Reduced oxygen (hypoxia) triggers the stabilization of transcription factors of the hypoxia-inducible factor (HIF) family and also prolongs signaling from the epidermal growth factor receptor (EGFR) from endosomes by slowing intracellular trafficking of the receptor. Shen et al. found that EGFR and a microRNA (miRNA)–processing protein, AGO2, interacted at endosomes in response to hypoxia and that AGO2 was phosphorylated by EGFR at Tyr393. Hypoxic conditions triggered an EGFR-dependent increase in the abundance of a specific set of immature, precursor miRNAs that was enriched in tumor-suppressing miRNAs, suggesting that processing of these miRNAs was impaired. Consistent with this interpretation, the abundance of the transcripts targeted by this set of miRNAs was increased in cells exposed to hypoxia. Hypoxia stimulated the phosphorylation of Tyr393 of AGO2 and reduced the interaction between AGO2 and proteins of the RNA-induced silencing complex (RISC)–loading complex. The miRNAs that exhibited reduced AGO2 Tyr393-dependent processing in response to hypoxia had a long-loop structure, and miR reporter assays indicated that the hypoxia-regulated miRNAs with this characteristic were not processed and their targets were derepressed. Conversion of this long-loop structure to a short-loop structure converted the miRNA to one that remained effectively processed in cells exposed to hypoxia and overexpressing wild-type AGO2, but not a Y393F AGO2 mutant. Cells expressing the Y393F mutant exhibited increased apoptotic cell death and reduced migration in response to hypoxia, compared with cells expressing wild-type AGO2. The abundance of phosphorylated AGO2 correlated with the abundance of HIFs in breast cancer samples, suggesting that phosphorylated AGO2 was associated with hypoxic tumors. Phosphorylated AGO2 abundance correlated with poor survival. Thus, by reducing the maturation of miRNAs that function as tumor suppressors, EGFR-mediated phosphorylation of AGO2 may contribute to carcinogenesis and, by enabling survival under hypoxic conditions, may contribute to cancer progression.
J. Shen, W. Xia, Y. B. Khotskaya, L. Huo, K. Nakanishi, S.-O. Lim, Y. Du, Y. Wang, W.-C. Chang, C.-H. Chen, J. L. Hsu, Y. Wu, Y. C. Lam, B. P. James, X. Liu, C.-G. Liu, D. J. Patel, M.-C. Hung, EGFR modulates microRNA maturation in response to hypoxia through phosphorylation of AGO2. Nature 497, 383–387 (2013). [PubMed]