Bisphenol A increases mammary cancer risk in two distinct mouse models of breast cancer

Bisphenol A (BPA) is an industrial plasticizer that leaches from food containers during normal usage, leading to human exposure. Early and chronic exposure to endocrine-disrupting environmental contaminants such as BPA elevates the potential for long-term health consequences. We examined the impact of BPA exposure on fetal programming of mammary tumor susceptibility as well as its growth promoting effects on transformed breast cancer cells in vivo. Fetal mice were exposed to 0, 25, or 250 μg/kg BPA by oral gavage of pregnant dams. Offspring were subsequently treated with the known mammary carcinogen, 7,12-dimethylbenz[a]anthracene (DMBA). While no significant differences in postnatal mammary development were observed, both low- and high-dose BPA cohorts had a statistically significant increase in susceptibility to DMBA-induced tumors compared to vehicle-treated controls. To determine if BPA also promotes established tumor growth, MCF-7 human breast cancer cells were subcutaneously injected into flanks of ovariectomized NCR nu/nu female mice treated with BPA, 17beta-estradiol, or placebo alone or combined with tamoxifen. Both estradiol- and BPA-treated cohorts formed tumors by 7 wk post-transplantation, while no tumors were detected in the placebo cohort. Tamoxifen reversed the effects of estradiol and BPA. We conclude that BPA may increase mammary tumorigenesis through at least two mechanisms: molecular alteration of fetal glands without associated morphological changes and direct promotion of estrogen-dependent tumor cell growth. Both results indicate that exposure to BPA during various biological states increases the risk of developing mammary cancer in mice.

Aberrant expression of LMO4 induces centrosome amplification and mitotic spindle abnormalities in breast cancer cells

The LIM-only protein, LMO4, is a transcriptional modulator overexpressed in breast cancer. It is oncogenic in murine mammary epithelium and is required for G2/M progression of ErbB2-dependent cells as well as growth and invasion of other breast cancer cell types. However, the mechanisms underlying the oncogenic activity of LMO4 remain unclear. Herein, we show that LMO4 is expressed in all breast cancer subtypes examined and its expression level correlates with the degree of proliferation of such tumours. In addition, we have determined that LMO4 silencing induces G2/M arrest in cells from various breast cancer subtypes, suggesting that LMO4 action in the cell cycle is not restricted to a single breast cancer subtype. This arrest was accompanied by increased cell death, amplification of centrosomes, and formation of abnormal mitotic spindles. Consistent with its ability to positively and negatively regulate the formation of active transcription complexes, overexpression of LMO4 also resulted in an increase in centrosome number. Centrosome amplification has been shown to prolong the G2/M phase of the cell cycle and induce apoptosis; thus, we conclude that supernumerary centrosomes mediate the G2/M arrest and cell death in LMO4-deficient cells. Furthermore, the correlation of centrosome amplification with genomic instability suggests that the impact of dysregulated LMO4 on the centrosome cycle may promote LMO4-induced tumour formation.

HER2/ErbB2-induced breast cancer cell migration and invasion require p120 catenin activation of Rac1 and Cdc42

Breast cancers that overexpress the receptor tyrosine kinase ErbB2/HER2/Neu result in poor patient outcome because of extensive metastatic progression. Herein, we delineate a molecular mechanism that may govern this malignant phenotype. ErbB2 induction of migration requires activation of the small GTPases Rac1 and Cdc42. The ability of ErbB2 to activate these small GTPases necessitated expression of p120 catenin, which is itself up-regulated by signaling through ErbB2 and the tyrosine kinase Src. Silencing p120 in ErbB2-dependent breast cancer cell lines dramatically inhibited migration and invasion as well as activation of Rac1 and Cdc42. In contrast, overexpression of constitutively active mutants of these GTPases reversed the effects of p120 silencing. Lastly, ectopic expression of p120 promoted migration and invasion and potentiated metastatic progression of a weakly metastatic, ErbB2-dependent breast cancer cell line. These results suggest that p120 acts as an obligate intermediate between ErbB2 and Rac1/Cdc42 to modulate the metastatic potential of breast cancer cells.

