Purine metabolism as a target for leukemia chemotherapy

Application of omics in the diagnosis, prognosis, and treatment of acute myeloid leukemia

Acute myeloid leukemia (AML) is the most frequent leukemia in adults with a high mortality rate. Current diagnostic criteria and selections of therapeutic strategies are generally based on gene mutations and cytogenetic abnormalities. Chemotherapy, targeted therapies, and hematopoietic stem cell transplantation (HSCT) are the major therapeutic strategies for AML. Two dilemmas in the clinical management of AML are related to its poor prognosis. One is the inaccurate risk stratification at diagnosis, leading to incorrect treatment selections. The other is the frequent resistance to chemotherapy and/or targeted therapies. Genomic features have been the focus of AML studies. However, the DNA-level aberrations do not always predict the expression levels of genes and proteins and the latter is more closely linked to disease phenotypes. With the development of high-throughput sequencing and mass spectrometry technologies, studying downstream effectors including RNA, proteins, and metabolites becomes possible. Transcriptomics can reveal gene expression and regulatory networks, proteomics can discover protein expression and signaling pathways intimately associated with the disease, and metabolomics can reflect precise changes in metabolites during disease progression. Moreover, omics profiling at the single-cell level enables studying cellular components and hierarchies of the AML microenvironment. The abundance of data from different omics layers enables the better risk stratification of AML by identifying prognosis-related biomarkers, and has the prospective application in identifying drug targets, therefore potentially discovering solutions to the two dilemmas. In this review, we summarize the existing AML studies using omics methods, both separately and combined, covering research fields of disease diagnosis, risk stratification, prognosis prediction, chemotherapy, as well as targeted therapy. Finally, we discuss the directions and challenges in the application of multi-omics in precision medicine of AML. Our review may inspire both omics researchers and clinical physicians to study AML from a different angle.

Serine Biosynthesis Is a Metabolic Vulnerability in FLT3-ITD-Driven Acute Myeloid Leukemia

Internal tandem duplication of the FMS-like tyrosine kinase 3 gene (FLT3-ITD) occurs in 30% of all acute myeloid leukemias (AML). Limited clinical efficacy of FLT3 inhibitors highlights the need for alternative therapeutic modalities in this subset of disease. Using human and murine models of FLT3-ITD-driven AML, we demonstrate that FLT3-ITD promotes serine synthesis and uptake via ATF4-dependent transcriptional regulation of genes in the de novo serine biosynthesis pathway and neutral amino acid transport. Genetic or pharmacologic inhibition of PHGDH, the rate-limiting enzyme of de novo serine biosynthesis, selectively inhibited proliferation of FLT3-ITD AMLs in vitro and in vivo. Moreover, pharmacologic inhibition of PHGDH sensitized FLT3-ITD AMLs to the standard-of-care chemotherapeutic cytarabine. Collectively, these data reveal novel insights into FLT3-ITD-induced metabolic reprogramming and reveal a targetable vulnerability in FLT3-ITD AML. SIGNIFICANCE: FLT3-ITD mutations are common in AML and are associated with poor prognosis. We show that FLT3-ITD stimulates serine biosynthesis, thereby rendering FLT3-ITD-driven leukemias dependent upon serine for proliferation and survival. This metabolic dependency can be exploited pharmacologically to sensitize FLT3-ITD-driven AMLs to chemotherapy.This article is highlighted in the In This Issue feature, p. 1307.

Adenosine deaminase inhibition

Adenosine deaminase is a critical enzyme in purine metabolism that regulates intra and extracellular adenosine concentrations by converting it to inosine. Adenosine is an important purine that regulates numerous physiological functions by interacting with its receptors. Adenosine and consequently adenosine deaminase can have pro or anti-inflammatory effects on tissues depending on how much time has passed from the start of the injury. In addition, an increase in adenosine deaminase activity has been reported for various diseases and the significant effect of deaminase inhibition on the clinical course of different diseases has been reported. However, the use of inhibitors is limited to only a few medical indications. Data on the increase of adenosine deaminase activity in different diseases and the impact of its inhibition in various cases have been collected and are discussed in this review. Overall, the evidence shows that many studies have been done to introduce inhibitors, however, in vivo studies have been much less than in vitro, and often have not been expanded for clinical use.

