Leveraging increased cytoplasmic nucleoside kinase activity to target mtDNA and oxidative phosphorylation in AML

Our current understanding of the biological impact of endometrial cancer mtDNA genome mutations and their potential use as a biomarker

Endometrial cancer (EC) is a devastating and common disease affecting women's health. The NCI Surveillance, Epidemiology, and End Results Program predicted that there would be >66,000 new cases in the United States and >13,000 deaths from EC in 2023, and EC is the sixth most common cancer among women worldwide. Regulation of mitochondrial metabolism plays a role in tumorigenesis. In proliferating cancer cells, mitochondria provide the necessary building blocks for biosynthesis of amino acids, lipids, nucleotides, and glucose. One mechanism causing altered mitochondrial activity is mitochondrial DNA (mtDNA) mutation. The polyploid human mtDNA genome is a circular double-stranded molecule essential to vertebrate life that harbors genes critical for oxidative phosphorylation plus mitochondrial-derived peptide genes. Cancer cells display aerobic glycolysis, known as the Warburg effect, which arises from the needs of fast-dividing cells and is characterized by increased glucose uptake and conversion of glucose to lactate. Solid tumors often contain at least one mtDNA substitution. Furthermore, it is common for cancer cells to harbor mixtures of wild-type and mutant mtDNA genotypes, known as heteroplasmy. Considering the increase in cancer cell energy demand, the presence of functionally relevant carcinogenesis-inducing or environment-adapting mtDNA mutations in cancer seems plausible. We review 279 EC tumor-specific mtDNA single nucleotide variants from 111 individuals from different studies. Many transition mutations indicative of error-prone DNA polymerase γ replication and C to U deamination events were present. We examine the spectrum of mutations and their heteroplasmy and discuss the potential biological impact of recurrent, non-synonymous, insertion, and deletion mutations. Lastly, we explore current EC treatments, exploiting cancer cell mitochondria for therapy and the prospect of using mtDNA variants as an EC biomarker.

Unveiling aging dynamics in the hematopoietic system insights from single-cell technologies

As the demographic structure shifts towards an aging society, strategies aimed at slowing down or reversing the aging process become increasingly essential. Aging is a major predisposing factor for many chronic diseases in humans. The hematopoietic system, comprising blood cells and their associated bone marrow microenvironment, intricately participates in hematopoiesis, coagulation, immune regulation and other physiological phenomena. The aging process triggers various alterations within the hematopoietic system, serving as a spectrum of risk factors for hematopoietic disorders, including clonal hematopoiesis, immune senescence, myeloproliferative neoplasms and leukemia. The emerging single-cell technologies provide novel insights into age-related changes in the hematopoietic system. In this review, we summarize recent studies dissecting hematopoietic system aging using single-cell technologies. We discuss cellular changes occurring during aging in the hematopoietic system at the levels of the genomics, transcriptomics, epigenomics, proteomics, metabolomics and spatial multi-omics. Finally, we contemplate the future prospects of single-cell technologies, emphasizing the impact they may bring to the field of hematopoietic system aging research.

MYC-driven increases in mitochondrial DNA copy number occur early and persist throughout prostatic cancer progression

Increased mitochondrial function may render some cancers vulnerable to mitochondrial inhibitors. Since mitochondrial function is regulated partly by mitochondrial DNA copy number (mtDNAcn), accurate measurements of mtDNAcn could help reveal which cancers are driven by increased mitochondrial function and may be candidates for mitochondrial inhibition. However, prior studies have employed bulk macrodissections that fail to account for cell type-specific or tumor cell heterogeneity in mtDNAcn. These studies have often produced unclear results, particularly in prostate cancer. Herein, we developed a multiplex in situ method to spatially quantify cell type-specific mtDNAcn. We show that mtDNAcn is increased in luminal cells of high-grade prostatic intraepithelial neoplasia (HGPIN), is increased in prostatic adenocarcinomas (PCa), and is further elevated in metastatic castration-resistant prostate cancer. Increased PCa mtDNAcn was validated by 2 orthogonal methods and is accompanied by increases in mtRNAs and enzymatic activity. Mechanistically, MYC inhibition in prostate cancer cells decreases mtDNA replication and expression of several mtDNA replication genes, and MYC activation in the mouse prostate leads to increased mtDNA levels in the neoplastic prostate cells. Our in situ approach also revealed elevated mtDNAcn in precancerous lesions of the pancreas and colon/rectum, demonstrating generalization across cancer types using clinical tissue samples.

Inhibition of mitochondria induces apoptosis and reduces telomere length and activity in acute myeloid leukemia stem cells

Acute myeloid leukemia (AML) is a highly lethal hematological malignancy in adults and children. Abnormal proliferation of leukemia stem cells (LSC) with CD34+ and CD38- phenotypes are the main clinical features of AML. Patients with AML face drug resistance and treatment failure due to a default in stem and progenitor cells. Therefore, defining LSC properties is necessary for targeting leukemia-initiating cells. Mitochondrial mass and activity increase in AML initiating cells compared with normal stem cells. This idea has offered the inhibition of the mitochondrial translation machinery to reduce the number of leukemia-initiating cells in patients with AML Tigecycline is an FDA-approved microbial antibiotic that inhibits oxidative phosphorylation in mitochondria, resulting in the suppression of leukemia cell proliferation with little toxicity to normal cells. Thus, the present study was conducted to evaluate whether LSC is influenced by mitochondrial inhibition. We measured the IC50 of tigecycline in KG-1a AML cell lines. KG-1a AML cell lines were separated into CD34+ and CD34- cells by MACS. In the following, these cells were treated with 20 µM (IC50) tigecycline. The expression of Annexin/PI, Caspase 3, apoptotic genes (BCL2, BCLX, BAX, BAD, and P53) and proteins (P53, BAX, BCL2 and Caspase 9) was evaluated in CD34+ , CD34- and KG-1a AML cells. In addition, the telomere length and expression of hTERT were evaluated in this study. The results indicated that BCl2 (gene and protein) and BCLX gene dramatically decreased. In addition, BAD, BAX, and P53 gene and protein expression significantly increased in CD34+ AML cells compared to CD34- AML cells. The results also suggested that tigecycline induced intrinsic (Cleaved-caspase 9/Pro-Caspase 9 ratio) and p53-mediated apoptosis. Furthermore, hTERT gene expression and telomere length decreased in the tigecycline-treated groups. Taken together, our findings indicate that inhibition of mitochondrial activity with tigecycline can induce apoptosis in cancer stem cells and can be used as a novel method for cancer therapy.

The role of bone marrow microenvironment (BMM) cells in acute myeloid leukemia (AML) progression: immune checkpoints, metabolic checkpoints, and signaling pathways

Acute myeloid leukemia (AML) comprises a multifarious and heterogeneous array of illnesses characterized by the anomalous proliferation of myeloid cells in the bone marrow microenvironment (BMM). The BMM plays a pivotal role in promoting AML progression, angiogenesis, and metastasis. The immune checkpoints (ICs) and metabolic processes are the key players in this process. In this review, we delineate the metabolic and immune checkpoint characteristics of the AML BMM, with a focus on the roles of BMM cells e.g. tumor-associated macrophages, natural killer cells, dendritic cells, metabolic profiles and related signaling pathways. We also discuss the signaling pathways stimulated in AML cells by BMM factors that lead to AML progression. We then delve into the roles of immune checkpoints in AML angiogenesis, metastasis, and cell proliferation, including co-stimulatory and inhibitory ICs. Lastly, we discuss the potential therapeutic approaches and future directions for AML treatment, emphasizing the potential of targeting metabolic and immune checkpoints in AML BMM as prognostic and therapeutic targets. In conclusion, the modulation of these processes through the use of directed drugs opens up new promising avenues in combating AML. Thereby, a comprehensive elucidation of the significance of these AML BMM cells' metabolic and immune checkpoints and signaling pathways on leukemic cells can be undertaken in the future investigations. Additionally, these checkpoints and cells should be considered plausible multi-targeted therapies for AML in combination with other conventional treatments in AML. Video Abstract.

