Thomas J. Spady

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 know ledge 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.

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-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×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 17β‐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. Mol. Carcinog. 26:239–253, 1999.

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

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 17β‐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. Mol. Carcinog. 23:86–95, 1998.

Leptin as a surrogate indicator of body fat in the American black bear

Abstract We investigated the potential use of leptin as a wildlife management tool to monitor adiposity in American black bears (Ursus americanus). Body mass (BM), body condition index (BCI), serum leptin concentration, and percent of body fat were measured in semi-free ranging (SFR) adult male (South Dakota, Oct 2003–Jun 2004) and free-ranging (FR) adult females (Colorado and Wyoming, 1997–99) in fall, winter, spring, and summer. These variables were analyzed by simple and multiple linear regression models to determine their relative value as predictors of body fat proportion over a wide range of adiposity (7.5–48% of total BM). Log-transformed serum leptin concentration (ln leptin) was the most consistent single surrogate predictor of percent body fat when compared with BM and BCI. For both SFR and FR cohorts, ln leptin was strongly predictive of percent body fat within the range of adiposity between 7.5–35%. Moreover, the best predictors of body fat were multiple regression models that included ln leptin in both SFR males (AICc  =  75.97, ω  =  0.61) and FR females (AICc  =  85.07, ω  =  0.54). Therefore, serum leptin markedly improves the resolution and accuracy of common field estimates of body condition in black bears. Because it is strongly associated with and predictive of bioelectrical impedance assay (BIA) estimation of body fat, further investigation of serum leptin as a predictor of adiposity is warranted.

Genetic bases of renal agenesis in the ACI rat: mapping of Renag1 to chromosome 14

Unilateral renal agenesis (URA) is a common developmental defect in humans, occurring at a frequency of approximately 1 in 500–1000 births. Several genetic syndromes include bilateral or unilateral renal agenesis as an associated phenotype. However, URA frequently occurs in individuals not afflicted by these syndromes and is often asymptomatic. Although it is clear that genetic factors contribute to the etiology of URA, the genetic bases of URA are poorly defined at this time. ACI rats, both males and females, exhibit URA at an incidence of 5%–15%. In this article we characterize the incidence of URA in female and male F1, F2, and backcross (BC) progeny from reciprocal genetic crosses between the ACI strain and the unaffected Brown Norway (BN) strain. Through interval mapping analyses of 353 phenotypically defined female F2 progeny, we mapped to rat Chromosome 14 (RNO14) a genetic locus, designated Renag1 (Renal agenesis 1), that serves as the major determinant of URA in these crosses. Further genotypic analyses of URA-affected female and male F2 and BC progeny localized Renag1 to a 14.4-Mb interval on RNO14 bounded by markers D14Rat50 and D14Rat12. The data from these genetic studies suggest that the ACI allele of Renag1 acts in an incompletely dominant and incompletely penetrant manner to confer URA.

The ACI Rat as a Genetically Defined Animal Model for the Study of Estrogen Induced Mammary Cancers

We have demonstrated that physiologic levels of 17β-estradiol (E2) rapidly induce mammary cancer development in ovary intact, but not ovariectomized, female ACI rats. Susceptibility to E2 induced mammary cancers in F1, F2 and backcross (BC) progeny from a genetic cross between the highly susceptible ACI strain and the highly resistant Copenhagen (COP) rat strain, has now been examined. Susceptibility in progeny from a cross between the ACI strain and a second highly resistant rat strain, Brown Norway (BN), is currently being evaluated. Data from these genetic studies strongly suggest that the unique susceptibility of the ACI rat strain to development of E2 induced mammary cancers is conferred by a limited number of genes, making mapping and eventual cloning of these genes experimentally tractable. The phenotypically defined F2 and BC progeny are being used in linkage studies to identify the genetic loci that confer and/or modulate susceptibility to E2 induced mammary cancers. Data accumulated to date indicate that: 1) the genetic etiology of estrogen induced mammary cancers in the ACI rat is distinct from that of dimethylbenz-a- anthracene induced mammary cancers; and 2) the rat homologs of the known human breast cancer susceptibility genes BRCA1 and BRCA2 do not confer susceptibility to estrogen induced mammary cancer in the ACI rat strain.

Dietary Energy Restriction Inhibits Estrogen Induced Pituitary Tumorigenesis in a Rat Strain Specific Manner

We are investigating modulation by dietary energy consumption of estrogen action in the regulation of cell proliferation and survival and induction of prolactin (PRL)-producing pituitary tumors in different inbred rat strains. Summarized herein are data which indicate that a 40% restriction of energy consumption virtually abolishes development of estrogen induced pituitary tumors in the inbred Fischer 344 (F344) rat strain and that this inhibition occurs through modulation of estrogen regulation of pituitary cell survival, not inhibition of estrogen stimulated cell proliferation. Data are also presented which indicate that the inhibitory effect of energy restriction on estrogen induced pituitary tumorigenesis is rat strain specific. Whereas energy restriction markedly inhibited development of pituitary tumors in the F344 and Copenhagen (COP) rat strains, no inhibitory effect was observed in the ACI strain. Genetic studies have been initiated to elucidate the molecular bases of the strain specific inhibitory actions of dietary energy restriction on development of estrogen induced pituitary tumors in the genetically related COP and ACI rat strains.

American black bear mating behavior and chemosensation of estrus

Abstract Quantitative documentation of bear (Ursidae) mating behavior is sparse and incomplete. Understanding subtleties of mating behavior and how it differs between successful and unsuccessful mate pairings is a crucial first step to permit empirical testing of sexual selection hypotheses and to inform captive breeding efforts. Because bears possess extremely sensitive olfactory systems and are well known for marking behaviors, it is probable that they are able to sense reproductive status of conspecifics using chemosensory cues. However, very little is currently known of these phenomena in most bear species, including American black bears (Ursus americanus). Our study documents the entire consummatory phase of mating behavior and provides behavioral evidence of chemosensory communication of estrus status in American black bears. Using quantitative ethological methods, we discovered that successful mounts (with ejaculation) averaged 29-fold longer duration than unsuccessful mounts, with no overlap. Male...