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)

NFAT5 binds to the TNF promoter distinctly from NFATp, c, 3 and 4, and activates TNF transcription during hypertonic stress alone

Tumor necrosis factor (TNF) is a pro-inflammatory cytokine that plays an important role in a variety of infectious and autoimmune disorders. Its transcription is regulated in a stimulus- and cell-type-specific manner via the recruitment of distinct DNA/activator complexes forming secondary structures or enhanceosomes. NFATp, a member of the nuclear factor of activated T cells (NFAT) family of transcription factors, plays a critical role in TNF gene regulation under a variety of conditions. In this study, we show that NFAT5, the most recently described NFAT family member, binds to the TNF promoter in a manner distinct from other NFAT proteins and is a key mediator in the activation of TNF gene transcription during hypertonic stress alone.

DNA damage and osmotic regulation in the kidney

Renal medullary cells normally are exposed to extraordinarily high interstitial NaCl concentration as part of the urinary concentrating mechanism, yet they survive and function. Acute elevation of NaCl to a moderate level causes transient cell cycle arrest in culture. Higher levels of NaCl, within the range found in the inner medulla, cause apoptosis. Recently, it was surprising to discover that even moderately high levels of NaCl cause DNA double-strand breaks. The DNA breaks persist in cultured cells that are proliferating rapidly after adaptation to high NaCl, and DNA breaks normally are present in the renal inner medulla in vivo. High NaCl inhibits repair of broken DNA both in culture and in vivo, but the DNA is rapidly repaired if the level of NaCl is reduced. The inhibition of DNA repair is associated with suppressed activity of some DNA damage-response proteins like Mre11, Chk1, and H2AX but not that of others, like GADD45, p53, ataxia telangiectasia-mutated kinase (ATM), and Ku86. In this review, we consider possible mechanisms by which the renal cells escape the known dangerous consequences of persistent DNA damage. Furthermore, we consider that the persistent DNA damage may be a sensor of hypertonicity that activates ATM kinase to provide a signal that contributes to protective osmotic regulation.

TRPV4 and the mammalian kidney

Transient receptor potential vanilloid 4 (TRPV4) was identified as the mammalian homologue of the Caenorhabditis elegans osmosensory channel protein, OSM-9. In mammals, TRPV4 is activated by a variety of stimuli including thermal stress, fatty acid metabolites, and hypotonicity. Two distinct mechanisms have been described through which TRPV4 may be activated by hypotonicity: one involves the Src family of nonreceptor protein tyrosine kinases, whereas a second is mediated via arachidonic acid metabolites. TRPV4 likely plays a role in systemic osmoregulation; accordingly, it is expressed in the blood-brain barrier-deficient osmosensory nuclei of the hypothalamus. TRPV4 is also abundantly expressed in the kidney, and its precisely demarcated distribution along the kidney tubule permits speculation about a physiological role in this tissue. TRPV4-expressing and TRPV4-negative tubule segments co-exist at all levels of the kidney, from the cortex through the inner medulla. It is conceivable that basolaterally expressed TRPV4 transmits signals arising in the interstitium (e.g, changing tonicity) to more-distal tubule segments where "fine-tuning" of the incipient urine takes place.

Intracellular water homeostasis and the mammalian cellular osmotic stress response

The cellular response to osmotic stress ensures that the concentration of water inside the cell is maintained within a range that is compatible with biologic function. Single cell organisms are particularly dependent on mechanisms that permit adaptation to osmotic stress because each individual cell is directly exposed to the external environment. Mammals, however, limit osmotic stress by establishing an internal aqueous environment in which intravascular water and electrolytes are subject to sensitive and dynamic, organism-based homeostatic regulation. Recent studies of NFAT5/TonEBP, an essential mammalian osmoregulatory transcription factor, demonstrate the unexpected yet critical significance of cell-based osmotic regulation in vivo. These results highlight the fundamental importance of maintaining intracellular water homeostasis in the face of varying cellular metabolic activity and distinct tissue microenvironments.

