Primate mammary development. Effects of hypophysectomy, prolactin inhibition, and growth hormone administration

Review of serum prolactin levels as an antipsychotic-response biomarker

Antipsychotics acting as antagonists at dopamine D2 receptors concentrated in the striatum are the cornerstone of effective treatment of psychosis. Substantial progress in treating persons with schizophrenia could be achieved by the identification of biomarkers which reliably determine the lowest efficacious dose of antipsychotics. Prolactin levels have been considered a promising treatment-response biomarker due to dopamine’s regulation of serum prolactin levels through D2 receptors in the hypothalamic-pituitary pathway. Prolactin secretion in response antipsychotic administration is associated with the antipsychotics affinity for D2 receptors. This review assesses the available literature on the use of serum prolactin levels as an antipsychotic-response biomarker. Articles were identified through PubMed as well as the reference lists of full text articles available online. Relevant publications were summarized briefly to define the limitations and utility of serum prolactin levels as a tool for improving antipsychotic dosing. Serum prolactin levels in combination with prolactin-inducing potencies for each antipsychotic may help identify the lowest effective dose of antipsychotic medications. , In addition to the fact that prolactin secretion is dependent on serum antipsychotic levels and not brain levels, recent findings show that prolactin release is independent of the β-arrestin-2 pathway and GSK3β regulation, one branch of the pathway that has been implicated in antipsychotic efficacy. Therefore, serum prolactin is an indirect biomarker for treatment response. Further investigations are warranted to characterize prolactin-antipsychotic dose-response curves and systematically test the utility of measuring prolactin levels in patients to identify a person’s lowest efficacious dose.

Estrogen/isoflavone interactions in cynomolgus macaques (Macaca fascicularis)

Soy isoflavones are phytoestrogenic components of dietary soy, which are widely consumed for their potential health benefits. Soy isoflavones appear to decrease breast and endometrial cancer risk in human observational studies, but paradoxically stimulate growth of breast cancer cells in culture and uterine enlargement in rodents. We have shown that these compounds are not estrogenic in cynomolgus monkeys even at relatively high doses, but that they reduce estrogen-induced proliferative responses of the breast and endometrium. This effect may be mediated through estrogen receptor interactions and/or modulation of endogenous estrogen metabolism. Interindividual variation in isoflavone absorption and metabolism contributes to the degree of estrogen antagonistic effect. Our recent studies have also shown that individual isoflavone metabolites such as glyceollins may have unique selective estrogen receptor modulator-like activity, acting as tissue-specific antagonists without agonist activity. Rodent studies and human epidemiologic data suggest that timing of exposure and dose relative to endogenous estrogen concentrations are important determinants of effect, and studies of dietary soy on breast development and pubertal maturation are under way. Because soy isoflavones are both abundant in standard monkey chow diets and widely available as dietary supplements for human beings, these findings have broad relevance to the health of human and nonhuman primates.

Growth hormone and insulin-like growth factor-I in the transition from normal mammary development to preneoplastic mammary lesions

Adult female mammary development starts at puberty and is controlled by tightly regulated cross-talk between a group of hormones and growth factors. Although estrogen is the initial driving force and is joined by luteal phase progesterone, both of these hormones require GH-induced IGF-I in the mammary gland in order to act. The same group of hormones, when experimentally perturbed, can lead to development of hyperplastic lesions and increase the chances, or be precursors, of mammary carcinoma. For example, systemic administration of GH or IGF-I causes mammary hyperplasia, and overproduction of IGF-I in transgenic animals can cause the development of usual or atypical hyperplasias and sometimes carcinoma. Although studies have clearly demonstrated the transforming potential of both GH and IGF-I receptor in cell culture and in animals, debate remains as to whether their main role is actually instructive or permissive in progression to cancer in vivo. Genetic imprinting has been shown to occur in precursor lesions as early as atypical hyperplasia in women. Thus, the concept of progression from normal development to cancer through precursor lesions sensitive to hormones and growth factors discussed above is gaining support in humans as well as in animal models. Indeed, elevation of estrogen receptor, GH, IGF-I, and IGF-I receptor during progression suggests a role for these pathways in this process. New agents targeting the GH/IGF-I axis may provide a novel means to block formation and progression of precursor lesions to overt carcinoma. A novel somatostatin analog has recently been shown to prevent mammary development in rats via targeted IGF-I action inhibition at the mammary gland. Similarly, pegvisomant, a GH antagonist, and other IGF-I antagonists such as IGF binding proteins 1 and 5 also block mammary gland development. It is, therefore, possible that inhibition of IGF-I action, or perhaps GH, in the mammary gland may eventually play a role in breast cancer chemoprevention by preventing actions of both estrogen and progesterone, especially in women at extremely high risk for developing breast cancer such as BRCA gene 1 or 2 mutations.

The Mammary Glands of Macaques

This review describes the normal biology and physiology of the mammary gland in macaques, including the typical histologic appearance across the life span (development, reproductive maturity, lactation, and senescence). The molecular events regulating breast morphogenesis are described, as well as systemic and local hormonal regulators of mammary gland proliferation, differentiation, and function. Similarities and differences to the human breast are described. Regulatory events are illuminated by discussion of genetically modified mouse models. Tissue response markers, including immunohistochemical markers of proliferation and other hormonally induced changes and studies to date, regarding the effects of exogenous hormones, are briefly summarized. In general, estrogens stimulate progesterone receptor expression and proliferation in the mammary gland, and combinations of estrogens and progestogens cause greater proliferation than estrogens alone. Evaluation of novel chemical agents in macaques requires careful evaluation of age and hormonal context to avoid the confounding effects of mammary gland development, past reproductive history, and other influences on mammary gland morphology. The expression of proliferation markers and progesterone receptors may be used as biomarkers to measure chemically induced hormonal effects.

IGF-I, GH, and sex steroid effects in normal mammary gland development

Although the pubertal surge of estrogen is the immediate stimulus to mammary development, the action of estrogen depends upon the presence of pituitary growth hormone and the ability of GH to stimulate production of IGF-I in the mammary gland. Growth hormone binds to its receptor in the mammary fat pad, after which production of IGF-I mRNA and IGF-I protein occurs. It is likely that IGF-I then works through paracrine means to stimulate formation of TEBs, which then form ducts by bifurcating or trifurcating and extending through the mammary fat pad. By the time pubertal development is complete a tree-like structure of branching ducts fills the rodent mammary fat pad. In addition to requiring IGF-I in order to act, estradiol also directly synergizes with IGF-I to enhance formation of TEBs and ductal morphogenesis. Together they increase IRS-1 phosphorylation and cell proliferation, and inhibit apoptosis. In fact, the entire process of ductal morphogenesis, in oophorectomized IGF-I(-/-) knockout female mice, can occur as a result of the combined actions of estradiol and IGF-I. IGF-I also permits progesterone action in the mammary gland. Together they have been shown to stimulate a form of ductal morphogenesis, which is anatomically different from the kind induced by IGF-I and estradiol. Although both progesterone and estradiol synergize with IGF-I by increasing IGF-I action parameters, there must be other, as yet unknown mechanisms that account for the anatomical differences in the different forms of ductal morphogenesis observed (hyperplasia in response to IGF-I plus estradiol and single layered ducts in response to IGF-I plus progesterone).

Assessing the mammary gland of nonhuman primates: effects of endogenous hormones and exogenous hormonal agents and growth factors

This review provides a summary of the normal biology, development, and morphology of the breast in nonhuman primates (macaques), and of the major published work addressing hormonally-induced changes in the breast of these animals. The mammary glands of macaques are anatomically, developmentally, and physiologically similar to the human breast, with similar expression of sex steroid receptors (estrogen receptors alpha and beta, progesterone receptor A and B, androgen receptors), estrogen dependent markers, and steroid metabolizing enzymes. Genetic similarity between human beings and macaques is high, varying from 95-99% depending on the sequence evaluated. Macaques develop hyperplastic and cancerous lesions of the breast spontaneously, which are similar in type and prevalence to those of human beings. They have a reproductive physiology typical of anthropoid primates, including a distinct menarche and menopause, and a 28-day menstrual cycle. These similarities give unique value to the macaque model for evaluation of the effectiveness and safety of hormonal agents. Such agents considered in this review include estrogens and progestogens, combined therapies such as oral contraceptives and post-menopausal hormone therapies, androgens, selective estrogen receptor modulators, phytoestrogens, prolactin, somatotropin, epidermal growth factor, and other novel agents with hormonal or growth factor-like activity. This review also includes a consideration of selected background changes and typical strategies and markers used for evaluation of experimentally-induced changes, including biopsy-based strategies designed to control for inter-individual variability and minimize numbers of animals used.

