title: The transcription factor ATF3 is upregulated during chondrocyte differentiation and represses cyclin D1 and A gene transcription.
abstract: BACKGROUND
  Coordinated chondrocyte proliferation and differentiation are required for normal endochondral bone growth. Transcription factors binding to the cyclicAMP response element (CRE) are known to regulate these processes. One member of this family, Activating Transcription Factor 3 (ATF3), is expressed during skeletogenesis and acts as a transcriptional repressor, but the function of this protein in chondrogenesis is unknown.
  
  
  RESULTS
  Here we demonstrate that Atf3 mRNA levels increase during mouse chondrocyte differentiation in vitro and in vivo. In addition, Atf3 mRNA levels are increased in response to cytochalasin D treatment, an inducer of chondrocyte maturation. This is accompanied by increased Atf3 promoter activity in cytochalasin D-treated chondrocytes. We had shown earlier that transcription of the cell cycle genes cyclin D1 and cyclin A in chondrocytes is dependent on CREs. Here we demonstrate that overexpression of ATF3 in primary mouse chondrocytes results in reduced transcription of both genes, as well as decreased activity of a CRE reporter plasmid. Repression of cyclin A transcription by ATF3 required the CRE in the cyclin A promoter. In parallel, ATF3 overexpression reduces the activity of a SOX9-dependent promoter and increases the activity of a RUNX2-dependent promoter.
  
  
  CONCLUSIONS
  Our data suggest that transcriptional induction of the Atf3 gene in maturing chondrocytes results in down-regulation of cyclin D1 and cyclin A expression as well as activation of RUNX2-dependent transcription. Therefore, ATF3 induction appears to facilitate cell cycle exit and terminal differentiation of chondrocytes.
authors.affiliation: CIHR Group in Skeletal Development and Remodeling, University of Western Ontario, London, ON, Canada. claudine.james@schulich.uwo.ca
authors.forename: Claudine G
authors.initials: CG
authors.lastname: James
authors.forename: Anita
authors.initials: A
authors.lastname: Woods
authors.forename: T Michael
authors.initials: TM
authors.lastname: Underhill
authors.forename: Frank
authors.initials: F
authors.lastname: Beier
mesh_terms: D000199:Actins / Q000037:antagonists & inhibitors / Q000378:metabolism; D051700:Activating Transcription Factor 3 / Q000235:genetics* / Q000378:metabolism; D000818:Animals; D002454:Cell Differentiation / Q000235:genetics*; D002478:Cells, Cultured; D019902:Chondrocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D050659:Core Binding Factor Alpha 1 Subunit / Q000235:genetics / Q000378:metabolism; D017362:Cyclic AMP Response Element-Binding Protein / Q000235:genetics / Q000378:metabolism; D019925:Cyclin A / Q000235:genetics*; D019938:Cyclin D1 / Q000235:genetics*; D015638:Cytochalasin D / Q000494:pharmacology; D005260:Female; D006609:High Mobility Group Proteins / Q000235:genetics / Q000378:metabolism; D008297:Male; D051379:Mice; D008954:Models, Biological; D009154:Mutation / Q000235:genetics; D020411:Oligonucleotide Array Sequence Analysis / Q000379:methods; D011247:Pregnancy; D011401:Promoter Regions, Genetic / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D020218:Response Elements / Q000235:genetics; D055758:SOX9 Transcription Factor; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects / Q000235:genetics*
chemical_list: D000199:Actins; D051700:Activating Transcription Factor 3; D050659:Core Binding Factor Alpha 1 Subunit; D017362:Cyclic AMP Response Element-Binding Protein; D019925:Cyclin A; D006609:High Mobility Group Proteins; D012333:RNA, Messenger; D055758:SOX9 Transcription Factor; C527933:Sox9 protein, mouse; D014157:Transcription Factors; D019938:Cyclin D1; D015638:Cytochalasin D