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.

Genetic data in the mouse have shown that endothelin 3 (ET3) and its receptor B (ETRB) are essential for the development of two neural crest (NC) derivatives, the melanocytes and the enteric nervous system. We report here the effects of ET3 in vitro on the differentiation of quail trunk NC cells (NCC) in mass and clonal cultures. Treatment with ET3 is highly mitogenic to the undifferentiated NCC population, which leads to expansion of the population of cells in the melanocytic, and to a lesser extent, the glial lineages. The effect of ET3 on these two NC derivatives was confirmed by the quantitative analysis of clones derived from individual NCC subjected to ET3: we found a large increase in the survival and proliferation of unipotent and bipotent precursors for glial cells and melanocytes, with no significant effect on multipotent cells generating neurons. ET3 first stimulates expression of both ETRB and ETRB2 by cultured NCC. Then, under prolonged exposure to ET3, ETRB expression decreases and switches toward an ETRB2-positive melanogenic cell population. We therefore propose that the present in vitro experiments (long-lasting exposure to a high concentration of ET3) mimic the environment encountered by NCC in vivo when they migrate to the skin under the ectoderm that expresses ET3.The neural crest (NC) appears dorsally to the neural primordium as it forms according to a craniocaudal gradient. The presumptive NC cells (NCC) undergo an epithelio-mesenchymal transition and, after a phase of migration, give rise to multiple cell types including melanocytes, neurons, and glial cells of peripheral nerves and ganglia, a large majority of the cephalic mesenchymal structures, and certain endocrine cells (1, 2). Because of its pluripotentiality and the fact that its constitutive cells become localized in various regions of the developing embryo, the NC is an ideal developmental system in which to study the mode of action of factors involved in differentiation choices.The importance of environmental influences on the development of NCC has been demonstrated by in vivo transplantation experiments in the chicken embryo and in vitro culture studies (1-6). At the onset of migration, the NCC population is composed of a mixture of pluripotent and more or less restricted progenitors that have been identified by various cell cloning experiments (7-14). These observations suggest that both selective and instructive mechanisms are involved in NCC diversification. Thus far, differentiation of definite lineages of NC-derived cells in clonal cultures has proved to be favored by factors such as brain-derived neurotrophic factor (15), glial growth factor (16), retinoic acid (17), and members of the transforming growth factor ␤ family (18). Other growth factors (and their receptors) encoded by loci that affect NC derivatives in mice have been shown to have an important role in NC ontogeny but their mode of action is not yet fully understood. Such is the case for the receptor-ligand system constituted by endothelin receptor B (E...

Endothelin 3 (EDN 3) and the endothelin receptor B (EDNRB) are involved in the development of neural crest and particularly of the melanocytes and the enteric nervous system. We reported previously that the avian EDNRB gene is expressed in the neural fold before crest cell migration and later on in all the neural crest derivatives except, at any developmental stage, in the melanocytic lineage. However, quail melanoblasts proliferate in response to EDN 3 stimulation in vitro. These observations prompted us to search for another type of endothelin receptor (EDNR). We report here the cloning by reverse transcriptase-PCR of an avian cDNA encoding a subtype of EDNR, which we have called EDNRB2, because its deduced amino acid sequence is more closely related to that of EDNRB than to either the mammalian EDNRA or to the Xenopus EDNRC. Its expression pattern differs from that of the ''classical'' avian EDNRB because it is strongly expressed in melanoblasts and melanocytes. EDNRB2 transcripts are also abundant in the liver and kidney. Our pharmacological studies showed that EDNRB2 binds with similar affinity to EDN 1, EDN 2, and EDN 3, further confirming that this receptor belongs to the B type, although it displays a low affinity for sarafotoxin-c, a known EDNRB-selective agonist.