Molecular cloning and expression of murine thromboxane synthase

The cytochromes P450 (CYP) response to allergic inflammation of the lung

The expression of the mouse Cyp family and key inflammatory mediators were examined in a model of ovalbumin (OVA)-induced allergic airway disease. The expression of IL-4, IL-13 and Ccl11 increased during the acute phase of allergic inflammation and decreased with its resolution. Interestingly, the expression of Ccl20 was increased during the resolution phase. The response of the Cyp gene family to the development of allergic inflammation was differential and correlated with the evolution of the inflammatory response. During the acute inflammatory phase the mRNA levels of Cyp2e1, Cyp2f2, Cyp2j6, Cyp4b1, Cyp8a1 and Cypor were decreased while the mRNA levels of Cyp4f18, Cyp5a1 and Cyp7b1 were elevated. With resolution of the inflammation the expression patterns returned to normal. These changes suggest that the Cyp family may play a role in the allergic inflammation by modulating the metabolism of xenobiotics and endogenous compounds such as LTB4, TXA1, PGI2 and native anti-glucocorticoids.

Expression of cyclooxygenase-2 and thromboxane synthase in non-neoplastic and neoplastic thyroid lesions

Cyclooxygenase-2 (COX-2) and thromboxane synthase (TBXAS) are important enzymes involved in the arachidonic acid pathway and synthesis of prostaglandins. We examined COX-2 and TBXAS immunoreactivity in 150 surgically resected thyroid specimens using immunohistochemistry to determine expression in benign and malignant thyroid lesions and to examine their roles in thyroid tumor progression. Papillary thyroid carcinomas and follicular carcinomas expressed higher levels of COX-2 compared to follicular adenomas and adenomatous nodules. We showed for the first time that TBXAS was expressed in thyroid tissues, with higher levels in papillary carcinomas compared to non-neoplastic and benign thyroid tissues. Western blot was performed on seven thyroid samples. These results indicate that both COX-2 and TBXAS are expressed in benign and malignant thyroid tissues. Although some malignant thyroid tumors showed higher levels of COX-2 expression, COX-2 and TBXAS are probably not useful in the immunohistochemical diagnosis of thyroid malignancies. However, the expression of both COX-2 and TBXAS by thyroid tissue may provide insight into the role of these enzymes in progression from benign to malignant thyroid tumors.

Transcriptional control of the human thromboxane synthase gene in vivo and in vitro

Thromboxane A(2), a potent mediator of vasoconstriction and platelet aggregation, is synthesized from prostaglandin H(2) by thromboxane synthase (TXAS). We report here on promoter analyses of human TXAS using in vitro transcription and in vivo transfection methods. The 39-bp core promoter, containing both TATA and initiator elements, accurately initiates transcription in an orientation-dependent manner in a cell-free transcription system. Mutation of either TATA or initiator abolished transcriptional activity, but the upstream sequence had no effect on TXAS promoter activities in vitro, suggesting that the core promoter is sufficient for transcriptional activity from a naked DNA template. In contrast, mutation of both elements drastically decreased the promoter activity in vivo. Furthermore, TXAS proximal promoter containing the nucleotides -90 to -56 relative to the transcriptional start site was necessary for promoter transactivation in vivo. Transcriptional activities from 5'-deletion mutants indicated that the effects of the nucleotides -90/-56 were more pronounced in stably transfected cells (a 150-fold difference) than in the transiently transfected cells (an 8-fold difference), reflecting the effects of TXAS transcriptional activation from replicating and nonreplicating DNA templates. Partial micrococcal nuclease digestion indicated that the sequence -90/-56, where the NF-E2 site is located, is associated with alterations of nucleosomal structure at the regions of promoter and reporter gene but not the region further downstream. Mutagenesis and forced expression studies demonstrated a critical role of p45 NF-E2 in controlling TXAS expression under native chromatin conditions. Band shifting and chromatin immunoprecipitation assays indicated that p45 NF-E2 was bound to the TXAS promoter in vitro and in vivo. Indirect end labeling and ligation-mediated PCR analyses further demonstrated that the occupation of TXAS promoter NF-E2 site was associated with disruption of nucleosomal structure. Collectively, these results indicate that binding of NF-E2 is critical both for alteration of the nucleosomal structure and for activation of the TXAS promoter, whereas the TATA and initiator elements are essential for transcription.

