Mutation of RRM2B, encoding p53-controlled ribonucleotide reductase (p53R2), causes severe mitochondrial DNA depletion

Mitochondrial DNA (mtDNA) depletion syndrome (MDS; MIM 251880) is a prevalent cause of oxidative phosphorylation disorders characterized by a reduction in mtDNA copy number. The hitherto recognized disease mechanisms alter either mtDNA replication (POLG (ref. 1)) or the salvage pathway of mitochondrial deoxyribonucleosides 5'-triphosphates (dNTPs) for mtDNA synthesis (DGUOK (ref. 2), TK2 (ref. 3) and SUCLA2 (ref. 4)). A last gene, MPV17 (ref. 5), has no known function. Yet the majority of cases remain unexplained. Studying seven cases of profound mtDNA depletion (1-2% residual mtDNA in muscle) in four unrelated families, we have found nonsense, missense and splice-site mutations and in-frame deletions of the RRM2B gene, encoding the cytosolic p53-inducible ribonucleotide reductase small subunit. Accordingly, severe mtDNA depletion was found in various tissues of the Rrm2b-/- mouse. The mtDNA depletion triggered by p53R2 alterations in both human and mouse implies that p53R2 has a crucial role in dNTP supply for mtDNA synthesis.

Herpes simplex virus type 1-induced ribonucleotide reductase activity is dispensable for virus growth and DNA synthesis: isolation and characterization of an ICP6 lacZ insertion mutant

Herpes simplex virus (HSV) encodes a ribonucleotide reductase consisting of two subunits (140 and 38 kilodaltons) whose genes map to coordinates 0.56 to 0.60 on the viral genome. Host cell lines containing the HpaI F fragment which includes the reductase subunit genes of HSV type 1 strain KOS (coordinates 0.535 to 0.620) were generated. Transfection of these cells with a plasmid containing the immediate-early ICP0 gene resulted in the expression of ICP6; interestingly, ICP4 plasmids failed to induce expression, indicating an unusual pattern of ICP6 regulation. One such cell line (D14) was used to isolate a mutant with the structural gene of lacZ inserted into the ICP6 gene such that the lacZ gene is read in frame with the N-terminal region of ICP6. This mutant generated a protein containing 434 amino acids (38%) of the N terminus of ICP6 fused to beta-galactosidase under control of the endogenous ICP6 promoter. Screening for virus recombinants was greatly facilitated by staining virus plaques with 5-bromo-4-chloro-3-indoyl-beta-D-galactoside (X-gal). Enzyme assays of infected BHK cells indicated that the mutant is incapable of inducing viral ribonucleotide reductase activity. Surprisingly, although plaque size was greatly reduced, mutant virus yield was reduced only four- to fivefold compared with that of the wild type grown in exponentially growing Vero cells. Mutant virus plaque size, yields, and ability to synthesize viral DNA were more severely compromised in serum-starved cells as compared with the wild type grown under the same condition. Although our evidence suggests that the HSV type 1 ribonucleotide reductase is not required for virus growth and DNA replication in dividing cells, it may be required for growth in nondividing cells.

Interferons regulate the phenotype of wild-type and mutant herpes simplex viruses in vivo

Mechanisms responsible for neuroattenuation of herpes simplex virus (HSV) have been defined previously by studies of mutant viruses in cultured cells. The hypothesis that null mutations in host genes can override the attenuated phenotype of null mutations in certain viral genes was tested. Mutants such as those in infected cell protein (ICP) 0, thymidine kinase, ribonucleotide reductase, virion host shutoff, and ICP34.5 are reduced in their capacity to replicate in nondividing cells in culture and in vivo. The replication of these viruses was examined in eyes and trigeminal ganglia for 1-7 d after corneal inoculation in mice with null mutations (-/-) in interferon receptors (IFNR) for type I IFNs (IFN-alpha/betaR), type II IFN (IFN-gammaR), and both type I and type II IFNs (IFN-alpha/beta/gammaR). Viral titers in eyes and ganglia of IFN-gammaR-/- mice were not significantly different from congenic controls. However, in IFN-alpha/betaR-/- or IFN-alpha/beta/gammaR-/- mice, growth of all mutants, including those with significantly impaired growth in cell culture, was enhanced by up to 1,000-fold in eyes and trigeminal ganglia. Blepharitis and clinical signs of infection were evident in IFN-alpha/betaR-/- and IFN-alpha/beta/gammaR-/- but not control mice for all viruses. Also, IFNs were shown to significantly reduce productive infection of, and spread from intact, but not scarified, corneas. Particularly striking was restoration of near-normal trigeminal ganglion replication and neurovirulence of an ICP34.5 mutant in IFN-alpha/betaR-/- mice. These data show that IFNs play a major role in limiting mutant and wild-type HSV replication in the cornea and in the nervous system. In addition, the in vivo target of ICP34.5 may be host IFN responses. These experiments demonstrate an unsuspected role for host factors in defining the phenotypes of some HSV mutants in vivo. The phenotypes of mutant viruses therefore cannot be interpreted based solely upon studies in cell culture but must be considered carefully in the context of host factors that may define the in vivo phenotype.

