Tissue-specific and ubiquitous promoters direct the expression of alternatively spliced transcripts from the calcitonin receptor gene

The gene encoding the murine calcitonin receptor (mCTR) was isolated, and the exon/intron structure was determined. Analysis of transcripts revealed novel cDNA sequences, new alternative exon splicing in the 5'-untranslated region, and three putative promoters (P1, P2, and P3). The longest transcription unit is greater than 67 kilobase pairs, and the location of introns within the coding region of the mCTR gene (exons E3-E14) are identical to those of the porcine and human CTR genes. We have identified novel cDNA sequences that form three new exons as well as others that add 512 base pairs to the 5' side of the previously published cDNA, thereby extending exon E1 to 682 base pairs. Two of these novel exons are upstream of exon E2 and form a tripartite exon E2 (E2a, E2b, and E2c) in which E2a is utilized by promoter P2 with variable splicing of E2b. The third new exon (E3b') lies between E3a and E3b and is utilized by promoter P3. Analysis of mCTR mRNAs has revealed that the three alternative promoters give rise to at least seven mCTR isoforms in the 5' region of the gene and generate 5'-untranslated regions of very different lengths. Analysis by reverse transcription-polymerase chain reaction shows that promoters P1 and P2 are utilized in osteoclasts, brain, and kidney, whereas promoter P3 appears to be osteoclast-specific. Using transiently transfected reporter constructs, promoter P2 has activity in both a murine kidney cell line (MDCT209) and a chicken osteoclast-like cell line (HD-11EM), whereas promoter P3 is active only in the osteoclast-like cell line. These transfection data confirm the osteoclast specificity of promoter P3 and provide the first evidence that the CTR gene is regulated in a tissue-specific manner by alternative promoter utilization.

The role of TNF-receptor family members and other TRAF-dependent receptors in bone resorption

The contribution of osteoclasts to the process of bone loss in inflammatory arthritis has recently been demonstrated. Studies in osteoclast biology have led to the identification of factors responsible for the differentiation and activation of osteoclasts, the most important of which is the receptor activator of NF-kappa B ligand/osteoclast differentiation factor (RANKL/ODF), a tumor necrosis factor (TNF)-like protein. The RANKL/ODF receptor, receptor activator of NF-kappa B (RANK), is a TNF-receptor family member present on both osteoclast precursors and mature osteoclasts. Like other TNF-family receptors and the IL-1 receptor, RANK mediates its signal transduction via TNF receptor-associated factor (TRAF) proteins, suggesting that the signaling pathways activated by RANK and other inflammatory cytokines involved in osteoclast differentiation and activation are interconnected.

Calcitonin-dependent down-regulation of the mouse C1a calcitonin receptor in cells of the osteoclast lineage involves a transcriptional mechanism

Although expression of the calcitonin (CT) receptor (CTR) decreases after CT binding, there has been no evidence that it occurs at the transcriptional level. In the present study we investigated the mechanism of CTR messenger RNA (mRNA) down-regulation by CT in mouse cocultures of bone marrow and osteoblasts. Ribonuclease protection analysis revealed that osteoclast-like cells purified from cocultures predominantly express the C1a isoform and do not express an appreciable amount of the brain-specific C1b mRNA (< 1% of C1a). Treatment of day 5 cocultures with CT caused a dose- and time-dependent decrease in the steady state level of C1a mRNA. This CT effect was mimicked by the cAMP agonists forskolin and (Bu)2cAMP. Prolonged suppression of C1a mRNA was observed after short treatment with CT, but not with (Bu)2cAMP, suggesting that persistent intracellular cAMP elevation is necessary for the prolonged CT effect. The half-life of the C1a mRNA in cocultures was 4-6 h and was not altered by CT or (Bu)2cAMP. Moreover, competitive RT-PCR analysis revealed that 1-h treatment with CT reduced the level of CTR heterogeneous nuclear RNA to 10% in a cycloheximide-independent manner. These results suggest that CT down-regulates C1a-CTR mRNA expression at least in part by a transcriptional mechanism, thereby contributing to the ligand-induced desensitization in cells of the osteoclast lineage.

