cgmp transport by vesicles from human and mouse erythrocytes

cGMP transport by vesicles from human and mouseerythrocytesCornelia J. F. de Wolf1, Hiroaki Yamaguchi1,*, Ingrid van der Heijden1, Peter R. Wielinga1,†,Stefanie L. Hundscheid1,‡, Nobuhito Ono1,§, George L. Scheffer2, Marcel de Haas1,John D. Schuetz3, Jan Wijnholds1,4 and Piet Borst1

1 Department of Molecular Biology, the Netherlands Cancer Institute, Amsterdam, the Netherlands

2 Department of Pathology, Free University Medical Center, Amsterdam, the Netherlands

3 Department of Pharmaceutical Sciences, St Jude Children’s Research Hospital, Memphis, TN, USA

4 Netherlands Institute for Neurosciences, Royal Netherlands Academy of Arts and Sciences (KNAW), Amsterdam, the Netherlands

Three ATP-binding cassette (ABC) proteins, the multi-

drug resistance-associated proteins (MRPs), MRP4,

MRP5 and MRP8, now known as ABCC4, ABCC5,

and ABCC11, have been reported to transport cGMP

out of cells in an ATP-dependent manner [1–9]. The

physiologic significance of cGMP transport by these

transporters has remained unclear, however, and the

reported affinity of ABCC4 and ABCC5 for cGMP

Keywords

ABCC4; ABCG2; cGMP; multidrug

resistance; multidrug resistance protein

(MRP)

Correspondence

P. Borst, Department of Molecular Biology,

the Netherlands Cancer Institute, 1066 CX,

Plesmanlaan 121, Amsterdam, the Netherlands

Fax: +31 20 6691383

Tel: +31 20 5122880

E-mail: [email protected]

Present address

*Department of Pharmaceutical Sciences,

Tohoku University Hospital, Sendai, Japan

†National Institute for Public Health and

Environment (RIVM), Microbiological

Laboratory for Health Protection (MGB),

Bilthoven, the Netherlands

‡Division of Diagnostic Oncology, the

Netherlands Cancer Institute, Amsterdam,

the Netherlands

§The 2nd Department of Internal Medicine,

Faculty of Medicine, Kagoshima University,

Kagoshima, Japan

(Received 13 September 2006, revised

20 October 2006, accepted 13 November

2006)

doi:10.1111/j.1742-4658.2006.05591.x

cGMP secretion from cells can be mediated by ATP-binding cassette

(ABC) transporters ABCC4, ABCC5, and ABCC11. Indirect evidence sug-

gests that ABCC4 and ABCC5 contribute to cGMP transport by erythro-

cytes. We have re-investigated the issue using erythrocytes from wild-type

and transporter knockout mice. Murine wild-type erythrocyte vesicles

transported cGMP with an apparent Km that was 100-fold higher than

their human counterparts, the apparent Vmax being similar. Whereas cGMP

transport into human vesicles was efficiently inhibited by the ABCC4-speci-

fic substrate prostaglandin E1, cGMP transport into mouse vesicles was

inhibited equally by Abcg2 and Abcc4 inhibitors ⁄ substrates. Similarly,

cGMP transport into vesicles from Abcc4– ⁄ – and Abcg2– ⁄ – mice was 42%

and 51% of that into wild-type mouse vesicles, respectively, whereas cGMP

transport into vesicles from Abcc4– ⁄ – ⁄Abcg2– ⁄ – mice was near background.

The knockout mice were used to show that Abcg2-mediated cGMP trans-

port occurred with lower affinity but higher Vmax than Abcc4-mediated

transport. Involvement of Abcg2 in cGMP transport by Abcc4– ⁄ – erythro-

cyte vesicles was supported by higher transport at pH 5.5 than at pH 7.4, a

characteristic of Abcg2-mediated transport. The relative contribution of

ABCC4 ⁄Abcc4 and ABCG2 ⁄Abcg2 in cGMP transport was confirmed with

a new inhibitor of ABCC4 transport, the protease inhibitor 4-(2-amino-

ethyl)benzenesulfonyl fluoride.

Abbreviations

ABC, ATP-binding cassette; AEBSF, 4-(2-aminoethyl)benzenesulfonyl fluoride; Bcrp, murine breast cancer resistance protein; BCRP, human breast

cancer resistance protein; KO, knockout; MRP, multidrug resistance-associated protein; MTX, methotrexate; PG, prostaglandin.

FEBS Journal 274 (2007) 439–450 ª 2006 The Authors Journal compilation ª 2006 FEBS 439

differs widely depending on the investigator and

experimental method used [1,2,4,9].

The group of Sager characterized cGMP efflux from

human erythrocytes [10–15]. Subsequent studies with

various MRP inhibitors suggested that the major cGMP

transport system (low affinity) of erythrocytes has prop-

erties similar to those reported for ABCC4 [16–18].

However, Boadu & Sager [19] recently suggested that

ABCC5 is the major cGMP transporter in human eryth-

rocytes, based on their findings in ABCC5-depleted

human erythrocyte proteoliposomes. To further explore

this issue, we have turned to murine erythrocytes.

As knockout (KO) mice lacking specific ABC trans-

porters are available, it should be possible to unambigu-

ously determine the contribution of each transporter to

cGMP transport in these mice, rather than relying on

more or less specific inhibitors. Mice lacking Abcc4 have

been described [20]. Here we report the generation of

Abcc5– ⁄ – mice. Using these and other KO mice, we

found that at a substrate concentration of 1.8 lm

cGMP, about half of the cGMP transport by murine

erythrocyte vesicles is mediated by Abcg2 [murine breast

cancer resistance protein 1 (Bcrp1)], a transporter previ-

ously not known to transport nucleotides. The other

half is mediated by Abcc4. Abcc5 makes either a minor

or no contribution to cGMP transport. In contrast, our

results support the conclusion [16,17] that the bulk of

cGMP transport by vesicles from human erythrocytes is

attributable to ABCC4 and not to ABCC5 or ABCG2

[human breast cancer resistance protein (BCRP)].

Results

ABC transporters in mouse erythrocytes

To determine which of the ABC transporters that are

able to transport cGMP are present in the erythrocyte

membrane, we analyzed freshly isolated mouse erythro-

cytes by immunoblot, using Abcc1 and Abcg2 as

positive controls. Abcc4 and 5 were detected (Fig. 1).

Mice lack the ortholog of the human ABCC11 gene

[21]. Figure 1 also shows blots for erythrocytes of each

of the KO mice tested. Each KO mouse had indeed

lost the corresponding transporter, and the loss of one

transporter had not resulted in major secondary altera-

tions of the level of other transporters. However, we

note that we have not done serial dilutions of the pro-

tein loaded to determine more precisely whether minor

alterations (two-fold) do occur. For comparison,

Fig. 2 shows results obtained with human erythrocytes.

ABCC1, ABCC4, ABCC5 and ABCG2 were readily

detected (Fig. 2A), but ABCC11 was not (Fig. 2B).

Slight interindividual variations in ABCC1, ABCC4

and ABCG2 levels were observed between the human

volunteers, whereas larger variations in ABCC5 pro-

tein levels were seen. Although interindividual differ-

ences may be caused by variation in transporter

degradation between samples, the differences in

ABCC5 levels between individuals were repeatedly seen

in independent samples.

cGMP transport into membrane vesicles from

mouse erythrocytes

At a substrate concentration of 1.8 lm, the rate and

affinity of cGMP transport into mouse erythrocyte

vesicles (Fig. 3A–C) were much lower than reported

for human erythrocyte vesicles [16] and confirmed here

(Km ¼ 132 ± 31 lm; Fig. 3D–F). This was due to the

low affinity of the murine transporters for cGMP, the

apparent Km being about 9 mm (9.0 ± 1.8 mm). This

is obviously a very rough estimate, as the maximal

concentration tested was 10 mm cGMP. The Vmax of

about 0.8 nmolÆ(mg protein)Æmin)1 [0.76 ± 0.24 nmolÆ(mg protein)Æmin)1] was comparable to that obtained

with human erythrocytes [0.39 ± 0.22 nmolÆ(mg protein)Æmin)1).

Fig. 1. Levels of Abccs and Abcg2 in erythrocytes from WT and

KO mice. Western blot analysis of 10 lg of protein from mouse

erythrocyte vesicles. Each protein was detected as described in

Experimental procedures.

cGMP transport by erythrocytes C. J. F. de Wolf et al.

440 FEBS Journal 274 (2007) 439–450 ª 2006 The Authors Journal compilation ª 2006 FEBS

Inhibition of cGMP transport by MRP-specific

inhibitors and substrates

To test whether similar transport systems mediate

cGMP transport in human and mouse erythrocytes,

the effect of MRP inhibitors on cGMP transport was

assessed. The results are summarized in Table 1. The

sensitivity of cGMP transport into human erythrocyte

vesicles to MRP inhibitors was consistent with that

found in earlier studies [16,18]. In addition, we found

inhibition by low concentrations of prostaglandin

(PG) E1 and PGE2. This is of interest, as these com-

pounds are relatively specific for ABCC4 and are not

detectably transported in vesicular transport experi-

ments by ABCC5 [22]. Less than 50% inhibition of

cGMP transport was obtained with the ABCG2

inhibitors Ko143 and GF120918. Significantly differ-

ent results were obtained with murine erythrocyte ves-

icles. On the one hand, PGE1 and PGE2 reduced

cGMP transport to only 57% and 59% of the con-

trol value, and the inhibitory effect of dipyridamole

and indomethacin was also less pronounced. On the

other hand, the Abcg2-specific inhibitor Ko143 inhib-

ited cGMP transport by the murine vesicles more

(52%) than cGMP transport by the human vesicles

(33%). These results raised the possibility that Abcg2

contributes to mouse erythrocyte cGMP transport (as

well as Abcc4), even though cGMP transport by

ABCG2 has not been reported before.

cGMP transport into erythrocyte membrane

vesicles from Abcc KO mice

Figure 4 shows the amount of cGMP transported after

30 min into vesicles from KO mice. Relative to wild-

type (WT) mice, the amounts obtained with Abcc4– ⁄ –,

Abcc1– ⁄ – ⁄Abcc4– ⁄ –, Abcc4– ⁄ – ⁄Abcc5– ⁄ –, Abcg2– ⁄ – and

Abcc4– ⁄ – ⁄Abcg2– ⁄ – mice were 42%, 42%, 39%, 51%

and 16%, respectively (P < 0.01, as determined by

one-way anova). The differences in cGMP transport

between Abcc1– ⁄ –, Abcc5– ⁄ – and WT mice were not sig-

nificant.

