SUMO modification regulates MafB-driven macrophage differentiation by enabling Myb-dependent transcriptional repression

MAFB: a key regulator of myeloid commitment involved in hematological diseases

The MAFB protein, a member of the MAF family of bZip transcription factors, plays a pivotal role in various biological processes, including cell differentiation, development, and homeostasis. Characterized by its selective expression in monocytes and macrophages, MAFB has been shown to play a crucial role during myeloid lineage differentiation, acting as a critical determinant in the transition from multipotent progenitors to fully differentiated monocytes. By modulating the expression of genes associated with immune activation and inflammation, MAFB plays a vital role in maintaining immune homeostasis and responding to pathogenic challenges. Dysregulation of MAFB expression or function has been implicated in several pathological conditions, including hematological malignancies and metabolic disorders. In particular, aberrant MAFB activity has been associated with the progression of diseases such as multiple myeloma and acute myeloid leukemia as well as other solid tumors, where it may contribute to the survival and proliferation of malignant cells, thereby promoting disease progression. MAFB and downstream targets of its transcriptional network are now being regarded as predictive biomarkers for certain types of tumors as well as being considered as potential therapeutic targets for cancer treatment. In this review, we summarize current knowledge on both physiological and pathological roles of MAFB and highlight the impact of its deregulation on hematological cancer initiation and progression.

A comprehensive pan-cancer examination of transcription factor MAFF: Oncogenic potential, prognostic relevance, and immune landscape dynamics

AIMS

Previous studies indicate that MAF BZIP Transcription Factor F (MAFF) facilitates ectopic metastasis and tumor cell migration. While its role in neoplasm progression is recognized, a thorough pan-cancer analysis of MAFF's impact remains pending.

MAIN METHODS

MAFF expression across normal and tumor tissues was analyzed using transcriptomic data from Genomic Data Commons (GDC) and UCSC XENA, with protein details from Human Protein Atlas (HPA) and GeneMANIA. Tumor Immune Single-cell Hub (TISCH) and Spatial Transcriptomics Omics DataBase (STOmics DB) identified MAFF expression in the tumor microenvironment (TME). MAFF's prognostic significance and immune-related gene associations were evaluated through univariate Cox regression, TIMER2.0 immune cell infiltration analysis, and Spearman correlation. Critical pathways were identified using Gene Set Enrichment Analysis (GSEA) and Gene Set Variation Analysis (GSVA), while molecular docking explored anticancer agent interactions.

KEY FINDINGS

MAFF expression varies across cancers, affecting tumor prognosis, notably in monocytes/macrophages and endothelial cells. Copy number variation (CNV) positively correlates with MAFF expression, while methylation shows inverse correlation. MAFF mutations significantly affect LGG patient prognosis and correlate with immune therapy responses. ESTIMATE and immune profiling linked MAFF to immunosuppression pathways. Molecular docking identified MAFF-targeted drugs, with validated effects on breast cancer and endometrial cancer cell survival and migration in vitro.

SIGNIFICANCE

Multi-omics analysis identified MAFF as a potential prognostic marker correlating with tumor immunity and microenvironment, suggesting its value for personalized cancer immunotherapy, particularly in BRCA and UCEC.

Macrophage niche imprinting as a determinant of macrophage identity and function

Macrophage niches are the anatomical locations within organs or tissues consisting of various cells, intercellular and extracellular matrix, transcription factors, and signaling molecules that interact to influence macrophage self-maintenance, phenotype, and behavior. The niche, besides physically supporting macrophages, imposes a tissue- and organ-specific identity on the residing and infiltrating monocytes and macrophages. In this review, we give examples of macrophage niches and the modes of communication between macrophages and surrounding cells. We also describe how macrophages, acting against their immune defensive nature, can create a hospitable niche for pathogens and cancer cells.

