Biomedical Engineering Reference
In-Depth Information
For functional characterization, ABCCs/MRPs are often recombinantly expressed
in polarized MDCK cells. These transfected cells are used either for the isolation of
plasma membrane vesicles for transport studies (Section 11.4) or for the measurement
of vectorial transport of substances in intact cells (Section 11.6). In these polarized
MDCK cells, human ABCCs/MRPs acquire a domain-specific plasma membrane
localization. Whereas ABCC2
70
,
105
and ABCC11
60
are localized in the apical mem-
brane, ABCC1,
86
ABCC3,
47
,
48
ABCC4,
50
ABCC5,
56
and ABCC6
155
are routed to the
basolateral membrane (Figure 11.2
a
). Hepatocytes are polarized cells that express en-
dogenously at least four different ABCC/MRP efflux pumps: that is, ABCC2 localized
in the apical (canalicular) membrane
8
,
41
,
156
and ABCC3,
47
,
48
ABCC4,
53
,
157
,
158
and
ABCC6
154
,
159
−
161
localized in the basolateral (sinusoidal) membrane (Figure 11.2
b
).
The molecular mechanisms of domain-specific ABCC/MRP targeting are incom-
pletely understood. Instead of a distinct linear sequence motif, which had been sug-
gested in one study for the apical localization of ABCC2,
162
targeting signals appear
to be composed of several motifs within different parts of the respective ABCC/MRP
that come together only in the intact protein. For instance, MSD0 is required for proper
localization, at least of ABCC1 and ABCC2.
163
,
164
Other regions also contribute to
the correct targeting of ABCC1 and ABCC2.
165
−
167
The interaction of ABCCs/MRPs
with other proteins may be required as well, as indicated by studies with radixin knock-
out mice.
168
Radixin belongs to the ezrin/radixin/moesin family and cross-links actin
with several integral membrane proteins.
169
The Abcc2 protein is selectively absent
from the hepatocyte canalicular membrane of these radixin knockout mice, which are
therefore characterized by conjugated hyperbilirubinemia.
168
Canalicular localization
of ABCC2 in human hepatocytes may also depend on the interaction with radixin.
170
Another class of scaffolding proteins comprises the PSD95/Dlg/ZO-1 (PDZ) proteins
that bind to specific consensus sequences at the C-termini of integral membrane pro-
teins. Some PDZ proteins interact with ABCC2 in vitro.
171
,
172
Whether an interaction
is also required in vivo is not clear, because Abcc2 is properly localized in the apical
membrane of kidney proximal tubule cells of Pdzk1 knockout mice.
173
11.5.1. ABCC1
The ABCC1 protein is detectable in a number of human cells and tissues with the
highest levels in lung, testis, kidney, and macrophages.
17
,
174
Normal human hepato-
cytes lack detectable amounts of ABCC1,
159
,
175
but ABCC1 seems to be up-regulated
in proliferating hepatocyte-derived cells.
175
Immunohistochemical and immunoflu-
orescence analyses have shown that ABCC1 is localized predominantly in cells of
blood-tissue barriers which form pharmacological sanctuaries in the body. ABCC1
is, for instance, present in the basolateral membrane of Sertoli cells of the blood-
testis barrier,
176
of choroid plexus cells of the blood-cerebrospinal fluid barrier,
177
in bronchial epithelium,
178
,
179
and in the apical syncytiotrophoblast membrane of
the placenta.
180
Whether ABCC1 also contributes to the function of the blood-
brain barrier is still controversial.
177
,
181-183
ABCC1 was localized in the luminal
membrane of human
182
brain capillary endothelial cells in one study, but not in
others.
177
,
183
Studies with Abcc1 knockout mice were also not conclusive, because
Search WWH ::
Custom Search