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

Regulation of Microtubule Stability in Pulmonary Microvascular Endothelial Cells in Rats with Severe Acute Pancreatitis: Qingyi Decoction is a Potential CDK5 Inhibitor

Purpose

Explore the therapeutic effects and regulatory mechanism of Qingyi Decoction (QYD) on severe acute pancreatitis (SAP) associated acute lung injury (ALI).

Methods

We identified the constituents absorbed into the blood of QYD based on a network pharmacological strategy. The differentially expressed genes from the GEO database were screened to identify the critical targets of QYD treatment of SAP-ALI. The SAP-ALI rat model was constructed.Some methods were used to evaluate the efficacy and mechanism of QYD in treating SAP-ALI. LPS-stimulated pulmonary microvascular endothelial cell injury simulated the SAP-induced pulmonary endothelial injury model. We further observed the therapeutic effect of QYD and CDK5 plasmid transfection on endothelial cell injury.

Results

18 constituents were absorbed into the blood, and 764 targets were identified from QYD, 25 of which were considered core targets for treating SAP-ALI. CDK5 was identified as the most critical gene. The results of differential expression analysis showed that the mRNA expression level of CDK5 in the blood of SAP patients was significantly up-regulated compared with that of healthy people. Animal experiments have demonstrated that QYD can alleviate pancreatic and lung injury inflammatory response and reduce the upregulation of CDK5 in lung tissue. QYD or CDK5 inhibitors could decrease the expression of NFAT5 and GEF-H1, and increase the expression of ACE-tub in SAP rat lung tissue. Cell experiments proved that QYD could inhibit the expression of TNF-α and IL-6 induced by LPS. Immunofluorescence results suggested that QYD could alleviate the cytoskeleton damage of endothelial cells, and the mechanism might be related to the inhibition of CDK5-mediated activation of NFAT5, GEF-H1, and ACE-tub.

Conclusion

CDK5 has been identified as a critical target for pulmonary endothelial injury of SAP-ALI. QYD may partially alleviate microtubule disassembly by targeting the CDK5/NFAT5/GEF-H1 signaling pathway, thus relieving SAP-induced pulmonary microvascular endothelial cell injury.

Unconventional tonicity-regulated nuclear trafficking of NFAT5 mediated by KPNB1, XPOT and RUVBL2

NFAT5 is the only known mammalian tonicity-responsive transcription factor with essential role in cellular adaptation to hypertonic stress. It is also implicated in diverse physiological and pathological processes. NFAT5 activity is tightly regulated by extracellular tonicity, but the underlying mechanisms remain elusive. We demonstrated that NFAT5 enters the nucleus via the nuclear pore complex. We found that NFAT5 utilizes a unique nuclear localization signal (NFAT5-NLS) for nuclear import. siRNA screening revealed that only karyopherin β1 (KPNB1), but not karyopherin alpha, is responsible for the nuclear import of NFAT5 via direct interaction with the NFAT5-NLS. Proteomics analysis and siRNA screening further revealed that nuclear export of NFAT5 under hypotonicity is driven by Exportin-T, where the process requires RuvB-Like AAA type ATPase 2 (RUVBL2) as an indispensable chaperone. Our findings have identified an unconventional tonicity-dependent nucleocytoplasmic trafficking pathway for NFAT5, a critical step in orchestrating rapid cellular adaptation to change in extracellular tonicity. These findings offer an opportunity for the development of novel NFAT5 targeting strategies that are potentially useful for the treatment of diseases associated with NFAT5 dysregulation.

The prolyl-isomerase PIN1 is essential for nuclear Lamin-B structure and function and protects heterochromatin under mechanical stress

Chromatin organization plays a crucial role in tissue homeostasis. Heterochromatin relaxation and consequent unscheduled mobilization of transposable elements (TEs) are emerging as key contributors of aging and aging-related pathologies, including Alzheimer's disease (AD) and cancer. However, the mechanisms governing heterochromatin maintenance or its relaxation in pathological conditions remain poorly understood. Here we show that PIN1, the only phosphorylation-specific cis/trans prolyl isomerase, whose loss is associated with premature aging and AD, is essential to preserve heterochromatin. We demonstrate that this PIN1 function is conserved from Drosophila to humans and prevents TE mobilization-dependent neurodegeneration and cognitive defects. Mechanistically, PIN1 maintains nuclear type-B Lamin structure and anchoring function for heterochromatin protein 1α (HP1α). This mechanism prevents nuclear envelope alterations and heterochromatin relaxation under mechanical stress, which is a key contributor to aging-related pathologies.

