Transcriptional regulation of surfactant proteins SP-A and SP-B by phorbol ester

Differential Regulation of Human Surfactant Protein A Genes, SFTPA1 and SFTPA2, and Their Corresponding Variants

The human SFTPA1 and SFTPA2 genes encode the surfactant protein A1 (SP-A1) and SP-A2, respectively, and they have been identified with significant genetic and epigenetic variability including sequence, deletion/insertions, and splice variants. The surfactant proteins, SP-A1 and SP-A2, and their corresponding variants play important roles in several processes of innate immunity as well in surfactant-related functions as reviewed elsewhere [1]. The levels of SP-A have been shown to differ among individuals both under baseline conditions and in response to various agents or disease states. Moreover, a number of agents have been shown to differentially regulate SFTPA1 and SFTPA2 transcripts. The focus in this review is on the differential regulation of SFTPA1 and SFTPA2 with primary focus on the role of 5′ and 3′ untranslated regions (UTRs) and flanking sequences on this differential regulation as well molecules that may mediate the differential regulation.

Comparison of the effects of betamethasone and dexamethasone on surfactant protein A mRNA expression in human lung cells

OBJECTIVE

While prenatal administration of synthetic corticosteroids stimulates both fetal lung development and expression of pulmonary surfactant, the specific effects may depend on the corticosteroid formulation used. We compared the dose-dependent effects of various concentrations of two synthetic corticosteroids, betamethasone and dexamethasone, on steady state levels of surfactant protein A (SP-A) mRNA in human lung cells.

METHODS

Cultured human NCI-H441 bronchoalveolar epithelial cells were exposed to varying concentrations of betamethasone or dexamethasone (10^-7 to 10^-12 M) for 48 h alone or in combination with dibutyryl cAMP (1 mM), which augments surfactant protein gene expression. RNA was harvested and SP-A mRNA levels were quantified by real-time quantitative reverse transcriptase polymerase chain reaction analysis. Results were compared using the Kruskal-Wallis test.

RESULTS

A dose-dependent modification in SP-A mRNA levels was demonstrated with both dexamethasone and betamethasone. Cells treated with cAMP expressed higher levels of SP-A mRNA than untreated cells. A biphasic curve in the SP-A mRNA response to corticosteroids was elicited only in the presence of cAMP: at lower concentrations (10^-10 through 10^-12 M), SP-A mRNA levels were upregulated, whereas at higher concentrations (10^-7 and 10^-8 M), SP-A mRNA levels were reduced. Dexamethasone was more effective than betamethasone in inducing these changes.

CONCLUSIONS

Our results support a biphasic effect on SP-A mRNA levels after exposure to corticosteroids in combination with cAMP. At higher corticosteroid concentrations, betamethasone is less inhibitory than dexamethasone on SP-A mRNA.

Signal transduction events involved in TPA downregulation of SP-A gene expression

Surfactant protein A (SP-A), the most abundant pulmonary surfactant protein, plays a role in innate host defense and blocks the inhibitory effects of serum proteins on surfactant surface tension-lowering properties. SP-A mRNA and protein are downregulated by phorbol esters (TPA) via inhibition of gene transcription. We evaluated the TPA signaling pathways involved in SP-A inhibition in a lung cell line, H441 cells. TPA caused sustained phosphorylation of p44/42 mitogen-activated protein kinase (MAPK), p38 MAPK, and c-Jun-NH2-terminal kinase. An inhibitor of conventional and novel isoforms of protein kinase C (PKC) and two inhibitors of p44/42 MAPK kinase partially or completely blocked the inhibitory effects of TPA on SP-A mRNA levels. In contrast, inhibitors of conventional PKC-α and -β, stress-activated protein kinases, protein phosphatases, protein kinase A, and the phosphatidylinositol 3-kinase pathway had no effect on the TPA-mediated inhibition of SP-A mRNA. TPA also stimulated the synthesis of c-Jun mRNA and protein in a time-dependent manner. Inhibitors of the p44/42 MAPK signaling pathway and PKC blocked the TPA-mediated phosphorylation of p44/42 MAPK and the increase in c-Jun mRNA. We conclude that TPA inhibits SP-A gene expression via novel isoforms of PKC, the p44/42 MAPK pathway, and the activator protein-1 complex.

