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

PGC-1 alpha 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 PPAR gamma- and TR alpha-activated brown adipose tissue-specific UCP-1 enhancer to investigate mechanistic aspects of PGC-1 alpha function. We first demonstrate a cellular role for the PGC-1 alpha-interacting MED1 subunit of Mediator in UCP-1 induction, as well as the accumulation of TR alpha, PPAR gamma, PGC-1 alpha, and MED1 on the UCP-1 enhancer in brown adipocytes. We then use biochemical assays to show that (i) PGC-1 alpha is recruited to the TR alpha-RXR alpha-UCP-1 enhancer complex through interaction of an N-terminal LXXLL domain with TR alpha, (ii) MED1/Mediator displaces PGC-1 alpha from TR alpha through LXXLL domain competition, and (iii) upon loss of PGC-1 alpha-TR alpha interactions, PGC-1 alpha remains associated with the enhancer complex through an interaction between PGC-1 alpha and MED1 C-terminal domains. These results indicate dynamic MED1-dependent PGC-1 alpha interactions related to functions in both chromatin remodeling and the transition to subsequent transcription initiation.