FGF-2 induces surfactant protein gene expression in foetal rat lung epithelial cells through a MAPK-independent pathway

3D Cell Culture Models Demonstrate a Role for FGF and WNT Signaling in Regulation of Lung Epithelial Cell Fate and Morphogenesis

FGF signaling plays an essential role in lung development, homeostasis, and regeneration. We employed mouse 3D cell culture models and imaging to study ex vivo the role of FGF ligands and the interplay of FGF signaling with epithelial growth factor (EGF) and WNT signaling pathways in lung epithelial morphogenesis and differentiation. In non-adherent conditions, FGF signaling promoted formation of lungospheres from lung epithelial stem/progenitor cells (LSPCs). Ultrastructural and immunohistochemical analyses showed that LSPCs produced more differentiated lung cell progeny. In a 3D extracellular matrix, FGF2, FGF7, FGF9, and FGF10 promoted lung organoid formation. FGF9 showed reduced capacity to promote lung organoid formation, suggesting that FGF9 has a reduced ability to sustain LSPC survival and/or initial divisions. FGF7 and FGF10 produced bigger organoids and induced organoid branching with higher frequency than FGF2 or FGF9. Higher FGF concentration and/or the use of FGF2 with increased stability and affinity to FGF receptors both increased lung organoid and lungosphere formation efficiency, respectively, suggesting that the level of FGF signaling is a crucial driver of LSPC survival and differentiation, and also lung epithelial morphogenesis. EGF signaling played a supportive but non-essential role in FGF-induced lung organoid formation. Analysis of tissue architecture and cell type composition confirmed that the lung organoids contained alveolar-like regions with cells expressing alveolar type I and type II cell markers, as well as airway-like structures with club cells and ciliated cells. FGF ligands showed differences in promoting distinct lung epithelial cell types. FGF9 was a potent inducer of more proximal cell types, including ciliated and basal cells. FGF7 and FGF10 directed the differentiation toward distal lung lineages. WNT signaling enhanced the efficiency of lung organoid formation, but in the absence of FGF10 signaling, the organoids displayed limited branching and less differentiated phenotype. In summary, we present lung 3D cell culture models as useful tools to study the role and interplay of signaling pathways in postnatal lung development and homeostasis, and we reveal distinct roles for FGF ligands in regulation of mouse lung morphogenesis and differentiation ex vivo.

Developmental pathways in the pathogenesis of lung fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive and terminal lung disease with no known cure. IPF is a disease of aging, with median age of diagnosis over 65 years. Median survival is between 3 and 5 years after diagnosis. IPF is characterized primarily by excessive deposition of extracellular matrix (ECM) proteins by activated lung fibroblasts and myofibroblasts, resulting in reduced gas exchange and impaired pulmonary function. Growing evidence supports the concept of a pro-fibrotic environment orchestrated by underlying factors such as genetic predisposition, chronic injury and aging, oxidative stress, and impaired regenerative responses may account for disease development and persistence. Currently, two FDA approved drugs have limited efficacy in the treatment of IPF. Many of the genes and gene networks associated with lung development are induced or activated in IPF. In this review, we analyze current knowledge in the field, gained from both basic and clinical research, to provide new insights into the disease process, and potential approaches to treatment of pulmonary fibrosis.

A role for FGF2 in visceral adiposity-associated mammary epithelial transformation

Obesity is a leading risk factor for post-menopausal breast cancer, and this is concerning as 40% of cancer diagnoses in 2014 were associated with overweight/obesity. Despite this epidemiological link, the underlying mechanism responsible is unknown. We recently published that visceral adipose tissue (VAT) releases FGF2 and stimulates the transformation of skin epithelial cells. Furthermore, obesity is differentially associated with many epithelial cancers, and this mechanistic link could be translational. As FGF2 and FGFR1 are implicated in breast cancer progression, we hypothesize that VAT-derived FGF2 plays a translational role in promoting adiposity-associated mammary epithelial cell transformation. In this brief report, data suggest that FGF2/FGFR1 signaling is a potential mechanistic link in VAT-stimulated transformation of breast epithelial cells.

Fibroblast growth factor receptor is a mechanistic link between visceral adiposity and cancer

Epidemiological evidence implicates excess adipose tissue in increasing cancer risk. Despite a steeply rising global prevalence of obesity, how adiposity contributes to transformation (stage a non-tumorigenic cell undergoes to become malignant) is unknown. To determine the factors in adipose tissue that stimulate transformation, we used a novel ex vivo system of visceral adipose tissue (VAT)-condition medium-stimulated epithelial cell growth in soft agar. To extend this system in vivo, we used a murine lipectomy model of ultraviolet light B-induced, VAT-promoted skin tumor formation. We found that VAT from mice and obese human donors stimulated growth in soft agar of non-tumorigenic epithelial cells. The difference in VAT activity was associated with fibroblast growth factor-2 (FGF2) levels. Moreover, human and mouse VAT failed to stimulate growth in soft of agar in cells deficient in FGFR-1 (FGF2 receptor). We also demonstrated that circulating levels of FGF2 were associated with non-melanoma tumor formation in vivo. These data implicate FGF2 as a major factor VAT releases to transform epithelial cells-a novel, potential pathway of VAT-enhanced tumorigenesis. Strategies designed to deplete VAT stores of FGF2 or inhibit FGFR-1 in abdominally obese individuals may be important cancer prevention strategies as well as adjuvant therapies for improving outcomes.

