Sp1 and c-Myc regulate transcription of BMI1 in nasopharyngeal carcinoma

B-lymphoma mouse Moloney leukemia virus insertion region1 (Bmi1), a member of the polycomb group, has elevated expression and is involved in the pathogenesis of various aggressive cancers, including nasopharyngeal carcinoma (NPC). To date, the mechanisms underlying the high expression of Bmi1 in NPC remain obscure. To gain new insights into the transcriptional regulation of BMI1, we cloned and characterized the promoter region of BMI1. Luciferase reporter assays demonstrated that the region from -783 to +375 showed significant promoter activity. With the use of a series of 5-deletion and 3-deletion promoter constructs in luciferase reporter assays, the +167/+232 and -536/-134 regions were found to be sufficient for full promoter activity. Transcriptional activity of the BMI1 promoter was dependent on the Sp1 binding site cluster (+181/+214) as well as the E-box elements (-181), and was abolished after mutation of the two cis-elements. Electrophoretic mobility shift assays and chromatin immunoprecipitation assays demonstrated that Sp1 bound to the region from +181 to +214 within the BMI1 promoter. In addition, gain-of-function and loss-of-function analyses revealed that Sp1 augmented Bmi1 expression. Further investigations using immunohistochemistry and quantitative RT-PCR disclosed a significant positive correlation between the expression of Sp1 and Bmi1 in normal nasopharyngeal epithelial cells, NPC cells, and NPC tissue specimens. In addition, Myc, the known transcription factor for BMI1 in neuroblastomas, also activated the transcription of BMI1 through binding to the E-box element (-181) within its promoter, and showed a positive correlation with the mRNA level of BMI1 in NPC. In conclusion, these findings provide valuable mechanistic insights into the role of Sp1 and c-Myc in BMI1 transcription in NPC, and suggest that targeting of Sp1 or c-Myc may be a potential therapeutic strategy for NPC.