Rapid Development of Tamoxifen-stimulated Mutant p53 Breast Tumors (T47D) in Athymic Mice

MCF-7 cells are used routinely to study tamoxifen-stimulated drug resistance in vivo . However, unlike MCF-7 cells, T47D cells express mutant p53 protein and lose the estrogen receptor (ER) during long-term estrogen deprivation in vitro [Pink et al. , Br. J. Cancer, 74: 1227–1236, 1996 (erratum, Br. J. Cancer, 75: 1557, 1997)]. As a result, T47D tumors may respond differently from MCF-7 tumors to long-term tamoxifen treatment. Ovariectomized athymic mice were given injections bilaterally with T47D cells (5 × 10 5 ) into the mammary fat pads. A rapidly growing estradiol responsive tumor (T47D:E2) was established and 0.5 mg of tamoxifen given daily blocked estrogen-stimulated growth. In subsequent experiments, low doses of tamoxifen (0.17 mg or 0.5 mg) did not produce tamoxifen-stimulated tumors at 14 weeks, whereas high-dose tamoxifen (1.5 mg) consistently produced tamoxifen-stimulated tumors (T47D:Tam; 17 tumors/20 sites) at 8 weeks. In contrast, 1.5 mg of tamoxifen produced tamoxifen-stimulated MCF-7 tumors (MCF-7:Tam2) at a slower rate (20 weeks) and less consistently (14 tumors/26 sites). When the T47D:Tam tumor was passaged, it grew maximally with either 1.5 mg of tamoxifen or a 1-cm estradiol (premenopausal levels) capsule, and similar results were obtained with MCF-7:Tam2 tumors. Interestingly, when T47D:Tam tumors were treated with the 0.5 mg of tamoxifen, tumors grew only to 50% maximum. All of the tumors originating from MCF-7 and T47D cells expressed ER at similar levels; therefore, tamoxifen did not select for an ER-negative tumor. In conclusion, we have shown that tamoxifen-stimulated T47D p53 mutant tumors can be developed rapidly with high-dose therapy (1.5 mg daily). The results from this model provide new opportunities to investigate the rapid development of drug resistance to adjuvant tamoxifen in patients with mutant p53 breast tumors.