P-glycoprotein expression and function in rat hepatocytes in culture

Evidence of Oatp and Mdr1 in cryopreserved rat hepatocytes

Transport proteins control uptake of drugs into the liver (e.g., organic anion transporting polypeptide (Oatp)) and excretion of drugs from the liver (e.g., multidrug resistance protein 1 (Mdr1)). In this study, cryopreserved rat hepatocytes were used to investigate the effect of different culture conditions (suspension, conventional culture and sandwich culture) on the uptake of [(3)H]-taurocholate+/-probenecid and the efflux of [(3)H]-vinblastine+/-ketoconazole; mRNA levels of Oatp1a1, Oatp1a4, Mdr1a and Mdr1b were determined using real-time reverse transcription polymerase chain reaction (RT-PCR) and protein expression of Mdr was assessed by immunocytochemistry. The uptake of [(3)H]-taurocholate was higher in cryopreserved rat hepatocytes maintained in suspension as compared to hepatocytes in culture. A significant time dependent decline in the uptake of [(3)H]-taurocholate was noticed from day 2 to day 4 in conventional and sandwich cultures. [(3)H]-taurocholate uptake was significantly reduced using the inhibitor probenecid. Oatp mRNA expression in hepatocytes in suspension was similar to that of liver, whereas much lower levels were detected in the cultures; this was in accordance with [(3)H]-taurocholate uptake results. Mdr1 activity was assessed by accumulation of the Mdr1 selective substrate, [(3)H]-vinblastine, in hepatocytes using ketoconazole as an inhibitor. The results showed Mdr1 activity in cryopreserved rat hepatocytes in conventional and sandwich cultures. A time dependent increase in Mdr1 activity was noticed from day 2 to day 4. Mdr1 activity was not found using hepatocytes in suspension. Mdr1 mRNA expression was high in cryopreserved hepatocytes from both culture systems. Immunocytochemistry showed the Mdr protein in membranes of hepatocytes in culture as well as in that of hepatocytes in liver sections. In conclusion, the present study showed that cryopreserved rat hepatocytes maintained canalicular transport activity (Mdr1) and basolateral transport activity. Hepatocytes in suspension had a higher uptake of taurocholate with a high Oatp (1a1 and 1a4) mRNA expression as compared to hepatocytes in culture. The presence of Mdr1 in both conventional and sandwich culture was confirmed at mRNA level, by protein expression as well as transport activity.

Cyclosporine a augments P-glycoprotein expression in the regenerating rat liver

In the liver, the multidrug resistance (MDR) protein P-glycoprotein (P-gp) is physiologically expressed at the bile canalicular membrane, where it participates in the biliary excretion of various lipophilic drugs and xenobiotics. Previous studies showed that the immunosuppressive agent cyclosporine A (CsA) modulates P-gp and exerts a hepatotrophic influence in the regenerating liver. Hepatocytes isolated from regenerating rat liver, after 2/3 partial hepatectomy (PH 2/3), were used as an in vivo experimental model of cells with high proliferating activity in order to investigate whether CsA influences cellular levels of P-gp in those cells. Male Wistar rats were treated with CsA (20 mg/kg body weight) for 4 d preoperatively and 1 d postoperatively, and regenerating hepatocytes were isolated by collagenase perfusion 12, 24 and 48 h after PH 2/3. Flow cytometry and Western blotting studies with the monoclonal antibodies C494 and C219 showed that after PH 2/3, cellular levels of P-gp were initially suppressed, 12 h after PH 2/3, by 23%, but were significantly elevated thereafter, 24 and 48 h after PH 2/3 by 28% and 73%, respectively. In CsA pretreated animals, P-gp levels were increased even in normal hepatocytes by 34%, and an additional augmentation was seen in hepatocytes from 24 and 48 h regenerating livers (60% and 56%, respectively). In summary, we demonstrate for the first time that CsA has an additive effect on the expression of P-glycoprotein during liver regeneration in the rat. Therefore, induction of P-gp might also be considered in patients receiving CsA after liver transplantation for hepatocellular carcinoma and chemotherapy as an adjuvant treatment for the prevention of tumor recurrence.

