The vertebrate transcription factors TCF (T cell factor) and LEF (lymphocyte enhancer binding factor) interact with beta-catenin and are hypothesized to mediate Wingless/Wnt signaling. We have cloned a maternally expressed Drosophila TCF family member, dTCF. dTCF binds a canonical TCF DNA motif and interacts with the beta-catenin homolog Armadillo. Previous studies have identified two regions in Armadillo required for Wingless signaling. One of these interacts with dTCF, while the other constitutes a transactivation domain. Mutations in dTCF and expression of a dominant-negative dTCF transgene cause a segment polarity phenotype and affect expression of the Wingless target genes engrailed and Ultrabithorax. Epistasis analysis positions dTCF downstream of armadillo. The Armadillo-dTCF complex mediates Wingless signaling as a bipartite transcription factor.

Mutations in APC or beta-catenin inappropriately activate the transcription factor Tcf4, thereby transforming intestinal epithelial cells. Here it is shown that one of the target genes of Tcf4 in epithelial cells is Tcf1. The most abundant Tcf1 isoforms lack a beta-catenin interaction domain. Tcf1(-/-) mice develop adenomas in the gut and mammary glands. Introduction of a mutant APC allele into these mice substantially increases the number of these adenomas. Tcf1 may act as a feedback repressor of beta-catenin-Tcf4 target genes and thus may cooperate with APC to suppress malignant transformation of epithelial cells.

Activation of the Wnt signaling pathway initiates the transformation of colorectal epithelial cells, although the transition to metastatic cancer requires angiogenesis. We have investigated the expression of the von Hippel-Lindau (VHL) tumor suppressor in the intestines from humans and mice. Here, we show that VHL expression is regulated by TCF4 and is restricted to the proliferative compartment at the bottom of intestinal crypts. Accordingly, VHL is completely absent from the proliferative intestinal pockets of Tcf4 À/À perinatal mice. We observed complementary staining of the hypoxia-inducible factor (HIF) 1a to VHL in normal intestinal epithelium as well as in all stages of colorectal cancer (CRC). To the best of our knowledge, this is the first report demonstrating the presence of nuclear HIF1a in normoxic healthy adult tissue. Although we observed upregulated levels of VHL in very early CRC lesions from sporadic and familial adenomatous polyposis patients -presumably due to activated Wnt signaling -a clear reduction of VHL expression is observed in later stages of CRC progression, coinciding with stabilization of HIF1a. As loss of VHL in later stages of CRC progression results in stabilization of HIF, these data provide evidence that selection for VHL downregulation provides a proangiogenic impulse for CRC progression.

SummarySequence specific HMG box factors bind and bend DNA via interactions in the minor groove.3D NMR analyses have provided the structural basis for this interaction. The cognate HMG domain DNA motif is generally believed to span 6 to 8 bases. However, alignment of promotor elements controlled by the yeast genes Ste11 and Rox1 has indicated strict conservation of a larger DNA motif. By site selection, we identify a highly specific 12 bp motif for Ste11:AGAACAAAGAAA. Similarly, we show that Tcf1, MatMc and Sox4 bind unique, highly specific DNA motifs of 12, 12 and 10 bp respectively. Footprinting with a deletion mutant of Ste11 reveals a novel interaction between the 3' base pairs of the extended DNA motif and amino acids Cterminal to the HMG domain. The sequence-specific interaction of Ste11 with these 3' base pairs contributes significantly to binding and bending of the DNA motif.