DRP1 Inhibition Enhances Venetoclax-Induced Mitochondrial Apoptosis in TP53-Mutated Acute Myeloid Leukemia Cells through BAX/BAK Activation.

Although TP53 mutations in acute myeloid leukemia (AML) are associated with poor response to venetoclax, the underlying resistance mechanism remains unclear. Herein, we investigated the functional role of dynamin-related protein 1 (DRP1) in venetoclax sensitivity in AML cells with respect to TP53 mutation status. Effects of DRP1 inhibition on venetoclax-induced cell death were compared in TP53-mutated (THP-1 and Kasumi-1) and TP53 wild-type leukemia cell lines (MOLM-13 and MV4-11), as well as in primary AML cells obtained from patients. Venetoclax induced apoptosis in TP53 wild-type AML cells but had limited effects in TP53-mutated AML cells. DRP1 expression was downregulated in MOLM-13 cells after venetoclax treatment but was unaffected in THP-1 cells. Cotreatment of THP-1 cells with venetoclax and a TP53 activator NSC59984 downregulated DRP1 expression and increased apoptosis. Combination treatment with the DRP1 inhibitor Mdivi-1 and venetoclax significantly increased mitochondria-mediated apoptosis in TP53-mutated AML cells. The combination of Mdivi-1 and venetoclax resulted in noticeable downregulation of MCL-1 and BCL-xL, accompanied by the upregulation of NOXA, PUMA, BAK, and BAX. These findings suggest that DRP1 is functionally associated with venetoclax sensitivity in TP53-mutated AML cells. Targeting DRP1 may represent an effective therapeutic strategy for overcoming venetoclax resistance in TP53-mutated AML.

Arsenic trioxide synergistically promotes the antileukaemic activity of venetoclax by downregulating Mcl-1 in acute myeloid leukaemia cells.

BACKGROUND: The evasion of apoptosis through dysregulated Bcl-2 family members is a hallmark of leukaemia stem cells (LSCs) in acute myeloid leukaemia (AML). Therefore, targeting Bcl-2 with venetoclax has been suggested as an attractive strategy for inducing apoptosis in AML LSCs. However, the selective inhibition of Bcl-2 in AML often leads to upregulation of Mcl-1, another dominant anti-apoptotic Bcl-2 family protein conferring venetoclax resistance. METHODS: We assessed the combined effect of venetoclax and arsenic trioxide (ATO) on leukaemic cell viability, apoptosis, combination index, and cell cycle in the human LSC-like KG1 and KG1a cells. The synergistic effect of venetoclax and ATO on apoptosis was also examined in primary CD34+ and CD34+CD38- LSCs from the bone marrow (BM) of AML patients, and compared with those from healthy donors. RESULTS: Venetoclax efficiently impaired cell viability and dose-dependently promoted apoptosis when combined with ATO; their synergism was aptly represented by the combination index. The combination of venetoclax and ATO impaired cell cycle progression by restricting cells within the sub-G1 phase and facilitating caspase-dependent apoptotic cell death associated with the loss of mitochondrial membrane potential, while sparing healthy BM haematopoietic stem cells. Mechanistically, ATO mitigated venetoclax-induced upregulation of Mcl-1 by the inhibition of AKT and ERK, along with activation of GSK-3ß. This led to the Mcl-1 destabilisation, triggering Noxa and Bim to facilitate apoptosis and the consequent activation of the apoptosis executioner protein Bak. Moreover, the combination promoted phosphorylation of ATM, Chk2, p38, and H2AX, indicating an active DNA damage response. CONCLUSIONS: Our findings demonstrate the synergistic, preferential antileukaemic effects of venetoclax and ATO on LSCs, providing a rationale for preclinical and clinical trials by combining these agents already being used in clinical practice to treat acute leukaemia.

All-Trans Retinoic Acid Synergizes with Enasidenib to Induce Differentiation of IDH2-Mutant Acute Myeloid Leukemia Cells.

