RSK Inhibition Induces Metaphase Arrest and Apoptosis in Acute Myeloid Leukemia Cells

The 90 kDa Ribosomal S6 Kinase (RSK) drives cell proliferation and survival in several cancers, although its oncogenic mechanism has not been well characterized. To examine the role of RSK in acute myeloid leukemia (AML), we analyzed apoptosis, DNA-damage, and the cell cycle profile following treatment with BI-D1870, a potent inhibitor of RSK. BI-D1870 treatment (5 µM, 12 h) of HL-60 and KG-1 cells increased the G2/M population and induced apoptosis. Previously, RSK has been shown to activate CDC2 for G2/M progression by dephosphorylating the negative regulatory Tyr14 through activation of CDC25 or inhibition of MYT1. Phosphorylation of CDC2 at Tyr14 was increased transiently during G2 phase following treatment with BI-D1870, while phosphorylation of the positive regulatory Ser198 of CDC25C was inhibited in G2 and mitotic populations. However, BI-D1870 treatment did not decrease CDC2/Cyclin B kinase activity, suggesting that inhibition of CDC2 is not a critical regulatory mechanism in BI-D1870-induced mitotic arrest. The mitotic population was further characterized by evaluation of expression levels of Cyclin A and Cyclin B with flow cytometry. Approximately 70% of mitotic cells accumulated in Cyclin A-negative/Cyclin B-high metaphase at 12 hours after 5 µM BI-D1870 treatment. Metaphase arrest was confirmed using a thymidine/nocodazole block to achieve synchronization at metaphase. Our results show that the metaphase/anaphase transition is significantly stalled by RSK inhibition as assessed by degradation of Cyclin B and Securin following release from mitotic arrest (Metaphase cells in mitotic population (%), control vs. BI-D1870, 0 h post-release: 80.53 ± 0.09% vs. 88.03 ± 0.67%; 2 h post-release: 56.27 ± 1.42% vs. 83.37 ± 0.32%; 4 h post-release: 23.53 ± 0.52% vs. 62.90 ± 0.62%, mean ± SEM, n=3, p< 0.01). We also assessed the efficacy of combination treatment of BI-D1870 with standard AML chemotherapeutic drugs cytarabine and daunorubicin. The combination of BI-D1870 with cytarabine or daunorubicin in HL-60 cells showed an additive effect. Furthermore, BI-D1870 induced apoptosis and metaphase arrest in primary AML cells and significantly suppressed the CD34⁺ subpopulation containing leukemia stem cells. Mitotic progression is controlled by the sequential degradation of cell cycle regulators including Cyclin A, Cyclin B, and Securin by the anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase. Since proper association of CDC20 with APC/C is critical to destroy Cyclin B and Securin for metaphase/anaphase progression, we examined the effect of BI-D1870 on the complex formation of CDC20 with APC/C. Co-IP experiments showed that treatment of BI-D1870 impeded the association of activator CDC20 with APC/C, but increased binding of inhibitor MAD2 to APC/C, preventing metaphase/anaphase progression. These data show that BI-D1870 inhibits metaphase/anaphase progression by inactivating APC/C, resulting in apoptosis of AML cells. We are currently studying the effects of RSK knockdown on AML cells. In summary, our study demonstrates that RSK plays an important role in maintaining AML cell survival and proliferation, and RSK inhibitors could be used therapeutically to treat AML by inducing apoptosis and cell cycle arrest through the inhibition of APC/C in AML cells.