Functional assessment of population and tumor-associated APE1 protein variants.

Apurinic/apyrimidinic endonuclease 1 (APE1) is the predominant AP site repair enzyme in mammals. APE1 also maintains 3'-5' exonuclease and 3'-repair activities, and regulates transcription factor DNA binding through its REF-1 function. Since complete or severe APE1 deficiency leads to embryonic lethality and cell death, it has been hypothesized that APE1 protein variants with slightly impaired function will contribute to disease etiology. Our data indicate that except for the endometrial cancer-associated APE1 variant R237C, the polymorphic variants Q51H, I64V and D148E, the rare population variants G241R, P311S and A317V, and the tumor-associated variant P112L exhibit normal thermodynamic stability of protein folding; abasic endonuclease, 3'-5' exonuclease and REF-1 activities; coordination during the early steps of base excision repair; and intracellular distribution when expressed exogenously in HeLa cells. The R237C mutant displayed reduced AP-DNA complex stability, 3'-5' exonuclease activity and 3'-damage processing. Re-sequencing of the exonic regions of APE1 uncovered no novel amino acid substitutions in the 60 cancer cell lines of the NCI-60 panel, or in HeLa or T98G cancer cell lines; only the common D148E and Q51H variants were observed. Our results indicate that APE1 missense mutations are seemingly rare and that the cancer-associated R237C variant may represent a reduced-function susceptibility allele.

Diverse small molecule inhibitors of human apurinic/apyrimidinic endonuclease APE1 identified from a screen of a large public collection.

The major human apurinic/apyrimidinic endonuclease APE1 plays a pivotal role in the repair of base damage via participation in the DNA base excision repair (BER) pathway. Increased activity of APE1, often observed in tumor cells, is thought to contribute to resistance to various anticancer drugs, whereas down-regulation of APE1 sensitizes cells to DNA damaging agents. Thus, inhibiting APE1 repair endonuclease function in cancer cells is considered a promising strategy to overcome therapeutic agent resistance. Despite ongoing efforts, inhibitors of APE1 with adequate drug-like properties have yet to be discovered. Using a kinetic fluorescence assay, we conducted a fully-automated high-throughput screen (HTS) of the NIH Molecular Libraries Small Molecule Repository (MLSMR), as well as additional public collections, with each compound tested as a 7-concentration series in a 4 µL reaction volume. Actives identified from the screen were subjected to a panel of confirmatory and counterscreen tests. Several active molecules were identified that inhibited APE1 in two independent assay formats and exhibited potentiation of the genotoxic effect of methyl methanesulfonate with a concomitant increase in AP sites, a hallmark of intracellular APE1 inhibition; a number of these chemotypes could be good starting points for further medicinal chemistry optimization. To our knowledge, this represents the largest-scale HTS to identify inhibitors of APE1, and provides a key first step in the development of novel agents targeting BER for cancer treatment.

S-glutathionylation of cysteine 99 in the APE1 protein impairs abasic endonuclease activity.

Human apurinic/apyrimidinic (AP) endonuclease 1 (APE1) is a central participant in the base excision repair pathway, exhibiting AP endonuclease activity that incises the DNA backbone 5' to an abasic site. Besides its prominent role as a DNA repair enzyme, APE1 was separately identified as a protein called redox effector factor 1, which is able to enhance the DNA binding activity of several transcription factors through a thiol-exchange-based reduction-oxidation mechanism. In the present study, we found that human APE1 is S-glutathionylated under conditions of oxidative stress both in the presence of glutathione in vitro and in cells. S-glutathionylated APE1 displayed significantly reduced AP endonuclease activity on abasic-site-containing oligonucleotide substrates, a result stemming from impaired DNA binding capacity. The combination of site-directed mutagenesis, biochemical assays, and mass spectrometric analysis identified Cys99 in human APE1 as the critical residue for the S-glutathionylation that leads to reduced AP endonuclease activity. This modification is reversible by reducing agents, which restore APE1 incision function. Our studies describe a novel posttranslational modification of APE1 that regulates the DNA repair function of the protein.

Variation in base excision repair capacity.