PURPOSE: Pharmacological inhibition of mutant isocitrate dehydrogenase (IDH) reduces R-2-hydroxyglutarate (2-HG) levels and restores cellular differentiation in vivo and in vitro. The IDH2 inhibitor enasidenib (AG-221) has been approved by the FDA as a first-in-class inhibitor for the treatment of relapsed or refractory (R/R) IDH2-mutant acute myeloid leukemia (AML). In this study, the effects of a combination of all-trans retinoic acid (ATRA) and AG-221 on AML cell differentiation was explored, along with the mechanisms employed by IDH2-mutant cells in AML. MATERIALS AND METHODS: We treated the human AML cell line, IDH2-mutant-TF-1, and primary human AML cells carrying IDH2 mutation with 30 µM AG-221 and 100 nM ATRA, alone or in combination. RESULTS: Combined treatment with AG-221 and ATRA inhibited 2-HG production and resulted in synergistic effects on differentiation among IDH2-mutant AML cells and primary AML cells expressing IDH2 mutation. Combined treatment with AG-221 and ATRA altered autophagic activity. AG-221 and ATRA treatment-induced differentiation of IDH2-mutant AML cells was associated with autophagy induction, without suppressing autophagy flux at maturation and degradation stages. A RAF-1/MEK/ERK pathway was founded to be associated with AG-221 and ATRA-induced differentiation in IDH2-mutant AML cells. IDH-associated changes in histone methylation markers decreased after AG-221 and ATRA combination treatment. CONCLUSION: Our preliminary evidence indicates that the addition of ATRA to treatments with IDH2 inhibitor may lead to further improvements or increases in response rates in IDH2-mutant AML patients who do not appear to benefit from treatments with IDH2 inhibitor alone.

ULK1 inhibition as a targeted therapeutic strategy for FLT3-ITD-mutated acute myeloid leukemia.

BACKGROUND: In acute myeloid leukemia (AML), internal tandem duplication mutations in the FLT3 tyrosine kinase receptor (FLT3-ITD) are associated with a dismal outcome. Although uncoordinated 51-like kinase 1 (ULK1), which plays a central role in the autophagy pathway, has emerged as a novel therapeutic target for various cancers, its role in FLT3-ITD AML remains elusive. In this study, we evaluated the effects of ULK1 inhibition on leukemia cell death in FLT3-ITD AML. METHOD: We evaluated ULK1 expression and the levels of apoptosis and autophagy following ULK1 inhibition in FLT3-ITD AML cell lines and investigated the mechanism underlying apoptosis induced by ULK1 inhibition. Statistical analysis was performed using GraphPad Prism 4.0 (GraphPad Software Inc). RESULTS: FLT3-ITD AML cells showed significantly higher ULK1 expression than FLT3-wild-type (WT) AML cells. Two ULK1 inhibitors, MRT 68921 and SBI-0206965, induced apoptosis in FLT3-ITD AML cells, with relatively minimal effects on FLT3-WT AML cells and normal CD34-positive cells. Apoptosis induction by ULK1 inhibition was associated with caspase pathway activation. Interestingly, ULK1 inhibition paradoxically also induced autophagy, showing synergistic interaction with autophagy inhibitors. Hence, autophagy may act as a prosurvival mechanism in FLT3-ITD AML cells. FLT3-ITD protein degradation and inhibition of the ERK, AKT, and STAT5 pathways were also observed in FLT3-ITD AML cells following treatment with ULK1 inhibitors. CONCLUSION: ULK1 is a viable drug target and ULK1 inhibition may represent a promising therapeutic strategy against FLT3-ITD AML.

PERK/NRF2 and autophagy form a resistance mechanism against G9a inhibition in leukemia stem cells.

