p45SKP2 promotes p27Kip1 degradation and induces S phase in quiescent cells

Cyclin-dependent kinase 4 overexpression is mostly independent of gene amplification and constitutes an independent prognosticator for nasopharyngeal carcinoma

Data mining in the public domain demonstrates that cyclin-dependent kinase 4 (CDK4) is highly expressed in nasopharyngeal carcinomas (NPC). Associated with cyclin-D, CDK4 phosphorylates and inactivates retinoblastoma (Rb) protein family members and mediates progression through the G1- to the S-phase of the cell cycle. Amplification and overexpression of CDK4 has been identified in various human malignancies. However, its expression and amplification has never been systemically evaluated in NPC. This study aimed to evaluate the amplification and expression status, correlation with clinicopathological features, and prognostic implications of CDK4 based on public domain dataset and in our well-defined cohort of NPC patients. The association between CDK4 transcript level and gene dosage was explored by analysis of an independent public domain dataset. We retrospectively assessed CDK4 immunoexpression in biopsies of 124 consecutive NPC patients devoid of initial distant metastasis and treated according to consistent guidelines. The results were correlated with clinicopathological features, local recurrence-free survival (LRFS), distant metastasis-free survival (DMeFS), and disease-specific survival (DSS). High levels of CDK4 protein were positively correlated with the T 3, 4 status (p = 0.024); N 2, 3 status (p < 0.001); and the American Joint Committee on Cancer stage 3, 4 (p < 0.001). Multivariate analysis suggested high CDK4 expression was an independent prognostic indicator of worse DMeFS (p = 0.001, hazard ratio (HR) = 3.226) and DSS (p = 0.037, HR = 1.838). Although CDK4 is frequently upregulated, its gene locus is very uncommonly amplified in NPC. CDK4 overexpression is mostly independent with gene amplification and represents a potential prognostic biomarker in NPC and may indicate tumor aggressiveness through cell cycle dysregulation.

Myc and cell cycle control

Soon after the discovery of the Myc gene (c-Myc), it became clear that Myc expression levels tightly correlate to cell proliferation. The entry in cell cycle of quiescent cells upon Myc enforced expression has been described in many models. Also, the downregulation or inactivation of Myc results in the impairment of cell cycle progression. Given the frequent deregulation of Myc oncogene in human cancer it is important to dissect out the mechanisms underlying the role of Myc on cell cycle control. Several parallel mechanisms account for Myc-mediated stimulation of the cell cycle. First, most of the critical positive cell cycle regulators are encoded by genes induced by Myc. These Myc target genes include Cdks, cyclins and E2F transcription factors. Apart from its direct effects on the transcription, Myc is able to hyperactivate cyclin/Cdk complexes through the induction of Cdk activating kinase (CAK) and Cdc25 phosphatases. Moreover, Myc antagonizes the activity of cell cycle inhibitors as p21 and p27 through different mechanisms. Thus, Myc is able to block p21 transcription or to induce Skp2, a protein involved in p27 degradation. Finally, Myc induces DNA replication by binding to replication origins and by upregulating genes encoding proteins required for replication initiation. Myc also regulates genes involved in the mitotic control. A promising approach to treat tumors with deregulated Myc is the synthetic lethality based on the inhibition of Cdks. Thus, the knowledge of the Myc-dependent cell cycle regulatory mechanisms will help to discover new therapeutic approaches directed against malignancies with deregulated Myc. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Roles of F-box proteins in cancer

F-box proteins, which are the substrate-recognition subunits of SKP1-cullin 1-F-box protein (SCF) E3 ligase complexes, have pivotal roles in multiple cellular processes through ubiquitylation and subsequent degradation of target proteins. Dysregulation of F-box protein-mediated proteolysis leads to human malignancies. Notably, inhibitors that target F-box proteins have shown promising therapeutic potential, urging us to review the current understanding of how F-box proteins contribute to tumorigenesis. As the physiological functions for many of the 69 putative F-box proteins remain elusive, additional genetic and mechanistic studies will help to define the role of each F-box protein in tumorigenesis, thereby paving the road for the rational design of F-box protein-targeted anticancer therapies.

Epidermal growth factor upregulates Skp2/Cks1 and p27kip1 in human extrahepatic cholangiocarcinoma cells

AIM: To evaluate the expression status of S-phase kinase-associated protein 2 (Skp2)/cyclin-dependent kinases regulatory subunit 1 (Cks1) and p27^kip1, and assess the prognostic significance of Skp2/Cks1 expression with p27^kip1 in patients with extrahepatic cholangiocarcinoma.

METHODS: Seventy-six patients who underwent curative resection for histologically confirmed extrahepatic cholangiocarcinoma at our institution from December 1994 to March 2008 were enrolled. Immunohistochemical staining for Skp2, Cks1, p27^kip1, and Ki67, along with other relevant molecular biologic experiments, were performed.

RESULTS: By Cox regression analyses, advanced age (> 65 years), advanced AJCC tumor stage, poorly differentiated histology, and higher immunostaining intensity of Skp2 were identified as independent prognostic factors in patients with extrahepatic cholangiocarcinoma. Exogenous epidermal growth factor (EGF, especially 0.1-10 ng/mL) significantly increased the proliferation indices by MTT assay and the mRNA levels of Skp2/Cks1 and p27^kip1 in SNU-1196, SNU-1079, and SNU-245 cells. The protein levels of Skp2/Cks1 (from nuclear lysates) and p27^kip1 (from cytosolic lysate) were also significantly increased in these cells. There were significant reductions in the protein levels of Skp2/Cks1 and p27^kip1 (from nuclear lysate) after the treatment of LY294002. By chromatin immunoprecipitation assay, we found that E2F1 transcription factor directly binds to the promoter site of Skp2.

CONCLUSION: Higher immunostaining intensity of Skp2/Cks1 was an independent prognostic factor for patients with extrahepatic cholangiocarcinoma. EGF upregulates the mRNA and protein levels of Skp2/Cks1 and p27^kip1via the PI3K/Akt pathway and direct binding of E2F1 transcription factor with the Skp2 promoter.

The evolutionary young miR-1290 favors mitotic exit and differentiation of human neural progenitors through altering the cell cycle proteins

Regulation of cellular proliferation and differentiation during brain development results from processes requiring several regulatory networks to function in synchrony. MicroRNAs are part of this regulatory system. Although many microRNAs are evolutionarily conserved, recent evolution of such regulatory molecules can enable the acquisition of new means of attaining specialized functions. Here we identify and report the novel expression and functions of a human and higher primate-specific microRNA, miR-1290, in neurons. Using human fetal-derived neural progenitors, SH-SY5Y neuroblastoma cell line and H9-ESC-derived neural progenitors (H9-NPC), we found miR-1290 to be upregulated during neuronal differentiation, using microarray, northern blotting and qRT-PCR. We then conducted knockdown and overexpression experiments to look at the functional consequences of perturbed miR-1290 levels. Knockdown of miR-1290 inhibited differentiation and induced proliferation in differentiated neurons; correspondingly, miR-1290 overexpression in progenitors led to a slowing down of the cell cycle and differentiation to neuronal phenotypes. Consequently, we identified that crucial cell cycle proteins were aberrantly changed in expression level. Therefore, we conclude that miR-1290 is required for maintaining neurons in a differentiated state.

Korean red ginseng extract induces proliferation to differentiation transition of human acute promyelocytic leukemia cells via MYC-SKP2-CDKN1B axis

ETHNOPHARMACOLOGICAL RELEVANCE

Korean red ginseng has been used as traditional medicine in East Asia. Recent scientific research revealed multiple effects of Korean red ginseng, including anticancer activity. To evaluate the effect of Korean red ginseng extract (KRGE) in acute promyelocytic leukemia (APL) and elucidate its molecular mechanism.

