Prostaglandin E receptors and the kidney

High salt induces cognitive impairment via the interaction of the angiotensin II-AT1 and prostaglandin E2-EP1 systems

BACKGROUND AND PURPOSE

High salt (HS) intake has been associated with hypertension and cognitive impairment. It is well known that the angiotensin II (Ang II)-AT1 receptor and prostaglandin E2 (PGE2)-EP1 receptor systems are involved in hypertension and neurotoxicity. However, the involvement of these systems in HS-mediated hypertension and emotional and cognitive impairments remains unclear.

EXPERIMENTAL APPROACH

Mice were loaded with HS solution (2% NaCl drinking water) for 12 weeks, and blood pressure was monitored. Subsequently, effects of HS intake on emotional and cognitive function and tau phosphorylation in the prefrontal cortex (PFC) and hippocampus (HIP) were investigated. The involvement of Ang II-AT1 and PGE2-EP1 systems in HS-induced hypertension and neuronal and behavioural impairments was examined by treatment with losartan, an AT1 receptor blocker (ARB), or EP1 gene knockout.

KEY RESULTS

We demonstrate that hypertension and impaired social behaviour and object recognition memory following HS intake may be associated with tau hyperphosphorylation, decreased phosphorylation of Ca2+ /calmodulin-dependent protein kinase II (CaMKII), and postsynaptic density protein 95 (PSD95) expression in the PFC and HIP of mice. These changes were blocked by pharmacological treatment with losartan or EP1 receptor gene knockout.

CONCLUSIONS AND IMPLICATIONS

Our findings suggest that the interaction of Ang II-AT1 receptor and PGE2-EP1 receptor systems could be novel therapeutic targets for hypertension-induced cognitive impairment.

The diuretic effect of ethyl acetate fractions of Artocarpus altilis, Artocarpus champeden, and Artocarpus heterophyllus leaves in normotensive Wistar rats

BACKGROUND

Artocarpus altilis, A. Champeden, and Artocarpus heterophylus are popular species in Indonesia, and are commonly used as traditional medicine.

OBJECTIVE

This study aims to evaluate the diuretic effects of the ethyl acetate fraction of these three species on normotensive Wistar rats.

METHODS

The ethyl acetate fraction was prepared by a liquid-liquid extraction method. To evaluate diuretic effects, the sixty rats were divided into normal (distilled water), negative (4.5% NaCl), positive control (furosemide 5 mg/kg), and the testing groups. The testing groups were orally given the ethyl acetate fraction of A. altilis, Artocarpus champeden, and A. heterophylus at three dose levels of 25, 50, and 100 mg/kg. All animals were orally given 4.5% NaCl at a dose of 2 mL/200 g except the normal group; then the animals were given drugs according to group doses. Urine volume and electrolyte levels produced by the testing groups were compared to those of the control group. The concentration and ratio of ions were calculated to determine the natriuretic and carbonic anhydrase activities.

RESULTS

The ethyl acetate fractions of the three Artocarpus species at 100 mg/kg dose were more active than the standard furosemide (p < 0.05) to increase urinary excretion. Furthermore, at doses of 50 and 100 mg/kg, the fraction significantly increased the excretion of Na+, K+, and Cl- ions more than the standard (p < 0.05). The testing groups showed good natriuretic and carbonic anhydrase activities.

CONCLUSIONS

The ethyl acetate fraction of A. altilis, A. champeden, and A. heterophylus leaves performed good diuretic and natriuretic activities. Therefore, the fractions can be considered as potential natural diuretic medicines.

Dynamics of differentiated-renal epithelial cell monolayer after calcium oxalate injury: The role of cyclooxygenase-2

AIMS

Calcium oxalate (Oxa), constituent of most common kidney stones, damages renal tubular epithelial cells leading to kidney disease. Most in vitro studies designed to evaluate how Oxa exerts its harmful effects were performed in proliferative or confluent non-differentiated renal epithelial cultures; none of them considered physiological hyperosmolarity of renal medullary interstitium. Cyclooxygenase 2 (COX2) has been associated to Oxa deleterious actions; however, up to now, it is not clear how COX2 acts. In this work, we proposed an in vitro experimental system resembling renal differentiated-epithelial cells that compose medullary tubular structures which were grown and maintained in a physiological hyperosmolar environment and evaluated whether COX2 → PGE2 axis (COX2 considered a cytoprotective protein for renal cells) induces Oxa damage or epithelial restitution.

MAIN METHODS

MDCK cells were differentiated with NaCl hyperosmolar medium for 72 h where cells acquired the typical apical and basolateral membrane domains and a primary cilium. Then, cultures were treated with 1.5 mM Oxa for 24, 48, and 72 h to evaluate epithelial monolayer restitution dynamics and COX2-PGE2 effect.

KEY FINDINGS

Oxa completely turned the differentiated phenotype into mesenchymal one (epithelial-mesenchymal transition). Such effect was partially and totally reverted after 48 and 72 h, respectively. Oxa damage was even deeper when COX2 was blocked by NS398. PGE2 addition restituted the differentiated-epithelial phenotype in a time and concentration dependence.

SIGNIFICANCE

This work presents an experimental system that approaches in vitro to in vivo renal epithelial studies and, more important, warns about NSAIDS use in patients suffering from kidney stones.

Vaptans or voluntary increased hydration to protect the kidney: how do they compare?

The adverse effects of vasopressin (AVP) in diverse forms of chronic kidney disease have been well described. They depend on the antidiuretic action of AVP mediated by V2 receptors (V2R). Tolvaptan, a selective V2R antagonist, is now largely used for the treatment of patients with autosomal dominant polycystic kidney disease. Another way to reduce the adverse effects of AVP is to reduce endogenous AVP secretion by a voluntary increase in fluid intake. These two approaches differ in several ways, including the level of thirst and AVP. With voluntary increased drinking, plasma osmolality will decline and so will AVP secretion. Thus, not only will V2R-mediated effects be reduced, but also those mediated by V1a and V1b receptors (V1aR and V1bR). In contrast, selective V2R antagonism will induce a loss of fluid that will stimulate AVP secretion and thus increase AVP's influence on V1a and V1b receptors. V1aR is expressed in the luminal side of the collecting duct (CD) and in inner medullary interstitial cells, and their activation induces the production of prostaglandins, mostly prostaglandin E2 (PGE2). Intrarenal PGE2 has been shown to reduce sodium and water reabsorption in the CD and increase blood flow in the renal medulla, both effects contributing to increase sodium and water excretion and reduce urine-concentrating activity. Conversely, non-steroidal anti-inflammatory drugs have been shown to induce significant water and sodium retention and potentiate the antidiuretic effects of AVP. Thus, during V2R antagonism, V1aR-mediated actions may be responsible for part of the diuresis observed with this drug. These V1aR-dependent effects do not take place with a voluntary increase in fluid intake. In summary, while both strategies may have beneficial effects, the information reviewed here leads us to assume that pharmacological V2R antagonism, with resulting stimulation of V1aR and increased PGE2 production, may provide greater benefit than voluntary high water intake. The influence of tolvaptan on the PGE2 excretion rate and the possibility to use somewhat lower tolvaptan doses than presently prescribed remain to be evaluated.

The influence of cyclooxygenase inhibitors on kynurenic acid production in rat kidney: a novel path for kidney protection?

BACKGROUND

Kidney diseases have become a global health problem, affecting about 15% of adults and being often under-recognized. Immunological system activation was shown to accelerate kidney damage even in inherited disorders. The kynurenine pathway is the main route of tryptophan degradation. A metabolite of kynurenine (KYN), kynurenic acid (KYNA), produced by kynurenine aminotransferases (KATs), was reported to affect fluid and electrolyte balance as a result of natriuresis induction. The accumulation of KYNA was shown in patients with impaired kidney function and its level was related to the degree of kidney damage. Cyclooxygenase (COX) inhibitors are well-known analgesics and most of them demonstrate an anti-inflammatory effect. Their main mechanism of action is prostaglandin synthesis blockade, which is also responsible for their nephrotoxic potential. Since the KYN pathway is known to remain under immunological system control, the purpose of this study was to analyze the effect of 9 COX inhibitors on KYNA production together with KATs' activity in rat kidneys in vitro.

METHODS

Experiments were carried out on kidney homogenates in the presence of L-KYN and the selected compound in 6 various concentrations.

RESULTS

Among the examined COX inhibitors only acetaminophen did not change KYNA production in rat kidneys in vitro. Additionally, acetaminophen did not affect the activity of KAT I and KAT II, whereas acetylsalicylic acid and ibuprofen inhibited only KAT II. The remaining COX inhibitors decreased the activity of both KATs in rat kidneys in vitro.