Human SP-A 3'-UTR variants mediate differential gene expression in basal levels and in response to dexamethasone

Human surfactant protein A (SP-A) is encoded by two genes (SP-A1, SP-A2), and each is identified with several alleles. SP-A is involved in normal lung function, innate immunity, inflammatory processes, and is regulated by glucocorticoids. We investigated the role of 3'-untranslated region (UTR) of 10 SP-A variants on gene expression using transient transfection of 3'-UTR constructs in the human lung adenocarcinoma cell line NCI-H441. We found: 1) both basal mRNA and protein levels of the reporter gene of SP-A 3'-UTR constructs are significantly (P < 0.01) reduced compared with controls (vector pGL3 and surfactant protein B pGL3) and that differences exist among alleles; and 2) after dexamethasone (Dex) treatment (100 nM for 16 h), mRNA was reduced (31-51%). Seven alleles showed a significant decrease (P < 0.05) in mRNA, and three did not. Reporter activity was also decreased, from 17% (1A(1)) to 38% (1A), with six alleles showing a significant decrease. The data indicate that the 3'-UTR of SP-As play a differential role in SP-A basal expression and in response to Dex. Therefore, a careful consideration of individual use of steroid treatment may be considered.

Role and regulation of activator protein-1 in toxicant-induced responses of the lung

Aberrant cell proliferation and differentiation after toxic injury to airway epithelium can lead to the development of various lung diseases including cancer. The activator protein-1 (AP-1) transcription factor, composed of mainly Jun-Jun and Jun-Fos protein dimers, acts as an environmental biosensor to various external toxic stimuli and regulates gene expression involved in various biological processes. Gene disruption studies indicate that the AP-1 family members c-jun, junB, and fra1 are essential for embryonic development, whereas junD, c-fos, and fosB are required for normal postnatal growth. However, broad or target-specific transgenic overexpression of the some of these proteins gives very distinct phenotype(s), including tumor formation. This implies that, although they are required for normal cellular processes, their abnormal activation after toxic injury can lead to the pathogenesis of the lung disease. Consistent with this view, various environmental toxicants and carcinogens differentially regulate Jun and Fos expression in cells of the lung both in vivo and in vitro. Moreover, Jun and Fos proteins distinctly bind to the promoter regions of a wide variety of genes to differentially regulate their expression in epithelial injury, repair, and differentiation. Importantly, lung tumors induced by various carcinogens display a sustained expression of certain AP-1 family members. Therefore a better understanding of the mechanisms of regulation and functional role(s), as well as identification of target genes of members of the AP-1 family in airway epithelial cells, will provide additional insight into toxicant-induced lung diseases. These studies might offer a unique opportunity to use AP-1 family members and transactivation as potential diagnostic markers or drug targets for early detection and/or prevention of various lung diseases.

BMK1 (ERK5) regulates squamous differentiation marker SPRR1B transcription in Clara-like H441 cells

