p160ROCK mediates RhoA activation of Na-H exchange

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Mislocalized scaffolding by the Na-H exchanger NHE1 dominantly inhibits fibronectin production and TGF-beta activation

Secretion and assembly of the extracellular matrix protein fibronectin regulates a number of normal cell and tissue functions and is dysregulated in disease states such as fibrosis, diabetes, and cancer. We found that mislocalized scaffolding by the plasma membrane Na-H exchanger NHE1 suppresses fibronectin expression, secretion, and assembly. In fibroblasts, wild-type NHE1 localizes to the distal margin of membrane protrusions or lamellipodia but a mutant NHE1-KRA2 lacking binding sites for PI(4,5)P2 and the ERM proteins ezrin, radixin, and moesin is mislocalized and found uniformly along the plasma membrane. Although NHE1 regulates intracellular pH homeostasis, fibronectin production is not regulated by changes in intracellular pH, nor is it attenuated in NHE1-deficient cells, indicating fibronectin expression is independent of NHE1 activity. However, fibronectin production is nearly absent in cells expressing NHE1-KRA2 because scaffolding by NHE1 is mislocalized. Additionally, secretion of active but not latent TGF-beta is reduced and exogenous TGF-beta restores fibronectin secretion and assembly. Our data indicate that scaffolding by NHE1-KRA2 dominantly suppresses fibronectin synthesis and TGF-beta activation, and they suggest that NHE1-KRA2 can be used for obtaining a mechanistic understanding of how fibronectin production is regulated and speculatively for therapeutic control of dysregulated production in pathological conditions.

Modulation of Na+-H+ exchange isoforms NHE1 and NHE3 by insulin-like growth factor-1 in isolated bovine articular chondrocytes

Incubation with serum modulates the transporters that regulate intracellular pH (pH(i)) in articular chondrocytes, upregulating acid extrusion by Na(+)-H(+) exchange (NHE). There is stimulation of NHE1, together with induction of NHE3 activity. These isoforms exhibit differential responses to components of mechanical load experienced by chondrocytes during joint loading. The identity of the component(s) of serum responsible is unknown. A possibility, however, is insulin-like growth factor-1 (IGF-1), present in normal cartilage and found at enhanced levels in osteoarthritic tissue. In the present study, the effects of IGF-1 on pH(i) regulation have been characterized using fluorescence measurements of bovine articular chondrocytes, and the sensitivity of pH(i) regulation to hyperosmotic shock and raised hydrostatic pressure determined. For cells isolated in the absence of IGF-1, pH(i) recovery following acidification was predominantly mediated by NHE1. Recovery was enhanced when cells were incubated for 18 h with 20 ng mL(-1) IGF; this effect represented increased acid extrusion by NHE1, supplemented by NHE3 activity. NHE3 activity was not detected in IGF-1-treated cells that had been incubated with the protein synthesis inhibitor cycloheximide, although NHE1 activity was unaffected. In the absence of IGF-1, suspension in hyperosmotic solutions or raised hydrostatic pressure enhanced pH(i) recovery of acidified cells. This response was missing in cells incubated with IGF-1. Unresponsiveness to hyperosmotic shock represented inhibition of NHE3 activity, and was prevented using the protein kinase A inhibitor KT5720. For raised hydrostatic pressure, a decrease in NHE1 activity was responsible, and was prevented by the protein kinase C inhibitor chelerythrine.

Protein kinase B/Akt phosphorylates and inhibits the cardiac Na+/H+ exchanger NHE1

Sarcolemmal Na(+)/H(+) exchanger (NHE) activity is mediated by NHE isoform 1 (NHE1), which is subject to regulation by protein kinases. Our objectives were to determine whether NHE1 is phosphorylated by protein kinase B (PKB), identify any pertinent phosphorylation site(s), and delineate the functional consequences of such phosphorylation. Active PKBalpha phosphorylated in vitro a glutathione S-transferase (GST)-NHE1 fusion protein comprising amino acids 516 to 815 of the NHE1 carboxyl-terminal regulatory domain. PKBalpha-mediated phosphorylation of GST-NHE1 fusion proteins containing overlapping segments of this region localized the targeted residues to the carboxyl-terminal 190 amino acids (625 to 815) of NHE1. Mass spectrometry and phosphorylation analysis of mutated (Ser-->Ala) GST-NHE1 fusion proteins revealed that PKBalpha-mediated phosphorylation of NHE1 occurred principally at Ser648. Far-Western assays demonstrated that PKBalpha-mediated Ser648 phosphorylation abrogated calcium-activated calmodulin (CaM) binding to the regulatory domain of NHE1. In adult rat ventricular myocytes, adenovirus-mediated expression of myristoylated PKBalpha (myr-PKBalpha) increased cellular PKB activity, as confirmed by increased glycogen synthase kinase 3beta phosphorylation. Heterologously expressed myr-PKBalpha was present in the sarcolemma, colocalized with NHE1 at the intercalated disc regions, increased NHE1 phosphorylation, and reduced NHE1 activity following intracellular acidosis. Conversely, pharmacological inhibition of endogenous PKB increased NHE1 activity following intracellular acidosis. Our data suggest that NHE1 is a novel PKB substrate and that its PKB-mediated phosphorylation at Ser648 inhibits sarcolemmal NHE activity during intracellular acidosis, most likely by interfering with CaM binding and reducing affinity for intracellular H(+).

Apoptosis-induced alkalinization by the Na+/H+ exchanger isoform 1 is mediated through phosphorylation of amino acids Ser726 and Ser729

Apoptosis is a complex process essential for normal tissue development and cellular homeostasis. While biochemical events that occur late in the apoptotic process are better characterized, early physiological changes that initiate the progression of cell death remain poorly understood. Previously, we observed that lymphocytes, undergoing apoptosis in response to growth factor withdrawal, experienced a rapid and transient rise in cytosolic pH. We found that the protein responsible was the pH-regulating, plasma membrane protein Na(+)/H(+) exchanger isoform 1 (NHE1), and that its activity was impeded by inhibition of the stress-activated kinase, p38 MAP kinase. In the current study, we examined how NHE1 is activated during apoptosis. We identified the phosphorylation sites on NHE1 that regulate its alkalinizing activity in response to a cell death stimulus. Performing targeted mutagenesis, we observed that substitution of Ser726 and Ser729 for alanines produced a mutant form of NHE1 that did not alkalinize in response to an apoptotic stimulus, and expression of which protected cells from serum withdrawal- induced death. In contrast, substitution of Ser726 and Ser729 for glutamic acids raised the basal pH and induced susceptibility to death. Analysis of serine phosphorylation showed that phosphorylation of NHE1 during apoptosis decreased upon mutation of Ser726 and Ser729. Our findings thus confirm a necessary function for NHE1 during apoptosis and reveal the critical regulatory sites that when phosphorylated mediate the alkalinizing activity of NHE1 in the early stages of a cell death response.

Nerve growth factor inhibits Na+/H+ exchange and formula absorption through parallel phosphatidylinositol 3-kinase-mTOR and ERK pathways in thick ascending limb

In the medullary thick ascending limb, inhibiting the basolateral NHE1 Na(+)/H(+) exchanger with nerve growth factor (NGF) induces actin cytoskeleton remodeling that secondarily inhibits apical NHE3 and transepithelial HCO(3)(-) absorption. The inhibition by NGF is mediated 50% through activation of extracellular signal-regulated kinase (ERK). Here we examined the signaling pathway responsible for the remainder of the NGF-induced inhibition. Inhibition of HCO(3)(-) absorption was reduced 45% by the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin or LY294002 and 50% by rapamycin, a specific inhibitor of mammalian target of rapamycin (mTOR), a downstream effector of PI3K. The combination of a PI3K inhibitor plus rapamycin did not cause a further reduction in the inhibition by NGF. In contrast, the combination of a PI3K inhibitor plus the MEK/ERK inhibitor U0126 completely eliminated inhibition by NGF. Rapamycin decreased NGF-induced inhibition of basolateral NHE1 by 45%. NGF induced a 2-fold increase in phosphorylation of Akt, a PI3K target linked to mTOR activation, and a 2.2-fold increase in the activity of p70 S6 kinase, a downstream effector of mTOR. p70 S6 kinase activation was blocked by wortmannin and rapamycin, consistent with PI3K, mTOR, and p70 S6 kinase in a linear pathway. Rapamycin-sensitive inhibition of NHE1 by NGF was associated with an increased level of phosphorylated mTOR in the basolateral membrane domain. These findings indicate that NGF inhibits HCO(3)(-) absorption in the medullary thick ascending limb through the parallel activation of PI3K-mTOR and ERK signaling pathways, which converge to inhibit NHE1. The results identify a role for mTOR in the regulation of Na(+)/H(+) exchange activity and implicate NHE1 as a possible downstream effector contributing to mTOR's effects on cell growth, proliferation, survival, and tumorigenesis.

