Loss of the Transfer RNA Wobble Uridine-Modifying Enzyme Elp3 Delays T Cell Cycle Entry and Impairs T Follicular Helper Cell Responses through Deregulation of Atf4.

The activation of T cells is accompanied by intensive posttranscriptional remodeling of their proteome. We observed that protein expression of enzymes that modify wobble uridine in specific tRNAs, namely elongator subunit 3 (Elp3) and cytosolic thiouridylase (Ctu)2, increased in the course of T cell activation. To investigate the role of these tRNA epitranscriptomic modifiers in T cell biology, we generated mice deficient for Elp3 in T cells. We show that deletion of Elp3 has discrete effects on T cells. In vitro, Elp3-deficient naive CD4+ T cells polarize normally but are delayed in entering the first cell cycle following activation. In vivo, different models of immunization revealed that Elp3-deficient T cells display reduced expansion, resulting in functional impairment of T follicular helper (TFH) responses, but not of other CD4+ effector T cell responses. Transcriptomic analyses identified a progressive overactivation of the stress-responsive transcription factor Atf4 in Elp3-deficient T cells. Overexpression of Atf4 in wild-type T cells phenocopies the effect of Elp3 loss on T cell cycle entry and TFH cell responses. Reciprocally, partial silencing of Atf4 or deletion of its downstream effector transcription factor Chop rescues TFH responses of Elp3-deficient T cells. Together, our results reveal that specific epitranscriptomic tRNA modifications contribute to T cell cycle entry and promote optimal TFH responses.

PERK leads a hub dictating pancreatic ß cell homoeostasis.

In humans, the pathogenesis of diabetes is characterised by two major pancreatic ß cell defects: a reduction in ß cell mass and the failure of ß cells to produce enough insulin. Over the past two decades, multiple studies involving cell cultures, animal models and human subjects have established the importance of the protein kinase RNA-like endoplasmic reticulum kinase (PERK) in the adaptive functional capacity of pancreatic ß cells during embryonic development and into adulthood. In this review, we will highlight major findings identifying PERK as a crucial player in ß cell physiology and in diabetes.

Functional block of IL-17 cytokine promotes bone healing by augmenting FOXO1 and ATF4 activity in cortical bone defect model.

We determine the effect of interleukin (IL)-17 neutralizing antibody on new bone regeneration. Anti-IL-17 antibody promoted new bone regeneration in cortical bone defect model by augmenting FOXO1 and ATF4 activity thereby decreasing oxidative stress. Our study demonstrates the bone healing and regeneration potential of neutralizing IL-17antibody in osteoporotic fractures. INTRODUCTION: The immune system plays important role in the fracture healing process. However, fracture healing is prolonged in disorders associated with systemic inflammation. Fracture healing is decelerated in osteoporosis, condition linked with systemic inflammation. Bone regeneration therapies like recombinant human BMP2 are associated with serious side effects. Studies have been carried out where agents like denosumab and infliximab enhance bone regeneration in osteoporotic conditions. Our previous studies show the osteoprotective and immunoprotective effects of neutralizing IL-17 antibody. Here, we determine the effect of IL-17 neutralizing antibody on new bone regeneration and compare its efficacy with known osteoporotic therapies. METHODS: For the study, female BALB/c mice were ovariectomized or sham operated and left for a month followed by a 0.6-mm drill-hole injury in femur mid-diaphysis. The treatment was commenced next day onwards with anti-IL-17, anti-RANKL (Receptor activator of nuclear factor kappa-B ligand), parathyroid hormone (PTH), or alendronate for a period of 3, 10, or 21 days. Animals were then autopsied, and femur bones were dissected out for micro-CT scanning, confocal microscopy, and gene and protein expression studies. RESULTS: Micro-CT analysis showed that anti-IL-17 antibody promoted bone healing at days 10 and 21, and the healing effect observed was significantly better than Ovx, anti-RANKL antibody, and ALN, and equal to PTH. Anti-IL-17 also enhanced new bone regeneration as assessed by calcein-labeling studies. Additionally, anti-IL-17 therapy enhanced expression of osteogenic markers and decreased oxidative stress at the injury site. CONCLUSION: Overall, our study demonstrates bone healing and regeneration potential of neutralizing IL-17 antibody in osteoporotic fractures.

