The p85 regulatory subunit of PI3K mediates TSH-cAMP-PKA growth and survival signals

Phosphatidylinositol 3-kinase (PI3K) is necessary for thyroid stimulating hormone (TSH)-induced cell cycle progression. To determine the molecular mechanism linking PI3K to TSH, we have identified a serine residue in p85aPI3K phosphorylated by protein kinase A (PKA) in vitro and in vivo. Expression of an alanine mutant (p85A) abolished cyclic AMP/TSH-induced cell cycle progression and was lethal in thyroid cells (FRTL-5). The aspartic version of the p85aPI3K (p85D) inhibited apoptosis following TSH withdrawal. The p85aPI3K wild type not the p85A bound PKA regulatory subunit RIIb in cells stimulated with cAMP or TSH. The binding of the aspartic version of p85aPI3K to RIIb was independent of cAMP or TSH stimulation. Similarly, binding of PI3K to p21Ras and activation of AKT, a downstreamPI3K target, were severely impaired in cells expressing the p85A mutant. Finally, we found that the catalytic activity of PI3K was stimulated by TSH in cells expressing the wildtype p85aPI3K but not in cells expressing p85A. This latter mutant did not affect the epidermal growth factorstimulated PI3K activity. We suggest that (1) TSH– cAMP-induced PKA phosphorylates p85aPI3K at serine 83, (2) phosphorylated p85aPI3K binds RIIb-PKA and targets PKAII to the membrane, and (3) PI3K activity and p21Ras binding to PI3K increase and activate PI3K downstreamtargets. This pathway is essential for the transmission of TSH–cAMP growth signals.