TCF: Lady Justice casting the final verdict on the outcome of Wnt signalling.
The Wnt signalling cascade plays an important role during embryonic patterning and cell fate determination and is highly conserved throughout evolution. Factors of the TCF/LEF HMG domain family (Tcfs) are the downstream effectors of this signal transduction pathway. Upon Wnt signalling, a cascade is initiated that results in the translocation of beta-catenin to the nucleus, where it interacts with Tcf to generate a transcriptionally active complex. This bipartite transcription factor is targeted to the upstream regulatory regions of Tcf target genes. In the absence of Wnt signals, beta-catenin is degraded in the cytoplasm via the ubiquitin-proteasome pathway. Several proteins are instrumental in achieving this tight regulation of beta-catenin levels in the cell, including adenomatous polyposis coli (APC), GSK3 beta, and Axin/Conductin. Deregulation of the Wnt signalling pathway is implicated in several forms of cancer, such as colon carcinoma and melanoma. This deregulation is achieved via mutation of APC, beta-catenin or Axin, resulting in elevated beta-catenin levels and the presence of constitutively active Tcf-beta-catenin complexes in the nucleus. The accompanying inappropriate activation of target genes is considered to be a critical, early event in this carcinogenesis. In addition to regulating beta-catenin levels, normal healthy cells have evolved a second level of regulation, by manipulating the activity of the Tcf proteins themselves. In the absence of Wnt signalling, Tcf complexes with several transcriptional repressor proteins ensuring active repression of Tcf target genes. In this review the dual role of Tcf proteins in the Wnt signalling cascade will be discussed.
Brantjes
Helen
H
Department of Immunology, University Hospital Utrecht, The Netherlands.
Barker
Nick
N
van Es
Johan
J
Clevers
Hans
H
D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins; D003598:Cytoskeletal Proteins / Q000235:genetics / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D004330:Drosophila / Q000196:embryology; D004625:Embryo, Nonmammalian / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D011518:Proto-Oncogene Proteins / Q000235:genetics* / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism*; D015533:Transcriptional Activation / Q000235:genetics*; D014407:Tumor Cells, Cultured; D051153:Wnt Proteins; D014981:Xenopus / Q000196:embryology; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin
D051792:Basic Helix-Loop-Helix Proteins; D003598:Cytoskeletal Proteins; D004268:DNA-Binding Proteins; D011518:Proto-Oncogene Proteins; D012097:Repressor Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D051153:Wnt Proteins; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; D051176:beta Catenin; C495266:CTNNB1 protein, Xenopus; C091023:gro protein, Drosophila