drug transporters || organic cation transporters

PGN PGNJWDD059-02 JWDD059-YOU June 13, 2007 7:25

2ORGANIC CATION TRANSPORTERS

Thomas J. Urban and Kathleen M. Giacomini

University of California San Francisco, San Francisco, California

2.1. Introduction

2.2. Structural–Functional Characteristics and Transport Mechanism

2.3. Substrate Selectivity

2.4. Tissue Distribution

2.5. Regulation of OCT Expression and Activity

2.5.1. OCT1

2.5.2. OCT2

2.5.3. OCT3

2.6. Animal Models

2.6.1. Oct1 Knockout Mice


2.6.3. Oct1/Oct2 Knockout Mice


2.7. Genetic Variation

2.7.1. OCT1

2.7.2. OCT2

2.7.3. OCT3

2.8. Conclusions

References

Drug Transporters: Molecular Characterization and Role in Drug Disposition, Edited byGuofeng You and Marilyn E. MorrisCopyright C© 2007 John Wiley & Sons, Inc.

11


12 ORGANIC CATION TRANSPORTERS

2.1. INTRODUCTION

Renal elimination, including both passive glomerular filtration and active tubularsecretion, is a major route of clearance for many drugs and drug metabolites. Activesecretion of drugs by the kidney takes place primarily in the renal proximal tubule,where drug transporters facilitate the flux of drug molecules from the blood to thetubular lumen for excretion. As early as 1947, it was recognized that transport systemsexist that aid in the elimination of a variety of structurally diverse organic cations,including drugs and toxic xenobiotics, as well as their metabolites.1 Prior to themolecular cloning of organic cation transporters, it had been shown that uptake of low-molecular-weight organic cations into the renal proximal tubule (the first step in activesecretion of organic cations) is facilitated by a polyspecific membrane potential–sensitive transport system. Efflux of organic cations into the tubular lumen, the secondstep in active tubular secretion, was shown to occur via an H+- or cation-exchangemechanism. Additionally, uptake of organic cations into other tissues, such as hepaticuptake, or transport across the blood–brain barrier, was known to display propertiesof protein-mediated transport across the plasma membrane (e.g., saturability, andsensitivity to temperature, membrane potential, and small-molecule inhibitors).

The first member of the organic cation transporter (OCT) family [rat OCT1(rOCT1)] was identified in rat by expression cloning from rat kidney.2 This was fol-lowed quickly by homology cloning of other OCT isoforms in rodents and human.3−13

Of the membrane transporters cloned to date, three have been identified as membersof the basolateral (membrane potential–driven) organic cation transporter system:OCT1 (SLC22A1), OCT2 (SLC22A2), and OCT3 (SLC22A3). All are members of theSLC22A family of solute carrier (SLC) transporters, which includes the novel organiccation transporters OCTN1 and OCTN2 (SLC22A4 and SLC22A5, discussed in de-tail in Chapter 3) and the organic anion transporters OAT1–4 (SLC22A6, SLC22A7,SLC22A8, and SLC22A11, discussed in detail in Chapter 4), as well as several trans-porters that are less well characterized. The molecular identification of the organiccation transporters (OCTs) has led to more detailed studies of the transport mecha-nism, substrate specificity, and tissue distribution of these transporters, and to a greaterunderstanding of their roles in organic cation disposition and elimination.

2.2. STRUCTURAL–FUNCTIONAL CHARACTERISTICSAND TRANSPORT MECHANISM

In terms of structural and functional characteristics, all three of the OCTs shareseveral common features: (1) a similar transmembrane topology, (2) a shared groupof preferred substrates, and (3) a common transport mechanism.14−20 The OCTs alsoshare a common genomic structure of 11 coding exons. The genes encoding the threeOCTs are located in close proximity on human chromosome 6q26, and presumablyarose via gene duplication from a single ancestral OCT gene.

The predicted secondary structure of the OCTs, based on sequence or hydropathyanalysis, consists of 12 transmembrane domains (TMDs) with cytoplasmic amino


STRUCTURAL–FUNCTIONAL CHARACTERISTICS AND TRANSPORT MECHANISM 13

Extracellular

Intracellular

FIGURE 2.1. Secondary structure predicted for OCT1. OCT1 shares a similar transmembranetopology to the other OCTs, consisting of 12 transmembrane domains (TMDs), and a largeextracellular loop between TMD1 and TMD2 which contains several putative N-glycosylationsites. TMD4 and TMD10 have been shown to contribute to substrate recognition by OCT1.

and carboxy termini. The OCTs share a large extracellular loop between TMDs 1 and2 which contains potential N-glycosylation sites, as well as a large intracellular loopbetween TMDs 6 and 7 which includes predicted phosphorylation sites. The poten-tial importance of N-glycosylation and phosphorylation for OCT activity is discussedbelow. Figure 2.1 shows the transmembrane topology of OCT1, which is typical ofthe OCTs. The transmembrane domains are believed to be important for substraterecognition by the OCTs; specifically, recent evidence suggests that the fourth andtenth transmembrane domains are critically involved in substrate recognition by theOCTs,21,22 and differences between isoforms in terms of substrate specificity (seeSection 2.3) may be related to differences in these critical regions. However, it is im-portant to note that given the broad substrate selectivity of the OCTs, the key domainsor residues involved in substrate recognition may be different from one substrate toanother, even within the same protein. For example, in mutational analysis studies ofrat OCT1, mutation of two residues in the fourth TMD (Trp218Tyr and Tyr222Leu)resulted in increased affinity for both tetraethylammonium (TEA) and 1-methyl-4-phenylpyridinium (MPP+), whereas a third mutant (Thr226Ala) had increased affinityfor MPP+ but no change in affinity for TEA.21 This suggests that OCT1, and probablyall of the OCTs, contain multiple overlapping but nonidentical recognition sites forthe various structurally diverse substrates.

Transport of organic cations by the OCTs occurs by facilitated diffusion and isdriven by the inside-negative membrane potential.14−20 Positively charged cationsare taken up into cells according to the electrochemical gradient (see Figure 2.2),and this process is membrane potential sensitive [i.e., artificially reducing the mem-brane potential (as through replacement of extracellular Na+ with K+, or treatmentwith ionophores such as valinomycin) reduces the rate of transport by OCTs].2,9



OCT1

(rodent)

OCT2

MDR1

OC+

(hydrophobic)

OC+

(hydrophobic)

OC+

Na+

−70 mV

Carnitine

H+ (or OC+)

H+ (Carnitine, OC+)

PROXIMAL TUBULE EPITHELIAL CELL

Basolateral Membrane

Apical Membrane

MATE1

H+

H+

OCTN2

OCTN1

OCTN2

MATE2

OC+

OC+

OC+

OC+

OC+

(a)

OCT1

MDR1

MDR3

OCT1

Bile Canaliculus

Canalicular (Apical)

Membrane

Sinusoidal (Basolateral)

Membrane

−70 mV

HEPATOCYTE

Blood

MATE1

OC+

OC+

ATP

ATP

H+

OC+

OC+

OC+

(b)

FIGURE 2.2. Model of organic cation transport in excretory organs. (a) Kidney: In humans,OCT2 is the primary transporter responsible for uptake of hydrophilic organic cations (OC+)across the basolateral membrane, in a process driven by the inside-negative membrane potential.In rodents, OCT1 and OCT2 are expressed to a similar degree in kidney. Efflux of organiccations across the luminal (apical) membrane for excretion occurs by an organic cation-H+

exchange mechanism, and several transporters are suspected to be involved in apical OC+

transport. Membrane-permeable organic cations may not require facilitative transport acrossthe basolateral membrane, and these hydrophobic cations may be substrates for the apicalefflux pump MDR1. (b) Liver: OCT1 is by far the most abundant organic cation transporter inthe liver, where it serves as a basolateral, potential-driven uptake transporter. After entry intothe hepatocyte, cationic drugs may be metabolized or excreted into bile via the primary activeefflux transporters MDR1 and MDR3, or via the organic cation-H+ exchanger MATE1.


SUBSTRATE SELECTIVITY 15

Electrophysiological studies using Xenopus laevis oocytes microinjected withOCT1, OCT2, or OCT3 cRNA show directly that transport of organic cations byOCTs is electrogenic (i.e., addition of substrate to the extracellular media resultsin inward currents).4,10,23−29 OCT activity is insensitive to changes in the Na+

gradient.17 Transport of weak bases by OCTs has also been shown to be sensi-tive to extracellular pH; however, this is explained by the decrease in fractionalionization of the substrate at high pH and does not indicate that transport of or-ganic cations is driven by proton exchange.30 OCT transport can also occur bycation–cation exchange. Preloading OCT1-injected X. laevis oocytes with unlabeledTEA enhances uptake of [3H]MPP+,26 and similar results have been found forOCT2.31

2.3. SUBSTRATE SELECTIVITY

Common substrates of all OCTs include low-molecular-weight relatively hydrophilicorganic cations such as the prototypical cation TEA, the neurotoxin MPP+, and theendogenous compound N -methylnicotinamide (NMN).18,20,32 Several clinically im-portant drugs have been shown to interact with all of the OCTs, including the antidia-betic drug metformin,18,33 and the peptic ulcer drug famotidine,34 demonstrating thebroad potential for influence of OCTs on drug disposition and drug action. Figure 2.3shows the structures of several compounds that interact with OCTs. Table 2.1 listsknown substrates and inhibitors of human OCTs.18,20,30,32−38

Included among OCT substrates are endogenous compounds such as the bio-genic amine neurotransmitters. Dopamine, epinephrine, norepinephrine, histamine,and serotonin have all been shown to interact with one or more OCT isoforms.20,32

Of particular interest in this regard is OCT3, which was cloned as the extraneu-ronal monoamine transporter (EMT) and is thought to comprise the uptake-2 systemof catecholamine transport, which aids in clearance of catecholamines in extraneu-ronal tissues.39 Despite the particular preference of OCT3 for biogenic amines, itis suggested that all three of the cloned OCT transporters combine to contribute toextraneuronal clearance of amine neurotransmitters.32,40

Although the OCT family shows broad overlap in substrate specificity, there areexamples of relatively isoform-specific substrates and inhibitors. Examples includecorticosterone, which has approximately 40-fold greater affinity than Oct1 for ratOct2, and conversely, mepiperphenidol and O-methylisoprenaline, which show a 70-fold greater affinity for rOct1 than for rOct2.23 OCT3 appears to have the mostunique substrate selectivity among the OCT family, with a preference for endogenousmonoamines such as dopamine and norepinephrine.

Notably, although most known substrates of the OCTs are cations, some OCTsubstrates are anionic or neutral compounds at physiological pH. For example, theanions prostaglandin E2 and prostaglandin F2α have been shown to be substrates forboth OCT1 and OCT2,41 and the neutral steroid β-estradiol interacts with OCT3 withhigh affinity.39 Thus, a net positive charge does not appear to be an absolute require-ment for interaction with the OCTs.



N+

CH3

CH3

CH3

H3C

CH3

CH3H3C

CH3

N+

Tetramethylammonium (TMA) Tetraethylammonium (TEA)

HO

OH

NH2

N

N

N

H

NH2

Dopamine Histamine

N

H3C

CH3

NH NH2

NHNH

N

NH

H3C

Metformin Desipramine

FIGURE 2.3. Representative substrates of organic cation transporters. OCTs are capable oftransporting a variety of structurally diverse substrates, including tetraalkylammonium com-pounds (TMA, TEA), endogenous compounds (dopamine, histamine) and clinically importantdrugs (desipramine, metformin).

Importantly, OCT orthologs from different species may exhibit differences in sub-strate specificity. For example, OCT1 orthologs from human, rabbit, mouse, andrat all transport the low-molecular-weight tetraalkylammonium compounds tetram-ethylammonium (TMA) and TEA. In contrast to human and rabbit, rat and mouseOct1 do not transport the larger tetrapropylammonium (TPA) or tetrabutylammonium(TBA),26 suggesting that molecular mass or hydrophobicity may effect differences inrecognition of OCT substrates across species.

2.4. TISSUE DISTRIBUTION

Despite the similarities in structure and function, there are important differences inthe tissue distribution of mRNA transcripts of the OCTs in human tissues. OCT1is expressed predominantly in human liver, with lower levels of expression in othertissues.4,9 OCT1 is thus characterized as a liver-specific OCT. Analogously, OCT2,which is expressed almost exclusively in kidney, is described as the kidney-specificOCT.4,42 OCT3, also referred to as the extraneuronal monoamine transporter (EMT),has a broad tissue distribution, with moderate levels of mRNA found in the aorta,


TA

BL

E2.

1.Su

bstr

ates

and

Inhi

bito

rsof

Hum

anO

CT

Isof

orm

sa

OC

T1

OC

T2

OC

T3

Com

poun

dIn

hibi

tor

Subs

trat

eIn

hibi

tor

Subs

trat

eIn

hibi

tor

Subs

trat

eR

efs.

Ace

buto

lol

+N

.D.

N.D

.N

.D.

N.D

.N

.D.

18,2

0A

cycl

ovir

++

N.D

.N

.D.

N.D

.N

.D.

18A

gmat

ine

++

++

++

32A

man

tadi

ne+

N.D

.+

+N

.D.

N.D

.20

Aqu

inav

ir+

N.D

.N

.D.

N.D

.N

.D.

N.D

.18

Cho

line

+N

.D.

++

N.D

.−

20,3

2C

imet

idin

e+

N.D

.+

++

+18

,20,

32C

loni

dine

+−

N.D

.N

.D.

+−

18,2

0C

ocai

neN

.D.

N.D

.+

N.D

.N

.D.

N.D

.18

Cor

ticos

tero

ne+

N.D

.+

N.D

.+

N.D

.20

,23

Cre

atin

ine

+N

.D.

N.D

.N

.D.

N.D

.−

20,3

2,36

Cya

nine

-863

N.D

.N

.D.

+N

.D.

N.D

.N

.D.

20D

ebri

soqu

ine

N.D

.N

.D.

++

N.D

.N

.D.

18D

ecyn

ium

-22

+N

.D.

+N

.D.

+N

.D.

20,3

7D

esip

ram

ine

+N

.D.

+N

.D.

+N

.D.

18,2

0D

isop

yram

ide

+N

.D.

N.D

.N

.D.

N.D

.N

.D.

18,2

0D

ispr

ocyn

ium

-24

N.D

.N

.D.

N.D

.N

.D.

+N

.D.

20D

opam

ine

+N

.D.

++

++

20,3

2,35

Epi

neph

rine

N.D

.N

.D.

N.D

.N

.D.

++

32β

-Est

radi

ol+

N.D

.+

N.D

.+

N.D

.37

Fam

otid

ine

++

+−

+−

34G

anci

clov

ir+

+N

.D.

N.D

.N

.D.

N.D

.38

Gua

nidi

neN

.D.

N.D

.N

.D.

N.D

.+

−36

His

tam

ine

+−

++

++

20In

dina

vir

+N

.D.

N.D

.N

.D.

N.D

.N

.D.

18,2

0M

eman

tine

N.D

.N

.D.

++

N.D

.N

.D.

18,2

0M

epip

erph

enid

olN

.D.

N.D

.+

N.D

.N

.D.

N.D

.18

,20

(Con

tinu

ed)

17


TA

BL

E2.

1.(C

ontin

ued)

OC

T1

OC

T2

OC

T3

Com

poun

dIn

hibi

tor

Subs

trat

eIn

hibi

tor

Subs

trat

eIn

hibi

tor

Subs

trat

eR

efs.

Met

form

in+

++

+N

.D.

N.D

.33

O-M

ethy

lisop

rena

line

+N

.D.

+N

.D.

+N

.D.

18M

idaz

olam

+N

.D.

N.D

.N

.D.

N.D

.N

.D.

18,2

0M

PP+

++

++

++

20,3

2N

elfin

avir

+N

.D.

N.D

.N

.D.

