Aroclor-1254-induced rat-liver S9 causes chromosomal aberrations in CHO cells but not human lymphocytes: a role for...

Incubation of CHO cells with Aroclor-1254 induced S9 mix gave higher than normal levels of chromosomal aberrations (CA), sometimes achieving exceptional levels (e.g. up to 40 CA, minus gaps, per 100 cells against a normal range of 0-10). The same batches of S9 gave normal CA frequencies in human lymphocytes from whole blood cultures, normal mutation frequencies in bacterial reversion (Ames) tests and mammalian cell hgprt mutation tests, and normal levels of repair in scheduled DNA synthesis tests. As cytochrome P-450 enzymes, in the absence of substrate, will cycle electrons and could produce active oxygen species (AOS), which are known to be clastogenic, this was investigated. Preliminary studies showed the complete S9 mix was required for the high levels of CA, and that CA levels could be dramatically reduced by co-incubation with catalase or vitamin E. Interestingly, uninduced S9 and phenobarbitone/beta-naphthoflavone induced S9 mixes did not cause high levels of CA, and presumably have different isozymes of P-450 that do not generate AOS as readily. These observations suggest that CHO cells are sensitive to AOS which are inactivated by blood components. This may have implications for the choice of cell system in routine clastogenicity testing of novel chemicals.