Endothelial cells are not required for specification of respiratory progenitors

Crosstalk between mesenchymal and epithelial cells influences organogenesis in multiple tissues, such as lung, pancreas, liver, and the nervous system. Lung mesenchyme comprises multiple cell types, however, and precise identification of the mesenchymal cell type(s) that drives early events in lung development remains unknown. Endothelial cells have been shown to be required for some aspects of lung epithelial patterning, lung stem cell differentiation, and regeneration after injury. Furthermore, endothelial cells are involved in early liver and pancreas development. From these observations we hypothesized that endothelial cells might also be required for early specification of the respiratory field and subsequent lung bud initiation. We first blocked VEGF signaling in E8.5 cultured foreguts with small molecule VEGFR inhibitors and found that lung specification and bud formation were unaltered. However, when we examined E9.5 mouse embryos carrying a mutation in the VEGFR Flk-1, which do not develop endothelial cells, we found that respiratory progenitor specification was impeded. Because the E9.5 embryos were substantially smaller than control littermates, suggesting the possibility of developmental delay, we isolated and cultured foreguts from mutant and control embryos on E8.5, when no size differences were apparent. We found that both specification of the respiratory field and lung bud formation occurred in mutant and control explants. These observations were unaffected by the presence or absence of serum. We also observed that hepatic specification and initiation occurred in the absence of endothelial cells, and that expansion of the liver epithelium in culture did not differ between mutant and control explants. Consistent with previously published results, we also found that pancreatic buds were not maintained in cultured foreguts when endothelial cells were absent. Our observations support the conclusion that endothelial cells are not required for early specification of lung progenitors and bud initiation, and that the diminished lung specification seen in E9.5 Flk-/- embryos is likely due to developmental delay resulting from the insufficient delivery of oxygen, nutrients, and other factors in the absence of a vasculature.

ATP-containing vesicles in stria vascular marginal cell cytoplasms in neonatal rat cochlea are lysosomes

We confirmed that ATP is released from cochlear marginal cells in the stria vascular but the cell organelle in which ATP stores was not identified until now. Thus, we studied the ATP-containing cell organelles and suggest that these are lysosomes. Primary cultures of marginal cells of Sprague-Dawley rats aged 1-3 days was established. Vesicles within marginal cells stained with markers were identified under confocal laser scanning microscope and transmission electron microscope (TEM). Then ATP release from marginal cells was measured after glycyl-L-phenylalanine-ß- naphthylamide (GPN) treatment using a bioluminescent assay. Quinacrine-stained granules within marginal cells were labeled with LysoTracker, a lysosome tracer, and lysosomal-associated membrane protein 1(LAMP1), but not labeled with the mitochondrial tracer MitoTracker. Furthermore, LysoTracker-labelled puncta showed accumulation of Mant-ATP, an ATP analog. Treatment with 200 μM GPN quenched fluorescently labeled puncta after incubation with LysoTracker or quinacrine, but not MitoTracker. Quinacrine-labeled organelles observed by TEM were lysosomes, and an average 27.7 percent increase in ATP luminescence was observed in marginal cells extracellular fluid after GPN treatment. ATP-containing vesicles in cochlear marginal cells of the stria vascular from neonatal rats are likely lysosomes. ATP release from marginal cells may be via Ca(2+)-dependent lysosomal exocytosis.

Fibroblast growth factor 2 is required for epithelial recovery, but not for pulmonary fibrosis, in response to bleomycin