CONCLUSION

Our study provides novel mechanisms of COX inhibitors action in the kidney, with possible implications for the treatment of kidney diseases.

Renal sympathetic activity: A key modulator of pressure natriuresis in hypertension

Hypertension is a complex disorder ensuing necessarily from alterations in the pressure-natriuresis relationship, the main determinant of long-term control of blood pressure. This mechanism sets natriuresis to the level of blood pressure, so that increasing pressure translates into higher osmotically driven diuresis to reduce volemia and control blood pressure. External factors affecting the renal handling of sodium regulate the pressure-natriuresis relationship so that more or less natriuresis is attained for each level of blood pressure. Hypertension can thus only develop following primary alterations in the pressure to natriuresis balance, or by abnormal activity of the regulation network. On the other hand, increased sympathetic tone is a very frequent finding in most forms of hypertension, long regarded as a key element in the pathophysiological scenario. In this article, we critically analyze the interplay of the renal component of the sympathetic nervous system and the pressure-natriuresis mechanism in the development of hypertension. A special focus is placed on discussing recent findings supporting a role of baroreceptors as a component, along with the afference of reno-renal reflex, of the input to the nucleus tractus solitarius, the central structure governing the long-term regulation of renal sympathetic efferent tone.

Prostanoid receptors in hyperfiltration-mediated glomerular injury: Novel agonists and antagonists reveal opposing roles for EP2 and EP4 receptors

Increased fluid-flow shear stress (FFSS) contributes to hyperfiltration-induced podocyte and glomerular injury resulting in progression of chronic kidney disease (CKD). We reported that increased FFSS in vitro and in vivo upregulates PGE2 receptor EP2 (but not EP4 expression), COX2-PGE₂ -EP2 axis, and EP2-linked Akt-GSK3β-β-catenin signaling pathway in podocytes. To understand and use the disparities between PGE2 receptors, specific agonists, and antagonists of EP2 and EP4 were used to assess phosphorylation of Akt, GSK3β and β-catenin in podocytes using Western blotting, glomerular filtration barrier function using in vitro albumin permeability (P_alb ) assay, and mitigation of hyperfiltration-induced injury in unilaterally nephrectomized (UNX) mice at 1 and 6 months. Results show an increase in P_alb by PGE₂ , EP2 agonist (EP2^AGO ) and EP4 antagonist (EP4^ANT ), but not by EP2 antagonist (EP2^ANT ) or EP4 agonist (EP4^AGO ). Pretreatment with EP2^ANT blocked the effect of PGE₂ or EP2^AGO on P_alb . Modulation of EP2 and EP4 also induced opposite effects on phosphorylation of Akt and β-Catenin. Individual agonists or antagonists of EP2 or EP4 did not induce significant improvement in albuminuria in UNX mice. However, treatment with a combination EP2^ANT  + EP4^AGO for 1 or 6 months caused a robust decrease in albuminuria. EP2^ANT  + EP4^AGO combination did not impact adaptive hypertrophy or increased serum creatinine. Observed differences between expression of EP2 and EP4 on the glomerular barrier highlight these receptors as potential targets for intervention. Safe and effective mitigating effect of EP2^ANT  + EP4^AGO presents a novel opportunity to delay the progression of hyperfiltration-associated CKD as seen in transplant donors.

Hormone-Dependent Regulation of Renin and Effects on Prorenin Receptor Signaling in the Collecting Duct

The production of renin by the principal cells of the collecting duct has widened our understanding of the regulation of intrarenal angiotensin II (Ang II) generation and blood pressure. In the collecting duct, Ang II increases the synthesis and secretion of renin by mechanisms involving the activation of Ang II type 1 receptor (AT1R) via stimulation of the PKCα, Ca²⁺, and cAMP/PKA/CREB pathways. Additionally, paracrine mediators, including vasopressin (AVP), prostaglandins, bradykinin (BK), and atrial natriuretic peptide (ANP), regulate renin in principal cells. During Ang II-dependent hypertension, despite plasma renin activity suppression, renin and prorenin receptor (RPR) are upregulated in the collecting duct and promote de novo formation of intratubular Ang II. Furthermore, activation of PRR by its natural agonists, prorenin and renin, may contribute to the stimulation of profibrotic factors independent of Ang II. Thus, the interactions of RAS components with paracrine hormones within the collecting duct enable tubular compartmentalization of the RAS to orchestrate complex mechanisms that increase intrarenal Ang II, Na+ reabsorption, and blood pressure.

Na+-Retaining Action of COX-2 (Cyclooxygenase-2)/EP1 Pathway in the Collecting Duct via Activation of Intrarenal Renin-Angiotensin-Aldosterone System and Epithelial Sodium Channel

BACKGROUND

The collecting duct (CD) is a major site of both biosynthesis and action of prostaglandin E₂ as highlighted by the predominant expression of COX-2 (cyclooxygenase-2) and some E-prostanoid (EP) subtypes at this nephron site. The purpose of this study was to determine the relevance and mechanism of CD COX-2/prostaglandin E₂/EP₁ signaling for the regulation of Na⁺ hemostasis during Na⁺ depletion.

METHODS

Mice with Aqp2Cre-driven deletion of COX-2 (COX-2^fl/flAqp2Cre⁺) or the EP₁ subtype (EP₁^fl/flAqp2Cre⁺) were generated and the Na⁺-wasting phenotype of these mice during low-salt (LS) intake was examined. EP subtypes responsible for prostaglandin E₂-induced local renin response were analyzed in primary cultured mouse inner medullary CD cells.

RESULTS

Following 28-day LS intake, COX-2^fl/flAqp2Cre⁺ mice exhibited a higher urinary Na⁺ excretion and lower cumulative Na⁺ balance, accompanied with suppressed intrarenal renin, AngII (angiotensin II), and aldosterone, expression of CYP11B2 (cytochrome P450 family 11 subfamily B member 2), and blunted expression of epithelial sodium channel subunits compared to floxed controls (COX-2^fl/flAqp2Cre^-), whereas no differences were observed for indices of systemic renin-angiotensin-aldosterone system. In cultured CD cells, exposure to prostaglandin E₂ stimulated release of soluble (pro)renin receptor, prorenin/renin and aldosterone and the stimulation was more sensitive to antagonism of EP₁ as compared other EP subtypes. Subsequently, EP₁^fl/flAqp2Cre⁺ mice largely recapitulated Na⁺-wasting phenotype seen in COX-2^fl/flAqp2Cre⁺ mice.

CONCLUSIONS

The study for the first time reports that CD COX-2/EP₁ pathway might play a key role in maintenance of Na⁺ homeostasis in the face of Na⁺ depletion, at least in part, through activation of intrarenal renin-angiotensin-aldosterone-system and epithelial sodium channel.

Intact prostaglandin signaling through EP2 and EP4 receptors in stromal progenitor cells is required for normal development of the renal cortex in mice

Cyclooxygenase (Cox) inhibitors are known to have severe side effects during renal development. These consist of reduced renal function, underdeveloped subcapsular glomeruli, interstitial fibrosis, and thinner cortical tissue. Global genetic deletion of Cox-2 mimics the phenotype observed after application of Cox inhibitors. This study aimed to investigate which cell types express Cox-2 and prostaglandin E2 receptors and what functions are mediated through this pathway during renal development. Expression of EP2 and EP4 mRNA was detected by RNAscope mainly in descendants of FoxD1+ stromal progenitors; EP1 and EP3, on the other hand, were expressed in tubules. Cox-2 mRNA was detected in medullary interstitial cells and macula densa cells. Functional investigations were performed with a cell-specific approach to delete Cox-2, EP2, and EP4 in FoxD1+ stromal progenitor cells. Our data show that Cox-2 expression in macula densa cells is sufficient to drive renal development. Deletion of EP2 or EP4 in FoxD1+ cells had no functional effect on renal development. Codeletion of EP2 and EP4 in FoxD1+ stromal cells, however, led to severe glomerular defects and a strong decline of glomerular filtration rate (1.316 ± 69.7 µL/min/100 g body wt in controls vs. 644.1 ± 64.58 µL/min/100 g body wt in FoxD1+/Cre EP2-/- EP4ff mice), similar to global deletion of Cox-2. Furthermore, EP2/EP4-deficient mice showed a significant increase in collagen production with a strong downregulation of renal renin expression. This study shows the distinct localization of EP receptors in mice. Functionally, we could identify EP2 and EP4 receptors in stromal FoxD1+ progenitor cells as essential receptor subtypes for normal renal development.NEW & NOTEWORTHY Cyclooxygenase-2 (Cox-2) produces prostaglandins that are essential for normal renal development. It is unclear in which cells Cox-2 and the receptors for prostaglandin E2 (EP receptors) are expressed during late nephrogenesis. This study identified the expression sites for EP subtypes and Cox-2 in neonatal mouse kidneys. Furthermore, it shows that stromal progenitor cells may require intact prostaglandin E2 signaling through EP2 and EP4 receptors for normal renal development.

