SUMO conjugation contributes to immune deviation in nonobese diabetic mice by suppressing c-Maf transactivation of IL-4

Molecular Mechanisms Driving IL-10- Producing B Cells Functions: STAT3 and c-MAF as Underestimated Central Key Regulators?

Regulatory B cells (Bregs) have been highlighted in very different pathology settings including autoimmune diseases, allergy, graft rejection, and cancer. Improving tools for the characterization of Bregs has become the main objective especially in humans. Transitional, mature B cells and plasma cells can differentiate into IL-10 producing Bregs in both mice and humans, suggesting that Bregs are not derived from unique precursors but may arise from different competent progenitors at unrestricted development stages. Moreover, in addition to IL-10 production, regulatory B cells used a broad range of suppressing mechanisms to modulate the immune response. Although Bregs have been consistently described in the literature, only a few reports described the molecular aspects that control the acquisition of the regulatory function. In this manuscript, we detailed the latest reports describing the control of IL-10, TGFβ, and GZMB production in different Breg subsets at the molecular level. We focused on the understanding of the role of the transcription factors STAT3 and c-MAF in controlling IL-10 production in murine and human B cells and how these factors may represent an important crossroad of several key drivers of the Breg response. Finally, we provided original data supporting the evidence that MAF is expressed in human IL-10- producing plasmablast and could be induced in vitro following different stimulation cocktails. At steady state, we reported that MAF is expressed in specific human B-cell tonsillar subsets including the IgD⁺ CD27⁺ unswitched population, germinal center cells and plasmablast.

SUMOylation of PDPK1 Is required to maintain glycolysis-dependent CD4 T-cell homeostasis

The immune system is finely tuned to fight against infections, eradicate neoplasms, and prevent autoimmunity. Protein posttranslational modification (PTM) constitutes a molecular layer of regulation to guarantee the proper intensity of immune response. Herein, we report that UBC9-mediated protein SUMOylation plays an essential role in peripheral CD4 T-cell proliferation, but without a perceptible impact on T-cell polarization. Both conventional T-cell (Tcon) and regulatory T-cell (Treg) maintenance are differentially affected, which was likely caused by a shared deficit in cell glycolytic metabolism. Mechanistically, PDPK1 (3-phosphoinositide-dependent protein-kinase 1) was identified as a novel SUMOylation substrate, which occurred predominantly at lysine 299 (K299) located within the protein-kinase domain. Loss of PDPK1 SUMOylation impeded its autophosphorylation at serine 241 (S241), thereby leading to hypoactivation of downstream mTORC1 signaling coupled with incompetence of cell proliferation. Altogether, our results revealed a novel regulatory mechanism in peripheral CD4 T-cell homeostatic proliferation, which involves SUMOylation regulation of PDPK1–mTORC1 signaling-mediated glycolytic process.

Post-Translational Modifications in Transcription Factors that Determine T Helper Cell Differentiation

CD4+ T helper (Th) cells play a crucial role in the modulation of innate and adaptive immune responses through the differentiation of Th precursor cells into several subsets, including Th1, Th2, Th17, and regulatory T (Treg) cells. Effector Th and Treg cells are distinguished by the production of signature cytokines and are important for eliminating intracellular and extracellular pathogens and maintaining immune homeostasis. Stimulation of naïve Th cells by T cell receptor and specific cytokines activates master transcription factors and induces lineage specification during the differentiation of Th cells. The master transcription factors directly activate the transcription of signature cytokine genes and also undergo post-translational modifications to fine-tune cytokine production and maintain immune balance through cross-regulation with each other. This review highlights the post-translational modifications of master transcription factors that control the differentiation of effector Th and Treg cells and provides additional insights on the immune regulation mediated by protein arginine-modifying enzymes in effector Th cells.

