Comparison of ovariectomy and retinyl acetate on the growth of established 7,12-dimethylbenz[a]anthracene-induced mammary tumors in the rat.

Prolonged exposure to retinyl acetate (RA) in the diet inhibits the development of 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary cancers in rats. The effectiveness of RA was examined when given 6 months after the administration of DMBA. Non-inbred female Sprague-Dawley rats with DMBA-induced mammary tumors were divided into 3 groups and treated for 4 weeks as follows: Group 1 served as controls, group 2 was ovariectomized, and group 3 received 328 mg RA/kg diet. Ovariectomy (OVX) markedly reduced both the number and size of the tumors. RA administration failed to induce any significant regression in tumor number but significantly retarded tumor growth when compared to tumor growth in group 1 controls. The levels of estradiol, progestin, and prolactin (PRL) receptors were significantly reduced after OVX, whereas only the levels of PRL receptors declined significantly after RA administration. Circulating progesterone concentrations were not affected in the RA-treated group but the plasma PRL level was significantly increased. The present studies show that if treatment with RA is delayed until 6 months after carcinogen administration, the protective effect of RA can still be observed although its effectiveness is less dramatic than when it is administered earlier.

N-nitroso-N-methylurea-induced mammary tumors in the rat: role of prolactin and a prolactin-lowering drug.

Female outbred Sprague-Dawley rats bearing N-nitroso-N-methylurea (NMU)-induced mammary tumors received various endocrine therapies 3 months after the first NMU injection. Rats were divided into 5 groups (15-20 rats/group) and received a 4-week treatment as follows: group 1, controls; group 2, ovariectomized; group 3, 0.5 mg 2-bromoergocryptine (CB-154) injected so twice daily; group 4a, pituitary implant under the kidney capsule; and group 4b, CB-154 injected during the last 2 weeks of the experiment in rats bearing a pituitary implant. Castration of rats with established NMU-induced tumors resulted in a decrease in both tumor number and size, but these parameters again started to increase 3 weeks post castration. CB-154 failed to reduce the tumor number but did arrest the increase in tumor size. In the animals with a pituitary implant, both tumor number and tumor size increased progressively at a greater rate than in control animals, whereas the addition of CB-154 (group 4b) stabilized the tumor growth. Ovariectomy (OVX) resulted in a significant decline of steroid receptor levels. Prolactin (PRL) receptor levels were significantly stimulated by the pituitary implant, and CB-154 prevented this increase. The present studies confirmed that NMU-induced mammary tumors are less hormone-dependent (response to OVX) than 7,12-dimethylbenz[a]anthracene (DMBA)-induced tumors. The role of PRL also appears to be less important, at least for established tumors, for NMU-induced mammary tumors than for DMBA-induced mammary tumors.

Characteristics of androgen binding in rat adrenal tissue using [3H]methyltrienolone (R1881) as tracer.

A high level of binding of [3H]methyltrienolone (R1881 = 17beta-hydroxy-17alpha-methyl-estra-4, 9, 11-trien-3-one) was found in cytosol prepared from adrenals of castrated male rats. Binding of [3H]R1881 was of high affinity (DK = 6.2 nM) and highly specific for androgens. The [3H]R1881 complex migrates at 7-9S on sucrose gradients in low ionic strength buffer and at 4-5S in buffer containing 0.4M KC1. All binding studies have been performed in parallel with rat ventral prostate and adrenal cytosol. The present data suggest the presence of an androgen binding component in rat adrenal tissue.

Prolactin receptors in N-nitroso-N-methylurea-induced rat mammary tumors: relationship to tumor age and down-regulation in short-term explant culture.

Mammary tumors were induced by N-nitrosomethylurea (NMU) in young female Sprague-Dawley rats and time of appearance of the tumors was recorded daily. A total of 41 tumors of different ages (7 to 110 days old) were removed and prolactin (PRL) receptors were measured using [125I] human growth hormone (hGH) (equipotent lactogenic activity to oPRL). All but one tumor contained high PRL receptor levels. However, there was a slight decline of both free and total PRL receptor levels with increasing tumor age, with total prolactin receptor levels in tumors 7-20 days old being 32.0 +/- 1.4% compared to 26.0 +/- 2.0% in tumors 66-110 days old (p less than 0.05). In addition, for 24 of these tumors which were of sufficient size, explant cultures were carried out up to 24 h in medium-199 containing insulin and cortisol in the presence or absence of 20 microgram/ml oPRL. At this concentration, an occupation of free and a reduction of total receptor levels was observed in all explant cultures. This down-regulation also appeared to vary as a function of tumor age. Total PRL receptors declined 24.9 +/- 2.3% in young tumors (7-35 days old) compared to 41.7 +/- 4.4% in tumors 50-110 days old (p less than 0.01). The present data are important in that they demonstrate that PRL receptor levels are relatively high regardless of the age of the tumor with, however, a slight decline with increased tumor age. In contrast, the responsiveness of the tumors measured by the ability of prolactin to down-regulate its own receptors is greater in older tumors.

Prolactin receptors in explant cultures of carcinogen-induced rat mammary tumors.

The turnover, down-regulation and role of intracellular organelles in the down-regulation of prolactin (PRL) receptors have been investigated in N-nitrosomethylurea (NMU)-induced rat mammary tumors cultured in short-term explants. Tumor explants are capable of maintaining PRL receptors for 24-48 hr. This maintenance reflects a dynamic phenomenon involving receptor synthesis, since addition of cycloheximide (1 microgram/ml) in the culture medium results within 12 hr in a marked decline of PRL receptor levels. A down-regulation of total PRL receptors (measured after exposure of membranes to 3M MgCl2) is observed in cultures containing concentrations of 20 micrograms/ml or greater of ovine PRL (oPRL). Lysosomotropic agents, such as chloroquine (100 microM) are ineffective in either increasing basal PRL receptor levels or in preventing the PRL-induced down-regulation in NMU-induced mammary tumor explants. Cytochalasin B (20 microM), without effect on basal PRL binding, prevents the down-regulation of PRL receptors, whereas colchicine (10 microM) results in a decline of PRL receptor levels both in the absence and in the presence of oPRL. The present data suggest a different pattern of PRL receptor regulation in vitro for tumors compared to normal rabbit mammary explants.