Effects of NaCl, glucose, and aldose reductase inhibitors on cloning efficiency of renal medullary cells

Long-term health outcomes associated with hydration status

Body water balance is determined by fundamental homeostatic mechanisms that maintain stable volume, osmolality and the composition of extracellular and intracellular fluids. Water balance is maintained by multiple mechanisms that continuously match water losses through urine, the skin, the gastrointestinal tract and respiration with water gains achieved through drinking, eating and metabolic water production. Hydration status is determined by the state of the water balance. Underhydration occurs when a decrease in body water availability, due to high losses or low gains, stimulates adaptive responses within the water balance network that are aimed at decreasing losses and increasing gains. This stimulation is also accompanied by cardiovascular adjustments. Epidemiological and experimental studies have linked markers of low fluid intake and underhydration - such as increased plasma concentration of vasopressin and sodium, as well as elevated urine osmolality - with an increased risk of new-onset chronic diseases, accelerated aging and premature mortality, suggesting that persistent activation of adaptive responses may be detrimental to long-term health outcomes. The causative nature of these associations is currently being tested in interventional trials. Understanding of the physiological responses to underhydration may help to identify possible mechanisms that underlie potential adverse, long-term effects of underhydration and inform future research to develop preventative and treatment approaches to the optimization of hydration status.

A comparative in vitro study of the effect of biospecific integrin recognition processes and substrate nanostructure on stem cell 3D spheroid formation

The in vitro study of the properties of the human mesenchymal stem cells as well as their manipulation in culture for clinical purposes depends on the elimination of artefacts caused by the lack of their natural environment. It is now widely accepted that mesenchymal stem cells should be studied when they are organised as 3D spheroids rather than fibroblast-like colonies. Although this can be achieved with the use of some extracellular matrix proteins or by non-adherent conditions these suffer of significant limitations. The recent development of synthetic substrates resembling the physicochemical and biochemical properties of the adult stem cell niche has prompted questions about the role played by nanotopography and receptor-mediated adhesion. In the present paper, the influence of two types of substrates bearing the same nanostructure, but exposing either a non-specific or an integrin-specific binding motif was studied. Carboxybetaine-tethered hyperbranched poly(ɛ-lysine) dendrons showed that the hyperbranched structure was fundamental to induce spheroid formation, but these were forming more slowly, were of reduced size and less stable than those growing on substrates based on the same hyperbranched structures that had been functionalised at their uppermost branching generation by a laminin amino acid sequence, i.e. YIGSR. The study shows that both nanostructure and biorecognition need to be combined to achieve a substrate for stem cell spheroid formation as that observed in vivo in the adult stem cell niche.

Effect of Hypertonic Stress on Amino Acid Levels and System a Activity in Rat Peritoneal Mesothelial Cells

Objective

Peritoneal mesothelial cells (PMC) are exposed to a hypertonic environment during peritoneal dialysis. When exposed to a hypertonic medium, many types of cells accumulate small osmotically active organic solutes, which are called osmolytes, to match the higher external osmolality. However, no information has been available concerning the osmolytes in PMC. To investigate osmoregulation in rat PMC, the levels of amino acids in the cells and the activity of system A, a major neutral amino acid transport, were measured after switching to a medium made hypertonic by the addition of NaCl. System A was measured by Na+-dependent [14C]-2-methylamino-isobutyric acid (MeAIB) uptake.

Results

Total amount of 20 amino acids increased from 306 to 757 nmol/mg protein after 12 hours of hypertonicity. The amount of neutral amino acids accounted for 81% of the increase in total amino acids. Glutamine, alanine, glycine, threonine, and serine were the major neutral amino acids that accumulated in the hypertonic mesothelial cells. The amount of neutral amino acids increased 2.9-fold after 12 hr of hypertonicity, and decreased thereafter. MeAIB uptake increased 36-fold relative to the uptake in isotonic cells after 4 – 8 hr of hypertonicity. When the culture medium was made hypertonic by adding raffinose or glucose, the activity of system A was also stimulated (raffinose > glucose > NaCl). System A was located on both the apical and basal sides of isotonic PMC, and extracellular hypertonicity stimulated the MeAIB uptake on both sides.

Conclusions

These data indicate that neutral amino acids and system A transport play an important role in earlyphase osmoregulation in rat peritoneal mesothelial cells.

Molecular basis of the osmolyte effect on protein stability: a lesson from the mechanical unfolding of lysozyme

Osmolytes are a class of small organic molecules that shift the protein folding equilibrium. For this reason, they are accumulated by organisms under environmental stress and find applications in biotechnology where proteins need to be stabilized or dissolved. However, despite years of research, debate continues over the exact mechanisms underpinning the stabilizing and denaturing effect of osmolytes. Here, we simulated the mechanical denaturation of lysozyme in different solvent conditions to study the molecular mechanism by which two biologically relevant osmolytes, denaturing (urea) and stabilizing (betaine), affect the folding equilibrium. We found that urea interacts favorably with all types of residues via both hydrogen bonds and dispersion forces, and therefore accumulates in a diffuse solvation shell around the protein. This not only provides an enthalpic stabilization of the unfolded state, but also weakens the hydrophobic effect, as hydrophobic forces promote the association of urea with nonpolar residues, facilitating the unfolding. In contrast, we observed that betaine is excluded from the protein backbone and nonpolar side chains, but is accumulated near the basic residues, yielding a nonuniform distribution of betaine molecules at the protein surface. Spatially resolved solvent–protein interaction energies further suggested that betaine behaves in a ligand- rather than solvent-like manner and its exclusion from the protein surface arises mostly from the scarcity of favorable binding sites. Finally, we found that, in the presence of betaine, the reduced ability of water molecules to solvate the protein results in an additional enthalpic contribution to the betaine-induced stabilization.