A Leukemic Target with a Thousand Faces: The Mitochondria

In the era of personalized medicine greatly improved by molecular diagnosis and tailor-made therapies, the survival rate of acute myeloid leukemia (AML) at 5 years remains unfortunately low. Indeed, the high heterogeneity of AML clones with distinct metabolic and molecular profiles allows them to survive the chemotherapy-induced changes, thus leading to resistance, clonal evolution, and relapse. Moreover, leukemic stem cells (LSCs), the quiescent reservoir of residual disease, can persist for a long time and activate the recurrence of disease, supported by significant metabolic differences compared to AML blasts. All these points highlight the relevance to develop combination therapies, including metabolism inhibitors to improve treatment efficacy. In this review, we summarized the metabolic differences in AML blasts and LSCs, the molecular pathways related to mitochondria and metabolism are druggable and targeted in leukemia therapies, with a distinct interest for Venetoclax, which has revolutionized the therapeutic paradigms of several leukemia subtype, unfit for intensive treatment regimens.

MYC-driven increases in mitochondrial DNA copy number occur early and persist throughout prostatic cancer progression

Increased mitochondrial function may render some cancers vulnerable to mitochondrial inhibitors. Since mitochondrial function is regulated partly by mitochondrial DNA copy number (mtDNAcn), accurate measurements of mtDNAcn could help reveal which cancers are driven by increased mitochondrial function and may be candidates for mitochondrial inhibition. However, prior studies have employed bulk macrodissections that fail to account for cell type-specific or tumor cell heterogeneity in mtDNAcn. These studies have often produced unclear results, particularly in prostate cancer. Herein, we developed a multiplex in situ method to spatially quantify cell type specific mtDNAcn. We show that mtDNAcn is increased in luminal cells of high-grade prostatic intraepithelial neoplasia (HGPIN), is increased in prostatic adenocarcinomas (PCa), and is further elevated in metastatic castration-resistant prostate cancer. Increased PCa mtDNAcn was validated by two orthogonal methods and is accompanied by increases in mtRNAs and enzymatic activity. Mechanistically, MYC inhibition in prostate cancer cells decreases mtDNA replication and expression of several mtDNA replication genes, and MYC activation in the mouse prostate leads to increased mtDNA levels in the neoplastic prostate cells. Our in situ approach also revealed elevated mtDNAcn in precancerous lesions of the pancreas and colon/rectum, demonstrating generalization across cancer types using clinical tissue samples.

Metabolism in acute myeloid leukemia: mechanistic insights and therapeutic targets

Metabolic rewiring and cellular reprogramming are trademarks of neoplastic initiation and progression in acute myeloid leukemia (AML). Metabolic alteration in leukemic cells is often genotype specific, with associated changes in epigenetic and functional factors resulting in the downstream upregulation or facilitation of oncogenic pathways. Targeting abnormal or disease-sustaining metabolic activities in AML provides a wide range of therapeutic opportunities, ideally with enhanced therapeutic windows and robust clinical efficacy. This review highlights the dysregulation of amino acid, nucleotide, lipid, and carbohydrate metabolism in AML; explores the role of key vitamins and enzymes that regulate these processes; and provides an overview of metabolism-directed therapies currently in use or development.

Structural and Molecular Basis for Mitochondrial DNA Replication and Transcription in Health and Antiviral Drug Toxicity

Human mitochondrial DNA (mtDNA) is a 16.9 kbp double-stranded, circular DNA, encoding subunits of the oxidative phosphorylation electron transfer chain and essential RNAs for mitochondrial protein translation. The minimal human mtDNA replisome is composed of the DNA helicase Twinkle, DNA polymerase γ, and mitochondrial single-stranded DNA-binding protein. While the mitochondrial RNA transcription is carried out by mitochondrial RNA polymerase, mitochondrial transcription factors TFAM and TFB2M, and a transcription elongation factor, TEFM, both RNA transcriptions, and DNA replication machineries are intertwined and control mtDNA copy numbers, cellular energy supplies, and cellular metabolism. In this review, we discuss the mechanisms governing these main pathways and the mtDNA diseases that arise from mutations in transcription and replication machineries from a structural point of view. We also address the adverse effect of antiviral drugs mediated by mitochondrial DNA and RNA polymerases as well as possible structural approaches to develop nucleoside reverse transcriptase inhibitor and ribonucleosides analogs with reduced toxicity.

FUNDC1 protects against doxorubicin-induced cardiomyocyte PANoptosis through stabilizing mtDNA via interaction with TUFM

Doxorubicin (DOX) is an effective anthracycline chemotherapeutic anticancer drug with its life-threatening cardiotoxicity severely limiting its clinical application. Mitochondrial damage-induced cardiomyocyte death is considered an essential cue for DOX cardiotoxicity. FUN14 domain containing 1 (FUNDC1) is a mitochondrial membrane protein participating in the regulation of mitochondrial integrity in multiple diseases although its role in DOX cardiomyopathy remains elusive. Here, we examined whether PANoptosis, a novel type of programmed cell death closely associated with mitochondrial damage, was involved in DOX-induced heart injury, and FUNDC1-mediated regulation of cardiomyocyte PANoptosis, if any. FUNDC1 was downregulated in heart tissues in patients with dilated cardiomyopathy (DCM) and DOX-challenged mice. FUNDC1 deficiency aggravated DOX-induced cardiac dysfunction, mitochondrial injury, and cardiomyocyte PANoptosis. Further examination revealed that FUNDC1 countered cytoplasmic release of mitochondrial DNA (mtDNA) and activation of PANoptosome through interaction with mitochondrial Tu translation elongation factor (TUFM), a key factor in the translational expression and repair of mitochondrial DNA, via its 96-133 amino acid domain. TUFM intervention reversed FUNDC1-elicited protection against DOX-induced mtDNA cytosolic release and cardiomyocyte PANoptosis. Our findings shed light toward a beneficial role of FUNDC1 in DOX cardiotoxicity and cardiomyocyte PANoptosis, thus offering therapeutic promises in DOX-induced cardiotoxicity.

Contribution of metabolic abnormalities to acute myeloid leukemia pathogenesis

Acute myeloid leukemia (AML) is a malignant disease of myeloid precursors. Somatic mutations have long been accepted as drivers of this malignancy. Over the past decade, unique mitochondrial and metabolic dependencies of AML and AML stem cells have been identified, including a reliance on oxidative phosphorylation. More recently, metabolic enzymes have demonstrated noncanonical roles in regulating gene expression in AML, controlling cell differentiation and stemness. These mitochondrial and metabolic adaptations occur independent of underlying genomic abnormalities and contribute to chemoresistance and relapse. In this opinion article, we discuss the current understanding of AML pathogenesis and whether mitochondrial and metabolic abnormalities drive leukemogenesis or are a non-contributory phenotype.

Inhibition of Mitochondrial Biosynthesis Using a "Right-Side-Out" Membrane-Camouflaged Micelle to Facilitate the Therapeutic Effects of Shikonin on Triple-Negative Breast Cancer

The mitochondria represent a potential target for the treatment of triple-negative breast cancer (TNBC) and shikonin (SK) has shown remarkable therapeutic effects on TNBC. Herein, it is found that SK possesses potent inhibitory effects on mitochondrial biogenesis via targeting polymerase gamma (POLG). However, its application is restricted by its poor aqueous solubility and stability, and therefore, a biomimetic micelle to aid with tumor lesion accumulation and mitochondria-targeted delivery of SK is designed. A folic acid (FA) conjugated polyethylene glycol derivative (FA-PEG-FA) is inserted onto the external membranes of red blood cells (FP-RBCm) to prepare a "right-side-out" RBCm-camouflaged cationic micelle (ThTM/SK@FP-RBCm). Both FP-RBCm coating and a triphenylphosphine (TPP) moiety on the periphery of micelles contribute to tumor lesion distribution, receptor-mediated cellular uptake, and electrostatic attraction-dependent mitochondrial targeting, thereby maximizing inhibitory effects on mitochondrial biosynthesis in TNBC cells. Intravenous administration of ThTM/SK@FP-RBCm leads to profound inhibition of tumor growth and lung metastasis in a TNBC mouse model with no obvious toxicity. This work highlights the mitochondria-targeted delivery of SK using a "right-side-out" membrane-camouflaged micelle for the inhibition of mitochondrial biogenesis and enhanced therapeutic effects on TNBC.

Single-cell transcriptomics profiling the compatibility mechanism of As2O3-indigo naturalis formula based on bone marrow stroma cells

Compound realgar natural indigo tablet is the only oral arsenic agent widely used in acute promyelocytic leukemia (APL) treatment. However, as a therapeutic drug for diseases of the blood system, the scientific knowledge of As₂O₃-indigo naturalis formula compatibility has not been studied in bone marrow stromal cells (BMSCs). We chose arsenic trioxide (As₂O₃: A), tanshinone IIA (T) and indirubin (I) as representative active compounds of realgar, indigo naturalis, and Salvia miltiorrhiza, respectively, to evaluated the pharmaceutical mechanism and the compatibility of ATI (drug combination) using single-cell RNA sequencing (scRNA-seq). The overlapped genes associated with both disease and drug were selected in BMSCs for in-depth analysis. Results show that joint applications of ATI had the strongest therapeutic efficacy in a murine APL model. Lepr-MSCs, OLCs and BMECs were the sensitive cell groups targeted by ATI in the murine APL model. ATI could regulate the related genes of osteogenic differentiation, adipogenic differentiation, and endothelial cell migration in bone marrow mesenchymal lineage cells in murine APL model and improve normal hematopoiesis-related gene expression and poor prognosis of Lepr-MSCs, OLCs and BMECs in mice with leukemia according to scRNA-seq data. The strongest regulatory effects were found in the joint applications of ATI. ATI combination had the potential mechanism to maintain the stability of the hematopoietic microenvironment and promote hematopoiesis to assist in the treatment of APL. This study illustrated the potential mechanism of ATI in regulating BMSCs from the overall perspective of the hematopoietic microenvironment, and broadened the scientific understanding of ATI compatibility in BMSCs.

Disruption of dNTP homeostasis by ribonucleotide reductase hyperactivation overcomes AML differentiation blockade

Differentiation blockade is a hallmark of acute myeloid leukemia (AML). A strategy to overcome such a blockade is a promising approach against the disease. The lack of understanding of the underlying mechanisms hampers development of such strategies. Dysregulated ribonucleotide reductase (RNR) is considered a druggable target in proliferative cancers susceptible to deoxynucleoside triphosphate (dNTP) depletion. Herein, we report an unanticipated discovery that hyperactivating RNR enables differentiation and decreases leukemia cell growth. We integrate pharmacogenomics and metabolomics analyses to identify that pharmacologically (eg, nelarabine) or genetically upregulating RNR subunit M2 (RRM2) creates a dNTP pool imbalance and overcomes differentiation arrest. Moreover, R-loop-mediated DNA replication stress signaling is responsible for RRM2 activation by nelarabine treatment. Further aggravating dNTP imbalance by depleting the dNTP hydrolase SAM domain and HD domain-containing protein 1 (SAMHD1) enhances ablation of leukemia stem cells by RRM2 hyperactivation. Mechanistically, excessive activation of extracellular signal-regulated kinase (ERK) signaling downstream of the imbalance contributes to cellular outcomes of RNR hyperactivation. A CRISPR screen identifies a synthetic lethal interaction between loss of DUSP6, an ERK-negative regulator, and nelarabine treatment. These data demonstrate that dNTP homeostasis governs leukemia maintenance, and a combination of DUSP inhibition and nelarabine represents a therapeutic strategy.

Targeting Mitochondrial Oxidative Phosphorylation Eradicates Acute Myeloid Leukemic Stem Cells

Acute myeloid leukemia (AML) is a heterogeneous hematologic malignancy characterized by multiple cytogenetic and molecular abnormalities, with a very poor prognosis. Current treatments for AML often fail to eliminate leukemic stem cells (LSCs), which perpetuate the disease. LSCs exhibit a unique metabolic profile, especially dependent on oxidative phosphorylation (OXPHOS) for energy production. Whereas, normal hematopoietic stem cells (HSCs) and leukemic blasts rely on glycolysis for adenosine triphosphate (ATP) production. Thus, understanding the regulation of OXPHOS in LSCs may offer effective targets for developing clinical therapies in AML. This review summarizes these studies with a focus on the regulation of the electron transport chain (ETC) and tricarboxylic acid (TCA) cycle in OXPHOS and discusses potential therapies for eliminating LSCs.

Exploring the Metabolic Landscape of AML: From Haematopoietic Stem Cells to Myeloblasts and Leukaemic Stem Cells

Despite intensive chemotherapy regimens, up to 60% of adults with acute myeloid leukaemia (AML) will relapse and eventually succumb to their disease. Recent studies suggest that leukaemic stem cells (LSCs) drive AML relapse by residing in the bone marrow niche and adapting their metabolic profile. Metabolic adaptation and LSC plasticity are novel hallmarks of leukemogenesis that provide important biological processes required for tumour initiation, progression and therapeutic responses. These findings highlight the importance of targeting metabolic pathways in leukaemia biology which might serve as the Achilles' heel for the treatment of AML relapse. In this review, we highlight the metabolic differences between normal haematopoietic cells, bulk AML cells and LSCs. Specifically, we focus on four major metabolic pathways dysregulated in AML; (i) glycolysis; (ii) mitochondrial metabolism; (iii) amino acid metabolism; and (iv) lipid metabolism. We then outline established and emerging drug interventions that exploit metabolic dependencies of leukaemic cells in the treatment of AML. The metabolic signature of AML cells alters during different biological conditions such as chemotherapy and quiescence. Therefore, targeting the metabolic vulnerabilities of these cells might selectively eradicate them and improve the overall survival of patients with AML.

Fluorescence Imaging of Mitochondrial DNA Base Excision Repair Reveals Dynamics of Oxidative Stress Responses

Mitochondrial function in cells declines with aging and with neurodegeneration, due in large part to accumulated mutations in mitochondrial DNA (mtDNA) that arise from deficient DNA repair. However, measuring this repair activity is challenging. We employ a molecular approach for visualizing mitochondrial base excision repair (BER) activity in situ by use of a fluorescent probe (UBER) that reacts rapidly with AP sites resulting from BER activity. Administering the probe to cultured cells revealed signals that were localized to mitochondria, enabling selective observation of mtDNA BER intermediates. The probe showed elevated DNA repair activity under oxidative stress, and responded to suppression of glycosylase activity. Furthermore, the probe illuminated the time lag between the initiation of oxidative stress and the initial step of BER. Absence of MTH1 in cells resulted in elevated demand for BER activity upon extended oxidative stress, while the absence of OGG1 activity limited glycosylation capacity.

