Binding of human apolipoprotein A-IV to human hepatocellular plasma membranes

We have investigated the binding of human apolipoprotein A-IV (apo A-IV) to human hepatocellular plasma membranes. Addition of increasing concentrations of radiolabeled apo A-IV to hepatic plasma membranes, in the presence and absence of a 25-fold excess of unlabeled apo A-IV, revealed saturation binding to the membranes with a KD of 154 nM and a binding maximum of 1.6 ng/microgram of membrane protein. The binding was temperature-insensitive, partially calcium-dependent, abolished when apo A-IV was denatured by guanidine hydrochloride or when the membranes were treated with Pronase and decreased when apo A-IV was incorporated into phospholipid/cholesterol proteoliposomes. In displacement studies using purified apolipoproteins and isolated lipoproteins, only unlabeled apo A-IV, apo A-I and high-density lipoproteins effectively competed with radiolabeled apo A-IV for membrane binding sites. We conclude that human apo A-IV exhibits high-affinity binding to isolated human hepatocellular plasma membranes which is saturable, reversible and specific.

Effects of phospholipid on the structure of human apolipoprotein A-IV

We have used fluorescence and circular dichroism spectroscopy to investigate the effect of phospholipid on the structure and molecular stability of human apolipoprotein A-IV (apo-A-IV). Binding of apo-A-IV to egg phosphatidylcholine vesicles was rapid and did not cause release of encapsulated 6-carboxyfluorescein. Fluorometric titration established that apo-A-IV bound to the vesicles with an association constant of 1.36 x 10(6) liters/mol and a binding maximum of 2 molecules per vesicle. Binding of apo-A-IV to the vesicle surface caused a progressive increase in alpha helicity from 43% at baseline to 83% at saturation; denaturation studies showed that the free energy of stabilization of binding was 6.31 kcal/mol. Fluorescence quenching studies revealed that binding of apo-A-IV to the vesicles was associated with a dramatic decrease in the fractional exposure of tyrosine to iodide, and a decrease in the efficiency of intramolecular tyrosine-tryptophan energy transfer. These findings suggest that the binding of apo-A-IV to the vesicle surface may involve a relaxation of the globular protein conformation in which the tyrosine containing alpha-helical domains surrounding the tryptophan "unfold" and reorient their hydrophobic faces toward the phospholipid monolayer, with a consequent induction of additional alpha-helical structure. However, our data also suggest that apo-A-IV does not penetrate deeply into the region of the phospholipid fatty acyl chains, but rather sits higher in the monolayer, intercalated between the charged phospholipid head groups. This characteristic may determine the labile interaction of apo-A-IV with high density lipoproteins.

Sensitivity of serum apolipoprotein A-IV levels to changes in dietary fat content

The distinctive biological properties of apolipoprotein A-IV suggest that its concentration in serum should be specifically regulated by dietary triglyceride. To test this hypothesis, serum lipids, lipoprotein cholesterol, and apolipoprotein levels were measured in 10 normolipidemic male subjects whose dietary fat intake was isocalorically modified over a range of 10%-50% of total calories. Serum apolipoprotein A-IV levels changed significantly from baseline in response to each dietary modification. Apolipoprotein A-IV levels decreased by 21% during the first week of the low-fat (10%) diet, increased to 12% over baseline during the first week of the moderate-fat (25%) diet, and increased further to 35% over baseline during the first week of the high-fat (50%) diet. Unexpectedly, during the second week of each dietary period, apolipoprotein A-IV levels demonstrated statistically significant trends back toward baseline, which were opposite in direction and proportional in magnitude to the changes during the first week. Nonetheless, serum apolipoprotein A-IV levels, but not apolipoprotein A-I or apolipoprotein B levels, were significantly and positively correlated with the percent of total daily caloric intake ingested as dietary fat. It was concluded that serum apolipoprotein A-IV levels are extremely sensitive to acute changes in dietary fat content and over the range of fat intake examined in this study are significantly correlated with the percent of total calories consumed as dietary triglyceride. Furthermore, the results also suggest that apolipoprotein A-IV may be subject to a rapidly acting autoregulatory mechanism.

