Mechanisms of apoptosis sensitivity and resistance to the BH3 mimetic ABT-737 in acute myeloid leukemia

BCL-2 proteins are critical for cell survival and are overexpressed in many tumors. ABT-737 is a small-molecule BH3 mimetic that exhibits single-agent activity against lymphoma and small-cell lung cancer in preclinical studies. We here report that ABT-737 effectively kills acute myeloid leukemia blast, progenitor, and stem cells without affecting normal hematopoietic cells. ABT-737 induced the disruption of the BCL-2/BAX complex and BAK-dependent but BIM-independent activation of the intrinsic apoptotic pathway. In cells with phosphorylated BCL-2 or increased MCL-1, ABT-737 was inactive. Inhibition of BCL-2 phosphorylation and reduction of MCL-1 expression restored sensitivity to ABT-737. These data suggest that ABT-737 could be a highly effective antileukemia agent when the mechanisms of resistance identified here are considered.

Triptolide induces caspase-dependent cell death mediated via the mitochondrial pathway in leukemic cells

Triptolide, a diterpenoid isolated from the Chinese herb Tripterygium wilfordii Hook.f, has shown antitumor activities in a broad range of solid tumors. Here, we examined its effects on leukemic cells and found that, at 100 nM or less, it potently induced apoptosis in various leukemic cell lines and primary acute myeloid leukemia (AML) blasts. We then attempted to identify its mechanisms of action. Triptolide induced caspase-dependent cell death accompanied by a significant decrease in XIAP levels. Forced XIAP overexpression attenuated triptolide-induced cell death. Triptolide also decreased Mcl-1 but not Bcl-2 and Bcl-X(L) levels. Bcl-2 overexpression suppressed triptolide-induced apoptosis. Further, triptolide induced loss of the mitochondrial membrane potential and cytochrome C release. Caspase-9 knock-out cells were resistant, while caspase-8-deficient cells were sensitive to triptolide, suggesting criticality of the mitochondrial but not the death receptor pathway for triptolide-induced apoptosis. Triptolide also enhanced cell death induced by other anticancer agents. Collectively, our results demonstrate that triptolide decreases XIAP and potently induces caspase-dependent apoptosis in leukemic cells mediated through the mitochondrial pathway at low nanomolar concentrations. The potent antileukemic activity of triptolide in vitro warrants further investigation of this compound for the treatment of leukemias and other malignancies.

Caspase-independent cell death in AML: caspase inhibition in vitro with pan-caspase inhibitors or in vivo by XIAP or Survivin does not affect cell survival or prognosis

Survivin and XIAP, members of the protein family known as the inhibitors of apoptosis, interfere with the activation of caspases, called the "cell death executioners." We examined Survivin (n = 116) and XIAP (n = 172) expression in primary acute myeloid leukemia (AML) blasts and assessed the impact of their expression on prognosis. They were detected in all samples analyzed. However, no correlation was observed with cytogenetics, remission attainment, or overall survival of patients with AML. To investigate the importance of caspases in chemotherapy-induced apoptosis in AML, we treated OCI-AML3 cells with Ara-C, doxorubicin, vincristine, and paclitaxel, which induced caspase cleavage and apoptosis. Blocking of caspase activation by pan-caspase inhibitor abolished poly(adenosine diphosphate [ADP]-ribose) polymerase cleavage and DNA fragmentation but did not prevent chemotherapy-induced cell death and did not inhibit, or only partially inhibited, mitochondrial release of cytochrome c, Smac, apoptosis-inducing factor (AIF), or loss of mitochondrial membrane potential. Caspase inhibition also did not protect AML blasts from chemotherapy-induced cell death in vitro. These results suggest that expression levels of Survivin or XIAP have no prognostic impact in AML patients. Although anticancer drugs induced caspase cleavage and apoptosis, cell killing was caspase independent. This may partially explain the lack of prognostic impact of XIAP and Survivin and may suggest caspase-independent mechanisms of cell death in AML.

Small-molecule XIAP inhibitors derepress downstream effector caspases and induce apoptosis of acute myeloid leukemia cells

