Increased E6/E7 transcription in HPV 18-immortalized human keratinocytes results from inactivation of E2 and...

We characterized the state of the viral genome and transcription of human papillomavirus 18 oncogenes, E6 and E7, in immortalized human keratinocytes. At passage 9 after transfection with HPV 18 a homogeneous population of immortal clones was present. These cells have the viral DNA integrated within the E2 orf, accompanied by its partial deletion, similarly to what has been found in cervical carcinoma specimens. Transcription of the E6 and E7 oncogenes is mediated by the major viral early promoter (P105). Interestingly, transcriptional activity from this promoter increased upon continued in vitro passage of the cells. This event is concomitant with an increase in the proliferation rate of the cells. Reintroduction of the HPV 18 E2 gene into these cells resulted in repression of P105. However, the amount of E2 was limited in the HPV 18-immortalized cells. These data suggest that both viral and cellular factors play a role in increasing levels of E6 and E7 transcription providing the host cell with a proliferation advantage necessary for tumor growth.