Sakina E. Eltom

Induction by transforming growth factor-β1 of epithelial to mesenchymal transition is a rare event in vitro

IntroductionTransforming growth factor (TGF)-β1 is proposed to inhibit the growth of epithelial cells in early tumorigenesis, and to promote tumor cell motility and invasion in the later stages of carcinogenesis through the induction of an epithelial to mesenchymal transition (EMT). EMT is a multistep process that is characterized by changes in cell morphology and dissociation of cell–cell contacts. Although there is growing interest in TGF-β1-mediated EMT, the phenotype is limited to only a few murine cell lines and mouse models.MethodsTo identify alternative cell systems in which to study TGF-β1-induced EMT, 18 human and mouse established cell lines and cultures of two human primary epithelial cell types were screened for TGF-β1-induced EMT by analysis of cell morphology, and localization of zonula occludens-1, E-cadherin, and F-actin. Sensitivity to TGF-β1 was also determined by [3H]thymidine incorporation, flow cytometry, phosphorylation of Smad2, and total levels of Smad2 and Smad3 in these cell lines and in six additional cancer cell lines.ResultsTGF-β1 inhibited the growth of most nontransformed cells screened, but many of the cancer cell lines were insensitive to the growth inhibitory effects of TGF-β1. In contrast, TGF-β1 induced Smad2 phosphorylation in the majority of cell lines, including cell lines resistant to TGF-β1-mediated cell cycle arrest. Of the cell lines screened only two underwent TGF-β1-induced EMT.ConclusionThe results presented herein show that, although many cancer cell lines have lost sensitivity to the growth inhibitory effect of TGF-β1, most show evidence of TGF-β1 signal transduction, but only a few cell lines undergo TGF-β1-mediated EMT.

Knockdown of aberrantly upregulated aryl hydrocarbon receptor reduces tumor growth and metastasis of MDA-MB-231 human breast cancer cell line

The aryl hydrocarbon receptor (AhR), a ligand‐activated transcription factor that belongs to the basic‐helix‐loop‐helix (bHLH)–Per‐ARNT‐Sim (PAS) superfamily of transcription factors, mediates toxic response induced by environmental chemicals such as polycyclic aromatic hydrocarbons (PAH). AhR is expressed at high levels in several human breast carcinoma cell lines in direct correlation with the degree of their malignancy. Recent studies suggest a possible role for AhR in cancer independent of PAH. Therefore, we established stable AhR knockdown cells of the human breast cancer cell line MDA‐MB‐231 and analyzed their tumorigenic properties in in vitro and in vivo model systems. In addition we analyzed their response to radiation and chemotherapeutic treatment. AhR knockdown attenuated these cells tumorigenic properties in vitro including proliferation, anchorage independent growth, migration and apoptosis and reduced orthotopic xenograft tumor growth and lung metastasis in vivo. Notably, we observed that AhR knockdown enhanced radiation‐induced apoptosis as well as significantly decreased cell clonogenic survival. Furthermore, AhR knockdown in MDA‐MB‐231 cells sensitized them to paclitaxel treatment, evident by a decrease in the required cytotoxic dose. Subsequent analysis revealed AhR knockdown significantly reduced phosphorylation of AKT, which impacts cell proliferation and survival. Apoptosis‐focused gene expression analyses revealed an altered expression of genes regulating apoptosis in MDA‐MB‐231 cells. Collectively, our data identify AhR as a potential novel therapeutic target in the treatment of metastatic breast cancer.

Calpain Mediates the Dioxin-Induced Activation and Down-Regulation of the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated basic-helix-loop-helix transcription factor that binds polyaromatic hydrocarbons (PAH), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and mediates their toxicity. Binding of PAH to AhR in the cytoplasm triggers a poorly defined transformation step of the receptor into a nuclear transcription factor. In this study, we show that the calcium-dependent cysteine protease calpain plays a major role in the ligand-induced transformation and signaling of AhR. Fluorescence imaging measurements showed that TCDD treatment elevates intracellular calcium, providing the trigger for calpain activation, as measured toward t-butoxycarbonyl-Leu-Met-chloromethylaminocoumarin, a calpain-specific substrate. Inhibition of calpain activity by the N-benzyloxycarbonyl-Val-Phe-aldehyde (MDL28170) blocked the TCDD-induced nuclear translocation of AhR in Hepa1c1c7 mouse hepatoma cell line. Treatment of the human metastatic breast carcinoma cell line MT-2 with MDL28170 and 3-(4-iodophenyl)-2-mercapto-(Z)-2-propenoic acid (PD 150606), two calpain-selective inhibitors, completely abolished the TCDD-induced transactivation of AhR as assessed by transcription of CYP1A1 gene. Previous studies have established that after TCDD-induced transactivation, the AhR undergoes a massive depletion; we show here that selective calpain inhibitors can block this step, which suggests that the ligand-induced down-regulation of the AhR is calpain-dependent. The data presented support a major role for calpain in the AhR transformation, transactivation, and subsequent down-regulation, and provide a possible explanation for many of the reported phenomena of ligand-independent activation of AhR.