NFAT5-Mediated Signalling Pathways in Viral Infection and Cardiovascular Dysfunction

The nuclear factor of activated T cells 5 (NFAT5) is well known for its sensitivity to cellular osmolarity changes, such as in the kidney medulla. Accumulated evidence indicates that NFAT5 is also a sensitive factor to stress signals caused by non-hypertonic stimuli such as heat shock, biomechanical stretch stress, ischaemia, infection, etc. These osmolality-related and -unrelated stimuli can induce NFAT5 upregulation, activation and nuclear accumulation, leading to its protective role against various detrimental effects. However, dysregulation of NFAT5 expression may cause pathological conditions in different tissues, leading to a variety of diseases. These protective or pathogenic effects of NFAT5 are dictated by the regulation of its target gene expression and activation of its signalling pathways. Recent studies have found a number of kinases that participate in the phosphorylation/activation of NFAT5 and related signal proteins. Thus, this review will focus on the NFAT5-mediated signal transduction pathways. As for the stimuli that upregulate NFAT5, in addition to the stresses caused by hyperosmotic and non-hyperosmotic environments, other factors such as miRNA, long non-coding RNA, epigenetic modification and viral infection also play an important role in regulating NFAT5 expression; thus, the discussion in this regard is another focus of this review. As the heart, unlike the kidneys, is not normally exposed to hypertonic environments, studies on NFAT5-mediated cardiovascular diseases are just emerging and rapidly progressing. Therefore, we have also added a review on the progress made in this field of research.

The Role of CDK5 in Tumours and Tumour Microenvironments

Cyclin-dependent kinase 5 (CDK5), which belongs to the protein kinase family, regulates neuronal function but is also associated with cancer development and has been proposed as a target for cancer treatment. Indeed, CDK5 has roles in cell proliferation, apoptosis, angiogenesis, inflammation, and immune response. Aberrant CDK5 activation triggers tumour progression in numerous types of cancer. In this review, we summarise the role of CDK5 in cancer and neurons and CDK5 inhibitors. We expect that our review helps researchers to develop CDK5 inhibitors as treatments for refractory cancer.

The evolving role of TonEBP as an immunometabolic stress protein

Tonicity-responsive enhancer-binding protein (TonEBP), which is also known as nuclear factor of activated T cells 5 (NFAT5), was discovered 20 years ago as a transcriptional regulator of the cellular response to hypertonic (hyperosmotic salinity) stress in the renal medulla. Numerous studies since then have revealed that TonEBP is a pleiotropic stress protein that is involved in a range of immunometabolic diseases. Some of the single-nucleotide polymorphisms (SNPs) in TONEBP introns are cis-expression quantitative trait loci that affect TONEBP transcription. These SNPs are associated with increased risk of type 2 diabetes mellitus, diabetic nephropathy, inflammation, high blood pressure and abnormal plasma osmolality, indicating that variation in TONEBP expression might contribute to these phenotypes. In addition, functional studies have shown that TonEBP is involved in the pathogenesis of rheumatoid arthritis, atherosclerosis, diabetic nephropathy, acute kidney injury, hyperlipidaemia and insulin resistance, autoimmune diseases (including type 1 diabetes mellitus and multiple sclerosis), salt-sensitive hypertension and hepatocellular carcinoma. These pathological activities of TonEBP are in contrast to the protective actions of TonEBP in response to hypertonicity, bacterial infection and DNA damage induced by genotoxins. An emerging theme is that TonEBP is a stress protein that mediates the cellular response to a range of pathological insults, including excess caloric intake, inflammation and oxidative stress.

Potential Role of Gene Regulator NFAT5 in the Pathogenesis of Diabetes Mellitus

Nuclear factor of activated T cells 5 (NFAT5), a Rel/nuclear factor- (NF-) κB family member, is the only known gene regulator of the mammalian adaptive response to osmotic stress. Exposure to elevated glucose increases the expression and nuclear translocation of NFAT5, as well as NFAT5-driven transcriptional activity in vivo and in vitro. Increased expression of NFAT5 is closely correlated with the progression of diabetes in patients. The distinct structure of NFAT5 governs its physiological and pathogenic roles, indicating its opposing functions. The ability of NFAT5 to maintain cell homeostasis and proliferation is impaired in patients with diabetes. NFAT5 promotes the formation of aldose reductase, pathogenesis of diabetic vascular complications, and insulin resistance. Additionally, NFAT5 activates inflammation at a very early stage of diabetes and induces persistent inflammation. Recent studies revealed that NFAT5 is an effective therapeutic target for diabetes. Here, we describe the current knowledge about NFAT5 and its relationship with diabetes, focusing on its diverse regulatory functions, and highlight the importance of this protein as a potential therapeutic target in patients with diabetes.

