ERJ
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by McElroy, M.C.
Right arrow Articles by Kasper, M.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by McElroy, M.C.
Right arrow Articles by Kasper, M.
Eur Respir J 2004; 24:664-673
Copyright ©ERS Journals Ltd 2004

The use of alveolar epithelial type I cell-selective markers to investigate lung injury and repair

M.C. McElroy1 and M. Kasper2

1 MRC Centre for Inflammation Research, School of Molecular and Clinical Medicine, University of Edinburgh, Edinburgh, UK. 2 Dept of Anatomy, Carl Gustav Carus Medical Faculty, Technical University of Dresden, Dresden, Germany.

CORRESPONDENCE: M.C. McElroy, MRC Centre for Inflammation Research, School of Molecular and Clinical Medicine, University of Edinburgh, Edinburgh, EH8 9AG, UK. Fax: 44 1316506528. E-mail: mmcelroy@ed.ac.uk

Keywords: Alveolar epithelial type I cell, cell differentiation, fibrosis, marker, primary acute respiratory distress syndrome, pulmonary injury

Received: August 21, 2003
Accepted April 29, 2004

M.C. McElroy is funded by the Norman Salvesen Emphysema Research Fund (Edinburgh, UK) and M. Kasper by the Deutsche Forschungsgemeinschaft (SFB 13) (Bonn, Germany).

Abstract

Alveolar epithelial type I cells cover most of the internal surface area of the lungs.

Ultrastructural studies demonstrate that alveolar epithelial type I cell damage is frequently observed in acute and chronic lung diseases.

This article discusses the use of cell-selective proteins as markers for the investigation of injury and repair of the alveolar epithelium. The utility of proteins specific to alveolar epithelial type I cells as diagnostic markers of alveolar epithelial injury in acute lung injury is considered, and expression of proteins selective for alveolar epithelial type I cells in lungs following injury and in fibrosis are discussed.

Epithelial damage is a major hallmark of many life-threatening pulmonary diseases, including acute lung injury (ALI) or acute respiratory distress syndrome (ARDS), severe acute respiratory syndrome, cystic fibrosis and idiopathic pulmonary fibrosis (IPF) 1–4. In addition, alveolar epithelial injury followed by abnormal epithelial repair appears to be a key pathological feature of IPF 4.

Two cell types populate the alveolar epithelium in normal adult lungs, alveolar epithelial type I (ATI) and alveolar epithelial type II (ATII) cells. ATI cells cover ~95% of the internal surface area of the lung. They are branched cells with multiple apical surfaces that extend into adjacent alveoli 5. The apical surface area of ATI cells is large in comparison with most cells (i.e. ~5,000 µm2 for human ATI cells), yet they are very thin cells (i.e. 0.2 µm in depth) 5, 6. The gas exchange barrier is composed of ATI and endothelial cells joined by fused basement membranes. ATI cells are important in the regulation of alveolar fluid balance and surfactant secretion by ATII cells in response to stretch 7–10. ATII cells cover the remaining 2–5% of the lung's surface area. They are cuboidal cells situated between ATI cells, and contain characteristic lamellar bodies and apical microvilli 5. ATII cells have many known functions, including the production, secretion and reuptake of pulmonary surfactant 11, regulation of alveolar fluid in normal lungs and during the resolution of pulmonary oedema 12, and synthesis and secretion of immunomodulatory proteins important for host defence, such as surfactant proteins A and D 13, 14.

Damaged ATI cells are replaced by ATII cells 15–19 (reviewed in 19) (fig. 1Go). The studies of Evans and coworkers 15, 20, in particular, provide the foundation of current knowledge concerning the response of the alveolar epithelium to injury. They examined ultrastructural changes to the alveolar epithelium following exposure to the oxidant gas nitrogen dioxide for 48 h. In alveoli adjacent to terminal bronchioles, ATI cells were maximally damaged after 8–12 h 20. After 48 h of exposure, the denuded basement membranes were repopulated by cuboidal epithelial cells, some of which were ATII cells. Evans et al. 15 followed the fate of proliferating ATII cells (after 48 h of NO2 exposure) using a combination of autoradiography and electron microscopic analysis. Proliferating cells were labelled with tritiated thymidine, which is incorporated into newly synthesised DNA during the S phase of the cell cycle. After 48 h of NO2 exposure, most labelled cells were ATII cells; however, following afurther 48-h recovery period, the number of labelled ATII cells had decreased, whereas the number of labelled ATI cells had increased. In addition, over the 48-h recovery period, there was a transient increase in label associated with alveolar epithelial cells with characteristics of both ATI and ATII cells (i.e. ATII-like cells but spread out over the basement membrane or ATI-like cells containing occasional microvilli and lamellar bodies) 15. These studies demonstrate three important aspects of alveolar epithelial injury and repair (in response to NO2). First, the loss of ATI cells can precede ATII cell proliferation. Secondly, alveolar epithelial repair involves the transformation ATII cells (or a subset thereof) into new ATI cells. Thirdly, ATII cells transforming to new ATI cells do so via intermediate cell types.



View larger version (29K):
[in this window]
[in a new window]
  		Fig. 1.— Alveolar epithelial repair. Following injury, the alveolar epithelium is repaired by the proliferation and transdifferentiation of alveolar epithelial type II (ATII) cells, via intermediate cells, into new alveolar epithelial type I (ATI) cells (solid arrows). Injured alveolar epithelial cells may also be replaced by bone marrow and airway progenitor cells. The role of ATI cells in alveolar epithelial repair is still speculative (dotted arrows).

 
More recent studies have refined the role of ATII cells in ATI cell repair and raised alternative mechanisms of ATI cell repair (fig. 1Go) 21–24. There may be subpopulations of ATII cells with different proliferative capabilities following injury 24. However, the potential for these putative ATII cell subpopulations to transdifferentiate into ATI cells is unknown. Damaged ATI cells may also be replaced by bone marrow and nonciliated bronchiolar epithelial progenitor cells 21, 22. It has been assumed for a long time that ATI cells were terminally differentiated. However, experiments designed to investigate the role of mechanical load in foetal lung development suggest that ATI cells have the potential to transdifferentiate into ATII cells 23. It is not known whether ATI cells are capable of replacing damaged ATII cells in injured adult lungs.

Investigating ATI cell injury and repair is difficult because they are not easily recognised using light microscopy. To date, most studies have relied on visualisation of ATI cells, and, in particular, necrosis of ATI cells, using electron microscopy. A number of proteins have been identified which are expressed by ATI cells but not ATII cells (reviewed in 25). The present review focuses on the application of ATI-cell-selective proteins as markers for the investigation of injury to and repair of ATI cells in experimental models and human lung diseases. First, known ATI-cell-selective proteins are described (see also 25). Secondly, the utility of ATI-cell-specific proteins in biochemical assays for quantification of the extent of ATI cell necrosis is discussed. Thirdly, the significance of changes in ATI-cell-selective proteins following injury and in lung fibrosis is evaluated.

Proteins expressed by alveolar epithelial type I cells

In the following section, the ATI-cell-selective proteins have been categorised into three groups (table 1Go). Group I contains proteins that are specifically expressed by ATI cells in the lung (ATI-cell-specific). Group II contains proteins expressed by ATI, and other lung cells, but not ATII cells (ATI-cell-selective), and group III contains proteins either expressed more abundantly by ATI than ATII cells or for which there is insufficient evidence to place them in group II (ATI-cell-selective/associated).


