Signaling by p38 MAPK Stimulates Nuclear Localization of the Microprocessor Component p68 for Processing of Selected Primary MicroRNAs

An early step in microRNA processing depends on signaling downstream of the p38 mitogen-activated protein kinase. Promoting MicroRNA Production MicroRNAs (miRNAs) are small noncoding RNAs that target specific mRNAs for degradation or block their translation, thus leading to knockdown of given gene products. Posttranscriptional generation of miRNAs requires the nuclear processing of primary miRNAs (pri-miRNAs) by components of the Drosha-containing complex followed by the cytosolic processing of the resulting precursor miRNAs (pre-miRNAs) by the Dicer complex to generate mature miRNAs. Hong et al. found that inhibition of the mitogen-activated protein kinase (MAPK) p38 and its effector kinase MK2 blocked the processing of a subset of pri-miRNAs. In the cytosol, MK2 phosphorylated the Drosha complex component p68, which was required for its translocation to the nucleus for pri-miRNA processing. Inhibition of p38 signaling in cells decreased the production of miR-145, which targets the mRNA encoding c-Myc, resulting in increased c-Myc abundance and enhanced proliferation. Together, these data suggest that p38 MAPK signaling is required for selected miRNA biogenesis by promoting the nuclear localization of p68. The importance of microRNAs (miRNAs) in biological and disease processes necessitates a better understanding of the mechanisms that regulate miRNA abundance. We showed that the activities of the mitogen-activated protein kinase (MAPK) p38 and its downstream effector kinase MAPK-activated protein kinase 2 (MK2) were necessary for the efficient processing of a subset of primary miRNAs (pri-miRNAs). Through yeast two-hybrid screening, we identified p68 (also known as DDX5), a key component of the Drosha complex that processes pri-miRNAs, as an MK2-interacting protein, and we found that MK2 phosphorylated p68 at Ser197 in cells. In wild-type mouse embryonic fibroblasts (MEFs) treated with a p38 inhibitor or in MK2-deficient (MK2−/−) MEFs, expression of a phosphomimetic mutant p68 fully restored pri-miRNA processing, suggesting that MK2-mediated phosphorylation of p68 was essential for this process. We found that, whereas p68 was present in the nuclei of wild-type MEFs, it was found mostly in the cytoplasm of MK2−/− MEFs. Nuclear localization of p68 depended on MK2-mediated phosphorylation of Ser197. In addition, inhibition of p38 MAPK promoted the growth of wild-type MEFs and breast cancer MCF7 cells by enhancing the abundance of c-Myc through suppression of the biogenesis of the miRNA miR-145, which targets c-Myc. Because pri-miRNA processing occurs in the nucleus, our findings suggest that the p38 MAPK–MK2 signaling pathway promotes miRNA biogenesis by facilitating the nuclear localization of p68.

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