MRP1 (
multidrug resistance protein 1) co-exports
glutathione (GSH) and
drug(s) and exports GSH,
glucuronide, and sulphate-conjugated drugs. Human Fly-eco
fibrosarcoma cells producing the MRP1-expressing retrovirus SF91MRP (Fly-eco MRP1), as well as 3T3 cells transduced with SF91MRP (3T3/
MRP1), presented a decrease in intracellular GSH levels, as measured by two different methods. The enhanced export of GSH caused by the overexpression of
MRP1 was partially counterbalanced by an increased rate of GSH synthesis. Fly-eco
MRP1 and 3T3/
MRP1 were hypersensitive to the GSH-depleting and cytotoxic activities of L-
buthionine-S,R-sulphoximine (BSO), compared with their parental counterparts. In addition, the potentiation by BSO of the cytotoxic activity of
chlorambucil and
doxorubicin in Fly-eco
MRP1 cells was greater than in parental Fly-eco cells. Although the turnover time of GSH, i.e. the theoretical time in which the entire GSH pool is resynthesised, was approximately 50% faster in Fly-eco
MRP1 cells than in parental cells, this was not sufficient to fully restore the intracellular GSH level. In addition,
mrp1 (-/-) mice were resistant to the GSH-depleting activity of intraperitoneally (i.p.) injected BSO, compared with
mrp1 (+/+) mice. Co-transfer of the cDNAs for
MRP1 and the heavy subunit of
gamma-glutamyl cysteine synthetase (GCS) resulted in increased intracellular GSH levels and in high-level resistance to the GSH-depleting and cytotoxic activities of BSO. These data, and in particular the elevated single-agent cytotoxicity of BSO, provide a new rationale for the use of BSO in the treatment of MRP1-overexpressing tumours.