BACKGROUND: The histone methyltransferase G9a has recently been identified as a potential target for epigenetic therapy of acute myeloid leukemia (AML). However, the effect of G9a inhibition on leukemia stem cells (LSCs), which are responsible for AML drug resistance and recurrence, is unclear. In this study, we investigated the underlying mechanisms of the LSC resistance to G9a inhibition. METHODS: We evaluated the effects of G9a inhibition on the unfolded protein response and autophagy in AML and LSC-like cell lines and in primary CD34+CD38- leukemic blasts from patients with AML and investigated the underlying mechanisms. The effects of treatment on cells were evaluated by flow cytometry, western blotting, confocal microscopy, reactive oxygen species (ROS) production assay. RESULTS: The G9a inhibitor BIX-01294 effectively induced apoptosis in AML cell lines; however, the effect was limited in KG1 LSC-like cells. BIX-01294 treatment or siRNA-mediated G9a knockdown led to the activation of the PERK/NRF2 pathway and HO-1 upregulation in KG1 cells. Phosphorylation of p38 and intracellular generation of reactive oxygen species (ROS) were suppressed. Pharmacological or siRNA-mediated inhibition of the PERK/NRF2 pathway synergistically enhanced BIX-01294-induced apoptosis, with suppressed HO-1 expression, increased p38 phosphorylation, and elevated ROS generation, indicating that activated PERK/NRF2 signaling suppressed ROS-induced apoptosis in KG1 cells. By contrast, cotreatment of normal hematopoietic stem cells with BIX-01294 and a PERK inhibitor had no significant proapoptotic effect. Additionally, G9a inhibition induced autophagy flux in KG1 cells, while autophagy inhibitors significantly increased the BIX-01294-induced apoptosis. This prosurvival autophagy was not abrogated by PERK/NRF2 inhibition. CONCLUSIONS: PERK/NRF2 signaling plays a key role in protecting LSCs against ROS-induced apoptosis, thus conferring resistance to G9a inhibitors. Treatment with PERK/NRF2 or autophagy inhibitors could overcome resistance to G9a inhibition and eliminate LSCs, suggesting the potential clinical utility of these unique targeted therapies against AML.

Early Cytomegalovirus Reactivation and Expansion of CD56brightCD16dim/-DNAM1+ Natural Killer Cells Are Associated with Antileukemia Effect after Haploidentical Stem Cell Transplantation in Acute Leukemia.

Cytomegalovirus (CMV) infection is a major complication after allogeneic hematopoietic stem cell transplantation but is suggested to exert a strong antileukemia effect in part due to alterations in the composition of natural killer (NK) cells. We evaluated the impact of early CMV reactivation and changes in NK cell subset recovery on relapse rate and survival after haploidentical stem cell transplantation (haploSCT) for acute leukemia. Fifty patients with acute leukemia who received haploSCT were analyzed. Expression of T cells and specific receptors (NKG2A, NKG2D, DNAM1, and CD57) on circulating NK cells (CD56brightCD16dim/- or CD56dimCD16+ cells) was serially measured using multiparametric flow cytometry. CMV reactivation during the first 100 days was observed in 41 patients (82%) at a median of 23 days after haploSCT. The incidence of acute graft-versus-host disease (GVHD) and chronic GVHD tended to be higher in patients with CMV reactivation, although this difference was not statistically significant. Multivariate analysis showed that CMV reactivation (P = .011) and a dose of infused T cells > 3.2 × 108/kg (P = .027) were independent predictors of a reduced relapse risk and only CMV reactivation (P = .029) was an independent predictor of improved leukemia-free survival. CD56brightCD16dim/-DNAM1+NK cell counts increased from day 30 to 90 in patients with CMV reactivation but decreased after day 30 in patients without CMV reactivation. An increase in CD56brightCD16dim/-DNAM1+ NK cells was not associated with the occurrence of chronic GVHD but was associated with a reduced cumulative relapse rate (16.4% versus 58.0%, P = .019). Multivariate analysis indicates that an increase in the CD56brightCD16dim/-DNAM1+NK cell count was an independent predictor of reduced relapse risk. Our study demonstrates a significant correlation between low relapse rates and CMV reactivation as well as the recovery of CD56brightCD16dim/-DNAM1+ NK cells, providing valuable information for understanding the plausible immunologic mechanism of the graft-versus-leukemia effect.