FOXA1 is an essential determinant of ERalpha expression and mammary ductal morphogenesis

FOXA1, estrogen receptor alpha (ERalpha) and GATA3 independently predict favorable outcome in breast cancer patients, and their expression correlates with a differentiated, luminal tumor subtype. As transcription factors, each functions in the morphogenesis of various organs, with ERalpha and GATA3 being established regulators of mammary gland development. Interdependency between these three factors in breast cancer and normal mammary development has been suggested, but the specific role for FOXA1 is not known. Herein, we report that Foxa1 deficiency causes a defect in hormone-induced mammary ductal invasion associated with a loss of terminal end bud formation and ERalpha expression. By contrast, Foxa1 null glands maintain GATA3 expression. Unlike ERalpha and GATA3 deficiency, Foxa1 null glands form milk-producing alveoli, indicating that the defect is restricted to expansion of the ductal epithelium, further emphasizing the novel role for FOXA1 in mammary morphogenesis. Using breast cancer cell lines, we also demonstrate that FOXA1 regulates ERalpha expression, but not GATA3. These data reveal that FOXA1 is necessary for hormonal responsiveness in the developing mammary gland and ERalpha-positive breast cancers, at least in part, through its control of ERalpha expression.

Kruppel-like factor 4 inhibits epithelial-to-mesenchymal transition through regulation of E-cadherin gene expression

The Krüppel-like factor 4 (KLF4) is a transcriptional regulator of proliferation and differentiation in epithelial cells, both during development and tumorigenesis. Although KLF4 functions as a tumor suppressor in several tissues, including the colon, the role of KLF4 in breast cancer is less clear. Here, we show that KLF4 is necessary for maintenance of the epithelial phenotype in non-transformed MCF-10A mammary epithelial cells. KLF4 silencing led to alterations in epithelial cell morphology and migration, indicative of an epithelial-to-mesenchymal transition. Consistent with these changes, decreased levels of KLF4 also resulted in the loss of E-cadherin protein and mRNA. Promoter/reporter analyses revealed decreased E-cadherin promoter activity with KLF4 silencing, while chromatin immunoprecipitation identified endogenous KLF4 binding to the GC-rich/E-box region of this promoter. Furthermore, forced expression of KLF4 in the highly metastatic MDA-MB-231 breast tumor cell line was sufficient to restore E-cadherin expression and suppress migration and invasion. These findings identify E-cadherin as a novel transcriptional target of KLF4. The clear requirement for KLF4 to maintain E-cadherin expression and prevent epithelial-to-mesenchymal transition in mammary epithelial cells supports a metastasis suppressive role for KLF4 in breast cancer.

The double-stranded RNA-binding protein, PACT, is required for postnatal anterior pituitary proliferation

PACT is a double-stranded RNA-binding protein that also binds and activates the latent protein kinase, PKR, which plays a major role in cellular antiviral defense in mammals. For evaluating PACT's contribution to the innate immune system, Pact(-/-) mice have been generated; these mice exhibit notable developmental abnormalities including microtia, with craniofacial, ear, and hearing defects. Here we report that, in addition, Pact(-/-) mice had smaller body size and fertility defects, both of which were caused by defective pituitary functions. Pact(-/-) mice exhibited anterior pituitary lobe (AL) hypoplasia, which developed postnatally, when the second phase of pituitary expansion occurs. Among the 5 cell types in AL, the numbers of corticotrophs, gonadotrophs, and somatotrophs were equally decreased in Pact(-/-) mice with a greater impact on lactotrophs and a lesser impact on thyrotrophs. PACT mRNA and protein were highly expressed in the pituitary of wild-type (Wt) mice during the postnatal wave of AL proliferation, the same period in which the hypoplasia developed in Pact(-/-) mice. During this time, the pituitaries of Pact(-/-) mice did not exhibit significantly increased apoptosis compared with Wt mice but showed a decrease in cell proliferation. The inhibition of cell proliferation observed in vivo could be recapitulated in vitro in GH3 somato/lactotroph and LbetaT2 gonadotroph cell lines; knockdown of PACT expression with siRNA diminished the rate of proliferation of these cells. Our study revealed a physiologically significant role for PACT in cell proliferation and an essential role of a dsRNA-binding protein in mammalian pituitary expansion.

