Changes of apolipoprotein A-IV in the human neonate: evidence for different inductions of apolipoproteins A-IV and A-I in the postpartum period

Progress in Mass Spectrometry-Based Proteomics in Hypoxia-Related Diseases and High-Altitude Medicine

Human migration, influenced by social conflict and natural disasters as well as global climate change, has become recognized as a major "planetary force." It has also brought to the forefront, new specialties of integrative biology-such as high-altitude medicine-and the impact of hitherto understudied environmental factors on human pathophysiology in these new geographical settings. For people migrating to or living in high-altitude regions, environmental hypoxia is a primary challenge. Decreased partial pressure of oxygen in environmental air, caused by lower barometric pressure, puts living organisms in a hypoxic state. When there is a serious inability to adapt, death may ensue. Research efforts over the past few years have applied mass spectrometry-based proteomics analyses to uncover the mechanisms of hypoxia-related high-altitude pathophysiology. The differential proteomic profiles in plasma and tissues under high-altitude hypoxia conditions, as compared with sea level controls, and the multitudinous hypoxia-specific proteins identified elucidate mechanisms underlying high-altitude hypoxia acclimatization and diseases, and provide a foundation for development of new therapeutic, prophylactic, and diagnostic approaches. In this expert review and innovation analysis, we highlight the current proteomics findings on high-altitude hypoxia, and suggest paths forward toward effective interventions to address this key challenge in high-altitude medicine.

Structure and function of the apoA-IV T347S and Q360H common variants

Human apolipoprotein A-IV (apoA-IV) is involved in chylomicron assembly and secretion, and in reverse cholesterol transport. Several apoA-IV isoforms exist, the most common in Caucasian populations being apoA-IV-1a (T347S) and apoA-IV-2 (Q360H). The objective of the present study was to investigate the impact of these common aminoacid substitutions on the ability of apoA-IV to bind lipids, to promote cell cholesterol efflux via ABCA1, and to maintain endothelial homeostasis. Recombinant forms of wild-type apoA-IV, apoA-IV Q360H, and apoA-IV T347S were produced in Escherichia coli. ApoA-IV Q360H and apoA-IV T347S showed a slightly higher alpha-helical content compared to wild-type apoA-IV, and associated with phospholipids faster than wild-type apoA-IV. The capacity to promote ABCA1-mediated cholesterol efflux was significantly greater for the apoA-IV T347S than the other apoA-IV isoforms. No differences were observed in the ability of apoA-IV isoforms to inhibit the production of VCAM-1 and IL-6 in TNFalpha-stimulated endothelial cells. In conclusion, the apoA-IV T347S common variant has increased lipid binding properties and cholesterol efflux capacity, while the apoA-IV Q360H variant has only slightly increased lipid binding properties. The two common aminoacid substitutions have no effect on the ability of apoA-IV to maintain endothelial homeostasis.

Global screening of human cord blood proteomes for biomarkers of toxic exposure and effect

BACKGROUND

Exposures of pregnant women to natural and manmade chemicals can lead to negative health effects in the baby, ranging from low birth weight to developmental defects. In some cases, diseases were postulated to have their basis in toxic exposure in utero or in early childhood. Therefore, an understanding of fetal responses to environmental exposures is essential. To that end, cord blood is a readily accessible biofluid whose proteomic makeup remains mostly unexplored when compared with that of adults.

OBJECTIVES

Our goal was an initial global assessment of the fetal serum proteome and for the identification of protein biomarkers indicative of toxic in utero exposures related to maternal cigarette smoking.

METHODS

Drawing from a repository of 300 samples, we selected umbilical cord blood sera from 12 babies born to six smokers and six nonsmokers and analyzed both sample pools by tandem mass spectrometry in conjunction with isobaric tags (iTRAQ) for protein quantification.

RESULTS

We identified 203 proteins, 17 of which were differentially expressed between the cigarette smoke-exposed and control populations. Most of the identified candidate biomarkers were biologically plausible, thereby underscoring the feasibility of screening neonates with global proteomic techniques for biomarkers of exposure and early biological effects triggered by in utero chemical exposures.

CONCLUSIONS

This validation study provides an initial view of the proteome of human cord blood sera; it demonstrates the feasibility of identifying therein by use of proteomics, biomarkers of environmental, toxic exposures.

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.

