Non-classical genomic estrogen receptor (ER)/specificity protein and ER/activating protein-1 signaling pathways

17beta-estradiol binds to the estrogen receptor (ER) to activate gene expression or repression and this involves both genomic (nuclear) and non-genomic (extranuclear) pathways. Genomic pathways include the classical interactions of ligand-bound ER dimers with estrogen-responsive elements in target gene promoters. ER-dependent activation of gene expression also involves DNA-bound ER that subsequently interacts with other DNA-bound transcriptions factors and direct ER-transcription factor (protein-protein) interactions where ER does not bind promoter DNA. Ligand-induced activation of ER/specificity protein (Sp) and ER/activating protein-1 [(AP-1); consisting of jun/fos] complexes are important pathways for modulating expression of a large number of genes. This review summarizes some of the characteristics of ER/Sp- and ER/AP-1-mediated transactivation, which are dependent on ligand structure, cell context, ER-subtype (ERalpha and ERbeta), and Sp protein (SP1, SP3, and SP4) and demonstrates that this non-classical genomic pathway is also functional in vivo.

Cancer chemotherapy with indole-3-carbinol, bis(3'-indolyl)methane and synthetic analogs

Indole-3-carbinol (I3C) conjugates are phytochemicals expressed in brassica vegetables and have been associated with the anticancer activities of vegetable consumption. I3C and its metabolite bis(3'-indolyl)methane (DIM) induce overlapping and unique responses in multiple cancer cell lines and tumors, and these include growth inhibition, apoptosis and antiangiogenic activities. The mechanisms of these responses are complex and dependent on cell context. I3C and/or DIM activate or inactivate multiple nuclear receptors, induce endoplasmic reticulum stress, decrease mitochondrial membrane potential, and modulate multiple signaling pathways including kinases. DIM has been used as a template to synthesize a series of 1,1-bis(3'indolyl)-1-(substituted aromatic)methanes (i.e. C-DIMs) which are also cytotoxic to cancer cells and tumors. Some of the effects of C-DIMs resemble those reported for DIM analogs; however, structure-activity studies with the aromatic ring has resulted in generation of highly unique receptor agonists. For example, p-trifluoromethylphenyl, p-t-butylphenyl and p-biphenyl analogs activate peroxisome proliferator-activated receptor gamma (PPARgamma), and p-methoxyphenyl and p-phenyl compounds activate nerve growth factor-induced-Balpha (NGFI-Balpha, Nur77) orphan nuclear receptor. The effects of C-DIMs on PPARgamma and Nur77 coupled with their receptor-independent activities has resulted in the development of a novel group of multi-targeted anticancer drugs with excellent potential for clinical treatment of cancer.

Ligand structure-dependent activation of estrogen receptor alpha/Sp by estrogens and xenoestrogens

This study investigated the effects of E2, diethylstilbestrol (DES), antiestrogens, the phytoestrogen resveratrol, and the xenoestrogens octylphenol (OP), nonylphenol (NP), endosulfan, kepone, 2,3,4,5-tetrachlorobiphenyl-4-ol (HO-PCB-Cl(4)), bisphenol-A (BPA), and 2,2-bis-(p-hydroxyphenyl)-1,1,1-trichloroethane (HPTE) on induction of luciferase activity in breast cancer cells transfected with a construct (pSp1(3)) containing three tandem GC-rich Sp binding sites linked to luciferase and wild-type or variant ERalpha. The results showed that induction of luciferase activity was highly structure-dependent in both MCF-7 and MDA-MB-231 cells. Moreover, RNA interference assays using small inhibitory RNAs for Sp1, Sp3 and Sp4 also demonstrated structure-dependent differences in activation of ERalpha/Sp1, ERalpha/Sp3 and ERalpha/Sp4. These results demonstrate for the first time that various structural classes of ER ligands differentially activate wild-type and variant ERalpha/Sp-dependent transactivation, selectively use different Sp proteins, and exhibit selective ER modulator (SERM)-like activity.

1,1-Bis(3'-indolyl)-1-(p-biphenyl)methane inhibits basal-like breast cancer growth in athymic nude mice

Introduction

1,1-Bis (3'-indolyl)-1-(p-biphenyl) methane (CDIM9) has been identified as a new peroxisome proliferator-activated receptor (PPAR)-γ agonist that exhibits both receptor dependent and independent antitumor activities. CDIM9 has not previously been studied with respect to its effects against basal-like breast cancer. Our goal in the present study was to investigate the anti-basal-like breast tumor activity of CDIM9 in vitro and in vivo.

