Intracellular acidification inhibits the proliferative response in BALB/c-3T3 cells

The Ph paradigm in cancer

Malignant tissues show a peculiar feature regarding pH: while normal tissues have a higher extracellular pH than intracellular pH, in cancer is exactly the opposite. This phenomenon is called the inversion of the pH gradient and is now considered a hallmark of malignancy. For some time, this inverted pH gradient was believed to be a secondary effect of cancer. Now, it is becoming clear that pH inversion is not an innocent consequence, but a key player in the etiopathogenesis of cancer. Therefore, addressing this issue as part of an integral treatment of neoplasia should be a necessary step for improving cancer patients' outcomes. However, the knowledge acquired in this regard through basic research has not reached bedside treatments. The most striking fact is that there are repurposed drugs and nutraceuticals with low or no toxicity that can modify the pH gradient inversion. However, these drugs have not even been tested in cancer treatment.

An Arabidopsis Mutant Over-Expressing Subtilase SBT4.13 Uncovers the Role of Oxidative Stress in the Inhibition of Growth by Intracellular Acidification

Intracellular acid stress inhibits plant growth by unknown mechanisms and it occurs in acidic soils and as consequence of other stresses. In order to identify mechanisms of acid toxicity, we screened activation-tagging lines of Arabidopsis thaliana for tolerance to intracellular acidification induced by organic acids. A dominant mutant, sbt4.13-1D, was isolated twice and shown to over-express subtilase SBT4.13, a protease secreted into endoplasmic reticulum. Activity measurements and immuno-detection indicate that the mutant contains less plasma membrane H⁺-ATPase (PMA) than wild type, explaining the small size, electrical depolarization and decreased cytosolic pH of the mutant but not organic acid tolerance. Addition of acetic acid to wild-type plantlets induces production of ROS (Reactive Oxygen Species) measured by dichlorodihydrofluorescein diacetate. Acid-induced ROS production is greatly decreased in sbt4.13-1D and atrboh-D,F mutants. The latter is deficient in two major NADPH oxidases (NOXs) and is tolerant to organic acids. These results suggest that intracellular acidification activates NOXs and the resulting oxidative stress is important for inhibition of growth. The inhibition of acid-activated NOXs in the sbt4.13-1D mutant compensates inhibition of PMA to increase acid tolerance.

The acid-base transport proteins NHE1 and NBCn1 regulate cell cycle progression in human breast cancer cells

Precise acid-base homeostasis is essential for maintaining normal cell proliferation and growth. Conversely, dysregulated acid-base homeostasis, with increased acid extrusion and marked extracellular acidification, is an enabling feature of solid tumors, yet the mechanisms through which intra- and extracellular pH (pH_i, pH_e) impact proliferation and growth are incompletely understood. The aim of this study was to determine the impact of pH, and specifically of the Na⁺/H⁺ exchanger NHE1 and Na⁺, HCO₃^- transporter NBCn1, on cell cycle progression and its regulators in human breast cancer cells. Reduction of pH_e to 6.5, a common condition in tumors, significantly delayed cell cycle progression in MCF-7 human breast cancer cells. The NHE1 protein level peaked in S phase and that of NBCn1 in G2/M. Steady state pH_i changed through the cell cycle, from 7.1 in early S phase to 6.8 in G2, recovering again in M phase. This pattern, as well as net acid extrusion capacity, was dependent on NHE1 and NBCn1. Accordingly, knockdown of either NHE1 or NBCn1 reduced proliferation, prolonged cell cycle progression in a manner involving S phase prolongation and delayed G2/M transition, and altered the expression pattern and phosphorylation of cell cycle regulatory proteins. Our work demonstrates, for the first time, that both NHE1 and NBCn1 regulate cell cycle progression in breast cancer cells, and we propose that this involves cell cycle phase-specific pH_i regulation by the two transporters.

