Apolipoprotein A-IV polymorphisms Q360H and T347S attenuate its endogenous inhibition of thrombosis

Platelets are small anucleate cells that play a key role in thrombosis and hemostasis. Our group previously identified apolipoprotein A-IV (apoA-IV) as an endogenous inhibitor of thrombosis by competitive blockade of the αIIbβ3 integrin on platelets. ApoA-IV inhibition of platelets was dependent on the N-terminal D5/D13 residues, and enhanced with absence of the C-terminus, suggesting it sterically hinders its N-terminal platelet binding site. The C-terminus is also the site of common apoA-IV polymorphisms apoA-IV-1a (T347S) and apoA-IV-2 (Q360H). Interestingly, both are linked with an increased risk of cardiovascular disease, however, the underlying mechanism remains unclear. Here, we generated recombinant apoA-IV and found that the Q360H or T347S polymorphisms dampened its inhibition of platelet aggregation in human platelet-rich plasma and gel-filtered platelets, reduced its inhibition of platelet spreading, and its inhibition of P-selectin on activated platelets. Using an ex vivo thrombosis assay, we found that Q360H and T347S attenuated its inhibition of thrombosis at both high (1800s-1) and low (300s-1) shear rates. We then demonstrate a conserved monomer-dimer distribution among apoA-IV WT, Q360H, and T347S and use protein structure modelling software to show Q360H and T347S enhance C-terminal steric hindrance over the N-terminal platelet-binding site. These data provide critical insight into increased cardiovascular risk for individuals with Q360H or T347S polymorphisms.

Apolipoprotein A-IV concentrations and cancer in a large cohort of chronic kidney disease patients: results from the GCKD study

BACKGROUND

Chronic kidney disease (CKD) is highly connected to inflammation and oxidative stress. Both favour the development of cancer in CKD patients. Serum apolipoprotein A-IV (apoA-IV) concentrations are influenced by kidney function and are an early marker of kidney impairment. Besides others, it has antioxidant and anti-inflammatory properties. Proteomic studies and small case-control studies identified low apoA-IV as a biomarker for various forms of cancer; however, prospective studies are lacking. We therefore investigated whether serum apoA-IV is associated with cancer in the German Chronic Kidney Disease (GCKD) study.

METHODS

These analyses include 5039 Caucasian patients from the prospective GCKD cohort study followed for 6.5 years. Main inclusion criteria were an eGFR of 30-60 mL/min/1.73m2 or an eGFR > 60 mL/min/1.73m2 in the presence of overt proteinuria.

RESULTS

Mean apoA-IV concentrations of the entire cohort were 28.9 ± 9.8 mg/dL (median 27.6 mg/dL). 615 patients had a history of cancer before the enrolment into the study. ApoA-IV concentrations above the median were associated with a lower odds for a history of cancer (OR = 0.79, p = 0.02 when adjusted age, sex, smoking, diabetes, BMI, albuminuria, statin intake, and eGFRcreatinine). During follow-up 368 patients developed an incident cancer event and those with apoA-IV above the median had a lower risk (HR = 0.72, 95%CI 0.57-0.90, P = 0.004). Finally, 62 patients died from such an incident cancer event and each 10 mg/dL higher apoA-IV concentrations were associated with a lower risk for fatal cancer (HR = 0.62, 95%CI 0.44-0.88, P = 0.007).

CONCLUSIONS

Our data indicate an association of high apoA-IV concentrations with reduced frequencies of a history of cancer as well as incident fatal and non-fatal cancer events in a large cohort of patients with CKD.