View this table:
[in this window]
[in a new window]
  		Table 1— Alveolar epithelial type I (ATI) cell-specific and -associated proteins in normal adult lung

 
Group I. alveolar epithelial type I cell-specific proteins
Rat type I cell 40-kDa protein/type I cell alpha protein
Rat type I cell 40-kDa protein (RTI40)/type I cell alpha (TI{alpha}) protein is an apical membrane protein expressed exclusively by ATI cells in the lung (fig. 2Go) 26, 52, 53. A monoclonal antibody directed against RTI40/TI{alpha} protein was used to identify and characterise the rat gene TI{alpha} in a bacterial expression library 53. The TI{alpha} gene encodes a membrane protein predicted to have a large extracellular domain, a transmembrane region and a short cytoplasmic tail 53. RTI40/TI{alpha} protein is also detected in nonpulmonary tissues, including the choroid plexus and ciliary epithelium of the eye 54. RTI40/TI{alpha} protein is homologous to a number of other proteins, including OTS-8 (mouse phorbol ester-treated osteoblast cell line 55), 38-kDa glycoprotein (gp38) (mouse thymic epithelial cells 56), E11 (rat lymphatic endothelial cells and osteoblasts 57), podoplanin (rat kidney podocytes 58), 40-kDa glycoprotein (gp40) (Madin-Darby canine kidney cells 59), PA2.26 antigen (mouse keratinocytes during epidermal carcinogenesis 60) and Aggrus (human colorectal tumours 27). Mouse and rat RTI40/TI{alpha} protein are 78% homologous on the basis of deduced amino acid sequences 53.



View larger version (71K):
[in this window]
[in a new window]
  		Fig. 2.— Use of alveolar epithelial type I (ATI) cell-specific antibodies to visualise rat ATI cells using immunofluorescence microscopy at a) low and b) high power. ATI cells are stained green with a monoclonal antibody directed against rat type I cell 40–kDa protein (RTI40)/type I cell alpha (TI{alpha}) protein. Alveolar epithelial type II cells and Clara cells are stained red with the MMC4 monoclonal antibody (arrow). Br: bronchiole; BV: blood vessel. *: airspace. (From 51.)

 
Human homologues of RTI40/TI{alpha} protein have been cloned and are called hTI{alpha}-2 and gp36 61, 62. hTI{alpha}-2 is 50% identical to TI{alpha} at the nucleotide level. Northern blot analysis of messenger ribonucleic acid (mRNA) demonstrates that the transcript for hTI{alpha}-2 is present in lung and not in other selected tissues (e.g. kidney and small intestine) 61. gp36 is 66% identical to TI{alpha} at the nucleotide level. Sequence alignment demonstrates that the greatest variability between RTI40/TI{alpha} protein and gp36 occurs in the extracellular domain, which might explain why antibodies that recognise rat ATI cells do not also recognise human ATI cells 62. Although a monoclonal antibody raised against gp36 is reported to recognise the alveolar epithelium, there is insufficient data at present to determine whether gp36 is ATI-cell-specific in human lungs 62.

Mice with targeted mutations in the TI{alpha} locus die at birth due to respiratory failure 63, 64. Histological analysis of the lungs demonstrates that mice deficient in RTI40/TI{alpha} protein exhibit fewer ATI cells, narrower and irregular airspaces, and defective formation of alveolar saccules in comparison to wild-type litter mates 63. In addition, there is abnormally high expression of proliferation markers associated with the alveolar epithelium and mesenchymal cells (e.g. fibroblasts) in the distal lung 63. These data demonstrate that the loss of TI{alpha} is important, although not absolutely required, for the formation of ATI cells during the late stages of gestation in the mouse. These data suggest that TI{alpha}, ATI cells or both regulate distal airway cell proliferation during lung development.

The role of RTI40/TI{alpha} protein in normal adult lung is unknown. RTI40/TI{alpha} protein is a sialoglycoprotein 52, 59, 62. Highly negatively charged proteins, such as sialoglycoproteins, are relatively resistant to proteases. Therefore, RTI40/TI{alpha} protein may act as a physical barrier and protect the surface of ATI cells from proteases 62. The overexpression of TI{alpha} in endothelial cells and keratinocytes in culture promotes phenotypic changes (i.e. long thin cell extensions) and increases the migration rate in wounding experiments 60, 64. In adult lungs, RTI40/TI{alpha} protein may, therefore, be important for initiating and maintaining the extended and flattened ATI cell phenotype. A similar role has been proposed for RTI40/TI{alpha} in protein kidney podocytes 58. However, other recognised functions of RTI40/TI{alpha} protein are harder to explain. RTI40/TI{alpha} protein has also been identified as a platelet aggregation-inducing factor expressed by colorectal tumours 27. It is not known whether RTI40/TI{alpha} protein on the surface of ATI cells facilitates platelet aggregation.

Human type I cell 56-kDa protein
Dobbs et al. 29 have also generated an antibody that specifically reacts with ATI cells in human lungs, human type I cell 56-kDa protein (HTI56). This antibody has been used to quantify ATI cell injury in patients with established ARDS (see later). It is not known whether HTI56 and RTI40 are homologous. At present, there are no data on the biological or functional properties of HTI56.

Group II. alveolar epithelial type I cell-selective proteins
Immunoelectron microscopy has been used to demonstrate that caveolin-1 and -2 30, 31, cytochrome P450 2B1 34, aquaporin (AQP) 5 33, 65 and the {alpha}2-isoform of sodium/potassium-exchanging adenosine triphosphatase (Na+/K+-ATPase) 32 are expressed by ATI but not ATII cells (table 1Go).

Caveolin-1 and -2
Caveolae are small flask-shaped microvesicles found in abundance in ATI and endothelial cells in the lung. The principle protein component of caveolae is caveolin-1, which is required for the biogenesis of these organelles 66, 67 (reviewed in 68, 69). In the lung, caveolin-2 is coexpressed with caveolin-1 (70 and references therein). Caveolae are also enriched with cholesterol and sphingolipids and may serve as specific microdomains or "rafts" in which cell signalling cascades are regulated (reviewed in 69). In particular, caveolin-1 may negatively regulate proliferation and apoptosis, and may, therefore, act as a tumour suppressor 69, 71. Mice deficient in either caveolin-1 or -2 are born healthy and are able to reproduce 66, 67, 70. However, by 4–5 months of age, they develop a lung pathology that resembles fibrosis. Specifically, caveolin-1 and -2 knockout mice show smaller airspaces and thickened alveolar walls 66, 67, 70, 72. The increased alveolar wall thickness is due to endothelial hyperproliferation and increased deposition of extracellular matrix proteins 69–72. Both types of knockout mice are also exercise-intolerant in comparison with their wild-type litter mates 66, 67, 70. The pulmonary defect may be caused solely by caveolin-2 deficiency 70. Caveolin-1-deficient mice show a dramatic reduction in caveolin-2 levels 66, 67, whereas caveolin-2 null mice show almost normal levels of caveolin-1 70. It has been hypothesised that caveolin-2 might be required for the mitigating mechanical forces that stimulate lung cell proliferation 70.

{alpha}2-Isoform of sodium/potassium-exchanging adenosine triphosphatase
The alveoli have to be maintained essentially fluid-free in order to ensure efficient gas exchange (reviewed in 12). In addition, the upregulation of fluid absorption following acute lung injury is an important factor in recovery from ALI or ARDS 12. Fluid is removed from the airspaces by active transport of sodium across the epithelium by a basolateral Na+/K+-ATPase. The resulting osmotic gradient promotes reabsorption of water 12. Until recently, it was believed that ATII cells were the most important cell type in the regulation of alveolar fluid clearance. However, it has now been shown that ATI cells contain apical sodium channels and can actively transport sodium ions 8, 32, 73. Immunoelectron microscopy suggests that the {alpha}2-isoform of Na+/K+-ATPase is expressed in ATI but not ATII cells and accounts for a significant percentage of basal alveolar fluid reabsorption 32.