Reversal of MYB-dependent suppression of MAFB expression overrides leukaemia phenotype in MLL-rearranged AML

The transcription factor MYB plays a pivotal role in haematopoietic homoeostasis and its aberrant expression is involved in the genesis and maintenance of acute myeloid leukaemia (AML). We have previously demonstrated that not all AML subtypes display the same dependency on MYB expression and that such variability is dictated by the nature of the driver mutation. However, whether this difference in MYB dependency is a general trend in AML remains to be further elucidated. Here, we investigate the role of MYB in human leukaemia by performing siRNA-mediated knock-down in cell line models of AML with different driver lesions. We show that the characteristic reduction in proliferation and the concomitant induction of myeloid differentiation that is observed in MLL-rearranged and t(8;21) leukaemias upon MYB suppression is not seen in AML cells with a complex karyotype. Transcriptome analyses revealed that MYB ablation produces consensual increase of MAFB expression in MYB-dependent cells and, interestingly, the ectopic expression of MAFB could phenocopy the effect of MYB suppression. Accordingly, in silico stratification analyses of molecular data from AML patients revealed a reciprocal relationship between MYB and MAFB expression, highlighting a novel biological interconnection between these two factors in AML and supporting new rationales of MAFB targeting in MLL-rearranged leukaemias.

The role and regulation of Maf proteins in cancer

The Maf proteins (Mafs) belong to basic leucine zipper transcription factors and are members of the activator protein-1 (AP-1) superfamily. There are two subgroups of Mafs: large Mafs and small Mafs, which are involved in a wide range of biological processes, such as the cell cycle, proliferation, oxidative stress, and inflammation. Therefore, dysregulation of Mafs can affect cell fate and is closely associated with diverse diseases. Accumulating evidence has established both large and small Mafs as mediators of tumor development. In this review, we first briefly describe the structure and physiological functions of Mafs. Then we summarize the upstream regulatory mechanisms that control the expression and activity of Mafs. Furthermore, we discuss recent studies on the critical role of Mafs in cancer progression, including cancer proliferation, apoptosis, metastasis, tumor/stroma interaction and angiogenesis. We also review the clinical implications of Mafs, namely their potential possibilities and limitations as biomarkers and therapeutic targets in cancer.

Protein sumoylation in normal and cancer stem cells

Stem cells with the capacity of self-renewal and differentiation play pivotal roles in normal tissues and malignant tumors. Whereas stem cells are supposed to be genetically identical to their non-stem cell counterparts, cell stemness is deliberately regulated by a dynamic network of molecular mechanisms. Reversible post-translational protein modifications (PTMs) are rapid and reversible non-genetic processes that regulate essentially all physiological and pathological process. Numerous studies have reported the involvement of post-translational protein modifications in the acquirement and maintenance of cell stemness. Recent studies underscore the importance of protein sumoylation, i.e., the covalent attachment of the small ubiquitin-like modifiers (SUMO), as a critical post-translational protein modification in the stem cell populations in development and tumorigenesis. In this review, we summarize the functions of protein sumoylation in different kinds of normal and cancer stem cells. In addition, we describe the upstream regulators and the downstream effectors of protein sumoylation associated with cell stemness. We also introduce the translational studies aiming at sumoylation to target stem cells for disease treatment. Finally, we propose future directions for sumoylation studies in stem cells.

SENP3 loss promotes M2 macrophage polarization and breast cancer progression

Tumor‐associated macrophages (TAM) play a crucial role in promoting cancer progression. Upon cytokine stimulation, TAM preferentially polarize to the anti‐inflammatory and pro‐tumor M2 subtype. The mechanism underlying such preferential polarization remains elusive. Here, we report that macrophage‐specific deletion of the SUMO‐specific protease Sentrin/SUMO‐specific protease 3 promotes macrophage polarization towards M2 in bone marrow‐derived macrophage (BMDM) induced by interleukin 4 (IL‐4)/IL‐13 and in an ex vivo model (murine Py8119 cell line), as well as in a mouse orthotopic tumor model. Notably, Sentrin/SUMO‐specific protease 3 (SENP3) loss in macrophages accelerated breast cancer malignancy in ex vivo and in vivo models. Mechanistically, we identified Akt Serine/threonine kinase 1 (Akt1) as the substrate of SENP3 and found that the enhanced Akt1 SUMOylation upon SENP3 loss resulted in Akt1 hyper‐phosphorylation and activation, which facilitates M2 polarization. Analysis of clinical data showed that a lower level of SENP3 in TAM has a strong negative correlation with the level of the M2 marker CD206, as well as with a worse clinical outcome. Thus, increased Akt1 SUMOylation as a result of SENP3 deficiency modulates polarization of macrophages to the M2 subtype within a breast cancer microenvironment, which in turn promotes tumor progression.

