Identification of inducible damage-recognition proteins that are overexpressed in HeLa cells resistant to cis-diamminedichloroplatinum (II)

Mechanisms of resistance to alkylating agents

Alkylating agents are the most widely used anticancer drugs whose main target is the DNA, although how exactly the DNA lesions cause cell death is still not clear. The emergence of resistance to this class of drugs as well as to other antitumor agents is one of the major causes of failure of cancer treatment. This paper reviews some of the best characterized mechanisms of resistance to alkylating agents. Pre- and post-target mechanisms are recognized, the former able to limit the formation of lethal DNA adducts, and the latter enabling the cell to repair or tolerate the damage. The role in the pre-target mechanisms of reduced drug accumulation and the increased detoxification or activation systems (such as DT-diaphorase, metallothionein, GST/GSH system, etc...) are discussed. In the post-target mechanisms the different DNA repair pathways, tolerance to alkylation damage and the 'downstream' effects (cell cycle arrest and/or apoptosis) are examined.

Identification of rat DDB1, a putative DNA repair protein, and functional correlation with its damaged-DNA recognition activity

Recognition and incision of UV-DNA adducts play key roles in the efficacy of nucleotide excision repair. Damaged-DNA recognition activity has been identified from primate cells as a complex of DDB1 (127-kD) and DDB2 (48-kD) subunits. However, the function of damaged-DNA binding proteins (DDBs) in damaged-DNA recognition is not well understood. To assess the functional correlation between DDBs and UV-damaged-DNA recognition activity, we identified UV-damaged-DNA recognition activities in rodent cell lines. There is a cell type-dependent expression of DDB1 and DDB2. Rodent cells had less abundant DDBs and lower UV-damaged-DNA recognition activity than did human tumor cells. Interestingly, the profusion of DDBs is associated with UV-damaged-DNA recognition activity in these cell lines. We also discovered tissue-dependent expression of DDBs and its functional correlation with UV-damaged-DNA recognition activity. cDNA (3850 nucleotides) from rat ddb1 was isolated. It contained the complete length of the open reading frame that encodes an 1140-amino-acid polypeptide with a predicted molecular weight of 126.8 kD. The predicted protein size from the rat ddb1 gene resembles that from human DDB1 (127 kD). Rat DDB1 shares highly conserved sequencing (greater than 98% similarity) with those of mouse, human, and monkey. Rat and fruit fly DDB1 exhibit 62.23% identity and 57.66% homology. The evolutionary conservation of the DDB1 sequence suggests that DDB1 may play a pivotal role in mammals as well as in other eukaryotes. However, overexpression of DDB1 did not augment UV-damaged-DNA recognition activity in human HeLa, hamster V79, or rat PC12 cells. In contrast, restricting DDB2 expression by antisense ddb2 partially inhibited UV-damaged-DNA recognition activity in cells, whereas overexpressing DDB2 through a recombinant ddb2 adenovirus partly restored the recognition activity of these cells. These findings support the notion that DDB abundance is functionally correlated with UV-damaged-DNA recognition activity. These results also suggest that the profusion of DDB2, but not DDB1, may moderate UV-damaged-DNA recognition activity.

Restoration of UV sensitivity in UV-resistant HeLa cells by antisense-mediated depletion of damaged DNA-binding protein 2 (DDB2)

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.

Cisplatin DNA adduct detection and depurination measured by 32P DNA radiolabeling and two-dimensional thin-layer chromatography: a time and concentration study

Platinum-based chemotherapies cause the formation of DNA adducts and have profound effects on DNA. This study measured cis-diamminedichloroplatinum II (cisplatin) DNA adducts by 32P-radiolabeling DNA, enzymatically digesting radiolabeled DNA, separating the formed adducts on two-dimensional thin-layer chromatography, and quantitating the adducts with autoradiography and densitometry. HeLa DNA was incubated with cisplatin at varying concentrations (6.25-325 nM) and times (0 min to 72 hr). Cisplatin rapidly depurinated dGMP and dAMP (90%, 15-min incubation with 325 nM cisplatin). Partial depurination of dGMP (15%) and dAMP (25%) occurred with lower cisplatin concentrations at equal incubation times. A minimum of four new adducts, with relatively rapid migratory patterns, were detected at high cisplatin concentrations with short incubation times. These results indicate that the depurination of DNA correlates with DNA adduct formation and that the quantification of these adducts may be applicable to monitoring tumor and host cell response to cisplatin chemotherapy.

Enhancement of excision repair of cisplatin-DNA adducts by cell-free extract from a cisplatin-resistant rat cell line

To characterize the enhanced repair synthesis of defined DNA lesions, oligodeoxyribonucleotides were synthesized and inserted into plasmid DNA. The inserted plasmid DNA was treated with cis-diamminedichloroplatinum(II) (cisplatin) and subjected to in vitro DNA repair assay with soluble extract from the rat liver cell line Ac2F. All cisplatin adducts tested stimulated DNA repair synthesis. Moreover, two cisplatin-resistant cell lines, Ac2F-CR4 and Ac2F-CR10, were established by stepwise exposure of Ac2F cells to this drug. The DNA repair synthesis was enhanced 3- to 4-fold in the extract from cisplatin-resistant Ac2F cells relative to that from Ac2F cells. Such repair synthesis was suppressed by the specific DNA polymerase inhibitor aphidicolin. The results of the present study suggested that the enhanced repair activity induced by a cisplatin adduct can be detected by in vitro DNA repair assay with soluble cell extract.

Reduction of cellular cisplatin resistance by hyperthermia--a review

Resistance to cisplatin (cDDP) is a major limitation to its clinical effectiveness. Review of literature data indicates that cDDP resistance is a multifactorial phenomenon. This provides an explanation why attempts to reverse or circumvent resistance using cDDP-analogues or combination therapy with modulators of specific resistance mechanisms have had limited success so far. It therefore provides a rationale to use hyperthermia, an agent with pleiotropic effects on cells, in trying to modulate cDDP resistance. In this review the effects of hyperthermia on cDDP cytotoxicity and resistance as well as underlying mechanisms are discussed. Hyperthermia is found to be a powerful modulator of cDDP cytotoxicity, both in sensitive and resistant cells. Relatively high heat doses (60 min 43 degrees C) seem to specifically interfere with cDDP resistance. The mechanism of interaction has not been fully elucidated so far, but seems to consist of multiple (simultaneous) effects on drug accumulation, adduct-formation and -repair. This may explain why hyperthermia seems to be so effective in increasing cDDP cytotoxicity, irrespective of the presence of resistance mechanisms. Therefore, the combination of hyperthermia and cDDP deserves further attention.

