Abstract
The major ganglioside NeuAcα2 → 3Galβ1 → 4Glc/gb1 → 1Cer (GM3) present in cultured rat granulosa cells was examined for potential function in the expression of luteinizing hormone (LH) receptor on the cell surface in response to follicle-stimulating hormone (FSH). Synthesis of GM3 was stimulated concentration-dependently by FSH, and the stimulation was enhanced synergistically by insulin, as revealed by metabolic labeling of glycosphingolipids with [ 3H]galactose. When granulosa cells were cultured in the media containing GM3 (0.2-20 μM), biphasic changes in FSH-dependent expression of LH receptor were observed, as measured by the binding of 125I-deglycosylated human choriogonadotropin to the intact cells. Exogenous GM3 suppressed expression of LH receptor in the cells treated with a low dose of FSH (20 ng/ml), which was characterized by a low GM3 level, to 30% of control at 10 μM, with a half-maximal inhibitory concentration of 8 μM. In contrast, in the cells treated with a high dose of FSH (100 ng/ml) and insulin, which was characterized by a high GM3 level, expression of LH, receptor was enhanced by exogenous GM3, to 148% of control at 10 μM, with a half-maximal effective concentration of 2 μM. Exogenous GM3 produced concomitant changes in the levels of extracellular cAMP. These effects of exogenous GM3 were not accompanied by changes in granulosa cell proliferation. Exogenous GM3 also modulated the LH receptor expression by the synergistic action of 12- O-tetradecanoylphorbol 13-acetate with insulin, with no significant changes in cellular DNA contents, suggesting that exogenous GM3 does not modulate directly the action of FSH at its receptor sites. The asialo form of GM3, CDH, had little effect. These findings indicate that GM3 may be a biphasic regulator of positive and negative signals for expression of the granulosa cell LH receptor or cell differentiation.
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