N.D

.N

.D.

18,2

0N

MN

++

++

+N

.D.

20,3

2N

orep

inep

hrin

eN

.D.

N.D

.+

++

+20

,32

Phen

form

in+

N.D

.+

N.D

.N

.D.

N.D

.18

Phen

oxyb

enza

min

e+

N.D

.+

N.D

.+

N.D

.18

Praz

osin

+N

.D.

+N

.D.

+N

.D.

18Pr

ocai

nam

ide

+N

.D.

+N

.D.

+N

.D.

18,2

0Pr

oges

tero

ne+

N.D

.+

N.D

.+

N.D

.37

Qui

nidi

ne+

N.D

.+

N.D

.N

.D.

N.D

.20

,30

Qui

nine

+N

.D.

++

N.D

.N

.D.

20,3

0R

aniti

dine

++

++

+−

34R

itona

vir

+N

.D.

N.D

.N

.D.

N.D

.N

.D.

18,2

0Sa

quin

avir

+N

.D.

N.D

.N

.D.

N.D

.N

.D.

20Se

roto

nin

N.D

.N

.D.

++

++

20,3

2Te

trab

utyl

amm

oniu

m+

++

N.D

.N

.D.

N.D

.20

,69

Tetr

aeth

ylam

mon

ium

++

++

++

20,3

2Te

trah

epty

lam

mon

ium

+N

.D.

N.D

.N

.D.

N.D

.N

.D.

20Te

tram

ethy

lam

mon

ium

++

+N

.D.

N.D

.N

.D.

20Te

trap

ropy

lam

mon

ium

++

+N

.D.

N.D

.N

.D.

20,6

9Ty

ram

ine

N.D

.N

.D.

N.D

.N

.D.

++

20,3

2V

ecur

oniu

m+

N.D

.N

.D.

N.D

.N

.D.

N.D

.18

,20

Ver

apam

il+

N.D

.+

N.D

.N

.D.

N.D

.18

,20

aIn

clud

esco

mpo

unds

that

have

been

test

edas

eith

ersu

bstr

ates

orin

hibi

tors

ofan

yO

CT

isof

orm

.Com

poun

dssh

own

tobe

subs

trat

esor

inhi

bito

rsar

ede

sign

ated

as“+

,”an

dth

ose

show

npo

sitiv

ely

not

tobe

subs

trat

esor

inhi

bito

rsar

ede

sign

ated

as“−

.”C

ompo

unds

that

have

not

been

test

edas

eith

ersu

bstr

ate

orin

hibi

tor

are

liste

das

“N.D

.”(n

otde

term

ined

)fo

rth

atis

ofor

m.

18


TISSUE DISTRIBUTION 19

TABLE 2.2. Tissue Distribution of Human OCT Isoforms

OCT1 OCT2 OCT3

Liver + + + − +Kidney − + + + +Lung + − +Trachea − − −Heart + − ++Skeletal Muscle ++ − ++Placenta + − ++Pancreas − − −Brain − − +Spinal cord − − +Adrenal gland + − −Testis − − −Ovary + − +Fetal liver + − −Fetal lung + − −Fetal brain + − −aThe expression level is estimated relative to other tissues for each isoform, based on Northern blotanalysis.4,9,12,42

skeletal muscle, prostate, adrenal gland, salivary gland, liver, term placenta, and fetallung.12 See Table 2.2 for details on tissue distribution of each OCT isoform.

Both OCT1 and OCT2 have been localized to the basolateral membrane in renalproximal tubular epithelial cells in rat43−45 as well as in cell culture models of kidney(GFP-rOCT2-transfected MDCK cells).46 In human proximal tubules, OCT2 expres-sion is restricted to the basolateral membrane; however, OCT1 is expressed at very lowlevels in human kidney.47 Despite early controversy regarding OCT2 localization,4,5,48

the emerging consensus is that OCT2 is indeed a basolateral organic cation uptaketransporter in kidney.15 The pharmacological significance of OCT2 thus appears pri-marily to be its role in active secretion of organic cations in the kidney, facilitatingthe first step (uptake of organic cations across the basolateral membrane) of activesecretion. Efflux of organic cations across the apical membrane is thought to occurby a separate transporter-mediated process, either by the primary active efflux pump,MDR1 (which generally transports bulky hydrophobic cationic compounds) or by anorganic cation-H+ exchange mechanism for small, hydrophilic organic cations (seeFigure 2.2).14,15,20,49 The molecular identity of the organic cation-H+ exchanger iscurrently unclear; the novel organic cation transporters (OCTN1 and OCTN2) andthe more recently discovered multidrug and toxin exclusion transporters (MATE1and MATE2) are likely candidates, as all of these are highly expressed in kidney andhave been shown to transport the typical cation TEA in a manner consistent with H+

exchange.14,15,20,49−51 It is possible that all of these transporters participate in apicalorganic cation efflux, with differential importance depending on the substrate underquestion. Among the apical organic cation transporters, only OCTN2 has been shown



to influence renal organic cation (TEA) secretion in vivo52; however, appropriate ani-mal models to study the in vivo role of OCTN1, MATE1, and MATE2 in renal organiccation secretion do not yet exist.

In liver, the primary site of OCT1 expression, OCT1 was found to be expressed atthe sinusoidal (basolateral) membrane by immunohistochemistry and Western blot.53

This suggests that OCT1 acts as a sinusoidal uptake transporter in liver, aiding inthe transfer of organic cations from blood into the hepatocyte for elimination bymetabolism or biliary excretion. In addition, for drugs that target the liver for theirpharmacological activity, OCT1 may be a limiting step in drug access to hepatocytesand 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 mostabundant OCT in terms of mRNA expression, rodent kidney expresses nearly similarlevels of OCT1 and OCT2. As with substrate-specificity differences, differences be-tween species in terms of tissue distribution of OCTs are important to consider wheninterpreting 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 conservedamong the several OCT isoforms. These include target sequences for protein kinaseA (PKA), protein kinase C (PKC), protein kinase G (PKG), and tyrosine kinase. Thereexists ample evidence for the modulation of activity of the OCTs by activation of thesekinases, presumably through phosphorylation of the OCTs at the protein level. SeveralN-glycosylation sites in the large extracellular loop between transmembrane domains1 and 2 are also potential sites of protein-level regulation. Additionally, hormonalregulation, which is thought to reflect effects on transcription, has been described forOCT2. Regulation of OCT expression is also modulated during development and incertain pathophysiological states. For an extensive review of the regulation of organiccation transport, see ref. 54. Current knowledge of the regulation of the OCTs will bediscussed individually for each isoform.

2.5.1. OCT1

Early studies of rat OCT1 showed that stimulation of either PKA or PKC results in asignificant 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 rOCT155 PKC stimulation also results in increasedaffinity for TEA, TPA, and quinine. The effect of PKC stimulation on rOCT1 activityis thought to occur by phosphorylation of the rOCT1 protein, resulting in a con-formational change that enhances substrate binding. rOCT1 activity is significantlyreduced by treatment with aminogenistein, a specific inhibitor of p56lck tyrosine


REGULATION OF OCT EXPRESSION AND ACTIVITY 21

kinase, suggesting that this kinase is an endogenous activator of OCT1; however,inhibitors of other tyrosine kinases do not affect OCT1 function.

In contrast to rOCT1, stimulation of PKA results in decreased activity of humanOCT1 in two different expression systems.56 In the same studies, PKC activationhas no appreciable effect on activity of human OCT1. However, as with rOCT1,human OCT1 appears to be positively regulated by the p56lck tyrosine kinase, asevidenced by reduced hOCT1 activity after treatment with aminogenistein. For bothrOCT1 and hOCT1, PKG signaling does not appear to influence OCT1 activity.Human OCT1 has further been shown to be regulated by the Ca2+–calmodulin com-plex. Specifically, inhibition of Ca2+- and calmodulin-dependent protein kinase II(CaMKII) results in reduced ASP+ transport by hOCT1, suggesting that this ki-nase consitutively stimulates hOCT1 activity. In both PKA stimulation and CaMKIIinhibition, the reduction in transport activity by hOCT1 is the result of decreasedaffinity for the substrates tested. These results suggest, again, that the observed ef-fects of protein kinases are due to phosphorylation of the OCT1 protein, causing aconformational change that results in either decreased or increased substrate-bindingaffinity.

