Indolo[3,2-b]carbazole: a dietary-derived factor that exhibits both antiestrogenic and estrogenic activity

BACKGROUND

Indole-3-carbinol (I3C) and related compounds have been identified in vegetables of the Brassica genus. I3C and its acid-derived condensation product, indolo[3,2-b]carbazole (ICZ), bind to the aryl hydrocarbon (Ah) receptor and induce CYP1A1/1A2 gene expression in both in vivo and in vitro models. I3C also inhibits mammary tumor development in rodent models.

PURPOSE

The major focus of this study was to investigate the induction of CYP1A1-dependent activity and antiestrogenic effects of ICZ in the MCF-7 human breast cancer cell line and determine if induction of CYP1A1 is required for observed antiestrogenic responses.

METHODS

The induction of CYP1A1 in MCF-7 cells was determined by measuring time- and concentration-dependent changes in ethoxyresorufin O-deethylase (EROD) activity in response to ICZ treatment. The effects of ICZ on occupied nuclear estrogen receptor (ER) levels and inhibition of estrogen (17 beta-estradiol [E2])-induced cell proliferation, [3H]thymidine uptake, secretion of the 52-kd protein, and nuclear progesterone receptor (PR) levels were also measured. Chloramphenicol acetyl transferase (CAT) activity was assayed in MCF-7 cells transiently transfected with an estrogen-responsive vit-CAT plasmid. Competitive binding to rat cytosolic ER was also examined.

RESULTS

ICZ (> or = 10 nM) induced CYP1A1 in MCF-7 human breast cancer cells. This compound also elicited a diverse spectrum of antiestrogenic responses, including inhibition of E2-induced cell proliferation, [3H]thymidine uptake, occupied nuclear PR binding, and CAT activity in cells transfected with the estrogen-responsive vit-CAT plasmid. In nuclear extracts from ICZ-treated cells, there was a decrease in ER levels and binding to an estrogen-responsive element in a gel shift assay. I3C also decreased nuclear ER binding in MCF-7 cells. ICZ bound with low affinity to the ER and exhibited weak estrogen-like activity.

CONCLUSIONS

Like other Ah receptor agonists, ICZ is antiestrogenic in human breast cancer cells, and this activity is consistent with the inhibitory activity of I3C on mammary tumor formation in rodents. ICZ-induced antiestrogenic responses can be observed at times or concentrations in which EROD activity is unchanged, indicating an interaction between the Ah receptor and ER-mediated endocrine pathways that is independent of P450-induced hormone metabolism. ICZ also is a weak estrogen in MCF-7 cells and binds to the ER.

IMPLICATIONS

The current focus on the role of dietary and environmental estrogens in human disease should take into account the possible contra-active effects of Ah receptor agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), ICZ, I3C, and related compounds that exhibit antiestrogenic activity.

Naringenin: a weakly estrogenic bioflavonoid that exhibits antiestrogenic activity

Treatment of immature 21-day-old female Sprague-Dawley rats with 17 beta-estradiol (E2) (0.5 microgram/rat) caused a significant increase in uterine wet weight, DNA synthesis, progesterone receptor (PR) binding, and peroxidase activity. At doses as high as 40 mg/rat, the bioflavonoid naringenin did not cause a significant increase in any of these E2-induced responses. However, in rats cotreated with E2 (0.5 microgram/rat) plus naringenin (30 mg/rat); there was a significant decrease in E2-induced uterine wet weight, DNA synthesis, PR binding, and peroxidase activity, indicating that naringenin exhibits antiestrogenic activity in the immature rodent uterus. The binding of uterine nuclear extracts to a 32P-labeled estrogen responsive element (ERE) or progesterone responsive element (PRE) was determined using gel electrophoretic band shift assays. Incubation of [32P]ERE with uterine nuclear extracts from rats treated with naringenin or E2 resulted in the formation of estrogen receptor (ER):ERE complexes; a higher mobility complex was prominent in the extracts from E2-treated rats, whereas a lower mobility complex was observed using nuclear extracts from naringenin-treated animals. There was a significant decrease in the intensity of the E2-induced complex using nuclear extracts from rats treated with E2 plus naringenin. In contrast, transformed cytosol from control rats gave an intense ER:ERE complex, whereas the intensity of the band was decreased markedly using transformed uterine cytosol from treated rats. Formation of a PR:PRE complex was also determined using transformed uterine cytosol. Cytosol from E2-treated rats gave an intense retarded band, whereas only weak bands were observed using cytosols from DMSO- (solvent), naringenin-, or naringenin plus E2-treated cells. The results of in vitro studies showed that 1 nM E2 increased (3- to 4-fold) the growth of MCF-7 human breast cancer cells, whereas 1-1000 nM naringenin had no effect on cell proliferation. In cells cotreated with 1 nM E2 plus 1000 nM naringenin, there was a significant decrease in E2-induced cell growth. In MCF-7 cells transiently transfected with a pS2 promoter-regulated luciferase reporter gene, naringenin exhibited weak estrogenic activity. In cells cotreated with 0.1 or 1.0 microM naringenin plus 1 nM E2, naringenin inhibited E2-induced luciferase activity. The results of these studies confirmed that naringenin is a weak estrogen that also exhibits partial antiestrogenic activity in the female rat uterus and MCF-7 human breast cancer cells.