Erythrocyte vesicles isolated from the Abcc4– ⁄ – ⁄Abcg2– ⁄ – mouse still transported cGMP at 16% of the

WT control level. As this value is close to background,

as reflected by the large standard deviation, its signifi-

cance is low. It may reflect a small contribution of

Abcc5 to cGMP transport, however, as the Abcc5– ⁄ –

mouse also displayed a slight (not statistically signifi-

cant) reduction in cGMP transport. The borderline

transport remaining in the Abcc4– ⁄ – ⁄Abcg2– ⁄ – vesicles

shows that the inside-in vesicles present in our vesicle

preparations do not interfere with the cGMP transport

measurements.

Abcc4 and Abcg2 transport cGMP into mouse

erythrocyte vesicles

The results with inhibitors (Table 1) and KO mice

(Fig. 4A) indicated that Abcc4 and Abcg2 contribute

about equally to cGMP transport into mouse erythro-

cyte vesicles at the low substrate concentration used,

1.8 lm. We therefore made an attempt to determine

the kinetic constants for Abcc4- and Abcg2-mediated

cGMP transport using erythrocyte vesicles from the

KO mice, assuming that the remaining cGMP trans-

port in the Abcc4– ⁄ – mouse is due to Abcg2, and the

remaining transport in the Abcg2– ⁄ – mouse is due to to

Abcc4. The results are presented in Fig. 4B. At the

cGMP concentration routinely used for vesicular

uptake assays, 1.8 lm, Abcc4 and Abcg2 indeed con-

tributed equally to cGMP transport. However, at milli-

molar cGMP concentrations, Abcc4-specific cGMP

transport was saturable [Vmax ¼ 0.20 ± 0.03 nmol

cGMPÆ(mg protein)Æmin)1], whereas saturation of

Abcg2-specific cGMP transport was not reached

[apparent Vmax about 1.4 nmol cGMPÆ(mg pro-

tein)Æmin)1]. Nonlinear regression analysis further

yielded an apparent Km of about 2.3 ± 0.9 mm for

cGMP transport by Abcc4, and an estimated apparent

Km > 10 mm for cGMP transport by Abcg2. This

shows that both murine transporters have a much

lower affinity for cGMP than human ABCC4.

A B

Fig. 2. Levels of ABCCs and ABCG2 in human erythrocytes.

(A) Western blot analysis of 10 lg of protein from human erythro-

cyte vesicles from five healthy volunteers (lanes 1–5). Each protein

was detected as described in Experimental procedures. (B) West-

ern blot analysis of 40 lg of protein from human erythrocyte vesi-

cles from three healthy volunteers (lanes 1–3). Lane 4: 10 lg of

protein from Sf9-hABCC11 cell lysate (positive control). Lane 5:

40 lg of protein from Sf9 WT cell lysate (negative control). Only

results obtained with monoclonal antibody M8II-16 are shown.

ABCC11 was detected as described in Experimental procedures.

h. ery ves, human erythrocyte vesicles.

C. J. F. de Wolf et al. cGMP transport by erythrocytes


The role of ABCG2/Abcg2 in cGMP transport into

human and mouse erythrocyte vesicles

To further characterize the contribution of Abcc4 and

ABCG2 ⁄Abcg2 to erythrocyte cGMP transport, vesi-

cular uptake assays performed at physiologic pH were

compared with those done at low pH. Recently, it was

shown that ABCG2 transports methotrexate (MTX)

and resveratrol at a much higher rate at pH 6.0 than

at pH 7.4 [23]. In Fig. 5, we compare MTX (Fig. 5A)

and cGMP (Fig. 5B) transport into erythrocyte vesi-

cles at physiologic pH (pH 7.4) with transport at low

pH (pH 5.5). We confirmed the pH effect for murine

Abcg2 by demonstrating that MTX transport was

increased at pH 5.5 compared with pH 7.4 in vesicles

from WT and Abcc4– ⁄ – mice, whereas MTX transport

into vesicles derived from Abcg2– ⁄ – mice was not affec-

ted by low pH (Fig. 5A). Similarly, cGMP transport

into WT and Abcc4– ⁄ – mouse erythrocyte vesicles was

increased at low pH, whereas this pH effect was

absent from vesicles from Abcg2– ⁄ – mice (Fig. 5B).

However, whereas MTX transport into WT mouse

erythrocyte vesicles was increased 12-fold by low-pH

assay conditions, cGMP transport was increased only

two-fold. In contrast, low pH drastically decreased

transport of cGMP and MTX into human erythrocyte

vesicles. These results are compatible with a substan-

tial role for Abcg2 in cGMP transport by mouse

Fig. 3. Transport of cGMP into mouse and human erythrocyte vesicles. Erythrocyte membrane vesicles from five WT mice (A) or five

healthy volunteers (D) were incubated for the specified times at 37 �C with 1.8 lM [3H]cGMP. Concentration-dependent transport of cGMP

into vesicles from four WT mice (B) or five healthy volunteers (E) was determined over a time span of 30 min. ATP-dependent transport

was calculated by subtracting the transport in the absence of ATP from that in the presence of ATP. Each point represents the mean ATP-

dependent cGMP transport ± SD. The background in the minus ATP control is illustrated in (C) and (F). Human erythrocyte vesicles 1–5

correspond to an individual subject, and are consistent throughout the figure (D, E). Erythrocyte vesicles isolated from a single mouse were

sufficient to perform a single experiment in triplicate. Therefore, mice 1–4 in (A) are not the same as mice 1–4 in (B).



erythrocytes and a negligible role for ABCG2 in

human erythrocytes.

