scholarly journals In vitro studies on cellular and molecular mechanisms of arsenic trioxide (As2O3) in the treatment of acute promyelocytic leukemia: As2O3 induces NB4 cell apoptosis with downregulation of Bcl-2 expression and modulation of PML-RAR alpha/PML proteins

Blood ◽  
1996 ◽  
Vol 88 (3) ◽  
pp. 1052-1061 ◽  
Author(s):  
GQ Chen ◽  
J Zhu ◽  
XG Shi ◽  
JH Ni ◽  
HJ Zhong ◽  
...  
Keyword(s):  
Gene Expression ◽  
Arsenic Trioxide ◽  
Acute Promyelocytic Leukemia ◽  
Cell Apoptosis ◽  
Molecular Mechanisms ◽  
Nuclear Dna ◽  
Staining Pattern ◽  
Promyelocytic Leukemia ◽  
Nb4 Cells

Abstract It has been shown recently in China that arsenic trioxide (As2O3) is a very effective treatment for acute promyelocytic leukemia (APL). APL patients resistant to all-trans retinoic acid (ATRA) and conventional chemotherapy can still respond to AS2O3. In this study, we addressed the possible cellular and molecular mechanisms of this treatment by using NB4 cells as a model. The results show that: (1) As2O3 triggers relatively specific NB4 cell apoptosis at micromolar concentration, as proved by morphology, histogramic related nuclear DNA contents, and DNA gel eletrophoresis. (2) As2O3 does not influence bax, bcl-x, c-myc, and p53 gene expression, but downregulates bcl-2 gene expression at both mRNA and protein levels. (3) As2O3 induces a significant modulation of the PML staining pattern in NB4 cells and HL-60 cells. The micropunctates characteristic of PML-RAR alpha in NB4 cells dissappear after treatment with As2O3, whereas a diffuse PML staining occurs in the perinuclear cytoplasmic region. In addition, a low percentage of untreated NB4 cells exhibits an accumulation of PML positive particles in a compartment of cytoplasm. The percentage of these cells can be significantly increased after As2O3 treatment. A similar PML staining pattern is observed in apoptotic cells. (4) ATRA pretreatment does not influence As2O3-induced apoptosis. These results suggest that induction of cell apoptosis can be one of the mechanisms of the therapeutic effect of As2O3. Moreover, this apoptosis induction occurs independently of the retinoid pathway and may be mediated, at least partly, through the modulation of bcl-2, as well as PML-RAR alpha and/ or PML proteins.

scholarly journals In vitro studies on cellular and molecular mechanisms of arsenic trioxide (As2O3) in the treatment of acute promyelocytic leukemia: As2O3 induces NB4 cell apoptosis with downregulation of Bcl-2 expression and modulation of PML-RAR alpha/PML proteins

Blood ◽  
1996 ◽  
Vol 88 (3) ◽  
pp. 1052-1061 ◽  
Author(s):  
GQ Chen ◽  
J Zhu ◽  
XG Shi ◽  
JH Ni ◽  
HJ Zhong ◽  
...  
Keyword(s):  
Gene Expression ◽  
Arsenic Trioxide ◽  
Acute Promyelocytic Leukemia ◽  
Cell Apoptosis ◽  
Molecular Mechanisms ◽  
Nuclear Dna ◽  
Staining Pattern ◽  
Promyelocytic Leukemia ◽  
Nb4 Cells

It has been shown recently in China that arsenic trioxide (As2O3) is a very effective treatment for acute promyelocytic leukemia (APL). APL patients resistant to all-trans retinoic acid (ATRA) and conventional chemotherapy can still respond to AS2O3. In this study, we addressed the possible cellular and molecular mechanisms of this treatment by using NB4 cells as a model. The results show that: (1) As2O3 triggers relatively specific NB4 cell apoptosis at micromolar concentration, as proved by morphology, histogramic related nuclear DNA contents, and DNA gel eletrophoresis. (2) As2O3 does not influence bax, bcl-x, c-myc, and p53 gene expression, but downregulates bcl-2 gene expression at both mRNA and protein levels. (3) As2O3 induces a significant modulation of the PML staining pattern in NB4 cells and HL-60 cells. The micropunctates characteristic of PML-RAR alpha in NB4 cells dissappear after treatment with As2O3, whereas a diffuse PML staining occurs in the perinuclear cytoplasmic region. In addition, a low percentage of untreated NB4 cells exhibits an accumulation of PML positive particles in a compartment of cytoplasm. The percentage of these cells can be significantly increased after As2O3 treatment. A similar PML staining pattern is observed in apoptotic cells. (4) ATRA pretreatment does not influence As2O3-induced apoptosis. These results suggest that induction of cell apoptosis can be one of the mechanisms of the therapeutic effect of As2O3. Moreover, this apoptosis induction occurs independently of the retinoid pathway and may be mediated, at least partly, through the modulation of bcl-2, as well as PML-RAR alpha and/ or PML proteins.

