Role of the F-box protein Skp2 in lymphomagenesis

Absence of SKP2 expression attenuates BCR-ABL-induced myeloproliferative disease

BCR-ABL is proposed to impair cell-cycle control by disabling p27, a tumor suppressor that inhibits cyclin-dependent kinases. We show that in cell lines p27 expression is inversely correlated with expression of SKP2, the F-box protein of SCF(SKP2) (SKP1/Cul1/F-box), the E3 ubiquitin ligase that promotes proteasomal degradation of p27. Inhibition of BCR-ABL kinase causes G(1) arrest, down-regulation of SKP2, and accumulation of p27. Ectopic expression of wild-type SKP2, but not a mutant unable to recognize p27, partially rescues cell-cycle progression. A similar regulation pattern is seen in cell lines transformed by FLT3-ITD, JAK2(V617F), and TEL-PDGFRbeta, suggesting that the SKP2/p27 conduit may be a universal target for leukemogenic tyrosine kinases. Mice that received transplants of BCR-ABL-infected SKP2(-/-) marrow developed a myeloproliferative syndrome but survival was significantly prolonged compared with recipients of BCR-ABL-expressing SKP2(+/+) marrow. SKP2(-/-) leukemic cells demonstrated higher levels of nuclear p27 than SKP2(+/+) counterparts, suggesting that the attenuation of leukemogenesis depends on increased p27 expression. Our data identify SKP2 as a crucial mediator of BCR-ABL-induced leukemogenesis and provide the first in vivo evidence that SKP2 promotes oncogenesis. Hence, stabilization of p27 by inhibiting its recognition by SCF(SKP2) may be therapeutically useful.

Deregulated proteolysis by the F-box proteins SKP2 and beta-TrCP: tipping the scales of cancer

The maintenance and preservation of distinct phases during the cell cycle is a highly complex and coordinated process. It is regulated by phosphorylation--through the activity of cyclin-dependent kinases (CDKs)--and protein degradation, which occurs through ubiquitin ligases such as SCF (SKP1-CUL1-F-box protein) complexes and APC/C (anaphase-promoting complex/cyclosome). Here, we explore the functionality and biology of the F-box proteins, SKP2 (S-phase kinase-associated protein 2) and beta-TrCP (beta-transducin repeat-containing protein), which are emerging as important players in cancer biogenesis owing to the deregulated proteolysis of their substrates.

Skp2/p27 expression profile is correlated with Epstein-Barr virus status in extranodal nasal-type natural killer cell lymphoma

Extranodal nasal-type natural killer cell lymphoma (ENKL) is a high-grade malignancy and is associated with Epstein-Barr virus (EBV) latent infection. Little is known about its molecular abnormalities. Here, we studied the expression of Skp2 and p27 proteins in 48 cases of ENKL, and we evaluated their correlations with EBV status and clinical outcomes. EBV infection was observed in 90% of the cases. In all, 71% of the ENKLs were positive to Skp2 and 73% were negative to p27. A significant negative correlation was observed between the expression of Skp2 and p27 proteins (P = 0.022). Fifty-eight percent of the cases were Skp2+/p27- phenotype and correlated with EBV status (P = 0.047). The overall survival was influenced by the expression of Skp2, p27, and Skp2/p27. Patients with Skp2+, p27-, and Skp2+/p27- phenotypes had worse overall survival (P < 0.01, P = 0.016, and P < 0.01, respectively). Multivariance analysis showed the Skp2/p27 expression profile was an independent prognostic factor for overall survival (RR = 3.09, P < 0.01, 95% CI: 1.27-7.51). In conclusion, the Skp2/p27 expression profile is a helpful prognostic factor for ENKL. Latent EBV infection may increase the expression levels of Skp2, and consequently, p27 protein degradation is accelerated. EBV may be a good target for treatment of EBV-associated ENKL.

Skp2 expression is associated with high risk and elevated Ki67 expression in gastrointestinal stromal tumours

Background

Gastrointestinal stromal tumors (GIST) exhibit an unpredictable clinical course and can rapidly progress to lethality. Predictions about the biological behavior of GIST are based on a number of canonical clinical and pathologic parameters whose validity in distinguishing between a benign and a malignant tumour is still imperfect. The aim of our study was to investigate the role of morphologic parameters and expression of cells cycle regulators as prognosticators in GIST.

Methods

We performed an immunohistochemical analysis for Ki67, p27^Kip1, Jab1, and Skp2, on a Tissue Microarray (TMA) containing 94 GIST. Expression of the above proteins was correlated to classically used prognosticators, as well as to risk groups. Clinical significance of histologic and immunohistochemical features were evaluated in 59 patients for whom follow-up information was available.

Results

Overexpression of Ki67 and Skp2, and p27^Kip1 loss directly correlated with the high risk group (p = 0.03 for Ki67 and Skp2, p = 0.05 for p27^Kip1). Jab1 expression did not exhibit correlation with risk. In 59 cases provided with clinical follow-up, high cellularity, presence of necrosis, and Ki67 overexpression were predictive of a reduced overall survival in a univariate model. The same parameters, as well as mitotic rate, tumour size, and p27^Kip1 loss were indicative of a shortened relapse free survival interval. High cellularity, and high mitotic rate retained their prognostic significance by multivariate analysis.

Conclusion

Our data suggest that a number of histologic parameters in combination with immunohistochemical expression of cell cycle regulators can facilitate risk categorization and predict biologic behavior in GIST. Importantly this study demonstrates, for the first time, that Skp2 expression correlates with Ki67 expression and high risk in GIST.

Oncogenic properties and prognostic implications of the ubiquitin ligase Skp2 in cancer

The expression of Skp2, the ubiquitin ligase subunit that targets p27(Kip1) for degradation, is commonly overexpressed in human cancers. p27(Kip1) is a negative regulator of the cell cycle that plays an important role in tumor suppression. Loss of p27(Kip1) secondary to enhanced ubiquitin-mediated degradation results in uncontrolled proliferation and promotes tumor progression. In the present study the prognostic implications of Skp2 are reviewed and the mechanisms that regulate its expression in different human cancers. A review and analysis of the English literature was undertaken. Overexpression of Skp2 mRNA and protein levels was observed in many aggressive cancers and was commonly associated with down-regulation of p27(Kip1) levels and loss of tumor differentiation. Skp2 is an independent prognostic marker for disease-free and overall survival and may provide additional predictive information to that provided by p27(Kip1) alone. Targeting Skp2 in experimental models resulted in up-regulation of p27(Kip1) and arrested cellular proliferation. Alterations in Skp2 expression have profound effects on cancer progression and may serve as an accurate and independent prognostic marker. Thus, determination of levels of Skp2 and p27(Kip1) by readily available immunohistochemical studies may be a useful tool in the assessment of prognosis, especially in patients with intermediate disease, and may potentially assist in the planning of adjuvant therapy. Skp2 may be an attractive target for the development of novel interventional therapy.

The Cdk inhibitor p27 in human cancer: prognostic potential and relevance to anticancer therapy

The cyclin-dependent kinase (Cdk) inhibitor p27 (also known as KIP1) regulates cell proliferation, cell motility and apoptosis. Interestingly, the protein can exert both positive and negative functions on these processes. Diverse post-translational modifications determine the physiological role of p27. Phosphorylation regulates p27 binding to and inhibition of cyclin-Cdk complexes, its localization and its ubiquitin-mediated proteolysis. In cancers, p27 is inactivated through impaired synthesis, accelerated degradation and by mislocalization. Moreover, studies in several tumour types indicate that p27 expression levels have both prognostic and therapeutic implications.

Skp2 suppresses p53-dependent apoptosis by inhibiting p300

The F box protein Skp2 is oncogenic, and its frequent amplification and overexpression correlate with the grade of malignancy of certain tumors. Conversely, depletion of Skp2 decreases cell growth and increases apoptosis. Here, we show that Skp2 counteracts the transactivation function of p53 and suppresses apoptosis mediated by DNA damage or p53 stabilization. We demonstrate that Skp2 forms a complex with p300 through the CH1 and the CH3 domains of p300 to which p53 is thought to bind and antagonizes the interaction between p300 and p53 in cells and in vitro. As Skp2 antagonizes the interaction between p300 and p53, Skp2 suppresses p300-mediated acetylation of p53 and the transactivation ability of p53. Conversely, ectopic expression of p300 rescues the transactivation function of p53 in cells overexpressing Skp2. Taken together, our results indicate that Skp2 controls p300-p53 signaling pathways in cancer cells, making Skp2 a potential molecular target for cancer therapy.

FOXP3 is a novel transcriptional repressor for the breast cancer oncogene SKP2

S-phase kinase-associated protein 2 (SKP2) is a component of the E3 ubiquitin ligase SKP1-Cul1-Fbox complex. Overexpression of SKP2 results in cell cycle dysregulation and carcinogenesis; however, the genetic lesions that cause this upregulation are poorly understood. We recently demonstrated that forkhead box P3 (FOXP3) is an X-linked breast cancer suppressor and an important repressor of the oncogene ERBB2/HER2. Since FOXP3 suppresses tumor growth regardless of whether the tumors overexpress ERBB2/HER2, additional FOXP3 targets may be involved in its tumor suppressor activity. Here, we show that mammary carcinomas from mice heterozygous for a Foxp3 mutation exhibited increased Skp2 expression. Ectopic expression of FOXP3 in mouse mammary cancer cells repressed SKP2 expression with a corresponding increase in p27 and polyploidy. Conversely, siRNA silencing of the FOXP3 gene in human mammary epithelial cells increased SKP2 expression. We also show that Foxp3 directly interacted with and repressed the Skp2 promoter. Moreover, the analysis of over 200 primary breast cancer samples revealed an inverse correlation between FOXP3 and SKP2 levels. Finally, we demonstrated that downregulation of SKP2 was critical for FOXP3-mediated growth inhibition in breast cancer cells that do not overexpress ERBB2/HER2. Our data provide genetic, biochemical, and functional evidence that FOXP3 is a novel transcriptional repressor for the oncogene SKP2.

The E3 ubiquitin ligase skp2 regulates neural differentiation independent from the cell cycle

Background

The SCF^skp2 complex is an E3 ubiquitin ligase that is known to target a number of cell cycle regulators, including cyclin-dependent kinase inhibitors, for proteolysis. While its role in regulation of cell division has been well documented, additional functions in differentiation, including in the nervous system, have not been investigated.

Results

Using Xenopus as a model system, here we demonstrate that skp2 has an additional role in regulation of differentiation of primary neurons, the first neurons to differentiate in the neural plate. Xenopus skp2 shows a dynamic expression pattern in early embryonic neural tissue and depletion of skp2 results in generation of extra primary neurons. In contrast, over-expression of skp2 inhibits neurogenesis in a manner dependent on its ability to act as part of the SCF^skp2 complex. Moreover, inhibition of neurogenesis by skp2 occurs upstream of the proneural gene encoding NeuroD and prior to cell cycle exit. We have previously demonstrated that the Xenopus cyclin dependent kinase inhibitor Xic1 is essential for primary neurogenesis at an early stage, and before these cells exit the cell cycle. We show that SCF^skp2 degrades Xic1 in embryos and this contributes to the ability of skp2 to regulate neurogenesis.

Conclusion

We conclude that the SCF^skp2 complex has functions in the control of neuronal differentiation additional to its role in cell cycle regulation. Thus, it is well placed to be a co-ordinating factor regulating both cell proliferation and cell differentiation directly.

p27Kip1 repression of ErbB2-induced mammary tumor growth in transgenic mice involves Skp2 and Wnt/beta-catenin signaling

Expression of the cyclin-dependent kinase (Cdk) inhibitor (p27(Kip1)) is frequently reduced in human tumors, often correlating with poor prognosis. p27(Kip1) functions as a haploinsufficient tumor suppressor; however, the mechanism by which one allele of p27(Kip1) regulates oncogenic signaling in vivo is not well understood. We therefore investigated the mechanisms by which p27(Kip1) inhibits mammary tumor onset. Using the common background strain of FVB, p27(Kip1) heterozygosity (p27(+/-)) accelerated ErbB2-induced mammary tumorigenesis. We conducted microarray analyses of mammary tumors developing in mice with genetic haploinsufficiency for p27(Kip1) expressing a mammary-targeted ErbB2 oncogene. Global gene expression profiling and Western blot analysis of ErbB2/p27(+/-) tumors showed that the loss of p27(Kip1) induced genes promoting lymphangiogenesis, cellular proliferation, and collaborative oncogenic signaling (Wnt/beta-catenin/Tcf, Cdc25a, Smad7, and Skp2). Skp2 expression was induced by ErbB2 and repressed by p27(Kip1). Degradation of p27(Kip1) involves an SCF-type E3 ubiquitin ligase, including Skp2. The Skp2 component of the SCF(SKP2) complex that degrades p27(Kip1) was increased in ErbB2 tumors correlating with earlier tumor onset. In both murine and human ErbB2-overexpressing breast cancers, p27(Kip1) levels correlated inversely with Skp2. p27(Kip1) haploinsufficiency activated Wnt/beta-catenin/hedgehog signaling. Reintroduction of p27(Kip1) inhibited beta-catenin induction of Tcf-responsive genes (Siamosis, c-Myc, and Smad7). p27(Kip1) is haploinsufficient for ErbB2 mammary tumor suppression in vivo and functions to repress collaborative oncogenic signals including Skp2 and Wnt/beta-catenin signaling.

Wrenches in the works: drug discovery targeting the SCF ubiquitin ligase and APC/C complexes

Recently, the ubiquitin proteasome system (UPS) has matured as a drug discovery arena, largely on the strength of the proven clinical activity of the proteasome inhibitor Velcade in multiple myeloma. Ubiquitin ligases tag cellular proteins, such as oncogenes and tumor suppressors, with ubiquitin. Once tagged, these proteins are degraded by the proteasome. The specificity of this degradation system for particular substrates lies with the E3 component of the ubiquitin ligase system (ubiquitin is transferred from an E1 enzyme to an E2 enzyme and finally, thanks to an E3 enzyme, directly to a specific substrate). The clinical effectiveness of Velcade (as it theoretically should inhibit the output of all ubiquitin ligases active in the cell simultaneously) suggests that modulating specific ubiquitin ligases could result in an even better therapeutic ratio. At present, the only ubiquitin ligase leads that have been reported inhibit the degradation of p53 by Mdm2, but these have not yet been developed into clinical therapeutics. In this review, we discuss the biological rationale, assays, genomics, proteomics and three-dimensional structures pertaining to key targets within the UPS (SCFSkp2 and APC/C) in order to assess their drug development potential. Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; http://www.targetedproteinsdb.com).