Arachidonic Acid as Mechanotransducer of Renin Cell Baroreceptor

For normal maintenance of blood pressure and blood volume a well-balanced renin-angiotensin-aldosterone system (RAS) is necessary. For this purpose, renin is secreted as the situation demands by the juxtaglomerular cells (also called as granular cells) that are in the walls of the afferent arterioles. Juxtaglomerular cells can sense minute changes in the blood pressure and blood volume and accordingly synthesize, store, and secrete appropriate amounts of renin. Thus, when the blood pressure and blood volume are decreased JGA cells synthesize and secrete higher amounts of renin and when the blood pressure and blood volume is increased the synthesis and secretion of renin is decreased such that homeostasis is restored. To decipher this important function, JGA cells (renin cells) need to sense and transmit the extracellular physical forces to their chromatin to control renin gene expression for appropriate renin synthesis. The changes in perfusion pressure are sensed by Integrin β1 that is transmitted to the renin cell’s nucleus via lamin A/C that produces changes in the architecture of the chromatin. This results in an alteration (either increase or decrease) in renin gene expression. Cell membrane is situated in an unique location since all stimuli need to be transmitted to the cell nucleus and messages from the DNA to the cell external environment can be conveyed only through it. This implies that cell membrane structure and integrity is essential for all cellular functions. Cell membrane is composed to proteins and lipids. The lipid components of the cell membrane regulate its (cell membrane) fluidity and the way the messages are transmitted between the cell and its environment. Of all the lipids present in the membrane, arachidonic acid (AA) forms an important constituent. In response to pressure and other stimuli, cellular and nuclear shape changes occur that render nucleus to act as an elastic mechanotransducer that produces not only changes in cell shape but also in its dynamic behavior. Cell shape changes in response to external pressure(s) result(s) in the activation of cPLA2 (cytosolic phospholipase 2)-AA pathway that stretches to recruit myosin II which produces actin-myosin cytoskeleton contractility. Released AA can undergo peroxidation and peroxidized AA binds to DNA to regulate the expression of several genes. Alterations in the perfusion pressure in the afferent arterioles produces parallel changes in the renin cell membrane leading to changes in renin release. AA and its metabolic products regulate not only the release of renin but also changes in the vanilloid type 1 (TRPV1) expression in renal sensory nerves. Thus, AA and its metabolites function as intermediate/mediator molecules in transducing changes in perfusion and mechanical pressures that involves nuclear mechanotransduction mechanism. This mechanotransducer function of AA has relevance to the synthesis and release of insulin, neurotransmitters, and other soluble mediators release by specialized and non-specialized cells. Thus, AA plays a critical role in diseases such as diabetes mellitus, hypertension, atherosclerosis, coronary heart disease, sepsis, lupus, rheumatoid arthritis, and cancer.

A Vasopressin-Induced Change in Prostaglandin Receptor Subtype Expression Explains the Differential Effect of PGE2 on AQP2 Expression

Arginine vasopressin (AVP) stimulates the concentration of renal urine by increasing the principal cell expression of aquaporin-2 (AQP2) water channels. Prostaglandin E₂ (PGE₂) and prostaglandin_2α (PGF_2α) increase the water absorption of the principal cell without AVP, but PGE₂ decreases it in the presence of AVP. The underlying mechanism of this paradoxical response was investigated here. Mouse cortical collecting duct (mkpCCD_c14) cells mimic principal cells as they endogenously express AQP2 in response to AVP. PGE₂ increased AQP2 abundance without desmopressin (dDAVP), while in the presence of dDAVP, PGE₂, and PGF_2α reduced AQP2 abundance. dDAVP increased the cellular PGD₂ and PGE₂ release and decreased the PGF_2α release. MpkCCD cells expressed mRNAs for the receptors of PGE₂ (EP1/EP4), PGF₂ (FP), and TxB₂ (TP). Incubation with dDAVP increased the expression of EP1 and FP but decreased the expression of EP4. In the absence of dDAVP, incubation of mpkCCD cells with an EP4, but not EP1/3, agonist increased AQP2 abundance, and the PGE₂-induced increase in AQP2 was blocked with an EP4 antagonist. Moreover, in the presence of dDAVP, an EP1/3, but not EP4, agonist decreased the AQP2 abundance, and the addition of EP1 antagonists prevented the PGE₂-mediated downregulation of AQP2. Our study shows that in mpkCCD_c14 cells, reduced EP4 receptor and increased EP1/FP receptor expression by dDAVP explains the differential effects of PGE₂ and PGF_2α on AQP2 abundance with or without dDAVP. As the V2R and EP4 receptor, but not the EP1 and FP receptor, can couple to Gs and stimulate the cyclic adenosine monophosphate (cAMP) pathway, our data support a view that cells can desensitize themselves for receptors activating the same pathway and sensitize themselves for receptors of alternative pathways.

Prostaglandin E2 EP receptors in cardiovascular disease: An update

This review article provides an update for the role of prostaglandin E2 receptors (EP1, EP2, EP3 and EP4) in cardiovascular disease. Where possible we have reported citations from the last decade although this was not possible for all of the topics covered due to the paucity of publications. The authors have attempted to cover the subjects of ischemia-reperfusion injury, arrhythmias, hypertension, novel protein binding partners of the EP receptors and their pathophysiological significance, and cardiac regeneration. These latter two topics bring studies of the EP receptors into new and exciting areas of research that are just beginning to be explored. Where there is peer-reviewed literature, the authors have placed particular emphasis on clinical studies although these are limited in number.

Role of prostaglandin E2 in tissue repair and regeneration

Tissue regeneration following injury from disease or medical treatment still represents a challenge in regeneration medicine. Prostaglandin E2 (PGE2), which involves diverse physiological processes via E-type prostanoid (EP) receptor family, favors the regeneration of various organ systems following injury for its capabilities such as activation of endogenous stem cells, immune regulation, and angiogenesis. Understanding how PGE2 modulates tissue regeneration and then exploring how to elevate the regenerative efficiency of PGE2 will provide key insights into the tissue repair and regeneration processes by PGE2. In this review, we summarized the application of PGE2 to guide the regeneration of different tissues, including skin, heart, liver, kidney, intestine, bone, skeletal muscle, and hematopoietic stem cell regeneration. Moreover, we introduced PGE2-based therapeutic strategies to accelerate the recovery of impaired tissue or organs, including 15-hydroxyprostaglandin dehydrogenase (15-PGDH) inhibitors boosting endogenous PGE2 levels and biomaterial scaffolds to control PGE2 release.

Kidney Microcirculation as a Target for Innovative Therapies in AKI

Acute kidney injury (AKI) is a serious multifactorial conditions accompanied by the loss of function and damage. The renal microcirculation plays a crucial role in maintaining the kidney’s functional and structural integrity for oxygen and nutrient supply and waste product removal. However, alterations in microcirculation and oxygenation due to renal perfusion defects, hypoxia, renal tubular, and endothelial damage can result in AKI and the loss of renal function regardless of systemic hemodynamic changes. The unique structural organization of the renal microvasculature and the presence of autoregulation make it difficult to understand the mechanisms and the occurrence of AKI following disorders such as septic, hemorrhagic, or cardiogenic shock; ischemia/reperfusion; chronic heart failure; cardiorenal syndrome; and hemodilution. In this review, we describe the organization of microcirculation, autoregulation, and pathophysiological alterations leading to AKI. We then suggest innovative therapies focused on the protection of the renal microcirculation and oxygenation to prevent AKI.

Prostaglandin E2 stimulates the epithelial sodium channel (ENaC) in cultured mouse cortical collecting duct cells in an autocrine manner

Prostaglandin E₂ (PGE₂) is the most abundant prostanoid in the kidney, affecting a wide range of renal functions. Conflicting data have been reported regarding the effects of PGE₂ on tubular water and ion transport. The amiloride-sensitive epithelial sodium channel (ENaC) is rate limiting for transepithelial sodium transport in the aldosterone-sensitive distal nephron. The aim of the present study was to explore a potential role of PGE₂ in regulating ENaC in cortical collecting duct (CCD) cells. Short-circuit current (I_SC) measurements were performed using the murine mCCD_cl1 cell line known to express characteristic properties of CCD principal cells and to be responsive to physiological concentrations of aldosterone and vasopressin. PGE₂ stimulated amiloride-sensitive I_SC via basolateral prostaglandin E receptors type 4 (EP₄) with an EC₅₀ of ∼7.1 nM. The rapid stimulatory effect of PGE₂ on I_SC resembled that of vasopressin. A maximum response was reached within minutes, coinciding with an increased abundance of β-ENaC at the apical plasma membrane and elevated cytosolic cAMP levels. The effects of PGE₂ and vasopressin were nonadditive, indicating similar signaling cascades. Exposing mCCD_cl1 cells to aldosterone caused a much slower (∼2 h) increase of the amiloride-sensitive I_SC. Interestingly, the rapid effect of PGE₂ was preserved even after aldosterone stimulation. Furthermore, application of arachidonic acid also increased the amiloride-sensitive I_SC involving basolateral EP₄ receptors. Exposure to arachidonic acid resulted in elevated PGE₂ in the basolateral medium in a cyclooxygenase 1 (COX-1)–dependent manner. These data suggest that in the cortical collecting duct, locally produced and secreted PGE₂ can stimulate ENaC-mediated transepithelial sodium transport.