Methods

The effects of CDIM9 on cell protein and DNA syntheses were determined in basal-like breast cancer MDA-MB231 and BT549 cells in vitro. Maximum tolerated dose and dose-limited toxicity were determined in BalB/c mice, and antitumor growth activities were assessed in MDA-MB231 basal-like breast tumor xenografts in athymic nude mice.

Results

CDIM9 exhibited selective cell cytotoxicity and anti-proliferation effects on basal-like breast cancer lines. In MDA-MB231 cell, CDIM9 induced caveolin-1 and p27 expression, which was significantly downregulated by co-treatment with the PPAR-γ antagonist GW9662. Nonsteroidal anti-inflammatory drug-activated gene-1 and activating transcription factor-3 were upregulated by CDIM9 through a PPAR-γ independent pathway. CDIM9 (40 mg/kg daily, intraperitoneally, for 35 days) inhibited the growth of subcutaneous MDA-MB231 tumor xenografts by 87%, and produced a corresponding decrease in proliferation index. Nearly half of the treated mice (46%) had complete durable remissions, confirmed by histology. The growth of an established tumor was inhibited by CDIM9 treatment (64 mg/kg daily, intraperitoneally, for 10 days), with a mean tumor growth inhibition of 67% as compared with controls. CDIM9 induced increases in tumor caveolin-1 and p27 in vivo, which may contribute to its antitumor activity in basal-like breast cancer.

Conclusion

CDIM9 showed potent antiproliferative effects on basal-like breast cancer cell in tissue culture and dramatic growth inhibition in animal models at safe doses. These findings justify further development of this drug for treatment of basal-like breast cancer.

Cobaltous chloride and hypoxia inhibit aryl hydrocarbon receptor-mediated responses in breast cancer cells

The aryl hydrocarbon receptor (AhR) is expressed in estrogen receptor (ER)-positive ZR-75 breast cancer cells. Treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces CYP1A1 protein and mRNA levels and also activates inhibitory AhR-ERalpha crosstalk associated with hormone-induced reporter gene expression. In ZR-75 cells grown under hypoxia, induction of these AhR-mediated responses by TCDD was significantly inhibited. This was not accompanied by decreased nuclear AhR levels or decreased interaction of the AhR complex with the CYP1A1 gene promoter as determined in a chromatin immunoprecipitation assay. Hypoxia-induced loss of Ah-responsiveness was not associated with induction of hypoxia-inducible factor-1alpha or other factors that sequester the AhR nuclear translocation (Arnt) protein, and overexpression of Arnt under hypoxia did not restore Ah-responsiveness. The p65 subunit of NFkappaB which inhibits AhR-mediated transactivation was not induced by hypoxia and was primarily cytosolic in ZR-75 cells grown under hypoxic and normoxic conditions. In ZR-75 cells maintained under hypoxic conditions for 24 h, BRCA1 (an enhancer of AhR-mediated transactivation in breast cancer cells) was significantly decreased and this contributed to loss of Ah-responsiveness. In cells grown under hypoxia for 6 h, BRCA1 was not decreased, but induction of CYP1A1 by TCDD was significantly decreased. Cotreatment of ZR-75 cells with TCDD plus the protein synthesis inhibitor cycloheximide for 6 h enhanced CYP1A1 expression in cells grown under hypoxia and normoxia. These results suggest that hypoxia rapidly induces protein(s) that inhibit Ah-responsiveness and these may be similar to constitutively expressed inhibitors of Ah-responsiveness (under normoxia) that are also inhibited by cycloheximide.

