EGF receptor regulation in normal mouse mammary gland

Testosterone inhibits estrogen/progestogen-induced breast cell proliferation in postmenopausal women

OBJECTIVE

During the past few years serious concern has been raised about the safety of combined estrogen/progestogen hormone therapy, in particular about its effects on the breast. Several observations suggest that androgens may counteract the proliferative effects of estrogen and progestogen in the mammary gland. Thus, we aimed to study the effects of testosterone addition on breast cell proliferation during postmenopausal estrogen/progestogen therapy.

DESIGN

We conducted a 6-month prospective, randomized, double-blind, placebo-controlled study. A total of 99 postmenopausal women were given continuous combined estradiol 2 mg/norethisterone acetate 1 mg and were equally randomly assigned to receive additional treatment with either a testosterone patch releasing 300 microg/24 hours or a placebo patch. Breast cells were collected by fine needle aspiration biopsy at baseline and after 6 months, and the main outcome measure was the percentage of proliferating breast cells positively stained by the Ki-67/MIB-1 antibody.

RESULTS

A total of 88 women, 47 receiving active treatment and 41 in the placebo group, completed the study. In the placebo group there was a more than fivefold increase (P<0.001) in total breast cell proliferation from baseline (median 1.1%) to 6 months (median 6.2%). During testosterone addition, no significant increase was recorded (1.6% vs 2.0%). The different effects of the two treatments were apparent in both epithelial and stromal cells.

CONCLUSIONS

Addition of testosterone may counteract breast cell proliferation as induced by estrogen/progestogen therapy in postmenopausal women.

Expression of nuclear hormone receptors, their coregulators and type I iodothyronine 5'-deiodinase gene in mammary tissue of nonlactating and postlactating rats

The aim of the study was to test the hypothesis that expression of retinoid receptors (RARalpha, RARbeta, RARgamma), rexinoid receptors (RXRalpha, RXRbeta), thyroid hormone receptors (TRalpha, TRbeta), estrogen receptors (ERalpha, ERbeta), nuclear receptor coregulators (N-CoR, SRC-1, SMRT), and in addition type I iodothyronine 5'-deiodinase (5'-DI), EGFR and erb-B2/neu would be different in mammary postlactating tissue in comparison with that of nonlactating mammary gland. Using RT-PCR, we have shown that expression of RARalpha, RXRalpha,TRalpha, ERalpha,ERbeta,N-CoR, SRC-1, SMRT and EGFR in rat was significantly increased in postlactating mammary gland when compared to that of nonlactating mammary tissue. Postlactating mammary glands were found to express all RAR and RXR subtypes studied when compared to nonlactating mammary tissues that express exclusively RARalpha and RXRalpha subtypes. Enhanced expression of a number of nuclear hormone receptors, their coregulators in mammary tissue of postlactating rats in comparison with nonlactating animals identify a potential role for retinoid, thyroid and estrogen signalling pathways also after lactation period.

Requirement for IGF-I in epidermal growth factor-mediated cell cycle progression of mammary epithelial cells

Induction of cyclin proteins is required for progression of cells through the G(1)-S and G(2)-M cell cycle checkpoints and is a primary mechanism by which mitogens regulate cell cycle progression. IGF-I and the epidermal growth factor (EGF)-related ligands are mitogens for mammary epithelial cells in vitro and are essential for growth of the mammary epithelium during development. We report here that IGF-I in combination with EGF or TGFalpha is synergistic in promoting DNA synthesis in mammary epithelial cells in the intact mammary gland cultured in vitro. We further investigated the role of IGF-I and EGF in cyclin expression and cell cycle progression in the mammary gland and demonstrate that IGF-I and EGF induce expression of early G(1) cyclins. However, we show that IGF-I, but not EGF, induces late G(1) and G(2) cyclins and is required for mammary epithelial cells to overcome the G(1)-S checkpoint. These data demonstrate that IGF-I is essential for cell cycle progression in mammary epithelial cells and that it is required for EGF-mediated progression past the G(1)-S checkpoint in these cells.

