Indole-3-carbinol stimulates transcription of the interferon gamma receptor 1 gene and augments interferon responsiveness in human breast cancer cells

Role of interferon-gamma (IFN-γ) and IFN-γ receptor 1/2 (IFNγR1/2) in regulation of immunity, infection, and cancer development: IFN-γ-dependent or independent pathway

IFN-γ, a soluble cytokine being produced by T lymphocytes, macrophages, mucosal epithelial cells, or natural killer cells, is able to bind to the IFN-γ receptor (IFNγR) and in turn activate the Janus kinase (JAK)-signal transducer and transcription protein (STAT) pathway and induce expression of IFN-γ-stimulated genes. IFN-γ is critical for innate and adaptive immunity and aberrant IFN-γ expression and functions have been associated with different human diseases. However, the IFN-γ/IFNγR signaling could be a double-edged sword in cancer development because the tissue microenvironments could determine its anti- or pro-tumorigenic activities. The IFNγR protein consists of two IFNγR1 and IFNγR2 chains, subunits of which play different roles under certain conditions. This review assessed IFNγR polymorphisms, expression and functions in development and progression of various human diseases in an IFN-γ-dependent or independent manner. This review also discussed tumor microenvironment, microbial infection, and vital molecules in the IFN-γ upstream signaling that might regulate IFNγR expression, drug resistance, and druggable strategy, to provide evidence for further application of IFNγR.

Dietary Natural Products for Prevention and Treatment of Breast Cancer

Breast cancer is the most common cancer among females worldwide. Several epidemiological studies suggested the inverse correlation between the intake of vegetables and fruits and the incidence of breast cancer. Substantial experimental studies indicated that many dietary natural products could affect the development and progression of breast cancer, such as soy, pomegranate, mangosteen, citrus fruits, apple, grape, mango, cruciferous vegetables, ginger, garlic, black cumin, edible macro-fungi, and cereals. Their anti-breast cancer effects involve various mechanisms of action, such as downregulating ER-α expression and activity, inhibiting proliferation, migration, metastasis and angiogenesis of breast tumor cells, inducing apoptosis and cell cycle arrest, and sensitizing breast tumor cells to radiotherapy and chemotherapy. This review summarizes the potential role of dietary natural products and their major bioactive components in prevention and treatment of breast cancer, and special attention was paid to the mechanisms of action.

All-Trans Retinoic Acid Ameliorates Arsenic-Induced Oxidative Stress and Apoptosis in the Rat Uterus by Modulating MAPK Signaling Proteins

Exposure to arsenic leads to inhibition of the anti-oxidant defense mechanism of the body. Reactive oxygen species generated in response to arsenic causes reproductive failures in exposed females and also acts as an inducer of apoptosis. As a prospective remedial agent, all-trans retinoic acid (ATRA) was assessed for reversing arsenic-induced oxidative stress and apoptosis. Rats exposed to arsenic for 28 days were allowed to recover naturally or were treated simultaneously with ATRA for 28 days or up to 56 days. Production of H₂ O₂ was detected using 2',7'-dichlorfluorescein diacetate (DCFCA) by flow cytometry. Catalase, superoxide dismutase, glutathione, ALT, and AST were estimated by biochemical assays and Western blot analyses. Detection of apoptosis was performed using annexin V-FITC/propidium iodide. Expressions of p53, p21, cleaved caspase 3, JNK/pJNK, and ERK/pERK levels were estimated using Western blot analysis. Elemental arsenic deposition in the rat uterus and liver was estimated by atomic absorption spectrophotometry. Our results confirmed that ATRA ameliorated sodium arsenite-induced ROS generation, restored redox balance, and prevented apoptosis. Concomitant recovery was observed to be more prominent for ATRA-treated rats as compared to the rats that were allowed to recover naturally for 56 days. Tissue arsenic deposition was significantly reduced in the uterus upon continuous ATRA treatment. The results revealed that ATRA reversed arsenic-induced free radical generation, activated the anti-oxidant defence system, and subsequently repressed p53-dependent apoptosis through inhibition of the MAPK signaling components. J. Cell. Biochem. 118: 3796-3809, 2017. © 2017 Wiley Periodicals, Inc.