The major DNA repair pathway for coping with spontaneous forms of DNA damage, such as natural hydrolytic products or oxidative lesions, is base excision repair (BER). In particular, BER processes mutagenic and cytotoxic DNA lesions such as non-bulky base modifications, abasic sites, and a range of chemically distinct single-strand breaks. Defects in BER have been linked to cancer predisposition, neurodegenerative disorders, and immunodeficiency. Recent data indicate a large degree of sequence variability in DNA repair genes and several studies have associated BER gene polymorphisms with disease risk, including cancer of several sites. The intent of this review is to describe the range of BER capacity among individuals and the functional consequences of BER genetic variants. We also discuss studies that associate BER deficiency with disease risk and the current state of BER capacity measurement assays.

Complementary non-radioactive assays for investigation of human flap endonuclease 1 activity.

FEN1, a key participant in DNA replication and repair, is the major human flap endonuclease that recognizes and cleaves flap DNA structures. Deficiencies in FEN1 function or deletion of the fen1 gene have profound biological effects, including the suppression of repair of DNA damage incurred from the action of various genotoxic agents. Given the importance of FEN1 in resolving abnormal DNA structures, inhibitors of the enzyme carry a potential as enhancers of DNA-interactive anticancer drugs. To facilitate the studies of FEN1 activity and the search for novel inhibitors, we developed a pair of complementary-readout homogeneous assays utilizing fluorogenic donor/quencher and AlphaScreen chemiluminescence strategies. A previously reported FEN1 inhibitor 3-hydroxy-5-methyl-1-phenylthieno[2,3-d]pyrimidine-2,4(1H,3H)-dione displayed equal potency in the new assays, in agreement with its published IC(50). The assays were optimized to a low 4 µl volume and used to investigate a set of small molecules, leading to the identification of previously-unreported FEN1 inhibitors, among which aurintricarboxylic acid and NSC-13755 (an arylstibonic derivative) displayed submicromolar potency (average IC(50) of 0.59 and 0.93 µM, respectively). The availability of these simple complementary assays obviates the need for undesirable radiotracer-based assays and should facilitate efforts to develop novel inhibitors for this key biological target.

Functional capacity of XRCC1 protein variants identified in DNA repair-deficient Chinese hamster ovary cell lines and the human population.

XRCC1 operates as a scaffold protein in base excision repair, a pathway that copes with base and sugar damage in DNA. Studies using recombinant XRCC1 proteins revealed that: a C389Y substitution, responsible for the repair defects of the EM-C11 CHO cell line, caused protein instability; a V86R mutation abolished the interaction with POLbeta, but did not disrupt the interactions with PARP-1, LIG3alpha and PCNA; and an E98K substitution, identified in EM-C12, reduced protein integrity, marginally destabilized the POLbeta interaction, and slightly enhanced DNA binding. Two rare (P161L and Y576S) and two frequent (R194W and R399Q) amino acid population variants had little or no effect on XRCC1 protein stability or the interactions with POLbeta, PARP-1, LIG3alpha, PCNA or DNA. One common population variant (R280H) had no pronounced effect on the interactions with POLbeta, PARP-1, LIG3alpha and PCNA, but did reduce DNA-binding ability. When expressed in HeLa cells, the XRCC1 variants-excluding E98K, which was largely nucleolar, and C389Y, which exhibited reduced expression-exhibited normal nuclear distribution. Most of the protein variants, including the V86R POLbeta-interaction mutant, displayed normal relocalization kinetics to/from sites of laser-induced DNA damage: except for E98K and C389Y, and the polymorphic variant R280H, which exhibited a slightly shorter retention time at DNA breaks.

A pheromone mutant of Schizosaccharomyces pombe displays nucleolar fragmentation.

Stresses and nutritional starvation are two main external signals for the induction of sex pheromones in the fission yeast Schizosaccharomyces pombe. In an attempt to identify the components involved in transduction of starvation signals, we screened 135 temperature-sensitive (ts) mutants and isolated 6 mutants that induced the pheromone even in the presence of a nitrogen source. These mutants exhibited two distinct induction phenotypes: pheromone induction at restrictive but not at permissive temperatures; and pheromone induction at both permissive and restrictive temperatures. The times required for the maximum pheromone induction at the restrictive temperature differed slightly in each mutant. In addition to the pheromone induction phenotype, the ts243 and ts304 mutants exhibited cell-division-cycle defects. The ts304 mutant cells showed an abnormal cytoplasmic DAPI staining pattern. The nucleolus of this mutant seemed to be fragmented, a phenomenon which is typically observed in aged yeast cells. The result of our genetic analysis indicated that the pheromone induction mutants belonged to 6 separate complementation groups. We designated these mutants pws1 to pws6.

DNA damage levels and biochemical repair capacities associated with XRCC1 deficiency.

Base excision repair (BER) is the major corrective pathway for most spontaneous, oxidative, and alkylation DNA base and sugar damage. X-ray cross-complementing 1 (XRCC1) has been suggested to function at nearly every step of this repair process, primarily through direct protein-protein interactions. Using whole cell extract (WCE) repair assays and DNA damage measurement techniques, we examined systematically the quantitative contribution of XRCC1 to specific biochemical steps of BER and single-strand break repair (SSBR). Our studies reveal that XRCC1-deficient Chinese hamster ovary WCEs exhibit normal base excision activity for 8-oxoguanine (8-OH-dG), 5-hydroxycytosine, ethenoadenine, and uracil lesions. Moreover, XRCC1 mutant EM9 cells possess steady-state levels of endogenous 8-OH-dG base damage similar to those of their wild-type counterparts. Abasic site incision activity was found to be normal in XRCC1-deficient cell extracts, as were the levels of abasic sites in isolated chromosomal DNA from mutant cells. While one- and five-nucleotide gap filling was not affected by XRCC1 status, a significant approximately 2-4-fold reduction in nick ligation activity was observed in EM9 WCEs. Our results herein suggest that the primary biochemical defect associated with XRCC1 deficiency is in the ligation step of BER/SSBR, and that XRCC1 plays no significant role in endogenous base damage and abasic site repair, or in promoting the polymerase gap-filling step.

Examination of the requirement for ucp-4, a putative homolog of mammalian uncoupling proteins, for stress tolerance and longevity in C. elegans.

Reactive oxygen species (ROS) are generated by mitochondrial respiration and can react with and damage cellular components. According to the free radical theory of aging, oxidative damage from mitochondrial ROS is a major cause of cellular decline during aging. Mitochondrial uncoupling proteins (UCPs) uncouple ATP production from electron transport and can be stimulated by free radicals, suggesting UCPs may perform a cytoprotective function. The nematode, Caenorhabditis elegans, contains one UCP-like protein, encoded by the ucp-4 gene. We have investigated the genetic requirement for ucp-4 in normal aging and stress resistance. Consistent with the hypothesis that ucp-4 encodes a putative uncoupling protein, animals lacking ucp-4 function contained elevated ATP levels. However, the absence of ucp-4 function did not affect adult lifespan or survival in the presence of thermal or oxidative stress. Together, these results demonstrate that ucp-4 is a negative regulator of ATP production in C. elegans, but is not required for normal lifespan.

Transcription of Schizosaccharomyces pombe thioltransferase-1 in response to stress conditions.

Thioltransferase, also known as glutaredoxin, is an enzyme that catalyzes the reduction of a variety of disulfide compounds. In Schizosaccharomyces pombe, two thioltransferases were reported and the cDNA of one of the thioltransferases (thioltransferase-1) was cloned. Using a Northern blot assay, we investigated the thioltransferase transcription in response to various stress conditions. When the culture was shifted to a high temperature, the thioltransferase transcription was not significantly changed compared to the unshifted 30 degrees culture. Treatment of zinc chloride to exponentially-growing cells remarkably increased the thioltransferase transcription, whereas the treatment of mercury chloride greatly reduced the transcription. Treatment of hydrogen peroxide and cadmium chloride caused no significant effects on the transcription of the thioltransferase. These results suggest that the transcription of thioltransferase-1 in S. pombe is induced in response to metal stress that is caused by zinc chloride, but not in response to heat stress or oxidative stress that is caused by hydrogen peroxide.

Characterization and regulation of a second gene encoding thioredoxin from the fission yeast.

A genomic DNA encoding a second thioredoxin (TRX2) was isolated from the chromosomal DNA of the fission yeast Schizosaccharomyces pombe. The cloned sequence contains 1823 bp and encodes a protein of 121 amino acids. It has extra N-terminal 17 amino acid residues compared to previously identified thioredoxin (TRX1), which are positively charged and hydrophobic amino acids. The additional N-terminal region contains a plausible prepeptidase cleavage site, indicating that the TRX2 protein exists in mitochondria. The cloned TRX2 gene produced functional TRX estimated with insulin reduction assay. The upstream region of the TRX2 gene was fused into the promoterless beta-galactosidase gene of the shuttle vector YEp357R. The 782 bp sequence in the region further upstream of the TRX2 gene was found to be inhibitory in its expression. Synthesis of beta-galactosidase from the fusion plasmid pYFX135-HRL was enhanced by the addition of aluminum chloride and ferrous chloride, indicating that the TRX2 protein is involved in stress response.

Pap1-mediated regulation of thioredoxin gene from Schizosaccharomyces pombe.

The genomic DNA encoding thioredoxin (TRX) was previously isolated from the fission yeast Schizosaccharomyces pombe. In this investigation, regulation of the S. pombe TRX gene was studied in lacZ translational fusions. The synthesis of beta-galactosidase from the fusion plasmid pYKT24 was significantly enhanced by treatments with cadmium chloride, zinc chloride, and high temperatures. Synthesis of beta-galactosidase from the fusion plasmid was significantly decreased by higher concentrations (5 microM, 10 microM) of mercuric chloride, whereas it was enhanced by its lower concentration (1 microM). Diamide affected the synthesis of beta-galactosidase in the same manner with mercuric chloride. However, high osmolarity had no effect on the beta-galactosidase synthesis from the fusion plasmid pYKT24. Various fusion plasmids were constructed to carry serially deleted upstream regions of the TRX gene. Pap1 mediates the regulation of the S. pombe TRX gene. The upstream region, between 987 and 1,270 bp from the translational initiation point, is responsible for the regulation.

The 5'-flanking AT-rich sequence of thioredoxin gene involved in high-frequency transformation of the fission yeast.

During the cloning of a genomic DNA encoding mitochondrial thioredoxin (TRX) from the fission yeast Schizosaccharomyces pombe, its 5' flanking sequence was involved in the high-frequency of transformation. The recombinant plasmid pYEX that was constructed in the 2 mu plasmid-derived vector pYES2 gave rise to a significant high-frequency of transformation in S. pombe, compared to the vector alone. Plasmid pYEX contains 1,090 bp 5'-flanking sequences of the TRX gene that are ahead of the open-reading frame. Similar 5'-flanking sequences, which were inserted in the lacZ fusion vector YEp357R that contained the 2 mu origin of replication, also gave a high-frequency of transformation. Dissection of the 5'-flanking sequence of the TRX gene by the HindIII restriction site showed that the 782 bp flanking sequence (5' upstream of the HindIII site) was responsible for the high-frequency of transformation by the 2 mu plasmid-derived vector DNAs. The putative sequence that is involved in the high-frequency of transformation contains a very high ratio of A-T pairs. No known functions were assigned on the sequence, which was estimated from the GenBank database.

A potential membrane protein involved in pre-tRNA splicing of Schizosaccharomyces pombe.

We had previously isolated six pre-tRNA splicing mutants of Schizosaccharomyces pombe named ptp1 to ptp6. To investigate the molecular mechanism of tRNA splicing, we cloned the ptp4(+) gene by complementation of the temperature-sensitive growth defect. The ptp4(+) gene consists of three exons and encodes a putative protein of 218 amino acids with a molecular mass of 24.4 kDa. Analysis of the amino acid sequence reveals that the protein is a potential membrane protein with four membrane-spanning regions. The ptp4(+) shows significant similarity to the Saccharomyces cerevisiae putative protein YOR311C. Expression of the ptp4(+) gene in the ptp4(-) mutant restores the ability to splice tRNA. Northern blot analysis showed that the ptp4(+) gene is expressed in both mating-type cells of S. pombe. These results suggest that the Ptp4(+) could be a component involved in tRNA splicing.