Immortalization of osteoclast precursors by targeting Bcl -XL and Simian virus 40 large T antigen to the osteoclast lineage in transgenic mice

Cellular and molecular characterization of osteoclasts (OCL) has been extremely difficult since OCL are rare cells, and are difficult to isolate in large numbers. We used the tartrate-resistant acid phosphatase promoter to target the bcl-XL and/or Simian Virus 40 large T antigen (Tag) genes to cells in the OCL lineage in transgenic mice as a means of immortalizing OCL precursors. Immunocytochemical studies confirmed that we had targeted Bcl-XL and/or Tag to OCL, and transformed and mitotic OCL were readily apparent in bones from both Tag and bcl-XL/Tag mice. OCL formation in primary bone marrow cultures from bcl-XL, Tag, or bcl-XL/Tag mice was twofold greater compared with that of nontransgenic littermates. Bone marrow cells from bcl-XL/Tag mice, but not from singly transgenic bcl-XL or Tag mice, have survived in continuous culture for more than a year. These cells form high numbers of bone-resorbing OCL when cultured using standard conditions for inducing OCL formation, with approximately 50% of the mononuclear cells incorporated into OCL. The OCL that form express calcitonin receptors and contract in response to calcitonin. Studies examining the proliferative capacity and the resistance of OCL precursors from these transgenic mice to apoptosis demonstrated that the increased numbers of OCL precursors in marrow from bcl-XL/Tag mice was due to their increased survival rather than an increased proliferative capacity compared with Tag, bcl-XL, or normal mice. Histomorphometric studies of bones from bcl-XL/Tag mice also confirmed that there were increased numbers of OCL precursors (TRAP + mononuclear cells) present in vivo. These data demonstrate that by targeting both bcl-XL and Tag to cells in the OCL lineage, we have immortalized OCL precursors that form bone-resorbing OCL with an efficiency that is 300-500 times greater than that of normal marrow.

Phospholipase D- and protein kinase C isoenzyme-dependent signal transduction pathways activated by the calcitonin receptor

The calcitonin receptor expressed by the porcine LLC-PK1 renal tubule cells is a seven-transmembrane domain, G protein-coupled receptor activating adenylyl-cyclase and phospholipase C. Salmon calcitonin stimulated dose- and time-dependent release of the phospholipase D-dependent phosphatidylcholine product [3H] choline with an EC50 = 2.5 +/-0.3 x 10(-8) M, similar to that determined for phosphoinositide metabolism (EC50 = 4.5 +/-1.0 x 10(-8)M). The hormone failed to induce release of [3H]phosphocholine and [3H]glycerophosphocholine, ruling out activation of phosphatydilcholine-specific phospholipase C and phospholipase A. Calcitonin stimulated phosphatidic acid, a product of phospholipase D-dependent phosphatydilcholine hydrolysis. Activation of phospholipase D was confirmed by release of [3H]phosphatydilethanol, a specific and stable product in the presence of a primary alcohol. Activation of calcitonin receptor induced diacylglycerol formation, with a rapid peak followed by a prolonged increase, due to activation of phospholipase C and of phospholipase D. Consequently, the protein kinase-C alpha, but not the delta isoenzyme, was cytosol-to-membrane translocated by approximately 50% after 20 min exposure to calcitonin, whereas protein kinase-C zeta, which was approximately 40% membrane-linked in unstimulated cells, translocated by approximately 19%. The human calcitonin receptor expressed by BIN-67 ovary tumor cells, although displaying higher affinity for calcitonin, failed to activate phospholipase D and protein kinase-C in response to the hormone. This receptor lacks the G protein binding consensus site due to the presence of a 48-bp cassette encoding for a 16-amino acid insert in the predicted first intracellular loop. This modification is likely to prevent the calcitonin receptor from associating to phospholipase-coupled signaling.

