Diseases of the Gallbladder and Bile Ducts - part 9 potx

348 Section 3: Specific conditions with PBC who are receiving corticosteroids [48]. However, in a randomized, double-blind, placebo-controlled study in 67 patients with PBC, etidronate did not cause any improve- ment of bone mass density (BMD). A more recent study, in- cluding 32 female patients with PBC, showed that both etidronate and alendronate increase BMD but the positive effect of alendronate was far superior [49]. So, most likely alendronate, rather than etidronate, should be recommen- ded to patients with PBC with osteoporosis, but further stud- ies are required to establish the role of this agent in the t re a t men t o f os t e op or os i s i n P BC . I t c a n not b e i g n o r e d t ha t i n patients with advanced PBC (who are also more likely to have osteoporosis) and who have esophageal varices, biphospho- nates may potentially cause esophagitis and increase the risk of variceal bleeding. Vitamin K plays a modulatory role on bone metabolism. Increased BMD and prevention of bone fractures were ob- served in patients with osteoporosis who were treated with vitamin K. A randomized study in female patients with PBC showed a signifi cant increase of BMD in subjects treated with vitamin K [50]. These results are promising but require con- firmation in studies including larger cohorts of patients. After publication of the results of the HERS II trial (heart and estrogen/progestin replacement study) HRT cannot be recommended anymore for the treatment of osteoporosis as it increases the risk of certain malignancies, hip fracture, and thromboembolism and does not have any signifi cant cardio- protective effect. Also UDCA and calcitonin seem to be of no use in the prevention or treatment of osteoporosis in patients with PBC. A signifi cant proportion of patients with PBC is deficient of fat soluble vitamins. This refers mostly to patients with ad- vanced disease, in particular in those who also have low serum albumin and cholesterol levels and an elevated biliru- bin. When the Mayo risk score for PBC is higher or equal to 5, patients may be found to be vitamin A deficient. It has been recommended that these patients should be screened for fat- soluble vitamin deficiencies and adequately supplemented if necessary. As with all patients who are found to have large eso- phageal varices, prophylactic nonselective beta blockade is recommended. Specific therapies for PBC There is no medical therapy that cures PBC. Ursodeoxycholic acid (UDCA) remains the only US Food and Drug Adminis- tration approved medication for PBC. Although the mecha- nisms involved in its hepatoprotective properties have been extensively studied over last 20 years, the mechanisms of its action are not yet fully explained. UDCA and its conjugates stimulate bile salt excretion and protect against bile-salt- induced mitochondrial damage, oxidative stress, and apop- tosis and also may have a membrane-stabilizing effect, protecting against bile-salt-induced solubilization. There is no doubt that UDCA plays the role of a signaling molecule with precise regulatory properties in specific signaling pathways [51]. UDCA causes signifi cant improvement in biochemical parameters of cholestasis, including bilirubin, alkaline phos- phatase, and GGT. It also decreases serum cholesterol levels. UDCA slows down the histological progression of the disease [52,53]. It may also decrease pruritus in a proportion of pa- tients (but rarely it may increase pruritus) but has no signifi - cant effect on fatigue. A beneficial effect on survival was observed when patients were treated for up to 4 years with the trial dosage of UDCA [54], although clearly some patients respond better than others. It has been shown that those pa- tients with noncirrhotic PBC treated with UDCA appear to have a 10-year surv ival that is no different from an age- and gender-matched population [55]. Overall the greatest effect on short-term survival was seen in those patients with more severe disease, for whom UDCA treatment may delay the need for transplantation. Treatment before transplant does not have a detrimental effect on the patients’ post-transplant outcome, despite the patients being older when they eventu- ally come to need a transplant. At the recommended dose of 13 to 15 mg/kg per day UDCA is extremely well tolerated with only a minority of patients complaining of diarrhea – but lower doses, that is 10 mg/kg, appear to be less effective. The beneficial effect of UDCA has been questioned [56] and no signifi cant impact of this drug on survival or time to liver transplantation was found, although a signifi cant re- duction in jaundice and ascites was noted. This report has been criticized as early studies, where patients were treated for short periods of time and with subtherapeutic doses of UDCA, were included in the meta-analysis. Although the trigger for the development of PBC remains to be elucidated, autoimmune phenomena may play a role in the hepatic damage. Thus different immunosuppressive drugs have been investigated in PBC. In a study published by Christensen et al, azathioprine was found to prolong survival by approximately 20 months [57]. A positive effect on sur- vival was also observed by another European group, in a pla- cebo controlled trial with cyclosporine A [58]. Unfortunately, this was associated with signifi cant side-effects of this calci- neurin inhibitor, including renal impairment and hyperten- sion. Anecdotal reports suggest that combined therapy of UDCA and mycophenolate mofetil (MMF) may be of partic- ular use in patients with a signifi cant infl ammatory compo- nent on their biopsies. The potential application of MMF in PBC is now the subject of an ongoing randomized study. With regard to other treatments, no convincing effect was observed with colchicine and methotrexate. Budesonide was found to increase the risk of portal vein thrombosis in cirrhotic pa- tients with PBC [59] and therefore its use in this subgroup of PBC patients should be discouraged. Drugs more recently in- vestigated in PBC include bezafibrate, pranlukast and sulin- dac [60]. Their role in the management of PBC remains to be established. Chapter 21: Primary biliary cirrhosis 349 As PBC progresses very slowly, it is extremely difficult to prove effi cacy of any therapeutic agent. It has been suggested that in order to demonstrate a clear effect of any medication on death rate in patients without liver transplantation, over 200 patients have to be treated for a period of 5 years [58]. For those who reach end-stage liver disease, liver transplanta- tion is the only option but, certainly, liver transplantation can not be reliably used as a primary end-point. The proportion of patients with PBC who require liver transplantation, either for end-stage liver failure or for poor quality of life, is diminishing [61]. PBC used to be the most common indication for liver transplantation in patients with end-stage liver disease, comprising up to 55% of all trans- planted, cirrhotic patients in some centers [45]. This propor- tion has now decreased to 10%; this is in part due to both increasing number of patients being transplanted for viral- and alcohol-related cirrhosis and possibly a protective effect of UDCA [45]. Indications for liver transplantation in PBC are summarized in Table 21.4. The Mayo Clinic model and the more recently introduced MELD score are useful in pre- dicting survival in patients with end-stage liver disease. It is recommended that in patients with PBC, once their bilirubin level has reached 100 µmol/L (5.9 mg/100 mL), they should be referred for liver transplant assessment [45]. Current 5-year survival after liver transplantation for PBC varies between 83 and 86%, making this disease an excellent indi- cation for grafting. Early mortality after surgery is caused mostly by multiorgan failure and sepsis [61]. Chronic rejec- tion, which is the most common indication for regrafting within 1 year of surgery, occurs signifi cantly less commonly in patients with PBC than in patients transplanted for auto- immune hepatitis and more commonly than in those trans- planted for alcohol-related liver disease [62]. PBC does recur after transplantation and the diagnosis of the recurrence can only be reliably established byhistological examination [45]. This phenomenon is rarely of clinical signifi cance and may potentially be associated with tacrolimus rather than cyclosporine-based immunosupression [63]. A vital issue, which has to be addressed in the near future, is the identifi cation of patients who are more likely to prog- ress into end-stage liver disease. Clinical practice clearly shows that some subjects develop cirrhosis despite being treated with UDCA whereas others show no clinical or histo- logical progression over many years, sometimes even with no treatment. The role of newly identified serum markers of dis- ease progression has to be validated. Certainly, genetics may play a crucial role in the natural history of this disease, affect- ing the rate of progression and response to treatment. This di- lemma has prompted several leading groups from all over the world to establish a collaborative project which will