Indole-3-Carbinol Selectively Uncouples Expression and Activity of Estrogen Receptor Subtypes in Human Breast Cancer Cells

The Aryl Hydrocarbon Receptor and Its Crosstalk: A Chemopreventive Target of Naturally Occurring and Modified Phytochemicals

The aryl hydrocarbon receptor (AhR) is an environmentally sensitive transcription factor (TF) historically associated with carcinogenesis initiation via the activation of numerous carcinogens. Nowadays, the AhR has been attributed to multiple endogenous functions to maintain cellular homeostasis. Moreover, crosstalk, often reciprocal, has been found between the AhR and several other TFs, particularly estrogen receptors (ERs) and nuclear factor erythroid 2-related factor-2 (Nrf2). Adequate modulation of these signaling pathways seems to be an attractive strategy for cancer chemoprevention. Several naturally occurring and synthetically modified AhR or ER ligands and Nrf2 modulators have been described. Sulfur-containing derivatives of glucosinolates, such as indole-3-carbinol (I3C), and stilbene derivatives are particularly interesting in this context. I3C and its condensation product, 3,3'-diindolylmethane (DIM), are classic examples of blocking agents that increase drug-metabolizing enzyme activity through activation of the AhR. Still, they also affect multiple essential signaling pathways in preventing hormone-dependent cancer. Resveratrol is a competitive antagonist of several classic AhR ligands. Its analogs, with ortho-methoxy substituents, exert stronger antiproliferative and proapoptotic activity. In addition, they modulate AhR activity and estrogen metabolism. Their activity seems related to a number of methoxy groups introduced into the stilbene structure. This review summarizes the data on the chemopreventive potential of these classes of phytochemicals, in the context of AhR and its crosstalk modulation.

Cruciferous vegetables and colorectal cancer risk: a hospital-based matched case-control study in Northeast China

BACKGROUND/OBJECTIVES

Conflicting results have been reported on the association of cruciferous vegetable intake and colorectal cancer risk. This study aimed to clarify the relationship of cruciferous vegetables and colorectal cancer among individuals in Northeast China, where large amounts of cruciferous vegetables are consumed habitually.

SUBJECTS/METHODS

We conducted a hospital-based case-control study in the First Hospital of China Medical University, the Shengjing Hospital of China Medical University and the First Hospital of Dalian Medical University from 2009 to 2011. Patients in the study were matched individually by age, gender, and city of residence. The study ultimately included 833 case-control pairs. A structured questionnaire was applied to collect data on general characteristics, dietary habits, and selected dietary intake. Differences between cases and controls were ascertained with the chi-square test or the Mann-Whitney U test. Unconditional logistic regression was employed to compute odds ratios (ORs) and 95% confidence intervals (CIs). Stratified analyses were conducted by gender.

RESULTS

In the total study cohort, no significant association was found between total cruciferous vegetable intake and colorectal cancer risk. The adjusted OR for the highest versus the lowest intake was 0.83 (95% CI: 0.59-1.18). In stratification analyses by gender, reduced colorectal cancer risk was related to higher consumption of total cruciferous vegetables in women but not in men. Significant inverse correlations were found in analyses of individual cruciferous vegetables, including greens (OR = 0.47; 95% CI: 0.32-0.68), cabbage (OR = 0.61; 95% CI: 0.44-0.86), and cauliflower (OR = 0.66; 95% CI: 0.48-0.92).

CONCLUSIONS

No significant association was found between total cruciferous vegetable intake and colorectal cancer risk. However, specific types of cruciferous vegetables might have protective roles against colorectal cancer.

Effects of indole-3-carbinol on steroid hormone profile and tumor progression in a mice model of canine inflammatory mammarycancer

Background

Indole-3-carbinol, derived from Cruciferous vegetables is an estrogen receptor antagonist considered a preventive agent that is naturally present in diet. There are no previous studies on its effects in human inflammatory breast cancer or canine inflammatory mammary cancer that is the most aggressive type of breast cancer.

Methods

The aim of this study was to analyze the effect of indole-3-carbinol on a SCID mice xenograft model of canine inflammatory mammary cancer, using equivalent human oral dose as a preventive therapy in humans for 3 weeks.

Results

Indole-3-carbinol treatment decreased tumor proliferation and increased apoptosis, although tumor embolization and liver metastasis were observed in some animals. There was a characteristic subpopulation of lipid-rich cells and increased contents of select steroid hormones in tumor homogenates and serum.

Conclusions

Our data reveal for the first time that the ingestion of indole-3-carbinol, as administered, diminishes proliferation and increases apoptosis of tumor cells in an experimental model of inflammatory breast cancer, although this effect could not be enough to avoid the appearance of tumor embolization and metastasis. Future clinical trials will be needed to clarify the usefulness of indole-3-carbinol in this cancer and to understand the molecular mechanisms involved.