Expression of the transcription factor, Spi-1 (PU.1), in differentiating murine erythroleukemia cells Is regulated post-transcriptionally. Evidence for differential stability of transcription factor mRNAs following inducer exposure

Increased expression of the transcription factor Spi-1 (PU.1) results from retroviral insertion in nearly all Friend spleen focus-forming virus-transformed murine erythroleukemia cell lines and exposure of these cells to Me2SO, induces their differentiation and decreases Spi-1 mRNA level by 4-5-fold. While these results suggest that alterations in Spi-1 expression have significant effects on erythroblast growth and differentiation, neither the cause nor the effect of the decrease in Spi-1 expression that follows Me2SO exposure has been established. The experiments described here demonstrate that the effect of inducers on Spi-1 expression is regulated post-transcriptionally. Nuclear run-off transcriptions demonstrated that Spi-1 transcription was not decreased following Me2SO exposure. Additionally, expression of a recombinant Spi-1 mRNA under transcriptional control of a constitutively active Rous sarcoma virus promoter was regulated identically to endogenous Spi-1 mRNA. The ability of Me2SO to destabilize Spi-1 mRNA was selective, as the stability of the erythroid transcription factors GATA-1 and NF-E2 were not similarly effected. The effect of Me2SO on the stability of Spi-1 mRNA provides a novel means of altering gene expression in these cells and is likely to have significance for the differentiation of these cells.

The orphan receptor hepatic nuclear factor 4 functions as a transcriptional activator for tissue-specific and hypoxia-specific erythropoietin gene expression and is antagonized by EAR3/COUP-TF1

The erythropoietin (Epo) gene is regulated by hypoxia-inducible cis-acting elements in the promoter and in a 3' enhancer, both of which contain consensus hexanucleotide hormone receptor response elements which are important for function. A group of 11 orphan nuclear receptors, transcribed and translated in vitro, were screened by the electrophoretic mobility shift assay. Of these, hepatic nuclear factor 4 (HNF-4), TR2-11, ROR alpha 1, and EAR3/COUP-TF1 bound specifically to the response elements in the Epo promoter and enhancer and, except for ROR alpha 1, formed DNA-protein complexes that had mobilities similar to those observed in nuclear extracts of the Epo-producing cell line Hep3B. Moreover, both anti-HNF-4 and anti-COUP antibodies were able to supershift complexes in Hep3B nuclear extracts. Like Epo, HNF-4 is expressed in kidney, liver, and Hep3B cells but not in HeLa cells. Transfection of a plasmid expressing HNF-4 into HeLa cells enabled an eightfold increase in the hypoxic induction of a luciferase reporter construct which contains the minimal Epo enhancer and Epo promoter, provided that the nuclear hormone receptor consensus DNA elements in both the promoter and the enhancer were intact. The augmentation by HNF-4 in HeLa cells could be abrogated by cotransfection with HNF-4 delta C, which retains the DNA binding domain of HNF-4 but lacks the C-terminal activation domain. Moreover, the hypoxia-induced expression of the endogenous Epo gene was significantly inhibited in Hep3B cells stably transfected with HNF-4 delta C. On the other hand, cotransfection of EAR3/COUP-TF1 and the Epo reporter either with HNF-4 into HeLa cells or alone into Hep3B cells suppressed the hypoxia induction of the Epo reporter. These electrophoretic mobility shift assay and functional experiments indicate that HNF-4 plays a critical positive role in the tissue-specific and hypoxia-inducible expression of the Epo gene, whereas the COUP family has a negative modulatory role.

