Molecular regulation of hepatocellular transport systems in cholestasis

Involvement of Mrp2 in Hepatic and Intestinal Disposition of Dinitrophenyl-S-glutathione in Partially Hepatectomized Rats

The ability of the liver and small intestine for secretion of dinitrophenyl-S-glutathione (DNP-SG), a substrate for multidrug resistance-associated protein 2 (Mrp2), into bile and lumen, respectively, as well as expression of Mrp2 in both tissues, were assessed in 70-75% hepatectomized rats. An in vivo perfused intestinal model was used. A single i.v. dose of 30 micromol/kg b.w. of 1-chloro-2,4-dinitrobenzene (CDNB) was administered and its glutathione conjugate, DNP-SG, was determined by HPLC in bile and intestinal perfusate. One and seven days after hepatectomy, biliary excretion of DNP-SG was decreased by 90 and 50% with respect to shams, respectively, when expressed per mass unit. In contrast, intestinal excretion was increased by 63% or unchanged one and seven days post-hepatectomy, respectively. Tissue content of DNP-SG 5 min after CDNB administration was substantially decreased in liver and significantly increased in intestine, one day post-hepatectomy. Western and immunofluorescence studies revealed preserved levels and localization of Mrp2 in both tissues from hepatectomized animals, irrespective of the time analyzed. In spite of preserved expression of Mrp2, the higher availability of DNP-SG in intestinal cells, likely as a consequence of increased glutathione-S-transferase-mediated conjugation of CDNB, may explain the in vivo findings. Further experiments in isolated hepatocytes suggested that decreased synthesis of DNP-SG rather than altered canalicular transport is responsible for the substantial impairment in excretion of this compound into bile. Taken together, these results indicate that the intestine may partially compensate for liver DNP-SG disposition, particularly shortly after surgery, while liver capability is recovering.

Do recommended textbooks contain adequate information about bile salt transporters for medical students?

Several studies have recently highlighted a number of limitations in medical textbooks. The aims of this study were to 1) to assess whether available medical textbooks provided students with adequate information about bile salt transporters, 2) compare the level of detail and the amount of information provided in current textbooks on hepatic transport mechanisms with those available in the literature, and 3) compare the amount of information provided in medical textbooks on hepatocyte transport mechanisms with those involving other transporters e.g., those found in the nephron. Seventy medical textbooks from disciplines including physiology, pathology, cell biology, medicine, pediatrics, pharmacology, pathophysiology, and histology published during the past six years were examined. The literature on bile salt transport has been searched mainly from the Internet (MEDLINE and PubMed). Most textbooks failed to provide any information on transporters found in the basolateral and canalicular membranes of hepatocytes. There are also deficiencies in information on bile salt transporters in the terminal ileum. However, up to the end of 2002, 3,610 articles and reviews had been published on hepatobiliary and enterocyte transport of bile salts. During the same period (from 1965), 10,757 articles had been published on renal transport. Thus the contents of textbooks may reflect the overall volume of research knowledge on renal transport. However, despite our current understanding of hepatic and intestinal transport of bile salts and extensive research, particularly over the past 12 years, there are major deficiencies in textbooks in this area. These findings indicate that there is an imbalance in the contents of current textbooks and a lack of information about hepatobiliary physiology, bile salt transporters, bile formation, and mechanisms underlying cholestasis and drug-induced injury. Authors, editors, and publishers of medical textbooks should consider the need to update the information provided on bile salt transporters.

Mitochondrially-mediated toxicity of bile acids

In the healthy hepatocyte, uptake of bile acids across the basolateral membrane and export via the canalicular export pump, are tightly coupled. Impairment of bile formation or excretion results in cholestasis, characterized by accumulation of bile acids in systemic blood and within the hepatocyte. When the concentration of bile acids exceeds the binding capacity of the binding protein located in the cytosol of the hepatocyte, bile acids induce apoptosis and necrosis, by damage to mitochondria. Mitochondria play a central role on the toxicity of bile acids. In this article, we review the published literature regarding bile acid effects on cell function, especially at the mitochondrial level. In patients with cholestatic liver disease, the extent of hepatocyte damage caused by intracellular accumulation of bile acids appears to be delayed by ingesting a hydrophilic bile acid. However, its effects on disease progression are not completely clarified. Therefore, identification of the mechanisms of cell injury will be of clinical utility, helping in the development of new therapeutic strategies. The goal of this review is to include a fresh consideration of all possible targets and integrating pathways that are involved in cholestasis, as well as in the benefits of bile acid therapy.

Reversible cholestasis induced by experimental partial obstruction of the bile duct

The aim of this investigation was to reproduce in rats a partial stenosis of the common bile duct to analyze early liver functional and morphometric changes. The hepatic transport kinetics of sulfobromophthalein (organic anion) and rhodamine B (organic cation) was also investigated, and compartmental analysis of both compounds was performed. The humoral parameters of liver function indicated a cholestasis after 2 days of surgery, which reverted to reach normal values on the seventh day. Tumor necrosis factor alpha serum levels showed a tendency to increase on the second day of stenosis (7 out of 14 rats) while white blood cells increased on the second day of stenosis, and turned to normal levels on the seventh day. Histological studies showed increased volume of portal areas and ductular proliferation, which did no revert during the time of the study (up to 7 days post-op). Conversely, a moderate fibrosis and leukocyte infiltrates in portal areas predominated on the second day of stenosis, but normalized on the seventh day. Bile flow was considerably diminished on the second day of partial obstruction as compared to controls. The mean recovery in bile of sulfobromophthalein after 1h of being injected was low on the second day of stenosis, but normalized on the seventh day. Conversely, that of rhodamine B was very low in all animals. Sulfobromophthalein kinetics showed that hepatic uptake and canalicular excretion were impaired during the second but normalized on the seventh day of stenosis. However, rhodamine B kinetics showed that this compound was poorly excreted in all groups although canalicular excretion increased on the second day. The results suggested a model of obstructive cholestasis induced by the experimental stenosis of the bile duct which was not only reversible but also implicates the role of hepatic inflammation.

Defying death: the hepatocyte's survival kit

Acute liver injury can develop as a consequence of viral hepatitis, drug- or toxin-induced toxicity or rejection after liver transplantation, whereas chronic liver injury can be due to long-term exposure to alcohol, chemicals, chronic viral hepatitis, metabolic or cholestatic disorders. During liver injury, liver cells are exposed to increased levels of cytokines, bile acids and oxidative stress. This results in death of hepatocytes. In contrast, stellate cells become active and are resistant against cell death. Eventually, acute and chronic liver injury is followed by loss of liver function for which no effective therapies are available. Hepatocytes are well equipped with protective mechanisms to prevent cell death. As long as these protective mechanisms can be activated, the balance will be in favour of cell survival. However, the balance between cell survival and cell death is delicate and can be easily tipped towards cell death during liver injury. Therefore understanding the cellular mechanisms controlling death of liver cells is of clinical and scientific importance and can lead to the identification of novel intervention targets. This review describes some of the mechanisms that determine the balance between cell death and cell survival during liver diseases. The strict regulation of apoptotic cell death allows therapeutic intervention strategies. In this light, receptor-mediated apoptosis and mitochondria-mediated cell death are discussed and strategies are provided to selectively interfere with these processes.

