A specific mutation abolishing Na+/H+ antiport activity in hamster fibroblasts precludes growth at neutral and acidic pH

(5-Arylfuran-2-ylcarbonyl)guanidines as Cardioprotectives through the Inhibition of Na+/H+ Exchanger Isoform-1

A series of (5-arylfuran-2-ylcarbonyl)guanidines was synthesized and evaluated for the NHE-1 inhibitory activity and cardiprotective efficacy against ischemia-reperfusion injury. Starting with (5-phenylfuran-2-ylcarbonyl)guanidine 47 with a moderate inhibitory effect on NHE-1, the compounds with various substituents at the phenyl ring were investigated with the aim to optimize the potency. In this study, the 2,5-disubstituted compounds appeared to have better activities than the other analogues, and the 2-methoxy-5-chlorophenyl compound 85 was found as a potent inhibitor of NHE-1 (IC(50) = 0.081 microM). Furthermore, 85 showed a marked reduction of infarct size in the rat myocardial infarction model in vivo and significant improvement of cardiac contractile function in the isolated rat heart ischemia model in vitro.

Evolutionary origins of eukaryotic sodium/proton exchangers

More than 200 genes annotated as Na+/H+ hydrogen exchangers (NHEs) currently reside in bioinformation databases such as GenBank and Pfam. We performed detailed phylogenetic analyses of these NHEs in an effort to better understand their specific functions and physiological roles. This analysis initially required examining the entire monovalent cation proton antiporter (CPA) superfamily that includes the CPA1, CPA2, and NaT-DC families of transporters, each of which has a unique set of bacterial ancestors. We have concluded that there are nine human NHE (or SLC9A) paralogs as well as two previously unknown human CPA2 genes, which we have named HsNHA1 and HsNHA2. The eukaryotic NHE family is composed of five phylogenetically distinct clades that differ in subcellular location, drug sensitivity, cation selectivity, and sequence length. The major subgroups are plasma membrane (recycling and resident) and intracellular (endosomal/TGN, NHE8-like, and plant vacuolar). HsNHE1, the first cloned eukaryotic NHE gene, belongs to the resident plasma membrane clade. The latter is the most recent to emerge, being found exclusively in vertebrates. In contrast, the intracellular clades are ubiquitously distributed and are likely precursors to the plasma membrane NHE. Yeast endosomal ScNHX1 was the first intracellular NHE to be described and is closely related to HsNHE6, HsNHE7, and HsNHE9 in humans. Our results link the appearance of NHE on the plasma membrane of animal cells to the use of the Na+/K(+)-ATPase to generate the membrane potential. These novel observations have allowed us to use comparative biology to predict physiological roles for the nine human NHE paralogs and to propose appropriate model organisms in which to study the unique properties of each NHE subclass.

The lateral mobility of NHE3 on the apical membrane of renal epithelial OK cells is limited by the PDZ domain proteins NHERF1/2, but is dependent on an intact actin cytoskeleton as determined by FRAP

The epithelial brush border (BB) Na+/H+ exchanger, NHE3, plays a major role in transcellular Na+ absorption in the renal proximal tubule. NHE3 activity is rapidly regulated by neurohumoral substances and growth factors via changes in its amount on the BB by a process partially involving vesicle trafficking. The PDZ domain-containing proteins, NHERF1/2, are scaffold proteins that link NHE3 to the actin cytoskeleton via their binding to both ezrin and NHE3. NHERF1/2 interact with both an internal C-terminal domain of NHE3 and the N-terminus of ezrin. We used fluorescence recovery after photobleaching (FRAP) to study the effect of NHERF1/2 on NHE3 mobility in the brush border of opossum kidney (OK) proximal tubule cells. A confocal microscope was used to allow the selective study of apical membrane versus intracellular NHE3. A chimera of NHE3-EGFP was transiently expressed in OK cells and its lateral diffusion in the apical membrane was measured with FRAP and confocal microscopy at 37 degrees C. The contribution of intracellular NHE3-EGFP to recovery on the OK surface not directly over the juxtanuclear area (non-JN) was negligible as exposure to the water soluble crosslinker BS3 (10 mM) at 4 degrees C resulted in no recovery of this component of surface NHE3-EGFP after photobleaching. The mobile fraction (Mf) of apical NHE3-EGFP was 47.5+/-2.2%; the effective diffusion coefficient (Deff) was (2.2+/-0.3) x10(-10) cm2/second. Overexpression of NHERF2 in OK cells decreased the Mf to 29.1+/-3.1% without changing Deff. In the truncation mutant, NHE3585-EGFP (aa 1-585), which lacks the NHERF1/2 binding domain, Mf increased to 66.4+/-2.2%, with no change in Deff, whereas NHE3660-EGFP, which binds NHERF1/2, had Mf (48.3+/-3.0%) and Deff both similar to full-length NHE3. These results are consistent with the PDZ domain proteins NHERF1 and NHERF2 scaffolding NHE3 in macromolecular complexes in the apical membrane of OK cells under basal conditions, which limits the lateral mobility of NHE3. It is probable that this is one of the mechanisms by which NHERF1/2 affects rapid regulation of NHE3 by growth factors and neurohumoral mediators. By contrast, disrupting the actin cytoskeleton by latrunculin B treatment (0.05 microM, 30 minutes) reduced the NHE3 Mf (21.9+/-4.5%) without altering the Deff. Therefore the actin cytoskeleton, independently of NHERF1/2 binding, is necessary for apical membrane mobility of NHE3.

The yeast endosomal Na+K+/H+ exchanger Nhx1 regulates cellular pH to control vesicle trafficking

The relationship between endosomal pH and function is well documented in viral entry, endosomal maturation, receptor recycling, and vesicle targeting within the endocytic pathway. However, specific molecular mechanisms that either sense or regulate luminal pH to mediate these processes have not been identified. Herein we describe the use of novel, compartment-specific pH indicators to demonstrate that yeast Nhx1, an endosomal member of the ubiquitous NHE family of Na+/H+ exchangers, regulates luminal and cytoplasmic pH to control vesicle trafficking out of the endosome. Loss of Nhx1 confers growth sensitivity to low pH stress, and concomitant acidification and trafficking defects, which can be alleviated by weak bases. Conversely, weak acids cause wild-type yeast to present nhx1Delta trafficking phenotypes. Finally, we report that Nhx1 transports K+ in addition to Na+, suggesting that a single mechanism may responsible for both pH and K+-dependent endosomal processes. This presents the newly defined family of eukaryotic endosomal NHE as novel targets for pharmacological inhibition to alleviate pathological states associated with organellar alkalinization.

An I?B-? COOH terminal region protein is essential for the proliferation of CHO cells under acidic stress

CHO-K1 cells were able to proliferate and maintain pHi homeostasis at pH 6.3. A novel acidic sensitive mutant, AS-5B, which proliferated at pH 7.4 but failed to either proliferate or maintain pHi homeostasis at pH 6.3, was derived from CHO-K1 using a replica method. The acidic-sensitivity of AS-5B was not due to deficiencies in sodium proton exchangers, HCO3- (co)transporters or H+-ATPases. A cDNA clone encoding a COOH terminal region of IkappaB-beta conferred partial acidic-resistance on AS-5B, and the encoded protein was present in CHO-K1, but was nearly absent from AS-5B. Our data demonstrated that the expression of this small protein was essential for the proliferation of CHO cells under acidic stress.

A mechanism for the activation of the Na/H exchanger NHE-1 by cytoplasmic acidification and mitogens

Eukaryotic cells constantly have to fight against internal acidification. In mammals, this task is mainly performed by the ubiquitously expressed electroneutral Na(+)/H(+) exchanger NHE-1, which activates in a cooperative manner when cells become acidic. Despite its biological importance, the mechanism of this activation is still poorly understood, the most commonly accepted hypothesis being the existence of a proton-sensor site on the internal face of the transporter. This work uncovers mutations that lead to a nonallosteric form of the exchanger and demonstrates that NHE-1 activation is best described by a Monod-Wyman-Changeux concerted mechanism for a dimeric transporter. During intracellular acidification, a low-affinity form of NHE-1 is converted into a form possessing a higher affinity for intracellular protons, with no requirement for an additional proton-sensor site on the protein. This new mechanism also explains the activation of the exchanger by growth signals, which shift the equilibrium towards the high-affinity form.

