Mitogen-independent activation of Na+/H+ exchange in human epidermoid carcinoma A431 cells: regulation by medium osmolarity

Acid-Base Basics

Although students initially learn of ionic buffering in basic chemistry, buffering and acid-base transport in biology often is relegated to specialized classes, discussions, or situations. That said, for physiology, nephrology, pulmonology, and anesthesiology, these basic principles often are critically important for mechanistic understanding, medical treatments, and assessing therapy effectiveness. This short introductory perspective focuses on basic chemistry and transport of buffers and acid-base equivalents, provides an outline of basic science acid-base concepts, tools used to monitor intracellular pH, model cellular responses to pH buffer changes, and the more recent development and use of genetically encoded pH-indicators. Examples of newer genetically encoded pH-indicators (pHerry and pHire) are provided, and their use for in vitro, ex vivo, and in vivo experiments are described. The continued use and development of these basic tools provide increasing opportunities for both basic and potentially clinical investigations.

Modulation of hypoxia-inducible factor-1 alpha in cultured primary cells by intracellular ascorbate

Control of the transcription factor hypoxia inducible factor (HIF)-1 is mediated by hydroxylation by proline and asparagine hydroxylases. These enzymes require ascorbate for optimal activity, but little attention has been given to the effect of ascorbate on HIF-1 activation. Furthermore, cells in culture are ascorbate deficient. We investigated the effect of intracellular ascorbate on HIF-1alpha protein levels and on HIF-1-mediated gene expression in two human primary cell lines (umbilical vein endothelial cells and skin fibroblasts) and one human cancer cell line (A431 epithelial cells). Under normal culture conditions the cells contained no ascorbate and adding ascorbate to the medium increased intracellular concentrations in a dose-dependent manner. A basal level of HIF-1alpha detected in nonsupplemented cells under normoxic conditions disappeared when 10 microM ascorbate was added to the medium. Induction of HIF-1alpha by hypoxia (1% O(2)) or by CoCl(2) was markedly inhibited by ascorbate and loading with physiological levels resulted in almost complete reversal of HIF-1alpha stabilisation. Gene expression was similarly affected, with VEGF mRNA and GLUT-1 up-regulation being inhibited by ascorbate. Hence intracellular ascorbate is a major regulator of the hypoxic response in normal cells and optimal levels of this vitamin will have a profound effect on HIF-1-regulated processes.

Macromolecular crowding and its role as intracellular signalling of cell volume regulation

Macromolecular crowding has been proposed as a mechanism by means of which a cell can sense relatively small changes in volume or, more accurately, the concentration of intracellular solutes. According to the macromolecular theory, the kinetics and equilibria of enzymes can be greatly influenced by small changes in the concentration of ambient, inert macromolecules. A 10% change in the concentration of intracellular proteins can lead to changes of up to a factor of ten in the thermodynamic activity of putative molecular regulatory species, and consequently, the extent to which such regulator(s) may bind to and activate membrane-associated ion transporters. The aim of this review is to examine the concept of macromolecular crowding and how it profoundly affects macromolecular association in an intact cell with particular emphasis on its implication as a sensor and a mechanism through which cell volume is regulated.

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.

Intracellular pH regulation and Na+/H+ exchange activity in human hepatic stellate cells: effect of platelet-derived growth factor, insulin-like growth factor 1 and insulin

BACKGROUND/AIMS

The Na+/H+ exchanger is involved in rat hepatic stellate cell (HSC) proliferation induced by platelet-derived growth factor (PDGF). We therefore evaluated in human HSC: (1) the mechanisms of intracellular pH regulation; (2) the relationship between Na+/H+ exchange activation and cell proliferation induced by PDGF, insulin-like growth factor 1 (IGF-1) and insulin.

METHODS/RESULTS

pH(i) regulation was mainly dependent on the activity of the Na+/H+ exchanger, which was evaluated by measuring pH(i) recovery from an acute acid load. PDGF (25 ng/ml) gradually increased the activity of the Na+/H+ exchanger which peaked at 18 h and remained stable until the 24th h. IGF-1 (10 nmol/l), but not insulin (100 nmol/l), slightly but significantly increased the activity of the Na+/H+ exchanger. Amiloride (100 micromol/l) and 20 micromol/l 5-N-ethyl-N-isopropyl-amiloride completely inhibited HSC proliferation (evaluated by measurement of bromodeoxyuridine incorporation) induced by PDGF and IGF-1, but did not affect proliferation of HSC induced by insulin. Finally, IGF-1 did not modify the activity of the Na+/Ca2+ exchanger.

CONCLUSIONS

The Na+/H+ exchanger is involved in HSC proliferation induced by PDGF and IGF-1, whereas the proliferative effect of insulin is mediated by intracellular pathways which are Na+/H+ exchange-independent.

pH(i) responses to osmotic cell shrinkage in the presence of open-system buffers

Changes in plasma volume in vivo cause rapid changes in extracellular pH by altering the plasma bicarbonate concentration at a constant Pco(2) (Garella S, Chang BS, and Kahn SI. Kidney Int 8: 279, 1975). Few studies have examined the possibility that changes in cell volume produce comparable changes in intracellular pH (pH(i)). In the present study, alveolar macrophages were exposed to hyperosmotic medium in the absence or presence of the open-system buffers CO(2)-HCO(3)(-), propionic acid-propionate, or NH(3)-NH(4)(+). In the absence of open-system buffers, exposure to twice-normal osmolarity (2T) produced a slow cellular alkalinization [change in pH(i) (DeltapH(i)) approximately 0.38; exponential time constant (tau) approximately 120 s]. In the presence of 5% CO(2), 2T caused a biphasic pH(i) response: a rapid increase (DeltapH(i) approximately 0.10, tau approximately 15 s) followed by a slower pH(i) increase. Identical rapid pH(i) increases were produced by 2T in the presence of propionic acid (20 mM). Conversely, 2T caused a rapid pH(i) decrease (DeltapH(i) approximately -0.21, tau approximately 10 s) in the presence of NH(3) (20 mM). Thus osmotic cell shrinkage caused rapid pH(i) changes of opposite direction in the presence of a weak acid buffer (contraction alkalosis with CO(2) or propionic acid) vs. a weak base buffer (contraction acidosis with NH(3)). Graded DeltapH(i) were produced by varying extracellular osmolarity in the presence of open-system buffers; osmolarity increases of as little as 5-10% produced significant DeltapH(i). The rapid pH(i) responses to 2T were insensitive to inhibitors of membrane H(+) transport (ethylisopropylamiloride and bafilomycin A(1)). The results are consistent with shrinkage-induced disequilibria in the total cellular buffer system (i.e., intrinsic buffers plus added weak acid-base buffer).

Intracellular pathways mediating Na+/H+ exchange activation by platelet-derived growth factor in rat hepatic stellate cells

BACKGROUND & AIMS

The Na+/H+ exchanger is the main intracellular pH regulator in hepatic stellate cells (HSCs), and its activity is increased by platelet-derived growth factor (PDGF). Amiloride, an Na+/H+ exchange inhibitor, reduces PDGF-induced HSC proliferation, suggesting that the Na+/H+ exchanger plays a role in regulating HSC proliferative response. The aim of this study was to characterize the intracellular pathways mediating activation of the Na+/H+ exchanger by PDGF in HSCs.

METHODS

The activity of the Na+/H+ exchanger and HSC proliferation rate were evaluated under control condition and after incubation with PDGF in the absence or presence of specific inhibitors of the main intracellular pathways of signal transduction. Na+/H+ exchange protein expression was evaluated by means of Western blot.

RESULTS

PDGF induced a significant increase in the activity of the Na+/H+ exchanger without modifying protein expression. Inhibition of the calcium/calmodulin- and protein kinase C-dependent pathways resulted in a significant inhibition of both Na+/H+ exchange activity and of PDGF-induced HSC proliferation. The involvement of the two pathways was confirmed by showing that incubation of HSCs with both phorbol-12-myristate-13-acetate, a potent protein kinase C activator, and thapsigargin, which increases intracellular calcium levels, significantly increased both the Na+/H+ exchanger activity and HSC proliferation rate. Inhibition of the protein kinase A pathway did not modify either PDGF-induced Na+/H+ exchange activation or PDGF-induced HSC proliferation. On the contrary, inhibition of the mitogen-activated protein kinase- and of phosphatidylinositol 3-kinase-dependent pathways significantly reduced PDGF-induced HSC proliferation without affecting the activity of the Na+/H+ exchanger.

CONCLUSIONS

Activation of the Na+/H+ exchanger by PDGF in HSCs is mediated by calcium/calmodulin- and protein kinase C-dependent pathways. PDGF-induced HSC proliferation is mediated by Na+/H+ exchange-dependent and -independent pathways.

Fluorescence correlation microscopy of cells in the presence of autofluorescence

Fluorescence correlation microscopy (FCM), the combination of fluorescence correlation spectroscopy (FCS) and digital microscopy (Brock and Jovin, 1998. Cell. Mol. Biol. 44:847-856), has been implemented for measuring molecular diffusion and association in living cells with explicit consideration of autocorrelations arising from autofluorescence. Autofluorescence excited at 532 nm colocalizes with mitochondria, has flavin-like spectral characteristics, exhibits relaxation times characteristic for the diffusion of high-molecular-weight proteins, and depends on the incubation conditions of the cells. These time- and location-dependent properties preclude the assignment of universal background parameters. The lower limit for detection of microinjected dextran molecules labeled with the carboxymethylindocyanine dye Cy3 was a few thousand molecules per cell, and the diffusion constant of 1.7 x 10(-7) cm2/s agreed well with values measured with other methods. Based on the fluorescence signal per molecule (fpm) and the molecule number derived from autocorrelation analysis, a new method is devised to define intracellular association states. We conclude that FCM is a powerful, noninvasive method for probing molecular interactions in femtoliter volume elements within defined subcellular locations in living cells.

pH regulation and proton signalling by glial cells

The regulation of H+ in nervous systems is a function of several processes, including H+ buffering, intracellular H+ sequestering, CO2 diffusion, carbonic anhydrase activity and membrane transport of acid/base equivalents across the cell membrane. Glial cells participate in all these processes and therefore play a prominent role in shaping acid/base shifts in nervous systems. Apart from a homeostatic function of H(+)-regulating mechanisms, pH transients occur in all three compartments of nervous tissue, neurones, glial cells and extracellular spaces (ECS), in response to neuronal stimulation, to neurotransmitters and hormones as well as secondary to metabolic activity and ionic membrane transport. A pivotal role for H+ regulation and shaping these pH transients must be assigned to the electrogenic and reversible Na(+)-HCO3-membrane cotransport, which appears to be unique to glial cells in nervous systems. Activation of this cotransporter results in the release and uptake of base equivalents by glial cells, processes which are dependent on the glial membrane potential. Na+/H+ and Cl-/HCO3-exchange, and possibly other membrane carriers, accomplish the set of tools in both glial cells and neurones to regulate their intracellular pH. Due to the pH dependence of a great variety of processes, including ion channel gating and conductances, synaptic transmission, intercellular communication via gap junctions, metabolite exchange and neuronal excitability, rapid and local pH transients may have signalling character for the information processing in nervous tissue. The impact of H+ signalling under both physiological and pathophysiological conditions will be discussed for a variety of nervous system functions.