Hormone/growth factor interactions mediating epithelial/stromal communication in mammary gland development and carcinogenesis

Epithelial/mesenchymal interactions begin during embryonic development of the mammary gland and continue throughout mammary gland development into adult life. Stromal and epithelial growth factors that may mediate interactions between these compartments of the mammary gland are reviewed. Since mammogenic hormones are the primary regulators of mammary gland development, special consideration is given to hormonal regulation of growth factors in order to explore the integration of hormones and growth factors in the regulation of mammary gland growth and neoplasia. Examination of hormonal regulation of the fibroblast growth factor (FGF)-7/FGFR2-IIIb receptor system in the mammary gland reveals that mammogenic hormones differentially regulate the synthesis of stromal growth factors and their epithelial receptors. These effects serve to optimize the action of estrogen and progesterone on mammary gland development and illustrate that the ratio of these two hormones is critical in regulating this growth factor axis. The role of stromal/epithelial mitogenic microenvironments in modulating the genotype and phenotype of preneoplastic and neoplastic lesions by chemical carcinogens is discussed. Finally, changes in growth factor expression during mammary tumor progression are described to illustrate the relative roles that stromally-derived and epithelial-derived growth factors may play during progression to hormone independent tumor growth.

Fibronectin and the alpha(5)beta(1) integrin are under developmental and ovarian steroid regulation in the normal mouse mammary gland

Extracellular matrix (ECM) proteins have been shown to regulate mammary epithelial cell proliferation, differentiation, and apoptosis in vitro. However, little is known about the hormonal regulation and functional role of ECM proteins and integrins during mammary gland development in vivo. We examined the temporal and spatial localization and hormone regulation of collagen I, collagen IV, laminin, and fibronectin. Among these ECM proteins only fibronectin changed appreciably. Fibronectin levels increased 3-fold between the onset of puberty and sexual maturity, remaining high during pregnancy and lactation. This increase occurred specifically in the epithelial basement membrane. Fibronectin was decreased 70% by ovariectomy and increased 1.5- and 2-fold by estrogen or estrogen plus progesterone treatment, respectively. The fibronectin-specific integrin, alpha(5)beta(1), was localized in myoepithelial cells; it increased 2.2-fold between puberty and sexual maturity and decreased in late pregnancy and lactation. The basal localization of alpha(5)beta(1) was notably increased in pubertal and adult virgin mice. alpha(5)beta(1) concentrations decreased 40-50% after ovariectomy in pubertal and adult mice, which was reversed by estrogen plus progesterone treatment in adult mice. The high basal expression of alpha(5)beta(1) during active proliferation and the low expression in nonproliferating and lactating glands indicate that fibronectin signaling may be required for hormone-dependent proliferation in the mammary gland.

Induction of mammary carcinomas by N-methyl-N-nitrosourea in ovariectomized rats treated with epidermal growth factor

The importance of epidermal growth factor (EGF) in both normal and malignant mammary gland development are presented in these studies. Initial findings demonstrated that in the absence of ovarian hormones, EGF had a significant proliferative effect on mammary epithelial cells. To determine whether mammary epithelial cells grown with EGF, in the absence of ovarian hormones, could be transformed by N-methyl-N-nitrosourea (MNU), female ovariectomized Lewis rats were implanted with pellets containing EGF for 1 week and then treated with MNU for initiation. Two days after MNU treatment, ovaries were implanted and EGF pellets were removed from all ovariectomized groups in order to promote carcinogenesis. The mammary carcinoma incidence of the EGF-stimulated group (90%) was not significantly different from the intact group (100%). The mammary cancer morphology of EGF-treated carcinomas was either ductal carcinoma or cribriform adenocarcinoma, whereas intact animals developed mainly papillary and occasional cribriform carcinomas. Fifty-eight percent of the carcinomas from the EGF group were ovarian hormone-independent compared with 10% of carcinomas from the intact group. These results demonstrate that EGF-induced proliferation during initiation with MNU was sufficient to induce the transformation of mammary carcinomas in the absence of ovarian hormones. The hormonal dependency of these EGF-induced carcinomas were different compared with MNU-initiated mammary carcinomas in intact rats.

Regulation of mammary gland growth and morphogenesis by the mammary fat pad: a species comparison

The growth and morphogenesis of mammary parenchyma varies substantially between species and is regulated by an array of systemic and local factors. Central to this regulation is the mammary fat pad, a matrix of adipose and connective tissue capable of mediating hormone action and synthesizing an array of growth regulatory molecules. In this article we highlight differences between the morphological development of the mammary parenchyma in rodents, humans, and ruminant dairy animals, placing emphasis on differences in the cellular composition and structure of the mammary fat pad. While a great deal remains to be understood about the ability of stroma to locally regulate mammary development, the significance of its contribution is becoming increasingly apparent. The actions of several steroid and peptide hormones appear to be mediated by an array of growth factors, proteases and extracellular matrix components synthesized by constituents of the mammary fat pad. Further, mammary adipose tissue represents a significant store of lipid which, by itself and through its derivatives, could influence the growth of mammary epithelium in diverse ways. This review describes the integral role of the mammary fat pad during mammogenesis, emphasizing the point that species differences must be addressed if local growth and morphogenic mechanisms within the mammary gland are to be resolved.

