Metabolic activation of 3-methylcholanthrene and benzo(a)pyrene to mutagens in the L5178Y/TK assay by cultured embryonic rodent cells

Mutant yields and mutational spectra of the heterocyclic amines MeIQ and PhIP at the S1 locus of human-hamster AL cells with activation by chick embryo liver (CELC) co-cultures

Cooking meat and fish at high temperature creates heterocyclic amines (HA) including 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Several HA are mutagens in the Ames' S9/Salmonella assay. While PhIP is a substantial Ames' test mutagen, it is 1000-fold less active than the extraordinarily potent MeIQ. In contrast, MeIQ is significantly less mutagenic than PhIP in several mammalian cell assays, especially in repair-deficient Chinese hamster ovary (CHO) cells. HA are suspect human carcinogens on the basis of (i) epidemiological evidence, (ii) induction of tumors in rodents and monkeys, (iii) DNA adduct formation and (iv) mutagenic capacity. In this study, MeIQ and PhIP were significant mutagens at the S1 locus of co-cultivated human/hamster hybrid AL cells following metabolic activation by beta-napthoflavone (betaNF)-induced chick embryonic liver cultures (CELC). MeIQ was more mutagenic than PhIP in the CELC+AL cell assay. The mutant response curves increase with dose and then plateau (PhIP), or decrease (MeIQ). The inflections in these response curves coincide with dose-dependent decreases in cytochrome CYP1A1 activity. Molecular analysis of S1- mutants indicates that a substantial fraction, >65%, of the mutations induced by PhIP are deletions of 4.2 to 133 (Mbp); half are larger than 21 Mbp. Mutations induced by MeIQ were smaller, most (56%) being less than 5.7 Mbp. When appropriate metabolic activation is combined with a target locus, which can detect both small and large chromosomal mutations, both MeIQ and PhIP are significant mutagens and clastogens in repair proficient mammalian cells.

Comparison of mutagenicities in a Salmonella reversion assay mediated by uninduced hepatocytes and hepatocytes from rats pretreated for 1 or 5 days with Aroclor 1254

Hepatocytes prepared from rats pretreated for 5 days with 500 mg/kg Aroclor 1254 were found to be unsuitable for use in a modified Salmonella mutagenicity assay. These hepatocytes exhibited low viability, did not readily attach to plastic culture dishes, and produced mutagenicity responses with benzo[a]pyrene (B[a]P) and 2-aminofluorene (2AF) that were greatly enhanced by the addition of an NADPH-regenerating system (NADPH-RS). Shortening the Aroclor pretreatment time to 1 day resulted in hepatocytes that exhibited high viability and readily attached to plastic culture dishes. These hepatocytes produced higher numbers of revertants when used to assay the mutagenicities of B[a]P and 2AF than were produced using hepatocytes from animals that were pretreated for 5 days. These reversion frequencies were also higher than those produced using uninduced hepatocytes and were much less affected by the addition of NADPH-RS than were the reversions mediated by the 5-day preinduced hepatocytes. Liver homogenate postmitochondrial fractions (S9s), which were prepared from rats pretreated with Aroclor for 1 or 5 days, were nearly equal in their ability to mediate the mutagenicity of B[a]P and 2AF in the Salmonella/microsome reversion assay. Qualitative differences between the S9- and hepatocyte-mediated mutagenicity of 2AF were found, however. These results indicate that employing hepatocytes from rats pretreated with Aroclor for 1 day, rather than 5 days, results in an enzymatically induced, more-intact cell population that is capable of detecting the mutagenicity of B[a]P and 2AF in a modified Salmonella reversion assay.

The activation of procarcinogens to mutagens by cultured rat hepatocytes in the L5178Y/TK mutation assay

Whole cell preparations derived from collagenase-treated rat liver were cocultivated overnight with stationary (non-shaking) cultures of L5178Y/TK+/- cells in the presence of 8 different chemicals selected as representative aromatic amine, polycyclic hydrocarbon, or nitrosamine procarcinogens. When tested in the presence of hepatocytes, 2-aminoanthracene, 2-aminofluorene, N-nitrosodimethylamine, N-nitrosodiethylamine, N-nitrosodipropylamine, 3-methylcholanthrene, and benzo[a]pyrene all produced substantial dose-dependent increases in trifluorothymidine-resistant variants compared to solvent controls after 20 h total exposure time. Only N-nitrosodipropylamine (DPrN) and N-nitrosodiethylamine (DEN) produced any dose-related mutagenic activity in similar experiments where hepatocytes were omitted; however, the response for the DPrN was quite variable at high doses in the absence of hepatocytes and the mutagenic response for the DEN was consistently enhanced at all dose levels by the presence of hepatocytes. Benzanthracene was not active in the presence of whole hepatocytes, even when tested with cells from a rat pretreated 24 h earlier with 20 mg/kg benzanthracene. Excepting benzanthracene, these data suggest that rat hepatocytes can be used to active 3 types of procarcinogens to mutagens in the L5178Y/TK gene mutation assay.