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Diabetes is a complex metabolic disease which most commonly has a polygenic origin; however, in rare cases, diabetes may be monogenic. This is indeed the case in both Maturity Onset Diabetes of the Young (MODY) and neonatal diabetes. These disease subtypes are believed to be simpler than Type 1 (T1D) and Type 2 Diabetes (T2D), which allows for more precise modelling. During the three last decades, many studies have focused on rodent models. These investigations provided a wealth of knowledge on both pancreas development and beta cell function. In particular, they allowed the establishment of a hierarchy of the transcription factors and highlighted the role of microenvironmental factors in the control of progenitor cell proliferation and differentiation. Transgenic mice also offered the possibility to decipher the mechanisms that define the functional identity of the pancreatic beta cells. Despite such interest in transgenic mice, recent data have also indicated that important differences exist between mice and human. To overcome these limitations, new human models are necessary. In the present review, we describe these ex vivo models, which are created using stem cells and organoids, and represent an important step toward islet cell therapy and drug discovery., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision., (Copyright © 2024 Ka, Hawkins, Pouponnot and Duvillié.)

Atypical teratoid rhabdoid tumors (ATRT) are divided into MYC, TYR and SHH subgroups, suggesting diverse lineages of origin. Here, we investigate the imaging of human ATRT at diagnosis and the precise anatomic origin of brain tumors in the Rosa26-Cre ERT2 ::Smarcb1 flox/flox model. This cross-species analysis points to an extra-cerebral origin for MYC tumors. Additionally, we clearly distinguish SHH ATRT emerging from the cerebellar anterior lobe (CAL) from those emerging from the basal ganglia (BG) and intra-ventricular (IV) regions. Molecular characteristics point to the midbrain-hindbrain boundary as the origin of CAL SHH ATRT, and to the ganglionic eminence as the origin of BG/IV SHH ATRT. Single-cell RNA sequencing on SHH ATRT supports these hypotheses. Trajectory analyses suggest that SMARCB1 loss induces a de-differentiation process mediated by repressors of the neuronal program such as REST, ID and the NOTCH pathway., (© 2023. Springer Nature Limited.)

Background: Intensive chemotherapeutic regimens with craniospinal irradiation have greatly improved survival in medulloblastoma patients. However, survival markedly differs among molecular subgroups and their biomarkers are unknown. Through unbiased screening, we found Schlafen family member 11 (SLFN11), which is known to improve response to DNA damaging agents in various cancers, to be one of the top prognostic markers in medulloblastomas. Hence, we explored the expression and functions of SLFN11 in medulloblastoma., Methods: SLFN11 expression for each subgroup was assessed by immunohistochemistry in 98 medulloblastoma patient samples and by analyzing transcriptomic databases. We genetically or epigenetically modulated SLFN11 expression in medulloblastoma cell lines and determined cytotoxic response to the DNA damaging agents cisplatin and topoisomerase I inhibitor SN-38 in vitro and in vivo., Results: High SLFN11 expressing cases exhibited significantly longer survival than low expressing cases. SLFN11 was highly expressed in the WNT-activated subgroup and in a proportion of the SHH-activated subgroup. While WNT activation was not a direct cause of the high expression of SLFN11, a specific hypomethylation locus on the SLFN11 promoter was significantly correlated with high SLFN11 expression. Overexpression or deletion of SLFN11 made medulloblastoma cells sensitive and resistant to cisplatin and SN-38, respectively. Pharmacological upregulation of SLFN11 by the brain-penetrant histone deacetylase-inhibitor RG2833 markedly increased sensitivity to cisplatin and SN-38 in SLFN11-negative medulloblastoma cells. Intracranial xenograft studies also showed marked sensitivity to cisplatin by SLFN11-overexpression in medulloblastoma cells., Conclusions: High SLFN11 expression is one factor which renders favorable outcomes in WNT-activated and a subset of SHH-activated medulloblastoma possibly through enhancing response to cisplatin., (© The Author(s) 2022. Published by Oxford University Press on behalf of the Society for Neuro-Oncology. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

While apneas are associated with multiple pathological and fatal conditions, the underlying molecular mechanisms remain elusive. We report that a mutated form of the transcription factor Mafa (Mafa 4A ) that prevents phosphorylation of the Mafa protein leads to an abnormally high incidence of breath holding apneas and death in newborn Mafa 4A/4A mutant mice. This apneic breathing is phenocopied by restricting the mutation to central GABAergic inhibitory neurons and by activation of inhibitory Mafa neurons while reversed by inhibiting GABAergic transmission centrally. We find that Mafa activates the Gad2 promoter in vitro and that this activation is enhanced by the mutation that likely results in increased inhibitory drives onto target neurons. We also find that Mafa inhibitory neurons are absent from respiratory, sensory (primary and secondary) and pontine structures but are present in the vicinity of the hypoglossal motor nucleus including premotor neurons that innervate the geniohyoid muscle, to control upper airway patency. Altogether, our data reveal a role for Mafa phosphorylation in regulation of GABAergic drives and suggest a mechanism whereby reduced premotor drives to upper airway muscles may cause apneic breathing at birth., (© 2022. The Author(s).)

The MITF transcription factor and the RAS/RAF/MEK/ERK pathway are two interconnected main players in melanoma. Understanding how MITF activity is regulated represents a key question since its dynamic modulation is involved in the phenotypic plasticity of melanoma cells and their resistance to therapy. By investigating the role of ARAF in NRAS-driven mouse melanoma through mass spectrometry experiments followed by a functional siRNA-based screen, we unexpectedly identified MITF as a direct ARAF partner. Interestingly, this interaction is conserved among the RAF protein kinase family since BRAF/MITF and CRAF/MITF complexes were also observed in the cytosol of NRAS-mutated mouse melanoma cells. The interaction occurs through the kinase domain of RAF proteins. Importantly, endogenous BRAF/MITF complexes were also detected in BRAF-mutated human melanoma cells. RAF/MITF complexes modulate MITF nuclear localization by inducing an accumulation of MITF in the cytoplasm, thus negatively controlling its transcriptional activity. Taken together, our study highlights a new level of regulation between two major mediators of melanoma progression, MITF and the MAPK/ERK pathway, which appears more complex than previously anticipated., (© 2022. The Author(s).)

Retinoblastoma is the most frequent intraocular malignancy in children, originating from a maturing cone precursor in the developing retina. Little is known on the molecular basis underlying the biological and clinical behavior of this cancer. Here, using multi-omics data, we demonstrate the existence of two retinoblastoma subtypes. Subtype 1, of earlier onset, includes most of the heritable forms. It harbors few genetic alterations other than the initiating RB1 inactivation and corresponds to differentiated tumors expressing mature cone markers. By contrast, subtype 2 tumors harbor frequent recurrent genetic alterations including MYCN-amplification. They express markers of less differentiated cone together with neuronal/ganglion cell markers with marked inter- and intra-tumor heterogeneity. The cone dedifferentiation in subtype 2 is associated with stemness features including low immune and interferon response, E2F and MYC/MYCN activation and a higher propensity for metastasis. The recognition of these two subtypes, one maintaining a cone-differentiated state, and the other, more aggressive, associated with cone dedifferentiation and expression of neuronal markers, opens up important biological and clinical perspectives for retinoblastomas., (© 2021. The Author(s).)

Purpose: Medulloblastoma is an important cause of mortality and morbidity in pediatric oncology. Here, we investigated whether the DNA repair inhibitor, AsiDNA, could help address a significant unmet clinical need in medulloblastoma care, by improving radiotherapy efficacy without increasing radiation-associated toxicity., Experimental Design: To evaluate the brain permeability of AsiDNA upon systemic delivery, we intraperitoneally injected a fluorescence form of AsiDNA in models harboring brain tumors and in models still in development. Studies evaluated toxicity associated with combination of AsiDNA with radiation in the treatment of young developing animals at subacute levels, related to growth and development, and at chronic levels, related to brain organization and cognitive skills. Efficacy of the combination of AsiDNA with radiation was tested in two different preclinical xenografted models of high-risk medulloblastoma and in a panel of medulloblastoma cell lines from different molecular subgroups and TP53 status. Role of TP53 on the AsiDNA-mediated radiosensitization was analyzed by RNA-sequencing, DNA repair recruitment, and cell death assays., Results: Capable of penetrating young brain tissues, AsiDNA showed no added toxicity to radiation. Combination of AsiDNA with radiotherapy improved the survival of animal models more efficiently than increasing radiation doses. Medulloblastoma radiosensitization by AsiDNA was not restricted to a specific molecular group or status of TP53 . Molecular mechanisms of AsiDNA, previously observed in adult malignancies, were conserved in pediatric models and resembled dose increase when combined with irradiation., Conclusions: Our results suggest that AsiDNA is an attractive candidate to improve radiotherapy in medulloblastoma, with no indication of additional toxicity in developing brain tissues., (©2020 American Association for Cancer Research.)

Epidemiologic analyses have shed light on an association between type 2 diabetes (T2D) and pancreatic ductal adenocarcinoma (PDAC). Recent data also suggest a potential relationship between T2D and insulinoma. Under rare circumstances, type 1 diabetes (T1D) can also be implicated in tumorigenesis. The biological mechanisms underlying such relationships are extremely complex. Some genetic factors contributing to the development of T2D are shared with pancreatic exocrine and endocrine tumors. Obesity and overweight can also contribute to the initiation and severity of T2D, while aging may influence both endocrine and exocrine tumors. Finally, pharmacological treatments of T2D may have an impact on PDAC. On the other hand, some treatments for insulinoma can trigger diabetes. In the present minireview, we discuss the cellular and molecular mechanisms that could explain these interactions. This analysis may help to define new potential therapeutic strategies., (Copyright © 2020 Duvillié, Kourdoughli, Druillennec, Eychène and Pouponnot.)

Rhabdoid tumors (RTs) are genomically simple pediatric cancers driven by the biallelic inactivation of SMARCB1, leading to SWI/SNF chromatin remodeler complex deficiency. Comprehensive evaluation of the immune infiltrates of human and mice RTs, including immunohistochemistry, bulk RNA sequencing and DNA methylation profiling studies showed a high rate of tumors infiltrated by T and myeloid cells. Single-cell RNA (scRNA) and T cell receptor sequencing highlighted the heterogeneity of these cells and revealed therapeutically targetable exhausted effector and clonally expanded tissue resident memory CD8 + T subpopulations, likely representing tumor-specific cells. Checkpoint blockade therapy in an experimental RT model induced the regression of established tumors and durable immune responses. Finally, we show that one mechanism mediating RTs immunogenicity involves SMARCB1-dependent re-expression of endogenous retroviruses and interferon-signaling activation., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Medulloblastoma (MB) is a pediatric tumor of the cerebellum divided into four groups. Group 3 is of bad prognosis and remains poorly characterized. While the current treatment involving surgery, radiotherapy, and chemotherapy often fails, no alternative therapy is yet available. Few recurrent genomic alterations that can be therapeutically targeted have been identified. Amplifications of receptors of the TGFβ/Activin pathway occur at very low frequency in Group 3 MB. However, neither their functional relevance nor activation of the downstream signaling pathway has been studied. We showed that this pathway is activated in Group 3 MB with some samples showing a very strong activation. Beside genetic alterations, we demonstrated that an ActivinB autocrine stimulation is responsible for pathway activation in a subset of Group 3 MB characterized by high PMEPA1 levels. Importantly, Galunisertib, a kinase inhibitor of the cognate receptors currently tested in clinical trials for Glioblastoma patients, showed efficacy on orthotopically grafted MB-PDX. Our data demonstrate that the TGFβ/Activin pathway is active in a subset of Group 3 MB and can be therapeutically targeted., (© 2019 The Authors. Published under the terms of the CC BY 4.0 license.)

The current consensus recognizes four main medulloblastoma subgroups (wingless, Sonic hedgehog, group 3 and group 4). While medulloblastoma subgroups have been characterized extensively at the (epi-)genomic and transcriptomic levels, the proteome and phosphoproteome landscape remain to be comprehensively elucidated. Using quantitative (phospho)-proteomics in primary human medulloblastomas, we unravel distinct posttranscriptional regulation leading to highly divergent oncogenic signaling and kinase activity profiles in groups 3 and 4 medulloblastomas. Specifically, proteomic and phosphoproteomic analyses identify aberrant ERBB4-SRC signaling in group 4. Hence, enforced expression of an activated SRC combined with p53 inactivation induces murine tumors that resemble group 4 medulloblastoma. Therefore, our integrative proteogenomics approach unveils an oncogenic pathway and potential therapeutic vulnerability in the most common medulloblastoma subgroup., (Copyright © 2018 Elsevier Inc. All rights reserved.)

FGFR3 alterations (mutations or translocation) are among the most frequent genetic events in bladder carcinoma. They lead to an aberrant activation of FGFR3 signaling, conferring an oncogenic dependence, which we studied here. We discovered a positive feedback loop, in which the activation of p38 and AKT downstream from the altered FGFR3 upregulates MYC mRNA levels and stabilizes MYC protein, respectively, leading to the accumulation of MYC, which directly upregulates FGFR3 expression by binding to active enhancers upstream from FGFR3 Disruption of this FGFR3/MYC loop in bladder cancer cell lines by treatment with FGFR3, p38, AKT, or BET bromodomain inhibitors (JQ1) preventing MYC transcription decreased cell viability in vitro and tumor growth in vivo A relevance of this loop to human bladder tumors was supported by the positive correlation between FGFR3 and MYC levels in tumors bearing FGFR3 mutations, and the decrease in FGFR3 and MYC levels following anti-FGFR treatment in a PDX model bearing an FGFR3 mutation. These findings open up new possibilities for the treatment of bladder tumors displaying aberrant FGFR3 activation., (© 2018 The Authors. Published under the terms of the CC BY 4.0 license.)

Cancer cells often express differentiation programs unrelated to their tissue of origin, although the contribution of these aberrant phenotypes to malignancy is poorly understood. An aggressive subgroup of medulloblastoma, a malignant pediatric brain tumor of the cerebellum, expresses a photoreceptor differentiation program normally expressed in the retina. We establish that two photoreceptor-specific transcription factors, NRL and CRX, are master regulators of this program and are required for tumor maintenance in this subgroup. Beyond photoreceptor lineage genes, we identify BCL-XL as a key transcriptional target of NRL and provide evidence substantiating anti-BCL therapy as a rational treatment opportunity for select MB patients. Our results highlight the utility of studying aberrant differentiation programs in cancer and their potential as selective therapeutic vulnerabilities., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Using mouse genetics, we recently showed that BRAF has a critical role in initiation of NRAS-driven melanoma that cannot be compensated by CRAF. In contrast, RAF proteins display compensatory functions in fully established tumors and ARAF can sustain proliferation in the absence of BRAF and CRAF, highlighting an addiction to RAF signaling in NRAS-driven melanoma.

NRAS and its effector BRAF are frequently mutated in melanoma. Paradoxically, CRAF but not BRAF was shown to be critical for various RAS-driven cancers, raising the question of the role of RAF proteins in NRAS-induced melanoma. Here, using conditional ablation of Raf genes in NRAS-induced mouse melanoma models, we investigate their contribution in tumour progression, from the onset of benign tumours to malignant tumour maintenance. We show that BRAF expression is required for ERK activation and nevi development, demonstrating a critical role in the early stages of NRAS-driven melanoma. After melanoma formation, single Braf or Craf ablation is not sufficient to block tumour growth, showing redundant functions for RAF kinases. Finally, proliferation of resistant cells emerging in the absence of BRAF and CRAF remains dependent on ARAF-mediated ERK activation. These results reveal specific and compensatory functions for BRAF and CRAF and highlight an addiction to RAF signalling in NRAS-driven melanoma.

Transcription factors operate in developmental processes to mediate inductive events and cell competence, and perturbation of their function or regulation can dramatically affect morphogenesis, organogenesis, and growth. We report that a narrow spectrum of amino-acid substitutions within the transactivation domain of the v-maf avian musculoaponeurotic fibrosarcoma oncogene homolog (MAF), a leucine zipper-containing transcription factor of the AP1 superfamily, profoundly affect development. Seven different de novo missense mutations involving conserved residues of the four GSK3 phosphorylation motifs were identified in eight unrelated individuals. The distinctive clinical phenotype, for which we propose the eponym Aymé-Gripp syndrome, is not limited to lens and eye defects as previously reported for MAF/Maf loss of function but includes sensorineural deafness, intellectual disability, seizures, brachycephaly, distinctive flat facial appearance, skeletal anomalies, mammary gland hypoplasia, and reduced growth. Disease-causing mutations were demonstrated to impair proper MAF phosphorylation, ubiquitination and proteasomal degradation, perturbed gene expression in primary skin fibroblasts, and induced neurodevelopmental defects in an in vivo model. Our findings nosologically and clinically delineate a previously poorly understood recognizable multisystem disorder, provide evidence for MAF governing a wider range of developmental programs than previously appreciated, and describe a novel instance of protein dosage effect severely perturbing development., (Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.)

How metabolism regulators play roles during early development remains elusive. Here we show that PFKFB4 (6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4), a glycolysis regulator, is critical for controlling dorsal ectoderm global patterning in gastrulating frog embryos via a non-glycolytic function. PFKFB4 is required for dorsal ectoderm progenitors to proceed towards more specified fates including neural and non-neural ectoderm, neural crest or placodes. This function is mediated by Akt signalling, a major pathway that integrates cell homeostasis and survival parameters. Restoring Akt signalling rescues the loss of PFKFB4 in vivo. In contrast, glycolysis is not essential for frog development at this stage. Our study reveals the existence of a PFKFB4-Akt checkpoint that links cell homeostasis to the ability of progenitor cells to undergo differentiation, and uncovers glycolysis-independent functions of PFKFB4.

Retinoblastoma is a non-hereditary as well as an inherited pediatric tumor of the developing retina resulting from the inactivation of both copies of the RB1 tumor suppressor gene. Familial retinoblastoma is a highly penetrant genetic disease that usually develops by carrying germline mutations that inactivate one allele of the RB1 gene, leading to multiple retinoblastomas. However, large and complete germline RB1 deletions are associated with low or no tumor risk for reasons that remain unknown. In this study, we define a minimal genomic region associated with this low penetrance. This region encompasses few genes including MED4 a subunit of the mediator complex. We further show that retinoblastoma RB1 -/- cells cannot survive in the absence of MED4, both in vitro and in orthotopic xenograft models in vivo, therefore identifying MED4 as a survival gene in retinoblastoma. We propose that the contiguous loss of the adjacent retinoblastoma gene, MED4, explains the low penetrance in patients with large deletions that include both RB1 and MED4. Our findings also point to another synthetic lethal target in tumors with inactivated RB1 and highlight the importance of collateral damage in carcinogenesis., (© The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Insulin biosynthesis is an essential β-cell function and inappropriate insulin secretion and biosynthesis contribute to the pathogenesis of diabetes mellitus type 2. Previous studies showed that the dual leucine zipper kinase (DLK) induces β-cell apoptosis. Since β-cell dysfunction precedes β-cell loss, in the present study the effect of DLK on insulin gene transcription was investigated in the HIT-T15 β-cell line. Downregulation of endogenous DLK increased whereas overexpression of DLK decreased human insulin gene transcription. 5'- and 3'-deletion human insulin promoter analyses resulted in the identification of a DLK responsive element that mapped to the DNA binding-site for the β-cell specific transcription factor MafA. Overexpression of DLK wild-type but not its kinase-dead mutant inhibited MafA transcriptional activity conferred by its transactivation domain. Furthermore, in the non-β-cell line JEG DLK inhibited MafA overexpression-induced human insulin promoter activity. Overexpression of MafA and DLK or its kinase-dead mutant into JEG cells revealed that DLK but not its mutant reduced MafA protein content. Inhibition of the down-stream DLK kinase c-Jun N-terminal kinase (JNK) by SP600125 attenuated DLK-induced MafA loss. Furthermore, mutation of the serine 65 to alanine, shown to confer MafA protein stability, increased MafA-dependent insulin gene transcription and prevented DLK-induced MafA loss in JEG cells. These data suggest that DLK by activating JNK triggers the phosphorylation and degradation of MafA thereby attenuating insulin gene transcription. Given the importance of MafA for β-cell function, the inhibition of DLK might preserve β-cell function and ultimately retard the development of diabetes mellitus type 2., Competing Interests: The authors state no actual or potential conflict of interest including any financial, personal or other relationships with other people or organizations. All authors have approved the final manuscript., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Pax4 and MafA (v-maf musculoaponeurotic fibrosarcoma oncogene homolog A) are two transcription factors crucial for normal functions of islet beta cells in the mouse. Intriguingly, recent studies indicate the existence of notable difference between human and rodent islet in terms of gene expression and functions. To better understand the biological role of human PAX4 and MAFA, we investigated their expression in normal and diseased human islets, using validated antibodies. PAX4 was detected in 43.0±5.0% and 39.1±4.0% of normal human alpha and beta cells respectively. We found that MAFA, detected in 88.3±6.3% insulin(+)cells as in the mouse, turned out to be also expressed in 61.2±6.4% of human glucagons(+) cells with less intensity than in insulin(+) cells, whereas MAFB expression was found not only in the majority of glucagon(+) cells (67.2±7.6%), but also in 53.6±10.5% of human insulin(+) cells. Interestingly, MAFA nuclear expression in both alpha and beta cells, and the percentage of alpha cells expressing PAX4 were found altered in a substantial proportion of patients with type 2 diabetes. Both MAFA and PAX4 display, therefore, a distinct expression pattern in human islet cells, suggesting more potential plasticity of human islets as compared with rodent islets.

B-Raf and C-Raf kinases have emerged as critical players in melanoma. However, little is known about their role during development and homeostasis of the melanocyte lineage. Here, we report that knockout of B-raf and C-raf genes in this lineage results in normal pigmentation at birth with no defect in migration, proliferation, or differentiation of melanoblasts in mouse hair follicles. In contrast, the double raf knockout mice displayed hair graying resulting from a defect in cell-cycle entry of melanocyte stem cells (MSCs) and their subsequent depletion in the hair follicle bulge. Therefore, Raf signaling is dispensable for early melanocyte lineage development, but necessary for MSC maintenance., (Copyright © 2012 The Authors. Published by Elsevier Inc. All rights reserved.)

Aberrant expression of Eph and ephrin proteins in human cancers is extensively documented. However, data are frequently limited to one gene and therefore incomplete and in some instances conflicting. We analysed expression of all Eph and ephrin genes in colorectal cancer (CRC) cell lines and 153 clinical specimens, providing for the first time a comprehensive analysis of this system in CRC. Eph/ephrin mRNA expression was assessed by quantitative real-time PCR and correlated with protein expression (flow cytometry, Western blotting and immunocytochemistry). These data show that EphA1, EphA2, EphB2 and EphB4 were significantly over expressed in CRC. In all cases, at least one Eph gene was found in normal colon (EphA1, EphA2, EphB2, EphB4), where expression was observed at high levels in most CRCs. However, other Eph gene expression was lost in individual CRCs compared to the corresponding normal, EphA7 being a striking example. Loss of expression was more common in advanced disease and thus correlated with poor survival. This is consistent with the redundant functionality of Eph receptors, such that expression of a single Eph gene is sufficient for effector function. Overall, the data suggest a progressive loss of expression of individual Eph genes suggesting that individual CRCs need to be phenotyped to determine which Eph genes are highly expressed. Targeted therapies could then be selected from a group of specific antibodies, such as those developed for EphA1., (Crown Copyright © 2011. Published by Elsevier Ltd. All rights reserved.)

Dorsal root ganglia proceed from the coalescence of cell bodies of sensory neurons, which have migrated dorsoventrally from the delaminating neural crest. They are composed of different neuronal subtypes with specific sensory functions, including nociception, thermal sensation, proprioception, and mechanosensation. In contrast to proprioceptors and thermonociceptors, little is known about the molecular mechanisms governing the early commitment and later differentiation into mechanosensitive neurons. This is mainly due to the absence of specific molecular markers for this particular cell type. Using knockout mice, we identified the bZIP transcription factor MafA as the first specific marker of a subpopulation of "early c-ret" positive neurons characterized by medium-to-large diameters. This marker will allow further functional characterization of these neurons.

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.

The Maf oncoproteins are b-Zip transcription factors of the AP-1 superfamily. They are involved in developmental, metabolic, and tumorigenic processes. Maf proteins are overexpressed in about 50% of human multiple myelomas. Here, we show that Maf-transforming activity is controlled by GSK-3-dependent phosphorylation and that phosphorylation by GSK-3 can increase the oncogenic activity of a protein. Using microarray analysis, we identify a gene-expression subprogram regulated by GSK-3-mediated Maf phosphorylation involved in extracellular matrix remodeling and relevant to cancer progression. We also demonstrate that GSK-3 triggers MafA sequential phosphorylation on residues S61, T57, T53, and S49, inducing its ubiquitination and degradation. Paradoxically, this phosphorylation increases MafA-transcriptional activity through the recruitment of the coactivator P/CAF. We further demonstrate that P/CAF protects MafA from ubiquitination and degradation, suggesting that, upon the release of the coactivator complex, MafA becomes polyubiquitinated and degraded to allow the response to terminate.

In order to identify molecular targets of environmental polycyclic aromatic hydrocarbons (PAHs), we have analysed regulation of integrin (ITG) expression in PAH-exposed human macrophages. Among ITG subunits, beta7 ITG was found to be markedly up-regulated at both mRNA and protein levels in response to the prototypical PAH benzo(a)pyrene (BP). Knock-down of the transcription factor c-maf, known to control beta7 ITG expression, markedly impaired BP-mediated beta7 ITG induction. Moreover, chromatin immunoprecipitation and electrophoretic mobility shift assays showed BP-triggered binding of c-maf to a specific maf-responsive element found in beta7 ITG promoter. Such a binding, and also beta7 ITG induction, were however abolished in response to chemical inhibition of the aryl hydrocarbon receptor (AhR), to which PAHs bind. Taken together, these data establish beta7 ITG as a new molecular target of PAHs, whose up-regulation by these environmental contaminants most likely requires activation of co-operative pathways involving both AhR and c-maf.

Basic-leucine zipper transcription factors of the Maf family are key regulators of various developmental and differentiation processes. We previously reported that the phosphorylation status of MafA is a critical determinant of its biological functions. Using Western blot and mass spectrometry analysis, we now show that MafA is phosphorylated by p38 MAP kinase and identify three phosphoacceptor sites: threonine 113 and threonine 57, evolutionarily conserved residues located in the transcription activating domain, and serine 272. Mutation of these residues severely impaired MafA biological activity. Furthermore, we show that p38 also phosphorylates MafB and c-Maf. Together, these findings suggest that the p38 MAP kinase pathway is a novel regulator of large Maf transcription factors.