Inhibition of chemical carcinogenesis: Increased activity of soluble RNA polymerase in the liver of rats protected against 3′MeDAB hepatocarcinogenesis by dietary chloramphenicol

Abstract

Soluble RNA polymerases (nucleotidyl transferases, EC 2.7.7.6) were isolated from liver nuclei of rats that had been pair-fed diets containing the hepatocarcinogen 3′methyl- 4-dimethylaminoazobenzene ( 3′MeDAB, 0·06%) either alone or in combination with chloramphenicol (CAP, 2%), an inhibitor of azo dye carcinogenesis. DEAE Sephadex A- 25 chromatography of crude nuclear extracts (fraction IV protein) revealed 2 major peaks of enzyme activity which were equated with RNA polymerases I and II on the basis of order of elution and sensitivity to α-amanitin. The specific activities of both enzymes, in particular RNA polymerase I, were significantly increased after 4 days on diets containing CAP or both 3′MeDAB and CAP. Rats protected from 3′MeDAB carcinogenesis by CAP showed the greatest increase, which exceeded that in the group fed dye alone by 292% for polymerase I and 116% for polymerase II. Increases in both RNA polymerase activities were probably not a consequence of altered affinity for the substrate UTP. RNA polymerase activity was not altered by feeding the 3′MeDAB diet for 4 days, but after 10 days there was a 340% increase in the specific activity of enzyme I and a 14% increase in the specific activity of enzyme II relative to levels in control rats. The total nuclear protein content was significantly increased in rats fed 3′MeDAB or both 3′MeDAB and CAP for 4 days. Crude polymerase extracts contained 4–6% of the total nuclear protein and 83–87% of the RNA polymerase activity present in isolated nuclei. There was a significant increase in the amount of nuclear sap protein extracted after feeding 3′MeDAB for 4 days. Differences in nuclear RNA synthesis between experimental groups were insensitive to presacrifice starvation and altered feeding and lighting schedules; neither were they due to differential losses of RNA polymerases during the nuclear isolation procedure or to preferential extraction of these enzymes from isolated nuclei in the various groups. We conclude that the differences in liver nuclear RNA synthesis between 3′MeDAB-fed rats and rats protected against 3′MeDAB-induced hepatocarcinogenesis by concurrent CAP administration are at least partly due to differences in the activity and/or amount of soluble RNA polymerases.