Upregulation of cyclooxygenase-1 and the PGE2 receptor EP2 in rat and human mesangioproliferative glomerulonephritis

Modulation of Enzyme-Catalyzed Synthesis of Prostaglandins by Components Contained in Kidney Microsomal Preparations

In the kidney, prostaglandins formed by cyclooxygenase 1 and 2 (COX-1 and COX-2) play an important role in regulating renal blood flow. In the present study, we report our observations regarding a unique modulatory effect of renal microsomal preparation on COX-1/2-mediated formation of major prostaglandin (PG) products in vitro. We found that microsomes prepared from pig and rat kidneys had a dual stimulatory–inhibitory effect on the formation of certain PG products catalyzed by COX-1 and COX-2. At lower concentrations, kidney microsomes stimulated the formation of certain PG products, whereas at higher concentrations, their presence inhibited the formation. Presence of kidney microsomes consistently increased the K_m values of the COX-1/2-mediated reactions, while the V_max might be increased or decreased depending on stimulation or inhibition observed. Experimental evidence was presented to show that a protein component present in the pig kidney microsomes was primarily responsible for the activation of the enzyme-catalyzed arachidonic acid metabolism leading to the formation of certain PG products.

Changes in lectin-binding patterns in the kidneys of canines with immune-complex mediated glomerulonephritis

We surveyed the kidneys of dogs with immune-complex mediated glomerulonephritis (ICGN) by lectin histochemistry using seven lectins—namely WGA, RCA-I, ConA, PNA, SBA, DBA, and UEA-I. Their binding patterns were compared with those from normal dogs. RCA-I signals became weak in the brush borders of the proximal tubules, whereas DBA signals became positive in Bowman’s capsules. Also, varying intensity of the UEA-I signal was noted in the distal tubules, especially in the macula densa. The binding pattern profiles varied among the cases; this diversity in the lectin-binding patterns might be induced as a result of the diverse pathologies seen in canine ICGN.

Usp2-69 overexpression slows down the progression of rat anti-Thy1.1 nephritis

Mesangial proliferative glomerulonephritis is characterized by proliferation of mesangial cells (MCs) and transforming growth factor-β (TGF-β)-dependent stimulation of abnormal extracellular matrix (ECM) accumulation. We previously showed that Decorin--a leucine-rich proteoglycan inhibiting the progression of glomerulonephritis and glomerular sclerosis--can be degraded by the ubiquitin-proteasome pathway and deubiquitinated and stabilized by ubiquitin-specific processing protease 2-69(Usp2-69). Usp2-69 is highly expressed in the kidney and has been implicated in the regulation of cell proliferation and apoptosis. However, its role in mesangial proliferative glomerulonephritis remains unclear. Here, we explored the effect of Usp2-69 on MC proliferation and ECM deposition by transfecting Usp2-69 plasmid into rat anti-Thy1.1 nephritis model and into cultured MCs, as well as detected Usp2-69 and Decorin in rat anti-Thy1.1 nephritis model by western blot. Overexpressing Usp2-69 at the early stage, but not advanced stage, of anti-Thy1.1 nephritis alleviated cell proliferation and ECM deposition, which was shown by decreased Ki-67, Collagen IV and Fibronectin detected by immunohistochemistry. Overexpression also increased Decorin and decreased TGF-β1 and Collagen IV both in vitro and in vivo. In conclusion, our findings suggest that Usp2-69 overexpression alleviates the progression of rat anti-Thy1.1 nephritis and, therefore, that exogenous plasmid injection via the renal artery enhanced by electrotransfer technology could be a promising avenue for glomerular disease research.

Lipidomics: new insight into kidney disease

Due to the incidence of type-2 diabetes and hypertension, chronic kidney disease (CKD) has emerged as a major public health problem worldwide. CKD results in premature death from accelerated cardiovascular disease and various other complications. Early detection, careful monitoring of renal function, and response to therapeutic intervention are critical for prevention of CKD progression and its complications. Unfortunately, traditional biomarkers of renal function are insufficiently sensitive or specific to detect early stages of disease when therapeutic intervention is most effective. Therefore, more sensitive biomarkers of kidney disease are needed for early diagnosis, monitoring, and effective treatment. CKD results in profound changes in lipid and lipoprotein metabolism that, in turn, contribute to progression of CKD and its cardiovascular complications. Lipids and lipid-derived metabolites play diverse and critically important roles in the structure and function of cells, tissues, and biofluids. Lipidomics is a branch of metabolomics, which encompasses the global study of lipids and their biologic function in health and disease including identification of biomarkers for diagnosis, prognosis, prevention, and therapeutic response for various diseases. This review summarizes recent developments in lipidomics and its application to various kidney diseases including chronic glomerulonephritis, IgA nephropathy, chronic renal failure, renal cell carcinoma, diabetic nephropathy, and acute renal failure in clinical and experimental research. Analytical technologies, data analysis, as well as currently known metabolic biomarkers of kidney diseases are addressed. Future perspectives and potential limitations of lipidomics are discussed.

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.

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.

