Antagonism of the cytocidal activity and uptake of melphalan by tamoxifen in human breast cancer cells in vitro.

The effect of the antiestrogen tamoxifen on the cytocidal activity and uptake of melphalan in human breast cancer cells was investigated. A clonogenic assay was used to obtain dose-survival curves of estrogen receptor-positive MCF-7 cells and of estrogen receptor-negative Evsa T cells following treatment with melphalan and/or tamoxifen. Isobolograms derived from these dose-survival curves were concave downward, suggesting that the drug interaction was antagonistic. The effect of tamoxifen on melphalan uptake by breast cancer cells was evaluated at steady-state conditions. Thin-layer chromatography revealed that the intracellular level of free intact melphalan (mean +/- S.E.) in control cells was 6.47 +/- 1.21 fmoles/cell and that in cells treated with tamoxifen was 3.60 +/- 0.35 fmoles/cell; this 44% reduction in cellular melphalan was statistically significant (P = 0.006). Thus, the antagonistic cytocidal effect of melphalan and tamoxifen against breast cancer cells appeared to be due to inhibition of melphalan uptake at the steady state by the antiestrogen. Further investigation revealed that tamoxifen inhibited unidirectional melphalan influx in human breast cancer cells both by the sodium-independent system L and by the sodium-dependent system ASC. Tamoxifen also appeared to stimulate melphalan efflux from human breast cancer cells. The first-order rate constant K for melphalan efflux from control cells was 0.085 +/- 0.008 and that from cells treated with tamoxifen was 0.129 +/- 0.005; the difference was highly significant (P less than 0.001). Therefore, the antagonistic effect of tamoxifen on the uptake and cytocidal activity of melphalan in breast cancer cells appeared to be due to inhibition of melphalan influx and stimulation of drug efflux.

Induction of DNA single- and double-strand breaks by diethylstilbestrol in murine L5178Y lymphoblasts in vitro.

The mechanism of action of the synthetic estrogen diethylstilbestrol (DES) was investigated in murine L5178Y lymphoblasts. The dose-survival curve of cells treated with DES in serum-free medium for 1 hr was characterized by a prominent shoulder followed by a simple exponential decline; the Do, the dose of DES reducing cell survival to 1/e, was 1.52 nmoles/ml. DNA single-strand breaks, as measured by the alkaline elution method, were observed in DES-treated cells, and these followed a dose-response relationship after an apparent threshold of 10 microM DES was exceeded. Protein-associated strand breaks, which represent the increment in single-strand breaks that occurs by exposing drug-treated cells to proteinase K, were also noted. DNA double-strand breaks as measured by filter elution technology at pH 9.6 were observed and increased markedly to reach a level of approximately 9000 rad equivalents at a DES concentration of 20 microM. The measured ratio (mean +/- S.E.) of single- to double-strand breaks induced by DES in L5178Y limphoblasts was 0.09 +/- 0.035. A comparison of the ratio of single- to double-strand breaks induced by DES to that observed following radiation suggested that all of the single-strand breaks produced by DES could be attributed to double-strand breaks. The close correspondence of the dose-response curve for cytocidal activity of DES with that obtained for induction of DNA double-strand breaks suggested that such breaks may play an important role in the mechanism of cell kill by DES.

Drug and hormone sensitivity of estrogen receptor-positive and -negative human breast cancer cells in vitro.

A clonogenic assay of long-term breast cancer cell cultures in vitro has been developed to provide a highly reproducible method with which to quantitate tumor cell killing by hormones and/or cytotoxic chemotherapeutic agents. Monolayer cultures of estrogen receptor-positive MCF-7 human breast cancer cells and of estrogen receptor-negative Evsa T cells are harvested by treatment with 0.01% trypsin:0.02% EDTA in Hanks' balanced salt solution. Cell suspensions are treated with drug or hormone in serum-free medium for 1 hr at 37 degrees; treated cells are washed, plated, and cultured for approximately 14 days; and colonies consisting of greater than or equal to 30 cells are counted. Compared to estrogen receptor-positive cells, estrogen receptor-negative cells were 2-fold more sensitive to melphalan but were conversely 1.9-fold more resistant to Adriamycin; these differences were statistically significant (p less than 0.001). Thus, response to cytotoxic chemotherapeutic agents appeared to be independent of estrogen receptor status. For cells treated with diethylstilbestrol, the dose of drug or hormone reducing the surviving cell fraction to 1/e (DO) for estrogen receptor-positive cells was 2.27 nmol/ml, and that for estrogen receptor-negative cells was 2.80 nmol/ml; this difference was not statistically significant. However, with tamoxifen therapy, the DO for estrogen receptor-positive cells was 0.601 nmol/ml, and that for estrogen receptor-negative cells was 3.64 nmol/ml; this 6-fold greater degree of resistance to tamoxifen of estrogen receptor-negative cells was highly significant (p less than 0.001). Treatment of cells for 24 hr with 17 beta-estradiol stimulated proliferation not only of estrogen receptor-positive cells but also of estrogen receptor-negative cells. However, estradiol at concentrations up to 200 microM had no apparent cytocidal activity, as measured by the clonogenic assay. Furthermore, treatment of MCF-7 cells simultaneously with estradiol and either diethylstilbestrol or tamoxifen failed to reverse the cytocidal activity of those two agents. These findings suggest that, in the clonogenic assay described herein, diethylstilbestrol and tamoxifen may kill human breast cancer cells by an independent mechanism of action and that the cytocidal activity of diethylstilbestrol and the proliferative effect of 17 beta-estradiol appear to be independent of estrogen receptor status.

A comparison of melphalan transport in human breast cancer cells and lymphocytes in vitro.

Evidence is presented that melphalan uptake by MCF-7 human breast cancer cells and peripheral blood lymphocytes from normal human subjects is an active process involving 2 amino acid carriers. Quantitative differences were observed in drug transport between the 2 cell types. Cell/medium distribution ratios of melphalan at equilibrium conditions were approx. 4-fold higher in breast cancer cells than in lymphocytes. The transport capacity, Vmax, for both carrier systems was at least 50-fold greater in MCF-7 cells than in lymphocytes, whereas the Michaelis constants were similar. These findings, in part, provide a rational basis for the use of melphalan in the chemotherapy of breast cancer.