MYC-DEPENDENT TRANSCRIPTIONAL PROGRAMS IN MAMMARY EPITHELIAL CELLS

The transcriptional programs governing the decision of mammary epithelial stem and progenitor cells to self-renew or differentiate are still not completely outlined. Previous evidences pointed out the role of Myc, and in particular of its repressive activity with Miz1, in these processes. Since the presence of stem-like cells within the tumor, the so-called cancer stem cells (CSCs), is now considered crucial for tumor initiation and maintenance, clarifying this aspect of Myc biology could be relevant in understanding its contribution to the genesis of breast cancer. Here, we used RNA-seq technology to profile the transcriptional programs regulated by Myc in two different settings. First, we studied immortalized mammary epithelial cells, in which we induced either Myc loss or gain of function. In this setting, Myc contributed to the positive and negative regulation of different sets of genes. Activated genes are involved in proliferation, metabolism, ribosomal biogenesis, mitochondrial organization, chromatin modification, RNA processing and modification. Repressed genes, on the other hand, were mainly involved in lysosome and vesicle-mediated transport, angiogenesis, cell death, extracellular matrix interaction, cell adhesion regulation, epithelial development and morphogenesis. Second, we studied the effect of Myc activation in mammosphere cultures, which provide a measure of stem cell activity. We demonstrate that Myc, when overexpressed, is able to promote self-renewal of mammary epithelial stem cells, as assessed by increased mammosphere expansion, and confirmed by mammary gland reconstitution assays in vivo. This activity of Myc is in part dependent on the interaction with the co-repressor protein Miz1, since the Myc mutant V394D (hereby Myc VD), impaired in Miz1 binding, is defective in promoting self-renewal. Overexpression of Myc in mammospheres was associated with the de-regulated expression of about three thousand genes, with similar numbers of up- and down-regulated genes. A group of around nine hundred genes was specifically repressed by Myc WT and not by the VD mutant. Surprisingly, the overlap between the groups of regulated genes in those mammary epithelial cells in adhesion or grown as mammospheres was limited, illustrating the context-dependency of Myc-dependent responses. Thus, transcriptional repression via Miz1 may constitute one of the mechanisms through which Myc sustains mammary epithelial cell self-renewal. We are currently setting a functional screen, among genes repressed in a Miz1-dependent manner, to identify those that are critical in this process. Our study shall shed light on the mechanisms through which Myc regulates self-renewal in mammary epithelial stem and progenitor cells. Understanding this could be crucial in order to clarify the physiopathological roles of Myc in the mammary gland.