Control of intracellular pH during regulatory volume decrease in HL-60 cells

Functional and molecular characterization of transmembrane intracellular pH regulators in human dental pulp stem cells

OBJECTIVE

Homeostasis of intracellular pH (pH_i) plays vital roles in many cell functions, such as proliferation, apoptosis, differentiation and metastasis. Thus far, Na⁺-H⁺ exchanger (NHE), Na⁺-HCO₃^- co-transporter (NBC), Cl^-/HCO₃^- exchanger (AE) and Cl^-/OH^- exchanger (CHE) have been identified to co-regulate pH_i homeostasis. However, functional and biological pH_i-regulators in human dental pulp stem cells (hDPSCs) have yet to be identified.

DESIGN

Microspectrofluorimetry technique with pH-sensitive fluorescent dye, BCECF, was used to detect pH_i changes. NH₄Cl and Na⁺-acetate pre-pulse were used to induce intracellular acidosis and alkalosis, respectively. Isoforms of pH_i-regulators were detected by Western blot technique.

RESULTS

The resting pH_i was no significant difference between that in HEPES-buffered (nominal HCO₃^--free) solution or CO₂/HCO₃-buffered system (7.42 and 7.46, respectively). The pH_i recovery following the induced-intracellular acidosis was blocked completely by removing [Na⁺]_o, while only slowed (-63%) by adding HOE694 (a NHE1 specific inhibitor) in HEPES-buffered solution. The pH_i recovery was inhibited entirely by removing [Na⁺]_o, while adding HOE 694 pulse DIDS (an anion-transporter inhibitor) only slowed (-55%) the acid extrusion. Both in HEPES-buffered and CO₂/HCO₃-buffered system solution, the pH_i recovery after induced-intracellular alkalosis was entirely blocked by removing [Cl^-]_o. Western blot analysis showed the isoforms of pH_i regulators, including NHE1/2, NBCe1/n1, AE1/2/3/4 and CHE in the hDPSCs.

CONCLUSIONS

We demonstrate for the first time that resting pH_i is significantly higher than 7.2 and meditates functionally by two Na⁺-dependent acid extruders (NHE and NBC), two Cl^--dependent acid loaders (CHE and AE) and one Na⁺-independent acid extruder(s) in hDPSCs. These findings provide novel insight for basic and clinical treatment of dentistry.

Suppression of mammalian bone growth by membrane transport inhibitors

Bone lengthening during skeletal growth is driven primarily by the controlled enlargement of growth plate (GP) chondrocytes. The cellular mechanisms are unclear but membrane transporters are probably involved. We investigated the role of the Na(+)/H(+) antiporter (NHE1) and anion exchanger (AE2) in bone lengthening and GP chondrocyte hypertrophy in Sprague-Dawley 7-day-old rat (P7) bone rudiments using the inhibitors EIPA (5-(N-ethyl-N-isopropyl)amiloride) and DIDS (4,4-diidothiocyano-2,2-stilbenedisulphonate), respectively. We have also determined cell-associated levels of these transporters along the GP using fluorescent immunohistochemistry (FIHC). Culture of bones with EIPA or DIDS inhibited rudiment growth (50% at approx. 250 and 25 µM, respectively). Both decreased the size of the hypertrophic zone (P < 0.05) but had no effect on overall length or cell density of the GP. In situ chondrocyte volume in proliferative and hypertrophic zones was decreased (P < 0.01) with EIPA but not DIDS. FIHC labeling of NHE1 was relatively high and constant along the GP but declined steeply in the late hypertrophic zone. In contrast, AE2 labeling was relatively low in proliferative zone cells but increased (P < 0.05) reaching a maximum in the early hypertrophic zone, before falling rapidly in the late hypertrophic zone suggesting AE2 might regulate the transition phase of chondrocytes between proliferative and hypertrophic zones. The inhibition of bone growth by EIPA may be due to a reduction to chondrocyte volume set-point. However the effect of DIDS was unclear but could result from inhibition of AE2 and blocking of the transition phase. These results demonstrate that NHE1 and AE2 are important regulators of bone growth.

PKA regulates vacuolar H+-ATPase localization and activity via direct phosphorylation of the a subunit in kidney cells

The vacuolar H(+)-ATPase (V-ATPase) is a major contributor to luminal acidification in epithelia of Wolffian duct origin. In both kidney-intercalated cells and epididymal clear cells, cAMP induces V-ATPase apical membrane accumulation, which is linked to proton secretion. We have shown previously that the A subunit in the cytoplasmic V(1) sector of the V-ATPase is phosphorylated by protein kinase A (PKA). Here we have identified by mass spectrometry and mutagenesis that Ser-175 is the major PKA phosphorylation site in the A subunit. Overexpression in HEK-293T cells of either a wild-type (WT) or phosphomimic Ser-175 to Asp (S175D) A subunit mutant caused increased acidification of HCO(3)(-)-containing culture medium compared with cells expressing vector alone or a PKA phosphorylation-deficient Ser-175 to Ala (S175A) mutant. Moreover, localization of the S175A A subunit mutant expressed in HEK-293T cells was more diffusely cytosolic than that of WT or S175D A subunit. Acute V-ATPase-mediated, bafilomycin-sensitive H(+) secretion was up-regulated by a specific PKA activator in HEK-293T cells expressing WT A subunit in HCO(3)(-)-free buffer. In cells expressing the S175D mutant, V-ATPase activity at the membrane was constitutively up-regulated and unresponsive to PKA activators, whereas cells expressing the S175A mutant had decreased V-ATPase activity that was unresponsive to PKA activation. Finally, Ser-175 was necessary for PKA-stimulated apical accumulation of the V-ATPase in a polarized rabbit cell line of collecting duct A-type intercalated cell characteristics (Clone C). In summary, these results indicate a novel mechanism for the regulation of V-ATPase localization and activity in kidney cells via direct PKA-dependent phosphorylation of the A subunit at Ser-175.

