Remodeling of the pulmonary microenvironment controls transforming growth factor-beta activation and alveolar type II epithelial to mesenchymal transition
Dysart, Marilyn Markowski
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Pulmonary fibrosis is a potentially deadly pathology characterized by excessive deposition of extracellular matrix (ECM), increased tissue stiffness, and loss of tissue structure and function. Recent evidence has suggested epithelial to mesenchymal transition (EMT), the transdifferentiation of an epithelial cell into a mesenchymal fibroblast, is one mechanism that results in the accumulation of myofibroblasts and excessive deposition of ECM. EMT is a highly orchestrated process involving the integration of biochemical signals from specific integrin mediated interactions with ECM proteins and soluble growth factors including TGFβ. TGFβ, a potent inducer of EMT, can be activated by cell contraction mediated mechanical release of the growth factor from a macromolecular latent complex. Therefore, TGFβ activity and subsequent EMT may be influenced by both the biochemical composition and biophysical state of the surrounding ECM. Based on these knowns it was first investigated how changes in the biochemical composition of the matrix and changes in tissue rigidity together modulate EMT due to changes in epithelial cell contraction and TGFβ activation. Here we show that integrin specific interactions with fibronectin (Fn) variants displaying both the RGD and PHSRN binding sites facilitate cell binding through α3β1 and α5β1 integrins, and that these interactions maintain an epithelial phenotype despite engagement of increased tissue rigidities. Conversely, Fn fragments that facilitate cell binding through αv integrins drive TGFβ activation and subsequent EMT even while engaging soft underlying substrates. Adding to the complexity of studying mechanisms that contribute to pulmonary fibrosis, is exposure of the lung to injuries from environmental particulates. Therefore, we investigated how EMT is altered in response to particulate matter (PM). Here we show that PM exposure further drives TGFβ activation, EMT, and increases intracellular levels of reactive oxygen species (ROS). Additionally, cells binding the ECM through α5β1 and α3β1 integrins only partially recover an epithelial phenotype, suggesting ROS may be a secondary driver of TGFβ and EMT. Taken together these results suggest dynamic changes to the ECM microenvironment are major contributors to the control of EMT responses and provide insights into the design of biomaterial-based microenvironments for control of epithelial cell phenotype.