Regulation of Inflammatory Functions of Macrophages and T Lymphocytes by NFAT5

The transcription factor NFAT5, also known as TonEBP, belongs to the family of Rel homology domain-containing factors, which comprises the NF-κB proteins and the calcineurin-dependent NFAT1 to NFAT4. NFAT5 shares several structural and functional features with other Rel-family factors, for instance it recognizes DNA elements with the same core sequence as those bound by NFAT1 to 4, and like NF-κB it responds to Toll-like receptors (TLR) and activates macrophage responses to microbial products. On the other hand, NFAT5 is quite unique among Rel-family factors as it can be activated by hyperosmotic stress caused by elevated concentrations of extracellular sodium ions. NFAT5 regulates specific genes but also others that are inducible by NF-κB and NFAT1 to 4. The ability of NFAT5 to do so in response to hypertonicity, microbial products, and inflammatory stimuli may extend the capabilities of immune cells to mount effective anti-pathogen responses in diverse microenvironment and signaling conditions. Recent studies identifying osmostress-dependent and -independent functions of NFAT5 have broadened our understanding of how NFAT5 may modulate immune function. In this review we focus on the role of NFAT5 in macrophages and T cells in different contexts, discussing findings from in vivo mouse models of NFAT5 deficiency and reviewing current knowledge on its mechanisms of regulation. Finally, we propose several questions for future research.

NFAT5 Has a Job in the Brain

Nuclear factor of activated T cells 5 (NFAT5) has recently been classified as a new member of the Rel family. In addition, there are 5 more well-defined members (NF-κB and NFAT1-4) in the Rel family, which participate in regulating the expression of immune and inflammatory response-related genes. NFAT5 was initially identified in renal medullary cells where it regulated the expression of osmoprotective-related genes during the osmotic response. Many studies have demonstrated that NFAT5 is highly expressed in the nuclei of neurons in fetal and adult brains. Additionally, its expression is approximately 10-fold higher in fetal brains. With the development of detection technologies (laser scanning confocal microscopy, transgene technology, etc.), recent studies suggest that NFAT5 is also expressed in glial cells and plays a more diverse functional role. This article aims to summarize the current knowledge regarding the expression of NFAT5, its regulation of activation, and varied biological functions in the brain.

Localization and Expression of Nuclear Factor of Activated T-Cells 5 in Myoblasts Exposed to Pro-inflammatory Cytokines or Hyperosmolar Stress and in Biopsies from Myositis Patients

Aims: Regeneration in skeletal muscle relies on regulated myoblast migration and differentiation, in which the transcription factor nuclear factor of activated T-cells 5 (NFAT5) participates. Impaired muscle regeneration and chronic inflammation are prevalent in myositis. Little is known about the impact of inflammation on NFAT5 localization and expression in this group of diseases. The goal of this study was to investigate NFAT5 physiology in unaffected myoblasts exposed to cytokine or hyperosmolar stress and in myositis. Methods: NFAT5 intracellular localization and expression were studied in vitro using a cell culture model of myositis. Myoblasts were exposed to DMEM solutions enriched with pro-inflammatory cytokines IFN-γ with IL-1β or hyperosmolar DMEM obtained by NaCl supplementation. NFAT5 localization was visualized using immunohistochemistry (IHC) and Western blotting (WB) in fractionated cell lysates. NFAT5 expression was assessed by WB and RT-qPCR. In vivo localization and expression of NFAT5 were studied in muscle biopsies of patients diagnosed with polymyositis (n = 6), dermatomyositis (n = 10), inclusion body myositis (n = 11) and were compared to NFAT5 localization and expression in non-myopathic controls (n = 13). Muscle biopsies were studied by means of quantitative IHC and WB of total protein extracts. Results: In unaffected myoblasts, hyperosmolar stress ensues in NFAT5 nuclear translocation and increased NFAT5 mRNA and protein expression. In contrast, pro-inflammatory cytokines did not lead to NFAT5 nuclear translocation nor increased expression. Cytokines IL-1β with IFN-γ induced colocalization of NFAT5 with histone deacetylase 6 (HDAC6), involved in cell motility. In muscle biopsies from dermatomyositis and polymyositis patients, NFAT5 colocalized with HDAC6, while in IBM, this was often absent. Conclusions: Our data suggest impaired NFAT5 localization and expression in unaffected myoblasts in response to inflammation. This disturbed myogenic NFAT5 physiology could possibly explain deleterious effects on muscle regeneration in myositis.

Early protection to stress mediated by CDK-dependent PI3,5P2 signaling from the vacuole/lysosome

Adaptation to stress is a critical strategy for all life. Known strategies involve signaling pathways that induce changes in gene expression. These changes take time and cannot protect against acute assaults. Jin et al. reveal an early stress protection pathway regulated by the vacuole/lysosome.

Adaptation to environmental stress is critical for cell survival. Adaptation generally occurs via changes in transcription and translation. However, there is a time lag before changes in gene expression, which suggests that more rapid mechanisms likely exist. In this study, we show that in yeast, the cyclin-dependent kinase Pho85/CDK5 provides protection against hyperosmotic stress and acts before long-term adaptation provided by Hog1. This protection requires the vacuolar/endolysosomal signaling lipid PI3,5P₂. We show that Pho85/CDK5 directly phosphorylates and positively regulates the PI3P-5 kinase Fab1/PIKfyve complex and provide evidence that this regulation is conserved in mammalian cells. Moreover, this regulation is particularly crucial in yeast for the stress-induced transient elevation of PI3,5P₂. Our study reveals a rapid protection mechanism regulated by Pho85/CDK5 via signaling from the vacuole/lysosome, which is distinct temporally and spatially from the previously discovered long-term adaptation Hog1 pathway, which signals from the nucleus.

