Characterization of 5' end of human thromboxane receptor gene. Organizational analysis and mapping of protein kinase C--responsive elements regulating expression in platelets

Regulation of the human thromboxane A2 receptor gene by Sp1, Egr1, NF-E2, GATA-1, and Ets-1 in megakaryocytes

The alpha and beta isoforms of the human thromboxane A(2) (TXA(2)) receptor (TP) are encoded by a single gene but are transcriptionally regulated by distinct promoters, termed promoter 1 (Prm1) and Prm3, respectively. Herein, it was sought to identify factors regulating Prm1 within the megakaryocytic human erythroleukemia 92.1.7 cell line. Through gene deletion and reporter assays, the core Prm1 was localized to between nucleotides -6,320 and -5,895, proximal to the transcription initiation site. Furthermore, two upstream repressor and two upstream activator regions were identified. Site-directed mutagenesis of four overlapping Sp1/Egr1 elements and an NF-E2/AP1 element within the proximal region substantially reduced Prm1 activity. Deletion/mutation of GATA and Ets elements disrupted the upstream activator sequence located between -7,962 and -7,717, significantly impairing Prm1 activity. Electrophoretic mobility shift assays and chromatin immunoprecipitations confirmed that Sp1, Egr1, and NF-E2 bind to elements within the core promoter, whereas GATA-1 and Ets-1 factors bind to the upstream activator sequence (between -7,962 and -7,717). Collectively, these data establish that Sp1, Egr1, and NF-E2 regulate core Prm1 activity in the megakaryocytic-platelet progenitor cells, whereas GATA-1 and Ets-1 act as critical upstream activators, hence providing the first genetic basis for the expression of the human TXA(2) receptor (TP) within the vasculature.

Characterization of the 5' untranslated region of alpha and beta isoforms of the human thromboxane A2 receptor (TP). Differential promoter utilization by the TP isoforms

In humans, thromboxane (TX) A2 signals through two TXA2 receptor (TP) isoforms, TPalpha and TPbeta, that diverge within their carboxyl terminal cytoplasmic (C) tail regions and arise by differential splicing. The human TP gene contains three exons E1-E3; while E1 exclusively encodes 5' untranslated region (UTR) sequence, E2 and E3 represent the main coding exons. An additional noncoding exon, E1b was identified within intron 1. Additionally, the TP gene contains two promoters P1 and P2 located 5' of E1 and E1b, respectively. Herein, we investigated the molecular basis of the differential expression of the TP isoforms by characterizing the 5' UTR of the TP transcripts. While E1 and E1b were found associated with TP transcript(s), their expression was mutually exclusive. 5' rapid amplification of cDNA ends (5' RACE) established that the major transcription initiation (TI) sites were clustered between -115 and -92 within E1 and at -99 within E1b. While E1 and E1b sequences were identified on TPalpha transcript(s), neither existed on TPbeta transcript(s). More specifically, TPalpha and TPbeta transcripts diverged within E2 and the major TI sites for TPbeta transcripts mapped to -12/-15 therein. Through genetic reporter assays, a previously unrecognized promoter, termed P3, was identified on the TP gene located immediately 5' of -12. The proximity of P3 to the TI site of TPbeta suggests a role for P3 in the control of TPbeta expression and implies that TPalpha and TPbeta, in addition to being products of differential splicing, are under the transcriptional control of distinct promoters.

The 5' region of the human thromboxane A(2) receptor gene

Thromboxane is an important modulator of hemostasis and smooth muscle tonus and signals via G-protein-coupled thromboxane receptor. Previously, we characterized the TP receptor gene and suggested the presence of three promoter regions within the gene. The aim of the present study was to examine the regulation of transcriptional gene expression. By primer extension experiments the major transcription initiation site was shown to be a doublet at -160/165 bp upstream of the ATG codon in human megakaryoblastic MEG-01 cells, endothelial ECV 304 cells and in human myometrium smooth muscle cells. In the erythroleukemic HEL 1 cells transcription initiation site was identified at -10 bp. Transcriptional activity of the three 5'flanking regions of TP receptor gene representing the putative promoter regions was evaluated by transfection of MEG-01 cells with chimeric constructs containing luciferase gene-encoding sequence. Promoter region I displayed highest transcriptional activity and RT-PCR analysis confirmed the transcription of TP receptor mRNA driven by promoter I. Although, weak transcriptional activity was also observed regarding promoter region II, we were unable to amplify cDNA fragments representing promoter II-driven mRNA synthesis. Considering promoter region III, transcriptional activity was barely detectable. Various deletions of the 3.9 kb promoter I region revealed a size-dependent transcriptional activity. Further, for full activity a 'core' promoter corresponding to the region from -160/165 to -588 bp appeared to be necessary for full transcriptional activity of promoter 1.

