The enzyme ribonucleotide reductase: target for antitumor and anti-HIV therapy

Ribonucleotide reductase: Implications of thiol S-nitrosylation and tyrosine nitration for different subunits

Ribonucleotide reductase (RNR) is a multi-subunit enzyme responsible for catalyzing the rate-limiting step in the production of deoxyribonucleotides essential for DNA synthesis and repair. The active RNR complex is composed of multimeric R1 and R2 subunits. The RNR catalysis involves the formation of tyrosyl radicals in R2 subunits and thiyl radicals in R1 subunits. Despite the quaternary structure and cofactor diversity, all the three classes of RNR have a conserved cysteine residue at the active site which is converted into a thiyl radical that initiates the substrate turnover, suggesting that the catalytic mechanism is somewhat similar for all three classes of the RNR enzyme. Increased RNR activity has been associated with malignant transformation, cancer cell growth, and tumorigenesis. Efforts concerning the understanding of RNR inhibition in designing potent RNR inhibitors/drugs as well as developing novel approaches for antibacterial, antiviral treatments, and cancer therapeutics with improved radiosensitization have been made in clinical research. This review highlights the precise and potent roles of NO in RNR inhibition by targeting both the subunits. Under nitrosative stress, the thiols of the R1 subunits have been found to be modified by S-nitrosylation and the tyrosyl radicals of the R2 subunits have been modified by nitration. In view of the recent advances and progresses in the field of nitrosative modifications and its fundamental role in signaling with implications in health and diseases, the present article focuses on the regulations of RNR activity by S-nitrosylation of thiols (R1 subunits) and nitration of tyrosyl residues (R2 subunits) which will further help in designing new drugs and therapies.

Musashi1 Contribution to Glioblastoma Development via Regulation of a Network of DNA Replication, Cell Cycle and Division Genes

RNA-binding proteins (RBPs) function as master regulators of gene expression. Alterations in their levels are often observed in tumors with numerous oncogenic RBPs identified in recent years. Musashi1 (Msi1) is an RBP and stem cell gene that controls the balance between self-renewal and differentiation. High Msi1 levels have been observed in multiple tumors including glioblastoma and are often associated with poor patient outcomes and tumor growth. A comprehensive genomic analysis identified a network of cell cycle/division and DNA replication genes and established these processes as Msi1's core regulatory functions in glioblastoma. Msi1 controls this gene network via two mechanisms: direct interaction and indirect regulation mediated by the transcription factors E2F2 and E2F8. Moreover, glioblastoma lines with Msi1 knockout (KO) displayed increased sensitivity to cell cycle and DNA replication inhibitors. Our results suggest that a drug combination strategy (Msi1 + cell cycle/DNA replication inhibitors) could be a viable route to treat glioblastoma.

The Antitumor Didox Acts as an Iron Chelator in Hepatocellular Carcinoma Cells

Ribonucleotide reductase (RR) is the rate-limiting enzyme that controls the deoxynucleotide triphosphate synthesis and it is an important target of cancer treatment, since it is expressed in tumor cells in proportion to their proliferation rate, their invasiveness and poor prognosis. Didox, a derivative of hydroxyurea (HU), is one of the most potent pharmaceutical inhibitors of this enzyme, with low in vivo side effects. It inhibits the activity of the subunit RRM2 and deoxyribonucleotides (dNTPs) synthesis, and it seems to show iron-chelating activity. In the present work, we mainly investigated the iron-chelating properties of didox using the HA22T/VGH cell line, as a model of hepatocellular carcinoma (HCC). We confirmed that didox induced cell death and that this effect was suppressed by iron supplementation. Interestingly, cell treatments with didox caused changes of cellular iron content, TfR1 and ferritin levels comparable to those caused by the iron chelators, deferoxamine (DFO) and deferiprone (DFP). Chemical studies showed that didox has an affinity binding to Fe³⁺ comparable to that of DFO and DFP, although with slower kinetic. Structural modeling indicated that didox is a bidentated iron chelator with two theoretical possible positions for the binding and among them that with the two hydroxyls of the catechol group acting as ligands is the more likely one. The iron chelating property of didox may contribute to its antitumor activity not only blocking the formation of the tyrosil radical on Tyr122 (such as HU) on RRM2 (essential for its activity) but also sequestering the iron needed by this enzyme and to the cell proliferation.

A mechanism for 1,4-Benzoquinone-induced genotoxicity

Benzene is a common environmental toxin and its metabolite, 1-4-Benzoquinone (BQ) causes hematopoietic cancers like myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). BQ has not been comprehensively assessed for its impact on genome maintenance, limiting our understanding of the true health risks associated with benzene exposure and our ability to identify people with increased sensitivity to this genotoxin. Here we analyze the impact BQ exposure has on wild type and DNA repair-defective mouse embryonic stem (ES) cells and wild type human cells. We find that double strand break (DSB) repair and replication fork maintenance pathways including homologous recombination (HR) and Fanconi anemia (FA) suppress BQ toxicity. BQ-induced damage efficiently stalls replication forks, yet poorly induces ATR/DNA-PKCS responses. Furthermore, the pattern of BQ-induced γH2AX and 53BP1foci is consistent with the formation of poly(ADP-ribose) polymerase 1 (PARP1)-stabilized regressed replication forks. At a biochemical level, BQ inhibited topoisomerase 1 (topo1)-mediated DNA ligation and nicking in vitro; thus providing mechanism for the cellular phenotype. These data are consistent with a model that proposes BQ interferes with type I topoisomerase's ability to maintain replication fork restart and progression leading to chromosomal instability that has the potential to cause hematopoietic cancers like MDS and AML.

