Fyn and p38 signaling are both required for maximal hypertonic activation of the osmotic response element-binding protein/tonicity-responsive enhancer-binding protein (OREBP/TonEBP)

Rac1/osmosensing scaffold for MEKK3 contributes via phospholipase C-gamma1 to activation of the osmoprotective transcription factor NFAT5

Separate reports that hypertonicity activates p38 via a Rac1-OSM-MEKK3-MKK3-p38 pathway and that p38α contributes to activation of TonEBP/OREBP led us to the hypothesis that Rac1 might activate TonEBP/OREBP via p38. The present studies examine that possibility. High NaCl is hypertonic. We find that siRNA knockdown of Rac1 reduces high NaCl-induced increase of TonEBP/OREBP transcriptional activity (by reducing its transactivating activity but not its nuclear localization). Similarly, siRNA knockdown of osmosensing scaffold for MEKK3 (OSM) also reduces high NaCl-dependent TonEBP/OREBP transcriptional and transactivating activities. Simultaneous siRNA knockdown of Rac1 and OSM is not additive in reduction of TonEBP/OREBP transcriptional activity, indicating a common pathway. However, siRNA knockdown of MKK3 does not reduce TonEBP/OREBP transcriptional activity, although siRNA knockdown of MKK6 does. Nevertheless, the effect of Rac1 on TonEBP/OREBP is also independent of MKK6 because it occurs in MKK6-null cells. Furthermore, we find that siRNA knockdown of Rac1 or OSM actually increases activity (phosphorylation) of p38, rather than decreasing it, as previously reported. Thus, the effect of Rac1 on TonEBP/OREBP is independent of p38. We find instead that phospholipase C-γ1 (PLC-γ1) is involved. When transfected into PLC-γ1-null mouse embryonic fibroblast cells, catalytically active Rac1 does not increase TonEBP/OREBP transcriptional activity unless PLC-γ1 is reconstituted. Similarly, dominant-negative Rac1 also does not inhibit TonEBP/OREBP in PLC-γ1-null cells unless PLC-γ1 is reconstituted. We conclude that Rac1/OSM supports TonEBP/OREBP activity and that this activity is mediated via PLC-γ1, not p38.

Src kinase pathway is involved in NFAT5-mediated S100A4 induction by hyperosmotic stress in colon cancer cells

S100A4 is associated with the progression of many types of cancers as well as several nonmalignant conditions. However, how it is regulated by intracellular signaling and/or at the transcriptional level has not been extensively studied. We recently demonstrated that S100A4 is partially regulated by nuclear factor in activated T cell 5 (NFAT5) downstream of integrin α6β4. NFAT5 is a mammalian osmotic regulator. To study the regulation of S100A4 by NFAT5 in a more readily inducible model, colon cancer cells were subjected to hyperosmotic stress. We found that S100A4 is induced in a subset of colon cancer cell lines, and the ability to induce S100A4 depends on the methylation status of S100A4. The osmotic stress response elements were identified in the first intron region of S100A4 by S100A4 luciferase reporter assays. Depletion of NFAT5 by small interfering RNA abolished S100A4 induction. Furthermore, chromatin immunoprecipitation assays showed that NFAT5 is induced to bind to the first intron region. Inhibition of Src kinase pathways reduced S100A4 induction by affecting NFAT5 transactivation and protein levels. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was used to study the function of S100A4 induction in colon cancer cells under the condition of hyperosmotic stress; the results suggest that S100A4 induction contributes to cell survival. In conclusion, this study demonstrates that hyperosmotic stress induces S100A4 through NFAT5, and Src and chromatin remodeling are involved. In addition, the induction of S100A4 contributes to cell survival. Given that the gastrointestinal tract is periodically exposed to hyperosmotic stress, this study may uncover a novel signaling pathway that could contribute to GI cancer progression.

Hypertonicity-enhanced TNF-α release from activated human monocytic THP-1 cells requires ERK activation

BACKGROUND

Hypertonic stress enhances tumor necrosis factor (TNF)-α expression in activated monocytes. However, the underlying mechanism is unknown. The produced TNF-α is primarily cleaved and released by TNF-α-converting enzyme (TACE), and the surface expression of TACE is down-regulated by endocytosis. As hypertonicity inhibits endocytosis, we evaluated the mechanism of hypertonicity-induced TNF-α release from activated human monocytic THP-1 cells.

METHODS

THP-1 cells were stimulated with lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA) in the presence or absence of hypertonic agents (150 mM sucrose or 150-300 mM NaCl). The amount of TNF-α mRNA and protein, surface expression of TACE and activation of signaling pathways (mitogen-activated protein kinase, Akt and NF-κB) were assayed.

RESULTS

Hypertonic sucrose and NaCl significantly enhanced TNF-α release from THP-1 cells upon LPS or PMA stimulation. Hypertonic sucrose and other endocytosis inhibitors increased surface expression of TACE, but their effects on TNF-α release were inconsistent. This enhancement effect by hypertonicity was not attenuated by inhibition of TACE or IκB kinase, but it was blocked by cycloheximide and a MAP/ERK kinase inhibitor. The LPS- or PMA-induced TNF-α mRNA expression was not increased; rather, it was inhibited by hypertonicity. ERK1/2 was re-activated after sucrose treatment in LPS-stimulated THP-1 cells.

CONCLUSIONS

Hypertonicity-enhanced TNF-α protein synthesis from LPS- or PMA-activated THP-1 cells requires ERK activation and may proceed without TACE.

GENERAL SIGNIFICANCE

A vast amount of TNF-α production was regulated by a crucial post-transcriptional manner in activated human monocytic leukemia cells, and it may possibly be contributed to the cachexia condition.

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

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

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

Ambient tonicity and intestinal cytochrome CYP3A

IMPORTANCE OF THE FIELD

Recently-discovered tonicity-dependence of human CYP3A expression in vitro may be a novel mechanism of CYP3A regulation in the intestinal epithelia, which exists in a dynamic osmotic environment influenced by food intake.

AREAS COVERED IN THIS REVIEW

A combination of focused and comprehensive literature searches to identify any relevant reports using Medline (from 1950 to 7 November 2009) through the OVID system.

WHAT THE READER WILL GAIN

An update on current knowledge on osmotic environment in the gastrointestinal (GI) tract and its impact on intestinal CYP3A expression and function with special emphasis on the tonicity-sensitive transcription factor nuclear factor of activated T cells 5 (NFAT5).

TAKE HOME MESSAGE

In vitro hypertonicity of ambient osmotic environment in cultured human cells increases expression of CYP3A through transcriptional enhancement by osmosensitive NFAT5. Although post-prandial osmolality in the GI lumen in vivo is substantially increased, NFAT5 activation has not been reported. Similarly, high-salt diet increases intestinal CYP3A function in humans, but it is not known whether these changes are mediated directly by NFAT5.

Tonicity-responsive microRNAs contribute to the maximal induction of osmoregulatory transcription factor OREBP in response to high-NaCl hypertonicity

Osmotic response element binding protein (OREBP) is a Rel-like transcription factor critical for cellular osmoresponses. Previous studies suggest that hypertonicity-induced accumulation of OREBP protein might be mediated by transcription activation as well as posttranscriptional mRNA stabilization or increased translation. However, the underlying mechanisms remain incompletely elucidated. Here, we report that microRNAs (miRNAs) play critical regulatory roles in hypertonicity-induced induction of OREBP. In renal medullary epithelial mIMCD3 cells, hypertonicity greatly stimulates the activity of the 3'-untranslated region of OREBP (OREBP-3'UTR). Furthermore, overexpression of OREBP-3'UTR or depletion of miRNAs by knocking-down Dicer greatly increases OREBP protein expression. On the other hand, significant alterations in miRNA expression occur rapidly in response to high NaCl exposure, with miR-200b and miR-717 being most significantly down-regulated. Moreover, increased miR-200b or miR-717 causes significant down-regulation of mRNA, protein and transcription activity of OREBP, whereas inhibition of miRNAs or disruption of the miRNA-3'UTR interactions abrogates the silencing effects. In vivo in mouse renal medulla, miR-200b and miR-717 are found to function to tune OREBP in response to renal tonicity alterations. Together, our results support the notion that miRNAs contribute to the maximal induction of OREBP to participate in cellular responses to osmotic stress in mammalian renal cells.