6β-Hydroxytestosterone Promotes Angiotensin II-Induced Hypertension via Enhanced Cytosolic Phospholipase A2α Activity

Supplemental Digital Content is available in the text.

This study was conducted to test the hypothesis that the CYP1B1 (cytochrome P450 1B1)-testosterone metabolite 6β-hydroxytestosterone contributes to angiotensin II-induced hypertension by promoting activation of group IV cPLA₂α (cytosolic phospholipase A₂α) and generation of prohypertensive eicosanoids in male mice. Eight-week-old male intact or orchidectomized cPLA₂α^+/+/Cyp1b1^+/+ and cPLA₂α^–/–/Cyp1b1^+/+ and intact cPLA₂α^+/+/Cyp1b1^–/– mice were infused with angiotensin II (700 ng/kg/min, subcutaneous) for 2 weeks and injected with 6β-hydroxytestosterone (15 μg/g/every third day, intraperitoneal). Systolic blood pressure was measured by tail-cuff and confirmed by radiotelemetry. Angiotensin II-induced increase in systolic blood pressure, cardiac and renal collagen deposition, and reactive oxygen species production were reduced by disruption of the cPLA₂α or Cyp1b1 genes or by administration of the arachidonic acid metabolism inhibitor 5,8,11,14-eicosatetraynoic acid to cPLA₂α^+/+/Cyp1b1^+/+ mice. 6β-hydroxytestosterone treatment restored these effects of angiotensin II in cPLA₂α^+/+/Cyp1b1^–/– mice but not in orchidectomized cPLA₂α^–/–/Cyp1b1^+/+ mice, which were lowered by 5,8,11,14-eicosatetraynoic acid in cPLA₂α^+/+/Cyp1b1^–/– mice. Antagonists of prostaglandin E₂-EP1/EP3 receptors and thromboxane A₂-TP receptors decreased the effect of 6β-hydroxytestosterone in restoring the angiotensin II-induced increase in systolic blood pressure, cardiac and renal collagen deposition, and reactive oxygen species production in cPLA₂α^+/+/Cyp1b1^–/– mice. These data suggest that 6β-hydroxytestosterone promotes angiotensin II-induced increase in systolic blood pressure and associated pathogenesis via cPLA₂α activation and generation of eicosanoids, most likely prostaglandin E₂ and thromboxane A₂ that exerts prohypertensive effects by stimulating EP1/EP3 and TP receptors, respectively. Therefore, agents that selectively block these receptors could be useful in treating testosterone exacerbated angiotensin II-induced hypertension and its pathogenesis.

Activation of the prostaglandin E2 EP2 receptor attenuates renal fibrosis in unilateral ureteral obstructed mice and human kidney slices

Aim

Renal fibrosis plays a pivotal role in the development and progression of chronic kidney disease, which affects 10% of the adult population. Previously, it has been demonstrated that the cyclooxygenase‐2 (COX‐2)/prostaglandin (PG) system influences the progression of renal injury. Here, we evaluated the impact of butaprost, a selective EP₂ receptor agonist, on renal fibrosis in several models of kidney injury, including human tissue slices.

Methods

We studied the anti‐fibrotic efficacy of butaprost using Madin‐Darby Canine Kidney (MDCK) cells, mice that underwent unilateral ureteral obstruction and human precision‐cut kidney slices. Fibrogenesis was evaluated on a gene and protein level by qPCR and Western blotting.

Results

Butaprost (50 μM) reduced TGF‐β‐induced fibronectin (FN) expression, Smad2 phosphorylation and epithelial‐mesenchymal transition in MDCK cells. In addition, treatment with 4 mg/kg/day butaprost attenuated the development of fibrosis in mice that underwent unilateral ureteral obstruction surgery, as illustrated by a reduction in the gene and protein expression of α‐smooth muscle actin, FN and collagen 1A1. More importantly, a similar anti‐fibrotic effect of butaprost was observed in human precision‐cut kidney slices exposed to TGF‐β. The mechanism of action of butaprost appeared to be a direct effect on TGF‐β/Smad signalling, which was independent of the cAMP/PKA pathway.

Conclusion

In conclusion, this study demonstrates that stimulation of the EP₂ receptor effectively mitigates renal fibrogenesis in various fibrosis models. These findings warrant further research into the clinical application of butaprost, or other EP₂ agonists, for the inhibition of renal fibrosis.

Arachidonic Acid Metabolism and Kidney Inflammation

As a major component of cell membrane lipids, Arachidonic acid (AA), being a major component of the cell membrane lipid content, is mainly metabolized by three kinds of enzymes: cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP450) enzymes. Based on these three metabolic pathways, AA could be converted into various metabolites that trigger different inflammatory responses. In the kidney, prostaglandins (PG), thromboxane (Tx), leukotrienes (LTs) and hydroxyeicosatetraenoic acids (HETEs) are the major metabolites generated from AA. An increased level of prostaglandins (PGs), TxA₂ and leukotriene B4 (LTB₄) results in inflammatory damage to the kidney. Moreover, the LTB₄-leukotriene B4 receptor 1 (BLT1) axis participates in the acute kidney injury via mediating the recruitment of renal neutrophils. In addition, AA can regulate renal ion transport through 19-hydroxystilbenetetraenoic acid (19-HETE) and 20-HETE, both of which are produced by cytochrome P450 monooxygenase. Epoxyeicosatrienoic acids (EETs) generated by the CYP450 enzyme also plays a paramount role in the kidney damage during the inflammation process. For example, 14 and 15-EET mitigated ischemia/reperfusion-caused renal tubular epithelial cell damage. Many drug candidates that target the AA metabolism pathways are being developed to treat kidney inflammation. These observations support an extraordinary interest in a wide range of studies on drug interventions aiming to control AA metabolism and kidney inflammation.

Salt Inducible Kinase Signaling Networks: Implications for Acute Kidney Injury and Therapeutic Potential

A number of signal transduction pathways are activated during Acute Kidney Injury (AKI). Of particular interest is the Salt Inducible Kinase (SIK) signaling network, and its effects on the Renal Proximal Tubule (RPT), one of the primary targets of injury in AKI. The SIK1 network is activated in the RPT following an increase in intracellular Na⁺ (Na⁺_in), resulting in an increase in Na,K-ATPase activity, in addition to the phosphorylation of Class IIa Histone Deacetylases (HDACs). In addition, activated SIKs repress transcriptional regulation mediated by the interaction between cAMP Regulatory Element Binding Protein (CREB) and CREB Regulated Transcriptional Coactivators (CRTCs). Through their transcriptional effects, members of the SIK family regulate a number of metabolic processes, including such cellular processes regulated during AKI as fatty acid metabolism and mitochondrial biogenesis. SIKs are involved in regulating a number of other cellular events which occur during AKI, including apoptosis, the Epithelial to Mesenchymal Transition (EMT), and cell division. Recently, the different SIK kinase isoforms have emerged as promising drug targets, more than 20 new SIK2 inhibitors and activators having been identified by MALDI-TOF screening assays. Their implementation in the future should prove to be important in such renal disease states as AKI.

The role of prostaglandin E2 receptor EP1 in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced neonatal hydronephrosis in mice

Prostaglandin E₂ (PGE₂) is a critical factor in the pathogenesis of dioxin-induced neonatal hydronephrosis. Since the PGE₂ receptor has four subtypes, EP1 - EP4, this study was aimed to challenge the hypothesis that at least one of the four subtypes is responsible for the pathogenesis of dioxin-induced hydronephrosis. To this end, we used mouse pups, with a C57BL/6 J background, genetically lacking EP1, EP2, or EP3, and wild-type pups in whom EP4 was suppressed by administering ONO-AE3-208 (ONO), an EP4 antagonist, from postnatal day 1 (PND 1) to PND 13. To expose the pups to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via lactation, the dams were administered TCDD at an oral dose of 20 μg/kg on PND 1. The pups' urine and kidneys were collected on PND 14 for urinalysis and histological examination, respectively. We found that the incidence of hydronephrosis was 80% in the EP1^+/+ group, but was markedly reduced to 28.6% in the EP1^-/- group despite the fact that PGE₂ concentration in the urine was similarly increased in the both groups. In contrast, the incidence of hydronephrosis was 80% and 100% in the EP2^+/+ and EP2^-/-groups, respectively, and 88.9% and 100% in the EP3^+/+ and EP3^-/- groups, respectively. With regard to EP4, the incidence of hydronephrosis in vehicle (saline)-treated groups and ONO-treated was 88.9% and 100%, respectively. Therefore, we concluded that among PGE₂ receptor subtypes, EP1 plays a predominant role in the onset of TCDD-induced neonatal hydronephrosis in mouse pups.

