Adaptive response of cultured fibroblasts to hyperosmolarity

Osmolar Modulation Drives Reversible Cell Cycle Exit and Human Pluripotent Cell Differentiation via NF-κВ and WNT Signaling

Terminally differentiated cells are commonly regarded as the most stable cell state in adult organisms, characterized by growth arrest while fulfilling their specialized functions. A better understanding of the mechanisms involved in promoting cell cycle exit will improve the ability to differentiate pluripotent cells into mature tissues for both pharmacological and therapeutic use. Here, it demonstrates that a hyperosmolar environment enforces a protective p53-independent quiescent state in immature hepatoma cells and in pluripotent stem cell-derived models of human hepatocytes and endothelial cells. Prolonged culture in hyperosmolar conditions stimulates changes in gene expression promoting functional cell maturation. Interestingly, hyperosmolar conditions do not only trigger growth arrest and cellular maturation but are also necessary to maintain this maturated state, as switching back to plasma osmolarity reverses the changes in expression of maturation and proliferative markers. Transcriptome analysis revealed sequential stages of osmolarity-regulated growth arrest followed by cell maturation, mediated by activation of NF-κВ, and repression of WNT signaling, respectively. This study reveals that a modulated increase in osmolarity serves as a biochemical signal to promote long-term growth arrest and cellular maturation into different lineages, providing a practical method to generate differentiated hiPSCs that resemble their mature counterpart more closely.

Increased levels of sodium chloride directly increase osteoclastic differentiation and resorption in mice and men

To better understand the association between high salt intake and osteoporosis, we investigated the effect of sodium chloride (NaCl) on mice and human osteoclastogenesis. The results suggest a direct, activating role of NaCl supplementation on bone resorption.

INTRODUCTION

High NaCl intake is associated with increased urinary calcium elimination and parathyroid hormone (PTH) secretion which in turn stimulates the release of calcium from the bone, resulting in increased bone resorption. However, while calciuria after NaCl loading could be shown repeatedly, several studies failed to reveal a significant increase in PTH in response to a high-sodium diet. Another possible explanation that we investigated here could be a direct effect of high-sodium concentration on bone resorption.

METHODS

Mouse bone marrow macrophage and human peripheral blood mononuclear cells (PBMC) driven towards an osteoclastogenesis pathway were cultivated under culture conditions mimicking hypernatremia environments.

RESULTS

In this study, a direct effect of increased NaCl concentrations on mouse osteoclast differentiation and function was observed. Surprisingly, in a human osteoclast culture system, significant increases in the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts, calcitonin receptor (CTR)-positive osteoclasts, nuclear factor-activated T cells c1 (NFATc1) gene expression, and areal and volumetric resorptions were observed for increasing concentrations of NaCl. This suggests a direct, activating, cell-mediated effect of increased concentrations of NaCl on osteoclasts.

CONCLUSIONS

The reported that enhanced bone resorption after high-sodium diets may not only be secondary to the urinary calcium loss but may also be a direct, cell-mediated effect on osteoclastic resorption. These findings allow us to suggest an explanation for the clinical findings independent of a PTH-mediated regulation.

The role of silicon in osteoblast-like cell proliferation and apoptosis

The optimal concentration at which Si induces cell functions has not been fully elucidated. In the present study the effects of Si concentration (0-6 mM) on the biological functions of MG63 cells were investigated. Cell proliferation in the presence of 2 mM Si- and 4 mM Si-containing media progressively increased with culture time, whereas that of 6mM Si treated MG63 cells was significantly (P<0.05) reduced. The unusually high Si concentration (6 mM) induced a significant (P<0.05) increase in the sub-G1 phase of cells from the original 3.60% up to 43.01% after culture for 12 h. In contrast, the other lower Si concentration treated MG63 cells in the sub-G1 phase were in the range 3-5% at all culture time points. 4 mM Si treated MG63 cells, but not 6 mM Si treated MG63 cells, showed remarkably enhanced collagen type I (COL I) gene expression and extracellular signal-regulated kinase (ERK) secretion, which were significantly (P<0.05) higher than those in the control medium. The activation of ERK was also stimulated in MG63 cells by 4 mM Si. Cells cultured in the presence of 4 mM Si were found to have calcium matrix formation on day 7 that was 15-fold greater than that in the control medium. The results obtained in this study may be useful in designing calcium silicate-based materials with optimal biological properties.