4-(2-Aminoethyl) benzenesulfonyl fluoride

(AEBSF) inhibits Abcc4-specific cGMP transport

but not Abcg2-specific cGMP transport

AEBSF is an irreversible serine protease inhibitor [24]

that functions through acylation of serine residues in

the active site of the protease, resulting in sulfonate

ester formation [25]. As such, it is frequently included

in buffers and assay mixtures to prevent protein degra-

dation in plasma samples, in cell lysates, or in the

course of an enzymatic assay. However, AEBSF has

also been shown to bind to serine residues of other

proteins, and to a lesser extent also to tyrosine, lysine

and histidine residues, as well as the protein ⁄peptideN-terminus [26–29].

While optimizing the procedure for vesicle prepara-

tion, we observed an inhibitory effect of AEBSF on

acetylcholinesterase activity (reported also for the rela-

ted protease inhibitor phenylmethanesulfonyl fluoride

[30]) and, unexpectedly, also on the transport of

cGMP. Figure 6A shows the effect of three protease

inhibitors on cGMP transport into inside-out vesicles

prepared from human erythrocytes. In the concentra-

tion range recommended for the inhibition of protease

activity, leupeptin and aprotonin had a negligible effect

on vesicular uptake of cGMP. In contrast, complete

inhibition of cGMP transport into human erythrocyte

vesicles was already achieved at an AEBSF concentra-

tion of 5 mgÆmL)1 (Fig. 6B). Preincubation at room

temperature of human inside-out erythrocyte vesicles

in transport assay buffer resulted in decreased cGMP

transport in incubations including 1 mg AEBSFÆmL)1

but not in incubations lacking AEBSF (Fig. 6C). The

experiments were also performed with erythrocyte vesi-

cles from WT and KO mice to determine whether the

inhibition was transporter-specific or due to an overall

effect of AEBSF on the vesicles. cGMP transport into

erythrocyte inside-out vesicles from WT mice was

inhibited down to the level of transport observed for

vesicles from Abcc4– ⁄ – mice. In agreement with this,

cGMP uptake into vesicles from Abcc4– ⁄ – mice was

not affected by AEBSF, whereas AEBSF inhibited

cGMP uptake by vesicles from Abcg2– ⁄ – mice to the

same extent as observed for WT vesicles (Fig. 6D).

cGMP efflux from intact human erythrocytes

With intact HEK293 cells, we have previously reported

cGMP efflux mediated by ABCC4 or ABCC5 [4]. In an

attempt to show in vivo cGMP production and excretion

by human erythrocytes, we measured cGMP content as

well as cGMP efflux from freshly isolated and sodium

nitroprusside-stimulated erythrocytes, but we were

repeatedly unable to demonstrate the presence of cGMP

inside the erythrocytes, or of cGMP from the stimulated

Table 1. Effect of ABCC inhibitors and substrates on cGMP transport. Membrane vesicles from human and WT mouse erythrocytes were

coincubated for 30 min at 37 �C with 1.8 lM [3H]cGMP and various established ABCC inhibitors ⁄ substrates. Each value was calculated by

subtracting ATP-dependent cGMP transport in the presence of inhibitor from that in the absence of inhibitor. Each value represents the

mean ± SD of duplicate measurements obtained from vesicles prepared from five individual mice or six human volunteers. Sample popula-

tions were tested for normality of distribution (Gaussian distribution). Student’s t-test, with Welch’s correction for unequal variance when

necessary, was performed to compare the degree of inhibition observed for each condition for mouse and human erythrocyte vesicles. The

Mann–Whitney test was performed when the sample size was too small (n ¼ 4) for an accurate estimation of sample distribution. NS, not

significant.

Inhibitor

Concentration

(lM)

Erythrocyte vesicles

Mouse (n ¼ 5)

Transport

(% of control)

Human (n ¼ 6)

Transport

(% of control)

Student’s t-test

Mouse versus human

Dipyridamole 10 39.1 ± 6.6 26.1 ± 5.9 P ¼ 0.01

50 20.6 ± 12.3 5.0 ± 1.8 P < 0.05

Indomethacin 10 62.2 ± 17.9 5.1 ± 1.0 P < 0.01

50 42.9 ± 3.3a 0.9 ± 1.3 P < 0.05

MK571 5 35.2 ± 5.7a 9.0 ± 0.9a P < 0.05

PGE1 20 57.4 ± 11.1 2.0 ± 1.0 P < 0.001

PGE2 20 59.2 ± 8.6 4.1 ± 1.8 P < 0.001

Ko143 5 47.8 ± 18.9 67.1 ± 14.7 NS

GF120918 5 55.9 ± 19.4 58.5 ± 10.9 NS

a Average of measurements from four individuals.



erythrocytes in the medium (results not shown). The

previously described HEK293 cells transfected with

ABCC4 cDNA [4] were included as a positive control,

and did secrete cGMP. It should be noted, however, that

the expression of MRP4 in the HEK293 cells is much

higher than the expression of MRP4 in erythrocytes;

5–10-fold as estimated from western blots.

Discussion

We have used murine erythrocytes to obtain more

insight into the nature of the cGMP transporters pre-

sent in the erythrocyte membrane. At low cGMP con-

centrations (1.8 lm), Abcc4 and Abcg2 contribute

equally to vesicular transport, as shown by the fact

that transport into vesicles from Abcc4– ⁄ – or Abcg2– ⁄ –

mice is about half that into WT vesicles (Fig. 4A).

