Sensitivity of bile acid transport by organic anion-transporting polypeptides to intracellular pH.

We investigated the influence of intracellular pH (pHi) on [14C]-glycocholate (GC) uptake by human hepatoblastoma HepG2 cells that express sodium-independent (mainly OATP-A and OATP-8), but not sodium-dependent, GC transporters. Replacement of extracellular sodium by choline (Chol) stimulated GC uptake but did not affect GC efflux from loaded cells. Amiloride or NaCl replacement by tetraethylammonium chloride (TeACl) or sucrose also increased GC uptake. All stimulating circumstances decreased pHi. By contrast, adding to the medium ammonium or imidazole, which increased pHi, had no effect on GC uptake. In Chinese hamster ovary (CHO) cells expressing rat Oatp1, acidification of pHi had the opposite effect on GC uptake, that is, this was reduced. Changes in extracellular pH (pHo) between 7.40 and 7.00 had no effect on GC uptake at pHi 7.30 or 7.45 when pHopHi. Inhibition was not proportional to the pHo-pHi difference. Intracellular acidification decreased V(max), but had no effect on K(m). In sum, sodium-independent GC transport can be affected by intracellular acidification, possibly due both to modifications in the driving forces and to the particular response to protonation of carrier proteins involved in this process.

Relationship between tumor cell load and sensitivity to the cytostatic effect of two novel platinum-bile acid complexes, Bamet-D3 and Bamet-UD2.

Based on the organotropic characteristics of bile acids towards the liver and the intestine, two novel compounds of the Bamet family, containing at least one bile acid moiety bound to platinum(II), have been synthesized and their cytostatic effect compared to their ability to become accumulated in tumor cells of hepato-intestinal origin. Bamet-UD2 [cis-diammine-bis-ursodeoxycholic platinum(II)] induced a marked inhibition of cell growth, which was more marked in human hepatoblastoma HepG2 and mouse hepatoma Hepa 1-6 cells than in rat hepatoma McA-RH7777 and human colon adenocarcinoma LS 174T cells. This effect was similar to that observed for cisplatin and stronger than that previously reported for other members of this family, such as Bamet-H2 and Bamet-R2. By contrast, Bamet-D3 [(N'N'' cis-dichloro N(3-3-amminepropylammine)propyl) glycocholamide platinum (II)] was only effective in reducing growth in human hepatoblastoma HepG2 cells. Because the in vitro DNA-reactivity was approximately 5-fold higher for Bamet-D3 than for Bamet-UD2, an additional cause for the difference in their cytostatic abilities was sought, investigating the relationship between cell load and the cytostatic effect of the drugs. Drug uptake by two cell lines, Hepa 1-6 and HepG2, with different sensitivities to these compounds was measured. The cellular uptake of Bamet-D3 and Bamet-UD2 was several-fold higher than that of cisplatin. No significant difference in the amount of both drugs taken up by these cell types was found. A study on sodium-dependency and substrate specificity indicated that Hepa 1-6 cells take up Bamet-D3 and Bamet-UD2 via similar mechanism(s), whereas these compounds do not seem to share the uptake pathways in HepG2 cells. Measurement of cell viability by formazan formation from tetrazolium salts and by neutral red uptake, after short-term (6 h) exposure to the desired drug, indicated that no acute toxic effect occurs in the presence of cisplatin or Bamet-D3 in either HepG2 or Hepa 1-6 cells. By contrast, in both cell lines Bamet-UD2 induced acute cell toxicity in a dose-dependent fashion. In sum, the results indicate that tumor cells efficiently take up these two novel compounds of the Bamet family. Although the exact uptake mechanism remains unknown, it seems to be dependent on the cell type. However, the cell load does not account for the differences in the anti-proliferative properties of the drugs. The strong and promising cytostatic activity of Bamet-UD2 is additionally related to its ability, absent in Bamet-D3, to acutely alter cellular functions other than proliferation.

Changes in the pool of bile acids in hepatocyte nuclei during rat liver regeneration.

BACKGROUND/AIMS: To investigate changes in nuclear bile acids (BAs) during rat liver regeneration. METHODS: Nuclei were isolated from control rat livers and after two-thirds partial hepatectomy (PH). BAs in bile, liver homogenate and nuclei were measured by gas chromatography-mass spectrometry. Nuclear translocation of radiolabeled BAs was determined using fresh isolated hepatocytes from control donors. RESULTS: Liver BA concentrations were transiently reduced after PH. Relative increases in: -MCA at 1 day, deoxycholic acid at 7 days and cholic acid (CA) at 3 and 14 days were found. Nuclear BAs accounted for <0.5% of liver BAs. Contamination with cytosolic BAs during nuclei isolation was <4%. Unconjugated- and conjugated-CA were able to reach the nucleus with similar efficiency. The pattern of nuclear BAs--CA (80%) and ursodeoxycholic acid (UDCA) (8.5%) being the most abundant--did not match that found in liver or bile. A transient decrease in CA/UDCA ratio, in absence of significant change in total BA content, was observed in nuclei after PH. "Flat" BA species were only detected in homogenate, but not in nuclei, at 1 day after PH. CONCLUSIONS: BA pool in nuclei of rat hepatocytes, whose composition is different to that of total liver BA pool, undergoes important changes during liver regeneration.

Evidence for dual effects of DNA-reactive bile acid derivatives (Bamets) on hepatitis B virus life cycle in an in vitro replicative system.

A liver targeting strategy to direct antiviral drugs toward hepatitis B virus (HBV) was investigated. As model drugs we used cisplatin-bile acid derivatives (Bamets) to determine the production of virions by HBV-transfected hepatoblastoma cells (HepG2 2.2.15). Drug uptake was determined using flameless atomic absorption spectrometry to measure platinum cell contents. Cytotoxic effect was determined by formazan formation and neutral red uptake tests. The release of viral surface protein was evaluated by ELISA. The abundance of HBV-DNA in the medium was determined by quantitative real-time PCR and its structure by Southern blot analysis. The uptake of Bamets by HepG2 2.2.15 cells was higher than that of cisplatin. At concentrations lower than 10 microM, distinct Bamets have no toxic effect on host cells, whereas cisplatin dramatically reduced cell viability at concentrations higher than 1 microM. All the drugs tested inhibited the release of viral proteins to the medium, but induced a marked and progressive dose-dependent increase in the amount of viral DNA in the medium. This was mainly due to the release of short fragments of HBV-DNA in the case of cisplatin. On the contrary, Bamets induced an enhanced release of circular forms of HBV-DNA. These findings suggest the existence of a dual effect of Bamets on HBV life-cycle by enhancing the production of DNA replicative intermediates but reducing the secretion of complete virions. Altogether these characteristics recommend consideration of these compounds as a useful experimental tool in the investigation of novel liver targeted therapeutic agents based on bile acid derivatives for the treatment of HBV infections, or to carry out further studies on the HBV life cycle.