Coactivation of estrogen receptor alpha (ER alpha)/Sp1 by vitamin D receptor interacting protein 150 (DRIP150)

Vitamin D receptor interacting protein (DRIP150) coactivates estrogen receptor alpha (ERalpha)-mediated transactivation in breast cancer cell lines transfected with a construct (pERE(3)) containing three estrogen responsive elements (EREs). In this study, we show that DRIP150 also coactivates ERalpha/Sp1-mediated transactivation in ZR-75, MCF-7, and MDA-MB-231 breast cancer cells transfected with a construct (pSp1(3)) containing three consensus GC-rich motifs. Studies on coactivation of wild-type and variant ERalpha/Sp1 by DRIP150 indicates that the DNA-binding domain and helix 12 in the ligand binding domain of ERalpha are required and the coactivation response is squelched by overexpressing an NR-box peptide that contains two LXXLL motifs from GRIP2. In contrast, coactivation of ERalpha/Sp1 by wild-type and mutant DRIP150 expression plasmids show that coactivation of ERalpha/Sp1 by DRIP150 is independent of the NR-boxes. Deletion analysis of DRIP150 demonstrates that coactivation requires an alpha-helical NIFSEVRVYN (amino acids 795-804) motif within 23 amino acid sequence (789-811) in the central region of DRIP150 and similar results were obtained for coactivation of ERalpha by DRIP150. Thus, although different domains of ERalpha are required for hormone-dependent activation of ERalpha and ERalpha/Sp1, coactivation of these transcription factors by DRIP150 requires the alpha-helical amino acids 795-804. This is the first report of a coactivator that enhances ERalpha/Sp1-mediated transactivation in breast cancer cells.

The role of xenoestrogenic compounds in the development of breast cancer

Lifetime exposure to endogenous steroidal estrogens is an established risk factor for breast cancer, and exposures to other estrogenic and antiestrogenic compounds might also modify the risk of breast cancer. It has been hypothesized that synthetic estrogenic industrial pollutants such as organochlorine compounds and plant-derived estrogenic compounds also modify breast cancer risks; however, recent studies show that levels of organochlorine pollutants are similar in breast cancer patients and controls. There is evidence that synthetic and plant-derived estrogens are selective estrogen receptor modulators, which implies that these compounds can induce tissue-specific, time- and dose-dependent estrogenic or antiestrogenic responses. Therefore, the effects of synthetic or plant-derived estrogens on the incidence of breast cancer depend on both the levels and the timing of exposure to these compounds, particularly during stages of mammary gland development that are extremely sensitive to hormone levels.

Sp transcription factor family and its role in cancer

Specificity protein 1 (Sp1) and other Sp and Krüppel-like factor (KLF) proteins are members of a family of transcription factors which bind GC/GT-rich promoter elements through three C(2)H(2)-type zinc fingers that are present at their C-terminal domains. Sp1-Sp4 proteins regulate expression of multiple genes in normal tissues and tumours. There is growing evidence that some Sp proteins play a critical role in the growth and metastasis of many tumour types by regulating expression of cell cycle genes and vascular endothelial growth factor. Sp/KLF proteins are also potential targets for cancer chemotherapy.

Development of selective aryl hydrocarbon receptor modulators for treatment of breast cancer

The aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix DNA-binding protein that forms a transcriptionally-active heterodimer with the AhR nuclear translocator (Arnt) protein. The nuclear AhR complex is a ligand-induced transcription factor and the environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a high affinity ligand for the AhR. TCDD induces a diverse spectrum of tissue-, sex- and species-specific biochemical and toxic responses in Ah-responsive cells/tissues including the inhibition of 17beta-oestradiol (E2)-induced gene expression in the rodent uterus and mammary and in human breast cancer cell lines. TCDD also inhibits spontaneous and carcinogen-induced mammary tumour formation and growth in rodent models. Research in this laboratory has utilised the AhR as a target for developing anticancer drugs for treatment of breast cancer and two different structural classes of selective AhR modulators (SAhRMs) have been developed. Alternate-substituted (1,3,6,8- and 2,4,6,8-) alkyl polychlorinated dibenzofurans (PCDFs) and substituted diindolylmethanes (DIMs) bind the AhR and induce a pattern of AhR-oestrogen receptor (ER) inhibitory cross-talk similar to that observed for TCDD including inhibition of mammary tumour growth at doses < 1.0 mg/kg/day. In contrast, effective doses of these compounds do not induce hepatic CYP1A1-dependent activity or other AhR-mediated toxic responses induced by TCDD. These results indicate that SAhRMs may be an important new class of drugs for clinical treatment of breast cancer via AhR-ER inhibitory cross-talk.

