Goueli Basem S, Janknecht Ralf
Department of Biochemistry and Molecular Biology, Mayo Clinic, Rochester, Minnesota 55905, USA.
Mol Cell Biol. 2004 Jan;24(1):25-35. doi: 10.1128/MCB.24.1.25-35.2004.
One hallmark of tumor formation is the transcriptional upregulation of human telomerase reverse transcriptase, hTERT, and the resultant induction of telomerase activity. However, little is presently understood about how hTERT is differentially activated in tumor cells versus normal somatic cells. Specifically, it is unclear if oncoproteins can directly elicit hTERT expression. To this end, we now show that three oncoproteins, HER2/Neu, Ras, and Raf, stimulate hTERT promoter activity via the ETS transcription factor ER81 and ERK mitogen-activated protein (MAP) kinases. Mutating ER81 binding sites in the hTERT promoter or suppression of ERK MAP kinase-dependent phosphorylation of ER81 rendered the hTERT promoter unresponsive to HER2/Neu. Further, expression of dominant-negative ER81 or inhibition of HER2/Neu significantly attenuated telomerase activity in HER2/Neu-overexpressing SKBR3 breast cancer cells. Moreover, HER2/Neu, Ras, and Raf collaborated with ER81 to enhance endogenous hTERT gene transcription and telomerase activity in hTERT-negative, nonimmortalized BJ foreskin fibroblasts. Accordingly, hTERT expression was increased in HER2/Neu-positive breast tumors and breast tumor cell lines relative to their HER2/Neu-negative counterparts. Collectively, our data elucidated a mechanism whereby three prominent oncoproteins, HER2/Neu, Ras, and Raf, may facilitate tumor formation by inducing hTERT expression in nonimmortalized cells via the transcription factor ER81.
肿瘤形成的一个标志是人类端粒酶逆转录酶（hTERT）的转录上调以及由此导致的端粒酶活性诱导。然而，目前对于hTERT在肿瘤细胞与正常体细胞中如何被差异激活了解甚少。具体而言，尚不清楚癌蛋白是否能直接引发hTERT表达。为此，我们现在表明，三种癌蛋白HER2/Neu、Ras和Raf通过ETS转录因子ER81和ERK丝裂原活化蛋白（MAP）激酶刺激hTERT启动子活性。突变hTERT启动子中的ER81结合位点或抑制ERK MAP激酶依赖性的ER81磷酸化会使hTERT启动子对HER2/Neu无反应。此外，显性负性ER81的表达或HER2/Neu的抑制显著减弱了HER2/Neu过表达的SKBR3乳腺癌细胞中的端粒酶活性。而且，HER2/Neu、Ras和Raf与ER81协同作用，增强了hTERT阴性、未永生化的BJ包皮成纤维细胞中的内源性hTERT基因转录和端粒酶活性。因此，相对于HER2/Neu阴性的对应物，HER2/Neu阳性的乳腺肿瘤和乳腺肿瘤细胞系中的hTERT表达增加。总体而言，我们的数据阐明了一种机制，即三种突出的癌蛋白HER2/Neu、Ras和Raf可能通过转录因子ER81在未永生化细胞中诱导hTERT表达来促进肿瘤形成。
