Post-Transcriptional Regulation of mRNA Metabolism during Differentiation of 3T3-L1 Cells: Role of HuR

Abstract

Our studies address early control points in the differentiation process that are necessary for expression as well as the maintenance of the adipocyte phenotype. We have focused on the post-transcriptional regulation of mRNA metabolism by the RNA binding protein HuR. Depending on the particular mRNA, HuR has been suggested to control polyadenylation, translocation to the cytosol, mRNA stability, and/or translational efficiency. Using the 3T3-L1 preadipocyte cell line, we have demonstrated a critical role for HuR in the differentiation process by use of siRNA mediated suppression which resulted in an inhibition of differentiation. We have identified several mRNAs that serve as HuR ligands early in the differentiation process, one of which is C/EBPβ, an important adipocyte transcription factor, whose expression is highly controlled and essential for proper acquisition of the adipocyte phenotype. Within minutes of induction of differentiation, HuR forms an mRNP complex with C/EBPβ mRNA in the nucleus followed by translocation to the cytoplasm. HuR remains associated with C/EBPβ mRNA suggesting a role in mRNA stability and translation efficiency. Our data suggest that formation of this complex in the nucleus serves as a regulator/attenuator of polyadenylation and that this interaction leads to a controlled metabolism of the C/EBPβ mRNA by determination of the quantity of message translocated to the cytosol and available for translation. Additionally, our data have directed us toward the Zfp206 mRNA as an important HuR ligand whose regulation is predicted to be responsible for maintenance of the differentiation potential of the cells and whose expression is terminated as the cells express PPARγ and establish the adipocyte phenotype. Our mechanistic analysis of these issues will identify novel control points in the initial stages of adipogenesis and thus fundamental to the pathological states of obesity and diabetes.