Expression of the bile salt export pump is maintained after chronic cholestasis in the rat

Regulation of the hepatic multidrug resistance gene expression by endotoxin and inflammatory cytokines in mice

P-glycoprotein (PGP), an ATP-dependent membrane transporter is found in epithelial tissues of the liver, kidneys, intestine and blood-brain barrier. In tumor cells, PGP is often overexpressed and confers multidrug resistance toward cancer chemotherapeutics. It has been previously shown in rats that induction of an inflammatory response evokes a decrease in hepatic expression of PGP. In order to identify the inflammatory mediators involved in this phenomenon, we examined the influence of experimentally induced inflammation and pro-inflammatory cytokines (interleukin (IL)-6, IL-1beta and tumor necrosis factor (TNF)-alpha) on the hepatic expression of PGP in mice. A significant reduction in the hepatic expression of mdr1a, mdr1b, mdr2 and spgp genes were seen in endotoxin (lipopolysaccharide (LPS)) and turpentine-treated mice. Similarly, IL-6-treated mice displayed a 70% reduction in protein expression and a 40-70% reduction in the mRNA levels of all PGP mdr isoforms. Administration IL-1beta caused an increase in both mdr1b mRNA and protein expression, however, mRNA levels of mdr1a, mdr2 and spgp were significantly reduced. Administration of TNF-alpha also caused increases in mdr1b mRNA. These findings indicate that IL-6 plays a principal role in the downregulation of PGP that is observed in the livers of mice during an inflammatory response.

Human bile salt export pump promoter is transactivated by the farnesoid X receptor/bile acid receptor

The bile salt excretory pump (BSEP, ABCb11) is critical for ATP-dependent transport of bile acids across the hepatocyte canalicular membrane and for generation of bile acid-dependent bile secretion. Recent studies have demonstrated that the expression of this transporter is sensitive to the flux of bile acids through the hepatocyte, possibly at the level of transcription of the BSEP gene. To determine the mechanisms underlying the regulation of BSEP by bile acids, the promoter of the BSEP gene was cloned. The sequence of the promoter contained an inverted repeat (IR)-1 element (5'-GGGACA T TGATCCT-3') at base pairs -63/-50 consisting of two nuclear receptor half-sites organized as an inverted repeat and separated by a single nucleotide. This IR-1 element has been shown in several recent studies to serve as a binding site for the farnesoid X receptor (FXR), a nuclear receptor for bile acids. FXR activity requires heterodimerization with RXR alpha, and when bound by bile acids, the complex effectively regulates the transcription of several genes involved in bile acid homeostasis. Gel mobility shift assays demonstrated specific binding of FXR/RXR alpha heterodimers to the IR-1 element in the BSEP promoter. In HepG2 cells, co-transfection of FXR and RXR alpha is required to attain full transactivation of the BSEP promoter by bile acids. Two FXR transactivation-deficient mutants (an AF-2 deletion and a W469A point mutant) failed to transactivate, indicating that the effect of bile acids is FXR-dependent. Further, mutational analysis confirms that the FXR/RXR alpha heterodimer activates transcription through the IR-1 site in the human BSEP promoter. These results demonstrate a mechanism by which bile acids transcriptionally regulate the activity of the bile salt excretory pump, a critical component involved in the enterohepatic circulation of bile acids.

Expression of hepatic transporters OATP-C and MRP2 in primary sclerosing cholangitis

BACKGROUND/AIMS

In chronic cholestatic liver diseases, biliary excretion of organic anions from blood into bile is impaired. The aim of this study was to identify the underlying mechanism.

METHODS

Expression of the basolateral organic anion transporting polypeptide OATP-C (SLC21A6) and the canalicular multidrug resistance protein 2 (MRP2) was studied in patients with primary sclerosing cholangitis (PSC) (n=4), a chronic cholestatic liver disease, and in non-cholestatic controls (n=4) (two with chronic hepatitis C, one with idiopathic liver cirrhosis and one with fatty liver). Total RNA was isolated from liver tissue, reverse transcribed and subjected to polymerase chain reaction (PCR) amplification using primers specific for OATP-C, MRP2 and beta-actin. PCR products were quantified densitometrically.