GT-094, a NO-NSAID, inhibits colon cancer cell growth by activation of a reactive oxygen species-microRNA-27a: ZBTB10-specificity protein pathway

Ethyl 2-((2,3-bis(nitrooxy)propyl)disulfanyl)benzoate (GT-094) is a novel nitric oxide (NO) chimera containing an nonsteroidal anti-inflammatory drug (NSAID) and NO moieties and also a disulfide pharmacophore that in itself exhibits cancer chemopreventive activity. In this study, the effects and mechanism of action of GT-094 were investigated in RKO and SW480 colon cancer cells. GT-094 inhibited cell proliferation and induced apoptosis in both cell lines and this was accompanied by decreased mitochondrial membrane potential (MMP) and induction of reactive oxygen species (ROS), and these responses were reversed after cotreatment with the antioxidant glutathione. GT-094 also downregulated genes associated with cell growth [cyclin D1, hepatocyte growth factor receptor (c-Met), epidermal growth factor receptor (EGFR)], survival (bcl-2, survivin), and angiogenesis [VEGF and its receptors (VEGFR1 and VEGFR2)]. Results of previous RNA interference studies in this laboratory has shown that these genes are regulated, in part, by specificity protein (Sp) transcription factors Sp1, Sp3, and Sp4 that are overexpressed in colon and other cancer cell lines and not surprisingly, GT-094 also decreased Sp1, Sp3, and Sp4 in colon cancer cells. GT-094-mediated repression of Sp and Sp-regulated gene products was due to downregulation of microRNA-27a (miR-27a) and induction of ZBTB10, an Sp repressor that is regulated by miR-27a in colon cancer cells. Moreover, the effects of GT-094 on Sp1, Sp3, Sp4, miR-27a, and ZBTB10 were also inhibited by glutathione suggesting that the anticancer activity of GT-094 in colon cancer cells is due, in part, to activation of an ROS-miR-27a:ZBTB10-Sp transcription factor pathway.

Characterization of the H4IIE rat hepatoma cell bioassay for evaluation of environmental samples containing polynuclear aromatic hydrocarbons (PAHs)

The H4IIE rat hepatoma cell bioassay has been extensively used to assess the toxic equivalents (TEQs) of complex mixtures of halogenated aromatic hydrocarbons in environmental samples. However, there is often a discrepancy between bioassay induction results and toxic equivalents calculated from chemical analysis of samples; the former generally yield higher bioassay-TEQs. Polynuclear aromatic hydrocarbons (PAHs) are a class of chemicals which can significantly contribute to induction-TEQs. Benzo(a)pyrene (BAP), dibenz(a, h)anthracene (DBA), benz(a)anthracene (BA), benzo(k)fluoranthene (BkF), benzo(b)fluoranthene (BbF), chrysene (Chr), and indeno(1,2,3-c,d) pyrene (IdP) are carcinogenic PAHs found in environmental samples, including oysters collected from Galveston Bay. The induction potency of these PAHs relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was determined individually in rat hepatoma H4IIE cells seeded in 6-well plates, and the induction-derived equivalency factors (EFs) relative to TCDD were 0. 000354, 0.00203, 0.000025, 0.00478, 0.00253, 0.00020, 0.0011 for BAP, DBA, BA, BkF, BbF, Chr, and IdP, respectively. Dilutions of a reconstituted PAH mixture containing 23 PAHs (744 to 4466 ng/g total PAHs) with constant percentages of BAP (4.5%), DBA (3.5%), BA (2.4%), BkF (3.7%), BbF (3.5%), Chr (4.7%), and IdP (4.2%) yielded bioassay-derived induction-EQs that ranged from 0.52 to 1.44 ng/g. Oysters exposed in the laboratory to the same PAH mixture for 30 days differentially accumulated the PAHs with time. Bioassay-EQs for these oyster extracts ranged from 0.94 to 5.79 ng/g. These results were similar to the chemically calculated EQs which varied from 0.81 to 3.13 ng/g.

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.