Artemisinin Represses Telomerase Subunits and Induces Apoptosis in HPV-39 Infected Human Cervical Cancer Cells

Artemisinin, a plant-derived antimalarial drug with relatively low toxicity on normal cells in humans, has selective anticancer activities in various types of cancers, both in vitro and in vivo. In the present study, we have investigated the anticancer effects of artemisinin in human cervical cancer cells, with special emphasis on its role in inducing apoptosis and repressing cell proliferation by inhibiting the telomerase subunits, ERα which is essential for maintenance of the cervix, and downstream components like VEGF, which is known to activate angiogenesis. Effects of artemisinin on apoptosis of ME-180 cells were measured by flow cytometry, DAPI, and annexin V staining. Expression of genes and proteins related to cell proliferation and apoptosis was quantified both at the transcriptional and translational levels by semi-quantitative RT-PCR and western blot analysis, respectively. Our findings demonstrated that artemisinin significantly downregulated the expression of ERα and its downstream component, VEGF. Antiproliferative activity was also supported by decreased telomerase activity and reduced expression of hTR and hTERT subunits. Additionally, artemisinin reduced the expression of the HPV-39 viral E6 and E7 components. Artemisinin-induced apoptosis was confirmed by FACS, nuclear chromatin condensation, annexin V staining. Increased expression of p53 with concomitant decrease in expression of the p53 inhibitor Mdm2 further supported that artemisinin-induced apoptosis was p53-dependent. The results clearly indicate that artemisinin induces antiproliferative and proapoptotic effects in HPV-39-infected ME-180 cells, and warrants further trial as an effective anticancer drug.

Arecoline augments cellular proliferation in the prostate gland of male Wistar rats

Areca nut chewing is the fourth most popular habit in the world due to its effects as a mild stimulant, causing a feeling of euphoria and slightly heightened alertness. Areca nuts contain several alkaloids and tannins, of which arecoline is the most abundant and known to have several adverse effects in humans, specially an increased risk of oral cancer. On evaluating the effects of arecoline on the male endocrine physiology in Wistar rats, it was found that arecoline treatment led to an overall enlargement and increase in the wet weight of the prostate gland, and a two-fold increase in serum gonadotropin and testosterone levels. Since the prostate is a major target for testosterone, the consequences of arecoline consumption were studied specifically in the prostate gland. Arecoline treatment led to an increase in the number of rough endoplasmic reticulum and reduction of secretory vesicles, signifying a hyperactive state of the prostate. Increased expression of androgen receptors in response to arecoline allowed for enhanced effect of testosterone in the prostate of treated animals, which augmented cell proliferation, subsequently confirmed by an increase in the expression of Ki-67 protein. Cellular proliferation was also the outcome of concomitant over expression of the G(1)-to-S cell cycle regulatory proteins, cyclin D1 and CDK4, both at the transcriptional and translational levels. Taken together, the findings provide the first evidence that regular use of arecoline may lead to prostatic hyperplasia and hypertrophy, and eventually to disorders associated with prostate enlargement.

Arsenic abrogates the estrogen-signaling pathway in the rat uterus

BACKGROUND

Arsenic, a major pollutant of water as well as soil, is a known endocrine disruptor, and shows adverse effects on the female reproductive physiology. However, the exact molecular events leading to reproductive dysfunctions as a result of arsenic exposure are yet to be ascertained. This report evaluates the effect and mode of action of chronic oral arsenic exposure on the uterine physiology of mature female albino rats.

METHODS

The effect of chronic oral exposure to arsenic at the dose of 4 microg/ml for 28 days was evaluated on adult female albino rats. Hematoxylin-eosin double staining method evaluated the changes in the histological architecture of the uterus. Circulating levels of gonadotropins and estradiol were assayed by enzyme-linked immunosorbent assay. Expression of the estrogen receptor and estrogen-induced genes was studied at the mRNA level by RT-PCR and at the protein level by immunohistochemistry and western blot analysis.