Apolipoprotein A-IV-Deficient Mice in 129/SvJ Background Are Susceptible to Obesity and Glucose Intolerance

Apolipoprotein A-IV (apoA-IV), synthesized by enterocytes, is potentially involved in regulating lipid absorption and metabolism, food intake, and glucose metabolism. In this study, we backcrossed apoA-IV knockout (apoA-IV-/-) mice onto the 129/SvJ background for eight generations. Compared to the wild-type (WT) mice, the 129/SvJ apoA-IV-/- mice gained more weight and exhibited delayed glucose clearance even on the chow diet. During a 16-week high-fat diet (20% by weight of fat) study, apoA-IV-/- mice were more obese than the WT mice, which was associated with their increased food intake as well as reduced energy expenditure and physical activity. In addition, apoA-IV-/- mice developed significant insulin resistance (indicated by HOMA-IR) with severe glucose intolerance even though their insulin levels were drastically higher than the WT mice. In conclusion, we have established a model of apoA-IV-/- mice onto the 129/SvJ background. Unlike in the C57BL/6J strain, apoA-IV-/- 129/SvJ mice become significantly more obese and insulin-resistant than WT mice. Our current investigations of apoA-IV in the 129/SvJ strain and our previous studies in the C57BL/6J strain underline the impact of genetic background on apoA-IV metabolic effects.

Apolipoprotein A-IV restrains fat accumulation in skeletal and myocardial muscles by inhibiting lipogenesis and activating PI3K-AKT signalling

BACKGROUND

One of the pathological characteristics of obesity is fat accumulation of skeletal muscles (SKM) and the myocardium, involving mechanisms of insulin resistance and abnormal lipid metabolism. Apolipoprotein A-IV (ApoA-IV) is an essential gene in both glucose and lipid metabolisms.

MATERIALS AND METHODS

Using high-fat diet (HFD) induced obese apoA-IV-knockout mice and subsequent introduction of exogenous recombinant-ApoA-IV protein and adeno-associated virus (AAV)-transformed apoA-IV, we examined lipid metabolism indicators of SKM and the myocardium, which include triglyceride (TG) content, RT-PCR for lipogenic indicators and western blotting for AKT phosphorylation. Similarly, we used high-glucose-fed or palmitate (Pal)-induced C2C12 cells co-cultured with ApoA-IV protein to evaluate glucose uptake, the phosphoinositide 3-kinase (PI3K)-AKT pathway, and lipid metabolisms.

RESULTS

In stable obese animal models, we find ApoA-IV-knockout mice show elevated TG content, enhanced expression of lipogenic enzymes and diminished phosphorylated AKT in SKM and the myocardium, but both stable hepatic expression of AAV-apoA-IV and brief ApoA-IV protein administration suppress lipogenesis and promote AKT phosphorylation. In a myoblast cell line C2C12, ApoA-IV protein suppresses Pal-induced lipid accumulation and lipogenesis but enhances AKT activation and glucose uptake, and the effect is abolished by a PI3K inhibitor.

CONCLUSION

We find that ApoA-IV reduces fat accumulation by suppressing lipogenesis and improves glucose uptake in SKM and the myocardium by regulating the PI3K-AKT pathway.

Serum apolipoprotein A-IV levels are associated with flow-mediated dilation in patients with type 2 diabetes mellitus

Background

Endothelial dysfunction is common in diabetes. Apolipoprotein (apo) A-IV functions to antagonize inflammation and oxidative stress. The present study aimed to investigate the relationship between flow-mediated dilation (FMD) and serum apoA-IV level in type 2 diabetes mellitus (T2DM) patients. 

Methods

A total of 84 T2DM patients with chest discomfort were enrolled in this study. Their baseline characteristics and clinical parameters were documented. Endothelial function of the participants was evaluated by examining FMD of brachial artery. The severity of coronary atherosclerosis was determined by quantitative coronary angiography. Serum apoA-IV levels were measured by ELISA.

Results

These diabetic patients were dichotomized into low FMD (n = 42) and high FMD (n = 42) groups. Serum apoA-IV levels were significantly higher in high FMD group than in low FMD group (29.96 ± 13.17 vs 17.69 ± 9.16 mg/dL, P < 0.001). Moreover, the patients were also categorized into three apoA-IV tertile groups. FMD was significantly different across three apoA-IV tertiles (P < 0.001). Serum apoA-IV levels were positively correlated to FMD (r = 0.469, P < 0.001). Logistic regression analysis was performed to determine risk factors for low FMD. apoA-IV levels together with the risk factor hsCRP remained significantly to be independent determinants of low FMD (P < 0.01). Linear regression analysis was performed, and apoA-IV levels together with total-to-HDL cholesterol ratio were independently correlated with FMD (P < 0.01).

