Effect of glucocorticoids on renal cortical NHE-3 and NHE-1 mRNA

The Post-Translational Modification Networking in WNK-Centric Hypertension Regulation and Electrolyte Homeostasis

The with-no-lysine (WNK) kinase family, comprising four serine-threonine protein kinases (WNK1-4), were first linked to hypertension due to their mutations in association with pseudohypoaldosteronism type II (PHAII). WNK kinases regulate crucial blood pressure regulators, SPAK/OSR1, to mediate the post-translational modifications (PTMs) of their downstream ion channel substrates, such as sodium chloride co-transporter (NCC), epithelial sodium chloride (ENaC), renal outer medullary potassium channel (ROMK), and Na/K/2Cl co-transporters (NKCCs). In this review, we summarize the molecular pathways dysregulating the WNKs and their downstream target renal ion transporters. We summarize each of the genetic variants of WNK kinases and the small molecule inhibitors that have been discovered to regulate blood pressure via WNK-triggered PTM cascades.

AMP-Activated Protein Kinase (AMPK)-Dependent Regulation of Renal Transport

AMP-activated kinase (AMPK) is a serine/threonine kinase that is expressed in most cells and activated by a high cellular AMP/ATP ratio (indicating energy deficiency) or by Ca²⁺. In general, AMPK turns on energy-generating pathways (e.g., glucose uptake, glycolysis, fatty acid oxidation) and stops energy-consuming processes (e.g., lipogenesis, glycogenesis), thereby helping cells survive low energy states. The functional element of the kidney, the nephron, consists of the glomerulus, where the primary urine is filtered, and the proximal tubule, Henle's loop, the distal tubule, and the collecting duct. In the tubular system of the kidney, the composition of primary urine is modified by the reabsorption and secretion of ions and molecules to yield final excreted urine. The underlying membrane transport processes are mainly energy-consuming (active transport) and in some cases passive. Since active transport accounts for a large part of the cell's ATP demands, it is an important target for AMPK. Here, we review the AMPK-dependent regulation of membrane transport along nephron segments and discuss physiological and pathophysiological implications.

Glucocorticoids Reduce Renal NHE8 Expression

The proximal tubule reabsorbs most of the filtered bicarbonate which is mediated in large part by Na⁺/H⁺ exchange (NHE). We have previously demonstrated that there is an isoform switch during postnatal maturation from NHE8 to NHE3 that is concordant with the postnatal increase in serum glucocorticoid levels. To examine if glucocorticoids may be responsible for this isoform switch, we administered dexamethasone daily to mice at 7–10 days of age, a time prior to the normal isoform switch. We show that compared to vehicle-treated controls, dexamethasone caused a premature increase in renal NHE3 and decrease in NHE8 mRNA, total protein, and brush border membrane protein abundance. To examine if there was a direct epithelial action of dexamethasone on NHE8, we studied normal rat kidney (NRK) cells in vitro which express NHE8 on their apical membrane. Dexamethasone decreased NHE8 mRNA, total protein, and apical protein abundance. Dexamethasone also decreased Na⁺/H⁺ exchanger activity. These studies provide evidence that glucocorticoids may play a role in the developmental isoform switch from NHE8 to NHE3 and cause a decrease in NHE8 expression and activity.

Chronic regulation of the renal Na(+)/H(+) exchanger NHE3 by dopamine: translational and posttranslational mechanisms

The intrarenal autocrine/paracrine dopamine (DA) system contributes to natriuresis in response to both acute and chronic Na(+) loads. While the acute DA effect is well described, how DA induces natriuresis chronically is not known. We used an animal and a cell culture model to study the chronic effect of DA on a principal renal Na(+) transporter, Na(+)/H(+) exchanger-3 (NHE3). Intraperitoneal injection of Gludopa in rats for 2 days elevated DA excretion and decreased total renal cortical and apical brush-border NHE3 antigen. Chronic treatment of an opossum renal proximal cell line with DA decreased NHE3 activity, cell surface and total cellular NHE3 antigen, but not NHE3 transcript. The decrease in NHE3 antigen was dose and time dependent with maximal inhibition at 16-24 h and half maximal effect at 3 × 10(-7) M. This is in contradistinction to the acute effect of DA on NHE3 (half maximal at 2 × 10(-6) M), which was not associated with changes in total cellular NHE3 protein. The DA-induced decrease in total NHE3 protein was associated with decrease in NHE3 translation and mediated by cis-sequences in the NHE3 5'-untranslated region. DA also decreased cell surface and total cellular NHE3 protein half-life. The DA-induced decrease in total cellular NHE3 was partially blocked by proteasome inhibition but not by lysosome inhibition, and DA increased ubiquitylation of total and surface NHE3. In summary, chronic DA inhibits NHE3 with mechanisms distinct from its acute action and involves decreased NHE3 translation and increased NHE3 degradation, which are novel mechanisms for NHE3 regulation.