Cross-talk between protein kinases Czeta and B in cyclic AMP-mediated sodium taurocholate co-transporting polypeptide translocation in hepatocytes

Cyclic AMP stimulates taurocholate (TC) uptake and sodium taurocholate co-transporting polypeptide (Ntcp) translocation in hepatocytes via the phosphoinositide-3 kinase (PI3K) signaling pathway. The aim of the present study was to determine whether protein kinase (PK) Czeta, one of the downstream mediators of the PI3K signaling pathway, is involved in cAMP-mediated stimulation of TC uptake. Studies were conducted in isolated rat hepatocytes and in HuH-7 cells stably transfected with rat liver Ntcp (HuH-Ntcp cells). Studies in hepatocytes showed that cAMP activates PKCzeta in a PI3K-dependent manner without inducing translocation of PKCzeta to the plasma membrane. Inhibition of cAMP-induced PKCzeta activity by myristoylated PKC (zeta/lambda) pseudosubstrate, a specific inhibitor of PKCzeta, and Gö 6850, a PKC inhibitor, resulted in inhibition of cAMP-induced increases in TC uptake and Ntcp translocation. Studies in HuH-Ntcp cells showed that inhibition of cAMP-induced PKCzeta activation by dominant-negative (DN) PKCzeta resulted in inhibition of cAMP-induced increases in TC uptake and Ntcp translocation. DN PKCzeta also inhibited wild-type PKCzeta-induced increases in PKCzeta activity, TC uptake, and Ntcp translocation. Myristoylated PKC (zeta/lambda) pseudosubstrate and DN PKCzeta also inhibited cAMP-induced activation of PKB in hepatocytes and HuH-Ntcp cells, respectively. Neither DN PKB nor constitutively active PKB affected cAMP-induced activation of PKCzeta, and wild-type PKCzeta did not activate PKB. Taken together, these results suggest that cAMP-induced activation of PKB is dependent on cAMP-induced stimulation of PKCzeta. It is proposed that cAMP-induced Ntcp translocation involves the activation of the PI3K/PKCzeta signaling pathway followed by the activation of the PI3K/PKB signaling pathway.

Hepatocellular apoptosis after hepatectomy in obstructive jaundice in rats

AIM: To investigate the hepatocellular apoptosis after hepatectomy in obstructive jaundice and biliary decompression rats.

METHODS: After bile duct ligation for 7 days, rats were randomly divided into OB group in which the rats underwent 70% hepatectomy, OB-CD group in which the rats underwent hepatectomy accompanied by choledochoduodenostomy, CD-Hx group in which the rats underwent choledochoduodenostomy and then received 70% hepatectomy on the fifth day after biliary decompression. The control group (Hx group) only underwent hepatectomy.

RESULTS: The level of total serum bilirubin and serum enzymes was significantly lower in CD-Hx group than in OB-CD and OB groups on day 1, 3 and 5 after hepatectomy. The apoptotic index was significantly lower in CD-Hx group than in OB-CD and OB groups on day 3 and 5. The oligonucleosomal DNA fragments and Caspase-3 activity were also lower in CD-Hx group than in OB-CD and OB groups 3 days after hepatectomy, without differences between CD-Hx and Hx groups.

CONCLUSION: Hepatocellular apoptosis plays vital roles in jaundice rats, and biliary decompression is more effective in treatment of patients with severe jaundice before operation.

Substrate specificities of rat oatp1 and ntcp: implications for hepatic organic anion uptake

Transport of a series of 3H-radiolabeled C23, C24, and C27 bile acid derivatives was compared and contrasted in HeLa cell lines stably transfected with rat Na+/taurocholate cotransporting polypeptide (ntcp) or organic anion transporting polypeptide 1 (oatp1) in which expression was under regulation of a zinc-inducible promoter. Similar uptake patterns were observed for both ntcp and oatp1, except that unconjugated hyodeoxycholate was a substrate of oatp1 but not ntcp. Conjugated bile acids were transported better than nonconjugated bile acids, and the configuration of the hydroxyl groups (alpha or beta) had little influence on uptake. Although cholic and 23 norcholic acids were transported by ntcp and oatp1, other unconjugated bile acids (chenodeoxycholic, ursodeoxycholic) were not. In contrast to ntcp, oatp1-mediated uptake of the trihydroxy bile acids taurocholate and glycocholate was four- to eightfold below that of the corresponding dihydroxy conjugates. Ntcp mediated high affinity, sodium-dependent transport of [35S]sulfobromophthalein with a Km similar to that of oatp1-mediated transport of [35S]sulfobromophthalein (Km = 3.7 vs. 3.3 muM, respectively). In addition, for both transporters, uptake of sulfobromophthalein and taurocholic acid showed mutual competitive inhibition. These results indicate that the substrate specificity of ntcp is considerably broader than previously suspected and caution the extrapolation of transport data obtained in vitro to physiological function in vivo.

Changes in the expression and localization of hepatocellular transporters and radixin in primary biliary cirrhosis

BACKGROUND/AIMS

Expression and localization of human hepatocellular transporters and of radixin, cross-linking actin with some membrane transporters, may change in cholestatic liver diseases.

METHODS

We investigated the uptake transporters OATP2 (SLC21A6), OATP8 (SLC21A8), and NTCP (SLC10A1), the export pumps MRP2 (ABCC2), MRP3 (ABCC3), MRP6 (ABCC6), and P-glycoproteins (ABCB1, ABCB4, ABCB11), and radixin, in non-icteric primary biliary cirrhosis (PBC stages I-III) and control human liver needle-biopsies using immunofluorescence microscopy and semi-quantitative RT-PCR.

RESULTS

Expression and localization of all transporters were unchanged in PBC I-II. Immunostaining intensities of uptake transporters decreased in PBC III with a concomitant decrease in mRNA levels. Immunostaining intensities and mRNA levels of export pumps were similar in controls and PBC I-III, however, irregular MRP2 immunostaining suggested redistribution of MRP2 into intracellular structures in PBC III. Areas of irregular MRP2 immunostaining showed largely reduced radixin immunostaining, whereas normal hepatocytes had MRP2 and radixin confined to the canalicular membrane. Disrupted localization of radixin and MRP2 supports the concept that radixin contributes to the canalicular localization of MRP2.

CONCLUSIONS

Down-regulation of uptake transporters may contribute to the impaired hepatobiliary elimination in advanced PBC, and partially altered localization of MRP2 may reflect the onset of changes leading to icteric PBC.

Role of nuclear bile acid receptor, FXR, in adaptive ABC transporter regulation by cholic and ursodeoxycholic acid in mouse liver, kidney and intestine

BACKGROUND/AIMS

Adaptive changes in transporter expression in liver and kidney provide alternative excretory pathways for biliary constituents during cholestasis and may thus attenuate liver injury. Whether adaptive changes in ATP-binding cassette (ABC) transporter expression are stimulated by bile acids and their nuclear receptor FXR is unknown.

METHODS

Hepatic, renal and intestinal ABC transporter expression was compared in cholic acid (CA)- and ursodeoxycholic acid (UDCA)-fed wild-type (FXR(+/+)) and FXR knock-out mice (FXR(-/-)). Expression was assessed by reverse transcription-polymerase chain reaction, immunoblotting and immunofluorescence microscopy.

RESULTS

CA feeding stimulated hepatic Mrp2, Mrp3, Bsep and renal Mrp2 as well as intestinal Mrp2 and Mrp3 expression. Lack of Bsep induction by CA in FXR(-/-) was associated with disseminated hepatocyte necrosis which was not prevented by compensatory induction of Mrp2 and Mrp3. With the exception of Bsep, UDCA stimulated expression of hepatic, renal and intestinal ABC transporters independent of FXR without inducing liver toxicity.

CONCLUSIONS

Toxic CA and non-toxic UDCA induce adaptive ABC transporter expression, independent of FXR with the exception of Bsep. Stimulation of hepatic Mrp3 as well as intestinal and renal Mrp2 by UDCA may contribute to its therapeutic effects by inducing alternative excretory routes for bile acids and other cholephiles.

Constitutive rat multidrug-resistance protein 2 gene transcription is down-regulated by Y-box protein 1

BACKGROUND/AIMS

Molecular mechanisms underlying transcriptional rat multidrug-resistance protein 2 (Mrp2, Abcc2) gene regulation are mostly unclear. Given the presence of putative binding sites for the Y-box binding protein YB-1 in the regulatory sequence, its trans-regulatory influence was analyzed.

METHODS

Reporter assays in HepG2 cells with various Mrp2 deletion constructs in the absence and presence of co-transfected YB-1 were performed. DNA binding studies with recombinant YB-1 protein and nuclear extracts obtained from HepG2 cells and rat liver tissue were carried out.

RESULTS

The minimal promoter sequence was confined to the proximal 186 bp. A YB-1 responsive element, Mrp2 YRE-1, was mapped at -186/-157, which exhibits specific YB-1 binding. YB-1 acts as a potent repressor of Mrp2 promoter activity in vitro.

CONCLUSIONS

Constitutive Mrp2 gene expression is conferred through the proximal -186 bp. YB-1 acts as a repressor in vitro by specific binding to a defined element in the proximal promoter sequence.