A bioactive withanolide Tubocapsanolide A inhibits proliferation of human lung cancer cells via repressing Skp2 expression

Withanolides are generally defined as C(28) steroidal lactones built on an intact or rearranged ergostane skeleton and have been shown to exhibit antiproliferative activity on various types of cancer cells. In this study, we investigated the effect of a new withanolide Tubocapsanolide A isolated from Tubocapsicum anomalum and addressed its molecular action. Tubocapsanolide A inhibited proliferation of A549, H358, and H226 human lung cancer cells via induction of G(1) growth arrest. We found that Tubocapsanolide A treatment led to up-regulation of cyclin E, p21, and p27, whereas other cyclins and cyclin-dependent kinases were not affected in A549 cells. Conversely, Skp2, the F-box protein that is implicated in the mediation of degradation of p21 and p27, was significantly down-regulated. Chromatin immunoprecipitation assay suggested that Tubocapsanolide A suppressed Skp2 expression by inhibiting the binding of Rel A to the nuclear factor-kappaB site of Skp2 gene promoter. In addition, we showed that inhibition of Skp2 is a critical step for the suppression of cell proliferation by Tubocapsanolide A because ectoexpression of Skp2 effectively reversed Tubocapsanolide A-induced p27 up-regulation and growth inhibition in human lung cancer cells. Collectively, we have identified Skp2 as a molecular target for Tubocapsanolide A and suggest that this withanolide may be useful for the prevention or treatment of cancer cells with Skp2 overexpression.

Oncogenic regulators and substrates of the anaphase promoting complex/cyclosome are frequently overexpressed in malignant tumors

The fidelity of cell division is dependent on the accumulation and ordered destruction of critical protein regulators. By triggering the appropriately timed, ubiquitin-dependent proteolysis of the mitotic regulatory proteins securin, cyclin B, aurora A kinase, and polo-like kinase 1, the anaphase promoting complex/cyclosome (APC/C) ubiquitin ligase plays an essential role in maintaining genomic stability. Misexpression of these APC/C substrates, individually, has been implicated in genomic instability and cancer. However, no comprehensive survey of the extent of their misregulation in tumors has been performed. Here, we analyzed more than 1600 benign and malignant tumors by immunohistochemical staining of tissue microarrays and found frequent overexpression of securin, polo-like kinase 1, aurora A, and Skp2 in malignant tumors. Positive and negative APC/C regulators, Cdh1 and Emi1, respectively, were also more strongly expressed in malignant versus benign tumors. Clustering and statistical analysis supports the finding that malignant tumors generally show broad misregulation of mitotic APC/C substrates not seen in benign tumors, suggesting that a "mitotic profile" in tumors may result from misregulation of the APC/C destruction pathway. This profile of misregulated mitotic APC/C substrates and regulators in malignant tumors suggests that analysis of this pathway may be diagnostically useful and represent a potentially important therapeutic target.

Skp2 expression is associated with down-regulation of p27 protein and cell proliferation in salivary adenoid cystic carcinoma

Adenoid cystic carcinoma (ACC) is a malignant salivary gland tumor, which shows frequent recurrence and metastasis, ultimately with a poor outcome. We previously demonstrated that p27 down-regulation is frequently found and is due to an enhancement of its degradation in ACC. In this study, we transfected nondegradable p27 mutant (T187A) and wild-type gene into ACC cell line. Transfection of T187A mutant gene was more effective on inhibition of cell growth of ACC cells, suggesting that aberration of p27 degradation may be present in ACC. As F-box protein S-phase kinase-associated protein 2 (Skp2), which is necessary for ubiquitin-mediated degradation of p27, is involved in p27 down-regulation in various cancers, we examined the Skp2 expression and its association with p27 expression in 50 ACC cases. We found Skp2 expression in 36% of ACC cases and inverse association between the expression of Skp2 and p27. Moreover, Skp2 small interfering ribonucleic acid (siRNA) transfection decreased Skp2 protein and accumulation of p27 protein and inhibited the cell growth of ACC cells in vitro. These findings, overall, suggest that Skp2 may play an important role in ACC development through the down-regulation of p27 and that Skp2 siRNA can be a novel modality of cancer gene therapy for suppression of p27 down-regulation in ACC.

Utility of SKP2 and MIB-1 in grading follicular lymphoma using quantitative imaging analysis

Follicular lymphoma is classified into grades (G)1, 2, and 3 based on the number of centroblasts in neoplastic follicles. However, the accuracy of manually counting these centroblasts is limited by certain cells (large centrocytes, follicular dendritic cells, and histiocytes) that could mimic centroblast morphology. The reproducibility of follicular lymphoma grading is dependent upon observer experience; therefore, significant variations occur. This study is to explore a more objective, reliable way of grading follicular lymphoma using a quantitative imaging system in conjunction with immunostains with antibodies to proliferation markers MIB-1 and S-phase kinase-associated protein 2 (SKP2). Fifty-eight follicular lymphomas (G1, n = 23; G2, n = 18; and G3, n = 17) were studied on formalin-fixed, paraffin-embedded sections. Positive nuclear staining of both Ki-67 and SKP2 was recorded using the quantitative Clarient ACIS II system (Aliso Viejo, CA, USA). Ten high-power fields (x400) from randomly selected neoplastic follicles were counted by a pathologist blinded to the previously assigned morphologic grade. The results show that the percentages of Ki-67+ and SKP2+ cells significantly differ among the different grades of follicular lymphoma. A higher grade of follicular lymphoma is associated with a higher percentage of Ki-67+ and SKP2+ cells. The overall SKP2+% cells are substantially lower than Ki-67+% cells in the same grade of follicular lymphoma. Statistical significance is observed in Ki-67+ cells between follicular lymphoma G1 and follicular lymphoma G3 and between G1 and G2. In contrast, statistical significance is noted in SKP2+% cells between follicular lymphoma G1 and follicular lymphoma G3 and between follicular lymphoma G2 and follicular lymphoma G3. The findings suggest that the SKP2 expression has better discrimination with grades of follicular lymphoma than Ki-67 expression. Compared with traditional methods, quantitation of SKP2 expression using a quantitative image analysis system might be a useful and objective approach in grading follicular lymphoma.

Increased expression of SKP2 and phospho-MAPK/ERK1/2 and decreased expression of p27 during tumor progression of cervical neoplasms

OBJECTIVE

The objective of this study was to investigate whether the expression of SKP2, p27 and phospho-MAPK/ERK1/2 is associated with the progression of human cervical neoplasia.

METHODS

We performed immunohistochemical detection to stain formalin-fixed paraffin-embedded cervical tissues with anti-SKP2 and anti-p27 monoclonal antibodies and anti-phospho-p42/44 MAPK antibody. The study sample included 23 normal cervical epithelium, 25 low-grade squamous intraepithelial lesion (LSIL), 19 high-grade squamous intraepithelial lesion (HSIL), and 31 squamous cell carcinomas (SCC). In addition, 14 frozen cervical biopsies, including 1 normal, 6 HSIL, 2 adenocarcinoma and 5 SCC, and a human cervical cancer cell line (HeLa), were analyzed the expression levels of mRNA and protein of SKP2 and p27 by RT-PCR and Western blot analysis, respectively.

RESULTS

The expression of SKP2, p27 and phospho-MAPK/ERK1/2 were strongly associated with cervical neoplastic progression (P<0.0001, P=0.006, P=0.003, respectively; Fisher's Exact Test). In addition, SKP2 expression was positively correlated with phospho-MAPK/ERK1/2 expression (Spearman correlation coefficient=0.480, P=0.0002). The association between SKP2 and phospho-MAPK/ERK1/2 was significant after controlling for the four histologic grades (P=0.038, Mantel-Haenszel test).

CONCLUSIONS

These results suggest that expression levels of SKP2, p27 and phospho-MAPK/ERK1/2 may serve as markers for progression in human cervical carcinoma and may also play roles in cervical carcinoma progression and cervical carcinogenesis.

