The History and Mystery of Alveolar Epithelial Type II Cells: Focus on Their Physiologic and Pathologic Role in Lung

Alveolar type II (ATII) cells are a key structure of the distal lung epithelium, where they exert their innate immune response and serve as progenitors of alveolar type I (ATI) cells, contributing to alveolar epithelial repair and regeneration. In the healthy lung, ATII cells coordinate the host defense mechanisms, not only generating a restrictive alveolar epithelial barrier, but also orchestrating host defense mechanisms and secreting surfactant proteins, which are important in lung protection against pathogen exposure. Moreover, surfactant proteins help to maintain homeostasis in the distal lung and reduce surface tension at the pulmonary air–liquid interface, thereby preventing atelectasis and reducing the work of breathing. ATII cells may also contribute to the fibroproliferative reaction by secreting growth factors and proinflammatory molecules after damage. Indeed, various acute and chronic diseases are associated with intensive inflammation. These include oedema, acute respiratory distress syndrome, fibrosis and numerous interstitial lung diseases, and are characterized by hyperplastic ATII cells which are considered an essential part of the epithelialization process and, consequently, wound healing. The aim of this review is that of revising the physiologic and pathologic role ATII cells play in pulmonary diseases, as, despite what has been learnt in the last few decades of research, the origin, phenotypic regulation and crosstalk of these cells still remain, in part, a mystery.

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Mechanical distension modulates pulmonary alveolar epithelial phenotypic expression in vitro

AJP Lung Cellular and Molecular Physiology ◽

10.1152/ajplung.1998.274.2.l196 ◽

1998 ◽

Vol 274 (2) ◽

pp. L196-L202 ◽

Cited By ~ 27

Author(s):

Jorge A. Gutierrez ◽

Robert F. Gonzalez ◽

Leland G. Dobbs

Keyword(s):

Phenotypic Expression ◽

Alveolar Type ◽

Transcriptional Level ◽

Type I ◽

Type Ii Cells ◽

Type Ii ◽

Alveolar Type Ii Cells ◽

Alveolar Epithelial ◽

Mrna Content

The pulmonary alveolar epithelium is composed of two distinct types of cells, type I and type II cells, both of which are critical for normal lung function. On the basis of experiments of both nature and in vivo studies, it has been hypothesized that expression of the type I or type II phenotype is influenced by mechanical factors. We have investigated the effects of mechanical distension on the expression of specific markers for the type I and type II cell phenotypes in cultured alveolar type II cells. Rat alveolar type II cells were tonically mechanically distended in culture. Cells were analyzed for a marker for the type I phenotype (rTI40, an integral membrane protein specific for type I cells) and for markers for the type II phenotype [surfactant protein (SP) A, SP-B, and SP-C] as well as for glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Mechanical distension caused a 68 ± 25% ( n = 3) increase in mRNA content of rTI40 relative to undistended controls. In contrast, mechanical distension resulted in a decrease in mRNA content of SP-B to 35 ± 19% ( n = 3) and of SP-C to 20 ± 6.7% ( n = 3) of undistended controls. There was no effect on mRNA content of SP-A or GAPDH. The differences in mRNA content of SP-B and SP-C were found to be primarily due to changes at the transcriptional level by nuclear run-on assays. The effects on rTI40 appear to be due to posttranscriptional events. These data show that mechanical distension influences alveolar epithelial phenotypic expression in vitro, at least in part, at the transcriptional level.

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β-Catenin/Lin28/let-7 regulatory network determines type II alveolar epithelial stem cell differentiation phenotypes following thoracic irradiation

Journal of Radiation Research ◽

10.1093/jrr/rraa119 ◽

2020 ◽

Vol 62 (1) ◽

pp. 119-132

Author(s):

Xiaozhuan Liu ◽

Tingting Zhang ◽

Jianwei Zhou ◽

Ziting Xiao ◽

Yanjun Li ◽

...