Phytohormone metabolism in human cells: Cytokinins are taken up and interconverted in HeLa cell culture

Cytokinins (CKs) encompass a group of phytohormones, known to orchestrate many critical processes in plant development. Excluding Archaea, CKs are pervasive among all kingdoms, but much less is reported about their metabolism beyond plants. Recent evidence from mammalian tissues indicates the presence of six additional CK forms beyond the previously identified, single mammalian CK, N6-isopentenyladenosine (i6A). There is limited understanding of CK biosynthesis pathways in mammalian systems; therefore, human cervical cancer (HeLa) cells were used to further characterize CK processing by tracking the interconversion of CKs into their various structural derivatives in mammalian cells in a time-course study. Through high-performance liquid chromatography-positive electrospray ionization-tandem mass spectrometry (HPLC-(+ESI)-MS/MS), we document changes in the functional profiles of endogenous CKs in a human cell line following metabolism by HeLa cell cultures. The nucleotide CK fraction (iPRP) was found exclusively within the cell pellet (0.34 pmol/106 cells), and the active free base (FB) form (iP) and riboside fraction (iPR) were found in greater abundance extracellularly (1.67 and 0.10 nmol/L respectively). For further confirmation, we demonstrate that HeLa cells metabolize an exogenously supplied CK, N6-benzyladenosine (BAR). In the HeLa culture supernatant, a 12-fold decrease in BAR concentration was observed within the first 24 hours of incubation accompanied by a fivefold increase in the FB form, N6-benzyladenine (BA). These findings support the hypothesis that HeLa cells have the enzymatic pathways required for the metabolism of both endogenous and exogenous CKs.

Cutting off the power: inhibition of leukemia cell growth by pausing basal ATP release and P2X receptor signaling?

T cells respond to antigen stimulation with the rapid release of cellular ATP, which stimulates an autocrine feedback mechanism that regulates calcium influx through P2X receptors. This autocrine purinergic feedback mechanism plays an essential role in the activation of T cells resulting in cell proliferation and clonal expansion. We recently reported that increases in mitochondrial ATP production drive this stimulation-induced purinergic signaling mechanism but that low-level mitochondrial ATP production fuels basal T cell functions required to maintain vigilance of unstimulated T cells. Here we studied whether defects in these purinergic signaling mechanisms are involved in the unwanted proliferation of leukemia T cells. We found that acute leukemia T cells (Jurkat) possess a larger number and more active mitochondria than their healthy counterparts. Jurkat cells have higher intracellular ATP concentrations and generat more extracellular ATP than unstimulated T cells from healthy donors. As a result, increased purinergic signaling through P2X1 and P2X7 receptors elevates baseline levels of cytosolic Ca(2+) in Jurkat cells. We found that pharmacological inhibition of this basal purinergic signaling mechanism decreases mitochondrial activity, Ca(2+) signaling, and cell proliferation. Similar results were seen in the leukemic cell lines THP-1, U-937, and HL-60. Combined treatment with inhibitors of P2X1 or P2X7 receptors and the chemotherapeutic agent 6-mercaptopurine completely blocked Jurkat cell proliferation. Our results demonstrate that increased mitochondrial metabolism promotes autocrine purinergic signaling and uncontrolled proliferation of leukemia cells. These findings suggest that deranged purinergic signaling can result in T cell malignancy and that therapeutic targeting aimed at purinergic signaling is a potential strategy to combat T cell leukemia.

