Formation and repair of furocoumarin adducts in alpha deoxyribonucleic acid and bulk deoxyribonucleic acid of monkey cells

An efficient new method for extraction, separation and purification of psoralen and isopsoralen from Fructus Psoraleae by supercritical fluid extraction and high-speed counter-current chromatography

Psoralen and isopsoralen were extracted from Fructus Psoraleae (Psoralea corylitolia L.) by supercritical CO2. The effect of various parameters, i.e., pressure, temperature and sample particle size on yield was investigated with an analytical-scale supercritical fluid extraction (SFE) system to find the optimal conditions. The process was then scaled up by 50 times with a preparative SFE system under the optimized conditions of pressure (26 MPa), temperature (60 degrees C) and a sample particle size of 40-60 mesh. The yield of the preparative SFE was 9.1% and the combined yield of psoralen and isopsoralen was 2.5 mg/g of dry seeds. Psoralen and isopsoralen in the extract were separated and purified by high-speed counter-current chromatography with a two-phase solvent system composed of n-hexane-ethyl acetate-methanol-water (1:0.7:1:0.8, v/v), and the fractions were analyzed by HPLC, MS, 1HNMR and 13C NMR. The structures of the products were further confirmed by comparison with authentic samples (National Institute of the Control of Pharmaceutical and Biological Products, Beijing, China).

Psoralen cross-linking as probe of torsional tension and topological domain size in vivo

DNA within a cell is organized with unrestrained torsional tension, and each molecule is divided into multiple individual topological domains. Psoralen photobinding can be used as an assay for supercoiling and topological domain size in living cells. Psoralen photobinds to DNA at a rate nearly linearly proportional to superhelical density. Comparison of the rate of photobinding to supercoiled and relaxed DNA in cells provides a measure of superhelical density. For this, in vivo superhelical tension is relaxed by the introduction of nicks by either ionizing radiation or photolysis of bromodeoxyuridine in the DNA. Since nicks are introduced in a random fashion, the distribution of nicks is described by a Poisson distribution. Thus, after nicking, the fraction of topological domains containing no nicks is described by the zero term of the Poisson distribution. From measurement of the number of nicks introduced in the DNA and the fraction of torsional tension remaining, an average topological domain size can be estimated. Using this logic, procedures were designed and described for measuring supercoiling and domain size at specific sites in eukaryotic genomes.

Mutagenic and carcinogenic properties of platinum-based anticancer drugs

Cisplatin (DDP) is currently one of the most effective drugs for the treatment of cancer. It causes primarily intrastrand DNA-DNA cross-links, and is highly mutagenic and carcinogenic in both in vitro and in vivo experimental models. There is, however, considerable variability between the response seen in different cellular systems, probably at least partly because of the different cellular DNA repair capacities. A number of analogues of cisplatin have been developed and one of these, carboplatin (CDDCA), is also in widespread clinical use. Although it is somewhat less toxic, there is no evidence that its mode of action differs from that of cisplatin. A limited amount of mutagenicity data suggests that it has similar mutagenic and carcinogenic consequences as the parent drug. Many further analogues of cisplatin are now in clinical trials, and some of these appear to have different DNA repair responses (and therefore possibly the development of clinical resistance). Although some (e.g., iproplatin and spiroplatin) are less mutagenic than either cisplatin or carboplatin, these appear to be the ones least likely to achieve wide use. There are insufficient data on several of the most promising clinical analogues (e.g., DWA2114R and ACDDP) to judge their relative mutagenic and carcinogenic potential. Detailed studies on the DNA repair and mutagenicity characteristics of these compounds will not only provide clinically relevant data, but may also aid in the selection of further useful antitumour agents in this series.

Phototoxicity of plant secondary metabolites: insect and mammalian perspectives

Phototoxic chemicals produced by plants have been studied in a number of contexts, most notably as protective agents against mammalian and insect herbivores. Although there are commonalities in the responses of these two groups of herbivores to plant phototoxins, there are differences as well. Whereas a greater range of chemical classes has been demonstrated to display phototoxicity against insects, considerably more information is available on symptomology of phototoxicity and mechanisms of action in mammals. The commonalities include alterations in behavior following ingestion, notably photophobia, disruption of integumentary integrity following contact or ingestion, and metabolic detoxification following ingestion, in the case of furanocoumarins involving cytochrome P450 monooxygenases. Not yet known to exist in insects are phototoxin-mediated effects on sensory (particularly visual) systems and phototoxicity resulting from abnormal chlorophyll metabolism. In order to gain greater understanding of the ecological significance of phototoxin-mediated plant defense against both insects and mammals, there is a need for more studies centered on natural associations.