Disorders of bile formation and biliary transport

A wide range of cholestatic liver diseases result from various primary defects in bile formation. Clinical features include jaundice, pruritus, failure to thrive, fat malabsorption, cholelithiasis, and variably progressive cirrhosis. Accurate diagnosis of these disorders is essential for determination of prognosis and selection of the most appropriate therapies. Severe genetic defects in canalicular bile acid and phospholipid excretion lead to progressive liver disease that often requires liver transplantation. Defects in bile acid biosynthesis and aminophospholipid transport may be responsive to medical or non-transplant surgical approaches.

Mechanisms by which cAMP increases bile acid secretion in rat liver and canalicular membrane vesicles

Bile acid secretion induced by cAMP and taurocholate is associated with recruitment of several ATP binding cassette (ABC) transporters to the canalicular membrane. Taurocholate-mediated bile acid secretion and recruitment of ABC transporters are phosphatidylinositol 3-kinase (PI3K) dependent and require an intact microtubular apparatus. We examined mechanisms involved in cAMP-mediated bile acid secretion. Bile acid secretion induced by perfusion of rat liver with dibutyryl cAMP was blocked by colchicine and wortmannin, a PI3K inhibitor. Canalicular membrane vesicles isolated from cAMP-treated rats manifested increased ATP-dependent transport of taurocholate and PI3K activity that were reduced by prior in vivo administration of colchicine or wortmannin. Addition of a PI3K lipid product, phosphoinositide 3,4-bisphosphate, but not its isomer, phosphoinositide 4,5-bisphosphate, restored ATP-dependent taurocholate in these vesicles. Addition of a decapeptide that activates PI3K to canalicular membrane vesicles increased ATP-dependent transport above baseline activity. In contrast to effects induced by taurocholate, cAMP-stimulated intracellular trafficking of the canalicular ABC transporters was unaffected by wortmannin, and recruitment of multidrug resistance protein 2, but not bile salt excretory protein (bsep), was partially decreased by colchicine. These studies indicate that trafficking of bsep and other canalicular ABC transporters to the canalicular membrane in response to cAMP is independent of PI3K activity. In addition, PI3K lipid products are required for activation of bsep in the canalicular membrane. These observations prompt revision of current concepts regarding the role of cAMP and PI3K in intracellular trafficking, regulation of canalicular bsep, and bile acid secretion.

Prevention by dietary (n-6) polyunsaturated phosphatidylcholines of intrahepatic cholestasis induced by cyclosporine A in animals

Previous findings showed that dietary (n-6) polyunsaturated phosphatidylcholines (vegetable lecithin) could efficiently prevent intrahepatic cholestasis induced by cyclosporine A in rats. Mechanistic studies showed that expressions in rat liver of Na(+), K(+)-ATPase, Ca(2+), Mg(2+)-ATPase and F-actin were both decreased by drug administration and both enhanced by (n-6) lecithin enriched diet. There is a possible direct effect of phosphatidylcholines, vectors of polyunsaturated fatty acids provided by the metabolism of the dietary lecithin, on the aforesaid hepatic parameters. Such modulations by drug and diet result in reversed modifications of membrane composition and fluidity. Final outcome is decreased and enhanced bile lipid secretion by cyclosporine and vegetable lecithin enriched diet respectively. Moreover, we advance the hypothesis of a bypass process including a separate and functional actin-independent way for the non micellar and phospholipid-dependent secretion of bile lipids. The relationships between the ATPases, the microfilament components such as F-actin and the different transporters still remain to be clarified. Furthermore, one can speculate on beneficial effects in humans of diets enriched in vegetable lecithins that might prevent cholestasis induced by cyclosporine A.

Gastrointestinal disorders of the critically ill. Cholestasis of sepsis

Cholestasis of sepsis is a form of hepatocellular cholestasis that occurs as a result of sepsis. Usually, prior to the development of cholestasis, the manifestations of sepsis dominate the clinical picture. The occurrence of cholestasis is without direct bacterial involvement of the biliary system and appears to be mediated systemically by pro-inflammatory cytokines. These cytokines are released in response to the vigorous inflammatory reaction mediated by endotoxinaemia and bacterial wall lipopolysaccharides. The principal cytokines involved are the pro-inflammatory tumour necrosis factor-alpha (TNF-alpha), interleukin (IL) 1-beta and IL-6. Interplay between these cytokines and a series of hepatocyte membrane transporters appears to result in the cholestasis. Management principles focus upon the control of sepsis.

Effect of ursodeoxycholic acid on the impairment induced by maternal cholestasis in the rat placenta-maternal liver tandem excretory pathway

We investigated the effects of ursodeoxycholic acid (UDCA; 60 microg/day/100 g b.wt.) on the impairment induced by maternal obstructive cholestasis during pregnancy (OCP) in the rat placenta-maternal liver tandem excretory pathway. A blunted catheter was implanted in the common bile duct on day 14 of pregnancy, and the tip was cut on day 21. [(14)C]Glycocholate (GC) was then administered through the umbilical artery of "in situ" perfused placenta (placental transfer test) or through the maternal jugular vein (biliary secretion test), and GC bile output was measured. OCP impaired both GC placental transfer and maternal biliary secretion. UDCA moderately improved the latter but had a more marked beneficial effect on GC placental transfer. Histological examination revealed trophoblast atrophy and structural alterations, e.g., loss of apical membrane microvilli in OCP placentas. Gene expression level was investigated by real-time quantitative reverse transcription-polymerase chain reaction and Western blot analysis. OCP reduced both placental lactogen II (a trophoblast-specific gene) mRNA and the functional amount of epithelial tissue, determined by transplacental diffusion of antipyrin. Using a rapid filtration technique, impairment in the ATP-dependent GC transport across trophoblast apical plasma membranes obtained from OCP placentas was found. UDCA partially prevented all these changes. The expression level of organic anion transporters Oatp1, Oatp2, and Oatp4, and multidrug resistance-associated proteins Mrp1, Mrp2, and Mrp3 in whole placenta were not affected or were moderately affected by OCP but greatly enhanced by UDCA. In summary, UDCA partially prevents deleterious effects of OCP on the rat placenta-maternal liver tandem excretory pathway, mainly by preserving trophoblast structure and function.

Bile salt transporters: molecular characterization, function, and regulation

Molecular medicine has led to rapid advances in the characterization of hepatobiliary transport systems that determine the uptake and excretion of bile salts and other biliary constituents in the liver and extrahepatic tissues. The bile salt pool undergoes an enterohepatic circulation that is regulated by distinct bile salt transport proteins, including the canalicular bile salt export pump BSEP (ABCB11), the ileal Na(+)-dependent bile salt transporter ISBT (SLC10A2), and the hepatic sinusoidal Na(+)- taurocholate cotransporting polypeptide NTCP (SLC10A1). Other bile salt transporters include the organic anion transporting polypeptides OATPs (SLC21A) and the multidrug resistance-associated proteins 2 and 3 MRP2,3 (ABCC2,3). Bile salt transporters are also present in cholangiocytes, the renal proximal tubule, and the placenta. Expression of these transport proteins is regulated by both transcriptional and posttranscriptional events, with the former involving nuclear hormone receptors where bile salts function as specific ligands. During bile secretory failure (cholestasis), bile salt transport proteins undergo adaptive responses that serve to protect the liver from bile salt retention and which facilitate extrahepatic routes of bile salt excretion. This review is a comprehensive summary of current knowledge of the molecular characterization, function, and regulation of bile salt transporters in normal physiology and in cholestatic liver disease and liver regeneration.

Chronic Staphylococcal enterotoxin B and lipopolysaccharide induce a bimodal pattern of hepatic dysfunction and injury

OBJECTIVE

To determine the effect of chronic exposure to endotoxin (lipopolysaccharide) and Staphylococcal enterotoxin B on hepatic injury and function.

DESIGN

Prospective, controlled trial.

SETTING

Research laboratory in a university hospital.

SUBJECTS

Male Sprague-Dawley rats weighing 325-350 g with chronic vascular and bile catheters.

