Autosomal dominant ApoA4 mutations present as tubulointerstitial kidney disease with medullary amyloidosis

Sporadic cases of apolipoprotein A-IV medullary amyloidosis have been reported. Here we describe five families found to have autosomal dominant medullary amyloidosis due to two different pathogenic APOA4 variants. A large family with autosomal dominant chronic kidney disease (CKD) and bland urinary sediment underwent whole genome sequencing with identification of a chr11:116692578 G>C (hg19) variant encoding the missense mutation p.L66V of the ApoA4 protein. We identified two other distantly related families from our registry with the same variant and two other distantly related families with a chr11:116693454 C>T (hg19) variant encoding the missense mutation p.D33N. Both mutations are unique to affected families, evolutionarily conserved and predicted to expand the amyloidogenic hotspot in the ApoA4 structure. Clinically affected individuals suffered from CKD with a bland urinary sediment and a mean age for kidney failure of 64.5 years. Genotyping identified 48 genetically affected individuals; 44 individuals had an estimated glomerular filtration rate (eGFR) under 60 ml/min/1.73 m2, including all 25 individuals with kidney failure. Significantly, 11 of 14 genetically unaffected individuals had an eGFR over 60 ml/min/1.73 m2. Fifteen genetically affected individuals presented with higher plasma ApoA4 concentrations. Kidney pathologic specimens from four individuals revealed amyloid deposits limited to the medulla, with the mutated ApoA4 identified by mass-spectrometry as the predominant amyloid constituent in all three available biopsies. Thus, ApoA4 mutations can cause autosomal dominant medullary amyloidosis, with marked amyloid deposition limited to the kidney medulla and presenting with autosomal dominant CKD with a bland urinary sediment. Diagnosis relies on a careful family history, APOA4 sequencing and pathologic studies.

Distinctive Clinical Correlates of Small Intestinal Bacterial Overgrowth with Methanogens

BACKGROUND AND AIMS

Most patients with small intestinal bacterial overgrowth (SIBO) produce hydrogen by fermentation of dietary carbohydrates; however, ∼30% of patients with SIBO are colonized with Archaea, anaerobic organisms that produce methane. SIBO is associated with a plethora of symptoms and conditions, but their diagnostic significance is unclear. We aimed to determine if specific symptoms and conditions are associated with methanogenic SIBO.

METHODS

This study received institutional review board approval (IRB00059873). In this retrospective cross-sectional study, we queried a database of glucose breath tests conducted for suspected SIBO at our tertiary care medical center, which included data on the presence or absence of gastrointestinal symptoms and conditions often associated with SIBO. All patients had undergone a standardized breath testing protocol.

RESULTS

In a cohort of 1461 patients, 33.1% were SIBO positive; of these, 49.8% produced only hydrogen, 38.8% produced only methane, and 11.4% produced both gases. The following factors distinguished patients with hydrogen-producing SIBO, but not methanogenic SIBO, from SIBO-negative patients: vitamin B₁₂ deficiency (odds ratio, 1.44; confidence interval [CI], 1.01-2.06; P = .046), Roux-en-Y gastric bypass (odds ratio, 2.14; CI, 1.09-4.18; P = .027), cholecystectomy (odds ratio, 1.42; CI, 1.06-1.91; P = .020), and diabetes (odds ratio, 1.59; CI, 1.13-2.24; P = .008). The absence of vitamin B₁₂ deficiency was the sole discriminating factor between methanogenic and hydrogenic SIBO (odds ratio, 0.57; CI, 0.34-0.97; P = .038).

CONCLUSIONS

Patients with SIBO caused by methane-producing Archaea display a different spectrum of associated symptoms and clinical conditions compared with patients with SIBO caused by hydrogen-producing bacteria, particularly a lower incidence of vitamin B₁₂ deficiency.

The Impact of a Mediterranean Diet on the Gut Microbiome in Healthy Human Subjects: A Pilot Study

BACKGROUND AND AIMS

Despite the reported salutary benefits of a Mediterranean diet (MD) on a wide variety of health conditions, the specific microbial changes associated with an MD within the gastrointestinal (GI) tract are not well studied. Specifically, although population and survey-based studies have shown microbial changes, there are no published data on how an MD alters the gut flora in a controlled setting.

METHODS

We recruited 10 healthy subjects, each of whom gave a stool sample at baseline and then was provided with prepared meals of a "typical" American diet; after 2 weeks, a second stool sample was collected. All subjects were then provided with prepared meals based on the MD for another 2 weeks, followed by a final stool sample collection. Stool samples were batch analyzed with DNA extraction, and sequencing libraries were generated. Measures of bacterial diversity, species richness, and enterotypes were performed.

RESULTS

All ten subjects tolerated the diets well. Bacterial diversity increased with an MD, as measured by alpha diversity via the Simpson index. Furthermore, there were significant differences in 5 bacterial genera between the 2 diets.

CONCLUSION

This small pilot study of controlled diets demonstrates that the MD can rapidly alter the gut microbiome in healthy subjects at the level of global microbial diversity and individual genera. These data confirm the findings of previous observational studies and establish the feasibility of conducting longer term studies on the impact of the MD on the flora of the GI tract and its relationship to digestive diseases.

Ensuring respect for persons in COMPASS: a cluster randomised pragmatic clinical trial

Cluster randomised clinical trials present unique challenges in meeting ethical obligations to those who are treated at a randomised site. Obtaining informed consent for research within the context of clinical care is one such challenge. In order to solve this problem it is important that an informed consent process be effective and efficient, and that it does not impede the research or the healthcare. The innovative approach to informed consent employed in the COMPASS study demonstrates the feasibility of upholding ethical standards without imposing undue burden on clinical workflows, staff members or patients who may participate in the research by virtue of their presence in a cluster randomised facility. The COMPASS study included 40 randomised sites and compared the effectiveness of a postacute stroke intervention with standard care. Each site provided either the comprehensive postacute stroke intervention or standard care according to the randomisation assignment. Working together, the study team, institutional review board and members of the community designed an ethically appropriate and operationally reasonable consent process which was carried out successfully at all randomised sites. This achievement is noteworthy because it demonstrates how to effectively conduct appropriate informed consent in cluster randomised trials, and because it provides a model that can easily be adapted for other pragmatic studies. With this innovative approach to informed consent, patients have access to the information they need about research occurring where they are seeking care, and medical researchers can conduct their studies without ethical concerns or unreasonable logistical impediments.

TRIAL REGISTRATION NUMBER

NCT02588664, recruiting. This article covers the development of consent process that is currentlty being employed in the study.