RESULTS

Sodium arsenite treatment decreased circulating levels of estradiol in a dose and time-dependent manner, along with decrease in the levels of both LH and FSH. Histological evaluation revealed degeneration of luminal epithelial cells and endometrial glands in response to arsenic treatment, along with reduction in thickness of the longitudinal muscle layer. Concomitantly, downregulation of estrogen receptor (ER alpha), the estrogen-responsive gene - vascular endothelial growth factor (VEGF), and G1 cell cycle proteins, cyclin D1 and CDK4, was also observed.

CONCLUSION

Together, the results indicate that arsenic disrupted the circulating levels of gonadotropins and estradiol, led to degeneration of luminal epithelial, stromal and myometrial cells of the rat uterus and downregulated the downstream components of the estrogen signaling pathway. Since development and functional maintenance of the uterus is under the influence of estradiol, arsenic-induced structural degeneration may be attributed to the reduction in circulating estradiol levels. Downregulation of the estrogen receptor and estrogen-responsive genes in response to arsenic indicates a mechanism of suppression of female reproductive functions by an environmental toxicant that is contra-mechanistic to that of estrogen.

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.

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.

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.

Negative feedback regulation of IFN-gamma pathway by IFN regulatory factor 2 in esophageal cancers

IFN-gamma is an antitumor cytokine that inhibits cell proliferation and induces apoptosis after engagement with the IFN-gamma receptors (IFNGR) expressed on target cells, whereas IFN regulatory factor 2 (IRF-2) is able to block the effects of IFN-gamma by repressing transcription of IFN-gamma-induced genes. Thus far, few studies have explored the influences of IFN-gamma on human esophageal cancer cells. In the present study, therefore, we investigated in detail the functions of IFN-gamma in esophageal cancer cells. The results in clinical samples of human esophageal cancers showed that the level of IFN-gamma was increased in tumor tissues and positively correlated with tumor progression and IRF-2 expression, whereas the level of IFNGR1 was decreased and negatively correlated with tumor progression and IRF-2 expression. Consistently, in vitro experiments showed that low concentration of IFN-gamma induced the expression of IRF-2 with potential promotion of cell growth, and moreover, IRF-2 was able to suppress IFNGR1 transcription in human esophageal cancer cells by binding a specific motif in IFNGR1 promoter, which lowered the sensitivity of esophageal cancer cells to IFN-gamma. Taken together, our results disclosed a new IRF-2-mediated inhibitory mechanism for IFN-gamma-induced pathway in esophageal cancer cells: IFN-gamma induced IRF-2 up-regulation, then up-regulated IRF-2 decreased endogenous IFNGR1 level, and finally, the loss of IFNGR1 turned to enhance the resistance of esophageal cancer cells to IFN-gamma. Accordingly, the results implied that IRF-2 might act as a mediator for the functions of IFN-gamma and IFNGR1 in human esophageal cancers.

S100A7 (Psoriasin), highly expressed in ductal carcinoma in situ (DCIS), is regulated by IFN-gamma in mammary epithelial cells

BACKGROUND

The aim of the present work was to explore signal transduction pathways used in the regulation of S100A7 (psoriasin). Members of the S100 gene family participate in many important cellular functions. Psoriasin, S100A8 (calgranulin A) and S100A9 (calgranulin B) are expressed in ductal carcinoma in situ (DCIS), as well as in the hyperproliferative skin disease, psoriasis. In the latter condition, a disturbance in the STAT pathway has recently been reported. This pathway is implicated in the regulation of IFN-gamma, widely recognized as a key cytokine in psoriasis. IFN-gamma also exerts anti-tumor action in a number of tumor cell types, including breast cancer. We therefore examined the effect of IFN-gamma and STAT-signaling on the psoriasin expression.

