Mutations in the p53 gene in acute myeloid leukemia patients correlate with poor prognosis

Outcomes in patients with newly diagnosed TP53-mutated acute myeloid leukemia with or without venetoclax-based therapy

BACKGROUND

Venetoclax (VEN) in combination with a hypomethylating agent (HMA) has become the standard of care for patients aged >75 years and for those not eligible for intensive chemotherapy who have newly diagnosed acute myeloid leukemia (AML). The benefit of VEN-based therapy in patients who have newly diagnosed AML with mutations in the TP53 gene (TP53^mut ) over standard therapy is undefined.

METHODS

In this single-institutional, retrospective analysis, the authors assessed the clinical outcomes of 238 patients with newly diagnosed TP53^mut AML and compared the clinical characteristics, response to different therapies, and outcomes of those who received VEN-based (n = 58) and non-VEN-based (n = 180) regimens.

RESULTS

Patients who received VEN-based regimens were older (aged >65 years: 81% vs 65%; P = .02) and had higher response rates (complete remission, 43% vs 32%; P = .06) than those who received non-VEN-based regimens. Compared with patients who received non-VEN-based regimens, no difference in overall survival (median, 6.6 vs 5.7 months; P = .4) or relapse-free survival (median, 4.7 vs 3.5 months; P = .43) was observed in those who received VEN-based regimens, regardless of age or intensity of treatment.

CONCLUSIONS

The addition of VEN to standard treatment regimens did not improve outcomes in younger or older patients who had TP53^mut AML. These data highlight the need for novel therapies beyond VEN to improve the outcome of patients with TP53^mut AML.

Potential Therapeutic Targets of Quercetin, a Plant Flavonol, and Its Role in the Therapy of Various Types of Cancer through the Modulation of Various Cell Signaling Pathways

Polyphenolic flavonoids are considered natural, non-toxic chemopreventers, which are most commonly derived from plants, fruits, and vegetables. Most of these polyphenolics exhibit remarkable antioxidant, anti-inflammatory, and anticancer properties. Quercetin (Qu) is a chief representative of these polyphenolic compounds, which exhibits excellent antioxidant and anticancer potential, and has attracted the attention of researchers working in the area of cancer biology. Qu can regulate numerous tumor-related activities, such as oxidative stress, angiogenesis, cell cycle, tumor necrosis factor, proliferation, apoptosis, and metastasis. The anticancer properties of Qu mainly occur through the modulation of vascular endothelial growth factor (VEGF), apoptosis, phosphatidyl inositol-3-kinase (P13K)/Akt (proteinase-kinase B)/mTOR (mammalian target of rapamycin), MAPK (mitogen activated protein kinase)/ERK1/2 (extracellular signal-regulated kinase 1/2), and Wnt/β-catenin signaling pathways. The anticancer potential of Qu is documented in numerous in vivo and in vitro studies, involving several animal models and cell lines. Remarkably, this phytochemical possesses toxic activities against cancerous cells only, with limited toxic effects on normal cells. In this review, we present extensive research investigations aimed to discuss the therapeutic potential of Qu in the management of different types of cancers. The anticancer potential of Qu is specifically discussed by focusing its ability to target specific molecular signaling, such as p53, epidermal growth factor receptor (EGFR), VEGF, signal transducer and activator of transcription (STAT), PI3K/Akt, and nuclear factor kappa B (NF-κB) pathways. The anticancer potential of Qu has gained remarkable interest, but the exact mechanism of its action remains unclear. However, this natural compound has great pharmacological potential; it is now believed to be a complementary—or alternative—medicine for the prevention and treatment of different cancers.

