Regulation of Intestinal and Hypothalamic Apolipoprotein A-IV

Apolipoprotein A4 Elevates Sympathetic Activity and Thermogenesis in Male Mice

Long-chain fatty acids induce apolipoprotein A4 (APOA4) production in the small intestine and activate brown adipose tissue (BAT) thermogenesis. The increase in BAT thermogenesis enhances triglyceride clearance and insulin sensitivity. Acute administration of recombinant APOA4 protein elevates BAT thermogenesis in chow-fed mice. However, the physiological role of continuous infusion of recombinant APOA4 protein in regulating sympathetic activity, thermogenesis, and lipid and glucose metabolism in low-fat-diet (LFD)-fed mice remained elusive. The hypothesis of this study was that continuous infusion of mouse APOA4 protein would increase sympathetic activity and thermogenesis in BAT and subcutaneous inguinal white adipose tissue (IWAT), attenuate plasma lipid levels, and improve glucose tolerance. To test this hypothesis, sympathetic activity, BAT temperature, energy expenditure, body weight, fat mass, caloric intake, glucose tolerance, and levels of BAT and IWAT thermogenic and lipolytic proteins, plasma lipids, and markers of fatty acid oxidation in the liver in mice with APOA4 or saline treatment were measured. Plasma APOA4 levels were elevated, BAT temperature and thermogenesis were upregulated, and plasma triglyceride (TG) levels were reduced, while body weight, fat mass, caloric intake, energy expenditure, and plasma cholesterol and leptin levels were comparable between APOA4- and saline-treated mice. Additionally, APOA4 infusion stimulated sympathetic activity in BAT and liver but not in IWAT. APOA4-treated mice had greater fatty acid oxidation but less TG content in the liver than saline-treated mice had. Plasma insulin in APOA4-treated mice was lower than that in saline-treated mice after a glucose challenge. In conclusion, continuous infusion of mouse APOA4 protein stimulated sympathetic activity in BAT and the liver, elevated BAT thermogenesis and hepatic fatty acid oxidation, and consequently attenuated levels of plasma and hepatic TG and plasma insulin without altering caloric intake, body weight gain and fat mass.

Old Paradoxes and New Opportunities for Appetite Control in Obesity

Human obesity is accompanied by alterations in the blood concentrations of multiple circulating appetite regulators. Paradoxically, most of the appetite-inhibitory hormones are elevated in nonsyndromic obesity, while most of the appetite stimulatory hormones are reduced, perhaps reflecting vain attempts of regulation by inefficient feedback circuitries. In this context, it is important to understand which appetite regulators exhibit a convergent rather than paradoxical behavior and hence are likely to contribute to the maintenance of the obese state. Pharmacological interventions in obesity should preferentially consist of the supplementation of deficient appetite inhibitors or the neutralization of excessive appetite stimulators. Here, we critically analyze the current literature on appetite-regulatory peptide hormones. We propose a short-list of appetite modulators that may constitute the best candidates for therapeutic interventions.

Changes in serum levels of Apo AIV in patients with newly diagnosed hyperthyroidism and hypothyroidism: a preliminary study

OBJECTIVES

Apolipoprotein AIV has a role in chylomicrons and lipid secretion and catabolism. Also, Apo-AIV plays a role in the regulation of appetite and satiety. Previous studies on rats have shown that hyperthyroidism and hypothyroidism are associated with significant changes in Apo-AIV serum levels. There has been no research on serum Apo-AIV changes in hyper and hypothyroidism in humans.

METHODS

This case-control study was performed on new patients with hyper and hypothyroidism. Eighteen patients with hyperthyroidism and 18 patients with hypothyroidism enrolled in the study. After 12 weeks treatment blood samples were recruited. If euthyroidism was achieved, serum Apo-AIV level was measured. Eighteen euthyroid healthy individuals without thyroid disease were chosen as the control group from general population.

