High intracellular pH accompanies mitotic activity in murine lymphocytes

pH-Channeling in Cancer: How pH-Dependence of Cation Channels Shapes Cancer Pathophysiology

Tissue acidosis plays a pivotal role in tumor progression: in particular, interstitial acidosis promotes tumor cell invasion, and is a major contributor to the dysregulation of tumor immunity and tumor stromal cells. The cell membrane and integral membrane proteins commonly act as important sensors and transducers of altered pH. Cell adhesion molecules and cation channels are prominent membrane proteins, the majority of which is regulated by protons. The pathophysiological consequences of proton-sensitive ion channel function in cancer, however, are scarcely considered in the literature. Thus, the main focus of this review is to highlight possible events in tumor progression and tumor immunity where the pH sensitivity of cation channels could be of great importance.

Synergy Between Low Dose Metronomic Chemotherapy and the pH-centered Approach Against Cancer

Low dose metronomic chemotherapy (MC) is becoming a mainstream treatment for cancer in veterinary medicine. Its mechanism of action is anti-angiogenesis by lowering vascular endothelial growth factor (VEGF) and increasing trombospondin-1 (TSP1). It has also been adopted as a compassionate treatment in very advanced human cancer. However, one of the main limitations of this therapy is its short-term effectiveness: 6 to 12 months, after which resistance develops. pH-centered cancer treatment (pHT) has been proposed as a complementary therapy in cancer, but it has not been adopted or tested as a mainstream protocol, in spite of existing evidence of its advantages and benefits. Many of the factors directly or indirectly involved in MC and anti-angiogenic treatment resistance are appropriately antagonized by pHT. This led to the testing of an association between these two treatments. Preliminary evidence indicates that the association of MC and pHT has the ability to reduce anti-angiogenic treatment limitations and develop synergistic anti-cancer effects. This review will describe each of these treatments and will analyze the fundamentals of their synergy.

In vitro chromatin self-association and its relevance to genome architecture

Chromatin in a eukaryotic nucleus is condensed through 3 hierarchies: primary, secondary, and tertiary chromatin structures. In vitro, when induced with cations, chromatin can self-associate and form large oligomers. This self-association process has been proposed to mimic processes involved in the assembly and maintenance of tertiary chromatin structures in vivo. In this article, we review 30 years of studies of chromatin self-association, with an emphasis on the evidence suggesting that this in vitro process is physiologically relevant.

Regulation of T-cell apoptosis by reactive oxygen species

To ensure that a constant number of T cells are preserved in the peripheral lymphoid organs, the production and proliferation of T cells must be balanced out by their death. Newly generated T cells exit the thymus and are maintained as resting T cells. Transient disruption of homeostasis occurs when naïve T cells undergo antigen-induced expansion, a process involving intracellular signaling events that lead to T cell proliferation, acquisition of effector functions, and, ultimately, either apoptosis or differentiation into long-lived memory cells. The last decision point (death vs. differentiation) is a crucial one: it resets lymphoid homeostasis, promotes protective immunity, and limits autoimmunity. Despite its importance, relatively little is known about the molecular mechanisms involved in this cell fate decision. Although multiple mechanisms are likely involved, recent data suggest an underlying regulatory role for reactive oxygen species in controlling the susceptibility of T cells to apoptosis. This review focuses on recent advances in our understanding of how reactive oxygen species modulate T-cell apoptosis.

A role for intracellular pH in membrane IgM-mediated cell death of human B lymphomas

We show that anti-IgM-induced cell death in a human B lymphoma cell line, B104, is associated with early intracellular acidification and cell shrinkage. In contrast, another human B cell lymphoma line, Daudi, less susceptible to B cell antigen receptor-mediated cell death, responded to anti-IgM with an early increase in intracellular pH (pH(i)). The anti-IgM-induced changes of pH(i) were associated with different levels of activation of the Na(+)/H(+) exchanger isoform 1 (NHE1) as judged by its phosphorylation status. Prevention of anti-IgM-induced cell death in B104 cells by the calcineurin phosphatase inhibitor, cyclosporin A, abrogated both intracellular acidification and cell shrinkage and was associated with an increase in the phosphorylation level of NHE1 within the first 60 min of stimulation. This indicates a key role for calcineurin in regulating pH(i) and cell viability. The potential role of pH(i) in cell viability was confirmed in Daudi cells treated with an Na(+)/H(+) exchanger inhibitor 5-(N,N-hexamethylene)amiloride. These observations indicate that the outcome of the anti-IgM treatment depends on NHE1-controlled pH(i). We suggest that inactivation of the NHE1 in anti-IgM-stimulated cells results in intracellular acidification and subsequently triggers or amplifies cell death.

