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Submitted on July 24, 2006
Accepted on September 28, 2006
-Mediated Signaling in Xenopus Laevis oocytes by Signaling Through Classical Steroid Receptors
Department of Internal Medicine, Division of Endocrinology and Metabolism, Department of Pharmacology, University of Texas Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd., Dallas, Texas 75390-8857, USA
* To whom correspondence should be addressed. E-mail: stephen.hammes{at}utsouthwestern.edu.
Many transcription-independent (nongenomic) steroid effects are regulated by G proteins. A well-established, biologically relevant example of steroid/G protein interplay is steroid-triggered oocyte maturation, or meiotic resumption, in Xenopus laevis. Oocyte maturation is proposed to occur through a release of inhibition mechanism whereby constitutive signaling by G
and other G proteins maintain oocytes in meiotic arrest. Steroids (androgens in vivo, and androgens and progesterone in vitro) overcome this inhibition to promote meiotic resumption. To test this model, we used G protein-sensitive inward rectifying potassium channels (GIRKs) as markers of G activity. Over-expression of GIRKs 1 and 2 in Xenopus oocytes resulted in constitutive potassium influx, corroborating the presence of basal G signaling in resting oocytes. Testosterone and progesterone rapidly reduced potassium influx, validating that steroids attenuate G activity. Interestingly, reduction of classical androgen receptor (AR) expression by RNA interference abrogated testosterone's effects on GIRK activity at low, but not high, steroid concentrations. Accordingly, androgens bound to the Xenopus progesterone receptor (PR) at high concentrations, suggesting that, in addition to the AR, the PR might mediate G protein signaling when androgens levels are elevated. In contrast, progesterone bound with high affinity to both the Xenopus PR and AR, indicating that progesterone might signal and promote maturation through both receptors, regardless of its concentration. In sum, these studies introduce a novel method for detecting nongenomic steroid effects on G proteins in live cells in real time, and demonstrate that cross talk may occur between steroids and their receptors during Xenopus oocyte maturation.
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