2,3-Diarylthiophenes as selective EP1 receptor antagonists

The synthesis and the EP(1) receptor binding affinity of 2,3-diarylthiophene derivatives are described. The evaluation of the structure-activity relationship (SAR) in this series led to the identification of compounds 4, 7, and 12a, which exhibit high affinity for the human EP(1) receptor and a selectivity greater than 100-fold against the EP(2), EP(3), EP(4), DP, FP, and IP receptors and greater than 25-fold versus the TP receptor. These three antagonists present good pharmacokinetics in rats and significant differences in the way they are distributed in the brain.     

Discovery of novel prostaglandin analogs of PGE2 as potent and selective EP2 and EP4 receptor agonists

Analogs of PGE(2) with introduction of diene groups at the omega-side chain have been synthesized and evaluated for their binding affinity for EP(2) and EP(4) receptors. An optimized analog (compound 9b) showed high potency and selectivity for the EP(4) receptor over other known receptors.     

Binding and receptor-mediated endocytosis of erythropoietin in Friend virus-infected erythroid cells

The binding of labeled erythropoietin (EP) to cell surface receptors and subsequent processing of the hormone within the cell was studied in erythroid cells procured from the spleens of mice infected with the anemia strain of Friend virus. These immature erythroid cells respond to EP in culture to differentiate into reticulocytes and erythrocytes. Radiolabeled EP (both iodinated and tritiated) binds to 800-1000 cell surface receptors on these cells at 4 degrees C. Using 125I-EP, we found that 300 of these cell surface receptors have a higher affinity for EP (Kd = 0.09 nM) than the remaining receptors (Kd = 0.57 nM). The number of molecules of EP bound per cell increased about 2-fold when binding was carried out at 37 degrees C. Treatment of the cell surface with pronase or removal of surface-bound EP with a low pH wash revealed that radiolabeled EP is internalized by the cells at 37 degrees C. Pulse chase experiments showed that degradation products of radiolabeled EP are released into the medium with a corresponding loss of label from the interior of the cell. Inhibitors of lysosomal function greatly reduced this degradation of 125I-EP. Since 180 of the 300 high affinity receptors and very few of the low affinity receptors are occupied at the concentration of EP which elicits the maximum biological response in these cells, we suggest that interaction of EP with the high affinity receptors are necessary for the full biological effect of the hormone. A different murine erythroleukemia cell line which does not differentiate in response to EP was found to have only the lower affinity binding sites for the hormone.     

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.     

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.     

Lactams as prostanoid receptor ligands. Part 4: 2-Piperidones as selective EP4 receptor agonists

2-Piperidones were prepared bearing heptanoic acid or a thioether heptanoic acid at the 1-position as well as appropriately substituted at the 6-position to mimic the structure of prostaglandins. The stereochemical purity at the 6-position was determined to be 95% ee for an advanced synthetic intermediate. The 2-piperidones were identified as potent agonists at the EP4 prostanoid receptor. They displayed a high affinity (Ki 5-130 nM) at EP4 and subtype selectivity.     

Discovery of a series of acrylic acids and their derivatives as chemical leads for selective EP3 receptor antagonists

A series of acrylic acids and their structurally related compounds were evaluated for their binding affinity to four EP receptor subtypes (EP1-4). Starting from the initial hit 3, which was discovered in our in-house library, compounds 4 and 5 were identified as new chemical leads as candidates for further optimization towards a selective EP3 receptor antagonist. The identification process of these compounds and their pharmacokinetic profiles are presented.     

Functional expression of human prostaglandin E2 receptor 4 (EP4) in E. coli and characterization of the binding property of EP4 with Gα proteins

Human prostaglandin E2 receptor 4 (EP4) is one of the four subtypes of prostaglandin E₂ (PGE₂) receptors and belongs to the rhodopsin-type G protein-coupled receptor (GPCR) family. Particularly, EP4 is expressed in various cancer cells and is involved in cancer-cell proliferation by a G protein signaling cascade. To prepare an active form of EP4 for biochemical characterization and pharmaceutical application, this study designed a recombinant protein comprising human EP4 fused to the P9 protein (a major envelope protein of phi6 phage) and overexpressed the P9-EP4 fusion protein in the membrane fraction of E. coli. The solubilized P9-EP4 with sarkosyl (a strong anionic detergent) was purified by affinity chromatography. The purified protein was stabilized with amphiphilic polymers derived from poly-γ-glutamate. The polymer-stabilized P9-EP4 showed specific interaction with the alpha subunits of G_s or G_i proteins, and a high content of α-helical structure by a circular dichroism spectroscopy. Furthermore, the polymer-stabilized P9-EP4 showed strong heat resistance compared with P9-EP4 in detergents. The functional preparation of EP4 and its stabilization with amphiphilic polymers could facilitate both the biochemical characterization and pharmacological applications targeting EP4.     

