Genetic bases of estrogen-induced pituitary growth in an intercross between the ACI and Copenhagen rat strains: dominant mendelian inheritance of the ACI phenotype.

Estrogens stimulate cell proliferation in a variety of tissues and are widely believed to be contributing factors in the etiology of certain cancer types in humans. The molecular mechanisms through which estrogens regulate cell proliferation are currently unknown. Estrogens stimulate proliferation of the PRL-producing lactotroph of the rat anterior pituitary gland and induce development of PRL-producing pituitary tumors in several inbred rat strains. Therefore, the lactotroph provides a well defined model for identifying the mechanisms through which estrogens regulate cell proliferation and/or survival. Data from our laboratory and others indicate that the relative sensitivity to the pituitary growth-promoting actions of estrogens is highly strain specific. This allows genetics-based approaches to be used to address the molecular mechanisms through which estrogens stimulate lactotroph proliferation and induce pituitary tumor development. In the present study we have examined the ability of diethylstilbestrol (DES) to induce pituitary growth in the genetically related AxC-Irish (ACI) and Copenhagen (COP) strains and their derived F1, F2, and backcross progeny. The data presented herein indicate that the anterior pituitary gland of the ACI strain displays approximately a 2-fold greater growth response to administered DES than does the pituitary gland of the COP strain. The average pituitary weight in male ACI rats was increased from 9.2 +/- 0.2 mg (mean +/- SD in untreated rats to 63.7 +/- 12.6 mg in rats treated with DES for 12 weeks, whereas in male COP rats, DES increased pituitary weight from 12.7 +/- 0.9 to 38.1 +/- 8.2 mg. The ACI phenotype was inherited in the F1, F2, and backcross progeny of an ACI x COP intercross as a dominant genetic trait, and the approximately 30 mg of additional pituitary growth displayed by the DES-treated ACI rat, relative to that of the treated COP rat, appeared to result from the actions of a single locus. Moreover, in F1 progeny from an ACI x Brown Norway intercross, the ACI phenotype was inherited as a dominant or incompletely dominant genetic trait. These data, when compared with findings of previous studies using the Fischer 344 rat strain, provide the first indication that distinct genetic pathways contribute to regulation of estrogen-induced pituitary growth and induction of PRL-producing pituitary tumors in the ACI and F344 rat strains.

Susceptibility to estrogen-induced mammary cancer segregates as an incompletely dominant phenotype in reciprocal crosses between the ACI and Copenhagen rat strains.

Estrogens have been inextricably linked to the etiology of breast cancer. We have demonstrated that the female ACI rat exhibits a unique propensity to develop mammary cancers when treated continuously with physiological levels of 17 beta-estradiol (E2). The E2-induced mammary cancers are estrogen dependent and exhibit genomic instability. In contrast, the genetically related Copenhagen (COP) rat strain is relatively resistant to E2-induced mammary cancers. In this study we evaluated susceptibility to E2-induced mammary cancers in first filial (F(1)), second filial (F(2)), and backcross (BC) progeny generated from reciprocal intercrosses between the ACI and COP strains. F(1) progeny resembled the parental ACI strain with respect to incidence of E2-induced mammary cancers. However, latency was significantly prolonged in the F(1) populations. These data indicate that susceptibility behaves as an incompletely dominant phenotype in these crosses. Analysis of phenotypes exhibited by the F(1), F(2), and BC populations suggests that mammary cancer susceptibility is modified by one or two genetic loci in the reciprocal intercrosses between the ACI and COP strains. Susceptibility to E2-induced mammary cancers did not correlate with E2-induced pituitary growth in the genetically diverse F(2) and BC populations, suggesting that the genetic bases for susceptibility to E2-induced mammary cancers differ from those for E2-induced lactotroph hyperplasia.

Estrogen-induced tumorigenesis in the Copenhagen rat: disparate susceptibilities to development of prolactin-producing pituitary tumors and mammary carcinomas.

The Copenhagen (COP) rat is unique among inbred rat strains in its high degree of resistance to spontaneously arising and induced mammary cancers. Hyperprolactinemia resulting from tumors of the anterior pituitary gland has been suggested to be the causative factor in the etiology of estrogen-induced mammary cancer in rats. Therefore, we have examined the ability of administered estrogens to induce development of PRL-producing pituitary tumors and mammary carcinomas in COP rats. Diethylstilbestrol (DES), administered to male COP rats for 12 weeks, beginning when the animals were 9 weeks of age, induced development of PRL-producing pituitary tumors, defined as grossly enlarged pituitary masses displaying lactotroph hyperplasia and associated hyperprolactinemia. When treated with 17beta-estradiol (E2), female COP rats developed pituitary tumors and hyperprolactinemia, but displayed a high degree of resistance to development of mammary carcinomas. These data indicate that E2-induced hyperprolactinemia is insufficient to induce development of mammary carcinomas in the female COP rat.