allow complex genetic analyses in a large, shared pool of samples. Undoubtedly, this effort will enable an unprecedented accel- eration in our knowledge on the genetic background of PBC and may have a signifi cant impact on combating this chronic and potentially lethal disease. Questions 1. Which sentence is true about the epidemiology of PBC? a. recent studies have shown that the prevalence of PBC is decreasing b. smoking does not increase the risk for developing PBC c. the rate of AMA seropositivity in the general population is significantly higher than the prevalence of PBC arising from epidemiological studies d. all these statements are false e. statements (b) and (c) are true 2. In the pathogenesis of PBC a. there is a clear relationship to herpes zoster infection b. apoptosis may play a role c. N. aromaticivorum may play a role in triggering the disease d. there is a very weak association with possible genetic factors e. statements (b) and (c) are true 3. With regard to the natural history of PBC which of the following statements is true? a. the majority of patients diagnosed today are jaundiced b. pruritus never precedes the onset of jaundice c. the asymptomatic phase usually lasts a couple of weeks d. AMA-positive patients who have normal biochemistry may have features of PBC on their biopsies e. the onset of the disease is usually acute Table 21.4 Indications for transplantation in patients with primary biliary cirrhosis. Symptoms Intractable and intolerable pruritus Overwhelming lethargy End-stage disease Clinical signs Increasing muscle wasting Increasing symptomatic osteopenia Encephalopathy Intractable ascites Recurrent, intractable variceal hemorrhage Spontaneous bacterial peritonitis Moderate hepatopulmonary syndrome Early hepatocellular carcinoma Biochemical Serum bilirubin > 170 µmol/L > 6 months Serum albumin < 25 g/L Patients should be referred to a liver transplant center when their bilirubin reaches 100 µmol/L. (Reproduced from MacQuillan GC and Neuberger J. Clin Liver Dis 2003;7:941–56, with permission from Elsevier.) 350 Section 3: Specific conditions 4. In the natural history of PBC a. factors predisposing to progression from asymptomatic to symptomatic disease include bilirubin and alkaline phosphatase b. as many as 90% of initially asymptomatic patients may develop symptoms of liver disease within 4 years of diagnosis c. PBC in children usually has a very aggressive course d. the diagnosis of PBC is most commonly established by the age of 20 e. all statements are false 5. Fatigue in PBC – which statement is true? a. the origin of fatigue in PBC is most likely central b. fatigue is a rare symptom in PBC c. there is a strong correlation between age and degree of fatigue d. there is a strong correlation between fatigue and hepatic histology e. fatigue in PBC is usually relieved by rest 6. Pruritus in PBC – which statement is true? a. pruritus occurs in about 30% of patients b. pruritus is usually worst in the morning c. the palms or soles are never affected d. the classical “butterfly” area delineates the area of most intense scratching e. scratching usually does not relieve this symptom 7. Diseases associated with PBC – which statement is true? a. sicca syndrome occurs in approximately 10% of patients b. a history of hypothyroidism is present in 90% of patients c. psoriasis is the commonest associated disease d. majority of patients are found to have antiendomysial antibodies e. all statements are false 8. Diagnosis of PBC – which statement is true? a. diagnosis is established on the presence of positive ANA b. AMA may b e p o s it i v e in abo ut 40 to 5 0 % o f p at i e n t s c. liver biopsy is essential for establishing the diagnosis d. immunochemistry for AMA may be falsely positive in patients with type 2 autoimmune hepatitis e. the presence of AMA confirms only advanced PBC 9. Treatment of PBC – which statement is true? a. the curative rate of ursodeoxycholic acid is approximately 60% b. questran should be avoided in the treatment of pruritus c. rifampicin is a first-line treatment of pruritus d. HRT is highly recommended to prevent osteoporosis only in postmenopausal women with PBC e. budesonide must not be used in cirrhotic patients with PBC 10. Liver transplantation in PBC – which statement is true? a. amongst patients with cirrhosis, PBC is currently the most common indication for liver transplantation b. the 5-year survival after liver transplantation for PBC is approximately 85% c. treatment with ursodeoxycholic acid before transplant significantly reduces survival following liver transplantation d. calcineurin inhibitors (tacrolimus and cyclosporin A) have to be avoided after liver transplantation as they increase the risk of acute rejection e. recurrence of the disease is the most challenging issue after liver transplantation References 1. Addison T, Gull W. On a certain affection of the skin, vitilgoidea- plana, tuberosa. Guy’s Hosp Rev 1851;265–76. 