SUCLG1 restricts POLRMT succinylation to enhance mitochondrial biogenesis and leukemia progression.

Mitochondria are cellular powerhouses that generate energy through the electron transport chain (ETC). The mitochondrial genome (mtDNA) encodes essential ETC proteins in a compartmentalized manner, however, the mechanism underlying metabolic regulation of mtDNA function remains unknown. Here, we report that expression of tricarboxylic acid cycle enzyme succinate-CoA ligase SUCLG1 strongly correlates with ETC genes across various TCGA cancer transcriptomes. Mechanistically, SUCLG1 restricts succinyl-CoA levels to suppress the succinylation of mitochondrial RNA polymerase (POLRMT). Lysine 622 succinylation disrupts the interaction of POLRMT with mtDNA and mitochondrial transcription factors. SUCLG1-mediated POLRMT hyposuccinylation maintains mtDNA transcription, mitochondrial biogenesis, and leukemia cell proliferation. Specifically, leukemia-promoting FMS-like tyrosine kinase 3 (FLT3) mutations modulate nuclear transcription and upregulate SUCLG1 expression to reduce succinyl-CoA and POLRMT succinylation, resulting in enhanced mitobiogenesis. In line, genetic depletion of POLRMT or SUCLG1 significantly delays disease progression in mouse and humanized leukemia models. Importantly, succinyl-CoA level and POLRMT succinylation are downregulated in FLT3-mutated clinical leukemia samples, linking enhanced mitobiogenesis to cancer progression. Together, SUCLG1 connects succinyl-CoA with POLRMT succinylation to modulate mitochondrial function and cancer development.

Inhibition of PLK4 remodels histone methylation and activates the immune response via the cGAS-STING pathway in TP53-mutated AML.

Acute myeloid leukemia (AML) with TP53 mutation is one of the most lethal cancers and portends an extremely poor prognosis. Based on in silico analyses of druggable genes and differential gene expression in TP53-mutated AML, we identified pololike kinase 4 (PLK4) as a novel therapeutic target and examined its expression, regulation, pathogenetic mechanisms, and therapeutic potential in TP53-mutated AML. PLK4 expression was suppressed by activated p53 signaling in TP53 wild-type AML and was increased in TP53-mutated AML cell lines and primary samples. Short-term PLK4 inhibition induced DNA damage and apoptosis in TP53 wild-type AML. Prolonged PLK4 inhibition suppressed the growth of TP53-mutated AML and was associated with DNA damage, apoptosis, senescence, polyploidy, and defective cytokinesis. A hitherto undescribed PLK4/PRMT5/EZH2/H3K27me3 axis was demonstrated in both TP53 wild-type and mutated AML, resulting in histone modification through PLK4-induced PRMT5 phosphorylation. In TP53-mutated AML, combined effects of histone modification and polyploidy activated the cGAS-STING pathway, leading to secretion of cytokines and chemokines and activation of macrophages and T cells upon coculture with AML cells. In vivo, PLK4 inhibition also induced cytokine and chemokine expression in mouse recipients, and its combination with anti-CD47 antibody, which inhibited the "don't-eat-me" signal in macrophages, synergistically reduced leukemic burden and prolonged animal survival. The study shed important light on the pathogenetic role of PLK4 and might lead to novel therapeutic strategies in TP53-mutated AML.

Proton export alkalinizes intracellular pH and reprograms carbon metabolism to drive normal and malignant cell growth.

Proton export is often considered a detoxifying process in animal cells, with monocarboxylate symporters coexporting excessive lactate and protons during glycolysis or the Warburg effect. We report a novel mechanism by which lactate/H+ export is sufficient to induce cell growth. Increased intracellular pH selectively activates catalysis by key metabolic gatekeeper enzymes HK1/PKM2/G6PDH, thereby enhancing glycolytic and pentose phosphate pathway carbon flux. The result is increased nucleotide levels, NADPH/NADP+ ratio, and cell proliferation. Simply increasing the lactate/proton symporter monocarboxylate transporter 4 (MCT4) or the sodium-proton antiporter NHE1 was sufficient to increase intracellular pH and give normal hematopoietic cells a significant competitive growth advantage in vivo. This process does not require additional cytokine triggers and is exploited in malignancy, where leukemogenic mutations epigenetically increase MCT4. Inhibiting MCT4 decreased intracellular pH and carbon flux and eliminated acute myeloid leukemia-initiating cells in mice without cytotoxic chemotherapy. Intracellular alkalization is a primitive mechanism by which proton partitioning can directly reprogram carbon metabolism for cell growth.