Twist1 and Twist2 Contribute to Cytokine Downregulation following Chronic NOD2 Stimulation of Human Macrophages through the Coordinated Regulation of Transcriptional Repressors and Activators.

Proper regulation of microbial-induced cytokines is critical to intestinal immune homeostasis. Acute stimulation of nucleotide-binding oligomerization domain 2 (NOD2), the Crohn's disease-associated sensor of bacterial peptidoglycan, induces cytokines. However, chronic NOD2 stimulation in macrophages decreases cytokines upon pattern recognition receptor (PRR) restimulation; cytokine attenuation to PRR stimulation is similarly observed in intestinal macrophages. The role for the transcriptional repressors Twist1 and Twist2 in regulating PRR-induced cytokine outcomes is poorly understood and has not been reported for NOD2. We found that Twist1 and Twist2 were required for optimal cytokine downregulation during acute and, particularly, chronic NOD2 stimulation of human macrophages. Consistently, Twist1 and Twist2 expression was increased after chronic NOD2 stimulation; this increased expression was IL-10 and TGF-ß dependent. Although Twist1 and Twist2 did not coregulate each other's expression, they cooperated to enhance binding to cytokine promoters after chronic NOD2 stimulation. Moreover, Twist1 and Twist2 contributed to enhance expression and promoter binding of the proinflammatory inhibitor c-Maf and the transcriptional repressor Bmi1. Restoring c-Maf and Bmi1 expression in Twist-deficient macrophages restored NOD2-induced cytokine downregulation. Furthermore, with chronic NOD2 stimulation, Twist1 and Twist2 contributed to the decreased expression and cytokine promoter binding of the transcriptional activators activating transcription factor 4, C/EBPα, Runx1, and Runx2. Knockdown of these transcriptional activators in Twist-deficient macrophages restored cytokine downregulation after chronic NOD2 stimulation. Finally, NOD2 synergized with additional PRRs to increase Twist1 and Twist2 expression and Twist-dependent pathways. Therefore, after chronic NOD2 stimulation Twist1 and Twist2 coordinate the regulation of both transcriptional activators and repressors, thereby mediating optimal cytokine downregulation.

The bZIP Transcription Factor ZIP-11 Is Required for the Innate Immune Regulation in Caenorhabditis elegans.

Innate immunity is the first line of host defense against pathogen infection in metazoans. However, the molecular mechanisms of the complex immune regulatory network are not fully understood. Based on a transcriptome profiling of the nematode Caenorhabditis elegans, we found that a bZIP transcription factor ZIP-11 was up-regulated upon Pseudomonas aeruginosa PA14 infection. The tissue specific RNAi knock-down and rescue data revealed that ZIP-11 acts in intestine to promote host resistance against P. aeruginosa PA14 infection. We further showed that intestinal ZIP-11 regulates innate immune response through constituting a feedback loop with the conserved PMK-1/p38 mitogen-activated protein signaling pathway. Intriguingly, ZIP-11 interacts with a CCAAT/enhancer-binding protein, CEBP-2, to mediate the transcriptional response to P. aeruginosa PA14 infection independently of PMK-1/p38 pathway. In addition, human homolog ATF4 can functionally substitute for ZIP-11 in innate immune regulation of C. elegans. Our findings indicate that the ZIP-11/ATF4 genetic program activates local innate immune response through conserved PMK-1/p38 and CEBP-2/C/EBPγ immune signals in C. elegans, raising the possibility that a similar process may occur in other organisms.

The eIF2α kinase HRI in innate immunity, proteostasis, and mitochondrial stress.

