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Molecular and Cellular Biology, March 2001, p. 1921-1929, Vol. 21, No. 6
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.6.1921-1929.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Coordinated Regulation of Rap1 and Thyroid Differentiation by Cyclic AMP and Protein Kinase A

Oxana M. Tsygankova,1 Arturo Saavedra,1 John F. Rebhun,2 Lawrence A. Quilliam,2 and Judy L. Meinkoth1,*

Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6084,1 and Department of Biochemistry and Molecular Biology and Walther Oncology Center, Indiana University School of Medicine, Indianapolis, Indiana 46202-51222

Received 11 August 2000/Returned for modification 21 September 2000/Accepted 18 December 2000

Originally identified as an antagonist of Ras action, Rap1 exhibits many Ras-independent effects, including a role in signaling pathways initiated by cyclic AMP (cAMP). Since cAMP is a critical mediator of the effects of thyrotropin (TSH) on cell proliferation and differentiation, we examined the regulation of Rap1 by TSH in a continuous line of rat thyroid-like cells. Both cAMP and protein kinase A (PKA) contribute to the regulation of Rap1 activity and signaling by TSH. TSH activates Rap1 through a cAMP-mediated and PKA-independent mechanism. TSH phosphorylates Rap1 in a PKA-dependent manner. Interference with PKA activity blocked phosphorylation but not the activation of Rap1. Rather, PKA inhibitors prolonged Rap1 activation, as did expression of a Rap1A mutant lacking a PKA phosphorylation site. These results indicate that PKA elicits negative feedback regulation on cAMP-stimulated Rap1 activity in some cells. The dual regulation of Rap1 by cAMP and PKA extends to downstream effectors. The ability of TSH to stimulate Akt phosphorylation was markedly enhanced by the expression of activated Rap1A and was repressed in cells expressing a putative dominant-negative Rap1A mutant. Although the expression of activated Rap1A was sufficient to stimulate wortmannin-sensitive Akt phosphorylation, TSH further increased Akt phosphorylation in a phosphatidylinositol 3-kinase- and PKA-dependent manner. The ability of TSH to phosphorylate Akt was impaired in cells expressing a Rap1A mutant that could be activated but not phosphorylated. These findings indicate that dual signals, Rap1 activation and phosphorylation, contribute to TSH-stimulated Akt phosphorylation. Rap1 plays an essential role in cAMP-regulated differentiation. TSH effects on thyroid-specific gene expression, but not its effects on proliferation, were markedly enhanced in cells expressing activated Rap1A and repressed in cells expressing a dominant-negative Rap1A mutant. These findings reveal complex regulation of Rap1 by cAMP including PKA-independent activation and PKA-dependent negative feedback regulation. Both signals appear to be required for TSH signaling to Akt.

* Corresponding author. Mailing address: Department of Pharmacology, University of Pennsylvania School of Medicine, 3620 Hamilton Walk, Philadelphia, PA 19104-6084. Phone: (215) 898-1909. Fax: (215) 573-2236. E-mail: meinkoth{at}pharm.med.upenn.edu.

Molecular and Cellular Biology, March 2001, p. 1921-1929, Vol. 21, No. 6
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.6.1921-1929.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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