Loss of sphingosine kinase 2 promotes the expansion of hematopoietic stem cells by improving their metabolic fitness.

Hematopoietic stem cells (HSCs) have reduced capacities to properly maintain and replenish the hematopoietic system during myelosuppressive injury or aging. Expanding and rejuvenating HSCs for therapeutic purposes has been a long-sought goal with limited progress. Here, we show that the enzyme Sphk2 (sphingosine kinase 2), which generates the lipid metabolite sphingosine-1-phosphate, is highly expressed in HSCs. The deletion of Sphk2 markedly promotes self-renewal and increases the regenerative potential of HSCs. More importantly, Sphk2 deletion globally preserves the young HSC gene expression pattern, improves the function, and sustains the multilineage potential of HSCs during aging. Mechanistically, Sphk2 interacts with prolyl hydroxylase 2 and the Von Hippel-Lindau protein to facilitate HIF1α ubiquitination in the nucleus independent of the Sphk2 catalytic activity. Deletion of Sphk2 increases hypoxic responses by stabilizing the HIF1α protein to upregulate PDK3, a glycolysis checkpoint protein for HSC quiescence, which subsequently enhances the function of HSCs by improving their metabolic fitness; specifically, it enhances anaerobic glycolysis but suppresses mitochondrial oxidative phosphorylation and generation of reactive oxygen species. Overall, targeting Sphk2 to enhance the metabolic fitness of HSCs is a promising strategy to expand and rejuvenate functional HSCs.

Metabolism in Hematopoiesis and Its Malignancy.

Hematopoietic stem cells (HSCs) are multipotent stem cells that can self-renew and generate all blood cells of different lineages. The system is under tight control in order to maintain a precise equilibrium of the HSC pool and the effective production of mature blood cells to support various biological activities. Cell metabolism can regulate different molecular activities, such as epigenetic modification and cell cycle regulation, and subsequently affects the function and maintenance of HSC. Upon malignant transformation, oncogenic drivers in malignant hematopoietic cells can remodel the metabolic pathways for supporting the oncogenic growth. The dysregulation of metabolism results in oncogene addiction, implying the development of malignancy-specific metabolism-targeted therapy. In this chapter, we will discuss the significance of different metabolic pathways in hematopoiesis, specifically, the distinctive metabolic dependency in hematopoietic malignancies and potential metabolic therapy.

Functions of idh1 and its mutation in the regulation of developmental hematopoiesis in zebrafish.

Isocitrate dehydrogenase 1 mutation (IDH1-R132H) was recently identified in acute myeloid leukemia with normal cytogenetics. The mutant enzyme is thought to convert α-ketoglutarate to the pathogenic 2-hydroxyglutarate (2-HG) that affects DNA methylation via inhibition of ten-eleven translocation 2. However, the role of wild-type IDH1 in normal hematopoiesis and its relevance to acute myeloid leukemia is unknown. Here we showed that zebrafish idh1 (zidh1) knockdown by morpholino and targeted mutagenesis by transcription activator-like effector nuclease might induce blockade in myeloid differentiation, as evident by an increase in pu.1 and decrease in mpo, l-plastin, and mpeg1 expression, and significantly reduce definitive hematopoiesis. Morpholino knockdown of zidh2 also induced a blockade in myeloid differentiation but definitive hematopoiesis was not affected. The hematopoietic phenotype of zidh1 knockdown was not rescuable by zidh2 messenger RNA, suggesting nonredundant functions. Overexpression of human IDH1-R132H or its zebrafish ortholog resulted in 2-HG elevation and expansion of myelopoiesis in zebrafish embryos. A human IDH1-R132H-specific inhibitor (AGI-5198) significantly ameliorated both hematopoietic and 2-HG responses in human but not zebrafish IDH1 mutant expression. The results provided important insights to the role of zidh1 in myelopoiesis and definitive hematopoiesis and of IDH1-R132H in leukemogenesis.