Post-Translational Modifications of Transcription Factors Harnessing the Etiology and Pathophysiology in Colonic Diseases

Defects in mucosal immune balance can lead to colonic diseases such as inflammatory bowel diseases and colorectal cancer. With the advancement of understanding for the immunological and molecular basis of colonic disease, therapies targeting transcription factors have become a potential approach for the treatment of colonic disease. To date, the biomedical significance of unique post-translational modifications on transcription factors has been identified, including phosphorylation, methylation, acetylation, ubiquitination, SUMOylation, and O-GlcNAcylation. This review focuses on our current understanding and the emerging evidence of how post-translational regulations modify transcription factors involved in the etiology and pathophysiology of colonic disease as well as the implications of these findings for new therapeutic approaches in these disorders.

c-MAF, a Swiss Army Knife for Tolerance in Lymphocytes

Beyond its well-admitted role in development and organogenesis, it is now clear that the transcription factor c-Maf has owned its place in the realm of immune-related transcription factors. Formerly introduced solely as a Th2 transcription factor, the role attributed to c-Maf has gradually broadened over the years and has extended to most, if not all, known immune cell types. The influence of c-Maf is particularly prominent among T cell subsets, where c-Maf regulates the differentiation as well as the function of multiple subsets of CD4 and CD8 T cells, lending it a crucial position in adaptive immunity and anti-tumoral responsiveness. Recent research has also revealed the role of c-Maf in controlling Th17 responses in the intestine, positioning it as an essential factor in intestinal homeostasis. This review aims to present and discuss the recent advances highlighting the particular role played by c-Maf in T lymphocyte differentiation, function, and homeostasis.

SUMO-defective c-Maf preferentially transactivates Il21 to exacerbate autoimmune diabetes

SUMOylation is involved in the development of several inflammatory diseases, but the physiological significance of SUMO-modulated c-Maf in autoimmune diabetes is not completely understood. Here, we report that an age-dependent attenuation of c-Maf SUMOylation in CD4+ T cells is positively correlated with the IL-21-mediated diabetogenesis in NOD mice. Using 2 strains of T cell-specific transgenic NOD mice overexpressing wild-type c-Maf (Tg-WTc) or SUMOylation site-mutated c-Maf (Tg-KRc), we demonstrated that Tg-KRc mice developed diabetes more rapidly than Tg-WTc mice in a CD4+ T cell-autonomous manner. Moreover, SUMO-defective c-Maf preferentially transactivated Il21 to promote the development of CD4+ T cells with an extrafollicular helper T cell phenotype and expand the numbers of granzyme B-producing effector/memory CD8+ T cells. Furthermore, SUMO-defective c-Maf selectively inhibited recruitment of Daxx/HDAC2 to the Il21 promoter and enhanced histone acetylation mediated by CREB-binding protein (CBP) and p300. Using pharmacological interference with CBP/p300, we illustrated that CBP30 treatment ameliorated c-Maf-mediated/IL-21-based diabetogenesis. Taken together, our results show that the SUMOylation status of c-Maf has a stronger regulatory effect on IL-21 than the level of c-Maf expression, through an epigenetic mechanism. These findings provide new insights into how SUMOylation modulates the pathogenesis of autoimmune diabetes in a T cell-restricted manner and on the basis of a single transcription factor.

The ubiquitin ligase HERC4 mediates c-Maf ubiquitination and delays the growth of multiple myeloma xenografts in nude mice

The transcription factor c-Maf is extensively involved in the pathophysiology of multiple myeloma (MM), a fatal malignancy of plasma cells. In the present study, affinity chromatography and mass spectrometry were used to identify c-Maf ubiquitination-associated proteins, from which the E3 ligase HERC4 was found to interact with c-Maf and catalyzed its polyubiquitination and subsequent proteasome-mediated degradation. HERC4 mediated polyubiquitination at K85 and K297 in c-Maf, and this polyubiquitination could be prevented by the isopeptidase USP5. Further analysis on the NCI-60 cell line collection revealed that RPMI 8226, a MM-derived cell line, expressed the lowest level of HERC4. Primary bone marrow analysis revealed HERC4 expression was high in normal bone marrow, but was steadily decreased during myelomagenesis. These findings suggested HERC4 played an important role in MM progression. Moreover, ectopic HERC4 expression decreased MM proliferation in vitro, and delayed xenograft tumor growth in vivo. Therefore, modulation of c-Maf ubiquitination by targeting HERC4 may represent a new therapeutic modality for MM.

Regulation of IL-4 Expression in Immunity and Diseases

IL-4 was first identified as a T cell-derived growth factor for B cells. Studies over the past several decades have markedly expanded our understanding of its cellular sources and function. In addition to T cells, IL-4 is produced by innate lymphocytes, such as NTK cells, and myeloid cells, such as basophils and mast cells. It is a signature cytokine of type 2 immune response but also has a nonimmune function. Its expression is tightly regulated at several levels, including signaling pathways, transcription factors, epigenetic modifications, microRNA, and long noncoding RNA. This chapter will review in detail the molecular mechanism regulating the cell type-specific expression of IL-4 in physiological and pathological type 2 immune responses.