Aquaporin 5
AQP5 is a water channel protein located in the apical membrane of ATI cells and bronchiolar epithelial cells in the lung 33, 73. AQP5-deficient mice demonstrate that AQP5 is important for the movement of water from the airspaces to the capillary bed in response to an osmotic gradient generated from the vascular bed 74. However, AQP5 does not appear to be important in water absorption out of the airspaces under normal conditions or under conditions in which alveolar fluid clearance is upregulated by specific agonists 74. Nor is AQP5 important in lung extravascular fluid accumulation in acute injury 75. It has been proposed that AQP5 may be important in the regulation of ATI cell volume 76.

Cytochrome P450 2B1 and carboxypeptidase M
Both cytochrome P450 2B1 and carboxypeptidase M are associated with ATI but not ATII cells in the lung 35, 77. Cytochrome P450 2B1 is also present in nonciliated bronchiolar epithelial cells and carboxypeptidase M in macrophages 34, 35. The specific role of these enzymes in ATI cell function is unknown.

Group III. Alveolar epithelial type I cell-selective/associated proteins
Intercellular adhesion molecule-1
Alveolar epithelial intercellular adhesion molecule-1 (ICAM-1) may specifically regulate alveolar macrophage phagocytic activity in the lung 78 or remodelling of the alveolar epithelium following injury 1. Immunoelectron micrographs demonstrate that ICAM-1 is present on the apical surface of ATI cells, and, in particular, that there is abundant expression on surfaces near cell junctions 36. ICAM-1 is also detected at lower levels on the surface of capillary endothelial cells 78. Immunoelectron microscopic studies demonstrate low levels of ICAM-1 on ATII cells 36, but studies using isolated ATII cells have not demonstrated ICAM-1 expression 37. It is possible that this discrepancy may reflect differences in the methods used to detect ICAM-1, the use of different primary antibodies or loss of ICAM-1 during ATII cell isolation. ICAM-1 is upregulated by ATII cells, but not by ATI cells, in response to hyperoxia and various cytokines 36, 79, 80.

Receptor for advanced glycation end-products
The receptor for advanced glycation end-products (RAGE) is a multiligand-binding protein that binds advanced glycation end-products 81. Advanced glycation end-products are a heterogeneous group of molecules that accumulate in plasma and other body fluids as a consequence of age and certain diseases 81. The mRNA transcript for RAGE is highly expressed in lung tissue 81. Immmunoelectron microscopic studies demonstrate that RAGE is expressed on the basal surface of ATI cells in the lung 41, 42. However, mRNA transcripts for RAGE have also been detected in ATII cells in the lung 43. The role of RAGE in ATI cells is unknown.

Lectins
Finally, a group of lectins are able to distinguish between the carbohydrate moieties on the surface of ATI and ATII cells. Lectins such as Ricinus communis agglutinin 82, Lycopersicon esculentum agglutinin 83, Soybean agglutinin 84 and Bauhinia purpurea agglutinin 85 selectively bind to the surface of ATI cells.

Expression of alveolar epithelial type I cell-selective proteins following injury

Following injury, the amount of ATI-cell-selective proteins in lung tissue may change because of damage to ATI cells. However, ATI-cell-selective proteins may also change because they are also expressed on alveolar epithelial intermediate cell types (i.e. morphology-dependent changes in regulation) or because of changes in regulation by ATI cells (i.e. morphology-independent). The use of ATI cell-selective markers in the detection of ATI cell necrosis in both animal and human studies is reviewed below. Data from animal studies that suggest ATI-cell-selective proteins are upregulated by ATI cells in response to toxic agents or expressed by alveolar epithelial intermediate cells are also reviewed.

Use of rat type I cell 40-kDa protein/type I cell alpha protein and human type I cell 56-kDa protein to quantify ATI cell necrosis in ALI
Ultrastructural studies on injured lungs demonstrate that necrotic ATI cells are sloughed from their basement membranes into the airspaces 86–88. Various experimental studies demonstrate that the amount of RTI40/TI{alpha} protein in distal airway fluid or bronchoalveolar lavage fluid (BALF) can be used to quantify the extent of ATI cell necrosis (table 2Go). Specifically, in a model of severe ALI, caused by Pseudomonas aeruginosa, RTI40/TI{alpha} protein levels are elevated 80-fold above control values in distal airway fluid 6 h after infection 89. Morphological analysis of infected lungs at this time point demonstrates widespread ATI cell necrosis and denuded alveolar basement membranes 89. In addition, the ability of the alveolar epithelium to clear fluid, a functional measurement of epithelial integrity, is completely abolished following P. aeruginosa-induced lung injury. In contrast, in a model of focal ATI cell necrosis, induced by nitrogen dioxide exposure, BALF concentrations of RTI40/TI{alpha} protein are elevated only two-fold above control values and alveolar fluid clearance is stimulated 90. BALF RTI40/TI{alpha} protein concentrations are also elevated, to varying degrees, in other acute injury models, including hyperoxia 91, Staphylococcus aureus-induced pneumonia 92, acid-instillation followed by ventilator-induced injury 96 and following intratracheal adminstration of bleomycin (table 2Go) 94. In the rat model of acid-instillation, RTI40/TI{alpha} protein concentrations in oedema fluid and plasma were significantly reduced in animals ventilated at low tidal, or protective, volumes in comparison with those ventilated at higher tidal volumes (i.e. 3 versus 6 mL·kg body weight–1) 96. Low tidal volumes were also associated with better alveolar function as measured by the rate of alveolar fluid clearance 96. Combined, these studies suggest that ATI cell membrane proteins, such as RTI40/TI{alpha} protein, may be sensitive markers of lung injury resulting from a wide range of different causes. More specifically, however, these studies also suggest that the extent of ATI cell necrosis depends on the exact nature of the damaging agent.


View this table:
[in this window]
[in a new window]
  		Table 2— Rat type I cell 40-kDa protein (RTI40) in rat lung injury models

 
ALI and ARDS are life-threatening conditions that may occur as a consequence of either direct (pneumonia and acid aspiration) or indirect (sepsis and trauma) injury to the lungs. As in experimental models of ALI, ultrastructural analysis of the lung in patients who have died due to ARDS demonstrates widespread epithelial damage, including ATI cell necrosis 97. In addition, clinical studies also demonstrate that preserved alveolar fluid clearance correlates with lower mortality in patients with ARDS 12. The concentration of various ATII-cell-selective proteins (e.g. surfactant protein A and D, and KL-6) in BALF and serum are altered in patients at risk of ARDS and with ARDS 98, 99. However, since the BALF concentration of these secreted proteins may well change independently of ATII cell necrosis, their usefulness in diagnostic assays may be limited.

At the present time, the antibodies that recognise the human homologue of RTI40 have not been used to quantify ATI cell injury in patients with ALI or ARDS. However, HTI56, a human ATI-cell-specific protein, has been measured in oedema fluid and plasma from patients with established ARDS. The concentration of HTI56 was significantly elevated in both oedema fluid and plasma from patients with ARDS in comparison to those from hydrostatic pulmonary oedema patients 100. These data suggest that ATI-cell-specific membrane proteins may be sensitive markers of alveolar epithelial injury in patients with, and at-risk of, ALI or ARDS.

Specific regulation of alveolar epithelial type I cell-selective proteins by toxic agents
Hyperoxia is frequently used as a model for the investigation of lung injury and repair. Hyperoxic exposure characteristically causes endothelial damage that is followed by ATI cell damage 101. Cao et al. 102 investigated the lung content of RTI40/TI{alpha} protein, AQP5 and caveolin-1 following hyperoxic exposure. Instead of a decrease in ATI-cell-selective protein levels (which might be expected if ATI cells were damaged), they found that RTI40/TI{alpha} protein and AQP5 were both upregulated by approximately five-fold, whereas caveolin-1 levels were unchanged. Immunohistochemical analysis, for RTI40/TI{alpha} protein, demonstrated that staining remained restricted to ATI cells and that there was no additional staining in other cell types. Moreover, using an alveolar epithelial cell line, Cao et al. 102 demonstrated that the transcriptional activity of the TI{alpha} promoter was directly regulated by hyperoxia. These data suggest that RTI40/TI{alpha} protein and AQP5 are regulated by ATI cells in response to hyperoxic stress.

In a mouse model of viral pneumonia, levels of AQP5 and its mRNA transcripts were both decreased 7 and 14 days following inoculation 103. Histochemical analysis of the alveolar wall suggested that this decrease in AQP5 expression was not due to ATI cell necrosis or increased numbers of ATII cells. Conversely, immunohistochemical analysis revealed that AQP5 staining was reduced throughout the lung, even in areas without overt inflammation. These data suggest that AQP5 is downregulated by ATI cells following viral pneumonia. AQP5 has also been reported to be downregulated in lung tissue following endotoxin-induced lung injury 104. The functional significance of AQP5 downregulation is not understood. Although viral and endotoxin-induced lung injury both cause extravascular fluid accumulation, studies in AQP5-deficient mice suggest that AQP5 does not play a role in fluid accumulation following injury 75.

Expression of alveolar epithelial type I cell-selective proteins in alveolar intermediate cells
Following injury to the lung, the alveolar walls are populated not only by ATI and ATII cells but potentially also by alveolar epithelial intermediate cells and airway or bone-marrow-derived progenitor cells. Very little is known about the proteins expressed by any of these cell types, even ATI and ATII cells, in the lungs following injury. However, data from a small number of in vivo studies suggest that alveolar epithelial intermediate cells may express certain ATI-cell-selective proteins.

Like NO2, ozone damages epithelial cells located at the junction between the terminal bronchioles and the first alveolar duct 105. Following ozone exposure, immunoelectron microscopy demonstrates that cytochrome P450 2B1 is located in ATI cells, as observed in noninjured lungs. However, P450 2B1 is also expressed in two other epithelial subtypes, which exhibit ultrastructural characteristics of both ATI and ATII cells 34. One of these epithelial cell types has microvilli, an extended and thick lateral cytoplasm, and large vacuoles that occasionally contain lamellar bodies. The other type exhibits a smooth surface and ATI-like lateral cytoplasmic extensions that sometimes show focal thickening 34.

Exogenous administration of keratinocyte growth factor stimulates ATII cell proliferation in vivo. Following keratinocyte growth factor administration, the number of ATII cells is maximal at 2–3 days; however, by 7 days, alveolar epithelial architecture appears normal 106. ATII cell numbers are reduced to control levels via both apoptosis and transdifferentiation to new ATI cells (interestingly, this process appears to be an example of ATII cell transdifferentiation that is not preceded by ATI cell injury). Prior to the return of the alveolar epithelium to normal, cells have been identified that exhibit both ATII and ATI characteristics. These characteristics include an elongated cytoplasm and expression of an ATI-cell-specific lectin, L. esculentum agglutinin, and an ATII-cell-selective cell protein, surfactant protein D 106.

Using confocal microscopy, the phenotype of the lung alveolar epithelium was investigated following intratracheal administration of S. aureus. A small population of cells that coexpress RTI40/TI{alpha} protein and an ATII-cell-selective antigen, MMC4, were identified 51, 107. RTI40/TI{alpha} protein/MMC4-positive cultured ATII cells have also been observed as they become ATI-like 108. These data suggest that RTI40/TI{alpha} protein/MMC4-positive cells are intermediates in the transition of ATII to new ATI cells (data not shown).

Changes in the surface expression of ICAM-1 have been investigated in a number of animal models using immunogold electron microscopic analysis. ICAM-1 levels on the surface of ATI cells decrease at cell boundaries following hyperoxia but do not change following pneumonia. However, ICAM-1 expression is dramatically increased on the surface of ATII cells following both pneumonia and hyperoxia 80, 109. Since ICAM-1 expression increases as ATII cells become ATI-cell-like in culture, the upregulation on ATII cells in vivo could be an early differentiation event 37. However, ICAM-1 expression is also upregulated in response to cytokines 79. Therefore, ICAM-1 expression may also be regulated by ATII cells independently of morphological changes associated with transdifferentiation to ATI cells.

To date, alveolar epithelial intermediate cells have been defined on the basis of their ultrastructural characteristics, and these characteristics appear to include at least two major intermediate cell types 20, 23, 34. The preliminary studies mentioned above suggest that alveolar epithelial intermediate cells may also be identifiable by investigation of the coexpression of ATI- and ATII-cell-selective markers. However, whether the identified coexpressing cells are ATII cells becoming ATI cells, or vice versa, remains unknown. Nor is it known whether different ultrastructural intermediates have different phenotypes. It is well known that the conditions under which isolated ATII cells are cultured influence whether or not they express ATII- or ATI-cell-selective proteins and mRNAs (reviewed in 25). Moreover, it has also been demonstrated that ATI-like cells can be made to re-express ATII-like phenotypic markers by varying the culture conditions, e.g. by going from submerged to air/liquid interface culture 110, 111. Studies designed to determine coexpression of ATI- and ATII-cell-selective markers in culture may help in the phenotyping of ATII-to-ATI intermediates, and in the determination of whether the same intermediates exist as ATI cells become ATII cells.

Alveolar epithelial type I cell-selective proteins and lung fibrosis

The term lung fibrosis encompasses a diverse group of lung diseases that share many clinical, radiographic and physiological characteristics. The most common and severe form of fibrosis is IPF. Morphologically, IPF is characterised by regions of fibrosis alternating with regions of normal lung with little evidence of inflammation 112. At the borders between fibrotic and normal lung are collections of fibroblasts/myofibroblasts, called fibrotic foci. These regions of active fibrosis are also characterised by the presence of different alveolar epithelial phenotypes 4. Immunohistochemical studies demonstrate that the alveolar epithelium in fibrotic regions expresses a number of profibrotic cytokines and growth factors 18. Based on these observations, it has been proposed that multiple cycles of alveolar injury and repair lead to aberrant alveolar epithelial cell populations that drive fibrosis 4. Although there is no disputing that the alveolar epithelium is not normal in IPF, the origin and specific functions of the different epithelial phenotypes remain unknown. It has been proposed that some of the distinct alveolar epithelial cells in IPF are alveolar epithelial intermediate cells 4, 113.