Intrinsic bias in breast cancer gene expression data sets

Background

While global breast cancer gene expression data sets have considerable commonality in terms of their data content, the populations that they represent and the data collection methods utilized can be quite disparate. We sought to assess the extent and consequence of these systematic differences with respect to identifying clinically significant prognostic groups.

Methods

We ascertained how effectively unsupervised clustering employing randomly generated sets of genes could segregate tumors into prognostic groups using four well-characterized breast cancer data sets.

Results

Using a common set of 5,000 randomly generated lists (70 genes/list), the percentages of clusters with significant differences in metastasis latencies (HR p-value < 0.01) was 62%, 15%, 21% and 0% in the NKI2 (Netherlands Cancer Institute), Wang, TRANSBIG and KJX64/KJ125 data sets, respectively. Among ER positive tumors, the percentages were 38%, 11%, 4% and 0%, respectively. Few random lists were predictive among ER negative tumors in any data set. Clustering was associated with ER status and, after globally adjusting for the effects of ER-α gene expression, the percentages were 25%, 33%, 1% and 0%, respectively. The impact of adjusting for ER status depended on the extent of confounding between ER-α gene expression and markers of proliferation.

Conclusion

It is highly probable to identify a statistically significant association between a given gene list and prognosis in the NKI2 dataset due to its large sample size and the interrelationship between ER-α expression and markers of proliferation. In most respects, the TRANSBIG data set generated similar outcomes as the NKI2 data set, although its smaller sample size led to fewer statistically significant results.

Cell cycle correlated genes dictate the prognostic power of breast cancer gene lists

Background

Numerous gene lists or "classifiers" have been derived from global gene expression data that assign breast cancers to good and poor prognosis groups. A remarkable feature of these molecular signatures is that they have few genes in common, prompting speculation that they may use distinct genes to measure the same pathophysiological process(es), such as proliferation. However, this supposition has not been rigorously tested. If gene-based classifiers function by measuring a minimal number of cellular processes, we hypothesized that the informative genes for these processes could be identified and the data sets could be adjusted for the predictive contributions of those genes. Such adjustment would then attenuate the predictive function of any signature measuring that same process.

Results

We tested this hypothesis directly using a novel iterative-subtractive approach. We evaluated five gene expression data sets that sample a broad range of breast cancer subtypes. In all data sets, the dominant cluster capable of predicting metastasis was heavily populated by genes that fluctuate in concert with the cell cycle. When six well-characterized classifiers were examined, all contained a higher than expected proportion of genes that correlate with this cluster. Furthermore, when the data sets were globally adjusted for the cell cycle cluster, each classifier lost its ability to assign tumors to appropriate high and low risk groups. In contrast, adjusting for other predictive gene clusters did not impact their performance.

Conclusion

These data indicate that the discriminative ability of breast cancer classifiers is dependent upon genes that correlate with cell cycle progression.

Rapamycin inhibits multiple stages of c-Neu/ErbB2 induced tumor progression in a transgenic mouse model of HER2-positive breast cancer

Amplification of the HER2 (ErbB2, c-Neu) proto-oncogene in breast cancer is associated with poor prognosis and high relapse rates. HER2/ErbB2, in conjunction with ErbB3, signals through the Akt/phosphatidylinositol 3-kinase pathway and leads to the activation of mammalian target of rapamycin (mTOR), a critical mRNA translation regulator that controls cell growth. Gene expression analysis of mammary tumors collected from mouse mammary tumor virus-c-Neu transgenic mice revealed that mRNA levels of several mTOR pathway members were either up-regulated (p85/phosphatidylinositol 3-kinase and p70S6 kinase) or down-regulated (eIF-4E-BP1) in a manner expected to enhance signaling through this pathway. Treatment of these mice with the mTOR inhibitor rapamycin caused growth arrest and regression of primary tumors with no evidence of weight loss or generalized toxicity. The treatment effects were due to decreased proliferation, associated with reduced cyclin D1 expression, and increased cell death in primary tumors. Whereas many of the dead epithelial cells had the histopathologic characteristics of ischemic necrosis, rapamycin treatment was not associated with changes in microvascular density or apoptosis. Rapamycin also inhibited cellular proliferation in lung metastases. In summary, data from this preclinical model of ErbB2/Neu-induced breast cancer show that inhibition of the mTOR pathway with rapamycin blocks multiple stages of ErbB2/Neu-induced tumorigenic progression.

