Prostanoids are the cyclooxygenase metabolites of arachidonic acid and include prostaglandin (PG) D(2), PGE(2), PGF(2alpha), PGI(2), and thromboxne A(2). They are synthesized and released upon cell stimulation and act on cells in the vicinity of their synthesis to exert their actions. Receptors mediating the actions of prostanoids were recently identified and cloned. They are G protein-coupled receptors with seven transmembrane domains. There are eight types and subtypes of prostanoid receptors that are encoded by different genes but as a whole constitute a subfamily in the superfamily of the rhodopsin-type receptors. Each of the receptors was expressed in cultured cells, and its ligand-binding properties and signal transduction pathways were characterized. Moreover, domains and amino acid residues conferring the specificities of ligand binding and signal transduction are being clarified. Information also is accumulating as to the distribution of these receptors in the body. It is also becoming clear for some types of receptors how expression of their genes is regulated. Furthermore, the gene for each of the eight types of prostanoid receptor has been disrupted, and mice deficient in each type of receptor are being examined to identify and assess the roles played by each receptor under various physiological and pathophysiological conditions. In this article, we summarize these findings and attempt to give an overview of the current status of research on the prostanoid receptors.

Allergic asthma is caused by the aberrant expansion in the lung of T helper cells that produce type 2 (TH2) cytokines and is characterized by infiltration of eosinophils and bronchial hyperreactivity. This disease is often triggered by mast cells activated by immunoglobulin E (IgE)-mediated allergic challenge. Activated mast cells release various chemical mediators, including prostaglandin D2 (PGD2), whose role in allergic asthma has now been investigated by the generation of mice deficient in the PGD receptor (DP). Sensitization and aerosol challenge of the homozygous mutant (DP-/-) mice with ovalbumin (OVA) induced increases in the serum concentration of IgE similar to those in wild-type mice subjected to this model of asthma. However, the concentrations of TH2 cytokines and the extent of lymphocyte accumulation in the lung of OVA-challenged DP-/- mice were greatly reduced compared with those in wild-type animals. Moreover, DP-/- mice showed only marginal infiltration of eosinophils and failed to develop airway hyperreactivity. Thus, PGD2 functions as a mast cell-derived mediator to trigger asthmatic responses.

Prostanoids are a group of bioactive lipids working as local mediators and include D, E, F and I types of prostaglandins (PGs) and thromboxanes. Prostacyclin (PGI2) acts on platelets and blood vessels to inhibit platelet aggregation and to cause vasodilatation, and is thought to be important for vascular homeostasis. Aspirin-like drugs, including indomethacin, which inhibit prostanoid biosynthesis, suppress fever, inflammatory swelling and pain, and interfere with female reproduction, suggesting that prostanoids are involved in these processes, although it is not clear which prostanoid is the endogenous mediator of a particular process. Prostanoids act on seven-transmembrane-domain receptors which are selective for each type. Here we disrupt the gene for the prostacyclin receptor in mice by using homologous recombination. The receptor-deficient mice are viable, reproductive and normotensive. However, their susceptibility to thrombosis is increased, and their inflammatory and pain responses are reduced to the levels observed in indomethacin-treated wild-type mice. Our results establish that prostacyclin is an antithrombotic agent in vivo and provide evidence for its role as a mediator of inflammation and pain.

Fever, a hallmark of disease, is elicited by exogenous pyrogens, that is, cellular components, such as lipopolysaccharide (LPS), of infectious organisms, as well as by non-infectious inflammatory insults. Both stimulate the production of cytokines, such as interleukin (IL)-1beta, that act on the brain as endogenous pyrogens. Fever can be suppressed by aspirin-like anti-inflammatory drugs. As these drugs share the ability to inhibit prostaglandin biosynthesis, it is thought that a prostaglandin is important in fever generation. Prostaglandin E2 (PGE2) may be a neural mediator of fever, but this has been much debated. PGE2 acts by interacting with four subtypes of PGE receptor, the EP1, EP2, EP3 and EP4 receptors. Here we generate mice lacking each of these receptors by homologous recombination. Only mice lacking the EP3 receptor fail to show a febrile response to PGE2 and to either IL-1beta or LPS. Our results establish that PGE2 mediates fever generation in response to both exogenous and endogenous pyrogens by acting at the EP3 receptor.