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Hyperosmotic stress response: comparison with other cellular stresses

Cellular responses induced by stress are essential for the survival of cells under adverse conditions. These responses, resulting in cell adaptation to the stress, are accomplished by a variety of processes at the molecular level. After an alteration in homeostatic conditions, intracellular signalling processes link the sensing mechanism to adaptive or compensatory changes in gene expression. The ability of cells to adapt to hyperosmotic stress involves early responses in which ions move across cell membranes and late responses characterized by increased synthesis of either membrane transporters essential for uptake of organic osmolytes or of enzymes involved in their synthesis. The goal of these responses is to return the cell to its normal size and maintain cellular homeostasis. The enhanced synthesis of molecular chaperones, such as heat shock proteins, is another important component of the adaptive process that contributes to cell survival. Some responses are common to different stresses, whereas others are specific. In the first part of the review, we illustrate the characteristic and specific features of adaptive response to hypertonicity; we then describe similarities to and differences from other cellular stresses, such as genotoxic agents, nutrient starvation and heat shock.

Effects of osmolarity, ions and compatible osmolytes on cell-free protein synthesis

To mimic what might happen in cells exposed to hypertonicity, the effects of increased osmolarity and ionic strength on cell-free protein synthesis have been examined. Translation of globin mRNA by rabbit reticulocyte lysate decreased by 30-60% when osmolality was increased from 0.35 to 0.53 osmol/kg of water by the addition of NaCl, KCl, CH(3)CO(2)Na or CH(3)CO(2)K. In contrast, equivalent additions of the compatible osmolytes betaine or myo -inositol caused a 40-50% increase in the rate of translation, whereas amino acids (50-135 mM) that are transported via system A had no significant effect. Addition of 75 mM KCl caused a dramatic fall in the amount of the 43 S pre-initiation complex, whereas it was totally preserved when osmolarity was similarly increased by the addition of 150 mM betaine. The formation of a non-enzymic initiation complex between rabbit [(3)H]Phe-tRNA, poly(U) and the 80 S ribosomes was unaffected by the addition of 75 mM NaCl or KCl, but translation of the complex decreased by 70%. Density-gradient centrifugation of reticulocyte extracts translating endogenous mRNA revealed that addition of 150 mM betaine had no effect, whereas addition of 75 mM KCl caused a marked decrease in the polysome peak, concomitant with an increase in the proportion of 80 S ribosomes and ribosomal subunits, even when elongation was inhibited with fragment A of diphtheria toxin. These results are consistent with the notion that both initiation and elongation are inhibited by unusually high concentrations of inorganic ions, but not by the compatible osmolytes betaine or myo -inositol.

The role of system A for neutral amino acid transport in the regulation of cell volume

System A is a secondary active, sodium dependent transport system for neutral amino acids. Strictly coupled with Na,K-ATPase, its activity determines the size of the intracellular amino acid pool, through a complex network of metabolic reaction and exchange fluxes. Many hormones and drugs affect system A activity in specific cell models or tissues. In all the cell models tested thus far the activity of the system is stimulated by amino acid starvation, cell cycle progression, and the incubation under hypertonic conditions. These three conditions produce marked alterations of cell volume. The stimulation of system A activity plays an important role in cell volume restoration, through an expansion of the intracellular amino acid pool. Under normal conditions, system A substrates represent a major fraction of cell compatible osmolytes, organic compounds that exert a protein stabilizing effect. It is, therefore, likely that the activation of system A represents a portion of a more complex response triggered by exposure to stresses of various nature. Since system A transporters have been recently cloned, the molecular bases of these regulatory mechanisms will probably be elucidated in a short time.

Induction of BGT-1 and amino acid system A transport activities in endothelial cells exposed to hyperosmolarity

We studied the responses to hypertonicity of cultured endothelial cells from swine pulmonary arteries. In 0.5 osmol/kgH(2)O medium, initial cell shrinkage was followed by a regulatory volume increase (RVI), complete after 1 h, concomitant with an increase in cellular K(+) content. Then the activity of amino acid transport System A increased, accompanied by an accumulation of ninhydrin-positive solutes (NPS), reaching a peak at approximately 6 h. The subsequent decline in System A activity was paralleled by an induction of the betaine-GABA transporter (BGT-1), detected as increases of BGT-1 mRNA and of transport activity, which peaked at approximately 24 h. Inhibitors of transcription or translation prevented induction of both transport activities. The increased expression of BGT-1, which involved activation of "tonicity-responsive enhancer," was inhibited by 5 mM extracellular betaine. Cellular K(+) concentration gradually declined after the accumulation of NPS and during the induction of BGT-1. This very effective adaptation to hypertonicity suggests it has a physiological role.

