Peptide transport by the multidrug resistance protein MRP1

Small hydrophobic peptides were studied as possible substrates of the multi drug resistance protein (MRP)-1 (ABCC1) transmembrane transporter molecule. As observed earlier for P-glycoprotein- (Pgp; ABCB1) overexpressing cells, MRP1-overexpressing cells, including cells stably transfected with the MRP 1 cDNA, showed distinct resistance to the cytotoxic peptide N-acetyl-Leu-Le u-norleucinal (ALLN). Resistance to this peptide and another toxic peptide derivative, which is based on a Thr-His-Thr-Nle-Glu-Gly backbone conjugated to butyl and benzyl groups (4A6), could be reversed by MRP1 inhibitors. Th e reduced toxicity of 4A6 in MRP1-overexpressing cells was found to be asso ciated with lower accumulation of a fluorescein-labeled derivative of this peptide. Glutathione (GSH) depletion had a clear effect on resistance to AL LN but hardly affected 4A6 resistance. In a limited structure-activity stud y using peptides that are analogous to 4A6, MRP1-overexpressing cells were found to be resistant to these peptides as well. Remarkably, when selecting A2780 ovarian cancer cells for resistance to ALLN, even in the absence of Pgp blockers, resulting cell lines had up-regulated MRP1, rather then any o f the other currently known multidrug resistance transporter molecules incl uding Pgp, MRP2 (ABCC2), MRP3 (ABCC3), MRP5 (ABCC5), and the breast cancer resistance protein ABCG2. ALLN-resistant, MRP1-overexpressing cells were fo und to be cross-resistant to 4A6 and the classical multidrug resistance dru gs doxorubicin, vincristine, and etoposide. This establishes MRP1 as a tran sporter for small hydrophobic peptides. More extensive structure-activity r elationship studies should allow the identification of clinically useful pe ptide antagonists of MRP1.