INDUCTION OF P16 DURING IMMORTALIZATION BY HPV-16 AND HPV-18 AND NOT DURING MALIGNANT TRANSFORMATION

The p16 (MTS1) tumour-suppressor gene is a cyclin-dependent kinase (cd k) inhibitor that decelerates the cell cycle by inactivating the cdks that phosphorylate the retinoblastoma tumour-suppressor gene (Rb) prot ein (pRb). In cervical cancers, pRb is inactivated by the HPV E7 oncop rotein or by mutations. The hypothesis of earlier reports was that the disruption of the p16/cdk-cyclin/Rb cascade is essential for malignan t cervical transformation/carcinogenesis. We previously established in vitro model systems of cervical cancer representing four steps of onc ogenic progression initiated by the two most common oncogenic HPVs in ectocervical and endocervical epithelial cells. This report used these systems to investigate the role of p16 in cervical cancers. A dramati c enhancement of the p16 RNA level was observed after immortalization by HPV 16 or 18. Furthermore, the p16 protein was newly observed follo wing immortalization. However, no further changes were found for RNA o r protein levels after serum selection or malignant transformation. Fo r three cervical carcinoma cell lines, similar high levels of p16 expr ession were seen. Point mutations or homozygous deletions of p16 were not observed in the in vitro systems or in clinical specimens. These r esults suggest that the inactivation of the p16/cdk-cyclin/Rb cascade does not occur during malignant transformation but occurs during the i mmortalization by HPV in HPV-harbouring premalignant lesions, the in s itu equivalent of immortalized cells. Also suggested is that p16 has n o role in the specific malignant transformation step from immortal pre malignant lesions during the carcinogenesis of HPV-initiated cervical cancers.