Stable intracellular acidification upon polyamine depletion induced by alpha-difluoromethylornithine or N1,N12-bis(ethyl)spermine in L1210 leukaemia cells

Polyamines play major roles in ionic and osmotic regulation, but their exact involvement in specific ion transport processes is poorly defined. Treatment of L1210 mouse leukaemia cells with either 5 mM alpha-difluoromethylornithine (DFMO), a suicide substrate of ornithine decarboxylase, or 25 microM N1,N12-bis(ethyl)spermine (BE-3-4-3), a dysfunctional polyamine analogue, caused a stable decreased in intracellular pH (pHi) by 0.1-0.4 unit from steady-state control values between 7.4 and 7.6, as measured either by partition of a weak acid or with a fluorescent pH-sensitive probe. This effect was not related to cell growth status or differences in metabolic acid generation, and was observed in either the presence or absence of HCO3-. Exogenous spermidine (10-25 microM) or putrescine (25-50 microM) fully reversed DFMO- or BE-3-4-3-induced acidification within 2 and 8 h respectively. Recovery of pHi in L1210 cells after a nigericin- or NH4(+)-mediated acid load in HCO3(-)-free buffers was mediated by Na+/H+ antiporter activity, in addition to a minor Na(+)-independent and amiloride-insensitive pathway. Decreased steady-state pHi was maintained in polyamine-depleted L1210 cells after recovery from acid stress. Moreover, the pHi-dependence of the rate of Na(+)-dependent H+ extrusion after an acid stress was altered by DFMO and BE-3-4-3, resulting in a set-point which was lower by 0.25-0.30 pH unit in polyamine-depleted cells. On the other hand, neither the rate nor the magnitude of Na+/H(+)-exchanger-mediated alkalinization induced by hypertonic shock was decreased by polyamine depletion. Thus polyamine depletion induces a persistent defect in pHi homeostasis which is due, at least in part, to a stable decrease in the pHi set-point of the Na+/H+ exchanger.

Chloride conductive pathways which support electrogenic H+ pumping by Leishmania major promastigotes

The proton extrusion mechanisms of Leishmania promastigotes were studied in terms of electrogenic movements of protons and anions (Cl- and HCO3-). Changes in membrane potential (Vm) and intracellular pH (pHi) were monitored fluorimetrically with the potential sensitive dye bis-oxonol and the pH-sensitive dye tetraacethoxymethyl 2',7'-bis-(carboxyethyl)-5,6-carboxyfluorescein, respectively. In nominal bicarbonate-free medium (pHe 7.4, 28 degrees C), Vm and pHi of Leishmania promastigotes were maintained at -113 +/- 4 mV and 6.75 +/- 0.02, respectively. In Cl- free (gluconate-based) medium, cells underwent a time-dependent acidification (0.3 pH units) and a long term membrane hyperpolarization (7-10 mV), both of which were greatly enhanced in the presence of the anion blocker, 4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonic acid (H2DIDS). Cells in Cl(-)-free medium underwent a marked depolarization upon treatment with the H(+)-ATPase inhibitor dicyclohexylcarbodiimide (DCCD), but hyperpolarized after repletion with Cl-. In Cl(-)-depleted cells, replenishment of Cl- led to a H2DIDS-sensitive cytoplasmic alkalinization and a small initial hyperpolarization. Cells exposed either to DCCD or to the H+ uncoupler carbonylcyanide chlorophenylhydrazone caused a marked cytoplasmic acidification and membrane depolarization. In the presence of 25 mM HCO3-, promastigotes maintained an almost neutral cytosol, irrespective of H+ pump action or ionic composition of the medium. The present observations provide evidence for the operation of a DCCD-sensitive electrogenic H(+)-ATPase which contributes to the maintenance of a highly hyperpolarized plasma membrane in Leishmania promastigotes. H+ pump activity required a parallel pathway of Cl- ions in order to dissipate the pump generated electrical potential. In nominally CO2-free media, the two electrogenic systems are implicated in the maintenance of cell pH and indirectly in electrochemically driven nutrient uptake. In physiological CO2/HCO3(-)-containing media, the H+ pump and Cl- channel play a role only secondary to that of HCO3- in pHi homeostasis.

Growth inhibition of rat glioma cells in vitro and in vivo by aspirin

The effect of non-steroidal anti-inflammatory drugs (NSAIDs), acetylsalicylic acid (commonly known as aspirin), salicylic acid, piroxicam and indomethacin on the growth of rat glioma cells (RG 2) in vitro and aspirin in vivo was studied. The in vitro studies reveal that aspirin and salicylic acid strongly inhibit growth of rat glioma (RG 2) cells in concentrations used in medicine for treatment of rheumatic diseases. On the other hand, indomethacin and piroxicam had no effect, indicating that the inhibitory effect on tumor growth is not due to the inhibition of prostaglandin synthesis. The synthesis of ATP was markedly reduced (34% of control) in the presence of drugs, whereas protein synthesis measured as 3H-leucine incorporation was slightly more inhibited (73% of control) than cell growth. Aspirin administered to Fischer 344 rats inhibited growth of RG 2 cells inoculated into the caudate nucleus in vivo, both when administered the day before inoculation of tumor cells and when tumors had formed, i.e. 5 days post inoculation.