The pathogenesis of pulmonary fibrosis involves lung epithelial injury and aberrant proliferation of fibroblasts, and results in progressive pulmonary scarring and declining lung function. In vitro, fibroblast growth factor (FGF) 2 promotes myofibroblast differentiation and proliferation in cooperation with the profibrotic growth factor, transforming growth factor-β1, but the in vivo requirement for FGF2 in the development of pulmonary fibrosis is not known. The bleomycin model of lung injury and pulmonary fibrosis was applied to Fgf2 knockout (Fgf2(-/-)) and littermate control mice. Weight loss, mortality, pulmonary fibrosis, and histology were analyzed after a single intranasal dose of bleomycin. Inflammation was evaluated in bronchoalveolar lavage (BAL) fluid, and epithelial barrier integrity was assessed by measuring BAL protein and Evans Blue dye permeability. Fgf2 is expressed in mouse and human lung epithelial and inflammatory cells, and, in response to bleomycin, Fgf2(-/-) mice have significantly increased mortality and weight loss. Analysis of BAL fluid and histology show that pulmonary fibrosis is unaltered, but Fgf2(-/-) mice fail to efficiently resolve inflammation, have increased BAL cellularity, and, importantly, deficient recovery of epithelial integrity. Fgf2(-/-) mice similarly have deficient recovery of club cell secretory protein(+) bronchial epithelium in response to naphthalene. We conclude that FGF2 is not required for bleomycin-induced pulmonary fibrosis, but rather is essential for epithelial repair and maintaining epithelial integrity after bleomycin-induced lung injury in mice. These data identify that FGF2 acts as a protective growth factor after lung epithelial injury, and call into question the role of FGF2 as a profibrotic growth factor in vivo.

Tissue specificity of decellularized rhesus monkey kidney and lung scaffolds

Initial steps in establishing an optimal strategy for functional bioengineered tissues is generation of three-dimensional constructs containing cells with the appropriate organization and phenotype. To effectively utilize rhesus monkey decellularized kidney scaffolds, these studies evaluated two key parameters: (1) residual scaffold components after decellularization including proteomics analysis, and (2) the use of undifferentiated human embryonic stem cells (hESCs) for recellularization in order to explore cellular differentiation in a tissue-specific manner. Sections of kidney and lung were selected for a comparative evaluation because of their similar pattern of organogenesis. Proteomics analysis revealed the presence of growth factors and antimicrobial proteins as well as stress proteins and complement components. Immunohistochemistry of recellularized kidney scaffolds showed the generation of Cytokeratin+ epithelial tubule phenotypes throughout the scaffold that demonstrated a statistically significant increase in expression of kidney-associated genes compared to baseline hESC gene expression. Recellularization of lung scaffolds showed that cells lined the alveolar spaces and demonstrated statistically significant upregulation of key lung-associated genes. However, overall expression of kidney and lung-associated markers was not statistically different when the kidney and lung recellularized scaffolds were compared. These results suggest that decellularized scaffolds have an intrinsic spatial ability to influence hESC differentiation by physically shaping cells into tissue-appropriate structures and phenotypes, and that additional approaches may be needed to ensure consistent recellularization throughout the matrix.

Disruption of sorting nexin 5 causes respiratory failure associated with undifferentiated alveolar epithelial type I cells in mice

Sorting nexin 5 (Snx5) has been posited to regulate the degradation of epidermal growth factor receptor and the retrograde trafficking of cation-independent mannose 6-phosphate receptor/insulin-like growth factor II receptor. Snx5 has also been suggested to interact with Mind bomb-1, an E3 ubiquitin ligase that regulates the activation of Notch signaling. However, the in vivo functions of Snx5 are largely unknown. Here, we report that disruption of the Snx5 gene in mice (Snx5^-/- mice) resulted in partial perinatal lethality; 40% of Snx5^-/- mice died shortly after birth due to cyanosis, reduced air space in the lungs, and respiratory failure. Histological analysis revealed that Snx5^-/- mice exhibited thickened alveolar walls associated with undifferentiated alveolar epithelial type I cells. In contrast, alveolar epithelial type II cells were intact, exhibiting normal surfactant synthesis and secretion. Although the expression levels of surfactant proteins and saturated phosphatidylcholine in the lungs of Snx5^-/- mice were comparable to those of Snx5^+/+ mice, the expression levels of T1α, Aqp5, and Rage, markers for distal alveolar epithelial type I cells, were significantly decreased in Snx5^-/- mice. These results demonstrate that Snx5 is necessary for the differentiation of alveolar epithelial type I cells, which may underlie the adaptation to air breathing at birth.

Structure and epitope distribution of heparan sulfate is disrupted in experimental lung hypoplasia: a glycobiological epigenetic cause for malformation?

BACKGROUND

Heparan sulfate (HS) is present on the surface of virtually all mammalian cells and is a major component of the extracellular matrix (ECM), where it plays a pivotal role in cell-cell and cell-matrix cross-talk through its large interactome. Disruption of HS biosynthesis in mice results in neonatal death as a consequence of malformed lungs, indicating that HS is crucial for airway morphogenesis. Neonatal mortality (~50%) in newborns with congenital diaphragmatic hernia (CDH) is principally associated with lung hypoplasia and pulmonary hypertension. Given the importance of HS for lung morphogenesis, we investigated developmental changes in HS structure in normal and hypoplastic lungs using the nitrofen rat model of CDH and semi-synthetic bacteriophage ('phage) display antibodies, which identify distinct HS structures.

