Prostaglandin Signaling in the Renal Collecting Duct

Nonmicrobial Activation of TLRs Controls Intestinal Growth, Wound Repair, and Radioprotection

TLRs, key components of the innate immune system, recognize microbial molecules. However, TLRs also recognize some nonmicrobial molecules. In particular, TLR2 and TLR4 recognize hyaluronic acid, a glycosaminoglycan in the extracellular matrix. In neonatal mice endogenous hyaluronic acid binding to TLR4 drives normal intestinal growth. Hyaluronic acid binding to TLR4 in pericryptal macrophages results in cyclooxygenase2- dependent PGE2 production, which transactivates EGFR in LGR5+ crypt epithelial stem cells leading to increased proliferation. The expanded population of LGR5+ stem cells leads to crypt fission and lengthening of the intestine and colon. Blocking this pathway at any point (TLR4 activation, PGE2 production, EGFR transactivation) results in diminished intestinal and colonic growth. A similar pathway leads to epithelial proliferation in wound repair. The repair phase of dextran sodium sulfate colitis is marked by increased epithelial proliferation. In this model, TLR2 and TLR4 in pericryptal macrophages are activated by microbial products or by host hyaluronic acid, resulting in production of CXCL12, a chemokine. CXCL12 induces the migration of cyclooxygenase2-expressing mesenchymal stem cells from the lamina propria of the upper colonic crypts to a site adjacent to LGR5+ epithelial stem cells. PGE2 released by these mesenchymal stem cells transactivates EGFR in LGR5+ epithelial stem cells leading to increased proliferation. Several TLR2 and TLR4 agonists, including hyaluronic acid, are radioprotective in the intestine through the inhibition of radiation-induced apoptosis in LGR5+ epithelial stem cells. Administration of exogenous TLR2 or TLR4 agonists activates TLR2/TLR4 on pericryptal macrophages inducing CXCL12 production with migration of cyclooxygenase2-expressing mesenchymal stem cells from the lamina propria of the villi to a site adjacent to LGR5+ epithelial stem cells. PGE2 produced by these mesenchymal stem cells, blocks radiation-induced apoptosis in LGR5+ epithelial stem cells by an EGFR mediated pathway.

Recent advances in studies of SLCO2A1 as a key regulator of the delivery of prostaglandins to their sites of action

Solute carrier organic anion transporter family member 2A1 (SLCO2A1, also known as PGT, OATP2A1, PHOAR2, or SLC21A2) is a plasma membrane transporter consisting of 12 transmembrane domains. It is ubiquitously expressed in tissues, and mediates the membrane transport of prostaglandins (PGs, mainly PGE₂, PGF_2α, PGD₂) and thromboxanes (e.g., TxB₂). SLCO2A1-mediated transport is electrogenic and is facilitated by an outwardly directed gradient of lactate. PGs imported by SLCO2A1 are rapidly oxidized by cytoplasmic 15-hydroxyprostaglandin dehydrogenase (15-PGDH, encoded by HPGD). Accumulated evidence suggests that SLCO2A1 plays critical roles in many physiological processes in mammals, and it is considered a potential pharmacological target for diabetic foot ulcer treatment, antipyresis, and non-hormonal contraception. Furthermore, whole-exome analyses suggest that recessive inheritance of SLCO2A1 mutations is associated with two refractory diseases, primary hypertrophic osteoarthropathy (PHO) and chronic enteropathy associated with SLCO2A1 (CEAS). Intriguingly, SLCO2A1 is also a key component of the Maxi-Cl channel, which regulates fluxes of inorganic and organic anions, including ATP. Further study of the bimodal function of SLCO2A1 as a transporter and ion channel is expected to throw new light on the complex pathology of human diseases. Here, we review and summarize recent information on the molecular functions of SLCO2A1, and we discuss its pathophysiological significance.

Hyaluronic acid promotes Lgr5+ stem cell proliferation and crypt fission through TLR4 and PGE2 transactivation of EGFR

Hyaluronic acid (HA), a glycosaminoglycan in the extracellular matrix, binds to CD44 and Toll-like receptor 4 (TLR4). We previously demonstrated that both CD44 and TLR4, but predominately TLR4, mediated HA stimulation of Lgr5⁺ stem cell proliferation, crypt fission, and intestinal growth in postnatal mice. Here we address the questions of which cell type expresses the relevant TLR4 in driving intestinal growth and what are the downstream events from TLR4 activation. Studies were done in 14-day-old mice: wild type (WT), mice deficient in cyclooxygenase 2 (COX2), mice deficient in myeloid cell TLR4, and mice deficient in epithelial cell epidermal growth factor receptor (EGFR). Biological end points included crypt fission and Lgr5 cell proliferation. In WT mice, treatment with NS-398 (a COX2 inhibitor), clodronate (a macrophage-depleting agent), or tyrphostin (an EGFR inhibitor) resulted in 30% reductions in crypt fission and Lgr5⁺ stem cell proliferation compared with control mice. Mice deficient in COX2 or myeloid TLR4 or epithelial cell EGFR all had 30% reductions in crypt fission and Lgr5⁺ stem cell proliferation compared with WT mice. Administration of dimethyl PGE₂, a stable PGE₂ analog, increased crypt fission and Lgr5⁺ stem cell proliferation. Administration of dimethyl PGE₂ reversed the effects of NS-398, clodronate, COX2 deficiency, and myeloid TLR4 deficiency but had no effect on mice treated with tyrphostin or mice deficient in epithelial cell EGFR. We conclude that, in postnatal mice, ~30% of intestinal growth as manifested by crypt fission and Lgr5⁺ stem cell proliferation is driven by a novel pathway: Extracellular HA binds TLR4 on pericryptal macrophages, inducing the production of PGE₂ through COX2. PGE₂ transactivates EGFR in Lgr5⁺ epithelial stem cells, resulting in Lgr5⁺ stem cell proliferation and crypt fission.NEW & NOTEWORTHY This study, in newborn mice, describes a novel molecular pathway regulating Lgr5⁺ epithelial stem cell proliferation and normal intestinal elongation, as assessed by crypt fission. In this pathway, endogenous extracellular hyaluronic acid binds to Toll-like receptor 4 on pericryptal macrophages releasing PGE₂ which binds to epidermal growth factor receptor on Lgr5⁺ stem cells resulting in proliferation. Lgr5⁺ stem cell proliferation leads to crypt fission and intestinal elongation. The demonstration that normal growth requires microbial-independent Toll-like receptor activation is novel.

Novel SLCO2A1compound heterozygous mutation causing primary hypertrophic osteoarthropathy with Bartter-like hypokalemia in a Chinese family

PURPOSE

Primary hypertrophic osteoarthropathy (PHO) is an inherited disease characterized by digital clubbing, periostosis and pachydermia with defects in the degradation of prostaglandin E2 (PGE2). Mutations in SLCO2A1 gene-encoding prostaglandin transporter (PGT) resulted in PHO, autosomal recessive 2 (PHOAR2). The spectrum of mutations and variable clinical complications of PHOAR2 has been delineated. In this study, we investigated a Chinese PHO family with a manifestation of Bartter-like hypokalemia.

METHODS

Clinical manifestations were collected and genetic analyses were performed in the PHO family.

RESULTS

The 33-year-old male proband had severe hypokalemia due to potassium loss from the kidney, while his brother had mild hypokalemia. After being treated with etoricoxib, the serum potassium level of the patient increased rapidly to the normal range which corresponded with the reduction in his serum PGE2 and PE2 metabolite (PGEM) levels. A novel SLCO2A1 compound heterozygous mutation of p.I284V and p.C459R was identified in two PHO patients in this family.

CONCLUSIONS

The present findings supported that the Bartter-like hypokalemia is a new complication of PHOAR2 caused by the high level of PGE2. Etoricoxib was demonstrated to be effective for the renal hypokalemia in PHO patients.

Integrated eicosanoid lipidomics and gene expression reveal decreased prostaglandin catabolism and increased 5-lipoxygenase expression in aggressive subtypes of endometrial cancer

Eicosanoids comprise a diverse group of bioactive lipids which orchestrate inflammation, immunity, and tissue homeostasis, and whose dysregulation has been implicated in carcinogenesis. Among the various eicosanoid metabolic pathways, studies of their role in endometrial cancer (EC) have very much been confined to the COX-2 pathway. This study aimed to determine changes in epithelial eicosanoid metabolic gene expression in endometrial carcinogenesis; to integrate these with eicosanoid profiles in matched clinical specimens; and, finally, to investigate the prognostic value of candidate eicosanoid metabolic enzymes. Eicosanoids and related mediators were profiled using liquid chromatography-tandem mass spectrometry in fresh frozen normal, hyperplastic, and cancerous (types I and II) endometrial specimens (n = 192). Sample-matched epithelia were isolated by laser capture microdissection and whole genome expression analysis was performed using microarrays. Integration of eicosanoid and gene expression data showed that the accepted paradigm of increased COX-2-mediated prostaglandin production does not apply in EC carcinogenesis. Instead, there was evidence for decreased PGE₂ /PGF_2α inactivation via 15-hydroxyprostaglandin dehydrogenase (HPGD) in type II ECs. Increased expression of 5-lipoxygenase (ALOX5) mRNA was also identified in type II ECs, together with proportional increases in its product, 5-hydroxyeicosatetraenoic acid (5-HETE). Decreased HPGD and elevated ALOX5 mRNA expression were associated with adverse outcome, which was confirmed by immunohistochemical tissue microarray analysis of an independent series of EC specimens (n = 419). While neither COX-1 nor COX-2 protein expression had prognostic value, low HPGD combined with high ALOX5 expression was associated with the worst overall and progression-free survival. These findings highlight HPGD and ALOX5 as potential therapeutic targets in aggressive EC subtypes. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Roles of Organic Anion Transporting Polypeptide 2A1 (OATP2A1/SLCO2A1) in Regulating the Pathophysiological Actions of Prostaglandins

Solute carrier organic anion transporter family member 2A1 (OATP2A1, encoded by the SLCO2A1 gene), which was initially identified as prostaglandin transporter (PGT), is expressed ubiquitously in tissues and mediates the distribution of prostanoids, such as PGE₂, PGF_2α, PGD₂ and TxB₂. It is well known to play a key role in the metabolic clearance of prostaglandins, which are taken up into the cell by OATP2A1 and then oxidatively inactivated by 15-ketoprostaglandin dehydrogenase (encoded by HPGD); indeed, OATP2A1-mediated uptake is the rate-limiting step of PGE₂ catabolism. Consequently, since OATP2A1 activity is required for termination of prostaglandin signaling via prostanoid receptors, its inhibition can enhance such signaling. On the other hand, OATP2A1 can also function as an organic anion exchanger, mediating efflux of prostaglandins in exchange for import of anions such as lactate, and in this context, it plays a role in the release of newly synthesized prostaglandins from cells. These different functions likely operate in different compartments within the cell. OATP2A1 is reported to function at cytoplasmic vesicle/organelle membranes. As a regulator of the levels of physiologically active prostaglandins, OATP2A1 is implicated in diverse physiological and pathophysiological processes in many organs. Recently, whole exome analysis has revealed that recessive mutations in SLCO2A1 cause refractory diseases in humans, including primary hypertrophic osteoarthropathy (PHO) and chronic non-specific ulcers in small intestine (CNSU). Here, we review and summarize recent information on the molecular functions of OATP2A1 and on its physiological and pathological significance.

Phenotypic and pharmacogenetic evaluation of patients with thiazide-induced hyponatremia

Thiazide diuretics are among the most widely used treatments for hypertension, but thiazide-induced hyponatremia (TIH), a clinically significant adverse effect, is poorly understood. Here, we have studied the phenotypic and genetic characteristics of patients hospitalized with TIH. In a cohort of 109 TIH patients, those with severe TIH displayed an extended phenotype of intravascular volume expansion, increased free water reabsorption, urinary prostaglandin E2 excretion, and reduced excretion of serum chloride, magnesium, zinc, and antidiuretic hormone. GWAS in a separate cohort of 48 TIH patients and 2,922 controls from the 1958 British birth cohort identified an additional 14 regions associated with TIH. We identified a suggestive association with a variant in SLCO2A1, which encodes a prostaglandin transporter in the distal nephron. Resequencing of SLCO2A1 revealed a nonsynonymous variant, rs34550074 (p.A396T), and association with this SNP was replicated in a second cohort of TIH cases. TIH patients with the p.A396T variant demonstrated increased urinary excretion of prostaglandin E2 and metabolites. Moreover, the SLCO2A1 phospho-mimic p.A396E showed loss of transporter function in vitro. These findings indicate that the phenotype of TIH involves a more extensive metabolic derangement than previously recognized. We propose one mechanism underlying TIH development in a subgroup of patients in which SLCO2A1 regulation is altered.

Impact of FDA-Approved Drugs on the Prostaglandin Transporter OATP2A1/SLCO2A1

To understand interaction of drugs with the prostaglandin transporter OATP2A1/SLCO2A1 that regulates disposition of prostaglandins, we explored the impact of 636 drugs in an FDA-approved drug library on 6-carboxyfluorescein (6-CF) uptake by OATP2A1-expressing HEK293 cells (HEK/2A1). Fifty-one and 10 drugs were found to inhibit and enhance 6-CF uptake by more than 50%, respectively. Effect of the 51 drugs on 6-CF uptake was positively correlated with that on PGE₂ uptake (r = 0.64, p < 0.001). Among those, 5 drugs not structurally related to prostaglandins, suramin, pranlukast, zafirlukast, olmesartan medoxomil, and losartan potassium, exhibited more than 90% PGE₂ uptake inhibition. Inhibitory affinity of suramin to OATP2A1 was the highest (IC_50,2A1 of 0.17 μM), and its IC₅₀ values to MRP4-mediated PGE₂ transport (IC_50,MRP4) and PGE₂ synthesis in human U-937 cells treated with phorbol 12-myristate 13-acetate (IC_50,Syn) were 73.6 and 336.7 times higher than IC_50,2A1, respectively. Moreover, structure-activity relationship study in 29 nonsteroidal anti-inflammatory drugs contained in the library displayed inhibitory activities of anthranilic acid derivatives, but enhancing effects of propionic acid derivatives. These results demonstrate that suramin is a potent selective inhibitor of OATP2A1, providing a comprehensive information about drugs in clinical use that interact with OATP2A1.

Prostaglandin transporter (OATP2A1/SLCO2A1) contributes to local disposition of eicosapentaenoic acid-derived PGE3

Eicosapentaenoic acid (EPA)-derived prostaglandin E3 (PGE3) possesses an anti-inflammatory effect; however, information for transporters that regulate its peri-cellular concentration is limited. The present study, therefore, aimed to clarify transporters involved in local disposition of PGE3. PGE3 uptake was assessed in HEK293 cells transfected with OATP2A1/SLCO2A1, OATP1B1/SLCO1B1, OATP2B1/SLCO2B1, OAT1/SLC22A6, OCT1/SLC22A1 or OCT2/SLC22A2 genes, compared with HEK293 cells transfected with plasmid vector alone (Mock). PGE3 uptake by OATP2A1-expressing HEK293 cells (HEK/2A1) was the highest and followed by HEK/1B1, while no significantly higher uptake of PGE3 than Mock cells was detected by other transporters. Saturation kinetics in PGE3 uptake by HEK/2A1 estimated the Km as 7.202 ± 0.595 μM, which was 22 times higher than that of PGE2 (Km=0.331 ± 0.131 μM). Furthermore, tissue disposition of PGE3 was examined in wild-type (WT) and Slco2a1-deficient (Slco2a1(-/-)) mice after oral administration of EPA ethyl ester (EPA-E) when they underwent intraperitoneal injection of endotoxin (e.g., lipopolysaccharide). PGE3 concentration was significantly higher in the lung, and tended to increase in the colon, stomach, and kidney of Slco2a1(-/-), compared to WT mice. Ratio of PGE2 metabolite 15-keto PGE2 over PGE2 concentration was significantly lower in the lung and colon of Slco2a1(-/-) than that of WT mice, suggesting that PGE3 metabolism is downregulated in Slco2a1(-/-) mice. In conclusion, PGE3 was found to be a substrate of OATP2A1, and local disposition of PGE3 could be regulated by OATP2A1 at least in the lung.

OATP2A1/SLCO2A1-mediated prostaglandin E2 loading into intracellular acidic compartments of macrophages contributes to exocytotic secretion

There is significant evidence that the inducible cyclooxygenase isoform (COX-2) regulates the pericellular concentration of PGE2; however, the mechanism of the secretory process remains unclear. The present study, therefore, aimed to evaluate the role of prostaglandin transporter (OATP2A1) in PGE2 secretion from macrophages. Immunofluorescence staining for Oatp2a1 (Slco2a1) was primarily detected in cytoplasmic domains, and was partially co-localized with anti-PGE2 antibody, LysoTracker®, and anti-lysosome-associated membrane protein (Lamp) 1 antibody in murine macrophage-derived RAW264 cells and peritoneal macrophages (PMs). PGE2 uptake by subcellular fraction containing light lysosomes was reduced significantly in the presence of an OATP inhibitor and in Slco2a1(+/-) PMs. Secretion of PGE2 and lysosome-specific N-acetyl-β-d-glucosaminidase was enhanced in activated macrophagic cells, and diminished significantly under the Ca(2+)-depleted condition. The amount of PGE2 secreted from lipopolysaccharide-activated Slco2a1(-/-) PMs was significantly lower than that from PMs from wild type (WT) mice. Expression of Cox-2 and 15-hydroxyprostaglandin dehydrogenase (15-Pgdh) was unchanged between PMs from Slco2a1(-/-) and WT mice. These results suggest that OATP2A1 is involved in PGE2-loading into intracellular acidic compartments, including light lysosomes. Thus, OATP2A1 contributes to PGE2 secretion by macrophages via exocytosis induced by Ca(2+) influx, independently of PGE2 synthesis and metabolism.

Interplay between the prostaglandin transporter OATP2A1 and prostaglandin E2-mediated cellular effects

Prostaglandins such as prostaglandin E2 (PGE2) play a pivotal role in physiological and pathophysiological pathways in gastric mucosa. Little is known about the interrelation of the prostaglandin E (EP) receptors with the prostaglandin transporter OATP2A1 in the gastric mucosa and gastric carcinoma. Therefore, we first investigated the expression of OATP2A1 and EP4 in normal and carcinoma gastric mucosa. Different PGE2-mediated cellular pathways and mechanisms were investigated using human embryonic kidney cells (HEK293) and the human gastric carcinoma cell line AGS stably transfected with OATP2A1. Colocalization and expression of OATP2A1 and EP4 were detected in mucosa of normal gastric tissue and of gastric carcinomas. OATP2A1 reduced the PGE2-mediated cAMP production in HEK293 and AGS cells overexpressing EP4 and OATP2A1. The expression of OATP2A1 in AGS cells resulted in a reduction of [(3)H]-thymidine incorporation which was in line with a higher accumulation of AGS-OATP2A1 cells in S-phase of the cell cycle compared to control cells. In contrast, the expression of OATP2A1 in HEK293 cells had no influence on the distribution in the S-phase compared to control cells. OATP2A1 also diminished the PGE2-mediated expression of interleukin-8 mRNA (IL-8) and hypoxia-inducible-factor 1α (HIF1α) protein in AGS-OATP2A1 cells. The expression of OATP2A1 increased the sensitivity of AGS cells against irinotecan which led to reduced cell viability. Taken together, these data show that OATP2A1 influences PGE2-mediated cellular pathways. Therefore, OATP2A1 needs to be considered as a key determinant for the understanding of the physiology and pathophysiology of prostaglandins in healthy and tumorous gastric mucosa.

Regulation of prostaglandin EP1 and EP4 receptor signaling by carrier-mediated ligand reuptake

After synthesis and release from cells, prostaglandin E₂ (PGE₂) undergoes reuptake by the prostaglandin transporter (PGT), followed by cytoplasmic oxidation. Although genetic inactivation of PGT in mice and humans results in distinctive phenotypes, and although experiments in localized environments show that manipulating PGT alters downstream cellular events, a direct mechanistic link between PGT activity and PGE₂ (EP) receptor activation has not been made. Toward this end, we created two reconstituted systems to examine the effect of PGT expression on PGE₂ signaling via two of its receptors (EP₁ and EP₄). In human embryonic kidney cells engineered to express the EP₁ receptor, exogenous PGE₂ induced a dose-dependent increase in cytoplasmic Ca²⁺. When PGT was expressed at the plasma membrane, the PGE₂ dose–response curve was right-shifted, consistent with reduction in cell surface PGE₂ availability; a potent PGT inhibitor acutely reversed this shift. When bradykinin was used to induce endogenous PGE₂ release, PGT expression similarly induced a reduction in Ca²⁺ responses. In separate experiments using Madin–Darby Canine Kidney cells engineered to express the PGE₂ receptor EP₄, bradykinin again induced autocrine PGE₂ signaling, as judged by an abrupt increase in intracellular cAMP. As in the EP₁ experiments, expression of PGT at the plasma membrane caused a reduction in bradykinin-induced cAMP accumulation. Pharmacological concentrations of exogenous PGE₂ induced EP₄ receptor desensitization, an effect that was mitigated by PGT. Thus, at an autocrine/paracrine level, plasma membrane PGT regulates PGE₂ signaling by decreasing ligand availability at cell surface receptors.

Prostaglandin E2 modifies SMAD2 and promotes SMAD2-SMAD4 complex formation

We report that PGE2 promotes Smad2-Smad4 complex formation and this phenomenon could be blocked by DIDS, an anion transporter inhibitor. Our data suggest that PGE2 had no effects on Smad2 phosphorylation, suggesting that PGE2-mediated Smad2-Smad4 complex formation is independent of TGF-β signaling and that PGE2 induced Smad2 modification which is different from TGF-β-mediated phosphorylation. We demonstrate that in primary human glomerular mesangial cells PGE2 caused modification of Smad2 as detected by Smad2N antibody, raised against a peptide near the N-terminus of Smad2. We hypothesize that Smad2 protein is post-translationaly modified by PGE2. Direct evidence of Smad2 modification by PGE2 was achieved by avidin pulldown assay which showed that endogenous Smad2 and recombinant Smad2 protein were attached by biotin-labeled PGE2. Taken together, our results provided evidence that post-translational modification of Smad2 could be a mechanism for the action of PGE2 in the pathogenesis of human pathologies.

Postnatal regulation of 15-hydroxyprostaglandin dehydrogenase in the rat kidney

Cyclooxygenase 2 (COX-2) has an established role in postnatal kidney development. 15-Hydroxyprostaglandin dehydrogenase (15-PGDH) is recently identified as an endogenous inhibitor of COX-2, limiting the production of COX-2-derived prostanoids in several pathological conditions. The present study was undertaken to examine the regulation of renal 15-PGDH expression during postnatal kidney development in rats compared with COX-2. qRT-PCR and immunoblotting demonstrated that 15-PGDH mRNA and protein in the kidney were present in neonates, peaked in the second postnatal week, and then declined sharply to very low level in adulthood. Immunostaining demonstrated that at the second postnatal week, renal 15-PGDH protein was predominantly found in the proximal tubules stained positive for Na/H exchanger 3 and brush borders (periodic acid-Schiff), whereas COX-2 protein was restricted to macular densa and adjacent thick ascending limbs. Interestingly, in the fourth postnatal week, 15-PGDH protein was redistributed to thick ascending limbs stained positive for the Na-K-2Cl cotransporter. After 6 wk of age, 15-PGDH protein was found in the granules in subsets of the proximal tubules. Overall, these results support a possibility that 15-PGDH may regulate postnatal kidney development through interaction with COX-2.

