Inhibition of nNOS expression in the macula densa by COX-2-derived prostaglandin E2

Collecting duct renin regulates potassium homeostasis in mice

AIM

The kaliuretic action of the renin-angiotensin-aldosterone system (RAAS) is well established as highlighted by hyperkalemia side effect of RAAS inhibitors but such action is usually ascribed to systemic RAAS. The present study addresses the involvement of intrarenal RAAS in K+ homeostasis with emphasis on locally generated renin within the collecting duct (CD).

METHODS

Wild-type (Floxed) and CD-specific deletion of renin (CD renin KO) mice were treated for 7 days with a high K+ (HK) diet to investigate the role of CD renin in kaliuresis regulation and further define the underlying mechanism with emphasis on analysis of intrarenal aldosterone biosynthesis.

RESULTS

In floxed mice, renin levels were elevated in the renal medulla and urine following a 1-week HK diet, indicating activation of the intrarenal renin. CD renin KO mice had blunted HK-induced intrarenal renin response and developed impaired kaliuresis and elevated plasma K+ level (4.45 ± 0.14 vs. 3.89 ± 0.04 mM, p < 0.01). In parallel, HK-induced intrarenal aldosterone and CYP11B2 expression along with expression of renal outer medullary K+ channel (ROMK), calcium-activated potassium channel subunit alpha-1 (α-BK), α-Na+ -K+ -ATPase, and epithelial sodium channel (β-ENaC and cleaved-γ-ENaC) expression were all significantly blunted in CD renin KO mice in contrast to the unaltered responses of plasma aldosterone and adrenal CYP11B2.

CONCLUSION

Taken together, these results support a kaliuretic action of CD renin during HK intake.

(Pro)renin receptor antagonist PRO20 attenuates nephrectomy-induced nephropathy in rats via inhibition of intrarenal RAS and Wnt/β-catenin signaling

Introduction

(Pro)renin receptor has emerged as a new member of the renin‐angiotensin system implicated in the pathogenesis of chronic kidney disease (CKD). Herein we report characterization of the therapeutic potential of (pro)renin receptor (PRR) antagonist PRO20 in 5/6 nephrectomy (5/6Nx) rats.

Methods

Male Wistar rats underwent 5/6Nx followed by treatment with vehicle or received daily injections of a PRR inhibitor PRO20 (700 μg/kg) via the 3 s.c. Sham group served as a control.

Results

As compared with the sham control, the 5/6Nx rats exhibited significant increases in proteinuria, glomerulosclerosis, tubular injury, and interstitial inflammation in the remnant kidneys. Treatment with PRO20 significantly attenuated these abnormalities, as evidenced by reduced expression of fibronectin, α‐SMA, collagen 1, TGF‐β1, IL‐6, IL‐8, IL‐1β, MCP‐1 and increased expression of E‐cadherin. Increased urinary/renal levels of renin activity, angiotensinogen (AGT), and Angiotensin II (Ang II) by 5/6Nx, which were all ameliorated by PRO20. Renal PRR, the secreted proteolytic fragment of PRR (sPRR) in renal and urinary, were all elevated in 5/6Nx rats. Moreover, our results revealed that renal Wnt3A and β‐catenin expression were upregulated during 5/6Nx, which were all attenuated by PRO20.

Conclusions

Overall we conclude that in vivo antagonism of PRR with PRO20 will improve 5/6Nx‐induced CKD mainly through inhibition of intrarenal RAS and Wnt/β‐catenin signaling pathway.

Angiotensin II receptor blockade alleviates calcineurin inhibitor nephrotoxicity by restoring cyclooxygenase 2 expression in kidney cortex

AIM

The use of calcineurin inhibitors such as cyclosporine A (CsA) for immunosuppression after solid organ transplantation is commonly limited by renal side effects. CsA-induced deterioration of glomerular filtration rate and sodium retention may be related to juxtaglomerular dysregulation as a result of suppressed cyclooxygenase 2 (COX-2) and stimulated renin biosynthesis. We tested whether CsA-induced COX-2 suppression is caused by hyperactive renin-angiotensin system (RAS) and whether RAS inhibition may alleviate the related side effects.

METHODS

Rats received CsA, the RAS inhibitor candesartan, or the COX-2 inhibitor celecoxib acutely (3 days) or chronically (3 weeks). Molecular pathways mediating effects of CsA and RAS on COX-2 were studied in cultured macula densa cells.

RESULTS

Pharmacological or siRNA-mediated calcineurin inhibition in cultured cells enhanced COX-2 expression via p38 mitogen-activated protein kinase and NF-kB signalling, whereas angiotensin II abolished these effects. Acute and chronic CsA administration to rats led to RAS activation along with reduced cortical COX-2 expression, creatinine clearance and fractional sodium excretion. Evaluation of major distal salt transporters, NKCC2 and NCC, showed increased levels of their activating phosphorylation upon CsA. Concomitant candesartan treatment blunted these effects acutely and completely normalized the COX-2 expression and renal functional parameters at long term. Celecoxib prevented the candesartan-induced improvements of creatinine clearance and sodium excretion.

CONCLUSION

Suppression of juxtaglomerular COX-2 upon CsA results from RAS activation, which overrides the cell-autonomous, COX-2-stimulatory effects of calcineurin inhibition. Angiotensin II antagonism alleviates CsA nephrotoxicity via the COX-2-dependent normalization of creatinine clearance and sodium excretion.

Expression of neuronal nitric oxide synthase and renin in dysplastic kidneys of young dogs

Renin and neuronal nitric oxide synthase in the kidney control the renin-angiotensin and tubuloglomerular feedback systems. The present study investigated the expression of renin and neuronal nitric oxide synthase in the dysplastic kidneys of three young dogs. Renin-immunoreactivity, which occurs in the juxtaglomerular and tubular cells of dysplastic kidneys, did not differ from that in the normal kidneys of young dogs. Macula densa cells in the normal kidneys showed neuronal nitric oxide synthase -immunoreactivity, but those in the dysplastic kidneys showed no apparent signals. This observation may be correlated with the pathological mechanisms of renal failure in young dogs.

Role of (pro)renin receptor in albumin overload-induced nephropathy in rats

Proteinuria is not only a common feature of chronic kidney diseases (CKD) but also an independent risk factor promoting CKD progression to end-stage renal failure. However, the underlying molecular mechanisms for protein overload-induced renal injury remain elusive. The present study examined the role of (pro)renin receptor (PRR) in pathogenesis of albumin overload (AO)-induced nephropathy and activation of the intrarenal renin-angiotensin system (RAS) in rats. Wistar rats underwent unilateral nephrectomy and were treated for 7 wk with vehicle, bovine serum albumin (5 g·kg^-1·day^-1 via a single ip injection), alone or in conjunction with the PRR decoy inhibitor PRO20 (500 μg·kg^-1·day^-1 via 3 sc injections). The AO rat model exhibited severe proteinuria, tubular necrosis, and interstitial fibrosis, oxidative stress, and inflammation, accompanied by elevated urinary N-acetyl-β-d-glucosaminidase activity and urinary β₂-microglobulin secretion, all of which were significantly attenuated by PRO20. Urinary and renal levels of renin, angiotensinogen, and ANG II were elevated by AO and suppressed by PRO20, contrasting to largely unaltered plasma levels of the RAS parameters. The AO model also showed increased renal expression of full-length PRR and soluble PRR (sPRR) and urinary excretion of sPRR. Taken together, we conclude that PRR antagonism with PRO20 alleviates AO-induced nephropathy via inhibition of intrarenal RAS.

(Pro)renin receptor mediates albumin-induced cellular responses: role of site-1 protease-derived soluble (pro)renin receptor in renal epithelial cells

Proteinuria is a characteristic of chronic kidney disease and also a causative factor that promotes the disease progression, in part, via activation of the intrarenal renin-angiotensin system (RAS). (Pro)renin receptor (PRR), a newly discovered component of the RAS, binds renin and (pro)renin to promote angiotensin I generation. The present study was performed to test the role of soluble PRR (sPRR) in albumin overload-induced responses in cultured human renal proximal tubular cell line human kidney 2 (HK-2) cells. Bovine serum albmuin (BSA) treatment for 24 h at 20 mg/ml induced renin activity and inflammation, both of which were attenuated by a PRR decoy inhibitor PRO20. BSA treatment induced a more than fivefold increase in medium sPRR due to enhanced cleavage of PRR. Surprisingly, this cleavage event was unaffected by inhibition of furin or a disintegrin and metalloproteinase 19. Screening for a novel cleavage enzyme led to the identification of site-1 protease (S1P). Inhibition of S1P with PF-429242 or siRNA remarkably suppressed BSA-induced sPRR production, renin activity, and inflammatory response. Administration of a recombinant sPRR, termed sPRR-His, reversed the effects of S1P inhibition. In HK-2 cells overexpressing PRR, mutagenesis of the S1P, but not furin cleavage site, reduced sPRR levels. Together, these results suggest that PRR mediates albumin-induced cellular responses through S1P-derived sPRR.