Very Low Density Lipoprotein Assembly Is Required for cAMP-responsive Element-binding Protein H Processing and Hepatic Apolipoprotein A-IV Expression

Hepatic apolipoprotein A-IV (apoA-IV) expression is correlated with hepatic triglyceride (TG) content in mouse models of chronic hepatosteatosis, and steatosis-induced hepatic apoA-IV gene expression is regulated by nuclear transcription factor cAMP-responsive element-binding protein H (CREBH) processing. To define what aspects of TG homeostasis regulate hepatic CREBH processing and apoA-IV gene expression, several mouse models of attenuated VLDL particle assembly were subjected to acute hepatosteatosis induced by an overnight fast or short term ketogenic diet feeding. Compared with chow-fed C57BL/6 mice, fasted or ketogenic diet-fed mice displayed increased hepatic TG content, which was highly correlated (r² = 0.95) with apoA-IV gene expression, and secretion of larger, TG-enriched VLDL, despite a lower rate of TG secretion and a similar or reduced rate of apoB100 secretion. When VLDL particle assembly and secretion was inhibited by hepatic shRNA-induced apoB silencing or genetic or pharmacologic reduction in microsomal triglyceride transfer protein (MTP) activity, hepatic TG content increased dramatically; however, CREBH processing and apoA-IV gene expression were attenuated compared with controls. Adenovirus-mediated reconstitution of MTP expression proportionately restored CREBH processing and apoA-IV expression in liver-specific MTP knock-out mice. These results reveal that hepatic TG content, per se, does not regulate CREBH processing. Instead, TG mobilization into the endoplasmic reticulum for nascent VLDL particle assembly activates CREBH processing and enhances apoA-IV gene expression in the setting of acute steatosis. We conclude that VLDL assembly and CREBH activation play key roles in the response to hepatic steatosis by up-regulating apoA-IV and promoting assembly and secretion of larger, more TG-enriched VLDL particles.

Abstract 540: Very Low Density Lipoprotein Assembly is Required for cAMP Responsive Element-binding Protein H Processing and Hepatic Apolipoprotein A-IV Expression in Mouse Models of Acute Steatosis

Background: Our previous studies have demonstrated that hepatic expression of apolipoprotein A-IV (apoA-IV) is increased in mouse models of chronic steatosis and is closely correlated with hepatic triglyceride (TG) content. We have also shown that steatosis-induced hepatic apoA-IV gene expression is regulated by processing of the nuclear transcription factor cAMP responsive element-binding protein H (CREBH). Herein, we explored the mechanisms that mediate steatosis-induced CREBH processing.

Methods: We measured hepatic CREBH processing, apoA-IV gene expression, and lipid content in several mouse models of attenuated or enhanced VLDL assembly that were subjected to acute steatosis induced by a 16 hour overnight fast or by feeding a ketogenic diet for 6 days.

Results: Both fasting and the ketogenic diet induced acute hepatic TG accumulation associated with increased CREBH processing and apoA-IV gene expression, which were associated with hepatic TG content in C57BL/6 mice. All mouse models of attenuated VLDL secretion (shRNA-induced apoB knock down, liver specific microsomal triglyceride transfer protein (MTP) knockout, treatment with the MTP inhibitor BMS212122, and comparative gene identification-58 (CGI-58) deficiency) had increased hepatic TG accumulation, but displayed repressed CREBH processing and reduced apoA-IV gene expression compared to controls. When deficient VLDL assembly in liver-specific MTP knockout mice was reconstituted by adenoviral infection with a human MTP transgene, steatosis-induced CREBH processing and apoA-IV expression were restored. In a mouse model of enhanced VLDL assembly (transgenic overexpression of human MTP), apoA-IV gene expression correlated with bulk hepatic TG accumulation, but not with VLDL secretion rate, indicating that other steatosis-related factors participate in apoA-IV gene regulation.

Conclusions: Taken together, these data provide compelling evidence that VLDL assembly and secretion is required for hepatic CREBH processing and enhanced apoA-IV gene expression in the setting of acute steatosis. These data further suggest that CREBH and apoA-IV play central roles in VLDL-mediated hepatic lipid efflux.

Abstract 4: Apolipoprotein A-IV Confers Protection Against Oxidant Stress and Adipose Tissue Inflammation in Mouse Models of Obesity

Background: Increased lipid oxidation may initiate the chronic inflammatory state which characterizes adipose tissue in obesity. Apo A-IV is a potent in vitro antioxidant, and has been reported to confer protection against multiple inflammatory stimuli in vivo. Apo A-IV may be an especially effective antioxidant in peripheral tissues, as its labile affinity for lipoproteins results in relatively high concentrations in interstitial fluid.

Hypothesis: Apo A-IV attenuates lipid peroxidation and adipose tissue inflammation in mouse models of obesity.

Methods: Double heterozygote A4+/- x Ob+/- mice on a C57BL6 background were bred to generate wild type (WT), apo A-IV knockout (A4KO), Ob/Ob, and double knockout (DKO) mice. Groups of age-matched male littermates were fed pro-inflammatory high fat (HF, 42% fat, 0.2% cholesterol) or high fat/high cholesterol (HFHC, 38% fat, 1.25% cholesterol) diets; fasting plasma apo A-IV levels were measured by immunoblotting; plasma total F2 isoprostane levels (a biomarker of global lipid peroxidation) were measured by GC-MS; and expression of genes for pro-inflammatory mediators in adipose tissue was measured in WT and A4KO mice by RT-PCR.

Results: Baseline plasma apo A-IV levels were higher in the Ob/Ob mice, but increased ~2-fold after 8 weeks on the HF diet. After 12 weeks on the HFHC diet, plasma total F2 isoprostane levels were elevated in Ob/Ob mice, but were highest in DKO mice, indicating that free radical lipid peroxidation in peripheral tissues was increased in the absence of intestinal apo A-IV synthesis. All mice gained weight, but DKO mice gained less than Ob/Ob mice. Adipose tissue expression of the genes for MCP1, IL6, and IL1b was elevated 2-3 fold in A4KO mice.

Conclusions: These data indicate that plasma apo A-IV levels are chronically elevated by a high fat intake, and that apo A-IV expression attenuates the pro-oxidant/pro-inflammatory state in diet-induced and genetic obesity, which may thereby modulate energy balance.

Acyl chain length, saturation, and hydrophobicity modulate the efficiency of dietary fatty acid absorption in adult humans

Intestinal fat absorption is known to be, overall, a highly efficient process, but much less is known about the efficiency with which individual dietary fatty acids (FA) are absorbed by the adult small intestine. We therefore measured the absorption efficiency of the major dietary FA using sucrose polybehenate (SPB) as a nonabsorbable marker and analyzed how it is modulated by acyl chain physicochemical properties and polymorphisms of proteins involved in chylomicron assembly. Dietary FA absorption efficiency was measured in 44 healthy subjects fed a standard diet containing 35% fat and 5% SPB. FA and behenic acid (BA) were measured in homogenized diets and stool samples by gas chromatography-mass spectroscopy, and coefficients of absorption for each FA were calculated as 1 - [(FA/BA)feces/(FA/BA)diet]. Absorption coefficients for saturated FA decreased with increasing chain length and hydrophobicity (mean ± SE) and ranged from 0.95 ± 0.02 for myristate (14:0), 0.80 ± 0.03 for stearate (18:0), to 0.26 ± 0.02 for arachidate (20:0). Absorption coefficients for unsaturated FA increased with increasing desaturation from 0.79 ± 0.03 for elaidic acid (18:1t), 0.96 ± 0.01 for linoleate (18:2), to near complete absorption for eicosapentaenoic (20:5) and docosahexaenoic (22:6) acids. Of several common genetic polymorphisms in key proteins involved in the chylomicron assembly pathway, only the intestinal fatty acid-binding protein-2 A54T allele (rs1799883) had any impact on FA absorption. We conclude that acyl chain length, saturation, and hydrophobicity are the major determinants of the efficiency with which dietary FA are absorbed by the adult small intestine.

Abstract 299: Intestinal Apolipoprotein A-IV Expression Conserves Visceral Adiposity in Mice Subjected to Cyclical Feeding and Fasting

Background. We recently observed that although C57BL6 and apo A-IV knockout (A4KO) mice had similar weight gain on rodent chow, C56BL6 mice gained more weight and had greater adiposity when fed a high fat diet for 12 weeks (J Lipid Res 52:1984-1994, 2011). As food intake and intestinal fat absorption were the same in both groups, these data suggest that intestinal apo A-IV expression increases the efficiency of dietary energy storage. Here we have investigated whether apo A-IV could conserve adiposity in the setting of energy deprivation.

Methods. Four month old male C56BL6 and A4KO mice (n=5) weighing ∼30-35 gm were subjected to repeated 3 day cycles of free access to a high fat (40%) diet alternating with access to only water. Intra-abdominal fat, fat free mass (FFM), and hepatic fat were measured at baseline and after 8 weeks by CT scanning. At the end of the study, livers and epidydimal fat pads were collected and weighed.

Results. After 8 weeks on the feed-fast regimen, C57BL6 and A4KO mice had lost similar amounts of total weight, 4.4 ± 0.7 vs 5.2 ± 0.5 g (mean ± SE) and fat free mass, 3.0 ± 0.5 vs 3.0 ± 0.4 g; however, C57BL6 mice had lost less fat than A4KO mice, 1.4 ± 0.3 vs 2.2 ± 0.2 g (p=0.05). C57BL6 mice displayed a smaller decrease in adiposity than A4KO mice, 13.8 ± 1.1 to 10.9 ± 1.1 % vs 13.1 ± 0.5 to 7.9 ± 0.8 % (p=0.02 for the difference in the decrease between groups). There was a trend towards lower hepatic fat content (estimated from CT Hounsfield units) in C57BL6 mice, but liver (1.4 ± 0.2 vs 1.7 ± 0.2 g) and fat pad (1.3 ± 0.7 vs 1.1 ± 0.7 g) weights were not significantly different between the strains, suggesting that cyclical fasting affected mainly visceral fat.

Conclusions. These data establish that intestinal apo A-IV expression conserves visceral adipose tissue during cyclical fasting. Because apo A-IV is a uniquely mammalian apolipoprotein which appeared at the time of the divergence of mammals from the avian-reptile lineages, and as the mammalian survival paradigm requires that sufficient stores of lipid energy are available to support thermogenesis, internal gestation, and lactation, these data suggest that a major role of apo A-IV is to promote energy storage during times of dietary energy surfeit and conserve lipid energy stores during periods of prolonged energy deprivation.

Bitter taster status predicts susceptibility to vection-induced motion sickness and nausea

BACKGROUND

Sensitivity to bitter taste and susceptibility to nausea are both protective mechanisms that guard against toxin ingestion, and both these traits vary within and between populations. Thus, we postulated that they may have co-evolved, such that they are associated.

METHODS

Bitter taster status was determined in 40 subjects (13 men, 27 women) by measuring the differential perceived taste intensity between salt and n-propylthiouracil using a labeled magnitude scale; susceptibility to vection-induced motion sickness and nausea was assessed using an optokinetic drum, a validated multi-symptom scoring scale, and electrogastrography.

KEY RESULTS

Taster status distribution was 25% non-tasters (NT), 40% tasters (T), and 35% supertasters (ST). Gender had no impact on this distribution, but females had a higher mean maximum symptom score than males (12.4 ± 1.4 vs 7.3 ± 2.0). Non-tasters displayed a faster and larger increase in mean symptom scores, had a higher percentage of subjects with high maximum symptom scores, and had a higher mean maximum score than T or ST, (14.8 ± 2.6 vs 7.1 ± 1.8, vs 9.8 ± 2.0). Taster status did not affect the gastric myoelectric frequency response to vection.

CONCLUSIONS & INFERENCES

Non-tasters are more susceptible to vection-induced motion sickness and nausea than T or ST, suggesting these two traits may have co-evolved in a reciprocal manner: in environments where the NT trait conferred an evolutionary advantage by enabling intake of fruits and vegetables containing bitter, yet beneficial, phytonutrients, increased nausea susceptibility may have arisen to maintain protection against ingested toxins.

ApoA-IV modulates the secretory trafficking of apoB and the size of triglyceride-rich lipoproteins

Although the evidence linking apoA-IV expression and triglyceride (TG)-rich lipoprotein assembly and secretion is compelling, the intracellular mechanisms by which apoA-IV could modulate these processes remain poorly understood. We therefore examined the functional impact of apoA-IV expression on endogenous apoB, TG, and VLDL secretion in stably transfected McA-RH7777 rat hepatoma cells. Expression of apoA-IV modified with the endoplasmic reticulum (ER) retention signal KDEL (apoA-IV-KDEL) dramatically decreased both the rate and efficiency of endogenous apoB secretion, suggesting a presecretory interaction between apoA-IV-KDEL and apoB or apoB-containing lipoproteins. Expression of native apoA-IV using either a constitutive or tetracycline-inducible promoter delayed the initial rate of apoB secretion and reduced the final secretion efficiency by ∼40%. However, whereas apoA-IV-KDEL reduced TG secretion by 75%, expression of native apoA-IV caused a 20-35% increase in TG secretion, accompanied by a ∼55% increase in VLDL-associated apoB, an increase in the TG:phospholipid ratio of secreted d < 1.006 lipoproteins, and a 10.1 nm increase in peak VLDL(1) particle diameter. Native apoA-IV expression had a negligible impact on expression of the MTP gene. These data suggest that by interacting with apoB in the secretory pathway, apoA-IV alters the trafficking kinetics of apoB-containing TG-rich lipoproteins through cellular lipidation compartments, which in turn, enhances particle expansion and increases TG secretion.

