Expression of transforming growth factor-alpha in mid-gestation human fetal lung

Mechanical stretch promotes fetal type II epithelial cell differentiation via shedding of HB-EGF and TGF-alpha

The mechanisms by which mechanical forces promote fetal lung development are not fully understood. Here, we investigated differentiation of fetal type II epithelial cells via the epidermal growth factor receptor (EGFR) in response to mechanical strain. First, we showed that incubation of embryonic day (E) 19 fetal type II cells with recombinant heparin-binding EGF-like growth factor (HB-EGF) or transforming growth factor (TGF)-alpha, but not with amphiregulin (AR), betacellulin (BTC) or epiregulin (EPR), increased fetal type II cell differentiation, as measured by surfactant protein B/C mRNA and protein levels. Next, we demonstrated that 5% cyclic stretch of E19 monolayers transfected with plasmid encoding alkaline phosphatase (AP)-tagged ligands shed mature HB-EGF and TGF-alpha into the supernatant and promoted type II cell differentiation. Release of these ligands was also observed in E19 cells subjected to higher degrees of cyclic strain, but not in cells exposed to continuous stretch. Interestingly, the addition of fibroblasts to type II cell cultures did not enhance release of HB-EGF. Whereas HB-EGF shedding was also detected in E18 cells exposed to 5% cyclic stretch, release of this ligand after 2.5% sustained stretch was restricted to cells isolated on E18 of gestation. In addition, mechanical stretch released EGF, AR and BTC. We conclude that mechanical stretch promotes fetal type II cell differentiation via ectodomain shedding of HB-EGF and TGF-alpha. The magnitude of shedding varied depending on gestational age, ligand, and strain protocol. These studies provide novel mechanistic information potentially relevant to fetal lung development and to mechanical ventilation-induced lung injury.

HB-EGF decelerates cell proliferation synergistically with TGFalpha in perinatal distal lung development

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is a member of the EGF family of growth factors that is suggested to be involved in distal lung development. In HB-EGF null (HB(del/del)) newborns, a histopathologic analysis revealed abnormally thick saccular walls occurring from embryonic day 18.5 that reduced the terminal saccular space area. HB-EGF gene deletion resulted in a significant increase in cell proliferation, indicating that HB-EGF suppresses distal lung cell proliferation. Furthermore, an analysis of saccular morphology and proliferation in HB-EGF and transforming growth factor-alpha (TGFalpha) double-mutant newborns revealed that HB-EGF and TGFalpha function synergistically in this suppression. Finally, crosses between HB(del/del) mice and waved 2 mice, a hypomorphic EGF receptor (EGFR) mutant strain, suggest that HB-EGF and EGFR cooperate in this process. Thus, HB-EGF has a novel suppressive function that contributes to decelerating distal lung cell proliferation synergistically with TGFalpha through EGFR in perinatal distal lung development.

VEGF induces airway epithelial cell proliferation in human fetal lung in vitro

Vascular endothelial growth factor (VEGF) is a potent endothelial cell mitogen involved in normal and abnormal angiogenesis. VEGF mRNA and protein are abundant in distal epithelium of midtrimester human fetal lung. In the present study, we identified immunoreactivity for KDR, a major VEGF-specific receptor, in distal lung epithelial cells of human fetal lung tissue, suggesting a possible autocrine or paracrine regulatory role for VEGF in pulmonary epithelial cell growth and differentiation. Addition of exogenous VEGF to human fetal lung explants resulted in increased epithelium volume density and lumen volume density in the tissues, both morphometric parameters of tissue differentiation. Cellular proliferation demonstrated by bromodeoxyuridine uptake was prominent in distal airway epithelial cells and increased in the VEGF-treated explants. VEGF-treated explants also demonstrated increased surfactant protein (SP) A mRNA, SP-C mRNA, and SP-A protein levels compared with controls. However, SP-B mRNA levels were unaffected by VEGF treatment. [(3)H]choline incorporation into total phosphatidylcholine was increased by VEGF treatment, but incorporation into disaturated phosphatidylcholine was not affected by exogenous VEGF. Based on these observations, we conclude that VEGF may be an important autocrine growth factor for distal airway epithelial cells in the developing human lung.

