The effects of selenium and the GPx-1 selenoprotein on the phosphorylation of H2AX

Background

Significant data supports the health benefits of selenium although supplementation trials have yielded mixed results. GPx-1, whose levels are responsive to selenium availability, is implicated in cancer etiology by human genetic data. Selenium's ability to alter the phosphorylation of the H2AX, a histone protein that functions in the reduction of DNA damage by recruiting repair proteins to the damage site, following exposure to ionizing radiation and bleomycin was investigated.

Methods

Human cell lines that were either exposed to selenium or were transfected with a GPx-1 expression construct were exposed to ionizing radiation or bleomycin. Phosphorylation of histone H2AX was quantified by flow cytometry and survival by the MTT assay. Phosphorylation of the Chk1 and Chk2 checkpoint proteins was quantified by western blotting.

Results

In colon-derived cells, selenium increases GPx-1 and attenuated H2AX phosphorylation following genotoxic exposures while the viability of these cells was unaffected. MCF-7 cells and transfectants that express high GPx-1 levels were exposed to ionizing radiation and bleomycin, and H2AX phosphorylation and cell viability were assessed. GPx-1 increased H2AX phosphorylation and viability following the induction of DNA damage while enhancing the levels of activated Chk1 and Chk2.

Conclusions

Exposure of mammalian cells to selenium can alter the DNA damage response and do so by mechanisms that are dependent and independent of its effect on GPx-1.

General significance

Selenium and GPx-1 may stimulate the repair of genotoxic DNA damage and this may account for some of the benefits attributed to selenium intake and elevated GPx-1 activity.     

GPx-1 modulates Akt and P70S6K phosphorylation and Gadd45 levels in MCF-7 cells

Selenium has been shown to prevent cancer in animal models, and recent data indicate it is likely to be effective in humans as well. One selenium-containing protein, the cytoplasmic form of glutathione peroxidase (GPx-1), has been implicated in cancer risk and development by genetic studies identifying at-risk alleles and loss of heterozygosity in tumors. In order to evaluate the biological consequences of GPx-1 overexpression, human MCF-7 cells were stably transfected with a GPx-1 expression construct and the effects of GPx-1 on protein kinases associated with stress responses were determined. GPx-1 overexpression affected phosphorylation of p70S6K, whereas Erk1/2 and p38 MAPK were not affected. Site-specific phosphorylation of Akt declined and the levels of Gadd45, a DNA damage response protein, increased significantly as a consequence of elevated GPx-1 expression. Effects on p70S6K and Gadd45 after selenium supplementation have been reported, and given previous data demonstrating a role for GPx-1 in cancer etiology, these results support the concept that the chemopreventive properties of selenium may be due, at least in part, to its role in regulating GPx-1.     

The oncogenic phosphatase WIP1 negatively regulates nucleotide excision repair

Nucleotide excision repair (NER) is the only mechanism in humans to repair UV-induced DNA lesions such as pyrimidine (6-4) pyrimidone photoproducts and cyclobutane pyrimidine dimers (CPDs). In response to UV damage, the ataxia telangiectasia mutated and Rad3-related (ATR) kinase phosphorylates and activates several downstream effector proteins, such as p53 and XPA, to arrest cell cycle progression, stimulate DNA repair, or initiate apoptosis. However, following the completion of DNA repair, there must be active mechanisms that restore the cell to a prestressed homeostatic state. An important part of this recovery must include a process to reduce p53 and NER activity as well as to remove repair protein complexes from the DNA damage sites. Since activation of the damage response occurs in part through phosphorylation, phosphatases are obvious candidates as homeostatic regulators of the DNA damage and repair responses. Therefore, we investigated whether the serine/threonine wild-type p53-induced phosphatase 1 (WIP1/PPM1D) might regulate NER. WIP1 overexpression inhibits the kinetics of NER and CPD repair, whereas WIP1 depletion enhances NER kinetics and CPD repair. This NER suppression is dependent on WIP1 phosphatase activity, as phosphatase-dead WIP1 mutants failed to inhibit NER. Moreover, WIP1 suppresses the kinetics of UV-induced damage repair largely through effects on NER, as XPD-deficient cells are not further suppressed in repairing UV damage by overexpressed WIP1. Wip1 null mice quickly repair their CPD and undergo less UV-induced apoptosis than their wild-type counterparts. In vitro phosphatase assays identify XPA and XPC as two potential WIP1 targets in the NER pathway. Thus WIP1 may suppress NER kinetics by dephosphorylating and inactivating XPA and XPC and other NER proteins and regulators after UV-induced DNA damage is repaired.     