Various toxicants and carcinogens upregulate the expression of small proline-rich protein 1B (SPRR1B), a squamous differentiation marker, in bronchial epithelial cells both in vivo and in vitro. We have recently shown that phorbol 13-myristate 12-acetate (PMA)-stimulated SPRR1B transcription in Clara-like H441 cells is mainly mediated by activator protein-1 (AP-1) and c-Jun N-terminal kinase-1 (JNK1). Though mitogen-activated protein kinase (MAPK) kinase (MEK)-1/2 pathway inhibitors strongly suppressed both basal and PMA-inducible SPRR1B transcription, overexpression of dominant negative (dn) forms of extracellular signal-regulated kinase (ERK)-1 and/or -2 did not have any significant effect indicating the involvement of another ERK-like MAPK in this pathway. Here, we report for the first time the involvement of ERK5 in PMA-inducible SPRR1B transcription in H441 cells. PMA significantly induced ERK5 activation in H441 cells. Overexpression of dn-ERK5 strongly suppressed both basal and PMA-inducible SPRR1B transcription, whereas wild-type ERK5 upregulated it. Consistent with this, a mutant form of MEK-5, an upstream activator of ERK5, strongly suppressed PMA-inducible promoter activity. However, coexpression of c-Jun restored promoter activation suppressed by dn-ERK5. Thus, in addition to JNK1, the activation of MEK5-ERK5 MAPK pathway probably plays a pivotal role in transcriptional regulation of AP-1-mediated SPRR1B expression in the distal bronchiolar region.

Human surfactant protein B promoter in transgenic mice: temporal, spatial, and stimulus-responsive regulation

Surfactant protein B (SP-B) is a developmentally and hormonally regulated lung protein that is required for normal surfactant function. We generated transgenic mice carrying the human SP-B promoter (-1,039/+431 bp) linked to chloramphenicol acetyltransferase (CAT). CAT activity was high in lung and immunoreactive protein localized to alveolar type II and bronchiolar epithelial cells. In addition, thyroid, trachea, and intestine demonstrated CAT activity, and each of these tissues also expressed low levels of SP-B mRNA. Developmental expression of CAT activity and SP-B mRNA in fetal lung were similar and both increased during explant culture. SP-B mRNA but not CAT activity decreased during culture of adult lung, and both were reduced by transforming growth factor (TGF)-beta(1). Treatment of adult mice with intratracheal bleomycin caused similar time-dependent decreases in lung SP-B mRNA and CAT activity. These findings indicate that the human SP-B promoter fragment directs tissue- and lung cell-specific transgene expression and contains cis-acting elements involved in regulated expression during development, fetal lung explant culture, and responsiveness to TGF-beta and bleomycin-induced lung injury.

JNK1 and AP-1 regulate PMA-inducible squamous differentiation marker expression in Clara-like H441 cells

Exposure of distal bronchiolar region to various toxicants and pollutants suppresses Clara cell differentiation marker expression and greatly enhances the induction of squamous cell differentiation (SCD). Here, we demonstrate for the first time phorbol 13-myristate 12-acetate (PMA)-inducible expression of SCD markers, SPRRs, in Clara-like H441 cells. The transcriptional stimulation of human SPRR1B expression is mainly mediated by a -150- to -84-bp region that harbors two critical activator protein (AP)-1 sites. In unstimulated cells, the -150- to -84-bp region is weakly bound by AP-1 proteins, mainly JunD and Fra1. However, PMA prominently induced the binding of JunB and Fra1. Consistent with this, overexpression of wild-type Jun proteins upregulated the SPRR1B promoter activity. Conversely, a c-jun mutant suppressed both basal and PMA-inducible reporter gene expression. Intriguingly, overexpression of fra2 suppressed PMA-inducible reporter activity, whereas fra1 significantly enhanced basal level activity, indicating an opposing role for these proteins in SPRR1B expression in a manner similar to that observed in proximal tracheobronchial epithelial cells (BEAS-2B clone S6). Interestingly, unlike in S6 cells, a catalytically inactive c-Jun NH(2)-terminal kinase (JNK) 1 mutant significantly reduced the PMA-inducible SPRR1B promoter activity in H441 cells. Thus either temporal expression and/or spatial activation of AP-1 proteins by JNK1 might contribute to the induction of SCD in Clara cells.

