A Novel Role for the p16INK4A Tumor Supressor in Regulating Cellular Oxidative Stress

Mutations, deletions, and epigenetic silencing of the cyclin-dependent kinase inhibitor p16INK4A are associated with several cancer types, but are more commonly associated with familial melanoma predisposition and melanoma tumors. p16INK4A functions as a tumor suppressor by negatively regulating the cell cycle, however several outstanding questions remain. It remains unclear why compromise of p16INK4A predisposes to melanoma over other cancers, and why several melanoma-associated p16INK4A mutations do not compromise CDK4-binding. This study describes a novel function of p16INK4A in regulating intracellular oxidative stress independently of its role in cell cycle inhibition, and analyzes these functions in several familial melanomaassociated p16INK4A point mutants. I also demonstrate that, due in part to the prooxidizing nature of melanogenesis, melanocytes have higher constitutive levels of intracellular reactive oxygen species (ROS) than other cell types, suggesting why genetic compromise of p16INK4A preferentially predisposes to melanoma. This dissertation demonstrates that p16INK4A was rapidly upregulated following ultraviolet-irradiation and H2O2-induced oxidative stress (Chapter 2). Depletion of p16INK4A increased ROS and oxidative DNA damage in several cell types, which was exacerbated by H2O2. Aberrant ROS levels in Cdkn2a-deficient fibroblasts were elevated relative to controls and normalized by expression of exogenous p16INK4A. Finally, p16INK4A-mediated suppression of ROS could not be attributed to the potential effects of iv p16 on cell cycle phase. I then constructed 12 different familial melanoma-associated point mutants and analyzed their capacity to restore normal cell-cycle phase and ROS levels in p16INK4A-deficient fibroblasts (Chapter 3). Whereas wild-type p16INK4A fully restored both functions, various p16INK4A mutants showed different abilities to normalize ROS and cell cycle profiles. Different mutations were found to affect both, neither, or only one of the functions of p16INK4A, indicating that these two regulatory functions can be uncoupled. Structural analysis indicated that these distinct functions may be mediated by distinct regions of the protein. Lastly, in normal melanocytes, inhibition of melanin was sufficient to decrease levels of intracellular ROS to levels constitutively observed in fibroblasts (Chapter 4), indicating that the unique process of melanin production may be responsible for high basal levels of ROS and preferential susceptibility to oncogenic transformation brought on by genetic compromise of p16INK4A.