Age-dependent redox status in the brain stem of NO-deficient hypertensive rats

BACKGROUND

The brain stem contains important nuclei that control cardiovascular function via the sympathetic nervous system (SNS), which is strongly influenced by nitric oxide. Its biological activity is also largely determined by oxygen free radicals. Despite many experimental studies, the role of AT1R-NAD(P)H oxidase-superoxide pathway in NO-deficiency is not yet sufficiently clarified. We determined changes in free radical signaling and antioxidant and detoxification response in the brain stem of young and adult Wistar rats during chronic administration of exogenous NO inhibitors.

METHODS

Young (4 weeks) and adult (10 weeks) Wistar rats were treated with 7-nitroindazole (7-NI group, 10 mg/kg/day), a specific nNOS inhibitor, with N^G-nitro-L-arginine-methyl ester (L-NAME group, 50 mg/kg/day), a nonspecific NOS inhibitor, and with drinking water (Control group) during 6 weeks. Systolic blood pressure was measured by non-invasive plethysmography. Expression of genes (AT1R, AT2R, p22phox, SOD and NOS isoforms, HO-1, MDR1a, housekeeper GAPDH) was identified by real-time PCR. NOS activity was detected by conversion of [3H]-L-arginine to [3H]-L-citrulline and SOD activity was measured using UV VIS spectroscopy.

RESULTS

We observed a blood pressure elevation and decrease in NOS activity only after L-NAME application in both age groups. Gene expression of nNOS (youngs) and eNOS (adults) in the brain stem decreased after both inhibitors. The radical signaling pathway triggered by AT1R and p22phox was elevated in L-NAME adults, but not in young rats. Moreover, L-NAME-induced NOS inhibition increased antioxidant response, as indicated by the observed elevation of mRNA SOD3, HO-1, AT2R and MDR1a in adult rats. 7-NI did not have a significant effect on AT1R-NADPH oxidase-superoxide pathway, yet it affected antioxidant response of mRNA expression of SOD1 and stimulated total activity of SOD in young rats and mRNA expression of AT2R in adult rats.

CONCLUSION

Our results show that chronic NOS inhibition by two different NOS inhibitors has age-dependent effect on radical signaling and antioxidant/detoxificant response in Wistar rats. While 7-NI had neuroprotective effect in the brain stem of young Wistar rats, L-NAME- induced NOS inhibition evoked activation of AT1R-NAD(P)H oxidase pathway in adult Wistar rats. Triggering of the radical pathway was followed by activation of protective compensation mechanism at the gene expression level.

Calcineurin inhibitor cyclosporine A activates renal Na-K-Cl cotransporters via local and systemic mechanisms

Calcineurin dephosphorylates nuclear factor of activated T cells transcription factors, thereby facilitating T cell-mediated immune responses. Calcineurin inhibitors are instrumental for immunosuppression after organ transplantation but may cause side effects, including hypertension and electrolyte disorders. Kidneys were recently shown to display activation of the furosemide-sensitive Na-K-2Cl cotransporter (NKCC2) of the thick ascending limb and the thiazide-sensitive Na-Cl cotransporter (NCC) of the distal convoluted tubule upon calcineurin inhibition using cyclosporin A (CsA). An involvement of major hormones like angiotensin II or arginine vasopressin (AVP) has been proposed. To resolve this issue, the effects of CsA treatment in normal Wistar rats, AVP-deficient Brattleboro rats, and cultured renal epithelial cells endogenously expressing either NKCC2 or NCC were studied. Acute administration of CsA to Wistar rats rapidly augmented phosphorylation levels of NKCC2, NCC, and their activating kinases suggesting intraepithelial activating effects. Chronic CsA administration caused salt retention and hypertension, along with stimulation of renin and suppression of renal cyclooxygenase 2, pointing to a contribution of endocrine and paracrine mechanisms at long term. In Brattleboro rats, CsA induced activation of NCC, but not NKCC2, and parallel effects were obtained in cultured cells in the absence of AVP. Stimulation of cultured thick ascending limb cells with AVP agonist restored their responsiveness to CsA. Our results suggest that the direct epithelial action of calcineurin inhibition is sufficient for the activation of NCC, whereas its effect on NKCC2 is more complex and requires concomitant stimulation by AVP.

(Pro)Renin receptor regulates potassium homeostasis through a local mechanism

(Pro)renin receptor (PRR) is highly expressed in the distal nephron, but it has an unclear functional implication. The present study was conducted to explore a potential role of renal PRR during high K⁺ (HK) loading. In normal Sprague-Dawley rats, a 1-wk HK intake increased renal expression of full-length PRR and urinary excretion of soluble PRR (sPRR). Administration of PRO20, a decoy peptide antagonist of PRR, in K⁺-loaded animals elevated plasma K⁺ level and decreased urinary K⁺ excretion, accompanied with suppressed urinary aldosterone excretion and intrarenal aldosterone levels. HK downregulated Na⁺-Cl^- cotransporter (NCC) expression but upregulated CYP11B2 (cytochrome P-450, family 11, subfamily B, polypeptide 2), renal outer medullary K⁺ channel (ROMK), calcium-activated potassium channel subunit α₁ (α-BK), α-Na⁺-K⁺-ATPase (α-NKA), and epithelial Na⁺ channel subunit β (β-ENaC), all of which were blunted by PRO20. After HK loading was completed, urinary, but not plasma renin, was upregulated, which was blunted by PRO20. The same experiments that were performed using adrenalectomized (ADX) rats yielded similar results. Interestingly, spironolactone treatment in HK-loaded ADX rats attenuated kaliuresis but promoted natriuresis, which was associated with the suppressed responses of β-ENaC, α-NKA, ROMK, and α-BK protein expression. Taken together, we discovered a novel role of renal PRR in regulation of K⁺ homeostasis through a local mechanism involving intrarenal renin-angiotensin-aldosterone system and coordinated regulation of membrane Na⁺- and K⁺-transporting proteins.

Collecting duct (pro)renin receptor targets ENaC to mediate angiotensin II-induced hypertension

The (pro)renin receptor (PRR) is abundantly expressed in the collecting duct (CD) and the expression is further induced by angiotensin II (ANG II). The present study was conducted to investigate the role of CD PRR during ANG II-induced hypertension and to further explore the underlying mechanism. Radiotelemetry demonstrated that a 1-wk ANG II infusion gradually and significantly induced hypertensive response in floxed mice and this response was significantly attenuated in mice lacking PRR in the CD (termed CD PRR KO). ANG II infusion in floxed mice increased urinary renin activity and selectively induced renal medullary α-epithelial sodium channel (α-ENaC) mRNA and protein expression, all of which were blunted in the null mice. In cultured mpkCCD cells grown in Transwells, transepithelial Na⁺ transport as measured by using a volt-ohmmeter was transiently stimulated by acute ANG II treatment, which was abolished by a PRR antagonist, PRO20. In a chronic setting, ANG II treatment induced α-ENaC mRNA expression in mpkCCD cells, which was similarly blocked by PRO20. Chronic intramedullary infusion of an ENaC inhibitor amiloride in rats significantly attenuated ANG II-induced hypertension. Overall, the present study suggests that CD PRR contributes to ANG II-induced hypertension at least partially via activation of renal medullary ENaC.

Renal medullary (pro)renin receptor contributes to angiotensin II-induced hypertension in rats via activation of the local renin-angiotensin system

BACKGROUND

(Pro)renin receptor (PRR) is a new component of the renin-angiotensin system and regulates renin activity in vitro. Within the kidney, PRR is highly expressed in the renal medulla where its expression is induced by angiotensin II infusion. The objective of the present study was to test a potential role of renal medullary PRR during angiotensin II-induced hypertension.

METHODS

A rat AngII infusion model (100 ng/kg/min) combined with renal intramedullary infusion of PRO20, a specific inhibitor of PRR, was builded. And the intravenous PRO20 infusion serve as control. Mean arterial pressure was recorded by radiotelemetry for one week. Further analysis of kidney injury, inflammation, biochemical indices and protein localization were performed in vivo or in vitro.

RESULTS

Radiotelemetry demonstrated that AngII infusion elevated the mean arteria pressure from 108 ± 5.8 to 164.7 ± 6.2 mmHg. Mean arterial pressure decreased to 128.6 ± 5.8 mmHg (P < 0.05) after intramedullary infusion of PRO20, but was only modestly affected by intravenous PRO20 infusion. Indices of kidney injury, including proteinuria, glomerulosclerosis, and interstitial fibrosis, inflammation, and increased renal medullary and urinary renin activity following angiotensin II infusion were all remarkably attenuated by intramedullary PRO20 infusion. Following one week of angiotensin II infusion, increased PRR immunoreactivity was found in vascular smooth muscle cells. In cultured rat vascular smooth muscle cells, angiotensin II induced parallel increases in soluble PRR and renin activity, and the latter was significantly reduced by PRO20.

CONCLUSION

Renal medullary PRR mediates angiotensin II-induced hypertension, likely by amplifying the local renin response.