Bleomycin is commonly used to induce pulmonary fibrosis in experimental animals, although, unlike IPF, bleomycin-induced fibrosis is classically associated with inflammation. Four weeks after bleomycin treatment, regions of the alveolar barrier stain positively for some ATI-cell-specific lectins (e.g. B. purpurea agglutinin) but demonstrate focal loss of RTI40/TI{alpha} protein 94. Other investigations into lung fibrosis also report the loss of ATI-cell-associated proteins, including caveolin-1, RTI40/TI{alpha} protein, AQP5 and RAGE 30, 95, 114. More studies are required in order to fully understand to what extent the phenotype of ATI cells change in fibrosis or whether the loss of ATI-cell-selective proteins reflects a decrease in the number of ATI cells. Data from knockout mice, and in particular TI{alpha}-deficient mice (see earlier) 63, suggest that TI{alpha} (or ATI cells) may play a role in the regulation of distal airway cell proliferation. Therefore, it is possible that loss of ATI cells in fibrosis not only leads to a decrease in surface area for gas exchange but may also contribute to the increased proliferation of other distal airway cells.

Conclusions

The present article has reviewed the use of cell-selective proteins as markers for the investigation of ATI cell injury and repair following injury (fig. 3Go). ATI-cell-selective proteins such as RTI40/TI{alpha} protein are useful for quantifying the extent of ATI cell necrosis in animal models of lung injury, and human homologues of RTI40/TI{alpha} protein may form the basis of diagnostic assays in conditions such as ALI or ARDS. The coexpression of ATI-cell-selective markers, such as RTI40/TI{alpha} protein, AQP5, caveolin-1 and 2, and cytochrome P450 2B1, may help identify ATI cells in immunohistochemistry- and immunofluorescence-based studies and facilitate investigation into ATI cell numbers at active sites of injury and repair. The coexpression of ATI cell-selective proteins with ATII- selective proteins may help identify alveolar epithelial intermediate cells and their role in lung repair and fibrosis.



View larger version (20K):
[in this window]
[in a new window]
  		Fig. 3.— Potential applications of cell-selective markers in the investigation of alveolar epithelial type I cell injury and repair. ATI: alveolar epithelial type I.

 
A better understanding of the regulation of alveolar epithelial type I cell numbers, and the function of proteins selective for alveolar epithelial type I cells, will lead to a greater understanding of the role of alveolar epithelial type I cells in both normal and diseased lungs.

Acknowledgements

The authors kindly thank L. Dobbs (University of California at San Francisco, San Francisco, CA, USA) for the monoclonal antibody directed against rat type I cell 40-kDa protein. They also thank J.V. Gray (University of Glasgow, Glasgow, UK) and S. McKechnie, L. Franklin and K. Tyrrell (all University of Edinburgh, Edinburgh, UK) for comments on the manuscript and G. Clegg (University of Edinburgh, Edinburgh, UK) for assistance in preparing figure 1Go.

