Protection of renal inner medullary epithelial cells from apoptosis by hypertonic stress-induced p53 activation

Neutral amino acid transporter SLC38A2 protects renal medulla from hyperosmolarity-induced ferroptosis

Hyperosmolarity of the renal medulla is essential for urine concentration and water homeostasis. However, how renal medullary collecting duct (MCD) cells survive and function under harsh hyperosmotic stress remains unclear. Using RNA-Seq, we identified SLC38A2 as a novel osmoresponsive neutral amino acid transporter in MCD cells. Hyperosmotic stress-induced cell death in MCD cells occurred mainly via ferroptosis, and it was significantly attenuated by SLC38A2 overexpression but worsened by Slc38a2-gene deletion or silencing. Mechanistic studies revealed that the osmoprotective effect of SLC38A2 is dependent on the activation of mTORC1. Moreover, an in vivo study demonstrated that Slc38a2-knockout mice exhibited significantly increased medullary ferroptosis following water restriction. Collectively, these findings reveal that Slc38a2 is an important osmoresponsive gene in the renal medulla and provide novel insights into the critical role of SLC38A2 in protecting MCD cells from hyperosmolarity-induced ferroptosis via the mTORC1 signalling pathway.

Can the cGAS-STING Pathway Play a Role in the Dry Eye?

Dry eye is one of the most common ocular surface diseases in the world and seriously affects the quality of life of patients. As an immune-related disease, the mechanism of dry eye has still not been fully elucidated. The cGAS-STING pathway is a recently discovered pathway that plays an important role in autoimmune and inflammatory diseases by recognizing dsDNA. As an important signal to initiate inflammation, the release of dsDNA is associated with dry eye. Herein, we focused on the pathophysiology of the immune-inflammatory response in the pathogenesis of dry eye, attempted to gain insight into the involvement of dsDNA in the dry eye immune response, and investigated the mechanism of the cGAS-STING pathway involved in the immune-inflammatory response. We further proposed that the cGAS-STING pathway may participate in dry eye as a new mechanism linking dry eye and the immune-inflammatory response, thus providing a new direction for the mechanistic exploration of dry eye.

Hypertonicity counteracts MCL-1 and renders BCL-XL a synthetic lethal target in head and neck cancer

Head and neck squamous cell carcinoma (HNSCC) is an aggressive and difficult-to-treat cancer entity. Current therapies ultimately aim to activate the mitochondria-controlled (intrinsic) apoptosis pathway, but complex alterations in intracellular signaling cascades and the extracellular microenvironment hamper treatment response. On the one hand, proteins of the BCL-2 family set the threshold for cell death induction and prevent accidental cellular suicide. On the other hand, controlling a cell's readiness to die also determines whether malignant cells are sensitive or resistant to anticancer treatments. Here, we show that HNSCC cells upregulate the proapoptotic BH3-only protein NOXA in response to hyperosmotic stress. Induction of NOXA is sufficient to counteract the antiapoptotic properties of MCL-1 and switches HNSCC cells from dual BCL-XL/MCL-1 protection to exclusive BCL-XL addiction. Hypertonicity-induced functional loss of MCL-1 renders BCL-XL a synthetically lethal target in HNSCC, and inhibition of BCL-XL efficiently kills HNSCC cells that poorly respond to conventional therapies. We identify hypertonicity-induced upregulation of NOXA as link between osmotic pressure in the tumor environment and mitochondrial priming, which could perspectively be exploited to boost efficacy of anticancer drugs.

Targeted toxicometabolomics of endosulfan sulfate in adult zebrafish (Danio rerio) using GC-MS/MS in multiple reaction monitoring mode

Endosulfan sulfate is a major oxidative metabolite of the chlorinated insecticide endosulfan. In this study, a targeted metabolomics approach was used to investigate the toxic mechanisms of endosulfan sulfate in adult zebrafish using the multiple reaction monitoring mode of a GC-MS/MS. The LC₅₀ of endosulfan sulfate in adult zebrafish was determined and then zebrafish were exposed to endosulfan sulfate at one-tenth the LC₅₀ (0.1LC₅₀) or the LC₅₀ for 24 and 48 h. After exposure, the fish were extracted, derivatized and analyzed by GC-MS/MS for 379 metabolites to identify 170 metabolites. Three experimental groups (control, 0.1LC₅₀ and LC₅₀) were clearly separated in PLS-DA score plots. Based on the VIP, ANOVA, and fold change results, 40 metabolites were selected as biomarkers. Metabolic pathways associated with those metabolites were identified using MetaboAnalyst 4.0 as follows: aminoacyl-tRNA biosynthesis, valine/leucine/isoleucine biosynthesis, citrate cycle, glycerolipid metabolism, and arginine/proline metabolism. Gene expression studies confirmed the activation of citrate cycle and glycerolipids metabolism. MDA levels of the exposed group significantly increased in oxidative toxicity assay tests. Such significant perturbations of important metabolites within key biochemical pathways must result in biologically hazardous effects in zebrafish.

NFAT5 up-regulates expression of the kidney-specific ubiquitin ligase gene Rnf183 under hypertonic conditions in inner-medullary collecting duct cells

We previously reported that among the 37 RING finger protein (RNF) family members, RNF183 mRNA is specifically expressed in the kidney under normal conditions. However, the mechanism supporting its kidney-specific expression pattern remains unclear. In this study, we elucidated the mechanism of the transcriptional activation of murine Rnf183 in inner-medullary collecting duct cells. Experiments with anti-RNF183 antibody revealed that RNF183 is predominantly expressed in the renal medulla. Among the 37 RNF family members, Rnf183 mRNA expression was specifically increased in hypertonic conditions, a hallmark of the renal medulla. RNF183 up-regulation was consistent with the activation of nuclear factor of activated T cells 5 (NFAT5), a transcription factor essential for adaptation to hypertonic conditions. Accordingly, siRNA-mediated knockdown of NFAT5 down-regulated RNF183 expression. Furthermore, the -3,466 to -3,136-bp region upstream of the mouse Rnf183 promoter containing the NFAT5-binding motif is conserved among mammals. A luciferase-based reporter vector containing the NFAT5-binding site was activated in response to hypertonic stress, but was inhibited by a mutation at the NFAT5-binding site. ChIP assays revealed that the binding of NFAT5 to this DNA site is enhanced by hypertonic stress. Of note, siRNA-mediated RNF183 knockdown increased hypertonicity-induced caspase-3 activation and decreased viability of mIMCD-3 cells. These results indicate that (i) RNF183 is predominantly expressed in the normal renal medulla, (ii) NFAT5 stimulates transcriptional activation of Rnf183 by binding to its cognate binding motif in the Rnf183 promoter, and (iii) RNF183 protects renal medullary cells from hypertonicity-induced apoptosis.

Transcriptome analysis identifies activated signaling pathways and regulated ABC transporters and solute carriers after hyperosmotic stress in renal MDCK I cells

Hyperosmolality is found under physiological conditions in the kidneys, whereas hyperosmolality in other tissues may be associated with pathological conditions. In such tissues an association between inflammation and hyperosmolality has been suggested. During hyperosmotic stress, an important phenomenon is upregulation of solute carriers (SLCs). We hypothesize that hyperosmolality affects the expression of many SLCs as well as ABC transporters. Through RNA-sequencing and topological pathway analysis, the cell cycle, the cytokine-cytokine receptor interaction pathway, and the chemokine-signaling pathway were significantly activated in MDCK I cells after hyperosmotic treatment (Δ200 mOsm) with raffinose or NaCl. 9065, 8052 and 5018 genes were significantly regulated by raffinose, NaCl or urea supplementation (500 mOsm), respectively, compared to control (300 mOsm). Cytokines, that have not previously been associated with hyperosmolality, were identified. We further provide an overview of transport proteins that could be of relevance in tissues exposed to hyperosmolality. Especially Slc5a8 was found highly up-regulated.

