Signalling and gene regulation by urea and NaCl in the renal medulla

Phosphoproteomic identification of vasopressin-regulated protein kinases in collecting duct cells

BACKGROUND AND PURPOSE

The peptide hormone vasopressin regulates water transport in the renal collecting duct largely via the V2 receptor, which triggers a cAMP-mediated activation of a PKA-dependent signalling network. The protein kinases downstream from PKA have not been fully identified or mapped to regulated phosphoproteins.

EXPERIMENTAL APPROACH

We carried out systems-level analysis of large-scale phosphoproteomic data quantifying vasopressin-induced changes in phosphorylation in aquaporin-2-expressing cultured collecting duct (mpkCCD) cells. Quantification was done using stable isotope labelling (SILAC method).

KEY RESULTS

Six hundred forty phosphopeptides were quantified. Stringent statistical analysis identified significant changes in response to vasopressin in 429 of these phosphopeptides. The corresponding phosphoproteins were mapped to known vasopressin-regulated cellular processes. The vasopressin-regulated sites were classified according to the sequences surrounding the phosphorylated amino acids giving 11 groups. Among the vasopressin-regulated phosphoproteins were 25 distinct protein kinases. Among these, six plus PKA appeared to account for phosphorylation of about 81% of the 313 vasopressin-regulated phosphorylation sites. The six downstream kinases were salt-inducible kinase 2 (Sik2), cyclin-dependent kinase 18 (Cdk18), calmodulin-dependent kinase kinase 2 (Camkk2), protein kinase D2 (Prkd2), mitogen-activated kinase 3 (Mapk3) and myosin light chain kinase (Mylk).

CONCLUSION AND IMPLICATIONS

In V2 receptor-mediated signalling, PKA is at the head of a complex network that includes at least six downstream vasopressin-regulated protein kinases that are prime targets for future study. The extensive phosphoproteomic data reported in this study are provided as a web-based data resource for future studies of GPCRs.

RNA-Seq and protein mass spectrometry in microdissected kidney tubules reveal signaling processes initiating lithium-induced nephrogenic diabetes insipidus

Lithium salts, used for treating bipolar disorder, frequently induce nephrogenic diabetes insipidus (NDI) thereby limiting therapeutic success. NDI is associated with loss of expression of the gene coding for the molecular water channel, aquaporin-2, in the renal collecting duct (CD). Here, we use systems biology methods in a well-established rat model of lithium-induced NDI to identify signaling pathways activated at the onset of polyuria. Using single-tubule RNA-Seq, full transcriptomes were determined in microdissected cortical collecting ducts (CCDs) of rats after 72 hours without or with initiation of lithium chloride administration. Transcriptome-wide changes in mRNA abundances were mapped to gene sets associated with curated canonical signaling pathways, showing evidence for activation of NF-κB signaling with induction of genes coding for multiple chemokines and most components of the Major Histocompatibility Complex Class I antigen-presenting complex. Administration of anti-inflammatory doses of dexamethasone to lithium chloride-treated rats countered the loss of aquaporin-2. RNA-Seq also confirmed prior evidence of a shift from quiescence into the cell cycle with arrest. Time course studies demonstrated an early (12 hour) increase in multiple immediate early response genes including several transcription factors. Protein mass spectrometry in microdissected CCDs provided corroborative evidence and identified decreased abundance of several anti-oxidant proteins. Thus, in the context of prior observations, our study can be best explained by a model in which lithium increases ERK activation leading to induction of NF-κB signaling and an inflammatory-like response that represses Aqp2 transcription.

