Mechanisms of Myofibroblast Differentiation

A distinct subpopulation of fibroblasts in fibrotic tissue is characterized by expression of α-smooth muscle actin (α-SMA) and consequently referred to as myofibroblasts. These cells express high levels of extracellular matrix and exhibit contractile properties that may be significant in pathological alteration of mechanical properties of affected fibrotic tissues. Moreover, they express high levels of profibrogenic cytokines such as transforming growth factor β (TGFβ). These are key properties of tissues undergoing fibrosis; thus, insights into the genesis of myofibroblasts should advance understanding of the pathogenesis of fibrotic diseases. There is compelling evidence, especially in vitro that myofibroblasts are derived from appropriately stimulated (e.g., with TGFβ) resident tissue fibroblasts, which normally do not express α-SMA. Recent studies also implicate epithelial and endothelial cells as potential additional sources through a process referred to as epithelial- or endothelial-mesenchymal transition (EMT). An additional potential source is the circulating “fibrocyte.” The relative contributions by these mechanisms to the overall myofibroblast population remain uncertain, especially in vivo. The mechanisms involved in myofibroblast differentiation from these diverse cell types are likely to have different components, although there may be similarities with respect to downstream TGFβ signaling, since this is a common agent found to be effective in inducing differentiation in all these cell types. Give that α-SMA is a key marker of myofibroblast differentiation, an obvious focus for studies into this process is directed at regulation of expression of this gene. TGFβ is a potent inducer of differentiation and regulates α-SMA gene expression via the canonical Smad and MAP kinase signaling pathways. Additionally, CArG elements, E-boxes, and a purine-rich motif are implicated in regulating gene expression. The corresponding transcription factors such as serum response factor (SRF) and transcription enhancer factor-1 (TEF-1) have been identified. A Smad binding element (SBE), a TGFβ hypersensitivity region (THR), and a TGFβ control element (TCE) are present in the α-SMA promoter and found to be essential for TGFβ-induced gene expression. Additionally, p53, Kruppel-like factors, C/EBPβ, Sp1, and Sp3 also influence α-SMA gene expression. Some of these may interact with each other to effect their influence on gene transcription. Existence of repressive factors, such as gut Kruppel-like factor (GKLF), Nkx2.5, YB-1, NFκB, PPARγ, and the liver-enriched inhibitory protein (LIP) isoform of C/EBPβ, suggests that de-repression may contribute to gene expression. Finally, the Notch signaling pathway is also found to be important in myofibroblast differentiation. The role of epigenetic regulation in myofibroblast differentiation adds another layer of complexity to this process. The importance of histone acetylation and DNA methylation in regulating myofibroblast differentiation is increasingly being recognized. In summary, the mechanisms underlying myofibroblast differentiation are multifactorial and complex.