Role of Endoplasmic Reticulum Stress in Epithelial–Mesenchymal Transition of Alveolar Epithelial Cells

Abstract

Endoplasmic reticulum (ER) stress has been implicated in alveolar epithelial type II (AT2) cell apoptosis in idiopathic pulmonary fibrosis. We hypothesized that ER stress (either chemically induced or due to accumulation of misfolded proteins) is also associated with epithelial-mesenchymal transition (EMT) in alveolar epithelial cells (AECs). ER stress inducers, thapsigargin (TG) or tunicamycin (TN), increased expression of ER chaperone, Grp78, and spliced X-box binding protein 1, decreased epithelial markers, E-cadherin and zonula occludens-1 (ZO-1), increased the myofibroblast marker, α-smooth muscle actin (α-SMA), and induced fibroblast-like morphology in both primary AECs and the AT2 cell line, RLE-6TN, consistent with EMT. Overexpression of the surfactant protein (SP)-C BRICHOS mutant SP-C(ΔExon4) in A549 cells increased Grp78 and α-SMA and disrupted ZO-1 distribution, and, in primary AECs, SP-C(ΔExon4) induced fibroblastic-like morphology, decreased ZO-1 and E-cadherin and increased α-SMA, mechanistically linking ER stress associated with mutant SP to fibrosis through EMT. Whereas EMT was evident at lower concentrations of TG or TN, higher concentrations caused apoptosis. The Src inhibitor, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4]pyramidine) (PP2), abrogated EMT associated with TN or TG in primary AECs, whereas overexpression of SP-C(ΔExon4) increased Src phosphorylation, suggesting a common mechanism. Furthermore, increased Grp78 immunoreactivity was observed in AT2 cells of mice after bleomycin injury, supporting a role for ER stress in epithelial abnormalities in fibrosis in vivo. These results demonstrate that ER stress induces EMT in AECs, at least in part through Src-dependent pathways, suggesting a novel role for ER stress in fibroblast accumulation in pulmonary fibrosis.

Endoplasmic reticulum stress in lung disease

“…It is therefore worth noting that ER stress is a well-recognised inducer of EMT in the lung. Expression of mutant SFTPC, for example in primary murine AECII or AECII-like cell lines triggers morphological and biochemical EMT, and similar changes occur following treatment with chemical inducers of ER stress [17].…”

Endoplasmic reticulum stress as a novel target to ameliorate epithelial-to-mesenchymal transition and apoptosis of human peritoneal mesothelial cells

“…*Po0.05 vs HPMCs isolated from the omentum. 19 Apoptosis of HPMCs may also be induced by a CHOPindependent pathway via the activation of JNK, caspase, or mitochondrial cytochrome C. Interestingly, in contrast to the role of smad2/3 in EMT, smad2/3 activation was mediated by neither ER stress-nor TGF-β1-induced apoptosis in HPMCs in our study ( Supplementary Figures S4 and S6). Beneficial effect of preconditioning with ER stress was demonstrated in an animal model of mesangioproliferative glomerulonephritis.…”

“…20 The role of the UPR and ER stress has been demonstrated in idiopathic pulmonary fibrosis, as well as glomerulonephritis, diabetic nephropathy and drug-induced renal damage. 17,19,43,44 Despite ample evidence regarding the role of ER stress in organ fibrosis, the about ER stress and peritoneal fibrosis remain very limited. Previous studies revealed ultrastructural alterations of rough ER in the parietal peritoneum of patients on PD.…”

“…Apoptosis of HPMCs using a higher concentration of ER stress inducers was associated with a persistent increase in the expression of C/EBP homologous protein (CHOP; Figure 6a), a UPR marker has been implicated in apoptosis. 19,37,38 A mild and transient upregulation of CHOP was observed with a lower concentration of TM or TG (Figure 6b).…”

“…[14][15][16][17] ER stress is reported to induce EMT of epithelial cells in the kidney and lung, which may result in fibroblast accumulation. 15,[18][19][20][21][22] The ER has a key role in the maintenance of protein homeostasis through its control of the content, structure, folding and trafficking of proteins. [23][24][25] The accumulation of unfolded proteins in the ER lumen results in a release of glucose-regulated protein (GRP)78/94, a central regulator of ER stress, from the ER membrane, which allows GRP78/94 to either dimerize or move to other locations within cells.…”