ATR/ATM-mediated phosphorylation of human Rad17 is required for genotoxic stress responses

Genotoxic stress triggers the activation of checkpoints that delay cell-cycle progression to allow for DNA repair. Studies in fission yeast implicate members of the Rad family of checkpoint proteins, which includes Rad17, Rad1, Rad9 and Hus1, as key early-response elements during the activation of both the DNA damage and replication checkpoints. Here we demonstrate a direct regulatory linkage between the human Rad17 homologue (hRad17) and the checkpoint kinases, ATM and ATR. Treatment of human cells with genotoxic agents induced ATM/ATR-dependent phosphorylation of hRad17 at Ser 635 and Ser 645. Overexpression of a hRad17 mutant (hRad17AA) bearing Ala substitutions at both phosphorylation sites abrogated the DNA-damage-induced G2 checkpoint, and sensitized human fibroblasts to genotoxic stress. In contrast to wild-type hRad17, the hRad17AA mutant showed no ionizing-radiation-inducible association with hRad1, a component of the hRad1-hRad9-hHus1 checkpoint complex. These findings demonstrate that ATR/ATM-dependent phosphorylation of hRad17 is a critical early event during checkpoint signalling in DNA-damaged cells.

Functional interactions between BRCA1 and the checkpoint kinase ATR during genotoxic stress

The BRCA1 gene encodes a tumor suppressor that is mutated in 50% of familial breast cancers. The BRCA1 protein has been implicated in the DNA damage response, as DNA damage induces the phosphorylation of BRCA1 and causes its recruitment into nuclear foci that contain DNA repair proteins. The ataxia-telangiectasia-mutated (ATM) gene product controls overall BRCA1 phosphorylation in response to gamma-irradiation (IR). In this study, we show that BRCA1 phosphorylation is only partially ATM dependent in response to IR and ATM independent in response to treatment with UV light, or the DNA replication inhibitors hydroxyurea (HU) and aphidicolin (APH). We provide evidence that the kinase responsible for this phosphorylation is the ATM-related kinase, ATR. ATR phosphorylates BRCA1 on six Ser/Thr residues, including Ser 1423, in vitro. Increased expression of ATR enhanced the phosphorylation of BRCA1 on Ser 1423 following cellular exposure to HU or UV light, whereas doxycycline-induced expression of a kinase-inactive ATR mutant protein inhibited HU- or UV light-induced Ser 1423 phosphorylation in GM847 fibroblasts, and partially suppressed the phosphorylation of this site in response to IR. Thus, ATR, like ATM, controls BRCA1 phosphorylation in vivo. Although ATR isolated from DNA-damaged cells does not show enhanced kinase activity in vitro, we found that ATR responds to DNA damage and replication blocks by forming distinct nuclear foci at the sites of stalled replication forks. Furthermore, ATR nuclear foci overlap with the nuclear foci formed by BRCA1. The dramatic relocalization of ATR in response to DNA damage points to a possible mechanism for its ability to enhance the phosphorylation of substrates in response to DNA damage. Together, these results demonstrate that ATR and BRCA1 are components of the same genotoxic stress-responsive pathway, and that ATR directly phosphorylates BRCA1 in response to damaged DNA or stalled DNA replication.

Inhibition of ATM and ATR kinase activities by the radiosensitizing agent, caffeine

Caffeine exposure sensitizes tumor cells to ionizing radiation and other genotoxic agents. The radiosensitizing effects of caffeine are associated with the disruption of multiple DNA damage-responsive cell cycle checkpoints. The similarity of these checkpoint defects to those seen in ataxia-telangiectasia (A-T) suggested that caffeine might inhibit one or more components in an A-T mutated (ATM)-dependent checkpoint pathway in DNA-damaged cells. We now show that caffeine inhibits the catalytic activity of both ATM and the related kinase, ATM and Rad3-related (ATR), at drug concentrations similar to those that induce radiosensitization. Moreover, like ATM-deficient cells, caffeine-treated A549 lung carcinoma cells irradiated in G2 fail to arrest progression into mitosis, and S-phase-irradiated cells exhibit radioresistant DNA synthesis. Similar concentrations of caffeine also inhibit gamma- and UV radiation-induced phosphorylation of p53 on Ser15, a modification that may be directly mediated by the ATM and ATR kinases. DNA-dependent protein kinase, another ATM-related protein involved in DNA damage repair, was resistant to the inhibitory effects of caffeine. Likewise, the catalytic activity of the G2 checkpoint kinase, hChk1, was only marginally suppressed by caffeine but was inhibited potently by the structurally distinct radiosensitizer, UCN-01. These data suggest that the radiosensitizing effects of caffeine are related to inhibition of the protein kinase activities of ATM and ATR and that both proteins are relevant targets for the development of novel anticancer agents.

A role for ATR in the DNA damage-induced phosphorylation of p53

Phosphorylation at Ser-15 may be a critical event in the up-regulation and functional activation of p53 during cellular stress. In this report we provide evidence that the ATM-Rad3-related protein ATR regulates phosphorylation of Ser-15 in DNA-damaged cells. Overexpression of catalytically inactive ATR (ATRki) in human fibroblasts inhibited Ser-15 phosphorylation in response to gamma-irradiation and UV light. In gamma-irradiated cells, ATRki expression selectively interfered with late-phase Ser-15 phosphorylation, whereas ATRki blocked UV-induced Ser-15 phosphorylation in a time-independent manner. ATR phosphorylated p53 at Ser-15 and Ser-37 in vitro, suggesting that p53 is a target for phosphorylation by ATR in DNA-damaged cells.

Inhibition of phosphoinositide 3-kinase related kinases by the radiosensitizing agent wortmannin

Members of the phosphatidylinositol-3 kinase related kinase (PIKK) family function in both cell cycle progression and DNA damage-induced cell cycle checkpoints. The fungal metabolite, wortmannin, is an effective radiosensitizer that irreversibly inhibits certain members of the PIKK family. Based on their roles in DNA damage responses, several PIKKs, DNA-dependent protein kinase (DNA-PK), ataxia telangiectasia mutated (ATM) and the ataxia- and Rad3-related protein (ATR), are potential targets for the radiosensitizing effect of wortmannin. In this report, we demonstrate that wortmannin is a relatively potent inhibitor of DNA-PK (IC50, 16 nM) and ATM (IC50, 150 nM) activities, whereas ATR activity is significantly less sensitive to this drug (IC50, 1.8 microM). In intact A549 lung adenocarcinoma cells, wortmannin inhibited both DNA-PK and ATM at concentrations that correlated closely with those required for radiosensitization. Furthermore, pretreatment of A549 cells with wortmannin resulted in radioresistant DNA synthesis, a characteristic abnormality of ATM-deficient cells. These results identify wortmannin as an inhibitor of ATM activity and suggest that ATM and DNA-PK are relevant targets for the radiosensitizing effect of this drug in cancer cells.

TcDJ1, a putative mitochondrial DnaJ protein in Trypanosoma cruzi

A full length cDNA encoding a novel Trypanosoma cruzi DnaJ protein was cloned and characterized. The 324 amino acid protein encoded by the cDNA (TcDJ1) displays a characteristics J-domain, but lacks the Gly-Phe and zinc finger regions present in some other DnaJ proteins. Relative to four other T. cruzi DnaJ proteins, TcDJ1 has an amino terminal extension containing basic and hydroxylated resides characteristic of mitochondrial import peptides. A T. cruzi transfectant expressing epitope-tagged TcDJ1 was generated and subcellular fractions were produced. Western blot analysis revealed that the protein has a molecular mass of 29 kDa and is found in the mitochondrial fraction. The expression of TcDJ1 is developmentally regulated since the levels of both mRNA and protein are much higher in epimastigotes (replicative form) than in metacyclic trypomastigotes (infective form). Thus it may participate in mitochondrial biosynthetic processes in this organism.

The DnaJ family of protein chaperones in Trypanosoma cruzi

We have molecularly cloned four members of the DnaJ (heat shock protein 40) family of protein chaperones of the protozoan parasite Trypanosoma cruzi--tcj1, tcj2, tcj3 and tcj4. While all the proteins contain defining J domains at their N-termini, only tcj2, tcj3 and tcj4 contain glycine/phenylalanine-rich and zinc finger domains common to many other DnaJ homologues. Furthermore, tcj2 and tcj4 contain C-terminal CaaX motifs, substrates for prenyl modifications, suggesting that they are associated with cellular membranes. tcj1 is a divergent member of the family, containing neither glycine/phenylalanine-rich nor zinc finger domains. All the T. cruzi DnaJ genes are single copy, in contrast to other T. cruzi heat shock genes, which are arranged in multicopy direct tandem arrays. Among the tcj mRNAs, only tcj2 is heat inducible, which may result from posttranscriptional regulation involving a sequence found in the 3' untranslated regions of all heat-inducible T. cruzi mRNAs described to date. Further study of this important family of protein chaperones will aid our understanding of the protein folding and assembly processes in protozoans.

Utility of recombinant flagellar calcium-binding protein for serodiagnosis of Trypanosoma cruzi infection

The protozoan Trypanosoma cruzi is the causative agent of Chagas' disease, a major public health problem in Latin America and of growing concern in the United States as the number of infected immigrants increases. There is currently no testing of U.S. blood products for T. cruzi infection, and the best tests available, although highly sensitive, are not of high enough specificity to be useful for widespread screening of the blood supply in this country. Among the parasite antigens detected by sera of infected humans and mice, those in the range of 24 to 26 kDa are particularly reactive. With an aim of developing a sensitive, specific, recombinant antigen-based serologic test for T. cruzi infection, we used two antibody reagents specific for these 24- to 26-kDa antigens to isolate cDNA clones from a T. cruzi expression library. One clone was found to encode a previously characterized T. cruzi antigen, a 24-kDa flagellar calcium-binding protein (FCaBP). Recombinant FCaBP was found to be a sensitive, specific reagent for distinguishing T. cruzi-infected individuals from uninfected persons, and it therefore could potentially be used for screening purposes, especially if combined with other recombinant T. cruzi antigens that have similarly high degrees of diagnostic sensitivity and specificity.

Molecular cloning and characterization of the 78-kilodalton glucose-regulated protein of Trypanosoma cruzi

The protozoan Trypanosoma cruzi is the etiologic agent of Chagas' disease, an illness responsible for morbidity and death among millions of Latin Americans. Mice also develop this disease when infected with T. cruzi and are a useful model organism for the study of parasite-specific immune responses. To identify immunogenic T. cruzi antigens, serum from an infected mouse was used to isolate clones from a T. cruzi epimastigote cDNA expression library. One of these clones was found to encode the 78-kDa glucose-regulated protein (grp78), the endoplasmic reticular member of the 70-kDa heat shock protein (hsp70) family. Like the mammalian and yeast grp78s, the T. cruzi protein contains an endoplasmic reticular leader peptide and a carboxyl-terminal endoplasmic reticular retention sequence. T. cruzi grp78 is encoded by a tandemly arranged family of three genes located on a chromosome of 1.6 Mb. The effects on grp78 expression of heat shock and tunicamycin treatment, the latter of which specifically stimulates mammalian grp78, were investigated. While the level of the grp78 protein remained constant under all circumstances, grp78 mRNA was unaffected by heat shock but induced fivefold by tunicamycin. Finally, we found that grp78 is the most immunogenic of the T. cruzi heat shock proteins we have characterized, reacting strongly in immunoblots with sera from infected mice.

Cardiac antigen-specific autoantibody production is associated with cardiomyopathy in Trypanosoma cruzi-infected mice

An inflammatory cardiomyopathy may develop in humans and experimental animals with chronic Trypanosoma cruzi infection (Chagas' disease). Among the possible mechanisms involved in the pathogenesis of Chagas' cardiomyopathy, induction of heart-specific autoimmune responses has recently received substantial experimental support. The goal of the current study was to determine whether cardiac Ag-specific antibodies are produced in T. cruzi-infected mice with heart disease and, if so, to determine their Ag specificities. Upon infection with the Brazil strain of T. cruzi, C57BL/6 mice develop a cardiomyopathy that is histologically similar to that observed in chronically infected humans. Antisera from these mice were found to react with three cardiac Ag, having relative molecular masses of 200, 150, and 53 kDa. p200 and p150 are specifically found in heart muscle, although p53 is found in skeletal muscle as well. C57BL/6 mice infected with the Guayas strain of T. cruzi, which do not develop cardiomyopathy, did not produce antibodies to p200, p150, or p53, indicating that these antibodies may be specific markers of cardiomyopathy. Finally, p200 and p53 were identified as the contractile protein myosin and the intermediate filament protein desmin, respectively. This last finding is of special interest, because antibodies specific for myosin or desmin have been detected in humans and experimental animals with other natural and experimental cardiomyopathies. This suggests that infection with particular strains of T. cruzi may lead to the development of a cardiac Ag-specific autoimmune disease, possibly involving one or more of the Ag identified in this study.

Cardiac antigen-specific autoantibody production is associated with cardiomyopathy in Trypanosoma cruzi-infected mice

An inflammatory cardiomyopathy may develop in humans and experimental animals with chronic Trypanosoma cruzi infection (Chagas' disease). Among the possible mechanisms involved in the pathogenesis of Chagas' cardiomyopathy, induction of heart-specific autoimmune responses has recently received substantial experimental support. The goal of the current study was to determine whether cardiac Ag-specific antibodies are produced in T. cruzi-infected mice with heart disease and, if so, to determine their Ag specificities. Upon infection with the Brazil strain of T. cruzi, C57BL/6 mice develop a cardiomyopathy that is histologically similar to that observed in chronically infected humans. Antisera from these mice were found to react with three cardiac Ag, having relative molecular masses of 200, 150, and 53 kDa. p200 and p150 are specifically found in heart muscle, although p53 is found in skeletal muscle as well. C57BL/6 mice infected with the Guayas strain of T. cruzi, which do not develop cardiomyopathy, did not produce antibodies to p200, p150, or p53, indicating that these antibodies may be specific markers of cardiomyopathy. Finally, p200 and p53 were identified as the contractile protein myosin and the intermediate filament protein desmin, respectively. This last finding is of special interest, because antibodies specific for myosin or desmin have been detected in humans and experimental animals with other natural and experimental cardiomyopathies. This suggests that infection with particular strains of T. cruzi may lead to the development of a cardiac Ag-specific autoimmune disease, possibly involving one or more of the Ag identified in this study.