Gadd45α Regulates p38-Dependent Dendritic Cell Cytokine Production and Th1 Differentiation

GADD45A: With or without you

The growth arrest and DNA damage inducible (GADD)45 family includes three small and ubiquitously distributed proteins (GADD45A, GADD45B, and GADD45G) that regulate numerous cellular processes associated with stress signaling and injury response. Here, we provide a comprehensive review of the current literature investigating GADD45A, the first discovered member of the family. We first depict how its levels are regulated by a myriad of genotoxic and non-genotoxic stressors, and through the combined action of intricate transcriptional, posttranscriptional, and even, posttranslational mechanisms. GADD45A is a recognized tumor suppressor and, for this reason, we next summarize its role in cancer, as well as the different mechanisms by which it regulates cell cycle, DNA repair, and apoptosis. Beyond these most well-known actions, GADD45A may also influence catabolic and anabolic pathways in the liver, adipose tissue and skeletal muscle, among others. Not surprisingly, GADD45A may trigger AMP-activated protein kinase activity, a master regulator of metabolism, and is known to act as a transcriptional coregulator of numerous nuclear receptors. GADD45A has also been reported to display a cytoprotective role by regulating inflammation, fibrosis and oxidative stress in several organs and tissues, and is regarded an important contributor for the development of heart failure. Overall data point to that GADD45A may play an important role in metabolic, neurodegenerative and cardiovascular diseases, and also autoimmune-related disorders. Thus, the potential mechanisms by which dysregulation of GADD45A activity may contribute to the progression of these diseases are also reviewed below.

Bacterial extracellular vesicles and their interplay with the immune system

The mammalian intestinal tract harbors trillions of microorganisms confined within this space by mucosal barriers. Despite these barriers, bacterial components may still be found elsewhere in the body, even in healthy subjects. Bacteria can release small lipid-bound particles, also named bacterial extracellular vesicles (bEV). While bacteria themselves cannot normally penetrate the mucosal defense, bEVs may infiltrate the barrier and disseminate throughout the body. The extremely diverse cargo that bEVs can carry, depending on their parent species, strain, and growth conditions, grant them an equally broad potential to interact with host cells and influence immune functions. Herein, we review the current knowledge of processes underlying the uptake of bEVs by mammalian cells, and their effect on the immune system. Furthermore, we discuss how bEVs could be targeted and manipulated for diverse therapeutic purposes.

Growth arrest and DNA damage-inducible proteins (GADD45) in psoriasis

The interplay between T cells, dendritic cells and keratinocytes is crucial for the development and maintenance of inflammation in psoriasis. GADD45 proteins mediate DNA repair in different cells including keratinocytes. In the immune system, GADD45a and GADD45b regulate the function and activation of both T lymphocytes and dendritic cells and GADD45a links DNA repair and epigenetic regulation through its demethylase activity. Here, we analyzed the expression of GADD45a and GADD45b in the skin, dendritic cells and circulating T cells in a cohort of psoriasis patients and their regulation by inflammatory signals. Thirty patients (17 male/13 female) with plaque psoriasis and 15 controls subjects (7 male/8 female), were enrolled. Psoriasis patients exhibited a lower expression of GADD45a at the epidermis but a higher expression in dermal infiltrating T cells in lesional skin. The expression of GADD45a and GADD45b was also higher in peripheral T cells from psoriasis patients, although no differences were observed in p38 activation. The expression and methylation state of the GADD45a target UCHL1 were evaluated, revealing a hypermethylation of its promoter in lesional skin compared to controls. Furthermore, reduced levels of GADD45a correlated with a lower expression UCHL1 in lesional skin. We propose that the demethylase function of GADD45a may account for its pleiotropic effects, and the complex and heterogeneous pattern of expression observed in psoriatic disease.

