DNA damage induced by 193-nm radiation in mammalian cells.

The contribution of DNA damage to the effects of 193-nm excimer laser radiation on mammalian cells in culture was studied in order to evaluate the mutagenic potential of this UV wavelength in vivo. Two approaches were taken: measurement of pyrimidine dimer-specific endonuclease-sensitive sites/megabase and comparison of the 193-nm radiation-induced cytotoxicity in normal versus DNA repair-deficient cells. The formation of pyrimidine dimer-specific endonuclease-sensitive sites/megabase was inversely related to the thickness of the cytoplasm overlying the nuclei of normal human fibroblasts (NHF) and Chinese hamster ovary cells. The results of these measurements and a calculation of the absorption coefficient of cytoplasm indicate that each 1 micron of cytoplasm attenuates the incident radiation by greater than 90% and, therefore, the nuclear DNA in tissue will be highly protected from 193-nm radiation by overlying cytoplasm. The reduction in colony-forming ability induced by 254-nm, 193-nm, and X-ray radiation was measured in NHF, xeroderma pigmentosum (group A) cells, and ataxia telangiectasia cells. Xeroderma pigmentosum (group A) cells were 16.5 times more sensitive to 254-nm radiation but only 3.5 times more sensitive to 193-nm radiation than NHF cells, indicating that cyclobutylpyrimidine dimers were not the major lethal lesion formed at 193 nm. AT cells were 3.4 times more sensitive to X-rays than NHF cells, but these cell types were almost equally sensitive to 193-nm radiation, indicating that 193 nm did not induce the same type of lethal lesions as X-rays.

Mechanism for 193-nm laser radiation-induced effects on mammalian cells.

The cellular sites for damage in mammalian cells caused by 193-nm radiation from an argon fluoride excimer laser were investigated. The ability of Chinese hamster ovary cells to reduce a tetrazolium dye (MTT) was decreased to 37% of unirradiated control by 2.5 x 10(3) J/m2 of 193-nm radiation when measured either 4 or 24 h after irradiation. In contrast, inhibition of MTT reduction by 254-nm radiation which primarily causes DNA damage was not measurable using this assay at 4 h after exposure; at 24 h 45 J/m2 inhibited MTT reduction to 37% of control. An increase in plasma membrane permeability, detected by 51Cr release, was observed within 15 min of exposure to 193-nm radiation, whereas exposure to 254-nm radiation did not cause this immediate release of 51Cr. In control experiments, the mitochondrial poison, carbonyl cyanide m-chlorophenyl hydrazone, did not cause 51Cr release in the dark, indicating that the 193-nm radiation-induced increase in plasma membrane permeability was not subsequent to loss of mitochondrial function. [3H]-Arachidonic acid was released from C3H10T1/2 cells using low 193-nm fluences, whereas release of [3H]arachidonic acid using UVB (290-32 nm) radiation required cytotoxic fluences. DNA does not appear to be a major site of 193 nm-induced cellular damage because alkali-labile sites were not detected in cells exposed on ice to up to 2 x 10(4) J/m2 of 193-nm radiation. These results indicate that 193-nm radiation produces primary damage on the level of the plasma membrane.

2,3,7,8-Tetrachlorodibenzo-p-dioxin sensitization of cultured human epidermal cells to carcinogenic heterocyclic amine toxicity.

Treatment of cultures of spontaneously immortalized human epidermal cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) sensitized them to carcinogen toxicity. While the tryptophan pyrolysis product 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) and the mycotoxin sterigmatocystin were highly toxic to the cultures at moderate concentration (1 microgram/ml), the potency of each agent was increased > or = 10-fold in the presence of TCDD. A toxicity increase was also evident in the several-fold stimulation by TCDD of protein and DNA adducts formed by Trp-P-1. In contrast, the cells were insensitive to toxicity from 3-amino-1-methyl-5H-pyrido[4,3-b]indole. DNA damage mediated by Trp-P-1 was capable of producing inheritable effects, as judged by the induction of hprt mutants in a TCDD-stimulated fashion. Northern blotting showed that TCDD strongly stimulated expression of P4501A1 and 1B1 in the cells, enzymes important for xenobiotic metabolism. These findings demonstrate the potential usefulness of SIK cultures as a model for studying keratinocyte responses to carcinogens activated by TCDD-induced cytochromes P450.

Aflatoxin toxicity in cultured human epidermal cells: stimulation by 2,3,7,8-tetrachlorodibenzo-p-dioxin.

Aflatoxin B1 (AFB1) at 1 microgram/ml was markedly toxic to human epidermal cells grown in the Rheinwald-Green 3T3 feeder layer system. At 0.1 microgram/ml, the toxicity was barely evident, as assessed by colony expansion during a 2 week exposure, but it was dramatically stimulated by 5 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which was non-toxic alone. Neither AFB1-dihydrodiol nor AFB2 were toxic in the presence or absence of TCDD, indicating that metabolism to the 8,9-epoxide was responsible for the AFB1 toxicity. Stimulation of AFB1 epoxidation by TCDD was also indicated by the > 20-fold increase in DNA adduct formation in cultures exposed to [14C]AFB1 and TCDD for 4 days as compared to AFB1 alone. Analysis of free metabolites in culture medium by reverse-phase HPLC revealed that confluent epidermal cultures metabolized AFB1 to AFM1, AFB2a and aflatoxicol. In the presence of TCDD, the levels of AFM1 were higher (14 versus 3% of dose) as were those of AFB2a (3 versus 0.5% of dose), while aflatoxicol levels were lower (0.8 versus 2% of dose). In the absence of irradiated 3T3, the toxicities of AFB1, AFB2, AFM1 and aflatoxicol to cells in serum-free medium (0.15 mM Ca2+) were similar to those in the feeder layer system. Although this moderately low calcium concentration appeared quite satisfactory for observing toxicity, the response was attenuated at a lower calcium concentration (0.09 mM Ca2+).

Identification of a novel cis-acting negative regulatory element affecting expression of the CYP1A1 gene in rat epidermal cells.

Polycyclic aromatic hydrocarbons such as 3-methylcholanthrene are toxic to rat epidermal cells in low passages (3 to 6), but cultures of high passage (>/=15) are resistant. Since such compounds can be metabolically activated by cytochrome P4501A1, we have examined the regulation of this gene in low and high passage cells. Consistent with this difference, little or no 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible P4501A1 mRNA or enzyme activity was observed in high passage as compared to low passage cultures. Similarly, transfection of a luciferase reporter construct containing -1317 to +256 base pairs of the 5'-flanking region of the murine CYP1A1 gene was TCDD-inducible in low but not high passage cells. Ligand binding and transfection experiments demonstrated the presence of functional Ah receptor complexes in both high and low passage cells. Deletion analysis identified a 26-base pair negative regulatory DNA (NeRD) element contained within the upstream regulatory region of the CYP1A1 gene responsible for this effect. Nuclear extracts from both low and high passage cells contain a protein which specifically binds to NeRD-containing DNA. Thus, the loss of polycyclic aromatic hydrocarbon sensitivity in high passage rat epidermal cells appears to be due to decreased expression of CYP1A1, and this effect may be mediated by an altered NeRD binding factor(s) present in these cells.