INTERVENTIONS

Chronically catheterized rats were treated daily with saline, 50 microg/kg Staphylococcal enterotoxin B alone, 1000 microg/kg lipopolysaccharide alone, 1000 microg/kg lipopolysaccharide with 50 microg/kg Staphylococcal enterotoxin B, or 100 microg/kg lipopolysaccharide with 50 microg/kg Staphylococcal enterotoxin B for 10 days. Serum and biliary measures of hepatic injury and dysfunction were measured before and then 6 hrs and 1, 2, 3, 7, and 10 days after the start of treatment. The animals were killed at 10 days and the livers examined histologically.

MEASUREMENTS AND MAIN RESULTS

Mean rates of bile flow, biliary indocyanine green excretion, and bile acid flux were significantly decreased immediately after treatment (6 hr, 1 and 2 days) and then at 10 days. Increases in biliary and serum gamma-glutamyltransferase and serum bile acids also occurred in a similar bimodal pattern. Animals treated with lipopolysaccharide or Staphylococcal enterotoxin B alone became tolerant and did not develop the bimodal pattern of hepatic dysfunction. Histologic examination of the liver at 10 days revealed periportal inflammation and fibrosis.

CONCLUSIONS

The combination of lipopolysaccharide and Staphylococcal enterotoxin B leads to late liver injury, whereas either toxin alone does not. These data may explain the frequent development of liver dysfunction in patients exposed to multiple bacterial toxins such as in sepsis, multiple-system organ failure, and other diseases with altered intestinal permeability.

Sinusoidal efflux of taurocholate correlates with the hepatic expression level of Mrp3

Multidrug resistance-associated protein 3 (Mrp3/ABCC3), which can mediate the cellular extrusion of bile acids, is induced on the hepatic sinusoidal membrane of Mrp2/ABCC2-deficient rats (Eisai hyperbilirubinemic rats; EHBRs) and phenobarbital-treated Sprague-Dawley rats. In the present study, the correlation between the sinusoidal efflux clearance (PS(eff)) of [3H]taurocholate (TC) and the hepatic expression of Mrp3 was investigated using perfused liver from these rats. A significant correlation was observed between the PS(eff) and the hepatic expression level of Mrp3, suggesting a contribution by Mrp3 to the sinusoidal efflux of TC. The results of the kinetic analysis also suggested that other transporter(s) on the sinusoidal plasma membrane may participate in the efflux of TC under physiological conditions. The contribution of Mrp3 to the sinusoidal efflux of TC in EHBRs and phenobarbital (80 and 40 mg/kg)-treated rats was revealed to be 58%, 48%, and 31%, respectively.

Effect of nitric oxide on the sinusoidal uptake of organic cations and anions by isolated hepatocytes

The issue of whether or not the presence NOx (NO and oxidized metabolites) in the hepatocytes at pathological levels affects the functional activity of transport systems within the sinusoidal membrane was investigated. For this purpose, the effect of the pretreatment of isolated hepatocytes with sodium nitroprusside (SNP), a spontaneous NO donor, on the sinusoidal uptake of tributylmethylammonium (TBuMA) and triethylmethyl ammonium (TEMA), representative substrates of the organic cation transporter (OCT), and taurocholate, a representative substrate of the Na+/taurocholate cotransporting polypeptide (NTCP), was measured. The uptake of TBuMA and TEMA was not affected by the pretreatment, as demonstrated by the nearly identical kinetic parameters for the uptake (i.e., Vmax, Km and CL(linear)). The uptake of mannitol into hepatocytes was not affected, demonstrating that the membrane integrity remained constant, irregardless of the SNP pretreatment. On the contrary, the uptake of taurocholate was significantly inhibited by the pretreatment, resulting in a significant decrease in Vmax, thus providing a clear demonstration that NOx preferentially affects the function of NTCP rather than OCT on the sinusoidal membrane. A direct interaction between NOx and NTCP or a decrease in Na+/K+ ATPase activity as the result of SNP pretreatment might be responsible for this selective effect of NOx.

Interleukin-1 beta-mediated suppression of RXR:RAR transactivation of the Ntcp promoter is JNK-dependent

Bile flow is rapidly and markedly reduced in hepatic inflammation, correlating with suppression of critical hepatic bile acid transporter gene expression, including the principal hepatic bile acid importer, the Na(+)/taurocholate co-transporting polypeptide (Ntcp, Slc10a1). Endotoxin treatment of rats and interleukin-1 beta (IL-1 beta) treatment of liver-derived HepG2 cells leads to a marked decline in the nuclear binding activity of a main Ntcp gene regulator, the nuclear receptor heterodimer retinoid X receptor:retinoic acid receptor (RXR:RAR). How IL-1 beta signaling leads to reduced RXR:RAR nuclear binding activity is unknown, and we sought to determine whether mitogen-activated protein kinase (MAPK) pathways were involved. IL-1 beta treatment of cultured primary rat hepatocytes markedly reduced Ntcp RNA levels and Ntcp promoter activity in transiently transfected HepG2 cells. Pretreatment with inhibitors of extracellular signal-regulated kinase (ERK, PD98059) or p38 MAPK (SB203580) did not affect IL-1 beta-mediated suppression of Ntcp gene expression, whereas curcumin, a derivative of the spice turmeric and a recently described inhibitor of c-Jun N-terminal kinase (JNK), completely ameliorated the effects of IL-1 beta. Co-transfection of a JNK expression plasmid inhibited RXR:RAR-mediated activation of the Ntcp promoter, while a dominant negative JNK expression plasmid completely blocked IL-1 beta-mediated suppression. Curcumin, but not PD98059 or SB203580, inhibited IL-1 beta-mediated suppression of nuclear RXR:RAR binding activity, which correlated with inhibition of JNK phosphorylation and phospho-JNK-mediated phosphorylation of RXR. Taken together, these data provide evidence supporting a novel player (JNK), as well as its inhibitor (curcumin), in inflammation-mediated regulation of hepatobiliary transporters and correlate JNK-dependent RXR phosphorylation with reduced RXR-dependent hepatic gene expression.

Protein kinase B/Akt mediates cAMP- and cell swelling-stimulated Na+/taurocholate cotransport and Ntcp translocation

Cyclic AMP and cell swelling stimulate hepatic Na+/TC cotransport and Ntcp translocation via the phosphoinositide 3-kinase signaling pathway. To determine the downstream target of the phosphoinositide 3-kinase action, we examined the role of protein kinase B (PKB)/Akt using SB203580 in hepatocytes as well as by transfection with a dominant negative (DN-PKB) or a constitutively active (CA-PKB) form of PKB in HuH-Ntcp cells. Both cAMP and cell swelling stimulated p38 mitogen-activated protein (MAP) kinase as well as PKB activity. Although 100 microm SB203580 inhibited cell swelling- and 8-chlorophenylthio-cAMP-induced activation of both p38 MAP kinase and PKB, 1 microm SB203580 inhibited activation of p38 MAP kinase, but not of PKB, in hepatocytes. 100 microm, but not 1 microm SB203580, inhibited cell swelling- and cAMP-induced increases in taurocholate (TC) uptake and Ntcp translocation in hepatocytes. TC uptake in HuH-Ntcp cells was more than 90% dependent on extracellular Na+. Cyclic AMP and cell swelling increased TC uptake by 50-100% and PKB activity 2-4-fold in HuH-Ntcp cells transfected with the empty vector and failed to increase PKB activity, TC uptake, and Ntcp translocation in DN-PKB-transfected HuH-Ntcp cells. Transfection with CA-PKB increased PKB activity, TC uptake, and Ntcp translocation in HuH-Ntcp cells compared with cells transfected with the empty vector. In contrast, transfection with DN-PKB did not affect basal PKB activity, TC uptake, or Ntcp translocation. Taken together, these results strongly suggest that cell swelling and cAMP-mediated stimulation of hepatic Na+/TC cotransport and Ntcp translocation requires activation of PKB and is mediated at least in part via a phosphoinositide 3-kinase/PKB-signaling pathway.

Hepatobiliary organic anion transporters are differentially regulated in acute toxic liver injury induced by carbon tetrachloride

BACKGROUND/AIMS

Hepatobiliary transporters are down-regulated in cholestasis, but their expression in acute, non-cholestatic, cytokine-mediated liver injury is unknown. Thus we studied the molecular mechanisms, by which sodium taurocholate cotransporting polypeptide (Ntcp), organic anion transporting polypeptide 1 (Oatp1), Oatp2, Oatp4, multidrug-resistance protein 2 (Mrp2) and bile salt export pump (Bsep) are regulated in liver injury induced by carbon tetrachloride (CCl(4)).

