Hyperosmolality causes growth arrest of murine kidney cells. Induction of GADD45 and GADD153 by osmosensing via stress-activated protein kinase 2

Melanoma differentiation associated gene-7, mda-7/IL-24, selectively induces growth suppression, apoptosis and radiosensitization in malignant gliomas in a p53-independent manner

Malignant gliomas are extremely aggressive cancers currently lacking effective treatment modalities. Gene therapy represents a promising approach for this disease. A requisite component for improving gene-based therapies of brain cancer includes tumor suppressor genes that exhibit cancer constrained inhibitory activity. Subtraction hybridization identified melanoma differentiation associated gene-7 (mda-7) as a gene associated with melanoma cell growth, differentiation and progression. Ectopic expression of mda-7 by means of a replication-incompetent adenovirus (Ad), Ad.mda-7, induces growth suppression and apoptosis selectively in diverse human cancers, without producing any apparent harmful effect in normal cells. We presently demonstrate that Ad.mda-7 induces growth inhibition and apoptosis in malignant human gliomas expressing both mutant and wild-type p53, and these effects correlate with an elevation in expression of members of the growth arrest and DNA damage (GADD) gene family. In contrast, infection with a recombinant Ad expressing wild-type p53, Ad.wtp53, specifically affects mutant p53 expressing gliomas. When tested in early passage normal and immortal human fetal astrocytes, growth inhibition resulting from infection with Ad.mda-7 or Ad.wtp53 is significantly less than in malignant gliomas and no toxicity is evident in these normal cells. Moreover, infection of gliomas with Ad.mda-7 or treatment with purified GST-MDA-7 protein sensitizes both wild-type and mutant p53 expressing tumor cells to the growth inhibitory and antisurvival effects of ionizing radiation, and this response correlates with increased expression of specific members of the GADD gene family. Since heterogeneity in p53 expression is common in evolving gliomas, the present findings suggest that Ad.mda-7 may, in many instances, prove more beneficial for the gene-based therapy of malignant gliomas than administration of wild-type p53.

Analysis of the role of GADD153 in the control of apoptosis in NS0 myeloma cells

Apoptosis can limit the maximum production of recombinant protein expression from cultured mammalian cells. This article focuses on the links between nutrient deprivation, ER perturbation, the regulation of (growth arrest and DNA damage inducible gene 153) GADD153 expression and apoptosis. During batch culture, decreases in glucose and glutamine correlated with an increase in apoptotic cells. This event was paralleled by a simultaneous increase in GADD153 expression. The expression of GADD153 in batch culture was suppressed by the addition of nutrients and with fed-batch culture the onset of apoptosis was delayed but not completely prevented. In defined stress conditions, glucose deprivation had the greatest effect on cell death when compared to glutamine deprivation or the addition of tunicamycin (an inhibitor of glycosylation), added to generate endoplasmic reticulum stress. However, the contribution of apoptosis to overall cell death (as judged by morphology) was smaller in conditions of glucose deprivation than in glutamine deprivation or tunicamycin treatment. Transient activation of GADD153 expression was found to occur in response to all stresses and occurred prior to detection of the onset of cell death. These results imply that GADD153 expression is either a trigger for apoptosis or offers a valid indicator of the likelihood of cell death arising from stresses of relevance to the bioreactor environment.

Three GADD45 isoforms contribute to hypertonic stress phenotype of murine renal inner medullary cells

Mammalian renal inner medullary (IM) cells routinely face and resist hypertonic stress. Such stress causes DNA damage to which IM cells respond with cell cycle arrest. We report that three growth arrest and DNA damage-inducible 45 (GADD45) isoforms (GADD45alpha, GADDD45beta, and GADD45gamma) are induced by acute hypertonicity in murine IM cells. Maximum induction occurs 16-18 h after the onset of hypertonicity. GADD45gamma is induced more strongly (7-fold) than GADD45beta (3-fold) and GADD45alpha (2-fold). GADD45alpha and GADD45beta protein induction is more pronounced and stable compared with the corresponding transcripts. Hypertonicity of various forms (NaCl, KCl, sorbitol, or mannitol) always induces GADD45 transcripts, whereas nonhypertonic hyperosmolality (urea) has no effect. Actinomycin D does not prevent hypertonic GADD45 induction, indicating that mRNA stabilization is the mechanism that mediates this induction. GADD45 induction patterns in IM cells exposed to 10 different stresses suggest isoform specificity, but similar functions, of individual isoforms during hypertonicity, heat shock, and heavy metal stress, when GADD45gamma induction is strongest (17-fold). These data associate all known GADD45 isoforms with the hypertonicity phenotype of renal IM cells.

Response of renal inner medullary epithelial cells to osmotic stress

As part of the urinary concentrating mechanism, renal inner medullary epithelial (IME) cells are normally exposed to variable and often very high interstitial levels of NaCl and urea, yet they survive and function. We have been studying the mechanisms involved, using an established cell line (mIMCD3). Acute increase of NaCl or urea from 300 to >500 mOsmol/kg causes cell cycle delay and apoptosis. High NaCl, but not high urea, causes DNA double strand breaks. At 500-600 mOsmol/kg inhibition of DNA replication following high NaCl depends on activation of the tumor suppressor protein, p53, and provides time for DNA repair. If p53 expression is suppressed, cells continue to replicate DNA, and many of those cells die. At higher levels of NaCl (>650 mOsmol/kg) the mitochondria rapidly depolarize and most cells die within a few hours despite a high level of p53 protein (which, however, is less phosphorylated than at 500 mOsmol/kg). Since the levels of NaCl and urea that kill mIMCD3 cells are much lower than those that exist in vivo, we investigated the difference, using early passage mouse IME cells under various conditions. Passage 2 IME cells survive higher levels of NaCl and urea than do mIMCD3 cells, but still not levels as high as in vivo. However, when the osmolality is increased linearly over 20 h, as occurs in vivo, rather than as a single step, cell survival increases to levels close to those found in vivo. We conclude that a more gradual increase in osmolality provides time for accumulation of organic osmolytes and activation of heat shock protein, previously known to be important for cell survival.

Adaptation of kidney medulla to hypertonicity: role of the transcription factor TonEBP

The osmolality of the mammalian kidney medulla is very high. The high osmolality provides the driving force for water reabsorption and urinary concentration, key functions of the kidney for maintaining proper body fluid volume and blood pressure. Salt and urea are the major solutes in the renal medullary interstitium. Unfortunately, high salt (hypertonicity) causes DNA damage and cell death. In response, the renal medullary cells adapt to the hypertonicity by accumulating compatible osmolytes. A regulatory protein, tonicity-responsive enhancer binding protein (TonEBP), plays a central role in the accumulation of these compatible osmolytes by stimulating genes whose products either actively transport or synthesize the appropriate osmolytes. TonEBP is active under isotonic conditions. It responds to both an increase and a decrease in ambient tonicity, in opposite directions, which involves changes in its abundance and nucleocytoplasmic distribution. In the kidney medulla, however, nucleocytoplasmic distribution is the major site of control, under normal conditions of diuresis and antidiuresis.

Osmotic stress sensitizes naturally resistant cells to TNF-alpha-induced apoptosis

Most cells are naturally resistant to TNF-alpha-induced cell death and become sensitized when NF-kappaB transactivation is blocked or in the presence of protein synthesis inhibitors that prevent the expression of anti-apoptotic genes. In this report we analyzed the role of osmotic stress on TNF-alpha-induced cell death. We found that it sensitizes the naturally resistant HeLa cells to TNF-alpha-induced apoptosis, with the involvement of an increase in the activity of several kinases, the inhibition of Bcl-2 expression, and a late increase on NF-kappaB activation. Cell death occurs regardless of the enhanced NF-kappaB activity, whose inhibition produces an increase in apoptosis. The inhibition of p38 kinase, also involved in NF-kappaB activation, significantly increases the effect of osmotic stress on TNF-alpha-induced cell death.

