DNA replication inhibitor hydroxyurea alters Fe-S centers by producing reactive oxygen species in vivo

Hydroxyurea in the management of sickle cell disease-associated priapism: a scoping review

Sickle cell disease (SCD) is an inherited hemolytic-hemoglobinopathy that affects the ability of erythrocytes to deliver oxygen in the body. Hydroxyurea is a pharmacologic agent that has been approved by the Food and Drug Administration (FDA) for the treatment of SCD crises since 1998. It has also been proposed as a therapy for recurrent ischemic priapism (RIP), recurrent prolonged or unwanted erections in the absence of sexual stimulation, which occurs commonly in SCD. The premise behind hydroxyurea's therapeutic effect is that it improves vascular function and hemoglobin metabolism. Its potential role in RIP is to normalize the nitric oxide (NO) signaling pathway, which is fundamental for the regulatory physiology of the erection response. Hydroxyurea has been investigated in diverse research studies, including a recent clinical trial, showing its potential benefit for treating RIP. In this article, we performed a scoping review of the effectiveness of hydroxyurea for the treatment of SCD-associated RIP.

Control of Replication Stress Response by Cytosolic Fe-S Cluster Assembly (CIA) Machinery

Accurate DNA replication is essential for the maintenance of genome stability and the generation of healthy offspring. When DNA replication is challenged, signals accumulate at blocked replication forks that elicit a multifaceted cellular response, orchestrating DNA replication, DNA repair and cell cycle progression. This replication stress response promotes the recovery of DNA replication, maintaining chromosome integrity and preventing mutations. Defects in this response are linked to heightened genetic instability, which contributes to tumorigenesis and genetic disorders. Iron-sulfur (Fe-S) clusters are emerging as important cofactors in supporting the response to replication stress. These clusters are assembled and delivered to target proteins that function in the cytosol and nucleus via the conserved cytosolic Fe-S cluster assembly (CIA) machinery and the CIA targeting complex. This review summarizes recent advances in understanding the structure and function of the CIA machinery in yeast and mammals, emphasizing the critical role of Fe-S clusters in the replication stress response.

Genome-wide conditional degron libraries for functional genomics

Functional genomics with libraries of knockout alleles is limited to non-essential genes and convoluted by the potential accumulation of suppressor mutations in knockout backgrounds, which can lead to erroneous functional annotations. To address these limitations, we constructed genome-wide libraries of conditional alleles based on the auxin-inducible degron (AID) system for inducible degradation of AID-tagged proteins in the budding yeast Saccharomyces cerevisiae. First, we determined that N-terminal tagging is at least twice as likely to inadvertently impair protein function across the proteome. We thus constructed two libraries with over 5,600 essential and non-essential proteins fused at the C-terminus with an AID tag and an optional fluorescent protein. Approximately 90% of AID-tagged proteins were degraded in the presence of the auxin analog 5-Ph-IAA, with initial protein abundance and tag accessibility as limiting factors. Genome-wide screens for DNA damage response factors revealed a role for the glucose signaling factor GSF2 in resistance to hydroxyurea, highlighting how the AID libraries extend the yeast genetics toolbox.

Characterizing the regulatory effects of H2A.Z and SWR1-C on gene expression during hydroxyurea exposure in Saccharomyces cerevisiae

Chromatin structure and DNA accessibility are partly modulated by the incorporation of histone variants. H2A.Z, encoded by the non-essential HTZ1 gene in S. cerevisiae, is an evolutionarily conserved H2A histone variant that is predominantly incorporated at transcription start sites by the SWR1-complex (SWR1-C). While H2A.Z has often been implicated in transcription regulation, htz1Δ mutants exhibit minimal changes in gene expression compared to wild-type. However, given that growth defects of htz1Δ mutants are alleviated by simultaneous deletion of SWR1-C subunits, previous work examining the role of H2A.Z in gene expression regulation may be confounded by deleterious activity caused by SWR1-C when missing its H2A.Z substrate (apo-SWR1-C). Furthermore, as H2A.Z mutants only display significant growth defects in genotoxic stress conditions, a more substantive role for H2A.Z in gene expression may only be uncovered after exposure to cellular stress. To explore this possibility, we generated mRNA transcript profiles for wild-type, htz1Δ, swr1Δ, and htz1Δswr1Δ mutants before and after exposure to hydroxyurea (HU), which induces DNA replication stress. Our data showed that H2A.Z played a more prominent role in gene activation than repression during HU exposure, and its incorporation was important for proper upregulation of several HU-induced genes. We also observed that apo-SWR1-C contributed to gene expression defects in the htz1Δ mutant, particularly for genes involved in phosphate homeostasis regulation. Furthermore, mapping H2A.Z incorporation before and after treatment with HU revealed that decreases in H2A.Z enrichment at transcription start sites was correlated with, but generally not required for, the upregulation of genes during HU exposure. Together this study characterized the regulatory effects of H2A.Z incorporation during the transcriptional response to HU.

Revised mechanism of hydroxyurea-induced cell cycle arrest and an improved alternative

Replication stress describes endogenous and exogenous challenges to DNA replication in the S-phase. Stress during this critical process causes helicase-polymerase decoupling at replication forks, triggering the S-phase checkpoint, which orchestrates global replication fork stalling and delayed entry into G2. The replication stressor most often used to induce the checkpoint response in yeast is hydroxyurea (HU), a clinically used chemotherapeutic. The primary mechanism of S-phase checkpoint activation by HU has thus far been considered to be a reduction of deoxynucleotide triphosphate synthesis by inhibition of ribonucleotide reductase (RNR), leading to helicase-polymerase decoupling and subsequent activation of the checkpoint, facilitated by the replisome-associated mediator Mrc1. In contrast, we observe that HU causes cell cycle arrest in budding yeast independent of both the Mrc1-mediated replication checkpoint response and the Psk1-Mrc1 oxidative signaling pathway. We demonstrate a direct relationship between HU incubation and reactive oxygen species (ROS) production in yeast and human cells and show that antioxidants restore growth of yeast in HU. We further observe that ROS strongly inhibits the in vitro polymerase activity of replicative polymerases (Pols), Pol α, Pol δ, and Pol ε, causing polymerase complex dissociation and subsequent loss of DNA substrate binding, likely through oxidation of their integral iron-sulfur (Fe-S) clusters. Finally, we present "RNR-deg," a genetically engineered alternative to HU in yeast with greatly increased specificity of RNR inhibition, allowing researchers to achieve fast, nontoxic, and more readily reversible checkpoint activation compared to HU, avoiding harmful ROS generation and associated downstream cellular effects that may confound interpretation of results.

