Toward a digital analysis of environmental impacts on rodent mammary gland density during critical developmental windows

While mammographic breast density is associated with breast cancer risk in humans, there is no comparable surrogate risk measure in mouse and rat mammary glands following various environmental exposures. In the current study, mammary glands from mice and rats subjected to reproductive factors and exposures to environmental chemicals that have been shown to influence mammary gland development and/or susceptibility to mammary tumors were evaluated for histologic density by manual and automated digital methods. Digital histological density detected changes due to hormonal stimuli/reproductive factors (parity), dietary fat, and exposure to environmental chemicals, such as benzophenone-3 and a combination of perfluorooctanoic acid and zeranol. Thus, digital analysis of mammary gland density offers a high throughput method that can provide a highly reproducible means of comparing a measure of histological density across independent experiments, experimental systems, and laboratories. This methodology holds promise for the detection of environmental impacts on mammary gland structure in mice and rats that may be comparable to human breast density, thus potentially allowing comparisons between rodent models and human breast cancer studies.

C/EBPβ LIP and c-Jun synergize to regulate expression of the murine progesterone receptor

CCAAT/enhancer binding protein β (C/EBPβ) is required for murine mammary ductal morphogenesis and alveologenesis. Progesterone is critical for proliferation and alveologenesis in adult mammary glands, and there is a similar requirement for progesterone receptor isoform B (PRB) in alveologenesis. We examined C/EBPβ regulation of PR expression. All three C/EBPβ isoforms, including typically inhibitory LIP, transactivated the PR promoter. LIP, particularly, strongly synergized with c-Jun to drive PR transcription. Endogenous C/EBPβ and c-Jun stimulated a PR promoter-reporter and these two factors showed promoter occupancy on the endogenous PR gene. Additionally, LIP overexpression elevated endogenous PR protein expression. In pregnancy, both PRB and the relative abundance of LIP among C/EBPβ isoforms increase. Consistent with a role in PRB expression, in vivo C/EBPβ and PR isoform A expression showed mutually exclusive localization in mammary epithelium, while C/EBPβ and PRB largely co-localized. We suggest a critical role for C/EBPβ, particularly LIP, in PRB expression.

Pubertal and adult windows of susceptibility to a high animal fat diet in Trp53-null mammary tumorigenesis

Premenopausal breast cancer is associated with increased animal fat consumption among normal weight, but not overweight women (Farvid et al., 2014). Our previous findings in obesity-resistant BALB/c mice similarly showed promotion of carcinogen-induced mammary tumorigenesis by a diet high in saturated animal fat (HFD). This effect was specific to pubertal versus adult HFD. This study identifies the effects of HFD during puberty versus adulthood in Trp53-null transplant BALB/c mice and investigates its mechanism of enhancing tumorigenesis. Either pubertal or adult HFD is sufficient to increase incidence of Trp53-null mammary tumors. Puberty-restricted HFD exposure promoted tumor cell proliferation, increased angiogenesis, and increased recruitment of total and M2 macrophages in epithelial tumors. Adult-restricted exposure to HFD similarly increased proliferation, angiogenesis, recruitment of total and M2 macrophages, and additionally reduced apoptosis. Adult HFD also increased incidence of spindle cell carcinomas resembling claudin-low breast cancer, and thus adult HFD in the Trp53-null transplantation system may be a useful model for human claudin low breast cancer. Importantly, these results on Trp53-null and our prior studies on DMBA-induced mammary tumorigenesis demonstrate a pubertal window of susceptibility to the promotional effects of HFD, indicating the potential of early life dietary intervention to reduce breast cancer risk.

Pubertally Initiated High-Fat Diet Promotes Mammary Tumorigenesis in Obesity-Prone FVB Mice Similarly to Obesity-Resistant BALB/c Mice

Premenopausal breast cancer is associated with increased animal fat consumption among normal-weight but not overweight women. Our previous findings in obesity-resistant BALB/c mice showed that a diet high in saturated animal fat (HFD) promotes mammary tumorigenesis in both DMBA carcinogenesis and Trp53-null transplant models. Having made these observations in BALB/c mice, which have very modest HFD weight gain, we determined the effects of HFD in FVB mice, which gain significant weight on HFD. Three-week-old FVB mice fed a low-fat diet or HFD were subjected to 7,12-dimethylbenz[a]anthracene-induced carcinogenesis. Like BALB/c mice, HFD promoted mammary tumorigenesis. Development of tumors largely occurred prior to mice becoming obese, indicating the role of animal-derived HFD rather than resulting obesity in tumor promotion. Also similar to BALB/c mice, early-occurring adenosquamous mammary tumors were abundant among HFD-fed FVB mice. Tumors from HFD mice also had increased intra-tumor M2 macrophages. Prior to tumor development, HFD accelerated normal mammary gland development and increased mammary M2 macrophages, similarly to BALB/c mice. The promotional effects of puberty-initiated HFD on carcinogen-induced mammary cancer are thus largely weight gain-independent. Like BALB/c mice, HFD promoted adenosquamous tumors, suggesting a role for early age HFD in promoting this subtype of triple negative mammary cancer. M2 macrophage recruitment was common to both mouse strains. We speculate that a similar effect of HFD on immune function may contribute to epidemiological findings of increased breast cancer risk in young, premenopausal, normal-weight women who consume a diet high in saturated animal fat.

Amphiregulin as a Novel Serum Marker of Puberty in Girls

STUDY OBJECTIVE

Amphiregulin is a member of the epidermal growth factor family. In breast tissue, amphiregulin is a mediator of estrogen and progesterone signaling. The objectives were to examine the relationship of amphiregulin levels during peripuberty with estrogen levels. DESIGN, SETTING, PARTICIPANTS, INTERVENTIONS, AND MAIN OUTCOME MEASURES: The participants in this analysis were a subset from a longitudinal study of pubertal maturation, the Breast Cancer and the Environment Research Program. They were recruited between ages 3 and 7 years. Blood specimens were selected for hormone analysis between 24 months before and 6 months after breast development. Serum amphiregulin levels were analyzed using enzyme-linked immunosorbent assay.

RESULTS

Amphiregulin levels were measured in 188 girls; 8.5% had a maternal history of breast cancer, and 30.9% of samples were below the limit of detection. Amphiregulin levels were greatest at 18 months before the onset of breast development (P < .006), and the rise in estrone levels between -24 and -18 months was correlated with the increase in amphiregulin levels in the same time period (P = .0002). After adjustment for time relative to breast development, amphiregulin levels were associated with maternal breast cancer (P = .024). Tracking of amphiregulin levels was highly significant (P < .0001) within a given individual.

CONCLUSION

Amphiregulin levels peaked at 18 months before the onset of breast development, were temporally related to the rise in serum estrone, and were significantly associated with maternal history of breast cancer. Elevated amphiregulin levels at puberty might be a predictor of increased breast cancer risk.

Puberty-specific promotion of mammary tumorigenesis by a high animal fat diet

INTRODUCTION

Increased animal fat consumption is associated with increased premenopausal breast cancer risk in normal weight, but not overweight, women. This agrees with our previous findings in obesity-resistant BALB/c mice, in which exposure to a high saturated animal fat diet (HFD) from peripuberty through adulthood promoted mammary tumorigenesis. Epidemiologic and animal studies support the importance of puberty as a life stage when diet and environmental exposures affect adult breast cancer risk. In this study, we identified the effects of peripubertal exposure to HFD and investigated its mechanism of enhancing tumorigenesis.

METHODS

Three-week-old BALB/c mice fed a low-fat diet (LFD) or HFD were subjected to 7,12-dimethylbenz[a]anthracene (DMBA)-induced carcinogenesis. At 9 weeks of age, half the mice on LFD were switched to HFD (LFD-HFD group) and half the mice on HFD were switched to LFD (HFD-LFD group). Tumor gene expression was evaluated in association with diet and tumor latency.

