Browsing by Subject "aryl hydrocarbon receptor"
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Item Differential mechanisms by which the aryl hydrocarbon receptor attenuates liver regeneration(2007-10-11) Courtney Alicia Lockhart; Kathleen O'Connor, Ph.D.; Wendy Mars, Ph.D.; Steve Weinman, M.D., Ph.D.; Randall Urban, M.D.; Jingwu Xie, Ph.D.; Chunming Liu, Ph.D.Liver regeneration is orchestrated by a series of autocrine and paracrine cues that function to restore hepatic tissue, however the precise cellular and molecular mechanisms that regulate these signaling events are poorly understood. Recent evidence demonstrates that hepatocyte proliferation following partial hepatectomy (PH) can be attenuated by the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor that is involved in hepatic organogenesis and cell cycle control. This growth suppression suggests that AhR modulates critical signaling processes of the regenerative program. In particular, the regeneration process is initiated by both cytokines and matrix enzymes and propagated by the potent mitogenic activity of two proteins, the c-Met transmembrane receptor and urokinase plasminogen activator (uPA). However, this growth response is limited by the expression of plasminogen activator inhibitor-1 (PAI-1) and TGF-?, which terminates hepatocyte proliferation. The goal of these studies was to determine the influence of AhR on these moieties in the context of the regenerative program. The hypothesis that AhR modulates these signaling molecules in a mito-inhibitory manner was tested using an in vivo model system of 70% PH in mice pre-treated with 2,3,7,8-tertachlorodibenzo-p-dioxin (TCDD), a potent, prototypical, and persistent AhR agonist. We demonstrate that AhR did not alter cytokine or matrix enzyme expression during the regenerative process, but markedly upregulated PAI-1 and TGF-? protein levels post-PH. As a consequence, both c-Met and uPA activation were greatly suppressed in an AhR-dependent fashion during liver regeneration as well. Conclusion: These observations suggest a novel mechanism of AhR-mediated attenuation of the regenerative response and identify a possible physiologic function of AhR in vivo.Item Inhibitory actions of Ah receptor agonists and indole-containing compounds in breast cancer cell lines and mouse models(Texas A&M University, 2005-08-29) Walker, Kelcey Manae BeckerThe aryl hydrocarbon receptor (AhR) binds synthetic and chemoprotective phytochemicals, and research in this laboratory has developed selective AhR modulators (SAhRMs) for treatment of breast cancer. Activation of the AhR through agonists such as TCDD inhibits hormone activation of several E2-responsive genes in breast cancer cell lines. In this study, inhibition of E2-induced proliferation and gene expression by TCDD has been investigated in the uterus of wildtype, ERKO and AhRKO mice. Cyclin D1, DNA polymerase ?, and VEGF mRNA levels are induced by E2 through ER? in the uterus as determined by in situ hybridization studies. TCDD down-regulated E2-induced cyclin D1 and DNA polymerase ? expression, but not E2-induced VEGF expression, in wild-type mice, but not AhRKO mice, confirming the role of the AhR. Furthermore, protein synthesis was not necessary for induction of cyclin D1 or DNA polymerase ?gene expression by E2 or inhibition of these responses by TCDD. Therefore, AhR-ER? crosstalk directly regulates the expression of genes involved in cell proliferation in vivo. AhR agonists induce down-regulation of ErbB family receptors in multiple tissues/organs suggesting possible inhibitory interactions with chemotherapeutic potential. Recently, it has been reported that the SAhRM 1,1??,2,2??-tetramethyldiindolylmethane inhibited DMBA-induced mammary tumor growth in rats and also inhibited MAPK and PI3-K pathways in human breast cancer cells. BT-474 and MDA-MB-453 cell lines are ErbB2-overexpressing breast cancer cells that express functional AhR and exhibit constitutive activation of MAPK and PI3-K pathways. Therefore, 1,1??,2,2??-tetramethyldiindolylmethane-induced inhibition of ErbB2 signaling was investigated in these cells lines and in the MMTV-c-neu mouse mammary tumor model, which overexpresses ErbB2 in the mammary gland. The growth of ErbB2 overexpressing cell lines and mammary tumors was inhibited by 1,1??,2,2??-tetramethyldiindolylmethane; however, modulation of MAPK or PI3-K pathways and cell cycle proteins nor induction of apoptosis by 1,1',2,2'-tetramethyldiindolylmethane was observed in the ErbB2overexpressing cell lines. Current studies are investigating mitochondrial effects of 1,1??,2,2??-tetramethyldiindolylmethane in the ErbB2-overexpressing cell lines, as well as continuing studies on gene expression profiles in the mammary glands of MMTV-c-neu mice to better understand and identify critical genes that are responsible for ErbB2-mediated transformation and growth of cancer cells/tumors.Item MOLECULAR INSIGHTS AND PHYSIOLOGICAL CONSEQUENCES OF ARYL HYDROCARBON RECEPTOR REGULATED PLASMINOGEN ACTIVATOR INHIBITOR-1 EXPRESSION(2013-06-03) Wilson, Shelly; Elferink, Cornelis; Barton, Michelle; Cicalese, Luca; Boor, Paul; Papaconstantinou, JohnThe aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, attenuates liver regeneration in vivo when activated by its prototypical agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) following 70% partial hepatectomy (PH). One reported target of the AhR that may account for suppression of the regenerative response is plasminogen activator inhibitor-1 (PAI-1), which negatively regulates the cleavage and activation of hepatocyte growth factor (HGF) from its latent form in the extracellular matrix. Once activated, HGF signalling through its receptor cMet is a crucial component early in regeneration. Recent studies identified a sequence distinct from the canonical AhR binding site, the ncXRE, which confers TCDD-inducible expression to the PAI-1 promoter. Since the ncXRE shares partial sequence homology with the Kruppel-like factor 6 (KLF6) consensus binding site; I hypothesize that the AhR interacts with KLF6 at the ncXRE, inducing transcription of PAI-1, suppressing HGF processing and its activation of cMet, inhibiting liver regeneration. To test this hypothesis, coimmunoprecipitation on liver nuclear extracts and recombinant proteins confirmed that KLF6 and the AhR interact, likely dependent on the C-terminus transactivating domain of AhR and the DNA binding domain of KLF6. Both proteins bind the ncXRE in vitro and deletion analyses revealed that the N-terminal 27 amino acids of hKLF6 were required for complex formation. Chromatin immunoprecipitation studies demonstrated that the AhR and KLF6 bind to the PAI-1 promoter in vivo. To assess the effects of AhR activation in vivo, C57BL/6 and PAI-1-/- mice were pretreated with TCDD, underwent PH, and liver samples and serum were collected at multiple time points post-PH to monitor PAI-1 expression, HGF processing, and cMet phosphorylation (activation) and DNA synthesis in the liver. I found that PAI-1 transcript and corresponding serum PAI-1 protein levels were markedly increased in TCDD-pretreated C57BL/6 mice, and this rise in PAI-1 levels inversely correlated to HGF processing and cMet phosphorylation. Hepatocytes in the periportal region of PAI-1-/- mice were able to overcome TCDD-mediated suppression of regeneration. The AhR-KLF6 interaction at the PAI-1 promoter, resulting in increased PAI-1 expression and decreased HGF processing and cMet activation, reveals a novel mechanism by which the AhR may contribute to liver homeostasis.