Project description:Converting epithelial into mesenchymal cells through epithelial-mesenchymal transition (EMT) requires massive changes in gene expression. How this is brought about is currently not clear. Here we examined the impact of the EMT master regulator SNAIL1 on the FOXA family of transcription factors which are distinguished by their particular competence to induce chromatin reorganization for the activation of transcriptional enhancer elements. We show that the expression of SNAIL1 and FOXA genes is anti-correlated in transcriptomes of colorectal tumors and cell lines. In two cellular EMT models, ectopically expressed Snail1 downregulates FOXA factors and directly represses FOXA1. To elucidate how FOXA factors contribute to the control of epithelial gene expression, we determined by ChIP-seq data analysis FOXA chromosomal distribution in relation to chromatin structural features characterizing distinct states of transcriptional activity. This revealed a preferential localization of FOXA1 and FOXA2 to transcriptional enhancers at signature genes that distinguish epithelial from mesenchymal colon tumors. To validate the significance of this association, we investigated the impact of FOXA factors on structure and function of transcriptional enhancers at the epithelial genes CDH1, CDX2 and EPHB3. Expression of dominant negative FOXA2 led to chromatin condensation at these enhancer elements. Site- directed mutagenesis of FOXA binding sites in reporter gene constructs and by genome- editing in situ impaired enhancer activity and completely abolished the active chromatin state of the EPHB3 enhancer. Conversely, expression of FOXA factors in cells with inactive CDX2 and EPHB3 enhancers led to chromatin opening and de novo deposition of the H3K4me1 and H3K27ac marks. These findings establish the pioneer function of FOXA factors at enhancer regions of epithelial genes and demonstrate their essential role in maintaining enhancer structure and function. Thus, by repressing FOXA family members, Snail1 targets transcription factors at strategically important positions in gene-regulatory hierarchies which may facilitate transcriptional reprogramming during EMT.

Project description:Phenotypic conversion of tumor cells through epithelial-mesenchymal transition (EMT) requires massive gene expression changes. How these are brought about is not clear. Here we examined the impact of the EMT master regulator SNAIL1 on the FOXA family of transcription factors which are distinguished by their particular competence to induce chromatin reorganization for the activation of transcriptional enhancer elements. We show that the expression of SNAIL1 and FOXA genes is anticorrelated in transcriptomes of colorectal tumors and cell lines. In cellular EMT models, ectopically expressed Snail1 directly represses FOXA1 and triggers downregulation of all FOXA family members, suggesting that loss of FOXA expression promotes EMT. Indeed, cells with CRISPR/Cas9-induced FOXA-deficiency acquire mesenchymal characteristics. Furthermore, ChIP-seq data analysis of FOXA chromosomal distribution in relation to chromatin structural features which characterize distinct states of transcriptional activity, revealed preferential localization of FOXA factors to transcriptional enhancers at signature genes that distinguish epithelial from mesenchymal colon tumors. To validate the significance of this association, we investigated the impact of FOXA factors on structure and function of enhancers at the CDH1, CDX2 and EPHB3 genes. FOXA-deficiency and expression of dominant negative FOXA2 led to chromatin condensation at these enhancer elements. Site-directed mutagenesis of FOXA binding sites in reporter gene constructs and by genome-editing in situ impaired enhancer activity and completely abolished the active chromatin state of the EPHB3 enhancer. Conversely, expression of FOXA factors in cells with inactive CDX2 and EPHB3 enhancers led to chromatin opening and de novo deposition of the H3K4me1 and H3K27ac marks. These findings establish the pioneer function of FOXA factors at enhancer regions of epithelial genes and demonstrate their essential role in maintaining enhancer structure and function. Thus, by repressing FOXA family members, SNAIL1 targets transcription factors at strategically important positions in gene-regulatory hierarchies, which may facilitate transcriptional reprogramming during EMT.

