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Molecular and Cellular Biology, February 1999, p. 1427-1437, Vol. 19, No. 2
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Regulation of Ribosomal S6 Protein Kinase-p90rsk, Glycogen Synthase Kinase 3, and beta -Catenin in Early Xenopus Development

Monica A. Torres,1,dagger Hagit Eldar-Finkelman,2 Edwin G. Krebs,1,2 and Randall T. Moon1,*

Howard Hughes Medical Institute and Department of Pharmacology1 and Department of Biochemistry,2 University of Washington School of Medicine, Seattle, Washington

Received 23 April 1998/Returned for modification 28 May 1998/Accepted 23 October 1998

beta -Catenin is a multifunctional protein that binds cadherins at the plasma membrane, HMG box transcription factors in the nucleus, and several cytoplasmic proteins that are involved in regulating its stability. In developing embryos and in some human cancers, the accumulation of beta -catenin in the cytoplasm and subsequently the nuclei of cells may be regulated by the Wnt-1 signaling cascade and by glycogen synthase kinase 3 (GSK-3). This has increased interest in regulators of both GSK-3 and beta -catenin. Searching for kinase activities able to phosphorylate the conserved, inhibitory-regulatory GSK-3 residue serine 9, we found p90rsk to be a potential upstream regulator of GSK-3. Overexpression of p90rsk in Xenopus embryos leads to increased steady-state levels of total beta -catenin but not of the free soluble protein. Instead, p90rsk overexpression increases the levels of beta -catenin in a cell fraction containing membrane-associated cadherins. Consistent with the lack of elevation of free beta -catenin levels, ectopic p90rsk was unable to rescue dorsal cell fate in embryos ventralized by UV irradiation. We show that p90rsk is a downstream target of fibroblast growth factor (FGF) signaling during early Xenopus development, since ectopic FGF signaling activates both endogenous and overexpressed p90rsk. Moreover, overexpression of a dominant negative FGF receptor, which blocks endogenous FGF signaling, leads to decreased p90rsk kinase activity. Finally, we report that FGF inhibits endogenous GSK-3 activity in Xenopus embryos. We hypothesize that FGF and p90rsk play heretofore unsuspected roles in modulating GSK-3 and beta -catenin.

* Corresponding author. Mailing address: Howard Hughes Medical Institute, Campus Box 357370, University of Washington School of Medicine, Seattle, WA 98195. Phone: (206) 543-1722. Fax: (206) 616-4230. E-mail: rtmoon{at}u.washington.edu.

dagger Present address: Department of Molecular and Cellular Physiology, Beckman Center for Molecular and Genetic Medicine, Stanford University School of Medicine, Stanford, CA 94305-5426.

Molecular and Cellular Biology, February 1999, p. 1427-1437, Vol. 19, No. 2
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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