1,25-dihydroxyvitamin D3 transcriptionally represses p45Skp2 expression via the Sp1 sites in human prostate cancer cells

Upregulation of p27Kip1 protein in 1,25-dihydroxyvitamin D3-treated cancer cells is mediated via enhancement of gene transcription and reduction of protein degradation. 1,25-dihydroxyvitamin D3 inhibits the expression of p45Skp2, the F-box protein which is implicated in p27Kip1 degradation, to reduce turnover of p27Kip1 protein. In this study, we elucidate the underlying mechanism by which 1,25-dihydroxyvitamin D3 inhibits p45Skp2 in human LNCaP prostate cancer cells. Western blot and RT-PCR analysis suggest that 1,25-dihydroxyvitamin D3 suppresses p45Skp2 via transcriptional repression. Promoter activity assays indicate that 1,25-dihydroxyvitamin D3 directly inhibits p45Skp2 promoter activity. Deletion analysis shows that 1,25-dihydroxyvitamin D3 response element is localized at -447/-291 bp region from the translational start site of the p45Skp2 promoter. Mutation analysis suggests that two Sp1 sites localized at -386/-380 and -309/-294 bp region are required for transcriptional repression. Chromatin immunoprecipitation (CHIP) assay demonstrates that VDR indirectly binds to these Sp1 sites in vivo and this binding is increased after 1,25-dihydroxyvitamin D3 treatment. Re-CHIP assay suggests that VDR and Sp1 form a complex to bind to the Sp1 sites. DNA affinity precipitation assay (DAPA) shows that histone deacetylase 1 (HDAC1) is recruited to the Sp1 sites after 1,25-dihydroxyvitamin D3 stimulation. Re-CHIP assay verifies that binding of Sp1 and HDAC1 to p45Skp2 promoter is enhanced after 1,25-dihydroxyvitamin D3 treatment. HDAC inhibitor trichostatin A (TSA) reverses the inhibition of p45Skp2 promoter activity by 1,25-dihydroxyvitamin D3. Collectively, our results suggest that 1,25-dihydroxyvitamin D3 induces the formation of VDR/Sp1 complex and acts via a Sp1- and HDAC1-depedent pathway to inhibit p45Skp2 transcription.

FOXO factors: a matter of life and death

Forkhead box O-class (FOXO) transcription factors, including FOXO1, FOXO3a and FOXO4, function as tumor-suppressor proteins by inhibiting cell proliferation, promoting apoptotic cell death and protecting cells from DNA damage and oxidative stress. The potency of these functions is regulated tightly by phosphorylation, acetylation and ubiquitination. Emerging evidence indicates that protein levels of FOXO1 are under dual regulation by Ak-mediated phosphorylation and Skp2-mediated ubiquitination. Given that Akt and Skp2 proteins are highly activated in human cancers due to the loss of phosphatase and tensin homolog (PTEN), deregulation of the FOXO1 protein appears to be a promising target for future drug discovery and cancer therapy.

E3 ubiquitin ligases as cancer targets and biomarkers

E3 ubiquitin ligases are a large family of proteins that are engaged in the regulation of the turnover and activity of many target proteins. Together with ubiquitin-activating enzyme E1 and ubiquitin-conjugating enzyme E2, E3 ubiquitin ligases catalyze the ubiquitination of a variety of biologically significant protein substrates for targeted degradation through the 26S proteasome, as well as for nonproteolytic regulation of their functions or subcellular localizations. E3 ubiquitin ligases, therefore, play an essential role in the regulation of many biologic processes. Increasing amounts of evidence strongly suggest that the abnormal regulation of some E3 ligases is involved in cancer development. Furthermore, some E3 ubiquitin ligases are frequently overexpressed in human cancers, which correlates well with increased chemoresistance and poor clinic prognosis. In this review, E3 ubiquitin ligases (such as murine double minute 2, inhibitor of apoptosis protein, and Skp1-Cullin-F-box protein) will be evaluated as potential cancer drug targets and prognostic biomarkers. Extensive study in this field would lead to a better understanding of the molecular mechanism by which E3 ligases regulate cellular processes and of how their deregulations contribute to carcinogenesis. This would eventually lead to the development of a novel class of anticancer drugs targeting specific E3 ubiquitin ligases, as well as the development of sensitive biomarkers for cancer treatment, diagnosis, and prognosis.

Balance of Yin and Yang: ubiquitylation-mediated regulation of p53 and c-Myc

Protein ubiquitylation has been demonstrated to play a vital role not only in mediating protein turnover but also in modulating protein activity. The stability and activity of the tumor suppressor p53 and of the oncoprotein c-Myc are no exception. Both are regulated through independent ubiquitylation by several E3 ubiquitin ligases. Interestingly, p53 and c-Myc are functionally connected by some of these E3 enzymes and their regulator ARF, although these proteins play opposite roles in controlling cell growth and proliferation. The balance of this complex ubiquitylation network and its disruption during oncogenesis will be the topics of this review.

Regulation of p27 degradation and S-phase progression by Ro52 RING finger protein

Ubiquitin-mediated degradation of the cyclin-dependent kinase inhibitor p27 provides a powerful route for enforcing normal progression through the mammalian cell cycle. According to a current model, the ubiquitination of p27 during S-phase progression is mediated by SCF(Skp2) E3 ligase that captures Thr187-phosphorylated p27 by means of the F-box protein Skp2, which in turn couples the bound substrate via Skp1 to a catalytic core complex composed of Cul1 and the Rbx/Roc RING finger protein. Here we identify Skp2 as a component of an Skp1-cullin-F-box complex that is based on a Cul1-Ro52 RING finger B-box coiled-coil motif family protein catalytic core. Ro52-containing complexes display E3 ligase activity and promote the ubiquitination of Thr187-phosphorylated p27 in a RING-dependent manner in vitro. The knockdown of Ro52 expression in human cells with small interfering RNAs causes the accumulation of p27 and the failure of cells to enter S phase. Importantly, these effects are abrogated by the simultaneous removal of p27. Taken together, these data suggest a key role for Ro52 RING finger protein in the regulation of p27 degradation and S-phase progression in mammalian cells and provide evidence for the existence of a Cul1-based catalytic core that utilizes Ro52 RING protein to promote ubiquitination.

Expression and prognostic significance of S-phase kinass-associated protein 2 in colorectal carcinoma tissues

AIM: To detect the expression of S-phase kinase associated protein 2 (SKP2) and assess its prognostic value in human colorectal carcinoma.

METHODS: SP immunohistochemical method was used to detect the expression of SKP2 P27 proteins in the tissues of colorectal carcinoma (n = 68). Survival analyses were performed using the Kaplan-Meier method and Cox regression model.

RESULTS: Of 68 cases with colorectal carcinoma, the positive rates of SKP2 and P27 expression were 41.2% (n = 28) and 52.9% (n = 36). The expression of SKP2 was significantly correlated with the differentiation of carcinoma (χ² = 14.073, P = 0.001), but it had no significant correlation with tumor AJCC staging, the gender and ages of patients (P > 0.05). There was a negative correlation between SKP2 and P27 expression (r = -0.528, P = 0.0001). With regard to prognosis, the overall survival rate for patients with the high-SKP2 expression was significantly lower as compared with that for the ones with low-SKP2 expression (31.5 ± 4.0 mo vs 54.5 ± 2.1 mo, P < 0.01). Furthermore, the expression of SKP2 was an independent predictor of prognosis by multivariate Cox regression model analysis (RR = 6.227, P = 0.033).

CONCLUSION: SKP2 expression can serve as a useful prognostic marker in colorectal carcinoma patients.