A functional approach reveals a genetic and physical interaction between ribonucleotide reductase and CHK1 in mammalian cells

Ribonucleotide reductase (RNR) enzyme is composed of the homodimeric RRM1 and RRM2 subunits, which together form a heterotetramic active enzyme that catalyzes the de novo reduction of ribonucleotides to generate deoxyribonucleotides (dNTPs), which are required for DNA replication and DNA repair processes. In this study, we show that ablation of RRM1 and RRM2 by siRNA induces G1/S phase arrest, phosphorylation of Chk1 on Ser345 and phosphorylation of γ-H2AX on S139. Combinatorial ablation of RRM1 or RRM2 and Chk1 causes a dramatic accumulation of γ-H2AX, a marker of double-strand DNA breaks, suggesting that activation of Chk1 in this context is essential for suppression of DNA damage. Significantly, we demonstrate for the first time that Chk1 and RNR subunits co-immunoprecipitate from native cell extracts. These functional genomic studies suggest that RNR is a critical mediator of replication checkpoint activation.

Deletion of BRCA2 exon 27 causes defects in response to both stalled and collapsed replication forks

BRCA2 is a tumor suppressor that maintains genomic integrity through double strand break (DSB) repair and replication fork protection. The BRC motifs and an exon 27-encoded domain (Ex27) of BRCA2 interact with the recombinase RAD51 to, respectively, facilitate the formation and stability of a RAD51 filament on single strand DNA. The BRC-RAD51 associations enable DSB repair while the Ex27-RAD51 association protects the nascent replication strand from MRE11-mediated degradation. MRE11 is a nuclease that facilitates the generation of 3' overhangs needed for homologous recombination (HR)-mediated DSB repair. Here we report the dynamics of replication fork maintenance in mouse embryonic stem (ES) cells deleted for Ex27 (brca2(lex1/lex2)) after exposure to hydroxyurea (HU) that depletes nucleotides. HU conditions were varied from mild to severe. Mild conditions induce an ATR-response to replication fork stalling while severe conditions induce a DNA-PKCS-response to replication fork collapse and a DSB. These responses were differentiated by replication protein A (RPA) phosphorylation. We found that Ex27 deletion reduced MRE11 localization to stalled, but not collapsed, replication forks and that Ex27-deletion caused a proportionately more severe phenotype with HU dose. Therefore, the BRCA2 exon 27 domain maintains chromosomal integrity at both stalled and collapsed replication forks consistent with involvement in both replication fork maintenance and double strand break repair.

The identification of novel 5'-amino gemcitabine analogs as potent RRM1 inhibitors

The ribonucleotide reductase (RNR) enzyme is a heteromer of RRM1 and RRM2 subunits. The active enzyme catalyzes de novo reduction of ribonucleotides to generate deoxyribonucleotides (dNTPs), which are required for DNA replication and DNA repair processes. Complexity in the generation of physiologically relevant, active RRM1/RRM2 heterodimers was perceived as limiting to the identification of selective RRM1 inhibitors by high-throughput screening of compound libraries and led us to seek alternative methods to identify lead series. In short, we found that gemcitabine, as its diphosphate metabolite, represents one of the few described active site inhibitors of RRM1. We herein describe the identification of novel 5'-amino gemcitabine analogs as potent RRM1 inhibitors through in-cell phenotypic screening.

A novel clade of unique eukaryotic ribonucleotide reductase R2 subunits is exclusive to apicomplexan parasites

Apicomplexa are protist parasites of tremendous medical and economic importance, causing millions of deaths and billions of dollars in losses each year. Apicomplexan-related diseases may be controlled via inhibition of essential enzymes. Ribonucleotide reductase (RNR) provides the only de novo means of synthesizing deoxyribonucleotides, essential precursors for DNA replication and repair. RNR has long been the target of antibacterial and antiviral therapeutics. However, targeting this ubiquitous protein in eukaryotic pathogens may be problematic unless these proteins differ significantly from that of their respective host. The typical eukaryotic RNR enzymes belong to class Ia, and the holoenzyme consists minimally of two R1 and two R2 subunits (α₂β₂). We generated a comparative, annotated, structure-based, multiple-sequence alignment of R2 subunits, identified a clade of R2 subunits unique to Apicomplexa, and determined its phylogenetic position. Our analyses revealed that the apicomplexan-specific sequences share characteristics with both class I R2 and R2lox proteins. The putative radical-harboring residue, essential for the reduction reaction by class Ia R2-containing holoenzymes, was not conserved within this group. Phylogenetic analyses suggest that class Ia subunits are not monophyletic and consistently placed the apicomplexan-specific clade sister to the remaining class Ia eukaryote R2 subunits. Our research suggests that the novel apicomplexan R2 subunit may be a promising candidate for chemotherapeutic-induced inhibition as it differs greatly from known eukaryotic host RNRs and may be specifically targeted.