Impact of murine intestinal apolipoprotein A-IV expression on regional lipid absorption, gene expression, and growth

Apolipoprotein A-IV (apoA-IV) is synthesized by intestinal enterocytes during lipid absorption and secreted into lymph on the surface of nascent chylomicrons. A compelling body of evidence supports a central role of apoA-IV in facilitating intestinal lipid absorption and in regulating satiety, yet a longstanding conundrum is that no abnormalities in fat absorption, feeding behavior, or weight gain were observed in chow-fed apoA-IV knockout (A4KO) mice. Herein we reevaluated the impact of apoA-IV expression in C57BL6 and A4KO mice fed a high-fat diet. Fat balance and lymph cannulation studies found no effect of intestinal apoA-IV gene expression on the efficiency of fatty acid absorption, but gut sac transport studies revealed that apoA-IV differentially modulates lipid transport and the number and size of secreted triglyceride-rich lipoproteins in different anatomic regions of the small bowel. ApoA-IV gene deletion increased expression of other genes involved in chylomicron assembly, impaired the ability of A4KO mice to gain weight and increase adipose tissue mass, and increased the distal gut hormone response to a high-fat diet. Together these findings suggest that apoA-IV may play a unique role in integrating feeding behavior, intestinal lipid absorption, and energy storage.

Biogenesis of apolipoprotein A-V and its impact on VLDL triglyceride secretion

Apolipoprotein A-V (apoA-V) is a potent regulator of intravascular triglyceride (TG) metabolism, yet its plasma concentration is very low compared with that of other apolipoproteins. To examine the basis for its low plasma concentration, the secretion efficiency of apoA-V was measured in stably transfected McA-RH7777 rat hepatoma cells. Pulse-chase experiments revealed that only ∼20% of newly synthesized apoA-V is secreted into culture medium within 3 h postsynthesis and that ∼65% undergoes presecretory turnover; similar results were obtained with transfected nonhepatic Chinese hamster ovary cells. ApoA-V secreted by McA-RH7777 cells was not associated with cell surface heparin-competable binding sites. When stably transfected McA-RH7777 cells were treated with oleic acid, the resulting increase in TG synthesis caused a reduction in apoA-V secretion, a reciprocal increase in cell-associated apoA-V, and movement of apoA-V onto cytosolic lipid droplets. In a stably transfected doxycycline-inducible McA-RH7777 cell line, apoA-V expression inhibited TG secretion by ∼50%, increased cellular TG, and reduced Z-average VLDL(1) particle diameter from 81 to 67 nm; however, no impact on apoB secretion was observed. These data demonstrate that apoA-V inefficiently traffics within the secretory pathway, that its intracellular itinerary can be regulated by changes in cellular TG accumulation, and that apoA-V synthesis can modulate VLDL TG mobilization and secretion.

Distinctive structure and interfacial activity of the human apolipoprotein A-IV 347S isoprotein

The T347S polymorphism in the human apolipoprotein (apo) A-IV gene is present at high frequencies among all the world's populations. Carriers of a 347S allele exhibit faster clearance of triglyceride-rich lipoproteins, greater adiposity, and increased risk for developing atherosclerosis, which suggests that this conservative amino acid substitution alters the structure of apo A-IV. Herein we have used spectroscopic and surface chemistry techniques to examine the structure, stability, and interfacial properties of the apo A-IV 347S isoprotein. Circular dichroism spectroscopy revealed that the 347S isoprotein has similar alpha-helical structure but lower thermodynamic stability than the 347T isoprotein. Fluorescence spectroscopy found that the 347S isoprotein exhibits an enhanced tyrosine emission and reduced tyrosine-->tryptophan energy transfer, and second derivative UV absorption spectra noted increased tyrosine exposure, suggesting that the 347S isoprotein adopts a looser tertiary conformation. Surface chemistry studies found that although the 347S isoprotein bound rapidly to the lipid interface, it has a lower interfacial exclusion pressure and lower elastic modulus than the 347T isoprotein. Together, these observations establish that the T347S substitution alters the conformation of apo A-IV and lowers its interfacial activity-changes that could account for the effect of this polymorphism on postprandial lipid metabolism.

Hepatic steatosis and subclinical cardiovascular disease in a cohort enriched for type 2 diabetes: the Diabetes Heart Study

OBJECTIVES

To explore mechanisms whereby hepatic steatosis may be associated with cardiovascular risk, we investigated cross-sectional relationships between hepatic steatosis, regional fat accumulation, inflammatory biomarkers, and subclinical measures of atherosclerosis in the Diabetes Heart Study.

METHODS

The Diabetes Heart Study is a family study of sibling pairs concordant for type 2 diabetes. A subset of 623 randomly selected participants was evaluated for hepatic steatosis, defined as a liver:spleen attenuation ratio of <1.0 by computed tomography. We quantified visceral fat, subcutaneous fat, coronary, aortic, and carotid artery calcium by computed tomography; and carotid atherosclerosis by ultrasound. Associations between the liver:spleen attenuation ratio and these factors were expressed as Spearman correlations.

RESULTS

After adjustment for age, race, gender, body mass index, and diabetes status, the liver:spleen attenuation ratio correlated with visceral fat (r =-0.22, P < 0.0001) and subcutaneous fat (r =-0.13, P= 0.031). Hepatic steatosis was associated with lower high-density lipoprotein (r = 0.21, P < 0.0001), higher triglycerides (r =-0.25, P < 0.0001), higher C-reactive protein (r =-0.095, P= 0.004), and lower serum adiponectin (r = 0.34, P < 0.0001). There were no significant associations between the liver:spleen attenuation ratio and coronary, aortic, or carotid calcium, or carotid intimal thickness.

CONCLUSIONS

This suggests that hepatic steatosis is less likely a direct mediator of cardiovascular disease and may best be described as an epiphenomenon. The strong correlations between pro-atherogenic biomarkers, visceral fat, and elements of the metabolic syndrome suggest that hepatic steatosis reflects more than general adiposity, but represents a systemic, inflammatory, pro-atherogenic adipose state.

Structural and dynamic interfacial properties of the lipoprotein initiating domain of apolipoprotein B

To better understand the earliest steps in the assembly of triglyceride (TG)-rich lipoproteins, we compared the biophysical and interfacial properties of two closely related apolipoprotein B (apoB) truncation mutants, one of which contains the complete lipoprotein initiating domain (apoB20.1; residues 1-912), and one of which, by virtue of a 50 amino acid C-terminal truncation, is incapable of forming nascent lipoproteins (apoB19; residues 1-862). Spectroscopic studies detected no major differences in secondary structure, and only minor differences in conformation and thermodynamic stability, between the two truncation mutants. Monolayer studies revealed that both apoB19 and apoB20.1 bound to and penetrated egg phosphatidylcholine (EPC) monolayers; however, the interfacial exclusion pressure of apoB20.1 was higher than apoB19 (25.1 mN/m vs. 22.8 mN/m). Oil drop tensiometry revealed that both proteins bound rapidly to the hydrophobic triolein/water interface, reducing interfacial tension by approximately 20 mN/m. However, when triolein drops were first coated with phospholipids (PL), apoB20.1 bound with faster kinetics than apoB19 and also displayed greater interfacial elasticity (26.9 +/- 0.8 mN/m vs. 22.9 +/- 0.8 mN/m). These data establish that the transition of apoB to assembly competence is accompanied by increases in surface activity and elasticity, but not by significant changes in global structure.

