Collaborative study of thresholds for mutagens: proposal of a typical protocol for detection of hormetic responses in cytotoxicity tests

BACKGROUND

According to the linear no-threshold model (LNT), even the smallest amount of radiation is hazardous. Although the LNT is not based on solid data, this hypothesis has been applied to mutagens and carcinogens. As a result, it has been postulated that there are no thresholds for these chemicals. To demonstrate negativity by experiments is practically impossible, because negative data may leave behind the possibility that additional data might make the resolution power high enough to change negativity to positivity. Furthermore, additional data collection may be endless and we may be trapped in agnosticism. When hormesis is established, in which biological responses are higher at low-doses and lower at high-doses than the control, thresholds could be established between the low- and high-doses. Before examination of thresholds in chemical mutagenesis, hormetic responses in cytotoxicity were tested using cultured mammalian cells.

METHOD

Human cells (HeLa S3 and TK6) or Chinese hamster cells (CHL/IU) were cultured in 96-well plates and treated with mitomycin C (MMC) or ethyl methanesulfonate (EMS) at various dose levels and optical density was measured after addition of a reagent to detect cellular activity. In hormetic responses, data might fluctuate to and fro; therefore, experimental conditions were examined from various aspects to eliminate confounding factors including cell numbers, detection time, the edge effect of 96-well plates, and measurement time after addition of the reagent for detection.

RESULTS

The dose response relationship was never linear. Cellular activities after treatment with MMC or EMS were generally higher at lower doses levels and lower at higher doses than the control, showing hormesis and allowing the establishment of thresholds. Dose response curves sometimes showed two or three peaks, probably reflecting different cellular responses.

CONCLUSION

Hormetic responses in cytotoxicity tests were observed and thresholds could be established. Based on the results of this investigation, we put forward a tentative protocol to detect chemical hormesis in cytotoxicity tests, i.e., inoculate 2000 cells per well, add various doses of a test chemical 48 h after inoculation, add a detection dye 10 h after treatment, and measure optical density 2 h after addition of the reagent for detection.

Mechanisms of chromosomal aberrations induced by sesamin metabolites in Chinese hamster lung cells

Sesamin is a major lignan in sesame seeds and oil. We previously demonstrated that sesamin induces chromosomal aberrations (CA) in Chinese hamster lung (CHL/IU) cells in the presence of a metabolic activation system (S9 mix), although no genotoxicity was detected in vivo. To clarify the mechanism of CA induction by sesamin, we identified its principal active metabolite. A mono-catechol derivative, [2-(3,4-methylenedioxyphenyl)-6-(3,4-dihydroxyphenyl)-3,7-dioxabi-cyclo[3.3.0]octane (SC-1)], was previously identified in culture medium when sesamin was incubated with S9 mix. In the present study, we show that SC-1 induces CA in CHL/IU cells but not in human hepatoblastoma (HepG2) cells. SC-1 was unstable in culture medium. Addition of glutathione (GSH) to the incubation mixture decreased the rate of decomposition and also suppressed induction of CA in CHL/IU cells. These results indicate that SC-1 itself may not contribute to the induction of CA. Two GSH adducts of SC-1 were identified when SC-1 was incubated with GSH, suggesting that SC-1 was converted to the semiquinone/quinone form and then conjugated with GSH in the culture medium. Sodium sulfite (a quinone-responsive compound) also suppressed CA induction by SC-1. These findings strongly suggest that SC-1 is oxidized to semiquinone/quinone derivatives extracellularly in culture medium, that these derivatives are responsible for the induction of CA in CHL/IU cells, and therefore that the positive results obtained with sesamin in in vitro CA tests using CHL/IU cells may not be relevant to the assessment of in vivo activity.