Characterization of signaling pathways to Na+/H+ exchanger activation with epidermal growth factor in hepatocytes

To investigate the signaling pathways to Na+/H+ exchanger activation with epidermal growth factor in hepatocytes, we measured changes in cytosolic free calcium and intracellular pH levels at the single-cell level using digital imaging fluorescence microscopy of fura-2- or BCECF-loaded hepatocytes in primary culture. Epidermal growth factor induced cytosolic free calcium oscillations consisting of periodic trains of spikes with a latency period of up to several minutes. These calcium responses were inhibited by tyrosine kinase inhibitor genistein (100 mumol/L) and abolished by emptying of intracellular Ca2+ pools with 3 mumol/L thapsigargin, an inhibitor of Ca(2+)-ATPase on the endoplasmic reticulum. Epidermal growth factor (1 nmol/L) induced an intracellular pH increase of 0.12 +/- 0.07 units from the basal level of 7.25 +/- 0.09 units after several minutes of latency. This effect was completely abolished by 1 mmol/L amiloride, an inhibitor of the Na+/H+ exchanger. The epidermal growth factor-induced intracellular pH increase was inhibited by pretreatment of hepatocytes with genistein (100 mumol/L), thapsigargin (3 mumol/L) or calmodulin inhibitor W-7 (25 mumol/L), but not with protein kinase C inhibitor H-7 (50 mumol/L) or with cyclic AMP-dependent kinase inhibitor H-8 (60 mumol/L). Phorbol ester PMA (phorbol 12-myristate 13-acetate), a potent activator of protein kinase C, induced a slight intracellular pH increase significantly smaller than that with epidermal growth factor, whereas this effect was completely blocked by pretreatment with H-7, indicating that PMA-induced intracellular pH increase is mediated by protein kinase C pathways, unlike epidermal growth factor.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperosmotic activation of the Na(+)-H+ exchanger in a rat bone cell line: temperature dependence and activation pathways

1. The hyperosmotic activation of the Na(+)-H+ exchanger was studied in an osteoblast-like rat cell line (RCJ 1.20). The activation was monitored by recording the intracellular pH (pHi) changes employing double excitation of the pH-sensitive fluorescent dye 2'7'-bis(carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM). 2. Exposure of the cells to a hyperosmotic HCO(3-)-free medium at 37 degrees C produced an initial cytosolic acidification of 0.05 pH units followed by a lag period and an alkalinization overshoot of about 0.2 pH units, without a concomitant change of the free cytosolic calcium [Ca2+]i by the use of Fura-2 calcium-sensitive probes. This response was completely inhibited by amiloride (0.33 mM) or by Na+ depletion from the external medium and insensitive to the extracellular Cl- replacement, indicating the involvement of a Na(+)-H+ exchanger in the hyperosmotic response. 3. Hyperosmotic stimuli (200 moSM sucrose) applied in the temperature range of 17-37 degrees C demonstrated a shortening of the lag period preceding alkalinization and an increased rate of proton extrusion upon temperature elevation. The biochemical reaction underlying the lag period and the proton extrusion resulted in apparent activation energies of 19 and 29 kcal mol-1, respectively, as calculated from the appropriate Arrhenius plots. 4. Stimulation of the exchanger under isosmotic conditions by 25 nM 4 beta-phorbol 12-myristate 13-acetate (PMA) and 0.1 mM vanadate resulted in an amiloride-sensitive pHi increase of about 0.08 pH units. The hyperosmotic stress was additive to the stimulatory effects of these agents, suggesting an independent hyperosmotic activation pathway. 5. The hyperosmotic activation of the Na(+)-H+ exchanger was independent of cAMP, cGMP, cytosolic Ca2+ and protein kinase C. Thus, none of the classical transduction mechanisms seem to be involved directly in the hyperosmotic activation of the antiporter. 6. The pHi response induced by the hyperosmotic stress was abolished by two calmodulin inhibitors, W-7 and chlorpromazine (50% inhibition, Ki at 28 and 20 microM, respectively), 20 microM cytochalasin B, but not by 10 microM colchicine. The results suggest the involvement of actin and calmodulin-like structural elements of the cytoskeleton in the transduction process leading to the activation of the Na(+)-H+ exchanger.

Na(+)-H+ exchanger subtypes: a predictive review

In recent years, many reports have appeared describing distinct heterogeneity of proteins that heretofore were considered to be a single species or type. The division of proteins into different classes or subtypes is aided by pharmacological tools such as selective ligands, functional measurements such as those examining kinetic or regulatory differences, and molecular biological approaches that have identified distinct genes coding for similar yet distinguishable gene products. Currently, much effort is directed toward understanding the significance of these sometimes subtle differences in terms of functional consequences for the cells in which they exist. Although most reports to date involve hormone and neurotransmitter receptor subtypes, it is also possible that other cell surface molecules such as ion transporters exist as multiple subtypes. In this paper we review the current evidence that Na(+)-H+ exchange activity is mediated by different Na(+)-H+ exchanger subtypes. Although subtypes have not been identified with certainty, we can predict certain distinguishing characteristics that these putative subtypes may have that may be of value in correlating predicted gene products obtained from cDNA cloning with previously characterized Na(+)-H+ exchangers.

