Kathleen M. Metters

The utilization of recombinant prostanoid receptors to determine the affinities and selectivities of prostaglandins and related analogs

Stable cell lines that individually express the eight known human prostanoid receptors (EP(1), EP(2), EP(3), EP(4), DP, FP, IP and TP) have been established using human embryonic kidney (HEK) 293(EBNA) cells. These recombinant cell lines have been employed in radioligand binding assays to determine the equilibrium inhibitor constants of known prostanoid receptor ligands at these eight receptors. This has allowed, for the first time, an assessment of the affinity and selectivity of several novel compounds at the individual human prostanoid receptors. This information should facilitate interpretation of pharmacological studies that employ these ligands as tools to study human tissues and cell lines and should, therefore, result in a greater understanding of prostanoid receptor biology.

Molecular cloning and characterization of the four rat prostaglandin E2 prostanoid receptor subtypes

We have characterized the rat prostanoid EP1, EP2, EP3alpha and EP4 receptor subtypes cloned from spleen, hepatocyte and/or kidney cDNA libraries. Comparison of the deduced amino acid sequences of the rat EP receptors with their respective homologues from mouse and human showed 91% to 98% and 82% to 89% identity, respectively. Radioreceptor binding assays and functional assays were performed on EP receptor expressing human embryonic kidney (HEK) 293 cells. The KD values obtained with prostaglandin E2 for the prostanoid receptor subtypes EP1, EP2, EP3alpha and EP4 were approximately 24, 5, 1 and 1 nM, respectively. The rank order of affinities for various prostanoids at the prostanoid receptor subtypes EP2, EP3alpha and EP4 receptor subtypes was prostaglandin E2 = prostaglandin E1 > iloprost > prostaglandin F2alpha > prostaglandin D2 > U46619. The rank order at the prostanoid EP1 receptor was essentially the same except that iloprost had the highest affinity of the prostanoids tested. Of the selective ligands, butaprost was selective for prostanoid EP2, M&B28767 and sulprostone were selective for EP3alpha and enprostil displayed dual selectivity, interacting with both prostanoid receptor subtypes EP1 and EP3alpha. All four receptors coupled to their predominant signal transduction pathways in HEK 293 cells. Notably, using a novel aequorin luminescence assay to monitor prostanoid EP1 mediated increases in intracellular calcium, both iloprost and sulprostone were identified as partial agonists. Finally, by Northern blot analysis EP3 transcripts were most abundant in liver and kidney whereas prostanoid EP2 receptor mRNA was expressed in spleen, lung and testis and prostanoid EP1 receptor mRNA transcripts were predominantly expressed in the kidney. The rat prostanoid EP1 probes also detected additional and abundant transcripts present in all the tissues examined. These were found to be related to the expression of a novel protein kinase gene and not the prostanoid EP1 gene [Batshake, B., Sundelin, J., 1996. The mouse genes for the EP1 prostanoid receptor and the novel protein kinase overlap. Biochem. Biophys. Res. Commun. 227. 1329-1333].

Distribution and regulation of cyclooxygenase-2 in carrageenan-induced inflammation.

We characterized the regulation of cyclooxygenase‐2 (COX‐2) at the mRNA, protein and mediator level in two rat models of acute inflammation, carrageenan‐induced paw ædema and mechanical hyperalgesia. Carrageenan was injected in the hind paw of rat at low (paw ædema) and high doses (hyperalgesia). COX‐2 and prostaglandin E2 (PGE2) levels were measured by RT–PCR and immunological assays. We also determined the distribution of COX‐2 by immunohistochemistry. The injection of carrageenan produced a significant and parallel induction of both COX‐2 and PGE2. This induction was significantly higher in hyperalgesia than in paw ædema. This was probably due to the 9 fold higher concentration of carrageenan used to provoke hyperalgesia. Immunohistochemical examination showed COX‐2 immunoreactivity in the epidermis, skeletal muscle and inflammatory cells of rats experiencing hyperalgesia. In paw ædema however, only the epidermis showed positive COX‐2 immunoreactivity. Pretreatment with indomethacin completely abolished the induction of COX‐2 in paw ædema but not in hyperalgesia. These results suggest that multiple mechanisms regulate COX‐2 induction especially in the more severe model. In carrageenan‐induced paw ædema, prostanoid production have been linked through the expression of the COX‐2 gene which suggest the presence of a positive feedback loop mechanism.