Peptide affinity analysis of proteins that bind to an unstructured region containing the transactivating domain of the osmoprotective transcription factor NFAT5

NFAT5 is a transcription factor originally identified because it is activated by hypertonicity and that activation increases expression of genes that protect against the adverse effects of the hypertonicity. However, its targets also include genes not obviously related to tonicity. The transactivating domain of NFAT5 is contained in its COOH-terminal region, which is predicted to be unstructured. Unstructured regions are common in transcription factors particularly in transactivating domains where they can bind co-regulatory proteins essential to their function. To identify potential binding partners of NFAT5 from either cytoplasmic or nuclear HEK293 cell extracts, we used peptide affinity chromatography followed by mass spectrometry. Peptide aptamer-baits consisted of overlapping 20 amino acid peptides within the predicted COOH-terminal unstructured region of NFAT5. We identify a total of 351 unique protein preys that associate with at least one COOH-terminal peptide bait from NFAT5 in either cytoplasmic or nuclear extracts from cells incubated at various tonicities (NaCl varied). In addition to finding many proteins already known to associate with NFAT5, we found many new ones whose function suggest novel aspects of NFAT5 regulation, interaction, and function. Relatively few of the proteins pulled down by peptide baits from NFAT5 are generally involved in transcription, and most, therefore, are likely to be specifically related to the regulation of NFAT5 or its function. The novel associated proteins are involved with cancer, effects of hypertonicity on chromatin, development, splicing of mRNA, transcription, and vesicle trafficking.

Peptide affinity analysis of proteins that bind to an unstructured NH2-terminal region of the osmoprotective transcription factor NFAT5

NFAT5 is an osmoregulated transcription factor that particularly increases expression of genes involved in protection against hypertonicity. Transcription factors often contain unstructured regions that bind co-regulatory proteins that are crucial for their function. The NH2-terminal region of NFAT5 contains regions predicted to be intrinsically disordered. We used peptide aptamer-based affinity chromatography coupled with mass spectrometry to identify protein preys pulled down by one or more overlapping 20 amino acid peptide baits within a predicted NH2-terminal unstructured region of NFAT5. We identify a total of 467 unique protein preys that associate with at least one NH2-terminal peptide bait from NFAT5 in either cytoplasmic or nuclear extracts from HEK293 cells treated with elevated, normal, or reduced NaCl concentrations. Different sets of proteins are pulled down from nuclear vs. cytoplasmic extracts. We used GeneCards to ascertain known functions of the protein preys. The protein preys include many that were previously known, but also many novel ones. Consideration of the novel ones suggests many aspects of NFAT5 regulation, interaction and function that were not previously appreciated, for example, hypertonicity inhibits NFAT5 by sumoylating it and the NFAT5 protein preys include components of the CHTOP complex that desumoylate proteins, an action that should contribute to activation of NFAT5.

How do kinases contribute to tonicity-dependent regulation of the transcription factor NFAT5?

NFAT5 plays a critical role in maintaining the renal functions. Its dis-regulation in the kidney leads to or is associated with certain renal diseases or disorders, most notably the urinary concentration defect. Hypertonicity, which the kidney medulla is normally exposed to, activates NFAT5 through phosphorylation of a signaling molecule or NFAT5 itself. Hypotonicity inhibits NFAT5 through a similar mechanism. More than a dozen of protein and lipid kinases have been identified to contribute to tonicity-dependent regulation of NFAT5. Hypertonicity activates NFAT5 by increasing its nuclear localization and transactivating activity in the early phase and protein abundance in the late phase. The known mechanism for inhibition of NFAT5 by hypotonicity is a decrease of nuclear NFAT5. The present article reviews the effect of each kinase on NFAT5 nuclear localization, transactivation and protein abundance, and the relationship among these kinases, if known. Cyclosporine A and tacrolimus suppress immune reactions by inhibiting the phosphatase calcineurin-dependent activation of NFAT1. It is hoped that this review would stimulate the interest to seek explanations from the NFAT5 regulatory pathways for certain clinical presentations and to explore novel therapeutic approaches based on the pathways. On the basic science front, this review raises two interesting questions. The first one is how these kinases can specifically signal to NFAT5 in the context of hypertonicity or hypotonicity, because they also regulate other cellular activities and even opposite activities in some cases. The second one is why these many kinases, some of which might have redundant functions, are needed to regulate NFAT5 activity. This review reiterates the concept of signaling through cooperation. Cells need these kinases working in a coordinated way to provide the signaling specificity that is lacking in the individual one. Redundancy in regulation of NFAT5 is a critical strategy for cells to maintain robustness against hypertonic or hypotonic stress.