Differential human nucleotide excision repair of paired and mispaired cisplatin-DNA adducts

In order to understand the action of the chemotherapeutic drug cisplatin, it is necessary to determine why some types of cisplatin-DNA intrastrand crosslinks are repaired better than others. Using cell extracts and circular duplex DNA, we compared nucleotide excision repair of uniquely placed 1,2-GG, 1,2-AG, and 1,3-GTG cisplatin-crosslinks, and a 2-acetylaminofluorene lesion. The 1,3 crosslink and the acetylaminofluorene lesion were repaired by normal cell extracts approximately 15-20 fold better than the 1,2 crosslinks. No evidence was found for selective shielding of 1,2 cisplatin crosslinks from repair by cellular proteins. Fractionation of cell extracts to remove putative shielding proteins did not improve repair of the 1,2-GG crosslink, and cell extracts did not selectively inhibit access of UvrABC incision nuclease to 1,2-GG crosslinks. The poorer repair of 1,2 crosslinks in comparison to the 1,3 crosslink is more likely a consequence of different structural alterations of the DNA helix. In support of this, a 1,2-GG-cisplatin crosslink was much better repaired when it was opposite one or two non-complementary thymines. Extracts from cells defective in the hMutSalpha mismatch binding activity also showed preferential repair of the 1,3 crosslink over the 1,2 crosslink, and increased repair of the 1,2 adduct when opposite thymines, showing that hMutSalphais not involved in the differential NER of these substrates in vitro. Mismatched cisplatin adducts could arise by translesion DNA synthesis, and improved repair of such adducts could promote cisplatin-induced mutagenesis in some cases.

Reduced expression of the ICE-related protease CPP32 is associated with radiation-induced cisplatin resistance in HeLa cells

Low-dose fractionated gamma-irradiation (three cycles of 5 x 2 Gy) induced cisplatin resistance in HeLa cells. The drug resistance was modest (Rf of about 2) and stable, similar to that found previously in murine cells after irradiation. In the drug-resistant HeLa-C3 cells, flow cytometric analysis revealed a decreased number of apoptotic cells compared with the parental cells. Drug resistance was associated with considerably enhanced expression of the p53 suppressor protein in HeLa-C3 cells after cisplatin exposure but seemed not to be regulated by the bcl-2-dependent pathway. Cisplatin resistance correlated with reduced expression of ICE-related proteases (interleukin-1beta-converting enzyme). Basal levels of the 45-kDa precursor ICE protein were reduced in HeLa-C3 cells, while those of the mature 60-kDa heterotetramer were similar. The CPP32 protease, a member of the ICE family with structural homology but different substrate specificity, was expressed at a lowered level. After drug exposure, there was a slight increase of CPP32 in HeLa-C3 cells, equivalent to about 45% of the level attained in the parental cells. This is in contrast to the CPP32 levels measured after irradiation, which were similar in sensitive and in resistant cells. As the radiosensitivity is unchanged in both cell lines, these results suggest that cisplatin resistance in HeLa-C3 cells is associated with alterations of a CPP32-linked apoptotic pathway, which is affected by the damage caused by cisplatin but not by irradiation. Whether these changes are dependent on the observed p53 modifications is now being studied in resistant clones.

Mutations specific to the xeroderma pigmentosum group E Ddb- phenotype

The activity of a damage-specific DNA-binding protein (DDB) is absent from a subset, Ddb-, of cell strains from patients with xeroderma pigmentosum group E (XP-E). DDB is a heterodimer of 127-kDa and 48-kDa subunits. We have now identified single-base mutations in the gene of the 48-kDa subunit in cells from the three known Ddb- individuals, but not in XP-E strains that have the activity. An A --> G transition causes a K244E change in XP82TO and a G --> A transition causes an R273H change in XP2RO and XP3RO. No mutations were found in the cDNA of the 127-kDa subunit. Overexpression of p48 in insect cells greatly increases DDB activity in the cells, especially if p127 is jointly overexpressed. These results demonstrate that p48 is required for DNA binding activity, but at the same time necessitate further definition of the genetic basis of XP group E.

Cross-resistance to cis-diamminedichloroplatinum(II) of a multidrug-resistant lymphoma cell line associated with decreased drug accumulation and enhanced DNA repair

HOB1/VCR, a multidrug-resistant subline of the immunoblastic B lymphoma cell line, was established by sequential selection in increasing concentrations of vincristine. The expression of the human mdr l gene, as analyzed by reverse transcription and polymerase-chain reaction (RT-PCR), revealed a 10-15-fold overexpression in this resistant cell line. A complete inhibition of vincristine resistance by verapamil was observed in the vincristine-resistant HOB1/VCR cells, which suggests that acquired resistance may be mainly due to P-glycoprotein. HOB1/VCR cells also developed a 67-fold cross-resistance to the anticancer drug cis-diamminedichloroplatinum (cisplatin). DNA repair of the resistant and the parental cell lines was investigated by in situ detection with a cisplatin-DNA adduct-specific antibody and by measurement of repair-associated host cell reactivation of damaged plasmid DNA. HOB1/VCR cells exhibited a 2-fold decrease in the level of cisplatin-DNA adducts, compared to the parental cells. The DNA repair rate following peak accumulation of cisplatin-DNA adducts (which took approximately 4 h) was also enhanced in the resistant cells. This was supported by the measurement of the cisplatin level remaining in cells by atomic absorption spectrophotometry, which showed a 2.7-fold reduction in the resistant cells. In addition, the acquired resistance and enhanced DNA repair in HOB1/VCR cells were partially reversed by nontoxic aphidicolin, a DNA polymerase-alpha and DNA repair inhibitor. Inhibition of the intracellular level of glutathione by DL-buthionine-[S,R]-sulfoximine demonstrated that cell viability was inhibited 4-fold more in the resistant cells than in the parental cells. The results suggest that the reduced formation of cisplatin-DNA adducts and the increased glutathione content of the multidrug-resistant cells play a major role in phenotypic cross-resistance to cisplatin.

