Expanding the frontiers of lung biology through the creative use of alveolar epithelial cells in culture

TAZ is required for lung alveolar epithelial cell differentiation after injury

The lung is a relatively quiescent organ during homeostasis, but has a remarkable capacity for repair after injury. Alveolar epithelial type I cells (AEC1s) line airspaces and mediate gas exchange. After injury, they are regenerated by differentiation from their progenitors - alveolar epithelial type II cells (AEC2s) - which also secrete surfactant to maintain surface tension and alveolar patency. While recent studies showed that the maintenance of AEC2 stemness is Wnt dependent, the molecular mechanisms underlying AEC2-AEC1 differentiation in adult lung repair are still incompletely understood. Here we show that WWTR1 (TAZ) plays a crucial role in AEC differentiation. Using an in vitro organoid culture system, we found that tankyrase inhibition can efficiently block AEC2-AEC1 differentiation, and this effect was due to the inhibition of TAZ. In a bleomycin induced lung injury model, conditional deletion of TAZ in AEC2s dramatically reduced AEC1 regeneration during recovery, leading to exacerbated alveolar lesions and fibrosis. In patients with idiopathic pulmonary fibrosis (IPF), decreased blood levels of RAGE, a biomarker of AEC1 health, were associated with more rapid disease progression. Our findings implicate TAZ as a critical factor involved in AEC2 to AEC1 differentiation, and hence the maintenance of alveolar integrity after injury.

Consequences of Hypoxia for the Pulmonary Alveolar Epithelial Cell Innate Immune Response

Pulmonary innate immune responses involve a highly regulated multicellular network to defend the enormous surface area of the lung. Disruption of these responses renders the host susceptible to pneumonia. Alveolar epithelial cells (AEC) are a critical source of innate immune molecules such as GM-CSF, which determine the functional maturation of alveolar macrophages. In many pulmonary diseases, heterogeneous ventilation leads to regional hypoxia in the lung. The effect of hypoxia on AEC innate immune function is unknown. We now report that exposure of primary murine AEC to hypoxia (1% oxygen) for 24 h results in significant suppression of key innate immune molecules, including GM-CSF, CCL2, and IL-6. This exposure did not cause toxicity but did induce stabilization of hypoxia-inducible factor 1α protein (HIF-1α) and shift to glycolytic metabolism. Focusing on GM-CSF, we found that hypoxia greatly decreased the rate of GM-CSF transcription. Hypoxia both decreased NF-κB signaling in AEC and induced chromosomal changes, resulting in decreased accessibility in the GM-CSF proximal promoter of target sequences for NF-κB binding. In mice exposed to hypoxia in vivo (12% oxygen for 2 d), lung GM-CSF protein expression was reduced. In vivo phagocytosis of fluorescent beads by alveolar macrophages was also suppressed, but this effect was reversed by treatment with GM-CSF. These studies suggest that in critically ill patients, local hypoxia may contribute to the susceptibility of poorly ventilated lung units to infection through complementary effects on several pathways, reducing AEC expression of GM-CSF and other key innate immune molecules.

Automated procedure for biomimetic de-cellularized lung scaffold supporting alveolar epithelial transdifferentiation

The optimal method for creating a de-cellularized lung scaffold that is devoid of cells and cell debris, immunologically inert, and retains necessary extracellular matrix (ECM) has yet to be identified. Herein, we compare automated detergent-based de-cellularization approaches utilizing either constant pressure (CP) or constant flow (CF), to previously published protocols utilizing manual pressure (MP) to instill and rinse out the de-cellularization agents. De-cellularized lungs resulting from each method were evaluated for presence of remaining ECM proteins and immunostimulatory material such as nucleic acids and intracellular material. Our results demonstrate that the CP and MP approaches more effectively remove cellular materials but differentially retain ECM proteins. The CP method has the added benefit of being a faster, reproducible de-cellularization process. To assess the functional ability of the de-cellularized scaffolds to maintain epithelial cells, intra-tracheal inoculation with GFP expressing C10 alveolar epithelial cells (AEC) was performed. Notably, the CP de-cellularized lungs were able to support growth and spontaneous differentiation of C10-GFP cells from a type II-like phenotype to a type I-like phenotype.