Standardized cell sources and recommendations for good cell culture practices in genotoxicity testing

Good cell culture practice and characterization of the cell lines used are of critical importance in in vitro genotoxicity testing. The objective of this initiative was to make continuously available stocks of the characterized isolates of the most frequently used mammalian cell lines in genotoxicity testing anywhere in the world ('IVGT' cell lines). This project was organized under the auspices of the International Life Sciences Institute (ILSI) Health and Environmental Sciences Institute (HESI) Project Committee on the Relevance and Follow-up of Positive Results in In Vitro Genetic Toxicity (IVGT) Testing. First, cell isolates were identified that are as close as possible to the isolate described in the initial publications reporting their use in genotoxicity testing. The depositors of these cell lines managed their characterization and their expansion for preparing continuously available stocks of these cells that are stored at the European Collection of Cell Cultures (ECACC, UK) and the Japanese Collection of Research Bioresources (JCRB, Japan). This publication describes how the four 'IVGT' cell lines, i.e. L5178Y TK^+/- 3.7.2C, TK6, CHO-WBL and CHL/IU, were prepared for deposit at the ECACC and JCRB cell banks. Recommendations for handling these cell lines and monitoring their characteristics are also described. The growth characteristics of these cell lines (growth rates and cell cycles), their identity (karyotypes and genetic status) and ranges of background frequencies of select endpoints are also reported to help in the routine practice of genotoxicity testing using these cell lines.

Genotoxicity and subchronic oral toxicity of L-ornithine monohydrochloride

L-Ornithine monohydrochloride was evaluated in two in vitro genotoxicity assays and a rat 90-day oral toxicity study. No evidence of genotoxicity was observed in the reverse bacterial mutation assay or the chromosome aberration test at doses of up to 5000 μg/plate or 1686 μg/mL, respectively, both in the presence and absence of metabolic activation. Rats were administered L-ornithine monohydrochloride at dietary concentrations of 0 (basal diet), 1.25%, 2.5%, or 5.0% for 90 days. No changes in body weight, food consumption, ophthalmoscopy, or hematology were observed. Transient increases in water intake and urinary volume, and a decrease in specific gravity were observed in males receiving 5.0% L-ornithine monohydrochloride; however, these were likely attributable to the central role of ornithine in the urea cycle and the consequent increase in urea production. A decrease in serum chloride concentration and an increase in urinary chloride excretion were observed; however, these were likely attributable to administration of the hydrochloride salt of ornithine and were not considered to be of any toxicological significance. No remarkable findings were noted at necropsy. Based on the results of the study, a no-observed-adverse effect level (NOAEL) of 3445 and 3986 mg/kg body weight/day was established for male and female rats.

Genotoxic effects of N-nitrosoketamine and ketamine as assessed by in vitro micronucleus test in Chinese hamster lung fibroblast cell line

OBJECTIVES

Ketamine hydrochloride (KT) is a secondary amine that has been safely used as an injectable anesthetic and analgesic to avoid the production of nitroso compounds in the stomach. However, ketamine in the tablet form has recently become an abused, recreational drug. The aim of this study was to investigate the genotoxic effects of N-nitrosoketamine (NKT) and KT on the basis of an in vitro micronucleus (MN) test using a Chinese hamster lung fibroblast cell line (CHL/IU).

METHODS

NKT was synthesized from KT in our laboratory. In the MN tests, CHL/IU cells were continuously treated with either NKT or KT for 24, 48, or 72 hours without the S9 mix. The cells were also treated with NKT or KT with or without the S9 mix for 6 hours, followed by a recovery period of 18, 42, or 66 hours (short-term treatment). The results were considered to be statistically significant when the p-values of both Fisher's exact test and the trend test were less than 0.05.

RESULTS

After the short-term treatment with either NKT or KT with and without the S9 mix, the frequency of micronuclei significantly increased. However, the frequency of micronuclei did not significantly increase after the continuous treatment with either NKT or KT. Both NKT and KT were determined to be genotoxic in the short-term treatment with or without the S9 mix, but they were determined to be nongenotoxic in continuous treatment.

CONCLUSION

Our findings suggest that NKT has a stronger genotoxic effect than KT.