2.5.2. OCT2

Studies on regulation of basolateral organic cation transport have been performedin isolated human proximal tubules, in which OCT2 is known to be the major OCTisoform based on mRNA expression and relative affinity for various substrates. Thesestudies have shown that activation of PKA, PKC, or PKG leads to reduced uptakeof the fluorescence organic cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium(ASP+) into renal proximal tubules, suggesting an inhibitory effect of these kinases onOCT2 activity.57 Inhibition of PKC by calphostin C reverses the inhibition of OCT2activity, demonstrating the specificity of the PKC-mediated pathway. However, whenheterologously expressed in HEK293 cells, hOCT2 was shown to be inhibited onlyby PKA activation, not by PKC.58 These conflicting results may be explained bydifferences between these experimental systems in terms of the expression of genesinvolved in the PKC pathway.54

Further studies using heterologously expressed OCT2 have identified a role for Gprotein–coupled receptor (GPCR) signaling in regulation of OCT2. The muscarinicreceptor agonist carbachol causes a dose-dependent decrease in OCT2 activity (i.e.,uptake of ASP+) that is not explained by direct inhibition of the OCT2 protein bycarbachol.58 Similarly, activation of the phospholipase C (PLC) pathway throughpurinergic receptor agonism by ATP leads to dose-dependent decreases in OCT2activity. As both of these pathways result in increased intracellular Ca2+ concentra-tions, it has been hypothesized that the mechanism for OCT2 regulation by GPCRsinvolves the Ca2+–calmodulin complex. Reduction of intracellular Ca2+ concentra-tion or treatment with a Ca2+ chelator results similarly in decreased OCT2 activity, andinhibition of calmodulin and its downstream targets CaMKII and myosin light-chainkinase (MLCK) produces a similar effect. Although the effect of muscarinic receptorstimulation on OCT2 activity is partially explained by its effects on intracellular Ca2+,



additional studies have shown that the majority of this effect is due to activation ofphospatidylinositol 3-kinase (PI3K), as inhibition of PI3K almost completely preventsthe inhibitory effect of carbachol on OCT2-mediated transport. These studies suggestthat PKA, PI3K, and the Ca2+–calmodulin complex are endogenous regulators ofOCT2; however, in contrast to the effect of protein kinases on OCT1, the inhibition oftransport activity is not explained by effects on substrate-binding affinity. It is there-fore thought that these proteins regulate other aspects of OCT2, such as maturationand trafficking of OCT2 to the plasma membrane, or stimulation of endocytosis.54

Glycosylation of OCT2 may be an additional mechanism for regulation of organiccation transport. The rabbit OCT2 isoform was shown to contain three N-glycosylationsites (Asn71, Asn96, and Asn112), each of which was found to be necessary for op-timal transport activity by mutational analysis.59 The Asn112Gln mutant resulted ina significant decrease in membrane expression (to approximately 20% of wild-type)in stably transfected Chinese hamster ovary (CHO) cells. Asn71Gln and Asn96Glndid not alter OCT2 surface expression, but the unglycosylated triple mutant(Asn71Gln/Asn96Gln/Asn112Gln) was retained intracellularly. Affinity for TEA wasincreased approximately twofold by mutation of each of the three glycosylation sites.Maximal transport rate was decreased fivefold by the Asn112Gln mutation (explainedby the reduced surface expression of Asn112Gln) and threefold by Asn96Gln (pre-sumably, due to a lower transporter turnover number). Thus, N-glycosylation mayregulate maturation and trafficking of OCT2 as well as substrate recognition andtransporter turnover.

There is also evidence for hormonal regulation of OCT2. Functional studies of ratrenal cortical slices showed higher basolateral transport of TEA in slices from malerats versus female rats, and rOCT2 mRNA and protein expression was also shown tobe higher in male rats than in female rats.60 Further, treatment of rats with testosteroneincreased rOCT2 and protein expression in the kidney of both male and female rats.61

In Madin–Darby canine kidney (MDCK) cells, treatment with dexamethasone, hydro-cortisone, or testosterone increased both OCT2 mRNA and protein levels as well asuptake of TEA across the basolateral membrane.62 Conversely, treatment with estra-diol led to decreased OCT2 expression and activity in male rats and in MDCK cells.The physiological significance of hormonal regulation of OCT2 is unclear but maybe related to gender-specific differences in clearance of endogenous metabolites.32

2.5.3. OCT3

Although OCT3 contains several conserved target sequences for PKA, PKC, andPKG, studies of heterologously expressed OCT3 have shown that activation of thesepathways has no apparent effect on OCT3 activity.63 However, inhibition of the p44/42mitogen-activated protein kinase (MAPK) has been shown to decrease MPP+ trans-port by OCT3, suggesting that MAPK is a positive regulator of OCT3. Similarly,inhibition of calmodulin as well as of the downstream effector CaMKII results indecreased OCT3 activity. Selective inhibition of the calmodulin-dependent phospho-diesterase PDE1 also results in decreased OCT3 activity, suggesting PDE1 stimulationas an additional mechanism for Ca2+- and calmodulin-dependent regulation of OCT3.


ANIMAL MODELS 23

2.6. ANIMAL MODELS

Targeted genetic disruption of the organic cation transporters in mice (i.e., OCTknockout mice) provided the first direct evidence of the significance of organic cationtransporters to drug disposition in vivo.


Oct1 –/– knockout mice were shown to be viable and fertile, showing no obviousphysiological abnormalities compared with their wild-type littermates, suggesting thatOct1 is dispensible for normal physiology.64 However, significant differences were ob-served in the disposition of organic cations in these mice. For example, when admini-stered the typical organic cation, TEA, Oct1 –/– mice showed significantly (ap-proximately fivefold) reduced uptake of TEA into the liver, the site of highest OCT1expression. Biliary excretion of TEA was 2.5-fold lower in Oct1 –/– mice, whichwas explained by the reduced hepatic uptake of TEA. Additionally, direct intesti-nal excretion of TEA was reduced by approximately 50%. Renal excretion of TEAwas paradoxically increased (∼50%) in Oct1 –/– mice; however, this was ex-plained by the lack of OCT1 in the liver of Oct1 –/– mice; that is, decreasedhepatic uptake of TEA in these mice resulted in increased availability to the kid-ney for renal excretion. Further, the coexpression of Oct1 and Oct2 in rodent kid-ney may have led to a confounding effect of Oct2 activity in Oct1 –/– mousekidney.

In addition to the pharmacokinetic differences observed for TEA, Oct1 –/– miceshowed similar decreases in hepatic uptake of other OCT1 substrates, including theneurotoxin MPP+ (∼60% reduced) and meta-iodobenzylguanidine (MIBG) (∼75%reduced).64 Cimetidine, an OCT1 inhibitor (but not a substrate), did not show signif-icant differences in hepatic uptake in Oct1 –/– mice compared with wild-type mice.Similar results were found after intravenous injection of [14C]choline, which is anOct1 substrate; however, it was suggested that rapid metabolism of the radiolabeledcompound to species that are not Oct1 substrates, as well as redundancy (i.e., othertransporters capable of transporting choline), may have masked any possible effectof Oct1 on the pharmacokinetics of this compound.

Further studies of Oct1 –/– mice have focused on the antidiabetic drug met-formin, which exerts its pharmacological effects in the liver. Oct1 –/– mice showeda greater than 30-fold decrease in metformin uptake into liver compared with wild-type littermates.65 Further studies investigated the role of Oct1 in the developmentof metformin-induced lactic acidosis, a leading toxicity from this drug. A signifi-cant increase in serum lactic acid concentration was observed after administrationof metformin to wild-type mice, but only slight elevations in serum lactate wereseen in Oct1 –/– mice, despite similar pharmacokinetic profiles between genotypegroups.66 Taken together, these results suggest that Oct1-mediated metformin trans-port is a limiting step in metformin uptake into liver, and that the lactic acido-sis induced by metformin is related to the availability of the drug to this targetorgan.