FoxM1 mediates the progenitor function of type II epithelial cells in repairing alveolar injury induced by Pseudomonas aeruginosa

The alveolar epithelium is composed of the flat type I cells comprising 95% of the gas-exchange surface area and cuboidal type II cells comprising the rest. Type II cells are described as facultative progenitor cells based on their ability to proliferate and trans-differentiate into type I cells. In this study, we observed that pneumonia induced by intratracheal instillation of Pseudomonas aeruginosa (PA) in mice increased the expression of the forkhead transcription factor FoxM1 in type II cells coincidentally with the induction of alveolar epithelial barrier repair. FoxM1 was preferentially expressed in the Sca-1(+) subpopulation of progenitor type II cells. In mice lacking FoxM1 specifically in type II cells, type II cells showed decreased proliferation and impaired trans-differentiation into type I cells. Lungs of these mice also displayed defective alveolar barrier repair after injury. Expression of FoxM1 in the knockout mouse lungs partially rescued the defective trans-differentiation phenotype. Thus, expression of FoxM1 in type II cells is essential for their proliferation and transition into type I cells and for restoring alveolar barrier homeostasis after PA-induced lung injury.

Gene expression of rat alveolar type II cells during hyperoxia exposure and early recovery

Alveolar epithelial cell (AEC) injury and repair during hyperoxia exposure and recovery have been investigated for decades, but the molecular mechanisms of these processes are not clear. To identify potentially important genes involved in lung injury and repair, we studied the gene expression profiles of isolated AEC II from control, 48-h hyperoxia-exposed (>95% O(2)), and 1-7 day recovering rats using a DNA microarray containing 10,000 genes. Fifty genes showed significant differential expression between two or more time points (P<0.05, fold change >2). These genes can be classified into 8 unique gene expression patterns. Real-time PCR verified 14 selected genes in three patterns related to hyperoxia exposure and early recovery. The change in the protein level for two of the selected genes, bmp-4 and retnla, paralleled that of the mRNA level. Many of these genes were found to be involved in cell proliferation and differentiation. In an in vitro AEC trans-differentiation culture model using AEC II isolated from control and 48-h hyperoxia-exposed rats, the expressions of the cell proliferation and differentiation genes identified above were consistent with their predicted roles in the trans-differentiation of AEC. These data indicate that a coordinated mechanism may control AEC differentiation during in vivo hyperoxia exposure and recovery as well as during in vitro AEC culture.

Alveolar type I cells protect rat lung epithelium from oxidative injury

The lung alveolar surface is covered by two morphologically and functionally distinct cells: alveolar epithelial cell types I and II (AEC I and II). The functions of AEC II, including surfactant release, cell differentiation and ion transport, have been extensively studied. However, relatively little is known regarding the physiological functions of AEC I. Global gene expression profiling of freshly isolated AEC I and II revealed that many genes were differentially expressed in AEC I. These genes have a diversity of functions, including cell defence. Nine out of 10 selected genes were verified by quantitative real-time PCR. Two genes, apolipoprotein E (Apo E) and transferrin, were further characterized and functionally studied. Immunohistochemistry indicated that both proteins were specifically localized in AEC I. Up-regulation of Apo E and transferrin was observed in hyperoxic lungs. Functionally, Apo E and transferrin play a protective role against oxidative stress in an animal model. Our studies suggest that AEC I is not just a simple barrier for gas exchange, but a functional cell that protects alveolar epithelium from injury.