Targeting AMPK-ULK1-mediated autophagy for combating BET inhibitor resistance in acute myeloid leukemia stem cells.

Bromodomain and extraterminal domain (BET) inhibitors are promising epigenetic agents for the treatment of various subsets of acute myeloid leukemia (AML). However, the resistance of leukemia stem cells (LSCs) to BET inhibitors remains a major challenge. In this study, we evaluated the mechanisms underlying LSC resistance to the BET inhibitor JQ1. We evaluated the levels of apoptosis and macroautophagy/autophagy induced by JQ1 in LSC-like leukemia cell lines and primary CD34+ CD38- leukemic blasts obtained from AML cases with normal karyotype without recurrent mutations. JQ1 effectively induced apoptosis in a concentration-dependent manner in JQ1-sensitive AML cells. However, in JQ1-resistant AML LSCs, JQ1 induced little apoptosis and led to upregulation of BECN1/Beclin 1, increased LC3 lipidation, formation of autophagosomes, and downregulation of SQSTM1/p62. Inhibition of autophagy by pharmacological inhibitors or knockdown of BECN1 using specific siRNA enhanced JQ1-induced apoptosis in resistant cells, indicating that prosurvival autophagy occurred in these cells. Independent of MTOR signaling, activation of the AMPK (p-Thr172)-ULK1 (p-Ser555) pathway was found to be associated with JQ1-induced autophagy in resistant cells. AMPK inhibition using the pharmacological inhibitor compound C or by knockdown of PRKAA/AMPKα suppressed autophagy and promoted JQ1-induced apoptosis in AML LSCs. These findings revealed that prosurvival autophagy was one of the mechanisms involved in the resistance of AML LSCs to JQ1. Targeting the AMPK-ULK1 pathway or inhibition of autophagy could be an effective therapeutic strategy for combating resistance to BET inhibitors in AML and other types of cancer.

AMPK-ULK1-Mediated Autophagy Confers Resistance to BET Inhibitor JQ1 in Acute Myeloid Leukemia Stem Cells.

Purpose: Bromodomain and extraterminal domain (BET) inhibitors are promising epigenetic agents for the treatment of various subsets of acute myeloid leukemia (AML). However, the resistance of leukemia stem cells (LSC) to BET inhibitors remains a major challenge. In this study, we evaluated the mechanisms underlying LSC resistance to the BET inhibitor JQ1.Experimental Design: We evaluated the levels of apoptosis and autophagy induced by JQ1 in LSC-like leukemia cell lines and primary CD34+CD38- leukemic blasts obtained from AML cases with normal karyotype without recurrent mutations.Results: JQ1 effectively induced apoptosis in a concentration-dependent manner in JQ1-sensitive AML cells. However, in JQ1-resistant AML LSCs, JQ1 induced little apoptosis and led to upregulation of beclin-1, increased LC3-II lipidation, formation of autophagosomes, and downregulation of p62/SQSTM1. Inhibition of autophagy by pharmacologic inhibitors or knockdown of beclin-1 using specific siRNA enhanced JQ1-induced apoptosis in resistant cells, indicating that prosurvival autophagy occurred in these cells. Independent of mTOR signaling, activation of the AMPK (pThr172)/ULK1 (pSer555) pathway was found to be associated with JQ1-induced autophagy in resistant cells. AMPK inhibition using the pharmacologic inhibitor compound C or by knockdown of AMPKα suppressed autophagy and promoted JQ1-induced apoptosis in AML LSCs.Conclusions: These findings revealed that prosurvival autophagy was one of the mechanisms involved in the resistance AML LSCs to JQ1. Targeting the AMPK/ULK1 pathway or inhibition of autophagy could be an effective therapeutic strategy for combating resistance to BET inhibitors in AML and other types of cancer. Clin Cancer Res; 23(11); 2781-94. ©2016 AACR.

Enhanced autophagy in cytarabine arabinoside-resistant U937 leukemia cells and its potential as a target for overcoming resistance.

