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Molecular and Cellular Biology, December 2000, p. 8720-8730, Vol. 20, No. 23
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

8-Bromo-Cyclic AMP Induces Phosphorylation of Two Sites in SRC-1 That Facilitate Ligand-Independent Activation of the Chicken Progesterone Receptor and Are Critical for Functional Cooperation between SRC-1 and CREB Binding Protein

Brian G. Rowan, Nefretiti Garrison, Nancy L. Weigel,* and Bert W. O'Malley

Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, Texas 77030

Received 21 July 2000/Returned for modification 11 September 2000/Accepted 19 September 2000

Elevation of intracellular 8-bromo-cyclic AMP (cAMP) can activate certain steroid receptors and enhance the ligand-dependent activation of most receptors. During ligand-independent activation of the chicken progesterone receptor (cPRA) with the protein kinase A (PKA) activator, 8-bromo-cAMP, we found no alteration in cPRA phosphorylation (W. Bai, B. G. Rowan, V. E. Allgood, B. W. O'Malley, and N. L. Weigel, J. Biol. Chem. 272:10457-10463, 1997). To determine if other receptor-associated cofactors were targets of cAMP-dependent signaling pathways, we examined the phosphorylation of steroid receptor coactivator 1 (SRC-1). We detected a 1.8-fold increase in SRC-1 phosphorylation in transfected COS-1 cells incubated with 8-bromo-cAMP. Phosphorylation was increased on two mitogen-activated protein kinase (MAPK) sites, threonine 1179 and serine 1185. PKA did not phosphorylate these sites in vitro. However, blockage of PKA activity in COS-1 cells with the PKA inhibitor (PKI) prevented the 8-bromo-cAMP-mediated phosphorylation of these sites. Incubation of COS-1 cells with 8-bromo-cAMP resulted in activation of the MAPK pathway, as determined by Western blotting with antibodies to the phosphorylated (active) form of Erk-1/2, suggesting an indirect pathway to SRC-1 phosphorylation. Mutation of threonine 1179 and serine 1185 to alanine in COS-1 cells coexpressing cPRA and the GRE2E1bCAT reporter resulted in up to a 50% decrease in coactivation during both ligand-independent activation and ligand-dependent activation. This was due, in part, to loss of functional cooperation between SRC-1 and CREB binding protein for coactivation of cPRA. This is the first demonstration of cross talk between a signaling pathway and specific phosphorylation sites in a nuclear receptor coactivator that can regulate steroid receptor activation.


* Corresponding author. Mailing address: Department of Molecular and Cellular Biology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030. Phone: (713) 798-6234. Fax: (713) 790-1275. E-mail: nweigel{at}bcm.tmc.edu.


Molecular and Cellular Biology, December 2000, p. 8720-8730, Vol. 20, No. 23
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.



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