8-Methoxypsoralen DNA interstrand cross-linking of the ribosomal RNA genes in Tetrahymena thermophila. Distribution, repair and effect on rRNA synthesis

The distribution and repair of 8-methoxypsoralen-DNA interstrand cross-links in the ribosomal RNA genes (rDNA) in Tetrahymena thermophila have been studied in vivo by Southern blot analysis. It is found that the cross-links at a density of < or = 1/2 x 10(4) base pairs (bp) are distributed equally between three domains (terminal spacer, transcribed region and central spacer) as defined by restriction enzyme analysis (BamHI and ClaI). It is furthermore shown that a dosage resulting in approximately one cross-link per rDNA molecule (21 kbp, two genes) is sufficient to block RNA synthesis. Finally, it is shown that the cross-links in the rDNA molecules are repaired at equal rate in all three domains within 24 h and that RNA synthesis is partly restored during this repair period. The majority of the cells also go through one to two cell divisions in this period but do not survive.

Repair of the two diastereoisomer photoadducts formed between 7-methylpyrido(3,4-c)psoralen (MePyPs) and thymidine in yeast cells: a chemical approach

When analysing the repair of psoralen plus UVA-induced photoadducts in DNA, it must be realized that, in most cases, different isomers are formed. The monofunctional psoralen derivative 7-methylpyrido(3,4-c)psoralen (MePyPs) is known for its high antiproliferative activity at the cellular level and interesting photochemotherapeutic properties. To understand its photobiological efficiency in more detail, the induction of specific photoadducts in DNA and their repair were analysed in a eukaryotic cell system, the yeast Saccharomyces cerevisiae. After photoaddition of MePyPs, two main diastereoisomers were characterized after enzymatic hydrolysis of the DNA and analysis by high performance liquid chromatography. One diastereoisomer was more effectively repaired in yeast than the other during post-treatment incubation, suggesting that the two diastereoisomers may be recognized differently by cellular enzymatic repair systems.

DNA sequence selectivity of guanine-N7 alkylation by nitrogen mustards is preserved in intact cells

Nitrogen mustard alkylating agents react with isolated DNA in a sequence selective manner, and the substituent attached to the drug reactive group can impose a distinct sequence preference. It is not clear however to what extent the observed DNA sequence preferences are preserved in intact cells. The highly reiterated sequence of human alpha DNA has been used to determine the sites of guanine-N7 alkylation following treatment of cells with three nitrogen mustards, mechlorethamine, uracil mustard and quinacrine mustard, known to react in isolated DNA with distinctly different sequence preferences. Alpha DNA from drug treated cells was extracted, purified, end-labeled, and a 296 base pair, singly end-labelled, fragment isolated. Following the quantitative conversion of alkylation sites to strand breaks the fragments were separated on DNA sequencing gels. Clear differences were observed between the alkylation patterns of the three compounds, and the selectivities were qualitatively similar to those predicted and observed in the same sequence alkylated in vitro. In particular the unique preferences of uracil and quinacrine mustards for 5'-PyGC-3' and 5'-GT/GPu-3' sequences, respectively, were preserved in intact cells suggesting that the pattern of sequence dependent reactivity is not grossly affected by the nuclear milieu.

Formation and removal of 8-MOP-DNA photoadducts in keratinocytes: effects of calcium concentration and retinoids

8-methoxypsoralen (8-MOP)-DNA photoadducts were quantified in freshly isolated human and murine keratinocytes and cultured keratinocyte cell lines after in vitro treatment with 8-MOP (1-200 ng/ml) and ultraviolet A (UVA; 0.2-24.0 J/cm2). Greater doses of 8-MOP and UVA led to proportionately greater numbers of photoadducts, with a dose reciprocity relationship between the amounts of 8-MOP and UVA. No significant difference in photoadduct formation was observed between basal and differentiated cells. However, the transformed keratinocyte cell lines showed fewer photoadducts than did normal keratinocytes, which appeared to be correlated with the finding that the adduct formation was inhibited in normal keratinocytes cultured with phorbol 12-myristate 13-acetate, because this agent leads to epidermal hyperproliferation. In viable keratinocytes that were treated with a sublethal dose of 8-MOP and UVA (15 ng/ml and 1 J/cm2, respectively), 54% of photoadducts formed were removed over a 20-h period. Adduct removal depended on the calcium concentration in the media; cells cultured in standard high calcium levels showed a higher removal rate than those cultured in low-calcium media. The addition of retinoids (etretinate, acitretin, and 13-cis retinoic acid) to the culture induced 55 to 80% of suppression of the adduct removal. The calcium ionophore A23187 partially restored the suppression of photoadduct removal induced by retinoids. The present studies suggest that calcium performs an important role in the photoadduct removal and raise the possibility that the synergism of systemic retinoids and psoralen plus UVA photochemotherapy relates to the former's inhibition of repair of 8-MOP photoadducts in DNA.

DNA damage by 8-methoxypsoralen plus near ultraviolet light (PUVA) and its repair in Escherichia coli: genetic analysis

Mutants of Escherichia coli, hyper-resistant and sensitive to 8-methoxypsoralen plus near ultraviolet light (PUVA) have been isolated and studied. Results show that a mutation, located at 57.2 min on the linkage map of E. coli, is responsible for the hyper-resistant phenotype. It is also responsible for the synthesis of a 55-kdal protein in high concentrations. In a wild-type cell the synthesis of this enzyme is inducible by mitomycin C. There are indications that the mutation may have occurred in a regulatory gene, puvR, and as a result the operon, including a putative puvA gene (the structural gene for the synthesis of the 55-kdal protein), is expressed constitutively. A model for the control of the PUV operon is proposed.

Molecular dosimetry of 8-MOP + UVA-induced DNA photoadducts in Saccharomyces cerevisiae: correlation of lesions number with genotoxic potential

Acid hydrolysis of purified DNA extracted from cells of a haploid repair-proficient (RAD) yeast strain that had been treated with 8-MOP + UVA revealed the existence of two major and one minor thymine photoproduct. At survival levels of the RAD strain between 100% and 1% furanside monoadducts constituted the major DNA lesion, followed by diadducts that, at the lowest survival level, nearly reached 50% of the thymine photoproducts; pyrone-side monoadducts were only detectable at the highest UVA exposure dose applied and clearly constitute a minority photoproduct. The number of induced diadducts was verified by determination of interstrand cross-links via denaturation and renaturation of 8-MOP + UVA-treated DNA from RAD and rad2 yeast strain. 8-MOP + UVA was shown to induce two types of locus-specific mutations: reversion of the lys1-1 ochre allele was between 20- to 50-fold higher than that of the his4-38 frameshift allele. Mutant yield for the lys 1-1 reversion was the same in RAD and excision repair-deficient rad2-20 strains whereas frameshift mutagenesis was about eightfold higher in the rad2-20 background.

UVA-induced binding of 8-methoxypsoralen to cells of Saccharomyces cerevisiae: separation and characterization of DNA photoadducts

We present methods for the determination of UVA-induced binding of 8-methoxypsoralen (8-MOP) to nucleic acids and protein and for a quantitative assay of radioactively labelled 8-MOP plus UVA induced DNA photoproducts in the yeast Saccharomyces cerevisiae. For the dose range up to 60 kJ m-2, with a wild-type survival of 1% or higher, binding to DNA is 100-fold and to RNA 10- to 20-fold more efficient than that to protein. Between 20% and 65% of the 8-MOP binds to macromolecules that are neither nucleic acids nor protein. The number of DNA-bound 8-MOP molecules for the haploid genome rises from 14 (unirradiated control) to 349 at the highest UVA exposure dose (60 kJ m-2). Gel chromatography reveals three types of DNA thymine photoproduct, the pyrone-side monoadducts, the furan-side monoadducts and the diadducts. Among these, pyrone-side monoadducts always constitute the smallest fraction, regardless of whether the treatment is with in vitro or in vivo 8-MOP plus UVA.

General method for quantifying base adducts in specific mammalian genes

A general method has been developed to measure the formation and removal of DNA adducts in defined sequences of mammalian genomes. Adducted genomic DNA is digested with an appropriate restriction enzyme, treated with Escherichia coli UvrABC excision nuclease (ABC excinuclease), subjected to alkaline gel electrophoresis, and probed for specific sequences by Southern hybridization. The ABC excinuclease incises DNA containing bulky adducts and thus reduces the intensity of the full-length fragments in Southern hybridization in proportion to the number of adducts present in the probed sequence. This method is similar to that developed by Bohr et al. [Bohr, V. A., Smith, C. A., Okumoto, D. S. & Hanawalt, P. C. (1985) Cell 40, 359-369] for quantifying pyrimidine dimers by using T4 endonuclease V. Because of the wide substrate range of ABC exinuclease, however, our method can be used to quantify a large variety of DNA adducts in specific genomic sequences.

Interstrand crosslinks due to 4,5',8-trimethylpsoralen and near ultraviolet light in specific sequences of animal DNA. Effect of constitutive chromatin structure and of induced transcription

We have used low-level photocrosslinkage to study chromatin effects on psoralen intercalation at specific DNA sequences of various complexities in intact, cultured, Drosophila cells. Alkali-denatured DNA connected in both strands to a 4,5',8-trimethylpsoralen (TMP) interstrand crosslink is insensitive to digestion by the single strand-specific nuclease S1 and does not hybridize to complementary DNA. Crosslink number at any ultraviolet light exposure increases in proportion to the concentration [PS] of TMP dark binding sites that are occupied. The crosslinking constant, K, is the increase in crosslink number per length DNA per increment [PS]. Many factors influence K, including sequence composition and ionic strength. We show here that the ratio of K at any specific sequence (Kh, from hybridization measurements) to Kh at any other specific sequence or to K of total DNA (Kf, from fluorimetry measurements) can be calculated from measurements of crosslinkage, the mass fraction of the sequence in question or of total DNA that is connected in both strands to a crosslink. When crosslinked and uncrosslinked DNAs fragmented by mechanical shear were mixed in known proportions, Kf exceeded Kh of a single-copy gene by 15%. We treated cells with TMP plus near ultraviolet light, then tested for crosslinkage and for hybridization. A single-copy, larval gene at 70D, and a 250-copy type 1 ribosomal DNA intervening sequence, neither of which is transcribed in these cells, were as sensitive to crosslinkage as total, cell DNA. However, single-copy, heat shock gene sequences from loci 63BC and 95D, and the 180-copy ribosomal DNA coding sequence were more sensitive to crosslinkage than total DNA in the same preparations. The excess was largest in the shortest fragments, indicating a localized effect. The same sequences were crosslinked less readily than total DNA in vitro; we calculate a 3.4 to 3.8-fold excess crosslink number in these sequences due to chromatin microenvironment. We tested for effect of transcriptional induction on crosslink sensitivity in the heat shock genes. At low [TMP], heat shock stimulated crosslinkage at or very near heat shock genes in cells, but not in other sequences or in naked DNA. However, overall crosslink sensitivity was unaffected by heat shock. This suggests that transcription increased the affinity of some heat shock gene DNA binding sites for TMP without increasing the number of such sites.

DNA repair by articular chondrocytes. IV. Measurement of Micrococcus luteus endonuclease-sensitive sites by alkaline elution in rabbit articular chondrocytes

The ability of resting and dividing rabbit articular chondrocytes to repair low doses of ultraviolet (UV) damage was measured through removal of UV endonuclease-sensitive sites (ESS, pyrimidines dimers) as measured by alkaline elution. The repair of damage was significantly (P less than 0.001) greater in dividing than non-dividing cells. An age-related decrease in repair capability was found in resting chondrocytes, but not in their dividing counterpart. These results support earlier findings of unscheduled DNA synthesis by the same cells. (Mech. Ageing Dev., 32: 39-55, 1985).

Processing of psoralen adducts in an active human gene: repair and replication of DNA containing monoadducts and interstrand cross-links

We have examined DNA repair in the dihydrofolate reductase (DHFR) gene in cultured human cells treated with 4'-hydroxymethyl-4,5',8-trimethylpsoralen (HMT) using a newly developed assay for interstrand DNA cross-linking in defined genomic sequences. Within 24 hr, 80% of the cross-links, but only 45% of the monoadducts, were removed from a 32 kb transcribed sequence, demonstrating that repair efficiency in an active human gene varies with the nature of the damage. HMT monoadducts were also detected in the replicated DHFR sequence at frequencies indicating little interference with replication. The existence of cross-linkable monoadduct sites in the replicated DNA implies strand continuity opposite those sites and a relatively error-free mechanism of bypass. Translesion replication could circumvent transcription blockage in a damaged gene. These findings have important implications for mechanisms of mutagenesis and DNA lesion tolerance in human cells.

Mutagenesis by 8-methoxypsoralen plus near-UV treatment: analysis of specificity in the lacI gene of Escherichia coli

We have studied the specificity of mutation induced by PUVA treatment in the lacI gene of E. coli. Cells were exposed to near UV (approximately 365 nm) in the presence of 8-methoxypsoralen under conditions yielding about 7% survival and a 10-fold increase in mutation frequency. The cloning and sequencing of 131 mutants recovered following PUVA treatment revealed that almost all classes of mutation including base substitutions, frameshifts and deletions were induced. The distribution of mutations was non-random and a region of the lacI gene was found to be virtually silent for all classes of mutation. Intriguingly, the broad spectrum of mutation is accompanied by the recovery of mutation at two spontaneous hotspots. We observed a 7-fold increase at a frameshift hotspot involving the gain or loss of a tetramer tandemly repeated 3 times at this site and a 23-fold increase at an A:T----G:C transition hotspot located at the +6 mutational spectrum recovered following PUVA treatment was unique and a detailed analysis of the different classes of mutations indicates a role for DNA repair of both monoadducts and cross-links in the production of mutation.

Differential repair of DNA damage in specific nucleotide sequences in monkey cells

An immunological method was developed that isolates DNA fragments containing bromouracil in repair patches from unrepaired DNA using a monoclonal antibody that recognizes bromouracil. Cultured monkey cells were exposed to either UV light or the activated carcinogen aflatoxin B1 and excision repair of damage in DNA fragments containing the integrated and transcribed E. coli gpt gene was compared to that in the genome overall. A more rapid repair, of both UV and AFB1 damage was observed in the DNA fragments containing the E. coli gpt genes. The more efficient repair of UV damage was not due to a difference in the initial level of pyrimidine dimers as determined with a specific UV endonuclease. Consistent with previous observations using different methodology, repair of UV damage in the alpha sequences was found to occur at the same rate as that in the genome overall, while repair of AFB1 damage was deficient in alpha DNA. The preferential repair of damage in the gpt gene may be related to the functional state of the sequence and/or to alterations produced in the chromatin conformation by the integration of plasmid sequences carrying the gene.

Cell-cycle-dependent repair of damage in alpha and bulk DNA of monkey cells

Excision repair of bulky chemical adducts in alpha DNA of confluent cultures of African green monkey cells has previously been shown to be deficient relative to that in the overall genome. We have compared the removal of adducts produced by treatment with aflatoxin B1 (AFB1) and N-acetoxy-2-acetylamino-fluorene (NA-AAF) from alpha DNA sequences in synchronized and exponentially growing cultures of monkey cells. Proficient removal of AFB1 adducts in alpha DNA was observed in exponentially growing cultures. However, as the cultures approached confluence, adduct removal from alpha DNA became deficient. Cells synchronized by subculturing confluent cultures exhibited proficient removal of adducts from both alpha and bulk DNA when treated in early G1 or late S/G2 while those cells treated in early S phase did not remove adducts from either alpha or bulk DNA. We conclude that the accessibility of chemical adducts to repair in alpha chromatin is influenced by the growth state and the cell cycle stage.

Stimulation of deoxyribonucleic acid excision repair in human fibroblasts pretreated with sodium butyrate

The effect of pretreatment with sodium butyrate on DNA excision repair was studied in intact and permeable confluent (i.e., growth-inhibited) diploid human fibroblasts. Exposure to 20 mM sodium butyrate for 48 h increased subsequent ultraviolet (UV)-induced [methyl-3H]thymidine incorporation by intact AG1518 fibroblasts by 1.8-fold and by intact IMR-90 fibroblasts by 1.2-1.3-fold. UV-induced incorporation of deoxy[5-3H]cytidine, deoxy[6-3H]cytidine, and deoxy[6-3H]uridine, however, showed lesser degrees of either stimulation or inhibition in butyrate-pretreated cells. This result suggested that measurements of butyrate's effect on DNA repair synthesis in intact cells are confounded by simultaneous changes in nucleotide metabolism. The effect of butyrate on excision repair was also studied in permeable human fibroblasts in which excision repair is dependent on exogenous nucleotides. Butyrate pretreatment stimulated UV-induced repair synthesis by 1.3-1.7-fold in permeable AG1518 cells and by 1.5-2-fold in permeable IMR-90 cells. This stimulation of repair synthesis was not due to changes in repair patch size or composition or in the efficiency of DNA damage production but rather resulted from a butyrate-induced increase in the rate of damage-specific incision of DNA. The increased rate of incision in butyrate-pretreated cells could be due either to increased levels of enzymes mediating steps in excision repair at or before incision or to alterations in chromatin structure making damage sites in DNA more accessible to repair enzymes.

Photoreaction of 8-methoxypsoralen with yeast-tRNAPhe. Identification of the major reaction sites

Yeast tRNAPhe was photoreacted with [3H]8-methoxypsoralen and the product was digested with ribonuclease T1, ribonuclease A or a combination of the two or cleaved with sodium borohydride/aniline. The oligonucleotides from these digestions were analyzed by polyacrylamide gel electrophoresis or high-pressure liquid chromatography and the psoralen-containing fragments were identified. The results indicate that one major and two minor photoreaction sites for 8-methoxypsoralen exist in yeast tRNAPhe. The major site (containing about 55% of the label) was determined as U50 in the T psi arm of the tRNA molecule while the minor sites were assigned to U59 (30% of the label) and C70 (15%) respectively. Our results suggest that psoralens may be used as photoprobes for studying conformational changes in tRNA molecules.