pRb is necessary for inhibition of N-myc expression by TGF-beta 1 in embryonic lung organ cultures

Inhibitory morphogens and monopodial branching of the embryonic chicken lung

BACKGROUND

Branching morphogenesis generates a diverse array of epithelial patterns, including dichotomous and monopodial geometries. Dichotomous branching can be instructed by concentration gradients of epithelial-derived inhibitory morphogens, including transforming growth factor-β (TGFβ), which is responsible for ramification of the pubertal mammary gland. Here, we investigated the role of autocrine inhibitory morphogens in monopodial branching morphogenesis of the embryonic chicken lung.

RESULTS

Computational modeling and experiments using cultured organ explants each separately revealed that monopodial branching patterns cannot be specified by a single epithelial-derived autocrine morphogen gradient. Instead, signaling by means of TGFβ1 and bone morphogenetic protein-4 (BMP4) differentially affect the rates of branching and growth of the airways. Allometric analysis revealed that development of the epithelial tree obeys power-law dynamics; TGFβ1 and BMP4 have distinct but reversible effects on the scaling coefficient of the power law.

CONCLUSIONS

These data suggest that although autocrine inhibition cannot specify monopodial branching, inhibitory morphogens define the dynamics of lung morphogenesis.

ALK-5 mediates endogenous and TGF-beta1-induced expression of connective tissue growth factor in embryonic lung

Transforming growth factor-beta1 (TGF-beta1) has been implicated as a major negative regulator of lung branching morphogenesis. Since connective tissue growth factor (CTGF) is a downstream mediator of TGF-beta1 effects on mesenchymal cells, we hypothesized that TGF-beta1 induces CTGF expression in mouse embryonic lung explants and that CTGF mediates TGF-beta1 inhibition of branching morphogenesis. We show that addition of TGF-beta1 to the serum-free medium of embryonic day (E)12.5 lung explant cultures inhibited branching morphogenesis and induced CTGF mRNA expression in time- and dose-dependent manners. In contrast to basal endogenous CTGF protein, which was exclusively localized in the distal airway epithelium, TGF-beta1-induced CTGF protein was localized in both the epithelium and the mesenchyme. Addition of exogenous CTGF to culture medium directly inhibited branching morphogenesis. To identify the signal transduction pathway through which TGF-beta1 induces CTGF, we used SB431542, a specific inhibitor for TGF-beta type I receptor (TbetaRI)/ALK-5 to block TGF-beta1-induced Smad2/3 phosphorylation. Consequently, SB431542 stimulated normal branching morphogenesis and blocked TGF-beta1 inhibition of branching. Furthermore, SB-431542 blocked both endogenous and TGF-beta1-induced expression of CTGF mRNA and protein. These results demonstrate for the first time that TGF-beta1 induces CTGF expression in mouse embryonic lung explants, that CTGF inhibits branching morphogenesis, and that both endogenous and TGF-beta1-induced CTGF expression are mediated by the TbetaRI/ALK-5-dependent Smad2 signaling pathway.

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.

Nkx2.1 transcription factor in lung cells and a transforming growth factor-beta1 heterozygous mouse model of lung carcinogenesis

The Nkx2.1 homeobox gene and transforming growth factor-beta1 (TGF-beta1) are essential for organogenesis and differentiation of the mouse lung. NKX2.1 is a marker of human lung carcinomas, but it is not known whether this gene participates in early tumorigenesis. Addition of TGF-beta1 to TGF-beta1-responsive nontumorigenic mouse lung cells cotransfected with a NKX2.1Luc luciferase reporter and either a Sp1 or Sp3 plasmid showed a significant increase or decrease, respectively, in NKX2.1Luc transcription. Cotransfection of Sp3 and dominant-negative TGF-beta type II receptor plasmids negated the effect of Sp1. Cotransfected Sp1 plasmid with either dominant-negative Smad2 or Smad3 or Smad4 plasmids significantly decreased NKX2.1Luc transcription. Electrophoretic mobility shift assays revealed binding of Sp1 and Smad4 to the NKX2.1 promoter. With a TGF-beta1 heterozygous mouse model, Nkx2.1 mRNA and protein in lungs of TGF-beta1 heterozygous mice were significantly lower compared to wildtype (WT) littermates. Competitive reverse transcription (RT)-polymerase chain reaction (PCR) and immunostaining showed that Nkx2.1 mRNA and protein decreased significantly in adenomas and adenocarcinomas compared to normal lung tissue. Our in vitro data showed that regulation of Nkx2.1 by TGF-beta1 occurs through TGF-beta type II receptor and Smad signaling, with Sp1 and Sp3 in lung cells. Our in vivo data showed reduced Nkx2.1 in lungs of TGF-beta1 heterozygous mice compared to WT mice, that is detectable in adenomas, and that is further reduced in carcinogenesis, and that correlates with reduction of Sp1, Sp3, and Smads in lung adenocarcinomas. Our findings suggest that reduced Nkx2.1 and TGF-beta1 signaling components may contribute to tumorigenesis in the lungs of TGF-beta1 heterozygous mice.