GADD45A is a protective modifier of neurogenic skeletal muscle atrophy

Neurogenic muscle atrophy is the loss of skeletal muscle mass and function that occurs with nerve injury and in denervating diseases, such as amyotrophic lateral sclerosis. Aside from prompt restoration of innervation and exercise where feasible, there are currently no effective strategies for maintaining skeletal muscle mass in the setting of denervation. We conducted a longitudinal analysis of gene expression changes occurring in atrophying skeletal muscle and identified growth arrest and DNA damage-inducible A (Gadd45a) as a gene that shows one of the earliest and most sustained increases in expression in skeletal muscle after denervation. We evaluated the role of this induction using genetic mouse models and found that mice lacking GADD45A showed accelerated and exacerbated neurogenic muscle atrophy, as well as loss of fiber type identity. Our genetic analyses demonstrate that, rather than directly contributing to muscle atrophy as proposed in earlier studies, GADD45A induction likely represents a protective negative feedback response to denervation. Establishing the downstream effectors that mediate this protective effect and the pathways they participate in may yield new opportunities to modify the course of muscle atrophy.

GADD45a and GADD45b Genes in Rheumatoid Arthritis and Systemic Lupus Erythematosus Patients

Background: GADD45 genes are stress sensors in response to cellular stress response, activated signal pathways leading to the stimulation of inflammatory cytokines. This study is to examine the associations of GADD45a and GADD45b genes with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) patients. Methods: 230 patients of RA, 140 patients of SLE, and 191 healthy controls were enrolled. Genomic DNA was extracted from peripheral blood mononuclear cells and gene polymorphisms were genotyped by TaqMan assay. RNA expression was quantitated with real-time polymerase chain reaction. Results: The RNA expression of the GADD45b gene was significantly lower in RA patients than the control cases (p = 0.03). The odds ratio of GADD45a genotype -589 CC (rs581000) was significantly low (OR = 0.36, 95% CI, 0.15–0.87) in DR4-negative RA patients. The odds ratio of GADD45b genotype -712CT (rs3795024) in DR4-negative RA patients was 0.41 (95% CI, 0.18–0.95). In clinical manifestation, the odds ratio of GADD45b -712CT genotype with anti-RNP antibody was 4.14 (95% CI, 1.10–15.63) in SLE patients. GADD45a genotype -589GG+GC was associated with rheumatoid factor (RF) in SLE patients. Conclusions: Genotypes GADD45a -589CC and GADD45b -712CT were shown to be less susceptible to RA and related to the disease state in SLE patients.

A Human Proteome Array Approach to Identifying Key Host Proteins Targeted by Toxoplasma Kinase ROP18

Toxoplasma kinase ROP18 is a key molecule responsible for the virulence of Toxoplasma gondii; however, the mechanisms by which ROP18 exerts parasite virulence via interaction with host proteins remain limited to a small number of identified substrates. To identify a broader array of ROP18 substrates, we successfully purified bioactive mature ROP18 and used it to probe a human proteome array. Sixty eight new putative host targets were identified. Functional annotation analysis suggested that these proteins have a variety of functions, including metabolic process, kinase activity and phosphorylation, cell growth, apoptosis and cell death, and immunity, indicating a pleiotropic role of ROP18 kinase. Among these proteins, four candidates, p53, p38, UBE2N, and Smad1, were further validated. We demonstrated that ROP18 targets p53, p38, UBE2N, and Smad1 for degradation. Importantly, we demonstrated that ROP18 phosphorylates Smad1 Ser-187 to trigger its proteasome-dependent degradation. Further functional characterization of the substrates of ROP18 may enhance understanding of the pathogenesis of Toxoplasma infection and provide new therapeutic targets. Similar strategies could be used to identify novel host targets for other microbial kinases functioning at the pathogen-host interface.

Gadd45 in stress signaling, cell cycle control, and apoptosis

The first identified Gadd45 gene, Gadd45a, encodes a ubiquitously expressed protein that is often induced by DNA damage and other stress signals associated with growth arrest and apoptosis. This protein and the other two members of this small gene family, Gadd45b and Gadd45g, have been implicated in a variety of the responses to cell injury including cell cycle checkpoints, apoptosis, and DNA repair. In vivo, many of the prominent roles for the Gadd45 proteins are associated with signaling mediated by p38 mitogen-activated protein kinases (MAPK). Gadd45 proteins can contribute to p38 activation either by activation of upstream kinase(s) or by direct interaction. In vivo, there are important tissue and cell-type-specific differences in the roles for Gadd45 in MAPK signaling. In addition to being p53-regulated, Gadd45a has been found to contribute to p53 activation via p38. Like other stress and signaling proteins, Gadd45 proteins show complex regulation and numerous effectors.

