2,3,7,8-Tetrachlorodibenzo-p-dioxin: molecular mechanism of carcinogenesis and its implication in human in vitro model

Gene expression and pathologic alterations in juvenile rainbow trout due to chronic dietary TCDD exposure

The goal of this project was to use functional genomic methods to identify molecular biomarkers as indicators of the impact of TCDD exposure in rainbow trout. Specifically, we investigated the effects of chronic dietary TCDD exposure on whole juvenile rainbow trout global gene expression associated with histopathological analysis. Juvenile rainbow trout were fed Biodiet starter with TCDD added at 0, 0.1, 1, 10 and 100 ppb (ngTCDD/g food), and fish were sampled from each group at 7, 14, 28 and 42 days after initiation of feeding. 100 ppb TCDD caused 100% mortality at 39 days. Fish fed with 100 ppb TCDD food had TCDD accumulation of 47.37 ppb (ngTCDD/g fish) in whole fish at 28 days. Histological analysis from TCDD-treated trout sampled from 28 and 42 days revealed that obvious lesions were found in skin, oropharynx, liver, gas bladder, intestine, pancreas, nose and kidney. In addition, TCDD caused anemia in peripheral blood, decreases in abdominal fat, increases of remodeling of fin rays, edema in pericardium and retrobulbar hemorrhage in the 100 ppb TCDD-treated rainbow trout compared to the control group at 28 days. Dose- and time-dependent global gene expression analyses were performed using the cGRASP 16,000 (16K) cDNA microarray. TCDD-responsive whole body transcripts identified in the microarray experiments have putative functions involved in various biological processes including growth, cell proliferation, metabolic process, and immune system processes. Nine microarray-identified genes were selected for QPCR validation. CYP1A3 and CYP1A1 were common up-regulated genes and HBB1 was a common down-regulated gene among each group based on microarray data, and their QPCR validations are consistent with microarray data for the 10 and 100 ppb TCDD treatment groups after 28 days exposure (p<0.05). In addition, in the 100 ppb group at 28 days, expression of complement component C3-1 and trypsin-1 precursor have a more than 10-fold induction from the microarray experiments, and their QPCR validations are consistent and showed significant induction in the 100 ppb group at 28 days (p<0.05). Overall, lesion in nasal epithelium is a novel and significant result in this study, and TCDD-responsive rainbow trout transcripts identified in the present study may lead to the development of new molecular biomarkers for assessing the potential impacts of environmental TCDD on rainbow trout populations.

Effects of polychlorinated biphenyls on the development of neuronal cells in growth period; structure-activity relationship

Polychlorinated biphenyls (PCBs) are accumulated in our body through food chain and cause a variety of adverse health effects including neurotoxicities such as cognitive deficits and motor dysfunction. In particular, neonates are considered as a high risk group for the neurotoxicity of PCBs exposure. The present study attempted to analyze the structure-activity relationship among PCB congeners and the mechanism of PCBs-induced neurotoxicity. We measured total protein kinase C (PKC) activities, PKC isoforms, reactive oxygen species (ROS), and induction of neurogranin (RC-3) and growth associated protein-43 (GAP-43) mRNA in cerebellar granule cells of neonatal rats with phorbol 12, 13-dibutyrate ([(3)H]PDBu) binding assay, western blot, ROS assay, and reverse transcription PCR (RT-PCR) analysis respectively following the different structural PCBs exposure. Only non-coplanar PCBs showed a significant increase of total PKC-α and βII activity as measured with [(3)H]PDBu binding assay. ROS were more increased with non-coplanar PCBs than coplanar PCBs. The mRNA levels of RC-3 and GAP-43 were more induced with non-coplanar PCBs than coplanar PCBs, indicating that these factors may be useful biomarkers for differentiating non-coplanar PCBs from coplanar PCBs. Non-coplanar PCBs may be more potent neurotoxic congeners than coplanar PCBs. This study provides evidences that non-coplanar PCBs, which have been neglected in the risk assessment processes, should be added in the future to improve the quality and accuracy of risk assessment on the neuroendocrinal adverse effects of PCBs exposures.

Dietary antioxidants in the prevention of hepatocarcinogenesis: a review

In this review, the role of dietary antioxidants in the prevention of hepatocarcinogenesis is examined. Both human and animal models are discussed. Vitamin C, vitamin E, and selenium are antioxidants that are essential in the human diet. A number of non-essential chemicals also contain antioxidant activity and are consumed in the human diet, mainly as plants or as supplements, including beta-carotene, ellagic acid, curcumin, lycopene, coenzyme Q(10), epigallocatechin gallate, N-acetyl cysteine, and resveratrol. Although some human and animal studies show protection against carcinogenesis with the consumption of higher amounts of antioxidants, many studies show no effect or an enhancement of carcinogenesis. Because of the conflicting results from these studies, it is difficult to make dietary recommendations as to whether consuming higher amounts of specific antioxidants will decrease the risk of developing hepatocellular carcinoma.

