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

Variations in fluid chemical potential induce fibroblast mechano-response in 3D hydrogels

Mechanical deformation of skin creates variations in fluid chemical potential, leading to local changes in hydrostatic and osmotic pressure, whose effects on mechanobiology remain poorly understood. To study these effects, we investigate the specific influences of hydrostatic and osmotic pressure on primary human dermal fibroblasts in three-dimensional hydrogel culture models. Cyclic hydrostatic pressure and hyperosmotic stress enhanced the percentage of cells expressing the proliferation marker Ki67 in both collagen and PEG-based hydrogels. Osmotic pressure also activated the p38 MAPK stress response pathway and increased the expression of the osmoresponsive genes PRSS35 and NFAT5. When cells were cultured in two-dimension (2D), no change in proliferation was observed with either hydrostatic or osmotic pressure. Furthermore, basal, and osmotic pressure-induced expression of osmoresponsive genes differed in 2D culture versus 3D hydrogels, highlighting the role of dimensionality in skin cell mechanotransduction and stressing the importance of 3D tissue-like models that better replicate in vivo conditions. Overall, these results indicate that fluid chemical potential changes affect dermal fibroblast mechanobiology, which has implications for skin function and for tissue regeneration strategies.

Impact of the -1T>C single-nucleotide polymorphism of the CD40 gene on the development of endothelial dysfunction in a pro-diabetic microenvironment

BACKGROUND AND AIMS

Hyperglycemia reinforces pro-inflammatory conditions that enhance CD40 expression in endothelial cells (EC). Thymine to cytosine transition (-1T > C) in the promoter of the CD40 gene (rs1883832) further increases the abundance of CD40 protein on the EC surface. This study examines potential associations of the -1T > C SNP of the CD40 gene with type 1 (T1D) or type 2 (T2D) diabetes. Moreover, it investigates the impact of a pro-inflammatory diabetic microenvironment on gene expression in human cultured umbilical vein EC (HUVEC) derived from CC- vs. TT-genotype donors.

METHODS

Tetra-ARMS-PCR was used to compare genotype distribution in 252 patients with diabetes. Soluble CD40 ligand (sCD40L) and soluble CD40 receptor (sCD40) plasma levels were monitored using ELISA. RNA-sequencing was performed with sCD40L-stimulated CC- and TT-genotype HUVEC. Quantitative PCR, Western blot, multiplex-sandwich ELISA array, and immunocytochemistry were used to analyse changes in gene expression in these cells.

RESULTS

Homozygosity for the C-allele was associated with a significant 4.3-fold higher odds of developing T2D as compared to individuals homozygous for the T-allele. Inflammation and endothelial-to-mesenchymal transition (EndMT) driving genes were upregulated in CC-genotype but downregulated in TT-genotype HUVEC when exposed to sCD40L. Expression of EndMT markers significantly increased while that of endothelial markers decreased in HUVEC following exposure to hyperglycemia, tumour necrosis factor-α and sCD40L.

CONCLUSIONS

The -1T > C SNP of the CD40 gene is a risk factor for T2D. Depending on the genotype, it differentially affects gene expression in human cultured EC. CC-genotype HUVEC adopt a pro-inflammatory and intermediate EndMT-like phenotype in a pro-diabetic microenvironment.

The NF-κB/FXR/TonEBP pathway protects renal medullary interstitial cells against hypertonic stress

Farnesoid X receptor (FXR), a ligand-activated transcription factor, plays an important role in maintaining water homeostasis by up-regulating aquaporin 2 (AQP2) expression in renal medullary collecting ducts; however, its role in the survival of renal medullary interstitial cells (RMICs) under hypertonic conditions remains unclear. We cultured primary mouse RMICs and found that the FXR was expressed constitutively in RMICs, and that its expression was significantly up-regulated at both mRNA and protein levels by hypertonic stress. Using luciferase and ChIP assays, we found a potential binding site of nuclear factor kappa-B (NF-κB) located in the FXR gene promoter which can be bound and activated by NF-κB. Moreover, hypertonic stress-induced cell death in RMICs was significantly attenuated by FXR activation but worsened by FXR inhibition. Furthermore, FXR increased the expression and nuclear translocation of hypertonicity-induced tonicity-responsive enhance-binding protein (TonEBP), the expressions of its downstream target gene sodium myo-inositol transporter (SMIT), and heat shock protein 70 (HSP70). The present study demonstrates that the NF-κB/FXR/TonEBP pathway protects RMICs against hypertonic stress.

TonEBP: A Key Transcription Factor in Microglia Following Intracerebral Hemorrhage Induced-Neuroinflammation

Transcription factors within microglia contribute to the inflammatory response following intracerebral hemorrhage (ICH). Therefore, we employed bioinformatics screening to identify the potential transcription factor tonicity-responsive enhancer-binding protein (TonEBP) within microglia. Inflammatory stimuli can provoke an elevated expression of TonEBP in microglia. Nevertheless, the expression and function of microglial TonEBP in ICH-induced neuroinflammation remain ambiguous. In our recent research, we discovered that ICH instigated an increased TonEBP in microglia in both human and mouse peri-hematoma brain tissues. Furthermore, our results indicated that TonEBP knockdown mitigates lipopolysaccharide (LPS)-induced inflammation and the activation of NF-κB signaling in microglia. In order to more deeply comprehend the underlying molecular mechanisms of how TonEBP modulates the inflammatory response, we sequenced the transcriptomes of TonEBP-deficient cells and sought potential downstream target genes of TonEBP, such as Pellino-1 (PELI1). PELI has been previously reported to mediate nuclear factor-κB (NF-κB) signaling. Through the utilization of CUT & RUN, a dual-luciferase reporter, and qPCR, we confirmed that TonEBP is the transcription factor of Peli1, binding to the Peli1 promoter. In summary, TonEBP may enhance the LPS-induced inflammation and activation of NF-κB signaling via PELI1.

High Salt Promotes Inflammatory and Fibrotic Response in Peritoneal Cells

Recent studies draw attention to how excessive salt (NaCl) intake induces fibrotic alterations in the peritoneum through sodium accumulation and osmotic events. The aim of our study was to better understand the underlying mechanisms. The effects of additional NaCl were investigated on human primary mesothelial cells (HPMC), human primary peritoneal fibroblasts (HPF), endothelial cells (HUVEC), immune cells (PBMC), as well as ex vivo on peritoneal tissue samples. Our results showed that a high-salt environment and the consequently increased osmolarity increase the production of inflammatory cytokines, profibrotic growth factors, and components of the renin-angiotensin-aldosterone system, including IL1B, IL6, MCP1, TGFB1, PDGFB, CTGF, Renin and Ace both in vitro and ex vivo. We also demonstrated that high salt induces mesenchymal transition by decreasing the expression of epithelial marker CDH1 and increasing the expression of mesenchymal marker ACTA2 and SNAIL1 in HPMCs, HUVECs and peritoneal samples. Furthermore, high salt increased extracellular matrix production in HPFs. We demonstrated that excess Na+ and the consequently increased osmolarity induce a comprehensive profibrotic response in the peritoneal cells, thereby facilitating the development of peritoneal fibrosis.

Alternations of NF-κB Signaling by Natural Compounds in Muscle-Derived Cancers

The NF-κB-signaling pathway plays a crucial role in cancer progression, including muscle-derived cancers such as rhabdomyosarcoma or sarcoma. Several natural compounds have been studied for their ability to alter NF-κB signaling in these types of cancers. This review paper summarizes the current knowledge on the effects of natural compounds, including curcumin, resveratrol, quercetin, epigallocatechin-3-gallate, and berberine, on NF-κB signaling in muscle-derived cancers. These compounds have been shown to inhibit NF-κB signaling in rhabdomyosarcoma cells through various mechanisms, such as inhibiting the activation of the IKK complex and the NF-κB transcription factor. These findings suggest that natural compounds could be potential therapeutic agents for muscle-derived cancers. However, further research is needed to fully understand their mechanisms of action and potential clinical applications.