Glycine increases preimplantation development of mouse oocytes following vitrification at the germinal vesicle stage

Ice-free cryopreservation, referred to as vitrification, is receiving increased attention in the human and animal assisted reproduction. However, it introduces the detrimental osmotic stress by adding and removing high contents of cryoprotectants. In this study, we evaluated the effects of normalizing cell volume regulation by adding glycine, an organic osmolyte, during vitrification of mouse germinal vesicle stage oocyte and/or subsequent maturation on its development. The data showed that glycine supplementation in either vitrification/thawing or maturation medium significantly improved the cytoplasmic maturation of MII oocytes manifested by spindle assembly, chromosomal alignment, mitochondrial distribution, euploidy rate, and blastocyst development following fertilization in vitro, compared to the control without glycine treatment. Furthermore, glycine addition during both vitrification/thawing and maturation further enhanced the oocyte quality demonstrated by various markers, including ATP contents and embryo development. Lastly, the effect of anti-apoptosis was also observed when glycine was added during vitrification. Our result suggests that reducing osmotic stress induced by vitrification could improve the development of vitrified mouse oocyte.

Quantitative evaluation of Oryeongsan and its action on water regulation in renal inner medullary collecting duct cells

ETHNOPHARMACOLOGICAL RELEVANCE

Oryeongsan (ORS, Wulingsan) has been reported to possess renal protective effects from renal diseases such as diabetes-induced renal damage, and nephrocalcinosis.

AIM OF THE STUDY

This study was conducted to evaluate the quantitative analysis and the inhibitory effect of ORS on hypertonic stress-induced water channel and apoptosis in murine inner medullary collecting duct cell line (mIMCD-3).

MATERIALS AND METHODS

Chromatographic and NMR spectroscopic analysis were performed and water balance regulation was determined by Western blot, RT-PCR, and immunofluorescnece.

RESULTS

Seven active principles (5-hydroxymethylfurfural, alismoxide, methyl(-)trans-cinnamate, adenine, guanosine, adenosine, and ferulic acid) in ORS were isolated and the structures were identified mainly by NMR spectroscopic analysis. In addition, contents of these metabolites in ORS were evaluated by HPLC analysis. Pretreatment with ORS significantly attenuated the hypertonic stress (175mM NaCl)-induced increase in protein levels of AQP2 and apical membrane insertion. ORS also attenuated osmolyte sodium-myo-inositol transporter (SMIT) expression and tonicity-responsive enhancer binding protein (TonEBP) mRNA under hypertonic stress. Those actions of ORS presented the similar effect of PKA inhibitor which AQP2 expression throughout the inhibition of vasopressin-mediated cAMP/PKA signal pathway. On the other hand, pretreatment with ORS attenuated hypertonic stress-induced cell death. Hypertonic stress-induced Bax or caspase-3 expression was decreased by ORS, resulting in anti-apoptotic effect.

CONCLUSIONS

The present data suggest that the beneficial effect of ORS in water balance and apoptosis against hypertonic stress of renal collecting ducts.

Role of nuclear factor of activated T-cells 5 in regulating hypertonic-mediated secretin receptor expression in kidney collecting duct cells

A growing body of evidence suggests that secretin (SCT) is an important element in the osmoregulatory pathway. It is interesting to note that both SCT and its receptor (SCTR) gene are activated upon hyperosmolality in the kidney. However, the precise molecular mechanisms underlying the induction of the SCTR gene expression in response to changes in osmolality have yet to be clarified. Detailed DNA sequence analysis of the promoter regions of the SCTR gene reveals the presence of multiple osmotic response elements (ORE). The ORE is the binding site of a key osmosensitive transactivator, namely, the nuclear factor of activated T-cells 5 (NFAT5). SCTR and NFAT5 are co-expressed in the kidney cortex and medulla collecting duct cells. We therefore hypothesize that NFAT5 is responsible for modulating SCTR expression in hypertonic environments. In this study, we found hypertonicity stimulates the promoter activities and endogenous gene expression of SCTR in mouse kidney cortex collecting duct cells (M1) and inner medulla collecting duct cells (mIMCD3). The overexpression and silencing of NFAT5 further confirmed it to be responsible for the up-regulation of the SCTR gene under hypertonic conditions. A significant increase in the interaction between NFAT5 and the SCTR promoter was also observed following chromatin immunoprecipitation assay. In vivo, osmotic stress up-regulates the SCTR gene in the kidney cortex and medulla of wild-type mice, but does not do so in NFAT5(+/-) animals. Hence, this study provides comprehensive information on how NFAT5 regulates SCTR expression in different osmotic environments.

TGRL Lipolysis Products Induce Stress Protein ATF3 via the TGF-β Receptor Pathway in Human Aortic Endothelial Cells

Studies have suggested a link between the transforming growth factor beta 1 (TGF-β1) signaling cascade and the stress-inducible activating transcription factor 3 (ATF3). We have demonstrated that triglyceride-rich lipoproteins (TGRL) lipolysis products activate MAP kinase stress associated JNK/c-Jun pathways resulting in up-regulation of ATF3, pro-inflammatory genes and induction of apoptosis in human aortic endothelial cells. Here we demonstrate increased release of active TGF-β at 15 min, phosphorylation of Smad2 and translocation of co-Smad4 from cytosol to nucleus after a 1.5 h treatment with lipolysis products. Activation and translocation of Smad2 and 4 was blocked by addition of SB431542 (10 μM), a specific inhibitor of TGF-β-activin receptor ALKs 4, 5, 7. Both ALK receptor inhibition and anti TGF-β1 antibody prevented lipolysis product induced up-regulation of ATF3 mRNA and protein. ALK inhibition prevented lipolysis product-induced nuclear accumulation of ATF3. ALKs 4, 5, 7 inhibition also prevented phosphorylation of c-Jun and TGRL lipolysis product-induced p53 and caspase-3 protein expression. These findings demonstrate that TGRL lipolysis products cause release of active TGF-β and lipolysis product-induced apoptosis is dependent on TGF-β signaling. Furthermore, signaling through the stress associated JNK/c-Jun pathway is dependent on TGF-β signaling suggesting that TGF-β signaling is necessary for nuclear accumulation of the ATF3/cJun transcription complex and induction of pro-inflammatory responses.

Regulation of p53 in NIH3T3 mouse fibroblasts following hyperosmotic stress

The aim of this project was to analyze the regulation of p53 expression in NIH3T3 fibroblasts under the influence of increasing hyperosmotic stress. Expression of p53 showed a biphasic response pattern in NIH3T3 cells under increasing osmotic stress (337 mOsm to 737 mOsm) with a maximum at 587 mOsm. Under isotonic conditions p53 expression increased after addition of the proteasome inhibitor MG132 indicating that cellular p53 levels in unperturbed cells is kept low by proteasomal degradation. However, under hypertonic conditions p53 synthesis as well as p53 degradation were significantly reduced and it is demonstrated that the increase in p53 expression observed when tonicity is increased from 337 to 587 mOsm reflects that degradation is more inhibited than synthesis, whereas the decrease in p53 expression at higher tonicities reflects that synthesis is more inhibited than degradation. The activity of the p53 regulating proteins p38 MAP kinase and the ubiquitin ligase MDM2 were studied as a function of increasing osmolarity. MDM2 protein expression was unchanged at all osmolarities, whereas MDM2 phosphorylation (Ser¹⁶⁶) increased at osmolarities up to 537 mOsm and remained constant at higher osmolarities. Phosphorylation of p38 increased at osmolarities up to 687 mOsm which correlated with an increased phosphorylation of p53 (Ser¹⁵) and the decreased p53 degradation. Caspase-3 activity increased gradually with hypertonicity and at 737 mOsm both Caspase-3 activity and annexin V binding are high even though p53 expression and activity are low, indicating that initiation of apoptosis under severe hypertonic conditions is not strictly controlled by p53.

