Skp2-Mediated Degradation of p27 Regulates Progression into Mitosis

Abstract

Although Skp2 has been thought to mediate the degradation of p27 at the G(1)-S transition, Skp2(-/-) cells exhibit accumulation of p27 in S-G(2) phase with overreplication. We demonstrate that Skp2(-/-)p27(-/-) mice do not exhibit the overreplication phenotype, suggesting that p27 accumulation is required for its development. Hepatocytes of Skp2(-/-) mice entered the endoduplication cycle after mitogenic stimulation, whereas this phenotype was not apparent in Skp2(-/-)p27(-/-) mice. Cdc2-associated kinase activity was lower in Skp2(-/-) cells than in wild-type cells, and a reduction in Cdc2 activity was sufficient to induce overreplication. The lack of p27 degradation in G(2) phase in Skp2(-/-) cells may thus result in suppression of Cdc2 activity and consequent inhibition of entry into M phase. These data suggest that p27 proteolysis is necessary for the activation of not only Cdk2 but also Cdc2, and that Skp2 contributes to regulation of G(2)-M progression by mediating the degradation of p27.

Mutant B-RAF signaling and cyclin D1 regulate Cks1/S-phase kinase-associated protein 2-mediated degradation of p27Kip1 in human melanoma cells

“…p27 Kip1 expression was increased upon knockdown of Skp2. Other Cdk inhibitor proteins, p21 Cip1 and p57 Kip2 , are putative targets of the SCF Skp2 complex Spruck et al, 2001;Kamura et al, 2003;Nakayama et al, 2004); however, we observed only minor changes in expression of these proteins following Skp2 knockdown in WM793 (Figure 5b) and WM278 cells (data not shown). Furthermore, Skp2 knockdown did not alter STAT1 levels (Sledz et al, 2003), indicating that changes in p27 Kip1 levels were not owing to an interferon response (data not shown).…”

“…Malek and co-workers showed that deficient proliferation in regenerating liver cells from Skp2 (À/À) mice was associated with a G1 arrest and lack of cyclin A induction (Kossatz et al, 2004). In contrast, Nakayama et al (2004) showed that mouse embryo fibroblasts derived from Skp2 (À/À) mice had decreased Cdk1 and Cdk2 activity and exhibited a defect in progression through G2/M. Future experiments will analyse the effect of Skp2 and Cks1 knockdown on different stages of the cell cycle in melanoma cells.…”

“…Degradation of p27 Kip1 involves the Skp1/Cul1/F-box (SCF)-S-phase kinase-associated protein 2 (Skp2) E3 ligase complex Sutterluty et al, 1999;Tsvetkov et al, 1999;Mirza et al, 2004). In this complex, the F-box component, Skp2 and its cofactor, Cdc kinase subunit 1 (Cks1), provide specificity by targeting substrates, such as p27 Kip1 that has been phosphorylated at residue threonine-187 Sutterluty et al, 1999;Spruck et al, 2001;Kamura et al, 2003;Nakayama et al, 2004). Structural analysis indicates that p27 Kip1 interacts with both Cks1 and Skp2 (Hao et al, 2005).…”

Skp2 regulates the antiproliferative function of the tumor suppressor RASSF1A via ubiquitin-mediated degradation at the G1–S transition

“…However, the phenotype of Skp2 -/-p27 -/-mice is not identical to that of p27 -/-mice. Although accumulation of p27 mainly contributes to the proliferation defect of Skp2 -/-MEFs, Skp2 appears to prevent the accumulation of p27 during S-G 2 rather than at the G 1 -S transition (Nakayama et al, 2004). We have now shown that the delay in G 1 -S progression induced by Skp2 depletion was normalized by the additional depletion of RASSF1A even in the presence of a high level of p27 (Figure 6c).…”

“…Skp2 regulates entry into S phase by mediating degradation of the Cdk inhibitor p27 (Reed, 2003). However, the observation that the phenotype of Skp2 -/-p27 -/-mice is similar but not identical to that of p27 -/-mice (Nakayama et al, 2004) suggests that, although p27 might be the main target of Skp2, Skp2 likely also mediates the ubiquitination of other substrates. We therefore investigated whether G 1 -S progression might be affected by Skp2-mediated degradation of RASSF1A.…”