A UV-resistant mutant without an increased repair synthesis activity, established from a UV-sensitive human clonal cell line

When cells of a human clonal cell line, RSa, with high sensitivity to UV lethality, were treated with the mutagen, ethyl methanesulfonate, a variant cell strain, UVr-1, was established as a mutant resistant to 254-nm far-ultraviolet radiation (UV). Cell proliferation studies showed that UVr-1 cells survived and actively proliferated at doses of UV-irradiation that greatly suppressed the proliferation of RSa cells. Colony-formation assays also confirmed the increased resistance of UVr-1 cells to UV. The recovery from a UV-induced inhibition in DNA synthesis, as [methyl-3H]thymidine uptake into cellular DNA, was more pronounced in UVr-1 cells than in RSa cells. Nevertheless, there was no significant difference in the activity of UV-induced DNA repair synthesis in either cell line, as estimated by the extent of unscheduled DNA synthesis and DNA repair replication. UVr-1 cells were also more refractory to 4-nitroquinoline 1-oxide (4NQO), but the activity of DNA repair synthesis induced by 4NQO in UVr-1 cells was much the same as in the RSa cells. Both N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) sensitivity and MNNG-induced DNA repair synthesis activity in UVr-1 cells were similar to that of RSa cells. These characteristics of UVr-1 cells are discussed in the light of a previously reported UV-resistant variant, UVr-10, which had an increased DNA repair synthesis activity.     

PUVA-induced apoptosis involves mitochondrial dysfunction caused by the opening of the permeability transition pore

Damaged DNA-binding activity comprises two major protein components, DDB1 and DDB2, which are implicated in the repair of ultraviolet (UV) radiation-induced DNA damage. The possible role of DDB2 as a determinant of cellular sensitivity to UV was investigated. The abundance of DDB2 in UV-resistant HeLa cell lines was increased compared with that in the parental UV-sensitive cells. Stable transfection of the resistant cells with DDB2 antisense cDNA resulted in marked depletion of DDB2 protein and restored cellular sensitivity to UV-induced apoptosis. Whereas the extent of UV-induced activation of apoptosis executioners, including DNA fragmentation factor, and caspase-3 were reduced in the UV-resistant cells compared with those apparent in the sensitive cells, depletion of DDB2 from the resistant cells restored the normal activation patterns for these proteins. In contrast, overexpressing DDB2 in DDB2-depleted cells with recombinant adenovirus, which carries ddb2 cDNA, markedly inhibited the extent of UV-induced activation of DNA fragmentation factor, and caspase-3. Interestingly, a mutated form of DDB2, which is defective in interacting with DDB1 and binding to UV-damaged DNA, also markedly inhibited the activation of apoptosis executioners. These results indicate that DDB2 is a modulator of UV-induced apoptosis, and that UV resistance can be overcome by inhibition of DDB2. The findings also suggest that modulation of UV-induced apoptosis by DDB2 may be independent of DNA repair.     

