Bone Marrow Stromal Cells Enhance Differentiation of Acute Myeloid Leukemia Induced by Pyrimidine Synthesis Inhibitors.

Acute myeloid leukemia (AML) is a heterogeneous group of hematologic malignancies characterized by differentiation arrest, high relapse rates, and poor survival. The bone marrow (BM) microenvironment is recognized as a critical mediator of drug resistance and a primary site responsible for AML relapse. Our previous study reported that 5-aminoimidazole-4-carboxamide ribonucleoside (AICAr) induces AML cell differentiation by inhibiting pyrimidine synthesis and activating Checkpoint kinase 1. While the protective effect of BM stroma on leukemia cells in response to cytotoxic drugs is well-documented, its effect on AML differentiation remains less explored. In this study, we investigated the impact of stromal cell lines and primary mesenchymal stromal cells (MSCs) on AML cell line differentiation triggered by AICAr and brequinar, a known dihydroorotate dehydrogenase (DHODH) inhibitor. Our findings indicate that the mouse MS-5 stromal cell line, known for its cytoprotective effects, does not inhibit AML cell differentiation induced by pyrimidine synthesis inhibitors. Interestingly, AICAr caused morphological changes and growth arrest in MS-5 stromal cells via an AMPK-dependent pathway. Human stromal cell lines HS-5 and HS-27, as well as primary MSCs isolated from patient bone marrow, were superior in promoting AML differentiation compared to mouse cells in response to AICAr and brequinar, with the inhibitors not significantly affecting the stromal cells themselves. In conclusion, our study highlights the supportive role of human BM MSCs in enhancing the differentiation effects of pyrimidine synthesis inhibitors on AML cells, suggesting that AML treatment strategies focusing on differentiation rather than cell killing may be successful in clinical settings.

Immunology demystified: A guide for transplant hepatologists.

Liver transplantation has become standard practice for treating end-stage liver disease. The success of the procedure relies on effective immunosuppressive medications to control the host's immune response. Despite the liver's inherent capacity to foster tolerance, the early post-transplant period is marked by significant immune reactivity. To ensure favorable outcomes, it is imperative to identify and manage various rejection types, encompassing T-cell-mediated, antibody-mediated, and chronic rejection. However, the approach to prescribing immunosuppressants relies heavily on clinical judgment rather than evidence-based criteria. Given that the majority of patients will require lifelong immuno suppression as the mechanisms underlying operational tolerance are still being investigated, healthcare providers must possess an understanding of immune responses, rejection mechanisms, and the pathways targeted by immunosuppressive drugs. This knowledge enables customization of treatments and improved patient care, even though a consensus on an optimal immunosuppressive regimen remains elusive.

Bone marrow stromal cells reduce low-dose cytarabine-induced differentiation of acute myeloid leukemia.

Low-dose cytarabine (LDAC) is a standard therapy for elderly acute myeloid leukemia (AML) patients unfit for intensive chemotherapy. While high doses of cytarabine induce cytotoxicity, the precise mechanism of action of LDAC in AML remains elusive. In vitro studies have demonstrated LDAC-induced differentiation; however, such differentiation is seldom observed in vivo. We hypothesize that this discrepancy may be attributed to the influence of bone marrow (BM) stromal cells on AML cells. Thus, this study aimed to investigate the impact of BM stromal cells on LDAC-induced differentiation of AML cell lines and primary samples. Our results demonstrate that the presence of MS-5 stromal cells prevented LDAC-induced cell cycle arrest, DNA damage signaling and differentiation of U937 and MOLM-13 cell lines. Although transcriptomic analysis revealed that the stroma reduces the expression of genes involved in cytokine signaling and oxidative stress, data obtained with pharmacological inhibitors and neutralizing antibodies did not support the role for CXCL12, TGF-ß1 or reactive oxygen species. The presence of stromal cells reduces LDAC-induced differentiation in primary samples from AML-M4 and myelodysplastic syndrome/AML patients. In conclusion, our study demonstrates that BM stroma reduces differentiation of AML induced by LDAC. These findings provide insights into the limited occurrence of terminal differentiation observed in AML patients, and suggest a potential explanation for this observation.

Cytarabine-induced differentiation of AML cells depends on Chk1 activation and shares the mechanism with inhibitors of DHODH and pyrimidine synthesis.