Progesterone signaling and mammary gland morphogenesis

Progesterone was identified as a mammogenic hormone several years ago but until now its precise role in mammary development has remained obscure. Recently with the generation of several transgenic mouse models and development of reagents for analysis of progesterone receptor expression, the role of progesterone signaling in mammary development is becoming more clear. The most significant observations to emerge from these studies are (1) progesterone receptors (PR) are present in a heterogeneous manner in the epithelial cells and undetectable in the surrounding fat pad; (2) they are essential for lobuloalveolar and not for ductal morphogenesis; (3) progesterone signaling through progesterone receptors, leading to lobuloalveolar development, is initiated in the epithelium and may occur through paracrine mechanisms; and (4) a regulated expression of the two isoforms of progesterone receptor is critical for maintaining appropriate responsiveness to progesterone and hence, epithelial cell replicative homeostasis. These studies also reveal that the consequences of progesterone signaling through progesterone receptor may depend on the cell context, cell-cell and cell-extracellular matrix interactions, the dynamics of PR turnover and the fate of PR positive cells.

Hormonal regulation of epidermal growth factor receptor content and signaling in bovine mammary tissue

Mammary tissue from midpregnant heifers was cultured with epidermal growth factor (EGF) or transforming growth factor alpha for 1-3 days. After 1 day, 10 nM EGF or transforming growth factor alpha doubled DNA synthesis, whereas lower concentrations (0.1 or 1 nM) increased DNA synthesis 2- to 3-fold after 2-3 days in culture. In other studies, bovine mammary tissue was transplanted to ovariectomized athymic mice and treated for 10 days with saline, estradiol (1 microg/day), progesterone (1 mg/day), or estradiol + progesterone. Subsequent explant culture of the bovine tissue indicated that estradiol + progesterone augmented the ability of EGF to stimulate DNA synthesis. The increased response to EGF was associated with increased EGF binding and with increased EGF-induced tyrosine kinase that paralleled the increased EGF binding. In other studies, athymic mice bearing xenografted bovine mammary tissue were primed for 10 days with estradiol and progesterone, followed by 2-day treatment with saline (control), hydrocortisone (200 microg/day), PRL (1 mg/day), or hydrocortisone + PRL. Hydrocortisone and PRL alone decreased, and PRL + hydrocortisone eliminated, EGF-induced DNA synthesis. EGF receptor content was unaffected by hydrocortisone but was reduced by PRL or hydrocortisone + PRL. Furthermore, the ability of EGF to induce tyrosine kinase activity was decreased by PRL and by hydrocortisone + PRL. The decreased kinase activity was greater than the decrease in receptor binding, suggesting a specific modulation of EGF receptor kinase activity in response to lactogenic hormones.

The role of mammary stroma in modulating the proliferative response to ovarian hormones in the normal mammary gland

Postnatal mammary gland development is highly dependent on the ovarian steroids, estrogen and progesterone. However, evidence from both in vitro and in vivo studies indicates that steroid-induced development occurs indirectly, requiring stromal cooperation in epithelial proliferation and morphogenesis. Stromal cells appear to influence epithelial cell behavior by secretion of growth factors and/or by altering the composition of the extracellular matrix in which epithelial cells reside. This review will discuss the requirement for stromal tissue in modulating proliferative responses to ovarian hormones during postnatal development and the potential role of the EGF, IGF, HGF and FGF3 growth factor families. Additionally, the roles of extracellular matrix proteins, including fibronectin, collagens and laminin, will be summarized.