2. Ahrens EH, Rayne MA, Kunkle HG. Primary biliary cirrhosis. Medicine 1950;29:299–364. 3. Scheuer PJ. Primary biliary cirrhosis. Proc R Soc Med 1967;60: 1257–60. 4. Doniach D, Roitt IM, Walker JG, Sherlock S. 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SECTION 3.3 Intrahepatic cholestasis Diseases of the Gallbladder and Bile Ducts: Diagnosis and Treatment, Second Edition Edited By Pierre-Alain Clavien, John Baillie Copyright © 2006 by Blackwell Publishing Ltd CHAPTER 22 Intrahepatic cholestasis Andrew Stolz and Neil Kaplowitz 22 OBJECTIVES • Recognize the major hepatic transporters responsible for the sinusoidal uptake of bile salts or organic anions or the efflux of bile salts, organic anions, or lipids into bile • Understand how transcription factors co-ordinate the response of hepatocyte transporters and detoxification enzymes to cholestatic liver injury • Learn about the major genetic causes of cholestatic diseases in children and adults • Identify the clinical features and molecular mechanisms for drug-induced cholestatic liver injury Multiple etiologies are responsible for cholestatic liver injury, which require vastly different diagnostic and therapeutic ap- proaches. In this chapter, we will review a diverse group of nonsurgical diseases that can mimic the presentation of other causes of cholestatic liver disease discussed elsewhere in this book. In order to understand how these diverse disorders can cause cholestatic liver injury, we will concisely review the current understanding of the key proteins that constitute the normal hepatic uptake and biliary excretory pathways. In human disease and animal models, the molecular regulation of these key transport proteins in response to cholestatic liver injury has provided detailed insight into the pathophysiolog- ical mechanisms in different cholestatic conditions. Clinical presentation, evaluation, treatment, and medical conditions associated with cholestasis will follow a review of the normal biliary physiology. A review of drug-induced cholestasis will also be provided, as inability to recognize this important eti- ology can lead to persistent exposure to the offending agent and unnecessary diagnostic or therapeutic interventions. We anticipate that enhanced understanding of molecular mech- anisms of cholestatic liver disease should lead to new thera- peutic strategies for these widely different diseases in the future. Mechanisms for hepatic bile formation The cellular organization of the liver combined with the unique endothelial structure of the sinusoids are ideally suit- ed for the efficient sinusoidal uptake and effl ux of molecules by the hepatocytes as well as their biliary excretion into the canaliculus, the most proximal portion of the biliary system 355 [1–4]. The hepatocytes, like other epithelial cells such as the enterocyte, are polarized cells with distinct membrane do- mains exposed to either the sinusoidal surface, referred to as the basal or sinusoidal domain, or to the canaliculus, referred to as the apical or canalicular domain. The membrane be- tween these regions forms the lateral domain. Cords of hepa- tocytes two cell thick are attached at their sinusoidal and apical domain by gap junctions, generating a physical barrier between the vascular sinusoidal space and the biliary excre- tory pathway. Ions and water can travel between these gaps and disruption of these anchors between hepatocytes can lead to regurgitation of biliary components into the vascular space during cholestatic conditions. Bile formation requires both the maintenance of these cell-to-cell contacts as well as vectorial uptake at the sinusoidal membrane, followed by se- cretion of biliary components at the canalicular membrane. Based on animal studies, the origins of bile flow are divid- ed into bile acid dependent and independent bile flow. Bile salts are the predominate solute in bile which, along with sodium ion, are responsible for water movement into the canalicular space, predominately by paracellular pathways between adjacent hepatocytes. Bile salts are also essential for biliary excretion of phospholipids, which are extracted from the outer leaflet of the canalicular membrane forming mixed micelles in the presence of bile salts. An absence of bile salts leads to