The STAT3-MYC axis promotes survival of leukemia stem cells by regulating SLC1A5 and oxidative phosphorylation

Acute myeloid leukemia (AML) is characterized by the presence of leukemia stem cells (LSCs), and failure to fully eradicate this population contributes to disease persistence/relapse. Prior studies have characterized metabolic vulnerabilities of LSCs, which demonstrate preferential reliance on oxidative phosphorylation (OXPHOS) for energy metabolism and survival. In the present study, using both genetic and pharmacologic strategies in primary human AML specimens, we show that signal transducer and activator of transcription 3 (STAT3) mediates OXPHOS in LSCs. STAT3 regulates AML-specific expression of MYC, which in turn controls transcription of the neutral amino acid transporter gene SLC1A5. We show that genetic inhibition of MYC or SLC1A5 acts to phenocopy the impairment of OXPHOS observed with STAT3 inhibition, thereby establishing this axis as a regulatory mechanism linking STAT3 to energy metabolism. Inhibition of SLC1A5 reduces intracellular levels of glutamine, glutathione, and multiple tricarboxylic acid (TCA) cycle metabolites, leading to reduced TCA cycle activity and inhibition of OXPHOS. Based on these findings, we used a novel small molecule STAT3 inhibitor, which binds STAT3 and disrupts STAT3-DNA, to evaluate the biological role of STAT3. We show that STAT3 inhibition selectively leads to cell death in AML stem and progenitor cells derived from newly diagnosed patients and patients who have experienced relapse while sparing normal hematopoietic cells. Together, these findings establish a STAT3-mediated mechanism that controls energy metabolism and survival in primitive AML cells.

The Importance of Cellular Metabolic Pathways in Pathogenesis and Selective Treatments of Hematological Malignancies

Despite recent advancements in the treatment of hematologic malignancies and the emergence of newer and more sophisticated therapeutic approaches such as immunotherapy, long-term overall survival remains unsatisfactory. Metabolic alteration, as an important hallmark of cancer cells, not only contributes to the malignant transformation of cells, but also promotes tumor progression and metastasis. As an immune-escape mechanism, the metabolic adaptation of the bone marrow microenvironment and leukemic cells is a major player in the suppression of anti-leukemia immune responses. Therefore, metabolic rewiring in leukemia would provide promising opportunities for newer therapeutic interventions. Several therapeutic agents which affect essential bioenergetic pathways in cancer cells including glycolysis, β-oxidation of fatty acids and Krebs cycle, or anabolic pathways such as lipid biosynthesis and pentose phosphate pathway, are being tested in various types of cancers. So far, numerous preclinical or clinical trial studies using such metabolic agents alone or in combination with other remedies such as immunotherapy are in progress and have demonstrated promising outcomes. In this review, we aim to argue the importance of metabolic alterations and bioenergetic pathways in different types of leukemia and their vital roles in disease development. Designing treatments based on targeting leukemic cells vulnerabilities, particularly in nonresponsive leukemia patients, should be warranted.

Venetoclax in Acute Myeloid Leukemia: Molecular Basis, Evidences for Preclinical and Clinical Efficacy and Strategies to Target Resistance

Venetoclax is a BH3-mimetics agent specifically interacting with the antiapoptotic protein BCL-2, facilitating cytochrome c release from mitochondria, subsequent caspases activation, and cell death. Utilization of venetoclax has profoundly changed the landscape of treatment for the poor-prognosis category of AML patients unfit for intensive chemotherapy. In the phase III VIALE-A study, Venetoclax, in combination with the hypomethylating agent azacitidine, showed a 65% overall response rate and 14.7-month overall survival, in comparison with 22% and 8 months in the control arm. These results led to the widespread use of venetoclax in this indication. Other combination regimens, consisting of low-intensity, intensive, or targeted therapies are currently under evaluation. Despite promising results, preventing relapses or resistance to venetoclax is still an unmet clinical need. Numerous studies have been conducted to identify and overcome venetoclax resistance in preclinical models or in clinical trials, including the inhibition of other antiapoptotic proteins, the induction of proapoptotic BH3-only proteins, and/or the targeting of the mitochondrial metabolism and machinery.

Mitochondrial complex II and V activity is enhanced in pediatric acute myeloid leukemia

BACKGROUND

Mitochondrial bioenergetic alterations are commonly observed metabolic adaptation in malignancies including acute myeloid leukemia (AML). Mitochondrial DNA alterations are well known in pediatric AML with possible prognostic significance; however, mitochondrial complex activity and its impact on disease outcome have not been previously explored. The aim of this study was to evaluate the mitochondrial complex II and complex V activity and its prognostic significance in pediatric AML patients.

METHODS

Consecutive 82 de novo pediatric (≤18 years) patients with AML were included in the study along with age and sex matched controls. Bone marrow mononuclear cells were isolated from baseline bone marrow samples from all patients and controls. DNA, RNA and proteins were extracted and relative expression of mitochondrial biogenesis genes TFAM, POLG, POLRMT were estimated along with mitochondrial DNA copy number. The mitochondrial complex II and V enzymes were immunocaptured and their activity was measured by substrate specific absorbance change by kinetic ELISA. The mitochondrial complex II and V activity was compared with controls and their association with clinico-pathological features and survival outcome were analysed. Complex activity was also correlated with relative expression of biogenesis genes.

RESULTS

The activity of mitochondrial complex II and V were found to be significantly enhanced (P = 0.010 and P = 0.0013 respectively) in pediatric AML patients compared to controls. The activity of mitochondrial complex II and V showed significant positive correlation with relative gene expression of mitochondrial biogenesis genes TFAM (P = 0.001 and P = 0.016 respectively) and POLG (P = 0.005 and P = 0.006 respectively). Neither of the two complex activities showed any significant association with baseline disease demographics or any clinico-pathological feature. Furthermore, the complex II and V activity did not show any impact on event free survival (P = 0.25 and P = 0.24 respectively) and overall survival (P = 0.14 and P = 0.17 respectively) in our cohort.

CONCLUSION

The activity of both mitochondrial complex II and V are significantly elevated in bone marrow mononuclear cells of children with AML compared to controls. The enhanced activity may be related to upregulation of mitochondrial biogenesis genes TFAM and POLG. The enhanced activity of either of the complexes did not impact disease biology or survival outcomes in pediatric AML.

The Combined Treatment With the FLT3-Inhibitor AC220 and the Complex I Inhibitor IACS-010759 Synergistically Depletes Wt- and FLT3-Mutated Acute Myeloid Leukemia Cells

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy with a high mortality rate and relapse risk. Although progress on the genetic and molecular understanding of this disease has been made, the standard of care has changed minimally for the past 40 years and the five-year survival rate remains poor, warranting new treatment strategies. Here, we applied a two-step screening platform consisting of a primary cell viability screening and a secondary metabolomics-based phenotypic screening to find synergistic drug combinations to treat AML. A novel synergy between the oxidative phosphorylation inhibitor IACS-010759 and the FMS-like tyrosine kinase 3 (FLT3) inhibitor AC220 (quizartinib) was discovered in AML and then validated by ATP bioluminescence and apoptosis assays. In-depth stable isotope tracer metabolic flux analysis revealed that IACS-010759 and AC220 synergistically reduced glucose and glutamine enrichment in glycolysis and the TCA cycle, leading to impaired energy production and de novo nucleotide biosynthesis. In summary, we identified a novel drug combination, AC220 and IACS-010759, which synergistically inhibits cell growth in AML cells due to a major disruption of cell metabolism, regardless of FLT3 mutation status.