Role of epidermal growth factor in the acquisition of ovarian steroid hormone responsiveness in the normal mouse mammary gland

The purpose of the present studies was to investigate the role of epidermal growth factor (EGF) in the acquisition of estrogen (E) and progestin (P) responsiveness in the mouse mammary gland in vivo. Using the Elvax 40P implant technique to introduce bioactive molecules directly into the mammary gland to produce a localized effect, we have made the novel observation that EGF implanted into glands of pubertal mice followed by E treatment resulted in the precocious acquisition of E-inducible progesterone receptors (PR). In sexually mature mice, EGF implants alone were able to increase PR. A neutralizing antibody specific for EGF blocked E-dependent stimulation of end-bud development and PR induction. Furthermore, the antiestrogen ICI 182,780 blocked the EGF-induced stimulation end-buds and PR induction, indicating that these EGF effects are mediated via estrogen receptors (ER). Immunohistochemical analysis showed that the endogenous EGF content of mammary glands of mature mice was higher than pubertal mice, that E implants caused a localized increase in mammary gland EGF content in both pubertal and mature mice, and that in mature mice E caused an increase in stromal cell EGF content. We have previously shown that the acquisition of E-inducible PR can be modulated by mammary stroma, and the present results indicate that mammary stroma could modulate hormonal responsiveness through control of local growth factor concentration. Taken together, these results provide evidence that E-dependent responses of mouse mammary gland in vivo, such as end-bud proliferation and PR regulation, may be mediated by EGF through an ER-dependent mechanism.

Mammary gland growth and development from the postnatal period to postmenopause: ovarian steroid receptor ontogeny and regulation in the mouse

Ovarian steroid hormones play a critical role in regulating mammary gland growth and development. The mammary gland sequentially acquires and cyclically exhibits proliferative responses to estrogen and/or progesterone from birth to postmenopause. The focus of this review is to present our current understanding of estrogen and progesterone receptor distribution in epithelial and stromal cells and their functions in relation to mammary gland development. Insights gained from the study of the normal mammary gland are relevant to our understanding of the conditions which may predispose women to the development of breast cancer as well as to alterations in hormonal regulation that occur in breast cancer.

EGF-related peptides and their receptors in mammary gland development

The discovery of multiple EGF-like ligands and erbB receptors offers the potential for a highly diverse signaling system allowing specific ligand/receptor complexes to be created in response to a certain hormone(s) or stage of mammary development. The known erbB receptors and several of the erbB-related ligands are synthesized by the normal mammary gland and have different temporal and spatial expression patterns. For instance, cumulative findings support the concepts that the EGF receptor has an essential role in morphogenesis of the mammary gland and that activation of this receptor occurs in response to estradiol-stimulated synthesis of an EGF receptor ligand in mammary stromal cells. The importance of both epithelial and stromal mammary cells in the hormonal activation of erbB-related pathways is underscored in this review. Current experimental protocols that utilize erbB mutant mice or enable detection of phosphorylated erbB members and their proximal substrates should permit more precise identification of the pathways operative in the mammary gland.

Relationship between receptors for epidermal growth factor and steroid hormones in normal, dysplastic and neoplastic canine mammary tissues

The concentrations of receptors for epidermal growth factor (EGF-R), oestrogen (ER) and progesterone (PR) were measured in 108 samples from canine mammary tumours and 132 samples of normal mammary tissue removed surgically from 84 bitches. The history and clinical signs were also recorded. Binding sites of high affinity were detected in 70 per cent of both types of tissue and no significant variations in EGF-R concentrations or positivity were observed with the histology, location, size or number of mammary tumours or the age of the animal. A significant direct correlation (P = 0.002) was observed between the concentrations of ER and EGF-R only in malignant tumours. The concentrations of EGF-R were significantly correlated (P = 0.04) in normal mammary tissues adjacent to and distant from the lesions, but not between normal tissue and tumour tissue. No significant differences were observed in the expression of EGF-R in normal and neoplastic tissues from the same bitches. The direct correlation between the concentrations of EGF-R and ER in malignant tumours could be related to an oestrogen-dependent expression of EGF-R or to a similar pattern of regulation of the receptors.

Emergence of salivary gland cell lineage diversity suggests a role for androgen-independent epidermal growth factor receptor signaling

Diversity of cell lineages within glandular organs is generated postnatally by differentiation of committed progenitor cells. Fundamental regulatory aspects of this process are not understood. The mouse submandibular salivary gland (SSG) served as model to assess the role of epidermal growth factor (EGF) receptor signaling during emergence of cell lineage diversity. Temporal fluctuations in EGF receptor mRNA levels coincident with crucial differentiative cell lineage transitions were revealed by RNase protection analyses. Between days 2 and 5, when proacinar cells are maturing and striated duct cells emerge, EGF receptor mRNA levels were highest and all differentiating cells exhibited EGF receptor immunoreactivity. EGF receptor mRNA levels then declined sharply and immunoreactivity became confined to ductal cells. At day 11 in male mice, and days 11 and 16 in females, a second increase in EGF receptor mRNA was detected coincident with emergence of granular convoluted tubule (GCT) cells. With completion of androgen-dependent GCT cell differentiation at the onset of puberty, EGF receptor mRNA levels and intensity of immunoreactivity decreased. Androgen effects on EGF receptor mRNA or immunoreactivity could not be detected. These temporally distinct patterns of EGF receptor expression suggest that this signaling pathway is a mechanism of potential importance in emergence of cell lineage diversity in a glandular organ.