At higher cGMP concentrations, Abcg2 contributes

more, as its apparent Vmax is higher than that of

Abcc4; 1.4 versus 0.2 nmol cGMPÆ(mg protein)Æmin)1,

Fig. 4. Transport of cGMP into erythrocyte vesicles from WT and

KO mice. (A) Erythrocyte membrane vesicles from WT and KO

mice were incubated for 30 min at 37 �C with 1.8 lM [3H]cGMP.

ATP-dependent transport of cGMP into vesicles from WT mice was

set to 100%. (B) Concentration-dependent transport of cGMP, 0.5–

10 mM, into vesicles from WT (h), Abcc4– ⁄ – (.), Abcg2– ⁄ – (d) and

Abcc4– ⁄ – ⁄ Abcg2 – ⁄ – (s) mice was determined over a time span of

30 min. ATP-dependent transport was calculated by subtracting the

transport in the absence of ATP from that in the presence of ATP.

Each value represents the mean ± SD of duplicate measurements

from at least three individual mice.Fig. 5. Effect of pH on MTX and cGMP transport into membrane

vesicles from humans and from WT and KO mice. (A) Effect of pH

on MTX transport. Erythrocyte membrane vesicles from humans

and WT and KO mice were incubated for 10 min at 37 �C with

1 lM [3H]MTX at either pH 7.4 (j) or pH 5.5 (h). (B) Effect of pH

on cGMP transport. Erythrocyte membrane vesicles from WT and

KO mice were incubated for 30 min at 37 �C with 1.8 lM

[3H]cGMP at either pH 7.4 (j) or pH 5.5 (h). For both panels, ATP-

dependent transport was calculated by subtracting the transport in

the absence of ATP from that in the presence of ATP. Substrate

transport into vesicles from WT mice at pH 7.4 was set to 100%.

The vesicle uptake buffer was 10 mM Tris at either pH 7.4 or

pH 5.5. The final pH was verified by measurement with a pH

meter. Each value represents the mean ± SD of duplicate measure-

ments from three individuals ⁄ mice. For these experiments, erythro-

cyte vesicles from human individuals 1, 2 and 3 from Fig. 3 were

used.



respectively (Fig. 4B). The ability of Abcg2 to trans-

port cGMP has not been noted before. This is sup-

ported not only by the experiments with the Abcg2– ⁄ –

erythrocyte vesicles, but also by the increased cGMP

transport at pH 5.5 (Fig. 5B), which is specific for

the Abcg2 fraction of cGMP transport. Increased

transport of MTX and resveratrol by human ABCG2

at acidic pH was first noted by Breedveld et al. [23],

but it is clear from Fig. 5A that it also applies to

murine Abcg2 and to the substrate cGMP (Fig. 5B),

although the pH effect on cGMP transport is less

pronounced than on MTX transport. Whether trans-

port of cGMP by Abcg2 has any physiologic signifi-

cance is doubtful, given the very low affinity of

Abcg2 for this substrate. The low rate of cGMP

transport by Abcg2 at substrate concentrations below

100 lm may also explain why this Abcg2 activity has

not been noted before. The ability of other ABC

transporters, such as ABCC4, ABCC5 and ABCC8,

to transport cyclic nucleotides is accompanied by the

ability to transport nucleotide analogs. Indeed, Wang

et al. [31,32] have reported that ABCG2 overexpres-

sion induces low-level resistance to some antiviral

nucleoside analogs, presumably through increased

excretion of the corresponding nucleotide analogs,

and we have recently found that Abcg2 confers high-

level resistance to the nucleoside analog cladribine

(unpublished results).

Our results for human erythrocyte vesicles confirm

and extend the conclusions of Klokouzas et al. [16]

and Wu et al. [18], in that cGMP transport by these

vesicles is attributable to ABCC4. We found >95%

inhibition by PGE1 and PGE2, at present the most

ABCC4-specific substrates known [22], and a complete

block of cGMP transport by the protease inhibitor

AEBSF, which seems to be relatively specific for

ABCC4, as we have not found inhibition by this com-

pound of ABCG2 ⁄Abcg2 (Fig. 6). We note in passing

that the inhibition of ABCC4 by AEBSF is a compli-

cation that should be kept in mind, as protease inhib-

itor cocktails are often used routinely in vesicular

transport experiments.

A C

BD

Fig. 6. Effect of AEBSF, aprotinin and leupeptin on cGMP transport into membrane vesicles from humans and from WT and KO mice.

(A) Effect of three different protease inhibitors on cGMP transport by human erythrocyte vesicles. Erythrocyte membrane vesicles were co-

incubated for 30 min at 37 �C with 1.8 lM [3H]cGMP and the indicated concentration of either AEBSF, leupeptin or aprotinin. (B) Concentra-

tion-dependent effect of AEBSF on cGMP transport by human erythrocyte vesicles. Erythrocyte membrane vesicles were coincubated for

30 min at 37 �C with 1.8 lM [3H]cGMP and AEBSF in the concentration range of 0.5–10 mg AEBSF per milliliter of incubation mix. (C) Effect

of preincubation of human erythrocyte vesicles with AEBSF on cGMP transport. Vesicles were preincubated at room temperature with (h)

or without (j) 1 mg of AEBSF per milliliter of incubation mix for either 0, 30 or 60 min. The length of preincubation time is shown on the

x-axis. Transport reactions were initiated by addition of 4 mM ATP. (D) Concentration-dependent effect of AEBSF on cGMP transport by WT

and KO mouse erythrocyte vesicles. Erythrocyte membrane vesicles from WT (j), Abcc4– ⁄ – (j), Abcg2– ⁄ – (h) and Abcc4– ⁄ – ⁄ Abcg2 – ⁄ – (j)

mice were coincubated for 30 min at 37 �C with 1.8 lM [3H]cGMP and 0, 0.1, 0.5 or 1 mg of AEBSF per milliliter of incubation mix. ATP-

dependent cGMP transport activity by vesicles from WT mice without addition of AEBSF were set to 100%; all other values are relative to

this value. All panels display the ATP-dependent transport of cGMP, which was calculated by subtracting the transport in the absence of

ATP from that in the presence of ATP. Each value represents the mean ± SD of duplicate measurements from three individuals ⁄ mice.