We tested the effects of small-molecule XIAP antagonists based on a polyphenylurea pharmacophore on cultured acute myelogenous leukemia (AML) cell lines and primary patient samples. X-linked inhibitor of apoptosis protein (XIAP) antagonist N-[(5R)-6-[(anilinocarbonyl)amino]-5-((anilinocarbonyl){[(2R)-1-(4-cyclohexylbutyl)pyrrolidin-2-yl]methyl}amino)hexyl]-N-methyl-N'-phenylurea (1396-12), but not a structurally related control compound, induced apoptosis of primary leukemia samples with a lethal dose (LD50) of less than 10 microM in 16 of 27 (60%) samples. In contrast, XIAP antagonist 1396-12 was not lethal to the normal hematopoietic cells in short-term cytotoxicity assays. Response of primary AML specimens to XIAP inhibitor correlated with XIAP protein levels, with higher levels of XIAP associated with sensitivity. The XIAP antagonist 1396-12 induced activation of downstream caspases 3 and 7 prior to the activation of upstream caspase 8 and caspase 9. Apoptosis induction was also independent of B-cell lymphoma protein-2 (Bcl-2) or caspase 8, indicative of a downstream effect on apoptotic pathways. Thus, polyphenylurea-based XIAP antagonsists directly induce apoptosis of leukemia cells and AML patient samples at low micromolar concentrations through a mechanism of action distinct from conventional chemotherapeutic agents.

Synergistic induction of apoptosis by simultaneous disruption of the Bcl-2 and MEK/MAPK pathways in acute myelogenous leukemia

Recent studies suggest that the Bcl-2 and mitogen-activated protein kinase (MAPK) pathways together confer an aggressive, apoptosis-resistant phenotype on acute myelogenous leukemia (AML) cells. In this study, we analyzed the effects of simultaneous inhibition of these 2 pathways. In AML cell lines with constitutively activated MAPK, MAPK kinase (MEK) blockade by PD184352 strikingly potentiated the apoptosis induced by the small-molecule Bcl-2 inhibitor HA14-1 or by Bcl-2 antisense oligonucleotides. Isobologram analysis confirmed the synergistic nature of this interaction. Moreover, MEK blockade overcame Bcl-2 overexpression-mediated resistance to the proapoptotic effects of HA14-1. Most importantly, simultaneous exposure to PD184352 significantly (P =.01) potentiated HA14-1-mediated inhibition of clonogenic growth in all primary AML samples tested. These findings show that the Bcl-2 and MAPK pathways are relevant molecular targets in AML and that their concurrent inhibition could be developed into a new therapeutic strategy for this disease.

Regulation and targeting of antiapoptotic XIAP in acute myeloid leukemia

XIAP is a member of the inhibitors-of-apoptosis family of proteins, which inhibit caspases and block cell death, with prognostic importance in AML. Here we demonstrate that cytokines regulate the expression of XIAP in leukemic cell lines and primary AML blasts. Inhibition of phosphatidylinositol-3 kinase (PI3K) with LY294002 and of the mitogen-activated protein kinase (MAPK) cascade by PD98059 resulted in decreased XIAP levels (34+/-8.7 and 23+/-5.7%, respectively). We then generated OCI-AML3 cells with constitutively phosphorylated Akt (p473-Akt) by retroviral gene transfer. Neither these nor Akt inhibitor-treated OCI-AML3 cells showed changes in XIAP levels, suggesting that XIAP expression is regulated by PI3K downstream effectors other than Akt. The induction of XIAP expression by cytokines through PI3K/MAPK pathways is consistent with its role in cell survival. Exposure of leukemic cells to chemotherapeutic agents decreased XIAP protein levels by caspase-dependent XIAP cleavage. Targeting XIAP by XIAP antisense oligonucleotide resulted in downregulation of XIAP, activation of caspases and cell death, and sensitized HL-60 cells to Ara-C. Our results suggest that XIAP is regulated by cytokines through PI3K, and to a lesser degree through MAPK pathways. Selective downregulation of XIAP expression might be of therapeutic benefit to leukemic patients.

Regulation of survivin expression through Bcr-Abl/MAPK cascade: targeting survivin overcomes imatinib resistance and increases imatinib sensitivity in imatinib-responsive CML cells

KBM5 cells, derived from a patient with blast crisis Philadelphia chromosome-positive (Ph+) chronic myelogenous leukemia (CML), and imatinib-resistant KBM5 (KBM5-STI571) cells were found to express high levels of survivin. Inhibition of Bcr-Abl by imatinib significantly decreased survivin expression and cell viability in KBM5, but much less so in KBM5-STI571 cells. Inhibition of MEK, downstream of the Bcr-Abl signaling cascade decreased survivin expression and cell viability in both KBM5 and KBM5-STI571 cells. In addition, down-regulation of survivin by a survivin antisense oligonucleotide (Sur-AS-ODN) inhibited cell growth and induced maximal G2M block at 48 hours, whereas cell death was observed only at 72 hours in both KBM5 and KBM5-STI571 cells as shown by annexin V staining. Further, the combination of Sur-AS-ODN and imatinib induced more cell death in KBM5 cells than did either treatment alone. Down-regulating survivin also decreased colony-forming units (CFUs) in blast crisis CML patient samples. Our data therefore suggest that survivin is regulated by the Bcr-Abl/MAPK cascade in Ph+ CML. The facts that down-regulating survivin expression induced cell-growth arrest and subsequent cell death regardless of the cell response to imatinib and enhanced the sensitivity to imatinib suggest the potential therapeutic utility of this strategy in patients with CML, both imatinib sensitive and resistant.