Autophagy is a lysosomal degradation mechanism that is essential for cell survival, differentiation, development, and homeostasis. Autophagy protects cells from various stresses, including protecting normal cells from harmful metabolic conditions, and cancer cells from chemotherapeutics. In the current study, a cytarabine arabinoside (Ara­C)­sensitive U937 leukemia cell line and an Ara­C­resistant U937 (U937/AR) cell line were assessed for baseline autophagy activity by investigating the LC3­I conversion to LC3­II, performing EGFP­LC3 puncta, an acidic autophagolysosome assay, and measuring the expression of various autophagy­related genes. The results demonstrated significantly higher autophagic activity in the U937/AR cells compared with the U937 cells, when the cells were cultured with or without serum. Furthermore, an increase in the autophagic activity in starved U937/AR cells was demonstrated, compared with that in the starved U937 cells. Administration of an autophagy inhibitor demonstrated no change in cell death in the two cell lines when cultured with serum, however, it induced cell death regardless of the Ara­C sensitivity when the cell lines were cultured without serum. In addition, the U937 cells demonstrated an Ara­C resistance when cultured without serum. Co­treatment with Ara­C and the autophagy inhibitor significantly induced cell death in the U937/AR and Ara­C­sensitive U937 cells. In conclusion, autophagy serves an important role in protecting U937 cells from Ara­C and in the development of Ara­C resistance. Inhibition of autophagy combined with the Ara­C treatment in the U937 cells augmented the anti­leukemic effect of Ara­C and overcame Ara­C resistance, suggesting that autophagy may be an important therapeutic target to further improve the treatment outcome in patients with acute myeloid leukemia.

Induction of cytosine arabinoside-resistant human myeloid leukemia cell death through autophagy regulation by hydroxychloroquine.

We investigated the effects of the autophagy inhibitor hydroxychloroquine (HCQ) on cell death of cytosine arabinoside (Ara-C)-resistant human acute myeloid leukemia (AML) cells. Ara-C-sensitive (U937, AML-2) and Ara-C-resistant (U937/AR, AML-2/AR) human AML cell lines were used to evaluate HCQ-regulated cytotoxicity, autophagy, and apoptosis as well as effects on cell death-related signaling pathways. We found that HCQ-induced dose- and time-dependent cell death in Ara-C-resistant cells compared to Ara-C-sensitive cell lines. The extent of cell death and features of HCQ-induced autophagic markers including increase in microtubule-associated protein light chain 3 (LC3) I conversion to LC3-II, beclin-1, ATG5, as well as green fluorescent protein-LC3 positive puncta and autophagosome were remarkably greater in U937/AR cells. Also, p62/SQSTM1 was increased in response to HCQ. p62/SQSTM1 protein interacts with both LC3-II and ubiquitin protein and is degraded in autophagosomes. Therefore, a reduction of p62/SQSTM1 indicates increased autophagic degradation, whereas an increase of p62/SQSTM1 by HCQ indicates inhibited autophagic degradation. Knock down of p62/SQSTM1 using siRNA were prevented the HCQ-induced LC3-II protein level as well as significantly reduced the HCQ-induced cell death in U937/AR cells. Also, apoptotic cell death and caspase activation in U937/AR cells were increased by HCQ, provided evidence that HCQ-induced autophagy blockade. Taken together, our data show that HCQ-induced apoptotic cell death in Ara-C-resistant AML cells through autophagy regulation.

Serum microRNA-21 as a potential biomarker for response to hypomethylating agents in myelodysplastic syndromes.