TonEBP/NFAT5 stimulates transcription of HSP70 in response to hypertonicity

While hyperosmolality of the kidney medulla is essential for urinary concentration, it imposes a great deal of stress. Cells in the renal medulla adapt to the stress of hypertonicity (hyperosmotic salt) by accumulating organic osmolytes. Tonicity-responsive enhancer (TonE) binding protein (TonEBP) (or NFAT5) stimulates transcription of transporters and a synthetic enzyme for the cellular accumulation of organic osmolytes. We found that dominant-negative TonEBP reduced expression of HSP70 as well as the transporters and enzyme. Near the major histocompatibility complex class III locus, there are three HSP70 genes named HSP70-1, HSP70-2, and HSC70t. While HSP70-1 and HSP70-2 were heat inducible, only HSP70-2 was induced by hypertonicity. In the 5' flanking region of the HSP70-2 gene, there are three sites for TonEBP binding. In cells transfected with a reporter plasmid containing this region, expression of luciferase was markedly stimulated in response to hypertonicity. Coexpression of the dominant-negative TonEBP reduced the luciferase expression. Mutating all three sites in the reporter plasmid led to a complete loss of induction by hypertonicity. Thus, TonEBP rather than heat shock factor stimulates transcription of the HSP70-2 gene in response to hypertonicity. We conclude that TonEBP is a master regulator of the renal medulla for cellular protection against high osmolality via organic osmolytes and molecular chaperones.

mda-7 (IL-24) Mediates selective apoptosis in human melanoma cells by inducing the coordinated overexpression of the GADD family of genes by means of p38 MAPK

Subtraction hybridization identified melanoma differentiation-associated gene-7 (mda-7) as a gene induced during terminal differentiation in human melanoma cells. On the basis of structure, chromosomal localization and cytokine-like properties, mda-7 is classified as IL-24. Administration of mda-7/IL-24 by means of a replication-incompetent adenovirus (Ad.mda-7) induces apoptosis selectively in diverse human cancer cells without inducing harmful effects in normal fibroblast or epithelial cells. The present studies investigated the mechanism underlying this differential apoptotic effect. Infection of melanoma cells, but not normal immortal melanocytes, with Ad.mda-7 induced a time- and dose-dependent increase in expression, mRNA and protein, of a family of growth arrest and DNA damage (GADD)-inducible genes, which correlated with induction of apoptosis. Among the members of the GADD family of genes, GADD153, GADD45 alpha, and GADD34 displayed marked, and GADD45 gamma showed minimal induction. Treatment of melanoma cells with SB203580, a selective inhibitor of the p38 mitogen-activated protein kinase (MAPK) pathway, effectively inhibited Ad.mda-7-induced apoptosis. Additional support for an involvement of the p38 MAPK pathway in Ad.mda-7-mediated apoptosis was documented by using an adenovirus expressing a dominant negative mutant of p38 MAPK. Infection with Ad.mda-7 increased the phosphorylation of p38 MAPK and heat shock protein 27 in melanoma cells but not in normal immortal melanocytes. In addition, SB203580 effectively inhibited Ad.mda-7-mediated induction of the GADD family of genes in a time- and dose-dependent manner, and it effectively blocked Ad.mda-7-mediated down-regulation of the antiapoptotic protein BCL-2. Inhibition of GADD genes by an antisense approach either alone or in combination also effectively blocked Ad.mda-7-induced apoptosis in melanoma cells. These results support the hypothesis that Ad.mda-7 mediates induction of the GADD family of genes by means of the p38 MAPK pathway, thereby resulting in the selective induction of apoptosis in human melanoma cells.

Secretion of interleukin-1beta and interleukin-6 by fragments of term human placental villi: signalling pathways and effects of tumour necrosis factor alpha and mode of delivery

The regulation of cytokine secretion has not been extensively investigated in placental tissue. Fragments of term human placenta were incubated in Tyrode's medium for 3h and cytokine concentrations were measured in the supernatant. IL-1beta secretion after vaginal delivery (VD) was (mean +/- SEM fmol/mg wet weight/3h) 0.193 +/- 0.005 (basal) and 0.549 +/- 0.18 (+1n M TNFalpha) and was more sensitive to TNFalpha dose after elective Caesarean section in the absence of clinical labour (CS) than VD. Secretion of IL-6 after VD was 2.3 +/- 0.47 (basal) and 3.01 +/- 0.34 (+1n M TNFalpha), was correlated with the secretion of IL-1beta and was more sensitive to TNFalpha dose after VD than CS. The inhibitors SB203580, PD98059, SN50, cycloheximide and D-ribofuranosylbenzimidazole each reduced the basal and TNFalpha-stimulated secretion of IL-1beta and also reduced IL-6 secretion with the exception of SN50. There were no interactions between effects of inhibitors and mode of delivery or TNFalpha. In summary we found that term placenta spontaneously secretes IL-1beta and IL-6 in vitro. Delivery after labour alters placental sensitivity to TNFalpha. Exposure to agents known to inhibit MAPK pathways, NF-kappaB, or synthesis of protein and mRNA reduces placental cytokine secretion.

A homolog of the E3 ubiquitin ligase Rbx1 is induced during hyperosmotic stress of salmon

Juvenile salmon migrating from freshwater to the marine environment confront a marked change in environmental osmolality. Using differential display of mRNA expression, we cloned a 1.9-kb cDNA upregulated in isolated tissues of salmon exposed to the hyperosmotic stress associated with transition to the dehydrating marine environment. The cDNA codes for a 21-kDa protein, salmon hyperosmotic protein 21 (Shop21), with 98% identity to Rbx1, an E3 ubiquitin ligase; the protein also contains a novel 81-amino acid domain at the NH(2) terminus not found in Rbx1. Moderate hyperosmotic stress (24 h at 550 mosmol/kg) increased Shop21 transcript 10-fold in branchial lamellae, whereas no upregulation was observed under more severe stress (> or = 800 mosmol/kg). Expression of the gene also was observed in heart and kidney. Replacement of NaCl with mannitol, but not glycerol, also elicited an increase in Shop21 mRNA. Inhibition of the mitogen-activated protein kinase and mitogen-activated extracellular regulated kinase kinase signal transduction pathways failed to blunt the Shop21 response during hyperosmotic stress. Shop21 mRNA also accumulated during thermal stress but to a lesser extent than heat shock protein 70 mRNA. The potential importance of Shop21 to the living animal is suggested by marked upregulation of the gene in salmon after transfer to seawater. The results of these investigations suggest that Shop21 may have a role in targeting selected proteins (e.g., in freshwater ionocytes) nonessential for adaptation to seawater for removal via the proteasome pathway.

Hyposmotic stress induces cell growth arrest via proteasome activation and cyclin/cyclin-dependent kinase degradation

Ordered cell cycle progression requires the expression and activation of several cyclins and cyclin-dependent kinases (Cdks). Hyperosmotic stress causes growth arrest possibly via proteasome-mediated degradation of cyclin D1. We studied the effect of hyposmotic conditions on three colonic (Caco2, HRT18, HT29) and two pancreatic (AsPC-1 and PaCa-2) cell lines. Hyposmosis caused reversible cell growth arrest of the five cell lines in a cell cycle-independent fashion, although some cell lines accumulated at the G(1)/S interface. Growth arrest was followed by apoptosis or by formation of multinucleated giant cells, which is consistent with cell cycle catastrophe. Hyposmosis dramatically decreased Cdc2, Cdk2, Cdk4, cyclin B1, and cyclin D3 expression in a time-dependent fashion, in association with an overall decrease in cellular protein synthesis. However, some protein levels remained unaltered, including cyclin E and keratin 8. Selective proteasome inhibition prevented Cdk and cyclin degradation and reversed hyposmotic stress-induced growth arrest, whereas calpain and lysosome enzyme inhibitors had no measurable effect on cell cycle protein degradation. Therefore, hyposmotic stress inhibits cell growth and, depending on the cell type, causes cell cycle catastrophe with or without apoptosis. The growth arrest is due to decreased protein synthesis and proteasome activation, with subsequent degradation of several cyclins and Cdks.

The cellular hydration state: a critical determinant for cell death and survival

Alterations in cellular hydration not only contribute to metabolic regulation, but also critically determine the cellular response to different kinds of stress. Whereas cell swelling triggers anabolic pathways and protects cells from heat and oxidative challenge, cellular dehydration contributes to insulin resistance and catabolism and increases the cellular susceptibility to stress-induced damage. Intracellular accumulation of organic osmolytes, cell cycle delay and the expression of heat shock proteins provide cellular tolerance to hyperosmolarity and protect against stressors under dehydrating conditions. This article discusses some mechanisms by which alterations in cell hydration contribute to cytoprotection or cell damage. In addition, the close relationship between osmotic and oxidative stress and the contribution of isoosmotic shrinkage to apoptotic cell death are considered.