Adipose tissue: a vital in vivo role in mammary gland development but not differentiation

Development and differentiation of the mammary gland occurs by means of critical stromal-epithelial interactions. Although many studies have attempted to understand these complex interactions, it has been difficult to demonstrate the essential role of adipose tissue in the development and function of the mammary gland. By using the A-ZIP/F-1 transgenic mice lacking in white adipose tissue (WAT), we have studied the role of adipocytes in mammary gland development and differentiation. In the absence of WAT, rudimentary mammary anlagen form but are unable to grow and branch normally, resulting in a few, short, severely distended ducts. However, during pregnancy, a tremendous amount of epithelial cell division and alveolar cell formation occurs even in the absence of adipocytes, illustrating that adipose tissue is not required for mammary gland differentiation. Mammary gland transplantation revealed that epithelial cells from these transgenic mice possess the potential for normal growth and differentiation when placed into a normal stromal environment. These experiments clearly demonstrate that the absence of adipocytes in the mammary gland results in disruption of stromal-epithelial interactions that prevent normal mammary gland development. The rudimentary epithelial anlage, however, contain mammary stem cells, which are fully capable of alveolar differentiation.

Are estrogens of import to primate/human ovarian folliculogenesis?

The notion that estrogens play a meaningful role in ovarian folliculogenesis stems from a large body of in vitro and in vivo experiments carried out in certain rodent models, (e.g., rats) wherein the stimulatory role of estrogen on granulosa cell growth and differentiation is undisputed. However, evidence derived from these polyovulatory species may not be readily generalizable to the monoovulatory subhuman primates, let alone the human. Only recently, significant observations on the ovarian role(s) of estrogen have been reported for the primate/human. It is thus the objective of this communication to review the evidence for and against a role for estrogens in primate/human ovarian follicular development with an emphasis toward the application of the concepts so developed to contemporary reproductive physiology and to the practice of reproductive medicine. The role(s) of estrogens will be examined not only by analyzing the physiological evidence to the effect that these hormones control ovarian function and follicular growth, but also by summarizing the molecular evidence for the existence and distribution of the cognate receptors.

Growth hormone receptor is expressed in human breast cancer

Several clinical observations and experimental studies indicate that pituitary hormones, including growth hormone, play a role in the development of human breast cancer. We analyzed 48 human breast carcinomas using reverse transcription polymerase chain reaction, immunohistochemistry, and Western blotting techniques to assess growth hormone receptor expression. In 17 of these cases, adjacent normal breast tissue was similarly analyzed. These analyses revealed that growth hormone receptor (GHR) is expressed in human breast cancer and appears to be up-regulated compared to adjacent normal breast tissue. GHR expression correlated inversely with tumor grade and MIB-1 index. Progesterone receptor expression correlated positively with GHR expression. These findings, along with our observation of GHR expression in breast cancer stromal cells and previous reports of local production of growth hormone in breast carcinoma, suggest that GHR-mediated signaling pathways are involved in the development of human breast cancer, possibly via autocrine or paracrine mechanisms.

Insulin-like growth factor I is essential for terminal end bud formation and ductal morphogenesis during mammary development

Previous studies from this laboratory have emphasized the essential role of GH in pubertal mammary development and shown that insulin-like growth factor I (IGF-I) was capable of substituting for GH in this process in rats and mice. The present study shows that, even when GH is present, no mammary development is possible unless IGF-I is present. IGF-I(-/-) null female animals were found to have significantly less mammary development than age matched wild-type controls (P <0.006) using several endpoints including the number of terminal end buds or TEBs (1.3 vs. 7.3), percent of the fat pad occupied by glandular elements (6.5 vs. 100), and number of ducts (15 vs. too numerous to count). That the deficiency in mammary gland development was related to the absence of IGF-I was underscored by the observation that des (1-3) IGF-I administration to IGF-I(-/-) null animals for 5 days caused significant mammary gland development as measured by TEB formation and branching of ducts. The number of TEBs rose from a mean of 1.3 in controls to 20.5 without added E2 (P < 0.009), and from 1.7 to 21 when des (1-3) IGF-I was given together with E2 (P < 0.006). The number of ducts increased significantly from a mean of 12 to 27 in response to IGF-I and E2, and from 15 to 24.5 with IGF-I alone. In contrast, administration of human GH with E2 had no stimulatory effect on mammary development in these animals, indicating that the full effect of GH in this process is mediated by IGF-I. To determine whether IGF-I was also responsible for further ductal morphogenesis, we administered des (1-3) IGF-I + E2 to the knockout animals for 14 days and compared the effects of this combination of hormones on mammary development with those observed after 5 days. We found that there was a significant increase from 5 to 14 days in the number of TEBs (mean: 21 vs. 41) and the area of the mammary fat pad occupied by glands (mean: 10 vs. 20%). There was elongation and thickening of the ducts which accounted for the increased area that was occupied by ductal structures. There was no significant increase in the number of ducts. However, there was the appearance of a large number of buds along the length of the ductal structures, suggesting the beginning of side branching. These results suggest that IGF-I, when given along with E2, is responsible for ductal morphogenesis.