Common variants of apolipoprotein A-IV differ in their ability to inhibit low density lipoprotein oxidation

Apolipoprotein A-IV (apoA-IV) inhibits lipid peroxidation, thus demonstrating potential anti-atherogenic properties. The aim of this study was to investigate how the inhibition of low density lipoprotein (LDL) oxidation was influenced by common apoA-IV isoforms. Recombinant wild type apoA-IV (100 microg/ml) significantly inhibited the oxidation of LDL (50 microg protein/ml) by 5 microM CuSO(4) (P<0.005), but not by 100 microM CuSO(4), suggesting that it may act by binding copper ions. ApoA-IV also inhibited the oxidation of LDL by the water-soluble free-radical generator 2,2'-azobis(amidinopropane) dihydrochloride (AAPH; 1 mM), as shown by the two-fold increase in the time for half maximal conjugated diene formation (T(1/2); P<0.05) suggesting it can also scavenge free radicals in the aqueous phase. Compared to wild type apoA-IV, apoA-IV-S347 decreased T(1/2) by 15% (P=0.036) and apoA-IV-H360 increased T(1/2) by 18% (P=0.046). All apoA-IV isoforms increased the relative electrophoretic mobility of native LDL, suggesting apoA-IV can bind to LDL and acts as a site-specific antioxidant. The reduced inhibition of LDL oxidation by apoA-IV-S347 compared to wild type apoA-IV may account for the previous association of the APOA4 S347 variant with increased CHD risk and oxidative stress.

The C Terminus of Apolipoprotein A-V Modulates Lipid-binding Activity

Human apolipoprotein A-V (apoA-V) is a potent modulator of plasma triacylglycerol (TG) levels. To probe different regions of this 343-amino-acid protein, four single Trp apoA-V variants were prepared. The variant with a Trp at position 325, distal to the tetraproline sequence at residues 293-296, displayed an 11-nm blue shift in wavelength of maximum fluorescence emission upon lipid association. To evaluate the structural and functional role of this C-terminal segment, a truncated apoA-V comprising amino acids 1-292 was generated. Far UV circular dichroism spectra of full-length apoA-V and apoA-V-(1-292) were similar, with approximately 50% alpha-helix content. In guanidine HCl denaturation experiments, both full-length and truncated apoA-V yielded biphasic profiles consistent with the presence of two structural domains. The denaturation profile of the lower stability component (but not the higher stability component) was affected by truncation. Truncated apoA-V displayed an attenuated ability to solubilize l-alpha-dimyristoylphosphatidylcholine phospholipid vesicles compared with full-length apoA-V, whereas a peptide corresponding to the deleted C-terminal segment displayed markedly enhanced kinetics. The data support the concept that the C-terminal region is not required for apoA-V to adopt a folded protein structure, yet functions to modulate apoA-V lipid-binding activity; therefore, this concept may be relevant to the mechanism whereby apoA-V influences plasma TG levels.

Self-association and lipid binding properties of the lipoprotein initiating domain of apolipoprotein B

The amino-terminal 20.1% of apolipoprotein B (apoB20.1; residues 1-912) is sufficient to initiate and direct the formation of nascent apoB-containing lipoprotein particles. To investigate the mechanism of initial lipid acquisition by apoB, we examined the lipid binding and interfacial properties of a carboxyl-terminal His6-tagged form of apoB20.1 (apoB20.1H). ApoB20.1H was expressed in Sf9 cells and purified by nickel affinity chromatography. ApoB20.1H was produced in a folded state as characterized by formation of intramolecular disulfide bonds and resistance to chemical reduction. Dynamic light scattering in physiological buffer indicated that purified apoB20.1H formed multimers, which were readily dissociable upon the addition of nonionic detergent (0.1% Triton X-100). ApoB20.1H was incapable of binding dimyristoylphosphatidylcholine multilamellar vesicles, unless its multimeric structure was first disrupted by guanidine hydrochloride. However, apoB20.1H multimers spontaneously dissociated and bound to the interface of naked and phospholipid-coated triolein droplets. These data reveal that the initiating domain of apoB contains solvent-accessible hydrophobic sequences, which, in the absence of a hydrophobic lipid interface or detergent, engage in self-association. The high affinity of apoB20.1H for neutral lipid is consistent with the membrane binding and desorption model of apoB-containing lipoprotein assembly.

Overexpression of apolipoprotein A-IV enhances lipid secretion in IPEC-1 cells by increasing chylomicron size

Intestinal apolipoprotein A-IV expression is highly regulated by dietary lipid in newborn swine, suggesting a role in lipid absorption. Constitutive overexpression of apoA-IV in newborn swine enterocytes enhances basolateral secretion of triacylglycerol (TG) in TG-rich lipoproteins 4.9-fold (Lu, S., Yao, Y., Meng, S., Cheng, X., and Black, D. D. (2002) J. Biol. Chem. 277, 31929-31937). To investigate the mechanism of this enhancement, IPEC-1 cells were transfected with a tetracycline-regulatable expression system (Tet-On). In cells incubated with oleic acid, a dose response relationship was observed between medium doxycycline concentration and basolateral apoA-IV and TG secretion. Similarly regulated expression of apoA-I did not enhance lipid secretion. The mean diameter of TG-rich lipoproteins secreted from doxycycline-treated cells was larger than from untreated cells (87.0 nm versus 53.4 nm). Basolateral apoB secretion decreased. Using the same expression system, full-length human apoA-IV (376 amino acids); a "pig-like" human apoA-IV, lacking the C-terminal EQQQ repeats (361 amino acids); and a "chicken-like" apoA-IV, further truncated to 343 amino acids, were expressed in IPEC-1 cells. With increasing protein secretion, cells expressing the full-length human apoA-IV displayed a 2-fold increase in TG secretion; in sharp contrast, cells expressing the pig-like human apoA-IV displayed a 25-fold increase in TG secretion and a 27-fold increase in lipoprotein diameter. When human apoA-IV was further truncated to yield a chicken-like protein, TG secretion was inhibited. We conclude that overexpression of swine apoA-IV enhances basolateral TG secretion in a dose-dependent manner by increasing the size of secreted lipoproteins. These data suggest that the region in the human apoA-IV protein from residues 344 to 354 is critical to its ability to enhance lipid secretion, perhaps by enabling the packaging of additional core TG into chylomicron particles. The EQQQ-rich region may play an inhibitory or modulatory role in chylomicron packaging in humans.

