Das Tumorsuppressor-Homologe p63 als Modulator des Wnt-Signalweges

The p63 gene encodes for different isoforms of p53 homologous proteins. p63, especially deltaNp63alpha, is expressed in stem cells of multilayered squamous epithelia and acts as a key regulator of proliferation and differentiation. However, in malignant tumors of the epidermis, the urothelium and the cervix uteri, p63 is expressed to high levels. Beside the regulation of p53 responsive genes, p63 proteins modulate the transcription of specific genes by largely unknown mechanisms. It was shown that deltaNp63alpha strengthens the activity of beta-Catenin-Tcf/Lef responsive promoters. beta-Catenin is a multifunctional protein. Among others, it is the effector of the canonical Wnt signaling pathway, an important regulator of cell proliferation during development and oncogenesis. In the absence of Wnt ligands, the association of ß-catenin with a multiprotein complex leads to its phosphorylation and proteasomal degradation. The association of Wnt ligands with their cell surface receptors triggers the activation of numerous downstream signaling events, leading to beta-Catenin intranuclear accumulation. In the nucleus, beta-Catenin associates with Tcf/Lef family members and activates Wnt responsive genes such as c-Myc und Cyclin D1. In this dissertation, we used a reporter assay system to quantify the impact of p63 on the activity of a beta-Catenin-Tcf-4 responsive promoter. We could show that tetrameric deltaNp63alpha but not p53 and deltaNp73alpha strengthens the transcription of the reporter gene synergistically with beta-Catenin and Tcf-4. Physiological relevant mutations, as known from patients with human syndromes caused by germline mutations within the p63 gene, abolish this function of deltaNp63alpha. In the presence of constitutively active beta-Catenin, deltaNp63alpha again synergistically activates the transcription of the reporter gene, while having no influence on beta-Catenin phosphorylation or total beta-Catenin levels. Co-immunoprecipitation studies revealed that deltaNp63alpha directly binds to Tcf-4. This interaction occured independently of the ability of Tcf-4 to associate with ß-catenin. Since our results provide evidence that Tcf-4 interacts with beta-Catenin and deltaNp63alpha simultaneously, we suggest that all three proteins, beta-Catenin, Tcf-4 and deltaNp63alpha, are found in complex. The formation of such a ternary complex could explain the Tcf-4-mediated accumulation of nuclear beta-Catenin in the presence of deltaNp63alpha. Our findings support a function of deltaNp63alpha as a direct activator of the beta-Catenin mediated transcription. We hypothesize that Tcf-4, beta-Catenin and deltaNp63alpha build a transcriptionally active complex and activate transcription of Wnt responsive genes by the recruitment of specific transcriptional cofactors. In accordance with this hypothesis, p63 could have the potential to engage in the canonical Wnt signaling pathway by Tcf-4 and thereby to regulate developmental processes as well as oncogenesis by deregulation of this pathway.