Photosensitized DNA breaks and DNA-to-protein crosslinks induced in human cells by antitumor agent gilvocarcin V

The antitumor agent gilvocarcin V (GV) is photoactivated to a genotoxic form by low fluences of near-ultraviolet radiation. Activation of GV by monochromatic 450-nm radiation causes two specific DNA changes in human P3 cells in culture as shown by alkaline elution techniques: single-strand breaks (i.e., alkali-labile sites plus frank strand scissions) and DNA-to-protein covalent bond crosslinks. When GV is present with the cells during irradiation, the yields of these damages are increased. Fluence and concentration studies show that the induction of both DNA lesions occurs at unusually low concentrations of drug and fluences of radiation. Both breaks and crosslinks are readily detectable after exposure to less than 100 kJ m-2 of 405 nm-radiation at a GV concentration of 7.5 X 10(-9) M. These results indicate a possible potential for use of GV in human tumor photochemotherapy.     

Phototherapy and photopharmacology

The activation of 8-methoxypsoralen (8-MOP) by long-wavelength ultraviolet A light (UVA, 320-400 nm) induces the formation of interstrand cross-links in DNA. Psoralen plus UVA (PUVA) is widely used in the treatment of psoriasis, a hyperproliferative disease of the skin. A new psoralen plus UVA therapy has been developed in which the 8-MOP-containing blood of cutaneous T cell lymphoma (CTCL) patients is irradiated with UVA light extracorporeally (i.e., extracorporeal photopheresis). The first group of patients had the leukemic variant of CTCL. A regimen of two treatments on successive days at monthly intervals produced a clinical response in eight of 11 patients. In this review the properties of several psoralens (both naturally occurring and synthetic derivatives) are compared, using several assays (DNA cross-linking, inhibition of lymphocyte response to mitogen stimulation, and cell viability). The development of a panel of monoclonal antibodies that recognize 8-MOP-modified DNA is also described. These antibodies have been used to quantitate 8-MOP photoadduct levels in human DNA samples. In addition to the psoralens, the light activation of two other compounds, gilvocarcin and an insulin-psoralen conjugate, is described.     

G-to-T transversions and small tandem base deletions are the hallmark of mutations induced by ultraviolet a radiation in mammalian cells

Ultraviolet A (UVA) radiation received from the sun and from the widespread use of tanning beds by populations residing in areas of northern latitude represents a potential risk factor for human health. The genotoxic and cancer-causing effects of UVA have remained controversial. A mutagenic role for UVA based on DNA damage formation by reactive oxygen species as well as by generation of photoproducts such as cyclobutane pyrimidine dimers (CPDs) has been suggested. Here, we investigated the mutagenicity of UVA in relation to its DNA damaging effects in transgenic Big Blue mouse embryonic fibroblasts. We determined the formation of a typical oxidative DNA lesion, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG), and of CPDs, as well as quantified the induction of mutations in the cII transgene in cells irradiated with a 2000 W UVA lamp. UVA irradiation at a dose of 18 J/cm(2) produced significant levels of 8-oxo-dG in DNA (P < 0.03) but did not yield detectable CPDs. UVA irradiation also increased the cII mutant frequency almost 5-fold over background (P < 0.01) while showing moderate cytotoxicity (70% cell viability). UVA-induced mutations were characterized by statistically significant increases in G-to-T transversions and small tandem base deletions (P = 0.0075, P = 0.008, respectively) relative to spontaneously derived mutations. This mutational spectrum differs from those previously reported for UVA in other test systems; however, it corresponds well with the known spectrum of mutations established for oxidative base lesions such as 8-oxo-dG. We conclude that UVA has the potential to trigger carcinogenesis owing to its mutagenic effects mediated through oxidative DNA damage.     