Cyclooxygenase-2 inhibitors decrease vascular endothelial growth factor expression in colon cancer cells by enhanced degradation of Sp1 and Sp4 proteins

Cyclooxygenase 2 (COX-2) inhibitors, such as celecoxib (Cel), nimesulfide (NM), and NS-398 [NS; N-[2-(cyclohexyloxy)-4-nitrophenyl]methanesulfonamide] and other nonsteroidal anti-inflammatory drugs inhibit colon cancer growth and angiogenesis; however, the mechanism of this response is not well defined. Treatment of SW-480 colon cancer cells with Cel, NS, or NM decreased vascular endothelial growth factor (VEGF) mRNA and immunoreactive protein expression. This was also accompanied by decreased transactivation in cells transfected with constructs containing VEGF gene promoter inserts. Deletion analysis of the VEGF promoter indicated that decreased VEGF expression by COX-2 inhibitors was associated with the proximal -131 to -47 GC-rich region of the VEGF promoter that binds Sp proteins. Treatment of SW-480 cells with Cel, NM, and NS also decreased Sp1 and Sp4 protein expression but not that of Sp2 or Sp3. Similar results were observed in RKO, HT-29, and DLD colon cancer cells demonstrating comparable responses in COX-2-expressing and -nonexpressing colon cancer cell lines. COX-2 inhibitors do not affect Sp1 or Sp4 mRNA levels in SW-480 cells; however, decreased expression of both proteins was accompanied by increased protein ubiquitination and inhibited by the proteasome inhibitor gliotoxin. These results suggest that the antiangiogenic activity of COX-2 inhibitors in colon cancer cells is linked to activation of proteasome-dependent degradation of Sp1 and Sp4 proteins.

Clinical correlates of environmental endocrine disruptors

Endocrine disrupting chemicals (EDCs), such as environmental estrogens, are hypothesized to be associated with a global decrease in sperm counts, other male reproductive tract problems and increasing rates of female breast cancer. Results of human population studies do not support the association between certain organochlorine EDCs and female breast cancer. Moreover, there is minimal evidence linking EDCs or exposure to other environmental chemicals with male reproductive tract problems. With the exception of the increasing incidence of testicular cancer, it is also questionable whether male reproductive tract problems are increasing, decreasing or unchanged. However, several studies report large differences in sperm count and quality and other endocrine-related problems within countries and regions, but the environmental, dietary and/or lifestyle factors responsible remain unknown.

Endocrine disruptors and human health: is there a problem

It has been hypothesized that endocrine-active chemicals (EACs) may be responsible for the increased incidence of breast cancer and disorders of the male reproductive tract. Synthetic chemicals with estrogenic activity (xenoestrogen) and the organochlorine environmental contaminants polychlorinated biphenyls (PCBs) and DDE have been the prime etiologic suspects. However, results of extensive research on PCBs and DDE does not show a correlation between PCB/DDE exposure and development of breast cancer. Studies also show that sperm count levels vary with demography, and the hypothesized coordinate global decrease in sperm counts and other disorders of the male reproductive tract is not supported by published data. In contrast, testicular cancer is increasing in most countries, and causal environmental/lifestyle factors for this disease are unknown.

Problems for risk assessment of endocrine-active estrogenic compounds

Estrogenic industrial compounds such as bisphenol A (BPA) and nonylphenol typically bind estrogen receptor (ER) alpha and ERBeta and induce transactivation of estrogen-responsive genes/reporter genes, but their potencies are usually greater than or equal to 1,000-fold lower than observed for 17Beta-estradiol. Risk assessment of estrogenic compounds on the basis of their potencies in simple reporter gene or binding assays may be inappropriate. For example, selective ER modulators (SERMs) represent another class of synthetic estrogens being developed for treatment of hormone-dependent problems. SERMs differentially activate wild-type ERalpha and variant forms expressing activation function 1 (ER-AF1) and AF2 (ER-AF2) in human HepG2 hepatoma cells transfected with an estrogen-responsive complement C3 promoter-luciferase construct, and these in vitro differences reflect their unique in vivo biologies. The HepG2 cell assay has also been used in our laboratories to investigate the estrogenic activities of the following structurally diverse synthetic and phytoestrogens: 4 -hydroxytamoxifen; BPA; 2 ,4 ,6 -trichloro-4-biphenylol; 2 ,3 ,4 ,5 -tetrachloro-4-biphenylol; p-t-octylphenol; p-nonylphenol; naringenin; kepone; resveratrol; and 2,2-bis(p-hydroxyphenyl)-1,1,1-trichloroethane. The results show that synthetic and phytoestrogens are weakly estrogenic but induce distinct patterns of ER agonist/antagonist activities that are cell context- and promoter-dependent, suggesting that these compounds will induce tissue-specific (in vivo(ER agonist or antagonist activities. These results suggest that other receptors, such as the aryl hydrocarbon receptor, that also bind structurally diverse ligands may exhibit unique responses in vivo that are not predicted by standard in vitro bioassays.