RESULTS

Serum levels of Apo-AIV before treatment in hypothyroidism, hyperthyroidism and in the control group were 85.61, 110.66 and 33.51 mg/dL respectively (p<0.001), which was significantly higher in hyperthyroid patients than hypothyroidism and control group. In patients with hyperthyroidism there was a significant decrease in serum levels of Apo-AIV after treatment (p=0.044). However in hypothyroidism a non-significant elevation in serum levels of Apo-AIV was observed (p=0.403). Furthermore, serum levels of Apo-AIV after treatment were significantly higher in both hyperthyroidism and hypothyroidism in comparison to control group (p<0.001).

CONCLUSIONS

The results of this study for the first time showed that the serum level of Apo-AIV is increased in patients with hyperthyroidism and is decreased in patients with hypothyroidism, and after treatment, there was a significant difference with the control group.

Apolipoprotein A-IV Enhances Fatty Acid Uptake by Adipose Tissues of Male Mice via Sympathetic Activation

Apolipoprotein A-IV (ApoA-IV) synthesized by the gut regulates lipid metabolism. Sympathetic innervation of adipose tissues also controls lipid metabolism. We hypothesized that ApoA-IV required sympathetic innervation to increase fatty acid (FA) uptake by adipose tissues and brown adipose tissue (BAT) thermogenesis. After 3 weeks feeding of either a standard chow diet or a high-fat diet (HFD), mice with unilateral denervation of adipose tissues received intraperitoneal administration of recombinant ApoA-IV protein and intravenous infusion of lipid mixture with radioactive triolein. In chow-fed mice, ApoA-IV administration increased FA uptake by intact BAT but not the contralateral denervated BAT or intact white adipose tissue (WAT). Immunoblots showed that, in chow-fed mice, ApoA-IV increased expression of lipoprotein lipase and tyrosine hydroxylase in both intact BAT and inguinal WAT (IWAT), while ApoA-IV enhanced protein levels of β3 adrenergic receptor, adipose triglyceride lipase, and uncoupling protein 1 in the intact BAT only. In HFD-fed mice, ApoA-IV elevated FA uptake by intact epididymal WAT (EWAT) but not intact BAT or IWAT. ApoA-IV increased sympathetic activity assessed by norepinephrine turnover (NETO) rate in BAT and EWAT of chow-fed mice, whereas it elevated NETO only in EWAT of HFD-fed mice. These observations suggest that, in chow-fed mice, ApoA-IV activates sympathetic activity of BAT and increases FA uptake by BAT via innervation, while in HFD-fed mice, ApoA-IV stimulates sympathetic activity of EWAT to shunt FAs into the EWAT.

The role of apolipoprotein A-IV in regulating glucagon-like peptide-1 secretion

Both glucagon-like peptide-1 (GLP-1) and apolipoprotein A-IV (apoA-IV) are produced from the gut and enhance postprandial insulin secretion. This study investigated whether apoA-IV regulates nutrient-induced GLP-1 secretion and whether apoA-IV knockout causes compensatory GLP-1 release. Using lymph-fistula-mice, we first determined lymphatic GLP-1 secretion by administering apoA-IV before an intraduodenal Ensure infusion. apoA-IV changed neither basal nor Ensure-induced GLP-1 secretion relative to saline administration. We then assessed GLP-1 in apoA-IV-/- and wild-type (WT) mice administered intraduodenal Ensure. apoA-IV-/- mice had comparable lymph flow, lymphatic triglyceride, glucose, and protein outputs as WT mice. Intriguingly, apoA-IV-/- mice had higher lymphatic GLP-1 concentration and output than WT mice 30 min after Ensure administration. Increased GLP-1 was also observed in plasma of apoA-IV-/- mice at 30 min. apoA-IV-/- mice had comparable total gut GLP-1 content relative to WT mice under fasting, but a lower GLP-1 content 30 min after Ensure administration, suggesting that more GLP-1 was secreted. Moreover, an injection of apoA-IV protein did not reverse the increased GLP-1 secretion in apoA-IV-/- mice. Finally, we assessed gene expression of GLUT-2 and the lipid receptors, including G protein-coupled receptor (GPR) 40, GPR119, and GPR120 in intestinal segments. GLUT-2, GPR40 and GPR120 mRNAs were unaltered by apoA-IV knockout. However, ileal GPR119 mRNA was significantly increased in apoA-IV-/- mice. GPR119 colocalizes with GLP-1 in ileum and stimulates GLP-1 secretion by sensing OEA, lysophosphatidylcholine, and 2-monoacylglycerols. We suggest that increased ileal GPR119 is a potential mechanism by which GLP-1 secretion is enhanced in apoA-IV-/- mice.