Roles of volume-activated Cl- currents and regulatory volume decrease in the cell cycle and proliferation in nasopharyngeal carcinoma cells

OBJECTIVES

Previously it has been shown, that the volume-activated plasma membrane chloride channel is associated with regulatory volume decrease (RVD) of cells and may play an important role in control of cell proliferation. We have demonstrated that both expression of the channel and RVD capacity are actively regulated in the cell cycle. In this study, we aimed to further study the role of the volume-activated chloride current and RVD in cell cycle progression and overall in cell proliferation.

MATERIALS AND METHODS

Whole-cell currents, RVD, cell cycle distribution, cell proliferation and cell viability were measured or detected with the patch-clamp technique, the cell image analysis technique, flow cytometry, the MTT assay and the trypan blue assay respectively, in nasopharyngeal carcinoma cells (CNE-2Z cells).

RESULTS

The Cl- channel blockers, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and tamoxifen, inhibit the volume-activated chloride current, RVD and proliferation of CNE-2Z cells in a dose-dependent manner. Analysis of relationships between the current, RVD and cell proliferation showed that both the current and RVD were positively correlated with cell proliferation. NPPB (100 microM) and tamoxifen (20 microM) did not significantly induce cell death, but inhibited cell proliferation, implying that the blockers may inhibit cell proliferation by affecting cell cycle progression. This was verified by the observation that tamoxifen (20 microM) and NPPB (100 microM) inhibited cell cycle progress and arrested cells at the G0/G1 phase boundary.

CONCLUSIONS

Activity of the volume-activated chloride channel is one of the important factors that regulate the passage of cells through the G1 restriction point and that the Cl- current associated with RVD plays an important role in cell proliferation.

Spectral imaging microscopy demonstrates cytoplasmic pH oscillations in glial cells

Glial cells exhibit distinct cellular domains, somata, and filopodia. Thus the cytoplasmic pH (pH(cyt)) and/or the behavior of the fluorescent ion indicator might be different in these cellular domains because of distinct microenvironments. To address these issues, we loaded C6 glial cells with carboxyseminaphthorhodafluor (SNARF)-1 and evaluated pH(cyt) using spectral imaging microscopy. This approach allowed us to study pH(cyt) in discrete cellular domains with high temporal, spatial, and spectral resolution. Because there are differences in the cell microenvironment that may affect the behavior of SNARF-1, we performed in situ titrations in discrete cellular regions of single cells encompassing the somata and filopodia. The in situ titration parameters apparent acid-base dissociation constant (pK'(a)), maximum ratio (R(max)), and minimum ratio (R(min)) had a mean coefficient of variation approximately six times greater than those measured in vitro. Therefore, the individual in situ titration parameters obtained from specific cellular domains were used to estimate the pH(cyt) of each region. These studies indicated that glial cells exhibit pH(cyt) heterogeneities and pH(cyt) oscillations in both the absence and presence of physiological HCO(3)(-). The amplitude and frequency of the pH(cyt) oscillations were affected by alkalosis, by acidosis, and by inhibitors of the ubiquitous Na(+)/H(+) exchanger- and HCO(3)(-)-based H(+)-transporting mechanisms. Optical imaging approaches used in conjunction with BCECF as a pH probe corroborated the existence of pH(cyt) oscillations in glial cells.

Anion exchanger isoform 2 operates in parallel with Na(+)/H(+) exchanger isoform 1 during regulatory volume decrease of human cervical cancer cells

Intracellular pH (pH(i)) homeostasis was investigated in human cervical cancer SiHa cells undergoing regulatory volume decrease (RVD) to determine which transport systems were involved. Using isoform-specific primers, mRNA transcripts of Na(+)/H(+) exchanger isoform 1 (NHE1) and isoform 3 were identified by reverse transcriptase polymerase chain reaction (RT-PCR) and the results confirmed by Western immunoblotting. From anion exchanger isoforms 1-3 (AE1-3), only the mRNA transcript of AE2 was identified by RT-PCR and the identity was confirmed by digestion with a specific restriction endonuclease. SiHa cells loaded with the fluorescent dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein and resuspended in isotonic media showed a stable pH(i). In contrast, a gradual internal acidification took place following resuspension in hypotonic media. The NHE inhibitors, HOE694 (10 microM) and amiloride (1 mM), showed a similar potency in enhancing the rate and extent of the hypotonicity-induced internal acidification. The absence of extracellular Na(+) also substantially enhanced the acidification during RVD. These results suggest that internal acidification during RVD is mainly compensated by the operation of NHE1. Extracellular Cl(-) was critically necessary for the pH(i) acidification during RVD. The hypotonicity-induced acidification was significantly attenuated by 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, a concentration inhibiting more than 90% AE activity. This indicates that AE2 mediates a net Cl(-) influx with compensating HCO(3)(-) efflux during RVD. We conclude that AE2 operates in parallel with NHE1 to regulate pH(i) during RVD of human cervical cancer cells.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Expression and localization of Na+/H+ exchangers in rat central nervous system