Murine double minute 2 inhibition alone or with cytarabine in acute myeloid leukemia: Results from an idasanutlin phase 1/1b study⋆

The prognosis remains poor for patients with relapsed or refractory (r/r) acute myeloid leukemia; thus, novel therapies are needed. We evaluated idasanutlin-a new, potent murine double minute 2 antagonist-alone or with cytarabine in patients with r/r acute myeloid leukemia, de novo untreated acute myeloid leukemia unsuitable for standard treatment or with adverse features, or secondary acute myeloid leukemia in a multicenter, open-label, phase 1/1b trial. Primary objectives were to determine the maximum tolerated dose (MTD) and recommended dose for expansion (RDE) and characterize the safety profile of idasanutlin monotherapy and combination therapy. Clinical activity and pharmacokinetics were secondary objectives. Two idasanutlin formulations were investigated: a microprecipitate bulk powder (MBP) and optimized spray-dried powder (SDP). Following dose escalation, patients (N = 122) received idasanutlin at the RDE in the extension cohorts. No formal MTD was identified. Idasanutlin was tolerable alone and in combination with cytarabine. The RDE was determined as 600 mg twice a day for the MBP formulation and 300 mg twice a day for the SDP formulation. Adverse events were mostly grade 1/2 (76.2 %). The most common any-grade adverse events were gastrointestinal (including diarrhea [90.2 %]). The early death rate across all patients was 14.8 %. Plasma idasanutlin exposure was dose related. In TP53 wild-type patients, composite complete remission rates were 18.9 % with monotherapy and 35.6 % with combination therapy. Based on these results, idasanutlin development continued with further investigation in the treatment of acute myeloid leukemia. ClinicalTrials.gov: NCT01773408.

MIRROS: a randomized, placebo-controlled, Phase III trial of cytarabine ± idasanutlin in relapsed or refractory acute myeloid leukemia

Patients with refractory or relapsed acute myeloid leukemia (R/R AML) have a poor prognosis, with a high unmet medical need. Idasanutlin is a small-molecule inhibitor of MDM2, a negative regulator of tumor suppressor p53. By preventing the p53–MDM2 interaction, idasanutlin allows for p53 activation, particularly in patients with TP53 wild-type (WT) status. MIRROS (NCT02545283) is a randomized Phase III trial evaluating idasanutlin + cytarabine versus placebo + cytarabine in R/R AML. The primary end point is overall survival in the TP53-WT population. Secondary end points include complete remission rate (cycle 1), overall remission rate (cycle 1) and event-free survival in the TP53-WT population. MIRROS has an innovative design that integrates a stringent interim analysis for futility; continuation criteria were met in mid-2017 and accrual is ongoing.

Trial registration number: NCT02545283

PARI (PARPBP) suppresses replication stress-induced myeloid differentiation in leukemia cells

Hyperproliferative cancer cells face increased replication stress, which can result in accumulation of DNA damage. As DNA damage can arrest proliferation, and, in the case of myeloid leukemia, induce differentiation of cancer cells, understanding the mechanisms that regulate the replication stress response is paramount. Here, we show that PARI, a replisome protein involved in regulating DNA repair and replication stress, suppresses differentiation of myeloid leukemia cells. We show that PARI is overexpressed in myeloid leukemia cells, and its knockdown reduces leukemia cell proliferation in vitro and in vivo in xenograft mouse models. PARI depletion enhances replication stress and DNA-damage accumulation, coupled with increased myeloid differentiation. Mechanistically, we show that PARI inhibits activation of the NF-κB pathway, which can initiate p21-mediated differentiation and proliferation arrest. Finally, we show that PARI expression negatively correlates with expression of differentiation markers in clinical myeloid leukemia samples, suggesting that targeting PARI may restore differentiation ability of leukemia cells and antagonize their proliferation.

NFκB regulates p21 expression and controls DNA damage-induced leukemic differentiation

DNA damage exposure is a major modifier of cell fate in both normal and cancer tissues. In response to DNA damage, myeloid leukemia cells activate a poorly understood terminal differentiation process. Here, we show that the NFκB pathway directly activates expression of the proliferation inhibitor p21 in response to DNA damage in myeloid leukemia cells. In order to understand the role of this unexpected regulatory event, we ablated the NFκB binding site we identified in the p21 promoter, using CRISPR/Cas9-mediated genome editing. We found that NFκB-mediated p21 activation controls DNA damage-induced myeloid differentiation. Our results uncover a p53-independent pathway for p21 activation involved in controlling hematopoietic cell fate.

