An AP-1 binding site mutation in HPV-16 LCR enhances E6/E7 promoter activity in human oral epithelial cells

Expression of the human papillomavirus (HPV) E6 and E7 oncoproteins is responsible for the transforming ability of the virus. The HPV long control region (LCR) and E6/E7 promoter regulate transcription of the E6 and E7 viral oncogenes. However, factors involved in the stimulation of E6/E7 promoter activity in carcinogenesis are unclear. We previously identified a point mutation in an HPV-16 immortalized human oral keratinocyte cell line subsequently exposed to a tobacco-specific carcinogen. This mutation was located in the LCR at nucleotide 7633 and contains binding sites for the transcription activator, AP-1, overlapping with putative binding regions for the transcription factor, C/EBP, which represses the E6/E7 promoter. In this study, this mutation was analyzed by both electrophoretic mobility shift analysis and luciferase assays. We found that the point mutation enhanced the binding affinity of AP-1 to the LCR, thus stimulating the E6/E7 promoter activity. Our results suggest that mutations in binding sites for crucial regulators may be the result of exposure to carcinogens and could induce expression of the E6 and E7 oncogenes.