Independent regulation of growth and steroid receptors in uterus and mammary tumors of rats.

Anti-estrogens and progestagens are synergistically active in the treatment of hormone dependent tumors. The combined action of both compounds in daily treatment schedules are analyzed in rat uterus and in DMBA- induced rat mammary tumors. Tamoxifen in contrast to estradiol does not significantly affect tissue growth, while PgR induction is considerably stimulated by Tamoxifen. It is suggested that the "estrogenic effects" of Tamoxifen and estradiol are separately modulated. When given in sequential combination with anti-estrogens, the anti-tumor response of progestagens is enhanced. Combinations of hormonal treatment based on careful analysis of the regulation processes at target cell level may greatly improve results of anti-tumor therapy.