Specific Sequences in the N and C Termini of Apolipoprotein A-IV Modulate Its Conformation and Lipid Association

Apolipoprotein (apoA-IV) is a 376-residue exchangeable apolipoprotein that may play a number of important roles in lipid metabolism, including chylomicron assembly, reverse cholesterol transport, and appetite regulation. In vivo, apoA-IV exists in both lipid-poor and lipid-associated forms, and the balance between these states may determine its function. We examined the structural elements that modulate apoA-IV lipid binding by producing a series of deletion mutants and determining their ability to interact with phospholipid liposomes. We found that the deletion of residues 333-343 strongly increased the lipid association rate versus native apoA-IV. Additional mutagenesis revealed that two phenylalanine residues at positions 334 and 335 mediated this lipid binding inhibitory effect. We also observed that residues 11-20 in the N terminus were required for the enhanced lipid affinity induced by deletion of the C-terminal sequence. We propose a structural model in which these sequences can modulate the conformation and lipid affinity of apoA-IV.

Apolipoprotein A-I helix 6 negatively charged residues attenuate lecithin-cholesterol acyltransferase (LCAT) reactivity

Apolipoprotein A-I (apoA-I), the major protein in high density lipoprotein (HDL) regulates cholesterol homeostasis and is protective against atherosclerosis. An examination of the amino acid sequence of apoA-I among 21 species shows a high conservation of positively and negatively charged residues within helix 6, a domain responsible for regulating the rate of cholesterol esterification in plasma. These observations prompted an investigation to determine if charged residues in helix 6 maintain a structural conformation for protein-protein interaction with lecithin-cholesterol acyltransferase (LCAT) the enzyme for which apoA-I acts as a cofactor. Three apoA-I mutants were engineered; the first, (3)/(4) no negative apoA-I, eliminated 3 of the 4 negatively charged residues in helix 6, no negative apoA-I (NN apoA-I) eliminated all four negative charges, while all negative (AN apoA-I) doubled the negative charge. Reconstituted phospholipid-containing HDL (rHDL) of two discrete sizes and compositions were prepared and tested. Results showed that LCAT activation was largely influenced by both rHDL particle size and the net negative charge on helix 6. The 80 A diameter rHDL showed a 12-fold lower LCAT catalytic efficiency when compared to 96 A diameter rHDL, apparently resulting from an increased protein-protein interaction, at the expense of lipid-protein association on the 80 A rHDL. When mutant apoproteins were compared bound to the two different sized rHDL, a strong inverse correlation (r = 0.85) was found between LCAT catalytic efficiency and apoA-I helix 6 net negative charge. These results support the concept that highly conserved negatively charged residues in apoA-I helix 6 interact directly and attenuate LCAT activation, independent of the overall particle charge.

apoA-IV tagged with the ER retention signal KDEL perturbs the intracellular trafficking and secretion of apoB

To examine the role of apolipoprotein A-IV (apoA-IV) in the intracellular trafficking and secretion of apoB, COS cells were cotransfected with microsomal triglyceride transfer protein (MTP), apoB-41 (amino terminal 41% of apoB), and either native apoA-IV or apoA-IV modified with the carboxy-terminal endoplasmic reticulum (ER) retention signal, KDEL (apoA-IV-KDEL). As expected, apoA-IV-KDEL was inefficiently secreted relative to native apoA-IV. Coexpression of apoB-41 with apoA-IV-KDEL reduced the secretion of apoB-41 by approximately 80%. The apoA-IV-KDEL effect was specific, as neither KDEL-modified forms of human serum albumin or apoA-I affected apoB-41 secretion. Similar results were observed in McA-RH7777 rat hepatoma cells, which express endogenous MTP. The full inhibitory effect of apoA-IV-KDEL on apoB secretion was observed only for forms of apoB containing a minimum of the amino-terminal 25% of the protein (apoB-25). However, apoA-IV-KDEL inhibited the secretion of both lipid-associated and lipid-poor forms of apoB-25. Dual-label immunofluorescence microscopy of cells transfected with native apoA-IV and apoB-25 revealed that both apolipoproteins were localized to the ER and Golgi, as expected. However, when apoA-IV-KDEL was cotransfected with apoB-25, both proteins localized primarily to the ER. These data suggest that apoA-IV may physically interact with apoB in the secretory pathway, perhaps reflecting a role in modulating the process of triglyceride-rich lipoprotein assembly and secretion.

Identification of the lipoprotein initiating domain of apolipoprotein B

We have explored the minimum sequence requirement for the initiation of apolipoprotein B (apoB)-mediated triglyceride-rich lipoprotein assembly. A series of apoB COOH-terminal truncation mutants, spanning a range from apoB34 (amino acid residues 1-1544 of apoB100) to apoB19 (residues 1-862) were transfected into COS cells with and without coexpression of the microsomal triglyceride transfer protein (MTP). ApoB34, -25, -23, -21, -20.5, and -20.1 underwent efficient conversion to buoyant lipoproteins when coexpressed with MTP. ApoB19.5 (amino acids 1-884) also directed MTP-dependent particle assembly, although at reduced efficiency. When apoB19.5 was truncated by another 22 amino acids to form apoB19, MTP-dependent lipoprotein assembly was abolished. Analysis of the lipid stoichiometry of secreted lipoproteins revealed that all apoB truncation mutants formed spherical particles containing a hydrophobic core. Even highly truncated assembly-competent forms of apoB, such as apoB19.5 and 20.1, formed lipoproteins with surface:core lipid ratios of <1. We conclude that the translation of the first approximately 884 amino acids of apoB completes a domain capable of initiating nascent lipoprotein assembly. The composition of lipids recruited into lipoproteins by this initiating domain is consistent with formation of small emulsion particles, perhaps by simultaneous desorption of both polar and neutral lipids from a saturated bilayer.

Structure and interfacial properties of human apolipoprotein A-V

Apolipoprotein A-V (apoA-V), the newest member of the plasma apolipoprotein family, was recently discovered by comparison of the mouse and human genomes. Studies in rodents and population surveys of human apoA-V polymorphisms have noted a strong effect of apoA-V on plasma triglyceride levels. Toward the elucidation of the biologic function of apoA-V, we used spectroscopic and surface chemistry techniques to probe its structure and interfacial activity. Computer-assisted sequence analysis of apoA-V predicts that it is very hydrophobic, contains a significant amount of alpha-helical secondary structure, and probably is composed of discrete structural regions with varying degrees of lipid affinity. Fluorescence spectroscopy of recombinant human apoA-V provided evidence of tertiary folding, and light scattering studies indicated that apoA-V transforms dimyristoylphosphatidylcholine vesicles into discoidal complexes with an efficiency similar to that of apoA-I. Surface chemistry techniques revealed that apoA-V displays high affinity, low elasticity, and slow binding kinetics at hydrophobic interfaces, properties we propose may retard triglyceride-rich particle assembly. Metabolic labeling and immunofluorescence studies of COS-1 cells transfected with human apoA-V demonstrated that apoA-V is poorly secreted, remains associated with the endoplasmic reticulum, and does not traffic to the Golgi. Given that overexpression of the apoA-V gene lowers plasma triglycerides in mice, these data together suggest that apoA-V may function intracellularly to modulate hepatic VLDL synthesis and/or secretion.