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

Urea transporter UT-B deletion induces DNA damage and apoptosis in mouse bladder urothelium

BACKGROUND

Previous studies found that urea transporter UT-B is abundantly expressed in bladder urothelium. However, the dynamic role of UT-B in bladder urothelial cells remains unclear. The objective of this study is to evaluate the physiological roles of UT-B in bladder urothelium using UT-B knockout mouse model and T24 cell line.

METHODOLOGY/PRINCIPAL FINDINGS

Urea and NO measurement, mRNA expression micro-array analysis, light and transmission electron microscopy, apoptosis assays, DNA damage and repair determination, and intracellular signaling examination were performed in UT-B null bladders vs wild-type bladders and in vitro T24 epithelial cells. UT-B was highly expressed in mouse bladder urothelium. The genes, Dcaf11, MCM2-4, Uch-L1, Bnip3 and 45 S pre rRNA, related to DNA damage and apoptosis were significantly regulated in UT-B null urothelium. DNA damage and apoptosis highly occurred in UT-B null urothelium. Urea and NO levels were significantly higher in UT-B null urothelium than that in wild-type, which may affect L-arginine metabolism and the intracellular signals related to DNA damage and apoptosis. These findings were consistent with the in vitro study in T24 cells that, after urea loading, exhibited cell cycle delay and apoptosis.

CONCLUSIONS/SIGNIFICANCE

UT-B may play an important role in protecting bladder urothelium by balancing intracellular urea concentration. Disruption of UT-B function induces DNA damage and apoptosis in bladder, which can result in bladder disorders.

Quantitative molecular phenotyping of gill remodeling in a cichlid fish responding to salinity stress

A two-tiered label-free quantitative (LFQ) proteomics workflow was used to elucidate how salinity affects the molecular phenotype, i.e. proteome, of gills from a cichlid fish, the euryhaline tilapia (Oreochromis mossambicus). The workflow consists of initial global profiling of relative tryptic peptide abundances in treated versus control samples followed by targeted identification (by MS/MS) and quantitation (by chromatographic peak area integration) of validated peptides for each protein of interest. Fresh water acclimated tilapia were independently exposed in separate experiments to acute short-term (34 ppt) and gradual long-term (70 ppt, 90 ppt) salinity stress followed by molecular phenotyping of the gill proteome. The severity of salinity stress can be deduced with high technical reproducibility from the initial global label-free quantitative profiling step alone at both peptide and protein levels. However, an accurate regulation ratio can only be determined by targeted label-free quantitative profiling because not all peptides used for protein identification are also valid for quantitation. Of the three salinity challenges, gradual acclimation to 90 ppt has the most pronounced effect on gill molecular phenotype. Known salinity effects on tilapia gills, including an increase in the size and number of mitochondria-rich ionocytes, activities of specific ion transporters, and induction of specific molecular chaperones are reflected in the regulation of abundances of the corresponding proteins. Moreover, specific protein isoforms that are responsive to environmental salinity change are resolved and it is revealed that salinity effects on the mitochondrial proteome are nonuniform. Furthermore, protein NDRG1 has been identified as a novel key component of molecular phenotype restructuring during salinity-induced gill remodeling. In conclusion, besides confirming known effects of salinity on gills of euryhaline fish, molecular phenotyping reveals novel insight into proteome changes that underlie the remodeling of tilapia gill epithelium in response to environmental salinity change.

ABCC1 is related to the protection of the distal nephron against hyperosmolality and high sodium environment: possible implications for cancer chemotherapy

Aims

Glutathione (GSH) plays an important role in protecting cells against oxidative damage. ABCC1 protein transports GSH. Although this protein is largely studied in cancer, due to multidrug resistance phenotype, its role in the tubular cells of the kidney is unknown. The goal of this study was to find out whether ABCC1 has a role in protecting cells from the distal nephron against the stress caused by high medullar osmolality.

Main Methods

MA104 cells were treated with high concentrations of sodium chloride, urea, or both to raise the osmolality of the culture medium. Cell viability was accessed by MTT and trypan blue assays. ABCC1 expression and extrusion of carboxi-fluorescein (CF), a fluorescent ABCC1 substrate, were measured by flow cytometry.

Key Findings

Incubation of MA104 cells in a high sodium concentration medium resulted in changes in cell granularity and altered expression and activity of ABCC1. Urea did not alter ABCC1 expression or activity, but reversed the observed NaCl effects. High sodium concentrations also had a negative effect on cell viability and urea also protected cells against this effect.

Significance

Our findings demonstrate that ABCC1 plays a significant role in the protection of kidney epithelial cells against the stress caused by high sodium environment present in renal medulla.

Environmental hyperosmolality regulates phospholipid biosynthesis in the renal epithelial cell line MDCK

Hyperosmolality is a key signal for renal physiology. On the one hand, it contributes to the differentiation of renal medullary structures and to the development of the urinary concentrating mechanism. On the other, it is a stress factor. In both cases, hyperosmolality activates processes that require an adequate extension of cellular membranes. In the present work, we examined whether hyperosmolality regulates phospholipid biosynthesis, which is needed for the membrane biogenesis in the renal epithelial cell line Madin-Darby canine kidney (MDCK). Because phospholipids are the structural determinants of all cell membranes, we evaluated their content, synthesis, and regulation in MDCK cultures subjected to different hyperosmotic concentrations of NaCl, urea, or both. Hyperosmolality increased phospholipid content in a concentration-dependent manner. Such an effect was exclusively due to changes in NaCl concentration and occurred at the initial stage of hyperosmolar treatment concomitantly with the expression of the osmoprotective protein COX-2. The hypertonic upregulation of phosphatidylcholine (PC) synthesis, the main constituent of all cell membranes, involved the transcriptional activation of two main regulatory enzymes, choline kinase (CK) and cytidylyltransferase α (CCTα) and required ERK1/2 activation. Considering that physiologically, renal medullary cells are constantly exposed to high and variable NaCl, these findings could contribute to explaining how renal cells could maintain cellular integrity even in a nonfavorable environment.

Effect of Poria cocos on hypertonic stress-induced water channel expression and apoptosis in renal collecting duct cells

ETHNOPHARMACOLOGICAL RELEVANCE

A major physiological role of the kidney is to regulate body water and urine concentration. Aquaporin-2 (AQP2), a family of water channels, plays an important role in the urinary concentrating process and regulation of water balance in the kidney. The dried sclerotia of Poria cocos Wolf has been known to have a diuretic effect and used for the treatment of chronic edema and nephrosis.

AIM OF THE STUDY

This study was conducted to evaluate the inhibitory effect of the sclerotia of Poria cocos (WPC) on hypertonic stress-induced AQP2 expression and apoptosis in inner medullary collecting duct cell lines (IMCD-3).