Shedding of soluble ICAM-1 into the alveolar space in murine models of acute lung injury

Intercellular adhesion molecule-1 (ICAM-1; CD54) is an adhesion molecule constitutively expressed in abundance on the cell surface of type I alveolar epithelial cells (AEC) in the normal lung and is a critical participant in pulmonary innate immunity. At many sites, ICAM-1 is shed from the cell surface as a soluble molecule (sICAM-1). Limited information is available regarding the presence, source, or significance of sICAM-1 in the alveolar lining fluid of normal or injured lungs. We found sICAM-1 in the bronchoalveolar lavage (BAL) fluid of normal mice (386 +/- 50 ng/ml). Additionally, sICAM-1 was spontaneously released by murine AEC in primary culture as type II cells spread and assumed characteristics of type I cells. Shedding of sICAM-1 increased significantly at later points in culture (5-7 days) compared with earlier time points (3-5 days). In contrast, treatment of AEC with inflammatory cytokines had limited effect on sICAM-1 shedding. BAL sICAM-1 was evaluated in in vivo models of acute lung injury. In hyperoxic lung injury, a reversible process with a major component of leak across the alveolar wall, BAL fluid sICAM-1 only increased in parallel with increased alveolar protein. However, in lung injury due to FITC, there were increased levels of sICAM-1 in BAL that were independent of changes in BAL total protein concentration. We speculate that after lung injury, changes in sICAM-1 in BAL fluid are associated with progressive injury and may be a reflection of type I cell differentiation during reepithelialization of the injured lung.

Differential expression of a V-type ATPase C subunit gene, Atp6v1c2, during culture of rat lung type II pneumocytes

The lung alveolar epithelium consists of type I and type II pneumocytes. In vivo, the type II cell is the progenitor cell from which the type I cell originates. When freshly-isolated type II cells are cultured under conventional conditions they rapidly lose their phenotypic properties and attain characteristics of type I cells. Taking advantage of this transdifferentiation, we sought to identify genes that are differentially expressed during culture of rat type II cells. Using suppression subtractive hybridization (SSH), a vacuolar-type H+-ATPase (V-ATPase) C2 subunit gene (Atp6v1c2) was found to be enriched in freshly isolated rat type II cells compared to those cultured for 4 days. Northern blotting and reverse-transcription polymerase chain reaction (RT-PCR) confirmed the differential expression of Atp6v1c2 during in vitro culture of isolated type II cells. Expression ofAtp6v1c2 was significantly reduced early during in vitro culture: almost 90% reduction was observed after 24 h of incubation as determined by real-time PCR. In situ hybridization showed that Atp6v1c2 is expressed in both bronchiolar and alveolar lung epithelial cells, an expression pattern similar to that of surfactant protein B (SP-B). Multi-tissue Northern blotting revealed a unique tissue distribution with Atp6v1c2 expression limited to lung, kidney and testis. The presence and expression of Atp6v1c2 gene transcript isoforms, resulting from alternative splicing, were also investigated. Elucidation of differential expression of Atp6v1c2 in type II cells and further studies of its regulation may provide information useful in understanding the molecular mechanism underlying phenotypic and functional changes during transdifferentiation of alveolar epithelial cells.

Paracrine stimulation of surfactant secretion by extracellular ATP in response to mechanical deformation

We developed a heterologous system to study the effect of mechanical deformation on alveolar epithelial cells. First, isolated primary rat alveolar type II (ATII) cells were plated onto silastic substrata coated with fibronectin and maintained in culture under conditions where they become alveolar type I-like (ATI) cells. This was followed by a second set of ATII cells labeled with the nontransferable, vital fluorescent stain 5-chloromethylfluorescein diacetate to distinguish them from ATI cells. By morphometric analysis, equibiaxial deformation (stretch) of the silastic substratum induced comparable changes in cell surface area for both ATII and ATI cells. Surfactant lipid secretion was measured using cells metabolically labeled with [(3)H]choline. In response to 21% tonic stretch for 15 min, ATII cells seeded with ATI cells secreted nearly threefold more surfactant lipid compared with ATII cells seeded alone. ATI cells did not secrete lipid in response to stretch. The enhanced lipid secretion by ATII plus ATI cocultures was inhibited by treatment with apyrase and adenosine deaminase, suggesting that ATP release by ATI cells enhanced surfactant lipid secretion at 21% stretch. This was confirmed using a luciferase assay where, in response to 21% stretch, ATI cells released fourfold more ATP than ATII cells. Because ATI cells release significantly more ATP at a lower level of stretch than ATII cells, this supports the hypothesis that ATI cells are mechanosensors in the lung and that paracrine stimulation of ATII cells by extracellular ATP released from ATI cells plays a role in regulating surfactant secretion.

