Molecular cloning and expression of a rat prostaglandin E2 receptor of the EP2 subtype

Prostaglandin E receptors and the kidney

Prostaglandin E(2) is a major renal cyclooxygenase metabolite of arachidonate and interacts with four G protein-coupled E-prostanoid receptors designated EP(1), EP(2), EP(3), and EP(4). Through these receptors, PGE(2) modulates renal hemodynamics and salt and water excretion. The intrarenal distribution and function of EP receptors have been partially characterized, and each receptor has a distinct role. EP(1) expression predominates in the collecting duct where it inhibits Na(+) absorption, contributing to natriuresis. The EP(2) receptor regulates vascular reactivity, and EP(2) receptor-knockout mice have salt-sensitive hypertension. The EP(3) receptor is also expressed in vessels as well as in the thick ascending limb and collecting duct, where it antagonizes vasopressin-stimulated salt and water transport. EP(4) mRNA is expressed in the glomerulus and collecting duct and may regulate glomerular tone and renal renin release. The capacity of PGE(2) to bidirectionally modulate vascular tone and epithelial transport via constrictor EP(1) and EP(3) receptors vs. dilator EP(2) and EP(4) receptors allows PGE(2) to serve as a buffer, preventing excessive responses to physiological perturbations.

Knockout of the murine prostaglandin EP2 receptor impairs osteoclastogenesis in vitro

Prostaglandin E2 (PGE2) stimulates the formation of osteoclast-like tartrate-resistant acid phosphatase-positive multinucleated cells (TRAP + MNC) in vitro. This effect likely results from stimulation of adenylyl cyclase, which is mediated by two PGE2 receptors, designated EP2 and EP4. We used cells from mice in which the EP2 receptor had been disrupted to test its role in the formation of TRAP + MNC. EP2 heterozygous (+/-) mice in a C57BL/6 x 129/SvEv background were bred to produce homozygous null (EP2 -/-) and wild-type (EP2 +/+) mice. PGE2, PTH, or 1,25 dihydroxyvitamin D increased TRAP+ MNC in 7-day cultures of bone marrow cells from EP2 +/+ mice. In cultures from EP2 -/- animals, responses to PGE2, PTH, and 1,25 dihydroxyvitamin D were reduced by 86%, 58%, and 50%, respectively. A selective EP4 receptor antagonist (EP4RA) further inhibited TRAP+ MNC formation in both EP2 +/+ and EP2 -/- cultures. In cocultures of spleen and calvarial osteoblastic cells, the response to PGE2 or PTH was reduced by 92% or 85% when both osteoblastic cells and spleen cells were from EP2 -/- mice, by 88% or 68% when only osteoblastic cells were from EP2 -/- mice and by 58% or 35% when only spleen cells were from EP2 -/- mice. PGE2 increased receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) messenger RNA expression in osteoblastic and bone marrow cell cultures from EP2 +/+ mice 2-fold but had little effect on cells from EP2 -/- mice. Spleen cells cultured with RANKL and macrophage colony stimulating factor produced TRAP+ MNC. PGE2 increased the number of TRAP+ MNC in spleen cell cultures from EP2 +/+ mice but not in cultures from EP2 -/- mice. EP4RA had no effect on the PGE2 response in spleen cell cultures. PGE2 decreased the expression of messenger RNA for granulocyte-macrophage colony stimulating factor in spleen cell cultures from EP2 +/+ mice but had little effect on cells from EP2 -/- mice. These data demonstrate that the prostaglandin EP2 receptor plays a role in the formation of osteoclast-like cells in vitro. A major defect in EP2 -/- mice appears to be in the capacity of osteoblastic cells to stimulate osteoclast formation. In addition, there appears to be a defect in the response of cells of the osteoclastic lineage to PGE2 in EP2 -/- mice.

Characterization of prostaglandin E(2) receptors and their role in 24,25-(OH)(2)D(3)-mediated effects on resting zone chondrocytes

