The thromboxane receptor (TP) also known as the prostanoid TP receptor is a protein that in humans is encoded by the TBXA2R gene, The thromboxane receptor is one among the five classes of prostanoid receptors and was the first eicosanoid receptor cloned. The TP receptor derives its name from its preferred endogenous ligand thromboxane A2.
Gene The gene responsible for directing the synthesis of the thromboxane receptor, TBXA2R, is located on human chromosome 19 at position p13.3, spans 15 kilobases, and contains 5 exons. TBXA2R codes for a member of the G protein-coupled super family of seven-transmembrane receptors.
Heterogeneity Molecular biology findings have provided definitive evidence for two human TP receptor subtypes. The originally cloned TP subtype from human placenta is known as the α isoform and the splice variant cloned from endothelium (with 407 amino acids) is termed the β isoform. The first 328 amino acids are the same for both isoforms, but the β isoform exhibits an extended C-terminal cytoplasmic domain. Both isoforms stimulate cells in part by activating the Gq family of G proteins. In at least certain cell types, however, TPα also stimulates cells by activating the Gs family of G proteins while TPβ also stimulates cells by activating the Gi class of G proteins. This leads to the stimulation or inhibition, respectively, of adenylate cyclase activity and thereby very different cellular responses. Differences in their C-terminal tail sequence also allow for significant differences in the two receptors internalization and thereby desensitization (i.e. loss of G protein- and therefore cell-stimulating ability) after activation by an agonist; TPβ but not TPα undergoes agonist-induced internalization.
The expression of α and β isoforms is not equal within or across different cell types. For example, platelets express high concentrations of the α isoform (and possess residual RNA for the β isoform), while expression of the β isoform has not been documented in these cells. The β isoform is expressed in human endothelium. Furthermore, each TP isoform can physically combine with: a) another of its isoforms to make TPα-TPα or TPβ-TPβ homodimers that promote stronger cell signaling than achieved by their monomer counterparts; b) their opposite isoform to make TPα-TPβ heterodimers that activate more cell signaling pathways than either isoform or homodimer; and c) with the prostacyclin receptor (i.e. IP receptor) to form TP-IP heterodimers that, with respect to TPα-IP heterodimers, trigger particularly intense activation of adenyl cyclase. The latter effect on adenyl cyclase may serve to suppress TPα's cell stimulating actions and thereby some of its potentially deleterious actions.
Mice and rats express only the TPα isoform. Since these rodents are used as animal models to define the functions of genes and their products, their failure to have two TP isoforms has limited understanding of the individual and different functions of each TP receptor isoform.
Tissue distribution Historically, TP receptor involvement in blood platelet function has received the greatest attention. However, it is now clear that TP receptors exhibit a wide distribution in different cell types and among different organ systems. For example, TP receptors have been localized in cardiovascular, reproductive, immune, pulmonary and neurological tissues, among others. Organ/Tissue Cells/Cell lines TP Receptor Distribution Lung, Spleen, Uterus, Placenta, Aorta, Heart, Intestine, Liver, Eye, Thymus, Kidney, Spinal Cord, Brain Platelets, Blood Monocytes, Glomerular mesangial cells, Oligodendrocytes, Cardiac myocytes, Afferent Sympathetic Nerve Endings in the Heart, Epithelial cells, Hela cells, Smooth muscle cells, Endothelial cells, Trophoblasts, Schwann cells, Astrocytes, Megakaryocytes, Kupffer cells, Human erythroleukemic megakaryocyte (HEL), K562 (Human chronic myelogenous leukemia) cells, Hepatoblastoma HepG2 cells, Immature thymocytes, EL-4 (mouse T cell line), astrocytoma cells
TP receptor ligands Activating ligands Standard prostanoids have the following relative efficacies as receptor ligands in binding to and activating TP: TXA2=PGH2>>PGD2=PGE2=PGF2alpha=PGI2. Since TXA2 is highly unstable, receptor binding and biological studies on TP are conducted with stable TXA2 analogs such as I-BOP and U46619. These two analogs have one-half of their maximal binding capacity and cell-stimulating potency at ~1 and 10-20 nanomolar, respectively; it is assumed that TXA2 and PGH2 (which also is unstable) have binding and cell-stimulating potencies within this range. PGD2, PGE2, PGF2alpha, and PGI2 have binding and stimulating potencies that are >1,000-fold weaker than I-BOP and therefore are assumed not to have appreciable ability to stimulate TP in vivo. 20-Hydroxyeicosatetraenoic acid (20-HETE) is a full agonist and certain isoprostanes, e.g. 8-iso-PGF2 alpha and 8-iso-PGE2, are partial agonists of the TP receptor. In animal models and human tissues, they act through TP to promote platelet responses and stimulate blood vessel contraction. Synthetic analogs of TXA2 that activate TP but are relatively resistant to spontaneous and metabolic degradation include SQ 26655, AGN192093, and EP 171, all of which have binding and activating potencies for TP similar to I-BOP.