Embryonic stem cells maintain high origin activity and slow forks to coordinate replication with cell cycle progression

Embryonic stem (ES) cells are pluripotent stem cells that can produce all cell types of an organism. ES cells proliferate rapidly and are thought to experience high levels of intrinsic replication stress. Here, by investigating replication fork dynamics in substages of S phase, we show that mammalian pluripotent stem cells maintain a slow fork speed and high active origin density throughout the S phase, with little sign of fork pausing. In contrast, the fork speed of non-pluripotent cells is slow at the beginning of S phase, accompanied by increased fork pausing, but thereafter fork pausing rates decline and fork speed rates accelerate in an ATR-dependent manner. Thus, replication fork dynamics within the S phase are distinct between ES and non-ES cells. Nucleoside addition can accelerate fork speed and reduce origin density. However, this causes miscoordination between the completion of DNA replication and cell cycle progression, leading to genome instability. Our study indicates that fork slowing in the pluripotent stem cells is an integral aspect of DNA replication.

Ulipristal-temozolomide-hydroxyurea combination for glioblastoma: in-vitro studies

BACKGROUND

Glioblastoma multiforme (GBM) is a brain malignancy with worst survival. Low dose progesterone stimulates GBM growth, while progesterone receptor (PR)-antagonist mifepristone was shown to reduce growth and to enhance temozolomide sensitivity in GBM cells. Mifepristone is not available in all countries due to ethical reasons and may cause adrenal insufficiency and pelvic infections. Ulipristal is also a PR-antagonist used in treatment of uterine leiomyomas with higher biosafety. Ulipristal is demonstrated to suppress growth of breast cancer, yet it is not tested as yet whether it can also block growth and sensitize to temozolomide in glioblastoma as it was previously shown with mifepristone. Our first aim was to detect whether ulipristal exerts antiproliferative and chemotherapy-sensitizing effects in glioblastoma. Hydroxyurea inhibits DNA replication via blocking ribonucleotide reductase (RR) and it was demonstrated to increase temozolomide antineoplasticity in GBM. Progesterone receptor-activation in the uterus enhances RR transcription. Hence, we have hypothesized that PR-inactivation with ulipristal would further enhance hydroxyurea antineoplasticity by shutting down DNA synthesis mechanisms through further suppression of RR. Lastly, there exists no study as yet whether ulipristal, hydroxyurea and temozolomide could exert ternary antineoplastic efficacy, which was our last aim to define.

METHODS

To reveal interactions between ulipristal, hydroxyurea and temozolomide, we treated human U251 GBM cell line with these agents alone and in combination and measured cell proliferation, total antioxidant capacity (TAC) and total oxidant status (TOS) in conditioned medium and cellular cytokine gene expressions.

RESULTS

All agents significantly reduced cell proliferation significantly, yet the most significant decrease of GBM cells occurred with the triple drug combination at the 96th hour. All agents significantly decreased TAC and increased TOS in culture media, which was mostly relevant for the triple combination at the 96th hour. All these three agents tend to reduce the expression of immunosuppressive and/or GBM-growth stimulating cytokines TGF-β, IL-10 and IL-17 while increasing the expression of GBM-growth suppressing cytokine IL-23.

CONCLUSIONS

Reproposal of these agents in treatment of GBM would be a plausible approach if future studies prove their efficacy.

Characteristic effect of hydroxyurea on the higher-order structure of DNA and gene expression

Hydroxyurea (HU; hydroxycarbamide) is a chemotherapy medication used to treat various types of cancer and other diseases such as sickle cell anemia. HU inhibits DNA synthesis by targeting ribonucleotide reductase (RNR). Recent studies have suggested that HU also causes oxidative stress in living systems. In the present study, we investigated if HU could directly affect the activity and/or conformation of DNA. We measured in vitro gene expression in the presence of HU by adapting a cell-free luciferase assay. HU exhibited a bimodal effect on gene expression, where promotion or inhibition were observed at lower or higher concentrations (mM range), respectively. Using atomic force microscopy (AFM), the higher-order structure of DNA was revealed to be partially-thick with kinked-branching structures after HU was added. An elongated coil conformation was observed by AFM in the absence of HU. Single DNA molecules in bulk aqueous solution under fluctuating Brownian motion were imaged by fluorescence microscopy (FM). Both spring and damping constants, mechanical properties of DNA, increased when HU was added. These experimental investigations indicate that HU directly interacts with DNA and provide new insights into how HU acts as a chemotherapeutic agent and targets other diseases.

Effect of Hydroxyurea on Morphology, Proliferation, and Protein Expression on Taenia crassiceps WFU Strain

Flatworms are known for their remarkable regenerative ability, one which depends on totipotent cells known as germinative cells in cestodes. Depletion of germinative cells with hydroxyurea (HU) affects the regeneration of the parasite. Here, we studied the reduction and recovery of germinative cells in T. crassiceps cysticerci after HU treatment (25 mM and 40 mM of HU for 6 days) through in vitro assays. Viability and morphological changes were evaluated. The recovery of cysticerci's mobility and morphology was evaluated at 3 and 6 days, after 6 days of treatment. The number of proliferative cells was evaluated using EdU. Our results show morphological changes in the size, shape, and number of evaginated cysticerci at the 40 mM dose. The mobility of cysticerci was lower after 6 days of HU treatment at both concentrations. On days 3 and 6 of recovery after 25 mM of HU treatment, a partial recovery of the proliferative cells was observed. Proteomic and Gene Ontology analyses identified modifications in protein groups related to DNA binding, DNA damage, glycolytic enzymes, cytoskeleton, skeletal muscle, and RNA binding.