Up-regulation of MDR1 and induction of doxorubicin resistance by histone deacetylase inhibitor depsipeptide (FK228) and ATRA in acute promyelocytic leukemia cells

The multidrug resistance 1 (MDR1) gene product P-glycoprotein (P-gp) is frequently implicated in cross-resistance of tumors to chemotherapeutic drugs. In contrast, acute promyelocytic leukemia (APL) cells do not express MDR1 and are highly sensitive to anthracyclines. The combination of ATRA and the novel histone deacetylase inhibitor (HDACI) depsipeptide (FK228) induced P-gp expression and prevented growth inhibition and apoptosis in NB4 APL cells subsequently exposed to doxorubicin (DOX). ATRA/FK228 treatment after exposure to DOX, however, enhanced apoptosis. Both agents, ATRA or FK228, induced MDR1 mRNA. This effect was significantly enhanced by ATRA/FK228 administered in combination, due in part to increased H4 and H3-Lys9 acetylation of the MDR1 promoter and recruitment of the nuclear transcription factor Y alpha (NFYA) transcription activator to the CCAAT box. Cotreatment with specific P-gp inhibitor PSC833 reversed cytoprotective effects of ATRA/FK228. G1 cell-cycle arrest and p21 mRNA induction were also observed in response to ATRA/FK228, which may restrict DOX-induced apoptosis of cells in G2 phase. These results indicate that epigenetic mechanisms involving NF-YA transcription factor recruitment and histone acetylation are activated by ATRA and HDACI, induce MDR1 in APL cells, and point to the critical importance of mechanism-based sequential therapy in future clinical trials that combine HDAC inhibitors, ATRA, and anthracyclines.

Concomitant inhibition of MDM2 and Bcl-2 protein function synergistically induce mitochondrial apoptosis in AML

Disruption of Mdm2-p53 interaction activates p53 signaling, disrupts the balance of antiapoptotic and proapoptotic Bcl-2 family proteins and induces apoptosis in acute myeloid leukemia (AML). Overexpression of Bcl-2 may inhibit this effect. Thus, functional inactivation of antiapoptotic Bcl-2 proteins may enhance apoptogenic effects of Mdm2 inhibition. We here investigate the potential therapeutic utility of combined targeting of Mdm2 by Nutlin-3a and Bcl-2 by ABT-737, recently developed inhibitors of protein-protein interactions. Nutlin-3a and ABT-737 induced Bax conformational change and mitochondrial apoptosis in AML cells in a strikingly synergistic fashion. Nutlin-3a induced p53-mediated apoptosis predominantly in S and G2/M cells, while cells in G1 were protected through induction of p21. In contrast, ABT-737 induced apoptosis predominantly in G1, the cell cycle phase with the lowest Bcl-2 protein levels and Bcl-2/Bax ratios. In addition, Bcl-2 phosphorylation on Ser70 was absent in G1 but detectable in G2/M, thus lower Bcl-2 levels and absence of Bcl-2 phosphorylation appeared to facilitate ABT-737-induced apoptosis of G1 cells. The complementary effects of Nutlin-3a and ABT-737 in different cell cycle phases could, in part, account for their synergistic activity. Our data suggest that combined targeting of Mdm2 and Bcl-2 proteins could offer considerable therapeutic promise in AML.

Blockade of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase and murine double minute synergistically induces Apoptosis in acute myeloid leukemia via BH3-only proteins Puma and Bim

Molecular aberrations of the Ras/Raf/mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK and/or Murine double minute (MDM2)/p53 signaling pathways have been reported in 80% and 50% of primary acute myeloid leukemia (AML) samples and confer poor outcome. In this study, antileukemic effects of combined MEK inhibition by AZD6244 and nongenotoxic p53 activation by MDM2 antagonist Nutlin-3a were investigated. Simultaneous blockade of MEK and MDM2 signaling by AZD6244 and Nutlin-3a triggered synergistic proapoptotic responses in AML cell lines [combination index (CI) = 0.06 +/- 0.03 and 0.43 +/- 0.03 in OCI/AML3 and MOLM13 cells, respectively] and in primary AML cells (CI = 0.52 +/- 0.01). Mechanistically, the combination upregulated levels of BH3-only proteins Puma and Bim, in part via transcriptional upregulation of the FOXO3a transcription factor. Suppression of Puma and Bim by short interfering RNA rescued OCI/AML3 cells from AZD/Nutlin-induced apoptosis. These results strongly indicate the therapeutic potential of combined MEK/MDM2 blockade in AML and implicate Puma and Bim as major regulators of AML cell survival.