Similar to Oct1 –/– mice, Oct2 –/– mice were viable and fertile, showing no apparentphysiological defects.67 However, unlike the Oct1 –/– mice, pharmacokinetics ofintravenous [14C]TEA were quite similar in Oct2 –/– mice and wild-type mice, withthe exception of a slight decrease in distribution of TEA into brain in Oct2-deficientmice. Renal clearance of TEA in Oct2 –/– mice was not different from that inwild-type mice, with active secretion being the primary mechanism of clearance. Theexplanation for these findings is that in mouse kidney, Oct1 expression is sufficientto maintain the capacity for active secretion of organic cations even in the absence ofOct2. Thus, to develop a mouse model of renal disposition of organic cations, it wasnecessary to generate an Oct1/2 –/– double-knockout mouse.

2.6.3. Oct1/Oct2 Knockout Mice

Generation of Oct1/2 –/– double-knockout mice revealed that the absence of bothgenes, as with each of the single knockouts, was compatible with normal physi-ology (i.e., normal viability, fertility, and life span were observed, with no appar-ent physiological abnormalities).67 However, unlike the single knockouts, Oct1/2–/– mice show significant impairment in the active tubular secretion of organiccations in the kidney. Specifically, renal tubular secretion of TEA was effectivelyabolished in Oct1/2 –/– mice, with renal clearance approximating glomerularfiltration. This resulted in significantly elevated plasma levels of TEA in thesemice compared with wild-type or Oct1 –/– single-knockout mice. After steady-state infusion of TEA, plasma levels were elevated approximately sixfold in theOct1/2 –/– double-knockout mice compared to Oct1 –/–, Oct2 –/–, or wild-typemice.


As mentioned earlier, OCT3 is believed to be primarily responsible for the transportof endogenous monoamines in extraneuronal tissues, also known as the uptake-2transport system. Zwart et al. generated an Oct3-deficient mouse in order to deter-mine the importance of this transporter in the peripheral disposition of monoamineneurotransmitters.68 Although the Oct3 –/– mice were viable and fertile and showedno deficiency in dopamine or norepinephrine metabolism, they did show impaireduptake-2 activity as evidenced by a >70% reduced uptake of the synthetic monoamineMPP+ into heart in Oct3 –/– mice compared to wild-type mice. Additionally, it wasshown that in Oct3 +/− heterozygous females impregnated in a heterozygous cross,intravenous injection of MPP+ resulted in a threefold reduction of MPP+ uptake inOct3 –/– embryos versus wild-type embryos, whereas the accumulation of MPP+ inthe placenta was not different between groups. These results suggest that OCT3 actsas an uptake transporter for monoamines at the fetoplacental interface. Apart fromthe distribution of MPP+ into heart and placental transfer of MPP+, no significantphenotypic differences have been observed in Oct3 –/– mice.


GENETIC VARIATION 25

2.7. GENETIC VARIATION

Much attention has recently been focused on the pharmacogenetics of drug trans-porters (i.e., the effect of natural human genetic variation in drug transporter geneson drug disposition and drug response). Research in this area has taken primarily asequence-based approach, beginning with identification of genetic variants likely toproduce a functional effect (e.g., amino acid substitutions, or insertions or deletionsin the coding region), followed in some cases by functional studies of the variantproteins in heterologous expression systems.

2.7.1. OCT1

Several functionally significant polymorphisms in OCT1 have been described. In asample of 57 healthy Caucasians, 25 genetic variants were discovered, of which eightresulted in a change in protein sequence.69 Of these, five (Arg61Cys, Cys88Arg,Phe160Leu, Gly401Ser, and Met420del) were tested for function by measurementof transport activity in X. laevis oocytes. Cys88Arg and Gly401Ser showed virtuallycomplete loss of activity toward MPP+, while Arg61Cys retained approximately 30%of wild-type activity. In contrast to MPP+, uptake of [3H]serotonin by Cys88Argand Gly401Ser was detectable (∼10% of wild-type), suggesting that these variantsinfluence the substrate selectivity of OCT1.

Shu et al. discovered 15 amino acid sequence–altering variants of OCT1 that wereidentified in a large sample of ethnically diverse healthy subjects, and tested thesefor function by uptake of MPP+ in X. laevis oocytes.70 Variants with significantly re-duced or complete loss of function included the Arg61Cys and Gly401Ser describedby Kerb et al.69, as well as Pro341Leu, Gly220Val, and Gly465Arg. An additionalvariant, Ser14Phe, showed a significant increase in activity; interestingly, this vari-ant corresponds to the likely ancestral allele, as the consensus mammalian OCT1sequence includes phenylalanine at this position. It was noted that all of the variantswith reduced function occurred at evolutionarily conserved amino acid residues, andthat variants with reduced function tended to be amino acid substitutions that resultin a large chemical change. Of the variants with significant functional differencesfrom the reference OCT1, five (Ser14Phe, Arg61Cys, Pro341Leu, Gly401Ser, andGly465Arg) occurred at >1% allele frequency in at least one ethnic group and are at-tractive candidates for association with drug response phenotypes for OCT1 substratedrugs.

Functional studies of several additional OCT1 variants have been performed bytwo independent groups using microinjected X. laevis oocytes71 or transiently trans-fected HEK293 cells.72 Three amino acid sequence variants, Pro283Leu, Arg287Gly,and the previously described Pro341Leu, showed significantly reduced transport ac-tivity, with Pro283Leu and Arg287Gly having no appreciable activity toward themodel substrates TEA and MPP+. The reduction in activity of these variants was notexplained by reduction in protein expression, as immunofluorescence studies showedall three variants to have membrane expression levels similar to that of the wild-typeOCT1.72



2.7.2. OCT2

Human genetic variants of OCT2 have been investigated comprehensively by rese-quencing of the coding region in a large ethnically diverse sample.35 Eight of thevariants identified in this screen result in amino acid substitutions, and one single-nucleotide insertion (134-135insA) leads to a premature stop codon at amino acid po-sition 48. Four of the nonsynonymous variants (Met165Ile, Ala270Ser, Arg400Cys,and Lys432Gln) were polymorphic, with ethnic-specific allele frequencies ≥1%. Theremaining four nonsynonymous variants (Pro54Ser, Phe161Leu, Met165Val, andAla297Gly) as well as the insertion variant were found on only one of 494 chro-mosomes screened.

Of the four polymorphic OCT2 protein sequence variants (Met165Ile, Ala270Ser,Arg400Cys, and Lys432Gln), all retained function as measured by uptake of theprototypical organic cation substrate, MPP+, in X. laevis oocytes expressing thevariant transporters. However, quantitative differences in MPP+ uptake activity andkinetic differences in interactions with organic cations were observed for the com-mon variants, with Lys432Gln having twofold increased affinity for MPP+, and theMet165Ile and Arg400Cys variants having lower maximal transport rates (Vmax)for MPP+ than the reference OCT2.35 In inhibition kinetics studies, Ala270Sershowed an increased Ki value for TBA, while Arg400Cys and Lys432Gln hadlower Ki values for TBA inhibition than did the reference OCT2. When the rarevariants of OCT2 were expressed in the same system, all of the amino acid sub-stitutions were found to have no effect on OCT2 function.30 However, as ex-pected, the single nucleotide insertion (frameshift) variant showed a complete loss offunction.

Additional single-nucleotide polymorphisms (SNPs) in the OCT2 gene were iden-tified by direct sequencing of genomic DNA from 48 unrelated Japanese persons73

and 116 arrhythmic Japanese patients.74 In the former study, 27 SNPs and five dele-tion polymorphisms in the OCT2 gene were identified.73 Two synonymous variants,also reported by Leabman et al.,35 were identified at amino acid positions 130 and150. The remaining SNPs and deletion polymorphisms appeared in introns, 3′ un-translated regions, and 3′ flanking regions of the OCT2 gene. In the latter study, 33genetic variants, including 14 novel ones, were found.74 Two nonsynonymous vari-ants were identified (Thr199Ile and Thr201Met) and other SNPs and insertion anddeletion polymorphisms were located in exons, the 3′-untranslated region of exon 11,introns, and the 3′-flanking region. The functional effects of these polymorphismshave not been investigated.