Oxytocin evokes a pulsatile PGE2 release from ileum mucosa and is required for repair of intestinal epithelium after injury

We measured the short-circuit current (I_sc) in rat ileum mucosa to identify the effect of oxytocin (OT) on mucosal secretion in small intestine. We identified a COX-2-derived pulsatile PGE2 release triggered by OT in rat ileum mucosa. OT receptors (OTR) are expressed in intestine crypt epithelial cells. Notably, OT evoked a dynamic change of [Ca²⁺]_i in ileum crypts, which was responsible for this pulsatile release of PGE2. OT ameliorated 5-FU-, radiation- or DSS- induced injury in vivo, including the improvement of weight loss, reduced villus height and impaired survival of crypt transit-amplifying cells as well as crypt. Moreover, these protective effects of OT against intestinal injury were eliminated by coadministration of a selective inhibitor of PGE2, AH6809. Our findings strongly suggest that OT, a novel and important regulator of intestine mucosa barrier, is required for repair of intestinal epithelium after injury. Considering that OT is an FDA-approved drug, this work reveals a potential novel and safe way to combat or prevent chemo-radiotherapy induced intestine injury or to treat IBD.

Renal Effects of Cyclooxygenase Inhibition When Nitric Oxide Synthesis Is Reduced and Angiotensin II Levels Are Enhanced

The involvement of both cyclooxygenase (COX) isoforms in regulating renal function is well known but their interactions with other regulatory mechanisms, such as angiotensin II (Ang II) and nitric oxide (NO), are not well defined. This study has evaluated the relative contribution of both COX isoforms in regulating renal function when NO synthesis is reduced with and without a simultaneous increment in Ang II levels. The renal responses to a nonselective (meclofenamate) or a selective COX2 (nimesulide) inhibitor were examined in dogs pretreated with L-NAME with or without an intrarenal Ang II infusion. Meclofenamate induced a greater (P < 0.05) renal vasoconstriction than nimesulide in dogs pretreated with L-NAME. This vasoconstriction seems to be Ang II-dependent because it was reduced (P < 0.05) by captopril administration. Meclofenamate also induced a greater (P < 0.05) renal vasoconstriction than that elicited by nimesulide in dogs with reduced NO synthesis and elevated Ang II levels. The renal vasoconstriction induced by nimesulide but not that elicited by meclofenamate in dogs pretreated with L-NAME and Ang II, decreased (P < 0.05) during an extracellular volume expansion. These results demonstrate that the nonselective COX inhibition induces a greater renal vasoconstriction than that elicited by the selective COX2 inhibition when NO synthesis is reduced, and when NO synthesis is reduced and Ang II levels are elevated.

Different Effects of 7-nitroindazole and L-NAME Administered Both Individually and Together on the Cardiovascular System of the Rat

We evaluated the effects of NG-nitro-L-arginine methylester (L-NAME) (50 mg/kg/day) and 7-nitroindazole (7NI) (10 mg/kg/day) administered from 10th-16th week of age either individually or together on cardiovascular system of Wistar rats and SHR. Systolic blood pressure (sBP) was measured weekly by the plethysmographic method. For morphological studies, the animals (n=10) were perfused with a fixative (120 mm Hg), and thoracic aorta and carotid and coronary arteries were processed for electron microscopy. For functional investigation (n=10), aortic rings were used in an organ bath. In Wistar rats, L-NAME evoked an increase of sBP; hypertrophy of the heart and arterial walls; an increase in cross-sectional areas (CSA) of endothelial cells (EC), muscle cells (SMC), extracellular matrix (ECM), and a decrease in acetylcholine-induced endothelial-dependent relaxation (EDR). 7NI evoked sBP-independent hypotrophy of the heart and arterial walls, a decrease in CSA of EC and SMC without affecting the CSA of ECM, and a mild decrease in acetylcholine-induced EDR. 7NI and L-NAME administered together evoked lower effect on BP and trophicity of the heart and all arteries, and a similar decrease in acetylcholine-induced EDR compared to L-NAME alone. In SHR, 7NI did not evoke any effect on the studied parameters.

COX-2 but not mPGES-1 contributes to renal PGE2 induction and diabetic proteinuria in mice with type-1 diabetes

Prostaglandin E2 (PGE2) has been implicated to play a pathogenic role in diabetic nephropathy (DN) but its source remains unlcear. To elucidate whether mPGES-1, the best characterized PGE2 synthase, was involved in the development of DN, we examined the renal phenotype of mPGES-1 KO mice subjected to STZ-induced type-1 diabetes. After STZ treatment, mPGES-1 WT and KO mice presented the similar onset of diabetes as shown by similar elevation of blood glucose. Meanwhile, both genotypes of mice exhibited similar increases of urinary and renal PGE2 production. In parallel with this comparable diabetic status, the kidney injury indices including the urinary albumin excretion, kidney weight and the kidney histology (PAS staining) did not show any difference between the two genotypes. By Western-blotting and quantitative qRT-PCR, mPGES-1, mPGES-2, cPGES and 15-hydroxyprostaglandin dehydrogenase (15-PGDH) remain unaltered following six weeks of diabetes. Finally, a selective COX-2 inhibitor celecoxib (50 mg/kg/day) was applied to the STZ-treated KO mice, which resulted in significant reduction of urinary albumin excretion (KO/STZ: 141.5±38.4 vs. KO/STZ + Celebrex: 48.7±20.8 ug/24 h, p<0.05) and the blockade of renal PGE2 induction (kidney: KO/STZ: 588.7±89.2 vs. KO/STZ + Celebrex: 340.8±58.7 ug/24 h, p<0.05; urine: KO/STZ 1667.6±421.4 vs. KO/STZ + Celebrex 813.6±199.9 pg/24 h, p<0.05), without affecting the blood glucose levels and urine volume. Taken together, our data suggests that an as yet unidentified prostaglanind E synthase but not mPGES-1 may couple with COX-2 to mediate increased renal PGE2 sythsesis in DN.

Prostaglandin E-prostanoid4 receptor mediates angiotensin II-induced (pro)renin receptor expression in the rat renal medulla

Angiotensin II (Ang II) stimulates (pro)renin receptor (PRR) expression in the renal collecting duct, triggering the local renin response in the distal nephron. Our recent study provided evidence for involvement of cyclooxygenase-2-prostaglandin E2 pathway in Ang II-dependent stimulation of PRR expression in the collecting duct. Here, we tested the role of E-prostanoid (EP) subtypes acting downstream of cyclooxygenase-2 in this phenomenon. In primary rat inner medullary collecting duct cells, Ang II treatment for 12 hours induced a 1.8-fold increase in the full-length PRR protein expression. To assess the contribution of EP receptor, the cell was pretreated with specific EP receptor antagonists: SC-51382 (for EP1), L-798106 (for EP3), L-161982 (for EP4), and ONO-AE3-208 (ONO, a structurally distinct EP4 antagonist). The upregulation of PRR expression by Ang II was consistently abolished by L-161982 and ONO and partially suppressed by SC-51382 but was unaffected by L-798106. The PRR expression was also significantly elevated by the EP4 agonist CAY10598 in the absence of Ang II. Sprague-Dawley rats were subsequently infused for 1 or 2 weeks with vehicle, Ang II alone, or in combination with ONO. Ang II infusion induced parallel increases in renal medullary PRR protein and renal medullary and urinary renin activity and total renin content, all of which were blunted by ONO. Both tail cuff plethysmography and telemetry demonstrated attenuation of Ang II hypertension by ONO. Overall, these results have established a crucial role of the EP4 receptor in mediating the upregulation of renal medullary PRR expression and renin activity during Ang II hypertension.

COX-2 mediates angiotensin II-induced (pro)renin receptor expression in the rat renal medulla

(Pro)renin receptor (PRR) is predominantly expressed in the distal nephron where it is activated by angiotensin II (ANG II), resulting in increased renin activity in the renal medulla thereby amplifying the de novo generation and action of local ANG II. The goal of the present study was to test the role of cycloxygenase-2 (COX-2) in meditating ANG II-induced PRR expression in the renal medulla in vitro and in vivo. Exposure of primary rat inner medullary collecting duct cells to ANG II induced sequential increases in COX-2 and PRR protein expression. When the cells were pretreated with a COX-2 inhibitor NS-398, ANG II-induced upregulation of PRR protein expression was almost completely abolished, in parallel with the changes in medium active renin content. The inhibitory effect of NS-398 on the PRR expression was reversed by adding exogenous PGE2. A 14-day ANG II infusion elevated renal medullary PRR expression and active and total renin content in parallel with increased urinary renin, all of which were remarkably suppressed by the COX-2 inhibitor celecoxib. In contrast, plasma and renal cortical active and total renin content were suppressed by ANG II treatment, an effect that was unaffected by COX-2 inhibition. Systolic blood pressure was elevated with ANG II infusion, which was attenuated by the COX-2 inhibition. Overall, the results obtained from in vitro and in vivo studies established a crucial role of COX-2 in mediating upregulation of renal medullary PRR expression and renin content during ANG II hypertension.