Dynamic regulation of a metabolic multi-enzyme complex by protein kinase CK2

The reversible association and dissociation of a metabolic multi-enzyme complex participating in de novo purine biosynthesis, the purinosome, was demonstrated in live cells to respond to the levels of purine nucleotides in the culture media. We also took advantage of in vitro proteomic scale studies of cellular substrates of human protein kinases (e.g. casein kinase II (CK2) and Akt), that implicated several de novo purine biosynthetic enzymes as kinase substrates. Here, we successfully identified that purinosome formation in vivo was significantly promoted in HeLa cells by the addition of small-molecule CK2-specific inhibitors (i.e. 4,5,6,7-tetrabromo-1H-benzimidazole, 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole, tetrabromocinammic acid, 4,4′,5,5′,6,6′-hexahydroxydiphenic acid 2,2′,6,6′-dilactone (ellagic acid) as well as by silencing the endogenous human CK2α catalytic subunit with small interfering RNA. However, 4,5,6,7-tetrabromobenzotriazole, another CK2-specific inhibitor, triggered the dissociation of purinosome clusters in HeLa cells. Although the mechanism by which 4,5,6,7-tetrabromobenzotriazole affects purinosome clustering is not clear, we were capable of chemically reversing purinosome formation in cells by the sequential addition of two CK2 inhibitors. Collectively, we provide compelling cellular evidence that CK2-mediated pathways reversibly regulate purinosome assembly, and thus the purinosome may be one of the ultimate targets of kinase inhibitors.

Metabolism of the antineoplastic and immunosuppressive drug 2-CdA (Leustatin®) in animals and humans

1. The in vivo metabolism of the antineoplastic and immunosuppressive drug 2-CdA (Leustatin) was investigated in mice, monkeys and humans after a single subcutaneous dose of cladribine 60 mg kg(-1) to eight male and eight female mice and 10 mg kg(-1) to one male and one female monkey, and an intravenous infusion dose of cladribine 22-45 mg(-1) per subject to 12 male patients. 2. Plasma (1 h), red blood cells (1 h) and faecal samples (0-24 h) were obtained from mice and monkeys, and urine samples (0-24 h) were obtained from these species and humans. 3. Unchanged cladribine (urine: 47% of the sample in human; 60% of the sample in mouse; 73% of the sample in monkey) and 10 metabolites, consisting of four phase I metabolites (M1-3, M7) and six phase II metabolites -- five glucuronides (M4, M6, M8-10) and one sulfate (M5) -- were profiled, characterized and tentatively identified in plasma, red blood cells, and faecal and urine samples on the basis of API ionspray-mass spectrometry (MS) and MS/MS data. 4. Metabolites were formed via the following three metabolic pathways: oxidative cleavage at the adenosine and deoxyribose linkage (A); oxidation at adenosine/deoxyribose (B); and conjugation (C). 5. Pathways A and B appear to be major steps, forming four oxidative/cleavage metabolites (M1-3, M7) (each 3-20% of the sample). 6. Pathway C along or in conjunction with pathways A and B produced cladribine glucuronide, cladribine sulfate and four glucuronides of oxidative/cleavage metabolites in minor/trace quantities (each < or = 5% of the sample). 7. In addition, the in vitro metabolism of cladribine was conducted using rat and human liver microsomal fractions in the presence of an beta-nicotinamide adenine dinucleotide phosphate-generating system. Unchanged cladribine (> or = 90% of the sample) plus three minor metabolites, M1-3 (each < 8% of the sample), were profiled and tentatively identified by thin-layer chromatography and MS data. 8. Cladribine is not extensively metabolized in vitro and in vivo in all species. However, humans appear to metabolize cladribine to a greater extent than other animals.

Molecular determinants of substrate specificity in plant 5'-methylthioadenosine nucleosidases