Comparison of the human and mouse erythropoietin genes shows extensive homology in the flanking regions

Considerable insights into important cis regulatory elements in a gene can be gleaned from the identification of sequence homologies among different species. To extend and optimize the sequence comparison between human and mouse erythropoietin (Epo) genes, we have obtained new human sequence from 5,547 to 385 bp upstream of the cap site and extended the 3' flank by 489 bp. In addition, we have obtained new sequence information on the mouse Epo gene extending from within the 3' untranslated region (UTR) to 1,001 bp downstream of the polyadenylation site. Analysis of these additional sequences shows considerable homology between human and mouse Epo genes as far as 4 kb (human) or 3 kb (mouse) upstream of the cap sites, as well as far more homology at the 3' end than was previously realized. In addition, both species were found to have a high frequency of short interspersed (SINE) repetitive sequences that interrupt homologies in both the 5' flank and within the transcription unit.

Mouse beta-globin DNA-binding protein B1 is identical to a proto-oncogene, the transcription factor Spi-1/PU.1, and is restricted in expression to hematopoietic cells and the testis

The hematopoietic-specific DNA-binding protein B1 binds to the DNA consensus sequence AAAGRGGAARYG located twice in intervening sequence 2 of both of the mouse beta-globin genes (D. L. Galson and D.E. Housman, Mol. Cell. Biol. 8:381-392, 1988). B1 was cloned by expression of a murine erythroleukemia (MEL) cell cDNA library in transfected COS cells and screening by electrophoretic mobility shift analysis. B1 is identical to the proto-oncogene Spi-1/PU.1 (Spi-1), an ets family member. Protein-DNA contacts are shown to resemble those of the helix-turn-helix homeodomain proteins. By Northern (RNA) analysis, we found that Spi-1 mRNA is present at low levels during murine CFU-E maturation and is at least 20-fold higher in uninduced MEL, a transformed proerythroblast-like cell line which contains an activating/transforming insertion of spleen focus-forming virus at the Spi-1 locus. Dimethyl sulfoxide-induced MEL cell differentiation decreases Spi-1 mRNA to approximately 20% of the uninduced level before commitment occurs. In addition to erythroid cells, Spi-1 mRNA is present in B cells, myelomonocytes, and mast cells but not in T cells and nonhematopoietic cell types. In situ hybridization demonstrated Spi-1 mRNA expression in bone marrow, spleen, interstitial nonhepatocytes of the liver, and interstitial nontubular cells of the testis. The Spi-1 locus was mapped on human chromosome 11 to the same interval as ACP2 (lysosomal acid phosphatase), between the anonymous DNA markers D11S33 and D11S14. This region has not yet been found to be associated with a human malignancy.

The proto-oncogene PU.1 regulates expression of the myeloid-specific CD11b promoter

The myeloid integrin CD11b is expressed selectively on the surface of mature monocytes, macrophages, granulocytes, and natural killer cells. Tissue-specific and developmentally regulated expression of CD11b is controlled at the level of mRNA transcription, and recent characterization of the human CD11b promoter indicates that the first 92 bp of 5'-flanking DNA are sufficient to direct tissue-specific expression of a reporter gene. Here we show that the sequence AAAAGGAGAAG at base pair -20 of the CD11b promoter binds the proto-oncogene PU.1 in vitro and that mutation of this site significantly reduces the ability of the CD11b promoter to direct expression of a reporter gene in myeloid cells but not in nonmyeloid cells. PU.1 may thus represent a major determinant of the myeloid expression of CD11b.

