Peptide inhibitors of mammalian ribonucleotide reductase

Delineating the glioblastoma stemness by genes involved in cytoskeletal rearrangements and metabolic alterations

Literature data on glioblastoma ongoingly underline the link between metabolism and cancer stemness, the latter is one responsible for potentiating the resistance to treatment, inter alia due to increased invasiveness. In recent years, glioblastoma stemness research has bashfully introduced a key aspect of cytoskeletal rearrangements, whereas the impact of the cytoskeleton on invasiveness is well known. Although non-stem glioblastoma cells are less invasive than glioblastoma stem cells (GSCs), these cells also acquire stemness with greater ease if characterized as invasive cells and not tumor core cells. This suggests that glioblastoma stemness should be further investigated for any phenomena related to the cytoskeleton and metabolism, as they may provide new invasion-related insights. Previously, we proved that interplay between metabolism and cytoskeleton existed in glioblastoma. Despite searching for cytoskeleton-related processes in which the investigated genes might have been involved, not only did we stumble across the relation to metabolism but also reported genes that were found to be implicated in stemness. Thus, dedicated research on these genes in GSCs seems justifiable and might reveal novel directions and/or biomarkers that could be utilized in the future. Herein, we review the previously identified cytoskeleton/metabolism-related genes through the prism of glioblastoma stemness.

TAS1553, a small molecule subunit interaction inhibitor of ribonucleotide reductase, exhibits antitumor activity by causing DNA replication stress

Ribonucleotide reductase (RNR) is composed of two non-identical subunits, R1 and R2, and plays a crucial role in balancing the cellular dNTP pool, establishing it as an attractive cancer target. Herein, we report the discovery of a highly potent and selective small-molecule inhibitor, TAS1553, targeting protein-protein interaction between R1 and R2. TAS1553 is also expected to demonstrate superior selectivity because it does not directly target free radical or a substrate binding site. TAS1553 has shown antiproliferative activity in human cancer cell lines, dramatically reducing the intracellular dATP pool and causing DNA replication stress. Furthermore, we identified SLFN11 as a biomarker that predicts the cytotoxic effect of TAS1553. Oral administration of TAS1553 demonstrated robust antitumor efficacy against both hematological and solid cancer xenograft tumors and also provided a significant survival benefit in an acute myelogenous leukemia model. Our findings strongly support the evaluation of TAS1553 in clinical trials.

A small-molecule protein-protein interaction inhibitor of ribonucleotide reductase subunit, TAS1553, is shown to inhibit growth of both hematological and solid cancer xenograft tumors following oral administration in mice.

PLEK2, RRM2, GCSH: A Novel WWOX-Dependent Biomarker Triad of Glioblastoma at the Crossroads of Cytoskeleton Reorganization and Metabolism Alterations

Glioblastoma is one of the deadliest human cancers. Its malignancy depends on cytoskeleton reorganization, which is related to, e.g., epithelial-to-mesenchymal transition and metastasis. The malignant phenotype of glioblastoma is also affected by the WWOX gene, which is lost in nearly a quarter of gliomas. Although the role of WWOX in the cytoskeleton rearrangement has been found in neural progenitor cells, its function as a modulator of cytoskeleton in gliomas was not investigated. Therefore, this study aimed to investigate the role of WWOX and its collaborators in cytoskeleton dynamics of glioblastoma. Methodology on RNA-seq data integrated the use of databases, bioinformatics tools, web-based platforms, and machine learning algorithm, and the obtained results were validated through microarray data. PLEK2, RRM2, and GCSH were the most relevant WWOX-dependent genes that could serve as novel biomarkers. Other genes important in the context of cytoskeleton (BMP4, CCL11, CUX2, DUSP7, FAM92B, GRIN2B, HOXA1, HOXA10, KIF20A, NF2, SPOCK1, TTR, UHRF1, and WT1), metabolism (MTHFD2), or correlation with WWOX (COL3A1, KIF20A, RNF141, and RXRG) were also discovered. For the first time, we propose that changes in WWOX expression dictate a myriad of alterations that affect both glioblastoma cytoskeleton and metabolism, rendering new therapeutic possibilities.

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.

Thioredoxin and glutaredoxin-mediated redox regulation of ribonucleotide reductase

Ribonucleotide reductase (RNR), the rate-limiting enzyme in DNA synthesis, catalyzes reduction of the different ribonucleotides to their corresponding deoxyribonucleotides. The crucial role of RNR in DNA synthesis has made it an important target for the development of antiviral and anticancer drugs. Taking account of the recent developments in this field of research, this review focuses on the role of thioredoxin and glutaredoxin systems in the redox reactions of the RNR catalysis.

Targeting ribonucleotide reductase for cancer therapy

INTRODUCTION

Ribonucleotide reductase (RR) is a unique enzyme, because it is responsible for reducing ribonucleotides to their corresponding deoxyribonucleotides, which are the building blocks required for DNA replication and repair. Dysregulated RR activity is associated with genomic instability, malignant transformation and cancer development. The use of RR inhibitors, either as a single agent or combined with other therapies, has proven to be a promising approach for treating solid tumors and hematological malignancies.

AREAS COVERED

This review covers recent publications in the area of RR, which include: i) the structure, function and regulation of RR; ii) the roles of RR in cancer development; iii) the classification, mechanisms and clinical application of RR inhibitors for cancer therapy and iv) strategies for developing novel RR inhibitors in the future.

EXPERT OPINION

Exploring the possible nonenzymatic roles of RR subunit proteins in carcinogenesis may lead to new rationales for developing novel anticancer drugs. Updated information about the structure and holoenzyme models of RR will help in identifying potential sites in the protein that could be targets for novel RR inhibitors. Determining RR activity and subunit levels in clinical samples will provide a rational platform for developing personalized cancer therapies that use RR inhibitors.

Trans-species activity of a nonself recognition domain

Background

The ability to distinguish nonself from self is a fundamental characteristic of biological systems. In the filamentous fungus Neurospora crassa, multiple incompatibility genes mediate nonself recognition during vegetative growth. One of these genes, un-24, encodes both nonself recognition function and the large subunit of a type I ribonucleotide reductase, an evolutionarily conserved enzyme that is essential for the conversion of NDP precursors into dNDPs for use in DNA synthesis. Previous studies have shown that co-expression of the two allelic forms of un-24, Oakridge (OR) and Panama (PA), in the same cell results in cell death.

Results

We identify a 135 amino acid nonself recognition domain in the C-terminus region of UN-24 that confers an incompatibility-like phenotype when expressed in the yeast, Saccharomyces cerevisiae. Low-level expression of this domain results in several cytological and phenotypic characteristics consistent with an incompatibility reaction in filamentous fungi. These incompatibility phenotypes are correlated with the presence of a non-reducible complex consisting of the PA incompatibility domain and Rnr1p, a large subunit of ribonucleotide reductase in yeast. When the PA incompatibility domain is switched to high-level expression, the incompatibility phenotype transitions to wild-type concomitant with the appearance of a complex containing the PA incompatibility domain and Ssa1p, an Hsp70 homolog.

Conclusions

Results from this study provide insights into the mechanism and control of vegetative nonself recognition mediated by ribonucleotide reductase in N. crassa, thus establishing the yeast system as a powerful tool to study fungal nonself recognition. Our work shows that heat shock proteins may function to deactivate vegetative incompatibility systems, as required for entry into the sexual cycle. Finally, our results suggest that variations on the PA incompatibility domain may serve as novel and specific antimicrobial peptides.

Nonself recognition through intermolecular disulfide bond formation of ribonucleotide reductase in neurospora

Type I ribonucleotide reductases (RNRs) are conserved across diverse taxa and are essential for the conversion of RNA into DNA precursors. In Neurospora crassa, the large subunit of RNR (UN-24) is unusual in that it also has a nonself recognition function, whereby coexpression of Oak Ridge (OR) and Panama (PA) alleles of un-24 in the same cell leads to growth inhibition and cell death. We show that coexpressing these incompatible alleles of un-24 in N. crassa results in a high molecular weight UN-24 protein complex. A 63-amino-acid portion of the C terminus was sufficient for un-24(PA) incompatibility activity. Redox active cysteines that are conserved in type I RNRs and essential for their catalytic function were found to be required for incompatibility activity of both UN-24(OR) and UN-24(PA). Our results suggest a plausible model of un-24 incompatibility activity in which the formation of a complex between the incompatible RNR proteins is potentiated by intermolecular disulfide bond formation.

The structural basis for the allosteric regulation of ribonucleotide reductase

Ribonucleotide reductases (RRs) catalyze a crucial step of de novo DNA synthesis by converting ribonucleoside diphosphates to deoxyribonucleoside diphosphates. Tight control of the dNTP pool is essential for cellular homeostasis. The activity of the enzyme is tightly regulated at the S-phase by allosteric regulation. Recent structural studies by our group and others provided the molecular basis for understanding how RR recognizes substrates, how it interacts with chemotherapeutic agents, and how it is regulated by its allosteric regulators ATP and dATP. This review discusses the molecular basis of allosteric regulation and substrate recognition of RR, and particularly the discovery that subunit oligomerization is an important prerequisite step in enzyme inhibition.

Targeting the Large Subunit of Human Ribonucleotide Reductase for Cancer Chemotherapy

Ribonucleotide reductase (RR) is a crucial enzyme in de novo DNA synthesis, where it catalyses the rate determining step of dNTP synthesis. RRs consist of a large subunit called RR1 (α), that contains two allosteric sites and one catalytic site, and a small subunit called RR2 (β), which houses a tyrosyl free radical essential for initiating catalysis. The active form of mammalian RR is an α_nβ_m hetero oligomer. RR inhibitors are cytotoxic to proliferating cancer cells. In this brief review we will discuss the three classes of RR, the catalytic mechanism of RR, the regulation of the dNTP pool, the substrate selection, the allosteric activation, inactivation by ATP and dATP, and the nucleoside drugs that target RR. We will also discuss possible strategies for developing a new class of drugs that disrupts the RR assembly.

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.