Circadian regulation of intestinal lipid absorption by apolipoprotein AIV involves forkhead transcription factors A2 and O1 and microsomal triglyceride transfer protein

Circadian Regulation of Apolipoproteins in the Brain: Implications in Lipid Metabolism and Disease

The circadian rhythm is a 24 h internal clock within the body that regulates various factors, including sleep, body temperature, and hormone secretion. Circadian rhythm disruption is an important risk factor for many diseases including neurodegenerative illnesses. The central and peripheral oscillators' circadian clock network controls the circadian rhythm in mammals. The clock genes govern the central clock in the suprachiasmatic nucleus (SCN) of the brain. One function of the circadian clock is regulating lipid metabolism. However, investigations of the circadian regulation of lipid metabolism-associated apolipoprotein genes in the brain are lacking. This review summarizes the rhythmic expression of clock genes and lipid metabolism-associated apolipoprotein genes within the SCN in Mus musculus. Nine of the twenty apolipoprotein genes identified from searching the published database (SCNseq and CircaDB) are highly expressed in the SCN. Most apolipoprotein genes (ApoE, ApoC1, apoA1, ApoH, ApoM, and Cln) show rhythmic expression in the brain in mice and thus might be regulated by the master clock. Therefore, this review summarizes studies on lipid-associated apolipoprotein genes in the SCN and other brain locations, to understand how apolipoproteins associated with perturbed cerebral lipid metabolism cause multiple brain diseases and disorders. This review describes recent advancements in research, explores current questions, and identifies directions for future research.

From worms to humans: Understanding intestinal lipid metabolism via model organisms

The intestine is responsible for efficient absorption and packaging of dietary lipids before they enter the circulatory system. This review provides a comprehensive overview of how intestinal enterocytes from diverse model organisms absorb dietary lipid and subsequently secrete the largest class of lipoproteins (chylomicrons) to meet the unique needs of each animal. We discuss the putative relationship between diet and metabolic disease progression, specifically Type 2 Diabetes Mellitus. Understanding the molecular response of intestinal cells to dietary lipid has the potential to undercover novel therapies to combat metabolic syndrome.

Bmal1 regulates production of larger lipoproteins by modulating cAMP-responsive element-binding protein H and apolipoprotein AIV

BACKGROUND AND AIMS

High plasma lipid/lipoprotein levels are risk factors for various metabolic diseases. We previously showed that circadian rhythms regulate plasma lipids and deregulation of these rhythms causes hyperlipidemia and atherosclerosis in mice. Here, we show that global and liver-specific brain and muscle aryl hydrocarbon receptor nuclear translocator-like 1 (Bmal1)-deficient mice maintained on a chow or Western diet developed hyperlipidemia, denoted by the presence of higher amounts of triglyceride-rich and apolipoprotein AIV (ApoAIV)-rich larger chylomicron and VLDL due to overproduction.

APPROACH AND RESULTS

Bmal1 deficiency decreased small heterodimer partner (Shp) and increased microsomal triglyceride transfer protein (MTP), a key protein that facilitates primordial lipoprotein assembly and secretion. Moreover, we show that Bmal1 regulates cAMP-responsive element-binding protein H (Crebh) to modulate ApoAIV expression and the assembly of larger lipoproteins. This is supported by the observation that Crebh-deficient and ApoAIV-deficient mice, along with Bmal1-deficient mice with knockdown of Crebh, had smaller lipoproteins. Further, overexpression of Bmal1 in Crebh-deficient mice had no effect on ApoAIV expression and lipoprotein size.

CONCLUSIONS

These studies indicate that regulation of ApoAIV and assembly of larger lipoproteins by Bmal1 requires Crebh. Mechanistic studies showed that Bmal1 regulates Crebh expression by two mechanisms. First, Bmal1 interacts with the Crebh promoter to control circadian regulation. Second, Bmal1 increases Rev-erbα expression, and nuclear receptor subfamily 1 group D member 1 (Nr1D1, Rev-erbα) interacts with the Crebh promoter to repress expression. In short, Bmal1 modulates both the synthesis of primordial lipoproteins and their subsequent expansion into larger lipoproteins by regulating two different proteins, MTP and ApoAIV, through two different transcription factors, Shp and Crebh. It is likely that disruptions in circadian mechanisms contribute to hyperlipidemia and that avoiding disruptions in circadian rhythms may limit/prevent hyperlipidemia and atherosclerosis.