Disrupting Mitochondrial Electron Transfer Chain Complex I Decreases Immune Checkpoints in Murine and Human Acute Myeloid Leukemic Cells

Oxidative metabolism is crucial for leukemic stem cell (LSC) function and drug resistance in acute myeloid leukemia (AML). Mitochondrial metabolism also affects the immune system and therefore the anti-tumor response. The modulation of oxidative phosphorylation (OxPHOS) has emerged as a promising approach to improve the therapy outcome for AML patients. However, the effect of mitochondrial inhibitors on the immune compartment in the context of AML is yet to be explored. Immune checkpoints such as ectonucleotidase CD39 and programmed dead ligand 1 (PD-L1) have been reported to be expressed in AML and linked to chemo-resistance and a poor prognosis. In the present study, we first demonstrated that a novel selective electron transfer chain complex (ETC) I inhibitor, EVT-701, decreased the OxPHOS metabolism of murine and human cytarabine (AraC)-resistant leukemic cell lines. Furthermore, we showed that while AraC induced an immune response regulation by increasing CD39 expression and by reinforcing the interferon-γ/PD-L1 axis, EVT-701 reduced CD39 and PD-L1 expression in vitro in a panel of both murine and human AML cell lines, especially upon AraC treatment. Altogether, this work uncovers a non-canonical function of ETCI in controlling CD39 and PD-L1 immune checkpoints, thereby improving the anti-tumor response in AML.

Mitochondrial metabolism as a target for acute myeloid leukemia treatment

Acute myeloid leukemias (AML) are a group of aggressive hematologic malignancies resulting from acquired genetic mutations in hematopoietic stem cells that affect patients of all ages. Despite decades of research, standard chemotherapy still remains ineffective for some AML subtypes and is often inappropriate for older patients or those with comorbidities. Recently, a number of studies have identified unique mitochondrial alterations that lead to metabolic vulnerabilities in AML cells that may present viable treatment targets. These include mtDNA, dependency on oxidative phosphorylation, mitochondrial metabolism, and pro-survival signaling, as well as reactive oxygen species generation and mitochondrial dynamics. Moreover, some mitochondria-targeting chemotherapeutics and their combinations with other compounds have been FDA-approved for AML treatment. Here, we review recent studies that illuminate the effects of drugs and synergistic drug combinations that target diverse biomolecules and metabolic pathways related to mitochondria and their promise in experimental studies, clinical trials, and existing chemotherapeutic regimens.

Mitochondrial DNA stress triggers autophagy-dependent ferroptotic death

Pancreatic cancer tends to be highly resistant to current therapy and remains one of the great challenges in biomedicine with very low 5-year survival rates. Here, we report that zalcitabine, an antiviral drug for human immunodeficiency virus infection, can suppress the growth of primary and immortalized human pancreatic cancer cells through the induction of ferroptosis, an iron-dependent form of regulated cell death. Mechanically, this effect relies on zalcitabine-induced mitochondrial DNA stress, which activates the STING1/TMEM173-mediated DNA sensing pathway, leading to macroautophagy/autophagy-dependent ferroptotic cell death via lipid peroxidation, but not a type I interferon response. Consequently, the genetic and pharmacological inactivation of the autophagy-dependent ferroptosis pathway diminishes the anticancer effects of zalcitabine in cell culture and animal models. Together, these findings not only provide a new approach for pancreatic cancer therapy but also increase our understanding of the interplay between autophagy and DNA damage response in shaping cell death.Abbreviations: ALOX: arachidonate lipoxygenase; ARNTL/BMAL1: aryl hydrocarbon receptor nuclear translocator-like; ATM: ATM serine/threonine kinase; ATG: autophagy-related; cGAMP: cyclic GMP-AMP; CGAS: cyclic GMP-AMP synthase; ER: endoplasmic reticulum; FANCD2: FA complementation group D2; GPX4: glutathione peroxidase 4; IFNA1/IFNα: interferon alpha 1; IFNB1/IFNβ: interferon beta 1; MAP1LC3B/LC3: microtubule-associated protein 1 light chain 3 beta; MDA: malondialdehyde; mtDNA: mitochondrial DNA; NCOA4: nuclear receptor coactivator 4; PDAC: pancreatic ductal adenocarcinoma; POLG: DNA polymerase gamma, catalytic subunit; qRT-PCR: quantitative polymerase chain reaction; RCD: regulated cell death; ROS: reactive oxygen species; SLC7A11: solute carrier family 7 member 11; STING1/TMEM173: stimulator of interferon response cGAMP interactor 1; TFAM: transcription factor A, mitochondrial.

Mitochondria: The metabolic switch of cellular oncogenic transformation

Mitochondria, well recognized as the "powerhouse" of cells, are maternally inherited organelles with bacterial ancestry that play essential roles in a myriad of cellular functions. It has become profoundly evident that mitochondria regulate a wide array of cellular and metabolic functions, including biosynthetic metabolism, cell signaling, redox homeostasis, and cell survival. Correspondingly, defects in normal mitochondrial functioning have been implicated in various human malignancies. Cancer development involves the activation of oncogenes, inactivation of tumor suppressor genes, and impairment of apoptotic programs in cells. Mitochondria have been recognized as the site of key metabolic switches for normal cells to acquire a malignant phenotype. This review outlines the role of mitochondria in human malignancies and highlights potential aspects of mitochondrial metabolism that could be targeted for therapeutic development.

Mitochondrial and Metabolic Pathways Regulate Nuclear Gene Expression to Control Differentiation, Stem Cell Function, and Immune Response in Leukemia

Mitochondria are involved in many biological processes including cellular homeostasis, energy generation, and apoptosis. Moreover, mitochondrial and metabolic pathways are interconnected with gene expression to regulate cellular functions such as cell growth, survival, differentiation, and immune recognition. Metabolites and mitochondrial enzymes regulate chromatin-modifying enzymes, chromatin remodeling, and transcription regulators. Deregulation of mitochondrial pathways and metabolism leads to alterations in gene expression that promote cancer development, progression, and evasion of the immune system. This review highlights how mitochondrial and metabolic pathways function as a central mediator to control gene expression, specifically on stem cell functions, differentiation, and immune response in leukemia. SIGNIFICANCE: Emerging evidence demonstrates that mitochondrial and metabolic pathways influence gene expression to promote tumor development, progression, and immune evasion. These data highlight new areas of cancer biology and potential new therapeutic strategies.

The antiretroviral 2',3'-dideoxycytidine causes mitochondrial dysfunction in proliferating and differentiated HepaRG human cell cultures

Nucleoside reverse transcriptase inhibitors (NRTIs) were the first drugs used to treat human immunodeficiency virus infection, and their use can cause mitochondrial toxicity, including mitochondrial DNA (mtDNA) depletion in several cases. The first-generation NRTIs, including 2',3'-dideoxycytidine (ddC), were originally and are still pursued as anticancer agents. NRTI-sensitive DNA polymerases localizing to mitochondria allow for the opportunity to poison proliferating cancer cell mtDNA replication as certain cancers rely heavily on mitochondrial functions. However, mtDNA replication is independent of the cell cycle creating a significant concern that toxicants such as ddC impair mtDNA maintenance in both proliferating and nonproliferating cells. To examine this possibility, we tested the utility of the HepaRG cell line to study ddC-induced toxicity in isogenic proliferating (undifferentiated) and nonproliferating (differentiated) cells. Following ddC exposures, we measured cell viability, mtDNA copy number, and mitochondrial bioenergetics utilizing trypan blue, Southern blotting, and extracellular flux analysis, respectively. After 13 days of 1 μM ddC exposure, proliferating and differentiated HepaRG harbored mtDNA levels of 0.9% and 17.9% compared with control cells, respectively. Cells exposed to 12 μM ddC contained even less mtDNA. By day 13, differentiated cell viability was maintained but declined for proliferating cells. Proliferating HepaRG bioenergetic parameters were severely impaired by day 8, with 1 and 12 μM ddC, whereas differentiated cells displayed defects of spare and maximal respiratory capacities (day 8) and proton-leak linked respiration (day 14) with 12 μM ddC. These results indicate HepaRG is a useful model to study proliferating and differentiated cell mitochondrial toxicant exposures.

