DRUG TRANSPORT IN THE LIVER - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 370

microtubule-associated vesicles together with Bsep and polymeric immunoglobulin
A receptor and undergoes vesicular transcytosis. 52 Mrp2 also undergoes trafficking
to the canalicular membrane in a microtubule-dependent manner upon stimulation by
cAMP. 53
Osmolarity plays a role in the localization of transport proteins. In perfused rat
livers, hypo-osmotic solutions increased taurocholate excretion in bile due to the
increased translocation of Bsep to the canalicular membrane. In contrast, a hyper-
osmotic environment caused a decrease in biliary excretion of taurocholate and the
endocytic retrieval of Bsep. 54 Under hyper-osomotic conditions, Mrp2 resides intra-
cellularly, whereas under hypo-osmotic conditions Mrp2 translocates to the plasma
membrane. 55 Lipopolysaccharide exposure also resulted in the retrieval of Mrp2 from
the apical membrane. 56
The role of the N-terminal domain in MRP2 trafficking has been examined. 57 The
specific apical localization of MRP2 is due to a sequence in the C-terminal tail not
present in basolaterally targeted MRP1. 58 Deletion of three amino acids from the C-
terminus of MRP2 results in localization predominantly at the basolateral membrane
in MDCK cells. In sandwich-cultured rat hepatocytes, glycosylation plays a role in
directing Mrp2 to the canalicular domain. 19
13.4.2. Basolateral Proteins
Less is known about the intracellular trafficking of basolateral transport proteins. It is
probable that transport proteins traffic via microtubule-dependent intracellular path-
ways to the basolateral domain. The PDZ consensus binding site at the C-terminal
domain of OATP proteins plays a role in basolateral membrane localization of this
family of proteins; oligomerization of rat Oatp1a1 with PDZK1 allows for proper
expression of this isoform at the basolateral membrane surface. In PDZK1 knockout
mice, Oatp1a1 protein expression was unaltered, but the protein was localized predom-
inantly in intracellular structures. 59 Another important sequence structure common
to all OATPs is a large extracellular loop between transmembrane domains IX and X,
with 10 conserved cysteines; this region of the protein plays a role in disulfide linkages
and glycosylation which is important for the trafficking and function of OATPs. The
absence or substitution of these cysteine residues in OATP2B1 expressed in CHO-K1
cells resulted in a decrease in membrane expression and function. 60 Glycosylation ap-
pears to be involved in the targeting of OAT to the basolateral membrane. A common
structural feature that is shared among all OATs is a consensus site for N-linked gly-
cosylation in the first extracellular loop within the current secondary structure model.
OAT1 remained intracellularly localized following treatment with tunicamycin, an
antibiotic that inhibits N-glycosylation. 303
The phosphorylation state of Ntcp determines whether this protein is local-
ized on the basolateral membrane or intracellularly in the endosomal compartment.
cAMP treatment causes Ntcp to translocate from the endosomes to the basolateral
membrane. 2 Translocation induced by cAMP involves dephosphorylation of Ntcp
by protein phosphatase 2B, which leads to increased retention of the protein in the
basolateral membrane. 61 , 62 cAMP-induced trafficking of Ntcp relies on the phospho-
inositide 3-kinase(PI3K)/protein kinase B pathway and intact microfilaments. 63 − 65