Cyclooxygenase metabolites in the kidney

In the mammalian kidney, prostaglandins (PGs) are important mediators of physiologic processes, including modulation of vascular tone and salt and water. PGs arise from enzymatic metabolism of free arachidonic acid (AA), which is cleaved from membrane phospholipids by phospholipase A2 activity. The cyclooxygenase (COX) enzyme system is a major pathway for metabolism of AA in the kidney. COX are the enzymes responsible for the initial conversion of AA to PGG2 and subsequently to PGH2, which serves as the precursor for subsequent metabolism by PG and thromboxane synthases. In addition to high levels of expression of the "constitutive" rate-limiting enzyme responsible for prostanoid production, COX-1, the "inducible" isoform of cyclooxygenase, COX-2, is also constitutively expressed in the kidney and is highly regulated in response to alterations in intravascular volume. PGs and thromboxane A2 exert their biological functions predominantly through activation of specific 7-transmembrane G-protein-coupled receptors. COX metabolites have been shown to exert important physiologic functions in maintenance of renal blood flow, mediation of renin release and regulation of sodium excretion. In addition to physiologic regulation of prostanoid production in the kidney, increases in prostanoid production are also seen in a variety of inflammatory renal injuries, and COX metabolites may serve as mediators of inflammatory injury in renal disease.

Prostaglandin E2 induces chloride secretion through crosstalk between cAMP and calcium signaling in mouse inner medullary collecting duct cells

Under conditions of high dietary salt intake, prostaglandin E2 (PGE2) production is increased in the collecting duct and promotes urinary sodium chloride (NaCl) excretion; however, the molecular mechanisms by which PGE2 increases NaCl excretion in this context have not been clearly defined. We used the mouse inner medullary collecting duct (mIMCD)-K2 cell line to characterize mechanisms underlying PGE2-regulated NaCl transport. When epithelial Na(+) channels were inhibited, PGE2 exclusively stimulated basolateral EP4 receptors to increase short-circuit current (Isc(PGE2)). We found that Isc(PGE2) was sensitive to inhibition by H-89 and CFTR-172, indicating that EP4 receptors signal through protein kinase A to induce Cl(-) secretion via cystic fibrosis transmembrane conductance regulator (CFTR). Unexpectedly, we also found that Isc(PGE2) was sensitive to inhibition by BAPTA-AM (Ca(2+) chelator), 2-aminoethoxydiphenyl borate (2-APB) (inositol triphosphate receptor blocker), and flufenamic acid (FFA) [Ca(2+)-activated Cl(-) channel (CACC) inhibitor], suggesting that EP4 receptors also signal through Ca(2+) to induce Cl(-) secretion via CACC. Additionally, we observed that PGE2 stimulated an increase in Isc through crosstalk between cAMP and Ca(2+) signaling; BAPTA-AM or 2-APB inhibited a component of Isc(PGE2) that was sensitive to CFTR-172 inhibition; H-89 inhibited a component of Isc(PGE2) that was sensitive to FFA inhibition. Together, our findings indicate that PGE2 activates basolateral EP4 receptors and signals through both cAMP and Ca(2+) to stimulate Cl(-) secretion in IMCD-K2 cells. We propose that these signaling pathways, and the crosstalk between them, may provide a concerted mechanism for enhancing urinary NaCl excretion under conditions of high dietary NaCl intake.

Intrauterine inhibition of prostaglandin transporter protein blocks release of luteolytic PGF2alpha pulses without suppressing endometrial expression of estradiol or oxytocin receptor in ruminants

In ruminants, prostaglandin F2 alpha (PGF2(alpha)) is synthesized and released in a pulsatile pattern from the endometria luminal epithelial (LE) cells during the process of luteolysis. Prostaglandin transporter (PGT) is a 12-transmembrane solute carrier organic anion transporter protein that facilitates transport of PGF2(alpha). The present study determined the effects of inhibition of PGT protein on pulsatile release of luteolytic PGF2(alpha) and the underlined cell-signaling mechanisms. The results indicate that intrauterine inhibition of the PGT protein inhibits the pulsatile release of PGF2(alpha) from the endometrium and maintains a functional corpus luteum. Surprisingly, inhibition of PGT-mediated luteolytic pulses is not associated with spatial regulation of estrogen and oxytocin receptors in the LE of the endometrium and is also not accompanied by decreased biosynthesis of PGF2(alpha) or increased catabolism of PGF2(alpha) by the endometrium. Importantly, PGT inhibitor increases expression of pERK1/2 proteins in the LE of the endometrium. Knock down of ERK1/2 genes in LE cells reverses the inhibitory effects of PGT inhibitor on release of PGF2(alpha). In conclusion, intrauterine inhibition of PGT inhibits the pulsatile release of PGF2(alpha) from the endometrium without modulating spatial expressions of estrogen and oxytocin receptor proteins and metabolism of PGF2(alpha) at the time of luteolysis. Activation of ERK1/2 pathways and interactions between ERK1/2 and PGT protein appear to be important cell-signaling mechanisms that control PGT-mediated efflux transport function. PGT emerges as an important final component in the luteolytic machinery that controls the release of luteolytic pulses of PGF2(alpha) from the endometrium in sheep.

Mutations in the SLCO2A1 gene and primary hypertrophic osteoarthropathy: a clinical and biochemical characterization

CONTEXT

We previously demonstrated that deficiency of the prostaglandin transporter (SLCO2A1) is a cause of primary hypertrophic osteoarthropathy (PHO). However, its clinical and metabolic characteristics have not been well defined.

OBJECTIVE

The objective of the study was to expand this mutational spectrum to better delineate the SLCO2A1 deficiency phenotype and investigate the clinical and metabolic characteristics of a cohort of subjects with PHO.

DESIGN, SETTING, PATIENTS, AND MAIN OUTCOME MEASURE

Eleven affected individuals and their available healthy family members from 9 unrelated Chinese families with PHO (7 of which were previously undescribed) were clinically studied. The SLCO2A1 gene was screened and analyzed. Urinary levels of prostaglandin E₂ (PGE₂) and prostaglandin E metabolite (PGE-M) were measured using competitive ELISAs. The serum levels of total T, estradiol, sex hormone-binding protein, LH, FSH, and fasting gastrin were detected.

RESULTS

Nine different SLCO2A1 mutations were identified in affected individuals in the 7 previously undescribed families, 7 of which (Glu165X, Ala286GlnfsX35, Gln356AlafsX77, Gly369Asp, Gly379Glu, Glu465Lys, and c.861+2T>C) were novel. The urinary levels of PGE₂ and PGE-M were much higher in the SLCO2A1-deficient individuals and decreased with age. There was no relationship between sex hormones and PGE₂ or PGE-M. There was no significant difference in the levels of fasting serum gastrin between PHO patients with watery diarrhea and their relatives.

CONCLUSIONS

The present findings broaden the allelic spectrum of SLCO2A1 mutations. The urinary levels of PGE₂ and PGE-M in the SLCO2A1-deficient individuals decreased with age. The measurement of the excreted PGE₂ and PGE-M may have implications in the differential diagnosis, treatment, and follow-up of PHO.

Distinct roles of central and peripheral prostaglandin E2 and EP subtypes in blood pressure regulation

Prostaglandin E(2) (PGE(2)) is a major prostanoid with a wide variety of biological activities. PGE(2) can influence blood pressure (BP) both positively and negatively. In particular, centrally administered PGE(2) induces hypertension whereas systemic administration of PGE(2) produces a hypotensive effect. These physiologically opposing effects are generated by the existence of multiple EP receptors, namely EP(1-4), which are G protein-coupled receptors with distinct signaling properties. This review highlights the distinct roles of PGE(2) in BP regulation and the involvement of specific EP receptor subtypes.