Inhibiting NFAT5 With KRN2 Mitigates Acute Allograft Rejection in a Murine Heart Transplantation Model

ABSTRACT

Despite advancements in immunosuppressive therapy, acute allograft rejection remains an important challenge for heart transplantation patients. Nuclear factor of activated T-cells 5 (NFAT5), a member of the family of Rel homology domain-containing factors that plays an important role in regulating immune responses of T lymphocytes, may be closely associated with cardiac rejection. KRN2, as a specific inhibitor of NFAT5, is injected intraperitoneally daily starting from day 0 after murine heart transplantation. When compared with saline treatment, KRN2 treatment can improve allograft survival. Histologic examination revealed that the KRN2 treatment group experienced less-severe rejection, and enzyme-linked immunosorbent assay revealed lower levels of inflammatory cytokines in circulating serum. The proportion and number of T-cell subpopulations in the spleens were analyzed by flow cytometry. We found that KRN2 treatment reduced the proportions of CD4 + IFN-γ + , CD4 + IL-17A + , and CD4 + IL-4 + Th cells, whereas increasing CD4 + Foxp3 + Treg cells compared with the control group. These findings suggest that KRN2 attenuates acute allograft rejection by regulating CD4 + T lymphocyte responses. NFAT5 could be a promising therapeutic target for preventing acute allograft rejection.

Interleukin-6 blockade reduces salt-induced cardiac inflammation and fibrosis in subtotal nephrectomized mice

Cardiovascular disease is the most common comorbidity in patients with chronic kidney disease (CKD), affecting both their prognosis and quality of life. Cardiac fibrosis is common in patients with CKD with left ventricular diastolic dysfunction, and it is associated with increased risk of heart failure and mortality. Recent evidence suggests that high salt intake activates immune responses associated with local accumulation of sodium. We reported that high salt intake promotes cardiac inflammation in subtotal nephrectomized (Nx) mice. We investigated the effects of administration of MR16-1, a rat anti-mouse monoclonal interleukin (IL)-6 receptor antibody, in Nx mice with salt loading (Nx-salt). Expression of monocyte chemoattractant protein-1, tumor necrosis factor-α, IL-1β, and IL-6 mRNAs and macrophage infiltration was significantly reduced in the heart of Nx-salt mice treated with MR16-1 (Nx-salt-MR16-1) compared with Nx-salt mice treated with control rat rat IgG1 (Nx-salt-rat IgG1). Correspondingly, cardiac fibrosis was significantly attenuated in Nx-salt-MR16-1 mice compared with Nx-salt-rat IgG1 mice. Furthermore, in the heart of Nx-salt-MR16-1 mice, expression of mRNA for nicotinamide adenine dinucleotide phosphate oxidase-2, an oxidative stress marker, was significantly downregulated compared with Nx-salt-rat IgG1 mice. Increases in cardiac metabolites, including histidine and γ-butyrobetaine, were also reversed by IL-6 blockade treatment. In conclusion, IL-6 blockade exerts anti-inflammatory, antifibrotic, and partial antioxidative effects in the heart of Nx-salt mice.NEW & NOTEWORTHY In the present study, IL-6 blockade exerted anti-inflammatory, antifibrotic, and partial antioxidative effects on the hearts of mice with CKD on a high-salt diet. Therefore, IL-6 potentially mediates cardiac fibrosis induced by high salt intake in patients with CKD, a finding with therapeutic implications. Of note, the next therapeutic implication may simply be the reinforcement of low-salt diets or diuretics and further research on the anti-inflammatory effects of these measures rather than IL-6 blockade with high-salt diet.

Epigenetics and tissue immunity-Translating environmental cues into functional adaptations

There is an increasing appreciation that many innate and adaptive immune cell subsets permanently reside within non-lymphoid organs, playing a critical role in tissue homeostasis and defense. The best characterized are macrophages and tissue-resident T lymphocytes that work in concert with organ structural cells to generate appropriate immune responses and are functionally shaped by organ-specific environmental cues. The interaction of tissue epithelial, endothelial and stromal cells is also required to attract, differentiate, polarize and maintain organ immune cells in their tissue niche. All of these processes require dynamic regulation of cellular transcriptional programmes, with epigenetic mechanisms playing a critical role, including DNA methylation and post-translational histone modifications. A failure to appropriately regulate immune cell transcription inevitably results in inadequate or inappropriate immune responses and organ pathology. Here, with a focus on the mammalian kidney, an organ which generates differing regional environmental cues (including hypersalinity and hypoxia) due to its physiological functions, we will review the basic concepts of tissue immunity, discuss the technologies available to profile epigenetic modifications in tissue immune cells, including those that enable single-cell profiling, and consider how these mechanisms influence the development, phenotype, activation and function of different tissue immune cell subsets, as well as the immunological function of structural cells.

Overcoming the Dependence on Animal Models for Osteoarthritis Therapeutics - The Promises and Prospects of In Vitro Models

Osteoarthritis (OA) is a musculoskeletal disease characterized by progressive degeneration of osteochondral tissues. Current treatment is restricted to the reduction of pain and loss of function of the joint. To better comprehend the OA pathophysiological conditions, several models are employed, however; there is no consensus on a suitable model. In this review, different in vitro models being developed for possible therapeutic intervention of OA are outlined. Herein, various in vitro OA models starting from 2D model, co-culture model, 3D models, dynamic culture model to advanced technologies-based models such as 3D bioprinting, bioassembly, organoids, and organ-on-chip-based models are discussed with their advantages and disadvantages. Besides, different growth factors, cytokines, and chemicals being utilized for induction of OA condition are reviewed in detail. Furthermore, there is focus on scrutinizing different molecular and possible therapeutic targets for better understanding the mechanisms and OA therapeutics. Finally, the underlying challenges associated with in vitro models are discussed followed by future prospective. Taken together, a comprehensive overview of in vitro OA models, factors to induce OA-like conditions, and intricate molecular targets with the potential to develop personalized osteoarthritis therapeutics in the future with clinical translation is provided.

The Role of Nuclear Factor of Activated T Cells 5 in Hyperosmotic Stress-Exposed Human Lens Epithelial Cells

We investigated the role of nuclear factor of activated T cells 5 (NFAT5) under hyperosmotic conditions in human lens epithelial cells (HLECs). Hyperosmotic stress decreased the viability of human lens epithelial B-3 cells and significantly increased NFAT5 expression. Hyperosmotic stress-induced cell death occurred to a greater extent in NFAT5-knockout (KO) cells than in NFAT5 wild-type (NFAT5 WT) cells. Bcl-2 and Bcl-xl expression was down-regulated in NFAT5 WT cells and NFAT5 KO cells under hyperosmotic stress. Pre-treatment with a necroptosis inhibitor (necrostatin-1) significantly blocked hyperosmotic stress-induced death of NFAT5 KO cells, but not of NFAT5 WT cells. The phosphorylation levels of receptor-interacting protein kinase 1 (RIP1) and RIP3, which indicate the occurrence of necroptosis, were up-regulated in NFAT5 KO cells, suggesting that death of these cells is predominantly related to the necroptosis pathway. This finding is the first to report that necroptosis occurs when lens epithelial cells are exposed to hyperosmolar conditions, and that NFAT5 is involved in this process.

NFAT5 Amplifies Antipathogen Responses by Enhancing Chromatin Accessibility, H3K27 Demethylation, and Transcription Factor Recruitment

The ability of innate immune cells to respond to pathogen-associated molecular patterns across a wide range of intensities is fundamental to limit the spreading of infections. Studies on transcription responses to pathogen-activated TLRs have often used relatively high TLR ligand concentrations, and less is known about their regulation under mild stimulatory conditions. We had shown that the transcription factor NFAT5 facilitates expression of antipathogen genes under TLR stimulation conditions corresponding to low pathogen loads. In this study, we analyze how NFAT5 optimizes TLR-activated responses in mouse macrophages. We show that NFAT5 was required for effective recruitment of central effectors p65/NF-κB and c-Fos to specific proinflammatory target genes, such as Nos2, Il6, and Tnf in primary macrophages responding to low doses of the TLR4 ligand LPS. By contrast, NFAT5 was not required for p65/NF-κB recruitment in response to high LPS doses. Using the transposase-accessible chromatin with high-throughput sequencing assay, we show that NFAT5 facilitated chromatin accessibility mainly at promoter regions of multiple TLR4-responsive genes. Analysis of various histone marks that regulate gene expression in response to pathogens identified H3K27me3 demethylation as an early NFAT5-dependent mechanism that facilitates p65 recruitment to promoters of various TLR4-induced genes. Altogether, these results advance our understanding about specific mechanisms that optimize antipathogen responses to limit infections.