MATERIALS AND METHODS

Hypertonic stress was induced by 175mM NaCl. Inhibitory effect of WPC on hypertonic stress-induced AQP2 expression and apoptosis were determined by western blot, RT-PCR, and immunofluorescence.

RESULTS

Hypertonic stress (175mM NaCl) increased in the levels of AQP2 expression by hypertonicity in IMCD-3 cells. WPC attenuated the hypertonicity-induced increase in protein and mRNA levels of AQP2 in a concentration-dependent manner. Pretreatment with WPC attenuated hypertonicity-induced cell death. Hypertonicity increased serum- and glucocorticoid-inducible protein kinase (Sgk1) phosphorylation, however, WPC attenuated the hypertonicity-induced Sgk1 activation. Tonicity-responsive enhancer binding protein (TonEBP) mRNA was also recovered by WPC under hypertonic stress. Pretreatment with WPC presented the similar effect of PKA inhibitor which decreased hypertonic stress-induced AQP2 expression. Hypertonicity increased cAMP levels and the changes were blocked by WPC. On the other hand, hypertonic stress-induced Bax or caspase-3 expression was decreased by WPC, resulting in anti-apoptotic effect.

CONCLUSIONS

These results provided evidence that the beneficial effect of WPC in water balance against in vitro hypertonic stress of renal collecting ducts. In addition, WPC exhibits anti-apoptotic property response to hypertonic stress. Thus, these data suggests that WPC has benefit for the therapeutic approach to the inhibition of renal disorder.

Placental TonEBP/NFAT5 osmolyte regulation in an ovine model of intrauterine growth restriction

TonEBP/NFAT5 (the tonicity-responsive enhancer binding protein/nuclear factor of activated T cells) modulates cellular response to osmotic changes by accumulating inositol and sorbitol inside the cells. Our objective was to assess placental osmolytes, TonEBP/NFAT5 RNA and protein expression, and signaling molecules across gestation between control and intrauterine growth restriction (IUGR) ovine pregnancies. Pregnant sheep were placed in hyperthermic conditions to induce IUGR. Placental tissues were collected at 55, 95, and 130 days gestational age (dGA) to measure inositol, sorbitol, TonEBP/NFAT5 (NFAT5), sodium-dependent myo-inositol transporter (SMIT; official symbol SLC5A3), aldose reductase (AR), and NADPH (official symbol DE-CR1). Placental weight was reduced in IUGR compared to controls at 95 and 130 dGA. Osmolyte concentrations were similar between control and IUGR placentas, but both groups demonstrated a significant decrease in inositol concentration and an increase in sorbitol concentration with advancing gestation. Cytosolic NFAT5 protein decreased significantly from 55 to 95 dGA in both groups, and nuclear NFAT5 protein increased only at 130 dGA in the IUGR group, but no differences were seen between groups for either cytosolic or nuclear NFAT5 protein concentrations. DE-CR1 concentrations were similar between groups and increased significantly with advancing gestational age. AR was lowest at 55dGA, and SLC5A3 increased with advancing gestational age. We conclude that both placental osmolytes inositol and sorbitol (and their corresponding proteins SLC5A3 and AR) change with gestational age and are regulated, at least in part, by NFAT5 and DE-CR1 (NADPH). The inverse relationship between each osmolyte across gestation (e.g., inositol higher in early gestation and sorbitol higher in late gestation) may reflect nutritional needs that change across gestation.

Transcriptional regulation of gene expression during osmotic stress responses by the mammalian target of rapamycin

Although stress can suppress growth and proliferation, cells can induce adaptive responses that allow them to maintain these functions under stress. While numerous studies have focused on the inhibitory effects of stress on cell growth, less is known on how growth-promoting pathways influence stress responses. We have approached this question by analyzing the effect of mammalian target of rapamycin (mTOR), a central growth controller, on the osmotic stress response. Our results showed that mammalian cells exposed to moderate hypertonicity maintained active mTOR, which was required to sustain their cell size and proliferative capacity. Moreover, mTOR regulated the induction of diverse osmostress response genes, including targets of the tonicity-responsive transcription factor NFAT5 as well as NFAT5-independent genes. Genes sensitive to mTOR-included regulators of stress responses, growth and proliferation. Among them, we identified REDD1 and REDD2, which had been previously characterized as mTOR inhibitors in other stress contexts. We observed that mTOR facilitated transcription-permissive conditions for several osmoresponsive genes by enhancing histone H4 acetylation and the recruitment of RNA polymerase II. Altogether, these results reveal a previously unappreciated role of mTOR in regulating transcriptional mechanisms that control gene expression during cellular stress responses.

Kinetics of hyperosmotically stimulated Na-K-2Cl cotransporter in Xenopus laevis oocytes

A detailed study of hypertonically stimulated Na-K-2Cl cotransport (NKCC1) in Xenopus laevis oocytes was carried out to better understand the 1 K(+):1 Cl(-) stoichiometry of transport that was previously observed. In this study, we derived the velocity equations for K(+) influx under both rapid equilibrium assumptions and combined equilibrium and steady-state assumptions and demonstrate that the behavior of the equations and curves in Lineweaver-Burke plots are consistent with a model where Cl(-) binds first, followed by Na(+), a second Cl(-), and then K(+). We further demonstrate that stimulation of K(+) movement by K(+) on the trans side is an intrinsic property of a carrier that transports multiple substrates. We also demonstrate that K(+) movement through NKCC1 is strictly dependent upon the presence of external Na(+), even though only a fraction of Na(+) is in fact transported. Finally, we propose that the larger transport of K(+), as compared with Na(+), is a result of the return of partially unloaded carriers, which masks the net 1Na(+):1K(+):2Cl(-) stoichiometry of NKCC1. These data have profound implications for the physiology of Na-K-2Cl cotransport, since transport of K-Cl in some conditions seems to be uncoupled from the transport of Na-Cl.

Osmoadaptation of Mammalian cells - an orchestrated network of protective genes

In mammals, the cells of the renal medulla are physiologically exposed to interstitial osmolalities several-fold higher that found in any other tissue. Nevertheless, these cells not only have the ability to survive in this harsh environment, but also to function normally, which is critical for maintenance of systemic electrolyte and fluid homeostasis. Over the last two decades, a substantial body of evidence has accumulated, indicating that sequential and well orchestrated genomic responses are required to provide tolerance to osmotic stress. This includes the enhanced expression and action of immediate-early genes, growth arrest and DNA damage inducible genes (GADDs), genes involved in cell cycle control and apoptosis, heat shock proteins, and ultimately that of genes involved in the intracellular accumulation of nonperturbing organic osmolytes. The present review summarizes the sequence of genomic responses conferring resistance against osmotic stress. In addition, the regulatory mechanisms mediating the coordinated genomic response to osmotic stress will be highlighted.