Hydroxyurea responsiveness in β-thalassemic patients is determined by the stress response adaptation of erythroid progenitors and their differentiation propensity

β-thalassemia is caused by mutations in the β-globin locus resulting in loss of, or reduced, hemoglobin A (adult hemoglobin, HbA, α2β2) production. Hydroxyurea treatment increases fetal γ-globin (fetal hemoglobin, HbF, α2γ2) expression in postnatal life substituting for the missing adult β-globin and is, therefore, an attractive therapeutic approach. Patients treated with hydroxyurea fall into three categories: i) 'responders' who increase hemoglobin to therapeutic levels; (ii) 'moderate-responders' who increase hemoglobin levels but still need transfusions at longer intervals; and (iii) 'non-responders' who do not reach adequate hemoglobin levels and remain transfusion-dependent. The mechanisms underlying these differential responses remain largely unclear. We generated RNA expression profiles from erythroblast progenitors of 8 responder and 8 non-responder β-thalassemia patients. These profiles revealed that hydroxyurea treatment induced differential expression of many genes in cells from non-responders while it had little impact on cells from responders. Part of the gene program up-regulated by hydroxyurea in non-responders was already highly expressed in responders before hydroxyurea treatment. Baseline HbF expression was low in non-responders, and hydroxyurea treatment induced significant cell death. We conclude that cells from responders have adapted well to constitutive stress conditions and display a propensity to proceed to the erythroid differentiation program.

New insights into the roles of ATM and DNA-PKcs in the cellular response to oxidative stress

Reactive oxygen species (ROS) are induced by a variety of endogenous and exogenous sources. At pathologically high levels, ROS cause damage to biological molecules, including DNA. The damage sustained by DNA likely plays a key role in the pathogenesis of aging and carcinogenesis. Extensive research has established in detail the mechanism of cellular response to oxidative stress. Attention is now focused on identifying the molecular contributions of the key DNA damage response kinases ataxia telangiectasia mutated (ATM), DNA-dependent protein kinase catalytic subunit (DNA-PKcs), and ATM- and Rad3-related (ATR) in the oxidative stress response. In this review, we will provide an update of the current evidence regarding the involvement of these related DNA damage response kinases in oxidative DNA lesion repair and signaling responses. The growing understanding of the involvement of ATM, DNA-PKcs, and ATR in the oxidative stress response will offer new possibilities for the treatment of ROS-related diseases.

HMGA-targeted phosphorothioate DNA aptamers increase sensitivity to gemcitabine chemotherapy in human pancreatic cancer cell lines

Elevated high mobility group A (HMGA) protein expression in pancreatic cancer cells is correlated with resistance to the chemotherapy agent gemcitabine. Here, we demonstrate use of HMGA-targeted AT-rich phosphorothioate DNA (AT-sDNA) aptamers to suppress HMGA carcinogenic activity. Cell growth of human pancreatic cancer cells (AsPC-1 and Miapaca-2) transfected with AT-sDNA were monitored after treatment with gemcitabine. Significant increases in cell death in AT-sDNA transfected cells compared to non-AT-rich sDNA treated cells were observed in both cell lines. The data indicate the potential use of HMGA targeted DNA aptamers to enhance chemotherapy efficacy in pancreatic cancer treatment.

Hydroxycarbamide: a user's guide for chronic myeloproliferative disorders

Hydroxycarbamide is a nonalkylating antiproliferative and antiviral agent that has been used for over 40 years to treat a variety of neoplastic and non-neoplastic conditions. Hydroxycarbamide is readily absorbed and widely distributed throughout the body. It acts primarily to inhibit DNA synthesis, which underpins its use in solid tumors, viral infections and chronic myeloproliferative disorders. Hydroxycarbamide is an effective treatment for preventing transient ischemic attacks associated with thrombocytosis in chronic myeloproliferative disorders because it is a nitric oxide donor. While its mechanism of action and side-effect profile are well defined, its potential for leukemic transformation as a single agent is still a matter of controversy. Based on a search of the Medline database, this article encompasses the pharmacokinetics, pharmacodynamics, clinical use and tolerability of hydroxycarbamide, plus its potential for mutagenicity with special reference to the chronic myeloproliferative disorders. The toxicity profile of hydroxycarbamide is also discussed to enable clinicians to balance potential risks with therapeutic benefits.

Effect of siRNAs on HSV-1 plaque formation and relative expression levels of RR mRNA

RNA interference (RNAi) is a process by which introduced small interfering RNA (siRNA) can cause the specific degradation of mRNA with identical sequences. The human herpes simplex virus type 1 (HSV-1) RR is composed of two distinct homodimeric subunits encoded by UL39 and UL40, respectively. In this study, we applied siRNAs targeting the UL39 and UL40 genes of HSV-1. We showed that synthetic siRNA silenced effectively and specifically UL39 and UL40 mRNA expression and inhibited HSV-1 replication. Our work offers new possibilities for RNAi as a genetic tool for inhibition of HSV-1 replication.

Structure-Based Design, Synthesis, and Evaluation of 2'-(2-Hydroxyethyl)-2'-deoxyadenosine and the 5'-Diphosphate Derivative as Ribonucleotide Reductase Inhibitors

Analysis of the recently solved X-ray crystal structures of Saccharomyces cerevisiae ribonucleotide reductase I (ScRnr1) in complex with effectors and substrates led to the discovery of a conserved water molecule located at the active site that interacted with the 2'-hydroxy group of the nucleoside ribose. In this study 2'-(2-hydroxyethyl)-2'-deoxyadenosine 1 and the 5'-diphosphate derivative 2 were designed and synthesized to see if the conserved water molecule could be displaced by a hydroxymethylene group, to generate novel RNR inhibitors as potential antitumor agents. Herein we report the synthesis of analogues 1 and 2, and the co-crystal structure of adenosine diphosphate analogue 2 bound to ScRnr1, which shows the conserved water molecule is displaced as hypothesized.

Function and regulation of class I ribonucleotide reductase-encoding genes in mycobacteria

Ribonucleotide reductases (RNRs) are crucial to all living cells, since they provide deoxyribonucleotides (dNTPs) for DNA synthesis and repair. In Mycobacterium tuberculosis, a class Ib RNR comprising nrdE- and nrdF2-encoded subunits is essential for growth in vitro. Interestingly, the genome of this obligate human pathogen also contains the nrdF1 (Rv1981c) and nrdB (Rv0233) genes, encoding an alternate class Ib RNR small (R2) subunit and a putative class Ic RNR R2 subunit, respectively. However, the role(s) of these subunits in dNTP provision during M. tuberculosis pathogenesis is unknown. In this study, we demonstrate that nrdF1 and nrdB are dispensable for the growth and survival of M. tuberculosis after exposure to various stresses in vitro and, further, that neither gene is required for growth and survival in mice. These observations argue against a specialist role for the alternate R2 subunits under the conditions tested. Through the construction of nrdR-deficient mutants of M. tuberculosis and Mycobacterium smegmatis, we establish that the genes encoding the essential class Ib RNR subunits are specifically regulated by an NrdR-type repressor. Moreover, a strain of M. smegmatis mc(2)155 lacking the 56-kb chromosomal region, which includes duplicates of nrdHIE and nrdF2, and a mutant retaining only one copy of nrdF2 are shown to be hypersensitive to the class I RNR inhibitor hydroxyurea as a result of depleted levels of the target. Together, our observations identify a potential vulnerability in dNTP provision in mycobacteria and thereby offer a compelling rationale for pursuing the class Ib RNR as a target for drug discovery.

The structural basis for peptidomimetic inhibition of eukaryotic ribonucleotide reductase: a conformationally flexible pharmacophore

Eukaryotic ribonucleotide reductase (RR) catalyzes nucleoside diphosphate conversion to deoxynucleoside diphosphate. Crucial for rapidly dividing cells, RR is a target for cancer therapy. RR activity requires formation of a complex between subunits R1 and R2 in which the R2 C-terminal peptide binds to R1. Here we report crystal structures of heterocomplexes containing mammalian R2 C-terminal heptapeptide, P7 (Ac-1FTLDADF7) and its peptidomimetic P6 (1Fmoc(Me)PhgLDChaDF7) bound to Saccharomyces cerevisiae R1 (ScR1). P7 and P6, both of which inhibit ScRR, each bind at two contiguous sites containing residues that are highly conserved among eukaryotes. Such binding is quite distinct from that reported for prokaryotes. The Fmoc group in P6 peptide makes several hydrophobic interactions that contribute to its enhanced potency in binding to ScR1. Combining all of our results, we observe three distinct conformations for peptide binding to ScR1. These structures provide pharmacophores for designing highly potent nonpeptide class I RR inhibitors.

Understanding Ribonucleotide Reductase Inactivation by Gemcitabine

This paper focuses on the inhibition of ribonucleotide reductase (RNR) by gemcitabine, 2',2'-difluoro-2'-deoxycytidine (dFdC), a deoxycytidine analogue that is a very active drug against solid tumors and is currently commercialized as gemzar. RNR inactivation is reductant-dependent and occurs in a very different way from that of other known substrate analogues. In the presence of reductants monomer R1 of RNR is inhibited, whereas in the absence of reductants the radical is lost and monomer R2 is inhibited. As inside the cell reductants are available, it is likely that R1 inactivation is the most favorable mechanism responsible for drug cytotoxicity. This inhibition pathway has been unknown to date, but we have conducted a theoretical study that has led us to the first proposal of a mechanism for RNR inhibition by dFdC in the presence of reductants.

Histone deacetylase inhibitors and hydroxyurea modulate the cell cycle and cooperatively induce apoptosis

Therapy resistance represents a major problem for disease management in oncology. Histone deacetylase inhibitors (HDACi) have been shown to modulate the cell cycle, to induce apoptosis and to sensitize cancer cells for other chemotherapeutics. Our study shows that the HDACi valproic acid (VPA) and the ribonucleotide reductase inhibitor hydroxyurea (HU) potentiate the pro-apoptotic effects of each other towards several cancer cell lines. This correlates with the HU-induced degradation of the cyclin-dependent kinase inhibitors (CDKI) p21 and p27, mediated by the proteasome or caspase-3. Moreover, we found that caspase-3 activation is required for VPA-induced apoptosis. Remarkably, p21 and p27 can confer resistance against VPA and HU. Both CDKI interact with caspase-3 and compete with other caspase-3 substrates. Hence, p21 and p27 may contribute to chemotherapy resistance as apoptosis inhibitors. Since the biological effects of VPA and HU could be achieved at concentrations used in current treatment protocols, the combined application of these compounds might be considered as a potential strategy for cancer treatment.