Project description:Snail1 is a transcriptional repressor required for a correct embryonic development. In cancer, Snail1 promotes the epithelial to mesenchymal transition in tumorigenic epithelial cells. In this work, we have analyzed the control of Snail1 in the differentiation of the 3T3-L1 cell line derived from murine embryo cells. The activation by snail of 3T3-L1 induced typical markers of cancer-activated fibroblasts as S100A4 or CD44. We generated 3T3-L1 cells stably over expressing Snail1 (3T3L1/Snail1) and control (3T3-L1/control) cells. We used SILAC quantitative approach to identify and characterize protein alterations induced by Snail1. Cells were fractionated in 5 subcellular fractions. The nuclear fraction of the cells was separated by 10% SDS-PAGE. Gels with forward and reverse experiments were stained with Coomassie Blue and cut into 18 slices prior to reduction, alkylation and digestion with trypsin. Tryptic peptides were scanned and fragmented with a linear ion trap-Orbitrap Velos (ThermoScientific). We identified a total of 3108 proteins, with 2572 quantified proteins, and 565 proteins modulated >1.5-fold by Snail1 overexpression. Among them, we found interesting up-regulated proteins associated to early differentiation of adipogenesis (C/EBPβ) and down-regulated proteins implicated in the final stages of differentiation to adipocytes (Fatty acid-binding protein or Fatty acid synthase). We also observed as down-regulated proteins important mediators of PPARγ pathway. We also observed downregulation of proteins implicated in mTOR, SRC and JAK/STAT pathway. We validated these proteomics data by western blot and qPCR in 3T3-L1 cells and other types of fibroblasts with capable to differentiate to terminal mesenchymal phenotypes, as well as in mesenchymal stem cells (MSC). This work provided insight into novel proteins with potential roles in the regulation of differentiation of the 3T3-L1 and MSCs as Nr2F6, ASC-1, Prrx1 or Cbx6. These candidates are down regulated due to the overexpression of Snail1 in 3T3-L1 cells. We next investigated the potential binding of Snail1 to promoter of these candidates. In silico analysis with MatInspector program revealed various putative E-box consensus motifs for Snail1. We performed ChIP and Luciferase assay to validate Snail1 binds to different E-box motifs of our candidates. Additionally, we analyzed the ability to prevent the differentiation to adipocytes of the 3T3-L1 cells using siRNAs. This work provided insight into novel proteins with potential roles in the regulation of differentiation to adipocytes of the 3T3-L1 and mMSC cells as Nr2F6, ASC-1, Prrx1 or Cbx6 controlled by Snail1.

Project description:At the molecular level, epithelial-to-mesenchymal transition (EMT) necessitates extensive transcriptional reprogramming which is orchestrated by a small group of gene regulatory proteins. The transcription factor SNAIL1 is a zinc-finger DNA-binding protein and well-known master regulator of EMT. However, knowledge of its immediate target genes is incomplete. Here, we performed ChIP-seq to chart genome-wide SNAIL1 chromosomal binding sites and to identify genes directly regulated by SNAIL1. For this we used the colorectal adenocarcinoma cell line LS174T which was engineered to express hemagglutinin epitope-tagged murine SNAIL1 in a doxycycline inducible fashion. In total, 1501 SNAIL1-HA ChIP-seq peaks were mapped which were linked to 1307 genes based on nearest neighbor relationships. The vast majority of SNAIL1-HA binding events occurred within 1 kb upstream of transcriptional start sites (44%), and in intergenic regions and promoter-distal introns (46%). De novo motif deduction showed that nearly all ChIP-seq peak regions harbored at least one sequence element corresponding to the cognate SNAIL1 DNA-recognition motif 5’-CAGGTG-3’. SNAIL1-HA ChIP-seq peaks included previously determined SNAIL1 binding sites at the FOXA1, EPHB3, ITGB4 and CLDN3 genes, thus validating the ChIP-Seq results. When comparing the genomic distribution of SNAIL1-HA to that of the intestinal stem cell (ISC) transcription factors ASCL2 and TCF7L2, we observed a significant overlap. Furthermore, SNAIL1-HA ChIP-seq peaks are associated with a substantial fraction of ISC signature genes. In two colorectal cancer cell lines, we verified that SNAIL1 decreases ISC marker expression. Likewise, SNAIL1-HA was found to occupy multiple regulatory elements at the proto-oncogene MYB, and the long non-coding RNA (lncRNA) WiNTRLINC1, a recently described regulator of ASCL2. Occupancy of the MYB and WiNTRLINC1 regulatory elements by SNAIL1-HA correlated with the downregulation of the two genes. We propose that SNAIL1-mediated repression of MYB and ISC markers like WiNTRLINC1 contributes to the decrease in proliferation known to be associated with EMT, while simultaneously abrogating stemness features of colorectal cancer cells.

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SNAIL1-mediated Downregulation of FOXA Proteins Facilitates the Inactivation of Transcriptional Enhancer Elements at Key Epithelial Genes in Colorectal Cancer Cells

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