RNA interference targeting WSSV ribonucleotide reductase 2 provides long-term protection against infection in Litopenaeus vannamei.

Many studies have demonstrated that long double-stranded RNAs (dsRNAs) targeting essential genes of white spot syndrome virus (WSSV) can induce a sequence-specific antiviral RNA interference (RNAi) response in shrimp, thereby offering protection against WSSV infection. However, further experimental data on the required dose of dsRNAs and the duration of protection from a single administration are necessary to establish RNAi-mediated methods as effective and practical antiviral measures. In this study, we evaluated the protective efficacy and the duration of protection provided by a single administration of various doses of long dsRNA targeting WSSV ribonucleotide reductase 2 (rr2) in white-leg shrimp Litopenaeus vannamei. The protective efficacy of long dsRNA targeting WSSV rr2 was not diminished by the reduction of the dose to 100 ng g-1 of body weight, suggesting that a relatively low dose can effectively induce an RNAi response in shrimp. Furthermore, shrimp were well-protected against WSSV challenges for up to 4 wk post-administration of the rr2-targeting long dsRNA, although the protective effect almost disappeared at 6 wk post-administration. These results suggest that long dsRNAs can provide protection against WSSV for at least 1 mo, and monthly administration of long dsRNAs could serve as a long-term protective strategy for shrimp against WSSV.

Nucleotide binding to the ATP-cone in anaerobic ribonucleotide reductases allosterically regulates activity by modulating substrate binding.

A small, nucleotide-binding domain, the ATP-cone, is found at the N-terminus of most ribonucleotide reductase (RNR) catalytic subunits. By binding adenosine triphosphate (ATP) or deoxyadenosine triphosphate (dATP) it regulates the enzyme activity of all classes of RNR. Functional and structural work on aerobic RNRs has revealed a plethora of ways in which dATP inhibits activity by inducing oligomerisation and preventing a productive radical transfer from one subunit to the active site in the other. Anaerobic RNRs, on the other hand, store a stable glycyl radical next to the active site and the basis for their dATP-dependent inhibition is completely unknown. We present biochemical, biophysical, and structural information on the effects of ATP and dATP binding to the anaerobic RNR from Prevotella copri. The enzyme exists in a dimer-tetramer equilibrium biased towards dimers when two ATP molecules are bound to the ATP-cone and tetramers when two dATP molecules are bound. In the presence of ATP, P. copri NrdD is active and has a fully ordered glycyl radical domain (GRD) in one monomer of the dimer. Binding of dATP to the ATP-cone results in loss of activity and increased dynamics of the GRD, such that it cannot be detected in the cryo-EM structures. The glycyl radical is formed even in the dATP-bound form, but the substrate does not bind. The structures implicate a complex network of interactions in activity regulation that involve the GRD more than 30 Å away from the dATP molecules, the allosteric substrate specificity site and a conserved but previously unseen flap over the active site. Taken together, the results suggest that dATP inhibition in anaerobic RNRs acts by increasing the flexibility of the flap and GRD, thereby preventing both substrate binding and radical mobilisation.

Copper(II) Complexes with Isomeric Morpholine-Substituted 2-Formylpyridine Thiosemicarbazone Hybrids as Potential Anticancer Drugs Inhibiting Both Ribonucleotide Reductase and Tubulin Polymerization: The Morpholine Position Matters.

The development of copper(II) thiosemicarbazone complexes as potential anticancer agents, possessing dual functionality as inhibitors of R2 ribonucleotide reductase (RNR) and tubulin polymerization by binding at the colchicine site, presents a promising avenue for enhancing therapeutic effectiveness. Herein, we describe the syntheses and physicochemical characterization of four isomeric proligands H2L3-H2L6, with the methylmorpholine substituent at pertinent positions of the pyridine ring, along with their corresponding Cu(II) complexes 3-6. Evidently, the position of the morpholine moiety and the copper(II) complex formation have marked effects on the in vitro antiproliferative activity in human uterine sarcoma MES-SA cells and the multidrug-resistant derivative MES-SA/Dx5 cells. Activity correlated strongly with quenching of the tyrosyl radical (Y•) of mouse R2 RNR protein, inhibition of RNR activity in the cancer cells, and inhibition of tubulin polymerization. Insights into the mechanism of antiproliferative activity, supported by experimental results and molecular modeling calculations, are presented.