RESULTS

The peripubertal HFD reduced the latency of DMBA-induced mammary tumors and was associated with tumor characteristics similar to those in mice fed a continuous HFD. Notably, short-latency tumors in both groups shared gene expression characteristics and were more likely to have adenosquamous histology. Both HFD-LFD and continuous HFD tumors showed similar gene expression patterns and early latency. Adult switch from HFD to LFD did not reverse peripubertal HFD tumor promotion. Increased proliferation, hyperplasia, and macrophages were present in mammary glands before tumor development, implicating these as possible effectors of tumor promotion. Despite a significant interaction between pubertal diet and carcinogens in tumor promotion, peripubertal HFD by itself produced persistent macrophage recruitment to mammary glands.

CONCLUSIONS

In obesity-resistant mice, peripubertal HFD is sufficient to irreversibly promote carcinogen-induced tumorigenesis. Increased macrophage recruitment is likely a contributing factor. These results underscore the importance of early life exposures to increased adult cancer risk and are consistent with findings that an HFD in normal weight premenopausal women leads to increased breast cancer risk. Notably, short-latency tumors occurring after peripubertal HFD had characteristics similar to human basal-like breast cancers that predominantly develop in younger women.

Pubertal high fat diet: effects on mammary cancer development

INTRODUCTION

Epidemiological studies linking dietary fat intake and obesity to breast cancer risk have produced inconsistent results. This may be due to the difficulty of dissociating fat intake from obesity, and/or the lack of defined periods of exposure in these studies. The pubertal mammary gland is highly sensitive to cancer-causing agents. We assessed how high fat diet (HFD) affects inflammation, proliferative, and developmental events in the pubertal gland, since dysregulation of these can promote mammary tumorigenesis. To test the effect of HFD initiated during puberty on tumorigenesis, we utilized BALB/c mice, for which HFD neither induces obesity nor metabolic syndrome, allowing dissociation of HFD effects from other conditions associated with HFD.

METHODS

Pubertal BALB/c mice were fed a low fat diet (12% kcal fat) or a HFD (60% kcal fat), and subjected to carcinogen 7,12-dimethylbenz[a]anthracene (DMBA)-induced tumorigenesis.

RESULTS

HFD elevated mammary gland expression of inflammatory and growth factor genes at 3 and 4 weeks of diet. Receptor activator of nuclear factor kappa-B ligand (RANKL), robustly induced at 4 weeks, has direct mitogenic activity in mammary epithelial cells and, as a potent inducer of NF-κB activity, may induce inflammatory genes. Three weeks of HFD induced a transient influx of eosinophils into the mammary gland, consistent with elevated inflammatory factors. At 10 weeks, prior to the appearance of palpable tumors, there were increased numbers of abnormal mammary epithelial lesions, enhanced cellular proliferation, increased growth factors, chemokines associated with immune-suppressive regulatory T cells, increased vascularization, and elevated M2 macrophages. HFD dramatically reduced tumor latency. Early developing tumors were more proliferative and were associated with increased levels of tumor-related growth factors, including increased plasma levels of HGF in tumor-bearing animals. Early HFD tumors also had increased vascularization, and more intra-tumor and stromal M2 macrophages.

CONCLUSIONS

Taken together in this non-obesogenic context, HFD promotion of inflammatory processes, as well as local and systemically increased growth factor expression, are likely responsible for the enhanced tumorigenesis. It is noteworthy that although DMBA mutagenesis is virtually random in its targeting of genes in tumorigenesis, the short latency tumors arising in animals on HFD showed a unique gene expression profile, highlighting the potent overarching influence of HFD.

Epidermal growth factor receptor (EGFR) signaling is a key mediator of hormone-induced leukocyte infiltration in the pubertal female mammary gland

It is well documented that macrophages and eosinophils play important roles in normal murine pubertal mammary gland development. Although it is accepted that estrogen (E) and progesterone (P) are key players in mammary gland development, the roles these hormones might play in regulating the actions of leukocytes in that process is an understudied area. We show here that P and E, respectively, induce unique, but overlapping, sets of proinflammatory and angiogenic cytokines and chemokines, in the pubertal female BALB/c mammary gland, as well as induce infiltration of macrophages and eosinophils to the mammary periepithelium. This extends earlier studies showing P induction of proinflammatory products in pubertal and adult mammary epithelial organoids and P-induced in vivo infiltration of leukocytes to the adult mammary periepithelium. Importantly, epidermal growth factor receptor-signaling, which is likely mediated by amphiregulin (Areg), a downstream mediator of E and P, is both necessary and sufficient for both E- and P-induced recruitment of macrophages and eosinophils to the pubertal mammary periepithelium. We further show that receptor activator of nuclear factor κB ligand (RANKL), although not sufficient of itself to cause macrophage and eosinophil recruitment, contributes to an optimal response to P. The potency of Areg is highlighted by the fact that it is sufficient to induce macrophage and eosinophil recruitment at levels equivalent to that induced by either E or P. Our finding of a dominant role for Areg in hormonally induced leukocyte recruitment to the pubertal mammary gland parallels its dominance in regulating ductal outgrowth and its role in P-induced proliferation in the pubertal gland.

Progesterone stimulates proliferation and promotes cytoplasmic localization of the cell cycle inhibitor p27 in steroid receptor positive breast cancers

Progestins are reported to increase the risk of more aggressive estrogen receptor positive, progesterone receptor positive (ER+ PR+) breast cancers in postmenopausal women. Using an in vivo rat model of ER+ PR + mammary cancer, we show that tumors arising in the presence of estrogen and progesterone exhibit increased proliferation and decreased nuclear expression of the cell cycle inhibitor p27 compared with tumors growing in the presence of estrogen alone. In human T47D breast cancer cells, progestin increased proliferation and decreased nuclear p27 expression. The decrease of nuclear p27 protein was dependent on activation of Src and PI3K by progesterone receptor isoforms PRA or PRB. Importantly, increased proliferation and decreased nuclear p27 expression were observed in invasive breast carcinoma compared with carcinoma in situ. These results suggest that progesterone specifically regulates intracellular localization of p27 protein and proliferation. Therefore, progesterone-activated pathways can provide useful therapeutic targets for treatment of more aggressive ER+ PR+ breast cancers.

Amphiregulin mediates progesterone-induced mammary ductal development during puberty

Introduction

Puberty is a period of increased susceptibility to factors that cause increased breast cancer risk in adulthood. Mammary end buds (EBs) that develop during puberty are believed to be the targets of breast cancer initiation. Whereas the role of estrogen (E) has been extensively studied in pubertal mammary gland development, the role of progesterone (P) during puberty is less defined.

Methods

Pubertal and prepubertal ovariectomized mice were treated with vehicle control (C), E, P, or E+P. Mammary glands from these mice were analyzed for changes in morphology, proliferation, and expression of the downstream targets amphiregulin (AREG) and receptor activator of NF-κB ligand (RANKL).

Results

P, acting specifically through the progesterone receptor, induced increases in mammary gland proliferation and EB formation that were associated with increased AREG expression in ducts and EBs. E, acting specifically through the estrogen receptor, produced similar responses also mediated by AREG. Blocking AREG action by treatment with an EGFR inhibitor completely abrogated the effect of P on EB formation and proliferation and significantly reduced proliferation within ducts. P also increased expression of RANKL, primarily in ducts. Treatment with RANK-Fc, an inhibitor of RANKL, reduced P-dependent proliferation in ducts and to a lesser extent in EB, but did not cause EB regression.