Cytotoxicity of pomegranate polyphenolics in breast cancer cells in vitro and vivo: potential role of miRNA-27a and miRNA-155 in cell survival and inflammation

Several studies have demonstrated that polyphenolics from pomegranate (Punica granatum L.) are potent inhibitors of cancer cell proliferation and induce apoptosis, cell cycle arrest, and also decrease inflammation in vitro and vivo. There is growing evidence that botanicals exert their cytotoxic and anti-inflammatory activities, at least in part, by decreasing specificity protein (Sp) transcription factors. These are overexpressed in breast tumors and regulate genes important for cancer cell survival and inflammation such as the p65 unit of NF-κB. Moreover, previous studies have shown that Pg extracts decrease inflammation in lung cancer cell lines by inhibiting phosphatidylinositol-3,4,5-trisphosphate (PI3K)-dependent phosphorylation of AKT in vitro and inhibiting the activation of NF-kB in vivo. The objective of this study was to investigate the roles of miR-27a-ZBTB10-Sp and miR-155-SHIP-1-PI3K on the anti-inflammatory and cytotoxic activity of pomegranate extract. Pg extract (2.5-50 μg/ml) inhibited growth of BT-474 and MDA-MB-231 cells but not the non-cancer MCF-10F and MCF-12F cells. Pg extract significantly decreased Sp1, Sp3, and Sp4 as well as miR-27a in BT474 and MDA-MB-231 cells and increased expression of the transcriptional repressor ZBTB10. A significant decrease in Sp proteins and Sp-regulated genes was also observed. Pg extract also induced SHIP-1 expression and this was accompanied by downregulation of miRNA-155 and inhibition of PI3K-dependent phosphorylation of AKT. Similar results were observed in tumors from nude mice bearing BT474 cells as xenografts and treated with Pg extract. The effects of antagomirs and knockdown of SHIP-1 by RNA interference confirmed that the anti-inflammatory and cytotoxic effects of Pg extract were partly due to the disruption of both miR-27a-ZBTB10 and miR-155-SHIP-1. In summary, the anticancer activities of Pg extract in breast cancer cells were due in part to targeting microRNAs155 and 27a. Both pathways play an important role in the proliferative/inflammatory phenotype exhibited by these cell lines.

The nuclear receptor TR3 regulates mTORC1 signaling in lung cancer cells expressing wild-type p53

The orphan nuclear receptor TR3 (NR41A and Nur77) is overexpressed in most lung cancer patients and is a negative prognostic factor for patient survival. The function of TR3 was investigated in non-small-cell lung cancer A549 and H460 cells, and knockdown of TR3 by RNA interference (siTR3) inhibited cancer cell growth and induced apoptosis. The prosurvival activity of TR3 was due, in part, to formation of a p300/TR3/ specificity protein 1 complex bound to GC-rich promoter regions of survivin and other Sp-regulated genes (mechanism 1). However, in p53 wild-type A549 and H460 cells, siTR3 inhibited the mTORC1 pathway, and this was due to activation of p53 and induction of the p53-responsive gene sestrin 2, which subsequently activated the mTORC1 inhibitor AMP-activated protein kinase α (AMPKα) (mechanism 2). This demonstrates that the pro-oncogenic activity of TR3 in lung cancer cells was due to inhibition of p53 and activation of mTORC1. 1,1-Bis(3'-indolyl)-1-(p-hydroxyphenyl)methane (DIM-C-pPhOH) is a recently discovered inhibitor of TR3, which mimics the effects of siTR3. DIM-C-pPhOH inhibited growth and induced apoptosis in lung cancer cells and lung tumors in murine orthotopic and metastatic models, and this was accompanied by decreased expression of survivin and inhibition of mTORC1 signaling, demonstrating that inactivators of TR3 represent a novel class of mTORC1 inhibitors.

Mechanism of action of phenethylisothiocyanate and other reactive oxygen species-inducing anticancer agents

Reactive oxygen species (ROS)-inducing anticancer agents such as phenethylisothiocyanate (PEITC) activate stress pathways for killing cancer cells. Here we demonstrate that PEITC-induced ROS decreased expression of microRNA 27a (miR-27a)/miR-20a:miR-17-5p and induced miR-regulated ZBTB10/ZBTB4 and ZBTB34 transcriptional repressors, which, in turn, downregulate specificity protein (Sp) transcription factors (TFs) Sp1, Sp3, and Sp4 in pancreatic cancer cells. Decreased expression of miR-27a/miR-20a:miR-17-5p by PEITC-induced ROS is a key step in triggering the miR-ZBTB Sp cascade leading to downregulation of Sp TFs, and this is due to ROS-dependent epigenetic effects associated with genome-wide shifts in repressor complexes, resulting in decreased expression of Myc and the Myc-regulated miRs. Knockdown of Sp1 alone by RNA interference also induced apoptosis and decreased pancreatic cancer cell growth and invasion, indicating that downregulation of Sp transcription factors is an important common mechanism of action for PEITC and other ROS-inducing anticancer agents.

Crosstalk between estrogen receptor alpha and the aryl hydrocarbon receptor in breast cancer cells involves unidirectional activation of proteasomes

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental toxin that activates the aryl hydrocarbon receptor (AhR) and disrupts multiple endocrine signaling pathways. T47D human breast cancer cells express a functional estrogen receptor alpha (ERalpha) and AhR, and treatment of these cells with 17beta-estradiol (E2) or TCDD resulted in a rapid proteasome-dependent decrease in immunoreactive ERalpha and AhR proteins (>60-80%), respectively. E2 did not affect the AhR, whereas TCDD induced proteasome-dependent degradation of both the AhR and ERalpha in T47D and MCF-7 human breast cancer cells, and these responses were specifically blocked by proteasome inhibitors. Thus, TCDD-induced degradation of ERalpha may contribute to the antiestrogenic activity of AhR agonists and this pathway may be involved in AhR-mediated disruption of other endocrine responses.