A highly temperature-sensitive proton current in mouse bone marrow-derived mast cells

Proton (H+) conductive pathways are suggested to play roles in the regulation of intracellular pH. We characterized temperature-sensitive whole cell currents in mouse bone marrow-derived mast cells (BMMC), immature proliferating mast cells generated by in vitro culture. Heating from 24 to 36 degrees C reversibly and repeatedly activated a voltage-dependent outward conductance with Q10 of 9.9 +/- 3.1 (mean +/- SD) (n = 6). Either a decrease in intracellular pH or an increase in extracellular pH enhanced the amplitude and shifted the activation voltage to more negative potentials. With acidic intracellular solutions (pH 5.5), the outward current was detected in some cells at 24 degrees C and Q10 was 6.0 +/- 2.6 (n = 9). The reversal potential was unaffected by changes in concentrations of major ionic constituents (K+, Cl-, and Na+), but depended on the pH gradient, suggesting that H+ (equivalents) is a major ion species carrying the current. The H+ current was featured by slow activation kinetics upon membrane depolarization, and the activation time course was accelerated by increases in depolarization, elevating temperature and extracellular alkalization. The current was recorded even when ATP was removed from the intracellular solution, but the mean amplitude was smaller than that in the presence of ATP. The H+ current was reversibly inhibited by Zn2+ but not by bafilomycin A1, an inhibitor for a vacuolar type H(+)-ATPase. Macroscopic measurements of pH using a fluorescent dye (BCECF) revealed that a rapid recovery of intracellular pH from acid-load was attenuated by lowering temperature, addition of Zn2+, and depletion of extracellular K+, but not by bafilomycin A1. These results suggest that the H+ conductive pathway contributes to intracellular pH homeostasis of BMMC and that the high activation energy may be involved in enhancement of the H+ conductance.

Effect of stilbene-type anion channel blockers on the immune response during experimental allergic neuritis (EAN)

We have studied the role of anion channel gating for the autoimmune response in experimental allergic neuritis (EAN) induced by bovine peripheral myelin (BPM). The influence of the stilbene-type anion channel blockers SITS and DIDS on T cell function was assessed by measurement of proliferation and by counting of interferon-gamma (IFN-gamma) secreting cells (IFN-gamma-sc) in response to BPM and phytohemagglutinin (PHA). SITS caused a dose-dependent increase of spontaneous proliferative activity as well as of proliferation in response to the antigenic stimulus BPM. In contrast, the drug caused a decrease of proliferation of cells stimulated with PHA. The number of cells induced to IFN-gamma secretion was reduced by SITS. The suppressive effect was dependent on the degree of activity of cells without drugs. Cultures showing high numbers of BPM reactive T cells were more easily suppressed than cultures with low numbers of BPM reactive T cells. Our results suggest that anion channel gating is involved in the triggering of T cells to IFN-gamma secretion. The anion channel signal pathway in lymphocytes could be a target for pharmacological intervention in inflammatory disorders. In the presently used autoimmune model, EAN, the net effect of in vivo treatment with SITS resulted in worsening of clinical signs and increased inflammatory cell infiltration in sciatic nerve, whereas the in vitro conductivity of sciatic nerve was not significantly affected by the drug. Thus anion channel gating seems to regulate activities of immune cells, and drugs with anion channel blocking properties may have effects that enhance autoimmune disease.