signifi cant reduction in bile formation and phospho- lipid excretion. On the other hand, bile salt excretion into the canalicular space in the absence of phospholipid excretion leads to extensive damage to cholangiocytes, due to potent, detergent and cytotoxic effects of free bile salts. A bile-salt- independent bile flow also exists in which effl ux of organic Diseases of the Gallbladder and Bile Ducts: Diagnosis and Treatment, Second Edition Edited By Pierre-Alain Clavien, John Baillie Copyright © 2006 by Blackwell Publishing Ltd 356 Section 3: Specific conditions anions and the important peptide antioxidant, glutathione (GSH), coupled with its cleavage by γ glutamyl transpepti- dase and dipeptidases into its component amino acids, gener- ate an osmotic gradient promoting water movement into the canalicular space [5]. Bicarbonate secretion is another im- portant component whose contribution is highly species de- pendent [6]. The bile canalicular space created by adjacent hepatocytes eventually empty at the periphery of the lobule into ductules lined by cholangiocytes which further modify ion and bicar- bonate content in bile, which is regulated by secretin. A cho- lehepatic shunt between the hepatocytes and cholangiocytes has been proposed to explain the bicarbonate-rich biliary ex- cretion caused by infusion of unconjugated bile acids such as ursodeoxycholate in animals. The role this plays in normal bile flow is controversial. It is postulated that certain uncon- jugated bile acids secreted by hepatocytes are reabsorbed by cholangiocytes in protonated form (leaving bicarbonate be- hind) and then returned to the hepatocytes via the ductular capillaries, emptying into the sinusoids in a repetitive recy- cling, leading to enhanced bicarbonate secretion associated with a hypercholeresis [7]. In addition to fl uid and electro- lyte movement, mechanical contraction of canalicular membrane by a microfilament network located at the apical domain of hepatocytes also promote bile flow by a “squeezing action” on the canalicular space. Within the last decade, identifi cation of cellular transport proteins mediating sinusoidal uptake and biliary secretion of the key solutes has greatly expanded our understanding of the cellular mechanism for bile formation and its dysregula- tion in cholestatic conditions. We will review the key trans- port proteins listed in Table 22.1 and illustrated in Fig. 22.1 at each domain and the nuclear receptors that regulate their ex- pression. The altered regulation of these transporters in re- sponse to cholestatic stimuli are now known to play a key role in the development and maintenance of cholestasis [3,4]. Table 22.1 Molecular and functional characteristics of human hepatocyte membrane transporters. Transporter Protein Gene Domain Transport Model substrates features Na + -taurcholate cotransporting NTCP SLC10A1 Sinusoidal Uni Bile acids polypeptide Microsomal epoxide hydrolyase mEH EPHX1 Sinusoidal/ Uni Cholate, taurocholate intracellular Organic anion transport peptide OATP1B1 SLCO1B1 Sinusoidal Bi Bile salts, HMG-Co reductase inhibitors, (OATP-C, LST-1, OATP2) organic anion Organic anion transport peptide OATP1B3 SLCO1B3 Basolateral Bi Bile salts, digoxin (OATP-8) Organic anion transport peptide OATP2B1 SLCO2B1 Basolateral Bi Estrone-3-sulfate, dehydroepiandrosterone- (OATP-B) sulfate Bile salt export pump BSEP ABCB11 Canaliculus Uni Taurocholate, bile acids ABCG5/ABCG8 ABCG5/ABC G8 ABCG5/ Canaliculus Uni Cholesterol ABCG8 Multidrug resistance 3 MDR3 ABCB4 Canaliculus Uni Phosphatidylcholine Familial intrahepatic FIC1 ATP8B1 Canaliculus Uni Unknown cholestasis 1 Multiresistance protein 2 (cMOAT) MRP2 ABCC2 Canaliculus Uni Leukotriene C4, GSH conjugates, conjugated bilirubin Multiresistance protein 3 MRP3 ABCC3 Basolateral Uni Bile acids, conjugated sex steroids Multiresistance protein 4 MRP4 ABCC4 Basolateral Uni Nucleotide analogs, organic anions, bile salts (with GSH) Multiresistance protein 5 MRP5 ABCC5 Basolateral Uni Nucleotide analogs, organic anions Multiresistance protein 6 MRP6 ABCC6 Basolateral Uni Organic anions Location (domain) of human hepatic transporters and some of their model substrates [3,21,28]. Transport features refers to the ability of the transporter to function as a unidirectional (Uni) or bidirectional (Bi) transporter. Chapter 22: Intrahepatic cholestasis 357 Sinusoidal membrane Uptake of bile acids by the sinusoidal membrane is a key step in the enterohepatic circulation of bile salts in which approx- imately 95% of the bile salt pool is efficiently recovered by the intestine and resecreted by the