Gadd45 proteins in immunity

The vertebrate immune system protects the host against invading pathogens such as viruses, bacteria and parasites. It consists of an innate branch and an adaptive branch that provide immediate and long-lasting protection, respectively. As the immune system is composed of different cell types and distributed throughout the whole body, immune cells need to communicate with each other. Intercellular communication in the immune system is mediated by cytokines, which bind to specific receptors on the cell surface and activate intracellular signalling networks. Growth arrest and DNA damage-inducible 45 (Gadd45) proteins are important components of these intracellular signalling networks. They are induced by a number of cytokines and by bacterial lipopolysaccharide. Within the innate immune system, Gadd45 proteins are crucial for the differentiation of myeloid cells as well as for the function of granulocytes and macrophages. Moreover, Gadd45β regulates autophagy, a catabolic pathway that also degrades intracellular pathogens. Regarding adaptive immunity, Gadd45 proteins are especially well characterized in T cells. For instance, Gadd45β and Gadd45γ regulate cytokine expression and Th1 differentiation, while Gadd45α inhibits p38 kinase activation downstream of the T cell receptor. Due to their many functions in the immune system, deficiency in Gadd45 proteins causes autoimmune diseases and less efficient tumour immunosurveillance.

p38 mitogen-activated protein kinase (p38 MAPK)-mediated autoimmunity: lessons to learn from ANCA vasculitis and pemphigus vulgaris

Evidence is beginning to accumulate that p38 mitogen activated protein kinase (p38 MAPK) signaling pathway plays an important role in the regulation of cellular and humoral autoimmune responses. The exact mechanisms and the degree by which the p38 MAPK pathway participates in the immune-mediated induction of diseases have started to emerge. This review discusses the recent advances in the molecular dissection of the p38 MAPK pathway and the findings generated by reports investigating its role in the pathogenesis of autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, and autoimmune hepatitis. Application of newly-developed protocols based on sensitive flow cytometric detection has proven to be a useful tool in the investigation of the phosphorylation of p38 MAPK within different peripheral blood mononuclear cell populations and may help us to better understand the enigmatic role of this signaling cascade in the induction of autoimmunity as well as its role in immunosuppressive-induced remission. Special attention is paid to reported data proposing a specific role for autoantibody-induced activation of p38 MAPK-mediated immunopathology in the pathogenesis of autoimmune blistering diseases and anti-neutrophilic antibody-mediated vasculitides.

Outer membrane vesicles of a human commensal mediate immune regulation and disease protection

Commensal bacteria impact host health and immunity through various mechanisms, including the production of immunomodulatory molecules. Bacteroides fragilis produces a capsular polysaccharide (PSA), which induces regulatory T cells and mucosal tolerance. However, unlike pathogens, which employ secretion systems, the mechanisms by which commensal bacteria deliver molecules to the host remain unknown. We reveal that Bacteroides fragilis releases PSA in outer membrane vesicles (OMVs) that induce immunomodulatory effects and prevent experimental colitis. Dendritic cells (DCs) sense OMV-associated PSA through TLR2, resulting in enhanced regulatory T cells and anti-inflammatory cytokine production. OMV-induced signaling in DCs requires growth arrest and DNA-damage-inducible protein (Gadd45α). DCs treated with PSA-containing OMVs prevent experimental colitis, whereas Gadd45α(-/-) DCs are unable to promote regulatory T cell responses or suppress proinflammatory cytokine production and host pathology. These findings demonstrate that OMV-mediated delivery of a commensal molecule prevents disease, uncovering a mechanism of interkingdom communication between the microbiota and mammals.