Cytotoxic T lymphocyte (CTL)-mediated cytolysis proceeds in the absence of Na+/H+ antiport activity: regulation of cytosolic pH by the Na+/H+ antiport in a cloned CTL

Cytotoxic T lymphocyte (CTL)-mediated cytolysis of specifically bound target cells (TC) is thought to be triggered by cross-linking the T-cell antigen receptor (TcR). Biochemical events associated with TcR cross-linking include increased intracellular calcium levels [Ca2+]i, hydrolysis of phosphatidylinositol (PI), and an increase in intracellular pH [pH]i. Whereas CTL-mediated cytolysis of some TC is calcium-dependent, and PI hydrolysis is speculated to trigger the CTL lethal hit via activation of PKC, little is known about changes in [pH]i relating to activation of the lethal hit stage. We report regulation of [pH]i in a cloned CTL by the electroneutral Na+/H+ antiport during activation with PMA and specific antigen-bearing TC. Furthermore, using 5-(N-methyl-N-isobutyl) amiloride (MIBA), a potent antiport inhibitor, we demonstrate that Na+/H+ exchange is not required for activation of CTL cytolytic activity.

Cytoplasmic alkalinization produced by the combination of anti-immunoglobulin antibody plus cytochalasin D in murine B lymphocytes

The intracellular pH of murine splenic B lymphocytes was measured using the pH-sensitive fluorescent dye, bis(carboxyethyl)carboxyfluorescein (BCECF). After stimulation of B lymphocytes with anti-immunoglobulin antibody plus cytochalasin D, two agents that act in synergy to promote S-phase entry, a late increase in pH was detected that occurred prior to the onset of DNA synthesis. The degree of alkalinization observed was comparable to that produced by two additional mitogenic regimens. Cytoplasmic alkalinization was not blocked by dimethylamiloride. Cytoplasmic alkalinization represents a sign of, and may play a role in, stimulation of B lymphocytes to enter S phase.

On the biophysics of transmembrane signalling

Transmembrane signalling involves a number of physical translocations, changes in proximity of membrane elements like receptor subunits, or sequestration of proteins from the membrane. The monitoring of such changes with flow cytometric energy transfer revealed a new putative subunit of the IL-2 receptor and a possible intermolecular interaction between HLA class I and class II antigens. Lateral diffusion of the components of the multi-subunit IL-2 receptor was also followed. Changes in the intracellular pH were considered as a measure of efficient signal transfer in a number of cases. An overview and critical comparison of data is presented in the paper.

Regulation of intracellular pH during the G1/S-phase transition of the neuroblastoma cell cycle

Changes in active K+ and Na+ influx during the cell cycle of neuroblastoma (clone Neuro-2A) have suggested activation of an Na+, H+ exchange system during the G1/S-phase transition. Here we report that pHi, measured by the digitonin null-point method, is constant during G1-phase and the G1/S-phase transition and decreases in early S-phase. In addition pHi is shown to be most sensitive to the diuretic amiloride in the G1/S-phase transition, in agreement with the ion influx data. It is concluded from these data, that pHi is tightly regulated during the early cell cycle phases by the Na+, H+ exchange system, in particular during the G1/S-phase transition.

Changes in intracellular pH and cell volume during the early phase of DMSO-induced differentiation of Friend erythroleukemia cells

Changes in intracellular pH and water volume were measured after treatment of Friend erythroleukemia cells with 1.5% DMSO. It was found that a continuous decrease in pHi occurred, beginning 1 h after induction and a decline in pHi of 0.18 was measured after 9 h. In addition a decline in cellular water volume, of 12% only 15 min after induction, and 23% after 9 h, was observed.

