Photobiological effects in Saccharomyces cerevisiae induced by the monofunctional furocoumarin 4,4',6-trimethylangelicin (TMA) and the bifunctional furocoumarin 8-methoxypsoralen (8-MOP)

Molecular Mechanism of Action of Trimethylangelicin Derivatives as CFTR Modulators

The psoralen-related compound, 4,6,4′-trimethylangelicin (TMA) potentiates the cAMP/PKA-dependent activation of WT-CFTR and rescues F508del-CFTR-dependent chloride secretion in both primary and secondary airway cells homozygous for the F508del mutation. We recently demonstrated that TMA, like lumacaftor (VX-809), stabilizes the first membrane-spanning domain (MSD1) and enhances the interface between NBD1 and ICL4 (MSD2). TMA also demonstrated anti-inflammatory properties, via reduction of IL-8 expression, thus making TMA a promising agent for treatment of cystic fibrosis. Unfortunately, TMA was also found to display potential phototoxicity and mutagenicity, despite the fact that photo-reactivity is absent when the compound is not directly irradiated with UVA light. Due to concerns about these toxic effects, new TMA analogs, characterized by identical or better activity profiles and minimized or reduced side effects, were synthesized by modifying specific structural features on the TMA scaffold, thus generating compounds with no mutagenicity and phototoxicity. Among these compounds, we found TMA analogs which maintained the potentiation activity of CFTR in FRT-YFP-G551D cells. Nanomolar concentrations of these analogs significantly rescued F508del CFTR-dependent chloride efflux in FRT-YFP-F508del, HEK-293 and CF bronchial epithelial cells. We then investigated the ability of TMA analogs to enhance the stable expression of varying CFTR truncation mutants in HEK-293 cells, with the aim of studying the mechanism of their corrector activity. Not surprisingly, MSD1 was the smallest domain stabilized by TMA analogs, as previously observed for TMA. Moreover, we found that TMA analogs were not effective on F508del-CFTR protein which was already stabilized by a second-site mutation at the NBD1-ICL4 interface. Altogether, our findings demonstrate that these TMA analogs mediate correction by modifying MSD1 and indirectly stabilizing the interface between NBD1 and CL4.

1,4,8-Trimethylfuro[2,3-H]quinolin-2(1H)-one, a new furocoumarin bioisoster

1,4,8-Trimethylfuro[2,3-h]quinolin-2(1H)-one (compound 5a) is the most interesting derivative among some new furoquinolinones prepared with the aim of moderating the strong toxic effects of 1,4,6,8-tetramethyl derivative (FQ), a powerful potential drug for photomedicine. Compound 5a showed a photobiological activity lower than FQ, but considerable higher than 8-MOP, the furocoumarin used in clinical photomedicine; contrary to classic furocoumarins, 5a induced a strong inhibition of protein synthesis in mammalian cells. Genotoxicity and skin erythema induction, the main side effects of both FQ and 8-MOP photosensitization, are virtually absent with 5a. This behavior seems to be connected to its particular reaction mechanism: differently from furocoumarin derivatives, 5a induced low levels of DNA-protein and no inter-strands cross-links, but formed covalent RNA-protein linkages, lesions not observed with known furocoumarins. Moreover, compound 5a generated reactive oxygen species to a considerable extent. For these features, compound 5a appears to be a new photosensitizing agent whose special activity deserves to be deeply investigated.

Photoinduction of micronuclei by 4,4',6-trimethylangelicin and 8-methoxypsoralen in different experimental models

The frequencies of micronuclei induced by treatment with 4,4',6-trimethylangelicin (TMA) and 8-methoxypsoralen (8-MOP) have been compared in the following experimental models: (1) peripheral normochromatic erythrocytes (NCE) during 10 days after single p.o. administration of TMA or 8-MOP in male and female mice; (2) peripheral NCE during photocarcinogenesis by TMA or 8-MOP topically administered to female mice; (3) primary cultures of human skin fibroblasts treated with TMA or 8-MOP. The frequency of micronuclei in peripheral NCE of mice (both sexes) was significantly enhanced after p.o. administration of TMA or 8-MOP. This latter was more active than TMA in inducing chromosomal damage. No increased frequencies of micronuclei in peripheral NCE were detected in mice subjected to TMA or 8-MOP photocarcinogenic treatment, even when malignancies developed. In human fibroblast cultures, at equimolar concentrations, the induction of lethal effects by TMA in the presence of 365-nm radiation was higher than that exerted by 8-MOP. At equal survival, however, TMA showed practically the same activity as 8-MOP in the induction of micronuclei. Our findings provide evidence of genotoxicity by TMA administered p.o. without irradiation and give further information about photogenotoxicity of these substances.

4,6,4'-Trimethylangelicin induces interstrand cross-links in mammalian cell DNA

4,6,4'-Trimethylangelicin, a well-known effective photosensitizer described as a pure monofunctional reactant with DNA, can induce interstrand cross-links in mammalian cell DNA in vivo (about 15% relative to 8-methoxypsoralen), as observed using alkaline elution and Chinese hamster ovary cells. Experiments performed using the two-step irradiation method and HeLa cells support these data. In contrast with 4,6,4'-trimethylangelicin, 4'-methylangelicin and 4,4'-dimethylangelicin do not form interstrand cross-links. These results are consistent with those recently reported by Chen et al. (X. Chen, J. Kagan, F. Dall'Acqua, D. Averbeck and E. Bisagni, J. Photochem. Photobiol. B: Biol, 22 (1994) 51-57) using pBR322 and M13 DNA. The cross-linking ability of 4,6,4'-trimethylangelicin does not seem to be related to a particular feature of these DNAs but to the compound itself.

Induction of the genes RAD54 and RNR2 by various DNA damaging agents in Saccharomyces cerevisiae

The relationship between the induction of the genes RAD54 and RNR2 and the induction and repair of specific DNA lesions was studied in the yeast Saccharomyces cerevisiae using Rad54-lacZ and RNR2-lacZ fusion strains. Gene induction was followed by measuring beta-galactosidase activity. At comparable levels of furocoumarin-DNA photoadducts, RAD54 was more effectively induced by bifunctional than by monofunctional furocoumarins indicating that mixtures of monoadducts (MA) and interstrand cross-links (CL) provide a stronger inducing signal than MA. RNR2 induction kinetics were measured in relation to cell growth and survival responses after treatment with the furocoumarins 8-methoxypsoralen (8-MOP), 5-methoxypsoralen (5-MOP), 3-carbethoxypsoralen (3-CPs), 7-methyl-pyrido[3,4-c]psoralen (MePyPs) and 4,4',6-trimethylangelicin (TMA), benzo[a]pyrene (B(a)P and 1,6-dioxapyrene (1,6-DP) plus UVA, 254 nm UV radiation and cobalt-60 gamma-radiation. Induction of RNR2 took place during the DNA repair period before resumption of cell growth and clearly increased with increasing equitoxic dose levels. Treatments with furocoumarin plus 365 nm radiation (UVA) and 254 nm (UV) radiation were effective inducers whereas gene induction was relatively weak after gamma-radiation and absent after the induction of oxidative damage by B(a)P and 1,6-DP and UVA. The results suggest that it is the specific processing of different DNA lesions that determines the potency of the induction signal. Apparently, DNA lesions such as CL, and probably also closely located MA or pyrimidine dimers in opposite DNA strands involving the formation of double-strand breaks as repair intermediates, are most effective inducers.

Photosensitized cross-linking and cleavage of pBR322 and M13 DNA: comparison of 4,4',6-trimethylangelicin and 3-carbethoxypsoralen

The furocourmarins 3-carbethoxypsoralen (3-CP) and 4,4',6-trimethylangelicin (TMA) were generally believed to be incapable of cross-linking DNA upon irradiation with ultraviolet light. Denaturation of photosensitized pBR322 DNA, either supercoiled or previously linearized with a restriction enzyme, proved that 3-CP was indeed monofunctional, but that TMA produced cross-links. Identical conclusions were reached with double stranded M13 DNA which had been linearized with EcoR 1. Both sensitizers also induced partial DNA cleavage. In contrast to 3-CP, photosensitization with TMA made the DNA resistant to enzymatic cleavage.

New aspects of the repair and genotoxicity of psoralen photoinduced lesions in DNA

Several approaches are described aiming at a better understanding of the genotoxicity of psoralen photoinduced lesions in DNA. Psoralens can photoinduce different types of photolesions including 3,4- and 4',5'-monoadducts and interstrand cross-links, oxidative damage (in the case of 3-carbethoxypsoralen (3-CPs)) and even pyrimidine dimers (in the case of 7-methylpyrido(3,4-c)psoralen (MePyPs)). The characterization and detection of different types of lesions has been essential for the analysis of their possible contributions to genotoxicity. For example, oxidative damage photoinduced by 3-CPs can be detected by the formamidopyrimidine glycosylase (FPG) protein. Furthermore, it is shown how the presence of MePyPs induced monoadducts may interfere with the photoreactivation of concomitantly induced pyrimidine dimers, how the ratio of monoadducts and interstrand cross-links (CL) affects the occurrence of double-strand breaks during the repair of photolesions and genotoxicity. In vitro treatment of yeast plasmids, followed by transformation, also indicates that the repair of photoadducts on exogenous DNA differs for 8-methoxy-psoralen (8-MOP) induced mono- and diadducts and for monoadducts alone. The recombinational rad52 dependent pathway is not needed for the repair of 8-MOP induced monoadducts. The results obtained suggest that the genotoxic effects of psoralens are conditioned by the nature, number, ratio and sequence distribution of the photolesions induced in DNA.

Angelicins, angular analogs of psoralens: chemistry, photochemical, photobiological and phototherapeutic properties

Angelicin and some of its derivatives are naturally occuring compounds which show interesting photobiological properties. In this review various aspects of angelicin and its derivatives have been reported. The natural occurrence and the chemical synthesis both of naturally occurring and synthetic angelicins have been reviewed. Photochemical and photophysical properties of angelicins have been considered with particular reference to the capacity to generate active forms of oxygen, photoreactions with nucleic acids, proteins and unsaturated fatty acids. Photobiological effects have been considered: skin phototoxicity, antiproliferative effects, genotoxicity, ability to induce hemolysis in erythrocytes, inactivation of prokaryotic and eukaryotic microorganism and of viruses. The ability of some angelicins to induce photocarcinogenesis has been reviewed as well as in the activity as photochemotherapeutic agents.

Photobinding of 8-methoxypsoralen, 4,6,4'-trimethylangelicin and chlorpromazine to Wistar rat epidermal biomacromolecules in vivo

Photoinduced binding of drugs to endogenous biomacromolecules may cause both toxic and therapeutic effects. For example, photobinding of certain phenothiazines to biomolecules possibly underlies their phototoxic and photoallergic potential, whereas photobinding of furocoumarins to epidermal DNA is held responsible for their advantageous effects in the photochemotherapy of psoriasis. Usually, the in vitro photobinding of drugs is investigated. However, under in vivo conditions, the metabolism and distribution of the drug and the light absorption by endogenous compounds will significantly affect the photobinding of drugs to biomolecules. Therefore, in the present study, the photobinding of 8-methoxypsoralen (8-MOP), 4,6,4'-trimethylangelicin (TMA) (two therapeutically used furocoumarins) and chlorpromazine (CPZ) (a member of the phenothiazines) was investigated in vivo. The compounds were applied topically on the shaven skin of Wistar rats; one group was exposed to UVA and the other was kept in a dimly lit environment. Immediately, and at certain time intervals after UVA exposure, members of the two groups were sacrificed. By separating epidermal lipids, DNA/RNA and proteins by a selective extraction method, irreversible binding of 8-MOP, TMA or CPZ to each of these biomacromolecules was determined. In contrast with in vitro experiments, photobinding of CPZ to epidermal DNA/RNA was not found in vivo; apparently the bioavailability in the nucleus is very low. Compared with TMA, 8-MOP was observed to bind more extensively to epidermal DNA/RNA (again in contrast with findings from in vitro experiments) and proteins, but less extensively to lipids. The rates of removal of photobound 8-MOP and TMA were comparable. Photobound CPZ was more slowly removed from epidermal proteins and lipids than the furocoumarins. The observed in vivo photobinding is discussed with respect to the UVA-induced (side) effects of these drugs.