WNT signaling : activation, repression and fine-tuning of TCF transcription factors

Abstract

In the absence of a Wnt signal ß-catenin is phosphorylated by GSK3-ß, in a complex also containing Axin and APC. Upon phosphorylation, ß-catenin is primed for ubiquitination and subsequent degradation by the proteasome. In the nucleus, Tcf proteins bind Groucho family members and repress target genes. When a Wnt protein reaches the cell, it associates with the transmembrane receptors Frizzled and LRP. The destruction complex of GSK3-?, APC and Axin is subsequently inactivated via Dishevelled, resulting in the accumulation of ß-catenin and its translocation to the nucleus. Here, ß-catenin associates with transcription factors of the TCF family to generate a functional transcription factor complex, resulting in transactivation of target genes. Deregulation of the WNT signaling pathway can have disastrous results. Mutations in this pathway are the cause of over 95% of primary colon carcinomas, mutations in APC, ß-catenin and Axin have been identified. Deregulation of ß-catenin activation is known to be implicated in a large number of other malignancies, such as melanoma, hepatocellular carcinoma, ovarian cancer, endometrial cancer and pilomatricoma.

Tcf/Lef family transcription factors are the downstream effectors of the Wingless/Wnt signal transduction pathway. They activate the pathway by interaction with ß-cateninand can also repress the pathway by interaction with members of the Groucho (Grg/TLE) family of transcriptional co-repressors. We have now tested all known mammalian Groucho family members for their ability to interact specifically with individual Tcf/Lef family members. We found that redundancy in Tcf/Grg interactions appears to be the rule. Every Grg factor can interact with each TCF family member. We study the de-repressor role of Grg-5 in more detail and show that this protein in contrast to the long- repressor family members does not bind to histone deacetylase-1.

In mouse there are five family members denoted Groucho-related genes: Grg-1, Grg-2, Grg-3, Grg-4 and Grg-5. Groucho repressors do not bind DNA directly, but affect transcription by binding transcription factors. A large variety of transcription factors including Hairy related genes, Runt domain proteins and TCF/LEF factors are known to use Groucho proteins to mediate repression. In an attempt to determine if there is any specificity in the interaction between Tcf and Grg proteins on the basis of their expression patterns in embryonic mouse tissues, we raised monoclonal antibodies specific to Grg-1, Grg-2, Grg-3, Grg-4 and Grg-5.

Two novel components of the Wingless signaling cascade in drosophila were recently identified in Drosophila, Pygopus and Legless. Here we study the function of their mammalian homologs, hPYGOPUS-1, hPYGOPUS-2 and BCL9-1 and BCL-9.2. We show that PYGOPUS-2 and BCL-9.2 show a ubiquitous expression pattern in a panel of cell lines, whereas PYGOPUS-1 and BCL-9.1 expression is more restricted. PYGOPUS and BCL-9 together enhance WNT signaling in a TCF reporter assay, only when the cascade is activated by WNT-1, not by any of the other stimuli tested, including co-transfection of ß-catenin or dominant negative Axin. Cellular localization studies suggest a role of BCL-9 in nuclear transport of ß-catenin.