METHODS

mRNA and protein levels were determined in rats 24 and 72h after CCl(4) injection. Transporter gene transcription and binding activities of Ntcp and Mrp2 transactivators were assessed by nuclear runoff and electrophoretic mobility shift assays.

RESULTS

mRNA levels significantly declined to 41+/-44% for Ntcp, 65+/-41% for Oatp1 and 64+/-28% for Oatp2, but remained unchanged for Oatp4, canalicular Mrp2 and Bsep. Protein levels declined only for Oatp4 (-50+/-17%) and Ntcp (-23+/-13%) at 24h. Reduced mRNA levels (Ntcp, Oatp1, Oatp2) were associated with decreased transcriptional activities. Binding activity of Ntcp transactivators (hepatocyte nuclear factor 1 alpha (HNF1alpha) and CAAT enhancer binding protein alpha (C/EBPalpha) were reduced by 24h, whereas retinoid X receptor alpha (RXRalpha):retinoid acid receptor alpha (RARalpha) as transactivator of both Ntcp and Mrp2 remained unaltered. Recovery of acute hepatitis and changes in gene expression occurred after 72h.

CONCLUSIONS

Acute liver injury results in down-regulation of basolateral organic anion transporters similar to liver regeneration after partial hepatectomy, but in contrast to endotoxin-induced cholestasis. Maintained binding activity of RXRalpha:RARalpha may explain differences in Mrp2 expression.

Role of PP2B in cAMP-induced dephosphorylation and translocation of NTCP

cAMP-mediated stimulation of hepatic bile acid uptake is associated with dephosphorylation and translocation of Na+-taurocholate (TC) cotransporting peptide (NTCP) to the plasma membrane. Although translocation of NTCP may be facilitated by dephosphorylation, the mechanism of dephosphorylation is unknown. The ability of cAMP to translocate and dephosphorylate NTCP is, in part, dependent on cAMP-mediated increases in cytosolic Ca2+ concentration ([Ca2+]), indicating that a Ca2+/calmodulin-dependent protein phosphatase (PP2B) may be involved. Thus we studied the role of PP2B using the inhibitor cypermethrin (CM). Freshly isolated hepatocytes were pretreated with 1-5 nM CM for 30 min followed by 15 min incubation with 10 microM 8-(4-chlorophenylthio)cAMP. CM (5 nM) and FK-506 (5 microM) inhibited cAMP-stimulated TC uptake by 80 and 75%, respectively, without affecting basal TC uptake. CM also reversed cAMP-mediated NTCP dephosphorylation and translocation to 80 and 15% of the basal level, respectively. cAMP stimulated PP2B activity by 60%, and this effect was completely inhibited by 5 nM CM. PP2B dephosphorylated NTCP immunoprecipitated from control but not from cAMP-treated hepatocytes. The effect of CM was not due to any changes in cAMP-mediated increases in cytosolic [Ca2+] or decreases in mitogen-activated protein kinase (extracellular regulated kinases 1 and 2) activity. Taken together, these results suggest that cAMP dephosphorylates NTCP by activating PP2B in hepatocytes, and PP2B-mediated dephosphorylation of NTCP may be involved in cAMP-mediated NTCP translocation to the plasma membrane.

Cyclin kinase inhibitor p21 potentiates bile acid-induced apoptosis in hepatocytes that is dependent on p53

Prolonged activation of the mitogen-activated protein kinase (MAPK) pathway enhances expression of the cyclin kinase inhibitor p21 that can promote growth arrest and cell survival in response to cytotoxic insults. Bile acids can also cause prolonged MAPK activation that is cytoprotective against bile acid-induced cell death. Here, we examined the impact of bile acid-induced MAPK signaling and p21 expression on the survival of primary mouse hepatocytes. Deoxycholic acid (DCA) caused prolonged activation of the MAPK pathway that weakly enhanced p21 protein expression. When DCA-induced MAPK activation was blocked using MEK1/2 inhibitors, both hepatocyte viability and expression of p21 were reduced. Surprisingly, constitutive overexpression of p21 in p21+/+ hepatocytes enhanced DCA-induced cell killing. In agreement with these findings, treatment of p21-/- hepatocytes with DCA and MEK1/2 inhibitors also caused less apoptosis than observed in wild-type p21+/+ cells. Expression of p21 in p21-/- hepatocytes did not modify basal levels of apoptosis but restored the apoptotic response of p21-/- cells to those of p21+/+ cells overexpressing p21. These findings suggest that basal expression of p21 plays a facilitating, proapoptotic role in DCA-induced apoptosis. Overexpression of p21 enhanced p53 protein levels. In agreement with a role for p53 in the enhanced apoptotic response, overexpression of p21 did not potentiate apoptosis in p53-/- hepatocytes but, instead, attenuated the death response in these cells. In conclusion, our data suggest that overexpression of p21 can promote apoptosis, leading to elevated sensitivity to proapoptotic stimuli.

Inhibition of the MAPK and PI3K pathways enhances UDCA-induced apoptosis in primary rodent hepatocytes

The mechanisms by which bile acids induce apoptosis in hepatocytes and the signaling pathways involved in the control of cell death are not understood fully. Here, we examined the impact of mitogen-activated protein kinase (MAPK) and phosphatidyl inositol 3-kinase (PI3K) signaling on the survival of primary hepatocytes exposed to bile acids. Treatment of hepatocytes with deoxycholic acid (DCA), chenodeoxycholic acid (CDCA) or ursodeoxycholic acid (UDCA) caused sustained MAPK activation that was dependent on activation of the epidermal growth factor receptor (EGFR). Activation of MAPK was partially blocked by inhibitors of PI3K. Inhibition of DCA-, CDCA-, and UDCA-stimulated MAPK activation resulted in approximately 20%, approximately 35%, and approximately 55% apoptosis, respectively. The potentiation of DCA- and CDCA-induced apoptosis by MEK1/2 inhibitors correlated with cleavage of procaspase 3, which was blocked by inhibitors of caspase 8 (ile-Glu-Thr-Asp-p-nitroanilide [IETD]) and caspase 3 (DEVD). In contrast, the potentiation of UDCA-induced apoptosis weakly correlated with procaspase 3 cleavage, yet this effect was also blocked by IETD and DEVD. Incubation of hepatocytes with the serine protease inhibitor AEBSF reduced the death response of cells treated with UDCA and MEK1/2 inhibitor to that observed for DCA and MEK1/2 inhibitor. The apoptotic response was FAS receptor- and neutral sphingomyelinase-dependent and independent of FAS ligand expression, and neither chelation of intracellular and extracellular Ca(2+) nor down-regulation of PKC expression altered the apoptotic effects of bile acids. In conclusion, bile acid apoptosis is dependent on the production of ceramide and is counteracted by activation of the MAPK and PI3K pathways.

Effects of epomediol on ethinyloestradiol-induced changes in glutathione homeostasis in the rat

Epomediol is a synthetic terpenoid compound that has been reported to reduce ethinyloestradiol-induced cholestasis. The choleretic action of epomediol is related to an increase in both the bile acid-dependent and independent fractions of bile flow, but the role of glutathione metabolism and transport is still unknown. This study was aimed to evaluate if changes in glutathione homeostasis could contribute to the beneficial effects of epomediol in rats with ethinyloestradiol-induced cholestasis. When compared to control animals, ethinyloestradiol treatment resulted in a significant decrease in the liver concentration of reduced (GSH) and oxidized glutathione. Both GSH and oxidized glutathione concentrations returned to normal in animals receiving ethinyloestradiol plus epomediol. Ethinyloestradiol administration induced a significant decrease in plasma and renal GSH and the tripeptide was almost absent from bile. Combined treatment with epomediol plus ethinyloestradiol normalised renal GSH and both biliary and liver cysteine were significantly increased. Liver and kidney gamma-glutamyltranspeptidase activities were higher in rats receiving ethinyloestradiol and still remained elevated in animals with the combined treatment. Liver gamma-glutamylcysteine synthetase activity rose significantly by administration of ethinyloestradiol plus epomediol but the corresponding mRNA levels were not modified. Changes in glutathione homeostasis and higher biliary levels of GSH amino acid constituents could contribute to the beneficial effects of epomediol in rats with ethinyloestradiol-induced cholestasis.