Pulmonary hypoplasia in mice lacking tumor necrosis factor-alpha converting enzyme indicates an indispensable role for cell surface protein shedding during embryonic lung branching morphogenesis

Many membrane-bound protein precursors, including cytokines and growth factors, are proteolytically shed to yield soluble intercellular regulatory ligands. The responsible protease, tumor necrosis factor-alpha converting enzyme (TACE/ADAM-17), is a transmembrane metalloprotease-disintegrin that cleaves multiple cell surface proteins, although it was initially identified for the enzymatic release of tumor necrosis factor-alpha (TNF-alpha). Mammalian lung growth and development are tightly controlled by cytokines and peptide growth factors. However, the biological function of the cell shedding mechanism during lung organogenesis is not understood. We therefore evaluated the role of TACE as a "sheddase" during lung morphogenesis by analyzing the developmental phenotypes of lungs in mice with an inactive TACE gene in both in vivo and ex vivo organ explant culture. Neonatal TACE-deficient mice had visible respiratory distress and their lungs failed to form normal saccular structures. These newborn mutant lungs had fewer peripheral epithelial sacs with deficient septation and thick-walled mesenchyme, resulting in reduced surface for gas exchange. At the canalicular stage of E16.5, the lungs of TACE mutant mice were impaired in branching morphogenesis, inhibited in epithelial cell proliferation and differentiation, and delayed in vasculogenesis. Embryonic TACE knockout mouse lungs (E12) branched poorly compared to wild-type lungs, when placed into serumless organ culture. Gene expression of both surfactant protein-C and aquaporin-5 were inhibited in cultured TACE-mutant embryonic lungs, indicating defects in both branching and peripheral epithelial cytodifferentiation in the absence of TACE protein. Furthermore, both the hypoplastic phenotype and the delayed cytodifferentiation in TACE-deficient lungs were rescued by exogenous addition of soluble stimulatory factors including either TNF-alpha or epidermal growth factor in embryonic lung culture. Thus, the impaired lung branching and maturation without TACE suggest a broad role for TACE in the processing of multiple membrane-anchored proteins, one or more of which is essential for normal lung morphogenesis. Taken together, our data indicate that the TACE-mediated proteolytic mechanism which enzymatically releases membrane-tethered proteins plays an indispensable role in lung morphogenesis, and its inactivation leads to abnormal lung development.

Chemokine expression in Th1 cell-induced lung injury: prominence of IFN-gamma-inducible chemokines

Proinflammatory responses generated by T helper type 1 (Th1) cells may contribute significantly to immune-mediated lung injury. We describe a murine model of Th1 cell-induced lung injury in which adoptive transfer of alloreactive Th1 cells produces pulmonary inflammation characterized by mononuclear cell vasculitis, alveolitis, and interstitial pneumonitis. To investigate the link between activation of Th1 cells in the lung and inflammatory cell recruitment, we characterized cytokine and chemokine mRNA expression in Th1 cells activated in vitro and in lung tissue after adoptive transfer of Th1 cells. Activated Th1 cells per se express mRNA for interferon (IFN)-gamma and several members of the tumor necrosis factor family as well as the C-C chemokine receptor-5 ligands regulated on activation normal T cells expressed and secreted and macrophage inflammatory protein-1alpha and -1beta. Additional chemokine genes were induced in the lung after Th1 cell administration, most notably IFN-gamma-inducible protein (IP-10) and monokine induced by IFN-gamma (MIG). Remarkable increases in IP-10- and MIG-immunoreactive proteins were present in inflammatory foci lung and identified in macrophages, endothelium, bronchial epithelium, and alveolar structures. The findings suggest that IFN-gamma-inducible chemokines are an important mechanism for amplifying inflammation initiated by Th1 cells in the lung.

Antisense inhibition of epidermal growth factor receptor decreases expression of human surfactant protein A

Epidermal growth factor (EGF) stimulates surfactant protein A (SP-A) synthesis in fetal lung tissue through ligand binding to the EGF receptor. We hypothesized that inhibition of EGF receptor messenger RNA (mRNA) would block SP-A expression in human fetal lung tissue during alveolar type II cell differentiation in vitro. Midtrimester human fetal lung explants were maintained in serum-free Waymouth's medium for 3 to 5 d in the presence or absence of an antisense 18-mer phosphorothioate oligonucleotide (ON) complementary to the initiation codon region of EGF receptor mRNA. Sense and scrambled ONs similarly modified were used as additional controls. The concentration of EGF receptor mRNA was semiquantitatively determined by reverse transcriptase/polymerase chain reaction (RT-PCR). We found a significant 3-fold decrease in EGF receptor mRNA levels in the antisense-treated groups compared with the control group with no effect in the sense condition. Immunohistochemical staining revealed a decrease in the amount of staining for EGF receptor protein in distal pulmonary epithelial cells in the antisense-treated groups compared with either control or sense conditions. Treatment with antisense EGF receptor ON decreased both SP-A mRNA and protein compared with controls with no effect in the sense condition. The ONs did not affect tissue viability as measured by the release of lactate dehydrogenase. We conclude that selective degradation of EGF receptor mRNA with antisense ON treatment results in a decrease in SP-A expression in human fetal lung. These findings support the critical importance of the EGF receptor for the regulation of SP-A gene expression during human alveolar type II cell differentiation.