Cyclooxygenase-2-dependent phosphorylation of the pro-apoptotic protein Bad inhibits tonicity-induced apoptosis in renal medullary cells

During antidiuresis, cell survival in the renal medulla requires cyclooxygenase-2 (COX-2) activity. We have recently found that prostaglandin E2 (PGE2) promotes cell survival by phosphorylation and, hence, inactivation of the pro-apoptotic protein Bad during hypertonic stress in Madin-Darby canine kidney (MDCK) cells in vitro. Here we determine the role of COX-2-derived PGE(2) on phosphorylation of Bad and medullary apoptosis in vivo using COX-2-deficient mice. Both wild-type and COX-2-knockout mice constitutively expressed Bad in tubular epithelial cells of the renal medulla. Dehydration caused a robust increase in papillary COX-2 expression, PGE2 excretion, and Bad phosphorylation in wild-type, but not in the knockout mice. The abundance of cleaved caspase-3, a marker of apoptosis, was significantly higher in papillary homogenates, especially in tubular epithelial cells of the knockout mice. Knockdown of Bad in MDCK cells decreased tonicity-induced caspase-3 activation. Furthermore, the addition of PGE2 to cells with knockdown of Bad had no effect on caspase-3 activation; however, PGE2 caused phosphorylation of Bad and substantially improved cell survival in mock-transfected cells. Thus, tonicity-induced COX-2 expression and PGE2 synthesis in the renal medulla entails phosphorylation and inactivation of the pro-apoptotic protein Bad, thereby counteracting apoptosis in renal medullary epithelial cells.

High NaCl-induced activation of CDK5 increases phosphorylation of the osmoprotective transcription factor TonEBP/OREBP at threonine 135, which contributes to its rapid nuclear localization

When activated by high NaCl, the transcription factor TonEBP/OREBP increases transcription of osmoprotective genes. High NaCl activates CDK5 kinase, which directly phosphorylates TonEBP/OREBP on threonine 135. This contributes to rapid nuclear translocation of TonEBP/OREBP, accelerating transcription of its osmoprotective target genes.

When activated by high NaCl, tonicity-responsive enhancer–binding protein/osmotic response element–binding protein (TonEBP/OREBP) increases transcription of osmoprotective genes. High NaCl activates TonEBP/OREBP by increasing its phosphorylation, nuclear localization, and transactivating activity. In HEK293 cells, mass spectrometry shows phosphorylation of TonEBP/OREBP-S120, -S134, -T135, and -S155. When those residues are individually mutated to alanine, nuclear localization is greater for S155A, less for S134A and T135A, and unchanged for S120A. High osmolality increases phosphorylation at T135 in HEK293 cells and in rat renal inner medullas in vivo. In HEK293 cells, high NaCl activates cyclin-dependent kinase 5 (CDK5), which directly phosphorylates TonEBP/OREBP-T135. Inhibition of CDK5 activity reduces the rapid high NaCl–induced nuclear localization of TonEBP/OREBP but does not affect its transactivating activity. High NaCl induces nuclear localization of TonEBP/OREBP faster (≤2 h) than it increases its overall protein abundance (≥6 h). Inhibition of CDK5 reduces the increase in TonEBP/OREBP transcriptional activity that has occurred by 4 h after NaCl is raised, associated with less nuclear TonEBP/OREBP at that time, but does not reduce either activity or nuclear TonEBP/OREBP after 16 h. Thus high NaCl–induced increase of the overall abundance of TonEBP/OREBP, by itself, eventually raises its effective level in the nucleus, but its rapid CDK5-dependent nuclear localization accelerates the process, speeding transcription of osmoprotective target genes.

Hypertonicity-induced expression of monocyte chemoattractant protein-1 through a novel cis-acting element and MAPK signaling pathways

MCP1 is upregulated by various stimuli, including LPS, high glucose, and hyperosmolality. However, the molecular mechanisms of transcriptional regulation of the MCP1 gene under hyperosmolar conditions are poorly understood. Treatment of NRK52E cells with NaCl or mannitol resulted in significant elevation of MCP1 mRNA and protein in a time- and dose-dependent manner. Treatment with a p38MAPK inhibitor (SB203580), an ERK inhibitor (PD98059), or an MEK inhibitor (U0126), suppressed the increase in MCP1 expression caused by hypertonic NaCl, whereas a JNK inhibitor (SP600125) and an AP1 inhibitor (curcumin) failed to attenuate MCP1 mRNA expression by NaCl. In the 5'-flanking region of the MCP1 gene, there is a sequence motif similar to the consensus TonE/ORE as well as the consensus C/E binding protein (BP), NF-kappaB, and AP1/Sp1 sites. Luciferase activity in cells transfected with reporter constructs containing a putative TonE/ORE element (MCP1-TonE/ORE) enhanced reporter gene expression under hypertonic stress. Results of electrophoretic gel mobility shift assay showed a slow migration of the MCP1-TonE/ORE probe, representing the binding of TonEBP/OREBP/NFAT5 to this enhancer element. These results indicate that the 5'-flanking region of MCP1 contains a hypertonicity-sensitive cis-acting element, MCP1-TonE/ORE, as a novel element in the MCP1 gene. Furthermore, p38MAPK and MEK-ERK pathways appear to be, at least in part, involved in hypertonic stress-mediated regulation of MCP1 expression through the MCP1-TonE/ORE.

Effect of osmotic shock and urea on phosphatidylserine scrambling in thrombocyte cell membranes

Blood passing the renal medulla enters a strongly hypertonic environment challenging functional properties and survival of blood cells. In erythrocytes, exposure to hyperosmotic shock stimulates Ca(2+) entry and ceramide formation with subsequent cell membrane scrambling, an effect partially reversed by high concentrations of Cl(-) or urea. Cell membrane scrambling with phosphatidylserine exposure is part of the procoagulant phenotype of platelets. Coagulation in the hypertonic renal medulla would jeopardize blood flow in the vasa recta. The present study thus explored whether hypertonic environment and urea modify phosphatidylserine exposure of human platelets. FACS analysis was employed to estimate cytosolic Ca(2+) activity with Fluo3 fluorescence, ceramide formation, P-selectin, and glycoprotein IIb/IIIa activation with fluorescent antibodies and phosphatidylserine exposure with annexin V-binding. The spontaneous platelet aggregation was measured by impedance aggregometry. Hyperosmotic shock (addition of 500 mM sucrose or 250 mM NaCl) significantly enhanced cytosolic Ca(2+) activity, ceramide formation, phosphatidylserine exposure, platelet degranulation, and aggregability. Addition of 500 mM urea to isotonic saline did not significantly modify cytosolic Ca(2+) activity, ceramide abundance, or annexin V-binding but significantly blunted the respective effects of hypertonic shock following addition of 500 mM sucrose. In isotonic solutions, both ceramide (20 microM) and Ca(2+) ionophore ionomycin (0.5 microM) increased annexin V-binding, effects again significantly blunted by 500 mM urea. Moreover, oxidative stress by addition of 0.5 mM peroxynitrite increased cytosolic Ca(2+) activity and triggered annexin V-binding, effects again blunted in the presence of 500 mM urea. The observations reveal that hyperosmotic shock and oxidative stress trigger a procoagulant platelet phenotype, an effect blunted by the presence of high urea concentrations.

Salinity stress results in rapid cell cycle changes of tilapia (Oreochromis mossambicus) gill epithelial cells

We have developed a technique for immunocytochemistry of fish gill cells that we used to quantify tilapia (Oreochromis mossambicus) mitochondria-rich cells (MRC) and other gill cells (non-MRC) within different cell cycle phases by laser scanning cytometry. Gill cells fixed on coverslips were triple stained with propidium iodide to distinguish G1 vs. G2 phases, Ser10-phosphorylated histone H3 antibody to label mitotic cells, and Na(+)/K(+) ATPase antibody to label MRC. These parameters were measured at 0 (control), 4, 8, 16, 24, 48, 72, and 168 hr (1 week) following exposure of freshwater (FW) acclimated fish to 2/3 seawater (SW). MRC increased mitotic activity very rapidly peaking at 8 hr following SW exposure. This change in mitotic MRC is indicative of epithelial reorganization during SW acclimation. In contrast to MRC, the proportion of non-MRC (likely pavement cells (PVC)) in mitosis did not change significantly in response to SW exposure. Moreover, twice as many MRC were in mitosis compared with non-MRC, suggesting that MRC turn over faster than other cell types during SW acclimation. Following the mitosis peak, MRC accumulated in G2 phase over a period of 16-72 hr post-SW exposure. We also observed G2 arrest with similar kinetics following SW exposure in tilapia non-MRC (likely PVC). We interpret the G2 arrest that occurs after an initial wave of transient increase in MRC mitosis as a means for conserving energy for dealing with the osmotic stress imposed during the exposure of FW fish to SW.

