Epigenetic Mechanism of the Synergistic Tumorigenic Effect of Arsenic and Benzo[a]pyrene Co-Exposure

  • Wang, Zhishan (PI)
  • Lee, Eun (CoI)
  • Yang, Chengfeng (CoI)

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Description

Arsenic and benzo[a]pyrene (BaP) are among the most common environmental pollutants that humans are exposed to. Contaminated drinking water is the main source of general population arsenic exposure. Cigarette smoke and cooked well-done meat on the barbecue contain high levels of BaP and are common sources of human BaP exposure. Therefore, human exposure to arsenic and BaP mixture is very common. Both arsenic and BaP are well-recognized carcinogens that cause lung cancer in humans. However, our knowledge on the effect of arsenic and BaP co-exposure on lung cancer risk and the underlying mechanism is very limited. The long-term goal of this study is to determine the mechanism of the effect of arsenic and BaP co-exposure and develop mechanism-based strategies for reducing the risk of lung cancer from arsenic and BaP co-exposure. Epigenetics refers to heritable alterations in the pattern of gene expression that are not caused by changes in DNA sequence, but are mediated by DNA methylation, histone posttranslational modifications, and non-coding RNAs. It is now known that epigenetics is often deregulated in cancer and that epigenetics deregulation plays important roles in carcinogenesis. Cancer stem cells (CSCs) or CSC-like cells are considered as cancer initiating and maintaining cells. Deregulation of epigenetics plays crucial roles in producing CSCs or CSC-like cells. While the mechanism of arsenic carcinogenesis is not well defined, it is believed that the carcinogenicity of BaP mainly depends on DNA adduct formation by its key metabolite BPDE. It is suspected that arsenic may enhance BaP carcinogenicity by increasing BPDE-DNA adduct formation, however, the reported effects of arsenic on BPDE-DNA adduct levels are inconsistent and controversial, suggesting that other mechanisms such as epigenetic mechanism may play an important role in the combined effect of arsenic and BaP co-exposure. Our preliminary studies found: (i) Arsenic and BaP co-exposure has a synergistic effect in inducing malignant transformation of immortalized human bronchial epithelial cells, CSC-like property, and mouse lung tumorigenesis; (ii) Arsenic and BaP co-exposure has no synergistic effect on the level of BPDE-DNA adduct and nucleotide excision repair (NER). In contrast, arsenic and BaP co-exposure shows a synergistic effect in epigenetic deregulations as evidenced by the increased levels of a histone H3K9 methyltransferase (HMTase) SUV39H1 and histone H3 repressive methylation mark H3K9me2. Moreover, arsenic and BaP co-exposure also shows synergistic effects in drastically reducing the level of suppressor of cytokine signaling 3 (SOCS3) and increasing Akt and Erk1/2 activation; (iii) ShRNA knockdown SUV39H1 in arsenic and BaP co-exposure-transformed cells drastically reduces their H3K9me2 level, increases SOCS3 level, reduces Akt and Erk1/2 activation and soft agar colony formation and CSC-like property; (iv) Stably re-expressing SOCS3 in arsenic and BaP co-exposure-transformed cells drastically reduces Akt, Erk1/2 activation, soft agar colony formation and CSC-like property; (v) Treatment of arsenic and BaP co-exposure-transformed cells with a natural compound Withaferin A inhibited both Akt and Erk1/2 activation and drastically reduced soft agar clone and suspension sphere formation with no significant cytotoxic effect, phenocopying the combined effect of Akt and Erk1/2 pharmacological inhibitors; and (vi) Arsenic and BaP co-exposure significantly increases aryl hydrocarbon receptor (AhR) nuclear localization and siRNA knockdown AhR greatly reduces the level of SUV39H1 in arsenic and BaP co-exposure-transformed cells. Based on our novel preliminary data and literature reports, our central hypothesis is: (i) Arsenic and BaP co-exposure up-regulates HMTase SUV39H1 via synergistically activating AhR; (ii) Up-regulation of SUV39H1 leads to SOCS3 down-regulation; and (iii) Down-regulation of SOCS3 causes a stronger Akt and Erk1/2 activation, which enhances cell transformation and CSC-like property thus increasing lung tumorigenesis. This proposal not only provides novel mechanistic insights for understanding the mechanism of the effect of arsenic and BaP co-exposure, it also has translational potential for developing preventive strategies against this co-exposure-caused lung cancer. Specific Aim 1: Arsenic and BaP co-exposure significantly enhances cell transformation and tumorigenesis by up-regulating HMTase SUV39H1 via synergistically activating AhR. (i) Determine the effect of SUV39H1 knockout using CRISPR/Cas9 technology on arsenic and BaP co-exposure-induced cell transformation, CSC-like property and tumorigenesis. (ii) Determine the mechanism of SUV39H1 up-regulation by arsenic and BaP co-exposure focusing on the role of the synergistic activation of AhR. Specific Aim 2: SUV39H1 up-regulation by arsenic and BaP co-exposure enhances cell transformation and tumorigenesis by down-regulating SOCS3 leading to stronger activation of Akt and Erk1/2. (i) Determine the protective effect and mechanism of SOCS3 overexpression on arsenic and BaP co-exposure-induced cell transformation, CSC-like property, and tumorigenesis. (ii) Determine the mechanism of SOCS3 down-regulation by arsenic and BaP co-exposure focusing on the role of DNA and H3 repressive methylation. Specific Aim 3: Simultaneously inactivating both Akt and Erk1/2 by a natural compound Withaferin A impairs the synergistic effect of arsenic and BaP co-exposure in inducing cell transformation and lung tumorigenesis in mice. (i) Determine the protective effect and mechanism of Withaferin A on arsenic and BaP co-exposure-induced cell transformation and CSC-like property; (ii) Determine the effect of Withaferin A oral administration on arsenic and BaP co-exposure-induced lung tumorigenesis in A/J mice.
StatusFinished
Effective start/end date8/15/182/28/21

Funding

  • National Institute of Environmental Health Sciences: $469,082.00

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