Although many organic anion transport protein (Oatp) family members have PDZ consensus binding sites at their C termini, the functional significance is unknown. In the present study, we utilized rat Oatp1a1 (NM_017111) as a prototypical member of this family to examine the mechanism governing its subcellular trafficking. A peptide corresponding to the C-terminal 16 amino acids of rat Oatp1a1 was used to affinity-isolate interacting proteins from rat liver cytosol. Protein mass fingerprinting identified PDZK1 as the major interacting protein. This was confirmed by immunoprecipitation of an Oatp1a1-PDZK1 complex from cotransfected 293T cells as well as from native rat liver membrane extracts. Oatp1a1 bound predominantly to the first and third PDZ binding domains of PDZK1, whereas the high density lipoprotein receptor, scavenger receptor B type I binds to the first domain. Although it is possible that PDZK1 forms a complex with these two integral membrane proteins, this did not occur, suggesting that as yet undescribed factors lead to selectivity in the interaction of these protein ligands with PDZK1. Oatp1a1 protein expression was near normal in PDZK1 knock-out mouse liver. However, it was located predominantly in intracellular structures, in contrast to its normal basolateral plasma membrane distribution. Plasma disappearance of the Oatp1a1 ligand [35S]sulfobromophthalein was correspondingly delayed in knock-out mice. These studies show a critical role for oligomerization of Oatp1a1 with PDZK1 for its proper subcellular localization and function. Because its ability to transport substances into the cell requires surface expression, this must be considered in any assessment of physiologic function.

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.

Bilirubin is a yellow pigment derived from the degradation of heme. Because of intramolecular hydrogen bonding, it is water insoluble (1). Following formation in various cells throughout the body, it is released into the circulation where it binds avidly to albumin (2, 3). Under normal circumstances, it is extracted from albumin and taken up rapidly by hepatocytes (4, 5). Although the kinetics of uptake suggests carrier mediation, the nature of this carrier has remained elusive (6, 7). The fact that bilirubin can pass rapidly through lipid bilayers has led some investigators to question the necessity of postulating the existence of a carrier (8, 9). A recent report (10) indicates that the human organic anion transport protein SLC21A6 (also known as OATP2, 1 LST-1, and OATP-C) mediates high affinity transport of bilirubin. That study was performed using HEK293 cells that had been stably transfected with OATP2. Cells were harvested from poly-D-lysine-coated plastic dishes, and bilirubin uptake studies were performed on cells in suspension. Nonspecific binding was not reported although it was used to correct uptake data, and we were concerned that the harvesting procedure could result in cell permeabilization. To validate the results in that report, we devised a system in which we could test the ability of OATP2-transfected cells to transport bilirubin without the necessity of detaching the cells from the culture dishes. Initially, we established an assay to quantify bilirubin transport in overnight cultured rat hepatocytes. This assay was then used to examine bilirubin transport by HeLa cells that had been stably transfected with OATP2 under the regulation of a metallothionein promoter. OATP2 expression was induced by incubation of these cells in zinc for 48 h. Uninduced cells did not express OATP2 and were used as controls. These studies showed that cultured rat hepatocytes transported bilirubin with kinetics virtually identical to transport of sulfobromophthalein (BSP). In contrast, OATP2-expressing HeLa cells transported BSP but not bilirubin. In further studies, OATP2-transfected HEK293 cells that were obtained from Cui et al. (10) were also studied using the methodology that they described. We were unable to reproduce their results with respect to bilirubin transport, although these cells transported BSP as they described. The existence of a bilirubin transporter has been an important field of study for many years. The current studies using a newly established assay for bilirubin transport indicate that a role for OATP2 in hepatocyte bilirubin transport is unlikely. EXPERIMENTAL PROCEDURES Preparation of Radiolabeled Ligands