Regulation of epithelial Na+ channels by adrenal steroids: mineralocorticoid and glucocorticoid effects

Epithelial Na+ channels (ENaC) can be regulated by both mineralocorticoid and glucocorticoid hormones. In the mammalian kidney, effects of mineralocorticoids have been extensively studied, but those of glucocorticoids are complicated by metabolism of the hormones and cross-occupancy of mineralocorticoid receptors. Here, we report effects of dexamethasone, a synthetic glucocorticoid, on ENaC in the rat kidney. Infusion of dexamethasone (24 μg/day) for 1 wk increased the abundance of αENaC 2.26 ± 0.04-fold. This was not accompanied by an induction of Na+ currents (I(Na)) measured in isolated split-open collecting ducts. In addition, hormone treatment did not increase the abundance of the cleaved forms of either αENaC or γENaC or the expression of βENaC or γENaC protein at the cell surface. The absence of hypokalemia also indicated the lack of ENaC activation in vivo. Dexamethasone increased the abundance of the Na+ transporters Na+/H+ exchanger 3 (NHE3; 1.36 ± 0.07-fold), Na(+)-K(+)-2Cl(-) cotransporter 2 (NKCC2; 1.49 ± 0.07-fold), and Na-Cl cotransporter (NCC; 1.72 ± 0.08-fold). Surface expression of NHE3 and NCC also increased with dexamethasone treatment. To examine whether glucocorticoids could either augment or inhibit the effects of mineralocorticoids, we infused dexamethasone (60 μg/day) together with aldosterone (12 μg/day). Dexamethasone further increased the abundance of αENaC in the presence of aldosterone, suggesting independent effects of the two hormones on this subunit. However, I(Na) was similar in animals treated with dexamethasone+aldosterone and with aldosterone alone. We conclude that dexamethasone can occupy glucocorticoid receptors in cortical collecting duct and induce the synthesis of αENaC. However, this induction is not sufficient to produce an increase in functional Na+ channels in the apical membrane, implying that the abundance of αENaC is not rate limiting for channel formation in the kidney.

Transcriptional regulation of the intestinal luminal Na⁺ and Cl⁻ transporters

The epithelial apical membrane Na+/H+ exchangers [NHE (sodium hydrogen exchanger)2 and NHE3] and Cl-/HCO3- exchangers [DRA (down-regulated in adenoma) and PAT-1 (putative anion transporter 1)] are key luminal membrane transporters involved in electroneutral NaCl absorption in the mammalian intestine. During the last decade, there has been a surge of studies focusing on the short-term regulation of these electrolyte transporters, particularly for NHE3 regulation. However, the long-term regulation of the electrolyte transporters, involving transcriptional mechanisms and transcription factors that govern their basal regulation or dysregulation in diseased states, has only now started to unfold with the cloning and characterization of their gene promoters. The present review provides a detailed analysis of the core promoters of NHE2, NHE3, DRA and PAT-1 and outlines the transcription factors involved in their basal regulation as well as in response to both physiological (butyrate, protein kinases and probiotics) and pathophysiological (cytokines and high levels of serotonin) stimuli. The information available on the transcriptional regulation of the recently identified NHE8 isoform is also highlighted. Therefore the present review bridges a gap in our knowledge of the transcriptional mechanisms underlying the alterations in the gene expression of intestinal epithelial luminal membrane Na+ and Cl- transporters involved in electroneutral NaCl absorption. An understanding of the mechanisms of the modulation of gene expression of these transporters is important for a better assessment of the pathophysiology of diarrhoea associated with inflammatory and infectious diseases and may aid in designing better management protocols.

A calcineurin homologous protein is required for sodium-proton exchange events in the C. elegans intestine

Caenorhabditis elegans defecation is a rhythmic behavior, composed of three sequential muscle contractions, with a 50-s periodicity. The motor program is driven by oscillatory calcium signaling in the intestine. Proton fluxes, which require sodium-proton exchangers at the apical and basolateral intestinal membranes, parallel the intestinal calcium flux. These proton shifts are critical for defecation-associated muscle contraction, nutrient uptake, and longevity. How sodium-proton exchangers are activated in time with intestinal calcium oscillation is not known. The posterior body defecation contraction mutant (pbo-1) encodes a calcium-binding protein with homology to calcineurin homologous proteins, which are putative cofactors for mammalian sodium-proton exchangers. Loss of pbo-1 function results in a weakened defecation muscle contraction and a caloric restriction phenotype. Both of these phenotypes also arise from dysfunctions in pH regulation due to mutations in intestinal sodium-proton exchangers. Dynamic, in vivo imaging of intestinal proton flux in pbo-1 mutants using genetically encoded pH biosensors demonstrates that proton movements associated with these sodium-proton exchangers are significantly reduced. The basolateral acidification that signals the first defecation motor contraction is scant in the mutant compared with a normal animal. Luminal and cytoplasmic pH shifts are much reduced in the absence of PBO-1 compared with control animals. We conclude that pbo-1 is required for normal sodium-proton exchanger activity and may couple calcium and proton signaling events.

Serum- and glucocorticoid-induced kinase 3 in recycling endosomes mediates acute activation of Na+/H+ exchanger NHE3 by glucocorticoids

SGK1 plays an important role in regulation of Na⁺/H⁺ exchanger (NHE) 3 in vivo. We show that SGK3 colocalizes with NHE3 in recycling endosomes. These studies identify SGK3 as the effector of the PI3K pathway that activates NHE3 and show that endosomal localization of SGK3 is essential for acute activation of NHE3.