Mediator of DNA damage checkpoint 1 (MDC1) contributes to high NaCl-induced activation of the osmoprotective transcription factor TonEBP/OREBP

Background

Hypertonicity, such as induced by high NaCl, increases the activity of the transcription factor TonEBP/OREBP whose target genes increase osmoprotective organic osmolytes and heat shock proteins.

Methodology

We used mass spectrometry to analyze proteins that coimmunoprecipitate with TonEBP/OREBP in order to identify ones that might contribute to its high NaCl-induced activation.

Principal Findings

We identified 20 unique peptides from Mediator of DNA Damage Checkpoint 1 (MDC1) with high probability. The identification was confirmed by Western analysis. We used small interfering RNA knockdown of MDC1 to characterize its osmotic function. Knocking down MDC1 reduces high NaCl-induced increases in TonEBP/OREBP transcriptional and transactivating activity, but has no significant effect on its nuclear localization. We confirm six previously known phosphorylation sites in MDC1, but do not find evidence that high NaCl increases phosphorylation of MDC1. It is suggestive that MDC1 acts as a DNA damage response protein since hypertonicity reversibly increases DNA breaks, and other DNA damage response proteins, like ATM, also associate with TonEBP/OREBP and contribute to its activation by hypertonicity.

Conclusions/Significance

MDC1 associates with TonEBP/OREBP and contributes to high NaCl-induced increase of that factor's transcriptional activity.

Osmoprotective transcription factor NFAT5/TonEBP modulates nuclear factor-kappaB activity

Tonicity responsive binding protein (TonEBP) is a transcription factor that plays a key role in osmoprotection. Here, we demonstrate enhanced activity of prosurvival NF-κB—at the onset of hypertonic challenge that depends on p38 kinase—and Akt-dependent formation of p65-TonEBP complexes that bind to elements of NF-κB-responsive genes.

Tonicity-responsive binding-protein (TonEBP or NFAT5) is a widely expressed transcription factor whose activity is regulated by extracellular tonicity. TonEBP plays a key role in osmoprotection by binding to osmotic response element/TonE elements of genes that counteract the deleterious effects of cell shrinkage. Here, we show that in addition to this “classical” stimulation, TonEBP protects cells against hypertonicity by enhancing nuclear factor-κB (NF-κB) activity. We show that hypertonicity enhances NF-κB stimulation by lipopolysaccharide but not tumor necrosis factor-α, and we demonstrate overlapping protein kinase B (Akt)-dependent signal transduction pathways elicited by hypertonicity and transforming growth factor-α. Activation of p38 kinase by hypertonicity and downstream activation of Akt play key roles in TonEBP activity, IκBα degradation, and p65 nuclear translocation. TonEBP affects neither of these latter events and is itself insensitive to NF-κB signaling. Rather, we reveal a tonicity-dependent interaction between TonEBP and p65 and show that NF-κB activity is considerably enhanced after binding of NF-κB-TonEBP complexes to κB elements of NF-κB–responsive genes. We demonstrate the key roles of TonEBP and Akt in renal collecting duct epithelial cells and in macrophages. These findings reveal a novel role for TonEBP and Akt in NF-κB activation on the onset of hypertonic challenge.

The Guanine Nucleotide Exchange Factor Brx: A Link between Osmotic Stress, Inflammation and Organ Physiology and Pathophysiology

Dehydration, and consequent intracellular hyperosmolarity, is a major challenge to land organisms, as it is associated with extraction of water from cells and disturbance of global cellular function. Organisms have thus developed a highly conserved regulatory mechanism that transduces the hyperosmolarity signal from the cell surface to the cell nucleus and adjusts the expression of cellular osmolarity-regulating genes. We recently found that the Rho-type guanine nucleotide exchange factor Brx, or AKAP13, is essential for osmotic stress-stimulated expression of nuclear factor of activated T-cells 5 (NFAT5), a key transcription factor of intracellular osmolarity. It accomplishes this by first attracting cJun kinase (JNK)-interacting protein (JIP) 4 and then coupling activated Rho-type small G-proteins to cascade components of the p38 MAPK signaling pathway, ultimately activating NFAT5. We describe the potential implications of osmotic stress and Brx activation in organ physiology and pathophysiology and connect activation of this system to key human homeostatic states.

c-Abl mediates high NaCl-induced phosphorylation and activation of the transcription factor TonEBP/OREBP

The transcription factor TonEBP/OREBP promotes cell survival during osmotic stress. High NaCl-induced phosphorylation of TonEBP/OREBP at tyrosine-143 was known to be an important factor in increasing its activity in cell culture. We now find that TonEBP/OREBP also is phosphorylated at tyrosine-143 in rat renal inner medulla, dependent on the interstitial osmolality. c-Abl seemed likely to be the kinase that phosphorylates TonEBP/OREBP because Y143 is in a consensus c-Abl phosphorylation site. We now confirm that, as follows. High NaCl increases c-Abl activity. Specific inhibition of c-Abl by imatinib, siRNA, or c-Abl kinase dead drastically reduces high NaCl-induced TonEBP/OREBP activity by reducing its nuclear location and transactivating activity. c-Abl associates with TonEBP/OREBP (coimmunoprecipitation) and phosphorylates TonEBP/OREBP-Y143 both in cell and in vitro. High NaCl-induced activation of ataxia telangiectasia mutated, previously known to contribute to activation of TonEBP/OREBP, depends on c-Abl activity. Thus, c-Abl is the kinase responsible for high NaCl-induced phosphorylation of TonEBP/OREBP-Y143, which contributes to its increased activity.

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

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

Determination of the NFAT5/TonEBP transcription factor in the human and ovine placenta

Osmotic stress results in the accumulation of osmolytes in tissues. Synthesis of these osmolytes is mediated by the transcription factor NFAT5/TonEBP in the human kidney. We tested for the presence of NFAT5 mRNA and protein in the human and ovine placenta and confirmed sorbitol and inositol osmolyte concentrations in these tissues. To determine NFAT5 protein, human and ovine placenta were tested for inositol, sorbitol and glucose using high performance liquid chromatography (HPLC). Additionally, RNA was extracted and cDNA was made from these tissues. PCR was performed and products were sequenced. Western blotting was used to assess the expression of the NFAT5 protein. Human and ovine placenta demonstrated: 1) high concentrations of sorbitol and inositol, 2) presence of NFAT5 mRNA, 3) confirmation by NFAT5 sequence identity, and 4) presence of NFAT5 protein. NFAT5 is present in the ovine and human placenta at the RNA and protein levels that suggest a role for this protein in the induction of these osmolytes. Further trophoblast studies of osmotic stress effects on osmolytes are planned.

Inducible nucleosome depletion at OREBP-binding-sites by hypertonic stress

Background

Osmotic Response Element-Binding Protein (OREBP), also known as TonEBP or NFAT5, is a unique transcription factor. It is hitherto the only known mammalian transcription factor that regulates hypertonic stress-induced gene transcription. In addition, unlike other monomeric members of the NFAT family, OREBP exists as a homodimer and it is the only transcription factor known to bind naked DNA targets by complete encirclement in vitro. Nevertheless, how OREBP interacts with target DNA, also known as ORE/TonE, and how it elicits gene transcription in vivo, remains unknown.

Methodology

Using hypertonic induction of the aldose reductase (AR) gene activation as a model, we showed that OREs contained dynamic nucleosomes. Hypertonic stress induced a rapid and reversible loss of nucleosome(s) around the OREs. The loss of nucleosome(s) was found to be initiated by an OREBP-independent mechanism, but was significantly potentiated in the presence of OREBP. Furthermore, hypertonic induction of AR gene was associated with an OREBP-dependent hyperacetylation of histones that spanned the 5′ upstream sequences and at least some exons of the gene. Nevertheless, nucleosome loss was not regulated by the acetylation status of histone.

Significance

Our findings offer novel insights into the mechanism of OREBP-dependent transcriptional regulation and provide a basis for understanding how histone eviction and transcription factor recruitment are coupled.

Hyperosmolarity regulates SOX9 mRNA posttranscriptionally in human articular chondrocytes

The transcription factor SOX9 regulates cartilage extracellular matrix gene expression and is essential for chondrocyte differentiation. We previously showed that activation of p38 MAPK by cycloheximide in human chondrocytes leads to stabilization of SOX9 mRNA (Tew SR and Hardingham TE. J Biol Chem 281: 39471–39479, 2006). In this study we investigated whether regulation of p38 MAPK caused by changes in osmotic pressure could control SOX9 mRNA levels expression by a similar mechanism. Primary human articular chondrocytes isolated from osteoarthritic cartilage at passage 2-4 showed significantly raised SOX9 mRNA levels when exposed to hyperosmotic conditions for 5 h. The effect was strongest and most reproducible when actin stress fibers were disrupted by the Rho effector kinase inhibitor Y27632, or by culturing the cells within alginate beads. Freshly isolated chondrocytes, used within 24–48 h of isolation, did not contain actin stress fibers and upregulated SOX9 mRNA in response to hyperosmolarity in the presence and absence of Y27632. In these freshly isolated chondrocytes, hyperosmolarity led to an increase in the half-life of SOX9 mRNA, which was sensitive to the p38 MAPK inhibitor SB202190. SOX9 protein levels were increased by hyperosmotic culture over 24 h, and, in passaged chondrocytes, the activity of a COL2A1 enhancer driven luciferase assay was upregulated. However, in freshly isolated chondrocytes, COL2A1 mRNA levels were reduced by hyperosmotic conditions and the half-life was decreased. The results showed that the osmotic environment regulated both SOX9 and COL2A1 mRNA posttranscriptionally, but in fresh cells resulted in increased SOX9, but decreased COL2A1.

Urea promotes TonEBP expression and cellular adaptation in extreme hypertonicity

The transcriptional activator TonEBP is a central regulator of osmolality in the renal medulla and whole body water homeostasis. In order to understand the regulation of TonEBP in the renal medulla, we examined MDCK cells, a kidney-derived epithelial cell line, under conditions mimicking the renal medulla. Moderate changes in ambient tonicity, which was tolerated without prior adaptation, displayed lasting effects on TonEBP in bidirectional manner-stimulated by hypertonicity and inhibited by hypotonicity. TonEBP expression was further enhanced by extreme hypertonicity observed in the inner medullae of antidiuretic animals. Urea stimulated TonEBP expression and promoted cellular proliferation under the conditions of extreme hypertonicity. On the other hand, the TonEBP activity was negatively modulated under these conditions presumably to temper the highly abundant TonEBP. We conclude that urea is critical to the cellular adaptation to extreme hypertonicity and the high level of TonEBP expression in the inner medulla.

Cellular stress response pathway system as a sentinel ensemble in toxicological screening

High costs, long test times, and societal concerns related to animal use have required the development of in vitro assays for the rapid and cost-effective toxicological evaluation and characterization of compounds in both the pharmaceutical and environmental arenas. Although the pharmaceutical industry has developed very effective, high-throughput in vitro assays for determining the therapeutic potential of compounds, the application of this approach to toxicological screening has been limited. A primary reason for this is that while drug candidate screens are directed to a specific target/mechanism, xenobiotics can cause toxicity through any of a myriad of undefined interactions with cellular components and processes. Given that it is not practical to design assays that can interrogate each potential toxicological target, an integrative approach is required if there is to be a rapid and low-cost toxicological evaluation of chemicals. Cellular stress response pathways offer a viable solution to the creation of a set of integrative assays as there is a limited and hence manageable set (a small ensemble of 10 or less) of major cellular stress response pathways through which cells mount a homoeostatic response to toxicants and which also participate in cell fate/death decisions. Further, over the past decades, these pathways have been well characterized at a molecular level thereby enabling the development of high-throughput cell-based assays using the components of the pathways. Utilization of the set of cellular stress response pathway-based assays as indicators of toxic interactions of chemicals with basic cellular machinery will potentially permit the clustering of chemicals based on biological response profiles of common mode of action (MOA) and also the inference of the specific MOA of a toxicant. This article reviews the biochemical characteristics of the stress response pathways, their common architecture that enables rapid activation during stress, their participation in cell fate decisions, the essential nature of these pathways to the organism, and the biochemical basis of their cross-talk that permits an assay ensemble screening approach. Subsequent sections describe how the stress pathway ensemble assay approach could be applied to screening potentially toxic compounds and discuss how this approach may be used to derive toxicant MOA from the biological activity profiles that the ensemble strategy provides. The article concludes with a review of the application of the stress assay concept to noninvasive in vivo assessments of chemical toxicants.

Transcription factors in the pathogenesis of diabetic nephropathy

Approximately a third of patients with diabetes develop diabetic kidney disease, and diabetes is the leading cause of end-stage renal disease in most developed countries. Hyperglycaemia is known to activate genes that ultimately lead to extracellular matrix accumulation, the hallmark of diabetic nephropathy. Several transcription factors have been implicated in glucose-mediated expression of genes involved in diabetic nephropathy. This review focuses on the transcription factors upstream stimulatory factors 1 and 2 (USF1 and 2), activator protein 1 (AP-1), nuclear factor (NF)-kappaB, cAMP-response-element-binding protein (CREB), nuclear factor of activated T cells (NFAT), and stimulating protein 1 (Sp1). In response to high glucose, several of these transcription factors regulate the gene encoding the profibrotic cytokine transforming growth factor beta, as well as genes for a range of other proteins implicated in inflammation and extracellular matrix turnover, including thrombospondin 1, the chemokine CCL2, osteopontin, fibronectin, decorin, plasminogen activator inhibitor 1 and aldose reductase. Identifying the molecular mechanisms by which diabetic nephropathy occurs has important clinical implications as therapies can then be tailored to target those at risk. Strategies to specifically target transcription factor activation and function may be employed to halt the progression of diabetic nephropathy.

Protein kinase Cmu plays an essential role in hypertonicity-induced heat shock protein 70 expression

Heat shock protein 70 (HSP70), which evidences important functions as a molecular chaperone and anti-apoptotic molecule, is substantially induced in cells exposed to a variety of stresses, including hypertonic stress, heavy metals, heat shock, and oxidative stress, and prevents cellular damage under these conditions. However, the molecular mechanism underlying the induction of HSP70 in response to hypertonicity has been characterized to a far lesser extent. In this study, we have investigated the cellular signaling pathway of HSP70 induction under hypertonic conditions. Initially, we applied a variety of kinase inhibitors to NIH3T3 cells that had been exposed to hypertonicity. The induction of HSP70 was suppressed specifically by treatment with protein kinase C (PKC) inhibitors (Gö6976 and GF109203X). As hypertonicity dramatically increased the phosphorylation of PKCmu, we then evaluated the role of PKCmu in hypertonicity-induced HSP70 expression and cell viability. The depletion of PKCmu with siRNA or the inhibition of PKCmu activity with inhibitors resulted in a reduction in HSP70 induction and cell viability. Tonicity-responsive enhancer binding protein (TonEBP), a transcription factor for hypertonicity-induced HSP70 expression, was translocated rapidly into the nucleus and was modified gradually in the nucleus under hypertonic conditions. When we administered treatment with PKC inhibitors, the mobility shift of TonEBP was affected in the nucleus. However, PKCmu evidenced no subcellular co-localization with TonEBP during hypertonic exposure. From our results, we have concluded that PKCmu performs a critical function in hypertonicity-induced HSP70 induction, and finally cellular protection, via the indirect regulation of TonEBP modification.

