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.

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.

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.