Na+/H+ exchanger 1 deficiency alters gene expression in mouse brain

Na(+)/H(+) exchanger 1 (NHE1) is well known to function as a major regulator of intracellular pH (pH(i)). It is activated by low pH(i) and exchanges extracellular Na(+) for intracellular H(+) to maintain cellular homeostasis. Despite the fact that we now have evidence suggesting other roles for NHE1, there has been no comprehensive study investigating its role as a signaling molecule. Toward this aim, we used in this study NHE1 null mutant mice and cDNA microarrays to investigate the effects of NHE1 on global gene expression in various regions of the brain, e.g., cortex, hippocampus, brain stem-diencephalon, and cerebellum. We found that a total of 35 to 79 genes were up- or downregulated in each brain region, with the majority being downregulated. The effect of NHE1 null mutation on gene expression is region specific, and only 11 genes were changed in all brain regions studied. Further analysis of the cis-regulatory regions of downregulated genes revealed that transcription suppressors, BCL6 and E4BP4, were probable candidates that mediated the inhibitory effect of NHE1 null mutation. One of the genes, MCT-13, was not only downregulated in the NHE1 null mutant brain but also in tissue cultures treated with an NHE1 inhibitor. We conclude that 1) a relatively small number of genes were altered in the NHE1 null mouse brain; 2) the effects of NHE1 null mutation on gene expression are region specific; and 3) several genes implicated in neurodegeneration have altered expression, potentially offering a molecular explanation for the phenotype of the NHE1 null mouse.

Proline residues in transmembrane segment IV are critical for activity, expression and targeting of the Na+/H+ exchanger isoform 1

NHE1 (Na+/H+ exchanger isoform 1) is a ubiquitously expressed integral membrane protein that regulates intracellular pH in mammalian cells. Proline residues within transmembrane segments have unusual properties, acting as helix breakers and increasing flexibility of membrane segments, since they lack an amide hydrogen. We examined the importance of three conserved proline residues in TM IV (transmembrane segment IV) of NHE1. Pro167 and Pro168 were mutated to Gly, Ala or Cys, and Pro178 was mutated to Ala. Pro168 and Pro178 mutant proteins were expressed at levels similar to wild-type NHE1 and were targeted to the plasma membrane. However, the mutants P167G (Pro167-->Gly), P167A and P167C were expressed at lower levels compared with wild-type NHE1, and a significant portion of P167G and P167C were retained intracellularly, possibly indicating induced changes in the structure of TM IV. P167G, P167C, P168A and P168C mutations abolished NHE activity, and P167A and P168G mutations caused markedly decreased activity. In contrast, the activity of the P178A mutant was not significantly different from that of wild-type NHE1. The results indicate that both Pro167 and Pro168 in TM IV of NHE1 are required for normal NHE activity. In addition, mutation of Pro167 affects the expression and membrane targeting of the exchanger. Thus both Pro167 and Pro168 are strictly required for NHE function and may play critical roles in the structure of TM IV of the NHE.

Expression profile of genes regulated by activity of the Na-H exchanger NHE1

BACKGROUND

In mammalian cells changes in intracellular pH (pHi), which are predominantly controlled by activity of plasma membrane ion exchangers, regulate a diverse range of normal and pathological cellular processes. How changes in pHi affect distinct cellular processes has primarily been determined by evaluating protein activities and we know little about how pHi regulates gene expression.

RESULTS

A global profile of genes regulated in mammalian fibroblasts by decreased pHi induced by impaired activity of the plasma membrane Na-H exchanger NHE1 was characterized by using cDNA microarrays. Analysis of selected genes by quantitative RT-PCR, TaqMan, and immunoblot analyses confirmed results obtained from cDNA arrays. Consistent with established roles of pHi and NHE1 activity in cell proliferation and oncogenic transformation, grouping regulated genes into functional categories and biological pathways indicated a predominant number of genes with altered expression were associated with growth factor signaling, oncogenesis, and cell cycle progression.

CONCLUSION

A comprehensive analysis of genes selectively regulated by pHi provides insight on candidate targets that might mediate established effects of pHi on a number of normal and pathological cell functions.

The NHE1 Na+/H+ exchanger recruits ezrin/radixin/moesin proteins to regulate Akt-dependent cell survival

Apoptosis results in cell shrinkage and intracellular acidification, processes opposed by the ubiquitously expressed NHE1 Na(+)/H(+) exchanger. In addition to mediating Na(+)/H(+) transport, NHE1 interacts with ezrin/radixin/moesin (ERM), which tethers NHE1 to cortical actin cytoskeleton to regulate cell shape, adhesion, motility, and resistance to apoptosis. We hypothesize that apoptotic stress activates NHE1-dependent Na(+)/H(+) exchange, and NHE1-ERM interaction is required for cell survival signaling. Apoptotic stimuli induced NHE1-regulated Na(+)/H(+) transport, as demonstrated by ethyl-N-isopropyl-amiloride-inhibitable, intracellular alkalinization. Ectopic NHE1, but not NHE3, expression rescued NHE1-null cells from apoptosis induced by staurosporine or N-ethylmaleimide-stimulated KCl efflux. When cells were subjected to apoptotic stress, NHE1 and phosphorylated ERM physically associated within the cytoskeleton-enriched fraction, resulting in activation of the pro-survival kinase, Akt. NHE1-associated Akt activity and cell survival were inhibited in cells expressing ERM binding-deficient NHE1, dominant negative ezrin constructs, or ezrin mutants with defective binding to phosphoinositide 3-kinase, an upstream regulator of Akt. We conclude that NHE1 promotes cell survival by dual mechanisms: by defending cell volume and pH(i) through Na(+)/H(+) exchange and by functioning as a scaffold for recruitment of a signalplex that includes ERM, phosphoinositide 3-kinase, and Akt.

Diversity of the mammalian sodium/proton exchanger SLC9 gene family

Sodium/proton antiporters or exchangers (NHE) are integral membrane proteins present in most, if not all, living organisms. In mammals, these transporters chiefly catalyze the electroneutral exchange of Na(+) and H(+) down their respective concentration gradients and are crucial for numerous physiological processes, ranging from the fine control of intracellular pH and cell volume to systemic electrolyte, acid-base and fluid volume homeostasis. NHE activity also facilitates the progression of other cellular events such as adhesion, migration, and proliferation. Thus far, eight distinct NHE genes (NHE1/SLC9A1-NHE8/SLC9A8) and several pseudogenes have been identified in the human genome. The functional genes encode proteins of varying primary sequence identity (25-70%), but share a common predicted secondary structure comprising 12 conserved membrane-spanning segments at the amino-terminus and a more divergent, cytoplasmically-oriented, carboxy-terminus. They show considerable heterogeneity in their patterns of tissue/cell expression and membrane localization. Functional studies have revealed further differences in their kinetic properties, sensitivity to pharmacological antagonists, and regulation by diverse hormonal and mechanical stimuli. Altered NHE activity has been linked to the pathogenesis of several diseases, including essential hypertension, congenital secretory diarrhea, diabetes, and tissue damage caused by ischemia/reperfusion. Further characterization of their functional properties should lead to a better understanding of their unique contributions to human health and disease.

Regulation of Expression of the Na+/H+ Exchanger by Thyroid Hormone

The Na+/H+ exchanger is a pH regulatory protein with a ubiquitous distribution in eukaryotic cells. Several isoforms of the Na+/H+ exchanger are known. The first isoform to be characterized and cloned, NHE1, is present on the plasma membrane of cells and functions to remove one intracellular proton in exchange for one extracellular sodium ion. It is involved in pH regulation, cell growth, differentiation, and cell migration. NHE1 is also involved in the cycle of damage that occurs in the heart with ischemic heart disease. Recent studies have shown that the Na+/H+ exchanger is regulated in response to thyroid hormone. Reduction in circulating thyroid hormone levels reduces the amount of both protein and mRNA of NHE1. Conversely, an elevation of thyroid hormone levels has the opposite effects. Transcriptional regulation of NHE1 expression has been demonstrated. The NHE1 promoter contains a TR alpha(1) binding site located between -841 to -800 bp. This element responds positively to TR alpha(1). This regulation of the NHE1 promoter by thyroid hormone is proposed to be responsible for postnatal changes in expression of the Na+/H+ exchanger.

Ca2+-dependent inhibition of NHE3 requires PKC alpha which binds to E3KARP to decrease surface NHE3 containing plasma membrane complexes

The intestinal brush border (BB) Na+/H+ exchanger isoform 3 (NHE3) is acutely inhibited by elevation in the concentration of free intracellular Ca2+ ([Ca2+]i) by the cholinergic agonist carbachol and Ca2+ ionophores in a protein kinase C (PKC)-dependent manner. We previously showed that elevating [Ca2+]i with ionomycin rapidly inhibited NHE3 activity and decreased the amount of NHE3 on the plasma membrane in a manner that depended on the presence of the PDZ domain-containing protein E3KARP (NHE3 kinase A regulatory protein, also called NHERF2). The current studies were performed in PS120 fibroblasts (NHE-null cell line) stably transfected with NHE3 and E3KARP to probe the mechanism of PKC involvement in Ca2+ regulation of NHE3. Pretreatment with the general PKC inhibitor, GF109203X prevented ionomycin inhibition of NHE3 without altering basal NHE3 activity. Similarly, the Ca2+-mediated inhibition of NHE3 activity was blocked after pretreatment with the conventional PKC inhibitor Gö-6976 and a specific PKCalpha pseudosubstrate-derived inhibitor peptide. [Ca2+]i elevation caused translocation of PKCalpha from cytosol to membrane. PKCalpha bound to the PDZ1 domain of GST-E3KARP in vitro in a Ca2+-dependent manner. PKCalpha and E3KARP coimmunoprecipitated from cell lysates; this occurred to a lesser extent at basal [Ca2+]i and was increased with ionomycin exposure. Biotinylation studies demonstrated that [Ca2+]i elevation induced oligomerization of NHE3 in total lysates and decreased the amount of plasma membrane NHE3. Treatment with PKC inhibitors did not affect the oligomerization of NHE3 but did prevent the decrease in surface amount of NHE3. These results suggest that PKCalpha is not necessary for the Ca2+-dependent formation of the NHE3 plasma membrane complex, although it is necessary for decreasing the membrane amounts of NHE3, probably by stimulating NHE3 endocytosis.