Induction of Oxidative Stress in Sirtuin Gene-Disrupted Ashbya gossypii Mutants Overproducing Riboflavin

AgHST1 and AgHST3 genes encode sirtuins that are NAD+-dependent protein deacetylases. According to previous reports, their disruption leads to the overproduction of riboflavin in Ashbya gossypii. In this study, we investigated the potential causes of riboflavin overproduction in the AgHST1Δ and AgHST3Δ mutant strains of A. gossypii. The generation of reactive oxygen species was increasd in the mutants compared to in WT. Additionally, membrane potential was lower in the mutants than in WT. The NAD+/NADH ratio in AgHST1Δ mutant strain was lower than that in WT; however, the NAD+/NADH ratio in AgHST3Δ was slightly higher than that in WT. AgHST1Δ mutant strain was more sensitive to high temperatures and hydroxyurea treatment than WT or AgHST3Δ. Expression of the AgGLR1 gene, encoding glutathione reductase, was substantially decreased in AgHST1Δ and AgHST3Δ mutant strains. The addition of N-acetyl-L-cysteine, an antioxidant, suppressed the riboflavin production in the mutants, indicating that it was induced by oxidative stress. Therefore, high oxidative stress resulting from the disruption of sirtuin genes induces riboflavin overproduction in AgHST1Δ and AgHST3Δ mutant strains. This study established that oxidative stress is an important trigger for riboflavin overproduction in sirtuin gene-disrupted mutant strains of A. gossypii and helped to elucidate the mechanism of riboflavin production in A. gossypii.

Oxidative replication stress induced by long-term exposure to hydroxyurea in root meristem cells of Vicia faba

KEY MESSAGE

Low concentrations of hydroxyurea, an inhibitor of DNA replication, induced oxidative and replicative stress in root apical meristem (RAM) cells of Vicia faba. Plant cells are constantly exposed to low-level endogenous stress factors that can affect DNA replication and lead to DNA damage. Long-term treatments of Vicia faba root apical meristems (RAMs) with HU leads to the appearance of atypical cells with intranuclear asynchrony. This rare form of abnormality was manifested by a gradual condensation of chromatin, from interphase to mitosis (so-called IM cells). Moreover, HU-treated root cells revealed abnormal chromosome structure, persisting DNA replication, and elevated levels of intracellular hydrogen peroxide (H2O2) and superoxide anion (O2∙-). Immunocytochemical studies have shown an increased number of fluorescent foci of H3 histones acetylated at lysine 56 (H3K56Ac; canonically connected with the DNA replication process). We show that continuous 3-day exposure to low concentrations (0.75 mM) of hydroxyurea (HU; an inhibitor of DNA replication) induces cellular response to reactive oxygen species and to DNA replication stress conditions.

Revised Mechanism of Hydroxyurea Induced Cell Cycle Arrest and an Improved Alternative

Replication stress describes various types of endogenous and exogenous challenges to DNA replication in S-phase. Stress during this critical process results in helicase-polymerase decoupling at replication forks, triggering the S-phase checkpoint, which orchestrates global replication fork stalling and delayed entry into G2. The replication stressor most often used to induce the checkpoint response is hydroxyurea (HU), a chemotherapeutic agent. The primary mechanism of S-phase checkpoint activation by HU has thus far been considered to be a reduction of dNTP synthesis by inhibition of ribonucleotide reductase (RNR), leading to helicase-polymerase decoupling and subsequent activation of the checkpoint, mediated by the replisome associated effector kinase Mrc1. In contrast, we observe that HU causes cell cycle arrest in budding yeast independent of both the Mrc1-mediated replication checkpoint response and the Psk1-Mrc1 oxidative signaling pathway. We demonstrate a direct relationship between HU incubation and reactive oxygen species (ROS) production in yeast nuclei. We further observe that ROS strongly inhibits the in vitro polymerase activity of replicative polymerases (Pols), Pol α, Pol δ, and Pol ε, causing polymerase complex dissociation and subsequent loss of DNA substrate binding, likely through oxidation of their integral iron sulfur Fe-S clusters. Finally, we present "RNR-deg," a genetically engineered alternative to HU in yeast with greatly increased specificity of RNR inhibition, allowing researchers to achieve fast, nontoxic, and more readily reversible checkpoint activation compared to HU, avoiding harmful ROS generation and associated downstream cellular effects that may confound interpretation of results.

Excessive reactive oxygen species induce transcription-dependent replication stress

Elevated levels of reactive oxygen species (ROS) reduce replication fork velocity by causing dissociation of the TIMELESS-TIPIN complex from the replisome. Here, we show that ROS generated by exposure of human cells to the ribonucleotide reductase inhibitor hydroxyurea (HU) promote replication fork reversal in a manner dependent on active transcription and formation of co-transcriptional RNA:DNA hybrids (R-loops). The frequency of R-loop-dependent fork stalling events is also increased after TIMELESS depletion or a partial inhibition of replicative DNA polymerases by aphidicolin, suggesting that this phenomenon is due to a global replication slowdown. In contrast, replication arrest caused by HU-induced depletion of deoxynucleotides does not induce fork reversal but, if allowed to persist, leads to extensive R-loop-independent DNA breakage during S-phase. Our work reveals a link between oxidative stress and transcription-replication interference that causes genomic alterations recurrently found in human cancer.

The identification of viral ribonucleotide reductase encoded by ORF23 and ORF141 genes and effect on CyHV-2 replication

Introduction

Ribonucleotide reductase (RR) is essential for the replication of the double-stranded DNA virus CyHV-2 due to its ability to catalyze the conversion of ribonucleotides to deoxyribonucleotides, and is a potential target for the development of antiviral drugs to control CyHV-2 infection.