The human prointerleukin 1 beta gene requires DNA sequences both proximal and distal to the transcription start site for tissue-specific induction

In these studies, we have identified DNA sequences and specific protein interactions necessary for transcriptional regulation of the human prointerleukin 1 beta (proIL-1 beta) gene. A cell-type-independent lipopolysaccharide (LPS)-responsive enhancer element located between -3757 and -2729 bp upstream from the transcription start site (cap site) consisted of at least six discrete subregions which were essential to the maximal induction by LPS in transfected monocytes. The enhancer also appeared to mediate phorbol myristate acetate induction in monocytes and IL-1 responsiveness in fibroblasts. Deletion and base substitution mutations along with DNA binding studies demonstrated that the enhancer contained a minimum of three functional protein binding sequences, two of which appeared to be important for gene induction. One of the essential proteins which bound to the enhancer was similar or identical to members of the C/EBP family of transcription factors required for both IL-1- and LPS-specific induction of the IL-6 gene (i.e., the NF-IL6 proteins). When ligated to the proIL-1 beta cap site-proximal region (located between -131 to +12), both the proIL-1 beta and the simian virus 40 enhancer elements functioned more efficiently in monocytes than in HeLa cells, which are not normally competent for IL-1 beta expression. When ligated to the murine c-fos promoter, however, the proIL-1 beta enhancer was inducible in phorbol myristate acetate-stimulated HeLa cells, suggesting the existence of a proIL-1 beta promoter-proximal requirement for tissue specificity.

Hypoxic induction of the human erythropoietin gene: cooperation between the promoter and enhancer, each of which contains steroid receptor response elements

Transcription of the human erythropoietin (Epo) gene is stimulated by exposure to hypoxia and/or cobalt in whole animals and in Hep3B cells. We have systematically investigated the promoter and 3' enhancer elements necessary for this induction by transient transfection of Hep3B cells. We define a promoter region of 53 bp and an enhancer region of 43 bp that confer hypoxia and cobalt inducibility. Each element gives rise to a 6- to 10-fold induction alone. In combination they produce a 50-fold induction after stimulation, similar to the 50- to 100-fold induction of the endogenous Epo gene. Two areas of DNA sequence homology are present in these regions. We demonstrate specific DNA-protein interactions in the enhancer and the ability of the promoter element to compete with these interactions in electrophoretic mobility shift assays. DNase I footprinting and methylation interference data further refine the cis-acting element in the 43-bp enhancer to a short region containing a direct repeat of a steroid/thyroid hormone receptor response element half-site separated by a 2-bp gap. Two half-site consensus sequences are also present in the 53-bp promoter. Site-specific mutation of the half-site sequences in the enhancer destroys the functional activity of the enhancer.

The Nucleic Acid Blot Analyzer II. ANALYZE, an image analysis software package for molecular biology

The Nucleic Acid Blot Analyzer, a new instrument providing high-speed imaging of 32P labeled nucleic acids, captures, stores and presents images in digital form, thus lending itself to rapid data handling and analysis as well as replacing conventional X-ray film autoradiography for many applications. A software package called ANALYZE has been specifically designed for the instrument in order to provide automatic or semi-automatic analysis for molecular biological techniques. The software includes image display manipulation, quantitative and positional analysis, as well as file maintenance utilities. The specific application of the software/hardware to various techniques is presented.

Detection of two tissue-specific DNA-binding proteins with affinity for sites in the mouse beta-globin intervening sequence 2

To identify proteins from uninduced murine erythroleukemia nuclear extracts which specifically bind to sequences from the DNase I-hypersensitive region within the mouse beta-globin intervening sequence 2 (IVS2), a gel electrophoretic mobility shift assay was used. Two distinct sequence-specific binding proteins were detected. The specific binding sites for these factors were delineated by both DNase I protection footprinting and methylation interference. Factor B1 bound specifically to two homologous sites, B1-A and B1-B, approximately 100 base pairs apart within the IVS2 and on opposite strands. These two regions could interact with factor B1 independently. Factor B1 was limited to cells of hematopoietic lineages. Factor B2 bound to a site approximately 5 base pairs away from the B1-A site and was limited to cells of the erythroid lineage. The limited tissue distribution of these factors and the locations of their binding sites suggest that one or both of these factors may be involved in the formation of the tissue-specific DNase I-hypersensitive site in the IVS2 of the mouse beta-globin gene.