Nuclear localization of leukotriene A4 hydrolase in type II alveolar epithelial cells in normal and fibrotic lung

Leukotriene A4 (LTA4) hydrolase catalyzes the final step in leukotriene B4 (LTB4) synthesis. In addition to its role in LTB4 synthesis, the enzyme possesses aminopeptidase activity. In this study, we sought to define the subcellular distribution of LTA4 hydrolase in alveolar epithelial cells, which lack 5-lipoxygenase and do not synthesize LTA4. Immunohistochemical staining localized LTA4 hydrolase in the nucleus of type II but not type I alveolar epithelial cells of normal mouse, human, and rat lungs. Nuclear localization of LTA4 hydrolase was also demonstrated in proliferating type II-like A549 cells. The apparent redistribution of LTA4 hydrolase from the nucleus to the cytoplasm during type II-to-type I cell differentiation in vivo was recapitulated in vitro. Surprisingly, this change in localization of LTA4 hydrolase did not affect the capacity of isolated cells to convert LTA4 to LTB4. However, proliferation of A549 cells was inhibited by the aminopeptidase inhibitor bestatin. Nuclear accumulation of LTA4 hydrolase was also conspicuous in epithelial cells during alveolar repair following bleomycin-induced acute lung injury in mice, as well as in hyperplastic type II cells associated with fibrotic lung tissues from patients with idiopathic pulmonary fibrosis. These results show for the first time that LTA4 hydrolase can be accumulated in the nucleus of type II alveolar epithelial cells and that redistribution of the enzyme to the cytoplasm occurs with differentiation to the type I phenotype. Furthermore, the aminopeptidase activity of LTA4 hydrolase within the nucleus may play a role in promoting epithelial cell growth.

Bleomycin-induced apoptosis of alveolar epithelial cells requires angiotensin synthesis de novo

Primary cultures of rat type II alveolar epithelial cells (AECs) or human AEC-derived A549 cells, when exposed to bleomycin (Bleo), exhibited concentration-dependent apoptosis detected by altered nuclear morphology, fragmentation of DNA, activation of caspase-3, and net cell loss over time. In both cell culture models, exposure to Bleo caused time-dependent increases in angiotensinogen (ANGEN) mRNA. Antisense oligonucleotides against ANGEN mRNA inhibited Bleo-induced apoptosis of rat AEC or A549 cells by 83 and 84%, respectively (P < 0.01 and P < 0.05), and prevented Bleo-induced net cell loss. Apoptosis of rat AECs or A549 cells in response to Bleo was inhibited 91% by the ANG-converting enzyme inhibitor captopril or 82%, respectively, by neutralizing antibodies specific for ANG II (both P < 0.01). Antagonists of ANG receptor AT(1) (losartan, L-158809, or saralasin), but not an AT(2)-selective blocker (PD-123319), inhibited Bleo-induced apoptosis of either rat AECs (79%, P < 0.01) or A549 cells (83%, P < 0.01) and also reduced the activity of caspase-3 by 52% (P < 0.05). These data indicate that Bleo, like Fas(L) or TNF-alpha, induces transactivation of ANG synthesis de novo that is required for AEC apoptosis. They also support the theory that ANG system antagonists have potential for the blockade of AEC apoptosis in situ.

Role of deformation-induced lipid trafficking in the prevention of plasma membrane stress failure

Cells experience plasma membrane stress failure when the matrix to which they adhere undergoes large deformations. In the lung, such a mechanism might explain mechanical ventilation-associated cell injury. We have previously shown that in alveolar epithelial cells, deformation induces lipid trafficking to the plasma membrane, thereby accommodating the required increase in the cell surface area. We now show that cell wounding is strain amplitude and rate dependent and that under conditions of impaired exocytosis strain-induced cell wounding is significantly increased. In addition, the susceptibility of cells to mechanical injury was not correlated with changes in cell stiffness. Using a dual-labeling technique, we differentiated between cell populations that were reversibly and irreversibly injured and showed that interventions that impair deformation-induced lipid trafficking also reduce the likelihood of plasma membrane resealing. Our findings suggest that cell plasticity and remodeling responses such as deformation-induced lipid trafficking are more important for cytoprotection from strain injury than are the innate mechanical properties of the cell. We also conclude that in deformation experiments, tests of cell membrane integrity cannot be interpreted as tests of cell viability because an intact plasma membrane after deformation does not mean that no injury had occurred.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.

