Search

ATP-binding cassette A3 (ABCA3) is a lipid transport protein required for synthesis and storage of pulmonary surfactant in type II cells in the alveoli. Abca3 was conditionally deleted in respiratory epithelial cells ([Abca3.sup.[DELTA]/[DELTA]]) in vivo. The majority of mice in which Abca3 was deleted in alveolar type II cells died shortly after birth from respiratory distress related to surfactant deficiency. Approximately 30% of the [Abca3.sup.[DELTA]/DELTA]] mice survived after birth. Surviving [Abca3.sup.[DELTA]/[DELTA]] mice developed emphysema in the absence of significant pulmonary inflammation. Staining of lung tissue and mRNA isolated from alveolar type II cells demonstrated that ~50% of alveolar type II cells lacked ABCA3. Phospholipid content and composition were altered in lung tissue, lamellar bodies, and bronchoalveolar lavage fluid from adult [Abca3.sup.[DELTA]/DELTA]] mice. In adult [Abca3.sup.[DELTA]/DELTA]] mice, cells lacking ABCA3 had decreased expression of mRNAs associated with lipid synthesis and transport. FOXA2 and CCAAT enhancer-binding protein-[alpha], transcription factors known to regulate genes regulating lung lipid metabolism, were markedly decreased in cells lacking ABCA3. Deletion of Abca3 disrupted surfactant lipid synthesis in a cell-autonomous manner. Compensatory surfactant synthesis was initiated in ABCA3-sufficient type II cells, indicating that surfactant homeostasis is a highly regulated process that includes sensing and coregulation among alveolar type II cells. gene regulation; lipids; lung; compensation doi: 10.1152/ajplung.00409.2009.

The ATP-binding cassette (ABC) ABCA3 gene encodes a lipid transporter critical for surfactant function at birth. To identify transcription factors that regulate ABCA3 expression in the lung, we identified by bioinformatic and functional analyses two positive regulatory regions, located between bp -2591 and -1102 and bp -1102 and +11, relative to the exon 1 of the Abca3 gene promoter. The distal cassette contains consensus sequences predicting binding to lung transcription factors including FOXA2, CCAAT/enhancer binding protein-[alpha] (C/EBP[alpha]), GATA-6, thyroid transcription factor-1 (TTF-1 or Nkx2.1), and nuclear factor of activated T cells-c3 (NFATc3). The activity of the distal region from bp -2591 to -1102 was assessed in HeLa and mouse lung epithelial MLE-15 cells. FOXA2, C/EBP[alpha], GATA-6, TTF-1, and NFATc3 increased the activity of the Abca3 luciferase construct in a dose-dependent manner. The distal cassette conferred activation by FOXA2, C/EBP[alpha], GATA-6, TTF-1, and NFATc3 in a position- and orientation-independent manner, serving as an enhancer-like regulatory element. The proximal Abca3 promoter region contained multiple sterol responsive element (SRE) binding sites. SRE binding protein (SREBP)-1c significantly increased the activity of the Abca3 luciferase construct in a dose-dependent manner, whereas SREBP-1a and SREBP-2 did not influence the Abca3 promoter activity. Chromatin immunoprecipitation (CHIP) analyses demonstrated the binding of SREBP-1c, C/EBP[alpha], and TTF-1 to their respective regulatory elements. Conditional deletion of SREBP cleavage-activating protein (Scap) in respiratory epithelial cells in the mouse lung in vivo inhibited the expression of SREBPs in concert with Abca3. Abca3 gene expression is mediated by discrete cis-acting cassettes that mediate pulmonary cell- and lipid-sensitive pathways regulating surfactant homeostasis. lung; development; Nkx2.1

An activated form of [beta]-catenin [[Catnb.sup.[DELTA](ex3)] was expressed in respiratory epithelial cells of the developing lung. Although morphogenesis was not altered at birth, air space enlargement and epithelial cell dysplasia were observed in the early postnatal period and persisted into adulthood. The [[Catnb.sup.[DELTA](ex3)] protein caused squamous, cuboidal, and goblet cell dysplasia in intrapulmonary conducting airways. Atypical epithelial cells that stained for surfactant pro protein C (pro-SP-C) and had morphological characteristics of alveolar type II cells were observed in bronchioles of the transgenic mice. [[Catnb.sup.[DELTA](ex3)] inhibited expression of Foxa2 and caused goblet cell hyperplasia associated with increased staining for mucins and the MUC5A/C protein. In vitro, both wild type and activated [beta]-catenin negatively regulated the expression of the Foxa2 promoter. [[Catnb.sup.[DELTA](ex3)] also caused pulmonary tumors in adult mice. Activation of [beta]-catenin caused ectopic differentiation of alveolar type II-like cells in conducting airways, goblet cell hyperplasia, and air space enlargement, demonstrating a critical role for the Wnt/[beta]-catenin signal transduction pathway in the differentiation of the respiratory epithelium in the postnatal lung. lung morphogenesis; tumorigenesis; goblet cell hyperplasia; air space enlargement