Acute myeloid leukemia (AML) is characterized by arrested differentiation making differentiation therapy a promising treatment strategy. Recent success of inhibitors of mutated isocitrate dehydrogenase (IDH) invigorated interest in differentiation therapy of AML so that several new drugs have been proposed, including inhibitors of dihydroorotate dehydrogenase (DHODH), an enzyme in pyrimidine synthesis. Cytarabine, a backbone of standard AML therapy, is known to induce differentiation at low doses, but the mechanism is not completely elucidated. We have previously reported that 5-aminoimidazole-4-carboxamide ribonucleoside (AICAr) and brequinar, a DHODH inhibitor, induced differentiation of myeloid leukemia by activating the ataxia telangiectasia and Rad3-related (ATR)/checkpoint kinase 1 (Chk1) via pyrimidine depletion. In this study, using immunoblotting, flow cytometry analyses, pharmacologic inhibitors and genetic inactivation of Chk1 in myeloid leukemia cell lines, we show that low dose cytarabine induces differentiation by activating Chk1. In addition, cytarabine induces differentiation ex vivo in a subset of primary AML samples that are sensitive to AICAr and DHODH inhibitor. The results of our study suggest that leukemic cell differentiation stimulated by low doses of cytarabine depends on the activation of Chk1 and thus shares the same pathway as pyrimidine synthesis inhibitors.

Bendamustine: A review of pharmacology, clinical use and immunological effects (Review).

Bendamustine is an alkylating agent classified into the group of nitrogen mustard analogues, synthesized almost sixty years ago. It was registered in former East Germany in 1971 and approved by the US Food and Drug Administration in 2008 for treatment of chronic lymphocytic leukemia and indolent B­cell non­Hodgkin lymphoma. Considering its beneficial properties in the therapy of relapsed or refractory hematological malignancies, synergistic effects with other antineoplastic agents and increasing recent reports on its immunomodulatory effects, bendamustine has once again gained its justified attention. The uniqueness of bendamustine­mediated effects should be observed keeping in mind its distinctive structure with structural similarities to both alkylating agents and purine analogs. In the present review, the current knowledge on the use of bendamustine in oncology, its pharmacokinetics, mechanism of action and toxicity was summarized. In addition, its immune­modulating effects that have not been fully elucidated so far are emphasized, hoping to encourage further investigations of this unique drug.

All-trans retinoic acid induces differentiation in primary acute myeloid leukemia blasts carrying an inversion of chromosome 16.

All-trans retinoic acid (ATRA)-based therapy for acute promyelocytic leukemia (APL), a subtype of acute myeloid leukemia (AML), is the most successful example of differentiation therapy. Although ATRA can induce differentiation in some non-APL AML cell lines and primary blasts, clinical results of adding ATRA to standard therapy in non-APL AML patients have been inconsistent, probably due to use of different regimens and lack of diagnostic tools for identifying which patients may be sensitive to ATRA. In this study, we exposed primary blasts obtained from non-APL AML patients to ATRA to test for differentiation potential in vitro. We observed increased expression of differentiation markers, indicating a response to ATRA, in four out of fifteen primary AML samples. Three samples in which CD11b increased in response to ATRA had an inversion of chromosome 16 as well as the CBFB-MYH11 fusion gene, and the fourth sample was from a patient with KMT2A-rearranged, therapy-related AML. In conclusion, we identified a subgroup of non-APL AML patients with inv(16) and CBFB-MYH11 as the most sensitive to ATRA-mediated differentiation in vitro, and our results can help identify patients who may benefit from ATRA treatment.

Covid-19 as occupational disease in healthcare workers: a brief review of cases in the Clinical Hospital Centre Rijeka, Croatia.

The coronavirus disease 19 (COVID-19) pandemic has had a tremendous impact on every facet of private life and work organisation in virtually all social and economic sectors worldwide. People who stand on the first line of defence are healthcare workers (HCWs) risking exposure to infected patients. However, even though they are often affected by COVID-19 and associated somatic and mental health problems, COVID-19 as a new illness was not immediately acknowledged as occupational disease. This is why several groups of HCWs contacted their occupational medicine physicians in 2020 with a request to register the infection with SARS-CoV-2 as occupational disease. In an attempt to support their appeals and show that hospital workers have a high occupational risk of COVID-19, this study presents COVID-19 incidence and symptoms in 100 employees working at 11 clinics of the Clinical Hospital Centre (CHC) Rijeka, Croatia from 1 June to end December 2020. All of them were infected with SARS-CoV-2 and took sick leave, which lasted 13.6±2.6 days in average. This study also looks into the role of occupational medicine physicians in prospective monitoring of acute and long-acting consequences of COVID-19 that might occur in HCWs.

