Characterization of a leukotriene C-4 export mechanism in human platelets:possible involvement of multidrug resistance-associated protein 1

Platelets express leukotriene (LT) C-4 synthase and can thus participate in the formation of bioactive LTC4. To further elucidate the relevance of thi s capability, we have flow determined the capacity of human platelets to ex port LTC4. Endogenously formed LTC4 was efficiently released from human pla telets after incubation with LTA(4) at 37 degrees C, whereas only 15% of pr oduced LTC4 was exported when the cells were incubated at 0 degrees C. The activation energy of the process war; calculated to 49.9 +/- 7.7 kJ/mol, in dicating carrier-mediated LTC4 export. This was also supported by the findi ng that the transport was saturable, reaching a maximal export rate of 470 +/- 147 pmol LTC4/min x 10(9) platelets, Furthermore, markedly suppressed L TC4 transport was induced by a combination of the metabolic inhibitors anti mycin A and 2-deoxyglucose, suggesting energy-dependent export.jlr The pres ence in platelets of multidrug resistance-associated protein 1 (MRP1), a pr otein described to be act energy-dependent LTC4 transporter in various cell types, was demonstrated at the mRNA and protein level. Additional support for a role of MRP1 in platelet LTC4 export was obtained by the findings tha t the process was inhibited by probenecid and the 5-lipoxygenase-activating protein (FLAP) inhibitor, MK-886, The present findings further support the physiological relevance of platelet LTC4 production.