Dynamic Interactions and Cooperative Functions of PGC-1α and MED1 in TRα-Mediated Activation of the Brown-Fat-Specific UCP-1 Gene

SummaryPGC-1α is an inducible nuclear receptor coactivator with direct functions in both p300-mediated chromatin remodeling and Mediator-dependent transcription in vitro. Here, we have employed the PPARγ- and TRα-activated brown adipose tissue-specific UCP-1 enhancer to investigate mechanistic aspects of PGC-1α function. We first demonstrate a cellular role for the PGC-1α-interacting MED1 subunit of Mediator in UCP-1 induction, as well as the accumulation of TRα, PPARγ, PGC-1α, and MED1 on the UCP-1 enhancer in brown adipocytes. We then use biochemical assays to show that (i) PGC-1α is recruited to the TRα-RXRα-UCP-1 enhancer complex through interaction of an N-terminal LXXLL domain with TRα, (ii) MED1/Mediator displaces PGC-1α from TRα through LXXLL domain competition, and (iii) upon loss of PGC-1α-TRα interactions, PGC-1α remains associated with the enhancer complex through an interaction between PGC-1α and MED1 C-terminal domains. These results indicate dynamic MED1-dependent PGC-1α interactions related to functions in both chromatin remodeling and the transition to subsequent transcription initiation.