Molecular pharmacology of the human prostaglandin D2 receptor, CRTH2

The recombinant human prostaglandin D2 (PGD2) receptor, hCRTH2, has been expressed in HEK293(EBNA) and characterized with respect to radioligand binding and signal transduction properties. High and low affinity binding sites for PGD2 were identified in the CRTH2 receptor population by saturation analysis with respective equilibrium dissociation constants (KD) of 2.5 and 109 nM. This revealed that the affinity of PGD2 for CRTH2 is eight times less than its affinity for the DP receptor. Equilibrium competition binding assays revealed that of the compounds tested, only PGD2 and several related metabolites bound with high affinity to CRTH2 (Ki values ranging from 2.4 to 34.0 nM) with the following rank order of potency: PGD2>13,14‐dihydro‐15‐keto PGD2>15‐deoxy‐Δ12,14‐PGJ2>PGJ2>Δ12‐PGJ2>15(S)‐15 methyl‐PGD2. This is in sharp contrast with the rank order of potency obtained at DP : PGD2>PGJ2>Δ12‐PGJ2>15‐deoxy‐Δ12,14‐PGJ2 >>>13,14‐dihydro‐15‐keto‐PGD2. Functional studies demonstrated that PGD2 activation of recombinant CRTH2 results in decrease of intracellular cAMP in a pertussis toxin‐sensitive manner. Therefore, we showed that CRTH2 can functionally couple to the G‐protein Gαi/o. PGD2 and related metabolites were tested and their rank order of potency followed the results of the membrane binding assay. By Northern blot analysis, we showed that, besides haemopoietic cells, CRTH2 is expressed in many other tissues such as brain, heart, thymus, spleen and various tissues of the digestive system. In addition, in situ hybridization studies revealed that CRTH2 mRNA is expressed in human eosinophils. Finally, radioligand binding studies demonstrated that two eosinophilic cell lines, butyric acid‐differentiated HL‐60 and AML 14.3D10, also endogenously express CRTH2.

Immunolocalization of cyclooxygenase-2 in the macula densa of human elderly

To gain insight into the role of prostanoids in human kidney function, we examined the distribution of cyclooxygenase (COX) 1 and COX‐2 by immunofluorescence and immunohistochemistry in human kidneys from adults of various age groups. COX‐1 was detected in the collecting ducts, thin loops of Henle and portions of the renal vasculature. COX‐2 was detected in the renal vasculature, medullary interstitial cells, and the macula densa. In addition, COX‐2 immunoreactivity was noted in afferent arteries and the macula densa of the renal cortex and was more evident in the kidneys of older adults.

Structure-activity relationship of cinnamic acylsulfonamide analogues on the human EP3 prostanoid receptor

Potent and selective antagonists of the human EP3 receptor have been identified. The structure-activity relationship of the chemical series was conducted and we found several analogues displaying sub-nanomolar K(i) values at the EP3 receptor and micromolar activities at the EP1, EP2 and EP4 receptors. The effect of added human serum albumin (HSA) on the binding affinity at the EP3 receptor was also investigated.

Cloning and expression of three isoforms of the human EP3 prostanoid receptor.

Functional cDNA clones coding for three isoforms of the human prostaglandin E receptor EP3 subtype have been isolated from kidney and uterus cDNA libraries. The three isoforms, designated hEP3‐I, hEP3‐II and hEP3‐III, have open reading frames corresponding to 390, 388 and 365 amino acids, respectively. They differ only in the length and amino acid composition of their carboxy‐terminal regions, beginning at position 360. The human EP3 receptor has seven predicted transmembrane spanning domains and therefore belongs to the G‐protein‐coupled receptor family. The rank order of potency for prostaglandins and related analogs in competition for [3H]PGE2 specific binding to membranes prepared from transfected COS cells was comparable for all three isoforms, and as predicted for the EP3 receptor, with PGE2 = PGE1 > PGF2α = iloprost > PGD2 ⪢ U46619. In addition, the EP3‐selective agonist MB28767 was a potent competing ligand with an IC50 value of 0.3 nM, whereas the EP1‐selective antagonist AH6909 gave IC50 values of 2–7 μM and the EP2‐selective agonist butaprost was inactive. In summary, we have cloned three isoforms of the human EP, receptor having comparable ligand binding properties.

STRUCTURE ACTIVITY RELATIONSHIPS OF TETRAHYDROCANNABINOL ANALOGUES ON HUMAN CANNABINOID RECEPTORS

A series of Δ8-tetrahydrocannabinol (THC) and biphenylic derivatives were prepared and their binding affinity for both human cannabinoid receptors hCB1 and hCB2 evaluated.

New class of potent ligands for the human peripheral cannabinoid receptor

Abstract A new class of potent ligand for the human peripheral cannabinoid (hCB 2 ) receptor is described. Two indole analogs 13 and 17 exhibited nanomolar potencies (K i ) with good selectivity for the hCB 2 receptor over the human central cannabinoid (hCB 1 ) receptor.

Discovery of MK-0476, a potent and orally active leukotriene D4 receptor antagonist devoid of peroxisomal enxyme induction

Abstract Structure-activity studies leading to the discovery of 1 (MK-0476) are described. The initial compound of this series, 2, was a potent leukotriene D4 (LTD4) antagonist, but was also a peroxisomal enzyme inducer in the mouse. Structure-activity relationships around the thioether chain were explored to remove this undesirable feature. It was found that alkyl substituents in the s position relative to the carboxylic acid reduce the potency as a peroxisomal enzyme inducer while preserving the LTD4 antagonistic properties. Dialkyl substitution essentially eliminates the enzyme induction. The optimal styryl quinoline 1 exhibited high in vitro potency and in vivo activity on oral dosing without significant liver enzyme induction in the mouse.