PGE2 stimulates Cl- secretion in murine M-1 cortical collecting duct cells in an autocrine manner

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.

Prostaglandin E2 receptor EP1 (PGE2/EP1) deletion promotes glomerular podocyte and endothelial cell injury in hypertensive TTRhRen mice

Prostaglandin E2 receptor EP1 (PGE₂/EP₁) promotes diabetic renal injury, and EP₁ receptor deletion improves hyperfiltration, albuminuria, and fibrosis. The role of EP₁ receptors in hypertensive kidney disease (HKD) remains controversial. We examined the contribution of EP₁ receptors to HKD. EP₁ null (EP₁^-/-) mice were bred with hypertensive TTRhRen mice (Htn) to evaluate kidney function and injury at 24 weeks. EP₁ deletion had no effect on elevation of systolic blood pressure in Htn mice (HtnEP₁^-/-) but resulted in pronounced albuminuria and reduced FITC-inulin clearance, compared with Htn or wild-type (WT) mice. Ultrastructural injury to podocytes and glomerular endothelium was prominent in HtnEP₁^-/- mice; including widened subendothelial space, subendothelial lucent zones and focal lifting of endothelium from basement membrane, with focal subendothelial cell debris. Cortex COX2 mRNA was increased by EP₁ deletion. Glomerular EP₃ mRNA was reduced by EP₁ deletion, and EP₄ by Htn and EP₁ deletion. In WT mice, PGE₂ increased chloride reabsorption via EP₁ in isolated perfused thick ascending limb (TAL), but PGE₂ or EP₁ deletion did not affect vasopressin-mediated chloride reabsorption. In WT and Htn mouse inner medullary collecting duct (IMCD), PGE₂ inhibited vasopressin-water transport, but not in EP₁^-/- or HtnEP₁^-/- mice. Overall, EP₁ mediated TAL and IMCD transport in response to PGE₂ is unaltered in Htn, and EP₁ is protective in HKD.

Prostaglandin E2 Induces Prorenin-Dependent Activation of (Pro)renin Receptor and Upregulation of Cyclooxygenase-2 in Collecting Duct Cells

BACKGROUND

Prostaglandin E2 (PGE₂) regulates renin expression in renal juxtaglomerular cells. PGE₂ acts through E-prostanoid (EP) receptors in the renal collecting duct (CD) to regulate sodium and water balance. CD cells express EP1 and EP4, which are linked to protein kinase C (PKC) and PKA downstream pathways, respectively. Previous studies showed that the presence of renin in the CD, and that of PKC and PKA pathways, activate its expression. The (pro)renin receptor (PRR) is also expressed in CD cells, and its activation enhances cyclooxygenase-2 (COX-2) through extracellular signal-regulated kinase (ERK). We hypothesized that PGE₂ stimulates prorenin and renin synthesis leading to subsequent activation of PRR and upregulation of COX-2.

METHODS

We used a mouse M-1 CD cell line that expresses EP1, EP3 and EP4 but not EP2.

RESULTS

PGE₂ (10^-6M) treatment increased prorenin and renin protein levels at 4 and 8 hours. No differences were found at 12-hour after PGE₂ treatment. Phospho-ERK was significantly augmented after 12 hours. COX-2 expression was decreased after 4 hours of PGE₂ treatment, but increased after 12 hours. Interestingly, the full-length form of the PRR was upregulated only at 12 hours. PGE₂-mediated phospho-ERK and COX-2 upregulation was suppressed by PRR silencing.

CONCLUSIONS

Our results suggest that PGE₂ induces biphasic regulation of COX-2 through renin-dependent PRR activation via EP1 and EP4 receptors. PRR-mediated increases in COX-2 expression may enhance PGE₂ synthesis in CD cells serving as a buffer mechanism in conditions of activated renin-angiotensin system.