Immunomodulatory Effects of CP-25 on Splenic T Cells of Rats with Adjuvant Arthritis

Rheumatoid arthritis (RA) is an autoimmune disease in which T cells play an important role. Paeoniflorin-6-oxy-benzenesulfonate (CP-25) shows a strong anti-inflammatory and immunomodulatory effect in the joint of adjuvant arthritis (AA) rats, but the role of the spleen function is still unclear. The aim of this study was to research how CP-25 regulated spleen function of AA rats. Male Sprague-Dawley rats were administered with CP-25 (50 mg/kg) orally from day 17 to 29 after immunization. The spleen histopathological changes were analyzed by hematoxylin-eosin staining. G protein-coupled receptor kinases (GRKs) and prostaglandin receptor subtypes (EPs) were screened by Western blot and immunohistochemistry. The co-expression of GRK2 and EP2 as well as GRK2 and EP4 was measured by immunofluorescence and co-immunoprecipitation. The expression of GRK2 and EP4 in splenic T cells was further detected by immunofluorescence. CP-25 was found to relieve the secondary paw swelling, attenuate histopathologic changes, and downregulate GRK2, EP2 and EP4 expression in AA rats. Additionally, CP-25 not only downregulated the co-expression of GRK2 and EP4 but also downregulated GRK2, EP4 expression in splenic T cells of AA rats. From these results, we can infer that CP-25 play an anti-inflammatory and immune function by affecting the function of the splenic T cells.

MicroRNA-132 controls water homeostasis through regulating MECP2-mediated vasopressin synthesis

Fine-tuning of the body’s water balance is regulated by vasopressin (AVP), which induces the expression and apical membrane insertion of aquaporin-2 water channels and subsequent water reabsorption in the kidney. Here we demonstrate that silencing of microRNA-132 (miR-132) in mice causes severe weight loss due to acute diuresis coinciding with increased plasma osmolality, reduced renal total and plasma membrane expression of aquaporin-2, and abrogated increase in AVP levels. Infusion with synthetic AVP fully reversed the antagomir-132-induced diuresis, and low-dose intracerebroventricular administration of antagomir-132 similarly caused acute diuresis. Central and intracerebroventricular antagomir-132 injection both decreased hypothalamic AVP mRNA levels. At the molecular level, antagomir-132 increased the in vivo and in vitro mRNA expression of methyl-CpG-binding protein-2 (MECP2), which is a miR-132 target and which blocks AVP gene expression by binding its enhancer region. In line with this, treatment of hypothalamic N6 cells with a high-salt solution increased its miR-132 levels, whereas it attenuated endogenous Mecp2 mRNA levels. In conclusion, we identified miR-132 as a first miRNA regulating the osmotic balance by regulating the hypothalamic AVP gene mRNA expression.

Impact of acute versus prolonged exercise and dehydration on kidney function and injury

Exercise and dehydration may be associated with a compromised kidney function and potential signs of kidney injury. However, the kidney responses to exercise of different durations and hypohydration levels are not yet known. Therefore, we aimed to compare the effects of acute versus prolonged exercise and dehydration on estimated glomerular filtration rate (eGFR) and kidney injury biomarkers in healthy male adults. A total of 35 subjects (23 ± 3 years) were included and invited for two study visits. Visit 1 consisted of a maximal cycling test. On Visit 2, subjects performed a submaximal exercise test at 80% of maximal heart rate until 3% hypohydration. Blood and urine samples were taken at baseline, after 30 min of exercise (acute effects; low level of hypohydration) and after 150 min of exercise or when 3% hypohydration was achieved (prolonged effects, high level of hypohydration). Urinary outcome parameters were corrected for urinary cystatin C, creatinine, and osmolality. Subjects dehydrated on average 0.6 ± 0.3% and 2.9 ± 0.7% after acute and prolonged exercise, respectively (P < 0.001). The eGFRcystatin C did not differ between baseline and acute exercise (118 ± 11 vs. 116 ± 12 mL/min/1.73 m2 , P = 0.12), whereas eGFRcystatin C was significantly lower after prolonged exercise (103 ± 16 mL/min/1.73 m2 , P < 0.001). We found no difference in osmolality corrected uKIM1 concentrations after acute and prolonged exercise (P > 0.05), and elevated osmolality corrected uNGAL concentrations after acute and prolonged exercise (all P-values < 0.05). In conclusion, acute exercise did barely impact on eGFRcystatin C and kidney injury biomarkers, whereas prolonged exercise is associated with a decline in eGFRcystatin C and increased biomarkers for kidney injury.

The acute diuretic effect of an ethanolic fraction of Phyllanthus amarus (Euphorbiaceae) in rats involves prostaglandins

Background

Phyllanthus amarus (Schum & Thonn), a plant belonging to the family of Euphorbiaceae is used in Ivorian traditional medicine to treat cardiovascular disorders such as hypertension. However, although this plant has been described as a diuretic agent, the underlying mechanism remains unclear. Therefore, the aim of the present study was to investigate the mechanism action of diuretic effects of an ethanolic fraction of Phyllanthus amarus (EFPA) in rats.

Methods

Effects of EFPA on urinary excretion were carried out for doses ranging from 5 to 80 mg/kg given by intraperitoneal injection (i.p.) and compared with that induced by furosemide (5 mg/kg) after 8 h. Thereafter, the diuretic activity of EFPA was also evaluated in the presence of indomethacin (5 mg/kg, i.p.) in order to determine the involvement of prostaglandins, after 24 h.

Results

Between 5 and 80 mg/kg, EFPA induced a significant urinary excretion. The profile of urinary excretion showed that after 2 h, the highest dose of 80 mg/kg induced a urinary volumetric excretion (UVE), which was similar to that induced by furosemide. After 24 h, EFPA at 10 mg/kg increased significantly UVE, Na⁺ (43 mEq) and Cl^¯ (97 mEq) urinary excretions without promoting kaliuresis. In rats pretreated with indomethacin, the urinary excretion and the natriuretic response of EFPA were significantly reduced.

Conclusion

Altogether, this study has shown that EFPA promotes a significant urinary excretion of water and Na⁺, confirming its diuretic activity. Moreover, the increased diuresis could be attributed, at least in part, to the involvement of prostaglandins.

Electronic supplementary material

The online version of this article (10.1186/s12906-018-2158-0) contains supplementary material, which is available to authorized users.

Prostaglandins in teleost ovulation: A review of the roles with a view to comparison with prostaglandins in mammalian ovulation

Prostaglandins are well known to be central regulators of vertebrate ovulation. Studies addressing the role of prostaglandins in mammalian ovulation have established that they are involved in the processes of oocyte maturation and cumulus oocyte complex expansion. In contrast, despite the first indication of the role of prostaglandins in teleost ovulation appearing 40 years ago, the mechanistic background of their role has long been unknown. However, studies conducted on medaka over the past decade have provided valuable information. Emerging evidence indicates an indispensable role of prostaglandin E₂ and its receptor subtype Ptger4b in the process of follicle rupture. In this review, we summarize studies addressing the role of prostaglandins in teleost ovulation and describe recent advances. To help understand differences from and similarities to ovulation in mammalian species, the findings on the roles of prostaglandins in mammalian ovulation are discussed in parallel.

Intermittent hypoxia alters dose dependent caffeine effects on renal prostanoids and receptors in neonatal rats

Caffeine, one of the most commonly prescribed drugs in preterm neonates, is given in standard or suprapharmacologic doses. Although known as a diuretic, its effects in the neonatal kidneys are not well studied. We tested the hypothesis that neonatal intermittent hypoxia (IH) and high caffeine doses (HCD) alter renal regulators of vasomotor tone and water balance. Newborn rats were randomized to room air, hyperoxia, or IH and treated with standard or high caffeine doses; or placebo saline. Renal prostanoids; histopathology; and cyclooxygenase (COX), prostanoid receptor, and aquaporin (AQP) immunoreactivity were determined. HCD in IH caused severe pathological changes in the glomeruli and proximal tubules, consistent with acute kidney injury. This was associated with reductions in anthropometric growth, PGI_2, and IP, DP, and AQP-4 immunoreactivity, well as a robust increase in COX-2, suggesting that the use of HCD should be avoided in preterm infants who experience frequent IH episodes.