Conclusions

Serum apoA-IV levels are associated with FMD, suggesting that apoA-IV protects endothelial function in patients with T2DM.

A systemic apolipoprotein A-IV-associated amyloidosis confirmed by proteome analysis

Amyloidosis is induced by extracellular deposition of certain proteins. Thirty-six proteins have so far been identified as amyloidogenic proteins in humans. Although it is very important to determine the specific amyloid protein type for the choice of therapy for amyloidosis patient, it might be difficult to identify specific proteins from amyloid-deposited tissue. Apolipoprotein A-IV is known as an amyloid-associated protein, but there have been few reports of apolipoprotein A-IV amyloidosis. Here we report a case of systemic apolipoprotein A-IV-associated amyloidosis that was confirmed by proteome analysis using formalin-fixed paraffin-embedded tissue and an immunohistochemical technique.

Identification and characterization of a rare variant in apolipoprotein A-IV, p.(V336M), and evaluation of HDL functionality in a Greek cohort with extreme HDL cholesterol levels

High-Density Lipoprotein cholesterol (HDL-C) levels do not correlate well with Coronary Artery Disease (CAD) risk, while HDL functionality affects atherogenesis and is a better prognostic marker for CAD. Often, the extreme HDL-C levels have a multigenic origin. Here, we searched for single-nucleotide polymorphisms (SNPs) in ten genes of HDL metabolism in a Greek cohort with very low (<10th percentile, n = 13) or very high (>90th percentile, n = 21) HDL-C. We also evaluated the association between HDL-C levels, HDL functionality (anti-oxidant capacity) and CAD in the subjects of this cohort. Individuals with low HDL-C levels had higher triglyceride levels, lower apoA-I levels, decreased HDL anti-oxidant capacity and higher incidence of CAD compared with individuals with control or high HDL-C levels. With next generation sequencing we identified 18 exonic SNPs in 6 genes of HDL metabolism and for selected amino acid changes we performed computer-aided structural analysis and modeling. A previously uncharacterized rare apolipoprotein A-IV variant, apoA-IV [V336M], present in a subject with low HDL-C (14 mg/dL) and CAD, was expressed in recombinant form and structurally and functionally characterized. ApoA-IV [V336M] had similar α-helical content to WT apoA-IV but displayed a small thermodynamic stabilization by chemical unfolding analysis. ApoA-IV [V336M] was able to associate with phospholipids but presented reduced kinetics compared to WT apoA-IV. Overall, we identified a rare apoA-IV variant in a subject with low HDL levels and CAD with altered biophysical and phospholipid binding properties and showed that subjects with very low HDL-C presented with HDL dysfunction and higher incidence of CAD in a Greek cohort.

Enrichment of apolipoprotein A-IV and apolipoprotein D in the HDL proteome is associated with HDL functions in diabetic kidney disease without dialysis

Background and aims

Diabetic kidney disease (DKD) is associated with lipid derangements that worsen kidney function and enhance cardiovascular (CVD) risk. The management of dyslipidemia, hypertension and other traditional risk factors does not completely prevent CVD complications, bringing up the participation of nontraditional risk factors such as advanced glycation end products (AGEs), carbamoylation and changes in the HDL proteome and functionality. The HDL composition, proteome, chemical modification and functionality were analyzed in nondialysis subjects with DKD categorized according to the estimated glomerular filtration rate (eGFR) and urinary albumin excretion rate (AER).

Methods

Individuals with DKD were divided into eGFR> 60 mL/min/1.73 m² plus AER stages A1 and A2 (n = 10) and eGFR< 60 plus A3 (n = 25) and matched by age with control subjects (eGFR> 60; n = 8).