Maintenance of differentiated function of the surfactant system in human fetal lung type II epithelial cells cultured on plastic

We report a simplified culture system for human fetal lung type II cells that maintains surfactant expression. Type II cells isolated from explant cultures of hormone-treated lungs (18-22 wk gestation) by collagenase + trypsin digestion were cultured on plastic for 4 days in serum-free medium containing dexamethasone (Dex, 10 nM) + 8-bromo-cAMP (0.1 mM + isobutylmethylxanthine (0.1 mM) or were untreated (control). Surfactant protein (SP) mRNAs decreased markedly in control cells between days 1 and 4 of culture, but mRNA levels were high in treated cells on day) 4 (SP-A, SP-B, SP-C, SP-D; 600%, 100%, 85%, 130% of day 0 content, respectively). Dex or cAMP alone increased SP-B, SP-C, and SP-D mRNAs and together had additive effects. The greatest increase in SP-A mRNA occurred with cAMP alone. Treated cells processed pro-SP-B and pro-SP-C proteins to mature forms and had a higher rate of phosphatidylcholine (PC) synthesis (2-fold) and higher saturation of PC (approximately 34% versus 27%) than controls. Only treated cells maintained secretagogue-responsive phospholipid synthesis. By electron microscopy, the treated cells retained lamellar bodies and extensive microvilli. We conclude that Dex and cAMP additively stimulate expression of surfactant components in isolated fetal type II cells, providing a simplified culture system for investigation of surfactant-related, and perhaps other, type II cell functions.

Role of transcription factors in fetal lung development and surfactant protein gene expression

Branching morphogenesis of the lung and differentiation of specialized cell populations is dependent upon reciprocal interactions between epithelial cells derived from endoderm of embryonic foregut and surrounding mesenchymal cells. These interactions are mediated by elaboration and concerted actions of a variety of growth and differentiation factors binding to specific receptors. Such factors include members of the fibroblast growth factor family, sonic hedgehog, members of the transforming growth factor-beta family, epidermal growth factor, and members of the platelet-derived growth factor family. Hormones that increase cyclic AMP formation, glucocorticoids, and retinoids also play important roles in branching morphogenesis, alveolar development, and cellular differentiation. Expression of the genes encoding these morphogens and their receptors is controlled by a variety of transcription factors that also are highly regulated. Several of these transcription factors serve dual roles as regulators of genes involved in early lung development and in specialized functions of differentiated cells. Targeted null mutations of genes encoding many of these morphogens and transcription factors have provided important insight into their function during lung development. In this chapter, the cellular and molecular mechanisms that control lung development are considered, as well as those that regulate expression of the genes encoding the surfactant proteins.

Transforming growth factor-beta(1) regulation of surfactant protein B gene expression is mediated by protein kinase-dependent intracellular translocation of thyroid transcription factor-1 and hepatocyte nuclear factor 3

The transforming growth factor-beta (TGF beta) polypeptides control a variety of cellular processes including organogenesis and cellular proliferation and differentiation. In the developing lung, TGF beta(1) treatment inhibits airway branching and expression of the genes for surfactant proteins (SP). Many effects of TGF beta are mediated at the level of gene transcription but there is limited information regarding signaling pathways and target transcription factors. In this study with human pulmonary adenocarcinoma H441 cells, we investigated TGF beta(1) effects on SP-B, a protein which is essential for normal function of pulmonary surfactant. TGF beta(1) (10 ng/ml) reduced SP-B mRNA content in a time-dependent fashion, and transient transfection studies localized responsiveness to the region of the SP-B promoter (-112/-72 bp) containing binding sites for thyroid transcription factor-1 (TTF-1) and hepatocyte nuclear factor 3 (HNF3), transcription factors that are important enhancers of SP gene expression. Using electrophoretic mobility shift assay and immunofluorescence, we demonstrated rapid accumulation of these transcription factors in the cytoplasm and subsequent loss from the nucleus on TGF beta(1) treatment of both adenocarcinoma cells and cultured human fetal lung. TGF beta(1) treatment caused intracellular translocation of protein kinase C and effects of TGF beta(1) were mostly abrogated in the presence of the protein kinase inhibitor calphostin C. We conclude that TGF beta(1), acting via protein phosphorylation, blocks nuclear translocation of TTF-1 and HNF3 which results in down-regulation of the SP-B gene and presumably other pulmonary genes which are transactivated by these factors.