Regulation of gene expression by NFAT transcription factors in hibernating ground squirrels is dependent on the cellular environment

Calcineurin is a calmodulin-stimulated phosphatase that regulates the nuclear translocation of nuclear factor of activated T cell (NFAT) c1-4 through dephosphorylation. We believe that this mechanism plays various roles in the remodeling and maintenance of Ictidomys tridecemlineatus skeletal muscle. During hibernation, bouts of torpor and arousal take place, and squirrels do not lose muscle mass despite being inactive. Protein expression of Ca(2+) signaling proteins were studied using immunoblotting. A DNA-protein interaction ELISA technique was created to test the binding of NFATs in the nucleus to DNA probes containing the NFAT response element under environmental conditions reflective of those during hibernation. Calcineurin protein levels increased by 3.08-fold during torpor (compared to euthermic control), whereas calpain1 levels also rose by 3.66-fold during torpor. Calmodulin levels were elevated upon entering torpor. NFATc4 binding to DNA showed a 1.4-fold increase during torpor, and we found that this binding was further enhanced when 600 nM of Ca(2+) was supplemented. We also found that decreasing the temperature of ELISAs resulted in progressive decreases in the binding of NFATs c1, c3, and c4 to DNA. In summary, calmodulin and calpain1 appear to activate calcineurin and NFATc4 during torpor. NFAT binding to target promoters is affected by intranuclear [Ca(2+)] and environmental temperatures. Therefore, Ca(2+) signaling and temperature changes play key roles in regulation of the NFAT-calcineurin pathway in skeletal muscle of hibernating 13-lined ground squirrels over the torpor-arousal cycle, and they may contribute to the avoidance of disuse-induced muscle atrophy that occurs naturally in these animals.

Nutritional strategies to alleviate heat stress in pigs

Pigs are comparatively less heat tolerant than other species of production animals, which poses challenges for stock productivity and management during seasonal heat waves that occur in summer. The issues surrounding heat and pig production are predicted to increase, based on the actions of climate change increasing the intensity, frequency and duration of heat waves. Furthermore, future growth areas of pig production are going to be in tropical regions such as South-east Asia and Latin America. Efforts by the pig to dissipate excess body heat come at a cost to health and divert energy away from growth, compromising efficient pig production. Management of heat stress requires multiple strategies, and recent research is improving the understanding of the application of nutritional strategies to ameliorate the effects of heat stress. In particular the use of feed additives is an important, flexible and economical method to alleviate heat stress and the intensive nature of pig production lends itself to the use of additives. Some specific examples include antioxidants, betaine and chromium, which have been proved effective or being tested in mitigating some certain impacts of heat stress in pigs. The aim of this review is to summarise recent advances in the nutritional management of heat stress in pigs.

Computational investigation on microsolvation of the osmolyte glycine betaine [GB (H(2)O)(1-7)]

The preferential interactions of glycine betaine (GB) with solvent components and the effect of solvent on its stability have been examined. In particular, the microsolvation of organic osmolyte and widely important osmoprotectant in nature as glycine betaine has been reported by using M06 method. A number of configurations (b(X) (a-z)) of the clusters for one to seven water molecules (× = 1-7) have been considered for the microsolvation. Structures of stable conformers are obtained and denoted as b1a, b2a, b3a, b4a, b5a, b6a and b7a. It is observed from the interaction energy difference (∆E) that only seven water molecules can be accommodated in the first solvation shell to stabilize GB. It is also observed that the calculated relative energy using M06 is in close agreement with calculations at the MP2 level of theory.

Determination of trigonelline in seeds and vegetable oils by capillary electrophoresis as a novel marker for the detection of adulterations in olive oils

A capillary electrophoresis method with UV detection was developed for the first time for the determination of the pyridine betaine trigonelline (N-methylnicotinic acid) in seeds and vegetable oils. Analytical characteristics of the method showed its good performance in terms of linearity (r > 0.999), precision (relative standard deviations < 5%), and limits of detection (up to 0.9 microM or 1 ng/g for oils). The developed method was applied to the analysis of soy and sunflower seeds, three varieties of olives, and sunflower, soy, and extra virgin olive oils. Trigonelline was determined in soy and sunflower seeds and their respective oils, whereas it was not detected in olives or olive oils. Different mixtures of extra virgin olive oil with seed oils were analyzed, detecting up to 10% of soy oil in olive oil. As a consequence, trigonelline is proposed in this work as a novel marker for the detection of adulterations of olive oils with other vegetable oils such as soy and sunflower oils.

The betaine content of sweat from adolescent females

Background

This study was developed to establish whether betaine was present in the sweat of females and to determine any correlations with other sweat components.

Methods

Sweat patches were placed on eight trained adolescent Highland dancers (age = 13.6 ± 2.3 yr), who then participated in a dance class for 2 hours. Patches were removed, and the sweat recovered via centrifugation. The sweat was subsequently analyzed for betaine, choline, sodium, potassium, chloride, lactate, glucose, urea and ammonia.

Results

Betaine was present in the sweat of all subjects (232 ± 84 μmol·L^-1), which is higher than typically found in plasma. The concentration of several sweat components were correlated, in particular betaine with most other measured components.

Conclusion

Betaine, an osmoprotectant and methyl donor, is a component of sweat that may be lost from the body in significant amounts.

Betaine structure and the presence of hydroxyl groups alters the effects on DNA melting temperatures

Betaine lowers the melting temperature of deoxyribonucleic acid (DNA) and decreases its dependence on base composition. The effects of synthetic betaine analogs on the melting of DNA samples with different GC content were measured. Since many polyhydroxy compounds also lower DNA melting temperatures, hydroxyl-substituted betaine analogs were included. Some synthetic sulfonate analogs of betaine lowered the DNA melting temperatures by twice as much at the same molar concentration. They were up to twice as effective at decreasing the base pair dependence. Some carboxylate homologs of betaine, substituted with hydroxyl groups, increased the melting temperature. This effect was greater with low GC content DNA. Sulfonate analogs of betaine with hydroxyl groups usually destabilize the DNA, while their carboxylate analogs stabilize the DNA. Distances between the charges of these synthetic zwitterionic solutes influence the effect on DNA, with the optimum separation being two or three methylene groups. A betaine with two hydroxyl groups on one N-alkyl group had a greater effect than an isomer with two hydroxyl groups on separate N-alkyl substituents. We suggest that the effect of these solutes depends on structuring the hydration water of DNA, as well as interactions with the DNA structure itself.