Multiscale Regulation of the Intervertebral Disc: Achievements in Experimental, In Silico, and Regenerative Research

Intervertebral disc (IVD) degeneration is a major risk factor of low back pain. It is defined by a progressive loss of the IVD structure and functionality, leading to severe impairments with restricted treatment options due to the highly demanding mechanical exposure of the IVD. Degenerative changes in the IVD usually increase with age but at an accelerated rate in some individuals. To understand the initiation and progression of this disease, it is crucial to identify key top-down and bottom-up regulations' processes, across the cell, tissue, and organ levels, in health and disease. Owing to unremitting investigation of experimental research, the comprehension of detailed cell signaling pathways and their effect on matrix turnover significantly rose. Likewise, in silico research substantially contributed to a holistic understanding of spatiotemporal effects and complex, multifactorial interactions within the IVD. Together with important achievements in the research of biomaterials, manifold promising approaches for regenerative treatment options were presented over the last years. This review provides an integrative analysis of the current knowledge about (1) the multiscale function and regulation of the IVD in health and disease, (2) the possible regenerative strategies, and (3) the in silico models that shall eventually support the development of advanced therapies.

Description of a Novel Mechanism Possibly Explaining the Antiproliferative Properties of Glucocorticoids in Duchenne Muscular Dystrophy Fibroblasts Based on Glucocorticoid Receptor GR and NFAT5

Glucocorticoids are drugs of choice in Duchenne muscular dystrophy (DMD), prolonging patients’ ambulation. Their mode of action at the protein level is not completely understood. In DMD, muscle tissue is replaced by fibrotic tissue produced by fibroblasts, reducing mobility. Nuclear factor of activated T-cells 5 (NFAT5) is involved in fibroblast proliferation. By treating one DMD fibroblast cell culture and one of unaffected skeletal muscle fibroblasts with methylprednisolone (MP) or hydrocortisone (HC) for 24 h or 12 d, the antiproliferative properties of glucocorticoids could be unraveled. NFAT5 localization and expression was explored by immunocytochemistry (ICC), Western blotting (WB) and RT-qPCR. NFAT5 and glucocorticoid receptor (GR) colocalization was measured by ImageJ. GR siRNA was used, evaluating GR’s influence on NFAT5 expression during MP and HC treatment. Cell proliferation was monitored by IncuCyte ZOOM. In DMD fibroblasts, treatment with MP for 24 h induced dots (ICC) positive for NFAT5 and colocalizing with GR. After 12 d of MP or HC in DMD fibroblasts, NFAT5 expression was decreased (RT-qPCR and WB) and growth arrest was observed (Incucyte ZOOM), whereas NFAT5 expression and cell growth remained unchanged in unaffected skeletal muscle fibroblasts. This study may help understand the antiproliferative properties of glucocorticoids in DMD fibroblasts.

Excessive salt intake increases peritoneal solute transport rate via local tonicity-responsive enhancer binding protein in subtotal nephrectomized mice

BACKGROUND

High peritoneal transport is associated with high mortality and technical failure in peritoneal dialysis (PD). Baseline peritoneal solute transport rate (PSTR) as measured by the peritoneal equilibration test (PET) within 6 months after PD initiation varies between patients. Sodium is reported to be stored in the skin or muscle of dialysis patients. This study investigated whether excessive salt intake in uremic mice caused peritoneal alterations without exposure to PD fluid.

METHODS

Sham-operated (Sham) and subtotal nephrectomized (Nx) mice were randomly given tap water or 1% sodium chloride (NaCl)-containing water for 8 weeks. PET was then performed to evaluate peritoneal function. Human mesothelial cell line Met-5A was used for in vitro studies.

RESULTS

We observed higher PSTR in Nx mice with 1% NaCl-containing drinking water (Nx + salt) compared with those with tap water (Nx + water), along with enhanced angiogenesis and inflammation in the peritoneum. Blockade of interleukin (IL)-6 signaling rescued peritoneal transport function in Nx + salt mice. In cultured Met-5A, additional NaCl in the medium upregulated IL-6 as well as vascular endothelial growth factor-A, associated with increased expression and nuclear translocation of tonicity-responsive enhancer binding protein (TonEBP). Knockdown of TonEBP lowered the induction caused by high tonicity. Peritoneal TonEBP expression was higher in Nx + salt mice, while removal of high-salt diet lowered TonEBP level and improved peritoneal transport function.

CONCLUSIONS

Excessive dietary salt intake caused peritoneal membrane functional and structural changes under uremic status. TonEBP regulated hypertonicity-related inflammatory changes and might play a crucial role in high baseline peritoneal transport.

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.

Arp2/3 inactivation causes intervertebral disc and cartilage degeneration with dysregulated TonEBP-mediated osmoadaptation

Extracellular matrix and osmolarity influence the development and homeostasis of skeletal tissues through Rho GTPase-mediated alteration of the actin cytoskeleton. This study investigated whether the actin-branching Arp2/3 complex, a downstream effector of the Rho GTPases Cdc42 and Rac1, plays a critical role in maintaining the health of matrix-rich and osmotically loaded intervertebral discs and cartilage. Mice with constitutive intervertebral disc- and cartilage-specific deletion of the critical Arp2/3 subunit Arpc2 (Col2-Cre; Arpc2fl/fl) developed chondrodysplasia and spinal defects. Since these mice did not survive to adulthood, we generated mice with inducible Arpc2 deletion in disc and cartilage (Acan-CreERT2; Arpc2fl/fl). Inactivation of Arp2/3 at skeletal maturity resulted in growth plate closure, loss of proteoglycan content in articular cartilage, and degenerative changes in the intervertebral disc at 1 year of age. Chondrocytes with Arpc2 deletion showed compromised cell spreading on both collagen and fibronectin. Pharmacological inhibition of Cdc42 and Arp2/3 prevented the osmoadaptive transcription factor TonEBP/NFAT5 from recruiting cofactors in response to a hyperosmolarity challenge. Together, these findings suggest that Arp2/3 plays a critical role in cartilaginous tissues through the regulation of cell-extracellular matrix interactions and modulation of TonEBP-mediated osmoadaptation.

Amelioration of Lipopolysaccharide-Induced Nephrotic Proteinuria by NFAT5 Depletion Involves Suppressed NF-κB Activity

Idiopathic nephrotic syndrome (INS) is characterized by proteinuria, in which podocyte dysfunction associated with NF-κB-mediated inflammation plays an important role. The nuclear factor of activated T cells 5 (NFAT5) has been shown to enhance NF-κB activity. However, whether NFAT5 is associated with proteinuria remains uncharacterized. NFAT5 is upregulated in the glomeruli in lipopolysaccharide (LPS)-induced mouse nephrotic proteinuria, as well as in LPS-treated podocytes in vitro. In addition, NFAT5 depletion improves filtration barrier function of LPS-treated podocytes in vitro. Mechanistically, NFAT5 depletion suppresses NF-κB activation and downstream proinflammatory reaction in LPS-treated podocytes, and moreover, NF-κB inhibition improves filtration barrier function of LPS-treated podocytes, suggesting that the suppressed NF-κB activity at least partly accounts for NFAT5 depletion-improved filtration barrier function. Furthermore, in vivo, depletion of NFAT5 suppresses NF-κB activity and ameliorates nephrotic proteinuria in LPS-treated mice. These findings suggest a protective role of NFAT5 depletion against LPS-induced nephrotic proteinuria and relate it to the suppression of NF-κB activity.

Alpha-Lipoic Acid Ameliorates Radiation-Induced Lacrimal Gland Injury through NFAT5-Dependent Signaling

Dry eye syndrome related to radiation therapy is relatively common and can severely impair a patient’s daily life. The nuclear factor of activated T cells 5(NFAT5) is well known for its osmoprotective effect under hyperosmolar conditions, and it also has immune-modulating functions. We investigated the role of NFAT5 and the protective effect of α-lipoic acid(ALA) on radiation-induced lacrimal gland (LG) injuries. Rats were assigned to control, ALA only, radiation only, and ALA administered prior to irradiation groups. The head and neck area, including the LG, was evenly irradiated with 2 Gy/minute using a photon 6-MV linear accelerator. NFAT5 expression was enhanced and localized in the LG tissue after irradiation and was related to cellular apoptosis. ALA had a protective effect on radiation-induced LG injury through the inhibition of NFAT5 expression and NFAT5-dependent signaling pathways. Functional radiation–induced damage of the LG and cornea was also restored with ALA treatment. NFAT5 expression and its dependent signaling pathways were deeply related to radiation-induced dry eye, and the condition was improved by ALA treatment. Our results suggest a potential role of NFAT5 and NF-κB in the proinflammatory effect in LGs and cornea, which offers a target for new therapies to treat dry eye syndrome.

