Induction of a dose-related increase in sulfobromophthalein uptake velocity in freshly isolated rat hepatocytes by phenobarbital

Organic anion transporting polypeptides: Pharmacology, toxicology, structure, and transport mechanisms

Organic anion transporting polypeptides (OATPs) are membrane proteins that mediate the uptake of a wide range of substrates across the plasma membrane of various cells and tissues. They are classified into 6 subfamilies, OATP1 through OATP6. Humans contain 12 OATPs encoded by 11 solute carrier of organic anion transporting polypeptide (SLCO) genes: OATP1A2, OATP1B1, OATP1B3, the splice variant OATP1B3-1B7, OATP1C1, OATP2A1, OATP2B1, OATP3A1, OATP4A1, OATP4C1, OATP5A1, and OATP6A1. Most of these proteins are expressed in epithelial cells, where they mediate the uptake of structurally unrelated organic anions, cations, and even neutral compounds into the cytoplasm. The best-characterized members are OATP1B1 and OATP1B3, which have an important role in drug metabolism by mediating drug uptake into the liver and are involved in drug-drug interactions. In this review, we aimed to (1) provide a historical perspective on the identification of OATPs and their nomenclature and discuss their phylogenic relationships and molecular characteristics; (2) review the current knowledge of the broad substrate specificity and their role in drug disposition and drug-drug interactions, with a special emphasis on human hepatic OATPs; (3) summarize the different experimental systems that are used to study the function of OATPs and discuss their advantages and disadvantages; (4) review the available experimental 3-dimensional structures and examine how they can help elucidate the transport mechanisms of OATPs; and (5) finally, summarize the current knowledge of the regulation of OATP expression, discuss clinically important single-nucleotide polymorphisms, and outline challenges of physiologically based pharmacokinetic modeling and in vitro to in vivo extrapolation. SIGNIFICANCE STATEMENT: Organic anion transporting polypeptides (OATPs) are a family of 12 uptake transporters in the solute carrier superfamily. Several members, particularly the liver-expressed OATP1B1 and OATP1B3, are important drug transporters. They mediate the uptake of several endobiotics and xenobiotics, including statins and numerous other drugs, into hepatocytes, and their inhibition by other drugs or reduced expression due to single-nucleotide polymorphisms can lead to adverse drug effects. Their recently solved 3-dimensional structures should help to elucidate their transport mechanisms and broad substrate specificities.

Conjugation metabolism of acetaminophen and bilirubin in extrahepatic tissues of rats

An anhepatic rat model was used to explore the extrahepatic conjugating metabolism of acetaminophen and serum bilirubin. The recovery of glucuronide- and sulfate-acetaminophen was 47.5% in normal control and 13.4% in model rats in the urine collected for 6 h after administration of acetaminophen 20 mg kg(-1). Following the increase of acetaminophen dose to 150 mg kg(-1), the recovery of urinary glucuronide-acetaminophen increased by 53.9% in normal control; but it decreased by 36.4% in model rats. In contrast to normal control, the pretreatment with phenobarbital did not affect acetaminophen and its metabolite levels in plasma and urine in model rats. After the establishment of anhepatic model the serum direct bilirubin rose dramatically. Urinary bilirubin test was positive in model rats, but not in normal control. No changes were observed in serum total bilirubin and ratio of direct/total bilirubin after the pretreatment with ranitidine or phenobarbital 50 mg kg (-1), i.p. for 5 days in model rats. The results indicate that the glucuronide- and sulfate-acetaminophen formed in the extrahepatic tissues of model rats is 28.2% of normal control, serum free bilirubin can be transformed into conjugated bilirubin in extrahepatic tissues, and the regulation mechanism of phase II conjugating enzymes is different between the hepatic and extrahepatic tissues.

Differential regulation of multidrug resistance-associated protein 2 (MRP2) and cytochromes P450 2B1/2 and 3A1/2 in phenobarbital-treated hepatocytes

Multidrug resistance-associated protein 2 (MRP2) is a drug efflux pump found at the biliary pole of hepatocytes. In the present study, we have investigated its expression in response to phenobarbital, a liver tumor promoter known to up-regulate hepatic cytochromes P450 (CYPs), such as CYP2B1/2 and CYP3A1/2. MRP2 mRNA and protein levels were found to be markedly increased in both primary rat and human hepatocytes exposed to phenobarbital. However, features of this up-regulation, especially the dose-response, were different from those of the induction of CYP2B1/2 and CYP3A1/2. In addition, hepatic MRP2 expression remained unaltered in rats treated by phenobarbital that, by contrast, increased CYP2B1/2 and CYP3A1/2 gene expression in the liver. Therefore, MRP2 and CYPs appeared differently regulated in response to phenobarbital in both in vivo and in vitro situations, suggesting that cellular and molecular mechanisms underlying up-regulation of MRP2 are, at least in part, unrelated to those operating for CYPs. Phenobarbital-related MRP2 induction in primary rat hepatocytes was associated with some phenotypic effects of the barbiturate, such as prolonged cell survival and inhibition of cell proliferation. Phenobarbital also inhibited growth of human hepatoma HepG(2) cells and increased their level of MRP2 gene expression. Such results may favor a putative relationship between phenobarbital-mediated MRP2 regulation in cultured liver parenchymal cells and alteration of cell cycle and survival.