Up-regulation of Human myo-Inositol Oxygenase by Hyperosmotic Stress in Renal Proximal Tubular Epithelial Cells

myo-Inositol oxygenase (MIOX) catalyzes the oxidative cleavage of myo-inositol (MI) to give d-glucuronic acid, a committed step in MI catabolism. d-Glucuronic acid is further metabolized to xylitol via the glucuronate-xylulose pathway. Although accumulation of polyols such as xylitol and sorbitol is associated with MI depletion in diabetic complications, no causal relationship has been established. Therefore we are examining the role of MIOX in diabetic nephropathy. Here we present evidence that the basis for the depletion of MI in diabetes is likely to be mediated by the increased expression of MIOX, which is induced by sorbitol, mannitol, and xylitol in a porcine renal proximal tubular epithelial cell line, LLC-PK1. To understand the molecular mechanism of regulation of MIOX expression by polyols, we have cloned the human MIOX gene locus of 10 kb containing 5.6 kb of the 5' upstream sequence. Analysis of the 5' upstream sequence led to the identification of an osmotic response element (ORE) in the promoter region, which is present approximately 2 kb upstream of the translation start site. Based on luciferase reporter and electrophoretic mobility shift assays, polyols increased the ORE-dependent expression of MIOX. In addition, we demonstrate that the activity of the promoter is dependent on the binding of the transcription factor, tonicity element-binding protein, or osmotic response element-binding protein, to the ORE site. These results suggest that the expression of MIOX is up-regulated by a positive feedback mechanism where xylitol, one of the products of MI catabolism via the glucuronate-xylulose pathway, induces an overexpression of MIOX.

Osmotic regulation of renal betaine transport: transcription and beyond

Cells in the kidney inner medulla are routinely exposed to high extracellular osmolarity during normal operation of the urinary concentrating mechanism. One adaptation critical for survival in this environment is the intracellular accumulation of organic osmolytes to balance the osmotic stress. Betaine is an important osmolyte that is accumulated via the betaine/gamma-aminobutyric acid transporter (BGT1) in the basolateral plasma membrane of medullary epithelial cells. In response to hypertonic stress, there is transcriptional activation of the BGT1 gene, followed by trafficking and membrane insertion of BGT1 protein. Transcriptional activation, triggered by changes in ionic strength and water content, is an early response that is a key regulatory step and has been studied in detail. Recent studies suggest there are additional post-transcriptional regulatory steps in the pathway leading to upregulation of BGT1 transport, and that additional proteins are required for membrane insertion. Reversal of this adaptive process, upon removal of hypertonic stress, involves a rapid efflux of betaine through specific release pathways, a reduction in betaine influx, and a slower downregulation of BGT1 protein abundance. There is much more to be learned about many of these steps in BGT1 regulation.

Ataxia telangiectasia-mutated, a DNA damage-inducible kinase, contributes to high NaCl-induced nuclear localization of transcription factor TonEBP/OREBP

High NaCl activates the transcription factor tonicity-responsive enhancer/osmotic response element binding protein (TonEBP/OREBP) by increasing its abundance and transactivation, the latter signaled by a variety of protein kinases. In addition, high NaCl causes TonEBP/OREBP to translocate into the nucleus, but little is known about the signals directing this translocation. The result is increased transcription of protective genes, including those involved in accumulation of organic osmolytes. High NaCl also damages DNA, and DNA damage activates ataxia telangiectasia-mutated (ATM) kinase through autophosphorylation on serine 1981. We previously found that ATM is involved in the high NaCl-induced increase in TonEBP/OREBP transactivation. The purpose of the present studies was to test whether ATM is also involved in high NaCl-induced TonEBP/OREBP nuclear translocation. We quantified TonEBP/OREBP in nuclear and cytoplasmic extracts from cultured cells by Western blot analysis. In COS-7 cells, wortmannin, an inhibitor of ATM, reduces high NaCl-induced nuclear translocation of TonEBP/OREBP. We used AT cells (in which ATM is inactive) to test the specificity of this effect. Nuclear translocation of native TonEBP/OREBP and of its recombinant NH2-terminal rel homology domain, which contains the nuclear localization signal, is reduced in AT cells and is restored when the cells are reconstituted with functional ATM. In conclusion, activation of ATM contributes to high NaCl-induced nuclear translocation of TonEBP/OREBP.

SRC family kinases in cell volume regulation

SRC family kinases are a group of nine cytoplasmic protein tyrosine kinases essential for many cell functions. Some appear to be ubiquitously expressed, whereas others are highly tissue specific. The ability of members of the SRC family to influence ion transport has been recognized for several years. Mounting evidence suggests a broad role for SRC family kinases in the cell response to both hypertonic and hypotonic stress, and in the ensuing regulatory volume increase or decrease. In addition, members of this tyrosine kinase family participate in the mechanotransduction that accompanies cell membrane deformation. Finally, at least one SRC family member operates in concert with the p38 MAPK to regulate tonicity-dependent gene transcription.

