Protein kinase A-dependent and -independent signaling pathways contribute to cyclic AMP-stimulated proliferation

The effects of cyclic AMP (cAMP) on cell proliferation are cell type specif ic. Although the growth-inhibitory effects of cAMP have been well studied, much less is known regarding how cAMP stimulates proliferation. We report t hat cAMP stimulates proliferation through both protein kinase A (PKA)-depen dent 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 p hosphorylation, 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 P KA inhibitors but not by wortmannin or microinjection of the N-terminal SH2 domain of the p85 regulatory subunit of PI3K, indicating that p70s6k and A kt can be regulated independently. Microinjection of highly specific inhibi tors of PI3K or Rad, or treatment with the p70s6k inhibitor rapamycin, impa ired cAMP-stimulated DNA synthesis, demonstrating that PKA-dependent and -i ndependent pathways contribute to cAMP-mediated mitogenesis. Direct elevati on of PI3K activity through microinjection of an antibody that stimulates P I3K 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 contrib ute to cAMP-stimulated mitogenesis, only some of which are PKA dependent. F urthermore, they demonstrate that the ability of cAMP to stimulate both p70 s6k- and PI3K-dependent pathways is an important facet of cAMP-regulated ce ll cycle progression.