Na⁺/H⁺ exchanger 3 (NHE3) is the major Na⁺ transporter in the intestine. Serum- and glucocorticoid-induced kinase (SGK) 1 interacts with NHE regulatory factor 2 (NHERF2) and mediates activation of NHE3 by dexamethasone (Dex) in cultured epithelial cells. In this study, we compared short-term regulation of NHE3 by Dex in SGK1-null and NHERF2-null mice. In comparison to wild-type mice, loss of SGK1 or NHERF2 significantly attenuated regulation of NHE3 by Dex but did not completely obliterate the effect. We show that transfection of SGK2 or SGK3 in PS120 cells resulted in robust activation of NHE3 by Dex. However, unlike SGK1 or SGK2, SGK3 rapidly activated NHE3 within 15 min of Dex treatment in both PS120 and Caco-2bbe cells. Immunofluorescence analysis showed that SGK3 colocalized with NHE3 in recycling endosomes, whereas SGK1 and SGK2 were diffusely distributed. Mutation of Arg-90 of SGK3 disrupted the endosomal localization of SGK3 and delayed NHE3 activation. Activation of SGK3 and NHE3 by Dex was dependent on phosphoinositide 3-kinase (PI3K) and phosphoinositide-dependent kinase 1 (PDK1), and Dex induced translocation of PDK1 to endosomes. Our study identifies SGK3 as a novel endosomal kinase that acutely regulates NHE3 in a PI3K-dependent mechanism.

Mechanisms of the regulation of the intestinal Na+/H+ exchanger NHE3

A major of Na⁺ absorptive process in the proximal part of intestine and kidney is electroneutral exchange of Na⁺ and H⁺ by Na⁺/H⁺ exchanger type 3 (NHE3). During the past decade, significant advance has been achieved in the mechanisms of NHE3 regulation. A bulk of the current knowledge on Na⁺/H⁺ exchanger regulation is based on heterologous expression of mammalian Na⁺/H⁺ exchangers in Na⁺/H⁺ exchanger deficient fibroblasts, renal epithelial, and intestinal epithelial cells. Based on the reductionist's approach, an understanding of NHE3 regulation has been greatly advanced. More recently, confirmations of in vitro studies have been made using animals deficient in one or more proteins but in some cases unexpected findings have emerged. The purpose of this paper is to provide a brief overview of recent progress in the regulation and functions of NHE3 present in the luminal membrane of the intestinal tract.

Luminal Na(+)/H (+) exchange in the proximal tubule

The proximal tubule is critical for whole-organism volume and acid-base homeostasis by reabsorbing filtered water, NaCl, bicarbonate, and citrate, as well as by excreting acid in the form of hydrogen and ammonium ions and producing new bicarbonate in the process. Filtered organic solutes such as amino acids, oligopeptides, and proteins are also retrieved by the proximal tubule. Luminal membrane Na(+)/H(+) exchangers either directly mediate or indirectly contribute to each of these processes. Na(+)/H(+) exchangers are a family of secondary active transporters with diverse tissue and subcellular distributions. Two isoforms, NHE3 and NHE8, are expressed at the luminal membrane of the proximal tubule. NHE3 is the prevalent isoform in adults, is the most extensively studied, and is tightly regulated by a large number of agonists and physiological conditions acting via partially defined molecular mechanisms. Comparatively little is known about NHE8, which is highly expressed at the lumen of the neonatal proximal tubule and is mostly intracellular in adults. This article discusses the physiology of proximal Na(+)/H(+) exchange, the multiple mechanisms of NHE3 regulation, and the reciprocal relationship between NHE3 and NHE8 at the lumen of the proximal tubule.

Effect of claudins 6 and 9 on paracellular permeability in MDCK II cells

The neonatal proximal tubule has a lower permeability to chloride, higher resistance, and higher relative sodium-to-chloride permeability (P(Na)/P(Cl)) than the adult tubule, which may be due to maturational changes in the tight junction. Claudins are tight-junction proteins between epithelial cells that determine paracellular permeability characteristics of epithelia. We have previously described the presence of two claudin isoforms, claudins 6 and 9, in the neonatal proximal tubule and subsequent reduction of these claudins during postnatal maturation. The question is whether changes in claudin expression are related to changes in functional characteristics in the neonatal tubule. We transfected claudins 6 and 9 into Madin-Darby canine kidney II (MDCK II) cells and performed electrophysiological studies to determine the resultant changes in physiological characteristics of the cells. Expression of claudins 6 and 9 resulted in an increased transepithelial resistance, decreased chloride permeability, and decreased P(Na)/P(Cl) and P(HCO3)/P(Cl). These findings constitute the first characterization of the permeability characteristics of claudins 6 and 9 in a cell model and may explain why the neonatal proximal tubule has lower permeability to chloride and higher resistance than the adult proximal tubule.

Developmental changes in proximal tubule NaCl transport

The proximal tubule reabsorbs two thirds of the filtered NaCl in an isoosmotic fashion. In the adult proximal tubule, active NaCl transport is mediated by the parallel operation of Na(+)/H(+) and Cl(-)/base exchangers, and a substantive amount of chloride transport occurs passively across the paracellular pathway. Studies in the neonatal proximal tubule have resulted in unexpected results. The isoform of the Na(+)/H(+) exchanger mediating proximal tubule sodium absorption, NHE3, is virtually absent in the neonatal rat kidney. NHE8, an isoform of the Na(+)/H(+) exchange, in low abundance on the apical membrane of the adult proximal tubule, is present in high abundance in the neonatal segment. Whereas chloride permeability is high in the adult, favoring passive paracellular chloride flux, the neonatal proximal tubule is virtually impermeable to chloride ions. This is again due to a developmental change in isoforms of proteins forming the tight junction. The permeability properties of epithelia are due to a family of tight junction proteins called claudins. Claudins 6 and 9 are expressed in the neonatal proximal tubule at a time when chloride permeability is low, but these claudin isoforms are virtually absent in the adult segment. The causes for these postnatal changes in proximal tubular transport and developmental isoform changes are also discussed in this review.

