Bile acid-induced modifications in DNA synthesis by the regenerating perfused rat liver

Abstract

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, bile acid-induced modifications in DNA synthesis by regenerating perfused rat liver were investigated. Two-thirds hepatectomy was carried out 24 hr before perfusion of liver with recirculating, erythrocyte-free Krebs-Henseleit solution. The viability of the preparations was maintained under all experimental conditions, as indicated by bile flow, oxygen 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-14C]thymidine were added to the perfusate at min 30. At the end of the experiments (min 60) the livers were washed, removed, weighed and homogenized to determine radioactivity in whole tissue, in DNA and in non-DNA-related fractions. Taurocholate and, to a lesser extent, taurodeoxycholate and dehydrocholate (but not ursodeoxycholate) were found to reduce 14C incorporation into DNA. This was not due to changes in the content of 14C in whole, regenerating liver tissue. Taurocholate, taurodeoxycholate, dehydrocholate and ursodeoxycholate had no effect on thymidine uptake; moreover, the proportion of 14C found in bile was negligible. However, bile acid-induced modification in the fate of intracellular thymidine was observed. In regenerating livers receiving no bile acid, the 14C carried by thymidine metabolites accounted for about 60% of 14C in whole liver tissue. Taurocholate markedly increased this proportion to about 80%. Reverse-phase high-pressure liquid chromatography revealed that most of this 14C (about 80%) was recovered at the elution time, corresponding to thymidine catabolites rather than to DNA precursors. These results suggest that bile acids induce enhancement of thymidine catabolism that reduces its incorporation into DNA; inhibition in the process of DNA synthesis itself, leading to a subsequent increase in the metabolism of DNA precursors; or both. Moreover, from the diversity in this property for bile acid species it might be inferred that changes in the composition and size of the bile acid pool during liver carcinogenesis or regeneration play a role in the modulation of the proliferative process.

Regulation of the proprotein convertase subtilisin/kexin type 9 in intestinal epithelial cells

“…In addition to their traditional role in dietary lipid absorption and cholesterol homeostasis, it has become clear now that BA constitute versatile signaling molecules endowed with systemic endocrine functions. In fact, BA are ligands for G protein-coupled receptors and modulate several nuclear hormone receptors (27,35,36). Through activation of these diverse signaling pathways, BA have been shown to regulate not only their own synthesis and enterohepatic recirculation, but also triglyceride, cholesterol, and glucose homeostasis (51).…”

“…Given their hydrophilic-hydrophobic properties, unconjugated and conjugated BA may differ in their various actions, including membrane permeability, lipid solubilization, intracellular signaling, and DNA synthesis (2,8,20,35,41,47). We have therefore assessed the modulation of unconjugated and conjugated BA to taurine or glycine.…”

Bile acids alter the subcellular localization of CNT2 (concentrative nucleoside cotransporter) and increase CNT2-related transport activity in liver parenchymal cells

“…There is no established link between CNT2 function and bile acid metabolism. However, it is known that TCA promotes either nucleoside catabolism or nucleoside incorporation into DNA, dependent upon nutritional status [15,16], and that CNT2 expression is up-regulated during liver cell proliferation [9][10][11]. Nevertheless, as previously described, CNT2 may have additional functions besides nucleoside salvage, as CNT2 is under purinergic control via K ATP channels in hepatocytes [14] and, at least in IEC-6 lines, CNT2-mediated adenosine uptake determines AMPK activation (I. Aymerich, F. Foufelle, P. Ferré, F. J. Casado and M. PastorAnglada, unpublished work).…”

“…Among a panel of putative modulators, bile acids were chosen since they regulate nucleoside metabolism in regenerating rat liver in which CNT2-related transport activity is up-regulated [10]. In particular TCA (taurocholate) promotes either nucleoside catabolism or incorporation into DNA, dependent upon nutritional status [15,16]. Moreover, bile acids are bioactive molecules.…”

Hepatic regeneration in the isolated perfused rat liver followed by liver transplantation

“…Because no replacement of bile acids was employed and liver weight was greater in group I than in groups II and III, this difference could explain a faster elimination of the pool of bile acids in the recirculating perfusate in group I. Studying bile acid-induced modifications in DNA synthesis by the regenerating perfused rat liver, Marin 36 showed that the basal bile flow per gram of liver tissue was slightly reduced 24 hours after partial hepatectomy.…”