Synergistic effects of particulate matter and substrate stiffness on epithelial-to-mesenchymal transition

Dysfunctional pulmonary homeostasis and repair, including diseases such as pulmonary fibrosis, chronic obstructive pulmonary disease (COPD*), and tumorigenesis, have been increasing steadily over the past decade, a fact that heavily implicates environmental influences. Several investigations have suggested that the lung "precursor cell"--the alveolar type II (ATII) epithelial cell--is central in the initiation and progression of pulmonary fibrosis. Specifically, ATII cells have been shown (Iwano et al. 2002) to be capable of undergoing an epithelial-to-mesenchymal transition (EMT). EMT, the de-differentiation of an epithelial cell into a mesenchymal cell, has been theorized to increase the number of extracellular matrix (ECM)-secreting mesenchymal cells, perpetuating fibrotic conditions and resulting in increased lung tissue stiffness. In addition, increased exposure to pollution and inhalation of particulate matter (PM) have been shown to be highly correlated with an increased incidence of pulmonary fibrosis. Although both of these events are involved in the progression of pulmonary fibrosis, the relationship between tissue stiffness, exposure to PM, and the initiation and course of EMT remains unclear. The hypothesis of this study was twofold: 1. That alveolar epithelial cells cultured on increasingly stiff substrates become increasingly contractile, leading to enhanced transforming growth factor beta (TGF-β) activation and EMT; and 2. That exposure of alveolar epithelial cells to PM with an aerodynamic diameter ≤ 2.5 μm (PM2.5; also known as fine PM) results in enhanced cell contractility and EMT. Our study focused on the relationship between the micromechanical environment and external environmental stimuli on the phenotype of alveolar epithelial cells. This relationship was explored by first determining how increased tissue stiffness affects the regulation of fibronectin (Fn)-mediated EMT in ATII cells in vitro. We cultured ATII cells on substrates of increasing stiffness and evaluated changes in cell contractility and EMT. We found that stiff, but not soft, Fn substrates were able to induce EMT and that this event depended on a contractile phenotype of the cell and the subsequent activation of TGF-β. In addition, we were able to show that activation or suppression of cell contractility by way of exogenous factors was sufficient to overcome the effect of substrate stiffness. Pulse-chase experiments indicated that the effect on cell contractility is dose- and time-dependent. In response to low levels of TGF-β on soft surfaces, either added exogenously or produced through contraction induced by the stiffness agonist thrombin, cells initiate EMT; on removal of the TGF-β, they revert to an epithelial phenotype. Overall, the results from this first part of our study identified matrix stiffness or cell contractility as critical targets for the control of EMT in fibrotic diseases. For the second part of our study, we wanted to investigate whether exposure to PM2.5, which might have higher toxicity than coarser PM because of its small size and large surface-to-mass ratio, altered the observed stiffness-mediated EMT. Again, we cultured ATII cells on increasingly stiff substrates with or without the addition of three concentrations of PM2.5. We found that exposure to PM2.5 was involved in increased stiffness-mediated EMT, as shown by increases in mesenchymal markers, cell contractility, and TGF-β activation. Most notably, on substrates with an elastic modulus (E) of 8 kilopascals (kPa), a physiologically relevant range for pulmonary fibrosis, the addition of PM2.5 resulted in increased mesenchymal cells and EMT; these were not seen in the absence of the PM2.5. Overall, this study showed that there is a delicate balance between substrate stiffness, TGF-β, and EMT. Furthermore, we showed that exposure to PM2.5 is able to further mediate this interaction. The higher levels of EMT seen with exposure to PM2.5 might have been a result of a positive feedback loop, in which enhanced exposure to PM2.5 through the loss of cell-cell junctions during the initial stages of EMT led to the cells being more susceptible to the effects of surrounding immune cells and inflammatory signals that can further activate TGF-β and drive additional EMT progression. Overall, our work--showing increased cell contractility, TGF-β activation, and EMT in response to substrate stiffness and PM2.5 exposure--highlights the importance of both the micromechanical and biochemical environments in lung disease. These findings suggest that already-fibrotic tissue might be more susceptible to further damage than healthy tissue when exposed to PM2.5.