Conclusions

These results demonstrate a novel P-specific effect through AREG to cause EB formation and proliferation in the developing mammary gland both before and during puberty. Thus, hormones and/or factors in addition to E that upregulate AREG can promote mammary gland development and have the potential to affect breast cancer risk associated with pubertal mammary gland development.

A potential role of progestin-induced laminin-5/α6-integrin signaling in the formation of side branches in the mammary gland

Mammary organoids from adult mice produce tubules, analogous to mammary ducts in vivo, in response to hepatocyte growth factor (HGF) when cultured in collagen gels. The combination of HGF plus progestin (R5020) causes reduced tubule number and length. We hypothesized that the inhibitory effect on tubulogenesis was due to progestin-mediated alteration of HGF/c-Met signaling. Using molecular inhibitors and short hairpin RNA, it was determined that HGF activation of Ras-related C3 botulinum toxin substrate (Rac1) was required for the formation of cytoplasmic extensions, the first step of tubulogenesis, and that Rac1 activity was Src kinase (Src) and focal adhesion kinase (FAK) dependent. The highly novel finding was that R5020 reduced tubulogenesis by up-regulating and increasing extracellular laminin and α6-integrin ligation to reduce activation of the Src, focal adhesion kinase, and Rac1 pathway. Receptor activator of nuclear factor-κB ligand, another progesterone-induced paracrine factor, did not replicate this effect of R5020. The inhibitory effect of R5020 on tubulogenesis was likely mediated through progesterone receptor (PR) isoform A (PRA), because PRA is the predominant PR isoform expressed in the organoids, and the progestin-induced effect was prevented by the PR antagonist RU486. These results provide a plausible mechanism that explains progestin/PRA-mediated blunting of HGF-induced tubulogenesis in vitro and is proposed to be relevant to progesterone/PRA-induced side-branching in vivo during pregnancy.

Perfluorooctanoic acid effects on ovaries mediate its inhibition of peripubertal mammary gland development in Balb/c and C57Bl/6 mice

Exposure to perfluorooctanoic acid (PFOA), a synthetic perfluorinated compound and an agonist of peroxisome proliferator-activated receptor α (PPARα), causes stunted mouse mammary gland development in various developmental stages. However, the underlying mechanisms remain poorly understood. We found that peripubertal PFOA exposure significantly inhibited mammary gland growth in both Balb/c and C57Bl/6 wild type mice, but not in C57Bl/6 PPARα knockout mice, and Balb/c mice were more sensitive to PFOA inhibition. PFOA caused (1) delayed or absence of vaginal opening and lack of estrous cycling during the experimental period; (2) decreases in ovarian steroid hormonal synthetic enzyme levels; and (3) reduced expression of estrogen- or progesterone-induced mammary growth factors. Supplementation with exogenous estrogen and/or progesterone reversed the PFOA inhibitory effect on mammary gland. These results indicate that PFOA effects on ovaries mediate its inhibition of mammary gland development in Balb/c and C57Bl/6 mice and that PPARα expression is a contributing factor.

Tip30 deletion in MMTV-Neu mice leads to enhanced EGFR signaling and development of estrogen receptor-positive and progesterone receptor-negative mammary tumors

Estrogen receptor-positive and progesterone receptor-negative (ER+/PR-) breast cancers account for 15% to 25% of all human breast cancers and display more aggressive malignant characteristics than ER+/PR+ cancers. However, the molecular mechanism underlying development of ER+/PR- breast cancers still remains elusive. We show here that Tip30 deletion dramatically accelerated the onset of mammary tumors in the MMTV-Neu mouse model of breast cancer. The mammary tumors arising in Tip30(-/-)/MMTV-Neu mice were exclusively ER+/PR-. The growth of these ER+/PR- tumors depends not only on estrogen but also on progesterone despite the absence of detectable PR. Tip30 is predominantly expressed in ER+ mammary epithelial cells, and its deletion leads to an increase in the number of phospho-ERα-positive cells in mammary glands and accelerated activation of Akt in MMTV-Neu mice. Moreover, we found that Tip30 regulates the EGFR pathway through controlling endocytic downregulation of EGFR protein level and signaling. Together, these findings suggest a novel mechanism in which loss of Tip30 cooperates with Neu activation to enhance the activation of Akt signaling, leading to the development of ER+/PR- mammary tumors.

Amphiregulin mediates estrogen, progesterone, and EGFR signaling in the normal rat mammary gland and in hormone-dependent rat mammary cancers

Both estrogen (E) and progesterone (P) are implicated in the etiology of human breast cancer. Defining their mechanisms of action, particularly in vivo, is relevant to the prevention and therapy of breast cancer. We investigated the molecular and cellular mechanisms of E and/or P-induced in vivo proliferation, in the normal rat mammary gland and in hormone-dependent rat mammary cancers which share many characteristics with the normal human breast and hormone-dependent breast cancers. We show that E+P treatment induced significantly greater proliferation in both the normal gland and mammary cancers compared to E alone. In both the normal gland and tumors, E+P-induced proliferation was mediated through the increased production of amphiregulin (Areg), an epidermal growth factor receptor (EGFR) ligand, and the activation of intracellular signaling pathways (Erk, Akt, JNK) downstream of EGFR that regulate proliferation. In vitro experiments using rat primary mammary organoids or T47D breast cancer cells confirmed that Areg and the synthetic progestin, R5020, synergize to promote cell proliferation through EGFR signaling. Iressa, an EGFR inhibitor, effectively blocked this proliferation. These results indicate that mediators of cross talk between E, P, and EGFR pathways may be considered as relevant molecular targets for the therapy of hormone-dependent breast cancers, especially in premenopausal women.

Signal transducer and activator of transcription 5a mediates mammary ductal branching and proliferation in the nulliparous mouse

Signal transducer and activator of transcription (Stat)5a is a critical regulator of mammary gland development. Previous studies have focused on Stat5a's role in the late pregnant and lactating gland, and although active Stat5a is detectable in mammary epithelial cells in virgin mice, little is known about its role during early mammary gland development. In this report, we compare mammary gland morphology in pubertal and adult nulliparous wild-type and Stat5a-/- mice. The Stat5a-null mammary glands exhibited defects in secondary and side branching, providing evidence that Stat5a regulates these processes. In addition, Stat5a-/- mammary glands displayed an attenuated proliferative response to pregnancy levels of estrogen plus progesterone (E+P), suggesting that it plays an important role in early pregnancy. Finally, we examined one potential mediator of Stat5a's effects, receptor activator of nuclear factor-kappaB ligand (RANKL). Stat5a-/- mammary glands were defective in inducing RANKL in response to E+P treatment. In addition, regulation of several reported RANKL targets, including inhibitor of DNA binding 2 (Id2), cyclin D1, and the cyclin-dependent kinase inhibitor p21(Waf1/Cip1), was altered in Stat5a-/- mammary cells, suggesting that one or more of these proteins mediate the effects of Stat5a in E+P-treated mammary epithelial cells.

