MULTIDRUG RESISTANCE PROTEINS OF THE ABCC SUBFAMILY - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 278

GSH or GSH derivatives ( S -methylglutathione and ophthalmate), and ATP-dependent
transport of GSH required the presence of monoanionic bile salts. 53 This cotransport
is an obligatory coefflux of both cosubstrates. 123 Thus, ABCC4 transports together
with GSH a wide range of bile salts, including unconjugated cholate as well as
taurine- and glycine-conjugated bile salts. 123 Because of its basolateral localization in
hepatocytes, 53 ABCC4 can mediate the efflux of GSH and bile salts from the hepatocy-
tes into blood.
Membrane vesicles from Sf9 insect cells expressing ABCC4 showed transport of
urate, the end product of purine catabolism. 124 In this cell system, ABCC4-mediated
cGMP transport was stimulated by urate by a complex allosteric interaction. 124 Trans-
port of PAH was also mediated by ABCC4, and to a lesser extent by ABCC2, indicating
that ABCC4 is the decisive export pump for PAH in kidney proximal tubules. 125 The
localization of ABCC4 in the apical membrane of proximal tubule epithelial cells 51
suggests that urate and PAH are excreted into urine via ABCC4.
Topotecan is apparently another ABCC4 substrate. 126 , 152 Abcc4-deficient mice
showed enhanced accumulation of topotecan in brain tissue and cerebrospinal
fluid. 152 Moreover, cells expressing recombinant murine Abcc4 conferred resistance
to topotecan, and Abcc4-mediated E 2 17 G transport was inhibited by topotecan. 152

Cells expressing recombinant human ABCC4 also showed reduced accumulation of
topotecan. 126 However, direct demonstration of topotecan as a substrate using mem-
brane vesicle transport studies is currently lacking.

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11.4.5. ABCC5

The initial functional studies were performed in intact cells expressing recombinant
human ABCC5. 54 , 56 In these cells, ABCC5 was able to mediate efflux of the anionic
dye fluorescein diacetate in an ATP-dependent and GSH-independent manner, 54 as
well as of DNP-SG and of GSH. 56 Low-level resistance mediated by ABCC5 was
reported against cadmium chloride and potassium antimonyl tartrate, 54 against the
purine analogs 6-mercaptopurine and 6-thioguanine, and against PMEA. 56
Studies in inside-out membrane vesicles demonstrated ATP-dependent transport of
cGMP and cAMP by ABCC5, describing for the first time an ABCC/MRP substrate
with a phosphate residue as the negatively charged moiety. 55 No significant trans-
port was detected for the glutathione and glucuronate conjugates LTC 4 and E 2 17 G,

respectively, as well as for GSSG. 55 This finding of ABCC5 as a cyclic nucleotide
export pump was subsequently confirmed in studies with intact cells. 153 The cyclic nu-
cleotide transport by ABCC5 was inhibited by several phosphodiesterase inhibitors.
Some of these compounds are structurally related to cGMP and able to enhance intra-
cellular cGMP concentrations by blocking degradation as well as export of cGMP. 55
Thus, ABCC5, together with ABCC4, may play an important role in the regulation of
the tissue levels of cGMP. However, the affinity of ABCC5 to cGMP and the potency
of inhibition by phosphodiesterase inhibitors seem to be less than observed originally
by Jedlitschky et al. 55 , 127 , 153
In cytotoxicity assays, ABCC5 conferred high-level resistance to 5-fluorouracil
and 6-thioguanine, and to a lesser extent, to methotrexate. 127 Measurements
of ATP-dependent transport in vesicles demonstrated ABCC5-mediated transport

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