Human renal 11beta-hydroxysteroid dehydrogenase 1 functions and co-localizes with COX-2

The local renal metabolism of glucocorticoids (GCs) by isoforms of 11beta-hydroxysteroid dehydrogenase (11beta-HSD1 and 11beta-HSD2) determines their biological effects. 11beta-HSD2, located in collecting duct epithelial cells of the mammalian and human kidney, serves as a putative "guardian" preventing GCs from binding to mineralocorticoid receptors. Various investigators have shown that both isoforms are present in kidney tissue from the rat, dog and other mammals. There is controversy as to whether 11beta-HSD1 exists and functions in human kidney. The current studies examine the locale and function of both isoforms in human kidney. The expression of 11beta-HSD1 was similar to that of 11beta-HSD2 by Western blot. Two distinct Lineweaver Burke plots could be drawn providing enzyme kinetics for both isoforms. The apparent Km for the NADP dependent 11beta-HSD1 enzyme was 0.42 muM while the apparent Km for the NAD dependent 11beta-HSD2 enzyme was 10.2 nM. Human renal 11beta-HSD1 appears to function as a dehydrogenase with no significant "reverse" reductase activity. Using immuno-histochemistry and Western blot analysis, 11beta-HSD1 was found to co-localize with COX-2 in proximal tubule cells; COX-2 was not seen with 11beta-HSD2 in cortical collecting duct. Thus, normal human kidney contains active 11beta-HSD1 and 11beta-HSD2. 11beta-HSD1 co-localizes with COX-2 in proximal tubule cells.

Effect of chronic elevated carbon dioxide on the expression of acid-base transporters in the neonatal and adult mouse

Several pulmonary and neurological conditions, both in the newborn and adult, result in hypercapnia. This leads to disturbances in normal pH homeostasis. Most mammalian cells maintain tight control of intracellular pH (pHi) using a group of transmembrane proteins that specialize in acid-base transport. These acid-base transporters are important in adjusting pHiduring acidosis arising from hypoventilation. We hypothesized that exposure to chronic hypercapnia induces changes in the expression of acid-base transporters. Neonatal and adult CD-1 mice were exposed to either 8% or 12% CO2for 2 wk. We used Western blot analysis of membrane protein fractions from heart, kidney, and various brain regions to study the response of specific acid-base transporters to CO2. Chronic CO2increased the expression of the sodium hydrogen exchanger 1 (NHE1) and electroneutral sodium bicarbonate cotransporter (NBCn1) in the cerebral cortex, heart, and kidney of neonatal but not adult mice. CO2increased the expression of electrogenic NBC (NBCe1) in the neonatal but not the adult mouse heart and kidney. Hypercapnia decreased the expression of anion exchanger 3 (AE3) in both the neonatal and adult brain but increased AE3 expression in the neonatal heart. We conclude that: 1) chronic hypercapnia increases the expression of the acid extruders NHE1, NBCe1 and NBCn1 and decreases the expression of the acid loader AE3, possibly improving the capacity of the cell to maintain pHiin the face of acidosis; and 2) the heterogeneous response of tissues to hypercapnia depends on the level of CO2and development.

Effect of osmotic shrinkage and hormones on the expression of Na+/H+ exchanger-1, Na+/K+/2Cl- cotransporter and Na+/K+ -ATPase in gill pavement cells of freshwater adapted Japanese eel, Anguilla japonica

It is well-known that gill epithelial cells are important in fish osmoregulation. However, studies on the effect of osmotic stress on the direct cellular responses of the gill epithelial cells are limited. In this paper, we aimed to determine the effects of osmotic hypertonicity, hormones and cellular signaling molecules on the expression of ion transporters in the cultured primary freshwater pavement cells (PVCs), prepared from freshwater-adapted eels (Anguilla japonica). Our data demonstrated that the hypertonic (500 mOsmol l(-1)) treatment of the isolated PVCs induced cell shrinkage, followed by regulatory volume increase (RVI). Application of blockers (i.e. ouabain, bumetanide and EIPA) demonstrated that Na+/K+ -ATPase, Na+/K+/2Cl- cotransporter (NKCC) and Na+/H+ exchanger-1 (NHE-1) were involved in RVI. Western blot analysis of the hypertonic-treated cells revealed a significant induction of NHE-1, NKCC and, alpha and beta subunits of Na+/K+ -ATPase. In nonshrunken cultured PVCs, we found that dexamethasone and dibutyryl cAMP treatments significantly stimulated the expression levels of the three ion transporters. Both prolactin and insulin-like growth factor-1, can only induce the expression of NKCC. The effect of thyroid hormone (T3) and dibutyryl cGMP was negligible. In this study, the induction of ion transporter expression was found to be post-transcriptionally regulated as no significant change in mRNA levels was detected. This observation implies that the regulation is rapid and is probably induced via nongenomic actions.

