Skp2-mediated p27(Kip1) degradation during S/G2 phase progession of a adipocyte hyperplasia

p27(Kip I), an important regulator of Cdk2 activity and G(1)/S transition, is tightly regulated in a cell-type and condition-specific manner to integrate mitogenic and differentiation signals governing cell cycle progression. We show that p27 protein levels progressively declined from mid-G, through late-G(2) phase as density-arrested 3T3-LI preadipocytes synchronously reentered the cell cycle during early stages of adipocyte differentiation. This dramatic fall in p27 protein accumulation was due, at least in part, to a decrease in protein stability. Specific inhibitors of the 26S proteasome were shown to completely block the decrease in p27 protein levels throughout G(1), increase the abundance of ubiquitylated p27 protein, and inhibit G(1)/S transition resulting in G(1) arrest. It is further demonstrated that p27 was phosphorylated on threonine 187 during S phase progression by Cdk2 and that phosphorylated p27 was polyubiquitylated and degraded. Furthermore, we demonstrate that Skp2 and CksI dramatically increased during S/G(2) phase progression concomitantly with the maximal fall in p27 protein. Complete knockdown of Skp2 with RNA interference partially prevented p27 degradation equivalent to that observed with Cdk2 blockade suggesting that the SCFSkp2 E3 ligase and other proteasome-dependent mechanisms contribute to p27 degradation during preadipocyte replication. Interestingly, Skp2-mediated p27 degradation was not essential for G(1)/S or S/G(2) transition as preadipocytes shifted from quiescence to proliferation during adipocyte hyperplasia. Finally, evidence is presented suggesting that elevated p27 protein in the absence of Skp2 was neutralized by sequestration of p27 protein into Cyclin DI/Cdk4 complexes.