Posttranscriptonal regulation of the microRNA-17-92 cluster

MicroRNAs (miRs) are endogenously expressed small non-coding RNAs that regulate gene expression by degradation or translational repression of target mRNAs. The polycistronic miR-17-92 cluster is highly expressed in endothelial cells. Although the seven cluster members are under the control of the same promoter, the individual members are differentially regulated after induction of hypoxia in vitro and ischemia in vivo. These findings suggest a tightly regulated processing and the presence of stability regulators of the individual miRs. In order to identify miR binding proteins that might potentially be involved in the posttranscriptional regulation we used in vitro RNA pull down assays. By mass spectrometry 38 proteins were identified that specifically bound to pre-miR-92a, but not to a control sequence. Among these proteins 18 are known to bind nucleotides and 6 are common RNA binding proteins like RNA helicases and splicing regulators. Moreover, the cytoskeletal proteins Filamin A and B were identified as miR-92a binding proteins. The influence of two of the identified proteins on miR-92a expression was analyzed. Preliminary data indicate that siRNA-mediated knock down of Filamin A results in a down-regulation of miR-92a expression. In contrast, knock down of the glutamyl-prolyl-tRNA synthesase (EPRS) up-regulates miR-92a expression. EPRS is part of the GAIT (IFN-Gamma-Activated Inhibitor of Translation) complex that is known to bind RNA. Interestingly, mimicking hypoxia by DFO treatment resulted in a significant down-regulation of EPRS in HUVEC suggesting that hypoxia might control the expression of proteins that control miR processing. Ongoing studies further address the biological implications of these proteins.