Identification of biomarkers that predict responses to hypomethylating agents (HMAs) will allow optimal strategies for epigenetic therapy in myelodysplastic syndromes (MDS) to be established. Serum miR-21 was quantitatively measured in 58 MDS patients treated with HMAs and 14 healthy controls. Serum miR-192 was an internal control, and diagnostic performance was evaluated according to receiver operating characteristics (ROCs). ROC analysis indicated that serum miR-21 levels differentiated responders from non-responders with an area under the curve of 0.648 (95% confidence, 0.49 to 0.72). The baseline level of serum miR-21 was significantly lower in the responder group than in the non-responder group (P = 0.041). The overall response rate (ORR) of the high miR-21 group was significantly lower than that of the low miR-21 group (41.2 vs. 73.2%, P = 0.021). Progression-free survival (PFS) was significantly inferior in the high group versus the low group (14.0 vs. 44.5 months, P = 0.001). Multivariate analyses revealed that the initial serum miR-21 level (P = 0.001) and circulating blasts (P = 0.007) were prognostic factors for PFS. Serum miR-21 level was significantly associated with ORR and PFS in MDS patients treated with HMAs. Although validation with a large prospective study is required, serum miR-21 is a potential biomarker of epigenetic therapy in MDS patients.

Aurora A kinase expression is increased in leukemia stem cells, and a selective Aurora A kinase inhibitor enhances Ara-C-induced apoptosis in acute myeloid leukemia stem cells.

BACKGROUND: The overexpression of Aurora A kinase (AurA) has been reported in various malignancies, including acute myeloid leukemia (AML). However, the expression of AurA and the effects of AurA inhibition in cancer stem cells are not yet fully understood. We investigated the expression and inhibition of AurA in AML stem cells (CD34(+)/CD38(-)). METHODS: Expression of AurA was investigated in cell lines (NB4 and KG1) that express high levels of CD34 and low levels of CD38. Primary AML cells were harvested from 8 patients. The expression of AurA and cell death induced by inhibition of AurA were analyzed in CD34(+)/CD38(-) cells. RESULTS: AurA was shown to be overexpressed in both primary AML cells and leukemia stem cells (LSCs) compared to normal hematopoietic stem cells. Inhibition of AurA plus cytarabine treatment in LSCs resulted in increased cytotoxicity compared to cytarabine treatment alone. Additional stimulation with granulocyte-colony stimulating factor (G-CSF) increased the cell death caused by AurA inhibition plus cytarabine treatment. CONCLUSION: To our knowledge, this is the first report describing increased expression of AurA in LSCs. Our results suggest that selective AurA inhibition may be used to reduce LSCs, and this reduction may be enhanced by stimulation with G-CSF. Further exploration of relationship between nuclear factor kappa-B and AurA inhibition and the potential of AurA inhibition for use in leukemia treatment is needed.

Resveratrol alters microRNA expression profiles in A549 human non-small cell lung cancer cells.

Resveratrol is a plant phenolic phytoalexin that has been reported to have antitumor properties in several types of cancers. In particular, several studies have suggested that resveratrol exerts antiproliferative effects against A549 human non-small cell lung cancer cells; however, its mechanism of action remains incompletely understood. Deregulation of microRNAs (miRNAs), a class of small, noncoding, regulatory RNA molecules involved in gene expression, is strongly correlated with lung cancer. In this study, we demonstrated that resveratrol treatment altered miRNA expression in A549 cells. Using microarray analysis, we identified 71 miRNAs exhibiting greater than 2-fold expression changes in resveratrol-treated cells relative to their expression levels in untreated cells. Furthermore, we identified target genes related to apoptosis, cell cycle regulation, cell proliferation, and differentiation using a miRNA target-prediction program. In conclusion, our data demonstrate that resveratrol induces considerable changes in the miRNA expression profiles of A549 cells, suggesting a novel approach for studying the anticancer mechanisms of resveratrol.

Inhibition of CK2{alpha} and PI3K/Akt synergistically induces apoptosis of CD34+CD38- leukaemia cells while sparing haematopoietic stem cells.

BACKGROUND/AIM: The CD34(+)CD38(-) leukaemia cell population contains leukaemia stem cells (LSCs) responsible for treatment failure in acute myeloid leukaemia (AML) and, thus, novel therapies are required to eradicate LSCs without harming healthy haematopoietic stem cells (HSC). MATERIALS AND METHODS: The present study evaluated the effects of co-treatment with LY294002 (a PI3K/Akt inhibitor) and apigenin (a CK2 inhibitor) (LY/Api) at subtoxic concentrations on leukaemia cell lines and primary AML cells. RESULTS: LY/Api synergistically induced apoptosis in leukaemia cells, especially CD34(+)CD38(-) leukaemia cells. However, these effects were negligible in HSCs. LY/Api-induced apoptosis was accompanied by activation of caspase cascades and disruption of mitochondrial membrane potential. Caspase inhibitor or Akt overexpression abrogated this synergistic induction in apoptosis by LY/Api. LY/Api also led to remarkable down-regulation of anti-apoptotic proteins including Bcl-xL and NF-κB in CD34(+)CD38(-) leukaemia cells, but not in healthy hematopoietic stem cells. CONCLUSION: Inhibition of both CK2 and PI3K/Akt pathways may be a promising LSCs-targeted therapeutic strategy for AML.

Myeloperoxidase expression as a potential determinant of parthenolide-induced apoptosis in leukemia bulk and leukemia stem cells.

Given that parthenolide (PTL) is an effective antileukemic agent, identifying molecular markers that predict response to PTL is important. We evaluated the role of myeloperoxidase (MPO) in determining the sensitivity of leukemia cells to PTL-induced apoptosis. In this study, the level of PTL-induced generation of reactive oxygen species (ROS) and apoptosis was significantly higher in the MPO-high leukemia cell lines compared with the MPO-low leukemia cell lines. Pretreatment of MPO-high leukemia cells with a MPO-specific inhibitor, 4-aminobenzoic acid hydrazide, or a MPO-specific small interfering RNA (siRNA) abrogated the PTL-induced ROS generation and apoptosis, indicating that MPO plays a crucial role in PTL-induced apoptosis in leukemia cells. PTL-induced apoptosis was accompanied by down-regulation of nuclear factor-κB, Bcl-xL, Mcl-1, X-linked inhibitor of apoptosis protein, and survivin and selectively observed in primary acute myeloid leukemia (AML) cells expressing higher levels of MPO (≥50%) while sparing both AML cells with lower MPO and normal CD34-positive (CD34+) normal bone marrow cells. The extent of PTL-induced apoptosis of the CD34+CD38- cell fraction was significantly greater in the MPO-high AML cases, compared with the MPO-low AML (P < 0.01) and normal CD34+ marrow cells (P < 0.01). Nonobese diabetic/severe combined immunodeficient human leukemia mouse model also revealed that PTL preferentially targets the MPO-high AML cells. Our data suggest that MPO plays a crucial role in determining the susceptibility of leukemia cells to PTL-induced apoptosis. PTL can be considered a promising leukemic stem cell-targeted therapy for AML expressing high levels of MPO.

Aurora-A kinase inhibition enhances the cytosine arabinoside-induced cell death in leukemia cells through apoptosis and mitotic catastrophe.

Aurora-A (Aur-A) is a centrosome-associated serine/threonine kinase that is overexpressed in various cancers and potentially correlated with chemoresistance. In the Ara-C-sensitive leukemia cell lines, silencing of Aur-A by small interfering RNA transfection led to a significant increase in the Ara-C-induced cell death rate through induction of mitochondria-mediated, caspase-dependent apoptosis. In contrast, combined treatment of the Ara-C-resistant leukemia cell lines with Aur-A siRNA and Ara-C remarkably enhanced the cell death rate via non-caspase-dependent mitotic catastrophe. Taken together, Aur-A inhibition was an effective treatment for both the Ara-C-sensitive and resistant leukemia cells by increasing apoptosis and mitotic catastrophe, respectively.

Apicidin potentiates the imatinib-induced apoptosis of Bcr-Abl-positive human leukaemia cells by enhancing the activation of mitochondria-dependent caspase cascades.