In addition to direct sequencing of the OCT2 gene, other lines of evidence existfor a role of genetic variation in OCT2 in variable drug response. Based on datafrom published literature, an estimate of the genetic component contributing to vari-ation in the renal clearance of metformin, which undergoes transporter-mediatedsecretion, was found to be particularly high (>90%).75 This finding suggests thatvariation in the renal clearance of metformin has a strong genetic component andthat genetic variation in OCT2 may explain a large part of this pharmacokineticvariability.


CONCLUSIONS 27

2.7.3. OCT3

In a study of genetic variation in 26 membrane transporter genes in a large sample (n =247) of ethnically diverse human subjects, Leabman et al. discovered 14 nucleotidesubstitutions in OCT3.76 The survey region included the exons and flanking intronicregion. Of the 14 variants identified, five were located in the coding exonic regions,and three of these (Thr44Met, Ala116Ser, and Thr400Ile) resulted in a change in theamino acid sequence. Only Ala116Ser was polymorphic, with an allele frequency of1.7% in the African-American subset of the sample. These variants have not beentested for effects on OCT3 function.

Genetic variation in OCT3 was investigated independently by resequencing thecore promoter, exons, and 3′ untranslated region of the gene in 100 healthy personsof European descent.77 Six nucleotide substitutions and one single base pair deletionwere discovered, including −29A>G in the OCT3 promoter and three synonymoussubstitutions in the coding region. The functional consequences of these variants havenot been studied. The authors suggest that the synonymous 1233G>A substitutionmay generate a cryptic 3′-splice acceptor site.

Additional data on genetic variation in organic cation transporter genes are avail-able on online public databases. The Pharmacogenetics and Genomics KnowledgeBase (PharmGKB, www.pharmgkb.org) is a repository for data and information re-lated to all areas of pharmacogenetics. PharmGKB includes well-annotated pages de-scribing genetic variation in OCTs, identified by the Pharmacogenetics of MembraneTransporters project (PMT, www.pharmacogenetics.ucsf.edu) in a large collectionof DNA samples from ethnically diverse populations. The NCBI single-nucleotidepolymorphism database (dbSNP, www.ncbi.nlm.nih.gov/SNP/) also includes data onOCT variants. The HapMap project (www.hapmap.org), which aims to take advan-tage of linkage between SNPs to increase power in genetic association studies, mayprove to be a useful tool for studying the genetics of response to OCT substrates.

2.8. CONCLUSIONS

The OCTs are a fairly well-studied family of multispecific organic cation transporters,with potential influence for a large number of endogenous and pharmacological com-pounds. The tissue distribution and subcellular localization of these transporters sug-gest that their primary role is in the excretion of toxic xenobiotic and endogenousorganic cations. OCT1 is a hepatic sinusoidal uptake transporter and appears to be mostimportant for drug distribution into liver, where it aids in presentation of substratedrugs to hepatic metabolizing enzymes or in biliary excretion. OCT2, the kidney-specific organic cation transporter, is primarily responsible for basolateral uptake oforganic cations into renal proximal tubules and acts as the first step in active tubularsecretion of its substrates. OCT3 has a more diffuse pattern of expression, as well as arelatively unique substrate selectivity profile, and is thought to be important for extra-neuronal clearance of monoamine neurotransmitters as well as uptake of monoaminesinto the heart and across the placenta. Knockout mouse models of the OCTs have



provided crucial information regarding the importance of these transporters to xeno-biotic and endobiotic disposition in vivo. The existence of genetic variants of OCTswith altered function in cellular assays suggests that genetic variation in OCTs maycontribute to interindividual variability in drug disposition or drug response; furtherstudy of the importance of these variants to human drug disposition may soon allowfor clinical pharmacogenetic testing and improvements in rational drug therapy.

Acknowledgments

This work was supported by National Institutes of Health grants GM36780 andGM61390.

REFERENCES

1. Rennick BR. 1981. Renal tubule transport of organic cations. Am J Physiol 240(2):F83–F89.

2. Grundemann D, Gorboulev V, Gambaryan S, Veyhl M, Koepsell H. 1994. Drug excretionmediated by a new prototype of polyspecific transporter. Nature 372(6506):549–552.

3. Okuda M, Saito H, Urakami Y, Takano M, Inui K. 1996. cDNA cloning and functionalexpression of a novel rat kidney organic cation transporter, OCT2. Biochem Biophys ResCommun 224(2):500–507.

4. Gorboulev V, Ulzheimer JC, Akhoundova A, Ulzheimer-Teuber I, Karbach U, Quester S,Baumann C, Lang F, Busch AE, Koepsell H. 1997. Cloning and characterization of twohuman polyspecific organic cation transporters. DNA Cell Biol 16(7):871–881.

5. Grundemann D, Babin-Ebell J, Martel F, Ording N, Schmidt A, Schomig E. 1997. Primarystructure and functional expression of the apical organic cation transporter from kidneyepithelial LLC-PK1 cells. J Biol Chem 272(16):10408–10413.

6. Mooslehner KA, Allen ND. 1999. Cloning of the mouse organic cation transporter 2 gene,Slc22a2, from an enhancer-trap transgene integration locus. Mamm Genome 10(3):218–224.

7. Schweifer N, Barlow DP. 1996. The Lx1 gene maps to mouse chromosome 17 and codesfor a protein that is homologous to glucose and polyspecific transmembrane transporters.Mamm Genome 7(10):735–740.

8. Terashita S, Dresser MJ, Zhang L, Gray AT, Yost SC, Giacomini KM. 1998. Molecularcloning and functional expression of a rabbit renal organic cation transporter. BiochimBiophys Acta 1369(1):1–6.

9. Zhang L, Dresser MJ, Gray AT, Yost SC, Terashita S, Giacomini KM. 1997. Cloningand functional expression of a human liver organic cation transporter. Mol Pharmacol51(6):913–921.

10. Kekuda R, Prasad PD, Wu X, Wang H, Fei YJ, Leibach FH, Ganapathy V. 1998. Cloning andfunctional characterization of a potential-sensitive, polyspecific organic cation transporter(OCT3) most abundantly expressed in placenta. J Biol Chem 273(26):15971–15979.

11. Grundemann D, Schechinger B, Rappold GA, Schomig E. 1998. Molecular identifica-tion of the corticosterone-sensitive extraneuronal catecholamine transporter. Nat Neurosci1(5):349–351.


REFERENCES 29

12. Verhaagh S, Schweifer N, Barlow DP, Zwart R. 1999. Cloning of the mouse and humansolute carrier 22a3 (Slc22a3/SLC22A3) identifies a conserved cluster of three organiccation transporters on mouse chromosome 17 and human 6q26–q27. Genomics 55(2):209–218.

13. Wu X, Huang W, Ganapathy ME, Wang H, Kekuda R, Conway SJ, Leibach FH, GanapathyV. 2000. Structure, function, and regional distribution of the organic cation transporterOCT3 in the kidney. Am J Physiol 279(3):F449–F458.

14. Wright SH. 2005. Role of organic cation transporters in the renal handling of therapeuticagents and xenobiotics. Toxicol Appl Pharmacol 204(3):309–319.

15. Wright SH, Dantzler WH. 2004. Molecular and cellular physiology of renal organic cationand anion transport. Physiol Rev 84(3):987–1049.

16. Koepsell H, Gorboulev V, Arndt P. 1999. Molecular pharmacology of organic cation trans-porters in kidney. J Membr Biol 167(2):103–117.

17. Koepsell H, Schmitt BM, Gorboulev V. 2003. Organic cation transporters. Rev PhysiolBiochem Pharmacol 150:36–90.

18. Koepsell H. 2004. Polyspecific organic cation transporters: their functions and interactionswith drugs. Trends Pharmacol Sci 25(7):375–381.