Update on the etiologies and management of neonatal cholestasis

The early detection of cholestatic liver disease is one of the major challenges facing pediatricians when evaluating the jaundiced infant. Early recognition of liver disease greatly facilitates the care and outcome of infants, because several serious life-threatening disorders may have cholestasis as a major presenting sign of underlying neonatal liver disease. A key component of the work-up is measurement of serum conjugated bilirubin levels, which if elevated should prompt the clinician to initiate a work-up to determine the cause of neonatal cholestasis. In general, if a patient is developing progressive jaundice soon after birth, is still jaundiced at 2 weeks of life, or develops jaundice within the first month of life, a work-up for neonatal cholestasis should begin. A number of previously undiagnosed causes of neonatal cholestasis are beginning to be assigned genetic and infectious etiologies, with significant implications for the work-up and management of cholestatic infants.

Molecular basis of neonatal cholestasis

At present, specific evidence regarding the molecular mechanisms of neonatal cholestasis is limited. The recent explosion in the understanding of the molecular physiology of bile formation has been fueled by the discovery of several genes that are involved in familial cholestasis. The ever-growing understanding of the functional immaturity of the neonatal liver is sure to be enhanced by the study of the ontogeny of important hepatobiliary transporters as they are discovered. The understanding of the functional differences between the immature and mature liver is key to the understanding of neonatal cholestasis.

Expression of liver plasma membrane transporters in gallstone-susceptible and gallstone-resistant mice

We tested the hypothesis that differential expression of liver plasma membrane transporters might account for variations in biliary lipid secretion rates between gallstone-susceptible C57L/J and gallstone-resistant AKR/J mice. Plasma membrane fractions and total RNA isolated from livers of mice fed with a control or lithogenic (15% fat/1.25% cholesterol/0.5% cholic acid) diet were used for measurements of steady-state gene expression of hepatobiliary transport systems for bile salts (Ntcp1/Slc10a1, Oatp1/Slc21a1 and Bsep/Abcb11), phospholipids (Mdr2/Abcb4), organic anions (Mrp2/Abcc2) and organic cations (Oct1/Slc22a1). Irrespective of the diet, the steady-state gene expression of hepatobiliary transporters did not differ significantly between the two strains. Despite a higher basal bile flow and bile-salt secretion in C57L mice, Mrp2 (Abcc2) and Bsep (Abcb11) expression did not differ between the two strains. Elevated biliary phospholipid secretion in response to the lithogenic diet was linked to increased Mdr2 (Abcb4) protein expression, whereas the induction of Oct1 (Slc22a1) might reflect an enhanced uptake of choline for augmented phospholipid synthesis. In response to the lithogenic diet, Bsep (Abcb11) protein expression was up-regulated only marginally and bile salt secretion did not increase. The down-regulation of Ntcp1 (Slc10a1) protein expression might protect hepatocytes from high intracellular bile-salt loads. We conclude that variations in protein function rather than in the gene expression of liver plasma membrane transporters might account for variations in biliary lipid secretion rates. Our findings support the concept that the formation of lithogenic bile is caused by the hypersecretion of bile salts as a result of augmented availability of canalicular membrane cholesterol, possibly amplified by bile-salt-phospholipid uncoupling due to the increased bile flow.

Hepatic transport systems

The identification of the genes responsible for various genetic liver disorders lead to a better understanding of basic physiology of hepatic transport systems. In this review we focus on transport systems involved in the generation of bile and in the maintenance of copper homeostasis. Abnormal function of these transporters results in diseases like Wilson's disease, progressive familial cholestasis syndromes, Dubin-Johnson syndrome and cystic fibrosis. Beyond these well defined diseases, functional impairments of transport proteins may predispose to non-genetic diseases ranging from intrahepatic cholestasis of pregnancy to neurodegenerative disorders including Alzheimer's disease.

Induction of short heterodimer partner 1 precedes downregulation of Ntcp in bile duct-ligated mice

Cholestasis is associated with retention of bile acids and reduced expression of the Na(+)/taurocholate cotransporter (Ntcp), the major hepatocellular bile acid uptake system. This study aimed to determine whether downregulation of Ntcp in obstructive cholestasis 1) is a consequence of bile acid retention and 2) is mediated by induction of the transcriptional repressor short heterodimer partner 1 (SHP-1). To study the time course for the changes in serum bile acid levels as well as SHP-1 and Ntcp steady-state mRNA levels, mice were subjected to common bile duct ligation (CBDL) for 3, 6, 12, 24, 72, and 168 h and compared with sham-operated controls. Serum bile acid levels were determined by radioimmunoassay. SHP-1 and Ntcp steady-state mRNA expression were assessed by Northern blotting. In addition, Ntcp protein expression was studied by Western blotting and immunofluorescence microscopy. Increased SHP-1 mRNA expression paralleled elevations of serum bile acid levels and was followed by downregulation of Ntcp mRNA and protein expression in CBDL mice. Maximal SHP-1 mRNA expression reached a plateau phase after 6-h CBDL (12-fold; P < 0.001) and preceded the nadir of Ntcp mRNA levels (12%, P < 0.001) by 6 h. In conclusion, bile acid-induced expression of SHP-1 may, at least in part, mediate downregulation of Ntcp in CBDL mice. These findings support the concept that downregulation of Ntcp in cholestasis limits intracytoplasmatic accumulation of potentially toxic bile acids.

Adaptive regulation of bile salt transporters in kidney and liver in obstructive cholestasis in the rat

BACKGROUND & AIMS

Cholestasis results in adaptive regulation of bile salt transport proteins in hepatocytes that may limit liver injury. However, it is not known if changes also occur in the expression of bile salt transporters that reside in extrahepatic tissues, particularly the kidney, which might facilitate bile salt excretion during obstructive cholestasis.

METHODS

RNA and protein were isolated from liver and kidney 14 days after common bile duct ligation in rats and assessed by RNA protection assays, Western analysis, and tissue immunofluorescence. Sodium-dependent bile salt transport was also measured in brush border membrane vesicles from the kidney.

RESULTS

After common bile duct ligation, serum bile salts initially rose and then declined to lower levels after 3 days. In contrast, urinary bile salt excretion rose progressively over the 2-week period. By that time, the ileal sodium-dependent bile salt transporter messenger RNA and protein expression in total liver had increased to 300% and 200% of controls, respectively, while falling to 46% and 37% of controls, respectively, in the kidney. Sodium-dependent uptake of (3)H-taurocholate in renal brush border membrane vesicles was decreased. In contrast, the multidrug resistance-associated protein 2 expression in the kidney was increased 2-fold, even 1 day after ligation. Immunofluorescent studies confirmed the changes in the expression of these transporters in liver and kidney.

CONCLUSIONS

These studies show that the molecular expression of bile salt transporters in the kidney and cholangiocytes undergo adaptive regulation after common bile duct obstruction in the rat. These responses may facilitate extrahepatic pathways for bile salt excretion during cholestasis.