RESULTS

The pulmonary pattern of elaborated HS structures is developmentally regulated. For example, the HS4E4V epitope is highly expressed in sub-epithelial mesenchyme of E15.5 - E17.5 lungs and at a lower level in more distal mesenchyme. However, by E19.5, this epitope is expressed similarly throughout the lung mesenchyme.We also reveal abnormalities in HS fine structure and spatiotemporal distribution of HS epitopes in hypoplastic CDH lungs. These changes involve structures recognised by key growth factors, FGF2 and FGF9. For example, the EV3C3V epitope, which was abnormally distributed in the mesenchyme of hypoplastic lungs, is recognised by FGF2.

CONCLUSIONS

The observed spatiotemporal changes in HS structure during normal lung development will likely reflect altered activities of many HS-binding proteins regulating lung morphogenesis. Abnormalities in HS structure and distribution in hypoplastic lungs can be expected to perturb HS:protein interactions, ECM microenvironments and crucial epithelial-mesenchyme communication, which may contribute to lung dysmorphogenesis. Indeed, a number of epitopes correlate with structures recognised by FGFs, suggesting a functional consequence of the observed changes in HS in these lungs. These results identify a novel, significant molecular defect in hypoplastic lungs and reveals HS as a potential contributor to hypoplastic lung development in CDH. Finally, these results afford the prospect that HS-mimetic therapeutics could repair defective signalling in hypoplastic lungs, improve lung growth, and reduce CDH mortality.

Eya1 controls cell polarity, spindle orientation, cell fate and Notch signaling in distal embryonic lung epithelium

Cell polarity, mitotic spindle orientation and asymmetric division play a crucial role in the self-renewal/differentiation of epithelial cells, yet little is known about these processes and the molecular programs that control them in embryonic lung distal epithelium. Herein, we provide the first evidence that embryonic lung distal epithelium is polarized with characteristic perpendicular cell divisions. Consistent with these findings, spindle orientation-regulatory proteins Insc, LGN (Gpsm2) and NuMA, and the cell fate determinant Numb are asymmetrically localized in embryonic lung distal epithelium. Interfering with the function of these proteins in vitro randomizes spindle orientation and changes cell fate. We further show that Eya1 protein regulates cell polarity, spindle orientation and the localization of Numb, which inhibits Notch signaling. Hence, Eya1 promotes both perpendicular division as well as Numb asymmetric segregation to one daughter in mitotic distal lung epithelium, probably by controlling aPKCζ phosphorylation. Thus, epithelial cell polarity and mitotic spindle orientation are defective after interfering with Eya1 function in vivo or in vitro. In addition, in Eya1(-/-) lungs, perpendicular division is not maintained and Numb is segregated to both daughter cells in mitotic epithelial cells, leading to inactivation of Notch signaling. As Notch signaling promotes progenitor cell identity at the expense of differentiated cell phenotypes, we test whether genetic activation of Notch could rescue the Eya1(-/-) lung phenotype, which is characterized by loss of epithelial progenitors, increased epithelial differentiation but reduced branching. Indeed, genetic activation of Notch partially rescues Eya1(-/-) lung epithelial defects. These findings uncover novel functions for Eya1 as a crucial regulator of the complex behavior of distal embryonic lung epithelium.

Cell polarity and spindle orientation in the distal epithelium of embryonic lung

A proper balance between self-renewal and differentiation of lung-specific progenitors at the distal epithelial tips is absolutely required for normal lung morphogenesis. Cell polarity and mitotic spindle orientation play a critical role in the self-renewal/differentiation of epithelial cells and can impact normal physiological processes, including epithelial tissue branching and differentiation. Therefore, understanding the behavior of lung distal epithelial progenitors could identify innovative solutions to restoring normal lung morphogenesis. Yet little is known about cell polarity, spindle orientation, and segregation of cell fate determinant in the embryonic lung epithelium, which contains progenitor cells. Herein, we provide the first evidence that embryonic lung distal epithelium is polarized and highly mitotic with characteristic perpendicular cell divisions. Consistent with these findings, mInsc, LGN, and NuMA polarity proteins, which control spindle orientation, are asymmetrically localized in mitotic distal epithelial progenitors of embryonic lungs. Furthermore, the cell fate determinant Numb is asymmetrically distributed at the apical side of distal epithelial progenitors and segregated to one daughter cell in most mitotic cells. These findings provide evidence for polarity in distal epithelial progenitors of embryonic lungs and provide a framework for future translationally oriented studies in this area.

