Intracellular acidosis activates c-Src

Inhibition of Pyk2 Improves Cx43 Intercalated Disc Localization, Infarct Size, and Cardiac Function in Rats With Heart Failure

BACKGROUND

Heart failure causes changes in Cx43 (Connexin43) regulation that are associated with arrhythmic heart disease. Pyk2 (proline-rich tyrosine kinase 2) is activated in cardiomyopathies and phosphorylates Cx43 to decrease intercellular communication. This study was designed to determine if Pyk2 inhibition improves cardiac function in a myocardial infarction (MI)-induced heart failure model in rats.

METHODS

MI (ligation of left anterior descending artery) rats were treated with the Pyk2 inhibitor PF4618433. Hemodynamic and structural parameters were monitored in Sham (n=5), MI-vehicle (n=5), and MI-PF4618433 (n=8) groups. Heart tissues were collected after 6 weeks to assess Pyk2 and Cx43 protein level and localization.

RESULTS

PF4618433 produced no observed adverse effects and inhibited ventricular Pyk2. PF4618433 reduced the MI infarct size from 34% to 17% (P=0.007). PF4618433 improved stroke volume (P=0.031) and cardiac output (P=0.009) in comparison to MI-vehicle with values similar to the Sham group. PF4618433 also led to an increase in the ejection fraction (P=0.002) and fractional shortening (P=0.006) when compared with the MI-vehicle (32% and 35% improvement, respectively) yet were lower in comparison with the Sham group. Pyk2 inhibition decreased Cx43 tyrosine phosphorylation (P=0.043) and maintained Cx43 at the intercalated disc in the distal ventricle 6 weeks post-MI.

CONCLUSIONS

Unlike other attempts to decrease Cx43 remodeling after MI-induced heart failure, inhibition of Pyk2 activity maintained Cx43 at the intercalated disc. This may have aided in the reduced infarct size (acute time frame) and improved cardiac function (chronic time frame). Additionally, we provide evidence that Pyk2 is activated following MI in human left ventricle, implicating a novel potential target for therapy in patients with heart failure.

GPR68 Contributes to Persistent Acidosis-Induced Activation of AGC Kinases and Tyrosine Phosphorylation in Organotypic Hippocampal Slices

Persistent acidosis occurs in ischemia and multiple neurological diseases. In previous studies, acidic stimulation leads to rapid increase in intracellular calcium in neurons. However, it remains largely unclear how a prolonged acidosis alters neuronal signaling. In our previous study, we found that GPR68-mediated PKC activities are protective against acidosis-induced injury in cortical slices. Here, we first asked whether the same principle holds true in organotypic hippocampal slices. Our data showed that 1-h pH 6 induced PKC phosphorylation in a GPR68-dependent manner. Go6983, a PKC inhibitor worsened acidosis-induced neuronal injury in wild type (WT) but had no effect in GPR68^−/− slices. Next, to gain greater insights into acid signaling in brain tissue, we treated organotypic hippocampal slices with pH 6 for 1-h and performed a kinome profiling analysis by Western blot. Acidosis had little effect on cyclin-dependent kinase (CDK) or casein kinase 2 activity, two members of the CMGC family, or Ataxia telangiectasia mutated (ATM)/ATM and RAD3-related (ATR) activity, but reduced the phosphorylation of MAPK/CDK substrates. In contrast, acidosis induced the activation of CaMKIIα, PKA, and Akt. Besides these serine/threonine kinases, acidosis also induced tyrosine phosphorylation. Since GPR68 is widely expressed in brain neurons, we asked whether GPR68 contributes to acidosis-induced signaling. Deleting GPR68 had no effect on acidosis-induced CaMKII phosphorylation, attenuated that of phospho-Akt and phospho-PKA substrates, while abolishing acidosis-induced tyrosine phosphorylation. These data demonstrate that prolonged acidosis activates a network of signaling cascades, mediated by AGC kinases, CaMKII, and tyrosine kinases. GPR68 is the primary mediator for acidosis-induced activation of PKC and tyrosine phosphorylation, while both GPR68-dependent and -independent mechanisms contribute to the activation of PKA and Akt.

Chronic lithium treatment induces novel patterns of pendrin localization and expression

Prolonged lithium treatment is associated with various renal side effects and is known to induce inner medullary collecting duct (IMCD) remodeling. In animals treated with lithium, the fraction of intercalated cells (ICs), which are responsible for acid-base homeostasis, increases compared with renal principal cells (PCs). To investigate the intricacies of lithium-induced IMCD remodeling, male Sprague-Dawley rats were fed a lithium-enriched diet for 0,1, 2, 3, 6, 9, or 12 wk. Urine osmolality was decreased at 1 wk, and from 2 to 12 wk, animals were severely polyuric. After 6 wk of lithium treatment, approximately one-quarter of the cells in the initial IMCD expressed vacuolar H⁺-ATPase, an IC marker. These cells were localized in portions of the inner medulla, where ICs are not normally found. Pendrin, a Cl^-/[Formula: see text] exchanger, is normally expressed only in two IC subtypes found in the convoluted tubule, the cortical collecting duct, and the connecting tubule. At 6 wk of lithium treatment, we observed various patterns of pendrin localization and expression in the rat IMCD, including a novel phenotype wherein pendrin was coexpressed with aquaporin-4. These observations collectively suggest that renal IMCD cell plasticity may play an important role in lithium-induced IMCD remodeling.

A c-Src Inhibitor Peptide Based on Connexin43 Exerts Neuroprotective Effects through the Inhibition of Glial Hemichannel Activity

The non-receptor tyrosine kinase c-Src is an important mediator in several signaling pathways related to neuroinflammation. Our previous study showed that cortical injection of kainic acid (KA) promoted a transient increase in c-Src activity in reactive astrocytes surrounding the neuronal lesion. As a cell-penetrating peptide based on connexin43 (Cx43), specifically TAT-Cx43_266–283, inhibits Src activity, we investigated the effect of TAT-Cx43_266–283 on neuronal death promoted by cortical KA injections in adult mice. As expected, KA promoted neuronal death, estimated by the reduction in NeuN-positive cells and reactive gliosis, characterized by the increase in glial fibrillary acidic protein (GFAP) expression. Interestingly, TAT-Cx43_266–283 injected with KA diminished neuronal death and reactive gliosis compared to KA or KA+TAT injections. In order to gain insight into the neuroprotective mechanism, we used in vitro models. In primary cultured neurons, TAT-Cx43_266–283 did not prevent neuronal death promoted by KA, but when neurons were grown on top of astrocytes, TAT-Cx43_266–283 prevented neuronal death promoted by KA. These observations demonstrate the participation of astrocytes in the neuroprotective effect of TAT-Cx43_266–283. Furthermore, the neuroprotective effect was also present in non-contact co-cultures, suggesting the contribution of soluble factors released by astrocytes. As glial hemichannel activity is associated with the release of several factors, such as ATP and glutamate, that cause neuronal death, we explored the participation of these channels on the neuroprotective effect of TAT-Cx43_266–283. Our results confirmed that inhibitors of ATP and NMDA receptors prevented neuronal death in co-cultures treated with KA, suggesting the participation of astrocyte hemichannels in neurotoxicity. Furthermore, TAT-Cx43_266–283 reduced hemichannel activity promoted by KA in neuron-astrocyte co-cultures as assessed by ethidium bromide (EtBr) uptake assay. In fact, TAT-Cx43_266–283 and dasatinib, a potent c-Src inhibitor, strongly reduced the activation of astrocyte hemichannels. In conclusion, our results suggest that TAT-Cx43_266–283 exerts a neuroprotective effect through the reduction of hemichannel activity likely mediated by c-Src in astrocytes. These data unveil a new role of c-Src in the regulation of Cx43-hemichannel activity that could be part of the mechanism by which astroglial c-Src participates in neuroinflammation.