Defective Sec61α1 underlies a novel cause of autosomal dominant severe congenital neutropenia

Background

The molecular cause of severe congenital neutropenia (SCN) is unknown in 30% to 50% of patients. SEC61A1 encodes the α-subunit of the Sec61 complex, which governs endoplasmic reticulum protein transport and passive calcium leakage. Recently, mutations in SEC61A1 were reported to be pathogenic in common variable immunodeficiency and glomerulocystic kidney disease.

Objective

Our aim was to expand the spectrum of SEC61A1-mediated disease to include autosomal dominant SCN.

Methods

Whole exome sequencing findings were validated, and reported mutations were compared by Western blotting, Ca2⁺ flux assays, differentiation of transduced HL-60 cells, in vitro differentiation of primary CD34 cells, quantitative PCR for unfolded protein response (UPR) genes, and single-cell RNA sequencing on whole bone marrow.

Results

We identified a novel de novo missense mutation in SEC61A1 (c.A275G;p.Q92R) in a patient with SCN who was born to nonconsanguineous Belgian parents. The mutation results in diminished protein expression, disturbed protein translocation, and an increase in calcium leakage from the endoplasmic reticulum. In vitro differentiation of CD34⁺ cells recapitulated the patient’s clinical arrest in granulopoiesis. The impact of Q92R-Sec61α1 on neutrophil maturation was validated by using HL-60 cells, in which transduction reduced differentiation into CD11b⁺CD16⁺ cells. A potential mechanism for this defect is the uncontrolled initiation of the unfolded protein stress response, with single-cell analysis of primary bone marrow revealing perturbed UPR in myeloid precursors and in vitro differentiation of primary CD34⁺ cells revealing upregulation of CCAAT/enhancer-binding protein homologous protein and immunoglobulin heavy chain binding protein UPR-response genes.

Conclusion

Specific mutations in SEC61A1 cause SCN through dysregulation of the UPR.

The Nuclear Remodeling Induced by Helicobacter Cytolethal Distending Toxin Involves MAFB Oncoprotein

Enterohepatic Helicobacters, such as Helicobacter hepaticus and Helicobacter pullorum, are associated with several intestinal and hepatic diseases. Their main virulence factor is the cytolethal distending toxin (CDT). In the present study, whole genome microarray-based identification of differentially expressed genes was performed in vitro in HT-29 intestinal cells while following the ectopic expression of the active CdtB subunit of H. hepaticus CDT. A CdtB-dependent upregulation of the V-maf musculoaponeurotic fibrosarcoma oncogene homolog B (MAFB) gene encoding the MAFB oncoprotein was found, as well as the CdtB-dependent regulation of several MAFB target genes. The transduction and coculture experiments confirmed MAFB mRNA and protein induction in response to CDT and its CdtB subunit in intestinal and hepatic cell lines. An analysis of MAFB protein subcellular localization revealed a strong nuclear and perinuclear localization in the CdtB-distended nuclei in intestinal and hepatic cells. MAFB was also detected at the cell periphery of the CdtB-induced lamellipodia in some cells. The silencing of MAFB changed the cellular response to CDT with the formation of narrower lamellipodia, a reduction of the increase in nucleus size, and the formation of less γH2AX foci, the biomarker for DNA double-strand breaks. Taken together, these data show that the CDT of enterohepatic Helicobacters modulates the expression of the MAFB oncoprotein, which is translocated in the nucleus and is associated with the remodeling of the nuclei and actin cytoskeleton.

Fetal monocytes possess increased metabolic capacity and replace primitive macrophages in tissue macrophage development

Tissue-resident macrophages (MΦ_TR ) originate from at least two distinct waves of erythro-myeloid progenitors (EMP) arising in the yolk sac (YS) at E7.5 and E8.5 with the latter going through a liver monocyte intermediate. The relative potential of these precursors in determining development and functional capacity of MΦ_TR remains unclear. Here, we studied development of alveolar macrophages (AM) after single and competitive transplantation of different precursors from YS, fetal liver, and fetal lung into neonatal Csf2ra^-/- mice, which lack endogenous AM. Fetal monocytes, promoted by Myb, outcompeted primitive MΦ (pMΦ) in empty AM niches and preferentially developed to mature AM, which is associated with enhanced mitochondrial respiratory and glycolytic capacity and repression of the transcription factors c-Maf and MafB. Interestingly, AM derived from pMΦ failed to efficiently clear alveolar proteinosis and protect from fatal lung failure following influenza virus infection. Thus, our data demonstrate superior developmental and functional capacity of fetal monocytes over pMΦ in AM development and underlying mechanisms explaining replacement of pMΦ in fetal tissues.