Methods

Bioinformatic analysis was conducted to identify potential homologues of RR in CyHV-2. The transcription and translation levels of ORF23 and ORF141, which showed high homology to RR, were measured during CyHV-2 replication in GICF. Co-localization experiments and immunoprecipitation were performed to investigate the interaction between ORF23 and ORF141. siRNA interference experiments were conducted to evaluate the effect of silencing ORF23 and ORF141 on CyHV-2 replication. The inhibitory effect of hydroxyurea, a nucleotide reductase inhibitor, on CyHV-2 replication in GICF cells and RR enzymatic activity in vitro was also evaluated.

Results

ORF23 and ORF141 were identified as potential viral ribonucleotide reductase homologues in CyHV-2, and their transcription and translation levels increased with CyHV-2 replication. Co-localization experiments and immunoprecipitation suggested an interaction between the two proteins. Simultaneous silencing of ORF23 and ORF141 effectively inhibited the replication of CyHV-2. Additionally, hydroxyurea inhibited the replication of CyHV-2 in GICF cells and the in vitro enzymatic activity of RR.

Conclusion

These results suggest that the CyHV-2 proteins ORF23 and ORF141 function as viral ribonucleotide reductase and their function makes an effect to CyHV-2 replication. Targeting ribonucleotide reductase could be a crucial strategy for developing new antiviral drugs against CyHV-2 and other herpesviruses.

Metabolic remodeling maintains a reducing environment for rapid activation of the yeast DNA replication checkpoint

Nucleotide metabolism fuels normal DNA replication and is also primarily targeted by the DNA replication checkpoint when replication stalls. To reveal a comprehensive interconnection between genome maintenance and metabolism, we analyzed the metabolomic changes upon replication stress in the budding yeast S. cerevisiae. We found that upon treatment of cells with hydroxyurea, glucose is rapidly diverted to the oxidative pentose phosphate pathway (PPP). This effect is mediated by the AMP-dependent kinase, SNF1, which phosphorylates the transcription factor Mig1, thereby relieving repression of the gene encoding the rate-limiting enzyme of the PPP. Surprisingly, NADPH produced by the PPP is required for efficient recruitment of replication protein A (RPA) to single-stranded DNA, providing the signal for the activation of the Mec1/ATR-Rad53/CHK1 checkpoint signaling kinase cascade. Thus, SNF1, best known as a central energy controller, determines a fast mode of replication checkpoint activation through a redox mechanism. These findings establish that SNF1 provides a hub with direct links to cellular metabolism, redox, and surveillance of DNA replication in eukaryotes.

Homologous Recombination as a Fundamental Genome Surveillance Mechanism during DNA Replication

Accurate and complete genome replication is a fundamental cellular process for the proper transfer of genetic material to cell progenies, normal cell growth, and genome stability. However, a plethora of extrinsic and intrinsic factors challenge individual DNA replication forks and cause replication stress (RS), a hallmark of cancer. When challenged by RS, cells deploy an extensive range of mechanisms to safeguard replicating genomes and limit the burden of DNA damage. Prominent among those is homologous recombination (HR). Although fundamental to cell division, evidence suggests that cancer cells exploit and manipulate these RS responses to fuel their evolution and gain resistance to therapeutic interventions. In this review, we focused on recent insights into HR-mediated protection of stress-induced DNA replication intermediates, particularly the repair and protection of daughter strand gaps (DSGs) that arise from discontinuous replication across a damaged DNA template. Besides mechanistic underpinnings of this process, which markedly differ depending on the extent and duration of RS, we highlight the pathophysiological scenarios where DSG repair is naturally silenced. Finally, we discuss how such pathophysiological events fuel rampant mutagenesis, promoting cancer evolution, but also manifest in adaptative responses that can be targeted for cancer therapy.

Biochemical Evaluation of the Effects of Hydroxyurea in Vitro on Red Blood Cells

Hydroxyurea (HU) is a low-cost, low-toxicity drug that is often used in diseases, such as sickle cell anemia and different types of cancer. Its effects on the red blood cells (RBC) are still not fully understood. The in vitro effects of HU were evaluated on the biochemical parameters of the RBC from healthy individuals that were treated with 0.6 mM or 0.8 mM HU for 30 min and 1 h. After 30 min, there was a significant increase in almost all of the parameters analyzed in the two concentrations of HU, except for the pyruvate kinase (PK) activity. A treatment with 0.8 mM HU for 1 h resulted in a reduction of the levels of lipid peroxidation, Fe³⁺, and in the activities of some of the enzymes, such as glutathione reductase (GR), glucose-6-phosphate dehydrogenase (G6PD), and PK. After the incubation for 1 h, the levels of H₂O₂, lipid peroxidation, reduced glutathione (GSH), enzymatic activity (hexokinase, G6PD, and superoxide dismutase (SOD) were reduced with the treatment of 0.8 mM HU when compared with 0.6 mM. The results have suggested that a treatment with HU at a concentration of 0.8 mM seemed to be more efficient in protecting against the free radicals, as well as in treating diseases, such as sickle cell anemia. HU appears to preferentially stimulate the pentose pathway over the glycolytic pathway. Although this study was carried out with the RBC from healthy individuals, the changes described in this study may help to elucidate the mechanisms of action of HU when administered for therapeutic purposes.

E. coli Ribonucleotide Reductase β2 Subunit Inactivation by Triapine Occurs through Binding of a Triapine-Fe(II) Adduct

Ribonucleotide reductase (RNR), which supplies the building blocks for DNA biosynthesis and its repair, has been linked to human diseases and is emerging as a therapeutic target. Here, we present a mechanistic investigation of triapine (3AP), a clinically relevant small molecule that inhibits the tyrosyl radical within the RNR β₂ subunit. Solvent kinetic isotope effects reveal that proton transfer is not rate-limiting for inhibition of Y122· of E. coli RNR β₂ by the pertinent 3AP-Fe(II) adduct. Vibrational spectroscopy further demonstrates that unlike inhibition of the β₂ tyrosyl radical by hydroxyurea, a carboxylate containing proton wire is not at play. Binding measurements reveal a low nanomolar affinity (K_d ∼ 6 nM) of 3AP-Fe(II) for β₂. Taken together, these data should prompt further development of RNR inactivators based on the triapine scaffold for therapeutic applications.