Regulation of expression and localisation of the Na+/H+ exchanger (NHE) 3 and the NHE regulatory factor 2 in baboon placental syncytiotrophoblast by oestrogen

Our understanding of the regulation of the expression of the sodium hydrogen exchangers (NHE) and their regulatory factors (NHERF), which play important roles in fetal-placental homeostasis, is incomplete. We previously showed that the expression and localisation of NHE3 and NHERF2 in the juxtanuclear compartment of the placental syncytiotrophoblast were markedly decreased between mid and late baboon pregnancy. In the current study, immunocytochemical fluorescence localisation and level of NHE3/NHE1 and NHERF1/NHERF2 proteins were determined in late gestation in baboons untreated or treated throughout the second half of gestation with an aromatase inhibitor CGS 20267 alone (reduced oestrogen levels by >95%) or with oestradiol to determine whether oestrogen regulated antiporter developmental expression. The immunocytochemical expression of NHE3 and NHERF2 in the juxtanuclear compartment was minimal in baboons untreated or treated with CGS 20267 plus oestradiol (i.e. oestrogen-replete) but extensive in oestrogen-suppressed animals. Moreover, the abundant expression of NHERF2 in fetal vascular endothelium of oestrogen-replete baboons was decreased in oestrogen-suppressed animals. In contrast, expression and localisation of NHE1 and NHERF1 in the placental syncytiotrophoblast were not altered by oestrogen deprivation in baboons. Based on our current and previous findings, we propose that oestrogen plays an important role in regulating localisation and expression of components of the NHE system within and consequently development and function of the primate placental syncytiotrophoblast.

Na+/H+ exchangers in renal regulation of acid-base balance

The kidney plays key roles in extracellular fluid pH homeostasis by reclaiming bicarbonate (HCO(3)(-)) filtered at the glomerulus and generating the consumed HCO(3)(-) by secreting protons (H(+)) into the urine (renal acidification). Sodium-proton exchangers (NHEs) are ubiquitous transmembrane proteins mediating the countertransport of Na(+) and H(+) across lipid bilayers. In mammals, NHEs participate in the regulation of cell pH, volume, and intracellular sodium concentration, as well as in transepithelial ion transport. Five of the 10 isoforms (NHE1-4 and NHE8) are expressed at the plasma membrane of renal epithelial cells. The best-studied isoform for acid-base homeostasis is NHE3, which mediates both HCO(3)(-) absorption and H(+) excretion in the renal tubule. This article reviews some important aspects of NHEs in the kidney, with special emphasis on the role of renal NHE3 in the maintenance of acid-base balance.

Glucocorticoids acutely increase cell surface Na+/H+ exchanger-3 (NHE3) by activation of NHE3 exocytosis

Glucocorticoids have important effects on renal function, including the modulation of renal acidification by the major proximal tubular Na(+)/H(+) exchanger, NHE3. While the chronic effect of glucocorticoids is considered to be primarily at the transcriptional level, with increases in NHE3 mRNA and protein expression driving increased transport activity, the mechanisms by which glucocorticoids activate NHE3 in an acute setting have not been investigated. Previous studies have shown that a glucocorticoid-stimulated increase in NHE3 activity can occur before any detectable change in NHE3 mRNA. The present study examines the acute effects of glucocorticoids on NHE3 using opossum kidney (OKP) cells as a cell model. In OKP cells, total NHE3 protein abundance was not changed by 3 h of treatment with dexamethasone (10(-6) M). However, the biotin-accessible fraction representing NHE3 at the apical membrane as well as Na(+)/H(+) exchange activity measured fluorimetrically using the pH-sensitive dye BCECF-AM were significantly increased. These effects were not prevented by the protein synthesis inhibitor cycloheximide. NHE3 insertion (biotinylatable NHE3 after sulfo-NHS-acetate blockade) was stimulated by dexamethasone incubation, with or without cycloheximide. The rate of NHE3 endocytic retrieval, assessed either by the avidin protection assay (early endocytosis) or by the sodium 2-mercaptoethane sulfonate (MesNa) cleavage assay (early and late endocytosis), was not affected by dexamethasone. These findings suggest that trafficking plays a key role in the acute stimulation of NHE3 by glucocorticoids, with exocytosis being the major contributor to the glucocorticoid-induced rapid increase in cell surface NHE3 protein abundance and Na(+)/H(+) exchange activity.

Activation of NHE3 by dexamethasone requires phosphorylation of NHE3 at Ser663 by SGK1

Glucocorticoids stimulate Na+ absorption by activation of the epithelial Na+/H+ exchanger NHE3 in the kidney and intestine. It has been thought that glucocorticoid-induced activation of NHE3 is solely dependent on transcriptional induction of the NHE3 gene. While the transcriptional regulation remains an essential part of the chronic effect of glucocorticoids, a previous study by us identified the serum- and glucocorticoid-inducible kinase 1 (SGK1) as an important component of the activation of NHE3 by glucocorticoids. In this work, we have demonstrated phosphorylation of NHE3 by SGK1 as the key mechanism for the stimulation of the transport activity by glucocorticoids. By using in vitro SGK1 kinase assay and site-directed mutagenesis, we have identified Ser663 of NHE3 to be the major site of phosphorylation by SGK1. Ser663 is invariantly conserved in all NHE3 proteins from several species, and the mutation of Ser663 to Ala blocks the effect of dexamethasone, demonstrating the importance of phosphorylation at Ser663. We also show that phosphorylation of NHE3 precedes the changes in NHE3 activity, and the increased activity is associated with an increased amount of NHE3 proteins in the surface membrane. These data reveal that dexamethasone activates NHE3 activity by phosphorylating the NHE3 protein, which initiates trafficking of the protein into the plasma membrane.