Perfluorooctanoic acid effects on steroid hormone and growth factor levels mediate stimulation of peripubertal mammary gland development in C57BL/6 mice

Perfluorooctanoic acid (PFOA) is a synthetic, widely used perfluorinated carboxylic acid and a persistent environmental pollutant. It is an agonist of peroxisome proliferator-activated receptor alpha (PPARalpha). Studies have shown that PFOA causes hepatocellular hypertrophy, tumorigenesis, and developmental toxicity in rodents, and some of its toxicity depends on the expression of PPARalpha. Our recent study revealed a stimulatory effect of peripubertal PFOA treatment (5 mg/kg) on mammary gland development in C57Bl/6 mice. The present study was designed to examine the underlying mechanism(s). It was found that mammary gland stimulation by PFOA was similarly observed in PPARalpha knockout and wild-type C57Bl/6 mice. The presence of ovaries was required for PFOA treatment (5 mg/kg) to stimulate mammary gland development with significant increases in the levels of enzymes involved in steroid hormone synthesis in both PFOA-treated wild-type and PPARalpha knockout mouse ovaries. PFOA treatment significantly increased serum progesterone (P) levels in ovary-intact mice and also enhanced mouse mammary gland responses to exogenous estradiol (E), P, and E + P. In addition, PFOA treatment resulted in elevated mammary gland levels of epidermal growth factor receptor (EGFR), estrogen receptor alpha, amphiregulin (Areg, a ligand of EGFR), hepatocyte growth factor, cyclin D1, and proliferating cell nuclear antigen (PCNA) in both wild-type and PPARalpha knockout mouse mammary glands. These results indicate that PFOA stimulates mammary gland development in C57Bl/6 mice by promoting steroid hormone production in ovaries and increasing the levels of a number of growth factors in mammary glands, which is independent of the expression of PPARalpha.

The breast cancer and the environment research centers: transdisciplinary research on the role of the environment in breast cancer etiology

OBJECTIVES

We introduce and describe the Breast Cancer and the Environment Research Centers (BCERC), a research network with a transdisciplinary approach to elucidating the role of environmental factors in pubertal development as a window on breast cancer etiology. We describe the organization of four national centers integrated into the BCERC network.

DATA SOURCES

Investigators use a common conceptual framework based on multiple levels of biologic, behavioral, and social organization across the life span. The approach connects basic biologic studies with rodent models and tissue culture systems, a coordinated multicenter epidemiologic cohort study of prepubertal girls, and the integration of community members of breast cancer advocates as key members of the research team to comprise the network.

DATA EXTRACTION

Relevant literature is reviewed that describes current knowledge across levels of organization. Individual research questions and hypotheses in BCERC are driven by gaps in our knowledge that are presented at genetic, metabolic, cellular, individual, and environmental (physical and social) levels.

DATA SYNTHESIS

As data collection on the cohort, animal experiments, and analyses proceed, results will be synthesized through a transdisciplinary approach.

CONCLUSION

Center investigators are addressing a large number of specific research questions related to early pubertal onset, which is an established risk factor for breast cancer. BCERC research findings aimed at the primary prevention of breast cancer will be disseminated to the scientific community and to the public by breast cancer advocates, who have been integral members of the research process from its inception.

Progesterone receptor A-regulated gene expression in mammary organoid cultures

Progesterone, through the progesterone receptor (PR), promotes development of the normal mammary gland and is implicated in the etiology of breast cancer. We identified PRA-regulated genes by microarray analysis of cultured epithelial organoids derived from pubertal and adult mouse mammary glands, developmental stages with differing progesterone responsiveness. Microarray analysis showed significant progestin (R5020)-regulation of 162 genes in pubertal organoids and 104 genes in adult organoids, with 68 genes regulated at both developmental stages. Greater induction of receptor activator of NFkappaB ligand and calcitonin expression was observed in adult organoids, suggesting possible roles in the differential progesterone responsiveness of the adult and pubertal mammary glands. Analysis of the R5020-responsive transcriptome revealed several enriched biological processes including cell adhesion, immune response, and survival. R5020 both induced Agtr1 and potentiated angiotensin II-stimulated proliferation, highlighting the functional significance of the latter process. Striking up-regulation of genes involved in innate immunity processes included the leukocyte chemoattractants serum amyloid A1, 2 and 3 (Saa1, 2, 3). In vivo analysis revealed that progesterone treatment increased SAA1 protein expression and leukocyte density in mammary gland regions undergoing epithelial expansion. These studies reveal novel targets of PRA in mammary epithelial cells and novel linkages of progesterone action during mammary gland development.

Strain-specific differences in the mechanisms of progesterone regulation of murine mammary gland development

Progesterone (P) is required for normal mammary gland development, and is implicated in the etiology of mammary cancer in rodents and humans. We analyzed mammary gland developmental responses to P and estrogen (E) in two strains of mice (BALB/c and C57BL/6) that exhibit differences in ductal development at sexual maturity and alveologenesis during pregnancy. C57BL/6 mice exhibited reduced proliferative and morphological responses to P. Analysis of known mediators of sidebranching and alveologenesis revealed that reduced P-induced expression of P receptor isoform B and receptor activator of nuclear factor-kappaB ligand (RANKL), as well as altered expression and regulation of cyclin D1, CCAAT/enhancer binding protein beta, and the downstream effectors of RANKL, nuclear Id2 and p21, contribute significantly to the reduced P responsiveness of the C57BL/6 mammary gland. In contrast, E responsiveness was greater in C57BL/6 than in BALB/c glands. E may play a compensatory role in C57BL/6 alveologenesis through its effect on the induction and activation of signal transducer and activator of transcription 5a, a known regulator of RANKL. These observations suggest that in human populations with heterogeneous genetic backgrounds, individuals may respond differentially to the same hormone. Thus, genetic diversity may have a role in determining the effects of P in normal mammary development and tumorigenesis.

Differential effects of peripubertal exposure to perfluorooctanoic acid on mammary gland development in C57Bl/6 and Balb/c mouse strains

Perfluorooctanoic acid (PFOA), a common and persistent industrial byproduct detected in human sera, has raised health concerns. PFOA is detrimental to lactational function and postnatal mammary gland development in CD-1 mice after gestational exposure. We have examined the peripubertal period (21 through 50 days of age) as an important window of mammary gland susceptibility to environmental exposures that may affect breast cancer risk later in life. The effects of PFOA (0.1-10mg/kg BW) were examined in Balb/c and C57BL/6 mice. PFOA treatment caused hepatocellular hypertrophy and delayed vaginal opening in both mouse strains. While Balb/c mice exhibited only inhibition of mammary gland and uterine development (5, 10mg/kg), C57BL/6 mice exhibited stimulatory effects in both organs at low dose (5mg/kg) and inhibition at higher dose (10mg/kg). This underscores the need for caution when drawing conclusions about the effects of PFOA and possibly other environmental pollutants on the basis of studies in a single mouse strain.

Progestin-regulated luminal cell and myoepithelial cell-specific responses in mammary organoid culture

Normal mammary gland development requires the coordinated proliferation and morphogenesis of both mammary luminal epithelial cells (LECs) and myoepithelial cells (MECs). Cell proliferation in cultured mammary organoids containing both LECs and MECs is not increased by progestin (R5020) or 17beta-estradiol (E2) alone or R5020+E2 but is increased by E2-regulated, mammary stroma-derived Hepatocyte growth factor (HGF) and further increased by HGF+R5020. We investigated the effects of HGF and/or R5020 on morphology and LEC- and MEC-specific in vitro proliferation in organoids. HGF-induced tubulogenesis was initiated and carried out by LECs starting with cellular extensions, followed by the formation of chains and cords, and culminating in tubule formation. MECs did not appear to have an active role in this process. Whereas HGF by itself caused maximal proliferation of LECs, HGF+R5020 produced a synergistic and specific increase in MEC proliferation. Because only LECs expressed progesterone receptors (PRs), we investigated the role of receptor activator of nuclear factor-kappaB ligand (RANKL), a progestin-induced paracrine factor, in mediating increased MEC proliferation. Quantitative RT-PCR showed that RANKL mRNA was induced by R5020 or HGF+R5020 and RANKL protein colocalized with PRs in LECs. The increased proliferation of MECs in response to HGF+R5020 could be blocked by neutralizing antibody to RANKL and reproduced by treatment with HGF plus exogenous RANKL in place of R5020. Neither R5020, nor exogenously administered RANKL increased proliferation of LECs. These results led us to conclude that RANKL, induced by progestin in PR-positive cells, is secreted and interacts with HGF to specifically increase proliferation of PR-negative MECs.

