Bailey P, Downes M, Lau P, Harris J, Chen S L, Hamamori Y, Sartorelli V, Muscat G E
University of Queensland, Centre for Molecular and Cellular Biology, Ritchie Research Laboratories, Brisbane, Australia.
Mol Endocrinol. 1999 Jul;13(7):1155-68. doi: 10.1210/mend.13.7.0305.
Classical ligand-activated nuclear receptors (e.g. thyroid hormone receptor, retinoic acid receptor), orphan nuclear receptors (e.g. Rev-erbAalpha/beta), Mad/Max bHLH (basic helix loop helix)-LZ proteins, and oncoproteins, PLZF and LAZ3/BCL6, bind DNA and silence transcription by recruiting a repressor complex that contains N-CoR (nuclear receptor corepressor)/SMRT (silencing 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 been investigated. We examined the functional role of N-CoR in myogenesis (muscle differentiation), an ideal paradigm for the analysis of the determinative events that govern the cell's decision to divide or differentiate. We observed that the mRNA encoding N-CoR was suppressed as proliferating myoblasts exited the cell cycle, and formed morphologically and biochemically differentiated myotubes. Exogenous expression of N-CoR (but not RIP13) in myogenic cells ablated 1) myogenic differentiation, 2) the expression of the myoD gene family that encode the myogenic specific bHLH proteins, and 3) the crucial cell cycle regulator, p21Waf-1/Cip-1 mRNA. Furthermore, N-CoR expression efficiently inhibits the myoD-mediated myogenic conversion of pluripotential C3H10T1/2 cells. We demonstrate that MyoD-mediated transactivation and activity are repressed by N-CoR. The mechanism involves direct interactions between MyoD and N-CoR; moreover, the interaction was dependent on the amino-terminal repression domain (RD1) of N-CoR and the bHLH region of MyoD. 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 corepressor and the recruitment of a histone deacteylase activity in the process. This work demonstrates that the corepressor N-CoR is a key regulator of MyoD activity and mammalian differentiation, and that N-CoR has a multifaceted role in myogenesis.
经典的配体激活核受体（如甲状腺激素受体、视黄酸受体）、孤儿核受体（如Rev-erbAα/β）、Mad/Max碱性螺旋-环-螺旋（bHLH）-亮氨酸拉链（LZ）蛋白以及癌蛋白PLZF和LAZ3/BCL6，通过募集包含N-CoR（核受体共抑制因子）/SMRT（视黄酸和甲状腺激素受体沉默介质）、Sin3A/B和HDAc-1/-2蛋白的抑制复合物来结合DNA并使转录沉默。共抑制因子N-CoR在细胞分化及相关表型获得过程中的功能尚未得到研究。我们研究了N-CoR在成肌作用（肌肉分化）中的功能作用，成肌作用是分析决定细胞分裂或分化的决定性事件的理想范例。我们观察到，随着增殖的成肌细胞退出细胞周期并形成形态和生化上分化的肌管，编码N-CoR的mRNA受到抑制。在成肌细胞中外源表达N-CoR（而非RIP13）消除了：1）成肌分化；2）编码成肌特异性bHLH蛋白的肌分化决定基因家族（myoD gene family）的表达；3）关键的细胞周期调节因子p21Waf-1/Cip-1 mRNA。此外，N-CoR表达有效抑制了多能C3H10T1/2细胞的MyoD介导的成肌转化。我们证明MyoD介导的反式激活和活性受到N-CoR的抑制。其机制涉及MyoD与N-CoR之间的直接相互作用；此外，这种相互作用依赖于N-CoR的氨基末端抑制结构域（RD1）和MyoD的bHLH区域。曲古抑菌素A处理显著刺激了MyoD的活性，使其提高了约10倍，并抑制了N-CoR抑制MyoD介导的反式激活的能力，这与共抑制因子的参与以及组蛋白去乙酰化酶活性在此过程中的募集一致。这项工作表明，共抑制因子N-CoR是MyoD活性和哺乳动物分化的关键调节因子，并且N-CoR在成肌作用中具有多方面的作用。