Amiloride-induced suppression of lymphocyte proliferation: inhibition of IL 2 receptor expression after blockade of early sodium influx

Increased rates of monovalent cation fluxes are implicated in the activation of lymphocytes by mitogens. Our report shows that amiloride and dimethylamiloride, two inhibitors of the Na+/H+ antiport, dose dependently prevent the proliferation of mitogen-activated human peripheral blood lymphocytes. The action of these drugs follows several mechanisms, since their inhibitory effect can be reversed by extensive washing when they are used at low concentrations (150 microM for amiloride, 40 microM for dimethylamiloride), while at higher non cytotoxic concentrations this reversibility is no longer observed. We have studied the mechanism whereby amiloride inhibits the blastogenesis by measuring their effect on: 1) IL 2 production, 2) acquisition of IL 2 responsiveness and induction of IL 2 receptors, 3) IL 2-induced proliferation. Unlike the expression of IL 2 receptors, IL 2 production and IL 2-dependent proliferation were not inhibited by the low drug concentrations which indeed prevent blastogenesis. Moreover under these conditions, an enhanced accumulation of IL 2 was observed in the supernatants of stimulated cells. These results show that the drugs interact differently with the different cell populations involved in T cell proliferation: increase of an amiloride-dependent sodium influx is an obligatory step required to induce the early increase of the ouabain-dependent potassium influx which is needed for the expression of IL 2 receptors. On the contrary, the influx of potassium necessary for the IL 2-dependent proliferation does not seem to be controlled by the amiloride-dependent sodium flux.

Flow cytometric measurement of cytoplasmic pH: a critical evaluation of available fluorochromes

Three pH-sensitive fluorochromes-4-methyl-umbelliferone(4MU),2, 3-dicyano-hydroquinone (DCH), and 2',7'-bis(carboxyethyl)-5,6-carboxy fluorescein (BCECF)--were evaluated for their resolution, range, and stability of cellular fluorescence. Flow cytometric techniques for determining cytoplasmic pH (pHi) have been fully described for 4MU and DCH; BCECF has previously been used for fluorimetric estimation of pHi, and was adapted to flow cytometry. For each fluorochrome, the ratio of fluorescence intensity at two wavelengths gives a measure of pHi, which may be calibrated by obtaining the fluorescence ratios for cells suspended in buffers of varying pH in the presence of a proton ionophore. Reliable calibration proved difficult using 4MU, partly because of poor retention within cells. Both DCH and BCECF could be calibrated using a fluorescence ratio and had resolutions of 0.2 and 0.4 pH units, respectively. The fluorescence of DCH is so strongly pH dependent that there were practical difficulties in its use over a wide pH range; however, pHi measurements are possible between pH 6.0 and pH 7.5 using either DCH or BCECF. Substantial dye leakage was found for 4MU and, to a lesser extent, DCH, while BCECF was retained by cells for up to 2 hours. Despite its lower resolution BCECF had a usable range of more than 1.5 pH units and this coupled with its stable fluorescence and excitation at 488 nm rather than UV suggests a wide application.

Evidence for a channel for the electrogenic transport of chloride ion in the rat hepatocyte

Chloride is the major inorganic anion in bile but its mechanism of passage from blood to bile is uncertain. Specific membrane channels account for most net inorganic anion flux in other cell types such as the proximal tubular cell and red blood cell; disulfonic stilbenes inhibit anion movement through these channels. Therefore, we have sought the presence of similar channels in the hepatocyte. Net inorganic anion flux or conductance was initiated in isolated rat hepatocytes by valinomycin in the presence of an outward potassium gradient. Potassium concentration in the extracellular medium increased from 2.75 +/- 0.02 in control cell suspensions to 3.15 +/- 0.04 in valinomycin-treated cell suspensions. Membrane potential difference (Em) (mV), determined as the distribution of [14C]tetraphenyl phosphonium ion was -28 mV in control cells and -42 mV in valinomycin-treated cells (p less than 0.05). Intracellular chloride concentration (36Cl-) (mEq per liter of cell water) decreased significantly from 38.6 in control cells to 32.0 in valinomycin-treated cells. The observed intracellular concentrations (36Cl-) in both control and valinomycin-treated cell suspensions closely approximates values predicted on the basis of the Nernst equation: 41 and 29 (mEq per liter of cell water), respectively, suggesting that the chloride ion is passively distributed on the basis of the membrane potential difference. Furthermore, net rate-limited cell water loss of approximately 15% of control values was associated with the above valinomycin-stimulated changes in ion distribution, as assessed using three methods of cell water volume determination.(ABSTRACT TRUNCATED AT 250 WORDS)

Interference of cyclosporin with lymphocyte activation maintenance of the cells in a non-cycling phase and with a low intracellular pH

Cyclosporin can block T cell activation by mitogens as evidenced by inhibition of exogenous thymidine integration into DNA and by blockage of the cells in the G0/G1 phase of the cell cycle. The mitogenic activation sequence shows at least one cyclosporin sensitive step at or prior to the rise of cytoplasmic pH (pHi), which normally occurs after exposure to mitogen and which seems to be a permissive condition for initiation of DNA synthesis (S phase of the cell cycle).