AICAr, a Widely Used AMPK Activator with Important AMPK-Independent Effects: A Systematic Review.

5-Aminoimidazole-4-carboxamide ribonucleoside (AICAr) has been one of the most commonly used pharmacological modulators of AMPK activity. The majority of early studies on the role of AMPK, both in the physiological regulation of metabolism and in cancer pathogenesis, were based solely on the use of AICAr as an AMPK-activator. Even with more complex models of AMPK downregulation and knockout being introduced, AICAr remained a regular starting point for many studies focusing on AMPK biology. However, there is an increasing number of studies showing that numerous AICAr effects, previously attributed to AMPK activation, are in fact AMPK-independent. This review aims to give an overview of the present knowledge on AMPK-dependent and AMPK-independent effects of AICAr on metabolism, hypoxia, exercise, nucleotide synthesis, and cancer, calling for caution in the interpretation of AICAr-based studies in the context of understanding AMPK signaling pathway.

Influence of delaying ocrelizumab dosing in multiple sclerosis due to COVID-19 pandemics on clinical and laboratory effectiveness.

OBJECTIVE: To evaluate clinical and laboratory effects of delaying ocrelizumab infusions during the COVID-19 pandemics in people with multiple sclerosis (pwMS). METHODS: We have retrospectively searched our electronic database and identified 33 pwMS who had a delay in treatment due to COVID-19 pandemics. The following data were extracted: age, sex, multiple sclerosis (MS) phenotype: relapsing-remitting (RRMS) or primary progressive multiple sclerosis (PPMS), disease duration, Expanded Disability Status scale (EDSS), previous disease modifying therapy (DMT), number of ocrelizumab cycles prior to the lockdown, dates of first ocrelizumab infusion, last ocrelizumab infusion prior to the lockdown and delayed ocrelizumab infusion after the lockdown. Flow cytometry results, relapses and EDSS progression prior to the delayed ocrelizumab infusion after the lockdown were extracted. RESULTS: The mean time between two ocrelizumab infusion during the lockdown was 7.72±0.64 (range 6.07 to 8.92) months. The mean time between last ocrelizumab infusion and the lymphocyte sampling prior to post COVID infusion was 6.59±0.95 (range 5.18 to 8.49) months. In this period, none of the studied patients had a relapse. In a multivariable linear regression analysis, time from last ocrelizumab infusion to lymphocyte sampling prior to the next infusion was the only significant predictor for CD19+ B cells count, when corrected for the number of previous ocrelizumab cycles and MS phenotype (RRMS or PPMS) (B=7.981, 95% C.I. 3.277-12.686, p=0.002). CONCLUSIONS: We have not shown clinical consequences of delaying ocrelizumab due to COVID-19 pandemics. However, the delay in dosing of ocrelizumab was an independent predictor of repopulation of B cells.

5-aminoimidazole-4-carboxamide ribonucleoside induces differentiation in a subset of primary acute myeloid leukemia blasts.

BACKGROUND: All-trans retinoic acid (ATRA)-based treatment of acute promyelocytic leukemia (APL) is the most successful pharmacological treatment of acute myeloid leukemia (AML). Recent development of inhibitors of mutated isocitrate dehydrogenase and dihydroorotate dehydrogenase (DHODH) has revived interest in differentiation therapy of non-APL AML. Our previous studies demonstrated that 5-aminoimidazole-4-carboxamide ribonucleoside (AICAr) induced differentiation of monocytic cell lines by activating the ATR/Chk1 via pyrimidine depletion. In the present study, the effects of AICAr on the viability and differentiation of primary AML blasts isolated from bone marrow of patients with non-APL AML were tested and compared with the effects of DHODH inhibitor brequinar and ATRA. METHODS: Bone marrow samples were obtained from 35 patients and leukemia blasts were cultured ex vivo. The cell viability was assessed by MTT assay and AML cell differentiation was determined by flow cytometry and morphological analyses. RNA sequencing and partial data analysis were conducted using ClusterProfiler package. Statistical analysis was performed using GraphPad Prism 6.0. RESULTS: AICAr is capable of triggering differentiation in samples of bone marrow blasts cultured ex vivo that were resistant to ATRA. AICAr-induced differentiation correlates with proliferation and sensitivity to DHODH inhibition. RNA-seq data obtained in primary AML blasts confirmed that AICAr treatment induced downregulation of pyrimidine metabolism pathways together with an upregulation of gene set involved in hematopoietic cell lineage. CONCLUSION: AICAr induces differentiation in a subset of primary non-APL AML blasts, and these effects correlate with sensitivity to a well-known, potent DHODH inhibitor.