5'-Methylthioadenosine (MTA)/S-adenosylhomocysteine (SAH) nucleosidase (MTAN) is essential for cellular metabolism and development in many bacterial species. While the enzyme is found in plants, plant MTANs appear to select for MTA preferentially, with little or no affinity for SAH. To understand what determines substrate specificity in this enzyme, MTAN homologues from Arabidopsis thaliana (AtMTAN1 and AtMTAN2, which are referred to as AtMTN1 and AtMTN2 in the plant literature) have been characterized kinetically. While both homologues hydrolyze MTA with comparable kinetic parameters, only AtMTAN2 shows activity towards SAH. AtMTAN2 also has higher catalytic activity towards other substrate analogues with longer 5'-substituents. The structures of apo AtMTAN1 and its complexes with the substrate- and transition-state-analogues, 5'-methylthiotubercidin and formycin A, respectively, have been determined at 2.0-1.8 A resolution. A homology model of AtMTAN2 was generated using the AtMTAN1 structures. Comparison of the AtMTAN1 and AtMTAN2 structures reveals that only three residues in the active site differ between the two enzymes. Our analysis suggests that two of these residues, Leu181/Met168 and Phe148/Leu135 in AtMTAN1/AtMTAN2, likely account for the divergence in specificity of the enzymes. Comparison of the AtMTAN1 and available Escherichia coli MTAN (EcMTAN) structures suggests that a combination of differences in the 5'-alkylthio binding region and reduced conformational flexibility in the AtMTAN1 active site likely contribute to its reduced efficiency in binding substrate analogues with longer 5'-substituents. In addition, in contrast to EcMTAN, the active site of AtMTAN1 remains solvated in its ligand-bound forms. As the apparent pK(a) of an amino acid depends on its local environment, the putative catalytic acid Asp225 in AtMTAN1 may not be protonated at physiological pH and this suggests the transition state of AtMTAN1, like human MTA phosphorylase and Streptococcus pneumoniae MTAN, may be different from that found in EcMTAN.

Apoptosis induced by N6-substituted derivatives of adenosine is related to intracellular accumulation of corresponding mononucleotides in HL-60 cells

The in vitro induction of apoptosis by N6-substituted derivatives of adenosine and adenine was investigated in HL-60 cells. Using reversed phase HPLC/MS analysis we demonstrated that both N6-substituted derivatives of adenosine and adenine are phosphorylated within cells to the monophosphate level. While N6-substituted derivatives of adenosine were phosphorylated by adenosine kinase and corresponding mononucleotides were produced in large quantities, N6-substituted derivatives of adenine were converted into the corresponding mononucleotides via the phosphoribosyl transferase pathway, which yielded 50-100 times lower amounts of the mononucleotides than the adenosine kinase pathway. Accordingly, N6-substituted derivatives of adenine were relatively inefficient inductors of apoptosis at the concentrations applied. Inhibitors of adenosine kinase that abrogated the formation of monophosphates from N6-substituted derivatives of adenosine completely prevented cells from going into apoptosis. These results consistently support the idea that pro-apoptotic effects of N6-substituted derivatives of adenosine are related to their intracellular conversion into corresponding mononucleotides which eventually trigger apoptosis when accumulated beyond certain level. Intracellular accumulation of mononucleotides derived from the corresponding N6-substituted derivatives of adenosine led to a rapid decrease in ATP production and consequently to apoptosis induction. Nevertheless, the detailed mechanism is unknown and must be further elucidated. Apoptosis, induced by N6-substituted derivatives of adenosine, was accompanied by a distinct caspase-3 activation. However, a broad spectrum caspase inhibitor, z-VAD-fmk, failed to prevent cells from death, thereby indicating that caspases alone were not mediators of cell death.

Hairy cell leukemia: an update

Hairy cell leukemia is an indolent, chronic B-cell lymphoproliferative disorder comprising approximately 2 to 3% of all adult leukemias in the United States. Hairy cells are clonal expansions of mature, activated B-cells. They co-express CD11c, CD19, CD20, CD22, CD25, and CD103. Hairy cells possess clonal immunoglobulin gene rearrangements and express monoclonal surface immunoglobulin of either IgG or multiple heavy-chain isotypes. Treatment of hairy cell leukemia should be considered for symptomatic patients. It is indicated in patients with significant neutropenia, anemia, thrombocytopenia, symptomatic splenomegaly, constitutional symptoms due to hairy cell leukemia, or recurrent serious infections. Many treatments exist, including cladribine, pentostatin, interferon-alpha, splenectomy, rituximab (mabthera), and BL-22 immunotoxin.