The presence of CO2/HCO3- is essential for hypoxic chemotransduction in the in vivo perfused carotid body

Carotid chemoreceptor activity was increased by the perfusion of the carotid body in vivo with hypoxic HEPES-buffered solution (HBS) containing CO2/HCO3- (HBA+), but not with hypoxic HBS without CO2/HCO3- (HBS-). When the perfusate was switched to hypoxic HBS+ during hypoxic HBS-perfusions, chemoreceptor activity increased immediately. Thus, CO2/HCO3- played a critical role in the hypoxic chemotransduction of the in vivo perfused carotid body.

Evidence for involvement of protein kinase C in regulation of intracellular pH by Cl-/HCO-3 antiport

The activity of the main base-extruding mechanism in Vero cells, the Na(+)-independent Cl-/HCO-3 antiport, increases 5- to 10-fold when the cytosolic pH (pHi) is increased over a narrow range close to neutrality. We have studied the effect on this regulation of stimulation and inhibition of protein kinase C by short-term and long-term treatment with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA). After short-term treatment with TPA to stimulate the kinase, the threshold value for activation of the antiport is shifted to a more acidic pH. After prolonged treatment with TPA to downregulate protein kinase C the sensitivity of the antiport to variation in proton concentration was lowered, possibly by reducing the number of essential proton-binding sites. Concomitantly, the steady state pHi of the cells was increased. The data indicate that protein kinase C is involved in the regulation of the Na(+)-independent Cl-/HCO-3 antiport.

Tumorigenic 3T3 cells maintain an alkaline intracellular pH under physiological conditions

One of the earliest events in the response of mammalian cells to mitogens is activation of Na+/H+ exchange, which increases intracellular pH (pHin) in the absence of HCO3- or at external pH values below 7.2. The proliferative response can be blocked by preventing the pHin increase; yet, the proliferative response cannot be stimulated by artificially raising pHin with weak bases or high medium pH. These observations support the hypothesis that optimal pHin is a necessary, but not sufficient, component of the proliferative-response sequence. This hypothesis has recently been challenged by the observation that transfection of NIH 3T3 cells with yeast H(+)-ATPase renders them tumorigenic. Although previous measurements indicated that these transfected cells maintain a higher pHin in the absence of HCO3-, whether H(+)-ATPase transfection raised the pHin under physiologically relevant conditions was not known. The current report shows that these transfected cells do maintain a higher pHin than control cells in the presence of HCO3-, supporting the possibility that elevated pHin is a proliferative trigger in situ. We also show that these cells are serum-independent for growth and that they glycolyze much more rapidly than phenotypically normal cells.

Ionic control of intracellular pH in rat cerebellar Purkinje cells maintained in culture

1. Intracellular pH (pHi) was measured in single rat cerebellar Purkinje cells maintained in primary culture using microspectrofluorescence analysis of the intracellularly trapped pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5 (and -6)-carboxyfluorescein (BCECF). 2. The ratio of the fluorescence signals measured at 530 nm in response to an alternating excitation at 450 and 490 nm was calibrated using the K(+)-H+ ionophore nigericin. This calibration gave a steady-state pHi of 7.06 +/- 0.02 (S.E.M., n = 17) when cells were perfused by a 5% CO2-25 mM-HCO3(-)-buffered solution at an external pH of 7.40 at 37 degrees C. 3. Replacement of external chloride with gluconate in the presence of bicarbonate induced a cytoplasmic alkalinization of about 0.3 pH unit. This alkalinization was independent of external sodium and was greatly reduced by 0.5 mM-DIDS, indicating the presence of a chloride-bicarbonate exchange. 4. In bicarbonate-free (HEPES-buffered) solution the steady-state pHi was 7.37 +/- 0.02 (n = 19), significantly higher than in bicarbonate-buffered solution. Recovery from an intracellular acid load brought about by the ammonium chloride pre-pulse technique was blocked by the removal of external sodium or the addition of 1.5 mM-amiloride, indicating the presence of a sodium-hydrogen exchange. 5. In bicarbonate-buffered solution pHi recovery after an acid load was also completely blocked by addition of 1.5 mM-amiloride indicating the absence of a bicarbonate-dependent acid extrusion mechanism. 6. Addition of 12-O-tetradecanoylphorbol-13-acetate (TPA, 100 nM) induced an amiloride-sensitive alkalinization of about 0.3 pH unit in bicarbonate-buffered solution but had no effect in HEPES-buffered solution. This observation suggests that in cultured Purkinje cells the sodium-hydrogen exchanger could be activated through a protein kinase C pathway only when pHi is maintained at a low physiological value by the activity of the chloride-bicarbonate exchange.

Regulation of Na+/H+ and Cl-/HCO3- antiports in Vero cells

The effect of serum, phorbol-12-myristate-13-acetate (TPA), and forskolin on the activity Na+/H+ antiport and the Na(+)-coupled and Na(+)-independent Cl-/HCO3- antiport was studied in Vero cells by measuring 22Na+ and 36Cl- fluxes and changes in cytosolic pH (pHi). The Na(+)-independent Cl-/HCO3- antiport, which acts as an acidifying mechanism, is strongly pH-sensitive. In serum-starved cells it is activated at alkaline cytosolic pH, with a half-maximal activity at pHi approximately 7.20. Incubation with serum increased the activity of the Na(+)-independent Cl-/HCO3- antiport at pHi values from 6.8 to 7.2. Thus serum appeared to alter the pHi sensitivity of this antiporter such that the threshold value for activation of the antiport was shifted to a more acidic value. Na+/H+ antiport was somewhat stimulated initially by addition of serum, but further incubation with serum (greater than 45 min) decreased its activity. The activity of the Na(+)-coupled Cl-/HCO3- antiport, which is the major alkalinizing antiport in Vero cells, was not altered by short-term incubation with serum (less than 10 min) but decreased after prolonged incubation (greater than 45 min). Our findings with TPA and forskolin indicate that the effect of serum is partly mediated by the protein kinase C pathway, whereas the cyclic adenosine monophosphate pathway does not appear to play an important role. The net effect of serum on the pHi-regulating antiports was a slight decrease in intracellular pH.