Roles of pH in control of cell proliferation

Precise spatiotemporal regulation of intracellular pH (pH_i ) is a prerequisite for normal cell function, and changes in pH_i or pericellular pH (pH_e ) exert important signalling functions. It is well established that proliferation of mammalian cells is dependent on a permissive pH_i in the slightly alkaline range (7.0-7.2). It is also clear that mitogen signalling in nominal absence of HCO3- is associated with an intracellular alkalinization (~0.3 pH unit above steady-state pH_i ), which is secondary to activation of Na⁺ /H⁺ exchange. However, it remains controversial whether this increase in pH_i is part of the mitogenic signal cascade leading to cell cycle entry and progression, and whether it is relevant under physiological conditions. Furthermore, essentially all studies of pH_i in mammalian cell proliferation have focused on the mitogen-induced G0-G1 transition, and the regulation and roles of pH_i during the cell cycle remain poorly understood. The aim of this review is to summarize and critically discuss the possible roles of pH_i and pH_e in cell cycle progression. While the focus is on the mammalian cell cycle, important insights from studies in lower eukaryotes are also discussed. We summarize current evidence of links between cell cycle progression and pH_i and discuss possible pH_i - and pH_e sensors and signalling pathways relevant to mammalian proliferation control. The possibility that changes in pH_i during cell cycle progression may be an integral part of the checkpoint control machinery is explored. Finally, we discuss the relevance of links between pH and proliferation in the context of the perturbed pH homoeostasis and acidic microenvironment of solid tumours.

Epi-reevesioside F inhibits Na+/K+-ATPase, causing cytosolic acidification, Bak activation and apoptosis in glioblastoma

Epi-reevesioside F, a new cardiac glycoside isolated from the root of Reevesia formosana, displayed potent activity against glioblastoma cells. Epi-reevesioside F was more potent than ouabain with IC₅₀ values of 27.3±1.7 vs. 48.7±1.8 nM (P < 0.001) and 45.0±3.4 vs. 81.3±4.3 nM (P < 0.001) in glioblastoma T98 and U87 cells, respectively. However, both Epi-reevesioside F and ouabain were ineffective in A172 cells, a glioblastoma cell line with low Na⁺/K⁺-ATPase α3 subunit expression. Epi-reevesioside F induced cell cycle arrest at S and G2 phases and apoptosis. It also induced an increase of intracellular concentration of Na⁺ but not Ca²⁺, cleavage and exposure of N-terminus of Bak, loss of mitochondrial membrane potential, inhibition of Akt activity and induction of caspase cascades. Potassium supplements significantly inhibited Epi-reevesioside F-induced effects. Notably, Epi-reevesioside F caused cytosolic acidification that was highly correlated with the anti-proliferative activity. In summary, the data suggest that Epi-reevesioside F inhibits Na⁺/K⁺-ATPase, leading to overload of intracellular Na⁺ and cytosolic acidification, Bak activation and loss of mitochondrial membrane potential. The PI3-kinase/Akt pathway is inhibited and caspase-dependent apoptosis is ultimately triggered in Epi-reevesioside F-treated glioblastoma cells.

Effects of Extracellular pH on Dental Pulp Cells In Vitro

INTRODUCTION

The proliferation and migration of dental pulp stem cells (DPSCs), a population comprised of dental pulp cells (DPCs), are important processes for pulp tissue repair. Dental pulp is exposed to changes in extracellular pH under various conditions, such as acidosis and exposure to caries-associated bacteria or a pulp capping agent. The objective of this study was to investigate the effects of extracellular pH on DPC proliferation and migration in vitro.

METHODS

To evaluate the proliferation potency of DPCs in various extracellular pH conditions, 2 × 10(4) cells were seeded into 35-mm dishes. The following day, we changed to NaHCO3-free medium, which was adjusted to different extracellular pH levels.