Although ABCG2 is present in human erythrocytes

(Fig. 2), it does not appear to significantly contribute

to cGMP transport, as there is no detectable transport

at pH 5.5 (Fig. 5B). The data in Fig. 5 indicate that

neither human ABCC4 nor murine Abcc4 transports

any cGMP at pH 5.5. Why the ABCG2-specific inhib-

itor Ko143 appears to inhibit cGMP transport into

human erythrocyte inside-out vesicles (Table 1) is

unclear. It seems likely that this is a nonspecific inhibi-

tory effect, like the inhibition by GF120918. Whether

the ABCC5 that is clearly present in human (Fig. 2)

and murine (Fig. 1) erythrocytes contributes at all to

cGMP transport is uncertain. There are no inhibitors

specific for ABCC5, and our results with the KO mice

(Fig. 4) are not unambiguous. Although the absence of

Abcc5 in the KO mice tends to lower the transport

rate somewhat, the effect is minimal and not statis-

tically significant. A substantial contribution of

ABCC5 ⁄Abcc5 to erythrocyte cGMP transport, as

postulated by Boadu & Sager [19], is therefore ruled

out by our results. Boadu & Sager [19] measured

cGMP transport by protein fractions immunoprecipi-

tated from a detergent extract of erythrocytes and

reconstituted in proteoliposomes. In our opinion, the

authors provide no evidence that this approach can be

used as a quantitative assay for transport activity.

The low rate of cGMP transport by murine erythro-

cyte vesicles relative to their human counterparts is

clearly not due to differences in Vmax, but to the low

affinity of the murine transporters for cGMP, resulting

in minimal transport at the cGMP concentration

(1.8 lm) used in Fig. 3. Figure 4 shows that this low

affinity holds for both the Abcc4 and the Abcg2 com-

ponents of cGMP transport by murine erythrocytes.

What could be the physiologic role of ABCC4 activity

in erythrocytes? We have been unable to detect cGMP

in erythrocytes or cGMP efflux from erythrocytes after

stimulation, ruling out a role for ABCC4 in cGMP

transport in mature erythrocytes. It is possible that

ABCC4 is involved in secretion of cGMP from an eryth-

roid precursor cell, and that the ABCC4 in mature

erythrocytes is just a leftover, caused by the long half-

life of ABCCs [33]. Given the very low (mm) affinity of

murine Abcc4 for cGMP (Fig. 4B), it seems unlikely,

however, that cGMP transport is a normal function of

ABCC4 at all. Further studies with the Abcc4 and

Abcc5 KO mice now available should help to settle the

question of whether these transporters have any physio-

logic role as cyclic nucleotide transporters [34].

Mouse models are routinely used for the purpose of

drug resistance testing in cancer and antiviral research.

Erythrocytes may function as a carrier system in the

transport of endogenous compounds and xenobiotics,

such as the anticancer agents 6-mercaptopurine and

thioguanine, through the body. Active low-affinity,

high-capacity efflux of these compounds and their

metabolites from the erythrocyte by ABCC4 might

affect the bioavailability of these drugs [35]. However,

our finding that murine and human Abcc4 ⁄ABCC4 and

Abcg2 ⁄ABCG2 differ greatly in their affinity for cGMP

raises the question of whether this also holds for other

substrates, such as nucleoside analog drugs. Hence, we

are performing in vitro experiments to further examine

potential differences in substrate affinity between human

and murine variants of the ABCC ⁄Abcc transporters.

Experimental procedures

Animals

Abcc4– ⁄ – [20], Abcc1– ⁄ – [36] and Abcg2– ⁄ – [37] mice were

generated previously. The Abcc5– ⁄ – mouse was generated

by J. Wijnholds through Abcc5 gene targeting. Briefly, a

sequenced 0.3 kb mouse Abcc5 cDNA fragment containing

sequences encoding the first ATP-binding domain of Abcc5

was used to screen an EMBL3 genomic 129 ⁄Ola DNA

phage library. Four identical phage clones were character-

ized by Southern blotting, and exon–intron boundaries

were mapped. A targeting vector was constructed by assem-

bling a 4.1 kb SacI–EcoRV 5¢-Abcc5 genomic fragment, a

fragment containing a hygromycin resistance gene driven

by the mouse phosphoglycerate kinase promoter in reverse

orientation, and a 3.4 kb SmaI–StuI 3¢ fragment of the

Abcc5 gene. Correct targeting deleted 1.5 kb of Abcc5

sequences containing exon 17 encoding amino acids 678–

745 of the first ATP-binding domain. Transfection of the

targeting construct into 129 ⁄Ola-derived E14 embryonic

stem (ES) cells resulted in 10% homologous recombinants.

Targeted clones with the predicted replacement event were

identified by using probes 5¢ and 3¢ to the homology region.

