The mechanisms of reversal of refractory dysplasia in the salivary gland: Testing the role of p53, RXR and PPARgamma agonists, and PD0332991

Loss of normal growth control is a hallmark of cancer progression. Therefore, understanding the early mechanisms of normal growth regulation and the changes that occur during preneoplasia may provide insights of both diagnostic and therapeutic importance. Models of dysplasia that help elucidate the mechanisms responsible for disease progression are useful in highlighting potential targets for prevention. An important strategy in cancer prevention treatment programs is to target the reversal of premalignant disease through re-differentiation. The studies presented here focused on investigating the mechanisms of reversal of dysplasia in tet-op-TAgMMTV-tTA mice, a well-established model of re-differentiation of dysplasia and multi-step carcinogenesis. In this experimental model of viral-oncoprotein-induced epithelial cancer, progression proceeds through sequential steps of "reversible" (4-months-of-age) and "irreversible dysplasia" (7-months-of-age) when TAg is down-regulated. This study identified abnormal up-regulation of cell cycle related proteins Cyclin D1, CDK4, CDK6 and phosphorylated pRb as mechanisms responsible for dysplasia persisting after the initiating viral oncoprotein Simian Virus 40 T Antigen was downregulated. Significantly, p53 was not required for successful re-differentiation of dysplasia. Additionally, we investigated 3 chemopreventive agents (UAB30, rosiglitazone and PD0332991) and tested their effect on reversal of dysplasia at the 7-month "irreversible dysplasia" timepoint. We determined that ligand-induced activation of RXR and PPAR gamma agonists attenuated Cyclin D1 and CDK6 but not CDK4 or phosphorylated pRb upregulation and dysplasia was not reverted. In contrast, administration of PD0332991, an orally available CDK4/6 inhibitor, was able to prevent upregulation of Cyclin D1, CDK6 as well as CDK4 and phosphorylated pRb and dysplasia was reverted. Moreover, the study distinguished CDK4 and phosphorylated pRb as targets for chemoprevention. These results reveal a candidate mechanism for interrupting cancer development at the dysplastic stage and highlight parameters to follow in evaluating potential biomarkers of response.