A metabonomic and proteomic analysis of changes in IMCD3 cells chronically adapted to hypertonicity

BACKGROUND

The genomic response to adaptation of IMCD3 cells to hypertonicity results in both upregulation and downregulation of a variety of genes.

METHOD

The present study was undertaken to assess the metabonomic and proteomic response of IMCD3 cells that have been chronically adapted to hypertonicity (600 and 900 mosm/kg H(2)O) as compared to cells under isotonic conditions.

RESULTS

Adaptation of IMCD3 cells to hypertonic conditions resulted in a change of a wide range of organic osmolytes, including sorbitol (+8,291%), betaine (+1,099%), myo-inositol (+669%), taurine (+113%) and glycerophosphorylcholine (+61%). Evaluation of the polyol pathway for sorbitol production revealed a reduction in sorbitol dehydrogenase and an increase in aldose reductase mRNA in adapted cells. Proteome analysis revealed increased expression of six glycolytic proteins, including malic enzyme and pyruvate carboxylase, indicating the activation of the pyruvate shunt and changes in glucose metabolism. This study showed that the observed reduction in cell replication could possibly reflect a redirection of cellular energy from cell growth and replication to maintenance of intracellular ion levels in chronically adapted cells.

CONCLUSION

The combined metabonomic and proteomic analysis was shown to be a very helpful tool for the analysis of the effects caused by chronic adaptation to hypertonicity. It made it possible to better evaluate the importance of certain changes that occur in the process of adaptation.

Osmotic and metabolic responses to dehydration and urea-loading in a dormant, terrestrially hibernating frog

Physiological responses to dehydration in amphibians are reasonably well documented, although little work has addressed this problem in hibernating animals. We investigated osmotic and metabolic responses to experimental manipulation of hydration state in the wood frog (Rana sylvatica), a terrestrial hibernator that encounters low environmental water potential during autumn and winter. In winter-conditioned frogs, plasma osmolality varied inversely with body water content (range 69-79%, fresh mass) primarily due to increases in sodium and chloride concentrations, as well as accumulation of glucose and urea. Decreased hydration was accompanied by a marked reduction in the resting rate of oxygen consumption, which was inversely correlated with plasma osmolality and urea concentration. In a separate experiment, resting rates of oxygen consumption in fully hydrated frogs receiving injections of saline or saline containing urea did not differ initially; however, upon dehydration, metabolic rates decreased sooner in the urea-loaded frogs than in control frogs. Our findings suggest an important role for urea, acting in concert with dehydration, in the metabolic regulation and energy conservation of hibernating R. sylvatica.

Genetic restoration of aldose reductase to the collecting tubules restores maturation of the urine concentrating mechanism

To investigate the underlying causes for aldose reductase deficiency-induced diabetes insipidus, we carried out studies with three genotypic groups of mice. These included wild-type mice, knockout mice, and a newly created bitransgenic line that was homozygous for both the aldose reductase null mutation and an aldose reductase knockin transgene driven by the kidney-specific cadherin promoter to direct transgene expression in the collecting tubule epithelial cells. We found that from early renal developmental stages onward, urine osmolality did not exceed 1,000 mosmol/kgH2O in aldose reductase-deficient mice. The functional defects were correlated with significant renal cellular and structural abnormalities that included cell shrinkage, apoptosis, disorganized tubular and vascular structures, and segmental atrophy. In contrast, the transgenic aldose reductase expression in the bitransgenic mice largely but incompletely rescued urine concentrating capacity and significantly improved renal cell survival, cellular morphology, and renal structures. Together, these results suggest that aldose reductase not only plays important roles in osmoregulation and medullary cell survival but may also be essential for the full maturation of the urine concentrating mechanism.

Ca2+ signaling, intracellular pH and cell volume in cell proliferation

Mitogens control progression through the cell cycle in non-transformed cells by complex cascades of intracellular messengers, such as Ca2+ and protons, and by cell volume changes. Intracellular Ca2+ and proton concentrations are critical for linking external stimuli to proliferation, motility, apoptosis and differentiation. This review summarizes the role in cell proliferation of calcium release from intracellular stores and the Ca2+ entry through plasma membrane Ca2+ channels. In addition, the impact of intracellular pH and cell volume on cell proliferation is discussed.

Organic osmolytes as compatible, metabolic and counteracting cytoprotectants in high osmolarity and other stresses

Organic osmolytes are small solutes used by cells of numerous water-stressed organisms and tissues to maintain cell volume. Similar compounds are accumulated by some organisms in anhydrobiotic, thermal and possibly pressure stresses. These solutes are amino acids and derivatives,polyols and sugars, methylamines, methylsulfonium compounds and urea. Except for urea, they are often called `compatible solutes', a term indicating lack of perturbing effects on cellular macromolecules and implying interchangeability. However, these features may not always exist, for three reasons. First, some of these solutes may have unique protective metabolic roles, such as acting as antioxidants (e.g. polyols, taurine, hypotaurine),providing redox balance (e.g. glycerol) and detoxifying sulfide (hypotaurine in animals at hydrothermal vents and seeps). Second, some of these solutes stabilize macromolecules and counteract perturbants in non-interchangeable ways. Methylamines [e.g. trimethylamine N-oxide (TMAO)] can enhance protein folding and ligand binding and counteract perturbations by urea (e.g. in elasmobranchs and mammalian kidney), inorganic ions, and hydrostatic pressure in deep-sea animals. Trehalose and proline in overwintering insects stabilize membranes at subzero temperatures. Trehalose in insects and yeast,and anionic polyols in microorganisms around hydrothermal vents, can protect proteins from denaturation by high temperatures. Third, stabilizing solutes appear to be used in nature only to counteract perturbants of macromolecules,perhaps because stabilization is detrimental in the absence of perturbation. Some of these solutes have applications in biotechnology, agriculture and medicine, including in vitro rescue of the misfolded protein of cystic fibrosis. However, caution is warranted if high levels cause overstabilization of proteins.