OATPs, OATs and OCTs: the organic anion and cation transporters of the SLCO and SLC22A gene superfamilies

The human organic anion and cation transporters are classified within two SLC superfamilies. Superfamily SLCO (formerly SLC21A) consists of organic anion transporting polypeptides (OATPs), while the organic anion transporters (OATs) and the organic cation transporters (OCTs) are classified in the SLC22A superfamily. Individual members of each superfamily are expressed in essentially every epithelium throughout the body, where they play a significant role in drug absorption, distribution and elimination. Substrates of OATPs are mainly large hydrophobic organic anions, while OATs transport smaller and more hydrophilic organic anions and OCTs transport organic cations. In addition to endogenous substrates, such as steroids, hormones and neurotransmitters, numerous drugs and other xenobiotics are transported by these proteins, including statins, antivirals, antibiotics and anticancer drugs. Expression of OATPs, OATs and OCTs can be regulated at the protein or transcriptional level and appears to vary within each family by both protein and tissue type. All three superfamilies consist of 12 transmembrane domain proteins that have intracellular termini. Although no crystal structures have yet been determined, combinations of homology modelling and mutation experiments have been used to explore the mechanism of substrate recognition and transport. Several polymorphisms identified in members of these superfamilies have been shown to affect pharmacokinetics of their drug substrates, confirming the importance of these drug transporters for efficient pharmacological therapy. This review, unlike other reviews that focus on a single transporter family, briefly summarizes the current knowledge of all the functionally characterized human organic anion and cation drug uptake transporters of the SLCO and the SLC22A superfamilies.

Identification of mutations in the prostaglandin transporter gene SLCO2A1 and its phenotype-genotype correlation in Japanese patients with pachydermoperiostosis

BACKGROUND

Pachydermoperiostosis (PDP) is a rare genetic disorder characterized by 3 major symptoms: pachydermia including cutis verticis gyrata (CVG), periostosis, and finger clubbing. Recently, a homozygous mutation in the gene HPGD, which encodes 15-hydroxyprostaglandin dehydrogenase (15-PGDH), was found to be associated with PDP. However, mutations in HPGD have not been identified in Japanese PDP patients.

OBJECTIVE

We aimed to identify a novel responsible gene for PDP using whole exome sequencing by next-generation DNA sequencer (NGS).

METHODS

Five patients, including 2 patient-parent trios were enrolled in this study. Entire coding regions were sequenced by NGS to identify candidate mutations associated with PDP. The candidate mutations were subsequently sequenced using the Sanger method. To determine clinical characteristics, we analyzed histological samples, as well as serum and urinary prostaglandin E2 (PGE2) levels for each of the 5 PDP patients, and 1 additional patient with idiopathic CVG.

RESULTS

From initial analyses of whole exome sequencing data, we identified mutations in the solute carrier organic anion transporter family, member 2A1 (SLCO2A1) gene, encoding prostaglandin transporter, in 3 of the PDP patients. Follow-up Sanger sequencing showed 5 different SLCO2A1 mutations (c.940+1G>A, p.E427_P430del, p.G104*, p.T347I, p.Q556H) in 4 unrelated PDP patients. In addition, the splice-site mutation c.940+1G>A identified in 3 of 4 PDP patients was determined to be a founder mutation in the Japanese population. Furthermore, it is likely that the combination of these SLCO2A1 mutations in PDP patients is also associated with disease severity.

CONCLUSION

We found that SLCO2A1 is a novel gene responsible for PDP. Although the SLCO2A1 gene is only the second gene discovered to be associated with PDP, it is likely to be a major cause of PDP in the Japanese population.

Prostaglandin transporter mutations cause pachydermoperiostosis with myelofibrosis

Pachydermoperiostosis, or primary hypertrophic osteoarthropathy (PHO), is an inherited multisystem disorder, whose features closely mimic the reactive osteoarthropathy that commonly accompanies neoplastic and inflammatory pathologies. We previously described deficiency of the prostaglandin-degrading enzyme 15-hydroxyprostaglandin dehydrogenase (HPGD) as a cause of this condition, implicating elevated circulating prostaglandin E(2) (PGE(2)) as causative of PHO, and perhaps also as the principal mediator of secondary HO. However, PHO is genetically heterogeneous. Here, we use whole-exome sequencing to identify recessive mutations of the prostaglandin transporter SLCO2A1, in individuals lacking HPGD mutations. We performed exome sequencing of four probands with severe PHO, followed by conventional mutation analysis of SLCO2A1 in nine others. Biallelic SLCO2A1 mutations were identified in 12 of the 13 families. Affected individuals had elevated urinary PGE(2), but unlike HPGD-deficient patients, also excreted considerable quantities of the PGE(2) metabolite, PGE-M. Clinical differences between the two groups were also identified, notably that SLCO2A1-deficient individuals have a high frequency of severe anemia due to myelofibrosis. These findings reinforce the key role of systemic or local prostaglandin excess as the stimulus to HO. They also suggest that the induction or maintenance of hematopoietic stem cells by prostaglandin may depend upon transporter activity.

Colonic epithelial response to injury requires Myd88 signaling in myeloid cells

Proper colonic injury response requires myeloid-derived cells and Toll-like receptor/Myd88 signaling. However, the precise role of Myd88 signaling specifically in myeloid-derived cells that occurs during tissue damage is unclear. Therefore, we created a mouse line with Myd88 expression restricted to myeloid lineages (Myd88(-/-); LysM(Cre/+); ROSA26(Myd88/+); herein Mlcr). In these mice, Myd88 was appropriately expressed and mediated responses to bacterial ligand exposure in targeted cells. Importantly, the severe colonic epithelial phenotype observed in dextran sodium sulfate-injured Myd88(-/-) mice was rescued by the genetic modification of Mlcr mice. During injury, myeloid cell activation and enrichment of Ptsg2-expressing stromal cells occurred within the mesenchyme that surrounded the crypt bases of Mlcr and Myd88(+/-) mice but not Myd88(-/-) mice. Interestingly, these cellular changes to the crypt base mesenchyme also occurred, but to a lesser extent in uninjured Mlcr mice. These results show that Myd88 expression in myeloid cells was sufficient to rescue intestinal injury responses, and surprisingly, these cells appear to require an additional Myd88-dependent signal from a non-myeloid cell type during homeostasis.

Aquaporin-2 promoter is synergistically regulated by nitric oxide and nuclear factor of activated T cells

Background/Aims

We have previously shown that aquaporin-2 (AQP2) is down-regulated in the renal medulla of rats made hypertensive by chronic inhibition of nitric oxide synthase. It has been shown that AQP2 expression is regulated by the calcineurin/nuclear factor of activated T cells (NFATc). Nitric oxide (NO) regulates the activity of NFATc via c-Jun-N-terminal kinase 2 (JNK2). Therefore, we hypothesized that increases in NO enhance NFATc-mediated up-regulation of AQP2 promoter activity.

Methods

AQP2 mRNA and protein expression were detected in mouse renal papilla. AQP2 promoter luciferase reporter- and NFAT luciferase reporter-transfected MDCK cells were used to determine AQP2 promoter activity and NFATc activity, respectively. Cells were incubated with classic activators and inhibitors of NFATc and the NO pathway.

Results

Our results demonstrate that both Ca²⁺ and NO have a synergistic effect resulting in an increase in AQP2 mRNA and protein in mouse papilla and activation of the AQP2 promoter in kidney-derived cells. In addition, NO enhances Ca²⁺-induced NFATc activation. The underlying mechanism involves increased NFATc nuclear import and decreased export via protein kinase G-mediated inhibition of JNK1/2.

Conclusions

This is the first study defining novel regulatory roles for NO and NFATc in the control of AQP2, which is an important renal protein.