Ovarian hyperstimulation induces centrosome amplification and aneuploid mammary tumors independently of alterations in p53 in a transgenic mouse model of breast cancer

Aneuploidy and genomic instability are common features of human cancers, including breast cancer; however, mechanisms by which such abnormalities develop are not understood. The exquisite dependence of the mammary gland on hormones for growth and development as well as hormonal contributions to breast cancer risk and progression suggest that tumorigenic mechanisms in the breast should be considered in the context of hormonal stimulation. We used transgenic mice that overexpress luteinizing hormone with subsequent ovarian hyperstimulation as a model to identify mechanisms involved in hormone-induced mammary cancer. Tumor pathology in these mice is highly variable, suggesting individual tumors undergo distinct initiating or promoting events. Supporting this notion, hormone-induced tumors display considerable chromosomal instability and aneuploidy, despite the presence of functional p53. The presence of extensive centrosome amplification in tumors and hyperplastic glands prior to tumor formation suggests that alterations in the ovarian hormonal milieu dysregulate the centrosome cycle in mammary epithelial cells, leading to aneuploidy and cancer.

The pleiotropic effects of excessive luteinizing hormone secretion in transgenic mice

Luteinizing hormone (LH) is member of the glycoprotein hormone family of gonadotropins, which also includes the highly related human chorionic gonadotropin and follicle-stimulating hormone. The necessity of these factors for sustaining human fertility has been known for decades. In addition, elevated serum levels of LH have been associated with polycystic ovarian syndrome, suggesting that the appropriate balance of LH is critical for maintaining reproductive function. To dissect the biological consequences of aberrant LH signaling in vivo, several genetically engineered mouse models have been developed that overexpress LH or have increased LH signaling. These models underscore the importance of tightly regulated LH levels for normal reproductive function, and reveal novel roles for LH and gonadal hormones in tumorigenesis of multiple tissues, including the ovary, mammary gland, and pituitary. Thus, mice with altered LH signaling provide valuable tools in understanding normal reproduction and various pathological conditions.

An evaluation of evidence for the carcinogenic activity of bisphenol A

The National Institutes of Health (NIEHS, NIDCR) and the United States Environmental Protection Agency convened an expert panel of scientists with experience in the field of environmental endocrine disruptors, particularly with knowledge and research on bisphenol A (BPA). Five subpanels were charged to review the published literature and previous reports in five specific areas and to compile a consensus report with recommendations. These were presented and discussed at an open forum entitled "Bisphenol A: An Expert Panel Examination of the Relevance of Ecological, In Vitro and Laboratory Animal Studies for Assessing Risks to Human Health" in Chapel Hill, NC on 28-30 November 2006. The present review consists of the consensus report on the evidence for a role of BPA in carcinogenesis, examining the available evidence in humans and animal models with recommendations for future areas of research.

Increases in luteinizing hormone are associated with declines in cognitive performance

Questions surrounding estrogen therapy for post-menopausal cognitive decline and dementia led us to examine the role of luteinizing hormone that becomes elevated after menopause. We examined hippocampal-associated cognitive performance, as measured with the Y-maze task, in two strains of transgenic mice, one (Tg-LHbeta) which over-expresses luteinizing hormone and another (LHRKO), which has increased circulating luteinizing hormone levels, but its receptors are silenced. Our results demonstrate that Tg-LHbeta, but not LHRKO mice, show decreased Y-maze performance when compared to aged-matched wild-type animals. These findings indicate that increased luteinizing hormone levels, in the presence of functional receptors may, at least in part, be responsible for cognitive decline after menopause. As such, modulation of luteinizing hormone or its receptor levels may prove to be useful therapeutic strategies for cognitive decline associated with aging and age-related neurodegenerative diseases such as Alzheimer disease.