LPS-induced inflammation desensitizes hepatocytes to Fas-induced apoptosis through Stat3 activation-The effect can be reversed by ruxolitinib.

Recent studies have established a concept of tumour necrosis factor-α (TNF-α)/Fas signalling crosstalk, highlighting TNF-α as a critical cytokine in sensitizing hepatocytes to death induced by Fas activation. However, in the exact inflammatory response, besides TNF-α, many other mediators, that might modulate apoptotic response differentially, are released. To resolve the issue, we studied the effects of lipopolysaccharide (LPS), one of the crucial inductors of inflammation in the liver, on apoptotic outcome. We show that LPS-induced inflammation diminishes the sensitivity of hepatocytes to Fas stimulus in vivo at caspase-8 level. Analysis of molecular mechanisms revealed an increased expression of various pro-inflammatory cytokines in non-parenchymal liver cells and hepatocyte-specific increase in Bcl-xL, associated with signal transducer and activator of transcription 3 (Stat3) phosphorylation. Pre-treatment with ruxolitinib, a selective Janus kinase (JAK) 1/2 inhibitor, prevented the LPS-induced Stat3 phosphorylation and restored the sensitivity of hepatocytes to Fas-mediated apoptosis. Furthermore, ruxolitinib pre-treatment diminished the LPS-induced Bcl-xL up-regulation without an inhibitory effect on LPS-induced expression of pro-inflammatory cytokines. In summary, although the reports are showing that the effects of isolated pro-inflammatory mediators, such as TNF-α or neutrophils, are pro-apoptotic, the overall effect of inflammatory milieu on hepatocytes in vivo is Stat3-dependent desensitization to Fas-mediated apoptosis.

Unfit for Work, Fit for Firearm or driving license - Is that Possible?

BACKGROUND: Psychiatric disorders are not compatible with carrying firearms or with driving a car. Persons with such disorders are often not employed and are persistent in demanding invalidity pensions, but some of them also insist on holding on to the mentioned licenses. In such cases, where persons are already in possession of firearms and driving licences, it never occurs to them, that they should surrender their permits back. AIM Pointing to the importance of OM controlling firearm/car driving licenses. CASE REPORTS: This paper discusses the problem of three cases that should be widely recognised as it is potentially life-threatening to other people. The first is the case of a war veteran in retirement with PTSD that had his application for firearms licence rejected by the authorities. The second is the case of a labourer who suffers from a depressive disorder, temporarily incapable of work. The third is the case of a war veteran, a chronic alcoholic with toxic epilepsy, who is applying for invalidity retirement but wants to keep his driving license. CONCLUSION: Occupational medicine assess every single worker by applying advanced methods and psycho tests that enable a thorough assessment of work capacity and fitness for carriage of firearms, driving as well as the assessment of psychiatric disorders, which are the most delicate to assess.

The Role of AMPK/mTOR Modulators in the Therapy of Acute Myeloid Leukemia.

Differentiation therapy of acute promyelocytic leukemia with all-trans retinoic acid represents the most successful pharmacological therapy of acute myeloid leukemia (AML). Numerous studies demonstrate that drugs that inhibit mechanistic target of rapamycin (mTOR) and activate AMP-kinase (AMPK) have beneficial effects in promoting differentiation and blocking proliferation of AML. Most of these drugs are already in use for other purposes; rapalogs as immunosuppressants, biguanides as oral antidiabetics, and 5-amino-4-imidazolecarboxamide ribonucleoside (AICAr, acadesine) as an exercise mimetic. Although most of these pharmacological modulators have been widely used for decades, their mechanism of action is only partially understood. In this review, we summarize the role of AMPK and mTOR in hematological malignancies and discuss the possible role of pharmacological modulators in proliferation and differentiation of leukemia cells.

Discrepancies in the evaluation of incapacity for work in a patient with epidermolysis bullosa acquisita between public pension fund and occupational medicine expert raise the issue of competencies.