Results

Targeted proteomic analyses quantified 28 proteins associated with HDL in all groups, although only 2 were more highly expressed in the eGFR< 60 + A3 group than in the controls: apolipoprotein D (apoD) and apoA-IV. HDL from the eGFR< 60 + A3 group presented higher levels of total AGEs (20%), pentosidine (6.3%) and carbamoylation (4.2 x) and a reduced ability to remove ¹⁴C-cholesterol from macrophages (33%) in comparison to HDL from controls. The antioxidant role of HDL (lag time for LDL oxidation) was similar among groups, but HDL from the eGFR< 60 + A3 group presented a greater ability to inhibit the secretion of IL-6 and TNF-alpha (95%) in LPS-elicited macrophages in comparison to the control group.

Conclusion

The increase in apoD and apoA-IV could contribute to counteracting the HDL chemical modification by AGEs and carbamoylation, which contributes to HDL loss of function in well-established DKD.

Low-arginine and low-protein diets induce hepatic lipid accumulation through different mechanisms in growing rats

Background

Dietary protein deficiency and amino acid imbalance cause hepatic fat accumulation. We previously demonstrated that only arginine deficiency or total amino acid deficiency in a diet caused significant hepatic triglyceride (TG) accumulation in young Wistar rats. In this study, we explored the mechanisms of fatty liver formation in these models.

Methods

We fed 6-week-old male Wistar rats a control diet (containing an amino acid mixture equivalent to 15% protein), a low-total-amino acid diet (equivalent to 5% protein; 5PAA), and a low-arginine diet (only the arginine content is as low as that of the 5PAA diet) for 2 weeks.

Results

Much greater hepatic TG accumulation was observed in the low-arginine group than in the low-total-amino acid group. The lipid consumption rate and fatty acid uptake in the liver did not significantly differ between the groups. In contrast, the low-total-amino acid diet potentiated insulin sensitivity and related signaling in the liver and enhanced de novo lipogenesis. The low-arginine diet also inhibited hepatic very-low-density lipoprotein secretion without affecting hepatic insulin signaling and lipogenesis.

Conclusions

Although the arginine content of the low-arginine diet was as low as that of the low-total-amino acid diet, the two diets caused fatty liver via completely different mechanisms. Enhanced lipogenesis was the primary cause of a low-protein diet-induced fatty liver, whereas lower very-low-density lipoprotein secretion caused low-arginine diet-induced fatty liver.

Downregulation of apolipoprotein A-IV in plasma & impaired reverse cholesterol transport in individuals with recent acts of deliberate self-harm

Background & objectives:

The major limiting factor in the prevention of suicide is the limited knowledge on molecular insights in individuals at risk. Identification of peripheral protein markers which can classify individuals at high-risk of suicide might aid in early diagnosis and effective medical intervention. The aim of the present study was, therefore, to analyze the differential regulation of plasma proteins in individuals with deliberate self-harm compared to controls.

Methods:

Using two-dimensional gel electrophoresis coupled with matrix-assisted laser desorption-ionization mass spectrometry, differentially expressed plasma proteins were identified in study participants with deliberate self-harm compared to age- and gender-matched controls. The finding was validated using mass spectrometry-based isotope-labelled relative quantification and Western blot analysis in a new set of individuals with deliberate self-harm and controls.

Results:

The plasma proteomic analysis showed that apolipoprotein A-IV (Apo A-IV) was downregulated by 2.63-fold (confidence interval: 1.52-4.54) in individuals with deliberate self-harm (n=10) compared to matched controls, which was consistent in mass spectrometry-based relative quantification and Western blot analysis performed in an independent set of individuals with deliberate self-harm (n=18). In addition, plasma levels of total cholesterol, esterified cholesterol and high-density lipoprotein (HDL) were observed to be significantly lower individuals with deliberate self-harm compared to controls.