Degree of lung maturity determines the direction of the interleukin-1- induced effect on the expression of surfactant proteins

Intra-amniotic interleukin (IL)-1 increases surfactant components in immature fetal lung, whereas high IL-1 after birth is associated with surfactant dysfunction. Our aim was to investigate whether the fetal age influences the responsiveness of surfactant proteins (SPs) to IL-1. Rabbit lung explants from fetuses at 19, 22, 27, and 30 d of gestation and 1-d-old newborns were cultured in serum-free medium in the presence of recombinant human (rh) IL-1alpha or vehicle. The influence of IL-1alpha on SP-A, -B, and -C messenger RNA (mRNA) content was dependent on the conceptional age. In very immature lung on Day 19, rhIL-1alpha (570 ng/ml for 20 h) increased SP-A, -B, and -C mRNA by 860+/-15%, 314+/-108%, and 64+/-17%, respectively. The increase in SP-A mRNA was evident within 4 to 6 h. IL-1alpha increased the SP-A concentration in alveolar epithelial cells and in the culture medium within 20 h. In contrast, at 27 to 30 d of gestation and in newborns, IL-1alpha decreased SP-C, -B, and -A mRNA by means of 64 to 67%, 48 to 59%, and 12 to 15%, respectively. SP-B protein decreased by 45 to 60%. The decrease in mRNA became evident within 8 to 12 h and was dependent on IL-1 concentration. On Day 27, IL-1alpha accelerated the degradation of SP-B mRNA in the presence of actinomycin D. IL-1 did not increase the degradation rate of SP-A mRNA unless both actinomycin D and cycloheximide were added to the explants. The present findings may explain some of the contrasting associations between inflammatory cytokines and lung diseases during the perinatal period. The determinants of the direction of the IL-1 effect on the expression of SPs remain to be identified.

Effect of phosphocholine cytidylyltransferase overexpression on phosphatidylcholine synthesis in alveolar type II cells and related cell lines

Disaturated phosphatidylcholine (DSPC) is the predominate phospholipid component of lung surfactant. In the alveolar type II cell, the cytidine diphosphocholine (CDP-choline) pathway is the major biosynthetic pathway for DSPC. To investigate the hypothesis that phosphocholine cytidylyltransferase (CT) is the rate-limiting enzyme in the CDP-choline pathway, rat alveolar type II cells or lung tumor-derived cell lines (A549 or H441) with type II cell features were transfected with CT complementary DNA (cDNA). Cell fractions were subsequently assayed for CT protein and activity, and cell rates of DSPC synthesis were determined. In all cases, cell CT protein and activity were increased after transfection with CT cDNA but not after control transfection. Rat type II cells, but not A549 or H441 cells, increased the rate of DSPC synthesis after transfection with CT cDNA. Exposure of type II cells transfected with CT cDNA to palmitic acid resulted in a further increase in CT protein and activity. Exposure to dexamethasone resulted in increased CT protein and activity and increased synthesis of DSPC. The results confirm that CT has a rate-limiting and regulatory role in the synthesis of type II cell DSPC, and raise possibilities for novel therapeutic interventions.

SP-A 3'-UTR is involved in the glucocorticoid inhibition of human SP-A gene expression

The synthetic glucocorticoid dexamethasone has a major inhibitory effect on human surfactant protein A1 (SP-A1) and SP-A2 gene expression that occurs at both the transcriptional and posttranscriptional levels. Toward the identification of cis-acting elements that may be involved in the dexamethasone regulation of SP-A mRNA stability, chimeric chloramphenicol acetyltransferase (CAT) constructs that contained various portions of SP-A1 or SP-A2 cDNA in place of the native CAT 3'-untranslated region (UTR) were transiently transfected into the lung adenocarcinoma cell line NCI-H441. CAT activity was reduced in NCI-H441 cells by exposure to 100 nM dexamethasone only for the chimeric CAT constructs that contained the SP-A 3'-UTR. Moreover, the inhibitory response seen with dexamethasone was greater for the 3'-UTR derived from the SP-A1 allele 6A3 than with the 3'-UTR derived from either the SP-A1 allele 6A2 or SP-A2 allele 1A0, indicating differential regulation between SP-A genes and/or alleles.

TGF-beta1 inhibits surfactant component expression and epithelial cell maturation in cultured human fetal lung

Transforming growth factor-beta1 (TGF-beta1) is a multifunctional cytokine shown to play a critical role in organ morphogenesis, development, growth regulation, cellular differentiation, gene expression, and tissue remodeling after injury. We examined the effect of exogenously administered TGF-beta1 on the expression of surfactant proteins (SPs) and lipids, fatty acid synthetase, and ultrastructural morphology in human fetal lung cultured for 5 days with and without dexamethasone (10 nM). Expression of the type II cell-specific marker surfactant proprotein C (proSP-C), studied by [35S]Met incorporation and immunoprecipitation, increased sevenfold with dexamethasone treatment. TGF-beta1 (0.1-100 ng/ml) in the presence of dexamethasone inhibited 21-kDa proSP-C expression in a dose-dependent manner (maximal inhibition 31% of control level at 100 ng/ml). There was no change in [35S]Met incorporation into total protein in any of the treatment groups vs. the control group. In immunoblotting experiments, TGF-beta1 blocked culture-induced accumulation of SP-A and SP-B. Under the same conditions, TGF-beta1 reduced mRNA content for SP-A, SP-B, and SP-C to 20, 38, and 41%, respectively, of matched control groups but did not affect levels of beta-actin mRNA. SP transcription rates after 24 h of exposure to TGF-beta1 were reduced to a similar extent (20-50% of control level). In both control and dexamethasone-treated explants, TGF-beta1 (10 ng/ml) also decreased fatty acid synthetase mRNA, protein, and enzyme activity and the rate of [3H]choline incorporation into phosphatidylcholine. By electron microscopy, well-differentiated type II cells lining potential air spaces were present in explants cultured with dexamethasone, whereas exposure to TGF-beta1 with or without dexamethasone resulted in epithelial cells lacking lamellar bodies. We conclude that exogenous TGF-beta1 disrupts culture-induced maturation of fetal lung epithelial cells and inhibits expression of surfactant components through effects on gene transcription.

Phorbol ester down-regulation of lung surfactant protein B gene expression by cytoplasmic trapping of thyroid transcription factor-1 and hepatocyte nuclear factor 3

The lung-specific surfactant protein B (SP-B) is essential for surfactant function and normal respiration. We investigated the role of thyroid transcription factor-1 (TTF-1) and hepatocyte nuclear factor 3 (HNF3) in the down-regulation of SP-B gene expression by phorbol ester in pulmonary adenocarcinoma H441 cells. Responsiveness to 12-O-tetradecanoylphorbol-13-acetate (TPA) localized to the SP-B proximal promoter (-140/-65 bp) and specifically to binding sites for TTF-1 and HNF3, which act as cell-specific enhancers of SP-B expression. Treatment of cells with TPA (10 nM) caused a time-dependent decrease in both TTF-1 and HNF3 in nuclear extracts and accumulation of both factors in the cytoplasm as assessed by electromobility shift, Western, Southwestern, and immunofluorescence assays. Treatment did not alter the mRNA content or DNA binding activity for either transcription factor. We conclude that down-regulation of SP-B gene expression by phorbol ester involves cytoplasmic trapping and loss of TTF-1 and HNF3 from the nucleus. This mechanism of action is independent of AP-1 and other transcription factors known to be influenced by phorbol ester.