Insulin synergistically amplified the stimulatory effect of low density lipoprotein (LDL) on progesterone biosynthesis by primary cultures of swine granulosa cells. The mechanisms subserving this facilitate interaction included the following: 1) insulin's synergism with LDL was profoundly attenuated by covalent modification of arginine residues in LDL by 1,2-cyclohexanedione treatment; 2) insulin increased by 2- to 6-fold the number of specific high affinity LDL receptors on granulosa cells, with no change in apparent binding affinity; 3) insulin augmented ratios of [125I]iodo-LDL internalization and degradation without enhancing nonspecific bulk fluid-phase pinocytosis (assessed with [125I]iodo-polyvinylpyrollidone); 4) insulin increased by 2.5 to 3-fold granulosa cell content of free and esterified cholesterol (measured by fluorometry) in response to treatment with unlabeled LDL; 5) insulin stimulated the intracellular accumulation of free [3H]cholesterol and [3H]cholesteryl ester, and amplified [3H]progesterone secretion by granulosa cells exposed to [3H]cholesteryl linoleate-labeled LDL; and 6) insulin action was specific in that it was not mimicked by desoctapeptide insulin, epidermal growth factor, fibroblast growth factor, or relaxin. We conclude that insulin and LDL synergistically enhance progesterone biosynthesis by swine granulosa cells via specific mechanisms that depend upon 1,2-cyclohexanedione-sensitive residues within LDL apoprotein. Insulin action results in significantly augmented binding, internalization, and degradation of LDL, which is accompanied by increased effectual delivery of cholesterol substrate into cellular sterol pools that participate in enhanced steroidogenesis.
|Original language||English (US)|
|Number of pages||12|
|State||Published - 1986|
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism