br Acknowledgment This work was supported by the National
Acknowledgment This work was supported by the National Science & Technology Major Special Project on Major New Drug Innovation, China [2018ZX09711003-006].
Introduction Polycyclic aromatic hydrocarbons (PAHs) are widely present in various aquatic ecosystems, particularly in sediments (Li et al., 2006, Guo et al., 2007, Oliva et al., 2010). They are derived from various sources, such as WAY 208466 dihydrochloride synthesis fuels, burning organic matter, and oil spill accidents (Banni et al., 2010). Due to persistence and bioaccumulation, they pose a threat against aquatic organisms. Benzo[a]pyrene (B[a]P) is a high molecular weight (5-ring) PAHs, and is a known carcinogen and/or mutagen (Shaw and Connell, 1994). In aquatic environments, B[a]P concentration is reported to be in the range of pg/L–ng/L in clean surface waters, and up to μg/L in polluted rivers (Maciel and Zaldivar, 2005). Several reports demonstrated that B[a]P has a negative impact on survival (Collier and Varanasi, 1991, Hawkins et al., 1991, Palalnikumar et al., 2012), growth (Jifa et al., 2006, Kim et al., 2008), behavior (Lawrence and Poulter, 2001, Oliveira et al., 2012), and reproduction (Monteverdi and DiGiulio, 2000, Choy et al., 2007) in aquatic organisms. B[a]P can easily diffuse within the cell through the cellular membrane, bind to aryl hydrocarbon receptor (AhR), and subsequently induce the transcription of many genes, including cytochrome P450 (CYP450) (Safe, 2001). B[a]P is metabolized by CYP450, a phase I enzyme, into a reactive metabolite, B[a]P-diolepoxide which forms a DNA adduct and leads to mutation (Wahidulla and Rajamanickam, 2009). B[a]P-diolepoxide is transformed into less harmful substances by phase II enzymes, such as glutathione S-transferase (GST) (Trushin et al., 2012). CYP450s are heme-containing monooxygenases constituting a conserved gene superfamily of heme-thiolate proteins, and are found in prokaryotes and eukaryotes (Martignoni et al., 2006). They play a key role in detoxification of endogenous (e.g., steroids) and exogenous compounds (e.g., drug, insecticides, PAHs, and polychlorinated biphenyls (PCBs)) (Werck-Reichhart and Feyereisen, 2000), and are involved in steroidogenesis. Among them, CYP 1–4 families are known to be mainly involved in detoxification of xenobiotics, and are transcriptionally activated by xenobiotic-AhR complex (Waxman, 1999). In particular, the CYP1 family (CYP1A, CYP1B, CYP1C, and CYP1D) has been extensively studied and considered as a reliable biomarker in aquatic vertebrates to evaluate metabolisms and/or biological effects of various environmental pollutants, including PAHs (reviewed by Guengerich, 2008, Iwamoto et al., 2012). However, little information on drug metabolism pathways in non-vertebrate is available (Solé and Livingstone, 2005, Rewitz et al., 2006). Until now, several CYP genes are identified in human (57 CYPs), mouse (108 CYPs), nematode (75 CYPs, Caenorhabiditis elegans), sea urchin (120 CYPs, Strongylocentrotus purpuratus), water flea (83 CYPs, Daphnia magna), and yeast (2–3 CYPs) (Nelson, 2013). In the phylum Ciliophora which is one of the major evolutionary lineage that constructs the Alveolates (Parfrey et al., 2006), CYPs from only freshwater ciliate Tetrahymena thermophila (40 CYPs) are identified (Fu et al., 2009), and one of these CYP genes (CYP5013C2) was characterized in response to dichlorodiphenyltrichloroethane (DDT) (Feng et al., 2014). Thus, identification and expression of CYP in response to xenobiotics are further required in ciliates to determine the function of CYPs in ciliates and to better understand evolution of CYPs in eukaryotes.