DNA damage and osmotic regulation in the kidney

Renal medullary cells normally are exposed to extraordinarily high interstitial NaCl concentration as part of the urinary concentrating mechanism, yet they survive and function. Acute elevation of NaCl to a moderate level causes transient cell cycle arrest in culture. Higher levels of NaCl, within the range found in the inner medulla, cause apoptosis. Recently, it was surprising to discover that even moderately high levels of NaCl cause DNA double-strand breaks. The DNA breaks persist in cultured cells that are proliferating rapidly after adaptation to high NaCl, and DNA breaks normally are present in the renal inner medulla in vivo. High NaCl inhibits repair of broken DNA both in culture and in vivo, but the DNA is rapidly repaired if the level of NaCl is reduced. The inhibition of DNA repair is associated with suppressed activity of some DNA damage-response proteins like Mre11, Chk1, and H2AX but not that of others, like GADD45, p53, ataxia telangiectasia-mutated kinase (ATM), and Ku86. In this review, we consider possible mechanisms by which the renal cells escape the known dangerous consequences of persistent DNA damage. Furthermore, we consider that the persistent DNA damage may be a sensor of hypertonicity that activates ATM kinase to provide a signal that contributes to protective osmotic regulation.

Counteraction of urea-induced protein denaturation by trimethylamine N-oxide: a chemical chaperone at atomic resolution

Proteins are very sensitive to their solvent environments. Urea is a common chemical denaturant of proteins, yet some animals contain high concentrations of urea. These animals have evolved an interesting mechanism to counteract the effects of urea by using trimethylamine N-oxide (TMAO). The molecular basis for the ability of TMAO to act as a chemical chaperone remains unknown. Here, we describe molecular dynamics simulations of a small globular protein, chymotrypsin inhibitor 2, in 8 M urea and 4 M TMAO/8 M urea solutions, in addition to other control simulations, to investigate this effect at the atomic level. In 8 M urea, the protein unfolds, and urea acts in both a direct and indirect manner to achieve this effect. In contrast, introduction of 4 M TMAO counteracts the effect of urea and the protein remains well structured. TMAO makes few direct interactions with the protein. Instead, it prevents unfolding of the protein by structuring the solvent. In particular, TMAO orders the solvent and discourages it from competing with intraprotein H bonds and breaking up the hydrophobic core of the protein.

Greater tolerance of renal medullary cells for a slow increase in osmolality is associated with enhanced expression of HSP70 and other osmoprotective genes

In tests of osmotic tolerance of renal inner medullary cells in tissue culture, osmolality has usually been increased in a single step, whereas in vivo the increase occurs gradually over several hours. We previously found that more passage 2 mouse inner medullary epithelial (p2mIME) cells survive a linear increase in NaCl and urea from 640 to 1,640 mosmol/kgH2O over 20 h (which is similar to the change that may occur in vivo) than they do a step increase. The present studies examine accompanying differences in gene expression. Among mRNAs of genes known to be protective, tonicity-responsive enhancer binding protein and aldose reductase increase with a linear but decrease with a step increase; betaine transporter BGT1 decreases with a step but not a linear increase; heat shock protein 70.1 (HSP70.1) and HSP70.3 increase more with a linear than a step increase; and osmotic stress protein 94 and heme oxygenase-1 increase with a linear but decrease with a step increase. mRNAs for known urea-responsive proteins, GADD153 and Egr-1, increase with both a step and linear increase. A step increase in urea alone reduces mRNAs, similar to the combination of NaCl and urea, but a step increase in NaCl alone does not. HSP70 protein increases substantially with a linear rise in osmolality but does not change significantly with a step rise. We speculate that poorer survival of p2mIME cells with a step than with linear increase in NaCl and urea is accounted for, at least in part, by urea-induced suppression of protective genes, particularly HSP70.

Compatible and counteracting solutes: protecting cells from the Dead Sea to the deep sea

Cells of many organisms accumulate certain small organic molecules--called compatible and counteracting solutes, compensatory solutes, or chemical chaperones--in response to certain physical stresses. These solutes include certain carbohydrates, amino acids, methylamine and methylsulphonium zwitterions, and urea. In osmotic dehydrating stress, these solutes serve as cellular osmolytes. Unlike common salt ions and urea (which inhibit proteins), some organic osmolytes are compatible; i.e., they do not perturb macromolecules such as proteins. In addition, some may protect cells through metabolic processes such as antioxidation reactions and sulphide detoxification. Other osmolytes, and identical or similar solutes accumulated in anhydrobiotic, heat and pressure stresses, are termed counteracting solutes or chemical chaperones because they stabilise proteins and counteract protein-destabilising factors such as urea, temperature, salt, and hydrostatic pressure. Stabilisation of proteins, not necessarily beneficial in the absence of a perturbant, may result indirectly from effects on water structure. Osmotic shrinkage of cells activates genes for chaperone proteins and osmolytes by mechanisms still being elucidated. These solutes have applications in agriculture, medicine and biotechnology.