Development of human dermal epithelial cell-based bioassay for the dioxins

None of bioassays is complete for assessing biological impact in humans upon the xenobiotic exposure due to species and organ-specific responsiveness. Thus, it is speculated that the human cell-based bioassay may be more appropriate system because of its direct relevance to humans. Here, we have developed a human epidermal cell-based bioassay for the dioxins and related compounds. The AD12-SV40-immortalized human keratinocyte cell line was stably transfected with a recombinant expression vector which contains the luciferase gene under dioxin-inducible control of four DREs. The tansfectants showed a consistent dose-response of luciferase activity upon dioxin exposure even after 120 passages. The maximal half effective dose (EC50) was 200 pM with a maximum of 32-fold induction of luciferase activity at 5 nM. The minimum detection limit was 10 pM. Optimal exposure time for the assay was 24h. When cells were treated with aryl hydrocarbon receptor agonists of different toxic equivalent factor (TEF) values, the shape of the dose-response curve for each compound was parallel to that of TCDD and the maximum response was similar, indicating that this bioassay system can be applied to generate the total toxic equivalency (TEQ) estimate from the samples. When relative induction potency of luciferase activities for each compound was calculated, it was similar to WHO-TEF values within an order of magnitude. This human cell system can be used as an efficient screening tool to quantify the TEQ values of dioxin-like chemicals in the samples and may help understand the interspecies difference between humans and animals.

Hepatic transcriptional networks induced by exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) serves as a prototype for a range of environmental toxicants and as a pharmacologic probe to study signal transduction by the aryl hydrocarbon receptor (AHR). Despite a detailed understanding of how TCDD exposure leads to the transcriptional up-regulation of cytochrome P450-dependent monooxygenases, we know little about how compounds like TCDD lead to a variety of AHR-dependent toxic end points such as liver pathology, terata, thymic involution, and cancer. Using an acute exposure protocol and the toxic response of the mouse liver as a model system, we have begun a detailed microarray analysis to describe the transcriptional changes that occur after various TCDD doses and treatment times. Through correlation analysis of time- and dose-dependent toxicological end points, we are able to identify coordinately responsive transcriptional events that can be defined as primary transcriptional events and downstream events that may represent mechanistically linked sequelae or that have potential as biomarkers of toxicity.

Expression of dioxin-responsive genes in human endometrial cells in culture

To investigate expression of dioxin-responsive genes in human endometrial cells with exposure to 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), human endometrial stromal cells immortalized with temperature-sensitive SV40 T antigen were used for the experiments. Cells were treated with 0.1% DMSO or 0.1, 1, 10, or 100 nM TCDD for 24 h. Induction of interleukin-1beta (IL-1beta) and plasminogen activator inhibitor-2 (PAI-2) mRNAs was analyzed by reverse-transcription polymerase chain reaction. Expression of IL-1beta or PAI-2 mRNA in response to TCDD was increased in a dose-dependent fashion. The maximum increases of PAI-2 and IL-1beta mRNAs were observed at 100 and 10 nM TCDD, respectively. While cycloheximide treatment did not show a significant difference of PAI-2 mRNA levels between control and TCDD-treated cells, mRNA stability assay using actinomycin D showed that PAI-2 mRNA in TCDD-treated cells was about twofold more stable than the control cells. While expression of CYP1A1 mRNA was not detected and levels of ARNT mRNA were not altered by TCDD exposure, the amount of AhR mRNA was decreased dose dependently. The present study represents an initial attempt to determine the responses of dioxin-responsive genes in human endometrial cells following TCDD exposure. The results demonstrated that IL-1beta and PAI-2 genes are induced dose dependently in human endometrial cells with exposure to TCDD and expression of PAI-2 mRNA is controlled at the posttranscriptional level.