Targeting MCL-1 dysregulates cell metabolism and leukemia-stroma interactions and resensitizes acute myeloid leukemia to BCL-2 inhibition

MCL-1 and BCL-2 are both frequently overexpressed in acute myeloid leukemia (AML) and critical for the survival of AML cells and AML stem cells. MCL-1 is a key factor in venetoclax resistance. Using genetic and pharmacological approaches, we discovered that MCL-1 regulates leukemia cell bioenergetics and carbohydrate metabolisms, including the TCA cycle, glycolysis and pentose phosphate pathway and modulates cell adhesion proteins and leukemia-stromal interactions. Inhibition of MCL-1 sensitizes to BCL-2 inhibition in AML cells and AML stem/progenitor cells, including those with intrinsic and acquired resistance to venetoclax through cooperative release of pro-apoptotic BIM, BAX, and BAK from binding to anti-apoptotic BCL- 2 proteins and inhibition of cell metabolism and key stromal microenvironmental mechanisms. The combined inhibition of MCL-1 by MCL-1 inhibitor AZD5991 or CDK9 inhibitor AZD4573 and BCL-2 by venetoclax greatly extended survival of mice bearing patient-derived xenografts established from an AML patient who acquired resistance to venetoclax/decitabine. These results demonstrate that co-targeting MCL-1 and BCL-2 improves the efficacy of and overcomes pre-existing and acquired resistance to BCL-2 inhibition. Activation of metabolomic pathways and leukemia-stroma interactions are newly discovered functions of MCL-1 in AML, which are independent from canonical regulation of apoptosis by MCL-1. Our data provide new mechanisms of synergy and a rationale for co-targeting MCL-1 and BCL-2 clinically in patients with AML and potentially other cancers.

Cancer cell metabolism: Rewiring the mitochondrial hub

To adapt to tumoral environment conditions or even to escape chemotherapy, cells rapidly reprogram their metabolism to handle adversities and survive. Given the rapid rise of studies uncovering novel insights and therapeutic opportunities based on the role of mitochondria in tumor metabolic programing and therapeutics, this review summarizes most significant developments in the field. Taking in mind the key role of mitochondria on carcinogenesis and tumor progression due to their involvement on tumor plasticity, metabolic remodeling, and signaling re-wiring, those organelles are also potential therapeutic targets. Among other topics, we address the recent data intersecting mitochondria as of prognostic value and staging in cancer, by mitochondrial DNA (mtDNA) determination, and current inhibitors developments targeting mtDNA, OXPHOS machinery and metabolic pathways. We contribute for a holistic view of the role of mitochondria metabolism and directed therapeutics to understand tumor metabolism, to circumvent therapy resistance, and to control tumor development.

Targeting mitochondrial respiration for the treatment of acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease with variable presentation, molecular phenotype, and cytogenetic abnormalities and has seen very little improvement in patient survival over the last few decades. This heterogeneity supports poor prognosis partially through the variability in response to the standard chemotherapy. Further understanding of molecular heterogeneity has promoted the development of novel treatments, some of which target mitochondrial metabolism and function. This review discusses the relative dependency that AML cells have on mitochondrial function, and the ability to pivot this reliance to target important subsets of AML cells, including leukemia stem cells (LSCs). LSCs are tumor-initiating cells that are resistant to standard chemotherapy and promote the persistence and relapse of AML. Historically, LSCs have been targeted based on immunophenotype, but recent developments in the understanding of LSC metabolism has demonstrated unique abilities to target LSCs while sparing normal hematopoietic stem cells (HSCs) through inhibition of mitochondrial function. Here we highlight the use of small molecules that have been demonstrated to effectively target mitochondrial function. IACS-010759 and ME-344 target the electron transport chain (ETC) to inhibit oxidative phosphorylation (OXPHOS). The imipridone family (ONC201, ONC206, ONC212) of inhibitors target mitochondria through activation of ClpP mitochondrial protease and reduce function of essential pathways. These molecules offer a new mechanism for developing clinical therapies in AML and support novel strategies to target LSCs in parallel with conventional therapies.

Nicotinamide Metabolism Mediates Resistance to Venetoclax in Relapsed Acute Myeloid Leukemia Stem Cells

We previously demonstrated that leukemia stem cells (LSCs) in de novo acute myeloid leukemia (AML) patients are selectively reliant on amino acid metabolism and that treatment with the combination of venetoclax and azacitidine (ven/aza) inhibits amino acid metabolism, leading to cell death. In contrast, ven/aza fails to eradicate LSCs in relapsed/refractory (R/R) patients, suggesting altered metabolic properties. Detailed metabolomic analysis revealed elevated nicotinamide metabolism in relapsed LSCs, which activates both amino acid metabolism and fatty acid oxidation to drive OXPHOS, thereby providing a means for LSCs to circumvent the cytotoxic effects of ven/aza therapy. Genetic and pharmacological inhibition of nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme in nicotinamide metabolism, demonstrated selective eradication of R/R LSCs while sparing normal hematopoietic stem/progenitor cells. Altogether, these findings demonstrate that elevated nicotinamide metabolism is both the mechanistic basis for ven/aza resistance and a metabolic vulnerability of R/R LSCs.

The role of mitochondrial proteases in leukemic cells and leukemic stem cells

The biological function of most mitochondrial proteases has not been well characterized. Moreover, most of the available information on the normal function of these proteases has been derived from studies in model organisms. Recently, the mitochondrial proteases caseinolytic protease P (CLPP) and neurolysin (NLN) have been identified as therapeutic targets in acute myeloid leukemia (AML). Both proteases are overexpressed in approximately 40% of AML patients. Mechanistically, CLPP and NLN maintain the integrity of the mitochondrial respiratory chain: CLPP cleaves defective respiratory chain proteins, while NLN promotes the formation of respiratory chain supercomplexes. In this review, we highlight the functional consequences of inhibiting and activating mitochondrial proteases and discuss their potential as therapeutic targets in AML.

An in Situ Atlas of Mitochondrial DNA in Mammalian Tissues Reveals High Content in Stem and Proliferative Compartments

Mitochondria regulate ATP production, metabolism, and cell death. Alterations in mitochondrial DNA (mtDNA) sequence and copy number are implicated in aging and organ dysfunction in diverse inherited and sporadic diseases. Because most measurements of mtDNA use homogenates of complex tissues, little is known about cell-type-specific mtDNA copy number heterogeneity in normal physiology, aging, and disease. Thus, the precise cell types whose loss of mitochondrial activity and altered mtDNA copy number that result in organ dysfunction in aging and disease have often not been clarified. Here, an in situ hybridization approach to generate a single-cell-resolution atlas of mtDNA content in mammalian tissues was validated. In hierarchically organized self-renewing tissues, higher levels of mtDNA were observed in stem/proliferative compartments compared with differentiated compartments. Striking zonal patterns of mtDNA levels in the liver reflected the known oxygen tension gradient. In the kidney, proximal and distal tubules had markedly higher mtDNA levels compared with cells within glomeruli and collecting duct epithelial cells. In mice, decreased mtDNA levels were visualized in renal tubules as a function of aging, which was prevented by calorie restriction. This study provides a novel approach for quantifying species- and cell-type-specific mtDNA copy number and dynamics in any normal or diseased tissue that can be used for monitoring the effects of interventions in animal and human studies.