Tumor necrosis factor (TNF)-[alpha] is a key molecule in lung inflammation. We have established the insulin-like growth factor binding protein 2 (IGFBP-2) as a marker associated with the growth arrest of lung alveolar epithelial cells (AEC). Here, we studied the effects of TNF-[alpha] on AEC proliferation and the putative protective role of retinoic acid (RA). We documented an antiproliferative action of TNF-[alpha] that was reversible only at 24 h and then became irreversible with induction of apoptosis. TNF-[alpha] treatment was associated with a dramatic induction of IGFBP-2. To discover the mechanism of action of IGFBP-2, we further tested the mitogenic potential of IGF-I to counteract TNF-[alpha] inhibition. Addition of IGF-I to the TNF-[alpha] containing medium did not stimulate proliferation, whereas des(1-3)IGF-I, an analog of IGF-I that bears low affinity for IGFBPs, was able to restore cell growth. Interestingly, we observed that RA abrogated TNF-[alpha]-induced growth arrest and that this effect was associated with a dramatic decrease in IGFBP-2 expression. These results suggest a protective role of RA from TNF-[alpha] antiproliferative action, through mechanisms involving modulation of IGFBP-2 production. lung epithelial cells; inflammation; insulin-like growth factor; proliferation; retinoic acid

A study was conducted to analyze the influence of retinoic acid (RA) on the proliferation of stem cells of the alveolar epithelium. Type 2 cell lines were obtained from rat primary neonatal type 2 cells while total cellular RNA was determined using the guanidium isothiocyanate procedure. Results indicated the correlation of RA with type 2 cell proliferation through mechanisms contradicting the transforming growth factor-beta-1 pathway.

Nabeyrat, Elodie, Sophie Corroyer, Ralph Epaud, Valerie Besnard, Veronique Cazals, and Annick Clement. Retinoic acid-induced proliferation of lung alveolar epithelial cells is linked to [p21.sup.CIP1] downregulation. Am. J. Physiol. Lung Cell. Mol. Physiol. 278: L42-L50, 2000. --Retinoids, including retinol and retinoic acid (RA) derivatives, have been shown to be involved in the processes of lung development as well as of lung repair after injury. Recently, we have provided evidence that RA could stimulate proliferation of lung alveolar type 2 epithelial cells (E. Nabeyrat, V. Besnard, S. Corroyer, V. Cazals, and A. Clement. Am. J. Physiol. Lung Cell. Mol. Physiol. 275: L71-L79, 1998). To gain some insight into the mechanisms involved in the mitogenic action of RA, we focused in the present study on the effects of RA on the expression of G1 phase cyclins and their cell cycle-dependent kinases (Cdks). Experiments were performed with serum-deprived cells cultured in the absence and presence of RA. The results showed no effects of RA on the expression of either cyclins or Cdks. In contrast, RA treatment was found to prevent the decrease in cyclin E-Cdk2 activity observed when cells were growth arrested by serum deprivation. The observation that changes in cyclin E-Cdk2 activity were not associated with modifications in the amount of complexes formed led to the suggestion that the Cdk inhibitory protein (CKI) was involved. Study of the CKI [p21.sup.CIP1] revealed marked differences in its expression in the absence and presence of RA, with a dramatic downregulation observed in RA-treated cells. Interestingly, immunoprecipitation experiments provided evidence that the decreased levels of [p21.sup.CIP1] were associated with a reduced interaction of this CKI with cyclin E-Cdk2 complexes. These data together with previous results obtained in various situations of type 2 cell growth arrest emphasize the role of [p21.sup.CIP1] in the control of lung alveolar epithelial cell proliferation. cell cycle; cyclin E; cyclin-dependent kinase-2; cyclin-dependent kinase inhibitor; mitogenesis

The ATP-binding cassette (ABC) ABCA3 gene encodes a lipid transporter critical for surfactant function at birth. To identify transcription factors that regulate ABCA3 expression in the lung, we identified by bioinformatic and functional analyses two positive regulatory regions, located between bp -2591 and -1102 and bp -1102 and +11, relative to the exon 1 of the Abca3 gene promoter. The distal cassette contains consensus sequences predicting binding to lung transcription factors including FOXA2, CCAAT/enhancer binding protein-α (CIEBPα), GATA-6, thyroid transcription factor-I (TTF-l or Nkx2.1), and nuclear factor of activated T cells-c3 (NFATc3). The activity of the distal region from bp -2591 to -1102 was assessed in HeLa and mouse lung epithelial MLE-15 cells. FOXA2, C/EBPα, GATA-6, TTF-I, and NFATc3 increased the activity of the Abca3 luciferase construct in a dose-dependent manner. The distal cassette conferred activation by FOXA2, C/EBPα, GATA-6, TTF-1, and NFATc3 in a position- and orientation-independent manner, serving as an enhancer-like regulatory element. The proximal Abca3 promoter region contained multiple sterol responsive element (SRE) binding sites. SRE binding protein (SREBP)-Ic significantly increased the activity of the Abca3 luciferase construct in a dose-dependent manner, whereas SREBP-1a and SREBP-2 did not influence the Abca3 promoter activity. Chromatin immunoprecipitation (ChIP) analyses demonstrated the binding of SREBP-1c, C/EBPα, and TTF-1 to their respective regulatory elements. Conditional deletion of SREBP cleavage-activating protein (Scap) in respiratory epithelial cells in the mouse lung in vivo inhibited the expression of SREBPs in concert with Abca3. Abca3 gene expression is mediated by discrete cis-acting cassettes that mediate pulmonary cell- and lipid-sensitive pathways regulating surfactant homeostasis. [ABSTRACT FROM AUTHOR]