Inhibition by arsenite of anticancer drug cis-diamminedichloroplatinum(II) induced DNA repair and drug resistance in HeLa cells

We have previously reported a cisplatin-resistant HeLa variant cell line (HeLa/CPR) which exhibited an enhancement in repairing cisplatin-DNA adducts (Chao, 1994, Mol. Pharmacol. 45, 1137-1144). In this study, using this cell line, we investigated the modification, by arsenite, of cisplatin-induced cytotoxicity and DNA repair in the resistant cell line. By a sublethal dose of arsenite, cytotoxicity of the resistant cells was enhanced by 2.5-fold, compared to 1.62-fold in the parental cells. Using enzyme-linked immunosorbent assay (ELISA) and a monoclonal antibody specific for cisplatin-DNA adducts, we found that the resistant cells showed a 5.15-fold decrease in the adduct formation compared to the parental cells. However, in the presence of arsenite, the resistant cells showed only a 1.47-fold decrease in the adduct formation, indicating a more than 3-fold modification. Using host cell reactivation of transfected plasmid DNA carrying cisplatin damage (an indirect detection of DNA repair), arsenite also revealed a ∼2-fold modification of adduct formation in the resistant cells. In addition, the time-dependent potentiation of cytotoxicity by arsenite in both cell lines was parallel to the increase of adduct formation. These results indicate that arsenite is an effective modifier of cisplatin-induced resistance and enhanced DNA repair in HeLa/CPR cells. The results are consistent with the notion that the cisplatin-resistant phenotype in HeLa cells is mainly mediated by enhancement of DNA repair.

Induction, not associated with host-cell re-activation of damaged plasmid DNA, of damaged-DNA-recognition proteins by retinoic acid and dibutyryl cyclic AMP in mammalian cells

Our previous studies [Chao (1992) Biochem. J. 282, 203-207; C.C.-K. Chao, unpublished work] has suggested a correlation between the levels of constitutive UV-damaged-DNA-recognitionproteins (UVDRP) and cellular DNA repair in different cell types. In the present study, UVDRP were induced in F9 and NIH3T3 cells by 0.1 microM retinoic acid (RA) and 1 mM dibutyryl cyclic AMP (dbcAMP), which is sufficient to induce differentiation in murine F9 stem cells. The induction of UVDRP in F9 and NIH3T3 cells was optimized after 6 and 2 days incubation with RA/dbcAMP respectively. Since NIH3T3 cells were not induced to differentiate by RA/dbcAMP, the upregulation of the UVDRP in mammalian cells would thus seem not to be mediated directly by differentiation. Using a plasmid re-activation assay to estimate DNA repair, we did not find a correlation between DNA repair and UVDRP in RA/dbcAMP-treated cells. The results suggest that UVDRP may have a function other than, or in addition to, its role in DNA repair.

Reversal of radiation-induced cisplatin resistance in murine fibrosarcoma cells by selective modulation of the cyclic GMP-dependent transduction pathway

Cisplatin resistance, induced in murine fibrosarcoma cells (SSK) in vitro or in vivo by low-dose irradiation, can be overcome by activation of the cyclic GMP(cGMP)-dependent transduction pathway. This is mediated either by stimulating cGMP formation with sodium nitroprusside or by replacing cGMP with a selective activator of the cGMP-dependent protein kinase, 8-bromo-cGMP. The cyclic AMP-dependent transduction pathway is not involved in cisplatin resistance. Instead, activation of cAMP sensitises both parental and resistant SSK cells equally to the action of cisplatin. There is a 1.8 to 2.5-fold increase in drug toxicity, depending on the activating agent. Enhancement of cisplatin sensitivity is induced by specific inhibition of cAMP hydrolysis, increase in cAMP formation or by increasing the activation potential to cAMP-dependent protein kinase by specific cAMP analogues. Cells that have lost cisplatin resistance respond to cGMP- or cAMP-elevating agents in the same way as the parental SSK cells. The radiation sensitivity is unchanged in all cell lines, even after activation of cAMP or cGMP. These results suggest that specific DNA repair pathways are altered by radiation but affected only in cisplatin damage repair, which is regulated by cGMP. Although there is ample cooperativity and interaction between the cAMP- and the cGMP-dependent transduction pathways, specific substrate binding by cGMP appears to play an important role in radiation-induced cisplatin resistance.

Preferential binding of cisplatin to mitochondrial DNA of Chinese hamster ovary cells

Some chemical carcinogens localize preferentially in mitochondrial DNA (mtDNA) when compared with genomic DNA (gDNA). Here we compare the ability of cisplatin (cis-diamminedichloroplatinum[II]) to induce DNA adducts in both genomic and mtDNA of Chinese hamster ovary (CHO) cells in culture. Cytotoxicity was examined by cell survival 4, 8 and 24 h after exposure to 50 microM cisplatin. Cisplatin-DNA adducts were measured in DNA from nuclear and mitochondrial fractions by dissociation-enhanced lanthanide fluoroimmunoassay (DELFIA), a sensitive competitive microtiter-based immunoassay utilizing antiserum elicited against cisplatin-modified DNA. An additional comparison of cisplatin-DNA binding in both compartments was performed by immunoelectron microscopy using the cisplatin-DNA antiserum and colloidal gold. DELFIA analysis of cisplatin-DNA adducts in gDNA and mtDNA showed a six-fold higher incorporation of drug into mtDNA as compared to gDNA. Morphometric studies of colloidal gold distribution in photomicrographs of CHO cells showed mtDNA to contain a four-fold higher concentration of cisplatin as compared to nuclear DNA. Therefore, both methods demonstrated a preferential binding of cisplatin to mtDNA versus gDNA.

Cisplatin inhibits chromatin remodeling, transcription factor binding, and transcription from the mouse mammary tumor virus promoter in vivo

The anticancer drug cis-diamminedichloro-platinum(II) (cisplatin) covalently modifies DNA, and these lesions are thought to lead to cell death by inhibiting DNA and RNA synthesis. By using in vivo analysis techniques, we have investigated the influence of cisplatin on hormone-induced transcription from the mouse mammary tumor virus (MMTV) promoter. Cisplatin substantially reduced glucocorticoid-induced expression from the MMTV promoter stably incorporated into mouse tumor cells. The glucocorticoid-receptor-dependent chromatin remodeling and loading of transcription factors that is a signature response of this promoter in the context of chromatin were significantly reduced by cisplatin but not by the clinically ineffective trans-isomer trans-diamminedichloroplatinum(II) (transplatin). Additional in vivo studies on transiently introduced nonchromatin MMTV templates demonstrated that cisplatin modification of DNA blocked binding of the transcription factor NF1. These results provide strong evidence that cisplatin influences transcription by interfering with the opening of repressive chromatin structures and by blocking transcription factor binding directly, each of which could contribute substantially to its toxicity.

