Cellular targets for activation by the E2F1 transcription factor include DNA synthesis- and G1/S-regulatory genes

Although a number of transfection experiments have suggested potential targets for the action of the E2F1 transcription factor, as is the case for many transcriptional regulatory proteins, the actual targets in their normal chromosomal environment have not been demonstrated. We have made use of a recombinant adenovirus containing the E2F1 cDNA to infect quiescent cells and then measure the activation of endogenous cellular genes as a consequence of E2F1 production. We find that many of the genes encoding S-phase-acting proteins previously suspected to be E2F targets, including DNA polymerase alpha, thymidylate synthase, proliferating cell nuclear antigen, and ribonucleotide reductase, are indeed induced by E2F1. Several other candidates, including the dihydrofolate reductase and thymidine kinase genes, were only minimally induced by E2F1. In addition to the S-phase genes, we also find that several genes believed to play regulatory roles in cell cycle progression, such as the cdc2, cyclin A, and B-myb genes, are also induced by E2F1. Moreover, the cyclin E gene is strongly induced by E2F1, thus defining an autoregulatory circuit since cyclin E-dependent kinase activity can stimulate E2F1 transcription, likely through the phosphorylation and inactivation of Rb and Rb family members. Finally, we also demonstrate that a G1 arrest brought about by gamma irradiation is overcome by the overexpression of E2F1 and that this coincides with the enhanced activation of key target genes, including the cyclin A and cyclin E genes.

Ribonucleotide reductase messenger RNA expression and survival in gemcitabine/cisplatin-treated advanced non-small cell lung cancer patients

PURPOSE

No chemotherapy regimen, including the widely used combination of gemcitabine/cisplatin, confers significantly improved survival over any other in metastatic non-small cell lung cancer (NSCLC); however, the selection of patients according to key genetic characteristics can help to tailor chemotherapy. Ribonucleotide reductase subunit M1 (RRM1) is involved in DNA synthesis and repair and in gemcitabine metabolism, and the excision repair cross-complementing group 1 (ERCC1) gene has been related to cisplatin activity.

EXPERIMENTAL DESIGN

Patients were part of a large randomized trial carried out from September 1998 to July 2000, comparing gemcitabine/cisplatin versus gemcitabine/cisplatin/vinorelbine versus gemcitabine/vinorelbine followed by vinorelbine/ifosfamide. We analyzed RRM1 and ERCC1 mRNA expression in paraffin-embedded samples obtained from bronchoscopy by real-time quantitative reverse transcription-PCR. Results were correlated with survival using the Kaplan-Meier method.

RESULTS

A total of 100 patients were assessed. There was a strong correlation between RRM1 and ERCC1 mRNA expression levels (Spearman r = 0.410; P < 0.001). In the gemcitabine/cisplatin arm, patients with low RRM1 mRNA expression levels had significantly longer median survival than those with high levels [13.7 versus 3.6 months; 95% confidence interval (CI), 9.6-17.8 months; P = 0.009]. Median survival was also significantly longer among patients with low mRNA expression levels of both RRM1 and ERCC1 (not reached), than among those with high levels of both genes (6.8 months; 95% CI, 2.6-11.1 months; P = 0.016).

CONCLUSIONS

RRM1 mRNA expression is a crucial predictive marker of survival in gemcitabine/cisplatin-treated patients. Genetic testing of RRM1 mRNA expression levels can and should be used to personalize chemotherapy.

Two genes differentially regulated in the cell cycle and by DNA-damaging agents encode alternative regulatory subunits of ribonucleotide reductase

Ribonucleotide reductase activity is essential for progression through the cell cycle, catalyzing the rate-limiting step for the production of deoxyribonucleotides needed for DNA synthesis. The enzymatic activity of the enzyme fluctuates in the cell cycle with an activity maximum in S phase. We have identified and characterized two Saccharomyces cerevisiae genes encoding the regulatory subunit of ribonucleotide reductase, RNR1 and RNR3. They share approximately 80% amino acid identity with each other and 60% with the mammalian homolog, M1. Genetic disruption reveals that the RNR1 gene is essential for mitotic viability, whereas the RNR3 gene is not essential. A high-copy-number clone of RNR3 is able to suppress the lethality of rnr1 mutations. Analysis of mRNA levels in cell-cycle-synchronized cultures reveals that the RNR1 mRNA is tightly cell-cycle regulated, fluctuating 15- to 30-fold, and is coordinately regulated with the POL1 mRNA, being expressed in the late G1 and S phases of the cell cycle. Progression from the alpha-factor-induced G1 block to induction of RNR1 mRNA is blocked by cycloheximide, further defining the requirement for protein synthesis in the G1- to S-phase transition. Both RNR1 and RNR3 transcripts are inducible by treatments that damage DNA, such as 4-nitroquinoline-1-oxide and methylmethanesulfonate, or block DNA replication, such as hydroxyurea. RNR1 is inducible 3- to 5-fold, and RNR3 is inducible greater than 100-fold. When MATa cells are arrested in G1 by alpha-factor, RNR1 and RNR3 mRNA is still inducible by DNA damage, indicating that the observed induction can occur outside of S phase. Inhibition of ribonucleotide reductase activity by hydroxyurea treatment results in arrest of the cell cycle in S phase as large budded, uninucleate cells. This specific cell-cycle arrest is independent of the RAD9 gene, defining a separate pathway for the coordination of DNA synthesis and cell-cycle progression.