Progesterone receptor isoform functions in normal breast development and breast cancer

Progesterone acting through two isoforms of the progesterone receptor (PR), PRA and PRB, regulates proliferation and differentiation in the normal mammary gland in mouse, rat, and human. Progesterone and PR have also been implicated in the etiology and pathogenesis of human breast cancer. The focus of this review is recent advances in understanding the role of the PR isoform-specific functions in the normal breast and in breast cancer. Also discussed is information obtained from rodent studies and their relevance to our understanding of the role of progestins in breast cancer etiology.

Estrogen and progesterone are critical regulators of Stat5a expression in the mouse mammary gland

Signal transducer and activator of transcription (Stat)5a is a well-established regulator of mammary gland development. Several pathways for activating Stat5a have been identified, but little is known about the mechanisms that regulate its expression in this tissue. In this report, we used immunofluorescent staining to examine Stat5a expression in mammary epithelial cells during normal development and in response to treatment with the ovarian hormones estrogen (E) and progesterone (P). Stat5a was present at very low levels in the prepubertal gland and was highly induced in a subset of luminal epithelial cells during puberty. The percentage of positive cells increased in adult virgin, pregnant, and lactating animals, dropped dramatically during involution, and then increased again after weaning. Ovariectomy ablated Stat5a expression in virgin animals, and treatment with both E and P was necessary to restore it. Double-labeling experiments in animals treated with E plus P for 3 d demonstrated that Stat5a was localized exclusively to cells containing both E and P receptors. Together, these results identify a novel role for E and P in inducing Stat5a expression in the virgin mammary gland and suggest that these hormones act at the cellular level through their cognate receptors.

Extracellular matrix, Rac1 signaling, and estrogen-induced proliferation in MCF-7 breast cancer cells

Estrogen receptor positive (ER+), estrogen (E) responsive MCF-7 breast cancer cells cultured on the extracellular matrix (ECM) protein laminin (LM), exhibit significantly reduced E-induced proliferation compared with cells cultured on collagen I (Col I) that is not due to a loss of ER. Based on reported differences in integrin-activated pathways on Col I vs. LM, we investigated the potential role of Rac1/c-jun-N-terminal kinase (JNK) activation and downstream regulation of cyclin D1 by E on Col I vs.LM. E-induced proliferation was increased on LM in MCF-7 cells expressing constitutively active Rac1 (CA Rac1) and decreased in dominant negative Rac1-(DN Rac1) expressing cells on Col I. siRNA knockdown established the specificity and requirement for Rac1 activation for E-induced regulation of cyclin D1. More robust c-Jun activation occurred on Col I than on LM and E-induced proliferation was abolished after treatment with a JNK inhibitor. These results provide evidence that Rac1/JNK/c-Jun activation promotes E-induced proliferation on Col I and reduced Rac1/JNK/c-Jun activation on LM contributes significantly to reduced E-induced proliferation in MCF-7 cells on LM. These results identify a novel role for extracellular matrix (ECM)-integrin regulation of Rac1-JNK pathway in E-regulated proliferation in ER+ breast cancer cells. These findings suggest that tumor stromal environment, i.e., ECM composition, may contribute to loss of E regulation in ER+ breast cancers. Defining molecular markers for early identification of ER+ tumors that are ER+ but antiestrogen resistant would allow the design and use of alternative therapies to inhibit tumor growth and improve survival.

Progesterone receptor isoforms and proliferation in the rat mammary gland during development

Progesterone (P), acting through progesterone receptor (PR) isoforms A and B, plays an important role in normal mammary gland development and is implicated in the etiology of breast cancer. Because of significant similarities between human and rat mammary gland development and hormonal responsiveness of mammary cancers, we investigated P action in the rat mammary gland. By immunohistochemical methods we determined PRA and PRB expression at puberty, sexual maturity, pregnancy, and lactation and after postlactational involution and their functional roles in the regulation of proliferation. PRA expression was restricted to luminal epithelial cells, whereas PRB was expressed in both luminal and myoepithelial cells, indicating a novel role of PRB in myoepithelial cell regulation. The majority of PRA-positive (PRA+) cells coexpressed PRB. In the pubertal and adult virgin mammary gland, PRA+PRB+ cells also expressed nuclear cyclin D1 but did not contain the proliferation marker bromodeoxyuridine. Based on a lack of phosphorylated retinoblastoma protein expression and the expression patterns of the cyclin-dependent kinase inhibitors p21 and p27 in these cells, we conclude that PRA+PRB+ cells appear to be cell cycle arrested and do not proliferate. PRA+ cells were decreased in the adult gland and during and after pregnancy. The percentage of PRB+ cells was relatively constant throughout development, and in a significant proportion of cells, only PRB was detected. During development, and especially during pregnancy, a high percentage of PRB+ cells were positive for bromodeoxyuridine. From this observation, we conclude that these cells proliferate and that P acting through PRB may directly stimulate proliferation.

Differential hormonal regulation and function of progesterone receptor isoforms in normal adult mouse mammary gland

In normal mouse mammary gland, the mitogenic action of progesterone (P) is mediated by two P receptor (PR) isoforms, PRA and PRB. PRA is predominantly expressed in the adult virgin, and PRB is predominantly expressed during pregnancy. To investigate hormonal regulation of PR isoform expression and isoform-specific functions in vivo, adult ovariectomized BALB/c mice were treated for 3, 5, or 10 d with estrogen (E), P, or estrogen plus progesterone (E+P). Using an immunohistochemical approach with isoform-specific antibodies, we investigated hormonal regulation of PRA and PRB and their functional roles in proliferation and morphogenesis. Significant E-induced proliferation was only observed after 5 d at the distal tips of ducts; there was no sidebranching or alveologenesis. P induced proliferation that resulted in sidebranching and alveologenesis, but E+P treatment produced more proliferation sooner and more extensive sidebranching and alveologenesis. PRA levels were increased by E and decreased by P. Increased PRB levels were induced by treatment with P or E+P and coincided with the formation of alveoli. PRA was the predominant PR isoform expressed during sidebranching, and colocalization of PRA with 5-bromo-2'-deoxyuridine revealed that proliferation of PRA-positive and -negative cells was responsible for P-induced sidebranching. PRB was the predominant PR isoform expressed during alveologenesis, and colocalization of PRB with 5-bromo-2'-deoxyuridine showed that both PRB-positive and -negative cells proliferated during alveolar expansion. These results demonstrate different hormonal regulation of PRA and PRB levels in vivo and suggest that P can induce proliferation through either PRA or PRB via direct and paracrine mechanisms.

Experimental mouse model of hormonal therapy effects on the postmenopausal mammary gland

Menopausal hormone therapy, in particular estrogen plus progestin therapy, has been associated with increased breast cancer risk. In order to understand the basis for increased breast cancer risk, more information is needed about the effects of menopausal hormone therapies on the breast. In this review we describe studies carried out in a mouse model of early vs. late postmenopausal states. We investigated the effects of 1) estrogen alone, 2) combined continuous estrogen + progestin, 3) systemically vs. locally administered estrogen and progestin, and 4) the effect of pregnancy on the response to hormonal therapies. We analysed the effects on mammary gland morphology and proliferation. Estrogen therapy started in late postmenopause caused a greater proliferative response than when started in early postmenopause. In parous, late postmenopausal mice the greater proliferative response to estrogen was not observed. Overall, the greatest proliferative response was observed with combined continuous estrogen + progestin hormone therapy and did not differ significantly in early vs. late nulliparous or parous postmenopausal mice. Both estrogen and progestin were found to act directly on the mammary gland rather than through systemically mediated effects. The possible implications of these findings for menopausal hormone therapy in women and breast cancer risk are discussed.

