Inhibition by cyclosporin A of adenosine triphosphate-dependent transport from the hepatocyte into bile

Hepatotoxicity of intravenous cyclosporin A in patients with acute ulcerative colitis on total parenteral nutrition

A group of 24 patients underwent a 7-14-day course of continuously infused Cyclosporin A (2 mg.kg-1.day-1) to treat a severe attack of ulcerative colitis. In 19 of them, including eight treated with total parenteral nutrition, we retrospectively analyzed the serum aminotransferase (AST/ALT) levels at the beginning and at the end of Cyclosporin infusion. The baseline levels of AST/ALT in the group were 19.9 +/- 3.2 and 31.4 +/- 6.4; on stopping Cyclosporin infusion, they were 43 +/- 15.8 and 119 +/- 56, respectively. Six patients showed an ALT change above 1.5 times the upper limit of reference. They included five of the eight patients treated with total parenteral nutrition (62.5%). In one of six, ALT rose to 1000 U/l and was accompanied by full-blown febrile cholangitis (proven by liver biopsy). This episode was preceded by excessive accumulation of Cyclosporin in blood. The development of liver toxicity was independent of the length of Cyclosporin treatment, nor did it impair drug efficacy. Thus, in these patients total parenteral nutrition and Cyclosporin were synergistic, causing twice the frequency of liver damage (62.5%) reported for ulcerative colitis patients on total parenteral nutrition alone (37%). Total parenteral nutrition should not be used to support patients needing Cyclosporin for autoimmune disease. However, too high a dose of Cyclosporin may cause liver disease per se.

Expression of the MRP gene-encoded conjugate export pump in liver and its selective absence from the canalicular membrane in transport-deficient mutant hepatocytes

We have previously shown that the multi-drug resistance protein (MRP) mediates the ATP-dependent membrane transport of glutathione S-conjugates and additional amphiphilic organic anions. In the present study we demonstrate the expression of MRP in hepatocytes where it functions in hepatobiliary excretion. Analysis by reverse transcription-PCR of human and normal rat liver mRNA resulted in two expected cDNA fragments of MRP. Four different antibodies against MRP reacted on immunoblots with the glycoprotein of about 190 kD from human canalicular as well as basolateral hepatocyte membrane preparations. A polyclonal antibody directed against the carboxy-terminal sequence of MRP detected the rat homolog of MRP in liver. Double immunofluorescence microscopy and confocal laser scanning microscopy showed the presence of human MRP and rat Mrp in the canalicular as well as in the lateral membrane domains of hepatocytes. The transport function of the mrp gene-encoded conjugate export pump was assayed in plasma membrane vesicles with leukotriene C4 as a high-affinity glutathione S-conjugate substrate. The deficient ATP-dependent conjugate transport in canalicular membranes from TR- mutant rat hepatocytes was associated with a lack of amplification of one of the mrp cDNA fragments and with a selective loss of Mrp on immunoblots of canalicular membranes. Double immunofluorescence microscopy of livers from transport-deficient TR- mutant rats localized Mrp only to the lateral but not to the canalicular membrane. Our results indicate that the absence of Mrp or an isoform of Mrp from the canalicular membrane is the basis for the hereditary defect of the hepatobiliary excretion of anionic conjugates by the transport-deficient hepatocyte.

Inhibition of ileal bile acid transport by cyclosporin A in rat

Chronic administration of cyclosporin A may induce cholestasis and this effect has been attributed to impaired hepatic bile salt synthesis, metabolism and transport. We investigated the effect of cyclosporin A on intestinal absorption of bile acids in the ileum of rat. Ileal bile acid absorption was measured by in vivo intestinal perfusion with cyclosporin A and the solvent Cremophor EL. During ileal perfusion with 25 microM glycocholic acid, the concentration of 2.8 mM cyclosporin A inhibited intestinal bile acid absorption on average by 34%. Additional experiments were performed with everted gut sacs of the distal ileum to evaluate active absorption. A dose and time dependent inhibition of the active intestinal absorption of bile acids was found, with a 50% transport inhibition at an average cyclosporin A concentration of 2.69 mmol L-1. Thus, cyclosporin A inhibits the active intestinal absorption of bile acids which may influence bile acid synthesis, turnover and secretion and may contribute to cyclosporin A induced cholestasis.

Evaluation of tumour metabolism and multidrug resistance in patients with treated malignant lymphomas

The management of patients with treated malignant lymphomas requires functional methods to differentiate a residual soft tissue mass. Patients with treated Hodgkin's lymphoma (HL, n = 20, 68 malignant lesions, three benign lesions) or non-Hodgkin's lymphoma (NHL, n = 26, 46 malignant lesions, one benign lesion) were studied with positron emission tomography (PET) and fluorine-18 deoxyglucose (FDG). Oxygen-15 labelled water was used (n = 14, 25 lesions) in addition to FDG in order to obtain information on the tissue perfusion. Long-term follow-up studies with PET and FDG were performed in nine patients up to 511 days after the initiation of second-line therapy. Fourteen patients underwent single-photon emission tomography (SPET) with technetium-99m sestamibi immediately prior to the first PET examination. PET with FDG displays a high sensitivity for the detection of viable tumour tissue, all the malignant lesions being correctly classified in this study. The possible limitations are inflammatory processes, which may obscure tumour detection due to increased FDG uptake, and malignant lesions with low FDG uptake due to reduced perfusion. Difficulties exist in the prognosis of long-term response, since the change in FDG uptake may be variable. Long-term therapy outcome was correlated with the slope values obtained from the standardized integral uptake (SIU) data, which provides a new approach for the evaluation of PET follow-up studies. 99mTc-sestamibi, which should reflect the multidrug resistance, was evaluated with respect to therapy outcome. A high uptake of 99mTc-sestamibi was observed in patients with stable disease or better. The data support the hypothesis that sestamibi may reflect multidrug resistance.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclosporine A hepatotoxicity: effect of prolonged treatment with cyclosporine on biliary lipid secretion in the rat

1. The effects of cyclosporine A (CyA) treatment on liver morphology, bile flow and biliary secretion of bile acid, cholesterol and phospholipid and some plasma biochemical indicators of liver function were examined. 2. Wistar rats were treated i.p. with 10 or 20 mg of CyA/kg per day for 1, 2, 3 or 4 weeks. 3. Treatment increased bile acid and bilirubin plasma concentration. Bile flow and biliary secretion of bile acid, cholesterol and phospholipid were reduced in CyA-treated animals. 4. All these effects of the drug appeared at 1 week after the start of treatment and were enhanced during prolonged treatment. Cyclosporine A-induced cholestasis was due to a decrease in both the bile acid-dependent and -independent fractions of bile flow. 5. The reduction in cholesterol and phospholipid biliary output may be secondary to the inhibition of the hepatobiliary flux of bile acid; however, perturbations in the removal of lipids from the canalicular membrane as well as intracanalicular interaction between CyA and lipid vesicles/micelles could also be involved.

Disruption of canalicular function in isolated rat hepatocyte couplets caused by cyclosporin A

Isolated rat hepatocyte couplets were used to study the effects of different concentrations of cyclosporin A in relation to canalicular function. Canalicular function was assessed by counting the percentage of couplets which were able to accumulate the fluorescent cholephile cholyl lysyl fluorescein (CLF) into the canalicular vacuole between the two cells, i.e. canalicular vacuole accumulation (CVA). At lower doses, the immunosuppressor increased the CVA, reaching 121 +/- 3.86% of control at 25 nM cyclosporin A. However, higher doses of cyclosporin A induced a concentration-dependent inhibition of CVA to 64.0 +/- 3.51% of control at 100 nM. Modifications in canalicular area (as % couplet area) were also observed. Image analysis of the fluorescent image showed that cyclosporin A (25 nM) increased canalicular area by 25% (of control); however, this parameter decreased to 36% of control at 100 nM cyclosporin A. In addition, at 100 nM, cyclosporin A reduced the proportion of couplets retaining CLF within the canaliculus to 75.0 +/- 6.59% of control. Treatment of couplets with cyclosporin A (0-2 microM) for 15 min revealed that reduced glutathione (GSH) intracellular content does not change significantly at these doses. However, alteration in pericanalicular F-actin at 100 nM cyclosporin A may be an important factor in the disruption of the canalicular function induced by higher doses of the immunosuppressor.