Resting zone chondrocyte differentiation is modulated by the vitamin D metabolite, 24,25-(OH)(2)D(3), via activation of protein kinase C (PKC). In previous studies, inhibition of prostaglandin production with indomethacin caused an increase in PKC activity, suggesting that changes in prostaglandin levels may mediate the 24, 25-(OH)(2)D(3)-dependent response and act as autocrine or paracrine regulators of chondrocyte metabolism. Supporting this hypothesis is the fact that resting zone cells respond directly to prostaglandin E(2) (PGE(2)). The aim of the present study was to identify which PGE(2) receptor subtypes (EP) mediate the effects of PGE(2) on resting zone cells. Using primers specific for EP1-EP4, reverse transcription-polymerase chain reaction (RT-PCR) amplified EP1 and EP2 cDNA in a RT-dependent manner. A variant form of the EP1 cDNA, EPlv, was also amplified in an RT-dependent manner. In parallel experiments, we used EP subtype-specific agonists to examine the role of EP receptors in 24,25-(OH)(2)D(3)-mediated cell proliferation and differentiation. 17-phenyl-trinor-PGE(2) (PTPGE(2)), an EP1 agonist, increased [(3)H]-thymidine incorporation in a dose-dependent manner and reversed the 24, 25-(OH)(2)D(2)-induced inhibition of [(3)H]-thymidine incorporation. SC-19220, an EP1 antagonist, caused a further dose-dependent decrease in 24,25-(OH)(2)D(3)-induced inhibition of [(3)H]-thymidine incorporation. PTPGE(2) also caused a biphasic increase in [(35)S]-sulfate incorporation and increased alkaline phosphatase enzyme activity at high concentrations (10(-8) M). 24, 25-(OH)(2)D(3)-induced alkaline phosphatase activity was synergistically stimulated in a dose-dependent manner by PTPGE(2). In contrast, 24,25-(OH)(2)D(3)-induced PKC activity was inhibited in a dose-dependent manner by PTPGE(2) and SC-19220, the EP1 antagonist, elevated PKC activity at high concentrations (10(-8) M). The EP2 agonist, misoprostol, only affected [(35)S]-sulfate incorporation, but in a dose-dependent manner. The EP3 and EP4 agonists had no effect on cell response. These results suggest that the EP1 receptor subtype mediates some of the PGE(2)-induced cellular responses in resting zone cells that lead to both increased proliferation and differentiation. Because 24,25-(OH)(2)D(3) inhibits PGE(2) synthesis in these cells, EP1-mediated induction of proliferation is blocked, encouraging cellular maturation and activation of PKC activity.

Prostaglandin receptors: their role in regulating renal function

Renal cyclooxygenase-1 and cyclooxygenase-2 actively metabolize arachidonate to metabolism five primary prostanoids: prostaglandin E2, prostaglandin F2a, prostaglandin I2, thromboxane A2, and prostaglandin D2. These lipid mediators interact with a family of distinct G-protein-coupled prostanoid receptors designated EP, FP, IP, TP, and DP, respectively, which exert important regulatory effects on renal function. The intrarenal distribution of these prostanoid receptors has been mapped and the consequences their activation are being characterized. The FP, TP, and EP1 receptors preferentially couple to increased cell Ca2+. EP2, EP4, DP, and IP receptors stimulate cyclic adenosine monophosphate, whereas the EP3 receptor preferentially couples to Gi, inhibiting cyclic adenosine monophosphate generation. EP1 and EP3 messenger RNA expression predominate in the collecting duct and thick limb, respectively, where their stimulation reduces sodium chloride and water absorption, promoting natriuresis and diuresis. Interestingly, only a mild change in renal water handling is seen in the EP3 receptor knockout mouse. Although only low levels EP2 receptor messenger RNA are detected in kidney and its precise intrarenal localization is uncertain, mice with targeted disruption of the EP2 receptor display salt-sensitive hypertension, suggesting it also plays an important role in salt excretion. In contrast, EP4 messenger RNA is readily detected in the glomerulus where it may contribute to the regulation of renin release and decrease glomerular resistance. TP receptors are also highly expressed in the glomerulus, where they may increase glomerular vascular resistance. The IP receptor messenger RNA is most highly expressed in the afferent arteriole and it may also modulate renal arterial resistance and renin release. At present there is little evidence for DP receptor expression in the kidney. Together these receptors act as physiologic buffers that protect the kidney from excessive functional changes during periods of physiologic stress. Loss of the combined effects of these receptors contributes to the side effects seen in the setting of nonsteroidal anti-inflammatory drug administration, whereas selective antagonists for these receptors may provide new therapeutic approaches in disease.

Cloning, structural characterization, and chromosomal localization of the gene encoding the human prostaglandin E(2) receptor EP2 subtype

Northern blot analysis of human placental RNA using a probe to the 5' end of the human prostaglandin E(2) (PGE(2)) EP2 receptor subtype coding region revealed the existence of a high abundance, low molecular weight transcript. To investigate the origin of this transcript, and its possible relationship to the human EP2 mRNA, we have cloned and characterized the gene encoding the human PGE(2) EP2 receptor subtype, identified transcriptional initiation and termination sites in two tissues (spleen and thymus), and determined its chromosomal localization. The human EP2 gene consists of two exons separated by a large intron, utilizes a common initiation site in both spleen and thymus at 1113 bp upstream of the translation initiation site, and has 3' transcript termini at 1140 bp and 1149 bp downstream of the translation stop site in spleen and thymus respectively. Southern and fluorescence in situ hybridization analysis demonstrated the human EP2 gene to be a single copy gene located in band 22 of the long arm of chromosome 14 (14q22). Though our initial interest in this gene was to investigate potential differential splicing of the human EP2 gene in placenta, this work demonstrates that the atypical transcript observed in placenta probably arises from a distinct, yet related, gene. Knowledge of the sequence, structure, and transcription events associated with the human EP2 gene will enable a broader understanding of its regulation and potential role in normal physiology and disease.