Role of prostaglandin E2 receptors in migration of murine and human breast cancer cells

Aberrant upregulation of COX-2 enzyme resulting in accumulation of PGE2 in a cancer cell environment is a marker for progression of many cancers, including breast cancer. Four subtypes of cell surface receptors (EP1, EP2, EP3, and EP4), which are coupled with different G-proteins, mediate PGE2 actions. Since migration is an essential step in invasion and metastasis, in the present study we defined the expression of EP receptors and their roles in migratory function of breast cancer cells of murine (C3L5) and human (MDA-MB-231 and MCF-7) origin. Highly metastatic C3L5 and MDA-MB-231 cells, found to be highly migratory in a Transwell migration assay, were shown to accumulate much higher levels of PGE2 in culture media in comparison with nonmetastatic and poorly migrating MCF-7 cells; the levels of PGF2alpha and 6-keto-PGF1alpha were low in all cases. The elevated PGE2 production by metastatic cancer cells was due to COX-2 activity since dual COX-1/2 inhibitor indomethacin and selective COX-2 inhibitor NS-398 equally suppressed both basal and inducible (by IFN-gamma/LPS or Ca2+-ionophores) PGE2 accumulation. RT-PCR analysis revealed that murine C3L5 cells expressed mRNA of EP1, EP3, and EP4 but not EP2 receptors. On the other hand, human MDA-MB-231 and MCF-7 cells expressed all the above receptors. High levels of expression of functional EP4 receptors coupled with Gs-protein was confirmed in C3L5 cells by biochemical assay showing a dose-dependent increase of intracellular cAMP synthesis in response to PGE2. EP receptor antagonists SC-19220, AH-6809, and AH-23848B, having highest affinity for EP1, EP1/EP2/DP, and EP4 receptors, respectively, variably inhibited migration of metastatic breast cancer cells. An autocrine PGE2-mediated migratory activity of these cells appeared to be associated predominantly with EP4 receptor-mediated signaling pathway, which uses cAMP as a second messenger. This conclusion is based on several observations: (1) selective EP4 antagonist AH-23848B effectively inhibited migration of both C3L5 and MDA-MB-231 cells in a dose-dependent manner; (2) exogenous PGE2 and EP4 agonist PGE1 alcohol increased migration of C3L5 cells; (3) forskolin, a potent activator of adenylate cyclase, as well as membrane-permeable analogues of cAMP (8-bromo-cAMP, dibutyryl-cAMP) stimulated migration of C3L5 cells; and (4) Rp-cAMPS, a selective protein kinase A inhibitor, reduced migration of C3L5 cells. Migration of poorly migratory MCF-7 cells remained unaffected with either PGE2 or EP4 antagonist. These findings are relevant for designing therapeutic strategies against breast cancer metastasis.     

Development of a highly selective EP2-receptor agonist. Part 1: identification of 16-hydroxy-17,17-trimethylene PGE2 derivatives

Design and synthesis of an EP2-receptor selective agonist began with the chemical modification of alpha- and omega-chains of butaprost 1a, which exhibits an affinity for the IP-receptor. Two series of prostaglandin (PG) analogues with a 16-hydroxy-17,17-trimethylene moiety as an omega-chain were identified. Among those tested, 4a,b,e,f,h and 6a,b,e,f,h were found to be highly selective EP2-receptor agonists. Structure-activity relationships are discussed.     

Molecular cloning and functional characterization of the canine prostaglandin E2 receptor EP4 subtype.

