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

p38/SAPK2-dependent gene expression in Jurkat T cells

The stress-activated protein kinase p38/SAPK2 is known to regulate the activity of transcription factors and to control expression of several genes at the transcriptional or post-transcriptional level. In order to identify genes whose expression is under the control of p38/SAPK2 activity, we have compared the mRNA levels of a pattern of 588 genes between human Jurkat T cells with anisomycin-activated p38/SAPK2 and cells in which p38/SAPK2 was inhibited by the compound SB203580. Genes strongly expressed at the transcript level as a result of p38/SAPK2 activation are the transcription factors c-jun, fos-related antigen 1 (fra-1), the growth-arrest and DNA-damage gene gadd153 and early-growth-related gene 1 (egr-1) as well as the c-srk kinase csk and the nucleotide exchange factor ras-GRF. mRNAs significantly down-regulated include the insulin receptor IR, the adapter grb2, the transcription factor c-myc and the defender against apoptotic death, dad-1. For six selected genes, p38/SAPK2-regulated expression was confirmed and further analysed by Northern blot experiments, demonstrating a complex regulation of these genes under stress conditions.

p53-independent induction of GADD45 and GADD153 in mouse lungs exposed to hyperoxia

Previous studies have shown that lungs of adult mice exposed to >95% oxygen have increased terminal deoxyribonucleotidyltransferase dUTP nick end-label staining and accumulate p53, the expression of which increases in cells exposed to DNA-damaging agents. The present study was designed to determine whether hyperoxia also increased expression of the growth arrest and DNA damage (GADD) gene 45 and GADD153, which are induced by genotoxic stress through p53-dependent and -independent pathways. GADD proteins have been shown to inhibit proliferation and stimulate DNA repair and/or apoptosis. GADD45 and GADD153 mRNAs were not detected in lungs exposed to room air but were detected after 48 and 72 h of exposure to hyperoxia. In situ hybridization and immunohistochemistry revealed that hyperoxia increased GADD45 and GADD153 expression in the bronchiolar epithelium and GADD45 expression predominantly in alveolar cells that were morphologically consistent with type II cells. Hyperoxia also increased GADD expression in p53-deficient mice. Terminal deoxyribonucleotidyltransferase dUTP nick end-label staining of lung cells from p53 wild-type and p53-null mice exposed to hyperoxia for 48 h revealed that hyperoxia-induced DNA fragmentation was not modified by p53 deficiency. These studies are consistent with the hypothesis that hyperoxia-induced DNA fragmentation is associated with the expression of GADD genes that may participate in DNA repair and/or apoptosis.

Ras signaling in the inner medullary cell response to urea and NaCl

The small guanine nucleotide-binding protein Ras, activated by peptide mitogens and other stimuli, regulates downstream signaling events to influence transcription. The role of Ras in solute signaling to gene regulation was investigated in the murine inner medullary collecting duct (mIMCD3) cell line. Urea treatment (100-200 mM), but not sham treatment, increased Ras activation 124% at 2 min; the effect of NaCl did not achieve statistical significance. To determine the contribution of Ras activation to urea-inducible signal transduction, mIMCD3 cells were stably transfected with an expression plasmid encoding a dominant negative-acting N17Ras mutant driven by a dexamethasone-inducible (murine mammary tumor virus) promoter. After 24 h of induction, selected cell lines exhibited sufficient N17Ras overexpression to abolish epidermal growth factor- and hypotonicity-mediated signaling to extracellular signal-regulated kinase (ERK) phosphorylation, as determined by immunoblotting. Conditional N17Ras overexpression inhibited urea- and NaCl-inducible ERK phosphorylation by 40-50%, but only at 15 min, and not 5 min, of treatment. N17Ras induction, however, almost completely inhibited urea-inducible Egr-1 transcription, as quantitated by luciferase reporter gene assay, but failed to influence tonicity-inducible (TonE-mediated) transcription. N17Ras overexpression also blocked urea-inducible expression of the transcription factor Gadd153 but did not influence osmotic or urea-inducible apoptosis. In addition, urea treatment induced recruitment of the Ras activator Sos to the plasma membrane. Taken together, these observations suggest a role for Ras signaling in the IMCD cell response to urea stress.

