Phototoxic effects of four psoralens on L1210 cells. The correlation with DNA interstrand cross-linking

Photochemical and photophysical studies of 3-amino-6-iodoacridine and the inactivation of lambda phage

The photochemistry and photophysics of 3-amino-6-iodoacridine (Acr-I) was studied. Photolysis (350 nm) of Acr-I (free base) generates products consistent with a free radical intermediate in methanol, benzene and carbon tetrachloride. The Acr-I hydrochloride is shown to bind to calf thymus DNA and to the self-complementary dinucleotide cytidylyl-(3'-5')-guanosine (CpG) miniduplex in a manner similar to that of proflavine (Acr-NH2), a known DNA intercalator. The Acr-I is shown to more efficiently nick supercoiled plasmid DNA pBR322 upon 350 nm or 420 nm photolysis than Acr-NH2. The efficiency of Acr-I-sensitized DNA nicking is not oxygen dependent. Photolysis of the Acr-I/(CpG)2 complex leads to cleavage of the dinucleotide and to cytidine base release by selective damage to a specific ribose moiety. Dinucleotide cleavage occurs equally well in the presence or absence of oxygen, thereby eliminating a singlet oxygen- or peroxyl radical-mediated process. Photolysis of Acr-I in the presence of a mononucleotide (GMP) or a non-self-complementary dinucleotide (uridylyl-[3'-5']-cytidine-UpC) does not lead to fragmentation and base release. Similarly, photolysis of the Acr-NH2/(CpG)2 complex does not lead to fragmentation and base release. The data indicate that photolysis of an iodinated intercalator bound to CpG or plasmid DNA generates an intercalated aryl radical and that the reactive intermediate initiates a sequence of reactions that efficiently nick nucleic acids. The inactivation of lambda phage sensitized by Acr-I with UV (350 nm) light is oxygen independent but with visible (420 nm) light is strongly oxygen dependent. The Acr-I fluoresces more intensely when excited at 446 than at 376 nm. Thus, UV photolysis may lead to C-I bond homolysis and free radical formation, a process that is not energetically feasible with visible light. The results demonstrate the difficulty of extrapolating model studies involving simple molecules and DNA to understanding the mechanism of viral inactivation with a particular sensitizer.

Relaxation of supercoiled DNA by aminomethyl trimethylpsoralen and UV photons: action spectrum

An action spectrum for the relaxation of supercoiled plasmid DNA (induction of the first single-strand break) by photoactivated 4'-aminomethyl-4,5',8-trimethylpsoralen (AMT) has been determined using monochromatic UV photons from 254 to 405 nm. The spectrum of AMT-induced plasmid DNA relaxation fits closely with the absorbance spectrum of AMT in the spectral region between 313 nm and 405 nm but deviates at wavelengths shorter than 313 nm. This assay also reveals that the psoralen photosensitization reaction with DNA also produces piperidine-labile sites. Addition of mannitol and azide partially quenches the supercoil relaxation reaction, evidence for a role of Type II photosensitization pathway.

Therapeutic potential of plant photosensitizers

Many bioactive phytochemicals have been shown in recent years to be photosensitizers, i.e. their toxic activities against viruses, micro-organisms, insects or cells are dependent on or are augmented by light of certain wavelengths. These activities are often selective, and this has led to the concept of therapeutic prospects in the control of infectious diseases, pests and cancer. Reaction mechanisms commonly involve singlet oxygen and radicals, which are thought to cause photodamage to membranes or macromolecules. The main classes of plant photosensitizers reviewed here are polyyines (acetylenes, thiophenes and related compounds); furanyl compounds; beta-carbolines and other alkaloids; and complex quinones. We propose that within each group of phytochemicals there are several representatives that merit further study for therapeutic abilities in appropriate animal models.

Lysosomes: a possible target for psoralen photodamage

Treatment in vitro of Ehrlich ascites tumor cells or human fibroblasts with 8-methoxypsoralen (8-MOP, 2.4 microM) and UVA irradiation results in a 30% and 60% respectively reduction in lysosomal beta-galactosidase activity in situ. Under identical conditions one 8-MOP adduct was formed per 2 X 10(4) bases of DNA, one 8-MOP adduct was formed per approximately 10(4) tRNA molecules and one per approximately 100 ribosomes. It is suggested that the decrease in lysosomal beta-galactosidase activity is a result of leakage through the lysosomal membrane caused by psoralen-UVA damage of the lipids in the membrane, since no effect was found on beta-galactosidase in vitro. These results indicate that the lysosomes may also be a target for cellular photodamage by 8-methoxy-psoralen.

Induction and removal of DNA interstrand cross-links in V-79 Chinese hamster cells measured by hydroxylapatite chromatography after treatments with bifunctional furocoumarins

DNA interstrand crosslinks (CL) photoinduced by bifunctional furocoumarins in V-79 Chinese hamster cells were measured by alkaline denaturation and hydroxylapatite chromatography. Treatments with 5-methoxypsoralen (5-MOP), 8-methoxypsoralen (8-MOP) and 4,5',8-trimethylpsoralen (4,5',8-TMP) and 365 nm irradiation (UVA) confer a dose-dependent linear increase in the amount of double-stranded DNA indicating the induction of CL. Determination in alkaline sucrose gradients of the molecular weight of the DNA and estimation of drug-induced strand breakage allowed quantification of the CL induced. 5-MOP was found to be slightly more effective than 8-MOP whereas 4,5',8-TMP was 9 times more effective for the induction of CL. The fate of CL during post-treatment incubation was also followed. Cells in exponential growth phase were found to be efficient in the removal of CL.

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.

Thermal chemistry of podophyllotoxin in ethanol and a comparison of the cytostatic activity of the thermolysis products

Podophyllotoxin (1) in buffered ethanolic solution is degraded by two pathways. One leads to (a) picropodophyllin (2), which undergoes dehydration to give alpha-apopicropodophyllin (5), which rearranges to give beta-apopicropodophyllin (6), (b) the ethyl ether of picropodophyllotoxin, 8, and (c) the ethyl ether of epipicropodophyllotoxin, 7. The other pathway leads directly to epipodophyllotoxin (10) and the corresponding ethyl ether, 9, and possibly, via a transient 3,4-dehydropodophyllotoxin (5'), to beta-apopicropodophyllin (6). The 1H NMR spectra of these compounds are described, their in vitro cytostatic activity compared, and their syntheses, including that of podophyllotoxin ethyl ether, reported.

Photochemical crosslinking of protein and DNA in chromatin. II. Synthesis and application of psoralen-cystamine-arylazido photocrosslinking reagents

The synthesis and testing of a new type of nucleic acid-protein photocrosslinking reagent is described. The reagents are composed of a psoralen ligand for nucleic acid photoattachment, which is linked to an azidobenzoyl group, for protein photoattachment. The linker contains a disulfide bridge which can be opened by reduction with mercaptans. The reagents were tested in a chromatin system, where it was found that they induced cleavable crosslinks between the histones and the DNA upon irradiation with long-wavelength ultraviolet light (lambda greater than 300 nm).

Repair of 8-methoxypsoralen induced DNA interstrand cross-links in Tetrahymena thermophila. The effect of inhibitors of macromolecular synthesis

The effect of several growth-inhibiting compounds on the repair of 8-methoxypsoralen-UVA-light-induced DNA interstrand cross-links has been studied in the protozoan Tetrahymena thermophila. The repair process was analyzed by the alkaline elution technique and could be divided into 3 phases: a protein-DNA complexing phase, a DNA-incision phase and finally a DNA-ligation phase. The incision was found to be completely inhibited by novobiocin (50 micrograms/ml), nalidixic acid (150 micrograms/ml), n-butyrate (15 mM) and cycloheximide (1 microgram/ml), while no effect was observed for cytosine-1-beta-D-arabinofuranoside (10 mM), puromycin (1 mM), hydroxyurea (5 mM) or 3-aminobenzamide (2.5 mM). None of the compounds showed any effect on the protein-DNA complexing step, and the ligation was partly inhibited only by nalidixic acid (150 micrograms/ml). The involvement of topoisomerases in the repair of psoralen-induced DNA interstrand cross-links is suggested.

Psoralen-DNA crosslink repair in human lymphocytes. Comparison of alkaline elution with electron microscopy

Much interest has surrounded the question of the removal of psoralen interstrand crosslinks in DNA of eukaryotic organisms. A commonly employed method for the study of psoralen repair is alkaline elution. In this study we have used alkaline elution to assess psoralen crosslink repair in human lymphocytes. The lymphocytes were treated with 8-methoxypsoralen or 4,5',8-trimethylpsoralen and allowed to repair for different periods of time. Analysis by alkaline elution showed elution patterns compatible with crosslink removal. When the crosslink removal under comparable conditions was studied by the use of electron microscopy under totally denaturing conditions, no repair of the crosslinks could be detected.