Lipid abnormalities in cyclosporine-prednisone-treated renal transplant recipients

Hyperlipidemia and hypertension, two major risk factors for accelerated atherosclerosis, undoubtedly contribute to the excessive cardiovascular morbidity and mortality experienced by renal transplant recipients. The present survey of posttransplant hyperlipidemia in 500 cyclosporine-treated patients documented a 37.6% incidence of hypercholesterolemia, which occurred within 6 months posttransplant in 82% of patients. An etiologic relation to corticosteroid therapy was suggested by the strong correlation between prednisone doses and cholesterol levels, by the reduced cholesterol levels in patients undergoing steroid withdrawal, and by the reduction in hypercholesterolemia to 13% by 3 years posttransplant when steroid doses were less than 10 mg daily. Hypertriglyceridemia, which was present in 14.7% of the patients, was more severe under CsA-prednisone compared with azathioprine-prednisone therapy. Hypertriglyceridemia, which occurred later in the posttransplant course than hypercholesterolemia, strongly correlated with an excessive percent relative weight and elevated serum creatinine but not with steroid or CsA doses. Increasing age, diabetes mellitus, beta-blockers and nephrotic syndrome contribute to posttransplant hyperlipidemia in the CsA-Pred era as they did in the azathioprine era of immunosuppression.

Short-term parenteral nutrition with glucose and Intralipid: effects on serum lipids and lipoproteins

We prospectively investigated the effect of a 3-wk course of parenteral nutrition with 20% glucose, 4.25% amino acids, and 10% Intralipid on plasma lipids and lipoproteins in a cohort of 12 nutritionally replete patients with inflammatory bowel disease. Mean total serum cholesterol and mean total serum phospholipids increased in parallel throughout the study; density gradient ultracentrifugation demonstrated these increases were due to the appearance of lipoprotein-X. The mean low-density lipoprotein (LDL) mass on the density gradients decreased during the study and high-density lipoprotein (HDL) cholesterol levels decreased by 28% by the end of the third week. Plasma free fatty acids decreased by 42%. These results demonstrate that the glucose and 10% Intralipid regimen caused modest decreases in serum HDL cholesterol and LDL mass and the prompt appearance of lipoprotein-X with attendant hypercholesterolemia and hyperphospholipidemia. Patients undergoing long-term treatment with glucose and Intralipid should be closely monitored for the occurrence of significant dyslipoproteinemia.

Serum apolipoprotein A-IV and lipoprotein cholesterol in patients undergoing total parenteral nutrition

The distinctive biological properties of apolipoprotein (apo) A-IV suggest that serum levels of this apoprotein should be profoundly and selectively depressed by nutritional modalities that totally bypass the intestine. To test this hypothesis we have measured serum lipid and apoprotein levels in 18 fasting patients receiving total parenteral nutrition and in a group of 31 normal controls. Measurement of total serum lipids revealed that the patients had significantly higher serum triglyceride levels and lower total serum cholesterol levels than the controls. The lower total serum cholesterol levels were a consequence of decreases in both low-density lipoprotein and high-density lipoprotein cholesterol. The mean serum level of apo A-IV in the patients was 15 +/- 13 AU/dl, compared with 93 +/- 29 AU/dl in the controls. This difference was almost threefold greater than the differences between levels of apo A-I, apo B, or lipoprotein subfractions. Moreover, apo A-IV was undetectable in 4 patients who had undergone significant small intestinal resection. A trend toward lower apo A-IV levels was seen in the resected patients as compared with those with intact small intestine. Linear regression analysis showed that levels of apo A-IV were not correlated with any apoprotein or lipoprotein parameter in either the patient or control group, with the exception of a positive correlation between serum apo A-IV levels and total serum cholesterol in the controls only, suggesting that apo A-IV synthesis is regulated independently of the synthesis of the major classes of plasma lipoproteins. We conclude that serum apo A-IV levels are especially sensitive to the interruption of enteral feeding and rapidly fall to very low values during prolonged fasting.