Progestins and breast cancer

Progesterone (P) regulates proliferation and differentiation in the normal mammary gland in mouse, rat and human. Progesterone has also been implicated in the etiology and pathogenesis of human breast cancer. The focus of this review is on recent advances in understanding the role of the progesterone receptor (PR) and functional significance of PR isoforms, PRA and PRB, in the normal mammary gland and in mammary cancer in mouse, rat and human.

Progesterone receptor isoforms A and B: temporal and spatial differences in expression during murine mammary gland development

Progesterone is a potent mitogen in the mammary gland. Based on studies using cells and animals engineered to express progesterone receptor (PR) isoforms A or B, PRA and PRB are believed to have different functions. Using an immunohistochemical approach with antibodies specific for PRA only or PRB only, we show that PRA and PRB expression in mammary epithelial cells is temporally and spatially separated during normal mammary gland development in the BALB/c mouse. In the virgin mammary gland when ductal development is active, the only PR protein isoform expressed was PRA. PRA levels were significantly lower during pregnancy, suggesting a minor role at this stage of development. PRB was abundantly expressed only during pregnancy, during alveologenesis. PRA and PRB colocalization occurred in only a small percentage of cells. During pregnancy there was extensive colocalization of PRB with 5-bromo-2'-deoxyuridine (BrdU) and cyclin D1; 95% of BrdU-positive cells and 83% of cyclin D1-positive cells expressed PRB. No colocalization of PRA with either BrdU or cyclin D1 was observed at pregnancy. In the virgin gland, PRA colocalization with BrdU or cyclin D1 was low; only 27% of BrdU-positive cells and 4% of cyclin D1-positive cells expressed PRA. The implication of these findings is that different actions of progesterone are mediated in PRB positive vs. PRA-positive cells in vivo. The spatial and temporal separation of PR isoform expression in mouse mammary gland provides a unique opportunity to determine the specific functions of PRA vs. PRB in vivo.

Estrogen increases brain expression of the mRNA encoding transthyretin, an amyloid β scavenger protein

Estrogen replacement therapy in postmenopausal women is associated with a reduced risk of Alzheimer's Disease (AD). The multiple mechanisms by which estrogen protects against AD are still unknown. To conduct a broad screen for estrogen-regulated AD-related genes in the brain, we used cDNA array assays of brain mRNA samples from ovariectomized (ovx) adult female mice treated with either 17beta-estradiol or vehicle at 1 or 5 weeks post-ovx. The gene encoding transthyretin (TTR), which has been reported to scavenge amyloid beta peptides and reduce amyloid plaque formation, is increased by estradiol treatment at both 1 and 5 weeks post-ovx. Northern blot analyses and RNase protection assays performed on whole brain samples obtained from estradiol- or vehicle-treated mice confirmed the cDNA array assays showing a significant increase in TTR mRNA with estradiol treatment. Qualitative in situ hybridization or immunocytochemistry performed on brain sections demonstrated that TTR mRNA is expressed only in choroid plexus and leptomeninges, and that both estrogen receptor proteins, alpha and beta, are present in choroid plexus cells. These novel findings suggest that estrogen may reduce the risk of AD by acting on choroid plexus cells to increase TTR gene expression, leading to enhanced sequestration and reduced aggregation of amyloid beta peptides.

Breast cancer among young African-American women: a summary of data and literature and of issues discussed during the Summit Meeting on Breast Cancer Among African American Women, Washington, DC, September 8-10, 2000

BACKGROUND

African-American women younger than age 45 years have a higher risk of incidence and mortality due to breast cancer than other women. The reason for this disparity in risk is not well understood.

METHODS

This review summarizes the literature on the topic of breast cancer in young women and presents a summary of a discussion on this topic during a national forum on breast cancer among African-American women.

RESULTS

The occurrence of breast cancer among African-American women younger than the age of 45 years has not been well studied. There is a clear and long-term pattern of higher incidence and mortality and poorer survival in this population subgroup.

CONCLUSION

Research is needed to understand the reasons for these disparities and to reduce or eliminate them. Studies focused on hormonal factors, genetic factors, diet and obesity, and timely access to state-of-the-art prevention, information, screening, diagnosis, and treatment are likely to produce important new knowledge in this area.

Host microenvironment in breast cancer development: epithelial-cell-stromal-cell interactions and steroid hormone action in normal and cancerous mammary gland

Mammary epithelial cells comprise the functional component of the normal gland and are the major target for carcinogenesis in mammary cancer. However, the stromal compartment of the normal gland and of tumors plays an important role in directing proliferative and functional changes in the epithelium. In vivo and in vitro studies of the murine mammary gland have provided insights into novel stroma-dependent mechanisms by which estrogen and progesterone action in the epithelium can be modulated by hepatocyte growth factor (HGF) and the extracellular matrix proteins, collagen type I, fibronectin and laminin. In vitro and in vivo studies of estrogen receptor positive, estrogen-responsive human breast cancer cells have also demonstrated that estrogen responsiveness of tumor cells can also be modulated by extracellular matrix proteins, collagen type I and laminin.

Estrogen mediates mammary epithelial cell proliferation in serum-free culture indirectly via mammary stroma-derived hepatocyte growth factor

Epithelial-stromal cell interactions are important for normal development and function of the mouse mammary gland. The steroid hormone estrogen is required for epithelial cell proliferation and ductal development in vivo. Recent studies of estrogen receptor alpha knockout mice indicate that estrogen-induced proliferation is dependent upon the presence of estrogen receptor in mammary stromal cells, but not in epithelial cells. The purpose of the present study was to identify the underlying mechanism of estrogen-dependent stroma-derived effects on mammary epithelium. We have developed a minimally supplemented serum-free medium, collagen gel primary mammary coculture system to address the issue of stroma-derived, estrogen-dependent effects on epithelial cell proliferation. Conditioned medium from mammary fibroblasts or coculture with mammary fibroblasts caused increased epithelial cell proliferation and produced tubular/ductal morphology. Hepatocyte growth factor (HGF) was identified as the mediator of this effect, as the proliferative activity in fibroblast-conditioned medium was completely abolished by neutralizing antibody to HGF, whereas neutralizing antibodies to either epidermal growth factor or IGF-I had no effect. Treatment of mammary fibroblasts with estrogen increased the production of HGF. From these results we conclude that estrogen may indirectly mediate mammary epithelial cell proliferation via the regulation of HGF in mammary stromal cells and that HGF plays a crucial role in estrogen-induced proliferation in vivo.

Hepatocyte growth factor is required for progestin-induced epithelial cell proliferation and alveolar-like morphogenesis in serum-free culture of normal mammary epithelial cells

The steroid hormones, estrogen and progesterone, are required for mammary epithelial cell proliferation and alveolar morphogenesis in vivo. We have developed a minimally supplemented, serum-free medium, collagen gel primary mammary culture system to determine the mechanism of progestin-induced proliferation and alveolar morphogenesis. In epithelial cells cultured alone, treatment with progestin (R5020) alone produced a lumen within the epithelial organoids, but did not stimulate epithelial cell proliferation. The formation of lumens was associated with increased apoptosis, targeted within the organoids. We have previously reported that in our culture system hepatocyte growth factor (HGF) increases epithelial cell proliferation and induces a tubulo-ductal morphological response. In the present report we show that treatment with HGF and progestin (R5020) further increases epithelial proliferation above that with HGF alone and also produces an alveolar-like morphology similar to that observed in vivo in response to progestin treatment. To the best of our knowledge this is the first in vitro demonstration of both progestin-induced proliferation and alveolar-like morphogenesis of normal nonpregnant mouse mammary epithelial cells in vitro. These results suggest that HGF may play a crucial role in progestin-induced proliferation and morphogenesis in vivo.

