title: Ligand-independent regulation of transforming growth factor beta1 expression and cell cycle progression by the aryl hydrocarbon receptor.
abstract: The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the toxic effects of its xenobiotic ligands and acts as an environmental checkpoint during the cell cycle. We expressed stably integrated, Tet-Off-regulated AHR variants in fibroblasts from AHR-null mice to further investigate the AHR role in cell cycle regulation. Ahr+/+ fibroblasts proliferated significantly faster than Ahr-/- fibroblasts did, and exposure to a prototypical AHR ligand or deletion of the ligand-binding domain did not change their proliferation rates, indicating that the AHR function in cell cycle was ligand independent. Growth-promoting genes, such as cyclin and cyclin-dependent kinase genes, were significantly down-regulated in Ahr-/- cells, whereas growth-arresting genes, such as the transforming growth factor beta1 (TGF-beta1) gene, extracellular matrix (ECM)-related genes, and cyclin-dependent kinase inhibitor genes, were up-regulated. Ahr-/- fibroblasts secreted significantly more TGF-beta1 into the culture medium than Ahr+/+ fibroblasts did, and Ahr-/- showed increased levels of activated Smad4 and TGF-beta1 mRNA. Inhibition of TGF-beta1 signaling by overexpression of Smad7 reversed the proliferative and gene expression phenotype of Ahr-/- fibroblasts. Changes in TGF-beta1 mRNA accumulation were due to stabilization resulting from decreased activity of TTP, the tristetraprolin RNA-binding protein responsible for mRNA destabilization through AU-rich motifs. These results show that the Ah receptor possesses interconnected intrinsic cellular functions, such as ECM formation, cell cycle control, and TGF-beta1 regulation, that are independent of activation by either exogenous or endogenous ligands and that may play a crucial role during tumorigenesis.
authors.affiliation: Department of Environmental Health, University of Cincinnati Medical Center, and Shriners Hospital for Children, Cincinnati, OH 45267-0056, USA.
authors.forename: Xiaoqing
authors.initials: X
authors.lastname: Chang
authors.forename: Yunxia
authors.initials: Y
authors.lastname: Fan
authors.forename: Saikumar
authors.initials: S
authors.lastname: Karyala
authors.forename: Sandy
authors.initials: S
authors.lastname: Schwemberger
authors.forename: Craig R
authors.initials: CR
authors.lastname: Tomlinson
authors.forename: Maureen A
authors.initials: MA
authors.lastname: Sartor
authors.forename: Alvaro
authors.initials: A
authors.lastname: Puga
mesh_terms: D000818:Animals; D000954:Antigens, Surface / Q000235:genetics / Q000378:metabolism; D002453:Cell Cycle / Q000502:physiology*; D049109:Cell Proliferation; D002478:Cells, Cultured; D051959:ELAV Proteins; D000067780:ELAV-Like Protein 1; D005109:Extracellular Matrix / Q000378:metabolism; D005347:Fibroblasts / Q000166:cytology / Q000502:physiology; D020869:Gene Expression Profiling; D005786:Gene Expression Regulation; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D008024:Ligands; D051379:Mice; D018345:Mice, Knockout; D020411:Oligonucleotide Array Sequence Analysis; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D018336:Receptors, Aryl Hydrocarbon / Q000235:genetics / Q000378:metabolism*; D051906:Smad7 Protein / Q000235:genetics / Q000378:metabolism; D053773:Transforming Growth Factor beta1 / Q000235:genetics / Q000378:metabolism*; D000099190:MAP-Kinase-Activated Kinase 2
chemical_list: D000954:Antigens, Surface; D051959:ELAV Proteins; D000067780:ELAV-Like Protein 1; D047908:Intracellular Signaling Peptides and Proteins; D008024:Ligands; D017346:Protein Serine-Threonine Kinases; D016601:RNA-Binding Proteins; D018336:Receptors, Aryl Hydrocarbon; D051906:Smad7 Protein; D053773:Transforming Growth Factor beta1; D000099190:MAP-Kinase-Activated Kinase 2; C098770:ELAVL1 protein, human; C497807:Smad7 protein, mouse