G Protein βγ Subunits Inhibit Nongenomic Progesterone-induced Signaling and Maturation in Xenopus laevis Oocytes: EVIDENCE FOR A RELEASE OF INHIBITION MECHANISM FOR CELL CYCLE PROGRESSION

Lindsey B Lutz,Bonnie Kim,David Jahani,Stephen R Hammes

doi:10.1074/jbc.m006757200

Abstract

Progesterone-induced maturation of Xenopus oocytes is a well known example of nongenomic signaling by steroids; however, little is known about the early signaling events involved in this process. Previous work has suggested that G proteins and G protein-coupled receptors may be involved in progesterone-mediated oocyte maturation as well as in other nongenomic steroid-induced signaling events. To investigate the role of G proteins in nongenomic signaling by progesterone, the effects of modulating Galpha and Gbetagamma levels in Xenopus oocytes on progesterone-induced signaling and maturation were examined. Our results demonstrate that Gbetagamma subunits, rather than Galpha, are the principal mediators of progesterone action in this system. We show that overexpression of Gbetagamma inhibits both progesterone-induced maturation and activation of the MAPK pathway, whereas sequestration of endogenous Gbetagamma subunits enhances progesterone-mediated signaling and maturation. These data are consistent with a model whereby endogenous free Xenopus Gbetagamma subunits constitutively inhibit oocyte maturation. Progesterone may induce maturation by antagonizing this inhibition and therefore allowing cell cycle progression to occur. These studies offer new insight into the early signaling events mediated by progesterone and may be useful in characterizing and identifying the membrane progesterone receptor in oocytes.

Highlights

Progesterone-induced maturation of Xenopus oocytes is a well known example of nongenomic signaling by steroids; little is known about the early signaling events involved in this process
Previous work has suggested that G proteins and G protein-coupled receptors may be involved in progesterone-mediated oocyte maturation as well as in other nongenomic steroid-induced signaling events
We show that overexpression of G␤␥ inhibits both progesterone-induced maturation and activation of the mitogenactivated protein kinase (MAPK) pathway, whereas sequestration of endogenous G␤␥ subunits enhances progesterone-mediated signaling and maturation

Summary

EVIDENCE FOR A RELEASE OF INHIBITION MECHANISM FOR CELL CYCLE PROGRESSION*

Previous work has suggested that G proteins and G protein-coupled receptors may be involved in progesterone-mediated oocyte maturation as well as in other nongenomic steroid-induced signaling events. Progesterone may induce maturation by antagonizing this inhibition and allowing cell cycle progression to occur These studies offer new insight into the early signaling events mediated by progesterone and may be useful in characterizing and identifying the membrane progesterone receptor in oocytes. Progesterone treatment of oocytes results in a rapid decrease in intracellular cAMP, perhaps through attenuation of adenylyl cyclase activity (26 –29) This suggests that G␣i, and a G␣i-coupled receptor, may be involved in progesterone-mediated signaling. When progesterone levels decrease at the end of pregnancy, this “release of inhibition” may allow oxytocin-mediated contractions, and labor, to begin These data are consistent with progesterone modulation of G protein-coupled receptor signaling. Ration by antagonizing constitutive G␤␥-mediated inhibition of cell cycle progression

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