Keyword(s):

Cell Differentiation ◽

Stem Cell Differentiation ◽

Alveolar Epithelial Cells ◽

Type I ◽

Type Ii ◽

Epithelial Stem Cells ◽

Alveolar Epithelial ◽

Thoracic Irradiation ◽

Radiation Induced ◽

Differentiation Marker

Abstract The contribution of type II alveolar epithelial stem cells (AEC II) to radiation-induced lung fibrosis (RILF) is largely unknown. Cell differentiation phenotypes are determined by the balance between Lin28 and lethal-7 microRNA (let-7 miRNA). Lin28 is activated by β-catenin. The aim of this study was to track AEC II phenotypes at different phases of injury following thoracic irradiation and examine the expression of β-catenin, Lin28 and let-7 to identify their role in AEC II differentiation. Results showed that coexpression of prosurfactant protein C (proSP-C, an AEC II biomarker) and HOPX (homeobox only protein X, an AEC I biomarker) or vimentin (a differentiation marker) was detected in AEC II post-irradiation. The protein expression levels of HOPX and proSP-C were significantly downregulated, but vimentin was significantly upregulated following irradiation. The expression of E-cadherin, which prevents β-catenin from translocating to the nucleus, was downregulated, and the expression of β-catenin and Lin28 was upregulated after irradiation (P < 0.05 to P < 0.001). Four let-7 miRNA members (a, b, c and d) were upregulated in irradiated lungs (P < 0.05 to P < 0.001), but let-7d was significantly downregulated at 5 and 6 months (P < 0.001). The ratios of Lin28 to four let-7 members were low during the early phase of injury and were slightly higher after 2 months. Intriguingly, the Lin28/let-7d ratio was strikingly increased after 4 months. We concluded that β-catenin contributed to RILF by promoting Lin28 expression, which increased the number of AEC II and the transcription of profibrotic molecules. In this study, the downregulation of let-7d miRNA by Lin28 resulted in the inability of AEC II to differentiate into type I alveolar epithelial cells (AEC I).

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Fluid transport across cultured rat alveolar epithelial cells: a novel in vitro system

AJP Lung Cellular and Molecular Physiology ◽

10.1152/ajplung.00176.2003 ◽

2004 ◽

Vol 287 (1) ◽

pp. L104-L110 ◽

Cited By ~ 26

Author(s):

Xiaohui Fang ◽

Yuanlin Song ◽

Rachel Zemans ◽

Jan Hirsch ◽

Michael A. Matthay

Keyword(s):

Epithelial Cells ◽

Fluid Transport ◽

Alveolar Epithelial Cells ◽

Lung Epithelial Cells ◽

Morphological Evidence ◽

Alveolar Type ◽

Type Ii ◽

In Vitro System ◽

Alveolar Epithelial

Previous studies have used fluid-instilled lungs to measure net alveolar fluid transport in intact animal and human lungs. However, intact lung studies have two limitations: the contribution of different distal lung epithelial cells cannot be studied separately, and the surface area for fluid absorption can only be approximated. Therefore, we developed a method to measure net vectorial fluid transport in cultured rat alveolar type II cells using an air-liquid interface. The cells were seeded on 0.4-μm microporous inserts in a Transwell system. At 96 h, the transmembrane electrical resistance reached a peak level (1,530 ± 115 Ω·cm2) with morphological evidence of tight junctions. We measured net fluid transport by placing 150 μl of culture medium containing 0.5 μCi of 131I-albumin on the apical side of the polarized cells. Protein permeability across the cell monolayer, as measured by labeled albumin, was 1.17 ± 0.34% over 24 h. The change in concentration of 131I-albumin in the apical fluid was used to determine the net fluid transported across the monolayer over 12 and 24 h. The net basal fluid transport was 0.84 μl·cm−2·h−1. cAMP stimulation with forskolin and IBMX increased fluid transport by 96%. Amiloride inhibited both the basal and stimulated fluid transport. Ouabain inhibited basal fluid transport by 93%. The cultured cells retained alveolar type II-like features based on morphologic studies, including ultrastructural imaging. In conclusion, this novel in vitro system can be used to measure net vectorial fluid transport across cultured, polarized alveolar epithelial cells.