Collagen accumulation is decreased in SPARC-null mice with bleomycin-induced pulmonary fibrosis

Secreted protein acidic and rich in cysteine (SPARC) has been shown to be coexpressed with type I collagen in tissues undergoing remodeling and wound repair. We speculated that SPARC is required for the accumulation of collagen in lung injury and that its absence would attenuate collagen accumulation. Accordingly, we have assessed levels of collagen in SPARC-null mice in an intratracheal bleomycin-injury model of pulmonary fibrosis. Eight- to ten-week-old SPARC-null and wild-type (WT) mice received bleomycin (0.0035 U/g) or saline intratracheally and were subsequently killed after 14 days. Relative levels of SPARC mRNA were increased 2.7-fold (P < 0.001) in bleomycin-treated WT lungs in comparison with saline-treated lungs. Protein from bleomycin-treated WT lung contained significantly more hydroxyproline (191.9 microg/lung) than protein from either bleomycin-treated SPARC-null lungs or saline-treated WT and SPARC-null lungs (147.4 microg/lung, 125.4 microg/lung, and 113. 0 microg/lung, respectively; P < 0.03). These results indicate that SPARC is increased in response to lung injury and that accumulation of collagen, as indicated by hydroxyproline content, is attenuated in the absence of SPARC. The properties of SPARC as a matricellular protein associated with cell proliferation and matrix turnover are consistent with its participation in the development of pulmonary fibrosis.

Transforming growth factor-alpha deficiency reduces pulmonary fibrosis in transgenic mice

Despite evidence that implicates transforming growth factor-alpha (TGF-alpha) in the pathogenesis of acute lung injury, the contribution of TGF-alpha to the fibroproliferative response is unknown. To determine whether the development of pulmonary fibrosis depends on TGF-alpha, we induced lung injury with bleomycin in TGF-alpha null-mutation transgenic mice and wild-type mice. Lung hydroxyproline content was 1.3, 1.2, and 1.6 times greater in wild-genotype mice than in TGF-alpha-deficient animals at Days 10, 21, and 28, respectively, after a single intratracheal injection of bleomycin. At Days 7 and 10 after bleomycin treatment, lung total RNA content was 1.5 times greater in wild-genotype mice than in TGF-alpha-deficient animals. There was no significant difference between mice of the two genotypes in lung total DNA content or nuclear labeling indices after bleomycin administration. Wild-genotype mice had significantly higher lung fibrosis scores at Days 7 and 14 after bleomycin treatment than did TGF-alpha-deficient animals. There was no significant difference between TGF-alpha-deficient mice and wild-genotype mice in lung inflammation scores after bleomycin administration. To determine whether expression of other members of the epidermal growth factor (EGF) family is increased after bleomycin-induced injury, we measured lung EGF and heparin-binding- epidermal growth factor (HB-EGF) mRNA levels. Steady-state HB-EGF mRNA levels were 321% and 478% of control values in bleomycin-treated lungs at Days 7 and 10, respectively, but were not significantly different in TGF-alpha-deficient and in wild-genotype mice. EGF mRNA was not detected in normal or bleomycin-treated lungs of mice of either genotype. These results show that TGF-alpha contributes significantly to the pathogenesis of pulmonary fibrosis after bleomycin-induced injury, and that compensatory increases in other EGF family members do not occur in TGF-alpha-deficient mice.