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

Background

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

Methodology/Principal Findings

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

Conclusions/Significance

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

Brx shines a light on the route from hyperosmolarity to NFAT5

Nuclear factor of activated T cells 5 (NFAT5) is a member of the Rel family of transcription factors and is an essential inducer of osmoprotective gene products in mammalian cells. Its activation by hypertonicity requires p38 mitogen-activated protein kinase (MAPK) signaling and other pathways. A study now elucidates a signaling cascade regulated by the guanine nucleotide exchange factor Brx that leads to the activation of p38alpha MAPK and the induction of nfat5 messenger RNA in response to osmotic stress in lymphocytes and renal medullary cells. Brx-deficient lymphocytes showed impaired responses to hypertonicity, and brx(+/-) mice exhibited immune defects similar to those of nfat5-deficient mice. These findings support a major role for Brx in regulating the osmoprotective function of NFAT5 in different cell types.

Physiology of cell volume regulation in vertebrates

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

EGF receptor signaling is involved in expression of osmoprotective TonEBP target gene aldose reductase under hypertonic conditions

Renal medullary cells adapt to their hyperosmotic environment by enhanced expression of various osmoprotective genes. Although it is clearly established that TonEBP contributes to the expression of these genes, neither the precise signaling mechanism by which hypertonicity activates TonEBP is completely understood, nor is it known whether a membrane-bound osmosenser, corresponding to yeast and bacteria, is present in mammalian cells. We found evidence that metalloproteinase (MMP)-dependent activation of the epidermal growth factor receptor (EGFR) signals to TonEBP and stimulates the expression of the TonEBP target gene aldose reductase (AR) under hypertonic conditions. Phosphorylation of EGFR and the downstream MAP kinases ERK1/2 and p38 was significantly enhanced by high NaCl in Madin-Darby canine kidney (MDCK) cells. Conversely, the broad-spectrum MMP inhibitor GM6001 or the EGFR inhibitor AG1478 diminished phosphorylation of EGFR, p38, and ERK1/2, the induction of AR mRNA and protein, and AR promoter reporter activity in response to hypertonicity. Accordingly, neutralizing antibodies against the putative EGFR ligand transforming growth factor-alpha (TGF-alpha) abolished AR induction during osmotic stress. Furthermore, tonicity-induced phosphorylation of p38 and ERK1/2 and expression of AR were reduced significantly in MDCK cells transfected with a dominant-negative Ras construct. These effects were not caused by reduced nuclear abundance of TonEBP during osmotic stress; however, inhibition of EGFR or p38 diminished TonEBP transactivation activity under hypertonic conditions. The contribution of MMP/EGFR signaling in vivo was confirmed in C57BL/6 mice, in which treatment with GM6001 was associated with reduced AR induction following dehydration. Taken together, these results indicate that osmotic stress induces MMP-dependent activation of EGFR, likely via shedding of TGF-alpha, and downstream activation of Ras and the MAP kinases p38 and ERK1/2, which stimulate TonEBP transactivation activity. This EGFR-Ras-MAPK pathway contributes to TonEBP transcriptional activation and targets gene expression during osmotic stress, thus establishing a membrane-associated signal input that contributes to the regulation of TonEBP activity.

Controlled aquaporin-2 expression in the hypertonic environment

The corticomedullary osmolality gradient is the driving force for water reabsorption occurring in the kidney. In the collecting duct, this gradient allows luminal water to move across aquaporin (AQP) water channels, thereby increasing urine concentration. However, this same gradient exposes renal cells to great osmotic challenges. These cells must constantly adapt to fluctuations of environmental osmolality that challenge cell volume and incite functional change. This implies profound alterations of cell phenotype regarding water permeability. AQP2 is an essential component of the urine concentration mechanism whose controlled expression dictates apical water permeability of collecting duct principal cells. This review focuses on changes of AQP2 abundance and trafficking in hypertonicity-challenged cells. Intracellular mechanisms governing these events are discussed and the biological relevance of altered AQP2 expression by hypertonicity is outlined.

Brx mediates the response of lymphocytes to osmotic stress through the activation of NFAT5

Extracellular hyperosmolarity, or osmotic stress, generally caused by differences in salt and macromolecule concentrations across the plasma membrane, occurs in lymphoid organs and at inflammatory sites. The response of immune cells to osmotic stress is regulated by nuclear factor of activated T cells 5 (NFAT5), a transcription factor that induces the expression of hyperosmolarity-responsive genes and stimulates cytokine production. We report that the guanine nucleotide exchange factor (GEF) Brx [also known as protein kinase A-anchoring protein 13 (AKAP13)] is essential for the expression of nfat5 in response to osmotic stress, thus transmitting the extracellular hyperosmolarity signal and enabling differentiation of splenic B cells and production of immunoglobulin. This process required the activity of p38 mitogen-activated protein kinase (MAPK) and NFAT5 and involved a physical interaction between Brx and c-Jun N-terminal kinase (JNK)-interacting protein 4 (JIP4), a scaffold molecule specific to activation of the p38 MAPK cascade. Our results indicate that Brx integrates the responses of immune cells to osmotic stress and inflammation by elevating intracellular osmolarity and stimulating the production of cytokines.

Does the intracellular ionic concentration or the cell water content (cell volume) determine the activity of TonEBP in NIH3T3 cells?

The transcription factor, tonicity-responsive enhancer binding protein (TonEBP), is involved in the adaptive response against hypertonicity. TonEBP regulates the expression of genes that catalyze the accumulation of osmolytes, and its transcriptional activity is increased by hypertonicity. The goal of the present investigation was to investigate whether cell shrinkage or high intracellular ionic concentration induced the activation of TonEBP. We designed a model system for isotonically shrinking cells over a prolonged period of time. Cells swelled in hypotonic medium and performed a regulatory volume decrease. Upon return to the original isotonic medium, cells shrank initially, followed by a regulatory volume increase. To maintain cell shrinkage, the RVI process was inhibited as follows: ethyl-isopropyl-amiloride inhibited the Na+/H+ antiport, bumetanide inhibited the Na+-K+-2Cl− cotransporter, and gadolinium inhibited shrinkage-activated Na+ channels. Cells remained shrunken for at least 4 h (isotonically shrunken cells). The activity of TonEBP was investigated with a Luciferase assay after isotonic shrinkage and after shrinkage in a high-NaCl hypertonic medium. We found that TonEBP was strongly activated after 4 and 16 h in cells in high-NaCl hypertonic medium, but not after 4 or 16 h in isotonically shrunken cells. Cells treated with high-NaCl hypertonic medium for 4 h had significantly higher intracellular concentrations of both K+ and Na+ than isotonically shrunken cells. This strongly suggested that an increase in intracellular ionic concentration and not cell shrinkage is involved in TonEBP activation.

Effects of hyperosmolarity on the Na+-myo-inositol cotransporter SMIT2 stably transfected in the Madin-Darby canine kidney cell line

Myo-inositol (MI) is a compatible osmolyte used by cells to compensate for changes in the osmolarity of their surrounding milieu. In kidney, the basolateral Na+-MI cotransporter (SMIT1) and apical SMIT2 proteins are homologous cotransporters responsible for cellular uptake of MI. It has been shown in the Madin-Darby canine kidney (MDCK) cell line that SMIT1 expression was under the control of the tonicity-sensitive transcription factor, tonicity-responsive enhancer binding protein (TonEBP). We used an MDCK cell line stably transfected with SMIT2 to determine whether variations in external osmolarity could also affect SMIT2 function. Hyperosmotic conditions (+200 mosM raffinose or NaCl but not urea) generated an increase in SMIT2-specific MI uptake by three- to ninefold in a process that required protein synthesis. Using quantitative RT-PCR, we have determined that hyperosmotic conditions augment both the endogenous SMIT1 and the transfected SMIT2 mRNAs. Transport activities for both SMIT1 and SMIT2 exhibited differences in their respective induction profiles for both their sensitivities to raffinose, as well as in their time course of induction. Application of MG-132, which inhibits nuclear translocation of TonEBP, showed that the effect of osmolarity on transfected SMIT2 was unrelated to TonEBP, unlike the effect observed with SMIT1. Inhibition studies involving the hyperosmolarity-related MAPK suggested that p38 and JNK play a role in the induction of SMIT2. Further studies have shown that hyperosmolarity also upregulates another transfected transporter (Na+-glucose), as well as several endogenously expressed transport systems. This study shows that hyperosmolarity can stimulate transport in a TonEBP-independent manner by increasing the amount of mRNA derived from an exogenous DNA segment.