TonEBP modulates the protective effect of taurine in ischemia-induced cytotoxicity in cardiomyocytes

Taurine, which is found at high concentration in the heart, exerts several protective actions on myocardium. Physically, the high level of taurine in heart is maintained by a taurine transporter (TauT), the expression of which is suppressed under ischemic insult. Although taurine supplementation upregulates TauT expression, elevates the intracellular taurine content and ameliorates the ischemic injury of cardiomyocytes (CMs), little is known about the regulatory mechanisms of taurine governing TauT expression under ischemia. In this study, we describe the TonE (tonicity-responsive element)/TonEBP (TonE-binding protein) pathway involved in the taurine-regulated TauT expression in ischemic CMs. Taurine inhibited the ubiquitin-dependent proteasomal degradation of TonEBP, promoted the translocation of TonEBP into the nucleus, enhanced TauT promoter activity and finally upregulated TauT expression in CMs. In addition, we observed that TonEBP had an anti-apoptotic and anti-oxidative role in CMs under ischemia. Moreover, the protective effects of taurine on myocardial ischemia were TonEBP dependent. Collectively, our findings suggest that TonEBP is a core molecule in the protective mechanism of taurine in CMs under ischemic insult.

Extracellular osmolarity regulates matrix homeostasis in the intervertebral disc and articular cartilage: evolving role of TonEBP

Degeneration of the intervertebral disc is characterized by changes in proteoglycan status, loss of bound water molecules, decreased tissue osmotic pressure and a resulting mechanical failure of the disc. A similar spectrum of changes is evident in osteoarthritic articular cartilage. When healthy, resident cells in these skeletal tissues respond to applied mechanical loads by regulating their own osmotic state and the hydration of the extracellular matrix. The transcription factor Tonicity-Responsive Enhancer Binding Protein (TonEBP or NFAT5) is known to mediate the osmoadaptive response in these and other tissues. While the molecular basis of how osmotic loading controls matrix homeostasis is not completely understood, TonEBP regulates the expression of aggrecan and β1,3-glucoronosyltransferase in nucleus pulposus cells, in addition to targets that allow for survival under hypertonic stress. Moreover, in chondrocytes, TonEBP controls expression of several collagen subtypes and Sox9, a master regulator of aggrecan and collagen II expression. Thus, TonEBP-mediated regulation of the matrix composition allows disc cells and chondrocytes to modify the extracellular osmotic state itself. On the other hand, TonEBP in immune cells induces expression of TNF-α, IL-6 and MCP-1, pro-inflammatory molecules closely linked to matrix catabolism and pathogenesis of both disc degeneration and osteoarthritis, warranting investigations of this aspect of TonEBP function in skeletal cells. In summary, the TonEBP system, through its effects on extracellular matrix and osmoregulatory genes can be viewed primarily as a protective or homeostatic response to physiological loading.

Silencing of the glycerophosphocholine phosphodiesterase GDPD5 alters the phospholipid metabolite profile in a breast cancer model in vivo as monitored by (31) P MRS

Abnormal choline phospholipid metabolism is an emerging hallmark of cancer, which is implicated in carcinogenesis and tumor progression. The malignant metabolic phenotype is characterized by high levels of phosphocholine (PC) and relatively low levels of glycerophosphocholine (GPC) in aggressive breast cancer cells. Phosphorus ((31) P) MRS is able to non-invasively detect these water-soluble metabolites of choline as well as ethanolamine phospholipid metabolism. Here we have investigated the effects of stably silencing glycerophosphoester diesterase domain containing 5 (GDPD5), which is an enzyme with glycerophosphocholine phosphodiesterase activity, in MDA-MB-231 breast cancer cells and orthotopic tumor xenografts. Tumors in which GDPD5 was stably silenced with GDPD5-specific shRNA contained increased levels of GPC and phosphoethanolamine (PE) compared with control tumors.

14-3-3-β and -{varepsilon} contribute to activation of the osmoprotective transcription factor NFAT5 by increasing its protein abundance and its transactivating activity