Phase I and pharmacokinetic study of Triapine, a potent ribonucleotide reductase inhibitor, in adults with advanced hematologic malignancies

Triapine, a potent inhibitor of ribonucleotide reductase, has demonstrated anti-leukemia activity in pre-clinical models. We conducted a Phase I study of Triapine administered as a 2 h infusion for 5 days in 25 adults with advanced leukemias. We established that Triapine at 96 mg/m2 once a day can be given safely on days 1-5 and 15-19 or 1-5 and 8-12 of a 4-week cycle. When administered twice a day on days 1-5 and 8-12, the maximum tolerated dose of Triapine appears to be 64 mg/m2, although the true criteria for DLT were not met by protocol definition. No CR or PR were observed, but 76% of patients had a >50% reduction in white blood cell counts. At all dose levels, the peak plasma concentration of Triapine (2.2-5.5 microM) was above levels required to achieve in vitro/in vivo leukemia growth inhibition. Based on these data, we conclude that Triapine warrants further investigation in hematologic malignancies.

Didox, a ribonucleotide reductase inhibitor, induces apoptosis and inhibits DNA repair in multiple myeloma cells

Ribonucleotide reductase (RR) is the enzyme that catalyses the rate-limiting step in DNA synthesis, the production of deoxynucleotides. RR activity is markedly elevated in tumour tissue and is crucial for cell division. It is therefore an excellent target for cancer chemotherapy. This study examined the anti-myeloma activity of Didox (3,4-Dihydroxybenzohydroxamic acid), a novel RR inhibitor (RRI). Our data showed that Didox induced caspase-dependent multiple myeloma (MM) cell apoptosis. Didox, unlike other RRIs that mainly target the pyrimidine metabolism pathway, targets both purine and pyrimidine metabolism pathways in MM, as demonstrated by transcriptional profiling using the Affymetrix U133A 2.0 gene chip. Specifically, a >or=2-fold downregulation of genes in these anabolic pathways was shown as early as 12 h after exposure to Didox. Furthermore, apoptosis was accompanied by downregulation of bcl family proteins including bcl-2, bcl(xl), and XIAP. Importantly, RR M1 component transcript was also downregulated, associated with decreased protein expression. Genes involved in DNA repair mechanisms, specifically RAD 51 homologue, were also downregulated. As Didox acts on MM cells by inhibiting DNA synthesis and repair, combination studies with melphalan, an agent commonly used in MM, were performed. A strong in vitro synergism was shown, with combination indices of <0.7 as determined by the Chou-Talalay method. These studies therefore provide the preclinical rationale for evaluation of Didox, alone and in combination with DNA-damaging agents, to improve patient outcome in MM.

Structures of eukaryotic ribonucleotide reductase I define gemcitabine diphosphate binding and subunit assembly

Ribonucleotide reductase (RNR) catalyzes the conversion of nucleoside diphosphates to deoxynucleoside diphosphates. Crucial for rapidly dividing cells, RNR is a target for cancer therapy. In eukaryotes, RNR comprises a heterooligomer of alpha(2) and beta(2) subunits. Rnr1, the alpha subunit, contains regulatory and catalytic sites; Rnr2, the beta subunit (in yeast, a heterodimer of Rnr2 and Rnr4), houses the diferric-tyrosyl radical crucial for catalysis. Here, we present three x-ray structures of eukaryotic Rnr1 from Saccharomyces cerevisiae: one bound to gemcitabine diphosphate (GemdP), the active metabolite of the mechanism-based chemotherapeutic agent gemcitabine; one with an Rnr2-derived peptide, and one with an Rnr4-derived peptide. Our structures reveal that GemdP binds differently from its analogue, cytidine diphosphate; because of unusual interactions of the geminal fluorines, the ribose and base of GemdP shift substantially, and loop 2, which mediates substrate specificity, adopts different conformations when binding to GemdP and cytidine diphosphate. The Rnr2 and Rnr4 peptides, which block RNR assembly, bind differently from each other but have unique modes of binding not seen in prokaryotic RNR. The Rnr2 peptide adopts a conformation similar to that previously reported from an NMR study for a mouse Rnr2-based peptide. In yeast, the Rnr2 peptide binds at subsites consisting of residues that are highly conserved among yeast, mouse, and human Rnr1s, suggesting that the mode of Rnr1-Rnr2 binding is conserved among eukaryotes. These structures provide new insights into subunit assembly and a framework for structure-based drug design targeting RNR.

Triapine and cytarabine is an active combination in patients with acute leukemia or myelodysplastic syndrome

Triapine, an iron chelator and a potent inhibitor of ribonucleotide reductase, has significant anti-leukemia activity. A phase I study of Triapine in combination with ara-C was conducted in 32 patients with refractory acute leukemia and high-risk MDS. Triapine (105 mg/m2/day 6-h infusion) was followed immediately by ara-C [100 (n=4), 200 (n=6), 400 (n=7), or 800 (n=8)mg/m2/day] as an 18-h infusion for 5 consecutive days. Dose-limiting toxicities (DLTs) were observed at the 800 mg/m2 ara-C dose level (one patient each with grade 4 mucositis; grade 4 neutropenic colitis, sepsis; grade 4 neuropathy; and grade 4 hyperbilirubinemia). Therefore, the study was amended to include an ara-C dose level of 600 mg/m2/day, no DLTs occurred in seven patients treated at this dose level. Mean Triapine C(max) and AUC were 1.13 microg/mL and 251.5 minmicrog/mL. Of 31 evaluable patients, 4 (13%) (3 AML, 1 Ph+ALL) achieved a CR (1 at a dose of 800 mg/m2; 2 at 600 mg/m2; 1 at 200mg/m2). The recommended phase II regimen is Triapine 105 mg/m2/day followed by ara-C 600 mg/m2/day for 5 consecutive days every 3-6 weeks.

Design, synthesis, and evaluation of octahydropyranopyrrole-based inhibitors of mammalian ribonucleotide reductase

Inhibitors of mammalian ribonucleotide reductase possessing a novel octahydropyranopyrrole scaffold based on a cyclic heptapeptide inhibitor have been designed, synthesized, and evaluated. Structure-function studies reveal that the bicyclic scaffold is indeed necessary to maintain inhibitory activity.

Electron transfer in N-hydroxyurea complexes with iron(III)

Redox behaviour of the iron(III) complex with the antitumour drug hydroxyurea was studied by cyclic voltammetry. The complex underwent a one-electron reduction, followed by an irreversible chemical reaction (EC mechanism) in which a ligand was released. In addition, it was found that the hydroxyurea gave up an electron to iron(III) in solution. Differential-pulse voltammetry revealed an increase in the concentration of the generated iron(II) species. Electron paramagnetic resonance (EPR) spectroscopy studies of the oxidative degradation of hydroxyurea confirmed formation of the radical species H2N-CO-NHO*. Electrochemical data for iron(III) complexes of hydroxyurea and its structural analogue 3-ethylhydroxyurea, which also exhibits antitumour activity, show the same mechanism involved in the electron transfer. The observed redox properties indicate that hydroxyurea may interfere with electron transfer processes in biological systems after binding to iron-containing ribonucleotide reductase.

Mechanisms of action of peptide inhibitors of mammalian ribonucleotide reductase targeting quaternary structure

Mammalian ribonucleotide reductase (mRR) is a chemotherapeutic target. The enzyme is composed of 2 subunits (mR1 and mR2) and is inhibited by Ac-FTLDADF (denoted P7), corresponding to the C-terminus of mR2, which competes with mR2 for binding to mR1. mRR has 2 physiologically important active forms, mR12mR22 and mR16(mR22)j (j = 1-3). Here we report on the mechanism of action of recently identified peptide derivatives having higher activities than P7 toward inhibition of one or both active forms. A significant feature of both P7 and these new inhibitors is that they are more potent vs. mR12mR22 than mR16(mR22)j. For some of these peptides, this is due in part to their preferential binding to the mR1 monomer. The possible application of these peptide derivatives in cancer chemotherapy is discussed.

Synergistic action of resveratrol, an ingredient of wine, with Ara-C and tiazofurin in HL-60 human promyelocytic leukemia cells

OBJECTIVE

Resveratrol, a naturally occurring stilbene derivative, is a potent free-radical scavenger causing a number of biochemical and antineoplastic effects. It was shown to induce differentiation and apoptosis in leukemia cells. Resveratrol was also identified as an inhibitor of ribonucleotide reductase (RR), a key enzyme of DNA synthesis. We report about the biochemical effects of resveratrol on the concentration of deoxyribonucleoside triphosphates (dNTPs), the products of RR, and on the incorporation of 14C-labeled cytidine into the DNA of HL-60 human promyelocytic leukemia cells.

MATERIALS AND METHODS

Incorporation of 14C-labeled cytidine into the DNA of resveratrol-treated HL-60 cells was measured. Concentration of dNTPs was determined by a HPLC method. Cytotoxic effects of resveratrol, Ara-C, and tiazofurin were analyzed using growth inhibition and clonogenic assays. Induction of apoptosis was studied using a Hoechst/propidium iodide staining method.

RESULTS

We found that resveratrol effectively inhibited incorporation of 14C-labeled cytidine into DNA. Furthermore, incubation of HL-60 cells with resveratrol significantly decreased intracellular dCTP, dTTP, dATP, and dGTP concentrations. Based on these results, we investigated the combination effects of resveratrol with Ara-C or tiazofurin, both antimetabolites, which are known to exhibit synergistic effects in combination with other inhibitors of RR. In growth inhibition, apoptosis, and clonogenic assays, resveratrol acted synergistically with both Ara-C and tiazofurin in HL-60 cells.

CONCLUSIONS

We conclude that resveratrol could become a viable candidate as one compound in the combination chemotherapy of leukemia and therefore deserves further testing.

The enantioselectivities of the active and allosteric sites of mammalian ribonucleotide reductase

Here we examine the enantioselectivity of the allosteric and substrate binding sites of murine ribonucleotide reductase (mRR). L-ADP binds to the active site and L-ATP binds to both the s- and a-allosteric sites of mR1 with affinities that are only three- to 10-fold weaker than the values for the corresponding D-enantiomers. These results demonstrate the potential of L-nucleotides for interacting with and modulating the activity of mRR, a cancer chemotherapeutic and antiviral target. On the other hand, we detect no substrate activity for L-ADP and no inhibitory activity for N3-L-dUDP, demonstrating the greater stereochemical stringency at the active site with respect to catalytic activity.