Effect of phenobarbital on the expression of bile salt and organic anion transporters of rat liver

BACKGROUND/AIMS

The hepatic clearance of drugs and cholephilic organic anions is stimulated by phenobarbital (PB). Our aim was to analyze the effects of PB on the expression of hepatocellular bile salt and organic anion transporters.

METHODS

Male Sprague-Dawley rats were treated intraperitoneally with PB (80 mg/kg/d) or saline for 5 days. Transporter expression was quantified by northern and western blot analysis and initial uptake rates of bromosulphophthalein (BSP) and digoxin were measured in isolated hepatocytes.

RESULTS

Compared to control rats, PB treatment increased expression of the organic anion transporting polypeptide 2 (Oatp2; Slc21aS) more than 2-fold on the RNA (P < 0.05) and protein (P < 0.001) levels. Expression of Oatpl (Slc21al), Oatp4 (Slc21a6) and the Na+-taurocholate cotransporting polypeptide (Ntcp; Slc10a1) was unaltered. At the canalicular pole, expression of the bile salt export pump (Bsep; ABCB11) and of the multidrug resistance proteins 2 (Mrp2; ABCC2) and 6 (Mrp6; ABCC6) was not significantly changed. Whereas hepatocellular BSP uptake was unaffected by PB, digoxin uptake was stimulated 4-fold.

CONCLUSIONS

The induction of digoxin uptake by PB correlates with Oatp2 expression. In contrast, the lack of increase of Oatpl and Oatp4 expression is in accordance with unchanged BSP uptake. These data challenge the previously held view that PB induces hepatocellular BSP uptake systems.

Characterization of inducible nature of MRP3 in rat liver

We found previously that expression of multidrug resistance-associated protein (MRP) 3 is induced in a mutant rat strain (Eisai hyperbilirubinemic rats) whose canalicular multispecific organic anion transporter (cMOAT/MRP2) function is hereditarily defective and in normal Sprague-Dawley (SD) rats after ligation of the common bile duct. In the present study, the inducible nature of MRP3 was examined, using Northern and Western blot analyses, in comparison with that of other secondary active [Na(+)-taurocholic acid cotransporting polypeptide (Ntcp), organic anion transporting polypeptide 1 (oatp1), and organic cation transporter (OCT1)] and primary active [P-glycoprotein (P-gp), cMOAT/MRP2, and MRP6] transporters. alpha-Naphthylisothiocyanate treatment and common bile duct ligation induced expression of P-gp and MRP3, whereas expression of Ntcp, oatp1, and OCT1 was reduced by the same treatment. Although expression of MRP3 was also induced by administration of phenobarbital, that of cMOAT/MRP2, MRP1, and MRP6 was not affected by any of these treatments. Moreover, the mRNA level of MRP3, but not that of P-gp, was increased in SD rats after administration of bilirubin and in Gunn rats whose hepatic bilirubin concentration is elevated because of a defect in the expression of UDP-glucuronosyl transferase. However, the MRP3 protein level was not affected by bilirubin administration. Although the increased MRP3 mRNA level was associated with the increased concentration of bilirubin and/or its glucuronides in mutant rats and in SD rats that had undergone common bile duct ligation or alpha-naphthylisothiocyanate treatment, we must assume that factor(s) other than these physiological substances are also involved in the increased protein level of MRP3.

High affinity uptake by isolated rat hepatocytes of a linear pseudo-hexapeptide, ditekiren

The hepatic elimination of many oligopeptides is both rapid and extensive, and often limits their potential as therapeutic agents. The linear, hydrophobic pseudo-hexapeptide ditekiren, a renin inhibitor, is one such example. The mechanism(s) involved in its hepatic clearance are largely unknown; accordingly, the characteristics of ditekiren's transport into isolated rat hepatocytes was investigated. In addition to a concentration-independent, linear process, uptake also involved a carrier-mediated component (Km = 0.2 +/- 0.05 microM; Vmax = 11.6 +/- 0.6 pmol (mg protein)[-1] min[-1]). Phenobarbital pretreatment in vivo resulted in marked induction of such transport. Negative results from cis-inhibition studies with substrates and/or inhibitors of well-established hepatic transport systems, e.g., sodium-dependent bile acid, sodium-independent multispecific bile acid and cation carriers, ruled out their involvement in ditekiren's uptake. By contrast, a number of cyclic and linear oligopeptides inhibited the uptake process to varying extents and in the case of EMD-59121, the most inhibitory compound, the interaction was competitive in nature. Collectively, these data suggest the presence of a novel high affinity, low capacity transporter in rat hepatocytes with specific affinity for ditekiren and possibly other oligopeptides.