The origins of the mesenchymal cells participating in tissue repair and pathological processes, notably tissue fibrosis, tumor invasiveness, and metastasis, are poorly understood. However, emerging evidence suggests that epithelial-mesenchymal transitions (EMTs) represent one important source of these cells. As we discuss here, processes similar to the EMTs associated with embryo implantation, embryogenesis, and organ development are appropriated and subverted by chronically inflamed tissues and neoplasias. The identification of the signaling pathways that lead to activation of EMT programs during these disease processes is providing new insights into the plasticity of cellular phenotypes and possible therapeutic interventions.

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The basics of epithelial-mesenchymal transition

The Epithelial-Mesenchymal Transition Generates Cells with Properties of Stem Cells

Without epithelial–mesenchymal transitions, in which polarized epithelial cells are converted into motile cells, multicellular organisms would be incapable of getting past the blastula stage of embryonic development. However, this important developmental programme has a more sinister role in tumour progression. Epithelial–mesenchymal transition provides a new basis for understanding the progression of carcinoma towards dedifferentiated and more malignant states.

Epithelial–mesenchymal transitions in tumour progression

The transdifferentiation of epithelial cells into motile mesenchymal cells, a process known as epithelial-mesenchymal transition (EMT), is integral in development, wound healing and stem cell behaviour, and contributes pathologically to fibrosis and cancer progression. This switch in cell differentiation and behaviour is mediated by key transcription factors, including SNAIL, zinc-finger E-box-binding (ZEB) and basic helix-loop-helix transcription factors, the functions of which are finely regulated at the transcriptional, translational and post-translational levels. The reprogramming of gene expression during EMT, as well as non-transcriptional changes, are initiated and controlled by signalling pathways that respond to extracellular cues. Among these, transforming growth factor-β (TGFβ) family signalling has a predominant role; however, the convergence of signalling pathways is essential for EMT.

Molecular mechanisms of epithelial–mesenchymal transition

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jor.1100090504

Mesenchymal stem cells

Purpose. To examine the possibility that retinoic acid (RA), a stabilizer of the epithelial phenotype, would inhibit formation of mesenchymal cells from avian lens epithelium in three-dimensional collagen. Methods. Lens epithelia from 11-day-old chick embryos were cultured for 6 days in collagen gels in the presence of RA. The number of mesenchymal cells emigrating into the gels was quantitatively compared with control cultures to which RA was not added. Results. It was found that few fibroblast-like cells form at the highest dose used (10" 5 M RA) and outgrowth approaches control levels at lower doses of RA. The mesenchymal cells that form after RA treatment are not ultrastructurally different from those of controls. Many have well-developed rough endoplasmic reticulum and undoubtedly produce the collagen fibrils that accumulate around the cells. Others, although spindle-shaped, still exhibit lenslike cytoplasm. New basement membrane is deposited on the former free surface of RA-treated lens epithelium, but is not present at the former free surface of control epithelium. Conclusions. It is possible that RA inhibition of lens transformation to fibroblast-like cells is at least partly due to the ability of RA to stimulate production of basement membrane components by epithelia. More studies of RA action on epithelial-mesenchymal transformation in collagen gels may reveal additional mechanisms. It is also suggested that mesenchymal genes similar to those activated in lens epithelium by suspension in collagen may turn on in pathologic transformations (ie, in anterior capsular cataract, fibroblast-like cells arise from lens epithelium). Invest Ophthalmol Vis Sci. 1993;34:2526-2537.

Retinoic Acid Inhibits Formation of Mesenchyme From Lens Epithelium in Collagen Gels

NEW EVIDENCE THAT NUCLEAR IMPORT OF ENDOGENOUS β-CATENIN IS LEF-1 DEPENDENT, WHILE LEF-1 INDEPENDENT IMPORT OF EXOGENOUS β-CATENIN LEADS TO NUCLEAR ABNORMALITIES