Apo A-IV: an update on regulation and physiologic functions

Apolipoprotein (apo) A-IV, first identified 28 years ago as a plasma lipoprotein moiety, is now known to participate in the regulation of various metabolic pathways. It is synthesized primarily in the enterocytes of the small intestine during fat absorption. After entry into the bloodstream, the 46-kDa glycoprotein apo A-IV appears associated with chylomicrons, high-density lipoproteins, and in the lipoprotein-free fraction. It has a role in lipid absorption, transport and metabolism, and may act as a post-prandial satiety signal, an anti-oxidant and a major factor in the prevention of atherosclerosis. After summarizing and discussing these functions for reader's comprehension, the current review focuses on the regulation of apo A-IV by nutrients, biliary components, drugs, hormones and gastrointestinal peptides. The understanding of the involved mechanisms that underline apo A-IV regulation may in the long run allow us to switch on its gene, which may confer multiple beneficial effects, including the protection from atherosclerosis.

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.

Control of synthesis and secretion of intestinal apolipoprotein A-IV by lipid

Apolipoprotein (apo) A-IV, a component of intestinally secreted, triacylglycerol-rich lipoproteins, has recently been proposed as a physiological controller of gastric function and food intake. Thus, it is important to understand the mechanisms involved in the control of expression, synthesis and secretion of apo A-IV. Apo A-IV is a member of a closely linked, multigene cluster which includes apolipoproteins A-I and C-III. Expression and synthesis of apo A-IV display marked variability with regard to species, tissue, stage of development and response to hormones, but intestinal apo A-IV is consistently stimulated by dietary lipid. The precise molecular mechanisms underlying the response of apo A-IV to lipid have not been clearly defined. Most evidence supports the hypothesis that some aspect of lipid transport is necessary for the apo A-IV response, but only part of this response may be due to a direct effect of intestinal lipid: recent findings suggest a connection between intestinal production of apo A-IV and hormonal and/or neural factors associated with operation of the "ileal brake." Thus, apo A-IV may play an integrative role in the modulation of both upper gastrointestinal function and ingestive behavior.

Lipoproteins containing apolipoprotein A-IV but not apolipoprotein A-I take up and esterify cell-derived cholesterol in plasma

Two-dimensional nondenaturing polyacrylamide gradient gel electrophoresis (2D-PAGGE) identifies distinct apoA-I-or apoE-containing subclasses of high-density lipoproteins (HDLs), each of which plays a different role in reverse cholesterol transport. In this study we used 2D-PAGGE to investigate the role of apoA-IV-containing lipoproteins in reverse cholesterol transport in native plasma. Incubation of 2D electrophoretograms with anti-apoA-IV antibodies identified up to three subclasses of particles. The smaller particle subclasses, LpA-IV-1 and LpA-IV-2, were found in every plasma sample. The largest particle subclass, LpA-IV-3, was observed in fewer than 10% of the plasmas analyzed. 2D-PAGGE of apoA-I-deficient plasma and apoA-I-depleted plasma and anti-apoA-I immunosubtracting 2D-PAGGE of normal plasma revealed that LpA-IV-1 and LpA-IV-2 do not contain apoA-I. The importance of LpA-IV-1 and LpA-IV-2 for uptake and esterification of cell-derived cholesterol was investigated using pulse-chase incubations of plasma with [3H]cholesterol-labeled fibroblasts followed by anti-apoA-I immunosubtracting 2D-PAGGE. During 1-minute pulse incubation with cells, [3H]cholesterol was taken up by gamma-LpE > LpA-IV-1 > pre-beta 1-LpA-I > LpA-IV-2 (">" denotes "more than"). During subsequent chase incubation without cells, proportionately less radioactivity disappeared from LpA-IV-1 and LpA-IV-2 than from pre-beta 1-LpA-I and gamma-LpE. During 5-minute pulse incubations, radioactive cholesteryl esters were formed in pre-beta 3-LpA-I > alpha-LpA-I > LpA-IV-1 > LpA-IV-2. The fractional estertification rate was highest in pre-beta 2-LpA-I and lowest in alpha-LpA-I.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex-specific effects of the glutamine/histidine polymorphism in apo A-IV on HDL metabolism

In Caucasians, a histidine for glutamine substitution (Gln-->His) at residue 360 in apolipoprotein (apo) A-IV leads to an electrophoretically detectable polymorphism whose contribution to lipid metabolism regulation is controversial. In this study of 426 male and 188 female coronary heart disease patients, we analyzed the impact of this polymorphism on lipid metabolism, particularly high-density lipoprotein (HDL). The frequency of the rarer apo A-IV (360:His) allele was .069. This polymorphism exerted opposite effects in men and women in terms of serum concentrations of total cholesterol; triglycerides; HDL cholesterol; LDL cholesterol; lipoprotein (Lp) A-I; and apo A-I, A-II, and B. Only the difference in Lp A-I levels between male apo A-IV (360:Gln/Gln) homozygotes and apo A-IV (360:Gln/His) heterozygotes was significant (P < .05). In randomly selected subgroups of 38 male and 15 female apo A-IV (360:Gln/His) heterozygotes and 104 male and 15 female apo A-IV (360:Gln) homozygotes, heterozygosity for apo A-IV (360:Gln/His) in both sexes was associated with lower plasma cholesteryl ester transfer protein (CETP) activity (P < .05) and higher serum apo A-IV concentrations (P < .01 in men). Moreover, only men had significantly higher mean plasma activity levels of lecithin:cholesterol acyltransferase (LCAT) (P < .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Lipoproteins containing apolipoprotein A-IV: composition and relation to cholesterol esterification