Organic anion transport protein 1a1 (oatp1a1), a prototypical member of the oatp family of highly homologous transport proteins, is expressed on the basolateral (sinusoidal) surface of rat hepatocytes. The organization of oatp1a1 within the plasma membrane has not been well defined, and computer-based models have predicted possible 12- as well as 10-transmembrane domain structures. Which of oatp1a1's four potential N-linked glycosylation sites are actually glycosylated and their influence on transport function have not been investigated in a mammalian system. In the present study, topology of oatp1a1 in the rat hepatocyte plasma membrane was examined by immunofluorescence analysis using an epitope-specific antibody designed to differentiate a 10- from a 12-transmembrane domain model. To map glycosylation sites, the asparagines at the each of the four N-linked glycosylation consensus sites were mutagenized to glutamines. Mutagenized oatp1a1 constructs were expressed in HeLa cells, and effects on protein expression and transport activity were assessed. These studies revealed that oatp1a1 is a 12-transmembrane-domain protein in which the second and fifth extracellular loops are glycosylated at asparagines 124, 135, and 492, whereas the potential glycosylation site at asparagine 62 is not utilized, consistent with its position in a transmembrane domain. Constructs in which more than one glycosylation site were eliminated had reduced transport activity but not necessarily reduced transporter expression. This was in accord with the finding that fully unglycosylated oatp1a1 was well expressed but located intracellularly with limited transport ability as a consequence of its reduced cell surface expression.

The roles of vascular binding, flow, transporters, and enzymes as determinants of the clearance of digoxin were examined in the rat liver. Digoxin is metabolized by Cyp3a and utilizes the organic anion transporting polypeptide 2 (Oatp2) and P-glycoprotein (Pgp) for influx and excretion, respectively. Uptake of digoxin was found to be similar among rat periportal (PP) and perivenous (PV) hepatocytes isolated by the digitonin-collagenase method. The K m values for uptake were 180 Ϯ 112 and 390 Ϯ 406 nM, V max values were 13 Ϯ 8 and 18 Ϯ 4.9 pmol/min/mg protein, and nonsaturable components were 9.2 Ϯ 1.3 and 10.7 Ϯ 2.5 l/min/mg for PP and PV, respectively. The evenness of distribution of Oatp2 and Pgp was confirmed by Western blotting and confocal immunofluorescent microscopy. When digoxin was recirculated to the rat liver preparation in Krebs-Henseleit bicarbonate (KHB) for 3 h in absence or presence of 1% bovine serum albumin (BSA) and 20% red blood cell (rbc) at flow rates of 40 and 10 ml/min, respectively, biexponential decays were observed. Fitted results based on compartmental analyses revealed a higher clearance (0.244 Ϯ 0.082 ml/min/g) for KHB-perfused livers over the rbc-albuminperfused livers (0.114 Ϯ 0.057 ml/min/g) (P Ͻ 0.05). We further found that binding of digoxin to 1% BSA was modest (unbound fraction ϭ 0.64), whereas binding to rbc was associated with slow on (0.468 Ϯ 0.021 min Ϫ1) and off (1.81 Ϯ 0.12 min Ϫ1 ) rate constants. We then used a zonal, physiologically based pharmacokinetic model to show that the difference in digoxin clearance was attributed to binding to BSA and rbc and not to the difference in flow rate and that clearance was unaffected by transporter or enzyme heterogeneity.Digoxin, an important cardiotonic drug of narrow therapeutic index, is cleared by both the kidney and liver, and exhibits a long half-life in vivo due to the large volume of distribution (Kramer et al., 1974;Rodin and Johnson, 1988). In the rat liver, digoxin is primarily metabolized by cytochrome P450 3a to the didigitoxoside and monodigitoxoside as well as digitoxigenin (Harrison and Gibaldi, 1976; Sal- ABBREVIATIONS:Pgp, P-glycoprotein; Oatp, rat organic anion transporting polypeptide; OAT, organic anion transporter; PP, periportal; PV, perivenous; rbc, red blood cell; KHB, Krebs-Henseleit bicarbonate buffer; BSA, bovine serum albumin; PBPK, physiologically based pharmacokinetic; Dg3, unlabeled digoxin; Dg2, digitoxigenin bis-digitoxoside; Dg1, digitoxigenin monodigitoxoside; Dg0, digitoxigenin; HPLC, high-performance liquid chromatography; HRP, horseradish peroxidase; Hct, hematocrit; AUC, area under the curve; A 2 , Dg3 amount in peripheral compartment; A bile and A met , amounts of Dg3 excreted into bile and metabolized, respectively; C 1 , Dg3 concentration in central compartment (compartmental modeling); C R (or Dg3 R ), Dg3 concentration in the reservoir; C rbc,R (or Dg3 rbc,R ), Dg3 concentration in rbc in the reservoir; C p,R (or Dg3 p,R ), Dg3 concentration in plasma, in the reservoir; C rbci...