Estrogen action in the regulation of cell proliferation, cell survival, and tumorigenesis in the rat anterior pituitary gland.

Estrogens act as important regulators of cell proliferation, cell survival, and differentiation in a variety of organ systems and tissues and have been implicated in the etiology of a variety of malignant cancers and benign tumors. The anterior pituitary gland of the rat provides an excellent model for the study of estrogen action in the regulation of cell proliferation and survival. Estrogens stimulate proliferation of the prolactin (PRL)-producing lactotroph and enhance lactotroph survival. Through these actions on lactotroph proliferation and survival, estrogens induce or contribute to the development of PRL-producing pituitary tumors in several rat strains. Data from our laboratory and others indicate that estrogen-induced pituitary growth is rat strain specific and segregates as a quantitative genetic trait in crosses between different rat strains. The purpose of this review is to summarize current knowledge pertaining to estrogen action in the regulation of cell proliferation, cell survival, and tumorigenesis in the anterior pituitary gland of the rat species, Rattus norvegicus, and to illustrate the advantages of the rat pituitary gland as a model for elucidating the mechanisms through which estrogens regulate these processes.

Ovary-intact, but not ovariectomized female ACI rats treated with 17beta-estradiol rapidly develop mammary carcinoma.

We have examined the ability of 17beta-estradiol (E2) to induce development of mammary cancers in the female ACI rat. Continuous treatment with E2, delivered through release from s.c. Silastic tubing implants containing 27.5 mg crystalline hormone, resulted in rapid development of palpable mammary tumors in ovary-intact ACI rats. In a population of 21 E2-treated rats, palpable tumors were first observed following 99 days treatment and 100% of the treated population developed tumors within 197 days. The median and mean times to appearance of first palpable tumor were 143 and 145 days respectively. All mammary tumors were classified as carcinomas and invasive features were observed. Circulating E2 levels in the treated animals at the time of sacrifice averaged 185 pg/ml serum. Mammary tumors were not observed in ovary-intact female ACI rats that were not treated with E2. This is the first report indicating that this naturally occurring estrogen is capable of inducing mammary cancers in the ACI rat strain. Mammary carcinoma did not develop in a population of 11 ovariectomized female ACI rats treated with E2 for a period of 140 days. Circulating E2 levels in the treated ovariectomized animals averaged 207 pg/ml. These data indicate that the ovary modulates estrogen-mediated mammary carcinogenesis in this rat strain. Both ovary-intact and ovariectomized female ACI rats displayed similar susceptibilities to E2-induced pituitary tumors and hyperprolactinemia. Pituitary weight was increased 6.0-fold in ovary-intact ACI rats and 5.3-fold in ovariectomized female rats. Circulating prolactin levels averaged 2318 ng/ml in E2-treated, ovary-intact rats and 2285 ng/ml in E2-treated, ovariectomized ACI rats. These data indicate that estrogen-induced hyperprolactinemia is not the sole factor leading to development of mammary cancers in the E2-treated ACI rat.

Estrogen-induced pituitary tumor development in the ACI rat not inhibited by dietary energy restriction.

We have demonstrated that a 40% restriction of dietary energy consumption virtually abolishes the development of prolactin (PRL)-producing pituitary tumors in Fischer 344 (F344) rats treated chronically with estrogen, apparently by inhibiting the ability of estrogen to enhance survival within a rapidly proliferating lactotroph population. The purpose of the study reported here was to determine whether energy restriction exerts a similar antitumorigenic action in another rat strain, August x Copenhagen-Irish (ACI), in which PRL-producing pituitary tumors develop in response to estrogen treatment. Ovariectomized female ACI rats were either allowed to consume a control diet ad libitum or were fed a modified diet that restricted energy consumption by 40% relative to the amount of energy consumed by animals fed the control diet. We also examined the ability of 17beta-estradiol (E2) administered for 20 wk via subcutaneous Silastic implants to induce development of PRL-producing pituitary tumors. Treatment with E2 increased pituitary weight as well as the pituitary weight-to-body weight ratio and induced gross hyperprolactinemia to the same extent in ACI rats fed either the control or the energy-restricted diet. Moreover, dietary energy restriction did not affect the ability of E2 to induce pituitary cell proliferation or inhibit apoptosis, as evidenced by quantification of two surrogate markers. These data provide compelling evidence that a 40% restriction of energy consumption does not inhibit the ability of E2 to induce pituitary tumor development in the ACI rat. In conjunction with our published studies of the F344 rat strain, the data presented herein indicate that the inhibitory effects of dietary energy restriction on estrogen-induced pituitary tumor development are rat-strain specific and suggest that sensitivity to specific antitumorigenic actions of energy restriction is strongly affected by genetic background.