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Disparate cytokine regulation of ICAM-1 in rat alveolar epithelial cells and pulmonary endothelial cells in vitro

AJP Lung Cellular and Molecular Physiology ◽

10.1152/ajplung.1995.269.1.l127 ◽

1995 ◽

Vol 269 (1) ◽

pp. L127-L135 ◽

Cited By ~ 5

Author(s):

W. W. Barton ◽

S. Wilcoxen ◽

P. J. Christensen ◽

R. Paine

Keyword(s):

Endothelial Cells ◽

Epithelial Cells ◽

Inflammatory Cytokines ◽

Alveolar Epithelial Cells ◽

Normal Lung ◽

Type I ◽

Type Ii Cells ◽

Type Ii ◽

Alveolar Epithelial

Intercellular adhesion molecule-1 (ICAM-1) is expressed at high levels on type I alveolar epithelial cells in the normal lung and is induced in vitro as type II cells spread in primary culture. In contrast, in most nonhematopoetic cells ICAM-1 expression is induced in response to inflammatory cytokines. We have formed the hypothesis that the signals that control ICAM-1 expression in alveolar epithelial cells are fundamentally different from those controlling expression in most other cells. To test this hypothesis, we have investigated the influence of inflammatory cytokines on ICAM-1 expression in isolated type II cells that have spread in culture and compared this response to that of rat pulmonary artery endothelial cells (RPAEC). ICAM-1 protein, determined both by a cell-based enzyme-linked immunosorbent assay and by Western blot analysis, and mRNA were minimally expressed in unstimulated RPAEC but were significantly induced in a time- and dose-dependent manner by treatment with tumor necrosis factor-alpha, interleukin-1 beta, or interferon-gamma. In contrast, these cytokines did not influence the constitutive high level ICAM-1 protein expression in alveolar epithelial cells and only minimally affected steady-state mRNA levels. ICAM-1 mRNA half-life, measured in the presence of actinomycin D, was relatively long at 7 h in alveolar epithelial cells and 4 h in RPAEC. The striking lack of response of ICAM-1 expression by alveolar epithelial cells to inflammatory cytokines is in contrast to virtually all other epithelial cells studied to date and supports the hypothesis that ICAM-1 expression by these cells is a function of cellular differentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

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Sulfation of extracellular matrices modifies responses of alveolar type II cells to fibroblast growth factors

AJP Lung Cellular and Molecular Physiology ◽

10.1152/ajplung.1996.271.5.l688 ◽

1996 ◽

Vol 271 (5) ◽

pp. L688-L697 ◽

Cited By ~ 9

Author(s):

P. L. Sannes ◽

J. Khosla ◽

P. W. Cheng

Keyword(s):

Growth Factors ◽

Biological Significance ◽

Alveolar Type ◽

Type I ◽

Type Ii Cells ◽

Type Ii ◽

Fibroblast Growth ◽

Alveolar Type Ii Cells ◽

Proliferation And Differentiation ◽

Type Ii Cell

The pulmonary alveolar basement membrane (BM) associated with alveolar type II cells has been shown to be significantly less sulfated than that of type I cells. To examine the biological significance of this observation, we measured the incorporation of 5-bromodeoxyuridine (BrdU) as an indicator of DNA synthesis in isolated rat type II cells cultured for 72-120 h on substrata that were naturally sulfated, not sulfated, or chemically desulfated in serum-free, hormonally defined media, with and without selected growth factors. The percentage of cells incorporating BrdU was significantly elevated by desulfated chondroitin sulfate in the presence of fibroblast growth factor-2 (FGF-2 or basic FGF) and depressed by heparin in the presence of either FGF-1 or acidic FGF or FGF-2. This depressive effect was lost by removing sulfate from the heparin. Some responses were dependent on the period of time in culture and concentration and molecular weight of the substrata. These observations support the notion that sulfation per se of certain components of BM is a key determinant of type II cell responses to select growth factors that may define patterns of proliferation and differentiation.

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Varied Cytochrome Oxidase Activities of the Alveolar Type I, Type II and Type III Cells in Rat Lungs: Quantitative Cytochemistry

Journal of Electron Microscopy ◽

10.1093/oxfordjournals.jmicro.a050762 ◽

1989 ◽

Keyword(s):

Cytochrome Oxidase ◽

Alveolar Type ◽

Type I ◽

Type Ii ◽

Type Iii ◽

Rat Lungs ◽

Quantitative Cytochemistry

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Differential Response of Primary Alveolar Type I and Type II Cells to LPS Stimulation

PLoS ONE ◽

10.1371/journal.pone.0055545 ◽

2013 ◽

Vol 8 (1) ◽

pp. e55545 ◽

Cited By ~ 36

Author(s):