Regional heterogeneity of expression of renal NPRs, TonEBP, and AQP-2 mRNAs in rats with acute kidney injury

To understand the pathophysiology of ischemia/reperfusion (I/R) - induced acute kidney injury (AKI), the present study defined changes in renal function, plasma renotropic hormones and its receptors in the kidney 2, 5, or 7 days after 45 min-renal ischemia in rats. Blood urea nitrogen, plasma creatinine, and osmolarity increased 2 days after I/R injury and tended to return to control level 7 days after I/R injury. Decreased renal function tended to return to control level 5 days after I/R injury. However, plasma concentrations of atrial natriuretic peptide and renin did not change. In control kidney, natriuretic peptide receptor (NPR)-A, -B and -C mRNAs were highly expressed in medulla (ME), inner cortex (IC), and outer cortex (OC), respectively, and tonicity-responsive enhancer binding protein (TonEBP), auqaporin-2 (AQP-2) and eNOS mRNAs were highly expressed in ME. NPR-A and -B mRNA expressions were markedly decreased 2 days after I/R injury. On 5 days after I/R injury, NPR-A mRNA expression increased in OC and recovered to control level in IC but not in ME. NPR-B mRNA expression was increased in OC, and recovered to control level in IC and ME. NPR-C mRNA expression was markedly decreased in OC 2 and 5 days after I/R injury. TonEBP, APQ-2 and eNOS mRNA expressions were markedly decreased 2 days after I/R injury and did not recover in ME 7 days after I/R injury. Therefore, we suggest that there is a regional heterogeneity of regulation of renal NPRs, TonEBP, and APQ-2 mRNA in AKI.

Upregulation of ANP and NPR-C mRNA in the kidney and heart of eNOS knockout mice

OBJECTIVES

The aim of the present studywas to examine the question of whether the atrial natriuretic peptide (ANP) system is altered by endothelial nitric-oxide synthase (eNOS).

METHODS

Male eNOS-deficient mice (eNOS-/-) and wild type control mice (eNOS+/+, C57B1/6J) were used. Blood pressure was measured in anesthetized mice by tail cuff plethysmography and renal function was measured. Expression of ANP, natriuretic peptide receptor (NPR)-A, NPR-C, and tonicity-responsive enhancer binding protein (TonEBP) mRNA was determined by real-time PCR. Localization of (125)I-ANP binding sites was measured using in vitro autoradiography.

RESULTS

In eNOS-/- mice, systolic blood pressure increased and left ventricular hypertrophy was observed. Urine volume and osmolarity did not change. Expression of ANP markedly increased in the heart and kidney of eNOS-/- mice. Expression of NPR-A and NPR-C increased in the heart and tended to increase in the kidney of eNOS-/- mice. In the renal medulla in particular, increased expression of NPR-C was more prominent. Expression of TonEBP mRNA was markedly decreased in the renal medulla, but not in the renal cortex. Maximum binding capacity (B(max)) of ANP and C-ANP increased in the renal medulla in eNOS-/- mice.

CONCLUSION

These results suggest that the eNOS-NO system may be partly involved in regulation of ANP, NPR-A, -C, and TonEBP mRNA expression in the kidney.

A multi-layer microfluidic device for efficient culture and analysis of renal tubular cells

We have developed a simple multi-layer microfluidic device by integrating a polydimethyl siloxane (PDMS) microfluidic channel and a porous membrane substrate to culture and analyze the renal tubular cells. As a model cell type, primary rat inner medullary collecting duct (IMCD) cells were cultured inside the channel. To generate in vivo-like tubular environments for the cells, a fluidic shear stress of 1 dyn/cm(2) was applied for 5 hours, allowing for optimal fluidic conditions for the cultured cells, as verified by enhanced cell polarization, cytoskeletal reorganization, and molecular transport by hormonal stimulations. These results suggest that the microfluidic device presented here is useful for resembling an in vivo renal tubule system and has potential applications in drug screening and advanced tissue engineering.

Osmoregulation of natriuretic peptide receptors in bromoethylamine-treated rat kidney