Exposure and electronic interaction of tyrosine and tryptophan residues in human apolipoprotein A-IV

We have investigated the exposure and electronic interaction of tyrosine and tryptophan residues in human apolipoprotein A-IV (apo A-IV). Differential absorption spectroscopy and chemical titration demonstrated that human apo A-IV contains six tyrosine residues, four of which are buried in the hydrophobic interior of the protein and two of which are exposed on the protein surface. Denaturation of the protein by guanidinium chloride caused progressive exposure of the buried tyrosines. The fluorescence emission spectra of apo A-IV were characterized by a blue-shifted tryptophan emission with a low relative quantum yield of 0.37 and a tyrosine emission with a relative quantum yield of 0.62. Fluorescence quenching studies demonstrated a low fractional exposure of tryptophan in the native state. Denaturation of apo A-IV was accompanied by an increase in the relative quantum yield which peaked at the denaturation midpoint. Fluorescence excitation techniques demonstrated energy transfer from tyrosine residues with a transfer efficiency of 0.40 in the native state; the efficiency was conformation dependent and decreased with protein unfolding. Fluorescence studies of tetranitromethane-modified apo A-IV suggested that a significant fraction of energy transfer proceeds from the exposed tyrosine residues. These data demonstrate the existence of intramolecular fluorescence energy transfer and tryptophan quenching in human apolipoprotein A-IV and suggest that the amino terminus of this protein is situated in a hydrophobic domain within energy-transfer range of nonvicinal tyrosine residues.

Differences in the hydrophobic properties of discrete alpha-helical domains of rat and human apolipoprotein A-IV

The primary structure of human apolipoprotein A-IV has been analyzed to identify amphipathic alpha-helical regions and compare their hydrophobic properties with analogous segments of rat apolipoprotein A-IV. Significant differences were found in the mean hydrophobicity and/or mean helical hydrophobic moment of several discrete alpha-helical domains of rat and human apolipoprotein A-IV, which may determine the differences in their biophysical and biological behavior.

Lipoprotein affinity of human apolipoprotein A-IV during cholesterol esterification

We have studied the lipoprotein distribution of human apo A-IV during cholesterol esterification by the action of endogenous lecithin-cholesterol acyltransferase. Using immunologic and radiotracer techniques at 4 degrees C, apo A-IV was found in two discrete monomeric and dimeric populations, unassociated with plasma lipoproteins. With incubation at 37 degrees C, apo A-IV initially associated with the high density lipoprotein-3 fraction, but thereafter dissociated from its surface, and reappeared as unbound protein and in association with a complex in the low density lipoprotein size range. Inclusion of LCAT inhibitors in the incubations abolished these changes. We conclude that the changes in lipoprotein distribution of human apo A-IV closely parallel the formation and exchange of plasma cholesteryl esters.

The self-association of human apolipoprotein A-IV. Evidence for an in vivo circulating dimeric form

We have investigated the self-association properties of human apolipoprotein A-IV using several complementary physical techniques. Sedimentation equilibrium analysis demonstrated that human apolipoprotein A-IV formed oligomeric species in aqueous solution at physiologic pH. Computer analysis established that the best model of self-association is a monomer-dimer-tetramer scheme, with an unusually large monomer-dimer association constant of 2.9 X 10(5) liters/mol. Fluorescence spectroscopy and electrophoretic analysis demonstrated that the rate of monomer-oligomer interconversion is sufficiently slow that a stable population of dimeric protein exists in solution, even at low total protein concentrations, and that the extent of dimerization is minimally influenced by pH. Moreover, these techniques established that the dissociation of oligomeric forms and the unfolding of the monomeric form are discrete and sequential events. In experiments where apolipoprotein A-IV was incubated with human high density lipoproteins, fractionated by gradient gel electrophoresis, and localized by immunoblotting, dimer formation occurred, but very little binding to lipoproteins was observed. Immunoblots of human serum fractionated on acrylamide gradient gels and isopycnic density gradients demonstrated an apolipoprotein A-IV band of size and density consistent with a circulating dimeric form, unassociated with lipid. We conclude that human apolipoprotein A-IV undergoes high affinity self-association in aqueous solutions, and that such self-association likely occurs in vivo. Self-association may thus be important in determining the biologic behavior of human apolipoprotein A-IV by influencing both the kinetics and distribution of its association with plasma lipoproteins.