Having previously found that high NaCl causes rapid exit of 14‐3‐3 isoforms from the nucleus, we used siRNA‐mediated knockdown to test whether 14‐3‐3s contribute to the high NaCl‐induced increase in the activity of the osmoprotective transcription factor NFAT5. We find that, when NaCl is elevated, knockdown of 14‐3‐3‐β and/or 14‐3‐3‐ε decreases NFAT5 transcriptional activity, as assayed both by luciferase reporter and by the mRNA abundance of the NFAT5 target genes aldose reductase and the sodium‐ and chloride‐dependent betaine transporter, BGT1. Knockdown of other 14‐3‐3 isoforms does not significantly affect NFAT5 activity. 14‐3‐3‐β and/or 14‐3‐3‐ε do not act by affecting the nuclear localization of NFAT5, but by at least two other mechanisms: (1) 14‐3‐3‐β and 14‐3‐3‐ε increase protein abundance of NFAT5 and (2) they increase NFAT5 transactivating activity. When NaCl is elevated, knockdown of 14‐3‐3‐β and/or 14‐3‐3‐ε reduces the protein abundance of NFAT5, as measured by Western blot, without affecting the level of NFAT5 mRNA, and the knockdown also decreases NFAT5 transactivating activity, as measured by luciferase reporter. The 14‐3‐3s increase NFAT5 protein, not by increasing its translation, but by decreasing the rate at which it is degraded, as measured by cycloheximide chase. It is not clear at this point whether the 14‐3‐3s affect NFAT5 directly or indirectly through their effects on other proteins that signal activation of NFAT5.

e12000

When NaCl is elevated, knockdown of 14‐3‐3‐β and/or 14‐3‐3‐ε reduces the protein abundance of NFAT5, as measured by Western blot, without affecting the level of NFAT5 mRNA, and the knockdown also decreases NFAT5 transactivating activity, as measured by luciferase reporter. The 14‐3‐3s increase NFAT5 protein, not by increasing its translation, but by decreasing the rate at which it is degraded, as measured by cycloheximide chase.

Rubbing salt into wounded endothelium: sodium potentiates proatherogenic effects of TNF-α under non-uniform shear stress

Increased consumption of sodium is a risk factor for hypertension and cardiovascular diseases. In vivo studies indicated that high dietary sodium may have a direct negative influence on endothelium. We investigated the effects of high sodium on the endothelial activation during early steps of atherogenesis. Endothelial cells (HUVECs) grown in a model of arterial bifurcations were exposed to shear stress in the presence of normal or high (+ 30 mmol/l) sodium. Adherent THP-1 cells, and the adhesion molecule expression were quantified. Sodium channel blockers, pathways' inhibitors, and siRNA against tonicity-responsive enhancer binding protein (TonEBP) were used to identify the mechanisms of sodium effects on endothelium. ApoE-deficient mice on low-fat diet received water containing normal or high salt (8% w/v) for four weeks, and the influence of dietary salt on inflammatory cell adhesion in the common carotid artery and carotid bifurcation was measured by intravital microscopy. In vitro, high sodium dramatically increased the endothelial responsiveness to tumour necrosis factor-α under non-uniform shear stress. Sodium-induced increase in monocytic cell adhesion was mediated by reactive oxygen species and the endothelial nitric oxygen synthase, and was sensitive to the knockdown of TonEBP. The results were subsequently confirmed in the ApoE-deficient mice. As compared with normal-salt group, high-salt intake significantly enhanced the adhesion of circulating CD11b+ cells to carotid bifurcations, but not to the straight segment of common carotid artery. In conclusion, elevated sodium has a direct effect on endothelial activation under atherogenic shear stress in vitro and in vivo, and promotes the endothelial-leukocyte interactions even in the absence of increased lipid concentrations.

High NaCl-induced inhibition of PTG contributes to activation of NFAT5 through attenuation of the negative effect of SHP-1

Activation of the transcription factor NFAT5 by high NaCl involves changes in phosphorylation. By siRNA screening, we previously found that protein targeting to glycogen (PTG), a regulatory subunit of protein phosphatase1 (PP1), contributes to regulation of high NaCl-induced NFAT5 transcriptional activity. The present study addresses the mechanism involved. We find that high NaCl-induced inhibition of PTG elevates NFAT5 activity by increasing NFAT5 transactivating activity, protein abundance, and nuclear localization. PTG acts via a catalytic subunit PP1γ. PTG associates physically with PP1γ, and NaCl reduces both this association and remaining PTG-associated PP1γ activity. High NaCl-induced phosphorylation of p38, ERK, and SHP-1 contributes to activation of NFAT5. Knockdown of PTG does not affect phosphorylation of p38 or ERK. However, PTG and PP1γ bind to SHP-1, and knockdown of either PTG or PP1γ increases high NaCl-induced phosphorylation of SHP-1-S591, which inhibits SHP-1. Mutation of SHP-1-S591 to alanine, which cannot be phosphorylated, increases inhibition of NFAT5 by SHP-1. Thus high NaCl reduces the stimulatory effect of PTG and PP1γ on SHP-1, which in turn reduces the inhibitory effect of SHP-1 on NFAT5. Our findings add to the known functions of PTG, which was previously recognized only for its glycogenic activity.