Bridge-Induced Translocation between NUP145 and TOP2 Yeast Genes Models the Genetic Fusion between the Human Orthologs Associated With Acute Myeloid Leukemia

In mammalian organisms liquid tumors such as acute myeloid leukemia (AML) are related to spontaneous chromosomal translocations ensuing in gene fusions. We previously developed a system named bridge-induced translocation (BIT) that allows linking together two different chromosomes exploiting the strong endogenous homologous recombination system of the yeast Saccharomyces cerevisiae. The BIT system generates a heterogeneous population of cells with different aneuploidies and severe aberrant phenotypes reminiscent of a cancerogenic transformation. In this work, thanks to a complex pop-out methodology of the marker used for the selection of translocants, we succeeded by BIT technology to precisely reproduce in yeast the peculiar chromosome translocation that has been associated with AML, characterized by the fusion between the human genes NUP98 and TOP2B. To shed light on the origin of the DNA fragility within NUP98, an extensive analysis of the curvature, bending, thermostability, and B-Z transition aptitude of the breakpoint region of NUP98 and of its yeast ortholog NUP145 has been performed. On this basis, a DNA cassette carrying homologous tails to the two genes was amplified by PCR and allowed the targeted fusion between NUP145 and TOP2, leading to reproduce the chimeric transcript in a diploid strain of S. cerevisiae. The resulting translocated yeast obtained through BIT appears characterized by abnormal spherical bodies of nearly 500 nm of diameter, absence of external membrane and defined cytoplasmic localization. Since Nup98 is a well-known regulator of the post-transcriptional modification of P53 target genes, and P53 mutations are occasionally reported in AML, this translocant yeast strain can be used as a model to test the constitutive expression of human P53. Although the abnormal phenotype of the translocant yeast was never rescued by its expression, an exogenous P53 was recognized to confer increased vitality to the translocants, in spite of its usual and well-documented toxicity to wild-type yeast strains. These results obtained in yeast could provide new grounds for the interpretation of past observations made in leukemic patients indicating a possible involvement of P53 in cell transformation toward AML.

Selective activation of TNFR1 and NF-κB inhibition by a novel biyouyanagin analogue promotes apoptosis in acute leukemia cells

BACKGROUND

Acquired resistance towards apoptosis is a hallmark of cancer. Elimination of cells bearing activated oncogenes or stimulation of tumor suppressor mediators may provide a selection pressure to overcome resistance. KC-53 is a novel biyouyanagin analogue known to elicit strong anti-inflammatory and anti-viral activity. The current study was designed to evaluate the anticancer efficacy and molecular mechanisms of KC-53 against human cancer cells.

METHODS

Using the MTT assay we examined initially how KC-53 affects the proliferation rates of thirteen representative human cancer cell lines in comparison to normal peripheral blood mononuclear cells (PBMCs) and immortalized cell lines. To decipher the key molecular events underlying its mode of action we selected the human promyelocytic leukemia HL-60 and the acute lymphocytic leukemia CCRF/CEM cell lines that were found to be the most sensitive to the antiproliferative effects of KC-53.

RESULTS

KC-53 promoted rapidly and irreversibly apoptosis in both leukemia cell lines at relatively low concentrations. Apoptosis was characterized by an increase in membrane-associated TNFR1, activation of Caspase-8 and proteolytic inactivation of the death domain kinase RIP1 indicating that KC-53 induced mainly the extrinsic/death receptor apoptotic pathway. Regardless, induction of the intrinsic/mitochondrial pathway was also achieved by Caspase-8 processing of Bid, activation of Caspase-9 and increased translocation of AIF to the nucleus. FADD protein knockdown restored HL-60 and CCRF/CEM cell viability and completely blocked KC-53-induced apoptosis. Furthermore, KC-53 administration dramatically inhibited TNFα-induced serine phosphorylation on TRAF2 and on IκBα hindering therefore p65/NF-κΒ translocation to nucleus. Reduced transcriptional expression of pro-inflammatory and pro-survival p65 target genes, confirmed that the agent functionally inhibited the transcriptional activity of p65.