Differential regulation of renal prostaglandin receptor mRNAs by dietary salt intake in the rat

BACKGROUND

In this study, we tested the hypothesis that prostaglandin (PG) receptor expression in the rat kidney is subject to physiological regulation by dietary salt intake.

METHODS

Rats were fed diets with 0.02 or 4% NaCl for two weeks. PG receptor expression was assayed in kidney regions and cells by ribonuclease protection assay and reverse transcription-polymerase chain reaction analysis. Functional correlates were studied by measurement of PGE2-induced cAMP formation and renin secretion in juxtaglomerular (JG) cells isolated from animals on various salt intakes.

RESULTS

EP1 and EP3 receptors were predominantly expressed, and the EP2 receptor was exclusively expressed in the rat kidney medulla. The EP4 receptor was strongly expressed in glomeruli and in renin-secreting JG granular cells. IP receptor transcripts were found mainly in cortex. Maintaining rats on a low- or high-NaCl diet did not affect the expression of EP1 or IP receptors, whereas EP4 transcripts in glomeruli were increased twofold by salt deprivation. Consistent with this, we found that PGE2-evoked cAMP production and renin secretion by JG cells from salt-deprived animals were significantly higher compared with cells obtained from salt-loaded animals. In the outer medulla, EP3 transcripts correlated directly with salt intake, and mRNA abundance was increased twofold by a high-NaCl diet.

CONCLUSIONS

Our results suggest that subtype-specific, regional changes in PG receptor expression are involved in the renal adaptation to changes in salt intake. The results are in accord with the general concept that renocortical PGE2 stimulates renin secretion and maintains renal blood flow during low-salt states, whereas medullary PGE2 promotes salt excretion in response to a high salt intake.

Roles of prostaglandin E receptors in mesangial cells under high-glucose conditions

BACKGROUND

High glucose reportedly stimulates prostaglandin (PG) E2 production and DNA synthesis in mesangial cells (MCs). However, the pathophysiological significance of PGE2 in MCs has remained unclear.

METHODS

The effects of prostanoids on [3H]-thymidine uptake and cAMP production in rat MCs cultured with 5.6 mM glucose, 25 mM glucose, or 5.6 mM glucose supplemented with 19.4 mM mannitol were examined. The gene expression of PGE2 receptor (EP) subtypes in MCs was analyzed with Northern blotting techniques.

RESULTS

Northern blotting indicated EP1 and EP4 gene expression in MCs. EP1 agonists and PGE2 stimulated [3H]-thymidine uptake in MCs. EP1 antagonists dose dependently attenuated high-glucose-induced [3H]-thymidine uptake, which suggests EP1 involvement, by an increase in intracellular Ca2+, in DNA synthesis of MCs. On the other hand, forskolin, db-cAMP, and 11-deoxy-PGE1, an EP4/EP3/EP2 agonist, significantly decreased DNA synthesis in MCs. These inhibitory effects are thought to be mediated via EP4 as a result of an increase in cAMP synthesis. The effects via EP4 seem to be particularly important because PGE2-induced cAMP synthesis was significantly attenuated in the high-glucose group compared with the mannitol group, in which [3H]-thymidine uptake did not increase in spite of augmented PGE2 production.

CONCLUSION

The increase in DNA synthesis in MCs under high-glucose conditions can be explained, at least in part, by the high-glucose-induced inhibition of cAMP production via EP4, which augments EP1 function in conjunction with the overproduction of PGE2.

Reproductive failure and reduced blood pressure in mice lacking the EP2 prostaglandin E2 receptor

Prostaglandins (PGs) are bioactive lipids that modulate a broad spectrum of biologic processes including reproduction and circulatory homeostasis. Although reproductive functions of mammals are influenced by PGs at numerous levels, including ovulation, fertilization, implantation, and decidualization, it is not clear which PGs are involved and whether a single mechanism affects all reproductive functions. Using mice deficient in 1 of 4 prostaglandin E2 (PGE2) receptors -- specifically, the EP2 receptor -- we show that Ep2(-/-) females are infertile secondary to failure of the released ovum to become fertilized in vivo. Ep2(-/-) ova could be fertilized in vitro, suggesting that in addition to previously defined roles, PGs may contribute to the microenvironment in which fertilization takes place. In addition to its effects on reproduction, PGE2 regulates regional blood flow in various vascular beds. However, its role in systemic blood pressure homeostasis is not clear. Mice deficient in the EP2 PGE2 receptor displayed resting systolic blood pressure that was significantly lower than in wild-type controls. Blood pressure increased in these animals when they were placed on a high-salt diet, suggesting that the EP2 receptor may be involved in sodium handling by the kidney. These studies demonstrate that PGE2, acting through the EP2 receptor, exerts potent regulatory effects on two major physiologic processes: blood pressure homeostasis and in vivo fertilization of the ovum.