PPARγ Agonist Rosiglitazone Suppresses Renal mPGES-1/PGE2 Pathway in db/db Mice

Evidence had shown the detrimental effect of prostaglandin (PG) E2 in diabetic nephropathy (DN) of STZ-induced type-1 diabetes but its role in the development of DN of type-2 diabetes remains uncertain. The present study was undertaken to investigate the regulation of PGE2 synthetic pathway and the interaction between peroxisome proliferator-activated receptor (PPAR) γ and PGE2 synthesis in the kidneys of db/db mice. Strikingly, urinary PGE2 was remarkably elevated in db/db mice paralleled with the increased protein expressions of COX-2 and mPGES-1. In contrast, the protein expressions of COX-1, mPGES-2, cPGES, and 15-hydroxyprostaglandin dehydrogenase (15-PGDH) were not altered. Following 1-week rosiglitazone (Rosi) therapy, urinary PGE2, but not other prostanoids, was reduced by 57% in parallel with significant reduction of mPGES-1 protein and EP4 mRNA expressions. By immunohistochemistry, mPGES-1 was significantly induced in the glomeruli of db/db mice, which was almost entirely abolished by Rosi. In line with the reduction of glomerular mPGES-1, the glomerular injury score showed a tendency of improvement after 1 week of Rosi therapy. Collectively, the present study demonstrated an inhibitory effect of PPAR γ activation on renal mPGES-1/PGE2/EP4 pathway in type-2 diabetes and suggested that mPGES-1 may potentially serve as a therapeutic target for treating type-2 diabetes-associated DN.

Reciprocal regulation of the nitric oxide and cyclooxygenase pathway in pathophysiology: relevance and clinical implications

The nitric oxide (NO) and cyclooxygenase (COX) pathways share a number of similarities. Nitric oxide is the mediator generated from the NO synthase (NOS) pathway, and COX converts arachidonic acid to prostaglandins, prostacyclin, and thromboxane A(2). Two major forms of NOS and COX have been identified to date. The constitutive isoforms critically regulate several physiological states. The inducible isoforms are overexpressed during inflammation in a variety of cells, producing large amounts of NO and prostaglandins, which may underlie pathological processes. The cross-talk between the COX and NOS pathways was initially reported by Salvemini and colleagues in 1993, when they demonstrated in a series of in vitro and in vivo studies that NO activates the COX enzymes to produce increased amounts of prostaglandins. Those studies led to the concept that COX enzymes represent important endogenous "receptor" targets for amplifying or modulating the multifaceted roles of NO in physiology and pathology. Since then, numerous studies have furthered our mechanistic understanding of these interactions in pathophysiological settings and delineated potential clinical outcomes. In addition, emerging evidence suggests that the canonical nitroxidative species (NO, superoxide, and/or peroxynitrite) modulate biosynthesis of prostaglandins through non-COX-related pathways. This article provides a comprehensive state-of-the art overview in this area.

Tubular control of renin synthesis and secretion

The intratubular composition of fluid at the tubulovascular contact site of the juxtaglomerular apparatus serves as regulatory input for secretion and synthesis of renin. Experimental evidence, mostly from in vitro perfused preparations, indicates an inverse relation between luminal NaCl concentration and renin secretion. The cellular transduction mechanism is initiated by concentration-dependent NaCl uptake through the Na-K-2Cl cotransporter (NKCC2) with activation of NKCC2 causing inhibition and deactivation of NKCC2 causing stimulation of renin release. Changes in NKCC2 activity are coupled to alterations in the generation of paracrine factors that interact with granular cells. Among these factors, generation of PGE2 in a COX-2-dependent fashion appears to play a dominant role in the stimulatory arm of tubular control of renin release. [NaCl] is a determinant of local PG release over an appropriate concentration range, and blockade of COX-2 activity interferes with the NaCl dependency of renin secretion. The complex array of local paracrine controls also includes nNOS-mediated synthesis of nitric oxide, with NO playing the role of a modifier of the intracellular signaling pathway. A role of adenosine may be particularly important when [NaCl] is increased, and at least some of the available evidence is consistent with an important suppressive effect of adenosine at higher salt concentrations.

Eplerenone inhibits aldosterone-induced renal expression of cyclooxygenase

INTRODUCTION

The upregulation of cyclooxygenase (COX) expression by aldosterone (ALDO) or high salt diet intake is very interesting and complex in the light of what is known about the role of COX in renal function. Thus, in this study, we hypothesize that apocynin (APC) and/or eplerenone (EPL) inhibit ALDO/salt-induced kidney damage by preventing the production of prostaglandin E₂ (PGE₂).

METHODS

Dahl salt-sensitive rats on either a low-salt or high-salt diet were treated with ALDO (0.2 mg pellet) in the presence of EPL (100 mg/kg/day) or APC (1.5 mM). Indirect blood pressure, prostaglandins and ALDO levels and histological changes were measured.

RESULTS

Cyclooxygenase-2 (COX-2) levels were upregulated in the renal tubules and peritubular vessels after high-salt intake, and APC attenuated renal tubular COX-2 protein expression induced by ALDO. Plasma PGE₂ levels were significantly reduced by ALDO in the rats fed a low-salt diet when compared to rats fed a high-salt diet. PGE₂ was blocked by EPL but increased in the presence of APC.

CONCLUSIONS

The beneficial effects of EPL may be associated with an inhibition of PGE₂. The mechanism underlying the protective effects of EPL is clearly distinct from that of APC and suggests that these agents can have differential roles in cardiovascular disease.

Synthesis and secretion of renin in mice with induced genetic mutations

The juxtaglomerular (JG) cell product renin is rate limiting in the generation of the bioactive octapeptide angiotensin II. Rates of synthesis and secretion of the aspartyl protease renin by JG cells are controlled by multiple afferent and efferent pathways originating in the CNS, cardiovascular system, and kidneys, and making critical contributions to the maintenance of extracellular fluid volume and arterial blood pressure. Since both excesses and deficits of angiotensin II have deleterious effects, it is not surprising that control of renin is secured by a complex system of feedforward and feedback relationships. Mice with genetic alterations have contributed to a better understanding of the networks controlling renin synthesis and secretion. Essential input for the setting of basal renin generation rates is provided by β-adrenergic receptors acting through cyclic adenosine monophosphate, the primary intracellular activation mechanism for renin mRNA generation. Other major control mechanisms include COX-2 and nNOS affecting renin through PGE2, PGI2, and nitric oxide. Angiotensin II provides strong negative feedback inhibition of renin synthesis, largely an indirect effect mediated by baroreceptor and macula densa inputs. Adenosine appears to be a dominant factor in the inhibitory arms of the baroreceptor and macula densa mechanisms. Targeted gene mutations have also shed light on a number of novel aspects related to renin processing and the regulation of renin synthesis and secretion.

Upregulation of cyclooxygenase-2 expression in porcine macula densa with chronic nitric oxide synthase inhibition

The objective of this study was to investigate the effects of chronic inhibition of nitric oxide synthase (NOS) on cyclooxygenase-2 (COX-2) expression in the macula densa (MD) of swine, as well as the effects on expression of related proteins. Adult female Yucatan swine were given either tap water (control, n = 6) or water with N (G)-nitro-L-arginine methyl ester (L-NAME, 100 mg/liter, n = 5) for a minimum of 30 days. Duplicate samples of kidney were fixed or snap frozen. There was a significant (P = .0082) upregulation of COX-2 mRNA expression in the MD of L-NAME, as well as an apparent increase in COX-2 protein. Plasma renin activity also increased with L-NAME treatment (control, 0.34 ± 0.08 ng/ml; L-NAME, 1.26 ± 0.03 ng/ml; P = .00000003). There were no differences between groups in expression of either inducible NOS or renin protein or in serum electrolyte concentrations. In conclusion, with chronic inhibition of NOS, COX-2 in MD is upregulated, perhaps to compensate for loss of nitric oxide. Increases in COX-2 products may counteract renal arteriolar constriction and sustain renin release.

mPGES-1 deletion impairs aldosterone escape and enhances sodium appetite

Aldosterone (Aldo) is a major sodium-retaining hormone that reduces renal sodium excretion and also stimulates sodium appetite. In the face of excess Aldo, the sodium-retaining action of this steroid is overridden by an adaptive regulatory mechanism, a phenomenon termed Aldo escape. The underlying mechanism of this phenomenon is not well defined but appeared to involve a number of natriuretic factors such prostaglandins (PGs). Here, we investigated the role of microsomal prostaglandin E synthase-1 (mPGES-1) in the response to excess Aldo. A 14-day Aldo infusion at 0.35 mg x kg(-1) x day(-1) via an osmotic minipump in conjunction with normal salt intake did not produce obvious disturbances in fluid metabolism in WT mice as suggested by normal sodium and water balance, plasma sodium concentration, hematocrit, and body weight, despite the evidence of a transient sodium accumulation on days 1 or 2. In a sharp contrast, the 14-day Aldo treatment in mPGES-1 knockoute (KO) mice led to increased sodium and water balance, persistent reduction of hematocrit, hypernatremia, and body weight gain, all evidence of fluid retention. The escaped wild-type (WT) mice displayed a remarkable increase in urinary PGE(2) excretion in parallel with coinduction of mPGES-1 in the proximal tubules, accompanied by a remarkable, widespread downregulation of renal sodium and water transporters. The increase in urinary PGE(2) excretion together with the downregulation of renal sodium and water transporters were all significantly blocked in the KO mice. Interestingly, compared with WT controls, the KO mice exhibited consistent increases in sodium and water intake during Aldo infusion. Together, these results suggest an important role of mPGES-1 in antagonizing the sodium-retaining action of Aldo at the levels of both the central nervous system and the kidney.