A comparative study of chronic kidney disease in dogs and cats: induction of cyclooxygenases

The present study investigated whether renal cyclooxygenase (COX) induction is associated with the severity of chronic kidney disease (CKD) in dogs and cats. The collected kidneys were examined histopathologically and immunohistochemically. The immunoreactivities of COX-1 and COX-2 were evaluated quantitatively, and the correlations to the plasma creatinine concentrations, glomerular size, glomerulosclerosis, interstitial fibrosis, and interstitial cell infiltration were evaluated statistically. Immunoreactivities for COX-1 were heterogeneously observed in the medullary distal tubules and collecting ducts; no correlations with the severity of renal damage were detected. Immunoreactivities for COX-2 were heterogeneously observed in the macula densa (MD) regions. In dogs, the percentage of COX-2-positive MD was significantly correlated with the glomerular size. In cats, glomeruli with COX-2-positive MD had significantly higher sclerosis scores than those with COX-2-negative MD. In conclusion, renal COX-2 is induced in canine and feline CKD, especially in relation to the glomerular changes.

Prostaglandin EP2 and EP4 receptors modulate expression of the chemokine CCL2 (MCP-1) in response to LPS-induced renal glomerular inflammation

The pro-inflammatory chemokine CCL2 [chemokine (Cys-Cys motif) ligand 2; also known as MCP-1 (monocyte chemotactic protein-1)] is up-regulated in the glomerular compartment during the early phase of LPS (lipopolysaccharide)-induced nephritis. This up-regulation also occurs in cultured MCs (mesangial cells) and is more pronounced in MCs lacking the PGE2 (prostaglandin E2) receptor EP2 or in MCs treated with a prostaglandin EP4 receptor antagonist. To examine a possible feedback mechanism of EP receptor stimulation on CCL2 expression, we used an in vitro model of MCs with down-regulated EP receptor expression. Selectively overexpressing the various EP receptors in these cells then allows the effects on the LPS-induced CCL2 expression to be examined. Cells were stimulated with LPS and CCL2 gene expression was examined and compared with LPS-stimulated, mock-transfected PTGS2 [prostaglandin-endoperoxide synthase 2, also known as COX-2 (cyclo-oxygenase-2)]-positive cells. Overexpression of EP1, as well as EP3, had no effect on LPS-induced Ccl2 mRNA expression. In contrast, overexpression of EP2, as well as EP4, significantly decreased LPS-induced CCL2 expression. These results support the hypothesis that PTGS2-derived prostaglandins, when strongly induced, counter-balance inflammatory processes through the EP2 and EP4 receptors in MCs.

Cyclooxygenase-2 and antagonists in pain management

Non-steroidal anti-inflammatory drugs are widely used for the treatment of pain and inflammation by inhibiting the formation of prostaglandins. However, their use is limited by their side-effects, including gastrointestinal, renal function, cardiovascular and platelet function. Cyclooxygenase activity is the principal target for the action of non-steroidal anti-inflammatory drugs. Two isoforms of cyclooxygenase have been characterized: (i) cyclooxygenase-1, which is found in many tissues and is generally constitutively expressed and synthesizes prostanoids that mediate homeostatic functions; and (ii) cyclooxygenase-2, the inducible isoform, which is mainly expressed at sites of injury or inflammation and synthesizes prostanoids that mediate inflammation, pain and fever. These findings led to the development of selective cyclooxygenase-2 inhibitors, with comparable anti-inflammatory and analgesic properties to traditional non-steroidal anti-inflammatory drugs, but with significantly fewer side-effects. However, these new selective cyclooxygenase-2 inhibitors are not risk free, and care should be taken when using these drugs, especially with elderly patients with multiple medical problems. Finally, the future is bright for the broader usage of these agents in the treatment of diseases other than inflammation and pain, such as Alzheimer's disease, colonic polyp and colon cancer, just to name a few.

Roles of Lipid Mediators in Kidney Injury

Small lipids such as eicosanoids exert diverse and complex functions. In addition to their role in regulating normal kidney function, these lipids also play important roles in the pathogenesis of kidney diseases. Increased glomerular cyclooxygenase (COX)1 or COX2 expression has been reported in patients with nephritis and in animal models of nephritis. COX inhibitors have shown beneficial effects on lupus nephritis and passive Heymann nephritis, but not anti-Thy1.1-induced nephritis. 5-Lipoxygenase-derived leukotrienes are involved in inflammatory glomerular injury. Lipoxygenase product 12-hydroxyeicosatetraenoic acid may mediate angiotensin II and transforming growth factor beta-induced mesangial cell abnormality in diabetic nephropathy. P450 arachidonic acid mono-oxygenase-derived 20-hydroxyeicosatetraenoic acid and epoxyeicosatrienoic acids are involved in several forms of kidney injury, including renal injury in metabolic syndrome. Ceramide also has been shown to be an important signaling molecule that is involved in the pathogenesis of acute kidney injury caused by ischemia/reperfusion and toxic insults. Those pathways should provide fruitful targets for intervention in the pharmacologic treatment of renal disease.