IKKalpha controls p52/RelB at the skp2 gene promoter to regulate G1- to S-phase progression

The IkappaB-inducing kinase (IKK) is composed of two catalytic subunits, IKKalpha and IKKbeta, and a regulatory subunit, IKKgamma. IKK-regulated signaling pathways are believed to promote the proliferation of normal cells as well as the aberrant proliferation of cancer cells. The molecular mechanisms linking the IKK signaling pathway components to the cell cycle machinery are not entirely understood. To study the function(s) of the catalytic subunits of the IKK complex, we used pancreatic cancer cells, with constitutive IKK activity. We show that the G1 phase of the cell cycle is specifically regulated by the IKKalpha subunit, which regulates the stability of the cyclin-dependent kinase inhibitor p27(Kip1). Increased p27(Kip1) protein levels following the transfection of IKKalpha-specific siRNAs are a result of the downregulation of the F-box protein S-phase kinase-associated protein 2 (skp2). Additionally, we demonstrate that IKKalpha signaling regulates the transcription of the skp2 gene by controlling the composition of a RelB-containing NF-kappaB complex. Together, this work defines a novel IKKalpha-regulated growth pathway involving the p52/RelB-dependent transcriptional regulation of the skp2 gene.

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

Levels of cyclins and cyclin-dependent kinase (Cdk) inhibitors are tightly controlled during normal cell proliferation and are frequently dysregulated in cancerous cells. In melanoma, cyclin D1 is highly expressed and downregulation of the Cdk inhibitor, p27(Kip1), is associated with a poor prognosis. Mutant B-RAF is frequently expressed in melanoma and overrides growth factor and matrix adhesion control of cyclin D1 and p27(Kip1) levels in human melanocytes. Here, we demonstrate that p27(Kip1) expression is regulated by multiple mechanisms in melanoma cells. B-RAF regulates p27(Kip1) mRNA abundance independently of cyclin D1. Additionally, B-RAF and cyclin D1 control the levels of S-phase kinase-associated protein 2 (Skp2) that directs ubiquitin-mediated proteolysis of p27(Kip1). The cofactor for Skp2, Cdc kinase subunit 1 (Cks1) controls levels of Skp2 in melanoma cells and acts jointly with Skp2 to regulate p27(Kip1) levels. Importantly, expression of Cks1 is regulated by B-RAF and cyclin D1 at the mRNA level. Reduced Cks1 or Skp2 expression and enhanced p27(Kip1) levels inhibit melanoma cell growth. In summary, p27(Kip1) expression in melanoma is regulated by B-RAF at the mRNA level, and via B-RAF and cyclin D1 control of Cks1/Skp2-mediated proteolysis.

F-box protein Skp2: a novel transcriptional target of E2F

The F-box-containing protein Skp2 plays a critical role in coordinating the G1/S transition and progression through the S phase of the mammalian cell cycle. Skp2 is overexpressed in a broad spectrum of human cancers and the expression level correlates with tumor malignancy. However, the Skp2 gene is neither amplified nor rearranged in most human cancers and the underlying mechanism of Skp2 overexpression remains poorly understood. We show here that the Skp2 gene contains a functional E2F response element (hSRE2). Ectopic expression of E2F1 induces expression of the endogenous Skp2 gene in human fibroblast cells, whereas antisense-mediated knockdown of E2F1 in human tumor cell lines reduces expression of endogenous Skp2 gene. The hSRE2 element not only participates in activation of Skp2 promoter function during normal cell cycle progression into S phase, it is also required for the high-level Skp2 gene expression in many human tumor cell lines. These results reveal Skp2 as a novel target for E2F regulation that is disrupted in several human tumor cell lines.

Ubiquitin ligases: cell-cycle control and cancer

A driving force of the cell cycle is the activation of cyclin-dependent kinases (CDKs), the activities of which are controlled by the ubiquitin-mediated proteolysis of key regulators such as cyclins and CDK inhibitors. Two ubiquitin ligases, the SKP1-CUL1-F-box-protein (SCF) complex and the anaphase-promoting complex/cyclosome (APC/C), are responsible for the specific ubiquitylation of many of these regulators. Deregulation of the proteolytic system might result in uncontrolled proliferation, genomic instability and cancer. Cumulative clinical evidence shows alterations in the ubiquitylation of cell-cycle regulators in the aetiology of many human malignancies. A better understanding of the ubiquitylation machinery will provide new insights into the regulatory biology of cell-cycle transitions and the development of anti-cancer drugs.

High-level Skp2 expression in pancreatic ductal adenocarcinoma: correlation with the extent of lymph node metastasis, higher histological grade, and poorer patient outcome

OBJECTIVES

Recent studies have shown that overexpression of S-phase kinase-associated protein 2 (Skp2) occurs in many cancers at an advanced stage. We examined the clinicopathologic significance and prognostic implication of Skp2 expression in pancreatic invasive ductal carcinoma.

METHODS

Tissue samples from 46 pancreatic carcinomas were examined immunohistochemically for Skp2. The proportion of constituent tumor cells with Skp2 expression was analyzed and classified as high-level nuclear expression when more than 20% of the cancer cells were positive, or low-level nuclear expression otherwise.

RESULTS

High-level Skp2 overexpression was detected in 13 (28.3%) of the 46 tumors. The incidence of high-level Skp2 was correlated with higher histological grade (P = 0.0056), the extent of lymph node metastasis (P = 0.0086), the level of lymphatic permeation (P = 0.0024), and poorer patient outcome (P = 0.0189). Multivariate analysis showed that high-level Skp2 expression was an independent predictor of overall patient survival (P = 0.0140).

CONCLUSIONS

It is suggested that examination of Skp2 expression might be clinically useful for prognostication in patients with pancreatic carcinoma and that Skp2 protein might be a novel therapeutic molecular target.

HSP27 favors ubiquitination and proteasomal degradation of p27 Kip1 and helps S‐phase re‐entry in stressed cells

Stress-inducible HSP27 protects cells from death through various mechanisms. We have recently demonstrated that HSP27 can also enhance the degradation of some proteins through the proteasomal pathway. Here, we show that one of these proteins is the cyclin-dependent kinase (Cdk) inhibitor p27Kip1. The ubiquitination and degradation of this protein that favors progression through the cell cycle was previously shown to involve either a Skp2-dependent mechanism,i.e., at the S-/G2-transition, or a KPC (Kip1 ubiquitination-promoting complex)-dependent mechanism, i.e.,at the G0/G1 transition. In this work, we demonstrate that, in response to serum depletion, p27Kip1 cellular content first increases then progressively decreases as cells begin to die. In this stressful condition, HSP27favors p27Kip1 ubiquitination and degradation by the proteasome. A similar observation was made in response to stress induced by the NO donor glyceryl trinitrate (GTN). HSP27-mediated ubiquitination ofp27Kip1 does not require its phosphorylation on Thr187 or Ser-10, nor does it depend on the SCFSkp2 ubiquitin ligase E3 complex. It facilitates the G1/S transition,which suggests that, in stressful conditions, HSP27might render quiescent cells competent to re-enter the cell cycle.

Knockdown of Skp2 by siRNA inhibits melanoma cell growth in vitro and in vivo

BACKGROUND

Low levels of p27Kip1 expression are associated with poor prognosis in various malignancies including malignant melanoma. Recently, it has been reported that S phase kinase-interacting protein 2 (Skp2), the specific ubiquitin ligase subunit that targets p27Kip1 for degradation, was overexpressed and was inversely related to p27Kip1 levels in malignant melanoma with poor prognosis.

OBJECTIVE

We investigated whether small interfering RNA (siRNA)-mediated gene silencing of Skp2 can be employed in order to inhibit p27Kip1 down-regulation and suppress melanoma cell growth as a consequence in vitro and in vivo.

METHODS

We constructed a plasmid vector, which synthesizes siRNAs to determine the effects of decreasing the high constitutive levels of Skp2 protein in melanoma cells. Western blot and real-time RT-PCR were performed to examine the decreases of Skp2 protein and mRNA in vitro. Furthermore, melanoma cells were injected into the back of nude mice subcutaneously to examine the suppression of tumorigenicity targeting Skp2 gene silencing in vivo.

