Is it the end of the line for the EMT?

E pithelial-to-mesenchymal transition (EMT) is an important biological concept that describes the reversible transition of differentiated epithelial cells into mesenchymal cells with increased motility and changes in gene expression. Three types of EMT have been proposed: Type 1 EMT describes the invasion of transition cells into the mesenchyme during development. Type 2 EMT describes the transition into mesenchymal cells, potentially including myofibroblasts during wound healing and repair. Type 3 EMT describes invasion of cancer cells, including those migrating into the circulation to generate distant metastases. Fibrosis studies conducted in different organs including the liver have demonstrated that myofibroblasts are the primary source of extracellular matrix. Myofibroblasts are immunophenotypically characterized by a stellate shape, expression of abundant pericellular matrix, and fibrotic genes (a-smooth muscle actin [a-SMA], nonmuscle myosin, fibronectin, vimentin). Ultrastructurally, myofibroblasts are defined by prominent rough endoplasmic reticulum, a Golgi apparatus producing collagen, peripheral myofilaments, fibronexus (no lamina), and gap junctions. Myofibroblasts are implicated in wound healing and fibroproliferative disorders. In response to fibrogenic stimuli, such as transforming growth factor b1 (TGF-b1), myofibroblasts express aSMA, secrete extracellular matrix (fibronectin, collagen type I and III), obtain high contractility, and change phenotype (production of the stress fibers). But where do these myofibroblasts come from? Hepatic stellate cells (HSCs) are considered to be a major source of fibrogenic myofibroblasts in the injured liver. Hepatic myofibroblasts may also originate from portal fibroblasts, bone marrow–derived mesenchymal cells, and fibrocytes, and by the transition of epithelial cells and endothelial cells to mesenchymal cells. What is the evidence for type 2 EMT being the etiology of the myofibroblasts in liver fibrosis? First, multiple studies in the kidney and in the lung were interpreted as showing EMT during fibrosis in those organs, and this concept was extrapolated to liver fibrosis (discussed in Zeisberg and Duffield). Second, primary cell culture studies have clearly demonstrated that cholangiocytes and hepatocytes undergo a change in the phenotype and gene expression toward a mesenchymal cell, especially after incubation with TGF-beta, which is the cytokine most closely associated with EMT. Third, immunohistochemical studies both in experimental and clinical liver fibrosis have reported the coexpression of mesenchymal markers (fibroblastspecific protein 1 [FSP1], a-SMA, vimentin, desmin) with the original epithelial markers (cytokeratin-19 for cholangiocytes and albumin for hepatocytes). These types of studies have been recently questioned, because the allegedly EMT-specific marker FSP1 (also called S100A4) has now been shown to be expressed by nonfibroblast cells in the liver, including by a subset of monocytes. Fourth, at least one study used lineage tracing in mouse liver (see description below) to demonstrate that cells that were originally hepatocytes (i.e., at one point expressed albumin) expressed FSP1 after a fibrogenic liver injury. However, a potential artifact in these studies is the specific lack of the anti–b-galactosidase antibodies used for cell fate mapping. The current report by the Wells laboratory in HEPA-