Extracellular osmolarity is known as an important factor for the regulation of natriuretic peptide receptors (NPRs). We investigated the intra-renal osmoregulation of NPRs using renal medullectomized rats with bromoethylamine hydrobromide (BEA, 200mg/kg). The administration of BEA caused the decreased food intake and body weight. Water intake was decreased on the first day and then increased from the second day. Urine volume was persistently increased from the first day and free water clearance was also increased from the second day. Urinary excretions of sodium and potassium were decreased on the second day and then recovered to control level. Plasma levels of atrial natriuretic peptide (ANP) and Dendroaspis natriuretic peptide (DNP) in BEA-treated rats were not different from control rats. The inactive renin was increased. The maximum binding capacities of (125)I-ANP as well as (125)I-DNP decreased in glomeruli and medulla of BEA-treated rat kidneys but the binding affinity was not changed. In renal cortex, the gene expressions of ANP, NPR-A, and NPR-B were not changed but that of NPR-C decreased. In renal medulla, the gene expressions of NPR-A, -B, and -C decreased without change in ANP mRNA. Both renal medullary osmolarity and sodium concentration by BEA treatment were lower than those in control kidney. The cGMP concentrations in renal medulla and urine in BEA-treated rats were higher than those in control rats. These results suggest that the increased cGMP production may be partly involved in the decrease in NPRs mRNA expression and their binding capacities by BEA-induced medullectomy.

Effects of the renal medullary pH and ionic environment on vasopressin binding and signaling

The kidney has a cortico-medullary interstitial gradient of decreasing pH and increasing concentrations of sodium chloride and urea, but the influence of these gradients on receptor signaling is largely unknown. Here, we measured G-protein coupled receptor function in LLC-PK1 cells acutely exposed to conditions mimicking different kidney regions. Signaling through the parathyroid hormone receptor, normally expressed in the cortex, was greatly reduced at an acidic pH similar to that of the inner medulla. Parathyroid hormone receptor, tagged with green fluorescent protein, showed no ligand-induced internalization. In contrast, under both acidic and hyperosmotic conditions, vasopressin increased intracellular cAMP, and upon binding to its type 2 receptor (V2R) was internalized and degraded. Dose-displacement binding assays with selective vasopressin/oxytocin receptor ligands under inner medullary conditions indicated a shift in the V2R pharmacological profile. Oxytocin did not bind to the V2R, as it does under normal conditions and the vasopressin type 1 receptor (V1R) had reduced affinity for vasopressin compared to the V2R in low pH and high osmolality. We suggest that the cortico-medullary gradient causes a receptor-specific selectivity in ligand binding that is of functional significance to the kidney. While the gradient is important for urinary concentration, it may also play a substantial role in fine-tuning of the vasopressin response through the V2R.

Regulatory volume decrease after swelling induced by urea in fibroblasts: prominent role of organic osmolytes

Cell swelling, regulatory volume decrease (RVD), volume-sensitive Cl(-) (Cl(-) (swell)) current and taurine efflux after exposure to high concentrations of urea were characterized in fibroblasts Swiss 3T3, and results compared to those elicited by hyposmotic (30%) swelling. Urea 70, 100, and 150 mM linearly increased cell volume (8.25%, 10.6%, and 15.7%), by a phloretin-inhibitable process. This was followed by RVD by which cells exposed to 70, 100, or 150 mM urea recovered 27.6%, 38.95, and 74.1% of their original volume, respectively. Hyposmolarity (30%) led to a volume increase of 25.9% and recovered volume in 32.5%. (3)H-taurine efflux was increased by urea with a sigmoid pattern, as 9.5%, 18.9%, 71.5%, and 89% of the labeled taurine pool was released by 70, 100, 150, or 200 mM urea, respectively. Only about 11% of taurine was released by 30% hyposmolarity reduction in spite of the high increase in cell volume. Urea-induced taurine efflux was suppressed by NPPB (100 microM) and markedly reduced by the tyrosine kinase-general blocker AG18. The Cl(-) (swell) current was more rapidly activated and higher in amplitude in the hyposmotic than in the isosmotic/urea condition (urea 150 mM), but this was not sufficient to accomplish an efficient RVD. These results showed that at similar volume increase, cells swollen by urea showed higher taurine efflux, lower Cl(-) (swell) current and more efficient RVD, than in those swollen by hyposmolarity. The correlation found between RVD efficiency and taurine efflux suggest a prominent role for organic over ionic osmolytes for RVD evoked by urea in isosmotic conditions.

Survival in Hostile Environments: Strategies of Renal Medullary Cells

Cells in the renal medulla exist in a hostile milieu characterized by wide variations in extracellular solute concentrations, low oxygen tensions, and abundant reactive oxygen species. This article reviews the strategies adopted by these cells to allow them to survive and fulfill their functions under these extreme conditions.