Prostaglandin E2 (PGE2) is an important mediator of diverse biologic functions in many tissues and binds with high affinity to four cell surface, seven-transmembrane domain, G protein-coupled receptors (EP1-EP4). The EP4 receptor subtype has a long intracellular carboxy-terminal region and is functionally coupled to adenylate cyclase, resulting in elevated intracellular cyclic adenosine 5' monophosphate (cAMP) levels upon activation. To further study EP4 receptor subtype function, a canine kidney cDNA library was screened and three clones were isolated and sequenced. The longest clone was 3,103 bp and contained a single open reading frame of 1,476 bp, potentially encoding a protein of 492 amino acids with a predicted molecular weight of 53.4 kDa. Sequence analysis of this open reading frame reveals 89% identity to the human EP4 protein coding region at the nucleotide level and 90% identity when the putative canine and human protein sequences are compared. Northern blot analysis showed relatively high levels of canine EP4 expression in heart, lung and kidney, while Southern blot analysis of canine genomic DNA suggests the presence of a single copy gene. Following transfection of canine EP4 into CHO-KI cells, Scatchard analysis revealed a dissociation constant of 24 nM for PGE, while competition binding studies using 3H-PGE2 as ligand demonstrated specific displacement by PGE2 prostaglandin E, (PGE1), and prostaglandin A3 (PGA3). Treatment with PGE2 also resulted in increased levels of cAMP in transfected, but not in parental, CHO-KI cells. In contrast, butaprost, an EP2 selective ligand, and sulprostone, an EP1/EP3 selective ligand, did not bind to this receptor at the maximal concentration used (320 nM). To further investigate secondary signaling, the canine EP4 cDNA was truncated to produce an 1,117 bp fragment encoding a 356 amino acid protein lacking the intracellular carboxy-terminus. When transfected, this truncated cDNA produced a protein with a dissociation constant of 11 nM for PGE2 and a binding and cAMP accumulation profile similar to that of the full-length protein. Both full-length and truncated canine EP4 underwent short term PGE2-induced desensitization as shown by a lack of continuing cAMP accumulation after the initial PGE2 stimulation, suggesting no involvement of the C-terminal intracellular tail. This result is in contrast to that reported for the human EP4 receptor, where residues within the C-terminal intracellular tail were shown to mediate short term, ligand induced desensitization.     

3-(2-Aminocarbonylphenyl)propanoic acid analogs as potent and selective EP3 receptor antagonists. Part 3: Synthesis,metabolic stability,and biological evaluation of optically active analogs

A series of 3-(2-aminocarbonylphenyl)propanoic acid analogs possessing the (1R)-1-(3,5-dimethylphenyl)-3-methylbutylamine moiety on the carboxyamide side chain were synthesized and evaluated for their binding affinity for the EP1-4 receptors and their antagonist activity for the EP3 receptor. Rational drug design based on the structure of the metabolites in human liver microsomes led us to the discovery of another series of analogs. Several compounds were further evaluated for their in vivo efficacy in rats after oral administration and also for their pharmacokinetic profiles including in vitro stability in the liver microsomes.     

Prostaglandin moieties that determine receptor binding specificity in the bovine corpus luteum.

This study provided a pharmacological evaluation of prostaglandin binding to bovine luteal plasma membrane. It was found that [3H]PGF2 alpha' [3H]PGE2' [3H]PGE1 and [3H]PGD2 all bound with high affinity to luteal plasma membrane but had different specificities. Binding of [3H]PGF2 alpha and [3H]PGD2 was inhibited by non-radioactive PGF2 alpha (IC50 values of 21 and 9 nmol l-1, respectively), PGD2 (35 and 21 nmol l-1), and PGE2 (223 and 81 nmol l-1), but not by PGE1 (> 10,000 and 5616 nmol l-1). In contrast, [3H]PGE1 was inhibited by non-radioactive PGE1 (14 nmol l-1) and PGE2 (7 nmol l-1), but minimally by PGD2 (2316 nmol l-1) and PGF2 alpha (595 nmol l-1). Binding of [3H]PGE2 was inhibited by all four prostaglandins, but slopes of the dissociation curves indicated two binding sites. Binding of [3H]PGE1 was inhibited, resulting in low IC50 values, by pharmacological agonists that are specific for EP3 receptor and possibly EP2 receptor. High affinity binding of [3H]PGF2 alpha required a C15 hydroxyl group and a C1 carboxylic acid that are present on all physiological prostaglandins. Specificity of binding for the FP receptor depended on the C9 hydroxyl group and the C5/C6 double bond. Alteration of the C11 position had little effect on affinity for the FP receptor. In conclusion, there is a luteal EP receptor with high affinity for PGE1' PGE2' agonists of EP3 receptors, and some agonists of EP2 receptors. The luteal FP receptor binds PGF2 alpha' PGD2 (high affinity), and PGE2 (moderate affinity) but not PGE1 due to affinity determination by the C9 and C5/C6 moieties, but not the C11 moiety.     

