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Molecular and Cellular Biology, September 1999, p. 5882-5891, Vol. 19, No. 9
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Protein Kinase A-Dependent and -Independent Signaling Pathways Contribute to Cyclic AMP-Stimulated Proliferation

Lisa A. Cass,1 Scott A. Summers,2 Gregory V. Prendergast,1 Jonathan M. Backer,3 Morris J. Birnbaum,2 and Judy L. Meinkoth1,*

Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461,3 and Howard Hughes Medical Institute and Departments of Medicine2 and Pharmacology,1 University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6084

Received 23 February 1999/Returned for modification 30 March 1999/Accepted 27 May 1999

The effects of cyclic AMP (cAMP) on cell proliferation are cell type specific. Although the growth-inhibitory effects of cAMP have been well studied, much less is known regarding how cAMP stimulates proliferation. We report that cAMP stimulates proliferation through both protein kinase A (PKA)-dependent and PKA-independent signaling pathways and that phosphatidylinositol 3-kinase (PI3K) is required for cAMP-stimulated mitogenesis. In cells where cAMP is a mitogen, cAMP-elevating agents stimulate membrane ruffling, Akt phosphorylation, and p70 ribosomal S6 protein kinase (p70s6k) activity. cAMP effects on ruffle formation and Akt were PKA independent but sensitive to wortmannin. In contrast, cAMP-stimulated p70s6k activity was repressed by PKA inhibitors but not by wortmannin or microinjection of the N-terminal SH2 domain of the p85 regulatory subunit of PI3K, indicating that p70s6k and Akt can be regulated independently. Microinjection of highly specific inhibitors of PI3K or Rac1, or treatment with the p70s6k inhibitor rapamycin, impaired cAMP-stimulated DNA synthesis, demonstrating that PKA-dependent and -independent pathways contribute to cAMP-mediated mitogenesis. Direct elevation of PI3K activity through microinjection of an antibody that stimulates PI3K activity or stable expression of membrane-localized p110 was sufficient to confer hormone-independent DNA synthesis when accompanied by elevations in p70s6k activity. These findings indicate that multiple pathways contribute to cAMP-stimulated mitogenesis, only some of which are PKA dependent. Furthermore, they demonstrate that the ability of cAMP to stimulate both p70s6k- and PI3K-dependent pathways is an important facet of cAMP-regulated cell cycle progression.

* Corresponding author. Mailing address: Department of Pharmacology, University of Pennsylvania School of Medicine, 36th St. and 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, September 1999, p. 5882-5891, Vol. 19, No. 9
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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