Cyclin D1 overexpression and p53 inactivation immortalize primary oral keratinocytes by a telomerase-independent mechanism

The immortalization of human cells is a critical step in multistep carcinog enesis. Oral-esophageal carcinomas, a model system to investigate molecular mechanisms underlying squamous carcinogenesis, frequently involve cyclin D 1 overexpression and inactivation of the p53 tumor suppressor. Therefore, o ur goal was to establish the functional role of cyclin D I overexpression a nd p53 inactivation in the immortalization of primary human oral squamous e pithelial cells (keratinocytes) as an important step toward malignant trans formation. Cyclin D I overexpression alone was found to induce extension of the replicative life span of normal oral keratinocytes, whereas the combin ation of cyclin DI overexpression and p53 inactivation led to their immorta lization. This study also demonstrates that immortalization of oral keratin ocytes can be independent of telomerase activation, involving an alternativ e pathway of telomere maintenance (ALT).