The thioredoxin system determines CHK1 inhibitor sensitivity via redox-mediated regulation of ribonucleotide reductase activity.

Checkpoint kinase 1 (CHK1) is critical for cell survival under replication stress (RS). CHK1 inhibitors (CHK1i's) in combination with chemotherapy have shown promising results in preclinical studies but have displayed minimal efficacy with substantial toxicity in clinical trials. To explore combinatorial strategies that can overcome these limitations, we perform an unbiased high-throughput screen in a non-small cell lung cancer (NSCLC) cell line and identify thioredoxin1 (Trx1), a major component of the mammalian antioxidant-system, as a determinant of CHK1i sensitivity. We establish a role for redox recycling of RRM1, the larger subunit of ribonucleotide reductase (RNR), and a depletion of the deoxynucleotide pool in this Trx1-mediated CHK1i sensitivity. Further, the TrxR inhibitor auranofin, an approved anti-rheumatoid arthritis drug, shows a synergistic interaction with CHK1i via interruption of the deoxynucleotide pool. Together, we show a pharmacological combination to treat NSCLC that relies on a redox regulatory link between the Trx system and mammalian RNR activity.

Physical interactions between specifically regulated subpopulations of the MCM and RNR complexes prevent genetic instability.

The helicase MCM and the ribonucleotide reductase RNR are the complexes that provide the substrates (ssDNA templates and dNTPs, respectively) for DNA replication. Here, we demonstrate that MCM interacts physically with RNR and some of its regulators, including the kinase Dun1. These physical interactions encompass small subpopulations of MCM and RNR, are independent of the major subcellular locations of these two complexes, augment in response to DNA damage and, in the case of the Rnr2 and Rnr4 subunits of RNR, depend on Dun1. Partial disruption of the MCM/RNR interactions impairs the release of Rad52 -but not RPA-from the DNA repair centers despite the lesions are repaired, a phenotype that is associated with hypermutagenesis but not with alterations in the levels of dNTPs. These results suggest that a specifically regulated pool of MCM and RNR complexes plays non-canonical roles in genetic stability preventing persistent Rad52 centers and hypermutagenesis.

Preparation and characterization of MnIIMnIII complexes with relevance to class Ib ribonucleotide reductases.

The Mn2 complex [MnII2(TPDP)(O2CPh)2](BPh4) (1, TPDP = 1,3-bis(bis(pyridin-2-ylmethyl)amino)propan-2-ol, Ph =phenyl) was prepared and subsequently characterized via single-crystal X-ray diffraction, X-ray absorption, electronic absorption, and infrared spectroscopies, and mass spectrometry. 1 was prepared in order to explore its properties as a structural and functional mimic of class Ib ribonucleotide reductases (RNRs). 1 reacted with superoxide anion (O2•-) to generate a peroxido-MnIIMnIII complex, 2. The electronic absorption and electron paramagnetic resonance (EPR) spectra of 2 were similar to previously published peroxido-MnIIMnIII species. Furthermore, X-ray near edge absorption structure (XANES) studies indicated the conversion of a MnII2 core in 1 to a MnIIMnIII state in 2. Treatment of 2 with para-toluenesulfonic acid (p-TsOH) resulted in the conversion to a new MnIIMnIII species, 3, rather than causing O-O bond scission, as previously encountered. 3 was characterized using electronic absorption, EPR, and X-ray absorption spectroscopies. Unlike other reported peroxido-MnIIMnIII species, 3 was capable of oxidative O-H activation, mirroring the generation of tyrosyl radical in class Ib RNRs, however without accessing the MnIIIMnIV state.

Evaluation of 1,10-phenanthroline-based hydroxamate derivative as dual histone deacetylases/ribonucleotide reductase inhibitor with antitumor activities.

