Characterization of the nuclear proteins binding the CACCC element of a glucocorticoid-responsive enhancer in the tyrosine aminotransferase gene

Mucin 2 (MUC2) promoter characterization: an overview

Transgenic livestock have been studied with a well-known interest in improving quantitative and qualitative traits. In order to direct heterologous gene expression, it is indispensable to identify and characterize a promoter suitable for directing the expression of the gene of interest (GOI) in a tissue-specific way. The gastrointestinal tract is a desirable target for gene expression in several mammalian models. Throughout the surface of the intestinal epithelium, there is an intricate polymer network, formed by gel-forming mucins (especially MUC2 and MUC5AC, of which MUC2 is the major one), which plays a protective role due to the formation of a physical, chemical and immunological barrier between the organism and the environment. The characterization of the gel-forming mucins is difficult because of their large size and repetitive DNA sequences and domains. The main mucin in the small and large intestine, mucin 2 (MUC2), is expressed specifically in goblet cells. MUC2 plays an important role in intestinal homeostasis and its disruption is associated with several diseases and carcinomas. This mucin is also an important marker for elucidating mechanisms that regulate differentiation of the secretory cell lineage. This review presents the state of the art of MUC2 promoter structure and functional characterization.

Col3.6-HSD2 transgenic mice: a glucocorticoid loss-of-function model spanning early and late osteoblast differentiation

The goal of this study was to characterize the bone phenotype and molecular alterations in Col3.6-HSD2 mice in which a 3.6-kb Col1a1 promoter fragment drives 11beta-HSD2 expression broadly in the osteoblast lineage to reduce glucocorticoid signaling. Serum corticosterone was unchanged in transgenic females excluding a systemic effect of the transgene. Adult transgenic mice showed reduced vertebral trabecular bone volume and reduced femoral and tibial sub-periosteal and sub-endosteal areas as assessed by microCT. In adult female transgenic mice, histomorphometry showed that vertebral bone mass and trabecular number were reduced but that osteoblast and osteoclast numbers and the mineral apposition and bone formation rates were not changed, suggesting a possible developmental defect in the formation of trabeculae. In a small sample of male mice, osteoblast number and percent osteoid surface were increased but the mineral apposition bone formation rates were not changed, indicating subtle sex-specific phenotypic differences in Col3.6-HSD2 bone. Serum from transgenic mice had decreased levels of the C-terminal telopeptide of alpha1(I) collagen but increased levels of osteocalcin. Transgenic calvarial osteoblast and bone marrow stromal cultures showed decreased alkaline phosphatase and mineral staining, reduced levels of Col1a1, bone sialoprotein and osteocalcin mRNA expression, and decreased cell growth and proliferation. Transgenic bone marrow cultures treated with RANKL and M-CSF showed greater osteoclast formation; however, osteoclast activity as assessed by resorption of a calcium phosphate substrate was decreased in transgenic cultures. Gene profiling of cultured calvarial osteoblasts enriched in the Col3.6-HSD2 transgene showed modest but significant changes in gene expression, particularly in cell cycle and integrin genes. In summary, Col3.6-HSD2 mice showed a low bone mass phenotype, with decreased ex vivo osteogenesis. These data further strengthen the concept that endogenous glucocorticoid signaling is required for optimal bone mass acquisition and highlight the complexities of glucocorticoid signaling in bone cell lineages.

