Effect of tauroursodeoxycholate on actin filament alteration induced by cholestatic agents. A study in isolated rat hepatocyte couplets

Taurolithocholate-induced MRP2 retrieval involves MARCKS phosphorylation by protein kinase Cϵ in HUH-NTCP Cells

Taurolithocholate (TLC) acutely inhibits the biliary excretion of multidrug-resistant associated protein 2 (Mrp2) substrates by inducing Mrp2 retrieval from the canalicular membrane, whereas cyclic adenosine monophosphate (cAMP) increases plasma membrane (PM)-MRP2. The effect of TLC may be mediated via protein kinase Cϵ (PKCϵ). Myristoylated alanine-rich C kinase substrate (MARCKS) is a membrane-bound F-actin crosslinking protein and is phosphorylated by PKCs. MARCKS phosphorylation has been implicated in endocytosis, and the underlying mechanism appears to be the detachment of phosphorylated myristoylated alanine-rich C kinase substrate (pMARCKS) from the membrane. The aim of the present study was to test the hypothesis that TLC-induced MRP2 retrieval involves PKCϵ-mediated MARCKS phosphorylation. Studies were conducted in HuH7 cells stably transfected with sodium taurocholate cotransporting polypeptide (HuH-NTCP cells) and in rat hepatocytes. TLC increased PM-PKCϵ and decreased PM-MRP2 in both HuH-NTCP cells and hepatocytes. cAMP did not affect PM-PKCϵ and increased PM-MRP2 in these cells. In HuH-NTCP cells, dominant-negative (DN) PKCϵ reversed TLC-induced decreases in PM-MRP2 without affecting cAMP-induced increases in PM-MRP2. TLC, but not cAMP, increased MARCKS phosphorylation in HuH-NTCP cells and hepatocytes. TLC and phorbol myristate acetate increased cytosolic pMARCKS and decreased PM-MARCKS in HuH-NTCP cells. TLC failed to increase MARCKS phosphorylation in HuH-NTCP cells transfected with DN-PKCϵ, and this suggested PKCϵ-mediated phosphorylation of MARCKS by TLC. In HuH-NTCP cells transfected with phosphorylation-deficient MARCKS, TLC failed to increase MARCKS phosphorylation or decrease PM-MRP2.

CONCLUSION

Taken together, these results support the hypothesis that TLC-induced MRP2 retrieval involves TLC-mediated activation of PKCϵ followed by MARCKS phosphorylation and consequent detachment of MARCKS from the membrane.

Activation of focal adhesion kinase and JNK contributes to the extracellular matrix and cAMP-GEF mediated survival from bile acid induced apoptosis in rat hepatocytes

BACKGROUND/AIMS

Adherence to an extracellular matrix (ECM) rescues hepatocytes from apoptosis, but how hepatocytes adhered to different ECM and respond to apoptotic and cytoprotective stimuli is unknown.

METHODS

Rat hepatocytes were plated on type 1 collagen (CI), laminin (LM) or polylysine (PL) and the amount of apoptosis induced by glycochenodeoxycholate (GCDC), deoxycholate (DCA), Fas ligand or serum withdrawal was determined by Hoechst staining. The response to cytoprotection by cAMP-guanine exchange factor (cAMP-GEF) activation was determined. Kinase activation was determined by immunoblotting with phosphospecific antibodies.

RESULTS

Hepatocytes on LM and PL had more apoptosis in response to all apoptotic stimuli. GCDC increased c-jun-N-terminal kinase (JNK) phosphorylation 2-fold in hepatocytes on CI, but 15- and 30-fold in hepatocytes on PL or LM. SP-600125, a JNK inhibitor, prevented LM and PL potentiation of bile acid apoptosis. GCDC induced dephosphorylation of focal adhesion kinase (FAK) was prevented by cAMP-GEF activation. Cytochalasin B which decreased FAK phosphorylation prevented cAMP-GEF cytoprotection.

CONCLUSIONS

JNK activation augments apoptosis in hepatocytes plated on PL and LM. Decreased FAK phosphorylation as seen in cells treated with bile acids or attached to PL and LM promotes hepatocyte apoptosis.

