Activated alveolar epithelial cells initiate fibrosis through autocrine and paracrine secretion of connective tissue growth factor

Abstract

Fibrogenesis involves a pathological accumulation of activated fibroblasts and extensive matrix remodeling. Profibrotic cytokines, such as TGF-β, stimulate fibroblasts to overexpress fibrotic matrix proteins and induce further expression of profibrotic cytokines, resulting in progressive fibrosis. Connective tissue growth factor (CTGF) is a profibrotic cytokine that is indicative of fibroblast activation. Epithelial cells are abundant in the normal lung, but their contribution to fibrogenesis remains poorly defined. Profibrotic cytokines may activate epithelial cells with protein expression and functions that overlap with the functions of active fibroblasts. We found that alveolar epithelial cells undergoing TGF-β-mediated mesenchymal transition in vitro were also capable of activating lung fibroblasts through production of CTGF. Alveolar epithelial cell expression of CTGF was dramatically reduced by inhibition of Rho signaling. CTGF reporter mice demonstrated increased CTGF promoter activity by lung epithelial cells acutely after bleomycin in vivo. Furthermore, mice with lung epithelial cell-specific deletion of CTGF had an attenuated fibrotic response to bleomycin. These studies provide direct evidence that epithelial cell activation initiates a cycle of fibrogenic effector cell activation during progressive fibrosis. Therapy targeted at epithelial cell production of CTGF offers a novel pathway for abrogating this progressive cycle and limiting tissue fibrosis.

Fibrocytes Are Not an Essential Source of Type I Collagen during Lung Fibrosis

“…Primary lung mesenchymal cells from wild-type mice were seeded on chamber slides(27, 37, 45, 46). After 24 hours, cells were fixed, permeabilized and stained for the proteins indicated as previously described.…”

“…BAL samples were obtained and quantified on a hemocytometer as previsously described.. Hydroxyproline was measured by methods previously described(37, 46, 47). Briefly, homogenates from both lungs were incubated in 12N HCl for at least 16 hours at 120°C.…”

The cytokines of pulmonary fibrosis: Much learned, much more to learn

“…On the other hand, in the absence of TGF-b or other profibrotic mediators, CTGF upregulates MMP1 by activating the Ets1 and ERK1/2 pathways [182]. Overexpression of CTGF in fibroblasts in vivo induced systemic tissue fibrosis [183], whereas an elegant study [184] has recently validated the functional contribution of CTGF production by alveolar epithelial cells to the development of pulmonary fibrosis in the BLM model. Mice with lung epithelial cell-specific deletion of CTGF had an attenuated fibrotic response to bleomycin.…”

Sunitinib, a Small-Molecule Kinase Inhibitor, Attenuates Bleomycin-Induced Pulmonary Fibrosis in Mice

“…During the progression of IPF, the chronic injuries incite the abnormally activated alveolar epithelial cells (AECs) to secrete TGF-β, which in turn promotes the EMT, by which AECs undergo mesenchymal transformation, lose their cell-cell adhesion, acquire migratory and invasive properties, and ultimately convert into mesenchymal cells, accounting for excessive deposition of ECM in parenchymal . TGF-β signaling is essential in a number of profibrotic events including EMT, fibroblast activation and eventual ECM deposition (Yang et al 2014;Eberlein et al 2015;Okumura et al 2015;Zerr et al 2016). Sunitinib profoundly inhibited TGF-β-induced EMT (Fig.…”