Structure, function, and mechanism of ribonucleotide reductases

Ribonucleotide reductase (RNR) is the enzyme responsible for the conversion of ribonucleotides to 2'-deoxyribonucleotides and thereby provides the precursors needed for both synthesis and repair of DNA. In the recent years, many new crystal structures have been obtained of the protein subunits of all three classes of RNR. This review will focus upon recent structural and spectroscopic studies, which have offered deeper insight to the mechanistic properties as well as evolutionary relationship and diversity among the different classes of RNR. Although the three different classes of RNR enzymes depend on different metal cofactors for the catalytic activity, all three classes have a conserved cysteine residue at the active site located on the tip of a protein loop in the centre of an alpha/beta-barrel structural motif. This cysteine residue is believed to be converted into a thiyl radical that initiates the substrate turnover in all three classes of RNR. The functional and structural similarities suggest that the present-day RNRs have all evolved from a common ancestral reductase. Nevertheless, the different cofactors found in the three classes of RNR make the RNR proteins into interesting model systems for quite diverse protein families, such as diiron-oxygen proteins, cobalamin-dependent proteins, and SAM-dependent iron-sulfur proteins. There are also significant variations within each of the three classes of RNR. With new structures available of the R2 protein of class I RNR, we have made a comparison of the diiron centres in R2 from mouse and Escherichia coli. The R2 protein shows dynamic carboxylate, radical, and water shifts in different redox forms, and new radical forms are different from non-radical forms. In mouse R2, the binding of iron(II) or cobalt(II) to the four metal sites shows high cooperativity. A unique situation is found in RNR from baker's yeast, which is made up of heterodimers, in contrast to homodimers, which is the normal case for class I RNR. Since the reduction of ribonucleotides is the rate-limiting step of DNA synthesis, RNR is an important target for cell growth control, and the recent finding of a p53-induced isoform of the R2 protein in mammalian cells has increased the interest for the role of RNR during the different phases of the cell cycle.

A ribonucleotide reductase homolog of cytomegalovirus and endothelial cell tropism

Human cytomegalovirus infects vascular tissues and has been associated with atherogenesis and coronary restenosis. Although established laboratory strains of human cytomegalovirus have lost the ability to grow on vascular endothelial cells, laboratory strains of murine cytomegalovirus retain this ability. With the use of a forward-genetic procedure involving random transposon mutagenesis and rapid phenotypic screening, we identified a murine cytomegalovirus gene governing endothelial cell tropism. This gene, M45, shares sequence homology to ribonucleotide reductase genes. Endothelial cells infected with M45-mutant viruses rapidly undergo apoptosis, suggesting that a viral strategy to evade destruction by cellular apoptosis is indispensable for viral growth in endothelial cells.

Rice virescent3 and stripe1 encoding the large and small subunits of ribonucleotide reductase are required for chloroplast biogenesis during early leaf development

The virescent3 (v3) and stripe1 (st1) mutants in rice (Oryza sativa) produce chlorotic leaves in a growth stage-dependent manner under field conditions. They are temperature-conditional mutants that produce bleached leaves at a constant 20 degrees C or 30 degrees C but almost green leaves under diurnal 30 degrees C/20 degrees C conditions. Here, we show V3 and St1, which encode the large and small subunits of ribonucleotide reductase (RNR), RNRL1, and RNRS1, respectively. RNR regulates the rate of deoxyribonucleotide production for DNA synthesis and repair. RNRL1 and RNRS1 are highly expressed in the shoot base and in young leaves, and the expression of the genes that function in plastid transcription/translation and in photosynthesis is altered in v3 and st1 mutants, indicating that a threshold activity of RNR is required for chloroplast biogenesis in developing leaves. There are additional RNR homologs in rice, RNRL2 and RNRS2, and eukaryotic RNRs comprise alpha(2)beta(2) heterodimers. In yeast, RNRL1 interacts with RNRS1 (RNRL1:RNRS1) and RNRL2:RNRS2, but no interaction occurs between other combinations of the large and small subunits. The interacting activities are RNRL1:RNRS1 > RNRL1:rnrs1(st1) > rnrl1(v3):RNRS1 > rnrl1(v3):rnrs1(st1), which correlate with the degree of chlorosis for each genotype. This suggests that missense mutations in rnrl1(v3) and rnrs1(st1) attenuate the first alphabeta dimerization. Moreover, wild-type plants exposed to a low concentration of an RNR inhibitor, hydroxyurea, produce chlorotic leaves without growth retardation, reminiscent of v3 and st1 mutants. We thus propose that upon insufficient activity of RNR, plastid DNA synthesis is preferentially arrested to allow nuclear genome replication in developing leaves, leading to continuous plant growth.