MATERIALS AND METHODS

NB4 cells were treated with 1mg/ml KRGE for 48 h and examined for cell proliferation and differentiation. Cell cycle distribution of KRGE-treated cells was analyzed and the expression level of G1 phase regulators was determined. MYC was overexpressed by retroviral transduction and its effect on SKP2 and CDKN1B gene expression, cell proliferation, cell cycle and differentiation was evaluated in KRGE-treated cells.

RESULTS

KRGE alone was sufficient to induce granulocytic differentiation accompanied with growth inhibition. KRGE treatment resulted in cell cycle arrest at the G1 phase with augmented Cdkn1b proteins without changes in transcript levels. Cycloheximide treatment revealed reduced degradation of Cdkn1b protein by KRGE. In addition, KRGE treatment reduced expression of MYC and SKP2 genes, both at mRNA and protein levels. Upon ectopic expression of MYC, the effect of KRGE was reversed with lesser reduction and induction of SKP2 gene and Cdkn1b protein, respectively. Taken together, these results suggest a sequential molecular mechanism from MYC reduction, SKP2 reduction, Cdkn1b protein stabilization, G1 phase arrest to granulocytic differentiation by KRGE in human APL.

CONCLUSIONS

KRGE induces leukemic proliferation to differentiation transition in APL through modulation of the MYC-SKP2-CDKN1B axis.

p27 is regulated independently of Skp2 in the absence of Cdk2

Cyclin-dependent kinase 2 (Cdk2) is dispensable for mitotic cell cycle progression and Cdk2 knockout mice are viable due to the compensatory functions of other Cdks. In order to assess the role of Cdk2 under limiting conditions, we used Skp2 knockout mice that exhibit increased levels of Cdk inhibitor, p27(Kip1), which is able to inhibit Cdk2 and Cdk1. Knockdown of Cdk2 abrogated proliferation of Skp2(-/-) mouse embryonic fibroblasts, encouraging us to generate Cdk2(-/-)Skp2(-/-) double knockout mice. Cdk2(-/-)Skp2(-/-) double knockout mice are viable and display similar phenotypes as Cdk2(-/-) and Skp2(-/-) mice. Unexpectedly, fibroblasts generated from Cdk2(-/-)Skp2(-/-) double knockout mice proliferated at normal rates. The increased stability of p27 observed in Skp2(-/-) MEFs was not observed in Cdk2(-/-)Skp2(-/-) double knockout fibroblasts indicating that in the absence of Cdk2, p27 is regulated by Skp2-independent mechanisms. Ablation of other ubiquitin ligases for p27 such as KPC1, DDB1, and Pirh2 did not restore stability of p27 in Cdk2(-/-)Skp2(-/-) MEFs. Our findings point towards novel and alternate pathways for p27 regulation.

Epstein-Barr virus latent membrane protein 2A enhances MYC-driven cell cycle progression in a mouse model of B lymphoma

Elevated expression of MYC is a shared property of many human cancers. Epstein-Barr virus (EBV) has been associated with lymphoid malignancies, yet collaborative roles between MYC and EBV in lymphomagenesis are unclear. EBV latent membrane protein 2A (LMP2A) functions as a B-cell receptor (BCR) mimic known to provide survival signals to infected B cells. Co-expression of human MYC and LMP2A in mice (LMP2A/λ-MYC) accelerates B lymphoma onset compared with mice expressing human MYC alone (λ-MYC mice). Here we show a novel role of LMP2A in potentiating MYC to promote G1-S transition and hyperproliferation by downregulating cyclin-dependent kinase inhibitor p27(kip1) in a proteasome-dependent manner. Expressing a gain-of-function S10A mutant of p27(kip1) has minor effect on tumor latency. However, pretumor B cells from λ-MYC mice expressing homozygous S10A mutant show a significant decrease in the percentage of S-phase cells. Interestingly, LMP2A is able to counteract the antiproliferative effect of the S10A mutant to promote S-phase entry. Finally, we show that LMP2A expression correlates with higher levels of MYC expression and suppression of p27(kip1) before lymphoma onset. Our study demonstrates a novel function of EBV LMP2A in maximizing MYC expression, resulting in hyperproliferation and cellular transformation into cancer cells in vivo.

A genetically engineered oncolytic adenovirus decoys and lethally traps quiescent cancer stem-like cells in S/G2/M phases

PURPOSE

Because chemoradiotherapy selectively targets proliferating cancer cells, quiescent cancer stem-like cells are resistant. Mobilization of the cell cycle in quiescent leukemia stem cells sensitizes them to cell death signals. However, it is unclear that mobilization of the cell cycle can eliminate quiescent cancer stem-like cells in solid cancers. Thus, we explored the use of a genetically-engineered telomerase-specific oncolytic adenovirus, OBP-301, to mobilize the cell cycle and kill quiescent cancer stem-like cells.

EXPERIMENTAL DESIGN

We established CD133(+) cancer stem-like cells from human gastric cancer MKN45 and MKN7 cells. We investigated the efficacy of OBP-301 against quiescent cancer stem-like cells. We visualized the treatment dynamics of OBP-301 killing of quiescent cancer stem-like cells in dormant tumor spheres and xenografts using a fluorescent ubiquitination cell-cycle indicator (FUCCI).

RESULTS

CD133(+) gastric cancer cells had stemness properties. OBP-301 efficiently killed CD133(+) cancer stem-like cells resistant to chemoradiotherapy. OBP-301 induced cell-cycle mobilization from G0-G1 to S/G2/M phases and subsequent cell death in quiescent CD133(+) cancer stem-like cells by mobilizing cell-cycle-related proteins. FUCCI enabled visualization of quiescent CD133(+) cancer stem-like cells and proliferating CD133(-) non-cancer stem-like cells. Three-dimensional visualization of the cell-cycle behavior in tumor spheres showed that CD133(+) cancer stem-like cells maintained stemness by remaining in G0-G1 phase. We showed that OBP-301 mobilized quiescent cancer stem-like cells in tumor spheres and xenografts into S/G2/M phases where they lost viability and cancer stem-like cell properties and became chemosensitive.

CONCLUSION

Oncolytic adenoviral infection is an effective mechanism of cancer cell killing in solid cancer and can be a new therapeutic paradigm to eliminate quiescent cancer stem-like cells.

Identification of small molecule inhibitors of p27(Kip1) ubiquitination by high-throughput screening

Dysregulation of p27(Kip1) due to proteolysis that involves the ubiquitin ligase (SCF) complex with S-phase kinase-associated protein 2 (Skp2) as the substrate-recognition component (SCF(Skp2)) frequently results in tumorigenesis. In this report, we developed a high-throughput screening system to identify small-molecule inhibitors of p27(Kip1) degradation. This system was established by tagging Skp2 with fluorescent monomeric Azami Green (mAG) and CDK subunit 1 (Cks1) (mAGSkp2-Cks1) to bind to p27(Kip1) phosphopeptides. We identified two compounds that inhibited the interaction between mAGSkp2-Cks1 and p27(Kip1): linichlorin A and gentian violet. Further studies have shown that the compounds inhibit the ubiquitination of p27(Kip1) in vitro as well as p27(Kip1) degradation in HeLa cells. Notably, both compounds exhibited preferential antiproliferative activity against HeLa and tsFT210 cells compared with NIH3T3 cells and delayed the G1 phase progression in tsFT210 cells. Our approach indicates a potential strategy for restoring p27(Kip1) levels in human cancers.