Enzymology and Molecular Biology of Glucocorticoid Metabolism in Humans

Glucocorticoids (GCs) are a vital class of steroid hormones that are secreted by the adrenal cortex and that are regulated by ACTH largely under the control of the hypothalamic-pituitary-adrenal axis. GCs mediate profound and diverse physiological effects in vertebrates, ranging from development, metabolism, neurobiology, anti-inflammation and programmed cell death to many other fuctions. Multiple factors "downstream" of GC secretion, such as glucocorticoid receptor (GR) number and the abundance of plasma binding proteins have originally been considered as modulators of GC action. However, in the last decade the role of tissue-specific GC activating and inactivating enzymes have been identified as additional determinants in GC signalling pathways. On the cellular level, they function as important pre-receptor regulators by acting as "molecular switches" for receptor-active and receptor-inactive GC hormones. According to their biologic activity to catalyze the interconversion of C11-hydroxyl and C11-oxo GCs these enzymes have been named 11beta-hydroxysteroid dehydrogenase (11beta-HSD; EC 1.1.1.146). Two isoforms of 11beta-HSD have been cloned and characterized so far. 11beta-HSD type 1 is found in a wide range of tissues, acts predominantly as a reductase in intact cells and tissues by regenerating active cortisol from cortisone, and has been described to regulate GC access to the GR. 11beta-HSD type 2 is found mainly in mineralocorticoid target tissues such as kidney and colon, acts only as a dehydrogenase by producing inactive cortisone, and has been found to protect the mineralocorticoid receptor from high levels of receptor-active cortisol. Recently, 11beta-HSD 1 has become highly topical due to the finding that 11beta-HSD 1 plays a pivotal role in the pathogenesis of central obesity and the appearance of the metabolic syndrome. This review provides an overview on the components involved in GC signalling of 11beta-HSD type 1 as an important pre-receptor control enzyme that modulates activation of the GR.

Identification and characterization of the MUC2 (human intestinal mucin) gene 5'-flanking region: promoter activity in cultured cells

The initiation point for MUC2 gene transcription is located within a 7000-base GC-rich region of the mucin gene cluster found on chromosome 11p15.5. The promoter activity of the 5'-flanking region of the MUC2 gene was examined following its cloning into the luciferase-producing pGL2-Basic reporter vector. A short segment comprising bases -91 to -73 relative to the start of transcription was found to be important for basal promoter activity in all cell lines tested. Electrophoretic mobility shift assays demonstrated nuclear protein binding to this region, which contains the consensus CACCC motif (5'-GCCACACCC). This element has been shown to be functionally important in several promoters that are active in diverse cell types. Competition experiments using an Sp1 oligonucleotide and antibody supershift experiments indicated that both Sp1 and other Sp1 family members bind to this element. Inclusion of the region between bases -228 and -171 in pGL2-Basic constructs increased normalized luciferase reporter activity by almost 3-fold in C1a cells, which produce relatively high levels of MUC2 mRNA. Significantly lower levels of normalized luciferase activity resulted when the same construct was transfected into cultured cell lines that express low or undetectable levels of MUC2, suggesting a possible role for this region in conferring cell-type specificity of expression. We also demonstrate, using actinomycin D, that the MUC2 mRNA is long-lived, at least in cultured cells. Moreover, no evidence was found that the MUC2 mRNA turned over more rapidly in LS174T cells, which produce relatively low levels of MUC2 mRNA, as compared with C1a cells, which produce high levels of mRNA. Thus a long mRNA half-life appears to be an important mechanism involved in achieving elevated levels of MUC2 mRNA.

The sequence of 5' flanking DNA from the mouse 11 beta-hydroxysteroid dehydrogenase type 1 gene and analysis of putative transcription factor binding sites

11 beta-Hydroxysteroid dehydrogenase type 1 (E.C. 1.1.1.146) (11 beta-HSD 1) is a key enzyme in the metabolism of glucocorticoids, catalysing their interconversion with physiologically inert 11-keto metabolites. To identify transcription factors which may be involved in the regulation of expression of mouse 11 beta-HSD 1 we have isolated and sequenced the 5' flanking DNA to 900 bp upstream from the major transcription start site.

Multiple factors regulate the rat liver basolateral sodium-dependent bile acid cotransporter gene promoter

