Regulation of cell volume and intracellular pH in hyposmotically swollen rat osteosarcoma cells

Acidic cellular microenvironment modifies carcinogen-induced DNA damage and repair

Chronic inflammation creates an acidic microenvironment, which plays an important role in cancer development. To investigate how low pH changes the cellular response to the carcinogen benzo[a]pyrene (B[a]P), we incubated human pulmonary epithelial cells (A549 and BEAS-2B) with nontoxic doses of B[a]P using culturing media of various pH’s (extracellular pH (pH_e) of 7.8, 7.0, 6.5, 6.0 and 5.5) for 6, 24 and 48 h. In most incubations (pH_e 7.0–6.5), the pH in the medium returned to the physiological pH 7.8 after 48 h, but at the lowest pH (pH_e < 6.0), this recovery was incomplete. Similar changes were observed for the intracellular pH (pH_i). We observed that acidic conditions delayed B[a]P metabolism and at t = 48 h, and the concentration of unmetabolized extracellular B[a]P and B[a]P-7,8-diol was significantly higher in acidic samples than under normal physiological conditions (pH_e 7.8) for both cell lines. Cytochrome P450 (CYP1A1/CYP1B1) expression and its activity (ethoxyresorufin-O-deethylase activity) were repressed at low pH_e after 6 and 24 h, but were significantly higher at t = 48 h. In addition, a DNA repair assay showed that the incision activity was ~80% inhibited for 6 h at low pH_e and concomitant exposure to B[a]P. However, at t = 48 h, the incision activity recovered to more than 100% of the initial activity observed at neutral pH_e. After 48 h, higher B[a]P-DNA adduct levels and γ-H2AX foci were observed at low pH samples than at pH_e 7.8. In conclusion, acidic pH delayed the metabolism of B[a]P and inhibited DNA repair, ultimately leading to increased B[a]P-induced DNA damage.

Uncoupling of K+ and Cl- transport across the cell membrane in the process of regulatory volume decrease

It is accepted that K(+) and Cl(-) flows are coupled tightly in regulatory volume decrease (RVD). However, using self referencing microelectrodes, we proved that K(+) and Cl(-) transport mainly by channels in RVD was uncoupled in nasopharyngeal carcinoma CNE-2Z cells, with the transient K(+) efflux activated earlier and sustained Cl(-) efflux activated later. Hypotonic challenges decreased intracellular pH (pH(i)), and activated a proton pump-dependent H(+) efflux, resulting in a decline of extracellular pH (pH(o)). Modest decreases of pH(o) inhibited the volume-activated K(+) outflow and RVD, but not the Cl(-) outflow, while inhibition of H(+) efflux or increase of pH(o) buffer ability promoted K(+) efflux and RVD. The results suggest that the temporal dynamics of K(+) channel activities is different from that of Cl(-) channels in RVD, due to differential sensitivity of K(+) and Cl(-) channels to pH(o). H(+) efflux may play important roles in cell volume regulation, and may be a therapeutic target for human nasopharyngeal carcinoma.

Sperm abnormalities in heterozygous acid sphingomyelinase knockout mice reveal a novel approach for the prevention of genetic diseases

Acid sphingomyelinase knockout mice are a model of the inherited human disorder types A and B Niemann-Pick disease. Herein, we show that heterozygous (ASMKO(+/-)) mice have two distinct sperm populations resembling those found in normal and mutant animals, respectively, and that these two populations could be distinguished by their morphology, ability to undergo capacitation or the acrosome reaction, and/or mitochondrial membrane potential (MMP). The abnormal morphology of the mutant sperm could be normalized by demembranation with detergents or by the addition of recombinant acid sphingomyelinase to the culture media, and the corrected sperm also had an enhanced fertilization capacity. Methods were then explored to enrich for normal sperm from the mixed ASMKO(+/-) population, and flow cytometric sorting based on MMP provided the best results. In vitro fertilization was performed using ASMKO(+/-) oocytes and sperm before and after MMP sorting, and it was found that the sorted sperm produced significantly more wild-type pups than nonsorted sperm. Sperm sorting is much less invasive and more cost-effective than egg isolation, and offers several advantages over the existing assisted reproduction options for Niemann-Pick disease carrier couples. It therefore could have a major impact on the prevention of this and perhaps other genetic diseases.

Stretch-induced PTH-related protein gene expression in osteoblasts

Mechanical forces play a critical role in regulating skeletal mass and structure. We report that mechanical loading induces PTHrP in osteoblast-like cells and that TREK-2 stretch-activated potassium channels seem to be involved in this induction. Our data suggest PTHrP as a candidate endogenous mediator of the anabolic effects of mechanical force on bone.

INTRODUCTION

Mechanical force has anabolic effects on bone. The PTH-related protein (PTHrP) gene is known to be mechanically inducible in smooth muscle cells throughout the organism, and N-terminal PTH and PTHrP products have been reported to have anabolic effects in bone. We explored the idea that PTHrP might be a candidate mediator of the effects of mechanical force on bone.

MATERIALS AND METHODS

Mechanical loading was applied by swelling osteoblast-like cells in hypotonic solution and/or by application of cyclical stretch through a FlexerCell apparatus. RNase protection assay and real-time quantitative PCR analysis were used to assay PTHrP gene expression.

RESULTS AND CONCLUSION

Stretching UMR201-10B osteoblast-like cells by swelling in hypotonic solutions rapidly increased PTHrP mRNA. This induction was insensitive to gadolinium and nifedipine, to the removal of extracellular calcium, and to depletion of endoplasmic reticulum calcium, indicating that neither stretch-activated cation channels, L-type calcium channels, nor ER calcium is involved in the induction of PTHrP. The TREK family potassium channels are activated by both stretch and intracellular acidosis, and we identified these channels in osteoblast-like cells by PCR. Intracellular acidification increased PTHrP mRNA expression in UMR-201-10B cells, and siRNA targeted against the TREK-2 gene reduced endogenous TREK-2 expression and dampened PTHrP mRNA induction. Cyclical stretch also induced PTHrP in UMR-201-10B osteoblast-like cells and in MLO-A5 post-osteoblast-pre-osteocyte cells, the latter a stage in the osteoblastic differentiation program that is likely to be a key target of force in vivo. Our evidence suggests PTHrP as a candidate mediator of the anabolic effects of mechanical force on bone.

Modulation of KCNQ4 channel activity by changes in cell volume

KCNQ4 channels expressed in HEK 293 cells are sensitive to cell volume changes, being activated by swelling and inhibited by shrinkage, respectively. The KCNQ4 channels contribute significantly to the regulatory volume decrease (RVD) process following cell swelling. Under isoosmotic conditions, the KCNQ4 channel activity is modulated by protein kinases A and C, G protein activation, and a reduction in the intracellular Ca2+ concentration, but these signalling pathways are not responsible for the increased channel activity during cell swelling.

Effects of hypotonic shock on intracellular pH in bovine articular chondrocytes

Chondrocytes inhabit an unusual environment, in which they are repeatedly subjected to osmotic challenges as fluid is expressed from the extracellular matrix during static joint loading. In the present study, the effects of hypotonic shock on intracellular pH, pH(i), have been studied in isolated bovine articular chondrocytes using the pH-sensitive fluroprobe BCECF. Cells subjected to a 50% dilution rapidly alkalinised, by approximately 0.2 pH units, a sustained plateau being achieved within 300 s. The effect was not altered by inhibitors of pH regulators, such as amiloride, bafilomycin and SITS, but was absent when cells were subjected to hypotonic shocks in solutions in which Na(+) ions were replaced by NMDG(+). The response was found to be sensitive to Gd(3+) ions, blockers of stretch-activated cation channels. Alkalinisation was also inhibited by treatment with Zn(2+) ions, at a concentration reported to block voltage-activated H(+) channels (VAHC). Depolarisation using high K(+) solutions supplemented with valinomycin also induced intracellular alkalinisation. Measurements using a membrane potential (E(m)) fluorescent dye showed that E(m) was approximately -44 mV, but was depolarised by over 50 mV following HTS. The depolarisation was also inhibited by Na(+) substitution with NMDG(+) or treatment with Gd(3+). We conclude that in response to HTS the opening of a stretch-activated cation channel leads to Na(+) influx, which results in a membrane depolarisation. Subsequent activation of VAHC permits H(+) ion efflux along the prevailing electrochemcial gradient, leading to the alkalinisation, which we record.

Potentiation of a voltage-gated proton current in acidosis-induced swelling of rat microglia

Microglia are equipped with a strong proton (H(+)) extrusion pathway, a voltage-gated H(+) channel, probably to compensate for the large amount of H(+) generated during phagocytosis; however, little is known about how this channel is regulated in pathological states. Because neural damage is often associated with intracellular and extracellular acidosis, we examined the regulatory mechanisms of the H(+) current of rat spinal microglia in acidic environments. More than 90% of round/amoeboid microglia expressed the H(+) current, which was characterized by slow activation kinetics, dependencies on both intracellular and extracellular pH, and blockage by Zn(2+). Extracellular lactoacidosis, pH 6.8, induced intracellular acidification and cell swelling. Cell swelling was also induced by intracellular dialysis with acidic pipette solutions, pH 5.5-6.8, at normal extracellular pH 7.3 in the presence of Na(+). The H(+) currents were increased in association with cell swelling as shown by shifts of the half-activation voltage to more negative potentials and by acceleration of the activation kinetics. The acidosis-induced cell swelling and the accompanying potentiation of the H(+) current required nonhydrolytic actions of intracellular ATP and were inhibited by agents affecting actin filaments (phalloidin and cytochalasin D). The H(+) current was also potentiated by swelling caused by hypotonic stress. These findings suggest that the H(+) channel of microglia can be potentiated via cell swelling induced by intracellular acidification. This potentiation might operate as a negative feedback mechanism to protect microglia from cytotoxic acidification and hence acidosis-induced swelling in pathological states of the CNS.

