Glycogen Synthase Kinase-3 Couples AKT-dependent Signaling to the Regulation of p21 super(Cip1) Degradation

Signaling via the phosphoinositide 3-kinase (PI3K)/AKT pathway is crucial for the regulation of endothelial cell (EC) proliferation and survival, which involves the AKT-dependent phosphorylation of the DNA repair protein p21 super(Cip1) at Thr-145. Because p21 super(Cip1) is a short-lived protein with a high proteasomal degradation rate, we investigated the regulation of p21 super(Cip1) protein levels by PI3K/AKT-dependent signaling. The PI3K inhibitors Ly294002 and wortmannin reduced p21 super(Cip1) protein abundance in human umbilical vein EC. However, mutation of the AKT site Thr-145 into aspartate (T145D) did not increase its protein half-life. We therefore investigated whether a kinase downstream of AKT regulates p21 super(Cip1) protein levels. In various cell types, AKT phosphorylates and inhibits glycogen synthase kinase-3 (GSK-3). Upon serum stimulation of EC, GSK-3 beta was phosphorylated at Ser-9. Site-directed mutagenesis revealed that GSK-3 in vitro phosphorylated p21 super(Cip1) specifically at Thr-57 within the Cdk binding domain. Overexpression of GSK-3 beta decreased p21 super(Cip1) protein levels in EC, whereas the specific inhibition of GSK-3 with lithium chloride interfered with p21 super(Cip1) degradation and increased p21 super(Cip1) protein about 10-fold in EC and cardiac myocytes (30 mM, p < 0.001). These data indicate that GSK-3 triggers p21 super(Cip1) degradation. In contrast, stimulation of AKT increases p21 super(Cip1) via inhibitory phosphorylation of GSK-3.