Ste20-Related Kinase SLK Phosphorylates Ser188 of RhoA to Induce Vasodilation in Response to Angiotensin II Type 2 Receptor Activation

The small G protein Rho signaling pathways are recognized as major regulators of cardiovascular functions, and activation of Rho proteins appears to be a common component for the pathogenesis of hypertension and vascular proliferative disorders. Recent evidence suggests that modulation of Rho protein signaling by phosphorylation of Rho proteins provides an additional simple mechanism for coordinating Rho protein functions. Phosphorylation of RhoA by cAMP- or cGMP-activated kinase on Ser188 induces cytosolic sequestration of RhoA through increased interaction with guanine dissociation inhibitor, thereby resulting in inhibition of RhoA-dependent functions. Here we show that stimulation of angiotensin II (Ang II) type 2 receptor (AT 2 R) in vascular smooth muscle cells induces Ser188 phosphorylation of RhoA independently of cAMP- or cGMP-activated kinase. We identify the Ser/Thr kinase Ste20-related kinase SLK as a new kinase phosphorylating RhoA on Ser188. Activation of the signaling cascade involving Src homology 2 domain–containing protein-tyrosine phosphatase 1, casein kinase II and SLK is responsible for RhoA phosphorylation and inhibition of RhoA-mediated arterial contraction induced by AT 2 R activation. These results thus identify the molecular mechanism linking AT 2 R to RhoA inhibition and vasodilation.

Regulation of cell survival by Na+/H+ exchanger-1

Na(+)/H(+) exchanger-1 (NHE1) is a ubiquitous plasma membrane Na(+)/H(+) exchanger typically associated with maintenance of intracellular volume and pH. In addition to the NHE1 role in electroneutral Na(+)/H(+) transport, in renal tubular epithelial cells in vitro the polybasic, juxtamembrane NHE1 cytosolic tail domain acts as a scaffold, by binding with ezrin/radixin/moesin (ERM) proteins and phosphatidylinositol 4,5-bisphosphate, which initiates formation of a signaling complex that culminates in Akt activation and opposition to initial apoptotic stress. With robust apoptotic stimuli renal tubular epithelial cell NHE1 is a caspase substrate, and proteolytic cleavage may permit progression to apoptotic cell death. In vivo, genetic or pharmacological NHE1 loss of function causes renal tubule epithelial cell apoptosis and renal dysfunction following streptozotocin-induced diabetes, ureteral obstruction, and adriamycin-induced podocyte toxicity. Taken together, substantial in vivo and in vitro data demonstrate that NHE1 regulates tubular epithelial cell survival. In contrast to connotations of NHE1 as an unimportant "housekeeping" protein, this review highlights that NHE1 activity is critical for countering tubular atrophy and chronic renal disease progression.

Hyaluronan-mediated CD44 activation of RhoGTPase signaling and cytoskeleton function promotes tumor progression

Hyaluronan (HA), a major component of the extracellular matrix (ECM), is enriched in many types of tumors. In cancer patients HA concentrations are usually higher in malignant tumors than in corresponding benign or normal tissues, and in some tumor types the level of HA is predictive of malignancy. HA is often bound to CD44 isoforms which are ubiquitous, abundant, and functionally important cell surface receptors. This article reviews the current evidence for HA/CD44-mediated activation of the ankyrin-based cytoskeleton and RhoGTPase signaling during tumor progression. A special focus is placed on the role of HA-mediated CD44 interaction with unique downstream effectors (e.g., the cytoskeletal protein, ankyrin and/or various GTPases (e.g., RhoA, Rac1 and Cdc42)) in coordinating intracellular signaling pathways (e.g., Ca(2+) mobilization, Rho signaling, PI3 kinase-AKT activation, NHE1-mediated cellular acidification, transcriptional upregulation and cytoskeletal function) and generating the concomitant onset of tumor cell activities (e.g., tumor cell adhesion, growth, survival, migration and invasion) and tumor progression. I believe this information will provide valuable new insights into poorly understood aspects of solid tumor malignancy. Furthermore, the new knowledge concerning HA/CD44-mediated oncogenic signaling events will have potentially important clinical utility, and could establish CD44 and its associated signaling molecules as important tumor markers for the early detection and evaluation of oncogenic potential. It could also serve as ground work for the future development of new drug targets to inhibit HA/CD44-mediated tumor metastasis and cancer progression.

Calcineurin B homologous protein 3 promotes the biosynthetic maturation, cell surface stability, and optimal transport of the Na+/H+ exchanger NHE1 isoform

Calcineurin B homologous protein (CHP) 1 and 2 are Ca(2+)-binding proteins that modulate several cellular processes, including cytoplasmic pH by positively regulating plasma membrane-type Na(+)/H(+) exchangers (NHEs). Recently another CHP-related protein, termed tescalcin or CHP3, was also shown to interact with the ubiquitous NHE1 isoform, but seemingly suppressed its activity. However, the precise physical and functional nature of this association was not examined in detail. In this study, biochemical and cellular studies were undertaken to further delineate this relationship. Glutathione S-transferase-NHE1 fusion protein pulldown assays revealed that full-length CHP3 binds directly to the proximal juxtamembrane C-terminal region (amino acids 505-571) of rat NHE1 in the same region that binds CHP1 and CHP2. The interaction was further validated by coimmunoprecipitation and coimmunolocalization experiments using full-length CHP3 and wild-type NHE1 in transfected Chinese hamster ovary AP-1 cells. Simultaneous mutation of four hydrophobic residues within this region ((530)FLDHLL(535)) to either Ala, Gln, or Arg (FL-A, FL-Q, or FL-R) abrogated this interaction both in vitro and in intact cells. The NHE1 mutants were sorted properly to the cell surface but showed markedly reduced (FL-A) or minimal (FL-R and FL-Q) activity. Interestingly, and contrary to an earlier finding, ectopic coexpression of CHP3 up-regulated the cell surface activity of wild-type NHE1. This stimulation was not observed with the CHP3 binding-defective mutants. Mechanistically, overexpression of CHP3 did not alter the H(+) sensitivity of wild-type NHE1 but rather promoted its biosynthetic maturation and half-life at the cell surface, thereby increasing the steady-state abundance of functional NHE1 protein.

Deficiency of the NHE1 gene prevents hypoxia-induced pulmonary hypertension and vascular remodeling

RATIONALE

Our previous studies found that Na(+)/H(+) exchanger (NHE) activity played an essential role in pulmonary artery smooth muscle cell (PASMC) proliferation and in the development of hypoxia-induced pulmonary hypertension and vascular remodeling. Other investigators recently observed increased expression of the NHE isoform 1 (NHE1) gene in rodents with pulmonary hypertension induced by hypoxia. However, a causal role for the NHE1 gene in pulmonary hypertension has not been determined.

OBJECTIVES

To determine the causal role of the NHE1 gene in pulmonary hypertension and vascular remodeling.

METHODS

We used NHE1-null mice to define the role of the NHE1 gene in the development of pulmonary hypertension and remodeling induced by hypoxia and to delineate the NHE1 regulatory pathway.

MEASUREMENTS AND MAIN RESULTS

After 2 weeks of exposure to hypoxia, in contrast to wild-type hypoxic littermates, there was no significant increase in right ventricular systolic pressure, in the ratio of right ventricular to left ventricular plus septal weight [RV/(LV + S)], or in medial wall thickness of the pulmonary arterioles in homozygous mice (NHE1(-/-)). There was a significant decrease in Rho kinase (ROCK1 and ROCK2) expression, accompanied by an increase in p27 expression in NHE1(-/-) mice.