Nonalcoholic fatty liver disease in CLOCK mutant mice

Nonalcoholic fatty liver disease (NAFLD) is becoming a major health issue as obesity increases around the world. We studied the effect of a circadian locomotor output cycles kaput (CLOCK) mutant (ClkΔ19/Δ19) protein on hepatic lipid metabolism in C57BL/6 Clkwt/wt and apolipoprotein E-deficient (Apoe-/-) mice. Both ClkΔ19/Δ19 and ClkΔ19/Δ19 Apoe-/- mice developed a full spectrum of liver diseases (steatosis, steatohepatitis, cirrhosis, and hepatocellular carcinoma) recognized in human NAFLD when challenged with a Western diet, lipopolysaccharide, or CoCl2. We identified induction of CD36 and hypoxia-inducible factor 1α (HIF1α) proteins as contributing factors for NAFLD. Mechanistic studies showed that WT CLOCK protein interacted with the E-box enhancer elements in the promoters of the proline hydroxylase domain (PHD) proteins to increase expression. In ClkΔ19/Δ19 mice, PHD levels were low, and HIF1α protein levels were increased. When its levels were high, HIF1α interacted with the Cd36 promoter to augment expression and enhance fatty acid uptake. Thus, these studies establish a regulatory link among circadian rhythms, hypoxia response, fatty acid uptake, and NAFLD. The mouse models described here may be useful for further mechanistic studies in the progression of liver diseases and in the discovery of drugs for the treatment of these disorders.

ApoB48 as an Efficient Regulator of Intestinal Lipid Transport

Fatty meals induce intestinal secretion of chylomicrons (CMs) containing apolipoprotein (Apo) B48. These CMs travel via the lymphatic system before entering the circulation. ApoB48 is produced after post-transcriptional RNA modification by Apobec-1 editing enzyme, exclusively in the small intestine of humans and most other mammals. In contrast, in the liver where Apobec-1 editing enzyme is not expressed (except in rats and mice), the unedited transcript encodes a larger protein, ApoB100, which is used in the formation of very low-density lipoproteins (VLDL) to transport liver-synthesized fat to peripheral tissues. Apobec-1 knockout (KO) mice lack the ability to perform ApoB RNA editing, and thus, express ApoB100 in the intestine. These mice, maintained on either a chow diet or high fat diet, have body weight gain and food intake comparable to their wildtype (WT) counterparts on the respective diet; however, they secrete larger triglyceride (TG)-rich lipoprotein particles and at a slower rate than the WT mice. Using a lymph fistula model, we demonstrated that Apobec-1 KO mice also produced fewer CMs and exhibited reduced lymphatic transport of TG in response to duodenal infusion of TG at a moderate dose; in contrast, the Apobec-1 KO and WT mice had similar lymphatic transport of TG when they received a high dose of TG. Thus, the smaller, energy-saving ApoB48 appears to play a superior role in comparison with ApoB100 in the control of intestinal lipid transport in response to dietary lipid intake, at least at low to moderate lipid levels.

Circadian Clock, Time-Restricted Feeding and Reproduction

The goal of this review was to seek a better understanding of the function and differential expression of circadian clock genes during the reproductive process. Through a discussion of how the circadian clock is involved in these steps, the identification of new clinical targets for sleep disorder-related diseases, such as reproductive failure, will be elucidated. Here, we focus on recent research findings regarding circadian clock regulation within the reproductive system, shedding new light on circadian rhythm-related problems in women. Discussions on the roles that circadian clock plays in these reproductive processes will help identify new clinical targets for such sleep disorder-related diseases.