METHODS

We established a TAC2 mouse mammary epithelial cell line with tetracycline-inducible psoriasin expression (Tet-Off). Viability in cell culture was estimated using MTS assay. Protein and gene expression were evaluated by Western blotting and quantitative real-time PCR. Statistical analyses were assessed using a one-tailed, paired t-test.

RESULTS

We report the downregulation of psoriasin by IFN-gamma in the MDA-MB-468 breast cancer cell line, as well as the downregulation of psoriasin induced by anoikis in cell lines derived from different epithelial tissues. In contrast, IFN-gamma had no suppressive effect on calgranulin A or calgranulin B. IFN-gamma is an important activator of the STAT1 pathway and we confirmed an active signaling pathway in the cell lines that responded to IFN-gamma treatment. In contrast, in the SUM190 breast carcinoma cell line, IFN-gamma did not suppress the expression of endogenous psoriasin. Moreover, a reduced phosphorylation of the STAT1 protein was observed. We showed that IFN-gamma treatment and the inhibition of the transcription factor NFkappaB had a synergistic effect on psoriasin levels. Finally, in TAC2 cells with tetracycline-induced psoriasin expression, we observed the increased viability of psoriasin-expressing cells after IFN-gamma treatment.

CONCLUSION

Our data support the possibility that psoriasin expression is transcriptionally suppressed by IFN-gamma and that this effect is likely to be mediated by the activation of the STAT1 signaling pathway. The increased viability of psoriasin-expressing cells after IFN-gamma exposure suggests that psoriasin expression leads to the development of an apoptosis-resistant phenotype.

Modulation of gamma interferon receptor 1 by Mycobacterium tuberculosis: a potential immune response evasive mechanism

Mycobacterium tuberculosis inhibits gamma interferon (IFN-gamma)-mediated antimycobacterial action by adopting diverse mechanisms. IFN-gamma binds to its receptor, IFN-gammaR, in order to initiate proper signaling. We have observed reduced surface expression levels of IFN-gamma receptor 1 (IFN-gammaR1) in untreated pulmonary tuberculosis patients compared to those in healthy individuals (P < 0.01). Following antitubercular therapy, the expression of IFN-gammaR1 was restored in these patients. To delineate the mechanism by which M. tuberculosis modulates IFN-gammaR1, in vitro experiments were designed, wherein the down modulation of IFN-gammaR1 surface expression was observed for CD14+ cells in peripheral blood mononuclear cells (PBMCs) cocultured with live M. tuberculosis compared to that for uninfected cells (P < 0.01). No modulation of IFN-gammaR1 expression was observed for CD14+ cells in PBMCs infected with Mycobacterium smegmatis. A time-dependent decrease in IFN-gammaR1 mRNA expression was observed for PBMCs infected with M. tuberculosis. Similar down modulation of IFN-gammaR1 protein and mRNA expression in phorbol myristate acetate-differentiated THP-1 cells (pdTHP-1) by M. tuberculosis was observed (P < 0.01). Using reporter gene analysis of 5' deletion constructs of the IFN-gammaR1 gene (IFNGR1) promoter, the decrease in IFN-gammaR1 mRNA in M. tuberculosis-infected pdTHP-1 cells was shown to be due to the decreased transcription of IFNGR1. By immunoblotting and electrophoretic mobility shift assays, the down regulation of stimulating protein 1 (Sp1) expression and its recruitment on the phorbol ester-responsive element of the IFNGR1 promoter in M. tuberculosis-infected pdTHP-1 cells was observed. This down regulation of Sp1 in pdTHP-1 cells cocultured with M. tuberculosis may be responsible for the down regulation of IFN-gammaR1 expression, thereby potentially altering its receptivity to IFN-gamma.