Upregulation of cyclooxygenases by retinoic acid in rat mesangial cells

All-trans retinoic acid (ATRA) increases the expression of COX-1 and COX-2 and the production of PGE2, a prostaglandin with anti-inflammatory effects in human mesangial cells (MC). COX-2 increased through a transcriptional mechanism independent of retinoic acid receptors (RAR) and retinoid X receptors (RXR) and dependent on extracellular regulated kinase-1/2 (ERK1/2), that became phosphorylated 5 min after ATRA addition. Here, in rat MC, ATRA also upregulated COX isoenzymes and PGE2 production, but not in the same way as in human MC: (1) PGE2 production increased only slightly; (2) RAR and RXR were involved in the transcriptional upregulation of COX-2 by ATRA since the RAR-pan-antagonist AGN193109 or the RXR-pan-antagonist HX531 abolished the induction of COX-2 mRNA whereas the RAR-pan-agonist TTNPB or the RXR-pan-agonist AGN194204 induced expression of COX-2, and (3) ERK1/2 phosphorylation, though important for COX-2 upregulation, took more than 1 h. Therefore the regulation of COX by ATRA exhibits striking differences between human and rat MC.

The role of the cylooxygenase pathway in nociception and pain

Cycloxygenase (COX) pathways have long been targeted for the treatment of inflammatory pain, initially through the use of NSAIDs. With the demonstration of two major COX isoforms, COX-1 and COX-2, involved in the production of prostanoids, but with different distribution and regulation, selective COX-2 inhibitors have been developed. This review covers factors influencing COX enzyme activity, the role of their products in the development and maintenance of pain and discusses recent safety concerns of COX-2 inhibitors.

Kinase-dependent, retinoic acid receptor-independent up-regulation of cyclooxygenase-2 by all-trans retinoic acid in human mesangial cells

BACKGROUND AND PURPOSE

Preliminary results in human mesangial cells (MC) suggested that all-trans retinoic acid (ATRA) increased the expression of COX-2 and the production of prostaglandin E2 (PGE2), a PG with anti-inflammatory effects in MC. The aim of this work is to confirm that ATRA increases the expression of COX-2 in MC and to examine the mechanisms involved.

EXPERIMENTAL APPROACH

Cultured MC were treated with ATRA. COX expression and kinase activity were analyzed by Western blot. Transcriptional mechanisms were analyzed by Northern blot, RT-PCR and promoter assays.

KEY RESULTS

COX-2 and COX-1 expression and PGE2 production were increased by ATRA. COX-2 played a role in PGE2 production as production was only partially inhibited by COX-1 inhibitor SC-560. COX-2 up-regulation by ATRA was due to transcriptional mechanisms as pre-incubation with actinomycin D abolished it and ATRA increased the expression of COX-2 mRNA and the activity of a human COX-2 promoter construct, whereas post-transcriptional mechanisms were not found. Retinoic acid receptors (RAR) were not involved in the up-regulation of COX-2 by ATRA since it was not inhibited by RAR-pan-antagonists and the RAR-pan-agonist TTNPB did not up-regulate COX-2. Instead ATRA might act through a sustained activation of extracellular signal-regulated kinase 1/2 (ERK1/2) since up-regulation of COX-2 was prevented by inhibition of the activation of ERK1/2 with PD098059. Also ERK1/2, as well as downstream signalling proteins from ERK1/2, remained phosphorylated when COX-2 increased 24 h later.

CONCLUSIONS AND IMPLICATIONS

These results highlight the relevance of RAR-independent mechanisms to the biological effects of ATRA.

Prostaglandin E2 inhibits its own renal transport by downregulation of organic anion transporters rOAT1 and rOAT3

Prostaglandin E2 (PGE2) is the principal mediator of fever and inflammation. Recently, evidence emerged that during febrile response, PGE2 that is generated in the periphery enters the hypothalamus and contributes to the maintenance of fever. In a rat model of fever generation, peripheral PGE2 is increased, whereas clearance by metabolism of peripheral PGE2 is downregulated. The major route of PGE2 excretion is via the renal proximal tubular organic anion secretory system, where basolateral uptake that is mediated by renal organic anion transporter 1 (rOAT1) and rOAT3 is rate limiting. Therefore, it was hypothesized that PGE2 itself will abolish its excretion by rOAT1 or rOAT3. Fluorescein was used as a prototypic organic anion, and NRK-52E cells from rat served as a proximal tubular model system. PGE2 time-dependently downregulates basolateral organic anion uptake, without affecting cell volume or cell protein, recirculation of counter ions, or proximal tubular transport systems in general. In addition, PGE2 diminishes expression of both rOAT1 and rOAT3. Both organic anion uptake and expression of rOAT1 and rOAT3 are dose-dependently downregulated by PGE2. These findings suggest that during fever or inflammation, renal secretory transport of PGE2 is reduced, contributing to elevated PGE2 levels in blood. These data fit into the hypothetical concept of peripheral PGE2's playing a significant role in fever. The described regulatory mechanism may also be of relevance in chronic inflammatory events. Moreover, the data presented could explain why increased plasma urate levels occur in diseases that go along with increased levels of PGE2.

