Multidrug resistance protein 1 (
MRP1) is an
ATP-binding cassette transporter that confers multidrug resistance on
tumor cells. Much convincing evidence has accumulated that
MRP1 transports most substances in a GSH-dependent manner. On the other hand, several reports have revealed that
MRP1 can transport some substrates independently of GSH; however, the importance of GSH-independent transport activity is not well established and the mechanistic differences between GSH-dependent and -independent transport by
MRP1 are unclear. We previously demonstrated that the
amino acids W261 and K267 in the L0 region of
MRP1 were important for
leukotriene C4 (
LTC4) transport activity of
MRP1 and for GSH-dependent photolabeling of
MRP1 with azidophenyl
agosterol-A (azidoAG-A). In this paper, we further tested the effect of W222L, W223L and R230A mutations in
MRP1, designated dmL0MRP1, on
MRP1 transport activity.
SN-38 is an active metabolic form of
CPT-11 that is one of the most promising anti-
cancer drugs. Membrane vesicles prepared from cells expressing dmL0MRP1 could transport
SN-38, but not
LTC4 or
estradiol-17 (beta-D-
glucuronate), and could not be photolabeled with azidoAG-A. These data suggested that
SN-38 was transported by a different mechanism than that of GSH-dependent transport. Understanding the GSH-independent transport mechanism of
MRP1, and identification of drugs that are transported by this mechanism, will be critical for combating MRP1-mediated drug resistance. We performed a pairwise comparison of compounds that are transported by
MRP1 in a GSH-dependent or -independent manner. These data indicated that it may be possible to predict compounds that are transported by
MRP1 in a GSH-independent manner.