Environmental estrogens: roles in male reproductive tract problems and in breast cancer

It has been hypothesized that endocrine disruptors and particularly synthetic estrogenic environmental contaminants (xenoestrogens) are etiologic factors in the global decrease of sperm counts and other problems of the male reproductive tract, including cryptorchidism, hypospadias, and testicular cancer. Xenoestrogens might also be linked to an increased incidence of breast cancer in women. These hypotheses have stimulated research addressing the current incidence rates and time-dependent changes in these human health problems, and the results do not support the hypotheses. With the exception of testicular cancer, the evidence does not indicate that sperm counts and other male reproductive tract problems are increasing, and correlations with xenoestrogen exposure have not been made. Moreover, initial observations that the level of certain organochlorine pollutants is higher in breast cancer patients vs. controls have not been duplicated in more recent studies. The results do not preclude an environmental eitology to some of these health problems or to susceptible subpopulations, and these areas require further research and critical scrutiny.

Toxicology of environmental estrogens

It has been hypothesized that environmental contaminants that modulate endocrine signaling pathways may be causally linked to adverse health effects in humans. There has been particular concern regarding synthetic estrogens and their role in disrupting normal development of the male reproductive tract. Most estrogenic industrial compounds, such as bisphenol A (BPA) and nonylphenol, typically bind estrogen receptors alpha (ERalpha) and beta (ERbeta) and induce transactivation of estrogen-responsive genes/reporter genes, but their potencies are usually > or = 1,000-fold lower than observed for 17beta-estradiol (E2). Selective estrogen receptor modulators (SERMs) represent another class of synthetic estrogens that are being developed for treatment of hormone-dependent problems. The SERMs differentially activate wild-type ERalpha and variant forms expressing activation function 1 (ER-AF1) and AF2 (ER-AF2) in human HepG2 hepatoma cells transfected with a pC3-luciferase construct, and these in vitro differences reflect their unique in vivo biologies. The HepG2 cell assay has also been used in our laboratories to investigate the estrogenic activities of the following structurally diverse synthetic and phytoestrogens: 4'-hydroxytamoxifen; BPA; 2',4',6'-trichloro-4-biphenylol; 2',3',4',5'-tetrachloro-4-biphenylol; p-t-octylphenol; p-nonylphenol; naringenin; kepone; resveratrol; and 2,2-bis(p-hydroxyphenyl)-1,1,1-trichloroethane (HPTE). The results show that synthetic and phytoestrogens induce distinct patterns of gene activation in HepG2 and U2 osteogenic sarcoma cells, suggesting that these compounds will induce tissue-specific in vivo ER agonist or antagonist activities. The predicted differences between these compounds, based on results of the in vitro bioassay, have been confirmed. For example, BPA inhibits E2-induced responses in the rodent uterus, and HPTE and structurally related compounds are ERalpha agonists and ERbeta antagonists in assays carried out in HepG2 and other cancer cell lines.

Mechanism of action and development of selective aryl hydrocarbon receptor modulators for treatment of hormone-dependent cancers (Review)

Ligand-activated receptors are extensively used as targets for developing tissue-selective drugs for treatment of multiple diseases including cancers. The aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix transcription factor that binds both synthetic chemicals such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and naturally-occurring phytochemicals, sterols and heme breakdown products. The high affinity ligand TCDD induces several AhR-mediated changes in gene expression, tissue/species-specific toxicities, and both tumorigenic and anticarcinogenic responses including inhibition of estrogen-dependent mammary and uterine tumor formation and growth. Research in this laboratory has demonstrated that TCDD inhibits E2-induced responses in the rodent uterus and mammary tumors (growth inhibition) and in breast and endometrial cancer cell lines through complex inhibitory AhR-estrogen receptor (ER) crosstalk. 6-Alkyl-1,3,8-trichlorodibenzofurans and substituted diindolylmethanes represent two structural classes of selective AhR modulators (SAhRMs). These compounds are relatively non-toxic and inhibit ER-positive and ER-negative mammary tumor growth, and synergize with tamoxifen to inhibit breast cancer growth and block tamoxifen-induced estrogenic activity in the uterus. Preliminary studies also indicate that SAhRMs inhibit prostate cancer cell growth, and there is evidence for inhibitory AhR-androgen receptor crosstalk. SAhRMs represent a novel class of drugs for treatment of hormone-dependent cancers, and combined therapies of SAhRMs with tamoxifen and other selective ER modulators (SERMs) provides a new approach for treating women with breast cancer.