Effects of curcumin on P-glycoprotein in primary cultures of rat hepatocytes

Curcumin is a natural phenolic compound found in the rhizomes of Curcuma longa and endowed with beneficial biological activities including antioxidant, anticarcinogenic and hepatoprotective effects. In this study curcumin was tested for its potential ability to interact in vitro with hepatic P-glycoprotein (Pgp), in a model system represented by primary cultures of rat hepatocytes, in which spontaneous overexpression of multidrug resistance (mdr) genes occurs. In both freshly-plated hepatocytes, containing low levels of Pgp, and 72 hour-cultured hepatocytes, containing high levels of Pgp, the Rhodamine-123 (R-123) efflux, which represents a specific functional test for Pgp-mediated transport, was inhibited by curcumin in a dose-dependent manner. Western blot analysis showed that 25microM curcumin, when included in the culture medium throughout the experimental observation (72 hours), was able to significantly lower the increase of mAb C219-immunoreactive protein spontaneously occurring in the cells during culture. Curcumin, at doses ranging from 50 to 150microM was cytotoxic for freshly-plated hepatocytes, as shown by the strong decrease in the cell ability to exclude trypan blue 24 hours later, but it was significantly less cytotoxic when added to 24 or 48 hour-cultured cells. The resistance to curcumin, progressively acquired by cells during culture, was significantly reduced by high concentrations of dexamethasone (DEX) or dimethyl-sulfoxide (DMSO), culture conditions known to inhibit the spontaneous overexpression of Pgp. In addition, in a concentration-dependent manner, verapamil reverted curcumin resistance in Pgp overexpressing hepatocytes. In photoaffinity labeling studies, curcumin competed with azidopine for binding to Pgp, suggesting a direct interaction with glycoprotein. These results suggest that curcumin is able to modulate in vitro both expression and function of hepatic Pgp and support the hypothesis that curcumin, a chemopreventive phytochemical, could reveal itself also as a compound endowed with chemosensitizing properties on mdr phenotype.

Modulation of anthracycline accumulation and metabolism in rat hepatocytes in culture by three revertants of multidrug resistance

The aim of this study was to compare the action of three multidrug resistance (MDR) modulators, cyclosporine A, S 9788, and verapamil, on the efflux of two anthracyclines, doxorubicin and daunorubicin, and of daunorubicinol, the C-13 alcohol metabolite of daunorubicin. Rat-hepatocyte primary cultures have been used as a model of P-glycoprotein (Pgp) expression. This model allows the study of MDR at different levels of Pgp expression, which increases in parallel with the time in culture; furthermore, the hepatocytes are capable of metabolizing drugs, which enables the determination of the role of Pgp on metabolite efflux. All modulators tested were incubated for 6 h at concentrations of 1, 5, and 15 microM with doxorubicin (0.5 microM) and at 1 and 15 microM with daunorubicin (0.5 microM) on hepatocytes grown for 4 and 48 h in culture. Daunorubicinol (0.5 microM) was tested with modulators at 48 h of culture. In fresh hepatocytes, the three MDR modulators did not induce an increase in the intracellular retention of anthracycline as compared with controls (no MDR modulator). At 48 h of culture, the three test drugs increased doxorubicin intracellular accumulation. In contrast, daunorubicin retention was not modified, but that of its metabolites was increased. Within the concentration range tested, cyclosporine was the most potent modulator without dose-dependent activity. The activity rank order was cyclosporine > S 9788 > verapamil. Cyclosporine and S 9788 were as active in coincubation as in preincubation with anthracyclines. Verapamil had no action when incubated before the addition of anthracyclines. Cyclosporine and S 9788 had an effect on the intracellular retention of daunorubicinol used alone whereas verapamil did not. The action of cyclosporine and S 9788 on the retention of daunorubicinol proves that at least a part of the efflux of C-13 alcohol metabolites of anthracyclines is mediated by Pgp. This study shows that S 9788, cyclosporine, and verapamil are MDR modulators in hepatocytes with high-level Pgp expression. This study also demonstrates that hepatocytes are a potent tool for the study of the action of new MDR modulators on cytostatic drugs as well as on their metabolites.