Molecular biology of blood pressure regulatory genes

Blood pressure is determined by vascular resistance and circulating volume. Activation of vascular angiotensin II or thromboxane receptor is mostly involved in the former, and function of renal prostagalandin EP3 receptor or thiazide-sensitive sodium-chloride co-transporter is also in the latter. We have cloned rat genes for these blood pressure regulatory factors, and studied their gene expression. Here we review the molecular biology of those genes, based on our observations.

Role of thromboxane A2 in mitogenesis of vascular smooth muscle cells

Thromboxane A2, a product of activated platelets, is a potent vasoconstrictor and promoter of vascular smooth muscle cell growth. Therefore, thromboxane has the potential to contribute to processes, such as restenosis following coronary angioplasty, characterized by both platelet activation and abnormal vascular smooth muscle growth. This article reviews the effects of thromboxane on growth of cultured vascular smooth muscle cells, discusses the mechanisms by which thromboxane transduces its growth promoting effects in tissue culture with an emphasis on the role of endogenously produced basic fibroblast growth factor, and reviews clinical studies of thromboxane synthesis inhibitors and/or receptor blockers in angioplasty restenosis.

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.

Structure and transcriptional function of the 5'-flanking region of rat thromboxane receptor gene

We cloned a cDNA for rat TX receptor, and observed its expression in the kidney, including vascular smooth muscle. The aim of the present study was to clone the 5'-flanking region (5'-FL) of rat TX receptor gene, and to examine its transcriptional gene expression regulation. The 5'-FL was cloned by a PCR method, and the nucleic acid structure of 5'-FL (approximately 1 Kb) was disclosed. The transcription initiation site was shown to be 63 bases upstream of the 5' end of the cDNA by the primer extension. In the 5'-FL, putative AP-1 binding sites, glucocorticoid-responsive elements, NF-kappa B binding sites, GATA box, and shear stress-responsive elements were identified. The 5'-FL was then fused upstream of firefly luciferase cDNA in an expression vector, and we examined its transcriptional activity in transiently transfected cultured vascular smooth muscle cells (VSMC). Luciferase expression was dependent on the length of 5'-FL, and it was significantly stimulated by phorbol 12-myristate 13-acetate (PMA), dexamethasone (Dex), tumor necrosis factor-alpha, and interleukin (IL). By a semi-quantitative RT-PCR method, TX receptor mRNA was shown to be induced by Dex, IL-6, and PMA in cultured VSMC. In conclusion, we have revealed the structure of transcription regulatory region of TX receptor. Expression of TX receptor gene is possibly up-regulated by activation of protein kinase C, glucocorticoid excess, and IL-6, in vascular smooth muscle.

Establishment and characterization of a conditionally immortalized smooth muscle/myometrial-like cell line

A novel smooth muscle/myometrial-like cell line, SMU1-10, has been generated from the uterus of a H-2Kb-tsA58 transgenic mouse carrying a thermolabile SV40 large T-antigen gene. These cells grow continuously when maintained at the permissive temperature (33 degrees C) for the SV40 large T-antigen but stop dividing when placed at the non-permissive temperature (39 degrees C) and ultimately die within 3 weeks. All of the SMU1-10 cells produce smooth muscle alpha-actin (SMAA) at both 33 degrees C and 39 degrees C. A subset of the cells also contain smooth muscle gamma-actin (SMGA), a hallmark of smooth muscle differentiation, and the fraction of cells staining for this actin increases from about 1% when maintained for three days at 33 degrees C to as much as 30% at 39 degrees C over the same length of time. However, the appearance of SMGA in SMU1-10 cells appears to be regulated mainly at a post-transcriptional level since in situ hybridization indicates that all cells contain SMGA mRNA at both 33 degrees C and 39 degrees C. SMU1-10 cultures also contain smooth muscle myosin heavy chain (SM-MHC) and SM22 alpha, both of which are only found in smooth muscle of the adult mouse. Three additional smooth muscle (myometrium)-related markers, connexin 43, the thromboxane A2 receptor, and the progesterone receptor also are present in these cells. At the nonpermissive temperature for SV40 large T-antigen, the both level of SMGA mRNA and the number of cells staining for this actin are significantly increased in the presence of progesterone, a process that is similar to the upregulation of SMGA in the myometrium late in pregnancy. Overall, SMU1-10 cells provides a potentially useful in vitro model system to study smooth muscle/myometrial differentiation.