Sonic hedgehog signaling regulates Gli3 processing, mesenchymal proliferation, and differentiation during mouse lung organogenesis

Lack of Sonic hedgehog (Shh) signaling, mediated by the Gli proteins, leads to severe pulmonary hypoplasia. However, the precise role of Gli genes in lung development is not well established. We show Shh signaling prevents Gli3 proteolysis to generate its repressor forms (Gli3R) in the developing murine lung. In Shh(-/-) or cyclopamine-treated wild-type (WT) lung, we found that Gli3R level is elevated, and this upregulation appears to contribute to defects in proliferation and differentiation observed in the Shh(-/-) mesenchyme, where Gli3 is normally expressed. In agreement, we found Shh(-/-);Gli3(-/-) lungs exhibit enhanced growth potential. Vasculogenesis is also enhanced; in contrast, bronchial myogenesis remains absent in Shh(-/-);Gli3(-/-) compared with Shh(-/-) lungs. Genes upregulated in Shh(-/-);Gli3(-/-) relative to Shh(-/-) lung include Wnt2 and, surprisingly, Foxf1 whose expression has been reported to be Shh-dependent. Cyclins D1, D2, and D3 antibody labelings also reveal distinct expression patterns in the normal and mutant lungs. We found significant repression of Tbx2 and Tbx3, both linked to inhibition of cellular senescence, in Shh(-/-) and partial derepression in Shh(-/-); Gli3(-/-) lungs, while Tbx4 and Tbx5 expressions are less affected in the mutants. Our findings shed light on the role of Shh signaling on Gli3 processing in lung growth and differentiation by regulating several critical genes.

E2F proteins regulate MYCN expression in neuroblastomas

Amplification of the MYCN gene, resulting in overexpression of MYCN, distinguishes a subset of neuroblastomas with poor prognosis. The transcription factors driving MYCN expression in neuroblastomas are unknown. In transient-transfection assays, E2F-1, E2F-2, and E2F-3 activate a MYCN reporter construct dependent on the presence of several putative E2F-binding sites. Using chromatin immunoprecipitation, we show that E2F-1, E2F-2, and E2F-3 bind to the proximal MYCN promoter in vivo, specifically in neuroblastoma cell lines expressing MYCN. Inhibition of E2F activity in MYCN-amplified cells by the overexpression of p16(INK4A) reduced MYCN expression. In addition, we provide evidence that E2F proteins are involved in the negative regulation of MYCN by TGF-beta and retinoic acid. These data suggest that E2F transcription factors are critical for both the full activation and the repression of MYCN in neuroblastomas.

Signaling to the epithelium is not sufficient to mediate all of the effects of transforming growth factor beta and bone morphogenetic protein 4 on murine embryonic lung development

Many studies have suggested that transforming growth factor beta (TGF-beta) and bone morphogenetic protein 4 (Bmp4) regulate early development of the lung. In this study, administration of growth factors directly into the lumen of lungs grown in organ culture was used to limit their activity to the epithelium and test the hypothesis that signaling to the epithelium is sufficient to mediate the known effects of TGF-beta and BMP-4 on early lung development. Addition of TGF-beta1, beta2, or beta3 to the medium surrounding lungs grown in organ culture resulted in decreased branching, reduced cell proliferation, accumulation of alpha-smooth muscle actin protein (alpha-SMA) in the mesenchyme, and decreased expression of a marker for respiratory epithelium, surfactant protein-C (Sp-C). When TGF-beta1 was restricted to the epithelium, accumulation of alpha-SMA and inhibition of Sp-C expression were not observed but branching and proliferation were inhibited. In contrast, branching was not inhibited in lungs where TGF-beta2 or TGF-beta3 were restricted to the epithelium suggesting differences in the mechanism of signaling by TGF-beta1, TGF-beta2 or TGF -beta3 in lung. Addition of Bmp4 to the medium surrounding lungs grown in organ culture stimulated cell proliferation and branching morphogenesis; however, direct injection of Bmp4 into the lung lumen had no effect on proliferation or branching. Based on these data and data from mesenchyme-free cultures, we propose that the mesenchyme influences growth factor signaling in the lung.

