p21, p27 and p53 in estrogen and antiprogestin-induced tumor regression of experimental mouse mammary ductal carcinomas

Metastatic mammary carcinoma tumor lines 59-2-HI and C7-2-HI originated in female BALB/c mice treated with medroxyprogesterone acetate and are maintained by syngeneic transplantation. Both lines express estrogen (ER) and progesterone receptors (PR) and regress completely after estradiol (E₂) or antiprogestin treatment. The BET tumor line, of similar origin and biological features, regresses only after E₂ treatment. To investigate possible differences between E₂- and antiprogestin-mediated effects we evaluated the morphological features, mitosis and apoptosis, and the differential expression of cell-cycle inhibitors associated with tumor regression. Treatments started when tumors reached 50-100 mm². After 24-96 h, tumors were excised and processed for morphological and immunohistochemical studies. Regression was associated with a significant and early decrease in the number of mitosis and with higher percentages of apoptotic cells. These phenomena were accompanied by an increase in p21 and p27 expression in the E₂ and antiprogestin-responsive lines treated with E₂, RU 38.486 or ZK 98.299 (P < 0.05). In BET tumors treated with E₂, p21 expression remained within basal levels and only p27 increased (P < 0.05). p53 was low in control 59-2-HI and C7-2-HI tumors and increased after treatment (P < 0.05) whereas BET untreated tumors already expressed high levels of p53 and MDM2. Although the immunohistochemical findings were compatible with alterations of p53, SSCP evaluation failed to disclose the presence of mutations, suggesting that the defective expression of p21 is related to an impaired p53 pathway. UV irradiation failed to increase p21 expression in BET, but was able to induce p53 and p21 in 59-2-HI and C7-2-HI tumors. The absence of an increased expression of p21 in E₂-regressing BET lesions suggests that this protein is not necessary for estrogen-induced regression, but may be essential for antiprogestin action. Our results also suggest that p53/MDM2 alterations may be one of the mechanisms responsible for selected hormone resistance in breast carcinomas.