Interfacial exclusion pressure determines the ability of apolipoprotein A-IV truncation mutants to activate cholesterol ester transfer protein

We used a panel of recombinant human apolipoprotein (apo) A-IV truncation mutants, in which pairs of 22-mer alpha-helices were sequentially deleted along the primary sequence, to examine the impact of protein structure and interfacial activity on the ability of apoA-IV to activate cholesterol ester transfer protein. Circular dichroism and fluorescence spectroscopy revealed that the secondary structure, conformation, and molecular stability of recombinant human apoA-IV were identical to the native protein. However, deletion of any of the alpha-helical domains in apoA-IV disrupted its tertiary structure and impaired its molecular stability. Surprisingly, determination of the water/phospholipid interfacial exclusion pressure of the apoA-IV truncation mutants revealed that, for most, deletion of amphipathic alpha-helical domains increased their affinity for phospholipid monolayers. All of the truncation mutants activated the transfer of fluorescent-labeled cholesterol esters between high and low density lipoproteins at a rate higher than native apoA-IV. There was a strong positive correlation (r = 0.790, p = 0.002) between the rate constant for cholesterol ester transfer and interfacial exclusion pressure. We conclude that molecular interfacial exclusion pressure, rather than specific helical domains, determines the degree to which apoA-IV, and likely other apolipoproteins, facilitate cholesterol ester transfer protein-mediated lipid exchange.

Apolipoprotein A-IV polymorphisms and diet-gene interactions

Apolipoprotein A-IV is a 46kDa glycoprotein that is synthesized by intestinal enterocytes and is incorporated into the surface of nascent chylomicrons. Considerable evidence suggests that apolipoprotein A-IV plays a role in intestinal lipid absorption and chylomicron assembly. We have proposed that polymorphisms that alter the interfacial behavior of apolipoprotein A-IV may modulate the physical properties and metabolic fate of plasma chylomicrons. Of the reported genetic polymorphisms of apolipoprotein A-IV, two, Q360H and T347S, are known to occur at high frequencies among the world populations. Biophysical studies have established that the Q360H isoprotein displays higher lipid affinity; conversely the T347S isoprotein is predicted to be less lipid avid. Recent studies have shown that the Q360H polymorphism is associated with increased postprandial hypertriglyceridemia, a reduced low-density lipoprotein response to dietary cholesterol in the setting of a moderate fat intake, an increased high-density lipoprotein response to changes in total dietary fat content, and lower body mass and adiposity; the T347S polymorphism appears to confer the opposite effects. Studies on the diet-gene interactions of other apolipoprotein A-IV alleles are needed, as are studies on the interactions between apolipoprotein A-IV alleles and other apolipoprotein polymorphisms.

Effects of a high polyunsaturated fat diet and vitamin E supplementation on high-density lipoprotein oxidation in humans

Oxidative modification of high-density lipoproteins (HDL) impairs several biologic functions critical to its role in reverse cholesterol transport. We therefore investigated the effect of dietary polyunsaturated fat and vitamin E on the kinetics of HDL oxidation. Ten subjects were fed sequentially: a baseline diet in which the major fat source was olive oil; a high polyunsaturated fat diet in which the major fat source was safflower oil; and the safflower oil diet plus 800 I.U. vitamin E per day. Plasma lipoprotein levels, vitamin E content, fatty acid composition, and oxidation lag time and rate were determined after 3 weeks on each diet. The polyunsaturated fat diet increased the mean HDL(2) lag time from 45.8+/-12.5 to 83.3+/-11.6 min with no change in oxidation rate. Addition of vitamin E further increased the HDL(2) lag time to 115.6+/-4.4 min and decreased the HDL(2) oxidation rate 10-fold. Neither the polyunsaturated diet alone nor the diet with vitamin E supplementation had any effect on HDL(3) oxidation. We conclude that under conditions of controlled dietary fat intake, a high polyunsaturated fat intake does not increase the oxidation susceptibility of HDL subfractions, and that in this setting, vitamin E supplementation reduces the oxidation susceptibility of HDL(2). These data suggest that antioxidants could influence HDL function in vivo.

Pro-oxidant effect of vitamin E in cigarette smokers consuming a high polyunsaturated fat diet

Dietary polyunsaturated fats and vitamin E are associated with reduced risk for atherosclerosis, but in smokers, they could promote lipid oxidation. Therefore, we examined the effects of a high polyunsaturated fat diet and vitamin E supplementation on measures of lipid oxidation in cigarette smokers. Ten subjects who smoked >1 pack of cigarettes per day were sequentially fed the following: a baseline diet in which the major fat source was olive oil, a diet in which the major fat source was high-linoleic safflower oil, and finally, the safflower oil diet plus 800 IU vitamin E per day. LDL oxidation lag time and rate and plasma total F(2)-isoprostanes and prostaglandin F(2alpha) (PGF(2alpha)) were determined after 3 weeks on each diet. The safflower oil diet increased total F(2)-isoprostanes from 53.0+/-7.2 to 116.2+/-11.2 nmol/L and PGF(2alpha) from 3.5+/-0.2 to 5.5+/-0.5 nmol/L, without changing LDL oxidation parameters. Addition of vitamin E prolonged mean LDL oxidation lag time but, paradoxically, further increased F(2)-isoprostanes to 188.2+/-10.9 nmol/L and PGF(2alpha) to 7.8+/-0.4 nmol/L. These data suggest that vitamin E may function as a pro-oxidant in cigarette smokers consuming a high polyunsaturated fat diet.

Effect of the apolipoprotein A-IV Q360H polymorphism on postprandial plasma triglyceride clearance

Apolipoprotein (apo)A-IV is synthesized in the small intestine during fat absorption and is incorporated onto the surface of nascent chylomicrons. In circulation, apoA-IV is displaced from the chylomicron surface by high density lipoprotein-associated C and E apolipoproteins; this exchange is critical for activation of lipoprotein lipase and chylomicron remnant clearance. The variant allele A-IV-2 encodes a Q360H polymorphism that increases the lipid affinity of the apoA-IV-2 isoprotein. We hypothesized that this would impede the transfer of C and E apolipoproteins to chylomicrons, and thereby delay the clearance of postprandial triglyceride-rich lipoproteins. We therefore measured triglycerides in plasma, S(f) > 400 chylomicrons, and very low density lipoproteins (VLDL) in 14 subjects heterozygous for the A-IV-2 allele (1/2) and 14 subjects homozygous for the common allele (1/1) who were fed a standard meal containing 50 gm fat per m(2) body surface area. All subjects had the apoE-3/3 genotype. Postprandial triglyceride concentrations in the 1/2 subjects were significantly higher between 2;-5 h in plasma, chylomicrons, and VLDL, and peaked at 3 h versus 2 h for the 1/1 subjects. The area under the triglyceride time curves was greater in the 1/2 subjects (plasma, P = 0.045; chylomicrons, P = 0.027; VLDL, P = 0.063). A post-hoc analysis of the frequency of the apoA-IV T347S polymorphism suggested that it had an effect on triglyceride clearance antagonistic to that of the A-IV-2 allele. We conclude that individuals heterozygous for the A-IV-2 allele display delayed postprandial clearance of triglyceride-rich lipoproteins.

