A license to kill: transcriptional activation and enhanced turnover of Myc by the SCF(kp2) ubiquitin ligase

Trace elements in pancreatic cancer

BACKGROUND

Pancreatic cancer (PCA) is an extremely aggressive malignant cancer with an increasing incidence and a low five-year survival rate. The main reason for this high mortality is that most patients are diagnosed with PCA at an advanced stage, missing early treatment options and opportunities. As important nutrients of the human body, trace elements play an important role in maintaining normal physiological functions. Moreover, trace elements are closely related to many diseases, including PCA.

REVIEW

This review systematically summarizes the latest research progress on selenium, copper, arsenic, and manganese in PCA, elucidates their application in PCA, and provides a new reference for the prevention, diagnosis and treatment of PCA.

CONCLUSION

Trace elements such as selenium, copper, arsenic and manganese are playing an important role in the risk, pathogenesis, diagnosis and treatment of PCA. Meanwhile, they have a certain inhibitory effect on PCA, the mechanism mainly includes: promoting ferroptosis, inducing apoptosis, inhibiting metastasis, and inhibiting excessive proliferation.

The role of ubiquitination in tumorigenesis and targeted drug discovery

Ubiquitination, an important type of protein posttranslational modification (PTM), plays a crucial role in controlling substrate degradation and subsequently mediates the "quantity" and "quality" of various proteins, serving to ensure cell homeostasis and guarantee life activities. The regulation of ubiquitination is multifaceted and works not only at the transcriptional and posttranslational levels (phosphorylation, acetylation, methylation, etc.) but also at the protein level (activators or repressors). When regulatory mechanisms are aberrant, the altered biological processes may subsequently induce serious human diseases, especially various types of cancer. In tumorigenesis, the altered biological processes involve tumor metabolism, the immunological tumor microenvironment (TME), cancer stem cell (CSC) stemness and so on. With regard to tumor metabolism, the ubiquitination of some key proteins such as RagA, mTOR, PTEN, AKT, c-Myc and P53 significantly regulates the activity of the mTORC1, AMPK and PTEN-AKT signaling pathways. In addition, ubiquitination in the TLR, RLR and STING-dependent signaling pathways also modulates the TME. Moreover, the ubiquitination of core stem cell regulator triplets (Nanog, Oct4 and Sox2) and members of the Wnt and Hippo-YAP signaling pathways participates in the maintenance of CSC stemness. Based on the altered components, including the proteasome, E3 ligases, E1, E2 and deubiquitinases (DUBs), many molecular targeted drugs have been developed to combat cancer. Among them, small molecule inhibitors targeting the proteasome, such as bortezomib, carfilzomib, oprozomib and ixazomib, have achieved tangible success. In addition, MLN7243 and MLN4924 (targeting the E1 enzyme), Leucettamol A and CC0651 (targeting the E2 enzyme), nutlin and MI-219 (targeting the E3 enzyme), and compounds G5 and F6 (targeting DUB activity) have also shown potential in preclinical cancer treatment. In this review, we summarize the latest progress in understanding the substrates for ubiquitination and their special functions in tumor metabolism regulation, TME modulation and CSC stemness maintenance. Moreover, potential therapeutic targets for cancer are reviewed, as are the therapeutic effects of targeted drugs.