Upregulation of CRM1 relates to neuronal apoptosis after traumatic brain injury in adult rats

Traumatic brain injury (TBI) initiates a complex series of neurochemical and signaling changes that leads to neuronal dysfunction and over-reactive astrocytes. There is increasing evidence that CRM1 mediated P27(Kip1), which is a potent inhibitor of G1 cyclin-dependent kinases complexes, nuclear export-dependent or -independent Jab1/CSN5, and cytoplasmic degradation in cells. Up to now, the function of CRM1 in central nervous system (CNS) is still with limited acquaintance. In our study, to investigate whether CRM1 is involved in CNS lesion, we performed a TBI model in adult rats. Western blot and RT-PCR analysis revealed that the level of protein and mRNA of CRM1 increased in ipsilateral brain cortex in comparison to the contralateral. Immunohistochemistry and immunofluorescence double labeling indicated that CRM1 was shutting into nucleus around the wound, and increased CRM1 co-localized with P27(Kip1). Terminal deoxynucleotidyl transferase deoxy-UTP-nick end labeling (TUNEL) staining suggested that CRM1 was involved in neuronal apoptosis after brain injury. We also investigated co-localization of CRM1 and active-caspase-3 in the ipsilateral brain cortex. In addition, the expression patterns of Bax and active-caspase-3 were parallel with that of CRM1. Based on our data, we suggested that CRM1 might play an important role in neuronal apoptosis following TBI, and might provide a basis for the further study on its role in regulating the expression of P27(Kip1) and cell cycle re-entry in TBI.

Identification of acetylation-dependent regulatory mechanisms that govern the oncogenic functions of Skp2

The Skp2 (S-phase kinase associated protein 2) oncoprotein is often highly expressed in various types of human cancers. However, the mechanistic basis of its oncogenic function, as well as the upstream regulatory pathway(s) that control Skp2 activities remains not fully understood. Recently, we reported that p300 acetylates Skp2 at two conserved lysine residues K68 and K71 within its NLS (Nuclear localization signal). This modification leads to increased Skp2 stability and cytoplasmic translocation, thus contributing to elevated Skp2 oncogenic potential. Moreover, we found that the SIRT3 tumor suppressor serves as the physiological deacetylase that antagonizes p300-mediated Skp2 acetylation. Furthermore, we showed that Skp2 governs E-cadherin ubiquitination and degradation in the cytosol. Consistent with this, we observed an inverse correlation between Skp2 and E-cadherin expression in clinical breast tumor samples. Therefore, our work elucidates a novel acetylation-dependent regulatory mechanism for Skp2 oncogenic functions.

The DREAM complex mediates GIST cell quiescence and is a novel therapeutic target to enhance imatinib-induced apoptosis

Gastrointestinal stromal tumors (GIST) can be successfully treated with imatinib mesylate (Gleevec); however, complete remissions are rare and patients frequently achieve disease stabilization in the presence of residual tumor masses. The clinical observation that discontinuation of treatment can lead to tumor progression suggests that residual tumor cells are, in fact, quiescent and, therefore, able to re-enter the cell-division cycle. In line with this notion, we have previously shown that imatinib induces GIST cell quiescence in vitro through the APC(CDH1)-SKP2-p27(Kip1) signaling axis. Here, we provide evidence that imatinib induces GIST cell quiescence in vivo and that this process also involves the DREAM complex, a multisubunit complex that has recently been identified as an additional key regulator of quiescence. Importantly, inhibition of DREAM complex formation by depletion of the DREAM regulatory kinase DYRK1A or its target LIN52 was found to enhance imatinib-induced cell death. Our results show that imatinib induces apoptosis in a fraction of GIST cells while, at the same time, a subset of cells undergoes quiescence involving the DREAM complex. Inhibition of this process enhances imatinib-induced apoptosis, which opens the opportunity for future therapeutic interventions to target the DREAM complex for more efficient imatinib responses.

The p27-Skp2 axis mediates glucocorticoid-induced cell cycle arrest in T-lymphoma cells

Glucocorticoid therapy is an important treatment modality of hematological malignancies, especially T-cell acute lymphoblastic leukemia (T-ALL). Glucocorticoids are known to induce a cell cycle arrest and apoptosis in T-lymphoma cells. We could demonstrate that the cell cycle arrest induced by the synthetic glucocorticoid dexamethasone (Dex) clearly precedes apoptosis in human CEM T-ALL and murine S49.1 T-lymphoma cells. Cyclin D3 is strongly downregulated, whereas the CDK inhibitor p27^Kip1 (p27) is strongly upregulated in response to dexamethasone in these cells. RNAi-mediated knockdown of p27 as well as overexpression of its negative regulator Skp2 revealed the critical function of p27 in the Dex-induced G₁ arrest of CEM cells. Our studies indicate that several mechanisms contribute to the increase of p27 protein in our T-lymphoma cell lines. We found a significant upregulation of p27 mRNA in S49.1 and CEM cells. In addition, Dex treatment activated the mouse p27 promotor in reporter gene experiments, indicating a transcriptional regulation. However, the relatively moderate induction of p27 mRNA levels by Dex did not explain the strong increase of p27 protein in CEM and S49.1 cells. We found clear evidence for a posttranslational mechanism responsible for the robust increase in p27 protein. Dex treatment of S49.1 and CEM cells increases the half-life of p27 protein, which indicates that decreased protein degradation is the primary mechanism of p27 induction by glucocorticoids. Interestingly, we found that Dex treatment decreased the protein and mRNA levels of the negative regulator of p27 protein and E3 ubiquitin ligase subunit Skp2. We conclude that the cell cycle inhibitor p27 and its negative regulator Skp2 are key players in the glucocorticoid-induced growth suppression of T-lymphoma cells and should be considered as potential drug targets to improve therapies of T-cell malignancies.

Sprouty2 but not Sprouty4 is a potent inhibitor of cell proliferation and migration of osteosarcoma cells

As negative regulators of receptor tyrosine kinase-mediated signalling, Sprouty proteins fulfil important roles during carcinogenesis. In this report, we demonstrate that Sprouty2 protein expression inhibits cell proliferation and migration in osteosarcoma-derived cells. Although earlier reports describe a tumour-promoting function, these results indicate that Sprouty proteins also have the potential to function as tumour suppressors in sarcoma. In contrast to Sprouty2, Sprouty4 expression failed to interfere with proliferation and migration of the osteosarcoma-derived cells, possibly due to a less pronounced interference with mitogen-activated protein kinase activity. Sequences within the NH2-terminus are responsible for the specific inhibitory function of Sprouty2 protein.

Mechanisms and function of substrate recruitment by F-box proteins

S phase kinase-associated protein 1 (SKP1)-cullin 1 (CUL1)-F-box protein (SCF) ubiquitin ligase complexes use a family of F-box proteins as substrate adaptors to mediate the degradation of a large number of regulatory proteins involved in diverse processes. The dysregulation of SCF complexes and their substrates contributes to multiple pathologies. In the 14 years since the identification and annotation of the F-box protein family, the continued identification and characterization of novel substrates has greatly expanded our knowledge of the regulation of substrate targeting and the roles of F-box proteins in biological processes. Here, we focus on the evolution of our understanding of substrate recruitment by F-box proteins, the dysregulation of substrate recruitment in disease and potential avenues for F-box protein-directed disease therapies.

Immunohistochemical expression of Skp2 protein in oral nevi and melanoma

Objective: The aim of this study was to analyze the immunohistochemical expression of Skp2 protein in 38 oral nevi and 11 primary oral melanomas. Study Design: Expression of this ubiquitin protein was evaluated by immunohistochemistry in 49 oral melanocytic lesions, including 38 intramucosal nevi and 11 primary oral melanomas. The labeling index (LI) was assessed considering the percentage of cells expressing nuclear positivity out of the total number of cells, counting 1000 cells per slide. Results: Skp2 protein was rarely expressed in intramucosal nevi, in contrast to oral melanomas, which showed high levels of this protein. Conclusion: These results indicate that Skp2 protein may play a role in the development and progression of oral melanomas, and it also could be useful as an immunohistochemical marker for differential diagnosis of oral benign and malignant melanocytic lesions.

Key words:Oral melanoma, oral nevi, Skp2, cell cycle, immunohistochemistry.

