Myc gene encodes for a transcription factor that activates the expression of a great number of genes through binding on Enhancer Box sequences (E-boxes) and recruiting histone acetyltransferases (HATs). It can also act as a transcriptional repressor, by binding Miz-1 transcription factor and displacing the p300 co-activator. Although Myc is ubiquitously expressed and is necessary during development, its expression is tighly regulated. Thus, a mutated version of Myc is found in many cancers which cause Myc to be persistently expressed. This leads to the unregulated expression of many genes some of which are involved in cell proliferation and results in the formation of cancer. A common translocation which involves Myc is t(8:14) is involved in the development of a lymphoma. Myc regulates the expression of 10-15% of human genes and includes the regulation of miRNAs, histone acetylases and demethylases. 60% of the target genes are up-regulated whereas 40% are down regulated. Thus, Myc looks more like a general transcription factor that regulates genes involved in numerous biological effects such as DNA synthesis, genomic instability, cell cycle progression, apoptosis, protein synthesis, cell size, immortalization as well as inhibition of differentiation. In general, Myc is repressed during differentiation and inhibits cell differentiation in cell cultures. However, Myc induces differentiation in vivo in certain circumstances such as on hematopoietic precursors. On the other hand, Myc is involved in remodelling de state of chromatin and activates fibroblasts reprogramming to iPS (induced pluripotent stem cells).

Inhibition of differentiation is a relevant mechanism for Myc-mediated tumorogenesis. The group of Dr. Javier Leon has focused his work in describing the mechanism by which Myc inhibits cellular differentiation in two cellular models. The first studied model has been a genetically defined erythroid differentiation model in human leukemia K562 cells by conditional expression of the cyclin-dependent kinase (Cdk) inhibitor p27 and Myc. The expression of p27 is frequently lost in human cancer and is associated with poor tumor prognosis. Transfection with constructs expressing p27 induces erythroid differentiation in K562 cells accompanied by Cdk inhibition and G1 cell cycle arrest. Conditional expression of p27 and Myc arrests cells at G1 phase but avoides erythroid differentiation. Myc effect on differentiation is independent from Cdk inhibition and does not reverse pRb phosphorylation. Thus, Dr. Leon analysed if Myc regulates genes that impede differentiation by microarrays. Network analysis confirms Myc’s role in the phenotypic change and represses erythroid genes induced by p27 including genes related to erythroid lineage (e.g. GATA1, NFE2 and JUNB). Moreover, Myc also blocked the upregulation of Mad1, a transcriptional antagonist of Myc that is able to induce erythroid differentiation. Co-transfection of K562 cells with p27/Myc/GATA1 or Mad1 partially recues the differentiated phenotype. On the other hand, Myc binds to promoter of skp2 and induces its expression. Skp2 is involved in proteasome-mediated degradation of p27 and is thus, a new Myc target gene. It may explain the inverse correlation of Skp2 and p27 in lymphomas and the direct correlation between p27 and Myc. In conclusion, this model demonstrates that Myc-mediated inhibition of differentiation depends on the regulation of a specific genetic program, which is independent of p27-mediated cell cycle arrest. Moreover, Myc is overexpressed during CML progression and a positive correlation is observed between high levels of Myc and poor response to treatment. Myc could be a prognostic marker of CML progression and a therapeutic target in this tumor.

The second studied model by Dr. Leon has been a genetically defined neuronal differentiation model in human UR61 cells (PC12-derived cells) by conditional expression of the Ras and Myc. In these cells, Ras activation induces neuronal-like differentiation by a process involving c-Jun activation. c-Myc inhibits Ras-mediated differentiation by a mechanism that involves the blockade of c-Jun upregulation in response to Ras signal (Figure 4.). Furthermore, Max is not required for c-Myc activity, as UR61 cells lack a functional Max gene. At the present the group of Dr. León is studying putative Myc-interacting proteins as well as analyzing the differential trascriptome profile of UR61 cells that express Myc vs Myc and Max which do not differentiate to neuronal phenotype.

In summary, Dr. Leon shows that Myc inhibits cell differentiation modifying the genetic program of cells to a not-differentiated state at two levels: i) as a transcription factors that repress genes codifying for transcription factors that master differentiation (e.g. c-Jun in neural differentiation of UR61 cells, GATA1 in erythroid differentitation of K562 cells, c/EBPalpha in adipocytic differentiation in 3T3L1 cells or ERG1 in macrophagic differentiation of M1 cells); and, ii) remodelling the state of chromatin. His studies support the hypothesis that differentiation inhibition is an important Myc tumorigenic mechanism that is independent of cell proliferation. Myc can, therefore, be a putative therapeutic target to obtain pro-differentaiting drugs.