Multiple sclerosis-linked and interferon-beta-regulated gene expression in plasmacytoid dendritic cells

The cause of multiple sclerosis (MS) is not known and the mechanism of interferon-beta, a disease-modifying treatment, is not well-understood. We studied gene expression in plasmacytoid dendritic cells (pDCs), antigen-presenting cells implicated in MS pathogenesis. PDCs were separated from healthy donors and MS patients at two time points: before and after initiation of treatment with interferon-beta. Expression of selected MS-linked and interferon-beta-regulated genes was validated with single assays. We have identified 60 genes which were abnormally expressed in MS patients and were corrected after treatment. These genes could be studied as potential MS biomarkers and possible therapeutic targets in MS.

Lack of the T cell-specific alternative p38 activation pathway reduces autoimmunity and inflammation

Stimulation via the T-cell receptor (TCR) activates p38α and p38β by phosphorylation of p38 Tyr-323 (p38(Y323)). Here we characterize knockin mice in which p38α and/or β Tyr-323 has been replaced with Phe. We find that p38α accounts for two-thirds and p38β the remainder of TCR-induced p38 activation. T cells from double knockin mice (p38αβ(Y323F)) had defects in TCR-mediated proliferation and Th1 and Th17 skewing, the former corresponding with an inability to sustain T-bet expression. Introduction of p38α(Y323F) into Gadd45α-deficient mice, in which the alternative p38 pathway is constitutively active, reversed T-cell hyperproliferation and autoimmunity. Furthermore, p38αβ(Y323F) mice had delayed onset and reduced severity of the inflammatory autoimmune diseases collagen-induced arthritis and experimental autoimmune encephalomyelitis. Thus, T cell-specific alternative activation of p38 is an important pathway in T-cell proliferation, Th skewing, and inflammatory autoimmunity, and may be an attractive tissue-specific target for intervention in these processes.

The regulation of Th1 responses by the p38 MAPK

IL-12 is a dimeric cytokine that is produced primarily by APCs. In this study we examined the role that the p38 MAPKs (MAPK/p38) play in regulating IL-12 production. We show that inhibition of p38 dramatically increased IL-12 production upon stimulation, while decreasing TNF-α. This reciprocal effect on these two cytokines following MAPK/p38 inhibition occurred in many different APCs, following a variety of different stimuli. IL-12 production was also increased in macrophages treated with small interfering RNA to limit p38α expression, and in macrophages deficient in MKK3, a kinase upstream of p38. The increase in IL-12 production following MAPK/p38 inhibition appears to be due to enhanced IL-12 (p40) mRNA stability. We show that MAPK/p38 inhibition can promote Th1 immune responses and thereby enhance vaccine efficacy against leishmaniasis. In a mouse model of Leishmania major infection, vaccination with heat-killed L. major plus CpG and SB203580 elicited complete protection against infection compared with heat-killed L. major plus CpG without SB203580. Thus, this work suggests that MAPK/p38 inhibitors may be applied as adjuvants to bias immune responses and improve vaccinations against intracellular pathogens.

Transcriptional profiles during the differentiation and maturation of monocyte-derived dendritic cells, analyzed using focused microarrays

Dendritic cells (DC) are professional antigen-presenting cells capable of initiating primary immune responses. They have been intensively studied and are used in both basic immunology research and clinical immunotherapy. However, the genetic pathways leading to DC differentiation and maturation remain poorly understood. Using focused microarrays with oligonucletotide probes for 120 genes encoding co-stimulatory molecules, chemokines, chemokine receptors, cytokines, cytokine receptors, TLRs, and several other related molecules, we analyzed the kinetics of gene expression for the overall differentiation process of monocytes into mature DC. In parallel, we compared the transcriptional profiles in DC maturation in the presence of LPS, TNF-alpha or trimeric CD40L. We found similar transcriptional profiles for early immature DC and immature DC, respectively generated by culturing monocytes with GM-CSF and IL-4 for three or six days. We identified sets of common and stimuli-specific genes, the expression of which changed following stimulation with LPS, TNF-alpha or CD40L. A dynamic analysis of the entire DC differentiation and maturation process showed that some important inflammatory and constitutive chemokines are transcribed in both immature and mature DC. The correlative expression kinetics of the gene pairs IL1R1/IL1R2, IL15/IL15RA, DC-SIGN/ICAM-2 and DC-SIGN/ICAM-3 imply that they all play crucial roles in mediating DC functions. Thus, our analysis with focused microarrays shed light on the transcriptional kinetics of DC differentiation and maturation, and this method may also prove useful for identifying novel marker genes involved in DC functions.