Protection of human endothelial cells from oxidative stress: role of Ras-ERK1/2 signaling

BACKGROUND

Reactive oxygen species play a critical role in inducing apoptosis. The small GTPase p21 Ras and the ERK1/2 MAPK have been proposed as key regulators of the signaling cascade triggered by oxidative stress (H2O2). Harvey-Ras (Ha-Ras) and Kirsten-Ras (Ki-Ras) isoforms are so far functionally indistinguishable, because they activate the same downstream effectors, including ERK1/2. Moreover, ERK1/2 signaling has been involved in both protection and induction of apoptosis.

METHODS AND RESULTS

Human umbilical vein endothelial cells (HUVECs) were subjected to H2O2, and apoptosis was detected by fluorescence-activated cell sorting analysis, fluorescence microscopy, and caspase-3 activation. Transfection of Ha-Ras and Ki-Ras genes in HUVECs was performed to evaluate the response to H2O2. We have found that, whereas Ha-Ras decreases tolerance to oxidative stress, Ki-Ras has a potent antiapoptotic activity. Both effects are mediated by ERK1/2. Tolerance to H2O2 is encoded by a unique stretch of lysines at the COOH terminus of the Ki-Ras, lacking in Ha-Ras, and it is relatively independent of the farnesylated anchor. Inhibition of p21 Ras signaling by farnesylation inhibitors increased the resistance to apoptosis in Ha-Ras-expressing cells.

CONCLUSIONS

These findings explain the opposite effects of ERK1/2 stimulation on apoptosis found in different cell types and suggest that local activation of ERK1/2 signaling may account for the opposing response to oxidative stress by Ha-Ras or Ki-Ras-expressing cells. Modulation of cell reactivity to oxidative stress by p21 Ras points to the specific and predictive effects of Ras inhibitors in vivo as potential therapeutic drugs in disorders produced by increase of reactive oxygen species inside the cells.

Osmoregulation of natriuretic peptide receptor signaling in inner medullary collecting duct. A requirement for p38 MAPK

In the inner medullary collecting duct of the terminal nephron, the type A natriuretic peptide receptor (NPR-A) plays a major role in determining urinary sodium content. This nephron segment, by virtue of its medullary location, is subject to very high levels of extracellular tonicity. We have examined the ability of medium tonicity to regulate the activity and expression of this receptor in cultured rat inner medullary collecting duct cells. We found that NaCl (75 mm) and sucrose (150 mm), but not urea (150 mm), increased natriuretic peptide receptor activity, gene expression, and promoter activity. The osmotic stimulus also activated extracellular signal-regulated kinase (ERK), c-Jun NH(2)-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK). In the latter instance the beta isoform was selectively activated. Inhibition of p38 MAPK with SB203580 blocked the osmotic induction of receptor activity and expression, as well as receptor gene promoter activity, whereas inhibition of ERK with PD98059 had no effect. Cotransfection of p38 beta MAPK together with the receptor gene promoter resulted in amplification of the osmotic stimulation of the latter, whereas cotransfection of dominant negative MKK6, but not dominant-negative MEK, completely blocked the osmotic induction of receptor promoter activity. Collectively, the data indicate that extracellular osmolality stimulates receptor activity and receptor gene expression through a specific p38 beta-dependent mechanism, raising the possibility that changes in medullary tonicity could play an important role in the regulation of renal sodium handling in the terminal nephron.

Rapid activation of G2/M checkpoint after hypertonic stress in renal inner medullary epithelial (IME) cells is protective and requires p38 kinase

Cells in the kidney medulla are subject to variable and often extreme osmotic stress during concentration of the urine. Previous studies showed that renal inner medullary epithelial (IME) cells respond to hypertonicity by G(2) arrest. The purpose of the present study was to investigate the mechanisms involved in initiation and maintenance of G(2) arrest. Rapid initiation of G(2) arrest after UV radiation is mediated by p38 kinase. Here we find that p38 kinase is responsible for rapid initiation of the G(2) delay in IME cells after the hypertonic stress created by adding NaCl. High NaCl, but not high urea, rapidly initiates G(2) arrest. Inhibition of p38 kinase by SB202190 (10 microM) blocks the rapid initiation of this checkpoint both in an immortalized cell line (mIMCD3) and in second-passage IME cells from mouse renal inner medulla. p38 inhibition does not affect exit from G(2) arrest. The rapid initiation of G(2) arrest is followed by inhibition of cdc2 kinase, which is also prevented by SB202190. To assess the possible protective role of G(2) arrest, we measured DNA strand breaks as reflected by immunostaining against phospho-histone H2AX, which becomes phosphorylated on Ser-139 associated with DNA breaks. Abrogation of rapid G(2)/M checkpoint activation by SB202190 increases the histone H2AX phosphorylation in G(2)/M cells. We propose that the rapid initiation of G(2) delay by p38 kinase after hypertonicity protects the cells by decreasing the level of DNA breaks caused by aberrant mitosis entry.

Involvement of multiple kinase pathways in stimulation of gene transcription by hypertonicity

Osmolality of the mammalian renal medulla is high because of the operation of the urinary concentrating mechanism. To understand molecular events during the early phase of cellular adaptation to hypertonicity, we performed comprehensive searches for genes induced in response to hypertonicity using a cell line (mIMCD3) derived from the inner medullary collecting duct of mouse kidney. PCR-based subtractive hybridization of cDNA pools and cDNA microarray analysis were used. We report 12 genes whose mRNA expression is significantly increased within 4 h after exposure to hypertonicity. The increase in mRNA expression was the result of increased transcription. Many are either stress response genes or growth regulatory genes, supporting the notion that hypertonicity evokes the stress response and growth regulation in cells. Experiments using inhibitors revealed that mitogen-activated protein kinases were commonly involved in signaling for the induction of genes by hypertonicity. Tyrosine kinases and phosphatidylinositol 3-kinase also play a significant role. Signaling pathways for stimulation of transcription appeared quite diverse in that each gene was sensitive to different combinations of inhibitors.

Urea and hypertonicity increase expression of heme oxygenase-1 in murine renal medullary cells

Epithelial cells derived from the mammalian kidney medulla are responsive to urea at the levels of signal transduction and gene regulation. Hybridization of RNA harvested from control- and urea-treated murine inner medullary collecting duct (mIMCD3) cells with a cDNA expression array encoding stress-responsive genes suggested that heme oxygenase (HO)-1 mRNA was upregulated by urea. RNase protection assay confirmed this upregulation; hypertonicity also increased HO-1 mRNA expression but neither hypertonic NaCl nor urea were effective in the nonrenal 3T3 cell line. The effect on HO-1 expression appeared to be transcriptionally mediated on the basis of mRNA half-life studies and reporter gene analyses using the promoters of both human and chicken HO-1. Although urea signaling resembles that of heavy metal signaling in other contexts, the effect of urea on HO-1 transcription was independent of the cadmium response element in this promoter. Urea-inducible HO-1 expression was sensitive to antioxidants but not to scavengers of nitric oxide. Urea also upregulated HO-1 protein expression and pharmacological inhibition of HO-1 action with zinc protoporphyrin-sensitized mIMCD3 cells to the adverse effects of hypertonicity but not to urea. Coupled with the prior observation of others that HO-1 expression increases along the renal corticomedullary gradient, these data suggest that HO-1 expression may comprise an element of the adaptive response to hypertonicity and/or urea in renal epithelial cells.