Homology-directed repair protects the replicating genome from metabolic assaults

Homology-directed repair (HDR) safeguards DNA integrity under various forms of stress, but how HDR protects replicating genomes under extensive metabolic alterations remains unclear. Here, we report that besides stalling replication forks, inhibition of ribonucleotide reductase (RNR) triggers metabolic imbalance manifested by the accumulation of increased reactive oxygen species (ROS) in cell nuclei. This leads to a redox-sensitive activation of the ATM kinase followed by phosphorylation of the MRE11 nuclease, which in HDR-deficient settings degrades stalled replication forks. Intriguingly, nascent DNA degradation by the ROS-ATM-MRE11 cascade is also triggered by hypoxia, which elevates signaling-competent ROS and attenuates functional HDR without arresting replication forks. Under these conditions, MRE11 degrades daughter-strand DNA gaps, which accumulate behind active replisomes and attract error-prone DNA polymerases to escalate mutation rates. Thus, HDR safeguards replicating genomes against metabolic assaults by restraining mutagenic repair at aberrantly processed nascent DNA. These findings have implications for cancer evolution and tumor therapy.

Hydroxyurea-The Good, the Bad and the Ugly

Hydroxyurea (HU) is mostly referred to as an inhibitor of ribonucleotide reductase (RNR) and as the agent that is commonly used to arrest cells in the S-phase of the cycle by inducing replication stress. It is a well-known and widely used drug, one which has proved to be effective in treating chronic myeloproliferative disorders and which is considered a staple agent in sickle anemia therapy and—recently—a promising factor in preventing cognitive decline in Alzheimer’s disease. The reversibility of HU-induced replication inhibition also makes it a common laboratory ingredient used to synchronize cell cycles. On the other hand, prolonged treatment or higher dosage of hydroxyurea causes cell death due to accumulation of DNA damage and oxidative stress. Hydroxyurea treatments are also still far from perfect and it has been suggested that it facilitates skin cancer progression. Also, recent studies have shown that hydroxyurea may affect a larger number of enzymes due to its less specific interaction mechanism, which may contribute to further as-yet unspecified factors affecting cell response. In this review, we examine the actual state of knowledge about hydroxyurea and the mechanisms behind its cytotoxic effects. The practical applications of the recent findings may prove to enhance the already existing use of the drug in new and promising ways.

Hydroxyurea Induces a Stress Response That Alters DNA Replication and Nucleotide Metabolism in Bacillus subtilis

Hydroxyurea (HU) is classified as a ribonucleotide reductase (RNR) inhibitor and has been widely used to stall DNA replication by depleting deoxyribonucleoside triphosphate (dNTP) pools. Recent evidence in Escherichia coli shows that HU readily forms breakdown products that damage DNA directly, indicating that toxicity is a result of secondary effects. Because HU is so widely used in the laboratory and as a clinical therapeutic, it is important to understand its biological effects. To determine how Bacillus subtilis responds to HU-induced stress, we performed saturating transposon insertion mutagenesis followed by deep sequencing (Tn-seq), transcriptome sequencing (RNA-seq) analysis, and measurement of replication fork progression. Our data show that B. subtilis cells elongate, and replication fork progression is slowed, following HU challenge. The transcriptomic data show that B. subtilis cells initially mount a metabolic response likely caused by dNTP pool depletion before inducing the DNA damage response (SOS) after prolonged exposure. To compensate for reduced nucleotide pools, B. subtilis upregulates the purine and pyrimidine biosynthetic machinery and downregulates the enzymes producing ribose 5-phosphate. We show that overexpression of the RNR genes nrdEF suppresses the growth interference caused by HU, suggesting that RNR is an important target of HU in B. subtilis. Although genes involved in nucleotide and carbon metabolism showed considerable differential expression, we also find that genes of unknown function (y-genes) represent the largest class of differentially expressed genes. Deletion of individual y-genes caused moderate growth interference in the presence of HU, suggesting that cells have several ways of coping with HU-induced metabolic stress. IMPORTANCE Hydroxyurea (HU) has been widely used as a clinical therapeutic and an inhibitor of DNA replication. Some evidence suggests that HU inhibits ribonucleotide reductase, depleting dNTP pools, while other evidence shows that toxic HU breakdown products are responsible for growth inhibition and genotoxic stress. Here, we use multiple, complementary approaches to characterize the response of Bacillus subtilis to HU. B. subtilis responds by upregulating the expression of purine and pyrimidine biosynthesis. We show that HU challenge reduced DNA replication and that overexpression of the ribonucleotide reductase operon suppressed growth interference by HU. Our results demonstrate that HU targets RNR and several other metabolic enzymes contributing to toxicity in bacteria.

The stabilized Pol31-Pol3 interface counteracts Pol32 ablation with differential effects on repair

DNA polymerase δ, which contains the catalytic subunit, Pol3, Pol31, and Pol32, contributes both to DNA replication and repair. The deletion of pol31 is lethal, and compromising the Pol3-Pol31 interaction domains confers hypersensitivity to cold, hydroxyurea (HU), and methyl methanesulfonate, phenocopying pol32Δ. We have identified alanine-substitutions in pol31 that suppress these deficiencies in pol32Δ cells. We characterize two mutants, pol31-T415A and pol31-W417A, which map to a solvent-exposed loop that mediates Pol31-Pol3 and Pol31-Rev3 interactions. The pol31-T415A substitution compromises binding to the Pol3 CysB domain, whereas Pol31-W417A improves it. Importantly, loss of Pol32, such as pol31-T415A, leads to reduced Pol3 and Pol31 protein levels, which are restored by pol31-W417A. The mutations have differential effects on recovery from acute HU, break-induced replication and trans-lesion synthesis repair pathways. Unlike trans-lesion synthesis and growth on HU, the loss of break-induced replication in pol32Δ cells is not restored by pol31-W417A, highlighting pathway-specific roles for Pol32 in fork-related repair. Intriguingly, CHIP analyses of replication forks on HU showed that pol32Δ and pol31-T415A indirectly destabilize DNA pol α and pol ε at stalled forks.