Mammalian osmolytes and S-nitrosoglutathione promote Delta F508 cystic fibrosis transmembrane conductance regulator (CFTR) protein maturation and function

In cystic fibrosis, the absence of functional CFTR results in thick mucous secretions in the lung and intestines, as well as pancreatic deficiency. Although expressed at high levels in the kidney, mutations in CFTR result in little or no apparent kidney dysfunction. In an effort to understand this phenomenon, we analyzed Delta F508 CFTR maturation and function in kidney cells under conditions that are common to the kidney, namely osmotic stress. Kidney cells were grown in culture and adapted to 250 mM NaCl and 250 mM urea. High performance liquid chromatography analysis of lysates from kidney cells adapted to these conditions identified an increase in the cellular osmolytes glycerophosphorylcholine, myo-inositol, sorbitol, and taurine. In contrast to isoosmotic conditions, hyperosmotic stress led to the proper folding and processing of Delta F508 CFTR. Furthermore, three of the cellular osmolytes, when added individually to cells, proved effective in promoting the proper folding and processing of the Delta F508 CFTR protein in both epithelial and fibroblast cells. Whole-cell patch clamping of osmolyte-treated cells showed that Delta F508 CFTR had trafficked to the plasma membrane and was activated by forskolin. Encouraged by these findings, we looked at other features common to the kidney that may impact Delta F508 maturation and function. Interestingly, a small molecule, S-nitrosoglutathione, which is a substrate for gamma glutamyltranspeptidase, an abundant enzyme in the kidney, likewise promoted Delta F508 CFTR maturation and function. S-Nitrosoglutathione-corrected Delta F508 CFTR exhibited a shorter half-life as compared with wild type CFTR. These results demonstrate the feasibility of a small molecule approach as a therapeutic treatment in promoting Delta F508 CFTR maturation and function and suggest that an additional treatment may be required to stabilize Delta F508 CFTR protein once present at the plasma membrane. Finally, our observations may help to explain why Delta F508 homozygous patients do not present with kidney dysfunction.

COX2 activity promotes organic osmolyte accumulation and adaptation of renal medullary interstitial cells to hypertonic stress

The mechanism by which COX2 inhibition decreases renal cell survival is poorly understood. In the present study we examined the effect of COX2 activity on organic osmolyte accumulation in renal medulla and in cultured mouse renal medullary interstitial cells (MMICs) and its role in facilitating cell survival. Hypertonicity increased accumulation of the organic osmolytes inositol, sorbitol, and betaine in cultured mouse medullary interstitial cells. Pretreatment of MMICs with a COX2-specific inhibitor (SC58236, 10 micromol/liter) dramatically reduced osmolyte accumulation (by 79 +/- 9, 57 +/- 12, and 96 +/- 10% for inositol, sorbitol, and betaine respectively, p < 0.05). Similarly, 24 h of dehydration increased inner medullary inositol, sorbitol, and betaine concentrations in vivo by 85 +/- 10, 197 +/- 28, and 190 +/- 24 pmol/microg of protein, respectively, but this increase was also blunted (by 100 +/- 5, 66 +/- 15, and 81 +/- 9% for inositol, sorbitol, and betaine, respectively, p < 0.05) by pretreatment with an oral COX2 inhibitor. Dehydrated COX2-/- mice also exhibited an impressive defect in sorbitol accumulation (88 +/- 9% less than wild type, p < 0.05) after dehydration. COX2 inhibition (COX2 inhibitor-treated or COX2-/- MMICs) dramatically reduced the expression of organic osmolyte uptake mechanisms including betaine (BGT1) and sodium-myo-inositol transporter and aldose reductase mRNA expression under hypertonic conditions. Importantly, preincubation of COX2 inhibitor-treated MMICs with organic osmolytes restored their ability to survive hypertonic stress. In conclusion, osmolyte accumulation in the kidney inner medulla is dependent on COX2 activity, and providing exogenous osmolytes reverses COX2-induced cell death. These findings may have implications for the pathogenesis of analgesic nephropathy.

pH dependence of Na+/myo-inositol cotransporters in rat thick limb cells

BACKGROUND

To balance medullary interstitium hypertonicity generated by transepithelial NaCl absorption, medullary thick ascending limb (MTAL) cells accumulate myo-inositol (MI). Expression of Na+-MI cotransporter (SMIT) mRNA in TAL is correlated with the NaCl absorption rate. Our present study aimed to determine the plasma membrane location and functional properties of the Na+-MI cotransporter in MTAL cells.

METHODS

Preparation of basolateral (BLMV) and luminal (LMV) membrane vesicles were simultaneously isolated from purified rat MTAL suspension, and uptake of [3H]myo-inositol ([3H]MI) was used to assess Na+-MI cotransport activity.

RESULTS

In the presence of an inside-negative membrane potential, imposing an inwardly-directed Na+-gradient versus tetramethylammonium (TMA) stimulated the initial [3H]MI uptake in BLMV and LMV. Phlorizin inhibited Na+ gradient-dependent initial [3H]MI uptake in both preparations, with IC50 values of 565 and 29 micromol/L in BLMV and LMV, respectively. 2-0,C-methylene myo-inositol (MMI), a competitive inhibitor of MI transport, only inhibited the BLMV Na+-MI cotransporter. Phlorizin-sensitive Na+ gradient-dependent initial [3H]MI uptake showed Michaelis-Menten kinetics in both preparations, with similar Vmax but different Km values of 51 and 107 micromol/L in BLMV and LMV, respectively. Finally, BLMV but not LMV Na+-MI cotransporter exhibited a marked pH dependence with sigmoidal patterns of activation, as intravesicular pH (pHi) was decreased from 8.0 to 6.0 at extravesicular pH (pHe) 8.0, and as pHe was increased from 6.0 to 8.0 at pHi 6.0. Maximal activation was observed at pHi 6.5 and pHe 7.5.

CONCLUSIONS

In rat MTAL cells, Na+-MI cotransporter activity is present in both BLM and LM, and has markedly different functional properties, indicating the presence of distinct transporters. Basolateral Na+-MI cotransporter activity is maximal at physiological pH values of MTAL cells and interstitium, and a powerful modulation of the transporter activity may be exerted by pHe and pHi.