Selectivity of cyclooxygenase isoform activity and prostanoid production in normal and diseased Han:SPRD-cy rat kidneys

Renal prostanoids are important regulators of normal renal function and maintenance of renal homeostasis. In diseased kidneys, renal cylooxygenase (COX) expression and prostanoid formation are altered. With the use of the Han:Sprague-Dawley-cy rat, the aim of this study was to determine the relative contribution of renal COX isoforms (protein, gene expression, and activity) on renal prostanoid production [thromboxane B(2) (TXB(2), stable metabolite of TXA(2)), prostaglandin E(2) (PGE(2)), and 6-keto-prostaglandin F(1alpha) (6-keto-PGF(1alpha), stable metabolite of PGI(2))] in normal and diseased kidneys. In diseased kidneys, COX-1-immunoreactive protein and mRNA levels were higher and COX-2 levels were lower compared with normal kidneys. In contrast, COX activities were higher in diseased compared with normal kidneys for both COX-1 [0.05 +/- 0.02 vs. 0.45 +/- 0.11 ng prostanoids x min(-1) x mg protein(-1) (P < 0.001)] and COX-2 [0.64 +/- 0.10 vs. 2.32 +/- 0.22 ng prostanoids x min(-1).mg protein(-1) (P < 0.001)]. As the relative difference in activity was greater for COX-1, the ratio of COX-1/COX-2 was higher in diseased compared with normal kidneys, although the predominant activity was still due to the COX-2 isoform in both genotypes. Endogenous and steady-state in vitro levels of prostanoids were approximately 2-10 times higher in diseased compared with normal kidneys. The differences between normal and diseased kidney prostanoids were in the order of TXB(2) > 6-keto-PGF(1alpha) > PGE(2), as determined by higher renal prostanoid levels and COX activity ratios of TXB(2)/6-keto-PGF(1alpha), TXB(2)/PGE(2), and 6-keto-PGF(1alpha)/PGE(2). This specificity in both the COX isoform type and for the prostanoids produced has implications for normal and diseased kidneys in treatments involving selective inhibition of COX isoforms.

Prostaglandin E2 EP2 and EP4 receptor activation mediates cAMP-dependent hyperpolarization and exocytosis of renin in juxtaglomerular cells

PGE(2) and PGI(2) stimulate renin secretion and cAMP accumulation in juxtaglomerular granular (JG) cells. We addressed, at the single-cell level, the receptor subtypes and intracellular transduction mechanisms involved. Patch clamp was used to determine cell capacitance (C(m)), current, and membrane voltage in response to PGE(2), EP2 and EP4 receptor agonists, and an IP receptor agonist. PGE(2) (0.1 micromol/l) increased C(m) significantly, and the increase was abolished by intracellular application of the protein kinase A antagonist Rp-8-CPT-cAMPS. EP2-selective ligands butaprost (1 micromol/l), AE1-259-01 (1 nmol/l), EP4-selective agonist AE1-329 (1 nmol/l), and IP agonist iloprost (1 micromol/l) significantly increased C(m) mediated by PKA. The EP4 antagonist AE3-208 (10 nmol/l) blocked the effect of EP4 agonist but did not alter the response to PGE(2). Application of both EP4 antagonist and EP2-antagonist AH-6809 abolished the effects of PGE(2) on C(m) and current. EP2 and EP4 ligands stimulated cAMP formation in JG cells. PGE(2) rapidly stimulated renin secretion from superfused JG cells and diminished the membrane-adjacent granule pool as determined by confocal microscopy. The membrane potential hyperpolarized significantly after PGE(2), butaprost, AE1-329 and AE1-259 and outward current was augmented in a PKA-dependent fashion. PGE(2)-stimulated outward current, but not C(m) change, was abolished by the BK(Ca) channel inhibitor iberiotoxin (300 nmol/l). EP2 and EP4 mRNA was detected in sampled JG cells, and the preglomerular and glomerular vasculature was immunopositive for EP4. Thus IP, EP2, and EP4 receptors are associated with JG cells, and their activation leads to rapid PKA-mediated exocytotic fusion and release of renin granules.

Action of EGF and PGE2on basolateral organic anion uptake in rabbit proximal renal tubules and hOAT1 expressed in human kidney epithelial cells

We recently showed that, in a proximal tubule cell line (opossum kidney cells), epithelial growth factor (EGF) stimulates basolateral organic anion transport (OAT) via ERK1/2, arachidonic acid, phospholipase A2, and generation of prostaglandins. PGE2binds the prostanoid receptor and, thus, activates adenylate cyclase and PKA, which stimulate basolateral organic anion uptake. In the present study, we investigated whether this regulatory cascade is also true 1) for ex vivo conditions in isolated renal proximal (S2) tubules from rabbit and 2) in a human renal epithelial cell line stably expressing human OAT1 (IHKE-hOAT1). EGF activated ERK1/2 in S2 tubules and IHKE-hOAT1, and, in both cases, inhibition of ERK activation (by U-0126) abolished this stimulation. In S2 tubules and IHKE-hOAT1, EGF led to an increase of organic anion uptake, which again was inhibited by U-0126. PGE2stimulated basolateral organic anion uptake in rabbit S2 tubules and IHKE-hOAT1. EGF- and PGE2-mediated stimulation of organic anion uptake was abolished by inhibition of PKA in rabbit S2 tubules and IHKE-hOAT1, respectively. We conclude that 1) stimulation of basolateral organic anion uptake by EGF or PGE2is a widespread (if not general) regulatory mechanism, 2) the signal transduction pathway involved seems to be general, 3) stimulation of basolateral organic anion uptake by EGF or PGE2is also present under ex vivo conditions and, thus, is not a cell culture artifact, 4) activation of OAT1 is sufficient to explain the stimulatory effects of EGF and PGE2in opossum kidney cells and rabbit S2 segments, and 5) stimulation of basolateral OAT1 by EGF or PGE2is also important in humans and, thus, may have clinical implications.