Effects of fibroblast growth factors on the differentiation of the pulmonary progenitors from murine embryonic stem cells

The fibroblast growth factors (FGFs) play an important role in the development of embryonic lung. In this study, we investigated the effects of mainly FGF 1, 2, and 10 at concentrations selected on the basis of data obtained from previous in vitro culture on the derivation of the pulmonary progenitors from murine embryonic stem cells cultured on gelatin or Matrigel-coated plates. For cells cultured on a gelatin-coated plate, high concentrations of FGF1 were found to enhance the expression of mRNAs for SPC and CC10, markers of distal airway epithelium, while high levels of FGF2 decreased the expression of RNAs for not only SPC, CC10 but also for the additional markers SPD and aquaporin 5. FGF10 at all tested concentrations was found to have no effect on the differentiation of pneumocytes when ESCs were grown on gelatin-coated plates. However, when differentiation was performed on Matrigel-coated plates, the addition of 60 ng/ml FGF10 enhanced the expression of pneumocyte markers, suggesting a synergic effect of FGF10 and extracellular matrix. In conclusion, growth factors were proven to be effective in the differentiation of pulmonary progenitors from mESCs. The need of signals from extracellular matrix proteins depends on the growth factors supplemented.

Elf5 is an epithelium-specific, fibroblast growth factor-sensitive transcription factor in the embryonic lung

Fibroblast growth factor (FGF) signaling has been shown to be essential for many aspects of normal lung development. To determine epithelial targets of FGF signaling, we cultured embryonic day (E) 11.5 mouse lungs for 24 hr in the presence or absence of the FGF receptor antagonist SU5402, which inhibited branching morphogenesis. Affymetrix gene chip analysis of treated and control epithelia identified several genes regulated by FGF signaling, including Elf5, a member of the Epithelial-specific Ets family of transcription factors. SU5402 reduced Elf5 expression in mesenchyme-free cultures of E12.5 epithelium, demonstrating that the inhibition was direct. In situ hybridization revealed that Elf5 had a dynamic pattern of expression during lung development. We found that expression of Elf5 was induced by FGF7 and FGF10, ligands that primarily bind FGFR2b. To further define the pathways by which FGFs activate Elf5 expression, we cultured E11.5 lung tips in the presence of compounds to inhibit FGF receptors (SU5402), PI3-Kinase/Akt-mediated signaling (LY294002), and MAP Kinase/Erk-mediated signaling (U0126). We found that SU5402 and LY294002 significantly reduced Elf5 expression, whereas U0126 had no effect. LY294002 also reduced Elf5 expression in cultures of purified epithelium. Finally, pAkt was coexpressed with Elf5 in the proximal epithelial airways of E17.5 lungs. These results demonstrate that Elf5 is an FGF-sensitive transcription factor in the lung with a dynamic pattern of expression and that FGF regulation of Elf5 by means of FGFR2b occurs through the PI3-Kinase/Akt pathway.

A tissue-engineered model of fetal distal lung tissue

In extending our previous studies toward development of an engineered distal lung tissue construct (M. J. Mondrinos, S. Koutzaki, E. Jiwanmall, M. Li, J. P. Dechadarevian, P. I. Lelkes, and C. M. Finck. Tissue Eng 12: 717-728, 2006), we studied the effects of exogenous fibroblast growth factors FGF10, FGF7, and FGF2 on mixed populations of embryonic day 17.5 murine fetal pulmonary cells cultured in three-dimensional collagen gels. The morphogenic effects of the FGFs alone and in various combinations were assessed by whole mount immunohistochemistry and confocal microscopy. FGF10/7 significantly increased epithelial budding and proliferation; however, only FGF10 alone induced widespread budding. FGF7 alone induced dilation of epithelial structures but not widespread budding. FGF2 alone had a similar dilation, but not budding, effect in epithelial structures, and, in addition, significantly enhanced endothelial tubular morphogenesis and network formation, as well as mesenchymal proliferation. The combination of FGF10/7/2 induced robust budding of epithelial structures and the formation of uniform endothelial networks in parallel. These data suggest that appropriate combinations of exogenous FGFs chosen to target specific FGF receptor isoforms will allow for control of lung epithelial and mesenchymal cell behavior in the context of an engineered system. We propose that tissue-engineered fetal distal lung constructs could provide a potential source of tissue or cells for lung augmentation in pediatric pulmonary pathologies, such as pulmonary hypoplasia and bronchopulmonary dysplasia. In addition, engineered systems will provide alternative in vitro venues for the study of lung developmental biology and pathobiology.