Inhibition of aldose reductase enhances HeLa cell sensitivity to chemotherapeutic drugs and involves activation of extracellular signal-regulated kinases

Changes in glucose metabolism during diabetes are linked to an increased risk for the development of cancer. Increased activity of aldose reductase, the rate-limiting polyol pathway enzyme that converts glucose into sorbitol, mediates pathologies associated with diabetes and is thought to be involved in increased resistance to chemotherapeutic drugs. Thus, increased intracellular sorbitol levels may serve a protective function in cancer cells. In these studies we determined whether an inhibitor of aldose reductase could enhance the effectiveness of anticancer agents. Our findings indicate that treatment with the aldose reductase inhibitor, ethyl 1-benzyl-3-hydroxy-2(5H)-oxopyrrole-4-carboxylate (EBPC), enhances the cytotoxic effects of the anticancer agents doxorubicin and cisplatin in HeLa cervical carcinoma cells. To establish a mechanistic basis for the increased cytotoxicity by EBPC, we examined the activity of the extracellular signal-regulated kinase (ERK) pathway, which is an important regulator of cell growth. Interestingly, treatment with EBPC in combination with the chemotherapeutic drugs increased ERK activity as compared to treatment with the chemotherapeutic drugs, suggesting a possible role for the ERK pathway in mediating doxorubicin- or cisplatin-induced cell death. Consistent with this possibility, inhibition of ERK activation by the MEK inhibitor, U0126, reversed the EBPC-mediated enhancement of cell death. In summary, these data provide evidence that adjuvant therapy with aldose reductase inhibitors improves the effectiveness of chemotherapeutic drugs, possibly through an ERK pathway-mediated mechanism.

The expression of the gamma subunit of Na-K-ATPase is regulated by osmolality via C-terminal Jun kinase and phosphatidylinositol 3-kinase-dependent mechanisms

The alpha and beta subunits of Na-K-ATPase are up-regulated by hypertonicity in inner-medullary collecting duct cells adapted to survive in hypertonic conditions. We examined the regulation of the gamma subunit by hypertonicity. Although cultured inner-medullary collecting duct cells lacked the gamma subunits, both variants gamma(a) and gamma(b) were expressed in cells adapted to 600 and 900 mosmol/KgH(2)O. This expression was reversible with a half-time of 17.2 +/- 0.5 h. The message of the gamma subunit was absent in isotonic conditions and increased with higher tonicity in adapted cells. In acute experiments the appearance of the gamma subunit was found to be both time-dependent (> or =24 h) and osmolality-dependent (> or =500 mosmol/KgH(2)O). No induction was noted with urea and only minimal induction with mannitol. Increasing concentrations of the phosphatidylinositol 3-kinase inhibitor LY294002 resulted in a dose-dependent decrement in the expression of the gamma subunit with total abolition at 10 microM. This was associated with a decrease in cell viability as <20% survived the treatment with 10 microM of LY294002. Neither inhibition of extracellular response kinase nor p38 mitogen-activated protein kinase inhibited osmotic induction of the gamma subunit. In contrast, cells transfected with a dominant negative c-Jun N-terminal kinase 2-APF construct displayed complete inhibition of the gamma subunit. Such cells have accelerated loss of viability in hypertonic conditions. This study describes the regulation of the gamma subunit of Na-K-ATPase by hypertonicity. This regulation is transcriptionally regulated and involves signaling mediated by phosphatidylinositol 3-kinase and c-Jun N-terminal kinase 2 pathways.

Near infra-red spectra of urea with glycine betaine or trimethylamine N-oxide are additive

Glycine betaine and trimethylamine-N-oxide counteract urea denaturation in solutions containing urea and the methylamine in the mole ratio of 2:1. Near infra-red difference spectra (water spectrum subtracted) of solutions containing both urea with either glycine betaine or trimethylamine-N-oxide can be predicted from the spectra of the single solutes, with r(2)>0.999 both using the spectrum from 1200 to 2100 nm (where most absorbance is attributable to hydrogen bonding) and using an extended range 1000 to 2500 nm, which includes solute specific bands. Thus urea and the kosmotropes appear to interact with water independently and the counteraction cannot be attributed to specific interactions between them. The spectrum of aqueous glycine betaine can be predicted from tetramethylammonium and formate ions (r(2)=0.998), suggesting that independent interactions of the quaternary amine, and of the carboxyl function, with water are dominant. The exceptional properties of glycine betaine do not arise from specific intramolecular interactions between the charged groups.

Urea increases tolerance of yeast inorganic pyrophosphatase activity to ethanol: the other side of urea interaction with proteins

Ethanol is the major product of yeast sugar fermentation and yet, at certain concentrations, it is very toxic to yeast cells. The major targets for ethanol's toxicity are the plasma membrane and the cytosolic enzymes: ethanol alters membrane organization and permeability and inactivates and unfolds globular cytosolic enzymes. The effects of ethanol on the plasma membrane are attenuated by the presence of trehalose, a disaccharide of glucose that is accumulated simultaneously with urea. The data presented in this paper show that trehalose is not effective at protecting yeast cytosolic inorganic pyrophosphatase against the inactivation of its catalytic activity promoted by alcohols. In contrast, 1 M trehalose increased the toxicity of alcohols against pyrophosphatase by at least 34%. On the other hand, 1.5 M urea attenuated the inactivation of pyrophosphatase promoted by alcohols by approximately 50%. Here we propose that, in the presence of alcohols, urea functions as a molecular filter, enriching the vicinity of the protein with water and excluding alcohol molecules. Conversely, trehalose tends to increase the interaction of alcohols with protein molecules, by withdrawing water, leading to a stronger inactivation promoted for a given concentration of alcohol in the bulk solution on pyrophosphatase activity.

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.