Functional mammary gland development and oncogene-induced tumor formation are not affected by the absence of the retinoblastoma gene

Loss of cell cycle regulation in mammary epithelium results in impaired mammary gland development and neoplasia. We investigated the consequences of the absence of pRb in mammary epithelial cells during normal development and in mice that express an oncogene in the mammary epithelium. Since pRb-deficiency results in embryonic lethality, we transplanted pRb-null mammary anlagen into wild hosts. pRb-deficient mammary epithelia were capable of functional differentiation in term animals and they regenerated a differentiated gland even after multiple pregnancies. In serial transplantations no significant differences were found in outgrowth of pRb-deficient and wild type epithelia indicating that the absence of pRb does not lead to transformation. Likewise the effect of a TGFbeta1 transgene was not altered in the absence of pRb. The susceptibility of mammary epithelium to form tumors was assessed in three different models. No differences in tumor incidence were found between wild type and Rb +/- WAP-int3, MMTV-PyMT transgenic and Brcal-/- epithelia. These results demonstrate that the absence of pRb does not affect normal mammary gland development and tumorigenesis in three different mouse models investigated and suggest that loss of more than one member of the pRb pathway is required to induce mammary tumors.

TGF-beta1 regulates the expression of multiple max-interacting transcription factors in Balb/MK cells: implications for understanding the mechanism of action of TGF-beta1

Appropriate transforming growth factor-beta1 (TGF-beta1) signaling is required to preserve homeostasis of diverse tissues during development. At the cellular level, one function of TGF-beta1 that is critical for preserving homeostasis is the ability to arrest cell growth. TGF-beta1 arrests growth by blocking the function of the c-myc proto-oncogene. c-myc function is determined by the level of c-myc expression relative to other Max-interacting transcription factors, and TGF-beta1 has been shown to inhibit c-myc expression by inhibiting c-myc transcription. However, whether TGF-beta1 might also increase the expression of a Max-interacting factor that blocks myc function by competing with myc for Max binding is not known. Therefore, we determined the effect of TGF-beta1 on the expression of Max-interacting transcription factors in Balb/MK cells. We found unexpectedly that Balb/MK cells express both N-myc and c-myc. The pattern of N-myc expression during the cell cycle differs from that of c-myc, indicating that mRNA accumulation is controlled by mechanisms specific to each gene. TGF-beta1 rapidly inhibits N-myc mRNA expression; thus N-myc is a novel target of TGF-beta1 in Balb/MK cells. More importantly, we found that TGF-beta1 induces the expression of the putative tumor suppressor genes Mad4 and Mxi1 in both the Balb/MK and Mv1Lu cell lines. Mad4 and Mxi1 are novel targets of TGF-beta1, known to inhibit cell growth by antagonizing the interaction of Myc with Max. Thus, our results suggest that the induction of Mad4 and Mxi1 may function in tandem with the inhibition of N-myc and c-myc to mediate the growth inhibitory function of TGF-beta1.

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.

Hepatitis C virus core protein transactivates insulin-like growth factor II gene transcription through acting concurrently on Egr1 and Sp1 sites

