Hepatocyte canalicular membrane

Muller, M., Mayer, R., Hero, U., Keppler, D., ATP-dependent transport of amphiphilic cations across the hepatocyte canalicular membrane mediated by mdrl P-glycoprotein, FEBS Lett. 1994, 343, 168-172. 

Breast Cancer Resistance Protein (BCRP, also known as MXR or ABCP), first cloned from mitoxantrone and anthracycline-resistant breast and colon cancer cells [188, 189] is a half-transporter efflux pump believed to function as a homo-or hetero-dimer. Following its identification, BCRP-mediated drug resistance was observed for topoisomerase inhibitors including camptothecins [190, 191] and in-dolocarbazoles [192]. In normal tissues, BCRP was detected in placental syncytio-trophoblasts, hepatocyte canalicular membrane, apical intestinal epithelia and vascular endothelial cells [193]. These findings support the important role BCRP plays in modulating topotecan bioavailability, fetal exposure and hepatic elimination [194]. Considering that the substrates and tissue distributions for BCRP overlap somewhat with MDR1 and MRPs [195], additional studies will be required to define the relative contribution of each of these transporters in the overall and tis-... 

The solubility of cholesterol in bile is determined by the relative proportions of bile acids, lecithin, and cholesterol. Although prolonged ursodiol therapy expands the bile acid pool, this does not appear to be the principal mechanism of action for dissolution of gallstones. Ursodiol decreases the cholesterol content of bile by reducing hepatic cholesterol secretion. Ursodiol also appears to stabilize hepatocyte canalicular membranes, possibly through a reduction in the concentration of other endogenous bile acids or through inhibition of immune-mediated hepatocyte destruction. 

The preparation of plasma membrane vesicles from liver canalicular membrane is highly enriched with the canalicular (apical) isoform MRP2 (Buchler et al. 1996). Methods for the isolation of hepatocyte canalicular membranes from liver tissue have been described in detail (Bohme et al. 1994 and Boyer and Meier et al. 1990). The percentage of inside-out-oriented vesicles in these preparations amounts to 32%. Alternatively, transfected I ILK and MDCK cells are often used to study ATP-dependent transport into inside-out vesicles (Cui et al. 1999 Leier et al. 2000). 

Bohme M, Buchler M, Muller M, Keppler D (1993) Differential inhibition by cyclosporins of primary-active ATP-dependent transporters in the hepatocyte canalicular membrane. FEBS Lett 333 193-196... 

Buchler M, Bohme M, Ortlepp H, Keppler D (1994) Functional reconstitution of ATP-dependent transporters from the solubilized hepatocyte canalicular membrane. Eur J Biochem 224 345-352... 

The liver plays an important role in determining the oral bioavailability of drags. Drag molecules absorbed into the portal vein are taken up by hepatocytes, and then metabolized and/or excreted into the bile. For hydrophilic drugs, transporters located on the sinusoidal membrane are responsible for the hepatic uptake [1, 2]. Biliary excretion of many drags is also mediated by the primary active transporters, referred to as ATP-binding cassette transmembrane (ABC) transporters, located on the bile canalicular membrane [1, 3-5], Recently, many molecular biological... 

MDCK II cells (Fig. 12.3) [93], Kinetic analysis revealed that the Km value for transcellular transport (24 pM) was similar to the Km for OATP2 (34 pM) [93], Moreover, the efflux across the bile canalicular membrane was not saturated under these experimental conditions. These in vitro observations are consistent with in vivo experimental results in rats which showed that the rate-determining process for the biliary excretion of pravastatin is uptake across the sinusoidal membrane. By normalizing the expression level between the double transfectant and human hepatocytes, it might be possible to predict in vivo hepatobiliary excretion. 

Figure 15.2 Transport proteins involved in the intestinal absorption and the renal and hepatic excretion of drugs. In the intestine, drugs are taken up from the luminal side into enterocytes before the subsequent elimination into blood. In hepatocytes, drugs are taken up from the blood over the basolateral membrane and excreted over the canalicular membrane into bile. In the renal epithelium, drugs undergo secretion (drugs are taken up from the blood and excreted into the urine) or reabsorption (drugs are taken up from the urine and are excreted back into blood). Uptake transporters belonging to the SLC transporter superfamily are shown in red and export pumps...

The phosphatidylcholine in bile is synthesised in the endoplasmic reticulum of the hepatocyte and must be transported to the canalicular membrane. One possibility involves the nonspecific phosphatidylcholine transfer protein but a mouse null for this protein did not show reduced phosphatidylcholine secretion into bile and there was no compensatory increase in other phospholipids transfer proteins. However, the plasma membrane would receive a ready supply of phospholipid by insertion of vesicles, and the MDR3 protein translocates this molecule from the inner leafiet to the outer surface where there is contact with bile acids, as suggested by Smit and colleagues. The role of this transporter is shown in Figure 2.2. 