Phosphorylation by casein kinase 1 regulates tonicity-induced osmotic response element-binding protein/tonicity enhancer-binding protein nucleocytoplasmic trafficking

The osmotic response element-binding protein (OREBP), also known as tonicity enhancer-binding protein (TonEBP) or NFAT5, is the only known osmo-sensitive transcription factor that mediates cellular adaptations to extracellular hypertonic stress. Although it is well documented that the subcellular localization and transactivation activity of OREBP/TonEBP are tightly regulated by extracellular tonicity, the molecular mechanisms involved remain elusive. Here we show that nucleocytoplasmic trafficking of OREBP/TonEBP is regulated by the dual phosphorylation of Ser-155 and Ser-158. Alanine scanning mutagenesis revealed that Ser-155 is an essential residue that regulates OREBP/TonEBP nucleocytoplasmic trafficking. Tandem mass spectrometry revealed that Ser-155 and Ser-158 of OREBP/TonEBP are both phosphorylated in living cells under hypotonic conditions. In vitro phosphorylation assays further suggest that phosphorylation of the two serine residues proceeds in a hierarchical manner with phosphorylation of Ser-155 priming the phosphorylation of Ser-158 and that these phosphorylations are essential for nucleocytoplasmic trafficking of the transcription factor. Finally, we have shown that the pharmacological inhibition of casein kinase 1 (CK1) abolishes the phosphorylation of Ser-158 and impedes OREBP/TonEBP nuclear export and that recombinant CK1 phosphorylates Ser-158. Knockdown of CK1alpha1L, a novel isoform of CK1, inhibits hypotonicity-induced OREBP/TonEBP nuclear export. Together these data highlight the importance of Ser-155 and Ser-158 in the nucleocytoplasmic trafficking of OREBP/TonEBP and indicate that CK1 plays a major role in regulating this process.

MKP-1 inhibits high NaCl-induced activation of p38 but does not inhibit the activation of TonEBP/OREBP: opposite roles of p38alpha and p38delta

High NaCl rapidly activates p38 MAPK by phosphorylating it, the phosphorylation presumably being regulated by a balance of kinases and phosphatases. Kinases are known, but the phosphatases are uncertain. Our initial purpose was to identify the phosphatases. We find that in HEK293 cells transient overexpression of MAPK phosphatase-1 (MKP-1), a dual-specificity phosphatase, inhibits high NaCl-induced phosphorylation of p38, and that overexpression of a dominant negative mutant of MKP-1 does the opposite. High NaCl lowers MKP-1 activity by increasing reactive oxygen species, which directly inhibit MKP-1, and by reducing binding of MKP-1 to p38. Because inhibition of p38 is reported to reduce hypertonicity-induced activation of the osmoprotective transcription factor, TonEBP/OREBP, we anticipated that MKP-1 expression might also. However, overexpression of MKP-1 has no significant effect on Ton EBP/OREBP activity. This paradox is explained by opposing effects of p38alpha and p38delta, both of which are activated by high NaCl and inhibited by MKP-1. Thus, we find that overexpression of p38alpha increases high NaCl-induced TonEBP/OREBP activity, but overexpression of p38delta reduces it. Also, siRNA-mediated knockdown of p38delta enhances the activation of TonEBP/OREBP. We conclude that high NaCl inhibits MKP-1, which contributes to the activation of p38. However, opposing actions of p38alpha and p38delta negate any effect on TonEBP/OREBP activity. Thus, activation of p38 isoforms by hypertonicity does not contribute to activation of TonEBP/OREBP because of opposing effects of p38alpha and p38delta, and effects of inhibitors of p38 depend on which isoform is affected, which can be misleading.

Analysis of the transcriptional activity of endogenous NFAT5 in primary cells using transgenic NFAT-luciferase reporter mice

Background

The transcription factor NFAT5/TonEBP regulates the response of mammalian cells to hypertonicity. However, little is known about the physiopathologic tonicity thresholds that trigger its transcriptional activity in primary cells. Wilkins et al. recently developed a transgenic mouse carrying a luciferase reporter (9xNFAT-Luc) driven by a cluster of NFAT sites, that was activated by calcineurin-dependent NFATc proteins. Since the NFAT site of this reporter was very similar to an optimal NFAT5 site, we tested whether this reporter could detect the activation of NFAT5 in transgenic cells.

Results

The 9xNFAT-Luc reporter was activated by hypertonicity in an NFAT5-dependent manner in different types of non-transformed transgenic cells: lymphocytes, macrophages and fibroblasts. Activation of this reporter by the phorbol ester PMA plus ionomycin was independent of NFAT5 and mediated by NFATc proteins. Transcriptional activation of NFAT5 in T lymphocytes was detected at hypertonic conditions of 360–380 mOsm/kg (isotonic conditions being 300 mOsm/kg) and strongly induced at 400 mOsm/kg. Such levels have been recorded in plasma in patients with osmoregulatory disorders and in mice deficient in aquaporins and vasopressin receptor. The hypertonicity threshold required to activate NFAT5 was higher in bone marrow-derived macrophages (430 mOsm/kg) and embryonic fibroblasts (480 mOsm/kg). Activation of the 9xNFAT-Luc reporter by hypertonicity in lymphocytes was insensitive to the ERK inhibitor PD98059, partially inhibited by the PI3-kinase inhibitor wortmannin (0.5 μM) and the PKA inhibitor H89, and substantially downregulated by p38 inhibitors (SB203580 and SB202190) and by inhibition of PI3-kinase-related kinases with 25 μM LY294002. Sensitivity of the reporter to FK506 varied among cell types and was greater in primary T cells than in fibroblasts and macrophages.

Conclusion

Our results indicate that NFAT5 is a sensitive responder to pathologic increases in extracellular tonicity in T lymphocytes. Activation of NFAT5 by hypertonicity in lymphocytes was mediated by a combination of signaling pathways that differed from those required in other cell types. We propose that the 9xNFAT-Luc transgenic mouse model might be useful to study the physiopathological regulation of both NFAT5 and NFATc factors in primary cells.

Activator protein-1 contributes to high NaCl-induced increase in tonicity-responsive enhancer/osmotic response element-binding protein transactivating activity

Tonicity-responsive enhancer/osmotic response element-binding protein (TonEBP/OREBP) is a Rel protein that activates transcription of osmoprotective genes at high extracellular NaCl. Other Rel proteins NFAT1-4 and NF-kappaB complex with activator protein-1 (AP-1) to transactivate target genes through interaction at composite NFAT/NF-kappaB.AP-1 sites. TonEBP/OREBP target genes commonly have one or more conserved AP-1 binding sites near TonEBP/OREBP cognate elements (OREs). Also, TonEBP/OREBP and the AP-1 proteins c-Fos and c-Jun are all activated by high NaCl. We now find, using an ORE.AP-1 reporter from the target aldose reductase gene or the same reporter with a mutated AP-1 site, that upon stimulation by high extracellular NaCl, 1) the presence of a wild type, but not a mutated, AP-1 site contributes to TonEBP/OREBP-dependent transcription and 2) AP-1 dominant negative constructs inhibit TonEBP/OREBP-dependent transcription provided the AP-1 site is not mutated. Using supershifts and an ORE.AP-1 probe, we find c-Fos and c-Jun present in combination with TonEBP/OREBP. Also, c-Fos and c-Jun coimmunoprecipitate with TonEBP/OREBP, indicating physical association. Small interfering RNA knockdown of either c-Fos or c-Jun inhibits high NaCl-induced increase of mRNA abundance of the TonEBP/OREBP target genes AR and BGT1. Furthermore, a dominant negative AP-1 also reduces high NaCl-induced increase of TonEBP/OREBP transactivating activity. Inhibition of p38, which is known to stimulate TonEBP/OREBP transcriptional activity, reduces high NaCl-dependent transcription of an ORE.AP-1 reporter only if the AP-1 site is intact. Thus, AP-1 is part of the TonEBP/OREBP enhanceosome, and its role in high NaCl-induced activation of TonEBP/OREBP may require p38 activity.

