Interaction of 8-methoxypsoralen and near-UV light causes mutation and cytotoxicity in mammalian cells

Photosensitizing compounds in the treatment of psoriasis

Photosensitizers were first used to treat psoriasis 15 years ago when the phototoxic reaction of psoralens and UVA was found to induce remissions of the disease. The effect of this reaction on DNA, particularly the formation of cross-links, was thought to be the decisive event. Strong cross-linking agents such as 8-MOP, TMP and 5-MOP are clinically effective whereas most compounds which produce only monofunctional adducts are virtually ineffective. Orally administered 8-methoxypsoralen (8-MOP) is the most widely used compound. 4,5',8-Trimethylpsoralen (TMP) is poorly absorbed from the intestine but has marked efficacy when applied topically. 5-MOP may be a useful alternative to 8-MOP because it is less erythemogenic and does not cause nausea. These three furocoumarins appear to be similar photochemically and may introduce similar risks. However, the photobiological properties of furocoumarins can be modified by altering one or more parts of the molecule. Such modifications might yield effective analogues with reduced cytogenetic hazards. Several psoralens and angular furocoumarins are being tested for effectiveness combined with fewest long-term side-effects, especially carcinogenesis. Encouraging preliminary results have been obtained with 7-methyl-pyridopsoralen and 4,6,4'-trimethylangelicin. Other important approaches to increasing the safety of photochemotherapy may be the use of different photoactivating wavelengths or the introduction of new classes of photosensitizers.

Cloning properties of T lymphocyte subpopulations after treatment with 8-methoxypsoralen and UVA irradiation

The cloning rate of PHA-stimulated T lymphocytes after treatment with 8-methoxypsoralen plus UVA irradiation described by Wunder and Reischmann (1983) gives a linear dose-effect relationship at low dosages. However, with increasing doses a flattening of the negative gradient occurs. This relationship deviates from the classical exponential curve which can be observed when fibroblasts are treated with mutagens and which is explainable by a 'recovery plateau' at lower dosages. In this study we show that some subpopulations of T lymphocytes, in particular the T-helper and T-suppressor cells, influence the overall dose-effect relationship. These isolated subpopulations exhibit varying sensitivities in comparison with their depleted cell populations. It may be assumed that heterogeneous cell populations exist within each isolated subpopulation which may be separated into further subclasses according to their specific sensitivity.

Mechanisms of photodynamic effects of furocoumarins

The photosensitizing action of furocoumarins on biological systems occurs by both an oxygen-independent pathway, which involves the photoaddition of the sensitizer to nucleic acids, proteins and lipids, and an oxygen-dependent pathway, which includes furocoumarins in the category of photodynamic sensitizers. The photodynamic action of furocoumarins, as studied using isolated biomolecules, human erythrocytes and human skin, appears to involve both activated oxygen species (singlet oxygen, superoxide anion, and hydroxyl radicals) and radical species formed by electron transfer from or to photoexcited furocoumarins. Another oxygen-dependent process involves the formation of photo-oxidized furocoumarin derivatives, which can react in the dark with several substrates (in particular, membrane components), causing an irreversible damage of cells. The latter type of process is temperature dependent. The relative importance of the different photosensitization mechanisms under various experimental conditions is discussed.

Mechanism of action of psoralens: isobologram analysis reveals that ultraviolet light potentiation of psoralen action is not additive but synergistic

The combination of psoralens and ultraviolet light (UVA, 320-400 nm), referred to as PUVA, inhibits proliferation of a variety of cell types. In the present studies, we used S-180 cells to investigate the mechanism underlying the antiproliferative actions of PUVA. We found that inhibition of growth of S-180 cells by PUVA was dependent on the concentration of psoralen as well as the dose of UVA light. Neither the psoralens nor UVA light by themselves inhibited cell growth. Several clinically important psoralen analogs inhibited cell growth. The potent phototoxin 4,5',8-trimethylpsoralen was the most active psoralen analog tested, followed by 5-methoxypsoralen and 8-methoxypsoralen. The angular furocoumarin, 5-methylangelicin, was the least active inhibitor of growth. Multivariate (isobologram) analysis of the growth-inhibition curves revealed that combinations of psoralens and UVA light were not simply additive but synergistic. Similar results were observed when inhibition of DNA synthesis was used as an endpoint for the biological effects of PUVA. These studies are the first to demonstrate that psoralens and UVA light act synergistically. Our results suggest that the synergism between the psoralens and UVA light may be an important property of PUVA that contributes to its therapeutic efficacy in proliferative diseases.

Side-effects of psoralen photochemotherapy (PUVA)

Although psoralen photochemotherapy (PUVA) is one of the most effective forms of therapy for psoriasis, the risk of potential long-term side-effects is, as yet, not clearly determined. Chronic degenerative and pigmentary skin changes similar to those of chronic solar exposure occur after long-term PUVA treatment; PUVA also causes non-melanoma skin cancers in man, although there is, as yet, no consensus as to what cumulative phototoxic PUVA dose is carcinogenic. Long-term multicentre studies from the U.S.A. indicate a definite risk of squamous cell carcinoma for long-term PUVA-treated patients, whereas European studies reveal no overall increase in skin cancers in similar patients except for those exposed to other carcinogens. Assignment to PUVA should be based on the risk:benefit ratio for the individual patient. Careful patient selection is therefore mandatory and should be limited to those who can be monitored and controlled by informed, competent and conscientious physicians.

Further evidence for photoinduced genotoxicity of dictamnine as shown by prophage induction

Photobiological activity of dictamnine, a furoquinoline alkaloid, to induce lytic phage development in a lysogen of Escherichia coli was measured as a line of evidence for the photoinduced genotoxicity. Since dictamnine forms the monoadducts to DNA but not the diadducts (DNA cross-links) by photoirradiation, the photobiological activity was compared with that of a cross-linking agent 5-methoxypsoralen, a structural analog, on the basis of relative quantum yield. The activity of dictamnine with respect to both phage induction and photoinduced lethal activity was weaker than the psoralen derivative. Any lethal DNA damage including monoadducts and diadducts of 5-methoxypsoralen appeared to contribute to prophage induction at the same level of efficiency.

Genotoxic effects and DNA photoadducts induced in Chinese hamster V79 cells by 5-methoxypsoralen and 8-methoxypsoralen

The induction of lethal effects and 6-thioguanine-resistant (6-TGr) mutants were studied in Chinese hamster V79 cells after treatment with the two bifunctional furocoumarins 5- and 8-methoxypsoralens (5-MOP, 8-MOP) in the presence of 365-nm radiation (UVA). The in vivo DNA-photobinding capacity of these two compounds was measured and in parallel the cross-linking capacities of 5-MOP and 8-MOP were determined using the alkaline elution technique. The results show that 5-MOP plus UVA was about 2.5 times more effective than 8-MOP plus UVA for inhibiting cell survival and for inducing the same frequency of 6-TGr mutants (10(-4]. The total number of photoinduced lesions by 5-MOP plus UVA was about 6 times higher than that induced by 8-MOP plus UVA. However, the cross-linking capacities of 5-MOP and 8-MOP were found to be within the same range at equal doses of UVA. At equal number of DNA photoadducts produced, the lesions induced by 5-MOP appeared to be less genetically active than those induced by 8-MOP. The apparently weaker genotoxicity of 5-MOP-induced lesions is likely to be due to the induction of a lower proportion of cross-links by 5-MOP at a given number of photoadducts.

Cytotoxicity and mutagenicity of coal oils in the CHO/HGPRT assay

We used the Chinese hamster ovary cell/hypoxanthine-guanine phosphoribosyl-transferase (CHO/HGPRT) assay to determine the cytotoxicity and mutagenicity of a crude coal oil, the neutral fraction of this crude, and the following three subfractions of the neutral fraction: aliphatic, neutral polar, and a subfraction composed of polycyclic aromatic hydrocarbons plus neutral nitrogen heterocyclics. We also studied the cytotoxicity and mutagenicity of a blend of light and heavy coal-derived fuel oils before and after hydrogenation. All seven mixtures were highly cytotoxic to CHO cells, but the addition of S9 reduced the cytotoxicity. Also, hydrogenation reduced the cytotoxicity of the blend of coal-derived fuel oils. Although highly cytotoxic, none of the seven mixtures induced a clear mutagenic response in the CHO/HGPRT assay. However, previous work has shown that all of the mixtures except the aliphatic subfraction and the blend after hydrogenation are mutagenic in the histidine-reversion assay in Salmonella typhimurium. Based on chemical analyses of the mixtures, the differential sensitivity of Salmonella and CHO cells to nonmutagenic cytotoxins, and studies of the neutral fraction to which additional benzo[a]pyrene had been added, we conclude that the disparity between the results in Salmonella and those obtained in the CHO/HGPRT assay is probably due to the much greater sensitivity of CHO cells (relative to Salmonella) to the cytotoxins in these coal oils. This sensitivity, coupled with the low concentrations of mutagens relative to nonmutagenic cytotoxins in the coal oils, prevents exposure of the cells to concentrations of the mutagens in the mixtures that are high enough to be quantified in the CHO/HGPRT assay.

Mutagenic effects of 3-carbethoxypsoralen and 8-methoxypsoralen plus 365-nm irradiation in mammalian cells

Cell survival, i.e. colony-forming ability, and the induction of 6-thioguanine-resistant (6-TGr) mutants were determined in Chinese hamster V79 cells by using two photoreactive furocoumarins of photochemotherapeutic interest: the bifunctional compound 8-methoxypsoralen (8-MOP) and the monofunctional compound 3-carbethoxypsoralen (3-CPs). To quantify the mutation induction in V79 cells mutants deficient in the enzyme hypoxanthine-guanine phosphoribosyl transferase (HGPRT) were selected with the purine analogue 6-thioguanine (6-TG). The effects of the compounds alone at 50 microM in the absence of light and those of 365-nm radiation (UVA) at doses of up to 6 kJm-2 were negligible. When exposed to equimolar concentrations of the compounds together with UVA, V79 cells were about 8 times more sensitive to 8-MOP-plus-UVA than to 3-CPs-plus-UVA. Per unit dose of UVA, 8-MOP was about 7 times more effective than 3-CPs for the induction of 6-TGr mutants. The induction followed about one-hit kinetics for 3-CPs and about two-hit kinetics for 8-MOP. At 50% survival the frequency of 6-TGr mutants induced by 8-MOP plus UVA and 3-CPs plus UVA differed by a factor of about 3.5. These results show a marked concordance with those obtained in the yeast Saccharomyces cerevisiae: both compound exhibited lethal and mutagenic activities but the monofunctional compound 3-CPs was less lethal and mutagenic than the bifunctional compound 8-MOP.

Utilization of Chinese hamster cells in vitro and in vivo in genetic toxicology: a multiphasic approach

An approach often used to study genetic risk is comparative mutagenesis in different organisms. We have begun the development and validation of a multiphasic genetic toxicity system using the Chinese hamster and its somatic cells to measure mutational events in the same (or similar) gene, the same chromosome derived from the same animal (Fig. 1). This system will eliminate much of the uncertainty generated when different mutational events in such evolutionally divergent organisms as bacteria, insects, and mammals are compared. Using CHO cells we have defined an assay, CHO/HGPRT, to determine mutation at the hgprt locus. Coupled with S9 metabolic activation system, the CHO/HGPRT assay can quantify gene mutation and cytotoxicity induced by various classes of chemicals, physical agents, and the combination thereof. The quantitative nature of this assay permits elucidation of the structure-activity relationship for a given class of direct-acting agents. By incorporating the cytogenetics of CHO cells into this assay we can simultaneously measure induced chromosome aberrations and SCE. Using the stable CHO/human hybrid cell line AL-J1 measurement of chromosome deletion and/loss can be also performed. In order to further expand the usefulness of this genetic toxicity system to the molecular and whole animal levels we have begun development and validation of two additional systems. To study the molecular events which may result in mutation we are developing a CHOpSVgpt system. A Chinese hamster system with treatment in vivo is being developed to study mutation at the hgprt locus, chromosome aberration, and SCE in spleen cells in vitro. The use of this multiphasic genetic toxicity system at the cellular, molecular, and animal levels may soon provide reliable and rapid identification of suspected environmental mutagens.

DNA replication and cell-cycle progression of cultured mouse FM3a cells after treatment with 8-methoxypsoralen plus near UV-radiation

To investigate the response of cells to one type of DNA damage--namely DNA crosslinks--cell-cycle progression and macromolecular synthesis were studied with cultured mouse FM3A cells. Treatment of the cells with low doses of 8-methoxypsoralen (8-MOP) plus near-UV radiation (0.1 micrograms/ml plus 5 kJ/m2 or 1.0 micrograms/ml plus 1-2.5 kJ/m2) halted the progression of cells through the cell cycle temporarily for the first several hours. Then the cells resumed progression through the cell cycle, and most of the cells reached, and were finally arrested at, the G2 phase of the cycle. There was a rapid decrease of incorporation of [3H]thymidine into cellular Dna immediately after the treatment. Then, after 8 h of incubation, the incorporation of [3H]thymidine recovered to some extent depending on the dose of 8-MOP plus near-UV radiation. Thus the decrease and recovery of the incorporation of [3H]thymidine were correlated with the halt and resumption in the cell-cycle process. Synthesis of RNA and protein was measured by determination of the amounts in the cells or by the incorporation of radioactive precursors after treatment. RNA and protein synthesis were stimulated by low doses of 8-MOP plus near-UV radiation, but inhibited severely by high doses.