Development of a high-affinity inhibitor of the prostaglandin transporter

Prostaglandin E(2) (PGE(2)) triggers a vast array of biological signals and physiological events. The prostaglandin transporter (PGT) controls PGE(2) influx and is rate-limiting for PGE(2) metabolism and signaling termination. PGT global knockout mice die on postnatal day 1 from patent ductus arteriosus. A high-affinity PGT inhibitor would thus be a powerful tool for studying PGT function in adult animals. Moreover, such an inhibitor could be potentially developed into a therapeutic drug targeting PGT. Based on structure-activity relationship studies that built on recently identified inhibitors of PGT, we obtained N-(2-(2-(2-azidoethoxy)ethoxy)ethyl)-4-((4-((2-(2-(2-benzamidoethoxy)ethoxy)ethyl)amino)-6-((4-hydroxyphenyl)amino)-1,3,5-triazin-2-yl)amino)benzamide (T26A), a competitive inhibitor of PGT, with a K(i) of 378 nM. T26A seems to be highly selective for PGT, because it neither interacts with a PGT homolog in the organic anion transporter family nor affects PGE(2) synthesis. In Madin-Darby canine kidney cells stably transfected with PGT, T26A blocked PGE(2) metabolism, resulting in retention of PGE(2) in the extracellular compartment and the negligible appearance of PGE(2) metabolites in the intracellular compartment. Compared with vehicle, T26A injected intravenously into rats effectively doubled the amount of endogenous PGE(2) in the circulation and reduced the level of circulating endogenous PGE(2) metabolites to 50%. Intravenous T26A was also able to slow the metabolism of exogenously injected PGE(2). These results confirm that PGT directly regulates PGE(2) metabolism and demonstrate that a high-affinity inhibitor of PGT can effectively prevent PGE(2) metabolism and prolong the half-life of circulating PGE(2).

Role of EP4 receptor and prostaglandin transporter in prostaglandin E2-induced alteration in colonic epithelial barrier integrity

Prostaglandin E(2) (PGE(2)) is a proinflammatory lipid mediator produced in excess in inflammatory bowel disease (IBD). PGE(2) couples to and signals via four different E-prostanoid (EP) receptors, namely EP1, EP2, EP3, and EP4. In this study, we determined a role for PGE(2) and EP4 receptors in altering colonic epithelial barrier integrity. In healthy colonic mucosa, EP4 receptors were localized on apical plasma membrane of epithelial cells at the tip of mucosal folds, whereas, in patients with IBD and in rats with dextran sodium sulfate (DSS)-induced colitis, they were diffusely overexpressed throughout the mucosa. Similarly, expression of EP4 receptor was polarized in T84 colonic epithelial monolayer and mimics the normal epithelium. Apical exposure of T84 monolayer with high levels of PGE(2) decreased barrier integrity, which was abrogated by an EP4 receptor antagonist. To reveal the mechanism of vectorial transport of basally produced PGE(2) toward apical EP4 receptors, we identified prostaglandin transporters (PGT) in human colonic epithelia. PGT were least expressed on epithelial cells at the colonic mucosal folds of control subjects but overexpressed in epithelial cells of patients with IBD or animals with DSS-induced colitis. T84 monolayer also expressed PGT, which increased twofold following stimulation with TNF-α. Importantly, in T84 monolayer stimulated with TNF-α, there was a corresponding increase in the uptake and vectorial transport of (3)H-PGE(2) to the apical surface. Knockdown or pharmacological inhibition of PGT significantly decreased vectorial transport of (3)H-PGE(2). These studies unravel a mechanism whereby EP4 receptor and PGT play a role in PGE(2)-induced alteration of epithelial barrier integrity in colitis.

Failure of postnatal ductus arteriosus closure in prostaglandin transporter-deficient mice

BACKGROUND

Prostaglandin E(2) (PGE(2)) plays a major role both in maintaining patency of the fetal ductus arteriosus and in closure of the ductus arteriosus after birth. The rate-limiting step in PGE(2) signal termination is PGE(2) uptake by the transporter PGT.

METHODS AND RESULTS

To determine the role of PGT in ductus arteriosus closure, we used a gene-targeting strategy to produce mice in which PGT exon 1 was flanked by loxP sites. Successful targeting was obtained because neither mice hypomorphic at the PGT allele (PGT Neo/Neo) nor global PGT knockout mice (PGT(-/-)) exhibited PGT protein expression; moreover, embryonic fibroblasts isolated from targeted mice failed to exhibit carrier-mediated PGE(2) uptake. Although born in a normal mendelian ratio, no PGT(-/-) mice survived past postnatal day 1, and no PGT Neo/Neo mice survived past postnatal day 2. Necropsy revealed patent ductus arteriosus with normal intimal thickening but dilated cardiac chambers. Both PGT Neo/Neo and PGT(-/-) mice could be rescued through the postnatal period by giving the mother indomethacin before birth. Rescued mice grew normally and had no abnormalities by gross and microscopic postmortem analyses. In accordance with the known role of PGT in metabolizing PGE(2), rescued adult PGT(-/-) mice had lower plasma PGE(2) metabolite levels and higher urinary PGE(2) excretion rates than wild-type mice.

CONCLUSIONS

PGT plays a critical role in closure of the ductus arteriosus after birth by ensuring a reduction in local and/or circulating PGE(2) concentrations.

Influence of cyclooxygenase inhibitors on the function of the prostaglandin transporter organic anion-transporting polypeptide 2A1 expressed in human gastroduodenal mucosa

The human organic anion-transporting polypeptide 2A1 (OATP2A1) is a prostaglandin transporter expressed in several tissues and plays an important role for local distribution of prostaglandins, which contribute to the integrity of gastric mucosa. Blockade of prostaglandin pathways by cyclooxygenase (COX) inhibitors has been associated with serious side effects such as gastrointestinal ulceration and bleeding. However, little is known regarding OATP2A1 expression in the upper gastrointestinal tract and the potential impact of cyclooxygenase inhibitors on OATP2A1 function. We first investigated the expression of OATP2A1 mRNA and protein in human gastroduodenal mucosa using human biopsy specimens obtained from antrum, corpus, and duodenum. The results indicate that OATP2A1 is expressed in the neck region and deep pyloric glands of antrum and in parietal cells of gastric corpus. Second, we examined various COX inhibitors for their effects on OATP2A1 transporter activity. Using HEK293 cells expressing OATP2A1, we found that diclofenac and lumiracoxib are potent inhibitors of OATP2A1-mediated transport of prostaglandin (PG) E(2) with IC(50) values of 6.2 +/- 1.2 and 3.1 +/- 1.2 microM. In contrast, indomethacin, ketoprofen, and naproxen led to significant stimulation of OATP2A1-mediated PGE(2) transport by 162.7 +/- 13.9, 77.2 +/- 3.6, and 32.3 +/- 4.9%, respectively. Taken together, our results suggest that various clinically used COX inhibitors have differential impact on the function of the prostaglandin transporter OATP2A1 in human stomach and that these effects may contribute to differences in the gastrointestinal side effects of COX inhibitors.

Dietary salt induces transcription of the prostaglandin transporter gene in renal collecting ducts

Prostaglandin E(2) (PGE(2)) plays an important role in maintaining body fluid homeostasis by activating its receptors on the renal collecting duct (CD) to stimulate renal Na(+) and water excretion. The PG carrier prostaglandin transporter (PGT) is expressed on the CD apical membrane, where it mediates PG reuptake as part of the termination of autocrine PG signaling. Here we tested the hypothesis that dietary salt loading regulates PGT gene transcription in renal CDs. We placed green fluorescence protein (GFP) under control of 3.3 kb of the mouse PGT promoter and injected this construct into the pronuclei of fertilized FVB mouse eggs. Four of thirty-eight offspring were GFP positive by genotyping. We extensively characterized one (no. 29) PGT-GFP transgenic mouse line. On microscopic examination, GFP was expressed in CDs as determined by their expression of aquaporin-2. We fed mice a low (0.03% NaCl)-, normal (0.3% NaCl)-, or high-salt (3% NaCl) diet for 2 wk and quantified CD GFP expression. The average number of GFP-positive CD cells per microscopic section varied directly with dietary salt intake. Compared with mice on the control (0.3% sodium) diet, mice on a low-sodium (0.03%) diet had reduced numbers of GFP-positive cells (71% of control, P < 0.001), whereas mice on a high-sodium (3%) diet had increased numbers of GFP-positive cells (139% of control, P < 0.001). This increase in apparent CD PGT transcription resulted in a 51-55% increase (P < 0.001) in whole kidney PGT mRNA levels as determined by real-time PCR. The regulation of PG signal termination via reuptake represents a new pathway for controlling renal Na(+) balance.