Role of IGF-I in normal mammary development

Growth hormone (GH) is now believed to be the pituitary factor that is responsible for mammary ductal morphogenesis. Mammary development at puberty occurs because of synergy between GH and estrogen on formation of terminal end buds (TEBs). TEBs extend into the substance of the mammary gland fat pad, resulting in ductal morphogenesis. Ultimately, the whole mammary fat pad accommodates a complex network of ducts. IGF-I or des(1-3) IGF-I mimic the actions of GH on TEB formation in hypophysectomized, gonadectomized rats. Since GH stimulates IGF-I mRNA within the mammary gland synergistically, we hypothesize that IGF-I partially mediates actions of GH in mammary gland development. Studies in transgenic mice overexpressing IGF-I, des(1-3) IGF-I, or IGFBP-3 show that IGF-I causes ductal hypertrophy in the lactating mouse and prevention of post-lactational mammary gland involution. One of the mechanisms for this effect involves apoptosis. The potential role of GH or IGF-I in mammary carcinogenesis, and the applicability of animal studies to humans, are discussed.

Growth hormone treatment induces mammary gland hyperplasia in aging primates

The decline of growth hormone (GH) and insulin-like growth factor I (IGF-I) production during aging has been likened to the decrease in gonadal steroids in menopause. The repletion of GH/IGF-I levels in aging individuals is suggested to restore the lean tissue anabolism characteristic of youth. In addition to anabolic effects on musculo-skeletal tissues, GH also stimulates mammary glandular growth in some species, although its effects on primate mammary growth remain unclear. Some clinical observations implicate GH in human mammary growth, for example, gynecomastia occurs in some children treated with GH (ref. 6), and tall stature and acromegaly are associated with an increased incidence of breast cancer. To investigate the effects of GH/IGF-I augmentation on mammary tissue in a model relevant to aging humans, we treated aged female rhesus monkeys with GH, IGF-I, GH + IGF-I or saline diluent for 7 weeks. IGF-I treatment was associated with a twofold increase, GH with a three- to fourfold increase, and GH + IGF-I with a four'-to fivefold increase in mammary glandular size and epithelial proliferation index. These mitogenic effects were directly correlated with circulating GH and IGF-I levels, suggesting that either GH or its downstream effector IGF-I stimulates primate mammary epithelial proliferation.

Early mammary development: growth hormone and IGF-1

The first step in pubertal mammary development is the appearance of terminal end buds arising from pleuropotent stem cells present in the immature ductal tree of the prepubertal animal. Work from this laboratory indicates that growth hormone is the pituitary hormone responsible for terminal end bud development. Growth hormone likely acts through the production of IGF-1. This minireview focuses on the hormonal control of early mammary development with special emphasis on the roles of growth hormone and IGF-1.

Intact and amino-terminally shortened forms of insulin-like growth factor I induce mammary gland differentiation and development

Growth hormone (GH) plays a role in regulating growth and differentiation of immature glandular structures in the mammary gland, but the mechanisms by which the hormone exerts these effects are unknown. We have previously found that GH stimulates insulin-like growth factor I (IGF-I) I mRNA production within the mammary glands of hypophysectomized rats. In this study we set out to determine if IGF-I administration could mimic the action of GH in initiating mammary gland differentiation and development. Two forms of IGF-I, intact and amino-terminally shortened [des-(1-3)-IGF-I], were found to induce the development of terminal end buds and the formation of alveolar structures in the mammary glands of hypophysectomized, castrated, and estradiol-treated sexually immature male rats. The effect of both forms of IGF-I was similar to that obtained with human GH, but the truncated form was at least 5 times more potent than intact IGF-I. These findings suggest that the inductive effect of GH on glandular differentiation is mediated by the GH-induced production of IGF-I or a related molecule within the mammary gland itself.

