Arginine 482 to glycine mutation in ABCG2/BCRP increases etoposide transport and resistance to the drug in HEK-293 cells

Resistance to etoposide has been associated with the overexpression of P-glycoprotein and MRP1 in human tumor cells. However, the role of BCRP in resistance to etoposide has not been clearly established, especially the significance of arginine 482 mutations in drug transport (cellular uptake and efflux). Different levels of resistance to etoposide have been recently observed in cells expressing BCRP in terms of cytotoxicity. The aim of this work was to study the effects of these mutations on the functional involvement of BCRP in etoposide transport. HEK293 cells were transfected with an empty vector (HEK/V), the vector bearing the wild-type BCRP (HEK/R482), the mutant arginine-482-glycine (HEK/R482G) or the mutant arginine-482-threonine (HEK/R482T). MTT assay was used to study the cytotoxic effect of etoposide and [3H]-etoposide was used to determine cellular drug uptake and efflux. Data show that HEK/R482G cells displayed the highest levels of resistance to etoposide. Cellular [3H]-etoposide uptake was lower in HEK/R482, HEK/R482G and HEK/R482T cells compared to HEK/V cells. In addition, cellular [3H]-etoposide uptake in HEK/R482G was the lowest. Drug efflux measurements showed that fumitremorgin C was able to increase the residual cellular [3H]-etoposide uptake in BCRP-transfected cells and especially in HEK/R482G ones. Our data show that the R482G mutation in BCRP is able to increase efflux of etoposide and that mutation analysis at codon 482 may be of clinical importance in cancers treated with etoposide.