BACKGROUND: Aberrant expression of histone deacetylases (HDACs) and ribonucleotide reductase (RR) enzymes are commonly observed in various cancers. Researchers are focusing on these enzymes in cancer studies with the aim of developing effective chemotherapeutic drugs for cancer treatment. Targeting both HDAC and RR simultaneously with a dual HDAC/RR inhibitor has exhibited enhanced effectiveness compared to monotherapy in cancer treatment, making it a promising strategy. OBJECTIVES: The objective of the study is to synthesize and assess the anti-cancer properties of a 1,10-phenanthroline-based hydroxamate derivative, characterizing it as a novel dual HDAC/RR inhibitor. METHODS: The N1-hydroxy-N8-(1,10-phenanthrolin-5-yl)octanediamide (PA), a 1,10-phenanthroline-based hydroxamate derivative, was synthesized and structurally characterized. The compound was subjected to in vitro assessments of its anti-cancer, HDAC, and RR inhibitory activities. In silico docking and molecular dynamics simulations were further studied to explore its interactions with HDACs and RRM2. RESULTS: The structurally confirmed PA exhibited antiproliferative activity in SiHa cells with an IC50 of 16.43 µM. It displayed potent inhibitory activity against HDAC and RR with IC50 values of 10.80 µM and 9.34 µM, respectively. Co-inhibition of HDAC and RR resulted in apoptosis-induced cell death in SiHa cells, mediated by the accumulation of reactive oxygen species (ROS). In silico docking studies demonstrated that PA can effectively bind to the active sites of HDAC isoforms and RRM2. Furthermore, PA demonstrated a more favorable interaction with HDAC7, displaying a docking score of -9.633 kcal/mol, as compared to the standard HDAC inhibitor suberoylanilide hydroxamic acid (SAHA), which exhibited a docking score of -8.244 kcal/mol against HDAC7. CONCLUSION: The present study emphasizes the prospect of designing a potential 1,10-phenanthroline hydroxamic acid derivative as a novel dual HDAC and RR-inhibiting anti-cancer molecule.

Protein engineering a PhotoRNR chimera based on a unifying evolutionary apparatus among the natural classes of ribonucleotide reductases.

Ribonucleotide reductases (RNRs) are essential enzymes that catalyze the de novo transformation of nucleoside 5'-di(tri)phosphates [ND(T)Ps, where N is A, U, C, or G] to their corresponding deoxynucleotides. Despite the diversity of factors required for function and the low sequence conservation across RNRs, a unifying apparatus consolidating RNR activity is explored. We combine aspects of the protein subunit simplicity of class II RNR with a modified version of Escherichia coli class la photoRNRs that initiate radical chemistry with light to engineer a mimic of a class II enzyme. The design of this RNR involves fusing a truncated form of the active site containing α subunit with the functionally important C-terminal tail of the radical-generating ß subunit to render a chimeric RNR. Inspired by a recent cryo-EM structure, a [Re] photooxidant is located adjacent to Y356[ß], which is an essential component of the radical transport pathway in class I RNRs. Combination of this RNR photochimera with cytidine diphosphate (CDP), adenosine triphosphate (ATP), and light resulted in the generation of Y356• along with production of deoxycytidine diphosphate (dCDP) and cytosine. The photoproducts reflect an active site chemistry consistent with both the consensus mechanism of RNR and chemistry observed when RNR is inactivated by mechanism-based inhibitors in the active site. The enzymatic activity of the RNR photochimera in the absence of any ß metallocofactor highlights the adaptability of the 10-stranded αß barrel finger loop to support deoxynucleotide formation and accommodate the design of engineered RNRs.

Therapeutic prime/pull vaccination of HSV-2-infected guinea pigs with the ribonucleotide reductase 2 (RR2) protein and CXCL11 chemokine boosts antiviral local tissue-resident and effector memory CD4+ and CD8+ T cells and protects against recurrent genital herpes.