Tonicity-regulated gene expression

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

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Hyperosmotic stress signaling to the nucleus disrupts the Ran gradient and the production of RanGTP

The RanGTP gradient depends on nucleocytoplasmic shuttling of Ran and its nucleotide exchange in the nucleus. Here we show that hyperosmotic stress signaling induced by sorbitol disrupts the Ran protein gradient and reduces the production of RanGTP. Ran gradient disruption is rapid and is followed by early (10-20 min) and late (30-60 min) phases of recovery. Results from SB203580 and siRNA experiments suggest the stress kinase p38 is important for Ran gradient recovery. NTF2 and Mog1, which are transport factors that regulate the nuclear localization of Ran, showed kinetics of delocalization and recovery similar to Ran. Microinjection of a nuclear localization signal reporter protein revealed that sorbitol stress decreases the rate of nuclear import. Sorbitol stress also slowed RCC1 mobility in the nucleus, which is predicted to reduce RCC1 dissociation from chromatin and RanGTP production. This was tested using a FRET biosensor that registers nuclear RanGTP levels, which were reduced in response to sorbitol stress. Although sorbitol alters nucleotide levels, we show that inverting the GTP/GDP ratio in cells is not sufficient to disrupt the Ran gradient. Thus, the Ran system is a target of hyperosmotic stress signaling, and cells use protein localization-based mechanisms as part of a rapid stress response.

MEK/ERK signaling controls osmoregulation of nucleus pulposus cells of the intervertebral disc by transactivation of TonEBP/OREBP

Earlier studies have shown that intervertebral disc cells express TonEBP, a transcription factor that permits adaptation to osmotic stress and regulates aggrecan gene expression. However, the mechanism of hyperosmotic activation of TonEBP in disc cells is not known. Results of this study show that hypertonic activation of ERK signaling regulates transactivation activity of TonEBP, modulating its function.

INTRODUCTION

In an earlier report, we showed that tonicity enhancer binding protein (TonEBP) positively regulates aggrecan gene expression in disc cells, thereby autoregulating its osmotic environment. Although these studies indicated that the cells of the nucleus pulposus were optimally adapted to a hyperosmotic state, the mechanism by which the cells transduce the osmotic stress was not delineated. The primary goal of this study was to test the hypothesis that, in a hyperosmotic medium, the extracellular signal-regulated kinase (ERK) signaling pathway regulated TonEBP activity.

MATERIALS AND METHODS

Nucleus pulposus cells were maintained in isotonic or hypertonic media, and MAPK activation and TonEBP expression were analyzed. To study the role of MAPK in regulation of TonEBP function, gel shift and luciferase reporter assays were performed. ERK expression in cells was modulated by using expression plasmids or siRNA, and transactivation domain (TAD)-TonEBP activity was studied.

RESULTS

We found that hypertonicity resulted in phosphorylation and activation of ERK1/2 proteins and concomitant activation of C terminus TAD activity of ELK-1, a downstream transcription factor. In hypertonic media, treatment with ERK and p38 inhibitors resulted in downregulation of TonE promoter activity of TauT and HSP-70 and decreased binding of TonEBP to TonE motif. Similarly, forced expression of DN-ERK and DN-p38 in nucleus pulposus cells suppressed TauT and HSP-70 reporter gene activity. Finally, we noted that ERK was needed for transactivation of TonEBP. Expression of DN-ERK significantly suppressed, whereas, WT-ERK and CA-MEK1 enhanced, TAD activity of TonEBP. Experiments performed with HeLa cells indicated that the ERK signaling pathway also served a major role in regulating the osmotic response in nondiscal cells.

CONCLUSIONS

Together, these studies showed that adaptation of the nucleus pulposus cells to their hyperosmotic milieu is dependent on activation of the ERK and p38- MAPK pathways acting through TonEBP and its target genes.

Proteomic identification of proteins associated with the osmoregulatory transcription factor TonEBP/OREBP: functional effects of Hsp90 and PARP-1

Hypertonicity (e.g., high NaCl) activates the transcription factor tonicity-responsive enhancer/osmotic response element-binding protein (TonEBP/OREBP), increasing transcription of protective genes. In the present studies, by stably expressing amino acids 1-547 of TonEBP/OREBP in HEK 293 cells and immunoprecipitating it plus associated proteins from the nuclei of cells exposed to high NaCl, we identify 14 proteins that are physically associated with TonEBP/OREBP. The associated proteins fall into several classes: 1) DNA-dependent protein kinase, both its catalytic subunit and regulatory subunit, Ku86; 2) RNA helicases, namely RNA helicase A, nucleolar RNA helicase II/Gu, and DEAD-box RNA helicase p72; 3) small or heterogeneous nuclear ribonucleoproteins (snRNPs or hnRNPs), namely U5 snRNP-specific 116 kDa protein, U5 snRNP-specific 200 kDa protein, hnRNP U, hnRNP M, hnRNP K, and hnRNP F; 4) heat shock proteins, namely Hsp90beta and Hsc70; and 5) poly(ADP-ribose) polymerase-1 (PARP-1). We confirm identification of most of the proteins by Western analysis and also demonstrate by electrophoretic mobility-shift assay that they are present in the large complex that binds specifically along with TonEBP/OREBP to its cognate DNA element. In addition, we find that PARP-1 and Hsp90 modulate TonEBP/OREBP activity. PARP-1 expression reduces TonEBP/OREBP transcriptional activity and the activity of its transactivating domain. Hsp90 enhances those activities and sustains the increased abundance of TonEBP/OREBP protein in cells exposed to high NaCl.

Regulation of ROMK (Kir 1.1) Channel Expression in Kidney Thick Ascending Limb by Hypertonicity: Role of TonEBP and MAPK Pathways

The present study assessed the mechanisms by which hypertonicity caused by NaCl enhances the renal outer medullary potassium channel (ROMK) mRNA abundance in rat kidney medullary thick ascending limb (MTAL) and in cultured mouse TAL cells. Using the run-off technique, we observed that the ROMK gene transcription rate in nuclei isolated from MTAL fragments was enhanced approximately 40% by a high NaCl medium. In MTAL fragments, hypertonicity (450 mosm) caused by NaCl, not by mannitol or urea, enhanced both ROMK mRNA abundance and tonicity-responsive enhancer binding protein (TonEBP) total abundance and nuclear localization. In an immortalized mouse TAL cell culture in which ROMK is apically expressed, hypertonicity caused by both NaCl and mannitol, not urea, enhanced both ROMK mRNA abundance and TonEBP total abundance and nuclear localization. Confocal microscopy confirmed an increased nuclear translocation of TonEBP in response to NaCl-induced hypertonicity. Finally, inhibition of the p38 MAPK pathway by SB203580 and of the ERK pathway by PD98059 abolished the NaCl-induced stimulation of TonEBP and ROMK. These results establish that mRNA expression of ROMK is augmented in the MTAL by NaCl-induced hypertonicity through stimulation of ROMK gene transcription, and that TonEBP and the p38 MAPK and ERK pathways are involved in this effect.