Activation of heat-shock transcription factor 1 by hypertonic shock in 3T3 cells

The exposure of 3T3 cells to a medium made hypertonic by the addition of NaCl induced activation of a heat-shock transcription factor (HSF). This activation, as monitored by gel-mobility-shift assays, occurred within 10 min of hypertonic shock and was dose-dependent in relation to the osmotic strength of the medium up to 0.7 osM. Competition analysis indicated that the effect of hypertonic shock on HSF binding activity was specific. The magnitude of the heat-shock element (HSE)-HSF binding induced by incubating the cells in a 0.7 osM medium was comparable in intensity and time course with that induced by a 44 degrees C heat shock. Following removal of the stressors, the decrease in HSF-HSE binding was more rapid in hypertonicity-shocked than in heat-shocked cells. Treatment of the cells with cycloheximide did not inhibit HSF-HSE binding, indicating that the activation was independent of new protein synthesis. By using a specifically directed polyclonal serum, HSF1 was identified as the transcription factor involved in the hypertonicity-induced activation. HSF was also activated when a membrane-impermeable osmolyte such as sucrose was used to increase the osmolarity of the medium. However, no HSF-HSE binding was observed after addition of glycerol (a freely membrane-permeable osmolyte) in excess. There was a temporal relationship between the hypertonicity-induced volume decrease, the increase in the intracellular K+ concentration and the induction of HSF-HSE binding. In contrast, an increase in the intracellular Na+ concentration was not required to induce HSF-HSE binding. However, unlike the heat-shock response, the activation of HSF by hypertonic shock did not lead to elongation of the RNA transcript of heat-shock protein 70.

Role of amino acid-induced changes in ion fluxes in the regulation of hepatic protein synthesis

Alanine is a powerful stimulator of hepatic protein synthesis whose mechanism of action has not yet been ascertained. The present work aimed to elucidate whether rate changes in ion fluxes accompanying the transport of this amino acid could play a role in the stimulation of protein synthesis. In perfused livers, the utilization of alanine produced a net uptake of K+ of 1.5 mumol/min/liver, a progressively increasing efflux of Ca2+ to reach a maximum of 0.9 mumol/min/liver, and alkalization of the extracellular medium. Inhibition of Na+/K+ exchange by ouabain reversed only the uptake of K+, indicating that this is the main way for the efflux of Na+ cotransported with alanine. In isolated hepatocytes, the uptake of alanine increased the intracellular content of K+ and the cell volume. The following observations suggest that these changes, and not an increased intracellular concentration of Na+, are associated with the stimulation of protein synthesis: 1) Ouabain inhibited the alanine stimulation of L-[3H]-valine incorporation into protein without altering the basal rate of protein labeling; 2) ouabain had no effects on alanine uptake indicating that Na+ influx is not involved in the alanine stimulation of protein synthesis; 3) disruption of Na+ gradient across the plasma membrane by specific ionophores, monensin and gramicidin D, inhibited both basal and alanine-stimulated protein synthesis, but substitution of extracellular Na+ by K+ did not prevent the stimulatory action of alanine. The observation that hypotonic buffer enhanced protein synthesis to the same degree than alanine in liver cells indicates that alanine-induced cell swelling could be sufficient to stimulate protein synthesis.

Adaptive cellular response to osmotic stress in pig articular chondrocytes

The authors studied the effects of a wide range of medium osmolarities (from 0.28 osM (physiological osmolarity of plasma and synovial fluid) to 0.58 osM) by altering Na+ concentration in high density cultures of pig articular chondrocytes in order to analyze the behaviour of some functional and structural parameters during cell adaptation to these imposed changes in the ionic environment. Biochemical and morphological results indicated that, even if isolated from the tissue matrix and cultured in vitro, chondrocytes maintained active osmoregulation systems which are present in living conditions. They showed a similar biochemical and morphological behavior when cultured at 0.28 osM and 0.38 osM but they were able, with regard to protein synthesis, aminoacid transport and proliferation rates, to respond quickly and to adapt to 0.48 osM medium as well. On the contrary, the treatment at the highest osmolarity (0.58 osM) early altered these biochemical parameters and was detrimental or even gave rise to lethal damage during long-term treatment. Furthermore, while chondrocytes cultured in 0.28-0.38 osM medium maintained phenotypic characteristics in culture, the higher osmolarities (0.48-0.58 osM) caused morphological changes in cell populations resulting in loss of phenotypic cell stability as demonstrated by their taking on a fibroblast-like shape as well as a lack of ability to assembly matrix proteoglycans.

Cell cycle effects of hypertonic stress on various human cells in culture

Long-term exposure to hypertonic (HT) culture media has been found to perturb the cell cycle and change gene expression in various animal cell types. A lower growth rate, with exit of cells from the cycling compartment has been observed previously in human transformed EUE cells. The aim of this study was to investigate if the kinetic changes after long-term HT stress, were typical of transformed cells or could be also found in primary cultures of normal cells. Human transformed cells from normal and neoplastic tissues, and normal human cells of epithelial and connective origin have been studied. After the incorporation of bromodeoxyuridine (BrdUrd), the frequency of S-phase cells was estimated by dual-parameter flow cytometry of DNA content versus BrdUrd immunolabelling; the total growth fraction was also estimated, after immunolabelling with an anti-PCNA antibody. We also investigated, by polyacrylamide gel electrophoresis, changes in the amount of a 35 kDa protein band, which increased in EUE cells grown in an HT medium, and which may be directly involved in cell resistance to hypertonicity. Lower BrdUrd labelling indices and higher frequencies of cells in the G0/1 range of DNA content were common features of all the cells in HT media, irrespective of their tissue of origin; other cycle phases may also be involved, depending on the cell type considered. The mechanisms by which cells cope with the HT environment could however differ, since only some cell types showed an increase of the 35 kDa stress protein found originally in HT EUE cells.

Osmotically inducible uptake of betaine via amino acid transport system A in SV-3T3 cells

The osmotically inducible uptake of betaine (NNN-trimethylglycine) by SV-3T3 cells has been studied and compared with the similar process in MDCK cells. Betaine uptake by SV-3T3 cells could be described in terms of a saturable, Na(+)-dependent, component plus a small non-saturable, Na(+)-independent, component. Transport was active, producing considerable accumulation of betaine in the cells. After exposure of the cells to hypertonic conditions for 6 h, there was a marked increase in betaine uptake. Kinetic analysis indicated that this increase resulted from an increase in the Vmax. value of the saturable component, from about 88 to 185 nmol of betaine/5 min per mg of protein, the corresponding Km values of about 15 and 10 mM not being significantly different. This induction of transport activity was detectable only after about 2 h exposure of the cells to hypertonic medium, closely paralleling an induction of influx of N-methylaminoisobutyric acid, and was prevented by the presence of cycloheximide. Betaine influx was markedly inhibited by several neutral amino acids, particularly those transported by system A, such as N-methylaminoisobutyric acid and the imino acid proline. A high concentration (25 mM) of betaine also significantly inhibited the uptake of proline by SV-3T3 cells. Although very similar results were obtained with MDCK cells, prolonged exposure of cells to hypertonic conditions revealed distinct differences. When the hypertonic incubation was extended from 6 h to 24 h, betaine transport in SV-3T3 cells either remained the same or decreased, whereas it showed a further marked increase in MDCK cells, and also became sensitive to inhibition by gamma-aminobutyric acid. mRNA for the betaine transporter BGT-1 [Yamauchi, Uchida, Kwon, Preston, Brooks Robey, Garcia-Perez, Burg and Handler (1992) J. Biol. Chem. 267, 649-652] was detectable in MDCK cells exposed to hypertonic medium for 24 h, but not in SV-3T3 cells under any conditions. It is concluded that SV-3T3 cells do not produce a specific inducible transporter analogous to BGT-1, but they can accumulate betaine via the amino acid transport system A.

Regulatory and molecular aspects of mammalian amino acid transport

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Effect of betaine on HSP70 expression and cell survival during adaptation to osmotic stress

Induced expression of the HSP70 gene in 3T3 and SV-3T3 cells was monitored by measurements of the synthesis of HSP70 and of the cellular contents of both HSP70 and its mRNA. The presence of betaine (N-trimethylglycine) at concentrations of 2.5-25 mM decreased the induction of HSP70 gene expression caused by incubation of 3T3 and SV-3T3 cells in hypertonic (0.5 osM) medium. This effect was accompanied by an enhancement of SV-3T3 cell adaptation, assayed by colony formation, to the hyperosmotic conditions. In contrast, the presence of betaine did not affect HSP70 gene expression induced in these cells by heat shock. After 6 h incubation with 25 mM betaine under hypertonic (0.5 osM) conditions the intracellular concentration of betaine in SV-3T3 cells was about 195 mM, compared with about 70 mM under isotonic (0.3 osM) conditions. Hence, with this concentration of extracellular betaine, the marked increase in the accumulation of betaine within the cells presumably counteracts the imposed osmotic pressure and eliminates the signal that otherwise initiates increased expression of the HSP70 gene.

Regulation of matrix synthesis rates by the ionic and osmotic environment of articular chondrocytes

Chondrocytes in cartilage are embedded in a matrix containing a high concentration of proteoglycans and hence of fixed negative charges. Their extracellular ionic environment is thus different from that of most cells, with extracellular Na+ being 250-350 mM and extracellular osmolality 350-450 mOsm. When chondrocytes are isolated from the matrix and incubated in standard culture medium (DMEM; osmolality 250-280 mOsm), their extracellular environment changes sharply. We incubated isolated bovine articular chondrocytes and cartilage slices in DMEM whose osmolality was altered over the range 250-450 mOsm by Na+ or sucrose addition. 35S-sulphate and 3H-proline incorporation rates were at a maximum when the extracellular osmolality was 350-400 mOsm for both freshly isolated chondrocytes and for chondrocytes in cartilage. The incorporation rate per cell of isolated chondrocytes was only 10% that of chondrocytes in situ both 4 and 24 hours after isolation. For freshly isolated chondrocytes, the rate increased 30-50% in DMEM to which NaCl or sucrose had been added to increase osmolality. In chondrocytes incubated overnight in DMEM, the rate was greatest in DMEM of normal osmolality and fell from the maximum in proportion to the change in osmolality. The effects of sucrose addition on incorporation rates were similar but not identical to those of Na+ addition. Changes in cell volume might be linked to changes in synthesis rates since the cell volume of chondrocytes (measured by Coulter-counter) increased 30-40% when the cells were removed from their in situ environment into DMEM. Synthesis rates can thus be partly regulated by changes in extracellular osmolality, which in cartilage is controlled by proteoglycan concentration. This provides a mechanism by which the chondrocytes can rapidly respond to changes in extracellular matrix composition.

Modulation by betaine of cellular responses to osmotic stress

Various solutes were tested to see if they could modify the responses of SV-3T3 cells to hyperosmotic (0.5 osM) conditions, which cause an inhibition of general cell protein synthesis and of the rate of cell proliferation, coupled with an induction of amino acid transport activity. The added solutes were glycerol, proline, taurine, betaine, dimethylglycine and sarcosine. Of these, betaine produced the most dramatic and consistent effects. Addition of 10-25 mM-betaine to the hyperosmotic medium largely prevented the 90% inhibition of cell proliferation that occurred in its absence. Whether it was added initially or after the cells were exposed to hyperosmotic medium, 25 mM-betaine also converted a 50% recovery of the rate of protein synthesis into 100%. Similarly, the same concentrations of betaine prevented a 30% decrease in cell volume and decreased the induction of amino acid transport via system A by 73%. Lower concentrations of betaine produced smaller but still significant changes in these functional responses. With chick-embryo fibroblasts, under identical hyperosmotic conditions, 25 mM-betaine completely counteracted a 75% inhibition of the rate of protein synthesis. At present it is not clear how betaine modulates these effects of hyperosmolarity on cell functions.

A 33 kDa protein band is enhanced during long-term adaptation of EUE cells to a hypertonic medium

A cell line derived from human embryonic epithelium (EUE cells) shows an enhanced expression of a 33 kDa protein when adapted to grow in a hypertonic medium containing 0.246 M NaCl (1.8 x the isotonic concentration). The maximum amount of this protein, followed by SDS-PAGE electrophoresis, was found after 4 days of adaptation; thereafter, the protein band remained fairly constant up to 30 days. When the cells were transferred back to a medium containing 0.137 M NaCl (isotonic medium), the protein pattern reverted to that of control cells. This protein is mainly localized in the cytosol, although a small part is associated with the 150,000 g pellet and needs detergents to be extracted. The molecular weight and the cellular location suggest a possible analogy with the so-called amphitropic proteins, that are known to interact with both the epidermal growth factor receptor and hydrophobic structures, such as the membrane phospholipids and the cytoskeletal components.

Differential adaptive response to hyperosmolarity of 3T3 and transformed SV3T3 cells

Both 3T3 and simian virus 40-transformed 3T3 (SV3T3) cells were used to investigate differences in population kinetics, protein synthesis, monovalent ion levels, and amino acid accumulations between normal and transformed cells exposed to hyperosmolarity at 0.5 Osm. Under similar culture conditions, SV3T3 cells were found to be more sensitive in their proliferative response than normal cells to the hyperosmolar treatment. In the normal 3T3 cells, the increase in transport of amino acids was less sustained and was associated with higher levels of accumulated amino acids. The equilibrium distribution of intracellular monovalent cations and the rate of protein synthesis also returned faster to baseline values in the normal cells than in the transformed cells. Two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) analysis revealed the induction of a 69-kDa polypeptide in the 3T3 cells but not in the SV3T3 cells after exposure to hyperosmolarity. On electrofocusing and relative mass analysis, this polypeptide closely migrated with the 70-kDa heat shock protein (hsp) family, although it was unrelated immunologically to the inducible 72-kDa hsp.

Induction of amino acid transport activity in chick embryo fibroblasts by replacement of extracellular sodium chloride with disaccharide

The activity of amino acid transport System A in avian fibroblasts was increased following incubation of the cells in a medium in which most of the NaCl normally present had been isoosmotically replaced by sucrose. This increase was detectable after 2 h of incubation, reached a maximum at about 4 h, and remained constant thereafter. Transfer of treated cells back to a normal medium resulted in decay of the induced transport activity, with a half-life of less than 2 h. Kinetic analysis revealed that the increase in transport activity arose from an increase in Vmax, with little change in Km. This induction of System A activity did not occur if an inhibitor of either RNA or protein synthesis was present in the modified medium. The use of various different solutes as replacements for NaCl in the incubation medium showed that, although each replacement caused a decrease in both cellular Na+ content and protein synthesis, only disaccharides produced the increase in amino acid transport activity. In addition, estimates of cell volume indicated that, even under iso-osmotic conditions, incubation in the sucrose-containing medium caused initial cell shrinkage, followed by swelling. It is concluded that this induction of System A activity is associated with a volume regulatory process and that this process probably accounts for the parallel responses previously observed when cells were incubated in hyperosmolar media. Induction of amino acid transport activity by this process is distinct from adaptive regulation, caused by amino acid starvation; but the two processes are not strictly additive, and so appear to converge at some step.

Anisosmotic cell volume regulation: a comparative view

A variety of organisms and cell types spanning the five taxonomic kingdoms are exposed, either naturally or through experimental means, to osmotic stresses. A common physiological response to these challenges is maintenance of cell volume through changes in the concentration of intracellular inorganic and organic solutes, collectively termed osmolytes. Research on the mechanisms by which the concentration of these solutes is regulated has proceeded along several experimental lines. Extensive studies on osmotically activated ion transport pathways have been carried out in vertebrate cells and tissues. Much of our knowledge on organic osmolytes has come from investigations on invertebrates, bacteria, and protists. The relative simplicity of bacterial genetics has provided a powerful and elegant tool to explore the modifications of gene expression during volume regulation. An implication of this diverse experimental approach is that phylogenetically divergent organisms employ uniquely adapted mechanisms of cell volume regulation. Given the probability that changes in extracellular osmolality were physiological stresses faced by the earliest organisms, it is more likely that cell volume regulation proceeds by highly conserved physiological processes. We review volume regulation from a comparative perspective, drawing examples from all five taxonomic kingdoms. Specifically, we discuss the role of inorganic and organic solutes in volume maintenance and the mechanisms by which the concentrations of these osmolytes are regulated. In addition, the processes that may transduce volume perturbations into regulatory responses, such as stretch activation of ion channels, intracellular signaling, and genomic regulation, are discussed. Throughout this review we emphasize areas we feel are important for future research.

