The RNA-Binding protein HuR and the members of miR-200 family play an unconventional role in the regulation of c-Jun mRNA stability

MiR-200 family (miR-200a, miR-200b, miR-200c, miR-141, and miR-429) is a cluster of miRNAs highly correlated with epithelial-mesenchymal transition (EMT). A biphasic role of miR-200 family is supported by a lot of studies that show that primary cancer cells downregulate miR-200 expression at the invasive front where they undergo EMT and upregulate miR-200s in the resulting metastasis where mesenchymal to epithelial transition (MET) facilitates colonization of a distant tissue . Therefore, the miR-200 family seems to have a dynamic role in tumor progression, following a still unclear mechanism. In order to clarify this mechanism, in this work we describe the regulation of c-Jun and Dual-specificity phosphatases 1 (DUSP1) mRNA mediated by two members of the miR-200 family: miR-200a and miR-200b. C-Jun is the main component of the AP-1 transcription factor that play an essential role in almost all areas of eukaryotic cellular behavior, DUSP1 is one of many phosphatases coded by the mammalian genome, its role is to dephosphorylate and therefore inactivate the MAP kinases such as ERKs and JNKs. By bioinformatics analysis we identified putative binding sites for both miR-200a and miR-200b in 3’UTR of mRNA of c-Jun and DUSP1 . Surprisingly, our reporter assay revealed an opposite action of the two miRNAs on this 3’UTRs: over expression of miR-200b reduces luciferase activity whereas miR-200a increase it. Interestingly, miR-200a target site is into the well-described ARE for both mRNAs, and this ARE is bounded by the HuR RBP. So we wondered whether this RBP could play a role in this non canonical regulation. After transfection of the siRNA against HuR, we lost the stabilizing effect of miR-200a, suggesting the presence of a potential interaction between the RBP and the miR-200a on the mRNAs 3’-UTR. These results suggest a new regulatory mechanism for microRNA in cooperation with HuR and highlight a new potential role for miR-200a in tumor progression.