ApoA-IV: current and emerging roles in intestinal lipid metabolism, glucose homeostasis, and satiety

Apolipoprotein A-IV (apoA-IV) is secreted by the small intestine on chylomicrons into intestinal lymph in response to fat absorption. Many physiological functions have been ascribed to apoA-IV, including a role in chylomicron assembly and lipid metabolism, a mediator of reverse-cholesterol transport, an acute satiety factor, a regulator of gastric function, and, finally, a modulator of blood glucose homeostasis. The purpose of this review is to update our current view of intestinal apoA-IV synthesis and secretion and the physiological roles of apoA-IV in lipid metabolism and energy homeostasis, and to underscore the potential for intestinal apoA-IV to serve as a therapeutic target for the treatment of diabetes and obesity-related disease.

Apolipoprotein A-IV: a protein intimately involved in metabolism

The purpose of this review is to summarize our current understanding of the physiological roles of apoA-IV in metabolism, and to underscore the potential for apoA-IV to be a focus for new therapies aimed at the treatment of diabetes and obesity-related disorders. ApoA-IV is primarily synthesized by the small intestine, attached to chylomicrons by enterocytes, and secreted into intestinal lymph during fat absorption. In circulation, apoA-IV is associated with HDL and chylomicron remnants, but a large portion is lipoprotein free. Due to its anti-oxidative and anti-inflammatory properties, and because it can mediate reverse-cholesterol transport, proposed functions of circulating apoA-IV have been related to protection from cardiovascular disease. This review, however, focuses primarily on several properties of apoA-IV that impact other metabolic functions related to food intake, obesity, and diabetes. In addition to participating in triglyceride absorption, apoA-IV can act as an acute satiation factor through both peripheral and central routes of action. It also modulates glucose homeostasis through incretin-like effects on insulin secretion, and by moderating hepatic glucose production. While apoA-IV receptors remain to be conclusively identified, the latter modes of action suggest that this protein holds therapeutic promise for treating metabolic disease.

Oral administration of omega-7 palmitoleic acid induces satiety and the release of appetite-related hormones in male rats

We have analyzed the effect of palmitoleic acid on short-term food intake in male rats. Administration of omega-7 palmitoleic acid by oral gavage significantly decreased food intake compared to palmitic acid, omega-9 oleic acid, or a vehicle control. Palmitoleic acid exhibited a dose-dependent effect in this context and did not cause general malaise. A triglyceride form of palmitoleate also decreased food intake, whereas olive oil, which is rich in oleic acid, did not. Palmitoleic acid accumulated within the small intestine in a dose-dependent fashion and elevated levels of the satiety hormone cholecystokinin (CCK). Both protein and mRNA levels of CCK were affected in this context. The suppression of food intake by palmitoleic acid was attenuated by intravenous injection of devazepide, a selective peripheral CCK receptor antagonist. Palmitoleic acid did not alter the expression of peroxisome proliferator-activated receptor alpha (PPARα) target genes, and a PPARα antagonist did not affect palmitoleic acid-induced satiety. This suggests that the PPARα pathway might not be involved in suppressing food intake in response to palmitoleic acid. We have shown that orally administered palmitoleic acid induced satiety, enhanced the release of satiety hormones in rats.