Effect of tyrosine kinase inhibition on surfactant protein A gene expression during human lung development

Epidermal growth factor (EGF) stimulates surfactant protein (SP) A synthesis in human fetal lung explants. Ligand binding to the EGF receptor stimulates an intrinsic receptor tyrosine kinase with subsequent activation of second messengers. We hypothesized that inhibition of EGF-receptor tyrosine kinase activity would block SP-A expression in spontaneously differentiating cultured human fetal lung tissue. Midtrimester fetal lung explants were exposed for 4 days to genistein (a broad-range inhibitor of tyrosine kinases) and tyrphostin AG-1478 (a specific inhibitor of EGF-receptor tyrosine kinase). Genistein significantly decreased SP-A and SP-A mRNA levels without affecting either tissue viability or the morphological differentiation of alveolar type II cells. Tyrphostin AG-1478 also decreased SP-A content and SP-A mRNA levels in cultured fetal lung explants. Treatment with EGF could not overcome the inhibitory effects of either genistein or tyrphostin on SP-A; however, only tyrphostin inhibited EGF-receptor tyrosine phosphorylation. We conclude that specific inhibition of EGF-receptor tyrosine kinase with tyrphostin AG-1478 blocks the expression of SP-A during spontaneous differentiation of cultured human fetal lung tissue. Furthermore, exposure to genistein also decreases SP-A expression and blocks the effects of EGF in human fetal lung tissue without inhibiting EGF-receptor tyrosine phosphorylation. These findings support the importance of tyrosine kinase-dependent signal transduction pathways in the regulation of SP-A during human fetal lung development.

H441 pulmonary epithelial cell mitogenic effects and signaling pathways in response to HGF and TGF-alpha

Pulmonary epithelial cells are important in lung growth, development, and injury. H441 pulmonary adenocarcinoma cells may be a useful model for studying pulmonary epithelial cell growth factor responses in vitro. Isolated pulmonary epithelial type II cells proliferate in response to transforming growth factor (TGF)-alpha via the epidermal growth factor (EGF) receptor. Type II cells also proliferate in response to hepatocyte growth factor (HGF). In the present study, H441 cell responses to these growth factors were examined, and compared to type II cells. Both the EGF-R and the c-met proto-oncogene receptor, to which HGF binds, were immunoprecipitated from H441 cells. In H441 cells, addition of TGF-alpha resulted in phosphorylation of the EGF receptor and increased cell number and tritiated thymidine incorporation. Incubation with HGF resulted in phosphorylation of its c-met proto-oncogene receptor in type II and H441 cells, and also increased cell number and tritiated thymidine incorporation. Both HGF and TGF-alpha stimulated phosphorylation of the intracellular signaling molecules p42 and p44 mitogen activated protein kinases in H441 cells. H441 cells exhibited responses to mitogenic growth factors similar to type II cells and may be useful as a model for type II cell growth factor responses and signal transduction.

Soluble transforming growth factor-alpha is present in the pulmonary edema fluid of patients with acute lung injury

Recent in vivo and in vitro experimental evidence indicates that transforming growth factor-alpha (TGF-alpha) is an important growth factor in the process of recovery and remodeling that occurs after acute lung injury. However, there are very little clinical data on TGF-alpha in patients with acute lung injury. Therefore, the purpose of this study was to determine if TGF-alpha is present in biologically significant concentrations in the pulmonary edema fluid from patients with acute lung injury, and to determine if the presence of TGF-alpha is specific for acute lung injury by including control patients with hydrostatic edema. Using an enzyme-linked immunosorbent assay, plasma and pulmonary edema fluid TGF-alpha levels were measured in 43 patients (34 with increased permeability edema, nine with hydrostatic edema). TGF-alpha was detected in 24 of 34 patients (71%) with increased permeability pulmonary edema (range, 0.035 to 2.57 ng/mL) compared with only two of nine patients with hydrostatic edema (p < 0.05). TGF-alpha was not detected in any plasma samples. These concentrations of TGF-alpha in pulmonary edema fluid have potent in vivo and in vitro effects on alveolar epithelial sodium transport and alveolar epithelial cell motility. In conclusion, biologically relevant concentrations of soluble TGF-alpha are present in the pulmonary edema fluid on day 1 of patients with acute lung injury, a remarkable finding with important implications for the repair and resolution of acute lung injury, particularly since TGF-alpha was detected so early in the course of acute lung injury.