Na-H exchange-dependent increase in intracellular pH times G2/M entry and transition

It is well established that activation of the Na-H exchanger NHE1 and increases in intracellular pH (pHi) are early and universal responses to mitogens and have permissive effects in promoting cell proliferation. Despite this evidence, a specific role for NHE1 or pHi in cell cycle progression remains undetermined. We now show that NHE1 activity and pHi regulate the timing of G2/M entry and transition. Prior to G2/M entry there is a rapid and transient increase in NHE1 activity and pHi, but in fibroblasts expressing a mutant NHE1 that lacks ion translocation activity, this increase in pHi is attenuated, S phase is delayed, and G2/M transition is impaired. In the absence of ion translocation by NHE1, expression of cyclin B1 and the kinase activity of Cdc2 are decreased and Wee1 kinase expression increases. Increasing pHi in the absence of NHE1 activity, however, is sufficient to restore Cdc2 activity and cyclin B1 expression and to promote G2/M entry and transition. These data indicate that a transient increase in pHi induced by NHE1 promotes the timing of G2/M, and they suggest that increases in pHi at the completion of S phase may constitute a previously unrecognized checkpoint for progression to G2 and mitosis.

Kinetic dissection of two distinct proton binding sites in Na+/H+ exchangers by measurement of reverse mode reaction

We examined the effect of intracellular acidification on the reverse mode of Na+/H+ exchange by measuring 22Na+ efflux from 22Na+-loaded PS120 cells expressing the Na+/H+ exchanger (NHE) isoforms NHE1, NHE2, and NHE3. The 5-(N-ethyl-N-isopropyl)amiloride (EIPA)- or amiloride-sensitive fraction of 22Na+ efflux was dramatically accelerated by cytosolic acidification as opposed to thermodynamic prediction, supporting the concept that these NHE isoforms are activated by protonation of an internal binding site(s) distinct from the H+ transport site. Intracellular pH (pHi) dependence of 22 Na+ efflux roughly exhibited a bell-shaped profile; mild acidification from pHi 7.5 to 7 dramatically accelerated 22Na+ efflux, whereas acidification from pHi 6.6 gradually decreased it. Alkalinization above pHi 7.5 completely suppressed EIPA-sensitive 22Na+ efflux. Cell ATP depletion and mutation of NHE1 at Arg440 (R440D) caused a large acidic shift of the pHi profile for 22Na+ efflux, whereas mutation at Gly455 (G455Q) caused a significant alkaline shift. Because these mutations and ATP depletion cause correspondingly similar effects on the forward mode of Na+/H+ exchange, it is most likely that they alter exchange activity by modulating affinity of the internal modifier site for protons. The data provide substantial evidence that a proton modifier site(s) distinct from the transport site controls activities of at least three NHE isoforms through cooperative interaction with multiple protons.

Ethyl isopropyl amiloride inhibits smooth muscle cell proliferation and migration by inducing apoptosis and antagonizing urokinase plasminogen activator activity

Amiloride inhibits activation of the Na(+)-H+ exchanger (NHE), a critical step in smooth muscle cell (SMC) growth. While amiloride treatment reduces SMC proliferation and migration, as well as experimental lesion formation, these effects are not exclusively due to NHE inhibition and remain incompletely understood. The purpose of this study was to examine the mechanisms involved in amiloride-induced attenuation of SMC proliferation and migration, looking specifically at the potential role of apoptosis and urokinase plasminogen activator (uPA) activity in these processes. Rabbit SMCs in tissue culture were exposed to 10-80 microM of the amiloride analogue ethyl isopropyl amiloride (EIPA). Compared with controls, EIPA reduced DNA synthesis, cell number, and mitochondrial respiration, but without toxic effects on quiescent or proliferating cells. In a Boyden chamber assay, EIPA reduced uPA-induced SMC migration. Moreover, in a SMC scratch assay EIPA treatment resulted in a 66% reduction in the number of repopulating cells, a 92% decrease in the number of proliferating cells, and a 37-fold increase in the number of apoptotic cells. SMC apoptosis was frequently localized to the scratch edges, where cell proliferation and bcl-2 expression were absent. Finally, uPA enzymatic activity in the cell culture media was lower for EIPA-treated versus control SMCs. Therefore, EIPA inhibits both SMC proliferation and migration by inducing apoptosis and antagonizing uPA activity, respectively, and requires further study as an agent for reducing vascular lesion formation.

Alternative splicing of NHE-1 mediates Na-Li countertransport and associates with activity rate

Sodium-lithium countertransport (SLC) is an ouabain-insensitive exchange of Na for Li found in the erythrocyte membrane of several mammalian species. Although increased SLC activity is presently the most consistent intermediate phenotype of essential hypertension and diabetic nephropathy in humans, the gene responsible for this membrane transport has not been identified. Because of functional similarities, SLC was suggested to represent an in vitro mode of operation of the Na-H exchanger (NHE). This hypothesis, however, has been long hampered by the total insensitivity of SLC to amiloride, which is an intrinsic inhibitor of the first isoform of NHE, the only NHE isoform detected in human erythrocytes. We describe here the identification in human reticulocytes and erythrocytes of an alternative splicing of NHE lacking the amiloride binding site. Transfection experiments with this spliced variant restore amiloride-insensitive, phloretin-sensitive SLC activity. Expression of both regular and spliced transcripts of NHE is increased in subjects with high SLC activity. Altogether, these findings, by extending to NHE the characteristics of inheritance and predictivity previously attributed to SLC, eventually restore the candidacy of NHE isoform 1 as a gene involved in the pathogenesis of essential hypertension and diabetic nephropathy.

Mechanism of acid adaptation of a fish living in a pH 3.5 lake

Despite unfavorable conditions, a single species of fish, Osorezan dace, lives in an extremely acidic lake (pH 3.5) in Osorezan, Aomori, Japan. Physiological studies have established that this fish is able to prevent acidification of its plasma and loss of Na(+). Here we show that these abilities are mainly attributable to the chloride cells of the gill, which are arranged in a follicular structure and contain high concentrations of Na(+)-K(+)-ATPase, carbonic anhydrase II, type 3 Na(+)/H(+) exchanger (NHE3), type 1 Na(+)-HCO(3)(-) cotransporter, and aquaporin-3, all of which are upregulated on acidification. Immunohistochemistry established their chloride cell localization, with NHE3 at the apical surface and the others localized to the basolateral membrane. These results suggest a mechanism by which Osorezan dace adapts to its acidic environment. Most likely, NHE3 on the apical side excretes H(+) in exchange for Na(+), whereas the electrogenic type 1 Na(+)-HCO(3)(-) cotransporter in the basolateral membrane provides HCO(3)(-) for neutralization of plasma using the driving force generated by Na(+)-K(+)-ATPase and carbonic anhydrase II. Increased expression of glutamate dehydrogenase was also observed in various tissues of acid-adapted dace, suggesting a significant role of ammonia and bicarbonate generated by glutamine catabolism.

Structure and function of the NHE1 isoform of the Na+/H+ exchanger

The Na+/H+ exchanger is a ubiquitous, integral membrane protein involved in pH regulation. It removes intracellular acid, exchanging a proton for an extracellular sodium ion. There are seven known isoforms of this protein that are the products of distinct genes. The first isoform discovered (NHE1) is ubiquitously distributed throughout the plasma membrane of virtually all tissues. It plays many different physiological roles in mammals, including important functions in regulation of intracellular pH, in heart disease, and in cytoskeletal organization. The first 500 amino acids of the protein are believed to consist of 12 transmembrane helices, a membrane-associated segment, and two reentrant loops. A C-terminal regulatory domain of approximately 315 amino acids regulates the protein and mediates cytoskeletal interactions. Studies are underway to determine the amino acid residues important in NHE1 function. At present, it is clear that transmembrane segment IV is important in NHE1 function and that transmembrane segments VII and IX are also involved in transport. Further experiments are required to elucidate the mechanism of transport and regulation of this multifunctional protein.