Transient increase in intracellular pH during Dictyostelium differentiation

The intracellular pH (pHi) of Dictyostelium discoideum amebae has been determined using the pH-dependent fluorescence of intracellularly trapped fluorescein (Thomas, J. A., R. N. Buschbaum, A. Zimiak, and E. Racker, Biochemistry, 18:2210-2218). The pHi of cells measured 45-60 min after initiation of differentiation was between 6.2 and 6.3. At approximately 2 h into differentiation cells underwent a transient intracellular alkalinization during which the pHi rose to 7.13 (+/- 0.3, n = 4), after which the pHi returned to approximately the original value (6.2-6.4). Cells that were removed from growth medium but were incubated in differentiation medium containing 3% dextrose did not exhibit this transient increase in pHi. The alkalinization event can also be prevented from occurring by differentiation in Na+-free solutions or by the addition of amiloride to sodium-containing buffer solutions, suggesting that the alkalinization is sodium dependent. When the alkalinization was prevented by amiloride treatment, cells did not progress normally into differentiation. This increase in pHi was initiated by the cells 2 h after removal from nutrient medium and it could be inhibited by several treatments that had been observed to delay the differentiation program, suggesting that it plays a major role in the initiation of the developmental program of this organism.

Acetazolamide teratogenesis: interaction of maternal metabolic and respiratory acidosis in the induction of ectrodactyly in C57BL/6J mice

Exposure of C57BL/6J mice to 20% CO2 for 8 hours on day 10 of gestation has been shown to produce right-sided postaxial forelimb ectrodactyly in 23% of the offspring. Carbon dioxide exposure produces a dramatic increase in maternal plasma CO2 accompanied by an inevitable decrease in plasma pH, both of which appear to be involved in the induction of ectrodactyly. However, the low incidence of ectrodactyly associated with NH4Cl-induced metabolic acidosis suggests that the primary teratogenic factor in respiratory acidosis is elevated CO2 tension. This conclusion is supported by the observation that moderation of maternal plasma pH in the face of sustained elevated PCO2 fails to reduce the incidence of ectrodactyly; moreover, there is a strong correlation between maternal serum CO2 content and the incidence of ectrodactyly.

The intracellular pH of quiescent and proliferating human and rat thymic lymphocytes

We compared the cytoplasmic pH (pHi) of quiescent and actively cycling thymic lymphocytes. Human and rat thymocyte suspensions were fractionated by centrifugation on one-step albumin density gradients. The pellet was composed of small, quiescent cells and the interphase contained mostly larger, actively cycling cells with a high proliferation index. When measured using [14C]-dimethyloxazolidinedione (DMO), pHi in the large cells of both species was approximately 0.15 units more alkaline than in the small cells. However, these differences were not detectable when pHi was measured with carboxylated fluorescein derivatives generated in situ by cytoplasmic enzymes. This apparent discrepancy can be explained by compartmentation of DMO, which accumulates in the alkaline mitochondrial matrix. Comparison of the mitochondrial content of quiescent and cycling thymocytes by several methods showed that the latter contained over 2.5-fold more mitochondria per unit cell volume. Assuming a constant intramitochondrial pH, this difference can account for the observed accumulation of DMO (i.e., apparent cytoplasmic alkalinity) in the actively proliferating cells. Therefore, no evidence was found for the existence of differences in pHi between quiescent and proliferating lymphocytes. Moreover, caution must be exercised when comparing DMO partition data in cells with varying relative mitochondrial content.