mPGES-1 protects against DOCA-salt hypertension via inhibition of oxidative stress or stimulation of NO/cGMP

Microsomal prostaglandin E synthase-1 (mPGES-1) is a recently characterized cytokine-inducible enzyme critically involved in pain and inflammatory response. However, its role in blood pressure regulation is still debatable. The present study was undertaken to examine the effect of mPGES-1 deletion on DOCA-salt hypertension. After 2 weeks of DOCA plus 1% NaCl as drinking fluid, hypertension and sodium retention were more severe in mPGES-1 knockout (KO) mice than in wild-type (WT) controls. The indices of oxidative stress including urinary 8-isprostane and renal thiobarbituric acid-reactive substances were only modestly increased or unchanged in the WT mice but more significantly increased in the KO mice after DOCA-salt. Conversely, in response to DOCA-salt, the indices of antioxidant systems including renal expression of superoxide dismutase-3 and urinary nitrate/nitrite excretion were all significantly elevated in the WT mice but remarkably suppressed in the KO mice. Tempol treatment (50 mg/kg per day) in DOCA-salt KO mice produced a marked attenuation of hypertension, sodium retention, and kidney injury. Immunoblotting demonstrated increased renal expression of mPGES-1 in DOCA-salt WT mice. DOCA-salt induced a nearly 5-fold increase in urinary PGE(2) excretion in the WT mice, and this increase was completely abolished in the KO mice. Together, these results suggest that mPGES-1-derived PGE(2) confers protection against DOCA-salt hypertension likely via inhibition of oxidative stress or stimulation of superoxide dismutase-3 and urinary nitrate/nitrite system.

NOX2 is the primary source of angiotensin II-induced superoxide in the macula densa

Macula densa (MD)-mediated regulation of renal hemodynamics via tubuloglomerular feedback is regulated by interactions between factors such as superoxide (O(2)(-)) and angiotensin II (ANG II). We have reported that NaCl-induced O(2)(-) in the MD is produced by the NOX2 isoform of NADPH oxidase (NOX); however, the source of ANG II-induced O(2)(-) in MD is unknown. Thus we determined the pathways by which ANG II increased O(2)(-) in the MD by measuring O(2)(-) in ANG II-treated MMDD1 cells, a MD-like cell line. ANG II caused MMDD1 O(2)(-) levels to increase by more than twofold (P < 0.01). This increase was blocked by losartan (AT(1) receptor blocker) but not PD-123319 (AT(2) receptor antagonist). Apocynin (a NOX inhibitor) decreased O(2)(-) by 86% (P < 0.01), whereas oxypurinol (a xanthine oxidase inhibitor) and NS-398 (a cyclooxygenase-2 inhibitor) had no significant effect. The NOX-dependent increase in O(2)(-) was due to the NOX2 isoform; a short interfering (si)RNA against NOX2 blunted ANG II-induced increases in O(2)(-), whereas the NOX4/siRNA did not. Finally, we found that inhibiting the Rac1 subunit of NOX blunted ANG II-induced O(2)(-) production in NOX4/siRNA-treated cells but did not further decrease it in NOX2/siRNA-treated cells. Our results indicate that ANG II stimulates O(2)(-) production in the MD primarily via AT(1)-dependent activation of NOX2. Rac1 is required for the full activation of NOX2. This pathway may be an important component of ANG II enhancement of tubuloglomerular feedback.

ATP as a mediator of macula densa cell signalling

Within each nephro-vascular unit, the tubule returns to the vicinity of its own glomerulus. At this site, there are specialised tubular cells, the macula densa cells, which sense changes in tubular fluid composition and transmit information to the glomerular arterioles resulting in alterations in glomerular filtration rate and blood flow. Work over the last few years has characterised the mechanisms that lead to the detection of changes in luminal sodium chloride and osmolality by the macula densa cells. These cells are true "sensor cells" since intracellular ion concentrations and membrane potential reflect the level of luminal sodium chloride concentration. An unresolved question has been the nature of the signalling molecule(s) released by the macula densa cells. Currently, there is evidence that macula densa cells produce nitric oxide via neuronal nitric oxide synthase (nNOS) and prostaglandin E(2) (PGE(2)) through cyclooxygenase 2 (COX 2)-microsomal prostaglandin E synthase (mPGES). However, both of these signalling molecules play a role in modulating or regulating the macula-tubuloglomerular feedback system. Direct macula densa signalling appears to involve the release of ATP across the basolateral membrane through a maxi-anion channel in response to an increase in luminal sodium chloride concentration. ATP that is released by macula densa cells may directly activate P2 receptors on adjacent mesangial cells and afferent arteriolar smooth muscle cells, or the ATP may be converted to adenosine. However, the critical step in signalling would appear to be the regulated release of ATP across the basolateral membrane of macula densa cells.

Mice lacking mPGES-1 are resistant to lithium-induced polyuria

Cyclooxygenase-2 activity is required for the development of lithium-induced polyuria. However, the involvement of a specific, terminal prostaglandin (PG) isomerase has not been evaluated. The present study was undertaken to assess lithium-induced polyuria in mice deficient in microsomal prostaglandin E synthase-1 (mPGES-1). A 2-wk administration of LiCl (4 mmol.kg(-1).day(-1) ip) in mPGES-1 +/+ mice led to a marked polyuria with hyposmotic urine. This was associated with elevated renal mPGES-1 protein expression and increased urine PGE(2) excretion. In contrast, mPGES-1 -/- mice were largely resistant to lithium-induced polyuria and a urine concentrating defect, accompanied by nearly complete blockade of high urine PGE(2) and cAMP output. Immunoblotting, immunohistochemistry, and quantitative (q) RT-PCR consistently detected a significant decrease in aquaporin-2 (AQP2) protein expression in both the renal cortex and medulla of lithium-treated +/+ mice. This decrease was significantly attenuated in the -/- mice. qRT-PCR detected similar patterns of changes in AQP2 mRNA in the medulla but not in the cortex. Similarly, the total protein abundance of the Na-K-2Cl cotransporter (NKCC2) in the medulla but not in the cortex of the +/+ mice was significantly reduced by lithium treatment. In contrast, the dowregulation of renal medullary NKCC2 expression was significantly attenuated in the -/- mice. We conclude that mPGES-1-derived PGE(2) mediates lithium-induced polyuria likely via inhibition of AQP2 and NKCC2 expression.

Effect of chronic nNOS inhibition on blood pressure, vasoactivity, and arterial wall structure in Wistar rats

While the unequivocal pattern of endothelial nitric oxide (NO) synthase (eNOS) inhibition in cardiovascular control has been recognised, the role of NO produced by neuronal NOS (nNOS) remains unclear. The purpose of the present study was to describe the cardiovascular effects of NO production interference by inhibition of nNOS with 7-nitroindazole (7-NI). Wistar rats (10 weeks old) were used: control and experimental rats were administered 7-NI 10 mg/kg b.w./day in drinking water for 6 weeks. Systolic blood pressure (BP) was measured by the tail-cuff plethysmographic method. Isolated thoracic aortas (TAs) were used to study vasomotor activity of the conduit artery in vitro. The BP response of anaesthetised animals was used to follow the cardiovascular-integrated response in vivo. Geometry of the TA was measured after perfusion fixation (120 mm Hg) by light microscopy. Expression of eNOS was measured in the TA by immunoblot analysis. Although 6 weeks of nNOS inhibition did not alter systolic BP, the heart/body weight ratio was decreased. Relaxation of the TA in response to acetylcholine (10(-9)-10(-5)mol/L) was moderately inhibited. However, no difference in the BP hypotensive response after acetylcholine (0.1, 1, 10 microg) was observed. The contraction of TA in response to noradrenaline (10(-10)-10(-5)mol/L), and the BP pressor response to noradrenaline (0.1, 1 microg) was attenuated. The inner diameter of the TA was increased, and the wall thickness, wall cross-sectional area, and wall thickness/inner diameter ratio were decreased. The expression of eNOS in the TA was increased. In summary, cardiac and TA wall hypotrophy, underlined by decreased contractile efficiency, were observed. The results suggested that two constitutive forms of NOS (nNOS, eNOS) likely participate in regulation of cardiovascular tone by different mechanisms.

