Reversal of hormone resistance in murine mammary carcinomas

VICTORIA WARGON, PAOLA ROJAS, VIRGINIA NOVARO, ALFREDO MOLINOLO AND CLAUDIA LANARI

Colby Sawyer, New London, NH, USA.

Congreso; 15/7-20/7/2007: Gordon Conference on Hormone Action in Development and Cancer; 2007

Gordon

By using selective pressure, we induced in a previously sensitive experimental mouse mammary carcinoma the acquisition of resistance to RU-486 treatment. This tumor variant, now termed C4-HIR, also developed a significantly more aggressive metastatic behavior, as compared with the parental tumor. C4-HIR originated from a hormone-independent ductal carcinoma, C4-HI, which had been induced by medroxyprogesterone acetate in a female BALB/c mouse, and is maintained by syngeneic passages. C4-HI tumors express high levels of estrogen (ER) and progesterone receptors (PR) and they are inhibited by antiprogestins, estrogens and tamoxifen. To further characterize this phenomenon we decided a) to evaluate if the acquired resistant phenotype could be reverted; b) to follow hormone receptor and EGF-R expression profile throughout the process; and c) to investigate whether estrogen treatment could modify RU-486 responsiveness of C4-HIR tumors. To evaluate, if hormone resistance could be reversed, C4-HIRs tumors were successively transplanted sc. in 4 animals, 2 of which were treated with RU-486. The tumors from untreated animals were used for further passages. During the 2 first passages tumors grew even in the presence of antiprogestins. After the 3rd passage (5 months) there was a significant (p<0.001) decrease in growth rate  in the presence of RU-486, and after 5 generations (8 months) the tumors were able to regress with RU 486 as the parental C4-HI tumors and were now named C4-HIRv. The acquisition of hormone resistance was associated with a significant decrease in ER and PR expression (p<0.05), especially of isoform A and an increased nuclear localization of EGFR, evaluated by western blot and immunofluorescence.  Interestingly, C4-HIRv reacquired the pattern of ER, PR and EGF-R expression of the original C4-HI while the highly aggressive metastatic phenotype did not revert. As E2 treatment (5 mg pellets, 48 hs) induced the re-expression of PR in C4-HIR, we decided to evaluate whether C4-HIR tumors would acquire sensitivity to RU 486 plus E2.   Mice bearing tumors of 75 mm2 were treated with E2 (pellets 5 mg) plus/or RU 486 (12mg/kg/day) during one month. The combined treatment induced a significant tumor regression, while single treatments induced a slight inhibition of tumor growth indicating that in the presence of estrogens, tumors had become sensitive to RU 486. Our data suggest that epigenetic mechanisms may be down regulating hormone receptors rendering the tumors insensitive to treatment. In the process of selective pressure, a genotypic selection of more aggressive clones may favor tumor progression; however, this increase in aggressiveness is unidirectional, and could be dissociated from the ability of tumors to respond to a hormone therapy.