A potential role for estrogen in cigarette smoke-induced microRNA alterations and lung cancer

Alteration in the expression of microRNAs (miRNAs) is associated with oncogenesis and cancer progression. In this review we aim to suggest that elevated levels of estrogens and their metabolites inside the lungs as a result of cigarette smoke exposure can cause widespread repression of miRNA and contribute to lung tumor development. Anti-estrogenic compounds, such as the components of cruciferous vegetables, can attenuate this effect and potentially reduce the risk of lung cancer (LC) among smokers.

Cooperative antiproliferative signaling by aspirin and indole-3-carbinol targets microphthalmia-associated transcription factor gene expression and promoter activity in human melanoma cells

Antiproliferative signaling of combinations of the nonsteroidal anti-inflammatory drug acetylsalicylic acid (aspirin) and indole-3-carbinol (I3C), a natural indolecarbinol compound derived from cruciferous vegetables, was investigated in human melanoma cells. Melanoma cell lines with distinct mutational profiles were sensitive to different extents to the antiproliferative response of aspirin, with oncogenic BRAF-expressing G361 cells and wild-type BRAF-expressing SK-MEL-30 cells being the most responsive. I3C triggered a strong proliferative arrest of G361 melanoma cells and caused only a modest decrease in the proliferation of SK-MEL-30 cells. In both cell lines, combinations of aspirin and I3C cooperatively arrested cell proliferation and induced a G1 cell cycle arrest, and nearly ablated protein and transcript levels of the melanocyte master regulator microphthalmia-associated transcription factor isoform M (MITF-M). In melanoma cells transfected with a -333/+120-bp MITF-M promoter-luciferase reporter plasmid, treatment with aspirin and I3C cooperatively disrupted MITF-M promoter activity, which accounted for the loss of MITF-M gene products. Mutational analysis revealed that the aspirin required the LEF1 binding site, whereas I3C required the BRN2 binding site to mediate their combined and individual effects on MITF-M promoter activity. Consistent with LEF1 being a downstream effector of Wnt signaling, aspirin, but not I3C, downregulated protein levels of the Wnt co-receptor LDL receptor-related protein-6 and β-catenin and upregulated the β-catenin destruction complex component Axin. Taken together, our results demonstrate that aspirin-regulated Wnt signaling and I3C-targeted signaling pathways converge at distinct DNA elements in the MITF-M promoter to cooperatively disrupt MITF-M expression and melanoma cell proliferation.

Indole-3-carbinol and its N-alkoxy derivatives preferentially target ERα-positive breast cancer cells

Indole-3-carbinol (I3C) is a natural anti-carcinogenic compound found at high concentrations in Brassica vegetables. I3C was recently reported to inhibit neutrophil elastase (NE) activity, while consequently limiting the proteolytic processing of full length cyclin E into pro-tumorigenic low molecular weight cyclin E (LMW-E). In this study, we hypothesized that inhibition of NE activity and resultant LMW-E generation is critical to the anti-tumor effects of I3C. LMW-E was predominately expressed by ERα-negative breast cancer cell lines. However, ERα-positive cell lines demonstrated the greatest sensitivity to the anti-tumor effects of I3C and its more potent N-alkoxy derivatives. We found that I3C was incapable of inhibiting NE activity or the generation of LMW-E. Therefore, this pathway did not contribute to the anti-tumor activity of I3C. Gene expression analyzes identified ligand-activated aryl hydrocarbon receptor (AhR), which mediated sensitivity to the anti-tumor effects of I3C in ERα-positive MCF-7 cells. In this model system, the reactive oxygen species (ROS)-induced upregulation of ATF-3 and pro-apoptotic BH3-only proteins (e.g. NOXA) contributed to the sensitivity of ERα-positive breast cancer cells to the anti-tumor effects of I3C. Overexpression of ERα in MDA-MB-231 cells, which normally lack ERα expression, increased sensitivity to the anti-tumor effects of I3C, demonstrating a direct role for ERα in mediating the sensitivity of breast cancer cell lines to I3C. Our results suggest that ERα signaling amplified the pro-apoptotic effect of I3C-induced AhR signaling in luminal breast cancer cell lines, which was mediated in part through oxidative stress induced upregulation of ATF-3 and downstream BH3-only proteins.