Degradation of NFAT5, a transcriptional regulator of osmotic stress-related genes, is a critical event for doxorubicin-induced cytotoxicity in cardiac myocytes

Nuclear factor-activated T cell 5 (NFAT5), a novel member of the NFAT family of proteins, was originally identified as a transcriptional factor responsible for adaptation to hyperosmotic stress. Though NFAT5 is ubiquitously expressed, the biological functions of NFAT5 remain to be clarified, especially in the tissues that are not exposed to hypertonicity, including hearts. In the present study, we focused on the cardioprotective roles of NFAT5 against the cardiotoxic anti-tumor agent doxorubicin (Dox). In cultured cardiomyocytes, transcripts of the hypertonicity-inducible genes, such as taurine transporter (TauT) and sodium/myo-inositol transporter, were down-regulated by Dox. Interestingly, NFAT5 protein, but not mRNA, was decreased in cardiomyocytes exposed to Dox. Treatment of proteasome inhibitors, MG-132 or proteasome-specific inhibitor 1, prevented the Dox-mediated decrease of NFAT5 protein. Further, ubiquitin-conjugated NFAT5 was not detected in cultured cardiomyocytes treated with MG-132 and/or Dox, as assessed by immunoprecipitation assay, suggesting Dox-induced degradation through ubiquitin-independent proteasome pathway. Importantly, inhibition of NFAT5 with overexpression of dominant-negative NFAT5 decreased cell viability and increased creatine kinase leakage into culture medium. Consistently, small interfering RNA targeting NFAT5 gene enhanced myocyte death. These findings suggest that Dox promoted the degradation of NFAT5 protein, reducing cell viability in cardiomyocytes. This is the first demonstration that NFAT5 is a positive regulator of cardiomyocyte survival.

Regulation of nucleocytoplasmic trafficking of transcription factor OREBP/TonEBP/NFAT5

The osmotic response element-binding protein (OREBP), also known as tonicity enhancer-binding protein (TonEBP) or NFAT5, regulates the hypertonicity-induced expression of a battery of genes crucial for the adaptation of mammalian cells to extracellular hypertonic stress. The activity of OREBP/TonEBP is regulated at multiple levels, including nucleocytoplasmic trafficking. OREBP/TonEBP protein can be detected in both the cytoplasm and nucleus under isotonic conditions, although it accumulates exclusively in the nucleus or cytoplasm when subjected to hypertonic or hypotonic challenges, respectively. Using immunocytochemistry and green fluorescent protein fusions, the protein domains that determine its subcellular localization were identified and characterized. We found that OREBP/TonEBP nuclear import is regulated by a nuclear localization signal. However, under isotonic conditions, nuclear export of OREBP/TonEBP is mediated by a CRM1-dependent, leucine-rich canonical nuclear export sequence (NES) located in the N terminus. Disruption of NES by site-directed mutagenesis yielded a mutant OREBP/TonEBP protein that accumulated in the nucleus under isotonic conditions but remained a target for hypotonicity-induced nuclear export. More importantly, a putative auxiliary export domain distal to the NES was identified. Disruption of the auxiliary export domain alone is sufficient to abolish the nuclear export of OREBP/TonEBP induced by hypotonicity. By using bimolecular fluorescence complementation assay, we showed that CRM1 interacts with OREBP/TonEBP, but not with a mutant protein deficient in NES. Our findings provide insight into how nucleocytoplasmic trafficking of OREBP/TonEBP is regulated by changes in extracellular tonicity.

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

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

Phosphatidylinositol 3-kinase mediates activation of ATM by high NaCl and by ionizing radiation: Role in osmoprotective transcriptional regulation

High NaCl causes DNA double-strand breaks and activates the transcription factor, TonEBP/OREBP, resulting in increased transcription of several protective genes, including those involved in accumulation of compatible organic osmolytes. Several kinases are known to contribute to signaling activation of TonEBP/OREBP, including ATM, which is a member of the phosphatidylinositol 3-kinase (PI3K)-like kinase family and is activated by DNA double-strand breaks. The purpose of the present studies was to investigate a possible role of PI3K Class IA (PI3K-IA). We found that high NaCl increases PI3K-IA lipid kinase activity. Inhibiting PI3K-IA either by expressing a dominant negative of its regulatory subunit, p85, or by small interfering RNA-mediated knockdown of its catalytic subunit, p110alpha, reduces high NaCl-induced increases in TonEBP/OREBP transcriptional activity and transactivation, but not nuclear translocation of TonEBP/OREBP, or increases in its abundance. Further, suppression of PI3K-IA inhibits the activation of ATM that is caused by either ionizing radiation or high NaCl. High NaCl-induced increase in TonEBP/OREBP activity is reduced equally by inhibition of ATM or PI3K-IA, and the effects are not additive. The conclusions are as follows: (i) PI3K-IA activity is necessary for both high NaCl- and ionizing radiation-induced activation of ATM and (ii) high NaCl activates PI3K-IA, which, in turn, contributes to full activation of TonEBP/OREBP via ATM.

MEKK3-mediated signaling to p38 kinase and TonE in hypertonically stressed kidney cells

Mitogen-activated protein kinase (MAPK) cascades contain a trio of kinases, MAPK kinase kinase (MKKK) --> MAPK kinase (MKK) --> MAPK, that mediate a variety of cellular responses to different signals including hypertonicity. The signaling response to hypertonicity is conserved across evolution from yeast to mammals in that it involves activation of p38/SAPK. However, very little is known about which upstream protein kinases mediate activation of p38 by hypertonicity in mammals. The MKKKs, MEKK3 and MEKK4, are upstream regulators of p38 in many cells. To investigate these signaling proteins as potential activators of p38 in the hypertonicity response, we generated stably transfected MDCK cells that express activated versions of MEKK3 or MEKK4, utilized RNA interference to deplete MEKK3, and employed pharmacological inhibition of p38 kinase. MEKK3-transfected cells demonstrated increased betaine transporter (BGT1) mRNA levels and upregulated tonicity enhancer (TonE)-driven luciferase activity under isotonic (basal) and hypertonic conditions compared with empty vector-transfected controls; small-interference RNA-mediated depletion of MEKK3 downregulated the activity of p38 kinase and decreased the expression of BGT1 mRNA. p38 Kinase inhibition abolished the effects of MEKK3 activation on BGT1 induction. In contrast, the response to hypertonicity in MEKK4-kA-transfected cells was similar to that observed in empty vector-transfected controls. Our data are consistent with the existence of an input from MEKK3 -->--> p38 kinase -->--> TonE.

Osmotic stress and the cytoskeleton: the R(h)ole of Rho GTPases

Hyperosmotic stress initiates a variety of compensatory and adaptive responses, which either serve to restore near-normal volume or remodel and reinforce the cell structure to withstand the physical challenge. The latter response is brought about by the reorganization of the cytoskeleton; however, the underlying mechanisms are not well understood. Recent research has provided major breakthroughs in our knowledge about the link between message and structure, i.e. between signalling and cytoskeletal remodelling, predominantly in the context of cell migration. The major components of this progress are the in-depth characterization of Rho family small GTPases, master regulators of the cytoskeleton, and the discovery of the actin-related protein 2/3 complex, a signalling-sensitive structural element of the actin polymerization machinery. The primary aim of this review is to find the place of these novel and crucial players in osmotically induced (volume-dependent) remodelling of the cytoskeleton. We aim to address three questions: (1) What are the major structural changes in the cytoskeleton under hyperosmotic conditions? (2) Are the Rho family small GTPases (Rho, Rac and Cdc42) regulated by osmotic stress, and if so, by what mechanisms? (3) Are Rho GTPases involved, as mediators, in major adaptive responses, including cytoskeleton rearrangement, changes in ion transport and genetic reprogramming? Our answers will show how fragmentary our current knowledge is in these areas. Therefore, this overview has been written with the hardly disguised intention that it might foster further research in this field by highlighting some intriguing questions.