The hepatic uptake of bile acids from the portal circulation is primarily dependent upon a sodium-dependent basolateral membrane transporter. In order to begin to investigate the factors controlling rat liver sodium-dependent bile acid cotransporter (ntcp) gene expression, we isolated approximately 30 kilobase pairs of rat genomic DNA in three overlapping lambdaphage clones. The rat ntcp gene is distributed over 16.5 kilobase pairs as five exons. Primer extension analysis revealed two closely spaced transcription initiation sites, 27 and 41 nucleotides downstream of a TATA sequence. Regulation of transcription was investigated first by transfection of primary rat hepatocytes by a series of 5'-deleted rat ntcp promoter-driven luciferase constructs (from approximately -6 kilobase pairs to -59 base pairs of upstream sequences, terminating at nucleotide +47), identifying a minimal promoter element: nucleotide -158 to +47. This minimal promoter was active in transfected HepG2, but inactive in NIH3T3, Caco-2, and Madin-Darby canine kidney cells, indicating that the determinants of hepatocyte-specific expression reside within this region. The individual elements within the minimal promoter were investigated via transfection of HepG2 cells by a series of 20 mutant plasmids, each containing a 10-base pair sequential block mutation. Eight mutant constructs profoundly suppressed promoter activity; encompassing sequences from -66 to +4 nt, and +15 to +24 nucleotides, while no other 10-base pair mutation significantly interfered with minimal promoter activity. Deoxyribonuclease I footprint analysis of the minimal promoter revealed three bound regions; -92 to -74 (footprint C), -50 to -37 (footprint B), and -17 to +12 (footprint A). Gel mobility shift assays provided evidence for hepatocyte nuclear factor 1 binding within footprint A and a liver-enriched factor(s) that binds within a novel palindrome in footprint B. These studies indicate that three elements direct the basal and tissue-restricted expression of the rat ntcp promoter; a TATA element, the liver-enriched transcription factor hepatocyte nuclear factor 1, and an unknown liver-enriched factor that binds within a novel palindrome in footprint B.

Determination of the molecular weight of DNA-binding proteins using UV-crosslinking and SDS-PAGE

We describe the use of UV-crosslinking in combination with SDS-PAGE to determine the approximate molecular weight of DNA-binding proteins. A 5-bromo-2'-deoxyuridine (5-BrdU)-substituted, radioactively labeled double-stranded oligonucleotide representing the protein binding site is incubated with a crude nuclear extract containing the protein of interest. Following irradiation with a UV light source, the DNA/protein complex is subjected to SDS-PAGE and its molecular weight determined by comparison with appropriate protein standards.

In vivo footprinting and functional analysis of the human c-sis/PDGF B gene promoter provides evidence for two binding sites for transcriptional activators

By in vivo DMS footprint and reporter gene analyses we identified two transcription factor binding sites in the human c-sis/PDGF B gene promoter. The low basal activity of the PDGF B promoter in HeLa and undifferentiated K562 cells, which express low PDGF B mRNA levels, and in PC3 cells, which express a high PDGF B mRNA level, results from binding of a weak transcriptional activator between positions -64 and -61 relative to the transcription start site. Cytotrophoblast-like JEG-3 cells, which do not express the 3.5 kb PDGF B mRNA, contain a transcriptional activator directed at the -64/-61 sequence, but DNA methylation may render the endogenous promoter inaccessible to this activator. A CCACCCAC element at position -61/-54 was identified as the in vivo binding site for a strong transcriptional activator in phorbol ester-treated megakaryocytic K562 cells, which express a high PDGF B mRNA level. Primary human fibroblasts, which do not transcribe the PDGF B gene, contain a transcriptional activator that recognizes an element between positions -60 and -45 but does not bind to the endogenous unmethylated promoter. Our results show that the complex expression pattern of the human PDGF B gene involves the cell type-specific expression of weak and strong transcriptional activators and regulation of promoter accessibility to these factors.

The efficiency of nuclear processing of the tyrosine aminotransferase mRNA transcript increases after partial hepatectomy

Following a two-thirds partial hepatectomy, an approximately fivefold increase in the levels of nuclear and total mRNA for tyrosine aminotransferase was observed at 1 h and 1.5 h, respectively, and a return to the levels of the quiescent state, i.e. the levels found in non-operated livers from adrenalectomized rats, was established 16 h post-hepatectomy. The increase in mRNA levels was not accounted for by a comparable change in the rate of transcription of the gene which, at 0.5 h post-hepatectomy, reached a maximum value that amounted to only 1.4-fold the value for quiescent liver. Subsequent changes in the transcription rate largely accounted for the changes in mRNA levels observed later on. Although tyrosine aminotransferase mRNA levels were equal in quiescent and 16-h-regenerating liver, the rate of transcription of the gene in quiescent liver was threefold higher than the rate in 16-h-regenerating liver. The maintenance of a higher rate of gene transcription in quiescent liver, as compared to regenerating liver, was shown to depend on ongoing protein synthesis. The possibility that the high rate of gene transcription was due to blockage or pausing during transcript elongation in quiescent liver was excluded. The inference is that the pronounced increase in tyrosine aminotransferase mRNA levels within 1 h of partial hepatectomy is largely due to a rapid increase in the efficiency of nuclear processing of the primary transcript.