Disease relevance of SKP2

• This study provides evidence of a role for an F-box protein in oncogenesis and establishes SKP2 as a protooncogene causally involved in the pathogenesis of lymphomas [1].
• Amplification and overexpression of SKP2 are associated with metastasis of non-small-cell lung cancers to lymph nodes [2].
• Our earlier studies revealed SKP2 as a target gene within the 5p13 amplicon that is often seen in small-cell lung cancers [2].
• Elevated expression of SKP2 correlated significantly with positive lymph node metastasis (P = 0.007), with stage II or higher of the international TNM classification (P = 0.014), with poor or moderate differentiation (P < 0.001), and with the presence of squamous cell carcinoma (P = 0.037) [2].
• Conversely, in human glioblastoma cells, ectopic PTEN expression leads to p27 accumulation, which is accompanied by a reduction of SKP2 [3].
• Thyroid cancer cell lines and tissues with high levels of Skp2 protein presented high p27 degradation activity and there was an inverse correlation between Skp2 and p27 expression in thyroid cancer tissues (n=68; P<0.05) [4].

High impact information on SKP2

• In vitro, recombinant Skp2 was able to bind hyperphosphorylated but not dephosphorylated p130 [5].
• The phosphorylated Thr187 side chain of p27(Kip1) is recognized by a Cks1 phosphate binding site, whereas the side chain of an invariant Glu185 inserts into the interface between Skp2 and Cks1, interacting with both [6].
• The regulation of antigen receptor gene assembly by Skp2-SCF provides an unexpected and direct mechanistic link between DNA recombination and the cell cycle [7].
• We have found that Skp2 interacts with c-Myc and participates in its ubiquitylation and degradation [8].
• These data suggest that Skp2 functions to connect Myc activity and destruction, and reveal an unexpected oncoprotein connection that may play an important role in controlling cell growth in normal and cancer cells [9].

Chemical compound and disease context of SKP2

• Here we used the human prostate cancer cell line LNCaP, which undergoes G1 cell cycle arrest in response to androgen, to examine the role of the SKP2 F-box protein in p27 regulation in prostate cancer [10].
• We found that both luminal and basal epithelial cells in normal prostate had very low Skp2 levels, but Skp2 levels and labeling frequency increased dramatically in both premalignant lesions of prostatic intraepithelial neoplasm (P = 0.0252) and in prostate cancer (P = 0.0037) [11].
• Dibutyryl cAMP stimulates the proliferation of SH-SY5Y human neuroblastoma cells by up-regulating Skp2 protein [12].
• MATERIALS AND METHODS: The expression of Skp2, Cks1, and p27Kip1 was examined by immunohistochemistry using highly specific antibodies on formalin-fixed, paraffin-embedded tissue sections from 80 patients with colorectal carcinoma [13].
• In conclusion, troglitazone attenuated Skp2 expression, thereby promoting p27(Kip1) accumulation in human hepatoma cells [14].

Biological context of SKP2

• SKP2 accumulation is often deregulated in cancer, which indicates that temporal control of SKP2 is essential for normal cell proliferation [15].
• CDK9 protein levels remained unchanged in human cells entering and progressing through the cell cycle from G(0), despite dramatic changes in SKP2 expression [16].
• Finally, downregulation of endogenous SKP2 gene expression by interfering RNA had no effect on CDK9 protein levels, whereas p27 protein levels increased dramatically [16].
• Activation of CDK2 and binding to SKP2 or p27(KIP1) were not affected by the phosphorylation of Ser-154 [17].
• Reduction of SKP2 expression by transfection of an anti-sense oligonucleotide inhibited invasion and migration of NSCLC cells in culture [2].

Anatomical context of SKP2

• SKP2 exhibited amplification in 5 (20%) of 25 cell lines derived from NSCLC, and the transcript was overexpressed in 11 (44%) of the 25 lines [2].
• Moreover, adenoviral-mediated expression of SKP2 accelerates downregulation of endogenous hyperphosphorylated p130 in mitogen-stimulated T98G cells and primary WI38 fibroblasts [18].
• We found that the supplementation of Skp2-Skp1 and substrate (along with further components necessary for substrate presentation to the ubiquitin ligase) to extracts of HeLa cells synergistically increased levels of neddylated Cul1 [19].
• RESULTS: Western blot analysis showed that high Skp2 expression was observed in 57 (57.6%) cases and significantly correlated with unfavorable cytogenetics (P = 0.035) but not with age, white blood cell count, serum lactic dehydrogenase level, and the French-American-British subtype [20].
• RESULTS: Skp2 staining was localized in the nuclei of the glandular cells of the proliferative phase endometrium, and endometrial hyperplasia and carcinoma cells [21].