The integrated stress response (ISR) is an evolutionary conserved stress response pathway that leads to a global arrest in translation as well as to the expression of specific genes, such as the transcription factor ATF4, to promote cellular recovery. The central nexus of this pathway is the phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2α) by one of the four eIF2α kinases that sense specific cellular stressors. The heme-regulated inhibitor (HRI) is one of these kinases, and it was initially reported to be activated in response to heme deprivation. Nevertheless, further studies have established that cytosolic proteotoxicity, resulting from oxidative or osmotic stress, heat shock, and proteasome inhibition, is the predominant trigger for HRI to induce the ISR. In this review, we present newly identified functions of HRI in innate immunity, proteostasis, and mitochondrial stress. Indeed, HRI-mediated signaling defines a novel cytosolic unfolded protein response (cUPR) required for the proper formation of some innate immune signalosomes and the control of toxic protein aggregates, and this eIF2α kinase also serves as a relay for mitonuclear communication after a mitochondrial stress.

ER stress-induced mediator C/EBP homologous protein thwarts effector T cell activity in tumors through T-bet repression.

Understanding the intrinsic mediators that render CD8+ T cells dysfunctional in the tumor microenvironment is a requirement to develop more effective cancer immunotherapies. Here, we report that C/EBP homologous protein (Chop), a downstream sensor of severe endoplasmic reticulum (ER) stress, is a major negative regulator of the effector function of tumor-reactive CD8+ T cells. Chop expression is increased in tumor-infiltrating CD8+ T cells, which correlates with poor clinical outcome in ovarian cancer patients. Deletion of Chop in T cells improves spontaneous antitumor CD8+ T cell immunity and boosts the efficacy of T cell-based immunotherapy. Mechanistically, Chop in CD8+ T cells is elevated primarily through the ER stress-associated kinase Perk and a subsequent induction of Atf4; and directly represses the expression of T-bet, a master regulator of effector T cell function. These findings demonstrate the primary role of Chop in tumor-induced CD8+ T cell dysfunction and the therapeutic potential of blocking Chop or ER stress to unleash T cell-mediated antitumor immunity.

Effects of hypoxia on transcription factor expression in human monocytes and macrophages.

The presence of multiple areas of hypoxia (low oxygen tension) is a hallmark feature of human and experimental tumours. Monocytes are continually recruited into tumours where they differentiate into tumour-associated macrophages (TAM) and often accumulate in hypoxic and/or necrotic areas. A number of recent studies have shown that macrophages respond to hypoxia by up-regulating transcription factors such as HIF-1alpha and HIF-2alpha, which in turn up-regulate the expression of a broad array of mitogenic, pro-invasive, pro-angiogenic and pro-metastatic genes. Here we show that primary human macrophages but not monocytes rapidly up-regulate HIF-1alpha and HIF-2alpha proteins upon exposure to hypoxia, and that these proteins then translocate to the nucleus. We also demonstrate differences in the temporal expression and responses to re-oxygenation for HIF-1alpha and HIF-2alpha in macrophages. Here we found that, compared to HIF-1alpha, HIF-2alpha expression was prolonged and persisted with re-oxygenation. ATF-4 and Egr-1 were also found to be hypoxia-responsive transcription factors in macrophages but not monocytes, but only early after exposure to hypoxia. Taken together, these findings indicate that a number of transcription factors work together in a tightly regulated fashion to control macrophage activities in ischaemic areas of diseased tissues.

ATF4 is directly recruited by TLR4 signaling and positively regulates TLR4-trigged cytokine production in human monocytes.

Toll-like receptors (TLRs) are sentinels of the host defense system, which recognize a large number of microbial pathogens. The host defense system may be inefficient or inflammatory diseases may develop if microbial recognition by TLRs and subsequent TLR-triggered cytokine production are deregulated. Activating transcription factor 4 (ATF4), a member of the ATF/CREB transcription factor family, is an important factor that participates in several pathophysiological processes. In this report, we found that ATF4 is also involved in the TLR-mediated innate immune response, which participates in TLR4 signal transduction and mediates the secretion of a variety of cytokines. We observed that ATF4 is activated and translocates to the nucleus following lipopolysaccharide (LPS) stimulation via the TLR4-MyD88-dependent pathway. Additionally, a cytokine array assay showed that some key inflammatory cytokines, such as IL-6, IL-8 and RANTES, are positively regulated by ATF4. We also demonstrate that c-Jun directly binds to ATF4, thereby promoting the secretion of inflammatory cytokines. Taken together, these results indicate that ATF4 acts as a positive regulator in TLR4-triggered cytokine production.