CONCLUSIONS

Our study demonstrated that deficiency of the NHE1 gene prevented the development of hypoxia-induced pulmonary hypertension and vascular remodeling in mice and revealed a novel regulatory pathway associated with NHE1 signaling.

Physical interactions and functional coupling between Daxx and sodium hydrogen exchanger 1 in ischemic cell death

Daxx, a death domain-associated protein, is implicated in ischemic cell death. To clarify the mechanism of cell death mediated by Daxx, a yeast two-hybrid assay was performed. Sodium hydrogen exchanger isoform 1 (NHE1) was identified as a Daxx-interacting protein. During ischemic stress, Daxx translocates from the nucleus to the cytoplasm, where it colocalizes with NHE1. Daxx binds to the ezrin/radixin/moesin-interacting domain of NHE1, in competition with ezrin. Consistent with this finding, transfection of the constitutively cytoplasmic mutant, Daxx(W621A), inhibited ezrin-mediated Akt-1 activation. Moreover, transfection of Daxx(W621A), but not the Daxx(S667A) mutant that is confined to the nucleus, accelerated pH(i) recovery from an acid load, indicating that the cytoplasmic protein activates NHE1. Based on the results, we propose that ischemic insult triggers the nucleocytoplasmic translocation of Daxx, following which cytoplasmic Daxx stimulates the NHE1 transporter activity and suppresses activation of the NHE1-ezrin-Akt-1 pathway. Our data support a novel molecular function of Daxx as an upstream regulator of NHE1 in ischemic cell death.

Rho-kinase (ROCK) in sea urchin sperm: its role in regulating the intracellular pH during the acrosome reaction

Sperm must undergo the acrosome reaction (AR) in order to fertilize the egg. In sea urchins, this reaction is triggered by the egg jelly (EJ) which, upon binding to its sperm receptor, induces increases in the ion permeability of the plasma membrane and changes in protein phosphorylation. Here, we demonstrated that the sperm expresses ROCK (approximately 135kDa), which is a serine/threonine protein kinase. ROCK localized, as RhoGTPase (Rho), in the acrosomal region, midpiece and flagellum. H-1152, a ROCK antagonist, inhibited the two cellular processes defining the AR: the acrosomal exocytosis and the actin polymerization. The ionophores nigericin and A23187 reversed the AR inhibition induced by H-1152, suggesting that ROCK functions at the level of the EJ-induced ion fluxes. Accordingly, H-1152 blocked 70% the intracellular alkalinization induced by EJ. These results indicate that EJ activates a Na+-H+ exchanger (NHE) in the sperm through a Rho/ROCK-dependent signaling pathway that culminates in the AR.

pH nanoenvironment at the surface of single melanoma cells

Extracellular pH and the Na(+)/H(+) exchanger (NHE1) modulate tumor cell migration. Yet, the pH nanoenvironment at the outer surface of the cell membrane (pH(em)) where cell/matrix interaction occurs and matrix metalloproteinases work was never measured. We present a method to measure this pH nanoenvironment using proton-sensitive dyes to label the outer leaflet of the plasma membrane or the glycocalyx of human melanoma cells. Polarized cells generate an extracellular proton gradient at their surface that increases from the rear end to the leading edge of the lamellipodium along the direction of movement. This gradient collapses upon NHE1 inhibition by HOE642. NHE1 stimulation by intracellular acidification increases the difference in pH(em) between the tips of lamellipodia and the cell body in a Na(+) dependent way. Thus, cells create a pH nanoenvironment that promotes cell migration by facilitating cell adhesion at their front and the release of cell/matrix contacts at their rear part.

Beyond ion translocation: structural functions of the sodium-hydrogen exchanger isoform-1

PURPOSE OF REVIEW

The sodium-hydrogen exchanger isoform-1 (NHE1) functions in intracellular pH and cell volume homeostasis by catalyzing an electroneutral exchange of extracellular sodium and intracellular hydrogen. Recent studies have revealed the structural functions of NHE1 as an anchor for actin filaments and a scaffold for an ensemble of signaling proteins. This review highlights how these functions contribute to NHE1 regulation of biochemical events and cell behaviors.

RECENT FINDINGS

New data confirming nontransport structural functions of NHE1 suggest reexamining how NHE1 regulates cell functions. Cell survival, cell substrate adhesion, and organization of the actin cytoskeleton are confirmed to be regulated through actin anchoring by NHE1 and likely by NHE1-dependent scaffolding of signaling proteins. A role for NHE1 in mechanotransduction is emerging and a challenge of future studies is to determine whether structural functions of NHE1 are important for mechanoresponsiveness.

SUMMARY

This review highlights evidence for the nontransport functions of NHE1 and describes how the structural functions are integrated with ion translocation to regulate a range of cellular processes. Nontransporting features of NHE1 are analogous to recently observed nonconducting actions of ion channels in regulating cell behaviors and represent an emerging paradigm of ion transporters as multifunctional proteins.

Physiological role and regulation of the Na+/H+ exchanger

In mammalian eukaryotic cells, the Na+/H+ exchanger is a family of membrane proteins that regulates ions fluxes across membranes. Plasma membrane isoforms of this protein extrude 1 intracellular proton in exchange for 1 extracellular sodium. The family of Na+/H+ exchangers (NHEs) consists of 9 known isoforms, NHE1-NHE9. The NHE1 isoform was the first discovered, is the best characterized, and exists on the plasma membrane of all mammalian cells. It contains an N-terminal 500 amino acid membrane domain that transports ions, plus a 315 amino acid C-terminal, the intracellular regulatory domain. The Na+/H+ exchanger is regulated by both post-translational modifications including protein kinase-mediated phosphorylation, plus by a number of regulatory-binding proteins including phosphatidylinositol-4,5-bisphosphate, calcineurin homologous protein, ezrin, radixin and moesin, calmodulin, carbonic anhydrase II, and tescalcin. The Na+/H+ exchanger is involved in a variety of complex physiological and pathological events that include regulation of intracellular pH, cell movement, heart disease, and cancer. This review summarizes recent advances in the understanding of the physiological role and regulation of this protein.

Rho kinase in the regulation of cell death and survival

Rho kinase (ROCK) belongs to a family of serine/threonine kinases that are activated via interaction with Rho GTPases. ROCK is involved in a wide range of fundamental cellular functions, such as contraction, adhesion, migration, and proliferation. Recent studies have shown that ROCK plays an important role in the regulation of apoptosis in various cell types and animal disease models. Two ROCK isoforms, ROCK1 and ROCK2, are assumed to be function redundant, this based largely on kinase construct overexpression and chemical inhibitors (Y27632 and fasudil) which inhibit both ROCK1 and ROCK2. Gene targeting and RNA interference approaches allow further dissection of distinct cellular, physiological, and patho-physiological functions of the two ROCK isoforms. This review, based on recent molecular, cellular, and animal studies, focuses on the current understanding of ROCK signaling in the regulation of apoptosis and highlights new findings from recently generated ROCK-deficient mice.

Structural and functional analysis of the Na+/H+ exchanger

The mammalian NHE (Na+/H+ exchanger) is a ubiquitously expressed integral membrane protein that regulates intracellular pH by removing a proton in exchange for an extracellular sodium ion. Of the nine known isoforms of the mammalian NHEs, the first isoform discovered (NHE1) is the most thoroughly characterized. NHE1 is involved in numerous physiological processes in mammals, including regulation of intracellular pH, cell-volume control, cytoskeletal organization, heart disease and cancer. NHE comprises two domains: an N-terminal membrane domain that functions to transport ions, and a C-terminal cytoplasmic regulatory domain that regulates the activity and mediates cytoskeletal interactions. Although the exact mechanism of transport by NHE1 remains elusive, recent studies have identified amino acid residues that are important for NHE function. In addition, progress has been made regarding the elucidation of the structure of NHEs. Specifically, the structure of a single TM (transmembrane) segment from NHE1 has been solved, and the high-resolution structure of the bacterial Na+/H+ antiporter NhaA has recently been elucidated. In this review we discuss what is known about both functional and structural aspects of NHE1. We relate the known structural data for NHE1 to the NhaA structure, where TM IV of NHE1 shows surprising structural similarity with TM IV of NhaA, despite little primary sequence similarity. Further experiments that will be required to fully understand the mechanism of transport and regulation of the NHE1 protein are discussed.