Multiple mechanisms of resistance to cisplatin toxicity in an Escherichia coli K12 mutant

The mechanisms underlying cellular resistance to the antitumor drug cis-diamminedichloro-platinum(II) (CDDP) were studied in Escherichia coli K12. A bacterial strain (MC4100/DDP) was selected from the MC4100 wild-type strain after growth for four cycles in CDDP. MC4100/DDP bacteria showed a high level of resistance and exhibited various modifications including (1) a decrease in drug uptake and platinum/DNA binding which only partly contributed to resistance, (2) an increase in glutathione content not involved in the resistant phenotype, (3) an increase in DNA repair capacity. Resistance was unmodified by introducing a uvrA mutation which neutralizes the excision-repair pathway. In contrast, it was abolished by deletion of the recA gene which abolishes recombination and SOS repair but also by a mutation in the recA gene leading to RecA co-protease minus (no SOS induction). RecA protein was unchanged in MC4100/DDP but the expression of RecA-dependent gene(s) was required for CDDP resistance. The regulation of genes belonging to the SOS regulon was analysed in MC4100/DDP by monitoring the expression of sfiA and recA::lacZ gene fusions after UV irradiation. These gene fusions were derepressed faster and the optimal expression was obtained for a lower number of UV lesions in MC4100/DDP, suggesting a role of RecA co-protease activity in the mechanism of resistance to CDDP in this E. coli strain.

Use of a monoclonal antibody to detect DNA damage caused by the anticancer drug cis-diamminedichloroplatinum (II) in vivo and in vitro

A monoclonal antibody, MAb62-5, was prepared and used to detect DNA damage due to the anticancer drug cis-diamminedichloroplatinum (II) (or cisplatin). ELISA competition indicated that the binding of MAb62-5 to cisplatin-DNA was competitively inhibited (50% control) by 210 nM of cisplatin bound to DNA, cisplatin/nucleotide (D/N) = 0.2. Using a DNA mobility shift assay, MAb62-5 binding activity was inhibited by 50% by approximately 50-fold molar excess of cisplatin-DNA adducts (D/N = 0.08), whereas there was less than 5% inhibition by UV-DNA adducts or mock-treated DNA. In addition, MAb62-5 showed a similar affinity to the cisplatin-DNA adducts as compared to an endogenous cisplatin-damaged DNA recognition protein. Using ELISA with this antibody, we have demonstrated a 2-fold enhancement in excision repair of cisplatin-DNA adducts in resistant HeLa cells. This is supported by the measurement of repair-associated DNA strand breaks using alkaline elution and host cell reactivation of transfected plasmid DNA carrying cisplatin damage. These findings also provide explanation for the complexicity of immunoassay in cells.

Base sequence-independent distorsions induced by interstrand cross-links in cis-diamminedichloroplatinum (II)-modified DNA

Physico-chemical and immunological studies have been done in order to further characterize the distorsions induced in DNA by the interstrand cross-links formed between the antitumor drug cis-diamminedichloroplatinum (II) (cis-DDP) and two guanines on the opposite strands of DNA at the d(GC/GC) sites. Bending (45 degrees) and unwinding (79 +/- 4 degrees) were determined from the electrophoretic mobility of multimers of 21- 24-base pairs double-stranded oligonucleotides containing an interstrand cross-link in the central sequence d(TGCT/AGCA). The distorsions induced by the interstrand cross-link in the three 22-base pairs oligonucleotides d(TGCT/AGCA), d(AGCT/AGCT) and d(CGCT/AGCG) were compared by means of gel electrophoresis, circular dichroism, phenanthroline-copper footprinting and antibodies specifically directed against cis-DDP interstrand cross-links. The four different technical approaches indicate that the distorsions are independent of the chemical nature of the base pairs adjacent to the interstrand cross-link. The general conclusion is that the interstrand cross-link induces a bending and in particular an unwinding larger than other platinum adducts and the distorsions are independent of the nature of the bases (purine or pyrimidine) adjacent to the d(GC/GC) site.

Enhanced excision repair of DNA damage due to cis-diamminedichloroplatinum(II) in resistant cervix carcinoma HeLa cells

We have previously reported a cisplatin-resistant HeLa cell line which exhibits overproduction of nuclear proteins preferential for cisplatin-modified DNA (Chao et al., Cancer Res. 51:601-605, 1991; Biochem. J. 277: 875-878, 1991). In this study, excision repair of cisplatin-DNA adducts in a resistant and a revertant cell lines was investigated using in situ detection of cisplatin-DNA adducts by an immunoassay and the measurement of repair-associated DNA strand breaks by a sensitive alkaline elution method. The resistant cells exhibited a 2-fold decrease in the accumulation of cisplatin-DNA adducts; whereas, the revertant cells showed a similar level of cisplatin-DNA adducts as the parental cells in the parallel experiment. Immediately following cisplatin treatment, the resistant and the revertant cells accumulated respectively approximately 50% and 90% cisplatin-DNA adducts of the parental cells. However, the kinetic patterns of repair rate following peak accumulation of cisplatin-DNA adducts (which took approximately 4 h) was the same in the three cell lines. This finding was supported by the measurement of repair-associated DNA strand breaks using alkaline elution which showed 1.6- and 1.5-fold increase in the resistant and the revertant cells respectively. In addition, following transfection with plasmid DNA carrying cisplatin damage, the resistant and the revertant cells displayed a 2.4- and 1.4-fold enhancement in host cell reactivation, respectively. Furthermore, the acquired resistance in HeLa cells was partially reversed by nontoxic aphidicolin, a DNA polymerase-alpha and DNA repair inhibitor. The results strongly suggest the improved excision repair of cisplatin-DNA adducts as a mechanism of phenotypic resistance of cells to cisplatin.

Correction of the DNA repair defect in xeroderma pigmentosum group E by injection of a DNA damage-binding protein

Cells from a subset of patients with the DNA-repair-defective disease xeroderma pigmentosum complementation group E (XP-E) are known to lack a DNA damage-binding (DDB) activity. Purified human DDB protein was injected into XP-E cells to test whether the DNA-repair defect in these cells is caused by a defect in DDB activity. Injected DDB protein stimulated DNA repair to normal levels in those strains that lack the DDB activity but did not stimulate repair in cells from other xeroderma pigmentosum groups or in XP-E cells that contain the activity. These results provide direct evidence that defective DDB activity causes the repair defect in a subset of XP-E patients, which in turn establishes a role for this activity in nucleotide-excision repair in vivo.

DNase I footprinting of cis- or trans-diamminedichloroplatinum(II)-modified DNA

DNase I has been used as an enzymatic probe to visualize the conformational alteration induced in DNA by the binding of either the antitumor drug cis-platinum (cis-DDP) or the therapeutically inactive derivatives, trans-platinum (trans-DDP) and chlorodiethylene-triamineplatinum(II) (dien-Pt). We have constructed double-stranded oligonucleotides (52-mer) containing a single adduct either at the d(GG) site (cis-DDP intrastrand cross-link) or at the d(GC/GC) site (cis-DDP interstrand cross-link) or at the d(G/C) site (trans-DDP interstrand cross-link) or at the d(G) site (dien-Pt adduct). The platinated oligonucleotides are differently recognized by DNase I. As judged by DNase I, the distortions induced in the DNA double helix by the cis-DDP and trans-DDP interstrand cross-links spread over more base-pairs than that induced by the cis-DDP intrastrand cross-link.

Proteins binding to cisplatin-damaged DNA in human cell lines

Cisplatin (CDDP) is a highly effective, frequently used cancer chemotherapeutic drug employed in the treatment of several human malignancies including ovarian, testicular, and bladder cancers. A common problem encountered with cisplatin therapy is intrinsic or acquired resistance to this drug. While the mechanisms of resistance to cisplatin, and other chemotherapeutic agents, are not fully understood, one factor affecting the cellular response to CDDP may result from differences in the level of specific proteins that recognize CDDP-damaged DNA. We have developed a damaged DNA affinity precipitation technique that allows the direct visualization and characterization of cellular proteins that bind to cisplatin-damaged DNA. In the present study we have utilized this method to analyze proteins present in several mammalian cell lines that bind to cisplatin-damaged DNA. We demonstrate that HeLa cells, resistant to CDDP cytotoxicity, contain high levels of high-mobility-group proteins 1 and 2, which bind to CDDP-DNA. We also show that xeroderma pigmentosum cells of different genetic complementation groups contain variable levels of a 45-kDa protein that binds to CDDP-DNA. Thus, our results indicate that different human cell lines demonstrate qualitative and quantitative differences in the expression of cisplatin-damaged DNA binding proteins.

Lack of DNA enzymatic photoreactivation in HeLa cell-free extracts

Using a DNA-protein binding assay, we have previously identified and characterized a UV-damaged DNA recognition protein (UVDRP) from HeLa cells [(1991) Nucleic Acids Res. 19, 6413-6418]. In this report, the photoreactivating activity of UVDRP from the yeast, Saccharomyces cerevisiae, and HeLa cells was investigated. Although yeast and human cells are evolutionarily different from each other, both UVDRPs were conserved in the sense of their biochemical characteristics except that the yeast UVDRP also exhibited an enzymatic photoreactivating activity. A mammalian expression vector plasmid DNA carrying the bacterial chloramphenicol acetyltransferase (CAT) gene was UV irradiated in vitro followed immediately by exposure to photoreactivating light, and its transient expression in repair-deficient xeroderma pigmentosum (XP) cells was investigated. More than 80% of the CAT activity was inhibited by UV irradiation, which was partially restored (> 60%) by a partially purified yeast photolyase. In contrast, HeLa cell extracts did not express a photoreactivatable recovery from UV-induced inhibition of the CAT activity tested in the same system. This study has demonstrated the potential use of the DNA-mobility shift assay to investigate enzymatic photoreactivation, and has indicated the absence of the repair mechanism in human cells.

New platinum antitumor complexes

Over the past two decades, platinum-based drugs (cisplatin and, latterly, the less toxic analogue carboplatin) have conferred significant therapeutic benefit to a large number of cancer sufferers. However, there remains scope for substantial improvement in the clinical utility of metal coordination complexes through the discovery of additional platinum-based complexes (or possibly alternative metals). Future drug discovery strategies should focus on tumor resistance and its circumvention. To date, only one series of compounds, those containing a 1,2-diaminocyclohexane carrier ligand (e.g., oxaliplatin, tetraplatin), has entered clinical trial based on their circumvention of acquired cisplatin resistance in some (mainly murine) preclinical tumor models. At present these agents are in early clinical trial and thus their true clinical utility in cisplatin/carboplatin refractory disease is not yet determinable (and may not be due to dose-limiting neurotoxicity). Over the past few years, our understanding of mechanisms of resistance to cisplatin and its interaction with DNA has vastly increased. This new information will undoubtedly guide the development of new strategies aimed at the circumvention of intrinsic and acquired tumor resistance to cisplatin. Approaches to circumvent resistance will probably involve not only the rational development of a new generation of platinum-based drugs (e.g., compounds designed to overcome reduced cisplatin accumulation or enhanced removal of cisplatin-induced DNA adducts) but also non-platinum drugs which are capable of modulating resistance (e.g., modulators of signal transduction pathways, ras and myc oncogene expression and glutathione biosynthesis). One may look forward with a great deal of optimism that these promising new approaches will result in clinical benefit by the end of the century. Nevertheless, cisplatin and carboplatin remain the standard anticancer drugs to which novel platinum-based complexes must be compared.

A damage-recognition protein which binds to DNA containing interstrand cross-links is absent or defective in Fanconi anemia, complementation group A, cells

A DNA binding protein with specificity for DNA containing interstrand cross-links induced by 4,5',8-trimethylpsoralen (TMP) plus long wavelength ultraviolet (UVA) light has been identified in normal human chromatin. Protein binding to DNA was determined using a gel mobility shift assay and an oligonucleotide containing a hot spot for formation of psoralen interstrand cross-links. Specificity of the damage-recognition protein for cross-links was demonstrated both by a positive correlation between level of cross-link formation in DNA and extent of protein binding and by effective competition by treated but not undamaged DNA for the binding protein. Chromatin protein extracts from cells from individuals with the genetic disorder, Fanconi anemia, complementation group A (FA-A), which have decreased ability to repair damage produced by TMP plus UVA light, failed to show any protein binding to TMP plus UVA treated DNA. We have previously shown that these chromatin protein extracts contain a DNA endonuclease complex, pI 4.6, which specifically recognizes and incises DNA containing interstrand cross-links and which in FA-A cells is defective in its ability to incise this damaged DNA (Lambert et al. (1992) Mutation Res., 273, 57-71). Together, these findings suggest that the DNA binding protein identified is involved in recognition and repair of DNA interstrand cross-links.

Apparent alterations in the early stage of excision repair of UV-induced DNA damages in a HeLa mutant cell line that is resistant to genotoxic stresses

We have previously reported a cisplatin-resistant HeLa cell line featuring a cross-resistance to genotoxic stresses including ultraviolet (UV) radiation and an enhancement of plasmid reactivation. In this study, excision repair of UV-DNA adducts in this resistant cell line was investigated. Using alkaline elution analysis, this resistant cell line showed a 2-fold enhancement in damage incision-associated DNA strand breaks. Using a gel mobility shift assay, the resistant cells exhibited a 3-fold increase in nuclear proteins which specifically recognize UV-damaged DNA. However, the rate of repair synthesis in the resistant cells appeared to be the same as in their parental counterparts. Thus, recognition and incision activities, the early stage of excision repair, are altered in the resistant cells. The results suggest that phenotypic cross-resistance of this cell line to UV is probably due to an improved excision repair of UV-DNA adducts which is defective in xeroderma pigmentosum group A cells. The results are consistent with the conclusion that the early stage, including recognition and incision, of excision repair is critical in determining cellular sensitivity or resistance to DNA damage.

Constitutive over-production of DNA-damage recognition proteins and acquired UV resistance in prolonged culture of F9 teratocarcinoma stem cells

An ultraviolet (UV)-resistant F9 variant cell line, termed F9Vc, was established from a prolonged culture of murine F9 embryonic stem cells. A 6-fold UV resistance was detected in F9V2 cells compared to the F9 parental cells, as determined by ID50 (36 J/m2 vs. 6 J/m2), the UV dose causing 50% growth inhibition. Using a DNA mobility-shift assay, a nuclear protein (termed UVDRP) that preferentially binds to UV-damaged DNA was detected in F9 and F9Vc cell extracts. The UVDRP in F9Vc cells was present at a 7-fold higher concentration than that of F9 cells. Interestingly, the F9 UVDRP was transiently induced following cellular differentiation by retinoic acid (RA)/cAMP, with optimum induction (15-fold) at 6 days. Although constitutively over-produced, UVDRP also remained inducible in F9Vc cells in response to RA/cAMP. Indirect DNA repair measurement by host cell reactivation of UV-damaged plasmid DNA demonstrated that F9Vc cells exhibited a slight increase or a similarity in repair ability compared to the F9 cells. Parallel experiments using the repair-defective xeroderma pigmentosum (XP) group A fibroblasts and the normal VA13 fibroblasts also indicated that over-production of UVDRP binding activity was associated with enhanced DNA repair and UV resistance. The findings indicate that prolonged culture of F9 cells can establish a condition sufficient to cause constitutive over-production of UVDRP binding activity and UV resistance. The results also suggest that the RA/cAMP-inducible UVDRP in F9 stem cells may be important for the sensitivity or resistance of the cells to UV damage.

Detection of DNA damage-recognition proteins using the band-shift assay and southwestern hybridization

We describe electrophoresis and biochemical conditions that allow detection of damaged DNA-binding proteins in cell extracts. In addition, we present an overview of the damage-recognition DNA-binding proteins from eukaryotic cells and discuss their hypothetical role in DNA repair.

Isolation and characterization of cDNA clones encoding the Drosophila homolog of the HMG-box SSRP family that recognizes specific DNA structures

Recently an HMG-box protein denoted SSRP1, for structure-specific recognition protein 1, has been discovered which binds to specific DNA structural elements such as the bent, unwound conformations that occur upon the formation of intrastrand crosslinks by the anticancer drug cisplatin. The SSRP family includes the mouse protein T160, which recognizes recombination signal sequences. In order to delineate functional domains more clearly, a homolog of SSRP1 was cloned from Drosophila melanogaster. This homolog maps to polytene region 60A (1-4) and shares 54% identity with human SSRP1. Comparison of the predicted amino acid sequences among SSRP family members reveals 48% identity, with structural conservation in the carboxy terminus of the HMG box as well as domains of highly charged residues. Interestingly, however, the most highly conserved regions of the protein are in the less well understood amino terminus, strongly suggesting that this portion of the protein is critical for its function.

Cisplatin-DNA damage recognition proteins in human tumour extracts

Enhanced repair of DNA adducts may be a cause of cis-diamminedichloroplatinum(II) resistance in solid malignancies. Binding of specific damage recognition proteins to the sites of DNA damage may be involved in the initial steps of DNA repair, or alternatively may block access of repair proteins to damaged DNA. Proteins which bind specifically to CDDP-modified DNA were identified in cell extracts from human ovarian carcinoma cell lines by two assays, the gel mobility shift assay and the southwestern blot. In the first assay, proteins complexed with CDDP-modified oligonucleotide and produced two retarded bands, B1 and B2. The B2 complex was partially purified from an ovarian cell extract by anion exchange FPLC, and was shown to bind to DNA damaged by CDDP but not by transDDP or UV irradiation. Using the southwestern blot, proteins of 97, 48, and 25 kD were identified; each of these bound to CDDP-modified but not undamaged oligonucleotide. The partially purified B2 protein fraction contained both the 97 and the 25 kD damage recognition proteins. A human ovarian carcinoma cell line selected in vitro for CDDP-resistance (OV1P/DDP), which is 5-fold more resistant to CDDP than the parental line (OV1P), showed an increase in binding of the 97 and 48 kD damage recognition proteins compared with the parental line. Twelve ovarian cell lines differed by up to 3-fold in their expression of these proteins, but there was no correlation between the amount of damage recognition protein in a cell extract and the cellular sensitivity to CDDP. Damage recognition proteins were also demonstrated in extracts prepared from biopsies of human ovarian, cervical, and testicular malignancies, but there was no apparent difference in the binding activity in extracts from tumours of different CDDP-sensitivity. The functional role of these damage recognition proteins remains to be established.

Differential expression of pyrimidine dimer-binding proteins in normal and UV light-treated vertebrate cells

The expression of UV damage-specific DNA-binding proteins was examined in various phylogenetically distant species with differing DNA repair phenotypes. Two distinct constitutive DNA-binding activities, one specific for cyclobutane pyrimidine dimers and the other for non-cyclobutane dimer photoproducts, were detected. The expression of these binding activities was found to be variable throughout the animal kingdom: cold-blooded vertebrates show a constitutive cyclobutane dimer-binding activity exclusively, and primates reveal only non-cyclobutane binding activity. In contrast, birds and marsupials appear to express both types of binding activities. The kinetics of expression (rather than the constitutive presence) of these UV damage-specific DNA-binding activities after UV treatment correlate with the cell's capacity for DNA repair. In addition, cyclobutane pyrimidine dimer-binding activities could be detected only in cells with established photoreactivating activity.

Binding activities of cis-platin-damage-recognition proteins in human tumour cell lines

Proteins can be detected by South-western analyses of human tumour-cell extracts binding to double-stranded oligonucleotide DNA treated in vitro with the chemotherapeutic drug cis-diamminedichloroplatinum (II) (CDDP), but not to untreated DNA. The relative molecular masses of proteins binding to the CDDP-treated double-stranded oligonucleotide are 25, 48 and 97 kDa. The binding activity of 2 of the CDDP-damage-recognition proteins, of relative molecular mass 48 and 97 kDa, is greater in a CDDP-resistant human ovarian tumour cell line than in the parental sensitive line. South-western analysis of a panel of human bladder cell lines and CDDP-sensitive testicular cell lines show consistent patterns of CDDP-damage-recognition proteins within each cell type, however with differences between the 2 cell types. Binding of the proteins to CDDP-damaged DNA and the altered binding activity detected in tumour cell lines suggests that alteration in damage-recognition proteins could play a role in tumour response to CDDP.

Characterization of a DNA-damage-recognition protein from F9 teratocarcinoma cells, which is inducible by retinoic acid and cyclic AMP

A nuclear protein that recognizes u.v.-damaged DNA was detected in extracts from murine F9 embryonic stem cells using a DNA-binding assay. The nuclear-protein-binding activity was increased in cells after treatment with retinoic acid/dibutyryl cyclic AMP (dbcAMP), with optimum induction at 6 days. In vitro treatment of nuclear extracts with agents that affect protein conformation (such as urea, Nonidet P40 and Ca2+) slightly modulated the damage-recognition activity. Furthermore, treatment of nuclear extracts with phosphatase dramatically inhibited the binding activity. In addition, damaged-DNA recognition of the nuclear extracts was effectively inhibited by damaged double- and single-stranded DNA. The expression of the nuclear protein with similar characteristics was abundant in HeLa cells and was increased in drug- or u.v.-resistant cells. The findings suggest that the recognition of a u.v.-DNA adduct is modulated, at least in part, by an activity that is induced during retinoic acid/dbcAMP-induced differentiation. These results also imply that the identified damage-recognition protein may be important for the sensitivity or resistance of mammalian cells to DNA damage.

DNA excision-repair synthesis is enhanced in a murine leukemia L1210 cell line resistant to cisplatin

Among various molecular mechanisms of cell resistance to antitumor agents such as cisplatin, it has recently been suggested that enhanced DNA-repair activity might be involved in the resistant phenotype of cell lines. Mouse leukemia-cisplatin-resistant cell lines L1210/10 (adapted in vitro) and L1210/DDP (adapted in vivo) have been reported to exhibit an increase DNA-repair activity, as determined by host-cell reactivation after transformation with damaged plasmids. In this paper, excision-repair activity was monitored by an in-vitro assay allowing quantification of DNA-repair synthesis in cell extracts from resistant and sensitive parental cells (L1210/10 versus L1210/0 and L1210/DDP versus L1210/S). Experimental conditions for optimal repair-synthesis activity were found to be different from these reported with human cell-line extracts. L1210/S sensitive cell line, grown in vivo by a weekly intraperitoneal graft in mice, displayed a repair activity about fourfold lower than the same cell line maintained in vitro or than L1210/0 cell grown in vitro. The repair activity was found similar in a L1210/10 and L1210/0 cell lines, but it was enhanced in L1210/DDP resistant cell line when compared with its parental line.

Characterization of high mobility group protein binding to cisplatin-damaged DNA

cis-Diamminedichloroplatinum (II) (cisplatin, CDDP) is a widely used chemotherapeutic agent. While many tumors are highly responsive to CDDP, certain tumors are resistant to this drug, limiting its efficacy. The anti-tumor activity of CDDP is believed to result from its coordination bonding to chromosomal DNA. Alterations in tumor cell sensitivity to CDDP may result from the presence or absence of protein(s) which specifically recognize CDDP-damaged DNA. We have developed a damaged-DNA affinity precipitation assay that allows the direct identification of cellular proteins that bind to CDDP-damaged DNA. Using this procedure, we have identified several proteins which specifically bind to CDDP-damaged DNA. Two of these proteins have been identified as high mobility group proteins (HMG) 1 and 2 in the current report, we have characterized the binding of these proteins to CDDP-DNA. The calculated Kd of binding to CDDP-damaged DNA was 3.27 x 10(-10) for HMG1 and 1.87 x 10(-10) for HMG2. Using highly specific chemical modifying reagents, we have determined that Cys residues play an important role in protein binding. We also observed that HMG2 will bind to DNA modified with carboplatin and iproplatin although to a lesser extent than to DNA damaged with CDDP. Thus, our results indicate that HMG 2 binds with high affinity to DNA modified with therapeutically active platinum compounds. In addition, our findings suggest that thiol groups play an essential role in the binding of HMG1 and HMG2 to CDDP-DNA.

An intrastrand d(GpG) platinum crosslink in duplex M13 DNA is refractory to repair by human cell extracts

We have examined the ability of human cell extracts to repair the most frequent DNA adduct caused by the cancer chemotherapeutic agent cis-diamminedichloroplatinum(II). A circular DNA duplex with an intrastrand d(GpG) crosslink positioned at a specific site was synthesized. Human cell extracts were unable to induce repair synthesis in a 29-base-pair region encompassing the adduct or in adjacent regions. The same extracts could repair a single defined 2-acetylaminofluorene lesion in a similar location. When molecules containing the platinum adduct were cleaved by Escherichia coli UvrABC enzyme, human cell extracts could perform repair synthesis at the damaged site, suggesting that human enzymes fail to make incisions near the d(GpG) crosslink but can complete repair once incisions are made. This result indicates that most repair synthesis in DNA damaged with multiple cis-diamminedichloroplatinum(II) adducts takes place at lesions other than the predominant d(GpG) crosslink. These data support the idea that the clinical effectiveness of cis-diamminedichloroplatinum(II) may be explained by the inefficient repair of the major DNA adduct caused by this drug.

Isolation and characterization of human cDNA clones encoding a high mobility group box protein that recognizes structural distortions to DNA caused by binding of the anticancer agent cisplatin

Human cDNA clones encoding a structure-specific recognition protein, SSRP1, that binds specifically to DNA modified with cisplatin have been isolated and characterized. The SSRP1 gene maps to human chromosome 11q12. The cDNA clones, obtained by using partial-length cDNAs described previously, predict an 81-kDa protein containing several highly charged domains and a stretch of 75 amino acids 47% identical to a portion of the high mobility group (HMG) protein HMG1. This HMG box most likely constitutes the structure recognition element for cisplatin-modified DNA, with the probable recognition motif being the local duplex unwinding and bending toward the major groove that occurs upon formation of intrastrand cis-[Pt(NH3)2]2+ d(GpG) and d(ApG) cross-links. Although the DNA recognition properties of members of the HMG-box family of proteins have been characterized with respect to their sequence specificity, the present work demonstrates that proteins with this domain can recognize particular DNA structures as well. The Pt-DNA SSRP described here is the human homolog of a recently identified mouse protein that binds to recombination signal sequences [Shirakata, M., Hüppi, K., Usuda, S., Okazaki, K., Yoshida, K. & Sakano, H. (1991) Mol. Cell. Biol. 11, 4528-4536]. These sequences have been postulated to form stem-loop structures, further implicating local bends and unwinding in DNA as a recognition target for HMG-box proteins. Expression analysis in a variety of tissues and cisplatin-resistant cell lines and the inability of cisplatin to induce the message in HeLa cells argue against a direct link between SSRP1 mRNA levels and the response of cells to the drug.

Specific binding of chromosomal protein HMG1 to DNA damaged by the anticancer drug cisplatin

The mechanism of action of the anticancer compound cis-diamminedichloroplatinum(II) (cisplatin) involves covalent binding to DNA. In an effort to understand the tumor-specific cytotoxicity of such DNA damage, the interactions of these lesions with cellular proteins have been studied. One such protein has been identified as the high-mobility group protein HMG1. Recombinant rat HMG1 binds specifically (dissociation constant 3.7 +/- 2.0 x 10(-7) molar) to DNA containing cisplatin d(GpG) or d(ApG) intrastrand cross-links, which unwind and bend DNA in a specific manner, but not to DNA modified by therapeutically inactive platinum analogs. These results suggest how HMG1 might bind to altered DNA structures and may be helpful in designing new antitumor drugs.

Damage-recognition proteins as a potential indicator of DNA-damage-mediated sensitivity or resistance of human cells to ultraviolet radiation

We have previously identified damage-recognition proteins that bind to cisplatin[cis-diamminedichloroplatinum(II), a DNA cross-linking agent]- or u.v.-modified DNA in HeLa cells [Chao, Huang, Huang & Lin-Chao (1991) Mol. Cell. Biol. 11, 2075-2080; Chao, Huang, Lee & Lin-Chao (1991) Biochem. J. 277, 875-878]. In the present study we compared damage-recognition proteins in cells expressing different sensitivities to DNA damage. An increase in damage-recognition proteins and an enhancement of plasmid re-activation were detected in HeLa cells resistant to cisplatin and u.v. However, repair-defective cells derived from xeroderma-pigmentosum (a rare skin disease) patients did not express less cisplatin damage-recognition proteins than repair-competent cells, suggesting that damage-recognition-protein expression may not be related to DNA repair. By contrast, cells resistant to DNA damage consistently expressed high levels of u.v.-modified-DNA damage-recognition proteins. The results support the notion that u.v. damage-recognition proteins are different from those that bind to cisplatin. These findings also suggest that the damage-recognition proteins identified could be used as potential indicators of the sensitivity or resistance of cells to u.v.

Characterization of a UV-damage recognition factor in vitro that is associated with UV resistance in HeLa cells

We have previously reported a cisplatin-selected HeLa cell line showing cross-resistance to ultraviolet (UV) radiation and overexpression of UV-damage recognition factors (Chao et al., Mol. Cell. Biol., 11, 2075-2080, 1991). Here, we further characterize a UV-damage recognition factor in vitro using a gel mobility shift assay. The results indicate that the damage-recognition factor is (i) localized mostly in the nucleus, (ii) protease-sensitive, (iii) RNA-independent, (iv) active in a wide range of ionic strengths (50-400 mM NaCl), (v) with a high affinity for UV-damaged DNA (50-fold molar excess competitor causes 50% recognition loss), and (vi) resistant to agents and that modify protein conformation (urea and NP-40), but slightly sensitive to CaCl2. The significance of the identified UV-damage recognition factor in the sensitivity or resistance of cells to UV is also discussed.

Ca(2+)-mediated inhibition of a nuclear protein that recognizes UV-damaged DNA and is constitutively overexpressed in resistant human cells: DNA-binding assay

A nuclear protein that recognizes UV-damaged DNA was detected from HeLa cells using DNA-binding assay. Treatment of cells with Ca2+ ionophore (A23187) caused a dramatic inhibition of the damage-recognition activity. In contrast, in vitro treatment of nuclear extracts with agents that affect protein conformation (such as urea, NP40 and Ca2+) did not significantly affect on the damage-recognition activity. The Ca(2+)-mediated inhibition of UV damage recognition was reconstituted by the addition of the cytosolic extracts, suggesting that the Ca2+ effect does not directly act on the UV damage-recognition protein. The expression of the detected nuclear protein was increased in UV-resistant HeLa cells. In contrast, the level of this protein was dramatically reduced in UV-sensitive xeroderma pigmentosum group A cells. In addition, UV damage-recognition protein is resistant to RNase, and is independent of the previously identified proteins that bind cisplatin-DNA adduct. These findings implied that the recognition of UV-DNA adduct is modulated by the intracellular level of Ca2+.