Apicidin, a histone deacetylase inhibitor, is a novel cyclic tetrapeptide with potent antiproliferative activity against various cancer cells. We examined whether apicidin potentiates the imatinib-induced apoptosis of Bcr-Abl-positive human leukaemia cells. In K562 cells, the co-administration of minimally toxic concentrations of imatinib and apicidin (imatinib/apicidin) for 48 h produced a marked increase in mitochondrial damage, processing of caspase cascades and apoptosis. Similar results were observed in leukaemic blasts obtained from patients with chronic myeloid leukaemia in blast crisis. Imatinib/apicidin co-treatment for 48 h resulted in a near complete loss of the full-length XIAP (X-linked inhibitor of apoptosis) protein, with a corresponding increase in the 29-kDa XIAP cleavage product. Both the degradation of XIAP and increased release of second mitochondria-derived activator of caspase/direct IAP-binding protein with low pI (Smac/DIABLO) into the cytosol were abrogated by pretreatment with the caspase-3 inhibitor DEVD-CHO. Imatinib/apicidin co-treatment for 48 h produced a prominent decrease in Bcr-Abl protein levels in a caspase-dependent manner. In summary, these data indicate that apicidin potentiates the imatinib-induced apoptosis of Bcr-Abl-positive leukaemia cells through the enhanced activation of the mitochondria-dependent caspase cascades, accompanied by caspase-dependent downregulation of Bcr-Abl and XIAP. These findings generate a rationale for further investigation of apicidin and imatinib as a potential therapeutic strategy in Bcr-Abl-positive leukaemias.

Induction of apoptosis by apicidin, a histone deacetylase inhibitor, via the activation of mitochondria-dependent caspase cascades in human Bcr-Abl-positive leukemia cells.

PURPOSE: Apicidin, a histone deacetylase inhibitor, is a novel cyclic tetrapeptide that exhibits potent antiproliferative activity against various cancer cell lines. The aim of this study was to examine the potential of apicidin to induce apoptosis in human Bcr-Abl-positive leukemia cells and to assess the mechanism of apicidin-induced apoptosis. EXPERIMENTAL DESIGN: Cells were exposed to various concentrations of apicidin for 2-72 h, after which the levels of apoptosis, histone acetylation, mitochondrial damage, caspase activation, and Bcr-Abl expression were assessed. RESULTS: Apicidin induced apoptosis in K562 cells in a concentration- and time-dependent manner. Similarly, apicidin notably induced the apoptosis in the primary leukemic blasts obtained from chronic myelogenous leukemia patients in blast crisis. The acetylated histone H4 levels increased in a concentration-dependent manner in the K562 cells. However, the timing of cell death caused by apicidin did not exactly correlate with the histone deacetylase inhibitory effect. The disruption of the mitochondrial membrane potential, cytochrome c release into the cytosol, and the mitochondrial Bax translocation were notably demonstrated after the apicidin treatment. Apicidin induced the proteolytic cleavage of procaspase-9, -3, -8, and poly(ADP-ribose) polymerase. Pretreatment of the K562 cells with the caspase-3 inhibitor, DEVD-CHO, completely inhibited the apicidin-induced apoptosis, suggesting that apicidin-induced apoptosis was caspase-dependent. The Fas/Fas ligand death receptor pathway was not involved in the apicidin-mediated apoptosis in K562 cells. Pretreatment of the cells with the caspase-9 inhibitor LEHD-fmk abrogated the apicidin- induced cleavage of procaspase-3, -8, and poly(ADP-ribose) polymerase. The p210 Bcr-Abl protein levels were notably decreased after the apicidin treatment, with near complete loss after 48 h. Reverse transcription-PCR assay demonstrated that the Bcr-Abl mRNA level was also remarkably decreased in a time-dependent manner. CONCLUSIONS: These results indicate that apicidin effectively induces the apoptosis of Bcr-Abl-positive leukemia cells through the activation of the mitochondrial pathway-dependent caspase cascades. The down-regulation of Bcr-Abl mRNA might also be one of the mechanisms implicated in the apicidin-mediated apoptosis in the K562 cells. This study provides the rationale to additionally investigate apicidin as a potential therapeutic agent for the drug-resistant Bcr-Abl-positive leukemia cells.