Mutations of Arg440 and Gly455/Gly456 oppositely change pH sensing of Na+/H+ exchanger 1

To identify important amino acid residues involved in intracellular pH (pH(i)) sensing of Na(+)/H(+) exchanger 1, we produced single-residue substitution mutants in the region of the exchanger encompassing the putative 11th transmembrane segment (TM11) and its adjacent intracellular (intracellular loop (IL) 5) and extracellular loops (extracellular loop 6). Substitution of Arg(440) in IL5 with other residues except positively charged Lys caused a large shift in pH(i) dependence of (22)Na(+) uptake to an acidic side, whereas substitution of Gly(455) or Gly(456) within the highly conserved glycine-rich sequence of TM11 shifted pH(i) dependence to an alkaline side. The observed alkaline shift was larger with substitution of Gly(455) with residues with increasing sizes, suggesting the involvement of the steric effect. Interestingly, mutation of Arg(440) (R440D) abolished the ATP depletion-induced acidic shift in pH(i) dependence of (22)Na(+) uptake as well as the cytoplasmic alkalinization induced by various extracellular stimuli, whereas with that of Gly(455) (G455Q) these functions were preserved. These mutant exchangers did not alter apparent affinities for extracellular transport substrates Na(+) and H(+) and the inhibitor 5-(N-ethyl-N-isopropyl)amiloride. These results suggest that positive charge at Arg(440) is required for normal pH(i) sensing, whereas mutation-induced perturbation of the TM11 structure may be involved in the effects of Gly mutations. Thus, both Arg(440) in IL5 and Gly residues in the conserved segment of TM11 appear to constitute important elements for proper functioning of the putative "pH(i) sensor" of Na(+)/H(+) exchanger 1.

Renal tubular epithelial cell apoptosis is associated with caspase cleavage of the NHE1 Na+/H+ exchanger

Renal tubular epithelial cell (RTC) apoptosis causes tubular atrophy, a hallmark of renal disease progression. Apoptosis is generally characterized by reduced cell volume and cytosolic pH, but epithelial cells are relatively resistant to shrinkage due to regulatory volume increase, which is mediated by Na(+)/H(+) exchanger (NHE) 1. We investigated whether RTC apoptosis requires caspase cleavage of NHE1. Staurosporine- and hypertonic NaCl-induced RTC apoptosis was associated with cell shrinkage and diminished cytosolic pH, and apoptosis was potentiated by amiloride analogs, suggesting NHE1 activity opposes apoptosis. NHE1-deficient fibroblasts demonstrated increased susceptibility to apoptosis, which was reversed by NHE1 reconstitution. NHE1 expression was markedly decreased in apoptotic RTC due to degradation, and preincubation with peptide caspase antagonists restored NHE1 expression, indicating that NHE1 is degraded by caspases. Recombinant caspase-3 cleaved the in vitro-translated NHE1 cytoplasmic domain into five distinct peptides, identical in molecular weight to NHE1 degradation products derived from staurosporine-stimulated RTC lysates. In vivo, NHE1 loss-of-function C57BL/6.SJL-swe/swe mice with adriamycin-induced nephropathy demonstrated increased RTC apoptosis compared with adriamycin-treated wild-type controls, thereby implicating NHE1 inactivation as a potential mechanism of tubular atrophy. We conclude that NHE1 activity is critical for RTC survival after injury and that caspase cleavage of RTC NHE1 may promote apoptosis and tubular atrophy by preventing compensatory intracellular volume and pH regulation.

Characterization of sabiporide, a new specific NHE-1 inhibitor exhibiting slow dissociation kinetics and cardioprotective effects

Sabiporide, a new benzoguanidine, was characterized on fibroblasts stably expressing the Na(+)/H(+) exchanger isoforms NHE-1, NHE-2 and NHE-3. 22Na(+) uptake experiments show that this compound possesses a K(i) of 5+/-1.2 x 10(-8) M for NHE-1, and discriminates efficiently between the NHE-1, -2 and -3 isoforms (K(i) for NHE-2: 3+/-0.9 x 10(-6) M, and K(i)>1 mM for NHE-3). Similar K(i) values are obtained on rat cardiomyocytes and human platelets expressing NHE-1 (K(i)'s of 7+/-1 x 10(-9) and 2.7+/-0.4 x 10(-8) M respectively). Interestingly, when compared with amiloride and cariporide, sabiporide inhibition persists even after this molecule had been rinsed out (half time of 7 h for sabiporide, and of 1 and 2.5 min for amiloride and cariporide, respectively), the decay of all these molecules exhibiting a complex multiexponential behavior. Thus, sabiporide, which possesses remarkable cardioprotective properties, is a specific NHE-1 inhibitor possessing unique binding kinetics.

Functional characterization of the human intestinal NaPi-IIb cotransporter in hamster fibroblasts and Xenopus oocytes

The recently cloned NaPi-IIb cotransporter is an apical membrane protein that is involved in the absorption of phosphate in the intestine. To expedite functional and structural studies, the human intestinal NaPi-IIb cotransporter was stably expressed in hamster fibroblast (PS120) cells. The hNaPi-IIb cDNA stably transfected cells exhibited a 1.8-fold higher sodium-dependent phosphate uptake than vector DNA transfected cells, and had a K(m) for Pi of approximately 106 microM and a K(m) for Na(+) of approximately 34 mM. The hNaPi-IIb cotransporter was also expressed in Xenopus oocytes and it exhibited a K(m) for Pi of approximately 113 microM and a K(m) for Na(+) of approximately 65 mM. The hNaPi-IIb cotransporter expressed in both PS120 cells and oocytes was inhibited by high external pH. Furthermore, the phosphate uptake mediated by the hNaPi-IIb cotransporter was inhibited by 5 mM phosphonoformic acid (PFA), 1 mM arsenate and 100 nM phorbol myristate acetate (PMA). These results demonstrate that the human intestinal NaPi-IIb cotransporter is functional when expressed in hamster fibroblasts, and that this model system may be useful in the future to identify NaPi-IIb cotransporter-specific inhibitors.

Upregulation of NHE3 is associated with compensatory cell growth response in young uninephrectomized rats

It is well established that after removal of renal mass, the remaining tissue undergoes compensatory growth. Several laboratories have reported that the activity of the apical membrane Na+ - H+ exchanger (NHE3) is increased after a reduction in renal mass. These studies were designed to determine whether NHE3 expression is altered early after loss of renal mass and to investigate the possible role of NHE3 activation in the compensatory tissue growth response. Experiments were performed in young male Wistar rats submitted to left nephrectomy or sham operation. At either 4 or 24 h after the surgery, the right kidney from each animal was removed and weighed. Significant increases in the wet weight of the remaining kidney were only observed 24 h after uninephrectomy (UNX). Western blot analysis of brush-border membranes and Northern blot analysis of cortex RNA showed that NHE3 protein abundance and NHE3 mRNA were greatly enhanced 4 and 24 h after UNX in relation to the sham kidney. To identify which growth pattern was mostly responsible for the enlargement of the remained kidney in our experimental models, we measured 5-bromo-2-deoxyuridine incorporation (BrdU) and protein-to-DNA ratio (protein/DNA ratio). The number of BrdU-positive nuclei increased and protein/DNA ratio slightly decreased, indicating that a hyperplastic response was the main component involved in the early compensatory renal growth in our animals. BrdU incorporation and protein/DNA were also assessed in rats treated with S3226, a selective blocker of NHE3. Neither the number of BrdU-positive nuclei nor the protein/DNA ratio was significantly altered 4 and 24 h after UNX in rats treated with S3226. In conclusion, UNX induced an upregulation of NHE3, which was evidenced at both functional and expression levels. The compensatory growth response in young UNX rats could be blocked by inhibiting NHE3 activity, suggesting that NHE3 activation may result in a facilitator state for the cell growth response in the renal proximal tubule.

Expression of calcineurin B homologous protein 2 protects serum deprivation-induced cell death by serum-independent activation of Na+/H+ exchanger

The calcineurin B homologous protein (designated CHP1) has been shown to be a common essential cofactor for the plasma membrane Na(+)/H(+) exchangers (NHEs) (Pang, T., Su, X., Wakabayashi, S., and Shigekawa, M. (2001) J. Biol. Chem. 276, 17367-17372). In this study, we characterized the function of another isoform of CHP (designated CHP2) that has a 61% amino acid identity with CHP1. CHP2, like CHP1, conferred the ability to NHEs 1-3 to express a high exchange activity by binding to the juxtamembrane region of the cytoplasmic domain of the exchanger, but it interacts more strongly (approximately 5-fold) with NHE1 than does CHP1. Although CHP1 is expressed ubiquitously at relatively high levels, CHP2 expression was extremely low in most human tissues but was higher in tumor cells. We produced stable cell clones overexpressing either CHP1 or CHP2 in which one of them is predominantly bound to NHE1. Serum (10%) induced a significant cytoplasmic alkalinization (0.1-0.2 pH unit) in cells co-expressing CHP1 and NHE1 but not in cells co-expressing CHP2 and NHE1. In the latter, pH(i) was high (7.4-7.5) even in the absence of serum, suggesting that NHE1 was already activated. Surprisingly, most (>80%) of CHP2/NHE1 cells unlike CHP1/NHE1 cells were viable even after long serum starvation (>7 days). Thus, the expression of CHP2 appears to protect cells from serum deprivation-induced death by increasing pH(i). These properties of CHP2/NHE1 cells are similar to those of malignantly transformed cells. We propose that serum-independent activation of NHE1 by bound CHP2 is one of the key mechanisms for the maintenance of high pH(i) and the resistance to serum deprivation-induced cell death in malignantly transformed cells.