SUMOylated MAFB promotes colorectal cancer tumorigenesis

The transcription factor, v-maf avian musculoaponeurotic fibrosarcoma oncogene homolog B (MAFB), promotes tumorigenesis in some cancers. In this study, we found that MAFB levels were increased in clinical colorectal cancer (CRC) samples, and higher expression correlated with more advanced TNM stage. We identified MAFB amplifications in a majority of tumor types in an assessment of The Cancer Genome Atlas database. Altered MAFB levels due to gene amplification, deletion, mutation, or transcription upregulation occurred in 9% of CRC cases within the database. shRNA knockdown experiments demonstrated that MAFB deficiency blocked CRC cell proliferation by arresting the cell cycle at G0/G1 phase in vitro. We found that MAFB could be SUMOylated by SUMO1 at lysine 32, and this modification was critical for cell cycle regulation by MAFB in CRC cells. SUMOylated MAFB directly regulated cyclin-dependent kinase 6 transcription by binding to its promoter. MAFB knockdown CRC cell xenograft tumors in mice grew more slowly than controls, and wild-type MAFB-overexpressing tumors grew more quickly than tumors overexpressing MAFB mutated at lysine 32. These data suggest that SUMOylated MAFB promotes CRC tumorigenesis through cell cycle regulation. MAFB and its SUMOylation process may serve as novel therapeutic targets for CRC treatment.

MAFB is dispensable for the fetal testis morphogenesis and the maintenance of spermatogenesis in adult mice

The transcription factor MAFB is an important regulator of the development and differentiation of various organs and tissues. Previous studies have shown that MAFB is expressed in embryonic and adult mouse testes and is expected to act as the downstream target of retinoic acid (RA) to initiate spermatogenesis. However, its exact localization and function remain unclear. Here, we localized MAFB expression in embryonic and adult testes and analyzed its gene function using Mafb-deficient mice. We found that MAFB and c-MAF are the only large MAF transcription factors expressed in testes, while MAFA and NRL are not. MAFB was localized in Leydig and Sertoli cells at embryonic day (E) 18.5 but in Leydig cells, Sertoli cells, and pachytene spermatocytes in adults. Mafb-deficient testes at E18.5 showed fully formed seminiferous tubules with no abnormal structure or differences in testicular somatic cell numbers compared with those of control wild-type mice. Additionally, the expression levels of genes related to development and function of testicular cells were unchanged between genotypes. In adults, the expression of MAFB in Sertoli cells was shown to be stage specific and induced by RA. By generating Mafb^fl/fl CAG-CreER^™ (Mafb-cKO) mice, in which Cre recombinase was activated upon tamoxifen treatment, we found that the neonatal cKO mice died shortly upon Mafb deletion, but adult cKO mice were alive upon deletion. Adult cKO mice were fertile, and spermatogenesis maintenance was normal, as indicated by histological analysis, hormone levels, and germ cell stage-specific markers. Moreover, there were no differences in the proportion of seminiferous stages between cKO mice and controls. However, RNA-Seq analysis of cKO Sertoli cells revealed that the down-regulated genes were related to immune function and phagocytosis activity but not spermatogenesis. In conclusion, we found that MAFB is dispensable for fetal testis morphogenesis and spermatogenesis maintenance in adult mice, despite the significant gene expression in different cell types, but MAFB might be critical for phagocytosis activity of Sertoli cells.

MafB enhances efferocytosis in RAW264.7 macrophages by regulating Axl expression

The transcription factor MafB is involved in cellular differentiation and phagocytosis in macrophages. Macrophages phagocytose apoptotic cells in vivo; this process, which is known as efferocytosis, requires Axl receptor tyrosine kinase (Axl) activity. However, the association between MafB and efferocytosis, as well as that between MafB and Axl, in macrophages is unknown. We hypothesized that MafB modulates macrophage efferocytosis by regulating Axl expression. Fluorescent-labeled apoptotic thymocytes were added to RAW264.7-MafB-shRNA and control cells, and the proportion of phagocytosis-positivey fluorescence microscopy and flow cytometry. In addition, Axl mRNA and protein were quantified by real-time PCR and western blotting in each group. RAW264.7-MafB-shRNA cells were transfected with a plasmid expressing green fluorescent protein (GFP)-tagged Axl or a control empty plasmid expressing only GFP. The capacity for phagocytosis of apoptotic cells was assessed in GFP-positive cells gated based on fluorescence intensity. In RAW264.7-MafB-shRNA cells, capacity for phagocytosis of apoptotic thymocytes was significantly reduced compared with that of control cells, as determined by fluorescence microscope and flow cytometry. Axl mRNA and protein expression was significantly reduced in RAW264.7-MafB-shRNA cells relative to control cells. Furthermore, the capacity of RAW264.7-MafB-shRNA cells, transfected with an Axl-expressing plasmid, for phagocytosis of apoptotic thymocytes was significantly greater than that of cells transfected with the control plasmid. Collectively, the present findings indicate that MafB enhances efferocytosis by regulating Axl expression in RAW264.7 macrophages.