Suppression of SOX7 by DNA methylation and its tumor suppressor function in acute myeloid leukemia.

SOX7 belongs to the SOX (Sry-related high-mobility group [HMG] box) gene family, a group of transcription factors containing in common a HMG box domain. Its role in hematologic malignancies and, in particular, acute myeloid leukemia (AML) is completely unknown. Here, we showed that SOX7 expression was regulated by DNA hypermethylation in AML but not in acute lymphoblastic leukemia or normal bone marrow cells. In cell lines (KG1, ML2, and K562) and in primary CD34(+) AML samples, SOX7 expression could be induced by the DNA demethylating agent 5-aza-2'-deoxycytidine. Overexpression of SOX7 in K562 cells inhibited cell proliferation, with cell cycle delay in S/G2/M phases and reduced clonogenic activity. Apoptosis was unaffected. Ectopic expression of SOX7 in K562 and THP-1 cells, as well as primary CD33(+)CD34(+) AML cells, abrogated leukemia engraftment in xenogeneic transplantation. SOX7 expression inhibited the Wnt/ß-catenin pathway through direct protein binding to ß-catenin, and the antileukemia effects of SOX7 in THP-1 cells were significantly reduced by deletion of its ß-catenin binding site. The results provided unequivocal evidence for a novel tumor suppressor role of SOX7 in AML via a negative modulatory effect on the Wnt/ß-catenin pathway.

A novel tescalcin-sodium/hydrogen exchange axis underlying sorafenib resistance in FLT3-ITD+ AML.

Internal tandem duplication (ITD) of fms-like tyrosine kinase 3 (FLT3) in acute myeloid leukemia (AML) is associated with inferior clinical prognosis. Sorafenib is effective in clearing leukemic blasts in chemorefractory FLT3-ITD(+) AML, but leukemia progression invariably occurs. Mechanisms of drug resistance are not completely understood. We hypothesized that a gene encoding tescalcin (TESC), known to be upregulated at leukemia progression during continuous sorafenib treatment and activate an Na(+)/H(+) exchanger type-1 (NHE1), may underlie tyrosine kinase inhibitor resistance. TESC was highly expressed in FLT3-ITD(+) AML lines MOLM-13 and MV4-11, and its knockdown by small-interfering RNA lowered intracellular pH (pHi) and induced apoptosis. The results were recapitulated by treatment with an NHE1 inhibitor, 5-(N,N-hexamethylene) amiloride (HMA). Induction of sorafenib resistance in the MOLM-13 cell line (M13-RE) significantly increased its sensitivity to HMA. The later also enhanced suppression of FLT3 signaling by sorafenib in otherwise resistant cell lines. HMA treatment of MOLM-13 and MV4-11 as well as primary FLT3-ITD(+) AML cells significantly reduced leukemia initiation in anti-CD122-primed NOD/SCID mouse xenotransplantation. These observations provided novel information about the pathogenetic role of a TESC-NHE1-pHi axis in mediating sorafenib resistance in AML.

Functions of flt3 in zebrafish hematopoiesis and its relevance to human acute myeloid leukemia.