Deficient thromboxane synthesis and response in platelets from premature infants

In vitro function of cord blood platelets from 35 premature infants (gestational age 32 +/- 3.2 wk) was compared with that of 12 full-term infants and 14 adult control subjects. In comparison with adult platelets, preterm platelets showed impaired aggregation, in response to thrombin, collagen, ADP, and U46619 [a stable analog of thromboxane A2 (TxA2)], and impaired [14C]serotonin secretion in response to collagen and U46619. The production of TxB2 (the stable TxA2 metabolite) in response to collagen was reduced in preterm platelets (30.2 +/- 5.5 ng/mL) compared with full-term (52.7 +/- 12.6 ng/mL) or adult control platelets (132.3 +/- 38.7 ng/mL). The deficient TxB2 production and U46619 response prompted further investigation of TxA2 receptor number and binding characteristics. Immunoblotting using an anti-TxA2 receptor antibody (anti-P2) identified a single, identical 55-kD band in solubilized membranes of control, full-term, and preterm platelets. Flow cytometry using anti-P2 produced histograms that did not differ between adults and neonates. Ligand binding studies using [3H]U46619 were carried out on 10 samples from each group. Scatchard analysis yielded a single class of binding sites with no significant difference among the Kd values (85 +/- 16 versus 99 +/- 12 versus 100 +/- 12 nM) or number of binding sites per platelet (1876 +/- 460 versus 2450 +/- 478 versus 2777 +/- 536) for preterm and full-term infants and adults. Therefore platelets of preterm infants show impaired TxA2 production and response. The poor response is not related to altered binding characteristics of the TxA2 receptor but may lie in the postreceptor signal transduction pathway.

Novel role for Sp1 in phorbol ester enhancement of human platelet thromboxane receptor gene expression

Expression of platelet thromboxane receptors is transcriptionally increased during megakaryocytic differentiation stimulated by phorbol 12-myristate 13-acetate (PMA). We previously cloned and characterized the promoter region of the human thromboxane receptor gene and localized PMA-responsive elements to a region between 1.84 and 1.95 kilobase pairs (kb) 5' of the transcription initiation site (D'Angelo, D. D., Davis, M. G., Houser, W. A., Eubank, J. J., Ritchie, M. E., and Dorn, G. W., II (1995) Circ. Res. 77, 466-474). Herein we report the localization of the PMA response element to a 14-nucleotide C-rich sequence, flanked by an octanucleotide inverted repeat, located -1.938 to -1.925 kb 5' of the transcription start site of this gene. We further identify the PMA-responsive enhancer factor that binds to this C-rich sequence as Sp1. Heterologous thromboxane receptor gene promoter/thymidilate kinase reporter constructs transfected into K562 cells exhibited PMA responsiveness when the C-rich element was included with additional 3' sequence from -1.924 to -1.84 kb. However, mutations of the C-rich element that disrupted a GC box located on the inverse strand eliminated PMA responsiveness and, in gel mobility shift assays, eliminated binding of Sp1. PMA treatment of K562 cells significantly increased, by 5-fold, Sp1 binding to the C-rich element and increased both phosphorylated and nonphosphorylated Sp1 protein levels by 2-fold. Furthermore, PMA treatment transiently increased Sp1 mRNA levels prior to increasing thromboxane receptor mRNA, suggesting that up-regulation of Sp1 contributes to up-regulation of thromboxane receptors. Finally, we have detected an unidentified K562 nuclear protein that binds specifically to the sense strand of the C-rich sequence overlapping the Sp1 binding site and that, by stabilizing a double stem-loop conformation of this DNA segment, may also play a role in Sp1 regulation of this gene. These studies are the first to describe regulatory and regulated roles for Sp1 in PMA-responsive gene expression and suggest that modulation of Sp1 levels controls thromboxane receptor expression during megakaryocytic differentiation.