Progesterone inhibits folic acid transport in human trophoblasts

The aim of this work was to test the putative involvement of members of the ABC superfamily of transporters on folic acid (FA) cellular homeostasis in the human placenta. [³H]FA uptake and efflux in BeWo cells were unaffected or hardly affected by multidrug resistance 1 (MDR1) inhibition (with verapamil), multidrug resistance protein (MRP) inhibition (with probenecid) or breast cancer resistance protein (BCRP) inhibition (with fumitremorgin C). However, [³H]FA uptake and efflux were inhibited by progesterone (200 μM). An inhibitory effect of progesterone upon [³H]FA uptake and efflux was also observed in human cytotrophoblasts. Moreover, verapamil and ß-estradiol also reduced [³H]FA efflux in these cells. Inhibition of [³H]FA uptake in BeWo cells by progesterone seemed to be very specific since other tested steroids (β-estradiol, corticosterone, testosterone, aldosterone, estrone and pregnanediol) were devoid of effect. However, efflux was also inhibited by β-estradiol and corticosterone and stimulated by estrone. Moreover, the effect of progesterone upon the uptake of [³H]FA by BeWo cells was concentration-dependent (IC₅₀ = 65 [range 9-448] μM) and seems to involve competitive inhibition. Also, progesterone (1-400 μM) did not affect either [³H]FA uptake or efflux at an external acidic pH. Finally, inhibition of [³H]FA uptake by progesterone was unaffected by either 4-acetamido-4′-isothiocyanato-2, 2′-stilbenedisulfonic acid (SITS), a known inhibitor of the reduced folate carrier (RFC), or an anti-RFC antibody. These results suggest that progesterone inhibits RFC. In conclusion, our results show that progesterone, a sterol produced by the placenta, inhibits both FA uptake and efflux in BeWo cells and primary cultured human trophoblasts.