Unscheduled DNA synthesis caused by norethindrone and related contraceptive steroids in short-term male rat hepatocyte cultures.

The genotoxic potential of oral contraceptive steroids such as norethindrone was investigated in short-term rat hepatocyte cultures by measurement of unscheduled DNA synthesis. Norethindrone caused a small dose-dependent increase in unscheduled DNA synthesis in male rat hepatocytes as judged by the incorporation of [methyl-3H]thymidine into DNA. This was assessed either by liquid scintillation counting following isolation of DNA or by autoradiography. No increase in unscheduled DNA synthesis could be detected in female rat hepatocytes treated with norethindrone. Pre-treatment of male rats with phenobarbitone prior to hepatocyte preparation decreased the norethindrone mediated unscheduled DNA synthesis relative to control hepatocyte cultures while 3-methylcholanthrene pre-treatment had little effect. Unscheduled DNA synthesis in norethindrone treated control male rat hepatocytes was reduced by the mixed function oxidase inhibitors SKF 525A or metyrapone. In 24- or 52-hour-old hepatocyte cultures in which the cytochrome P-450 content was lower than in freshly prepared cells, or in a hepatocyte-derived cell line lacking cytochrome P-450, unscheduled DNA synthesis due to norethindrone was either decreased or abolished. Structure activity studies showed that only steroids containing a 17 alpha-ethynyl substituent caused an increase in unscheduled DNA synthesis.