BioDiscovery : Review Article
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Corresponding author: Sandra E. Ghayad (sandra.ghayad@ul.edu.lb)
Academic editor: Jean-Christophe Bourdon
Received: 22 Aug 2018 | Accepted: 07 Feb 2019 | Published: 13 Feb 2019
© 2019 Myriam El Helou, Pascale A. Cohen, Mona Diab-Assaf, Sandra Ghayad
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: El Helou M, Cohen P, Diab-Assaf M, Ghayad S () Environmental pollutants-dependent molecular pathways and carcinogenesis. . https://doi.org/
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Exposure to environmental pollutants can modulate many biological and molecular processes such as gene expression, gene repair mechanisms, hormone production and function and inflammation, resulting in adverse effects on human health including the occurrence and development of different types of cancer. Carcinogenesis is a complex and long process, taking place in multiple stages and is affected by multiple factors. Some environmental molecules are genotoxic, able to damage the DNA or to induce mutations and changes in gene expression acting as initiators of carcinogenesis. Other molecules called xenoestrogens can promote carcinogenesis by their mitogenic effects by possessing estrogenic-like activities and consequently acting as endocrine disruptors causing multiple alterations in cellular signal transduction pathways. In this review, we focus on recent research on environmental chemicals-driven molecular functions in human cancers. For this purpose, we will be discussing the case of two receptors in mediating environmental pollutants effects: the established nuclear receptor, the Aryl hydrocarbon receptor (AhR) and the emerging membrane receptor, G-protein coupled estrogen receptor 1 (GPER1).
environmental pollutants, genotoxic, endocrine disruptors, GPER1, AhR, carcinogenesis
The environment presents all the elements that surround us (
The International Agency for Research on Cancer (IARC) evaluated the carcinogenic risks to humans and has classified around 120 agents as carcinogenic, where the chemical substances represent the majority (
A "genotoxic" agent is able to damage the genetic material by inducing DNA damage, mutation or both (
Endocrine disruptors or endocrine disrupting chemicals (EDC) are pseudo-persistent compounds present in the environment at very low concentrations; however, these low levels are able to interfere with hormonal regulation pathways causing effects leading to a variety of health problems, such as cancer, specifically the hormone-dependent type (breast, ovarian, endometrial, prostate, testicular) (
In Table
List of the most common environmental molecules (genotoxics and endocrine disruptors) and the different types of cancers developed following their exposure.
Class |
Source |
Compound |
Mechanism of action |
Target organs |
References |
N-Nitrosamines |
Contaminated water, Preserved foods, Tobacco smoke |
N-Nitrosodimethylamine (NDMA) |
Genotoxic |
Liver |
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Esophagus (squamous cell carcinoma) |
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N-Butyl-N-(4-hydroxybutyl)nitrosamine (BBN) |
Genotoxic |
Bladder |
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Polycyclic Aromatic Hydrocarbon (PAH) |
Grilled meat, Tobacco smoke, Combustion of organic substances |
2-Acetylaminofluorene (2-AAF) |
Genotoxic |
Liver |
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7,12-Dimethylbenz[a]anthracene (DMBA) |
Genotoxic |
Breast |
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Mouth (Buccal cheek pouch) |
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Benzo[a]pyrene (B[a]P) |
Genotoxic |
Lung |
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Stomach |
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Colon |
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Compounds of natural origin |
Aspergillus flavus, Aspergillus parasitus |
Aflatoxines |
Genotoxic |
Liver |
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Aristolochia clematitis | Aristolochic acid | Genotoxic | Urethra |
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Pesticides |
Insecticides, Acaricides |
1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT) |
Endocrine disruptors |
Breast |
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Liver |
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Plasticiser |
Plastic products, Epoxy resin |
Bisphenol A (BPA) |
Endocrine disruptors |
Breast |
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Cervical |
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Prostate |
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Ovaries |
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Lung |
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Larynx |
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Dioxin |
Formed during industrial process, present in dust, soil and water |
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) |
Endocrine disruptors |
Lung |
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Phthalates |
Plastics products, Perfumes, Cosmetics and Care products |
Di-n-butyl phthalate (DBP) |
Endocrine disruptors |
Breast |
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Mono-2-ethylhexyl phthalate (MEHP) | Endocrine disruptors | Ovaries |
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Previous studies have suggested that environmental factors are able to induce deleterious effects within the cells through the activation of cellular receptors (
Nuclear receptors are activated by both intracellular and extracellular signals and act as transcription factors of target genes (
Membrane receptors are transmembrane proteins that serve as a communication interface between cells and their external and internal environments (
GPCRs represent one of the largest and most diverse families of membrane proteins. They are encoded by more than 800 genes and constitute the largest class of drug targets in the human genome (
GPCRs are involved in many diseases including cancer (
The present review will highlight the recent research advances regarding carcinogenic mechanisms with the focus on two receptors in mediating environmental pollutants effects: the established nuclear receptor the Aryl hydrocarbon receptor (AhR), known to have a major role in the metabolism of toxic compounds and the promotion of tumours and the emerging membrane receptor G-protein coupled estrogen receptor 1 (GPER1), known to mediate estrogenic activity of environmental xeno-estrogens in different cell types (
AhR is a cytosolic nuclear receptor that, after binding with its ligand, moves to the nucleus and acts as a transcription factor (
Constitutive activation of AhR. Studies have shown that AhR can be constitutively active, presumably because of endogenous ligands and plays an important role in the biology of several cell types when exogenous ligands (environmental molecules) are absent.