scholarly journals Arsenic Trioxide Promotes Histone H3 Phosphoacetylation at the Chromatin ofCASPASE-10in Acute Promyelocytic Leukemia Cells

2002 ◽  
Vol 277 (51) ◽  
pp. 49504-49510 ◽  
Author(s):  
Ji Li ◽  
Peili Chen ◽  
Natasha Sinogeeva ◽  
Myriam Gorospe ◽  
Robert P. Wersto ◽  
...  
Keyword(s):  
Arsenic Trioxide ◽  
Acute Promyelocytic Leukemia ◽  
Molecular Mechanisms ◽  
Expression Profiles ◽  
Histone H3 ◽  
Gene Expression Profiles ◽  
Promyelocytic Leukemia ◽  
Therapeutic Effects ◽  
Acid Therapy ◽  
Nb4 Cells

Arsenic trioxide (As2O3) is highly effective for the treatment of acute promyelocytic leukemia, even in patients who are unresponsive to all-trans-retinoic acid therapy. As2O3is believed to function primarily by promoting apoptosis, but the underlying molecular mechanisms remain largely unknown. In this report, using cDNA arrays, we have examined the changes in gene expression profiles triggered by clinically achievable doses of As2O3in acute promyelocytic leukemia NB4 cells.CASPASE-10expression was found to be potently induced by As2O3. Accordingly, caspase-10 activity also substantially increased in response to As2O3treatment. A selective inhibitor of caspase-10, Z-AEVD-FMK, effectively blocked caspase-3 activation and significantly attenuated As2O3-triggered apoptosis. Interestingly, the treatment of NB4 cells with As2O3markedly increased histone H3 phosphorylation at serine 10, an event that is associated with acetylation of the lysine 14 residue. Chromatin immunoprecipitation assays revealed that As2O3potently enhances histone H3 phosphoacetylation at theCASPASE-10locus. These results suggest that the effect of As2O3on histone H3 phosphoacetylation at theCASPASE-10gene may play an important role in the induction of apoptosis and thus contribute to its therapeutic effects on acute promyelocytic leukemia.

scholarly journals Arsenic Trioxide Potentiates Extracellular Traps-Formation Cell Death in Acute Promyelocytic Leukemia through mTOR-Regulated Autophagy

Blood ◽  
2016 ◽  
Vol 128 (22) ◽  
pp. 1575-1575
Author(s):  
Tao Li ◽  
Muhua Cao ◽  
Ruishuang Ma ◽  
Xiaoming Wu ◽  
Lu Zhao ◽  
...  
Keyword(s):  
Cell Death ◽  
Arsenic Trioxide ◽  
Acute Promyelocytic Leukemia ◽  
Promyelocytic Leukemia ◽  
Extracellular Dna ◽  
Apoptosis Resistance ◽  
Dna Complexes ◽  
Nb4 Cells ◽  
Wide Range