Prolactin and epidermal growth factor regulation of the proliferation, morphogenesis, and functional differentiation of normal rat mammary epithelial cells in three dimensional primary culture

The epithelial cell-specific effects of prolactin and epidermal growth factor (EGF) on the development of normal rat mammary epithelial cells (MEC) were evaluated using a three dimensional primary culture model developed in our laboratory. Non-milk-producing MEC were isolated as spherical end bud-like mammary epithelial organoids (MEO) from pubescent virgin female rats. The cultured MEO developed into elaborate multilobular and lobuloductal alveolar organoids composed of cytologically and functionally differentiated MEC. Prolactin (0.01-10 micrograms/ml) and EGF (1-100 ng/ml) were each required for induction of cell growth, extensive alveolar, as well as multilobular branching morphogenesis, and casein accumulation. MEO cultured without prolactin for 14 days remained sensitive to the mitogenic, morphogenic, and lactogenic effects of prolactin upon subsequent exposure. Similarly, cells cultured in the absence of EGF remained sensitive to the mitogenic and lactogenic effects of EGF, but were less responsive to its morphogenic effects when it was added on day 14 of a 21-day culture period. If exposure to prolactin was terminated after the first week, the magnitude of the mitogenic and lactogenic effects, but not the morphogenic response was decreased. Removal of EGF on day 7 also reduced the mitogenic response, but did not have any effect on the magnitude of the lactogenic or morphogenic responses. These studies demonstrate that physiologically relevant development of normal MEC can be induced in culture and that this model system can be used to study the mechanisms by which prolactin and EGF regulate the complex developmental pathways operative in the mammary gland.

Changes in oestrogen, progesterone and epidermal growth factor receptor concentrations and affinities during the oestrous cycle in the normal mammary gland and uterus of dogs

Changes in the concentrations and affinities of receptors for oestrogen (ER), progesterone (PR) and epidermal growth factor (EGF-R) were studied in mammary glands of healthy bitches with regard to age, the location in the mammary chain and the stage of the oestrous cycle. Uterus was used as the reference tissue for the evaluation of steroid receptors. Mammary and uterine samples from 7 healthy bitches were taken at five stages of the oestrous cycle in such a way that all the locations in the mammary chain were represented at each stage of the cycle (10 samples/dog). ER, PR and EGF-R were detected by biochemical assays using increasing concentrations of tritiated (steroids) or iodinated (EGF) ligands. A significant direct correlation was found between the ER and PR concentrations for mammary and uterine samples. No significant correlation was found between the steroid receptors and EGF-R concentrations. Mammary ER concentrations were significantly higher in bitches of 5 years of age or older than in younger ones; in posterior glands (4th and 5th pairs) than in anterior glands; and in the mid-luteal phase. Mammary PR did not vary significantly with age or location but was significantly lower in the early luteal phase than in other phases. A similar decrease in PR concentrations was observed in the uterus during the early luteal phase and uterine ER and PR concentrations were very low in the mid-luteal phase. Mammary EGF-R were not significantly higher in the early or mid-luteal phase than in pro-oestrus or anoestrus. The differences observed between the uterine and mammary steroid receptor concentrations during the oestrous cycle could be due to different mechanisms for regulating steroid receptor expression in the two tissues. Mammary EGF-R concentrations may be linked, as in other species, to cellular proliferation and/or to the serum progesterone concentrations.

