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 beta (C/EBPbeta) message is a ligand for HuR. Within 2 
h of initiation of the differentiation process, HuR complexes containing 
C/EBPbeta 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/EBPbeta, 
was not found in the immunoprecipitated messenger ribonucleoprotein complexes. 
The proximity of this event to adipogenic stimuli and the importance of 
C/EBPbeta 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/EBPbeta protein expression and an attenuation of the 
differentiation process.