TRANSPORT-PROPERTIES OF THE MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN (MRP) IN HUMAN TUMOR-CELLS

In this paper we demonstrate that the expression of the multidrug resi stance-associated protein (MRP) in a variety of intact human tumour ce lls results in the ATP-dependent, mutually exclusive extrusion of both the acetoxymethyl ester and the free anion forms of the fluorescent d ye calcein, as well as that of a fluorescent pyrenemaleimide-glutathio ne conjugate. The MRP-dependent transport of all these three model com pounds closely correlates with the expression level of MRP and is cros s-inhibited by hydrophobic anticancer drugs, by reversing agents for M DR1, and also by compounds not influencing MDR1, such as hydrophobic a nions, alkylating agents, and inhibitors of organic anion transporters . Cellular glutathione depletion affects neither the MRP-dependent ext rusion of calcein AM or free calcein, nor its modulation by most hydro phobic or anionic compounds, although eliminating the cross-inhibitory effect of glutathione conjugates. These results suggest that the outw ard pumping of both hydrophobic uncharged and water-soluble anionic co mpounds, including glutathione conjugates, is an inherent property of MRP, and offer sensitive methods for the functional diagnostics of thi s transport protein as well as for the rapid screening of drug-resista nce modulating agents.