Splice variants of mIAP1 have an enhanced ability to inhibit apoptosis

c-IAP1 is a member of the Inhibitor of Apoptosis protein family. Functions ascribed to c-IAP1 include inhibition of apoptosis and activation of NF-kappaB. Herein, we show that murine c-IAP1 (mIAP1) undergoes alterative splicing, generating two truncated proteins; one that lacks the CARD and RING domains (mIAP1-DeltaCARDDeltaRING) and the other that lacks only the CARD domain (mIAP1-DeltaCARD). mIAP1-DeltaCARDDeltaRING mRNA is expressed at 2-3% of the levels of full-length mIAP1 (FL-mIAP1) in mouse tissues, yet it encodes a protein that accumulates at 50-fold higher levels than the FL-mIAP1 in cultured cells. This protein has an enhanced ability to inhibit Bax-induced apoptosis, but does not activate an NF-kappaB reporter. In contrast to mIAP1-DeltaCARDDeltaRING, the mIAP1-DeltaCARD mRNA displays distinct tissue variation, ranging from 5% to 15% of the FL-mIAP1 mRNA levels and its levels increase in the mammary gland during involution. This isoform also has enhanced anti-apoptotic activity, but diminished NF-kappaB activation. In summary, mIAP1 is alternatively spliced, generating protein isoforms with distinct functional characteristics.

Sustained trophism of the mammary gland is sufficient to accelerate and synchronize development of ErbB2/Neu-induced tumors

Epidemiological studies indicate that parity enhances HER2/ErbB2/Neu-induced breast tumorigenesis. Furthermore, recent studies using multiparous, ErbB2/Neu-overexpressing mouse mammary tumor virus (MMTV-Neu) mice have shown that parity induces a population of cells that are targeted for ErbB2/Neu-induced transformation. Although parity accelerates mammary tumorigenesis, the pattern of tumor development in multiparous MMTV-Neu mice remains stochastic, suggesting that additional events are required for ErbB2/Neu to cause mammary tumors. Whether such events are genetic in nature or reflective of the dynamic hormonal control of the gland that occurs with pregnancy remains unclear. We postulated that young age at pregnancy initiation or chronic trophic maintenance of mammary epithelial cells might provide a cellular environment that significantly increases susceptibility to ErbB2/Neu-induced tumorigenesis. MMTV-Neu mice that were maintained pregnant or lactating beginning at 3 weeks of age demonstrated accelerated tumorigenesis, but this process was still stochastic, indicating that early pregnancy does not provide the requisite events of tumorigenesis. However, bitransgenic mice that were generated by breeding MMTV-Neu mice with a luteinizing hormone-overexpressing mouse model of ovarian hyperstimulation developed multifocal mammary tumors in an accelerated, synchronous manner compared to virgin MMTV-Neu animals. This synchrony of tumor development in the bitransgenic mice suggests that trophic maintenance of the mammary gland provides the additional events required for tumor formation and maintains the population of cells that are targeted by ErbB2/Neu for transformation. Both the synchrony of tumor appearance and the ability to characterize a window of commitment by ovariectomy/palpation studies permitted microarray analysis to evaluate changes in gene expression over a defined timeline that spans the progression from normal to preneoplastic mammary tissue. These approaches led to identification of several candidate genes whose expression changes in the mammary gland with commitment to ErbB2/Neu-induced tumorigenesis, suggesting that they may either be regulated by ErbB2/Neu and/or contribute to tumor formation.

EB1089, a vitamin D receptor agonist, reduces proliferation and decreases tumor growth rate in a mouse model of hormone-induced mammary cancer

1,25-Dihydroxyvitamin D3 and several of its analogs, such as EB1089, induce growth arrest and apoptosis of breast cancer cells in culture. EB1089 has also been shown to limit growth of xenografts in nude mice and carcinogen-induced mammary tumors in rats. Coupled with the fact that the vitamin D receptor is highly expressed in a large proportion of breast tumors, these data suggest that it may be a broad spectrum therapeutic target. We utilized a transgenic model of hormone-induced mammary cancer, the LH-overexpressing mouse, to assess, for the first time, the efficacy of EB1089 in a spontaneous tumor model. Similar to human breast cancers, the pre-neoplastic mammary glands and mammary tumors in these mice express high levels of vitamin D receptor. Treatment with EB1089 decreased proliferation of mammary epithelial cells in pre-neoplastic glands by 35%. Moreover, half of hormone-induced mammary tumors treated with EB1089 demonstrated a decreased rate of growth, with a subset of these tumors even regressing, suggesting that 1,25-dihydroxyvitamin D3 analogs may be effective chemopreventive and chemotherapeutic agents for breast cancer.

