The influence of coordinate overexpression of glutathione phase II detoxification gene products on drug resistance

Glutathione (GSH), glutathione S-transferases (GSTs), and the multidrug res istance-associated protein 1 (MRP1) have been independently studied for the ir contributions to drug resistance. Single cDNA transfection experiments h ave provided inconsistent and disparate conclusions with respect to the imp ortance of GSH and GST in conferring a resistant phenotype. Because these t hree proteins can act as a concerted coordinated pathway, we reasoned that equivalent increases may be required for enhanced resistance to be expresse d. We have assembled these proteins together, or in various combinations, t o determine whether they show cooperativity in determining drug response. I ncreased expression through single cDNA transfection of GST pi, gamma-gluta mylcysteine synthetase (gamma-GCS) (regulatory plus catalytic subunits), or MRP1 enhanced resistance to a number of anticancer drugs. Cotransfection o f GST pi and GCS, gave higher resistance to doxorubicin, etoposide, and vin cristine than with either alone. Resistance toward chlorambucil and ethacry nic acid was similar in cells overexpressing either component or overexpres sing GST alone. Coexpression of GST pi with MRP1 conferred significant resi stance above that seen for MRP1 alone to chlorambucil, etoposide, ethacryni c acid, and vincristine. The combination of GCS and MRP1 did not afford add itional resistance above MRP1 alone. When all three were transfected, signi ficantly higher levels of resistance were found for doxorubicin and etoposi de. These results support the concept that coordinate enhancement of focal thiol elements of detoxification pathways provides a more efficient protect ive phenotype than do single components alone.