Following acute herpes simplex virus type 2 (HSV-2) infection, the virus undergoes an asymptomatic latent infection of sensory neurons of dorsal root ganglia (DRG). Chemical and physical stress cause intermittent virus reactivation from latently infected DRG and recurrent virus shedding in the genital mucosal epithelium causing genital herpes in symptomatic patients. While T cells appear to play a role in controlling virus reactivation from DRG and reducing the severity of recurrent genital herpes, the mechanisms for recruiting these T cells into DRG and the vaginal mucosa (VM) remain to be fully elucidated. The present study investigates the effect of CXCL9, CXCL10, and CXCL11 T-cell-attracting chemokines on the frequency and function of DRG- and VM-resident CD4+ and CD8+ T cells and its effect on the frequency and severity of recurrent genital herpes in the recurrent herpes guinea pig model. HSV-2 latent-infected guinea pigs were immunized intramuscularly with the HSV-2 ribonucleotide reductase 2 (RR2) protein (Prime) and subsequently treated intravaginally with the neurotropic adeno-associated virus type 8 expressing CXCL9, CXCL10, or CXCL11 chemokines to recruit CD4+ and CD8+ T cells into the infected DRG and VM (Pull). Compared to the RR2 therapeutic vaccine alone, the RR2/CXCL11 prime/pull therapeutic vaccine significantly increased the frequencies of functional tissue-resident and effector memory CD4+ and CD8+ T cells in both DRG and VM tissues. This was associated with less virus in the healed genital mucosal epithelium and reduced frequency and severity of recurrent genital herpes. These findings confirm the role of local DRG- and VM-resident CD4+ and CD8+ T cells in reducing virus shedding at the vaginal site of infection and the severity of recurrent genital herpes and propose the novel prime-pull vaccine strategy to protect against recurrent genital herpes.IMPORTANCEThe present study investigates the novel prime/pull therapeutic vaccine strategy to protect against recurrent genital herpes using the latently infected guinea pig model. In this study, we used the strategy that involves immunization of herpes simplex virus type 2-infected guinea pigs using a recombinantly expressed herpes tegument protein-ribonucleotide reductase 2 (RR2; prime), followed by intravaginal treatment with the neurotropic adeno-associated virus type 8 expressing CXCL9, CXCL10, or CXCL11 T-cell-attracting chemokines to recruit T cells into the infected dorsal root ganglia (DRG) and vaginal mucosa (VM) (pull). We show that the RR2/CXCL11 prime-pull therapeutic vaccine strategy elicited a significant reduction in virus shedding in the vaginal mucosa and decreased the severity and frequency of recurrent genital herpes. This protection was associated with increased frequencies of functional tissue-resident (TRM cells) and effector (TEM cells) memory CD4+ and CD8+ T cells infiltrating latently infected DRG tissues and the healed regions of the vaginal mucosa. These findings shed light on the role of tissue-resident and effector memory CD4+ and CD8+ T cells in DRG tissues and the VM in protection against recurrent genital herpes and propose the prime-pull therapeutic vaccine strategy in combating genital herpes.

Synergistic therapeutics: Co-targeting histone deacetylases and ribonucleotide reductase for enhanced cancer treatment.

The development of cancer is influenced by several variables, including altered protein expression, and signaling pathways. Cancers are inherently heterogeneous and exhibit genetic and epigenetic aberrations; therefore, developing therapies that act on numerous biological targets is encouraged. To achieve this, two approaches are employed: combination therapy and dual/multiple targeting chemotherapeutics. Two enzymes, histone deacetylases (HDACs) and ribonucleotide reductase (RR), are crucial for several biological functions, including replication and repair of DNA, division of cells, transcription of genes, etc. However, it has been noted that different cancers exhibit abnormal functions of these enzymes. Potent inhibitors for each of these proteins have been extensively researched. Many medications based on these inhibitors have been successfully food and drug administration (FDA) approved, and the majority are undergoing various stages of clinical testing. This review discusses various studies of HDAC and RR inhibitors in combination therapy and dual-targeting chemotherapeutics.

Probing Nonadiabaticity of Proton-Coupled Electron Transfer in Ribonucleotide Reductase.

The enzyme ribonucleotide reductase, which is essential for DNA synthesis, initiates the conversion of ribonucleotides to deoxyribonucleotides via radical transfer over a 32 Å pathway composed of proton-coupled electron transfer (PCET) reactions. Previously, the first three PCET reactions in the α subunit were investigated with hybrid quantum mechanical/molecular mechanical (QM/MM) free energy simulations. Herein, the fourth PCET reaction in this subunit between C439 and guanosine diphosphate (GDP) is simulated and found to be slightly exoergic with a relatively high free energy barrier. To further elucidate the mechanisms of all four PCET reactions, we analyzed the vibronic and electron-proton nonadiabaticities. This analysis suggests that interfacial PCET between Y356 and Y731 is vibronically and electronically nonadiabatic, whereas PCET between Y731 and Y730 and between C439 and GDP is fully adiabatic and PCET between Y730 and C439 is in the intermediate regime. These insights provide guidance for selecting suitable rate constant expressions for these PCET reactions.

A novel alkane monooxygenase evolved from a broken piece of ribonucleotide reductase in Geobacillus kaustophilus HTA426 isolated from Mariana Trench.