Effect of tauroursodeoxycholate and S-adenosyl-L-methionine on 17beta-estradiol glucuronide-induced cholestasis

BACKGROUND/AIMS

S-adenosyl-L-methionine (SAMe) and tauroursodeoxycholate (TUDC) exert an additive ameliorating effect on taurolithocholate (TLC)-induced cholestasis. The aims were to investigate the protective effect of SAMe on 17beta-estradiol-glucuronide (17betaEG) cholestasis and to find out whether SAMe and TUDC may exert an additive, ameliorating effect.

METHODS

Hepatocyte couplet function was assessed by canalicular vacuolar accumulation (cVA) of cholyllysylfluorescein (CLF). Cells were co-treated with 17betaEG and SAMe, TUDC, or both (protection study), or treated with 17betaEG and then with SAMe, TUDC or both (reversion study) before CLF uptake. Couplets were also co-treated with SAMe and dehydroepiandrosterone (DHEA), a competitive substrate for the sulfotransferase involved in 17betaEG detoxification. The effects of 17betaEG, SAMe and TUDC were also examined on intracellular distribution of F-actin.

RESULTS

Both SAMe and TUDC significantly protected against, and reversed, 17betaEG-induced cholestasis, but their effects were not additive. DHEA abolished the protective effect of SAMe. 17BetaEG did not affect the uptake of CLF into hepatocytes at the concentrations used, and also, it did not affect the intracellular distribution of F-actin.

CONCLUSIONS

17BetaEG does not affect the uptake of CLF into hepatocytes. SAMe and TUDC protect and reverse 17betaEG-induced cholestasis, but without an additive effect. Protection by SAMe may involve facilitating the sulfation of 17betaEG.

Role of glutathione and oxidative stress in phalloidin-induced cholestasis

BACKGROUND/AIMS

Biliary glutathione is an important generator of the bile-salt independent flow, and is known to be regulated by the hepatic glutathione availability. We investigated, in an acute model of phalloidin-induced cholestasis, biliary glutathione secretion and the role of hepatic glutathione, oxidative stress, and protein kinase c activation, which have been implicated in many hepatotoxin-induced hepatic dysfunctions.

METHODS

Rats were given a single dose of 80 microg/100 g body weight of phalloidin and the hepatic thiols and glutathione content, redox state and vesicular activity were evaluated during both development of and recovery from cholestasis. The prophylactic effect of N-acetylcysteine (a precursor of glutathione synthesis and an antioxidant) was also examined. In addition, in the isolated perfused rat liver, we studied the prophylactic effect of the PKc inhibitor H7 on phalloidin-induced cholestasis.

RESULTS

In the early stages of cholestasis, phalloidin induced a decline in bile flow, mainly related to a disruption of biliary glutathione secretion. The decline in biliary glutathione content was not associated with increased glutathione degradation, indicated by a parallel decline in biliary non-protein thiols and by the lack of an increase in biliary gamma-glutamyltranspeptidase. There was also no evidence of hepatic depletion of glutathione or of oxidative stress, as measured by the oxidized-to-reduced glutathione ratio. Moreover, phalloidin resulted in inhibition of vascular transcytosis as assessed by horseradish peroxidase labeling. Pre-treatment with N-acetylcysteine did not counteract the decline in biliary glutathione secretion and bile flow produced by phalloidin, supporting the view that the hepatic availability of glutathione and oxidative stress injury are not implicated in the early stages of cholestatic injury. Moreover, treatment with H-7 did not alter the biliary glutathione output, or the decline in bile flow induced by the toxin.

CONCLUSIONS

This study suggests that the phalloidin-induced inhibition of bile formation may be attributed to rapid disruption of the hepatocanalicular transport of glutathione.