The transcription factor NFAT5 is required for cyclin expression and cell cycle progression in cells exposed to hypertonic stress

Background

Hypertonicity can perturb cellular functions, induce DNA damage-like responses and inhibit proliferation. The transcription factor NFAT5 induces osmoprotective gene products that allow cells to adapt to sustained hypertonic conditions. Although it is known that NFAT5-deficient lymphocytes and renal medullary cells have reduced proliferative capacity and viability under hypertonic stress, less is understood about the contribution of this factor to DNA damage responses and cell cycle regulation.

Methodology/Principal Findings

We have generated conditional knockout mice to obtain NFAT5^−/− T lymphocytes, which we used as a model of proliferating cells to study NFAT5-dependent responses. We show that hypertonicity triggered an early, NFAT5-independent, genotoxic stress-like response with induction of p53, p21 and GADD45, downregulation of cyclins, and cell cycle arrest. This was followed by an NFAT5-dependent adaptive phase in wild-type cells, which induced an osmoprotective gene expression program, downregulated stress markers, resumed cyclin expression and proliferation, and displayed enhanced NFAT5 transcriptional activity in S and G2/M. In contrast, NFAT5^−/− cells failed to induce osmoprotective genes and exhibited poorer viability. Although surviving NFAT5^−/− cells downregulated genotoxic stress markers, they underwent cell cycle arrest in G1/S and G2/M, which was associated with reduced expression of cyclins E1, A2 and B1. We also show that pathologic hypertonicity levels, as occurring in plasma of patients and animal models of osmoregulatory disorders, inhibited the induction of cyclins and aurora B kinase in response to T cell receptor stimulation in fresh NFAT5^−/− lymphocytes.

Conclusions/Significance

We conclude that NFAT5 facilitates cell proliferation under hypertonic conditions by inducing an osmoadaptive response that enables cells to express fundamental regulators needed for cell cycle progression.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Analysis of DNA breaks, DNA damage response, and apoptosis produced by high NaCl

We previously reported that, both in cell culture and in the renal inner medulla in vivo, elevating NaCl increased the number of DNA breaks, which persisted as long as NaCl remained high but were rapidly repaired when NaCl was lowered. Furthermore, those breaks did not induce the DNA repair protein gammaH2AX or cause activation of the MRN (Mre11, Rad50, Nbs1) complex. In contrast, others recently reported that high NaCl does induce gammaH2AX and MRN complex formation and concluded that these activities are associated with repair of the DNA (Sheen MR, Kim SW, Jung JY, Ahn JY, Rhee JG, Kwon HM, Woo SK. Am J Physiol Renal Physiol 291: F1014-F1020, 2006). The purpose of the present studies was to resolve the disparity. The important difference is that HeLa cells, which were the main subject of the later report, are much less tolerant of high NaCl than are the mIMCD3 cells, which were our main subject. mIMCD3 cells survive levels of NaCl that kill HeLa cells by apoptosis. Here we demonstrate that in both cell types raising NaCl to a level that the cells survive (higher for mIMCD3 than HeLa) increases DNA breaks without inducing gammaH2AX or activating the MRN complex and that the DNA breaks persist as long as NaCl remains elevated, but are rapidly repaired when it is lowered. Importantly, in both cell types, raising NaCl further to cause apoptosis activates these DNA damage response proteins and greatly fragments DNA, associated with cell death. We conclude that gammaH2AX induction and MRN activation in response to high NaCl are associated with apoptosis, not DNA repair.

Gene expression profiling in brain following acute systemic hypertonicity: novel genes possibly involved in osmoadaptation

In brain osmoprotective genes known to be involved in cellular osmoadaptation to hypertonicity, as well as the related transcription factor tonicity-responsive enhancer binding protein (TonEBP) are only expressed in some cell subsets. In the search for other genes possibly involved in osmoadaptation of brain cells we have analyzed, through microarray, the transcriptional profile of forebrain from rats subjected to 45 min, 90 min, and 6 h systemic hypertonicity. Microarray data were validated by quantitative real-time PCR. Around 23 000 genes gave a reliable hybridization signal. The number of genes showing a higher expression increased from around 15 (45 min) up to nearly 200 (6 h). Among about 30 immediate early genes (IEGs) encoding transcription factors, only Atf3, Verge, and Klf4 showed a rapid increased expression. TonEBP-mRNA tissue level and TonEBP-mRNA labeling in neurons remained unchanged whereas TonEBP labeling was rapidly increased in neurons. Sodium-dependent neutral amino acid transporter-2 (SNAT2) encoded by gene Slc38a2 showed a delayed increased expression. The rapid tonicity-induced activation of Atf3, Verge, and Klf4 may regulate genes involved in osmoadaptation. Nfat5 encoding TonEBP is not an IEG and the early tonicity-induced expression of TonEBP in neurons may result from translational activation. Increased expression of sodium-dependent neutral amino-acid transporter 2 may lead to the cellular accumulation of amino acids for adaptation to hypertonicity.

Tonicity-regulated gene expression

Hypertonicity activates several different transcription factors, including TonEBP/OREBP, that in turn increase transcription of numerous genes. Hypertonicity elevates TonEBP/OREBP transcriptional activity by moving it into the nucleus, where it binds to its cognate DNA element (ORE), and by increasing its transactivational activity. This chapter presents protocols for measuring the transcriptional activity of TonEBP/OREBP and determining its subcellular localization, its binding to OREs, and activity of its transactivation domain.

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.

Specific TSC22 domain transcripts are hypertonically induced and alternatively spliced to protect mouse kidney cells during osmotic stress

We recently cloned a novel osmotic stress transcription factor 1 (OSTF1) from gills of euryhaline tilapia (Oreochromis mossambicus) and demonstrated that acute hyperosmotic stress transiently increases OSTF1 mRNA and protein abundance [Fiol DF, Kültz D (2005) Proc Natl Acad Sci USA102, 927-932]. In this study, a genome-wide search was conducted to identify nine distinct mouse transforming growth factor (TGF)-beta-stimulated clone 22 domain (TSC22D) transcripts, including glucocorticoid-induced leucine zipper (GILZ), that are orthologs of OSTF1. These nine TSC22D transcripts are encoded at four loci on chromosomes 14 (TSC22D1, two splice variants), 3 (TSC22D2, four splice variants), X (TSC22D3, two splice variants), and 5 (TSC22D4). All nine mouse TSC22D transcripts are expressed in renal cortex, medulla and papilla, and in the mIMCD3 cell line. The two TSC22D3 transcripts (including GILZ) are upregulated by aldosterone but not by hyperosmolality in mIMCD3 cells. In contrast, TSC22D4 is stably upregulated by hyperosmolality in mIMCD3 cells and increased in renal papilla compared with cortex. Moreover, all four TSC22D2 transcripts are transiently upregulated by hyperosmolality and resemble tilapia OSTF1 in this regard. All TSC22D2 transcripts depend on hypertonicity as the signal for their upregulation and are unresponsive to increases in cell-permeable osmolytes. mRNA stabilization is the mechanism for TSC22D2 upregulation by hyperosmolality. Overexpression of TSC22D2-4 in mIMCD3 cells confers protection towards osmotic stress, as evidenced by a 2.7-fold increase in cell survival after 3 days at 600 mOsmol x kg(-1). Based on variable responsiveness to aldosterone and hyperosmolality in kidney cells we conclude that mouse TSC22D genes have diverse physiological functions. TSC22D2 and TSC22D4 are involved in adaptation of renal cells to hypertonicity suggesting that they represent important elements of osmosensory signal transduction in mouse kidney cells.

