Cell Proliferation Kinetics of MCF-7 Human Mammary Carcinoma Cells in Culture and Effects of Tamoxifen on Exponentially Growing and Plateau-Phase Cells

Abstract MCF-7 human mammary carcinoma cells were inoculated into 150-sq cm flasks at 3 × 10 5 cells/flask, and after a lag period of about 48 hr, these cells grew exponentially for 5 days with a mean population doubling time of about 24 hr. During exponential growth, 80 to 90% of cells were in the “rapidly cycling” pool, the clonogenic fraction was 50 to 60%, and the mean percentage of cells in the G 0 –G 1 , S, and G 2 + M phases of the cell cycle was 48.9 ± 0.6% (S.E.), 39.4 ± 0.6%, and 11.6 ± 0.3%, respectively. These parameters changed rapidly between Days 7 and 13 when plateau phase was reached. Between Days 13 and 18, 74.8 ± 0.7% of cells were in G 0 –G 1 , 15.3 ± 0.4% were in S, and 9.8 ± 0.6% were in G 2 + M phase. Only about 30% of these cells were cycling rapidly, and the clonogenic fraction had fallen to less than 10%. Tamoxifen induced a dose-dependent decrease in the growth rate of exponentially growing cells, which was accompanied by a dose-dependent increase in percentage of G 0 –G 1 -phase cells, and a decline in percentage of S-phase cells. At doses ⩾10 µm, a 24-hr pulse of tamoxifen was cytotoxic to exponentially growing cells. Plateau-phase cells were less sensitive to these effects of tamoxifen. In an attempt to define the kinetic basis of the G 0 –G 1 accumulation induced by tamoxifen, asynchronous MCF-7 cells were pretreated for 42 hr with various doses of tamoxifen, and the rate of efflux of cells from the G 0 –G 1 phase of the cell cycle was assessed by blocking their reentry into G 1 with ICRF 159. Following treatment of control cultures with ICRF 159, two populations of cells were distinguished by their rates of efflux from G 0 –G 1 phase. The majority of cells left G 0 –G 1 rapidly with a mean t ½ of 2.3 hr (“rapidly cycling” cells). However, about 18% of cells had a much slower rate of exit with a mean t ½ of about 30 hr (“slowly cycling” cells). Pretreatment with tamoxifen resulted in a dose-dependent decrease in the proportion of rapidly cycling cells and an increase in the proportion of cells with slow G 1 transit times. Although this appeared to be the predominant effect, tamoxifen also decreased the rate at which the slowly cycling cells traversed G 1 . Simultaneous treatment with estradiol returned these parameters to control values at doses of tamoxifen ≤5 µm, partially reversed the effect of 7.5 µm tamoxifen, but was without effect on the arrest of cell cycle progression induced by 10 µm tamoxifen. It is concluded that cells accumulate in G 0 –G 1 following tamoxifen treatment due to an increase in the proportion of slowly cycling cells at the expense of a population of rapidly cycling cells, which appear to be relatively uninfluenced by the drug.