Targeted disruption of Skp2 results in accumulation of cyclin E and p27Kip1, polyploidy and centrosome overduplication

Abstract

The ubiquitin–proteasome pathway plays an important role in control of the abundance of cell cycle regulators. Mice lacking Skp2, an F‐box protein and substrate recognition component of an Skp1–Cullin–F‐box protein (SCF) ubiquitin ligase, were generated. Although Skp2−/− animals are viable, cells in the mutant mice contain markedly enlarged nuclei with polyploidy and multiple centrosomes, and show a reduced growth rate and increased apoptosis. Skp2−/− cells also exhibit increased accumulation of both cyclin E and p27Kip1. The elimination of cyclin E during S and G2 phases is impaired in Skp2−/− cells, resulting in loss of cyclin E periodicity. Biochemical studies showed that Skp2 interacts specifically with cyclin E and thereby promotes its ubiquitylation and degradation both in vivo and in vitro. These results suggest that specific degradation of cyclin E and p27Kip1 is mediated by the SCFSkp2 ubiquitin ligase complex, and that Skp2 may control chromosome replication and centrosome duplication by determining the abundance of cell cycle regulators.

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

“…Skp2 is the substrate-recognition component of the SCF Skp2 ubiquitin ligase complex and is a key regulator of S-phase entry (Nakayama et al, 2000;Harper, 2002;Cardozo and Pagano, 2004). Overexpression or Regulation of RASSF1A by Skp2 at G 1 -S transition MS Song et al amplification of Skp2 has been reported in a large number of human cancers (Yokoi et al, 2002) and transgenic expression of Skp2 in mice leads to tumor formation, suggesting that Skp2 is oncogenic (Gstaiger et al, 2001;Latres et al, 2001).…”

“…Overexpression or Regulation of RASSF1A by Skp2 at G 1 -S transition MS Song et al amplification of Skp2 has been reported in a large number of human cancers (Yokoi et al, 2002) and transgenic expression of Skp2 in mice leads to tumor formation, suggesting that Skp2 is oncogenic (Gstaiger et al, 2001;Latres et al, 2001). Skp2 targets p27, cyclin E and FOXO1 for degradation in a phosphorylationdependent manner (Marti et al, 1999;Nakayama et al, 2000;Huang et al, 2005). Here we provide several lines of evidence that RASSF1A is also targeted by Skp2 at the G 1 -S transition: (1) the level of RASSF1A during cell cycle progression is inversely related to that of Skp2; (2) Skp2 directly interacts with and ubiquitinates RASSF1A in a manner dependent on phosphorylation of RASSF1A on Ser 203 in G 1 -S transition; (3) overexpression of Skp2 results in a decrease in the cellular abundance of RASSF1A, thereby elevating cell viability and proliferation; (4) deletion of Skp2 or depletion of Skp2 increases both the level and half-life of RASSF1A as well as induces a delay in G 1 -S progression; and (5) entry into S phase is accelerated in RASSF1A -/-MEFs.…”

“…The SCF complex consists of the essential components Skp1, Cul1, Rbx1 and an F-box protein, the latter of which binds to Skp1 through its F-box motif and serves to recognize and recruit target proteins (Cardozo and Pagano, 2004). The F-box protein Skp2 has been implicated in the ubiquitin-dependent degradation of p27, E2F-1, free cyclin E and FOXO1 (Marti et al, 1999;Nakayama et al, 2000;Huang et al, 2005). The APC-Cdh1 complex degrades Skp2 at the onset of G 1 phase, after which Skp2 begins to accumulate late in G 1 and its abundance peaks during S and G 2 phases (Bashir et al, 2004;Wei et al, 2004).…”

Deregulated degradation of the cdk inhibitor p27 and malignant transformation

“…Knockout mouse models of components of the ubiquitin ligase that targets p27, Skp2 and Cks1, further elucidate p27's role in cell cycle control [26,37]. Skp2 −/− mice are smaller than littermate controls [37].…”

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

“…Structural analysis indicates that p27 Kip1 interacts with both Cks1 and Skp2 (Hao et al, 2005). Knockout of either Skp2 or Cks1 in mice leads to increased p27 Kip1 levels (Nakayama et al, 2000;Spruck et al, 2001) and, notably, expression of Skp2 and Cks1 is enhanced in several cancers (Gstaiger et al, 2001;Latres et al, 2001;Inui et al, 2003;Shapira et al, 2004).…”