liver [8]. Initial characteriza- tion of bile acid and bile salt uptake in isolated hepatocytes and enriched sinusoidal plasma membrane vesicles identi- fied both sodium dependent and independent transport sys- tems for bile salts. In addition, hydrophobic secondary bile acids or unconjugated bile acids may also enter into the hepa- tocytes by passive diffusion. Using an expression cloning strategy, a specific sodium-dependent bile salt transporter has been identifi ed as well as sodium-independent organic anion transporters, some of which can also mediate sodium- independent bile acid uptake. Sodium-dependent bile acid transporter The sodium-dependent taurocholate carrier protein, Ntcp, is a 55-kD glycoprotein composed of 362 amino acids in the rat which strictly mediates sodium-dependent bile salt transport favoring taurine-conjugated, trihydroxy-bile acids [9]. This protein is exclusively expressed in the hepatocyte at the sinu- soidal domain throughout the hepatic acinus. Like other so- dium cotransporters, the concentrative uptake of bile salts is thermodynamically favored by coupling to a signifi cant out- side-to-inside sodium gradient maintained by the activity of Na + , K + ATPase, assuring efficient uptake of bile salts at the sinusoidal surface. Two sodium ions are transported for each molecule of bile salt. Activity of the ntcp transporter is regu- lated by both its levels of expression at the sinusoidal surface and by gene transcription, which is an active area of investi- gation [3,4,10]. Increased cAMP rapidly increases the con- tent of ntcp at the sinusoidal membrane by mobilizing an intracellular pool of transporters, the translocation being dependent on microtubule and microfilament activity [11]. The ntcp expression is rapidly lost in primary cultured hepa- tocytes and all tumor-derived liver cell lines, suggesting tight regulation of this transporter, but it has been identified in human hepatocellular carcinoma samples. Elevated serum levels of bile salts are associated with decreased expression whereas increased expression of the transporter gene has been noted postpartum due to prolactin [12,13]. Increased Figure 22.1 Transport proteins of the basolateral (sinusoidal) and canalicular surface of human hepatocytes. Proteins involved in the hepatocellular uptake of organic anions include: Na + -taurocholate cotransporting polypeptide (NTCP) and microsomal epoxide hydrolase (mEH) (secondary active unidirectional transporters) and organic anion transporting polypeptides OATP1B1, OATP1B3, and OATP2B1 functioning as bidirectional antiporters. Multidrug resistance proteins (MRPs), bile salt export pump (BSEP), and multidrug resistant gene product (MDR) are unidirectional primary active transporters involved in the efflux of specific substrates listed with their respective transporters. The substrate for FIC 1 is unknown. Na + ,K + ATPase is responsible for maintaining the low sodium, high potassium within the cell. BS − = bile s a l t s, OA − = organic anions, PC = phosphatidylcholine, BDG = bilirubin diglucuronide, CHOL = cholesterol. 358 Section 3: Specific conditions bile salt fl ux does not regulate its expression in the rat [14]. Teleologically, decreased expression of ntcp protects the liver in cholestasis in the face of elevated serum bile salts by reduc- ing hepatic accumulation and potential toxicity of increased concentration of intrahepatic bile salts. Human NTCP is a 349 amino acid protein, which shares 77% sequence identity with the rat [15]. Of note, NTCP shares 50% sequence identi- ty with the sodium-dependent ileal bile acid transporter, re- ferred to as apical sodium dependent bile acid transporter (ASBT) which is expressed at the lumenal (apical) domain of the enterocyte [16,17]. ASBT has also been identified in the apical domain of large cholangiocytes, where it participates in cholehepatic shunting, and in renal tubules cells [18,19]. Microsomal epoxide hydrolase (mEH), a key enzyme in de- toxifi cation of reactive epoxides expressed at both the plasma membrane and endoplasmic reticulum in the rat, has also been implicated as a sodium dependent bile acid transporter as its expression in a cell line can confer sodium-dependent bile salt uptake [20]. mEH expression seems to favor glycine- conjugated bile salts. Organic anion and sodium-independent bile salt transport In addition to sodium-dependent bile salt transport, dihy- droxy-bile salts and unconjugated bile acids can enter hepa- tocytes via a sodium-independent transport mechanism mediated by members of the organic anion transporting pep- tides (OATP) superfamily. Members