3-(2-Bromoethyl)-indole inhibits the growth of cancer cells and NF-κB activation

Indole-3-carbinol (I3C) and diindolylmethane (DIM), found in cruciferous vegetables, have chemopreventive and anticancer properties. In the present study, 14 substituted indoles were tested for activity against SW480 colon cancer cells. Among these, 3-(2-bromoethyl)-indole, named BEI-9, showed the greatest inhibition. The effects of BEI-9 on cancer cells were analyzed by MTS and CellTiter-Glo assays for effects on cell viability, by microscopy for phenotypic changes, by scratch wound assays for effects on migration, by flow cytometry for changes in the cell cycle, by immunoblotting for cyclin D and A to assess effects on cell cycle regulation, and by NF-κB reporter assays for effects on basal and drug-induced NF-κB activation. BEI-9 inhibited the growth of SW480 and HCT116 colon cancer cells at concentrations of 12.5 and 5 µM, respectively. BEI-9 also inhibited cell motility as determined with scratch wound assays, and reduced the levels of cyclin D1 and A. Furthermore, in reporter cells, BEI-9 (0.8 µM) inhibited basal and induced NF-κB activation and increased cell death when combined with the cytokine TNFα or the drug camptothecin (CPT), both of which activate NF-κB. Preliminary experiments to identify a safe dose range for immunodeficient mice showed that BEI-9, administered intraperitoneally, was tolerable at doses below 10 mg/kg. Thus, BEI-9 and other indole derivatives may be useful in chemoprevention or as chemosensitizers. Since NF-κB activation is implicated in carcinogenesis and in reducing sensitivity to anticancer drugs, BEI-9 should be investigated in combination with drugs such as CPT, which activate NF-κB.

Subglutinol A, an immunosuppressive α-pyrone diterpenoid from Fusarium subglutinans, acts as an estrogen receptor antagonist

Subglutinol A is an immunosuppressive α-pyrone diterpenoid isolated from Fusarium subglutinans that exhibits osteogenic activity. Several non-steroid mycotoxins isolated from various strains of Fusarium fungi exhibit female steroid hormone activities. In this study, we characterized the estrogenic activity of subglutinol A (1). Subglutinol A blocked the 17β-estradiol-induced activation of reporter plasmids and endogenous estrogen-responsive target genes in a dose-dependent manner and efficiently destabilized ER proteins as shown using the estrogen receptor antagonist ICI 182,780. Subglutinol A also displaced the specific binding of [(3)H]17β-estradiol from ER in MCF-7 whole-cell ligand binding assays. These data demonstrate the potential of subglutinol A as an ER antagonist though its competition with 17β-estradiol for direct ER association.

The role of estrogen receptor β in transplacental cancer prevention by indole-3-carbinol

In the present study, the efficacy of indole-3-carbinol (I3C), a key bioactive component of cruciferous vegetables, for prevention of cancer in offspring exposed in utero to the environmental carcinogen dibenzo[def,p]chrysene (DBC) was evaluated using an estrogen receptor β (ERβ) knockout mouse model. I3C was provided either through the maternal diet coincident with carcinogen exposure during pregnancy or directly to offspring postinitiation with DBC. I3C was effective at reducing T-cell acute lymphoblastic lymphoma/leukemia (T-ALL)-related mortality in offspring only if provided via the maternal diet, although a gender difference in the role of ERβ in mediating this response was evident. In female offspring, chemoprevention of T-ALL by maternal dietary I3C required expression of ERβ; survival in Esr2 wild-type and heterozygous female offspring was more than 90% compared with 66% in Esr2 null females. Alternatively, ERβ status did not significantly impact the transplacental chemoprevention by I3C in males. The possible role of ERβ in mediating lung carcinogenesis or chemoprevention by I3C was similarly complicated. Lung tumor incidence was unaltered by either dietary intervention, whereas lung tumor multiplicity was substantially reduced in Esr2 null females on the control diet and marginally lower in Esr2 null males exposed to I3C via the maternal diet compared with their wild-type and heterozygous counterparts. These findings suggest that I3C may act via ERβ to prevent or suppress DBC-initiated transplacental carcinogenesis but that the involvement of this receptor seems to differ depending on the cancer type and gender of the offspring.

Indole-3-carbinol disrupts estrogen receptor-alpha dependent expression of insulin-like growth factor-1 receptor and insulin receptor substrate-1 and proliferation of human breast cancer cells

We previously established that Indole-3-Carbinol (I3C), a natural hydrolysis product of glucobrassicin in cruciferous vegetables, arrests the proliferation of estrogen-dependent human breast cancer cells and induces protein degradation of Estrogen Receptor-alpha (ERα). We demonstrate in human MCF-7 breast cancer cells that I3C ablates expression of Insulin-like Growth Factor Receptor-1 (IGF1R) and Insulin Receptor Substrate-1 (IRS1), downstream effectors of the IGF1 signaling pathway. Exogenous ERα reversed the I3C mediated loss of IGF1R and IRS1 gene expression demonstrating that down-regulation of ERα is functionally linked to I3C control of IGF1R and IRS1 expression. I3C disrupted binding of endogenous ERα, but not Sp1, to ERE-Sp1 composite elements within the IGF1R/IRS1 promoters. Exogenous ERα abrogated, and combined expression of IGF1R and IRS1 attenuated, the I3C mediated cell cycle arrest. Therefore, I3C inhibits proliferation of estrogen-sensitive breast cancer cells through disruption of ERα-mediated transcription of cell signaling components within the IGF1 cascade.