Effect of iron deficiency and desferrioxamine on DNA synthesis in human cells

Desferrioxamine (10(-3) M) caused a fall in the deoxyadenosine triphosphate level after 4 h incubation in normal phytohaemagglutinin-stimulated lymphocytes. There was a rise in the concentrations of the other three deoxyribonucleoside triphosphates (deoxythymidine-,deoxycytidine-and deoxyguanosine-triphosphate). The changes are similar to those caused by hydroxyurea, a known inhibitor of ribonucleotide reductase. Desferrioxamine (10(-3 M) was found to inhibit human lymphocyte ribonucleotide reductase to a mean of 11% of control activity after 45 min incubation. Both drugs, desferrioxamine and hydroxyurea, inhibited incorporation of [3H]thymidine DNA into lymphocytes in the presence or absence of deoxyuridine, and inhibited production of lymphocytic thymidine kinase, having opposite effects to methotrexate on both [3H]thymidine incorporation and thymidine kinase activity. Phytohaemagglutinin-stimulated lymphocytes from patients with chronic iron deficiency showed lower levels of all our deoxyribonucleoside triphosphates than normal lymphocytes. It is suggested that this may be due to reduced ribnucleotide reductase activity of the iron-deficient cells.

A ribonucleotide reductase gene is a transcriptional target of p53 and p73

Many p53-inducible genes have been identified that might play a role in mediating the various downstream activities of p53. We have identified a close relative of ribonucleotide reductase, recently named p53R2, as a p53-inducible gene, and show that this gene is activated by several stress signals that activate a p53 response, including DNA damaging agents and p14(ARF). p53R2 expression was induced by p53 mutants that are defective for the activation of apoptosis, but retain cell cycle arrest function, although no induction of p53R2 was seen in response to p21(WAF1/CIP1)-mediated cell cycle arrest. Several isoforms of the p53 family member p73 were also shown to induce p53R2 expression. Transient ectopic expression of either wild type p53R2 or p53R2 targeted to the nucleus, did not significantly alter cell cycle progression in unstressed cells. The identification of this gene as a p53 target supports a direct role for p53 in DNA repair, in addition to inhibition of growth of damaged cells. Oncogene (2000) 19, 4283 - 4289

Two additional glutaredoxins exist in Escherichia coli: glutaredoxin 3 is a hydrogen donor for ribonucleotide reductase in a thioredoxin/glutaredoxin 1 double mutant

Thioredoxin (Trx) and glutaredoxin (Grx1) are hydrogen donors for ribonucleotide reductase, the key enzyme for deoxyribonucleotide biosynthesis. The viability of a double mutant lacking both Trx and Grx1 implies the presence of a third, unknown hydrogen donor. This paper reports the purification and characterization of two proteins with glutaredoxin activity (using hydroxyethyl disulfide as a substrate) from an Escherichia coli mutant lacking Trx and Grx1 (delta trxA, grx::kan). Affinity chromatography was used to bind glutaredoxin on a glutathione-containing thiol-Sepharose column. The molecular weight of Grx2, 27,000, was atypical for glutaredoxins, whereas Grx3 had a molecular weight of 10,000. Amino acid sequence analysis revealed novel structures with putative active sites typical of glutaredoxins: Cys-Pro-Tyr-Cys. The proteins are therefore referred to as Grx2 and Grx3. The low hydrogen donor activity for ribonucleotide reductase in the crude extract was recovered in the purification of Grx3, whereas Grx2 was inactive. As a hydrogen donor for E. coli ribonucleotide reductase, Grx3 showed approximately the same Km value (0.35 microM) as Grx1, whereas its Vmax value was only 5% that of Grx1. The combination of the Grx3 hydrogen donor activity and a 25-fold induction of ribonucleotide reductase activity in a delta trxA, grx double mutant provides an explanation for its viability and deoxyribonucleotide biosynthesis. The physiological functions of Grx2 and Grx3 remain to be determined.

