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Frankfurt am Main, Germany

Winkler C.,Goethe University Frankfurt | Doller D.,Goethe University Frankfurt | Doller D.,University Tsklinikum Frankfurt | Doller D.,University Tsklinikum Frankfurt | And 10 more authors.
Cell Death and Disease | Year: 2014

Caspase-2 represents the most conserved member of the caspase family, which exhibits features of both initiator and effector caspases. Using ribonucleoprotein (RNP)-immunoprecipitation assay, we identified the proapoptotic caspase-2L encoding mRNA as a novel target of the ubiquitous RNA-binding protein HuR in DLD-1 colon carcinoma cells. Unexpectedly, crosslinking- RNP and RNA probe pull-down experiments revealed that HuR binds exclusively to the caspase-2-50 untranslated region (UTR) despite that the 30 UTR of the mRNA bears several adenylate- and uridylate-rich elements representing the prototypical HuR binding sites. By using RNAi-mediated loss-of-function approach, we observed that HuR regulates the mRNA and in turn the protein levels of caspase-2 in a negative manner. Silencing of HuR did not affect the stability of caspase-2 mRNA but resulted in an increased redistribution of caspase-2 transcripts from RNP particles to translational active polysomes implicating that HuR exerts a direct repressive effect on caspase-2 translation. Consistently, in vitro translation of a luciferase reporter gene under the control of an upstream caspase-2-50UTR was strongly impaired after the addition of recombinant HuR, whereas translation of caspase-2 coding region without the 50UTR is not affected by HuR confirming the functional role of the caspase-2-50UTR. Functionally, an elevation in caspase-2 level by HuR knockdown correlated with an increased sensitivity of cells to apoptosis induced by staurosporine- and pore-forming toxins as implicated by their significant accumulation in the sub G1 phase and an increase in caspase-2, -3 and poly ADP-ribose polymerase cleavage, respectively. Importantly, HuR knockdown cells remained insensitive toward STS-induced apoptosis if cells were additionally transfected with caspase-2-specific siRNAs. Collectively, our findings support the hypothesis that HuR by acting as an endogenous inhibitor of caspase-2-driven apoptosis may essentially contribute to the antiapoptotic program of adenocarcinoma cells by HuR. © 2014 Macmillan Publishers Limited All rights reserved 2041-4889/14. Source

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