Tonicity-responsive enhancer binding protein (TonEBP) regulates TNF-α-induced hypothalamic inflammation

Tonicity-responsive enhancer binding protein (TonEBP) is a widely expressed transcription factor and is important in the regulation of inflammatory cytokines. Here, we have identified TonEBP expression in the hypothalamus, which is particularly high in proopiomelanocortin (POMC) neurons. TonEBP overexpression stimulates POMC transcription, and TonEBP haploinsufficiency in TonEBP (+/-) mice results in a decrease in hypothalamic POMC expression. TonEBP (+/-) mice show reduced sickness responses, which include anorexia and hyperthermia, that are initially induced by tumor necrosis factor (TNF)-α. TonEBP (+/-) mice also show lower levels of TNF-α-induced hypothalamic expression of POMC and pro-inflammatory cytokines. These results suggest that TonEBP is an important molecular regulator in the development of inflammatory sickness responses through the control of POMC and pro-inflammatory cytokine expression in the hypothalamus.

TonEBP Suppresses the HO-1 Gene by Blocking Recruitment of Nrf2 to Its Promoter

TonEBP is a key transcriptional activator in macrophages with an M1 phenotype. High expression of TonEBP is associated with many inflammatory diseases. Heme oxygenase-1 (HO-1), a stress-inducible protein, is induced by various oxidative and inflammatory signals, and its expression is regarded as an adaptive cellular response to inflammation and oxidative injury. Here, we show that TonEBP suppresses expression of HO-1 by blocking Nrf2 binding to the HO-1 promoter, thereby inducing polarization of macrophages to the M1 phenotype. Inhibition of HO-1 expression or activity significantly reduced the inhibitory responses on M1 phenotype and stimulatory effects on M2 phenotype by TonEBP knockdown. Additional experiments showed that HO-1 plays a role in the paracrine anti-inflammatory effects of TonEBP knockdown in macrophages. Identification of HO-1 as a downstream effector of TonEBP provides new possibilities for improved therapeutic approaches to inflammatory diseases.

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.

Sequential and synchronized hypertonicity-induced activation of Rel-family transcription factors is required for osmoprotection in renal cells

NF-κB and TonEBP belong to the Rel-superfamily of transcription factors. Several specific stimuli, including hypertonicity which is a key factor for renal physiology, are able to activate them. It has been reported that, after hypertonic challenge, NF-κB activity can be modulated by TonEBP, considered as the master regulator of transcriptional activity in the presence of changes in environmental tonicity. In the present work we evaluated whether hypertonicity-induced gene transcription mediated by p65/RelA and TonEBP occurs by an independent action of each transcription factor or by acting together. To do this, we evaluated the expression of their specific target genes and cyclooxygenase-2 (COX-2), a common target of both transcription factors, in the renal epithelial cell line Madin-Darby canine kidney (MDCK) subjected to hypertonic environment. The results herein indicate that hypertonicity activates the Rel-family transcription factors p65/RelA and TonEBP in MDCK cells, and that both are required for hypertonic induction of COX-2 and of their specific target genes. In addition, present data show that p65/RelA modulates TonEBP expression and both colocalize in nuclei of hypertonic cultures of MDCK cells. Thus, a sequential and synchronized action p65/RelA → TonEBP would be necessary for the expression of hypertonicity-induced protective genes.

NFAT5 up-regulates expression of the kidney-specific ubiquitin ligase gene Rnf183 under hypertonic conditions in inner-medullary collecting duct cells

We previously reported that among the 37 RING finger protein (RNF) family members, RNF183 mRNA is specifically expressed in the kidney under normal conditions. However, the mechanism supporting its kidney-specific expression pattern remains unclear. In this study, we elucidated the mechanism of the transcriptional activation of murine Rnf183 in inner-medullary collecting duct cells. Experiments with anti-RNF183 antibody revealed that RNF183 is predominantly expressed in the renal medulla. Among the 37 RNF family members, Rnf183 mRNA expression was specifically increased in hypertonic conditions, a hallmark of the renal medulla. RNF183 up-regulation was consistent with the activation of nuclear factor of activated T cells 5 (NFAT5), a transcription factor essential for adaptation to hypertonic conditions. Accordingly, siRNA-mediated knockdown of NFAT5 down-regulated RNF183 expression. Furthermore, the -3,466 to -3,136-bp region upstream of the mouse Rnf183 promoter containing the NFAT5-binding motif is conserved among mammals. A luciferase-based reporter vector containing the NFAT5-binding site was activated in response to hypertonic stress, but was inhibited by a mutation at the NFAT5-binding site. ChIP assays revealed that the binding of NFAT5 to this DNA site is enhanced by hypertonic stress. Of note, siRNA-mediated RNF183 knockdown increased hypertonicity-induced caspase-3 activation and decreased viability of mIMCD-3 cells. These results indicate that (i) RNF183 is predominantly expressed in the normal renal medulla, (ii) NFAT5 stimulates transcriptional activation of Rnf183 by binding to its cognate binding motif in the Rnf183 promoter, and (iii) RNF183 protects renal medullary cells from hypertonicity-induced apoptosis.

Induction of Osmolyte Pathways in Skeletal Muscle Inflammation: Novel Biomarkers for Myositis

We recently identified osmolyte accumulators as novel biomarkers for chronic skeletal muscle inflammation and weakness, but their precise involvement in inflammatory myopathies remains elusive. In the current study, we demonstrate in vitro that, in myoblasts and myotubes exposed to pro-inflammatory cytokines or increased salt concentration, mRNA levels of the osmolyte carriers SLC5A3, SLC6A6, SLC6A12, and AKR1B1 enzyme can be upregulated. Induction of SLC6A12 and AKR1B1 was confirmed at the protein level using immunofluorescence and Western blotting. Gene silencing by specific siRNAs revealed that these factors were vital for muscle cells under hyperosmotic conditions. Pro-inflammatory cytokines activated mitogen-activated protein kinases, nuclear factor κB as well as nuclear factor of activated T-cells 5 mRNA expression. In muscle biopsies from patients with polymyositis or sporadic inclusion body myositis, osmolyte pathway activation was observed in regenerating muscle fibers. In addition, the osmolyte carriers SLC5A3 and SLC6A12 localized to subsets of immune cells, most notably to the endomysial macrophages and T-cells. Collectively, this study unveiled that muscle cells respond to osmotic and inflammatory stress by osmolyte pathway activation, likely orchestrating general protection of the tissue. Moreover, pro-inflammatory properties are attributed to SLC5A3 and SLC6A12 in auto-aggressive macrophages and T-cells in inflamed skeletal muscle.

NFAT5 regulates renal gene expression in response to angiotensin II through Annexin-A2-mediated posttranscriptional regulation in hypertensive rats

The aim was to identify new targets that regulate gene expression at the posttranscriptional level in angiotensin II (ANGII)-mediated hypertension. Heparin affinity chromatography was used to enrich nucleic acid-binding proteins from kidneys of two-kidney, one-clip (2K1C) hypertensive Wistar rats. The experiment was repeated with 14-day ANGII infusion using Alzet osmotic mini pumps, with or without ANGII receptor AT1a inhibition using losartan in the drinking water. Mean arterial pressure increased after 2K1C or ANGII infusion and was inhibited with losartan. Heparin affinity chromatography and mass spectrometry were used to identify Annexin-A2 (ANXA2) as having differential nucleic acid-binding activity. Total Annexin-A2 protein expression was unchanged, whereas nucleic acid-binding activity was increased in both kidneys of 2K1C and after ANGII infusion through AT1a stimulation. Costaining of Annexin-A2 with α-smooth muscle actin and aquaporin 2 showed prominent expression in the endothelia of larger arteries and the cells of the inner medullary collecting duct. The nuclear factor of activated T cells (NFAT) transcription factor was identified as a likely Annexin-A2 target using enrichment analysis on a 2K1C microarray data set and identifying several binding sites in the regulatory region of the mRNA. Expression analysis showed that ANGII increases NFAT5 protein but not mRNA level and, thus, indicated that NFAT5 is regulated by posttranscriptional regulation, which correlates with activation of the RNA-binding protein Annexin-A2. In conclusion, we show that ANGII increases Annexin-A2 nucleic acid-binding activity that correlates with elevated protein levels of the NFAT5 transcription factor. NFAT signaling appears to be a major contributor to renal gene regulation in high-renin states.