19. Koepsell H, Endou H. 2004. The SLC22 drug transporter family. Pflugers Arch 447(5):666–676.

20. Dresser MJ, Leabman MK, Giacomini KM. 2001. Transporters involved in the eliminationof drugs in the kidney: organic anion transporters and organic cation transporters. J PharmSci 90(4):397–421.

21. Popp C, Gorboulev V, Muller TD, Gorbunov D, Shatskaya N, Koepsell H. 2005. Aminoacids critical for substrate affinity of rat organic cation transporter 1 line the substratebinding region in a model derived from the tertiary structure of lactose permease. MolPharmacol 67(5):1600–1611.

22. Gorboulev V, Shatskaya N, Volk C, Koepsell H. 2005. Subtype-specific affinityfor corticosterone of rat organic cation transporters rOCT1 and rOCT2 depends onthree amino acids within the substrate binding region. Mol Pharmacol 67(5):1612–1619.

23. Arndt P, Volk C, Gorboulev V, Budiman T, Popp C, Ulzheimer-Teuber I, Akhoundova A,Koppatz S, Bamberg E, Nagel G, Koepsell H. 2001. Interaction of cations, anions, andweak base quinine with rat renal cation transporter rOCT2 compared with rOCT1. Am JPhysiol Renal Physiol 281(3):F454–F468.

24. Budiman T, Bamberg E, Koepsell H, Nagel G. 2000. Mechanism of electrogenic cationtransport by the cloned organic cation transporter 2 from rat. J Biol Chem 275(38):29413–29420.

25. Busch AE, Quester S, Ulzheimer JC, Waldegger S, Gorboulev V, Arndt P, Lang F, KoepsellH. 1996. Electrogenic properties and substrate specificity of the polyspecific rat cationtransporter rOCT1. J Biol Chem 271(51):32599–32604.

26. Dresser MJ, Gray AT, Giacomini KM. 2000. Kinetic and selectivity differences betweenrodent, rabbit, and human organic cation transporters (OCT1). J Pharmacol Exp Ther292(3):1146–1152.

27. Gorboulev V, Volk C, Arndt P, Akhoundova A, Koepsell H. 1999. Selectivity of the polyspe-cific cation transporter rOCT1 is changed by mutation of aspartate 475 to glutamate. MolPharmacol 56(6):1254–1261.



28. Nagel G, Volk C, Friedrich T, Ulzheimer JC, Bamberg E, Koepsell H. 1997. A reevaluationof substrate specificity of the rat cation transporter rOCT1. J Biol Chem 272(51):31953–31956.

29. Sweet DH, Pritchard JB. 1999. rOCT2 is a basolateral potential-driven carrier, not anorganic cation/proton exchanger. Am J Physiol 277(6 Pt 2):F890–F898.

30. Fujita T, Urban TJ, Leabman MK, Fujita K, Giacomini KM. 2006. Transport of drugs inthe kidney by the human organic cation transporter, OCT2 and its genetic variants. J PharmSci 95(1):25–36.

31. Busch AE, Karbach U, Miska D, Gorboulev V, Akhoundova A, Volk C, Arndt P, UlzheimerJC, Sonders MS, Baumann C, et al. 1998. Human neurons express the polyspecific cationtransporter hOCT2, which translocates monoamine neurotransmitters, amantadine, andmemantine. Mol Pharmacol 54(2):342–352.

32. Jonker JW, Schinkel AH. 2004. Pharmacological and physiological functions of thepolyspecific organic cation transporters: OCT1, 2, and 3 (SLC22A1-3). J Pharmacol ExpTher 308(1):2–9.

33. Kimura N, Masuda S, Tanihara Y, Ueo H, Okuda M, Katsura T, Inui K. 2005. Metforminis a superior substrate for renal organic cation transporter OCT2 rather than hepatic OCT1.Drug Metab Pharmacokinet 20(5):379–386.

34. Bourdet DL, Pritchard JB, Thakker DR. 2005. Differential substrate and inhibitory ac-tivities of ranitidine and famotidine toward human organic cation transporter 1 (hOCT1;SLC22A1), hOCT2 (SLC22A2), and hOCT3 (SLC22A3). J Pharmacol Exp Ther 315(3):1288–1297.

35. Leabman MK, Huang CC, Kawamoto M, Johns SJ, Stryke D, Ferrin TE, DeYoung J, TaylorT, Clark AG, Herskowitz I, Giacomini KM. 2002. Polymorphisms in a human kidneyxenobiotic transporter, OCT2, exhibit altered function. Pharmacogenetics 12(5):395–405.

36. Grundemann D, Liebich G, Kiefer N, Koster S, Schomig E. 1999. Selective substrates fornon-neuronal monoamine transporters. Mol Pharmacol 56(1):1–10.

37. Hayer-Zillgen M, Bruss M, Bonisch H. 2002. Expression and pharmacological profileof the human organic cation transporters hOCT1, hOCT2 and hOCT3. Br J Pharmacol136(6):829–836.

38. Takeda M, Khamdang S, Narikawa S, Kimura H, Kobayashi Y, Yamamoto T, Cha SH,Sekine T, Endou H. 2002. Human organic anion transporters and human organic cationtransporters mediate renal antiviral transport. J Pharmacol Exp Ther 300(3):918–924.

39. Wu X, Kekuda R, Huang W, Fei YJ, Leibach FH, Chen J, Conway SJ, Ganapathy V.1998. Identity of the organic cation transporter OCT3 as the extraneuronal monoaminetransporter (uptake2) and evidence for the expression of the transporter in the brain. J BiolChem 273(49):32776–32786.

40. Eisenhofer G. 2001. The role of neuronal and extraneuronal plasma membrane transportersin the inactivation of peripheral catecholamines. Pharmacol Ther 91(1):35–62.

41. Kimura H, Takeda M, Narikawa S, Enomoto A, Ichida K, Endou H. 2002. Human or-ganic anion transporters and human organic cation transporters mediate renal transport ofprostaglandins. J Pharmacol Exp Ther 301(1):293–298.

42. Urakami Y, Akazawa M, Saito H, Okuda M, Inui K. 2002. cDNA cloning, functionalcharacterization, and tissue distribution of an alternatively spliced variant of organic cationtransporter hOCT2 predominantly expressed in the human kidney. J Am Soc Nephrol13(7):1703–1710.


REFERENCES 31

43. Urakami Y, Okuda M, Masuda S, Saito H, Inui KI. 1998. Functional characteristics andmembrane localization of rat multispecific organic cation transporters, OCT1 and OCT2,mediating tubular secretion of cationic drugs. J Pharmacol Exp Ther 287(2):800–805.

44. Karbach U, Kricke J, Meyer-Wentrup F, Gorboulev V, Volk C, Loffing-Cueni D, KaisslingB, Bachmann S, Koepsell H. 2000. Localization of organic cation transporters OCT1 andOCT2 in rat kidney. Am J Physiol Renal Physiol 279(4):F679–F687.

45. Sugawara-Yokoo M, Urakami Y, Koyama H, Fujikura K, Masuda S, Saito H, Naruse T,Inui K, Takata K. 2000. Differential localization of organic cation transporters rOCT1 andrOCT2 in the basolateral membrane of rat kidney proximal tubules. Histochem Cell Biol114(3):175–180.

46. Sweet DH, Miller DS, Pritchard JB. 2000. Basolateral localization of organic cation trans-porter 2 in intact renal proximal tubules. Am J Physiol Renal Physiol 279(5):F826–F834.

47. Motohashi H, Sakurai Y, Saito H, Masuda S, Urakami Y, Goto M, Fukatsu A, Ogawa O,Inui K. 2002. Gene expression levels and immunolocalization of organic ion transportersin the human kidney. J Am Soc Nephrol 13(4):866–874.

48. Dudley AJ, Bleasby K, Brown CD. 2000. The organic cation transporter OCT2 mediates theuptake of beta-adrenoceptor antagonists across the apical membrane of renal LLC-PK(1)cell monolayers. Br J Pharmacol 131(1):71–79.

