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 (E(2)) or antiprogestin treatment. The BET
tumor line, of similar origin and
biological features, regresses only after E(2) treatment. To investigate possible differences between E(2)- 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 mm(2). 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 E(2) and antiprogestin-responsive lines treated with E(2), RU 38.486 or
ZK 98.299 (P < 0.05). In BET
tumors treated with E(2), 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 E(2)-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.