Maintenance of genomic integrity in mammalian kidney cells exposed to hyperosmotic stress

Changes in environmental salinity/osmolality impose an osmotic stress upon cells because, if left uncompensated, such changes will alter the conserved intracellular ionic milieu and macromolecular density, for which cell metabolism in most extant cells has been optimized. Cell responses to osmotic stress include rapid posttranslational and slower transcriptional events for the compensatory regulation of cell volume, intracellular electrolyte concentrations, and protein stability/activity. Changes in external osmolality are perceived by osmosensors that control the activation of signal transduction pathways giving rise to the above responses. We have recently shown that the targets of such pathways include cell cycle-regulatory and DNA damage-inducible genes (reviewed in Kültz, D., 2000. Environmental stressors and gene responses, Elsevier, Amsterdam. pp 157-179). Moreover, recent evidence suggests that hyperosmotic stress causes chromosomal aberrations and DNA double-strand breaks in mammalian cells. We propose that the modulation of cell cycle checkpoints and the preservation of genomic integrity are important aspects of cellular osmoprotection and as essential for cellular osmotic stress resistance as the capacity for cell volume regulation and maintaining inorganic ion homeostasis and protein stability/activity.

Cell cycle delay and apoptosis in response to osmotic stress

As part of the urinary concentrating mechanism, renal inner medulla cells may be exposed to extremely variable NaCl and urea concentrations that can reach very high levels. A number of studies, reviewed herein, aim to understand how such osmotic stress affects the cells and what protective mechanisms might exist. The majority of these studies are done on inner medullary epithelial cells that grow continuously in tissue culture (mIMCD3). Cells grown at 300 mosmol/kg survive increase to 500 mosmol/kg by adding NaCl or urea, but only after a growth arrest of approximately 24 h. At a higher osmolality (650-700 mosmol/kg) most cells die within hours by apoptosis. The cells both in vitro and in vivo adapt to high osmolality by a number of mechanisms, including accumulation of variety of organic osmolytes and induction of heat shock proteins. The cell cycle delay results from blocks at the G1 and G2/M checkpoints and slowing during S. After adding NaCl, but not urea, the amount and transcriptional activity of p53 (the tumor suppressor protein) increases. The p53 is phosphorylated on ser-15 and is transcriptionally active at 500 mosmol/kg (associated with cell survival), but not at 700 mosmol/kg (associated with apoptosis). Reduction of p53 expression by p53 antisense oligonucleotide increases sensitivity of renal cells in culture to hyperosmotic stress caused by NaCl. The possible mechanisms of the protection action of p53 against hypertonic stress are discussed.

p53 Protects renal inner medullary cells from hypertonic stress by restricting DNA replication

We previously found that p53 upregulation by hypertonicity protected renal inner medullary collecting duct (mIMCD3) cells from apoptosis. The purpose of the present study was to investigate the mechanism by which p53 protects the cells. We now find that hypertonicity (NaCl added to a total of 500 mosmol) inhibits DNA replication and delays G(1)-S transition as concluded from analysis of cell cycle distributions and bromodeoxyuridine (BrDU) incorporation rates. Lowering of p53 with p53 antisense oligonucleotide attenuated such effects of hypertonicity, resulting in an increased number of apoptotic cells in S phase and cells with >4 N DNA. Results with synchronized cells are similar, showing that cells in the early S phase are more sensitive to hypertonicity. Immunocytochemistry revealed that p53 becomes phosphorylated on Ser(15) and translocates to the nucleus in S both in isotonic and hypertonic conditions. Caffeine (2 mM) greatly reduces the p53 level and Ser(15) phosphorylation, followed by a remarkable increase of DNA replication rate, by failure of hypertonicity to inhibit it, and by reduction of cell number during hypertonicity. Finally, inhibition of DNA replication by the DNA polymerase inhibitor aphidicolin significantly improves cell survival, confirming that keeping cells in G(1) and decreasing the rate of DNA replication is protective and that these actions of p53 most likely are what normally help protect cells against hypertonicity.

Transcriptional regulation by changes in tonicity

Most organisms respond to a hypertonic environment by accumulating small organic solutes. In contrast to high concentrations of electrolytes, the small organic solutes do not perturb the activity of enzymes and other macromolecules within the cell. When the renal medulla becomes hypertonic during antidiuresis, multiple signaling pathways are activated. Here, we review the role of tonicity responsive enhancers (TonE) binding protein (TonEBP), a transcription factor activated in hypertonic cells. The activation of TonEBP by hypertonicity results from its translocation to the nucleus as well as an increase in TonEBP mRNA and protein. TonEBP may have a role beyond the response to tonicity since it is highly expressed in activated lymphocytes and in developing tissues.

Hyperosmolality activates Akt and regulates apoptosis in renal tubular cells

BACKGROUND

The novel serine-threonine kinase Akt is a critical enzyme in cell survival. We investigated the roles of the Akt pathway and apoptotic signals in (1) Madin-Darby canine kidney (MDCK) cells in a hyperosmotic condition in vitro and (2) in the inner medulla of dehydrated rat in vivo.

METHODS

The in vivo experiments were performed in 24- and 48-hour water-restricted rats. Hyperosmolality-stimulated Akt phosphorylation was examined in MDCK cells. Phosphatidylinositol 3-kinase (PI3-K) inhibitors, the dominant-negative mutant of PI3-K, the dominant-negative mutant of Akt, and the dominant-active form of Akt were used to examine the roles of the PI3-K/Akt pathways in renal tubular cell apoptosis.

RESULTS

The amount of phosphorylated Akt protein was increased in the inner medulla of dehydrated rats. Hyperosmolality induced by the addition of NaCl, urea, and raffinose phosphorylated Akt in MDCK cells in an osmolality-dependent manner. PI3-K inhibitors and the dominant-negative mutant of PI3-K inhibited the hyperosmolality-induced phosphorylation of Akt. Raising the media osmolality from a normal level to 500 or 600 mOsm/kg H2O final osmolality elicited apoptotic changes such as nucleosomal laddering of DNA and an increment of caspase-3 activity and increased activity in the cell death enzyme-linked immunosorbent assay. Dominant-active Akt prevented the mild hyperosmolality-induced apoptosis, while inhibition of the PI3-K/Akt pathways promoted apoptosis.

CONCLUSION

The Akt pathway is activated by hyperosmolality in vitro and in vivo, and activation of Akt prevents the mild hyperosmolality-induced apoptotic changes in MDCK cells. PI3-K/Akt pathways are involved in a hypertonic condition that confers the balance between cell survival and apoptosis.

Regulation of interleukin-8 expression by tumor-associated stress factors

Tumor and host cells frequently express interleukin-8 (IL-8). IL-8 has been shown to be motogenic, mitogenic, and angiogenic and to play important roles in human tumor progression. IL-8 expression can be induced by numerous stress factors present in the tumor environment, such as hypoxia, acidosis, hyperglycemia, hyperosmotic pressure, high cell density, hyperthermia, radiation, and chemotherapeutic agents. Understanding the mechanisms of IL-8 expression and regulation will be helpful in designing potential therapeutic modalities targeting IL-8 to control tumor growth and metastasis.

Sensitive determination of cell number using the CyQUANT cell proliferation assay

We describe here the development and characterization of the CyQUANT cell proliferation assay, a highly sensitive, fluorescence-based microplate assay for determining numbers of cultured cells. The assay employs CyQUANT GR dye, which produces a large fluorescence enhancement upon binding to cellular nucleic acids that can be measured using standard fluorescein excitation and emission wavelengths. The fluorescence emission of the dye-nucleic acid complexes correlated linearly with cell number over a large range using a wide variety of cell types. Under the recommended assay conditions, standard curves were linear (r(2)>0.995), detecting as few as 10-50 cells and as many as 25,000-50,000 cells with a single dye concentration, depending on cell type. Increasing the dye concentration extended the linear range of the assay to 100,000-250,000 cells. Results of cell proliferation and growth inhibition studies with the assay were similar to those obtained in published studies using other standard assays. CyQUANT assay measurements of serum-stimulated cell proliferation correlated well with measurements made using [3H]-thymidine. Also, the assay was used to analyze cellular DNA or RNA content, with the addition of a nuclease digestion step to the protocol. The assay procedure is simple and convenient, with no wash steps, and is readily amenable to automation.