IL-18 reduces ultraviolet radiation-induced DNA damage and thereby affects photoimmunosuppression

UV-induced DNA damage has been recognized as the major molecular trigger for photoimmunosuppression. IL-12 prevents UV-induced immunosuppression via its recently discovered capacity to reduce DNA damage presumably via induction of DNA repair. Because IL-18 shares some biological activities with IL-12 we studied the effect of IL-18 on UV-induced DNA damage and immunosuppression. IL-18 reduced UV-induced apoptosis of keratinocytes and supported long-term cell survival on UV exposure. Injection of IL-18 into mice that were exposed to UV radiation significantly lowered the number of apoptotic keratinocytes. Accordingly, radiation immunohistochemistry revealed reduced amounts of DNA damage in epidermal cells upon injection of IL-18. These effects were not observed in DNA repair-deficient (XpaKO) mice, indicating that IL-18 like IL-12 reduces DNA damage via DNA repair. UV-mediated suppression of the induction of contact hypersensitivity, which is known to be primarily triggered by DNA damage, was prevented upon injection of IL-18 before UV exposure in wild-type but not in XpaKO mice. In contrast to IL-12, IL-18 was not able either in wild-type or in XpaKO mice to break UV-induced immunotolerance that is mediated via regulatory T cells rather than in a DNA damage-dependent fashion. This result indicates that IL-12 is still unique in its capacity to restore immune responses because of its effect on regulatory T cells. Together, these data identify IL-18 as a further cytokine that exhibits the capacity to affect DNA repair. Though being primarily a proinflammatory cytokine through this capacity, IL-18 can also foster an immune response that is suppressed by UV radiation.     

Interaction between UV-damaged DNA binding activity proteins and the c-Abl tyrosine kinase

The c-Abl tyrosine kinase is activated by some forms of DNA damage, including ionizing radiation, but not UV radiation. The functions of this activation in the damage response pathways remain obscure. To identify potential targets of c-Abl kinase, we utilized the yeast two-hybrid system to screen a murine cDNA library. One c-Abl binding protein of particular interest was the large subunit (DDB1) of the heterodimeric complex with UV-damaged DNA binding activity (UV-DDB). This complex binds with high specificity to DNA damaged by UV, is absent in a subset of xeroderma pigmentosum group E cells, and is required for global genomic repair of UV-induced damage. The association of c-Abl with DDB1 required the kinase domain of c-Abl and preserved the interaction between DDB1 and the small subunit (DDB2) of the UV-DDB complex. Significantly, overexpression of c-Abl increased tyrosine phosphorylation of DDB2 and suppressed UV-DDB activity. Conversely, a dominant negative, kinase-deficient allele of c-Abl decreased tyrosine phosphorylation of DDB2 and dramatically stimulated UV-DDB activity. These results suggest that one role of c-Abl may be to negatively regulate UV-DDB activity by phosphorylation of DDB2.     

The novel gene LRP15 is regulated by DNA methylation and confers increased efficiency of DNA repair of ultraviolet-induced DNA damage

LRP15 is a novel gene cloned from lymphocytic cells, and its function is still unknown. Bioinformatic data showed that LRP15 might be regulated by DNA methylation and had an important role in DNA repair. In this study, we investigate whether the expression of LRP15 is regulated by DNA methylation, and whether overexpression of LRP15 increases efficiency of DNA repair of UV-induced DNA damage in HeLa cells. The results showed (1) the promoter of LRP15 was hypermethylated in HeLa cells, resulting a silence of its expression. Gene expression was restored by a demethylating agent, 5-aza-2'-deoxycytidine, but not by a histone deacetylase inhibitor, trichostatin A; (2) overexpression of LRP15 inhibited HeLa cell proliferation, and the numbers of cells in the G2/M phase of the cell cycle in cells transfected with LRP15 increased about 10% compared with controls; (3) cyclin B1 level was much lower in cells overexpressing LRP15 than in control cells; and (4) after exposure to UV radiation, the LRP15-positive cells showed shorter comet tails compared with the LRP15-negative cells. From these results we conclude that the expression of LRP15 is controlled by methylation in its promoter in HeLa cells, and LRP15 confers resistance to UV damage and accelerates the DNA repair rate.     

