Abstract A20: Investigating the role of cell cycle proteins and CDK4/6 inhibitor PD0332991 in the redifferentiation of irreversible dysplasia

Background : An important strategy in cancer prevention treatment programs is to target the reversal of premalignant disease through re-differentiation. Furthermore, understanding early changes occurring during preneoplastic dysplasia may provide diagnostic and therapeutic insight for cancer prevention programs. Aberrant regulation of the CDK4/6-CyclinD-Rb signaling pathway is one of the most common aberrations found in human cancer; however, there are few published reports describing the status of these cell cycle proteins in preneoplasia. Identifying biomarkers of reversal of dysplasia as well as mechanisms of malignant transformation from preneoplastic disease to cancer will be essential in drug development for chemoprevention strategies. Preclinical mouse models of reversible and refractory dysplasia help elucidate mechanisms responsible for irreversible disease progression and highlight potential targets for its reversal and prevention. This study examined and compared the potential roles of altered expression and/or activation of cell cycle regulatory proteins. Additionally, we examined the effect of a CDK4/6 inhibitor PD0332991 in a system of early dysplasia. Specific Aims : 1. Evaluate expression patterns of cell cycle regulatory network proteins in reversible and irreversible stages of epithelial dysplasia. 2. Test if inhibition of CDK4/6 through PD0332991 can initiate re-differentiation of refractory epithelial dysplasia. 3. Evaluate cell cycle protein expression changes after inhibition of CDK4/6. Methods : A conditional transgenic mouse model, MMTV-tTA/tet-op-TAg (tTA/TAg) demonstrating time dependent reversible and refractory irreversible stages of salivary epithelial dysplasia following down regulation of the initiating oncogene was used. Previous studies demonstrated refractory dysplasia is a field effect without chromosomal rearrangements. Cell cycle regulatory protein expression patterns were compared by western blot and IHC analyses at the 4 (reversible) and 7 (irreversible) month stages. To test the chemopreventive role of CDK4/6 inhibition, PD0332991 inhibitor was administered to at the 7-month (irreversible) stage of dysplasia. Doxycycline was administered with the maximum recommended dose of PD0339221 (150 mg/kg) for 10 days via gavage. Results and Conclusions : Cell cycle proteins were differentially expressed at the reversible 4-month vs. irreversible 7-month time-point. The pattern of expression suggests that irreversible refractory dysplasia is maintained independently from the initiating oncogene due to persistent up-regulation of Cyclin D1 and CDK4/6 with consequent Rb phosphorylation and increased levels of DP-1 and E2F in a self-renewing cyclical manner. Treatment with PD0332991 at the 7 month (irreversible) stage resulted in a significant histological reversal in preliminary studies. The data indicate that cell cycle proteins play an important role in maintenance of dysplasia and are valid targets for cancer chemoprevention. Citation Information: Cancer Prev Res 2011;4(10 Suppl):A20.