Inhibiting ligands Several synthetic compounds bind to, but do not activate, TP and thereby inhibit its activation by activating ligands. These receptor antagonists include I-SAP, SQ-29548, S-145, domitroban, and vapiprost, all of which have affinities for binding TP similar to that of I-BOP. Other notable TP receptor antagonists are Seratrodast (AA-2414), Terutroban (S18886), PTA2, 13-APA, GR-32191, Sulotroban (BM-13177), SQ-29,548, SQ-28,668, ONO-3708, Bay U3405, EP-045, BMS-180,291, and S-145. Many of these TP receptor antagonists have been evaluated as potential therapeutic agents for asthma, thrombosis and hypertension. These evaluations indicate that TP receptor antagonists can be more effective than drugs which selectively block the production of TXA2 thromboxane synthase inhibitors. This seemingly paradoxical result may reflect the ability of PGH2, whose production is not blocked by the inhibitors, to substitute for TXA2 in activating TP. Novel TP receptor antagonists that also have activity in reducing TXA2 production by inhibiting cyclooxygenases have been discovered and are in development for testing in animal models.
Mechanism of cell stimulation TP is classified as a contractile type of prostenoid receptor based on its ability to contract diverse types of smooth muscle-containing tissues such as those of the lung, intestines, and uterus. TP contracts smooth muscle and stimulates various response in a wide range of other cell types by coupling with and mobilizing one or more families of the G protein class of receptor-regulated cell signaling molecules. When bound to TXA2, PGH2, or other of its agonists, TP mobilizes members of the: a) Gq alpha subunit family (i.e. G11, G15, and G16 types of Gq proteins) which activates phospholipase C, IP3, cell Ca2+ mobilization, protein kinase Cs, calmodulin-modulated myosin light chain kinase, Mitogen-activated protein kinases, and Calcineurin; b) G12/G13 family which activates Rho GTPases that control cell migration and intracellular organelle movements; c) Gs alpha subunit family which stimulates adenyl cyclase to raise intracellular levels of cAMP and thereby activate cAMP-regulated protein kinases A and thereby protein kinases A-dependent cell signaling pathways (see PKA) d) atypical G protein complex Gh/transglutaminase-2-calreticulin which activates phospholipase C, IP3, cell Ca2+ mobilization, protein kinase C, and Mitogen-activated protein kinase but inhibits adenyl cyclase.
Following its activation of these pathways, the TP receptors's cell-stimulating ability rapidly reverses by a process termed homologous desensitization, i.e. TP is no longer able to mobilize its G protein targets or further stimulate cell function. Subsequently, the β but not α isoform of TP undergoes receptor internalization. These receptor down regulating events are triggered by the G protein-coupled receptor kinases mobilized during TP receptor activation. TP receptor-independent agents that stimulate cells to activate protein kinases C or protein kinases A can also down-regulate TP in a process termed heterologous desensitization. For example, prostacyclin I2 (PGI2)-induced activation of its prostacyclin receptor (IP) and prostaglandin D2-induced activation of its prostaglandin DP1 receptor cause TP receptor desensitization by activating protein kinases A while prostaglandin F2alpha-induced activation of its prostaglandin F receptor and prostaglandin E2-induced activation of its prostaglandin EP1 receptor receptor desensitizes TP by activating protein kinases C. These desensitization responses serve to limit the action of receptor agonists as well as the overall extent of cell excitation.
In addition to its ability to down-regulate TPα, the IP receptor activates cell signaling pathways that counteract those activated by TP. Furthermore, the IP receptor can physically unite with the TPα receptor to form an IP-TPα heterodimer complex which, when bound by TXA2, activates predominantly IP-coupled cell signal pathways. The nature and extent of many cellular responses to TP receptor activation are thereby modulated by the IP receptor and this modulation may serve to limit the potentially deleterious effects of TP receptor activation (see following section on Functions).
Functions Studies using animals genetically engineered to lack the TP receptor and examining the actions of this receptor's agonists and antagonists in animals and on animal and human tissues indicate that TP has various functions in animals and that these functions also occur, or serve as a paradigm for further study, in humans.
Platelets Human and animal platelets stimulated by various agents such as thrombin produce TXA2. Inhibition of this production greatly reduces the platelets final adhesion aggregation and degranulation (i.e. secretion of its granule contents) responses to the original stimulus. In addition, the platelets of mice lacking TP receptors have similarly defective adhesion, aggregation, and degranulation responses and these TP deficient mice cannot form stable blood clots and in consequence exhibit bleeding tendencies. TP, as studies show, is part of a positive feedback loop that functions to promote platelet adhesion, aggregation, degranulation, and platelet-induced blood clotting-responses in vitro and in vivo. The platelet-directed functions of TP are in many respects opposite to those of the IP receptor. This further indicates (see previous section) that the balance between the TXA2-TP and PGI2-IP axes contribute to regulating platelet function, blood clotting, and bleeding.
Cardiovascular system