MicroRNAs: From Decay to Decoy

MicroRNAs interact with Argonaute proteins to guide posttranscriptional gene silencing. Eiring et al., 2010Eiring A.M. Harb J.G. Neviani P. Garton C. Oaks J. Spizzo R. Liu S. Schwind S. Santhanam R. Hickey C. Cell. 2010; (this issue)PubMed Google Scholar now show that miR-328 has a second function, acting as a decoy by binding to hnRNP E2 and lifting its translational repression of an mRNA involved in myeloid cell differentiation. MicroRNAs interact with Argonaute proteins to guide posttranscriptional gene silencing. Eiring et al., 2010Eiring A.M. Harb J.G. Neviani P. Garton C. Oaks J. Spizzo R. Liu S. Schwind S. Santhanam R. Hickey C. Cell. 2010; (this issue)PubMed Google Scholar now show that miR-328 has a second function, acting as a decoy by binding to hnRNP E2 and lifting its translational repression of an mRNA involved in myeloid cell differentiation. Although microRNAs (miRNAs) were discovered a decade ago, a detailed understanding of these tiny gene regulators is still in its infancy. Genes encoding miRNAs are transcribed as primary miRNA transcripts (pri-miRNAs) by RNA polymerase II. Pri-miRNAs are processed by Drosha to produce stem-loop-structured miRNA precursors (pre-miRNAs). Pre-miRNAs are exported to the cytoplasm, where Dicer generates ∼21 nucleotide double-stranded RNA intermediates. Such double-stranded RNAs are processed further, and one strand, the mature miRNA, interacts with Argonaute (Ago) proteins to form miRNA-protein complexes (miRNPs) (Bartel, 2009Bartel D.P. Cell. 2009; 136: 215-233Abstract Full Text Full Text PDF PubMed Scopus (14311) Google Scholar, Carthew and Sontheimer, 2009Carthew R.W. Sontheimer E.J. Cell. 2009; 136: 642-655Abstract Full Text Full Text PDF PubMed Scopus (3435) Google Scholar). miRNAs are able to silence gene expression posttranscriptionally by binding to partially complementary target sites in the 3′ untranslated region (UTR) of target messenger RNAs (mRNAs), leading to repression of translation or destabilization of the mRNA by deadenylation. Almost perfectly complementary target sites in the mRNA can be cleaved by the miRNA through an RNA interference-like mechanism (Filipowicz et al., 2008Filipowicz W. Bhattacharyya S.N. Sonenberg N. Nat. Rev. Genet. 2008; 9: 102-114Crossref PubMed Scopus (3856) Google Scholar). In contrast to these inhibitory effects on gene expression, miRNAs can also stimulate the expression of target genes (Ørom et al., 2008Ørom U.A. Nielsen F.C. Lund A.H. Mol. Cell. 2008; 30: 460-471Abstract Full Text Full Text PDF PubMed Scopus (955) Google Scholar, Vasudevan et al., 2007Vasudevan S. Tong Y. Steitz J.A. Science. 2007; 318: 1931-1934Crossref PubMed Scopus (2032) Google Scholar). In this issue, Eiring et al., 2010Eiring A.M. Harb J.G. Neviani P. Garton C. Oaks J. Spizzo R. Liu S. Schwind S. Santhanam R. Hickey C. Cell. 2010; (this issue)PubMed Google Scholar report a new function for the human miRNA, miR-328. They show that miR-328 can act as a decoy by binding to a regulatory RNA binding protein and preventing it from blocking translation of mRNAs. Thus, miR-328 has a dual role in the regulation of gene expression. The RNA binding protein hnRNP E2, also called poly(rC) binding protein, interacts with a conserved C-rich binding site in the 5′ UTR of the C/EBPα mRNA and inhibits its translation (Figure 1A ). C/EBPα mRNA encodes a transcription factor that is important for the differentiation of myeloid progenitor cells into white blood cells called granulocytes in the bone marrow. The activity of hnRNP E2 is induced by the kinase activity of BCR/ABL, a fusion protein derived from the reciprocal translocation of chromosomes 9 and 22. This chromosomal abnormality (called the Philadelphia chromosome) and the BCR-ABL oncoprotein it encodes are present in about 95% of all patients with chronic myeloid leukemia (CML). BCR/ABL prevents granulocytic differentiation, and thus allows myeloid progenitor cells to proliferate. The authors analyzed miRNA expression profiles in myeloid progenitor cells from CML patients in blast crisis that strongly express the BCR/ABL oncoprotein and found reduced expression of miR-328. They realized that the sequence of miR-328 is very similar to the hnRNP E2 binding sequence found in