We have investigated interrelationship between histological changes and the expression of DNA topoisomerases(Topo I and II) and proto-oncogenes, c-myc during the course of hepatocarcinogenesis induced by 3'-methyl dimethylaminoazobenzene.
1) Oval cell proliferation and vacuolated changes, liver cirrhosis, cholangiocarcinoma or hepatocellular carcinoma were observed in the 2nd, 8th, 12th weeks, respectively during hepatocarcinogenesis.
2) Specific activity of Topo I from normal liver was 203 unit/mg protein/min and that of Topo II was absent. The activity of Topo I and II was increased in the 2nd week during hepatocarcinogenesis, and the maximal increment of them occured in
the
4th
week. In case of Topo I activity. maximal increment was 2.6 folds, in comparison with that of normal liver. After 8 weeks the enzyme activity was gradually decreased. Topo II was sharply decreased and the disappeared after 10 weeks.
3) The level of Topo I mRNA was increased in the 4th week and showed maximal increment in the 6th week. After 8 weeks the transcript was decreased significantly, and then maintained as normal level.
4) Protein level of Topo I of hepatoma was higher than normal liver.
5) The level of c-myc mRNA was highly increased in the 4th week, respectively.
6) ATP, GTP inhibited the activity of Topo I from normal and cancer tissue of rat liver in the presence of Mg2+, K+, while in the absence of them, the activity was activated.
7) ddATP and ddGTP inhibited the activity of Topo I, while ddCTP activated.
8) Camptothecin and 10-OH-camptothecin inhibited the enzymes of normal and cancer tissue. and ID50 was 20μM and 5μM, respectively.
9) Etoposide and adriamycin inhibited Topo II, and the ID50 was 25μM and 6.25μM, respectively.
These results suggest that c-myc and DNA topoisomerase I may play a important role during hepatocarcinogenesis induced by 3'-MeDAB, and the possible induction of differentiation of oval cells by DNA topoisomerases inhibitors was discussed.
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