The possibility that hepatitis C virus core gene product (HCV-core) acts as a transactivator in insulin-like growth factor II (IGF-II) gene transcription was tested. HCV-core protein increases endogenous IGF-II expression from promoter 4 (P4) of the IGF-II gene through two cis-acting elements: Sp1 and Egr1 binding sites. Sp1 and Egr1 both bind to IGF-II P4 and functionally cooperate in mediating the maximal activity of IGF-II P4. HCV-core protein induced the binding of Sp1 and Egr1 on its binding sites on IGF-II P4. In addition, Sp1 and Egr1 were stimulated to phosphorylate by HCV-core, and its DNA binding activity was up-regulated upon HCV-core transfection. Transfection with HCV-core in HepG2 cells stimulated the membrane translocation of protein kinase C (PKC) and the treatment of HCV-core transfected cells with calphostin C, a PKC inhibitor, blocked induction of Sp1 and Egr1 DNA binding activity, and eventually transcriptional transactivations of the IGF-II gene. Increasing the DNA binding activity of the phosphorylated form of Sp1 and Egr1 might be an important mechanism for regulating IGF-II gene expression and for promoting cell division during hepatic carcinogenesis. These results indicate that HCV-core functions as a positive regulator of IGF-II transcription through the PKC pathway and that Sp1 and Egr1 are direct targets of the transcriptional regulation of the IGF-II gene which plays an important role in hepatitis C virus pathogenesis during the formation of hepatocellular carcinoma (HCC).

Mesenchymal expression of nuclear factor-kappaB inhibits epithelial growth and branching in the embryonic chick lung

It is becoming increasingly recognized that the ubiquitous, inducible transcription factor nuclear factor-kappaB (NF-kappaB) is involved in developmental processes. For example, NF-kappaB acts as a mediator of epithelial-mesenchymal interactions in the developing chick limb. We investigated the role of NF-kappaB in directing the branching morphogenesis of the developing chick lung, a process which relies on epithelial-mesenchymal communication. High level expression of relA was found in the mesenchyme surrounding the nonbranching structures of the lung but was not detected either in the mesenchyme surrounding the branching structures of the distal lung or in the developing lung epithelium. Specific inhibition of mesenchymal NF-kappaB in lung cultures resulted in increased epithelial budding. Conversely, expression of a trans-dominant activator of NF-kappaB in the lung mesenchyme repressed budding. Ectopic expression of RelA was sufficient to inhibit the ability of the distal mesenchyme to induce epithelial bud formation. Cellular proliferation in the mesenchyme was inhibited by hyperactivation of NF-kappaB in the mesenchyme of lung cultures. Interestingly, increased NF-kappaB activity in the mesenchyme also decreased the proliferation of the associated epithelium, while inhibition of NF-kappaB activity increased cellular proliferation in lung cultures. Expression patterns of several genes which are known to influence lung branching morphogenesis were altered in response to changes in mesenchymal NF-kappaB activity, including fgf10, bmp-4, and tgf-beta1. Thus NF-kappaB represents the first transcription factor reported to function within the lung mesenchyme to limit growth and branching of the adjacent epithelium.

Smad7 and Smad6 differentially modulate transforming growth factor beta -induced inhibition of embryonic lung morphogenesis

Transforming growth factors beta (TGF-beta) are known negative regulators of lung development, and excessive TGF-beta production has been noted in pulmonary hypoplasia associated with lung fibrosis. Inhibitory Smad7 was recently identified to antagonize TGF-beta family signaling by interfering with the activation of TGF-beta signal-transducing Smad complexes. To investigate whether Smad7 can regulate TGF-beta-induced inhibition of lung morphogenesis, ectopic overexpression of Smad7 was introduced into embryonic mouse lungs in culture using a recombinant adenovirus containing Smad7 cDNA. Although exogenous TGF-beta efficiently reduced epithelial lung branching morphogenesis in control virus-infected lung culture, TGF-beta-induced branching inhibition was abolished after epithelial transfer of the Smad7 gene into lungs in culture. Smad7 also prevented TGF-beta-mediated down-regulation of surfactant protein C gene expression, a marker of bronchial epithelial differentiation, in cultured embryonic lungs. Moreover, we found that Smad7 transgene expression blocked Smad2 phosphorylation induced by exogenous TGF-beta ligand in lung culture, indicating that Smad7 exerts its inhibitory effect on both lung growth and epithelial cell differentiation through modulation of TGF-beta pathway-restricted Smad activity. However, the above anti-TGF-beta signal transduction effects were not observed in cultured embryonic lungs with Smad6 adenoviral gene transfer, suggesting that Smad7 and Smad6 differentially regulate TGF-beta signaling in developing lungs. Our data therefore provide direct evidence that Smad7, but not Smad6, prevents TGF-beta-mediated inhibition of both lung branching morphogenesis and cytodifferentiation, establishing the mechanistic basis for Smad7 as a novel target to ameliorate aberrant TGF-beta signaling during lung development, injury, and repair.