Deoxycholic acid (DCA) causes ligand-independent activation of epidermal growth factor receptor (EGFR) and FAS receptor in primary hepatocytes: inhibition of EGFR/mitogen-activated protein kinase-signaling module enhances DCA-induced apoptosis

Previous studies have argued that enhanced activity of the epidermal growth factor receptor (EGFR) and the mitogen-activated protein kinase (MAPK) pathway can promote tumor cell survival in response to cytotoxic insults. In this study, we examined the impact of MAPK signaling on the survival of primary hepatocytes exposed to low concentrations of deoxycholic acid (DCA, 50 microM). Treatment of hepatocytes with DCA caused MAPK activation, which was dependent upon ligand independent activation of EGFR, and downstream signaling through Ras and PI(3) kinase. Neither inhibition of MAPK signaling alone by MEK1/2 inhibitors, nor exposure to DCA alone, enhanced basal hepatocyte apoptosis, whereas inhibition of DCA-induced MAPK activation caused approximately 25% apoptosis within 6 h. Similar data were also obtained when either dominant negative EGFR-CD533 or dominant negative Ras N17 were used to block MAPK activation. DCA-induced apoptosis correlated with sequential cleavage of procaspase 8, BID, procaspase 9, and procaspase 3. Inhibition of MAPK potentiated bile acid-induced apoptosis in hepatocytes with mutant FAS-ligand, but did not enhance in hepatocytes that were null for FAS receptor expression. These data argues that DCA is causing ligand independent activation of the FAS receptor to stimulate an apoptotic response, which is counteracted by enhanced ligand-independent EGFR/MAPK signaling. In agreement with FAS-mediated cell killing, inhibition of caspase function with the use of dominant negative Fas-associated protein with death domain, a caspase 8 inhibitor (Ile-Glu-Thr-Asp-p-nitroanilide [IETD]) or dominant negative procaspase 8 blocked the potentiation of bile acid-induced apoptosis. Inhibition of bile acid-induced MAPK signaling enhanced the cleavage of BID and release of cytochrome c from mitochondria, which were all blocked by IETD. Despite activation of caspase 8, expression of dominant negative procaspase 9 blocked procaspase 3 cleavage and the potentiation of DCA-induced apoptosis. Treatment of hepatocytes with DCA transiently increased expression of the caspase 8 inhibitor proteins c-FLIP-(S) and c-FLIP-(L) that were reduced by inhibition of MAPK or PI(3) kinase. Constitutive overexpression of c-FLIP-(s) abolished the potentiation of bile acid-induced apoptosis. Collectively, our data argue that loss of DCA-induced EGFR/Ras/MAPK pathway function potentiates DCA-stimulated FAS-induced hepatocyte cell death via a reduction in the expression of c-FLIP isoforms.

Endotoxin-induced reduction in biliary indocyanine green excretion rate in a chronically catheterized rat model

Using a nonstressed chronically catheterized rat model in which the common bile duct was cannulated, we studied endotoxin-induced alterations in hepatic function by measuring changes in the maximal steady-state biliary excretion rate of the anionic dye indocyanine green (ICG). Biliary excretion of ICG was calculated from direct measurements of biliary ICG concentrations and the bile flow rate during a continuous vascular infusion of ICG. Despite significant elevations in mean peak serum tumor necrosis factor-alpha (TNF-alpha) concentrations (90.9 +/- 16.2 ng/ml), there was no effect on mean rates of bile flow or biliary ICG clearance after administration of 100 microg/kg endotoxin at 6 or 24 h. Significant differences from mean baseline rates of bile flow and biliary ICG excretion did occur after administration of 1,000 microg/kg endotoxin (mean peak TNF-alpha 129.6 +/- 24.4 ng/ml). Furthermore, when rats were treated with up to 16 microg/kg of recombinant TNF-alpha, there was no change in mean rates of bile flow or ICG biliary clearance compared with baseline values. These data suggest that the complex regulation of biliary excretion is not mediated solely by TNF-alpha.

Staphylococcal enterotoxin B potentiates LPS-induced hepatic dysfunction in chronically catheterized rats

Most models of liver dysfunction in sepsis use endotoxin (lipopolysaccharide; LPS) to induce a pathophysiological response. In our study published in this issue (Beno DWA, Uhing MR, Goto M, Chen Y, Jiyamapa-Serna VA, and Kimura RE. Am J Physiol Gastrointest Liver Physiol 280: G858-G865, 2001), the adverse effect of LPS on hepatic function in vivo was only significant at relatively high LPS doses despite high tumor necrosis factor-alpha concentrations. However, many patients with sepsis are exposed to multiple bacterial toxins that may augment the immune response, resulting in increased hepatic dysfunction. We have developed a model of polymicrobial sepsis by parentally administering a combination of staphylococcal enterotoxin B (SEB) and LPS. Using this model, we demonstrate that SEB (50 microg/kg) potentiates the effect of LPS-induced hepatic dysfunction as measured by decreased rates of biliary indocyanine green clearance and bile flow. These increases were most pronounced with doses of 10 and 100 microg/kg LPS, doses that by themselves do not induce hepatic dysfunction. This may explain the seemingly increased incidence and severity of liver dysfunction in sepsis, and it suggests that the exclusive use of LPS for replicating septic shock may not be relevant for studies of hepatic dysfunction.

Reversible induction of rat hepatoma cell polarity with bile acids

A dynamic model for inducing and isolating polarized cell colonies from differentiated rat hepatoma was established with chenodeoxycholic acid (CDCA). Cells were treated with 75 microM CDCA in a 1% solvent mix (DMSO/ethanol: 0.5%/0.5%) for 11 days and positive Fao-BA1 and C2rev7-BA1 clones were isolated, respectively, from Fao and C2rev7. Cell polarization in these two clones was demonstrated by (i) the detection of (gamma)-glutamyl transpeptidase activity (gamma)-GT) and the presence of specific proteins, namely aminopeptidase N (APN), bile acid export pump (Bsep), multidrug resistance-associated protein 2 (Mrp2) at the canalicular pole, (ii) the expression of tight junction (ZO-1) and basolateral (1–18) marker proteins, (iii) the presence of regular microvilli in the cavities sealed by tight junctions, and (iv) functional bile canaliculi-like structures with the capacity to metabolise and secrete carboxyfluorescein diacetate dye. The polarized phenotype was maintained for more than 200 cell generations in the presence of CDCA and could be modulated by cell density or omitting the inducing agent. Hence this cellular model is well suited for studies on hepatic differentiation, polarization and bile salt trafficking with therapeutic implications.

Differential expression of canalicular membrane Ca2+/Mg(2+)-ecto-ATPase in estrogen-induced and obstructive cholestasis in the rat

Extracellular adenosine triphosphate (ATP) may regulate hepatocyte and cholangiocyte functions, and under some conditions it may have deleterious effects on bile secretion and cause cholestasis. The canalicular membrane enzyme Ca2+/Mg2+-ecto-ATPase (ecto-ATPase) hydrolyzes ATP/adenosine diphosphate (ATP/ADP) and regulates hepatic extracellular ATP concentration. Changes in liver ecto-ATPase in estrogen-induced cholestasis were examined in male rats receiving 17alpha-ethinylestradiol (E groups) for 1, 3, or 5 days (5 mg/kg/day, sc) and compared with changes in rats subjected to obstructive cholestasis (O groups) for 1, 3, or 8 days. Activity of ecto-ATPase, protein mass in canalicular membranes and bile (estimated by Western blotting), steady state mRNA levels (by Northern blotting), and cellular and acinar distributions of the enzyme (histochemistry and immunocytochemistry) were assessed in these groups. Activity of ecto-ATPase, protein mass in isolated canalicular membranes, and enzyme mRNA levels were significantly increased in E group rats as compared with controls. In contrast, these parameters were markedly decreased in O group rats, and the enzyme protein was undetectable in bile. The ecto-ATPase histochemical reaction was markedly increased in the canalicular membrane of E group rats, extending from acinar zone 2 to zone 1, whereas it decreased in the O group. The ecto-ATPase immunocytochemical reaction was present in the canalicular membrane and pericanalicular vesicles in control and E group hepatocytes, but it decreased in obstructive cholestasis and was localized only to the canalicular membrane. Thus, significant changes in liver ecto-ATPase were apparent in 17alpha-ethinylestradiol-induced cholestasis that were opposite to those observed in obstructive cholestasis. Assuming that the alterations observed in obstructive cholestasis are the result of the cholestatic phenomenon, we conclude that changes in ecto-ATPase in 17alpha-ethinylestradiol-treated rats might be either primary events or part of an adaptive response in 17alpha-ethinylestradiol-induced cholestasis.

Biliary excretory function is regulated by canalicular membrane fluidity associated with phospholipid fatty acyl chains in the bilayer: implications for the pathophysiology of cholestasis

BACKGROUND AND AIMS

Bile canalicular membrane fluidity is modulated by phospholipid molecular species within membrane lipid bilayers. Thus, organellar membrane lipid composition is a determinant of canalicular function. In this study, the effect of phalloidin-induced cholestasis on bile lipid composition and liver subcellular membrane fraction composition in rats was examined to clarify the relationship between cholestasis and hepatic lipid metabolism.