Neurons in the central nervous system regulate their intracellular pH using particular membrane proteins of which two, namely the Na+-dependent Cl-/HCO3- exchanger and the Na+/H+ exchanger, are essential. In this study we examined messenger RNA expression and distribution of Na+/H+ exchanger in the newborn rat central nervous system and with maturation using Northern blot analysis and in situ hybridization. Our study clearly shows that each Na+/H+ exchanger has a different expression pattern in the rat central nervous system. As in non-excitable tissues, Na+/H+ exchanger 1 is by far the most abundant of all Na+/H+ exchangers in the rat central nervous system. Its expression is ubiquitous although its messenger RNA appears at higher levels in the hippocampus, in the 2nd/3rd layers of periamygdaloid cortex and in the cerebellum. The low level of messenger RNAs encoding Na+/H+ exchanger 2 and 4 is mainly expressed in the cerebral cortex and in the brainstem-diencephalon, while Na+/H+ exchanger 3 transcripts are found only in the cerebellar Purkinje cells. From a developmental point of view, Na+/H+ exchanger 1, 2 and 4 showed an increased level in their transcripts in the cerebral cortex while an opposite trend existed in the cerebellum from postnatal day 0 to postnatal day 30. The messenger RNA for Na+/H+ exchanger 3, however, increased its level with age in cerebellum. From our data we conclude that: i) the expression of the Na+/H+ exchanger is age-, region-, and subtype-specific, with Na+/H+ exchanger 1 being the most prevalent in the rat central nervous system; ii) specialization of groups of neurons with respect to the type of Na+/H+ exchanger is clearly illustrated by Na+/H+ exchanger 3 which is almost totally localized in cerebellar Purkinje cells; and iii) the developmental increase in the messenger RNA for Na+/H+ exchanger 1 in the cerebral cortex and hippocampus is consistent with our previous studies on intracellular pH physiology in neonatal and mature neurons. Together this study indicates that expression of each Na+/H+ exchanger messenger RNA is differentially regulated both during development and in the different regions of rat central nervous system.

Activation of Na+/H+ exchange is required for regulatory volume decrease after modest "physiological" volume increases in jejunal villus epithelial cells

Epithelial cell volume increases that occur because of the uptake of Na+-cotransported solutes or hypotonic dilution are followed by a regulatory volume decrease (RVD) due to the activation of K+ and Cl- channels. We studied the relationship of Na+/H+ exchange (NHE) to this RVD in suspended guinea pig jejunal villus cells, using electronic sizing to measure cell volume changes and fluorescent spectroscopy of cells loaded with 2', 7'-bis(carboxyethyl)-5()-carboxyfluorescein to monitor intracellular pH (pHi). When the volume increase achieved by these cells during Na+ solute absorption was duplicated by a modest 5-7% hypotonic dilution, their pHi first acidified and then alkalinized. This alkalinization was blocked by 5-(N-methyl-N-isobutyl) amiloride (MIA; 1 microM), an inhibitor of NHE. The RVD subsequent to 5-7% hypotonic dilution was prevented by Na+-free medium and by amiloride and non-amiloride derivatives. The order of potency of these inhibitors was as follows: MIA > 5-(N,N-dimethyl) amiloride > cimetidine > clonidine, in keeping with the pattern attributable to NHE-1 as the isoform of NHE responsible for increase in pHi after modest volume increases. A substantial 30% hypotonic dilution caused acidification, and RVD following this larger volume increase was not affected by MIA. To assess the effect of hypotonicity on the activity of NHE, we measured the rate of MIA-sensitive pHi recovery from an acid load (dpHi/dt) in 5 and 30% hypotonic media. pHi recovery was faster in 5% hypotonic medium compared with isotonic medium and slowest in 30% hypotonic medium, which suggested that the activity of NHE was stimulated in the slightly hypotonic medium, but inhibited in the 30% hypotonic medium. To determine the role of activated NHE in RVD after a modest volume increase, cells were hypotonically diluted 7% in MIA to prevent RVD and then alkalinized by NH4Cl or acidified by propionic acid addition. Only after alkalinization was there complete volume regulation. We conclude that in Na+-absorbing enterocytes, the NHE-1 isoform of Na+/H+ exchange is stimulated by volume increases that duplicate the "physiological" volume increase occurring when these cells absorb Na+-cotransported solutes. The subsequent alkalinization of pHi is a required determinant of the osmolyte loss that underlies this distinct volume regulatory mechanism.