Abstrat A 50-year-old female patient suffering from a severe form of epidermolysis bullosa acquisita (EBA) took legal action against the Croatian Pension Insurance Institute (CPII) in an attempt to overturn their assessment that she was no longer capable of working as a seamstress but still capable of doing administrative jobs. Her claim was that she was not capable of doing any job at all. She was first diagnosed EBA in 2000, and the disease progressed slowly with intermittent remissions. In 2012, skin erosions appeared on her feet, followed by the loss of all toenails and lesions and infiltrations on the tongue and oral mucosa. Her whole body was covered in oozing wounds, she was in pain, and parts of her skin would stick to fabric while changing clothes or bandages. The most recent findings showed oesophageal stricture. She can consume only liquid food and is on the waiting list for receiving a feeding tube. The occupational health expert witness confirmed that the patient was generally incapable of work and was fighting her life. The judge and CPII lawyers fully accepted this report and the earlier assessment was overturned. To avoid incompetent assessments of working (in)capacity in the future, CPII and similar institutions should engage occupational medicine specialists to work in their assessment teams.

5-Aminoimidazole-4-carboxamide ribonucleoside-induced autophagy flux during differentiation of monocytic leukemia cells.

Pharmacological modulators of AMP-dependent kinase (AMPK) have been suggested in treatment of cancer. The biguanide metformin and 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) have been reported to inhibit proliferation of solid tumors and hematological malignancies, but their role in differentiation is less explored. Our previous study demonstrated that AICAR alone induced AMPK-independent expression of differentiation markers in monocytic U937 leukemia cells, and no such effects were observed in response to metformin. The aim of this study was to determine the mechanism of AICAR-mediated effects and to test for the possible role of autophagy in differentiation of leukemia cells. The results showed that AICAR-mediated effects on the expression of differentiation markers were not mimicked by A769662, a more specific direct AMPK activator. Long-term incubation of U937 cells with AICAR and other differentiation agents, all-trans-retinoic acid (ATRA) and phorbol 12-myristate 13-acetate, increased the expression of the autophagy marker LC3B-II, and these effects were not observed in response to metformin. Western blot and immunofluorescence analyses of U937 cells treated with bafilomycin A1 or transfected with mRFP-GFP-LC3 proved that the increase in the expression of LC3B-II was due to an increase in autophagy flux, and not to a decrease in lysosomal degradation. 3-Methyladenine inhibited the expression of differentiation markers in response to all inducers, but had stimulatory effects on autophagy flux at dose that effectively inhibited the production of phosphatidylinositol 3-phosphate. The small inhibitory RNA-mediated down-modulation of Beclin 1 and hVPS34 had no effects on AICAR and ATRA-mediated increase in the expression of differentiation markers. These results show that AICAR and other differentiation agents induce autophagy flux in U937 cells and that the effects of AICAR and ATRA on the expression of differentiation markers do not depend on the normal levels of key proteins of the classical or canonical autophagy pathway.

Expert assessments of war casualties.

By describing three different war casualty cases, this paper highlights the harshness of current legal regulations in Croatia, and the possible consequences of their strict implementation. In two of the three cases the second instance expert witness has ultimately found that, due to legal procedures being followed too strictly, the expert witness initially assessing the cases had exceeded the legal framework and actually assessed patients, that is, individuals claiming disability benefits, inadequately. However, in the third case the expert witnesses employed by the Disability Pension Insurance Institute were right in deciding that the claimant was not entitled to a higher category of disability. Assessment of ability to work thus continues to be a subject of disagreement between experts of various profiles in legal and medical circles. Similar assessment issues appear not to be uncommon in other countries also. Therefore, the time has perhaps come for the existing rules and regulations to be re-evaluated. In fact, this may apply not only to those countries that share the same values and interests, such as EU member states, but also further abroad, particularly within a wider context of international recognition of basic human rights.

Are psychologists and psychiatrists assessing work capacity part of the problem or solution?

The aim of this case study was to emphasise the importance of coordination between the members of occupational medicine teams who assess work capacity in persons whose jobs may involve responsibility for other people's safety and health. We have picked out four cases - three visiting nurses and one applicant for driving and firearms licence extension - where psychiatrists/psychologists and occupational health specialist disagreed in their assessment entirely. These cases illustrate how psychologists and psychiatrists tend to support patients' wishes to either remain at their workplace or take disability retirement, whereas occupational health specialists take a different, less easy course, relying on the medical condition of the patient, specific job requirements, and broader implications for public safety. It appears that this is not a problem only in Croatia, but in a number of developed countries as well. This problem calls for additional training of all members of a work capacity assessment team.