Mitogen-activated protein kinase-dependent activation of the Na+/H+ exchanger is mediated through phosphorylation of amino acids Ser770 and Ser771

We investigated regulation of the type 1 isoform of the Na(+)/H(+) exchanger by phosphorylation. Four specific groups of serine and threonine residues in the regulatory carboxyl-terminal tail were mutated to alanine residues: group 1, S693A; group 2, T718A and S723A/S726A/S729A; group 3, S766A/S770A/S771A; and group 4, T779A and S785A. The proteins were expressed in Na(+)/H(+) exchanger-deficient cells, and the activity was characterized. All of the mutants had proper expression, localization, and normal basal activity relative to wild type NHE1. Sustained intracellular acidosis was used to activate NHE1 via an ERK-dependent pathway that could be blocked with the MEK inhibitor U0126. Immunoprecipitation of (32)P-labeled Na(+)/H(+) exchanger from intact cells showed that sustained intracellular acidosis increased Na(+)/H(+) exchanger phosphorylation in vivo. This was blocked by U0126. The Na(+)/H(+) exchanger activity of mutants 1 and 2 was stimulated similar to wild type Na(+)/H(+) exchanger. Mutant 4 showed a partially reduced level of activation. However, mutant 3 was not stimulated by sustained intracellular acidosis, and loss of stimulation of activity correlated to a loss of sustained acidosis-mediated phosphorylation in vivo. Mutation of the individual amino acids within mutant 3, Ser(766), Ser(770), and Ser(771), showed that Ser(770) and Ser(771) are responsible for mediating increases in NHE1 activity through sustained acidosis. Both intact Ser(770) and Ser(771) were required for sustained acidosis-mediated activation of NHE1. Our results suggest that amino acids Ser(770) and Ser(771) mediate ERK-dependent activation of the Na(+)/H(+) exchanger in vivo.

Rho, Rho-kinase, and the actin cytoskeleton regulate the Na+–H+ exchanger in sea urchin eggs

At fertilization, the sea urchin egg undergoes an internal pH (pHi) increase mediated by a Na+ -H+ exchanger. We used antibodies against the mammalian antiporters NHE1 and NHE3 to characterize this exchanger. In unfertilized eggs, only anti-NHE3 cross-reacted specifically with a protein of 81-kDa, which localized to the plasma membrane and cortical granules. Cytochalasin D, C3 exotoxin (blocker of RhoGTPase function), and Y-27632 (inhibitor of Rho-kinase) prevented the pHi change in fertilized eggs. These inhibitors blocked the first cleavage division of the embryo, but not the cortical granule exocytosis. Thus, the sea urchin egg has an epithelial NHE3-like Na+ -H+ exchanger which can be responsible for the pHi change at fertilization. Determinants of this pHi change can be: (i) the increase of exchangers in the plasma membrane (via cortical granule exocytosis) and (ii) Rho, Rho-kinase, and optimal organization of the actin cytoskeleton as regulators, among others, of the intrinsic activity of the exchanger.

Ca2+-Calmodulin and Janus Kinase 2 Are Required for Activation of Sodium-Proton Exchange by the Gi-Coupled 5-Hydroxytryptamine1aReceptor

The type 1 sodium-proton exchanger (NHE-1) is expressed ubiquitously and regulates key cellular functions, including mitogenesis, cell volume, and intracellular pH. Despite its importance, the signaling pathways that regulate NHE-1 remain incompletely defined. In this work, we present evidence that stimulation of the 5-hydroxytryptamine 1A (5-HT1A) receptor results in the formation of a signaling complex that includes activated Janus kinase 2 (Jak2), Ca2+/calmodulin (CaM), and NHE-1, and which involves tyrosine phosphorylation of CaM. The signaling pathway also involves rapid agonist-induced association of CaM and NHE-1 as assessed by coimmunoprecipitation studies and by bioluminescence resonance energy transfer studies in living cells. We propose that NHE-1 is activated through this pathway: 5-HT1A receptor --> G(i2)alpha and/or G(i3)alpha --> Jak2 activation --> tyrosine phosphorylation of CaM --> increased binding of CaM to NHE-1 --> induction of a conformational change in NHE-1 that unmasks an obscured proton-sensing and/or proton-transporting region of NHE-1 --> activation of NHE-1. The G(i/o)-coupled 5-HT1A receptor now joins a handful of Gq-coupled receptors and hypertonic shock as upstream activators of this emerging pathway. In the course of this work, we have presented clear evidence that CaM can be activated through tyrosine phosphorylation in the absence of a significant role for elevated intracellular Ca2+. We have also shown for the first time that the association of CaM with NHE-1 in living cells is a dynamic process.

Rho/ROCK-dependent pseudopodial protrusion and cellular blebbing are regulated by p38 MAPK in tumour cells exhibiting autocrine c-Met activation

BACKGROUND INFORMATION

The c-Met-dependent, beta-actin-rich, blebbed pseudopodia of MSV-MDCK-INV (invasive Moloney-sarcoma-virus-transformed Madin-Darby canine kidney) cells are induced by Rho/ROCK (Rho kinase) activation, and are morphologically distinct from flat extended lamellipodia.

RESULTS

Microtubules were shown to extend to these actin-rich pseudopodial domains, and microtubule depolymerization by nocodazole treatment resulted in progressive cellular blebbing, initiating in the pseudopodial domains and resulting in transient cellular rounding and blebbing after 30 min. The blebbing response was dependent on autocrine HGF (hepatocyte growth factor) activation of c-Met and prevented by inhibition of RhoA, ROCK and p38 MAPK (p38 mitogen-activated protein kinase), but not ERK (extracellular-signal-regulated kinase) or PI3K (phosphoinositide 3-kinase). Phospho-p38 MAPK was present in pseudopodia, localizing activation of this signalling pathway to this protrusive membrane structure. In serum-starved cells, LPA (lysophosphatidic acid) activation of RhoA induced p38 MAPK-dependent pseudopodial protrusions, and inhibition of p38 MAPK prevented pseudopodial protrusion and displacement of MSV-MDCK-INV cells. MSV-MDCK-INV cells exhibited intermittent blebbing and rounding, which may represent an integral part of their motile behaviour.

CONCLUSIONS

The localized activation of an autocrine HGF/c-Met loop regulates Rho/ROCK activation of p38 MAPK signalling to stimulate both membrane blebbing and pseudopod formation.

Phosphoryl transfer is not rate-limiting for the ROCK I-catalyzed kinase reaction

Rho-associated coiled-coil kinase, ROCK, is implicated in Rho-mediated cell adhesion and smooth muscle contraction. Animal models suggest that the inhibition of ROCK can ameliorate conditions, such as vasospasm, hypertension, and inflammation. As part of our effort to design novel inhibitors of ROCK, we investigated the kinetic mechanism of ROCK I. Steady-state bisubstrate kinetics, inhibition kinetics, isotope partition analysis, viscosity effects, and presteady-state kinetics were used to explore the kinetic mechanism. Plots of reciprocals of initial rates obtained in the presence of nonhydrolyzable ATP analogues and the small molecule inhibitor of ROCK, Y-27632, against the reciprocals of the peptide concentrations yielded parallel lines (uncompetitive pattern). This pattern is indicative of an ordered binding mechanism, with the peptide adding first. The staurosporine analogue K252a, however, gave a noncompetitive pattern. When a pulse of (33)P-gamma-ATP mixed with ROCK was chased with excess unlabeled ATP and peptide, 0.66 enzyme equivalent of (33)P-phosphate was incorporated into the product in the first turnover. The presence of ATPase activity coupled with the isotope partition data is a clear evidence for the existence of a viable [E-ATP] complex in the kinase reaction and implicates a random binding mechanism. The k(cat)/K(m) parameters were fully sensitive to viscosity (viscosity effects of 1.4 +/- 0.2 and 0.9 +/- 0.3 for ATP and peptide 5, respectively), and therefore, the barriers to dissociation of either substrate are higher than the barrier for the phosphoryl transfer step. As a consequence, not all the binding steps are at fast equilibrium. The observation of a burst in presteady-state kinetics (k(b) = 10.2 +/- 2.1 s(-)(1)) and the viscosity effect on k(cat) of 1.3 +/- 0.2 characterize the phosphoryl transfer step to be fast and the release of product and/or the enzyme isomerization step accompanying it as rate-limiting at V(max) conditions. From the multiple kinetic studies, most of the rate constants for the individual steps were either evaluated or estimated.