Involvement of thromboxane A(2) in airway mucous cells in asthma-related cough

The aim of this study was to elucidate the role of thromboxane A(2) (TxA(2)) on asthma-related cough in guinea pigs. Animals were immunosensitized and repeatedly challenged with ovalbumin as an antigen. Coughs were induced by the inhalation of 10(-5) M capsaicin solution for 10 min. Thromboxane synthetase (TxS) inhibitor OKY-046 and thromboxane-receptor antagonist AA-2414 significantly inhibited cough responses in repeatedly challenged animals. Inhalation of TxA(2) mimic STA-2- potentiated cough responses in normal and immunosensitized animals but not in repeatedly challenged ones. Moreover, STA-2-potentiated coughs were inhibited by administration of neurokinin-receptor antagonist FK-224. In repeatedly challenged animals, concentration of TxB(2) in airway lavage fluid, expression of TxS mRNA in tracheal epithelia, and the immunostaining intensity against TxS in mucous cells of the epithelium significantly increased compared with normal and sensitized animals. These results suggest that TxA(2) derived from mucous cells potentiated cough responses to capsaicin in allergic airway inflammation.

Identification of genetic variants in the human thromboxane synthase gene (CYP5A1)

Thromboxane synthase (CYP5A1) catalyzes the conversion of prostaglandin H2 to thromboxane A2, a potent mediator of platelet aggregation, vasoconstriction and bronchoconstriction. It has been implicated in the patho-physiological process of a variety of diseases, such as atherosclerosis, myocardial infarction, stroke and asthma. On the basis of the hypothesis that variations of the CYP5A1 gene may play an important role in human diseases, we performed a screening for variations in the human CYP5A1 gene sequence. We examined genomic DNA from 200 individuals, for mutations in the promoter region, the protein encoding sequences and the 3'-untranslated region of the CYP5A1. Eleven polymorphisms have been identified in the CYP5A1 gene including eight missense mutations R61H, D161E, N246S, L357V, Q417E, E450K, T451N and R466Q. This is the first report of genetic variants in the human CYP5A1 altering the protein sequence. The effect of these variants on the metabolic activity of CYP5A1 remains to be further evaluated.

Suppression of rat thromboxane synthase gene transcription by peroxisome proliferator-activated receptor gamma in macrophages via an interaction with NRF2

We have studied the transcription regulation of the rat thromboxane synthase (TXS) gene by peroxisome proliferator-activated receptor gamma (PPARgamma) in macrophages. The transcription activity of a cloned 5'-flanking region (1.6 kilobases) of the rat TXS gene (5'FL-TXS) was examined by luciferase reporter gene assay. TXS mRNA expression and the transcription activity of 5'FL-TXS were inhibited by PPARgamma ligands, 15-deoxy-Delta(12,14)-prostaglandin J(2) (PGJ(2)), and the thiazolidinedione troglitazone (TRO) in a dose-dependent manner. Overexpression of PPARgamma also significantly suppressed transcription, and further addition of PGJ(2) or TRO augmented the suppression. Deletion analysis showed that the element responsible for the PPARgamma effect is located in a region containing the nuclear factor E2 (NF-E2)/AP-1 site (-98/-88), which was indicated to be the major promoter of the TXS gene. By electrophoretic mobility shift assay using the NF-E2/AP-1 site and nuclear extracts from macrophages, we observed a specific protein-DNA complex formation, which was inhibited by a specific antibody against the transcription factor NRF2 (NF-E2-related factor 2). Moreover, the complex was decreased with PGJ(2), TRO, or in vitro translated PPARgamma. The transcription suppression by PPARgamma was confirmed using this truncated NRF2-binding element (-98/-88) by the reporter gene assay. Finally, a direct interaction between PPARgamma and NRF2 was confirmed by glutathione S-transferase pull-down assay. In conclusion, the NRF2-binding site (-98/-88) is the major promoter of 5'FL-TXS which can be suppressed by activated PPARgamma via a protein-protein interaction with NRF2 in macrophages.