Short-Term Regulation of Basolateral Organic Anion Uptake in Proximal Tubular Opossum Kidney Cells: Prostaglandin E2 Acts via Receptor-Mediated Activation of Protein Kinase A

ABSTRACT. It was shown previously that EGF induces release of the important prostanoid prostaglandin E2 (PGE2) in proximal tubular opossum kidney (OK) cells and PGE2 then stimulates initial basolateral uptake of organic anions (OA) dose dependently. PGE2 is a receptor agonist and a known substrate for the basolateral exchanger mediating OA uptake (OAT1 and/or OAT3). This study investigated the mechanism of short-term PGE2 action on initial basolateral OA uptake in OK cells. PGE2 stimulation of OA uptake was abolished by selective inhibition of adenylate cyclase (by MDL-12, 330A) or protein kinase A (PKA; by H89). PGE2 stimulation of OA uptake persisted after preloading the cells with glutarate and was still abolished by inhibition of PKA. Selective activation of adenylate cyclase by forskolin led to identical results. These data contradicted the hypothesis that PGE2 action on OA uptake is due to its action as a counter ion. Therefore, we tested whether the PGE2 receptors (EP1 to 4) are involved in stimulation of OA uptake in OK cells by PGE2. Because of their intracellular signaling profile, EP1 and EP3 were not taken into account as possible receptors for mediation of PGE2-induced OA uptake. With the use of selective agonists (11-deoxy PGE1 and butaprost), EP4 was pharmacologically identified as the receptor responsible for PGE2-mediated stimulation of OA uptake. By reverse transcription–PCR, cloning, and subsequent sequencing, a homologue fragment to EP4 was identified in OK cells. EGF-induced stimulation of basolateral organic anion uptake was abolished by inhibition of adenylate cyclase or PKA. This indicates that EGF action is mediated by generation of PGE2. The following model is proposed: PGE2 generated in the cells does not act as a counter ion but activates adenylate cyclase. This is mediated by a homologue of EP4 receptor. cAMP then activates PKA, which stimulates initial basolateral uptake of OA in OK cells by a not-yet-known mechanism. PGE2 is an organic anion, a potential stimulator of organic anion excretion, and an important mediator of inflammation all at once. Thus, the mechanism presented here may contribute to a limitation of inflammatory events in the kidney cortex interstitium.

COX-2 inhibition and prostaglandin receptors in experimental nephritis

BACKGROUND

Renal cyclooxygenases (COX) produce the prostaglandins (PG) E2, I2 and thromboxane (TxA2), which interact with distinct G protein-coupled receptors. We investigated the expression of the three EP receptors EP2, EP3 and EP4 and the receptors for PGI2 (IP) and TxA2 (TP) in rats with passive Heymann nephritis (PHN). We studied their regulation by COX-2 inhibition with celecoxib.

MATERIALS AND METHODS

Four groups of Wistar rats were studied: healthy rats (group A), healthy rats treated with celecoxib (group B), rats with PHN (group C), and rats with PHN receiving celecoxib (group D). Expression of the mRNA for all receptors in the renal cortex and for the EP3 receptor in cultured mesangial cells (MCs) was determined by semiquantitative reverse transcriptase polymerase chain reaction. Stable prostaglandin metabolites were measured in the urine by radioimmunoassay.

RESULTS

Rats with PHN (group C) showed an 1.8-fold increase of cortical EP3 receptor mRNA expression as compared with controls (group A). In celecoxib-treated PHN rats (group D) the mRNA expression of the EP3 and EP4 receptors was significantly reduced to 1.0-fold and 0.7-fold induction, respectively. Furthermore, the excretion of bicyclo-prostaglandin E2 (PGE2) was inhibited by celecoxib. No changes were observed in the expression of the other PG-receptors. In cultured MC, PGE2 enhanced the EP3 mRNA expression.

CONCLUSIONS

These data suggest a predominant role of the EP3 receptor in the transduction of PGE2-actions in PHN. It was concluded that COX-2-dependent PGE2 is able to potentiate its effects in the kidney by up-regulating its own receptors.

Glomerular and renal vascular structural changes in alpha8 integrin-deficient mice

Integrins are matrix receptors that regulate cell-matrix interactions during development and in adult tissue. In the adult kidney, the alpha8 chain is specifically expressed in glomerular mesangial cells and vascular smooth muscle cells. alpha8-deficient (alpha8-/-) mice demonstrate reductions in renal mass, which can range from complete renal agenesis to the development of kidneys that are only slightly smaller than wild-type kidneys. No histologic abnormalities of these kidneys have been described. However, considering the prominent expression of alpha8 in glomeruli and renal vessels, it seemed unlikely that the kidneys of alpha8-/- mice would be completely normal. Therefore, the renal phenotype of adult alpha8-/- mice was investigated, for assessment of more subtle morphologic alterations in kidney tissue. alpha8-/- mice displayed a significant reduction in nephron number and an increase in glomerular volume, compared with wild-type control animals. Albuminuria was not different in wild-type and alpha8-/- mice. Quantitative morphologic analyses revealed that the glomeruli of alpha8-/- mice were hypercellular, with an increased number of mesangial cells, compared with wild-type mice. Mesangial matrix deposition (as demonstrated for collagen IV and the alpha8 ligand fibronectin) was expanded in alpha8-/- mice, compared with wild-type mice. Collagens I and III, which are not normally present in glomeruli, were detected in the glomeruli of alpha8-/- mice. Staining for other glomerular integrins demonstrated an increased abundance of the collagen receptor alpha2 integrin in alpha8-/- mice. The glomerular capillary length density was significantly greater in alpha8-/- mice than in wild-type mice. Cortical arterial vessel walls were not altered in alpha8-/- mice, but the capillaries of the peritubular network were widened. Despite the strong mesangial and vascular expression of alpha8, glomerular and renal vascular alterations in alpha8-/- mice were relatively mild. Only aged alpha8-/- mice demonstrated increased glomerular capillary widening, compared with control animals. The results suggest that the lack of alpha8 can be largely compensated for, at least in younger alpha8-/- mice. It is not yet clear whether the occurrence of collagens that are not normally present in glomeruli and the increased abundance of the collagen receptor alpha2 contribute to maintaining the glomerular structure in alpha8-/- mice. The compensatory mechanisms involved will be the subject of future research.