PGE2 upregulates renin through E-prostanoid receptor 1 via PKC/cAMP/CREB pathway in M-1 cells

During the early phase of ANG II-dependent hypertension, tubular PGE₂ is increased. Renin synthesis and secretion in the collecting duct (CD) are upregulated by ANG II, contributing to further intratubular ANG II formation. However, what happens first and whether the triggering mechanism is independent of tubular ANG II remain unknown. PGE₂ stimulates renin synthesis in juxtaglomerular cells via E-prostanoid (EP) receptors through the cAMP/cAMP-responsive element-binding (CREB) pathway. EP receptors are also expressed in the CD. Here, we tested the hypothesis that renin is upregulated by PGE₂ in CD cells. The M-1 CD cell line expressed EP1, EP3, and EP4 but not EP2. Dose-response experiments, in the presence of ANG II type 1 receptor blockade with candesartan, demonstrated that 10^-6 M PGE₂ maximally increases renin mRNA (approximately 4-fold) and prorenin/renin protein levels (approximately 2-fold). This response was prevented by micromolar doses of SC-19220 (EP1 antagonist), attenuated by the EP4 antagonist, L-161982, and exacerbated by the highly selective EP3 antagonist, L-798106 (~10-fold increase). To evaluate further the signaling pathway involved, we used the PKC inhibitor calphostin C and transfections with PKCα dominant negative. Both strategies blunted the PGE₂-induced increases in cAMP levels, CREB phosphorylation, and augmentation of renin. Knockdown of the EP1 receptor and CREB also prevented renin upregulation. These results indicate that PGE₂ increases CD renin expression through the EP1 receptor via the PKC/cAMP/CREB pathway. Therefore, we conclude that during the early stages of ANG II-dependent hypertension, there is augmentation of PGE₂ that stimulates renin in the CD, resulting in increased tubular ANG II formation and further stimulation of renin.

Regulation and function of renal medullary cyclooxygenase-2 during high salt loading

Prostaglandins (PGs) are important autocrine/paracrine regulators that contribute to sodium balance and blood pressure control. Along the nephron, the highest amount of PGE₂ is found in the distal nephron, an important site for fine-tuning of urinary sodium and water excretion. Cylooxygenase-2 (COX-2) is abundantly expressed in the renal medulla and its expression along with urinary PGE₂ excretion is highly induced by chronic salt loading. Factors involved in high salt-induced COX-2 expression in the renal medulla include the hypertonicity, fluid shear stress (FSS), and hypoxia-inducible factor-1 alpha (HIF-1 alpha). Site-specific inhibition of COX-2 in the renal medulla of Sprague-Dawley rats causes sodium retention and salt-sensitive hypertension. Together, these results support the concept that renal medullary COX-2 functions an important natriuretic mediator that is activated by salt loading and its products promote sodium excretion and contribute to maintenance of sodium balance and blood pressure.

Salt-losing nephropathy in mice with a null mutation of the Clcnk2 gene

AIM

The basolateral chloride channel ClC-Kb facilitates Cl reabsorption in the distal nephron of the human kidney. Functional mutations in CLCNKB are associated with Bartter's syndrome type 3, a hereditary salt-losing nephropathy. To address the function of ClC-K2 in vivo, we generated ClC-K2-deficient mice.

METHODS

ClC-K2-deficient mice were generated using TALEN technology.

RESULTS

ClC-K2-deficient mice were viable and born in a Mendelian ratio. ClC-K2-/- mice showed no gross anatomical abnormalities, but they were growth retarded. The 24-h urine volume was increased in ClC-K2-/- mice (4.4 ± 0.6 compared with 0.9 ± 0.2 mL per 24 h in wild-type littermates; P = 0.001). Accordingly, ambient urine osmolarity was markedly reduced (590 ± 39 vs. 2216 ± 132 mosmol L^-1 in wild types; P < 0.0001). During water restriction (24 h), urinary osmolarity increased to 1633 ± 153 and 3769 ± 129 mosmol L^-1 in ClC-K2-/- and wild-type mice (n = 12; P < 0.0001), accompanied by a loss of body weight of 12 ± 0.4 and 8 ± 0.2% respectively (P < 0.0001). ClC-K2-/- mice showed an increased renal sodium excretion and compromised salt conservation during a salt-restricted diet. The salt-losing phenotype of ClC-K2-/- mice was associated with a reduced plasma volume, hypotension, a slightly reduced glomerular filtration rate, an increased renal prostaglandin E2 generation and a massively stimulated renin-angiotensin system. Clckb-/- mice showed a reduced sensitivity to furosemide and were completely resistant to thiazides.

CONCLUSION

Loss of ClC-K2 compromises TAL function and abolishes salt reabsorption in the distal convoluted tubule. Our data suggest that ClC-K2 is crucial for renal salt reabsorption and concentrating ability. ClC-K2-deficient mice in most aspects mimic patients with Bartter's syndrome type 3.

Vascular Smooth Muscle-Specific EP4 Receptor Deletion in Mice Exacerbates Angiotensin II-Induced Renal Injury

AIMS

Cyclooxygenase inhibition by non-steroidal anti-inflammatory drugs is contraindicated in hypertension, as it may reduce glomerular filtration rate (GFR) and renal blood flow. However, the identity of the specific eicosanoid and receptor underlying these effects is not known. We hypothesized that vascular smooth muscle prostaglandin E2 (PGE2) E-prostanoid 4 (EP4) receptor deletion predisposes to renal injury via unchecked vasoconstrictive actions of angiotensin II (AngII) in a hypertension model. Mice with inducible vascular smooth muscle cell (VSMC)-specific EP4 receptor deletion were generated and subjected to AngII-induced hypertension.

RESULTS

EP4 deletion was verified by PCR of aorta and renal vessels, as well as functionally by loss of PGE2-mediated mesenteric artery relaxation. Both AngII-treated groups became similarly hypertensive, whereas albuminuria, foot process effacement, and renal hypertrophy were exacerbated in AngII-treated EP4^VSMC-/- but not in EP4^VSMC+/+ mice and were associated with glomerular scarring, tubulointerstitial injury, and reduced GFR. AngII-treated EP4^VSMC-/- mice exhibited capillary damage and reduced renal perfusion as measured by fluorescent bead microangiography and magnetic resonance imaging, respectively. NADPH oxidase 2 (Nox2) expression was significantly elevated in AngII-treated EP4^-/- mice. EP4-receptor silencing in primary VSMCs abolished PGE2 inhibition of AngII-induced Nox2 mRNA and superoxide production.

INNOVATION

These data suggest that vascular EP4 receptors buffer the actions of AngII on renal hemodynamics and oxidative injury.

CONCLUSION

EP4 agonists may, therefore, protect against hypertension-associated kidney damage. Antioxid. Redox Signal. 25, 642-656.

Pharmacogenomics of Prostaglandin and Leukotriene Receptors

Individual genetic background together with environmental effects are thought to be behind many human complex diseases. A number of genetic variants, mainly single nucleotide polymorphisms (SNPs), have been shown to be associated with various pathological and inflammatory conditions, representing potential therapeutic targets. Prostaglandins (PTGs) and leukotrienes (LTs) are eicosanoids derived from arachidonic acid and related polyunsaturated fatty acids that participate in both normal homeostasis and inflammatory conditions. These bioactive lipid mediators are synthesized through two major multistep enzymatic pathways: PTGs by cyclooxygenase and LTs by 5-lipoxygenase. The main physiological effects of PTGs include vasodilation and vascular leakage (PTGE2); mast cell maturation, eosinophil recruitment, and allergic responses (PTGD2); vascular and respiratory smooth muscle contraction (PTGF2), and inhibition of platelet aggregation (PTGI2). LTB4 is mainly involved in neutrophil recruitment, vascular leakage, and epithelial barrier function, whereas cysteinyl LTs (CysLTs) (LTC4, LTD4, and LTE4) induce bronchoconstriction and neutrophil extravasation, and also participate in vascular leakage. PTGs and LTs exert their biological functions by binding to cognate receptors, which belong to the seven transmembrane, G protein-coupled receptor superfamily. SNPs in genes encoding these receptors may influence their functionality and have a role in disease susceptibility and drug treatment response. In this review we summarize SNPs in PTGs and LTs receptors and their relevance in human diseases. We also provide information on gene expression. Finally, we speculate on future directions for this topic.

