Isabel López de Silanes

HuR, a protein that binds to specific mRNA subsets, is increasingly recognized as a pivotal posttranscriptional regulator of gene expression. Here, HuR was immunoprecipitated under conditions that preserved HuR-RNA interactions, and HuR-bound target mRNAs were identified by cDNA array hybridization. Analysis of primary sequences and secondary structures shared among HuR targets led to the identification of a 17- to 20-base-long RNA motif rich in uracils. This HuR motif was found in almost all mRNAs previously reported to be HuR targets, was located preferentially within 3′ untranslated regions of all unigene transcripts examined, and was conserved in >50% of human and mouse homologous genes. Importantly, the HuR motif allowed the successful prediction and subsequent validation of novel HuR targets from gene databases. This study describes an HuR target RNA motif and presents a general strategy for identifying target motifs for RNA-binding proteins.

Exposure to short-wavelength UV light (UVC) strongly induces p53 expression. In human RKO colorectal carcinoma cells, this increase was not due to elevated p53 mRNA abundance, cytoplasmic export of p53 mRNA, or UVC-triggered stabilization of the p53 protein. Instead, p53 translation was potently enhanced after UVC irradiation. The 3' UTR of p53 was found to be a target of the RNA-binding protein HuR in a UVC-dependent manner in vitro and in vivo. HuR-overexpressing RKO cells displayed elevated p53 levels, whereas cells expressing reduced HuR showed markedly diminished p53 abundance and p53 translation. Our results demonstrate a role for HuR in binding to the p53 mRNA and enhancing its translation.

The RNA-binding protein HuR regulates the stability and translation of target mRNAs. While no HuR mutations have been found in cancer, a link between HuR and malignant transformation has been suggested in cancers of the breast, colon, lung and ovary. We describe a paradigm consistent with a central role of HuR in oncogenesis.

Cytoplasmic export of the RNA-binding protein HuR, a process that critically regulates its function, was recently shown to be inhibited by the AMP-activated protein kinase (AMPK). In the present investigation, treatment of human fibroblasts with AMPK activators such as 5-amino-imidazole-4-carboxamide riboside, antimycin A, and sodium azide inhibited cell growth and lowered the expression of proliferative genes. As anticipated, AMPK activation also decreased both the cytoplasmic HuR levels and the association of HuR with target radiolabeled transcripts encoding such proliferative genes. HuR function was previously shown to be implicated in the maintenance of a “young cell” phenotype in models of replicative cellular senescence. We therefore postulated that AMPK activation in human fibroblasts might contribute to the implementation of the senescence phenotype through mechanisms that included a reduction in HuR cytoplasmic presence. Indeed, AMP:ATP ratios in fibroblasts with in senescence was by a in AMPK that AMPK to the implementation of the phenotype was through of AMPK activators senescence in such as the of and of with that AMPK with that AMPK decreased that AMPK activation senescence through mechanisms that HuR Cytoplasmic export of the RNA-binding protein HuR, a process that critically regulates its function, was recently shown to be inhibited by the AMP-activated protein kinase (AMPK). In the present investigation, treatment of human fibroblasts with AMPK activators such as 5-amino-imidazole-4-carboxamide riboside, antimycin A, and sodium azide inhibited cell growth and lowered the expression of proliferative genes. As anticipated, AMPK activation also decreased both the cytoplasmic HuR levels and the association of HuR with target radiolabeled transcripts encoding such proliferative genes. HuR function was previously shown to be implicated in the maintenance of a “young cell” phenotype in models of replicative cellular senescence. We therefore postulated that AMPK activation in human fibroblasts might contribute to the implementation of the senescence phenotype through mechanisms that included a reduction in HuR cytoplasmic presence. Indeed, AMP:ATP ratios in fibroblasts with in senescence was by a in AMPK that AMPK to the implementation of the phenotype was through of AMPK activators senescence in such as the of and of with that AMPK with that AMPK decreased that AMPK activation senescence through mechanisms that HuR AMP-activated protein kinase AMP-activated protein 5-amino-imidazole-4-carboxamide AMP-activated protein 5-amino-imidazole-4-carboxamide that to as a a of the cell was of its and function to be AMPK a of and and AMPK of AMPK as as AMPK by in and inhibited by of that the AMP:ATP in such as growth in in as as treatment with and the the of activation of AMPK In AMPK in shown to expression in a of in the AMPK a of expression of of AMPK in also AMPK was shown to the its to with AMPK also to the levels of and to the of In AMPK was recently to protein with decreased activation of the target of and its AMPK shown to by the cytoplasmic export of the RNA-binding protein HuR HuR in the of that the of HuR its to 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