Inositol pyrophosphates modulate cell cycle independently of alteration in telomere length.

Synthesis of inositol pyrophosphates through activation of Kcs1 plays an important role in the signalling response required for cell cycle progression after mating pheromone arrest. Overexpression of Kcs1 doubled the level of inositol pyrophosphates when compared to wild type cells and 30 min following the release from α-factor block further increase in inositol pyrophosphates was observed, which resulted that cells overexpressing Kcs1 reached G2/M phase earlier than wild type cells. Similar effect was observed in ipk1Δ cells, which are unable to synthesize IP6-derived inositol pyrophosphates (IP7 and IP8) but will synthesize IP5-derived inositol pyrophosphates (PP-IP4 and (PP)2-IP3). Although ipk1Δ cells have shorter telomeres than wild type cells, overexpression of Kcs1 in both strains have similar effect on cell cycle progression. As it is known that PP-IP4 regulates telomere length through Tel1, inositol polyphosphates, cell cycle and telomere length were determined in tel1Δ cells. The release of the cells from α-factor block and overexpression of Kcs1 in tel1Δ cells produced similar effects on inositol pyrophosphates level and cell cycle progression when compared to wild type cells, although tel1Δ cells possesses shorter telomeres than wild type cells. It can be concluded that telomere length does not affect cell cycle progression, since cells with short telomeres (ipk1Δ and tel1Δ) progress through cell cycle in a similar manner as wild type cells and that overexpression of Kcs1 in cells with either short or normal telomeres will increase S phase progression without affecting telomere length. Furthermore, IP5-derived inositol pyrophosphates can compensate for the loss of IP6-derived inositol pyrophosphates, in modulating S phase progression of the cell cycle.

The mechanism of synergistic effects of arsenic trioxide and rapamycin in acute myeloid leukemia cell lines lacking typical t(15;17) translocation.

Arsenic trioxide (ATO) has potent clinical activity in the treatment of patients with acute promyelocytic leukemia (APL), but is much less efficacious in acute myeloid leukemia (AML) lacking t(15;17) translocation. Recent studies have indicated that the addition of mammalian target of rapamycin (mTOR) inhibitors may increase the sensitivity of malignant cells to ATO. The aim of the present study was to test for possible synergistic effects of ATO and rapamycin at therapeutically achievable doses in non-APL AML cells. In HL-60 and U937 cell lines, the inhibitory effects of low concentrations of ATO and rapamycin were synergistic and more pronounced in U937 cells. The combination of drugs increased apoptosis in HL-60 cells and increased the percentage of cells in G(0)/G(1) phase in both cell lines. In U937 cells, rapamycin alone increased the activity of mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) and the addition of ATO decreased the level of phosphorylated ERK, Ser473 phosphorylated Akt and anti-apoptotic Mcl-1 protein. Primary AML cells show high sensitivity to growth-inhibitory effects of rapamycin alone or in combination with ATO. The results of the present study reveal the mechanism of the synergistic effects of two drugs at therapeutically achievable doses in non-APL AML cells.

5-Aminoimidazole-4-carboxamide ribonucleoside induces differentiation of acute myeloid leukemia cells.

Adenosine monophosphate (AMP)-activated kinase (AMPK) modulators have been shown to exert cytotoxic activity in hematological malignancies, but their role in the differentiation of acute myeloid leukemia (AML) is less explored. In this study, the effects of AMPK agonists on all-trans retinoic acid (ATRA)-mediated differentiation of acute promyelocytic leukemia (APL) and non-APL AML cell lines were investigated. The results show that AMPK agonists inhibit the growth of myeloblastic HL-60, promyelocytic NB4 and monocytic U937 cells. 5-Aminoimidazole-4-carboxamide ribonucleoside (AICAR), an AMPK activator, enhances ATRA-mediated differentiation of NB4 cells. In U937 cells, AICAR alone induces the expression of cell surface markers associated with mature monocytes and macrophages. In both cell lines, AICAR increases the activity of mitogen-activated protein kinase (MAPK), and the presence of a MAPK inhibitor reduces the expression of differentiation markers. These results reveal beneficial effects of AICAR in AML, including differentiation of non-APL AML cells.