Dietary energy restriction abolishes development of prolactin-producing pituitary tumors in Fischer 344 rats treated with 17beta-estradiol.

Reduction in energy consumption is known to inhibit development of a variety of spontaneous, carcinogen-induced, and hormone-dependent cancers, but the mechanism or mechanisms by which this occurs remain unknown. We hypothesize that energy consumption may modulate development of estrogen-dependent neoplasms by altering the manner in which target cells respond to estrogens. To test this hypothesis, ovariectomized female Fischer 344 rats were fed diets that allowed consumption of different amounts of energy, and the ability of 17beta-estradiol (E2), administered for 10 wk from subcutaneous Silastic implants, to promote development of prolactin-producing pituitary tumors was examined. A 40% restriction of energy consumption virtually abolished the ability of E2 to promote development of pituitary tumors and associated hyperprolactinemia. A 25% restriction of energy consumption appeared to slightly inhibit E2-induced pituitary growth and hyperprolactinemia, but the observed degree of inhibition was not statistically significant. Interestingly, dietary energy restriction did not inhibit induction by E2 of pituitary cell proliferation and lactotroph hyperplasia. Furthermore, E2 treatment inhibited expression of testosterone-repressed prostate message-2 mRNA, a cellular marker of apoptosis, and this inhibitory effect of E2 was blocked by 40% energy restriction. These data suggest that dietary energy restriction virtually abolished E2-induced development of prolactin-producing pituitary tumors, not by blocking the ability of E2 to induce cell proliferation but rather by blocking the ability of E2 to enhance cell survival. This study and the accompanying paper provide the first indication that dietary energy consumption may modulate estrogen action at the level of the target cell.

Estrogen induction of prolactin-producing pituitary tumors in the Fischer 344 rat: modulation by dietary-energy but not protein consumption.

Our laboratory is examining the hypothesis that diet may modulate the ability of estrogens to regulate cell proliferation and survival, either of which could affect development of neoplasms in estrogen-responsive tissues. In this study, we examined whether the amount of energy and protein consumed in the diet modulates the ability of the synthetic estrogen diethylstilbestrol (DES) to induce development of prolactin-producing pituitary tumors in two strains of rat, Fischer 344 (F344) and Holtzman, that differ in their propensity to develop pituitary tumors when treated with estrogens. Male F344 rats treated with DES for 8 wk developed pituitary tumors (defined as grossly enlarged pituitary masses that displayed diffuse lactotroph hyperplasia but lacked adenomatous foci). In contrast, male Holtzman rats displayed only a modest increase in pituitary weight in response to DES. Energy consumption but not protein consumption modulated DES-induced pituitary tumorigenesis in the male F344 rat. Relative to that observed in untreated animals, pituitary weights in F344 rats treated with DES increased 11.2- and 9.2-fold in animals fed either the control diet or an equicaloric high-protein diet, respectively, but only 3.5-fold in animals fed an energy-restricted diet. In contrast, neither the amount of energy nor protein consumed in the diet affected the modest pituitary growth response of male Holtzman rats to administered DES. Energy restriction had no apparent effect on pituitary cell proliferation, either basal or DES stimulated, in these rat strains. We concluded that dietary energy restriction inhibits the ability of administered DES to induce pituitary tumor development in the F344 rat by acting at a step after induction of pituitary cell proliferation.

Modulation of estrogen action in the rat pituitary and mammary glands by dietary energy consumption.

We are investigating the mechanisms through which estrogens induce development of prolactin (PRL)-producing pituitary tumors and mammary carcinomas in rats and how these mechanisms are affected by dietary energy consumption. The hypothesis under examination is that dietary energy restriction inhibits tumorigenesis in estrogen-responsive tissues by altering cellular responsiveness to estrogenic hormones. In the Fischer 344 (F344) rat strain, a 40% restriction of energy consumption virtually abolishes development of estrogen-induced pituitary tumors. Inhibition of pituitary tumorigenesis in the F344 strain by energy restriction results from modulation of estrogen regulation of cell survival, not cell proliferation. In contrast, energy restriction has no inhibitory effect on estrogen-induced pituitary tumor development in the ACI rat strain. However, energy restriction markedly inhibits induction of mammary carcinomas in female ACI rats treated with 17beta-estradiol. Data presented herein indicate that dietary energy restriction modulates the responsiveness of specific cell populations to estrogenic hormones and thereby inhibits estrogen-induced tumorigenesis in a manner specific to both rat strain and tissue.