EGFR and Src are involved in indole-3-carbinol-induced death and cell cycle arrest of human breast cancer cells

Indole-3-carbinol (I3C), a dietary chemopreventive compound, induced marked reduction in epidermal growth factor receptor (EGFR) prior to cell death in cells representing three breast cancer subtypes. Signalling pathways, linking these events were investigated in detail. I3C modulated tyrosine phosphorylation from 30 min in four cell lines. In MDA-MB-468 and HBL100 cells, it induced Src activation after 5 h. In MDA-MB-468 cells, I3C induced signalling between 4.5 and 7 h, which involved sequential activation of Src, EGFR, STAT-1 and STAT-3, followed by EGFR degradation. It also induced physical association between activated Src and EGFR. In MCF7 and MDA-MB-231 cells, I3C modulated expression of cell cycle-related proteins, p21Cip1, p27Kip1, cyclin E, cyclin D1 and CDK6, with upregulation of p21Cip1 and cyclin E being dependent on Src. Inhibition of EGFR by specific inhibitors PD153035 or ZD1839 increased susceptibility to I3C-induced apoptosis of MCF7, MDA-MB-468 and MDA-MB-231 cells. Inhibition of Src sensitized MDA-MB-468 and MDA-MB-231 cells to I3C, whereas overexpression of c-Src increased resistance to I3C in MDA-MB-468 and HBL100 cells. Modulation of Src in MDA-MB-468 cells influenced the basal level of EGFR expression and cell viability; the latter being positively correlated with EGFR activation levels. Therefore, EGFR and Src activities are essential for I3C-induced cell cycle arrest and death; however, I3C-induced pathways depend on specific features of breast cancer cells. The cancer types, which rely on 'EGFR addiction' or Src deregulation, are likely to be susceptible to I3C.

Cytokines in breast cancer

In recent decades many advances have occurred in the understanding of the role of cytokines in breast cancer. New signalling pathways of interleukin (IL)-1 family, IL-6, IL-11, IL-18, interferons (IFNs) and interferon regulatory factors 1 (IRF-1) and 2 (IRF-2) have been found within tumour microenvironments and in metastatic sites. Some cytokines (IL-1, IL-6, IL-11, TGFbeta) stimulate while others (IL-12, IL-18, IFNs) inhibit breast cancer proliferation and/or invasion. Similarly, high circulating levels of some cytokines seem to be favourable (soluble IL-2R) while others are unfavourable (IL-1beta, IL-6, IL-8, IL-10, IL-18, gp130) prognostic indicators. So far IL-2, IFNalpha, IFNbeta and occasionally IFNgamma, IL-6, IL-12 have been the cytokines used for anti tumour treatment of advanced breast cancer either to induce or increase hormone sensitivity and/or to stimulate cellular immunity. Disappointing results occurred in most trials; however, two long-term pilot studies suggest that IL-2 and IFNbeta, when used appropriately can have a positive effect on clinical benefit and overall survival of patients with minimal residual disease after chemotherapy or with disseminated disease controlled by conventional endocrine therapy.

Activation and potentiation of interferon-gamma signaling by 3,3'-diindolylmethane in MCF-7 breast cancer cells

3,3'-Diindolylmethane (DIM), a natural autolytic product in plants of the Brassica genus, including broccoli, cauliflower, and Brussels sprouts, exhibits promising cancer protective activities, especially against mammary neoplasia in animal models. We observed previously that DIM induced a G(1) cell-cycle arrest and strong induction of cell-cycle inhibitor p21 expression and promoter activity in both estrogen-responsive and -independent breast cancer cell lines. We showed recently that DIM up-regulates the expression of interferon gamma (IFNgamma) in human MCF-7 breast cancer cells. This novel effect may contribute to the anticancer effects of DIM because IFNgamma plays an important role in preventing the development of primary and transplanted tumors. In this study, we observed that DIM activated the IFNgamma signaling pathway in human breast cancer cells. DIM activated the expression of the IFNgamma receptor (IFNGR1) and IFNgamma-responsive genes p56- and p69-oligoadenylate synthase (OAS). In cotreatments with IFNgamma, DIM produced an additive activation of endogenous p69-OAS and of an OAS-Luc reporter and a synergistic activation of a GAS-Luc reporter. DIM synergistically augmented the IFNgamma induced phosphorylation of signal transducer and activator of transcription factor 1, further evidence of DIM activation of the IFNgamma pathway. DIM and IFNgamma produced an additive inhibition of cell proliferation and a synergistic increase in levels of major histocompatibility complex class-1 (MHC-1) expression, accompanied by increased levels of mRNAs of MHC-1-associated proteins and transporters. These results reveal novel immune activating and potentiating activities of DIM in human tumor cells that may contribute to the established effectiveness of this dietary indole against various tumors types.