Effect of apolipoprotein A-IV genotype and dietary fat on cholesterol absorption in humans

We investigated the effect of the A-IV-2 allele, which encodes a Q360H substitution in apolipoprotein (apo) A-IV, and dietary fat on cholesterol absorption in humans. In three separate studies we compared fractional intestinal cholesterol absorption between groups of subjects heterozygous for the A-IV-2 allele (1/2) and homozygous for the common allele (1/1) receiving high cholesterol ( approximately 800 mg/day) diets with different fatty acid compositions. All subjects had the apoE 3/3 genotype. There was no difference in cholesterol absorption between the two genotype groups receiving a high saturated fat diet (33% of total energy as fat; 18% saturated, 3% polyunsaturated, 12% monounsaturated) or a low fat diet (22% of total energy as fat; 7% saturated, 7% polyunsaturated, 8% monounsaturated) diet. However, on a high polyunsaturated fat diet (32% of total energy as fat; 7% saturated, 13% polyunsaturated, 12% monounsaturated) mean fractional cholesterol absorption was 56. 7% +/- 1.9 in 1/1 subjects versus 47.5% +/- 2.1 in 1/2 subjects (P = 0.004). A post hoc analysis of the effect of the apoA-IV T347S polymorphism across all diets revealed a Q360H x T347S interaction on cholesterol absorption, and suggested that the A-IV-2 allele lowers cholesterol only in subjects with the 347 T/T genotype. We conclude that a complex interaction between apoA-IV genotype and dietary fatty acid composition modulates fractional intestinal cholesterol absorption in humans.

Dynamic interfacial properties of human apolipoproteins A-IV and B-17 at the air/water and oil/water interface

Viscoelastic behavior of proteins at interfaces is a critical determinant of their ability to stabilize emulsions. We therefore used air bubble surfactometry and drop volume tensiometry to examine the dynamic interfacial properties of two plasma apolipoproteins involved in chylomicron assembly: apolipoprotein A-IV and apolipoprotein B-17, a recombinant, truncated apolipoprotein B. At the air/water interface apolipoproteins A-IV and B-17 displayed wide area - tension loops with positive phase angles indicative of viscoelastic behavior, and suggesting that they undergo rate-dependent changes in surface conformation in response to changes in interfacial area. At the triolein/water interface apolipoprotein A-IV displayed maximal surface activity only at long interface ages, with an adsorption rate constant of 1.0 3 10(-)(3) sec(-)(1), whereas apolipoprotein B-17 lowered interfacial tension even at the shortest interface ages, with an adsorption rate constant of 9.3 3 10(-)(3) sec(-)(1). Apolipoprotein A-IV displayed an expanded conformation at the air/water interface and a biphasic compression isotherm, suggesting that its hydrophilic amphipathic helices move in and out of the interface in response to changes in surface pressure. We conclude that apolipoproteins A-IV and B-17 display a combination of interfacial activity and elasticity particularly suited to stabilizing the surface of expanding triglyceride-rich particles.

Structure and interfacial properties of chicken apolipoprotein A-IV

To gain insight into the evolution and function of apolipoprotein A-IV (apoA-IV) we compared structural and interfacial properties of chicken apoA-IV, human apoA-IV, and a recombinant human apoA-IV truncation mutant lacking the carboxyl terminus. Circular dichroism thermal denaturation studies revealed that the thermodynamic stability of the alpha-helical structure in chicken apoA-IV (DeltaH = 71.0 kcal/mol) was greater than that of human apoA-IV (63.6 kcal/mol), but similar to that of human apoA-I (73.1 kcal/mol). Fluorescence chemical denaturation studies revealed a multiphasic red shift with a 65% increase in relative quantum yield that preceded loss of alpha-helical structure, a phenomenon previously noted for human apoA-IV. The elastic modulus of chicken apoA-IV at the air/water interface was 13.7 mN/m, versus 21.7 mN/m for human apoA-IV and 7.6 mN/m for apoA-I. The interfacial exclusion pressure of chicken apoA-IV for phospholipid monolayers was 31.1 mN/m, versus 33.0 mN/m for human A-I and 28.5 mN/m for apoA-IV. We conclude that the secondary structural features of chicken apoA-IV more closely resemble those of human apoA-I, which may reflect the evolution of apoA-IV by intraexonic duplication of the apoA-I gene. However, the interfacial properties of chicken apoA-IV are intermediate between those of human apoA-I and apoA-IV, which suggests that chicken apoA-IV may represent an ancestral prototype of mammalian apoA-IV, which subsequently underwent further structural change as an evolutionary response to the requisites of mammalian lipoprotein metabolism.

Apolipoprotein A-IV-2 allele: association of its worldwide distribution with adult persistence of lactase and speculation on its function and origin

Apolipoprotein A-IV (apo A-IV) is a 46-Kd plasma glycoprotein that may play a major role in intestinal lipid absorption. A genetic polymorphism in the apo A-IV gene, apo A-IV-2, encodes a His-->Gln substitution at codon 360 that alters the biological function of this apolipoprotein. As the worldwide distribution of the apo A-IV-2 allele appeared similar to the frequency of a genetic polymorphism that determines the persistence of lactase into adulthood, we examined the relationship between the apo A-IV-2 and lactase persistence polymorphisms by compiling the prevalence of adult lactase persistence in all populations in which the frequency of the apo A-IV-2 allele has been determined. Across 29 groups, there was an extremely strong correlation (4 = 0.937, P < 0.000001) between apo A-IV-2 allele frequency and the prevalence of adult lactase persistence. Apo A-IV-2 allele frequency was highest in Iceland, an ancient Viking colony, and decreased across Europe in a north-to-south and west-to-east gradient, generally following hypothetical isoclines for the lactase persistence gene. There were no correlations between the population frequencies of the apo E2, E3, or E4 alleles and either the prevalence of lactase persistence or the frequency of the apo A-IV-2 allele. In light of the effects of the apo A-IV-2 polymorphism on lipid metabolism, we speculate that the apo A-IV-2 allele may have originated in ancient Scandinavia, spread by conferring a nutritional advantage in the setting of a lifelong high milkfat intake, and was later carried southwards by the Viking incursions into Europe.