Distinct endocytotic pathways in epidermal growth factor-stimulated human carcinoma A431 cells

Addition of EGF to human epidermoid carcinoma A431 cells increases the rate of fluid-phase pinocytosis 6-10-fold as measured by horseradish peroxidase uptake (Haigler, H.T., J. A. McKanna, and S. Cohen. 1979. J. Cell Biol. 83:82-90). We show here that in the absence of extracellular Na+ or in the presence of amiloride the stimulation of pinocytosis by EGF is substantially reduced. Amiloride had no effect on the endocytosis of EGF itself or of transferrin, demonstrating that the receptor-mediated endocytotic pathway operated normally under conditions that blocked stimulated pinocytosis. Amiloride blocked EGF-stimulated pinocytosis in both HCO3(-)-containing and HCO3(-)-free media. The EGF-stimulated pinocytotic activity can frequently be localized to areas of the cell where membrane spreading and ruffling are taking place. These results demonstrate that (a) EGF induces a distinct amiloride-sensitive endocytotic pathway on A431 cells; (b) occupied EGF receptors do not utilize this pathway for their own entry; (c) endocytosis of occupied EGF receptors is not in itself sufficient to stimulate pinocytosis.

Heterogeneity of the changes in cytoplasmic pH upon serum stimulation of quiescent fibroblasts

Addition of mitogens to quiescent cells results in rapid ionic changes in the cytoplasm, including pH. We studied the changes in cytoplasmic pH in single Swiss 3T3 cells upon serum stimulation using fluorescence ratio imaging microscopy. Quiescence was attained using two approaches, serum deprivation of subconfluent cells and confluence. All measurements were made in the presence of bicarbonate and the absence of other organic buffers. We also used BCECF coupled to dextran to avoid several artifacts associated with using BCECF-AM, including leakage and phototoxicity. Analysis of the changes in cytoplasmic pH demonstrated a dramatic heterogeneity in the responses of single cells. There were six basic classes of responses, 1) a fast alkalinization, reaching a maximum pH in approximately 2-5 min; 2) a slow alkalinization, reaching a maximum pH in 10-20 min; 3) a very slow alkalinization, not reaching a plateau pH within the measurement time; 4) no apparent change in pH during the measurement time; 5) an early transient acidification, followed by either a fast or slow alkalinization; and 6) an acidification, followed by alkalinization and then by a decrease to some intermediate pH. Subconfluent cells exhibited greater heterogeneity in response than confluent cells, with no single dominant class of response. The dominant (55%) response for confluent cells was a gradual alkalinization of approximately 0.01 pH units/min. A larger proportion (52%) of subconfluent cells exhibited an early transient acidification compared to confluent cells (7%). A significant proportion of both types of cells (23% subconfluent, 36% confluent) exhibited no change in cytoplasmic pH upon stimulation. In general, the kinetics of changes in cytoplasmic pH were significantly different from the published results with population averaging methods.

Bicarbonate-dependent and -independent intracellular pH regulatory mechanisms in rat hepatocytes. Evidence for Na+-HCO3- cotransport

Using the pH-sensitive dye 2,7-bis(carboxyethyl)-5(6)-carboxy-fluorescein and a continuously perfused subconfluent hepatocyte monolayer cell culture system, we studied rat hepatocyte intracellular pH (pHi) regulation in the presence (+HCO3-) and absence (-HCO3-) of bicarbonate. Baseline pHi was higher (7.28 +/- 09) in +HCO3- than in -HCO3- (7.16 +/- 0.14). Blocking Na+/H+ exchange with amiloride had no effect on pHi in +HCO3- but caused reversible 0.1-0.2-U acidification in -HCO3- or in +HCO3- after preincubation in the anion transport inhibitor 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene (DIDS). Acute Na+ replacement in +HCO3- alos caused acidification which was amiloride independent but DIDS inhibitible. The recovery of pHi from an intracellular acid load (maximum H+ efflux rate) was 50% higher in +HCO3- than in -HCO3-. Amiloride inhibited H+ effluxmax by 75% in -HCO3- but by only 27% in +HCO3-. The amiloride-independent pHi recovery in +HCO3- was inhibited 50-63% by DIDS and 79% by Na+ replacement but was unaffected by depletion of intracellular Cl-, suggesting that Cl-/HCO3- exchange is not involved. Depolarization of hepatocytes (raising external K+ from 5 to 25 mM) caused reversible 0.05-0.1-U alkalinization, which, however, was neither Na+ nor HCO3- dependent, nor DIDS inhibitible, findings consistent with electroneutral HCO3- transport. We conclude that Na+-HCO3- cotransport, in addition to Na+/H+ exchange, is an important regulator of pHi in rat hepatocytes.

