Role of hepatocyte nuclear factor 4alpha (HNF4alpha) in hepatic lipid homeostasis: Adaptation of liver to nutritional signals is regulated by several transcription factors that are modulated by intracellular metabolites. A transcription factor network under the control of HNF4alpha that coordinates the reciprocal expression of fatty acid transport and metabolizing enzymes during fasting and feeding conditions. Hes6 is identified as a novel HNF4alpha target, which in normally fed animals, together with HNF4alpha, maintains PPARgamma expression at low levels and represses several PPARalpha-regulated genes. During fasting, Hes6 expression is diminished, and peroxisome proliferator-activated receptor alpha (PPARalpha) replaces the HNF4alpha/Hes6 complex on regulatory regions of target genes to activate transcription. Gene expression and promoter occupancy analyses confirmed that HNF4alpha is a direct activator of the Pparalpha gene in vivo and that its expression is subject to feedback regulation by PPARalpha and Hes6 proteins. These results establish the fundamental role of dynamic regulatory interactions between HNF4alpha, Hes6, PPARalpha, and PPARgamma in the coordinated expression of genes involved in fatty acid transport and metabolism. Role of HNF4alpha in the intestine: HNF4alpha is highly expressed in the intestinal epithelium from duodenum to colon and from crypt to villus. The homeostasis of this constantly renewing epithelium relies on an integrated control of proliferation, differentiation, and apoptosis, as well as on the functional architecture of the epithelial cells. In order to determine the consequences of HNF4alpha loss in the adult intestinal epithelium, we used a tamoxifen-inducible Cre-loxP system to inactivate the Hnf4alpha gene. Loss of HNF4alpha led to increased proliferation in crypts and increased expression of several genes controlled by the Wnt/beta-catenin system. This control of the Wnt/beta-catenin signaling pathway by HNF4alpha was confirmed in vitro. Cell lineage was affected, as indicated by an increased number of goblet cells and an impairment of enterocyte and enteroendocrine cell maturation. In the absence of HNF4alpha, cell-cell junctions were destabilized and paracellular intestinal permeability increased. These results showed that HNF4alpha modulates Wnt/beta-catenin signaling and controls intestinal epithelium homeostasis, cell function, and cell architecture. HNF4alpha regulates the intestinal balance between proliferation and differentiation, and that it might act as a tumor suppressor in the gut. Role of C/EBPalpha in lung development: The link between respiratory complications in prematurely born infants and susceptibility for developing chronic obstructive pulmonary disease (COPD) is receiving increasing attention. We have previously found that CCAAT/enhancer binding protein (C/EBP) activity in airway epithelial cells of COPD patients is decreased compared to healthy smokers, suggesting a previously unknown role for C/EBPs in COPD pathogenesis. To investigate the role of the transcription factor C/EBPalpha in lung development and its potential role in COPD, mice with a lung epithelial-specific disruption of the C/EBPalpha gene were generated. Cebpa(dLE) mice exhibit impaired lung development and epithelial differentiation, as well as affected vascularity. Cebpa(dLE) mice that survive until adulthood develop a severe pathological picture with irregular emphysema;bronchiolitis, including goblet cell hyperplasia, bronchiolar metaplasia, fibrosis and mucus plugging;and an inflammatory cell and gene expression profile similar to COPD. Cebpa(dLE) mice display lung immaturity during development, and adult Cebpa(DeltaLE) mice develop a majority of the histopathological and inflammatory characteristics of COPD. Cebpa(dLE) mice could thus provide new valuable insights into understanding the long-term consequences of lung immaturity and the link to susceptibility of developing COPD. Role of the aryl hydrocarbon receptor (AhR) in hepatic steatosis: To determine the endobiotic role of AhR in hepatic steatosis, wild-type, constitutively activated AhR transgenic, AhR null and CD36/fatty acid translocase null mice were used to investigate whether AhR influences steatosis and to determine the involvement of CD36 in the steatotic effect of AhR. Activation of AhR induced spontaneous hepatic steatosis characterized by the accumulation of triglycerides. The steatotic effect of AhR likely is owing to the combined up-regulation of CD36 and fatty acid transport proteins, suppression of fatty acid oxidation, inhibition of hepatic export of triglycerides, increase in peripheral fat mobilization, and increased hepatic oxidative stress. Promoter analysis established CD36 as a novel transcriptional target of AhR. Activation of AhR in liver cells induced CD36 gene expression and enhanced fatty acid uptake. The steatotic effect of an AhR agonist was inhibited in CD36-/- mice. CONCLUSIONS: Our study reveals a novel link between AhR-induced steatosis and the expression of CD36. Industrial or military exposures to dioxin and related compounds have been linked to increased prevalence of fatty liver in human beings. Results from this study may help to establish AhR and its target CD36 as novel therapeutic and preventive targets for fatty liver disease. Copyright (c) 2010 AGA Institute. Published by Elsevier Inc. All rights reserved. Role of AhR in the immune system: AhR KO mice are hypersensitive to lipopolysaccharide (LPS)-induced septic shock, mainly due to the dysfunction of their macrophages. In response to LPS, bone marrow-derived macrophages (BMDM) of AhR KO mice secreted an enhanced amount of interleukin-1beta (IL-1beta). Since the enhanced IL-1beta secretion was suppressed by supplementing plasminogen activator inhibitor-2 (Pai-2) expression through transduction with Pai-2-expressing adenoviruses, reduced Pai-2 expression could be a cause of the increased IL-1beta secretion by AhR KO mouse BMDM. Analysis of gene expression revealed that AhR directly regulates the expression of Pai-2 through a mechanism involving NF-kappaB but not Arnt, in an LPS-dependent manner. Together with the result that administration of the AhR ligand 3-methylcholanthrene partially protected mice with wild-type AhR from endotoxin-induced death, these results raise the possibility that an appropriate AhR ligand may be useful for treating patients with inflammatory disorders. Role of AhR is skin carcinogenesis: Benzo[a]pyrene (B[a]P) is a ligand for the AhR. To study the role of AhR/Arnt is skin, mice were developed in which the Arnt gene was disrupted specifically in adult skin epidermis. There was no overt pathological effect of lack of AhR activity in the skin;the epidermis developed normally. TO study the role of Arnt in skin cancer, the mutant nicotinamide adenine dinucleotide (phosphate): quinone oxidoreductase (NAPQ1) allele was introduced into the Arnt skin-null mouse strain to render it more susceptible to tumor initiation by B[a]P. Lack of AhR/Arnt activity in the epidermis of this strain completely prevented the induction of skin tumors in a tumor initiation-promotion protocol in which a single topical application of B[a]P acted as the tumor-initiating event, and tumor promotion was provided by repeated topical applications of 12-O-tetradecanoyl phorbol-13-acetate (TPA). In contrast, disruption of Arnt did not prevent the induction of skin tumors in a protocol also using TPA as the promoter but using as the initiator N-methyl-N'-nitro-N-nitrosoguanidine, whose activity is unlikely to be affected by the activity of AhR, Arnt or their target genes. These observations demonstrate that Arnt is required for tumor initiation by B[a]P in the skin.
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