Proteolysis of Xenopus Cip-type CDK inhibitor, p16Xic2, is regulated by PCNA binding and CDK2 phosphorylation

Background

Cell division is positively regulated by cyclin-dependent kinases (CDKs) partnered with cyclins and negatively regulated by CDK inhibitors. In the frog, Xenopus laevis, three types of CDK inhibitors have been described: p27^Xic1 (Xic1) which shares sequence homology with both p21^Cip1 and p27^Kip1 from mammals, p16^Xic2 (Xic2) which shares sequence homology with p21^Cip1, and p17^Xic3 (Xic3) which shares sequence homology with p27^Kip1. While past studies have demonstrated that during DNA polymerase switching, Xic1 is targeted for protein turnover dependent upon DNA, Proliferating Cell Nuclear Antigen (PCNA), and the ubiquitin ligase CRL4^Cdt2, little is known about the processes that regulate Xic2 or Xic3.

Methods

We used the Xenopus interphase egg extract as a model system to examine the regulation of Xic2 by proteolysis and phosphorylation.

Results

Our studies indicated that following primer synthesis during the initiation of DNA replication, Xic2 is targeted for DNA- and PCNA-dependent ubiquitin-mediated proteolysis and that Cdt2 can promote Xic2 turnover. Additionally, during interphase, Xic2 is phosphorylated by CDK2 at Ser-98 and Ser-131 in a DNA-independent manner, inhibiting Xic2 turnover. In the presence of double-stranded DNA ends, Xic2 is also phosphorylated at Ser-78 and Ser-81 by a caffeine-sensitive kinase, but this phosphorylation does not alter Xic2 turnover. Conversely, in the presence or absence of DNA, Xic3 was stable in the Xenopus interphase egg extract and did not exhibit a shift indicative of phosphorylation.

Conclusions

During interphase, Xic2 is targeted for DNA- and PCNA-dependent proteolysis that is negatively regulated by CDK2 phosphorylation. During a response to DNA damage, Xic2 may be alternatively regulated by phosphorylation by a caffeine-sensitive kinase. Our studies suggest that the three types of Xenopus CDK inhibitors, Xic1, Xic2, and Xic3 appear to be uniquely regulated which may reflect their specialized roles during cell division or early development in the frog.

Mutational analysis of SKP2 and P27 in Chinese Han women with premature ovarian failure

P27 and SKP2, a major regulator of P27, play a crucial role in ovarian function in mice. Both P27-deficient and SKP2-deficient female mice develop premature ovarian failure (POF). The coding regions of SKP2 and P27 were examined in 200 Chinese women with POF and 200 control volunteers. This study is the first to investigate SKP2 in POF. No plausible pathogenic mutations were detected. The results suggest that mutations in SKP2 and P27 are not common in Chinese Han women with POF.

Role of SKP1-CUL1-F-box-protein (SCF) E3 ubiquitin ligases in skin cancer

Many biological processes such as cell proliferation, differentiation, and cell death depend precisely on the timely synthesis and degradation of key regulatory proteins. While protein synthesis can be regulated at multiple levels, protein degradation is mainly controlled by the ubiquitin-proteasome system (UPS), which consists of two distinct steps: (1) ubiquitylation of targeted protein by E1 ubiquitin-activating enzyme, E2 ubiquitin-conjugating enzyme and E3 ubiquitin ligase, and (2) subsequent degradation by the 26S proteasome. Among all E3 ubiquitin ligases, the SCF (SKP1-CUL1-F-box protein) E3 ligases are the largest family and are responsible for the turnover of many key regulatory proteins. Aberrant regulation of SCF E3 ligases is associated with various human diseases, such as cancers, including skin cancer. In this review, we provide a comprehensive overview of all currently published data to define a promoting role of SCF E3 ligases in the development of skin cancer. The future directions in this area of research are also discussed with an ultimate goal to develop small molecule inhibitors of SCF E3 ligases as a novel approach for the treatment of human skin cancer. Furthermore, altered components or substrates of SCF E3 ligases may also be developed as the biomarkers for early diagnosis or predicting prognosis.

Role of polyamines at the G1/S boundary and G2/M phase of the cell cycle

The role of polyamines at the G1/S boundary and in the G2/M phase of the cell cycle was studied using synchronized HeLa cells treated with thymidine or with thymidine and aphidicolin. Synchronized cells were cultured in the absence or presence of α-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, plus ethylglyoxal bis(guanylhydrazone) (EGBG), an inhibitor of S-adenosylmethionine decarboxylase. When polyamine content was reduced by treatment with DFMO and EGBG, the transition from G1 to S phase was delayed. In parallel, the level of p27(Kip1) was greatly increased, so its mechanism was studied in detail. Synthesis of p27(Kip1) was stimulated at the level of translation by a decrease in polyamine levels, because of the existence of long 5'-untranslated region (5'-UTR) in p27(Kip1) mRNA. Similarly, the transition from the G2/M to the G1 phase was delayed by a reduction in polyamine levels. In parallel, the number of multinucleate cells increased by 3-fold. This was parallel with the inhibition of cytokinesis due to an unusual distribution of actin and α-tubulin at the M phase. Since an association of polyamines with chromosomes was not observed by immunofluorescence microscopy at the M phase, polyamines may have only a minor role in structural changes of chromosomes at the M phase. In general, the involvement of polyamines at the G2/M phase was smaller than that at the G1/S boundary.

The ubiquitin proteasome system - implications for cell cycle control and the targeted treatment of cancer

Two families of E3 ubiquitin ligases are prominent in cell cycle regulation and mediate the timely and precise ubiquitin-proteasome-dependent degradation of key cell cycle proteins: the SCF (Skp1/Cul1/F-box protein) complex and the APC/C (anaphase promoting complex or cyclosome). While certain SCF ligases drive cell cycle progression throughout the cell cycle, APC/C (in complex with either of two substrate recruiting proteins: Cdc20 and Cdh1) orchestrates exit from mitosis (APC/C(Cdc20)) and establishes a stable G1 phase (APC/C(Cdh1)). Upon DNA damage or perturbation of the normal cell cycle, both ligases are involved in checkpoint activation. Mechanistic insight into these processes has significantly improved over the last ten years, largely due to a better understanding of APC/C and the functional characterization of multiple F-box proteins, the variable substrate recruiting components of SCF ligases. Here, we review the role of SCF- and APC/C-mediated ubiquitylation in the normal and perturbed cell cycle and discuss potential clinical implications of SCF and APC/C functions. This article is part of a Special Issue entitled: Ubiquitin-Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.

The Role of F-Box Proteins during Viral Infection

The F-box domain is a protein structural motif of about 50 amino acids that mediates protein–protein interactions. The F-box protein is one of the four components of the SCF (SKp1, Cullin, F-box protein) complex, which mediates ubiquitination of proteins targeted for degradation by the proteasome, playing an essential role in many cellular processes. Several discoveries have been made on the use of the ubiquitin–proteasome system by viruses of several families to complete their infection cycle. On the other hand, F-box proteins can be used in the defense response by the host. This review describes the role of F-box proteins and the use of the ubiquitin–proteasome system in virus–host interactions.