NFAT5 in cellular adaptation to hypertonic stress - regulations and functional significance

The Nuclear Factor of Activated T Cells-5 (NFAT5), also known as OREBP or TonEBP, is a member of the nuclear factors of the activated T cells family of transcription factors. It is also the only known tonicity-regulated transcription factor in mammals. NFAT5 was initially known for its role in the hypertonic kidney inner medulla for orchestrating a genetic program to restore the cellular homeostasis. Emerging evidence, however, suggests that NFAT5 might play a more diverse functional role, including a pivotal role in blood pressure regulation and the development of autoimmune diseases. Despite the growing significance of NFAT5 in physiology and diseases, our understanding of how its activity is regulated remains very limited. Furthermore, how changes in tonicities are converted into functional outputs via NFAT5 remains elusive. Therefore, this review aims to summarize our current knowledge on the functional roles of NFAT5 in osmotic stress adaptation and the signaling pathways that regulate its activity.

Inhibitory phosphorylation of GSK-3β by AKT, PKA, and PI3K contributes to high NaCl-induced activation of the transcription factor NFAT5 (TonEBP/OREBP)

High NaCl activates the transcription factor nuclear factor of activated T cells 5 (NFAT5), leading to increased transcription of osmoprotective target genes. Kinases PKA, PI3K, AKT1, and p38α were known to contribute to the high NaCl-induced increase of NFAT5 activity. We now identify another kinase, GSK-3β. siRNA-mediated knock-down of GSK-3β increases NFAT5 transcriptional and transactivating activities without affecting high NaCl-induced nuclear localization of NFAT5 or NFAT5 protein expression. High NaCl increases phosphorylation of GSK-3β-S9, which inhibits GSK-3β. In GSK-3β-null mouse embryonic fibroblasts transfection of GSK-3β, in which serine 9 is mutated to alanine, so that it cannot be inhibited by phosphorylation at that site, inhibits high NaCl-induced NFAT5 transcriptional activity more than transfection of wild-type GSK-3β. High NaCl-induced phosphorylation of GSK-3β-S9 depends on PKA, PI3K, and AKT, but not p38α. Overexpression of PKA catalytic subunit α or of catalytically active AKT1 reduces inhibition of NFAT5 by GSK-3β, but overexpression of p38α together with its catalytically active upstream kinase, MKK6, does not. Thus, GSK-3β normally inhibits NFAT5 by suppressing its transactivating activity. When activated by high NaCl, PKA, PI3K, and AKT1, but not p38α, increase phosphorylation of GSK-3β-S9, which reduces the inhibitory effect of GSK-3β on NFAT5, and thus contributes to activation of NFAT5.

Fat-specific protein 27 modulates nuclear factor of activated T cells 5 and the cellular response to stress

Fat-specific protein 27 (FSP27), a member of the cell death-inducing DNA fragmentation factor α-like effector (Cide) family, is highly expressed in adipose tissues and is a lipid droplet (LD)-associated protein that induces the accumulation of LDs. Using a yeast two-hybrid system to examine potential interactions of FSP27 with other proteins, a direct interaction with the N-terminal region of nuclear factor of activated T cells 5 (NFAT5) was identified. NFAT5 is a transcription factor that induces osmoprotective and inflammatory genes after its translocation to the nucleus. The interaction between FSP27 and NFAT5 was confirmed by bimolecular fluorescence complementation and coimmunoprecipitation. Using immunocytochemistry, NFAT5 is detected in the cytoplasm and in the nucleus under isotonic conditions; however, overexpression of FSP27 inhibited the hypertonic-induced nuclear translocation of NFAT5. Consistent with the suppression of NFAT5 nuclear translocation, in cells transfected with a reporter construct containing the NFAT5 response element from the monocyte chemoattractant protein 1 (MCP1) promoter, FSP27 overexpression repressed hypertonic-induced luciferase activity and the expression of NFAT5 target genes. Knockdown of FSP27 in differentiated 3T3-L1 adipocytes increased the NFAT5-mediated rise in MCP1. These results suggest that FSP27 not only modulates LD homeostasis but also modulates the response to osmotic stress via a physical interaction with NFAT5 at the LD surface.

Mutations that reduce its specific DNA binding inhibit high NaCl-induced nuclear localization of the osmoprotective transcription factor NFAT5