CELL SURVIVAL IN THE HOSTILE ENVIRONMENT OF THE RENAL MEDULLA

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

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

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

MAP kinases and the adaptive response to hypertonicity: functional preservation from yeast to mammals

The adaptation to hypertonicity in mammalian cells is driven by multiple signaling pathways that include p38 kinase, Fyn, the catalytic subunit of PKA, ATM, and JNK2. In addition to the well-characterized tonicity enhancer (TonE)-TonE binding protein interaction, other transcription factors (and their respective cis elements) can potentially respond to hypertonicity. This review summarizes the current knowledge about the signaling pathways that regulate the adaptive response to osmotic stress and discusses new insights from yeast that could be relevant to the osmostress response in mammals.

Stem cell antigen-1 is necessary for cell-cycle withdrawal and myoblast differentiation in C2C12 cells

Extracellular signaling pathways regulating myoblast differentiation and cell-cycle withdrawal are not completely understood. Stem cell antigen-1 (Sca-1/Ly-6A/E) is a glycosylphosphatidylinositol-anchored membrane protein known for its role in T-cell activation, and recently described as a marker for regeneration-competent myoblasts. We previously determined that expression of Sca-1/Ly-6A is transiently upregulated during myocyte cell-cycle withdrawal; however, a specific function for Sca-1 in myogenesis has not been described. Here, we show that Sca-1 expression on the surface of a subpopulation of differentiating C2C12 myoblasts is maximal at the time of cell-cycle withdrawal, and that blocking Sca-1 with monoclonal antibodies or downregulating Sca-1 expression by antisense both promotes proliferation and inhibits myotube formation. Downregulating Sca-1 expression derepresses Fyn at the time of myoblast cell-cycle withdrawal, and dominant-negative and constitutively active Fyn mutants rescue and recapitulate the Sca-1 antisense phenotype, respectively. This suggests a Fyn-mediated mechanism for Sca-1 action. Thus, we demonstrate an unprecedented role for Sca-1 in early myogenesis in C2C12 cells, and propose a novel pathway from the myoblast cell surface to intracellular signaling networks controlling proliferation versus differentiation in mammalian muscle. These findings suggest that, beyond its role as a marker for muscle progenitors, Sca-1 may be an important therapeutic target for promoting muscle regeneration.

ATM, a DNA damage-inducible kinase, contributes to activation by high NaCl of the transcription factor TonEBP/OREBP

High NaCl activates the transcription factor tonicity-responsive enhancer/osmotic response element-binding protein (TonEBP/OREBP), resulting in increased transcription of several protective genes, including the glycine betaine/gamma-aminobutyric acid transporter (BGT1). High NaCl damages DNA, and DNA damage activates ataxia telangiectasia mutated (ATM) kinase through autophosphorylation on Ser-1981. TonEBP/OREBP contains ATM consensus phosphorylation sites at Ser-1197, Ser-1247, and Ser-1367. The present studies test whether ATM is involved in activation of TonEBP/OREBP by high NaCl. We find that raising osmolality from 300 to 500 mosmol/kg by adding NaCl activates ATM, as indicated by phosphorylation at Ser-1981. High urea and radiation also activate ATM, but they do not increase TonEBP/OREBP transcriptional activity like high NaCl does. Wortmannin, which inhibits ATM, reduces NaCl-induced TonEBP/OREBP transcriptional activation and BGT1 mRNA increase. Overexpression of wild-type TonEBP/OREBP increases ORE/TonE reporter activity much more than does overexpression of TonEBP/OREBP S1197A, S1247A, or S1367A. In AT cells (which express nonfunctional ATM), TonEBP/OREBP transcriptional and transactivating activity are further increased by expression of wild-type ATM but not of S1981A ATM. TonEBP/OREBP reciprocally coimmunoprecipitates with ATM kinase, demonstrating physical association. Additionally, antibody to ATM kinase supershifts TonEBP/OREBP bound to its cognate ORE/TonE DNA element. In AT cells, wortmannin further decreases high NaCl-induced increase in transcriptional activity, consistent with participation of signaling kinase(s) in addition to ATM. In conclusion, signaling via ATM is necessary for full activation of TonEBP/OREBP by high NaCl, but it is not sufficient.