Mre11-Rad50-Nbs1 complex is activated by hypertonicity

When exposed to hypertonic conditions, cells accumulate double-strand DNA breaks (DSBs) like they are exposed to ionizing radiation. It has been proposed that inactivation of the Mre11-Rad50-Nbs1 (MRN) complex due to nuclear exit is responsible for the accumulation of DSBs as cells fail to repair DSBs produced during normal cellular activity. In this study, we examined the MRN complex in cells switched to hypertonicity. Surprisingly, we found that the MRN complex stayed in the nucleus and remained intact in response to hypertonicity. In fact, the MRN complex was dramatically activated after 4 h of switch to hypertonicity in a dose-dependent manner as shown by formation of foci. Activation of ATM and the MRN complex by hypertonicity and bleomycin was additive as was activation of their downstream targets including gammaH2AX and Chk2 indicating that the cellular response to DSB was intact in hypertonic conditions. Activation of Chk2 in response to hypertonicity was not observed in mutant cells with functionally impaired MRN complex confirming that they are in the same pathway. After 20 h of a switch to hypertonicity, MRN foci and gammaH2AX returned to a control level, suggesting that cells adapted to hypertonicity by repairing DNA. We conclude that cells respond normally to DSB and repair the DNA damages induced by hypertonicity.

Regulation of the hypertonic stress response and other cellular functions by the Rel-like transcription factor NFAT5

Stress, be it from environmental factors or intrinsic to the cell as result of growth and metabolism, can be harmful to cells. Mammalian cells have developed numerous mechanisms to respond to diverse forms of stress. These mechanisms combine signaling cascades and activation of gene expression programs to orchestrate an adaptive response that will allow the cell to survive and resume its normal functioning. In this review we will focus on the transcription factor NFAT5, a fundamental regulator of the response to osmotic stress in mammalian cells. Identified in 1999, NFAT5 is the latest addition to the Rel family, which comprises the NF-kappaB and NFATc proteins. Though in some of its structural and functional features NFAT5 is a hybrid between these two major groups of Rel proteins, it has unique characteristics that make it stand on its own as a third type of Rel transcription factor. Since its discovery, NFAT5 has been studied mostly in the context of the hypertonicity stress response. The advent of mouse models deficient in NFAT5 and other recent advances have confirmed a fundamental osmoprotective role for this factor in mammals, but also revealed features that suggest it may have a wider range of functions.

Effects of expression of p53 and Gadd45 on osmotic tolerance of renal inner medullary cells

The response of renal inner medullary (IM) collecting duct cells (mIMCD3) to high NaCl involves increased expression of Gadd45 and p53, both of which have important effects on growth and survival of the cells. However, mIMCD3 cells, being immortalized by SV40, proliferate rapidly, which is known to sensitize cells to high NaCl, whereas IM cells in situ proliferate very slowly and survive much higher levels of NaCl. In the present studies, we have examined the importance of Gadd45 and p53 for survival of normal IM cells in their usual high-NaCl environment by using more slowly proliferating second-passage mouse inner medullary epithelial (p2mIME) cells and comparing cells from wild-type and gene knockout mice. Acutely elevating NaCl (and/or urea) reduces Gadd45a, but increases Gadd45b and Gadd45g mRNA, depending on the mix of NaCl and urea and the rate of increase of osmolality. Nevertheless, p2mIME cells from Gadd45b−/−, Gadd45g−/−, and Gadd45bg−/− mice survive elevation of NaCl (or urea) essentially the same as do wild-type cells. p53−/− Cells do not tolerate as high a concentration of NaCl (or urea) as p53+/+ cells, but urinary concentrating ability of p53−/− mice is normal, as is the histology of inner medullas from p53−/− and Gadd45abg−/− mice. Thus although Gadd45 and p53 may play roles in osmotically stressed mIMCD3 cells, we do not find that their expression makes an important difference, either for Gadd45 in slower proliferating p2mIME cells or for Gadd45 or p53 in normal inner medullary epithelial cells in situ.

DNA damage evaluated by gammaH2AX foci formation by a selective group of chemical/physical stressors

It has been reported that the phosphorylated form of histone variant H2AX (gammaH2AX) plays an important role in the recruitment of DNA repair and checkpoint proteins to sites of DNA damage, particularly at double strand breaks (DSBs). Using gammaH2AX foci formation as an indicator for DNA damage, several chemicals/stress factors were chosen to assess their ability to induce gammaH2AX foci in a 24h time frame in a human amnion FL cell line. Two direct-acting genotoxins, methyl methanesulfonate (MMS) and N-ethyl-N-nitrosourea (ENU), can induce gammaH2AX foci formation in a time- and dose-dependent manner. Similarly, an indirect-acting genotoxin, benzo[a]pyrene (BP), also induced the formation of gammaH2AX foci in a time- and dose-dependent manner. Another indirect genotoxin, 2-acetyl-aminofluorene (AAF), did not induce gammaH2AX foci formation in FL cells; however, AAF can induce gammaH2AX foci formation in Chinese hamster CHL cells. Neutral comet assays also revealed the induction of DNA strand breaks by these agents. In contrast, epigenetic carcinogens azathioprine and cyclosporine A, as well as non-carcinogen dimethyl sulfoxide, did not induce gammaH2AX foci formation in FL cells. In addition, heat shock and hypertonic saline did not induce gammaH2AX foci. Cell survival analyses indicated that the induction of gammaH2AX is not correlated with the cytotoxic effects of these agents/factors. Taken together, these results suggest that gammaH2AX foci formation could be used for evaluating DNA damage; however, the different cell types used may play an important role in determining gammaH2AX foci formation induced by a specific agent.

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.

Cell shrinkage as a signal to apoptosis in NIH 3T3 fibroblasts

Cell shrinkage is a hallmark of the apoptotic mode of programmed cell death, but it is as yet unclear whether a reduction in cell volume is a primary activation signal of apoptosis. Here we studied the effect of an acute elevation of osmolarity (NaCl or sucrose additions, final osmolarity 687 mosmol l(-1)) on NIH 3T3 fibroblasts to identify components involved in the signal transduction from shrinkage to apoptosis. After 1.5 h the activity of caspase-3 started to increase followed after 3 h by the appearance of many apoptotic-like bodies. The caspase-3 activity increase was greatly enhanced in cells expressing a constitutively active G protein, Rac (RacV12A3 cell), indicating that Rac acts upstream to caspase-3 activation. The stress-activated protein kinase, p38, was significantly activated by phosphorylation within 30 min after induction of osmotic shrinkage, the phosphorylation being accelerated in fibroblasts overexpressing Rac. Conversely, the activation of the extracellular signal-regulated kinase (Erk1/2) was initially significantly decreased. Subsequent to activation of p38, p53 was activated through serine-15 phosphorylation, and active p53 was translocated from the cytosol to the nucleus. Inhibition of p38 in Rac cells reduced the activation of both p53 and caspase-3. After 60 min in hypertonic medium the rate constants for K+ and taurine efflux were increased, particular in Rac cells. We suggest the following sequence of events in the cell shrinkage-induced apoptotic response: cellular shrinkage activates Rac, with activation of p38, followed by phosphorylation and nuclear translocation of p53, resulting in permeability increases and caspase-3 activation.