Mitogen-activated protein kinases are in vivo transducers of osmosensory signals in fish gill cells

The abundance and activity of three subgroups of mitogen-activated protein (MAP) kinases, the extracellular signal regulated kinase 1 (ERK1), stress-activated protein kinase 1/ Jun N-terminal kinase (SAPK1), and stress-activated protein kinase 2/ p38 (SAPK2), were measured in gill epithelium of the euryhaline teleost Fundulus heteroclitus exposed for 1 h to 4 weeks to hyper- and hyposmotic stress. The abundance of ERK1, SAPK1 and SAPK2 was analyzed by standard Western immunodetection. MAP kinase activity is a function of phosphorylation and was measured using phospho-specific and MAP kinase subgroup-specific antibodies. The abundance of the 63 kDa fish isoform of SAPK2 increases significantly during hyper- but not hyposmotic stress while ERK1 and SAPK1 protein levels remain unchanged during both types of osmotic stress. In contrast to this small effect of osmotic stress on MAP kinase abundance, the activity of all MAP kinases decreases significantly in response to hyperosmotic stress and increases significantly during hyposmotic stress. These results demonstrate for the first time that the activity of all major MAP kinases is osmoregulated in gill epithelium of euryhaline fish. Based on these results we conclude that MAP kinases are important components of salinity adaptation and participate in osmosensory signaling pathways in gill epithelium of euryhaline fishes.

Involvement of JNK-mediated pathway in EGF-mediated protection against paclitaxel-induced apoptosis in SiHa human cervical cancer cells

We investigated the signalling pathways by which epidermal growth factor (EGF) modulates paclitaxel-induced apoptosis in SiHa human cervical cancer cells. SiHa cells exposed to paclitaxel underwent apoptosis, which was strongly inhibited by EGF. This inhibition of apoptosis by EGF was not altered by pharmacological blockade of phosphatidylinositol 3'-OH kinase (PI-3K) with the PI-3K specific inhibitor LY294002 or blockade of the mitogen-activated protein kinase (MAPK) kinase (MEK) with the MEK specific inhibitor PD98059, or by transfection of the cells with PI-3K or MEK dominant-negative expression vectors. EGF did not stimulate PI-3K/Akt, MEK/MAPK, or p38 MAPK activity in SiHa cells but did transiently activate the c-Jun NH2-terminal kinase (JNK). Co-exposure of SiHa cells to SB202190 at concentrations that inhibit JNK abolished the protective effect of EGF on SiHa cells against paclitaxel-induced apoptosis. Our findings indicate that the JNK signaling pathway plays an important role in EGF-mediated protection from paclitaxel-induced apoptosis in SiHa cells.

Autoregulation of cell-specific MAP kinase control of the tryptophan hydroxylase promoter

The neurotransmitter serotonin controls a wide range of biological systems, including its own synthesis and release. As the rate-limiting enzyme in serotonin biosynthesis, tryptophan hydroxylase (TPH) is a potential target for this autoregulation. Using the serotonergic neuron-like CA77 cell line, we have demonstrated that treatment with a 5-hydroxytryptamine autoreceptor agonist, CGS 12066A, can lower TPH mRNA levels and promoter activity. We reasoned that this repression might involve inhibition of MAP kinases, since 5-HT1 receptors can increase mitogen-activated protein (MAP) kinase phosphatase levels. To test this hypothesis, we first showed that the TPH promoter can be activated 20-fold by mitogen-activated extracellular-signal regulated kinase kinase kinase (MEKK), an activator of MAP kinases. This activation was then blocked by CGS 12066A. The maximal MAP kinase and CGS repression regulatory region was mapped to between -149 and -45 base pairs upstream of the transcription start site. The activation by MEKK appears to be cell-specific, because MEKK did not activate the TPH promoter in nonneuronal cell lines. At least part, but not all, of the MAP kinase responsiveness was mapped to an inverted CCAAT box that binds the transcription factor NF-Y. These data suggest a model for the autoregulation of serotonin biosynthesis by repression of MAP kinase stimulation of the TPH promoter.

Tumor necrosis factor-alpha selectively induces MnSOD expression via mitochondria-to-nucleus signaling, whereas interleukin-1beta utilizes an alternative pathway

Mitochondrial levels of the anti-oxidant enzyme, manganese superoxide dismutase (MnSOD), are dramatically elevated in response to stimulation with tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and lipopolysaccharide (LPS). However, the precise intracellular signaling pathways responsible for this inducible expression are poorly understood. MnSOD expression in pulmonary epithelial and endothelial cells, treated with inflammatory mediators and various inhibitors, was studied by Northern analysis. The mitochondrial electron transport chain inhibitors, antimycin A and myxothiazol, selectively blocked TNF-alpha-inducible expression of MnSOD but not that of IL-1beta or LPS, indicating different signaling pathways. N-Acetylcysteine could reliably decrease inducible MnSOD expression by TNF-alpha, but not IL-1, linking reactive oxygen species (ROS) to the TNF-alpha signaling pathway. Elevated levels of arachidonic acid have been demonstrated previously to generate mitochondrial ROS. A specific cytoplasmic phospholipase A(2) inhibitor reduced stimulated MnSOD expression by TNF-alpha, but not by IL-1beta, further supporting the role of ROS. Other investigators have shown that MnSOD expression may be regulated by NF-kappaB. Our results with a specific inhibitory kappa-kinase inhibitor indicate that NF-kappaB modulates IL-1beta signaling but not the TNF-alpha pathway. Thus, we have demonstrated that although inducible MnSOD transcription may appear similar at the messenger RNA level, the intracellular signaling pathways are differentially regulated.

Differential cAMP gating of glutamatergic signaling regulates long-term state changes in the suprachiasmatic circadian clock

We investigated a role for cAMP/protein kinase A (PKA) in light/glutamate (GLU)-stimulated state changes of the mammalian circadian clock in the suprachiasmatic nucleus (SCN). Nocturnal GLU treatment elevated [cAMP]; however, agonists of cAMP/PKA did not mimic the effects of light/GLU. Coincident activation of cAMP/PKA enhanced GLU-stimulated state changes in early night but blocked light/GLU-induced state changes in the late night, whereas inhibition of cAMP/PKA reversed these effects. These responses are distinct from those mediated by mitogen-activated protein kinase (MAPK). MAPK inhibitors attenuated both GLU-induced state changes. Although GLU induced mPer1 mRNA in both early and late night, inhibition of PKA blocked this event only in early night, suggesting that cellular mechanisms regulating mPer1 are gated by the suprachiasmatic circadian clock. These data support a diametric gating role for cAMP/PKA in light/GLU-induced SCN state changes: cAMP/PKA promotes the effects of light/GLU in early night, but opposes them in late night.

Cytostatic p21 G protein-activated protein kinase gamma-PAK

The p21-activated protein kinase gamma-PAK, also known as PAK2, has very different properties from the other two highly conserved isoforms of the PAK family, alpha-PAK (PAK1) and beta-PAK (PAK3). gamma-PAK has cytostatic activity, as shown by inhibition of cleavage of early frog embryos following microinjection of gamma-PAK and by inhibition of growth when expressed in mammalian cells. gamma-PAK is activated in response to a variety of stresses including radiation- and chemically-induced DNA damage, hyperosmolarity, addition of sphingosine, serum starvation, and contact inhibition. Activation occurs through at least two signaling pathways, depending on the type of stress, one of which requires phosphoinositide 3-kinase and/or tyrosine kinase activity. During apoptosis gamma-PAK is cleaved by caspase 3 and activated and appears to have a role in the apoptotic response. gamma-PAK is present in the cytosol, associated with the membrane and in secretory granules. A wide variety of substrates have been identified for gamma-PAK. We propose gamma-PAK may be involved in coordinating the stress response, possibly in conjunction with other stress response proteins.

Urea inhibits hypertonicity-inducible TonEBP expression and action

Tonicity-responsive genes are regulated by the TonE enhancer element and the tonicity-responsive enhancer binding protein (TonEBP) transcription factor with which it interacts. Urea, a permeant solute coexistent with hypertonic NaCl in the mammalian renal medulla, activates a characteristic set of signaling events that may serve to counteract the effects of NaCl in some contexts. Urea inhibited the ability of hypertonic stressors to increase expression of TonEBP mRNA and also inhibited tonicity-inducible TonE-dependent reporter gene activity. The permeant solute glycerol failed to reproduce these effects, as did cell activators including peptide mitogens and phorbol ester. The inhibitory effect of urea was evident as late as 2 h after the application of hypertonicity. Pharmacological inhibitors of known urea-inducible signaling pathways failed to abolish the inhibitory effect of urea. TonEBP action is incompletely understood, but evidence supports a role for proteasome function and p38 action in regulation; urea failed to inhibit proteasome function or p38 signaling in response to hypertonicity. Consistent with its effect on TonEBP expression and action, urea pretreatment inhibited the effect of hypertonicity on expression of the physiological effector gene, aldose reductase. Taken together, these data 1) define a molecular mechanism of urea-mediated inhibition of tonicity-dependent signaling, and 2) underscore a role for TonEBP abundance in regulating TonE-mediated gene transcription.