In vivo induction of resistance to gemcitabine results in increased expression of ribonucleotide reductase subunit M1 as the major determinant

Gemcitabine is a deoxycytidine (dCyd) analogue with activity against several solid cancers. Gemcitabine is activated by dCyd kinase (dCK) and interferes, as its triphosphate dFdCTP, with tumor growth through incorporation into DNA. Alternatively, the metabolite gemcitabine diphosphate (dFdCDP) can interfere with DNA synthesis and thus tumor growth through inhibition of ribonucleotide reductase. Gemcitabine can be inactivated by the enzyme dCyd deaminase (dCDA). In most in vitro models, resistance to gemcitabine was associated with a decreased dCK activity. In all these models, resistance was established using continuous exposure to gemcitabine with increasing concentrations; however, these in vitro models have limited clinical relevance. To develop in vivo resistance to gemcitabine, we treated mice bearing a moderately sensitive tumor Colon 26-A (T/C = 0.25) with a clinically relevant schedule (120 mg/kg every 3 days). By repeated transplant of the most resistant tumor and continuation of gemcitabine treatment for >1 year, the completely resistant tumor Colon 26-G (T/C = 0.96) was created. Initial studies focused on resistance mechanisms known from in vitro studies. In Colon 26-G, dCK activity was 1.7-fold decreased; dCDA and DNA polymerase were not changed; and Colon 26-G accumulated 1.5-fold less dFdCTP, 6 hours after a gemcitabine injection, than the parental tumor. Based on in vitro studies, these relative minor changes were considered insufficient to explain the completely resistant phenotype. Therefore, an expression microarray was done with Colon 26-A versus Colon 26-G. Using independently grown nonresistant and resistant tumors, a striking increase in expression of the RRM1 subunit gene was found in Colon 26-G. The expression of RRM1 mRNA was 25-fold increased in the resistant tumor, as measured by real-time PCR, which was confirmed by Western blotting. In contrast, RRM2 mRNA was 2-fold decreased. However, ribonucleotide reductase enzyme activity was only moderately increased in Colon 26-G. In conclusion, this is the first model with in vivo induced resistance to gemcitabine. In contrast to most in vitro studies, dCK activity was not the most important determinant of gemcitabine resistance. Expression microarray identified RRM1 as the gene with the highest increase in expression in the Colon 26-G, which might clarify its complete gemcitabine-resistant phenotype. This study is the first in vivo evidence for a key role for RRM1 in acquired gemcitabine resistance.

Inducible overexpression of a toxic protein by an adenovirus vector with a tetracycline-regulatable expression cassette

We have constructed two new adenovirus expression cassettes that expand both the range of genes which can be expressed with adenovirus vectors (AdV) and the range of cells in which high-level expression can be attained. By inclusion of a tetracycline-regulated promoter in the transfer vector pAdTR5, it is now possible to generate recombinant adenoviruses expressing proteins that are either cytotoxic or that interfere with adenovirus replication. We have used this strategy to generate a recombinant adenovirus encoding a deletion in the R1 subunit [R1(delta2-357)] of the herpes simplex virus type 2 ribonucleotide reductase. Cell lines expressing the tetracycline-regulated transactivator (tTA) from an integrated vector or following infection with an AdV expressing tTA are able to produce deltaR1 protein at a level approaching 10% total cell protein (TCP) when infected with Ad5TR5 deltaR1 before they subsequently die. To our knowledge, this is the first report of the overexpression of a toxic gene product with AdV. We have also constructed a new constitutive adenovirus expression cassette based on an optimized cytomegalovirus immediate-early promoter-enhancer that allows the expression of recombinant proteins at a level greater than 20% TCP in nonpermissive cell lines. Together, these new expression cassettes significantly improve the utility of the adenovirus system for high-level expression of recombinant proteins in animal cells and will undoubtedly find useful applications in gene therapy.

Ribonucleotide reductase induced by herpes simplex virus has a virus-specified constituent

Ribonucleotide reductase, an enzyme found in all prokaryotic and eukaryotic cells that synthesize DNA, is induced by herpes simplex virus (HSV). In this study the effect of anti-HSV antiserum on the induced ribonucleotide reductase has been examined and the ability of different temperature-sensitive (ts) mutants of HSV-1 to induce the enzyme has been investigated. The HSV-1-induced ribonucleotide reductase was inhibited by antiserum raised against infected cell lysates but not by preimmune serum. The wild-type (ts+) virus induced similar levels of ribonucleotide reductase at 31 degrees C and 38.5 degrees C (the permissive and non-permissive temperatures respectively for the ts mutants). All ts mutants induced approximately wild-type levels of the enzyme at 31 degrees C. At 38.5 degrees C, two of the four ts mutants studied also induced wild-type levels of enzyme but ts G failed to induce any activity while ts K induced variable but low levels. The enzyme activity induced by ts G at 31 degrees C was thermolabile both in vivo and in vitro. These results provide the first strong evidence that the induced ribonucleotide reductase activity is at least partially virus-coded.