Apolipoprotein A-IV improves glucose homeostasis by enhancing insulin secretion

Apolipoprotein A-IV (apoA-IV) is secreted by the small intestine in response to fat absorption. Here we demonstrate a potential role for apoA-IV in regulating glucose homeostasis. ApoA-IV-treated isolated pancreatic islets had enhanced insulin secretion under conditions of high glucose but not of low glucose, suggesting a direct effect of apoA-IV to enhance glucose-stimulated insulin release. This enhancement involves cAMP at a level distal to Ca(2+) influx into the β cells. Knockout of apoA-IV results in compromised insulin secretion and impaired glucose tolerance compared with WT mice. Challenging apoA-IV(-/-) mice with a high-fat diet led to fasting hyperglycemia and more severe glucose intolerance associated with defective insulin secretion than occurred in WT mice. Administration of exogenous apoA-IV to apoA-IV(-/-) mice improved glucose tolerance by enhancing insulin secretion in mice fed either chow or a high-fat diet. Finally, we demonstrate that exogenous apoA-IV injection decreases blood glucose levels and stimulates a transient increase in insulin secretion in KKAy diabetic mice. These results suggest that apoA-IV may provide a therapeutic target for the regulation of glucose-stimulated insulin secretion and treatment of diabetes.

Apolipoprotein A-IV, a putative satiety/antiatherogenic factor, rises after gastric bypass

Roux-en-Y gastric bypass surgery (RYGBP) leads to improvements in satiety and obesity-related comorbidities. The mechanism(s) underlying these improvements are not known but may be revealed in part by discovery proteomics. Therefore, fasting plasma was collected from 12 subjects (mean BMI >45) during RYGBP and during a second procedure approximately 17 months later. Body weight, obesity-related comorbidities, and medication use were decreased after RYGBP. Mass spectrometry-based proteomic analysis was performed on a subset of seven samples using isobaric isotope-coded affinity tags (four plex iTRAQ). Initial proteomic analysis (n = 7) quantified and identified hundreds of plasma proteins. Manual inspection of the data revealed a 2.6 +/- 0.5-fold increase in apolipoprotein A-IV (apo A-IV, gene designation: APOA4), a approximately 46-kDa glycoprotein synthesized mainly in the bypassed small bowel and liver after RYGBP. The change in apo A-IV was significantly greater than other apolipoproteins. Immunoblot analysis of the full longitudinal sample set (n = 12) indicated even higher increases (8.3 +/- 0.2 fold) in apo A-IV. Thus iTRAQ may underestimate the changes in protein concentrations compared to western blotting of apo A-IV. Apo A-IV inhibits gastric emptying and serves as a satiety factor whose synthesis and secretion are increased by the ingestion of dietary fat. It also possesses anti-inflammatory and antiatherogenic properties. Based on these functions, we speculate changes in apo A-IV may contribute to weight loss as well as the improvements in inflammation and cardiovascular disease after RYGBP. In addition, the findings provide evidence validating the use of iTRAQ proteomics in discovery-based studies of post-RYGBP improvements in obesity-related medical comorbidities.

Apolipoprotein A-IV interacts synergistically with melanocortins to reduce food intake

Apolipoprotein (apo) A-IV is an anorexigenic gastrointestinal peptide that is also synthesized in the hypothalamus. The goal of these experiments was to determine whether apo A-IV interacts with the central melanocortin (MC) system in the control of feeding. The third ventricular (i3vt) administration of a subthreshold dose of apo A-IV (0.5 microg) potentiated i3vt MC-induced (metallothionein-II, 0.03 nmol) suppression of 30-min feeding in Long-Evans rats. A subthreshold dose of the MC antagonist (SHU9119, 0.1 nmol, i3vt) completely attenuated the anorectic effect of i3vt apo A-IV (1.5 microg). The i3vt apo A-IV significantly elevated the expression of c-Fos in neurons of the paraventricular nucleus of the hypothalamus, but not in the arcuate nucleus or median eminence. In addition, c-Fos expression was not colocalized with proopiomelanocortin-positive neurons. These data support a synergistic interaction between apo A-IV and melanocortins that reduces food intake by acting downstream of the arcuate.