A malignant transformation of human cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin exhibits altered expressions of growth regulatory factors

The neoplastic transformation of human cells in culture with exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has recently been reported. In this study, expressions of growth regulatory factors were analyzed to examine their possible roles in TCDD-induced malignant transformation of human cells. Reverse transcription-polymerase chain reaction (RT-PCR) and immunoblot analysis were performed to detect altered expressions of genes associated with dioxin responses. The RT-PCR analysis showed that expressions of the growth regulatory factors, such as transforming growth factor-beta1 (TGF-beta1), plasminogen activator inhibitor-2 (PAI-2) and tumor necrosis factor-alpha (TNF-alpha), were significantly changed in the transformed cells as compared with the parental cells. Whereas parental cells showed a dose-dependent increase of PAI-2 mRNA levels following TCDD exposure, the transformed cells did not show any significant induction. In addition, constitutive levels of PAI-2 mRNA were 25 times lower in the transformed cells than in the parental cells. The mRNA stability assay suggests that downregulation of PAI-2 mRNA in the transformed cells may be associated with the posttranscriptional control. Expression of TGF-beta1 mRNA in the transformed cells was also four times lower than the parental cells. However, levels of TNF-alpha mRNA in the transformed cells were increased 3-fold. These results suggest that dysregulation of growth regulatory factors may be involved in TCDD-induced cellular transformation. Whereas plenty of studies demonstrated a number of immediate toxic effects by TCDD, this study revealed an initial evidence that altered expression of growth regulatory genes, such as PAI-2, TGF-beta1 or TNF-alpha, are some of the genetic events fixed in the genome following the successive cell divisions of TCDD-damaged cells. It is suggested that these changes may be associated with TCDD-induced malignant transformation of human cells.

Alterations of signal transduction pathways involved in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced malignant transformation of human cells in culture

Effects of signal transduction pathways in TCDD-induced neoplastic transformation of human cells were assessed with respect to PLC-coupled signaling pathways, adenylyl cyclase-mediated responses and PKC isozyme expressions. A lower stimulation of the intracellular free calcium levels with exposure to extracellular ATP or histamine was observed in the transformed cells, as compared to the parental cells. While the steady-state level of IP3 was higher in the transformed cells, the magnitude of stimulation of IP3 generation by ATP or histamine was significantly lower in the transformed cells than the parental cells. These results indicate that a downregulation PLC-coupled signaling pathways may be involved in the TCDD-induced transformation of human cells. While the steady-state levels of cAMP accumulation were similar between the two cell lines, treatment of PGE2, a potent differentiation inducer, stimulated a higher accumulation of cAMP in the parental cells but isoproterenol, a typical beta-adrenergic agonist, did not induce a significant difference between the two cell lines. These results suggest that desensitization of cAMP-mediated response to extracellular signals including differentiation signals may be associated with a possible mechanism of the carcinogenesis. Elevated expression of PKC-alpha, -gamma, -zeta, -epsilon, -lambda, and -tau were observed in TCDD-transformed cells, indicating a possible association of altered expression of PKC isozymes with TCDD-induced transformation of human cells. The present study demonstrates that alterations of signal transduction pathways are involved in the TCDD-induced transformation of human cells and provides a valuable basis to investigate effects of signaling pathway as a possible mechanism of TCDD-induced carcinogenesis in human cells.

2-(4-Aminophenyl)benzothiazoles: novel agents with selective profiles of in vitro anti-tumour activity

2-(4-Aminophenyl)benzothiazole (CJM 126) elicits biphasic growth-inhibitory effects against a panel of oestrogen receptor-positive (ER+) and oestrogen receptor-negative (ER-) human mammary carcinoma cell lines in vitro, yielding IC50 values in the nM range. Substitutions adjacent to the amino group in the 2-phenyl ring with a halogen atom or methyl group enhance potency in sensitive breast lines (pM IC50 values). Transient biphasic dose responses were induced but rapidly eradicated after specific drug exposure periods. Two human prostate carcinoma cell lines were refractory to the growth-inhibitory properties of 2-(4-aminophenyl)benzothiazoles; IC50 values > 30 microM were obtained. Potency and selectivity were confirmed when compounds were examined in the National Cancer Institute's Developmental Therapeutics screen; the spectrum of activity included specific ovarian, renal, colon as well as breast carcinoma cell lines. Moreover, comparing 6-day and 48-h incubations, the exposure time-dependent nature of the biphasic response was corroborated. Differential perturbation of cell cycle distribution followed treatment of MCF-7 and MDA 468 cells with substituted 2-(4-aminophenyl)benzothiazoles. In MDA 468 populations only, accumulation of events in G2/M phase was observed. Two MCF-7 cell lines were established with acquired resistance to CJM 126 (IC50 values > 20 microM), which exhibit cross-resistance to substituted benzothiazoles, but equal sensitivity to tamoxifen and doxorubicin. Compared with standard anti-tumour agents evaluated in the National Cancer Institute in vitro cell panel, benzothiazoles revealed unique profiles of growth inhibition, suggesting a mode(s) of action shared with no known clinically active class of chemotherapeutic agents.