The transcription factor nuclear factor of activated T cell 5 (NFAT5) is activated by the stress of hypertonicity (e.g., high NaCl). Increased expression of NFAT5 target genes causes accumulation of protective organic osmolytes and heat shock proteins. Under normotonic conditions (∼300 mosmol/kgH(2)O), NFAT5 is distributed between the nucleus and cytoplasm, hypertonicity causes it to translocate into the nucleus, and hypotonicity causes it to translocate into the cytoplasm. The mechanism of translocation is complex and not completely understood. NFAT5-T298 is a known contact site of NFAT5 with its specific DNA element [osmotic response element (ORE)]. In the present study, we find that mutation of NFAT5-T298 to alanine or aspartic acid not only reduces binding of NFAT5 to OREs (EMSA) but also proportionately reduces high NaCl-induced nuclear translocation of NFAT5. Combined mutation of other NFAT5 DNA contact sites (R293A/E299A/R302A) also greatly reduces both specific DNA binding and nuclear localization of NFAT5. NFAT5-T298 is a potential phosphorylation site, but, using protein mass spectrometry, we do not find phosphorylation at NFAT5-T298. Further, decreased high NaCl-induced nuclear localization of NFAT5 mutated at T298 does not involve previously known regulatory mechanisms, including hypotonicity-induced export of NFAT5, regulated by phosphorylation of NFAT5-S155, XPO1 (CRM1/exportin1)-mediated export of NFAT5 from the nucleus, or hypertonicity-induced elevation of NUP88, which enhances nuclear localization of NFAT5. We conclude that specific DNA binding of NFAT5 contributes to its nuclear localization, by mechanisms, as yet undetermined, but independent of ones previously described to regulate NFAT5 distribution.

The role of hyperosmotic stress in inflammation and disease

Hyperosmotic stress is an often overlooked process that potentially contributes to a number of human diseases. Whereas renal hyperosmolarity is a well-studied phenomenon, recent research provides evidence that many non-renal tissues routinely experience hyperosmotic stress that may contribute significantly to disease initiation and progression. Moreover, a growing body of evidence implicates hyperosmotic stress as a potent inflammatory stimulus by triggering proinflammatory cytokine release and inflammation. Under physiological conditions, the urine concentrating mechanism within the inner medullary region of the mammalian kidney exposes cells to high extracellular osmolarity. As such, renal cells have developed many adaptive strategies to compensate for increased osmolarity. Hyperosmotic stress is linked to many maladies, including acute and chronic, as well as local and systemic, inflammatory disorders. Hyperosmolarity triggers cell shrinkage, oxidative stress, protein carbonylation, mitochondrial depolarization, DNA damage, and cell cycle arrest, thus rendering cells susceptible to apoptosis. However, many adaptive mechanisms exist to counter the deleterious effects of hyperosmotic stress, including cytoskeletal rearrangement and up-regulation of antioxidant enzymes, transporters, and heat shock proteins. Osmolyte synthesis is also up-regulated and many of these compounds have been shown to reduce inflammation. The cytoprotective mechanisms and associated regulatory pathways that accompany the renal response to hyperosmolarity are found in many non-renal tissues, suggesting cells are commonly confronted with hyperosmotic conditions. Osmoadaptation allows cells to survive and function under potentially cytotoxic conditions. This review covers the pathological consequences of hyperosmotic stress in relation to disease and emphasizes the importance of considering hyperosmolarity in inflammation and disease progression.

Extraneuronal activities and regulatory mechanisms of the atypical cyclin-dependent kinase Cdk5

Cyclin-dependent kinase, Cdk5, is an atypical but essential member of the Cdk family of proline-directed serine/threonine kinases with no evident role in cell cycle progression. Cdk5 is present in post-mitotic and terminally differentiated neuronal/glial cells and is also known to arrest cell cycle. Also atypical is the activation of Cdk5 by binding of a non-cyclin activator protein, namely, the Cdk5 regulatory proteins Cdk5R1 (p35), truncated Cdk5R1 (p25), or Cdk5R2 (p39). Despite its ubiquitous presence in all cells and tissues, Cdk5 is often referred to as a neuron-specific kinase largely due to the abundant presence of the activator proteins in neuronal cells. Recently, this concept of a canonical neuronal function of Cdk5 has been extended, if not challenged, by the observation of p35 and p39 expression, as well as Cdk5 activity, in multiple non-neuronal cells. Extraneuronal Cdk5 regulates critical biological processes including transcript-selective translation control for regulation of macrophage gene expression, glucose-inducible insulin secretion, hematopoietic cell differentiation, vascular angiogenesis, cell migration, senescence, and wound-healing, among others. Recent advances in the extraneuronal functions of Cdk5 are reviewed and discussed here in the context of their physiological activities and pathophysiological implications with some speculative comments on the endogenous control mechanisms that might "turn on" Cdk5 activity. The potential importance of targeted inhibition of Cdk5 as therapeutic agents against glucotoxicity, diabetes, cardiovascular diseases, and cancer is also discussed.

Going out of the brain: non-nervous system physiological and pathological functions of Cdk5

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that is mostly active in the nervous system, where it regulates several processes such as neuronal migration, actin and microtubule dynamics, axonal guidance, and synaptic plasticity, among other processes. In addition to these known functions, in the past few years, novel roles for Cdk5 outside of the nervous system have been proposed. These include roles in gene transcription, vesicular transport, apoptosis, cell adhesion, and migration in many cell types and tissues such as pancreatic cells, muscle cells, neutrophils, and others. In this review, we will summarize the recently studied non-neuronal functions of Cdk5, with a thorough analysis of the biological consequences of these novel roles.