Molecular cloning and characterization of the canine prostaglandin E receptor EP2 subtype

Prostaglandin E2 (PGE2) binds to four G-protein coupled cell surface receptors (EP1-EP4) and has been implicated as a local mediator of bone anabolism via a cyclic AMP mediated pathway following activation of the EP2 and/or EP4 receptor subtype. A canine kidney cDNA library was screened using a human EP2 probe, and a clone with an open reading frame of 1083 bp, potentially encoding a protein of 361 amino acids, was characterized. This open reading frame has 89% identity to the human EP2 cDNA at the nucleotide level and 87% identity at the predicted protein level. Scatchard analysis of a CHO cell line stably transfected with canine EP2 yielded a dissociation constant of 22 nM for PGE2. Competition binding studies, using 3H-PGE2 as ligand, demonstrated specific displacement by PGE2, Prostaglandin E1, Prostaglandin A3, and butaprost (an EP2 selective ligand), but not by ligands with selectivity for the related DP, FP, IP, or TP receptors. Specific ligand binding also resulted in increased levels of cAMP in EP2 transfected cells with no evidence of short-term, ligand-induced desensitization. Northern blot analysis revealed two transcripts of 3300 and 2400 bp in canine lung, and reverse-transcription polymerase chain reaction showed expression in all tissues examined. Southern blot analysis suggests the presence of a single-copy gene for EP2 in the dog.

The anabolic effect of PGE2 in rat bone marrow cultures is mediated via the EP4 receptor subtype

Prostaglandin E2 (PGE2) is an anabolic agent in vivo that stimulates bone formation by recruiting osteoblasts from bone marrow precursors. To understand which of the known PGE2 receptors (EP1-4) is involved in this process, we tested the effect of PGE2 and various EP agonists and/or antagonists on osteoblastic differentiation in cultures of bone marrow cells by counting bone nodules and measuring alkaline phosphatase activity. PGE2 increased both parameters, peaking at 100 nM, an effect that was mimicked by forskolin and was abolished by 2',3'-dideoxyadenosine (an adenylate cyclase inhibitor) and was thus cAMP dependent, pointing to the involvement of EP2 or EP4. Consistently, 17-phenyl-omega-trinor PGE2 (EP1 agonist) and sulprostone (EP3/EP1 agonist) lacked any anabolic activity. Furthermore, butaprost (EP2 agonist) was inactive, 11-deoxy-PGE1 (EP4/EP2 agonist) was as effective as PGE2, and the PGE2 effect was abolished dose dependently by the selective EP4 antagonist AH-23848B, suggesting the involvement of EP4. We also found that PGE2 increased nodule formation and AP activity when added for the initial attachment period of 24 h only. Thus this study shows that PGE2 stimulates osteoblastic differentiation in bone marrow cultures, probably by activating the EP4 receptor, and that this effect may involve recruitment of noncommitted (nonadherent) osteogenic precursors, in agreement with its suggested mode of operation in vivo.

Prostaglandin E2 inhibits renal collecting duct Na+ absorption by activating the EP1 receptor

PGE2 exerts potent diuretic and natriuretic effects on the kidney. This action is mediated in part by direct inhibition of collecting duct Na+ absorption via a Ca++-coupled mechanism. These studies examine the role the Ca++-coupled PGE-E EP1 receptor plays in mediating these effects of PGE2 on Na+ transport. Rabbit EP1 receptor cDNA was amplified from rabbit kidney RNA. Nuclease protection assays demonstrated highest expression of EP1 mRNA in kidney, followed by stomach, adrenal, and ileum. In situ hybridization, demonstrated renal expression of EP1 mRNA was exclusively over the collecting duct. In fura-2-loaded microperfused rabbit cortical collecting duct, EP1 active PGE analogs were 10-1, 000-fold more potent in raising intracellular Ca++ than EP2, EP3, or EP4-selective compounds. Two different EP1 antagonists, AH6809 and SC19220, completely blocked the PGE2-stimulated intracellular calcium increase. AH6809 also completely blocked the inhibitory effect of PGE2 on Na+ absorption in microperfused rabbit cortical collecting ducts. These studies suggest that EP1 receptor activation mediates PGE2-dependent inhibition of Na+ absorption in the collecting duct, thereby contributing to its natriuretic effects.