Multiple functions of Maf in the regulation of cellular development and differentiation

Cellular muscular aponeurotic fibrosarcoma (c-Maf) is a member of the large macrophage-activating factor family. C-Maf plays important roles in the morphogenetic processes and cellular differentiation of the lens, kidneys, liver, T cells and nervous system, and it is particularly important in pancreatic islet and erythroblastic island formation. However, the exact role of c-Maf remains to be elucidated. In this review, we summarize the research to clarify the functions of c-Maf in the cellular development and differentiation. The expression of c-Maf is higher in pancreatic duct cells than in pancreatic islet cells. Therefore, we suggest that pancreatic duct cells may be converted to the functional insulin-secreting cells by regulating c-Maf.

Ubiquitination of the transcription factor c-MAF is mediated by multiple lysine residues

The transcription factor c-MAF could be polyubiquitinated and subsequently degraded in the proteasomes. Theoretically, any lysine residues in c-MAF could be ubiquitinated. In the present study, we tried to find out the specific lysine residue(s) mediating c-MAF ubiquitination. Through a series of mutational screens from lysine (K) to arginine (R), we found that any single lysine mutation (K to R) failed to prevent c-MAF ubiquitination, and any single lysine residue alone could not mediate c-MAF ubiquitination, which indicated that multiple lysine residues were required for c-MAF ubiquitination. Bioinformatics and computing analyses revealed that K85 and K350 could mediate c-MAF ubiquitination, which was confirmed by the cell-based assays. However, this duo was not the only pair because the K85R/K350R mutant could also be ubiquitinated. Functionally, both M12 (K85/K350) and W12 (K85R/K350R) mutants increased cyclin D2 promoter-driven luciferase activity, but they were less potent than the lysine-free counterpart (M14). In addition, M14 induced a higher level of expression of cyclin D2 at both mRNA and protein levels. Therefore, we demonstrated that c-MAF ubiquitination is mediated by multiple lysine residues, of which K85 and K350 were sufficient but not the only residues in mediating c-MAF ubiquitination. Moreover, c-MAF was found to be degraded by lysosomes. This study added a novel insight for c-MAF ubiquitination and degradation, suggesting that c-MAF stability is strictly regulated.

Sumoylation modulates oxidative stress relevant to the viability and functionality of pancreatic beta cells

Sumoylation is an evolutionarily conserved regulatory mechanism to play an important role in various cellular processes through modulation of protein localization, stability and functionality. Recent studies including ours have consistently demonstrated that sumoylation provides protection for cells against oxidative stress. Given that pancreatic beta cells are a vulnerable target of oxidative stress, we thus in this minireview, updated the advancement of sumoylation in the regulation of ROS generation, and discussed its impact on several critical signaling pathways relevant to beta cells against oxidative stress and maintenance of functionality. Specifically, we bring together how sumoylation represses intracellular ROS formation, and protects beta cells against oxidative stress through regulating IκB/NFκB, JNK/c-Jun, and Maf/Nrf2 pathways. The tight implication of sumoylation in oxidative stress reflects that it could be an essential mechanism for beta cells to adapt to the detrimental cellular microenvironment.

Anti-TCR therapy combined with fingolimod for reversal of diabetic hyperglycemia by β cell regeneration in the LEW.1AR1-iddm rat model of type 1 diabetes

The therapeutic capacity of an antibody directed against the T cell receptor (anti-TCR) of the TCR/CD3 complex alone or in combination with fingolimod (FTY720) to reverse the diabetic metabolic state through suppression of autoimmunity and stimulation of β cell regeneration was analyzed in the LEW.1AR1-iddm (IDDM) rat, an animal model of human type 1 diabetes. Animals were treated with anti-TCR (0.5 mg/kg body weight for 5 days) monotherapy or in combination with fingolimod (1 mg/kg body weight for 40 days). Metabolic changes and β cell morphology were analyzed before, immediately after, and 60 days after end of therapy. Both therapies were started early after disease manifestation and led to normoglycemia in parallel with an increase of the C-peptide concentration. Combination therapy increased the β cell mass reaching a range of normoglycemic controls, decreased the apoptosis rate fivefold, and increased the proliferation rate threefold. Additionally, at 60 days after therapy, islets were virtually free of T cells, macrophages, and cytokine expression. In contrast, after anti-TCR monotherapy, β cell mass remained low with an activated immune cell infiltrate. A concomitant fivefold increased β cell apoptosis rate resulted in a complete loss of β cells. Only combination therapy yielded sustained normoglycemia with full reversal of islet infiltration and restoration of pancreatic β cell mass.