Peptide inhibitors of mammalian ribonucleotide reductase

Mammalian ribonucleotide reductase (mRR) is a chemotherapeutic target. In common with other class Ia RRs, the enzyme is composed of two subunits (mR1 and mR2), with mR1 containing both the active site and allosteric effector sites and mR2 containing a stable tyrosyl radical that is essential for enzymatic activity. mRR is inhibited by Ac-FTLDADF (denoted P7), corresponding to the C-terminus of mR2, which competes with mR2 for binding to mR1. The enzyme has two physiologically important active forms, mR12mR22 and mR16(mR22)j (j=1-3), with high ATP concentrations favoring the latter. Here, we report on our progress in using structural and functional studies in conjunction with library screening to identify derivatives of tri-, tetra- and hexapeptides, and cyclic heptapeptides, having equal or significantly higher activities than P7 toward inhibition of one or both active forms. These identifications were made by screening candidate peptides both for their abilities to bind to mR1 competitively with P7 and to inhibit ribonucleotide reductase activity. A significant feature of both P7 and the newly identified derivatives is that they are stronger inhibitors of mR12mR22 than of mR16(mR22)j. For the tetrapeptides, this is due in part to their preferential binding to mR1 monomer. The possible application of these peptide derivatives in cancer chemotherapy, exploiting their preferential inhibition of mR12mR22, is considered.

More potent linear peptide inhibitors of mammalian ribonucleotide reductase

Mammalian ribonucleotide reductase (mRR) is a chemotherapeutic target. The enzyme is composed of two subunits (mR1 and mR2) and is inhibited by Ac-FTLDADF (denoted P7), corresponding to the C-terminus of mR2, which disrupts mRR quaternary structure by competing with mR2 for binding to mR1. The tripeptide FmocWFF acts similarly. Here we report on the use of small, focused libraries to identify Fmoc derivatives of tetra and hexapeptides having comparable or considerably higher activities than P7 toward inhibition of mRR.

A systematic method for identifying small-molecule modulators of protein-protein interactions

Discovering small-molecule modulators of protein-protein interactions is a challenging task because of both the generally noncontiguous, large protein surfaces that form these interfaces and the shortage of high-throughput approaches capable of identifying such rare inhibitors. We describe here a robust and flexible methodology that couples disruption of protein-protein complexes to host cell survival. The feasibility of this approach was demonstrated through monitoring a small-molecule-mediated protein-protein association (FKBP12-rapamycin-FRAP) and two cases of dissociation (homodimeric HIV-1 protease and heterodimeric ribonucleotide reductase). For ribonucleotide reductase, we identified cyclic peptide inhibitors from genetically encoded libraries that dissociated the enzyme subunits. A solid-phase synthetic strategy and peptide ELISAs were developed to characterize these inhibitors, resulting in the discovery of cyclic peptides that operate in an unprecedented manner, thus highlighting the strengths of a functional approach. The ability of this method to process large libraries, coupled with the benefits of a genetic selection, allowed us to identify rare, uniquely active small-molecule modulators of protein-protein interactions at a frequency of less than one in 10 million.

Oligopeptide inhibition of class I ribonucleotide reductases

Class I ribonucleotide reductases (RRs), which are well-recognized targets for cancer chemotherapeutic and antiviral agents, are composed of two different subunits, R1 and R2, and are inhibited by oligopeptides corresponding to the C-terminus of R2, which compete with R2 for binding to R1. These peptides specifically inhibit the RRs from which they are derived, and closely homologous RRs, but do not inhibit less homologous RRs. Here we review results obtained for oligopeptide inhibition of RRs from several sources, including related x-ray, NMR, and modeling results. The most extensive studies have been performed on herpes simplex virus-RR (HSV-RR) and mammalian-RR (mRR). A common model fits the data obtained for both enzymes, in which the C-terminal residue of the oligopeptide (Leu for HSV-RR, Phe for mRR) binds with high specificity to a narrow and deep hydrophobic subsite, and two or more hydrophobic groups at the N-terminal portion of the peptide bind to a broad and shallow second hydrophobic subsite. The studies have led to the development of highly potent and specific inhibitors of HSV-RR and promising inhibitors of mRR, and indicate possible directions for the development of inhibitors of bacterial and fungal RRs.

Phase I and pharmacodynamic study of Triapine, a novel ribonucleotide reductase inhibitor, in patients with advanced leukemia

In a phase I study, 24 patients with refractory leukemia received Triapine, a novel ribonucleotide reductase (RR) inhibitor, as a continuous intravenous infusion over 96 h beginning on days 1 and 15 or days 1 and 8. On the days 1 and 15 regimen, the starting dose was 120 mg/m(2) per day, and the maximum tolerated dose (MTD) was 160 mg/m(2) per day. Three of eight patients receiving 160 mg/m(2) per day in the first course, and one patient escalated to this dose in a second course, developed hepatic dose-limiting toxicity (DLT). For the days 1 and 8 regimen, the first 96 h infusion was administered at a fixed dose of 140 mg/m(2) per day. The dose of the second infusion beginning on day 8 was escalated from 120 to 160 mg/m(2) per day without observing DLT. No objective responses occurred. Over 70% of patients had a >50% reduction in white blood cell counts. The steady-state levels of Triapine were between 0.6 and 1 microM. As expected from the in vitro studies, at these plasma concentrations there was a decline in dATP and dGTP pools and a decrease in DNA synthetic capacity of the circulating leukemia cells. Based on these clinical, pharmacokinetic, and pharmacodynamic data, Triapine warrants further study in patients with hematologic malignancies.