References

  1. Kasper M, Haroske G. Alterations in the alveolar epithelium after injury leading to pulmonary fibrosis. Histol Histopathol 1996;11:463–483.[ISI][Medline] [Order article via Infotrieve]
  2. Kuwano K, Hagimoto N, Hara N. Molecular mechanisms of pulmonary fibrosis and current treatment. Curr Mol Med 2001;1:551–573.[CrossRef][Medline] [Order article via Infotrieve]
  3. Witschi H. Role of the epithelium in lung repair. Chest 1991;99: Suppl. 3, 22S–25S.[Free Full Text]
  4. Selman M, King TE, Pardo A. Idiopathic pulmonary fibrosis: prevailing and evolving hypotheses about its pathogenesis and implications for therapy. Ann Intern Med 2001;134:136–151.[Abstract/Free Full Text]
  5. Weibel ER. The Pathway for OxygenCambridge, MA, London, Harvard University Press, 1984; pp. 231–271.
  6. Pinkerton KE, Gehr P, Crapo JD. Architecture and cellular composition of the air blood barrier. In: Parent RA, ed. Comparative Biology of the Normal Lung Boca Raton, FL, CRC Press, 1992; pp. 121–128.
  7. Dobbs LG, Gonzalez R, Matthay MA, Carter EP, Allen L, Verkman AS. Highly water-permeable type I alveolar epithelial cells confer high water permeability between the airspace and vasculature in rat lung. Proc Natl Acad Sci U S A 1998;95:2991–2996.[Abstract/Free Full Text]
  8. Johnson MD, Widdicombe JH, Allen L, Barbry P, Dobbs LG. Alveolar epithelial type I cells contain transport proteins and transport sodium, supporting an active role for type I cells in regulation of lung liquid homeostasis. Proc Natl Acad Sci U S A 2002;99:1966–1971.[Abstract/Free Full Text]
  9. Sloniewsky DE, Ridge KM, Adir Y, et al. Leukotriene D4 activates alveolar epithelial Na,K-ATPase and increases alveolar fluid clearance. Am J Respir Crit Care Med 2004;169:407–412.[Abstract/Free Full Text]
  10. Ashino Y, Ying X, Dobbs LG, Bhattacharya J. [Ca(2+)](i) oscillations regulate type II cell exocytosis in the pulmonary alveolus. Am J Physiol Lung Cell Mol Physiol 2000;279:L5–L13.[Abstract/Free Full Text]
  11. Fehrenbach H. Alveolar epithelial type II cell: defender of the alveolus revisited. Respir Res 2001;2:33–46.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  12. Matthay MA, Folkesson HG, Clerici C. Lung epithelial fluid transport and the resolution of pulmonary edema. Physiol Rev 2002;82:569–600.[Abstract/Free Full Text]
  13. Crouch E, Wright JR. Surfactant proteins A and D and pulmonary host defense. Annu Rev Physiol 2001;63:521–554.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  14. McCormack FX, Whitsett JA. The pulmonary collectins, SP-A and SP-D, orchestrate innate immunity in the lung. J Clin Invest 2002;109:707–712.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  15. Evans MJ, Cabral LJ, Stephens RJ, Freeman G. Transformation of alveolar type 2 cells to type 1 cells following exposure to NO2. Exp Mol Pathol 1975;22:142–150.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  16. Adamson IY, Bowden DH. The type 2 cell as progenitor of alveolar epithelial regeneration. A cytodynamic study in mice after exposure to oxygen. Lab Invest 1974;30:35–42.[ISI][Medline] [Order article via Infotrieve]
  17. Adamson IY, Bowden DH. Derivation of type 1 epithelium from type 2 cells in the developing rat lung. Lab Invest 1975;32:736–745.[ISI][Medline] [Order article via Infotrieve]
  18. Geiser T. Mechanisms of alveolar epithelial repair in acute lung injury – a translational approach. Swiss Med Wkly 2003;133:586–590.[Medline] [Order article via Infotrieve]
  19. Uhal BD. Cell cycle kinetics in the alveolar epithelium. Am J Physiol 1997;272:L1031–1045.
  20. Evans MJ, Stephens RJ, Cabral LJ, Freeman G. Cell renewal in the lungs of rats exposed to low levels of NO2. Arch Environ Health 1972;24:180–188.[ISI][Medline] [Order article via Infotrieve]
  21. Daly HE, Baecher-Allan CM, Paxhia AT, Ryan RM, Barth RK, Finkelstein JN. Cell-specific gene expression reveals changes in epithelial cell populations after bleomycin treatment. Lab Invest 1998;78:393–400.[ISI][Medline] [Order article via Infotrieve]
  22. Kotton DN, Ma BY, Cardoso WV, et al. Bone marrow-derived cells as progenitors of lung alveolar epithelium. Development 2001;128:5181–5188.
  23. Flecknoe SJ, Wallace MJ, Harding R, Hooper SB. Determination of alveolar epithelial cell phenotypes in fetal sheep: evidence for the involvement of basal lung expansion. J Physiol 2002;542:245–253.[Abstract/Free Full Text]
  24. Reddy R, Buckley S, Doerken M, et al. Isolation of a putative progenitor subpopulation of alveolar epithelial type 2 cells. Am J Physiol Lung Cell Mol Physiol 2004;286:L658–L667.[Abstract/Free Full Text]
  25. Williams MC. Alveolar type I cells: molecular phenotype and development. Annu Rev Physiol 2003;65:669–695.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  26. Dobbs LG, Williams MC, Gonzalez R. Monoclonal antibodies specific to apical surfaces of rat alveolar type I cells bind to surfaces of cultured, but not freshly isolated, type II cells. Biochim Biophys Acta 1988;970:146–156.[Medline] [Order article via Infotrieve]
  27. Kato Y, Fujita N, Kunita A, et al. Molecular identification of Aggrus/T1 alpha as a platelet aggregation-inducing factor expressed in colorectal tumors. J Biol Chem 2003;278:51599–51605.[Abstract/Free Full Text]
  28. Kasper M, Takeya M, Takahashi K, Grossmann H, Schuh D, Muller M. Immunohistochemistry of new type I alveolar epithelial cell markers of the rat. Histol Histopathol 1996;11:145–152.[ISI][Medline] [Order article via Infotrieve]
  29. Dobbs LG, Gonzalez RF, Allen L, Froh DK. HTI56, an integral membrane protein specific to human alveolar type I cells. J Histochem Cytochem 1999;47:129–137.[Abstract/Free Full Text]
  30. Kasper M, Reimann T, Hempel U, et al. Loss of caveolin expression in type I pneumocytes as an indicator of subcellular alterations during lung fibrogenesis. Histochem Cell Biol 1998;109:41–48.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  31. Newman GR, Campbell L, von Ruhland C, Jasani B, Gumbleton M. Caveolin and its cellular and subcellular immunolocalisation in lung alveolar epithelium: implications for alveolar epithelial type I cell function. Cell Tissue Res 1999;295:111–120.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  32. Ridge KM, Olivera WG, Saldias F, et al. Alveolar type 1 cells express the {alpha}2 Na,K-ATPase, which contributes to lung liquid clearance. Circ Res 2003;92:453–460.[Abstract/Free Full Text]
  33. Nielsen S, King LS, Christensen BM, Agre P. Aquaporins in complex tissues. II. Subcellular distribution in respiratory and glandular tissues of rat. Am J Physiol 1997;273:C1549–C1561.
  34. Takahashi Y, Aida S, Suzuki E, Ito Y, Miura T, Kimula Y. Cytochrome P450 2B1 immunoreactivity in bronchiolar and alveolar epithelial cells after exposure of rats to ozone. Toxicol Appl Pharmacol 1994;128:207–215.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  35. Nagae A, Abe M, Becker RP, Deddish PA, Skidgel RA, Erdos EG. High concentration of carboxypeptidase M in lungs: presence of the enzyme in alveolar type I cells. Am J Respir Cell Mol Biol 1993;9:221–229.
  36. Kang BH, Crapo JD, Wegner CD, Letts LG, Chang LY. Intercellular adhesion molecule-1 expression on the alveolar epithelium and its modification by hyperoxia. Am J Respir Cell Mol Biol 1993;9:350–355.
  37. Christensen PJ, Kim S, Simon RH, Toews GB, Paine R 3rd. Differentiation-related expression of ICAM-1 by rat alveolar epithelial cells. Am J Respir Cell Mol Biol 1993;8:9–15.
  38. Cunningham AC, Kirby JA. Regulation and function of adhesion molecule expression by human alveolar epithelial cells. Immunology 1995;86:279–286.[ISI][Medline] [Order article via Infotrieve]
  39. Abraham V, Chou ML, George P, et al. Heterocellular gap junctional communication between alveolar epithelial cells. Am J Physiol Lung Cell Mol Physiol 2001;280:L1085–L1093.[Abstract/Free Full Text]
  40. Campbell L, Abulrob AN, Kandalaft LE, et al. Constitutive expression of p-glycoprotein in normal lung alveolar epithelium and functionality in primary alveolar epithelial cultures. J Pharmacol Exp Ther 2003;304:441–452.[Abstract/Free Full Text]
