A CLASS OF ONCOGENE-REGULATED MICRORNAS COUPLING DIFFERENTIATION AND CELL CYCLE CONTROL

MicroRNAs are an evolutionary conserved epigenetic component, which negatively regulates gene expression in many cellular processes. Originally, microRNAs were discovered as developmental regulators in worms (i.e. lin-4/let-7) and even in higher organisms many microRNAs display a tissue- or lineage-specific pattern of expression, underlying a direct role in tissue differentiation. In human tumors microRNAs are often deregulated or lost. A widespread down-regulation of microRNAs (and in particular tissue specific microRNAs) often occurs in human tumors, which correlates with advanced diseases in breast and ovarian cancer and favors tumor progression in vivo. We, therefore, hypothesized that differentiation associated microRNAs could also act as negative regulator of proliferation and behave as a tumor-suppressor pathway during oncogene induced cellular transformation. We tested this hypothesis by exploiting the ability of the adenoviral oncogene E1A to overcome the proliferative block of terminally differentiated (TD) myotubes by mimicking proliferative and dedifferentiative pathways, which are primary involved in cancer. By using genome-wide approaches, we deeply characterized the transcriptional reprogramming induced by E1A on TD-cells at both transcriptional (mRNA) and post-transcriptional (microRNA) level, focusing on the role of differentiation-associated microRNA in the control of cell-cycle exit and differentiation. We isolated two genetic independent components in the oncogene-induce reprogramming. On one hand, E1A induces the up-regulation of genes essential for proliferation (Dna replication, mitosis, cell cycle) through the interference with the Retinoblastoma (Rb) tumor suppressor pathway. Concomitantly, the E1A-oncogene down-regulates microRNAs accumulated over differentiation by a mechanism, which is, by and large, genetically independent on Rb, and involves tissue specific transcription factors, c-myc and p53. Importantly, the two components (messages and miRNAs) are functionally coupled, since microRNA participates in the control of cell cycle exit, by negatively regulating the expression of Rb-dependent proliferative genes and by promoting differentiation. Indeed, differentiation microRNAs also interferes with oncogene induced cell cycle re-entry by acting as a ”brake” against the transcriptional reprogramming by E1A.