Multiple domains of TonEBP cooperate to stimulate transcription in response to hypertonicity

Tonicity-responsive enhancer binding protein (TonEBP), also known as NFAT5, belongs to the Rel family of transcriptional activators. In the kidney medulla and thymus, TonEBP plays a major role in protecting renal cells and T cells from the deleterious effects of ambient hypertonicity. TonEBP is stimulated by hypertonicity via several pathways: increased expression of protein, nuclear translocation, and increased transactivation. In this study, we identified five domains of TonEBP involved in transactivation. The two conserved glutamine repeats were not involved in transactivation. There were three activation domains that could stimulate transcription independently. In addition, there were two modulation domains that potentiated the activity of the activation domains. One of the activation domains is unique to a splice isoform that is more active than others, indicating that alternative splicing can affect the activity of TonEBP. Another activation domain and one of the modulation domains were stimulated by hypertonicity. All the five domains acted in synergy in every combination. Although overall phosphorylation of TonEBP increased in response to hypertonicity, phosphorylation of the activation and modulation domains did not increase in isolation. In sum, TonEBP possesses far more elaborate domains involved in transactivation compared with other Rel proteins.

Fyn is required for oxidative- and hyperosmotic-stress-induced tyrosine phosphorylation of caveolin-1

Caveolin-1 is phosphorylated on Tyr(14) in response to both oxidative and hyperosmotic stress. In the present paper, we show that this phosphorylation requires activation of the Src family kinase Fyn. Stress-induced caveolin phosphorylation was abolished by three Src kinase inhibitors, SU6656, PP2 and PD180970, and was not observed in fibroblasts derived from a Src, Yes and Fyn triple-knockout mouse (SYF-/-). Using cell lines derived from single-kinase-knockout mice (Src-/-, Yes-/- and Fyn-/-), we show that expression of Fyn, but not Src or Yes, is required for stress-induced caveolin phosphorylation. Heterologous expression of Fyn in the SYF-/- and Fyn-/- cells was sufficient to reconstitute stress-induced caveolin phosphorylation, and overexpression of Fyn in wild-type cells induced hyperphosphorylation of caveolin. Fyn was autophosphorylated following oxidative stress, verifying activation of this kinase. Interestingly, there was a concomitant increase in the phosphorylation of Fyn on its Csk (C-terminal Src kinase) site, indicating feedback inhibition. Csk binds to phosphocaveolin [Cao, Courchesne and Mastick (2002) J. Biol. Chem. 277, 8771-8774] and should phosphorylate any co-localized Src-family kinases. Oxidative-stress-induced phosphorylation of caveolin-1 also requires expression of Abl [Sanguinetti and Mastick (2003) Cell Signal. 15, 289-298]. Using inhibitors and cells derived from knockout mice, we verified a requirement for both Abl and Fyn in stress-induced caveolin phosphorylation in a single cell type. Our data suggest a novel mechanism for attenuation of Src-kinase activity by Abl: stable tyrosine phosphorylation of a scaffolding protein, caveolin, and recruitment of Csk. Paxillin, a substrate of both Abl and Src, organizes a similar regulatory complex.

Hyperosmotic stress activates Rho: differential involvement in Rho kinase-dependent MLC phosphorylation and NKCC activation

Hyperosmotic stress initiates adaptive responses, including phosphorylation of myosin light chain (MLC) and concomitant activation of Na+-K+-Cl- cotransporter (NKCC). Because the small GTPase Rho is a key regulator of MLC phosphorylation, we investigated 1) whether Rho is activated by hyperosmotic stress, and if so, what the triggering factors are, and 2) whether the Rho/Rho kinase (ROK) pathway is involved in MLC phosphorylation and NKCC activation. Rho activity was measured in tubular epithelial cells by affinity pulldown assay. Hyperosmolarity induced rapid (<1 min) and sustained (>20 min) Rho activation that was proportional to the osmotic concentration and reversed within minutes upon restoration of isotonicity. Both decreased cell volume at constant ionic strength and elevated total ionic strength at constant cell volume were capable of activating Rho. Changes in [Na+] and [K+] at normal total salinity failed to activate Rho, and Cl- depletion did not affect the hyperosmotic response. Thus alterations in cellular volume and ionic strength but not individual ion concentrations seem to be the critical triggering factors. Hyperosmolarity induced mono- and diphosphorylation of MLC, which was abrogated by the Rho-family blocker Clostridium toxin B. ROK inhibitor Y-27632 suppressed MLC phosphorylation under isotonic conditions and prevented its rise over isotonic levels in hypertonically stimulated cells. ML-7 had a smaller inhibitory effect. In contrast, it abolished the hypertonic activation of NKCC, whereas Y-27632 failed to inhibit this response. Thus hyperosmolarity activates Rho, and Rho/ROK pathway contributes to basal and hyperosmotic MLC phosphorylation. However, the hypertonic activation of NKCC is ROK independent, implying that the ROK-dependent component of MLC phosphorylation can be uncoupled from NKCC activation.