S Phase-specific transcription of the mouse ribonucleotide reductase R2 gene requires both a proximal repressive E2F-binding site and an upstream promoter activating region

Ribonucleotide reductase is essential for supplying a balanced pool of the four deoxyribonucleotides required for DNA synthesis and repair. The active enzyme consists of two non-identical subunits called proteins R1 and R2. There are multiple levels of regulation of ribonucleotide reductase activity, which is highest during the S and G(2) phases of an unperturbed cell cycle in mammalian cells. Previous reports in the literature have indicated that the S phase-specific transcription of the mammalian R2 gene is regulated by a transcriptional block, is dependent on the transcription factor E2F1, or is simply regulated by proteins that bind to promoter CCAAT boxes plus the TATA box. Here, we demonstrate that the S phase-specific transcription of the mouse R2 gene is dependent on an upstream promoter activating region (located at nucleotides (nt) -672 to -527 from the transcription start site) and one proximal promoter repressive element (located at nt -112 to -107) that binds E2F4. Binding to the E2F site is modulated by binding of nuclear factor-Y to an adjacent CCAAT element (nt -79 to -75). The upstream activating region is crucial for overall R2 promoter activity. Mutation of the E2F-binding site leads to premature promoter activation in G(1) and increases overall promoter activity but only when the upstream activating region is present and intact. Therefore, E2F-dependent repression is essential for cell cycle-specific R2 transcription.

Partial purification and characterization of the ribonucleotide reductase induced by herpes simplex virus infection of mammalian cells

In this report we confirm and further characterize the induction of a novel ribonucleotide reductase after herpes simplex virus infection of mammalian cells. Induction of the enzyme was observed at a multiplicity of infection of 1 PFU/cell or greater and was found to be maximal (three- to sixfold the activity in mock-infected controls at 6 to 8 h postinfection at a multiplicity of infection of 10 PFU/cell. Partial purification and subsequent characterization of the reductase activity from infected cells demonstrated the existence of two enzymes which could be separated by precipitation with ammonium sulfate. One of the activities precipitated at between 35 and 55% salt saturation, as did the enzyme from control cells, whereas the novel activity precipitated at 0 to 35% saturation. This latter enzyme was similar to the herpes simplex virus-induced reductase described by others in its lack of requirement for Mg2 and its resistance to inhibition by dTTP and dATP; in addition, we found that it was inhibited by ATP, whereas the enzyme from control cells displayed an absolute requirement for the nucleotide. Both enzymes were equally inhibited by pyridoxal phosphate and showed similar cold and heat stability. The enzyme induced by herpes simplex virus infection, however, was much more labile than the control enzyme upon purification.

A novel human gene similar to the protein kinase (PK) coding domain of the large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10) codes for a serine-threonine PK and is expressed in melanoma cells

The large subunit of herpes simplex virus type 2 ribonucleotide reductase (ICP10) is a multifunctional protein that contains a serine-threonine protein kinase (PK) activity (Nelson, J. W., Zhu, J. , Smith, C. C., Kulka, M., and Aurelian, L. (1996) J. Biol. Chem. 271, 17021-17027). Phylogenetic analyses indicated that ICP10 PK belongs to a distinct subfamily of growth factor receptor serine-threonine PKs that are characterized by their ability to function with a limited number of conserved catalytic motifs (Hunter, J. C. R., Smith, C. C., and Aurelian, L. (1995) Int. J. Onc. 7, 515-522). Here, we report the isolation and characterization of a novel gene, designated H11, that contains an open reading frame of 588 nucleotides, which encodes a protein similar to ICP10 PK. The H11 protein has Mn(2+)-dependent serine-threonine-specific PK activity as determined with a GST-H11 fusion protein and by immununocomplex PK/immunoblotting assays of 293 cells transfected with a H11 eukaryotic expression vector. PK activity is ablated by mutation of Lys(113) within the presumtive catalytic motif II (invariant Lys). 293 cells stably transfected with H11 acquire anchorage-independent growth. Endogenous H11 RNA and the H11 phosphoprotein are expressed in melanoma cell lines and primary melanoma tissues at levels higher than in normal melanocytes and in benign nevi. Melanoma cell proliferation is inhibited by treatment with antisense oligonucleotides that inhibit H11 translation, suggesting that H11 expression is associated with cell growth.