Expression of cyclooxygenase-2 in biopsies obtained from human transplanted kidneys undergoing rejection

BACKGROUND

The inducible cyclooxygenase (COX)-2 is a target of immunosuppressive drugs routinely administered to patients after transplantation. This study investigates a potential involvement of COX-2 in transplant rejection. Therefore, we examined the expression of COX-2 in biopsies obtained for diagnostic purposes.

METHODS

COX-2 was detected by immunohistochemistry and in situ hybridization. Congruent staining was obtained by both methods: in specimens of a kidney explanted as the result of vascular rejection, tubular epithelial cells and endothelial cells stained positively for COX-2. Furthermore, in appendiceal specimens obtained at surgery, epithelial cells of the crypts, interstitial cells, and mesothelial cells were positive by both methods, affirming the specificity of the antibody.

RESULTS

Compared with healthy control subjects, intensive staining of COX-2 was observed in most of the 28 biopsies obtained from patients diagnosed with vascular rejection combined with cellular interstitial rejection and tubulitis. Glomeruli and the macula densa area were essentially negative compared with prominent staining in cortical and medullary epithelial cells of the tubuli. Staining was distinct with individual positive cells in the tubular cross sections. Few arteries expressed COX-2 in intimal cells. Less prominent expression of COX-2 was detected in the biopsies of six kidneys obtained from patients diagnosed with acute tubular necrosis.

CONCLUSION

This is the first report to show the up-regulation of COX-2 in human transplanted kidneys, despite ongoing immunosuppressive treatment. It remains to be established whether the up-regulation of COX-2 is part of the rejection process or has to be considered implicated in renal preservative mechanisms.

Key enzymes for renal prostaglandin synthesis: site-specific expression in rodent kidney (rat, mouse)

Prostanoids derived from endogenous cylooxygenase (COX)-mediated arachidonic acid metabolism play important roles in the maintenance of renal blood flow and salt and water homeostasis. The relative importance of COX-1 and COX-2 isoforms is under active investigation. We have performed a comprehensive histochemical analysis by comparing rat and mouse kidneys for cellular and subcellular localization of COX-1 and -2 and microsomal-type PGE synthase (PGES), the rate-limiting biosynthetic enzyme in PGE2 synthesis. A choice of different sera was compared, and the results were confirmed by antigen-retrieval techniques, in situ hybridization, RT-PCR, and the use of COX knockout mice. In the glomerulus, significant COX-1 expression was detected in a subset of mesangial cells. Along the renal tubule, the known COX-2 expression in cTAL and macula densa was paralleled by PGES staining. In the terminal distal convoluted tubule, connecting tubule, and cortical and medullary collecting ducts, a significant COX-1 signal was colocalized with PGES; COX-2 was not found in these sites. Intercalated cells were generally negative. Cortical fibroblasts were COX-1 and PGES positive in mice, whereas in rats only PGES could be reliably detected. Lipid-laden interstitial cells of the inner medulla were COX-1, -2, and PGES positive. Vascular smooth muscle cells were not stained. The present data support prominent functions of renal prostanoids, predominantly PGE2, by defining expression sites of the key enzymes for their biosynthesis in the rat and mouse. Results define the renal cell types involved in prostaglandin autacoid functions within spatially restricted sites such as the juxtaglomerular apparatus, mesangium, distal convolutions and collecting duct, and in compartments of the renal interstitium.

Role of macula densa cyclooxygenase-2 in renovascular hypertension

Upregulation of the inducible cyclooxygenase (COX-2) in the macula densa accompanies the activation of the juxtaglomerular apparatus in many high-renin conditions. The functional role of COX-2 in these disease states is poorly understood. We tested whether COX-2 is required to increase renin in renovascular hypertension. Rats with established two-kidney, one-clip (2K1C) hypertension were treated for 2 wk with two different inhibitors of COX-2, NS-398 and rofecoxib, respectively. Hypertension in 2K1C rats was not affected or slightly enhanced by COX-2 inhibition, as measured intra-arterially in conscious animals. The increase in plasma renin activity was also unchanged by both rofecoxib and NS-398. The number of glomeruli with a renin-positive juxtaglomerular apparatus was elevated in clipped kidneys and decreased in contralateral kidneys of 2K1C rats. This pattern was unaltered by COX-2 inhibition. To test the effects of COX-2 blockade on a primarily macula densa-mediated stimulus, we studied salt depletion for comparison. A low-salt diet induced a significant increase in plasma renin activity, which was partially inhibited by treatment with NS-398. We conclude that inhibition of COX-2 in established renovascular hypertension does not affect renin synthesis or release. Thus either COX-2 is not necessary for the macula densa mechanism or the macula densa is not important for maintaining high renin in renovascular hypertension.