The endocrine prevention of breast cancer

A clear explanation for the high incidence of breast cancer in modern women is now possible. The risk of breast cancer rises steeply from menarche until menopause. Associated with the reproductive process, the ovary, including the corpus luteum, produces substantial amounts of estrogen and progesterone, both of which induce growth of the breast epithelium. This sex-steroid-driven breast epithelial cell proliferation increases the risk of carcinogenesis by accelerating the occurrence of somatic genetic errors. Postmenopausally, as there is little cell proliferation, the breast epithelium is more "resistant" to mutagenic effects, and breast cancer risk rises at a low rate. Unfortunately, the genetic errors accumulated during the premenopausal period are not lost following menopause, and breast cancer risk remains high. Sex-steroid antagonists, such as tamoxifen, may reduce breast cancer incidence both by blocking breast epithelial cell proliferation and by direct antitumor effects on clinically occult breast cancers. The rationale for a contraceptive designed to reduce breast cell proliferation by decreasing premenopausal sex-steroid exposure is presented.

Role of mesenchymal-epithelial interactions in normal and abnormal development of the mammary gland and prostate

Development of the mammary gland (MG) and prostate occurs via mesenchymal-epithelia interactions. Epithelial MG buds are induced in ventral epidermis by mammary mesenchyme, which ultimately specifies the functional expression of the ability to produce milk. Mammary ductal branching is induced by embryonic mammary mesenchyme and is promoted by the mammary fat pad postnatally. These influences of connective tissue on the differentiation of mammary epithelium (ME) begin prenatally, but in adulthood, the connective tissue environment of adult ME profoundly influences epithelial growth, ductal branching, epithelial differentiation, and the ability of adult ME to produce milk. In a similar fashion, prostatic development occurs via mesenchymal-epithelial interactions in which urogenital sinus mesenchyme (UGM) induces epithelial morphogenesis, regulates epithelial proliferation, and evokes the expression of epithelial androgen receptors and prostate-specific secretory proteins. Although prostatic development is induced by androgens, androgenic effects on epithelial development are elicited via androgen receptors of UGM. As in MG, mesenchymal-epithelial interactions in the prostate begin during fetal periods, but continue into adulthood. The responsiveness of adult epithelial cells from various glands to stroma raises the possibility that carcinomas also may be regulated by connective tissue. Indeed, UGM can induce a rat prostatic carcinoma (Dunning tumor) to undergo striking changes in differentiation, which are accompanied by a reduction in growth rate and an apparent loss of tumorigenesis. Although the mechanism of mesenchymal-epithelial interactions remains unknown, the communication between the epithelium and stroma undoubtedly is multifactorial, involving the extracellular matrix, soluble growth or differentiation, and angiogenesis.

Breast cancer prevention through modulation of endogenous hormones

The use of exogenous sex-steroids for hormonal contraception is important to the way of life of many modern women. The widespread use of hormonal contraceptives represents a unique opportunity to have a substantial positive impact on women's health. The observation that users of oral combination type contraceptives have a reduced risk of ovarian cancer should encourage the extension of contraceptive development to address the most important malignancy facing modern women, breast cancer. Epidemiological evidence strongly suggests that both estrogens and progestogens contribute to breast cancer risk, and account for the steep rise in risk seen during the premenopausal years. Studies of normal breast epithelial cell proliferation confirm that progestogens are breast mitogens, and explain why current contraceptives, which are progestogen dominant, do not prevent breast cancer. A long-acting depot contraceptive can be developed which releases: 1) an agonist of gonadotropin releasing hormone to suppress ovarian function; and 2) sex-steroids at doses below those in current contraceptives, and below those associated with ovulation. Such a contraceptive should provide substantial life-time protection against both breast and ovarian cancer, and would retain many of the other health benefits of current contraceptives.

Effects of epidermal growth factor, estrogen, and progestin on DNA synthesis in mammary cells in vivo are determined by the developmental state of the gland

Estrogen (E), progesterone (P), and epidermal growth factor (EGF) are involved in the growth and development of the normal mammary gland. While studies have been carried out to investigate the in vivo effects of EGF in the immature mammary gland, nothing is known about the growth effects of EGF or its potential interactions with E and/or P in the adult mammary gland. The present studies were undertaken to investigate the effects of EGF, E, and P on mammary cell proliferation in immature, peripubertal vs. adult, sexually mature mice. We have found that EGF promotes epithelial and stromal cell proliferation in both the immature and adult mammary glands. In the immature gland, the end bud epithelium is most responsive to the proliferative effects of EGF and there is no apparent interaction between EGF, E, and/or P. In contrast, in the mature gland EGF adds to the proliferative effects of E+P in the ductal epithelium resulting in more extensive ductal sidebranching. Thus these results demonstrate that the developmental state of the mammary gland determines the nature and extent of the interactions between EGF, E, and P in growth and development.