The nuclear receptor corepressor N-CoR regulates differentiation: N-CoR directly interacts with MyoD

Classical ligand-activated nuclear receptors (e.g. thyroid hormone receptor , retinoic acid receptor), orphan nuclear receptors (e.g. Rev-erbA alpha/be ta), Mad/Max bHLH (basic helix loop helix)-LZ proteins, and oncoproteins, P LZF and LAZ3/BCL6, bind DNA and silence transcription by recruiting a repre ssor complex that contains N-CoR (nuclear receptor corepressor)/SMRT (silen cing mediator of retinoic acid and thyroid hormone receptor), Sin3A/B, and HDAc-1/-2 proteins. The function of the corepressor, N-CoR, in the process of cellular differentiation and coupled phenotypic acquisition, has not bee n investigated. We examined the functional role of N-CoR in myogenesis (mus cle differentiation), an ideal paradigm for the analysis of the determinati ve events that govern the cell's decision to divide or differentiate. We ob served that the mRNA encoding N-CoR was suppressed as proliferating myoblas ts exited the cell cycle, and formed morphologically and biochemically diff erentiated myotubes. Exogenous expression of N-CoR (but not RIP13) in myoge nic cells ablated 1) myogenic differentiation, 2) the expression of the myo D gene family that encode the myogenic specific bHLH proteins, and 3) the c rucial cell cycle regulator, p21(Waf-1/Cip-1) mRNA. Furthermore, N-CoR expr ession efficiently inhibits the myoD-mediated myogenic conversion of plurip otential C3H10T1/2 cells. We demonstrate that MyoD-mediated transactivation and activity are repressed by N-CoR. The mechanism involves direct interac tions between MyoD and N-CoR; moreover, the interaction was dependent on th e amino-terminal repression domain (RD1) of N-CoR and the bHLH region of My oD. Trichostatin A treatment significantly stimulated the activity of MyoD by approximately 10-fold and inhibited the ability of N-CoR to repress MyoD -mediated transactivation, consistent with the involvement of the corepress or and the recruitment of a histone deacteylase activity in the process. Th is work demonstrates that the corepressor N-CoR is a key regulator of MyoD activity and mammalian differentiation, and that N-CoR has a multifaceted r ole in myogenesis.