miRNAs regulate miRNAs: coordinated transcriptional and post-transcriptional regulation.

microRNAs are short single-stranded RNA molecules that negatively regulate gene expression at post-transcriptional level. Recently it has been demonstrated that microRNAs are involved in many physiological processes, including differentiation and development. Moreover, this class of tiny regulators is drammatically involved in many pathological processes, like tumors and genetic diseases. A main issue is how microRNA processing and expression are regulated: thanks to recent findings, we are beginning to get insight into this critical aspect of microRNA biology, but we are far from understanding these processes in all their complexity. In particular, microRNA coordinated expression with other regulatory molecules, such as transcription factors, is still under investigation. We depleted microRNA 221 and 222 and found that the microRNA signature of Human Umbilical Vein Endothelial Cells was affected as a consequence: 9 microRNAs were up-regulated and 23 microRNAs were down-regulated. We propose a model in which a complex network involving coexpressed microRNAs and transcription factors is affected by single microRNA-variations, with the result that microRNA expression profile is in turn modified.