Heparin and fibroblast growth factors affect surfactant protein gene expression in type II cells

The stimulation and maintenance of the pulmonary alveolar type II cell's capacity to biosynthesize, store, and secrete surfactant proteins (SPs) are modulated to a great extent by growth factors, extracellular matrix (ECM) components, and hormones. It is possible that differences in ECM composition, as exist between type I and II cells normally or as might occur with excessive cell surface shedding during inflammation or injury states, may specifically alter SP expression. Here, isolated type II cells were exposed to the model sulfated ECM heparin; desulfated heparin; and/or fibroblast growth factor (FGF)-1, -2, or -7 for 24 h to examine by quantitative real-time polymerase chain reaction their effects on SP gene expression. Aquaporin 5 (AQP-5) gene expression was also examined as a phenotypic marker for the type I cell. SP-B mRNA abundance was increased 4- to 8-fold by all three FGFs. Heparin at low concentrations (5 microg/ml) or desulfated heparin at high concentrations (500 microg/ml) enhanced the effects of FGF-2 and -7, while high heparin concentrations (500 microg/ml) were inhibitory. In contrast, SP-B mRNA abundance was increased by heparin in a dose- and sulfation-dependent manner when used in combination with FGF-1. SP-C and AQP-5 mRNA levels were increased by heparin alone in a dose- and sulfation-dependent manner, while all FGFs lacked effect on SP-C or AQP-5 mRNA levels. These data indicate that heparin can be stimulatory to SP gene expression depending on concentration, degree of sulfation, and surrounding FGF environment, and that heparin plays a significant role in modulating alveolar epithelial cell phenotype in vitro.

Growth factors in lung development

Organized and coordinated lung development follows transcriptional regulation of a complex set of cell-cell and cell-matrix interactions resulting in a blood-gas interface ready for physiologic gas exchange at birth. Transcription factors, growth factors, and various other signaling molecules regulate epithelial-mesenchymal interactions by paracrine and autocrine mechanisms. Transcriptional control at the earliest stages of lung development results in cell differentiation and cell commitment in the primitive lung bud, in essence setting up a framework for pattern formation and branching morphogenesis. Branching morphogenesis results in the formation of the conductive airway system, which is critical for alveolization. Lung development is influenced at all stages by spatial and temporal distribution of various signaling molecules and their receptors and also by the positive and negative control of signaling by paracrine, autocrine, and endocrine mechanisms. Lung bud formation, cell differentiation, and its interaction with the splanchnic mesoderm are regulated by HNF-3beta, Shh, Nkx2.1, HNF-3/Forkhead homolog-8 (HFH-8), Gli, and GATA transcription factors. HNF-3beta regulates Nkx2.1, a transcription factor critical to the formation of distal pulmonary structures. Nkx2.1 regulates surfactant protein genes that are important for the development of alveolar stability at birth. Shh, produced by the foregut endoderm, regulates lung morphogenesis signaling through Gli genes expressed in the mesenchyme. FGF10, produced by the mesoderm, regulates branching morphogenesis via its receptors on the lung epithelium. Alveolization and formation of the capillary network are influenced by various factors that include PDGF, vascular endothelial growth factor (VEGF), and retinoic acid. Epithelial-endothelial interactions during lung development are important in establishing a functional blood-gas interface. The effects of various growth factors on lung development have been demonstrated by gain- or loss-of-function studies in null mutant and transgenic mice models. Understanding the role of growth factors and various other signaling molecules and their cellular interactions in lung development will provide us with new insights into the pathogenesis of bronchopulmonary dysplasia and disorders of lung morphogenesis.

Hypoxia-induced mitogenic factor modulates surfactant protein B and C expression in mouse lung

Previous studies have demonstrated a robust pulmonary expression of hypoxia-induced mitogenic factor (HIMF) during the perinatal period, when surfactant protein (SP) synthesis begins. We hypothesized that HIMF modulates SP expression and participates in lung development and maturation. The temporal-spatial expression of HIMF, SP-B, and SP-C in developing mouse lungs was examined by immunohistochemical staining, Western blot, and RT-PCR. The expression and localization of SP-B and SP-C were investigated in mouse lungs after intratracheal instillation of HIMF in adult mice. The effects of HIMF on SP-B and SP-C transcription activity, and on mRNA degradation, were investigated in mouse lung epithelial (MLE)-12 and C10 cells using the promoter-luciferase reporter assay and actinomycin D incubation. The activation of Akt, extracellular signal-regulated kinase (ERK)1/2, and p38 mitogen-activated protein kinase was explored by Western blot. Intratracheal instillation of HIMF resulted in significant increases of SP-B and SP-C production, predominantly localized to alveolar type II cells. In MLE-12 and C10 cells, HIMF enhanced SP-B and SP-C mRNA levels in a dose-dependent manner. Meanwhile, HIMF increased transcription activity and prevented actinomycin D-facilitated SP-B and SP-C mRNA degradation in MLE-12 cells. Incubation of cells with LY294002, PD098059, or U0126 abolished HIMF-induced Akt and ERK1/2 phosphorylation and suppressed HIMF-induced SP-B and SP-C production, whereas SB203580 had no effect. These results indicate that HIMF induces SP-B and SP-C production in mouse lungs and alveolar type II-like cell lines via activations of phosphatidylinositol 3-kinase/Akt and ERK1/2 mitogen-activated protein kinase, suggesting that HIMF plays critical roles in lung development and maturation.