the C/EBPα mRNA and hypothesized that the mature miR-328 might bind to hnRNP E2. Using several experimental approaches, they demonstrate that miR-328 interacts with hnRNP E2 under physiological conditions and that surprisingly this interaction is independent of Ago proteins and other proteins associated with the gene silencing machinery. The C/EBPα protein drives myeloid cell differentiation, and hnRNP E2 inhibits this differentiation by blocking the translation of C/EBPα mRNA. Consequently, the authors surmised that ectopic expression of miR-328 should influence myeloid differentiation. Using a number of in vitro and in vivo assays, they demonstrate that miR-328 is able to lift translational inhibition of C/EBPα mRNA and drive CML blast cells to differentiate into granulocytes. The ability of miR-328 to rescue the differentiation of CML blast cells is due to its interaction with hnRNP E2 leading to the sequestration of hnRNP E2 and the restoration of C/EBPα mRNA translation. Furthermore, the authors identified a positive regulatory feedback loop, which highlights the complexity of miRNA regulatory networks. They show not only that miR-328 regulates C/EBPα expression, but also that the C/EBPα protein induces miR-328 expression by binding directly to the miR-328 promoter (Figure 1B). Is miR-328 a typical miRNA, or does it just act as a decoy for hnRNP E2? Searching for targets using common miRNA target prediction algorithms, Eiring and colleagues validated the oncogene PIM1 as a direct target of miR-328. PIM1 is a regulator of the cell cycle and apoptosis and is therefore important for the survival of leukemic blast cells expressing BCR/ABL. Therefore, miR-328 has a dual function: on the one hand, it guides silencing of target genes, and on the other, it acts as a decoy blocking translational inhibition mediated by hnRNP E2. It is well established that RNA binding proteins can modulate the function of miRNAs in different systems. For example, proteins like Deadend 1 or HuR (Hu antigen R, also known as ELAV1) can compete with miRNA binding sites on mRNAs and modulate miRNA function (Bhattacharyya et al., 2006Bhattacharyya S.N. Habermacher R. Martine U. Closs E.I. Filipowicz W. Cell. 2006; 125: 1111-1124Abstract Full Text Full Text PDF PubMed Scopus (1018) Google Scholar, Kedde et al., 2007Kedde M. Strasser M.J. Boldajipour B. Oude Vrielink J.A. Slanchev K. le Sage C. Nagel R. Voorhoeve P.M. van Duijse J. Ørom U.A. et al.Cell. 2007; 131: 1273-1286Abstract Full Text Full Text PDF PubMed Scopus (537) Google Scholar). The Eiring et al. findings are intriguing because an miRNA-mediated regulatory function associated with RNA binding proteins has not been reported before. Moreover, the general view of miRNA functions entails a model in which miRNAs serve as guides for Ago protein complexes, the actual mediators of posttranscriptional gene silencing. Eiring and coworkers show that miR-328 can act independently of Ago proteins by interacting directly with hnRNP E2. The discovery of this new function for miRNAs raises a number of fascinating questions. How many other miRNAs are similar to the binding sequences of RNA binding proteins? What is the impact of miRNAs acting as decoys in the pathogenesis of cancer and other diseases? How are miRNAs released from Ago proteins and how is the interplay between the two pathways regulated? Future studies will be necessary to unravel the mechanistic details underlying the diverse functions of miRNAs both in healthy as well as diseased tissue. miR-328 Functions as an RNA Decoy to Modulate hnRNP E2 Regulation of mRNA Translation in Leukemic BlastsEiring et al.CellMarch 05, 2010In BriefMicroRNAs and heterogeneous ribonucleoproteins (hnRNPs) are posttranscriptional gene regulators that bind mRNA in a sequence-specific manner. Here, we report that loss of miR-328 occurs in blast crisis chronic myelogenous leukemia (CML-BC) in a BCR/ABL dose- and kinase-dependent manner through the MAPK-hnRNP E2 pathway. Restoration of miR-328 expression rescues differentiation and impairs survival of leukemic blasts by simultaneously interacting with the translational regulator poly(rC)-binding protein hnRNP E2 and with the mRNA encoding the survival factor PIM1, respectively. Full-Text PDF Open Archive