FMS-like tyrosine kinase 3 (FLT3) is expressed in human hematopoietic stem and progenitor cells (HSPCs) but its role during embryogenesis is unclear. In acute myeloid leukemia (AML), internal tandem duplication (ITD) of FLT3 at the juxtamembrane (JMD) and tyrosine kinase (TKD) domains (FLT3-ITD(+)) occurs in 30% of patients and is associated with inferior clinical prognosis. TKD mutations (FLT3-TKD(+)) occur in 5% of cases. We made use of zebrafish to examine the role of flt3 in developmental hematopoiesis and model human FLT3-ITD(+) and FLT3-TKD(+) AML. Zebrafish flt3 JMD and TKD were remarkably similar to their mammalian orthologs. Morpholino knockdown significantly reduced the expression of l-plastin (pan-leukocyte), csf1r, and mpeg1 (macrophage) as well as that of c-myb (definitive HSPCs), lck, and rag1 (T-lymphocyte). Expressing human FLT3-ITD in zebrafish embryos resulted in expansion and clustering of myeloid cells (pu.1(+), mpo(+), and cebpα(+)) which were ameliorated by AC220 and associated with stat5, erk1/2, and akt phosphorylation. Human FLT3-TKD (D835Y) induced significant, albeit modest, myeloid expansion resistant to AC220. This study provides novel insight into the role of flt3 during hematopoiesis and establishes a zebrafish model of FLT3-ITD(+) and FLT3-TKD(+) AML that may facilitate high-throughput screening of novel and personalized agents.

Sorafenib treatment of FLT3-ITD(+) acute myeloid leukemia: favorable initial outcome and mechanisms of subsequent nonresponsiveness associated with the emergence of a D835 mutation.

Internal tandem duplication (ITD) of the fms-related tyrosine kinase-3 (FLT3) gene occurs in 30% of acute myeloid leukemias (AMLs) and confers a poor prognosis. Thirteen relapsed or chemo-refractory FLT3-ITD(+) AML patients were treated with sorafenib (200-400 mg twice daily). Twelve patients showed clearance or near clearance of bone marrow myeloblasts after 27 (range 21-84) days with evidence of differentiation of leukemia cells. The sorafenib response was lost in most patients after 72 (range 54-287) days but the FLT3 and downstream effectors remained suppressed. Gene expression profiling showed that leukemia cells that have become sorafenib resistant expressed several genes including ALDH1A1, JAK3, and MMP15, whose functions were unknown in AML. Nonobese diabetic/severe combined immunodeficiency mice transplanted with leukemia cells from patients before and during sorafenib resistance recapitulated the clinical results. Both ITD and tyrosine kinase domain mutations at D835 were identified in leukemia initiating cells (LICs) from samples before sorafenib treatment. LICs bearing the D835 mutant have expanded during sorafenib treatment and dominated during the subsequent clinical resistance. These results suggest that sorafenib have selected more aggressive sorafenib-resistant subclones carrying both FLT3-ITD and D835 mutations, and might provide important leads to further improvement of treatment outcome with FLT3 inhibitors.

Inhibition of NOTCH3 signalling significantly enhances sensitivity to cisplatin in EBV-associated nasopharyngeal carcinoma.

Nasopharyngeal carcinoma (NPC) is an EBV-associated epithelial malignancy which is prevalent in south-east Asia and southern China. Despite the multiple genetic and epigenetic changes reported, the contribution of dysregulated signalling pathways to this distinct type of head and neck cancer is not well understood. Here we demonstrate the up-regulation of NOTCH ligands (JAG1 or DLL4) and effector (HEY1) in the majority of EBV-positive tumour lines and primary tumours. Among the NOTCH receptors, NOTCH3 was over-expressed in all EBV-positive tumour lines and 92.5% of primary tumours. Aberrant activation of NOTCH3 signalling was consistently detected in all these samples. These findings imply that NOTCH3 may play an crucial role in the development of NPC. By NOTCH3 specific siRNA, NOTCH3 signalling was suppressed and thereby significant growth inhibition and apoptosis induction occurred in NPC cells. Down-regulation of a number of targets involved in cell proliferation, eg CCND1, C-MYC,NFKB1, and survival, eg BCL2, BCL-XL, SURVIVIN, was confirmed in the NOTCH3 knockdown NPC cells. Importantly, NOTCH3 knockdown highly enhanced the sensitivity of NPC cells to cisplatin treatment. Furthermore, we revealed that the ability of NPC cells to form spheroids in vitro and tumours in nude mice was also significantly decreased after knockdown of NICD3 expression. These findings indicate that activation of NOTCH3 pathway is a critical oncogenic event in NPC tumourigenesis. Targeting NOTCH3 signalling may serve as a potential therapeutic approach for treating patients suffering from EBV-associated NPC.