PI3K-AKT pathway mediates growth and survival signals during development of fetal mouse lung

We examined the roles of the PI3K-AKT signalling pathway in fetal lung development. By Western blotting, phosphorylated AKT (pAKT) was highly expressed in fetal days 12 and 14 with decreased expression thereafter. By immunohistochemistry, pAKT was expressed mainly in the respiratory epithelium of early fetal days. We examined the effects of fibroblast growth factor 1 (FGF1), PI3K inhibitors (LY294002 and wortmannin), MAPK inhibitor (PD98059) and both of FGF1 and each inhibitor on lung morphogenesis, BrdU incorporation and apoptosis. In the FGF1-treated explants, the number of terminal buds and BrdU-labelled cells increased significantly, while the LY294002-, wortmannin-, PD98059-treated explants demonstrated obvious decreases. The effects by FGF1 were inhibited by LY294002, wortmannin and PD98059. Regardless of the presence of FGF1, the LY294002-, wortmannin- and PD98059-treated explants increased apoptosis revealed by terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling assay in the mesenchyme of the explants. At the same time, the effect of LY294002, wortmannin, PD98059 on expression of surfactant apoprotein C (SPC) were also studied. The LY294002 and wortmannin treatments showed decreased expression of SPC. These findings suggest that the PI3K-AKT signalling pathway plays a pivotal role in mouse lung development through various biological processes.

Hypoxia-responsive growth factors upregulate periostin and osteopontin expression via distinct signaling pathways in rat pulmonary arterial smooth muscle cells

This study tested the hypothesis that expression of the novel adhesion molecule periostin (PN) and osteopontin (OPN) is increased in lung and in isolated pulmonary arterial smooth muscle cells (PASMCs) in response to the stress of hypoxia and explored the signaling pathways involved. Adult male rats were exposed to 10% O2 for 2 wk, and growth-arrested rat PASMCs were incubated under 1% O2 for 24 h. Hypoxia increased PN and OPN mRNA expression in rat lung. In PASMCs, hypoxia increased PN but not OPN expression. The hypoxia-responsive growth factors fibroblast growth factor-1 (FGF-1) and angiotensin II (ANG II) caused dose- and time-dependent increases in PN and OPN expression in PASMCs. FGF-1-induced PN expression was blocked by the FGF-1 receptor antagonist PD-166866 and by inhibitors of phosphatidylinositol 3-kinase (PI3K) (LY-294002, wortmannin), p70S6K (rapamycin), MEK1/2 (U-0126, PD-98059), and p38MAPK (SB-203580) but not of JNK (SP-600125). ANG II-induced PN expression was blocked by the AT(1)-receptor antagonist losartan and by inhibitors of PI3K and MEK1/2. In contrast, FGF-1-induced OPN expression was blocked by inhibitors of JNK or MEK1/2 but not of PI3K, p70S6K, or p38MAPK. Activation of p70S6K and p38MAPK by anisomycin robustly stimulated PN but not OPN expression. This study is the first to demonstrate that growth factor-induced expression of PN in PASMCs is mediated through PI3K/p70S6K, Ras/MEK1/2, and Ras/p38MAPK signaling pathways, whereas the expression of OPN is mediated through Ras/MEK1/2 and Ras/JNK signaling pathways. These differences in signaling suggest that PN and OPN may play different roles in pulmonary vascular remodeling under pathophysiological conditions.

Effect of eosinophil adhesion on intracellular signaling in cholinergic nerve cells

Eosinophil localization to cholinergic nerves occurs in a variety of inflammatory conditions, including asthma. This localization is mediated by interactions between eosinophil integrins and neuronal vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1). Eosinophil-nerve cell interactions lead to generation of neuronal reactive oxygen species and release of eosinophil proteins. The effects of eosinophil adhesion on neuronal intracellular signaling pathways were investigated. Eosinophil adhesion to IMR32 cholinergic nerves led to a rapid and sustained activation of the nuclear transcription factors nuclear factor (NF)-kappaB and activator protein (AP)-1 in the nerve cells. Eosinophil binding to neuronal ICAM-1 led to a rapid activation of ERK1/2 in nerve cells. Inhibition of ERK1/2 prevented NF-kappaB activation. Eosinophil adhesion to VCAM-1 resulted in AP-1 activation, mediated partially by rapid activation of the p38 mitogen-activated protein kinase. These data show that adhesion of eosinophils induces mitogen-activated protein kinase-dependent activation of the transcription factors NF-kappaB and AP-1 in nerve cells, indicating that eosinophil adhesion may control nerve growth and phenotype.

