Protein kinase C and progesterone-induced maturation in Xenopus oocytes

Though progesterone-induced maturation has been studied extensively in Xenopus oocytes, the mechanism whereby the prophase block arrest is released is not well understood. The current hypothesis suggests that a reduction in cAMP and subsequent inactivation of cAMP-dependent protein kinase is responsible for reentry into the cell cycle. However, several lines of evidence indicate that maturation can be induced without a concomitant reduction in cAMP. We show that the mass of diacylglycerol in whole oocytes and plasma membranes decreases 29% and 10% respectively, within the first 15 sec after the addition of progesterone. Diacylglycerol in plasma membranes further decreased 59% by 5 min. We also show that the protein kinase C inhibitors sphingosine and staurosporine can induce oocyte maturation. In addition, the synthetic diglyceride, DiC8, and microinjected PKC can inhibit or delay progesterone-induced maturation. These results together suggest that a transient decrease in protein kinase C activity may regulate entry into the cell cycle. The mechanism whereby DAG is decreased in response to progesterone is unclear. Initial studies show that progesterone leads to a decrease in IP3 suggesting that progesterone may act by reducing the hydrolysis of PIP2. On the other hand, progesterone caused a decrease in the amount of [3H]arachidonate labelling in DAG during the same time suggesting that progesterone may stimulate lipase activity. The relationship between postulated changes in the PKC pathway and those hypothesized for the PKA pathway are discussed.