FoxM1 involvement in astrocyte proliferation after spinal cord injury in rats

The Forkhead box M1 (FoxM1) protein is a proliferation-associated transcription factor that plays a key role in controlling both the G1/S and G2/M transitions of the cell cycle and regulates transcription of cell cycle genes, including cyclin-dependent kinase inhibitors p27(kip1) and p21(waf1/cip1). The expression levels of FoxM1 directly correlated with the proliferation index, cancer survival, genomic instability rate, and microvessel density, and inversely correlated with apoptosis. Furthermore, FoxM1 is determined to play a role in tissue repair following injury in the lungs and liver. However, the signaling of FoxM1, involved in its expression and its role in central nervous system lesion and repair is poorly known. In this study, we performed a spinal cord injury (SCI) model in adult Sprague-Dawley rats and investigated the dynamic changes and role of FoxM1 expression in the spinal cord. Western blot analysis revealed that FoxM1 was lowly presented in normal spinal cord. It gradually increased, reached a peak at day 3, and then declined to basal levels at 14 days after spinal cord injury. Immunohistochemistry further confirmed that FoxM1 was expressed at low levels in gray and white matters in normal condition and increased after SCI. Double immunofluorescence staining showed that FoxM1was co-expressed with NeuN (neuronal marker) and GFAP (astrocytic marker), and FoxM1 expression was increased predominantly in astrocytes after injury, which were regenerating axons and largely proliferated after injury. Furthermore, co-immunoprecipitation studies demonstrated increased interactions among FoxM1, Skp2, and p27(kip1) in the spinal cord after injury. Taken together, these results provide new insights into the molecular mechanisms underlying astrocyte proliferation during SCI and suggest that FoxM1 might play crucial roles in CNS pathophysiology after SCI.

Role of ubiquitin ligases and the proteasome in oncogenesis: novel targets for anticancer therapies

The ubiquitin proteasome system (UPS) regulates the ubiquitination, and thus degradation and turnover, of many proteins vital to cellular regulation and function. The UPS comprises a sequential series of enzymatic processes using four key enzyme families: E1 (ubiquitin-activating enzymes), E2 (ubiquitin-carrier proteins), E3 (ubiquitin-protein ligases), and E4 (ubiquitin chain assembly factors). Because the UPS is a crucial regulator of the cell cycle, and abnormal cell-cycle control can lead to oncogenesis, aberrancies within the UPS pathway can result in a malignant cellular phenotype and thus has become an attractive target for novel anticancer agents. This article will provide an overall review of the mechanics of the UPS, describe aberrancies leading to cancer, and give an overview of current drug therapies selectively targeting the UPS.

SKP2 overexpression is associated with a poor prognosis of rectal cancer treated with chemoradiotherapy and represents a therapeutic target with high potential

The S-phase kinase-associated protein 2 (SKP2) oncoprotein is an E3 ubiquitin ligase. Overexpression of SKP2 was found in various human cancers, including colorectal cancers, but its potential role as a prognostic marker after neoadjuvant chemoradiotherapy (CRT) and for therapeutic intervention in rectal cancers is unknown. This study examined the correlation of SKP2 expression in the prognosis of rectal cancer patients and the viability of colorectal cancer cells treated with CRT. SKP2 immunoexpression was retrospectively assessed in pretreatment biopsies of 172 rectal cancer patients treated with neoadjuvant CRT followed by surgery. Results were correlated with clinicopathological features, therapeutic responses, and patient survival. Pharmacologic assays were used to evaluate the therapeutic relevance of Bortezomib in two colorectal cancer cell lines (HT-29 and SW480). High expression of SKP2 was correlated with the advanced Post-Tx nodal status (p = 0.002), Post-Tx International Union for Cancer Control stage (p = 0.002), and a lower-degree tumor regression grade (p < 0.001). Moreover, high expression of SKP2 (p = 0.027, hazard ratio 3.21) was an independent prognostic factor for local recurrence-free survival. In vitro, Bortezomib downregulated SKP2 expression, induced caspase activation, and decreased the viability of colorectal cancer cells with or without a combination with fluorouracil. Bortezomib also promoted caspase activation and gamma-H2AX formation in colorectal cancer cells concurrently treated with CRT. High expression of SKP2 was associated with a poor therapeutic response and adverse outcomes in rectal cancer patients treated with neoadjuvant CRT. In the presence of chemotherapy with or without radiotherapy, the promoted sensitivity of colorectal cancer cells to Bortezomib with an SKP2-repressing effect indicated that it is a potential therapeutic target.

Roles of the Skp2/p27 axis in the progression of chronic nephropathy

S-phase kinase-associated protein 2 (Skp2) is an F-box protein component of the Skp/Cullin/F-box-type E3 ubiquitin ligase that targets several cell cycle regulatory proteins for degradation through the ubiquitin-dependent pathway. Skp2-mediated degradation of p27, a cyclin-dependent kinase inhibitor, is involved in cell cycle regulation. Tubular epithelial cell proliferation is a characteristic feature of renal damage that is apparent in the early stages of nephropathy. The p27 level is associated with the progression of renal injury, and increased Skp2 expression in progressive nephropathy is implicated in decreases of p27 expression. In Skp2^−/− mice, renal damage caused by unilateral ureteral obstruction (UUO) was ameliorated by p27 accumulation, mainly in tubular epithelial cells. However, the amelioration of UUO-induced renal injury in Skp2^−/− mice was prevented by p27 deficiency in Skp2^−/−/p27^−/− mice. These results suggest that the Skp2-mediated reduction in p27 is a pathogenic activity that occurs during the progression of nephropathy. Here, we discuss the roles of the Skp2/p27 axis and/or related signaling pathways/components in the progression of chronic nephropathy.

p53, SKP2, and DKK3 as MYCN Target Genes and Their Potential Therapeutic Significance

Neuroblastoma is the most common extra-cranial solid tumor of childhood. Despite significant advances, it currently still remains one of the most difficult childhood cancers to cure, with less than 40% of patients with high-risk disease being long-term survivors. MYCN is a proto-oncogene implicated to be directly involved in neuroblastoma development. Amplification of MYCN is associated with rapid tumor progression and poor prognosis. Novel therapeutic strategies which can improve the survival rates whilst reducing the toxicity in these patients are therefore required. Here we discuss genes regulated by MYCN in neuroblastoma, with particular reference to p53, SKP2, and DKK3 and strategies that may be employed to target them.

Functional characterization of SAG/RBX2/ROC2/RNF7, an antioxidant protein and an E3 ubiquitin ligase

SAG (Sensitive to Apoptosis Gene), also known as RBX2 (RING box protein 2), ROC2 (Regulator of Cullins 2), or RNF7 (RING Finger Protein 7), was originally cloned in our laboratory as a redox inducible antioxidant protein and later characterized as the second member of the RBX/ROC RING component of the SCF (SKP1-CUL-F-box Proteins) E3 ubiquitin ligase. When acting alone, SAG scavenges oxygen radicals by forming inter- and intra-molecular disulfide bonds, whereas by forming a complex with other components of the SCF E3 ligase, SAG promotes ubiquitination and degradation of a number of protein substrates, including c-JUN, DEPTOR, HIF-1α, IκBα, NF1, NOXA, p27, and procaspase-3, thus regulating various signaling pathways and biological processes. Specifically, SAG protects cells from apoptosis, confers radioresistance, and plays an essential and non-redundant role in mouse embryogenesis and vasculogenesis. Furthermore, stress-inducible SAG is overexpressed in a number of human cancers and SAG overexpression correlates with poor patient prognosis. Finally, SAG transgenic expression in epidermis causes an early stage inhibition, but later stage promotion, of skin tumorigenesis triggered by DMBA/TPA. Given its major role in promoting targeted degradation of tumor suppressive proteins, leading to apoptosis suppression and accelerated tumorigenesis, SAG E3 ligase appears to be an attractive anticancer target.

MYC antagonizes the differentiation induced by imatinib in chronic myeloid leukemia cells through downregulation of p27(KIP1.)