Target protein interactions of indole-3-carbinol and the highly potent derivative 1-benzyl-I3C with the C-terminal domain of human elastase uncouples cell cycle arrest from apoptotic signaling

Elastase is the only currently identified target protein for indole-3-carbinol (I3C), a naturally occurring hydrolysis product of glucobrassicin in cruciferous vegetables such as broccoli, cabbage, and Brussels sprouts that induces a cell cycle arrest and apoptosis of human breast cancer cells. In vitro elastase enzymatic assays demonstrated that I3C and at lower concentrations its more potent derivative 1-benzyl-indole-3-carbinol (1-benzyl-I3C) act as non-competitive allosteric inhibitors of elastase activity. Consistent with these results, in silico computational simulations have revealed the first predicted interactions of I3C and 1-benzyl-I3C with the crystal structure of human neutrophil elastase, and identified a potential binding cluster on an external surface of the protease outside of the catalytic site that implicates elastase as a target protein for both indolecarbinol compounds. The Δ205 carboxyterminal truncation of elastase, which disrupts the predicted indolecarbinol binding site, is enzymatically active and generates a novel I3C resistant enzyme. Expression of the wild type and Δ205 elastase in MDA-MB-231 human breast cancer cells demonstrated that the carboxyterminal domain of elastase is required for the I3C and 1-benzyl-I3C inhibition of enzymatic activity, accumulation of the unprocessed form of the CD40 elastase substrate (a tumor necrosis factor receptor family member), disruption of NFκB nuclear localization and transcriptional activity, and induction of a G1 cell cycle arrest. Surprisingly, expression of the Δ205 elastase molecule failed to reverse indolecarbinol stimulated apoptosis, establishing an elastase-dependent bifurcation point in anti-proliferative signaling that uncouples the cell cycle and apoptotic responses in human breast cancer cells.

Indole-3-carbinol downregulation of telomerase gene expression requires the inhibition of estrogen receptor-alpha and Sp1 transcription factor interactions within the hTERT promoter and mediates the G1 cell cycle arrest of human breast cancer cells

Indole-3-carbinol (I3C), a naturally occurring hydrolysis product of glucobrassicin from cruciferous vegetables such as broccoli, cabbage and Brussels sprouts, is an anticancer phytochemical that triggers complementary sets of antiproliferative pathways to induce a cell cycle arrest of estrogen-responsive MCF7 breast cancer cells. I3C strongly downregulated transcript expression of the catalytic subunit of the human telomerase (hTERT) gene, which correlated with the dose-dependent indole-mediated G(1) cell cycle arrest without altering the transcript levels of the RNA template (hTR) for telomerase elongation. Exogenous expression of hTERT driven by a constitutive promoter prevented the I3C-induced cell cycle arrest and rescued the I3C inhibition of telomerase enzymatic activity and activation of cellular senescence. Time course studies showed that I3C downregulated expression of estrogen receptor-alpha (ERα) and cyclin-dependent kinase-6 transcripts levels (which is regulated through the Sp1 transcription factor) prior to the downregulation of hTERT suggesting a mechanistic link. Chromatin immunoprecipitation assays demonstrated that I3C disrupted endogenous interactions of both ERα and Sp1 with an estrogen response element-Sp1 composite element within the hTERT promoter. I3C inhibited 17β-estradiol stimulated hTERT expression and stimulated the production of threonine-phosphorylated Sp1, which inhibits Sp1-DNA interactions. Exogenous expression of both ERα and Sp1, but not either alone, in MCF7 cells blocked the I3C-mediated downregulation of hTERT expression. These results demonstrate that I3C disrupts the combined ERα- and Sp1-driven transcription of hTERT gene expression, which plays a significant role in the I3C-induced cell cycle arrest of human breast cancer cells.