Inhibiting p90 ribosomal S6 kinase prevents (Na+)-H+ exchanger-mediated cardiac ischemia-reperfusion injury

BACKGROUND

Pharmacological and genetic studies indicate that the (Na+)-H+ exchanger isoform 1 (NHE1) plays a critical role in myocardial ischemia and reperfusion (I/R) injury. We found that p90 ribosomal S6 kinase (RSK) phosphorylated serine 703 of NHE1, stimulating 14-3-3 binding and NHE1 activity. Therefore, we hypothesized that inhibiting RSK in cardiomyocytes would prevent NHE1 activation and decrease I/R-mediated injury.

METHODS AND RESULTS

To examine the role of RSK in vivo, we generated transgenic mice with cardiac-specific overexpression of dominant negative RSK (DN-RSK-TG). DN-RSK-TG hearts demonstrated normal basal cardiac function and morphology. However, myocardial infarction (left coronary artery occlusion for 45 minutes) in DN-RSK-TG hearts was significantly reduced at 24 hours of reperfusion from 46.9+/-5.6% area at risk in nontransgenic littermate controls to 26.0+/-4.2% in DN-RSK-TG (P<0.01). Cardiomyocyte apoptosis was significantly reduced after I/R in DN-RSK (0.9+/-0.2%) compared with nontransgenic littermate controls (6.2+/-2.6%). Importantly, activation of RSK and interaction of 14-3-3 with NHE1, necessary for agonist-stimulated NHE1 activity, were increased by I/R and inhibited by 70% in DN-RSK-TG (P<0.01). Next, we transduced rat neonatal cardiomyocytes with adenovirus-expressing DN-RSK (Ad.DN-RSK) and measured NHE1 activity. The baseline rate of pH recovery in acid-loaded cells was equal in cells expressing LacZ or DN-RSK. However, NHE1 activation by 100 micromol/L H2O2 was significantly inhibited in cells expressing DN-RSK (0.16+/-0.02 pH units/min) compared with Ad.LacZ (0.49+/-0.13 pH units/min). Apoptosis induced by 12 hours of anoxia followed by 24 hours' reoxygenation was significantly reduced in cells expressing Ad.DN-RSK (18.6+/-2.0%) compared with Ad.LacZ (29.3+/-5.4%).

CONCLUSIONS

In summary, RSK is a novel regulator of cardiac NHE1 activity by phosphorylating NHE1 serine 703 and a new pathological mediator of I/R injury in the heart.

Role of the Na/H exchanger NHE1 in cell migration

Cell migration plays a basic role in many physiological and pathophysiological processes such as embryogenesis, immune defence, wound healing or metastasis. The activity of the ubiquitously expressed NHE1 isoform of the plasma membrane Na+/H+ exchanger is one of the requirements for directed locomotion of migrating cells and also contributes to cell adhesion. The mechanisms by which NHE1 is involved in cell migration are multiple. NHE1 contributes to cell migration by affecting the cell volume, by regulating the intracellular pH and thereby the assembly and activity of cytoskeletal elements, by anchoring the cytoskeleton to the plasma membrane, by signalling, by regulating gene expression and by controlling cell adhesion. The present article gives a review of the different ways in which NHE1 is involved in and contributes to cell migration. These different mechanisms complement one another forming an intricate, integrative process.

The Na+/H+ exchanger NHE1 in stress-induced signal transduction: implications for cell proliferation and cell death

The ubiquitous plasma membrane Na+/H+ exchanger NHE1 is highly conserved across vertebrate species and is extensively characterized as a major membrane transport mechanism in the regulation of cellular pH and volume. In recent years, the understanding of the role of NHE1 in regulating cell function has expanded from one of a household protein involved in ion homeostasis to that of a multifaceted regulator and/or modulator of a wide variety of cell functions. NHE1 plays pivotal roles in response to a number of important physiological stress conditions which, in addition to cell shrinkage and acidification, include hypoxia and mechanical stimuli, such as cell stretch. It has recently become apparent that NHE1-mediated modulation of, e.g., cell migration, morphology, proliferation, and death results not only from NHE1-mediated changes in pHi, cell volume, and/or [Na+]i, but also from direct protein-protein interactions with, e.g., ezrin/radixin/moesin (ERM) proteins and regulation of cellular signaling events, including the activity of mitogen-activated protein kinases (MAPKs) and Akt/protein kinase B (PKB). The aim of this review is to present and discuss new findings implicating NHE1 activation as a central signaling event activated by stress conditions and modulating cell proliferation and death. The pathophysiological importance of NHE1 in modulating the balance between cell proliferation and cell death in cancer and in ischemia/severe hypoxia will also be briefly addressed.

Ca2+ signaling, intracellular pH and cell volume in cell proliferation

Mitogens control progression through the cell cycle in non-transformed cells by complex cascades of intracellular messengers, such as Ca2+ and protons, and by cell volume changes. Intracellular Ca2+ and proton concentrations are critical for linking external stimuli to proliferation, motility, apoptosis and differentiation. This review summarizes the role in cell proliferation of calcium release from intracellular stores and the Ca2+ entry through plasma membrane Ca2+ channels. In addition, the impact of intracellular pH and cell volume on cell proliferation is discussed.

Overexpression of RhoA-GTP induces activation of the Epidermal Growth Factor Receptor, dephosphorylation of focal adhesion kinase and increased motility in breast cancer cells

Rho GTPases are overexpressed in human tumors and are involved in a variety of cellular processes such as organization of the actin cytoskeleton, cell-cell contact and malignant transformation. EGFR activation plays a key role in the acquisition of motile properties in carcinoma cells, and it has been proposed that downregulation of FAK activity is one of its most relevant consequences. In the present study, using mammary MCF-7 cells, we demonstrated that overexpression of the active form of the small GTPase RhoA induced the activation of EGFR by a phenomenon that depends on the activity of a metalloproteinase (MMP), which presumably cleaves a membrane-bound EGFR ligand. The EGFR tyrosine phosphorylation correlates with ERK1,2 activation and the stimulation of urokinase production. An aggressive mammary cell line (MDA-MB-231) that overexpresses both RhoA and EGFR in their active forms also displayed an MMP-dependent activation mechanism of EGFR. RhoA-GTP-transfected cells showed a cortical array of F-actin, rounded morphology, reduced spreading potential and a dephosphorylation of FAK that was released by integrin-dependent fibronectin adhesion and a specific EGFR tyrosine kinase inhibitor. Our results suggest that the MMP-dependent EGFR activation observed in V14 RhoA cells represents the starting point of a signaling route that promotes cell motility by activation of ERK1,2 and further enhancement of proteases production.

ROCK-I and ROCK-II cooperatively regulate closure of eyelid and ventral body wall in mouse embryo

Rho-associated kinase (ROCK) is a serine/threonine kinase working in the Rho signaling to actin cytoskeleton. We previously reported that loss of ROCK-I results in the eyelid open at birth (EOB) and omphalocele phenotype in mice, while loss of ROCK-II results in placental dysfunction leading to intrauterine growth retardation and fetal death. Here, we report that after backcross to the C57BL/6 N genetic background, ROCK-II knockout (KO) neonates are born also with open eyelid and umbilical hernia, a phenotype similar to that of ROCK-I KO mice. ROCK-II KO embryos show impaired extension of the eyelid epithelial sheet with disorganized actin bundles in the leading edge of the sheet. These results suggest that ROCK-I and ROCK-II cooperatively regulates the assembly of actin bundles essential for closure of the eyelid and ventral body wall in mouse embryos. Consistently, ROCK-I(+/-)ROCK-II(+/-) double heterozygous mice also show the EOB and omphalocele phenotype.

Signal transduction mechanisms involved in the proliferation of C6 glioma cells induced by lysophosphatidic acid

We studied pathways involved in the proliferation of rat C6 glioma cells induced by lysophosphatidic acid (LPA), a phospholipid with diverse biological functions. LPA induced a dose-responsive proliferation of C6 cells after 48 h. Proliferation was blocked by inhibitors of the sodium/proton exchanger type 1 (NHE1), Rho-associated kinase, the phosphatidylinositol 3-kinase/Akt pathway (PI3K/Akt), protein kinase C (PKC) and extracellular signal regulated kinase kinase (MEK). Phospho-specific antibodies were used to investigate the pathways involved. LPA induced transient (10 min) phosphorylations of ERK 1/2, Akt and the transcription factor CREB. The LPA-induced phosphorylation of ERK 1/2 and CREB was blocked by inhibition of PI3K, PKC and MEK, but that of Akt was only inhibited by wortmannin, the PI3K inhibitor. Inhibition of Rho kinase or NHE1 did not reduce the LPA-induced phosphorylation of ERK, Akt or CREB. The results were compared with the effects of LPA on transduction pathways in other cell types.

The role of disturbed pH dynamics and the Na+/H+ exchanger in metastasis

Recent research has highlighted the fundamental role of the tumour's extracellular metabolic microenvironment in malignant invasion. This microenvironment is acidified primarily by the tumour-cell Na(+)/H(+) exchanger NHE1 and the H(+)/lactate cotransporter, which are activated in cancer cells. NHE1 also regulates formation of invadopodia - cell structures that mediate tumour cell migration and invasion. How do these alterations of the metabolic microenvironment and cell invasiveness contribute to tumour formation and progression?

Control of basement membrane remodeling and epithelial branching morphogenesis in embryonic lung by Rho and cytoskeletal tension

Local alterations in the mechanical compliance of the basement membrane that alter the level of isometric tension in the cell have been postulated to influence tissue morphogenesis. To explore whether cell tension contributes to tissue pattern formation in vivo, we modulated cytoskeletal force generation in embryonic mouse lung (embryonic days 12-14) rudiments using inhibitors of Rho-associated kinase (ROCK), myosin light chain kinase, myosin ATPase, and microfilament integrity, or a Rho stimulator (cytotoxic necrotizing factor-1). Tension inhibition resulted in loss of normal differentials in basement membrane thickness, inhibition of new terminal bud formation, and disorganization of epithelial growth patterns as well as disruption of capillary blood vessels. In contrast, increasing cell tension through Rho activation, as confirmed by quantitation of myosin light chain phosphorylation and immunohistocytochemical analysis of actin organization, accelerated lung branching and increase capillary elongation. These data suggest that changes in cytoskeletal tension mediated by Rho signaling through ROCK may play an important role in the establishment of the spatial differentials in cell growth and extracellular matrix remodeling that drive embryonic lung development.

Protein kinases in vascular smooth muscle tone--role in the pulmonary vasculature and hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an adaptive mechanism that in the normal animal diverts blood away from poorly ventilated areas of the lung, thereby maintaining optimal ventilation-perfusion matching. In global hypoxia however, such as in respiratory disease or at altitude, it causes detrimental increases in pulmonary vascular resistance and pulmonary artery (PA) pressure. The precise intracellular pathways and mechanisms underlying HPV remain unclear, although it is now recognised that both an elevation in smooth muscle intracellular [Ca2+] and a concomitant increase in Ca2+ sensitivity are involved. Several key intracellular protein kinases have been proposed as components of the signal transduction pathways leading to development of HPV, specifically Rho kinase, non-receptor tyrosine kinases (NRTK), p38 mitogen activated protein (MAP) kinase, and protein kinase C (PKC). All of these have been implicated to a greater or lesser extent in pathways leading to Ca2+ sensitisation, and in some cases regulation of intracellular [Ca2+] as well. In this article, we review the role of these key protein kinases in the regulation of vascular smooth muscle (VSM) constriction, applying what is known in the systemic circulation to the pulmonary circulation and HPV. We conclude that the strongest evidence for direct involvement of protein kinases in the mechanisms of HPV concerns a central role for Rho kinase in Ca2+ sensitisation, and a potential role for Src-family kinases in both modulation of Ca2+ entry via capacitative Ca2+ entry (CCE) and activation of Rho kinase, though others are likely to have indirect or modulatory influences. In addition, we speculate that Src family kinases may provide a central interface between the proposed hypoxia-induced generation of reactive oxygen species by mitochondria and both the elevation in intracellular [Ca2+] and Rho kinase mediated Ca2+ sensitisation.

Two G protein-coupled receptors activate Na+/H+ exchanger isoform 1 in Chinese hamster lung fibroblasts through an ERK-dependent pathway

The sodium hydrogen exchanger isoform 1 (NHE1) is present in nearly all cells. Regulation of proton flux via the exchanger is a permissive step in cell growth and tumorgenesis and is vital in control of cell volume. The regulation of NHE1 by growth factors involves the Ras-extracellular signal regulated kinase (ERK) pathway, however, the mechanism for G protein-coupled receptor (GPCR) activation of NHE1 is not well established. In this report, the relationship between GPCRs, ERK, and NHE1 in CCL39 cells is investigated. We give evidence that two agonists, the specific alpha(1)-adrenergic agonist, phenylephrine and the water-soluble lipid mitogen, lysophosphatidic acid (LPA) activate NHE1 in CCL39 cells. Activation of ERK by phenylephrine and LPA occurs in a dose- and time-dependent manner. Optimal ERK activation was observed at 10 min and displayed a maximum stimulation at 100 microM phenylephrine and 10 microM LPA. alpha(1)-Adrenergic stimulation also led to a rise in steady-state pH(i) of 0.16+/-0.02 pH units, and incubation with LPA induced a 0.43+/-0.06 pH unit increase in pH(i). Phenylephrine-induced activation of NHE1 transport and ERK activity was inhibited by pretreating the cells with the MEK inhibitor PD98059. While only half of the LPA activatable exchange activity was abolished by PD98059 and U0126. To further demonstrate the specificity of the phenylephrine and LPA regulation of NHE1 and ERK, CCL39 cells were transfected with a kinase inactive MEK. The data indicate that ERK activation is essential for phenylephrine stimulation of NHE1, and that ERK and RhoA are involved in LPA stimulation of NHE1 by more than one mechanism. In addition, evidence of the convergence of these two pathways is shown by the loss of NHE1 activity when both pathways are inhibited and by the partial additivity of the two agonists on ERK and NHE1 activity. These studies indicate a direct involvement of ERK in the alpha(1)-adrenergic activation of NHE1 and a significant role for both ERK and RhoA in LPA stimulation of NHE1 in CCL39 fibroblasts.

Phospholipase C-β1mediates α1-adrenergic receptor-stimulated activation of the sodium–hydrogen exchanger in Chinese hamster lung fibroblasts (CCL39)

The activation of the Na+–H+exchanger 1 (NHE1) and extracellular-signal regulated kinase (ERK) phospho rylation in Chinese hamster lung fibroblasts (CCL39) was characterized in response to the specific α1-adrenergic agonist, phenylephrine (PE). Addition of 100 µmol PE/L increased the steady-state intracellular pH (pHi) by 0.16 ± 0.03 pH units, as well as increasing the phosphorylation of ERK. The response of NHE1 to PE in CCL39 cells was determined by the use of specific antagonists. Use of 2 specific chemical inhibitors of phosphoinositide-specific phospholipase C (PLC) reduced the ability of PE to activate either the exchanger or ERK. Studies were conducted in PLCβ-deficient cell lines derived from parental CCL39 cells. NHE1 activity in both mutant cell lines was increased in response to phorbal esters or lysophosphatidic acid, whereas the addition of PE only caused a minimal change in either pHior ERK phosphorylation. These results, combined with reconstitution experiments with exogenously expressed PLCβ1, PLCβ2, or PLCβ3, revealed that stimulation of NHE1 activity by PE in CCL39 cells is a PLCβ1-coupled event. Furthermore, the data indicate that α1-adrenergic signaling of PLCβ is upstream of ERK activation. These data demonstrate that PLCβ1is primarily involved in the activation of NHE1 in CCL39 fibroblasts.Key words: CCL39, sodium hydrogen exchanger, ERK, α1-adrenergic receptor, phospholipase Cβ.

Protein Phosphatase Regulation of Na+/H+ Exchanger Isoform I

We investigated regulation of Na(+)/H(+) exchanger isoform 1 (NHE1) by dephosphorylation. Treatment of primary cultures of cardiomyocytes with the phosphatase inhibitor okadaic acid increased the rate of recovery from an acid load, suggesting that the okadaic acid sensitive PP1 may be involved in NHE1 regulation in vivo. We examined the ability of purified protein phosphatases PP1, PP2A, and PP2B to dephosphorylate the regulatory cytoplasmic tail. NHE1 was completely dephosphorylated by PP1, poorly dephosphorylated by PP2A, and not dephosphorylated by PP2B. Examination of NHE1 binding to PP1 or PP2B revealed that an association occurs between NHE1 and PP1 both in vitro and in vivo, but NHE1 did not associate with full-length PP2B. We expressed PP1 or inhibitor 2, a specific PP1 inhibitor, in cell lines to examine the effect of PP1 on NHE1 activity in vivo. Overexpression of PP1 causes a decrease in NHE1 activity but does not affect stimulation by thrombin. Cell lines expressing the specific PP1 inhibitor, inhibitor 2, had elevated proton efflux rates and could not be further stimulated by the Na(+)/H(+) exchanger agonist thrombin. The results suggest that PP1 is an important regulatory phosphatase of NHE1, that it can bind to and dephosphorylate the protein, and that it regulates NHE1 activity in vivo.

The Na+/H+ exchanger isoform 1

The Na+/H+ exchanger (NHE) isoform 1 is a ubiquitously expressed integral membrane protein which regulates intracellular pH in mammalian cells. Nine isoforms of the Na+/H+ exchanger have been identified. The isoform first discovered has two domains: an N-terminal membrane domain containing approximately 500 amino acids and a C-terminal regulatory domain containing approximately 315 amino acids. The exchanger, which resides in the plasma membrane, exchanges an intracellular proton for an extracellular sodium, thereby regulating intracellular pH. It is involved in cell growth and differentiation, cell migration, and regulation of sodium fluxes. The Na+/H+ exchanger plays an important role in myocardial damage during ischemia and reperfusion and has recently been implicated as a mediator of cardiac hypertrophy. Inhibitors of the Na+/H+ exchanger, which may prove useful in the clinical treatment of these conditions, are currently being developed and clinical trials are underway.

The cardiac Na-H exchanger: a key downstream mediator for the cellular hypertrophic effects of paracrine, autocrine and hormonal factors

The major mechanism by which the heart cell regulates intracellular pH is the Na+–H+exchanger (NHE) with the NHE-1 isoform as the primary cardiac subtype. Although NHE-1 has been implicated in mediating ischemic injury, more recent evidence implicates the antiporter as a key mediator of hypertrophy, which is produced by various autocrine, paracrine and hormonal factors such as endothelin-1, angiotensin II, and α1adrenoceptor agonists. These agonists activate the antiporter via phosphorylation-dependent processes. NHE-1 inhibition is likely conducive to attenuating the remodelling process after myocardial infarction. These effects probably occur independently of infarct size reduction and involve attenuation of subsequent postinfarction heart failure. As such, inhibitors of NHE offer substantial promise for clinical development that will attenuate acute responses to myocardial postinfarction and chronic pos t infarction, which evolve toward heart failure. The regulation of NHE-1 is discussed as is its potential role in mediating cardiomyocyte hypertrophy.Key words: NHE-1, cardiac hypertrophy, heart failure, myocardial remodelling.

Na+/H+exchanger NHE1 as plasma membrane scaffold in the assembly of signaling complexes

The plasma membrane Na+/H+exchanger NHE1 has an established function in intracellular pH and cell volume homeostasis by catalyzing electroneutral influx of extracellular Na+and efflux of intracellular H+. A second function of NHE1 as a structural anchor for actin filaments through its direct binding of the ezrin, radixin, and moesin (ERM) family of actin-binding proteins was recently identified. ERM protein binding and actin anchoring by NHE1 are necessary to retain the localization of NHE1 in specialized plasma membrane domains and to promote cytoskeleton-dependent processes, including actin filament bundling and cell-substrate adhesions. This review explores a third function of NHE1, as a plasma membrane scaffold in the assembly of signaling complexes. Through its coordinate functions in H+efflux, actin anchoring, and scaffolding, we propose that NHE1 promotes protein interactions and activities, assembles signaling complexes in specialized plasma membrane domains, and coordinates divergent signaling pathways.

The Na+-H+ exchanger-1 induces cytoskeletal changes involving reciprocal RhoA and Rac1 signaling, resulting in motility and invasion in MDA-MB-435 cells

Introduction

An increasing body of evidence shows that the tumour microenvironment is essential in driving neoplastic progression. The low serum component of this microenvironment stimulates motility/invasion in human breast cancer cells via activation of the Na⁺–H⁺ exchanger (NHE) isoform 1, but the signal transduction systems that underlie this process are still poorly understood. We undertook the present study to elucidate the role and pattern of regulation by the Rho GTPases of this serum deprivation-dependent activation of both NHE1 and subsequent invasive characteristics, such as pseudopodia and invadiopodia protrusion, directed cell motility and penetration of normal tissues.

Methods

The present study was performed in a well characterized human mammary epithelial cell line representing late stage metastatic progression, MDA-MB-435. The activity of RhoA and Rac1 was modified using their dominant negative and constitutively active mutants and the activity of NHE1, cell motility/invasion, F-actin content and cell shape were measured.

Results

We show for the first time that serum deprivation induces NHE1-dependent morphological and cytoskeletal changes in metastatic cells via a reciprocal interaction of RhoA and Rac1, resulting in increased chemotaxis and invasion. Deprivation changed cell shape by reducing the amount of F-actin and inducing the formation of leading edge pseudopodia. Serum deprivation inhibited RhoA activity and stimulated Rac1 activity. Rac1 and RhoA were antagonistic regulators of both basal and stimulated tumour cell NHE1 activity. The regulation of NHE1 activity by RhoA and Rac1 in both conditions was mediated by an alteration in intracellular proton affinity of the exchanger. Interestingly, the role of each of these G-proteins was reversed during serum deprivation; basal NHE1 activity was regulated positively by RhoA and negatively by Rac1, whereas RhoA negatively and Rac1 positively directed the stimulation of NHE1 during serum deprivation. Importantly, the same pattern of RhoA and Rac1 regulation found for NHE1 activity was observed in both basal and serum deprivation dependent increases in motility, invasion and actin cytoskeletal organization.

Conclusion

Our findings suggest that the reported antagonistic roles of RhoA and Rac1 in cell motility/invasion and cytoskeletal organization may be due, in part, to their concerted action on NHE1 activity as a convergence point.

Jak2 and Ca2+/calmodulin are key intermediates for bradykinin B2 receptor-mediated activation of Na+/H+ exchange in KNRK and CHO cells

Na(+)/H(+) exchangers are ubiquitous in mammalian cells, carrying out key functions, such as cell volume defense, acid-base homeostasis, and regulation of the cytoskeleton. We used two screening technologies (FLIPR and microphysiometry) to characterize the signal transduction pathway used by the bradykinin B(2) receptor to activate Na(+)/H(+) exchange in two cell lines, KNRK and CHO. In both cell types, B(2) receptor activation resulted in rapid increases in the rate of proton extrusion that were sodium-dependent and could be blocked by the Na(+)/H(+) exchange inhibitors EIPA and MIA or by replacing extracellular sodium with TMA. Activation of Na(+)/H(+) exchange by bradykinin was concentration-dependent and could be blocked by the selective B(2) receptor antagonist HOE140, but not by the B(1) receptor antagonist des-Arg10-HOE140. Inhibitors of Jak2 tyrosine kinase (genistein and AG490) and of CAM (W-7 and calmidazolium) attenuated bradykinin-induced activation of Na(+)/H(+) exchange. Bradykinin induced formation of a complex between CAM and Jak2, supporting a regulatory role for Jak2 and CAM in the activation of Na(+)/H(+) exchange in KNRK and CHO cells. We propose that this pathway (B(2) receptor --> Jak2 --> CAM --> Na(+)/H(+) exchanger) is a fundamental regulator of Na(+)/H(+) exchange activity.

Comparative biology of the ubiquitous Na+/H+ exchanger, NHE1: lessons from erythrocytes

By virtue of their electroneutral exchange of intracellular H+ for extracellular Na+, the Na+/H+ exchangers (NHE1-NHE8) play a pivotal role in many physiological processes. This review focuses on the ubiquitous plasma membrane isoform, NHE1. Particular attention is given to the roles and regulation of NHE1 in erythrocytes, in their own right and as model systems, but pertinent findings from non-erythroid cells are also discussed. NHE1 plays a key role in the regulation of cell volume and pH, and consequently in the control of such diverse processes as blood O2/CO2 transport, and cell proliferation, motility, and survival. Disturbances in NHE1 function are involved in important pathological states such as hypoxic cell damage and cancer development. NHE1 has a predicted topology of 12 transmembrane domains, and a hydrophilic C-terminus thought to be the major site for NHE1 regulation. NHE1 is highly conserved throughout the vertebrate phylum, particularly in the transmembrane region and the proximal part of the C-terminus. In non-erythroid, and probably also in erythroid cells, this part of the hydrophilic C-terminus interacts with multiple binding partners important for NHE1 function. Erythrocyte NHE1s from mammalian, amphibian, and teleost species are activated by cell shrinkage, decreased pH(i), inhibition of Ser/Thr protein phosphatases, and activation of Ser/Thr protein kinases, i.e., many of the stimuli activating NHE1 in non-erythroid cells. In erythrocytes of many lower vertebrates, NHE1 is activated during hypoxia and is an important modulator of hemoglobin oxygen affinity. Sensitivity of NHE1 to oxygenation status has recently been described also in non-erythroid mammalian cells.

Rho localization in cells and tissues

Rho family small GTPases regulate cytoskeletal organization. Although their spatiotemporal activities appear to be important for cellular morphogenesis, there has been little characterization of the localization of Rho family GTPases in cells and tissues. Here we show precise localization of Rho subfamily proteins in mammalian cultured cells and tissues through evaluation of anti-Rho antibodies and fixation protocols. Although Rho is not a structural protein but functions as a switching molecule, it often localizes at several distinct domains or structures of cells. In cultured epithelial cells, Rho was highly accumulated at lateral membranes. However, in fibroblastic cells, Rho appeared to be distributed evenly in the cytoplasm. Rho concentration at the cleavage furrow at cytokinesis was generally observed. In A431 cells, Rho translocation from the cytoplasm to elongating microvilli at the apical membrane within 30 s after EGF stimulation was clearly demonstrated. Also, Myc- or GFP-tagged RhoA did not always reflect the localization of endogenous Rho, indicating a drawback of protein-tagging methods for localization research. In mouse tissues, Rho localization differed depending on cell type, probably reflecting the functional differences of each cell type.

CD44 interaction with Na+-H+ exchanger (NHE1) creates acidic microenvironments leading to hyaluronidase-2 and cathepsin B activation and breast tumor cell invasion

We have explored CD44 (a hyaluronan (HA) receptor) interaction with a Na(+)-H(+) exchanger (NHE1) and hyaluronidase-2 (Hyal-2) during HA-induced cellular signaling in human breast tumor cells (MDA-MB-231 cell line). Immunological analyses demonstrate that CD44s (standard form) and two signaling molecules (NHE1 and Hyal-2) are closely associated in a complex in MDA-MB-231 cells. These three proteins are also significantly enriched in cholesterol and ganglioside-containing lipid rafts, characterized as caveolin and flotillin-rich plasma membrane microdomains. The binding of HA to CD44 activates Na(+)-H(+) exchange activity which, in turn, promotes intracellular acidification and creates an acidic extracellular matrix environment. This leads to Hyal-2-mediated HA catabolism, HA modification, and cysteine proteinase (cathepsin B) activation resulting in breast tumor cell invasion. In addition, we have observed the following: (i) HA/CD44-activated Rho kinase (ROK) mediates NHE1 phosphorylation and activity, and (ii) inhibition of ROK or NHE1 activity (by treating cells with a ROK inhibitor, Y27632, or NHE1 blocker, S-(N-ethyl-N-isopropyl) amiloride, respectively) blocks NHE1 phosphorylation/Na(+)-H(+) exchange activity, reduces intracellular acidification, eliminates the acidic environment in the extracellular matrix, and suppresses breast tumor-specific behaviors (e.g. Hyal-2-mediated HA modification, cathepsin B activation, and tumor cell invasion). Finally, down-regulation of CD44 or Hyal-2 expression (by treating cells with CD44 or Hyal-2-specific small interfering RNAs) not only inhibits HA-mediated CD44 signaling (e.g. ROK-mediated Na(+)-H(+) exchanger reaction and cellular pH changes) but also impairs oncogenic events (e.g. Hyal-2 activity, hyaluronan modification, cathepsin B activation, and tumor cell invasion). Taken together, our results suggest that CD44 interaction with a ROK-activated NHE1 (a Na(+)-H(+) exchanger) in cholesterol/ganglioside-containing lipid rafts plays a pivotal role in promoting intracellular/extracellular acidification required for Hyal-2 and cysteine proteinase-mediated matrix degradation and breast cancer progression.

The tumor invasion inhibitor dihydromotuporamine C activates RHO, remodels stress fibers and focal adhesions, and stimulates sodium-proton exchange

The motuporamines are macrocyclic alkaloids that inhibit tumor cell invasion by an, as yet, unknown mechanism. A structure-activity study recently identified dihydromotuporamine C (dhMotC) as a highly active and readily synthesized analogue. Here, we show that dhMotC causes subtle cytoskeletal alterations in highly invasive MDA231 breast tumor cells that include an increase in the thickness and number of cytoplasmic actin stress fibers. Experiments with serum-starved Swiss 3T3 fibroblasts showed that micromolar concentrations of dhMotC that inhibit tumor cell invasion induce the formation of new stress fibers and large focal adhesion complexes that are dispersed around the entire cell periphery. dhMotC treatment of Swiss 3T3 cells also initiates a strong, long-lived activation of the small GTP-binding protein Rho, and it stimulates Rho kinase-dependent sodium-proton exchanger activity. Liposome-mediated cell loading of C3 exoenzyme prevents dhMotC-mediated Rho activation and stress fiber formation in 3T3 cells. C3 exoenzyme loading also reestablishes elongated MDA231 breast tumor cell invasion in the presence of dhMotC. Taken together, these results indicate that the ability to activate Rho is one important determinant of the anti-invasive activity of dhMotC.

DIP (mDia interacting protein) is a key molecule regulating Rho and Rac in a Src-dependent manner

Cell movement is driven by the coordinated regulation of cytoskeletal reorganization through Rho GTPases downstream of integrin and growth-factor receptor signaling. We have reported that mDia, a target protein of Rho, interacts with Src and DIP. Here we show that DIP binds to p190RhoGAP and Vav2, and that DIP is phosphorylated by Src and mediates the phosphorylation of p190RhoGAP and Vav2 upon EGF stimulation. When endogenous DIP was inhibited by expressing dominant-negative mutants of DIP or siRNA, phosphorylation of p190RhoGAP and Vav2 upon EGF stimulation was diminished, and EGF-induced actin organization, distribution of p190RhoGAP and Vav2, and cell movement were affected. Therefore, DIP seems to transfer the complex of the three proteins from cytosol to beneath the membrane, and the three proteins, in turn, can be phosphorylated by Src. DIP inactivated Rho and activated Rac following EGF stimulation in the membrane fraction. Thus, DIP acts as a regulatory molecule causing Src kinase-dependent feedback modulation of Rho GTPases downstream of Rho-mDia upon EGF stimulation, and plays an important role in cell motility.