Chronic myeloid leukemia (CML) progresses from a chronic to a blastic phase where the leukemic cells are proliferative and undifferentiated. The CML is nowadays successfully treated with BCR-ABL kinase inhibitors as imatinib and dasatinib. In the CML-derived K562 cell line, low concentrations of imatinib induce proliferative arrest and erythroid differentiation. We found that imatinib upregulated the cell cycle inhibitor p27(KIP1) (p27) in a time- and -concentration dependent manner, and that the extent of imatinib-mediated differentiation was severely decreased in cells with depleted p27. MYC (c-Myc) is a transcription factor frequently deregulated in human cancer. MYC is overexpressed in untreated CML and is associated to poor response to imatinib. Using K562 sublines with conditional MYC expression (induced by Zn(2+) or activated by 4-hydroxy-tamoxifen) we show that MYC prevented the erythroid differentiation induced by imatinib and dasatinib. The differentiation inhibition is not due to increased proliferation of MYC-expressing clones or enhanced apoptosis of differentiated cells. As p27 overexpression is reported to induce erythroid differentiation in K562, we explored the effect of MYC on imatinib-dependent induction of p27. We show that MYC abrogated the imatinib-induced upregulation of p27 concomitantly with the differentiation inhibition, suggesting that MYC inhibits differentiation by antagonizing the imatinib-mediated upregulation of p27. This effect occurs mainly by p27 protein destabilization. This was in part due to MYC-dependent induction of SKP2, a component of the ubiquitin ligase complex that targets p27 for degradation. The results suggest that, although MYC deregulation does not directly confer resistance to imatinib, it might be a factor that contributes to progression of CML through the inhibition of differentiation.

Coordination of cell growth and division by the ubiquitin-proteasome system

The coupling of cellular growth and division is crucial for a cell to make an accurate copy of itself. Regulated protein degradation by the ubiquitin-proteasome system (UPS) plays an important role in the coordination of these two processes. Many ubiquitin ligases, in particular the Skp1-Cullin-F-box (SCF) family and the Anaphase-Promoting Complex (APC), couple growth and division by targeting cell cycle and metabolic regulators for degradation. However, many regulatory proteins are targeted by multiple ubiquitin ligases. As a result, we are only just beginning to understand the complexities of the proteolytic regulatory network that connects cell growth and the cell cycle.

PKC-dependent phosphorylation of p27 at T198 contributes to p27 stabilization and cell cycle arrest

In this manuscript, we present experimental evidence that PKCs phosphorylate p27 at T198 in vitro and in vivo, resulting in p27 stabilization and cell cycle arrest in MCF-7 and HeLa cells. Our findings indicate that (1) recombinant PKCα, βII, δ, η and θ isoforms phosphorylate, in in vitro kinase assays, wild-type recombinant p27 protein expressed in E. coli and wild-type p27 protein immunoprecpitated from transfected HEK-293 cells but not the T198A mutant, (2) adoptive expressed PKCα and δ phosphorylate both transfected and endogenous p27 at T198 in HEK-293 cells, (3) T198 phosphorylation of transfected and endogenous p27 is increased by PKC activators [Phorbol 12-myristate 13-acetate (PMA)] and suppressed by PKC inhibitors (Rottlerin A, G06976, Calphostin C), (4) in parallel with increased T198 phosphorylation, PMA induces stabilization of p27 protein in HeLa cells, whereas PKC inhibitors induce a decrease in p27 stability and, finally, (5) PMA-induced p27 upregulation is necessary for growth arrest of HeLa and MCF-7 cells induced by PKC activation by PMA.   Overall, these results suggest that PKC-dependent upregulation of p27 induced by its phosphorylation at T198 represents a mechanism that mediates growth arrest promoted by PMA and provide novel insights on the ability of different PKC isoforms to play a role in controlling cell cycle progression.

FOXP3 expression in cancer cells and anthracyclines efficacy in patients with primary breast cancer treated with adjuvant chemotherapy in the phase III UNICANCER-PACS 01 trial

BACKGROUND

Predictive markers of response to chemotherapy are lacking in breast cancer patients. Forkhead Box Protein 3 (FOXP3) is an anti-oncogene whose absence in cancer cells could confer resistance to DNA damaging agent. So we made the hypothesis that FOXP3 expression predicts the response to anthracyclines in breast cancer patients and that adjuvant chemotherapy adding taxanes to anthracyclines confers an overall survival (OS) benefit over anthracyclines alone, in patients with FOXP3-negative tumors.

PATIENTS AND METHODS

Expression of FOXP3 in cancer cells was evaluated by immunohistochemistry in tumor samples from 1097 patients who participated in the PACS01 randomized trial that evaluated in adjuvant setting the adjunction of docetaxel (Taxotere) to anthracyclines in patients with localized breast cancer. Kaplan-Meier analysis and Cox regression model were used to assess OS according to the presence or absence of FOXP3 expression in tumor cells.

RESULTS

Four hundred and five tumors were found to express FOXP3 (37%). FOXP3 expression in breast cancer cells was associated with better OS (P = 0.003). Uni- and multivariate survival analyses according to treatment arm revealed that FOXP3 expression in breast cancer cells is independently associated with improved OS in patients treated with anthracycline-based adjuvant chemotherapy, but not in patients treated with sequential anthracycline-taxane. Moreover, in vitro experiments showed that FOXP3 induction in breast cancer cell lines using histone deacetylase inhibitor enhances anthracyclines efficacy.

CONCLUSION

FOXP3 expression in tumor cells may be an accurate predictive biomarker of anthracycline efficacy in breast cancer.

Akt: a double-edged sword in cell proliferation and genome stability

The Akt family of serine/threonine protein kinases are key regulators of multiple aspects of cell behaviour, including proliferation, survival, metabolism, and tumorigenesis. Growth-factor-activated Akt signalling promotes progression through normal, unperturbed cell cycles by acting on diverse downstream factors involved in controlling the G1/S and G2/M transitions. Remarkably, several recent studies have also implicated Akt in modulating DNA damage responses and genome stability. High Akt activity can suppress ATR/Chk1 signalling and homologous recombination repair (HRR) via direct phosphorylation of Chk1 or TopBP1 or, indirectly, by inhibiting recruitment of double-strand break (DSB) resection factors, such as RPA, Brca1, and Rad51, to sites of damage. Loss of checkpoint and/or HRR proficiency is therefore a potential cause of genomic instability in tumor cells with high Akt. Conversely, Akt is activated by DNA double-strand breaks (DSBs) in a DNA-PK- or ATM/ATR-dependent manner and in some circumstances can contribute to radioresistance by stimulating DNA repair by nonhomologous end joining (NHEJ). Akt therefore modifies both the response to and repair of genotoxic damage in complex ways that are likely to have important consequences for the therapy of tumors with deregulation of the PI3K-Akt-PTEN pathway.

Glypican-1 stimulates Skp2 autoinduction loop and G1/S transition in endothelial cells

The heparan sulfate proteoglycan glypican-1 (GPC1) is involved in tumorigenesis and angiogenesis and is overexpressed frequently in tumor and endothelial cells (ECs) in human gliomas. We demonstrated previously that in brain EC, GPC1 regulates mitotic cyclins and securin as well as mitosis and that GPC1 is required for progression through the cell cycle. To characterize the molecular mechanism underlying cell cycle regulation by GPC1, we systematically investigated its effects on key G(1)/S checkpoint regulators and on major signaling pathways reportedly activated by Dally (Division abnormally delayed) the Drosophila GPC1 homologue. We found that elevated GPC1 affected a wide range of G(1)/S checkpoint regulators, leading to inactivation of the G(1)/S checkpoint and increased S phase entry, apparently by activating the mitogen-independent Skp2 autoinduction loop. Specifically, GPC1 suppressed CDK inhibitors (CKIs), including p21, p27, p16, and p19, and the D cyclins, and induced CDK2 and Skp2. GPC1 may trigger the Skp2 autoinduction loop at least partially by suppressing p21 transcription as knockdown of p21 by RNAi can mimic the effect of GPC1 on the cell cycle regulators related to the loop. Moreover, multiple mitogenic signaling pathways, including ERK MAPK, Wnt and BMP signaling, were significantly stimulated by GPC1 as has been reported for Dally in Drosophila. Notably, the c-Myc oncoprotein, which is frequently up-regulated by both ERK and Wnt signaling and functions as a potent transcription repressor for CKIs as well as D cyclins, was also significantly induced by GPC1. These findings provide mechanistic insights into how GPC1 regulates the cell cycle and proliferation.

Artemis interacts with the Cul4A-DDB1DDB2 ubiquitin E3 ligase and regulates degradation of the CDK inhibitor p27

Artemis, a member of the SNM1 gene family, is a multifunctional phospho-protein that has been shown to have important roles in V(D)J recombination, DNA double strand break repair, and stress-induced cell-cycle checkpoint regulation. We show here that Artemis interacts with the Cul4A-DDB1 E3 ubiquitin ligase via a direct interaction with the substrate-specificity receptor DDB2. Furthermore, Artemis also interacts with the CDK inhibitor and tumor suppressor p27, a substrate of the Cul4A-DDB1 ligase, and both DDB2 and Artemis are required for the degradation of p27 mediated by this complex. We also show that the regulation of p27 by Artemis and DDB2 is important for cell cycle progression in normally proliferating cells and in response to serum deprivation. These findings thus define a function for Artemis as an effector of Cullin-based E3 ligase-mediated ubiquitylation, demonstrate a novel pathway for the regulation of p27, and show that Cul4A-DDB1(DDB2-Artemis) regulates G1 phase cell cycle progression in mammalian cells.

Adhesion-dependent Skp2 transcription requires selenocysteine tRNA gene transcription-activating factor (STAF)

Cell adhesion is essential for cell cycle progression in most normal cells. Loss of adhesion dependence is a hallmark of cellular transformation. The F-box protein Skp2 (S-phase kinase-associated protein 2) controls G(1)-S-phase progression and is subject to adhesion-dependent transcriptional regulation, although the mechanisms are poorly understood. We identify two cross-species conserved binding elements for the STAF (selenocysteine tRNA gene transcription-activating factor) in the Skp2 promoter that are essential for Skp2 promoter activity. Endogenous STAF specifically binds these elements in EMSA (electrophoretic mobility-shift assay) and ChIP (chromatin immunoprecipitation) analysis. STAF is sufficient and necessary for Skp2 promoter activity since exogenous STAF activates promoter activity and expression and STAF siRNA (small interfering RNA) inhibits Skp2 promoter activity, mRNA and protein expression and cell proliferation. Furthermore, ectopic Skp2 expression completely reverses the inhibitory effects of STAF silencing on proliferation. Importantly, STAF expression and binding to the Skp2 promoter is adhesion-dependent and associated with adhesion-dependent Skp2 expression in non-transformed cells. Ectopic STAF rescues Skp2 expression in suspension cells. Taken together, these results demonstrate that STAF is essential and sufficient for Skp2 promoter activity and plays a role in the adhesion-dependent expression of Skp2 and ultimately cell proliferation.

Overexpression of SKP2 promotes the radiation resistance of esophageal squamous cell carcinoma

SKP2 is the substrate recognition subunit of the SCF(SKP2) ubiquitin ligase complex. It is implicated in ubiquitin-mediated degradation of the cyclin-dependent kinase (CDK) inhibitor p27(KIP1) and positively regulates the G(1)/S transition. Overexpression of SKP2 has been found in many kinds of tumors. In the present study, we found that SKP2 expression levels increased in esophageal squamous cell carcinoma tissues. Elevated expression of SKP2 correlated significantly with tumor stage and positive lymph node metastasis (P < 0.05). Moreover, a significantly negative correlation was found between SKP2 expression and the survival of patients who received radiotherapy (P < 0.05). At the molecular level, induced expression of SKP2 promoted the radioresistance of EC9706 cells. Knockdown of SKP2 expression sensitized cancer cells to radiation, and a wobble mutant of SKP2 that was resistant to SKP2 siRNA was able to rescue this effect. Increased or decreased expression levels of SKP2 had effects on Rad51 expression after irradiation. These results demonstrate for the first time that overexpression of SKP2 was correlated with the increased radioresistance of esophageal squamous cell carcinoma. Elevated expression of SKP2 promoted the radioresistance of cancer cells, and this effect was mediated at least in part by the Rad51 pathway.

Modulation of AKT activity is associated with reversible dormancy in ascites-derived epithelial ovarian cancer spheroids

Epithelial ovarian cancer (EOC) metastasis is a direct contributor to high recurrence and low survival for patients with this disease. Metastasis in EOC occurs by cell exfoliation from the primary tumor into the fluid-filled peritoneal cavity, persistence of these cells as non-adherent multicellular aggregates or spheroids and reattachment of spheroids to form secondary lesions. We have recovered native spheroids from ascites fluid and demonstrated that EOC cells within these structures exhibit reduced proliferation, yet regain the capacity to attach and reinitiate cell division. To model this process in vitro for further investigation, primary EOC cells from patient peritoneal fluid were cultured under non-adherent conditions. Here we show that these cells naturally form spheroids resembling those observed in ascites. Spheroids exhibit reduced cell proliferation and a protein expression pattern consistent with cellular quiescence: specifically, decreased phospho-AKT and p45/SKP2 with a concomitant increase in p130/RBL2 and p27(Kip1). However, when spheroids are seeded to an adherent surface, reattachment occurs rapidly and is followed by reinitiation of AKT-dependent cell proliferation. These results were strikingly consistent among numerous clinical specimens and were corroborated in the EOC cell line OVCAR3. Therefore, our data reveal that EOC cells become quiescent when forming spheroids, but reactivate proliferative mechanisms upon attachment to a permissive substratum. Overall, this work utilizes a novel in vitro model of EOC metastasis that employs primary human EOC cells and introduces the important concept of reversible dormancy in EOC pathogenesis.

Proteolytic control of the oncoprotein transcription factor Myc

The c-Myc oncogene encodes a multifunctional transcription factor that directs the expression of genes required for cell growth and proliferation. Consistent with its potent growth-promoting properties, cells have evolved numerous mechanisms that limit the expression and activity of Myc. One of the most prominent of these mechanisms is proteolysis, which destroys Myc within minutes of its synthesis. The rapid and controlled destruction of Myc keeps its levels low and precisely tied to processes that regulate Myc production. In this review, we discuss how Myc protein stability is regulated and the influence of Myc proteolysis on its function. We describe what is known about how Myc is destroyed by ubiquitin (Ub)-mediated proteolysis, attempt to rationalize the role of different Ub-protein ligases and deubiquitylating enzymes (dUbs) in the regulation of Myc stability, and detail how these processes go awry in cancer. Finally, we discuss how our understanding of Myc regulation by the ubiquitin-proteasome system (UPS) can expose strategies for therapeutic intervention in human malignancies.

Skp2 is required for incretin hormone-mediated β-cell proliferation

The glucoincretin hormone glucagon-like peptide-1 (GLP-1) and its analog exendin-4 (Ex-4) promote β-cell growth and expansion. Here we report an essential role for Skp2, a substrate recognition component of SCF (Skp, Cullin, F-box) ubiquitin ligase, in promoting glucoincretin-induced β-cell proliferation by regulating the cellular abundance of p27. In vitro, GLP-1 treatment increases Skp2 levels, which accelerates p27 degradation, whereas in vivo, loss of Skp2 prevents glucoincretin-induced β-cell proliferation. Using inhibitors of phosphatidylinositol 3-kinase and Irs2 silencing RNA, we also show that the effects of GLP-1 in facilitating Skp2-dependent p27 degradation are mediated via the Irs2-phosphatidylinositol-3 kinase pathway. Finally, we show that down-regulation of p27 occurs in islets from aged mice and humans, although in these islets, age-dependent accumulation of p16(Ink4a) prevent glucoincretin-induced β-cell proliferation; however, ductal cell proliferation is maintained. Taken together, these data highlight a critical role for Skp2 in glucoincretin-induced β-cell proliferation.

AMPKα2 deletion exacerbates neointima formation by upregulating Skp2 in vascular smooth muscle cells

RATIONALE

Adenosine monophosphate-activated protein kinase (AMPK), a metabolic and redox sensor, is reported to suppress cell proliferation of nonmalignant and tumor cells. Whether AMPKα alters vascular neointima formation induced by vascular injury is unknown.