Effect of 4-methoxyindole-3-carbinol on the proliferation of colon cancer cells in vitro, when treated alone or in combination with indole-3-carbinol

Consumption of cruciferous vegetables and cancer prevention seem to be positively associated. We present an easy two-step synthesis for 4-methoxyindole-3-carbinol (4MeOI3C), the expected breakdown product of 4-methoxyglucobrassicin during ingestion. 4MeOI3C inhibited the proliferation of human colon cancer cells DLD-1 and HCT 116 with IC(50) values of 116 microM and 96 microM, respectively, after 48 h in vitro, and is therefore a more potent inhibitor than indole-3-carbinol (I3C). 4MeOI3C and I3C combined in different molar ratios inhibited proliferation in a nearly additive to slightly synergistic manner. Proliferation was inhibited by 100 microM 4MeOI3C after 48 h without affecting cell cycle phase distribution, indicating an overall-slowdown effect on the cell cycle. However, 200 microM 4MeOI3C caused a very high level of cell death and an accumulation of living cells in the G(0)/G(1) phase, indicating a concentration-dependent mode of action. We conclude that 4MeOI3C might play a role in the cancer preventive effect of cruciferous vegetables.

Direct inhibition of elastase activity by indole-3-carbinol triggers a CD40-TRAF regulatory cascade that disrupts NF-kappaB transcriptional activity in human breast cancer cells

Treatment of highly tumorigenic MDA-MB-231 human breast cancer cells with indole-3-carbinol (I3C) directly inhibited the extracellular elastase-dependent cleavage of membrane-associated CD40, a member of the tumor necrosis factor (TNF) receptor superfamily. CD40 signaling has been implicated in regulating cell survival, apoptosis, and proliferation, as well as in sensitizing breast cancer cells to chemotherapy, and is therefore an important potential target of novel breast cancer treatments. The I3C-dependent accumulation of full-length unprocessed CD40 protein caused a shift in CD40 signaling through TNF receptor-associated factors (TRAF), including the TRAF1/TRAF2 positive regulators and TRAF3 negative regulator of NF-kappaB transcription factor activity. Because TRAF1 is a transcriptional target gene of NF-kappaB, I3C disrupted a positive feedback loop involving these critical cell survival components. siRNA ablation of elastase expression mimicked the I3C inhibition of CD40 protein processing and G(1) cell cycle arrest, whereas siRNA knockdown of TRAF3 and the NF-kappaB inhibitor IkappaB prevented the I3C-induced cell cycle arrest. In contrast, siRNA knockdown of PTEN had no effect on the I3C control of NF-kappaB activity, showing the importance of CD40 signaling in regulating this transcription factor. Our study provides the first direct in vitro evidence that I3C directly inhibits the elastase-mediated proteolytic processing of CD40, which alters downstream signaling to disrupt NF-kappaB-induced cell survival and proliferative responses. Furthermore, we have established a new I3C-mediated antiproliferative cascade that has significant therapeutic potential for treatment of human cancers associated with high levels of elastase and its CD40 membrane substrate.

1-Benzyl-indole-3-carbinol is a novel indole-3-carbinol derivative with significantly enhanced potency of anti-proliferative and anti-estrogenic properties in human breast cancer cells

Indole-3-carbinol (I3C), a natural autolysis product of a gluccosinolate present in Brassica vegetables such as broccoli and cabbage, has anti-proliferative and anti-estrogenic activities in human breast cancer cells. A new and significantly more potent I3C analogue, 1-benzyl-I3C was synthesized, and in comparison to I3C, this novel derivative displayed an approximate 1000-fold enhanced potency in suppressing the growth of both estrogen responsive (MCF-7) and estrogen-independent (MDA-MB-231) human breast cancer cells (I3C IC(50) of 52 microM, and 1-benzyl-I3C IC(50) of 0.05 microM). At significantly lower concentrations, 1-benzyl-I3C induced a robust G1 cell cycle arrest and elicited the key I3C-specific effects on expression and activity of G1-acting cell cycle genes including the disruption of endogenous interactions of the Sp1 transcription factor with the CDK6 promoter. Furthermore, in estrogen responsive MCF-7 cells, with enhanced potency 1-benzyl-I3C down-regulated production of estrogen receptor-alpha protein, acts with tamoxifen to arrest breast cancer cell growth more effectively than either compound alone, and inhibited the in vivo growth of human breast cancer cell-derived tumor xenografts in athymic mice. Our results implicate 1-benzyl-I3C as a novel, potent inhibitor of human breast cancer proliferation and estrogen responsiveness that could potentially be developed into a promising therapeutic agent for the treatment of indole-sensitive cancers.

Indole-3-carbinol triggers aryl hydrocarbon receptor-dependent estrogen receptor (ER)alpha protein degradation in breast cancer cells disrupting an ERalpha-GATA3 transcriptional cross-regulatory loop

We have established in human breast cancer cells that indole-3-carbinol (I3C), a promising anti-cancer phytochemical from Brassica vegetables, ablates ERα expression by stimulating the Rbx-1 E3 ligase mediated degradation of ERα protein and disruption of a cross-regulatory positive feedback loop involving ERα and the GATA3 transcription factor.