KEY MESSAGE

Combination therapy of anti-TCR and fingolimod was effective in the reversal of T1D. Combination therapy increased the pancreatic β cell mass to normoglycemic control levels. Combination therapy leads to a full reversal of pancreatic islet infiltration. Anti-TCR monotherapy did not abolish islet infiltration. Combination therapy was successful only immediately after diabetes manifestation.

Tyrosine phosphorylation of c-Maf enhances the expression of IL-4 gene

Maf proteins are involved in a variety of biological processes, such as oncogenesis, lens development, and differentiation. In immune system, c-Maf transactivates IL-4 promoter, and ectopic expression of c-Maf skews primary T cell response toward the Th2 pathway. Numerous transcription factors are subjected to posttranslational modification. In this study, to our knowledge, we show for the first time that c-Maf is subjective to tyrosine phosphorylation in Th cells and that the level of its tyrosine phosphorylation positively correlates with IL-4 expression by peripheral Th cells, but is negatively associated with the severity of disease in NOD mice. c-Maf undergoes tyrosine phosphorylation at Tyr(21), Tyr(92), and Tyr(131) residues in Th2 cells. Furthermore, tyrosine phosphorylation at these three residues is critical for the recruitment of c-Maf to IL-4 promoter and IL-4 production in Th cells. Taken together, this study sheds new light on the role of posttranslational modification of c-Maf in IL-4 production and Th cell-mediated autoimmune diseases.

SUMOylation negatively regulates transcriptional and oncogenic activities of MafA

Dysregulated expression of Maf proteins (namely c-Maf, MafA and MafB) leads to multiple myeloma in humans and oncogenic transformation of chicken embryonic fibroblasts. Maf proteins are transcriptional activators of tissue-specific gene expression and regulators of cell differentiation. For example, MafA is a critical regulator of crystallin genes and the lens differentiation program in chickens. In mammals, MafA is essential for the development of mature insulin-producing beta-cells of pancreas. It has been shown that MafA protein stability is regulated by phosphorylations at multiple serine and threonine residues. Here, we report that Maf proteins are also post-translationally modified by small ubiquitin-like modifier (SUMO) proteins at a conserved lysine residue in the amino-terminal transactivator domain. A SUMOylation-deficient mutant of MafA (K32R) was more potent than wild-type MafA in transactivating luciferase reporter construct driven by alphaA-crystallin or insulin gene promoter. In ovo electroporation into developing chicken embryo showed that the K32R mutant induced ectopic delta-crystallin gene expression more efficiently than the wild-type MafA. We also demonstrated that the K32R mutant had enhanced ability to induce colony formation of a chicken fibroblast cell line DF-1. Therefore, SUMOylation is a functional post-translational modification of MafA that negatively regulates its transcriptional and transforming activities.

Expression analysis of Ubc9, the single small ubiquitin-like modifier (SUMO) E2 conjugating enzyme, in normal and malignant tissues

Unlike ubiquitination, which targets proteins for degradation, sumoylation modulates protein-protein interactions of target proteins. Although there are multiple E2 enzymes required for ubiquitination, there is only one E2-conjugating enzyme for sumoylation, which is Ubc9. In line with increasing evidence that sumoylation plays an important role in tumorigenesis, we recently demonstrated that Ubc9 is expressed at high levels in advanced melanomas and that blocking expression of Ubc9 sensitizes melanomas to the cytotoxic effects of chemotherapeutic drugs. To determine whether and to what extent Ubc9 is expressed in other malignancies and their normal tissue counterparts, we undertook a detailed analysis of colon, lung, prostate, and breast cancer tissue microarrays. The findings, presented here, document that in primary colon and prostate cancer, Ubc9 expression is increased compared with their normal tissue counterparts, whereas in metastatic breast, prostate, and lung cancer, it is decreased in comparison with their corresponding normal and primary adenocarcinoma tissues. We also provide evidence that Ubc9 expression correlates positively with Dukes' stage and negatively with the Gleason score as well as breast cancer grade and that Ubc9 expression is substantially higher in the luminal than in the nonluminal type of breast cancer.