Interpretation & conclusions:

Apo A-IV, which plays a crucial role in the esterification of free cholesterol, was found to be downregulated with concomitantly decreased levels of HDL, esterified cholesterol and total cholesterol in individuals with deliberate self-harm compared to matched controls. The present findings might provide a link between the differential regulation of plasma proteins and the previously reported results on altered cholesterol levels in individuals with deliberate self-harm.

Apolipoprotein A-IV: A potential therapeutic target for atherosclerosis

Apolipoprotein A-IV is lipid-binding protein, which is synthesized by the intestine and secreted into mesenteric lymph. ApoA-IV is correlated with chylomicrons and high density lipoprotein, but a large portion is free-lipoprotein, in circulation. Studies showed that apoA-IV has anti-inflammatory and anti-oxidative properties, and is able to mediate reverse cholesterol transport, which suggest that it may has anti-atherosclerotic effects and be related to protection from atherosclerotic cardiovascular disease. This article focus on current studies and the possible anti-atherogenic mechanism related to apoA-IV, in order to provide a new therapeutic target for atherosclerotic cardiovascular diseases.

Apolipoprotein A-IV enhances cholecystokinnin secretion

Cholecystokinin (CCK) and apolipoprotein A-IV (ApoA-IV) are gastrointestinal peptides that play an important role in controlling energy homeostasis. Lymphatic ApoA-IV and plasma CCK secretion are mediated via a chylomicron formation-dependent pathway during a dietary lipid infusion. Given their similar roles as satiating proteins, the present study examines how the two peptides interact in their function. Specifically, this study sought to understand how ApoA-IV regulates CCK secretion. For this purpose, Cck gene expression in the small intestines of ApoA-IV knockout (ApoA-IV-KO) and wild-type (WT) mice were compared under an array of feeding conditions. When fed with a chow or high-fat diet (HFD), basal levels of Cck transcripts were significantly reduced in the duodenum of ApoA-IV-KO mice compared to WT mice. Furthermore, after an oral gavage of a lipid mixture, Cck gene expression in the duodenum was significantly reduced in ApoA-IV-KO mice relative to the change seen in WT mice. To determine the mechanism by which ApoA-IV modulates Cck gene expression, STC-1 cells were transfected with predesigned mouse lysophosphatidic acid receptor 5 (LPAR5) small interfering RNA (siRNA) to knockdown Lpar5 gene expression. In this in-vitro study, mouse recombinant ApoA-IV protein increased Cck gene expression in enteroendocrine STC-1 cells and stimulated CCK release from the STC-1 cells. However, the levels of CCK protein and Cck expression were attenuated when Lpar5 was knocked down in the STC-1 cells. Together these observations suggest that dietary lipid-induced ApoA-IV is associated with Cck synthesis in the duodenum and that ApoA-IV protein directly enhances CCK release through the activation of a LPAR5-dependent pathway.

Biliopancreatic Diversion Decreases Postprandial Apolipoprotein A-IV Levels in Mildly Obese Individuals with Type 2 Diabetes Mellitus: a Prospective Study

BACKGROUND

Bariatric surgery usually leads to improvement on the general lipid profile, but its role in the levels of apolipoprotein A-IV (Apo-AIV) is not completely understood. Apo-AIV is a gut-released lipoprotein which is enrolled in satiety regulation and presents anti-inflammatory, anti-atherogenic, and anti-oxidative properties. The objective of this study was to determine the influence of biliopancreatic diversion (BPD) in the levels of Apo-AIV.

METHODS

This is a prospective exploratory study which evaluated eight obese individuals with type 2 diabetes mellitus (T2DM) who underwent BPD (Scopinaro operation) and were followed-up for 12 months. Apo-AIV levels were determined by means of serial dosages through a standard meal tolerance test (MTT) in the immediate preoperative period and then 12 months later.

RESULTS

There was a significant change in the Apo-AIV curve following MTT before and after surgery. At 0 and 45 min, the Apo-AIV levels did not significantly differ before and after surgery; at 120 and 180 min, Apo-AIV levels were significantly lower following BPD.