Targeting Survivin expression induces cell proliferation defect and subsequent cell death involving mitochondrial pathway in myeloid leukemic cells

Survivin, a member of inhibitor of apoptosis family of proteins, plays important roles in both cell proliferation and cell death. We previously observed that Survivin is overexpressed in leukemic cell lines and blasts from patients with acute myelogenous leukemia (AML). To understand the roles of Survivin in AML and search for new approaches to the treatment of AML, we inhibited Survivin expression in HL-60 cells with a Survivin anti-sense oligonucleotide (sur-AS-ODN) (ISIS 23722). This blocked significant numbers of HL-60 cells in G2/M phase, and halted cell proliferation at 24 hrs and progressing over time. There was only a slight increase in the number of apoptotic cells at 24 hrs compared with cells treated with nonsense oligonucleotide (NS-ODN). At 48 hrs, however, there were significant increases in sub-G1 phase and annexin V+ cells, suggesting that cell division defects caused cell death. This was supported by the finding that a reduction in the Survivin protein by sur-AS-ODN in cells under serum-free medium did not induce G2/M block and cell death compared to cells treated with NS-ODN. The formation of polyploid cells was observed 48 hrs after sur-AS-ODN treatment, as was the activation of caspase 3, which suggested that apoptotic cell death had occurred. The mitochondrial release of cytochrome C and Smac and the nuclear translocation of the apoptosis-inducing factor were also detected. Our results suggest that Survivin is essential for cell cycle progression in leukemic cells. Reduced Survivin expression causes a cell-cycle defect that leads to cell death through a mitochondrial pathway. This finding has potential utility for therapy of patients with AML.

Inhibition of KSP by ARRY-520 induces cell cycle block and cell death via the mitochondrial pathway in AML cells

Kinesin spindle protein (KSP), a microtubule-associated motor protein essential for cell cycle progression, is overexpressed in many cancers and is a potential anti-tumor target. We found that inhibition of KSP by a selective inhibitor, ARRY-520, blocked cell cycle progression, leading to apoptosis in acute myeloid leukemia cell lines that express high levels of KSP. Knockdown of p53, overexpression of XIAP and mutation in caspase-8 did not significantly affect sensitivity to ARRY-520, suggesting that the response is independent of p53, XIAP and the extrinsic apoptotic pathway. Although ARRY-520 induced mitotic arrest in both HL-60 and Bcl-2-overexpressing HL-60Bcl-2 cells, cell death was blunted in HL-60Bcl-2 cells, suggesting that the apoptotic program is executed through the mitochondrial pathway. Accordingly, inhibition of Bcl-2 by ABT-737 was synergistic with ARRY-520 in HL-60Bcl-2 cells. Furthermore, ARRY-520 increased Bim protein levels prior to caspase activation in HL-60 cells. ARRY-520 significantly inhibited tumor growth of xenografts in SCID mice and inhibited AML blast but not normal colony formation, supporting a critical role for KSP in proliferation of leukemic progenitor cells. These results demonstrate that ARRY-520 potently induces cell cycle block and subsequent death in leukemic cells via the mitochondrial pathway and has the potential to eradicate AML progenitor cells.

Regulation and targeting of Eg5, a mitotic motor protein in blast crisis CML: overcoming imatinib resistance

Patients with blast crisis (BC) CML frequently become resistant to Imatinib, a Bcr-Abl tyrosine kinase-targeting agent. Eg5, a microtubule-associated motor protein has been described to be highly expressed in BC CML by microarray analysis (Nowicki et al., Oncogene 2003; 22:3952-63). We investigated the regulation of Eg5 by Bcr-Abl tyrosine kinase and its potential as a therapeutic target in BC CML. Eg5 was highly expressed in all Philadelphia chromosome positive (Ph(+)) cell lines and BC CML patient samples. Inhibition of Bcr-Abl by Imatinib downregulated Eg5 expression in Imatinib-sensitive KBM5 and HL-60p185 cells, but not in Imatinib-resistant KBM5-STI571, harboring a T315I mutation, and Bcr-Abl-negative HL-60 cells. Blocking Eg5 expression with antisense oligonucleotide (Eg5-ASO) or inhibiting its activity with the small-molecule Eg5 inhibitor, S-trityl-L-cysteine induced G(2)/M cell cycle block and subsequent cell death in both Imatinib-sensitive and -resistant cells. Further, Eg5-ASO treatment of SCID mice harboring KBM5 cell xenografts significantly prolonged the median survival of the animals (p = 0.03). Our findings suggest that Eg5 is downstream of and regulated by Bcr-Abl tyrosine kinase in Philadelphia chromosome positive cells. Inhibition of Eg5 expression or its activity blocks cell cycle progression and induces cell death independent of the cellular response to Imatinib. Therefore, Eg5 could be a potential therapeutic target for the treatment of BC CML, in particular Imatinib-resistant BC CML.