A high-salt diet stimulates thick ascending limb eNOS expression by raising medullary osmolality and increasing release of endothelin-1

A high-salt diet increases renal endothelin (ET) production and thick ascending limb (THAL) endothelial nitric oxide synthase (eNOS) expression. ET stimulates THAL eNOS expression via ET(B) receptors. The tonicity of the renal medulla is highly variable, and hyperosmolality stimulates ET-1 synthesis by endothelial cells. We hypothesized that a high-salt diet raises medullary osmolality, increases ET release by the THAL, and thus enhances eNOS expression. Seven days of high salt (1% NaCl in drinking water) increased eNOS expression in THALs by 125 +/- 31%. High salt increased outer medullary osmolality from 362 +/- 13 to 423 +/- 6 mosmol/kg H(2)O (P < 0.05). Bosentan, a dual-ET receptor antagonist, blocked the increase in THAL eNOS expression caused by high salt (2.66 +/- 0.44 absorbance units with bosentan vs. 5.15 +/- 0.67 for vehicle; P < 0.05). Conscious systolic blood pressure did not differ between the two groups. In primary cultures of medullary THALs, raising osmolality from 300 to 350 and 400 mosmol/kg H(2)O using NaCl increased eNOS expression by 39 +/- 11% (P < 0.05) and 71 +/- 16%, respectively (P < 0.05). In primary cultures of THALs, raising osmolality from 300 to 400 mosmol/kg H(2)O for 1 h increased ET-1 release from 62 +/- 7 to 113 +/- 2 pg/mg protein (P < 0.05). BQ-788, an ET(B) receptor antagonist (1 muM), blocked the stimulatory effect of 400 mosmol/kg H(2)O on eNOS expression (70 +/- 13% vs. -5 +/- 10%; paired difference, 74 +/- 15%; P < 0.05). BQ-788 alone had no significant effect. We concluded that high salt stimulates THAL eNOS expression by increasing outer medullary osmolality, ET-1 release by the THAL and ET(B) receptor activation. This may be an important regulatory mechanism of THAL NaCl absorption when dietary salt intake is increased.

High urea and NaCl carbonylate proteins in renal cells in culture and in vivo, and high urea causes 8-oxoguanine lesions in their DNA

Urea and NaCl are elevated in the renal inner medulla. We now find that a high concentration of urea or NaCl increases reactive oxygen species (ROS) in mouse renal inner medullary (mIMCD3) cells in culture. Previously, high NaCl, but not high urea, was found to cause DNA double-strand breaks. We now tested whether high urea or NaCl causes oxidative damage to DNA or cellular proteins. We find that high urea increases mIMCD3 cell DNA single-strand breaks and 8-oxoguanine lesions. High NaCl does not cause detectable 8-oxoguanine lesions. High urea or NaCl also greatly increases carbonylation of proteins in mIMCD3 cells. Carbonylation occurs within 5 min and with as little as 5 mM urea, a normal plasma level. It increases as urea is raised over the range in uremia. A high raffinose level increases ROS and carbonylation. High sorbitol and glycerol levels do not increase ROS or carbonylation. Carbonyl content is high in mouse renal inner medullas where interstitial NaCl and urea concentrations are normally high. There, numerous proteins are carbonylated, and carbonylation occurs in both collecting ducts and thin limbs.

CONCLUSIONS

(i) Oxidative stress, associated with high urea, causes 8-oxoguanine DNA lesions in mIMCD3 cell DNA. (ii) High urea or NaCl carbonylates proteins in mIMCD3 cells and in renal inner medullary cells in vivo. (iii) In mIMCD3 cells a normal plasma concentration of urea causes carbonylation, and carbonylation increases over the uremic range of urea concentration, indicating that urea can contribute directly to the carbonylation found in uremia.

