Elevated APEX1 Disrupts G2/M Checkpoint, Contributing to Evolution and Survival of Myeloma Cells

Cell cycle checkpoints provide the cell with time to repair chromosomal DNA damage before its replication (G1) and also prior to its segregation (G2), thus ensuring integrity, maintenance and protection of genome. Although proper functioning of both checkpoints is essential, G2/M has a special significance as a potentially lethal double-strand break in DNA escape repair and persist from G₂ into mitosis, it may recombine in G₁ to produce gene rearrangements. Moreover, G2 is the phase where homologous recombination (HR) can utilize a sister chromatid as a template to provide error-free repair. There is ample evidence that supports the role of defective G2/M checkpoint and dysregulated HR in genomic rearrangements and evolution in cancer. Previously, we have shown that elevated APEX1 contributes to dysregulated HR and genome stability in multiple myeloma (MM), and its upregulation leads to genomic instability and tumorigenesis in animal model. To further understand the role of APEX1 in myeloma, we investigated the impact of elevated APEX1 on cell cycle checkpoint/s and in the cellular response to genotoxic exposure. Our investigation using antibody array and subsequent confirmation with immunoprecipitation experiments demonstrated that APEX1 interacts with cyclin B and PLK in myeloma cells. A key step in progression from G2 to mitosis is the activation of cyclin B-CDK1 complex, which subsequently activates PLK to ensure G2/M progression. Based on observed interaction of APEX1 with cyclin B/PLK, we hypothesized that elevated APEX1 disrupts G2 checkpoint by mediating progression into mitosis. To test this, we inhibited APEX1 in myeloma cells by a small molecule as well as by shRNA targeting this gene, and investigated the impact on cell cycle checkpoints using a unique phospho-antibody array which allows investigation of 238 relevant proteins and their phosphorylation status. APEX1 inhibition by small molecule led to downregulation (> 2-fold) of many proteins/phosphorylations involved in the activation of cyclin B-CDK1 complex and other mediators of G2/M progression (including CDC25A, CDC25A, CDK1, ABL1), and upregulation of proteins/phosphorylations involved in G2/M arrest, including CDK1-phospho-Tyr15, 14-3-3 zeta-phospho-Ser58, p53-phospho-Ser15, and MYT and WEE which are involved in negative regulation of cyclin B-CDK1 complex. To further investigate the role of APEX1 in G2/M progression, myeloma cell lines (ARP, RPMI 8226, MM1S, LR5, H929) were treated with APEX1 inhibitor and subjected to cell cycle analysis using flow cytometery. Compared to control cells, all five APEX1-inhibitor treated cell lines showed a strong G2/M block, ranging from 5- to 10-fold increase in the fraction of cells in G2 phase in a dose dependent manner. The G2/M cell cycle arrest of APEX1-treated myeloma cells was further supported by reduced cell viability of treated myeloma cells (RPMI, H929, MM1S, ARP and U266); IC50 of inhibitor in myeloma cell lines ranged from 1.2 to 4 µM. Co-treatment with APEX1 inhbitor also sensitized myeloma cells to Melphalan. Consistent with these data, shRNA-mediated knockdown (KD) of APEX1 in RPMI cells was associated with 4-fold increase in the fraction of cells in G2, relative to control cells. APEX1-KD was also associated with reduction in cell viability (by 40%) and sensitization to melphalan. Our results therefore suggest that elevated APEX1 disrupts G2 checkpoint and sets a stage for genomic rearrangements by allowing persistance of DNA damage from G₂ into mitosis. Dysfunctional G2 checkpoint, combined with APEX1-mediated dysregulation of HR, could be attributed to APEX1 associated genomic instability and oncogenic transformation. Therefore, inhibitors of APEX1, alone or in combination with other agents, have potential to make myeloma cells static.