Biomed Res Int. 2015;2015:573210.
Breakdown of Epithelial Barrier Integrity and Overdrive Activation of Alveolar Epithelial Cells in the Pathogenesis of Acute Respiratory Distress Syndrome and Lung Fibrosis.
- 1Division of Neurology, Respirology, Endocrinology and Metabolism, Department of Internal Medicine, Faculty of Medicine, University of Miyazaki, Kiyotake, Miyazaki 889-1692, Japan.
Individual alveolar epithelial cells (AECs) collaboratively form a tight barrier between atmosphere and fluid-filled tissue to enable normal gas exchange. The tight junctions of AECs provide intercellular sealing and are integral to the maintenance of the AEC barrier integrity. Disruption and failure of reconstitution of AEC barrier result in catastrophic consequences, leading to alveolar flooding and subsequent devastating fibrotic scarring. Recent evidences reveal that many of the fibrotic lung diseases involve AECs both as a frequent target of injury and as a driver of ongoing pathological processes. Aberrantly activated AECs express most of the growth factors and chemokines responsible for the proliferation, migration, and activation of fibroblasts. Current evidences suggest that AECs may acquire overdrive activation in the initial step of fibrosis by several mechanisms, including abnormal recapitulation of the developmental pathway, defects of the molecules essential for epithelial integrity, and acceleration of aging-related properties. Among these initial triggering events, epithelial Pten, a multiple phosphatase that negatively regulates the PI3K/Akt pathway and is crucial for lung development, is essential for the prevention of alveolar flooding and lung fibrosis through the regulation of AEC barrier integrity after injury. Reestablishment of AEC barrier integrity also involves the deployment of specialized stem/progenitor cells.
Supplement Figure 1: Aberrant activation of alveolar epithelial cells (AECs) in lung fibrosis.
- Recapitulation of the developmental pathway of AECs in lung fibrosis: (1) Wnt/β signaling: type II AEC (AEC II) and fibroblast isolated from idiopathic pulmonary fibrosis (IPF) patients overexpress the members of the Wnt/wingless pathway and exhibit excessive nuclear accumulations of β-catenin. Several Wnt ligands render fibroblasts more active. WNT1-inducible signaling protein-1 (WISP1) is upregulated in AEC II in both a murine bleomycin-induced lung fibrosis model and IPF lungs. WISP1 activates AEC II and fibroblasts. Depletion of WISP1 attenuates bleomycin-induced lung fibrosis in mice. Inhibition of Wnt/β-catenin/CREB binding protein (CBP) also ameliorates lung fibrosis of bleomycin-treated mice. (2) Sonic hedgehog (Shh) signaling: a high expression of Shh protein is seen in the epithelial cells lining honeycomb cysts in lungs affected by IPF. TGF-β induced Shh expression in cultured murine AECs, whereas Shh induced TGF-β and alpha-smooth muscle actin (α-SMA) expression in cultured murine lung fibroblasts.
- Defect of molecules essential for epithelial integrity: (1) Shp2: AEC-specific deletion of Shp2 in mice induces deregulated surfactant homeostasis, increased AEC II apoptosis, and spontaneous inflammation-independent lung fibrosis. (2) CD151: tetraspanin CD151 is expressed at the basolateral surface of AECs and is important in maintaining AEC integrity via rigid adhesion to the basement membrane (BM). CD151-deleted cultured AECs show attenuated adhesion on the BM. CD151-KO mice exhibit spontaneous age-related lung fibrosis.
- Acceleration of aging-related properties: (1) cell biological characteristics of aging in the IPF lungs and (2) PTEN-induced putative kinase 1 (PINK1). AEC II in IPF lungs exhibits marked accumulation of dysmorphic and dysfunctional mitochondria, defective autophagy, and decreased PINK1 expression. Lung epithelial cell-specific PINK1-deleted mice show dysmorphic and dysfunctional mitochondria and vulnerability to apoptosis. PINK1 KO mice also show increased susceptibility to lung fibrosis after both MHV68 (a murine gammaherpesvirus homologous to EBV) infection and bleomycin treatment. EMT: epithelial mesenchymal transition; ECM: extracellular matrix; TCF: T-cell factor.
Supplement Figure 2: Exacerbated alveolar flooding, failure of reconstitution of alveolar epithelial cell (AEC) integrity, and subsequent augmented fibrotic scarring in epithelial cell-specific Pten-deficient lungs compared to the normal repair and resolution of acute lung injury in wild-type lungs.
- Normal condition of the alveolus: type I AEC (AEC I) and type II AEC (AEC II) establish close contacts with neighbors through laterally located intercellular junctional complexes (i.e., tight junctions and adherens junctions) and reside on basement membrane (BM). Paracellular permeability depends on claudin-family tight junction proteins. Though AEC I and AEC II have distinct patterns of claudin expression, claudin-4 expression is comparable between the two types of AECs. AECs barrier integrity is primarily regulated by claudin-4.
- Acute injury phase in wild-type lungs: injury to the lung parenchyma and loss of AECs lead to a deposition of provisional extracellular matrix (ECM), which is conducive to the ingrowth of fibroblasts. To establish the normal reconstitution of AEC integrity, AEC II spreads and migrates on a denuded BM.
- Resolution of acute lung injury: if the BM is intact and the loss of AECs integrity is limited, the provisional ECM is reabsorbed and the reepithelialization of AEC integrity occurs.
- Acute injury phase in epithelial cell-specific Pten-deficient lungs: the intensified disruption of tight junctions leads to a greater deposition of intra-alveolar provisional ECM and a subsequent induction of greater numbers of fibroblast ingrowths. Epithelial cell-specific Pten-deficient lungs also show considerable degradation of the BM after injury. Loss of the BM leads to a failure of the normal reconstitution of AEC integrity through deprivation of the stable scaffolding from AECs which is necessary for normal spatial orientation in cell spreading and migration.
- Fibrotic scarring in epithelial cell-specific Pten-deficient lungs: failure of the reconstitution of AEC integrity and persistent alveolar flooding cause the expansion and activation of fibroblasts and myofibroblasts, which results in excessive collagen-rich matrix and fibrotic scarring.
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The authors thank Sumie Tajiri for her invaluable assistance in preparing the paper and Chie Tanaka for her excellent work with the illustrations. This study was supported by Grants-in-Aid for Young Scientists (B) (no. 20790573 and no. 22790763) from the Japan Society for Promotion of Science and by the Takeda Science Foundation and Japan Intractable Disease Research Foundation.
Shigehisa Yanagi, MD, PhD.
Division of Neurology, Respirology, Endocrinology and Metabolism,
Department of Internal Medicine, Faculty of Medicine,
University of Miyazaki, Kihara 5200, Kiyotake, Miyazaki 889-1692, Japan.