RESULTS

After 120 hours, DPCs cultured in media from a pH of 3.5 to 5.5 showed cell death, those cultured in conditions from a pH of 6.5 to 7.5 showed growth arrest or cell death, and those grown at a pH of 9.5 showed mild proliferation. The migratory activity of living DPCs was not affected by extracellular pH. For histologic analysis, human teeth possessing a small abscess in the coronal pulp chamber were sliced for histologic analysis. Proliferating cell nuclear antigen (PCNA) immunolocalization was used as an index of cell proliferation for the sections and cultured cells. Acidic extracellular pH conditions resulted in reduced numbers of PCNA-positive DPCs in the dishes. As for pulp tissue affected by a small abscess, a PCNA-negative pulp cell layer was observed in close proximity to the infectious lesion.

CONCLUSIONS

Together, these results suggest that an acidic extracellular pH condition is associated with DPC growth arrest or cell death.

Decreased astroglial monocarboxylate transporter 4 expression in temporal lobe epilepsy

Efflux of monocaroxylates like lactate, pyruvate, and ketone bodies from astrocytes through monocarboxylate transporter 4 (MCT4) supplies the local neuron population with metabolic intermediates to meet energy requirements under conditions of increased demand. Disruption of this astroglial-neuron metabolic coupling pathway may contribute to epileptogenesis. We measured MCT4 expression in temporal lobe epileptic foci excised from patients with intractable epilepsy and in rats injected with pilocarpine, an animal model of temporal lobe epilepsy (TLE). Cortical MCT4 expression levels were significantly lower in TLE patients compared with controls, due at least partially to MCT4 promoter methylation. Expression of MCT4 also decreased progressively in pilocarpine-treated rats from 12 h to 14 days post-administration. Underexpression of MCT4 in cultured astrocytes induced by a short hairpin RNA promoted apoptosis. Knockdown of astrocyte MCT4 also suppressed excitatory amino acid transporter 1 (EAAT1) expression. Reduced MCT4 and EAAT1 expression by astrocytes may lead to neuronal hyperexcitability and epileptogenesis in the temporal lobe by reducing the supply of metabolic intermediates and by allowing accumulation of extracellular glutamate.

Glutamate release from astrocyte cell-line GL261 via alterations in the intracellular ion environment

Astrocytes modify and maintain neural activity and functions via gliotransmitter release such as, glutamate. They also change their properties and functions in response to alterations of ion environment resulting from neurotransmission; however, the direct evidence for whether intracellular ion alteration in astrocytes triggers gliotransmitter release is not indicated. Recent studies have reported that channelrhodopsin-2 (ChR2) is useful for alteration of intracellular ion environment in several types of cells with blue light exposure. Here, we show that ChR2-expressing GL261 (GLChR2) cells, clonal astrocytes, change their properties by photo-activation. Increased intracellular sodium and calcium ion concentrations and an altered membrane potential were observed in GLChR2 cells with blue light exposure. Alterations in the intracellular ion environment caused intracellular acidification and the inhibition of proliferation. In addition, it triggered glutamate release from GLChR2 cells. Glutamate from GLChR2 cells acted on N18 cells, clonal neuronal cells, as both a transmitter and neurotoxin depending on photo-activation. Our results show that the properties of ChR2-expressing astrocytes can be controlled by blue light exposure, and cation influx through photo-activated ChR2 might trigger functional cation influx via endogenous channels and result in the increase of glutamate release. Further, our results suggest that ChR2-expressing glial cells could become a useful tool in understanding the roles of glial cell activation and neural communication in the regulation of brain functions.