Biomechanical Stretch Induces Inflammation, Proliferation, and Migration by Activating NFAT5 in Arterial Smooth Muscle Cells

The increasing wall stress as is elicited by arterial hypertension promotes their reorganization in the vessel wall which may lead to arterial stiffening and contractile dysfunction. The nuclear factor of activated T cells 5 (NFAT5) pathway plays a role in regulating growth and differentiation in various cell types. We investigated whether the NFAT5 pathway was involved in the regulation of biomechanical stretch-induced human arterial smooth muscle cell (HUASMC) proliferation, inflammation, and migration. Herein, we showed that stretch promoted the expression of NFAT5 in human arterial smooth muscle cells and regulated through activation of c-Jun N-terminal kinase under these conditions. This may contribute to an improved activity of HUASMCs and thus promote reorganization in vascular remodeling processes such as hypertension-induced arterial stiffening and contractile dysfunction.

Cleavage of osmosensitive transcriptional factor NFAT5 by Coxsackieviral protease 2A promotes viral replication

Nuclear factor of activated T cells 5 (NFAT5)/Tonicity enhancer binding protein (TonEBP) is a transcription factor induced by hypertonic stress in the kidney. However, the function of NFAT5 in other organs has rarely been studied, even though it is ubiquitously expressed. Indeed, although NFAT5 was reported to be critical for heart development and function, its role in infectious heart diseases has remained obscure. In this study, we aimed to understand the mechanism by which NFAT5 interferes with infection of Coxsackievirus B3 (CVB3), a major cause of viral myocarditis. Our initial results demonstrated that although the mRNA level of NFAT5 remained constant during CVB3 infection, NFAT5 protein level decreased because the protein was cleaved. Bioinformatic prediction and verification of the predicted site by site-directed mutagenesis experiments determined that the NFAT5 protein was cleaved by CVB3 protease 2A at Glycine 503. Such cleavage led to the inactivation of NFAT5, and the 70-kDa N-terminal cleavage product (p70-NFAT5) exerted a dominant negative effect on the full-length NFAT5 protein. We further showed that elevated expression of NFAT5 to counteract viral protease cleavage, especially overexpression of a non-cleavable mutant of NFAT5, significantly inhibited CVB3 replication. Ectopic expression of NFAT5 resulted in elevated expression of inducible nitric oxide synthase (iNOS), a factor reported to inhibit CVB3 replication. The necessity of iNOS for the anti-CVB3 effect of NFAT5 was supported by the observation that inhibition of iNOS blocked the anti-CVB3 effect of NFAT5. In a murine model of viral myocarditis, we observed that treatment with hypertonic saline or mannitol solution upregulated NFAT5 and iNOS expression, inhibited CVB3 replication and reduced tissue damage in the heart. Taken together, our data demonstrate that the anti-CVB3 activity of NFAT5 is impaired during CVB3 infection due to 2A-mediated cleavage of NFAT5. Thus induction of NFAT5 by hypertonic agents may be a promising strategy for the development of anti-CVB3 therapeutics.

TNF-α promotes nuclear enrichment of the transcription factor TonEBP/NFAT5 to selectively control inflammatory but not osmoregulatory responses in nucleus pulposus cells

Intervertebral disc degeneration (IDD) causes chronic back pain and is linked to production of proinflammatory molecules by nucleus pulposus (NP) and other disc cells. Activation of tonicity-responsive enhancer-binding protein (TonEBP)/NFAT5 by non-osmotic stimuli, including proinflammatory molecules, occurs in cells involved in immune response. However, whether inflammatory stimuli activate TonEBP in NP cells and whether TonEBP controls inflammation during IDD is unknown. We show that TNF-α, but not IL-1β or LPS, promoted nuclear enrichment of TonEBP protein. However, TNF-α-mediated activation of TonEBP did not cause induction of osmoregulatory genes. RNA sequencing showed that 8.5% of TNF-α transcriptional responses were TonEBP-dependent and identified genes regulated by both TNF-α and TonEBP. These genes were over-enriched in pathways and diseases related to inflammatory response and inhibition of matrix metalloproteases. Based on RNA-sequencing results, we further investigated regulation of novel TonEBP targets CXCL1, CXCL2, and CXCL3 TonEBP acted synergistically with TNF-α and LPS to induce CXCL1-proximal promoter activity. Interestingly, this regulation required a highly conserved NF-κB-binding site but not a predicted TonE, suggesting cross-talk between these two members of the Rel family. Finally, analysis of human NP tissue showed that TonEBP expression correlated with canonical osmoregulatory targets TauT/SLC6A6, SMIT/SLC5A3, and AR/AKR1B1, supporting in vitro findings that the inflammatory milieu during IDD does not interfere with TonEBP osmoregulation. In summary, whereas TonEBP participates in the proinflammatory response to TNF-α, therapeutic strategies targeting this transcription factor for treatment of disc disease must spare osmoprotective, prosurvival, and matrix homeostatic activities.

TonEBP/NFAT5 haploinsufficiency attenuates hippocampal inflammation in high-fat diet/streptozotocin-induced diabetic mice

Recent studies have shown that overexpression of tonicity-responsive enhancer binding protein (TonEBP) is associated with many inflammatory diseases, including diabetes mellitus, which causes neuroinflammation in the hippocampus as well as hepatic steatosis. However, the exact mechanism in diabetic neuroinflammation is unknown. We report that haploinsufficiency of TonEBP inhibits hepatic and hippocampal high-mobility group box-1 (HMGB1) expression in diabetic mice. Here, mice were fed a high-fat diet (HFD) for 16 weeks and received an intraperitoneal injection of 100 mg/kg streptozotocin (STZ) and followed by continued HFD feeding for an additional 4 weeks to induce hyperglycemia and hepatic steatosis. Compared with wild-type diabetic mice, diabetic TonEBP^+/- mice showed decreased body weight, fat mass, hepatic steatosis, and macrophage infiltration. We also found that adipogenesis and HMGB1 expression in the liver and hippocampus were lower in diabetic TonEBP^+/- mice compared with the wild type. Furthermore, iba-1 immunoreactivity in the hippocampus was decreased in diabetic TonEBP^+/- mice compared with that in the wild type. Our findings suggest that TonEBP haploinsufficiency suppresses diabetes-associated hepatic steatosis and neuroinflammation.

Hyperosmotic stress enhances cytotoxicity of SMAC mimetics

Inhibitors of apoptosis (IAP) proteins contribute to cell death resistance in malignancies and emerged as promising targets in cancer therapy. Currently, small molecules mimicking the IAP-antagonizing activity of endogenous second mitochondria-derived activator of caspases (SMAC) are evaluated in phase 1/2 clinical trials. In cancer cells, SMAC mimetic (SM)-mediated IAP depletion induces tumor necrosis factor (TNF) secretion and simultaneously sensitizes for TNF-induced cell death. However, tumor cells lacking SM-induced autocrine TNF release survive and thus limit therapeutic efficacy. Here, we show that hyperosmotic stress boosts SM cytotoxicity in human and murine cells through hypertonicity-induced upregulation of TNF with subsequent induction of apoptosis and/or necroptosis. Hypertonicity allowed robust TNF-dependent killing in SM-treated human acute lymphoblastic leukemia cells, which under isotonic conditions resisted SM treatment due to poor SM-induced TNF secretion. Mechanistically, hypertonicity-triggered TNF release bypassed the dependency on SM-induced TNF production to execute SM cytotoxicity, effectively reducing the role of SM to TNF-sensitizing, but not necessarily TNF-inducing agents. Perspectively, these findings could extend the clinical application of SM.