Alterations in renal cytosolic phospholipase A2 and cyclooxygenases in polycystic kidney disease

Cytosolic phospholipase A2 (cPLA2), cyclooxygenase-1 (COX-1), and cyclooxygenase-2 (COX-2) regulate the formation of physiologically active prostaglandins, the production of which is known to be elevated in several renal disorders. We studied the relevance of these enzymes in polycystic kidney disease (PKD) by using two models of the disease: a model in which decline in renal function begins in adulthood (CD1-pcy/pcy mouse) and one in which it occurs early, during growth (Han:SPRD-cy rat). Immunoblotting analyses of cytosolic and particulate kidney fractions revealed that cPLA2 levels are significantly higher (by 34-131%) in the latter stages of the disease in both models. Renal COX enzymes were found only in the particulate fractions, with COX-1 87% higher in 6-month-old CD1-pcy/pcy mice compared with normal controls, and 110% higher in male 70-day-old Han:SPRD-cy rats with cystic kidneys compared with controls. Renal COX-2 was detected only in the rats and was 58% lower in diseased kidneys of 70-day-old male Han:SPRD-cy rats, indicating that cPLA2 is coupled to COX-1 in the kidney. The altered levels of these eicosanoid-regulating enzymes has implications for the use of NSAIDS and specific COX inhibitors in individuals with this disorder.

Placental restriction increases the expression of prostaglandin endoperoxide G/H synthase-2 and EP2 mRNA in the fetal sheep kidney during late gestation

There is evidence that fetal growth restriction is associated with impaired nephrogenesis and reduced numbers of mature nephrons at birth. It has been proposed that such impairment of renal growth may contribute to increased blood pressure in later life. Although prostaglandins (PG) play a key role in kidney development, it is unknown whether a poor fetal substrate supply alters the synthesis or actions of PG within the fetal kidney. Using real-time reverse transcriptase PCR, we have measured the effect of chronic placental restriction (PR) on the renal expression of PG endoperoxide G/H synthase-2 (PGHS-2), PGE(2) receptors EP(2) and EP(4), and renin mRNA in the sheep fetus in late gestation. Restriction of placental growth reduced fetal body weight (PR: 3.2 +/- 0.2 kg, control: 4.8 +/- 0.2 kg) and total kidney weight (PR: 19.7 +/- 1.8 g, control: 25.1 +/- 1.3 g). Mean fetal arterial PO(2) was reduced by PR (PR: 15.03 +/- 0.67 mm Hg, control: 21.3 +/- 0.87 mm Hg). Renal PGHS-2 mRNA was increased in the PR group (PR: 2.26 +/- 0.38, control: 1.20 +/- 0.31) and was inversely related to mean fetal arterial PO(2) in the PR and control groups [PGHS-2: -0.17 (PO(2)) + 4.69, r(2) = 0.26]. PR also increased renal EP(2) (PR: 1.57 + 0.24, control: 0.82 + 0.13) but not EP(4) mRNA. Renin mRNA was directly related to renal EP(2) [renin = 0.37 (EP(2)) + 0.97, r(2) = 0.29] and EP(4), [renin = 0.75 (EP(4)) + 0.44, r(2) = 0.38] mRNA expression. Thus, the restriction of placental growth and associated chronic hypoxemia appear to increase the renal capacity to synthesize and respond to PG, which may play an important role in maintaining renin mRNA expression in the growth-restricted fetus.

Gene expression of prostanoid forming enzymes along the rat nephron

BACKGROUND

To obtain information about the general capability of nephron segments to elaborate prostanoids, we determined the gene expression of key enzymes for prostanoid formation.

METHODS

For this goal mRNAs were assayed for cyclooxygenases-1 and -2 as well as for the synthases of prostaglandin D2 (PGD2), prostaglandin E2 (PGE2), prostacyclin (PGI2) and thromboxane A2 (TXA2) in microdissected rat nephron segments by RT-PCR.

RESULTS

Cyclooxygenase-1 (COX-1) mRNA was strongly expressed in all segments of the collecting ducts and to a lesser extent in glomeruli. COX-2 mRNA was found in the cortical thick ascending limb of Henle, and weaker expression also was detected in glomeruli. The lipocalin-type PGD synthase mRNA displayed a broad expression pattern in the cortex and outer medulla, including proximal convoluted tubule, thick ascending limb of Henle, distal convoluted tubule, and cortical and outer medullary collecting duct. The hematopoietic PGD synthase mRNA was restricted to the outer medullary collecting duct, and the membrane-associated PGE-synthase mRNA was exclusively expressed in the whole collecting duct system. Prostacylin-synthase mRNA was found in the whole kidney, but not in any microdissected nephron segment analyzed in this study. TXA-synthase mRNA was expressed in glomeruli.