Environmental molecules that deregulate cell cycle control via AhR pathway. As cited previously, the TCDD and B[a]P represent high-affinity xenobiotic ligands for the AhR. Emerging evidence has demonstrated the role of the AhR and its ligands in cancer. A study showed that the treatment of rat liver normal cells with TCDD leads to the activation of the transcription factor JUN-D, via AhR, resulting in the transcriptional induction of the cell cycle regulator proto-oncogene Cyclin A, that provokes a release from contact inhibition (
Environmental molecules that influence apoptosis. Inhibition of apoptosis is also a factor for tumour promotion/progression. In a model for studying hepatocarcinogenesis, TCDD stimulates the clonal expansion of pre-neoplastic hepatocytes by inhibiting apoptosis (
Environmental molecules that affect cellular plasticity. Deregulation of cell–cell contact and tumour malignancy is associated with increased AhR expression. For instance,
Environmental molecules that are able to induce DNA damage leading to genetic mutations. It is well known that the environmental molecule B[a]P induces, via AhR, the expression of CYP1A1, which is involved in the biotransformation of B[a]P, a procarcinogen, into B[a]P-diol-epoxide (BPDE), an ultimate mutagen with a strong electrophilic power that allows it to form DNA adducts that cause cytogenetic alterations, DNA breaks, DNA damage and mutations in oncogenes and tumour suppressor genes (
AhR activity maintains cancer stem cells (CSC) capacity. AhR has also been reported to affect CSC, a subtype of cancerous cells and to lead to the initiation, progression and development of metastases in the carcinogenesis (
The possible AhR biomarker value in cancer. Few studies investigated the prognosis value of AhR in cancer. In upper urinary tract tumours, the high levels of nuclear AhR expression predicted a higher tumour grade (
The molecular mechanisms affected by the activation of AhR by environmental pollutants, discussed above, are represented in Fig.
GPER1 is a seven transmembrane-domain G protein-coupled receptor that shares, with other GPCR, a similar global architecture which consists of a transmembrane canonical part formed of seven helices α with various sequences serving as a communication link between the ligands and the G protein coupling region; the extracellular part consists of three extracellular loops containing the N-terminus and the intracellular part consisting of three intracellular loops with the C-terminus (
GPER1 may promote carcinogenesis. The chemical structure of BPA that looks like E2 provides estrogenic properties to BPA (
GPER1 is implicated in pathways that lead to the activation of the transcriptional machinery. Studies have demonstrated the involvement of GPER1 in cell proliferation, cell survival and cell migration mechanisms by inducing the transcription of genes such as cyclin D2, Bcl-2, connective tissue growth factor (CTGF) and the oncogene c-fos etc. (
The possible GPER1 biomarker value in cancer. Several studies have highlighted the use of GPER1 as a cancer biomarker. The results of a clinical study showed that the expression of GPER-1 might correlate with clinical and pathological-poor outcome biomarkers, by showing an association with metastasis, human epidermal growth factor receptor 2 (HER2) expression and tumour size (
The molecular mechanisms affected by the activation of GPER1 by environmental pollutants, discussed above, are represented in Fig.
Several risk factors were identified playing important roles in carcinogenesis. Some major factors were attributed to the exposure to environmental molecules. In this review, we showed that exposure to environmental molecules can play a crucial role in the process of carcinogenesis. These molecules have the ability to interact with cellular receptors and act as either initiators of carcinogenesis by their genotoxic effect or agents promoting carcinogenesis via their estrogenic-like activities (xenoestrogens). Amongst cellular receptors, we highlighted two main receptors AhR and GPER1, where many studies demonstrated their implication in carcinogenesis. As discussed, studies reported that environmental pollutants exert estrogenic effects. The established nuclear receptor AhR, has long been identified as a receptor that mediates environmental pollutants effects. Acting as a transcription factor that responds to xenobiotics and play significant roles in the development and progression of cancer cells such as proliferation and differentiation, genetic damage, toxins metabolism, angiogenesis and survival, where its overexpression and constitutive activation have been observed in various tumour types. Some studies have also suggested that higher AhR activity could be correlated with increased aggressiveness and a poor prognosis. Previously, mechanistic studies focused on their actions mediated by the ER pathway and gave less importance to their effects mediated by the GPER1 pathway. However, GPER1 proved to be an emerging membrane receptor in mediating environmental pollutants impact. The currently available data suggest that GPER1 is a potential target for xenoestrogens in the human body. There is now good evidence that GPER1 may contribute separately to estrogen-induced carcinogenesis due to its ability to activate transcriptional machinery and employ different intracellular signalling mechanisms that promote cancer progression such as cell proliferation, migration, escape from apoptosis and cell cycle arrest. Moreover, several studies do suggest that GPER1 measurement alone may be a significant biomarker in cancer and therefore may hold a prognostic significance.
In this context, more studies are needed to fully establish the role of pollutants that we are chronically (daily) exposed to, in inducing carcinogenesis and to develop a better understanding of how cellular receptors cooperate with these molecules to drive the biology of cancer. In fact, this type of research encounters important barriers to progress; for instance, some chemicals are rapidly metabolised, many exposures are complex mixtures of chemicals that have varied mechanisms of action, thus, there is a great challenge to reconstruct environmental exposures to assess pollutants effects. Furthermore, research needs to include continued support of cohorts with prospective exposure measurements from early life, so that further follow-ups would be informative. Finally, epidemiological studies highlight the need for better chemical testing and risk assessment approaches that are relevant to cancer that could be essential for cancer prediction and prevention. Clearly, the current scientific challenge is to identify new molecular biomarkers for environmental exposure that could be used to develop candidate prevention strategies for the environmental carcinogenesis induced by molecules with different mechanisms of action.
MEH was funded by a PhD grant from the Lebanese University.
SEG planned the different sections, edited and corrected the review. MEH wrote and edited the review. PC and MDA corrected the review.
No conflict of interest to declare.