1 Eight types and subtypes of the mouse prostanoid receptor, the prostaglandin D (DP) receptor, the prostaglandin F (FP) receptor, the prostaglandin I (IP) receptor, the thromboxane A (TP) receptor and the EP 1 , EP 2 , EP 3 and EP 4 subtypes of the prostaglandin E receptor, were stably expressed in Chinese hamster ovary cells. Their ligand binding characteristics were examined with thirty two prostanoids and their analogues by determining the K i values from the displacement curves of radioligand binding to the respective receptors. 2 The DP, IP and TP receptors showed high ligand binding speci®city and only bound their own putative ligands with high anity such as PGD 2 , BW245C and BW868C for DP, cicaprost, iloprost and isocabacyclin for IP, and S-145, I-BOP and GR 32191 for TP. 3 The FP receptor bound PGF 2a and¯uprostenol with K i values of 3 ± 4 nM. In addition, PGD 2 , 17-phenyl-PGE 2 , STA 2 , I-BOP, PGE 2 and M&B Á -28767 bound to this receptor with K i values less than 100 nM. 4 The EP 1 receptor bound 17-phenyl-PGE 2 , sulprostone and iloprost in addition to PGE 2 and PGE 1 , with K i values of 14 ± 36 nM. 16,16-dimethyl-PGE 2 and two putative EP 1 antagonists, AH6809 and SC-19220, did not show any signi®cant binding to this receptor. M&B-28767, a putative EP 3 agonist, and misoprostol, a putative EP 2 /EP 3 agonist, also bound to this receptor with K i values of 120 nM. 5 The EP 2 and EP 4 receptors showed similar binding pro®les. They bound 16,16-dimethyl PGE 2 and 11-deoxy-PGE 1 in addition to PGE 2 and PGE 1 . The two receptors were discriminated by butaprost, AH-13205 and AH-6809 that bound to the EP 2 receptor but not to the EP 4 receptor, and by 1-OH-PGE 1 that bound to the EP 4 but not to the EP 2 receptor. 6 The EP 3 receptor showed the broadest binding pro®le, and bound sulprostone, M&B-28767, GR63799X, 11-deoxy-PGE 1 , 16,16-dimethyl-PGE 2 and 17-phenyl-PGE 2 , in addition to PGE 2 and PGE 1 , with K i values of 0.6 ± 3.7 nM. In addition, three IP ligands, iloprost, carbacyclin and isocarbacyclin, and one TP ligand, STA 2 , bound to this receptor with K i values comparable to the K i values of these compounds for the IP and TP receptors, respectively. 7 8-Epi-PGF 2a showed only weak binding to the IP, TP, FP, EP 2 and EP 3 receptor at 10 mM concentration.

Arachidonic acid is metabolized to prostaglandin H(2) (PGH(2)) by cyclooxygenase (COX). COX-2, the inducible COX isozyme, has a key role in intestinal polyposis. Among the metabolites of PGH(2), PGE(2) is implicated in tumorigenesis because its level is markedly elevated in tissues of intestinal adenoma and colon cancer. Here we show that homozygous deletion of the gene encoding a cell-surface receptor of PGE(2), EP2, causes decreases in number and size of intestinal polyps in Apc(Delta 716) mice (a mouse model for human familial adenomatous polyposis). This effect is similar to that of COX-2 gene disruption. We also show that COX-2 expression is boosted by PGE(2) through the EP2 receptor via a positive feedback loop. Homozygous gene knockout for other PGE(2) receptors, EP1 or EP3, did not affect intestinal polyp formation in Apc(Delta 716) mice. We conclude that EP2 is the major receptor mediating the PGE2 signal generated by COX-2 upregulation in intestinal polyposis, and that increased cellular cAMP stimulates expression of more COX-2 and vascular endothelial growth factor in the polyp stroma.

Thromboxane A2 is a very unstable arachidonate metabolite, yet a potent stimulator of platelet aggregation and a constrictor of vascular and respiratory smooth muscles. It has been implicated as a mediator in diseases such as myocardial infarction, stroke and bronchial asthma. Using a stable analogue of this compound we recently purified the human platelet thromboxane A2 receptor to apparent homogeneity. Using an oligonucleotide probe corresponding to its partial amino-acid sequence, we have obtained a complementary DNA clone encoding this receptor from human placenta and a partial clone from cultured human megakaryocytic leukaemia cells. The placenta cDNA encodes a protein of 343 amino acids with seven putative transmembrane domains. The protein expressed in COS-7 cells binds drugs with affinities identical to those of the platelet receptor, and that in Xenopus oocytes opens Ca2(+)-activated Cl- channel on agonist stimulation. Northern blot analysis and nucleotide sequences of the two clones suggest that an identical species of the thromboxane A2 receptor is present in platelets and vascular tissues. This first report on the molecular structure of an eicosanoid receptor will promote the molecular pharmacology and pathophysiology of these bioactive compounds.

Mice lacking the gene encoding the receptor for prostaglandin F2alpha (FP) developed normally but were unable to deliver normal fetuses at term. Although these FP-deficient mice showed no abnormality in the estrous cycle, ovulation, fertilization, or implantation, they did not respond to exogenous oxytocin because of the lack of induction of oxytocin receptor (a proposed triggering event in parturition), and they did not show the normal decline of serum progesterone concentrations that precedes parturition. Ovariectomy at day 19 of pregnancy restored induction of the oxytocin receptor and permitted successful delivery in the FP-deficient mice. These results indicate that parturition is initiated when prostaglandin F2alpha interacts with FP in ovarian luteal cells of the pregnant mice to induce luteolysis.