Global Interactome Mapping of Mitochondrial Intermembrane Space Proteases Identifies a Novel Function for HTRA2

A number of unique proteases localize to specific sub-compartments of the mitochondria, but the functions of these enzymes are poorly defined. Here, in vivo proximity-dependent biotinylation (BioID) is used to map the interactomes of seven proteases localized to the mitochondrial intermembrane space (IMS). In total, 802 high confidence proximity interactions with 342 unique proteins are identified. While all seven proteases co-localized with the IMS markers OPA1 and CLPB, 230 of the interacting partners are unique to just one or two protease bait proteins, highlighting the ability of BioID to differentiate unique interactomes within the confined space of the IMS. Notably, high-temperature requirement peptidase 2 (HTRA2) interacts with eight of 13 components of the mitochondrial intermembrane space bridging (MIB) complex, a multiprotein assembly essential for the maintenance of mitochondrial cristae structure. Knockdown of HTRA2 disrupts cristae in HEK 293 and OCI-AML2 cells, and leads to increased intracellular levels of the MIB subunit IMMT. Using a cell-free assay it is demonstrated that HTRA2 can degrade recombinant IMMT but not two other core MIB complex subunits, SAMM50 and CHCHD3. The IMS protease interactome thus represents a rich dataset that can be mined to uncover novel IMS protease biology.

Aurora kinase inhibitor restrains STAT5-activated leukemic cell proliferation by inducing mitochondrial impairment

Current chemotherapy regimens on acute myeloid leukemia (AML) still have some drawbacks, such as intolerance and drug resistance, which calls need for the development of targeted therapy. Signal transducer and activator of transcription 5 (STAT5) is often overexpressed or abnormally activated in leukemia and involved in cell self-renewal, proliferation, and stress adaptation. Overexpressed Aurora A (AURKA) is associated with poor prognosis in tumors, and inhibitors against AURKA are already in clinical trials. However, it has rarely been reported whether AURKA inhibitors restrain STAT5-activated leukemia cells. In this study, we constructed STAT5 constitutively activated (cS5) cells and found that STAT5 promoted cell proliferation and colony formation. Moreover, cS5 cells showed elevated reactive oxygen species (ROS) and adenosine triphosphate (ATP) levels, which indicated higher mitochondrial metabolism in cS5 cells. A novel AURKA inhibitor AKI604 was synthesized and showed significant inhibitory effects to the proliferation and colony formation in both STAT5 constitutively activated and nonactivated AML cells. AKI604 induced mitochondrial impairment, leading to the disruption of mitochondrial membrane potential and the elevation of ROS as well as cellular calcium (Ca²⁺ ) levels. AKI604 could also decline basal oxygen consumption rate and ATP biosynthesis, indicating the damage of oxidative phosphorylation. Furthermore, AKI604 exhibited significant antitumor effect in the HL-60 cS5 xenograft model of the BALB/c nude mice without an obvious influence on mice body weight and other healthy indicators. This study suggested that AKI604 was a potential strategy to overcome STAT5-induced leukemic proliferation in AML treatment by inducing mitochondrial impairment.

Alterations of Mitochondria and Related Metabolic Pathways in Leukemia: A Narrative Review

Dysregulation of mitochondrial function often precedes malignant transformation of hematopoietic stem cells (HSCs). Mitochondria have a direct role in the maintenance of HSC functions. For example, D-2-hydroxyglutarate, generated due to the activity of mutated mitochondrial isocitrate dehydrogenase (IDH), has been implicated in the pathogenesis of leukemia. Furthermore, disturbances in the fatty acid breakdown and pyruvate oxidation are often seen in leukemic cells. These and other abnormalities expedite leukemogenesis and chemoresistance of leukemic cells. However, it needs to be elucidated whether these aberrations are the result or cause of leukemogenesis. Accordingly, for this review, a search was carried out in PubMed and Google Scholar databases until June 2019 to assess the relationship between metabolic pathways in altered mitochondria and leukemia development. In the present review, an overview of mitochondria-related mechanisms and their abnormalities in leukemia is presented, with mitochondrial pathways and factors, such as mitophagy, intermediary metabolism enzymes, oncometabolites and reactive oxygen species' generation, discussed as potential diagnostic and therapeutic targets in leukemia.

Can we selectively target AML stem cells?

De novo acute myeloid leukemia (AML) leukemia stem cell (LSC) populations are uniquely dependent on amino acid metabolism to drive the TCA cycle and oxidative phosphorylation. Oxidative phosphorylation can be selectively downregulated in de novo AML LSC populations by perturbing amino acid metabolism via BCL2 inhibition with venetoclax. While venetoclax-based therapies have shown high response rates, not all patients achieve remission. It may be possible to prospectively identify the patients who will most likely respond to venetoclax-based treatment by analyzing the metabolic properties of individual patients. Specifically, it appears that patients who are likely to be resistant to venetoclax-based therapy are able to employ alternate metabolic pathways to drive oxidative phosphorylation.

Metabolic Plasticity of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is one of the most common and life-threatening leukemias. A highly diverse and flexible metabolism contributes to the aggressiveness of the disease that is still difficult to treat. By using different sources of nutrients for energy and biomass supply, AML cells gain metabolic plasticity and rapidly outcompete normal hematopoietic cells. This review aims to decipher the diverse metabolic strategies and the underlying oncogenic and environmental changes that sustain continuous growth, mediate redox homeostasis and induce drug resistance in AML. We revisit Warburg's hypothesis and illustrate the role of glucose as a provider of cellular building blocks rather than as a supplier of the tricarboxylic acid (TCA) cycle for energy production. We discuss how the diversity of fuels for the TCA cycle, including glutamine and fatty acids, contributes to the metabolic plasticity of the disease and highlight the roles of amino acids and lipids in AML metabolism. Furthermore, we point out the potential of the different metabolic effectors to be used as novel therapeutic targets.

Exploiting metabolic vulnerabilities for personalized therapy in acute myeloid leukemia

Changes in cell metabolism and metabolic adaptation are hallmark features of many cancers, including leukemia, that support biological processes involved into tumor initiation, growth, and response to therapeutics. The discovery of mutations in key metabolic enzymes has highlighted the importance of metabolism in cancer biology and how these changes might constitute an Achilles heel for cancer treatment. In this Review, we discuss the role of metabolic and mitochondrial pathways dysregulated in acute myeloid leukemia, and the potential of therapeutic intervention targeting these metabolic dependencies on the proliferation, differentiation, stem cell function and cell survival to improve patient stratification and outcomes.

Mubritinib Targets the Electron Transport Chain Complex I and Reveals the Landscape of OXPHOS Dependency in Acute Myeloid Leukemia

To identify therapeutic targets in acute myeloid leukemia (AML), we chemically interrogated 200 sequenced primary specimens. Mubritinib, a known ERBB2 inhibitor, elicited strong anti-leukemic effects in vitro and in vivo. In the context of AML, mubritinib functions through ubiquinone-dependent inhibition of electron transport chain (ETC) complex I activity. Resistance to mubritinib characterized normal CD34⁺ hematopoietic cells and chemotherapy-sensitive AMLs, which displayed transcriptomic hallmarks of hypoxia. Conversely, sensitivity correlated with mitochondrial function-related gene expression levels and characterized a large subset of chemotherapy-resistant AMLs with oxidative phosphorylation (OXPHOS) hyperactivity. Altogether, our work thus identifies an ETC complex I inhibitor and reveals the genetic landscape of OXPHOS dependency in AML.