MAFB Determines Human Macrophage Anti-Inflammatory Polarization: Relevance for the Pathogenic Mechanisms Operating in Multicentric Carpotarsal Osteolysis

Macrophage phenotypic and functional heterogeneity derives from tissue-specific transcriptional signatures shaped by the local microenvironment. Most studies addressing the molecular basis for macrophage heterogeneity have focused on murine cells, whereas the factors controlling the functional specialization of human macrophages are less known. M-CSF drives the generation of human monocyte-derived macrophages with a potent anti-inflammatory activity upon stimulation. We now report that knockdown of MAFB impairs the acquisition of the anti-inflammatory profile of human macrophages, identify the MAFB-dependent gene signature in human macrophages and illustrate the coexpression of MAFB and MAFB-target genes in CD163⁺ tissue-resident and tumor-associated macrophages. The contribution of MAFB to the homeostatic/anti-inflammatory macrophage profile is further supported by the skewed polarization of monocyte-derived macrophages from multicentric carpotarsal osteolysis (Online Mendelian Inheritance in Man #166300), a pathology caused by mutations in the MAFB gene. Our results demonstrate that MAFB critically determines the acquisition of the anti-inflammatory transcriptional and functional profiles of human macrophages.

Tissue macrophage identity and self-renewal

Macrophages are cellular components of the innate immune system that reside in virtually all tissues and contribute to immunity, repair, and homeostasis. The traditional view that all tissue-resident macrophages derive from the bone marrow through circulating monocyte intermediates has dramatically shifted recently with the observation that macrophages from embryonic progenitors can persist into adulthood and self-maintain by local proliferation. In several tissues, however, monocytes also contribute to the resident macrophage population, on which the local environment can impose tissue-specific macrophage functions. These observations have raised important questions: What determines resident macrophage identity and function, ontogeny or environment? How is macrophage proliferation regulated? In this review, we summarize the current knowledge about the identity, proliferation, and turnover of tissue-resident macrophages and how they differ from freshly recruited short-lived monocyte-derived cells. We examine whether macrophage proliferation can be qualified as self-renewal of mature differentiated cells and whether the concepts and molecular pathways are comparable to self-renewal mechanisms in stem cells. Finally, we discuss how improved understanding of macrophage identity and self-renewal could be exploited for therapeutic intervention of macrophage-mediated pathologies by selectively targeting freshly recruited or resident macrophages.

Design of a bZip transcription factor with homo/heterodimer-induced DNA-binding preference

The ability of basic leucine zipper transcription factors for homo- or heterodimerization provides a paradigm for combinatorial control of eukaryotic gene expression. It has been unclear, however, how facultative dimerization results in alternative DNA-binding repertoires on distinct regulatory elements. To unravel the molecular basis of such coupled preferences, we determined two high-resolution structures of the transcription factor MafB as a homodimer and as a heterodimer with c-Fos bound to variants of the Maf-recognition element. The structures revealed several unexpected and dimer-specific coiled-coil-heptad interactions. Based on these findings, we have engineered two MafB mutants with opposite dimerization preferences. One of them showed a strong preference for MafB/c-Fos heterodimerization and enabled selection of heterodimer-favoring over homodimer-specific Maf-recognition element variants. Our data provide a concept for transcription factor design to selectively activate dimer-specific pathways and binding repertoires.

Beyond stem cells: self-renewal of differentiated macrophages

In many mammalian tissues, mature differentiated cells are replaced by self-renewing stem cells, either continuously during homeostasis or in response to challenge and injury. For example, hematopoietic stem cells generate all mature blood cells, including monocytes, which have long been thought to be the major source of tissue macrophages. Recently, however, major macrophage populations were found to be derived from embryonic progenitors and to renew independently of hematopoietic stem cells. This process may not require progenitors, as mature macrophages can proliferate in response to specific stimuli indefinitely and without transformation or loss of functional differentiation. These findings suggest that macrophages are mature differentiated cells that may have a self-renewal potential similar to that of stem cells.

Transcriptional control of macrophage identity, self-renewal, and function

Macrophages not only are prominent effector cells of the immune system that are critical in inflammation and innate immune responses but also fulfill important functions in tissue homeostasis. Transcription factors can define macrophage identity and control their numbers and functions through the induction and maintenance of specific transcriptional programs. Here, we review the mechanisms employed by lineage-specific transcription factors to shape macrophage identity during the development from hematopoietic stem and progenitor cells. We also present current insight into how specific transcription factors control macrophage numbers, by regulating coordinated proliferation and differentiation of myeloid progenitor cells and self-renewal of mature macrophages. We finally discuss how functional specialization of mature macrophages in response to environmental stimuli can be induced through synergistic activity of lineage- and stimulus-specific transcription factors that plug into preexisting transcriptional programs. Understanding the mechanisms that define macrophage identity, numbers, and functions will provide important insights into the differential properties of macrophage populations under various physiological and pathological conditions.

The small ubiquitin-like modifier-deconjugating enzyme sentrin-specific peptidase 1 switches IFN regulatory factor 8 from a repressor to an activator during macrophage activation

Macrophages, when activated by IFN-γ and TLR signaling, elicit innate immune responses. IFN regulatory factor 8 (IRF8) is a transcription factor that facilitates macrophage activation and innate immunity. We show that, in resting macrophages, some IRF8 is conjugated to small ubiquitin-like modifiers (SUMO) 2/3 through the lysine residue 310. SUMO3-conjugated IRF8 failed to induce IL12p40 and other IRF8 target genes, consistent with SUMO-mediated transcriptional repression reported for other transcription factors. SUMO3-conjugated IRF8 showed reduced mobility in live nuclei and bound poorly to the IL12p40 gene. However, macrophage activation caused a sharp reduction in the amount of SUMOylated IRF8. This reduction coincided with the induction of a deSUMOylating enzyme, sentrin-specific peptidase 1 (SENP1), in activated macrophages. In transfection analysis, SENP1 removed SUMO3 from IRF8 and enhanced expression of IL12p40 and other target genes. Conversely, SENP1 knockdown repressed IRF8 target gene expression. In parallel with IRF8 deSUMOylation, macrophage activation led to the induction of proteins active in the SUMO pathway and caused a global shift in nuclear protein SUMOylation patterns. Together, the IRF8 SUMO conjugation/deconjugation switch is part of a larger transition in SUMO modifications that takes place upon macrophage activation, serving as a mechanism to trigger innate immune responses.

Integrated genome-wide chromatin occupancy and expression analyses identify key myeloid pro-differentiation transcription factors repressed by Myb

To gain insight into the mechanisms by which the Myb transcription factor controls normal hematopoiesis and particularly, how it contributes to leukemogenesis, we mapped the genome-wide occupancy of Myb by chromatin immunoprecipitation followed by massively parallel sequencing (ChIP-Seq) in ERMYB myeloid progenitor cells. By integrating the genome occupancy data with whole genome expression profiling data, we identified a Myb-regulated transcriptional program. Gene signatures for leukemia stem cells, normal hematopoietic stem/progenitor cells and myeloid development were overrepresented in 2368 Myb regulated genes. Of these, Myb bound directly near or within 793 genes. Myb directly activates some genes known critical in maintaining hematopoietic stem cells, such as Gfi1 and Cited2. Importantly, we also show that, despite being usually considered as a transactivator, Myb also functions to repress approximately half of its direct targets, including several key regulators of myeloid differentiation, such as Sfpi1 (also known as Pu.1), Runx1, Junb and Cebpb. Furthermore, our results demonstrate that interaction with p300, an established coactivator for Myb, is unexpectedly required for Myb-mediated transcriptional repression. We propose that the repression of the above mentioned key pro-differentiation factors may contribute essentially to Myb's ability to suppress differentiation and promote self-renewal, thus maintaining progenitor cells in an undifferentiated state and promoting leukemic transformation.

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.

The transcription factor MafB antagonizes antiviral responses by blocking recruitment of coactivators to the transcription factor IRF3

Viral infection induces type I interferons (IFN-alpha and IFN-beta) that recruit unexposed cells in a self-amplifying response. We report that the transcription factor MafB thwarts auto-amplification by a metastable switch activity. MafB acted as a weak positive basal regulator of transcription at the IFNB1 promoter through activity at transcription factor AP-1-like sites. Interferon elicitors recruited the transcription factor IRF3 to the promoter, whereupon MafB acted as a transcriptional antagonist, impairing the interaction of coactivators with IRF3. Mathematical modeling supported the view that prepositioning of MafB on the promoter allows the system to respond rapidly to fluctuations in IRF3 activity. Higher expression of MafB in human pancreatic islet beta cells might increase cellular vulnerability to viral infections associated with the etiology of type 1 diabetes.

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.

MafB/c-Maf deficiency enables self-renewal of differentiated functional macrophages

In metazoan organisms, terminal differentiation is generally tightly linked to cell cycle exit, whereas the undifferentiated state of pluripotent stem cells is associated with unlimited self-renewal. Here, we report that combined deficiency for the transcription factors MafB and c-Maf enables extended expansion of mature monocytes and macrophages in culture without loss of differentiated phenotype and function. Upon transplantation, the expanded cells are nontumorigenic and contribute to functional macrophage populations in vivo. Small hairpin RNA inactivation shows that continuous proliferation of MafB/c-Maf deficient macrophages requires concomitant up-regulation of two pluripotent stem cell-inducing factors, KLF4 and c-Myc. Our results indicate that MafB/c-MafB deficiency renders self-renewal compatible with terminal differentiation. It thus appears possible to amplify functional differentiated cells without malignant transformation or stem cell intermediates.

MafB protein stability is regulated by the JNK and ubiquitin-proteasome pathways

MafB is a basic leucine zipper transcription factor that plays important roles in development and differentiation processes. During osteoclastogenesis, its expression is downregulated at the transcriptional level via the JNK and p38 MAP kinase pathways. In the present study, we demonstrated that MafB protein stability is regulated by JNK and identified a phosphorylation site, Thr62. The expression of a constitutively active form of JNK (a fusion protein MKK7alpha1-JNK1beta1) promoted the degradation of MafB in COS7 cells, and a T62A substitution significantly reduced the instability of MafB. The introduction of a fourfold (T58A/T62A/S70A/S74A) substitution in an acidic transcription-activating domain almost protected the instability resulting from the activation of JNK. Furthermore, treatment with proteasome inhibitors increased the MafB level, and a high-molecular-weight smear, characteristic of polyubiquitination, was observed in lysates from cells in which MafB, ubiquitin, and MKK7alpha1-JNK1beta1 were co-expressed. These results suggest that phosphorylation of MafB by JNK confers susceptibility to proteasomal degradation.

Regulatory interdependence of myeloid transcription factors revealed by Matrix RNAi analysis

The knockdown of 78 transcription factors in differentiating human THP-1 cells using matrix RNAi reveals their interdependence

Background

With the move towards systems biology, we need sensitive and reliable ways to determine the relationships between transcription factors and their target genes. In this paper we analyze the regulatory relationships between 78 myeloid transcription factors and their coding genes by using the matrix RNAi system in which a set of transcription factor genes are individually knocked down and the resultant expression perturbation is quantified.

Results

Using small interfering RNAs we knocked down the 78 transcription factor genes in monocytic THP-1 cells and monitored the perturbation of the expression of the same 78 transcription factors and 13 other transcription factor genes as well as 5 non-transcription factor genes by quantitative real-time RT-PCR, thereby building a 78 × 96 matrix of perturbation and measurement. This approach identified 876 cases where knockdown of one transcription factor significantly affected the expression of another (from a potential 7,488 combinations). Our study also revealed cell-type-specific transcriptional regulatory networks in two different cell types.

Conclusions

By considering whether the targets of a given transcription factor are naturally up- or downregulated during phorbol 12-myristate 13-acetate-induced differentiation, we could classify these edges as pro-differentiative (229), anti-differentiative (76) or neither (571) using expression profiling data obtained in the FANTOM4 study. This classification analysis suggested that several factors could be involved in monocytic differentiation, while others such as MYB and the leukemogenic fusion MLL-MLLT3 could help to maintain the initial undifferentiated state by repressing the expression of pro-differentiative factors or maintaining expression of anti-differentiative factors.

