Rabbit lung surfactant protein A gene: identification of a lung-specific DNase I hypersensitive site

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.

Glucocorticoid inhibition of SP-A gene expression in lung type II cells is mediated via the TTF-1-binding element

Induction of surfactant protein-A (SP-A) gene expression in fetal lung type II cells by cAMP and IL-1 is mediated by increased binding of thyroid transcription factor-1 (TTF-1) and NF-B proteins p50 and p65 to the TTF-1-binding element (TBE) at -183 bp. In type II cell transfections, dexamethasone (Dex) markedly inhibits cAMP-induced expression of rabbit SP-A:human growth hormone (hGH) fusion genes containing as little as 300 bp of the SP-A 5'-flanking sequence. Dex inhibition is blocked by RU-486, suggesting a role of the glucocorticoid receptor (GR). The present study was undertaken to define the mechanisms for GR inhibition of SP-A expression. Cotransfection of primary cultures of type II cells with a GR expression vector abrogated cAMP induction of SP-A promoter activity while, at the same time, causing a 60-fold induction of cotransfected mouse mammary tumor virus (MMTV) promoter. In lung cells transfected with a fusion gene containing three TBEs fused to the basal SP-A promoter, Dex prevented the stimulatory effect of IL-1 on TTF-1 induction of SP-A promoter activity, suggesting that the GR inhibits SP-A promoter activity through the TBE. In gel shift assays using nuclear extracts from human fetal type II cells cultured in the absence or presence of cAMP, Dex markedly reduced binding of nuclear proteins to the TBE and blocked the stimulatory effect of cAMP on TBE-binding activity. Our finding that Dex increased expression of the NF-kappaB inhibitory partner IkappaB-alpha suggests that the decrease in TBE-binding activity may be caused, in part, by GR inhibition of NF-kappaB interaction with this site.

Transcription factor USF2 is developmentally regulated in fetal lung and acts together with USF1 to induce SP-A gene expression

Expression of the pulmonary surfactant protein A (SP-A) gene is lung specific, developmentally regulated, and enhanced by hormones and factors that increase cAMP. We previously identified two E-box-like enhancers termed distal binding element (DBE) and proximal binding element (PBE) in the 5'-flanking region of the rabbit (r) SP-A gene that are essential for cAMP induction of rSP-A promoter activity (Gao E, Alcorn JL, and Mendelson CR. J Biol Chem 268: 19697-19709, 1993). We also found that DBE and PBE serve as binding sites for the basic helix-loop-helix-leucine zipper transcription factor, upstream stimulatory factor-1 (USF1) (Gao E, Wang Y, Alcorn JL, and Mendelson CR. J Biol Chem 272: 23398-23406, 1997). In the present study, PBE was used to screen a rabbit fetal lung cDNA expression library; a cDNA insert encoding the structurally related rabbit upstream stimulatory factor-2 (rUSF2) was isolated. The levels of rUSF2 mRNA reach peak levels in fetal rabbit lung at 28 days of gestation, in concert with the time of maximal induction of SP-A gene transcription. In yeast two-hybrid analysis, rUSF2 was found to preferentially form heterodimers, compared with homodimers, with rUSF1. Binding complexes of nuclear proteins isolated from fetal rabbit lung type II cells with the DBE and PBE were supershifted by anti-rUSF2 antibodies. Binding activity was enriched in nuclear proteins from type II cells compared with fibroblasts. Overexpression of rUSF2 in transfected lung A549 cells increased rSP-A promoter activity and acted synergistically with rUSF1. We suggest that heterodimers of USF2 and USF1 bound to two E-box elements in the SP-A gene 5'-flanking region serve a key role in developmental and hormonal regulation of SP-A gene expression in pulmonary type II cells.

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.