In vivo genome-wide CRISPR screening in murine acute myeloid leukemia uncovers microenvironmental dependencies.

Genome-wide CRISPR screens have been extremely useful in identifying therapeutic targets in diverse cancers by defining genes that are essential for malignant growth. However, most CRISPR screens were performed in vitro and thus cannot identify genes that are essential for interactions with the microenvironment in vivo. Here, we report genome-wide CRISPR screens in 2 in vivo murine models of acute myeloid leukemia (AML) driven by the KMT2A/MLLT3 fusion or by the constitutive coexpression of Hoxa9 and Meis1. Secondary validation using a focused library identified 72 genes specifically essential for leukemic growth in vivo, including components of the major histocompatibility complex class I complex, Cd47, complement receptor Cr1l, and the ß-4-galactosylation pathway. Importantly, several of these in vivo-specific hits have a prognostic effect or are inferred to be master regulators of protein activity in human AML cases. For instance, we identified Fermt3, a master regulator of integrin signaling, as having in vivo-specific dependency with high prognostic relevance. Overall, we show an experimental and computational pipeline for genome-wide functional screens in vivo in AML and provide a genome-wide resource of essential drivers of leukemic growth in vivo.

COVID-19 metabolism: Mechanisms and therapeutic targets.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) dysregulates antiviral signaling, immune response, and cell metabolism in human body. Viral genome and proteins hijack host metabolic network to support viral biogenesis and propagation. However, the regulatory mechanism of SARS-CoV-2-induced metabolic dysfunction has not been elucidated until recently. Multiomic studies of coronavirus disease 2019 (COVID-19) revealed an intensive interaction between host metabolic regulators and viral proteins. SARS-CoV-2 deregulated cellular metabolism in blood, intestine, liver, pancreas, fat, and immune cells. Host metabolism supported almost every stage of viral lifecycle. Strikingly, viral proteins were found to interact with metabolic enzymes in different cellular compartments. Biochemical and genetic assays also identified key regulatory nodes and metabolic dependencies of viral replication. Of note, cholesterol metabolism, lipid metabolism, and glucose metabolism are broadly involved in viral lifecycle. Here, we summarized the current understanding of the hallmarks of COVID-19 metabolism. SARS-CoV-2 infection remodels host cell metabolism, which in turn modulates viral biogenesis and replication. Remodeling of host metabolism creates metabolic vulnerability of SARS-CoV-2 replication, which could be explored to uncover new therapeutic targets. The efficacy of metabolic inhibitors against COVID-19 is under investigation in several clinical trials. Ultimately, the knowledge of SARS-CoV-2-induced metabolic reprogramming would accelerate drug repurposing or screening to combat the COVID-19 pandemic.

Regulation of proton partitioning in kinase-activating acute myeloid leukemia and its therapeutic implication.

Gain-of-function kinase mutations are common in AML and usually portend an inferior prognosis. We reported a novel mechanism whereby kinase mutants induced intracellular alkalization characteristic in oncogenesis. Thirteen kinases were found to activate sodium/hydrogen exchanger (NHE1) in normal hematopoietic progenitors, of which FLT3-ITD, KRASG12D, and BTK phosphorylated NHE1 maintained alkaline intracellular pH (pHi) and supported survival of AML cells. Primary AML samples with kinase mutations also showed increased NHE1 phosphorylation and evidence of NHE1 addiction. Amiloride enhanced anti-leukemic effects and intracellular distribution of kinase inhibitors and chemotherapy. Co-inhibition of NHE1 and kinase synergistically acidified pHi in leukemia and inhibited its growth in vivo. Plasma from patients taking amiloride for diuresis reduced pHi of leukemia and enhanced cytotoxic effects of kinase inhibitors and chemotherapy in vitro. NHE1-mediated intracellular alkalization played a key pathogenetic role in transmitting the proliferative signal from mutated-kinase and could be exploited for therapeutic intervention in AML.

