Increased active transport of LTC(4) observed frequently in multidrug-resistant cancer cells have been attributed to ABC-transporter proteins particularly, MRP1. We have demonstrated recently that a novel non-ABC transporter, RLIP76 (RALBP1) can also mediate ATP-dependent transport of GSH-conjugates (GS-E) as well as doxorubicin (DOX). We demonstrate RLIP76 reconstituted in artificial liposomes can catalyze ATP-dependent transport of LTC(4), which can be modulated by PKC-alpha. The ATPase activity of E. coli expressed homogenous RLIP76 was stimulated in a saturable fashion by LTC(4) with half maximal stimulation at 130 nM. Proteoliposomes reconstituted with RLIP76 catalyzed temperature and osmolar sensitive ATP-dependent transport of LTC(4) with K(m) values of 5.1 mM and 210 nM for ATP and LTC(4), respectively. V(max) for transport was found to be 3.2 nmol/min/mg. Colchicine inhibited LTC(4) transport to 50% at 5.8 microM. PKC-alpha catalyzed phosphorylation of RLIP76 and increased its transport activity by 2-3-fold. Membrane vesicles prepared from the small (SCLC) and non-small (NSCLC) lung cancer cell lines as well as HL-60 (leukemia) and U937 (lymphoma) cell lines exhibited ATP-dependent transport of LTC(4), which was inhibited by anti-RLIP76 antibodies. The rate of transport of LTC(4) in SCLC (H69, H378) was half of that observed in NSCLC cell lines but after transfection with RLIP76, the transport rate of LTC(4) in H69 became comparable to that in NSCLC cell lines. Anti-RLIP76 antibodies inhibited LTC(4) transport by 67-81% in all 8 cell lines examined, whereas N-19 anti-MRP1 antibodies inhibited transport of LTC(4) by only 11-26%. These results suggest that RLIP76 is the major LTC(4) transporter in cancer cells and that its transport activity is regulated by PKC-alpha-mediated phosphorylation.
(c) 2004 Wiley-Liss, Inc