CONCLUSIONS

We observed a decrease of postprandial levels of Apo-AIV following MTT in mildly obese individuals with T2DM. This finding appears to be related to the suppression in the Apo-AIV response that obese individuals tend to present. Weight reduction itself, endotoxemia, and the large segments of bypassed intestine may be enrolled in this impaired response.

Genomic characterization and expression analysis of four apolipoprotein A-IV paralogs in Senegalese sole (Solea senegalensis Kaup)

The apolipoprotein A-IV (ApoA-IV) plays a key role in lipid transport and feed intake regulation. In this work, four cDNA sequences encoding ApoA-IV paralogs were identified. Sequence analysis revealed conserved structural features including the common 33-codon block and nine repeated motifs. Gene structure analysis identified four exons and three introns except for apoA-IVAa1 (with only 3 exons). Synteny analysis showed that the four paralogs were structured into two clusters (cluster A containing apoA-IVAa1 and apoA-IVAa2 and cluster B with apoA-IVBa3 and apoA-IVBa4) linked to an apolipoprotein E. Phylogenetic analysis clearly separated the paralogs according to their cluster organization as well as revealed four subclades highly conserved in Acanthopterygii. Whole-mount analyses (WISH) in early larvae (0 and 1day post-hatch (dph)) showed that the four paralogs were mainly expressed in yolk syncytial layer surrounding the oil globules. Later, at 3 and 5dph, the four paralogs were mainly expressed in liver and intestine although with differences in their relative abundance and temporal expression patterns. Diet supply triggered the intensity of WISH signals in the intestine of the four paralogs. Quantification of mRNA abundance by qPCR using whole larvae only detected the induction by diet at 5dph. Moreover, transcript levels increased progressively with age except for apoA-IVAa2, which appeared as a low-expressed isoform. Expression analysis in juvenile tissues confirmed that the four paralogs were mainly expressed in liver and intestine and secondary in other tissues. The role of these ApoA-IV genes in lipid transport and the possible role of apoA-IVAa2 as a regulatory form are discussed.

Role of Conserved Proline Residues in Human Apolipoprotein A-IV Structure and Function

Apolipoprotein (apo)A-IV is a lipid emulsifying protein linked to a range of protective roles in obesity, diabetes, and cardiovascular disease. It exists in several states in plasma including lipid-bound in HDL and chylomicrons and as monomeric and dimeric lipid-free/poor forms. Our recent x-ray crystal structure of the central domain of apoA-IV shows that it adopts an elongated helical structure that dimerizes via two long reciprocating helices. A striking feature is the alignment of conserved proline residues across the dimer interface. We speculated that this plays important roles in the structure of the lipid-free protein and its ability to bind lipid. Here we show that the systematic conversion of these prolines to alanine increased the thermodynamic stability of apoA-IV and its propensity to oligomerize. Despite the structural stabilization, we noted an increase in the ability to bind and reorganize lipids and to promote cholesterol efflux from cells. The novel properties of these mutants allowed us to isolate the first trimeric form of an exchangeable apolipoprotein and characterize it by small-angle x-ray scattering and chemical cross-linking. The results suggest that the reciprocating helix interaction is a common feature of all apoA-IV oligomers. We propose a model of how self-association of apoA-IV can result in spherical lipoprotein particles, a model that may have broader applications to other exchangeable apolipoprotein family members.

TNF-alpha and IL-6 inhibit apolipoprotein A-IV production induced by linoleic acid in human intestinal Caco2 cells

Background

Apolipoprotein A-IV (apoA-IV) is a protein mainly synthesized by enterocytes in the intestine. Its gene expression is suppressed during fasting and stimulated during active fat absorption. Chronic feeding of a high-fat (HF) diet abolishes the differential expression between fasting and fat-feeding and therefore may contribute to diet-induced obesity since apoA-IV is a potent satiety factor. It is well established that the circulating pro-inflammatory cytokines TNF-α and IL-6 are increased by HF feeding.

