Cooperativity between endogenous glutamylcysteinylglycine-based oligopeptides and drugs determines the function of the multidrug transporter ABCC1

The multidrug transporter ABCC1 confers resistance against many anticancer drugs, often together with endogenous glutamylcysteinylglycine. Neither highly potent inhibition of ABCC1-mediated drug transport, nor enhancement of ABCC1-mediated glutamylcysteinylglycine efflux and stress-related cell death, have worked out clinically as strategies to overcome multidrug resistance. This study sought to analyze the relationship between (i) the ABCC1 ATPase, which provides the energy for the transport process; (ii) ABCC1 co-substrates, such as glutamylcysteinylglycine derivatives; and (iii) ABCC1-targeting drugs. Glutamylcysteinylglycine disulfide showed strongest ABCC1 ATPase activation, which was further enhanced in combination with the ABCC1 ATPase activator indomethacin. Indomethacin and ABCC1 inhibitors exhibited reduced half-maximal activation and inhibition effect values of the ABCC1 ATPase in combination with glutamylcysteinylglycine disulfide. Kinetic and functional analyses provided evidence of a non-essential activation / allosteric competitive stimulation exerted by ABCC1 activators and inhibitors. Increase of both ABCC1 ATPase and ABCC1 transport activities triggered by targeting distinct ABCC1 binding sites through mutual affinity increase by glutamylcysteinylglycine derivative-drug combinations appears as a third, novel way to overcome ABCC1-mediated multidrug resistance.