OBJECTIVE

The aim of this study was to determine the roles of AMPKα in the development of vascular neointima hyperplasia and to elucidate the underlying mechanisms.

METHODS AND RESULTS

Vascular smooth muscle cell (VSMC) proliferation and neointimal hyperplasia were evaluated in cultured VSMCs and wire-injured mouse carotid arteries from wild-type (WT, C57BL/6J), AMPKα2(-/-), and AMPKα1(-/-) mice. Mouse VSMCs derived from aortas of AMPKα2(-/-) mice exhibited increased proliferation compared with either WT or AMPKα1(-/-) VSMCs. Further, deletion of AMPKα2 but not AMPKα1 reduced the level of p27(Kip1), a cyclin-dependent kinase inhibitor, and increased the level of S-phase kinase-associated protein 2 (Skp2), a known E3 ubiquitin ligase for p27(Kip1), through activation of p52 nuclear factor kappa B (NF-κB)-2. Moreover, either pharmacological (ie, through compound C) or genetical (ie, through AMPKα2-specific siRNA) inhibition of AMPK decreased p27(Kip1) levels but increased the abundance of Skp2 in human VSMCs. Furthermore, gene silencing of Skp2 reversed the levels of p27(Kip1) and VSMCs proliferation. Finally, neointima formation after mechanical arterial injury was increased in AMPKα2(-/-) but not AMPKα1(-/-) mice.

CONCLUSIONS

These findings indicate that deletion of AMPKα2 through p52-Skp2-mediated ubiquitination and degradation of p27(Kip1) accentuates neointimal hyperplasia in response to wire injury.

Dynamic changes of PIRH2 and p27kip1 expression in injured rat sciatic nerve

p53-induced ring-H2 protein (PIRH2), a newly identified E3 ubiquitin ligase, has been reported to be interacted with p27Kip1 and promote ubiquitination of p27Kip1 independently of p53. p27kip1, a member of the Cip/Kip family of cyclin-dependent kinases inhibitors (CKIs), was shown to control cell cycle progression and promote cell proliferation. While the distribution and function of PIRH2 and p27kip1 in nervous system lesion and regeneration remains unclear. Here, we performed a sciatic nerve injury model in adult rats and studied the dynamic changes of PIRH2 and p27kip1 expression by western blot and RT-PCR in injured rat sciatic nerve. Sciatic nerve crush resulted in a significant up-regulation of PIRH2 and a down-regulation of p27kip1. Besides, we observed that they were expressed widely in both Schwann cells and axons in adult rat sciatic nerve by double immunofluorescence staining. Results obtained by coimmunoprecipitation and double labeling further showed their interaction in the regenerating process. Thus, these results indicate that PIRH2 and p27kip1 likely play an important role in peripheral nerve injury and regeneration.

The role of Skp2 in hematopoietic stem cell quiescence, pool size, and self-renewal

Although the maintenance of HSC quiescence and self-renewal are critical for controlling stem cell pool and transplantation efficiency, the mechanisms by which they are regulated remain largely unknown. Understanding the factors controlling these processes may have important therapeutic potential for BM failure and cancers. Here, we show that Skp2, a component of the Skp2 SCF complex, is an important regulator for HSC quiescence, frequency, and self-renewal capability. Skp2 deficiency displays a marked enhancement of HSC populations through promoting cell cycle entry independently of its role on apoptosis. Surprisingly, Skp2 deficiency in HSCs reduces quiescence and displays increased HSC cycling and proliferation. Importantly, loss of Skp2 not only increases HSC populations and long-term reconstitution ability but also rescues the defect in long-term reconstitution ability of HSCs on PTEN inactivation. Mechanistically, we show that Skp2 deficiency induces Cyclin D1 gene expression, which contributes to an increase in HSC cycling. Finally, we demonstrate that Skp2 deficiency enhances sensitivity of Lin(-) Sca-1(+) c-kit(+) cells and leukemia cells to chemotherapy agents. Our findings show that Skp2 is a novel regulator for HSC quiescence and self-renewal and that targeting Skp2 may have therapeutic implications for BM transplantation and leukemia stem cell treatment.

Roles of COP9 signalosome in cancer

The constitutive photomorphogenesis 9 signalosome (COP9 or CSN) is an evolutionarily conserved multiprotein complex found in plants and animals. Because of the homology between the COP9 signalosome and the 19S lid complex of the proteosome, COP9 has been postulated to play a role in regulating the degradation of polyubiquitinated proteins. Many tumor suppressor and oncogene products are regulated by ubiquitination- and proteosome-mediated protein degradation. Therefore, it is conceivable that COP9 plays a significant role in cancer, regulating processes relevant to carcinogenesis and cancer progression (e.g., cell cycle control, signal transduction and apoptosis). In mammalian cells, it consists of eight subunits (CSN1 to CSN8). The relevance and importance of some subunits of COP9 to cancer are emerging. However, the mechanistic regulation of each subunit in cancer remains unclear. Among the CSN subunits, CSN5 and CSN6 are the only two that each contain an MPN (Mpr1p and Pad1p N-terminal) domain. The deneddylation activity of an MPN domain toward cullin-RING ubiquitin ligases (CRL) may coordinate CRL-mediated ubiquitination activity. More recent evidence shows that CSN5 and CSN6 are implicated in ubiquitin-mediated proteolysis of important mediators in carcinogenesis and cancer progression. Here, we discuss the mechanisms by which some CSN subunits are involved in cancer to provide a much needed perspective regarding COP9 in cancer research, hoping that these insights will lay the groundwork for cancer intervention.

Transcriptional regulation of S phase kinase-associated protein 2 by NR4A orphan nuclear receptor NOR1 in vascular smooth muscle cells

Members of the NR4A subgroup of the nuclear hormone receptor superfamily have emerged as key transcriptional regulators of proliferation and inflammation. NOR1 constitutes a ligand-independent transcription factor of this subgroup and induces cell proliferation; however, the transcriptional mechanisms underlying this mitogenic role remain to be defined. Here, we demonstrate that the F-box protein SKP2 (S phase kinase-associated protein 2), the substrate-specific receptor of the ubiquitin ligase responsible for the degradation of p27(KIP1) through the proteasome pathway, constitutes a direct transcriptional target for NOR1. Mitogen-induced Skp2 expression is silenced in vascular smooth muscle cells (VSMC) isolated from Nor1-deficient mice or transfected with Nor1 siRNA. Conversely, adenovirus-mediated overexpression of NOR1 induces Skp2 expression in VSMC and decreases protein abundance of its target p27. Transient transfection experiments establish that NOR1 transactivates the Skp2 promoter through a nerve growth factor-induced clone B response element (NBRE). Electrophoretic mobility shift and chromatin immunoprecipitation assays further revealed that NOR1 is recruited to this NBRE site in the Skp2 promoter in response to mitogenic stimulation. In vivo Skp2 expression is increased during the proliferative response underlying neointima formation, and this transcriptional induction depends on the expression of NOR1. Finally, we demonstrate that overexpression of Skp2 rescues the proliferative arrest of Nor1-deficient VSMC. Collectively, these results characterize Skp2 as a novel NOR1-regulated target gene and detail a previously unrecognized transcriptional cascade regulating mitogen-induced VSMC proliferation.

The predator becomes the prey: regulating the ubiquitin system by ubiquitylation and degradation

Ubiquitylation (also known as ubiquitination) regulates essentially all of the intracellular processes in eukaryotes through highly specific modification of numerous cellular proteins, which is often tightly regulated in a spatial and temporal manner. Although most often associated with proteasomal degradation, ubiquitylation frequently serves non-proteolytic functions. In light of its central roles in cellular regulation, it has not been surprising to find that many of the components of the ubiquitin system itself are regulated by ubiquitylation. This observation has broad implications for pathophysiology.