Ribonucleotide reductase encoded by herpes simplex virus is a determinant of the pathogenicity of the virus in mice and a valid antiviral target

The role of the herpes simplex virus (HSV)-encoded ribonucleotide reductase (RR) in the pathogenicity of the virus has been examined by use of mutants with lesions in either the large or small subunit of the enzyme. The virulence of the mutants in mice was reduced by about 10(6)-fold when compared with that of the parental virus (HSV type 1 strain 17), while the virulence of a revertant of one of the mutants was restored to within about 100-fold of that of the parent virus. These experiments demonstrate that activity of the HSV RR is essential for virus pathogenicity in mice and suggests that the enzyme is a valid target for specific antiviral compounds.

The herpes simplex virus type 2 R1 protein kinase (ICP10 PK) blocks apoptosis in hippocampal neurons, involving activation of the MEK/MAPK survival pathway

Herpes simplex virus type 1 (HSV-1) and HSV-2 trigger or counteract apoptosis by a cell-specific mechanism. Our studies are based on previous findings that the protein kinase (PK) domain of the large subunit of HSV-2 ribonucleotide reductase (ICP10) activates the Ras/MEK/MAPK pathway (Smith et al., J. Virol. 74:10417, 2000). Because survival pathways can modulate apoptosis, we used cells that are stably or transiently transfected with ICP10 PK, an HSV-2 mutant deleted in ICP10 PK (ICP10DeltaPK) and the MEK-specific inhibitor U0126 to examine the role of ICP10 PK in apoptosis. Apoptosis was induced by staurosporine or D-mannitol in human (HEK293) cells or HEK293 cells stably transfected with the ICP10 PK-negative mutant p139 (JHL15), as determined by morphology, DNA fragmentation, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL), caspase-3 activation, and poly(ADP-ribose) polymerase (PARP) cleavage. HEK293 cells stably transfected with ICP10 (JHLa1) were protected from apoptosis. ICP10 but not p139 protected neuronally differentiated PC12 cells from death due to nerve growth factor withdrawal, and apoptosis (determined by TUNEL) and caspase-3 activation were seen in primary hippocampal cultures infected with ICP10DeltaPK but not with HSV-2 or a revertant virus [HSV-2(R)]. The data indicate that ICP10 has antiapoptotic activity under both paradigms and that it requires a functional PK activity. The apoptotic cells in primary hippocampal cultures were neurons, as determined by double immunofluorescence with fluorescein-labeled dUTP (TUNEL) and phycoerythrin-labeled antibodies specific for neuronal proteins (TuJ1 and NF-160). Protection from apoptosis was associated with MEK/MAPK activation, as evidenced by (i) increased levels of activated (phosphorylated) MAPK in HSV-2- but not ICP10DeltaPK-infected cultures and (ii) inhibition of MAPK activation by the MEK-specific inhibitor U0126. MEK and MAPK were activated by infection with UV-inactivated but not antibody-neutralized HSV-2, suggesting that activation requires cellular penetration but is independent of de novo viral protein synthesis.

The use of thiols by ribonucleotide reductase

Ribonucleotide reductase (RNR) catalyzes the rate-limiting de novo synthesis of 2'-deoxyribonucleotides from the corresponding ribonucleotides and thereby provides balanced deoxyribonucleotide pools required for error-free DNA replication and repair. The essential role of RNR in DNA synthesis and the use of DNA as genetic material has made it an important target for the development of anticancer and antiviral agents. The most well known feature of the universal RNR reaction in all kingdoms of life is the involvement of protein free radicals. Redox-active cysteines, thiyl radicals, and thiol redox proteins of the thioredoxin superfamily play major roles in the catalytic mechanism. The involvement of cysteine residues in catalysis is common to all three classes of RNR. Taking account of the recent progress in this field of research, this review focuses on the use of thiols in the redox mechanism of RNR enzymes.

p53R2-dependent ribonucleotide reduction provides deoxyribonucleotides in quiescent human fibroblasts in the absence of induced DNA damage