Isoform heterogeneity and lipid affinity of human lymph and plasma apolipoprotein A-IV

We have compared the physical properties and lipid affinity of apolipoprotein A-IV isolated from lymph chylomicrons and from lipoprotein-depleted plasma. Lymph and plasma apolipoprotein A-IV demonstrated distinctly different charge properties as assessed by anion exchange chromatography and isoelectric focusing. These differences were not attributable to disparities of amino acid or sialic acid content. Lymph apolipoprotein A-IV displayed a significantly higher affinity than plasma apolipoprotein A-IV for particles of a phospholipid-triglyceride emulsion. We conclude that the charge properties of human lymph and plasma apolipoprotein A-IV may determine conformational states which alter its ability to bind to the surface of lipid particles.

Structural properties and lipid binding of human apolipoprotein A-IV

The in vivo affinity of human apolipoprotein A-IV (apo-A-IV) for plasma lipoproteins is considerably less than that of other apolipoproteins. We have therefore studied its spectroscopic properties and its association with model chylomicrons to investigate its structural characteristics and to define their influence upon its affinity for lipids. Fluorescence emission spectra of apo-A-IV in dilute aqueous solution revealed that its single tryptophan residue resides in a pH-sensitive hydrophobic domain, which is maximally protected from iodide quenching at pH 7.5. Denaturation of apo-A-IV by guanidine hydrochloride caused a multiphasic fluorescence emission red shift, with an unusual enhancement of quantum yield. Circular dichroism spectroscopy of apo-A-IV demonstrated negative ellipticity maxima at 210 and 222 nm, consistent with 54% alpha-helical structure. The alpha-helicity of apo-A-IV as measured by [theta]222 was also pH-sensitive and displayed a distinctive decrease between pH 7.0 and 8.0. Apo-A-IV was exquisitely sensitive to denaturation by guanidine hydrochloride, and its estimated free energy of stabilization in aqueous solution was near zero. Apo-A-IV bound to the surface of Sf greater than 400 particles of a phospholipid-triglyceride emulsion in a noncooperative, concentration-dependent manner. The affinity of apo-A-IV for these model chylomicrons was influenced by changes in pH or addition of guanidine hydrochloride in a manner which correlated well with the structural changes observed under similar conditions. We conclude that human apolipoprotein A-IV possesses several biophysical properties characteristic of the better studied plasma apolipoproteins, yet, apo-A-IV appears to be marginally stable in aqueous solution and its structural characteristics and lipid binding properties are particularly sensitive to environment.

Human apolipoprotein A-IV: displacement from the surface of triglyceride-rich particles by HDL2-associated C-apoproteins

Human apolipoprotein A-IV rapidly dissociates from the surface of lymph chylomicrons following their entry into circulation by an unknown mechanism. We have therefore investigated the binding of human apoA-IV to triglyceride-rich particles and the interaction of these apoA-IV/lipid complexes with human HDL2. Human apoA-IV was purified from lipoprotein depleted serum (J. Lipid Res. 1983. 24:52-59). Triglyceride-rich particles of well-defined properties were isolated from Intralipid, a commercially available phospholipid-triglyceride emulsion. Various concentrations of radiolabeled human apoA-IV were incubated at 24 degrees C with a fixed quantity of lipid particles; the particles were reisolated by centrifugation, and bound and free apoA-IV were quantitated. In 50 mM Tris, pH 7.4, apoA-IV bound to the triglyceride-rich particles in a non-cooperative manner, with a Kd of 2.0 microM. The calculated maximal binding was 4.96 X 10(-4) mol of apoA-IV bound per mol of phospholipid. The addition of increasing amounts of human HDL2 to the incubations caused the progressive dissociation of apoA-IV from the triglyceride-rich particles. Analysis of the reisolated particles by isoelectric focusing demonstrated the presence of C-apoproteins, suggesting their transfer from HDL2. Addition of purified apoC-III-1 to the incubations at concentrations equivalent to those present in HDL2 caused a similar dissociation of apoA-IV. HDL2 was modified to selectively remove C-apoproteins, without alteration of other physical characteristics. This modified HDL2 was four times less effective in causing apoA-IV dissociation. These results demonstrate that the lipid binding properties of human apoA-IV may be quantitatively examined using triglyceride-rich particles as model chylomicrons. This approach reproduces in vitro the dissociation of apoA-IV that occurs in vivo when mesenteric lymph chylomicrons enter the circulation, and suggests that the primary mechanism for this phenomenon is the transfer of C-apoproteins from high density lipoproteins to the triglyceride-rich particle surface. We hypothesize that this mechanism may play an important role in the modulation of chylomicron apoA-IV content in man.

