Mechanisms subserving the steroidogenic synergism between follicle-stimulating hormone and insulin-like growth factor I (somatomedin C). Alterations in cellular sterol metabolism in swine granulosa cells.

Swine granulosa cells respond to follicle-stimulating hormone (FSH) and the insulin-like growth factor, IGF-I (somatomedin C), with synergistic increases in progesterone production. This facilitative interaction was not attributable to decreased catabolism of progesterone to 20 alpha-hydroxypregn-4-en-3-one, but rather to enhanced pregnenolone biosynthesis observed in response to provision of 25-hydroxycholesterol as exogenous sterol substrate. The latter evidence of increased functional cholesterol side-chain cleavage activity was accompanied by augmented incorporation of [35S]methionine into specific immunoisolated components of the cholesterol side-chain cleavage apparatus, viz. cytochrome P-450scc and adrenodoxin. The synergism between FSH and IGF-I could be sustained over 4 days of serum-free monolayer culture. Under these conditions, compactin, a competitive inhibitor of de novo endogenous cholesterol biosynthesis, suppressed stimulated progesterone production by approximately equal to 50%. However, synergism was not expressed at the levels of [14C]acetate incorporation into nonsaponifiable lipids or endogenous 3-hydroxy-3-methylglutaryl coenzyme A reductase activity per se. Conversely, exogenous sterol substrate provided in the form of low-density lipoprotein (LDL)-borne cholesterol increased the absolute magnitude of the combined actions of IGF-I and FSH by 3-6-fold. This increase in steroidogenesis in response to LDL was associated with enhanced surface binding, internalization, and degradation of [125I] iodo-LDL. In addition, when granulosa cells were incubated with [3H]cholesteryl linoleate-labeled LDL, FSH and IGF-I synergistically augmented the intracellular accumulation of [3H]cholesterol and [3H]cholesteryl ester and the production of [3H]progesterone. Moreover, FSH and IGF-I coordinately increased the total mass of free and esterified cholesterol contained in granulosa cells. We conclude that FSH and IGF-I can augment absolute rates of progestin biosynthesis by granulosa cells by activating dual mechanisms: stimulation of functional cholesterol side chain cleavage activity and enhancement of effective cellular uptake and utilization of low-density lipoprotein-borne sterol substrate.