Type I insulin-like growth factor (IGF-I) stimulated high density lipoprotein (HDL)-promoted progesterone production by swine granulosa cells cultured under serum-free conditions in vitro. In the presence of pure human IGF-I (50 ngsol; ml), the half-maximally effective concentration of swine HDL was 16 µgsol;ml (67% confidence limits; 15-17 µg/ml) after 2 days of exposure to this growth factor, 5.4 (2.6-9.8) µgsol;ml after 4 days, and 3.8 (1.2-4.8) µg/ml after 6 days. Maximal progesterone production increased approximately 10-fold in the presence of IGF-I and HDL on day 2, 125-fold on day 4, and 330-fold on day 6. The facilitative action of IGF-I on HDL-supported progesterone biosynthesis was accompanied by time-dependent stimulatory effects of IGF-I on trypsin-releasable HDL, trypsinresistant cell-associated HDL, and degraded HDL (P ≪ 0.01). Moreover, incubation of swine granulosa cells with [3H]cholesteryl oleate-labeled HDL demonstrated that IGF-I exerted a time-dependent stimulatory effect on [3H]free cholesterol and [3H]cholesteryl ester accumulation in granulosa cells, and significantly augmented the secretion of [3H]progesterone (separated by two-dimensional TLC). In addition to the ability of IGF-I to amplify the cellular acquisition of radiolabeled sterol, this growth factor also increased the total mass of cellular cholesteryl ester and total cellular cholesterol as measured by microfluorometric assay (P ≪ 0.01). We conclude that IGF-I facilitates the effective delivery of HDL-derived sterol substrate into the steroidogenic pool of ovarian cells. Such observations offer an additional role for the differentiative actions of this somatomedin in the expression of full steroidogenic potential by granulosa-luteal cells.
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