apoA-IV-2 bound to egg phospholipid vesicles with a Ka of 3.3 x 10(6) liter/mol, 2.4-fold greater than the affinity of apoA-IV-1. Fluorescence quenching studies revealed that, unlike apoA-IV-1, binding of apoA-IV-2 to phospholipid vesicles induced strong shielding of the amino-terminal tryptophan against iodide quenching. Enzyme kinetic studies using both saturated and unsaturated phospholipid substrates demonstrated that apoA-IV-2 was 36-71% more efficient in activating lecithin-cholesterol acyltransferase than apoA-IV-1. We conclude that apoA-IV-2 has more alpha-helical structure, is more stable in solution, and is more hydrophobic than apoA-IV-1, and that these distinctive structural features are associated with a higher affinity for phospholipid surfaces and an increased catalytic efficiency of lecithin:cholesterol acyltransferase activation. The biophysical basis for this latter characteristic may be the ability of apoA-IV-2 to penetrate phospholipid surfaces to a greater depth than apoA-IV-1. These molecular properties may be responsible for the increased levels of high density lipoproteins which have been observed in apoA-IV-2 heterozygotes.