Sumoylation of bZIP transcription factor NRL modulates target gene expression during photoreceptor differentiation

Development of rod photoreceptors in the mammalian retina is critically dependent on the basic motif-leucine zipper transcription factor NRL (neural retina leucine zipper). In the absence of NRL, photoreceptor precursors in mouse retina produce only cones that primarily express S-opsin. Conversely, ectopic expression of NRL in post-mitotic precursors leads to a rod-only retina. To explore the role of signaling molecules in modulating NRL function, we identified putative sites of post-translational modification in the NRL protein by in silico analysis. Here, we demonstrate the sumoylation of NRL in vivo and in vitro, with two small ubiquitin-like modifier (SUMO) molecules attached to the Lys-20 residue. NRL-K20R and NRL-K20R/K24R sumoylation mutants show reduced transcriptional activation of Nr2e3 and rhodopsin promoters (two direct targets of NRL) in reporter assays when compared with wild-type NRL. Consistent with this, in vivo electroporation of the NRL-K20R/K24R mutant into newborn Nrl(-/-) mouse retina leads to reduced Nr2e3 activation and only a partial rescue of the Nrl(-/-) phenotype in contrast to the wild-type NRL that is able to convert cones to rod photoreceptors. Although PIAS3 (protein inhibitor of activated STAT3), an E3-SUMO ligase implicated in photoreceptor differentiation, can be immunoprecipitated with NRL, there appears to be redundancy in E3 ligases, and PIAS3 does not seem to be essential for NRL sumoylation. Our studies suggest an important role of sumoylation in fine-tuning the activity of NRL and thereby incorporating yet another layer of control in gene regulatory networks involved in photoreceptor development and homeostasis.

SUMOylation attenuates c-Maf-dependent IL-4 expression

The function of transcription factors can be critically regulated by SUMOylation. c-Maf, the cellular counterpart of v-maf oncogene, is a potent transactivator of the IL-4 gene in Th2 cells. We found in a yeast two-hybrid screen that c-Maf can interact with Ubc9 and PIAS1, two key enzymes of the SUMOylation pathway. In this study, we report that c-Maf co-localized with these two SUMO (small ubiquitin-like modifier) ligases in the nucleus and that c-Maf can be SUMOylated in vitro and also in primary Th2 cells. We also demonstrated that lysine-33 is the dominant, if not the only, SUMO acceptor site of c-Maf. SUMOylation of c-Maf attenuated its transcriptional activity. Reciprocally, a SUMOylation resistant c-Maf was more potent than WT-c-Maf in driving IL-4 production in c-Maf-deficient Th2 cells. Furthermore, we showed that ablation of the SUMO site did not alter the subcellular localization or the stability of c-Maf protein but instead enhanced its recruitment to the Il4-promoter. We conclude that SUMOylation at lysine-33 is a functionally critical post-translational modification event of c-Maf in Th cells.

SUMOylation and cell signalling

SUMOylation is a highly transient post-translational protein modification. Attachment of SUMO to target proteins occurs via a number of specific activating and ligating enzymes that form the SUMO-substrate complex, and other SUMO-specific proteases that cleave the covalent bond, thus leaving both SUMO and target protein free for the next round of modification. SUMO modification has major effects on numerous aspects of substrate function, including subcellular localisation, regulation of their target genes, and interactions with other molecules. The modified SUMO-protein complex is a very transient state, and it thus facilitates rapid response and actions by the cell, when needed. Like phosphorylation, acetylation and ubiquitination, SUMOylation has been associated with a number of cellular processes. In addition to its nuclear role, important sides of mitochondrial activity, stress response signalling and the decision of cells to undergo senescence or apoptosis, have now been shown to involve the SUMO pathway. With ever increasing numbers of reports linking SUMO to human disease, like neurodegeneration and cancer metastasis, it is highly likely that novel and equally important functions of components of the SUMOylation process in cell signalling pathways will be elucidated in the near future.