Evaluation of apoptosis induction by concomitant inhibition of MEK, mTOR, and Bcl-2 in human acute myelogenous leukemia cells

Aberrant activation of multiple signaling pathways is common in acute myelogenous leukemia (AML) cells, which can be linked to a poor prognosis for patients with this disease. Previous research with mTOR or MEK inhibitors revealed cytostatic, rather than cytotoxic, effects in in vitro and in vivo AML models. We evaluated the combination effect of the mTOR inhibitor AZD8055 and the MEK inhibitor selumetinib on human AML cell lines and primary AML samples. This combination demonstrated synergistic proapoptotic effects in AML cells with high basal activation of MEK and mTOR. We next incorporated the BH3 mimetic ABT-737 into this combination regimen to block Bcl-2, which further enhanced the apoptogenic effect of MEK/mTOR inhibition. The combination treatment also had a striking proapoptotic effect in CD33(+)/CD34(+) AML progenitor cells from primary AML samples with NRAS mutations. Mechanistically, upregulation of the proapoptotic protein Bim, accompanied by the downregulation of the antiapoptotic protein Mcl-1 (mainly via protein degradation), seemed to play critical roles in enhancing the combination drug effect. Furthermore, the modulation of survivin, Bax, Puma, and X-chromosome-linked inhibitor of apoptosis protein (XIAP) expression suggested a role for mitochondria-mediated apoptosis in the cytotoxicity of the drug combination. Consequently, the concomitant blockade of prosurvival MEK/mTOR signaling and the deactivation of Bcl-2 could provide a mechanism-based integrated therapeutic strategy for the eradication of AML cells.

Triptolide induces cell death independent of cellular responses to imatinib in blast crisis chronic myelogenous leukemia cells including quiescent CD34+ primitive progenitor cells

The advent of Bcr-Abl tyrosine kinase inhibitors (TKI) has revolutionized the treatment of chronic myelogenous leukemia (CML). However, resistance evolves due to BCR-ABL mutations and other mechanisms. Furthermore, patients with blast crisis CML are less responsive and quiescent CML stem cells are insensitive to these inhibitors. We found that triptolide, a diterpenoid, at nanomolar concentrations, promoted equally significant death of KBM5 cells, a cell line derived from a Bcr-Abl-bearing blast crisis CML patient and KBM5STI571 cells, an imatinib-resistant KBM5 subline bearing the T315I mutation. Similarly, Ba/F3 cells harboring mutated BCR-ABL were as sensitive as Ba/F3Bcr-Abl(p210wt) cells to triptolide. Importantly, triptolide induced apoptosis in primary samples from blast crisis CML patients, who showed resistance to Bcr-Abl TKIs in vivo, with less toxicity to normal cells. Triptolide decreased X-linked inhibitor of apoptosis protein, Mcl-1, and Bcr-Abl protein levels in K562, KBM5, and KBM5STI571 cells and in cells from blast crisis CML patients. It sensitized KBM5, but not KBM5STI571, cells to imatinib. More importantly, triptolide also induced death of quiescent CD34(+) CML progenitor cells, a major problem in the therapy of CML with TKIs. Collectively, these results suggest that triptolide potently induces blast crisis CML cell death independent of the cellular responses to Bcr-Abl TKIs, suggesting that triptolide could eradicate residual quiescent CML progenitor cells in TKI-treated patients and benefit TKI-resistant blast crisis CML patients.

Peroxisome proliferator-activated receptor γ and retinoid X receptor ligands are potent inducers of differentiation and apoptosis in leukemias