Gene expression profiling of cancer progression reveals intrinsic regulation of transforming growth factor-beta signaling in ErbB2/Neu-induced tumors from transgenic mice

Upregulation of HER2/ErbB2/Neu occurs in 15-30% of human breast cancers and correlates with poor prognosis. Identification of ErbB2/Neu transcriptional targets should facilitate development of novel therapeutic approaches. Development of breast cancer is a multistep process; thus, to identify the transcriptomes associated with different stages of progression of tumorigenesis, we compared expression profiles of mammary tumors and preneoplastic mammary tissue from MMTV-Neu transgenic mice to expression profiles of wild-type mammary glands using Affymetrix microarrays. We identified 324 candidate genes that were unique to ErbB2/Neu-induced tumors relative to normal mammary gland tissue from wild-type controls. Expression of a subset of these genes (82) was also changed in the preneoplastic mammary glands compared to wild-type controls, indicating that they may play a pivotal role during early events of ErbB2/Neu-initiated mammary tumorigenesis. Further analysis of the microarray data revealed that expression of several known transforming growth factor (TGF)-beta target genes was altered, suggesting that the TGF-beta signaling cascade is downregulated in ErbB2/Neu-induced tumors. Western blot analysis for TGF-beta-Receptor-I/ALK5 and immunohistochemistry for TGF-beta-Receptor-I/ALK5 and phosphorylated/activated Smad2 confirmed that the Smad-dependent TGF-beta signaling cascade was inactive in these tumors. Although absent in most of the tumor, phosphorylated Smad2 was present in the periphery of tumors. Interestingly, presence of phosphorylated/activated Smad2 correlated with expression of Activin-Receptor-IB/ALK4, suggesting that although Smad-dependent TGF-beta signaling is absent in ErbB2/Neu-induced tumors, Activin signaling may be active at the leading edge of these tumors. Cumulatively, these data indicate that the TGF-beta pathway is intrinsically suppressed in ErbB2/Neu tumors via a mechanism involving loss of TGF-beta-Receptor-I/ALK5.

Obesity in transgenic female mice with constitutively elevated luteinizing hormone secretion

Transgenic (TG) female mice, expressing a chimeric bovine luteinizing hormone (LH) beta-subunit/human chorionic gonadotropin beta-subunit COOH-terminal extension (bLHbeta-CTP) gene, produce high levels of circulating LH and serve as a model for functional ovarian hyperandrogenism and follicular cysts. We report here that obesity is a typical feature of these female mice. The mean body weight of the bLHbeta-CTP females was significantly higher than in controls at, and beyond 5 wk of age, and at 5 mo, it was 32% increased. At this age, the amount of white adipose tissue in the bLHbeta-CTP females was significantly increased, as reflected by the weight difference of the retroperitoneal fat pad. In addition, the expression of leptin mRNA in white adipose tissue of the TG females was elevated about twofold. Serum leptin and insulin levels, and food intake, were also increased significantly in the TG females. Brown adipose tissue (BAT) thermogenic activity, as measured by GDP binding to BAT mitochondria, was reduced (P < 0.05). Ovariectomy at the age of 3 wk totally prevented the development of obesity. In summary, the present results show that intact female bLHbeta-CTP mice are obese, have increased food consumption, and reduced BAT thermogenic activity. The weight gain can be explained partly by elevated androgens but is probably also contributed to the increased adrenal steroidogenesis. Hence, the bLHbeta-CTP mice provide a useful model for studying obesity related to elevated LH secretion, with consequent alterations in ovarian and adrenal function.