Identification of a herpes simplex virus type 1 polypeptide which is a component of the virus-induced ribonucleotide reductase

We have characterized a temperature-sensitive mutant of herpes simplex virus type 1 (HSV-1), 17tsVP1207, that induces a thermolabile ribonucleotide reductase activity. This mutant was derived from the multiple mutant tsG. Fine-structure mapping studies showed that the defect in 17tsVP1207 lies within an 800 bp sequence between genome map coordinates 0.580 and 0.585 in the gene encoding a polypeptide of 140 000 mol. wt. (Vmw136, ICP6). Since the mutation in this polypeptide produced a temperature-sensitive ribonucleotide reductase activity, Vmw136 must be a component of the herpes simplex virus-induced ribonucleotide reductase. The mRNA of Vmw136 has a common 3' terminus with an mRNA encoding a 38 000 mol. wt. polypeptide (Vmw38). Although the polypeptide-coding sequences of these mRNAs do not overlap, monoclonal antibodies against Vmw136 immunoprecipitated Vmw38 as well as Vmw136 from wild-type HSV-1-infected cells. Our data do not exclude the possibility that Vmw38 is part of the ribonucleotide reductase complex but suggest that this species on its own is not responsible for the HSV-induced enzyme activity.

Ribonucleotide reductase R2 component is a novel malignancy determinant that cooperates with activated oncogenes to determine transformation and malignant potential

Ribonucleotide reductase is a highly regulated cell cycle-controlled activity that is essential for DNA synthesis and repair. A retroviral vector for the R2 component of mammalian ribonucleotide reductase, the rate-limiting protein for enzyme activity and DNA synthesis in proliferating cells, was constructed and introduced into mammalian cells. Expression of Myc epitope-tagged R2 protein in benign BALB/c 3T3 and NIH 3T3 cells leads to a greatly increased frequency of focus formation in cooperation with H-ras transformation. Four lines of H-ras-transformed mouse 10T1/2 fibroblasts showed increased growth efficiency in soft agar after infection with the recombinant R2 expression virus vector. Furthermore, cells with altered R2 expression also exhibited significantly reduced subcutaneous tumor latency and increased tumor growth rates in syngeneic mice, and showed markedly elevated metastatic potential in lung metastasis assays. The results indicate that altered R2 gene expression cooperates with ras in mechanisms of malignant progression. A major Ras pathway involves the Raf-1 protein, which is recruited to the plasma membrane for activation. We show that recombinant R2 expression leads to significant increases in membrane-associated Raf-1 protein and mitogenactivating protein kinase-2 activity suggesting a mechanism for the observed Ras/R2 synergism. In support of this finding, we observed that activated Rac-1, which operates parallel to Raf-1 and cooperates with Raf-1 in Ras activated pathways, also cooperates with R2 in cellular transformation. These studies demonstrate that the R2 protein can participate in other critical cellular functions in addition to ribonucleotide reduction, and that deregulated R2 is a novel tumor progressor determinant that cooperates in oncogene-mediated mechanisms, which control malignant potential.

Cloning, sequencing, and expression of the adenosylcobalamin-dependent ribonucleotide reductase from Lactobacillus leichmannii

Ribonucleoside-triphosphate reductase (RTPR, EC 1.17.4.2) from Lactobacillus leichmannii, a monomeric adenosylcobalamin-requiring enzyme, catalyzes the conversion of nucleoside triphosphates to deoxynucleoside triphosphates. The gene for this enzyme has been cloned and sequenced. In contrast to expectations based on mechanistic considerations, there is no statistically significant sequence homology with the Escherichia coli reductase that requires a dinuclear-iron center and tyrosyl radical cofactor. The RTPR has been overexpressed and purified to homogeneity, yielding 90 mg of protein from 2.5 g of bacteria. Initial characterization of the recombinant RTPR indicates that its properties are identical to those of the RTPR isolated from L. leichmannii.

Generation of the R2 Subunit of Ribonucleotide Reductase by Intein Chemistry: Insertion of 3-Nitrotyrosine at Residue 356 as a Probe of the Radical Initiation Process

Escherichia coli ribonucleotide reductase (RNR) catalyzes the conversion of nucleoside diphosphates to deoxynucleoside diphosphates. The enzyme is composed of two subunits: R1 and R2. R1 contains the active site for nucleotide reduction and the allosteric effector sites that regulate the specificity and turnover rate. R2 contains the diferric-tyrosyl (Y(*)) radical cofactor that initiates nucleotide reduction by a putative long-range proton-coupled electron transfer (PCET) pathway over 35 A. This pathway is thought to involve specific amino acid radical intermediates (Y122 to W48 to Y356 within R2 to Y731 to Y730 to C439 within R1). In an effort to study radical initiation, R2 (375 residues) has been synthesized semisynthetically. R2 (residues 1-353), attached to an intein and a chitin binding domain, was constructed, and the protein was expressed (construct 1). This construct was then incubated with Fe(2+) and O(2) to generate the diferric-Y(*) cofactor, and the resulting protein was purified using a chitin affinity column. Incubation of construct 1 with 2-mercaptoethanesulfonic acid (MESNA) resulted in the MESNA thioester of R2 (1-353) (construct 2). A peptide containing residues 354-375 of R2 was generated using solid-phase peptide synthesis where 354, a serine in the wild-type (wt) R2, was replaced by a cysteine. Construct 2 and this peptide were ligated, and the resulting full-length R2 was separated from truncated R2 by anion-exchange chromatography. The purified protein had a specific activity of 350 nmol min(-1) mg(-1), identical to the same protein generated by site-directed mutagenesis when normalized for Y(*). As a first step in studying the radical initiation by PCET, R2 was synthesized with Y356 replaced by 3-nitrotyrosine (NO(2)Y). The protein is inactive (specific activity 1 x 10(-4) that of wt-R2), which permitted a determination of the pK(a) of the NO(2)Y in the R1/R2 complex in the presence of substrate and effectors. Under all conditions, the pK(a) was minimally perturbed. This has important mechanistic implications for the radical initiation process.

Optimization of the PAXgene blood RNA extraction system for gene expression analysis of clinical samples

One major problem associated with collecting whole blood from patients for use as a source of RNA in gene expression studies is that the RNA degrades during collection and storage. Preservation of RNA quality is vital in such studies because the stability of the RNA ultimately affects analysis of gene expression. In this study the PAXgene blood collection system was compared with a standard erythrocyte lysis method for isolating RNA from blood samples. The methods were compared in terms of RNA yield, RNA stabilization, and DNA contamination. The study also included the downstream application to RT-PCR analysis for relative mRNA expression levels of the ribonucleotide reductase subunits R1 and R2. The results show that blood collection in conventional collection tubes, and leukocyte isolation by erythrocyte lysis lead to significant degradation of RNA. Our findings confirm the ability of PAXgene to stabilize RNA in whole blood; however, RNA extracted by the PAXgene method contained significant DNA contamination. Given the low basal expression of the target genes analyzed in this study, contaminating DNA could potentially affect accurate interpretation of RT-PCR data. As a result, the PAXgene protocol was optimized to include off-column DNase treatments, which yielded high-quality RNA suitable for gene expression studies. Furthermore, the results suggest that RNA isolation with PAXgene is advantageous compared to traditional extraction methods for RT-PCR analysis of large or different-sized amplicons.

The ribonucleotide reductase induced by herpes simplex virus type 1 involves minimally a complex of two polypeptides (136K and 38K)

Herpes simplex virus type 1 (HSV-1) encodes a polypeptide of apparent mol. wt. 136 000 (Vmw136) known to be a component of the virus-specified ribonucleotide reductase. Monoclonal antibodies that precipitate this polypeptide also precipitate a polypeptide of mol. wt. 38 000 (Vmw38) from extracts of HSV-1-infected cells. The basis for this co-precipitation has been investigated using a monoclonal antibody directed against Vmw136 and an oligopeptide-induced antiserum directed against the carboxy terminus of Vmw38. We have also made use of a temperature-sensitive (ts) mutant of HSV-1 which maps within the sequences encoding Vmw136 and which induces a thermolabile ribonucleotide reductase. Our experiments show (i) Vmw136 and Vmw38 form a complex in infected cells and (ii) the mutation in the ts mutant results in the two polypeptides being unable to form the complex at the non-permissive temperature. We speculate that association of the two polypeptides is necessary for ribonucleotide reductase activity. No evidence was found for involvement of host proteins in the proposed virus-induced ribonucleotide reductase complex. The terms RR1 and RR2 are suggested for the large and small subunits of the HSV-induced enzyme.

Analysis of transcription of the Staphylococcus aureus aerobic class Ib and anaerobic class III ribonucleotide reductase genes in response to oxygen

Staphylococcus aureus is a gram-positive facultative aerobe that can grow in the absence of oxygen by fermentation or by using an alternative electron acceptor. To investigate the mechanism by which S. aureus is able to adapt to changes in oxygen concentration, we analyzed the transcriptional regulation of genes that encode the aerobic class Ib and anaerobic class III ribonucleotide reductase (RNR) systems that are responsible for the synthesis of deoxyribonucleotides needed for DNA synthesis. The S. aureus class Ib RNR nrdIEF and class III RNR nrdDG genes and their regulatory regions were cloned and sequenced. Inactivation of the nrdDG genes showed that the class III RNR is essential for anaerobic growth. Inhibition of aerobic growth by hydroxyurea showed that the class Ib RNR is an oxygen-dependent enzyme. Northern blot analysis and primer extension analysis demonstrated that transcription of class III nrdDG genes is regulated by oxygen concentration and was at least 10-fold higher under anaerobic than under aerobic conditions. In contrast, no significant effect of oxygen concentration was found on the transcription of class Ib nrdIEF genes. Disruption or deletion of S. aureus nrdDG genes caused up to a fivefold increase in nrdDG and nrdIEF transcription under anaerobic conditions but not under aerobic conditions. Similarly, hydroxyurea, an inhibitor of the class I RNRs, resulted in increased transcription of class Ib and class III RNR genes under aerobic conditions. These findings establish that transcription of class Ib and class III RNR genes is upregulated under conditions that cause the depletion of deoxyribonucleotide. Promoter analysis of class Ib and class III RNR operons identified several inverted-repeat elements that may account for the transcriptional response of the nrdIEF and nrdDG genes to oxygen.

Enhancement of gene therapy specificity for diffuse colon carcinoma liver metastases with recombinant herpes simplex virus

OBJECTIVE

The authors determined whether a recombinant herpes simplex virus (HSV) vector could destroy human colon carcinoma cells in vitro and whether the vector would selectively replicate in colon carcinoma liver metastases but not surrounding hepatocytes in vivo.

BACKGROUND

The HSV vector hrR3 is defective in the gene encoding ribonucleotide reductase and contains the lacZ reporter gene. Ribonucleotide reductase is expressed in actively dividing cells and generates deoxyribonucleotides for DNA synthesis. hrR3 replicates only in actively dividing cells that can provide ribonucleotide reductase in complementation, but not in quiescent cells such as normal hepatocytes.

METHODS

hrR3-mediated lysis of HT29 human colon carcinoma cells was first determined in vitro. For in vivo studies, athymic BALB/c nude mice underwent intrasplenic injection of HT29 and intrasplenic injection of hrR3 7 days later, and were killed 7 days after viral injection. Their livers were examined histochemically for lacZ expression.

RESULTS

All the HT29 cells were destroyed in vitro when hrR3 was added at a titer of 1 plaque-forming unit per 10 tumor cells. One hundred one of 105 tumor nodules examined in liver sections from mice treated by intrasplenic injection of hrR3 demonstrated lacZ expression. Minimal beta-galactosidase activity was present in normal liver.

CONCLUSIONS

The hrR3 HSV vector effectively destroys HT29 human colon carcinoma cells at very low multiplicities of infection. Differential expression of ribonucleotide reductase between liver metastases and normal liver allows hrR3 to selectively replicate in tumor cells with minimal replication in surrounding normal liver. Further investigation of HSV-based vectors as oncolytic agents for liver metastases is warranted.

Promoter identification and expression analysis of Salmonella typhimurium and Escherichia coli nrdEF operons encoding one of two class I ribonucleotide reductases present in both bacteria

Salmonella typhimurium and Escherichia coli cells have two different class I ribonucleotide reductases encoded by the nrdEF and nrdAB operons. Despite the presence of one additional ribonucleotide reductase, the nrdAB-encoded enzyme is essential to the aerobic growth of the cell because nrdAB-defective mutants of both species are not viable in the presence of oxygen. Several factors controlling nrdAB gene transcription have been analysed intensively. Nothing is known about the expression of the nrdEF genes. To study this subject, and after cloning of E. coli nrdEF genes and sequencing of their 5' ends, the promoter of this operon has been identified by primer extension in both bacterial species. The +1 position was 691 bp and 692 bp upstream of the translational start points of the nrdE genes of S. typhimurium and E. coli, respectively. Downstream of the +1 position, and before the nrdE gene, two open reading frames (ORFs) of 81 and 136 amino acid residues are present in both bacteria. The synthesis of a polypeptide with a molecular mass of 9 kDa, corresponding to the first of these two ORFs, was observed by using the T7 RNA polymerase expression system. Comparison of the amino acid predicted sequence of this ORF reveals a significant similarity with glutaredoxin proteins. Competitive, reverse-transcription polymerase chain reaction experiments indicate that transcription from the nrdEF promoter normally takes place in wild-type cells. nrdEF transcription is increased by hydroxyurea, which inhibits class I ribonucleotide reductase activity, in both RecA+ and RecA- cells. nrdA(ts) mutants show a higher level of nrdEF transcription than wild-type cells at either the permissive or the restrictive temperature. nrdEF expression was unaffected by changes in DNA supercoiling whether caused by the introduction of either topA::Tn10 and hns::Tn10 mutations or by the inhibition of DNA gyrase with the antibiotic novobiocin. In contrast to the nrdAB genes, the nrdEF operon is not essential to the cells because nrdEF-defective mutants are viable under both aerobic and anaerobic conditions.

Characterization of ribonucleotide reductase induction in BHK-21/C13 Syrian hamster cell line upon infection by herpes simplex virus (HSV)

Ribonucleotide reductase is an essential enzyme in mammalian DNA replication. In quiescent BHK-21/C13 cells exhibiting a low level of ribonucleotide reductase activity, infection with herpes simplex virus (HSV) resulted in the early induction of an altered ribonucleotide reductase. The extent of the induction was dependent upon the m.o.i. and could be diminished or prevented by u.v. treatment of the viral stock, or by inhibitors of mRNA synthesis or protein synthesis. The induction followed the same course of synthesis as viral thymidine kinase and DNA polymerase, and could thus be classified with them as a beta polypeptide. These results suggested that the new activity was produced as a consequence of the virus genome expression. Comparisons of the properties of ribonucleotide reductase extracted from exponentially growing BHK-21/C13 cells showed that the HSV-induced enzyme differed from the cellular isozyme by its insensitivity to inhibition by dTTP, dATP or araATP and its resistance to high salt concentrations. On the other hand, the virus-induced enzyme and the cellular isozyme exhibited a similar sensitivity to hydroxyurea. Therefore, the reported inhibition of HSV DNA replication by hydroxyurea could be the result of inhibition of both HSV-induced and cellular reductase activities.

Immunocytochemical evidence for the cytoplasmic localization and differential expression during the cell cycle of the M1 and M2 subunits of mammalian ribonucleotide reductase

Mammalian ribonucleotide reductase consists of two non-identical subunits, proteins M1 and M2. We have produced and characterized rat polyclonal and monoclonal antibodies directed against protein M2 of mouse ribonucleotide reductase. Using these antibodies for immunocytochemical studies, an exclusively cytoplasmic localization of protein M2 was demonstrated both in cultured parent and hydroxyurea-resistant, M2-over-producing mouse TA3 cells, and in cells from various mouse tissues. These data, together with the previously demonstrated cytoplasmic localization of the M1 subunit, clearly show that ribonucleotide reductase is a cytoplasmic enzyme. Combining the anti-M2 antibodies with a monoclonal anti-M1 antibody allowed for double-labelling immunofluorescence studies of the two subunits in individual cells. Only approximately 50% of the cells in a logarithmically growing culture contained immunodetectable protein M2, while the M1-specific staining was present in all cells. The M2 staining correlates well with the proportion of cells in the S-phase of the cell cycle. In tissues, only actively dividing cells stained with either antibody and there were always fewer cells stained with the M2-antibodies than with the M1-antibody. Our data therefore present independent evidence for the earlier proposed model of a differential regulation during the cell cycle of the M1 and M2 subunits of ribonucleotide reductase.

Ribonucleotide reductase small subunit M2B prognoses better survival in colorectal cancer

Ribonucleotide reductase subunit RRM2B (p53R2) has been reported to suppress invasion and metastasis in colorectal cancer (CRC). Here, we report that high levels of RRM2B expression are correlated with markedly better survival in CRC patients. In a fluorescence-labeled orthotopic mouse xenograft model, we confirmed that overexpression of RRM2B in nonmetastatic CRC cells prevented lung and/or liver metastasis, relative to control cells that did metastasize. Clinical outcome studies were conducted on a training set with 103 CRCs and a validation set with 220 CRCs. All participants underwent surgery with periodic follow-up to determine survivability. A newly developed specific RRM2B antibody was employed to carry out immunohistochemistry for determining RRM2B expression levels on tissue arrays. In the training set, the Kaplan-Meier and multivariate Cox analysis revealed that RRM2B is associated with better survival of CRCs, especially in stage IV patients (HR = 0.40; 95% CI = 0.18-0.86, P = 0.016). In the validation set, RRM2B was negatively related to tumor invasion (OR = 0.45, 95% CI = 0.19-0.99, P = 0.040) and lymph node involvement (OR = 0.48, 95% CI = 0.25-0.92, P = 0.026). Furthermore, elevated expression of RRM2B was associated with better prognosis in this set as determined by multivariate analyses (HR = 0.48, 95% CI = 0.26-0.91, P = 0.030). Further investigations revealed that RRM2B was correlated with better survival of CRCs with advanced stage III and IV tumors rather than earlier stage I and II tumors. Taken together, our findings establish that RRM2B suppresses invasiveness of cancer cells and that its expression is associated with a better survival prognosis for CRC patients.

Evolution of mobile group I introns: recognition of intron sequences by an intron-encoded endonuclease

Mobile group I introns are hypothesized to have arisen after invasion by endonuclease-encoding open reading frames (ORFs), which mediate their mobility. Consistent with an endonuclease-ORF invasion event, we report similarity between exon junction sequences (the recognition site for the mobility endonuclease) and intron sequences flanking the endonuclease ORF in the sunY gene of phage T4. Furthermore, we have demonstrated the ability of the intron-encoded endonuclease to recognize and cleave these intron sequences when present in fused form in synthetic constructs. These observations and accompanying splicing data are consistent with models in which the invading endonuclease ORF is provided safe haven within a splicing element. In turn the intron is afforded immunity to the endonuclease product, which imparts mobility to the intron.