Inorganic arsenic modulates the expression of selenoproteins in mouse embryonic stem cell
Introduction
Inorganic arsenic (iAs) is a well-known human carcinogen, which is associated with an increased risk for tumors of the skin, bladder, liver, and other tissues (NRC, 2001, IARC, 2004, Chen et al., 2003). Although the molecular mechanism of iAs carcinogenesis remains poorly understood, it has been attributed to the genotoxicity associated with reactive oxygen species (ROS) (Hei et al., 1998). Selenium (Se) is an essential micronutrient with antitumorigenic properties (Clark et al., 1996). The best studied function for selenium is its incorporation into selenoproteins that play important roles in defense against ROS and detoxification (Driscoll and Copeland, 2003). Selenium can prevent the arsenic's toxicity and carcinogenesis through its antioxidative activity (Biswas et al., 1999, Rossman and Uddin, 2005, Uddin et al., 2005). Thus, the modulation of the expression of selenoproteins by iAs exposure may highlight the molecular mechanism for the arsenic toxicity.
Previous studies of arsenic–selenium interactions focused mainly on the metabolism of arsenic and biological activities of selenium. It has been confirmed that selenium could modulate the metabolism of arsenic (Gregus et al., 1998, Miyazaki et al., 2005). The “mutual sparing effect” between arsenic and selenium has been revealed using the cultured cell models (Styblo and Thomas, 2001, Walton et al., 2003). Formation of seleno-bis (S-glutathionyl) arsinium ion [(GS)2AsSe]− in vivo indicated a molecular basis to the direct interaction between the two metalloids (Gailer et al., 2000). The dietary selenium supplement decreases whereas selenium deficiency increases the risk of skin lesions in arsenic-exposed population (Yang et al., 2002, Chen et al., 2007, Huang et al., 2008). Selenium supply could restore some oxidative stress-related genes induced by arsenic exposure (Kibriya et al., 2007). However, information on the modulation of the expression of selenoproteins by acute or chronic iAs exposure is often lacking.
At least 25 selenoproteins in humans and 24 members in rodents have been identified (Kryukov et al., 2003). Selenoenzymes contain glutathione peroxidases (Gpx1–4), thioredoxin reductases (Tr1–3), iodothyronine deiodinases (Dio1–3) and methionine sulfoxide reductase (SelR). The endoplasmic reticulum (ER) associated selenoproteins include the 15 kDa selenoprotein (15-Sep), selenoprotein K (SelK), selenoprotein M (SelM), selenoprotein N (SelN), selenoprotein S (SelS), and selenoprotein T (SelT). Other selenoproteins such as selenoprotein H (SelH), selenoprotein I (SelI), selenoprotein O (SelO), selenoprotein P (SelP), selenoprotein V (SelV), and selenoprotein W (SelW) share little sequence homology but perform very broad functions (Gromer et al., 2005). Selenoproteins Tr1 and Gpx2, contain the antioxidant response element (ARE) in their promoter regions (Banning et al., 2005, Sakurai et al., 2005). Arsenic can induce oxidative stress and increase the expression of the transcription factor NF-E2-related factor 2 (Nrf2) (Pi et al., 2003). It has been reported that arsenite caused an increase in Tr1 levels in keratinocytes likely due to activation of ARE by Nrf2 (Rea et al., 2003, Ganyc et al., 2007). It requires clarifying the effects of iAs exposure on the expression of all known selenoproteins to further elucidate the interactions between arsenic and selenium.
In the present study, we investigated how iAs exposure modulated the expression of all known selenoproteins in the embryonic stem (ES) cell line, CGR8. We also explored the selenium supplement on those selenoproteins in response to iAs exposure. Our results provide an insight into the mechanisms for the molecular interactions between arsenic and selenium.
Section snippets
Materials
Sodium selenite (SeIV), sodium arsenite (iAsIII) and sodium arsenate (iAsV), tetrazolium dye [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT)], 2′,7′-dichlrofluorescin diacetate (DCF-DA), tertiary butyl hydroperoxide, 5,5′-dithio-bis nitrobenzoic acid, NADPH, glutathione, and glutathione reductase were purchased from Sigma (Sigma Chemical Co., St. Louis, USA). All other reagents were of analytical grade unless mentioned.
Cell culture
The mouse ES CGR8 cell line was used in the present
Proliferation of CGR8 ES cells
The iAs species inhibited the cell proliferation in a dose-dependent manner (Fig. 1). A significant decrease of the cell proliferation was found in treatments of iAsIII (≥0.5 μM) or iAsV (≥2.0 μM) (P < 0.01). There were no significant changes of the cell proliferation in treatments of 0.25 μM iAsIII and 1.0 μM iAsV.
Treatments with SeIV (0.5 μM) caused a significant increase of the cell proliferation (P < 0.05). However, treatments with SeIV at 3.0 μM and higher led to a significant decrease of the cell
Discussion
Gene expression profiling in responses to arsenic has been investigated in keratinocytes (Hamadeh et al., 2002), fibroblasts (Burnichon et al., 2003), urothelial cells (Su et al., 2006), lung epithelial L2 cells (Posey et al., 2008) and in peripheral lymphocytes of arsenic-exposed population (Argos et al., 2006, Andrew et al., 2008). Hundreds genes were identified to be differentially induced by arsenic exposure. These genes have functions directly or indirectly related to metabolism, oxidative
Conflict of interest
The authors declare that there are no conflicts of interest.
Acknowledgments
We are grateful to Prof. Pei Duanqing for providing the mouse ES cell line, CGR8. Also, we thank research assistants Huang Luyuan and Zhang Jin for their help in developing reagents and protocols and Yao Dong for his assistance with cell culture. This work was supported by the National Natural Science Foundation of China (20535020 and 20675045).
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