The role of the canalicular multispecific organic anion transporter in the disposal of endo- and xenobiotics

Bile is an important excretory route for the elimination of amphiphilic organic anions, and hepatocytes are the primary secretory units of bile formation. The hepatocytic basolateral and canalicular membranes are equipped with various carrier proteins. Transport across the canalicular membrane represents a major concentrative step. Various ATP-dependent transporters have been identified, such as a multispecific organic anion transporter (canalicular multispecific organic ion transporter, cMOAT), a bile acid transporter and several P-glycoproteins. TR- rats, which lack cMOAT activity, have been valuable in defining the substrate specificity of cMOAT. A wide range of glucuronide-, glutathione- and sulfate-conjugates are transported by this system.

Hepatobiliary elimination of the peroxisome proliferator nafenopin by conjugation and subsequent ATP-dependent transport across the canalicular membrane

Amphiphilic carboxylates acting as peroxisome proliferators and hypolipidemic drugs induce enzymes of peroxisomal lipid beta-oxidation, certain drug-metabolizing enzymes in the liver, and a number of additional proteins. The peroxisome proliferators represent a well-established class of non-genotoxic hepatocarcinogens. In this study we characterized the hepatic elimination of the peroxisome proliferator nafenopin. In the rat in vivo, 1 hr after intravenous administration of [3H]nafenopin, approx. 40% of injected radioactivity was recovered in bile. HPLC analysis of bile samples revealed that only about 10% of the radioactivity recovered in bile was associated with non-metabolized nafenopin and approx. 90% with more polar metabolites. One of the main metabolites formed in the liver and excreted into bile was identified as nafenopin glucuronide by beta-glucuronidase-catalysed reconversion to nafenopin. In mutant rats deficient in the canalicular transport of leukotriene C4 and related amphiphilic anion conjugates, recovery of [3H]nafenopin-derived radioactivity in bile was reduced to 4% of the injected dose. Although nafenopin glucuronide could not be detected in bile, it was a major metabolite in the liver from these mutant rats. Using membrane vesicles enriched in bile canalicular membranes from normal rats, transport of nafenopin glucuronide was shown to be a primary-active ATP-dependent process which was inhibited by leukotriene C4 and S-dinitrophenyl glutathione with IC50 values of 0.2 and 12 microM, respectively. ATP-dependent transport was not detectable for non-conjugated nafenopin. In canalicular membrane vesicles prepared from the mutant rats, the rate of ATP-dependent transport of nafenopin glucuronide was less than 10% of the transport observed in vesicles from normal rats. These data indicate that conjugation and subsequent transport by the ATP-dependent export carrier for leukotriene C4 and related conjugates is a major pathway for the elimination of nafenopin and structurally-related peroxisome proliferators.

Functional reconstitution of ATP-dependent transporters from the solubilized hepatocyte canalicular membrane

The hepatocyte canalicular membrane contains several primary-active ATP-dependent export carriers including one for bile salts and one for leukotriene C4 and related conjugates. The molecular identity of both transporters has not been fully elucidated. To establish a transport assay that allows the purification and identification of the proteins involved in ATP-dependent bile salt transport and in leukotriene C4 transport, we reconstituted solubilized hepatocyte canalicular membranes into phospholipid bilayers using a rapid dilution method. The proteoliposomes formed exhibited both [3H]taurocholate and [3H]leukotriene C4 uptake, which was much higher in the presence of ATP than in the presence of the non-hydrolyzable ATP-analog AdoPP[CH2]P or in the absence of nucleotides. Nucleotide requirement and osmotic sensitivity of [3H]taurocholate transport indicates true transport into the vesicle lumen. Optimized conditions for reconstitution included the addition of a high concentration of an osmolyte (glycerol) and the presence of exogenous phospholipids (0.3%) during solubilization. Highest transport rates were obtained by reconstitution into acetone/ether-precipitated Escherichia coli phospholipid supplemented with 20% cholesterol and by use of octylglucoside concentrations between 30 mM and 50 mM. Taurocholate transport was non-competitively inhibited by vanadate (Ki = 39 microM). The kinetic parameters of cyclosporin A inhibition (Ki = 2.6 microM for taurocholate and 4.3 microM for leukotriene C4 transport) as well as the affinities of taurocholate (Km = 12 microM) and leukotriene C4 (Km = 0.5 microM) in the proteoliposome system indicate that the reconstitution resulted in functionally active transport systems, which are representative of ATP-dependent transport in the intact plasma membrane.

