Toxicological potential of 2-alkylcyclobutanones – specific radiolytic products in irradiated fat-containing food – in bacteria and human cell lines

Photoautotrophically Grown Chlorella vulgaris Shows Genotoxic Potential but No Apoptotic Effect in Epithelial Cells

This study investigated the effect of Chlorella vulgaris (C. vulgaris) on genotoxicity, cytotoxicity, and apoptosis in Caco-2 and HT-29 cells. C. vulgaris significantly induced DNA damage in both cell lines at a concentration of 200 μg dry matter/mL (comet tail intensity CTI: 24.6 ± 4.7% for Caco-2, 16.6 ± 0.9% for HT-29). The application of processing (sonication, ball-milling) did not affect the genotoxicity negatively and lowered the lipid peroxidation in C. vulgaris preparations. C. vulgaris-induced intracellular formation of reactive oxygen species in human cell lines and might be responsible for the genotoxic effect. A solid fraction mainly triggered the observed DNA damage (CTI: 41.5 ± 1.9%), whereas a hydrophilic (CTI: 7.9 ± 1.7%) and lipophilic (CTI: 10.2 ± 2.1%) fraction revealed a significantly lower tail intensity. C. vulgaris significantly induced DNA damage in both cell lines possibly through intracellular formation of reactive oxygen species; however, it was repaired after a 2 h recovery time or was even avoided at lower concentrations. In addition, none of the preparations indicated an adverse effect on cell proliferation or revealed apoptotic activity.

Toxicological evaluation of 2-dodecylcyclobutanone, a unique radiolytic compound of palmitic acid

This study was conducted to evaluate the toxic effects and potency of 2-dodecylcyclobutanone (2-dDCB), a unique compound derived from palmitic acid via irradiation. In a series of assays of bacterial reverse-mutation, in vitro chromosomal aberration, and in vivo micronucleus, negative responses were found by the treatment of 2-dDCB comparing vehicle control, dimethyl sulfoxide or corn oil. In the acute oral toxicity test, all of the mice administrated 2-dDCB survived, and there were no clinical and necropsy signs observed at any doses (0, 300, and 2000 mg/kg body weight) during the experimental period of 14 days. These results suggested that 2-dDCB is a relatively non-toxic substance with median lethality dose higher than 2000 mg/kg body weight. Moreover, there were no adverse effects noted in rats orally administrated 2-dDCB everyday via gavage for 28 days, even at the highest dose (2.0 mg/kg body weight/day) tested, which is 1000-times higher than the human daily intake of 2-dDCB estimated through an extreme exposure scenario. Overall, these results indicate that 2-dDCB is not likely to raise any human health concerns and irradiated foods containing palmitic acid can be recognized as safe for human consumption under the current international regulation systems for food irradiation.

Flow synthesis of cyclobutanones via [2 + 2] cycloaddition of keteneiminium salts and ethylene gas

A robust and safe flow method for the synthesis of mono-substituted cyclobutanones.

Modifications of azoxymethane-induced carcinogenesis and 90-day oral toxicities of 2-tetradecylcyclobutanone as a radiolytic product of stearic acid in F344 rats

A 90-day oral toxicity test in rats was performed to evaluate the toxicity of 2-tetradecylcyclobutanone (2-tDCB), a unique radiolytic product of stearic acid. Six-week-old male and female F344 rats (n=15/group) were given 2-tDCB at concentrations of 0, 12, 60 and 300 ppm in a powder diet for 13 weeks. Slight dose-dependent increases in serum total protein and albumin in male rats were found, but these changes were not considered to be a toxic effect. The fasting, but not non-fasting, blood glucose levels of the male rats in the 300 ppm group and female rats in the 60 and 300 ppm groups were lower than those of the controls. Gas chromatography-mass spectrometry analysis showed dose-dependent accumulation of 2-tDCB in adipose tissue, notably in males. Next, we performed an azoxymethane (AOM)-induced two-stage carcinogenesis study. After injection of 6-week-old male F344 rats (n=30/group) once a week for 3 weeks, the animals received 2-tDCB at concentrations of 0, 10, 50 and 250 ppm in a powder diet for 25 weeks. The incidences of colon tumors for the 2-tDCB dosages were 34%, 45%, 40% and 37%, respectively, and were not statistically significant. These data suggest that 2-tDCB shows no toxic or tumor-modifying effects under the present conditions, and that the no-observed-adverse-effect level for 2-tDCB is 300 ppm in both sexes, equivalent to 15.5 mg/kg b.w./day in males and 16.5 mg/kg b.w./day in females.

Genotoxic potential and in vitro tumour-promoting potential of 2-dodecylcyclobutanone and 2-tetradecylcyclobutanone, two radiolytic products of fatty acids

The DNA-damaging and tumour-promoting effects of two 2-alkylcyclobutanones (2-ACBs), which are found in irradiated fat-containing foods, were investigated by use of the comet assay and in an azoxymethane (AOM)-induced colon-carcinogenesis study in rats, respectively. We conducted genotoxicity tests of 2-dodecylcyclobutanone (2-dDCB) and 2-tetradecylcyclobutanone (2-tDCB) according to the test guidelines for chemicals or drugs. In addition, a cell-transformation assay with Bhas 42 cells was performed to investigate their promoting potential in vitro. The Salmonella typhimurium mutagenicity assay (Ames test), conducted with five tester strains, revealed that neither 2-dDCB nor 2-tDCB possessed mutagenic activity. Moreover, both in the in vitro chromosomal aberration test on CHL/IU cells and the in vivo bone-marrow micronucleus test where mice were given 2-dDCB and 2-tDCB (orally, up to 2000 mg/kg bw/day), we did not detect any clastogenic effects. Furthermore, DNA strand-breaks were not detected in the in vitro comet assay with CHL/IU cells, and DNA adducts derived from 2-dDCB and 2-tDCB were not detected in the colon tissues of the mice used for the micronucleus tests, in rats from a repeated dose 90-day oral toxicity test (0.03% 2-tDCB in the diet), or in rats from the AOM-induced carcinogenesis study (0.025% 2-tDCB in the diet). An in vitro tumour-promotion assay with Bhas 42 cells revealed that the number of transformed foci increased significantly following treatment of cells in the stationary phase with 2-dDCB or 2-tDCB for 10 days. Our results indicate that neither 2-dDCB nor 2-tDCB were genotoxic chemicals. However, they exhibited promoting activity, at least in vitro, when Bhas 42 cells were continuously exposed to these chemicals at toxic doses.

Molecular mechanisms of apoptosis induction by 2-dodecylcyclobutanone, a radiolytic product of palmitic acid, in human lymphoma U937 cells

The irradiation of fat-containing food forms 2-dodecylcyclobutanone (2-DCB) from palmitic acid (PA). In this study, we investigated whether 2-DCB and PA induce apoptosis in human lymphoma U937 cells. We found that cell viability decreased by 2-DCB and apoptosis was induced by 2-DCB and PA. 2-DCB and PA significantly enhanced the formation of intracellular reactive oxygen species (ROS). Apoptosis induced by 2-DCB and PA was strongly prevented by an antioxidant, N-acetyl-L: -cysteine. The treatment with 2-DCB and PA resulted in the loss of mitochondrial membrane potential, and Fas, caspase-8 and caspase-3 activation. Pretreatment with a pan-caspase inhibitor (z-VAD) significantly inhibited apoptosis induced by 2-DCB and PA. Moreover, 2-DCB and PA also induced Bax up-regulation, the reduction in Bcl-2 expression level, Bid cleavage and the release of cytochrome c from the mitochondria to the cytosol. In addition, an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) was observed after the treatment with 2-DCB and PA. Our results indicated that intracellular ROS generation, the modulation of the Fas-mitochondrion-caspase-dependent pathway and the increase in [Ca(2+)](i) involved in apoptosis are induced by 2-DCB and PA in U937 cells.

In vitro and in vivo metabolism of the radiolytic compound 2-dodecylcyclobutanone

Our knowledge about the metabolism of alkylcyclobutanones (2-ACBs) is limited, and the lack of literature on the metabolism of 2-ACBs causes consumers to doubt the safety of irradiated foods. The objectives of this study were to evaluate the metabolism of 2-dodecylcyclobutanone (2-DCB) and identify any possible metabolite. The 2-DCB was mixed with rat S9 (postmitochondrial supernatant fraction) and beta-nicotinamide adenine dinucleotide phosphate (NADPH) in phosphate buffer (pH 7.4) and incubated for 2 h at 37 degrees C. Then, the incubation mixture was mixed with sodium sulfate and extracted with n-hexane by using a Soxhlet apparatus. The hexane extract was concentrated under nitrogen and injected into the gas chromatography-mass spectrometry (GC-MS) machine running in selective ion monitoring mode (SIM) to measure 2-DCB concentration. The hexane extract from the in vitro and in vivo studies was also derivatized with a silylation reagent and injected into a GC-MS running in full scan mode. The average percentage of 2-DCB recovered from the test incubations was 23%, compared with 50% from the controls. The GC-MS chromatograms of the derivatized samples showed a unique peak in the in vitro test incubations and in the hexane extract of the rat feces that were given 2-DCB. This peak was later identified as 2-doecylcyclobutanol.

DNA oxidation: investigating its key role in environmental mutagenesis with the comet assay

DNA oxidation, which can have potentially serious mutagenic consequences, commonly accompanies exposure to environmental mutagens. Oxidised bases can be measured chromatographically, but spurious oxidation during sample preparation leads to serious over-estimation of low levels of damage. A more reliable approach is to employ endonucleases specific for oxidised bases, to introduce breaks in cellular DNA that are then most commonly measured using the comet assay (alkaline single cell gel electrophoresis). The two enzymes in general use are formamidopyrimidine DNA glycosylase, which detects primarily 8-oxo-7,8-dihydroguanine (8-oxoGua), and endonuclease III which recognises oxidised pyrimidines. We give a brief account of the recommended experimental procedures, and then describe applications in various areas of environmental research. Cultured cell lines or white blood cells have been exposed to a range of environmental mutagens, including natural products, industrial chemicals, radiation and nanoparticles. In vivo exposure of animals and humans to pollutants is more challenging but can give particularly valuable information in relation to real life exposure. Possibly the most useful application is in biomonitoring of human population groups suffering actual exposure to environmental or occupational mutagens. Finally, the potential use of this technique to monitor effects of contaminants in the natural environment has yet to be fully exploited.