Biomedical Engineering Reference
In-Depth Information
full-length sequences from human, rat, mouse, chicken, and zebrafish are known and
their amino acid sequence identities vary from 87% for rat and mouse Abcc4 to 70%
for zebrafish Abcc4 in comparison to the human ABCC4. Furthermore, members of
the ABCC subfamily have been also described in plants, such as in
Arabidopsis
with
15 ABCCs and in rice with 17 ABCCs.
75
11.4. SUBSTRATE SPECIFICITY AND MULTIDRUG RESISTANCE
PROFILES OF HUMAN ABCCs/MRPs
The ABCCs/MRPs are ATP-dependent unidirectional efflux pumps for conjugated
and unconjugated organic anions. Many of these pumps have been identified to con-
fer drug resistance to natural-product drugs as well as to purine and pyrimidine 5
-
nucleoside monophosphate analogs. The expression of a defined ABCC in a cell
system is an important tool for the specific characterization of its substrate speci-
ficity and multidrug resistance profile. This has been achieved using cells, including
polarized cell lines, such as Madin-Darby canine kidney (MDCK), human HepG2,
and pig LLC-PK
1
cells, as well as nonpolarized cell lines, such as human HEK293,
human HeLa, hamster V79, and insect Sf9 cells. The substrate specificity can be char-
acterized in the intact cell system. However, the studies in intact cell systems have
a number of limitations [e.g., an endogenous uptake transporter for the compound
under study must be present (section 11.6)]. In contrast, studies using inside-out mem-
brane vesicles provide direct evidence for ABCC-mediated ATP-dependent transport
of compounds, the only restriction being that compounds must be radiolabeled or
fluorescent.
76
In the following sections we provide an overview of the substrate speci-
ficity as well as the multidrug resistance profiles of the human ABCC/MRP efflux
pumps (Table 11.2).
11.4.1. ABCC1
In 1994, the cysteinyl leukotriene LTC
4
was identified as the first physiological sub-
strate for ABCC1.
77
,
78
This finding was the result of the search for the molecu-
lar identity of the ATP-dependent efflux pump that mediates the release of LTC
4
from mastocytoma cells.
80
In addition, several cell systems allowed the identifica-
tion of ABCC1 and its substrate specificity, especially ABCC1-overexpressing drug-
selected cell lines, such as the doxorubicin-selected small cell lung cancer cell lines
H69/AR
15
and GLC4/ADR,
81
as well as the doxorubicin-selected human leukemia
HL60/ADR cells.
14
In addition, HeLa cells transfected with the recombinant
ABCC1
allowed direct evidence supporting the results obtained with the drug-selected cell
lines.
78
After the discovery of LTC
4
as an ABCC1 substrate, further compounds
were identified as ABCC1 substrates, including LTD
4
,LTE
4
,
S
-glutathionyl 2,4-
dinitrobenzene (DNP-SG), 17
β
-glucuronosyl estradiol (E
2
17
β
G), lithocholyltaurine
3-sulfate, 6
-glucuronosylhyodeoxycholate, oxidized glutathione (GSSG), and bili-
rubin glucuronosides.
78
,
82
−
84
In addition, glutathione conjugates from ethacrynic
acid, prostaglandin A
1
, melphalan, chlorambucil, and aflatoxin B
1
are also transported
α
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