ORGANIC ANION–TRANSPORTING POLYPEPTIDES - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 82

of OATPs also support a role for N-glycosylation in regulating transport function.
For example, OATP1B1 variants found on the cell surface in a mammalian het-
erologous expression system were mainly glycosylated proteins, whereas those con-
fined intracellularly were underglycosylated. 38 Moreover, the degree of expression
of glycosylated plasma membrane-localized OATP1B1 mirrored transport activity.
In human liver, glycosylated and unglycosylated OATP1B1 are present 16 and there
exists significant variability in the expression of both forms among individuals, 39
but the functional significance of these findings is unclear. A polymorphic variant of
OATP1A2 ( ∗ 6) results in an asparagine-to-isoleucine transversion in amino acid 135 of
the second extracellular loop. 13 Consistent with an important role of N-glycosylation,
the OATP1A2 ∗ 6 variant is poorly glycosylated, retained intracellularly, and exhibits
significantly reduced transport function in vitro. It is also interesting to note that
the glycosylation state of OATP1A2 appears to be tissue dependent since the trans-
porter has a larger apparent molecular mass in liver than in brain capillary endothelial
cells. 40 , 41 Finally, investigations with OATP2B1 showed that directed mutations of
cysteine residues in extracellular loop IX-X was associated with reduced cell surface
expression and transport function, due perhaps to incomplete glycosylation. 42
Adapter Protein Interactions With the exception of OATP2A1, which resides largely
in intracellular spaces, the OATPs are localized on plasma membranes. Members
of the PDZ domain-containing proteins have been shown to regulate plasma mem-
brane sorting as well as to modulate transport function by direct interactions with
C-terminal amino acids of various solute transporters. 43 Several human OATPs, in-
cluding OATP1A2 and OATP2B1, contain potential PDZ consensus sequences. 14
Indeed, protein interaction studies have demonstrated that OATP1A2, OATP3A1,
and OATP1C1 bind directly to members of the PDZ proteins, including PDZK1,
IKEPP, NHERF1, and NHERF2. 44 A convincing role for PDZ proteins on OATP
functions was demonstrated elegantly in the cellular distribution and transport func-
tion of Oatp1a1 in PDZK1 knockout mice. 14 Although such mice expressed signifi-
cant amounts of hepatic Oatp1a1, the transporter was largely localized intracellularly.
This lack of functional membrane transporter expression translated in vivo to reduced
solute (bromosulfophthalein) clearance after intravenous administration. It remains
to be determined what role PDZ proteins play in the regulation of human OATP
transporter expression and function at both the molecular and physiological levels.
Furthermore, it would be of interest to understand whether other protein interactions
are required for cell surface sorting of OATPs, particularly for those transporters that
do not possess consensus PDZ domains.
5.3.3. Transcriptional Regulation
Basal and adaptive expression of OATPs in various tissues is controlled, in part, by
transcriptional mechanisms. At present, much of what is known about transcriptional
regulation of OATPs arises from work aimed to understand transporter expression in
liver and hence focused on the roles of hepatic transcription factors.