The molecular basis of lung morphogenesis

To form a diffusible interface large enough to conduct respiratory gas exchange with the circulation, the lung endoderm undergoes extensive branching morphogenesis and alveolization, coupled with angiogenesis and vasculogenesis. It is becoming clear that many of the key factors determining the process of branching morphogenesis, particularly of the respiratory organs, are highly conserved through evolution. Synthesis of information from null mutations in Drosophila and mouse indicates that members of the sonic hedgehog/patched/smoothened/Gli/FGF/FGFR/sprouty pathway are functionally conserved and extremely important in determining respiratory organogenesis through mesenchymal-epithelial inductive signaling, which induces epithelial proliferation, chemotaxis and organ-specific gene expression. Transcriptional factors including Nkx2.1, HNF family forkhead homologues, GATA family zinc finger factors, pou and hox, helix-loop-helix (HLH) factors, Id factors, glucocorticoid and retinoic acid receptors mediate and integrate the developmental genetic instruction of lung morphogenesis and cell lineage determination. Signaling by the IGF, EGF and TGF-beta/BMP pathways, extracellular matrix components and integrin signaling pathways also directs lung morphogenesis as well as proximo-distal lung epithelial cell lineage differentiation. Soluble factors secreted by lung mesenchyme comprise a 'compleat' inducer of lung morphogenesis. In general, peptide growth factors signaling through cognate receptors with tyrosine kinase intracellular signaling domains such as FGFR, EGFR, IGFR, PDGFR and c-met stimulate lung morphogenesis. On the other hand, cognate receptors with serine/threonine kinase intracellular signaling domains, such as the TGF-beta receptor family are inhibitory, although BMP4 and BMPR also play key inductive roles. Pulmonary neuroendocrine cells differentiate earliest in gestation from among multipotential lung epithelial cells. MASH1 null mutant mice do not develop PNE cells. Proximal and distal airway epithelial phenotypes differentiate under distinct transcriptional control mechanisms. It is becoming clear that angiogenesis and vasculogenesis of the pulmonary circulation and capillary network are closely linked with and may be necessary for lung epithelial morphogenesis. Like epithelial morphogenesis, pulmonary vascularization is subject to a fine balance between positive and negative factors. Angiogenic and vasculogenic factors include VEGF, which signals through cognate receptors flk and flt, while novel anti-angiogenic factors include EMAP II.

TGF-beta3-null mutation does not abrogate fetal lung maturation in vivo by glucocorticoids

Newborn transforming growth factor (TGF)-beta3-null mutant mice exhibit defects of palatogenesis and pulmonary development. Glucocorticoids, which play a central role in fetal lung maturation, have been postulated to mediate their stimulatory effects on tropoelastin mRNA expression through TGF-beta3 in cultured lung fibroblasts. In the present study, we analyzed the abnormally developed lungs in TGF-beta3-null mutant mice and compared the effects of glucocorticoids on gene expression and lung morphology between TGF-beta3 knockout and wild-type mice. Lungs of TGF-beta3-null mutant mice on embryonic day 18.5 did not form normal saccular structures and had a thick mesenchyme between terminal air spaces. Moreover, the number of surfactant protein C-positive cells was decreased in TGF-beta3-null mutant lungs. Interestingly, glucocorticoids were able to promote lung maturation and increased expression of both tropoelastin and fibronectin but decreased the relative number of surfactant protein C-positive cells in fetal lungs of both genotypes. This finding provides direct evidence that glucocorticoid signaling in the lung can use alternative pathways and can exert its effect without the presence of TGF-beta3.

Cloning of rat fibrillin-2 cDNA and its role in branching morphogenesis of embryonic lung