Fibroblast growth factor mediates hypoxia-induced endothelin-- a receptor expression in lung artery smooth muscle cells

We have previously demonstrated that endothelin (ET)-1 and its subtype A receptor (ET-AR) expression are increased in lung under hypoxic conditions and that activation of ET-AR by ET-1 is a major mediator of hypoxia-induced pulmonary hypertension in the rat. The present study tested the hypothesis that the hypoxia-responsive tyrosine kinase receptor-activating growth factors fibroblast growth factor (FGF)-1, FGF-2, and platelet-derived growth factor (PDGF)-BB stimulate expression of the ET-AR in pulmonary arterial smooth muscle cells (PASMCs). Quiescent rat PASMCs were incubated under hypoxia (1% O2), or with FGF-1, FGF-2, PDGF-BB, vascular endothelial growth factor, ET-1, angiotensin II, or atrial natriuretic peptide under normoxic conditions for 24 h. FGF-1 and -2 and PDGF-BB, but not hypoxia, vascular endothelial growth factor, ET-1, angiotensin II, or atrial natriuretic peptide, significantly increased ET-AR mRNA levels. FGF-1-induced ET-AR expression was inhibited by FGF-receptor inhibitor PD-166866, MEK inhibitor U-0126, transcription inhibitor actinomycin D, and translation inhibitor cycloheximide. In contrast, the stimulatory effect of FGF-1 on ET-AR mRNA expression was not altered by PI3 kinase, PKA, PKC, or adenylate cyclase inhibitors. PASMC ET-AR gene transcription, assessed by nuclear-runoff analysis, was increased by FGF-1. These results provide novel finding that ET-AR in PASMCs in vitro is unresponsive to hypoxia per se but is robustly simulated by tyrosine kinase receptor-associated growth factors (FGF-1, FGF-2, PDGF-BB) that themselves are stimulated by hypoxia in lung. This observation suggests a novel signaling mechanism that may be responsible for overexpression of ET-AR in lung, and may contribute to the hypoxia-induced pulmonary vasoconstriction, hypertension, and vascular remodeling in hypoxia-adapted animal.

BMP4 modulates fibroblast growth factor-mediated induction of proximal and distal lung differentiation in mouse embryonic tracheal epithelium in mesenchyme-free culture

Lung morphogenesis and differentiation require interaction between the epithelium and mesenchyme, which is mediated by diffusible molecules such as fibroblast growth factors (FGFs), bone morphogenetic protein 4 (BMP4), and Shh. We have used mesenchyme-free culture to study the effects of these molecules on lung epithelial differentiation. We have tested the individual abilities of FGF1, FGF2, FGF7, FGF9, FGF10, and FGF18, as well as BMP4 and Shh to promote growth and specify distal lung differentiation in mouse tracheal epithelium. The different FGFs exhibited distinct abilities to induce epithelial growth and the expression of the distal lung epithelial marker, surfactant protein C (SP-C), although all FGFs were able to induce expression of BMP4. Tracheal epithelium treated with FGF10 showed little growth and failed to express SP-C as measured by whole-mount in situ hybridization and quantitative real-time polymerase chain reaction. FGF1 treatment resulted in the strongest induction of SP-C. Treatment with BMP4 inhibited epithelial growth and differentiation and antagonized the stimulatory effects of FGF1. In contrast, inhibition of endogenous BMP4 signaling with Noggin protein did not inhibit growth or expression of SP-C but did increase the expression of the proximal lung markers CCSP and HFH4. Expression of Shh was not affected by any of the conditions tested. These results suggest that BMP4 does not signal epithelial cells to adopt a distal fate but may regulate the expansion of proximal epithelial cells in the lung.

Molecular regulation of lung development

There is increasing evidence suggesting that formation of the tracheobronchial tree and alveoli results from heterogeneity of the epithelial-mesenchymal interactions along the developing respiratory tract. Recent genetic data support this idea and show that this heterogeneity is likely the result of activation of distinct networks of signaling molecules along the proximal-distal axis. Among these signals, fibroblast growth factors, retinoids, Sonic hedgehog, and transforming growth factors appear to play prominent roles. We discuss how these and other pattern regulators may be involved in initiation, branching, and differentiation of the respiratory system.

Phosphoinositide 3-kinase: a critical signalling event in pulmonary cells

Phosphoinositide 3-kinases (PI-3Ks) are enzymes that generate lipid second messenger molecules, resulting in the activation of multiple intracellular signalling cascades. These events regulate a broad array of cellular responses including survival, activation, differentiation and proliferation and are now recognised to have a key role in a number of physiological and pathophysiological processes in the lung. PI-3Ks contribute to the pathogenesis of asthma by influencing the proliferation of airways smooth muscle and the recruitment of eosinophils, and affect the balance between the harmful and protective responses in pulmonary inflammation and infection by the modulation of granulocyte recruitment, activation and apoptosis. In addition they also seem to exert a critical influence on the malignant phenotype of small cell lung cancer. PI-3K isoforms and their downstream targets thus provide novel therapeutic targets for intervention in a broad spectrum of respiratory diseases.