Hyperosmolality in the form of elevated NaCl but not urea causes DNA damage in murine kidney cells

This study demonstrates, by using neutral comet assay and pulsed field gel electrophoresis, that hyperosmotic stress causes DNA damage in the form of double strand breaks (dsb). Different solutes increase the rate of DNA dsb to different degrees at identical strengths of hyperosmolality. Hyperosmolality in the form of elevated NaCl (HNa) is most potent in this regard, whereas hyperosmolality in the form of elevated urea (HU) does not cause DNA dsb. The amount of DNA dsb increases significantly as early as 15 min after the onset of HNa. By using neutral comet and DNA ladder assays, we show that this rapid induction of DNA damage is not attributable to apoptosis. We demonstrate that renal inner medullary cells are able to efficiently repair hyperosmotic DNA damage within 48 h after exposure to hyperosmolality. DNA repair correlates with cell survival and is repressed by 25 microM LY294002, an inhibitor of DNA-activated protein kinases. These results strongly suggest that the hyperosmotic stress resistance of renal inner medullary cells is based not only on adaptations that protect cellular proteins from osmotic damage but, in addition, on adaptations that compensate DNA damage and maintain genomic integrity.

Cell and molecular biology of organic osmolyte accumulation in hypertonic renal cells

When the renal medulla becomes hypertonic in association with the formation of concentrated urine, the cells of the medulla avoid the stress of high intracellular salts by accumulating small non-perturbing organic osmolytes. The response has been studied in most detail in cultured kidney-derived cells, and confirmed in studies of the intact kidney. The non-perturbing osmolytes, myo-inositol, betaine, and sorbitol, are accumulated because of stimulation of the transcription of the genes for the proteins that catalyze their accumulation by transport or synthesis. The genes involved have all been cloned and sequenced and contain tonicity responsive regulatory elements (TonEs) in their 5' region. During hypertonicity, the elements are occupied by TonE-binding protein, a transacting factor that has been cloned and characterized. Current efforts focus on identifying the mechanism by which cells sense hypertonicity and how that leads to activation of TonE-binding protein.

Peroxynitrite induces GADD34, 45, and 153 VIA p38 MAPK in human neuroblastoma SH-SY5Y cells

Peroxynitrite, one of the most reactive radicals, is produced from superoxide anion and nitric oxide. A peroxynitrite generator, 3-morpholinosydonimine (SIN-1), was found to induce the expression of three different growth arrest and DNA damage-inducible (GADD) mRNA, GADD34, GADD45, and GADD153, at the early phase during cell death in human neuroblastoma SH-SY5Y cells. In addition, peroxynitrite activated p38 MAPK just before induction of three GADD mRNA. A specific inhibitor of p38 MAPK, SB202190, markedly suppressed peroxynitrite-induced expression of three GADD mRNA in SH-SY5Y cells. The expression of three GADD genes and also p38 MAPK phosphorylation were suppressed by treatment with radical scavengers, superoxide dismutase plus catalase and glutathione. Glutathione depletion by L-buthionine-S, R-sulfoximine (BSO), increased the vulnerability of the cells to peroxynitrite. These findings indicate that peroxynitrite-mediated oxidative stress activated p38 MAPK to induce three GADD genes.

Cellular osmoregulation: beyond ion transport and cell volume

All cells are characterized by the expression of osmoregulatory mechanisms, although the degree of this expression is highly variable in different cell types even within a single organism. Cellular osmoregulatory mechanisms constitute a conserved set of adaptations that offset antagonistic effects of altered extracellular osmolality/environmental salinity on cell integrity and function. Cellular osmoregulation includes the regulation of cell volume and ion transport but it does not stop there. We know that organic osmolyte concentration, protein structure, cell turnover, and other cellular parameters are osmoregulated as well. In this brief review two important aspects of cellular osmoregulation are emphasized: 1) maintenance of genomic integrity, and 2) the central role of protein phosphorylation. Novel insight into these two aspects of cellular osmoregulation is illustrated based on two cell models, mammalian kidney inner medullary cells and teleost gill epithelial cells. Both cell types are highly hypertonicity stress-resistant and, therefore, well suited for the investigation of osmoregulatory mechanisms. Damage to the genome is discussed as a newly discovered aspect of hypertonic threat to cells and recent insights on how mammalian kidney cells deal with such threat are presented. Furthermore, the importance of protein phosphorylation as a core mechanism of osmosensory signal transduction is emphasized. In this regard, the potential roles of the 14-3-3 family of phospho-protein adaptor molecules for cellular osmoregulation are highlighted primarily based on work with fish gill epithelial cells. These examples were chosen for the reader to appreciate the numerous and highly specific interactions between stressor-specific and non-specific pathways that form an extensive cellular signaling network giving rise to adaptive compensation of hypertonicity. Furthermore, the example of 14-3-3 proteins illustrates that a single protein may participate in several pathways that are non-specific with regard to the type of stress and, at the same time, in stress-specific pathways to promote cell integrity and function during hypertonicity.

Eicosanoid activation of extracellular signal-regulated kinase1/2 in human epidermoid carcinoma cells

12(S)-Hydroxyeicosatetraenoic acid (12(S)-HETE), a 12-lipoxygenase metabolite of arachidonic acid, has multiple effects on tumor and endothelial cells, including stimulation of invasion and angiogenesis. However, the signaling mechanisms controlling these physiological processes are poorly understood. In a human epidermoid carcinoma cell line (i.e. A431), 12(S)-HETE activates extracellular signal-regulated kinases 1/2 (ERK1/2), which is mediated by upstream kinases MEK and Raf. 12(S)-HETE stimulates phosphorylation of phospholipase Cgamma1 and activity of protein kinase Calpha (PKCalpha). In addition, independent of PKC 12(S)-HETE increases tyrosine phosphorylation of Shc, and Grb2, stimulates association between Shc and Src, and increases the activity of Ras, via Src family kinases. Furthermore, at low (10-100 nm) concentrations 12(S)-HETE counteracts epidermal growth factor-stimulated activation of ERK1/2 via stimulating protein tyrosine phosphatases. We also present evidence that 12(S)-HETE stimulates ERK1/2 via G proteins and that A431 cells have multiple binding sites for 12(S)-HETE. Finally, inhibition of 12-lipoxygenase induced apoptosis of A431 cells, which was reversed by addition of exogenous 12(S)-HETE. Collectively we demonstrate that the activation of ERK1/2 by 12(S)-HETE may be regulated by multiple receptors triggering PKC-dependent and PKC-independent pathways in A431 cells.

p38 mitogen-activated protein kinase mediates tumor necrosis factor-alpha-induced apoptosis in rat fetal brown adipocytes

Tumor necrosis factor-alpha (TNFalpha) induces apoptosis and cell growth inhibition in primary rat fetal brown adipocytes. Here, we examine the role played by some members of the mitogen-activated protein kinase (MAPK) superfamily. TNFalpha activates extracellular regulated kinase-1/2 (ERK1/2) and p38MAPK. Inhibition of p38MAPK by either SB203580 or SB202190 highly reduces apoptosis induced by TNFalpha, whereas ERK inhibition potentiates it. Moreover, cotransfection of an active MKK3 mutant and p38MAPK induces apoptosis. p38MAPK inhibition also prevents TNFalpha-induced cell cycle arrest, whereas MEK1 inhibition enhances this effect, which correlates with changes in proliferating cell nuclear antigen expression, but not in cyclin D1. c-Jun and activating transcription factor-1 are potential downstream effectors of p38MAPK and ERKs upon TNFalpha treatment. Thus, TNFalpha-induced c-Jun messenger RNA expression requires ERKs activation, whereas p38MAPK inhibition enhances its expression. In addition, TNFalpha-induced activating transcription factor-1 phosphorylation is extensively decreased by SB203580. However, TNFalpha-induced NF-kappaB DNA-binding activity is independent of p38MAPK and ERK activation. On the other hand, C/EBP homology protein does not appear to mediate the actions of TNFalpha, because its expression is almost undetectable and even reduced by TNFalpha. Finally, although TNFalpha induces c-Jun N-terminal kinase (JNK) activation, transfection of a dominant negative of either JNK1 or JNK2 had no effect on TNFalpha-induced apoptosis. These results suggest that p38MAPK mediates TNFalpha-induced apoptosis and cell cycle arrest, whereas ERKs do the opposite, and JNKs play no role in this process of apoptosis.

