Single-cell analysis of the intracellular pH and its regulation during the monocytic differentiation of U937 human leukemic cells

Potassium channel Kir 4.1 regulates oligodendrocyte differentiation via intracellular pH regulation

In humans, loss-of-function mutations of Kcnj10 in SeSAME/EAST syndrome, which encodes the inwardly rectifying K⁺ channel 4.1 (K_ir 4.1), causes progressive neurological decline. Despite its rich expression in oligodendrocyte (OL) lineage cells and an emerging link with demyelinating disease, the function of K_ir 4.1 in OLs is unclear. Here we show a novel role of K_ir 4.1 in OL development. K_ir 4.1 expression is markedly greater in OLs than in OL precursor cells (OPCs), and the down-regulation of K_ir 4.1 impairs OL maturation by affecting OPC differentiation. Interestingly, K_ir 4.1 regulates the intracellular pH of OPCs and OLs via the Na⁺ /H⁺ exchanger, which underlies impeded OPC differentiation by K_ir 4.1 inhibition. Furthermore, K_ir 4.1 regulates GSK3β and SOX10, two molecules critical to OPC development. Collectively, our work opens a new avenue to understanding the functions of K_ir 4.1 and intracellular pH in OLs.

Application of Spectral Phasor analysis to sodium microenvironments in myoblast progenitor cells

Sodium ions (Na+) are key regulators of molecular events in many cellular processes, yet the dynamics of this ion remain poorly defined. Developing approaches to identify and characterise Na+ microenvironments will enable more detailed elucidation of the mechanisms of signal transduction. Here we report the application of Spectral Phasor analysis to the Na+ fluorophore, CoroNa Green, to identify and spatially map spectral emissions that report Na+ microenvironments. We use differentiating stem cells where Na+ fluxes were reported as an antecedent. Myoblast stem cells were induced to differentiate by serum starvation and then fixed at intervals between 0 and 40-minutes of differentiation prior to addition of CoroNa Green. The fluorescent intensity was insufficient to identify discrete Na+ microenvironments. However, using Spectral Phasor analysis we identified spectral shifts in CoroNa Green fluorescence which is related to the Na+ microenvironment. Further, spectral-heterogeneity appears to be contingent on the distance of Na+ from the nucleus in the early stages of differentiation. Spectral Phasor analysis of CoroNa Green in fixed stem cells demonstrates for the first time that CoroNa Green has unique spectral emissions depending on the nature of the Na+ environment in differentiating stem cells. Applying Spectral Phasor analysis to CoroNa Green in live stem cells is likely to further elucidate the role of Na+ microenvironments in the differentiation process.

Increase in Intracellular Ca2+ Concentrations and the Corresponding Intracellular Acidification Are Early Steps for Induction of Apoptosis by Geranylgeraniol in HL60 Cells

To elucidate the mechanism of induction of apoptosis by geranylgeraniol (GGO), which is a potent inducer of apoptosis in various lines of human cancer cells, we examined the role of intracellular acidification during GGO-induced apoptosis using human leukemia HL60 cells. Flow cytometry analysis revealed that apoptosis induced in human leukemia HL60 cells by GGO was associated with intracellular acidification. Both GGO-induced intracellular acidification and apoptosis as analyzed by DNA fragmentation were inhibited by phorbol myristate acetate (TPA) and O'-bis(2-aminophenyl)ethyleneglycol-N,N,N',N-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM), an intracellular Ca(2+) chelator, but not by ethyleneglycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA). These results suggest that the early concentration change of intracellular Ca(2+) and the corresponding decrease in intracellular pH are required for the induction of apoptosis in HL60 cells by GGO.

RXR-induced TNF-alpha suppression is reversed by morphine in activated U937 cells

Deficiency in vitamin A has been associated with adverse clinical outcomes in drug users with HIV-1 infection. Retinoids have been demonstrated to suppress proinflammatory cytokine production by immune cells in vitro. These effects are induced by ligand-mediated activation of the retinoid receptors--retinoic acid receptor (RAR) and retinoid X receptor (RXR). In these studies, the effects of all-trans-retinoid acid (ATRA, a RAR agonist), 9-cis-retinoic acid (9cis RA; RAR and RXR agonist), LG101305 (RXR agonist), LG100815 (RAR antagonist) and LG101208 (RXR antagonist) on TNF-alpha production by phytohemagglutanin-activated U937 cells and the modulation of these effects by morphine were examined. TNF-alpha production was suppressed in all cultures exposed to retinoid agonist and antagonist agents. For cells exposed to RXR agonists or RAR antagonist, incubation with morphine resulted in the reversal of TNF-alpha suppression and this effect was inhibited by naloxone. These data suggest that interactions between RXR and morphine are involved in the immune effects of retinoids on TNF-alpha production by activated U937 cells. Such information may be important for understanding interactions between drugs of abuse and immune function in individuals with chronic proinflammatory states such as HIV-1 infection.

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

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

Transcriptional regulation of Na+/H+ exchanger expression in the intact mouse

We examined regulation of the Na+/H+ exchanger (NHE1 isoform) in the developing mouse. We generated transgenic mice with the Na+/H+ exchanger promoter directing expression of the beta-Galactosidase reporter. We found that expression of the Na+/H+ exchanger was maximum in the heart and liver of 12-day-old embryonic mice. Similar results were found in mice using the green fluorescent protein reporter driven by the Na+/H+ exchanger promoter. Detailed examination of the myocardium revealed that the GFP reporter protein was expressed in the cytoplasm of cardiomyocyte cells. We examined NHE1 protein expression in transgenic mice lacking the transcription factors AP-2alpha or the transcription factor COUP-TF1. Eighteen-day-old AP-2alpha heterozygote mice show no large changes in NHE1 expression in heart, lung, liver, kidney and brain. In contrast, 18-day-old embryos from AP-2alpha null mice showed a large increase in Na+/H+ exchanger protein expression in the brain. NHE1 protein levels in COUP-TF1 knockout embryos did not differ from wild type embryos. The results suggest that AP-2alpha and COUP-TF1 are not critical to NHE1 expression in the late stage embryo and that other related transcription factors may function in regulation of the Na+/H+ exchanger.

Developmental regulation of Na(+)/H(+) exchanger expression in fetal and neonatal mice

We examined the hypothesis that Na(+)/H(+) exchanger expression is regulated during fetal and neonatal development and differentiation. To examine transcriptional regulation of the NHE1 isoform of the Na(+)/H(+) exchanger, transgenic mice were created that contained the mouse NHE1 promoter driving expression of green fluorescent protein. The level of NHE1 transcription varied between tissues and with the stage of embryonic development. The highest expression was in the heart and liver of 12- to 15-day-old mice, and this declined with age. To examine Na(+)/H(+) exchanger protein levels, we immunoblotted mouse tissues from 18-day-old embryos, neonates, and adults. Protein levels increased after embryonic day 18 and peaked at 14 days of age in the heart, lung, liver, kidney, and brain. The greatest rise in NHE1 protein expression occurred in the heart, whereas the smallest increase was in the brain. The results suggest that Na(+)/H(+) exchanger transcription and protein levels are controlled in a tissue-specific and time-dependent manner during development.

Functional analysis of polar amino-acid residues in membrane associated regions of the NHE1 isoform of the mammalian Na+/H+ exchanger

The NHE1 isoform of the Na+/H+ exchanger is a ubiquitous plasma membrane protein that regulates intracellular pH in mammalian cells. Site-specific mutagenesis was used to examine the functional role of conserved, polar amino-acid residues occurring in segments of the protein associated with the membrane. Seventeen mutant proteins were assessed by characterization of intracellular pH changes in stably transfected cells that lacked an endogenous Na+/H+ exchanger. All of the mutant proteins were targeted correctly to the plasma membrane and were expressed at similar levels. Amino-acid residues Glu262 and Asp267 were critical to Na+/H+ exchanger activity while mutation of Glu391 resulted in only a partial reduction in activity. The Glu262-->Gln mutant was expressed partially as a deglycosylated protein with increased sensitivity to trypsin treatment in presence of Na+. Substitution of mutated Glu262, Asp267 and Glu391 with alternative acidic residues restored Na+/H+ exchanger activity. The Glu262-->Asp mutant had a decreased affinity for Li+, but its activity for Na+ and H+ ions was unaffected. The results support the hypothesis that side-chain oxygen atoms in a few, critically placed amino acids are important in Na+/H+ exchanger activity and the acidic amino-acid residues at positions 262, 267 and 391 are good candidates for being involved in Na+ coordination by the protein.

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

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

The Na+/H+ antiporter potentiates growth and retinoic acid-induced differentiation of P19 embryonal carcinoma cells

The Na+/H+ exchanger is a ubiquitous plasma membrane protein that is responsible for pH regulation and is activated by growth factors. We examined the role of the Na+/H+ exchanger in cell growth and differentiation. Treatment of P19 cells with the Na+/H+ exchanger inhibitor Hoe 694 eliminated retinoic acid-induced differentiation in this cell line. We developed a P19 embryonal carcinoma cell line that was deficient in the Na+/H+ antiporter. Na+/H+ exchanger-deficient cells were reduced in the rate of cell growth and this effect was enhanced by the removal of added HCO3- and by reducing extracellular pH. The antiporter-deficient cells were also markedly deficient in their ability to differentiate to neuronal-like cells and recovered this ability when the Na+/H+ antiporter was reintroduced. The results show that the absence of Na+/H+ antiport as a pH regulatory mechanism can result in deficiencies in both cell growth and differentiation in embryonal carcinoma cells.

Regulation of NHE1 expression in L6 muscle cells

We examined regulation of expression of the NHE1 promoter on rat L6 cells. Transient transfection of these cells showed that there are two regions critical for basal expression in this cell type. One is from bp -155 to -171 and second more proximal region is between bp -92 and -125. When cells were induced to differentiate by serum withdrawal, mRNA levels rose 2-3-fold. To investigate the mechanisms of this phenomenon a series of stable transfectants were made of the NHE1 promoter in L6 cells. Muscle differentiation caused a significant stimulation of transcriptional activity in the stable cells containing the more distal regions of the promoter. The results show that basal expression of the NHE1 promoter is mediated largely by two proximal regions of the gene. However, during the process of differentiation more distal regions of the gene are involved in elevation of the level of expression.

Interleukin-1 increases vacuolar-type H+-ATPase activity in murine peritoneal macrophages

Maintenance of cytoplasmic pH (pHi) within a narrow physiological range is crucial to normal cellular function. This is of particular relevance to phagocytic cells within the acidic inflammatory microenvironment where the pHi tends to be acid loaded. We have previously reported that a vacuolar-type H(+)-ATPase (V-ATPase) situated in the plasma membrane of macrophages and poised to extrude protons from the cytoplasmic to the extracellular space is an important pHi regulatory mechanism within the inflammatory milieu. Since this microenvironment is frequently characterized by the influx of cells known to release inflammatory cytokines, we performed studies to examine the effect of one such mediator molecule, interleukin-1 (IL-1), on pHi regulation in peritoneal macrophages. IL-1 caused a time- and dose-dependent increase in macrophage pHi recovery from an acute acid load. This effect was specific to IL-1 and was due to enhanced plasmalemmal V-ATPase activity. The increased V-ATPase activity by IL-1 occurred following a lag period of several hours and required de novo protein and mRNA synthesis. However, Northern blot analysis revealed that IL-1 did not exert its effect via alterations in the levels of mRNA transcripts for the A or B subunits of the V-ATPase complex. Finally, stimulation of both cAMP-dependent protein kinase and protein kinase C was required for the stimulatory effect of IL-1 on V-ATPase activity. Thus, cytokines present within the inflammatory milieu are able to modulate pHi regulatory mechanisms. These data may represent a novel mechanism whereby cytokines may improve cellular function at inflammatory sites.

Intracellular alkalinization suppresses lovastatin-induced apoptosis in HL-60 cells through the inactivation of a pH-dependent endonuclease

Protein isoprenylation is a post-translational modification essential for the biological activity of G-proteins. Inhibition of protein isoprenylation by lovastatin (LOV) induces apoptosis in HL-60 cells, a process of active cell death characterized by the internucleosomal degradation of genomic DNA. In this article we show that LOV-induced apoptosis is associated with intracellular acidification and that activation of the Na+/H+ antiporter induces a raise in pHi which is sufficient to prevent or arrest DNA digestion. First, LOV induced a decrease in pHi which was dose-dependent and correlated with the extent of DNA degradation. Flow cytometry analysis revealed that this acidification was due to the appearance of a subpopulation of cells whose pHi was 0.9 pH units below control values. Cell sorting experiments demonstrated that DNA degradation had occurred only in those cells which had suffered intracellular acidification. LOV-induced apoptosis could be suppressed by mevalonate supplementation, inhibition of protein synthesis, and protein kinase C activation by phorbol myristate acetate. In all three cases, intracellular acidification was abolished. Inhibition of the Na+/H+ antiporter by 5-N-ethyl-N-isopropyl amiloride induced DNA degradation in HL-60 cells per se and suppressed the protective effect of phorbol myristate acetate. LOV-induced intracellular acidification was not due to a complete inhibition of the Na+/H+ antiporter. In fact, LOV-treated cells were able to respond to phorbol myristate acetate stimulation of the Na+/H+ antiporter with a marked increase in pHi. This effect was accompanied by a rapid arrest of DNA digestion. These observations illustrate the strong pH dependence of LOV-induced DNA degradation, thus providing a connection between the activation of the Na+/H+ antiporter and the suppression of apoptosis.

Relative roles of Na+/H+ exchange and vacuolar-type H+ ATPases in regulating cytoplasmic pH and function in murine peritoneal macrophages

Two distinct mechanisms have been shown to mediate cytoplasmic pH (pHi) recovery in acid-loaded peritoneal macrophages (M phi s): Na+/H+ exchange and H+ extrusion by vacuolar-type (V-type) H+ ATPases. The present studies examined the relative roles of these two systems in maintaining pHi and cell function. Measurements of M phi pHi and superoxide (O2-) production in response to stimulation with 12-O-tetradecanoyl phorbol 13-acetate (TPA) were made at physiological or acidic extracellular pH (pHo) levels. The V-type H+ ATPase inhibitor, bafilomycin A1, and the potent Na+/H+ exchange inhibitor, N-ethyl-N-propylamino amiloride (EPA), were used to examine the contributions of these ion transporters to pHi regulation and cell function. At pHo 7.35, the complementary activities of the Na+/H+ antiport and the V-type H+ ATPase mediate pHi homeostasis. At pHo 6.7, maintenance of pHi depends primarily on H+ ATPase activity: bafilomycin A1 reduced pHi from 6.8 +/- 0.02 in control cells to 6.59 +/- 0.01 (P < 0.01) while EPA was without effect. The functional importance of V-type H+ ATPase-activity in preserving pHi homeostasis at acidic extracellular pH levels was reflected by the impairment of O2- production at pHo 6.70 when H+ ATPase activity was inhibited: bafilomycin A1 reduced O2- production from 13.9 +/- 1.0 to 9.3 +/- 0.6 nmoles/10(6) cells/40 min, in control and bafilomycin A1-treated cells, respectively (P < 0.05), while EPA had no effect. In subsequent studies, pHi was independently manipulated using the ionophore nigericin. Lowering pHi from 6.80 to 6.60 reduced O2- production from 15.3 +/- 1.8 to 9.8 +/- 1.6 nmoles/10(6) cells/40 min (P < 0.05), indicating that the cytoplasmic acidification resulting from inhibition of H+ ATPases at low pHo could account for the associated impairment of O2- production. In a more profoundly acidic environment (pHo 6.35), H+ ATPases remained active in regulating pHi, but could not preserve a sufficiently physiological pHi to support respiratory burst activity. V-type H+ ATPases constitute the dominant mechanism by which the pHi of peritoneal M phi s is maintained in an acidic extracellular environment.

Hyperoxia induces alkalinization and dome formation in MDCK epithelial cells

We observed that confluent Madin-Darby canine kidney (MDCK) epithelial cells exposed to 95% O2 showed intensive dome formation, a manifestation of cell differentiation and transepithelial fluid transport, whereas cells exposed to 40% O2 or to normoxia did not. Hyperoxia-induced dome formation (HIDF) was preceded and accompanied by a significant rise in intracellular pH (pHi) on days 2 (7.53 vs. 7.42) and 3 (7.62 vs. 7.45), as compared with controls. Inhibition of the Na(+)-H+ exchanger by 0.1 or 1.0 mM amiloride caused 29 or 69% reduction of HIDF and completely abolished hyperoxia-induced alkalinization of the cells. HIDF was altered by modification of extracellular pH (pHo); there was a decrease by 84% with pHo 6.8, while pHo 7.8 led to earlier and more intensive dome formation (day 2, +472%; day 3, +27%). We also found that adenosine 3',5'-cyclic monophosphate (cAMP) was increased in hyperoxic cells, a change that was independent from the rise of pHi. We conclude that high-level hyperoxia induces dome formation in MDCK epithelial monolayers by a process involving activation of the Na(+)-H+ exchanger, together with increased intracellular cAMP.

Nitric oxide derived from L-arginine impairs cytoplasmic pH regulation by vacuolar-type H+ ATPases in peritoneal macrophages

The ability of macrophages (Møs) to function within an acidic environment has been shown to depend on cytoplasmic pH (pHi) regulation by vacuolar-type H+ ATPases. Møs metabolize L-arginine via an oxidative pathway that generates nitric oxide, nitrate, and nitrite. Since each of these products could potentially inhibit vacuolar-type H+ ATPases, we investigated the effect of L-arginine metabolism on Mø pHi regulation in thioglycolate-elicited murine peritoneal Møs. H+ ATPase-mediated pHi recovery from an imposed cytoplasmic acid load was measured fluorometrically. When Møs were incubated with L-arginine (0.25-2.0 mM), their rate of pHi recovery declined progressively from 2 to 6 h of incubation. By contrast, the recovery rate of cells incubated in arginine-free medium remained stable over the same period. The impairment of pHi recovery was specific for L-arginine, and was blocked competitively by NG-monomethyl-L-arginine, demonstrating its dependence on L-arginine metabolism. In addition, the inhibition of pHi recovery was enhanced by lipopolysaccharide, an agent known to stimulate L-arginine metabolism by Møs. Scavenging the L-arginine metabolite nitric oxide with either ferrous sulphate or ferrous myoglobin prevented the inhibition of pHi recovery, implying that L-arginine-derived nitric oxide was the species responsible for the inhibition. This concept was supported by the finding of elevated nitrite levels in the supernatant of cells incubated in L-arginine. Furthermore, incubation of Møs with sodium nitroprusside mimicked the L-arginine-dependent inhibition of H+ ATPase activity. Treatment with the cyclic GMP analogue, 8-bromoguanosine 3':5'-cyclic monophosphate, similarly impaired Mø pHi recovery, suggesting that a nitric oxide-stimulated elevation of cyclic GMP may contribute to the L-arginine-dependent inhibition of pHi regulation.

Intracellular pH in individual pituitary cells: measurement with a dual emission pH indicator

Intracellular pH (pHi) can now be measured at the single cell level using dual emission wavelength microspectrofluorimetry with the fluorescent pH indicator SNARF 1 and its membrane permeant acetoxymethyl ester (SNARF 1/AM). We measured pHi of individual pituitary cells under both basal and stimulated conditions. The emitted fluorescence of SNARF 1 probe was calibrated following experimental manipulations of pHi in two types of rat pituitary cells. The calibration curves obtained in the two cell types were identical. We observed a Gaussian distribution of individual pHi with a wide dispersion (6.95 to 8) in the two cell populations. TRH (10(-7) M) and ionomycin (5 microM) induced a transient acidification followed by a sustained alkalinization, whereas K+ (50 mM) depolarization only exerted a transient acidification. These results show that the dual emission pH indicator SNARF 1 can be used to reliably investigate changes in pHi in individual endocrine cells.

Internalization and acidification of surface HLA-DR molecules by epidermal Langerhans cells: a paradigm for antigen processing

CD4+ T lymphocytes recognize multi-molecular complexes, formed by major histocompatibility complex class II molecules and exogenous antigens, on the surface of antigen-presenting cells (APC). For most protein antigens, processing is required to produce immunogenic peptide fragments that can then form stable associations with class II molecules. These two processes, the modification of antigen and its coupling to class II molecules, are thought to occur in acidic endosomal compartments. Furthermore, membrane class II molecules are endocytosed in APC and may provide ligands for the immunogenic peptides. To gain insight into these processes, we examined the internalization and acidification of membrane HLA-DR molecules by three APC populations: 1) freshly isolated Langerhans cells (LC), 2) LC after 48-72 h of bulk epidermal cell culture, and 3) peripheral blood monocytes (PBM). Using FITC-conjugated anti-HLA-DR monoclonal antibodies (MoAb), endocytosis was studied by fluorescence microscopy and by flow cytometry (pulse width analysis), while acidification was assessed by exploiting the pH sensitivity of fluorescein fluorescence. We observed both freshly isolated LC and PBM to internalize surface HLA-DR molecules into acidic compartments with great efficiency. Endocytosis was inhibited by the addition of azide and 2-deoxy-D-glucose, whereas acidification was partially blocked by treatment with ammonium chloride or chloroquine. The degree of internalization and acidification of HLA-DR molecules was greatly influenced by the degree of Ab cross-linking. On the other hand, cultured LC were capable of internalizing HLA-DR molecules, but were not able to acidify the environments to which these molecules were delivered; this loss of acidification capacity was partially restored by treatment with phorbol 12-myristate 13-acetate.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of cytoplasmic pH in resident and activated peritoneal macrophages

Cytoplasmic pH (pHi) has been shown to be an important determinant of the activity of the NADPH oxidase in phagocytic cells. We hypothesized that a difference in pHi and/or its regulation existed between activated and resident macrophages (RES MOs) which might explain the increased NADPH oxidase activity observed in the former. The pHi of RES and lipopolysaccharide (LPS)-elicited MOs was examined using the fluorescent dye BCECF. Resting pHi did not differ between resident (RES) and elicited (ELI) MOs (7.16 +/- 0.05 and 7.20 +/- 0.05, respectively). pHi recovery after intracellular acid loading was partially dependent on the presence of Na+ in the extracellular medium, and was partially inhibited by the Na+/H+ antiport inhibitor, amiloride. At comparable pHi, the rate of acid extrusion during recovery was not different in RES and ELI MOs (1.48 +/- 0.12 and 1.53 +/- 0.06 mM/min, respectively). In both RES and ELI MOs, approx. 40% of total pHi recovery was insensitive to amiloride and independent of extracellular Na+. In both RES and ELI MOs, stimulation with TPA resulted in a biphasic pHi response: an initial acidification followed by a sustained alkalinization to a new steady-state pHi. This alkalinization was Na(+)-dependent and amiloride-sensitive, consistent with a TPA-induced increase in Na+/H+ antiport activity. The new steady-state pHi attained after TPA stimulation was equivalent in RES and ELI MOs (7.28 +/- 0.04 and 7.31 +/- 0.06, respectively), indicating comparable stimulated Na+/H+ antiport activity. However, the initial acidification induced by TPA was greater in ELI than in RES MOs (0.18 +/- 0.02 vs. 0.06 +/- 0.02 pH unit, respectively, P less than 0.05). The specific NADPH oxidase inhibitor diphenylene iodonium (DPI) completely inhibited the respiratory burst but reduced the magnitude of this pHi reduction by only about 50%. This suggested that the TPA-induced pHi reduction was due in part to acid produced via the respiratory burst, and in part to other acid-generating pathways stimulated by TPA.

Phorbol esters and chemotactic factor induce distinct changes in cytoplasmic Ca2+ and pH in granulocytic like HL60 cells

Differentiation of HL60 cells into neutrophil-like cells after exposure to dimethylsulfoxide is accompanied by an increase in intracellular pH (pHi) which results from an increased activity of the Na+/H+ antiporter at physiological pHi value, but not at acidic pHi values. The functional responses of differentiated HL60 cells to the chemotactic peptide, N-formylmethionylleucylphenylalanine (fMLP), and to phorbol myristate acetate (PMA) were studied. In differentiated cells fMLP produced a large increase in cytosolic Ca2+ levels and a small biphasic change in pHi, whereas PMA produced cellular acidification, which was potentiated by ethylisopropylamiloride and no change in [Ca2+]i. In undifferentiated HL60 cells, PMA produced the opposite effect on pHi, i.e., a cellular alkalinization. The extent of the acidification produced by PMA in differentiated HL60 cells correlated with the production of reactive oxygen species.

Intracellular Ca2+ potentiates Na+/H+ exchange and cell differentiation induced by phorbol ester in U937 cells

The human cell line U937 differentiates to monocyte macrophage-like cells in response to tumour-promoting phorbol esters. This effect is attributed to activation of protein kinase C. We show here that U937 cell differentiation induced by 12-O-tetradecanoylphorbol 13-acetate (TPA) is associated with cytoplasmic alkalinization. Ethyl-isopropyl-amiloride (EIPA), a potent inhibitor of Na+/H+ exchange, blocked both cytoplasmic alkalinization and cell differentiation. Cell acidification by addition of 2-4 mM sodium propionate also blocked TPA-induced U937 cell differentiation. These results suggest that a sustained cell alkalinization mediated by activation of Na+/H+ exchange is essential for TPA-induced differentiation in U937 cells. The increase of cytoplasmic free calcium concentration ([Ca2+]i) by addition of the calcium ionophore ionomycin enhanced TPA-induced alkalinization by increasing the apparent affinity of the Na+/H+ antiporter for intracellular H+. Treatment with ionomycin also potentiated differentiation of U937 cells induced by TPA. This synergism suggests that [Ca2+]i either potentiates the activation of protein kinase C or triggers additional transducing mechanisms. The key events of this interaction occur during the first 30 min of treatment, even though cell differentiation manifests much later.