Hepatocytes make up 60-70% of the total number of liver cells. They have a well-organized intracellular structure with huge numbers of cell organelles to maintain the high metabolic profile. At the apical side or canalicular membrane the cell is specialized for the secretion of bile components. There are several ATP-dependent transport carriers located on this side of the membrane, which transport bile salts, lipids and xenobiotics into the canaliculus. On the sinusoidal side, the cells specialize in uptake and secretion of a wide variety of components. To increase the surface of the membrane for this exchange with the bloodstream, the sinusoidal domain of the membrane is equipped with irregular microvilli. The microvilli are embedded into the fluid and matrix components of the space of Disse and are in close contact with the sinusoidal blood because of the discontinuous and fenestrated SECs. To facilitate its metabolic functions numerous membrane transport mechanisms and receptors are situated in the membrane. 

Many drugs that are taken up and metabolized by hepatocytes are excreted via the bile canahcuh into the bile. One of the remaining topics in liver slice research is the question of whether liver shces are capable of bile excretion via the bile canaliculus. Thompson et aZ.[93] showed that shces are capable of excreting bile acids, however, there is a need for more experiments to determine whether this excretion takes place across the bile canalicular membrane. 

BCRP (ABCG2) is similar to pGp but has only one ATP-binding domain. It is found in stem cells, where it may function to protect them against toxicants, and also in placenta and the canalicular membrane in hepatocytes. It is upregulated by low oxygen levels. 

Once mobilized in the hepatocyte, chemicals can contact and interact with biotransformation enzymes (Chapter 7). These enzymes generally increase the polarity of the chemical, thus reducing its ability to passively diffuse across the sinusoidal membrane back into the blood. Bio transformation reactions also typically render the xenobiotics susceptible to active transport across the canalicular membrane into the bile canaliculus and, ultimately, the bile duct (Figure 10.3). The bile duct delivers the chemicals, along with other constituents of bile, to the gall bladder that excretes the bile into the intestines for fecal elimination. 

Parenchymal cells (PC), or hepatocytes, originate from epithelial cells and represent most of the total number of liver cells (65%). Because of their relatively large size, hepatocytes are microscopically clearly visible after staining liver sections with hematoxylin and eosin (HE). Also, the hepatocytes store glycogen, which can be identified histochemically with periodic acid-Schiff (PAS) reagent. The hepatocytes are primarily responsible for the uptake of endogenous products and xenobiotics at the sinusoidal membrane of the cell and their subsequent metabolism and excretion into bile by means of the canalicular membrane. 

BSEP also known as sister-P-glycoprotein (SPGP) was originally cloned from pig liver (185). BSEP is localized on the canalicular membrane of hepa-tocytes and is responsible for the secretion of bile salts across the canalicular membrane into bile. BSEP appears to be the predominant bile salt efflux system for hepatocytes, and is a critical component in the enterohepatic circulation of bile acids. A number of mutations in the transporter were found to the basis for progressive familial intrahepatic cholestasis type 2 (PFIC2) (186-188). Mutations found in PFIC2 patients include frameshifts, missense mutations, and premature termination codons. Most PFIC2 patients lack immunohistochemically detectable BSEP in their liver. Recently, seven... 

BCRP is classified in ABCG subfamily other members of this subfamily are involved in sterol transport (269). Unlike P-gp and MRPs, BCRP consists of a single ABC cassette in the amino terminal followed by six putative transmembrane domains however, it forms a homodimer linked by a disulfide bond in the plasma membrane (270,271). Initially, ABCG2 was identified as an mRNA expressed in placenta (272) and as a non-MDRl- and non-MRP-type resistance factor from cell lines selected in the presence of anthracy-clines and mitoxantrone (273). BCRP is expressed widely in the normal tissues (274) and localized on the canalicular membrane of the hepatocytes and apical membranes of epithelial cells (274,275) and brain capillary endothelial cells (276,277). 

Figure 22-3. Transport and hepatic metabolism of bilirubin. Bilirubin that is produced in phagocytes is transported to liver as an albumin-bilirubin complex. Uptake into the hepatocytes takes place in liver sinusoids. Within the hepatocyte, bilirubin is transported to the endoplasmic reticulum (microsomes) bound to glutathione S-transferase (GST). Bilirubin is made water soluble by addition of one or two glucuronic acid moieties obtained from UPD-glucuronic acid, catalyzed by bilirubin-UDP-glucuronyltransferase. The product, conjugated bilirubin, is transported across the bile canalicular membrane for secretion into the biliary system, with subsequent movement into the intestines.

It has been suggested that multidrug resistance proteins (MRPs) play an important role in the transport and detoxification of a wide range of endogenous compounds and xenobiotics. They are predominantly expressed at the apical membrane of the small intestine, proximal tubules of the kidney and canalicular membrane of hepatocytes involved in intestinal, renal and hepatobiliary excretion of compounds. 

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