Osmotic stress regulates the stability of cyclin D1 in a p38SAPK2-dependent manner

We report here that different cell stresses regulate the stability of cyclin D1 protein. Exposition of Granta 519 cells to osmotic shock, oxidative stress, and arsenite induced the post-transcriptional down-regulation of cyclin D1. In the case of osmotic shock, this effect was completely reversed by the addition of p38(SAPK2)-specific inhibitors (SB203580 or SB220025), indicating that this effect is dependent on p38(SAPK2) activity. Moreover, the use of proteasome inhibitors prevented this down-regulation. Thus, osmotic shock induces proteasomal degradation of cyclin D1 protein by a p38(SAPK2)-dependent pathway. The effect of p38(SAPK2) on cyclin D1 stability might be mediated by direct phosphorylation at specific sites. We found that p38(SAPK2) phosphorylates cyclin D1 in vitro at Thr(286) and that this phosphorylation triggers the ubiquitination of cyclin D1. These results link for the first time a stress-induced MAP kinase pathway to cyclin D1 protein stability, and they will help to understand the molecular mechanisms by which stress transduction pathways regulate the cell cycle machinery and take control over cell proliferation.

Hyposmolality stimulates Na(+)/H(+) exchange and HCO(3)(-) absorption in thick ascending limb via PI 3-kinase

The signal transduction mechanisms that mediate osmotic regulation of Na(+)/H(+) exchange are not understood. Recently we demonstrated that hyposmolality increases HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through stimulation of the apical membrane Na(+)/H(+) exchanger NHE3. To investigate the mechanism of this stimulation, MTALs from rats were isolated and perfused in vitro with 25 mM HCO(3)(-)-containing solutions. The phosphatidylinositol 3-kinase (PI 3-K) inhibitors wortmannin (100 nM) and LY-294002 (20 microM) blocked completely the stimulation of HCO(3)(-) absorption by hyposmolality. In tissue strips dissected from the inner stripe of the outer medulla, the region of the kidney highly enriched in MTALs, hyposmolality increased PI 3-K activity 2. 2-fold. Wortmannin blocked the hyposmolality-induced PI 3-K activation. Further studies examined the interaction between hyposmolality and vasopressin, which inhibits HCO(3)(-) absorption in the MTAL via cAMP and often is involved in the development of plasma hyposmolality in clinical disorders. Pretreatment with arginine vasopressin, forskolin, or 8-bromo-cAMP abolished hyposmotic stimulation of HCO(3)(-) absorption, due to an effect of cAMP to inhibit hyposmolality- induced activation of PI 3-K. In contrast to their effects to block stimulation by hyposmolality, PI 3-K inhibitors and vasopressin have no effect on inhibition of apical Na(+)/H(+) exchange (NHE3) and HCO(3)(-) absorption by hyperosmolality. These results indicate that hyposmolality increases NHE3 activity and HCO(3)(-) absorption in the MTAL through activation of a PI 3-K-dependent pathway that is inhibited by vasopressin and cAMP. Hyposmotic stimulation and hyperosmotic inhibition of NHE3 are mediated through different signal transduction mechanisms.

MAPK signaling and the kidney

Following an overview of the biochemistry of mitogen-activated protein kinase (MAPK) pathways, the relevance of these signaling events to specific models of renal cell function and pathophysiology, both in vitro and in vivo, will be emphasized. In in vitro model systems, events activating the principal MAPK families [extracellular signal-regulated and c-Jun NH(2)-terminal kinase and p38] have been best characterized in mesangial and tubular epithelial cell culture systems and include peptide mitogens, cytokines, lipid mediators, and physical stressors. Several in vivo models of proliferative or toxic renal injury are also associated with aberrant MAPK regulation. It is anticipated that elucidation of downstream effector signaling mechanisms and a clearer understanding of the immediate and remote upstream activating pathways, when applied to these highly clinically relevant model systems, will ultimately provide much greater insight into the basis for specificity now seemingly absent from these signaling events.

Stress renders T cell blasts sensitive to killing by activated syngeneic NK cells

Exposure of primary T cell blasts to stress in the forms of heat, hydrogen peroxide, or high-density growth conditions resulted in a state of enhanced susceptibility to killing by syngeneic IL-2-activated NK cells or lymphokine-activated killer cells, but not to killing by CTL. Cytotoxicity was perforin mediated and was not due to decreased target expression of total MHC class I. The levels of stress used had little effect on cell viability. For thermal stress, sensitization increased with temperature, required a minimum exposure time, and disappeared when cells were given a long enough recovery time. Our data support a model that predicts that activated NK cells play a role in the immunosurveillance of nontransformed stressed cells in normal animals.

Cloning and characterization of salmon hsp90 cDNA: upregulation by thermal and hyperosmotic stress

Accumulating evidence suggests that glucocorticoids are essential for development of hypoosmoregulatory capacity in salmon during adaptation to seawater. Heat shock protein (hsp)90 has been reported to function in signal transduction and the maturation and affinity of glucocorticoid receptors. We sought to determine whether this hsp might be upregulated by thermal and hyperosmotic stress in salmon, a species that migrates between the freshwater and marine environments. A 2625-bp cDNA cloned from a salmon cDNA library was found to code for a protein of 722 amino acids exhibiting a high degree of identity with zebra fish (92%) and human (89%) hsp90beta. Accumulation of hsp90 mRNA was observed in isolated branchial lamellae incubated under hyperosmotic conditions and in branchial lamellae of salmon exposed to hyperosmotic stress in vivo. In contrast, exposure of kidney to hyperosmotic stress in vitro and in vivo failed to elicit an increase in the quantity of hsp90 mRNA. By way of comparison, accumulation of hsp90 mRNA was observed in both branchial lamellae and kidney tissue subjected to thermal stress in vitro and in vivo. Western blot analyses of proteins isolated from tissues under identical conditions in vitro revealed that the pool of hsp90 increased with thermal stress but not with osmotic stress. The results suggest that accumulation of hsp90 mRNA in response to osmotic stress is unrelated to cellular protein denaturation and that synthesis of hsp90 may be regulated at both the level of transcription and translation.

Tissue engineering of heart valves: in vitro experiences

BACKGROUND

Tissue engineering is a new approach, whereby techniques are being developed to transplant autologous cells onto biodegradable scaffolds to ultimately form new functional tissue in vitro and in vivo. Our laboratory has focused on the tissue engineering of heart valves, and we have fabricated a trileaflet heart valve scaffold from a biodegradable polymer, a polyhydroxyalkanoate. In this experiment we evaluated the suitability of this scaffold material as well as in vitro conditioning to create viable tissue for tissue engineering of a trileaflet heart valve.

METHODS

We constructed a biodegradable and biocompatible trileaflet heart valve scaffold from a porous polyhydroxyalkanoate (Meatabolix Inc, Cambridge, MA). The scaffold consisted of a cylindrical stent (1 x 15 x 20 mm inner diameter) and leaflets (0.3 mm thick), which were attached to the stent by thermal processing techniques. The porous heart valve scaffold (pore size 100 to 240 microm) was seeded with vascular cells grown and expanded from an ovine carotid artery and placed into a pulsatile flow bioreactor for 1, 4, and 8 days. Analysis of the engineered tissue included biochemical examination, enviromental scanning electron microscopy, and histology.

RESULTS

It was possible to create a trileaflet heart valve scaffold from polyhydroxyalkanoate, which opened and closed synchronously in a pulsatile flow bioreactor. The cells grew into the pores and formed a confluent layer after incubation and pulsatile flow exposure. The cells were mostly viable and formed connective tissue between the inside and the outside of the porous heart valve scaffold. Additionally, we demonstrated cell proliferation (DNA assay) and the capacity to generate collagen as measured by hydroxyproline assay and movat-stained glycosaminoglycans under in vitro pulsatile flow conditions.

CONCLUSIONS

Polyhydroxyalkanoates can be used to fabricate a porous, biodegradable heart valve scaffold. The cells appear to be viable and extracellular matrix formation was induced after pulsatile flow exposure.

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

Acute hypertonicity causes cell cycle delay and apoptosis in mouse renal inner medullary collecting duct cells (mIMCD3) and increases GADD45 expression. Because the tumor suppressor protein p53 may be involved in these effects, we have investigated the role of p53 in mIMCD3 response to hyperosmotic stress. Acute elevation of osmolality with NaCl addition from the control level of 320 mosmol/kg to 500-600 mosmol/kg greatly increased the levels of total and Ser(15)-phosphorylated p53 within 15 min. However, similar elevation of osmolality with urea did not increase p53 levels. Our studies indicate that induced p53 is transcriptionally active because NaCl addition to 500-600 mosmol/kg stimulated transcription of a luciferase reporter containing a p53 consensus element and appropriately altered mRNA levels of known transcriptional targets of p53, i.e. increased MDM-2 and decreased BCL-2 levels. Elevating NaCl further to 700-800 mosmol/kg rapidly killed most of the cells by apoptosis. At these higher NaCl concentrations, p53 levels were further increased although Ser(15) phosphorylation and transcriptional activity were significantly lower than levels at 500-600 mosmol/kg. At NaCl-induced 500 mosmol/kg, apoptosis was rare in the presence of control, nonspecific oligonucleotide but highly prevalent upon addition of p53 antisense oligonucleotide that substantially reduced p53 levels. We conclude that induction of active p53 in mIMCD3 cells by hypertonic stress contributes to cell survival.

p21-activated protein kinase gamma-PAK is translocated and activated in response to hyperosmolarity. Implication of Cdc42 and phosphoinositide 3-kinase in a two-step mechanism for gamma-PAK activation

A member of the family of p21-activated protein kinases, gamma-PAK, has cytostatic properties and is activated during apoptosis and in response to DNA damage. To determine whether gamma-PAK is activated by other types of cell stress and to assess its mechanism of activation, the response of gamma-PAK to hyperosmotic stress was examined. In 3T3-L1 mouse fibroblasts, there are two pools of gamma-PAK: the majority of the protein kinase is soluble and has low specific activity, whereas gamma-PAK associated with the particulate fraction has significantly higher specific activity. Hyperosmolarity promotes translocation of gamma-PAK from the soluble to the particulate fraction; this parallels activation of the protein kinase. Activation but not translocation of gamma-PAK is wortmannin-sensitive, suggesting the involvement of a phosphoinositide 3-kinase-related activity. gamma-PAK translocation in response to hyperosmolarity parallels Cdc42 translocation to the particulate fraction in vivo and can be induced in vitro by guanosine 5'-3-O-(thio)triphosphate. Cotransfection of gamma-PAK with constitutively active Cdc42 induces gamma-PAK activation and translocation, whereas inactive Cdc42 inhibits both processes in response to hyperosmotic stress, suggesting that Cdc42 has a role in the translocation and activation of gamma-PAK. alpha-PAK is not activated in response to hyperosmolarity in 3T3-L1 cells. A two-step model of gamma-PAK activation is presented.

Extension of osmolality-induced podia is observed from fluorescently labeled hematopoietic cell lines in hyperosmotic medium

BACKGROUND

Since the description of long podia extended by hematopoietic cells and cell lines, the reliable elicitation of podia extensions is needed to study these podia systemically. In this study, hyperosmotic stress was considered as an elicitor.

METHODS

Using two fluorescent membrane dyes PKH2 and PKH26, and an automated fluorescence microscopy system, morphological changes of seven human cell lines (six hematopoietic, one fibrosarcoma) at different osmolalities were monitored. Presence of surface molecules on the hyperosmolality-induced podia (osmopodia) was examined.

RESULTS

In hyperosmotic medium, cells shrank rapidly, followed by osmopodia extension. Cells exhibited variable number (up to five) and length (up to longer than 100 microm) of osmopodia in about 1 h. Dead cells did not extend podia. Frequency, length, and number of podia were variable among cell lines studied. CD44 and CD45 were not present on the osmopodia, although they were present on the cell surface, showing that osmopodia characteristics differ from the podia observed previously in isotonic media. The osmopodia extension process was shown to be reversible upon repeated osmolality changes.

CONCLUSIONS

Osmopodia extended by human hematopoietic cell lines display a newly observed cellular morphology and provide a tool for investigation of dynamic cellular response to environmental changes.

Inhibition of p38 MAP kinase as a therapeutic strategy

Since the discovery of p38 MAP kinase in 1994, our understanding of its biology has progressed dramatically. The key advances include (1) identification of p38 MAP kinase homologs and protein kinases that act upstream and downstream from p38 MAP kinase, (2) identification of interesting and potentially important substrates, (3) elucidation of the role of p38 MAP kinase in cellular processes and (4) the establishment of the mechanism by which the pyridinylimidazole p38 MAP kinase inhibitors inhibit enzyme activity. It is now known that there are four members of the p38 MAP kinase family. They differ in their tissue distribution, regulation of kinase activation and subsequent phosphorylation of downstream substrates. They also differ in terms of their sensitivities toward the p38 MAP kinase inhibitors. The best-studied isoform is p38 alpha, whose activation has been observed in many hematopoietic and non-hematopoietic cell types upon treatment with appropriate stimuli. The pyridinylimidazole compounds, exemplified by SB 203580, were originally prepared as inflammatory cytokine synthesis inhibitors that subsequently were found to be selective inhibitors of p38 MAP kinase. SB 203580 inhibits the catalytic activity of p38 MAP kinase by competitive binding in the ATP pocket. X-ray crystallographic studies of the target enzyme complexed with inhibitor reinforce the observations made from site-directed mutagenesis studies, thereby providing a molecular basis for understanding the kinase selectivity of these inhibitors. The p38 MAP kinase inhibitors are efficacious in several disease models, including inflammation, arthritis and other joint diseases, septic shock, and myocardial injury. In all cases, p38 activation in key cell types correlated with disease initiation and progression. Treatment with p38 MAP kinase inhibitors attenuated both p38 activation and disease severity. Structurally diverse p38 MAP kinase inhibitors have been tested extensively in preclinical studies.

Hypoosmotic stress activates p38, ERK 1 and 2, and SAPK/JNK in rat hepatocytes

BACKGROUND

Following hepatocyte injury, changes in the perihepatocyte milieu modulate cell volume and influence growth. Hypoosmotic stress activates nuclear factor-kappa B (NF-kappaB), a transcription factor believed to prime cell cycle progression in hepatocytes. In this study, we investigate the role of mitogen-activated protein kinases (MAPKs) in the activation of NF-kappaB.

MATERIALS AND METHODS

Quiescent primary hepatocytes were exposed to hypoosmotic serum-free William's E (WE) medium (200 mOsm/liter), with or without a 1-h pretreatment with either PD 98059 (15 microM) or SB 202190 (3 microM). Parallel experiments were conducted using hepatocyte growth factor (HGF) at 0.1 mg/ml and normoosmotic WE medium as positive and negative controls, respectively (n = 3). Relative densitometries of Western blots measured phosphorylated cytoplasmic p38, ERK 1 and 2, and SAPK/JNK. Electromobility shift assays examined nuclear NF-kappaB activation.

RESULTS

(i) Hypoosmolar WE medium phosphorylated p38, ERK 1 and 2, and SAPK/JNK by 5 min. (ii) Hypoosmolar WE medium activated NF-kappaB at 60 min. (iii) HGF phosphorylated all three MAPKs and activated NF-kappaB with profiles similar to those of hypoosmotic stress. (iv) Both PD 98059 and SB 202190 abrogated the activation of NF-kappaB in HGF-stimulated cells but not in hypoosmotically stressed cells.

CONCLUSION

(i) Both hypoosmotic cell swelling and HGF phosphorylate p38, ERK 1 and 2, and SAPK/JNK, and (ii) HGF, but not hypoosmotic stress, activates NF-kappaB via p38 and ERK 1 and 2 phosphorylation. These data suggest that cell swelling activates NF-kappaB through a pathway separate from that of growth factors.