Chloride secretion by renal collecting ducts

PURPOSE OF REVIEW

Renal collecting ducts maintain NaCl homeostasis by fine-tuning urinary excretion to balance dietary salt intake. This review focuses on recent studies on transcellular Cl secretion by collecting ducts, its regulation and its role in cyst growth in autosomal dominant polycystic kidney disease (ADPKD).

RECENT FINDINGS

Lumens of nonperfused rat medullary collecting ducts collapse in control media but expand with fluid following treatment with cAMP, demonstrating the capacity for both salt absorption and secretion. Recently, inhibition of apical epithelial Na channels (ENaC) unmasked Cl secretion in perfused mouse cortical collecting ducts (CCDs), involving Cl uptake by basolateral NKCC1 and efflux through apical Cl channels. AVP, the key hormone for osmoregulation, promotes cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl secretion. In addition, prostaglandin E2 stimulates Cl secretion through both CFTR and Ca-activated Cl channels.

SUMMARY

Renal Cl secretion has been commonly overlooked because of the overwhelming capacity for the nephron to reabsorb NaCl from the glomerular filtrate. In ADPKD, Cl secretion plays a central role in the accumulation of cyst fluid and the remarkable size of the cystic kidneys. Investigation of renal Cl secretion may provide a better understanding of NaCl homeostasis and identify new approaches to reduce cyst growth in PKD.

Antagonism of the prostaglandin E2 EP1 receptor in MDCK cells increases growth through activation of Akt and the epidermal growth factor receptor

The actions of prostaglandin E2 (PGE2) in the kidney are mediated by G protein-coupled E-prostanoid (EP) receptors, which affect renal growth and function. This report examines the role of EP receptors in mediating the effects of PGE2 on Madin-Darby canine kidney (MDCK) cell growth. The results indicate that activation of Gs-coupled EP2 and EP4 by PGE2 results in increased growth, while EP1 activation is growth inhibitory. Indeed, two EP1 antagonists (ONO-8711 and SC51089) stimulate, rather than inhibit, MDCK cell growth, an effect that is lost following an EP1 knockdown. Similar observations were made with M1 collecting duct and rabbit kidney proximal tubule cells. ONO-8711 even stimulates growth in the absence of exogenous PGE2, an effect that is prevented by ibuprofen (indicating a dependence upon endogenous PGE2). The involvement of Akt was indicated by the observation that 1) ONO-8711 and SC51089 increase Akt phosphorylation, and 2) MK2206, an Akt inhibitor, prevents the increased growth caused by ONO-8711. The involvement of the EGF receptor (EGFR) was indicated by 1) the increased phosphorylation of the EGFR caused by SC51089 and 2) the loss of the growth-stimulatory effect of ONO-8711 and SC51089 caused by the EGFR kinase inhibitor AG1478. The growth-stimulatory effect of ONO-8711 was lost following an EGFR knockdown, and transduction of MDCK cells with a dominant negative EGFR. These results support the hypothesis that 1) signaling via the EP1 receptor involves Akt as well as the EGFR, and 2), EP1 receptor pharmacology may be employed to prevent the aberrant growth associated with a number of renal diseases.

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

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

Direct visualization and quantitative analysis of water diffusion in complex biological tissues using CARS microscopy

To date, it has not been possible to measure microscopic diffusive water movements in epithelia and in the interstitial space of complex tissues and organs. Diffusive water movements are essential for life because they convey physiologically important small molecules, e.g. nutrients and signaling ligands throughout the extracellular space of complex tissues. Here we report the development of a novel method for the direct observation and quantitative analysis of water diffusion dynamics in a biologically organized tissue using Coherent Anti-Stokes Raman Scattering (CARS) microscopy. Using a computer simulation model to analyze the CARS O-H bond vibration data during H₂O/D₂O exchange in a 3D epithelial cyst, we succeeded in measuring the diffusive water permeability of the individual luminal and basolateral water pathways and also their response to hormonal stimulation. Our technique will be applicable to the measurement of diffusive water movements in other structurally complex and medically important tissues and organs.

