Retinoic acid (RA) signaling is mediated by its
nuclear receptors RXR and RAR, which bind to their cognate response elements as a heterodimer, RXR/RAR, and act in concert with coregulatory factors to regulate gene transcription on
ligand binding. To identify specific cofactors that interact with the RXR/RAR heterodimer in
acute promyelocytic leukemia (APL) cells, a double cistronic construct was used that allowed coexpression of the RXR LBD (
ligand binding domain) with the RAR LBD as an affinity matrix to pull down interacting
proteins from nuclear extracts prepared from a human APL cell line, NB4. A group of
proteins was detected whose interaction with RXR/RAR is
ligand inducible. The molecular weight pattern of these
proteins is similar to that of a complex of
proteins previously identified as DRIP or TRAP, which are
ligand-dependent transcription activators of VDR and TR, respectively. The RXR/RAR-interacting
proteins from NB4 were confirmed to be identical to the DRIP subunits by comparative electrophoresis, Western blot analysis, and in vitro
protein interaction assay. In addition to RXR/RAR, the DRIP component can interact directly with the APL-specific
PML-RARalpha fusion
protein. The same
DRIP complex is present in RA-resistant APL cells and in a variety of
cancer cell lines, supporting its global role in transcriptional regulation.