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Novel epigenetic therapies targeting bromodomain and extra-terminal (BET) family proteins have shown therapeutic efficacy in diverse hematologic malignancies and solid cancers. However, the mechanism of resistance remains poorly understood. In the present study, we evaluated the mechanism of resistance to the BET inhibitor I-BET151 and its signaling pathway to overcome resistance in U937 cells. Treatment with 10 μM I-BET151 significantly induced growth inhibition, apoptosis, and cell cycle modulation, including increases in sub-G1 and G1 phases and decreases in S and G2/M phases, in U937 cells. However, no significant changes in these factors were detected in I-BET151-resistant U937 (U937R) cells. Combined treatment with I-BET151 and IKK inhibitor VII synergistically induced apoptosis in U937 and U937R cells. Increased expression of bromodomain-containing protein (BRD) 2, BRD4, and nuclear NF-κBp65 proteins was detected in U937R cells. IKK inhibitor VII inhibited the activation of NF-κBp65 protein in the nuclear fraction of U937R cells. These findings suggest that resistance to I-BET151 in U937R cells is related to constitutive activation of the NF-κB signaling pathway via increased expression of both BRD2 and BRD4. Targeting the NF-κB signaling pathway may be an effective therapeutic strategy to enhance or restore the sensitivity to I-BET151 in U937 cells., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

To maintain the fidelity and integrity of blood formation, the cell cycle is under strict regulation during haematopoietic cell differentiation. To elucidate the molecular mechanisms of cell cycle regulation during haematopoiesis, we examined cell cycle control gene expression during lineage-specific differentiation from CD34+ progenitor cells. Expression of cyclin-dependent kinases (cdks) and cyclins, except cdk4, was generally suppressed in CD34+ cells freshly isolated from the bone marrow of healthy volunteers. Among four major cdk inhibitors, p16 was expressed more highly in CD34+ cells than in CD34-negative bone marrow mononuclear cells, whereas the amounts of p21 and p27 transcripts increased in the CD34- population. The behaviour of cell cycle control genes during haematopoietic differentiation was classified into four patterns: (i) universal upregulation (cdc2, cdk2, cyclin A, cyclin B and p21); (ii) upregulation in specific lineages (cyclin D1, cyclin D3 and p15); (iii) no induction or stable expression (cdk4, cyclin D2, cyclin E and p27); and (iv) universal downregulation (p16). Lineage-specific changes included the sustained elevation of cdc2 and cyclin A during erythroid differentiation, cyclin D1 and p15 induction in myeloid lineage and selective upregulation of cyclin D3 in megakaryocytes. Blocking induction of cyclin D3 resulted in the inhibition of megakaryocytic differentiation. These results suggest that the expression of cell cycle control genes is distinctively regulated in a lineage-dependent manner, reflecting the cell cycle characteristics of each lineage. Some of these genes play an essential role in the process of differentiation itself. [ABSTRACT FROM AUTHOR]