Effects of osmotic and cold shock on adherent human mesenchymal stem cells during cryopreservation

Cryopreservation is one of the most practical methods for the long-term storage of cell-matrix systems to ensure off-shelf availability in tissue engineering, stem cell therapy and drug testing. The aim of this study is to investigate the effects of osmotic and cold shock caused by the procedures of cryoprotectant agent addition/removal and freezing during cryopreservation on cell viability, intracellular properties, such as filamentous actin distribution, mitochondria localization and intracellular pH, and further recovery of adherent human mesenchymal stem cells. Our results shows a significant decrease in cell viability around 30% after cryopreservation at the cooling rates of 1, 5 and 10°C/min in comparison to the adherent cells and the cells in suspension, implicating that the adherent cells are more vulnerable than the suspension cells. The osmotic shock and cold shock induced by freezing lead to dramatic changes in the intracellular properties. The cooling rate of 10°C/min results in acidification of intracellular pH, distortion and accumulation of filamentous actin, and aggregation of mitochondria. Our findings also suggest that the cooling rate of 1°C/min helps to maintain cell morphology and attachment, integrity and uniformity of filamentous actin, and leads to better cell recovery after cryopreservation.

Acidic pH conditions mimicking degenerative intervertebral discs impair the survival and biological behavior of human adipose-derived mesenchymal stem cells

This study was designed to examine the survival and biological behavior of adipose-derived mesenchymal stem cells (ADMSCs) under an intervertebral disc (IVD)-like acidic environment. Human ADMSCs isolated from two age groups were cultured under four different pH levels (pH 7.4, 7.1, 6.8 and 6.5) which mimicked the standard condition and the normal, mildly degenerated and severely degenerated IVD. Cell viability was measured by fluorescein isothiocyanate-Annexin-V/propidium iodide staining, and cell proliferation was measured by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide) assay. The expression of aggrecan, collagen-I, collagen-II, matrix metalloproteinase-2 (MMP-2), tissue inhibitor of metalloproteinase-3 (TIMP-3), p53 and caspase-3 at the mRNA level was examined by realtime quantitative polymerase chain reaction, and the expression of aggrecan, collagen-I, collagen-II, MMP-2 and TIMP-3 at the protein level was measured by enzyme-linked immunosorbent assay. Acidic pH inhibited the viability and proliferation, and the expression of aggrecan, collagen-I and collagen-II of ADMSCs from both age groups. ADMSCs harvested from young and mature donors exhibited similar responses to the acidic pH, although cells from young donors appeared less sensitive to the low pH levels. The results demonstrated that acidic pH in IVD may be an important deleterious factor for ADMSC-based IVD regeneration. ADMSCs harvested from young donors may be more suitable to be utilized for the implantation into degenerated IVD, and the implantations may be more effective at an early stage of IVD degeneration when the pH of matrix acidity is higher than 6.8.

Effect of varying osmotic conditions on the response of bovine nucleus pulposus cells to growth factors and the activation of the ERK and Akt pathways

Intervertebral disc and especially nucleus pulposus is characterized by low cellularity. Additionally, extreme variations in osmolality are observed in this tissue, as a result of its specific physicochemical environment, daily activities, or degeneration. In this study, we investigated the role of osmotic fluctuations in the proliferative response of nucleus pulposus cells to exogenous growth factors. In particular, we examined the effect of platelet-derived growth factor (PDGF) and insulin-like growth factor-I (IGF-I) on the proliferation of bovine nucleus pulposus cells and on the activation of the MEK/ERK and PI-3-K/Akt pathways under varying osmotic conditions, in an effort to understand the mechanisms regulating cell proliferation in the intact and the degenerated intervertebral disc. Exposure of cells to high osmolality restrained novel DNA synthesis induced by PDGF or IGF-I in a dose-dependent manner and reduced ERK and Akt activation stimulated by serum or isolated growth factors. Our findings indicate that hyperosmolality imposes a strict control in intervertebral disc cells' proliferation, while hypo-osmotic conditions prevailing in degenerated discs may offer a more permissive environment for cellular proliferation.

Osmolarity effects on bovine articular chondrocytes during three-dimensional culture in alginate beads

OBJECTIVE

With the development of engineered cartilage, the determination of the appropriate culture conditions is vital in order to maximize extracellular matrix synthesis. As osmolarity could affect the fate of chondrocytes, the purpose of this study was to determine the effects of osmolarity on chondrocytes during relatively long-term culture.

