BILE ACID-INDUCED MODIFICATIONS IN DNA-SYNTHESIS BY THE REGENERATING PERFUSED-RAT-LIVER

Liver cell proliferation is a complex process that can be affected by a large number of factors such as bile acids, which have been reported to be associated to the pathogenesis of liver cancer. In this work, b ile acid-induced modifications in DNA synthesis by regenerating perfus ed rat liver were investigated. Two-thirds hepatectomy was carried out 24 hr before perfusion of livers with recirculating, erythrocyte-free Krebs-Henseleit solution. The viability of the preparations was maint ained under all experimental conditions, as indicated by bile flow, ox ygen uptake, perfusion pressure, perfusion flow and release of lactate dehydrogenase and potassium into the perfusate. Livers received (min 10 to min 60) bile acid infusion at a rate of 25 nmol/min/gm liver (i. e., maximal secretion rate/2) in regenerating livers as calculated for taurocholate in separate experiments). Trace amounts of [methyl-C-14] thymidine were added to the perfusate at min 30. At the end of the exp eriments (min 60) the livers were washed, removed, weighed and homogen ized to determine radioactivity in whole tissue, in DNA and in non-DNA -related fractions. Taurocholate and, to a lesser extent, taurodeoxych olate and dehydrocholate (but not ursodeoxycholate) were found to redu ce C-14 incorporation into DNA. This was not due to changes in the con tent of C-14 in whole, regenerating liver tissue. Taurocholate, taurod eoxycholate, dehydrocholate and ursodeoxycholate had no effect on thym idine uptake; moreover, the proportion of C-14 found in bile was negli gible. However, bile acid-induced modification in the fate of intracel lular thymidine was observed. In regenerating livers receiving no bile acid, the C-14 carried by thymidine metabolites accounted for about 6 0% of C-14 in whole liver tissue. Taurocholate markedly increased this proportion to about 80%. Reverse-phase high-pressure liquid chromatog raphy revealed that most of this C-14 (about 80%) was recovered at the elution time, corresponding to thymidine catabolites rather than to D NA precursors. These results suggest that bile acids induce enhancemen t of thymidine catabolism that reduces its incorporation into DNA; inh ibition in the process of DNA synthesis itself, leading to a subsequen t increase in the metabolism of DNA precursors; or both. Moreover, fro m the diversity in this property for bile acid species it might be inf erred that changes in the composition and size of the bile acid pool d uring liver carcinogenesis or regeneration play a role in the modulati on of the proliferative process.