The peroxisome proliferator-activated receptor γ (PPARγ) is a member of the nuclear receptor family that forms heterodimers with retinoid X receptor. These heterodimers bind to DNA and activate the transcription of target genes. Here, we report that the PPARγ receptor protein is expressed in primary myeloid and lymphoid leukemias and in lymphoma and myeloma cell lines. In this study, we compared the activity of several PPARγ ligands including BRL49653 (rosiglitazone), 15-deoxy-Δ12,14-prostaglandin J2, and the novel triterpenoid 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid on leukemia cells. Exposure to these PPARγ ligands induced apoptosis in myeloid (U937 and HL-60) and lymphoid (Su-DHL, Sup-M2, Ramos, Raji, Hodgkin's cell lines, and primary chronic lymphocytic leukemia) cells. A similar exposure to these PPARγ ligands induced the differentiation of myeloid leukemic cells. A combination of PPARγ ligands with a retinoid X receptor agonist (i.e., LG100268) or a retinoic acid receptor agonist (i.e., all trans-retinoic acid) enhanced differentiating and growth-inhibitory effects. 2-Cyano-3,12-dioxooleana-1,9-dien-28-oic acid induced differentiation and apoptosis with much greater potency than the other PPARγ ligands in established cell lines and primary chronic lymphocytic leukemia samples. Exposure to 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid induced mitochondrial depolarization and caspase activation, which was associated with apoptosis induction. In Bcl-2-overexpressing chronic lymphocytic leukemia cells, the small-molecule Bcl-2 inhibitor HA14-1 sensitized these cells to 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid–induced apoptosis. These results suggest that PPARγ ligation alone and in combination with retinoids holds promise as novel therapy for leukemias by activating the transcriptional activity of target genes that control apoptosis and differentiation in leukemias.

Targeting of BMI-1 expression by the novel small molecule PTC596 in mantle cell lymphoma

Despite the development of the novel Bruton tyrosine kinase inhibitor ibrutinib, mantle cell lymphoma (MCL) remains an incurable B-cell non-Hodgkin lymphoma. BMI-1 is required for the self-renewal and maintenance of MCL-initiating stem cells. Upregulation of BMI-1 has been reported in MCL patients, especially in those with refractory/relapsed disease. We studied the effects of a novel small-molecule selective inhibitor of BMI1 expression, PTC596, in MCL cells. Eight MCL cell lines and patient-derived samples were exposed to PTC596. PTC596 induced mitochondrial apoptosis, as evidenced by loss of mitochondrial membrane potential, caspase-3 cleavage, BAX activation, and phosphatidylserine externalization. There was a positive correlation between baseline BMI-1 protein levels and PTC596-induced apoptosis. p53 status did not affect sensitivity to PTC596. PTC596 effectively decreased BMI-1-expressing and tumor-initiating side population MCL cells (IC₅₀: 138 nM) compared with ibrutinib, which modestly decreased side population cells. Interestingly, PTC596, reported to target cancer stem cells, decreased MCL-1 expression levels and antagonized ibrutinib-induced increase in MCL-1 expression, leading to synergistic apoptosis induction in MCL cells. There are currently no drugs that specifically target cancer stem cell fractions, and a reduction in BMI-1 protein by PTC596 may offer a novel therapeutic strategy for MCL.

Tetra-O-methyl nordihydroguaiaretic acid inhibits growth and induces death of leukemia cells independent of Cdc2 and survivin

Tetra-O-methyl nordihydroguaiaretic acid (M4N) was shown to induce G2 arrest and suppress human xenograft tumor growth by inhibiting Cdc2 and survivin. We examined the effect of M4N on leukemia and found that M4N inhibited growth and induced cell death in leukemic cell lines and blasts from AML patients. However, no significant changes in Cdc2 and survivin levels and G2 arrest were observed. Cell death and growth inhibition were dependent neither on XIAP, Bcl-2, and Bcl-X(L) levels nor on caspase-8. M4N did not promote cell differentiation in HL-60 cells. Interestingly, significant inhibition of AKT phosphorylation was observed in M4N treated OCI-AML3 cells. Collectively, our data showed that M4N inhibited cell growth and induced cell death in both leukemic cell lines and AML patient sample via a mechanism not mediated by Cdc2 and survivin inhibition and suggested that the extrinsic and the mitochondrial apoptotic pathways are not essential.

Optimized fixation and storage conditions for FISH analysis of single-cell suspensions

In our protocol to isolate and identify fetal cells in maternal peripheral blood, antibody (Ab)-stained cells are preserved with paraformaldehyde (PF) before batch flow cytometric sorting. However, PF fixation compromises the quality of subsequent interphase fluorescence in situ hybridization (FISH). We therefore examined the effect of PF concentrations and storage time in phosphate-buffered saline (PBS) on the quality of FISH signals. Cells were analyzed for changes in light scatter, morphology, and accessibility of target cell DNA. Fixation in 3% PF for 1 hr was ideal for both flow cytometry and subsequent FISH detection. However, beyond 10 days of storage, FISH quality deteriorated. (J Histochem Cytochem 46:971-973, 1998)

Abstract 4264: NFκB activation in mesenchymal stromal cells induced by leukemia-stroma interaction plays a central role in stroma-mediated chemo-resistance of leukemic cells

