Schneikert Jean, Grohmann Annette, Behrens Jürgen
Nikolaus-Fiebiger-Center for Molecular Medicine, University Erlangen-Nürnberg, Glückstrasse 6, 91054 Erlangen, Germany.
Hum Mol Genet. 2007 Jan 15;16(2):199-209. doi: 10.1093/hmg/ddl464. Epub 2006 Dec 22.
Most colon cancer cells express truncated versions of the tumour suppressor Adenomatous Polyposis Coli (APC). These molecules are selected during tumourigenesis for impaired beta-catenin degrading activity. In this study, we describe that truncated APC can still control the activity of beta-catenin in colon cancer cell lines via its first 20 amino acid repeat. First, we show that both endogenous and ectopically expressed truncated APC molecules can bind to beta-catenin. Second, reduction of the levels of truncated APC by RNA interference increases the activity of a beta-catenin-dependent reporter gene and stimulates the expression of the beta-catenin target gene AXIN2/conductin. This occurs without alterations of the amounts of cytosolic beta-catenin. Conversely, ectopic expression of truncated APC decreases beta-catenin-dependent transcription without affecting the intensity of immunofluorescence staining of beta-catenin in transfected cells. Third, we reveal that the APC level increases when cells reach the G1-S boundary during cell cycle progression. Simultaneously, the amount of beta-catenin bound to APC increases and the transcriptional activity of beta-catenin drops in an APC-dependent manner. Again, this occurs independently of the amounts of either total or phosphorylated cytosolic beta-catenin. Together, these results indicate that truncated APC controls the ability of beta-catenin to activate transcription. As we also show that the inhibition involves the first 20 amino acid repeat of APC, our data suggest that colon cancer cells retain a truncated APC molecule containing at least the first 20 amino acid repeat to modulate the transcriptional activity of beta-catenin in a cell cycle-dependent manner.
大多数结肠癌细胞表达肿瘤抑制因子腺瘤性息肉病大肠杆菌（APC）的截短形式。这些分子在肿瘤发生过程中因β-连环蛋白降解活性受损而被选择。在本研究中，我们描述了截短的APC仍可通过其前20个氨基酸重复序列控制结肠癌细胞系中β-连环蛋白的活性。首先，我们表明内源性和异位表达的截短APC分子均可与β-连环蛋白结合。其次，通过RNA干扰降低截短APC的水平会增加β-连环蛋白依赖性报告基因的活性，并刺激β-连环蛋白靶基因AXIN2/传导蛋白的表达。这一过程中胞质β-连环蛋白的量没有改变。相反，截短APC的异位表达降低了β-连环蛋白依赖性转录，而不影响转染细胞中β-连环蛋白免疫荧光染色的强度。第三，我们发现细胞在细胞周期进程中到达G1-S边界时，APC水平会升高。同时，与APC结合的β-连环蛋白量增加，且β-连环蛋白的转录活性以APC依赖的方式下降。同样，这一过程独立于总胞质β-连环蛋白或磷酸化胞质β-连环蛋白的量。总之，这些结果表明截短的APC控制β-连环蛋白激活转录的能力。由于我们还表明这种抑制作用涉及APC的前20个氨基酸重复序列，我们的数据表明结肠癌细胞保留了至少包含前20个氨基酸重复序列的截短APC分子，以细胞周期依赖的方式调节β-连环蛋白的转录活性。