Inhibition of mitochondrial complex-1 restores the downregulation of aquaporins in obstructive nephropathy

Obstructive kidney disease is a common complication in the clinic. Downregulation of aquaporins (AQPs) in obstructed kidneys has been thought as a key factor leading to the polyuria and impairment of urine-concentrating capability after the release of kidney obstruction. The present study was to investigate the role of mitochondrial complex-1 in modulating AQPs in obstructive nephropathy. Following 7-day unilateral ureteral obstruction (UUO), AQP1, AQP2, AQP3, and vasopressin 2 (V₂) receptor were remarkably reduced as determined by qRT-PCR and/or Western blotting. Notably, inhibition of mitochondrial complex-1 by rotenone markedly reversed the downregulation of AQP1, AQP2, AQP3, and V₂ In contrast, AQP4 was not affected by kidney obstruction or rotenone treatment. In a separate study, rotenone also attenuated AQPs' downregulation after 48-h UUO. To study the potential mechanisms in mediating the rotenone effects on AQPs, we examined the regulation of the COX-2/microsomal prostaglandin E synthase (mPGES)-1/PGE₂/EP pathway and found that COX-2, mPGES-1, and renal PGE₂ content were all significantly elevated in obstructive kidneys, which was not affected by rotenone treatment. For EP receptors, EP₂ and EP₄ but not EP₁ and EP₃ were upregulated in obstructive kidneys. Importantly, rotenone strikingly suppressed EP₁ and EP₄ but not EP₂ and EP₃ receptors. However, treatment of EP₁ antagonist SC-51322 could not affect AQPs' reduction in obstructed kidneys. Collectively, these findings suggested an important role of mitochondrial dysfunction in modulating AQPs and V₂ receptor in obstructive nephropathy possibly via prostaglandin-independent mechanisms.

Pathogenesis of Type 2 Epithelial to Mesenchymal Transition (EMT) in Renal and Hepatic Fibrosis

Epithelial to mesenchymal transition (EMT), particularly, type 2 EMT, is important in progressive renal and hepatic fibrosis. In this process, incompletely regenerated renal epithelia lose their epithelial characteristics and gain migratory mesenchymal qualities as myofibroblasts. In hepatic fibrosis (importantly, cirrhosis), the process also occurs in injured hepatocytes and hepatic progenitor cells (HPCs), as well as ductular reaction-related bile epithelia. Interestingly, the ductular reaction contributes partly to hepatocarcinogenesis of HPCs, and further, regenerating cholangiocytes after injury may be derived from hepatic stellate cells via mesenchymal to epithelia transition, a reverse phenomenon of type 2 EMT. Possible pathogenesis of type 2 EMT and its differences between renal and hepatic fibrosis are reviewed based on our experimental data.

Physiology and pathophysiology of cyclooxygenase-2 and prostaglandin E2 in the kidney

The cyclooxygenase (COX) enzyme system is the major pathway catalyzing the conversion of arachidonic acid into prostaglandins (PGs). PGs are lipid mediators implicated in a variety of physiological and pathophysiological processes in the kidney, including renal hemodynamics, body water and sodium balance, and the inflammatory injury characteristic in multiple renal diseases. Since the beginning of 1990s, it has been confirmed that COX exists in 2 isoforms, referred to as COX-1 and COX-2. Even though the 2 enzymes are similar in size and structure, COX-1 and COX-2 are regulated by different systems and have different functional roles. This review summarizes the current data on renal expression of the 2 COX isoforms and highlights mainly the role of COX-2 and PGE2 in several physiological and pathophysiological processes in the kidney.

Neonates with Bartter syndrome have enormous fluid and sodium requirements

AIM

Managing neonatal Bartter syndrome by achieving adequate weight gain is challenging. We assessed the correlation between weight gain in neonatal Bartter syndrome and the introduction of fluid and sodium supplementations and indomethacin during the first 4 weeks of life.

METHODS

Daily fluid and electrolytes requirements were analysed using linear regression and Spearman correlation coefficients. The weight gain coefficient was calculated as daily weight gain after physiological neonatal weight loss.

RESULTS

We studied seven infants. The highest weight gain coefficients occurred between weeks two and four in the five neonates who either received prompt amounts of fluid (maximum 810 mL/kg/day) and sodium (maximum 70 mmol/kg/day) or were treated with indomethacin. For the two patients with the highest weight gain coefficient, water and sodium supplementations were decreased in weeks two to four leading to a significant negative Spearman correlation between weight gain and fluid supplements (r = -0.55 and -0.68) and weight gain and sodium supplementations (r = -0.96 and -0.72). The two patients with the lowest weight gain coefficient had positive Spearman correlation coefficients between weight gain and fluid and sodium supplementations.

CONCLUSION

Infants with neonatal Bartter syndrome required rapid and enormous fluid and sodium supplementations or the early introduction of indomethacin treatment to achieve adequate weight gain during the early postnatal period.

Disruption of prostaglandin E2 receptor EP4 impairs urinary concentration via decreasing aquaporin 2 in renal collecting ducts

The antidiuretic hormone arginine vasopressin is a systemic effector in urinary concentration. However, increasing evidence suggests that other locally produced factors may also play an important role in the regulation of water reabsorption in renal collecting ducts. Recently, prostaglandin E2 (PGE2) receptor EP4 has emerged as a potential therapeutic target for the treatment of nephrogenic diabetes insipidus, but the underlying mechanism is unknown. To evaluate the role of EP4 in regulating water homeostasis, mice with renal tubule-specific knockout of EP4 (Ksp-EP4(-/-)) and collecting duct-specific knockout of EP4 (AQP2-EP4(-/-)) were generated using the Cre-loxP recombination system. Urine concentrating defect was observed in both Ksp-EP4(-/-) and AQP2-EP4(-/-) mice. Decreased aquaporin 2 (AQP2) abundance and apical membrane targeting in renal collecting ducts were evident in Ksp-EP4(-/-) mice. In vitro studies demonstrated that AQP2 mRNA and protein levels were significantly up-regulated in mouse primary inner medullary collecting duct (IMCD) cells after pharmacological activation or adenovirus-mediated overexpression of EP4 in a cAMP/cAMP-response element binding protein-dependent manner. In addition, EP4 activation or overexpression also increased AQP2 membrane accumulation in a mouse IMCD cell line (IMCD3) stably transfected with the AQP2 gene, mainly through the cAMP/protein kinase A and extracellular signal-regulated kinase pathways. In summary, the EP4 receptor in renal collecting ducts plays an important role in regulating urinary concentration under physiological conditions. The ability of EP4 to promote AQP2 membrane targeting and increase AQP2 abundance makes it a potential therapeutic target for the treatment of clinical disorders including acquired and congenital diabetes insipidus.

Evaluation of the diuretic activity in two Mexican medicinal species: Selaginella nothohybrida and Selaginella lepidophylla and its effects with ciclooxigenases inhibitors

ETHNOPHARMACOLOGICAL RELEVANCE

Doradilla is a plant that has a long history in the Mexican traditional system of medicine for gall and renal stones, diuresis, stomach and liver inflammation among other diseases. Major components isolated from these plants include biflavonoids as amentoflavone (1), robustaflavone (2) and (S)-2,3-dihydrorobustaflavone (3) and the carbohydrate trehalose (4). The aim of this study was to evaluate the diuretic effect of the decoction of Selaginella nothohybrida Valdespino and Selaginella lepidophylla (Hook & Grev) Spring (Selaginellaceae), and compounds 1-4. We also explored the probable mode of action comparing the effects when using nonspecific and specific COX׳s inhibitors.

MATERIALS AND METHODS

Three biflavonoids (1-3) were isolated from the ethyl acetate extraction of the aqueous decoction and the carbohydrate trehalose (4) from the aqueous phase. The structures of all compounds were elucidated by spectroscopic methods and comparisons were made against published data. The diuretic activity was assessed in mice by oral administration of the decoctions in doses of 1000 and 2000mg/kg and biflavonoids 1-3 and trehalose (4) in a dose range of 10mg/kg using furosemide as a standard drug. Inhibitors of COX׳s such as acetyl salicylic acid, sodium naproxen, indomethacin and Celebrex were also assayed to analyze the involvement of renal prostaglandins in diuresis. Water excretion rate, pH, density, conductivity, and contents of Na(+) and K(+) were measured in the urine of mice.

RESULTS

Decoction of Selaginella lepidophylla showed lower effect in the urine output at doses of 1000 and 2000mg/kg, while decoction of Selaginella nothohybrida produced an increase at 2000mg/kg (P<0.05). Urinary electrolytes excretion was also affected by this last extract and pure compounds: decoction diminished urinary excretion of sodium and potassium ions, so as compounds 1 and 4; compounds 2 and 3 observed just a natriuretic effect. Pretreated mice with COX׳s inhibitors and then with test compounds 1, 2, 4 and decoction showed inhibition of diuresis in all cases exception for treatment with trehalose (4); natriuretic effect was observed in all cases except for biflavonoid robustaflavone (2) which behaved as the reference compound furosemide. Selaginella nothohybrida decoction behaved similarly to COX-2 inhibitor Celebrex (8), inhibiting diuresis.

