ABSTRACT
The p53 homologue p63/TP73L is required for the proper development of squamous epithelia, mammary glands and limb buds, with some of these tissues also displaying strong canonical Wnt signalling activity. It was previously suggested that DeltaNp63alpha, the predominant isoform of p63 in epithelia, positively regulates beta-Catenin through inhibition of GSK3beta. Results reported in this communication show that, upon transient overexpression, DeltaNp63alpha indeed promotes Wnt-inducible reporter gene activity in human cells, as well as secondary axis formation in Xenopus embryos. However, in apparent contradiction to these observations, siRNA-mediated knockdown of endogenous p63 equally enhanced the expression of Wnt-responsive genes. While p63 knockdown did not detectably affect beta-Catenin levels or phosphorylation, DeltaNp63alpha was found in a complex with members of the TCF/LEF family of Wnt-responsive transcription factors. On The basis of these findings, we propose that DeltaNp63alpha has a function in recruiting transcriptional repressors to Wnt-responsive genes. Overexpression of p63 may lead to sequestration of such repressors (squelching), resulting in a similar effect like siRNA-mediated removal of p63, i.e., activation of Wnt-responsive genes. The role of p63 as a negative Wnt-regulator thus matches with the frequently observed downregulation of p63 during tumor progression, when cancer cells adopt a more mesenchymal, invasive phenotype.
Subject(s)
Phosphoproteins/metabolism , Trans-Activators/metabolism , Transcription, Genetic , Tumor Suppressor Proteins/metabolism , Wnt Proteins/metabolism , Xenopus Proteins/metabolism , Xenopus/metabolism , Animals , Body Patterning , Embryo, Nonmammalian/metabolism , Gene Knockdown Techniques , Genes, Reporter , Humans , Lymphoid Enhancer-Binding Factor 1/metabolism , Models, Biological , Phosphorylation , Protein Binding , RNA, Small Interfering/metabolism , Signal Transduction , Transcription Factors , Transcriptional Activation/genetics , Xenopus/embryology , beta Catenin/metabolismABSTRACT
Canonical Wnt signalling is known to be involved in the regulation of differentiation and proliferation in the context of endodermal organogenesis. Wnt mediated beta-catenin activation is understood to be modulated by secreted Frizzled-related proteins, such as XsFRP5, which is dynamically expressed in the prospective liver/ventral pancreatic precursor cells during late neurula stages, becoming liver specific at tailbud stages and shifting to the posterior stomach/anterior duodenum territory during tadpole stages of Xenopus embryogenesis. These expression characteristics prompted us to analyse the function of XsFRP5 in the context of endodermal organogenesis. We demonstrate that XsFRP5 can form a complex with and inhibit a multitude of different Wnt ligands, including both canonical and non-canonical ones. Knockdown of XsFRP5 results in transient pancreatic hypoplasia as well as in an enlargement of the stomach. In VegT-injected animal cap explants, XsFRP5 can induce expression of exocrine but not endocrine pancreatic marker genes. Both, its expression characteristics as well as its interactions with XsFRP5, define Wnt2b as a putative target for XsFRP5 in vivo. Knockdown of Wnt2b results in a hypoplastic stomach as well as in hypoplasia of the pancreas. On the basis of these findings we propose that XsFRP5 exerts an early regulatory function in the specification of the ventral pancreas, as well as a late function in controlling stomach size via inhibition of Wnt signalling.