Cytokines and production of surfactant components

The production of pulmonary surfactant, a complex of lipids and proteins that reduces surface tension at the alveolar air-liquid interface, is developmentally regulated. Several hormones, most notably glucocorticoids, are known to accelerate maturation of the surfactant system. Cytokines are polypeptides that act mostly in a paracrine fashion and possess a wide spectrum of activities on multiple types of cells. Many cytokines are produced by different lung cells a various stages of fetal development or under pathological conditions affecting the fetus. In addition, cytokines present in amniotic fluid or in the blood stream may reach the fetal lungs. Some cytokines, including epidermal growth factor, transforming growth factor-alpha, and interferon-gamma have been shown to stimulate the production of surfactant components. On the other hand, tumor necrosis factor and transforming growth factor-beta downregulate the production of surfactant lipids and proteins. We have recently shown that the proinflammatory cytokine interleukin-1 (IL-I) enhances the expression of surfactant protein A (SP-A) in fetal rabbit lung explants. In addition, injection of IL-I into the amniotic fluid of fetal rabbits enhances the expression of surfactant proteins and improves the lung compliance of preterm animals. Preterm delivery is often associated with subclinical intraamniotic infection. In these cases, amniotic fluid concentrations of IL-I are often elevated. We propose that this cytokine accelerates maturation of the surfactant system in fetal lungs and thus prepares the fetus for extrauterine life.

Inhibition of TGF-alpha gene expression by vitamin A in airway epithelium

The autocrine/paracrine growth mechanism has been implicated in the regulation of bronchial epithelial cell proliferation. By inhibiting the expression of the transforming growth factor-alpha (TGF-alpha) gene product, vitamin A is able to suppress the proliferation of tracheobronchial epithelial cells in culture. Similar repressions in TGF-alpha mRNA levels by retinol were observed in airway explant cultures and in a cell line immortalized from normal human bronchial epithelial cells. Both the nuclear run-on transcriptional assay and the transfection study with the chimeric construct of the TGF-alpha promoter and chloramphenicol acetyltransferase reporter gene partly suggest a transcriptional downregulation mechanism of TGF-alpha gene expression by the retinol treatment; however, this inhibition at the transcriptional level cannot account for the total inhibition at the mRNA level. These results suggest that a downregulation of the expression of the TGF-alpha gene at the transcriptional and post-transcriptional levels by vitamin A may precede the essential event associated with the homeostasis of normal conducting airway epithelium.

Expression and localization of epidermal growth factor, transforming growth factor-alpha, and localization of their common receptor in fetal human lung development

Transforming growth factor-alpha (TGF-alpha), epidermal growth factor (EGF), and their common EGF receptor have been shown to be involved in cell proliferation and lung maturation. The aim of the study was to determine the site of production of TGF-alpha and EGF mRNA and the cellular distribution of TGF-alpha/EGF proteins and EGF receptor, in fetal human lung. By using in situ hybridization with 35S-labeled cDNA probes in frozen sections from eight lungs from fetuses ranging from 12 to 33 wk of gestation, TGF-alpha and EGF mRNA transcripts appeared to be confined to the mesenchymal cells and mainly found in the dense connective tissue along the pleura, bronchi, and large vessels, but undetected in bronchial epithelial cells. The streptavidin-biotin immunoperoxidase method, applied to paraffin-embedded specimens from 39 fetuses ranging from 10 to 41 wk, showed that TGF-alpha, EGF, and EGF receptor exhibited a similar cellular distribution during the whole period of gestation. They were detected in the undifferentiated cells of the airway surface epithelium, mesothelial cells, smooth muscle, and a few mesenchymal cells, as early as 10 wk. After 12 wk, the immunoreactivity was strong in the ciliated, secretory, and basal cells, and in growing glands along the large airways, but proved lower in the distal airways. After 24 wk, the immunoreactivity remained in the airway epithelium, but was mainly localized in the apical domain of ciliated cells, in alveolar cells, and in the serous cells of the glands. The presence of TGF-alpha, EGF, and EGF receptor during the whole period of fetal lung development suggests that these factors are not only mitogenic, but can also be involved in epithelial maturation, through paracrine secretion, as most TGF-alpha and EGF mRNA transcripts are expressed in mesenchymal cells.

Localization of epidermal growth factor receptor in alveolar epithelium during human fetal lung development in vitro

Epidermal growth factor (EGF) enhances alveolar type II cell differentiation. In human fetal lung explants, EGF stimulates surfactant protein A (SP-A) synthesis. This effect may occur through a direct interaction of the ligand on EGF receptors located within distal pulmonary epithelium during alveolar type II cell differentiation. To determine if EGF receptor is present in alveolar epithelium, immunostaining for EGF receptor and in situ hybridization for EGF receptor mRNA were performed in human fetal lung explants undergoing alveolar type II cell differentiation in vitro. After 4 days in culture, EGF receptor immunostaining was present in alveolar epithelium from human fetal lung explants compared to minimal immunostaining in undifferentiated human fetal lung epithelium prior to culture. In situ hybridization revealed increased EGF receptor mRNA in differentiated type II cells from cultured explants, with minimal EGF receptor mRNA detected in undifferentiated epithelium from tissue prior to culture. Immunogold staining revealed EGF receptors on the cytoplasmic membranes of epithelial cells lining the prealveolar ducts in human fetal lung explants after 2 days in culture. Alveolar type II cell differentiation in vitro was confirmed ultrastructurally by the presence of lamellar bodies and biochemically by an increase in SP-A content. Thus, EGF receptor is found in alveolar epithelium during differentiation, which suggests an important role for EGF during human fetal lung development.

