Inhibitory effects of prostaglandin A1 on membrane transport of folates mediated by both the reduced folate carrier and ATP-driven exporters

Studies are reported that describe the multifaceted inhibitory effects of prostaglandin A(1)(PGA(1)) on processes that govern the transport of folates across the plasma membrane of Chinese hamster ovary (CHO) cells: the reduced folate carrier, RFC1, and ATP-dependent exporters. PGA(1) was a noncompetitive inhibitor of MTX influx mediated by RFC1 with a K-i of similar to 21 mu M. The onset of inhibition was virtually instantaneous, not reversible, and appeared to require the incorporation of PGA(1) into the lipid membrane; surface adsorption alone was insufficient for inhibition of RFC1 transport activity. In contrast, the effect of PGA(1) on folic acid transport was small (similar to 20% inhibition of total influx), consistent with the observation that the major portion of folic acid transport in CHO cells is mediated by a low pH mechanism distinct from RFC1. PGA(1) was also a potent inhibitor of the ATP-driven efflux of both MTX and folic acid. At a concentration of 7 mu M PGA(1) the efflux rate constants for these folates were depressed by similar to 70 and similar to 50%, respectively. The net effects of PGA(1) on the bidirectional folate fluxes translated into marked alterations in net transport. The addition of 7 mu M PGA(1) to cells at steady state with 1 mu M MTX produced a rapid onset of net uptake and the achievement of an similar to 3-fold increase in the steady state free MTX level as compared with untreated CHO cells. The addition of 7 mu M PGA(1) to cells at steady state with 1 mu M folic acid produced an similar to 5-fold increase in the free folate level. These studies establish PGA(1) as a potent inhibitor of both the reduced folate carrier and ATP-driven folate exporter(s). The noncompetitive nature of the inhibition of RFC1 is unique among anionic compounds, which are usually competitive inhibitors of the carrier. (C) 1999 ELsevier Science Inc.