Role of Ca2+ and protein kinase C in the receptor-mediated activation of Na+/H+ exchange in isolated liver cells

Calcium influx, a new potential therapeutic target in the control of neutrophil-dependent inflammatory diseases in bovines

Neutrophils are the first line of defense against pathogens in bovines; however, they are also one of the most aggressive cells during the inflammatory process, causing injury in surrounding tissues. At present, anti-inflammatory drugs are limited in acute diseases, such as pneumonia, mastitis and endometritis, because neutrophils are mostly insensitive. One of the earliest events during neutrophil activation is the increase in intracellular calcium concentration. The calcium movement is attributed to the release from intracellular stores and influx through the calcium channels in the plasma membrane, a process called store operated calcium entry (SOCE). Recently, several calcium influx blockers have been shown to have strong effects on bovine neutrophils, and this suggests that the manipulation of this pathway can be useful in the control of neutrophil functions during acute inflammatory processes. In this paper, we will review the role of calcium influx as a potential anti-inflammatory target and summarize the most recent evidences for this in bovine neutrophils.

Interdependence of Ca2+ and proton movements in trout hepatocytes

This study was undertaken to investigate possible interrelationships between Ca2+ homeostasis and pH regulation in trout hepatocytes. Exposure of cells to Ca2+ mobilizing agents ionomycin (0.5 μmol l–1) and thapsigargin (0.1 μmol l–1)induced an increase in intracellular pH (pHi) that was dependent on Ca2+ influx from the extracellular medium as well as Ca2+ release from intracellular pools. Surprisingly, this increase in pHi and intracellular Ca2+ concentration,[Ca2+]i, was not accompanied by any change in proton secretion. By contrast, removal of extracellular Ca2+(Ca2+e) using EGTA (0.5 mmol l–1)briefly increased proton secretion rate with no apparent effect on pHi, while chelation of Ca2+i using BAPTA-AM (25 μmol l–1) resulted in a drop in pHi and a sustained increase in proton secretion rate. [Ca2+]i therefore affected intracellular proton distribution and/or proton production and also affected the distribution of protons across the cell membrane. Accordingly, changes in pHi were not always compensated for by proton secretion across the cell membrane.

Alteration in pHe below and above normal values induced a slow,continuous increase in [Ca2+]i with a tendency to stabilize upon exposure to high pHe values. Rapid pHi increase induced by NH4Cl was accompanied by an elevation in[Ca2+]i from both extracellular and intracellular compartments. Ca2+e appeared to be involved in pHi regulation following NH4Cl-induced alkalinization whereas neither removal of Ca2+e nor chelation of Ca2+i affected pHi recovery following Na-propionate exposure. Similarly, [Ca2+]i increase induced by hypertonicity appeared to be a consequence of the changes in pHi as Na-free medium as well as cariporide diminished the hypertonicity-induced increase in[Ca2+]i. These results imply that a compensatory relationship between changes in pHi and proton secretion across cell plasma membrane is not always present. Consequently, calculating proton extrusion from buffering capacity and rate of pHi change cannot be taken as an absolute alternative for measuring proton secretion rate, at least in response to Ca2+ mobilizing agents.

Store-operated calcium entry mediates intracellular alkalinization, ERK1/2, and Akt/PKB phosphorylation in bovine neutrophils

Neutrophil's responses to G protein-coupled chemoattractants are highly dependent on store-operated calcium (Ca(2+)) entry (SOCE). Platelet-activating factor (PAF), a primary chemoattractant, simultaneously increases cytosolic-free Ca(2+), intracellular pH (pH(i)), ERK1/2, and Akt/protein kinase B (PKB) phosphorylation. In this study, we looked at the efficacy of several putative SOCE inhibitors and whether SOCE mediates intracellular alkalinization, ERK1/2, and Akt/PKB phosphorylation in bovine neutrophils. We demonstrated that the absence of external Ca(2+) and the presence of EGTA reduced the intracellular alkalinization and ERK1/2 phosphorylation induced by PAF, apparently via SOCE influx inhibition. Next, we tested the efficacy of several putative SOCE inhibitors such as 2-aminoethoxydiphenyl borate (2-APB), capsaicin, flufenamic acid, 1-{beta-[3-(4-methoxy-phenyl)propoxy]-4-methoxyphenethyl}-1H-imidazole hydrochloride (SK&F 96365), and N-(4-[3,5-bis(trifluoromethyl)-1H-pyrazol-1-yl]phenyl)-4-methyl-1,2,3-thiadiazole-5-carboxamide (BTP2) on Ca(2+) entry induced by PAF or thapsigargin. 2-APB was the most potent SOCE inhibitor, followed by capsaicin and flufenamic acid. Conversely, SK&F 96365 reduced an intracellular calcium ([Ca(2+)](i)) peak but SOCE partially. BTP2 did not show an inhibitory effect on [Ca(2+)](i) following PAF stimuli. 2-APB strongly reduced the pH(i) recovery, whereas the effect of flufenamic acid and SK&F 96365 was partial. Capsaicin and BTP2 did not affect the pH(i) changes induced by PAF. Finally, we observed that 2-APB reduced the ERK1/2 and Akt phosphorylation completely, whereas the inhibition with flufenamic acid was partial. The results suggest that 2-APB is the most potent SOCE inhibitor and support a key role of SOCE in pH alkalinization and PI-3K-ERK1/2 pathway control. Finally, 2-APB could be an important tool to characterize Ca(2+) signaling in neutrophils.

Signalling pathways involved in hypertonicity- and acidification-induced activation of Na+/H+ exchange in trout hepatocytes

In trout hepatocytes, hypertonicity and cytosolic acidification are known to stimulate Na+/H+ exchanger (NHE) activity, which contributes to recovery of cell volume and intracellular pH (pHi),respectively. The present study investigated the signalling mechanisms underlying NHE activation under these conditions. Exposing trout hepatocytes to cariporide, a specific inhibitor of NHE-1, decreased baseline pHi,completely blocked the hypertonicity-induced increase of pHi and reduced the hypertonicity-induced proton secretion by 80%. Changing extracellular pH (pHe)above and below normal values, and allowing cells to adjust pHi accordingly,significantly delayed alkalinization during hypertonic exposure, whereas following an acid load an enhanced pHi recovery with increasing pHe was seen. Chelating Ca2+, and thereby preventing the hypertonicity-induced increase in intracellular Ca2+ ([Ca2+]i), significantly diminished hypertonic elevation of pHi, indicating that Ca2+signalling might be involved in NHE activation. A reduction in alkalinization and proton secretion was also observed in the presence of the protein kinase A(PKA) inhibitor H-89 or the calmodulin (CaM) inhibitor calmidazolium. A complete inhibition of hypertonic- and acidification-induced changes of pHi concurrent with an increase in hypertonically induced proton efflux was seen with the protein kinase C (PKC) inhibitor chelerythrine. Recovery of pHi following sodium propionate addition was reduced by more than 60% in the presence of cariporide, was sensitive to PKA inhibition, and tended to be reduced by CaM inhibition. In conclusion, we showed that NHE-1 is the main acid secretion mechanism during hypertonicity and recovery following acid loading. In addition, Ca2+-, PKA- and CaM-dependent pathways are involved in NHE-1 activation for recovery of cell volume and pHi. On the other hand, PKC appeared to have an impact on NHE-independent pathways affecting intracellular acid-base homeostasis.

Agonist-induced aggregation of Chinese hamster ovary cells coexpressing the human receptors for fibrinogen (integrin alphaIIbbeta3) and the platelet-activating factor: dissociation between adhesion and aggregation

This work reports the establishment of a Chinese hamster ovary (CHO) cell line stably coexpressing the human alphaIIbbeta3 integrin and the platelet-activating factor receptor (PAFR). These cells aggregate in response to PAF in a Ca(++), alphaIIbbeta3, and soluble fibrinogen (Fg)-dependent manner that is prevented by PAF antagonists or alphaIIbbeta3 blockade. The aggregating response is accompanied by enhanced binding of fibrinogen and the activation-dependent IgM PAC1. This model has permitted us to identify, for the first time, intracellular signals distinctly associated with either alphaIIbbeta3-mediated adhesion or aggregation. Nonreceptor activation of protein kinase C (PKC) by phorbol ester produced cellular adhesion and spreading onto immobilized Fg, but it was not a sufficient signal to provoke cellular aggregation. Moreover, inhibition of PKC impeded the PAF stimulation of cellular adhesion, whereas the aggregation was not prevented. The PAF-induced cellular aggregation was distinctly associated with signaling events arising from the liganded Fg receptor and the agonist-induced stimulation of a calcium/calmodulin-dependent signaling pathway. Sustained tyrosine phosphorylation of both mitogen-activated protein kinase (MAPK) and an approximately 100-kd protein was associated with the PAF-induced aggregation, whereas phosphorylation of focal adhesion kinase (FAK) was preferably associated with cellular adherence and spreading onto immobilized Fg.

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

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

Novel green fluorescent protein-based ratiometric indicators for monitoring pH in defined intracellular microdomains

To measure pH in defined intracellular microdomains of living cells, we developed ratiometric indicators based on fusing in tandem two green fluorescent protein (GFP) variants having different pH sensitivities. The indicators function in a single-excitation/dual-emission mode involving fluorescence resonance energy transfer, as well as in a dual-excitation/single-emission mode. The fluorescence ratio from GFpH and YFpH showed pH dependency and pK(a) values were 6.1 and 6.8, respectively. Using these indicators expressed in cultured cells, we measured and visualized pH changes in the cytosol and nucleus. Furthermore, by tethering the indicator to a membrane protein (the alpha(1B) adrenergic receptor), we visualized the pH in the vicinity of the protein during internalization caused by endocytosis after agonist stimulation. These novel probes will serve as a useful tool for monitoring pH in the defined organelle and in the microenvironment of a target protein, to analyze cellular function.

A zinc-sensing receptor triggers the release of intracellular Ca2+ and regulates ion transport

Changes in extracellular zinc concentration participate in modulating fundamental cellular processes such as proliferation, secretion, and ion transport in a mechanism that is not well understood. Here, we show that a micromolar concentration of extracellular zinc triggers a massive release of calcium from thapsigargin-sensitive intracellular pools in the colonocytic cell line HT29. Calcium release was blocked by a phospholipase-C inhibitor, indicating that formation of inositol 1,4,5-triphosphate is required for zinc-dependent calcium release. Zinc influx was not observed, indicating that extracellular zinc triggered the release. The Ca(i)2+ release was zinc specific and could not be triggered by other heavy metals. Furthermore, zinc failed to activate the Ca(2+)-sensing receptor heterologously expressed in HEK293 cells. The zinc-induced Ca(i)2+ rise stimulated the activity of the Na(+)/H(+) exchanger in HT29 cells. Our results indicate that a previously uncharacterized extracellular, G protein-coupled, Zn(2+)-sensing receptor is functional in colonocytes. Because Ca(i)2+ rise is known to regulate key cellular and signal-transduction processes, the zinc-sensing receptor may provide the missing link between extracellular zinc concentration changes and the regulation of cellular processes.

Rapid acid extrusion response triggered by alpha(1) adrenoceptor in CHO cells

1. Using a microphysiometer with synchronized valve switching, we investigated real-time acid extrusion from Chinese hamster ovary (CHO) cells in which human alpha(1) adrenoceptor (AR) is stably expressed, in response to noradrenaline (NA). 2. In the cells expressing alpha(1a) AR, the time course of extracellular acidification after stimulation had two phases; in the first phase it transiently reached a rate several times greater than the base rate with a peak at around 10 s, and in the second it increased to 2 times the base rate and reached a plateau in 2 min. Both phases showed a concentration-dependent increase of acidification rate in response to NA, but had distinct pEC(50) values; 5.6 for the transient phase and 7.2 for the steady phase. 3. In the cells expressing alpha(1b) AR, the transient phase was not detected but the steady phase was observed. The pEC(50) value was 7.1, although the magnitude of the response was much smaller than that with alpha(1a) AR. 4. Both 5-(N-ethyl-N-isopropyl)amiloride (EIPA) and HOE642 inhibited the acid extrusion response by either AR in a concentration-dependent manner. EIPA and HOE642 had high pIC(50) values (7.4 and 7.3, respectively) for inhibition of the transient phase response via alpha(1a) AR. In the inhibition of the steady phase response via alpha(1a) AR, both drugs revealed the presence of two components in the response; one had high pIC(50) values (8.1 and 8.2 for EIPA and HOE642, respectively) and the other had low pIC(50) values (5.6 and 6.0, respectively). In contrast, the steady phase response via alpha(1b) AR was inhibited by EIPA and HOE642 with low pIC(50) values (5.3 and 5.9, respectively). 5. As Ca2+ was depleted, the alpha(1a) AR-induced transient phase disappeared, while the steady phase was not affected. 6. These results suggest that alpha(1a) AR drives two acid extrusion systems in CHO cells upon stimulation; one elicits the transient response, which is largely mediated by an EIPA/HOE642-sensitive and Ca(2+)-dependent Na(+)-H(+) exchanger (NHE), presumably NHE1, and the other induces the steady acid extrusion that is mediated by NHE1 and another NHE which has low sensitivity to both EIPA and HOE642. alpha(1b) AR drives only the steady phase acid extrusion response, which is mainly mediated by NHEs other than NHE1.

Enhanced proliferation of lymphoblasts from patients with Alzheimer dementia associated with calmodulin-dependent activation of the na+/H+ exchanger

We have recently reported that lymphoblasts from late onset Alzheimer's disease (AD) patients show distinct intracellular pH homeostatic features than those obtained from age-matched healthy donors. Here we report that another distinct feature of AD lymphoblasts is their increased rate of proliferation in serum containing medium, suggesting a different responsiveness of AD cells to serum activators. The increased proliferation of AD cells was accompanied by intracellular alkalinization and was prevented by blockers of the plasma membrane Na+/H+ antiporter (NHE), indicating that the exchanger had to be activated to elicit the cellular responses. The activity of this exchanger can be controlled through several signaling pathways, but only the inhibition of calmodulin activity impeded the serum-induced intracellular alkalinization and enhanced proliferation of AD cells. In contrast, the inhibition of calmodulin did not alter the rate of proliferation of normal cells. Thus, it seems plausible to conclude that the enhanced proliferation of AD cells is the result of a surface receptor-mediated activation of the Ca(2+)-calmodulin signaling pathway. Our observations add further support in favor that AD may be considered a systemic disease which underlying etiopathogenic mechanism may be an altered responsiveness to cell activating agents. Thus, the use of lymphoblastoid cells from AD patients may be a useful model to investigate cell biochemical aspects of this disease.

Role of diacylglycerol (DAG) in hormonal induction of S phase in hepatocytes: the DAG-dependent protein kinase C pathway is not activated by epidermal growth factor (EGF), but is involved in mediating the enhancement of responsiveness to EGF by vasopressin, angiotensin II, and norepinephrine

The role of diacylglycerol (DAG) in hormonal induction of S phase was investigated in primary cultures of rat hepatocytes. In this model, several agonists that bind to G protein-coupled receptors act as comitogens when added to the cells soon after plating (i.e., in Go/early Gl phase), while the cells are most responsive to the mitogenic effect of epidermal growth factor (EGF) at 24-48 h of culturing (i.e., mid/late Gl). It was found that the cellular concentration of DAG rose markedly and progressively during the first 24 h of culturing. Exposure of the hepatocytes at 3 h to alpha1-adrenergic stimulation (norepinephrine with timolol), vasopressin, or angiotensin II further increased this rise, producing a sustained increase in the DAG level. Norepinephrine, which was the most efficient comitogen, produced the most prolonged DAG elevation. In contrast, no significant increase of DAG was found in response to EGF, neither at 3 nor at 24 h, using concentrations that markedly stimulated the ERK subgroup of the mitogen-activated protein kinases (MAPK) and DNA synthesis. Addition of Bacillus cereus phosphatidylcholine-specific phospholipase C (PC-PLC) strongly elevated DAG, while Streptomyces phospholipase D (PLD) increased phosphatidic acid (PA) but not DAG. B. cereus PC-PLC and the protein kinase C (PKC) activator tetradecanoyl phorbol-acetate (TPA), like norepinephrine, vasopressin, and angiotensin II, stimulated MAPK and enhanced the stimulatory effect of EGF on DNA synthesis. The PKC inhibitor GF109203X did not diminish the effect of EGF on MAPK or DNA synthesis, but strongly inhibited the effects of norepinephrine, vasopressin, angiotensin II, TPA and B. cereus PC-PLC on MAPK and almost abolished the enhancement by these agents of EGF-stimulated DNA synthesis. These results suggest that although generation of DAG is not a direct downstream response mediating the effects of the EGF receptor in hepatocytes, a sustained elevation of DAG with activation of PKC markedly increases the responsiveness to EGF. Mechanisms involving DAG and PKC seem to play a role in the comitogenic effects of various agents that bind to G protein-coupled receptors and activate the cells early in Gl, such as norepinephrine, angiotensin II, and vasopressin.