Mandi H. Wong ◽

Meshell D. Johnson

Keyword(s):

Differential Response ◽

Alveolar Type ◽

Type I ◽

Type Ii Cells ◽

Type Ii

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Paracrine Regulation of Alveolar Epithelial Damage and Repair Responses by Human Lung-Resident Mesenchymal Stromal Cells

Cells ◽

10.3390/cells10112860 ◽

2021 ◽

Vol 10 (11) ◽

pp. 2860

Author(s):

Dennis M. L. W. Kruk ◽

Marissa Wisman ◽

Jacobien A. Noordhoek ◽

Mehmet Nizamoglu ◽

Marnix R. Jonker ◽

...

Keyword(s):

Epithelial Cells ◽

Mesenchymal Stromal Cells ◽

Lung Tissue ◽

Stromal Cells ◽

Wound Repair ◽

Lung Cells ◽

Alveolar Type ◽

Type Ii ◽

Epithelial Damage ◽

Alveolar Epithelial

COPD is characterized by irreversible lung tissue damage. We hypothesized that lung-derived mesenchymal stromal cells (LMSCs) reduce alveolar epithelial damage via paracrine processes, and may thus be suitable for cell-based strategies in COPD. We aimed to assess whether COPD-derived LMSCs display abnormalities. LMSCs were isolated from lung tissue of severe COPD patients and non-COPD controls. Effects of LMSC conditioned-medium (CM) on H2O2-induced, electric field- and scratch-injury were studied in A549 and NCI-H441 epithelial cells. In organoid models, LMSCs were co-cultured with NCI-H441 or primary lung cells. Organoid number, size and expression of alveolar type II markers were assessed. Pre-treatment with LMSC-CM significantly attenuated oxidative stress-induced necrosis and accelerated wound repair in A549. Co-culture with LMSCs supported organoid formation in NCI-H441 and primary epithelial cells, resulting in significantly larger organoids with lower type II-marker positivity in the presence of COPD-derived versus control LMSCs. Similar abnormalities developed in organoids from COPD compared to control-derived lung cells, with significantly larger organoids. Collectively, this indicates that LMSCs’ secretome attenuates alveolar epithelial injury and supports epithelial repair. Additionally, LMSCs promote generation of alveolar organoids, with abnormalities in the supportive effects of COPD-derived LMCS, reflective of impaired regenerative responses of COPD distal lung cells.

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Role of collagenase in mediating in vitro alveolar epithelial wound repair

Journal of Cell Science ◽

10.1242/jcs.112.2.243 ◽

1999 ◽

Vol 112 (2) ◽

pp. 243-252

Author(s):

E. Planus ◽

S. Galiacy ◽

M. Matthay ◽

V. Laurent ◽

J. Gavrilovic ◽

...

Keyword(s):

Cell Migration ◽

Cell Adhesion ◽

Epithelial Cell ◽

Type I Collagen ◽

Alveolar Epithelial Cell ◽

Cell Monolayer ◽

Type I ◽

Type Ii ◽

Alveolar Epithelial

Type II pneumocytes are essential for repair of the injured alveolar epithelium. The effect of two MMP collagenases, MMP-1 and MMP-13 on alveolar epithelial repair was studied in vitro. The A549 alveolar epithelial cell line and primary rat alveolar epithelial cell cultures were used. Cell adhesion and cell migration were measured with and without exogenous MMP-1. Wound healing of a cell monolayer of rat alveolar epithelial cell after a mechanical injury was evaluated by time lapse video analysis. Cell adhesion on type I collagen, as well as cytoskeleton stiffness, was decreased in the presence of exogenous collagenases. A similar decrease was observed when cell adhesion was tested on collagen that was first incubated with MMP-1 (versus control on intact collagen). Cell migration on type I collagen was promoted by collagenases. Wound healing of an alveolar epithelial cell monolayer was enhanced in the presence of exogenous collagenases. Our results suggest that collagenases could modulate the repair process by decreasing cell adhesion and cell stiffness, and by increasing cell migration on type I collagen. Collagen degradation could modify cell adhesion sites and collagen degradation peptides could induce alveolar type II pneumocyte migration. New insights regarding alveolar epithelial cell migration are particularly relevant to investigate early events during alveolar epithelial repair following lung injury.

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