The Role of Mitochondria in Syncytiotrophoblast Cells: Bioenergetics and Steroidogenesis

Human placenta maintains pregnancy. The mitochondria of this tissue synthesize pregnenolone (P5) from cholesterol through a transport chain formed by adrenodoxin, adrenodoxin reductase and cytochrome P450scc (CYP11A1; EC 1.14.15.6) , which breaks up the lateral chain of cholesterol. P5 is transformed into progesterone (P4) within mitochondria by the 3-OH-steroid-dehydrogenase-5-6isomerase (3HSD). The particular hormone(s) or substance(s) that modulate P4 synthesis during pregnancy is currently unknown (Strauss et al., 1996; Martinez & Strauss, 1997); nevertheless, the presence of cAMP analogues stimulated P4 synthesis in trophoblastic cells, suggesting that a hormonal signal or another kind of signal may modulate the concentration of this second messenger into the cells (Ringler et al., 1989; Strauss et al., 1992). Although P4 synthesis was suggested to be the main function of the placenta, the analysis of P450scc cytochrome concentration shows that placental mitochondria have a lower content of P450scc than respiratory chain cytochromes (Table 1), even when it is compared to adrenal gland mitochondria, suggesting that placental mitochondria participate in other functions different to that from steroidogenesis.