Two of the ES cell clones with normal karyotype were

injected into mouse blastocysts, and both resulted in

chimeric mice that transmitted the Abcc5 mutant allele

through the germline of F1 offspring. The homozygous

mice were backcrossed to 100% Friend virus B-type (FVB)

genetic background. Double-KO mice, Abcc1– ⁄ – ⁄Abcc4– ⁄ –,

Abcc4– ⁄ – ⁄Abcc5– ⁄ – and Abcc4– ⁄ – ⁄Abcg2– ⁄ –, were generated

by crossbreeding of the single-KO mice. Male and

female Abcc1– ⁄ –, Abcc4– ⁄ –, Abcc5– ⁄ –, Abcc1 ⁄ 4– ⁄ –, Abcc4– ⁄ – ⁄Abcc5– ⁄ –, Abcg2– ⁄ – and Abcc4– ⁄ – ⁄Abcg2– ⁄ – mice and WT

mice were of comparable genetic background (FVB or

mixed Ola ⁄B6 and FVB) and were killed between 9 and

14 weeks of age. Animals were kept in a temperature-con-

trolled environment with a 12 h light ⁄ 12 h dark cycle. They

received a standard diet and acidified water ad libitum.

Mice were housed and handled according to institutional

guidelines complying with Dutch legislation.



Blood sampling

Five milliliters of whole blood (heparin) was drawn from

healthy Caucasian volunteers by vein puncture. One milli-

liter of whole blood (heparin) was drawn from mice by

heart blood sampling under methoxyflurane anesthesia,

after which the mice were killed. Mouse handling and

experimental procedures were conducted in accordance with

institutional guidelines for animal care and use. All human

volunteers had given their consent for vein puncture.

cGMP efflux from intact cells

cGMP efflux from intact stimulated erythrocytes and

erythrocytic cGMP contents were measured with the direct

cGMP enzyme immunoassay (Assay Designs, Ann Arbor,

MI, USA) according to the manufacturer’s instructions.

Preparation of membrane vesicles from mouse

and human erythrocytes

Membrane vesicles from human and mouse erythrocytes

were prepared as previously described, with minor modifi-

cations [16]. Briefly, red blood cells were washed three times

with five volumes of isotonic medium (80 mm KCl, 70 mm

NaCl, 0.2 mm MgCl2, 10 mm Hepes, 0.1 mm EGTA,

pH 7.5). The buffy coat and top layer were removed after

each wash. The packed cells were lysed in 50 volumes of

ice-cold solution L (2 mm Hepes, 0.1 mm EGTA, pH 7.5)

and subsequently centrifuged at 20 000 g for 20 min at

4 �C. The supernatant was removed, and the pelleted ghosts

were resuspended in ice-cold solution L. This step was

repeated until most erythrocytes were lysed, as checked by

microscopy. The pellets were subsequently resuspended in

twice the packed red blood cell volume of solution L and

incubated at 37 �C for 30 min with occasional vortexing.

After incubation, the suspension was washed once with

solution L and twice with vesicle buffer (10 mm Tris ⁄HCl,

pH 7.4). The final pellet was resuspended in vesicle buffer,

and the protein concentration was determined using the

Bio-Rad protein assay (Bio-Rad, Hercules, CA, USA). All

vesicles were prepared without protease inhibitors, unless

otherwise indicated. Membrane vesicles were frozen and

stored at ) 80 �C until use. To estimate the proportion of

inside-out vesicles, the activity of the ectoenzyme acetylcho-

linesterase was determined [11]. Routinely, 32–40% of vesi-

cles were inside-out, and there was no difference between

the inside-out ratios of vesicles from human or mouse

origin.

Vesicular transport assay

[8-3H]cGMP and [3¢,5¢,7-3H(N)]MTX (Moravek Biochemi-

cals, Brea, CA, USA) were used as substrates in vesicular

transport experiments. Substrate uptake into inside-out

erythrocyte vesicles was studied by use of the rapid filtra-

tion method as described previously [38]. Briefly, vesicles

containing 10 lg of protein were incubated with the indica-

ted concentration of substrate in a final volume of 25 lLof vesicle buffer containing 10 mm MgCl2, 10 mm creatine

phosphate and creatine kinase (100 lgÆmL)1) (both from

Boehringer Mannheim, Almere, the Netherlands) in the

presence or absence of 4 mm ATP. Vesicular transport

assays were either performed at physiologic pH (pH 7.4) or

at pH 5.5. For the experiments at low pH, all reaction

components were prepared in 10 mm Tris (pH 5.5). The

pH of the final incubation mix was verified with a pH

meter. (We realize that the buffering capacity of this

pH 5.5 mix is very low; it was used to keep the conditions

of the transport experiment as similar as possible to the

conditions at pH 7.4.) At the indicated time, the reaction

was terminated by adding 2 mL of ice-cold vesicle buffer,

and the mixture was immediately filtered through a pure

cellulose ME25 (cGMP) or OE67 (MTX) filter (0.45 lmpore size; Schleicher and Schuell, Dassel, Germany). The

filter was washed three times with 2 mL of ice-cold vesicle

buffer, and the radioactivity retained on the filter was

measured by liquid scintillation counting. The ATP-

dependent transport was calculated by subtracting the

transport in the absence of ATP from that in its presence.