Maturation of the Na+/H+ antiporter (NHE3) in the proximal tubule of the hypothyroid adrenalectomized rat

In previous studies examining the role of glucocorticoids and thyroid hormone on the maturation of the Na(+)/H(+) antiporter (NHE3), we found attenuation in the maturational increase in proximal tubule apical Na(+)/H(+) antiporter activity but no change in NHE3 mRNA abundance in either glucocorticoid-deficient or hypothyroid rats. In addition, prevention of the maturational increase in either hormone failed to totally prevent the maturational increase in Na(+)/H(+) antiporter activity. We hypothesized that one hormone played a compensatory role when the other was deficient. The present study examined whether combined deficiency of thyroid and glucocorticoid hormones would completely prevent the maturation of the Na(+)/H(+) antiporter. Adrenalectomy was performed in 9-day-old hypothyroid Sprague-Dawley rats, a time before the normal postnatal maturational increase in these hormones occurs. Nine- and 30-day-old adrenalectomized (ADX), hypothyroid rats had comparable NHE3 mRNA abundance, which was 5- to 10-fold less than 30-day-old ADX, hypothyroid rats that received corticosterone-thyroxine replacement and 30-day-old sham control rats (P < 0.05). Brush-border membrane NHE3 protein abundance was comparable in 9- and 30-day-old ADX, hypothyroid groups and approximately 20-fold lower than both the 30-day replacement and 30-day sham groups (P < 0.05). Similarly, the replacement and sham groups had higher sodium-dependent proton secretion than 9- and 30-day-old ADX, hypothyroid groups (P < 0.05). We conclude that combined deficiency of both hormones totally prevents the maturational increase in NHE3 mRNA and protein abundance and Na(+)/H(+) antiporter activity.

Regulation of butyrate uptake in Caco-2 cells by phorbol 12-myristate 13-acetate

Butyrate and the other short-chain fatty acids (SCFAs) are the most abundant anions in the colonic lumen. Also, butyrate is the preferred energy source for colonocytes and has been shown to regulate colonic electrolyte and fluid absorption. Previous studies from our group have demonstrated that the HCO(3)(-)/SCFA(-) anion exchange process is one of the major mechanisms of butyrate transport across the purified human colonic apical membrane vesicles and the apical membrane of human colonic adenocarcinoma cell line Caco-2 and have suggested that it is mainly mediated via monocarboxylate transporter-1 (MCT-1) isoform. However, little is known regarding the regulation of SCFA transport by various hormones and signal transduction pathways. Therefore, the present studies were undertaken to examine whether hydrocortisone and phorbol 12-myristate 13-acetate (PMA) are involved in a possible regulation of the butyrate/anion exchange process in Caco-2 cells. The butyrate/anion exchange process was assessed by measuring a pH-driven [(14)C]butyrate uptake in Caco-2 cells. Our results demonstrated that 24-h incubation with PMA (1 microM) significantly increased [(14)C]butyrate uptake compared with incubation with 4alphaPMA (inactive form). In contrast, incubation with hydrocortisone had no significant effect on butyrate uptake in Caco-2 cells compared with vehicle (ethanol) alone. Induction of butyrate uptake by PMA appeared to be via an increase in the maximum velocity (V(max)) of the transport process with no significant changes in the K(m) of the transporter for butyrate. Parallel to the increase in the V(max) of [(14)C]butyrate uptake, the MCT-1 protein level was also increased in response to PMA incubation. Our studies demonstrated that the butyrate/anion exchange was increased in response to PMA treatment along with the induction in the level of MCT-1 expression in Caco-2 cells.

Dopamine acutely decreases apical membrane Na/H exchanger NHE3 protein in mouse renal proximal tubule

BACKGROUND

Dopamine is a principal natriuretic hormone in mammalian Na+ homeostasis. Dopamine acutely alters glomerular filtration rate (GFR) and decreases Na+ absorption in both the proximal and distal nephron. Proximal tubule natriuresis is effected through inhibition of the apical membrane Na/H exchanger NHE3.

METHODS

We examined whether dopamine directly and acutely decreases apical membrane NHE3 protein using renal tissue in two in vitro systems: renal cortical slices and in vitro perfused single tubules. After incubation with dopamine, NHE3 activity was measured by 22Na flux and NHE3 antigen was measured by immunoblot in apical membrane and total cellular membranes.

RESULTS

Direct application of dopamine to either cortical slices or microperfused tubules acutely decreases NHE3 activity and antigen at the apical membrane of the proximal tubule. No change in total cellular NHE3 was detected.

CONCLUSION

One mechanism by which dopamine causes natriuresis is via direct and acute reduction of NHE3 protein at the apical membrane via changes in NHE3 protein trafficking.

Concerted roles of SGK1 and the Na+/H+ exchanger regulatory factor 2 (NHERF2) in regulation of NHE3

Na+/H+ exchanger regulatory factors, NHERF1 and NHERF2, are structurally related proteins and highly expressed in epithelial cells. These proteins are initially identified as accessory proteins in the regulation of Na+/H+ exchanger isoform 3, NHE3. In addition to regulation of NHE3, recent studies demonstrate the importance of NHERF1 and NHERF2 in recycling and localization of membrane receptors, ion channels and transporters. Recent studies show that serum- and glucocorticoid-induced kinase 1 (SGK1) specifically interacts with NHERF2 but not with NHERF1, adding to the growing number of differences between the two proteins. The association of SGK1 with NHERF2 is necessary for stimulation of NHE3 activity by glucocorticoids. In addition, SGK1 together with NHERF2 stimulates the K+ channel ROMK1, suggesting a broader role of SGK1 in regulation of ion transport.

