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Normal human oral keratinocytes (NHOK) undergo a limited number of replication in vitro and enter replicative senescence without spontaneous immortalization. Escape from senescence is therefore one of the earliest steps in cellular immortalization and tumorigenesis. The current study was undertaken to investigate the role of polycomb group (PcG) oncogene, Bmi-1, in control of replicative life span of NHOK. Secondary NHOK culture was infected with retroviral vector expressing full-length Bmi-1 or empty vector and was maintained in serial subcultures after selection with puromycin. Parental NHOK and those infected with the empty vector senesced after 16 population doublings (PDs), while the Bmi-1 expressing NHOK continued replication beyond 44 PDs, after which time the cells demonstrated features resembling senescent NHOK. Parental NHOK showed enhanced expression of p16INK4A during senescence, and p16INK4A expression remained at the elevated level in exponentially replicating NHOK expressing Bmi-1. During the Bmi-1-induced extended life span, telomerase activity was not detected, and the cells replicated with notable telomere shortening. Also, super infection of Bmi-1 expressing NHOK with a retroviral vector expressing E6 but not E7 oncoprotein of human papillomavirus (HPV) type 16 further enhanced their replicative life span beyond that induced by Bmi-1 alone. These results indicate that Bmi-1 extends the life span of NHOK independently of the p16INK4A level but cannot immortalize them. This study was supported in part by the grants DE 15316 and DE 14147 from NIDCR/NIH.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]