Epidermal growth factor activates calcium-permeable channels in A 431 cells

The patch clamp technique in a cell-attached configuration was used to search for calcium-permeable channels in human carcinoma A 431 cells. Unitary inward currents were recorded with 100 mM CaCl2 in a pipette, with the mean slope conductance of 2.8 pS and a reversal potential (obtained by extrapolation) of +25.5 mV. Application of epidermal growth factor (EGF) into the extracellular solution produced a transient increase in the probability of these channels being open. The effect develops with delay of about 20 s and lasts thereafter for 36 s (mean values). We propose that these channels mediate an EGF-induced increase in the concentration of cytosolic free calcium.

Arginine vasopressin enhances pHi regulation in the presence of HCO3- by stimulating three acid-base transport systems

Growth factors raise intracellular pH (pHi) by stimulating Na+/H+ exchange in the absence of HCO3-. In mutant cells that lack the Na+/H+ exchange activity, this alkalinization does not occur, and the cells do not proliferate without artificial elevation of pHi. It has therefore been widely suggested that an early pHi increase is a necessary signal for mitogenesis. In the presence of HCO3- however, growth factors fail to raise pHi in A431 cells, renal mesangial cells and 3T3 fibroblasts. In mesangial cells, arginine vasopressin (AVP) raises pHi in the absence of HCO3-, but lowers it when HCO3- is present; growth is stimulated under both conditions. We report here that, in the presence of HCO3-, AVP stimulates two potent HCO3- transporters, as well as the Na+/H+ exchanger. These are the Na+-dependent and Na+-independent Cl-/HCO3- exchangers. Our results indicate that AVP causes acidification in the presence of HCO3- because, at the resting pHi, it stimulates Na+-independent Cl-/HCO3- exchange (which lowers pHi) more than it stimulates the sum of Na+/H+ exchange and Na+-dependent Cl-/HCO3- exchange (both of which raise pHi). The stimulation of three acid-base transporters by the growth factor AVP greatly enhances the ability of the cell to regulate pHi.

Bicarbonate abolishes intracellular alkalinization in mitogen-stimulated 3T3 cells

An increase in intracellular pH (pHi) following mitogenic stimulation has been reported in a variety of mammalian cells (W. Moolenaar, Annu. Rev. Physiol., 48:363-376, 1986; E. Rozengurt, Science, 234:161-166, 1986). This increase is currently believed to constitute a "permissive" signal in the process of cell activation (A.E. Lagarde and J.M. Pouyssegur, Cancer Biochem. Biophys. 9:1-14, 1986). Since the majority of studies of this phenomenon have been conducted in the nonphysiological milieu of bicarbonate-free solutions, we have undertaken a study of the effects of bicarbonate and CO2 on mitogen-induced intracellular alkalinization in NIH 3T3 cells. Using nuclear magnetic resonance (NMR) spectroscopy and novel 31P NMR pH indicators (2-amino-phosphono-carboxylic acids) we found that mitogen induces an increase in pHi of 0.16 units only in cells bathed in medium containing low concentrations of bicarbonate (less than 1 mM) and not in cells bathed in medium containing physiological levels of bicarbonate (10-30 mM). In addition to abolishing the mitogen-induced alkalinization, bicarbonate stabilizes pHi at 7.25 units as the external pH (pHe) is varied from 7.0 to 7.6. In contrast, in a bicarbonate-free medium pHi increases from 6.9 to 7.3 over the same range of external pHs. At a constant external pH, increasing the bicarbonate/CO2 concentration results in an increase in pHi from 6.9 in bicarbonate-free solution to 7.25 in a bicarbonate-buffered medium. This relationship is hyperbolic with half-maximal effect occurring at a concentration of 0.4 mM bicarbonate at pH 7.05 and 37 degrees C. Our results suggest that the observations of mitogen-induced alkalinization may be due to the use of nonphysiological bicarbonate-free media. Since this increase in pHi is not observed in physiological media where bicarbonate concentrations are usually greater than 20 mM, we conclude that an increase in pHi is not an obligatory or usual part of the cellular response to growth factors in vivo.

Cytomegalovirus: sodium entry and development of cytomegaly in human fibroblasts

A possible relationship between net Na+ entry and the development of CMV-induced cytomegaly (cell enlargement) was investigated in human fibroblasts derived from skin-muscle and thyroid tissue. We found that inhibiting cellular Na+ uptake, either by pharmacological means (amiloride, an inhibitor of Na+/H+ exchange) or by replacement of extracellular Na+ (by N-methyl-D-glucamine or choline), inhibited the development of cytomegaly. Furthermore, we noted a temporal parallelism between the development of cytomegaly and enhancement of ouabain-sensitive (O-S) 86Rb+ uptake. O-S 86Rb+ uptake is a monitor for the activity of the sodium pump resident in the plasmalemma of the fibroblasts. The enhanced O-S 86Rb+ uptake reflects either an increased intracellular Na+ concentration or an increased number of sodium pump complexes per fibroblast. Amiloride inhibited the enhancement of O-S 86Rb+ uptake, as well as cytomegaly development. Addition of amiloride at selected times after infection suggested that the same phase of virus replication was sensitive to the inhibitory effect of this drug on the enhancement of O-S 86Rb+ uptake and on the development of cytomegaly. There was also a similar pattern of inhibition of O-S 86Rb+ uptake and cytomegaly with increasing concentrations of amiloride. Thus, there may be a relationship between CMV-induced Na+ entry through activation of the Na+/H+ exchanger and development of cytomegaly.