Is there a role for PGE2 in urinary concentration?

Prostanoids are prominent, yet complex, components in the maintenance of body water homeostasis. Recent functional and molecular studies have revealed that the local lipid mediator PGE2 is involved both in water excretion and absorption. The biologic actions of PGE2 are exerted through four different G-protein-coupled receptors; designated EP1-4, which couple to separate intracellular signaling pathways. Here, we discuss new developments in our understanding of the actions of PGE2 that have been uncovered utilizing receptor specific agonists and antagonists, EP receptor and PG synthase knockout mice, polyuric animal models, and the new understanding of the molecular regulation of collecting duct water permeability. The role of PGE2 in urinary concentration comprises a variety of mechanisms, which are not fully understood and likely depend on which receptor is activated under a particular physiologic condition. EP3 and microsomal PG synthase type 1 play a role in decreasing collecting duct water permeability and increasing water excretion, whereas EP2 and EP4 can bypass vasopressin signaling and increase water reabsorption through two different intracellular signaling pathways. PGE2 has an intricate role in urinary concentration, and we now suggest how targeting specific prostanoid receptor signaling pathways could be exploited for the treatment of disorders in water balance.

AQP2 exocytosis in the renal collecting duct -- involvement of SNARE isoforms and the regulatory role of Munc18b

Vasopressin regulates the fusion of the water channel aquaporin 2 (AQP2) to the apical membrane of the renal collecting-duct principal cells and several lines of evidence indicate that SNARE proteins mediate this process. In this work MCD4 renal cells were used to investigate the functional role of a set of Q- and R-SNAREs, together with that of Munc18b as a negative regulator of the formation of the SNARE complex. Both VAMP2 and VAMP3 were associated with immunoisolated AQP2 vesicles, whereas syntaxin 3 (Stx3), SNAP23 and Munc18 were associated with the apical plasma membrane. Co-immunoprecipitation experiments indicated that Stx3 forms complexes with VAMP2, VAMP3, SNAP23 and Munc18b. Protein knockdown coupled to apical surface biotinylation demonstrated that reduced levels of the R-SNAREs VAMP2 and VAMP3, and the Q-SNAREs Stx3 and SNAP23 strongly inhibited AQP2 fusion at the apical membrane. In addition, knockdown of Munc18b promoted a sevenfold increase of AQP2 fused at the plasma membrane without forskolin stimulation. Taken together these findings propose VAMP2, VAMP3, Stx3 and SNAP23 as the complementary set of SNAREs responsible for AQP2-vesicle fusion into the apical membrane, and Munc18b as a negative regulator of SNARE-complex formation in renal collecting-duct principal cells.

Hydrogen peroxide stimulates chloride secretion in primary inner medullary collecting duct cells via mPGES-1-derived PGE2

We investigated the role and mechanism of H2O2 in regulation of NaCl transport in primary inner medullary collecting duct (IMCD) cells. IMCD cells were isolated from wild-type mice and grown onto semipermeable membranes, and short-circuit current (Isc) was determined by Ussing chamber. Exposure of IMCD cells to H2O2 at a range of 100-300 microM caused a rapid increase in Isc in a time- and dose-dependent manner. This increase was almost abolished by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel inhibitors diphenylamine-2-carboxylic acid (DPC) and CFTR inhibitor-172. In contrast, the magnitude of stimulation was unaffected by the epithelial Na+ channel (ENaC) inhibitor amiloride. The H2O2-induced Cl(-) secretion was significantly inhibited by indomethacin, as well as by microsomal PGE synthase-1 (mPGES-1) deficiency. Like H2O2, PGE2 treatment induced a twofold increase in Isc that was reduced by the protein kinase A (PKA) inhibitors H-89 and KT5720. These data suggest that H2O2 stimulates CFTR Cl(-) channel-mediated Cl(-) secretion through cyclooxygenase- and mPGES-1-dependent release of PGE2 and subsequent activation of PKA.

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

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