Epidermal growth factor regulation of rat NHE2 gene expression

Epidermal growth factor (EGF) is involved in acute regulation of Na(+)/H(+) exchangers (NHEs), but the effect of chronic EGF administration on NHE gene expression is unknown. The present studies showed that EGF treatment increased NHE2-mediated intestinal brush-border membrane vesicle Na(+) absorption and NHE2 mRNA abundance by nearly twofold in 19-day-old rats. However, no changes were observed in renal NHE2 mRNA or intestinal and renal NHE3 mRNA abundance. To understand the mechanism of this regulation, we developed the rat intestinal epithelial (RIE) cell as an in vitro model to study the effect of EGF on NHE2 gene expression. EGF increased functional NHE2 activity and mRNA abundance in cultured RIE cells, and this stimulation could be blocked by actinomycin D (a transcriptional inhibitor). Additionally, NHE2 promoter reporter gene assays in transiently transfected RIE cells showed an almost twofold increase in promoter activity after EGF treatment. We conclude that rat NHE2 activity can be stimulated by chronic EGF treatment and that this response is at least partially mediated by gene transcription.

Role of glucocorticoids in the maturation of the rat renal Na+/H+ antiporter (NHE3)

BACKGROUND

Neonates have a lower Na+/H+ antiporter activity on the apical membrane of proximal tubule than that of adults. The maturational increase in Na+/H+ antiporter activity occurs at the time when there is a rise in serum glucocorticoid levels in rats. The purpose of the present study was to examine whether glucocorticoids are responsible for the postnatal increase in Na+/H+ antiporter activity.

METHODS

Nine-day-old Sprague-Dawley rats were compared with rats studied at 30 days of age who had either a sham operation or adrenalectomy (ADX) at nine days of age and with rats that had an adrenalectomy and physiologic corticosterone replacement (ADX-Cort) to determine whether glucocorticoid deficiency prevented the maturational increase in Na+/H+ antiporter activity. Na+/H+ antiporter activity was measured in proximal convoluted tubules perfused in vitro by the change in cell pH (pHi) following luminal sodium removal. NHE3 mRNA abundance was measured using Northern blot analysis, and NHE3 protein abundance was measured by immunoblot.

RESULTS

Na+/H+ antiporter activity was 93.8 +/- 17.7, 157.0 +/- 18.0, 356.7 +/- 29.9, and 402.5 +/- 14.5 pmol/mm. min in nine-day-old, ADX, ADX-Cort, and sham control groups, respectively. The ADX-Cort and sham control were higher than the 9-day-old and the 30-day-old ADX group (P < 0.05). Brush-border membrane NHE3 protein abundance in the nine-day-old and ADX groups were sixfold less than ADX-Cort and sham control groups (P < 0.001). Nine-day-old neonates had fivefold less renal cortical NHE3 mRNA than the ADX, ADX-Cort, and sham-operated control groups (P < 0.01).

CONCLUSIONS

These data demonstrate that glucocorticoids play a role in the postnatal maturation of the proximal tubule Na+/H+ antiporter activity and brush-border membrane NHE3 protein abundance. Glucocorticoid deficiency does not completely prevent the maturational increase in Na+/H+ antiporter activity and does not affect NHE3 mRNA abundance.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Effect of angiotensin II on rat renal cortical 11beta-hydroxysteroid dehydrogenase

Renal 11beta-hydroxysteroid dehydrogenases (11beta-HSDs) are subject to modulation by various endogenous factors. 11beta-HSDs convert glucocorticoids into inactive 11-ketones and thereby determine tissue levels of active glucocorticoids and thus the extent of glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) activation. As such, modulation of the activity of renal 11beta-HSDs may contribute to the cascade of regulatory events involved in renal electrolyte water handling. We investigated whether renal 11beta-HSDs are modulated by elevated circulating angiotensin II. In rats infused for 2 wk with angiotensin II (250 ng/[kg x min] subcutaneously), plasma angiotensin II, aldosterone, and corticosterone were raised 5.1-, 10.7-, and 2.3-fold, respectively, compared with control rats. Angiotensin II infusion raised corticosterone 11beta-oxidation 1.46- and 1.35-fold in renal cortical proximal and distal tubules (enriched by Percoll centrifugation), respectively, but had no effect on 11beta-HSD1 and 11beta-HSD2 mRNA levels (semiquantitative reverse transcriptase polymerase chain reaction), except for distal tubular 11beta-HSD1 mRNA, which was decreased to 50%. In vitro treatment of freshly isolated tubules with angiotensin II for 45 min prior to assessment of 11beta-HSD activity showed no direct acute effects of angiotensin II on tubular corticosterone 11beta-oxidation. The enhanced renal tubular corticosterone 11beta-oxidation in vivo may partly protect renal GR and MR from elevated plasma corticosterone on angiotensin II infusion.