Fibrillin-2 is an extracellular matrix protein. It is associated with elastic fibers in several tissues and is believed to serve as a ligand for alphavbeta3 integrin, the latter being a known morphogen. In this study, the role of fibrillin-2 in lung development was investigated. Also, rat fibrillin-2 cDNA was isolated and sequenced and its spatiotemporal expression determined. It had approximately 88% homology with human fibrillin-2 and had Ca(2+) binding epidermal growth factor-like domains, transforming growth factor-beta binding protein motifs, and two RGD binding sites. Northern blot analysis revealed an approximately 10-kb transcript, and fibrillin-2 expression was developmentally regulated, and it paralleled that of tropoelastin. At day 13 of gestation, fibrillin-2 was expressed in the mesenchyme and at the epithelial:mesenchymal interface. From day 13 to 19 of gestation, its expression intensified and was confined around the tracheobronchial airways, while it lessened during the postnatal period. Immunoprecipitation revealed an approximately 350-kDa band by SDS-PAGE. Treatment with fibrillin-2 antisense oligodeoxynucleotide induced dysmorphogenesis of the lung explants. They were smaller and had rudimentary lung bud branches, collapsed conducting airways, and loose expanded mesenchyme. Concomitantly, fibrillin-2 mRNA, antibody reactivity in the explants, and fibrillin-2-specific radioincorporation were reduced. Anti-alphav and -laminin antibody reactivity and their respective incorporated specific radioactivities were unaltered. These data indicate that fibrillin-2 modulates organogenesis of the lung in the context of epithelial:mesenchymal interactions. Conceivably, the collapse of the conducting airways may also be related to the perturbed biology of the fibrillin-2 interacting protein, i.e., elastin, the latter being critical for the normal biophysiology of the lungs.

Commitment and differentiation of lung cell lineages

To form a large diffusible interface capable of conducting respiratory gases to and from the circulation, the lung must undergo extensive cell proliferation, branching morphogenesis, and alveolar saccule formation, to generate sufficient surface area. In addition, the cells must differentiate into at least 40 distinct lung cell lineages. Specific transcriptional factors, peptide growth factor receptor-mediated signaling pathways, extracelluar matrix components, and integrin-signaling pathways interact to direct lung morphogenesis and lung cell lineage differentiation. Branching mutants of the respiratory tracheae in Drosophila have identified several functionally conserved genes in the fibroblast growth factor signaling pathway that also regulate pulmonary organogenesis in mice and probably also in man. Key transcriptional factors including Nkx2.1, hepatocyte nuclear factor family forkhead homologues, GATA family zinc finger factors, pou and homeodomain proteins, as well as basic helix-loop-helix factors, serve as master genes to integrate the developmental genetic instruction of lung morphogenesis and cell lineage determination. Key words: lung branching morphogenesis, lung cell proliferation, lung cell differentiation, alveolization, master genes, peptide growth factor signaling, extracellular matrix signaling, mesenchyme induction, alveolar epithelial cells, pulmonary neuroendocrine cells, stem cells, retinoic acid.

Abrogation of betaglycan attenuates TGF-beta-mediated inhibition of embryonic murine lung branching morphogenesis in culture

Although betaglycan (TGF-beta type III receptor) is known to enhance TGF-beta ligand binding to its type II receptor in murine lung epithelial cell lines, the biological significance of this phenomenon in the process of lung organogenesis is not understood. Betaglycan gene expression was detected in embryonic murine lungs undergoing branching morphogenesis in ex vivo culture. Antisense betaglycan oligodeoxynucleotides (ODN) resulted in up to 56% stimulation of lung branching morphogenesis in culture, while betaglycan mRNA and protein expression levels were suppressed by 90 and 82%, respectively. Following abrogation of betaglycan expression with antisense oligodeoxynucleotide, embryonic lungs were relatively insensitive to TGF-beta: TGF-beta2 (0.5 ng/ml) and TGF-beta1 (20 ng/ml), respectively, down-regulated lung morphogenesis by 38 and 34% in control cultures, whereas TGF-beta-induced inhibition was attenuated to 13 and 26% respectively, in the presence of betaglycan antisense oligodeoxynucleotides. TGF-beta neutralizing antibodies also prevented TGF-beta-mediated inhibition of lung branching in culture, supporting the speculation that autocrine/paracrine TGF-beta signaling is minimal in the absence of betaglycan. Betaglycan was immunolocalized mainly to the epithelial cells in developing airways, a spatial distribution which overlaps with that of TGF-beta type II receptor. Furthermore, abrogation of endogenous betaglycan gene expression prevented the characteristic down-regulation of cyclin A and surfactant protein C (SP-C) mRNAs by exogenous TGF-beta ligands. These results show that betaglycan expression is essential for optimal TGF-beta signaling during embryonic lung development. We therefore conclude that the abrogation of endogenous betaglycan attenuates endogenous autocrine and/or paracrine TGF-beta-mediated negative regulation of lung organogenesis.