MafB restricts M-CSF-dependent myeloid commitment divisions of hematopoietic stem cells

While hematopoietic stem cell (HSC) self-renewal is well studied, it remains unknown whether distinct control mechanisms enable HSC divisions that generate progeny cells with specific lineage bias. Here, we report that the monocytic transcription factor MafB specifically restricts the ability of M-CSF to instruct myeloid commitment divisions in HSCs. MafB deficiency specifically enhanced sensitivity to M-CSF and caused activation of the myeloid master-regulator PU.1 in HSCs in vivo. Single-cell analysis revealed that reduced MafB levels enabled M-CSF to instruct divisions producing asymmetric daughter pairs with one PU.1(+) cell. As a consequence, MafB(-/-) HSCs showed a PU.1 and M-CSF receptor-dependent competitive repopulation advantage specifically in the myelomonocytic, but not T lymphoid or erythroid, compartment. Lineage-biased repopulation advantage was progressive, maintained long term, and serially transplantable. Together, this indicates that an integrated transcription factor/cytokine circuit can control the rate of specific HSC commitment divisions without compromising other lineages or self-renewal.

Sumoylation regulates the transcriptional activity of MafA in pancreatic beta cells

MafA is a transcriptional regulator expressed primarily in pancreatic beta cells. It binds to the RIPE3b/C1-binding site within the ins gene promoter, which plays a critical role in regulating ins gene expression in response to glucose. Here, we show that MafA is post-translationally modified by the small ubiquitin-related modifiers SUMO-1 and -2. Mutation of a single site in MafA, Lys(32), blocks its sumoylation in beta cells. Incubation of beta cells in low glucose (2 mm) or exposure to hydrogen peroxide increases sumoylation of endogenous MafA. Forced sumoylation of MafA results in reduced transcriptional activity toward the ins gene promoter and increased suppression of the CHOP-10 gene promoter. Sumoylation of MafA has no apparent effect on either its nuclear localization in beta cells or its ubiquitin-dependent degradation. This study suggests that modification of MafA by SUMO modulates gene transcription and thereby beta cell function.

Synergistic induction of gene expression during the differentiation into mature macrophage in human myeloblastic leukemia cells treated with TPA and KH1060

Treatment of human myeloblastic leukemia ML-1 cells with the phorbol ester TPA in combination with the vitamin D(3) analogue KH1060 will induce a synergistic differentiation to mature macrophage with multinuclei. To investigate the mechanism underlying this differentiation and the synergistic effect, a cDNA microarray and Northern blot analysis were used to examine gene expression profiles of ML-1 cells treated with TPA and/or KH1060. Results show that KH1060 enhanced several TPA-induced gene expressions and that TPA enhanced several KH1060-induced gene expressions. Studies with inhibitors of signaling molecules suggested that PKC and MAPK pathways play an important role in the differentiation induced by TPA and KH1060, and that they are associated with the synergistic induction of genes. The results of this study indicate the possibility that the expression of various genes are induced synergistically by cross-talk between TPA and KH1060 signals. It is likely that the synergistic effect on gene expression leads to the synergistic induction of differentiation by both reagents.

Blood monocytes: development, heterogeneity, and relationship with dendritic cells

Monocytes are circulating blood leukocytes that play important roles in the inflammatory response, which is essential for the innate response to pathogens. But inflammation and monocytes are also involved in the pathogenesis of inflammatory diseases, including atherosclerosis. In adult mice, monocytes originate in the bone marrow in a Csf-1R (MCSF-R, CD115)-dependent manner from a hematopoietic precursor common for monocytes and several subsets of macrophages and dendritic cells (DCs). Monocyte heterogeneity has long been recognized, but in recent years investigators have identified three functional subsets of human monocytes and two subsets of mouse monocytes that exert specific roles in homeostasis and inflammation in vivo, reminiscent of those of the previously described classically and alternatively activated macrophages. Functional characterization of monocytes is in progress in humans and rodents and will provide a better understanding of the pathophysiology of inflammation.

A new MAFia in cancer

Like JUN and FOS, the Maf transcription factors belong to the AP1 family. Besides their established role in human cancer--overexpression of the large Maf genes promotes the development of multiple myeloma--they can display tumour suppressor-like activity in specific cellular contexts, which is compatible with their physiological role in terminal differentiation. However, their oncogenic activity relies mostly on the acquisition of new biological functions relevant to cell transformation, the most striking characteristic of Maf oncoproteins being their ability to enhance pathological interactions between tumour cells and the stroma.