Hyperosmotic stress stimulates autophagy via polycystin-2

Various intracellular mechanisms are activated in response to stress, leading to adaptation or death. Autophagy, an intracellular process that promotes lysosomal degradation of proteins, is an adaptive response to several types of stress. Osmotic stress occurs under both physiological and pathological conditions, provoking mechanical stress and activating various osmoadaptive mechanisms. Polycystin-2 (PC2), a membrane protein of the polycystin family, is a mechanical sensor capable of activating the cell signaling pathways required for cell adaptation and survival. Here we show that hyperosmotic stress provoked by treatment with hyperosmolar concentrations of sorbitol or mannitol induces autophagy in HeLa and HCT116 cell lines. In addition, we show that mTOR and AMPK, two stress sensor proteins involved modulating autophagy, are downregulated and upregulated, respectively, when cells are subjected to hyperosmotic stress. Finally, our findings show that PC2 is required to promote hyperosmotic stress-induced autophagy. Downregulation of PC2 prevents inhibition of hyperosmotic stress-induced mTOR pathway activation. In conclusion, our data provide new insight into the role of PC2 as a mechanosensor that modulates autophagy under hyperosmotic stress conditions.

Suppression of NFAT5-mediated Inflammation and Chronic Arthritis by Novel κB-binding Inhibitors

Nuclear factor of activated T cells 5 (NFAT5) has been implicated in the pathogenesis of various human diseases, including cancer and arthritis. However, therapeutic agents inhibiting NFAT5 activity are currently unavailable. To discover NFAT5 inhibitors, a library of > 40,000 chemicals was screened for the suppression of nitric oxide, a direct target regulated by NFAT5 activity, through high-throughput screening. We validated the anti-NFAT5 activity of 198 primary hit compounds using an NFAT5-dependent reporter assay and identified the novel NFAT5 suppressor KRN2, 13-(2-fluoro)-benzylberberine, and its derivative KRN5. KRN2 inhibited NFAT5 upregulation in macrophages stimulated with lipopolysaccharide and repressed the formation of NF-κB p65-DNA complexes in the NFAT5 promoter region. Interestingly, KRN2 selectively suppressed the expression of pro-inflammatory genes, including Nos2 and Il6, without hampering high-salt-induced NFAT5 and its target gene expressions. Moreover, KRN2 and KRN5, the latter of which exhibits high oral bioavailability and metabolic stability, ameliorated experimentally induced arthritis in mice without serious adverse effects, decreasing pro-inflammatory cytokine production. Particularly, orally administered KRN5 was stronger in suppressing arthritis than methotrexate, a commonly used anti-rheumatic drug, displaying better potency and safety than its original compound, berberine. Therefore, KRN2 and KRN5 can be potential therapeutic agents in the treatment of chronic arthritis.

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We identify a novel NFAT5 suppressor KRN2, 13-(2-fluoro)-benzylberberine, and its derivative KRN5 to inhibit NFAT5 activity.

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KRN2 inhibits the transcriptional activation of NFAT5 and the pro-inflammatory responses.

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KRN2 and KRN5 suppress experimentally induced arthritis in mice.

NFAT5 has been implicated in the pathogenesis of arthritis. However, therapeutic agents specifically inhibiting NFAT5 activity are currently unavailable. To discover NFAT5 inhibitors, a library of > 40,000 chemicals was screened, leading to the discovery of novel berberine-based NFAT5 suppressors, KRN2 and its oral derivative KRN5. KRN2 inhibited the transcriptional activation of NFAT5 by blocking NF-κB binding to the NFAT5 promoter region, thereby reducing the expression of pro-inflammatory genes. Moreover, KRN2 and KRN5 ameliorated experimentally induced arthritis in mice without serious adverse effects. Therefore, we propose that KRN2 and KRN5 may be potential therapeutic agents in the treatment of chronic arthritis.

Ultraviolet B Radiation Stimulates the Interaction between Nuclear Factor of Activated T Cells 5 (NFAT5) and Nuclear Factor-Kappa B (NF-κB) in Human Lens Epithelial Cells

PURPOSE

Nuclear factor-kappa B (NF-κB) has been proposed as a therapeutic target for the treatment of cataracts. The authors investigated the relationship between nuclear factor of activated T cells 5 (NFAT5) and NF-κB in ultraviolet B (UVB)-irradiated human lens epithelial (HLE) cells.

METHODS

Human lens epithelial B-3 (HLE-B3) cells were exposed to UVB light at a dose of 10 mJ/cm² and then incubated for 24 h. Cell viability was assessed by using the Cell Counting Kit-8 (CCK-8) assay. Gene expression level of NFAT5 was determined using real-time quantitative polymerase chain reaction (qPCR). Protein expression levels of NFAT5, NF-κB p65, and α-smooth muscle actin (α-SMA) and the association of NFAT5 with the NF-κB p65 subunit were measured by Western blot analysis and a co-immunoprecipitation assay, respectively. The cellular distribution of NFAT5 and NF-κB p65 was examined by triple immunofluorescence staining.

RESULTS

At 24 h after UVB exposure, cell viability significantly decreased in a dose-dependent manner, and UVB light (15 and 20 mJ/cm²) significantly increased the ROS generation. UVB irradiation increased NFAT5 mRNA and protein levels and increased phosphorylation of NF-κB in HLE-B3 cells. α-SMA protein levels were increased in the irradiated cells. In addition, NFAT5 and NF-κB translocated from the cytoplasm to the nucleus, and binding between the p65 subunit and NFAT5 was increased.

CONCLUSIONS

Exposure to UVB radiation induces nuclear translocation and stimulates binding between NFAT5 and NF-κB proteins in HLE-B3 cells. These interactions may form part of the biochemical mechanism of cataractogenesis in UVB-irradiated HLECs.

Protein Kinase R Mediates the Inflammatory Response Induced by Hyperosmotic Stress

High extracellular osmolarity results in a switch from an adaptive to an inflammatory gene expression program. We show that hyperosmotic stress activates the protein kinase R (PKR) independently of its RNA-binding domain. In turn, PKR stimulates nuclear accumulation of nuclear factor κB (NF-κB) p65 species phosphorylated at serine-536, which is paralleled by the induction of a subset of inflammatory NF-κB p65-responsive genes, including inducible nitric oxide synthase (iNOS), interleukin-6 (IL-6), and IL-1β. The PKR-mediated hyperinduction of iNOS decreases cell survival in mouse embryonic fibroblasts via mechanisms involving nitric oxide (NO) synthesis and posttranslational modification of proteins. Moreover, we demonstrate that the PKR inhibitor C16 ameliorates both iNOS amplification and disease-induced phenotypic breakdown of the intestinal epithelial barrier caused by an increase in extracellular osmolarity induced by dextran sodium sulfate (DSS) in vivo Collectively, these findings indicate that PKR activation is an essential part of the molecular switch from adaptation to inflammation in response to hyperosmotic stress.

Elementary immunology: Na+ as a regulator of immunity

The skin can serve as an interstitial Na+ reservoir. Local tissue Na+ accumulation increases with age, inflammation and infection. This increased local Na+ availability favors pro-inflammatory immune cell function and dampens their anti-inflammatory capacity. In this review, we summarize available data on how NaCl affects various immune cells. We particularly focus on how salt promotes pro-inflammatory macrophage and T cell function and simultaneously curtails their regulatory and anti-inflammatory potential. Overall, these findings demonstrate that local Na+ availability is a promising novel regulator of immunity. Hence, the modulation of tissue Na+ levels bears broad therapeutic potential: increasing local Na+ availability may help in treating infections, while lowering tissue Na+ levels may be used to treat, for example, autoimmune and cardiovascular diseases.

RNA Sequencing Reveals a Role of TonEBP Transcription Factor in Regulation of Pro-inflammatory Genes in Response to Hyperosmolarity in Healthy Nucleus Pulposus Cells: A HOMEOSTATIC RESPONSE?

