MED14 Tethers Mediator to the N-Terminal Domain of Peroxisome Proliferator-Activated Receptor γ and Is Required for Full Transcriptional Activity and Adipogenesis

The Mediator subunit MED1/TRAP220/DRIP205/PBP interacts directly with many nuclear receptors and was long thought to be responsible for tethering Mediator to peroxisome proliferator-activated receptor (PPAR)-responsive promoters. However, it was demonstrated recently that PPAR gamma can recruit Mediator by MED1-independent mechanisms. Here, we show that target gene activation by ectopically expressed PPAR gamma and PPAR gamma is independent of MED1. Consistent with this finding, recruitment of PPAR gamma, MED6, MED8, TATA box-binding protein (TBP), and RNA polymerase II (RNAPII) to the enhancer and proximal promoter of the PPAR gamma target gene Fabp4 is also independent of MED1. Using a small interfering RNA (siRNA)-based approach, we identify MED14 as a novel critical Mediator component for PPAR gamma-dependent transactivation, and we demonstrate that MED14 interacts directly with the N terminus of PPAR gamma in a ligand-independent manner. Interestingly, MED14 knockdown does not affect the recruitment of PPAR gamma, MED6, and MED8 to the Fabp4 enhancer but does reduce their occupancy of the Fabp4 proximal promoter. In agreement with the necessity of MED14 for PPAR gamma transcriptional activity, we show that knockdown of MED14 impairs adipogenesis of 3T3-L1 cells. Thus, MED14 constitutes a novel anchoring point between Mediator and the N-terminal domain of PPAR gamma that is necessary for functional PPAR gamma-mediated recruitment of Mediator and transactivation of PPAR gamma subtype-specific target genes.