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 A₁ (PGA₁) 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. PGA₁ was a noncompetitive inhibitor of MTX influx mediated by RFC1 with a K(i) of ~21 μM. The onset of inhibition was virtually instantaneous, not reversible, and appeared to require the incorporation of PGA₁ into the lipid membrane; surface adsorption alone was insufficient for inhibition of RFC1 transport activity. In contrast, the effect of PGA₁ on folic acid transport was small (~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. PGA₁ was also a potent inhibitor of the ATP-driven efflux of both MTX and folic acid. At a concentration of 7 μM PGA₁, the efflux rate constants for these folates were depressed by ~70 and ~50%, respectively. The net effects of PGA₁ on the bidirectional folate fluxes translated into marked alterations in net transport. The addition of 7 μM PGA₁ to cells at steady state with 1 μM MTX produced a rapid onset of net uptake and the achievement of an ~3-fold increase in the steady-state free MTX level as compared with untreated CHO cells. The addition of 7 μM PGA₁ to cells at steady state with l μM folic acid produced an ~5-fold increase in the free folate level. These studies establish PGA₁ 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. Copyright (C) 1999 Elsevier Science Inc.