22P Emerging role of histone acetyltransferase CBP in breast cancer cells undergoing DNA damage

DNA damage response and repair defects resulted from genetic and epigenetic alterations are frequently observed among breast cancer patients and eventually affect their response to chemo- and radiotherapies. Among the epigenetic alterations, the histone acetyltransferase CREB-binding protein (CBP) was found to be dysregulated in breast cancer cells. Despite of many reports that demonstrated the role of CBP in DNA damage repair, the involvement of CBP in the initial events of activating DNA damage response pathways and the potential of targeting CBP in breast cancer therapy remains poorly understood. Here we set out to explore the role of CBP in DNA damage response in breast cancer and normal cells. Cancer and normal breast cell lines were used to examine the expression, stability, and activity of CBP and its interaction with other proteins after DNA damage induction by chemotherapeutic agent or radiation. In addition, immunofluorescence, western blot, comet assay, colony formation assay and proximity ligation assay were performed after CBP inhibition or downregulation under DNA damage. Our results showed that CBP is stabilized and recruited at the sites of DNA double strand breaks. The depletion of CBP impaired the DNA repair capacity and subsequently increased the sensitivity of breast cancer cells to chemo- and radiotherapy, without influencing the behavior of normal cells. Mechanistically, CBP was found to form a stable complex with ATM, a central regulator of DNA damage response, after treatment with DNA damaging agent. Furthermore, CBP depletion or inhibition impaired the autophosphorylation of ATM in breast cancer cells, suggesting it’s pivotal role in ATM activation. The impact of CBP on ATM's kinase activity was further augmented by the observed reduction in the phosphorylation of downstream proteins including Chk2, Chk1 and p53 in CBP-depleted cells. Interestingly, CBP downregulation did not interfere with the activation of ATM in non-cancerous breast cells. Our data highlights the functional role of CBP in the response of breast cancer cells to DNA damage, particularly in ATM activation. Our results suggest that CBP could be a useful target to modulate the cellular response to DNA damaging agents in breast cancer.