The orphan nuclear receptor NR4A1 (Nur77) regulates oxidative and endoplasmic reticulum stress in pancreatic cancer cells

NR4A1 (Nur77, TR3) is an orphan nuclear receptor that is overexpressed in pancreatic cancer and exhibits pro-oncogenic activity. RNA interference of NR4A1 expression in Panc-1 cells induced apoptosis and subsequent proteomic analysis revealed the induction of several markers of endoplasmic reticulum stress, including glucose-related protein 78 (GRP78), CCAAT/enhancer-binding protein-homologous protein (CHOP), and activating transcription factor-4 (ATF-4). Treatment of pancreatic cancer cells with the NR4A1 antagonist 1,1-bis(3'-indolyl)-1-(p-hydroxyphenyl)methane (DIM-C-pPhOH) gave similar results. Moreover, both NR4A1 knockdown and DIM-C-pPhOH induced reactive oxygen species (ROS), and induction of ROS and endoplasmic reticulum stress by these agents was attenuated after cotreatment with antioxidants. Manipulation of NR4A1 expression coupled with gene expression profiling identified a number of ROS metabolism transcripts regulated by NR4A1. Knockdown of one of these transcripts, thioredoxin domain containing 5 (TXNDC5), recapitulated the elevated ROS and endoplasmic reticulum stress; thus, demonstrating that NR4A1 regulates levels of endoplasmic reticulum stress and ROS in pancreatic cancer cells to facilitate cell proliferation and survival. Finally, inactivation of NR4A1 by knockdown or DIM-C-pPhOH decreased TXNDC5, resulting in activation of the ROS/endoplasmic reticulum stress and proapoptotic pathways.

IMPLICATIONS

The NR4A1 receptor is pro-oncogenic, regulates the ROS/endoplasmic reticulum stress pathways, and inactivation of the receptor represents a novel pathway for inducing cell death in pancreatic cancer.

2-Cyano-3,12-dioxooleana-1,9-dien-28-imidazolide (CDDO-Im) directly targets mitochondrial glutathione to induce apoptosis in pancreatic cancer

Surgical resection is the only curative strategy for pancreatic cancer (PC). Unfortunately, >80% of pancreatic cancer patients bear inoperable, locally advanced, chemoresistant tumors demonstrating the urgent need for development of novel therapeutic approaches to treat this disease. Here we report that the synthetic triterpenoid 2-cyano-3,12 dioxooleana-1,9 dien-28-imidazolide (CDDO-Im) antagonizes PC cell growth by inducing apoptosis at submicromolar concentrations. Notably, we demonstrate for the first time that the cytotoxicity of CDDO-Im is accompanied by the rapid and selective depletion of mitochondrial glutathione that results in accumulation of reactive oxygen species, oxidation of the cellular glutathione pool, loss of mitochondrial membrane potential, and phosphatidylserine externalization. The parent compound CDDO as well as the methyl ester of CDDO also depleted mitochondrial glutathione, demonstrating that this effect is mediated by the triterpenoid nucleus of these agents. Co-treatment with sulfhydryl nucleophiles completely prevented apoptosis and loss of viability induced by CDDO-Im, whereas alkylation of intracellular thiols by diethylmaleate or co-treatment with dithiothreitol decreased the accumulation of a biotinylated derivative of CDDO, TP-301, in PC cells, suggesting that intracellular reduced thiols are functional targets of the electrophilic triterpenoid nucleus of CDDO and its derivatives. In conclusion, our report is the first to identify mitochondrial glutathione as a target of CDDO and its derivatives and demonstrates that depletion of this antioxidant in the mitochondria is an effective strategy to induce cell death in PC cells. These results suggest that CDDO and its derivatives may offer a clinical benefit for the treatment of PC.

Resveratrol and quercetin in combination have anticancer activity in colon cancer cells and repress oncogenic microRNA-27a

Resveratrol and quercetin (RQ) in combination (1:1 ratio) previously inhibited growth in human leukemia cells. This study investigated the anticancer activity of the same mixture in HT-29 colon cancer cells. RQ decreased the generation of reactive oxygen species (ROS) by up to 2.25-fold and increased the antioxidant capacity by up to 3-fold in HT-29 cells (3.8-60 μg/mL), whereas IC50 values for viability were 18.13, 18.73, and 11.85 μg/mL, respectively. RQ also induced caspase-3-cleavage (2-fold) and increased PARP cleavage. Specificity protein (Sp) transcription factors are overexpressed in colon and other cancers and regulate genes required for cell proliferation survival and angiogenesis. RQ treatment decreased the expression of Sp1, Sp3, and Sp4 mRNA and this was accompanied by decreased protein expression. Moreover, the Sp-dependent antiapoptotic survival gene survivin was also significantly reduced, both at mRNA and protein levels. RQ decreased microRNA-27a (miR-27a) and induced zinc finger protein ZBTB10, an Sp-repressor, suggesting that interactions of RQ with the miR-27a-ZBTB10-axis play a role in Sp downregulation. This was confirmed by transfection of cells with the specific mimic for miR-27a, which partially reversed the effects of RQ. These findings are consistent with previous studies on botanical anticancer agents in colon cancer cells.

Regulation of vascular endothelial growth factor receptor-1 expression by specificity proteins 1, 3, and 4 in pancreatic cancer cells

Vascular endothelial growth factor receptor-1 (VEGFR1) is expressed in cancer cell lines and tumors and, in pancreatic and colon cancer cells, activation of VEGFR1 is linked to increased tumor migration and invasiveness. Tolfenamic acid, a nonsteroidal anti-inflammatory drug, decreases Sp protein expression in Panc-1 and L3.6pl pancreatic cancer cells, and this was accompanied by decreased VEGFR1 protein and mRNA and decreased luciferase activity on cells transfected with constructs (pVEGFR1) containing VEGFR1 promoter inserts. Comparable results were obtained in pancreatic cancer cells transfected with small inhibitory RNAs for Sp1, Sp3, and Sp4 and all three proteins bound to GC-rich elements in the VEGFR1 promoter. These results show that VEGFR1 is regulated by Sp proteins and that treatment with tolfenamic acid decreases expression of this critical angiogenic factor. Moreover, in vitro studies in Panc-1 cells show that activation of VEGFR1 by VEGFB to increase mitogen-activated protein kinase 1/2 phosphorylation and cell migration on collagen-coated plates is also inhibited by tolfenamic acid. Thus, targeted degradation of Sp proteins is highly effective for inhibiting VEGFR1 and associated angiogenic responses in pancreatic cancer.

Differential interaction of the methoxychlor metabolite 2,2-bis-(p-hydroxyphenyl)-1,1,1-trichloroethane with estrogen receptors alpha and beta

Concern that some chemicals in our environment may affect human health by disrupting normal endocrine function has prompted research on interactions of environmental contaminants with steroid hormone receptors. We compared the activity of 2,2-bis-(p-hydroxyphenyl)-1,1,1-trichloroethane (HPTE), an estrogenic metabolite of the organochlorine pesticide methoxychlor, at estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta). Human hepatoma cells (HepG2) were transiently transfected with either human or rat ERalpha or ERbeta plus an estrogen-responsive, complement 3-luciferase construct containing a complement 3 gene promoter sequence linked to a luciferase reporter gene. After transfection, cells were treated with various concentrations of HPTE in the presence (for detecting antagonism) or absence (for detecting agonism) of 17beta-estradiol. HPTE was a potent ERalpha agonist in HepG2 cells, with EC50 values of approximately 5 x 10(-8) and 10(-8) M for human and rat ERalpha, respectively. In contrast, HPTE had minimal agonist activity with either human or rat ERbeta and almost completely abolished 17beta-estradiol-induced ERbeta-mediated activity. Moreover, HPTE behaved as an ERalpha agonist and an ERbeta antagonist with other estrogen-responsive promoters (ERE-MMTV and vtERE) in HepG2 and HeLa cells. This study demonstrates the complexity involved in determining the mechanism of action of endocrine-active chemicals that may act as agonists or antagonists through one or more hormone receptors.

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.

Immunosuppressive activities of polychlorinated biphenyls in C57BL/6N mice: structure-activity relationships as Ah receptor agonists and partial antagonists

The immunosuppressive activity of polychlorinated biphenyl (PCB) congeners is structure-dependent and 2 classes of compounds, namely the coplanar (class I) and monoortho coplanar (class II) congeners exhibit immunotoxicity. This study extends the structure-immunotoxicity relationships for PCBs by investigating representative congeners from the following structural classes of PCBs: monoortho coplanar (2,3,3',4,4',5-hexachlorobiphenyl, class II); monoortho coplanar minus a single parachloro group (2,3,3',4,5,5'-hexachlorobiphenyl and 2,3,3',4,5'-pentachlorobiphenyl, class III); diortho coplanar (2,3',4,4',5',6-hexachlorobiphenyl, class IV); triortho coplanar (2,2',4,4',5,6'-hexachlorobiphenyl, class V) and a tetraortho-substituted PCB (2,2',4,4',6,6'-hexachlorobiphenyl, class VI). The effects of these compounds on the splenic plaque forming cell response to sheep red blood cells was determined in 7-8 week old male C57BL/6N mice. The results showed that the class II-IV congeners were immunotoxic and with only one exception these compounds also induced hepatic microsomal aryl hydrocarbon hydroxylase and ethoxyresorufin O-deethylase activities and displaced [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) from the cytosolic aryl hydrocarbon (Ah) receptor in competitive binding assays. These results thus extend the structure-activity relationships for PCBs as Ah receptor agonists. The interaction of these PCB congeners with an ED70-90 dose of TCDD (3.7 nmol/kg) showed that only one structural class of compounds, namely class III, partially antagonized TCDD-mediated immunotoxicity.