Circadian Clock Regulation on Lipid Metabolism and Metabolic Diseases

The basic helix-loop-helix-PAS transcription factor (CLOCK, Circadian locomotor output cycles protein kaput) was discovered in 1994 as a circadian clock. Soon after its discovery, the circadian clock, Aryl hydrocarbon receptor nuclear translocator-like protein 1 (ARNTL, also call BMAL1), was shown to regulate adiposity and body weight by controlling on the brain hypothalamic suprachiasmatic nucleus (SCN). Farther, circadian clock genes were determined to exert several of lipid metabolic and diabetes effects, overall indicating that CLOCK and BMAL1 act as a central master circadian clock. A master circadian clock acts through the neurons and hormones, with expression in the intestine, liver, kidney, lung, heart, SCN of brain, and other various cell types of the organization. Among circadian clock genes, numerous metabolic syndromes are the most important in the regulation of food intake (via regulation of circadian clock genes or clock-controlled genes in peripheral tissue), which lead to a variation in plasma phospholipids and tissue phospholipids. Circadian clock genes affect the regulation of transporters and proteins included in the regulation of phospholipid metabolism. These genes have recently received increasing recognition because a pharmacological target of circadian clock genes may be of therapeutic worth to make better resistance against insulin, diabetes, obesity, metabolism syndrome, atherosclerosis, and brain diseases. In this book chapter, we focus on the regulation of circadian clock and summarize its phospholipid effect as well as discuss the chemical, physiology, and molecular value of circadian clock pathway regulation for the treatment of plasma lipids and atherosclerosis.

Quantitative Assays of Plasma Apolipoproteins

The apolipoproteins are well known for their roles in both health and disease, as components of plasma lipoprotein particles, such as high-density lipoprotein (HDL), low-density lipoprotein (LDL), very-low-density lipoprotein (VLDL), chylomicrons, and metabolic, vascular- and inflammation-related disorders, such as cardiovascular disease, atherosclerosis, metabolic syndrome, and diabetes. Increasingly, their roles in neurovascular and neurodegenerative disorders are also being elucidated. They play major roles in lipid and cholesterol transport between blood and organs and are, therefore, critical to maintenance and homeostasis of the lipidome, with apolipoprotein-lipid interactions, including cholesterol, fatty acids, triglycerides, phospholipids, and isoprostanes. Further, they have important pleiotropic roles related to aging and longevity, which are largely managed through their many structural variants, including multiple isoforms, and a diversity of post-translational modifications. Consequently, tools for the characterization and accurate quantification of apolipoproteins, including their diverse array of variant forms, are required to understand their salutary and disease related roles. In this chapter we outline three distinct quantitative approaches suitable for targeting apolipoproteins: (1) multiplex immunoassays, (2) mass spectrometric immunoassay, and (3) multiple reaction monitoring, mass spectrometric quantification. We also discuss management of pre-analytical and experimental design variables.

BMAL1 controls glucose uptake through paired-homeodomain transcription factor 4 in differentiated Caco-2 cells

The transcription factor aryl hydrocarbon receptor nuclear translocator-like protein-1 (BMAL1) is an essential regulator of the circadian clock, which controls the 24-h cycle of physiological processes such as nutrient absorption. To examine the role of BMAL1 in small intestinal glucose absorption, we used differentiated human colon adenocarcinoma cells (Caco-2 cells). Here, we show that BMAL1 regulates glucose uptake in differentiated Caco-2 cells and that this process is dependent on the glucose transporter sodium-glucose cotransporter 1 (SGLT1). Mechanistic studies show that BMAL1 regulates glucose uptake by controlling the transcription of SGLT1 involving paired-homeodomain transcription factor 4 (PAX4), a transcriptional repressor. This is supported by the observation that clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated endonuclease Cas9 (Cas9) knockdown of PAX4 increases SGLT1 and glucose uptake. Chromatin immunoprecipitation (ChIP) and ChIP-quantitative PCR assays show that the knockdown or overexpression of BMAL1 decreases or increases the binding of PAX4 to the hepatocyte nuclear factor 1-α binding site of the SGLT1 promoter, respectively. These findings identify BMAL1 as a critical mediator of small intestine carbohydrate absorption and SGLT1.

Circadian rhythms: a regulator of gastrointestinal health and dysfunction

Circadian rhythms regulate much of gastrointestinal physiology including cell proliferation, motility, digestion, absorption, and electrolyte balance. Disruption of circadian rhythms can have adverse consequences including the promotion of and/or exacerbation of a wide variety of gastrointestinal disorders and diseases. Areas covered: In this review, we evaluate some of the many gastrointestinal functions that are regulated by circadian rhythms and how dysregulation of these functions may contribute to disease. This review also discusses some common gastrointestinal disorders that are known to be influenced by circadian rhythms as well as speculation about the mechanisms by which circadian rhythm disruption promotes dysfunction and disease pathogenesis. We discuss how knowledge of circadian rhythms and the advent of chrono-nutrition, chrono-pharmacology, and chrono-therapeutics might influence clinical practice. Expert opinion: As our knowledge of circadian biology increases, it may be possible to incorporate strategies that take advantage of circadian rhythms and chronotherapy to prevent and/or treat disease.

Oleoylethanolamide differentially regulates glycerolipid synthesis and lipoprotein secretion in intestine and liver

Dietary fat absorption takes place in the intestine, and the liver mobilizes endogenous fat to other tissues by synthesizing lipoproteins that require apoB and microsomal triglyceride transfer protein (MTP). Dietary fat triggers the synthesis of oleoylethanolamide (OEA), a regulatory fatty acid that signals satiety to reduce food intake mainly by enhancing neural PPARα activity, in enterocytes. We explored OEA's roles in the assembly of lipoproteins in WT and Ppara ^-/- mouse enterocytes and hepatocytes, Caco-2 cells, and human liver-derived cells. In differentiated Caco-2 cells, OEA increased synthesis and secretion of triacylglycerols, apoB secretion in chylomicrons, and MTP expression in a dose-dependent manner. OEA also increased MTP activity and triacylglycerol secretion in WT and knockout primary enterocytes. In contrast to its intestinal cell effects, OEA reduced synthesis and secretion of triacylglycerols, apoB secretion, and MTP expression and activity in human hepatoma Huh-7 and HepG2 cells. Also, OEA reduced MTP expression and triacylglycerol secretion in WT, but not knockout, primary hepatocytes. These studies indicate differential effects of OEA on lipid synthesis and lipoprotein assembly: in enterocytes, OEA augments glycerolipid synthesis and lipoprotein assembly independent of PPARα. Conversely, in hepatocytes, OEA reduces MTP expression, glycerolipid synthesis, and lipoprotein secretion through PPARα-dependent mechanisms.

CREBH Regulates Systemic Glucose and Lipid Metabolism

The cyclic adenosine monophosphate (cAMP)-responsive element-binding protein H (CREBH, encoded by CREB3L3) is a membrane-bound transcriptional factor that primarily localizes in the liver and small intestine. CREBH governs triglyceride metabolism in the liver, which mediates the changes in gene expression governing fatty acid oxidation, ketogenesis, and apolipoproteins related to lipoprotein lipase (LPL) activation. CREBH in the small intestine reduces cholesterol transporter gene Npc1l1 and suppresses cholesterol absorption from diet. A deficiency of CREBH in mice leads to severe hypertriglyceridemia, fatty liver, and atherosclerosis. CREBH, in synergy with peroxisome proliferator-activated receptor α (PPARα), has a crucial role in upregulating Fgf21 expression, which is implicated in metabolic homeostasis including glucose and lipid metabolism. CREBH binds to and functions as a co-activator for both PPARα and liver X receptor alpha (LXRα) in regulating gene expression of lipid metabolism. Therefore, CREBH has a crucial role in glucose and lipid metabolism in the liver and small intestine.

The role of the daily feeding rhythm in the regulation of the day/night rhythm in triglyceride secretion in rats

Plasma triglyceride (TG) levels show a clear daily rhythm, however, thus far it is still unknown whether this rhythm results from a daily rhythm in TG production, TG uptake or both. Previous studies have shown that feeding activity affects plasma TG concentrations, but it is not clear how the daily rhythm in feeding activity affects plasma TG concentrations. In the present study, we measured plasma TG concentrations and TG secretion rates in rats at 6 Zeitgeber times to investigate whether plasma TG concentrations and TG secretion show a daily rhythm. We found that plasma TG concentrations and TG secretion show a significant day/night rhythm. Next, we removed the daily rhythm in feeding behavior by introducing a 6-meals-a-day (6M) feeding schedule to investigate whether the daily rhythm in feeding behavior is necessary to maintain the daily rhythm in TG secretion. We found that the day/night rhythm in TG secretion was abolished under 6M feeding conditions. Hepatic apolipoprotein B (ApoB) and microsomal TG transfer protein (Mttp), which are both involved in TG secretion, also lost their daily rhythmicity under 6M feeding conditions. Together, these results indicate that: (1) the daily rhythm in TG secretion contributes to the formation of a day/night rhythm in plasma TG levels and (2) a daily feeding rhythm is essential for maintaining the daily rhythm in TG secretion.

Global and hepatocyte-specific ablation of Bmal1 induces hyperlipidaemia and enhances atherosclerosis

Circadian rhythms controlled by clock genes affect plasma lipids. Here we show that global ablation of Bmal1 in Apoe-/- and Ldlr-/- mice and its liver-specific ablation in Apoe-/- (L-Bmal1-/-Apoe-/-) mice increases, whereas overexpression of BMAL1 in L-Bmal1-/-Apoe-/- and Apoe-/-mice decreases hyperlipidaemia and atherosclerosis. Bmal1 deficiency augments hepatic lipoprotein secretion and diminishes cholesterol excretion to the bile. Further, Bmal1 deficiency reduces expression of Shp and Gata4. Reductions in Shp increase Mtp expression and lipoprotein production, whereas reductions in Gata4 diminish Abcg5/Abcg8 expression and biliary cholesterol excretion. Forced SHP expression normalizes lipoprotein secretion with no effect on biliary cholesterol excretion, while forced GATA4 expression increases cholesterol excretion to the bile and reduces plasma lipids in L-Bmal1-/-Apoe-/- and Apoe-/- mice. Thus, our data indicate that Bmal1 modulates lipoprotein production and biliary cholesterol excretion by regulating the expression of Mtp and Abcg5/Abcg8 via Shp and Gata4.

Targeting microsomal triglyceride transfer protein and lipoprotein assembly to treat homozygous familial hypercholesterolemia

Homozygous familial hypercholesterolemia (HoFH) is a polygenic disease arising from defects in the clearance of plasma low-density lipoprotein (LDL), which results in extremely elevated plasma LDL cholesterol (LDL-C) and increased risk of atherosclerosis, coronary heart disease, and premature death. Conventional lipid-lowering therapies, such as statins and ezetimibe, are ineffective at lowering plasma cholesterol to safe levels in these patients. Other therapeutic options, such as LDL apheresis and liver transplantation, are inconvenient, costly, and not readily available. Recently, lomitapide was approved by the Federal Drug Administration as an adjunct therapy for the treatment of HoFH. Lomitapide inhibits microsomal triglyceride transfer protein (MTP), reduces lipoprotein assembly and secretion, and lowers plasma cholesterol levels by over 50%. Here, we explain the steps involved in lipoprotein assembly, summarize the role of MTP in lipoprotein assembly, explore the clinical and molecular basis of HoFH, and review pre-clinical studies and clinical trials with lomitapide and other MTP inhibitors for the treatment of HoFH. In addition, ongoing research and new approaches underway for better treatment modalities are discussed.

Circadian Regulation of Macronutrient Absorption

Various intestinal functions exhibit circadian rhythmicity. Disruptions in these rhythms as in shift workers and transcontinental travelers are associated with intestinal discomfort. Circadian rhythms are controlled at the molecular level by core clock and clock-controlled genes. These clock genes are expressed in intestinal cells, suggesting that they might participate in the circadian regulation of intestinal functions. A major function of the intestine is nutrient absorption. Here, we will review absorption of proteins, carbohydrates, and lipids and circadian regulation of various transporters involved in their absorption. A better understanding of circadian regulation of intestinal absorption might help control several metabolic disorders and attenuate intestinal discomfort associated with disruptions in sleep-wake cycles.

New Insights into the Regulation of Chylomicron Production

Dietary lipids are efficiently absorbed by the small intestine, incorporated into triglyceride-rich lipoproteins (chylomicrons), and transported in the circulation to various tissues. Intestinal lipid absorption and mobilization and chylomicron synthesis and secretion are highly regulated processes. Elevated chylomicron production rate contributes to the dyslipidemia seen in common metabolic disorders such as insulin-resistant states and type 2 diabetes and likely increases the risk for atherosclerosis seen in these conditions. An in-depth understanding of the regulation of chylomicron production may provide leads for the development of drugs that could be of therapeutic utility in the prevention of dyslipidemia and atherosclerosis. Chylomicron secretion is subject to regulation by various factors, including diet, body weight, genetic variants, hormones, nutraceuticals, medications, and emerging interventions such as bariatric surgical procedures. In this review we discuss the regulation of chylomicron production, mechanisms that underlie chylomicron dysregulation, and potential avenues for future research.

