Inhibitory effects of prostaglandin A(1) 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 p rostaglandin A(1)(PGA(1)) on processes that govern the transport of folates across the plasma membrane of Chinese hamster ovary (CHO) cells: the reduc ed folate carrier, RFC1, and ATP-dependent exporters. PGA(1) was a noncompe titive 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 membran e; surface adsorption alone was insufficient for inhibition of RFC1 transpo rt activity. In contrast, the effect of PGA(1) on folic acid transport was small (similar to 20% inhibition of total influx), consistent with the obse rvation that the major portion of folic acid transport in CHO cells is medi ated by a low pH mechanism distinct from RFC1. PGA(1) was also a potent inh ibitor of the ATP-driven efflux of both MTX and folic acid. At a concentrat ion of 7 mu M PGA(1) the efflux rate constants for these folates were depre ssed by similar to 70 and similar to 50%, respectively. The net effects of PGA(1) on the bidirectional folate fluxes translated into marked alteration s 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 compar ed with untreated CHO cells. The addition of 7 mu M PGA(1) to cells at stea dy state with 1 mu M folic acid produced an similar to 5-fold increase in t he 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 n oncompetitive nature of the inhibition of RFC1 is unique among anionic comp ounds, which are usually competitive inhibitors of the carrier. (C) 1999 EL sevier Science Inc.