RESULTS

When normalized for beta-actin expression, the level of OATP-C mRNA in liver tissue of patients with PSC was 49% of controls (OATP-C/beta-actin 1.60+/-0.25 vs. 3.24+/-0.69; p<0.05) and the level of MRP2 mRNA was 27% of controls (MRP2/beta-actin 0.70+/-0.36 vs. 2.54+/-0.56; p<0.01).

CONCLUSIONS

Both OATP-C and MRP2 are decreased as measured by mRNA level in PSC. Downregulation of OATP-C might be the consequence of impaired canalicular secretion of organic anions and could serve to reduce the organic anion load of cholestatic hepatocytes.

Colchicine inhibits taurodeoxycholate transport in pericentral but not in periportal hepatocytes

Indirect evidence for a microtubule-dependent vesicular hepatocellular transport of bile acids has accumulated. Since inhibition of this transport by colchicine can be achieved only at high but not at low bile acid infusion rates we were wondering whether this transport pathway shows a hepatic zonation or not. To answer this question we perfused isolated rat livers antegradely or retrogradely, respectively, with unlabeled and labeled taurocholate or taurodeoxycholate. Inhibition of microtubule-dependent bile acid transport was aimed at co-infusion of colchicine. Periportal cells eliminated the likewise hydrophobic taurodeoxycholate as fast as the more hydrophilic taurocholate. In contrast, pericentral cells excreted taurodeoxycholate much slower than taurocholate. Colchicine did not change the biliary taurocholate excretion profile in periportal and pericentral cells. However, colchicine reduced significantly taurodeoxycholate excretion in pericentral but not in periportal cells. It is concluded that a microtubule-dependent vesicular, colchicine-sensitive transport pathway seems to be involved in the translocation of taurodeoxycholate in pericentral but not in periportal cells. Since such a vesicular bile acid transport is regarded to be much slower than transcellular transport by diffusion, this observation may explain the much slower excretion of hydrophobic bile acids like taurodeoxycholate in pericentral than in periportal cells under physiological conditions.

Transport of bile acids in hepatic and non-hepatic tissues

Bile acids are steroidal amphipathic molecules derived from the catabolism of cholesterol. They modulate bile flow and lipid secretion, are essential for the absorption of dietary fats and vitamins, and have been implicated in the regulation of all the key enzymes involved in cholesterol homeostasis. Bile acids recirculate through the liver, bile ducts, small intestine and portal vein to form an enterohepatic circuit. They exist as anions at physiological pH and, consequently, require a carrier for transport across the membranes of the enterohepatic tissues. Individual bile acid carriers have now been cloned from several species. Na(+)-dependent transporters that mediate uptake into hepatocytes and reabsorption from the intestine and biliary epithelium and an ATP-dependent transporter that pumps bile acids into bile comprise the classes of transporter that are specific for bile acids. In addition, at least four human and five rat genes that code for Na(+)-independent organic anion carriers with broad multi-substrate specificities that include bile acids have been discovered. Studies concerning the regulation of these carriers have permitted identification of molecular signals that dictate eventual changes in the uptake or excretion of bile acids, which in turn have profound physiological implications. This overview summarizes and compares all known bile acid transporters and highlights findings that have identified diseases linked to molecular defects in these carriers. Recent advances that have fostered a more complete appreciation for the elaborate disposition of bile acids in humans are emphasized.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Expression and regulation of hepatic drug and bile acid transporters

Transport across hepatocyte plasma membranes is a key parameter in hepatic clearance and usually occurs through different carrier-mediated systems. Sinusoidal uptake of compounds is thus mediated by distinct transporters, such as Na(+)-dependent or Na(+)-independent anionic transporters and by some cationic transporters. Similarly, several membrane proteins located at the apical pole of hepatocytes have been incriminated in the excretion of compounds into the bile. Indeed, biliary elimination of anionic compounds, including glutathione S-conjugates, is mediated by MRP2, whereas bile salts are excreted by a bile salt export pump (BSEP) and Class I-P-glycoprotein (P-gp) is involved in the secretion of amphiphilic cationic drugs, whereas class II-P-gp is a phospholipid transporter. The expression of hepatic transporters and their activity are regulated in various situations, such as ontogenesis, carcinogenesis, cholestasis, cellular stress and after treatment by hormones and xenobiotics. Moreover, a direct correlation between a defect and the absence of transporter with hepatic disease has been demonstrated for BSEP, MDR3-P-gp and MRP2.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

The pathophysiology of cholestasis with special reference to primary biliary cirrhosis