The CREB-binding protein inhibitor ICG-001 suppresses pancreatic cancer growth

Pancreatic ductal adenocarcinoma (PDAC) is a highly lethal cancer due in part to a lack of highly robust cytotoxic or molecular-based therapies. Recent studies investigating ligand-mediated Wnt/β-catenin signaling have highlighted its importance in pancreatic cancer initiation and progression, as well as its potential as a therapeutic target in PDAC. The small-molecule ICG-001 binds cAMP-responsive element binding (CREB)-binding protein (CBP) to disrupt its interaction with β-catenin and inhibit CBP function as a coactivator of Wnt/β-catenin-mediated transcription. Given its ability to inhibit Wnt/β-catenin-mediated transcription in vitro and in vivo, as well as its efficacy in preclinical models of colorectal cancer and other Wnt-driven diseases, we examined ICG-001 and its potential role as a therapeutic in PDAC. ICG-001 alone significantly inhibited anchorage-dependent and -independent growth of multiple PDAC lines, and augmented in vitro growth inhibition when used in combination with gemcitabine. ICG-001 had only variable modest effects on PDAC apoptosis and instead mediated PDAC growth inhibition primarily through robust induction of G₁ cell-cycle arrest. These effects, however, seemed decoupled from its inhibition of Wnt/β-catenin-mediated transcription. DNA microarrays performed on PDAC cells in the context of ICG-001 treatment revealed ICG-001 altered the expression of several genes with well-established roles in DNA replication and cell-cycle progression, including direct actions on SKP2 and CDKN1A. ICG-001 also significantly prolonged survival in an in vivo orthotopic xenograft model of PDAC, indicating ICG-001 or derived compounds that disrupt CBP activity are potentially useful small-molecule therapeutics for pancreatic cancer.

E3-ligase Skp2 regulates β-catenin expression and maintains hematopoietic stem cell homing

The homing ability of hematopoietic stem cells (HSCs) was a critical step for transplantation and subsequent hematopoiesis. Although the HSC transplantation was widely used for many diseases, the mechanism by which HSC homing was regulated remained poorly understood. F-box protein S-phase kinase associated protein2 (Skp2), a component of the Skp2-SCF E3 ligase complex, was regarded as a cell cycle regulator by controlling the level of p21 and p27 through ubiquitination. We recently reported an important role of Skp2 in maintaining HSC pool size, quiescent stage and self-renewal ability. In this current study, we showed that Skp2 was a novel and critical regulator for maintaining the homing of HSCs as well as their residence in the endosteal niche. Microarray analysis together with biochemical validations revealed that Skp2 deficiency profoundly reduced the expression of β-catenin and its target genes. Knockdown of β-catenin mimicked the decline of HSC homing upon Skp2 deficiency, suggesting that Skp2 may regulate β-catenin and its target gene expression to orchestrate HSC homing. Our study not only identified Skp2 as a new regulator for maintaining β-catenin expression and HSC homing, but also suggested that Skp2 may serve as a predictive marker for monitoring the transplantation efficiency.

Kaposi sarcoma-associated herpesvirus vIRF-3 protein binds to F-box of Skp2 protein and acts as a regulator of c-Myc protein function and stability

The Kaposi sarcoma-associated herpesvirus (KSHV) has been linked to Kaposi sarcoma, body cavity-based lymphoma, and Castleman disease. vIRF-3 is a KSHV latent gene that is critical for proliferation of KSHV-positive lymphoid cells. Furthermore, vIRF-3 contributes to KSHV-associated pathogenesis by stimulating c-Myc transcription activity. Here we show that vIRF-3 can associate with Skp2, a key component of the SCF(skp2) ubiquitin ligase complex. Skp2 is a transcriptional co-factor for c-Myc that was shown to regulate the stability of c-Myc protein as well as c-Myc-dependent transcription. In this study, we show that vIRF-3 binds to the F-box of Skp2 and recruits it to c-Myc-regulated promoters to activate c-Myc-dependent transcription. Additionally, cells overexpressing vIRF-3 exhibit higher levels of c-Myc ubiquitylation, suggesting that ubiquitylation is necessary for c-Myc-mediated transcription. Moreover, vIRF-3 can stabilize the c-Myc protein by increasing its half-life. Collectively, these results indicate that vIRF-3 can effectively manipulate c-Myc stability and function and thus contribute to c-Myc-induced KSHV-associated lymphomagenesis.

Deregulation of the cell cycle machinery by Epstein-Barr virus nuclear antigen 3C

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus associated with a large number of lymphoid and epithelial malignancies. As a successful pathogen it has co-evolved with its human host for millions of years. EBV has the unique ability to establish life-long latent infection in primary human B lymphocytes. During latent infection, a small subset of viral proteins is expressed. These proteins are essential for maintenance of the EBV genome as well as the deregulation of various signaling pathways that facilitate the proliferation and survival of the infected cells. Epstein-Barr nuclear antigen (EBNA)3C is one of the latent proteins shown to be essential for transformation of primary human B lymphocytes in vitro. EBNA3C primarily functions as a transcriptional regulator by interacting with a number of well known cellular and viral transcriptional factors. We have recently identified several binding partners for EBNA3C including proteins that regulate cell cycle and chromatin remodeling. We are actively engaged in discerning the biological significance of these interactions. This review summarizes our current understanding of how EBNA3C usurps cellular pathways that promote B-cell transformation.