  41. Fehrenbach H, Kasper M, Tschernig T, Shearman MS, Schuh D, Muller M. Receptor for advanced glycation endproducts (RAGE) exhibits highly differential cellular and subcellular localisation in rat and human lung. Cell Mol Biol (Noisy-le-grand) 1998;44:1147–1157.
  42. Shirasawa M, Fujiwara N, Hirabayashi S, et al. Receptor for advanced glycation end-products is a marker of type I lung alveolar cells. Genes Cells 2004;9:165–174.[Abstract/Free Full Text]
  43. Katsuoka F, Kawakami Y, Arai T, et al. Type II alveolar epithelial cells in lung express receptor for advanced glycation end products (RAGE) gene. Biochem Biophys Res Commun 1997;238:512–516.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  44. Qiao R, Zhou B, Liebler JM, Li X, Crandall ED, Borok Z. Identification of three genes of known function expressed by alveolar epithelial type I cells. Am J Respir Cell Mol Biol 2003;29:98–105.[Abstract/Free Full Text]
  45. Sloan JL, Grubb BR, Mager S. Expression of the amino acid transporter ATB 0+ in lung: possible role in luminal protein removal. Am J Physiol Lung Cell Mol Physiol 2003;284:L39–L49.[Abstract/Free Full Text]
  46. Saarikoski ST, Husgafvel-Pursiainen K, Hirvonen A, Vainio H, Gonzalez FJ, Anttila S. Localization of CYP1A1 mRNA in human lung by in situ hybridization: comparison with immunohistochemical findings. Int J Cancer 1998;77:33–39.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  47. Ganzalo JA, Jia GQ, Aguirre V, et al. Mouse eotaxin expression parallels eosinophil accumulation during lung allergic inflammation but it is not restricted to a Th2-type response. Immunity 1996;4:1–14.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  48. Ingbar DH, Hepler K, Dowin R, et al. {gamma}-Glutamyl transpeptidase is a polarized alveolar epithelial membrane protein. Am J Physiol 1995;269:L261–L271.
  49. Joyce-Brady M, Takahashi Y, Oakes SM, et al. Synthesis and release of amphipathic {gamma}-glutamyl transferase by the pulmonary alveolar type 2 cell. Its redistribution throughout the gas exchange portion of the lung indicates a new role for surfactant. J Biol Chem 1994;269:14219–14226.[Abstract/Free Full Text]
  50. Maitre B, Clement A, Williams MC, Brody JS. Expression of insulin-like growth factor receptors 1 and 2 in the developing lung and their relation to epithelial cell differentiation. Am J Respir Cell Mol Biol 1995;13:262–270.[Abstract]
  51. Boylan GM, Pryde JG, Dobbs LG, McElroy MC. Identification of a novel antigen on the apical surface of rat alveolar epithelial type II and Clara cells. Am J Physiol Lung Cell Mol Physiol 2001;280:L1318–L1326.[Abstract/Free Full Text]
  52. Gonzalez RF, Dobbs LG. Purification and analysis of RTI40, a type I alveolar epithelial cell apical membrane protein. Biochim Biophys Acta 1998;1429:208–216.[CrossRef][Medline] [Order article via Infotrieve]
  53. Rishi AK, Joyce-Brady M, Fisher J, et al. Cloning, characterization, and development expression of a rat lung alveolar type I cell gene in embryonic endodermal and neural derivatives. Dev Biol 1995;167:294–306.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  54. Williams MC, Cao Y, Hinds A, Rishi AK, Wetterwald A. T1 alpha protein is developmentally regulated and expressed by alveolar type I cells, choroid plexus, and ciliary epithelia of adult rats. Am J Respir Cell Mol Biol 1996;14:577–585.[Abstract]
  55. Nose K, Saito H, Kuroki T. Isolation of a gene sequence induced later by tumor-promoting 12-O-tetradecanoylphorbol-13-acetate in mouse osteoblastic cells (MC3T3-E1) and expressed constitutively in ras-transformed cells. Cell Growth Differ 1990;1:511–518.[Abstract]
  56. Farr AG, Berry ML, Kim A, Nelson AJ, Welch MP, Aruffo A. Characterization and cloning of a novel glycoprotein expressed by stromal cells in T-dependent areas of peripheral lymphoid tissues. J Exp Med 1992;176:1477–1482.[Abstract/Free Full Text]
  57. Wetterwald A, Hoffstetter W, Cecchini MG, et al. Characterization and cloning of the E11 antigen, a marker expressed by rat osteoblasts and osteocytes. Bone 1996;18:125–132.[Medline] [Order article via Infotrieve]
  58. Breiteneder-Geleff S, Matsui K, Soleiman A, et al. Podoplanin, novel 43-kd membrane protein of glomerular epithelial cells, is down-regulated in puromycin nephrosis. Am J Pathol 1997;151:1141–1152.[Abstract]
  59. Zimmer G, Lottspeich F, Maisner A, Klenk HD, Herrler G. Molecular characterization of gp40, a mucin-type glycoprotein from the apical plasma membrane of Madin-Darby canine kidney cells (type I). Biochem J 1997;326:99–108.
  60. Scholl FG, Gamallo C, Vilaro S, Quintanilla M. Identification of PA2.26 antigen as a novel cell-surface mucin-type glycoprotein that induces plasma membrane extensions and increased motility in keratinocytes. J Cell Sci 1999;112:4601–4613.[Abstract]
  61. Ma T, Yang B, Matthay MA, Verkman AS. Evidence against a role of mouse, rat, and two cloned human T1{alpha} isoforms as a water channel or a regulator of aquaporin-type water channels. Am J Respir Cell Mol Biol 1998;19:143–149.[Abstract/Free Full Text]
  62. Zimmer G, Oeffner F, Von Messling V, et al. Cloning and characterization of gp36, a human mucin-type glycoprotein preferentially expressed in vascular endothelium. Biochem J 1999;341:277–284.
  63. Ramirez MI, Millien G, Hinds A, Cao Y, Seldin DC, Williams MC. T1{alpha}, a lung type I cell differentiation gene, is required for normal lung cell proliferation and alveolus formation at birth. Dev Biol 2003;256:61–72.[ISI][Medline] [Order article via Infotrieve]
  64. Schacht V, Ramirez MI, Hong YK, et al. T1{alpha}/podoplanin deficiency disrupts normal lymphatic vasculature formation and causes lymphedema. EMBO J 2003;22:3546–3556.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  65. Kreda SM, Gynn MC, Fenstermacher DA, Boucher RC, Gabriel SE. Expression and localization of epithelial aquaporins in the adult human lung. Am J Respir Cell Mol Biol 2001;24:224–234.[Abstract/Free Full Text]
  66. Drab M, Verkade P, Elger M, et al. Loss of caveolae, vascular dysfunction, and pulmonary defects in caveolin-1 gene-disrupted mice. Science 2001;293:2449–2452.[Abstract/Free Full Text]
  67. Razani B, Engelman JA, Wang XB, et al. Caveolin-1 null mice are viable but show evidence of hyperproliferative and vascular abnormalities. J Biol Chem 2001;276:38121–38138.[Abstract/Free Full Text]
  68. Razani B, Lisanti MP. Caveolin-deficient mice: insights into caveolar function human disease. J Clin Invest 2001;108:1553–1561.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  69. Razani B, Woodman SE, Lisanti MP. Caveolae: from cell biology to animal physiology. Pharmacol Rev 2002;54:431–467.[Abstract/Free Full Text]
  70. Razani B, Wang XB, Engelman JA, et al. Caveolin-2-deficient mice show evidence of severe pulmonary dysfunction without disruption of caveolae. Mol Cell Biol 2002;22:2329–2344.[Abstract/Free Full Text]
  71. Razani B, Schlegel A, Liu J, Lisanti MP. Caveolin-1, a putative tumour suppressor gene. Biochem Soc Trans 2001;29:494–499.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  72. Park DS, Cohen AW, Frank PG, et al. Caveolin-1 null (-/-) mice show dramatic reductions in life span. Biochemistry 2003;42:15124–15131.[CrossRef][Medline] [Order article via Infotrieve]
  73. Borok Z, Verkman AS. Lung edema clearance: 20 years of progress: invited review: role of aquaporin water channels in fluid transport in lung and airways. J Appl Physiol 2002;93:2199–2206.[Abstract/Free Full Text]
  74. Ma T, Fukuda N, Song Y, Matthay MA, Verkman AS. Lung fluid transport in aquaporin-5 knockout mice. J Clin Invest 2000;105:93–100.[ISI][Medline] [Order article via Infotrieve]
  75. Song Y, Fukuda N, Bai C, Ma T, Matthay MA, Verkman AS. Role of aquaporins in alveolar fluid clearance in neonatal and adult lung, and in oedema formation following acute lung injury: studies in transgenic aquaporin null mice. J Physiol 2000;525:771–779.[Abstract/Free Full Text]