Polybrominated dibenzo-p-dioxins and related compounds: quantitative in vivo and in vitro structure-activity relationships

The effects of structure on the in vitro receptor binding affinities, aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) induction potencies in rat hepatoma cells were determined for the following compounds: 2-bromo-, 2,7/2,8-dibromo-, 2,3,7-tribromo-, 2,4,6,8/1,3,7,9-tetrabromo-, 2,3,7,8-tetrabromo-, 1,3,7,8-tetrabromo-, 1,2,3,7,8-pentabromo-, 1,2,4,7,8-pentabromo-, 2,3-dibromo-7,8-dichloro-, 2,8-dibromo-3,7-dichloro- and 2-bromo-3,7,8-trichlorodibenzo-p-dioxin. The structure-activity relationships (SARs) for the polybrominated dibenzo-p-dioxins (PBDDs) were comparable for both in vitro responses: the most active compounds were substituted only in the lateral 2,3,7 and 8 position and the addition of non-lateral or removal of lateral halogen substituents reduced the activity of the resultant compound. The biologic and toxic effects of 2,3,7,8-tetrabromo-, 1,3,7,8-tetrabromo-, 1,2,4,7,8-pentabromo-1,2,3,7,8-pentabromo-, 2-bromo-3,7,8-trichloro- and 2,3-dibromo-7,8-dichlorodibenzo-p-dioxin on several receptor-mediated responses (thymic atrophy, body weight loss, hepatic microsomal AHH and EROD induction) were determined in a dose-response fashion in immature male Wistar rats. A comparison of the ED50 values for the in vivo responses demonstrated that the SARs for the PBDDs and brominated polychlorinated dibenzo-p-dioxins were comparable to those observed for in vitro receptor binding and AHH induction. Moreover, there was an excellent linear correlation between the -log EC50 (in vitro AHH induction) vs. the in vivo -log ED50 (thymic atrophy) and -log ED50 (body wt loss) correlation coefficient, r = 0.97 for all 2 correlations).

Differential gene expression in response to methoxychlor and estradiol through ERalpha, ERbeta, and AR in reproductive tissues of female mice

The reproductive and developmental effects of 17beta-estradiol (E2) and methoxychlor (MXC) observed in treated rodents appear to be linked to some unique but also overlapping patterns of gene expression. The MXC metabolite 2,2-bis(p-hydroxyphenyl)-1,1,1-trichloroethane (HPTE) was previously shown to have selective agonist activity through estrogen receptor alpha (ERalpha) and antagonist activity through ERbeta and androgen receptor (AR). To discover gene families regulated by HPTE and E2, and to characterize similarities and differences in patterns of gene expression induced by these selective ER ligands, we analyzed tissues from mice treated for 3 days with a combined treatment of E2 and HPTE (E2 + HPTE), or the antiandrogen flutamide (FLU). RNA from uteri and ovaries was analyzed with cDNA microarrays and real-time RT-PCR. Results indicate that HPTE and E2 acted similarly to regulate most gene families in the uterus, which expresses predominantly ERalpha. However, in both the uterus and the ovary, there were a few genes that displayed differential patterns of gene regulation by E2 or HPTE treatment, presumably through ERbeta, AR, or other unidentified pathways. In the uterus, progesterone receptor, ERalpha, AR, insulin-like growth factor 1, insulin-like growth factor binding protein 5, and clusterin mRNAs were significantly reduced with both E2 or HPTE treatments, whereas cathepsin B was induced. Conversely, in the ovary, induction of cathepsin B by E2 was reversed after cotreatment with HPTE, and ERbeta expression was induced similarly by HPTE and FLU but not by E2. In addition, E2 uniquely regulated glutathione peroxidase 3, glutathione S-transferase, and cytochrome P450 17alpha-hydroxylase, with no effect of HPTE or FLU treatments. This analysis demonstrated several gene families that appear to be regulated in a ligand-specific pattern, which may explain the unique but overlapping reproductive tissue pathologies following exposure to E2 and MXC.

Functional and physical interactions between the estrogen receptor Sp1 and nuclear aryl hydrocarbon receptor complexes

17beta-Estradiol (E2) induces cathepsin D gene expression in MCF-7 human breast cancer cells and previous analyses of the proximal promoter region of this gene identified two functional enhancer sequences; namely an Sp1(N)23estrogen-responsive element (ERE) half-site (-199 to -165) and an imperfect palindromic ERE (-119 to -107). A third region of the cathepsin D gene promoter (CD/L, -145 to -119) was also E2 responsive in transient transfection assays. A GC-rich sequence which contains two overlapping Sp1 binding sites (-145 to -135) was responsible for ER-mediated transactivation and required formation of an ER/Sp1 complex in which only the Sp1 protein bound DNA. E2 responsiveness of the CD/L sequence was also dependent on an adjacent overlapping GCGTG motif corresponding to the dioxin-responsive element (DRE) core binding sequence, which is the cognate response element for the heterodimeric aryl hydrocarbon receptor (AhR)/AhR nuclear translocator (ARNT) transcription factor complex. The results show that ER-mediated transactivation of CD/L was associated with the Sp1(N)2-4DRE (core) motif and involved formation of a multiprotein ER/Sp1-AhR/ARNT complex. These results illustrate a unique example of an endogenous role for AhR/ARNT in the absence of added AhR agonist and indicate that the cathepsin D gene proximal promoter region contains at least three different functional motifs associated with ER-mediated transactivation.

Estrogen regulates transcription of the ovine oxytocin receptor gene through GC-rich SP1 promoter elements

Establishment of pregnancy in ruminants results from paracrine signaling by interferon tau (IFNT) from the conceptus to uterine endometrial luminal epithelia (LE) that prevents release of luteolytic prostaglandin F(2alpha) pulses. In cyclic and pregnant ewes, progesterone down-regulates progesterone receptor (PGR) gene expression in LE. In cyclic ewes, loss of PGR allows for increases in estrogen receptor alpha (ESR1) and then oxytocin receptor (OXTR) gene expression followed by oxytocin-induced prostaglandin F(2alpha) pulses. In pregnant ewes, IFNT inhibits transcription of the ESR1 gene, which presumably inhibits OXTR gene transcription. Alternatively, IFNT may directly inhibit OXTR gene transcription. The 5' promoter/enhancer region of the ovine OXTR gene was cloned and found to contain predicted binding sites for activator protein 1, SP1, and PGR, but not for ESR1. Deletion analysis showed that the basal promoter activity was dependent on the region from -144 to -4 bp that contained only SP1 sites. IFNT did not affect activity of the OXTR promoter. In cells transfected with ESR1, E2, and ICI 182,780 increased promoter activity due to GC-rich SP1 binding sites at positions -104 and -64. Mutation analyses showed that the proximal SP1 sites mediated ESR1 action as well as basal activity of the promoter. In response to progesterone, progesterone receptor B also increased OXTR promoter activity. SP1 protein was constitutively expressed and abundant in the LE of the ovine uterus. These results support the hypothesis that the antiluteolytic effects of IFNT are mediated by direct inhibition or silencing of ESR1 gene transcription, thereby precluding ESR1/SP1 from stimulating OXTR gene transcription.

1,1-Bis(3'-indolyl)-1-(p-chlorophenyl)methane activates the orphan nuclear receptor Nurr1 and inhibits bladder cancer growth

Nurr1 is an orphan nuclear receptor and a member of the nerve growth factor I-B subfamily of transcription factors with no known endogenous ligand or stimulator. We show, for the first time, evidence that Nurr1 is expressed in a panel of 11 human bladder cancer cell lines. A new class of methylene-substituted diindolylmethanes (C-DIM) were screened and 1,1-bis(3'-indolyl)-1-(p-chlorophenyl)methane (DIM-C-pPhCl) activated the ligand-binding domain of Nurr1. Treatment of bladder cancer cells with Nurr1-active C-DIM resulted in decreased cell survival (MTT assay) and induction of cell death pathways, resulting in poly(ADP-ribose) polymerase cleavage and DNA fragmentation. The specificity of the Nurr1-active compound was shown using RNA interference in 253J B-V cells, whereby small interfering RNA against Nurr1 attenuated ligand-dependent activation of Nurr1 and poly(ADP-ribose) polymerase cleavage. Furthermore, activation of Nurr1 resulted in stimulation of tumor necrosis factor-related apoptosis-inducing ligand and small interfering RNA experiments attenuated tumor necrosis factor-related apoptosis-inducing ligand production. In an orthotopic model of human bladder tumors established in nude mice, administration of a Nurr1-active C-DIM suppressed bladder cancer growth. These results identify Nurr1 as a potential target for bladder cancer therapy and also identify a novel agent for activating Nurr1.

Identification of 3'-methoxy-4'-nitroflavone as a pure aryl hydrocarbon (Ah) receptor antagonist and evidence for more than one form of the nuclear Ah receptor in MCF-7 human breast cancer cells

