The transcription factor SNAIL1 is a master regulator of epithelial‐to‐mesenchymal transition (EMT), a process entailing massive gene expression changes. To better understand SNAIL1‐induced transcriptional reprogramming we performed time‐resolved transcriptome analysis upon conditional SNAIL1 expression in colorectal cancer cells. Gene set variation analyses indicated that SNAIL1 strongly affected features related to cell cycle and Wnt/β‐Catenin signalling. This correlated with upregulation of LEF1, a nuclear binding partner of β‐Catenin. Likewise, transcriptomes of cell lines and colorectal cancers, including poor‐prognosis mesenchymal tumours, exhibit positively correlated SNAI1 and LEF1 expression, and elevated LEF1 levels parallel increased patient mortality. To delineate the functional contribution of LEF1 to SNAIL1‐induced EMT, we used the CRISPR/Cas9 system to knock‐out LEF1 in colorectal cancer cells, and to engineer cells that express LEF1 mutants unable to interact with β‐Catenin. Both complete LEF1‐deficiency and prevention of the β‐Catenin‐LEF1 interaction impaired the ability of SNAIL1 to elicit expression of an alternative set of Wnt/β‐catenin targets, and to promote cancer cell invasion. Conversely, overexpression of wildtype, but not of mutant LEF1, stimulated alternative Wnt/β‐Catenin target gene expression, and caused cell‐cycle arrest. Moreover, like SNAIL1, LEF1 retarded tumour growth in xenotransplantations. Thus, LEF1 phenocopies SNAIL1 with respect to several critical aspects of EMT. Indeed, comparative transcriptomics suggested that 35% of SNAIL1‐induced transcriptional changes are attributable to LEF1. However, LEF1 did not autonomously induce EMT. Rather, LEF1 appears to be a strictly β‐Catenin‐dependent downstream effector of SNAIL1. Apparently, SNAIL1 employs β‐Catenin‐LEF1 complexes to redirect Wnt/β‐Catenin pathway activity towards pro‐invasive and anti‐proliferative gene expression.

Transcriptional silencing is a major cause for the inactivation of tumor suppressor genes, however, the underlying mechanisms are only poorly understood. The EPHB2 gene encodes a receptor tyrosine kinase that controls epithelial cell migration and allocation in intestinal crypts. Through its ability to restrict cell spreading, EPHB2 functions as a tumor suppressor in colorectal cancer whose expression is frequently lost as tumors progress to the carcinoma stage. Previously we reported that EPHB2 expression depends on a transcriptional enhancer whose activity is diminished in EPHB2 non-expressing cells. Here we investigated the mechanisms that lead to EPHB2 enhancer inactivation. We show that expression of EPHB2 and SNAIL1 - an inducer of epithelial-mesenchymal transition (EMT) - is anti-correlated in colorectal cancer cell lines and tumors. In a cellular model of Snail1-induced EMT, we observe that features of active chromatin at the EPHB2 enhancer are diminished upon expression of murine Snail1. We identify the transcription factors FOXA1, MYB, CDX2 and TCF7L2 as EPHB2 enhancer factors and demonstrate that Snail1 indirectly inactivates the EPHB2 enhancer by downregulation of FOXA1 and MYB. In addition, Snail1 induces the expression of Lymphoid enhancer factor 1 (LEF1) which competitively displaces TCF7L2 from the EPHB2 enhancer. In contrast to TCF7L2, however, LEF1 appears to repress the EPHB2 enhancer. Our findings underscore the importance of transcriptional enhancers for gene regulation under physiological and pathological conditions and show that SNAIL1 employs a combinatorial mechanism to inactivate the EPHB2 enhancer based on activator deprivation and competitive displacement of transcription factors.

An apparatus for high resolution photoelectron spectrometry of negative ions is described. With this machine, energy spectra of electrons detached from OH , SH , and SD-ions by 488.0 nm and 514.5 nm radiation have been measured with a resolution of 5-7 meV. The rotational branches have been clearly resolved and the electron angular distributions have been determined for the different branches. The measured rotational structure has been reproduced in model calculations; for SH-, our results confirm the threshold model of Walker. The following rotational constants for the respective negative ions have been derived: B0(OH-)=18.75(15)c m 1, B0(SH )=9.39(3)cm 1, B0(SD-)=4.87(2)cm-1The electron affinities of SH, SD have been determined to be EA(SH)=2.317(2)eV and E A(SD) = 2.315 (2)eV; the negative ion vibrational frequencies have been found to agree with those in the parent neutrals to within about 100 cm-1

The energy spectra of electrons detached from O-and S ions by 488.0nm and 514.5 nm photons have been measured with high electron energy resolution (5 meV) in order to determine the branching ratios for the various 2P3/2.1/2 --" aP2, 1.o fine-structure transitions together with their respective angular distributions. Detailed information on the relative importance of the s-and d-wave continuum is obtained.

Epithelial-mesenchymal transition (EMT) is a pivotal process in development and disease. In carcinogenesis, various signaling pathways are known to trigger EMT by inducing the expression of EMT transcription factors (EMT-TFs) like SNAIL1, ultimately promoting invasion, metastasis and chemoresistance. However, how EMT is executed downstream of EMT-TFs is incompletely understood. Here, using human colorectal cancer (CRC) and mammary cell line models of EMT, we demonstrate that SNAIL1 critically relies on bone morphogenetic protein (BMP) signaling for EMT execution. This activity requires the transcription factor SMAD4 common to BMP/TGFβ pathways, but is TGFβ signaling-independent. Further, we define a signature of BMP-dependent genes in the EMT-transcriptome, which orchestrate EMT-induced invasiveness, and are found to be regulated in human CRC transcriptomes and in developmental EMT processes. Collectively, our findings substantially augment the knowledge of mechanistic routes whereby EMT can be effectuated, which is relevant for the conceptual understanding and therapeutic targeting of EMT processes.