Serine Threonine Receptor-Associated Protein (STRAP) plays a role in the maintenance of mesenchymal morphology

The stromal tissue, made of extracellular matrix and mesenchymal cells, is vital for the functional design of all complex tissues. Fibroblasts are key components of stromal tissue and play a crucial role during organ development, wound repair, angiogenesis and fibrosis. We have previously reported the identification of a novel WD-domain protein, STRAP11STRAP: Serine Threonine Receptor-Associated Protein, TGF-β: transforming growth factor-β, MEF: mouse embryonic fibroblast, LEF1: lymphoid enhancer-binding factor 1, WT1: Wilms tumor homolog 1, EMT: epithelial-to-mesenchymal transition, EGF: epidermal growth factor, MET: mesenchymal-to-epithelial transition, cdc2: cell division cycle 2, ZEB1: zinc finger E-box binding homeobox 1, SIP1: survival of motor neuron protein interacting protein 1, TβRI and TβRII: TGF-β receptors type I and type II, PDK1: pyruvate dehydrogenase kinase, isozyme 1, ERK: extracellular signal-regulated kinase, EWS: Ewing sarcoma breakpoint region 1 protein, SMN: survival of motor neurons, NFX1: nuclear transcription factor, X-box binding 1, MAP1B: microtubule-associated protein 1B, 5-FU: 5-fluorouracil, GFP: green fluorescent protein, CMV: cytomegalovirus, RT-PCR: reverse transcription-polymerase chain reaction, MMLV: Moloney Murine Leukemia Virus, GAPDH: glyceraldehyde-3-phosphate dehydrogenase, PBS: phosphate-buffered saline, FITC: fluorescein isothiocyanate, PDGF: platelet-derived growth factor, MMP9: matrix metallopeptidase 9, TEM: transmission electron microscopy, SOX9: SRY-box containing gene 9, TIMP3: tissue inhibitor of metalloproteinase 3, IGF2: insulin-like growth factor 2. that inhibits transforming growth factor-β (TGF-β) signaling and enhances tumorigenicity via TGF-β-dependent and TGF-β-independent mechanisms. Here, we report, for the first time, that deletion of STRAP from Mouse Embryonic Fibroblasts (MEFs) results in a loss of mesenchymal morphology. These cells lose their spindle shape and exhibit features of an epithelial morphology. Gene expression profiling has confirmed that deletion of STRAP affects expression of sets of genes important for diverse functions including cell–cell adhesion and cell polarization, and upregulates E-cadherin expression leading to the formation of adherens junctions, subsequent localization of β-catenin to the cell membrane and downregulation of the mesenchymal markers like LEF1 (lymphoid enhancer-binding factor 1). Upregulation of WT1 (Wilms tumor homolog 1) in STRAP null MEFs plays a role in E-cadherin induction. Finally, stable expression of STRAP in these cells results in a loss of WT1 and E-cadherin expressions, and a reversal from epithelial to the mesenchymal morphology. Thus, these results provide a novel TGF-β-independent function of STRAP and describe a mechanism for the role of STRAP in the maintenance of mesenchymal morphology.