How tonicity regulates genes: story of TonEBP transcriptional activator

TonEBP stimulates genes whose products drive cellular accumulation of organic osmolytes and HSP70, which protect cells from the deleterious effects of hypertonicity and urea, respectively. Mice deficient in the TonEBP gene display severe atrophy of the renal medulla because cells failed to adapt to the hyperosmolality. Emerging data suggest that TonEBP plays a key role in the urinary concentrating mechanism by stimulating the UT-A urea transporters and possibly AQP2 water channel. Thus, TonEBP is an essential regulator in the urinary concentrating mechanism. Studies on structural basis of TonEBP function have revealed the structure of the DNA binding domain, and defined the transactivation domains. Molecular mechanisms underlying the nucleocytoplasmic trafficking, transactivation, and phosphorylation in response to changes in tonicity need to be understood in molecular detail. Such knowledge is needed for the identification of the sensor that detects changes in ambient tonicity and signals to TonEBP.

Elevated activity of transcription factor nuclear factor of activated T-cells 5 (NFAT5) and diabetic nephropathy

The expression of aldose reductase is tightly regulated by the transcription factor tonicity response element binding protein (TonEBP/NFAT5) binding to three osmotic response elements (OREs; OREA, OREB, and OREC) in the gene. The aim was to investigate the contribution of NFAT5 to the pathogenesis of diabetic nephropathy. Peripheral blood mononuclear cells (PBMCs) were isolated from the following subjects: 44 Caucasoid patients with type 1 diabetes, of whom 26 had nephropathy and 18 had no nephropathy after a diabetes duration of 20 years, and 13 normal healthy control subjects. In addition, human mesangial cells (HMCs) were isolated from the normal lobe of 10 kidneys following radical nephrectomy for renal cell carcinoma. Nuclear and cytoplasmic proteins were extracted from PBMCs and HMCs and cultured in either normal or high-glucose (31 mmol/l D-glucose) conditions for 5 days. NFAT5 binding activity was quantitated using electrophoretic mobility shift assays for each of the OREs. Western blotting was used to measure aldose reductase and sorbitol dehydrogenase protein levels. There were significant fold increases in DNA binding activities of NFAT5 to OREB (2.06 +/- 0.03 vs. 1.33 +/- 0.18, P = 0.033) and OREC (1.94 +/- 0.21 vs. 1.39 +/- 0.11, P = 0.024) in PBMCs from patients with diabetic nephropathy compared with diabetic control subjects cultured under high glucose. Aldose reductase and sorbitol dehydrogenase protein levels in the patients with diabetic nephropathy were significantly increased in PBMCs cultured in high-glucose conditions. In HMCs cultured under high glucose, there were significant increases in NFAT5 binding activities to OREA, OREB, and OREC by 1.38 +/- 0.22-, 1.84 +/- 0.44-, and 2.38 +/- 1.15-fold, respectively. Similar results were found in HMCs exposed to high glucose (aldose reductase 1.30 +/- 0.06-fold and sorbitol dehydrogenease 1.54 +/- 0.24-fold increases). Finally, the silencing of the NFAT5 gene in vitro reduced the expression of the aldose reductase gene. In conclusion, these results show that aldose reductase is upregulated by the transcriptional factor NFAT5 under high-glucose conditions in both PBMCs and HMCs.

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

Hypertonicity activates the transcription factor tonicity-responsive enhancer/osmotic response element binding protein (TonEBP/OREBP), resulting in increased expression of genes involved in osmoprotective accumulation of organic osmolytes, including glycine betaine, and in increased expression of osmoprotective heat shock proteins. Our previous studies showed that high NaCl increases reactive oxygen species (ROS), which contribute to activation of TonEBP/OREBP. Mitochondria are a major source of ROS. The purpose of the present study was to examine whether mitochondria produce the ROS that contribute to activation of TonEBP/OREBP. We inhibited mitochondrial ROS production in HEK293 cells with rotenone and myxothiazol, which inhibit mitochondrial complexes I and III, respectively. Rotenone (250 nM) and myxothiazol (12 nM) reduce high NaCl-induced ROS over 40%, whereas apocynin (100 microM), an inhibitor of NADPH oxidase, and allopurinol (100 microM), an inhibitor of xanthine oxidase, have no significant effect. Rotenone and myxothiazol reduce high NaCl-induced increases in TonEBP/OREBP transcriptional activity (ORE/TonE reporter assay) and BGT1 (betaine transporter) mRNA abundance ranging from 53 to 69%. They inhibit high NaCl-induced TonEBP/OREBP transactivating activity, but not its nuclear translocation. Release of ATP into the medium on hypertonic stress has been proposed to be a signal that triggers cellular osmotic responses. However, we do not detect release of ATP into the medium or inhibition of high NaCl-induced ORE/TonE reporter activity by an ATPase, apyrase (20 U/ml), indicating that high NaCl-induced activation of TonEBP/OREBP is not mediated by release of ATP. We conclude that high NaCl increases mitochondrial ROS production, which contributes to the activation of TonEBP/OREBP by increasing its transactivating activity.

Hypertonic Induction of COX-2 in Collecting Duct Cells by Reactive Oxygen Species of Mitochondrial Origin

Our previous studies have documented MAPK mediation of the hypertonicity-induced stimulation of COX-2 expression in cultured renal medullary epithelial cells. The present study extends this observation by examining the role of reactive oxygen species (ROSs). ROS levels, determined using dichlorodihydrofluorescence diacetate and cytochrome c, were rapidly and significantly increased following exposure of mIMCD-K2 cells to media made hypertonic by adding NaCl. Hypertonic treatment (550 mosmol/kg) for 16 h induced a 5.6-fold increase in COX-2 protein levels and comparable increases in prostaglandin E(2) release, both of which were completely abolished by the NADPH oxidase inhibitor diphenyleneiodonium (25-50 microM). The general antioxidant N-acetyl-l-cysteine (6 mM), and the superoxide dismutase mimetic TEMPO (2.0 mm) reduced COX-2 levels by 75.6 and 79.8%, respectively. Exposure of mIMCD-K2 cells to exogenous O(2)(-.) generated by the xanthine/xanthine oxidase system mimicked the effect of hypertonicity on COX-2 expression and prostaglandin E(2) release. The increases in phosphorylation of ERK1/2 and p38 were detected 20 min following the hypertonic treatment and were both prevented by N-acetyl-l-cysteine. The increases in ROSs in response to hypertonic treatment were completely blocked by any one of the mitochondrial inhibitors tested, such as rotenone, thenoyltrifluoroacetone, or carbonyl cyanide m-chlorophenylhydrazone, associated with remarkable inhibition of COX-2 expression. In contrast, the increases in ROSs were not significantly altered in IMCD cells deficient in either gp91(phox) or p47(phox), nor were the increases in COX-2 expression. We conclude that ROSs derived from mitochondria, but not NADPH oxidase, mediate the hypertonicity-induced phosphorylation of MAPK and the stimulation of COX-2 expression.

Hyperosmotic mannitol induces Src kinase-dependent phosphorylation of beta-catenin in cerebral endothelial cells

Mannitol, which is a cell-impermeable and nontoxic polyalcohol, has been shown to be a useful tool for reversible opening of the blood-brain barrier (BBB). Despite successful clinical trials, the molecular mechanism of the mannitol-induced changes in cerebral endothelial cells (CECs) are poorly understood. For our experiments, we used CECs in culture, which were treated with different, clinically relevant concentrations of mannitol. We found that mannitol induced a rapid, concentration-dependent, and reversible tyrosine phosphorylation of a broad range of proteins between 50 and 190 kDa. One of the targets of tyrosine phosphorylation turned out to be the adherens junction protein beta-catenin. Phosphorylation of beta-catenin on tyrosine residues caused its subcellular redistribution and its dissociation from cadherin and alpha-catenin as shown by coimmunoprecipitation studies. All these effects could be inhibited by the Src kinase inhibitor PP-1 but not by the Erk inhibitor U0126, the Rho kinase inhibitor Y27632, or the calcium channel blocker verapamil. Because beta-catenin is a key component of the junctional complex, its Src-mediated phpsphorylation may play an important role in the mannitol induced reversible opening of the BBB.