Bipyrimidine photoproducts rather than oxidative lesions are the main type of DNA damage involved in the genotoxic effect of solar UVA radiation

Exposure to solar UV radiation gives rise to mutations that may lead to skin cancer. UVA (320-340 nm) constitutes the large majority of solar UV radiation but is less effective than UVB (290-320 nm) at damaging DNA. Although UVA has been implicated in photocarcinogenesis, its contribution to sunlight mutagenesis has not been elucidated, and DNA damage produced by UVA remains poorly characterized. We employed HPLC-MS/MS and alkaline agarose gel electrophoresis in conjunction with the use of specific DNA repair proteins to determine the distribution of the various classes and types of DNA lesions, including bipyrimidine photoproducts, in Chinese hamster ovary cells exposed to pure UVA radiation, as well as UVB and simulated sunlight (lambda > 295 nm) for comparison. At UVA doses compatible with human exposure, oxidative DNA lesions are not the major type of damage induced by UVA. Indeed, single-strand breaks, oxidized pyrimidines, oxidized purines (essentially 8-oxo-7,8-dihydroguanine), and cyclobutane pyrimidine dimers (CPDs) are formed in a 1:1:3:10 ratio. In addition, we demonstrate that, in contrast to UVB and sunlight, UVA generates CPDs with a large predominance of TT CPDs, which strongly suggests that they are formed via a photosensitized triplet energy transfer. Moreover, UVA induces neither (6-4) photoproducts nor their Dewar isomers via direct absorption. We also show that UVA photons contained in sunlight, rather than UVB, are implicated in the photoisomerization of (6-4) photoproducts, a quickly repaired damage, into poorly repaired and highly mutagenic Dewar photoproducts. Altogether, our data shed new light on the deleterious effect of UVA.     

Engineered biosynthesis of gilvocarcin analogues with altered deoxyhexopyranose moieties

A combinatorial biosynthetic approach was used to interrogate the donor substrate flexibility of GilGT, the glycosyltransferase involved in C-glycosylation during gilvocarcin biosynthesis. Complementation of gilvocarcin mutant Streptomyces lividans TK24 (cosG9B3-U(-)), in which the biosynthesis of the natural sugar donor substrate was compromised, with various deoxysugar plasmids led to the generation of six gilvocarcin analogues with altered saccharide moieties. Characterization of the isolated gilvocarcin derivatives revealed five new compounds, including 4-β-C-D-olivosyl-gilvocarcin V (D-olivosyl GV), 4-β-C-D-olivosyl-gilvocarcin M (D-olivosyl GM), 4-β-C-D-olivosyl-gilvocarcin E (D-olivosyl GE), 4-α-C-L-rhamnosyl-gilvocarcin M (polycarcin M), 4-α-C-L-rhamnosyl-gilvocarcin E (polycarcin E), and the recently characterized 4-α-C-L-rhamnosyl-gilvocarcin V (polycarcin V). Preliminary anticancer assays showed that D-olivosyl-gilvocarcin and polycarcin V exhibit antitumor activities comparable to that of their parent drug congener, gilvocarcin V, against human lung cancer (H460), murine lung cancer (LL/2), and breast cancer (MCF-7) cell lines. Our findings demonstrate GilGT to be a moderately flexible C-glycosyltransferase able to transfer both D- and L-hexopyranose moieties to the unique angucyclinone-derived benzo[D]naphtho[1,2b]pyran-6-one backbone of the gilvocarcins.     

Zinc protects against ultraviolet A1-induced DNA damage and apoptosis in cultured human fibroblasts

Ultraviolet Al (UVA1) radiation generates reactive oxygen species and the oxidative stress is known as a mediator of DNA damage and of apoptosis. We exposed cultured human cutaneous fibroblasts to UVA1 radiation (wavelengths in the 340–450-nm range with emission peak at 365 nm) and, using the alkaline unwinding method, we showed an immediate significant increase of DNA strand breaks in exposed cells. Apoptosis was determined by detecting cytoplasmic nucleosomes (enzyme-linked immunosorbent assay method) at different time points in fibroblasts exposed to different irradiation doses. In our conditions, UVA1 radiation induced an early (8 h) and a delayed (18 h) apoptosis. Delayed apoptosis increased in a UVA dosedependent manner. Zinc is an important metal for DNA protection and has been shown to have inhibitory effects on apoptosis. The addition of zinc (6.5 mg/L) as zinc chloride to the culture medium significantly decreased immediate DNA strand breaks in human skin fibroblasts. Moreover, zinc chloride significantly decreased UVA1-induced early and delayed apoptosis. Thus, these data show for the first time in normal cutaneous cultured cells that UVA1 radiation induces apoptosis. This apoptosis is biphasic and appears higher 18 h after the stress. Zinc supplementation can prevent both immediate DNA strand breakage and early and delayed apoptosis, suggesting that this metal could be of interest for skin cell protection against UVA1 irradiation.     

Light-induced modifications of DNA by gilvocarcin V and its aglycone

Gilvocarcins are antitumor agents that have been reported to damage DNA upon activation by visible light. This activation is dependent on interaction with DNA. Here it is shown that gilvocarcin V and its synthetic aglycone analogue can both introduce single-strand scission into plasmid DNA. Light irradiation is required for the reaction. The binding of gilvocarcin V to plasmid DNA in the absence of light decreased the DNA linking number in a fashion similar to known intercalating agents such as ethidium bromide. The use of oligonucleotides as substrates for gilvocarcin V demonstrated that one of the steps of the reaction following binding of gilvocarcin V to DNA involves covalent modification at thymidine and to a lesser extent, cytosine residues.     

Binding of ellipticine base and ellipticinium cation to calf-thymus DNA. A thermodynamic and kinetic study

The acid-basic properties of ellipticine have been re-estimated. The apparent pK of protonation at 3 microM drug concentration is 7.4 +/- 0.1. The ellipticine free base (at pH 9, I = 25 mM) intercalates into calf-thymus DNA with an affinity constant of 3.3 +/- 0.2 X 10(5) M-1, and a number of binding sites per phosphate of 0.23. The ellipticinium cation (pH 5, I = 25 mM) binds also to DNA with a constant of 8.3 +/- 0.2 x 10(5) M-1 and at a number of binding sites (n = 0.19). It is postulated that the binding of the drug to DNA at pH 9 is driven by hydrophobic and/or dipolar effects. Even at pH 5, where ellipticine exists as a cation, it is thought that the hydrophobic interaction is the main contribution to binding. The neutral and cationic forms share common binding within DNA sites but yield to structurally different complexes. The free base has 0.04 additional specific binding sites per phosphate. As determined from temperature-jump experiments, the second-order rate constant of the binding of the free base (pH 9) is 3.4 x 10(7) M-1 s-1 and the residence time of the base within the DNA is 8 ms. The rate constant for the binding of the ellipticinium cation is 9.8 x 10(7) M-1 s-1 when it is assumed that drug attachment occurs via a pathway in which the formation of an intermediate ionic complex is not involved (competitive pathway).     

Cleavage of DNA by electrochemically activated MnIII and FeIII complexes of meso-tetrakis (N-methyl-4-pyridiniumyl)porphine

Electrochemical methods were used to activate MnIII and FeIII complexes of meso-tetrakis(N-methyl-4-pyridiniumyl)porphine (H2TMPyP) to cause cleavage of pBR322 DNA and to study their interaction with sonicated calf thymus DNA. Electrochemical reduction of MnIIITMPyP and FeIIITMPyP (at low concentrations) in the presence of O2 was required to activate these complexes. However, FeIIITMPyP at 1 x 10(-6) M produced DNA strand breakage without being electrochemically reduced. At low concentrations, FeIITMPyP was more efficient at cleaving DNA than MnIITMPyP. Reduction of O2 at a platinum electrode also produced some cleavage but to a much smaller extent. The oxidized form of MnIIITMPyP (charge 5+) has higher affinity for sonicated calf thymus (CT) DNA than the reduced form (charge 4+), as determined by the negative shift in E degrees' for the voltammetric wave in the presence of DNA. Both forms of FeIIITMPyP (charge 4+) interact with DNA to about the same extent. Differential pulse voltammetry was used to determine binding constants (K) and binding-site sizes (s) of the interaction of these metalloporphyrins with sonicated CT DNA. The data were analyzed assuming both mobile and static equilibria. MnIIITMPyP binds to DNA (5 mM Tris, 50 mM NaCl, pH 7) with K = 5 (+/- 2) x 10(6) M-1, s = 3 bp (mobile) or K = 3.6 (+/- 0.3) x 10(6) M-1, s = 4 bp (static). FeIIITMPyP at that ionic strength caused DNA precipitation. At higher ionic strength (0.1 M Tris, 0.1 M NaCl, pH 7), FeIIITMPyP associates to DNA with K = 4.4 (+/- 0.2) x 10(4) M-1, s = 5 bp (mobile) or K = 1.9 (+/- 0.1) x 10(4) M-1, s = 6 bp (static).     

Co-exposure to benzo[a]pyrene and UVA induces phosphorylation of histone H2AX

Phosphorylation of histone H2AX (termed gamma-H2AX) was recently identified as an early event after induction of DNA double strand breaks (DSBs). We have previously shown that co-exposure to benzo[a]pyrene (BaP), a wide-spread environmental carcinogen, and ultraviolet A (UVA), a major component of solar UV radiation, induced DSBs in mammalian cells. In the present study, we examined whether co-exposure to BaP and UVA generates gamma-H2AX in CHO-K1 cells. Single treatment with BaP (10(-9)-10(-7)M) or UVA ( approximately 2.4 J/cm(2)) did not result in gamma-H2AX, however, co-exposure drastically induced foci of gamma-H2AX in a dose-dependent manner. gamma-H2AX could be detected even at very low concentration of BaP (10(-9)M) plus UVA (0.6J/cm(2)), which did not change cell survival rates. NaN(3) effectively inhibited the formation of gamma-H2AX induced by co-exposure, indicating the contribution of singlet oxygen. This is the first evidence that co-exposure to BaP and UVA induced DSBs, involving gamma-H2AX.     

Interaction of SV40 T-antigen with homologous and heterologous DNA

Using the DNA filter binding assay, the effects of ionic strength and pH on SV40 T-antigen interaction with viral DNA were studied. The apparent association constants for T-antigen binding to SV40 DNA in Scatchard coordinates in the presence of 40 mM NaCl are equal to 0.67 . 10(6) M-1 (pH 6.0) and 0.86 x 10(7) M-1 (pH 7.4). These data indicate that the interaction between T-antigen and SV40 DNA is more specific at pH 7.4. The coincident values of association constants for T-antigen binding to viral and cellular DNAs (Ka = 0.9 x 10(7) M-1 for cellular DNA) at pH 7.4 and the absence of competition between the two DNA species upon binding with T-antigen suggest that viral and cellular DNAs possess similar sites for T-antigen binding. Denatured DNA competes with viral DNA only at pH 6.0, when the T-antigen--SV40 DNA interaction is less specific.     

Effects of in vitro exposure to power frequency magnetic fields on UV-induced DNA damage of rat lymphocytes

The mechanisms of biological effects of 50/60 Hz (power frequency) magnetic fields (MF) are still poorly understood. There are a number of studies indicating that MF affect biochemical processes in which free radicals are involved, such as the biological objects' response to ultraviolet radiation (UVA). Therefore, the present study was aimed to assess the effect of 50 Hz MFs on the oxidative deterioration of DNA in rat lymphocytes irradiated in vitro by UVA. UVA radiation (150 J/m2) was applied for 5 min for all groups and 50 Hz MF (40 microT rms) exposure was applied for some of the groups for 5 or 60 min. The level of DNA damage was assessed using the alkaline comet assay, the fluorescence microscope, and image analysis. It has been found that the 1 h exposure to MF caused an evident increase in all parameters consistent with damaged DNA. This suggest that MF affects the radical pairs generated during the oxidative or enzymatic processes of DNA repair.     

Ultraviolet B and A irradiation induces fibromodulin expression in human fibroblasts in vitro

Ultraviolet (UV) radiation affects the extracellular matrix (ECM) of the human skin. The small leucine-rich repeat protein fibromodulin interacts with type I and II collagen fibrils, thereby affecting ECM assembly. The aim of this study was to evaluate whether short wave UV (UVB) or long wave UV (UVA) irradiation influences fibromodulin expression. Exponentially growing human fibroblasts (IMR-90 cells) were exposed to increasing doses of UVB (2.5–60 mJ/cm²) or UVA (0.5–10 J/cm²). After UV irradiation fibromodulin, p21 and GADD45 levels were evaluated as well as cell viability, reactive oxygen species formation (ROS) and DNA damage. We found that fibromodulin expression: (i) increased after UVB and UVA irradiation; (ii) was 10-fold higher after UVA (10 J/cm²) versus 5-fold with UVB (10 mJ/cm²); (iii) correlated with reactive oxygen species formation, particularly after UVA; and (iv) was linked to the DNA damage binding protein (DDB1) translocation in the nucleus, particularly after UVB. These results further suggest that the UV-induced fibromodulin increase could counteract the UV-induced connective tissue damage, promoting the assembly of new collagen fibrils.     

The DNA guanyl radical: kinetics and mechanisms of generation and repair

The one-electron oxidation of DNA bases and single-stranded DNA was studied by pulse radiolysis of aqueous solutions from pH 7-7.4 at 20 degrees C. Thallic ions, Tl(II), were found to rapidly oxidize the purine nucleotides, deoxyguanosine 5'-monophosphate, k[Tl(II) + dGMP2-] = 3.4.10(9) M-1.s-1, and deoxyadenosine 5'-monophosphate, k[Tl(II) + dAMP2-] = 1.3.10(8) M-1.s-1. The reactivities of Tl(II) ions with model pyrimidine DNA bases, 1-methylcytosine and 1-methylthymine, were too low to be measured by pulse radiolysis, k less than 10(7) M-1.s-1. The Tl(II)-mediated oxidation of ssDNA, k = 2.8.10(8) M-1.s-1, produces DNA-guanyl radical, DNA-G.(-H), exclusively. The DNA-guanyl radical is found to be a potent oxidant in neutral media, E7 = 1.04 +/- 0.05 V. It rapidly oxidizes the aromatic amino acids in glycyl-tryptophan and tyrosine methyl ester, k = 3.6.10(7) M-1.s-1 and k = 1.7.10(8) M-1.s-1, respectively. These electron transfer processes indicate that a positive 'hole' may be transferred from DNA to a DNA-associated protein. The positive 'hole' in DNA can also be repaired by antioxidants, which are electron donors. The chemical repair of the DNA-guanyl radical by negatively charged antioxidants is slower than that by positively charged and neutral antioxidants.     

Structural analysis of DNA-chlorophyll complexes by Fourier transform infrared difference spectroscopy

Porphyrins and metalloporphyrins are strong DNA binders. Some of these compounds have been used for radiation sensitization therapy of cancer and are targeted to interact with cellular DNA. This study was designed to examine the interaction of calf thymus DNA with chlorophyll a (CHL) in aqueous solution at physiological pH with CHL/DNA(phosphate) ratios (r) of 1/160, 1/80, 1/40, 1/20, 1/10, and 1/5. Fourier transform infrared (FTIR) difference spectroscopy was used to characterize the nature of DNA-pigment interactions and to establish correlations between spectral changes and the CHL binding mode, binding constant, sequence selectivity, DNA secondary structure, and structural variations of DNA-CHL complexes in aqueous solution. Spectroscopic results showed that CHL is an external DNA binder with no affinity for DNA intercalation. At low pigment concentration (r = 1/160, 1/80, and 1/40), there are two major binding sites for CHL on DNA duplex: 1) Mg-PO2 and 2) Mg-N7 (guanine) with an overall binding constant of K = 1.13 x 10(4) M-1. The pigment distributions are 60% with the backbone PO2 group and 20% with the G-C base pairs. The chlorophyll interaction is associated with a major reduction of B-DNA structure in favor of A-DNA. At high chlorophyll content (r = 1/10), helix opening occurs, with major spectral alterations of the G-C and A-T bases. At high chlorophyll concentration (1/5), pigment aggregation is observed, which does not favor CHL-DNA complexation.     

Action mechanism of Escherichia coli DNA photolyase. I. Formation of the enzyme-substrate complex

Escherichia coli DNA photolyase (photoreactivating enzyme) is a flavoprotein. The enzyme binds to DNA containing pyrimidine dimers in a light-independent step and, upon illumination with 300-600 nm radiation, catalyzes the photosensitized cleavage of the cyclobutane ring thus restoring the integrity of the DNA. We have studied the binding reaction using the techniques of nitrocellulose filter binding and flash photolysis. The enzyme binds to dimer-containing DNA with an association rate constant k1 estimated by two different methods to be 1.4 X 10(6) to 4.2 X 10(6) M-1 S-1. The dissociation of the enzyme from dimer-containing DNA displays biphasic kinetics; for the rapidly dissociating class of complexes k2 = 2-3 X 10(-2) S-1, while for the more slowly dissociating class k2 = 1.3 X 10(-3) to 6 X 10(-4) S-1. The equilibrium association constant KA, as determined by the nitrocellulose filter binding assay and the flash photolysis assay, was 4.7 X 10(7) to 6 X 10(7) M-1, in reasonable agreement with the values predicted from k1 and k2. From the dependence of the association constant on ionic strength we conclude that the enzyme contacts no more than two phosphodiester bonds upon binding; this strongly suggests that the pyrimidine dimer is the main structural determinant of specific photolyase-DNA interaction and that nonspecific ionic interactions do not contribute significantly to substrate binding.     

Inhibition of radiation-induced DNA damage in plasmid pBR322 by chlorophyllin and possible mechanism(s) of action

Naturally occurring compounds capable of protecting DNA against ionizing radiation and chemical mutagens have considerable potential for prevention of mutation-based health impairment including cancer and other degenerative diseases. Chlorophyllin (CHL), a water-soluble derivative of chlorophyll, has been examined for its ability to protect DNA against radiation induced strand breaks using an in vitro plasmid DNA system. Gamma-radiation, up to a dose of 6 Gy (dose rate 1.25 Gy/min), induced a dose-dependent increase in single-strand breaks (ssbs) in plasmid pBR322 DNA. CHL per se did not induce, but inhibited radiation-induced ssbs in a concentration-dependent manner; 500 microM giving about 90% protection. The protection afforded by CHL was comparatively less than that of trolox, a water-soluble analogue of alpha-tocopherol. To elucidate the underlying mechanism(s), reaction of CHL with the radiation-derived hydroxyl radical (.OH) and deoxyribose peroxyl radical (ROO.) was studied by pulse radiolysis. CHL exhibited a rate constant of 6.1+/-0.4x109 M-1 s-1 with.OH and 5.0+/-1.3x107 M-1 s-1 with ROO. To our knowledge, this is the first report providing direct evidence of free radical-scavenging properties of CHL. The results showed that CHL, effectively protects plasmid DNA against ionizing radiation, in an in vitro system independent of DNA repair or other cellular defense mechanisms. The ability of CHL to scavenge. OH and ROO., may contribute to its protective effects against radiation induced DNA damage in the pBR322 system.     

DNA tris-intercalation: first acridine trimer with DNA affinity in the range of DNA regulatory proteins. Kinetic studies

A trimer made up of three acridine chromophores linked by a positively charged poly(aminoalkyl) chain was synthesized as a potential tris-intercalating agent. The length of the linking chain was selected to allow intercalation of each chromophore according to the excluded site model. 1H NMR studies have shown that, at 5 mM sodium, pH 5, the acridine trimer occurred under a folded conformation stabilized by stacking interactions between the three aromatic rings. DNA tris-intercalation of the dye at a low dye/base pair ratio was shown by measurements of both the unwinding of PM2 DNA and the lengthening of sonicated rodlike DNA. The trimer exhibits a high DNA affinity for poly[d(A-T)] (Kapp = 8 X 10(8) M-1, 1 M sodium) as shown by competition experiments with ethidium dimer. Kinetic studies of both the association with poly[d(A-T)] and the exchange between poly[d(A-T)] and sonicated calf thymus DNA have been performed as a function of the ionic strength. In 0.3 M sodium the on-rate constant (k1 = 2.6 X 10(7) M-1 s-1) is similar to that reported for other monoacridines or bis(acridines), whereas the off-rate constant is much smaller (k-1 = 1.2 X 10(-4) s-1), leading to an equilibrium binding constant as large as Kapp = 2.2 X 10(11) M-1. A plot of log (k1/k-1) as a function of log [Na+] yielded a straight line whose slope shows that 5.7 ion pairs (out of 7 potential) are formed upon the interaction with DNA. From this linear relationship a Kapp value of 10(14) M-1 in 0.1 M sodium can be estimated. Such a value reaches and even goes beyond that of some DNA regulatory proteins. This acridine trimer appears to be the first synthetic ligand with such a high DNA affinity.