Thromboxanes: synthase and receptors

Thromboxane A2 is a biologically potent arachidonate metabolite through the cyclooxygenase pathway. It induces platelet aggregation and smooth muscle contraction and may promote mitogenesis and apoptosis of other cells. Its roles in physiological and pathological conditions have been widely documented. The enzyme that catalyzes its synthesis, thromboxane A2 synthase, and the receptors that mediate its actions, thromboxane A2 receptors, are the two key components critical for the functioning of this potent autacoid. Recent molecular biological studies have revealed the structure-function relationship and gene organizations of these proteins as well as genetic and epigenetic factors modulating their gene expression. Future investigation should shed light on detailed molecular signaling events specifying thromboxane A2 actions, and the genetic underpinning of the enzyme and the receptors in health and disease.

In situ localization and regulation of thromboxane A(2) synthase in normal and LPS-primed lungs

Thromboxane (Tx) A(2) synthase catalyzes the conversion of prostaglandin H(2) to the unstable metabolite TxA(2), which is a potent mediator of vasoconstriction and bronchoconstriction. The cellular localization of TxA(2) synthase was examined by immunohistochemistry and in situ hybridization in human and rat lung tissues. Bronchial epithelial cells, bronchial smooth muscle cells, peribronchial nerve fibers, single cells of bronchus-associated lymphoid tissue, single cells located in the alveolar septum, and alveolar macrophages exhibited positive immunostaining for TxA(2) synthase protein in lung tissue of both species. In addition, vascular smooth muscle cells of muscular and partially muscular vessels displayed strong (rat) and moderate (human) immunostaining for TxA(2) synthase. In situ hybridization performed in the rat lungs demonstrated TxA(2) synthase mRNA localization in accordance with the immunostaining pattern. Perfusing isolated rat lungs with endotoxin for 1 and 2 h resulted in a marked increase in TxA(2) synthase protein staining intensity in most cell types as measured by quantitative image analysis, whereas the in situ hybridization signal was unchanged. We conclude that the pulmonary distribution of TxA(2) synthase displays close similarity between rat and human lung tissues and matches well with the previously described immunolocalization of cyclooxygenase-1 and cyclooxygenase-2 in this tissue. Endotoxin challenge is suggested to cause a rapid upregulation of TxA(2) synthase at the posttranscriptional level. These data provide a morphological basis for the understanding of the role of TxA(2) in the regulation of lung bronchial and vascular tone and in immunologic events.

Expression, purification, and spectroscopic characterization of human thromboxane synthase