Cellular biomechanics in the lung

Mechanical forces affect both the function and phenotype of cells in the lung. In this symposium, recent studies were presented that examined several aspects of biomechanics in lung cells and their relationship to disease. Wound healing and recovery from injury in the airways involve epithelial cell spreading and migration on a substrate that undergoes cyclic mechanical deformation; enhanced green fluorescent protein-actin was used in a stable cell line to examine cytoskeletal changes in airway epithelial cells during wound healing. Eosinophils migrate into the airways during asthmatic attacks and can also be exposed to cyclic mechanical deformation; cyclic mechanical stretch caused a decrease in leukotriene C(4) synthesis that may be dependent on mechanotransduction mechanisms involving the production of reactive oxygen species. Recent studies have suggested that proinflammatory cytokines are increased in ventilator-induced lung injury and may be elevated by overdistention of the lung tissue; microarray analysis of human lung epithelial cells demonstrated that cyclic mechanical stretch alone profoundly affects gene expression. Finally, airway hyperresponsiveness is a basic feature of asthma, but the relationship between airway hyperresponsiveness and changes in airway smooth muscle (ASM) function remain unclear. New analysis of the behavior of the ASM cytoskeleton (CSK) suggests, however, that the CSK may behave as a glassy material and that glassy behavior may account for the extensive ASM plasticity and remodeling that contribute to airway hyperresponsiveness. Together, the presentations at this symposium demonstrated the remarkable and varied roles that mechanical forces may play in both normal lung physiology as well as pathophysiology.

ICAM-1 facilitates alveolar macrophage phagocytic activity through effects on migration over the AEC surface

We postulate that intercellular adhesion molecule-1 (ICAM-1) on type I alveolar epithelial cells (AEC) facilitates phagocytic activity of alveolar macrophages (AM) in the alveolus. When wild-type and ICAM-1-deficient mice were inoculated intratracheally with FITC-labeled microspheres, AM phagocytosis of beads (after 1 and 4 h) was significantly reduced in ICAM-1-/- mice compared with controls. To focus on ICAM-1-mediated interactions specifically involving AM and AEC, rat AM were placed in culture with rat AEC treated with neutralizing anti-ICAM-1 F(ab')(2) fragments. Blocking ICAM-1 significantly decreased the AM phagocytosis of beads. Planar chemotaxis of AM over the surface of AEC was also significantly impaired by neutralization of AEC ICAM-1. ICAM-1 in rat AEC is associated with the actin cytoskeleton. Planar chemotaxis of AM was also significantly reduced by pretreatment of the AEC monolayer with cytochalasin B to disrupt the actin cytoskeleton. These studies indicate that ICAM-1 on the AEC surface promotes mobility of AM in the alveolus and is critically important for the efficient phagocytosis of particulates by AM.

Mechanical properties of alveolar epithelial cells in culture

With the use of magnetic twisting cytometry, we characterized the mechanical properties of rat type II alveolar epithelial (ATII) cells in primary culture and examined whether the cells' state of differentiation and the application of deforming stresses influence their resistance to shape change. Cells were harvested from rat lungs as previously described (Dobbs LG. Am J Physiol Lung Cell Mol Physiol 258: L134-L147, 1990) and plated at a density of 1 x 10(6) cells/cm(2) in fibronectin-coated 96 Remova wells, and their mechanical properties were measured 2-9 days later. We show 1) that ATII cells form much stronger bonds with RGD-coated beads than they do with albumin- or acetylated low-density lipoprotein-coated beads, 2) that RGD-mediated bonds seemingly "mature" during the first 60 min of bead contact, 3) that the apparent stiffness of ATII cells increases with days in culture, 4) that stiffness falls when the RGD-coated beads are intermittently oscillated at 0.3 Hz, and 5) that this fall cannot be attributed to exocytosis-related remodeling of the subcortical cytoskeleton. Although the mechanisms of force transfer between basement membrane, cytoskeleton, and plasma membrane of ATII cells remain to be resolved, such analyses undoubtedly require definition of the cell's mechanical properties. To our knowledge, the results presented here provide the first data on this topic.