Estrogen receptor (ER)α is a critical target of therapeutic strategies to control the proliferation of hormone-dependent breast cancers. Preferred clinical options have significant adverse side effects that can lead to treatment resistance due to the persistence of active estrogen receptors. We have established the cellular mechanism by which indole-3-carbinol (I3C), a promising anticancer phytochemical from Brassica vegetables, ablates ERα expression, and we have uncovered a critical role for the GATA3 transcription factor in this indole-regulated cascade. I3C-dependent activation of the aryl hydrocarbon receptor (AhR) initiates Rbx-1 E3 ligase-mediated ubiquitination and proteasomal degradation of ERα protein. I3C inhibits endogenous binding of ERα with the 3′-enhancer region of GATA3 and disrupts endogenous GATA3 interactions with the ERα promoter, leading to a loss of GATA3 and ERα expression. Ectopic expression of GATA3 has no effect on I3C-induced ERα protein degradation but does prevent I3C inhibition of ERα promoter activity, demonstrating the importance of GATA3 in this I3C-triggered cascade. Our preclinical results implicate I3C as a novel anticancer agent in human cancers that coexpress ERα, GATA3, and AhR, a combination found in a large percentage of breast cancers but not in other critical ERα target tissues essential to patient health.

Anticancer activities of artemisinin and its bioactive derivatives

Artemisinin, a sesquiterpene lactone derived from the sweet wormwood plant Artemisia annua, and its bioactive derivatives exhibit potent anticancer effects in a variety of human cancer cell model systems. The pleiotropic response in cancer cells includes growth inhibition by cell cycle arrest, apoptosis, inhibition of angiogenesis, disruption of cell migration, and modulation of nuclear receptor responsiveness. These effects of artemisinin and its derivatives result from perturbations of many cellular signalling pathways. This review provides a comprehensive discussion of these cellular responses, and considers the ramifications for the potential development of artemisinin-based compounds in anticancer therapeutic and preventative strategies.

Minireview: modulation of hormone receptor signaling by dietary anticancer indoles

Indole-3-carbinol and its diindole condensation product 3-3'-diindolylmethane are dietary phytochemicals that have striking anticarcinogenic properties in human cancer cells. Molecular, cellular, physiological, and clinical studies have documented that both indole-3-carbinol and 3-3'-diindolylmethane have potent endocrine modulating activities through a myriad of mechanisms. The focus of this review is to discuss the evidence that directly links the anticancer actions of these two indole compounds to the control of steroid receptor and growth factor receptor signaling.

Indole-3-carbinol inhibits MDA-MB-231 breast cancer cell motility and induces stress fibers and focal adhesion formation by activation of Rho kinase activity

Indole-3-carbinol (I3C), a phytochemical derived from cruciferous vegetables such as broccoli and Brussels sprouts, has potent antiproliferative effects in human breast cancer cells and has been shown to decrease metastatic spread of tumors in experimental animals. Using chemotaxis and fluorescent-bead cell motility assays, we demonstrated that I3C significantly decreased the in vitro migration of MDA-MB-231 cells, a highly invasive breast cancer cell line. Immunofluorescence staining of the actin cytoskeleton revealed that concurrent with the loss of cell motility, I3C treatment significantly increased stress fiber formation. Furthermore, I3C induced the localization of the focal adhesion component vinculin and tyrosine-phosphorylated proteins to the cell periphery, which implicates an indole-dependent enhancement of focal adhesions within the outer boundary of the cells. Coimmunoprecipitation analysis of focal adhesion kinase demonstrated that I3C stimulated the dynamic formation of the focal adhesion protein complex without altering the total level of individual focal adhesion proteins. The RhoA-Rho kinase pathway is involved in stress fiber and focal adhesion formation, and I3C treatment stimulated Rho kinase enzymatic activity and cofilin phosphorylation, which is a downstream target of Rho kinase signaling, but did not increase the level of active GTP-bound RhoA. Exposure of MDA-MB-231 cells to the Rho kinase inhibitor Y-27632, or expression of dominant negative RhoA ablated the I3C induced formation of stress fibers and of peripheral focal adhesions. Expression of constitutively active RhoA mimicked the I3C effects on both processes. Taken together, our data demonstrate that I3C induces stress fibers and peripheral focal adhesions in a Rho kinase-dependent manner that leads to an inhibition of motility in human breast cancer cells.