Cholestasis caused by inhibition of the adenosine triphosphate-dependent bile salt transport in rat liver

BACKGROUND/AIMS

Inhibition of bile salt transport across the hepatocyte during cholestasis induced by cyclosporin A has been shown. However, the contribution of the different bile salt transport systems in liver to cholestasis has remained controversial.

METHODS

The sensitivity of different bile salt transport systems in liver to cyclosporin-induced inhibition was determined by transport assays in plasma membrane vesicles and by in vivo studies in the rat.

RESULTS

Cyclosporin A--induced inhibition of sodium-dependent uptake of bile salts across the sinusoidal membrane, of potential-dependent, and of adenosine triphosphate (ATP)-dependent bile salt transport across the canalicular membrane exhibited inhibition constants (Ki) of 5, 70, and 0.2 mumol/L, respectively. The nonimmunosuppressive cyclosporin analogue PSC 833 also preferentially inhibited the ATP-dependent bile salt transport with an inhibition constant of 0.6 mumol/L. Cyclosporin A and its analogue PSC 833 [(3'-oxo-4-butenyl-4-methyl-Thr1)-(Val2)-cyclosporin] (25 mg/kg each) served as tools to interfere with [14C]taurocholate secretion into bile in vivo, causing an accumulation of [14C]-taurocholate in liver and reducing bile flow to 50%. In mutant rats deficient in the transport of leukotriene C4 and related conjugates across the canalicular membrane, bile flow was reduced to 14%.

CONCLUSIONS

The cyclosporins preferentially inhibit the ATP-dependent bile salt export carrier in the canalicular membrane. This inhibition reduces bile salt-dependent bile flow and causes intrahepatic cholestasis.

ATP-dependent export pumps and their inhibition by cyclosporins

Cyclosporins are potent tools to inhibit several primary-active, ATP-dependent export carriers. This has been demonstrated in membrane vesicle transport assays for CsA and for its non-immunosuppressive analog PSC 833. Inhibition in the low micromolar and in the nanomolar concentration range is shown for the three distinct ATP-dependent export carriers in the liver canalicular membrane mediating the secretion into bile of leukotrienes (LTC4, other cysteinyl leukotrienes, and related conjugates), bile salts (taurocholate), and amphiphilic, mostly cationic substances (daunorubicin and other P-glycoprotein substrates). Competitive inhibition by cyclosporins is most potent for ATP-dependent taurocholate transport with Ki values of 0.2 and 0.6 microM for CsA and PSC 833, respectively. This inhibition is in agreement with in vivo studies in the rat demonstrating a block at the canalicular membrane in the hepatobiliary elimination of labeled taurocholate. The data suggest that cholestasis, as a side effect during CsA therapy, is largely due to inhibition of the ATP-dependent bile salt export carrier in the canalicular membrane. Inhibition by cyclosporins is less effective with respect to ATP-dependent leukotriene transport, both during biosynthetic release from mastocytoma cells and during hepatobiliary excretion. The Ki values for the former were 4.5 and 30 microM, and the Km/Ki ratios only 0.015 and 0.002 for CsA and PSC 833, respectively. Distinct transporters are inhibited by the cyclosporins with different potency and structurally modified cyclosporins may serve to induce preferential inhibition of a selected transporter. This is illustrated by the inhibition of the multidrug export carrier with daunorubicin as substrate using PSC 833 as inhibitor with a Ki value of 0.3 microM in an in vitro membrane transport system.

Differential inhibition by cyclosporins of primary-active ATP-dependent transporters in the hepatocyte canalicular membrane

The distinct ATP-dependent transporters for taurocholate, leukotriene C4, and daunorubicin, studied in rat liver canalicular membrane vesicles, are sensitive to inhibition by cyclosporin A and its non-immunosuppressive analog PSC 833. Ki values for cyclosporin A were 0.2, 3.4 and 1.5 microM for the transport of taurocholate, leukotriene C4, and daunorubicin, respectively. The corresponding Ki values for PSC 833 were 0.6, 29, and 0.3 microM. Both inhibitors were competitive with respect to the three substrates. The cyclosporins serve as new and potent tools to interfere with different potency with the distinct ATP-dependent export carriers in the hepatocyte canalicular membrane.