title: Peroxisome proliferator-activated receptor gamma activation modulates cyclin D1 transcription via beta-catenin-independent and cAMP-response element-binding protein-dependent pathways in mouse hepatocytes.
abstract: Activation of peroxisome proliferator-activated receptor gamma (PPARgamma) following exposure to PPARgamma-specific ligands resulted in growth inhibition in various carcinoma cell lines. Our aim was to elucidate the pathway of PPARgamma2 activation-mediated modulation of cyclin D1 transcription in mouse hepatocytes. To address this we utilized stable control and PPARgamma hepatocyte cell lines created via retroviral overexpression utilizing AML-12 hepatocytes. Addition of PPARgamma ligand troglitazone (TZD) activated PPARgamma2 in proliferating hepatocytes and resulted in growth arrest accompanied by a down-regulation of proliferating cell nuclear antigen, cyclin D1, and beta-catenin expression. Furthermore activation of PPARgamma2 attenuated cyclin D1 promoter activity indicating a transcriptional regulation of cyclin D1. Since beta-catenin plays a pivotal role in regulating cyclin D1 transcription, we studied whether PPARgamma2-mediated inhibition of cyclin D1 transcription involved beta-catenin. Interestingly overexpression of either wild-type or S37A mutant beta-catenin was unable to rescue PPARgamma2-mediated suppression of cyclin D1 transcription, whereas overexpression of cAMP-response element-binding protein (CREB) was capable of antagonizing this inhibitory effect of PPARgamma2. Additionally pretreatment with okadaic acid antagonized PPARgamma2-mediated inhibition of cyclin D1 transcription without any effect on beta-catenin expression. These studies also showed a TZD-mediated inhibition of total and phospho-CREB(Ser133) levels, CREB promoter activity, and cAMP-response element-mediated transcription in PPARgamma hepatocytes. Pretreatment of PPARgamma hepatocytes with okadaic acid, however, maintained higher total and phospho-CREB(Ser133) levels in the presence of TZD. These results indicated that PPARgamma2 activation inhibited cyclin D1 transcription in hepatocytes via CREB-dependent and beta-catenin-independent pathways.
authors.affiliation: Division of Molecular Cardiology, The Texas A&M University System Health Science Center, College of Medicine, Temple, Texas 76504, USA.
authors.forename: Chandan
authors.initials: C
authors.lastname: Sharma
authors.forename: Anamika
authors.initials: A
authors.lastname: Pradeep
authors.forename: Richard G
authors.initials: RG
authors.lastname: Pestell
authors.forename: Basabi
authors.initials: B
authors.lastname: Rana
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002455:Cell Division; D002460:Cell Line; D002839:Chromans / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D017362:Cyclic AMP Response Element-Binding Protein / Q000378:metabolism*; D019938:Cyclin D1 / Q000096:biosynthesis* / Q000378:metabolism; D003598:Cytoskeletal Proteins / Q000378:metabolism*; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D017930:Genes, Reporter; D022781:Hepatocytes / Q000378:metabolism*; D006801:Humans; D007004:Hypoglycemic Agents / Q000494:pharmacology; D008024:Ligands; D008099:Liver / Q000378:metabolism; D008156:Luciferases / Q000378:metabolism; D051379:Mice; D009154:Mutation; D019319:Okadaic Acid / Q000494:pharmacology; D010957:Plasmids / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D012694:Serine / Q000737:chemistry; D045162:Thiazolidinediones / Q000494:pharmacology; D015534:Trans-Activators / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism*; D014158:Transcription, Genetic; D014162:Transfection; D000077288:Troglitazone; D051176:beta Catenin
chemical_list: C495270:CTNNB1 protein, human; C495267:CTNNB1 protein, mouse; D002839:Chromans; D017362:Cyclic AMP Response Element-Binding Protein; D003598:Cytoskeletal Proteins; D004791:Enzyme Inhibitors; D007004:Hypoglycemic Agents; D008024:Ligands; D018160:Receptors, Cytoplasmic and Nuclear; D045162:Thiazolidinediones; D015534:Trans-Activators; D014157:Transcription Factors; D051176:beta Catenin; D019938:Cyclin D1; D019319:Okadaic Acid; D012694:Serine; D000242:Cyclic AMP; D008156:Luciferases; D000077288:Troglitazone