Ectopic expression

Abstract: Epithelial to mesenchymal transition (EMT) facilitates tissue remodelling during embryonic development and is viewed as an essential early step in tumour metastasis. We found that all five members of the microRNA-200 family (miR-200a, miR-200b, miR-200c, miR-141 and miR-429) and miR-205 were markedly downregulated in cells that had undergone EMT in response to transforming growth factor (TGF)-β or to ectopic expression of the protein tyrosine phosphatase Pez. Enforced expression of the miR-200 family alone was sufficient to prevent TGF-β-induced EMT. Together, these microRNAs cooperatively regulate expression of the E-cadherin transcriptional repressors ZEB1 (also known as δEF1) and SIP1 (also known as ZEB2), factors previously implicated in EMT and tumour metastasis. Inhibition of the microRNAs was sufficient to induce EMT in a process requiring upregulation of ZEB1 and/or SIP1. Conversely, ectopic expression of these microRNAs in mesenchymal cells initiated mesenchymal to epithelial transition (MET). Consistent with their role in regulating EMT, expression of these microRNAs was found to be lost in invasive breast cancer cell lines with mesenchymal phenotype. Expression of the miR-200 family was also lost in regions of metaplastic breast cancer specimens lacking E-cadherin. These data suggest that downregulation of the microRNAs may be an important step in tumour progression. MicroRNAs are small, non-coding RNAs that modulate gene expression post-transcriptionally. In metazoa, they act predominantly to inhibit translation of their specific targets, but they also typically cause a modest reduction in the level of their target mRNAs 1,2 . Hundreds of microRNAs have been identified in vertebrates, with varying patterns of expression that range from ubiquitous to highly tissue- or developmental-stage-restricted. In some cases, an individual microRNA can act as a developmental switch by regulating a key target mRNA 3 . Speculating that switching between cell phenotypes that occurs during EMT may be specified to some extent by microRNAs, we searched for microRNAs whose expression changed during EMT. To this end, we used an in vitro model of EMT, which was generated by stable transfection of Madin Darby canine kidney (MDCK) epithelial cells with the protein tyrosine phosphatase Pez (PTP-Pez). Overexpression of PTP-Pez caused MDCK cells to undergo EMT, as indicated by loss of E-cadherin expression, gain in expression of the mesenchymal markers fibronectin, ZEB1 and SIP1, loss of cohesion, induction of cell motility and a change in cell morphology

Abstract: MicroRNAs (miRNAs) are an abundant class of ∼22-nucleotide regulatory RNAs found in plants and animals. Some miRNAs of plants, Caenorhabditis elegans, and Drosophila play important gene-regulatory roles during development by pairing to target mRNAs to specify posttranscriptional repression of these messages. We identify three miRNAs that are specifically expressed in hematopoietic cells and show that their expression is dynamically regulated during early hematopoiesis and lineage commitment. One of these miRNAs, miR-181, was preferentially expressed in the B-lymphoid cells of mouse bone marrow, and its ectopic expression in hematopoietic stem/progenitor cells led to an increased fraction of B-lineage cells in both tissue-culture differentiation assays and adult mice. Our results indicate that microRNAs are components of the molecular circuitry that controls mouse hematopoiesis and suggest that other microRNAs have similar regulatory roles during other facets of vertebrate development.