RESULTS

Skp2 protein was decreased and the p27Kip1 protein was accumulated in Skp2 siRNA transfected melanoma cells. Skp2 siRNA inhibited the cell growth of melanoma cells in vitro. Moreover, Skp2 siRNA also suppressed tumor proliferation in vivo.

CONCLUSION

Our results suggest that siRNA-mediated gene silencing of Skp2 can be a potent tool of cancer gene therapy for suppression of p27Kip1 degradation in malignant melanoma.

How to decrease p27Kip1 levels during tumor development

p27Kip1, a cyclin-cdk inhibitor, is a tumor suppressor. An overwhelming amount of data correlate p27 abundance to tumor prognosis in humans. Mouse models have supported the importance of decreasing p27 to tumor incidence. Inactivation of most tumor suppressors occurs at the level of gene mutation or silencing, but p27 is regulated posttranscriptionally, and how its level is reduced in cancer is largely unknown. Reports on a series of allelic mice with p27 mutations affecting different posttranscriptional regulatory pathways are emerging and being used to examine which pathways are necessary for p27 turnover associated with tumor development, with surprising results.

Effective inhibition of cell growth and invasion of melanoma by combined suppression of BRAF (V599E) and Skp2 with lentiviral RNAi

p27Kip1 that regulates the G1/S transition of cell cycle and inhibits Rho A signaling is frequently lost in several cancers leading to the deregulation of cell growth and cell motility. Mitogen-activated protein kinases (MAPK) regulate the export of p27Kip1 from nucleus to cytoplasm, followed by the degradation with proteases. Skp-2, a subunit of an SCF ubiquitin-protein ligase complex responsible for the ubiquitination of p27Kip1, is upregulated frequently in several cancers, leading to the decrease of p27Kip1. We applied human immunodeficiency virus (HIV) lentivirus-mediated RNA interference (RNAi) to melanoma cells with the BRAF mutation (V599E) and overexpressed Skp-2 and found that the simultaneous suppression of these activated oncogenes resulted in the effective inhibition of in vitro cell growth and invasive ability of melanoma cells accompanied by the additional increase of p27Kip1. Our results suggest that gene therapy against melanoma with the enhanced MAPK and ubiquitin-proteasomal pathways could be a specific and effective therapeutic strategy for cancers.

Structural basis of the Cks1-dependent recognition of p27(Kip1) by the SCF(Skp2) ubiquitin ligase

The ubiquitin-mediated proteolysis of the Cdk2 inhibitor p27(Kip1) plays a central role in cell cycle progression, and enhanced degradation of p27(Kip1) is associated with many common cancers. Proteolysis of p27(Kip1) is triggered by Thr187 phosphorylation, which leads to the binding of the SCF(Skp2) (Skp1-Cul1-Rbx1-Skp2) ubiquitin ligase complex. Unlike other known SCF substrates, p27(Kip1) ubiquitination also requires the accessory protein Cks1. The crystal structure of the Skp1-Skp2-Cks1 complex bound to a p27(Kip1) phosphopeptide shows that Cks1 binds to the leucine-rich repeat (LRR) domain and C-terminal tail of Skp2, whereas p27(Kip1) binds to both Cks1 and Skp2. 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. The structure and biochemical data support the proposed model that Cdk2-cyclin A contributes to the recruitment of p27(Kip1) to the SCF(Skp2)-Cks1 complex.

Cooperation of ERK and SCFSkp2 for MKP-1 destruction provides a positive feedback regulation of proliferating signaling

The dual-specificity MAPK phosphatase MKP-1/CL100/DUSP1 is an inducible nuclear protein controlled by p44/42 MAPK (ERK1/2) in a negative feedback mechanism to inhibit kinase activity. Here, we report on the molecular basis for a novel positive feedback mechanism to sustain ERK activation by triggering MKP-1 proteolysis. Active ERK2 docking to the DEF motif (FXFP, residues 339-342) of N-terminally truncated MKP-1 in vitro initiated phosphorylation at the Ser(296)/Ser(323) domain, which was not affected by substituting Ala for Ser at Ser(359)/Ser(364). The DEF and Ser(296)/Ser(323) sites were essential for ubiquitin-mediated MKP-1 proteolysis stimulated by MKK1-ERK signaling in H293 cells, whereas the N-terminal domain and Ser(359)/Ser(364) sites were dispensable. ERK activation by serum increased the endogenous level of ubiquitinated phospho-Ser(296) MKP-1 and the degradation of MKP-1. Intriguingly, active ERK-promoted phospho-Ser(296) MKP-1 bound to SCF(Skp2) ubiquitin ligase in vivo and in vitro. Forced expression of Skp2 enhanced MKP-1 polyubiquitination and proteolysis upon ERK activation, whereas depletion of endogenous Skp2 suppressed such events. The kinetics of ERK signaling stimulated by serum correlated with the endogenous MKP-1 degradation rate in a Skp2-dependent manner. Thus, MKP-1 proteolysis can be achieved via ERK and SCF(Skp2) cooperation, thereby sustaining ERK activation.

Relation of overexpression of S phase kinase-associated protein 2 with reduced expression of p27 and PTEN in human gastric carcinoma

AIM: To investigate the significance of S phase kinase-associated protein 2 (Skp2) expression in human gastric carcinoma and the relation between expressions of Skp2, p27 and PTEN.

METHODS: Immunohistochemical analysis was performed on 138 gastric carcinoma specimens, their paired adjacent mucosa specimens, 102 paired lymphatic metastatic carcinoma tissue specimens, 30 dysplasia specimens, 30 intestinal metaplasia specimens, 10 chronic superficial gastritis specimens and 5 normal gastric mucosa specimens for Skp2 expression and on 138 gastric carcinoma specimens for p27 and PTEN expression.

RESULTS: Skp2 labeling frequency was significantly higher in intestinal metaplasia (12.68±0.86) and adjacent mucosa (19.32±1.22) than in normal gastric mucosa (0.53±0.13) and chronic superficial gastritis (0.47±0.19) (P = 0.000); in dysplasia (16.74±0.82) than in intestinal metaplasia (P = 0.000); in gastric primary carcinoma (31.34±2.17) than in dysplasia and adjacent mucosa (P = 0.000); in metastasis gastric carcinoma in lymph nodes (39.76±2.00) than in primary gastric carcinoma (P = 0.037), respectively. Skp2 labeling frequency was positively associated with differentiation degree (rho = 0.315, P = 0.000), vessel invasion (rho = 0.303, P = 0.000) and lymph node metastasis (rho = 0.254, P = 0.000) of gastric cancer. Expression of Skp2 was negatively associated with p27 (rho = -0.451, P = 0.000) and PTEN (rho = -0.480, P = 0.000) expression in gastric carcinoma. p27 ex-pression was positively associated with PTEN expression in gastric carcinoma (rho = 0.642, P = 0.000).

CONCLUSION: Skp2 overexpression may be involved in carcinogenesis and progression of human gastric carcinoma in vivo, possibly via p27 proteolysis. PTEN may regulate the expression of p27 by negatively regulating Skp2 expression.

Down-regulation of S-phase kinase associated protein 2 (Skp2) induces apoptosis in oral cancer cells

S-phase kinase associated protein 2 (Skp2) is a member of an F-box family of substrate-recognition subunits of SCF ubiquitin-protein ligase complexes that has been implicated in the ubiquitin-mediated degradation of several key regulators of mammalian G1 progression, including the cyclin-dependent kinase inhibitor p27Kip1, a dosage-dependent tumor suppressor protein. The anti-sense effect was confirmed in two cell lines of oral cancer cells that also exhibited over-expression of the Skp2 protein. In this study, we examined the mechanism responsible for anti-sense-mediated growth inhibition of oral cancer cells in vitro and in vivo. Skp2-anti-sense treatment induced apoptosis characterized by an increase in the early apoptosis, fragmentation of nuclei and activation of caspase-3, -8 and -9. Moreover, the growth of xenograft tumors was markedly suppressed by Skp2-anti-sense treatment. Furthermore, histological specimen revealed apoptotic cell death was increased in Skp2-anti-sense treated tumors. Our results suggest that down-regulation of Skp2 appears to induce apoptosis in oral cancer cells, targeting this molecule could represent a promising new therapeutic approach for this type of cancer.

Cancer therapeutics: targeting the dark side of Myc

The potent Myc oncoprotein plays a pivotal role as a regulator of tumorigenesis in numerous human cancers of diverse origin. Experimental evidence shows that inhibiting Myc significantly halts tumour cell growth and proliferation. This review summarises recent progress in understanding the function of Myc as a transcription factor, with emphasis on key protein interactions and target gene regulation. In addition, major advances in drug development aimed at eliminating Myc are described, including antisense and triple helix forming oligonucleotides, porphyrins and siRNA. Future anti-Myc strategies are also discussed that inhibit Myc at the level of expression and/or function. Targeting the dark side of Myc with novel therapeutic agents promises to have a profound impact in combating cancer.

Oncogenic Ras blocks transforming growth factor-beta-induced cell-cycle arrest by degradation of p27 through a MEK/Erk/SKP2-dependent pathway

OBJECTIVE

To examine whether oncogenic Ras affects transforming growth factor (TGF)-beta-mediated cell-cycle arrest in hematopoietic cells and the downstream signal transduction pathway involved in the interference with TGF-beta-induced cell-cycle arrest.

MATERIALS AND METHODS

Two leukemic cell lines bearing N-Ras(L61) mutations; HL-60 and TF-1, and the M1 cell line with wt Ras were investigated for their response to TGF-beta. Signal transduction inhibitors, overexpression and RNA interference studies were performed to investigate the involvement of the various proteins.

RESULTS

Although TGF-beta signal transduction was not affected, G0-G1 arrest was absent in HL-60 and TF-1 cells due to the absence of p27. Overexpression of p27 restored TGF-beta-induced cell-cycle arrest, as well as interfering in Ras-mediated signaling. The farnesyl transferase inhibitor L744832 and the MEK inhibitor U0126 both restored p27 levels and cell-cycle arrest in response to TGF-beta. The absence of p27 protein is due to elevated levels of the ubiquitin ligase SKP2, which complexes with and targets p27 for degradation. RNA interference for SKP2 and treatment of these cells with the proteasome inhibitor MG132 restored p27 levels, corresponding with decreasing SKP2 levels after interfering in N-Ras signal transduction. P27, phosphorylated at threonine 187, is nuclear localized in N-Ras-containing cells. Mutation of this residue to alanine rendered p27 insensitive to degradation.

CONCLUSION

N-Ras(L61) transformed cells lack a G0-G1 arrest upon TGF-beta treatment due to absence of p27. p27 is degraded through a MapK-, and SKP2-dependent pathway. Overexpression of p27 results in restoration of cell-cycle arrest upon TGF-beta treatment.

Notch1 modulates timing of G1-S progression by inducing SKP2 transcription and p27 Kip1 degradation

Cyclin-dependent kinase inhibitors (CKIs) and Notch receptor activation have been shown to influence adult stem cells and progenitors by altering stem cell self-renewal and proliferation. Yet, no interaction between these molecular pathways has been defined. Here we show that ligand-independent and ligand-dependent activation of Notch1 induces transcription of the S phase kinase–associated protein 2 (SKP2), the F-box subunit of the ubiquitin-ligase complex SCF^SKP2 that targets proteins for degradation. Up-regulation of SKP2 by Notch signaling enhances proteasome-mediated degradation of the CKIs, p27^Kip1 and p21^Cip1, and causes premature entry into S phase. Silencing of SKP2 by RNA interference in G₁ stabilizes p27^Kip1 and p21^Cip1 and abolishes Notch effect on G₁-S progression. Thus, SKP2 serves to link Notch1 activation with the cell cycle machinery. This novel pathway involving Notch/SKP2/CKIs connects a cell surface receptor with proximate mediators of cell cycle activity, and suggests a mechanism by which a known physiologic mediator of cell fate determination interfaces with cell cycle control.

The ubiquitin-proteasome pathway and its role in cancer

Critical cellular processes are regulated, in part, by maintaining the appropriate intracellular levels of proteins. Whereas de novo protein synthesis is a comparatively slow process, proteins are rapidly degraded at a rate compatible with the control of cell cycle transitions and cell death induction. A major pathway for protein degradation is initiated by the addition of multiple 76-amino acid ubiquitin monomers via a three-step process of ubiquitin activation and substrate recognition. Polyubiquitination targets proteins for recognition and processing by the 26S proteasome, a cylindrical organelle that recognizes ubiquitinated proteins, degrades the proteins, and recycles ubiquitin. The critical roles played by ubiquitin-mediated protein turnover in cell cycle regulation makes this process a target for oncogenic mutations. Oncogenes of several common malignancies, for example colon and renal cell cancer, code for ubiquitin ligase components. Cervical oncogenesis by human papillomavirus is also mediated by alteration of ubiquitin ligase pathways. Protein degradation pathways are also targets for cancer therapy, as shown by the successful introduction of bortezomib, an inhibitor of the 26S proteasome. Further work in this area holds great promise toward our understanding and treatment of a wide range of cancers.

Down-regulation of Cdk inhibitor p27 in oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) is the most frequent malignant neoplasm of the head and neck region. Conversion of normal cells to cancer cells is achieved through a multi-step process that is closely associated with the accumulation of multiple gene changes including both oncogenes and tumour suppressor genes. The proliferation and progression of cancer may be caused by abnormalities of various positive and negative cell cycle regulators. Cell cycle progression is positively regulated by multiple cyclins and cyclin-dependent kinases (Cdks) and cyclin/Cdk complexes are negatively regulated by a number of Cdk inhibitors including p27. p27 is a Cdk inhibitor and plays an important role in negative regulation of the cell cycle during G0 and G1 phases. Degradation of p27 is a critical event for the G1/S transition and occurs through ubiquitination by SCF(Skp2) and subsequent degradation by the 26S proteasome. It has been revealed that down-regulation of p27 is frequently found in various cancers, including OSCC, and is due to an enhancement of its degradation. Importantly, down-regulation of p27 is well associated with its malignancy including poor prognosis in various cancers. Moreover, aggressive human cancers express low levels of p27 because of its decreased stability. More recent evidence suggests that Skp2 and Cks1, the specific recognition factors for p27 ubiquitination, have oncogenic properties. This review will focus on down-regulation of p27 and mechanism of its down-regulation in OSCC.

RNA silencing of S-phase kinase-interacting protein 2 inhibits proliferation and centrosome amplification in lung cancer cells

The S-phase kinase-associated protein-2 (SKP2) plays a key role in ubiquitin-mediated proteolysis, which results in the progression of cells from a quiescence to proliferative state. SKP2 is overexpressed in a variety of tumors. In this study, we used small interfering RNAs (siRNAs) to inhibit the SKP2 expression in lung cancer cells and thereby investigate the role of SKP2 in lung tumorigenesis. Three lung cancer cell lines were transfected with siRNAs targeted against SKP2. SKP2-siRNAs specifically and efficiently reduced the levels of the SKP2 protein by 90% 48 h after transfection in all cell lines. In the A549 and H1792 cells, p27 expression was increased and the increase was inversely proportional to the level of SKP2; cell proliferation was reduced to 12 and 28%, respectively; apoptosis was increased to 36 and 30%, respectively; 36 and 28% of cells accumulated in the sub-G1 phase, respectively; and the population of cells in the G1 phase was decreased to 37 and 41%, respectively. In addition, the SKP2-depleted A549 and H1792 cells showed decreased levels of cyclin E/CDK2. Correspondingly, only 4 and 6% of the treated A549 and H1792 cells had multiple centrosomes, respectively, compared with 43 and 46% of the control cells, respectively. These results imply that SKP2 plays an oncogenic role in lung cancer and that SKP2 silencing may be useful in the treatment of lung cancer.

