ROLE OF PHOSPHOPROTEIN PHOSPHATASES IN THE CORPUS-LUTEUM .2. CONTROL OF PROGESTERONE SECRETION BY ISOLATED RAT LUTEAL CELLS

The key role of protein kinases and protein phosphorylation in the reg ulation of luteal steroidogenesis is well documented. However the role of phosphoprotein phosphatases (PP) and dephosphorylation in the regu lation of luteal cell progesterone secretion is as yet unknown. We hav e recently demonstrated the presence and activity of PP1 and PP2A in r at luteal cells and the present study was undertaken to determine the consequences of inhibiting PP activity in terms of progesterone secret ion. Three structurally dissimilar inhibitors of PP1/2A, okadaic acid, calyculin A and cantharidin each caused a dose-dependent inhibition o f LH-induced progesterone secretion without affecting cyclic AMP accum ulation. The less potent derivative of okadaic acid, norokadaone, had no effect on either parameter, suggesting that the inhibitory actions on progesterone secretion are due to their specific actions on PP acti vity and that this inhibition occurs principally at a locus which is d istal to the generation of cyclic AMP. In contrast to the inhibitory e ffects of PP1/2A inhibitors on progesterone biosynthesis, a PP2B inhib itor, cypermethrin, had no effect on LH-stimulated steroidogenesis. Th e three PP1/2A inhibitors also caused a concentration-dependent inhibi tion of dibutyryl cyclic AMP-stimulated progesterone secretion. Howeve r, none of the inhibitors affected 22R-hydroxycholesterol-supported st eroidogenesis, clearly demonstrating that the inhibitors did not inter fere vith the activity of steroidogenic enzymes. These results suggest that cycles of phosphorylation/dephosphorylation of specific proteins are required for the sustained production of progesterone. Whilst the precise location and function of putative PP substrates is uncertain, the present results indicate that they are involved in regulating the availability of free cholesterol to steroidogenic enzymes within mito chondria.