Methods

To determine whether pro-inflammatory cytokines are involved in the diminished response of apoA-IV gene expression to fat-feeding, different concentrations of linoleic acid (LA), an important dietary fatty acid, was used to stimulate apoA-IV expression in human intestinal Caco2 cells. Cells were pre-treated with or without human recombinant TNF-α, IL-6 or their combination before the addition of LA. Real-time PCR and ELISA were used to detect and quantify RNA transcripts and proteins of apoA-IV and the cytokines.

Results

LA stimulated gene and protein expression of apoA-IV in a dose and time dependent manner. Pre-treatment with the cytokines for 72 h significantly inhibited the increased expression of apoA-IV gene and protein induced by LA. Furthermore, the cytokines, especially TNF-α, also positively up-regulate the cytokine themselves in Caco2 cells.

Conclusions

Our data indicate that the pro-inflammatory cytokines may be responsible for the reduced apoA-IV production in response to fat feeding. Because of apoA-IV’s role in satiety, we propose the inhibitory effect of circulating pro-inflammatory cytokines on apoA-IV production contributes to diet-induced obesity.

Zebrafish as a model for apolipoprotein biology: comprehensive expression analysis and a role for ApoA-IV in regulating food intake

Improved understanding of lipoproteins, particles that transport lipids throughout the circulation, is vital to developing new treatments for the dyslipidemias associated with metabolic syndrome. Apolipoproteins are a key component of lipoproteins. Apolipoproteins are proteins that structure lipoproteins and regulate lipid metabolism through control of cellular lipid exchange. Constraints of cell culture and mouse models mean that there is a need for a complementary model that can replicate the complex in vivo milieu that regulates apolipoprotein and lipoprotein biology. Here, we further establish the utility of the genetically tractable and optically clear larval zebrafish as a model of apolipoprotein biology. Gene ancestry analyses were implemented to determine the closest human orthologs of the zebrafish apolipoprotein A-I (apoA-I), apoB, apoE and apoA-IV genes and therefore ensure that they have been correctly named. Their expression patterns throughout development were also analyzed, by whole-mount mRNA in situ hybridization (ISH). The ISH results emphasized the importance of apolipoproteins in transporting yolk and dietary lipids: mRNA expression of all apolipoproteins was observed in the yolk syncytial layer, and intestinal and liver expression was observed from 4-6 days post-fertilization (dpf). Furthermore, real-time PCR confirmed that transcription of three of the four zebrafish apoA-IV genes was increased 4 hours after the onset of a 1-hour high-fat feed. Therefore, we tested the hypothesis that zebrafish ApoA-IV performs a conserved role to that in rat in the regulation of food intake by transiently overexpressing ApoA-IVb.1 in transgenic larvae and quantifying ingestion of co-fed fluorescently labeled fatty acid during a high-fat meal as an indicator of food intake. Indeed, ApoA-IVb.1 overexpression decreased food intake by approximately one-third. This study comprehensively describes the expression and function of eleven zebrafish apolipoproteins and serves as a springboard for future investigations to elucidate their roles in development and disease in the larval zebrafish model.

The structure of human apolipoprotein A-IV as revealed by stable isotope-assisted cross-linking, molecular dynamics, and small angle x-ray scattering

Apolipoprotein (apo)A-IV plays important roles in dietary lipid and glucose metabolism, and knowledge of its structure is required to fully understand the molecular basis of these functions. However, typical of the entire class of exchangeable apolipoproteins, its dynamic nature and affinity for lipid has posed challenges to traditional high resolution structural approaches. We previously reported an x-ray crystal structure of a dimeric truncation mutant of apoA-IV, which showed a unique helix-swapping molecular interface. Unfortunately, the structures of the N and C termini that are important for lipid binding were not visualized. To build a more complete model, we used chemical cross-linking to derive distance constraints across the full-length protein. The approach was enhanced with stable isotope labeling to overcome ambiguities in determining molecular span of the cross-links given the remarkable similarities in the monomeric and dimeric apoA-IV structures. Using 51 distance constraints, we created a starting model for full-length monomeric apoA-IV and then subjected it to two modeling approaches: (i) molecular dynamics simulations and (ii) fitting to small angle x-ray scattering data. This resulted in the most detailed models yet for lipid-free monomeric or dimeric apoA-IV. Importantly, these models were of sufficient detail to direct the experimental identification of new functional residues that participate in a "clasp" mechanism to modulate apoA-IV lipid affinity. The isotope-assisted cross-linking approach should prove useful for further study of this family of apolipoproteins in both the lipid-free and -bound states.

