The orphan nuclear receptor, COUP-TF II, inhibits myogenesis by post-transcriptional regulation of MyoD function: COUP-TF II directly interacts with p300 and MyoD

COUP-TF II is an orphan nuclear receptor that has no known ligand in the 'c lassical sense'. COUP-TF interacts with the compressors N-CoR, SMRT end RIP 13, and silences transcription by active repression and transrepression. Fo rced expression of the orphan nuclear receptor COUP-TF II in mouse C2 myoge nic cells has been demonstrated to inhibit morphological differentiation, a nd to repress the expression of: (i) the myoD gene family which encodes myo genic basic helix-loop-helix (bHLH) proteins; and (ii) the cell cycle regul ator, p21(Waf-1/Cip-1). In the present study, we show that COUP-TF II effic iently inhibits the myoD-mediated myogenic conversion of pluripotential C3H 10T1/2 cells by post-transcriptional mechanisms. Furthermore, repression of MyoD-dependent transcription by COUP-TF II occurs in the absence of the nu clear receptor cognate binding motif, The inhibition of MyoD-mediated trans -activation involves the direct binding of the DNA binding domain/C-region and hinge/D-regions [i,e, amino acid (aa) residues 78-213] of COUP-TF II to the N-terminal activation domain of nnyoD, Over-expression of the cofactor p300, which functions as a coactivator of myoD-mediated transcription, all eviated repression by COUP-TF Il,Further binding analysis demonstrated that COUP-TF II interacted With the N-terminal 149 aa residues of p300 which en coded the receptor interaction domain of the coactivator, Finally we observ ed that COUP-TF II, MyoD and p300 interact in a competitive manner, and tha t increasing amounts of COUP-TF II have the ability to reduce the interacti on between myoD and p300 in vitro, The experiments presented herein suggest that COUP-TF II post-transcriptionally regulates myoD activity/function, a nd that crosstalk between orphan nuclear receptors and the myogenic:bHLH pr oteins has functional consequences for differentiation.