Role of OAT4 in Uptake of Estriol Precursor 16α-Hydroxydehydroepiandrosterone Sulfate Into Human Placental Syncytiotrophoblasts From Fetus

Abstract

Estriol biosynthesis in human placenta requires the uptake of a fetal liver-derived estriol precursor, 16α-hydroxydehydroepiandrosterone sulfate (16α-OH DHEAS), by placental syncytiotrophoblasts at their basal plasma membrane (BM), which faces the fetal circulation. The aim of this work is to identify the transporter(s) mediating 16α-OH DHEAS uptake at the fetal side of syncytiotrophoblasts by using human placental BM-enriched vesicles and to examine the contribution of the putative transporter to estriol synthesis at the cellular level, using choriocarcinoma JEG-3 cells. Organic anion transporter (OAT)-4 and organic anion transporting polypeptide 2B1 proteins were enriched in human placental BM vesicles compared with crude membrane fraction. Uptake of [(3)H]16α-OH DHEAS by BM vesicles was partially inhibited in the absence of sodium but was significantly increased in the absence of chloride and after preloading glutarate. Uptake of [(3)H]16α-OH DHEAS by BM vesicles was significantly inhibited by OAT4 substrates such as dehydroepiandrosterone sulfate, estrone-3-sulfate, and bromosulfophthalein but not by cyclosporin A, tetraethylammonium, p-aminohippuric acid, or cimetidine. These characteristics of vesicular [(3)H]16α-OH DHEAS uptake are in good agreement with those of human OAT4-transfected COS-7 cells as well as forskolin-differentiated JEG-3 cells. Estriol secretion from differentiated JEG-3 cells was detected when the cells were incubated with 16α-OH DHEAS for 8 hours but was inhibited in the presence of 50 μM bromosulfophthalein. Our results indicate that OAT4 at the BM of human placental syncytiotrophoblasts plays a predominant role in the uptake of 16α-OH DHEAS for placental estriol synthesis.

Placental control of drug delivery

“…On the other hand, entrapment of efflux transporter substrates into the core of inside-out vesicles represents a unique method to characterize efflux transporter activity. While the activity of efflux transporters is determined in the presence of ATP, the activity of transport via some organic anion transporters can be assessed in the presence of α-ketoglutarate [28,32,33]. …”

“…OAT4 is capable of bidirectional transport and has broad specificity, as evidenced by its involvement in the transport of endogenous and exogenous compounds. It has been shown that OAT4 is involved in the transport of 16-α-hydroxydehydroepiandrosterone sulfate, a metabolite of dehydroepiandrosterone sulfate, into the syncytiotrophoblast for estriol synthesis [32], as well as the transport of estrone-3-sulfate. OAT4 is responsible for the transport of exogenous compounds including olmesartan [33], perfluoroalkyl acids [59], and others [48].…”

“…OAT4 is responsible for the transport of exogenous compounds including olmesartan [33], perfluoroalkyl acids [59], and others [48]. Interestingly, OAT4 transporter function is enhanced in the presence of ions or ionic gradients, with an inwardly directed chloride gradient increasing substrate transport out of the syncytiotrophoblast, and transport function generally being reduced in the absence of sodium [32,33]. The mechanism by which the presence of ions affects the function of the transporter has yet to be elucidated.…”

Reclassification of SLC22 Transporters: Analysis of OAT, OCT, OCTN, and other Family Members Reveals 8 Functional Subgroups

“…URAT1 is associated with very few metabolites and is best understood for its role in uric acid reabsorption in the kidney proximal tubule, making it relatively mono-specific (52). OAT4, on the other hand, has been shown to transport prostaglandins and conjugated sex hormones in addition to uric acid, making it oligo-specific (56)(57)(58). URAT1 is almost exclusively expressed in the kidney, and OAT4 is expressed in the kidney, placenta, and epididymis ( Table 4).…”

Cellular Uptake of Levocetirizine by Organic Anion Transporter 4

“…7 In the kidney, OAT4 is located at the apical membrane of proximal tubules. 8 OAT4 is unique because it works as an uptake transporter for some substrates, such as urate and steroid sulfates, 9,10 but as an efflux transporter for others, such as p-aminohippurate and olmesartan. 9,11 A meeting abstract reported that cellular uptake of levocetirizine was enhanced in OAT4-expressing cells.…”