49. Lee W, Kim RB. 2004. Transporters and renal drug elimination. Annu Rev PharmacolToxicol 44:137–166.

50. Masuda S, Terada T, Yonezawa A, Tanihara Y, Kishimoto K, Katsura T, Ogawa O, InuiK. 2006. Identification and functional characterization of a new human kidney-specificH+/organic cation antiporter, kidney-specific multidrug and toxin extrusion 2. J Am SocNephrol 17(8):2127–2135.

51. Otsuka M, Matsumoto T, Morimoto R, Arioka S, Omote H, Moriyama Y. 2005. A humantransporter protein that mediates the final excretion step for toxic organic cations. ProcNatl Acad Sci U S A 102(50):17923–17928.

52. Ohashi R, Tamai I, Nezu Ji J, Nikaido H, Hashimoto N, Oku A, Sai Y, Shimane M, TsujiA. 2001. Molecular and physiological evidence for multifunctionality of carnitine/organiccation transporter OCTN2. Mol Pharmacol 59(2):358–366.

53. Meyer-Wentrup F, Karbach U, Gorboulev V, Arndt P, Koepsell H. 1998. Membrane lo-calization of the electrogenic cation transporter rOCT1 in rat liver. Biochem Biophys ResCommun 248(3):673–678.

54. Ciarimboli G, Schlatter E. 2005. Regulation of organic cation transport. Pflugers Arch449(5):423–441.

55. Mehrens T, Lelleck S, Cetinkaya I, Knollmann M, Hohage H, Gorboulev V, Boknik P,Koepsell H, Schlatter E. 2000. The affinity of the organic cation transporter rOCT1 isincreased by protein kinase C–dependent phosphorylation. J Am Soc Nephrol 11(7):1216–1224.

56. Ciarimboli G, Struwe K, Arndt P, Gorboulev V, Koepsell H, Schlatter E, Hirsch JR. 2004.Regulation of the human organic cation transporter hOCT1. J Cell Physiol 201(3):420–428.

57. Pietig G, Mehrens T, Hirsch JR, Cetinkaya I, Piechota H, Schlatter E. 2001. Properties andregulation of organic cation transport in freshly isolated human proximal tubules. J BiolChem 276(36):33741–33746.

58. Cetinkaya I, Ciarimboli G, Yalcinkaya G, Mehrens T, Velic A, Hirsch JR, Gorboulev V,Koepsell H, Schlatter E. 2003. Regulation of human organic cation transporter hOCT2 by



PKA, PI3K, and calmodulin-dependent kinases. Am J Physiol Renal Physiol 284(2):F293–F302.

59. Pelis RM, Suhre WM, Wright SH. 2006. Functional influence of N-glycosylation in OCT2-mediated tetraethylammonium transport. Am J Physiol Renal Physiol 290(5):F1118–F1126.

60. Urakami Y, Nakamura N, Takahashi K, Okuda M, Saito H, Hashimoto Y, Inui K. 1999.Gender differences in expression of organic cation transporter OCT2 in rat kidney. FEBSLett 461(3):339–342.

61. Slitt AL, Cherrington NJ, Hartley DP, Leazer TM, Klaassen CD. 2002. Tissue distributionand renal developmental changes in rat organic cation transporter mRNA levels. DrugMetab Dispos 30(2):212–219.

62. Shu Y, Bello CL, Mangravite LM, Feng B, Giacomini KM. 2001. Functional characteristicsand steroid hormone–mediated regulation of an organic cation transporter in Madin-Darbycanine kidney cells. J Pharmacol Exp Ther 299(1):392–398.

63. Martel F, Keating E, Calhau C, Grundemann D, Schomig E, Azevedo I. 2001. Regu-lation of human extraneuronal monoamine transporter (hEMT) expressed in HEK293cells by intracellular second messenger systems. Naunyn-Schmiedebergs Arch Pharmacol364(6):487–495.

64. Jonker JW, Wagenaar E, Mol CA, Buitelaar M, Koepsell H, Smit JW, Schinkel AH. 2001.Reduced hepatic uptake and intestinal excretion of organic cations in mice with a tar-geted disruption of the organic cation transporter 1 (Oct1 [Slc22a1]) gene. Mol Cell Biol21(16):5471–5477.

65. Wang DS, Jonker JW, Kato Y, Kusuhara H, Schinkel AH, Sugiyama Y. 2002. Involvement oforganic cation transporter 1 in hepatic and intestinal distribution of metformin. J PharmacolExp Ther 302(2):510–515.

66. Wang DS, Kusuhara H, Kato Y, Jonker JW, Schinkel AH, Sugiyama Y. 2003. Involvementof organic cation transporter 1 in the lactic acidosis caused by metformin. Mol Pharmacol63(4):844–848.

67. Jonker JW, Wagenaar E, Van Eijl S, Schinkel AH. 2003. Deficiency in the organic cationtransporters 1 and 2 (Oct1/Oct2 [Slc22a1/Slc22a2]) in mice abolishes renal secretion oforganic cations. Mol Cell Biol 23(21):7902–7908.

68. Zwart R, Verhaagh S, Buitelaar M, Popp-Snijders C, Barlow DP. 2001. Impaired activityof the extraneuronal monoamine transporter system known as uptake-2 in Orct3/Slc22a3-deficient mice. Mol Cell Biol 21(13):4188–4196.

69. Kerb R, Brinkmann U, Chatskaia N, Gorbunov D, Gorboulev V, Mornhinweg E, KeilA, Eichelbaum M, Koepsell H. 2002. Identification of genetic variants of the humanorganic cation transporter hOCT1 and their functional consequences. Pharmacogenetics12(8):591–595.

70. Shu Y, Leabman MK, Feng B, Mangravite LM, Huang CC, Stryke D, Kawamoto M,Johns SJ, DeYoung J, Carlson E, et al. 2003. Evolutionary conservation predicts functionof variants of the human organic cation transporter, OCT1. Proc Natl Acad Sci U S A100(10):5902–5907.

71. Sakata T, Anzai N, Shin HJ, Noshiro R, Hirata T, Yokoyama H, Kanai Y, Endou H.2004. Novel single nucleotide polymorphisms of organic cation transporter 1 (SLC22A1)affecting transport functions. Biochem Biophys Res Commun 313(3):789–793.


REFERENCES 33

72. Takeuchi A, Motohashi H, Okuda M, Inui K. 2003. Decreased function of genetic variants,Pro283Leu and Arg287Gly, in human organic cation transporter hOCT1. Drug MetabPharmacokinet 18(6):409–412.

73. Saito S, Iida A, Sekine A, Ogawa C, Kawauchi S, Higuchi S, Nakamura Y. 2002. Catalogof 238 variations among six human genes encoding solute carriers ( hSLCs) in the Japanesepopulation. J Hum Genet 47(11):576–584.

74. Fukushima-Uesaka H, Maekawa K, Ozawa S, Komamura K, Ueno K, Shibakawa M,Kamakura S, Kitakaze M, Tomoike H, Saito Y, Sawada J. 2004. Fourteen novel single nu-cleotide polymorphisms in the SLC22A2 gene encoding human organic cation transporter(OCT2). Drug Metab Pharmacokinet 19(3):239–244.

75. Leabman MK, Giacomini KM. 2003. Estimating the contribution of genes and environmentto variation in renal drug clearance. Pharmacogenetics 13(9):581–584.

76. Leabman MK, Huang CC, DeYoung J, Carlson EJ, Taylor TR, de la Cruz M, Johns SJ,Stryke D, Kawamoto M, Urban TJ, et al. 2003. Natural variation in human membranetransporter genes reveals evolutionary and functional constraints. Proc Natl Acad Sci U SA 100(10):5896–5901.

77. Lazar A, Grundemann D, Berkels R, Taubert D, Zimmermann T, Schomig E. 2003. Geneticvariability of the extraneuronal monoamine transporter EMT (SLC22A3). J Hum Genet48(5):226–230.

drug transporters || organic cation transporters

Documents