Indole-3-carbinol activates the ATM signaling pathway independent of DNA damage to stabilize p53 and induce G1 arrest of human mammary epithelial cells

The phytochemical indole-3-carbinol (I3C), from cruciferous vegetables such as broccoli, has been shown to elicit a potent anti-proliferative response in human breast cancer cell lines. Treatment of the immortalized human mammary epithelial cell line MCF10A with I3C induced a G1 cell cycle arrest, elevated p53 tumor suppressor protein levels and stimulated expression of downstream transcriptional target, p21. I3C treatment also elevated p53 levels in several breast cancer cell lines that express mutant p53. I3C did not arrest MCF10A cells stably transfected with dominant-negative p53, establishing a functional requirement for p53. Cell fractionation and immunolocalization studies revealed a large fraction of stabilized p53 protein in the nucleus of I3C-treated MCF10A cells. With I3C treatment, phosphatidyl-inositol-3-kinase family member ataxia telangiectasia-mutated (ATM) was phosphorylated, as were its substrates p53, CHK2 and BRCA1. Phosphorylation of p53 at the N-terminus has previously been shown to disrupt the interaction between p53 and its ubiquitin ligase, MDM2, and therefore stabilizing p53. Coimmunoprecipitation analysis revealed that I3C reduced by 4-fold the level of MDM2 protein that associated with p53. The p53-MDM2 interaction and absence of p21 production were restored in cells treated with I3C and the ATM inhibitor wortmannin. Significantly, I3C does not increase the number of 53BP1 foci or H2AX phosphorylation, indicating that ATM is activated independent of DNA double-strand breaks. Taken together, our results demonstrate that I3C activates ATM signaling through a novel pathway to stimulate p53 phosphorylation and disruption of the p53-MDM2 interaction, which releases p53 to induce the p21 CDK inhibitor and a G1 cell cycle arrest.

Indole-3-Carbinol (I3C) Inhibits Cyclin-dependent Kinase-2 Function in Human Breast Cancer Cells by Regulating the Size Distribution, Associated Cyclin E Forms, and Subcellular Localization of the CDK2 Protein Complex

Indole-3-carbinol (I3C), a dietary compound found in cruciferous vegetables, induces a robust inhibition of CDK2 specific kinase activity as part of a G1 cell cycle arrest of human breast cancer cells. Treatment with I3C causes a significant shift in the size distribution of the CDK2 protein complex from an enzymatically active 90 kDa complex to a larger 200 kDa complex with significantly reduced kinase activity. Co-immunoprecipitations revealed an increased association of both a 50 kDa cyclin E and a 75 kDa cyclin E immunoreactive protein with the CDK2 protein complex under I3C-treated conditions, whereas the 90 kDa CDK2 protein complexes detected in proliferating control cells contain the lower molecular mass forms of cyclin E. I3C treatment caused no change in the level of CDK2 inhibitors (p21, p27) or in the inhibitory phosphorylation states of CDK2. The effects of I3C are specific for this indole and not a consequence of the cell cycle arrest because treatment of MCF-7 breast cancer cells with either the I3C dimerization product DIM or the anti-estrogen tamoxifen induced a G1 cell cycle arrest with no changes in the associated cyclin E or subcellular localization of the CDK2 protein complex. Taken together, our results have uncovered a unique effect of I3C on cell cycle control in which the inhibition of CDK2 kinase activity is accompanied by selective alterations in cyclin E composition, size distribution, and subcellular localization of the CDK2 protein complex.