Inhibitory effect of p21 in MCF-7 cells is overcome by its coordinated stabilization with D-type cyclins

Coordinated accumulation of cyclin D1 and D3 is observed in 15% of primary breast cancers and in the breast cancer cell line MCF-7 this simultaneous overexpression is due to a defect in their ubiquitin-mediated proteolysis. The F-box protein Skp2 is a component of an SCF ubiquitin ligase complex and can associate with cyclin D1 and the cdk inhibitor p21 (Zhong-Kang et al., 1998). We extend this observation and show that cyclin D3 can also associate with Skp2 suggesting that cyclins D1, D3 and p21 may share the same SCF complex. In agreement with this hypothesis we report here that in primary breast cancers and in MCF-7 cells where cyclins D1 and D3 are elevated the level of p21 is also elevated. Further, we demonstrate that the turnover of p21 protein is reduced in MCF-7 cells. We show that p21 is active as a cdk inhibitor in this cell line but that the presence of elevated levels of cyclin D3 titrates p21 away from cyclin D1-cdk4/6 complexes and cdk2 complexes resulting in increased kinase activities. Our results suggest that a defect in the SCF complex may occur in 15-20% of breast cancers and that the resulting coordinated elevation of cyclins D1 and D3 overcomes the inhibition of cell cycle progression by p21. We propose that in the context of cyclins D1 and D3 overexpression, p21 may promote cell cycle progression.