ORGANIC CATION TRANSPORTERS - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 20

to influence renal organic cation (TEA) secretion in vivo 52 ; however, appropriate ani-
mal models to study the in vivo role of OCTN1, MATE1, and MATE2 in renal organic
cation secretion do not yet exist.
In liver, the primary site of OCT1 expression, OCT1 was found to be expressed at
the sinusoidal (basolateral) membrane by immunohistochemistry and Western blot. 53
This suggests that OCT1 acts as a sinusoidal uptake transporter in liver, aiding in
the transfer of organic cations from blood into the hepatocyte for elimination by
metabolism or biliary excretion. In addition, for drugs that target the liver for their
pharmacological activity, OCT1 may be a limiting step in drug access to hepatocytes
and may therefore influence drug action.
It is important to note that species differences exist in the tissue-specific expres-
sion of OCTs. Most notably, whereas in human kidney OCT2 is by far the most
abundant OCT in terms of mRNA expression, rodent kidney expresses nearly similar
levels of OCT1 and OCT2. As with substrate-specificity differences, differences be-
tween species in terms of tissue distribution of OCTs are important to consider when
interpreting the results of studies performed in animal models.

2.5. REGULATION OF OCT EXPRESSION AND ACTIVITY

As mentioned previously, the large intracellular loop between transmembrane do-
mains 6 and 7 contains several predicted phosphorylation sites that are conserved
among the several OCT isoforms. These include target sequences for protein kinase
A (PKA), protein kinase C (PKC), protein kinase G (PKG), and tyrosine kinase. There
exists ample evidence for the modulation of activity of the OCTs by activation of these
kinases, presumably through phosphorylation of the OCTs at the protein level. Several
N-glycosylation sites in the large extracellular loop between transmembrane domains
1 and 2 are also potential sites of protein-level regulation. Additionally, hormonal
regulation, which is thought to reflect effects on transcription, has been described for
OCT2. Regulation of OCT expression is also modulated during development and in
certain pathophysiological states. For an extensive review of the regulation of organic
cation transport, see ref. 54. Current knowledge of the regulation of the OCTs will be
discussed individually for each isoform.

2.5.1. OCT1

Early studies of rat OCT1 showed that stimulation of either PKA or PKC results in a
significant increase in organic cation transport, as measured by uptake of the fluores-
cent compound 4-[4-(dimethylamino)-styryl]- N -methylpyridinium (ASP + ) in HEK-
293 cells stably transfected with rOCT1 55 PKC stimulation also results in increased
affinity for TEA, TPA, and quinine. The effect of PKC stimulation on rOCT1 activity
is thought to occur by phosphorylation of the rOCT1 protein, resulting in a con-
formational change that enhances substrate binding. rOCT1 activity is significantly
reduced by treatment with aminogenistein, a specific inhibitor of p56 lck tyrosine