The regulation of the intracellular pH in cells from vertebrates

Eukaryotic cells control their intracellular pH using ion-transporting systems that are situated in the plasma membrane. This paper describes the different mechanisms that are involved and how their activity is regulated.

Leucine transport-induced activation of the Na+/H+ exchanger in human peripheral lymphocytes

Adjustment of amino-acid-induced cytoplasmic pH decrease by the Na+/H+ exchange system in human lymphocytes has been studied using a fluorometric technique to monitor the intracellular pH change. When the interior of lymphocytes is acidified by addition of nigericin to medium, cytoplasmic pH is immediately corrected toward its resting value. This recovery of the cytoplasmic pH depends on extracellular Na+ and is inhibited by amiloride. A temporary (less than 2 min) decrease in the cytoplasmic pH, followed by a slow recovery phase, was observed in incubation with 1.0 mM leucine in Na+-containing medium. This leucine-dependent decrease of cytoplasmic pH persisted longer when amiloride was added to the medium. Cytoplasmic pH recovery from the leucine-induced acidification depends on external Na+ concentration. Amiloride-sensitive Na+/H+ exchanger was stimulated by 12-O-tetradecanoylphorbol 13-acetate (TPA) in the lymphocytes and preincubation of the cells with TPA partially prevented the leucine-induced cytoplasmic acidification. We conclude that human peripheral lymphocytes are provided with an amino acid-H+ cotransport system, which is cooperatively coupled to the amiloride-sensitive Na+/H+ exchanger to correct the cytoplasmic pH anomaly.

Effect of harvesting methods, growth conditions and growth phase on diacylglycerol levels in cultured human adherent cells

The cellular mass of sn-1,2-diacylglycerols, which are intracellular second messengers which activate protein kinase C, were quantitatively determined with an enzymatic assay. The method employed to harvest cultured human skin fibroblasts or human epidermal A431 cells prior to extraction of lipid into chloroform/methanol affected diacylglycerol (DAG) levels. Scraping or trypsinization significantly increased DAG levels. A method was devised to allow reliable and reproducible DAG measurements from adherent cells. The addition of methanol prior to scraping was shown to stop cellular metabolism and to permit accurate quantitation. Importantly, this solvent was compatible with cultures grown on plastic. Using this method, growth conditions which could affect DAG levels were investigated. Changes in the osmolality of the culture medium did not affect the DAG levels of A431 cells; exposure of A431 cells to acidic pH or elevated temperature lowered DAG levels. In contrast to fibroblasts, the total DAG levels of A431 cells continued to increase during serum deprivation. The highest DAG levels, normalized to phospholipids, were observed during the exponential growth phase. This ratio dropped when the cultures reached confluency. These experiments also demonstrated that A431 cells possess higher DAG levels than do normal fibroblasts. The function of DAG in cellular regulation is discussed.

Amiloride and amiloride analogs inhibit Na+/K+-transporting ATPase and Na+-coupled alanine transport in rat hepatocytes

Amiloride, a commonly used inhibitor of Na+-H+ exchange, has been shown to exhibit a variety of nonspecific effects. Recently, the more potent amiloride analogs, 5-(N,N-dimethyl)amiloride hydrochloride (DMA) and 5-(N-ethyl-N-isopropyl)amiloride (EIA), have been used to control for the nonspecific effects of the parent compound. In the present study, we have explored the effects of these analogs on Na+/K+-transporting ATPase (Na+/K+-ATPase) and Na+-coupled alanine transport in primary rat hepatocyte cultures and rat liver plasma membranes, and we have compared the effects of these analogs with the effects of amiloride and ouabain. Amiloride, DMA, and EIA increased steady-state Na+ content and inhibited ouabain-sensitive 86Rb+ uptake in a reversible, concentration-dependent, ouabain-like manner, with estimated 50% inhibitory concentrations (IC50) of 3.0.10(-3) M, 5.2.10(-4) M, and 1.2.10(-4) M, respectively. Amiloride, DMA and EIA also inhibited ouabain-sensitive ATP hydrolysis in rat liver plasma membranes with similar potency (IC50 values of 2.2.10(-3) M, 2.2.10(-3) M, and 1.7.10(-4) M, respectively). In separate experiments, amiloride (5.10(-3) M), DMA (10(-3) M), and EIA (2.5.10(-4) M) decreased the uptake into hepatocytes of alanine by 20%, 61%, and 59%, respectively, and further studies with DMA (10(-3) M) demonstrated that this inhibition was largely due to a decrease in the Na+-dependent fraction of alanine uptake. These findings indicate that amiloride, DMA, and EIA inhibit hepatic Na+/K+-ATPase directly, reversibly, and with a relative rank order potency of EIA greater than DMA greater than amiloride. All three compounds also inhibit the hepatic uptake of alanine, and presumably could indirectly inhibit other Na+-coupled transport processes as well.

Control of ion fluxes by mitogenic polypeptides

Several ion fluxes are stimulated when mitogenic polypeptides are added to cells. The precise mechanism by which this activation takes place is not understood, but compelling evidence exists in the case of the activation of sodium-hydrogen exchange that it requires the tyrosine kinase activity associated with the mitogen receptor. The activation of sodium-hydrogen exchange by mitogens is associated with changes in intracellular pH that appear to be permissive but not causal in allowing cells to proceed through the cell cycle. When added to cells, mitogens also activate protein kinase C, which acts as part of a feedback loop to control the activity of the mitogen receptor. Possible mechanisms for this control are discussed.