Molecular biology of the Ah receptor and its role in carcinogenesis

The aryl hydrocarbon receptor (AhR) is a ligand-activated nuclear transcription factor that mediates responses to toxic halogenated aromatic toxins such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polynuclear aromatic hydrocarbons, combustion products, and numerous phytochemicals such as flavonoids and indole-3-carbinol (I3C). The nuclear AhR complex is a heterodimer containing the AhR and AhR nuclear translocator (Arnt) proteins, and the molecular mechanism of AhR action is associated with binding of the heterodimer to dioxin responsive elements (DREs) in regulatory regions of Ah-responsive genes. TCDD, a 'xenodioxin', is a multi-site carcinogen in several species and possibly in humans, whereas natural AhR ligands including I3C and flavonoids tend to protect against cancer. Both TCDD and phytochemicals inhibit estrogen-induced breast and endometrial cancer, and the molecular mechanisms of this common response will be described.

Bisphenol A and related endocrine disruptors

The article highlighted in this issue is "Bisphenol A-Induced Increase in Uterine Weight and Alterations in Uterine Morphology in Ovariectomized B6C3F1 Mice: Role of the Estrogen Receptor" by Andriana D. Papaconstantinou, Thomas H. Umbreit, Benjamin R. Fisher, Peter L. Goering, Nicholas T. Lappas, and Ken M. Brown (pp. 332-339).

Mechanisms of inhibitory aryl hydrocarbon receptor-estrogen receptor crosstalk in human breast cancer cells

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that forms a functional heterodimeric complex with the AhR nuclear translocator (Arnt) protein. The environmental toxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is a high affinity ligand for the AhR and has been extensively used to investigate AhR-mediated biochemical and toxic responses. TCDD modulates several endocrine pathways including inhibition of 17beta-estradiol-induced responses in the immature and ovariectomized rodent uterus and mammary gland and in human breast cancer cell lines. TCDD inhibits formation and growth of mammary tumors in carcinogen-induced rodent models and relatively nontoxic selective AhR modulators (SAhRMs) are being developed for treatment of breast cancer. The mechanisms of inhibitory AhR-estrogen receptor (ER) crosstalk have been investigated in MCF-7 breast cancer cells by analysis of promoter regions of genes induced by E2 and inhibited by TCDD. AhR-mediated inhibition of E2-induced cathepsin D, pS2, c-fos, and heat shock protein 27 gene expression involves direct interaction of the AhR complex with inhibitory pentanucleotide (GCGTG) dioxin responsive elements (iDREs) resulting in disruption of interactions between proteins binding DNA elements required for ER action and the basal transcription machinery. Mechanisms of inhibitory AhR-ER crosstalk indicate that functional iDREs are required for inhibition of some genes; however, results indicate that other interaction pathways are important including AhR-mediated proteasome-dependent degradation of the ER.

Ah receptor agonists as endocrine disruptors: antiestrogenic activity and mechanisms

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds induce a broad spectrum of biochemical and toxic responses and disrupt multiple endocrine pathways. Research in this laboratory has focused on characterizing aryl hydrocarbon receptor (AhR)-mediated antiestrogenicity in the rodent uterus and mammary and in human breast cancer cells. TCDD inhibits multiple estrogen (E2)-induced responses in these tissues including development or growth of human mammary and endometrial cancer cells, carcinogen-induced mammary cancer in rats, and mammary cancer in mice bearing breast cancer cell xenografts. The mechanisms of AhR-mediated antiestrogenicity are complex; however, studies on the molecular biology of cross-talk between the AhR and estrogen-receptor (ER) signaling pathways have been initiated using several E2-regulated genes as models. The results indicate that the nuclear AhR complex targets specific genomic core inhibitory dioxin responsive elements (iDREs) in promoter regions of some E2-responsive target genes to inhibit hormone-induced transactivation. The pS2, cathepsin and c-fos genes have functional iDREs, whereas the iDRE in the progesterone receptor gene promoter was not functional. Research has also focused on development of AhR-based antiestrogens which inhibit mammary tumor development and growth but do not exhibit prototypical AhR-induced toxic responses.