CELL SURVIVAL IN THE HOSTILE ENVIRONMENT OF THE RENAL MEDULLA

The countercurrent system in the medulla of the mammalian kidney provides the basis for the production of urine of widely varying osmolalities, but necessarily entails extreme conditions for medullary cells, i.e., high concentrations of solutes (mainly NaCl and urea) in antidiuresis, massive changes in extracellular solute concentrations during the transitions from antidiuresis to diuresis and vice versa, and low oxygen tension. The strategies used by medullary cells to survive in this hostile milieu include accumulation of organic osmolytes and heat shock proteins, the extensive use of the glycolysis for energy production, and a well-orchestrated network of signaling pathways coordinating medullary circulation and tubular work.

Transgenic mice expressing dominant-negative osmotic-response element-binding protein (OREBP) in lens exhibit fiber cell elongation defect associated with increased DNA breaks

Osmotic-response element-binding protein (OREBP), also known as TonEBP or NFAT5, is thought to be responsible for the induction of osmolyte-accumulating genes when cells are under hypertonic stress. Recent studies suggest that OREBP also plays a role in water reabsorption in the kidney, T-cell proliferation, and embryonic development. We developed transgenic mice that express the dominant-negative OREBP (OREBPdn) specifically in the lens because our earlier studies showed that it is particularly sensitive to osmotic stress. The transgenic mice developed nuclear cataract soon after birth, suggesting defects in lens development. The developing transgenic lenses showed incomplete elongation of fiber cells and formation of vacuoles. This is accompanied by evidence of DNA strand breaks, activation of p53, and induction of checkpoint kinase, suggesting that the developing fiber cells lacking OREBP are in a similar physiological state as cells experiencing hypertonic stress. These results indicate that OREBP-mediated accumulation of osmolytes is essential during elongation of the lens fiber cells.

Increased reactive oxygen species contribute to high NaCl-induced activation of the osmoregulatory transcription factor TonEBP/OREBP

The signaling pathways leading to high NaCl-induced activation of the transcription factor tonicity-responsive enhancer binding protein/osmotic response element binding protein (TonEBP/OREBP) remain incompletely understood. High NaCl has been reported to produce oxidative stress. Reactive oxygen species (ROS), which are a component of oxidative stress, contribute to regulation of transcription factors. The present study was undertaken to test whether the high NaCl-induced increase in ROS contributes to tonicity-dependent activation of TonEBP/OREBP. Human embryonic kidney 293 cells were used as a model. We find that raising NaCl increases ROS, including superoxide. N-acetylcysteine (NAC), an antioxidant, and MnTBAP, an inhibitor of superoxide, reduce high NaCl-induced superoxide activity and suppress both high NaCl-induced increase in TonEBP/OREBP transcriptional activity and high NaCl-induced increase in expression of BGT1mRNA, a transcriptional target of TonEBP/OREBP. Catalase, which decomposes hydrogen peroxide, does not have these effects, whether applied exogenously or overexpressed within the cells. Furthermore, NAC and MnTBAP, but not catalase, blunt high NaCl-induced increase in TonEBP/OREBP transactivation. N(G)-monomethyl-l-arginine, a general inhibitor of nitric oxide synthase, has no significant effect on either high NaCl-induced increase in superoxide or TonEBP/OREBP transcriptional activity, suggesting that the effects of ROS do not involve nitric oxide. Ouabain, an inhibitor of Na-K-ATPase, attenuates high NaCl-induced superoxide activity and inhibits TonEBP/OREBP transcriptional activity. We conclude that the high NaCl-induced increase in ROS, including superoxide, contributes to activation of TonEBP/OREBP by increasing its transactivation.

Modulation of Hypertonicity-Induced Aquaporin-1 by Sodium Chloride, Urea, Betaine, and Heat Shock in Murine Renal Medullary Cells

Aquaporin-1 (AQP1) expression is induced by hypertonicity in renal medullary cells. The purpose of the present study was to elucidate the role of sodium chloride (NaCl), urea, betaine, and heat shock on hypertonicity-induced AQP1 expression in cultured murine renal medullary-K2 (mIMCD-K2) cells. AQP1 expression was maximally induced under mild hypertonic medium supplemented with 100 mM NaCl (N100), whereas severe hypertonic medium supplemented with 150 mM NaCl (N150) caused little AQP1 induction. The reduction of AQP1 expression in N150 was associated with reduced cell viability. When cells were exposed continuously to N100, hypertonicity-induced AQP1 expression was elevated, whereas the return to isotonic medium reduced AQP1 expression in a time-dependent manner. The half-life of AQP1 protein in isotonic conditions was approximately 4 h, whereas hypertonicity markedly increased its half-life. These results indicate that hypertonicity plays an important role in AQP1 induction, stability, and degradation. On the contrary, urea inhibited hypertonicity-induced AQP1 expression in a dose-dependent manner. The addition of organic osmolyte betaine in N150 enhanced hypertonicity-induced AQP1 expression, whereas it decreased AQP1 expression in N100. This suggests that the excessive accumulation of betaine may counteract hypertonic stress and thus attenuate hypertonicity-induced AQP1 expression. Heat shock treatment promoted hypertonicity-induced AQP1 and heat shock protein 70 (HSP70) expression in both N100 and N150, suggesting an effect on the stability of hypertonicity-induced AQP1 expression. Taken together, NaCl, urea, betaine, and heat shock that regulate hypertonicity-induced AQP1 expression are potentially important factors in urinary concentration and contribute to the steady-state level of AQP1 expression.

Hypertonic stress response

Mammalian renal inner medullary cells are normally exposed to extremely high NaCl concentrations. Remarkably, under these normal conditions, the high NaCl causes DNA damage and inhibits its repair, yet the cells survive and function both in cell culture and in vivo. The interstitial NaCl concentration in parts of a normal renal medulla can be 500 mM or more, depending on the species. Studies of how the cells survive and function despite this extreme stress have led to the discovery of protective adaptations, including accumulation of large amounts of organic osmolytes, which normalize cell volume and intracellular ionic strength, despite the hypertonicity of the high NaCl. Those adaptations, however, do not prevent DNA damage. High NaCl induces DNA breaks rapidly, and the DNA breaks persist even after the cells become adapted to the high NaCl. The adapted cells proliferate rapidly in cell culture and function adequately in vivo despite the DNA breaks. Both in cell culture and in vivo the breaks are rapidly repaired if the NaCl concentration is lowered. Although acute elevation of NaCl causes transient cell cycle arrest and, when the elevation is too extreme, apoptosis, proliferation of adapted cells is not arrested in culture and apoptosis is not evident either in culture or in vivo. Further, high NaCl impairs activation of several components of the classical DNA damage response such as Mre11, H2AX and Chk1 leading to inhibition of DNA repair. Nevertheless, other regular participants in the DNA damage response, such as Gadd45a, Gadd153, p53, Hsp70, and ATM are still upregulated by high NaCl. How high NaCl causes the DNA breaks and how the cells survive them is conjectural at this point. We discuss possible answers to these questions, based on current knowledge about induction and processing of DNA breaks.