Consequences of elevated luteinizing hormone on diverse physiological systems: use of the LHbetaCTP transgenic mouse as a model of ovarian hyperstimulation-induced pathophysiology

Chronically elevated luteinizing hormone (LH) induces significant pathology in the LHbetaCTP transgenic mouse model, which uses the bovine gonadotropin alpha (alpha)-subunit promoter to direct transgene expression specifically to gonadotropes in the anterior pituitary. Previously, it was shown that female LHbetaCTP mice are infertile due to anovulation, develop granulosa cell tumors, and undergo precocious puberty from elevated LH and steroid hormones that fail to completely repress the alpha-subunit promoter. This chapter will discuss recent studies that further elucidate the impact of chronically elevated LH on diverse physiological systems. Granulosa cell tumors induced by elevated LH are strain dependent and prevented when transgenics are treated with human chorionic gonadotropin (hCG) surges. A granulosa cell tumor-associated transcriptome is generated, revealing several possible gene candidates for ovarian granulosa cell tumorigenesis. Primordial follicles in LHbetaCTP transgenics become depleted and oocytes exhibit increased rates of meiotic segregation defects, although meiotic competency is acquired normally. Anovulation can be rescued in transgenics by superovulation, though pregnancy fails at midgestation due to maternal factors. Uterine receptivity defects prevent implantation of normal embryos following induction of pseuodpregnancy. Transgenics develop Cushing-like adrenocortical hyperfunction with increased corticosterone production following induction of adrenal LH receptor expression. Elevated LH acts as a tumor promoter in the gonads and the adrenal gland, when expressed in conjunction with the inhibin-alpha SV40 transgene. Finally, chronic elevated LH promotes mammary tumorigenesis. The understanding of multiple clinical pathologies--including ovarian cancer, perimenopausal reproductive aging, premature ovarian failure, polycystic ovarian syndrome, Cushing's syndrome, and breast cancer--may be enhanced through further study of this useful transgenic mouse model.

High levels of luteinizing hormone analog stimulate gonadal and adrenal tumorigenesis in mice transgenic for the mouse inhibin-alpha-subunit promoter/Simian virus 40 T-antigen fusion gene

Transgenic (TG) mice expressing the Simian virus 40 T-antigen under the control of the murine inhibin-alpha promoter (Inhalpha/Tag) develop granulosa and Leydig cell tumors at the age of 5-6 months, with 100% penetrance. When these mice are gonadectomized, they develop adrenocortical tumors. Suppression of gonadotropin secretion inhibits the tumorigenesis in the gonads of intact animals and in the adrenals after gonadectomy. To study further the role of luteinizing hormone (LH) in gonadal and adrenal tumorigenesis, a double TG mouse model was generated by crossing the Inhalpha/Tag mice with mice producing constitutively elevated levels of LH (bLHbeta-CTP mice). Our results show that in double TG mice (bLHbeta-CTP/Inhalpha/Tag), gonadal tumorigenesis starts earlier and progresses faster than in Inhalpha/Tag mice. Both ovarian and testicular tumors were histologically comparable with the tumors found in Inhalpha/Tag mice. In addition, adrenal tumorigenesis was found in intact double TG females, but not in Inhalpha/Tag females. Inhibin-alpha and LH receptor (LHR) were highly expressed in tumorigenic gonadal tissues, and the elevated LH levels were shown to be associated with ectopic LHR and high inhibin-alpha expression in the female adrenals. We conclude that in the Inhalpha/Tag tumor mouse model, elevated LH levels act as a tumor promoter, advancing gonadal and adrenal tumorigenesis.

Ovarian hyperstimulation by LH leads to mammary gland hyperplasia and cancer predisposition in transgenic mice

Many risk factors for breast cancer are associated with hormonally regulated events. Although numerous mouse models of mammary cancer exist, few address the roles of hormones in spontaneous tumor formation. Here we report that transgenic mice that overexpress LH, resulting in ovarian hyperstimulation, undergo precocious mammary gland development. A significant increase in proliferation leads to ovary-dependent mammary gland hyperplasia. Transgenic glands morphologically mimic those of wild-type pregnant mice and expression levels of multiple milk protein genes are comparable with what is observed at d 14 of pregnancy. In addition to sustained hyperplasia, spontaneous mammary tumors were observed with a mean latency of 41 wk, indicating that chronic hormonal stimulation causes mammary cancer. Although hormonally induced, these tumors lack expression of progesterone receptor, suggesting that following initiating events, the tumors may become hormone independent. This mouse model likely holds great potential as a tool for discovery of hormone-mediated mechanisms of breast cancer and identification of future targets for breast cancer prevention and treatment.

