Several eukaryotic E enzymes including
Several eukaryotic E3 enzymes, including BRE1 , RNF8 , and RAD18 , , have been reported to partner with UBE2B in the ubiquitination of various targets. We examined the expression levels of these E3 enzymes in HONE1 and TW01 cells; RAD18 was highly expressed in these ddhC (data not shown). Therefore, we selected RAD18 as a candidate E3 for further study. Previous studies have shown that UBE2B interacts with RAD18 to form the UBE2B-RAD18 complex  for the ubiquitination of the DNA repair protein PCNA , , . In this study, we demonstrated for the first time that the ubiquitination of MGMT in NPC cells also requires UBE2B and RAD18. In addition, similar results of Co-IP and cell viability assay were found in human colorectal adenocarcinoma HT-29 cells (data not shown), indicating that the regulation of MGMT by UBE2B might be a general phenomenon in various cancer types. Our results corroborated that ubiquitination is not only critical for protein degradation, but is also necessary to avoid the accumulation of dysfunctional protein that may be harmful to cells. Although this study demonstrated that UBE2B and RAD18 are involved in MGMT ubiquitination, the detailed interaction mechanism of UBE2B, RAD18, and MGMT requires further investigation. Furthermore, to complete the analysis of the mechanism of MGMT protein degradation, we also explored whether BCNU induces lysosomal degradation of MGMT by adding a lysosomal inhibitor, chloroquine (data not shown). Our findings showed that chloroquine was not able to rescue BCNU-mediated MGMT degradation, indicating that lysosomal-mediated proteolysis is not involved in BCNU-mediated MGMT degradation.
We further compared the UBE2B expression levels between normal nasopharynx tissues and clinical nasopharyngeal carcinoma specimens from public databases Oncomine (www.oncomine.org)  The results demonstrated that UBE2B is overexpressed in nasopharyngeal tumor tissues compared to normal tissues (GEO dataset, https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE12452) and correlates with poor clinical outcome in head and neck cancers (TCGA dataset, https://www.proteinatlas.org/ENSG00000119048-UBE2B/pathology/tissue/head+and+neck+cancer) , suggesting that UBE2B may be an important determinant of tumorigenesis. To analyze whether UBE2B generally regulates MGMT in alkylating-agent therapies, another alkylating agent, cisplatin was used. Methylene blue cytotoxicity assays showed that UBE2B-depleted HONE-1 cells exhibited increased cisplatin sensitivity, while overexpression of MGMT restored cisplatin resistance in UBE2B knockdown HONE-1 cells (data not shown). Furthermore, analysis of data from the NCBI Gene Expression Omnibus (GEO) showed that the UBE2B level is negatively associated with the overall survival of lung cancer patients receiving cisplatin treatment (data not shown). Taken together, these findings indicate that UBE2B is a potential marker of poor prognosis in cancer patients receiving -alkylating chemotherapeutic drugs.
In summary, our study provides a novel insight into the regulation of MGMT in cancer cells. We demonstrated for the first time that UBE2B and its partner RAD18 are involved in regulating the ubiquitination and activity of MGMT. The regulation of MGMT by UBE2B can affect the sensitivity of cancer cells to alkylating agents. Therefore, our results support the development of a potential co-treatment strategy for improving the anti-cancer effect of -alkylating chemotherapeutic drugs.
Introduction Vaccination through inoculation is an efficient route for defending against pathogenic infection and the spread of infectious disease. Whereas traditional vaccines use whole-attenuated or killed pathogens, subunit vaccines differ in that they contain only part of the microbial components (Moyle, 2015; Moyle and Toth, 2013). Although subunit vaccines offer improved safety, they are generally less immunogenic compared with whole pathogens. How to enhance the immunogenicity of antigens from viruses or pathogenic bacteria is an important consideration in subunit vaccine design. In general, a subunit antigen candidate with excellent immune properties is selected and incorporated along with a potent immunostimulant(s), which improves its recognition, processing and delivery to the immune system to elicit an effective immune response (Bachmann and Jennings, 2010; Chauhan et al., 2017; Rehm, 2017).