Nodules of less-differentiated tumor within or adjacent to hepatocellular carcinoma: relative expression of transforming growth factor-alpha and its receptor in the different areas of tumor

Expression of transforming growth factor-alpha (TGF-alpha) and its receptor, the epidermal growth factor receptor (EGFR), in hepatocellular carcinomas (HCCs) and adjacent nontumorous livers from 25 Japanese patients were examined using immunoperoxidase staining of paraffin-embedded sections. TGF-alpha was detected in 24 of 25 (96%) HCCs and 23 of 24 (96%) available adjacent nontumorous livers. EGFR was detected in 16 of 25 (64%) HCCs and 17 of 24 (71%) adjacent nontumorous livers. TGF-alpha and EGFR were not detected by immunohistochemical staining in normal livers. Fifteen of 25 HCCs contained an apparent area of a second tumor (two of the 15 also contained a third tumor) that had a less-differentiated histological grade developing within or adjacent to the first tumor. In those cases, staining in the less-differentiated area of tumor was usually less intense than in the more highly differentiated area (80% of cases for TGF-alpha; 91% for EGFR). These data confirm that increased expression of TGF-alpha and EGFR occur frequently in human HCC. Furthermore, the detection of greater staining in more highly differentiated portions of the tumors suggests that increased expression of TGF-alpha and EGFR may be events of the early stages of human hepatocarcinogenesis.

Respiratory epithelial cell expression of human transforming growth factor-alpha induces lung fibrosis in transgenic mice

Increased production of EGF or TGF-alpha by the respiratory epithelial cells has been associated with the pathogenesis of various forms of lung injury. Growth factors and cytokines are thought to act locally, via paracrine and autocrine mechanisms, to stimulate cell proliferation and matrix deposition by interstitial lung cells resulting in pulmonary fibrosis. To test whether TGF-alpha mediates pulmonary fibrotic responses, we have generated transgenic mice expressing human TGF-alpha under control of regulatory regions of the human surfactant protein C (SP-C) gene. Human TGF-alpha mRNA was expressed in pulmonary epithelial cells in the lungs of the transgenic mice. Adult mice bearing the SP-C-TGF-alpha transgene developed severe pulmonary fibrosis. Fibrotic lesions were observed in peribronchial, peribronchiolar, and perivascular regions, as well as subjacent to pleural surfaces. Lesions consisted of fibrous tissue that included groups of epithelial cells expressing endogenous SP-C mRNA, consistent with their identification as distal respiratory epithelial cells. Peripheral fibrotic regions consisted of thickened pleura associated with extensive collagen deposition. Alveolar architecture was disrupted in the transgenic mice with loss of alveoli in the lung parenchyma. Pulmonary epithelial cell expression of TGF-alpha in transgenic mice disrupts alveolar morphogenesis and produces fibrotic lesions mediated by paracrine signaling between respiratory epithelial and interstitial cells of the lung.

Epidermal growth factor-like activity in bronchoalveolar lavage fluid in experimental silicosis

We investigated whether development of pulmonary fibrosis following inhalational exposure of mice to silica (quartz) dust was accompanied by enhanced secretion of activity resembling epidermal growth factor (EGF). Mitogenic activity for pulmonary fibroblasts was assessed in bronchoalveolar lavage fluid (BALF) using a serum-free bioassay. Activity in BALF from mice exposed to nonfibrogenic titanium dioxide dust was comparable to that in BALF from normal animals. In contrast, mitogenic activity was significantly increased at 6 and 12 weeks after inhalation of silica particles, coinciding with the appearance of collagenised lesions in the lung. BALF from mice exposed to silica 6 weeks previously had significantly higher concentrations of growth factor(s) able to bind to EGF receptors on pulmonary fibroblasts. In parallel, macrophages within inflammatory lesions in the airspaces acquired immunoreactivity for EGF. The presence of an increased concentration of EGF-like growth factor(s) in BALF might constitute a marker of particle-induced pulmonary fibrosis.