Role of Sodium Bicarbonate Cotransporters in Intracellular pH Regulation and Their Regulatory Mechanisms in Human Submandibular Glands

Sodium bicarbonate cotransporters (NBCs) are involved in the pH regulation of salivary glands. However, the roles and regulatory mechanisms among different NBC isotypes have not been rigorously evaluated. We investigated the roles of two different types of NBCs, electroneutral (NBCn1) and electrogenic NBC (NBCe1), with respect to pH regulation and regulatory mechanisms using human submandibular glands (hSMGs) and HSG cells. Intracellular pH (pH_i) was measured and the pH_i recovery rate from cell acidification induced by an NH₄Cl pulse was recorded. Subcellular localization and protein phosphorylation were determined using immunohistochemistry and co-immunoprecipitation techniques. We determined that NBCn1 is expressed on the basolateral side of acinar cells and the apical side of duct cells, while NBCe1 is exclusively expressed on the apical membrane of duct cells. The pH_i recovery rate in hSMG acinar cells, which only express NBCn1, was not affected by pre-incubation with 5 μM PP2, an Src tyrosine kinase inhibitor. However, in HSG cells, which express both NBCe1 and NBCn1, the pH_i recovery rate was inhibited by PP2. The apparent difference in regulatory mechanisms for NBCn1 and NBCe1 was evaluated by artificial overexpression of NBCn1 or NBCe1 in HSG cells, which revealed that the pH_i recovery rate was only inhibited by PP2 in cells overexpressing NBCe1. Furthermore, only NBCe1 was significantly phosphorylated and translocated by NH₄Cl, which was inhibited by PP2. Our results suggest that both NBCn1 and NBCe1 play a role in pH_i regulation in hSMG acinar cells, and also that Src kinase does not regulate the activity of NBCn1.

DIDS inhibits Na-K-ATPase activity in porcine nonpigmented ciliary epithelial cells by a Src family kinase-dependent mechanism

The anion transport inhibitor DIDS is known to reduce aqueous humor secretion but questions remain about anion dependence of the effect. In some tissues, DIDS is reported to cause Na-K-ATPase inhibition. Here, we report on the ability of DIDS to inhibit Na-K-ATPase activity in nonpigmented ciliary epithelium (NPE) and investigate the underlying mechanism. Porcine NPE cells were cultured to confluence on permeable supports, treated with drugs added to both sides of the membrane, and then used for (86)Rb uptake measurements or homogenized to measure Na-K-ATPase activity or to detect protein phosphorylation. DIDS inhibited ouabain-sensitive (86)Rb uptake, activated Src family kinase (SFK), and caused a reduction of Na-K-ATPase activity. PP2, an SFK inhibitor, prevented the DIDS responses. In BCECF-loaded NPE, DIDS was found to reduce cytoplasmic pH (pHi). PP2-sensitive Na-K-ATPase activity inhibition, (86)Rb uptake suppression, and SFK activation were observed when a similar reduction of pHi was imposed by low-pH medium or an ammonium chloride withdrawal maneuver. PP2 and the ERK inhibitor U0126 prevented robust ERK1/2 activation in cells exposed to DIDS or subjected to pHi reduction, but U0126 did not prevent SFK activation or the Na-K-ATPase activity response. The evidence points to an inhibitory influence of DIDS on NPE Na-K-ATPase activity by a mechanism that hinges on SFK activation associated with a reduction of cytoplasmic pH.

Acidosis-induced V-ATPase trafficking in salivary ducts is initiated by cAMP/PKA/CREB pathway via regulation of Rab11b expression

Changes in systemic acid-base homeostasis cause a series of organ-specific cellular responses, among them changes of acid-base transporter activities, and recruitment or retrieval of these transporters from intracellular pools to the plasma membrane and vice versa. The purpose of this study was to investigate the impact of protein phosphorylation in the acidosis-induced translocation of vacuolar-type H(+)-ATPase (V-ATPase) in salivary ducts and to identify molecular targets. Therefore, the human submandibular gland cell line HSG was exposed to acidosis and V-ATPase trafficking was investigated in the presence or absence of inhibitors and activators of sAC/PKA and Src/ERK signaling pathways. Putative target genes have been identified by RT-PCR and immunoblotting, and validated by loss-of-function experiments. Acidosis caused activation of cAMP/PKA and Src signaling and inhibition of either pathway significantly impaired acidosis-induced V-ATPase redistribution and incorporation into the plasma membrane. Activation of ERK1/2 was Src-independent, whereas activation of PKA caused phosphorylation of cAMP response element-binding (CREB) and activation to regulate Rab11b transcription. Loss-of-function of CREB down-regulated Rab11b transcript and protein and significantly impaired acidosis-induced V-ATPase translocation in HSG cells. These data demonstrate that the cAMP/PKA/CREB signaling pathway initiates acidosis-induced V-ATPase trafficking in salivary ducts via regulation of Rab11b expression and provide first evidence for a molecular mechanism underlying cAMP/PKA-dependent transporter trafficking that could account for accumulation and activity of transporters in other cellular systems as well.

Acidosis environment promotes osteoclast formation by acting on the last phase of preosteoclast differentiation: a study to elucidate the action points of acidosis and search for putative target molecules

Acidosis promoted tartaric acid-resistant acid phosphatase-positive multinuclear cell (TRAP+MNC) or osteoclast formation. Large osteoclast or TRAP+LMNC formation was observed far more in an acidosis environment than in a physiologically neutral environment. One of the major action points of acidosis was determined to be located in the last phase of preosteoclast differentiation using a co-culture system and a soluble RANKL-dependent bone marrow cell culture system. On-going osteoclast formation in an acidosis environment markedly deteriorated when the medium was replaced with physiologically neutral medium within the first 6h; however, bone marrow cells previously stimulated in an acidosis environment for 9h differentiated into TRAP+LMNC in pH 7.4 medium. Messenger RNA (mRNA) expression levels of DC-STAMP, a key molecule in cell fusion, and NFATc1 did not increase in the acidosis environment compared with those under physiologically neutral conditions. Ruthenium red, a general TRP antagonist, deteriorated acidosis-promoted TRAP+LMNC formation. 4-Alpha-PDD, a TRPV4-specific agonist, added in the last 21 h of preosteoclast differentiation, potentiated TRAP+LMNC formation in a mild acidosis environment, showing synergism between TRPV4 activation and acidosis. RN1734, a TRPV4-specific antagonist, partly inhibited acidosis-promoted TRAP+LMNC formation. We thus narrowed down the major action points of acidosis in osteoclast formation and elucidated the characteristics of this system in detail. Our results show that acidosis effectively uses TRPV4 to drive large-scale cell fusion and also utilizes systems independently of TRPV4.

Physiological carbon dioxide, bicarbonate, and pH sensing

In biological systems, carbon dioxide exists in equilibrium with bicarbonate and protons. The individual components of this equilibrium (i.e., CO₂, HCO₃⁻, and H(+)), which must be sensed to be able to maintain cellular and organismal pH, also function as signals to modulate multiple physiological functions. Yet, the molecular sensors for CO₂/HCO₃⁻/pH remained unknown until recently. Here, we review recent progress in delineating molecular and cellular mechanisms for sensing CO₂, HCO₃⁻, and pH.

Structural and molecular mechanisms of gap junction remodeling in epicardial border zone myocytes following myocardial infarction

Lateralization of the ventricular gap junction protein connexin 43 (Cx43) occurs in epicardial border zone myocytes following myocardial infarction (MI) and is arrhythmogenic. Alterations in Cx43 protein partners have been hypothesized to play a role in lateralization although mechanisms by which this occurs are unknown. To examine potential mechanisms we did nuclear magnetic resonance, yeast 2-hybrid, and surface plasmon resonance studies and found that the SH3 domain of the tyrosine kinase c-Src binds to the Cx43 scaffolding protein zonula occludens-1 (ZO-1) with a higher affinity than does Cx43. This suggests c-Src outcompetes Cx43 for binding to ZO-1, thus acting as a chaperone for ZO-1 and causing unhooking from Cx43. To determine whether c-Src/ZO-1 interactions affect Cx43 lateralization within the epicardial border zone, we performed Western blot, immunoprecipitation, and immunolocalization for active c-Src (p-cSrc) post-MI using a canine model of coronary occlusion. We found that post-MI p-cSrc interacts with ZO-1 as Cx43 begins to decrease its interaction with ZO-1 and undergo initial loss of intercalated disk localization. This indicates that the molecular mechanisms by which Cx43 is lost from the intercalated disk following MI includes an interaction of p-cSrc with ZO-1 and subsequent loss of scaffolding of Cx43 leaving Cx43 free to diffuse in myocyte membranes from areas of high Cx43, as at the intercalated disk, to regions of lower Cx43 content, the lateral myocyte membrane. Therefore shifts in Cx43 protein partners may underlie, in part, arrhythmogenesis in the post-MI heart.

RhoA required for acid-induced stress fiber formation and trafficking and activation of NHE3

Exposure to an acid load increases apical membrane Na+/H+ antiporter (NHE3) activity, a process that involves exocytic trafficking of the transporter to the apical membrane. We have previously shown that an intact microfilament structure is required for this exocytic process (Yang X, Amemiya M, Peng Y, Moe OW, Preisig PA, Alpern RJ. Am J Physiol Cell Physiol 279: C410–C419, 2000). The present studies demonstrate that acid-induced stress fiber formation is required for stimulation of NHE3 activity. Formation of stress fibers is associated with acid-induced tyrosine phosphorylation and increases in protein abundance of two focal adhesion proteins, p125FAK and paxillin. The Rho kinase inhibitor Y27632 completely blocks acid-induced stress fiber formation and the increases in apical membrane NHE3 abundance and activity, but it has no effect on acid-induced tyrosine phosphorylation of p125FAK or paxillin. Herbimycin A completely blocks acid-induced tyrosine phosphorylation of p125FAK and paxillin but only partially blocks stress fiber formation and NHE3 activation. These studies demonstrate that Rho kinase mediates acid-induced stress fiber formation, which is required for NHE3 exocytosis, and increases in NHE3 activity. Acid-induced tyrosine phosphorylation of the focal adhesion proteins p125FAK and paxillin is not Rho kinase dependent. Thus these two acid-mediated effects are associated, yet independent processes.

The acid-activated signaling pathway: starting with Pyk2 and ending with increased NHE3 activity

On a typical Western diet, the body is faced with the generation of a metabolically derived acid load that must be excreted to maintain systemic acid-base balance. The kidney is responsible for this task and matches daily acid excretion with daily acid production. Multiple nephron segments are involved in the process, including the proximal tubule cell. This review discusses the acid-activated signaling pathway in the proximal tubule that senses a decrease in cell pH and then mediates stimulation of the apical membrane Na/H antiporter, isoform NHE3. NHE3 mediates secretion of the majority of protons involved in bicarbonate reclamation, is involved in ammonium secretion, and provides a source of luminal protons for titrating filtered titratable acids and secreted ammonia to ammonium.

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

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

c-Src: bridging the gap between phosphorylation- and acidification-induced gap junction channel closure

Gap junctions are a unique type of intercellular junction that mediate the direct exchange of small molecules between neighboring cells and play critical roles in the normal function of numerous organs. Mutations in the connexin proteins that make up gap junctions have been implicated in numerous human skin and neurosensory disorders. The ability of gap junctions to transmit molecules between cells is regulated by intracellular pH, the phosphorylation state of connexin, and the interaction of connexin with other cellular proteins. This Perspective focuses on the novel and complex events initiated by intracellular acidification resulting from tissue ischemia or hypoxia that lead to the interruption of intercellular communication between astrocytes. These events include alterations in connexin43 (Cx43) phosphorylation, disruption of beta-actin binding to Cx43, and the induced interaction of Cx43 with the c-Src tyrosine kinase, extracellular signal-regulated kinase 1 and 2, and mitogen-activated protein kinase phosphatase 1.

Acid sensing in renal epithelial cells

The kidney adjusts net acid excretion to match production with exquisite precision, despite little or no change in the plasma bicarbonate concentration. The acid-sensing pathway that signals the kidney to increase acid secretion involves activation of the proto-oncogene c-Src. A new study in this issue shows that proline-rich tyrosine kinase 2 (Pyk2) is responsible for acid-induced activation of c-Src and is essential for acid sensing in renal epithelial cells. The findings implicate a broader role for Pyk2 in acid-base homeostasis in bone and other tissues beyond the kidney.

Pyk2 activation is integral to acid stimulation of sodium/hydrogen exchanger 3

The present study examines the role of Pyk2 in acid regulation of sodium/hydrogen exchanger 3 (NHE3) activity in OKP cells, a kidney proximal tubule epithelial cell line. Incubation of OKP cells in acid media caused a transient increase in Pyk2 phosphorylation that peaked at 30 seconds and increased Pyk2/c-Src binding at 90 seconds. Pyk2 isolated by immunoprecipitation and studied in a cell-free system was activated and phosphorylated at acidic pH. Acid activation of Pyk2 (a) was specific for Pyk2 in that acid did not activate focal adhesion kinase, (b) required calcium, and (c) was associated with increased affinity for ATP. Transfection of OKP cells with dominant-negative pyk2(K457A) or small interfering pyk2 duplex RNA blocked acid activation of NHE3, while neither had an effect on glucocorticoid activation of NHE3. In addition, pyk2(K457A) blocked acid activation of c-Src kinase, which is also required for acid regulation of NHE3. The present results demonstrate that Pyk2 is directly activated by acidic pH and that Pyk2 activation is required for acid activation of c-Src kinase and NHE3. Given that partially purified Pyk2 can be activated by acid in a cell-free system, Pyk2 may serve as the pH sensor that initiates the acid-regulated signaling cascade involved in NHE3 regulation.

Regulation of Connexin43 Protein Complexes by Intracellular Acidification

Ischemia-induced acidification of astrocytes or cardiac myocytes reduces intercellular communication by closing gap junction channels and subsequently internalizing gap junction proteins. To determine whether such coupling changes might be attributable to altered interactions between connexin43 (Cx43) and other proteins, we applied the nigericin/high K + method to vary intracellular pH (pHi) in cultured cortical astrocytes. Intracellular acidification was accompanied by internalization of Cx43 with retention of Cx43 scaffolding protein Zonula Occludens-1 (ZO-1) at cell surfaces, suggesting that ZO-1 and Cx43 dissociate at low pHi. Coimmunoprecipitation studies revealed decreased binding of ZO-1 and increased binding of c-Src to Cx43 at low pHi. Resonant mirror spectroscopy was used to quantify binding of the SH3 domain of c-Src and the PDZ domains of ZO-1 to the carboxyl terminal domain of Cx43 (Cx43CT). Data indicate that the c-Src/Cx43CT interaction is highly pH dependent whereas the ZO-1/Cx43CT interaction is not. Moreover, binding of c-Src to Cx43CT prevented and reversed ZO-1/Cx43CT binding. We hypothesize that increased affinity of c-Src for Cx43 at low pHi aids in separation of Cx43 from ZO-1 and that this may facilitate internalization of Cx43. These data suggest that protracted acidification may remodel protein-protein interactions involving Cx43 and thus provide an important protective mechanism to limit lesion spread after ischemic injury.

Troglitazone acts by PPARγ and PPARγ-independent pathways on LLC-PK1-F+acid-base metabolism

Troglitazone was studied in pH-sensitive LLC-PK1-F+cells to determine the effect on pHiand glutamine metabolism as well as the role of peroxisome proliferator-activated receptor (PPARγ)-dependent and PPARγ-independent signaling pathways. Troglitazone induces a dose-dependent cellular acidosis that occurs within 4 min and persists over 18 h as a result of inhibiting Na+/H+exchanger-mediated acid extrusion. Cellular acidosis was associated with glutamine-dependent augmented [15N]ammonium production and decreased [15N]alanine formation from15N-labeled glutamine. The shift in glutamine metabolism from alanine to ammoniagenesis appears within 3 h and is associated after 18 h with both a reduction in assayable alanine aminotransferase (ALT) activity as well as cellular acidosis. The relative contribution of troglitazone-induced cellular acidosis vs. the decrease in assayable ALT activity to alanine production could be demonstrated. The PPARγ antagonist bisphenol A diglycide ether (BADGE) reversed both the troglitazone-induced cellular acidosis and ammoniagenesis but enhanced the troglitazone reduction of assayable ALT activity; BADGE also blocked troglitazone induction of peroxisome proliferator response element-driven firefly luciferase activity. The protein kinase C (PKC) inhibitor chelerythrine mimics troglitazone effects, whereas phorbol ester reverses the effects on ammoniagenesis consistent with troglitazone negatively regulating the DAG/PKC/ERK pathway. Although functional PPARγ signaling occurs in this cell line, the major troglitazone-induced acid-base responses appear to be mediated by pathway(s) involving PKC/ERK.

Extracellular alkalosis activates ERK mitogen-activated protein kinase of vascular smooth muscle cells through NADPH-mediated formation of reactive oxygen species

Extracellular alkalosis induced phosphorylation of extracellular signal-regulated kinase (ERK) and enhanced serum-induced ERK phosphorylation in cultured rat aortic smooth muscle cells. While extracellular alkalinization increased verapamil-sensitive (45)Ca(2+) uptake into the cells, ERK phosphorylation induced by extracellular alkalosis was not affected by verapamil. On the other hand, probes for oxidant signaling, such as superoxide dismutase, 4,5-dihydroxy-1,3-benzene-disulfonic acid, a cell-permeable antioxidant, and diphenyliodonium, a NADPH oxidase inhibitor, inhibited extracellular alkalosis-induced phosphorylation of ERK. These results suggest that activation of ERK induced by extracellular alkalosis is not dependent on transplasmalemmal Ca(2+) entry but is caused by reactive oxygen species derived from an activation of NADPH oxidase.

Stimulation of the plasma membrane Na+/H+ exchanger NHE1 by sustained intracellular acidosis. Evidence for a novel mechanism mediated by the ERK pathway

Activity of the Na+/H+ exchanger (NHE) isoform 1 (NHE1) is increased by intracellular acidosis through the interaction of intracellular H+ with an allosteric modifier site in the transport domain. Additional regulation is achieved via kinase-mediated modulation of the NHE1 regulatory domain. To determine if intracellular acidosis stimulates NHE1 activity solely by the allosteric mechanism, we subjected cultured neonatal rat ventricular myocytes (NRVM) with native NHE1 expression to intracellular acidosis (pHi approximately 6.6) for up to 6 min by transient exposure to NH4Cl and its washout in the presence of NHE inhibition (by zero [Na+]o or the NHE1 inhibitor cariporide) in HCO3- -free medium. After the desired duration of acidosis, NHE was reactivated (by reintroduction of [Na+]o or removal of cariporide), and the rate of recovery of pHi (dpHi/dt) was measured as the index of NHE activity. Regardless of the method used when intracellular acidosis was sustained for > or =3 min, subsequent NHE activity was significantly increased (>4-fold). Similar NHE stimulatory effects of sustained acidosis were observed in adult rat ventricular myocytes and COS-7 cells. Sustained (3 min) intracellular acidosis activated several NHE1 kinases in NRVM, in an in-gel kinase assay using as substrate a glutathione S-transferase fusion protein of the NHE1 regulatory domain. Detailed investigation of ERK and its downstream effector p90RSK, two putative NHE1 kinases, revealed time-dependent activation of both by intracellular acidosis in NRVM. Furthermore, inhibition of MEK1/2 by pretreatment of NRVM with two structurally distinct inhibitors, PD98059 (30 microM) or UO126 (3 microM), inhibited the activation of ERK and p90RSK and abolished the stimulation of NHE activity by sustained (3 min) intracellular acidosis. Our data show that not only the extent but also the duration of intracellular acidosis regulates NHE1 activity and suggest that the stimulatory effect of sustained intracellular acidosis occurs through a novel mechanism mediated by activation of the ERK pathway.

Terminal differentiation of intercalated cells: the role of hensin

During the response to metabolic acidosis, the intercalated cell of the collecting tubule converts from one that secretes HCO3(-) to one that absorbs HCO3(-) by H(+) secretion. The molecular basis of this complex change in phenotype was studied in an immortalized intercalated cell line. We found that it was induced by secretion, polymerization, and deposition of a protein, which we termed hensin, into the extracellular matrix. Surprisingly, this change in phenotype is identical to terminal differentiation of epithelial cells in that it recapitulated all the characteristics of terminal differentiation, including a change in cell shape, acquisition of specialized apical structures (microvilli and ruffles), and the ability to secrete and endocytose materials in a regulated manner from the apical membrane. Hensin is expressed in most epithelia, and others have discovered that it is deleted in a large number of epithelial tumors. These results suggest that conversion of polarity of the intercalated cells represents a process of terminal differentiation.

Terminal differentiation of epithelia from trophectoderm to the intercalated cell: the role of hensin

The intercalated cells of the collecting tubules of mammalian kidneys were discovered by Haggege and Richet to change their morphology in response to a variety of physiologic stimuli related to changes in acid base status. Recent studies showed that the conversion of beta to alpha intercalated cell under the influence of acidification of the medium is due to the deposition of hensin in the extracellular matrix of these cells and activation of a novel inductive signal transduction pathway. The conversion of beta to alpha cells is shown to be a process of terminal differentiation. Hensin is secreted as a monomer, and activation of the cell induces two activities that convert it to a dimer by folding and into a fiber by bundling of the folded dimers by galectin 3. Only the fiber is functional. Hensin is expressed in most epithelial cells, and its staining pattern suggests that it might be involved in the terminal differentiation of most epithelia. There is loss of heterozygosity of hensin in a large number of epithelial and neural tumors, making it likely that it is a tumor suppressor gene.

Terminal Differentiation of Epithelia

All epithelia form sheets of cells connected by tight and adherent junctions and exhibit polarized distribution of membrane proteins and lipids. During their development, epithelia progress from this 'generic' phenotype to terminally differentiated states characterized by the development of apical structures such as microvilli, apical endocytosis and regulated exocytosis as well as characteristic cell shapes. We have identified an extracellular matrix protein, hensin, which when polymerized into a fiber induces the terminal differentiation of renal cells. Hensin is expressed in most epithelia where it exists in tissue-specific alternately spliced forms. Many epithelial tumors have deletions in the human ortholog of hensin. We propose that hensin mediates terminal differentiation of these epithelia.

p38 MAPK mediates acid-induced transcription of PEPCK in LLC-PK(1)-FBPase(+) cells

LLC-PK(1)-FBPase(+) cells are a gluconeogenic and pH-responsive renal proximal tubule-like cell line. On incubation with acidic medium (pH 6.9), LLC-PK(1)-FBPase(+) cells exhibit an increased rate of ammonia production as well as increases in glutaminase and phosphoenolpyruvate carboxykinase (PEPCK) mRNA levels and enzyme activities. The increase in PEPCK mRNA is due to an enhanced rate of transcription that is initiated in response to intracellular acidosis. The involvement of known MAPK activities (ERK1/2, SAPK/JNK, p38) in the associated signal transduction pathway was examined by determining the effects of specific MAPK activators and inhibitors on basal and acid-induced PEPCK mRNA levels. Transfer of LLC-PK(1)-FBPase(+) cultures to acidic medium resulted in specific phosphorylation, and thus activation, of p38 and of activating transcription factor-2 (ATF-2), respectively. Anisomycin (AI), a strong p38 activator, increased PEPCK mRNA to levels comparable to those observed with acid stimulation. AI also induced a time-dependent phosphorylation of p38 and ATF-2. SB-203580, a specific p38 inhibitor, blocked both acid- and AI-induced PEPCK mRNA levels. Western blot analyses revealed that the SB-203580-sensitive p38alpha isoform is strongly expressed. The octanucleotide sequence of the cAMP-response element-1 site of the PEPCK promotor is a perfect match to the consensus element for binding ATF-2. The specificity of ATF-2 binding was proven by ELISA. We conclude that the SB-203580-sensitive p38alpha-ATF-2 signaling pathway is a likely mediator of the pH-responsive induction of PEPCK mRNA levels in renal LLC-PK(1)-FBPase(+) cells.

Glitazones regulate glutamine metabolism by inducing a cellular acidosis in MDCK cells

We studied the effect of the antihyperglycemic glitazones, ciglitazone, troglitazone, and rosiglitazone, on glutamine metabolism in renal tubule-derived Madin-Darby canine kidney (MDCK) cells. Troglitazone (25 microM) enhanced glucose uptake and lactate production by 108 and 92% (both P < 0.001). Glutamine utilization was not inhibited, but alanine formation decreased and ammonium formation increased (both P < 0.005). The decrease in net alanine formation occurred with a change in alanine aminotransferase (ALT) reactants, from close to equilibrium to away from equilibrium, consistent with inhibition of ALT activity. A shift of glutamine's amino nitrogen from alanine into ammonium was confirmed by using L-[2-(15)N]glutamine and measuring the [(15)N]alanine and [(15)N]ammonium production. The glitazone-induced shift from alanine to ammonium in glutamate metabolism was dose dependent, with troglitazone being twofold more potent than rosiglitazone and ciglitazone. All three glitazones induced a spontaneous cellular acidosis, reflecting impaired acid extrusion in responding to both an exogenous (NH) and an endogenous (lactic acid) load. Our findings are consistent with glitazones inducing a spontaneous cellular acidosis associated with a shift in glutamine amino nitrogen metabolism from predominantly anabolic into a catabolic pathway.

A central role for Pyk2-Src interaction in coupling diverse stimuli to increased epithelial NBC activity

Regulation of renal Na-HCO cotransporter (NBC1) activity by cholinergic agonists, ANG II, and acute acidosis (CO(2)) requires both Src family kinase (SFK) and classic MAPK pathway activation. The nonreceptor tyrosine kinase proline-rich tyrosine kinase 2 (Pyk2) couples discrete G protein-coupled receptor and growth factor receptor signaling to SFK activation. We examined the role of Pyk2-SFK interaction in coupling these stimuli to increased NBC1 activity in opossum kidney cells. Carbachol increased tyrosine autophosphorylation of endogenous Pyk2 and ectopically expressed wild-type Pyk2 and were abrogated by kinase-dead mutant (Pyk2-KD) overexpression. Pyk2 phosphorylation was calcium/calmodulin dependent, and Pyk2 associated with Src by means of SH2 domain interaction. Pyk2 phosphorylation and Pyk2-Src interaction by carbachol were mimicked by both ANG II and CO(2). To correlate Pyk2 autophosphorylation and Pyk2-Src interaction with NBC activity, cotransporter activity was measured in untransfected cells and in cells overexpressing Pyk2-KD in the presence or absence of carbachol, ANG II, or CO(2). In Pyk2-KD-overexpressing cells, the effect of carbachol, ANG II, and CO(2) was abolished. We conclude that Pyk2 plays a central role in coupling carbachol, ANG II, and CO(2) to increased NBC activity. This coupling is mediated by Pyk2 autophosphorylation and Pyk2-Src interaction.

Role of c-SRC and ERK in acid-induced activation of NHE3

BACKGROUND

In the renal proximal tubule, chronic acidosis causes increases in apical membrane NHE3 activity, which serve to increase transepithelial H+ secretion and return systemic pH to normal levels. Incubation of cultured renal epithelial cells in acid media activates c-Src.

METHODS

OKP cells were incubated in control (pH 7.4) or acid (7.0) media, and NHE3 activity measured as cytoplasmic pH (pHi) recovery from an acid load using BCECF. c-Src, ERK, and JNK kinase activities were measured by immune complex kinase assays with enolase, MBP, and GST-c-Jun, respectively, as substrates in the in vitro assays. To determine the role of c-Src in acid-induced NHE3 activation, cells were transfected with vector alone or a dominant negative c-Src (c-SrcK295M).

RESULTS

Expression of dominant negative c-srcK295M in OKP cells prevented acid-induced activation of NHE3. Incubation of OKP cells in acid media increased ERK activity and c-fos expression, but did not increase JNK activity. Acidosis in vivo also activated renal cortical c-Src and ERK kinases, whereas incubation of 3T3 cells in acid media activated c-Src but not ERK kinase. Expression of c-srcK295M did not affect ERK or c-fos activation by acid incubation. Inhibition of MEK with PD98059 inhibited activation of NHE3 by acid incubation.

CONCLUSIONS

These studies suggest that acidosis activates c-Src and MEK/ERK/c-fos. While both pathways are necessary for activation of NHE3, they are activated independently.

Troglitazone induces a cellular acidosis by inhibiting acid extrusion in cultured rat mesangial cells

We studied the effect of troglitazone on cellular acid-base balance and alanine formation in isolated rat mesangial cells. Mesangial cells were grown to confluency in RPMI 1640 media on 30-mm chambers used to monitor both cellular pH using the pH-sensitive dye 2'7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein and metabolic acid production as well as glutamine metabolism. Troglitazone (10 microM) induced a spontaneous cellular acidosis (6.95 +/- 0.02 vs. 7.47 +/- 0.04, respectively; P < 0.0001) but without an increase in lactic acid production. Alanine production was reduced 64% (P < 0.01) consistent with inhibition of the glutamate transamination. These findings pointed to a decrease in acid extrusion rather than an increase in acid production as the underlying mechanism leading to the cellular acidosis. To test their acid extrusion capabilities, mesangial cells were acid loaded with NH and then allowed to recover in Krebs-Henseleit media or in Krebs-Henseleit media minus bicarbonate (HEPES substituted), and the recovery response (Delta pH(i)/min) was monitored. In the presence of 10 microM troglitazone, the recovery response to the NH acid load was virtually eliminated in the bicarbonate-buffered media (0.00 +/- 0.001 vs. 0.06 +/- 0.02 pH(i)/min, P < 0.0001 vs. control) and reduced 75% in HEPES-buffered media (0.01 +/- 0.01 vs. 0.04 +/- 0.02 pH(i)/min, P < 0.002 vs. control). These results show that troglitazone induces a spontaneous cellular acidosis resulting from a reduction in cellular acid extrusion.

Effects of protein tyrosine kinase and protein tyrosine phosphatase on apical K(+) channels in the TAL

We have previously demonstrated that the protein level of c-Src, a nonreceptor type of protein tyrosine kinase (PTK), was higher in the renal medulla from rats on a K-deficient (KD) diet than that in rats on a high-K (HK) diet (Wang WH, Lerea KM, Chan M, and Giebisch G. Am J Physiol Renal Physiol 278: F165-F171, 2000). We have now used the patch-clamp technique to investigate the role of PTK in regulating the apical K channels in the medullary thick ascending limb (mTAL) of the rat kidney. Inhibition of PTK with herbimycin A increased NP(o), a product of channel number (N) and open probability (P(o)), of the 70-pS K channel from 0.12 to 0.42 in the mTAL only from rats on a KD diet but had no significant effect in tubules from animals on a HK diet. In contrast, herbimycin A did not affect the activity of the 30-pS K channel in the mTAL from rats on a KD diet. Moreover, addition of N-methylsulfonyl-12,12-dibromododec-11-enamide, an agent that inhibits the cytochrome P-450-dependent production of 20-hydroxyeicosatetraenoic acid, further increased NP(o) of the 70-pS K channel in the presence of herbimycin A. Furthermore, Western blot detected the presence of PTP-1D, a membrane-associated protein tyrosine phosphatase (PTP), in the renal outer medulla. Inhibition of PTP with phenylarsine oxide (PAO) decreased NP(o) of the 70-pS K channel in the mTAL from rats on a HK diet. However, PAO did not inhibit the activity of the 30-pS K channel in the mTAL. The effect of PAO on the 70-pS K channel was due to indirectly stimulating PTK because pretreatment of the mTAL with herbimycin A abolished the inhibitory effect of PAO. Finally, addition of exogenous c-Src reversibly blocked the activity of the 70-pS K channel in inside-out patches. We conclude that PTK and PTP have no effect on the low-conductance K channels in the mTAL and that PTK-induced tyrosine phosphorylation inhibits, whereas PTP-induced tyrosine dephosphorylation stimulates, the apical 70-pS K channel in the mTAL.

SFKs, Ras, and the classic MAPK pathway couple muscarinic receptor activation to increased Na-HCO(3) cotransport activity in renal epithelial cells

Cholinergic agents are known to affect the epithelial transport of H2O and electrolytes in the kidney. In proximal tubule cells, cholinergic agonists increase basolateral Na-HCO(3) cotransport activity via M(1) muscarinic receptor activation. The signaling intermediates that couple these G protein-coupled receptors to cotransporter activation, however, are not well defined. We therefore sought to identify distal effectors of muscarinic receptor activation that contribute to increased NBC activity in cultured proximal tubule cells. As demonstrated previously for acute CO2-regulated cotransport activity, we found that inhibitors of Src family kinases (SFKs) or the classic mitogen-activated protein kinase (MAPK) pathway prevented the stimulation of NBC activity by carbachol. The ability of carbachol to activate Src, as well as the proximal (Raf) and distal [extracellular signal-regulated kinases 1 and 2 (ERK1/2)] elements of the classic MAPK module, was compatible with these findings. Cholinergic stimulation of ERK1/2 activity was also completely prevented by overexpression of a dominant negative mutant of Ras (N17-Ras). Taken together, these findings suggest a requirement for the sequential activation of SFKs, Ras, and the classic MAPK pathway [Raf-->MAPK/ERK kinase (MEK)-->ERK]. These findings provide important insights into the molecular mechanisms underlying cholinergic regulation of NBC activity in renal epithelial cells. They also suggest a specific mechanism whereby cholinergic stimulation of the kidney can contribute to pH homeostasis.

Protein-tyrosine phosphatase reduces the number of apical small conductance K+ channels in the rat cortical collecting duct

Previous studies have demonstrated that an increase in the activity of protein-tyrosine kinase (PTK) is involved in the down-regulation of the activity of apical small conductance K(+) (SK) channels in the cortical collecting duct (CCD) from rats on a K(+)-deficient diet (). We used the patch clamp technique to investigate the role of protein-tyrosine phosphatase (PTP) in the regulation of the activity of SK channels in the CCD from rats on a high K(+) diet. Western blot analysis indicated that PTP-1D is expressed in the renal cortex. Application of 1 microm phenylarsine oxide (PAO) or 1 mm benzylphosphonic acid, agents that inhibit PTP, reversibly reduced channel activity by 95%. Pretreatment of CCDs with PAO for 30 min decreased the mean NP(o) reversibly from control value 3.20 to 0.40. Addition of 1 microm herbimycin A, an inhibitor of PTK, had no significant effect on channel activity in the CCDs from rats on a high K(+) diet. However, herbimycin A abolished the inhibitory effect of PAO, indicating that the effect of PAO is the result of interaction between PTK and PTP. Addition of brefeldin A, an agent that blocks protein trafficking from Golgi complex to the membrane, had no effect on channel activity. Moreover, application of colchicine, a microtubule inhibitor, or paclitaxel, a microtubule stabilizer, had no effect on channel activity. In contrast, PAO still reduced channel activity in the presence of brefeldin A, colchicine, or paclitaxel. Furthermore, the effect of PAO on channel activity was absent when the tubules were bathed in 16% sucrose-containing bath solution or treated with concanavalin A. We conclude that PTP is involved in the regulation of the activity of SK channels and that inhibition of PTP may facilitate the internalization of the SK channels.

Identification of a protein kinase activity that phosphorylates connexin43 in a pH-dependent manner

The carboxyl-terminal (CT) domain of connexin43 (Cx43) has been implicated in both hormonal and pH-dependent gating of the gap junction channel. An in vitro assay was utilized to determine whether the acidification of cell extracts results in the activation of a protein kinase that can phosphorylate the CT domain. A glutathione S-transferase (GST)-fusion protein was bound to Sephadex beads and used as a target for protein kinase phosphorylation. A protein extract produced from sheep heart was allowed to bind to the fusion protein-coated beads. The bound proteins were washed and then incubated with 32P-ATP. Phosphorylation was assessed after the proteins were resolved by SDS-PAGE. Incubation at pH 7.5 resulted in a minimal amount of phosphorylation while incubation at pH 6.5 resulted in significant phosphorylation reaction. Maximal activity was achieved when both the binding and kinase reactions were performed at pH 6.5. The protein kinase activity was stronger when the incubations were performed with manganese rather than magnesium. Mutants of Cx43 which lack the serines between amino acids 364-374 could not be phosphorylated in the in vitro kinase reaction, indicating that this is a likely target of this reaction. These results indicate that there is a protein kinase activity in cells that becomes more active at lower pH and can phosphorylate Cx43.

Protein tyrosine kinase regulates the number of renal secretory K channels

The apical small conductance (SK) channel plays a key role in K secretion in the cortical collecting duct (CCD). A high-K intake stimulates renal K secretion and involves a significant increase in the number of SK channels in the apical membrane of the CCD. We used the patch-clamp technique to examine the role of protein tyrosine kinase (PTK) in regulating the activity of SK channels in the CCD. The application of 100 microM genistein stimulated SK channels in 11 of 12 patches in CCDs from rats on a K-deficient diet, and the mean increase in NP(o), a product of channel number (N) and open probability (P(o)), was 2.5. In contrast, inhibition of PTK had no effect in tubules from animals on a high-K diet in all 10 experiments. Western blot analysis further shows that the level of cSrc, a nonreceptor type of PTK, is 261% higher in the kidneys from rats on a K-deficient diet than those on a high-K diet. However, the effect of cSrc was not the result of direct inhibition of channel itself, because addition of exogenous cSrc had no effect on SK channels in inside-out patches. In cell-attached patches, application of herbimycin A increased channel activity in 14 of 16 patches, and the mean increase in NP(o) was 2.4 in tubules from rats on a K-deficient diet. In contrast, herbimycin A had no effect on channel activity in any of 15 tubules from rats on a high-K diet. Furthermore, herbimycin A pretreatment increased NP(o) per patch from the control value (0.4) to 2.25 in CCDs from rats on a K-deficient diet, whereas herbimycin failed to increase channel activity (NP(o): control, 3.10; herbimycin A, 3.25) in the CCDs from animals on a high-K diet. We conclude that PTK is involved in regulating the number of apical SK channels in the kidney.

Cell shrinkage regulates Src kinases and induces tyrosine phosphorylation of cortactin, independent of the osmotic regulation of Na+/H+ exchangers

The signaling pathways by which cell volume regulates ion transporters, e.g. Na+/H+ exchangers (NHEs), and affects cytoskeletal organization are poorly understood. We have previously shown that shrinkage induces tyrosine phosphorylation in CHO cells, predominantly in an 85-kDa band. To identify volume-sensitive kinases and their substrates, we investigated the effect of hypertonicity on members of the Src kinase family. Hyperosmolarity stimulated Fyn and inhibited Src. Fyn activation was also observed in nystatin-permeabilized cells, where shrinkage cannot induce intracellular alkalinization. In contrast, osmotic inhibition of Src was prevented by permeabilization or by inhibiting NHE-1. PP1, a selective Src family inhibitor, strongly reduced the hypertonicity-induced tyrosine phosphorylation. We identified one of the major targets of the osmotic stress-elicited phosphorylation as cortactin, an 85-kDa actin-binding protein and well known Src family substrate. Cortactin phosphorylation was triggered by shrinkage and not by changes in osmolarity or pHi and was abrogated by PP1. Hyperosmotic cortactin phosphorylation was reduced in Fyn-deficient fibroblasts but remained intact in Src-deficient fibroblasts. To address the potential role of the Src family in the osmotic regulation of NHEs, we used PP1. The drug affected neither the hyperosmotic stimulation of NHE-1 nor the inhibition of NHE-3. Thus, members of the Src family are volume-sensitive enzymes that may participate in the shrinkage-related reorganization of the cytoskeleton but are probably not responsible for the osmotic regulation of NHE.

Incubation of OKP cells in low-K+ media increases NHE3 activity after early decrease in intracellular pH

Chronic hypokalemia increases the activity of proximal tubule apical membrane Na+/H+ antiporter NHE3. The present study examined the effect of the incubation of OKP cells (an opossum kidney, clone P cell line) in control medium (K+ concn ([K+]) = 5.4 mM) or low-K+ medium ([K+] = 2.7 mM) on NHE3. The activity of an ethylisopropyl amiloride-resistant Na+/H+ antiporter, whose characteristics were consistent with those of NHE3, was increased in low-K+ cells beginning at 8 h. NHE3 mRNA and NHE3 protein abundance were increased 2.2-fold and 62%, respectively, at 24 h but not at 8 h. After incubation in low-K+ medium, intracellular pH (pHi) decreased by 0.27 pH units (maximum at 27 min) and then recovered to the control level. Intracellular acidosis induced by 5 mM sodium propionate increased Na+/H+ antiporter activity at 8 and 24 h. Herbimycin A, a tyrosine kinase inhibitor, blocked low-K+- and sodium propionate-induced activation of the Na+/H+ antiporter at 8 and 24 h. Our results demonstrate that low-K+ medium causes an early decrease in pHi, which leads to an increase in NHE3 activity via a tyrosine kinase pathway.

Adaptation of NHE-3 in the rat thick ascending limb: effects of high sodium intake and metabolic alkalosis

The present studies examined the effects of chronic NaCl administration and metabolic alkalosis on NHE-3, an apical Na+/H+ exchanger of the rat medullary thick ascending limb of Henle (MTAL). NaCl administration had no effect on NHE-3 mRNA abundance as assessed by competitive RT-PCR, as well as on NHE-3 transport activity estimated from the Na+-dependent cell pH recovery of Na+-depleted acidified MTAL cells, in the presence of 50 microM Hoe-694, which specifically blocks NHE-1 and NHE-2. Two models of metabolic alkalosis were studied, one associated with high sodium intake, i.e., NaHCO3 administration, and one not associated with high sodium intake, i.e., chloride depletion alkalosis (CDA). In both cases, the treatment induced a significant metabolic alkalosis that was associated with a decrease in NHE-3 transport activity (-27% and -25%, respectively). Negative linear relationships were observed between NHE-3 activity and plasma pH or bicarbonate concentration. NHE-3 mRNA abundance and NHE-3 protein abundance, assessed by Western blot analysis, also decreased by 35 and 25%, respectively, during NaHCO3-induced alkalosis, and by 47 and 33%, respectively, during CDA. These studies demonstrate that high sodium intake has per se no effect on MTAL NHE-3. In contrast, chronic metabolic alkalosis, regardless of whether it is associated with high sodium intake or not, leads to an appropriate adaptation of NHE-3 activity, which involves a decrease in NHE-3 protein and mRNA abundance.

Stimulation by in vivo and in vitro metabolic acidosis of expression of rBSC-1, the Na+-K+(NH4+)-2Cl- cotransporter of the rat medullary thick ascending limb

To assess whether metabolic acidosis per se regulates rBSC-1, the rat medullary thick ascending limb (MTAL) apical Na+-K+(NH4+)-2Cl- cotransporter, rat MTALs were incubated for 16 h in an acid 1:1 mixture of Ham's nutrient mixture F-12 and Dulbecco's modified Eagle's medium. Cotransport activity was estimated in intact cells and membrane vesicles by intracellular pH and 22Na+ uptake measurements, respectively; rBSC-1 protein was quantified by immunoblotting analysis and mRNA by quantitative reverse transcription-polymerase chain reaction. As compared with incubation at pH approximately 7.35, acid incubation (pH approximately 7.10) up-regulated by 35-100% rBSC-1 transport activity in cells and membrane vesicles, and rBSC-1 protein and mRNA abundance. In contrast, acid incubation did not alter alkaline phosphatase and Na+/K+-ATPase enzyme activities or beta-actin protein abundance. After 3 h of in vivo chronic metabolic acidosis (CMA) rBSC-1 mRNA abundance increased in freshly harvested MTALs, which was accompanied after 1-6 days of CMA with enhanced rBSC-1 protein abundance. These results demonstrate that both in vivo and in vitro CMA stimulate rBSC-1 expression, which would contribute to the adaptive increase in MTAL absorption and urinary excretion of NH4+ in response to CMA.

Dominant negative c-Src inhibits angiotensin II induced activation of NHE3 in OKP cells

BACKGROUND

Angiotensin II is a potent stimulator of the proximal tubule apical membrane Na/H antiporter, encoded by NHE3. The nonreceptor tyrosine kinase, c-Src, plays a key role in regulation of NHE3 by acidosis in the proximal tubule, and in signaling effects of angiotensin II in vascular smooth muscle.

METHODS

The present studies examined the role of c-Src in mediating angiotensin II-induced NHE3 activation in cultured OKP cells. c-Src was inhibited with herbimycin A, a tyrosine kinase inhibitor, and expression of a dominant negative c-Src, c-SrcK295M.

RESULTS

Herbimycin A blocked angiotensin II induced increases in Na/H antiporter activity. In two clonal cell lines expressing vector alone, angiotensin II increased Na/H antiporter activity, while in three clones expressing c-SrcK295M, angiotensin II had no effect. Cyclic AMP and protein kinase A have been proposed to be key mediators in regulation of NHE3 by angiotensin II. 10(-4) M 8-bromo cAMP induced a 40 to 50% inhibition of Na/H antiporter activity in cells expressing c-SrcK295M, similar to that seen in wild-type OKP cells. In addition, cells expressing c-SrcK295M responded normally to 10(-7) M dexamethasone with a 50 to 80% increase in Na/H antiporter activity.

CONCLUSIONS

These studies demonstrate that c-Src is required for angiotensin II-induced increases in NHE3 activity. Thus, c-Src plays a key role in antiporter activation by acidosis and angiotensin II.

Carbonic anhydrase II plays a major role in osteoclast differentiation and bone resorption by effecting the steady state intracellular pH and Ca2+

Carbonic anhydrase II (CA II) expression in characteristic for the early stage of osteoclast differentiation. To study how CA II, which is crucial in proton generation in mature osteoclasts, influences the osteoclast differentiation process we performed rat bone marrow cultures. In this model, acetazolamide, a specific CA inhibitor, decreased the 1,25 (OH)2D3-induced formation of multinucleated tartrate-resistant acid phosphatase (TRAP)-positive cells, in a dose-dependent manner. We then performed intracellular pH (pHi) and Ca2+ (Cai2+) measurements for cultured osteoclasts and noticed that addition of acetazolamide caused a rapid, transient increase of both parameters. The increase in pHi was dependent neither on the culture substrate nor on the extracellular pH (pHe) but the increase could be diminished by DIDS or by bicarbonate removal. Membrane-impermeable CA inhibitors (benzolamide and pd5000) did not have this effect. Addition of CA II antisense oligonucleotides into the cultures reduced the pHi increase significantly. CA II inhibition was also found to neutralize the intracellular vesicles at extracellular pH (pHe) of 7.4, but at less extent at pHe 7.0. In mouse calvaria cultures, bone resorption was inhibited dose dependently by acetazolamide at pHe 7.4 while inhibition was smaller at pHe 7.0. We conclude that CA II is essential not only in bone resorption but also in osteoclast differentiation. In both processes, however, the crucial role of CA II is at least partially due to the effect on the osteoclast pHi regulation.