Associations of SKP2 with chemical compounds

• At the same time, androgen decreased expression of SKP2, but did not affect other components of SCFSKP2 [10].
• Using phosphopeptide mapping and mutagenesis studies, we found that threonine 29 within the N terminus of Cdt1 is phosphorylated by Cdk2 and required for interaction with Skp2 [22].
• Retinoic acid-mediated growth arrest requires ubiquitylation and degradation of the F-box protein Skp2 [23].
• Identification of a novel Skp2-like mammalian protein containing F-box and leucine-rich repeats [24].
• In addition, 1,25-(OH)2D3 reduced Skp2 protein levels in LNCaP cells [25].

Physical interactions of SKP2

• We show that Skp1 can bind to Skp2 in vitro using recombinant proteins, and in vivo using the yeast two-hybrid system [26].
• PURPOSE: Skp2 interacts with the degradation of cyclin-dependent kinase inhibitor p27 [21].
• In vitro, SKP2 specifically interacted with the cyclin E peptide containing the phosphorylated-Thr380 but not with a cognate nonphosphorylated peptide [27].
• Here we show that the Skp2-binding site of Cks1 is located on a region including the alpha2- and alpha1-helices and their immediate vicinity, well separated from the other two binding sites [28].
• We showed that the F-box protein Skp2 specifically interacted with human Cdt1 in a phosphorylation-dependent manner [29].
• We also provide evidence that Skp2 interacts physiologically with E2F-1 and stimulates its transcriptional activity toward the cyclin E promoter [30].

Enzymatic interactions of SKP2

• The F-box protein SKP2 binds to the phosphorylated threonine 380 in cyclin E and regulates ubiquitin-dependent degradation of cyclin E [27].
• This is the first demonstration that Skp2 is induced and phosphorylated in the late G1 and S phase of hepatocytes in vivo in regenerating liver as well as in vitro in mitogen-stimulated hepatocytes [31].

Regulatory relationships of SKP2

• In vivo, expression of SKP2 induced cyclin E polyubiquitination and degradation [27].
• The F-box protein Skp2 (Fbl1) is a positive regulator of G1-S transition and promotes ubiquitin-mediated proteolysis of the cyclin-dependent kinase inhibitor p27 [11].
• Selective inhibition of cyclooxygenase 2 induces p27kip1 and skp2 in oral squamous cell carcinoma [32].
• The F-box protein SKP2 promotes the G1-S transition by targeting key regulators for proteasomal degradation via its capacity to function as the specificity factor for the SKP1 Cullin F-box SCF(SKP2) ubiquitin ligase [15].
• The X protein of hepatitis B virus binds to the F box protein Skp2 and inhibits the ubiquitination and proteasomal degradation of c-Myc [33].
• B-RAF and cyclin D1 control the levels of S-phase kinase-associated protein 2 (Skp2) that directs ubiquitin-mediated proteolysis of p27(Kip1) [34].

Other interactions of SKP2

• SKP2 is required for ubiquitin-mediated degradation of the CDK inhibitor p27 [35].
• Skp1, together with F-box proteins like Skp2, are part of ubiquitin-ligase E3 complexes that target many cell cycle regulators for ubiquitination-mediated proteolysis [36].
• CDK9 is constitutively expressed throughout the cell cycle, and its steady-state expression is independent of SKP2 [16].
• Moreover, we provide evidence for a physical association between Cul4A, DDB1, and Skp2 [37].
• Exogenous Skp2 prevented growth arrest of MCF-7 cells by antiestrogen, coinciding with decreased p27Kip1 expression [38].

Analytical, diagnostic and therapeutic context of SKP2

• The Skp2 overexpression was significantly associated with shorter disease-free survival and overall survival (P = 0.0386 and P = 0.0369, respectively) [20].
• Kaplan-Meier analysis revealed that a higher Skp2 labeling index (>10) was a significant predictor of shorter biochemical recurrence-free survival time after radical prostatectomy (P < 0.0363, log-rank test) [11].
• EXPERIMENTAL DESIGN: Three hundred and thirty-eight breast cancer specimens were analyzed for Skp2 and p27 expression by immunohistochemistry [39].
• The transfection of pcDNA3.1-p27Kip1 mt or treatment with Skp2 AS and Jab1 AS induced a strong growth inhibition of xenograft tumors [40].
• The detection of Skp2 protein either by flow cytometry or by immunohistochemistry represents a simple method to precisely assess the S phase of lymphomas [41].

References