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Medulloblastoma (MB) is the most prevalent brain cancer in children. Four subgroups of MB have been identified; of these, Group 3 is the most metastatic. Its genetics and biology remain less clear than the other groups, and it has a poor prognosis and few effective treatments available. Tumor hypoxia and the resulting metabolism are known to be important in the growth and survival of tumors but, to date, have been only minimally explored in MB. Here we show that Group 3 MB tumors do not depend on the canonical transcription factor hypoxia-inducible factor-1α (HIF-1α) to mount an adaptive response to hypoxia. We discovered that HIF-1α is rendered inactive either through post-translational methylation, preventing its nuclear localization specifically in Group 3 MB, or by a low expression that prevents modulation of HIF-target genes. Strikingly, we found that HIF-2 takes over the role of HIF-1 in the nucleus and promotes the activation of hypoxia-dependent anabolic pathways. The exclusion of HIF-1 from the nucleus in Group 3 MB cells enhances the reliance on HIF-2's transcriptional role, making it a viable target for potential anticancer strategies. By combining pharmacological inhibition of HIF-2α with the use of metformin, a mitochondrial complex I inhibitor to block respiration, we effectively induced Group 3 MB cell death, surpassing the effectiveness observed in Non-Group 3 MB cells. Overall, the unique dependence of MB cells, but not normal cells, on HIF-2-mediated anabolic metabolism presents an appealing therapeutic opportunity for treating Group 3 MB patients with minimal toxicity., (© 2024. The Author(s).)

MafB, a member of the large Maf transcription factor family, is essential for the embryonic and terminal differentiation of pancreatic α- and β-cells. However, the role of MafB in the control of adult islet-cell proliferation remains unknown. Considering its oncogenic potential in several other tissues, we investigated the possible alteration of its expression in adult mouse β-cells under different conditions of proliferation. We found that MafB, in general silenced in these cells, was reexpressed in ∼30% of adaptive β-cells both in gestational female mice and in mice fed with a high-fat diet. Importantly, reactivated MafB expression was also observed in the early β-cell lesions and insulinomas that developed in β-cell specific Men1 mutant mice, appearing in >80% of β-cells in hyperplasic or dysplastic islets from the mutant mice >4 months of age. Moreover, MafB expression could be induced by glucose stimulation in INS-1 rat insulinoma cells. The induction was further reinforced following Men1 knockdown by siRNA. Furthermore, MafB overexpression in cultured βTC3 cells enhanced cell foci formation both in culture medium and on soft agar, accompanied with the increased expression of Cyclin B1 and D2. Conversely, MafB downregulation by siRNA transfection reduced BrdU incorporation in INS-1E cells. Taken together, our data reveal that Men1 inactivation leads to MafB reexpression in mouse β-cells in vivo, and provides evidence that deregulated ectopic MafB expression may have a hitherto unknown role in adult β-cell proliferation and Men1-related tumorigenesis.

Maf b-Zip transcription factors are involved in both terminal differentiation and oncogenesis. To investigate this apparent contradiction, we used two different primary cell types and performed an extensive analysis of transformation parameters induced by Maf proteins. We show that MafA and c-Maf are potent oncogenes in chicken embryo fibroblasts, while MafB appears weaker. We also provide the first evidence that MafA can confer growth factor independence and promote cell division at low density. Moreover, using MafA as a model, we identified several parameters that are critical for Maf transforming activities. Indeed, MafA ability to induce anchorage-independent cell growth was sensitive to culture conditions. In addition, the transforming activity of MafA was dependent on its phosphorylation state, since mutation on Ser65 impaired its ability to induce growth at low density and anchorage-independent growth. We next examined transforming activity of large Maf proteins in embryonic neuroretina cells, where they are known to induce differentiation. Unlike v-Jun, MafA, MafB and c-Maf did not show oncogenic activity in these cells. Moreover, they counteracted transformation induced by constitutive activation of the Ras/Raf/MEK pathway. Taken together, our results show that Maf proteins could display antagonistic functions in oncogenesis depending on the cellular context, and support a dual role for Maf as both oncogenes and tumor suppressor-like proteins.

The neuroretina is a functional unit of the central nervous system which arises through successive steps of division, growth arrest and differentiation of neuroectodermal precursors. Postmitotic quail neuroretina (QNR) cells are conditionally induced to divide upon infection with temperature sensitive mutants of Rous sarcoma virus (RSV), since QNR cell division can be arrested by either inactivating p60v-Src at the nonpermissive temperature (41 degrees C) or by serum deprivation at 37 degrees C. We are studying the transcriptional control of QR1, a neuroretina specific gene, whose expression is down-regulated in proliferating cells at 37 degrees C and is fully restored when these cells are made quiescent. We previously showed that this quiescence specific upregulation implicates a promoter region named A box, which binds Maf transcription factors. We report the identification of the C box, a second promoter sequence that activates QR1 transcription in non dividing cells. This sequence is able to form two DNA-protein complexes, one of which (C4) is predominantly detected in growth arrested NR cells. We identified the DNA binding site for C4 and described mutations that abolish both C4 binding and promoter activity in quiescent cells. Moreover, we show that a multimerized C box is able to stimulate a heterologous promoter in non dividing cells and constitutes, therefore, a novel quiescence responsive enhancer. Finally, we report that QR1 transcriptional response to cell quiescence requires cooperation between the C box and A box.

Transcription factors of the Maf proto-oncogene family have been shown to participate in the regulation of several differentiation specific genes. We previously reported that a member(s) of this family is involved in the regulation of the neuroretina specific gene, QR1, through a promoter region, designated the A box, that is closely related to the Maf recognition element (MARE). We undertook an identification of Maf family genes expressed in the quail neuroretina (QNR) and we report the isolation of mafA, a gene encoding a novel member of the large Maf proteins subgroup. Expression of this gene is developmentally regulated in the neuroretina. MafA is able to bind to MARE sequence and to heterodimerize with v-Maf, MafB, Jun and Fos, but not with the small MafF and MafK proteins. Accordingly, it is able to transactivate the QR1 promoter A box. We also show that increased expression of mafA induces sustained proliferation of postmitotic QNR cells.