Involvement of b-catenin in signal transduction and activation of TCF/ LEF regulated target genes

Sandra Urbach (sur@biobase.de), Edgar Wingender (ewi@biobase.de)
Biobase Biological Databases GmbH, Mascheroder Weg 1b, D-38124 Braunschweig

Introduction

Among vertebrates b-catenin is a highly conserved ubiquitous protein. It shows at least two functions in mammalian cells: as a component of adherens junctions it interacts with E-cadherin and binds a-catenin to the cytoskeleton or it complexes - among others - with the APC-(adenomatous polyposis coli) protein and thus is a pivotal part of the wnt-signalling pathway (fig. 1) which is mainly involved in control of embryonic patterning and cell-fate decision in development.
In the presence of a wnt-signal the cytoplasmatic pool of b-catenin rises and translocates to the nucleus where it transactivates transcription factors of the TCF/ LEF family. In the absence of wnt-signals a multiprotein complex promotes the degradation of b-catenin and thus no activation of TCF/ LEF target genes can take place.
In this work a short summary on the signalling pathway of b-catenin and its involvement in transcriptional activation of TCF/ LEF target genes is given.
In respect to the TRANSFAC database main targets of the work have been to complete the TCF/ LEF class of transcriptional factors, extracting binding sites and generating weight matrices based on available native binding sites. Furthermore all molecules that are engaged in the complex wnt-signalling pathway will soon be available in the TRANSPATH database.

A simple model of the wnt-pathway

Fig. 1

b-catenin besides the wnt pathway

b-catenin is also involved in the formation of adherens junctions of mammalian epithelia where it links a-catenin to E-cadherin. E-Cadherin and APC compete for interaction with b-catenin.

b-catenin in the wnt pathway

Binding of wnts (secreted glycoproteins) to frizzled receptors activate dish-evelled which blocks the function of a complex assembled over certain scaffold proteins (axin/ conduction).
These complexes can promote the phosphorylation of b-catenin by GSG-3b (glycogen synthase kinase 3b) in the absence of wnts    (-wnt). Thus phosphorylated b-catenin becomes ubiquiti-nated and degraded by proteasomes.
In the presence of wnt signals (+wnt) the cytosolic amount of b-catenin increases and thus translocates to the nucleus where it associates with transcription factors of the TCF/ LEF family. Due to its transactivating ability the b-catenin-transcription-factor-co mplex binds to DNA and activates wnt target genes.

TCF/ LEF transcription factors and target genes

The founding members of the TCF/ LEF family of transcription factors were independently cloned in 1991: human T-cell factor 1 (TCF-1), mouse lymphoid enhancer factor (LEF-1) and human LEF-1 showed high binding affinity to (A/T)(A/T)CAA(A/T)GG-sequences.
According to the fact that these factors comprise a common DNA binding motif of about 80 amino acid length (HMG-box) which binds to DNA´s minor groove these factors where grouped into the HMG box family (TRANSFAC class 4.7).
Until now 70 binding sites for TCF/ LEF transcription factors have been recorded in TRANSFAC (see table 1).

Figure 3 shows binding of a HMG box (red) to DNA (green) which can dramatically bend DNA. Thus TCF/ LEF are also called äarchitecturalô transcription factors.

Table 1 shows native binding sites for some members of the LEF/ TCF family from mice and humans.

Nuclear regulation of TCF/ LEF

Some interacting factors which have been identified in diverse organisms are shown in fig. 2. The activating factor (co-activator) b-catenin is mapped as well as diverse repressing factors such as groucho and CBP from fruit fly or CtBP from clawed frog. Furthermore a protein called pontin52 has recently been cloned which seems to be the missing link between TCF/LEF-b-catenin and the Tata-box binding protein (TBP).

Fig. 2

Entries TRANSPATH database (Matrch 2000)

At present the first entries have been made in TRANSPATH concerning b -catenin and its interacting partners such as a-catenin, E-cadherin and APC.

Entries in TRANSFAC database (March 2000)

At present TRANSFAC contains 29 different factors of the TCF/ LEF family which belong to 6 species whereas most of these factors belong to mouse or man. 70 binding sites for TCF/ LEF factors could be extracted from literature. 64 of these comprise native DNA fragments and 6 are artificial. Additionally 10 interacting factors such as repressors or activators are available and are still being extended. 3 weight matrices for TCF-1, TCF-4 and LEF-1 could be generated which will be proofed in respect of their reliability and will soon be available in TRANSFAC (see table 2).

(March 2000)

entries

factors

29

Sites

70

co-factors/ interacting factors

10

matrices

3

references

74

Table 2 shows the current number of entries which have been registered  in the TRANSFAC.

Perspectives TRANSFAC database

Concerning native binding sites for transcription factor TCF/LEF family TRANSFAC is up-to-date with scientific publications. Regarding interacting factors such as co-activators or repressors, artificial binding sites and homologous TCF/ LEF transcription factors of other species it still needs to be completed.
As scientific research on b-catenin, TCF/ LEF transcription factors and wnt-signalling pathway is rapidly extending and many new interactions where proved in the last months (e.g. b-catenin seems to be somehow connected with presenilins, proteins altered in Alzheimer disease) work on this field should be further enlarged in both databases TRANSFAC and TRANSPATH. Available data concerning the shown pathways could lead to the invention of new pharmaceutical drugs and drug targets in order to prevent or treat cancer diseases.

Perspectives TRANSPATH database

All molecules engaged in the wnt-pathway will be entered into TRANSPATH and it is scheduled that the entire wnt-pathway will soon be available. Special emphasis in this respect will be put on b-catenin and its interactions.