Malic enzyme 2 connects the Krebs cycle intermediate fumarate to mitochondrial biogenesis.

Mitochondria have an independent genome (mtDNA) and protein synthesis machinery that coordinately activate for mitochondrial generation. Here, we report that the Krebs cycle intermediate fumarate links metabolism to mitobiogenesis through binding to malic enzyme 2 (ME2). Mechanistically, fumarate binds ME2 with two complementary consequences. First, promoting the formation of ME2 dimers, which activate deoxyuridine 5'-triphosphate nucleotidohydrolase (DUT). DUT fosters thymidine generation and an increase of mtDNA. Second, fumarate-induced ME2 dimers abrogate ME2 monomer binding to mitochondrial ribosome protein L45, freeing it for mitoribosome assembly and mtDNA-encoded protein production. Methylation of the ME2-fumarate binding site by protein arginine methyltransferase-1 inhibits fumarate signaling to constrain mitobiogenesis. Notably, acute myeloid leukemia is highly dependent on mitochondrial function and is sensitive to targeting of the fumarate-ME2 axis. Therefore, mitobiogenesis can be manipulated in normal and malignant cells through ME2, an unanticipated governor of mitochondrial biomass production that senses nutrient availability through fumarate.

Follistatin is a novel therapeutic target and biomarker in FLT3/ITD acute myeloid leukemia.

Internal tandem duplication of Fms-like tyrosine kinase 3 (FLT3/ITD) occurs in about 30% of acute myeloid leukemia (AML) and is associated with poor response to conventional treatment and adverse outcome. Here, we reported that human FLT3/ITD expression led to axis duplication and dorsalization in about 50% of zebrafish embryos. The morphologic phenotype was accompanied by ectopic expression of a morphogen follistatin (fst) during early embryonic development. Increase in fst expression also occurred in adult FLT3/ITD-transgenic zebrafish, Flt3/ITD knock-in mice, and human FLT3/ITD AML cells. Overexpression of human FST317 and FST344 isoforms enhanced clonogenicity and leukemia engraftment in xenotransplantation model via RET, IL2RA, and CCL5 upregulation. Specific targeting of FST by shRNA, CRISPR/Cas9, or antisense oligo inhibited leukemic growth in vitro and in vivo. Importantly, serum FST positively correlated with leukemia engraftment in FLT3/ITD AML patient-derived xenograft mice and leukemia blast percentage in primary AML patients. In FLT3/ITD AML patients treated with FLT3 inhibitor quizartinib, serum FST levels correlated with clinical response. These observations supported FST as a novel therapeutic target and biomarker in FLT3/ITD AML.

Homoharringtonine (omacetaxine mepesuccinate) as an adjunct for FLT3-ITD acute myeloid leukemia.

An in vitro drug-screening platform on patient samples was developed and validated to design personalized treatment for relapsed/refractory acute myeloid leukemia (AML). Unbiased clustering and correlation showed that homoharringtonine (HHT), also known as omacetaxine mepesuccinate, exhibited preferential antileukemia effect against AML carrying internal tandem duplication of fms-like tyrosine kinase 3 (FLT3-ITD). It worked synergistically with FLT3 inhibitors to suppress leukemia growth in vitro and in xenograft mouse models. Mechanistically, the effect was mediated by protein synthesis inhibition and reduction of short-lived proteins, including total and phosphorylated forms of FLT3 and its downstream signaling proteins. A phase 2 clinical trial of sorafenib and HHT combination treatment in FLT3-ITD AML patients resulted in complete remission (true or with insufficient hematological recovery) in 20 of 24 patients (83.3%), reduction of ITD allelic burden, and median leukemia-free and overall survivals of 12 and 33 weeks. The regimen has successfully bridged five patients to allogeneic hematopoietic stem cell transplantation and was well tolerated in patients unfit for conventional chemotherapy, including elderly and heavily pretreated patients. This study validated the principle and clinical relevance of in vitro drug testing and identified an improved treatment for FLT3-ITD AML. The results provided the foundation for phase 2/3 clinical trials to ascertain the clinical efficacy of FLT3 inhibitors and HHT in combination.