Emerging Therapies for Acute Myelogenus Leukemia Patients Targeting Apoptosis and Mitochondrial Metabolism

Acute Myelogenous Leukemia (AML) is a malignant disease of the hematopoietic cells, characterized by impaired differentiation and uncontrolled clonal expansion of myeloid progenitors/precursors, resulting in bone marrow failure and impaired normal hematopoiesis. AML comprises a heterogeneous group of malignancies, characterized by a combination of different somatic genetic abnormalities, some of which act as events driving leukemic development. Studies carried out in the last years have shown that AML cells invariably have abnormalities in one or more apoptotic pathways and have identified some components of the apoptotic pathway that can be targeted by specific drugs. Clinical results deriving from studies using B-cell lymphoma 2 (BCL-2) inhibitors in combination with standard AML agents, such as azacytidine, decitabine, low-dose cytarabine, provided promising results and strongly support the use of these agents in the treatment of AML patients, particularly of elderly patients. TNF-related apoptosis-inducing ligand (TRAIL) and its receptors are frequently deregulated in AML patients and their targeting may represent a promising strategy for development of new treatments. Altered mitochondrial metabolism is a common feature of AML cells, as supported through the discovery of mutations in the isocitrate dehydrogenase gene and in mitochondrial electron transport chain and of numerous abnormalities of oxidative metabolism existing in AML subgroups. Overall, these observations strongly support the view that the targeting of mitochondrial apoptotic or metabolic machinery is an appealing new therapeutic perspective in AML.

Comparison of HepaRG cells following growth in proliferative and differentiated culture conditions reveals distinct bioenergetic profiles

HepaRG is a proliferative human hepatoma-derived cell line that can be differentiated into hepatocyte-like and biliary-like cells. Differentiated HepaRG cultures maintain key hepatic functions including drug transporters and xenobiotic-metabolizing enzymes. To gain insight into proliferative and differentiated HepaRG metabolism we profiled various bioenergetic parameters and investigated cell culture levels of adenosine triphosphate (ATP), lactate, and lactate dehydrogenase (LDH) activity. Compared to differentiated-derived HepaRG, cells from proliferative cultures had increased basal and ATP-linked respiration and decreased maximal and spare respiratory capacities. Basal ATP levels but not lactate or LDH activity were increased in samples from proliferative-derived compared to differentiated-derived HepaRG. Further extracellular acidification rate (ECAR) experiments revealed parameters associated with glycolysis and oxidative phosphorylation. Under basal conditions, cells derived from both cultures had similar ECARs; however, under stressed conditions, proliferative-derived HepaRG had increases in ECAR capacity and apparent glycolytic reserve. The biguanide metformin has been reported to protect differentiated HepaRG against acetaminophen (APAP)-induced cell injury, as well as offer protection against bioenergetic deficiencies; therefore, we studied the outcome of exposure to these drugs in both culture conditions. Proliferative- and differentiated-derived cells were found to have distinct mitochondrial bioenergetic alterations when exposed to the hepatotoxic drug APAP. Metformin offered protection against loss of APAP-induced cellular viability and prevented APAP-induced decreases in bioenergetics in differentiated- but not proliferative-derived HepaRG. Distinguishingly, treatment with metformin alone reduced ATP-linked respiration, maximal respiratory capacity, and basal respiration in proliferative-derived HepaRG. Our results support that HepaRG represents an appropriate model to study drug-induced bioenergetic dysfunction.

The thymidine dideoxynucleoside analog, alovudine, inhibits the mitochondrial DNA polymerase γ, impairs oxidative phosphorylation and promotes monocytic differentiation in acute myeloid leukemia

Mitochondrial DNA encodes 13 proteins that comprise components of the respiratory chain that maintain oxidative phosphorylation. The replication of mitochondrial DNA is performed by the sole mitochondrial DNA polymerase γ. As acute myeloid leukemia (AML) cells and stem cells have an increased reliance on oxidative phosphorylation, we sought to evaluate polymerase γ inhibitors in AML. The thymidine dideoxynucleoside analog, alovudine, is an inhibitor of polymerase γ. In AML cells, alovudine depleted mitochondrial DNA, reduced mitochondrial encoded proteins, decreased basal oxygen consumption, and decreased cell proliferation and viability. To evaluate the effects of polymerase γ inhibition with alovudine in vivo, mice were xenografted with OCI-AML2 cells and then treated with alovudine. Systemic administration of alovudine reduced leukemic growth without evidence of toxicity and decreased levels of mitochondrial DNA in the leukemic cells. We also showed that alovudine increased the monocytic differentiation of AML cells. Genetic knockdown and other chemical inhibitors of polymerase γ also promoted AML differentiation, but the effects on AML differentiation were independent of reductions in oxidative phosphorylation or respiratory chain proteins. Thus, we have identified a novel mechanism by which mitochondria regulate AML fate and differentiation independent of oxidative phosphorylation. Moreover, we highlight polymerase γ inhibitors, such as alovudine, as novel therapeutic agents for AML.

Metabolic Flexibility in Leukemia-Adapt or Die

In this issue of Cancer Cell, Jones et al. demonstrate that LSCs are metabolically inflexible. LSCs rely on amino acid metabolism to fuel oxidative phosphorylation and cannot compensate with other fuel sources following amino acid depletion. Combined venetoclax and azacitidine reduces amino acid uptake, partly explaining the anti-LSC effects.

Synthesis and Cytotoxic and Antiviral Profiling of Pyrrolo- and Furo-Fused 7-Deazapurine Ribonucleosides

Three series of isomeric pyrrolo- and furo-fused 7-deazapurine ribonucleosides were synthesized and screened for cytostatic and antiviral activity. The synthesis was based on heterocyclizations of hetaryl-azidopyrimidines to form the tricyclic heterocyclic bases, followed by glycosylation and final derivatizations through cross-coupling reactions or nucleophilic substitutions. The pyrrolo[2',3':4,5]pyrrolo[2,3- d]pyrimidine and furo[2',3':4,5]pyrrolo[2,3- d]pyrimidine ribonucleosides were found to be potent cytostatics, whereas the isomeric pyrrolo[3',2',4,5]pyrrolo[2,3- d]pyrimidine nucleosides were inactive. The most active were the methyl, methoxy, and methylsulfanyl derivatives exerting submicromolar cytostatic effects and good selectivity toward cancer cells. We have shown that the nucleosides are activated by intracellular phosphorylation and the nucleotides get incorporated to both RNA and DNA, where they cause DNA damage. They represent a new type of promising candidates for preclinical development toward antitumor agents.

AMPK/FIS1-Mediated Mitophagy Is Required for Self-Renewal of Human AML Stem Cells

Leukemia stem cells (LSCs) are thought to drive the genesis of acute myeloid leukemia (AML) as well as relapse following chemotherapy. Because of their unique biology, developing effective methods to eradicate LSCs has been a significant challenge. In the present study, we demonstrate that intrinsic overexpression of the mitochondrial dynamics regulator FIS1 mediates mitophagy activity that is essential for primitive AML cells. Depletion of FIS1 attenuates mitophagy and leads to inactivation of GSK3, myeloid differentiation, cell cycle arrest, and a profound loss of LSC self-renewal potential. Further, we report that the central metabolic stress regulator AMPK is also intrinsically activated in LSC populations and is upstream of FIS1. Inhibition of AMPK signaling recapitulates the biological effect of FIS1 loss. These data suggest a model in which LSCs co-opt AMPK/FIS1-mediated mitophagy as a means to maintain stem cell properties that may be otherwise compromised by the stresses induced by oncogenic transformation.