Molecular cloning and characterization of FBXO47, a novel gene containing an F-box domain, located in the 17q12 band deleted in papillary renal cell carcinoma

Genetic alterations of chromosome arm 17q occur in numerous tumor types, including breast and ovarian tumors, suggesting the presence of a tumor-suppressor gene on the long arm of chromosome 17 that is critical for carcinogenesis. Previous studies have shown an allelic imbalance (70% gain or loss) of 17q in papillary renal cell carcinoma (pRCC). In this study, we analyzed 15 cases of pRCC for loss of heterozygosity with the use of 7 microsatellite markers between 17q11 and 17q23. We identified a minimal deleted region in which the D17S250 marker (17q12) was deleted in 50% (7 of 14) of informative cases. We isolated the cDNA of a novel gene named FBXO47, which is near D17S250. Human FBXO47 is composed of 11 exons and spans approximately 30 kb of genomic DNA. FBXO47 cDNA consists of 2,269 bp with a 1,359-bp open-reading frame. Of note is that FBXO47 is preferentially expressed in normal tissue relative to the corresponding tumor tissue, particularly in the kidney, liver, and pancreas and to a lesser extent in the thyroid gland, stomach, and small intestine. The putative protein encoded by this gene is made up of 453 amino acids and belongs to the F-box family, most of whose members, such as SKP2 and FBW7, have been implicated in carcinogenesis. Together, these results indicate that FBX047 has a potential role as a tumor-suppressor gene.

SCF-mediated protein degradation and cell cycle control

The regulatory step in ubiquitin (Ub)-mediated protein degradation involves recognition and selection of the target substrate by an E3 Ub-ligase. E3 Ub-ligases evoke sophisticated mechanisms to regulate their activity temporally and spatially, including multiple post-translational modifications, combinatorial E3 Ub-ligase pathways, and subcellular localization. The phosphodegrons of many substrates incorporate the activities of multiple kinases, and ubiquitination only occurs when all necessary phosphorylation signals have been incorporated. In this manner, the precise timing of degradation can be controlled. Another way that the Ub pathway tightly controls the timing of proteolysis is with multiple E3 Ub-ligases acting upon a single target. Lastly, subcellular localization can either promote or prevent degradation by regulating the accessibility of kinases and E3 Ub-ligases. This review highlights recent findings that exemplify these emerging themes in the regulation of E3 Ub-ligase substrate recognition.

Cell cycle, proteolysis and cancer

Research in the past 15 years has shown that the mammalian cell cycle is controlled by the action of cyclin-dependent kinases (CDKs). A crucial substrate of the CDKs in G1-phase is the retinoblastoma tumor suppressor (pRB), which restrains proliferation largely by repressing the activity of the E2F transcription factors. More recent work has shown that the cell cycle is also a tale of two classes of ubiquitin ligases, referred to as SCF and APC/C ligases. CDKs, E2F and ubiquitin ligases reciprocally regulate each other, resulting in complex feedback loops. Perturbation of this network of molecular machines is associated with proliferative diseases, including cancer.

Expression of p27 and its ubiquitin ligase subunit Skp2 in upper urinary tract transitional cell carcinoma

OBJECTIVES

To analyze p27 and S-phase kinase-associated protein 2 (Skp2) expression in upper urinary tract transitional cell carcinoma (TCC) with respect to biologic significance. p27 (p27/kip1) is involved in cell cycle control, and loss of p27 protein expression may result in tumor development and/or progression. The association of p27 with the ubiquitin ligase subunit Skp2 targets p27 for degradation.

METHODS

A total of 53 upper urinary tract TCC specimens were investigated immunohistochemically using a tissue microarray technique. The immunoreactivity of p27 and Skp2 was analyzed with respect to associations with pT stage, grade, and prognosis.

RESULTS

Non-neoplastic renal tissue showed p27 immunoreactivity in tubule epithelium and pelvic urothelium, but lacked immunoreactivity for Skp2. In the TCC specimens, p27 immunoreactivity was noted in 47 (89%) of 53 cases. High p27 expression (50% or greater of tumor cell nuclei) tended to decrease with rising tumor stage (14 [45%] of 31 with pT1-pT2 versus 4 [18%] of 22 with pT3; P = 0.076), but was independent of tumor grade (11 [39%] of 28 grade 2 versus 7 [28%] of 25 grade 3-4; P = 0.56). Skp2 immunoreactivity was noted in 32 (60%) of 53 tumors. Skp2 expression increased with rising tumor stage (9 [41%] of 22 pT1 versus 23 [74%] of 31 pT2-pT3; P = 0.023) and tumor grade (12 [43%] of 28 grade 2 versus 20 [80%] of 25 grade 3; P = 0.043) and was associated with angioinvasion (P = 0.017). In multivariate analysis, tumor stage proved to be the only independent prognostic factor regarding disease-free survival.

CONCLUSIONS

p27 and Skp2 are additional biomarkers in urogenital pathologic findings. The statistically significant association of Skp2 expression with high-grade TCC, as well as the lack of expression in non-neoplastic tissue, suggests that Skp2 could be a promising target for future cancer therapy strategies.

SCFSkp2 complex targeted by Epstein-Barr virus essential nuclear antigen

The stability of cell cycle checkpoint and regulatory proteins is controlled by the ubiquitin-proteasome degradation machinery. A critical regulator of cell cycle molecules is the E3 ubiquitin ligase SCFSkp2, known to facilitate the polyubiquitination and degradation of p27, E2F, and c-myc. SCFSkp2 is frequently deregulated in human cancers. In this study, we have revealed a novel link between the essential Epstein-Barr virus (EBV) nuclear antigen EBNA3C and the SCFSkp2 complex, providing a mechanism for cell cycle regulation by EBV. EBNA3C associates with cyclin A/cdk2 complexes, disrupting the kinase inhibitor p27 and enhancing kinase activity. The recruitment of SCFSkp2 activity to cyclin A complexes by EBNA3C results in ubiquitination and SCFSkp2-dependent degradation of p27. This is the first report of a viral protein usurping the function of the SCFSkp2 cell cycle regulatory machinery to regulate p27 stability, establishing the foundation for a mechanism by which EBV regulates cyclin/cdk activity in human cancers.

p107 inhibits G1 to S phase progression by down-regulating expression of the F-box protein Skp2

Cell cycle progression is negatively regulated by the pocket proteins pRb, p107, and p130. However, the mechanisms responsible for this inhibition are not fully understood. Here, we show that overexpression of p107 in fibroblasts inhibits Cdk2 activation and delays S phase entry. The inhibition of Cdk2 activity is correlated with the accumulation of p27, consequent to a decreased degradation of the protein, with no change of Thr187 phosphorylation. Instead, we observed a marked decrease in the abundance of the F-box receptor Skp2 in p107-overexpressing cells. Reciprocally, Skp2 accumulates to higher levels in p107^−/− embryonic fibroblasts. Ectopic expression of Skp2 restores p27 down-regulation and DNA synthesis to the levels observed in parental cells, whereas inactivation of Skp2 abrogates the inhibitory effect of p107 on S phase entry. We further show that the serum-dependent increase in Skp2 half-life observed during G1 progression is impaired in cells overexpressing p107. We propose that p107, in addition to its interaction with E2F, inhibits cell proliferation through the control of Skp2 expression and the resulting stabilization of p27.