Transcription factor tonicity-responsive enhancer-binding protein (TonEBP/NFAT5) is critical for osmo-adaptation and extracellular matrix homeostasis of nucleus pulposus (NP) cells in their hypertonic tissue niche. Recent studies implicate TonEBP signaling in inflammatory disease and rheumatoid arthritis pathogenesis. However, broader functions of TonEBP in the disc remain unknown. RNA sequencing was performed on NP cells with TonEBP knockdown under hypertonic conditions. 1140 TonEBP-dependent genes were identified and categorized using Ingenuity Pathway Analysis. Bioinformatic analysis showed enrichment of matrix homeostasis and cytokine/chemokine signaling pathways. C-C motif chemokine ligand 2 (CCL2), interleukin 6 (IL6), tumor necrosis factor (TNF), and nitric oxide synthase 2 (NOS2) were studied further. Knockdown experiments showed that TonEBP was necessary to maintain expression levels of these genes. Gain- and loss-of-function experiments and site-directed mutagenesis demonstrated that TonEBP binding to a specific site in the CCL2 promoter is required for hypertonic inducibility. Despite inhibition by dominant-negative TonEBP, IL6 and NOS2 promoters were not hypertonicity-inducible. Whole-disc response to hypertonicity was studied in an ex vivo organ culture model, using wild-type and haploinsufficient TonEBP mice. Pro-inflammatory targets were induced by hypertonicity in discs from wild-type but not TonEBP-haploinsufficient mice. Mechanistically, NF-κB activity increased with hypertonicity and was necessary for hypertonic induction of target genes IL6, TNF, and NOS2 but not CCL2 Although TonEBP maintains transcription of genes traditionally considered pro-inflammatory, it is important to note that some of these genes also serve anabolic and pro-survival roles. Therefore, in NP cells, this phenomenon may reflect a physiological adaptation to diurnal osmotic loading of the intervertebral disc.

NFAT5 participates in seawater inhalation‑induced acute lung injury via modulation of NF-κB activity

Nuclear factor of activated T cells 5 (NFAT5) is a transcription factor that can be activated by extracellular tonicity. It has been reported that NFAT5 may increase the transcription of certain osmoprotective genes in the renal system, and the aim of the current study was to explore the role of NFAT5 in seawater inhalation-induced acute lung injury. Though establishing the model of seawater inhalation-induced acute lung injury, it was demonstrated that seawater inhalation enhanced the transcription and protein expression of NFAT5 (evaluated by reverse transcription-polymerase chain reaction, immunohistochemistry stain and western blotting) and activation of nuclear factor (NF)-κB (evaluated by western blotting and mRNA expression levels of three NF-κB-dependent genes) both in lung tissue and rat alveolar macrophage cells (NR8383 cells). When expression of NFAT5 was reduced in NR8383 cells using an siRNA targeted to NFAT5, the phosphorylation of NF-κB and transcription of NF-κB-dependent genes were significantly reduced. In addition, the elevated content of certain inflammatory cytokines [tumor necrosis factor α, interleukin (IL)-1 and IL-8] were markedly reduced. In conclusion, NFAT5 serves an important pathophysiological role in seawater inhalation-induced acute lung injury by modulating NF-κB activity, and these data suggest that NFAT5 may be a promising therapeutic target.

In Vitro Inhibition of NFAT5-Mediated Induction of CCL2 in Hyperosmotic Conditions by Cyclosporine and Dexamethasone on Human HeLa-Modified Conjunctiva-Derived Cells

Purpose

To investigate the pro-inflammatory intracellular mechanisms induced by an in vitro model of dry eye disease (DED) on a Hela-modified conjunctiva-derived cells in hyperosmolarity (HO) stress conditions. This study focused on CCL2 induction and explored the implications of the nuclear factor of activated T-cells 5 (NFAT5) as well as mitogen-activated protein kinases (MAPK) and nuclear factor kappa B (NFĸB). This work was completed by an analysis of the effects of cyclosporine A (CsA), dexamethasone (Dex) and doxycycline (Dox) on HO-induced CCL2 and NFAT5 induction.

Methods

A human HeLa-modified conjunctiva-derived cell line was cultured in NaCl-hyperosmolar medium for various exposure times. Cellular viability, CCL2 secretion, NFAT5 and CCL2 gene expression, and intracytoplasmic NFAT5 were assessed using the Cell Titer Blue^® assay, enzyme-linked immunosorbent assay (ELISA), RT-qPCR and immunostaining, respectively. In selected experiments, inhibitors of MAPKs or NFκB, therapeutic agents or NFAT5 siRNAs were added before the hyperosmolar stimulations.

Results

HO induced CCL2 secretion and expression as well as NFAT5 gene expression and translocation. Adding NFAT5-siRNA before hyperosmolar stimulation led to a complete inhibition of CCL2 induction and to a decrease in cellular viability. p38 MAPK (p38), c-Jun NH₂-terminal kinase (JNK) and NFĸB inhibitors, CsA and Dex induced a partial inhibition of HO-induced CCL2, while Dox and extracellular signal-regulated kinase (ERK) inhibitor did not. Dex also induced a partial inhibition of HO-induced NFAT5 gene expression but not CsA or Dox.

Conclusions

These in vitro results suggest a potential role of CCL2 in DED and highlight the crucial role of NFAT5 in the pro-inflammatory effect of HO on HeLa-modified conjunctiva-derived cells, a rarely studied cellular type. This inflammatory pathway involving NFAT5 and CCL2 could offer a promising target for developing new therapies to treat DED, warranting further investigations to fully grasp the complete intracellular mechanisms.

Activation of osmolyte pathways in inflammatory myopathy and Duchenne muscular dystrophy points to osmoregulation as a contributing pathogenic mechanism

Alongside well-known nuclear factor κB (NFκB) and its associated cytokine networks, nuclear factor of activated T cells 5 (NFAT5), the master regulator of cellular osmoprotective programs, comes forward as an inflammatory regulator. To gain insight into its yet unexplored role in muscle disease, we studied the expression of NFAT5 target proteins involved in osmolyte accumulation: aldose reductase (AR), taurine transporter (TauT), and sodium myo-inositol co-transporter (SMIT). We analyzed idiopathic inflammatory myopathy and Duchenne muscular dystrophy muscle biopsies and myotubes in culture, using immunohistochemistry, immunofluorescence, and western blotting. We report that the level of constitutive AR was upregulated in patients, most strongly so in Duchenne muscular dystrophy. TauT and SMIT expression levels were induced in patients' muscle fibers, mostly representing regenerating and atrophic fibers. In dermatomyositis, strong staining for AR, TauT, and SMIT in atrophic perifascicular fibers was accompanied by staining for other molecular NFAT5 targets, including chaperones, chemokines, and inducible nitric oxide synthase. In these fibers, NFAT5 and NFκB p65 staining coincided, linking both transcription factors with this important pathogenic hallmark. In sporadic inclusion body myositis, SMIT localized to inclusions inside muscle fibers. In addition, SMIT was expressed by a substantial subset of muscle-infiltrating macrophages and T cells in patient biopsies. Our results indicate that osmolyte pathways may contribute to normal muscle functioning, and that activation of AR, TauT, and SMIT in muscle inflammation possibly contributes to the tissue's failing program of damage control.

Autocrine MCP-1/CCR2 signaling stimulates proliferation and migration of renal carcinoma cells

The chemokine monocyte chemoattractant protein-1 [MCP-1; also known as chemokine (C-C motif) ligand 2] is an important mediator of monocyte recruitment during inflammatory processes. Pathologically high expression levels of MCP-1 by tumor cells have been observed in a variety of cancer types. In the majority of cases, high MCP-1 expression is associated with a poor prognosis, as infiltration of the tumor with inflammatory monocytes promotes tumor progression and metastasis. MCP-1 is also expressed in renal cell carcinoma (RCC). In the present study, the function and the regulation of MCP-1 was investigated in two RCC cell lines, CaKi-1 and 786-O. In both cell lines, expression of MCP-1 was significantly enhanced compared with non-cancerous control cells. As expected, secretion of MCP-1 into the medium facilitated the recruitment of peripheral blood monocytes via the chemokine (C-C motif) receptor type 2 (CCR2). As expression of CCR2 was also detected in 786-O and CaKi-1 cells, the effect of autocrine MCP-1/CCR2 signaling was evaluated in these cells. In proliferation assays, administration of an MCP-1 neutralizing antibody or of a CCR2 antagonist to CaKi-1 and 786-O cells significantly decreased cell growth; supplementation of the growth medium with recombinant human MCP-1 had no additional effect on proliferation. The migration ability of RCC cells was impaired by MCP-1 neutralization or pharmacological CCR2 inhibition, while it was stimulated by the addition of recombinant human MCP-1, compared with untreated control cells. Finally, substantial differences in the regulation of MCP-1 expression were observed between RCC cell lines. In CaKi-1 cells, expression of MCP-1 appears to be largely mediated by the transcription factor nuclear factor of activated T cells 5, while in 786-O cells, deletion of the tumor suppressor gene Von-Hippel-Lindau appeared to be responsible for MCP-1 upregulation, as suggested by previous studies. Taken together, the results of the current study indicate that expression of MCP-1 in RCC cells promotes tumor progression and metastasis not only by paracrine, but also by autocrine, MCP-1/CCR2 signaling events, enhancing cell proliferation and migration ability. Therefore, the present findings suggest the MCP-1/CCR2 axis is a potential target for future therapeutic strategies in the treatment of metastatic RCC.