Clathrin-mediated endocytosis and recycling of the neuron-specific Na+/H+ exchanger NHE5 isoform. Regulation by phosphatidylinositol 3'-kinase and the actin cytoskeleton

Mammalian Na+/H+ exchangers (NHEs) are a family of integral membrane proteins that play central roles in sodium, acid-base, and cell volume homeostasis. The recently cloned NHE5 isoform is expressed predominantly in brain, but its functional and cellular properties are poorly understood. To facilitate its characterization, an epitope-tagged construct of NHE5 was ectopically expressed in nonneuronal and neuronal cells. In NHE-deficient Chinese hamster ovary AP-1 cells, NHE5 localized at the plasmalemma, but a significant fraction accumulated intracellularly in vesicles that concentrated in a juxtanuclear region. Similarly, in nerve growth factor-differentiated neuroendocrine PC12 cells and primary hippocampal neurons, immunolabeling of NHE5 was detected in endomembrane vesicles in the perinuclear region of the cell body but also along the processes. More detailed characterization in AP-1 cells using organelle-specific markers showed that NHE5 co-localized with internalized transferrin, a marker of recycling endosomes. Transient transfection of a dominant negative mutant of dynamin-1, which inhibits clathrin-mediated endocytosis, blocked uptake of transferrin as well as internalization of NHE5. Likewise, wortmannin inhibition of phosphatidylinositol 3'-kinase, a lipid kinase implicated in endosomal traffic, induced coalescence of vesicles containing NHE5 and caused a pronounced inhibition of plasmalemmal Na+/H+ exchange. By contrast, disruption of the F-actin cytoskeleton with cytochalasin D increased cell surface NHE5 activity and abundance. These observations demonstrate that NHE5 is localized to the recycling endosomal pathway and is dynamically regulated by phosphatidylinositol 3'-kinase and by the state of F-actin assembly.

NHE3 serves as a molecular tool for cAMP-mediated regulation of receptor-mediated endocytosis

Receptor-mediated, clathrin-dependent endocytosis (RME) is important for macromolecular transport and regulation of cell-surface protein expression. Pharmacological studies have shown that the plasma membrane transport protein Na(+)/H(+) exchanger 3 (NHE3), which shuttles between the plasma membrane and the early endosomal compartment by means of clathrin-mediated endocytosis, contributes to endosomal pH homeostasis and endocytic fusion events. Furthermore, it is known that NHE3 is phosphorylated and inhibited by cAMP-dependent kinase (protein kinase A). Here, we show, in a cellular knockout/retransfection approach, that NHE3 supports RME and confers cAMP sensitivity to RME, using megalin/cubilin-mediated albumin uptake in opossum kidney cells. RME, but not fluid-phase endocytosis, was dependent on NHE3 activity and expression. Furthermore, NHE3 deficiency or inhibition reduced the relative surface expression of megalin without altering total expression. In wild-type cells, cAMP inhibits NHE3 activity, leads to endosomal alkalinization, and reduces RME. In NHE3-deficient cells, endosomal pH is not sensitive to NHE3 inhibition, and cAMP does not affect endosomal pH or RME. NHE3 transfection into deficient cells restores RME and the effects of cAMP. Thus our data show that NHE3 is important for cAMP sensitivity of clathrin-dependent RME.

The Role of auxin, pH, and stress in the activation of embryogenic cell division in leaf protoplast-derived cells of alfalfa

Culturing leaf protoplast-derived cells of the embryogenic alfalfa (Medicago sativa subsp. varia A2) genotype in the presence of low (1 microM) or high (10 microM) 2, 4-dichlorophenoxyacetic acid (2,4-D) concentrations results in different cell types. Cells exposed to high 2,4-D concentration remain small with dense cytoplasm and can develop into proembryogenic cell clusters, whereas protoplasts cultured at low auxin concentration elongate and subsequently die or form undifferentiated cell colonies. Fe stress applied at nonlethal concentrations (1 mM) in the presence of 1 microM 2,4-D also resulted in the development of the embryogenic cell type. Although cytoplasmic alkalinization was detected during cell activation of both types, embryogenic cells could be characterized by earlier cell division, a more alkalic vacuolar pH, and nonfunctional chloroplasts as compared with the elongated, nonembryogenic cells. Buffering of the 10 microM 2,4-D-containing culture medium by 10 mM 2-(N-morpholino)ethanesulfonic acid delayed cell division and resulted in nonembryogenic cell-type formation. The level of endogenous indoleacetic acid (IAA) increased transiently in all protoplast cultures during the first 4 to 5 d, but an earlier peak of IAA accumulation correlated with the earlier activation of the division cycle in embryogenic-type cells. However, this IAA peak could also be delayed by buffering of the medium pH by 2-(N-morpholino)ethanesulfonic acid. Based on the above data, we propose the involvement of stress responses, endogenous auxin synthesis, and the establishment of cellular pH gradients in the formation of the embryogenic cell type.

The changing face of the Na+/H+ exchanger, NHE1: structure, regulation, and cellular actions

The NHE family of ion exchangers includes six isoforms (NHE1-NHE6) that function in an electroneutral exchange of intracellular H(+) for extracellular Na(+). This review focuses on the only ubiquitously expressed isoform, NHE1, which is localized at the plasma membrane where it plays a critical role in intracellular pH (pHi) and cell volume homeostasis. All NHE isoforms share a similar topology: an N-terminus of 12 transmembrane (TM) alpha-helices that collectively function in ion exchange, and a C-terminal cytoplasmic regulatory domain that modulates transport activity by the TM domain. Extracellular signals, mediated by diverse classes of cell-surface receptors, regulate NHE1 activity through distinct signaling networks that converge to directly modify the C-terminal regulatory domain. Modifications in the C-terminus, including phosphorylation and the binding of regulatory proteins, control transport activity by altering the affinity of the TM domain for intracellular H(+). Recently, it was determined that NHE1 also functions as a membrane anchor for the actin-based cytoskeleton, independently of its role in ion translocation. Through its effects on pHi homeostasis, cell volume, and the actin cortical network, NHE1 regulates a number of cell behaviors, including adhesion, shape determination, migration, and proliferation.

Immunological detection of Na(+)/H(+) exchangers in the gills of a hagfish, Myxine glutinosa, an elasmobranch, Raja erinacea, and a teleost, Fundulus heteroclitus

Na(+)/H(+) exchangers (NHE) are a family of ion exchangers with diverse functions that are well defined in mammals. NHE-1 is expressed in the plasma membrane of most mammalian cells where it regulates intracellular pH, and usually in the basolateral membrane of epithelial cells. It has also been detected in teleost gills where it may participate in systemic pH regulation. NHE-3 is usually expressed in the apical membrane of mammalian epithelial cells where it helps reabsorb Na(+) and HCO(3)(-); it has also been detected in teleost gills. We used Western blotting and heterologous antibodies to screen for expression of NHE-1 and NHE-3 in gills of an agnathan (Myxine glutinosa) and an elasmobranch (Raja erinacea), and NHE-3 in gills of a teleost (Fundulus heteroclitus). Positive NHE-1 bands were detected in gills from the agnathan and elasmobranch. Using the NHE-3 antibody, bands were detected in the gills of the elasmobranch and teleost. These data are some of the first direct evidence of NHEs in the gills of an agnathan and elasmobranch, and confirm the presence of NHEs in the gills of teleosts.

Role of the cytoskeleton in mediating cAMP-dependent protein kinase inhibition of the epithelial Na+/H+ exchanger NHE3

The Na(+)/H(+) exchanger NHE3 isoform mediates the entry of Na(+) into epithelial cells of the kidney and gastrointestinal tract. Hormones and pharmacological agents that activate cAMP-dependent protein kinase A (PKA) are potent inhibitors of native and ectopically expressed NHE3 in epithelial and Chinese hamster ovary AP-1 cells, respectively. Previous studies have shown that acute inhibition is coupled to direct phosphorylation of the exchanger, but this only partly accounts for the observed effects. In this report, we show that inhibition of NHE3 activity by forskolin, an activator of adenylate cyclase, occurs without changes in surface expression of the exchanger but is associated with altered cytoskeletal structure. This effect resembles that obtained with cytochalasin D or latrunculin B, actin disrupting agents that also inhibit NHE3. Such similarities prompted us to further investigate the relationship between PKA-induced inhibition of the exchanger and changes in the actin cytoskeleton. Inhibition of NHE3 by cytochalasin D does not require PKA, because the inhibitory effect is preserved in a mutant NHE3 that is not phosphorylated by PKA and in cells pretreated with the PKA inhibitor H89. In contrast, involvement of actin in the effect of cAMP on the exchanger is supported by the following observations: (i) jasplakinolide, an F-actin stabilizer, prevents the inhibition caused by forskolin, and (ii) constitutively active forms of RhoA and Rho kinase interfere with actin disruption by forskolin and also decrease inhibition of the transporter. These results suggest that reorganization of the cytoskeleton by PKA is involved in mediating inhibition of NHE3.

