209. P-glycoprotein limits activation of the glucocorticoid receptor in placental BeWo cells

Abstract

Inadequate placental growth and function are key determinants of fetal growth retardation. Glucocorticoids potently inhibit fetal and placental growth via activation of the glucocorticoid receptor (GR). Placental and fetal glucocorticoid exposure is minimised by the ‘placental glucocorticoid barrier’, which consists primarily of placental 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) converting maternal glucocorticoids to inactive metabolites. Recent studies in the rodent brain show that P-glycoprotein (P-gp) is also an important physiological regulator of glucocorticoid access to the GR in target cells. Therefore, we hypothesised that placental P-gp may serve to exclude maternal glucocorticoids from the placenta and fetus, and thereby augment the barrier. We have used the placental choriocarcinoma cell line BeWo, and MDR-BeWo, a daughter cell line virally transduced with P-gp, to assess whether P-gp regulates access of dexamethasone to the GR. Quantitative PCR analysis showed that MDR-BeWo cells express ~10-fold higher levels of P-gp mRNA than BeWo cells. Syncytialisation of BeWo and MDR-BeWo cells with 20μM forskolin also increased P-gp mRNA by ~7-fold in each cell line. The elevated P-gp expression in MDR-BeWo cells resulted in a reduced activation of the GR with 1μM dexamethasone by ~50% (P<0.001) in comparison to BeWo cells. Accordingly, dexamethasone-induced apoptosis was reduced in MDR-BeWo cells, as indicated by a lack of induction of cleaved caspase 3 protein. Additionally, the P-glycoprotein inhibitor cyclosporin A (10μM) did not increase the level of dexamethasone-induced GR activation in the low P-gp expressing BeWo cells, but potentiated GR activation by ~2-fold in the MDR-BeWo cells, to a level comparable to that in BeWo cells. These data support the hypothesis that P-glycoprotein contributes to the placental glucocorticoid barrier. Thus, 11β-HSD2 and P-glycoprotein are likely to act in unison to reduce fetal and placental exposure to maternal glucocorticoids and minimise their growth inhibitory actions.