Changes in plasma apolipoproteins A-I, A-II, and B, and lipoprotein cholesterol after jejunoileal bypass

In view of the role of the small intestine in lipoprotein synthesis and bile salt reabsorption, we have determined plasma lipid fractions and apolipoproteins A-I, A-II, and B at regular intervals in 26 patients who underwent jejunoileal bypass surgery for morbid obesity. After surgery, no change in plasma triglycerides was noted, but there was a prompt and significant fall in total plasma cholesterol and low density lipoprotein cholesterol, which persisted beyond 2 yr. Plasma apolipoprotein B levels did not fall concurrently, and hence the protein/lipid composition of low density lipoprotein was permanently altered by the surgery. Levels of high density lipoprotein cholesterol were low before bypass surgery, decreased after surgery, and rose thereafter toward the normal range. Similar changes in apolipoprotein A-I and A-II values were noted. These postoperative alterations in plasma lipid and apoprotein levels demonstrate the importance of intestinal absorptive integrity in the regulation of lipoprotein metabolism.

Isolation and characterization of human apolipoprotein A-IV from lipoprotein-depleted serum

Human apolipoprotein A-IV is an acidic polypeptide of molecular weight 46,000 that is secreted into lymph on the surface of nascent chylomicrons, but which exists in circulation unassociated with lipoproteins. Previous studies of this protein have utilized material isolated from a d < 1.006 g/ml fraction of human serum and from human lymph. Although it has been suggested that apoA-IV circulating in plasma is the product of dissociation from the surface of nascent chylomicrons, it has been characterized by immunological techniques only. We have isolated human apoA-IV on a preparative scale from lipoprotein-depleted serum using a technique of adsorption to a phospholipid-triglyceride emulsion, followed by delipidation and preparative gel electrophoresis. The molecular weight, pI, and amino acid composition of material thus prepared agree with literature values for apoA-IV derived from chylomicrons. We have determined that apoA-IV is a glycoprotein containing 6% carbohydrate by weight (mannose 1.8%, galactose 1.55%, N-acetyl glucosamine 1.55%, sialic acid 1.1%). Electroimmunoassay of human serum using a monospecific rabbit antibody to serum-derived apoA-IV found 13.1 +/- 1.8 mg/dl, a value in agreement with determinations using antibodies to chylomicron-derived apoA-IV. We conclude that apoA-IV may be easily purified from normal human serum, and that the material thus isolated has identical chemical, physical, and immunological properties to apoA-IV obtained from human lymph. It is therefore likely that the dissociation of apoA-IV from the surface of nascent chylomicrons following their entry into circulation is not attended by changes in the structure or composition of this apoprotein.-Weinberg, R. B., and A. M. Scanu. Isolation and characterization of human apolipoprotein A-IV from lipoprotein-depleted serum.

Treatment of hyperlipidemic pancreatitis in pregnancy with total parenteral nutrition

A 29-yr-old pregnant woman presented in her third trimester with severe pancreatitis, hyperlipidemia, and small-for-dates fetus. Studies suggested that her pancreatitis was caused by profound hypertriglyceridemia, which was the result of an underlying lipoprotein disorder exacerbated by pregnancy. Throughout the first 7 wk of hospitalization, attempts to refeed the patient with solid food and various elemental diets resulted in the induction of hypertriglyceridemia and relapses of pancreatitis. Concern for the nutritional status of the mother and the possibility of further growth retardation of the fetus prompted the use of total parenteral nutrition for the last 2 wk of gestation. This treatment was well tolerated by the mother, promptly reversed maternal weight loss, caused a rapid resolution of her pancreatitis, and did not induce further hypertriglyceridemia. This report documents that total parenteral nutrition may be safely and effectively used in the management of hyperlipidemic pancreatitis in pregnancy.