Endocrine Abstracts

Background: Tamoxifen is a partial estrogen agonist used to treat estrogen receptor positive breast cancer. It has been discovered however that resistance to tamoxifen develops in some cancers. In order to investigate the mechanisms underlying the development of resistance we have created a tamoxifen resistant breast cancer cell line.

Aims: To develop a breast cancer cell line that is resistant to the anti-growth effects of tamoxifen.

Methods: Cells from the MCF-7 breast cancer cell line were obtained from ATCC. The cells were cultured in phenol red free RPMI 1640 (Gibco) plus 10% charcoal stripped fetal calf serum and 1% penicillin/streptomycin to which 4 hydroxy-tamoxifen was added at a concentration of 10^-7M. Media was changed every four days and cells split when they became confluent. Cells were grown for four months, then tamoxifen resistance confirmed by performing dose response experiments at concentrations of 10^-10 to 10^-6M 4 hydroxy-tamoxifen in both tamoxifen resistant (TR) and wild type (WT) cells, with cell proliferation assessed every three days by MTS assay.

Results: Initially the TR cells showed a decreased growth rate compared to WT cells grown in media without tamoxifen. After four months, the TR cell growth rate had increased to a similar rate as the WT cells. Subsequent tamoxifen dose response experiments after 12 days in the two cell types confirmed the resistance to this agent (results expressed as percentage of media only, mean of three experiments);WT vs TR; 10^-10M - 77% vs 112.6% (p<0.05); 10^-9M - 72.2% vs 107.2% (p<0.05); 10^-8M - 51.5% vs 102.4% (p<0.05); 10^-7M - 41.5% vs 105.1% (p<0.05); 10^-6M - 28% vs 91.7% (p<0.05).

Conclusion: After four months of culture in tamoxifen the TR cells have become resistant to tamoxifen as unlike the WT cells, they no longer show a decrease in cell proliferation when cultured with increasing concentrations of 4 hydroxy-tamoxifen.