CONCLUSIONS

Our findings demonstrate, for the first time, the selective anticancer properties of KC-53 towards leukemic cell lines and provide a detailed understanding of the molecular events underlying its dual anti-proliferative and pro-apoptotic properties. These results provide new insights into the development of innovative and targeted therapies for the treatment of some forms of leukemia.

Rational Combinations of Targeted Agents in AML

Despite modest improvements in survival over the last several decades, the treatment of AML continues to present a formidable challenge. Most patients are elderly, and these individuals, as well as those with secondary, therapy-related, or relapsed/refractory AML, are particularly difficult to treat, owing to both aggressive disease biology and the high toxicity of current chemotherapeutic regimens. It has become increasingly apparent in recent years that coordinated interruption of cooperative survival signaling pathways in malignant cells is necessary for optimal therapeutic results. The modest efficacy of monotherapy with both cytotoxic and targeted agents in AML testifies to this. As the complex biology of AML continues to be elucidated, many “synthetic lethal” strategies involving rational combinations of targeted agents have been developed. Unfortunately, relatively few of these have been tested clinically, although there is growing interest in this area. In this article, the preclinical and, where available, clinical data on some of the most promising rational combinations of targeted agents in AML are summarized. While new molecules should continue to be combined with conventional genotoxic drugs of proven efficacy, there is perhaps a need to rethink traditional philosophies of clinical trial development and regulatory approval with a focus on mechanism-based, synergistic strategies.

Prognostic significance of TP53 mutations and single nucleotide polymorphisms in acute myeloid leukemia: a case series and literature review

BACKGROUND

The response to treatment and overall survival (OS) of patients with acute myeloid leukemia (AML) is variable, with a median ranging from 6 months to 11.5 years. TP53 is associated with old age, chemotherapy resistance, and worse OS. Using genetic sequencing, we set out to look at our own experience with AML, and hypothesized that both TP53 mutations and SNPs at codon 72 would mimic the literature by occurring in a minority of patients, and conferring a worse OS.

MATERIALS AND METHODS

We performed a pilot study of randomly selected, newly diagnosed AML patients at Mount Sinai Medical Center, diagnosed from 2005-2008 (n=10). TP53 PCR sequencing was performed using DNA from bone marrow smears. Analysis was accomplished using Mutation Surveyor software with confirmation of the variants using the COSMIC and dbSNP databases.

RESULTS

Fewer than half of the patients harbored TP53 mutations (40%). There was no significant difference in OS based on gender, AML history, risk-stratified karyotype, or TP53 mutation. There were possible trends toward improved survival among patients less than 60 (11 vs 4 months, p=0.09), Hispanics (8 vs 1 months, p=0.11), and those not harboring SNP P72R (8 vs 2 months, p=0.10). There was a significant improvement in survival among patients with better performance status (28 vs 4 months, p=0.01) and those who did not have a complex karyotype (8 vs 1 months, p=0.03). The most commonly observed TP53 mutation was a missense N310K (40%) and the most commonly observed SNP was P72R (100.0%).

CONCLUSIONS

Our study confirms previous reports that poor PS and the presence of a complex karyotype are associated with a decreased OS. In our cohort, TP53 mutations were relatively common, occurring more frequently in male patients with an adverse karyotype. Although there was no significant difference in survival between TP53 mutated and un-mutated patients, there was a possible trend toward worse OS among patients with SNP P72R. Larger studies are needed to validate these findings.

