The multidrug resistance protein 5 functions as an ATP-dependent export pump for cyclic nucleotides

Cellular export of cyclic nucleotides has been observed in various tissues and may represent an elimination pathway for these signaling molecules, in addition to degradation by phosphodiesterases. In the present study we provide evidence that this export is mediated by the multidrug resistance protein isoform MRP5 (gene symbol ABCC5). The transport function of MRP5 was studied in V79 hamster lung fibroblasts transfected with a human MRP5 cDNA. An MRP5-specific antibody detected an overexpression of the glycoprotein of 185 +/- 15 kDa in membranes from MRP5-transfected cells and a low basal expression of hamster Mrp5 in control membranes. ATP-dependent transport of 3',5'-cyclic GMP at a substrate concentration of 1 mu M was 4-fold higher in membrane vesicles from MRP5-transfected cells than in control membranes. This transport was saturable with a K-m value of 2.1 mu M. MRP5-mediated transport was also detected for 3',5'-cyclic AMP at a lower affinity, with a K-m value of 379 mu M. A potent inhibition of MRP5-mediated transport was observed by several compounds, known as phosphodiesterase modulators, including trequinsin, with a K-i of 240 nM, and sildenafil, with a K-i value of 267 nM. Thus, cyclic nucleotides are physiological substrates for MRP5; moreover, MRP5 may represent a novel pharmacological target for the enhancement of tissue levels of cGMP.