METHODS AND RESULTS

Each rat received one phalloidin dose (400 microg/kg, i.v.). After the bile was collected, liver microsomes and canalicular membranes were analysed. The bile flow rate decreased by 50% 3.5 h after phalloidin administration. Although the bile acid output remained almost the same, the phospholipid and cholesterol output were significantly decreased (by 40.3+/-5.97% and 76.9+/-5.56%, respectively). Thus, the cholesterol:phospholipid (C:P) ratio in bile was significantly decreased by 80.4+/-10.1%. Phalloidin administration also increased the saturated: unsaturated fatty acid ratio (S:U) in bile for phosphatidylcholine by 25.5+/-3.2%. In the canalicular membrane, the C:P and S:U ratios for phosphatidylcholine were increased (24.8+/-4.2% and 34.4+/-6.9%, respectively), while the S:U for sphingomyelin was decreased by 61.0+/-6.2%. In microsomes, the C:P was decreased by 41.0+/-6.0%, but the S:U for both phosphatidylcholine and sphingomyelin were unaffected. Canalicular membrane fluidity, assayed by 1,6-diphenyl-1,3,5-hexatriene fluorescence depolarization, decreased significantly. Therefore, increased secretion of hydrophobic phosphatidylcholine into bile was associated with more hydrophobic canalicular membrane phosphatidylcholine, while sphingomyelin in the canalicular membrane was less hydrophobic.

CONCLUSIONS

These results indicate that phalloidin uncouples secretion of cholesterol and phospholipids, which causes a redistribution of fatty acyl chain species among canalicular membrane phospholipids that alters membrane fluidity. These changes may be a homeostatic response mediated by the phospholipid translocator in the canalicular membrane, although direct evidence for this is unavailable.

Cholestatic syndromes

Continued advances in the field of liver cell biology and molecular biology have provided further insights into the normal physiology of bile secretion and the pathogenesis and therapy for cholestatic syndromes. Important new data have also been published about pathogenesis, clinical features, and treatment of primary biliary cirrhosis, primary sclerosing cholangitis, drug-induced cholestasis, and cholestatic syndromes caused by viral infections.

Pathology of the liver

At the close of the 20th century, a selection of articles published in 1999 with relevance to liver pathology reflects the wealth of technological and intellectual progress made during the span of the century. Immunohistochemical staining for hepatitis B virus antigens focused attention on a correlation between cytoplasmic expression of core antigen in individuals with precore mutants and higher activity of hepatitis. Infection of ducklings with a presurface mutant strain of duck hepatitis B virus produced cytopathic liver cell damage. Fibrosing cholestatic hepatitis, originally described as an unusual form of recurrent hepatitis B after liver transplantation, has now been described in hepatitis C virus-positive patients with renal transplants. It may be related to the emergence or selection of hepatitis C virus quasispecies. In biliary tract disease, researchers investigated the canal of Hering as a possible source of hepatic stem cells, sporadic mutations in the JAGGED1 gene (involved in cell differentiation) in Alagille syndrome, and several models of nonsuppurative destructive cholangitis. Further work was accomplished on nonalcoholic steatohepatitis, including a proposal of a grading and staging system as well as its detection in workers exposed to volatile petrochemicals. Among hepatic neoplasms and proliferative disorders, epithelioid hemangioendothelioma, angiomyolipoma and Langerhans' cell histiocytosis received coverage in articles describing the diagnostic pathology in collected series of patients.

Human liver class I alcohol dehydrogenase gammagamma isozyme: the sole cytosolic 3beta-hydroxysteroid dehydrogenase of iso bile acids

3beta-Hydroxy (iso) bile acids are formed during enterohepatic circulation from 3alpha-hydroxy bile acids and constitute normal compounds in plasma but are virtually absent in bile. Isoursodeoxycholic acid (isoUDCA) is a major metabolite of UDCA. In a recent study it was found that after administration of isoUDCA, UDCA became the major acid in bile. Thus, epimerization of the 3beta-hydroxy to a 3alpha-hydroxy group, catalyzed by 3beta-hydroxysteroid dehydrogenases (HSD) and 3-oxo-reductases must occur. The present study aims to characterize the human liver bile acid 3beta-HSD. Human liver cytosol and recombinant alcohol dehydrogenase (ADH) betabeta and gammagamma isozymes were subjected to native polyacrylamide gel electrophoresis (PAGE) and isoelectric focusing. Activity staining with oxidized nicotinamide adenine dinucleotide (NAD(+)) or oxidized nicotinamide adenine dinucleotide phosphate (NADP(+)) as cofactors and various iso bile acids as substrates was used to screen for 3beta-HSD activity. Reaction products were identified and quantified by gas chromotography/mass spectrometry (GC/MS). Computer-assisted substrate docking of isoUDCA to the active site of a 3-dimensional model of human class I gammagamma ADH was performed. ADH gammagamma isozyme was identified as the iso bile acid 3beta-HSD present in human liver cytosol, with NAD(+) as a cofactor. Values for k(cat)/K(m) were in the rank order isodeoxycholic acid (isoDCA), isochenodeoxycholic acid (isoCDCA), isoUDCA, and isolithocholic acid (isoLCA) (0.10, 0.09, 0.08, and 0. 05 min(-1) x micromol/L(-1), respectively). IsoUDCA fits as substrate to the 3-dimensional model of the active-site of ADH gammagamma. ADH gammagamma isozyme was defined as the only bile acid 3beta-HSD in human liver cytosol. Hydroxysteroid dehydrogenases are candidates for the binding and transport of 3alpha-hydroxy bile acids. We assume that ADH gammagamma isozyme is involved in cytosolic bile acid binding and transport processes as well.

Bile acids and progesterone metabolites in intrahepatic cholestasis of pregnancy

The pathogenesis of intrahepatic cholestasis of pregnancy (ICP) can be related to abnormalities in the metabolism and disposition of sex hormones and/or bile acids, determined by a genetic predisposition interacting with environmental factors. The total amount of oestrogens and progesterone circulating in the blood or excreted in the urine of ICP patients is similar to normal pregnancies. Thus, the search for the cause has been focused on abnormal hormone metabolites. The cholestatic potential of some D-ring oestrogen metabolites is supported by experimental and clinical data. Similar observations with regard to bile acids and progesterone metabolites are still scarce. This article reviews current knowledge in this field, including our own data. Bile acid synthesis appears to be reduced in patients with ICP, in whom primary conjugated bile acids are retained in blood. The major bile acid in blood and urine of these patients is cholic acid instead of chenodeoxycholic acid present in normal pregnancies. Hydroxylation and sulfation of bile acids are enhanced, while glucuronidation appears to be of lesser importance. The synthesis of progesterone appears unimpaired, while the profiles of progesterone metabolites in plasma and urine are different from normal pregnancies, with a larger proportion of mono- and disulfated metabolites, mainly 3alpha,5alpha isomers. Glucuronidated metabolites, however, are unchanged. With the administration of ursodeoxycholic acid (UDCA) to patients with ICP, pruritus and serum liver values are improved, the concentration of bile acids in blood is diminished and the proportion of their conjugated metabolites returned to normal. Simultaneously, the concentration of sulfated progesterone metabolites in blood and their urinary excretion are reduced. The serum levels of bile acids and progesterone metabolites before UDCA administration and their decrease during treatment do not correlate with each other. We propose that patients with ICP have a selective defect in the secretion of sulfated progesterone metabolites into bile and speculate that this may be caused by genetic polymorphism of canalicular transporter(s) for steroid sulfates or their regulation. Interaction with oestrogen metabolites and/or some exogenous compounds may further enhance the process triggering ICP in genetically predisposed individuals.

Inflammation-induced cholestasis

Inflammatory cytokines produced in response to various infectious and non-infectious stimuli are potent inducers of intrahepatic cholestasis (inflammation-induced cholestasis). The cholestatic effect of cytokines results mainly from inhibition of expression and function of hepatocellular transport systems which normally mediate hepatic uptake and biliary excretion of bile salts and various non-bile salt organic anions (e.g. bilirubin). These cytokine effects are reversible and bile secretory function is restored upon disappearance of the inflammatory injury. This review summarizes the clinical, pathophysiological and molecular aspects of inflammation-induced cholestasis.