Isoforms and functions of NAD(P)H oxidase at the macula densa

Macula densa cells produce superoxide (O2-) during tubuloglomerular feedback primarily via NAD(P)H oxidase (NOX). The purpose of the present study was to determine NOXs expressed by the macula densa and the role of each one in NaCl-induced O2- production. To identify which isoforms are expressed, we applied single-cell RT-PCR to macula densa cells isolated by laser capture microdissection and to MMDD1 cells (a macula densa-like cell line). The captured cells expressed neuronal NOS (marker of macula densa), NOX2, and NOX4 but not NOX1. Expression of the NOXs and neuronal NOS was essentially identical in the MMDD1 cells. Thus, we used MMDD1 cells to investigate which isoform is responsible for NaCl-induced O2- production. We used small-interfering RNA to knock down NOX2 or NOX4 in MMDD1 cells and measured O2- exposed to low-salt solution (LS; 70 mmol/L of NaCl) or high-salt solution (HS; 140 mmol/L of NaCl). Exposing control cells (scrambled small-interfering RNA) to HS increased O2- concentrations from 0.75+/-0.28 to 1.48+/-0.46 U/min per 10(5) cells in LS and HS, respectively (P<0.001). Inhibiting NOX2 blocked the HS-induced increase in O2- (0.62+/-0.39 versus 0.76+/-0.31 U/min per 10(5) cells in LS and HS groups, respectively). Blocking NOX4 did not affect HS-induced O2- levels. O2- levels in the control cells during LS and HS were 0.80+/-0.30 and 1.56+/-0.49 U/min per 10(5) cells, respectively (P<0.001); whereas O2- levels in NOX4-small-interfering RNA-treated cells during LS and HS were 0.40+/-0.25 and 1.26+/-0.51 U/min per 10(5) cells, respectively (P<0.001). We conclude that, whereas macula densa cells express the NOX2 and NOX4 isoforms, NOX2 is primarily responsible for NaCl-induced O2- generation.

Cyclooxygenase 2 inhibition suppresses tubuloglomerular feedback: roles of thromboxane receptors and nitric oxide

Thromboxane (TxA(2)) and nitric oxide (NO) are potent vasoactive autocoids that modulate tubuloglomerular feedback (TGF). Each is produced in the macula densa (MD) by cyclooxygenase-2 (COX-2) and neuronal nitric oxide synthase (nNOS), respectively. Both enzymes are similarly regulated in the MD and their interaction may be an important factor in the regulation of TGF and glomerular filtration rate. We tested the hypothesis that TGF is modified by the balance between MD nNOS-dependent NO and MD COX-2-dependent TxA(2). We measured maximal TGF during perfusion of the loop of Henle (LH) by continuous recording of the proximal tubule stopped flow pressure response to LH perfusion of artificial tubular fluid (ATF) at 0 and 40 nl/min. The response to inhibitors of COX-1 (SC-560), COX-2 [parecoxib (Pxb)], and nNOS (l-NPA) added to the ATF solution was measured in separate nephrons. COX-2 inhibition with Pxb reduced TGF by 46% (ATF + vehicle vs. ATF + Pxb), whereas COX-1 inhibition with SC-560 reduced TGF by only 23%. Pretreatment with intravenous infusion of SQ-29,548, a selective thromboxone/PGH(2) receptor (TPR) antagonist, blocked all of the SC-560 effect on TGF, suggesting that this effect was due to activation of TPR. However, SQ-29,548 only partially diminished the effect of Pxb (-66%). Specific inhibition of nNOS with l-NPA increased TGF, as expected. However, the ability of Pxb to reduce TGF was significantly impaired with comicroperfusion of l-NPA. These data suggest that COX-2 modulates TGF by two proconstrictive actions: generation of TxA(2) acting on TPR and by simultaneous reduction of NO.

Diminished NO generation by injured endothelium and loss of macula densa nNOS may contribute to sustained acute kidney injury after ischemia-reperfusion

In postischemic acute kidney injury (AKI) or acute renal failure, a dissipation of glomerular filtration pressure is associated with an altered renal vascular tone and reactivity, as well as a loss of vascular autoregulation. To test the hypothesis that renal nitric oxide (NO) generation reflects endothelial damage in the kidney after ischemia-reperfusion, we quantified the urinary NO levels and identified the site of its generation in postischemic AKI. Subjects were 50 recipients of cadaveric renal allografts: 15 with sustained AKI and 35 with recovering renal function. Urine and blood samples were obtained after transplant, and intraoperative allograft biopsies were performed to examine NO synthases (NOSs) in the kidney. In the sustained AKI group, urinary nitrite and nitrate excretion (in mumol/g urine creatinine) was lower (12.3 +/- 1.8 and 10.0 +/- 1.4 on postoperative days 0 and 3) than in the recovery group [20.0 +/- 3.6 and 35.1 +/- 5.3 (P < 0.005 vs. sustained AKI on days 0 and 3) on postoperative days 0 and 3]. Endothelial NOS expression diminished from the peritubular capillaries of 6 of 7 subjects in the sustained AKI group but from only 6 of 16 subjects in the recovery group. No differences were observed in the inducible NOS staining pattern between the two groups. Neuronal NOS staining was rarely observed in the macula densae of subjects but was prominent in control tissues. These findings suggest that a diminished NO generation by injured endothelium and loss of macula densa neuronal NOS could impair the vasodilatory ability of the renal vasculature and contribute to the reduction in the glomerular filtration rate in postischemic AKI.

Vasopressin regulates the renin-angiotensin-aldosterone system via V1a receptors in macula densa cells

The neuropeptide hormone arginine-vasopressin (AVP) is well known to exert its antidiuretic effect via the vasopressin V2 receptor (V2R), whereas the role of the vasopressin V1a receptor (V1aR) in the kidney remains to be clarified. Previously, we reported decreased plasma volume and blood pressure in V1a receptor-deficient (V1aR-/-) mice (Koshimizu T, Nasa Y, Tanoue A, Oikawa R, Kawahara Y, Kiyono Y, Adachi T, Tanaka T, Kuwaki T, Mori T. Proc Natl Acad Sci USA 103: 7807-7812, 2006). In this study, we investigated the role of V1aR in urine concentration, renal function, and the renin-angiotensin system (RAS) using V1aR-/- mice. Urine volume of V1aR-/- mice was greater than that of wild-type mice, particularly when water was loaded, while the glomerular filtration rate (GFR), urinary NaCl excretion, AVP-dependent cAMP generation, V2R, and aquaporin 2 (AQP2) expression in the kidney were lower, indicating that the diminished GFR and V2R-AQP2 system led to impaired urinary concentration in V1aR-/- mice. Since the GFR and V2R-AQP2 system are regulated by RAS, we analyzed renin and angiotensin II in V1aR-/- mice and found that the plasma renin and angiotensin II were decreased. The expression of renin in granule cells was decreased in V1aR-/- mice, which led to a decreased level of plasma renin. In addition, the expression of renin stimulators such as neuronal nitric oxide synthase and cyclooxygenase-2 in macula densa (MD) cells, where V1aR was specifically expressed, was decreased in V1aR-/- mice. These data indicate that AVP regulates body fluid homeostasis and GFR via the V1aR in MD cells by activating RAS and subsequently the V2R-AQP2 system.

Different effects of 26-week dietary intake of rapeseed oil and soybean oil on plasma lipid levels, glucose-6-phosphate dehydrogenase activity and cyclooxygenase-2 expression in spontaneously hypertensive rats

We intended to determine whether or not dietary canola oil (CO) elevates plasma lipids and oxidative stress, since both of these are, possibly, related to the CO-induced life shortening through exacerbation of hypertension-associated vascular lesions found in stroke-prone spontaneously hypertensive rats (SHRSP). Spontaneously hypertensive rats (SHR) were used in this study to avoid a potential bias in the results due to the irregular death by stroke seen in SHRSP. SHR were fed for 26 weeks on a chow containing either, 10 wt/wt% of CO or soybean oil (SO), i.e., the control. Elevated plasma lipids and glucose-6-phosphate dehydrogenase (G6PD) activation in the liver and erythrocyte were found in SHR fed CO compared to that fed SO, while anti-oxidative enzymes other than G6PD were not activated. The CO diet brought about significant vascular lesions in the kidney, in which abundant cyclooxygenase-2 (COX-2) positive foci were immunochemically located in the juxtaglomerular apparatus. These results suggest that dietary CO induces a hyperlipidemic condition, in which G6PD may serve as an NADPH provider, and aggravates genetic diseases in SHR (also, probably, in SHRSP). The increased COX-2 expression indicates a role of renin-angiotensin-aldosterone system activation in the increased vascular lesions, whereas the effects of oxidative stress remain unclear.

