miR-200c overexpression led to a increase in apoptosis, as compared to cells transfected with scrambled pre-miRs. In contrast, in cells with Noxa knocked down the increase in apoptosis. To further investigate the relationship between miR-200c, Noxa and bortezomib-induced cell death, we went on to ectopically express a Noxa construct lacking the miR-200c target site. When Noxa was overexpressed in cells left untreated with bortezomib, only a minor effect on apoptosis could be observed. However, overexpression of Noxa potentiated the positive effect of miR-200c on bortezomib-induced apoptosis, showing that artificially maintaining high Noxa levels in cells increases the pro-apoptotic effects of miR-200c even further. In summary, these data show that miR-200c sensitizes cells to bortezomib treatment. However, at the same time it represses Noxa, which leads to an attenuated bortezomib response. In this study we identify and validate miR-200c as a regulator of the proapoptotic BH3-only member Noxa. Much is known regarding the transcriptional regulation of Noxa. Several types of cellular SBI-0640756 stress, such as DNA damage and hypoxia, lead to Noxa induction in both a p53-dependent and independent fashion. However, nothing has so far been reported concerning possible microRNA regulation of Noxa. The identification of miR-200c as a Noxa regulator was facilitated by a methodology that combines a luciferase-based screening with mining of microRNA expression data. This method is broadly applicable to the identification of other microRNA:target interactions. Obviously, other mechanisms than 1252003-15-8 microRNAs exist that regulate gene expression through the 39UTR. Several recent studies have demonstrated the importance of for example RNA-binding proteins in posttranscriptional gene regulation. However, it has also been shown that in many cases there is extensive interplay between microRNAs and RNA-binding proteins. For example, miR-16 is necessary for the regulated turnover of AU-rich element containing mRNAs by the ARE-binding protein tristetraprolin. The fact that microRNA-mediated gene repression makes up a substantial part of 39UTR-mediated regulation was substantiated in a recent report investigating the impact or shortened 39UTRs on oncogenic transformation. When isoforms of varying