In order to investigate the relationship of lipid and apolipoprotein composition to cholesterol esterification in lipoproteins containing apolipoprotein (apo) A-IV, apo A-containing lipoprotein particles were isolated from fresh human plasma using a system of sequential immunoaffinity chromatography. Plasma was first depleted of apo B- and apo E-containing lipoproteins. Four major subpopulations of apo A-containing lipoprotein particles were separated: Lp A-I, Lp A-I: A-II, Lp A-IV and Lp A-I: A-IV: A-II. Lp A-IV and Lp A-I: A-IV: A-II contained less total lipid, less cholesterol and more triacylglycerol than Lp A-I and Lp A-I: A-II. Lp A-IV and Lp A-I: A-IV: A-II contained more sphingomyelin and less phosphatidylcholine than Lp A-I and Lp A-I: A-II and were richer in (16:0 + 18:0) saturated fatty acids. Among these isolated lipoprotein particles, Lp A-IV contained the highest lecithin: cholesterol acyltransferase (LCAT) activity per micrograms of protein. Cholesterol esterification rates were 2.6 +/- 0.5, 5.3 +/- 0.4 and 0.8 +/- 0.2 mumol of cholesterol per hour per mg of lipoproteins for Lp A-IV, Lp A-I and Lp A-I: A-II, respectively. The apolipoprotein and lipid composition and LCAT activity of Lp A-IV suggest that this lipoprotein may be a source of cholesterol esterification in plasma.

Rapid in vivo transport and catabolism of human apolipoprotein A-IV-1 and slower catabolism of the apoA-IV-2 isoprotein

Apolipoprotein (apo) A-IV is a polymorphic, intestinally derived apolipoprotein that is genetically linked to and similar in structure to apoA-I, the major apolipoprotein in high density lipoproteins (HDL). ApoA-IV plays a potentially important role in lipoprotein metabolism and reverse cholesterol transport, but its in vivo metabolism is poorly understood. In order to gain insight into factors modulating apoA-IV metabolism in humans, the in vivo kinetics of the two major human apoA-IV isoproteins apoA-IV-1 and apoA-IV-2 were investigated in normolipidemic human subjects. 131I-apoA-IV-1 and 125I-apoA-IV-2 were reassociated with autologous plasma and injected into study subjects. Analysis of the kinetic data revealed a rapid mean fractional catabolic rate (FCR) for apoA-IV-1 of 2.42 +/- 0.11 d-1. The mean production, or transport, rate of apoA-IV-1 was 16.3 +/- 1.4 mg/kg per d. Plasma apoA-IV concentrations were highly correlated with apoA-IV production rate (r = 0.84, P < 0.001) and not correlated with apoA-IV fractional catabolic rate (r = 0.25, P = NS). The mean FCR of apoA-IV-2 was 2.21 +/- 0.10 d-1. In the ten subjects in whom 131I-apoA-IV-1 and 125I-apoA-IV-2 were simultaneously injected, the FCR of apoA-IV-2 was significantly slower by paired t test (P = 0.003). The FCR of apoA-IV-2 in an apoA-IV-2/2 homozygote was only 1.49 d-1, substantially slower than in all other subjects. We conclude that: (a) apoA-IV is a rapidly catabolized apolipoprotein in humans, with a fractional catabolic rate more than 10 times greater than that of apoA-I; (b) apoA-IV has a high absolute transport rate similar to that of apoA-I; (c) plasma levels of apoA-IV are primarily determined by apoA-IV production rate in normolipidemic subjects; and (d) the fractional catabolic rate of the common variant apoA-IV-2 is slower than that of the wild-type apoA-IV-1.

Characterization of apoA-IV-containing lipoprotein particles isolated from human plasma and interstitial fluid

Apolipoprotein (apo) A-IV has been proposed to play a role in reverse cholesterol transport. ApoA-IV-containing lipoprotein particles (A-IVLp) were isolated from human plasma and interstitial fluid and characterized by immunoaffinity chromatography. Two major A-IVLp subpopulations, lipoprotein particles containing apoA-IV with apoA-I (LpA-I:A-IV) and lipoprotein particles containing apoA-IV without apoA-I (LpA-IV), were identified. The larger subpopulation of A-IVLp is the LpA-IV that represents 70% (protein mass) of the initial particles. Only 5.8% of apoA-IV was recovered in the retained fraction after affinity chromatography with an anti-apoA-I immunosorbent. ApoA-I, apoA-II, apoA-IV, apoB, apoC-III, apoD, apoE, apoH, lecithin: cholesterol acyltransferase (LCAT), cholesteryl ester transfer (CET) protein, proline-rich protein, and a protein of Mr 59,000 were detected in the A-IVLp. These particles contain more than 20% triglycerides (lipid mass). ApoA-IV-containing particles that were isolated from plasma are heterogeneous in size, consisting of two major populations with Stokes' diameters of 10.3 nm and 9.3 nm. Both subpopulations of A-IVLp contain LCAT and CET activities and promote cholesterol efflux from cholesterol-preloaded adipose cells. These data support the hypothesis that A-IVLp particles may be involved in reverse cholesterol transport.

Severe type III hyperlipoproteinemia associated with unusual apolipoprotein E1 phenotype and epsilon 1/'null' genotype

A 60-year-old white male (KH) was diagnosed to suffer from severe type III hyperlipoproteinemia (HLP) and premature cardiovascular disease. Biochemical analysis revealed an unusual apolipoprotein (apo) E phenotype and genotype. All clinical characteristics of type III HLP were present in the patient. His very low density lipoprotein (VLDL) cholesterol to plasma triglyceride (TG) ratio was elevated at 0.97 without therapy which is unusually high (normal ratio about 0.18). By contrast his plasma apo E level was only moderately elevated (6.8 mg dl-1). The patient's apo E migrated in the apo E1 position on isoelectric focusing gels. Chemical modification with cysteamine and treatment with neuraminidase confirmed the presence of two cysteine residues in the patient's apo E and a normal sialylation pattern. Pedigree analysis suggested that the patient was a compound heterozygote with one apo epsilon 1 allele and another allele whose product did not appear in the plasma compartment ('null' allele). Direct sequencing of polymerase chain reaction (PCR) amplified segments of the apo E gene as well as restriction fragment length polymorphism (RFLP) analysis with the endonuclease Taq I identified an adenosine for guanosine (G-->A) exchange in the second base of codon 127 that is predictive for an Asp for Gly substitution in the encoded apo E amino acid sequence. This mutation is the structural basis for the apo E1 isoform identified upon isoelectric focusing. Five other family members are also carriers of the mutant apo epsilon 1 allele. Two of those were hyperlipidemic and exhibited biochemical characteristics of type III HLP. A second mutation, a deletion of a G in codon 31, is predictive for a reading frameshift that encodes for a premature stop in codon 60. Our inability to identify the product of a second apo E allele in the plasma of the patient and two other members of the KH family corresponds with the heterozygous presence of this mutation in the affected individuals. Both relatives (like the index case) had an increased VLDL cholesterol to plasma TG ratio, which indicates the presence of cholesterol-enriched VLDL particles. We propose that the single base deletion in the apo E gene which is the cause of a non-functional 'null' allele in addition to a probably dominant apo E1 (Gly127-->Asp, Arg158-->Cys) variant of late or incomplete penetrance are the primary genetic defects in this kindred leading to severe dysbetalipoproteinemia.

Low-dose oral contraceptives lower plasma levels of apolipoprotein E

Three different oral contraceptive preparations were studied before and after a 3 month treatment period with respect to their effects on plasma lipoprotein parameters. A total of 58 healthy women requesting oral contraception were randomly assigned to three groups. Each woman received either monophasic preparations containing ethinylestradiol and desogestrel (M-DG); ethinylestradiol and gestodene (M-GD); or a triphasic preparation of ethinylestradiol and levonorgestrel (T-LN). As has been reported in other studies, the concentrations of total plasma cholesterol and apolipoproteins B and A-IV did not change significantly in any group. HDL cholesterol, triglycerides, apolipoproteins A-I and A-II increased or tended to increase. Despite the effects of the three hormone preparations on these lipoprotein parameters, however, each led to a highly significant decrease in apolipoprotein E plasma levels. Considering the recently reported observations that oral contraceptives increase the hepatic uptake of cholesterol-rich remnants, this decrease in apo-E plasma levels may in women that take oral contraceptives be directly correlated with increased hepatic lipoprotein metabolism.

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.

Detection of human serum amyloid A protein in very low density--and high density lipoproteins of patients after acute myocardial infarction

Two acute-phase proteins have been identified in very low density (VLDL)- and high density lipoproteins (HDL) of patients after acute myocardial infarction. Both proteins have a relative molecular weight of 11,000 and isoelectric points pI 6.08 and 6.27, and do not contain cysteine or sugar residues. Polyclonal antibodies to these acute phase reactants did not cross-react with other serum apolipoproteins. Evidence is given that both proteins are polymorphic forms of the human serum amyloid A protein.