Developmental regulation of calcineurin isoforms in the rodent kidney: association with COX-2

Calcineurin (Cn)-Aα-deficient mice develop abnormalities of postnatal kidney development, similar to that of cyclooxygenase (COX)-2-deficient mice. The present study was undertaken to examine expression and regulation of Cn isoforms in the developing kidney during the postnatal period and further characterize the relationship between Cn and COX-2. The protein expressions of all three Cn isoforms, including Cn-Aα, -Aβ, and -B, as determined by immunoblotting, increased in parallel in the first postnatal week and declined gradually with age. Renal Cn-Aα and -Aβ mRNA expressions were both developmentally regulated in the same fashion as their protein expressions, whereas renal Cn-B1 mRNA was not obviously induced in the first postnatal week. Immunohistochemistry demonstrated colocalization of Cn-Aα, Cn-Aβ, and COX-2 in the same cells of thick ascending limb and macula densa. Administration with cyclosporine A (2.5 mg·kg−1·day−1) during the postnatal period remarkably suppressed renal COX-2 expression as assessed by both immunoblotting and immunohistochemistry. Deletion of Cn-Aα but not Cn-Aβ in mice significantly reduced renal COX-2 expression at the postnatal period. Together, these data suggest that renal Cn isoforms are subject to normal developmental regulation and they may play a role in postnatal kidney development via interaction with COX-2.

Mouse models and the urinary concentrating mechanism in the new millennium

Our understanding of urinary concentrating and diluting mechanisms at the end of the 20th century was based largely on data from renal micropuncture studies, isolated perfused tubule studies, tissue analysis studies and anatomical studies, combined with mathematical modeling. Despite extensive data, several key questions remained to be answered. With the advent of the 21st century, a new approach, transgenic and knockout mouse technology, is providing critical new information about urinary concentrating processes. The central goal of this review is to summarize findings in transgenic and knockout mice pertinent to our understanding of the urinary concentrating mechanism, focusing chiefly on mice in which expression of specific renal transporters or receptors has been deleted. These include the major renal water channels (aquaporins), urea transporters, ion transporters and channels (NHE3, NKCC2, NCC, ENaC, ROMK, ClC-K1), G protein-coupled receptors (type 2 vasopressin receptor, prostaglandin receptors, endothelin receptors, angiotensin II receptors), and signaling molecules. These studies shed new light on several key questions concerning the urinary concentrating mechanism including: 1) elucidation of the role of water absorption from the descending limb of Henle in countercurrent multiplication, 2) an evaluation of the feasibility of the passive model of Kokko-Rector and Stephenson, 3) explication of the role of inner medullary collecting duct urea transport in water conservation, 4) an evaluation of the role of tubuloglomerular feedback in maintenance of appropriate distal delivery rates for effective regulation of urinary water excretion, and 5) elucidation of the importance of water reabsorption in the connecting tubule versus the collecting duct for maintenance of water balance.

Inflammation and hepatic encephalopathy: ibuprofen restores learning ability in rats with portacaval shunts

One of the neurological alterations in patients with minimal or overt hepatic encephalopathy is cognitive impairment. This impairment is reproduced in rats with chronic liver failure due to portacaval shunt (PCS). These rats show decreased ability to learn a conditional discrimination task in a Y-maze, likely due to reduced function of the glutamate-nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) pathway in brain. It has been proposed that inflammation exacerbates the neuropsychological alterations induced by hyperammonemia, suggesting that inflammation-associated alterations may contribute to cognitive impairment in hepatic encephalopathy. This study assessed whether treatment with an anti-inflammatory drug, ibuprofen, is able to restore the function of the glutamate-NO-cGMP pathway in cerebral cortex in brain in vivo and/or learning ability in PCS rats. We show that PCS rats have increased levels of interleukin-6 and increased activities of cyclooxygenase and of inducible NO synthase in cerebral cortex, indicating the presence of inflammation. Chronic treatment with ibuprofen normalizes cyclooxygenase and inducible NO synthase activities but not interleukin-6 levels. Moreover, ibuprofen normalizes the function of the glutamate-NO-cGMP pathway in cerebral cortex in vivo and completely restores the ability of rats with chronic liver failure to learn the Y-maze task. This supports that inflammation contributes to the cognitive impairment in hepatic encephalopathy.

CONCLUSION

the results reported point to the possible therapeutic utility of decreasing inflammation in the treatment of the cognitive deficits in patients with minimal or overt hepatic encephalopathy.

Prostaglandin E2 inhibits vasotocin-induced osmotic water permeability in the frog urinary bladder by EP1-receptor-mediated activation of NO/cGMP pathway

PGE(2) is a well-known inhibitor of the antidiuretic hormone-induced increase of osmotic water permeability (OWP) in different osmoregulatory epithelia; however, the mechanisms underlying this effect of PGE(2) are not completely understood. Here, we report that, in the frog Rana temporaria urinary bladder, EP(1)-receptor-mediated inhibition of arginine-vasotocin (AVT)-induced OWP by PGE(2) is attributed to increased generation of nitric oxide (NO) in epithelial cells. It was shown that the inhibitory effect of 17-phenyl-trinor-PGE(2) (17-ph-PGE(2)), an EP(1) agonist, on AVT-induced OWP was significantly reduced in the presence of 7-nitroindazole (7-NI), a neuronal NO synthase (nNOS) inhibitor. NO synthase (NOS) activity in both lysed and intact epithelial cells measured as a rate of conversion of l-[(3)H]arginine to l-[(3)H]citrulline was Ca(2+) dependent and inhibited by 7-NI. PGE(2) and 17-ph-PGE(2), but not M&B-28767 (EP(3) agonist) or butaprost (EP(2) agonist), stimulated NOS activity in epithelial cells. The above effect of PGE(2) was abolished in the presence of SC-19220, an EP(1) antagonist. 7-NI reduced the stimulatory effect of 17-ph-PGE(2) on NOS activity. 17-ph-PGE(2) increased intracellular Ca(2+) concentration and cGMP in epithelial cells. Western blot analysis revealed an nNOS expression in epithelial cells. These results show that the inhibitory effect of PGE(2) on AVT-induced OWP in the frog urinary bladder is based at least partly on EP(1)-receptor-mediated activation of the NO/cGMP pathway, suggesting a novel cross talk between AVT, PGE(2), and nNOS that may be important in the regulation of water transport.

Molecular mechanisms involved in the reciprocal regulation of cyclooxygenase and nitric oxide synthase enzymes

The nitric oxide (NO) and cyclooxygenase (COX) pathways share a number of similarities. NO is the mediator generated from the NO synthase (NOS) pathway and COX converts arachidonic acid to prostaglandins (PGs), prostacyclin, and thromboxane A2. Two major forms of NOS and COX have been identified to date. The constitutive isoforms of these enzymes play an important role in the regulation of several physiological states. On the other hand, in an inflammatory setting, the inducible isoforms of these enzymes are induced in a variety of cells resulting in the production of large amounts NO and PGs, which play pathological roles in several disease states. An important link between the NOS and COX pathways was made by our group when we demonstrated that NO activates the COX enzymes, an event leading to overt production of PGs, suggesting that COX enzymes represent important endogenous 'receptor' targets for modulating the multifaceted roles of NO. More importantly, mechanistic studies of how NO activates the COX enzymes have been undertaken and additional pathways through which NO modulates PG production unraveled. The purpose of this article is to cover the advances, which have occurred over the years and in particular to summarize experimental data that outline how the discovery that NO modulates PG production has impacted and extended our understanding of these two systems in physiopathological events.

Low plasma renin and reduced renin secretory responses to acute stimuli in conscious COX-2-deficient mice

In the current experiments, we determined the response of plasma renin concentration (PRC) to acute intraperitoneal administration of furosemide (40 mg/kg), hydralazine (2 mg/kg), isoproterenol (10 mg/kg), candesartan (50 μg), or quinaprilate (50 μg) in conscious wild-type (WT) and cyclooxygenase (COX)-2−/− mice on three different genetic backgrounds (mixed, C57BL/6, 129J). PRC was measured in plasma obtained by tail vein puncture. Basal PRC was significantly lower in COX-2−/− than WT mice independent of genetic background (51, 10, and 17% of WT in mixed, 129J, and C57BL/6). All five acute interventions caused significant increases of PRC in both COX-2+/+ and −/− mice, but the response was consistently less in COX-2-deficient mice (e.g., ΔPRC in ng ANG I·ml−1·h−1 caused by furosemide, isoproterenol, hydralazine, quinaprilate, or candesartan 4,699 ± 544, 3,534 ± 957, 2,522 ± 369, 9,453 ± 1,705, 66,455 ± 21,938 in 129J WT, and 201 ± 78, 869 ± 275, 140 ± 71, 902 ± 304, 2,660 ± 954 in 129J COX-2−/−). A low-NaCl diet and enalapril for 1 wk caused a 14-fold elevation of PRC in COX-2−/− mice and was associated with a greatly increased PRC response to acute furosemide (ΔPRC 201 ± 78 before and 15,984 ± 2,397 after low Na/enalapril). As measured by radiotelemetry, blood pressure and heart rate responses to furosemide, hydralazine, isoproterenol, candesartan, or quinaprilate were not different between COX-2 genotypes. In conclusion, chronic absence of COX-2 reduces renin expression, release, and PRC and is associated with a reduced ability to alter PRC during acute stimulation regardless of the nature of the stimulus. COX-2 activity does not appear to be a mandatory and specific requirement for furosemide-stimulated renin secretion.