Gadd45 modulation of intrinsic and extrinsic stress responses in myeloid cells

Gadd45 proteins modulate signaling in response to physiological and environmental stressors. Expression of gadd45 genes is rapidly induced by different stressors, including differentiation-inducing cytokines and genotoxic stress. Induction of gadd45 genes at the onset of myeloid differentiation suggested that Gadd45 protein(s) play a role in hematopoiesis, yet no apparent abnormalities were observed in either the bone marrow (BM) or peripheral blood compartments of mice deficient for either gadd45a or gadd45b. However, under conditions of hematological stress, including acute stimulation with cytokines, myelo-ablation and inflammation, both gadd45a-deficient and gadd45b-deficient mice exhibited deficiencies. This is discussed within the context of what is known about Gadd45 proteins in stress signaling, hematopoietic development and the innate immune response. Furthermore, myeloid enriched BM cells from gadd45a and gadd45b deficient mice were observed to be more sensitive to ultraviolet radiation (UVC), VP-16 and daunorubicin (DNR) induced apoptosis compared to wild-type (WT) cells, displaying defective G2/M arrest following exposure to UVC and VP-16, but not to DNR. Novel mechanisms that mediate the pro-survival functions of Gadd45 in hematopoietic cells following UV irradiation were demonstrated, involving activation of the Gadd45a-p38-NF-kappaB survival pathway and Gadd45b mediated inhibition of the stress response MKK4-JNK apoptotic pathway. The ramifications regarding the pathogenesis of different leukemias and the response of normal and malignant hematopoietic cells to chemo- and radiation-therapy, as well as other challenges to the hematopoietic compartment, are discussed.

Genetic disruption of p38alpha Tyr323 phosphorylation prevents T-cell receptor-mediated p38alpha activation and impairs interferon-gamma production

T cells possess a p38 activation alternative pathway in which stimulation via the antigen receptor (T-cell receptor [TCR]) induces phosphorylation of p38alpha and beta on Tyr323. To assess the contribution of this pathway to normal T-cell function, we generated p38alpha knockin mice in which Tyr323 was replaced with Phe (p38alpha(Y323F)). TCR-mediated stimulation failed to activate p38alpha(Y323F) as measured by phosphorylation of the Thr-Glu-Tyr activation motif and p38alpha catalytic activity. Cell-cycle entry was delayed in TCR-stimulated p38alpha(Y323F) T cells, which also produced less interferon (IFN)-gamma than wild-type T cells in response to TCR-mediated but not TCR-independent stimuli. p38alpha(Y323F) mice immunized with T-helper 1 (Th1)-inducing antigens generated normal Th1 effector cells, but these cells produced less IFN-gamma than wild-type cells when stimulated through the TCR. Thus, the Tyr323-dependent pathway and not the classic mitogen-activated protein (MAP) kinase cascade is the physiologic means of p38alpha activation through the TCR and is necessary for normal Th1 function but not Th1 generation.

Role of gadd45 in myeloid cells in response to hematopoietic stress

The gadd45 family of genes is rapidly induced by different stressors, including differentiation-inducing cytokines, and there is a large body of evidence that their cognate proteins are key players in cellular stress responses. Induction of gadd45 genes at the onset of myeloid differentiation suggested that Gadd45 protein(s) play a role in hematopoiesis, yet no apparent abnormalities were observed in either the bone marrow or peripheral blood compartments of mice deficient for either gadd45a or gadd45b. However, under conditions of hematological stress, including acute stimulation with cytokines, myelo-ablation and inflammation, both gadd45a-deficient and gadd45b-deficient mice exhibited deficiencies. This topic is discussed within the context of what is known about Gadd45 proteins in stress signaling, hematopoietic development and the innate immune response.