The competitive binding of 3'-methoxy-4'-nitro, 4'-amino-3'-methoxy, 4'-methoxy-3'-nitro, and 3'-amino-4'-methoxyflavone (compounds 1 to 4, respectively) to the rat cytosolic aryl hydrocarbon (Ah) receptor gave IC50 values of 2.27, 86.1, 872, and 19.4 nM. Flavones 3 and 4 were characterized as Ah receptor agonists in MCF-7 human breast cancer cells and induced CYP1A1 gene expression, whereas the 3-methoxy-substituted flavones (1 and 2) were inactive. All four compounds inhibited induction of ethoxyresorufin O-deethylase (EROD) activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in MCF-7 cells; moreover, in vitro studies with TCDD-induced rat liver microsomes showed that flavones 1 to 4 inhibited EROD activity in the presence or absence of NADPH. In MCF-7 cells cotreated with flavones 1 or 2 (0.01 to 10 microM) plus TCDD or [3H]TCDD, there was a concentration-dependent inhibition of TCDD-induced CYP1A1 mRNA levels and formation of radiolabeled nuclear Ah receptor complex. Velocity sedimentation analysis of nuclear extracts from MCF-7 cells treated with [3H]TCDD plus 1 or 10 microM concentrations of flavones 1 and 2 showed that an early eluting specifically bound nuclear Ah receptor complex was present. However, under these same conditions the flavones inhibited TCDD-induced CYP1A1 gene expression. The apparent molecular mass of this nuclear complex was 190 kDa as determined by cross-linking to a 32P-labeled bromodeoxyuridine-substituted consensus dioxin-responsive element. Similar cross-linking results were obtained using the nuclear extract from MCF-7 cells treated with [3H]TCDD alone. The results of this study suggest that there are at least two forms of the nuclear Ah receptor complex in MCF-7 cells; the major transcriptionally active form binds [3H]TCDD and flavones 1 or 2 inhibit nuclear uptake of this receptor complex. The other form of the nuclear Ah receptor complex appears to be transcriptionally inactive and ligand binding with [3H]TCDD is not competitively inhibited by flavones 1 and 2.

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.

Aryl Hydrocarbon Receptor (AHR) Ligands as Selective AHR Modulators (SAhRMs)

The aryl hydrocarbon receptor (AhR) was first identified as the intracellular protein that bound and mediated the toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and dioxin-like compounds (DLCs). Subsequent studies show that the AhR plays an important role in maintaining cellular homeostasis and in pathophysiology, and there is increasing evidence that the AhR is an important drug target. The AhR binds structurally diverse compounds, including pharmaceuticals, phytochemicals and endogenous biochemicals, some of which may serve as endogenous ligands. Classification of DLCs and non-DLCs based on their persistence (metabolism), toxicities, binding to wild-type/mutant AhR and structural similarities have been reported. This review provides data suggesting that ligands for the AhR are selective AhR modulators (SAhRMs) that exhibit tissue/cell-specific AhR agonist and antagonist activities, and that their functional diversity is similar to selective receptor modulators that target steroid hormone and other nuclear receptors.

Polychlorinated biphenyls (PCBs): mutagenicity and carcinogenicity

The potential mutagenicity and carcinogenicity of commercial PCBs has been investigated in both in vivo and in vitro systems and several conclusions can be drawn from these studies. (1) PCBs can covalently adduct DNA both in vivo and in vitro (using a source of metabolic activation); the more highly chlorinated biphenyls are poorly metabolized and these compounds tend to exhibit very low binding to DNA. Based on the structure-activity relationships for PCBs (Safe, 1984) it is unlikely that the more toxic compounds such as 3,3',4,4',5-penta- and 3,3',4,4',5,5'-hexachlorobiphenyl, would form covalent adducts with DNA. (2) PCB mixtures and individual compounds exhibit minimal mutagenic activity in most assay systems. (3) The more highly chlorinated PCB mixtures (i.e. greater than 50% Cl by weight) are hepatocarcinogens in rodents whereas data from a limited number of studies suggest that the lower chlorinated mixtures are not carcinogenic. (4) In some model systems, the higher chlorinated PCB mixtures act as promoters of preneoplastic lesions and hepatocellular carcinomas in rodents treated with a variety of initiators. (5) Aroclor 1254 acts as a promoter of skin papilloma formation in HRS/J hairless mice and structure-activity and genetic studies suggest that the Ah receptor is necessary but not sufficient for the activity of halogenated aryl hydrocarbons as promoters in hairless mice. (6) Individual PCB congeners and higher chlorinated commercial mixtures also exhibit anti-carcinogenic activity in the CD-1 mouse skin cancer model. (7) Results from occupational studies suggest that individuals exposed to PCBs may have an excess of cancer at some sites, however, the most comprehensive study (Brown, 1987) suggests that there are no significant increases in the overall cancer rate in workers exposed to PCBs. Follow-up and continuing epidemiological studies on the PCB-exposed workers are required to further clarify the potential carcinogenic effects of PCBs on humans. In several strains of rats and mice, there is a high incidence of hepatic preneoplastic lesions and carcinomas and these lesions can be induced by diverse promoting agents (Schulte-Hermann et al., 1983; Weinstein, 1984). Since PCBs are not mutagenic and do not readily form covalent adducts with cellular DNA, it is likely that the higher chlorinated biphenyls are not genotoxic and act as promoters of carcinogenesis in rodents. A comparable mechanism has been suggested for 2,3,7,8-TCDD (Shu et al., 1987; Weinstein, 1984). For PCBs, the role of the Ah receptor in mediating their activity as promoters has not been delineated.(ABSTRACT TRUNCATED AT 400 WORDS)