Hsi L C, Angerman-Stewart J, Eling T E
National Institute of Environmental Health Sciences, Eicosanoid Biochemistry Section, Laboratory of Molecular Carcinogenesis, PO Box 12233, Research Triangle Park, NC 27709, USA.
Carcinogenesis. 1999 Nov;20(11):2045-9. doi: 10.1093/carcin/20.11.2045.
Mutation of the adenomatous polyposis coli (APC) gene is associated with the earliest stages of colorectal tumorigenesis and appears to be responsible for the hereditary condition familial adenomatous polyposis (FAP). Evidence indicates that cyclooxygenase-2 (COX-2) is induced and at elevated levels in human colorectal cancers and in the polyps of mouse FAP models. We have used HT-29 cells, a human colorectal carcinoma cell line with a mutant carboxy-truncated APC gene, in which intact APC gene has been introduced under the control of an inducible promoter. These HT-29-APC cells provide a suitable model system to examine how COX-2 expression becomes dysregulated after loss of APC function. Induction of full-length APC causes the HT-29-APC cells to undergo apoptosis. However, differentiation, as measured by alkaline phosphatase activity, is not induced upon expression of full-length APC. Full-length APC protein has been shown to bind the intracellular protein beta-catenin and, as a result, the Lef/Tcf transcription factors are down-regulated. Analysis of APC immunoprecipitates demonstrate a time-dependent increase of beta-catenin interacting with full-length APC. Thus, the Lef/Tcf signaling pathway is intact at this point in these cells. Furthermore, upon expression of full-length APC, COX-2 protein expression is down-regulated while COX-2 mRNA levels remain the same. These data indicate that APC plays a role, either directly or indirectly, in the translational regulation of COX-2. Treatment of the HT-29-APC cells with sodium butyrate, an inducer of apoptosis, does not alter COX-2 protein expression. Thus, COX-2 down-regulation appears to be APC specific and not just due to apoptotic induction. APC appears to uniquely regulate COX-2 expression. The mechanism by which COX-2 protein expression is down-regulated in the HT-29-APC cells is under investigation.
腺瘤性结肠息肉病（APC）基因的突变与结直肠癌发生的最早阶段相关，并且似乎是遗传性疾病家族性腺瘤性息肉病（FAP）的病因。有证据表明，环氧化酶-2（COX-2）在人类结直肠癌和小鼠FAP模型的息肉中被诱导且水平升高。我们使用了HT-29细胞，这是一种具有突变型羧基截短APC基因的人结肠癌细胞系，其中完整的APC基因已在可诱导启动子的控制下导入。这些HT-29-APC细胞提供了一个合适的模型系统，以研究APC功能丧失后COX-2表达如何失调。全长APC的诱导导致HT-29-APC细胞发生凋亡。然而，通过碱性磷酸酶活性测量的分化在全长APC表达时并未被诱导。全长APC蛋白已被证明可结合细胞内蛋白β-连环蛋白，结果，Lef/Tcf转录因子被下调。对APC免疫沉淀物的分析表明，与全长APC相互作用的β-连环蛋白呈时间依赖性增加。因此，此时这些细胞中的Lef/Tcf信号通路是完整的。此外，在全长APC表达时，COX-2蛋白表达下调，而COX-2 mRNA水平保持不变。这些数据表明，APC在COX-2的翻译调控中直接或间接发挥作用。用凋亡诱导剂丁酸钠处理HT-29-APC细胞不会改变COX-2蛋白表达。因此，COX-2下调似乎是APC特异性的，而不仅仅是由于凋亡诱导。APC似乎独特地调节COX-2表达。HT-29-APC细胞中COX-2蛋白表达下调的机制正在研究中。
