FSH amplified the stimulatory effect of low density lipoprotein (LDL) on progesterone biosynthesis by primary cultures of swine granulosa cells in a synergistic manner. The mechanisms subserving this synergism included the following. 1) FSH increased by 2.1-fold the number of specific high affinity low density lipoprotein (LDL) receptors on granulosa cells, with no change in apparent binding affinity. 2) FSH augmented by 1.8- and 3.6-fold the maximal rates of [125I]iodo-LDL internalization and degradation, respectively, without altering half-maximally effective concentrations of LDL supporting these processes. 3) FSH increased significantly the mass (micrograms) of free and esterified cholesterol (measured by fluorometry) contained in granulosa cells. 4) FSH stimulated the intracellular accumulation of free [3H]cholesterol and [3H]cholesteryl ester from exogenous [3H]cholesteryl linoleate-labeled LDL and amplified [3H]progesterone secretion by granulosa cells exposed to this source of lipoprotein-borne sterol. 5) The stimulatory effects of FSH on LDL degradation and cholesterol side-chain cleavage activity were mimicked by other activators of granulosa cell cAMP production, e.g. forskolin and cholera toxin.We conclude that FSH and LDL synergistically enhance progesterone biosynthesis by swine granulosa cells via cellular mechanisms that result in significantly augmented binding, internalization, and degradation of LDL in concert with increased effectual utilization of sterol substrate in the cholesterol sidechain cleavage reaction. These observations are consistent with a key trophic role for FSH in regulating the sterol-metabolizing ability and steroidogenic differentiation of granulosa cells and indicate that such actions of FSH can be accounted for by activation of the cAMP-effector pathway.
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