Molecular targets for antifungals in amino acid and protein biosynthetic pathways

Fungi cause death of over 1.5 million people every year, while cutaneous mycoses are among the most common infections in the world. Mycoses vary greatly in severity, there are long-term skin (ringworm), nail or hair infections (tinea capitis), recurrent like vaginal candidiasis or severe, life-threatening systemic, multiorgan infections. In the last few years, increasing importance is attached to the health and economic problems caused by fungal pathogens. There is a growing need for improvement of the availability of antifungal drugs, decreasing their prices and reducing side effects. Searching for novel approaches in this respect, amino acid and protein biosynthesis pathways appear to be competitive. The route that leads from amino acid biosynthesis to protein folding and its activation is rich in enzymes that are descriptive of fungi. Blocking the action of those enzymes often leads to avirulence or growth inhibition. In this review, we want to trace the principal processes of fungi vitality. We present the data of genes encoding enzymes involved in amino acid and protein biosynthesis, potential molecular targets in antifungal chemotherapy, and describe the impact of inhibitors on fungal organisms.

Fe-S coordination defects in the replicative DNA polymerase delta cause deleterious DNA replication in vivo and subsequent DNA damage in the yeast Saccharomyces cerevisiae

B-type eukaryotic polymerases contain a [4Fe-4S] cluster in their C-terminus domain, whose role is not fully understood yet. Among them, DNA polymerase delta (Polδ) plays an essential role in chromosomal DNA replication, mostly during lagging strand synthesis. Previous in vitro work suggested that the Fe-S cluster in Polδ is required for efficient binding of the Pol31 subunit, ensuring stability of the Polδ complex. Here we analyzed the in vivo consequences resulting from an impaired coordination of the Fe-S cluster in Polδ. We show that a single substitution of the very last cysteine coordinating the cluster by a serine is responsible for the generation of massive DNA damage during S phase, leading to checkpoint activation, requirement of homologous recombination for repair, and ultimately to cell death when the repair capacities of the cells are overwhelmed. These data indicate that impaired Fe-S cluster coordination in Polδ is responsible for aberrant replication. More generally, Fe-S in Polδ may be compromised by various stress including anti-cancer drugs. Possible in vivo Polδ Fe-S cluster oxidation and collapse may thus occur, and we speculate this could contribute to induced genomic instability and cell death, comparable to that observed in pol3-13 cells.

Hydroxyurea and Caffeine Impact pRb-like Protein-Dependent Chromatin Architecture Profiles in Interphase Cells of Vicia faba

The survival of cells depends on their ability to replicate correctly genetic material. Cells exposed to replication stress can experience a number of problems that may lead to deregulated proliferation, the development of cancer, and/or programmed cell death. In this article, we have induced prolonged replication arrest via hydroxyurea (HU) treatment and also premature chromosome condensation (PCC) by co-treatment with HU and caffeine (CF) in the root meristem cells of Vicia faba. We have analyzed the changes in the activities of retinoblastoma-like protein (RbS807/811ph). Results obtained from the immunocytochemical detection of RbS807/811ph allowed us to distinguish five unique activity profiles of pRb. We have also performed detailed 3D modeling using Blender 2.9.1., based on the original data and some final conclusions. 3D models helped us to visualize better the events occurring within the nuclei and acted as a high-resolution aid for presenting the results. We have found that, despite the decrease in pRb activity, its activity profiles were mostly intact and clearly recognizable, with some local alterations that may correspond to the increased demand in transcriptional activity. Our findings suggest that Vicia faba’s ability to withstand harsh environments may come from its well-developed and highly effective response to replication stress.

Kinetics of DNA Repair in Vicia faba Meristem Regeneration Following Replication Stress

The astonishing survival abilities of Vicia faba, one the earliest domesticated plants, are associated, among other things, to the highly effective replication stress response system which ensures smooth cell division and proper preservation of genomic information. The most crucial pathway here seems to be the ataxia telangiectasia-mutated kinase (ATM)/ataxia telangiectasia and Rad3-related kinase (ATR)-dependent replication stress response mechanism, also present in humans. In this article, we attempted to take an in-depth look at the dynamics of regeneration from the effects of replication inhibition and cell cycle checkpoint overriding causing premature chromosome condensation (PCC) in terms of DNA damage repair and changes in replication dynamics. We were able to distinguish a unique behavior of replication factors at the very start of the regeneration process in the PCC-induced cells. We extended the experiment and decided to profile the changes in replication on the level of a single replication cluster of heterochromatin (both alone and with regard to its position in the nucleus), including the mathematical profiling of the size, activity and shape. The results obtained during these experiments led us to the conclusion that even “chaotic” events are dealt with in a proper degree of order.

Deletion of the GAPDH gene contributes to genome stability in Saccharomyces cerevisiae

Cellular metabolism is directly or indirectly associated with various cellular processes by producing a variety of metabolites. Metabolic alterations may cause adverse effects on cell viability. However, some alterations potentiate the rescue of the malfunction of the cell system. Here, we found that the alteration of glucose metabolism suppressed genome instability caused by the impairment of chromatin structure. Deletion of the TDH2 gene, which encodes glyceraldehyde 3-phospho dehydrogenase and is essential for glycolysis/gluconeogenesis, partially suppressed DNA damage sensitivity due to chromatin structure, which was persistently acetylated histone H3 on lysine 56 in cells with deletions of both HST3 and HST4, encoding NAD⁺-dependent deacetylases. tdh2 deletion also restored the short replicative lifespan of cells with deletion of sir2, another NAD⁺-dependent deacetylase, by suppressing intrachromosomal recombination in rDNA repeats increased by the unacetylated histone H4 on lysine 16. tdh2 deletion also suppressed recombination between direct repeats in hst3∆ hst4∆ cells by suppressing the replication fork instability that leads to both DNA deletions among repeats. We focused on quinolinic acid (QUIN), a metabolic intermediate in the de novo nicotinamide adenine dinucleotide (NAD⁺) synthesis pathway, which accumulated in the tdh2 deletion cells and was a candidate metabolite to suppress DNA replication fork instability. Deletion of QPT1, quinolinate phosphoribosyl transferase, elevated intracellular QUIN levels and partially suppressed the DNA damage sensitivity of hst3∆ hst4∆ cells as well as tdh2∆ cells. qpt1 deletion restored the short replicative lifespan of sir2∆ cells by suppressing intrachromosomal recombination among rDNA repeats. In addition, qpt1 deletion could suppress replication fork slippage between direct repeats. These findings suggest a connection between glucose metabolism and genomic stability.

Ribonucleotide Reductases: Structure, Chemistry, and Metabolism Suggest New Therapeutic Targets

Ribonucleotide reductases (RNRs) catalyze the de novo conversion of nucleotides to deoxynucleotides in all organisms, controlling their relative ratios and abundance. In doing so, they play an important role in fidelity of DNA replication and repair. RNRs' central role in nucleic acid metabolism has resulted in five therapeutics that inhibit human RNRs. In this review, we discuss the structural, dynamic, and mechanistic aspects of RNR activity and regulation, primarily for the human and Escherichia coli class Ia enzymes. The unusual radical-based organic chemistry of nucleotide reduction, the inorganic chemistry of the essential metallo-cofactor biosynthesis/maintenance, the transport of a radical over a long distance, and the dynamics of subunit interactions all present distinct entry points toward RNR inhibition that are relevant for drug discovery. We describe the current mechanistic understanding of small molecules that target different elements of RNR function, including downstream pathways that lead to cell cytotoxicity. We conclude by summarizing novel and emergent RNR targeting motifs for cancer and antibiotic therapeutics.

The Aspartic Protease Ddi1 Contributes to DNA-Protein Crosslink Repair in Yeast

Naturally occurring or drug-induced DNA-protein crosslinks (DPCs) interfere with key DNA transactions if not repaired in a timely manner. The unique family of DPC-specific proteases Wss1/SPRTN targets DPC protein moieties for degradation, including stabilized topoisomerase-1 cleavage complexes (Top1ccs). Here, we describe that the efficient DPC disassembly requires Ddi1, another conserved predicted protease in Saccharomyces cerevisiae. We found Ddi1 in a genetic screen of the tdp1 wss1 mutant defective in Top1cc processing. Ddi1 is recruited to a persistent Top1cc-like DPC lesion in an S phase-dependent manner to assist in the eviction of crosslinked protein from DNA. Loss of Ddi1 or its putative protease activity hypersensitizes cells to DPC trapping agents independently from Wss1 and 26S proteasome, implying its broader role in DPC repair. Among the potential Ddi1 targets, we found the core component of Pol II and show that its genotoxin-induced degradation is impaired in ddi1. We propose that the Ddi1 protease contributes to DPC proteolysis.

Yeast AP-1 like transcription factors (Yap) and stress response: a current overview

Yeast adaptation to stress has been extensively studied. It involves large reprogramming of genome expression operated by many, more or less specific, transcription factors. Here, we review our current knowledge on the function of the eight Yap transcription factors (Yap1 to Yap8) in Saccharomyces cerevisiae, which were shown to be involved in various stress responses. More precisely, Yap1 is activated under oxidative stress, Yap2/Cad1 under cadmium, Yap4/Cin5 and Yap6 under osmotic shock, Yap5 under iron overload and Yap8/Arr1 by arsenic compounds. Yap3 and Yap7 seem to be involved in hydroquinone and nitrosative stresses, respectively. The data presented in this article illustrate how much knowledge on the function of these Yap transcription factors is advanced. The evolution of the Yap family and its roles in various pathogenic and non-pathogenic fungal species is discussed in the last section.

Translesion polymerase kappa-dependent DNA synthesis underlies replication fork recovery

DNA replication stress is often defined by the slowing or stalling of replication fork progression leading to local or global DNA synthesis inhibition. Failure to resolve replication stress in a timely manner contribute toward cell cycle defects, genome instability and human disease; however, the mechanism for fork recovery remains poorly defined. Here, we show that the translesion DNA polymerase (Pol) kappa, a DinB orthologue, has a unique role in both protecting and restarting stalled replication forks under conditions of nucleotide deprivation. Importantly, Pol kappa-mediated DNA synthesis during hydroxyurea (HU)-dependent fork restart is regulated by both the Fanconi Anemia (FA) pathway and PCNA polyubiquitination. Loss of Pol kappa prevents timely rescue of stalled replication forks, leading to replication-associated genomic instability, and a p53-dependent cell cycle defect. Taken together, our results identify a previously unanticipated role for Pol kappa in promoting DNA synthesis and replication stress recovery at sites of stalled forks.

Transcriptional regulation of FeS biogenesis genes: A possible shield against arsenate toxicity activated by Yap1

In the eukaryotic model yeast Saccharomyces cerevisiae, arsenic (As) detoxification is regulated by two transcriptional factors, Yap8 and Yap1. Yap8 specifically controls As extrusion from the cell, whether Yap1 avoids arsenic-induced oxidative damages. Accordingly, cells lacking both Yap1 and Yap8 are more sensitive to arsenate than cells lacking each regulator individually. Strikingly enough, the same sensitivity pattern was observed under anoxia, suggesting that Yap1 role in As detoxification might not be restricted to the regulation of the oxidative stress response. This finding prompted us to study the transcriptomic profile of wild-type and yap1 mutant cells exposed to arsenate. Interestingly, we found that, under such conditions, several genes involved in the biogenesis of FeS proteins were upregulated in a Yap1-dependent way. In line with this observation, arsenate treatment decreases the activity of the mitochondrial aconitase, Aco1, an FeS cluster-containing enzyme, this effect being even more pronounced in the yap1 mutant. Reinforcing the relevance of FeS cluster biogenesis in arsenate detoxification, the overexpression of several ISC and CIA machinery genes alleviates the deleterious effect of arsenate caused by the absence of Yap1 and Yap8. Altogether our data suggest that the upregulation of FeS biogenesis genes regulated by Yap1 might work as a cellular shield against arsenate toxicity.

Replication Rapidly Recovers and Continues in the Presence of Hydroxyurea in Escherichia coli

In both prokaryotes and eukaryotes, hydroxyurea is suggested to inhibit DNA replication by inactivating ribonucleotide reductase and depleting deoxyribonucleoside triphosphate pools. In this study, we show that the inhibition of replication in Escherichia coli is transient even at concentrations of 0.1 M hydroxyurea and that replication rapidly recovers and continues in its presence. The recovery of replication does not require the alternative ribonucleotide reductases NrdEF and NrdDG or the translesion DNA polymerases II (Pol II), Pol IV, and Pol V. Ribonucleotides are incorporated at higher frequencies during replication in the presence of hydroxyurea. However, they do not contribute significantly to the observed synthesis or toxicity. Hydroxyurea toxicity was observed only under conditions where the stability of hydroxyurea was compromised and by-products known to damage DNA directly were allowed to accumulate. The results demonstrate that hydroxyurea is not a direct or specific inhibitor of DNA synthesis in vivo and that the transient inhibition observed is most likely due to a general depletion of iron cofactors from enzymes when 0.1 M hydroxyurea is initially applied. Finally, the results support previous studies suggesting that hydroxyurea toxicity is mediated primarily through direct DNA damage induced by the breakdown products of hydroxyurea, rather than by inhibition of replication or depletion of deoxyribonucleotide levels in the cell.IMPORTANCE Hydroxyurea is commonly suggested to function by inhibiting DNA replication through the inactivation of ribonucleotide reductase and depleting deoxyribonucleoside triphosphate pools. Here, we show that hydroxyurea only transiently inhibits replication in Escherichia coli before replication rapidly recovers and continues in the presence of the drug. The recovery of replication does not depend on alternative ribonucleotide reductases, translesion synthesis, or RecA. Further, we show that hydroxyurea toxicity is observed only in the presence of toxic intermediates that accumulate when hydroxyurea breaks down, damage DNA, and induce lethality. The results demonstrate that hydroxyurea toxicity is mediated indirectly by the formation of DNA damage, rather than by inhibition of replication or depletion of deoxyribonucleotide levels in the cell.

Fe-S Clusters Emerging as Targets of Therapeutic Drugs

Fe-S centers exhibit strong electronic plasticity, which is of importance for insuring fine redox tuning of protein biological properties. In accordance, Fe-S clusters are also highly sensitive to oxidation and can be very easily altered in vivo by different drugs, either directly or indirectly due to catabolic by-products, such as nitric oxide species (NOS) or reactive oxygen species (ROS). In case of metal ions, Fe-S cluster alteration might be the result of metal liganding to the coordinating sulfur atoms, as suggested for copper. Several drugs presented through this review are either capable of direct interaction with Fe-S clusters or of secondary Fe-S clusters alteration following ROS or NOS production. Reactions leading to Fe-S cluster disruption are also reported. Due to the recent interest and progress in Fe-S biology, it is very likely that an increasing number of drugs already used in clinics will emerge as molecules interfering with Fe-S centers in the near future. Targeting Fe-S centers could also become a promising strategy for drug development.

Common Chemical Inductors of Replication Stress: Focus on Cell-Based Studies

DNA replication is a highly demanding process regarding the energy and material supply and must be precisely regulated, involving multiple cellular feedbacks. The slowing down or stalling of DNA synthesis and/or replication forks is referred to as replication stress (RS). Owing to the complexity and requirements of replication, a plethora of factors may interfere and challenge the genome stability, cell survival or affect the whole organism. This review outlines chemical compounds that are known inducers of RS and commonly used in laboratory research. These compounds act on replication by direct interaction with DNA causing DNA crosslinks and bulky lesions (cisplatin), chemical interference with the metabolism of deoxyribonucleotide triphosphates (hydroxyurea), direct inhibition of the activity of replicative DNA polymerases (aphidicolin) and interference with enzymes dealing with topological DNA stress (camptothecin, etoposide). As a variety of mechanisms can induce RS, the responses of mammalian cells also vary. Here, we review the activity and mechanism of action of these compounds based on recent knowledge, accompanied by examples of induced phenotypes, cellular readouts and commonly used doses.

The Cell Killing Mechanisms of Hydroxyurea

Hydroxyurea is a well-established inhibitor of ribonucleotide reductase that has a long history of scientific interest and clinical use for the treatment of neoplastic and non-neoplastic diseases. It is currently the staple drug for the management of sickle cell anemia and chronic myeloproliferative disorders. Due to its reversible inhibitory effect on DNA replication in various organisms, hydroxyurea is also commonly used in laboratories for cell cycle synchronization or generating replication stress. However, incubation with high concentrations or prolonged treatment with low doses of hydroxyurea can result in cell death and the DNA damage generated at arrested replication forks is generally believed to be the direct cause. Recent studies in multiple model organisms have shown that oxidative stress and several other mechanisms may contribute to the majority of the cytotoxic effect of hydroxyurea. This review aims to summarize the progress in our understanding of the cell-killing mechanisms of hydroxyurea, which may provide new insights towards the improvement of chemotherapies that employ this agent.