TonEBP suppresses IL-10-mediated immunomodulation

TonEBP is a key transcriptional activator of M1 phenotype in macrophage, and its high expression is associated with many inflammatory diseases. During the progression of the inflammatory responses, the M1 to M2 phenotypic switch enables the dual role of macrophages in controlling the initiation and resolution of inflammation. Here we report that in human and mouse M1 macrophages TonEBP suppresses IL-10 expression and M2 phenotype. TonEBP knockdown promoted the transcription of the IL-10 gene by enhancing chromatin accessibility and Sp1 recruitment to its promoter. The enhanced expression of M2 genes by TonEBP knockdown was abrogated by antagonism of IL-10 by either neutralizing antibodies or siRNA-mediated silencing. In addition, pharmacological suppression of TonEBP leads to similar upregulation of IL-10 and M2 genes. Thus, TonEBP suppresses M2 phenotype via downregulation of the IL-10 in M1 macrophages.

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.

The potential role of NFAT5 and osmolarity in peritoneal injury

A rise in osmotic concentration (osmolarity) activates the transcription factor Nuclear Factor of Activated T Cells 5 (NFAT5, also known as Tonicity-responsive Enhancer Binding Protein, TonEBP). This is part of a regulatory mechanism of cells adjusting to environments of high osmolarity. Under physiological conditions these are particularly important in the kidney. Activation of NFAT5 results in the modulation of various genes including some which promote inflammation. The osmolarity increases in patients with renal failure. Additionally, in peritoneal dialysis the cells of the peritoneal cavity are repeatedly exposed to a rise and fall in osmotic concentrations. Here we review the current information about NFAT5 activation in uremic patients and patients on peritoneal dialysis. We suggest that high osmolarity promotes injury in the “uremic” milieu, which results in inflammation locally in the peritoneal membrane, but most likely also in the systemic circulation.

Dual effect of lithium on NFAT5 activity in kidney cells

Lithium salts are used widely for treatment of bipolar and other mental disorders. Lithium therapy is accompanied frequently by renal side effects, such as nephrogenic diabetes insipidus or chronic kidney disease (CKD), but the molecular mechanisms underlying these effects are still poorly understood. In the present study we examined the effect of lithium on the activity of the osmosensitive transcriptional activator nuclear factor of activated T cells 5 (NFAT5, also known as TonEBP), which plays a key role in renal cellular osmoprotection and urinary concentrating ability. Interestingly, we found different effects of lithium on NFAT5 activity, depending on medium osmolality and incubation time. When cells were exposed to lithium for a relative short period (24 h), NFAT5 activity was significantly increased, especially under isosmotic conditions, resulting in an enhanced expression of the NFAT5 target gene heat shock protein 70 (HSP70). Further analysis revealed that the increase of NFAT5 activity depended primarily on an enhanced activity of the c-terminal transactivation domain (TAD), while NFAT5 protein abundance was largely unaffected. Enhanced activity of the TAD is probably mediated by lithium-induced inhibitory phosphorylation of glycogen synthase kinase 3β (GSK-3β), which is in accordance with previous studies. When cells were exposed to lithium for a longer period (96 h), cellular NFAT5 activity and subsequently expression of HSP70 significantly decreased under hyperosmotic conditions, due to diminished NFAT5 protein abundance, also resulting from GSK-3β inhibition. Taken together, our results provide evidence that lithium has opposing effects on NFAT5 activity, depending on environmental osmolality and exposure duration. The potential impacts of these observations on the diverse effects of lithium on kidney function are discussed.

Different effects of ZO-1, ZO-2 and ZO-3 silencing on kidney collecting duct principal cell proliferation and adhesion

Coordinated cell proliferation and ability to form intercellular seals are essential features of epithelial tissue function. Tight junctions (TJs) classically act as paracellular diffusion barriers. More recently, their role in regulating epithelial cell proliferation in conjunction with scaffolding zonula occludens (ZO) proteins has come to light. The kidney collecting duct (CD) is a model of tight epithelium that displays intense proliferation during embryogenesis followed by very low cell turnover in the adult kidney. Here, we examined the influence of each ZO protein (ZO-1, -2 and -3) on CD cell proliferation. We show that all 3 ZO proteins are strongly expressed in native CD and are present at both intercellular junctions and nuclei of cultured CD principal cells (mCCDcl1). Suppression of either ZO-1 or ZO-2 resulted in increased G0/G1 retention in mCCDcl1 cells. ZO-2 suppression decreased cyclin D1 abundance while ZO-1 suppression was accompanied by increased nuclear p21 localization, the depletion of which restored cell cycle progression. Contrary to ZO-1 and ZO-2, ZO-3 expression at intercellular junctions dramatically increased with cell density and relied on the presence of ZO-1. ZO-3 depletion did not affect cell cycle progression but increased cell detachment. This latter event partly relied on increased nuclear cyclin D1 abundance and was associated with altered β1-integrin subcellular distribution and decreased occludin expression at intercellular junctions. These data reveal diverging, but interconnected, roles for each ZO protein in mCCDcl1 proliferation. While ZO-1 and ZO-2 participate in cell cycle progression, ZO-3 is an important component of cell adhesion.

TonEBP suppresses adipogenesis and insulin sensitivity by blocking epigenetic transition of PPARγ2

TonEBP is a key transcription factor in cellular adaptation to hypertonic stress, and also in macrophage activation. Since TonEBP is involved in inflammatory diseases such as rheumatoid arthritis and atherosclerosis, we asked whether TonEBP played a role in adipogenesis and insulin resistance. Here we report that TonEBP suppresses adipogenesis and insulin signaling by inhibiting expression of the key transcription factor PPARγ2. TonEBP binds to the PPARγ2 promoter and blocks the epigenetic transition of the locus which is required for the activation of the promoter. When TonEBP expression is reduced, the epigenetic transition and PPARγ2 expression are markedly increased leading to enhanced adipogenesis and insulin response while inflammation is reduced. Thus, TonEBP is an independent determinant of adipose insulin sensitivity and inflammation. TonEBP is an attractive therapeutic target for insulin resistance in lieu of PPARγ agonists.

Activation of nuclear factor of activated T cells 5 in the peritoneal membrane of uremic patients

Peritoneal inflammation and fibrosis are responses to the uremic milieu and exposure to hyperosmolar dialysis fluids in patients on peritoneal dialysis. Cells respond to high osmolarity via the transcription factor nuclear factor of activated T cells (NFAT5). In the present study, the response of human peritoneal fibroblasts to glucose was analyzed in vitro. Expression levels of NFAT5 and chemokine (C-C motif) ligand (CCL2) mRNA were quantified in peritoneal biopsies of five nonuremic control patients, five uremic patients before PD (pPD), and eight patients on PD (oPD) using real-time PCR. Biopsies from 5 control patients, 25 pPD patients, and 25 oPD patients were investigated using immunohistochemistry to detect the expression of NFAT5, CCL2, NF-κB p50, NF-κB p65, and CD68. High glucose concentrations led to an early, dose-dependent induction of NFAT5 mRNA in human peritoneal fibroblasts. CCL2 mRNA expression was upregulated by high concentrations of glucose after 6 h, but, most notably, a concentration-dependent induction of CCL2 was present after 96 h. In human peritoneal biopsies, NFAT5 mRNA levels were increased in uremic patients compared with nonuremic control patients. No significant difference was found between the pPD group and oPD group. CCL2 mRNA expression was higher in the oPD group. Immunohistochemistry analysis was consistent with the results of mRNA analysis. CD68-positive cells were significantly increased in the oPD group. In conclusion, uremia results in NFAT5 induction, which might promote early changes of the peritoneum. Upregulation of NFAT5 in PD patients is associated with NFκB induction, potentially resulting in the recruitment of macrophages.