Control of protein synthesis by extracellular Na+ in cultured fibroblasts

In chick embryo fibroblasts (CEFs), a partial substitution of extracellular Na+ with other cations or carbohydrates decreased the intracellular Na+ content without altering the K+ level. Concomitantly, a significant decrease in the serum-dependent rate of protein synthesis occurred. This phenomenon appeared to be quickly reversible upon reconstitution of the correct extracellular Na+ concentration in the culture medium. The presence of a transcriptional inhibitor such as actinomycin D during the treatment did not inhibit the reversibility of the phenomenon. The presence in the culture medium of K+ in such excess as to dissipate the membrane potential did not alter the observed relationship between the protein synthesis rate and the internal Na+ content. Analysis of the amino acid pool indicated that the observed inhibition of the rate of protein synthesis in CEFs incubated in low Na+ medium was not caused by an unbalanced availability of intracellular amino acids. In addition, intracellular pH, as estimated by the measurement of the equilibrium distribution of benzoic acid, did not show any significant alteration in cells incubated in the presence of bicarbonate buffer and in low extracellular Na+. Moreover, the relationship between the rate of protein synthesis and the internal Na+ content was still observed in CEFs cultured in bicarbonate-containing media, but at lower or higher than physiological pH. Analysis by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) of the proteins synthesized by CEFs cultured at a reduced extracellular Na+ concentration showed that specific alterations of gene expression occurred.

Regulation of proteoglycan synthesis rate in cartilage in vitro: influence of extracellular ionic composition

Load-bearing cartilages regularly experience changes in fluid content as the result of changing load. It has been found that these changes in fluid content influence proteoglycan synthesis. The mechanism for this effect is not known. We have measured the influence of changes in cartilage hydration on the [35S]sulphate incorporation rate in both bovine nasal and human articular cartilage in medium whose concentration varied over the range 0.2-2-times physiological strength. In physiological medium the incorporation rate fell in proportion to fluid loss with a 10% fall in cartilage hydration resulting in a 30-50% decrease in 35S-incorporation rates. However, in medium of 0.5-times physiological strength, where the incorporation rate was only 40% of control values, the incorporation rate increased initially rather than falling as the cartilage lost fluid. These changes in hydration and hence proteoglycan content resulted in changes in the extracellular ionic composition of cartilage. When this was monitored in terms of [Na+]c, the internal sodium concentration, as a marker for changes in cartilage ionic composition, we found that incorporation rate varied with [Na+]c rather than directly with hydration.

The role of the iris-ciliary complex in the control of protein synthesis by the organ cultured rabbit ocular lens

When freshly isolated rabbit lenses were co-cultured with iris-ciliary body (IC), the protein synthetic activity of these lenses decreased significantly. The inhibitory effect is temporal as longer incubation showed greater inhibition. it was also found that the presence of IC in the culture medium increases the sodium ion levels in the lenses and causes some decrease in the potassium ion levels. Both the protein synthetic activity and cation levels of the lenses cocultured with IC are at least partially reversible. It seems that the IC-derived factor(s) has a molecular weight of more than 1 kDa. It is not clear from these experiments whether the altered cation balance is responsible for the inhibition of the protein synthesis.

Variation in the relative synthesis of some proteins in mammalian cells exposed to hypertonic medium

Exposure of a number of quiescent murine and human cell lines to low-graded doses of cycloheximide (CXM) results in a pattern of protein synthesis consisting of enhanced and induced species. This pattern is reminiscent of but not identical to that observed after several stress treatments [V. Sorrentino et al. (1985) J. Cell. Physiol. 125, 313]. A pattern identical to that seen after exposure to CXM is synthesized when cells are exposed to an hypertonic growth medium resulting in a full and reversible block of the initiation of polypeptide chain. This suggests that this kind of response is triggered by a reduction of overall protein synthesis rather than by a slow-down of the elongation step. Analysis of the synthesis of histones and ribosomal proteins during these two nonphysiological treatments (CXM or high salt) shows that these classes of proteins are neither stimulated nor preferentially retained. In contrast, greatly enhanced levels of steady-state histone mRNAs have been observed which have been translated in a reticulocyte lysate system, but are not apparently translated in vivo.