of this large transporter family are characterized by 12 membrane-spanning do- mains containing a distinctive, superfamily peptide signa- ture [21]. These transporters may share common substrates or be highly selective and are located in diverse tissues, in- cluding the blood–brain barrier, liver, lung, intestine, and kidneys. Members of subfamilies are now defined by their se- quence homology with proteins in humans, designated as OATPs with the corresponding SLCO gene symbol. As a class, OATPs function as organic anion exchangers promoting up- take of organic anions by exchanging them with effl ux of other anions such as bicarbonate and glutathione [22,23]. In human liver, three OATP family members are expressed on the sinusoidal domain [3,21]. OATP1B1 is only expressed in hepatocytes and facilitates the transport of organic anions bound to albumin, including bile salts and their conjugates and bilirubin, into the liver. The restricted site of expression of OATP1B1 in hepatocytes, coupled with its known poly- morphisms, are likely to have a major impact on the metabo- lism of pharmaceutical agents. OATP1B3 is also expressed in hepatocytes and shares 80% sequence identity with OATP1B1, but is expressed in multiple tissues as well as in cancer cell lines. In addition to organic anions, it can also transport small peptides such as cholecystokinin 8, which differentiates it from OATP1B1. OATP2B1 is the other family member expressed in the liver, as well as in spleen, placenta, and lungs. It has a narrower range of substrates as compared to OATP1B family members and favors transport of sulfobromophthalein (SBP) and estrone sulfate. Molecu- lar cloning of other oatp/OATP family members reveals a complex pattern of substrate specificity and distinct organ distribution in different species, suggesting that these trans- porters are involved with organ-specific uptake of specific organic anions [3,21]. Transcellular movement Little is known about the transcellular movement of the key constituents of bile [24,25]. Intracellular binding proteins with high affinity for bile acids, organic anions, fatty acids, and phosphatidylcholine have been identified, but their physiological function is still speculative [2,26]. These pro- teins are assumed to target their hydrophobic ligands to dif- ferent intracellular components of the cell, including their respective canalicular transporters. Vesicular-mediated transport of bile acids has also been implicated, based on in- creased transcellular movement of bile acids in response to cAMP and enhanced phospholipid excretion in response to infused bile acids. These effects may also be due to increased insertion of canalicular transporters into the membrane. Detailed studies with fluorescent bile acids have failed to demonstrate evidence for vesicular transport in isolated hepatocytes although these modified bile acids may not accurately reflect processing of bile salts [27]. Canalicular excretion Canalicular membrane transport is the rate-limiting step in the vectorial movement of biliary constituents from the sinu- soidal space into bile. Excretion of biliary components occurs across a relative concentration gradient of 100- to 1000-fold excess, requiring an active transport process. Prior biochem- ical characterization of transport activity in canalicular- enriched plasma membrane vesicles has recently been advanced by the molecular identifi cation of specific canalic- ular transporters. Detailed analysis of these individual trans- porters, coupled with loss of activities in rare human cholestatic syndromes and genetically engineered mice, has revealed their role in normal biliary physiology. To date, all the canalicular transporters involved with biliary excretion are members of the diverse ATP binding cassette (ABC) class of membrane transporters, in which transport of substrate is dependent on ATP hydrolysis [28,29]. These proteins share either a single or dual magnesium-dependent ATPase region contained within a six-membrane-spanning domain with substrate specificity dictated by other regions of the trans- porters. Major subclasses within this super gene family whose members play essential roles in biliary excretion in- clude the multi drug resistant (MDR) proteins, also known as P-glycoproteins, and the multidrug resistant proteins (MRP). Besides these ABC transporters with two ATP binding and catalysis domains, transporters with a single nucleotide binding domain also exist and function as key effl ux pumps. [...]... of classifications have been proposed including the Ohi classification [21], the one proposed by Kimura [22] (Table 23.1), and the one by Gautier and Eliot [23] The most common subset includes atresia of the gallbladder and all external bile ducts Variants include atresia of the proximal hepatic ducts with patency of the gallbladder and distal duct, or atresia of the distal ducts and patency of the gallbladder. .. nearer to the portal plate excluding the gallbladder, and the fundus of the gallbladder is approximated to the dissection at the hilum of the liver In this way, the gallbladder and bile duct act as the conduit through which bile may flow into the duodenum Theoretically, the presence of a functioning sphincter should prevent or reduce the incidence of cholangitis in the postoperative period Other modifications... In 199 9, the primary bile acids, chenodeoxycholate and cholate, were found to be the endogenous ligands for the previously identified FXR receptor [42,47] FXR is responsible for mediating bile- acid-induced down regulation of its own synthesis, by indirectly reducing the expression of CYP 7A1, the rate-limiting enzyme for the synthesis of primary bile acids in the neutral pathway [42,47] FXR induces the. .. regulating the de novo synthesis of primary bile acids from cholesterol, expression of bile salts, and organic anion transporters in both hepatocytes and enterocytes, and in the response of the liver to cholestatic injury Beside their essential roles in intestinal fat absorption and bile formation, synthesis of the primary bile acids has a major impact on global lipid metabolism and fatty acid synthesis in the. .. gallbladder and proximal common hepatic ducts The first task of the surgeon is to confirm the diagnosis by performing a cholangiogram A small right upper quadrant incision exposes the edge of the liver and the fundus of the gallbladder The fundus is opened and a small cannula inserted With fluoroscopy visualization, contrast is slowly injected into the gallbladder and monitored In cases of biliary atresia, the gallbladder. .. sutures The depth of the dissection at the portal plate should be deep enough to expose a thin layer of hepatocytes A 40-cm Roux-en-Y loop is then constructed and preferentially brought up to the liver behind the transverse colon An enterotomy at the end of the loop is created and approximated to the capsule of the liver posterior first with absorbable sutures The right and left hepatic arteries define the. .. absorb bile acids at their apical domain via the ileal sodium-dependent bile acid transporter J Clin Invest 199 7;100:2714–21 19 Christie DM, Dawson PA, Thevananther S, et al Comparative analysis of the ontogeny of a sodium-dependent bile acid transporter in rat kidney and ileum Am J Physiol 199 6;271: G377–G385 20 von Dippe P, Amoui M, Stellwagen RH, et al The functional expression of sodium-dependent bile. .. that the proximal ducts dilate in response to obstruction [32] The operative strategy required will depend on the age of the child, the size of the ducts, and the location of the perforation Isolated perforations in the common bile duct or common hepatic duct should be drained and observed The perforation may seal spontaneously, and bile flow resume in an unimpeded fashion In many instances, the bile. .. linked to the development of choledochal cysts The normal common channel between the two ducts is usually no longer than 1 cm A common channel of more than 1 cm between the ampulla of Vater and the junction of the pancreatic and biliary ducts has been linked to the development of type I cysts [ 49, 50] The pressure differential between the high pressure pancreatic duct and the lower pressure bile duct... the creation of a valve in the jejunal conduit to prevent reflux of intestinal contents into the bile ducts and theoretically to decrease the chances of cholangitis [24–27] There is no evidence to support this practice Other methods to prevent cholangitis and increase survival include the creation of a diverting stoma in the jejunal loop [28, 29] , which serves to collect bile, monitor the excretion of . and cytotoxic effects of free bile salts. A bile- salt- independent bile flow also exists in which effl ux of organic Diseases of the Gallbladder and Bile Ducts: Diagnosis and Treatment, Second. mediating bile- acid-induced down regulation of its own synthesis, by indirectly reducing the expression of CYP 7A1, the rate-limiting enzyme for the synthesis of primary bile acids in the neutral. as choles- tatic bile acids. Farsenoid X receptor (FXR) (NR1H4) In 199 9, the primary bile acids, chenodeoxycholate and cho- late, were found to be the endogenous ligands for the previ- ously identified