Normal mammary gland morphology in pubertal female mice following in utero and lactational exposure to genistein at levels comparable to human dietary exposure

The objective of the study was to determine the effect of in utero and lactational exposure to genistein (0, 0.1, 0.5, 2.5 and 10 mg/kg/day) on mammary gland morphology in female B6D2F1 mice at levels comparable to or greater than human exposures. The effect of diethylstilbestrol (DES; 0, 0.1, 1, 10 microg/kg/day) on the mammary gland was also examined as a positive estrogenic control. Pregnant females were treated by daily gavage from gestational day 12 to postnatal day (PND) 20. Female offspring were weaned on PND21 and mammary gland whole mounts were examined for growth (length and area of the epithelial tree), proliferation (number of terminal end buds (TEBs)), and differentiation (density of alveolar buds (ABs)) on PND49. The highest dose of DES induced a significant increase in mammary gland growth (P<0.05) and also decreased the number of TEBs (P<0.06). The density of ABs was not significantly affected by DES. By contrast to DES, genistein had no effect on mammary gland morphology at any dose. These results suggest that in utero and lactational exposure to genistein at levels comparable to or greater than human exposures do not adversely affect mammary gland development in pubertal female B6D2F1 mice.

Postmenopausal hormone replacement therapy: effects on normal mammary gland in humans and in a mouse postmenopausal model

Endogenous estrogen exposure has long been implicated in the causation of breast cancer through a mechanism of epithelial cell proliferation. Whether estrogen, progesterone, or both exhibit mitogenic activity and promote carcinogenesis in the human breast has been the subject of considerable debate. The purpose of this review article is to examine the evidence for the effects of hormone replacement therapy in its various forms on the biology of the postmenopausal breast both in humans and in an animal model, and to identify the gaps in knowledge that research will need to address to further understand this complex issue.

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.

Estrogen and estrogen plus progestin act directly on the mammary gland to increase proliferation in a postmenopausal mouse model

Hormone replacement therapy (HRT) with ovarian hormones is an important therapeutic modality for postmenopausal women. However, a negative side effect of HRT is an increased risk of breast cancer. Surgical induction of menopause by ovariectomy (OVX) in mice is an experimental model that may provide insights into the effects of hormone replacement therapy on the human breast. We have developed a mouse model of early and late postmenopausal states to investigate the effects of HRT on the normal mammary gland. The purpose of this study was to determine if HRT-induced proliferation was due to the direct action of the hormones on the mammary gland, or mediated systemically by hormones or growth factors produced elsewhere in the body. Estrogen (E) or E plus the synthetic progestin, R5020, were implanted directly into the mammary glands of early (1 week post OVX) and late (5 week post OVX) postmenopausal mice instead of administration by injection. We report that responses of early and late postmenopausal mice to implanted hormones were the same as those observed previously with systemically administered hormones. Implanted E conferred an enhanced proliferative response in the late postmenopausal gland characterized morphologically by enlarged duct ends. E+R5020 implants induced similar degrees of cell proliferation in both postmenopausal states but the morphological responses differed. Ductal sidebranching was observed in early postmenopausal mice, whereas duct end enlargement was observed in late postmenopausal mice. The differences in morphological response to E+R5020 in 5 week post OVX were associated with an inability of E to induce progesterone receptors (PR) in the late postmenopausal gland. The responses of the late postmenopausal glands to E and E+P were very similar to that observed previously in immature pubertal glands in ovary-intact mice. In pubertal mice, PR cannot be induced by E unless the mammary gland is pre-treated with EGF-containing implants. Similarly, herein pre-treatment of the late postmenopausal mammary gland with EGF-containing implants restored PR induction by E. Thus, EGF may determine the sensitivity of the mammary gland to E and E+P in late postmenopause and at puberty.

Proliferative effects of combination estrogen and progesterone replacement therapy on the normal postmenopausal mammary gland in a murine model

OBJECTIVE

The aim of the study was to analyze the proliferative response of the normal mammary gland to combination hormone replacement therapy with estrogen and progesterone in a murine model of early versus late postmenopausal states.

STUDY DESIGN

Ovariectomized mice were injected daily for up to 56 days with estrogen plus progesterone, starting at either 1 or 5 weeks after ovariectomy to simulate early and late menopausal periods, respectively. At various times after treatment, proliferation was analyzed by deoxyribonucleic acid histoautoradiography and whole-mount preparations. The induction of progesterone receptor by estrogen was also analyzed. To distinguish between estrogen- and progesterone-specific responses, we tested the effects of the antiprogesterone mifepristone (RU 486) and the antiestrogen ICI 182,780.

RESULTS

The acute response to estrogen-progesterone therapy in the early postmenopausal period resulted in duct-end enlargement, ductal side branching, alveolar bud formation, and a 100-fold increase in epithelial cell proliferation. This was caused by the dominant effect of progesterone acting through the progesterone receptor. In the late postmenopausal period the acute response produced only duct-end enlargement; the 100-fold increase in epithelial cell proliferation resulted from the dominant effect of estrogen. After long-term treatment, both early and late postmenopausal glands exhibited similar morphologic features and a 9-fold higher steady-state proliferation rate than was found in control-treated groups.

CONCLUSIONS

Starting combined estrogen and progesterone hormone replacement therapy in either early or late postmenopause produced a persistent, steady-state 9-fold increase in epithelial cell proliferation, which could be a contributing factor to increased breast cancer risk. The acute response in the late postmenopausal period mimics the hormonal response of the pubertal mammary gland, which in rodents is the stage most susceptible to carcinogen-induced mammary tumorigenesis. These observations raise questions about increased susceptibility of the late postmenopausal gland to carcinogenesis and a role for hormone replacement therapy in the promotion of tumorigenesis.

Reciprocal regulation of extracellular matrix proteins and ovarian steroid activity in the mammary gland

Despite the critical importance of ovarian steroids in the treatment of breast cancer, little is known about the acquisition or loss of estrogen and progesterone responsiveness in either the normal or neoplastic mammary gland. This review focuses on the interactions among mammary stroma-derived extracellular matrix (ECM) proteins, integrins and ovarian hormone-dependent proliferation in normal and neoplastic mammary cells both in vivo and in vitro. In vitro studies show that fibronectin is required for progesterone-induced proliferation of normal mammary epithelial cells and that specific ECM proteins also regulate interactions between growth factors and ovarian hormones. Studies with human breast cancer cell lines have shown that laminin inhibits estrogen-induced proliferation and estrogen-response-element-mediated transcription in vitro and also inhibits estrogen-induced proliferation in vivo. Reciprocally, ovarian steroids regulate the expression of ECM proteins and their cellular receptors, integrins, during mammary gland development in vivo. The fibronectin-specific integrin, alpha5beta1 is regulated by ovarian steroids and its expression is positively correlated with developmental stages of peak proliferation. These studies suggest that the coordinated regulation of ovarian hormone responsiveness and ECM/integrin expression may be critical to normal mammary gland development and breast cancer growth and progression.

Proliferation of mouse mammary epithelial cells in vitro: interactions among epidermal growth factor, insulin-like growth factor I, ovarian hormones, and extracellular matrix proteins

The purpose of the present study was to investigate the role of extracellular matrix proteins (ECMs; collagens I and IV, fibronectin, and laminin) in modulating proliferative responses of normal mammary epithelial cells in serum-free culture to epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I). As EGF and IGF-I can alter steroid responses, the interactions among growth factors, estrogen, and R5020 were also investigated. We report the novel finding that all ECMs tested, but not a nonspecific attachment factor, poly-L-lysine (PL), promoted a highly synergistic proliferative response to EGF plus IGF-I. EGF receptors were significantly increased with culture time on all ECMs, but not on PL. IGF receptor expression was significantly 2- to 4-fold higher on all ECMs compared with PL. EGF decreased IGF-binding protein-2 (IGFBP-2) and IGFBP-3 by more than 50% in the presence of IGF-I on PL or collagen I. These results indicate that ECM-specific IGF-I/EGF synergism occurs in response to ECM up-regulation of growth factor receptors and EGF down-regulation of inhibitory IGFBPs. Growth factors did not synergize with estrogen and/or R5020. Instead, estrogen plus R5020 decreased EGF-plus IGF-I-induced proliferation in an ECM-dependent manner. These studies demonstrate that proliferation of normal mammary epithelial cells involves complex interactions among steroids, growth factors, binding proteins, and ECMs.