Regulation by glucocorticoids and osmolality of expression of ROMK (Kir 1.1), the apical K channel of thick ascending limb

Mechanisms of regulation of ROMK channel mRNA and protein expression in medullary thick ascending limb (MTAL) were assessed in rat MTAL fragments incubated for 7 h. ROMK mRNA was quantified by quantitative RT-PCR and ROMK protein by immunoblotting analysis of crude membranes. Medium hyperosmolality (450 mosmol/kgH(2)O; NaCl plus urea added to isoosmotic medium) increased ROMK mRNA (P < 0.04) and protein (P < 0.006), and 10 nM dexamethasone also increased ROMK mRNA (P < 0.02). Hyperosmolality and dexamethasone had no additive effects on ROMK mRNA. NaCl alone, but not urea or mannitol, reproduced the hyperosmolality effect on ROMK mRNA. 1-Deamino-(8-d-arginine) vasopressin (1 nM) or 0.5 mM 8-bromo-cAMP had no effect per se on ROMK mRNA and protein. However, 8-bromo-cAMP abolished the stimulatory effect of dexamethasone on ROMK mRNA in the isoosmotic but not in the hyperosmotic medium (P < 0.004). In in vivo studies, the abundance of ROMK protein and mRNA increased in adrenalectomized (ADX) rats infused with dexamethasone compared with ADX rats (P < 0.02). These results establish glucocorticoids and medium NaCl concentration as direct regulators of MTAL ROMK mRNA and protein expression, which may be modulated by cAMP-dependent factors.

Hypertonicity-induced p38MAPK activation elicits recovery of corneal epithelial cell volume and layer integrity

In hypertonicity-stressed (i.e., 600 mOsm) SV40-immortalized rabbit and human corneal epithelial cell layers (RCEC and HCEC, respectively), we characterized the relationship between time-dependent changes in translayer resistance, relative cell volume and modulation of MAPK superfamily activities. Sulforhodamine B permeability initially increased by 1.4- and 2-fold in RCEC and HCEC, respectively. Subsequently, recovery to its isotonic level only occurred in RCEC. Light scattering revealed that in RCEC 1) regulatory volume increase (RVI) extent was 20% greater; 2) RVI half-time was 2.5-fold shorter. However, inhibition of Na-K-2Cl cotransporter and Na/K-ATPase activity suppressed the RVI response more in HCEC. MAPK activity changes were as follows: 1) p38 was wave-like and faster as well as larger in RCEC than in HCEC (90- and 18-fold, respectively); 2) increases in SAPK/JNK activity were negligible in comparison to those of p38; 3) Erk1/2 activity declined to 30-40% of their basal values. SB203580, a specific p38 inhibitor, dose dependently suppressed the RVI responses in both cell lines. However, neither U0126, which inhibits MEK, the kinase upstream of Erk, nor SP600125, inhibitor of SAPK/JNK, had any effect on this response. Taken together, sufficient activation of the p38 limb of the MAPK superfamily during a hypertonic challenge is essential for maintaining epithelial cell volume and translayer resistance. On the other hand, Erk1/2 activity restoration seems to be dependent on cell volume recovery.

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.

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.

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.

Urea protects from the proapoptotic effect of NaCl in renal medullary cells

Hypertonic NaCl upregulated two sensitive and specific biochemical indices of apoptosis, caspase-3 activation and annexin V binding, in a time- and dose-dependent fashion in renal medullary mIMCD3 cells. Pretreatment with urea (200 mM for 30 min) protected from the proapoptotic effect of hypertonic stress (200 mosmol/kgH(2)O) in this model. The protective effect of urea was dose dependent and was effective even when applied a short time (< or =1 h) following NaCl exposure; this protective effect was not observed in the nonrenal 3T3 cell line. In both mIMCD3 and 3T3 cells, urea failed to protect from the proapoptotic stressor, ultraviolet (UV)-B irradiation. The ability of urea to protect from hypertonic stress was approximately comparable to the protective effect of peptide mitogens epidermal growth factor and insulin-like growth factor (IGF), but it potentiated the IGF effect. Interestingly, the tyrosine kinase inhibitor, genistein, potentiated the proapoptotic effect of urea yet abrogated the proapoptotic effect of hypertonic stress. In aggregate, these data indicate that urea protects from the proapoptotic effect of hypertonic stress in a potentially cell type-specific and stimulus-specific fashion.

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.