CONCLUSIONS

Selaginella nothohybrida presents a moderate diuretic effect, which appears to be in partly mediated by the presence of biflavonoids and trehalose. Renal prostaglandins may be involved in the mechanism of diuresis. The present results provide a quantitative basis explaining the traditional folk medicine use of Selaginella nothohybrida as a diuretic agent by Mexican population.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Crosstalk between (Pro)renin receptor and COX-2 in the renal medulla during angiotensin II-induced hypertension

Angiotensin II (AngII) is an octapeptide hormone that plays a central role in regulation of sodium balance, plasma volume, and blood pressure. Its role in the pathogenesis of hypertension is highlighted by the wide use of inhibitors of the renin-angiotensin system (RAS) as the first-line antihypertensive therapy. However, despite intensive investigation, the mechanism of AngII-induced hypertension is still incompletely understood. Although diverse pathways are likely involved, increasing evidence suggests that the activation of intrarenal RAS may represent a dominant mechanism of AngII-induced hypertension. (Pro)renin receptor (PRR), a potential regulator of intrarenal RAS, is expressed in the intercalated cells of the collecting duct (CD) and induced by AngII, in parallel with increased renin in the principal cells of the CD. Activation of PRR elevated PGE2 release and COX-2 expression in renal inner medullary cells whereas COX-2-derived PGE2via the EP4 receptor mediates the upregulation of PRR during AngII infusion, thus forming a vicious cycle. The mutually stimulatory relationship between PRR and COX-2 in the distal nephron may play an important role in mediating AngII-induced hypertension.

The urinary levels of prostanoid metabolites predict acute kidney injury in heterogeneous adult Japanese ICU patients: a prospective observational study

BACKGROUND

Acute kidney injury (AKI) is frequently observed in critically ill patients in the intensive care unit (ICU) and is associated with increased mortality. Prostanoids regulate numerous biological functions, including hemodynamics and renal tubular transport. We herein investigated the ability of urinary prostanoid metabolites to predict the onset of AKI in critically ill adult patients.

METHODS

The current study was conducted as a prospective observational study. Urine of patients admitted to the ICU at Okayama University Hospital was collected and the urinary levels of prostaglandin E2 (PGE2), PGI2 metabolite (2,3-dinor-6-OXO-PGF1α), thromboxane A2 (TXA2) metabolite (11-dehydro-TXB2) were determined.

RESULTS

Of the 93 patients, 24 developed AKI (AKIN criteria). Surgical intervention (93, 75 %) was the leading cause of ICU admission. Overall, the ratio of the level of serum Cr on Day 1 after ICU admission to that observed at baseline positively correlated with the urinary 2,3-dinor-6-OXO-PGF1α/Cr (r = 0.57, p < 0.0001) and 11-dehydro-TXB2/Cr (r = 0.47, p < 0.0001) ratios. In 16 cases of de novo AKI, the urinary 2,3-dinor-6-OXO-PGF1α/Cr and 11-dehydro-TXB2/Cr values were significantly elevated compared with that observed in the non-AKI group, whereas the urinary PGE2/Cr values were not. The urinary 2,3-dinor-6-OXO-PGF1α/Cr ratio exhibited the best diagnostic and predictive performance among the prostanoid metabolites according to the receiver operating characteristic (ROC) analysis [ROC-area under the curve (AUC): 0.75].

CONCLUSIONS

Taken together, these results demonstrate that the urinary 2,3-dinor-6-OXO-PGF1α/Cr and 11-dehydro-TXB2/Cr ratios are associated with the subsequent onset of AKI and poor outcomes in adult heterogeneous ICU patients.

Role of COX-2/mPGES-1/prostaglandin E2 cascade in kidney injury

COX-2/mPGES-1/PGE2 cascade plays critical roles in modulating many physiological and pathological actions in different organs. In the kidney, this cascade is of high importance in regulating fluid metabolism, blood pressure, and renal hemodynamics. Under some disease conditions, this cascade displays various actions in response to the different pathological insults. In the present review, the roles of this cascade in the pathogenesis of kidney injuries including diabetic and nondiabetic kidney diseases and acute kidney injuries were introduced and discussed. The new insights from this review not only increase the understanding of the pathological role of the COX-2/mPGES-1/PGE2 pathway in kidney injuries, but also shed new light on the innovation of the strategies for the treatment of kidney diseases.

Leukemia and ABC transporters

Acute myeloid leukemia (AML) is a heterogeneous disease caused by aberrant proliferation and/or differentiation of myeloid progenitors. However, only ~65% of AML patients respond to induction chemotherapy and the overall survival rate for AML remains low (~24% for 5-year survival). The conventional view suggests that ATP-binding cassette (ABC) transporters contribute to treatment failure due to their drug-effluxing capabilities. This might be overly simplistic. Some ABC transporters export endogenous substrates that have defined roles in normal hematopoietic progenitors. It is conceivable that these substances also provide an advantage to leukemic progenitors. This review will highlight how certain endogenous substrates impact normal hematopoietic cells and suggest that ABC transporters facilitate export of these substances to affect both normal hematopoietic and leukemic progenitors. For example, the ability to export certain endogenous ligands may facilitate leukemogenesis by modifying leukemic progenitor cell proliferation or survival. If so, the addition of ABC transporter inhibitors to traditional chemotherapy might improve therapeutic efficacy by not just increasing intracellular drug accumulation but also blocking the beneficial effects ABC transporter ligands have on cell survival.

Collecting duct principal cell transport processes and their regulation

The principal cell of the kidney collecting duct is one of the most highly regulated epithelial cell types in vertebrates. The effects of hormonal, autocrine, and paracrine factors to regulate principal cell transport processes are central to the maintenance of fluid and electrolyte balance in the face of wide variations in food and water intake. In marked contrast with the epithelial cells lining the proximal tubule, the collecting duct is electrically tight, and ion and osmotic gradients can be very high. The central role of principal cells in salt and water transport is reflected by their defining transporters-the epithelial Na(+) channel (ENaC), the renal outer medullary K(+) channel, and the aquaporin 2 (AQP2) water channel. The coordinated regulation of ENaC by aldosterone, and AQP2 by arginine vasopressin (AVP) in principal cells is essential for the control of plasma Na(+) and K(+) concentrations, extracellular fluid volume, and BP. In addition to these essential hormones, additional neuronal, physical, and chemical factors influence Na(+), K(+), and water homeostasis. Notably, a variety of secreted paracrine and autocrine agents such as bradykinin, ATP, endothelin, nitric oxide, and prostaglandin E2 counterbalance and limit the natriferic effects of aldosterone and the water-retaining effects of AVP. Considerable recent progress has improved our understanding of the transporters, receptors, second messengers, and signaling events that mediate principal cell responses to changing environments in health and disease. This review primarily addresses the structure and function of the key transporters and the complex interplay of regulatory factors that modulate principal cell ion and water transport.

Direct regulation of ENaC by bradykinin in the distal nephron. Implications for renal sodium handling

PURPOSE OF REVIEW

Locally produced peptide hormones kinins, such as bradykinin, are thought to oppose many of the prohypertensive actions of the renin-angiotensin-aldosterone system. In the kidney, bradykinin, via stimulation of B2 receptors (B2R), favors natriuresis mostly due to the inhibition of tubular Na reabsorption. Recent experimental evidence identifies the epithelial Na channel (ENaC) as a key end effector of bradykinin actions in the distal tubular segments. The focus of this review is the physiological relevance and molecular details of the bradykinin signal to ENaC.

RECENT FINDINGS

The recent epidemiological GenSalt study demonstrated that genetic variants of the gene encoding B2R show significant associations with the salt sensitivity of blood pressure. Bradykinin was shown to have an inhibitory effect on the distal nephron sodium transport via stimulation of B2 receptor-phospholipase C (B2R-PLC) cascade to decrease ENaC open probability. Genetic ablation of bradykinin receptors in mice led to an augmented ENaC function, particularly during elevated sodium intake, likely contributing to the salt-sensitive hypertensive phenotype. Furthermore, augmentation of bradykinin signaling in the distal nephron was demonstrated to be an important component of the natriuretic and antihypertensive effects of angiotensin converting enzyme inhibition.

SUMMARY

Salt-sensitive inhibition of ENaC activity by bradykinin greatly advances our understanding of the molecular mechanisms that are responsible for shutting down distal tubule sodium reabsorption during volume expanded conditions to avoid salt-sensitive hypertension.