Prostaglandin PGE2 at very low concentrations suppresses collagen cleavage in cultured human osteoarthritic articular cartilage: this involves a decrease in expression of proinflammatory genes, collagenases and COL10A1, a gene linked to chondrocyte hypertrophy

Suppression of type II collagen (COL2A1) cleavage by transforming growth factor (TGF)-beta2 in cultured human osteoarthritic cartilage has been shown to be associated with decreased expression of collagenases, cytokines, genes associated with chondrocyte hypertrophy, and upregulation of prostaglandin (PG)E2 production. This results in a normalization of chondrocyte phenotypic expression. Here we tested the hypothesis that PGE2 is associated with the suppressive effects of TGF-beta2 in osteoarthritic (OA) cartilage and is itself capable of downregulating collagen cleavage and hypertrophy in human OA articular cartilage. Full-depth explants of human OA knee articular cartilage from arthroplasty were cultured with a wide range of concentrations of exogenous PGE2 (1 pg/ml to 10 ng/ml). COL2A1 cleavage was measured by ELISA. Proteoglycan content was determined by a colorimetric assay. Gene expression studies were performed with real-time PCR. In explants from patients with OA, collagenase-mediated COL2A1 cleavage was frequently downregulated at 10 pg/ml (in the range 1 pg/ml to 10 ng/ml) by PGE2 as well as by 5 ng/ml TGF-beta2. In control OA cultures (no additions) there was an inverse relationship between PGE2 concentration (range 0 to 70 pg/ml) and collagen cleavage. None of these concentrations of added PGE2 inhibited the degradation of proteoglycan (aggrecan). Real-time PCR analysis of articular cartilage from five patients with OA revealed that PGE2 at 10 pg/ml suppressed the expression of matrix metalloproteinase (MMP)-13 and to a smaller extent MMP-1, as well as the proinflammatory cytokines IL-1beta and TNF-alpha and type X collagen (COL10A1), the last of these being a marker of chondrocyte hypertrophy. These studies show that PGE2 at concentrations much lower than those generated in inflammation is often chondroprotective in that it is frequently capable of selectively suppressing the excessive collagenase-mediated COL2A1 cleavage found in OA cartilage. The results also show that chondrocyte hypertrophy in OA articular cartilage is functionally linked to this increased cleavage and is often suppressed by these low concentrations of added PGE2. Together these initial observations reveal the importance of very low concentrations of PGE2 in maintaining a more normal chondrocyte phenotype.

Renal localization and regulation of 15-hydroxyprostaglandin dehydrogenase

Tissue prostaglandin levels are determined by both biosynthesis and catabolism. The current studies report the expression and localization of 15-hydroxyprostaglandin dehydrogenase (15-PGDH), a key enzyme in prostaglandin catabolism in the kidneys. We also investigated potential interactions between 15-PGDH and cyclooxygenase (COX), a key enzyme in prostaglandin biosynthesis. Both 15-PGDH mRNA and protein levels were significantly higher in kidney cortex than in papilla, which is opposite to the expression pattern of COX-2. In situ hybridization indicated that 15-PGDH mRNA was mainly localized to the tubular epithelial cells in kidney cortex and outer medulla but not in the glomerulus or papilla. Dual immunofluorescent staining indicated that 15-PGDH was expressed in the proximal tubule, cortical, and outer medullary thick ascending limb and collecting duct but not in the macula densa or papilla. 15-PGDH levels were significantly lower in a macula densa cell line (MMDD1) than in a proximal tubule cell line. Although a high-salt diet decreased COX-2 expression in macula densa, it increased macula densa 15-PGDH expression in both mouse and rat kidneys. In MMDD1 cells, a COX-2 inhibitor increased 15-PGDH, whereas a COX-1 inhibitor had no effect. Furthermore, intense 15-PGDH immunofluorescent staining was found in both macula densa and glomerulus in COX-2 knockout mice. The intrarenal distribution of 15-PGDH and its interactions with COX-2 suggest that differential regulation of COX-2 and 15-PGDH may play an important role in determining levels of prostaglandins involved in regulation of salt, volume, and blood pressure homeostasis.

PGE2 inhibits apical K channels in the CCD through activation of the MAPK pathway

We used the patch-clamp technique and Western blot analysis to explore the effect of PGE(2) on ROMK-like small-conductance K (SK) channels and Ca(2+)-activated big-conductance K channels (BK) in the cortical collecting duct (CCD). Application of 10 microM PGE(2) inhibited SK and BK channels in the CCD. Moreover, either inhibition of PKC or blocking mitogen-activated protein kinase (MAPK), P38 and ERK, abolished the effect of PGE(2) on SK channels in the CCD. The effect of PGE(2) on SK channels was completely blocked in the presence of SC-51089, a specific EP1 receptor antagonist, and mimicked by application of sulprostone, an agonist for EP1 and EP3 receptors. To determine whether PGE(2) stimulates the phosphorylation of P38 and ERK, we treated mouse CCD cells (M-1) with PGE(2). Application of PGE(2) significantly stimulated the phosphorylation of P38 and ERK within 5 min. The dose-response curve of PGE(2) effect shows that 1, 5, and 10 microM PGE(2) increased the phosphorylation of P38 and ERK by 20-21, 50-80, and 80-100%, respectively. The stimulatory effect of PGE(2) on MAPK phosphorylation was not affected by indomethacin but abolished by inhibition of PKC. This suggests that the effect of PGE(2) on MAPK phosphorylation is PKC dependent. Also, the expression of cyclooxygenase II and PGE(2) concentration in renal cortex and outer medulla was significantly higher in rats fed a K-deficient diet than those on a normal-K diet. We conclude that PGE(2) inhibits SK and BK channels and that there is an effect of PGE(2) on SK channels in the CCD through activation of EP1 receptor and MAPK pathways. Also, high concentrations of PGE(2) induced by K restriction may be partially responsible for increasing MAPK activity during K restriction.

Differential expression and regulation of prostaglandin transporter and metabolic enzymes in mouse uterus during blastocyst implantation

OBJECTIVE

To examine the spatiotemporal expression and regulation of prostaglandin transporter (PGT), 15-hydroxy-PG dehydrogenase (15-PGDH), and carbonyl reductase 1 (CBR1) messenger RNA (mRNA) and protein in the mouse uterus during embryo implantation and in related models.

DESIGN

Experimental animal study.

SETTING

University research laboratory.

ANIMAL(S)

Sexually mature female Kunming strain white mice.

INTERVENTION(S)

Delayed and activated implantation, artificial decidualization, and subcutaneous injection of progesterone (P) and E(2) in ovariectomized mouse.

MAIN OUTCOME MEASURE(S)

The expression of mRNA and protein were detected by in situ hybridization and immunohistochemistry in mouse uterus.

RESULT(S)

Prostaglandin transporter mRNA and protein were expressed in the subluminal stroma at implantation site on day 5 of pregnancy and then in decidua but were not detected at the interimplantation sites and in preimplantation or pseudopregnant uterus. The presence of an active blastocyst was required for PGT expression at the implantation site. Both 15-PGDH and CBR1 mRNA were detected in glandular epithelium on day 4 of pregnancies. The expression of 15-PGDH and CBR1 mRNA was also detected in postimplantation embryos.

CONCLUSION(S)

These data suggest that differentially expressed PGT and 15-PGDH may participate in PG signaling in mouse uterus during implantation and decidualization.