The dietary phytochemical indole-3-carbinol is a natural elastase enzymatic inhibitor that disrupts cyclin E protein processing

Indole-3-carbinol (I3C), a naturally occurring component of Brassica vegetables, such as broccoli, cabbage, and Brussels sprouts, induces a G(1) cell-cycle arrest of human breast cancer cells, although the direct cellular targets that mediate this process are unknown. Treatment of highly invasive MDA-MB-231 breast cancer cells with I3C shifted the stable accumulation of cyclin E protein from the hyperactive lower-molecular-mass 35-kDa form that is associated with cancer cell proliferation and poor clinical outcomes to the 50-kDa cyclin E form that typically is expressed in normal mammary tissue. An in vitro cyclin E processing assay, in combination with zymography, demonstrated that I3C, but not its natural dimer, 3,3'-diindolylmethane, disrupts proteolytic processing of the 50-kDa cyclin E into the lower-molecular-mass forms by direct inhibition of human neutrophil elastase enzymatic activity. Analysis of elastase enzyme kinetics using either cyclin E or N-methoxysuccinyl-Ala-Ala-Pro-Val-p-nitroanalide as substrates demonstrated that I3C acts as a noncompetitive inhibitor of elastase activity with an inhibitory constant of approximately 12 microM. Finally, siRNA ablation of neutrophil elastase protein production in MDA-MB-231 cells mimicked the I3C-disrupted processing of the 50-kDa cyclin E protein and the indole-induced cell-cycle arrest. Taken together, our results demonstrate that elastase is the first identified specific target protein for I3C and that the direct I3C inhibition of elastase enzymatic activity implicates the potential use of this indole, or related compounds, in targeted therapies of human breast cancers where high elastase levels are correlated with poor prognosis.

Artemisinin selectively decreases functional levels of estrogen receptor-alpha and ablates estrogen-induced proliferation in human breast cancer cells

MCF7 cells are an estrogen-responsive human breast cancer cell line that expresses both estrogen receptor (ER) alpha and ERbeta. Treatment of MCF7 cells with artemisinin, an antimalarial phytochemical from the sweet wormwood plant, effectively blocked estrogen-stimulated cell cycle progression induced by either 17beta-estradiol (E(2)), an agonist for both ERs, or by propyl pyrazole triol (PPT), a selective ERalpha agonist. Artemisinin strongly downregulated ERalpha protein and transcripts without altering expression or activity of ERbeta. Transfection of MCF7 cells with ERalpha promoter-linked luciferase reporter plasmids revealed that the artemisinin downregulation of ERalpha promoter activity accounted for the loss of ERalpha expression. Artemisinin treatment ablated the estrogenic induction of endogenous progesterone receptor (PR) transcripts by either E(2) or PPT and inhibited the estrogenic stimulation of a luciferase reporter plasmid driven by consensus estrogen response elements (EREs). Chromatin immunoprecipitation assays revealed that artemisinin significantly downregulated the level of endogeneous ERalpha bound to the PR promoter, whereas the level of bound endogeneous ERbeta was not altered. Treatment of MCF7 cells with artemisinin and the pure antiestrogen fulvestrant resulted in a cooperative reduction of ERalpha protein levels and enhanced G(1) cell cycle arrest compared with the effects of either compound alone. Our results show that artemisinin switches proliferative human breast cancer cells from expressing a high ERalpha:ERbeta ratio to a condition in which ERbeta predominates, which parallels the physiological state linked to antiproliferative events in normal mammary epithelium.

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.

Indole-3-carbinol as a chemopreventive and anti-cancer agent

During the course of oncogenesis and tumor progression, cancer cells constitutively upregulate signaling pathways relevant to cell proliferation and survival as a strategy to overcome genomic instability and acquire resistance phenotype to chemotherapeutic agents. In light of this clinical and molecular heterogeneity of human cancers, it is desirable to concomitantly target these genetic abnormalities by using an agent with pleiotropic mode of action. Indole-3-carbinol and its metabolite 3,3'-diindoylmethane (DIM) target multiple aspects of cancer cell-cycle regulation and survival including Akt-NF kappa B signaling, caspase activation, cyclin-dependent kinase activities, estrogen metabolism, estrogen receptor signaling, endoplasmic reticulum stress, and BRCA gene expression. This broad spectrum of anti-tumor activities in conjunction with low toxicity underscores the translational value of indole-3-carbinol and its metabolites in cancer prevention/therapy. Furthermore, novel anti-tumor agents with overlapping underlying mechanisms have emerged via structural optimization of indole-3-carbinol and DIM, which may provide considerable therapeutic advantages over the parental compounds with respect to chemical stability and anti-tumor potency. Together, these agents might foster new strategies for cancer prevention and therapy.

Determining the efficacy of dietary phytochemicals in cancer prevention

Accumulating data suggest that dietary phytochemicals have the potential to moderate deregulated signalling or reinstate checkpoint pathways and apoptosis in damaged cells, while having minimal impact on healthy cells. These are ideal characteristics for chemopreventive and combination anticancer strategies, warranting substantial research effort into harnessing the biological activities of these agents in disease prevention and treatment. However, this requires further investigation into their mode of action and novel approaches to the development of reliable biomarkers.