Human fibroblasts in culture obtain deoxynucleotides by de novo ribonucleotide reduction or by salvage of deoxynucleosides. In cycling cells the de novo pathway dominates, but in quiescent cells the salvage pathway becomes important. Two forms of active mammalian ribonucleotide reductases are known. Each form contains the catalytic R1 protein, but the two differ with respect to the second protein (R2 or p53R2). R2 is cell cycle-regulated, degraded during mitosis, and absent from quiescent cells. The recently discovered p53-inducible p53R2 was proposed to be linked to DNA repair processes. The protein is not cell cycle-regulated and can provide deoxynucleotides to quiescent mouse fibroblasts. Here we investigate the in situ activities of the R1-p53R2 complex and two other enzymes of the de novo pathway, dCMP deaminase and thymidylate synthase, in confluent quiescent serum-starved human fibroblasts in experiments with [5-(3)H]cytidine, [6-(3)H]deoxycytidine, and [C(3)H(3)]thymidine. These cells had increased their content of p53R2 2-fold and lacked R2. From isotope incorporation, we conclude that they have a complete de novo pathway for deoxynucleotide synthesis, including thymidylate synthesis. During quiescence, incorporation of deoxynucleotides into DNA was very low. Deoxynucleotides were instead degraded to deoxynucleosides and exported into the medium as deoxycytidine, deoxyuridine, and thymidine. The rate of export was surprisingly high, 25% of that in cycling cells. Total ribonucleotide reduction in quiescent cells amounted to only 2-3% of cycling cells. We suggest that in quiescent cells an important function of p53R2 is to provide deoxynucleotides for mitochondrial DNA replication.

The R1 component of mammalian ribonucleotide reductase has malignancy-suppressing activity as demonstrated by gene transfer experiments

Our recent studies have shown that deregulated expression of R2, the rate-limiting component of ribonucleotide reductase, enhances transformation and malignant potential by cooperating with activated oncogenes. We now demonstrate that the R1 component of ribonucleotide reductase has tumor-suppressing activity. Stable expression of a biologically active ectopic R1 in ras-transformed mouse fibroblast 10T(1/2) cell lines, with or without R2 overexpression, led to significantly reduced colony-forming efficiency in soft agar. The decreased anchorage independence was accompanied by markedly suppressed malignant potential in vivo. In three ras-transformed cell lines, R1 overexpression resulted in abrogation or marked suppression of tumorigenicity. In addition, the ability to form lung metastases by cells overexpressing R1 was reduced by >85%. Metastasis suppressing activity also was observed in the highly malignant mouse 10T(1/2) derived RMP-6 cell line, which was transformed by a combination of oncogenic ras, myc, and mutant p53. Furthermore, in support of the above observations with the R1 overexpressing cells, NIH 3T3 cells cotransfected with an R1 antisense sequence and oncogenic ras showed significantly increased anchorage independence as compared with control ras-transfected cells. Finally, characteristics of reduced malignant potential also were demonstrated with R1 overexpressing human colon carcinoma cells. Taken together, these results indicate that the two components of ribonucleotide reductase both are unique malignancy determinants playing opposing roles in its regulation, that there is a novel control point important in mechanisms of malignancy, which involves a balance in the levels of R1 and R2 expression, and that alterations in this balance can significantly modify transformation, tumorigenicity, and metastatic potential.

The herpes simplex virus type 2 R1 protein kinase (ICP10 PK) functions as a dominant regulator of apoptosis in hippocampal neurons involving activation of the ERK survival pathway and upregulation of the antiapoptotic protein Bag-1

Herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) can trigger or block apoptosis in a cell type-dependent manner. We have recently shown that the protein kinase activity of the large subunit of the HSV-2 ribonucleotide reductase (R1) protein (ICP10 PK) blocks apoptosis in cultured hippocampal neurons by activating the extracellular signal-regulated kinase (ERK) survival pathway (Perkins et al., J. Virol. 76:1435-1449, 2002). The present studies were designed to better elucidate the mechanism of ICP10 PK-induced neuroprotection and determine whether HSV-1 has similar activity. The data indicate that apoptosis inhibition by ICP10 PK involves a c-Raf-1-dependent mechanism and induction of the antiapoptotic protein Bag-1 by the activated ERK survival pathway. Also associated with neuroprotection by ICP10 PK are increased activation/stability of the transcription factor CREB and stabilization of the antiapoptotic protein Bcl-2. HSV-1 and the ICP10 PK-deleted HSV-2 mutant ICP10DeltaPK activate JNK, c-Jun, and ATF-2, induce the proapoptotic protein BAD, and trigger apoptosis in hippocampal neurons. c-Jun activation and apoptosis are inhibited in hippocampal cultures infected with HSV-1 in the presence of the JNK inhibitor SP600125, suggesting that JNK/c-Jun activation is required for HSV-1-induced apoptosis. Ectopically delivered ICP10 PK (but not its PK-negative mutant p139) inhibits apoptosis triggered by HSV-1 or ICP10DeltaPK. Collectively, the data indicate that ICP10 PK-induced activation of the ERK survival pathway results in Bag-1 upregulation and overrides the proapoptotic JNK/c-Jun signal induced by other viral proteins.