Within the bone marrow (BM) microenvironment, BM mesenchymal stromal cells (BM-MSC) produce cytokines and chemokines and initiate cellular adhesion-mediated signals that tightly regulate normal and malignant hematopoietic cell development. This rich environment serves as a sanctuary for malignant hematopoietic cells, offering protection from chemotherapeutic agents and promoting residual disease with greater potential for developing acquired drug resistance. In this study we utilized gene expression profiling (GEP) as a genome-wide exploratory approach to characterize changes caused by the interaction between leukemic cells and BM-MSC, and how these changes can contribute to the observed microenvironment-mediated chemoresistance. Co-culture of REH, a pre-B-ALL cell line, with normal-donor BM-MSCs for 48 hours upregulated a variety of genes encoding cytokines and chemokines in BM-MSCs such as CXCR6, CCL5, CCL2, IL32, IL34, IL8, IL6, CXCL12, and the VLA4 ligand VCAM-1. These microarray-based GEP findings were confirmed by qRT-PCR. Gene Set Enrichment Analysis (GSEA) implicated activation of NFκB in BM-MSCs as a potential cause of these changes. Similar upregulation of NFκB target genes was found in MSC co-cultured with the AML cell line OCI-AML3, suggesting that NFκB activation is a common consequence of leukemia-stroma interation. Given known paracrine functions of NFκB-regulated target genes such as IL-8, we hypothesized that stromal NFκB activation contributes to stroma-mediated chemo-resistance of leukemic cells. Stable transduction of BM-MSC to overexpress a super-repressor form of IαBβ (IαBβ-SR), to block canonical-pathway NFκB activation in the stromal compartment, significantly reduced the stroma-mediated chemo-resistance to Vincristine (VCR) of REH cells. NFκB inhibition using a specific IKKα inhibitor (MLN120B), affecting both BM-MSC and tumor cells, highly increased the apoptotic effects in leukemia-stroma co-cultures of VCR in REH cells and NALM6, another pre-B-ALL cell line. Similar results were obtained with CDDO-Me, another NFκB inhibitor. Altogether these results indicate that the interaction of leukemic cells with BM stromal cells activates NFκB in the stromal compartment, with subsequent transcriptional upregulation of a number of cytokines and chemokines ultimately favoring the survival of leukemic cells. We propose that targeting NFκB may ameliorate stroma-mediated chemo-resistance and help in eliminating bone-marrow resident leukemic cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4264. doi:1538-7445.AM2012-4264

Abstract 4536: Simultaneous blockade of MEK and mTOR kinase signaling synergizes pro-apoptotic effects in AML cells, which is further potential by interfering with dimerization of BH3 domains

Simultaneous activation of PI3K/Akt/mTOR and Raf/MEK/ERK signaling pathways is common in acute myeloid leukemia (AML) and is associated with worse prognosis(1). Several small molecule inhibitors targeting mTOR or MEK/ERK signaling are being evaluated in clinical trials in AML, including allosteric mTOR inhibitors (CCI-779, RAD001) and MEK inhibitors (CI-1040, AZD6244 [ARRY-142886]), and demonstrated encouraging pre-clinical results(2-4). However, the efficacy of these agents as single agents is variable in AML(3, 5, 6). The predominant effects of these inhibitors are cytostatic rather than cytotoxic in AML. AZD8055 is a novel mTOR kinase inhibitor, acting against both mTORC1 and mTORC2, and is currently in Phase 1 clinical trials in solid tumors.

In the present study, we investigated the anti-leukemia efficacy of the AZD8055, alone or in combination with AZD6244 in human leukemic cells and in primary AML samples. AZD8055 potently suppressed p-S6K and p-4E-BP1 in AML cells (70-85% inhibition), significantly blocked p-AKT(473) in OCI-AML3 and MOLM13 cells, and inhibited both, p-AKT(473) and p-AKT(308) in PTEN-mutant U937 cells. This resulted in profound inhibition of cell growth at lower IC50s (40-147nM) in all these AML cells. However, AZD8055 did not induce apoptosis in the cells even up to 10μM concentrations. The combination of AZD8055 with AZD6244 triggered synergistic pro-apoptotic responses in U937 (CI = 0.22 ± 0.04) and MOLM13 (CI = 0.66 ± 0.06) cells, but not in OCI/AML3 and KG-1 cells. MEK inhibition resulted in partial blockade of S6K phosphorylation in OCI/AML3 and KG-1 cells, suggesting that S6K activity can also be regulated by MAPK pathways in some AML cell lines. Further analysis demonstrated that sensitive cells (U937 and MOLM13) express lower basal levels of Bcl-2 protein compared to resistant cell lines (OCI/AML3 and KG-1), and showed pronounced upregulation of pro-apoptotic Bim after AZD6244 and AZD8055/AZD6244 combination treatment. Furthermore, interference with dimerization of Bcl2 family proteins by BH3 mimetic ABT-737 dramatically enhanced the pro-apoptotic responses of the combination in resistant cells even when all three agents are used at sub-micromolar concentrations. Depleting Bim levels by siRNA rescued sensitive cells from AZD8055/AZD6244-induced apoptosis, indicating a key role of downstream Bim in cytotoxicity of these agents. In primary AML samples AZD8055/AZD6244 induced apoptosis in an additive fashion, while simultaneous use of ABT-737 significantly increased the pro-apoptotic effects.