The Aryl Hydrocarbon Receptor: A Target for Breast Cancer Therapy.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates a battery of genes in response to exposure to a broad class of environmental poly aromatic hydrocarbons (PAH). AhR is historically characterized for its role in mediating the toxicity and adaptive responses to these chemicals, however mounting evidence has established a role for it in ligand-independent physiological processes and pathological conditions, including cancer. The AhR is overexpressed and constitutively activated in advanced breast cancer cases and was shown to drive the progression of breast cancer. In this article we will review the current state of knowledge on the possible role of AhR in breast cancer and how it will be exploited in targeting AhR for breast cancer therapy.

The impact of low-dose carcinogens and environmental disruptors on tissue invasion and metastasis

The purpose of this review is to stimulate new ideas regarding low-dose environmental mixtures and carcinogens and their potential to promote invasion and metastasis. Whereas a number of chapters in this review are devoted to the role of low-dose environmental mixtures and carcinogens in the promotion of invasion and metastasis in specific tumors such as breast and prostate, the overarching theme is the role of low-dose carcinogens in the progression of cancer stem cells. It is becoming clearer that cancer stem cells in a tumor are the ones that assume invasive properties and colonize distant organs. Therefore, low-dose contaminants that trigger epithelial-mesenchymal transition, for example, in these cells are of particular interest in this review. This we hope will lead to the collaboration between scientists who have dedicated their professional life to the study of carcinogens and those whose interests are exclusively in the arena of tissue invasion and metastasis.

Depletion of the Aryl Hydrocarbon Receptor in MDA-MB-231 Human Breast Cancer Cells Altered the Expression of Genes in Key Regulatory Pathways of Cancer

The aryl hydrocarbon receptor (AhR), a transcription factor that is best known for its role in mediating the toxic responses elicited by poly aromatic hydrocarbons as well as many other environmental factors; is also involved in breast cancer progression. We previously reported that stable knockdown of AhR decreased the tumorigenic properties of the highly metastatic MDA-MB-231 breast cancer cell line; whereas ectopic overexpression of AhR was sufficient to transform immortalized human mammary epithelial cells to exhibit malignant phenotypes. In the present study we investigated the genes that are differentially regulated by AhR and are controlling cellular processes linked to breast cancer. We used Affymetrix Human GeneChip 1.0-ST whole transcriptome arrays to analyze alterations of gene expression resulting from stable AhR knockdown in the MDA-MB-231 breast cancer cell line. The expression of 144 genes was significantly altered with a ≥2.0-fold change and a multiple test corrected p-value ≤0.05, as a result of AhR knockdown. We demonstrate that AhR knockdown alters the expression of several genes known to be linked to cancer. These genes include those involved in tryptophan metabolism (KYNU), cell growth (MUC1 and IL8), cell survival (BIRC3 and BCL3), cell migration and invasion (S100A4 and ABI3), multi-drug resistance (ABCC3) and angiogenesis (VEGFA and CCL2). The identification of the genes and pathways affected by AhR depletion provides new insight into possible molecular events that could explain the reported phenotypic changes. In conclusion AhR knockdown alters the expression of genes known to enhance or inhibit cancer progression; tipping the balance towards a state that counteracts tumor progression.

Stability of the aryl hydrocarbon receptor and its regulated genes in the low activity variant of Hepa-1 cell line.

We examined the expression kinetics of some of the aryl hydrocarbon receptor (AhR)-regulated genes in LA1 variant cells compared to wild type (WT) Hepa-1 mouse hepatoma cell lines, and we investigated the stability of AhR protein as a key step in the function of this receptor. Treatment of both cell types with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) resulted in increased CYP1A1 and CYP1B1 mRNA with a subsequent down regulation of AhR. We show here that co-treatment with transcription inhibitor actinomycin D (ActD) has reversed the TCDD-induced depletion of AhR protein in WT. However, the proteolytic degradation of AhR in absence of TCDD was significantly higher in LA1 cells than in WT, and ActD treatment reduced this loss. Induction of CYP1A1 and CYP1B1 mRNA by TCDD in WT cells each exhibited bursts of activity in the initial hour which were about 3-fold greater than in LAI cells. The induced mRNA levels in LA1 exhibited a slow and sustained increase approximating the WT levels by 20 h. The induction of two other AhR-regulated genes also showed comparable turnover differences between the two cell types. Thus, altered regulation of the AhR responsive genes in LA1 may result from a difference in AhR stability.