Glucocorticoids enhance acid activation of the Na+/H+ exchanger 3 (NHE3)

In the absence of exogenous glucocorticoids, decreasing media pH (from 7.4 to 6.8) for 24 hours increased the Na+/H+ exchanger 3 (NHE3) activity in opossum kidney (OKP) cells. 10(-7) M and 10(-8) M hydrocortisone increased NHE3 activity, and in their presence, acid incubation further increased NHE3 activity. Hydrocortisone (10(-9) M) had no effect on NHE3 activity, but in its presence, the effect of acid incubation on NHE3 activity increased twofold. Aldosterone (10(-8) M) had no effect. In the absence of hydrocortisone, acid incubation increased NHE3 protein abundance by 47%; in the presence of 10(-9) M hydrocortisone, acid incubation increased NHE3 protein abundance by 132%. The increase in NHE3 protein abundance was dependent on protein synthesis. However, 10(-9) M hydrocortisone did not modify the effect of acid incubation to cause a twofold increase in NHE3 mRNA abundance. In the absence of protein synthesis, 10(-9) M hydrocortisone did potentiate an effect of acid on NHE3 activity, which was due to trafficking of NHE3 to the apical membrane. These results suggest that glucocorticoids and acid interact synergistically at the level of NHE3 translation and trafficking.

Renal Na/H exchanger NHE-3 and Na-PO4 cotransporter NaPi-2 protein expression in glucocorticoid excess and deficient states

Administration of pharmacologic doses of glucocorticoid in vivo increases renal proximal tubule apical membrane Na/H exchange and decreases Na/PO4 cotransport activity (1). Current data suggest that the NHE-3 and NaPi-2 proteins mediate significant fractions of proximal tubule apical membrane Na/H exchange and Na/PO4 cotransport, respectively. This study examines whether glucocorticoid excess or deficiency affects NHE-3 and NaPi-2 protein abundance and the intrarenal distribution of these transporters. Protein abundance of NHE-3 and NaPi-2 in control rats was compared to rats rendered glucocorticoid-deficient by bilateral adrenalectomy, and to rats receiving pharmacologic doses of dexamethasone using immunoblots and immunohistochemistry. Adrenalectomy had modest effects on NHE-3 protein abundance, but dexamethasone administration to either adrenalectomized or sham-operated rats significantly increased NHE-3 protein abundance in both the proximal tubule and thick ascending limb, but not the thin descending limb. Adrenalectomy increased NaPi-2 protein abundance in the proximal tubule, whereas dexamethasone administration dramatically suppressed NaPi-2 protein on the apical membrane in both adrenalectomized and sham-operated animals. No significant reciprocal increase in subapical NaPi-2 staining was seen in the dexamethasone-treated rats. The present study shows that glucocorticoids regulate proximal tubule apical membrane Na/H exchange and NaPi cotransport by changes in protein abundance of NHE-3 and NaPi-2, respectively.

Activation of protein kinase A acutely inhibits and phosphorylates Na/H exchanger NHE-3

In the mammalian renal proximal tubule, protein kinase A (PKA) plays an important role in mediating hormonal regulation of apical membrane Na/H exchanger activity. This exchanger is likely encoded by NHE-3. The present studies examined regulation of NHE-3 by PKA. NHE-3 was stably expressed in Na/H exchanger-deficient fibroblasts (AP-1/NHE-3 cells). PKA activation (0.1 mM 8-BrcAMP x 20 min) inhibited NHE-3 activity by 39% (P < 0.01) with no change in NHE-3 protein abundance in the plasma membrane. To define the structural requirements for PKA-mediated inhibition, full-length NHE-3 and a cytoplasmic domain-truncated mutant (NHE-3 delta cyto) were expressed in Xenopus laevis oocytes. 8-BrcAMP inhibited NHE-3 activity by 27% (P < 0.05), an effect that was blocked by 10(-7) M PKA inhibitor peptide. NHE-3 delta cyto had baseline activity similar to that of full-length NHE-3 but its activity was not regulated by 8-BrcAMP. The purified recombinant cytoplasmic domain of NHE-3 was phosphorylated in vitro by the catalytic subunit of PKA on serine residues. In AP-1/NHE-3 cells, NHE-3 was immunoprecipitated as a approximately 87-kD phosphoprotein. Addition of 0.1 mM 8-BrcAMP increased the phosphocontent of NHE-3 by threefold. In summary, acute activation of PKA inhibits NHE-3 activity, an effect that is likely mediated by phosphorylation of its cytoplasmic domain.

Overexpression of csk inhibits acid-induced activation of NHE-3

Opossum kidney OKP cells express an apical membrane Na+/H+ antiporter that is encoded by NHE-3 (for Na+/H+ exchanger 3) and is similar in many respects to the renal proximal tubule apical membrane Na+/H+ antiporter. Chronic incubation of OKP cells in acid medium for 24 hr increases Na+/H(+)-antiporter activity and NHE-3 mRNA abundance. The increase in Na+/H(+)-antiporter activity was not prevented by H7, a protein kinase C/protein kinase A inhibitor, but was prevented by herbimycin A, a tyrosine kinase inhibitor. Incubation of cells in acid medium increased c-src activity, and this was inhibited by herbimycin A. To determine the role of the src family of nonreceptor protein-tyrosine kinases, Csk (for carboxyl-terminal src kinase), a physiologic inhibitor of these kinases, was overexpressed in OKP cells. In three clones overexpressing csk, acid-induced increases in Na+/H(+)-antiporter activity and NHE-3 mRNA abundance were inhibited. In these clones, inhibition of acid activation of Na+/H(+)-antiporter activity paralleled inhibition of acid activation of c-src. Neither herbimycin A nor overexpression of csk inhibited dexamethasone-induced increases in Na+/H(+)-antiporter activity. These studies show that decreases in pH activate c-src and that the src family nonreceptor protein-tyrosine kinases play a key role in acid activation of NHE-3.