Endocrine and exocrine secretion of leptin by the gastric mucosa

Leptin is a hormone that plays important roles in nutritional status and in obesity. By means of immunocytochemistry, two populations of leptin-secreting cells were found in the lower half of the gastric mucosa. One consists of numerous large cells located around the gastric pits, the Chief epithelial cells, whereas the second refers to much smaller cells, strongly stained, few in number, and scattered between the gastric pits, the endocrine cells. By double immunostaining, leptin and pepsinogen were colocalized in the Chief cells, whereas the endocrine cells were positive only for leptin. Immunoelectron microscopy showed that leptin is present along the rough endoplasmic reticulum-Golgi-granules secretory pathways of the Chief and endocrine cells. On the other hand, leptin-receptor (long and short forms) immunolabelings, although absent in the gastric epithelial cell plasma membranes, were present in enterocytes at the level of their apical and basolateral membranes. Duodenal, jejunal, and ileal enterocytes displayed similar labelings for the leptin receptor. Thus, exocrine and endocrine secretions of leptin together with the presence of leptin receptors on enterocyte plasma membranes constitute a gastroenteric axis that coordinates the role played by leptin in the digestive tract.

Intestinal apolipoprotein A-IV gene transcription is controlled by two hormone-responsive elements: a role for hepatic nuclear factor-4 isoforms

In the small intestine, the expression of the apolipoprotein (apo) C-III and A-IV genes is restricted to the enterocytes of the villi. We have previously shown that, in transgenic mice, specific expression of the human apo C-III requires a hormone-responsive element (HRE) located in the distal region of the human apoA-IV promoter. This HRE binds the hepatic nuclear factors (HNF)-4alpha and gamma. Here, intraduodenal injections in mice and infections of human enterocytic Caco-2/TC7 cells with an adenovirus expressing a dominant-negative form of HNF-4alpha repress the expression of the apoA-IV gene, demonstrating that HNF-4 controls the apoA-IV gene expression in enterocytes. We show that HNF-4alpha and gamma functionally interact with a second HRE present in the proximal region of the human apoA-IV promoter. New sets of transgenic mice expressing mutated forms of the promoter, combined with the human apo C-III enhancer, demonstrate that, whereas a single HRE is sufficient to reproduce the physiological cephalo-caudal gradient of apoA-IV gene expression, both HREs are required for expression that is restricted to villi. The combination of multiple HREs may specifically recruit regulatory complexes associating HNF-4 and either coactivators in villi or corepressors in crypts.

Peptide YY and Neuropeptide Y Induce Villin Expression, Reduce Adhesion, and Enhance Migration in Small Intestinal Cells through the Regulation of CD63, Matrix Metalloproteinase-3, and Cdc42 Activity

Peptide YY (PYY) and neuropeptide Y (NPY) are regulatory peptides synthesized in the intestine and brain, respectively, that modify physiological functions affecting nutrient assimilation and feeding behavior. Because PYY and NPY also alter the expression of intestine-specific differentiation marker proteins and the tetraspanin CD63, which is involved in cell adhesion, we investigated whether intestinal cell differentiation could be linked to mucosal cell adhesion and migration through these peptides. PYY and NPY significantly decreased cell adhesion and increased cell migration in a dose-dependent manner prior to cell confluency in our model system, non-tumorigenic small intestinal hBRIE 380i cells. Both peptides reduced CD63 expression and CD63-dependent cell adhesion. CD63 overexpression increased and antisense CD63 cDNA decreased intestinal cell adhesion. In parallel, both PYY and NPY increased expression of matrix metalloproteinase-3 (MMP-3) to a level sufficient to induce cell migration by activating the Rho GTPase Cdc42. The effects of both peptides on cell migration were blocked in cells constitutively overexpressing dominant-negative Cdc42. PYY and NPY also significantly induced the expression of the differentiation marker villin, which could be eliminated by an MMP inhibitor at a concentration that inhibits cell migration. Increased MMP-3 activity, which enhanced cell migration, also induced villin mRNA levels. Therefore, these data indicate that the alteration of adhesion and migration by PYY and NPY occurs in part by synchronous modulation of three proteins that are involved in extracellular matrix-basolateral membrane interactions, CD63, MMP-3 and Cdc42, and that PYY/NPY regulation of expression of mucosal proteins such as villin is linked to the process of cell migration and adhesion.

Estrogen-related receptor alpha (ERRalpha) is a transcriptional regulator of apolipoprotein A-IV and controls lipid handling in the intestine

The estrogen-related receptor alpha (ERRalpha) is an orphan member of the superfamily of nuclear receptors involved in the control of energy metabolism. In particular, ERRalpha induces a high energy expenditure in the presence of the coactivator PGC-1alpha. However, ERRalpha knockout mice have reduced fat mass and are resistant to diet-induced obesity. ERRalpha is expressed in epithelial cells of the small intestine, and because the intestine is the first step in the energy chain, we investigated whether ERRalpha plays a function in dietary energy handling. Gene expression profiling in the intestine identified a subset of genes involved in oxidative phosphorylation that were down-regulated in the absence of ERRalpha. In support of the physiological role of ERRalpha in this pathway, isolated enterocytes from ERRalpha knockout mice display lower capacity for beta-oxidation. Microarray results also show altered expression of genes involved in dietary lipid digestion and absorption, such as pancreatic lipase-related protein 2 (PLRP2), fatty acid-binding protein 1 and 2 (L-FABP and I-FABP), and apolipoprotein A-IV (apoA-IV). In agreement, we found that ERRalpha-/- pups exhibit significant lipid malabsorption. We further show that the apoA-IV promoter is a direct target of ERRalpha and that its presence is required to maintain basal level but not feeding-induced regulation of the apoA-IV gene in mice. ERRalpha, in cooperation with PGC-1alpha, activates the apoA-IV promoter via interaction with the apoC-III enhancer in both human and mouse. Our results demonstrate that apoA-IV is a direct ERRalpha target gene and suggest a function for ERRalpha in intestinal fat transport, a crucial step in energy balance.

HNF-4-dependent induction of apolipoprotein A-IV gene transcription by an apical supply of lipid micelles in intestinal cells

Apolipoprotein (apo) A-IV, a component of triglyceride-rich lipoproteins secreted by the small intestine, has been shown to play an important role in the control of lipid homeostasis. Numerous studies have described the induction of apoA-IV gene expression by lipids, but the molecular mechanisms involved in this process remain unknown. In this study, we have demonstrated that a lipid bolus induced transcription of the apoA-IV gene in transgenic mice and that the regulatory region of the apoA-IV gene, composed of the apoC-III enhancer and the apoA-IV promoter (eC3-A4), was responsible for this induction. In enterocyte Caco-2/TC7 cells, a permanent supply of lipids at the basal pole induced expression of the apoA-IV gene both at the transcriptional level and through mRNA stabilization. ApoA-IV gene transcription and protein secretion were further induced by an apical supply of complex lipid micelles mimicking the composition of duodenal micelles, and this effect was not reproduced by apical delivery of different combinations of micelle components. Only induction of the apoA-IV gene by lipid micelles involved the participation of hepatic nuclear factor (HNF)-4, as demonstrated using a dominant negative form of this transcription factor. Accordingly, lipid micelles increased the DNA binding activity of HNF-4 on the eC3-A4 region. These results emphasize the importance of physiological delivery of dietary lipids on apoA-IV gene expression and the implication of HNF-4 in this regulation.