Elucidation of mechanisms underlying p53-independent growth-suppressive activity for tumor suppressor proteins with p53-activating function.

1)ARF介导的P53非依赖性肿瘤抑制部分通过BCL6发生。ARF肿瘤抑制基因拮抗多种肿瘤的发生。ARF介导的肿瘤抑制以P53不依赖的方式发生，也以P53依赖的方式发生。在这项研究中，我们证明了BCL6是ARF肿瘤抑制因子的靶点。无论是小鼠p19^&lt；ARF&gt；还是人p14^&lt；ARF&gt；都能与BCL6结合并下调BCL6诱导的转录抑制。P19^&lt；ARF&gt；N端37个氨基酸是ARF介导的BCL6转录抑制下调的必要条件和充分条件。因此，我们得出结论，ARF介导的BCL6活性下调可能是ARF介导的肿瘤抑制的部分原因。此外，我们还发现Bax是ARF诱导的P53非依赖性细胞凋亡的主要介导者。2)Ik3-1/Ik3-2诱导的P53非依赖性细胞凋亡部分是由CR.CR(CDC7表达抑制因子)介导的。我们发现CR/外周蛋白是一种广泛表达的核蛋白，呈斑点状分布。CR/外围素过表达诱导S期细胞周期阻滞。对参与DNA复制的调节因子的表达分析表明，作为DNA复制起始和持续调节因子的CDC7的mRNA和蛋白表达均显著下调，受CR/外周蛋白过表达的影响。此外，我们还发现，CR与组蛋白脱乙酰酶I和mSin3A结合，这两种酶对多种转录抑制物起协同抑制作用，这表明CR也是一种协同抑制物。综上所述，CR可能通过转录共抑制因子的作用来介导由ik3-1/ik3-2诱导的p53非依赖性细胞凋亡。

1)ARF-mediated p53-independent tumor suppression occurs in part through BCL6.The ARF tumor' suppressor gene antagonizes generation of various tumors. ARF-mediated tumor suppression occurs in a p53-independent manner as well as in a p53-dependent manner. In this study, we demonstrate that BCL6 is a target of the ARF tumor suppressor. Either mouse p19^<ARF> or human p14^<ARF> binds to BCL6 and downregulates BCL6-induced transcriptional repression. The N-terminal 37 amino acids of p19^<ARF> are necessary and sufficient for ARF-mediated downregulation of BCL6 transcriptional repression. Thus, we have concluded that ARF-mediated downregulation of the BCL6 activity may account in part for ARF-mediated tumor suppression. In addition, we have found that Bax is the main mediator of ARF-induced p53-independent apoptosis.2)p53-independent apoptosis induced by ik3-1/ik3-2 is mediated in part by mediated by CR.CR(Cdc7 expression repressor)/periphilin has been originally cloned as an interactor with periplakin, a precursor of the cornified cell envelope, and suggested to constitute a new type of nuclear matrix. We have found that CR/periphilin is a ubiquitously expressed nuclear protein with speckled distribution. Overexpression of CR/periphiilin induces S-phase arrest. Analysis of expression of regulators involved in DNA replication, has revealed that both mRNA and protein expression of Cdc7, a regulator of the initiation and continuation of DNA replication, are markedly downregulated by overexpression of CR/periphilin. In addition, we have found that CR associates with histone deacetylase I and mSin3A that act as co-reppressors for various transcriptional reppressors, suggesting that CR is also a co-repressor. In conclusion, it appears that CR mediates p53-independent apoptosis induced by ik3-1/ik3-2 by acting as a transcriptional co-repressor.