Ribonucleotide reduction in Mycobacterium tuberculosis: function and expression of genes encoding class Ib and class II ribonucleotide reductases

Mycobacterium tuberculosis, the causative agent of tuberculosis, possesses a class Ib ribonucleotide reductase (RNR), encoded by the nrdE and nrdF2 genes, in addition to a putative class II RNR, encoded by nrdZ. In this study we probed the relative contributions of these RNRs to the growth and persistence of M. tuberculosis. We found that targeted knockout of the nrdF2 gene could be achieved only in the presence of a complementing allele, confirming that this gene is essential under normal, in vitro growth conditions. This observation also implied that the alternate class Ib small subunit encoded by the nrdF1 gene is unable to substitute for nrdF2 and that the class II RNR, NrdZ, cannot substitute for the class Ib enzyme, NrdEF2. Conversely, a DeltanrdZ null mutant of M. tuberculosis was readily obtained by allelic exchange mutagenesis. Quantification of levels of nrdE, nrdF2, nrdF1, and nrdZ gene expression by real-time, quantitative reverse transcription-PCR with molecular beacons by using mRNA from aerobic and O(2)-limited cultures showed that nrdZ was significantly induced under microaerophilic conditions, in contrast to the other genes, whose expression was reduced by O(2) restriction. However, survival of the DeltanrdZ mutant strain was not impaired under hypoxic conditions in vitro. Moreover, the lungs of B6D2/F(1) mice infected with the DeltanrdZ mutant had bacterial loads comparable to those of lungs infected with the parental wild-type strain, which argues against the hypothesis that nrdZ plays a significant role in the virulence of M. tuberculosis in this mouse model.

Transferrin ensures survival of ovarian carcinoma cells when apoptosis is induced by TNFα, FasL, TRAIL, or Myc

The activation of Myc induces apoptosis of human ovarian adenocarcinoma N.1 cells when serum factors are limited. However, the downstream mechanism that is triggered by Myc is unknown. Myc-activation and treatment with the proapoptotic ligands TNFalpha, FasL, and TRAIL induced H-ferritin expression under serum-deprived conditions. H-ferritin chelates intracellular iron and also intracellular iron sequestration by deferoxamine-induced apoptosis of N.1 cells. Supplementation of serum-free medium with holo-transferrin blocked apoptosis of N.1 cells that was induced by Myc-activation or by treatment with TNFalpha, FasL, and TRAIL, whereas apotransferrin did not prevent apoptosis. This suggests that intracellular iron depletion was a trigger for apoptosis and that transferrin-bound iron rescued N.1 cells. Furthermore, apoptosis of primary human ovarian carcinoma cells, which was induced by TNFalpha, FasL, and TRAIL, was also inhibited by holo-transferrin. The data suggest that Myc-activation, FasL, TNFalpha, and TRAIL disturbed cellular iron homeostasis, which triggered apoptosis of ovarian carcinoma cells and that transferrin iron ensured survival by re-establishing this homeostasis.

Understanding the Antitumor Activity of Novel Hydroxysemicarbazide Derivatives as Ribonucleotide Reductase Inhibitors Using CoMFA and CoMSIA

Three-dimensional quantitative structure-activity relationship (3D-QSAR) studies were performed on a series of Schiff bases of hydroxysemicarbazide analogues using comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) methods with their antitumor activities against L1210 cells. The models were generated using 24 molecules, out of which one molecule was a commercially available ribonucleotide reductase (RR) inhibitor, hydroxyurea (HU), and the predictive ability of the resulting each model was evaluated against a test set of four molecules. Maximum common substructure (MCS)-based method was used for alignment and compared with the known alignment methods. The QSAR models from both methods exhibited considerable correlative and predictive properties. Inclusion of additional descriptor ClogP improved the statistics of CoMFA model significantly. Both methods strongly suggest the necessity of lipophilicity for antitumor activity. CoMFA and CoMSIA methods predicted HU optimally, indicating a similar mechanism of action for the molecules considered for generating the models and HU to inhibit the tumor cells. The analysis of CoMFA contour maps provided insight into the possible modification of the molecules for better activity.

Peptides with anticancer use or potential

This review is an attempt to illustrate the diversity of peptides reported for a potential or an established use in cancer therapy. With 612 references, this work aims at covering the patents and publications up to year 2000 with many inroads in years 2001-2002. The peptides are classed according to four categories of effective (or plausible) biological mechanisms of action: receptor-interacting compounds; inhibitors of protein-protein interaction; enzymes inhibitors; nucleic acid-interacting compounds. The fifth group is made of the peptides for which no mechanism of action has been found yet. Incidentally this work provides an overview of many of the modern targets of anticancer research.