Factors affecting counteraction by methylamines of urea effects on aldose reductase

The concentration of urea in renal medullary cells is high enough to affect enzymes seriously by reducing Vmax or raising Km, yet the cells survive and function. The usual explanation is that the methylamines found in the renal medulla, namely glycerophosphocholine and betaine, have actions opposite to those of urea and thus counteract its effects. However, urea and methylamines have the similar (not counteracting) effects of reducing both the Km and Vmax of aldose reductase (EC 1.1.1.21), an enzyme whose function is important in renal medullas. Therefore, we examined factors that might determine whether counteraction occurs, namely different combinations of assay conditions (pH and salt concentration), methylamines (glycerophosphocholine, betaine, and trimethylamine N-oxide), substrates (DL-glyceraldehyde and D-xylose), and a mutation in recombinant aldose reductase protein (C298A). We find that Vmax of both wild-type and C298A mutant generally is reduced by urea and/or the methylamines. However, the effects on Km are much more complex, varying widely with the combination of conditions. At one extreme, we find a reduction of Km of wild-type enzyme by urea and/or methylamines that is partially additive, whereas at the other extreme we find that urea raises Km for D-xylose of the C298A mutant, betaine lowers the Km, and the two counteract in a classical fashion so that at a 2:1 molar ratio of betaine to urea there is no net effect. We conclude that counteraction of urea effects on enzymes by methylamines can depend on ion concentration, pH, the specific methylamine and substrate, and identity of even a single amino acid in the enzyme.

Cis- and trans-acting factors regulating transcription of the BGT1 gene in response to hypertonicity

We have previously identified a tonicity-responsive enhancer (TonE) in the promoter region of the canine BGT1 gene. TonE mediates hypertonicity-induced stimulation of transcription. Here, we characterize TonE and TonE binding proteins (TonEBPs) to provide a biochemical basis for cloning of the TonEBPs. Mutational analysis applied to both hypertonicity-induced stimulation of transcription and TonEBP binding reveals that TonE is 11 base pairs in length, with the consensus sequence of (C/T)GGAAnnn(C/T)n(C/T). Activity of the TonEBPs increases in response to hypertonicity with a time course similar to that of transcription of the BGT1 gene. Studies with inhibitors indicate that translation, but not transcription, is required for activation of the TonEBPs. Phosphorylation is required for the stimulation of transcription but not for activation of DNA binding by the TonEBPs. In vivo methylation by dimethyl sulfate reveals that the TonE site of the BGT1 gene is protected with a time course like that of activity of the TonEBPs and activation of transcription. Ultraviolet cross-linking indicates that the TonEBPs share a DNA binding subunit of 200 kDa.

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.

Kidney cell survival in high tonicity

The kidney medulla of mammals undergoes large changes in tonicity in parallel with the tonicity of the final urine that emerges from the kidney at the tip of the medulla. When the medulla is hypertonic, its cells accumulate the compatible osmolytes myo-inositol, betaine, taurine, sorbitol and glycerophosphorylcholine. The mechanisms by which the compatible osmolytes are accumulated have been explored extensively in kidney-derived cells in culture. Myo-inositol, betaine and taurine are accumulated by increased activity of specific sodium-coupled transporters, sorbitol by increased synthesis of aldose reductase that catalyses the synthesis of sorbitol from glucose. Glycerophosphorylcholine accumulates primarily because its degradation is reduced in cells in hypertonic medium. cDNAs for the cotransporters and for aldose reductase have been cloned and used to establish that hypertonicity increases the transcription of the genes for the cotransporters for myo-inositol, betaine and for aldose reductase. The region 5' to the promoter of the gene for the betaine cotransporter and for aldose reductase confer osmotic responsiveness to a heterologous promoter. The 12-bp sequence responsible for the transcriptional response to hypertonicity has been identified in the 5' region of the gene for the betaine cotransporter.

The differential osmoregulation and localization of taurine transporter mRNA and Na+/myo-inositol cotransporter mRNA in rat eyes

We studied the cellular localization and osmotic regulation of taurine transporter (TauT) mRNA in the rat eyes using in situ hybridization. TauT mRNA signals were expressed in the ciliary body, and the outer part of the inner nuclear layer (INL), the outer nuclear layer (ONL) and the inner segment (IS) of the adult rat retina. Chronic hypernatrema, induced by gavaging with 1 ml/100 g body weight of 5% NaCl every other day for 7 days, markedly increased in TauT mRNA in the retina compared with control rats. However, there was little change in TauT mRNA in the eyes in acute hypernatremic state that is induced by single injection of high concentration of NaCl. On the contrary, acute hypernatremic rats displayed markedly elevated Na+/myo-inositol cotransporter (SMIT) mRNA in the retina and the iris-ciliary body and the lens epithelium. Under chronic hypernatremic conditions, there was no significant increase in SMIT mRNA in rat eyes. These findings suggest that TauT mRNA is osmotically regulated in vivo to protect retinal neuronal function, especially against chronic hypernatremic conditions, in contrast to rapid up-regulation of SMIT mRNA in acute hypernatremic rats.

Regulation of gene expression by hypertonicity

Adaptation of cells to hypertonicity often involves changes in gene expression. Since the concentration of salt in the interstitial fluid surrounding renal inner medullary cells varies with operation of the renal concentrating mechanism and generally is very high, the adaptive mechanisms of these cells are of special interest. Renal medullary cells compensate for hypertonicity by accumulating variable amounts of compatible organic osmolytes, including sorbitol, myo-inositol, glycine betaine, and taurine. In this review we consider how these solutes help relieve the stress of hypertonicity and the nature of transporters and enzymes responsible for their variable accumulation. We emphasize recent developments concerning the molecular basis for osmotic regulation of these genes, including identification and characterization of osmotic response elements. Although osmotic stresses are much smaller in other parts of the body than in the renal medulla, similar mechanisms operate throughout, yielding important physiological and pathophysiological consequences.