Deletion of microsomal prostaglandin E synthase-1 increases sensitivity to salt loading and angiotensin II infusion

Microsomal prostaglandin E synthase-1 (mPGES-1), a membrane-associated protein, is critically involved in the inflammatory response and may be involved in physiological processes as well. The present study examined the role of mPGES-1 in regulation of sodium balance and blood pressure in the settings of salt loading and angiotensin II infusion. mPGES-1 -/- mice developed severe and progressive hypertension associated with an inappropriate increase in sodium balance when fed a high-salt diet. These mice exhibited a significantly impaired ability to excrete an acute enteral load of NaCl. Under these 2 settings of salt loading, urinary excretion of prostaglandin E(2) and nitrate/nitrite were remarkably increased in wild-type animals but not in mPGES-1 -/- mice. The changes of urinary cGMP paralleled that of urinary nitrate/nitrite. mPGES-1 -/- mice exhibited a remarkable inhibition of high salt-induced increase in gene expression of all 3 NO synthase isoforms, whereas these mice had upregulated expression of NO synthase III but not NO synthase I and NO synthase II at basal state. Chronic salt loading remarkably induced mPGES-1 protein expression exclusively in the distal nephron. In primary cultures of CD cells, mPGES-1 expression was significantly increased following exposure to hypertonic NaCl, in parallel with increased prostaglandin E(2) release. These findings have revealed a mPGES-1/prostaglandin E(2)/NO/cGMP pathway that appears to be critically important for salt adaptation. In addition, we provide evidence that mPGES-1 deficiency sensitized the hypertensive effect of angiotensin II. Overall, this study has characterized the natriuretic and antihypertensive role of mPGES-1 that likely contributes to blood pressure homeostasis.

Role of renal cortical cyclooxygenase-2 expression in hyperfiltration in rats with high-protein intake

Renal cortical cyclooxygenase-2 (COX-2) is restricted to the macula densa and adjacent cortical thick ascending limbs (MD/cTALH). Renal cortical COX-2 increases in response to diabetes and renal ablation, both of which are characterized by hyperfiltration and reduced NaCl delivery to the MD due to increased proximal NaCl reabsorption. High-protein intake also induces hyperfiltration and decreases NaCl delivery to the MD due to increased NaCl reabsorption proximally. We investigated whether high protein induces cortical COX-2 and whether cortical COX-2 contributes to high protein-induced hyperfiltration and increased intrarenal renin biosynthesis. Cortical COX-2 increased after protein loading but decreased after protein restriction. COX-2 inhibition attenuated high protein-induced hyperfiltration but had no effect on high protein-induced intrarenal renin elevation. Therefore, induction of cortical COX-2 contributed to high protein-induced hyperfiltration but not intrarenal renin elevation. In the kidney cortex, neuronal nitric oxide synthase (nNOS) is also localized to the MD, and interactions between intrarenal nNOS and COX-2 systems have been proposed. Cortical COX-2 elevation seen in salt restriction was blocked by nNOS inhibiton. Cortical nNOS expression also increased after protein loading, and inhibition of nNOS activity completely reversed high protein-induced cortical COX-2 elevation and hyperfiltration. These results indicate that NO is a mediator of high protein-induced cortical COX-2 elevation and suggest that both intrarenal nNOS and COX-2 systems appear to regulate afferent arteriolar tone and subsequent hyperfiltration seen in high-protein intake.

Renal cortical regulation of COX-1 and functionally related products in early renovascular hypertension (rat)

Renal volume regulation is modulated by the action of cyclooxygenases (COX) and the resulting generation of prostanoids. Epithelial expression of COX isoforms in the cortex directs COX-1 to the distal convolutions and cortical collecting duct, and COX-2 to the thick ascending limb. Partly colocalized are prostaglandin E synthase (PGES), the downstream enzyme for renal prostaglandin E(2) (PGE(2)) generation, and the EP receptors type 1 and 3. COX-1 and related components were studied in two kidney-one clip (2K1C) Goldblatt hypertensive rats with combined chronic ANG II or bradykinin B(2) receptor blockade using candesartan (cand) or the B(2) antagonist Hoechst 140 (Hoe). Rats (untreated sham, 2K1C, sham + cand, 2K1C + cand, sham + Hoe, 2K1C + Hoe) were treated to map expression of parameters controlling PGE(2) synthesis. In 2K1C, cortical COX isoforms did not change uniformly. COX-2 changed in parallel with NO synthase 1 (NOS1) expression with a raise in the clipped, but a decrease in the nonclipped side. By contrast, COX-1 and PGES were uniformly downregulated in both kidneys, along with reduced urinary PGE(2) levels, and showed no clear relations with the NO status. ANG II receptor blockade confirmed negative regulation of COX-2 by ANG II but blunted the decrease in COX-1 selectively in nonclipped kidneys. B(2) receptor blockade reduced COX-2 induction in 2K1C but had no clear effect on COX-1. We suggest that in 2K1C, COX-1 and PGES expression may fail to oppose the effects of renovascular hypertension through reduced prostaglandin signaling in late distal tubule and cortical collecting duct.

Nitric oxide stimulates cyclooxygenase-2 in cultured cTAL cells through a p38-dependent pathway

To examine the interaction of nitric oxide (NO) and cyclooxygenase (COX-2) and the signaling pathway involved, primary cultured rabbit cortical thick ascending limb (cTAL) were used. In these cells, immunoreactive COX-2 and vasodilatory prostaglandins were increased by a NO donor, S-nitros- N-acetylpenicillamine (SNAP; 2.5 ± 0.3-fold control, n = 6, P < 0.01). SNAP increased expression of phosphorylated p38 (pp38; 2.4 ± 0.3-fold control; n = 5; P < 0.01), which was inhibited by the p38 inhibitor SB-203580 (1.3 ± 0.1-fold control, n = 5, P < 0.01). SB-203580 inhibited SNAP-induced COX-2 expression [1.4 ± 0.2-fold control, n = 6, not significant (NS) vs. control] and levels of PGE2significantly. In cTAL cells transfected with a luciferase reporter driven by the wild-type mouse COX-2 promoter, SNAP stimulated luciferase activity, which was reversed by SB-203580 (control vs. SNAP vs. SNAP + SB-203580: 1.4 ± 0.2-, 8.3 ± 1.4-, and 0.4 ± 0.1-fold control, respectively, n = 4, P < 0.01). Electrophoretic mobility shift assay indicated that SNAP stimulated nuclear factor (NF)-κB binding activity in cTAL that was also inhibited by the p38 inhibitor. SNAP was not able to stimulate a mutant COX-2 promoter construct that is not activated by NF-κB (0.9 ± 0.1, 1.2 ± 0.1, and 1.0 ± 0.2 respectively, n = 4, NS). Low chloride increased COX-2 expression (2.7 ± 0.4-fold control, n = 6, P < 0.01) and pp38 expression (2.8 ± 0.3-fold; n = 5, P < 0.01), which were reversed by the specific NO synthase (NOS) inhibitor 7-nitroindazole. Administration of a low-salt diet increased immunoreactive COX-2 and neuronal NOS (nNOS) in the macula densa and surrounding cTAL of kidneys of wild-type mice but did not significantly elevate COX-2 expression in nNOS−/−mice. In summary, these studies indicate that, in cTAL, NO can increase COX-2 expression in cTAL and macula densa through p38-dependent signaling pathways via activation of NF-κB.

Nitric oxide stimulates COX-2 expression in cultured collecting duct cells through MAP kinases and superoxide but not cGMP

Collecting ducts are a major site of renal production and action of both prostaglandins and nitric oxide. Experiments were undertaken to examine whether nitric oxide regulates cyclooxygenase (COX)-2 expression and PGE(2) release in cultured collecting duct cells. In mIMCD-K2 cells, sodium nitroprusside (SNP) in the 50- to 800-microM range induced a marked dose- and time-dependent increase in COX-2 protein levels, determined by immunoblotting, and the induction was detectable at 4 h. This was preceded by induction of COX-2 mRNA as determined by real-time-RT-PCR. The COX-2 induction was accompanied by a significant rise in PGE(2) release as determined by enzyme immunoassay. S-nitroso-N-acetylpenicillamine (SNAP) had a similar stimulatory effect on COX-2 expression and PGE(2) release. 8-bromo-cGMP (200 microM) had no effect on COX-2 expression. The SNP-stimulated COX-2 expression was not affected by the guanylyl cyclase inhibitor methylene blue or the protein kinase G inhibitor KT-5823 (2.0 microM). In contrast, the SNP-stimulated COX-2 expression was significantly reduced by either the Erk1/2 inhibitor PD-98059 or the P38 inhibitor SB-203580 and was abolished by combination of the two kinase inhibitors. The stimulation was also significantly blocked by the SOD mimetic tempol. Thus we conclude that NO stimulates COX-2 expression in collecting duct cells through mechanisms involving MAP kinase and superoxide, but not cGMP.