Multitarget effects of quercetin in leukemia

This study proposes to investigate quercetin antitumor efficacy in vitro and in vivo, using the P39 cell line as a model. The experimental design comprised leukemic cells or xenografts of P39 cells, treated in vitro or in vivo, respectively, with quercetin; apoptosis, cell-cycle and autophagy activation were then evaluated. Quercetin caused pronounced apoptosis in P39 leukemia cells, followed by Bcl-2, Bcl-xL, Mcl-1 downregulation, Bax upregulation, and mitochondrial translocation, triggering cytochrome c release and caspases activation. Quercetin also induced the expression of FasL protein. Furthermore, our results demonstrated an antioxidant activity of quercetin. Quercetin treatment resulted in an increased cell arrest in G1 phase of the cell cycle, with pronounced decrease in CDK2, CDK6, cyclin D, cyclin E, and cyclin A proteins, decreased Rb phosphorylation and increased p21 and p27 expression. Quercetin induced autophagosome formation in the P39 cell line. Autophagy inhibition induced by quercetin with chloroquine triggered apoptosis but did not alter quercetin modulation in the G1 phase. P39 cell treatment with a combination of quercetin and selective inhibitors of ERK1/2 and/or JNK (PD184352 or SP600125, respectively), significantly decreased cells in G1 phase, this treatment, however, did not change the apoptotic cell number. Furthermore, in vivo administration of quercetin significantly reduced tumor volume in P39 xenografts and confirmed in vitro results regarding apoptosis, autophagy, and cell-cycle arrest. The antitumor activity of quercetin both in vitro and in vivo revealed in this study, point to quercetin as an attractive antitumor agent for hematologic malignancies.

Clinical impact of SNP of P53 genes pathway on the adult AML patients

INTRODUCTION

Acute myeloid leukemia (AML) is a highly heterogeneous disease, with biologically and prognostically different subtypes.

AIM

To study the impact of p53, p21, and mdm2 gene polymorphisms on the clinical outcome in adult AML patients treated at the National Cancer Institute (NCI) - Cairo University.

METHODS

Forty-eight adult AML patients presented to the Medical Oncology Department, NCI, from April 2010 till November 2011. Clinical data and bone marrow samples were obtained. Molecular genetic analysis involving P53, MDM2, and P21 single-nucleotide gene polymorphisms was done using polymerase chain reaction-restriction fragment length polymorphism coupled analysis.

RESULTS

The mean age was 35.7 years. After a median follow-up period of 12 months, 28 patients (58.4%) achieved complete remission (CR) and the overall survival (OS) was 8.7 months. Patients with homozygous Arg/arg at codon 72 of P53 had a better median OS months than Arg/Pro and Pro/Pro (13.4 vs. 8.4 vs. 1.5 months, respectively; P = 0.045). P53/p21 combination had a better median OS and disease-free survival (DFS) of 12.1 and 13.7 months for wild type cases (GG + Ser/ser) and 20.3 and 20.7 months for patients with either variant genes (GC + Ser/arg) compared with 1.1 and 1.9 months for patients with both variant genes (CC + arg/arg), (P = 0.037 and 0.004). The presence of wild genotype of either P21 or MDM2 may abolish the effect of P53 homozygous variant genotype on the OS. Neither p21nor mdm2 polymorphism alone showed an impact on OS or DFS. CR was not affected by any of the three gene polymorphisms.

CONCLUSION

The p53 pathway gene polymorphisms may affect the OS of adult AML patients.