Abstract 698: Overexpression of the aryl hydrocarbon receptor correlates with high tumor grade in human breast invasive carcinomas

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL The aryl hydrocarbon receptor (AhR) is a ligand-activated basic helix-loop-helix transcription factor which binds environmental poly aromatic hydrocarbons (PAH) and mediates their carcinogenic effects. Recent reports, including data from our laboratory implicate AhR in breast cancer development and progression independent of the receptor occupancy by PAH. In this study we investigated the potential of AhR as a stage specific marker of breast cancer. We examined the expression of AhR by immunohistochemistry in tissue microarrays (TMA) containing 192 specimens of clinically defined three stages of invasive breast cancer: node-negative (NN), node-positive (NP) and metastatic (Met) carcinoma. The TMA were obtained from the National Cancer Institute Cooperative Breast Cancer Tissue Resource [NCI-CBCTR] and were stained for AhR using high affinity antibody. The AhR staining was then scored by three evaluators, including two pathologists who were blinded to the study. Statistical analysis showed a significant correlation between the AhR expression and the carcinoma case type (NN, NP or Met) (p-value of ANOVA is < 0.0003), and although there is a significant difference of AhR expression among carcinoma case type of white women (p< 0.0017) there is no significant difference for black women (p<0.0968). There is also a strong positive correlation between the receptor expression and the tumor grade (p <0.0001). While there is no correlation with the status of estrogen or progesterone receptors (p= 0.1643 and 0.1884, respectively), there is strong correlation of high AhR expression and invasive breast carcinoma in women over 50 years old (p<0.0043). In conclusion, our findings identify the AhR as a new predictive clinical marker for metastatic breast cancer and a unique target for the design of novel selective inhibitors for therapeutic intervention of metastatic breast cancer. More importantly, the AhR overexpression identifies a subset of patients who could benefit from therapy targeting this receptor. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 698. doi:1538-7445.AM2012-698

Abstract 4077: Knockdown of the aryl hydrocarbon receptor altered tumorigenic properties of human breast carcinoma cell lines

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC Previous studies have found that AhR is overexpressed and constitutively activated in several breast carcinoma cell lines and the expression levels showed strong correlation with the degree of the tumor malignancy. The objective of this study is to examine the effect of reducing the expression levels of AhR in metastatic human breast carcinoma (HBC) cell lines on their tumorigenic properties; specifically cellular growth, invasion into matrigel, and migration. We designed a series of short hairpin RNA (shRNA) targeting AhR and cloned them in pSuper retroviral vectors, and viruses expressing shRNA were used to infect a battery of HBC cell lines including MDA-MB231, MDA-MB468, BT549 and MT2 to generate cell lines that permanently express siRNA targeting AhR. Clonal cell lines with 50%-90% knockdown of AhR were isolated and characterized in vitro for their tumorigenic properties in comparison to the scramble RNA-expressing control cell lines. Cell cycle analysis by fluorescence-activated cell sorting showed that knock down of AhR expression decreases transition of cells from G1 to S and their progression to G2/M phases of cell cycle. This was seen as an increase of cells in G0/G1 (49% to 61%) and a decreased number of cells in S (26% to 21%) and G2/M phases (25% to 17%). This effect was coupled with a prolonged doubling time and a decreased cellular proliferation. Cells with AhR knock-down exhibited a decreased anchorage independent growth; forming less numbers of colonies in soft agar than control cells. Modified Boyden Chamber invasion assay coupled with fluorescence detection demonstrated that clones of HBC with AhR knock down levels exhibited substantially reduced invasiveness and diminished potential to invade matrigel. These clonal cell lines further exhibited reduced ability to migrate, as measured by scratch wound test. These observations collectively demonstrate a major role for the AhR in regulating tumorigenic phenotypes of metastatic breast cancer. Currently breast cancer prevention research focuses on the identification of novel approaches and/or novel targets. Our findings may establish the potential of targeting AhR as one of the prevention strategies for breast cancer. Supported by RR03032 15, CA91408-01 and DAMD17-02-01-0483 grants. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4077.

Response to Comments on “Calpain Mediates the Dioxin-Induced Activation and Down-Regulation of the Aryl Hydrocarbon Receptor”

In response to the comments by Dr. Richard S. Pollenz in reference to our article ([Dale and Eltom, 2006][1]), we provide the following points for discussion. Indeed, our study has built on some significant reports published by leading scientists in the AhR field, as outlined in our manuscript.