p45SKP2 promotes p27Kip1 degradation and induces S phase in quiescent cells

Abstract

The F-box protein p45SKP2 is the substrate-targeting subunit of the ubiquitin-protein ligase SCFSKP2 and is frequently overexpressed in transformed cells. Here we report that expression of p45SKP2 in untransformed fibroblasts activates DNA synthesis in cells that would otherwise growth-arrest. Expression of p45SKP2 in quiescent fibroblasts promotes p27Kip1 degradation, allows the generation of cyclin-A-dependent kinase activity and induces S phase. Coexpression of a degradation-resistant p27Kip1 mutant suppresses p45SKP2-induced cyclin-A-kinase activation and S-phase entry. We propose that p45SKP2 is important in the progression from quiescence to S phase and that the ability of p45SKP2 to promote p27Kip1 degradation is a key aspect of its S-phase-inducing function. In transformed cells, p45SKP2 may contribute to deregulated initiation of DNA replication by interfering with p27Kip1 function.

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

“…Notably, expression of Skp2 was enhanced in all four human melanoma cell lines compared to NHEM (Figure 4a). Two Skp2 immunoreactive bands were observed consistent with others' reports Sutterluty et al, 1999;Garriga et al, 2003). We have previously shown that serum-starved NHEM express high levels of p27 Kip1 that are downregulated in an adhesion and growth factor-dependent manner (Bhatt et al, 2005).…”

“…p27 Kip1 levels in melanoma cells are also regulated by the proteasome. Phosphorylation at threonine 187 is permissive for p27 Kip1 degradation (Vlach et al, 1997;Carrano et al, 1999;Sutterluty et al, 1999). However, as threonine 187-phosphorylated p27 Kip1 levels increased in parallel with total p27 Kip1 (data not shown), we focused on possible defects in the SCF complex containing Skp2 and its cofactor, Cks1, that recognizes phosphorylated p27 Kip1 and targets it for degradation (Ganoth et al, 2001;Spruck et al, 2001).…”

“…The SCF Skp2 ubiquitin E3 ligase complex is known to mediate p27 Kip1 degradation in several cell types Sutterluty et al, 1999). Initially, we analysed levels of Skp2 in serum-starved melanoma cell lines (WM793, WM115, SK-MEL-5 and SK-MEL-28) and normal human epidermal melanocytes (NHEM) by Western blotting.…”

“…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).…”

Deregulated degradation of the cdk inhibitor p27 and malignant transformation

“…SCF Skp2 was identified as the E3 that targets p27 phosphorylated on Thr-187 for ubiquitination [21][22][23]. SCF complexes represent an evolutionarily conserved class of E3 enzymes containing four subunits: Skp1, Cul1, one of many F-box proteins and Roc1/Rbx1 (reviewed in [24]).…”