pH homeostasis in human lymphocytes: modulation by ions and mitogen

Quiescent human peripheral blood lymphocytes have been shown to maintain a relatively constant intracellular pH of 7.0-7.2 over an extracellular pH range of 6.9-7.4. Two methods of measuring intracellular pH were used in these studies, 19F nuclear magnetic resonance and [14C]5,5-dimethyloxazolidine-2,4-dione (DMO) equilibrium distributions. When ATP levels were decreased in these cells, actively maintained pH regulation was abolished and cells exhibited a constant pH gradient of 0.2 pH unit (acid inside relative to outside). Possible mechanisms for pH regulation are discussed. The effects of the Na+ and K+ composition of the medium on pH regulation showed no correlation with their effects on mitogen-induced proliferative response, which we have previously determined (Deutsch, C., and M. Price, 1982, J. Cell. Physiol., 111:73-79). In low-Na+ mannitol medium, pH regulation was similar to that observed for lymphocytes in normal medium, whereas mitogen-induced proliferation was severely inhibited in low-Na+ mannitol. In contrast, high-K+, low Na+ medium caused loss of pH homeostasis, whereas it restored the proliferative response. Loss of pH homeostasis was also observed on prolonged exposure of lymphocytes to mitogen (greater than 6 h in culture). However, mitogen stimulation led to little or no change in intracellular pH in the first few hours of cell culture. Therefore, a shift in intracellular pH is not a necessary or general event in mitogen-stimulated proliferation of lymphocytes.

Cytoplasmic pH regulation in thymic lymphocytes by an amiloride-sensitive Na+/H+ antiport

The mechanisms underlying cytoplasmic pH (pHi) regulation in rat thymic lymphocytes were studied using trapped fluorescein derivatives as pHi indicators. Cells that were acid-loaded with nigericin in choline+ media recovered normal pHi upon addition of extracellular Na+ (Nao+). The cytoplasmic alkalinization was accompanied by medium acidification and an increase in cellular Na+ content and was probably mediated by a Nao+/Hi+ antiport. At normal [Na+]i, Nao+/Hi+ exchange was undetectable at pHi greater than or equal to 6.9 but was markedly stimulated by internal acidification. Absolute rates of H+ efflux could be calculated from the Nao+-induced delta pHi using a buffering capacity of 25 mmol X liter-1 X pH-1, measured by titration of intact cells with NH4+. At pHi = 6.3, pHo = 7.2, and [Na+]o = 140 mM, H+ extrusion reached 10 mmol X liter-1 X min-1. Nao+/Hi+ exchange was stimulated by internal Na+ depletion and inhibited by lowering pHo and by addition of amiloride (apparent Ki = 2.5 microM). Inhibition by amiloride was competitive with respect to Nao+. Hi+ could also exchange for Lio+, but not for K+, Rb+, Cs+, or choline+. Nao+/Hi+ countertransport has an apparent 1:1 stoichiometry and is electrically silent. However, a small secondary hyperpolarization follows recovery from acid-loading in Na+ media. This hyperpolarization is amiloride- and ouabain-sensitive and probably reflects activation of the electrogenic Na+-K+ pump. At normal Nai+ values, the Nao+/Hi+ antiport of thymocytes is ideally suited for the regulation of pHi. The system can also restore [Na+]i in Na+-depleted cells. In this instance the exchanger, in combination with the considerable cytoplasmic buffering power, will operate as a [Na+]i-regulatory mechanism.

Intracellular pH and the cell cycle of mitogen-stimulated murine lymphocytes

Rapidly proliferating, polyclonally stimulated mouse spleen lymphocytes were separated by density-gradient unit-gravity sedimentation. The following measurements were made on each fraction: the average intracellular water volume, the distribution of DNA content by flow microfluorometry, the rate of 3H-thymidine incorporation, and the intracellular pH. Fractions of cells with a small average intracellular volume were predominantly in G0 or G1 phase of the cell cycle, while fractions of larger cells had higher proportions of cells in S or G2. Multiple regression analysis of the data for both T and B lymphocytes indicated that the intracellular pH of cells in G0, G1, or G2 is around pH 7.2, and that the intracellular pH of cells in S phase of the cell cycle is around pH 7.4.