Constitutively Photomorphogenic 1 Reduces the Sensitivity of Chronic Lymphocytic Leukemia Cells to Fludarabine Through Promotion of Ubiquitin-Mediated P53 Degradation

Background/Aims: Chronic Lymphocytic leukemia (CLL) is characterized by accumulation of cells in the G0/G1 phase of the cell cycle and resistance to apoptosis due to gene mutation or abnormal gene expression. In our previous study, constitutively photomorphogenic 1 (COP1) was shown to be upregulated in Binet C-phase CLL patients. Based on the negative regulation of COP1 in the repair of DNA damage, we further studied the function of COP1 in CLL cell apoptosis induced by fludarabine in vitro and in vivo. Methods: We analyzed the sensitivity of primary CLL cells to the fludarabine by CCK-8, and detected the expression of p53 in cells after drug treatment by western blot. Next, we constructed COP1 overexrpessing CLL cell line HG3, and analyzed the effect of COP1 overexpression on the HG3 cell’s apoptosis, and HG3 transplant mice survival with drug treatment. Results: Here, we found that primary CLL cells with high expression of COP1 showed low sensitivity to the drug and presented delayed enrichment of p53 protein than cells with low COP1 expressed. COP1 overexpression reduced HG3 cell sensitivity to the fludarabine treatment and inhibited cell apoptosis, and also retarded itself via autoubiquitination. The further study showed that COP1 promoted ubiquitin-dependent p53 degradation, which further disrupts the formation of the p53-Brn-3a complex and activation of Bcl-2 transcription. Moreover, mice engrafted with cells overexpressing COP1 showed a shortened survival, increased tumor cells burden in spleen and bone marrow (BM), and reduced tumor cell apoptosis even when fludarabine combined cyclophosphamide (F+C) therapy was administered. Conclusion: This study demonstrates that COP1 contributes to drug resistance of CLL cells to the fludarabine treatment in vitro and in vivo.