Thromboxane A2 (TXA2) is a potent inducer of vasoconstriction and platelet aggregation. Large scale expression of TXA2 synthase (TXAS) is very useful for studies of the reaction mechanism, structural/functional relationships, and drug interactions. We report here a heterologous system for overexpression of human TXAS. The TXAS cDNA was modified by replacing the sequence encoding the first 28 amino acid residues with a CYP17 amino-terminal sequence and by adding a polyhistidine tag sequence prior to the stop codon; the cDNA was inserted into the pCW vector and co-expressed with chaperonins groES and groEL in Escherichia coli. The resulting recombinant protein was purified to electrophoretic homogeneity by affinity, ion exchange, and hydrophobic chromatography. UV-visible absorbance (UV-Vis), magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) spectra indicate that TXAS has a typical low spin cytochrome P450 heme with an oxygen-based distal ligand. The UV-Vis and EPR spectra of recombinant TXAS were essentially identical to those of TXAS isolated from human platelets, except that a more homogenous EPR spectrum was observed for the recombinant TXAS. The recombinant protein had a heme:protein molar ratio of 0.7:1 and a specific activity of 12 micromol of TXA2/min/mg of protein at 23 degreesC. Furthermore, it catalyzed formation of TXA2, 12-hydroxy-5,8,10-heptadecatrienoic acid, and malondialdehyde in a molar ratio of 0.94:1.0:0.93. Spectral binding titrations showed that bulky heme ligands such as clotrimazole bound strongly to TXAS (Kd approximately 0.5 microM), indicating ample space at the distal face of the heme iron. Analysis of MCD and EPR spectra showed that TXAS was a typical low spin hemoprotein with a proximal thiolate ligand and had a very hydrophobic distal ligand binding domain.

Genomic organization, chromosomal localization, and expression of the murine thromboxane synthase gene

Thromboxane synthase (TS) is a membrane-bound cytochrome P450 enzyme catalyzing the synthesis of TxA2, a potent modulator of vascular smooth muscle contraction and platelet aggregation. TS plays an important role in hemostasis and may be intimately involved in the etiology of cardiovascular, renal, and immune diseases. Restriction enzyme mapping, subcloning, and DNA sequencing analysis of recombinant phage lambda and P1 clones revealed that exons encoding the 1.9-kb mouse TS mRNA are dispersed over >150 kb genomic DNA. Determination of the intron-exon splicing junctions established that the mouse TS gene (Tbxas1) is encoded by 13 exons ranging in size from 53 (exon III) to 315 bp (exon IX). Genomic Southern analysis and fluorescence in situ hybridization suggested that the gene is a single-copy gene, located on chromosome 6 near the midpoint between the centromere and the Igkappa gene. An alternatively spliced variant of the Tbxas1 transcript, lacking the exon XII-encoded sequence, has been detected in normal mouse tissues. Ribonuclease protection and 5'-RACE assays identified at least five major transcription start sites clustered within 31 bp of the Tbxas1 promoter. The 5'-most start site is not preceded by a TATA box, suggesting transcription can be initiated in a TATA-independent manner. Transfection analyses indicated that the expression of Tbxas1 is controlled by a short (70-bp) positive regulatory sequence and several upstream repressive elements. Mutational studies further demonstrated that NF-E2/AP-1 and Sp1 exerted activating and repressive, respectively, effects on the promoter. These studies provide the genetic tools and information for TS research in mice, which should expedite understanding of the genetic contribution of TS in normal physiology as well as in disease states.

Inverse gene expression of prostacyclin and thromboxane synthases in resident and activated peritoneal macrophages

Prostacyclin and thromboxane A2 produced from prostaglandin H2 are known to be important modulators with opposite biological activities. To examine possible roles of these prostanoids in immune responses, we have studied the gene expression of prostacyclin synthase (PGIS) and thromboxane synthase (TXS) in murine resident macrophages or in macrophages elicited with casein or bacillus Calmette-Guérin (BCG). Northern blot analyses showed that the PGIS mRNA was expressed in a decreasing order in the resident, and casein- and BCG-elicited macrophages. In contrast, the TXS mRNA was expressed in an increasing order in the resident, and casein- and BCG-elicited macrophages. On the other hand, the mRNA for cyclooxygenase-2, which produces PGH2 and participates in the production of prostanoids in inflammation, was expressed in both the resident and BCG-elicited macrophages but barely in the casein-elicited cells. In situ hybridization analysis showed that the expression of mRNAs for PGIS and TXS was ascribable not only to the alteration of the expression levels of both mRNAs in the each macrophage but also to the changes in subpopulations of the cells expressing these mRNAs. These observations suggested that the inverse gene expression of PGIS and TXS in macrophages contributes to immune responses by modulating the relative levels of prostacyclin and thromboxane A2.