Deformation-induced lipid trafficking in alveolar epithelial cells

Mechanical ventilation with a high tidal volume results in lung injury that is characterized by blebbing and breaks both between and through alveolar epithelial cells. We developed an in vitro model to simulate ventilator-induced deformation of the alveolar basement membrane and to investigate, in a direct manner, epithelial cell responses to deforming forces. Taking advantage of the novel fluorescent properties of BODIPY lipids and the fluorescent dye FM1-43, we have shown that mechanical deformation of alveolar epithelial cells results in lipid transport to the plasma membrane. Deformation-induced lipid trafficking (DILT) was a vesicular process, rapid in onset, and was associated with a large increase in cell surface area. DILT could be demonstrated in all cells; however, only a small percentage of cells developed plasma membrane breaks that were reversible and nonlethal. Therefore, DILT was not only involved in site-directed wound repair but might also have served as a cytoprotective mechanism against plasma membrane stress failure. This study suggests that DILT is a regulatory mechanism for membrane trafficking in alveolar epithelia and provides a novel biological framework within which to consider alveolar deformation injury and repair.

Connexin expression by alveolar epithelial cells is regulated by extracellular matrix

Extracellular matrix (ECM) proteins promote attachment, spreading, and differentiation of cultured alveolar type II epithelial cells. The present studies address the hypothesis that the ECM also regulates expression and function of gap junction proteins, connexins, in this cell population. Expression of cellular fibronectin and connexin (Cx) 43 increase in parallel during early type II cell culture as Cx26 expression declines. Gap junction intercellular communication is established over the same interval. Cells plated on a preformed, type II cell-derived, fibronectin-rich ECM demonstrate accelerated formation of gap junction plaques and elevated gap junction intercellular communication. These effects are blocked by antibodies against fibronectin, which cause redistribution of Cx43 protein from the plasma membrane to the cytoplasm. Conversely, cells cultured on a laminin-rich ECM, Matrigel, express low levels of Cx43 but high levels of Cx26, reflecting both transcriptional and translational regulation. Cx26 and Cx43 thus demonstrate reciprocal regulation by ECM constituents.

Invited review: plasma membrane stress failure in alveolar epithelial cells

In this review, we examine the hypothesis that plasma membrane stress failure is a central event in the pathophysiology of injury from alveolar overdistension. This hypothesis leads us to consider alveolar micromechanics and specifically the mechanical interactions between lung matrix and alveolar epithelial cell cytoskeleton and plasma membrane. We then explore events that are central to the regulation of plasma membrane tension and detail the lipid-trafficking responses of in vitro deformed and/or injured cells. We conclude with a reference to upregulation of stress-responsive genes after membrane injury and resealing.

Apoptosis of lung epithelial cells in response to TNF-alpha requires angiotensin II generation de novo