Cloning of equine prostaglandin dehydrogenase and its gonadotropin-dependent regulation in theca and mural granulosa cells of equine preovulatory follicles during the ovulatory process

The mammalian ovulatory process is accompanied by a gonadotropin-dependent increase in follicular levels of prostaglandin E2 (PGE2) and PGF2α, which are metabolized by 15-hydroxy prostaglandin dehydrogenase (PGDH). Little is known about ovarian PGDH regulation in non-primate species. The objectives of this study were to characterize the structure of equine PGDH and its regulation in follicles during human chorionic gonadotropin (hCG)-induced ovulation. The full-length equine PGDH was obtained by RT-PCR, 5′- and 3′-rapid amplification of cDNA ends (RACE). Its open reading frame encodes a 266-amino acid protein that is 72–95% homologous to other species. Semi-quantitative RT-PCR/Southern blot were used to study PGDH regulation in follicles isolated 0–39 h post-hCG. Results showed that PGDH mRNA expression was low in follicles obtained at 0 h, increased at 12 and 24 h (P< 0.05), and decreased at 36-h post-hCG. This induction of expression was biphasic, with elevated abundance of transcripts at 12 and 33 h post-hCG (P< 0.05) in mural granulosa and theca cells. Immunohistochemistry and immunoblotting confirmed regulated expression of PGHD protein in both cell types of preovulatory follicles after hCG. High levels of PGDH mRNA were observed in corpus luteum and other non-ovarian tissues tested, except kidney, muscle, brain, and heart. Thus, this study is the first to report the gonadotropin-dependent regulation of PGDH during ovulation in a non-primate species. PGDH induction was biphasic in theca and mural granulosa cells differing from primates in which this induction was monophasic and limited to granulosa cells, suggesting species-specific differences in follicular control of PGDH expression during ovulation.

Myd88-dependent positioning of Ptgs2-expressing stromal cells maintains colonic epithelial proliferation during injury

We identified cellular and molecular mechanisms within the stem cell niche that control the activity of colonic epithelial progenitors (ColEPs) during injury. Here, we show that while WT mice maintained ColEP proliferation in the rectum following injury with dextran sodium sulfate, similarly treated Myd88(-/-) (TLR signaling-deficient) and prostaglandin-endoperoxide synthase 2(-/-) (Ptgs2(-/-)) mice exhibited a profound inhibition of epithelial proliferation and cellular organization within rectal crypts. Exogenous addition of 16,16-dimethyl PGE(2) (dmPGE(2)) rescued the effects of this injury in both knockout mouse strains, indicating that Myd88 signaling is upstream of Ptgs2 and PGE(2). In WT and Myd88(-/-) mice, Ptgs2 was expressed in scattered mesenchymal cells. Surprisingly, Ptgs2 expression was not regulated by injury. Rather, in WT mice, the combination of injury and Myd88 signaling led to the repositioning of a subset of the Ptgs2-expressing stromal cells from the mesenchyme surrounding the middle and upper crypts to an area surrounding the crypt base adjacent to ColEPs. These findings demonstrate that Myd88 and prostaglandin signaling pathways interact to preserve epithelial proliferation during injury using what we believe to be a previously undescribed mechanism requiring proper cellular mobilization within the crypt niche.

Role of Mouse Organic Anion Transporter 3 (mOat3) as a Basolateral Prostaglandin E2 Transport Pathway

Renal organic anion transporters play an important role in the handling of a number of endogenous and exogenous anionic substances in the kidney. In this study, we investigated prostaglandin E(2) (PGE(2)) transport properties and intrarenal localization of mouse organic anion transporter 3 (mOat3). When expressed in Xenopus oocytes, mOat3 mediated the time- and concentration-dependent transport of PGE(2) (K(m): 1.48 microM). PGE(2) transport mediated by mOat3 was trans-stimulated by intracellular glutarate injected into the oocytes. PGE(2) efflux via mOat3 was also trans-stimulated by extracellular glutarate. Thus, mOat3 was shown to mediate the bidirectional transport of PGE(2), partly coupled to the dicarboxylate exchange mechanism. Immunohistochemical study revealed that mOat3 protein was localized at the basolateral membrane of renal proximal and distal tubules. Furthermore, diffuse expression of mOat3, including expression in the basolateral membrane in macula densa (MD) cells, was observed. These results indicate that mOat3 plays an important role as a basolateral transport pathway of PGE(2) in the distal nephron including MD cells that may constitute one of the indispensable steps for renin release and regulation of the tubuloglomerular feedback mechanism.

Interaction between prostaglandin E2 and l-cis-diltiazem, a specific blocker of cyclic nucleotide gated channels in bovine aortic endothelial cells

Prostaglandins are known to transduce their signals via 7 transmembrane prostanoid receptors, which typically signal through coupling to G proteins and downstream second messenger molecules and protein kinase activation. Recently we have shown that cyclic nucleotides affect prostaglandins binding to bovine aortic endothelial cells independent of protein kinases. Here we show that incubation of bovine aortic endothelial cells with permeable analogs of cAMP or cGMP leads to a rapid and reversible reduction in PGE(2) binding to the cells. Since cyclic nucleotides are known modulators of cyclic nucleotide gated channels, we examined the effect of a specific cyclic nucleotide gated channel blocker l-cis-diltiazem on prostaglandin E(2) (PGE(2)) binding to bovine aortic endothelial cells. L-cis-diltiazem is shown to displace PGE(2) binding to bovine aortic endothelial cells in a dose dependent manner. In addition the effect of PGE(2) and l-cis-diltiazem on thapsigargin induced calcium elevation in the cells was compared. Both agents reduced in bovine aortic endothelial cells the thapsigargin induced calcium elevation by about half. PGE(2) also retarded the time course of the response to thapsigargin. Simultaneous treatment of the cells with both PGE(2) and l-cis-diltiazem did not yield an inhibitory effect beyond that observed with l-cis-diltiazem alone. Together our data point at the cyclic nucleotide gated channels as a feasible candidate for association with the PGE(2) binding site in bovine aortic endothelial cells.

15-Hydroxyprostaglandin dehydrogenase in the bovine endometrium during the oestrous cycle and early pregnancy

Prostaglandins (PG) are primary regulators of reproductive function. In ruminants, the relative production of PGE2and PGF2αdetermines the return to a new oestrous cycle or to the establishment of pregnancy in response to a viable embryo. PG action depends on biosynthesis, transport and interaction with their receptors, which are all expressed differentially during the oestrous cycle. PGs are, however, local mediators and thus the onsite degradation by enzymes such as 15-hydroxyprostaglandin dehydrogenase (HPGD), also known as 15-PGDH, is another factor to consider in the regulation of physiological action. Little information is available on PG catabolism in the endometrium during the oestrous cycle or early pregnancy. The purpose of this study was to clone the bovine 15-PGDH, produce the recombinant protein and generate a specific antibody to study its activity and its expression in the endometrium during the oestrous cycle. We have found that the bovine 15-PGDH is highly homologous to the rat and human isoforms. 15-PGDH is localized principally in the glandular epithelium and to a lesser extent in stromal and luminal epithelial cells. The enzyme expression is regulated during the oestrous cycle and it reaches its maximal level on days 16–18. Transient expression is observed in luminal epithelial and trophoblast cells on day 21 of pregnancy. The mRNA is expressed at a constant high level throughout the cycle. The activity of the recombinant 15-PGDH was also tested and was found comparable for PGF2αand PGE2. These data suggest that 15-PGDH contributes to the tight regulation of PG action in the endometrium especially at the critical period of recognition of pregnancy.

Functional characterization of prostaglandin transporter and terminal prostaglandin synthases during decidualization of human endometrial stromal cells

BACKGROUND

Decidualization of endometrial stromal cells is essential for successful implantation and pregnancy. Prostaglandins (PG) have been shown to be required for the initiation and maintenance of decidualization in animal models. The transport of PG across the plasma membrane is mediated by carriers such as prostaglandin transporter (PGT). Our recent data have shown the expression of human PGT (hPGT) in the endometrium during the menstrual cycle. The objective of the present study was to characterize hPGT in decidualized stromal cells.

METHODS AND RESULTS

Human endometrial stromal cells were treated with a combination of cAMP and medroxyprogesterone acetate to induce decidualization. Decidualization was confirmed by morphological differentiation and increased secretion of prolactin. A large increase in hPGT mRNA level, as measured by real-time PCR analysis, was observed in decidual cells compared with control. Similarly, a 2-fold up-regulation of hPGT and 3-12-fold increase in PG biosynthetic enzymes were obtained at the protein level. Decidual cells exhibited a higher isotopic PGE2 uptake and greater intracellular PG levels than control.

CONCLUSIONS

The higher uptake of PG by decidual cells is highly likely to be mediated via hPGT. PGT is a newly identified regulator of PG action at the cellular level and likely contributes to the regulation of PG action in female reproductive processes.