Phosphorylation of the plasma membrane calcium pump at high ATP concentration. On the mechanism of ATP hydrolysis

The plasma membrane calcium ATPase (PMCA) reacts with ATP to form acid-stable phosphorylated intermediates (EP) that can be measured using (gamma-32P)ATP. However, the steady-state level of EP at [ATP] higher than 100 microM has not yet been studied due to methodological problems. Using a microscale method and a purified preparation of PMCA from human red blood cells, we measured the steady-state concentration of EP as a function of [ATP] up to 2 mM at different concentrations of Mg2+, both at 4 and 25 degrees C. We have measured the Ca2+-ATPase activity (v) under the same conditions as those used for phosphorylation experiments. While the curves of ATPase activity vs [ATP] were well described by the Michaelis-Menten equation, the corresponding curves of EP required more complex fitting equations, exhibiting at least a high- and a low-affinity component. Mg2+ increases the apparent affinity for ATP of this latter component, but it shows no significant effect on its high-affinity one or on the Ca2+-ATPase activity. We calculated the turnover of EP (k(pEP)) as the ratio v/EP. At 1 mM Mg2+, k(pEP) increases hyperbolically with [ATP], while at 8 microM Mg2+, it exhibits a behavior that cannot be explained by the currently accepted mechanism for ATP hydrolysis. These results, together with measurements of the rate of dephosphorylation at 4 degrees C, suggest that ATP is acting in additional steps involving the interconversion of phosphorylated intermediates during the hydrolysis of the nucleotide.     

Reaction mechanism of (Ca2+, Mg2+)-ATPase of sarcoplasmic reticulum vesicles. I. Phosphoenzyme with bound Ca2+ which is exposed to the external medium

Sarcoplasmic reticulum vesicles were phosphorylated with [gamma-32P]ATP in the presence of external Ca2+ without added Mg2+. The phosphoenzyme (EP) formed had tightly bound Ca2+ and was dephosphorylated by ADP. When the external Ca2+ was chelated after phosphorylation, Ca2+ dissociated from the EP and ADP addition no longer induced dephosphorylation. Subsequent addition of CaCl2 caused rapid recombination of Ca2+ and restoration of the ADP sensitivity. These findings show that the dissociation and recombination of Ca2+ took place on the outer surface of the membranes, indicating the existence of EP with bound Ca2+ which was exposed to the external medium (Caout.EP). The Ca2+ affinity of the Ca2+ binding site in Caout.EP was comparable to that of the high affinity Ca2+ binding site in the dephosphoenzyme (E). This shows that phosphorylation is not accompanied by an appreciable reduction in the Ca2+ affinity of the Ca2+ binding site, provided this site is exposed to the external medium. The transition from ADP-sensitive EP to ADP-insensitive induced by Ca2+ chelation was unaffected by Mg2+ in the medium. Mg2+ did not activate hydrolysis of the ADP-sensitive EP with bound Ca2+, whereas it markedly accelerated hydrolysis of the ADP-insensitive EP without bound Ca2+.     

Gastric mucosal protection against ethanol by EP2 and EP4 signaling through the inhibition of leukotriene C4 production

Prostaglandin (PG)E derivatives are widely used for treating gastric mucosal injury. PGE receptors are classified into four subtypes, EP(1), EP(2), EP(3), and EP(4). We have tested which EP receptor subtypes participate in gastric mucosal protection against ethanol-induced gastric mucosal injury and clarified the mechanisms of such protection. The gastric mucosa of anesthetized rats was perfused at 2 ml/min with physiological saline, agonists for EP(1), EP(2), EP(3), and EP(4), or 50% ethanol, using a constant-rate pump connected to a cannula placed in the esophagus. The gastric microcirculation of the mucosal base of anesthetized rats was observed by transillumination through a window made by removal of the adventitia and muscularis externa. PGE(2) and subtype-specific EP agonists were applied to the muscularis mucosae at the window. Application of 50% ethanol dilated the mucosal arterioles and constricted the collecting venules. Collecting venule constriction by ethanol was completely inhibited by PGE(2) and by EP(2) and EP(4) agonists (100 nM) but not by an EP(1) or an EP(3) agonist. Ethanol-induced mucosal injury was also inhibited by EP(2) and EP(4) agonists. When leukotriene (LT)C(4) levels in the perfusate of the gastric mucosa were determined by ELISA, intragastric ethanol administration elevated the LTC(4) levels sixfold from the basal levels. These elevated levels were significantly (60%) reduced by both EP(2) and EP(4) agonists but not by other EP agonists. Since LTC(4) application at the window constricted collecting venules strongly, and an LTC antagonist reduced ethanol-induced mucosal injury, reductions in LTC(4) generation in response to EP(2) and EP(4) receptor signaling may be relevant to the protective action of PGE(2). The present results indicate that EP(2) and EP(4) receptor signaling inhibits ethanol-induced gastric mucosal injury through cancellation of collecting venule constriction by reducing LTC(4) production.     

