Function of MM-1, a novel c-Myc-binding protein, as a tumor suppressor

我们发现MM-1与转录辅抑制因子TIF1β结合，MM-1将包括HDAC1和mSin3在内的辅抑制因子复合物募集到c-Myc上，从而抑制c-Myc的转录活性。为了确定这一途径的靶基因，首先建立了一个MycER细胞系，该细胞系含有TIF1β的显性阴性形式，其中辅抑制因子复合物未被招募到c-Myc。然后应用dna微阵列方法鉴定与MycER细胞相比，该细胞系中表达上调的基因。通过该系统，我们鉴定出c-fms癌基因为c-Myc-MM-1抑制基因。此外，我们还发现Wint-4基因是MM-1的转录抑制靶点。在mm -1敲低的细胞中，Wint信号被激活，导致c-myc表达被激活(2)。我们还鉴定了长链蛋白B、26蛋白酶体亚基Rpt3和Rpn12是mm -1相关蛋白。由于这些蛋白具有泛素化和蛋白质降解的功能，我们随后测试了MM-1在c-Myc降解中发挥作用的可能性。结果表明，MM-1通过新的E3泛素连接酶复合物（包括Spk2、Cullin 1、Elongon B）刺激c-Myc降解，这些现象在体内通过靶向每个基因的sirna被证实。这些发现表明MM-1是负调控c-Myc功能的关键蛋白，MM-1的这些功能因突变而丧失导致c-Myc形成肿瘤。

1).Identification of a novel transrepression pathway of c-Myc and its target geneWe have found that MM-1 bound to TIF1β, a transcriptional corepressor, and that MM-1 recruited a corepressor complex, including HDAC1 and mSin3, to c-Myc, thereby leading to repressing c-Myc transcription activity. To identify target genes to this pathway, a MycER cell line harboring a dominant-negative form of TIF1β, in which the corepressor complex was not recruited to c-Myc, was first established. DNA-microarray method was then applied to identify genes whose expressions were upregulated in this line compared to those in MycER cells. By this system we identified c-fms oncogene as the c-Myc-MM-1 repressed gene. Furthermore, we also identified the Wint-4 gene as a transcriptional repression target of MM-1. In MM-1-knockdown cells, the Wint signal was found to be activated, resulting in activation of c-myc expression.2).Identification of a novel degradation pathway of c-MycWe have also identified Elongin B, 26Sproteasome subunits Rpt3 and Rpn12 as MM-1-associated proteins. Since these proteins functions for ubiquitination and degradation of proteins, we then tested a possibility that MM-1 plays a role in c-Myc degradation. The results indicate that MM-1 stimulated c-Myc degradation through the novel E3 ubiquitin ligase complex, including Spk2, Cullin 1, Elongon B, and these phenomena were confirmed to occur in vivo using siRNAs targeting each genes.These findings indicate that MM-1 is a key protein that negatively regulates c-Myc function and that lost of these functions of MM-1 by mutations leads to tumor formation by c-Myc.