Bioactivities of IGFs in various cells are often potentiated in the presence of other hormones. In previous studies, we showed that pretreatment of rat FRTL-5 thyroid cells with TSH or other cAMP-generating agents markedly potentiated DNA synthesis induced by IGF-I. Under these conditions, we found that phosphatidylinositol (PI) 3-kinase was activated in response to either cAMP or IGF stimulus, and both activation modes were indispensable for the potentiation of DNA synthesis. The present studies were undertaken to elucidate how cAMP and/or IGF-I stimulus regulated the G1 cyclin-cyclin dependent kinase (CDK)-inhibitor system, and to determine the roles of PI 3-kinase activation by cAMP or IGF-I stimulus in this system. We found that cAMP pretreatment enhanced IGF-I-dependent increases in cyclin D1, due to synergistic increases in mRNA and elevation of translation rates. Furthermore, cAMP pretreatment enhanced IGF-I-induced protein degradation of the CDK inhibitor, p27^Kip1. These changes well explained an increase in cyclin E leading to marked activation of G1 CDKs followed by Rb phosphorylation. Our results using a PI 3-kinase inhibitor showed that cAMP-dependent PI 3-kinase activation plays an important role in the increase in cyclin D1 translation. In contrast, IGF-I-dependent PI 3-kinase activation was required for the increase in cyclin D1 mRNA levels and degradation of p27^Kip1. Taken together, the present study elucidates the role of cAMP and IGF-I in differentially activating PI 3-kinase as a mediator of multiple molecular events. These events converge in the regulation of cyclin D1 and p27^Kip1 leading to cAMP-dependent potentiation of IGF-I-dependent CDK activation and DNA synthesis.