DESIGN

Bovine articular chondrocytes were cultured in alginate beads in a biocarbonate free system at 280, 380 and 550 mOsm at pH 7.4 for up to 12 days, respectively. Cell volume, intracellular pH (pH(i)), cell number, glucosaminoglycan (GAG) and collagen retention were measured at day 5 and 12. Cell viability and volume were monitored over the 12 days of culture.

RESULTS

By day 5 and 12, compared to the cell volume at 380 mOsm, around 20% (P<0.01) swelling and 15% (P<0.05) shrinkage were observed when the cells were cultured at 280 and 550 mOsm. The pH(i) over the 12 days of culture varied with osmolarity of the culture medium. In comparison with fresh cells, pH(i) became slightly more acidic by 0.15 pH units at 280 mOsm at day 5. However, by day 12, an alkalization of pH(i), by 0.2 pH units, was noted. A higher proliferation rate was seen at 280 mOsm than at other osmolarities while less GAG was produced.

CONCLUSIONS

Chronic exposure to anisotonic conditions results in cell swelling at 280 mOsm and shrinkage at 550 mOsm. The osmolarity of 280 mOsm appears to encourage proliferation of chondrocytes, but inhibits matrix production.

Intracellular pH homeostasis during cell-cycle progression and growth state transition in Schizosaccharomyces pombe

Accurate measurement of intracellular pH in unperturbed cells is fraught with difficulty. Nevertheless, using a variety of methods, intracellular pH oscillations have been reported to play a regulatory role in the control of the cell cycle in several eukaryotic systems. Here, we examine pH homeostasis in Schizosaccharomyces pombe using a non-perturbing ratiometric pH sensitive GFP reporter. This method allows for accurate intracellular pH measurements in living, entirely undisturbed, logarithmically growing cells. In addition, the use of a flow cell allows internal pH to be monitored in real time during nutritional, or growth state transition. We can find no evidence for cell-cycle-related changes in intracellular pH. By contrast, all data are consistent with a very tight homeostatic regulation of intracellular pH near 7.3 at all points in the cell cycle. Interestingly, pH set point changes are associated with growth state. Spores, as well as vegetative cells starved of either nitrogen, or a carbon source, show a marked reduction in their internal pH compared with logarithmically growing vegetative cells. However, in both cases, homeostatic regulation is maintained.

Effect of hyperoxia and exogenous oxidant stress on pulmonary artery endothelial cell Na+/H+ antiport activity

Little is known about the mechanisms of altered cell membrane function after hyperoxic exposure. We determined the effects of hyperoxic exposure and exogenous oxidant stress with xanthine/xanthine oxidase (X/XO) on Na+/H+ antiport activity. Pulmonary artery endothelial cell monolayers were incubated in 95% O2/5% CO2 (24 to 72 hours) simultaneously with controls placed in 21 % O2/5% CO2. Monolayers were then incubated for 2 hours in MEM with or without X/XO (100 micromol/L X; 0.01 U/ml XO). Antiport activity was determined as the rate of recovery from intracellular acidosis by measurement of intracellular pH (pH,) with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). Hyperoxic exposure (72 hours) decreased Na+/H+ antiport activity as compared with that in control monolayers. Exogenous oxidant stress also decreased antiport activity in both control and hyperoxic cells, but this effect was more pronounced in hyperoxic cells at all time points. These changes occurred in the absence of overt cytotoxicity. Incubation with antioxidants (polyethylene glycol-superoxide dismutase (PEG-SOD), PEG-catalase, vitamin E), N-acetylcysteine, or phospholipase A2 (PLA2) inhibitors did not prevent the decrease in antiport activity after hyperoxic exposure. Conditioned medium experiments demonstrated that the diminished antiport activity was not related to release of a soluble mediator after hyperoxic exposure. These findings suggest that the diminished Na+/H+ antiport activity represents a sublethal form of membrane dysfunction that may be a component of the increased endothelial cell susceptibility to injury after hyperoxic exposure.

Genesis and acid/base

Perhaps the enigmatic etiology of cell specialization ultimately leading to organogenesis can be explained by the unusual combine application of some common mechanisms. Perhaps the combination of control of variable pH through compartmentalization, dialysis/diffusion gradients, and an electrophoresis plane impacts cells in zones causing the developmental patterns. Possibly pH is even manipulated at times to change the charge on molecules in order that attraction of opposite charges can be utilized.

Diseased cells and pH

Diseased cells may alter their intracellular pH. This could explain such diverse activities as resistance to hormones, resistance to medications, and preferential metastasis to the same locations.

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.

Effects of amiloride analogues on the production of prostacyclin by aortic endothelial cells

1. The release of prostacyclin (PGI2) from bovine aortic endothelial cells stimulated by adenosine 5'-triphosphate (ATP) was decreased by amiloride analogues bearing alkyl groups on the 5-amino nitrogen atom, like 5-(N-ethyl-N-isopropyl)amiloride (EIPA), which are inhibitors of the Na+/H+ exchanger. Analogues substituted on a terminal guanidino nitrogen atom were not inhibitory. 2. The release of PGI2 induced by ATP was not significantly depressed in a Na+-poor medium or in a medium acidified to pH 6.9, two conditions known to inhibit the Na+/H+ exchanger. 3. Cytoplasmic alkalinization by ammonium chloride did not suppress the inhibitory action of EIPA. By itself, ammonium chloride decreased the response of endothelial cells to ionophore A23187 and ATP, whereas sodium acetate had no effect. 4. EIPA did not decrease the mobilization of free arachidonic acid induced by ATP. It inhibited the conversion of exogenous arachidonate into PGI2 and prostaglandin E2 (PGE2). 5. Although the intracellular pH was not measured in this study, it seems unlikely that cytoplasmic alkalinization via the activation of the Na+/H+ exchanger plays a significant role in the stimulatory action of ATP on the release of PGI2 from endothelial cells. The inhibition of that release by EIPA and other amiloride analogues might involve a direct effect on cyclo-oxygenase, although an action on the reacylation of free arachidonic acid cannot be excluded.

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.

Sphingosine inhibits phorbol 12-myristate 13-acetate-, but not serum-induced, activation of Na+/H+ exchange in mammalian cells

Addition of serum to quiescent mammalian cells in culture initiates a series of events which culminates in DNA replication and cell division. One of the earliest events in this sequence of events is activation of Na+/H+ exchange, which can result in an increase in intracellular pH (pHin). The regulation of this change in activity is not known. Since treatment of 3T3 cells with activators of protein kinase C (kinase C) can result in an increased pHin, it has been hypothesized that serum stimulation of kinase C is responsible for activation of Na+/H+ exchange. Recently, sphingolipids have been discovered to inhibit kinase C both in vitro and in vivo. Therefore, we undertook the present study to ask whether or not inhibition of kinase C using sphingolipids prevents mitogen-induced alkalinization in 3T3 cells. Our results indicate that activators of kinase C stimulate Na+/H+ exchange in normal human fibroblasts (BoGi), but not in mouse embryo (3T3) cells. Addition of serum to BoGi cells, on top of saturating doses of phorbol 12-myristate 13-acetate (PMA), results in a further cytoplasmic alkalinization. Furthermore, sphingosine prevents the PMA-induced increase in pHin in BoGi cells, and phosphorylation of an 80 kDa protein in 3T3 cells, but not the serum-induced alkalinization in either BoGi or 3T3 cells. These data indicate that activation of kinase C does not participate in the physiological activation of Na+/H+ exchange in human fibroblasts or mouse embryo cells by serum.