Cladribine in the treatment of hairy-cell leukaemia

Cladribine, a purine nucleoside analogue, is a safe and effective treatment for patients with hairy-cell leukaemia. It is administered at a dose of 0.09 mg/kg daily as a continuous intravenous infusion over 7 days. This chapter discusses the history, rationale, chemical structure and mechanism of action of cladribine. The indications for therapy and guidelines for clinical usage are reviewed. The response of hairy-cell leukaemia to cladribine, the acute and chronic complications and the risk for second malignancies are summarized. The chapter concludes with a section on salvage therapy.

Anthracyclines potentiate activity against murine leukemias L1210 and P388 in vivo and in vitro

The interactions of 2-chlorodeoxyadenosine (2-CdA, cladribine) and three anthracyclines: doxorubicin (DOX), idarubicin (IDA) and mitoxantrone (MIT) were evaluated on murine leukemias P388 and L1210. Prolongation of survival time of animals receiving drugs in combination compared to mice treated with drugs in monotherapy was tested. We have also evaluated interactions of the cytostatics on murine leukemias in vitro by measuring their inhibitory effects on P388 and L1210 cell proliferation. We have observed a synergistic effect of MIT and IDA in combination with 2-CdA on P388 leukemia resulting in an increase of life span (ILS)=226% in case of MIT+2-CdA and ILS=126% in the case of IDA+2-CdA, whereas 2-CdA used as a sole drug resulted in an ILS=47%. The survival time of animals inoculated with P388 leukemic cells and treated with DOX+ 2-CdA was similar to ILS gained by DOX monotherapy (178% and 200% respectively). The mice bearing L1210 leukemia receiving combined chemotherapy lived significantly longer than the animals on single agent regimens. The animals treated with schedule 2-CdA+MIT lived significantly longer (P=0.004) as compared to the groups receiving drugs in monotherapy (ILS of 2-CdA+MIT group=60%, ILS of MIT group 33%, and 2-CdA group 33%). Finally, combination of DOX or IDA with 2-CdA resulted in ILS =73% (2-CdA+DOX regimen), and ILS=60% in case of 2-CdA+IDA regimen, which is significantly higher than ILS gained on monotherapy schedules. In vitro tests revealed that all tested anthracyclines enhance the antiproliferative activity of 2-CdA against L1210 and P388 leukemic cells (P<0.05). Our study has shown that all anthracyclines potentiate 2-CdA antileukemic activity, both in vivo and in vitro. It failed however to point the best one to be combined with cladribine. We suggests that further clinical trials with such combinations are needed.

Inhibition of 6-phosphofructo-2-kinase activity by mercaptopurines

The activity of 6-phosphofructo-2-kinase (PFK-2), the enzyme that catalyses the synthesis of fructose 2,6-bisphosphate (Fru-2,6-P2), was inhibited by mercaptopurines in vitro. Inhibition was observed with the purified enzyme from rat liver and bovine heart, and in extracts from rat lymphocytes and hepatoma cells, chick embryo fibroblasts, and human HeLa and lymphoblastoid cells. Half-maximal effect was obtained with 0.1-0.2 mM mercaptopurine and maximal inhibition ranged between 50 and 90% depending on the enzyme preparation. The inhibition resulted from a decrease in Vmax with no change in Km for ATP. The inhibition was relieved by treatment of the enzyme with thiol reducing agents, suggesting that it involves the formation of a mixed disulfide between mercaptopurine and thiol group(s) essential for enzyme activity. Incubation of intact lymphocytes or lymphoblastoid cells with 2- or 6-mercaptopurine resulted in a decrease in Fru-2,6-P2 content and lactate release. A decrease in Fru-2,6-P2 content but no change in lactate release was observed in HeLa cells and fibroblasts treated with 6-mercaptopurine but not with 2-mercaptopurine. Treatment of HeLa cells with 6-mercaptopurine resulted in a decreased PFK-2 activity which could be restored by treatment of the cell extract with dithiothreitol. In isolated rat hepatocytes and perfused rat hearts mercaptopurines had little or no effect on the Fru-2,6-P2 content and lactate release. These results suggest that the effect of 6-mercaptopurine of arresting growth in lymphoid cells might involve the inhibition of glycolysis in addition to the known inhibition of de novo purine nucleotide synthesis.