Abstract Background:Emerging clinical data shows that arsenic trioxide (ATO) exerts selective cytotoxicity against acute promyelocytic leukemia (APL) without severe side effects that mainly ascribed to nonspecific induction of apoptosis. It is attractive to speculate whether other uncovered APL cell death exists which can be specifically sparked by ATO administration. We have recently demonstrated that APL cells can undergo a previously unrecognized pathway for death by releasing extracellular DNA traps (ETs), termed ETosis (Ma R et al, Cell Death Dis 2016). However, besides apoptosis, whether ATO induces this APL-specific ETotic cell death remains to be explored. We wereto investigated the effects of a wide range of concentrations of ATO on ETotic death in APL cells and elucidate the underlying molecular mechanisms. Methods: Bone marrow samples were obtained from sixteen APL patients before and after the continuous administration of ATO for two weeks. APL cells were isolated and cultured in the presence and absence of ATRA for 3 days. We used confocal microscopy to image ET formation by APL cells and the percentage of ETotic cells was simultaneously quantified. ELISA was used to measure the concentration of myeloperoxidase (MPO)-DNA complexes in the supernant. We also assessed the effects of a wide range of concentrations (0.1, 0.25, 0.5, 0.75, 1.0, 2.0 μM) of ATO treatment for 24, 48 and 72 hours on ETosis in APL-cell line NB4 cells in vitro. Autophagy activation and leukemia-initiating cell (LIC) activity were evaluated by immunoblotting and imaged by immunostaining. LICs were analyzed using colony-forming unit (CFU) assays, and identified and quantified by flow cytometry. Results: APL cells isolated from ATO-treated APL patients underwent significantly increased spontaneous (P = 0.005) and ATRA-stimulated (P = 0.002) ETosis compared to those from newly diagnosed patients (n = 16). In vitro ATO treatment showed that the percentage of ETotic NB4 cells dramatically increased at 0.5 μM (8 ± 1.6%), peaked at 0.75 μM (15 ± 2.4%) and was gradually suppressed at higher concentrations. The concentration of MPO-DNA complexes, an indirect marker of ETosis, parallelled the dose-dependent change in the percentage of ETotic cells. Interestingly, inhibition experiments indicated that ATO caused concentration-dependent APL cell death: ATO primarily triggered ETosis at moderate concentrations (0.5 to 0.75 μM) and switched to apoptosis at relatively high doses (1.0 to 2.0 μM). Furthermore, We found that ATO induced ETosis through mammalian target of rapamycin (mTOR)-regulated autophagy. Surprisingly, inhibition of ETosis spared LIC activity from ATO reduction, while combined treatment with ATO and rapamycin further increased ETosis-mediated LIC loss (~3.5-fold). Conclusion s : This is the first study to show that ATO potentiates ETotic death in APL cells through mTOR-regulated autophagy. Importantly, further investigation suggests that ATO specifically targets the APL LICs to ETosis. This implies that, in combination with ATO, therapy-triggered ETosis by targeting the corresponding signaling pathways could be a novel and effective strategy to improve a long relapse-free survival through LIC clearance, avoid lethal apoptosis-related complications and overcome apoptosis resistance. Conflict of interest statement: None. Disclosures No relevant conflicts of interest to declare.

Evaluation of Mechanisms of Resistance to Arsenic Trioxide In Patients with Acute Promyelocytic Leukemia

Blood ◽  
2010 ◽  
Vol 116 (21) ◽  
pp. 2746-2746
Author(s):  
Ezhilarasi Chendamarai ◽  
Ansu Abu Alex ◽  
Saravanan Ganesh ◽  
Kavitha M Lakshmi ◽  
Sachin Jadhav ◽  
...  
Keyword(s):  
Gene Expression ◽  
Bone Marrow ◽  
Arsenic Trioxide ◽  
Acute Promyelocytic Leukemia ◽  
Promyelocytic Leukemia ◽  
Newly Diagnosed ◽  
Mechanisms Of Resistance ◽  
Induced Apoptosis ◽  
The Impact

Abstract Abstract 2746 Introduction: Newly diagnosed and relapsed acute promyelocytic leukemia (APL) patients respond to therapy with arsenic trioxide (ATO) based regimens. Significantly more patients with relapsed APL have disease recurrence after ATO based therapy than newly diagnosed cases. We undertook a series of experiments to evaluate the potential mechanisms to explain this increased recurrence rate in patients with relapsed APL. Patients and methods: From April 2007 to March 2009 bone marrow samples from newly diagnosed and relapsed cases admitted at our center were utilized for these studies. If required the bone marrow blasts and promyelocyte component was enriched to above 90% using a lineage depletion cocktail and VarioMACS (Miltenyi Biotec, Germany). For in-vitro intracellular ATO concentration measurement, 2 × 107 cells were washed and suspended in RPMI media with 0.5 μM concentration of ATO and incubated for 24 hours. Cells were then washed, made into a pellet and solubilized with HNO3 and H2O2 and ATO content measured using an atomic absorption spectrophotometer. An in-vitro sensitivity assay of malignant cells as previously reported was standardized using an MTT assay system. The impact of co-culture of mesenchymal stromal cells (MSC) and malignant promyelocytes on ATO induced apoptosis was studied with 7AAD and Annexin staining using a flowcytometer. A gene expression array using 44k human microarray chip analysis (Agilent technologies) was done on 8 newly diagnosed and 8 relapsed cases. Results: Sixty five patients were included in this study. Of these 47 (72%) were newly diagnosed and 18 (28%) were relapsed cases. On immunophenotyping, CD34 was positive (>20%) in 3.6% of newly diagnosed and 50% of relapsed cases (P=0.001). The mean MFI of the relapsed cases for expression of CD38, VLA-5 and CD13 was significantly lower in the relapsed group. The ability of both newly diagnosed and relapsed primary APL cells to concentrate ATO intracellular was not significantly different (15.2±9 nG/107 cell Vs. 16.3±9.7 nG/107 cell). Similarly the in-vitro IC-50 assay was not significantly different between the two groups (5.5±3.8 Vs. 4.7±4.5 μM). Neither of these assays correlate with clinical parameters such as relapse, event free (EFS) or overall survival (OS). Evaluation of the impact of MSC on ATO induced apoptosis demonstrates a protective effect in newly diagnosed and relapsed cases (Fig 1). This effect was mediated partly by the MSC conditioning media and could not be overcome by addition of VLA-4 or VLA-5 blocking antibodies (data not shown). Gene expression studies comparing the two cohorts revealed 1744 genes that were differentially expressed (>2 fold) between samples at diagnosis and at relapse. Quantitative Real-time RT-PCR using SYBR- Green detection system was done to confirm the gene expression results obtained from microarray analysis. Using ΔΔCT method the fold difference was calculated for five selected genes which validated the microarray data. Conclusion: Relapsed patients have significant immuno-phenotypic differences from newly diagnosed cases. Mechanisms of resistance to ATO are probably multi-factorial but are unlikely to be related to intra-cellular ATO concentration. In-vitro IC-50 does not appear to predict clinical outcomes. Stromal interaction protects malignant promyelocytes from the apoptotic action of ATO which appears more pronounced in relapsed than in newly diagnosed cases. Further evaluation of parameters that enhance such stromal interaction and protect malignant promyelocytes from the apoptotic effect of ATO along with evaluation of dysregulated genes and pathways are required. Disclosure: No relevant conflicts of interest to declare.

Combined effect of all-trans retinoic acid and arsenic trioxide in acute promyelocytic leukemia cells in vitro and in vivo

Blood ◽  
2001 ◽  
Vol 97 (1) ◽  
pp. 264-269 ◽  
Author(s):  
Yongkui Jing ◽  
Long Wang ◽  
Lijuan Xia ◽  
Guo-qiang Chen ◽  
Zhu Chen ◽  
...  
Keyword(s):  
Retinoic Acid ◽  
Arsenic Trioxide ◽  
Acute Promyelocytic Leukemia ◽  
Promyelocytic Leukemia ◽  
Nb4 Cells ◽  
Trans Retinoic Acid ◽  
All Trans Retinoic Acid ◽  
Induced Apoptosis

Abstract All-trans retinoic acid (tRA) and arsenic trioxide (As2O3) induce non–cross-resistant complete clinical remission in patients with acute promyelocytic leukemia with t(15;17) translocation and target PML-RARα, the leukemogenic protein, by different pathways suggesting a possible therapeutic synergism. To evaluate this possibility, this study examined the effect of As2O3 on tRA-induced differentiation and, conversely, the effect of tRA on As2O3-induced apoptosis. As2O3 at subapoptotic concentrations (0.5 μM) decreased tRA-induced differentiation in NB4 cells but synergized with atRA to induce differentiation in tRA-resistant NB4 subclones MR-2 and R4 cells as measured by nitroblue tetrazolium reduction and tRA-inducible genes (TTGII, RARβ, RIG-E). tRA cleaved PML-RARα into distinct fragments in NB4 but not in tRA-resistant MR-2 or R4 cells, whereas As2O3 completely degraded PML-RARα in all 3 cell lines. As2O3-induced apoptosis was decreased by tRA pretreatment of NB4 cells but not of R4 cells and was associated with a strong induction of Bfl-1/A1 expression, a Bcl-2 protein family member. Severe combined immunodeficient mice bearing NB4 cells showed an additive survival effect after sequential treatment, but a toxic effect was observed after simultaneous treatment with tRA and As2O3. These data suggest that combined As2O3 and tRA treatment may be more effective than single agents in tRA-resistant patients. Although in vitro data do not always translate to in vivo response, toxicity and potential drug antagonism may be diminished by decreasing the concentration of As2O3 when given at the same time with therapeutic levels of tRA.

Src Family Kinase Inhibitor PP2 Has Different Effects On ATRA or Arsenic Trioxide-Induced Differentiation of Acute Promyelocytic Leukemia Cell Line

Blood ◽  
2012 ◽  
Vol 120 (21) ◽  
pp. 4800-4800
Author(s):  
Kyoung Ha Kim ◽  
Hee-Jeong Cheong ◽  
Sook-Ja Kim ◽  
Young Woo Jeon ◽  
Jina Yoon ◽  
...  
Keyword(s):  
Gene Expression ◽  
Retinoic Acid ◽  
Synergistic Effect ◽  
Arsenic Trioxide ◽  
Acute Promyelocytic Leukemia ◽  
Myeloid Leukemia ◽  
Leukemia Cell ◽  
Promyelocytic Leukemia ◽  
Induced Differentiation ◽  
Nb4 Cells

Abstract Abstract 4800 Acute promyelocytic leukemia (APL) is a subtype of acute myeloid leukemia (AML) with distinctive biologic and clinical features that is now highly curable. Leukemic promyelocytes have the unique ability to undergo differentiation after exposure to retinoic acid and both differentiation and apoptosis after exposure to arsenic trioxide (As2O3, ATO). Emerging evidence has implicated Src family kinases (SFKs) as regulators of proliferation and survival of myeloid lineage cells. Recent studies showed that inhibition of SFKs resulted in enhancement of retinoic acid-induced myeloid differentiation. In this study, we demonstrated that the SFK inhibitor PP2 enhanced the differentiation of NB4 cells when combined with ATO as well when combined with as all-trans-retinoic acid (ATRA) and the difference in the retinoic acid-induced gene expression between the cells treated with PP2 in combination with ATRA and the cells treated with PP2 in combination with ATO. SFK inhibitor PP2 significantly enhanced ATRA- or ATO-induced differentiation of NB4 cells. The synergistic effect was significantly stronger when PP2 was combined with ATRA than when PP2 was combined with ATO. Flow cytometric analysis demonstrated a significant increase in CD11b-positive granulocytes up to 60.73% and 31.58%, respectively. These results were confirmed by morphologic analysis using Wright stain and NBT staining. These effects were not related to apoptosis. Annexin-V-fluorescein staining revealed that PP2 combined with ATRA or PP2 combined with ATO did not induce apoptosis in NB4 cells. Retinoic acid-induced gene expression was different in both groups. ICAM-1 expression was significantly increased in the cells treated with PP2 in combination with ATRA whereas cathepsin D expression was significantly increased in the cells treated with PP2 in combination with ATO. These findings suggested that the synergistic effect of SFK inhibitor PP2 in combination with ATRA was significantly stronger than that of PP2 in combination with ATO on NB4 myeloid leukemia cell differentiation and this difference was related to the disparate activation of retinoic acid-induced genes. Disclosures: No relevant conflicts of interest to declare.

scholarly journals Fucoidan enhances the therapeutic potential of arsenic trioxide and all-trans retinoic acid in acute promyelocytic leukemia, in vitro and in vivo

Oncotarget ◽  
2016 ◽  
Vol 7 (29) ◽  
pp. 46028-46041 ◽  
Author(s):  
Farzaneh Atashrazm ◽  
Ray M. Lowenthal ◽  
Joanne L. Dickinson ◽  
Adele F. Holloway ◽  
Gregory M. Woods
Keyword(s):  
Retinoic Acid ◽  
Arsenic Trioxide ◽  
Acute Promyelocytic Leukemia ◽  
Therapeutic Potential ◽  
Promyelocytic Leukemia ◽  
Trans Retinoic Acid ◽  
All Trans Retinoic Acid

scholarly journals A Retinoid-Resistant Acute Promyelocytic Leukemia Subclone Expresses a Dominant Negative PML-RARα Mutation

Blood ◽  
1997 ◽  
Vol 89 (12) ◽  
pp. 4282-4289 ◽  
Author(s):  
Wenlin Shao ◽  
Laura Benedetti ◽  
William W. Lamph ◽  
Clara Nervi ◽  
Wilson H. Miller
Keyword(s):  
Gene Expression ◽  
Retinoic Acid ◽  
Ligand Binding ◽  
Cell Line ◽  
Acute Promyelocytic Leukemia ◽  
Regulation Of Gene Expression ◽  
Promyelocytic Leukemia ◽  
Dominant Negative