Apolipoprotein A-IV reduces hepatic gluconeogenesis through nuclear receptor NR1D1

We showed recently that apoA-IV improves glucose homeostasis by enhancing pancreatic insulin secretion in the presence of elevated levels of glucose. Therefore, examined whether apolipoprotein A-IV (apoA-IV) also regulates glucose metabolism through the suppression of hepatic gluconeogenesis. The ability of apoA-IV to lower gluconeogenic gene expression and glucose production was measured in apoA-IV(-/-) and wild-type mice and primary mouse hepatocytes. The transcriptional regulation of Glc-6-Pase and phosphoenolpyruvate carboxykinase (PEPCK) by apoA-IV was determined by luciferase activity assay. Using bacterial two-hybrid library screening, NR1D1 was identified as a putative apoA-IV-binding protein. The colocalization and interaction between apoA-IV and NR1D1 were confirmed by immunofluorescence, in situ proximity ligation assay, and coimmunoprecipitation. Enhanced recruitment of NR1D1 and activity by apoA-IV to Glc-6-Pase promoter was verified with ChIP and a luciferase assay. Down-regulation of apoA-IV on gluconeogenic genes is mediated through NR1D1, as illustrated in cells with NR1D1 knockdown by siRNA. We found that apoA-IV suppresses the expression of PEPCK and Glc-6-Pase in hepatocytes; decreases hepatic glucose production; binds and activates nuclear receptor NR1D1 and stimulates NR1D1 expression; in cells lacking NR1D1, fails to inhibit PEPCK and Glc-6-Pase gene expression; and stimulates higher hepatic glucose production and higher gluconeogenic gene expression in apoA-IV(-/-) mice. We conclude that apoA-IV inhibits hepatic gluconeogenesis by decreasing Glc-6-Pase and PEPCK gene expression through NR1D1. This novel regulatory pathway connects an influx of energy as fat from the gut (and subsequent apoA-IV secretion) with inhibition of hepatic glucose production.

Specific sequences in N termini of apolipoprotein A-IV modulate its anorectic effect

Rodent apoA-IV is expressed predominantly in small intestine and also expressed to a small extent in liver and hypothalamus. ApoA-IV has been shown to inhibit food intake in rats when injected centrally. In the current study, we hypothesize that a specific sequence within rat apoA-IV is responsible for mediating the anorectic effect. We use a bacterial expression system to generate truncation mutants (Δ249-371, Δ117-371 and Δ1-61) of rat apoA-IV and assess the ability of various regions of the molecule to inhibit food intake. The results indicate that a responsible sequence exists within the N-terminal 61 amino acids of rat apoA-IV. Synthetic peptides (1-30 EVTSDQVANVMWDYFTQLSNNAKEAVEQLQ, 1-15 EVTSDQVANVMWDYF and 17-30 QLSNNAKEAVEQLQ) were used to specify the region in between residues 1 and 30. A 14-mer peptide (17-30) encompassing this sequence was capable of reducing food intake in a dose-dependent manner whereas a peptide designed on a more C-terminal region (211-232) of apoA-IV (QEKLNHQMEGLAFQMKKNAEEL) failed to exhibit the dose-dependent anorectic effect. The isolation of this sequence provides a valuable tool for future work directed at identifying apoA-IV binding proteins and is a key step for exploring the potential of therapeutic manipulation of food intake via this pathway.