CFTR modulates programmed cell death by decreasing intracellular pH in Chinese hamster lung fibroblasts

To study the potential influence of cystic fibrosis conductance regulator (CFTR) on intracellular pH regulation during apoptosis induction, we used PS120 Chinese hamster lung fibroblasts devoid of the Na(+)/H(+) exchanger (NHE1 isoform) transfected with constructs, allowing the expression of CFTR and/or NHE1. Kinetics of lovastatin-induced apoptosis were measured by orcein staining, double staining with Hoechst-33258, propidium iodide, DNA fragmentation, and annexin V labeling. In PS120 control cells, the percentage of apoptotic cells after 40 h of lovastatin treatment was 23 +/- 3%, whereas in PS120 CFTR-transfected cells, this percentage was 40 +/- 4%. In PS120 NHE1 cells, the transfection with CFTR did not modify the percentage of apoptotic cells after 40 h (control: 19 +/- 3%, n = 8; CFTR: 17 +/- 1%, n = 8), indicating that blocking intracellular acidification by overexpressing the Na(+)/H(+) exchanger inhibited the enhancement of apoptosis induced by CFTR. In all cell lines, the initial pH values were identical (pH = 7.46 +/- 0.04, n = 9), and treatment with lovastatin led to intracellular acidification. However, the pH value after 40 h was lower in PS120 CFTR-transfected cells (pH = 6.85 +/- 0.02, n = 10) than in PS120 cells (pH = 7.15 +/- 0.03, n = 10). To further investigate the origin of this increased intracellular acidification observed in CFTR-transfected cells, the activity of the DIDS-inhibitable Cl(-)/HCO exchanger was studied. 8-Bromoadenosine 3',5'-cyclic monophosphate incubation resulted in Cl(-)/HCO exchanger activation in PS120 CFTR-transfected cells but had no effect on PS120 cells. Together, our results suggest that CFTR can enhance apoptosis in Chinese hamster lung fibroblasts, probably due to the modulation of the Cl(-)/HCO exchanger, resulting in a more efficient intracellular acidification.

An activating mutation in the gamma1 subunit of the AMP-activated protein kinase

The AMP-activated protein kinase (AMPK) is a heterotrimeric protein composed of a catalytic alpha subunit and two regulatory subunits, beta and gamma. The gamma subunit is essential for enzyme activity by virtue of its binding to the C-terminus of the alpha subunit and appears to play some role in the determination of AMP sensitivity. We demonstrate that a gamma1R70Q mutation causes a marked increase in AMPK activity and renders it largely AMP-independent. This activation is associated with increased phosphorylation of the alpha subunit activation loop T172. These in vitro characteristics of AMPK are also reflected in increased intracellular phosphorylation of one of its major substrates, acetyl-CoA carboxylase. These data illustrate the importance of the gamma1 subunit in the regulation of AMPK and its modulation by AMP.

Functional role of polar amino acid residues in Na+/H+ exchangers

Na(+)/H(+) exchangers are a family of ubiquitous membrane proteins. In higher eukaryotes they regulate cytosolic pH by removing an intracellular H(+) in exchange for an extracellular Na(+). In yeast and Escherichia coli, Na(+)/H(+) exchangers function in the opposite direction to remove intracellular Na(+) in exchange for extracellular H(+). Na(+)/H(+) exchangers display an internal pH-sensitivity that varies with the different antiporter types. Only recently have investigations examined the amino acids involved in pH-sensitivity and in cation binding and transport. Histidine residues are good candidates for H(+)-sensing amino acids, since they can ionize within the physiological pH range. Histidine residues have been shown to be important in the function of the E. coli Na(+)/H(+) exchanger NhaA and in the yeast Na(+)/H(+) exchanger sod2. In E. coli, His(225) of NhaA may function to interact with, or regulate, the pH-sensory region of NhaA. In sod2, His(367) is also critical to transport and may be a functional analogue of His(225) of NhaA. Histidine residues are not critical for the function of the mammalian Na(+)/H(+) exchanger, although an unusual histidine-rich sequence of the C-terminal tail has some influence on activity. Other amino acids involved in cation binding and transport by Na(+)/H(+) exchangers are only beginning to be studied. Amino acids with polar side chains such as aspartate and glutamate have been implicated in transport activity of NhaA and sod2, but have not been studied in the mammalian Na(+)/H(+) exchanger. Further studies are needed to elucidate the mechanisms involved in pH-sensitivity and cation binding and transport by Na(+)/H(+) exchangers.

A role for protein kinase CK2 in cell proliferation: evidence using a kinase-inactive mutant of CK2 catalytic subunit alpha

Protein kinase CK2 is an ubiquitous and pleiotropic Ser/Thr protein kinase composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits generally combined to form alpha(2)beta(2), alphaalpha'beta(2), or alpha'(2)beta(2) heterotetramers. To gain more insight into the role of CK2 in the control of proliferation in mammalian cells, overexpression of isolated CK2 subunits alpha, alpha', or beta was carried out in two fibroblast cell lines: NIH3T3 and CCL39. To interfere with CK2 cellular functions, cells were also transfected with a kinase-inactive mutant of CK2alpha catalytic subunit: CK2alpha-K68A. In NIH3T3 cells, overexpression of either wild-type subunit (alpha, alpha' or beta) had no effect on cell proliferation. In contrast, overexpression of the CK2alpha kinase-deficient mutant induced a marked inhibition of cell proliferation. This resulted from a defect in G1/S progression as demonstrated in transient transfection experiments in both NIH3T3 and CCL39 cells using BrdU incorporation measurements and in CCL39 clones stably overexpressing the CK2alpha-K68A mutant by growth curve analysis. We demonstrated that the kinase-negative mutant has the capacity to integrate the endogenous CK2 subunit pool both as an isolated kinase-inactive alpha subunit and as associated to the beta subunit in a kinase-inactive tetramer. Finally we showed that expression of the kinase-inactive mutant interferes with phosphorylation of an endogenous CK2 substrate; we speculate that optimal phosphorylation of target proteins by CK2 is required to achieve optimal cell cycle progression.

Regulatory interaction between the cystic fibrosis transmembrane conductance regulator and HCO3- salvage mechanisms in model systems and the mouse pancreatic duct

The pancreatic duct expresses cystic fibrosis transmembrane conductance regulator (CFTR) and HCO3- secretory and salvage mechanisms in the luminal membrane. Although CFTR plays a prominent role in HCO3- secretion, the role of CFTR in HCO3- salvage is not known. In the present work, we used molecular, biochemical, and functional approaches to study the regulatory interaction between CFTR and the HCO3- salvage mechanism Na+/H+ exchanger isoform 3 (NHE3) in heterologous expression systems and in the native pancreatic duct. We found that CFTR regulates NHE3 activity by both acute and chronic mechanisms. In the pancreatic duct, CFTR increases expression of NHE3 in the luminal membrane. Thus, luminal expression of NHE3 was reduced by 53% in ducts of homozygote DeltaF508 mice. Accordingly, luminal Na+-dependent and HOE694- sensitive recovery from an acid load was reduced by 60% in ducts of DeltaF508 mice. CFTR and NHE3 were co-immunoprecipitated from PS120 cells expressing both proteins and the pancreatic duct of wild type mice but not from PS120 cells lacking CFTR or the pancreas of DeltaF508 mice. The interaction between CFTR and NHE3 required the COOH-terminal PDZ binding motif of CFTR, and mutant CFTR proteins lacking the C terminus were not co-immunoprecipitated with NHE3. Furthermore, when expressed in PS120 cells, wild type CFTR, but not CFTR mutants lacking the C-terminal PDZ binding motif, augmented cAMP-dependent inhibition of NHE3 activity by 31%. These findings reveal that CFTR controls overall HCO3- homeostasis by regulating both pancreatic ductal HCO3- secretory and salvage mechanisms.

Potent and selective inhibition of the human Na+/H+ exchanger isoform NHE1 by a novel aminoguanidine derivative T-162559

We isolated Na+/H+ exchanger (NHE)-deficient Chinese hamster ovary (CHO-K1) cells stably expressing human NHE isoforms (hNHE1, hNHE2 and hNHE3) and established an assay system for measuring their Na+/H+ exchange activity by monitoring intracellular pH alterations. Using this assay system, we demonstrated that the acylguanidine derivatives, cariporide and eniporide, cause selective inhibition of hNHE1 (IC50 value of 30 nM for cariporide, IC50 value of 4.5 nM for eniporide). Furthermore, we found that a novel synthetic aminoguanidine derivative, T-162559 ((5E,7S)-[7-(5-fluoro-2-methylphenyl)-4-methyl-7,8-dihydro-5(6H)-quinolinylideneamino] guanidine dimethanesulfonate), causes a selective inhibition of hNHE1 with more potent activity than cariporide and eniporide (IC50 value of 0.96 nM). This compound did not affect Na+/HCO3- cotransport and Na+/Ca2+ exchange.