Massive breast enlargement in an infant girl with central nervous system dysfunction

A 6-month-old female is described who presented with severe idiopathic macromastia. The breast enlargement began at 2 months of age and progressed such that subtotal mastectomies were necessary at 23 months. Extensive hormonal evaluation prior to surgery revealed no evidence of estrogenization or precocious puberty. There was no galactorrhea. A breast biopsy showed immature mammary tissue. In vitro analysis of the patient's serum using a mouse mammary thymidine incorporation assay revealed similar mitogenic activity in the patient's serum compared to adult controls. Post surgical follow up of this patient, 3.5 years later, has revealed no breast enlargement, precocious sexual development, or growth acceleration. Of interest, however, she has manifested an idiopathic degenerative neurologic condition characterized by psychomotor delay, ataxia, and seizures. Remarkably, hormone studies at age 5.5 years showed an exaggerated gonadotrophin response to intravenous gonadotrophin releasing hormone and prepubertal estrogen levels. While this case may represent an extraordinary example of idiopathic premature thelarche, the severe nature of this infant's macromastia in association with neurologic dysfunction and increased gonadotrophins suggests that central nervous system factors were etiologic.

Endocrine pathology of estrogens: species differences

The clinical uses of estrogens are associated with serious adverse effects, so the experimental toxicology of these compounds is under continuous review. Structurally different estrogens have qualitatively similar effects in animals when given in amounts way above the rodent uterotrophic dose. Toxicity still tends, however, to be related to estrogenic potency. Carnivores are more susceptible than rodents. Changes in reproductive, mammary and endocrine tissues are consistent with hyperestrogenism. Growth rate is decreased in rats and mice, but weight gains have been reported in other species. The weights of the liver, spleen, thymus and other organs are changed. Liver damage can occur. Susceptibility declines in the order cat, ferret, rat and mouse, dog. Clotting changes seen in the rat are secondary to liver damage. Moderate doses elicit anemia in rats, but lethal bone marrow depression in dogs and ferrets. Death is associated with hemorrhage. Antiestrogens modify aspects of estrogen toxicity in the rat, but not in the ferret. The predictive value of animal studies for humans has been disappointing. Interspecies variations at the hypothalamic-pituitary axis appear to have an important bearing on the differential activities of estrogens and antiestrogens across the species.

Differential responses of pituitary kallikrein and prolactin to tamoxifen and chlorotrianisene

Glandular kallikrein, a trypsin-like serine protease, and prolactin (PRL) are both estrogen-induced proteins in rat anterior pituitary lactotrophs. The estrogen agonist and antagonist effects of tamoxifen (TAM, a triphenylethylene antiestrogen) and chlorotrianisene (TACE, a triphenylethylene estrogen) on anterior pituitary glandular kallikrein and PRL were examined to see if TAM and TACE differentially affect these estrogen response of lactotrophs after in vivo dosing of rats. TAM and TACE acted as partial agonists on PRL and uterine weight induction. In contrast, on glandular kallikrein induction TAM acted as a pure estrogen antagonist and TACE acted as an almost pure antagonist. The results document that both TAM and TACE exhibit protein-specific estrogen agonist and antagonist efficacies in lactotrophs, with the estrogen induction of glandular kallikrein being particularly sensitive to antagonism by TAM in vivo. The marked antiestrogen character of TACE was surprising since TACE has been classified and clinically used as an estrogen.

Evidence for a nonprolactin, non-growth-hormone mammary mitogen in the human pituitary gland

To determine whether the human pituitary contains a previously unidentified, nonprolactin (non-hPRL), non-growth-hormone (non-hGH) factor capable of stimulating mammary development, we tested the effects of whole human pituitary extract (hPE) and pituitary extracts depleted of hPRL and hGH ("stripped hPE") in hypophysectomized, castrated estradiol (E2)-treated male rats and rhesus monkeys. Both whole and stripped hPE significantly stimulated rat mammary development (mean scores = 3.3 and 2.0, respectively, on a scale ranging from 0 to 4) in comparison with controls (mean score = 1.0). Mammary development was not due to minute concentrations of hGH or hPRL remaining in stripped hPE because 30- to 100-fold higher concentrations of hGH (Genentech) and 1000-fold higher concentrations of hPRL were required to stimulate significant mammary development. Non-pituitary extracts of human ovary, muscle, and serum, and bovine serum albumin did not stimulate rat mammary gland growth. Trypsin destroyed the mammary mitogenic activity of whole hPE, indicating that the unidentified factor is likely a protein. Mammary growth and development were also stimulated in hypophysectomized, E2-treated monkeys by stripped hPE (mean histological score = 3.25 vs. 1.35 in control animals). Monkeys receiving stripped hPE had undetectable levels of hGH and hPRL in serum sampled over a 24-hr period. These findings suggest that the human pituitary contains a non-hPRL, non-hGH factor that stimulates mammary growth and may be important in normal mammary growth and development and perhaps in breast cancer.