We have accidentally found that a thermophilic Geobacillus kaustophilus HTA426 is capable of degrading alkanes although it has no alkane oxygenating enzyme genes. Our experimental results revealed that a putative ribonucleotide reductase small subunit GkR2loxI (GK2771) gene encodes a novel heterodinuclear Mn-Fe alkane monooxygenase/hydroxylase. GkR2loxI protein can perform two-electron oxidations similar to homonuclear diiron bacterial multicomponent soluble methane monooxygenases. This finding not only answers a long-standing question about the substrate of the R2lox protein clade, but also expands our understanding of the vast diversity and new evolutionary lineage of the bacterial alkane monooxygenase/hydroxylase family.

Ribonucleotide reductase inhibition improves the symptoms of a Caenorhabditis elegans model of Alzheimer's disease.

Alzheimer's disease is the main cause of aging-associated dementia, for which there is no effective treatment. In this work, we reanalyze the information of a previous genome wide association study, using a new pipeline design to identify novel potential drugs. With this approach, ribonucleoside-diphosphate reductase gene (RRM2B) emerged as a candidate target and its inhibitor, 2', 2'-difluoro 2'deoxycytidine (gemcitabine), as a potential pharmaceutical drug against Alzheimer's disease. We functionally verified the effect of inhibiting the RRM2B homolog, rnr-2, in an Alzheimer's model of Caenorhabditis elegans, which accumulates human Aß1-42 peptide to an irreversible paralysis. RNA interference against rnr-2 and also treatment with 200 ng/ml of gemcitabine, showed an improvement of the phenotype. Gemcitabine treatment increased the intracellular ATP level 3.03 times, which may point to its mechanism of action. Gemcitabine has been extensively used in humans for cancer treatment but at higher concentrations. The 200 ng/ml concentration did not exert a significant effect over cell cycle, or affected cell viability when assayed in the microglia N13 cell line. Thus, the inhibitory drug of the RRM2B activity could be of potential use to treat Alzheimer's disease and particularly gemcitabine might be considered as a promising candidate to be repurposed for its treatment.

Checkpoint activation by Spd1: a competition-based system relying on tandem disordered PCNA binding motifs.

DNA regulation, replication and repair are processes fundamental to all known organisms and the sliding clamp proliferating cell nuclear antigen (PCNA) is central to all these processes. S-phase delaying protein 1 (Spd1) from S. pombe, an intrinsically disordered protein that causes checkpoint activation by inhibiting the enzyme ribonucleotide reductase, has one of the most divergent PCNA binding motifs known. Using NMR spectroscopy, in vivo assays, X-ray crystallography, calorimetry, and Monte Carlo simulations, an additional PCNA binding motif in Spd1, a PIP-box, is revealed. The two tandemly positioned, low affinity sites exchange rapidly on PCNA exploiting the same binding sites. Increasing or decreasing the binding affinity between Spd1 and PCNA through mutations of either motif compromised the ability of Spd1 to cause checkpoint activation in yeast. These results pinpoint a role for PCNA in Spd1-mediated checkpoint activation and suggest that its tandemly positioned short linear motifs create a neatly balanced competition-based system, involving PCNA, Spd1 and the small ribonucleotide reductase subunit, Suc22R2. Similar mechanisms may be relevant in other PCNA binding ligands where divergent binding motifs so far have gone under the PIP-box radar.

Class Ib Ribonucleotide Reductases: Activation of a Peroxido-MnIIMnIII to Generate a Reactive Oxo-MnIIIMnIV Oxidant.

In the postulated catalytic cycle of class Ib Mn2 ribonucleotide reductases (RNRs), a MnII2 core is suggested to react with superoxide (O2·-) to generate peroxido-MnIIMnIII and oxo-MnIIIMnIV entities prior to proton-coupled electron transfer (PCET) oxidation of tyrosine. There is limited experimental support for this mechanism. We demonstrate that [MnII2(BPMP)(OAc)2](ClO4) (1, HBPMP = 2,6-bis[(bis(2 pyridylmethyl)amino)methyl]-4-methylphenol) was converted to peroxido-MnIIMnIII (2) in the presence of superoxide anion that converted to (µ-O)(µ-OH)MnIIIMnIV (3) via the addition of an H+-donor (p-TsOH) or (µ-O)2MnIIIMnIV (4) upon warming to room temperature. The physical properties of 3 and 4 were probed using UV-vis, EPR, X-ray absorption, and IR spectroscopies and mass spectrometry. Compounds 3 and 4 were capable of phenol oxidation to yield a phenoxyl radical via a concerted PCET oxidation, supporting the proposed mechanism of tyrosyl radical cofactor generation in RNRs. The synthetic models demonstrate that the postulated O2/Mn2/tyrosine activation mechanism in class Ib Mn2 RNRs is plausible and provides spectral insights into intermediates currently elusive in the native enzyme.