Ovulatory surges of human CG prevent hormone-induced granulosa cell tumor formation leading to the identification of tumor-associated changes in the transcriptome

Granulosa cell tumors comprise approximately 10% of ovarian tumors and, although rare, are clinically important due to their potential for malignancy and recurrence. Although their morphological features have been carefully described, the global changes in gene expression associated with their formation remain undetermined. To initiate this characterization, we used a transgenic mouse model in which granulosa cell tumors occur with 100% penetrance in CF-1 mice that harbor a novel transgene encoding a chimeric LHbeta subunit. When this transgene is expressed in other strains of mice, including (C57BL/6 female symbol x CF-1 male symbol,Tg) F1 hybrids, luteomas develop even though levels of LH remain high. This dichotomous response permits a longitudinal comparison of global changes in transcriptomes uniquely associated with either granulosa cell tumors or luteomas. Herein we report numerous changes in the transcriptome, including a decrease in LH receptor mRNA and increases in several mRNAs that encode secreted proteins previously associated with granulosa cell tumors. Furthermore, we identified a constellation of mRNAs that encode proteins that may serve as new markers for this tumor phenotype. Additional experiments indicated that periodic treatment with human CG prevented formation of granulosa cell tumors in mice genetically predisposed to tumor development and, instead, led to the appearance of luteomas. More importantly, ovarian transcriptomes from the luteomas induced by ovulatory doses of human CG permitted refined confirmation of gene expression changes that were uniquely associated with either granulosa cell tumors in the permissive CF-1 genetic background or in luteomas in the F1 hybrids. Together, these dynamic changes in the ovarian transcriptome indict various signaling pathways potentially involved in mediating the actions of LH over time and, depending on genetic background, the formation of either a luteoma or a granulosa cell tumor.

A single Pitx1 binding site is essential for activity of the LHbeta promoter in transgenic mice

Reproduction depends on regulated expression of the LHbeta gene. Tandem copies of regulatory elements that bind early growth response protein 1 (Egr-1) and steroidogenic factor 1 (SF-1) are located in the proximal region of the LHbeta promoter and make essential contributions to its activity as well as mediate responsiveness to GNRH: Located between these tandem elements is a single site capable of binding the homeodomain protein Pitx1. From studies that employ overexpression paradigms performed in heterologous cell lines, it appears that Egr-1, SF-1, and Pitx1 interact cooperatively through a mechanism that does not require the binding of Pitx1 to its site. Since the physiological ramifications of these overexpression studies remain unclear, we reassessed the requirement for a Pitx1 element in the promoter of the LHbeta gene using homologous cell lines and transgenic mice, both of which obviate the need for overexpression of transcription factors. Our analysis indicated a striking requirement for the Pitx1 regulatory element. When assayed by transient transfection using a gonadotrope-derived cell line (LbetaT2), an LHbeta promoter construct harboring a mutant Pitx1 element displayed attenuated transcriptional activity but retained responsiveness to GNRH: In contrast, analysis of wild-type and mutant expression vectors in transgenic mice indicated that LHbeta promoter activity is completely dependent on the presence of a functional Pitx1 binding site. Indeed, the dependence on an intact Pitx1 binding site in transgenic mice is so strict that responsiveness to GnRH is also lost, suggesting that the mutant promoter is inactive. Collectively, our data reinforce the concept that activity of the LHbeta promoter is determined, in part, through highly cooperative interactions between SF-1, Egr-1, and Pitx1. While Egr-1 can be regarded as a key downstream effector of GnRH, and Pitx1 as a critical partner that activates SF-1, our data firmly establish that the Pitx1 element plays a vital role in permitting these functions to occur in vivo.