Abstract The unique t(15; 17) of acute promyelocytic leukemia (APL) fuses the PML gene with the retinoic acid receptor α (RARα) gene. Although retinoic acid (RA) inhibits cell growth and induces differentiation in human APL cells, resistance to RA develops both in vitro and in patients. We have developed RA-resistant subclones of the human APL cell line, NB4, whose nuclear extracts display altered RA binding. In the RA-resistant subclone, R4, we find an absence of ligand binding of PML-RARα associated with a point mutation changing a leucine to proline in the ligand-binding domain of the fusion PML-RARα protein. In contrast to mutations in RARα found in retinoid-resistant HL60 cells, in this NB4 subclone, the coexpressed RARα remains wild-type. In vitro expression of a cloned PML-RARα with the observed mutation in R4 confirms that this amino acid change causes the loss of ligand binding, but the mutant PML-RARα protein retains the ability to heterodimerize with RXRα and thus to bind to retinoid response elements (RAREs). This leads to a dominant negative block of transcription from RAREs that is dose-dependent and not relieved by RA. An unrearranged RARα engineered with this mutation also lost ligand binding and inhibited transcription in a dominant negative manner. We then found that the mutant PML-RARα selectively alters regulation of gene expression in the R4 cell line. R4 cells have lost retinoid-regulation of RXRα and RARβ and the RA-induced loss of PML-RARα protein seen in NB4 cells, but retain retinoid-induction of CD18 and CD38. Thus, the R4 cell line provides data supporting the presence of an RARα-mediated pathway that is independent from gene expression induced or repressed by PML-RARα. The high level of retinoid resistance in vitro and in vivo of cells from some relapsed APL patients suggests similar molecular changes may occur clinically.

scholarly journals Arsenic trioxide inhibits ATRA-induced prostaglandin E2 and cyclooxygenase-1 in NB4 cells, a model of acute promyelocytic leukemia

Leukemia ◽  
2008 ◽  
Vol 22 (6) ◽  
pp. 1125-1130 ◽  
Author(s):  
A Habib ◽  
E Hamade ◽  
R Mahfouz ◽  
M S Nasrallah ◽  
H de Thé ◽  
...  
Keyword(s):  
Prostaglandin E2 ◽  
Arsenic Trioxide ◽  
Acute Promyelocytic Leukemia ◽  
Promyelocytic Leukemia ◽  
Nb4 Cells ◽  
Cyclooxygenase 1

scholarly journals Retinoic acid activates interferon regulatory factor-1 gene expression in myeloid cells

Blood ◽  
1996 ◽  
Vol 88 (1) ◽  
pp. 114-123 ◽  
Author(s):  
S Matikainen ◽  
T Ronni ◽  
M Hurme ◽  
R Pine ◽  
I Julkunen
Keyword(s):  
Gene Expression ◽  
Retinoic Acid ◽  
Growth Inhibition ◽  
Molecular Mechanisms ◽  
Gene Activation ◽  
Oligoadenylate Synthetase ◽  
Nb4 Cells ◽  
Drug Of Choice

All-trans-retinoic acid (ATRA) is the drug of choice in the treatment of acute promyelocytic leukemia (APL). ATRA induces both in vitro and in vivo differentiation of APL cells into mature granulocytes. However, the molecular mechanisms involved in ATRA-dependent growth inhibition and cellular differentiation are not presently understood. The NB4 cell line, which is derived from the bone marrow of a patient with APL during relapse, can be used as a model system to study the growth and differentiation of APL cells. Because interferon (IFN) regulatory factors (IRF-1 and IRF-2) and other IFN-inducible gene products regulate cell growth, we analyzed the effects of ATRA on the expression of these genes. We show that ATRA directly activates IRF-1 gene expression, followed by activation of IRF-2 and 2′–5′ oligoadenylate synthetase (OAS) gene expression with slower kinetics. In addition to NB4 cells, ATRA also activated IRF-1 gene expression in HL-60, U937, and THP-1 cells, which all respond to ATRA by growth inhibition. A more than additive increase in IRF-1 gene expression was seen with ATRA and IFN-gamma in NB4 cells. ATRA did not activate nuclear factor kappa B or signal transducer and activator of transcription (STAT) activation pathways, suggesting that an alternate mechanism is involved in IRF-1 gene activation. The ATRA-induced expression of IRF-1, an activator of transcription and repressor of transformation, may be one of the molecular mechanisms of ATRA-induced growth inhibition, and the basis for the synergistic actions of ATRA and IFNs in myeloid leukemia cells.

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