Catalytic and structural modifications of sarcoplasmic reticulum and plasma membrane (Ca(2+) + Mg2+) ATPases induced by organic solutes that accumulate in living systems

Organic solutes such as urea, methylamines, polyols and amino acid can accumulate in the cytoplasm of cells to compensate for hyperosmotic conditions in the external medium. Whereas urea is considered to be typical of solutes that destabilize structure and function of proteins, methylamines, polyols and some amino acids appear to have the opposite effect, and can also compensate for the perturbing effects of urea. These effects have been extensively analyzed for a variety of proteins in terms of global changes in enzyme structure and acceleration or inhibition of overall reaction rates. Here the influence of these solutes on sarcoplasmic reticulum and plasma membrane (Ca(2+) + Mg2+) ATPases is reviewed. The focus is on the changes induced by "perturbing" and "stabilizing" solutes at specific steps of the catalytic cycles of these enzymes, which can run forward (leading to ATP hydrolysis) and backward (leading to ATP synthesis). Structural changes promoted by osmolytes are correlated with functional changes, especially those that are related to energy coupling.

Expression of Na+/myo-inositol cotransporter mRNA in normal and hypertonic stress rat eyes

We studied the localization of Na+/myo-inositol cotransporter (SMIT) mRNA in normal and hypertonic stress rat eyes by in situ hybridization histochemistry using cRNA probes. SMIT mRNA signals were observed in the iris-ciliary body, the lens epithelial cells, and the ganglion cell layer and the inner nuclear layer of the retina. There was a rapid increase on SMIT mRNA in the retina of hypertonic stress rats compared with control rats. These findings suggest that Na+/myo-inositol cotransporter gene expression is osmotically regulated in vivo to protect retinal neuronal function against hypertonic stress.

Time-dependent aspects of osmolyte changes in rat kidney, urine, blood and lens with sorbinil and galactose feeding

Sorbitol plus myo-inositol, betaine and glycerophosphorylcholine (GPC) are cellular osmolytes in the mammalian renal medulla. Galactosemia and hyperglycemia can cause excessive levels of galactitol or sorbitol in several organs via aldose reductase (AR) catalysis. AR inhibitors can reduce these polyols. To examine osmolyte responses to polyol perturbations, male Wistar rats were fed normal diet, the AR inhibitor sorbinil (at 40 mg/kg/d), 25% galactose, or a combination, for 10, 21 and 42 days. All animals at 21 days had higher apparent renal AR activity than at 10 or 42 days, possibly providing resistance to sorbinil. Sorbinil feeding alone tended to increase urinary, plasma and renal urea levels. It reduced AR activity and sorbitol contents in renal inner medulla, though less so at 21 days; other renal osmolytes, especially betaine, were elevated. Galactose feeding caused little change in renal AR activity, and resulted in high galactose and galactitol contents in renal medulla, urine, blood and lens (and higher renal Na+ contents at 10 days). Renal sorbitol, inositol and GPC decreased, while betaine contents trended higher at all times. Sorbinilgalactose feeding reduced renal AR activities and galactitol contents (again less so at 21 days), urine, blood and lens galactitol, and further reduced renal sorbitol contents. At 10 and 21 days it tended to raise renal betaine more, and restore inositol (but not GPC) contents to control levels. At 42 days it reduced renal and urinary Na+ and galactose, and decreased renal betaine to control levels. Under most conditions, total renal (non-urea) organic osmolyte contents (presumed to be mostly intracellular) and Na+ plus galactose contents (presumed mostly extracellular) changed together such that cell volumes may have been maintained. The exception was 10 days on galactose, where total osmolytes appeared too low. In galactose-fed animals, urine/plasma ratios suggest some renal galactitol efflux, and cellular galactitol probably helps maintain osmotic balance rather than cause swelling.

Molecular basis for osmoregulation of organic osmolytes in renal medullary cells

Renal medullary cells are naturally exposed to extremely high and variable interstitial concentrations of NaCl and urea, consequent to operation of the urinary concentrating mechanism. They respond by accumulating large and variable amounts of sorbitol, glycerophosphocholine (GPC), glycine betaine (betaine), myo-inositol (inositol), and taurine both in vivo and in cell cultures. Sorbitol is synthesized from glucose, catalyzed by aldose reductase. Hypertonicity increases aldose reductase activity by raising this enzyme's transcription, mRNA level, and translation, and thereby increases production of sorbitol. GPC is synthesized from choline via phosphatidylcholine. A combination of high NaCl plus urea does not increase GPC synthesis, but does reduce its degradation by inhibiting GPC:choline phosphodiesterase. Betaine, inositol and taurine are taken up into the cells, each by a different sodium-dependent transporter. Hypertonicity increases mRNAs of all three transporters. This is due to increased transcription (at least of the inositol and betaine transporters). The eventual result is greater betaine, inositol and taurine uptake and accumulation. Osmoregulation of net sorbitol and GPC synthesis and of betaine, inositol and taurine transport is slow, requiring hours to days. However, following an acute fall in tonicity, these organic osmolytes exit from the cells within minutes, via specialized efflux mechanisms. As demonstrated by cloning efficiency studies, renal cell survival and growth following hypertonicity depend on the sum of all organic osmolytes that are accumulated; altering one experimentally changes the others to maintain a nearly constant total. Methylamine accumulation protects these cells against high urea; the methylamine that is preferentially accumulated in response to high urea is GPC.

Effects on rat renal osmolytes of extended treatment with an aldose reductase inhibitor

1. The mammalian renal medulla uses sorbitol, myo-inositol, betaine and glycerophosphorylcholine as intracellular osmolytes. 2. Sorbitol synthesis was inhibited by feeding male Wistar rats the aldose reductase inhibitor sorbinil at 40 mg/kg/day for 71 d, and renal inner medullas were extracted for analysis. 3. Aldose reductase activities and sorbitol contents were greatly reduced in sorbinil-treated animals, while betaine contents increased significantly (with no other osmolytes changing). 4. The betaine increase compensated for the sorbitol decrease such that the total organic osmolytes maintained the same ratio to sodium contents as controls. 5. These results are identical to the pattern previously reported for sorbinil treatment of rats for 10 d, but not for 21 d.