PML-RARα induces all-trans retinoic acid-dependent transcriptional activation through interaction with MED1

Transcriptional activation by PML-RAR alpha, an acute promyelocytic leukemia-related oncofusion protein, requires pharmacological concentrations of all-trans retinoic acid (ATRA). However, the mechanism by which the liganded PML-RAR alpha complex leads to the formation of the preinitiation complex has been unidentified. Here we demonstrate that the Mediator subunit MED1 plays an important role in the ATRA-dependent activation of the PML-RAR alpha-bound promoter. Luciferase reporter assays showed that PML-RAR alpha induced significant transcription at pharmacological doses (1 mu M) of ATRA; however, this was submaximal and equivalent to the level of transcription driven by intact RAR alpha at physiological doses (1 nM) of ATRA. Transcription depended upon the interaction of PML-RAR alpha with the two LxxLL nuclear receptor recognition motifs of MED1, and LxxLL -> LxxAA mutations led to minimal transcription. Mechanistically, MED1 interacted ATRA-dependently with the RAR alpha portion of PML-RAR alpha through the two LxxLL motifs of MED1. These results suggest that PML-RAR alpha initiates ATRA-induced transcription through its interaction with MED1.