Vaccinia virus-encoded ribonucleotide reductase subunits are differentially required for replication and pathogenesis

Ribonucleotide reductases (RRs) are evolutionarily-conserved enzymes that catalyze the rate-limiting step during dNTP synthesis in mammals. RR consists of both large (R1) and small (R2) subunits, which are both required for catalysis by the R1₂R2₂ heterotetrameric complex. Poxviruses also encode RR proteins, but while the Orthopoxviruses infecting humans [e.g. vaccinia (VACV), variola, cowpox, and monkeypox viruses] encode both R1 and R2 subunits, the vast majority of Chordopoxviruses encode only R2 subunits. Using plaque morphology, growth curve, and mouse model studies, we investigated the requirement of VACV R1 (I4) and R2 (F4) subunits for replication and pathogenesis using a panel of mutant viruses in which one or more viral RR genes had been inactivated. Surprisingly, VACV F4, but not I4, was required for efficient replication in culture and virulence in mice. The growth defects of VACV strains lacking F4 could be complemented by genes encoding other Chordopoxvirus R2 subunits, suggesting conservation of function between poxvirus R2 proteins. Expression of F4 proteins encoding a point mutation predicted to inactivate RR activity but still allow for interaction with R1 subunits, caused a dominant negative phenotype in growth experiments in the presence or absence of I4. Co-immunoprecipitation studies showed that F4 (as well as other Chordopoxvirus R2 subunits) form hybrid complexes with cellular R1 subunits. Mutant F4 proteins that are unable to interact with host R1 subunits failed to rescue the replication defect of strains lacking F4, suggesting that F4-host R1 complex formation is critical for VACV replication. Our results suggest that poxvirus R2 subunits form functional complexes with host R1 subunits to provide sufficient dNTPs for viral replication. Our results also suggest that R2-deficient poxviruses may be selective oncolytic agents and our bioinformatic analyses provide insights into how poxvirus nucleotide metabolism proteins may have influenced the base composition of these pathogens.

Efficient genome replication is central to the virulence of all DNA viruses, including poxviruses. To ensure replication efficiency, many of the more virulent poxviruses encode their own nucleotide metabolism machinery, including ribonucleotide reductase (RR) enzymes, which act to provide ample DNA precursors for replication. RR enzymes require both large (R1) and small (R2) subunit proteins for activity. Curiously, some poxviruses only encode R2 subunits. Other poxviruses, such as the smallpox vaccine strain, vaccinia virus (VACV), encode both R1 and R2 subunits. We report here that the R2, but not the R1, subunit of VACV RR is required for efficient replication and virulence. We also provide evidence that several poxvirus R2 proteins form novel complexes with host R1 subunits and this interaction is required for efficient VACV replication in primate cells. Our study explains why some poxviruses only encode R2 subunits and identifies a role for these proteins in poxvirus pathogenesis. Furthermore, we provide evidence that mutant poxviruses unable to generate R2 proteins may become entirely dependent upon host RR activity. This may restrict their replication to cells that over-express RR proteins such as cancer cells, making them potential therapeutics for human malignancies.

The pathogenicity of ribonucleotide reductase-null mutants of herpes simplex virus type 1 in mice

The pathogenicity of ribonucleotide reductase (RR)-null mutants (hrR3 and ICP6 delta) of herpes simplex virus (HSV) type 1 was studied after intracerebral and corneal inoculation in newborn and adult ICR mice. ICP6 delta failed to replicate in brains of mice greater than or equal to 8 days old but exhibited significant virulence for newborn mice as a result of viral replication in the brains. The RR- and a thymidine kinase (TK)-deficient mutant of HSV-1 strain KOS could grow in eye tissues of adult ICR mice. Viral DNA of hrR3 was detected in brain tissues of intracerebrally infected mice or in the trigeminal ganglia of corneally infected mice greater than or equal to 50 days after infection, and infectious hrR3 could be recovered from these tissues by superinfection of the mice with wild-type HSV-2. These observations indicate that pathogenicity of RR- mutants in mice is highly dependent on the physiologic state of tissues infected and that RR- mutants have the ability to establish latency in nervous system tissues of mice by either the peripheral or intracerebral route. It was also demonstrated that the inability of the RR- mutants to invade the central nervous system was efficiently complemented by simultaneous infection with another defective virus, the TK- mutant of KOS.

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

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

A mutant type 2 herpes simplex virus deleted for the protein kinase domain of the ICP10 gene is a potent oncolytic virus

Replication-selective oncolytic herpes simplex virus (HSV) has shown considerable promise as an antitumor agent. Although the current oncolytic HSVs were exclusively constructed from HSV-1, HSV-2 has several unique features that could be exploited to convert the virus to an oncolytic agent. The N-terminus of the HSV-2 ICP10 gene product contains a well-defined serine/threonine protein kinase (PK) domain, which can activate the Ras/MEK/MAPK mitogenic pathway and thus facilitate efficient HSV-2 replication. Because the Ras signaling pathway is a key regulator of normal cell growth and malignant transformation, it is aberrantly activated in many human tumors. Here we report that a mutant HSV-2 (FusOn-H2), constructed by replacing the PK domain of ICP10 with the gene encoding the green fluorescent protein, can selectively replicate in and thus lyse tumor cells. Moreover, infection of FusOn-H2 led to syncytia formation in tumor cells, providing an additional tumor-destroying mechanism. A single moderate-dose injection of FusOn-H2 into established breast cancer xenografts completely eradicated the tumors in more than 80% of the animals, leading to their long-term survival. We conclude that this HSV-2 mutant is a safe and potent oncolytic agent useful for the treatment of malignant solid tumors such as breast cancer.

Bacteriophage T4 anaerobic ribonucleotide reductase contains a stable glycyl radical at position 580

It has been recently recognized that the class III anaerobic ribonucleotide reductase requires the presence of a second activating gene product, NrdG. We have proposed that the role for NrdG involves the generation of an oxygen sensitive glycyl free radical within the NrdD enzyme. In this article we present the generation of such a glycyl free radical within the T4 NrdD subunit and its dependence upon the phage NrdG subunit. Initially, an overexpression system was created that allowed the joint production of T4 NrdD and T4 NrdG. With this system and in the presence of T4 NrdG, an oxygen-sensitive cleavage of NrdD was observed that mimicked the cleavage observed in phage infected Escherichia coli extracts. Under anaerobic conditions the presence of T4 NrdD with NrdG revealed a strong doublet EPR signal (g = 2.0039). Isotope labeling of the NrdD with [2H]glycine and [13C]glycine, respectively, confirmed the presence of a stabilized glycine radical. The unpaired electron is strongly coupled to C-2 in glycine and the doublet splitting originates from one of the alpha-protons. The glycine residue at position 580 was determined to be the radical containing residue through site-directed mutagenesis studies involving a G580A NrdD mutant. The glycyl radical generation was specific for the T4 NrdG, and the host E. coli NrdG was found to be unable to activate the phage reductase. Finally, anaerobic purification revealed the holoenzyme complex to contain iron, whereas the NrdD polypeptide was found to lack the metal. Our results suggest a tetrameric structure for the T4 anaerobic ribonucleotide reductase containing one homodimer each of NrdD and NrdG, with a single glycyl radical present.

Mechanism of inactivation of human ribonucleotide reductase with p53R2 by gemcitabine 5'-diphosphate

Ribonucleotide reductases (RNRs) catalyze the conversion of nucleoside 5'-diphosphates to the corresponding deoxynucleotides supplying the dNTPs required for DNA replication and DNA repair. Class I RNRs require two subunits, alpha and beta, for activity. Humans possess two beta subunits: one involved in S phase DNA replication (beta) and a second in mitochondrial DNA replication (beta' or p53R2) and potentially DNA repair. Gemcitabine (F(2)C) is used clinically as an anticancer agent, and its phosphorylated metabolites target many enzymes involved in nucleotide metabolism, including RNR. The present investigation with alpha (specific activity of 400 nmol min(-1) mg(-1)) and beta' (0.6 Y./beta'2 and a specific activity of 420 nmol min(-1) mg(-1)) establishes that F(2)CDP is a substoichiometric inactivator of RNR. Incubation of this alpha/beta' with [1'-(3)H]-F(2)CDP or [5-(3)H]-F(2)CDP and reisolation of the protein by Sephadex G-50 chromatography resulted in recovery 0.5 equiv of covalently bound sugar and 0.03 equiv of tightly associated cytosine to alpha2. SDS-PAGE analysis (loaded without boiling) of the inactivated RNR showed that 60% of alpha migrates as a 90 kDa protein and 40% as a 120 kDa protein. Incubation of [1'-(3)H]-F(2)CDP with active site mutants C444S/A, C218S/A, and E431Q/D-alpha and the C-terminal tail C787S/A and C790S/A mutants reveals that no sugar label is bound to the active site mutants of alpha and that, in the case of C218S-alpha, alpha migrates as a 90 kDa protein. Analysis of the inactivated wt-alpha/beta' RNR by size exclusion chromatography indicates a quaternary structure of alpha6beta'6. A mechanism of inactivation common with halpha/beta is presented.

Expression of an altered ribonucleotide reductase activity associated with the replication of murine cytomegalovirus in quiescent fibroblasts

Ribonucleotide reductase (RNR) is an essential enzyme for the de novo synthesis of both cellular and viral DNA and catalyzes the conversion of ribonucleoside diphosphates into the corresponding deoxyribonucleoside diphosphates. The enzyme consists of two nonidentical subunits, termed R1 and R2, whose expression is very low in resting cells and maximal in S-phase cells. Here we show that murine cytomegalovirus (MCMV) replication depends on ribonucleotide reduction since it is prevented by the RNR inhibitor hydroxyurea. MCMV infection of quiescent fibroblasts markedly induces both mRNA and protein corresponding to the cellular R2 subunit, whereas expression of the cellular R1 subunit does not appear to be up-regulated. The increase in R2 gene expression is due to an increase in gene transcription, since the activity of a reporter gene driven by the mouse R2 promoter is induced following virus infection. Cotransfection experiments revealed that expression of the viral immediate-early 1 protein was sufficient to mediate the increase in R2 promoter activity. It was found that the viral gene M45, encoding a putative homologue of the R1 subunit, is expressed 24 and 48 h after infection. Meanwhile, we observed an expansion of the deoxyribonucleoside triphosphate pool between 24 and 48 h after infection; however, neither CDP reduction nor viral replication was inhibited by treatment with 10 mM thymidine. These findings indicate the induction of an RNR activity with an altered allosteric regulation compared to the mouse RNR following MCMV infection and suggest that the virus R1 homologue may complex with the induced cellular R2 protein to reconstitute a new RNR activity.

Identification of drug-regulated genes in osteosarcoma cells

The introduction of systemic chemotherapy improved significantly the prognosis of osteosarcoma. Despite this success, approximately 30-40% of patients will relapse. Cytotoxic drugs have been shown to induce apoptosis in the target cells independent of their primary effects. The underlying molecular mechanisms and the intracellular mediators, however, are still largely unknown. Therefore, the purpose of our study was to identify drug-regulated genes in osteosarcoma cells useful as prognostic factors and for the development of new therapeutic strategies. Using suppressive subtractive hybridization (SSH) the gene expression pattern of untreated Saos-2 cells was compared to cells treated with cisplatin, methotrexate and doxorubicin, respectively. We identified 8 genes that are regulated >2-fold in drug-treated osteosarcoma cell lines. Expression of ferritin light chain, rhoA, inosine monophosphatdgehydrogenase II, ribonucleotide reductase M2, pro2000 and pro1859 were increased after drug treatment, whereas prohibitin and alpha-actinin expressions were significantly downregulated. Differential expression of the identified genes was verified by Northern blot analysis of 3 different osteosarcoma cell lines. In addition, the effects on chemosensitivity of 4 selected genes was analyzed by overexpression of recombinant constructs in Saos-2 cells and subsequent quantification of drug-induced apoptosis. Overexpression of prohibitin and rhoA reduced significantly drug sensitivity to approximately 52% and 59% indicating a crucial role in the modulation of drug-induced cell death.

Review of recent studies on resistance to cytotoxic deoxynucleoside analogues

Cytotoxic deoxynucleoside analogues are widely used in the treatment of haematological malignancies and solid tumours. Their metabolism and mechanisms of action are relatively well known, but with ongoing technological development, a continuous flow of scientific data is constantly adding new knowledge to this field. Thus, what was already a well-developed area some years ago has continued its expansion and become a better understood part of medical sciences. In order to keep abreast of the latest advances on cellular and clinical resistance to deoxynucleoside analogues, we have reviewed the recent literature and provide here an update on the subject. We have particularly focused on changes in gene products involved in the metabolic pathway of these drugs, such as membrane transporters, kinases, deaminases and 5'-nucleotidases. We also gave an overview on the chemical and biological development of modified deoxynucleoside analogues such as conjugates and pronucleotides.

Excess ribonucleotide reductase R2 subunits coordinate the S phase checkpoint to facilitate DNA damage repair and recovery from replication stress

Ribonucleotide reductase (RNR), which consists of R1 and R2 subunits, catalyzes a key step of deoxyribonucleoside triphosphate (dNTP) synthesis for DNA replication and repair. The R2 subunit is controlled in a cell cycle-specific manner for timely DNA synthesis and is negatively regulated by p53 in response to DNA damage. Herein we demonstrate that the presence of excess R2 subunits in p53(-/-) HCT-116 human colon cancer cells protects against DNA damage and replication stress. siRNA-mediated stable knockdown (>80%) of excess R2 subunits has no effect on proliferative growth but results in enhanced accumulation of gamma-H2Ax and delayed recovery from DNA lesions inflicted by exposure to cisplatin and Triapine. This accentuated induction of gamma-H2Ax in R2-knockdown cells is attributed to reduced ability to repair damaged DNA and overcome replication blockage. The lack of excess R2 subunits consequently augments chk1 activation and cdc25A degradation, causing impeded cell progression through the S phase and enhanced apoptosis in response to DNA damage and replication stress. In contrast, the level of R1 subunits appears to be limiting, since depletion of the R1 subunit directly activates the S phase checkpoint due to replication stress associated with impaired RNR activity. These findings suggest that excess R2 subunits facilitate DNA damage repair and recovery from replication stress through coordination with the S phase checkpoint in the absence of functional p53. Thus, the level of the R2 subunit constitutes an important determinant of the chemosensitivity of cancer cells and serves as a potential target for enhancement of DNA-damage based therapy.

Aerobic-type ribonucleotide reductase in the anaerobe Bacteroides fragilis

Bacteroides fragilis, a component of the normal intestinal flora, is an obligate anaerobe capable of long-term survival in the presence of air. Survival is attributed to an elaborate oxidative stress response that controls the induction of more than 28 peptides, but there is limited knowledge concerning the identities of these peptides. In this report, RNA fingerprinting by arbitrarily primed PCR identified five new genes whose expression increased following exposure to O2. Nucleotide sequence analysis of the cloned genes indicated that they encoded an outer membrane protein, an aspartate decarboxylase, an efflux pump, heat shock protein HtpG, and an NrdA ortholog constituting the large subunit of a class Ia ribonucleotide reductase (RRase). Attention was focused on the nrdA gene since class I RRases are obligate aerobic enzymes catalyzing the reduction of ribonucleoside 5'-diphosphates by a mechanism that requires molecular oxygen for activity. Sequence analysis of the nrd locus showed that two genes, nrdA and nrdB, are located in the same orientation in a 4.5-kb region. Northern hybridization and primer extension experiments confirmed induction of the genes by O2 and suggested they are an operon. The B. fragilis nrdA and nrdB genes were overexpressed in Escherichia coli, and CDP reductase assays confirmed that they encoded an active enzyme. The enzyme activity was inhibited by hydroxyurea, and ATP was shown to be a positive effector of CDP reductase activity, while dATP was an inhibitor, indicating that the enzyme was a class Ia RRase. A nrdA mutant was viable under anaerobic conditions but had decreased survival following exposure to O2, and it could not rapidly resume growth after O2 treatment. The results presented indicate that during aerobic conditions B. fragilis NrdAB may have a role in maintaining deoxyribonucleotide pools for DNA repair and growth recovery.

Role of the C terminus of the ribonucleotide reductase large subunit in enzyme regeneration and its inhibition by Sml1

Ribonucleotide reductase maintains cellular deoxyribonucleotide pools and is thus tightly regulated during the cell cycle to ensure high fidelity in DNA replication. The Sml1 protein inhibits ribonucleotide reductase activity by binding to the R1 subunit. At the completion of each turnover cycle, the active site of R1 becomes oxidized and subsequently regenerated by a cysteine pair (CX2C) at its C-terminal domain (R1-CTD). Here we show that R1-CTD acts in trans to reduce the active site of its neighboring monomer. Both Sml1 and R1-CTD interact with the N-terminal domain of R1 (R1-NTD), which involves a conserved two-residue sequence motif in the R1-NTD. Mutations at these two positions enhancing the Sml1-R1 interaction cause SML1-dependent lethality. These results point to a model whereby Sml1 competes with R1-CTD for association with R1-NTD to hinder the accessibility of the CX2C motif to the active site for R1 regeneration.

Two proteins mediate class II ribonucleotide reductase activity in Pseudomonas aeruginosa: expression and transcriptional analysis of the aerobic enzymes

The opportunistic human pathogen Pseudomonas aeruginosa is one of a few microorganisms that code for three different classes (I, II, and III) of the enzyme ribonucleotide reductase (RNR). Class II RNR of P. aeruginosa differs from all hitherto known class II enzymes by being encoded by two consecutive open reading frames denoted nrdJa and nrdJb and separated by 16 bp. Split nrdJ genes were also found in the few other gamma-proteobacteria that code for a class II RNR. Interestingly, the two genes encoding the split nrdJ in P. aeruginosa were co-transcribed, and both proteins were expressed. Exponentially growing aerobic cultures were predominantly expressing the class I RNR (encoded by the nrdAB operon) compared with the class II RNR (encoded by the nrdJab operon). Upon entry to stationary phase, the relative amount of nrdJa transcript increased about 6-7-fold concomitant with a 6-fold decrease in the relative amount of nrdA transcript. Hydroxyurea treatment known to knock out the activity of class I RNR caused strict growth inhibition of P. aeruginosa unless 5'-deoxyadenosylcobalamin, a cofactor specifically required for activity of class II RNRs, was added to the rich medium. Rescue of the hydroxyurea-treated cells in the presence of the vitamin B12 cofactor strongly implies that P. aeruginosa produces a functionally active NrdJ protein. Biochemical studies showed for the first time that presence of both NrdJa and NrdJb subunits were absolutely essential for enzyme activity. Based on combined genetic and biochemical results, we suggest that the two-component class II RNR in P. aeruginosa is primarily used for DNA repair and/or possibly DNA replication at low oxygen tension.