Regulation and functional significance of ATP binding cassette transporters in human placenta

Abstract

The aim of this project was to study ATP binding cassette (ABC) transporters in the human placenta, in particular their regulation and role in trophoblast differentiation and survival. The presence and localisation of four major placental drug transporters, multidrug resistance gene product 1 and 3 (MDR1 and 3)/ABC subfamily B members 1 and 4 (ABCB1 and 4), multidrug resistance associated proteins 1 and 2 (MRP1 and 2)/ABCC1 and 2 and breast cancer resistance protein (BCRP)/ABCG2 was initially studied in term human placenta, cultured primary trophoblast and BeWo and Jar trophoblast-like cell lines. Jar cells were found to be more similar to nondifferentiated cytotrophoblast with respect to their ABC protein expression profile, whereas BeWo cells more closely reflected differentiated syncytiotrophoblast. Treatment of primary term trophoblasts in vitro with cytokines (TNF- or IL-1) decreased expression and activity of apical transporters ABCB1/MDR1 and ABCG2/BCRP. Growth factors, on the other hand, increased BCRP expression and activity, while estradiol stimulated BCRP, MDR1 and MDR3 expression MDR1/3 functional activity. The ability of BCRP/ABCG2 to abrogate the apoptotic effects of TNF- and ceramides was studied in primary trophoblast and BeWo cells using pharmacological and molecular (siRNA) approaches. The results suggest that BCRP/ABCG2 contributes to the resistance of trophoblast cells to cytokine-induced (extrinsic) apoptosis, whereas its effects on apoptosis activated via the intrinsic mitochondrial pathway is minimal. This altered resistance was associated with increased intracellular accumulation of ceramides and reduced ability to maintain phosphatidylserine in the inner leaflet of the plasma membrane. A role for BCRP/ABCG2 in cell protection from differentiation-induced stressors was also demonstrated during the process of cell fusion associated with transient loss of plasma membrane lipid asymmetry. Finally, expression of BCRP/ABCG2 (and 9 other genes) was studied in 50 placentas from normal pregnancy and pregnancies complicated with fetal growth restriction (FGR). A marked reduction of BCRP/ABCG2 and MDR1/ABCB1 expression was observed in FGR placentas, while other transporter genes were unaffected. Collectively these data suggest that BCRP/ABCG2 and probably other ABC transporters may play a hitherto unrecognised survival role in the placenta, conferring a “stress resistance” to trophoblast cells.