It may come as a surprise to you that Keith Porter had a deep interest in the structure and biogenesis of extracellular matrix. As early as 1949, Porter and Vanamee began to use the electron microscope (EM) to follow the formation of extracellular matrix by fibroblasts (1), and, in 1959, Porter and George Pappas called attention to the close association with the cell surface of extracellular fibrils that they believed to be composed of collagen in the process of polymerizing on the fibroblast plasmalemma (2). Observations such as these (2–5) led Porter to propose and defend for some time the theory that collagen fibrils form by a process of ecdysis or direct shedding of cytoplasmic filaments through the plasmalemma onto the mesenchymal cell surface. This hypothesis required newly synthesized collagen to be released from ribosomes directly into the cytoplasm, rather then to take the route from the RER through the Golgi apparatus that was being demonstrated at the same time and the same institute for other secretory proteins by Porter’s colleagues, Philip Seikevitz and George Palade (6). Inspired by the latter study (6), Jean Paul Revel and I did an autoradiographic study at the EM level in 1963 using H proline as a precursor for collagen, a protein that consists of 25% proline and hydroxyproline (7). Our data showing tritium localization to the RER, followed by transport to the Golgi apparatus, were consistent with the idea that chondrocytes use the same secretory pathways as do pancreatic acinar cells, and, indeed, it was subsequently shown by EM immunohistochemistry that procollagen is present only in the RER, Golgi apparatus, and secretory vacuoles/vesicles of mesenchymal cells (8). While quantitation of autoradiographic studies of intracellular collagen transport proved difficult (9), the conclusion that the newly synthesized proline-rich products do not significantly accumulate on the cell surface was unquestionable. Rather, the tritium-labeled extracellular matrix proteins seem to diffuse through the cartilage matrix to polymerize some distance from the cells (7), and a similar sequence of events was shown by autoradiography for deposition of matrix by fibroblasts (9, 10) and odontoblasts (11, 12). Thus, it seems clear that newly forming collagen is not polymerizing on the plasmalemma of mesenchymal cells to any significant degree. What are we to conclude from the convincing demonstration by Porter and others of the close association of extracellular matrix fibrils with the fibroblast cell surface? Surely, what was being observed for the first time at the EM level was the phenomenon that we now call ‘cell-matrix interaction’ (13, 14). In oblique sections across the plasmalemma, the extracellular fibrils appear to be continuous with filamentous cortical cytoplasmic material (2–5), and this observation was extended by Hynes and Destree (15) using immunohistochemistry to demonstrate codistribution with intracellular actin of fibronectin fibrils on the fibroblast cell surface. The functional consequences of actin-matrix interaction were subsequently demonstrated in mesenchymal cells by Tomasek and Hay (16) and Harkin and Hay (17), and in epithelium by Sugrue and Hay (18) and others (13, 19). The major family of transmembrane extracellular matrix receptors that link actin filaments to the matrix are termed integrins, and they form cytoplasmic complexes with focal adhesion and Src kinases that transduce signals to the nucleus via MAP kinases and other proteins (19, 20). Thus, cell and extracellular matrix form a mutually dependent continuum in the body of the multicellular organism. The manner in which epithelial cells might control the polymerization of collagen fibrils at some distance from their cell surface was a subject that also intrigued Porter. In the 1950s, Paul Weiss called attention to the elegant orthogonal lattice of collagen fibrils in the acellular dermis created by the epidermis of aquatic vertebrates. Later, Porter extended this observation to include the subepidermal matrix of the lamprey eel and collagenous cuticle of the annelid worm (5, 13). We showed in 1963 by EM autoradiography that the newly synthesized collagen of the amphibian subepidermal orthogonal gridwork is deposited near the basal lamina (basement membrane) and is subsequently displaced inward by unlabeled matrix after the tritiated pulse has passed (21). Weiss et al. (13) proposed that a cell-switching mechanism allows each layer to be deposited in a plywood fashion at right angles to the one previously formed, but Porter pointed out the many shortcomings of this hypothesis. With his colleagues, Joe Nadol and John Gibbons, he proposed in 1969 (22) the ‘shingle’ or ‘scindulene’ hypothesis,

https://faseb.onlinelibrary.wiley.com/doi/pdf/10.1096/fasebj.13.9002.S281

Elizabeth D. Hay

Papers

Retinoic Acid Inhibits Formation of Mesenchyme From Lens Epithelium in Collagen Gels

NEW EVIDENCE THAT NUCLEAR IMPORT OF ENDOGENOUS β-CATENIN IS LEF-1 DEPENDENT, WHILE LEF-1 INDEPENDENT IMPORT OF EXOGENOUS β-CATENIN LEADS TO NUCLEAR ABNORMALITIES

Biogenesis and organization of extracellular matrix.

Carboxyfluorescein as a marker at both light and electron microscope levels to follow cell lineage in the embryo.

SDS gel analysis of muscle proteins in embryonic cells.