Because the ubiquitin system is indispensable for immune

Because the ubiquitin system is indispensable for immune responses, it has become an important target for pathogens to escape the host immune defenses (Kim et al., 2005; Li et al., 2016). For example, effector OspG of Shigella flexneri could bind many ubiquitinated E2 proteins, such as UbcH5 and Ubch7, to prevent the ubiquitination and inhibit NF-κB signaling pathway (Kim et al., 2005; Pruneda et al., 2014). OspI in S. flexneri could inactivate Ubc13 to inhibit TRAF6 polyubiquitination (Sanada et al., 2012). In the present study, the ROS level was found to be significantly decreased after CgUbe2g1 RNAi. ROSs are produced during normal metabolism of aerobic organisms (Aguirre et al., 2005), and they are highly induced when the organism is attacked by invaders or exposing to contaminant (Aguirre et al., 2005; Winterbourn, 2008). Oysters have evolved antioxidant defense system to rapidly and efficiently remove ROS to protect Cinobufagin from damaging effect (Wang et al., 2018). The lower ROS level might affect the generation of antioxidant enzymes, which could be utilized by pathogens to escape the immune system (Severo et al., 2013; Li et al., 2016; Wang et al., 2018). This result demonstrated that E2s (such as CgUbe2g1) and the ubiquitin system were critical for immune responses and pathogens elimination in oysters. In conclusion, an ubiquitin-conjugating enzyme E2, CgUbe2g1, was identified from Pacific oyster C. gigas. It was mainly expressed in hemocyte, mantle, hepatopancreas, and male gonad. The rCgUbe2g1 protein was found to induce the ubiquitination in vitro via binding E1-activated and un-activated Ub. CgUbe2g1 could play vital roles in innate immune system of C. gigas, including activation of inflammatory response and regulation of cellular immune response after pathogen stimulation.
Acknowledgments This research was supported by a grant (No. U1706204) from National Science Foundation of China, Aoshan Talent Cultivation Program (No. 2017ASTCP-OS13) supported by Qingdao National Laboratory for Marine Science and Technology, Dalian High Level Talent Innovation Support Program (2015R020), Earmarked Fund (CARS-49) from Modern Agro-industry Technology Research System, Outstanding Talents and Innovative Teams of Agricultural Scientific Research in the Ministry of Agriculture, and the Research Foundation for Distinguished Professor in Liaoning (to L. S.) and Talented Scholars in Dalian Ocean University (to L. W.).
Introduction O6-Methylguanine-DNA methyltransferase (MGMT) is a DNA-repair enzyme that specifically transfers alkyl adducts from the O6 position of guanine to the cysteine residue (Cys145) in its active site. In cancer cells, this ability of MGMT disrupts the cytotoxic actions of alkylating anticancer agents. Hence, MGMT is expressed at high levels in patients with various cancers [1] and is associated with resistance to -alkylating chemotherapeutic drugs [2]. Inhibition of MGMT through promoter methylation [3], [4] or shRNA [5] leads to increased drug sensitivity in multiple human cancers. However, several lines of evidence have revealed that treatment with the alkylating agent 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) leads to decreased MGMT proteins levels in various cancer types [6], [7], suggesting that BCNU mediates MGMT turnover. However, the underlying mechanism is unclear.