Laminin inhibits estrogen action in human breast cancer cells

Breast tumors that lack estrogen responsiveness have a poor prognosis. Despite the critical importance to breast cancer treatment, little is known about the loss of estrogen responsiveness and the development of antiestrogen resistance. We have examined the regulation of estrogen-induced proliferation, estrogen regulation of progesterone receptor (PR) expression, and estrogen signaling pathways in estrogen receptor (ER) positive (MCF-7 and T47D) breast cancer cell lines by specific extracellular matrix proteins (ECM) under serum-free conditions. Estrogen, supplemented with submaximal concentrations of insulin-like growth factor I (IGF-I) and epidermal growth factor (EGF), stimulated DNA synthesis of MCF-7 cells 7- to 10-fold and T47D cells 2-fold on collagen I or fibronectin. However, estrogen-induced proliferation was greatly reduced on laminin. In contrast, IGF-I or EGF, alone, stimulated proliferation of MCF-7 and T47D cells on all ECM. Thus, ER+ breast cancer cells were not refractory to mitogens when cultured on laminin. Similarly, estrogen induction of PR occurred on fibronectin or collagen I, but not on laminin. While ER content was similar on all ECM, estrogen stimulation of estrogen response element (ERE)-luciferase activity was significantly lower in MCF-7 cells cultured on laminin. Therefore, changes in ECM composition that occur in breast cancer may alter estrogen-responsiveness and the effectiveness of antiestrogen therapies in ER+ breast cancer cells.

Hormone replacement therapy with estrogen or estrogen plus medroxyprogesterone acetate is associated with increased epithelial proliferation in the normal postmenopausal breast

The relative effects of postmenopausal hormone replacement therapy (HRT) with estrogen alone vs. estrogen+progestin on breast cell proliferation and on breast cancer risk are controversial. A cross-sectional observational study was carried out to examine the proliferative effects of HRT with estrogen or estrogen plus the progestin, medroxyprogesterone acetate, in breast tissue of postmenopausal women. Benign breast biopsies from 86 postmenopausal women were analyzed with antiproliferating cell nuclear antigen (anti-PCNA) and Ki67 antibodies to measure relative levels of cell proliferation. Epithelial density and estrogen and progesterone receptor status were also determined. The women were categorized either as users of: 1) estrogen (E) alone; 2) estrogen+medroxyprogesterone acetate (E+P); or 3) no HRT. Compared with no HRT, the breast epithelium of women who had received either E+P or E alone had significantly higher PCNA proliferation indices, and treatment with E+P had a significantly higher index (PCNA and Ki67) than treatment with E alone. Breast epithelial density was significantly greater in postmenopausal women treated with E and E+P, compared with no HRT. Thus, the present study shows that postmenopausal HRT with E+P was associated with greater breast epithelial cell proliferation and breast epithelial cell density than E alone or no HRT. Furthermore, with E+P, breast proliferation was localized to the terminal duct-lobular unit of the breast, which is the site of development of most breast cancers. Further studies are needed to assess the possible association between the mitogenic activity of progestins and breast cancer risk.

A mouse model to study the effects of hormone replacement therapy on normal mammary gland during menopause: enhanced proliferative response to estrogen in late postmenopausal mice

Hormone replacement therapy (HRT) with estrogen alleviates menopausal symptoms and is effective in reducing osteoporosis and cardiovascular disease when taken in early postmenopause. Older, late postmenopausal women who never previously received HRT are also believed to benefit from estrogen treatment. On the other hand, increased lifetime exposure of the mammary gland to estrogen may increase the risk of breast cancer. The development of suitable experimental animal model systems can advance our understanding of the effects of estrogen and the timing of HRT on the postmenopausal breast. Toward this end, early and late postmenopausal states were induced in mice by short vs. long term ovariectomy (1 vs. 5 weeks), and the effects of 17beta-estradiol (E) on mammary gland morphology, cell proliferation, and progesterone receptor (PR) levels were investigated. We report that in late postmenopausal mice, E caused a pronounced enlargement of duct ends and 6.5- and 4-fold greater mitogenic responses in the duct end epithelium and adjacent stromal cells, respectively, compared with the response in early postmenopausal mice. Furthermore, after long term, daily treatment with E, steady state levels of proliferation remained 2-fold higher than those of similarly treated, early postmenopausal mice. E failed to increase mammary PR levels in late postmenopausal, but not in early postmenopausal mice. Stimulation of duct ends by E and lack of PR inducibility are characteristics of the immature pubertal mammary gland and indicate that the late postmenopausal mammary gland resembled the immature state. In contrast, minimal E-induced proliferation and increased PR inducibility, characteristics of the adult, sexually mature mammary gland, were retained in early postmenopausal mice. The lack of difference in the numbers of estrogen receptor-positive epithelial or stromal cells or in estrogen receptor cellular concentration after short vs. long term ovariectomy indicates that the observed greater efficacy of E is mediated at a step beyond receptor-ligand binding. This mouse model of experimentally induced early vs. late postmenopausal states should prove useful in better understanding alterations in hormone responsiveness and their implications for timing of HRT on the human breast.

Estrogenic effects of genistein on the growth of estrogen receptor-positive human breast cancer (MCF-7) cells in vitro and in vivo

Genistein, found in soy products, is a phytochemical with several biological activities. In the current study, our research focused on the estrogenic and proliferation-inducing activity of genistein. We have demonstrated that genistein enhanced the proliferation of estrogen-dependent human breast cancer (MCF-7) cells in vitro at concentrations as low as 10 nM, with a concentration of 100 nM achieving proliferative effects similar to those of 1 nM estradiol. Expression of the estrogen-responsive gene pS2 was also induced in MCF-7 cells in response to treatment with a concentration of genistein as low as 1 microM. At higher concentrations (above 20 microM), genistein inhibits MCF-7 cell growth. In vivo, we have shown that dietary treatment with genistein (750 ppm) for 5 days enhanced mammary gland growth in 28-day-old ovariectomized athymic mice, indicating that genistein acts as an estrogen in normal mammary tissue. To evaluate whether the estrogenic effects observed in vitro with MCF-7 cells could be reproduced in vivo, MCF-7 cells were implanted s.c. in ovariectomized athymic mice, and the growth of the estrogen-dependent tumors was measured weekly. Negative control animals received the American Institute of Nutrition (AIN)-93G diet, the positive control group received a new s.c. estradiol (2 mg) pellet plus the AIN-93G diet, and the third group received genistein at 750 ppm in the AIN-93G diet. Tumors were larger in the genistein (750 ppm)-treated group than they were in the negative control group, demonstrating that dietary genistein was able to enhance the growth of MCF-7 cell tumors in vivo. Increased uterine weights were also observed in the genistein-treated groups. In summary, genistein can act as an estrogen agonist in vivo and in vitro, resulting in the proliferation of cultured human breast cancer cells (MCF-7) and the induction of pS2 gene expression. Here we present new information that dietary genistein stimulates mammary gland growth and enhances the growth of MCF-7 cell tumors in ovariectomized athymic mice.

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.