  76. King LS, Agre P. Pathophysiology of the aquaporin water channels. Annu Rev Physiol 1996;58:619–648.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  77. Takahashi Y, Miura T. Responses of cytochrome P-450 isozymes of rat lung to in vivo exposure to ozone. Toxicol Lett 1990;54:327–335.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  78. Paine R 3rd, Morris SB, Jin H, Baleeiro CE, Wilcoxen SE. ICAM-1 facilitates alveolar macrophage phagocytic activity through effects on migration over the AEC surface. Am J Physiol Lung Cell Mol Physiol 2002;283:L180–L187.[Abstract/Free Full Text]
  79. Kang BH, Manderschied BD, Huang YC, Crapo JD, Chang LY. Contrasting response of lung parenchymal cells to instilled TNF alpha and IFN gamma: the inducibility of specific cell ICAM-1 in vivo. Am J Respir Cell Mol Biol 1996;15:540–550.[Abstract]
  80. Piedboeuf B, Frenette J, Petrov P, Welty SE, Kazzaz JA, Horowitz S. In vivo expression of intercellular adhesion molecule 1 in type II pneumocytes during hyperoxia. Am J Respir Cell Mol Biol 1996;15:71–77.[Abstract]
  81. Stern D, Yan SD, Yan SF, Schmidt AM. Receptor for advanced glycation endproducts: a multiligand receptor magnifying cell stress in diverse pathologic settings. Adv Drug Deliv Rev 2002;54:1615–1625.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  82. Dobbs LG, Williams MC, Brandt AE. Changes in biochemical characteristics and pattern of lectin binding of alveolar type II cells with time in culture. Biochim Biophys Acta 1985;846:155–166.[Medline] [Order article via Infotrieve]
  83. Bankston PW, Porter GA, Milici AJ, Palade GE. Differential and specific labeling of epithelial and vascular endothelial cells of the rat lung by Lycopersicon esculentum and Griffonia simplicifolia I lectins. Eur J Cell Biol 1991;54:187–195.[ISI][Medline] [Order article via Infotrieve]
  84. Kasper M, Haroske G, Muller M. Species differences in lectin binding to pulmonary cells: soybean agglutinin (SBA) as a marker of type I alveolar epithelial cells and alveolar macrophages in mini pigs. Acta Histochem 1994;96:63–73.[ISI][Medline] [Order article via Infotrieve]
  85. Kasper M, Schuh D, Muller M. Bauhinia purpurea lectin (BPA) binding of rat type I pneumocytes: alveolar epithelial alterations after radiation-induced lung injury. Exp Toxicol Pathol 1994;46:361–367.[ISI][Medline] [Order article via Infotrieve]
  86. Kawanami O, Jiang HX, Mochimaru H, et al. Alveolar fibrosis and capillary alteration in experimental pulmonary silicosis in rats. Am J Respir Crit Care Med 1995;151:1946–1955.[Abstract]
  87. Tatrai E, Brozik M, Adamis Z, Meretey K, Ungvary G. In vivo pulmonary toxicity of cellulose in rats. J Appl Toxicol 1996;16:129–135.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  88. Woods LW, Wilson DW, Schiedt MJ, Giri SN. Structural and biochemical changes in lungs of 3-methylindole-treated rats. Am J Pathol 1993;142:129–138.[Abstract]
  89. McElroy MC, Pittet JF, Hashimoto S, Allen L, Wiener-Kronish JP, Dobbs LG. A type I cell-specific protein is a biochemical marker of epithelial injury in a rat model of pneumonia. Am J Physiol 1995;268:L181–L186.
  90. McElroy MC, Pittet JF, Allen L, Wiener-Kronish JP, Dobbs LG. Biochemical detection of type I cell damage after nitrogen dioxide-induced lung injury in rats. Am J Physiol 1997;273:L1228–L1234.
  91. McElroy MC, Wiener-Kronish JP, Miyazaki H, et al. Nitric oxide attenuates lung endothelial injury caused by sublethal hyperoxia in rats. Am J Physiol 1997;272:L631–L638.
  92. McElroy MC, Harty HR, Hosford GE, Boylan GM, Pittet JF, Foster TJ. Alpha-toxin damages the air-blood barrier of the lung in a rat model of Staphylococcus aureus-induced pneumonia. Infect Immun 1999;67:5541–5544.[Abstract/Free Full Text]
  93. McElroy MC, Cain DJ, Tyrrell C, Foster TJ, Haslett C. Increased virulence of a fibronectin-binding protein mutant of Staphylococcus aureus in a rat model of pneumonia. Infect Immun 2002;70:3865–3873.[Abstract/Free Full Text]
  94. Koslowski R, Dobbs LG, Wenzel KW, Schuh D, Muller M, Kasper M. Loss of immunoreactivity for RTI40, a type I cell-specific protein in the alveolar epithelium of rat lungs with bleomycin-induced fibrosis. Eur Respir J 1998;12:1397–1403.[Abstract]
  95. Kasper M, Seidel D, Knels L, et al. Early signs of lung fibrosis after in vitro treatment of rat lung slices with CdCl(2) and TGF-ß(1). Histochem Cell Biol 2004;121:131–140.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  96. Frank JA, Gutierrez JA, Jones KD, Allen L, Dobbs L, Matthay MA. Low tidal volume reduces epithelial and endothelial injury in acid-injured rat lungs. Am J Respir Crit Care Med 2002;165:242–249.[Abstract/Free Full Text]
  97. Bachofen M, Weibel ER. Alterations of the gas exchange apparatus in adult respiratory insufficiency associated with septicemia. Am Rev Respir Dis 1977;116:589–615.[ISI][Medline] [Order article via Infotrieve]
  98. Eisner MD, Parsons P, Matthay MA, Ware L, Greene K. Plasma surfactant protein levels and clinical outcomes in patients with acute lung injury. Thorax 2003;58:983–988.[Abstract/Free Full Text]
  99. Ishizaka A, Matsuda T, Albertine KH, et al. Elevation of KL-6, a lung epithelial cell marker, in plasma and epithelial lining fluid in the acute respiratory distress syndrome. Am J Physiol Lung Cell Mol Physiol 2003;286:L1088–L1094.
  100. Newman V, Gonzalez RF, Matthay MA, Dobbs LG. A novel alveolar type I cell-specific biochemical marker of human acute lung injury. Am J Respir Crit Care Med 2000;161:990–995.[Abstract/Free Full Text]
  101. Crapo JD. Morphologic changes in pulmonary oxygen toxicity. Annu Rev Physiol 1986;48:721–731.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  102. Cao YX, Ramirez MI, Williams MC. Enhanced binding of Sp1/Sp3 transcription factors mediates the hyperoxia-induced increased expression of the lung type I cell gene T1{alpha}. J Cell Biochem 2003;89:887–901.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  103. Towne JE, Harrod KS, Krane CM, Menon AG. Decreased expression of aquaporin (AQP)1 and AQP5 in mouse lung after acute viral infection. Am J Respir Cell Mol Biol 2000;22:34–44.[Abstract/Free Full Text]
  104. Jiao G, Li E, Yu R. Decreased expression of AQP1 and AQP5 in acute injured lungs in rats. Chin Med J (Engl) 2002;115:963–967.
  105. Barr BC, Hyde DM, Plopper CG, Dungworth DL. Distal airway remodeling in rats chronically exposed to ozone. Am Rev Respir Dis 1988;137:924–938.[ISI][Medline] [Order article via Infotrieve]
  106. Fehrenbach H, Kasper M, Tschernig T, et al. Keratinocyte growth factor-induced hyperplasia of rat alveolar type II cells in vivo is resolved by differentiation into type I cells and by apoptosis. Eur Respir J 1999;14:534–544.[Abstract]
  107. Clegg GR, Tyrrell C, Harrison D, McElroy MC. A novel alveolar epithelial cell phenotype in repairing lungs. FASEB J 2003;17:A830.
  108. Clegg R, Harrison DJ, McElroy MC. The phenotype of cultured alveolar epithelial cells. FASEB J 2003;17:A562.
  109. Burns AR, Takei F, Doerschuk CM. Quantitation of ICAM-1 expression in mouse lung during pneumonia. J Immunol 1994;153:3189–3198.[Abstract]
  110. Danto SI, Shannon JM, Borok Z, Zabski SM, Crandall ED. Reversible transdifferentiation of alveolar epithelial cells. Am J Respir Cell Mol Biol 1995;12:497–502.[Abstract]
  111. Dobbs LG, Pian MS, Maglio M, Dumars S, Allen L. Maintenance of the differentiated type II cell phenotype by culture with an apical air surface. Am J Physiol 1997;273:L347–L354.
  112. Thannickal VJ, Toews GB, White ES, Lynch IJ, Martinez FJ. Mechanisms of pulmonary fibrosis. Annu Rev Med 2004;55:395–417.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  113. Kasper M, Fehrenbach H. Immunohistochemical evidence for the occurrence of similar epithelial phenotypes during lung development and radiation-induced fibrogenesis. Int J Radiat Biol 2000;76:493–501.[CrossRef][ISI][Medline] [Order article via Infotrieve]
  114. Kasper M, Koslowski R, Luther T, Schuh D, Muller M, Wenzel KW. Immunohistochemical evidence for loss of ICAM-1 by alveolar epithelial cells in pulmonary fibrosis. Histochem Cell Biol 1995;104:397–405.[CrossRef][ISI][Medline] [Order article via Infotrieve]



This article has been cited by other articles: