Correlation between luminescence intensity and cytotoxicity in cell-based cytotoxicity assay using luciferase

The luciferase reporter assay has become one of the conventional methods for cytotoxicity evaluation. Typically, the decrease of luminescence expressed by a constitutive promoter is used as an index of cytotoxicity. However, to our knowledge, there have been no reports of the correlation between cytotoxicity and luminescence intensity. In this study, to accurately verify the correlation between them, beetle luciferase was stably expressed in human hepatoma HepG2 cells harboring the multi-integrase mouse artificial chromosome vector. We showed that the cytotoxicity assay using luciferase does not depend on the stability of luciferase protein and the kind of constitutive promoter. Next, HepG2 cells in which green-emitting beetle luciferase was expressed under the control of CAG promoter were exposed to 58 compounds. The luminescence intensity and cytotoxicity curves of cells exposed to 48 compounds showed similar tendencies, whereas those of cells exposed to 10 compounds did not do so, although the curves gradually approached each other with increasing exposure time. Finally, we demonstrated that luciferase expressed under the control of a constitutive promoter can be utilized both as an internal control reporter for normalizing a test reporter and for monitoring cytotoxicity when two kinds of luciferases are simultaneously used in the cytotoxicity assay.

Evaluation of the mouse lymphoma tk assay (microwell method) as an alternative to the in vitro chromosomal aberration test

In order to evaluate the utility of the mouse lymphoma assay (MLA) for detecting in vitro clastogens and spindle poisons and to compare it with the in vitro chromosomal aberration test (CA), we conducted an international collaborative study of the MLA that included 45 Japanese laboratories and seven overseas laboratories under the cooperation of the Ministry of Health and Welfare of Japan and the Japanese Pharmaceutical Manufacturer's Association. We examined 40 chemicals; 33 were reportedly positive in the CA but negative in the bacterial reverse mutation assay, six were negative in both assays and one was positive in both. We assayed mutations of the thymidine kinase (TK) locus (tk) of L5178Y tk +/- mouse lymphoma cells using the microwell method. According to our standard protocol, cells were exposed to the chemical for 3 h, cultured for 2 days and TK-deficient mutants were expressed in 96-well plates under trifluorothymidine. Each chemical was coded and tested by two or three laboratories. Among the 34 CA-positive chemicals, positive MLA results were obtained for 20 and negative results were obtained for nine. The remaining five chemicals were inconclusive or equivocal because of discrepant inter-laboratory results or reproduced discrepant results, respectively. Among the six CA-negative chemicals, one was negative in the MLA, two were positive and three were inconclusive. Thus, the MLA could detect only 59% (20/34) of CA-positive chemicals. We concluded that the MLA was not as sensitive as the CA. Some MLA-negative chemicals evoked positive responses in the CA only after long continuous treatment. These might also be genotoxic in the MLA with long continuous treatment. Improvement of the MLA protocol, including alteration of the duration of the treatment, might render the MLA as sensitive as the CA.

The photogenotoxicity of titanium dioxide particles

We employed a series of in vitro genotoxicity assays--a single cell gel (SCG) assay with mouse lymphoma L5178Y cells, a microbial mutation assay with Salmonella typhimurium, a mammalian cell mutation assay with L5178Y cells, and a chromosomal aberration assay with Chinese hamster CHL/IU cells--to evaluate the photogenotoxicity of titanium dioxide (TiO2) particles. Without UV/visible light irradiation, TiO2 particles exhibited no or weak genotoxicity. With irradiation, however, TiO2 particles exhibited significant genotoxicity in the SCG and chromosomal aberration assays. Therefore, we concluded that TiO2 particles are photogenotoxic.

Re-evaluation of chromosomal aberration induction on nine mouse lymphoma assay "unique positive' NTP carcinogens

In a collaborative study organized under the JEMS MMS, nine mouse lymphoma assay (MLA) "unique positive' NTP rodent carcinogens were re-evaluated by an in vitro chromosomal aberration assay using Chinese hamster lung fibroblast cells (CHL/IU). Six of nine chemicals induced chromosomal aberrations; bromodichloromethane, chlorendic acid and isophorone induced structural aberrations, and chlorodibromomethane, pentachloroethane and 1,1,1,2-tetrachloroethane induced numerical aberrations (polyploidy). These six chemicals, therefore, are not uniquely positive in the MLA. The difference between the NTP results and ours might be due to the use of different cell lines and protocols, and in some cases, to different interpretations of polyploidy. The remaining three chemicals, benzyl acetate, cinnamyl anthranilate and trichloroethylene, were negative in this study.

Detection of in vitro clastogens and spindle poisons by the mouse lymphoma assay using the microwell method: interim report of an international collaborative study

Under the auspices of the Ministry of Health and Welfare of Japan and the Japanese Pharmaceutical Manufacturer Association, a collaborative study of the mouse lymphoma assay (MLA) was conducted by 42 Japanese laboratories and seven overseas laboratories to clarify the performance of the MLA for the detection of in vitro clastogens and spindle poisons. Twenty-one chemicals that were positive in in vitro chromosomal aberration assays (CA) but negative in bacterial reverse mutation assays (BRM) were examined by the MLA using the microwell method. All chemicals were coded, and each chemical was tested by two or three laboratories. Positive responses were obtained with 14 chemicals: mitomycin C (an internal positive control), arsenic trioxide, cadmium sulphate, chlorendic acid, cytosine arabinoside, diethylstilbestrol, eugenol, 5-fluorouracil, griseofulvin, hexamethyl phosphoramide, hydroxyurea, methotrexate, monocrotaline and pentachloroethane. Two chemicals (benzene and chlorodibromomethane) showed positive responses in one of two laboratories and were judged probably positive chemicals. Three chemicals (bromodichloromethane, isophorone and tetrachloroethane) were inconclusive because of a marginal response in one laboratory and a negative response in the other. Urethane was judged probably negative because two laboratories out of three showed clear negative responses. Dideoxycytidine (DDC) was a clear negative chemical in this study. The present results showed that 75.0% of the test chemicals (15/20, excluding mitomycin C) were positive, 15.0% (3/20) were inconclusive, and 10.0% (2/20) were negative. This suggests that the MLA may detect a majority of CA-positive chemicals. The inconclusive chemicals, however, are critical for the judgement of the MLA potential to detect clastogens. The findings that DDC was clearly negative suggests that the MLA may not be able to detect some clastogens. To clarify these issues, we began the second phase of the collaborative study with other BRM-negative and CA-positive chemicals.

Stimulation of nerve growth factor synthesis and secretion by fellutamide A in vitro

Fellutamide A, a tripeptide derivative from Penicillium fellutanum was found to be a potent enhancer of nerve growth factor (NGF) synthesis and secretion in vitro. This compound enhanced production of NGF in L-M cells, rat brain cells, and rat glial cells. The mode of action of this compound was suggested to be different from that of a known NGF inducer, epinephrine, as inducing activities of fellutamide A and epinephrine were additive when they were admixed at the concentration which gave saturation in its respective activity. Furthermore, fellutamide A but not epinephrine induced production of NGF in the rat brain cells. NGF-inducing activity of several tripeptides and other compounds related to fellutamide A was examined.