低氧環境對人類胎盤絨毛膜癌BeWo細胞上第二新型有機陽離子轉運蛋白(OCTN2)表現影響之研究

The syncytiotrophoblast of human placenta is an important interface mediating substance transfer between the mother and the fetus. Preeclampsia, a serious complication during pregnancy, is associated with impaired syncytialization. Carnitine is responsible for the transport of long-chain fatty acids into mitochondria, which is then undergoing β-oxidation for cellular energy production. However, the fetus cannot synthesize adequate amount of carnitine, and the active transfer of carnitine from the mother to the fetus is important. The novel organic cation transporter 2 (OCTN2) is a high-affinity carnitine transporter in human placenta. It was reported that plasma carnitine concentrations in pregnant women with preeclampsia increased about 50 % compared with healthy pregnant women. Hence, the aim of this study was to investigate the effects of hypoxic condition on the regulation of OCTN2 in human choriocarcinoma BeWo cells. BeWo cells were cultured under 0.5 % O2 for mimicking hypoxic condition. The protein expression of syncytin, OCTN2, and PDZK1 in crude membrane of BeWo cells was analyzed by Western blot analysis, and the nuclear expression of HIF-1α, HIF-2α, PPARα and RXRα was also measured to explore possible mechanisms in regulating OCTN2 under hypoxic condition. Under forskolin-induced syncytialization, syncytin and OCTN2 were increased, whereas both of them were decreased in hypoxic condition. PDZK1 was significantly downregulated after syncytialization, whereas it was slightly increased under hypoxia. HIF-1α and HIF-2α were upregulated at 4 hours and then decreased at 24 hours after hypoxia treatment. Both of PPARα and RXRα were significantly downregulated at 24 hours. However, OCTN2 expression was increased upon treatment PPARα agonist, WY14643. The activation of OCTN2 expression was also upregulated by WY14643 under hypoxic condition. Afterward, cellular uptake of 3H-labeled L-carnitine was measured after hypoxia and WY14643 treatment. The results showed that the values of carnitine uptake was increased upon WY14643 treatment, however it was decreased under hypoxia condition. We also observed human normal and preeclamptic placental tissue by immunohistochemistry. Both of OCTN2 and PPARα protein expression were decreased in human preeclamptic placentas. In conclusion, the process of syncytialization, OCTN2 protein expression and carnitine uptake was inhibited under hypoxic condition. According to the results of immunohistochemistry, we speculate the plasma carnitine concentrations accumulation in preeclamptic pregnant women can be due to the downregulated OCTN2 expression. Furthermore, we also provide evidences that PPARα is one of the important factors that regulate the expression of OCTN2. If there are any other factors involved in regulating OCTN2 should be verified.