Tauroursodeoxycholate and S-adenosyl-L-methionine exert an additive ameliorating effect on taurolithocholate-induced cholestasis: a study in isolated rat hepatocyte couplets

The monohydroxy bile acid, taurolithocholate (TLC), causes cholestasis in vivo and in isolated perfused livers. It is also cholestatic in vitro and, in this study using isolated rat hepatocyte couplets, causes a reduction of the accumulation of (fluorescent) bile acid in the canalicular vacuoles (cVA) of this polarized cell preparation. The hepatoprotective bile acid, tauroursodeoxycholate (TUDCA), partially protects against the action of TLC when added at the same time. It also partially reverses the cholestatic effect if added after the cells have been exposed to TLC. A second hepatoprotective compound, S-adenosyl-L-methionine (SAMe) also not only partially protects against the action of TLC when added at the same time, but it too is able to partially reverse the cholestatic effect. Neither hepatoprotective agent is fully effective alone, but their effects are additive. In combination, a full restoration of cVA is observed in moderate cholestasis, but not in severe cholestasis. We discuss briefly some possible mechanisms involved in the additive mode of action of both hepatoprotective compounds. In summary, we show for the first time that SAMe and TUDCA can exert an additive effect in the amelioration of TLC-induced cholestasis in isolated rat hepatocyte couplets. This finding may be of possible clinical relevance.

Hepatotoxicity in drug development: detection, significance and solutions

Despite considerable progress in the understanding of the mechanism of liver toxicity we are not yet able to design non-hepatotoxic molecules rationally. Also, there is no "universal" in vitro primary screening approach for early identification of hepatotoxic molecules. In most cases hepatotoxicity is detected at later stages of drug development in animal toxicity studies or clinical trials. Although the liver is the most common target organ for drug candidates in animal toxicity studies, hepatotoxicity rarely leads to cessation of drug development during the preclinical phase. Indeed, contrary to other target organs, liver toxicity is usually reversible and can be monitored in man by sensitive serum enzyme tests. Therefore in many cases a compound found hepatotoxic in an animal species will be tested in man for a definitive assessment of its hepatotoxic potential. Liver toxicity in man may be acceptable when a drug has major therapeutic potential. In this situation mechanistic studies are essential to assess the risk in man and in some cases to identify protective agents. When liver toxicity leads to project termination a secondary screening approach may be envisaged if biologically active analogs are available.

Different protective effects of tauroursodeoxycholate, ursodeoxycholate, and 23-methyl-ursodeoxycholate against taurolithocholate-induced cholestasis

The coinfusion of tauroursodeoxycholate (TUDC) prevents taurolithocholate (TLC) -induced cholestasis. 23-Methyl-ursodeoxycholate (MUDC) is a side-chain derivative of ursodeoxycholate (UDC). If conjugation with taurine is important for the protective effect of UDC, the MUDC may not be as able as TUDC to prevent TLC-induced cholestasis since it is poorly amidated by the liver. To answer this question, isolated livers of adult Sprague-Dawley rats were coinfused with MUDC (UDC, TUDC) and TLC. After 15 min, inflow rates of the bile acids were doubled. In further experiments taurine in excess was added to the coinfused bile acids. The uptake of bile acids was >90% in all groups, irrespective of whether they were perfused alone or in combination. Single perfusion of TLC caused a rapid decrease in bile flow. UDC and MUDC were hypercholeretic; TUDC moderately choleretic. During coinfusion experiments, TUDC not only completely abolished cholestasis but in addition increased bile flow and biliary bile acid secretion. UDC did prevent TLC cholestasis at the lower inflow rates. At high inflow rates, bile flow decreased significantly. Addition of taurine to this bile acid combination did not significantly improve the anticholestatic effect of UDC. At low and high infusion rates of MUDC, cholestasis induced by TLC was reduced very little. Cumulative bile flow over 30 min fell by approximately 70% as compared to that of singly perfused MUDC. Addition of taurine to the coinfused MUDC/TLC slightly, but less significantly, improved the anticholestatic effect of MUDC. Since MUDC is by far less protective than UDC (and TUDC) despite similar physiochemical properties, it is concluded that taurine conjugation of UDC seems to be a prerequisite to prevent TLC-induced cholestasis. The results imply that treatment of cholestatic liver diseases with taurine-conjugated UDC might be more appropriate than with unconjugated UDC in cases where taurine conjugation is defective or where taurine depletion has occurred.

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.