An Early Event in Adipogenesis, the Nuclear Selection of the CCAAT Enhancer-binding Protein ß (C/EBP ß ) mRNA by HuR and its Translocation to the Cytosol2006
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HuR is a ligand for nuclear mRNAs containing adenylate-uridylate-rich elements in the 3_-untranslated region. Once bound to the mRNA, HuR is recognized by adapter proteins that then facilitate nuclear export of the complex. In the cytosol, HuR is thought to function to control stability and translation of its ligand message. In the 3T3-L1 cells HuR is constitutively expressed and localized predominantly to the nucleus in the preadipocytes. However, within 30 min of exposure to the differentiation stimulus the HuR content in the cytosol increases, consistent with HuR regulating the availability of relevant mRNAs for translation. Using in vitro RNA gel shifts, we have demonstrated that the CCAAT enhancer binding protein ß (C/EBP ß) message is a ligand for HuR. Within 2 h of initiation of the differentiation process, HuR complexes containing C/EBP ß mRNA could be isolated from the cytosolic compartment. Importantly, the process appears to be highly selective, as cyclin D1, which contains a putative HuR binding site and is expressed on the same time frame as C/EBP ß, was not found in the immunoprecipitated messenger ribonucleoprotein complexes. The proximity of this event to adipogenic stimuli and the importance of C/EBP ß to the differentiation process have led us to hypothesize a role for HuR in the regulation of the onset of adipogenesis. In support of this hypothesis, small interfering RNA suppression of HuR protein content resulted in an inhibition of C/EBP ß protein expression and an attenuation of the differentiation process.