Recent work from this laboratory demonstrated that apoptosis of pulmonary alveolar epithelial cells (AEC) in response to Fas requires angiotensin II (ANGII) generation de novo and binding to its receptor (Wang et al., 1999b, Am J Physiol Lung Cell Mol Physiol 277:L1245-L1250). These findings led us to hypothesize that a similar mechanism might be involved in the induction of AEC apoptosis by TNF-alpha. Apoptosis was detected by assessment of nuclear and chromatin morphology, increased activity of caspase 3, binding of annexin V, and by net cell loss inhibitable by the caspase inhibitor ZVAD-fmk. Purified human TNF-alpha induced dose-dependent apoptosis in primary type II pneumocytes isolated from rats or in the AEC-derived human lung carcinoma cell line A549. Apoptosis in response to TNF-alpha was inhibited in a dose-dependent manner by the nonselective ANGII receptor antagonist saralasin or by the nonthiol ACE inhibitor lisinopril; the inhibition of TNF-induced apoptosis was maximal at 50 microgram/ml saralasin (101% inhibition) and at 0.5 microgram/ml lisinopril (86% inhibition). In both cell culture models, purified TNF-alpha caused a significant increase in the mRNA for angiotensinogen (ANGEN), which was not expressed in unactivated cells. Transfection of primary cultures of rat AEC with antisense oligonucleotides against ANGEN mRNA inhibited the subsequent induction of TNF-stimulated apoptosis by 72% (P < 0.01). Exposure to TNF-alpha increased the concentration of ANGII in the serum-free extracellular medium by fivefold in A549 cell cultures and by 40-fold in primary AEC preparations; further, exposure to TNF-alpha for 40 h caused a net cell loss of 70%, which was completely abrogated by either the caspase inhibitor ZVAD-fmk, lisinopril, or saralasin. Apoptosis in response to TNF-alpha was also completely inhibited by neutralizing antibodies specific for ANGII (P < 0.01), but isotype-matched nonimmune immunoglobulins had no significant effect. These data indicate that the induction of AEC apoptosis by TNF-alpha requires a functional renin/angiotensin system (RAS) in the target cell. They also suggest that therapeutic control of AEC apoptosis in response to TNF-alpha is feasible through pharmacologic manipulation of the local RAS.

Mechanical stretch stimulates macrophage inflammatory protein-2 secretion from fetal rat lung cells

Ventilation-induced lung injury has been related to cytokine production. Immaturity and barotrauma are important contributors to the development of bronchopulmonary dysplasia in infants. In the present study, stretch of organotypic cultured fetal rat lung cells was used to simulate ventilation of preterm newborns. Cells were stimulated with lipopolysaccharide (LPS; 100 ng/ml) and/or mechanical stretch. After 4 h, stretch enhanced LPS-induced macrophage inflammatory protein (MIP)-2 production in a force- and frequency-dependent manner. The maximal effect of stretch was seen with 5% elongation at 40 cycles/min. In contrast, after 1 h of stimulation, stretch alone significantly increased MIP-2 production, which was not blocked by cycloheximide, an inhibitor of protein synthesis. At both the 1- and 4-h time points, only LPS increased MIP-2 mRNA levels. Stretch-induced MIP-2 release was associated with cell injury as measured by lactate dehydrogenase release and was not inhibited by gadolinium, a stretch-activated ion channel blocker. Taken together, these results suggest that the major effect of stretch on MIP-2 production from fetal rat lung cells is to stimulate its secretion.

Abrogation of bleomycin-induced epithelial apoptosis and lung fibrosis by captopril or by a caspase inhibitor

Angiotensin-converting enzyme is involved in apoptosis of alveolar epithelial cells (Wang R, Zagariya A, Ang E, Ibarra-Sunga O, and Uhal BD. Am J Physiol Lung Cell Mol Physiol 277: L1245-L1250, 1999). This study tested the ability of the angiotensin-converting enzyme inhibitor captopril or the caspase inhibitor Z-Val-Ala-Asp-fluoromethylketone (ZVAD-fmk) to block alveolar epithelial cell apoptosis and lung fibrosis in vivo in response to bleomycin (Bleo). Male Wistar rats received 8 U/kg of Bleo (bleomycin sulfate) or vehicle intratracheally. Subgroups of Bleo-treated rats received captopril, ZVAD-fmk, or vehicle alone. Lung collagen was assessed by picrosirius red or hydroxyproline assay at 1, 7, and 14 days post-Bleo, and apoptosis was detected by in situ end labeling (ISEL). Bleo increased alveolar septal and peribronchial collagen by 100 and 133%, respectively (both P < 0.01), by day 14 but not earlier. In contrast, ISEL was increased in alveolar and airway cells at all time points. Captopril or ZVAD-fmk inhibited collagen accumulation by 91 and 85%, respectively (P < 0. 01). Both agents also inhibited ISEL in alveoli by 99 and 81% and in airways by 67 and 63%, respectively. These data suggest that the efficacy of captopril to inhibit experimental lung fibrogenesis is related to inhibition of apoptosis. They also demonstrate the antifibrotic potential of a caspase inhibitor.