Genetic Disruption Of The P27kip1 Confers Antiestrogen Insensitivity And Metastasis Potential On Human Breast Cancer Cells

1530 The genetic changes and molecular mechanisms underlying the progression of estrogen-dependent breast cancers to estrogen-independent and antiestrogen-resistant breast cancers, a major problem of endocrine therapy, are unclear. Here, we used an enhanced retroviral mutagen (ERM) system to screen the entire genome for identifying the loss- or gain-of-function mutations that could confer this critical progression. We show that the T47D estrogen-dependent human breast cancer cell line contains only one p27kip1 allele coding for the p27 cyclin-dependent kinase inhibitor. An ERM insertion into the p27kip1 locus of T47D cells disrupted the p27kip1 gene and caused estrogen-independent and antiestrogen-resistant breast cancer cell growth even these cells were still ERα positive. Disruption of p27 in T47D cells resulted in several changes. First, CDK2 activity was increased in the absence of estrogen or presence of tamoxifen or ICI. Second, AIB1, a cancer-overexpressed transcriptional coactivator, was hyper-phosphorylated, which made AIB1 a super-coactivator for E2F1. Third, overactivation of E2F indirectly stimulated Akt activation, which in turn promoted cell survival, motility and invasion. Both T47D and p27-deficient T47D cells formed solid tumors after injected into the mammary fat pads of ovariectomized nude mice treated with estradiol, but failed to develop tumors in placebo-treated mice, suggesting that inactivation of p27 alone is not sufficient to develop estrogen-independent tumors in the mammary gland although it is sufficient to maintain estrogen-independent cell growth in culture. Nevertheless, when the p27-deficient T47D cells were intravenously injected into the nude mice, they formed metastatic lung tumors in 3 weeks in an ovarian hormone-independent manner. In contrast, no lung tumors were observed in nude mice injected with T47D cells. In summary, out results demonstrate that p27 is required not only for estrogen dependency and antiestrogen sensitivity of ER-positive breast cancer cells but also for inhibition of cell motility, invasion and metastasis of breast cancer cells.