New evidence for ATP binding induced catalytic subunit interactions in pig kidney Na/K-ATPase

Pig kidney Na/K-ATPase preparations showed a positive cooperative effect for pNPP in Na-pNPPase activity. Measurements of the Na-pNPPase activity, Na-ATPase activity and the accumulation of phosphoenzyme (EP) under conditions of pNPP saturation showed several different ATP affinities. The presence of pNPP reduced both the maximum amount of EP and Na-ATPase activity to half showing a value of 4 and a 3,700-fold reduced ATP affinity for EP formation, and a 7 and 1,300-fold reduced affinity for Na-ATPase activity. The presence of low concentrations of ATP in the phosphorylation induced a 2-fold enhancement in Na-pNPPase activity despite a reduction in available pNPP sites. However, higher concentrations of ATP inhibited the Na-pNPPase activity and a much higher concentration of ATP increased both the phosphorylation and Na-ATPase activity to the maximum levels. The maximum Na-pNPPase activity was 1.7 and 3.4-fold higher without and with ATP, respectively, than the maximum Na-ATPase activity. These data and the pNPP dependent reduction in both Na-ATPase activity and the amount of enzyme bound ATP provide new evidence to show that ATP, pNPP and ATP with pNPP, respectively, induce different subunit interactions resulting a difference in the maximum Na(+)-dependent catalytic activity in tetraprotomeric Na/K-ATPase.     

Binding of monovalent cations to Na+,K+-dependent ATPase purified from porcine kidney. III. Marked changes in affinities for monovalent cations induced by formation of an ADP-insensitive but no an ADP-Sensitive phosphoenzyme

We measured the amounts of Rb+ ions (a K+ congener) as well as Na+ and K+ ions bound to the ATPase during the ATPase reaction at pH 7.5 and 0 degrees C. The affinity of the Na+-binding sites for three Na+ ions decreased markedly but that of the K+-binding sites for two K+ or Rb+ ions increased markedly upon formation of an ADP-insensitive phosphorylated intermediate. Furthermore, the present experiment did not give any indication of a change in the Hill coefficient of 2, and showed an increase in the affinity of the K+-binding sites for Rb+ ions of about 28 times upon the formation of an ADP-insensitive EP. The enzyme state with a high affinity for Rb+ was maintained after the disappearance of EP. When the ATPase was treated with N-ethylmaleimide (NEM), almost all the EP formed was ADP-sensitive. The formation of an ADP-sensitive EP with the NEM-treated enzyme induced no change in the affinities of the ATPase for Na+ and Rb+ ions.     

Central PGE2 exhibits anxiolytic-like activity via EP1 and EP4 receptors in a manner dependent on serotonin 5-HT1A, dopamine D1 and GABAA receptors

We found that centrally administered prostaglandin (PG) E(2) exhibited anxiolytic-like activity in the elevated plus-maze and open field test in mice. Agonists selective for EP(1) and EP(4) receptors, among four receptor subtypes for PGE(2), mimicked the anxiolytic-like activity of PGE(2). The anxiolytic-like activity of PGE(2) was blocked by an EP(1) or EP(4) antagonist, as well as in EP(4) but not EP(1) knockout mice. Central activation of either EP(1) or EP(4) receptors resulted in anxiolytic-like activity. The PGE(2)-induced anxiolytic-like activity was inhibited by antagonists for serotonin 5-HT(1A), dopamine D(1) and GABA(A) receptors. Taken together, PGE(2) exhibits anxiolytic-like activity via EP(1) and EP(4) receptors, with downstream involvement of 5-HT(1A), D(1) and GABA(A) receptor systems.