Note that, initially, we determined ATP-dependent trans-

port by replacing ATP with 5¢-AMP; this gave the same

background as reactions performed in the absence of ATP.

cGMP was stable for 4 h at 37 �C, with intact cells trans-

porting cGMP into the medium, as measured with a valid-

ated HPLC method [4]. For inhibition studies, cGMP

uptake in the absence and presence of inhibitors was com-

pared. The MRP inhibitors MK571 (Biomol, Plymouth

Meeting, PA, USA), GF120918 (Glaxo Wellcome,

Research Triangle Park, NC, USA), Ko143 [39], PGE1 and

PGE2 (Sigma Aldrich, Zwindrecht, the Netherlands),

dipyridamole (Sigma Aldrich) and indomethacin (Sigma

Aldrich) were used. The inhibitory effect of the protease

inhibitors AEBSF, leupeptin and aprotinin (all from Roche

Applied Science, Indianapolis, IN, USA) on the vesicular

uptake of cGMP was determined in the concentration ran-

ges of 0–10 mg AEBSFÆmL)1, 0–5 lg leupeptinÆmL)1, and

0–2 lg aprotininÆmL)1. The vesicles were not preincubated

with inhibitors, the only exception being the experiment

shown in Fig. 6C. Kinetic parameters were calculated using

the equation V ¼ Vmax · S ⁄ (Km + S), where V is the

transport rate [pmolÆ(mg protein)Æmin)1], S is the substrate

concentration in the buffer, Km is the Michaelis–Menten

constant, and Vmax is the extrapolated maximum velocity

[pmolÆ(mg protein)Æmin)1] at infinite S. The data were

fitted to the equation by nonlinear least-squares regression

analysis.



Generation of ABCC11 antibodies

Fusion genes consisting of the gene for the Escherichia coli

maltose-binding protein and fragments of human ABCC11

were constructed in the pMAL-c vector as previously des-

cribed [40]. The ABCC11 segment in the expression plasmid

encoded either amino acids 1–83 (FP M8I) or amino acids

455–526 (FP M8II). Production and purification of the

fusion proteins was performed as previously described [41].

Polyclonal rabbit anti-(human ABCC11) serum was

obtained from a rabbit immunized with FP M8I. For the

generation of monoclonal antibodies, a 12-week-old female

Wistar rat received approximately 30 lg of either FP M8I

or a 1 : 7 mix of FP M8II fusion protein and a synthetic

ABCC11 peptide (amino acids 475–526) per injection.

Three booster injections were given. Cells obtained from

draining lymph nodes and the spleen were fused with Sp20

mouse myeloma cells as previously described [42,43]. Rat

monoclonal antibodies M8I-74 and M8II-16 were selected

by screening hybridoma supernatants on ELISA plates coa-

ted with FP M8II and, as a control, on plates coated with

irrelevant fusion protein. Antibody binding was detected

using horseradish peroxidase-labeled rabbit anti-rat serum

(1 : 500; Dako, Glostrup, Denmark) and 5-amino-

2-hydroxybenzoic acid (Merck, Darmstadt, Germany) with

0.02% H2O2 as a chromogen. Human recombinant

ABCC11 expressed in Sf9 insect cells was specifically detec-

ted with both the rabbit polyclonal anti-(human ABCC11)

serum and the rat monoclonal antibodies M8I-74 and M8II-

16 (Fig. 2B and results not shown).

Western blot analysis

Membrane vesicles (10 lg of protein) were fractionated on

a denaturing 7.5% polyacrylamide gel and transferred onto

a nitrocellulose membrane. Forty micrograms of vesicular

protein was loaded onto a polyacrylamide gel for the detec-

tion of ABCC11. Equal loading and transfer of protein was

routinely checked by Ponceau S staining of the nitrocellu-

lose membrane. After blocking for 1 h in NaCl ⁄Pi contain-

ing 1% nonfat dry milk, 1% BSA, and 0.05% Tween-20,

the membrane was incubated for 1 h at room temperature

with the first antibody. ABCC (Abcc) 1, 4 and 5 and

ABCG2 (Abcg2) were detected with the monoclonal anti-

bodies ABCC-r1 [44] (1 : 1000), M4I-10 [20] (1 : 500), NKI-

12C5 [45] (1 : 1) and BXP-53 [37] (1 : 400), respectively.

For the detection of ABCC11, the polyclonal (1 : 1) and

monoclonal (1 : 5) ABCC11 antibodies described in the

previous section were used. As secondary antibody, horse-

radish peroxidase-conjugated rabbit anti-(rat IgG) or swine

anti-(rabbit IgG) was used at a dilution of 1 : 1000 (Dako).

Enhanced chemiluminescence was used for detection by

incubating the membrane for 1 min with freshly mixed

1.25 mm 3-aminophtalhydrazide, 0.2 mm p-coumaric acid,

and 0.01% v ⁄ v H2O2 in 0.1 m Tris (pH 8.5).

Acknowledgements

We thank A. Schinkel (Netherlands Cancer Institute)

for providing us with the Abcg2– ⁄ – mouse, and K. van

de Wetering of our group for the other mice. This

research was supported by grants from the Uehara

Memorial Foundation to H. Yamaguchi, the Dutch

Cancer Society to P. Borst (NKI 98-1794, and NKI

2001-2473) and J. Wijnholds (NKI 2001-2473), and NIH

research grants GM60904, ES058571, and CA23099,

Cancer Center Support Grant P30 CA21745, and a

grant from the American Lebanese Syrian Associated

Charities (ALSAC) to J. Schuetz. A major part of this

work was presented at the FEBS special meeting on

ABC proteins (Innsbruck, Austria, 4–10 March 2006).

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cgmp transport by vesicles from human and mouse erythrocytes

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