Genetic materials in cells are under constant bombardment by various DNA damaging agents such as ultraviolet radiation which generates bulky DNA adducts like 6-4 photoproducts and cyclobutane pyrimidine dimers. Failure to repair DNA damage produces mutation which is important for carcinogenesis of cancers. The highly compact chromatin structure is always an obstacle for DNA repair especially for global genomic repair (GGR). We have shown that tumor suppressor ING1b enhanced NER. ING1b also induced chromatin relaxation, histone acetylation and the recruitment of repair factor XPA to chromatin in melanoma MMRU cells. To further elucidate the mechanism by which ING1b regulates NER, we investigated the accessibility of different factors to chromatin after ING1b overexpression. We found that ING1b increased triton-insoluble proliferating cell nuclear antigen (PCNA) in MMRU cells which represents the chromatin bound form of the protein. PCNA is an essential factor in NER and has previously been shown to interact with ING1b as ING1b contains a putative PIP domain. UV irradiation increased triton-insoluble PCNA. ING1b knockdown by siRNA decreased triton-insoluble PCNA which was restored by histone deacetylase inhibitor treatment. This suggests that ING1b is essential for PCNA recruitment to chromatin after UV irradiation through chromatin relaxation as histone hyperacetylation and chromatin relaxation induced by histone deacetylatase inhibitor treatment bypassed the requirement of ING1b for PCNA to access chromatin after UV irradiation. Furthermore, p300 overexpression increased PCNA binding to chromatin which was abrogated by ING1b siRNA treatment, implicating that ING1b may cooperate with p300 as chromatin accessibility factor for NER. As p53 has also been reported to be a chromatin accessibility factor for NER, we investigated if p53 requires ING1b in this function. Histone H4 acetylation increased after UV irradiation in HCT116 p53+/+ but not in HCT116 p53-/- cells. ING1b siRNA decreased histone H4 acetylation in both p53+/+ and p53-/- cells Thus, it seems that p53 requires ING1b to induce chromatin relaxation through histone H4 acetylation after UV irradiation. The present result suggests that ING1b may enhance nucleotide excision repair through enhancing chromatin accessibility of PCNA to chromatin by chromatin remodeling in a p53 dependent manner. This sheds light on the role of tumor suppressor ING1b on nucleotide excision repair which is an essential component to protect cells from mutagenesis.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA