Direct inhibiting effects of [D-Trp6]gonadotropin-releasing hormone on the estrogen-sensitive progression of polyoma virus-induced mammary tumors in athymic mice.

The direct antitumoral effects of gonadotropin-releasing hormone (GnRH) analogues on breast tumors have been surmised from clinical observations and in vitro studies. The present study aimed to determine the effects of the GnRH agonist [D-Trp6]GnRH (Decapeptyl) on steps of experimental mammary carcinogenesis, and the mechanisms, other than the chemical castration, involved. We chose a recent model, i.e., mammary tumors induced by wild-type A2 polyoma (Py) virus in BALB/c female nu/nu mice, which displays the following characteristics. Tumors are mammary adenocarcinomas similar to well differentiated breast carcinomas. Tumor promotion period ends 20 days after Py virus inoculation and is estradiol dependent. The first palpable tumors occur 60 days after Py virus inoculation, and tumor growth is ovarian hormone independent. The effects of Decapeptyl treatment on tumor induction and tumor growth were studied in normal or ovariectomized 6-week-old nude mice inoculated with 10(7) plaque-forming units Py virus (day 0 of experiments). Normal mice and ovariectomized mice percutaneously supplemented with 0.6 micrograms 17 beta-estradiol every other day until day 30 (OvE2 mice) were treated with monthly s.c. injections of the sustained release form of Decapeptyl (5 mg/kg) until the end of 180-day experiments. Overall values for latency periods were included within a day 60 to day 130 time interval. Hormone-independent outgrowth was not affected. We focused on tumor progression before the outgrowth. Incidences on tumor appearance kinetics account for effects at this stage. 17 beta-Estradiol repletion strongly antagonized (P less than 0.001) the slowing effect of ovariectomy on the tumor appearance kinetics, indicating that tumor progression is estradiol sensitive in its early stages. [D-Trp6]GnRH treatment antagonized tumor appearance profiles, inducing similar kinetics in both normal and OvE2 mice. In normal mice, the antagonism (P less than 0.01) was concomitant with significant decreases (P less than 0.05) in serum levels of estradiol and prolactin, which are critical hormones for mammary tumor development in mice, suggesting a pituitary-mediated effect. In OvE2 mice, the antagonism (P less than 0.01) occurs independently of estradiol and prolactin, suggesting a direct effect at the mammary cell level. Because of alterations in kinetics, this effect is exerted at the early stages of tumor progression on Py virus-transformed, ovarian hormone-sensitive cells in the mammary tissue. This new animal model of breast cancer is shown to be useful in characterizing direct antitumoral effects of GnRH analogues and studying the basic mechanisms of mammary carcinogenesis.