Cell-type dependence of necroptosis pathways triggered by viral infection.

Necroptosis, a potent host defense mechanism, limits viral replication and pathogenesis through three distinct initiation pathways. Toll-like receptor 3 (TLR3) via TIR-domain-containing adapter-inducing interferon-ß (TRIF), Z-DNA-binding protein 1 (ZBP1) and tumor necrosis factor (TNF)α mediate necroptosis, with ZBP1 and TNF playing pivotal roles in controlling viral infections, with the role of TLR3-TRIF being less clear. ZBP1-mediated necroptosis is initiated when host ZBP1 senses viral Z-form double stranded RNA and recruits receptor-interacting serine/threonine-protein kinase 3 (RIPK3), driving a mixed lineage kinase domain-like pseudokinase (MLKL)-dependent necroptosis pathway, whereas TNF-mediated necroptosis is initiated by TNF signaling, which drives a RIPK1-RIPK3-MLKL pathway, resulting in necroptosis. Certain viruses (cytomegalovirus, herpes simplex virus and vaccinia) have evolved to produce proteins that compete with host defense systems, preventing programmed cell death pathways from being initiated. Two engineered viruses deficient of active forms of these proteins, murine cytomegalovirus M45mutRHIM and vaccinia virus E3∆Zα, trigger ZBP1-dependent necroptosis in mouse embryonic fibroblasts. By contrast, when bone-marrow-derived macrophages are infected with the viruses, necroptosis is initiated predominantly through the TNF-mediated pathway. However, when the TNF pathway is blocked by RIPK1 inhibitors or a TNF blockade, ZBP1-mediated necroptosis becomes the prominent pathway in bone-marrow-derived macrophages. Overall, these data implicate a cell-type preference for either TNF-mediated or ZBP1-mediated necroptosis pathways in host responses to viral infections. These preferences are important to consider when evaluating disease models that incorporate necroptosis because they may contribute to tissue-specific reactions that could alter the balance of inflammation versus control of virus, impacting the organism as a whole.

Impact of N221S missense mutation in human ribonucleotide reductase small subunit b on mitochondrial DNA depletion syndrome.

The impact of N221S mutation in hRRM2B gene, which encodes the small subunit of human ribonucleotide reductase (RNR), on RNR activity and the pathogenesis of mitochondrial DNA depletion syndrome (MDDS) was investigated. Our results demonstrate that N221 mutations significantly reduce RNR activity, suggesting its role in the development of MDDS. We proposed an allosteric regulation pathway involving a chain of three phenylalanine residues on the αE helix of RNR small subunit ß. This pathway connects the C-terminal loop of ß2, transfers the activation signal from the large catalytic subunit α to ß active site, and controls access of oxygen for radical generation. N221 is near this pathway and likely plays a role in regulating RNR activity. Mutagenesis studies on residues involved in the phenylalanine chain and the regulation pathway were conducted to confirm our proposed mechanism. We also performed molecular dynamic simulation and protein contact network analysis to support our findings. This study sheds new light on RNR small subunit regulation and provides insight on the pathogenesis of MDDS.

Theoretical Insights into the Generation Mechanism of the Tyr122 Radical Catalyzed by Intermediate X in Class Ia Ribonucleotide Reductase.

