Breast Cancer Cells in Tissue Culture Hormonal Regulation of Net DNA Synthesis in MCF-7 Human

Estrogens and anti-estrogens have striking effects on growth of target tissues in vivo. In order to examine these effects in vitro, experimental methods for the rigorous assessment of rates of net DMA synthesis in tissue culture systems are re quired. The quantitation of DNA synthesis with inorganic [32P]orthophosphate is described. The limitations of more con ventional [3H]thymidine labeling are illustrated in this hormonally responsive system. Estrogen administration increases the specific activity of acid-soluble phosphate and increases net DNA synthesis relative to controls in MCF-7 human breast cancer cells. Stimulation is most evident after 36 hr of hormone treatment. Conversely, tamoxifen produces substantial inhibi tion of net DNA synthesis after 36 hr of hormone treatment. Thymidine availability modulates the effect of estrogens and anti-estrogens on DNA synthesis and constitutes an independ ent experimental variable.