Rat kidney thromboxane synthase: cDNA cloning and gene expression regulation in hydronephrotic kidney

We isolated a rat homolog of thromboxane (TX) synthase cDNA (-1.8 kb) from the kidney with a fragment of human TX synthase cDNA amplified by polymerase chain reaction with placenta cDNA as a template. Northern blot analysis has shown that rat TX synthase gene is expressed abundantly in lung, liver, and uterus; moderately in kidney. TX synthase mRNA expression was up-regulated in hydronephrotic kidney made by ureter ligation. In conclusion, we have revealed the structure of rat kidney TX synthase. Up-regulation of renal TX synthase, which may cause stimulation of TX synthesis, is possibly implicated in the tissue injury in hydronephrotic kidney.

Multiple factors regulating the expression of human thromboxane synthase gene

Characterization of the 5.5 kb promoter of human thromboxane synthase (TS) gene revealed a proximal positive regulatory sequence (PPRS, -90 to -25 bp) and several distal repressive elements. The maximal promoter activity was found to reside within the first 285 bp, approximately 75% of which was contributed by the PPRS. The sequence between -365 and -665 bp exerted a strong repressive effect (approximately 55%) on reporter gene expression independent of orientation and position, consistent with properties expected for a silencer. The sequence upstream of -665 bp to -5.5 kb contains mainly repressive elements which further reduce the promoter activity by 30%. The 65 bp PPRS worked in an orientation-independent, but position-dependent, manner and could be further divided into two independent elements, PPRS1 (-90 to -50 bp) and PPRS2 (-50 to -25 bp). While similar nuclear factor(s) from different cell types interact with PPRS2, those interacting with PPRS1 exhibit cell specificity. Internal sequence deletion and oligonucleotide competition established that a binding sequence for NF-E2 in PPRS1 (-60 tgctgattcat -50) was important for enhancing TS promoter activity in HL-60 cells. The presence of NF-E2 mRNA in HL-60 cells was demonstrated by reverse-transcription PCR amplification of the cDNA and Northern blot analysis. A 9-fold transactivation of luciferase (luc) reporter gene expression had been detected when NF-E2 cDNA was co-expressed with a TS promoter/luc construct. Despite the fact that NF-E2 and the cis-elements could alter the efficiency of TS transcription, they were not sufficient for restricting cell-specific TS expression. Analysis of the methylation status at the TS promoter in several human cell lines reveals cell-specific patterns of methylation that might correlate with TS expression. Taken together, these results suggest that the expression of human TS gene is modulated by multiple factors including cis-elements, trans-activator(s), and possibly genomic methylation.

Genomic structure and polymorphism of the human thromboxane synthase-encoding gene

Thromboxane synthase (TS) is a cytochrome P-450 (CYP450) enzyme catalyzing the conversion of prostaglandin endoperoxide (PGH2) into thromboxane A2 (TxA2) which plays a crucial role in hemostasis and cardiovascular diseases. Twelve genomic clones containing the DNA encoding the human TS gene (hTS) were isolated and characterized to determine the exon/intron boundaries and restriction maps of the nearly contiguous structure of the gene. The hTS contains 13 exons spanning more than 150 kb. Its first five exons, divided by relatively large introns, spread over 100 kb, but encode less than one third of the full-length TS transcript. Southern analysis indicates that the human haploid genome contains a single copy of the TS gene. Although multiple transcription start points (tsp) are utilized, transcription of hTS is primarily TATA-independent, as determined by promoter-directed reporter gene expression in transfected cells. A dinucleotide (CA) repetitive sequence identified in the ninth intron of the gene exhibits allelic polymorphism. At least four distinctive alleles, containing from 13 to 20 copies of the CA repeats, have been detected.

