cAMP-induced degradation of cyclin D3 through association with GSK-3

In this study we report a new mechanism whereby cyclic AMP (cAMP) regulates the cell-cycle machinery. We demonstrate that elevation of intracellular levels of cAMP promotes degradation of cyclin D3 in proteasomes, and that this occurs via glycogen synthase kinase-3 (GSK-3)-mediated phosphorylation of cyclin D3 at Thr-283. Elevation of cAMP did not change the subcellular distribution of either cyclin D3 or GSK-3. However, cAMP promoted the interaction between cyclin D3 and GSK-3 both in vitro and in vivo, indicating that GSK-3-mediated phosphorylation of cyclin D3 might require the association between the two proteins. These results demonstrate how cAMP enhances degradation of cyclin D3. Furthermore, we provide evidence for a novel mechanism by which GSK-3 might phosphorylate unprimed substrates in vivo.