Methods for xenoestrogen testing

Research in our laboratories has focused on development of a battery of in vivo and in vitro bioassays for determining estrogenic activity and potency of different classes of natural and synthetic industrial-derived estrogenic compounds (xenoestrogens) including food/beverage extracts, phytoestrogens, phenolic compounds, organochlorine pesticides and pollutants. For many of the weak estrogenic compounds, their activity as estrogen receptor (ER) agonists or antagonists is dependent on the gene/gene promoter, cell context and expression of ER(alpha) or ER(beta) isoform. For example, extracts of red wine, bound to the ER, exhibited estrogenic activity in T47D, MCF-7 (breast) and Hep G2 (liver) human cancer cell lines, whereas reconstituted organochlorine pesticide residues found in food were active only in Hep G2 cells that transiently expressed ER(alpha) or ER(beta). The relative potencies of red wine extracts versus reconstituted organochlorine pesticides were assay-dependent; however, estrogen equivalent daily intakes from a glass of red wine (approximately 0.5-2 microg estrogen equivalents/day) were at least 10(3) higher than observed for the reconstituted organochlorine pesticide mixtures. Risk assessment of xenoestrogens and other synthetic chemicals which modulate endocrine responses must take into account high dietary levels of natural products in food, drugs and health food store extracts which also modulate endocrine responses.

Interactions between hormones and chemicals in breast cancer

Development of breast cancer in women is dependent on diverse factors, including genetic predisposition, exposure to both exogenous and endogenous chemicals, which can modulate initiation, promotion and progression of this disease, and the timing of exposure to these agents. Several compounds--including 16 alpha-hydroxyestrone (16 alpha-OHE1), catecholestrogens, and aromatic amines--have been proposed as initiators of mammary carcinogenesis in humans; however, their role as genotoxins is unconfirmed. Lifetime exposure to estrogens has been established as an important risk factor for breast cancer, and it has been suggested that xenoestrogens may directly add to the hormonal risk or indirectly increase risk by decreasing 2-hydroxyestrone (2-OHE1)/16 alpha-OHE1 metabolite ratios. Results of recent studies suggest that chemical-induced modulation of 2-OHE1/16 alpha-OHE1 metabolite ratios is not predictive of xenoestrogens or mammary carcinogens. Moreover, based on current known dietary intakes of natural and xenoestrogenic/antiestrogenic chemicals, it is unlikely that xenoestrogens contribute significantly to a woman's overall lifetime exposure to estrogens. More information is required on the identities and serum levels of both natural and xenoendocrine active compounds, their concentrations in serum, and the mammary gland and levels of these compounds at critical periods of exposure.

Estrogen- and antiestrogen-responsiveness of HEC1A endometrial adenocarcinoma cells in culture

HEC1A endometrial cancer cells express the wild-type form of the estrogen receptor (ER) and 17beta-estradiol (E2) induces proliferation of these cells. In contrast, tamoxifen only causes a minimal increase (<20%) in cell proliferation. In HEC1A cells transiently transfected with the C3-Luc plasmid derived from the complement C3 gene, both E2 and tamoxifen exhibited ER agonist activity and tamoxifen was also a partial antagonist for this response. The relative ER agonist/antagonist activities of E2, tamoxifen and ICI 182,780 were also investigated in HEC1A1 cells transiently transfected with two E2-responsive plasmids, pCATHD-CAT and pCKB-CAT which contain 5'-promoter inserts from the cathepsin D and creatine kinase B genes, respectively. The results showed that E2 and tamoxifen induced reporter gene activity in cells transiently transfected with both constructs. ICI 182,780 exhibited partial ER agonist activity only in cells transiently transfected with pCKB-CAT and antagonized E2-induced reporter gene activity using both the CKB- and CATHD-derived constructs. These results demonstrate that HEC1A endometrial cancer cells are E2-responsive and represent a useful cell culture model for understanding hormone/antihormone-induced endometrial cell responses.