Bax, Bak and Mitochondrial Oxidants are Involved in Hypoxia-reoxygenation-induced Apoptosis in Human Placenta

Although apoptosis is prominent in placental cells in pregnancy complications such as preeclampsia, the cause is unknown. We surmised that hypoxia-reoxygenation (HR) is the mechanism and hypothesized that mitochondrial oxidants and Bcl-2 proteins cause HR-induced placental apoptosis. Our goal was studying expression of five Bcl-2 proteins-Bcl-2, Bcl-xL, Bax, Bak, Bad-and testing effects of diazoxide and cyclosporine A on oxidative stress and apoptosis in villous tissues subjected to HR. Term human placentas were obtained from normal pregnancies following elective caesarean deliveries. Villous tissues were subjected to "repetitive HR" (one hour at 2% O₂ then one hour at 8% O₂, alternatively, for a total of 6 h) or "prolonged HR" (3 h at 2% O₂ then 3 h of 8% O₂). Samples maintained at 2% and 8% O₂ served as hypoxic and normoxic controls, respectively. Prolonged HR caused the most severe villous apoptotic changes, increased the expression of Bax and Bak mRNA and protein and reduced the expression of Bcl-2 mRNA. Pre-administration of diazoxide and cyclosporine A reduced TUNEL-positive nuclei and levels of nitrotyrosine and 4-hydroxy-2-nonenol after prolonged HR. Thus, duration of hypoxia and reoxygenation is important in determining severity of HR-induced apoptosis in placenta. These apoptotic changes are closely associated with Bax and Bak effects and oxidative stress in mitochondria.