Effects of hyperosmolarity on ligand processing and receptor recycling in the hepatic galactosyl receptor system

Binding, endocytosis, and degradation of asialo-orosomucoid (ASOR) mediated by the galactosyl (Gal) receptor were examined in isolated rat hepatocytes in complete media supplemented with an osmolite. The specific binding of 125I-ASOR to cells at 4 degrees C was unaffected by up to 0.4 M sucrose or NaCl. Unlike sucrose or NaCl, mannitol stimulated 125I-ASOR binding at low concentrations but inhibited binding at higher concentrations. Continuous internalization at 37 degrees C, which requires receptor recycling, was completely blocked at 0.2 M sucrose or 0.15 M NaCl, corresponding in each case to a total osmolality of about 550 mmol/kg. This effect was reversed and endocytic function was restored by washing the cells, indicating that cell viability was unaffected. The rate of degradation of internalized 125I-ASOR was also inhibited by increasing sucrose concentrations. This inhibition is due to a block in the delivery of ligand to lysosomes and not an effect on degradation per se. In the presence of 0.2 M sucrose, the rate and extent of endocytosis of surface-bound 125I-ASOR were, respectively, 33.0 +/- 8.1% and 69.4 +/- 10.5% (n = 8) of the control without sucrose. Under these conditions, the dissociation of internalized receptor-ASOR complexes was completely inhibited. When sucrose was added, the effect on the endocytosis of surface-bound 125I-ASOR was virtually immediate. Previous studies showed that about 40% of the surface-bound 125I-ASOR which is internalized can return to the cell surface still bound to receptor (Weigel and Oka: J Biol Chem 259:1150, 1984). If 0.2 M sucrose was added after endocytosis occurred, 125I-ASOR still returned to the cell surface, although the rate and extent of return were inhibited by more than 50%. Interestingly, hyperosmolarity is the only treatment we have found which can reversibly inhibit, although only partially, the endocytosis of surface-bound 125I-ASOR.

Hyperosmolarity-induced stress proteins in chick embryo fibroblasts

The effects of a short exposure of chick embryo fibroblasts to a hyperosmolar medium on monovalent cation content, rate of protein synthesis, and polypeptide pattern expression were studied. The hyperosmolar shock gave an immediate and pronounced inhibition of the protein-synthesis rate temporally related to a marked alteration of the intracellular Na+ content. Following the return of the cells to an osmolar environment, the internal Na+ content quickly resumed its previous level, while the recovery of the protein-synthesis rate was more gradual. During the recovery period, there was enhanced expression of at least 12 proteins. The 4 major induced proteins exhibited apparent molecular weights of 96, 87, 70, and 48 kDa. A reduction in the synthesis of five protein bands including three large polypeptides of 220, 160, and 140 kDa was also observed. A comparison with the 3 major proteins induced by a 44 degrees C heat shock indicated an apparent similarity with only two of the hyperosmolarity-inducible polypeptides. Moreover, evidence has been also obtained of the close similarity between the 96 and 75 kDa glucose-regulated proteins and the 96 and 75 kDa proteins inducible by a hyperosmolar shock or by a continuous hyperosmolar treatment, respectively. The kinetics of the stress-proteins appearance indicated nonsimultaneous induction. The presence of actinomycin D during the exposure of the cells to the stress and the recovery period suggested that the expression of some hyperosmolarity-enhanced proteins is regulated at the transcriptional level.

Induction of stress proteins by hyperosmolarity in normal and transformed cells

The synthesis of at least three proteins, with molecular weights of approximately 87, 70, and 53 kd, was enhanced following the exposure of chick embryo fibroblasts to hyperosmolar shock of 30 min at 0.6 osM. Two of these proteins, the 87 and 70 kd, comigrated on one-dimensional gel electrophoresis with the stress proteins induced by heat shock after 30 min at 44 degrees C. In 3T3 cells, the hyperosmolar shock enhanced the expression of two proteins of 88 and 52 kd, whereas the heat shock increased the synthesis of several new polypeptides including the 88 and 52 kd mw. In SV40-transformed 3T3 cells the synthesis of two proteins of 72 and 69 kd was enhanced by heat shock, but no change of the protein pattern was recorded after the hyperosmolar shock.