Cholestasis in primary biliary cirrhosis results from impairment of bile flow either by reduced transport at the level of the canaliculi or by disturbed bile flow through damaged intrahepatic bile ductules. Whatever its cause, the expression of hepatic transport proteins will be affected. In cholestatic rats: the expression of the multispecific organic anion transporter mrp2 is decreased; the bile salt export pump bsep and the phospholipid transporter mdr2 are less affected; the carrier protein for hepatic uptake of bile salts ntcp is sharply down-regulated; Mrp3, a basolateral ATP-dependent transporter for glucuronides and bile salts, is upregulated. Thus, bile salts that cannot exit the hepatocyte because of the cholestasis are effectively removed across the basolateral membrane. These may be adaptive responses in defence against overloading of hepatocytes with cytotoxic bile salts. These responses show that the expression of hepatic transporter proteins is highly regulated. This occurs by transcriptional and post-transcriptional mechanisms. Primary biliary cirrhosis starts as a disease of the small intrahepatic bile ducts and therefore the experimental evidence for 'cross-talk' between hepatocytes and cholangiocytes is of great interest for this disease and needs to be further investigated. New insights in bile physiology may enable the development of new therapies for cholestatic liver diseases as primary biliary cirrhosis.

Cholestasis with altered structure and function of hepatocyte tight junction and decreased expression of canalicular multispecific organic anion transporter in a rat model of colitis

Cholestasis is frequently associated with inflammatory bowel disease. Because some cholestasis is resulted from altered hepatocyte tight junctions (TJs) or the canalicular multispecific organic anion transporter, we have investigated the following topics in a rat model of inflammatory bowel disease: (1) alterations in hepatocyte TJs and in the canalicular multispecific organic anion transporter, (2) etiologic factors for cholestasis, and (3) effects of antibiotics on cholestasis. Rats with trinitrobenzene sulfonic acid-induced colitis were studied 24 hours after treatment. Hepatocyte TJs and the canalicular multispecific organic anion transporter were evaluated by immunostaining for TJ-associated proteins, 7H6 and ZO-1, and multidrug resistance protein 2 (mrp2). To investigate etiologic factors causing cholestasis, portal endotoxin and proinflammatory cytokines were examined. The effects of polymyxin B, penicillin G, or metronidazole on immunostaining for 7H6, ZO-1, mrp2, and cholestasis were investigated. (1) Immunostaining for 7H6 and ZO-1 colocalized outlining the bile canaliculi and immunostaining for mrp2 localized on the canalicular membrane in controls. Treatment with trinitrobenzene sulfonic acid induced significant cholestasis and caused translocation of immunostaining for 7H6, but not that for ZO-1, to the cytoplasm and diminished immunostaining for mrp2 on the canaliculus membrane. (2) The levels of portal endotoxin, but not proinflammatory cytokines, was increased. (3) Polymyxin B, but not the other antibiotics, prevented alterations in immunostaining for both 7H6 and mrp2, and cholestasis. We described that both hepatocyte TJs and the canalicular multispecific organic anion transporter were altered and that gut-derived endotoxin levels in the portal blood were increased in this rat colitis model.

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.

Mechanisms of cholestasis

From the multiple mechanisms of cholestasis presented in this article, a unifying hypothesis may be deduced by parsimony. The disturbance of the flow of bile must inevitably lead to the intracellular retention of biliary constituents. Alternatively, the lack of specific components of bile unmasks the toxic potential of other components, as in the case of experimental mdr2 deficiency. In the sequence of events that leads to liver injury, the cytotoxic action of bile salts is pivotal to all forms of cholestasis. The inhibition of the bsep by drugs, sex steroids, or monohydroxy bile salts is an example of direct toxicity to the key mediator in canalicular bile salt excretion. In other syndromes, the dysfunction of distinct hepatocellular transport systems is the primary pathogenetic defect leading to cholestasis. Such dysfunctions include the genetic defects in PFIC and the direct inhibition of gene transcription by cytokines. Perturbations in the short-term regulation of transport protein function are exemplified by the cholestasis of endotoxinemia. The effect of bile salts on signal transduction, gene transcription, and transport processes in hepatocytes and cholangiocytes has become the focus of intense research in recent years. The central role of bile salts in the pathogenesis of cholestasis has, ironically, become all the more evident from the improvement of many cholestatic syndromes with oral bile salt therapy.