Cell cycle delay and apoptosis are induced by high salt and urea in renal medullary cells

We investigated the effects of hyperosmolality on survival and proliferation of subconfluent cultures of mIMCD3 mouse renal collecting duct cells. High NaCl and/or urea (but not glycerol) reduces the number of viable cells, as measured with 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT). Raising osmolality from a normal level (300 mosmol/kg) to 550-1,000 mosmol/kg by adding NaCl and/or urea greatly increases the proportion of cells in the G(2)M phase of the cell cycle within 8 h, as measured by flow cytometry. Up to 600 mosmol/kg the effect is only transient, and by 12 h at 550 mosmol/kg the effect reverses and most cells are in G(1). Flow cytometry with 5-bromodeoxyuridine (BrdU) pulse-chase demonstrates that movement through the S phase of the cell cycle slows, depending on the concentrations of NaCl and/or urea, and that the duration of G(2)M increases greatly (from 2.5 h at 300 mosmol/kg to more than 16 h at the higher osmolalities). Addition of NaCl and/or urea to total osmolality of 550 mosmol/kg or more also induces apoptosis, as demonstrated by characteristic electron microscopic morphological changes, appearance of a subdiploid peak in flow cytometry, and caspase-3 activation. The number of cells with subdiploid DNA and activated caspase-3 peaks at 8-12 h. Caspase-3 activation occurs in all phases of the cell cycle, but to a disproportionate degree in G(0)/G(1) and S phases. We conclude that elevated NaCl and/or urea reduces the number of proliferating mIMCD3 cells by slowing the transit through the S phase, by cell cycle delay in the G(2)M and G(1), and by inducing apoptotic cell death.

Domain 5 of high molecular weight kininogen (kininostatin) down-regulates endothelial cell proliferation and migration and inhibits angiogenesis

We have demonstrated that high molecular weight kininogen (HK) binds specifically on endothelial cells to domain 2/3 of the urokinase receptor (uPAR). Inhibition by vitronectin suggests that kallikrein-cleaved HK (HKa) is antiadhesive. Plasma kallikrein bound to HK cleaves prourokinase to urokinase, initiating cell-associated fibrinolysis. We postulated that HK cell binding domains would inhibit angiogenesis. We found that recombinant domain 5 (D5) inhibited endothelial cell migration toward vitronectin 85% at 0.27 μM with an IC50 (concentration to yield 50% inhibition) = 0.12 μM. A D5 peptide, G486-K502, showed an IC50 = 0.2 μM, but a 25-mer peptide from a D3 cell binding domain only inhibited migration 10% at 139 μM (IC50 > 50 μM). D6 exhibited weaker inhibitory activity (IC50 = 0.50 μM). D5 also potently inhibited endothelial cell proliferation with an IC50 = 30 nM, while D3 and D6 were inactive. Using deletion mutants of D5, we localized the smallest region for full activity to H441-D474. To further map the active region, we created a molecular homology model of D5 and designed a series of peptides displaying surface loops. Peptide 440-455 was the most potent (IC50 = 100 nM) in inhibiting proliferation but did not inhibit migration. D5 inhibited angiogenesis stimulated by fibroblast growth factor FGF2 (97%) in a chicken chorioallantoic membrane assay at 270 nM, and peptide 400-455 was also inhibitory (79%). HK D5 (for which we suggest the designation, “kininostatin”) is a potent inhibitor of endothelial cell migration and proliferation in vitro and of angiogenesis in vivo.

