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We investigated the effect of the modulation of Na/H exchanger 1 (NHE1) on apoptosis, differentiation, and chemoresistance in acute myeloid leukemia (AML) cells to evaluate the possibility of NHE1 modulation as a novel therapeutic strategy for AML. The pHi of leukemia cell lines except KG1a was higher than that of normal bone marrow mononuclear cells (BM MNCs). Notably, in K562, cytarabine (AraC)-resistant OCI-AML2, and primary leukemia cells, pHi was significantly higher than that of normal BM MNCs. Western blotting and real-time quantitative PCR confirmed that the increased NHE1 expression was responsible for the higher pHi. Specifically, compared to CD34 + CD38 + leukemia cells, the mean fluorescence intensity of NHE1 was significantly higher in CD34 + CD38 - leukemic stem cells. The out of range in pHi by treatment with an NHE inhibitor, the amiloride analogue 5-(N,N-hexamethylene) amiloride (HMA), or an NHE activator, phorbol 12-myristate 13-acetate (PMA), resulted in dose- and time-dependent inhibition of leukemia cell proliferation. PMA induced CD14 + differentiation of leukemia cells, whereas HMA induced cell cycle arrest at the G1 phase. HMA could induce apoptosis of leukemia cells even in AraC-resistant cells and showed an additive effect on apoptosis in AraC-sensitive cells. Our result revealed that AML cells prefer more alkalic intracellular moiety than normal BM MNCs following increased NHE1 expression and that NHE1 modulation can induce apoptosis and differentiation of AML cells. These findings imply that NHE1 is a potential target in cytotoxic or differentiation-induction treatment for AML., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Cdc20 is an activator of the anaphase-promoting complex (APC/C), and APC/C(Cdc20) is essential for metaphase-anaphase transition. To allow progression beyond mitosis, Cdc20 is degraded through KEN-box-dependent APC/C(Cdh1) activity. Mammalian Cdc20 contains the CRY box, a second APC/C(Cdh1)-dependent degron, but the molecular mechanism in degradation process remains undefined. Polo-like kinase-1 (Plk1) is an essential mitotic kinase regulating various targets in kinetochore, centrosome, and midbody for proper mitotic progression. Plk1 directly bound to Cdc20 and phosphorylates it on serine-170 located in CRY-box. Whereas wild-type Cdc20 was degraded according to progress cell cycle beyond mitosis, the phosphorylation-defective mutant, which serine-170 was changed into alanine, was not destroyed in early G1 phase. The phosphorylation on serine-170 by Plk1 was important for ubiquitination and Cdh1-dependent proteolysis. However, this modification by Plk1 on CRY box had no effect on the subcellular localization of Cdc20 and the formation of APC/C-inhibitory checkpoint complexes under spindle assembly checkpoint. This mechanism will be the first finding of inhibitory phosphorylation related to Cdc20 instability., (Copyright © 2013 Elsevier Inc. All rights reserved.)

DNA damage induces multiple checkpoint pathways to arrest cell cycle progression until damage is repaired. In our previous reports, when DNA damage occurred in prometaphase, cells were accumulated in 4 N-DNA G1 phase, and mitosis-specific kinases were inactivated in dependent on ATM/Chk1 after a short incubation for repair. We investigated whether or not mitotic DNA damage causes cells to skip-over late mitotic periods under prolonged incubation in a time-lapse study. 4 N-DNA-damaged cells re-replicated without cell division and accumulated in 8 N-DNA content, and the activities of apoptotic factors were increased. The inhibition of DNA replication reduced the 8 N-DNA cell population dramatically. Induction of replication without cell division was not observed upon depletion of Chk1 or ATM. Finally, mitotic DNA damage induces mitotic slippage and that cells enter G1 phase with 4 N-DNA content and then DNA replication is occurred to 8 N-DNA content before completion of mitosis in the ATM/Chk1-dependent manner, followed by caspase-dependent apoptosis during long-term repair., (Copyright © 2012 Elsevier Inc. All rights reserved.)