Circadian regulators of intestinal lipid absorption

Among all the metabolites present in the plasma, lipids, mainly triacylglycerol and diacylglycerol, show extensive circadian rhythms. These lipids are transported in the plasma as part of lipoproteins. Lipoproteins are synthesized primarily in the liver and intestine and their production exhibits circadian rhythmicity. Studies have shown that various proteins involved in lipid absorption and lipoprotein biosynthesis show circadian expression. Further, intestinal epithelial cells express circadian clock genes and these genes might control circadian expression of different proteins involved in intestinal lipid absorption. Intestinal circadian clock genes are synchronized by signals emanating from the suprachiasmatic nuclei that constitute a master clock and from signals coming from other environmental factors, such as food availability. Disruptions in central clock, as happens due to disruptions in the sleep/wake cycle, affect intestinal function. Similarly, irregularities in temporal food intake affect intestinal function. These changes predispose individuals to various metabolic disorders, such as metabolic syndrome, obesity, diabetes, and atherosclerosis. Here, we summarize how circadian rhythms regulate microsomal triglyceride transfer protein, apoAIV, and nocturnin to affect diurnal regulation of lipid absorption.

Expressions of tight junction proteins Occludin and Claudin-1 are under the circadian control in the mouse large intestine: implications in intestinal permeability and susceptibility to colitis

Background & Aims

The circadian clock drives daily rhythms in behavior and physiology. A recent study suggests that intestinal permeability is also under control of the circadian clock. However, the precise mechanisms remain largely unknown. Because intestinal permeability depends on tight junction (TJ) that regulates the epithelial paracellular pathway, this study investigated whether the circadian clock regulates the expression levels of TJ proteins in the intestine.

Methods

The expression levels of TJ proteins in the large intestinal epithelium and colonic permeability were analyzed every 4, 6, or 12 hours between wild-type mice and mice with a mutation of a key clock gene Period2 (Per2; mPer2^m/m). In addition, the susceptibility to dextran sodium sulfate (DSS)-induced colitis was compared between wild-type mice and mPer2^m/m mice.

Results

The mRNA and protein expression levels of Occludin and Claudin-1 exhibited daily variations in the colonic epithelium in wild-type mice, whereas they were constitutively high in mPer2^m/m mice. Colonic permeability in wild-type mice exhibited daily variations, which was inversely associated with the expression levels of Occludin and Claudin-1 proteins, whereas it was constitutively low in mPer2^m/m mice. mPer2^m/m mice were more resistant to the colonic injury induced by DSS than wild-type mice.

Conclusions

Occludin and Claudin-1 expressions in the large intestine are under the circadian control, which is associated with temporal regulation of colonic permeability and also susceptibility to colitis.

Plasticity of gastro-intestinal vagal afferent endings

Vagal afferents are a vital link between the peripheral tissue and central nervous system (CNS). There is an abundance of vagal afferents present within the proximal gastrointestinal tract which are responsible for monitoring and controlling gastrointestinal function. Whilst essential for maintaining homeostasis there is a vast amount of literature emerging which describes remarkable plasticity of vagal afferents in response to endogenous as well as exogenous stimuli. This plasticity for the most part is vital in maintaining healthy processes; however, there are increased reports of vagal plasticity being disrupted in pathological states, such as obesity. Many of the disruptions, observed in obesity, have the potential to reduce vagal afferent satiety signalling which could ultimately perpetuate the obese state. Understanding how plasticity occurs within vagal afferents will open a whole new understanding of gut function as well as identify new treatment options for obesity.

Transcriptional regulation of apolipoprotein A-IV by the transcription factor CREBH

cAMP responsive element-binding protein H (CREBH) is an endoplasmic reticulum (ER) anchored transcription factor that is highly expressed in the liver and small intestine and implicated in nutrient metabolism and proinflammatory response. ApoA-IV is a glycoprotein secreted primarily by the intestine and to a lesser degree by the liver. ApoA-IV expression is suppressed in CREBH-deficient mice and strongly induced by enforced expression of the constitutively active form of CREBH, indicating that CREBH is the major transcription factor regulating Apoa4 gene expression. Here, we show that CREBH directly controls Apoa4 expression through two tandem CREBH binding sites (5'-CCACGTTG-3') located on the promoter, which are conserved between human and mouse. Chromatin immunoprecipitation and electrophoretic mobility-shift assays demonstrated specific association of CREBH with the CREBH binding sites. We also demonstrated that a substantial amount of CREBH protein was basally processed to the active nuclear form in normal mouse liver, which was further increased in steatosis induced by high-fat diet or fasting, increasing apoA-IV expression. However, we failed to find significant activation of CREBH in response to ER stress, arguing against the critical role of CREBH in ER stress response.

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.