Nerve growth factor in pituitary development and pituitary tumors

Cells in the anterior pituitary originate from a common pluripotent precursor whose phenotypic development is determined by intrapituitary transcription factors as well as by hypothalamic and peripheral signals. A rapidly growing body of evidence revealed that essential to the differentiation and proliferation of pituitary cells are an array of growth factors that are produced within the pituitary and act mainly through autocrine mechanisms. Growth factors are polypeptides that are released in carefully measured amounts by some cells to regulate cell growth and differentiation by activating specific tyrosine kinase receptors in the plasma membrane of target cells. Both overproduction of mitogenic growth factors and loss of factors inhibiting cell proliferation result in uncontrolled cell growth and tumor development. There is now increasing evidence that disruption of the calibrated signalling network activated by pituitary growth factors plays a central role in pituitary tumorigenesis. This paper is focussed on the role of nerve growth factor (NGF) in pituitary physiology and pathology. In particular, we propose that NGF plays a dual role in the gland: a local one as a stimulator of differentiation and proliferation of lactotrope cells during pituitary development and a systemic one as a neurohormone which is cosecreted with prolactin into the bloodstream. Furthermore, we discuss the evidence that NGF is an autocrine differentiation factor for prolactin-secreting cells. Escape from NGF control appears to be one of the mechanisms involved in the development and progression of prolactinomas. Along the same line, exposure of prolactinomas refractory to dopaminergic therapy to exogenous NGF results in their differentiation into lactotrope-like cells reexpressing the D2 receptor protein. This observation may open the way to a sequential therapy with NGF and bromocriptine for patients refractory to the conventional therapy.

Pituitary cytokine and growth factor expression and action

The complex range of pituitary regulatory mechanisms reviewed here underlies the critical function of the pituitary in sustaining all higher life forms. Thus, the ultimate net secretion of pituitary hormones is determined by signal integration from all three tiers of pituitary control. It is clear from our current knowledge that the trophic hormone cells of the anterior pituitary are uniquely specialized to respond to these signals. Unravelling their diversity and complexity will shed light upon the normal function of the master gland. Understanding these control mechanisms will lead to novel diagnosis and therapy of disordered pituitary function (357).

2-Aminopurine unravels a role for pRB in the regulation of gene expression by transforming growth factor beta

Transforming growth factor type beta (TGFbeta) is a pleiotropic factor that regulates different cellular activities including cell growth, differentiation, and extracellular matrix deposition. All the known effects of TGFbeta appear to be mediated by its interaction with cell surface receptors that possess a serine/threonine kinase activity. However, the intracellular signals that follow receptor activation and lead to the different cellular responses to TGFbeta are still largely unknown. On the basis of the different sensitivity to the protein kinase inhibitor 2-aminopurine and the phosphatase inhibitor okadaic acid, we identified two distinct pathways through which TGFbeta activates a genomic response. Consistently, 2-aminopurine prevented and okadaic acid potentiated the induction of JE by TGFbeta. The induction of PAI-1 and junB was instead potentiated by 2-aminopurine, after a transient inhibition and was unaffected by okadaic acid. The superinducing effect of 2-aminopurine required the presence of a functional RB protein since it was abolished in SV40 large T antigen-transfected cells, absent in the BT549 and Saos-2 RB-defective cell lines, and restored in BT549 and Saos-2 cells after reintroduction of pRB. The effects of 2-aminopurine on the TGFbeta inducible junB expression occur in all the cell lines examined suggesting that junB, and possibly other genes, can be regulated by TGFbeta through a distinct pRB-dependent pathway.

Definitive hematopoiesis is autonomously initiated by the AGM region

The adult hematopoietic system of mammals is a dynamic hierarchy of cells with the hematopoietic stem cell at its foundation. During embryonic development, the source and expansion potential of this cell remain unclear. Two sites of hematopoietic activity, the yolk sac and aorta-gonad-mesonephros (AGM) region, function in mouse ontogeny at the pre-liver stage of hematopoiesis. However, cellular interchange between these tissues obscures the embryonic site of hematopoietic stem cell generation. Here we present the results of a novel in vitro organ culture system demonstrating that, at day 10 in gestation, hematopoietic stem cells initiate autonomously and exclusively within the AGM region. Furthermore, we provide evidence for the in vitro expansion of hematopoietic stem cells within the AGM region. These results strongly suggest that the AGM region is the source of the definitive adult hematopoietic system, which subsequently colonizes the liver.