Use of low-density and high-density lipoproteins in undifferentiated porcine granulosa cells.

Abstract

Granulosa cells aspirated from medium-sized porcine ovarian follicles (3-5 mm) in short-term incubation responded to the addition of both low-density lipoprotein (LDL) and high-density lipoprotein (HDL) with increased accumulation of progesterone. HDL was more potent than LDL in enhancing progesterone secretion. When granulosa cells were cultured under serum-free conditions for 72 h, HDL but not LDL exhibited a dose-dependent enhancement of progesterone secretion. Addition of insulin to the cells greatly potentiated the stimulatory effect of LDL on progesterone accumulation, while the response to HDL was only slightly increased. Granulosa cells in culture degraded LDL but not HDL. Addition of insulin enhanced LDL degradation. Exposure of cells in culture to chloroquine, an inhibitor of lysosomal function, completely blocked LDL degradation and LDL-induced stimulation of steroidogenesis. The stimulatory effect of HDL was not affected by chloroquine. We interpret these findings to indicate that granulosa cells derive cholesterol from LDL by means of lysosomal degradation, which is not required for use of cholesterol from HDL. Monensin, a carboxylic ionophore that interrupts recycling of LDL receptors, prevented LDL-enhanced progesterone accumulation but not HDL-induced stimulation. This provides evidence that HDL-induced stimulation of steroidogenesis does not involve LDL receptors. We conclude that HDL present in follicular fluid is capable of providing cholesterol to granulosa cells for steroidogenesis. The stimulatory effect of HDL is not due to the presence of apoprotein E, an HDL component that binds to the LDL receptor. A unique HDL pathway that does not involve LDL receptors and lysosomal degradation may operate in porcine granulosa cells.