Cell shrinkage triggers the activation of mitogen-activated protein kinases by hypertonicity in the rat kidney medullary thick ascending limb of the Henle's loop. Requirement of p38 kinase for the regulatory volume increase response

The kidney medulla is exposed to very high interstitial osmolarity leading to the activation of mitogen-activated protein kinases (MAPK). However, the respective roles of increased intracellular osmolality and of cell shrinkage in MAPK activation are not known. Similarly, the participation of MAPK in the regulatory volume increase (RVI) following cell shrinkage remains to be investigated. In the rat medullary thick ascending limb of Henle (MTAL), extracellular hypertonicity produced by addition of NaCl or sucrose increased the phosphorylation level of extracellular signal-regulated kinase (ERK) and p38 kinase and to a lesser extent c-Jun NH(2)-terminal kinase with sucrose only. Both hypertonic solutions decreased the MTAL cellular volume in a dose- and time-dependent manner. In contrast, hypertonic urea had no effect. The extent of MAPK activation was correlated with the extent of MTAL cellular volume decrease. Increasing intracellular osmolality without modifying cellular volume did not activate MAPK, whereas cell shrinkage without variation in osmolality activated both ERK and p38. In the presence of 600 mosmol/liter NaCl, the maximal cell shrinkage was observed after 10 min at 37 degrees C and the MTAL cellular volume was reduced to 70% of its initial value. Then, RVI occurred and the cellular volume progressively recovered to reach about 90% of its initial value after 30 min. SB203580, a specific inhibitor of p38, almost completely inhibited the cellular volume recovery, whereas inhibition of ERK did not alter RVI. In conclusion, in rat MTAL: 1) cell shrinkage, but not intracellular hyperosmolality, triggers the activation of both ERK and p38 kinase in response to extracellular hypertonicity; and 2) RVI is dependent on p38 kinase activation.

Stress-induced JNK activation is independent of Gadd45 induction

DNA damage and environmental stress activate signaling and induce genes involved in cell cycle and cell death. Expression of the Gadd45 protein is induced following DNA damage and other stress. Gadd45 is believed to play a role in growth arrest and possibly in cell death. The JNK signaling pathway is also activated by some DNA-damaging agents. This activation leads to phosphorylation and activation of transcription factors, such as c-Jun/AP-1 and ATF2, which mediate immediate early gene induction. Recently Gadd45 was suggested to be involved in JNK activation. However, as this suggestion relied on in vitro experiments and ectopic overexpression of Gadd45 protein, we examined whether physiological levels of Gadd45 that are induced following exposure to DNA damaging agents and stress can lead to JNK induction. We found that JNK activation by UV irradiation and anisomycin treatment precedes the induction of gadd45 mRNA by these agents. Gadd45 protein induction by methyl methanesulfonate also lagged behind JNK activation. The use of protein synthesis inhibitors suggested that newly synthesized proteins, including the stress-induced Gadd45, make only a marginal contribution to JNK activation. We also found that stresses such as gamma irradiation induce Gadd45 and do not activate JNK in mouse fibroblasts. Therefore, stress-induced JNK does not depend on Gadd45 induction.

Urea-associated oxidative stress and Gadd153/CHOP induction

Urea treatment (100-300 mM) increased expression of the oxidative stress-responsive transcription factor, Gadd153/CHOP, at the mRNA and protein levels (at >/=4 h) in renal medullary mIMCD3 cells in culture, whereas other solutes did not. Expression of the related protein, CCAAT/enhancer-binding protein (C/EBP-beta), was not affected, nor was expression of the sensor of endoplasmic reticulum stress, grp78. Urea modestly increased Gadd153 transcription by reporter gene analysis but failed to influence Gadd153 mRNA stability. Importantly, upregulation of Gadd153 mRNA and protein expression by urea was antioxidant sensitive. Accordingly, urea treatment was associated with oxidative stress, as quantitated by intracellular reduced glutathione content in mIMCD3 cells. In addition, antioxidant treatment partially inhibited the ability of urea to activate transcription of an Egr-1 luciferase reporter gene. Therefore oxidative stress represents a novel solute-signaling pathway in the kidney medulla and, potentially, in other tissues.