Fps1p controls the accumulation and release of the compatible solute glycerol in yeast osmoregulation

The accumulation of compatible solutes, such as glycerol, in the yeast Saccharomyces cerevisiae, is a ubiquitous mechanism in cellular osmoregulation. Here, we demonstrate that yeast cells control glycerol accumulation in part via a regulated, Fps1p-mediated export of glycerol. Fps1p is a member of the MIP family of channel proteins most closely related to the bacterial glycerol facilitators. The protein is localized in the plasma membrane. The physiological role of Fps1p appears to be glycerol export rather than uptake. Fps1 delta mutants are sensitive to hypo-osmotic shock, demonstrating that osmolyte export is required for recovery from a sudden drop in external osmolarity. In wild-type cells, the glycerol transport rate is decreased by hyperosmotic shock and increased by hypo-osmotic shock on a subminute time scale. This regulation seems to be independent of the known yeast osmosensing HOG and PKC signalling pathways. Mutants lacking the unique hydrophilic N-terminal domain of Fps1p, or certain parts thereof, fail to reduce the glycerol transport rate after a hyperosmotic shock. Yeast cells carrying these constructs constitutively release glycerol and show a dominant hyperosmosensitivity, but compensate for glycerol loss after prolonged incubation by glycerol overproduction. Fps1p may be an example of a more widespread class of regulators of osmoadaptation, which control the cellular content and release of compatible solutes.

Calcium ions and tyrosine phosphorylation interact coordinately with actin to regulate cytoprotective responses to stretching

The actin-dependent sensory and response elements of stromal cells that are involved in mechanical signal transduction are poorly understood. To study mechanotransduction we have described previously a collagen-magnetic bead model in which application of well-defined forces to integrins induces an immediate (&lt; 1 second) calcium influx. In this report we used the model to determine the role of calcium ions and tyrosine-phosphorylation in the regulation of force-mediated actin assembly and the resulting change in membrane rigidity. Collagen-beads were bound to cells through the focal adhesion-associated proteins talin, vinculin, alpha 2-integrin and beta-actin, indicating that force application was mediated through cytoskeletal elements. When force (2 N/m2) was applied to collagen beads, confocal microscopy showed a marked vertical extension of the cell which was counteracted by an actin-mediated retraction. Immunoblotting showed that force application induced F-actin accumulation at the bead-membrane complex but vinculin, talin and alpha 2-integrin remained unchanged. Atomic force microscopy showed that membrane rigidity increased 6-fold in the vicinity of beads which had been exposed to force. Force also induced tyrosine phosphorylation of several cytoplasmic proteins including paxillin. The force-induced actin accumulation was blocked in cells loaded with BAPTA/AM or in cells preincubated with genistein, an inhibitor of tyrosine phosphorylation. Repeated force application progressively inhibited the amplitude of force-induced calcium ion flux. As force-induced actin reorganization was dependent on calcium and tyrosine phosphorylation, and as progressive increases of filamentous actin in the submembrane cortex were correlated with increased membrane rigidity and dampened calcium influx, we suggest that cortical actin regulates stretch-activated cation permeable channel activity and provides a desensitization mechanism for cells exposed to repeated long-term mechanical stimuli. The actin response may be cytoprotective since it counteracts the initial force-mediated membrane extension and potentially strengthens cytoskeletal integrity at force-transfer points.

Cell volume and the metabolic actions of insulin

Insulin increases the volume of isolated hepatocytes and cells in perfused livers, but effects of the hormone on the volume of fat or muscle cells have not been demonstrated. Exogenous amino acids may stimulate swelling of liver cells and induce insulin-like effects on hepatic protein metabolism; however, swelling of liver cells can be induced by some treatment that do not induce insulin-like metabolic responses. Exogenous amino acids also influence protein metabolism of fat and muscle cells, but no relationship with cell volume has been established and no corresponding effects on metabolism of carbohydrates or lipids have been observed. Three families of mitogen-activated protein kinases are activated after changes in extracellular osmolarity but they appear to play little or no role in the metabolic actions of insulin. Direct evidence against a metabolic role for the extracellular signal-regulated kinases ERK-1 and ERK-2 is discussed. The c-Jun N-terminal kinases (also called stress-activated protein kinases) and the mammalian homologs of the yeast Hog protein kinase are strongly activated by environmental stresses associated with catabolic metabolism. We conclude that cell volume and protein metabolism may be correlated in liver but there is no compelling evidence that the effects of insulin on metabolism of liver, fat, or muscle cells can be accounted for by changes in cell volume. The effects of insulin on cell volume may represent a discrete aspect of the complete physiological response rather than an obligatory intermediate step in metabolic signalling.