Low-density lipoproteins (LDL) have been consistently reported to stimulate ovarian steroidogenesis, apparently by the provision of cholesterol as a steroidogenic substrate. Recent studies suggest that high-density lipoproteins (HDL) can also deliver cholesterol to support progesterone synthesis in human granulosa-lutein cells. Therefore, this study investigated the contributions of (i) cholesterol delivery, (ii) cyclic AMP and (iii) protein kinase C (PKC) in the steroidogenic responses of human granulosa-lutein cells to HDL and LDL. Over a 24-h treatment incubation, HDL stimulated a larger increase in progesterone output than did LDL at equivalent cholesterol concentrations. Moreover, at equal protein concentrations (100 microg protein/ml), HDL doubled progesterone production by cells co-treated with a maximally effective concentration of 22R-hydroxycholesterol, whereas LDL had no effect on the progesterone response to this membrane-permeable sterol. These observations indicate that the progesterone response to HDL is not solely due to the delivery of cholesterol as a steroidogenic substrate. Over 24 h, the stimulation of progesterone synthesis by HDL was additive with the response to a maximally effective concentration of dibutyryl-cAMP, but was unaffected by the down-regulation of PKC activity (by chronic pre-treatment with a tumour-promoting phorbol ester). We have concluded that HDL appears to stimulate progesterone production in human granulosa-lutein cells by a mechanism not solely reliant on cholesterol delivery.
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