Expression of the regulatory cell cycle proteins p21, p27, p14, p16, p53, mdm2, and cyclin E in bone marrow biopsies with acute myeloid leukemia. Correlation with patients' survival

Cell cycle control is a crucial event in normal hematopoiesis, and abnormalities of regulatory cell cycle genes have been found to contribute to the development of many hematologic malignancies. The present study investigates the immunohistochemical expression of seven essential cell cycle proteins (p21, p27, p14, p16, p53, mdm2, and cyclin E) in paraffin-embedded sections from 42 bone marrow biopsies obtained from an equal number of patients with newly diagnosed acute myeloid leukemia (AML). This study revealed (i) a high frequency of p53+/mdm2-/p21-phenotype, which is probably a result of p53 gene mutation and/or inhibition of mdm2 action by p14(ARF); (ii) expression of p27+/cyclinE-phenotype in most cases, suggesting that p27 may act as a potent cyclin-dependent kinase inhibitor; (iii) expression of p16 only in very few cases; and (iv) no relationship between the expression of any of the above proteins and survival as well as histologic subtype.

Protein lysine acetylation in normal and leukaemic haematopoiesis: HDACs as possible therapeutic targets in adult AML

Several new therapeutic strategies are now considered for acute myelogenous leukaemia (AML), including modulation of protein lysine acetylation through inhibition of histone deacetylases (HDACs): a large group of enzymes that alters the acetylation and, thereby, the function of a wide range of nuclear and cytoplasmic proteins. Firstly, HDACs can deacetylate histones as well as transcription factors, and can modulate gene expression through both these mechanisms. Secondly, acetylation is an important post-translational modulation of several proteins involved in the regulation of cell proliferation, differentiation and apoptosis (e.g., p53, tubulin, heat-shock protein 90). The only HDAC inhibitors that have been investigated in clinical studies of AML are butyrate derivatives, valproic acid and depsipeptide. In the first studies, the drugs have usually been used as continuous therapy for several weeks or months, and in most studies the drugs were used alone or in combination with all-trans retinoic acid for treatment of patients with relapsed or primary resistant AML. Neurological toxicity and gastrointestinal side effects seem to be common for all three drugs. Complete haematological remission lasting for several months has been reported for a few patients (< 5% of included patients), whereas increased peripheral blood platelet counts seem more common and have been described both for patients with AML and myelodysplastic syndromes. Taken together, these studies suggest that HDAC inhibition can mediate antileukaemic effects in AML, but for most patients the clinical benefit seems limited and further studies of combination therapy are required.

Trp53 loss during in vitro selection contributes to acquired Ara-C resistance in acute myeloid leukemia

OBJECTIVE

Chemoresistance remains a major clinical obstacle to curative chemotherapy of acute myeloid leukemia (AML), but the molecular mechanisms underlying resistance to chemotherapeutic agents used in AML are largely unknown. We have attempted to investigate genetic mechanisms causing resistance to Ara-C [1-beta-D-arabinofuranosyl-cytosine (cytarabine)], one mainstay in AML chemotherapy for decades.

MATERIAL AND METHODS

Highly Ara-C-resistant murine BXH-2 strain AML cell lines were generated, and their molecular changes were compared to their sensitive parental lines. The causative changes were confirmed using a genetic approach.

RESULTS

We derived nine highly Ara-C-resistant murine BXH-2 strain AML sublines via in vitro selection. p21Cip1 was dramatically downregulated and p53 protein accumulation induced by Ara-C treatment was impaired in one resistant line. In this line, repeated Ara-C exposure had selected for cells that harbor a genomic deletion affecting the splicing of Trp53 mRNA. This deletion produces an aberrant Trp53 mRNA, in which exon 4 is skipped, producing a protein lacking parts of both the transactivation and DNA-binding domains. Retroviral transduction of the sensitive parental cells with a dominant-negative Trp53 cDNA caused changes in the protein levels of p21Cip1, BAX, and cleaved caspase-3, but not bcl-XL, and rendered the cells more resistant to Ara-C. Unexpectedly, we found that pifithrin-alpha (PFTalpha), a compound that has been proposed to regulate p53 protein activity, induced apoptosis in both Ara-C-sensitive and -resistant lines, and decreased Ara-C resistance in cells with either normal or mutant Trp53 genes.

CONCLUSIONS

These data indicate that Trp53 loss-of-function could partly explain the acquisition of AML chemoresistance, and suggest that PFTalpha could be useful in treatment of relapsed AML.

Sub-attomole oligonucleotide and p53 cDNA determinations via a high-resolution surface plasmon resonance combined with oligonucleotide-capped gold nanoparticle signal amplification

Oligonucleotide (ODN)-capped gold nanoparticles (Au-NPs) were used in a sandwich assay of ODN or polynucleotide by a flow injection surface plasmon resonance (SPR). A carboxylated dextran film was immobilized onto the SPR sensor surface to eliminate nonspecific adsorption of ODN-capped Au-NPs. The tandem use of signal amplification via the adlayer of the ODN-capped Au-NPs and the differential signal detection by the bicell detector on the SPR resulted in a remarkable DNA detection level. A 39-mer target at a quantity as low as 2.1 x 10(-20)mol, corresponding to 1.38 fM in a 15 microl solution, can be measured. To our knowledge, both the concentration and quantity detection levels are the lowest among all the gene analyses conducted with SPR to this point. The method is shown to be reproducible (relative standard deviation values <16%) and to possess high sequence specificity. It is also demonstrated to be viable for sequence-specific p53 cDNA analysis. The successful elimination of nonspecific adsorption of, and the signal amplification by, ODN-capped Au-NPs renders the SPR attractive for cases where the DNA concentration is extremely low and the sample availability is severely limited.

Boswellic acid acetate induces apoptosis through caspase-mediated pathways in myeloid leukemia cells

The mechanism of the cytotoxic effect of boswellic acid acetate, a 1:1 mixture of alpha-boswellic acid acetate and beta-boswellic acid acetate, isolated from Boswellia carterri Birdw on myeloid leukemia cells was investigated in six human myeloid leukemia cell lines (NB4, SKNO-1, K562, U937, ML-1, and HL-60 cells). Morphologic and DNA fragmentation assays indicated that the cytotoxic effect of boswellic acid acetate was mediated by induction of apoptosis. More than 50% of the cells underwent apoptosis after treatment with 20 mug/mL boswellic acid for 24 hours. This apoptotic process was p53 independent. The levels of apoptosis-related proteins Bcl-2, Bax, and Bcl-XL were not modulated by boswellic acid acetate. Boswellic acid acetate induced Bid cleavage and decreased mitochondrial membrane potential without production of hydrogen peroxide. A general caspase inhibitor (Z-VAD-FMK) and a specific caspase-8 inhibitor II (Z-IETD-FMK) blocked boswellic acid acetate-induced apoptosis. The mRNAs of death receptors 4 and 5 (DR4 and DR5) were induced in leukemia cells undergoing apoptosis after boswellic acid acetate treatment. These data taken together suggest that boswellic acid acetate induces myeloid leukemia cell apoptosis through activation of caspase-8 by induced expression of DR4 and DR5, and that the activated caspase-8 either directly activates caspase-3 by cleavage or indirectly by cleaving Bid, which in turn decreases mitochondria membrane potential.

Acute promyelocytic leukemia: PML/RARalpha and the leukemic stem cell

Acute promyelocytic leukemia (APL) is distinguished from other acute myeloid leukemias (AMLs) by cytogenetic, clinical, as well as biological characteristics. The hallmark of APL is the t(15;17), which leads to the expression of the PML/RARalpha fusion protein. PML/RARalpha is the central leukemia-inducing lesion in APL and is directly targeted by all trans retinoic acid (t-RA) as well as by arsenic, both compounds able to induce complete remissions. This review focuses on potential stem cell involvement in APL outlining the knowledge about the APL-initiating stem cell and the influence of PML/RARalpha on the biology of the hematopoietic stem cell. Moreover, the importance of the blockage of t-RA signaling by the PML/RARalpha for the pathogenesis of APL is discussed, taking the relevance of the t-RA signaling pathway for the global hematopoiesis into account.