Intracellular pH regulation and Na+/H+ exchange activity in human hepatic stellate cells: effect of platelet-derived growth factor, insulin-like growth factor 1 and insulin

BACKGROUND/AIMS

The Na+/H+ exchanger is involved in rat hepatic stellate cell (HSC) proliferation induced by platelet-derived growth factor (PDGF). We therefore evaluated in human HSC: (1) the mechanisms of intracellular pH regulation; (2) the relationship between Na+/H+ exchange activation and cell proliferation induced by PDGF, insulin-like growth factor 1 (IGF-1) and insulin.

METHODS/RESULTS

pH(i) regulation was mainly dependent on the activity of the Na+/H+ exchanger, which was evaluated by measuring pH(i) recovery from an acute acid load. PDGF (25 ng/ml) gradually increased the activity of the Na+/H+ exchanger which peaked at 18 h and remained stable until the 24th h. IGF-1 (10 nmol/l), but not insulin (100 nmol/l), slightly but significantly increased the activity of the Na+/H+ exchanger. Amiloride (100 micromol/l) and 20 micromol/l 5-N-ethyl-N-isopropyl-amiloride completely inhibited HSC proliferation (evaluated by measurement of bromodeoxyuridine incorporation) induced by PDGF and IGF-1, but did not affect proliferation of HSC induced by insulin. Finally, IGF-1 did not modify the activity of the Na+/Ca2+ exchanger.

CONCLUSIONS

The Na+/H+ exchanger is involved in HSC proliferation induced by PDGF and IGF-1, whereas the proliferative effect of insulin is mediated by intracellular pathways which are Na+/H+ exchange-independent.

Calcineurin homologous protein as an essential cofactor for Na+/H+ exchangers

The Na+/H+ exchangers (NHEs) comprise a family of transporters that catalyze cell functions such as regulation of the pH and volume of a cell and epithelial absorption of Na+ and bicarbonate. Ubiquitous calcineurin B homologous protein (CHP or p22) is co-localized and co-immunoprecipitated with expressed NHE1, NHE2, or NHE3 independently of its myristoylation and Ca2+ binding, and its binding site was identified as the juxtamembrane region within the carboxyl-terminal cytoplasmic domain of exchangers. CHP binding-defective mutations of NHE1-3 or CHP depletion by injection of the competitive CHP-binding region of NHE1 into Xenopus oocytes resulted in a dramatic reduction (>90%) in the Na+/H+ exchange activity. The data suggest that CHP serves as an essential cofactor, which supports the physiological activity of NHE family members.

Inhibition of the NA(+)/H(+) exchanger reduces rat hepatic stellate cell activity and liver fibrosis: an in vitro and in vivo study

BACKGROUND & AIMS

The Na(+)/H(+) exchanger is the main intracellular pH (pH(i)) regulator in hepatic stellate cells (HSCs) and plays a key role in regulating proliferation and gene expression. We evaluated the effect of specific inhibition of this exchanger on HSC proliferation and collagen synthesis in vivo and in vitro.

METHODS

Rat HSCs were incubated in the presence of platelet-derived growth factor (PDGF), transforming growth factor (TGF)-beta1, iron ascorbate (FeAsc), and ferric nitrilotriacetate solution (FeNTA) with or without the Na(+)/H(+) exchanger inhibitor 5-N-ethyl-N-isopropyl-amiloride (EIPA). pH(i) and Na(+)/H(+) exchanger activity, cell proliferation, and type I collagen accumulation were measured by using the fluorescent dye 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein, by immunohistochemistry for bromodeoxyuridine, and by enzyme-linked immunosorbent assay, respectively. In vivo liver fibrosis was induced by dimethylnitrosamine administration and bile duct ligation (BDL) in rats treated or not treated with amiloride.

RESULTS

PDGF, FeAsc, and FeNTA increased Na(+)/H(+) exchange activity and induced HSC proliferation. TGF-beta1 had no effect on the Na(+)/H(+) exchanger and was able, as for FeAsc and FeNTA, to induce type I collagen accumulation. EIPA inhibited all the effects determined by PDGF, FeAsc, and FeNTA and had no effect on TGF-beta1-induced collagen accumulation. In vivo, amiloride reduced HSC proliferation, activation, collagen deposition, and collagen synthesis.

CONCLUSIONS

The Na(+)/H(+) exchanger can play a key role in the development of liver fibrosis and in HSC activation in vivo.

Reactive oxygen intermediates involved in cellular regulation

Reactive oxygen intermediates (ROIs) in low concentration, as released permanently by nonphagocytic cells, possess important functions in inter- and intracellular signalling. They lead to alterations in the phosphorylation pattern followed by gene activation, including the expression of proto-oncogenes. Redox-sensitive sites in membrane molecules may trigger adhesion and chemotaxis or open ion channels and activate transport processes across the cytoplasma membrane. ROIs shift the ratio of cyclic GMP to cyclic AMP giving signals to proliferation and differentiation processes. Senescence, apoptosis, and cell death can also be modulated by ROIs, depending on concentration and cell type.

Second mutations rescue point mutant of the Na(+)/H(+) exchanger NHE1 showing defective surface expression

We studied the effect of point mutation within the putative 11th transmembrane domain (TM11) of the Na(+)/H(+) exchanger NHE1 on the plasma membrane expression. Of the 19 mutants tested, two mutants (Tyr454 or Arg458 replaced by Cys) were retained in the endoplasmic reticulum. Interestingly, Y454C was expressed on the cell surface when one of the endogenous cysteine residues at position 8, 133, 421, or 477 was substituted with alanine. Random mutagenesis at Cys8 and its surrounding residues in the cytosolic N-tail revealed that replacement of Cys8 with Ala was the only identified single residue mutation that rescued Y454C. These results suggest that the abnormal conformation of the region of TM11 containing the Y454C mutation is compensated by the second mutation within other domains such as the N-tail. This approach may provide evidence for the interdomain interaction in NHE1.

Direct binding of the Na--H exchanger NHE1 to ERM proteins regulates the cortical cytoskeleton and cell shape independently of H(+) translocation

The association of actin filaments with the plasma membrane maintains cell shape and adhesion. Here, we show that the plasma membrane ion exchanger NHE1 acts as an anchor for actin filaments to control the integrity of the cortical cytoskeleton. This occurs through a previously unrecognized structural link between NHE1 and the actin binding proteins ezrin, radixin, and moesin (ERM). NHE1 and ERM proteins associate directly and colocalize in lamellipodia. Fibroblasts expressing NHE1 with mutations that disrupt ERM binding, but not ion translocation, have impaired organization of focal adhesions and actin stress fibers, and an irregular cell shape. We propose a structural role for NHE1 in regulating the cortical cytoskeleton that is independent of its function as an ion exchanger.

Quantitation of plasma membrane expression of a fusion protein of Na/H exchanger NHE3 and green fluorescence protein (GFP) in living PS120 fibroblasts

We developed a confocal morphometric analysis to quantitate the relative plasma membrane (PM) expression of the Na/H exchanger NHE3 in living PS120 fibroblasts. NHE3 is a membrane transport protein that is acutely regulated by changes in the number of molecules expressed at the PM. To quantitate the PM expression of NHE3 under various experimental conditions, we stably expressed a chimera of rabbit NHE3 and green fluorescent protein (NHE3-GFP) in PS120 fibroblasts. A three-dimensional (3D) map of the intracellular distribution of NHE3-GFP was obtained by confocal laser scanning microscopy (CLSM) of cells superfused with a styryl dye, FM 4-64. This fluorophore rapidly and reversibly labeled the outer lipid layer of the PM, which allowed generation of a digital mask of the PM and calculation of the fraction of a total cellular NHE3-GFP expressed at the PM. This analysis was successfully used to quantitate the relative PM expression of NHE3-GFP in control cells (25%) and a decrease in the expression caused by subsequent exposure of cells to wortmannin (5.1%). Reliability of the method was confirmed by cell surface biotinylation, which yielded very similar results. Confocal morphometric analysis is fast and reproducible and could potentially be used for investigations on regulation of expression of other membrane proteins.

Regulation of the formation of tumor cell pseudopodia by the Na(+)/H(+) exchanger NHE1

The Na(+)/H(+) exchanger NHE1 is involved in intracellular pH homeostasis and cell volume regulation and accumulates with actin in the lamellipodia of fibroblasts. In order to determine the role of NHE1 following epithelial transformation and the acquisition of motile and invasive properties, we studied NHE1 expression in polarized MDCK cells, Moloney Sarcoma virus (MSV) transformed MDCK cells and an invasive MSV-MDCK cell variant (MSV-MDCK-INV). Expression of NHE1 was significantly increased in MSV-MDCK-INV cells relative to MSV-MDCK and MDCK cells. NHE1 was localized with b-actin to the tips of MSV-MDCK-INV cell pseudopodia by immunofluorescence. Sensitivity of NHE1-mediated (22)Na uptake to ethylisopropylamiloride, a specific inhibitor of NHE1, was increased in MSV-MDCK cells relative to MDCK cells. Changes in intracellular pH induced upon EIPA treatment were also of higher magnitude in MSV-MDCK and MSV-MDCK-INV cells compared to wild-type MDCK cells, especially in Hepes-buffered DMEM medium. Inhibition of NHE1 by 50 microM ethylisopropylamiloride induced the disassembly of actin stress fibers and redistribution of the actin cytoskeleton in all cell types. However, in MSV-MDCK-INV cells, the effect of ethylisopropylamiloride treatment was more pronounced and associated with the increased reversible detachment of the cells from the substrate. Videomicroscopy of MSV-MDCK-INV cells revealed that within 20 minutes of addition, ethylisopropylamiloride induced pseudopodial retraction and inhibited cell motility. The ability of ethylisopropylamiloride to prevent nocodazole-induced formation of actin stress fibers in MSV-MDCK cells was more pronounced in Hepes medium relative to NaHCO(3) medium, showing that NHE1 can regulate actin stress fiber assembly in transformed MSV-MDCK cells via its intracellular pH regulatory effect. These results implicate NHE1 in the regulation of the actin cytoskeleton dynamics necessary for the adhesion and pseudopodial protrusion of motile, invasive tumor cells.

RhoA and rho kinase regulate the epithelial Na+/H+ exchanger NHE3. Role of myosin light chain phosphorylation

The activity of the Na(+)/H(+) exchanger NHE3 isoform, which is found primarily in epithelial cells, is sensitive to the state of actin polymerization. Actin assembly, in turn, is controlled by members of the small GTPase Rho family, namely Rac1, Cdc42, and RhoA. We therefore investigated the possible role of these GTPases in modulating NHE3 activity. Cells stably expressing NHE3 were transiently transfected with inhibitory forms of Rac1, Cdc42, or RhoA and transport activity was assessed using microfluorimetry. NHE3 activity was not adversely affected by either dominant-negative Rac1 or Cdc42. By contrast, the inhibitory form of RhoA greatly depressed NHE3 activity, without noticeably altering its subcellular distribution. NHE3 activity was equally reduced by inhibiting p160 Rho-associated kinase I (ROK), a downstream effector of RhoA, with the selective antagonist Y-27632 and a dominant-negative form of ROK. Furthermore, inhibition of ROK reduced the phosphorylation of myosin light chain. A comparable net dephosphorylation was achieved by the myosin light chain kinase inhibitor ML9, which similarly inhibited NHE3. These data suggest that optimal NHE3 activity requires a functional RhoA-ROK signaling pathway which acts, at least partly, by controlling the phosphorylation of myosin light chain and, ultimately, the organization of the actin cytoskeleton.

Stress-activated protein kinases (JNK and p38/HOG) are essential for vascular endothelial growth factor mRNA stability

Stability of the vascular endothelial growth factor (VEGF) mRNA is tightly regulated through its 3'-untranslated region (3'-UTR). Here, we demonstrate that VEGF mRNA levels are increased by anisomycin, a strong activator of stress-activated protein kinases. Hence, VEGF mRNA induction is inhibited by SB202190, an inhibitor of JNK and p38/HOG kinase. Furthermore, VEGF mRNA expression is increased in cells that overexpress JNK and p38/HOG by an increase in its stability. We show by two different approaches that anisomycin exerts its effect on the VEGF mRNA 3'-UTR. First, by using an in vitro mRNA degradation assay, the half-life of the VEGF mRNA 3'-UTR region transcript was found to be increased when incubated with extracts from anisomycin-treated cells; and second, the 3'-UTR was also sufficient to confer mRNA instability to the Nhe3 (Na(+)/H(+) exchanger 3) heterologous reporter gene, and anisomycin treatment stabilized the chimeric mRNA (Nhe3 fused to the VEGF mRNA 3'-UTR). This chimeric mRNA is also more stable in cells overexpressing p38/HOG and JNK that have been stimulated by anisomycin. We show that such regulation is mediated through an AU-rich region of the 3'-UTR contained within a stable hairpin structure. By RNA electrophoretic mobility shift assays, we show that this region binds proteins specifically induced by anisomycin treatment. These findings clearly demonstrate a major role of stress-activated protein kinases in the post-transcriptional regulation of VEGF.

Kinetic and pharmacological properties of cloned human equilibrative nucleoside transporters, ENT1 and ENT2, stably expressed in nucleoside transporter-deficient PK15 cells. Ent2 exhibits a low affinity for guanosine and cytidine but a high affinity for inosine

We stably transfected the cloned human equilibrative nucleoside transporters 1 and 2 (hENT1 and hENT2) into nucleoside transporter-deficient PK15NTD cells. Although hENT1 and hENT2 are predicted to be 50-kDa proteins, hENT1 runs as 40 kDa and hENT2 migrates as 50 and 47 kDa on SDS-polyacrylamide gel electrophoresis. Peptide N-glycosidase F and endoglycosidase H deglycosylate hENT1 to 37 kDa and hENT2 to 45 kDa. With hENT1 being more sensitive, there is a 7000-fold and 71-fold difference in sensitivity to nitrobenzylthioinosine (NBMPR) (IC(50), 0.4 +/- 0.1 nM versus 2.8 +/- 0.3 microM) and dipyridamole (IC(50), 5.0 +/- 0.9 nM versus 356 +/- 13 nM), respectively. [(3)H]NBMPR binds to ENT1 cells with a high affinity K(d) of 0.377 +/- 0.098 nM, and each ENT1 cell has 34,000 transporters with a turnover number of 46 molecules/s for uridine. Although both transporters are broadly selective, hENT2 is a generally low affinity nucleoside transporter with 2.6-, 2.8-, 7. 7-, and 19.3-fold lower affinity than hENT1 for thymidine, adenosine, cytidine, and guanosine, respectively. In contrast, the affinity of hENT2 for inosine is 4-fold higher than hENT1. The nucleobase hypoxanthine inhibits [(3)H]uridine uptake by hENT2 but has minimal effect on hENT1. Taken together, these results suggest that hENT2 might be important in transporting adenosine and its metabolites (inosine and hypoxanthine) in tissues such as skeletal muscle where ENT2 is predominantly expressed.

A novel topology model of the human Na(+)/H(+) exchanger isoform 1

The membrane topology of the human Na(+)/H(+) exchanger isoform 1 (NHE1) was assessed by substituted cysteine accessibility analysis. Eighty-three cysteine residues were individually introduced into a functional cysteineless NHE1, and these mutants were expressed in the exchanger-deficient PS120 cells. The topological disposition of introduced cysteines was determined by labeling with a biotinylated maleimide in the presence or absence of preincubation with the membrane-impermeable sulfhydryl reagent, 2-trimethylammoniumethyl-methanethiosulfonate in streptolysin O-permeabilized or nonpermeabilized cells. We proposed a new model for the topology of NHE1 that is significantly different from the model derived from hydropathy analysis. In this model, NHE1 is composed of 12 transmembrane segments (TMs) with the N and C termini located in the cytosol. The large, last extracellular loop in the membrane domain of the original model was suggested to comprise an intracellular loop, a new transmembrane segment (TM11), and an extracellular loop in the new model. Interestingly, cysteines at 183 and 184 and at 324 and 325 mapped to intracellular loops connecting TMs 4 and 5 (IL2) and TMs 8 and 9 (IL4), respectively, were accessible to sulfhydryl reagents from the outside. Furthermore, exchange activities of two mutants, R180C and Q181C, within IL2 were markedly inhibited by external MTSET. These data suggest that part of IL2 or IL4 may be located in a pore-lining region that is accessible from either side of the membrane and involved in ion transport.

Apoptosis of leukemic cells accompanies reduction in intracellular pH after targeted inhibition of the Na+/H+exchanger

The Na+/H+ exchanger isoform 1 (NHE1) is primarily responsible for the regulation of intracellular pH (pHi). It is a ubiquitous, amiloride-sensitive, growth factor–activatable exchanger whose role has been implicated in cell-cycle regulation, apoptosis, and neoplasia. Here we demonstrate that leukemic cell lines and peripheral blood from primary patient leukemic samples exhibit a constitutively and statistically higher pHi than normal hematopoietic tissue. We then show that a direct correlation exists between pHi and cell-cycle status of normal hematopoietic and leukemic cells. Advantage was taken of this relationship by treating leukemic cells with the Na+/H+ exchanger inhibitor, 5-(N, N-hexamethylene)-amiloride (HMA), which decreases the pHiand induces apoptosis. By incubating patient leukemic cells in vitro with pharmacologic doses of HMA for up to 5 hours, we show, using flow cytometry and fluorescent ratio imaging microscopy, that when the pHi decreases, apoptosis—measured by annexin-V and TUNEL methodologies—rapidly increases so that more than 90% of the leukemic cells are killed. The differential sensitivity exhibited between normal and leukemic cells allows consideration of NHE1 inhibitors as potential antileukemic agents.