Relationship between serum estrogen and level of apolipoprotein E in human ovarian follicular fluid

Apolipoprotein E (apoE) is an important effector of plasma lipoprotein due to its interaction with cell-surface receptors. In addition, it is secreted by rat and human ovarian granulosa cells in tissue culture, and its production is hormonally modulated. Such observations indicate that apoE should be found in human follicular fluid (FF) and that its abundance therein may be subject to hormonal regulation. The authors have collected FF from seven normally cycling women by needle puncture at the time of laparoscopy and from hyperstimulated ovaries of four women undergoing in vitro fertilization (IVF). Apolipoprotein E is present in human FF. The ratio of apoE concentration in FF to that in serum, as determined by western blot analysis, ranges from 0.43 to 4.2 for the normally cycling patients and from 0.16 to 0.77 for the IVF patients. Serum estrogen levels (range, 42 to 477 pg/ml) for the cycling patients are inversely correlated with the FF-to-serum ratio of apoE (r = 0.91). The content of apoE in FF, relative to serum, thus falls dramatically as the follicle approaches ovulation. These data, plus the known ability of apoE to carry cholesterol and other lipids, suggest an important role for apoE in the ovary.