Role of the novel tumor suppressor p33ING2 in cellular stress response to ultraviolet irradiation in melanoma cells

P33ING2 is a candidate tumor suppressor, which shares 58.9% homology with p33ING1b, the founding member of the inhibitor of growth (ING) family, and has been shown to be involved in the regulation of gene transcription, cell cycle arrest and apoptosis in a p53-dependent manner for maintaining the genomic stability. Previously, we demonstrated that p33ING2 promoted UV-induced apoptosis via activation of both the mitochondria/intrinsic and death-receptor/extrinsic apoptotic pathways upon high UV irradiation. Recently, studies have shown that p33ING2 may complex with acetyltransferase (FAT) or histone deacetylase (HDAC). To further reveal the role of p33ING2 in the cellular stress response to UV irradiation, we hypothesized that p33ING2 may enhance the repair of UV-damaged DNA, similar to its homologue p33INGlb, through facilitating histone acetylation and chromatin modification, which are believed to be essential for gene transcription regulation. Using luciferase reporter assays, overexpression of p33ING2 is shown to significantly enhance nucleotide excision repair of UV-induced DNA damage in melanoma cells in a p53-dependent manner. Furthermore, DNA repair is completely abolished in cells treated with p33ING2 siRNA, suggesting that a physiological level of p33ING2 is required for nucleotide excision repair. In addition, p33ING2 is found to be an essential factor for UV-induced rapid histone H4 acetylation, chromatin relaxation, and the recruitment of damage recognition protein XPA to the photolesions. These observations suggest that p33ING2 is required for initial DNA damage sensing and chromatin remodeling in the nucleotide excision repair process. As a member of the ING family, p33ING2 contains a number of highly conserved regions with other ING members, including the plant homeodomain (PHD), leucine zipper-like domain (LZL), and potential chromatin regulatory domain (PCR). Previous studies focused on PHD region of ING proteins since it complexes with histone acetyltransferase (HAT) in its yeast homologue. However, the specific biological functions of other motifs are poorly understood. In this study, using a panel of p33ING2 deletion mutants, we performed functional analyses of different domains of p33ING2. We found that the LZL domain is critical for a functional p33ING2 in both UV-induced DNA repair and apoptosis. In summary, the evidence presented in this thesis that p33ING2 plays an essential role in DNA repair, apoptosis and chromatin remodeling strongly supports the notion that p33ING2 acts as a tumor suppressor.Medicine, Faculty ofMedicine, Department ofExperimental Medicine, Division ofGraduat