In summary, our results indicate that parallel blockade of PI3K/MAPK signaling and manipulation of the mitochondrial Bcl-2 rheostat potently enhances apoptosis induction in AML cells and may provide a viable therapeutic strategy for resistant/refractory AML patients.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4536.

Peroxisome proliferator-activated receptor gamma and retinoid X receptor ligands are potent inducers of differentiation and apoptosis in leukemias

The peroxisome proliferator-activated receptor gamma (PPAR gamma) is a member of the nuclear receptor family that forms heterodimers with retinoid X receptor. These heterodimers bind to DNA and activate the transcription of target genes. Here, we report that the PPAR gamma receptor protein is expressed in primary myeloid and lymphoid leukemias and in lymphoma and myeloma cell lines. In this study, we compared the activity of several PPAR gamma ligands including BRL49653 (rosiglitazone), 15-deoxy-Delta 12,14-prostaglandin J(2), and the novel triterpenoid 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid on leukemia cells. Exposure to these PPAR gamma ligands induced apoptosis in myeloid (U937 and HL-60) and lymphoid (Su-DHL, Sup-M2, Ramos, Raji, Hodgkin's cell lines, and primary chronic lymphocytic leukemia) cells. A similar exposure to these PPAR gamma ligands induced the differentiation of myeloid leukemic cells. A combination of PPAR gamma ligands with a retinoid X receptor agonist (i.e., LG100268) or a retinoic acid receptor agonist (i.e., all trans-retinoic acid) enhanced differentiating and growth-inhibitory effects. 2-Cyano-3,12-dioxooleana-1,9-dien-28-oic acid induced differentiation and apoptosis with much greater potency than the other PPAR gamma ligands in established cell lines and primary chronic lymphocytic leukemia samples. Exposure to 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid induced mitochondrial depolarization and caspase activation, which was associated with apoptosis induction. In Bcl-2-overexpressing chronic lymphocytic leukemia cells, the small-molecule Bcl-2 inhibitor HA14-1 sensitized these cells to 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid-induced apoptosis. These results suggest that PPAR gamma ligation alone and in combination with retinoids holds promise as novel therapy for leukemias by activating the transcriptional activity of target genes that control apoptosis and differentiation in leukemias.

Targeting Survivin expression induces cell proliferation defect and subsequent cell death involving mitochondrial pathway in myeloid leukemic cells

Survivin, a member of inhibitor of apoptosis family of proteins, plays important roles in both cell proliferation and cell death. We previously observed that Survivin is overexpressed in leukemic cell lines and blasts from patients with acute myelogenous leukemia (AML). To understand the roles of Survivin in AML and search for new approaches to the treatment of AML, we inhibited Survivin expression in HL-60 cells with a Survivin anti-sense oligonucleotide (sur-AS-ODN) (ISIS 23722). This blocked significant numbers of HL-60 cells in G2/M phase, and halted cell proliferation at 24 hrs and progressing over time. There was only a slight increase in the number of apoptotic cells at 24 hrs compared with cells treated with nonsense oligonucleotide (NS-ODN). At 48 hrs, however, there were significant increases in sub-G1 phase and annexin V+ cells, suggesting that cell division defects caused cell death. This was supported by the finding that a reduction in the Survivin protein by sur-AS-ODN in cells under serum-free medium did not induce G2/M block and cell death compared to cells treated with NS-ODN. The formation of polyploid cells was observed 48 hrs after sur-AS-ODN treatment, as was the activation of caspase 3, which suggested that apoptotic cell death had occurred. The mitochondrial release of cytochrome C and Smac and the nuclear translocation of the apoptosis-inducing factor were also detected. Our results suggest that Survivin is essential for cell cycle progression in leukemic cells. Reduced Survivin expression causes a cell-cycle defect that leads to cell death through a mitochondrial pathway. This finding has potential utility for therapy of patients with AML.