title: Rac1 GTPase and the Rac1 exchange factor Tiam1 associate with Wnt-responsive promoters to enhance beta-catenin/TCF-dependent transcription in colorectal cancer cells.
abstract: BACKGROUND
  beta-catenin is a key mediator of the canonical Wnt pathway as it associates with members of the T-cell factor (TCF) family at Wnt-responsive promoters to drive the transcription of Wnt target genes. Recently, we showed that Rac1 GTPase synergizes with beta-catenin to increase the activity of a TCF-responsive reporter. This synergy was dependent on the nuclear presence of Rac1, since inhibition of its nuclear localization effectively abolished the stimulatory effect of Rac1 on TCF-responsive reporter activity. We hypothesised that Rac1 plays a direct role in enhancing the transcription of endogenous Wnt target genes by modulating the beta-catenin/TCF transcription factor complex.
  
  
  RESULTS
  We employed chromatin immunoprecipitation studies to demonstrate that Rac1 associates with the beta-catenin/TCF complex at Wnt-responsive promoters of target genes. This association served to facilitate transcription, since overexpression of active Rac1 augmented Wnt target gene activation, whereas depletion of endogenous Rac1 by RNA interference abrogated this effect. In addition, the Rac1-specific exchange factor, Tiam1, potentiated the stimulatory effects of Rac1 on the canonical Wnt pathway. Tiam1 promoted the formation of a complex containing Rac1 and beta-catenin. Furthermore, endogenous Tiam1 associated with endogenous beta-catenin, and this interaction was enhanced in response to Wnt3a stimulation. Intriguingly, Tiam1 was recruited to Wnt-responsive promoters upon Wnt3a stimulation, whereas Rac1 was tethered to TCF binding elements in a Wnt-independent manner.
  
  
  CONCLUSIONS
  Taken together, our results suggest that Rac1 and the Rac1-specific activator Tiam1 are components of transcriptionally active beta-catenin/TCF complexes at Wnt-responsive promoters, and the presence of Rac1 and Tiam1 within these complexes serves to enhance target gene transcription. Our results demonstrate a novel functional mechanism underlying the cross-talk between Rac1 and the canonical Wnt signalling pathway.
authors.affiliation: Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Ontario, M5T 3L9, Canada. pinella.buongiorno@utoronto.ca
authors.forename: Pinella
authors.initials: P
authors.lastname: Buongiorno
authors.forename: Vaijayanti V
authors.initials: VV
authors.lastname: Pethe
authors.forename: George S
authors.initials: GS
authors.lastname: Charames
authors.forename: Susmita
authors.initials: S
authors.lastname: Esufali
authors.forename: Bharati
authors.initials: B
authors.lastname: Bapat
mesh_terms: D045744:Cell Line, Tumor; D047369:Chromatin Immunoprecipitation; D020662:Guanine Nucleotide Exchange Factors / Q000502:physiology*; D006801:Humans; D011401:Promoter Regions, Genetic; D000075684:T-Lymphoma Invasion and Metastasis-inducing Protein 1; D051820:TCF Transcription Factors / Q000502:physiology*; D014158:Transcription, Genetic / Q000502:physiology*; D051153:Wnt Proteins / Q000502:physiology*; D051176:beta Catenin / Q000502:physiology*; D020830:rac1 GTP-Binding Protein / Q000502:physiology*
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D000075684:T-Lymphoma Invasion and Metastasis-inducing Protein 1; D051820:TCF Transcription Factors; C087334:TIAM1 protein, human; D051153:Wnt Proteins; D051176:beta Catenin; D020830:rac1 GTP-Binding Protein