Identification of thromboxane A2 synthase active site residues by molecular modeling-guided site-directed mutagenesis

Human thromboxane A2 synthase (TXAS) exhibits spectral characteristics of cytochrome P450 but lacks monooxygenase activity. Its distinctive amino acid sequence makes TXAS the sole member of family 5 in the P450 superfamily. To better understand the structure-function relationship of this unusual P450, we have recently constructed a three-dimensional model for TXAS using P450BM-3 as the template (Ruan, K.-H., Milfeld, K., Kulmacz, R. J., and Wu, K. K. (1994) Protein Eng. 7, 1345-1551) and have identified a potential active site region. The catalytic roles of several putative active site residues were evaluated using selectively mutated recombinant TXAS expressed in COS-1 cells. Mutation of Ala-408 to Glu or Arg-413 to Gly led to a complete loss of enzyme activity despite expression of mutant protein levels equivalent to that of the wild-type TXAS. Mutation of Ala-408 to Gly or Leu retained the enzyme activity at levels of 30 or 40%, respectively. This suggests that Ala-408 provides a hydrophobic environment for substrate binding. Mutation of Arg-413 to Lys or Gln completely abolished the enzyme activity, indicating that this residue is essential to catalytic activity and supports its identification as an active site residue. Mutation of Arg-410 to Gly or Glu-433 to Ala resulted in >50% reduction in the enzyme activity without appreciably altering mutant protein expression, consistent with a more subtle effect of these residues on TXAS catalytic efficiency. Mutation of residues predicted to be involved in binding the heme prosthetic group, including the heme thiolate ligand Cys-480, Arg-478, Phe-127, and Asn-110, each markedly reduced the expressed protein level and abolished enzyme activity. This suggests that proper heme binding is important to synthesis or stability of recombinant TXAS. Mutation of Ile-346, which corresponds to P450cam-Thr-252, an essential amino acid involved in dioxygen bond scission, to Thr increased the enzymatic activity by 40%, suggesting that oxygen bond cleavage is not a rate-limiting step in thromboxane A2 biosynthesis. The present results from site-directed mutagenesis support the overall structure of the TXAS active site predicted by homology modeling and have allowed refinement of the position of bound substrate.

The porcine thromboxane synthase-encoding cDNA: sequence, mRNA expression and enzyme production in Sf9 insect cells

A full-length cDNA encoding porcine thromboxane synthase (TS) was isolated and sequenced. The open reading frame encodes a 534-amino acid (aa) protein (M(r) 60,451) which shares more than 75% identity with TS from other species and is 30% homologous to several enzymes of the cytochrome P-450 III family. Sequence comparison among porcine (p), human (h), and murine (m) TS indicated conservation of eight Cys residues and one putative N-glycosylation site. Several highly conserved regions were identified at the near N terminus, middle and C terminus. The most divergent region lies at aa residues 290-325, within which a Lys308 residue was unique to pTS. Between aa residues 70 and 90, considerable divergence was observed in mTS. Northern analysis showed that the pTS gene was expressed as a 2.3-kb transcript primarily in lung, kidney and thymus. A high-titer recombinant (re-) baculovirus containing pTS cDNA was developed to conduct a time course study of enzyme production in Spodoptera frugiperda (Sf9) cells. TS activity was detectable in the microsomes of Sf9 cells 12-h post-infection and reached maximum by 48 h. The produced TS resembles purified pTS in catalysis, as well as inhibition by a substrate analog inhibitor.

Abundant expression of thromboxane synthase in rat macrophages

The cloned cDNA for rat thromboxane (TX) synthase with a size of 1851 bp contained a 1599-bp open reading frame which encoded a 533-amino acid protein sharing 79.7% identity with human TX synthase. RNA blot analysis was carried out with rat cells and tissues. Rat peritoneal macrophages most abundantly expressed mRNA for TX synthase, and its level was not changed by in vivo stimulation of casein. Bone marrow, spleen, lung and thymus also expressed the TX synthase gene. These findings suggest the possibility that TXA2 plays a role in the immune system.