Ribonucleotide reductase (RNR) catalyzes the reduction of ribonucleotides to deoxyribonucleotides in all organisms. There is an ∼35 Å long-range electron-hole transfer pathway during the catalytic process of class Ia RNR, which can be described as Tyr122ß â†” [Trp48ß]? ↔ Tyr356ß â†” Tyr731α ↔ Tyr730α ↔ Cys439α. The formation of the Y122• radical initiates this long-range radical transfer process. However, the generation mechanism of Y122• is not yet clear due to confusion over the intermediate X structures. Based on the two reported X structures, we examined the possible mechanisms of Y122• generation by density functional theory (DFT) calculations. Our examinations revealed that the generation of the Y122• radical from the two different core structures of X was via a similar two-step reaction, with the first step of proton transfer for the formation of the proton receptor of Y122 and the second step of a proton-coupled long-range electron transfer reaction with the proton transfer from the Y122 hydroxyl group to the terminal hydroxide ligand of Fe1III and simultaneously electron transfer from the side chain of Y122 to Fe2IV. These findings provide an insight into the formation mechanism of Y122• catalyzed by the double-iron center of the ß subunit of class Ia RNR.

Ribonucleotide reductase as a therapeutic target for drug repurposing as anthelmintics.

Visceral cestodiases, like echinococcoses and cysticercoses, are zoonoses of worldwide distribution and are responsible for public health problems in many countries, especially in underdeveloped regions. Current treatments have low efficiency and there are few drugs currently in use for chemotherapy, making the development of new anthelmintics an urgent matter. The nucleotide salvage pathways are the only ones available for nucleotide synthesis in cestodes and other parasitic helminths, and, here, we used in silico approaches to assess the potential of the enzymes in these pathways as targets for drug repurposing as anthelminthics. First, a genomic survey allowed to identify a repertoire of 28 enzymes of the purine and pyrimidine salvage pathways from the cestode Echinococcus granulosus sensu stricto. Regarding purines, the parasite relies on salvaging free bases rather than salvaging nucleosides. Pyrimidines, on the other hand, can be salvaged from both bases and nucleosides. Druggability of the parasite enzymes was assessed, as well as the availability of commercial inhibitors for them. Druggable enzymes were then ranked according to their potential for drug repurposing and the 17 most promising enzymes were selected for evolutionary analyses. The constructed phylogenetic trees allowed to assess the degree of conservation among ortholog enzymes from parasitic helminths and their mammalian hosts. Positive selection is absent in all assessed flatworm enzymes. A potential target enzyme for drug repurposing, ribonucleotide reductase (RNR), was selected for further assessment. RNR 3D-modelling showed structural similarities between the E. granulosus and the human orthologs suggesting that inhibitors of the human RNR should be effective against the E. granulosus enzyme. In line with that, E. granulosus protoscolices treated in vitro with the inhibitor hydroxyurea had their viability and DNA synthesis reduced. These results are consistent with nucleotide synthesis inhibition and confirm the potential of a nucleotide salvage inhibitors for repurposing as an anthelmintic.

3-AP inhibits the growth of human osteosarcoma by decreasing the activity of the iron-dependent pathway.

3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) has broad-spectrum antitumor activity. However, its role in osteosarcoma (OS) remains unclear. Therefore, this study explored the effects of 3-AP on OS in vitro and in vivo using three human OS cell lines (MG-63, U2-OS, and 143B) and a nude mice model generated by transplanting 143B cells. The cells and mice were treated with DMSO (control) or gradient concentrations of 3-AP. Then, various assays (e.g., cell counting kit-8, flow cytometry, immunohistochemistry, and western blotting) were performed to assess cell viability and apoptosis levels, as well as γH2A.X (DNA damage correlation), ribonucleotide reductase catalytic subunit M1 and M2 (RRM1 and RRM2, respectively) protein levels (iron-dependent correlation). 3-AP time- and dose-dependably suppressed growth and induced apoptosis in all three OS cell lines, and ferric ammonium citrate (FAC) blocked these effects. Moreover, 3-AP decreased RRM2 and total ribonucleotide reductase (RRM1 plus RRM2) protein expression but significantly increased γH2A.X expression; treatment did not affect RRM1 expression. Again, FAC treatment attenuated these effects. In vivo, the number of apoptotic cells in the tumor slices increased in the 3-AP-treated mice compared to the control mice. 3-AP treatment also decreased Ki-67 and p21 expression, suggesting inhibited OS growth. Furthermore, the expression of RRM1, RRM2, and transferrin receptor protein 1 (i.e., Tfr1) indicated that 3-AP inhibited OS growth via an iron-dependent pathway. In conclusion, 3-AP exhibits anticancer activity in OS by decreasing the activity of iron-dependent pathways, which could be a promising therapeutic strategy for OS.