Role and regulation of p53 during an ultraviolet radiation-induced G1 cell cycle arrest.

p53 can play a key role in response to DNA damage by activating a G1 cell cycle arrest. However, the importance of p53 in the cell cycle response to UV radiation is unclear. In this study, we used normal and repair-deficient cells to examine the role and regulation of p53 in response to UV radiation. A dose-dependent G1 arrest was observed in normal and repair-deficient cells exposed to UV. Expression of HPV16-E6, or a dominant-negative p53 mutant that inactivates wildtype p53, caused cells to become resistant to this UV-induced G1 arrest. However, a G1 to S-phase delay was still observed after UV treatment of cells in which p53 was inactivated. These results indicate that UV can inhibit G1 to S-phase progression through p53-dependent and independent mechanisms. Cells deficient in the repair of UV-induced DNA damage were more susceptible to a G1 arrest after UV treatment than cells with normal repair capacity. Moreover, no G1 arrest was observed in cells that had completed DNA repair prior to monitoring their movement from G1 into S-phase. Finally, p53 was stabilized under conditions of a UV-induced G1 arrest and unstable when cells had completed DNA repair and progressed from G1 into S-phase. These results suggest that unrepaired DNA damage is the signal for the stabilization of p53, and a subsequent G1 phase cell cycle arrest in UV-irradiated cells.     

Inhibition of nucleotide excision repair by high mobility group protein HMGA1

The mammalian non-histone "high mobility group" A (HMGA) proteins are the primary nuclear proteins that bind to the minor groove of AT-rich DNA. They may, therefore, influence the formation and/or repair of DNA lesions that occur in AT-rich DNA, such as cyclobutane pyrimidine dimers (CPDs) induced by UV radiation. Employing both stably transfected lines of human MCF7 cells containing tetracycline-regulated HMGA1 transgenes and primary Hs578T tumor cells, which naturally overexpress HMGA1 proteins, we have shown that cells overexpressing HMGA1a protein exhibit increased UV sensitivity. Moreover, we demonstrated that knockdown of intracellular HMGA1 concentrations via two independent methods abrogated this sensitivity. Most significantly, we observed that HMGA1a overexpression inhibited global genomic nucleotide excision repair of UV-induced CPD lesions in MCF-7 cells. Consistent with these findings in intact cells, DNA repair experiments employing Xenopus oocyte nuclear extracts and lesion-containing DNA substrates demonstrated that binding of HMGA1a markedly inhibits removal of CPDs in vitro. Furthermore, UV "photo-foot-printing" demonstrated that CPD formation within a long run of Ts (T(18)-tract) in a DNA substrate changes significantly when HMGA1 is bound prior to UV irradiation. Together, these results suggest that HMGA1 directly influences both the formation and repair of UV-induced DNA lesions in intact cells. These findings have important implications for the role that HMGA protein overexpression might play in the accumulation of mutations and genomic instabilities associated with many types of human cancers.     

Ataxia telangiectasia mutated (ATM) kinase and ATM and Rad3 related kinase mediate phosphorylation of Brca1 at distinct and overlapping sites. In vivo assessment using phospho-specific antibodies

Recent studies have provided evidence that breast cancer susceptibility gene products (Brca1 and Brca2) suppress cancer, at least in part, by participating in DNA damage signaling and DNA repair. Brca1 is hyperphosphorylated in response to DNA damage and co-localizes with Rad51, a protein involved in homologous-recombination, and Nbs1.Mre11.Rad50, a complex required for both homologous-recombination and nonhomologous end joining repair of damaged DNA. Here, we report that there is a qualitative difference in the phosphorylation states of Brca1 between ionizing radiation (IR) and UV radiation. Brca1 is phosphorylated at Ser-1423 and Ser-1524 after IR and UV; however, Ser-1387 is specifically phosphorylated after IR, and Ser-1457 is predominantly phosphorylated after UV. These results suggest that different types of DNA-damaging agents might signal to Brca1 in different ways. We also provide evidence that the rapid phosphorylation of Brca1 at Ser-1423 and Ser-1524 after IR (but not after UV) is largely ataxia telangiectasia mutated (ATM) kinase-dependent. The overexpression of catalytically inactive ATM and Rad3 related (ATR) kinase inhibited the UV-induced phosphorylation of Brca1 at these sites, indicating that ATR controls Brca1 phosphorylation in vivo after the exposure of cells to UV light. Moreover, ATR associates with Brca1; ATR and Brca1 foci co-localize both in cells synchronized in S phase and after exposure of cells to DNA-damaging agents. ATR can itself phosphorylate the region of Brca1 phosphorylated by ATM (Ser-Gln cluster in the C terminus of Brca1, amino acids 1241-1530). However, there are additional uncharacterized ATR phosphorylation site(s) between residues 521 and 757 of Brca1. Taken together, our results support a model in which ATM and ATR act in parallel but somewhat overlapping pathways of DNA damage signaling but respond primarily to different types of DNA lesion.     

Ultraviolet radiation-induced interleukin 6 release in HeLa cells is mediated via membrane events in a DNA damage-independent way

Evidence exists that ultraviolet radiation (UV) affects molecular targets in the nucleus or at the cell membrane. UV-induced apoptosis was found to be mediated via DNA damage and activation of death receptors, suggesting that nuclear and membrane effects are not mutually exclusive. To determine whether participation of nuclear and membrane components is also essential for other UV responses, we studied the induction of interleukin-6 (IL-6) by UV. Exposing HeLa cells to UV at 4 degrees C, which inhibits activation of surface receptors, almost completely prevented IL-6 release. Enhanced repair of UV-mediated DNA damage by addition of the DNA repair enzyme photolyase did not affect UV-induced IL-6 production, suggesting that in this case membrane events predominant over nuclear effects. UV-induced IL-6 release is mediated via NFkappaB since the NFkappaB inhibitor MG132 or transfection of cells with a super-repressor form of the NFkappaB inhibitor IkappaB reduced IL-6 release. Transfection with a dominant negative mutant of the signaling protein TRAF-2 reduced IL-6 release upon exposure to UV, indicating that UV-induced IL-6 release is mediated by activation of the tumor necrosis factor receptor-1. These data demonstrate that UV can exert biological effects mainly by affecting cell surface receptors and that this is independent of its ability to induce nuclear DNA damage.     

A regulatory role for Sec tRNA[Ser]Sec in selenoprotein synthesis

Selenium is biologically active through the functions of selenoproteins that contain the amino acid selenocysteine. This amino acid is translated in response to in-frame UGA codons in mRNAs that include a SECIS element in its 3' untranslated region, and this process requires a unique tRNA, referred to as tRNA([Ser]Sec). The translation of UGA as selenocysteine, rather than its use as a termination signal, is a candidate restriction point for the regulation of selenoprotein synthesis by selenium. A specialized reporter construct was used that permits the evaluation of SECIS-directed UGA translation to examine mechanisms of the regulation of selenoprotein translation. Using SECIS elements from five different selenoprotein mRNAs, UGA translation was quantified in response to selenium supplementation and alterations in tRNA([Ser]Sec) levels and isoform distributions. Although each of the evaluated SECIS elements exhibited differences in their baseline activities, each was stimulated to a similar extent by increased selenium or tRNA([Ser]Sec) levels and was inhibited by diminished levels of the methylated isoform of tRNA([Ser]Sec) achieved using a dominant-negative acting mutant tRNA([Ser]Sec). tRNA([Ser]Sec) was found to be limiting for UGA translation under conditions of high selenoprotein mRNA in both a transient reporter assay and in cells with elevated GPx-1 mRNA. This and data indicating increased amounts of the methylated isoform of tRNA([Ser]Sec) during selenoprotein translation indicate that it is this isoform that is translationally active and that selenium-induced tRNA methylation is a mechanism of regulation of the synthesis of selenoproteins.     

Selenoproteins Are Essential for Proper Keratinocyte Function and Skin Development

Dietary selenium is known to protect skin against UV-induced damage and cancer and its topical application improves skin surface parameters in humans, while selenium deficiency compromises protective antioxidant enzymes in skin. Furthermore, skin and hair abnormalities in humans and rodents may be caused by selenium deficiency, which are overcome by dietary selenium supplementation. Most important biological functions of selenium are attributed to selenoproteins, proteins containing selenium in the form of the amino acid, selenocysteine (Sec). Sec insertion into proteins depends on Sec tRNA; thus, knocking out the Sec tRNA gene (Trsp) ablates selenoprotein expression. We generated mice with targeted removal of selenoproteins in keratin 14 (K14) expressing cells and their differentiated descendents. The knockout progeny had a runt phenotype, developed skin abnormalities and experienced premature death. Lack of selenoproteins in epidermal cells led to the development of hyperplastic epidermis and aberrant hair follicle morphogenesis, accompanied by progressive alopecia after birth. Further analyses revealed that selenoproteins are essential antioxidants in skin and unveiled their role in keratinocyte growth and viability. This study links severe selenoprotein deficiency to abnormalities in skin and hair and provides genetic evidence for the role of these proteins in keratinocyte function and cutaneous development.     

Protein phosphatase 5 is necessary for ATR-mediated DNA repair

Several recent studies have shown that protein phosphatase 5 (PP5) participates in cell cycle arrest after DNA damage, but its roles in DNA repair have not yet been fully characterized. We investigated the roles of PP5 in the repair of ultraviolet (UV)- and neocarzinostatin (NCS)-induced DNA damage. The results of comet assays revealed different repair patterns in UV- and NCS-exposed U2OS-PS cells. PP5 is only essential for Rad3-related (ATR)-mediated DNA repair. Furthermore, the phosphorylation of 53BP1 and BRCA1, important mediators of DNA damage repair, and substrates of ATR and ATM decreased in U2OS-PS cells exposed to UV radiation. In contrast, the cell cycle arrest proteins p53, CHK1, and CHK2 were normally phosphorylated in U2OS and U2OS-PS cells exposed to UV radiation or treated with NCS. In view of these results, we suggest that PP5 plays a crucial role in ATR-mediated repair of UV-induced DNA damage.     

Telomerase-immortalized human fibroblasts retain UV-induced mutagenesis and p53-mediated DNA damage responses

Immortalized cells frequently have disruptions of p53 activity and lack p53-dependent nucleotide excision repair (NER). We hypothesized that telomerase immortalization would not alter p53-mediated ultraviolet light (UV)-induced DNA damage responses. DNA repair proficient primary diploid human fibroblasts (GM00024) were immortalized by transduction with a telomerase expressing retrovirus. Empty retrovirus transduced cells senesced after a few doublings. Telomerase transduced GM00024 cells (tGM24) were cultured continuously for 6 months (>60 doublings). Colony forming ability after UV irradiation was dose-dependent between 0 and 20J/m2 UVC (LD50=5.6J/m2). p53 accumulation was UV dose- and time-dependent as was induction of p48(XPE/DDB2), p21(CIP1/WAF1), and phosphorylation on p53-S15. UV dose-dependent apoptosis was measured by nuclear condensation. UV exposure induced UV-damaged DNA binding as monitored by electrophoretic mobility shift assays using UV irradiated radiolabeled DNA probe was inhibited by p53-specific siRNA transfection. p53-Specific siRNA transfection also prevented UV induction of p48 and improved UV survival measured by colony forming ability. Strand-specific NER of cyclobutane pyrimidine dimers (CPD) within DHFR was identical in tGM24 and GM00024 cells. CPD removal from the transcribed strand was nearly complete in 6h and from the non-transcribed strand was 73% complete in 24h. UV-induced HPRT mutagenesis in tGM24 was indistinguishable from primary human fibroblasts. These wide-ranging findings indicate that the UV-induced DNA damage response remains intact in telomerase-immortalized cells. Furthermore, telomerase immortalization provides permanent cell lines for testing the immediate impact on NER and mutagenesis of selective genetic manipulation without propagation to establish mutant lines.     

Involvement of DNA polymerase delta in DNA repair synthesis in human fibroblasts at late times after ultraviolet irradiation

DNA repair synthesis following UV irradiation of confluent human fibroblasts has a biphasic time course with an early phase of rapid nucleotide incorporation and a late phase of much slower nucleotide incorporation. The biphasic nature of this curve suggests that two distinct DNA repair systems may be operative. Previous studies have specifically implicated DNA polymerase delta as the enzyme involved in DNA repair synthesis occurring immediately after UV damage. In this paper, we describe studies of DNA polymerase involvement in DNA repair synthesis in confluent human fibroblasts at late times after UV irradiation. Late UV-induced DNA repair synthesis in both intact and permeable cells was found to be inhibited by aphidicolin, indicating the involvement of one of the aphidicolin-sensitive DNA polymerases, alpha or delta. In permeable cells, the process was further analyzed by using the nucleotide analogue (butylphenyl)-2'-deoxyguanosine 5'-triphosphate, which inhibits DNA polymerase alpha several hundred times more strongly than it inhibits DNA polymerase delta. The (butylphenyl)-2'-deoxyguanosine 5'-triphosphate inhibition curve for late UV-induced repair synthesis was very similar to that for polymerase delta. It appears that repair synthesis at late times after UV irradiation, like repair synthesis at early times, is mediated by DNA polymerase delta.     

The nucleotide excision repair pathway is required for UV-C-induced apoptosis in Caenorhabditis elegans

Ultraviolet (UV) radiation is a mutagen of major clinical importance in humans. UV-induced damage activates multiple signaling pathways, which initiate DNA repair, cell cycle arrest and apoptosis. To better understand these pathways, we studied the responses to UV-C light (254 nm) of germ cells in Caenorhabditis elegans. We found that UV activates the same cellular responses in worms as in mammalian cells. Both UV-induced apoptosis and cell cycle arrest were completely dependent on the p53 homolog CEP-1, the checkpoint proteins HUS-1 and CLK-2, and the checkpoint kinases CHK-2 and ATL-1 (the C. elegans homolog of ataxia telangiectasia and Rad3-related); ATM-1 (ataxia telangiectasia mutated-1) was also required, but only at low irradiation doses. Importantly, mutation of genes encoding nucleotide excision repair pathway components severely disrupted both apoptosis and cell cycle arrest, suggesting that these genes not only participate in repair, but also signal the presence of damage to downstream components of the UV response pathway that we delineate here. Our study suggests that whereas DNA damage response pathways are conserved in metazoans in their general outline, there is significant evolution in the relative importance of individual checkpoint genes in the response to specific types of DNA damage.     

Opposing effects of the UV lesion repair protein XPA and UV bypass polymerase eta on ATR checkpoint signaling

An essential component of the ATR (ataxia telangiectasia-mutated and Rad3-related)-activating structure is single-stranded DNA. It has been suggested that nucleotide excision repair (NER) can lead to activation of ATR by generating such a signal, and in yeast, DNA damage processing through the NER pathway is necessary for checkpoint activation during G1. We show here that ultraviolet (UV) radiation-induced ATR signaling is compromised in XPA-deficient human cells during S phase, as shown by defects in ATRIP (ATR-interacting protein) translocation to sites of UV damage, UV-induced phosphorylation of Chk1 and UV-induced replication protein A phosphorylation and chromatin binding. However, ATR signaling was not compromised in XPC-, CSB-, XPF- and XPG-deficient cells. These results indicate that damage processing is not necessary for ATR-mediated S-phase checkpoint activation and that the lesion recognition function of XPA may be sufficient. In contrast, XP-V cells deficient in the UV bypass polymerase eta exhibited enhanced ATR signaling. Taken together, these results suggest that lesion bypass and not lesion repair may raise the level of UV damage that can be tolerated before checkpoint activation, and that XPA plays a critical role in this activation.