Hyperosmotic-induced protein kinase N 1 activation in a vesicular compartment is dependent upon Rac1 and 3-phosphoinositide-dependent kinase 1

Protein kinase N 1 (PKN1), which in part resembles yeast protein kinase C, has been shown to be under the control of Rho GTPases and 3-phosphoinositide-dependent kinase 1 (PDK1). We show here that green fluorescent protein-tagged PKN1 has the ability to translocate in a reversible manner to a vesicular compartment following hyperosmotic stress. PKN1 kinase activity is not necessary for this translocation, and in fact the PKN inhibitor HA1077 is also shown to induce PKN1 vesicle accumulation. PKN1 translocation is dependent on Rac1 activation, although the GTPase binding HR1abc domain is not sufficient for this recruitment. The PKN1 kinase domain, however, localizes constitutively to this compartment, and we demonstrate that this behavior is selective for PKNs. Associated with vesicle recruitment, PKN1 is shown to undergo activation loop phosphorylation and activation. It is established that this activation pathway involves PDK1, which is shown to be recruited to this PKN1-positive compartment upon hyperosmotic stress. Taken together, our findings present a pathway for the selective hyperosmotic-induced Rac1-dependent PKN1 translocation and PDK1-dependent activation.

The role of NFAT5/TonEBP in establishing an optimal intracellular environment

NFAT5/TonEBP, the most recently described member of the rel/NFkappaB/NFAT family of signal-dependent transcription factors, is activated by extracellular hypertonicity-a cellular stress of particular and perhaps unique physiologic relevance to cells of the renal medulla. Accumulating evidence suggests that NFAT5/TonEBP also functions in vivo under isotonic conditions as part of a ubiquitous regulatory mechanism that senses and adjusts available intracellular volume during cell growth to establish an intracellular environment appropriate for optimal cell proliferation.

cAMP-independent role of PKA in tonicity-induced transactivation of tonicity-responsive enhancer/ osmotic response element-binding protein

Hypertonicity-induced increase in activity of the transcription factor tonicity-responsive enhancer/osmotic response element-binding protein (TonEBP/OREBP) protects renal cells by increasing transcription of genes, including those involved in increased accumulation of organic osmolytes. We previously showed that hypertonicity increases transactivating activity of TonEBP/OREBP. Assay with a binary GAL4 transactivation system showed that the 984 C-terminal amino acids of TonEBP/OREBP (amino acids 548-1531) contain a tonicity-dependent transactivation domain (TAD). Also, amino acids 548-1531 undergo tonicity-dependent phosphorylation, and some inhibitors of protein kinases reduce the tonicity-dependent transactivation. In the present studies we examined the role of protein kinase A (PKA).

RESULTS

(i) An inhibitor of PKA (H89) reduces tonicity-dependent increases in transactivation, ORE/TonE reporter activity, and induction of aldose reductase and betaine transporter mRNAs. (ii) Overexpression of the catalytic subunit of PKA (PKAc) increases transactivation activity of amino acids 548-1531 and activity of an ORE/TonE reporter. The increases are much greater under isotonic than under hypertonic conditions. (iii) A dominant-negative PKAc reduces activity of an ORE/TonE reporter. (iv) PKAc activity increases with tonicity but cAMP does not. (v) TonEBP/OREBP and PKAc coimmunoprecipitate. (vi) amino acids 872-1271, including N- and C-terminal polyglutamine stretches, demonstrate tonicity-dependent transactivation, albeit less than amino acids 548-1531, and a similar role for PKA.

CONCLUSIONS

(i) PKA plays an important role in TonEBP/OREBP activation of tonicity-dependent gene expression; (ii) PKA activation of TonEBP/OREBP appears to be cAMP-independent; and (iii) amino acids 872-1271 are sufficient for tonicity-dependent transactivation of TonEBP/OREBP.