Estrogen receptor alpha and beta in uterine fibroids: a basis for altered estrogen responsiveness

OBJECTIVE

To investigate the relative expression and the DNA-binding status of estrogen receptors alpha and beta in fibroids and normal myometrial tissue to explore the molecular basis of altered estrogen responsiveness of leiomyomas.

DESIGN

Biopsy samples from uterine fibroids and adjacent normal myometrial tissue at the follicular phase of the menstrual cycle.

SETTING

Aretaieio University Hospital and the National Hellenic Research Foundation, Athens, Greece.

PATIENT(S)

Thirty-five patients who underwent hysterectomy or myomectomy because of myoma symptoms.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Deoxyribonucleic acid-binding status of estrogen receptors alpha and beta.

RESULT(S)

The level of messenger RNA expression of estrogen receptor alpha and beta and the level of estrogen receptor as a whole are increased on average to a similar extent in leiomyomas compared with normal myometrium. Occasionally, however, estrogen receptor alpha is disproportionately increased in leiomyomas, and this appears to increase the amount of estrogen receptor alpha that binds to the estrogen-responsive element of estrogen target genes as homodimer rather than as heterodimer with estrogen receptor beta.

CONCLUSION(S)

The estrogen receptor alpha-to-estrogen receptor beta expression ratio rather than the individual expression levels determines the fraction of DNA-binding homodimers of estrogen receptor alpha and possibly the growth potential of myomas.

N-Alkoxy derivatization of indole-3-carbinol increases the efficacy of the G1 cell cycle arrest and of I3C-specific regulation of cell cycle gene transcription and activity in human breast cancer cells

Indole-3-carbinol (I3C), a naturally occurring component of Brassica vegetables, such as cabbage, broccoli, and Brussels sprouts, induces a G1 cell cycle arrest of human breast cancer cells. Structure-activity relationships of I3C that mediate this anti-proliferative response were investigated using synthetic and natural I3C derivatives that contain substitutions at the indole nitrogen. Nitrogen substitutions included N-alkoxy substituents of one to four carbons in length, which inhibit dehydration and the formation of the reactive indolenine. Analysis of growth and cell cycle arrest of indole-treated human breast cancer cells revealed a striking increase in efficacy of the N-alkoxy I3C derivatives that is significantly enhanced by the presence of increasing carbon lengths of the N-alkoxy substituents. Compared to I3C, the half maximal growth arrest responses occurred at 23-fold lower indole concentration for N-methoxy I3C, 50-fold lower concentration for N-ethoxy I3C, 217-fold lower concentration for N-propoxy I3C, and 470-fold lower concentration for N-butoxy I3C. At these lower concentrations, each of the N-alkoxy substituted compounds induced the characteristic I3C response in that CDK6 gene expression, CDK6 promoter activity, and CDK2 specific enzymatic activity for its retinoblastoma protein substrate were strongly down-regulated. 3-Methoxymethylindole and 3-ethoxymethylindole were approximately as bioactive as I3C, whereas both tryptophol and melatonin failed to induce the cell cycle arrest, showing the importance of the C-3 hydroxy methyl substituent on the indole ring. Taken together, our study establishes the first I3C structure-activity relationship for cytostatic activities, and implicates I3C-based N-alkoxy derivatives as a novel class of potentially more potent experimental therapeutics for breast cancer.

Predicting the physiological relevance of in vitro cancer preventive activities of phytochemicals

There is growing interest in the ability of phytochemicals to prevent chronic diseases, such as cancer and heart disease. However, some of these agents have poor bioavailability and many of the in-depth studies into their mechanisms of action have been carried out in vitro using doses which are unachievable in humans. In order to optimize the design of chemopreventive treatment, it is important to determine which of the many reported mechanisms of action are clinically relevant. In this review we consider the physiologically achievable doses for a few of the best studied agents (indole-3-carbinol, diindolylmethane, curcumin, epigallocatechin-3-gallate and resveratrol) and summarize the data derived from studies using these low concentrations in cell culture. We then cite examples of in vitro effects which have been observed in vivo. Finally, the ability of agent combinations to act synergistically or antagonistically is considered. We conclude that each of the compounds shows an encouraging range of activities in vitro at concentrations which are likely to be physiologically relevant. There are also many examples of in vivo studies which validate in vitro observations. An important consideration is that combinations of agents can result in significant activity at concentrations where any single agent is inactive. Thus, for each of the compounds reviewed here, in vitro studies have provided useful insights into their mechanisms of action in humans. However, data are lacking on the full range of activities at low doses in vitro and the benefits or otherwise of combinations in vivo.