Goulet Brigitte, Watson Peter, Poirier Madeleine, Leduy Lam, Bérubé Ginette, Meterissian Sarkis, Jolicoeur Paul, Nepveu Alain
Molecular Oncology Group, McGill University Health Center, Montreal, Quebec, QC H3A 1A1, Canada.
Cancer Res. 2002 Nov 15;62(22):6625-33.
Two isoforms of the CCAAT-displacement protein/cut homeobox (CDP/Cux) transcription factor have been characterized thus far. The full length protein, p200, which contains four DNA binding domains, transiently binds to DNA and carries the CCAAT-displacement activity. The p110 isoform is generated by proteolytic processing at the G1-S transition and is capable of stable interaction with DNA. Here we demonstrate the existence of a shorter CDP/Cux isoform, p75, which contains only two DNA binding domains, Cut repeat 3 and the Cut homeodomain, and binds more stably to DNA. CDP/Cux p75 was able to repress a reporter carrying the promoter for the cyclin-dependent kinase inhibitor p21 gene and to activate a DNA polymerase alpha gene reporter. Expression of CDP/Cux p75 involved a novel mechanism: transcription initiation within intron 20. The intron 20-initiated mRNA (I20-mRNA) was expressed at higher level in the thymus and in CD4+/CD8+ and CD4+ T cells. I20-mRNA was expressed only weakly or not at all in normal human mammary epithelial cells and normal breast tissues but was detected in many breast tumor cells lines and breast tumors. In invasive tumors a significant association was established between higher I20-mRNA expression and a diffuse infiltrative growth pattern (n = 41, P = 0.0137). In agreement with these findings, T47D breast cancer cells stably expressing p75 could not form tubule structures in collagen but rather developed as solid undifferentiated aggregates of cells. Taken together, these results suggest that aberrant expression of the CDP/Cux p75 isoform in mammary epithelial cells may be associated with the process of tumorigenesis in breast cancer.
迄今为止，已鉴定出CCAAT位移蛋白/切割同源框（CDP/Cux）转录因子的两种亚型。全长蛋白p200含有四个DNA结合结构域，可短暂结合DNA并具有CCAAT位移活性。p110亚型是在G1-S期转换时通过蛋白水解加工产生的，能够与DNA稳定相互作用。在此，我们证明了存在一种较短的CDP/Cux亚型p75，它仅包含两个DNA结合结构域，即切割重复序列3和切割同源结构域，并且与DNA的结合更稳定。CDP/Cux p75能够抑制携带细胞周期蛋白依赖性激酶抑制剂p21基因启动子的报告基因，并激活DNA聚合酶α基因报告基因。CDP/Cux p75的表达涉及一种新机制：在第20内含子内起始转录。由第20内含子起始的mRNA（I20-mRNA）在胸腺以及CD4+/CD8+和CD4+ T细胞中表达水平较高。I20-mRNA在正常人乳腺上皮细胞和正常乳腺组织中仅微弱表达或根本不表达，但在许多乳腺癌细胞系和乳腺肿瘤中可检测到。在浸润性肿瘤中，较高的I20-mRNA表达与弥漫性浸润性生长模式之间存在显著关联（n = 41，P = 0.0137）。与这些发现一致，稳定表达p75的T47D乳腺癌细胞在胶原蛋白中不能形成小管结构，而是发展为未分化的实体细胞聚集体。综上所述，这些结果表明乳腺上皮细胞中CDP/Cux p75亚型的异常表达可能与乳腺癌的肿瘤发生过程有关。