CONCLUSION

Given that the existence of cyclooxygenase in combination with the different PG-synthases is a prerequisite for the formation of prostanoids, our data suggest that PGD2 is mainly formed in the thick ascending limb and in the collecting duct, while PGE2 appears to be mainly generated by the collecting ducts. Probably no formation of PGI2 occurs within the nephron. Whether TXA2 can be formed by nephron segments remains questionable.

Prostanoids and pain: unraveling mechanisms and revealing therapeutic targets

Advances in our understanding of the synthesis, regulation and function of prostanoids have led to a new appreciation of their actions in health and disease. Prostanoid synthesis is essential for the generation of inflammatory pain and this depends not only on prostanoid production at the site of inflammation, but also on the actions of prostanoids synthesized within the central nervous system (CNS). Moreover, central prostanoid synthesis is controlled both by neural and humoral signals, the latter being a novel form of input to the CNS. Diverse compounds that act along the pathway of prostanoid synthesis and action, both in the periphery and in the CNS, might provide increased benefit for treating inflammatory pain hypersensitivity and its associated sickness syndrome, with a reduced risk of adverse effects.

Distinct functions of COX-1 and COX-2

The enzymes that convert arachidonic acid to prostaglandin H2 are named cyclooxygenase-1 (COX-1) and COX-2. The properties of COX-1 are different from those of COX-2. It was originally thought that the function of COX-1 was involved in physiological phenomena, whereas that of COX-2 was involved in various pathologies. However, studies with COX-2 knockout mouse suggest that COX-2 also plays important roles in development and homeostasis. This chapter focuses on the distinct functions of COX-1 and COX-2.

Cyclooxygenase-2: a therapeutic target

Cyclooxygenase (COX), also known as prostaglandin endoperoxide synthase, is the key enzyme required for the conversion of arachidonic acid to prostaglandins. Two COX isoforms have been identified, COX-1 and COX-2. In many situations, the COX-1 enzyme is produced constitutively (e.g., in gastric mucosa), whereas COX-2 is highly inducible (e.g., at sites of inflammation and cancer). Traditional nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit both enzymes, and a new class of COX-2 selective inhibitors (COXIBs) preferentially inhibit the COX-2 enzyme. This review summarizes our current understanding of the role of COX-1 and COX-2 in normal physiology and disease.

Selective cyclooxygenase-2 inhibition impairs glomerular capillary healing in experimental glomerulonephritis

Selective cyclooxygenase-2 (COX-2) inhibitors have anti-inflammatory activity and reduce proteinuria in experimental membranous glomerulonephritis. Antiangiogenic properties of COX-2 inhibitors were recently reported. Whether these properties are relevant to the glomerular healing process in inflammatory glomerular diseases was investigated. For evaluation of the effects of selective COX-2 inhibitors on the glomerular healing process in a rat model of mesangioproliferative glomerulonephritis (induced by anti-Thy 1.1 antibody), a selective COX-2 inhibitor (rofecoxib or celecoxib) or vehicle was administered daily from day 1 after disease induction until euthanasia on day 6. Additional nephritic rats were treated with rofecoxib or vehicle from day 1 to day 10 and were monitored until day 28. Selective COX-2 inhibition led to significant increases in mesangiolysis (up to +71%) on days 2 and 6 and in albuminuria (up to 3.1-fold) on day 6. This augmentation of glomerular capillary damage was associated with rarefaction of glomerular endothelial cells, whereas the proliferation and activation of mesangial cells were not affected. No significant effects on the glomerular influx of polymorphonuclear neutrophils or the infiltration and proliferation of monocytes/macrophages at day 2 were noted. These effects were independent of systemic hemodynamic features, because rofecoxib did not affect systolic BP on day 2 or 5. Nephritic rats treated with rofecoxib for 10 d demonstrated persistent glomerular injury at day 28, as indicated by increased albuminuria (10-fold) and mesangial type IV collagen deposition (+24%). In normal rats, 5-d administration of rofecoxib failed to induce albuminuria or morphologic renal damage. In conclusion, selective COX-2 inhibitors impair glomerular capillary repair after mesangiolysis in rats with anti-Thy 1.1 glomerulonephritis. These data suggest that selective COX-2 inhibitors should be used with caution among patients with inflammatory endocapillary glomerular disorders.

Localization of prostaglandin E(2) EP2 and EP4 receptors in the rat kidney

We investigated the localization of cAMP-coupled prostaglandin E(2) EP2 and EP4 receptor expression in the rat kidney. EP2 mRNA was restricted to the outer and inner medulla in rat kidney, as determined by RNase protection assay. RT-PCR analysis of microdissected resistance vessels and nephron segments showed EP2 expression in descending thin limb of Henle's loop (DTL) and in vasa recta of the outer medulla. The EP4 receptor was expressed in distal convoluted tubule (DCT) and cortical collecting duct (CCD) in preglomerular vessels, and in outer medullary vasa recta. Butaprost, an EP2 receptor-selective agonist, dose dependently raised cAMP levels in microdissected DTL and outer medullary vasa recta specimens but had no effect in EP2-negative outer medullary collecting duct segments. Dietary salt intake did not alter EP2 expression in the kidney medulla. These results suggest that PGE(2) may act in the resistance vessels and in the DTL and DCT-CCD segments as a paracrine, cAMP-dependent regulator of vascular resistance and tubular transport, respectively.