MYC proteins promote neuronal differentiation by controlling the mode of progenitor cell division.

The role of MYC proteins in somatic stem and progenitor cells during development is poorly understood. We have taken advantage of a chick in vivo model to examine their role in progenitor cells of the developing neural tube. Our results show that depletion of endogenous MYC in radial glial precursors (RGPs) is incompatible with differentiation and conversely, that overexpression of MYC induces neurogenesis independently of premature or upregulated expression of proneural gene programs. Unexpectedly, the neurogenic function of MYC depends on the integrity of the polarized neural tissue, in contrast to the situation in dissociated RGPs where MYC is mitogenic. Within the polarized RGPs of the neural tube, MYC drives differentiation by inhibiting Notch signaling and by increasing neurogenic cell division, eventually resulting in a depletion of progenitor cells. These results reveal an unexpected role of MYC in the control of stemness versus differentiation of neural stem cells in vivo.

[The myc oncogene and transdifferentiation of the retinal pigment epithelium]. / Oncogène myc et transdifferenciation de l'épithélium pigmentaire de la rétine.

The retinal pigment epithelium (RPE) develops from the same sheet of neuroepithelium as the neuroretina. When infected with MC29, a v-myc expressing virus, the RPE cells can be induced to transdifferentiate and to take a neuroretinal epithelium fate. After a PCR-based differential screening from these cells we have identified three genes of interest. Qath5, a quail basic helix-loop-helix (bHLH) gene that is closely related to the Drosophila atonal, and whose expression is found in the developing neuroretina. A Chx10-related homeobox gene also expressed in the developing neuroretina and HuD, a RNA-binding protein not expressed in the RPE but expressed during neurogenesis. Beside these genes whose function is involved in regulating neuronal differentiation myc also induced a transient Mitf expression. Mitf is expressed in the entire optic cup, later restricted to the pigmented retina. Mitf is involved in the regulation of the pigmented differentiation. We conclude that v-myc can reverse the RPE to the bipotential retinal primordia.

Carcinoembryonic antigen, a human tumor marker, cooperates with Myc and Bcl-2 in cellular transformation.

Carcinoembryonic antigen (CEA) is a tumor marker that is overexpressed in many human cancers and functions in vitro as a homotypic intercellular adhesion molecule. We have investigated the possibility of synergy between CEA, v-Myc, and Bcl-2 in the transformation of cells with differentiation capacity. We find that v-Myc increases the cell division rate and maximum density of rat L6 myoblasts but also markedly stimulates both apoptosis and surprisingly, differentiation, thus preventing transformation. The superposition of Bcl-2 blocks the apoptotic stimulation of v-Myc and independently promotes further cell division at confluence, but still allows differentiation. The further expression of CEA has a dominant effect in blocking differentiation, regardless of the presence of the other activated oncogenes, generating cells that enter a reversible quiescent G0-like state in medium promoting differentiation. Transfectants expressing CEA with or without v-myc and bcl-2 allow the emergence of cells with the property of heritable, efficient, anchorage-independent growth in soft agar and the ability to markedly reduce the latency for tumor formation in nude mice. We propose that by prolonging cell survival in the presence of differentiation signals, CEA represents a novel class of dominant differentiation-blocking oncogene.

A novel function for Myc: inhibition of C/EBP-dependent gene activation.

We have investigated the effect of the v-Myc oncoprotein on gene expression in myelomonocytic cells. We find that v-Myc dramatically down-regulates the expression of myelomonocytic-specific genes, such as the chicken mim-1 and lysozyme genes, both of which are known targets for C/EBP transcription factors. We present evidence that Myc downregulates these genes by inhibiting the function of C/EBP transcription factors. Detailed examination of the inhibitory mechanism shows that amino-terminal sequences of v-Myc, but not its DNA-binding domain, are required for the suppression of C/EBP-dependent transactivation. Our findings identify a new function for Myc and reveal a novel mechanism by which Myc affects the expression of other genes.

An identical effect mediated by thyroid deficiency or oncogene v-erbA in the chick embryo.

We have shown earlier that the association of v-myc and v-erbA (MAHEVA construct) is responsible for the appearance of a specific phenotype in chick embryos inoculated at E3. This phenotype comprises rapidly growing heart rhabdomyomas (induced by v-myc alone) and within these tumors secondarily appearing cartilage nodules (Bachnou et al., Oncogene 6: 1041-1047, 1991). Here we report that v-erbA can be replaced by thyroid deficiency. When decapitated embryos were inoculated with virus MC29 (v-myc alone) or when v-myc inoculated embryos were treated with thiourea, 100% of the embryos reaching E17 to E19 displayed tumoral hearts bearing cartilage nodules. We thus report in vivo evidence that v-erbA acts by antagonizing the effects of thyroid hormones. Remarkably, thyroid deficiency rendered embryos more sensitive to the effect of v-myc, since 100% developed heart rhabdomyomas and cartilage nodules, versus about 70% affected when either v-myc or MAHEVA were inoculated. Thyroid deficiency did not alter the species-specific character of transdifferentiation, since only chick but not quail embryos developed cartilage nodules after thyroidectomy or MAHEVA infection.

Differentiation of the immortalized adult neuronal progenitor cell line HC2S2 into neurons by regulatable suppression of the v-myc oncogene.

A regulatable retroviral vector in which the v-myc oncogene is driven by a tetracycline-controlled transactivator and a human cytomegalovirus minimal promoter fused to a tet operator sequence was used for conditional immortalization of adult rat neuronal progenitor cells. A single clone, HC2S2, was isolated and characterized. Two days after the addition of tetracycline, the HC2S2 cells stopped proliferating, began to extend neurites, and expressed the neuronal markers tau, NeuN, neurofilament 200 kDa, and glutamic acid decarboxylase in accordance with the reduced production of the v-myc oncoprotein. Differentiated HC2S2 cells expressed large sodium and calcium currents and could fire regenerative action potentials. These results suggest that the suppression of the v-myc oncogene may be sufficient to make proliferating cells exit from cell cycles and induce terminal differentiation. The HC2S2 cells will be valuable for studying the differentiation process of neurons.

The transcription activation domains of v-Myc and VP16 interact with common factors required for cellular transformation and proliferation.

The amino terminus of the avian myelocytomatosis virus MC29 v-Myc oncoprotein contains sequences that are essential for cellular transformation (S. Farina, et al. J. Virol., 66: 2698-2708, 1992; S. Min and E. J. Taparowsky. Oncogene, 7:1531-1540, 1992) and for the ability to activate gene transcription (S. Min and E. J. Taparowsky. Oncogene, 7:1531-1540, 1992). To investigate the molecular interactions that mediate these v-Myc-associated activities, we performed competition assays in which various regions of the v-Myc amino terminal transcription activation domain (TAD) were examined for their ability to inhibit transcription activation by v-Myc, VP16, and the myogenic regulatory factor MyoD. Overexpression of these transcriptional activators also was used to investigate whether Myc-interacting proteins were required for cellular transformation and cell proliferation events. Our results demonstrate that at least two distinct cellular activities interact with the v-Myc TAD and that it is the synergism between these activities that is required for v-Myc to function fully as a transcriptional activator. In addition, v-Myc activators squelch VP16- and MyoD-dependent transcription activation, suggesting that the v-Myc TAD interacts with a component of the general transcription machinery. In support of this observation, we found that overexpression of the v-Myc TAD inhibits ras-mediated cellular transformation as well as cell proliferation, underscoring the critical role these amino terminal Myc-interacting factors play in regulating the physiology of both normal and transformed cells.