Heat-shock enhanced reactivation of a UV-damaged reporter gene in human cells involves the transcription coupled DNA repair pathway

Decreased transcription-coupled nucleotide excision repair capacity is associated with increased p53- and MLH1-independent apoptosis in response to cisplatin

Background

One of the most commonly used classes of anti-cancer drugs presently in clinical practice is the platinum-based drugs, including cisplatin. The efficacy of cisplatin therapy is often limited by the emergence of resistant tumours following treatment. Cisplatin resistance is multi-factorial but can be associated with increased DNA repair capacity, mutations in p53 or loss of DNA mismatch repair capacity.

Methods

RNA interference (RNAi) was used to reduce the transcription-coupled nucleotide excision repair (TC-NER) capacity of several prostate and colorectal carcinoma cell lines with specific defects in p53 and/or DNA mismatch repair. The effect of small inhibitory RNAs designed to target the CSB (Cockayne syndrome group B) transcript on TC-NER and the sensitivity of cells to cisplatin-induced apoptosis was determined.

Results

These prostate and colon cancer cell lines were initially TC-NER proficient and RNAi against CSB significantly reduced their DNA repair capacity. Decreased TC-NER capacity was associated with an increase in the sensitivity of tumour cells to cisplatin-induced apoptosis, even in p53 null and DNA mismatch repair-deficient cell lines.

Conclusion

The present work indicates that CSB and TC-NER play a prominent role in determining the sensitivity of tumour cells to cisplatin even in the absence of p53 and DNA mismatch repair. These results further suggest that CSB represents a potential target for cancer therapy that may be important to overcome resistance to cisplatin in the clinic.

Photoprotection in human skin--a multifaceted SOS response

Human skin has developed elaborate defense mechanisms for combating a wide variety of potentially damaging environmental factors; principal among these is UV light. Despite these defenses, short-term damage may include painful sunburn and long-term UV damage results in both accelerated skin aging and skin cancers such as basal cell carcinoma, squamous cell carcinoma and even malignant melanoma. While UV radiation damages many cellular constituents, its most lasting effects involve DNA alteration. The following sections briefly review UV-inducible protective responses in bacteria and in skin, thymidine dinucleotides (pTT) as a powerful probe of DNA damage responses, and potential means of harnessing these inducible responses therapeutically to reduce the now enormous burden of cutaneous photodamage in our society.

Monitoring repair of DNA damage in cell lines and human peripheral blood mononuclear cells

We introduce a method to follow DNA repair that is suitable for both clinical and laboratory samples. An episomal construct with a unique 8-oxoguanine (8-oxoG) base at a defined position was prepared in vitro using single-stranded phage harboring a 678-bp tract from exons 5 to 9 of the human P53 gene. Mixing curve experiments showed that the real-time PCR method has a linear response to damage, suggesting that it is useful for DNA repair studies. The episomal construct with a unique 8-oxoG base was introduced into AD293 cells or human peripheral blood mononuclear cells, and plasmids were recovered as a function of time. The quantitative real-time PCR assay demonstrated that repair of the 8-oxoG was 80% complete in less than 48 h in AD293 cells. Transfection of small interfering RNAs down-regulated OGG1 expression in AD293 cells and reduced the repair of 8-oxoG to 30%. Transfection of the episome into unstimulated white blood cells showed that 8-oxoG repair had a half-life of 2 to 5h. This method is a rapid, reproducible, and robust way to monitor repair of specific adducts in virtually any cell type.

The tale of the telomere: implications for prevention and treatment of skin cancers

Work in many laboratories over the past decade has established a central role for the telomere in maintaining genomic integrity. Available data may be interpreted to indicate that telomere disruption, whether due to acute DNA damage or progressive telomere shortening, is the initial event that triggers multiple DNA damage responses. The specific initiating event is likely exposure of the otherwise concealed single-stranded 3' overhang, tandem repeats of TTAGGG, a signal that can be provided to cells in the absence of DNA damage by exogenously provided T-oligos. The ability of T-oligo treatment to trigger SOS-like responses and/or to cause selective apoptosis of already malignantly transformed cells may provide an important new means of cancer prevention and treatment.

Pre-UV-Treatment of Cells Results in Enhanced Host Cell Reactivation of a UV Damaged Reporter Gene in CHO-AA8 Chinese Hamster Ovary Cells but Not in Transcription-Coupled Repair Deficient CHO-UV61 Cells

We have used a non-replicating recombinant adenovirus, Ad5MCMVlacZ, which expresses the β-galactosidase reporter gene, to examine both constitutive and inducible repair of UV-damaged DNA in repair proficient CHO-AA8 Chinese hamster ovary cells and in mutant CHO-UV61 cells which are deficient in the transcription-coupled repair (TCR) pathway of nucleotide excision repair. Host cell reactivation (HCR) of β-galactosidase activity for UV-irradiated Ad5MCMVlacZ was significantly reduced in non-irradiated CHO-UV61 cells compared to that in non-irradiated CHO-AA8 cells suggesting that repair in the transcribed strand of the UV-damaged reporter gene in untreated cells utilizes TCR. Prior UV-irradiation of cells with low UV fluences resulted in a transient enhancement of HCR for expression of the UV-damaged reporter gene in CHO-AA8 cells but not in TCR deficient CHO-UV61 cells. These results suggest the presence of an inducible DNA pathway in CHO cells that results from an enhancement of TCR or a mechanism that involves the TCR pathway.

XP-A cells complemented with Arg228Gln and Val234Leu polymorphic XPA alleles repair BPDE-induced DNA damage better than cells complemented with the wild type allele

Functional effects of Arg228Gln and Val2343Leu XPA polymorphisms on benzo[a]pyrene-r-7,t-8-dihydrodiol-t-9,10-epoxide-(+/-)-anti (BPDE) survival and repair were investigated in SV40 immortalized XP12RO cells complemented with wild type and polymorphic XPA cDNAs in an inducible cDNA expression system. In contrast to previous studies showing little impact of XPA polymorphisms on UV survival and repair, cells complemented with polymorphic XPAs displayed improved BPDE survival and repair as compared to wild type XPA-complemented cells. Survival after BPDE treatment was measured using AlamarBlue reduction and colony forming ability. Cells expressing low levels of either polymorphic XPA had equivalent or improved survival compared to wild type XPA-complemented cells (XPAwt cells). XPA induction improved BPDE survival in Arg228Gln (R228Q cells) and Val234Leu (V234L cells) complemented cells, but not XPAwt cells. BPDE-induced DNA damage repair was measured both by reactivation after transfection of a luciferase reporter plasmid reacted with BPDE in vitro, and by removal of adducts from genomic DNA of BPDE-treated cells. BPDE-induced DNA damage repair in R228Q and V234L cells expressing XPA at very low levels was similar to repair in XPAwt cells expressing XPA at normal levels. XPA induction improved repair in R228Q and V234L cells but not in XPAwt cells. Our findings suggest that both Arg228Gln and Val234Leu XPAs function better than wild type XPA for BPDE adduct removal. These observations differ from UV repair results suggesting that the differences are lesion specific. The location of the polymorphisms within the putative poly(ADP-ribose) binding domain suggests that poly(ADP-ribose) interaction is important in repair.

Infection of replication-deficient adenoviral vector enhances interleukin-8 production in small airway epithelial cells more than in large airway epithelial cells

OBJECTIVE

In clinical trials or experiments of gene therapy, airway administration of an adenoviral-based vector (E1A-deleted) elicits a dose-dependent inflammatory response with limitation in the duration of transgene expression. The purpose of this study was to evaluate the possibility that the adenoviral-based vector directly enhances IL-8 production independent of adenoviral E1A in normal human airway epithelial cells and to examine the different responses between primary human bronchial epithelial cells (HBE) and primary human small airway epithelial cells (HSAE) in production of IL-8 following exposure to an adenovirus vector.

METHODOLOGY

Interleukin (IL)-8 levels were evaluated in the culture medium from HBE and HSAE treated with increasing doses of E1A-deleted adenoviral vector contained the Escherichia coli LacZ reporter gene (AdCMVLacZ). To clarify the mechanism of enhancing IL-8 production in airway epithelial cells by infection with adenovirus vector, alphavbeta5 agonistic antibody as an analogue of adenoviral capsid and adenoviral capsid vector denatured by exposure to ultraviolet (UV) light were used in the present study.

RESULTS

Inoculation of HBE with AdCMVLacZ at a multiplicity of infection (MOI) of between 1 and 200 resulted in a dose-dependent expression of LacZ, and maximal expression was observed at a MOI of 100. In contrast, inoculation of HSAE with AdCMVLacZ resulted in maximum expression of LacZ at a MOI of 10. Interleukin-8 levels in culture media from the same experiments revealed significantly greater production of IL-8 in HSAE inoculated with AdCMVLacZ at a MOI of 50, compared to HBE under the same conditions. The capsid-denatured adenoviral vector did not enhance IL-8 production, and alphavbeta5 agonistic antibody induced IL-8 enhancement.

CONCLUSION

These results suggest that the adenoviral vector directly induces the expression of airway epithelial inflammatory cytokines in the pathogenesis of inflammation and that small airway cells have a greater affinity for adenovirus than other airway epithelial cells.

Insulin-like growth factor-I (IGF-I) receptor activation rescues UV-damaged cells through a p38 signaling pathway. Potential role of the IGF-I receptor in DNA repair

The activated insulin-like growth factor-I receptor (IGF-IR) is implicated in mitogenesis, transformation, and anti-apoptosis. To investigate the role of the IGF-IR in protection from UV-mimetic-induced DNA damage, 4-nitroquinoline N-oxide (4-NQO) was used. In this study we show that the activation of the IGF-IR is capable of rescuing NWTb3 cells overexpressing normal IGF-IRs from 4-NQO-induced DNA damage as demonstrated by cellular proliferation assays. This action was specific for the IGF-IR since cells expressing dominant negative IGF-IRs were not rescued from 4-NQO UV-mimetic treatment. DNA damage induced by 4-NQO in NWTb3 cells was significantly decreased after IGF-IR activation as measured by comet assay. IGF-I was also able to overcome the cell cycle arrest, observed after 4-NQO treatment, thereby enhancing the ability of NWTb3 cells to enter S phase. Interestingly, the p38 mitogen-activated protein kinase pathway was shown to represent the main signaling pathway involved in the IGF-IR-mediated rescue of UV-like damaged cells. The ability of the IGF-IR to induce DNA repair was also demonstrated by infecting NWTb3 cells with UV-irradiated adenovirus. Activation of the IGF-IR resulted in enhanced beta-galactosidase reporter gene activity demonstrating repair of the damaged DNA. This study indicates a direct role of the IGF system in the rescue of damaged cells via DNA repair.

Partial complementation of the DNA repair defects in cells from xeroderma pigmentosum groups A, C, D and F but not G by the denV gene from bacteriophage T4

Endonuclease V (denV) from bacteriophage T4 was examined for its ability to complement the DNA repair defect in xeroderma pigmentosum (XP) cells from complementation groups A, C, D, F and G. The denV gene was introduced into SV40-transformed normal and XP cells using a retroviral vector. Expression of denV resulted in partial correction of UV sensitivity and increased host cell reactivation (HCR) of a UV-damaged reporter gene for XP cells from groups A, C and D, but not those from group G. Expression of denV in XP-F cells resulted in enhanced HCR of a UV-damaged reporter but did not affect UV sensitivity. The observed partial complementation is thought to reflect denV-mediated repair of cyclobutane-pyrimidine dimers (CPD), and is incomplete as denV does not recognize other UV-induced lesions, and may not even efficiently remove all CPD. As XP-F cells are believed to retain near-normal levels of CPD repair in the bulk of the genome, we believe that the disparity in the ability of denV to complement the repair deficiency in these cells results from an increased rate, but not level, of CPD repair. Furthermore, we suggest that the lack of correction in the XP-G cells examined results from an inability to process denV-incised CPD by the base excision repair pathway, as has been suggested for cells from the related genetic disorder, Cockayne syndrome. Expression of denV in repair proficient normal cells also resulted in increased HCR of the UV-damaged reporter construct, possibly arising from an increased rate of CPD repair in these cells.

Adaptive enhancement and kinetics of nucleotide excision repair in humans

An adaptive response, low doses of a mutagen rendering cells more able to subsequently cope with higher doses of that or a related challenging mutagen, enhances nucleotide excision repair in human fibroblasts. After fibroblasts were flashed with 20 J/m2 of UVC, the cyclopyrimidine dimer frequency at any single dinucleotide position remained unchanged for several hours before abruptly displaying first order kinetics of repair. These kinetics were determined by ligation-mediated PCR along exon 9 of the human p53 gene. When a chronic dose of quinacrine mustard (QM) preceded the UVC challenge, the duration of the cyclobutane pyrimidine dimer (CPD) repair lags were reduced by a factor of three and the kinetic half-lives for CPD repair were reduced by a factor of three. The observed repair kinetics are consistent with the following model. The UVC dose required (K(m)) to generate a substrate concentration which half-saturates the cell's repair capacity is 3 J/m2 for the high affinity (6-4) photoproducts and greater than 100 J/m2 for the low affinity cyclobutane dimers. After 20 J/m2 of UVC, the repair enzyme is saturated with (6-4) photoproducts; these competitively inhibit CPD repair by binding all available repair enzyme. After the (6-4)s are repaired, the CPD concentration is less than K(m)(CPD) and so CPD repair kinetics initiate with first order kinetics. QM-induced enhancement, by increasing the concentration, Vmax, of repair enzyme, shortens the duration of (6-4) saturation and increases the rate constant for cyclobutane dimer repair. The data exactly fit the expectations from Michaelis kinetics. Transcription coupled repair is less amenable to Michaelis interpretations and enhanced global repair was almost as rapid as the slightly enhanced transcription coupled repair. We infer that repair enhancement is unable to proportionally increase the number of matrix attachment sites necessary for transcription coupled repair. Understanding competitive inhibition between adduct classes and adaptive enhancement of Vmax is important to understanding the effects of high doses of mutagen mixtures.

Do sunscreens increase or decrease melanoma risk: an epidemiologic evaluation

Ultraviolet adiation is an important cause of melanoma, so the use of sunscreen lotions has been advocated for melanoma prevention. Several arguments have been raised in opposition to this inference. Sunscreen use may interfere with cutaneous vitamin D synthesis, which some have hypothesized may lower melanoma risk. Sunscreen users may compensate for their sunscreen use by staying out much longer in the sun, or may use sunscreen lotions inconsistantly. Published melanoma case-control studies have not consistantly demonstrated a protective effect of sunscreens; however, these studies do not provide strong evidence, ultraviolet radiation is a known cause of melanoma, and ultraviolet B may be particularly potent, so on balance the evidence supports continued advocacy of sunscreen lotion use as part of an overall sun-protection regimen. Uncertainty will remain, however, until the action spectrum of melanoma is convincingly demonstrated or the methodologic limitations of existing epidemiologic evidence are overcome. The latter may require another decade or more of experience with sunscreen use.

Role for p53 in the recovery of transcription and protection against apoptosis induced by ultraviolet light

We have previously suggested that the inhibition of RNA polymerase II-mediated transcription after exposure to UV light promotes the accumulation of p53 and the induction of apoptosis (Oncogene 13, 823-831). However, it was not clear whether p53 induction was contributing to apoptosis. Here we report that apoptosis is triggered at lower UV doses in p53-deficient Li-Fraumeni syndrome (LFS) and human papillomavirus (HPV) E6 expressing fibroblasts than in normal cells, suggesting that p53 can be protective against UV-induced apoptosis. There is no significant difference in the effect of UV-irradiation on the cell cycle distribution of normal and primary LFS fibroblasts. Importantly, the recovery of nascent mRNA synthesis in all p53-deficient fibroblasts is significantly impaired compared with control cells after exposure to relevant doses of UV light. Taken together, our results suggest that wild-type p53 can protect cells against UV-induced apoptosis by facilitating the recovery of transcription. Furthermore, we suggest that the capacity of cells to recover transcription after genotoxic damage is an important determinant of sensitivity to apoptosis.

Age-associated decreases in human DNA repair capacity: Implications for the skin

Multiple pathways are involved in accurate synthesis and distribution of DNA during replication, repair and maintenance of genomic integrity. An increased error rate, abovethe spontaneous mutation baseline, has been implicated in carcinogenesis and aging. Moreover, cytogenetic abnormalities are increased in Down's, Edwards', Patau's, and Klinefelter's syndromes with increasing maternal age, and in Marfan's and Apert's syndromes with paternal age. In response to DNA damage, multiple overlapping systems of DNA repair have evolved, preferentially repairing the transcribed strand within transcriptionally-active regions of the genome. These include direct reversal of dimers and specific adducts and pathways for base excision, nucleotide excision, and mismatch repair. A consensus has emerged that some DNA repair capacities decline with organism age, contradictory reports notwithstanding. As is the case for inborn defects in humans, knockout mice lacking components of nucleotide excision repair or DNA-damage checkpoint arrest have increased frequencies of skin and internal cancers, whereas mice overexpressing DNA repair genes have fewer spontaneous cancers. Oxidative stress and resultant free radicals can damage genomic and mitochondrial DNA; damage increases with age but decreases with caloric restriction. We review recent studies of long-lived C. elegans mutants which appear to involve metabolic attenuation, the role of telomere shortening and telomerase in cellular senescence, and the genetic bases of progeroid syndromes in humans. Finally, we discuss roles of extrinsic and intrinsic factors in skin aging, and their association with DNA damage, emphasizing preventive and protective measures and prospects for intervention by modulating DNA repair pathways in the skin.

Enhanced repair of benzo(a)pyrene-induced DNA damage in human cells treated with thymidine dinucleotides

The small DNA fragment thymidine dinucleotide (pTpT) stimulates photoprotective responses in mammalian cells and intact skin. These responses include increased melanogenesis (tanning) and enhanced repair of DNA damage induced by ultraviolet (UV) light. Here we show that pTpT treatment of human keratinocytes enhances their repair of DNA damaged by the chemical carcinogen benzo(a)pyrene (BP), as determined by increased expression of a transfected BP-damaged reporter plasmid containing the chloramphenicol acetyltransferase (CAT) gene. The pTpT-enhanced repair of this BP-damaged plasmid is accomplished at least in part through activation of the p53 tumor suppressor protein and transcription factor, because p53-null H1299 cells showed enhanced repair only if previously transfected with a p53-expression vector. To elucidate the mechanism of this enhanced DNA repair, we examined the expression of p21 and proliferating cell nuclear antigen (PCNA), proteins known to be regulated by p53, as well as the XPA protein, which is mutated in the inherited repair-deficient disorder xeroderma pigmentosum (XP) group A and is necessary for the recognition of UV-induced DNA photoproducts. The p53, PCNA and XPA proteins were all up-regulated within 48 h after the addition of pTpT. Taken together, these data demonstrate that pTpT-enhanced repair of DNA damaged by either UV irradiation or chemical mutagens can be achieved in human cells by exposure to small DNA fragments at least in part through the activation of p53 and increased expression of p53-regulated genes.

Expression and nucleotide excision repair of a UV-irradiated reporter gene in unirradiated human cells

It has been suggested that reactivation of damaged reporter genes introduced into cultured mammalian cells reflects transcription-coupled nucleotide excision repair. To evaluate this possibility directly, we introduced a UV-irradiated shuttle vector, pCMV beta, into unirradiated human cells and compared expression of the reporter gene (lacZ) with repair of cyclobutane pyrimidine dimers (CPDs). Expression of the irradiated reporter gene was more UV resistant in XPC cells, which are deficient in global genome repair, than in CSB cells, which are deficient in transcription-coupled repair. These results are consistent with the idea that repair of the reporter gene is primarily dependent upon transcription-coupled repair. However, when the plasmid DNA was analyzed for removal of CPDs, no clear evidence was obtained for transcription-coupled repair either in XPC cells or in cells with normal repair capacity.

UV-enhanced reactivation of a UV-damaged reporter gene suggests transcription-coupled repair is UV-inducible in human cells

The genetic disorders xeroderma pigmentosum (XP) and Cockayne syndrome (CS) exhibit deficiencies in the repair of UV-induced DNA damage. CS fibroblasts retain proficient nucleotide excision repair (NER) of inactive (or bulk) DNA, but are deficient in the transcription-coupled repair (TCR) of active genes. In contrast, XP complementation group C (XP-C) fibroblasts retain proficient TCR, but are deficient in bulk DNA repair. The remaining NER-deficient XP groups exhibit deficiencies in both repair pathways. Ad5HCMVsp1lacZ is a recombinant adenovirus vector that is unable to replicate in human fibroblasts, but can efficiently infect and express the beta-galactosidase reporter gene in these cells. We have examined the host cell reactivation (HCR) of beta-galactosidase activity for UV-irradiated Ad5HCMVsp1lacZ in non-irradiated and UV-irradiated normal, XP-B, XP-C, XP-D, XP-F, XP-G, CS-A and CS-B fibroblasts. HCR of beta-galactosidase activity for UV-irradiated Ad5HCMVsp1lacZ was reduced in non-irradiated cells from each of the repair-deficient groups examined (including XP-C) relative to that in non-irradiated normal cells. Prior irradiation of cells with low UV fluences resulted in an enhancement of HCR for normal and XP-C strains, but not for the remaining XP and CS strains. HCR of the UV-damaged reporter gene in UV-irradiated XP and CS strains was similar to measurements of TCR reported previously for these cells. These results suggest that UV treatment results in an induced repair of UV-damaged DNA in the transcribed strand of an active gene in XP-C and normal cells through an enhancement of TCR or a mechanism which involves the TCR pathway.

Capacity of UV-irradiated human fibroblasts to support adenovirus DNA synthesis correlates with transcription-coupled repair and is reduced in SV40-transformed cells and cells expressing mutant p53

We have examined the capacity of UV-irradiated human diploid fibroblasts to support adenovirus (Ad) DNA synthesis in order to assess repair of UV-damaged DNA. The capacity of UV-irradiated xeroderma pigmentosum group C (XP-C) fibroblasts to support Ad DNA synthesis was similar to that of UV-irradiated normal diploid fibroblasts, following UV exposures of greater than 9 J/m2. In contrast, XP-A, Cockayne syndrome groups A and B (CS-A and CS-B) fibroblasts were reduced in their capacity to support Ad DNA synthesis compared to normal diploid fibroblasts following a similar UV treatment. These results demonstrate that the capacity of UV-irradiated fibroblasts to support Ad DNA synthesis correlates with their ability to remove UV-induced DNA damage from active genes by transcription-coupled repair (TCR). We also demonstrate that simian virus 40 (SV40)-transformed human fibroblasts, Li-Fraumeni syndrome (LFS) fibroblasts, heterozygous for mutations in one allele of the p53 gene and immortalized LFS cell lines expressing only mutant p53 are reduced in their capacity to support Ad DNA replication following similar UV treatments. These results suggest that the capacity of UV-irradiated cells to support viral DNA synthesis involves TCR of UV-damaged DNA and is disrupted by SV40 transformation and expression of mutant p53. We propose a model in which p53-dependent TCR regulates p53 stability in response to UV.

Incomplete complementation of the DNA repair defect in cockayne syndrome cells by the denV gene from bacteriophage T4 suggests a deficiency in base excision repair

Endonuclease V (denV) from bacteriophage T4 has been examined for its ability to complement the repair defect in Cockayne syndrome (CS) cells of complementation groups A and B. CS is an autosomal recessive disorder characterized by hypersensitivity to UV light and a defect in the preferential repair of UV-induced lesions in transcriptionally active DNA by the nucleotide excision repair (NER) pathway. The denV gene was introduced into non-transformed normal and CS fibroblasts transiently via a recombinant adenovirus (Ad) vector and into SV40-transformed normal and CS cells via a retroviral vector. Expression of denV in CS-A cells resulted in partial correction of the UV-sensitive phenotype in assays of gene-specific repair and cell viability, while correction of CS-B cells by expression of denV in the same assays was minimal or non-existent. In contrast, denV expression led to enhanced host cell reactivation (HCR) of viral DNA synthesis in both CS complementation groups to near normal levels. DenV is a glycosylase which is specific for cyclobutane-pyrimidine dimers (CPDs) but does not recognize other UV-induced lesions. Previous work has indicated that CS cells can efficiently repair all non-CPD UV-induced transcription blocking lesions (S.F. Barrett et al.. Mutation Res. 255 (1991) 281-291 [1]) and that denV incised lesions are believed to be processed via the base excision repair (BER) pathway. The inability of denV to complement the NER defect in CS cells to normal levels implies an impaired ability to process denV incised lesions by the BER pathway, and suggests a role for the CS genes, particularly the CS-B gene, in BER.