To test estrogen’s possible regulation of lipoprotein metabolism by granulosa cells, swine granulosa cells were cultured under serum-free conditions in the presence or absence of estradiol. Treatment with estradiol significantly enhanced high affinity, saturable, [125I]iodo-low density lipoprotein (LDL) binding with a median 2.85-fold (range 2.3- to 5.6-fold, n = six experiments) increase in the calculated number of LDL receptors and no change in the apparent dissociation constant (Kd for LDL binding (Kd = 3.4 ± 0.92 Mg/ml in control and 4.0 ± 0.87 fig/ml human LDL in estradiol-treated cultures). Estradiol also significantly increased [125I]iodo-LDL internalization by granulosa cells and augmented the maximal rate of LDL degradation by 2.0 to 2.5-fold without altering the apparent Michaelis- Menten constant (Km) for this process. Estrogen’s dose-dependent enhancement of [125I]iodo-LDL binding, internalization, and degradation could be observed at minimum estradiol concentrations of approximately 100 ng/ml and was accompanied by increased progesterone secretion by granulosa cells. Further studies indicated that estrogen’s stimulation of LDL internalization and degradation was not simply attributable to increased rates of nonspecific bulk fluid-phase pinocytosis (assessed with [125I]iodo-polyvinylpyrrolidone) or increased steroidogenesis per se (tested by blocking cholesterol side-chain cleavage with aminoglutethimide). We conclude that estradiol amplifies LDL binding by swine granulosa cells by increasing the number of high affinity, saturable LDL receptors with no alteration in their apparent affinity. Moreover, estrogen action is accompanied by enhanced rates of progesterone production in the presence of LDL, and increased rates of LDL internalization and degradation, which could not be accounted for simply by accelerated nonspecific bulk fluid-phase pinocytosis. We suggest that the significant facilitative actions of estradiol on lipoprotein binding and metabolism are likely to assist in preparing granulosa cells for the increased rates of progesterone biosynthesis ultimately required in functional corpora lutea.
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