Effect of okadaic acid on utilization of lipoprotein-derived cholesteryl esters by rat steroidogenic cells

This study examines various functional, biochemical, and structural changes in rat adrenocortical and ovarian granulosa cells that could account for the decline in lipoprotein-supported hormone production after cell treatment with the protein phosphatase inhibitor, okadaic acid. Although the steroidogenic pathway enzymes in these cells are not in themselves affected by okadaic acid, the intracellular transport of cholesterol to important cellular processing sites is defective. That is, okadaic acid does not interfere with the internalization of lipoprotein-derived cholesteryl esters, but the mitochondrial utilization of cholesterol obtained from intracellular cholesterol storage sites is 50% reduced as compared to control cells. Two-dimensional electrophoresis gels from okadaic acid-treated cells demonstrate a number of hyperphosphorylated proteins. Morphological examination of the affected cells reveal completely disrupted Golgi complexes with attendant structures, but otherwise the cells appear unchanged. The results suggest that some necessary sterol transport protein (or cofactor or associated membrane) is adversely phosphorylated by okadaic acid, and is rendered dysfunctional.