The Identification of Cellular Genes Altered by Htert and Hpv-16 E6 in the Immortalization of Human Keratinocytes

Studies have shown that wild-type hTERT protein can functionally replace the HPV16E6 protein, which cooperates with the viral E7 protein in the immortalization of primary keratinocytes. Previously, we made the surprising finding that catalytically inactive hTERT (hTERT), elongation-defective hTERT (hTERT-HA), and telomere recruitment-defective (hTERT N+T) also cooperate with E7 in cell immortalization, indicating that hTERT has immortalizing activities independent of its telomere maintenance functions. Since reports show an hTERT role in gene activation, we performed microarray studies to discover that E6, hTERT and hTERT mutated proteins altered the expression of highly overlapping sets of cellular genes. Pursuing in-depth studies of these targets shared by E6 and hTERT, we focused on AIB1, a nuclear coactivator known to be elevated in some cancers, and BMI1, the core subunit of the Polycomb Group Repressor Complex (PRC) 1 which is known to play a role in iii immortalization and determining cell fate. We proved that AIB1 levels were increased in a number of cervical cancer cell lines. Additionally, both AIB1 and BMI1 are elevated in HPV-immortalized cell lines. We showed further that BMI1 can substitute for E6 or hTERT in cell immortalization. Finally, in vivo tissue studies revealed expression of AIB1 and BMI1 increase with the severity of cervical dysplasia, suggesting a potential role in cervical cancer. Whereas BMI1 appears to be a marker of progression differentiating between pre-neoplastic and neoplastic lesions, AIB1 appears to delineate invasion from earlier stages, significantly increasing in invasive carcinoma. Together, these data demonstrate that AIB1 may be a novel cellular target of the HPV E6 oncoprotein and that hTERT has extratelomeric activities in cell immortalization and that its induction of BMI1 is a potential mechanism for mediating this activity.