Over-expression of Skp2 is associated with resistance to preoperative doxorubicin-based chemotherapy in primary breast cancer

INTRODUCTION

Preoperative chemotherapy is often used in patients with locally advanced breast cancer. However, commonly used clinical and pathological parameters are poor predictors of response to this type of therapy. Recent studies have suggested that altered regulation of the cell cycle in cancer may be involved in resistance to chemotherapy. Over-expression of the ubiquitin ligase Skp2 results in loss of the cell cycle inhibitor p27Kip1 and is associated with poor prognosis in early breast cancer. The purpose of the present study was to examine the role of these proteins as predictors of clinical outcome and response to chemotherapy in locally advanced breast cancer.

METHODS

The expression levels of Skp2 and p27Kip1 were determined by immunohistochemistry both before and after preoperative chemotherapy in 40 patients with locally advanced breast cancer. All patients were treated with cyclophosphamide/doxorubicin (adriamycin)/5-fluorouracil (CAF) and some patients received additional treatment with docetaxel. Expression data were compared with patients' clinical and pathological features, clinical outcome, and response to chemotherapy.

RESULTS

Skp2 expression before preoperative chemotherapy was inversely related to p27Kip1 levels, tumor grade, and expression of estrogen and progesterone receptors. Both Skp2 and p27Kip1 were found to be accurate prognostic markers for disease-free and overall survival. High preoperative expression of Skp2 was associated with resistance to CAF therapy in 94% of patients (P < 0.0001) but not with resistance to docetaxel.

CONCLUSION

Skp2 expression may be a useful marker for predicting response to doxorubicin-based preoperative chemotherapy and clinical outcome in patients with locally advanced breast cancer.

Epstein-Barr virus nuclear antigen 3C interacts with and enhances the stability of the c-Myc oncoprotein

Epstein-Barr virus (EBV) was the first human DNA virus to be associated with cancer. Its oncogenic potential was further demonstrated by its ability to transform primary B lymphocytes in vitro. EBV nuclear antigen 3C (EBNA3C) is one of a small subset of latent antigens critical for the transformation of human primary B lymphocytes. Although EBNA3C has been shown to modulate several cellular functions, additional targets involved in cellular transformation remain to be explored. EBNA3C can recruit key components of the SCF(Skp2) ubiquitin ligase complex. In this report, we show that EBNA3C residues 130 to 190, previously shown to bind to the SCF(Skp2) complex, also can strongly associate with the c-Myc oncoprotein. Additionally, the interaction of EBNA3C with c-Myc was mapped to the region of c-Myc that includes the highly conserved Skp2 binding domain. Skp2 has been shown to regulate c-Myc stability and also has been shown to function as a coactivator of transcription for c-Myc target genes. We now show that the EBV latent oncoprotein EBNA3C can stabilize c-Myc and that the recruitment of both c-Myc and its cofactor Skp2 to c-Myc-dependent promoters can enhance c-Myc-dependent transcription. This same region of EBNA3C also recruits and modulates the activity of retinoblastoma and p27, both major regulators of the mammalian cell cycle. The inclusion of c-Myc in the group of cellular targets modulated by this domain further accentuates the importance of these critical residues of EBNA3C in bypassing the cell cycle checkpoints.

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.

B-MYB, a transcription factor implicated in regulating cell cycle, apoptosis and cancer

B-MYB belongs to the MYB family of transcription factors that include A-MYB and c-MYB. While A-MYB and c-MYB are tissue-specific, B-MYB is broadly expressed in rapidly dividing cells of developing or adult mammals. B-MYBs liaisons with important players of the cell cycle and transcription machinery, such as E2F and retinoblastoma proteins, suggest that its essential function in stem cell formation and mammalian development could be related to its ability to directly or indirectly impinge on gene expression. Besides its role in the cell cycle, B-MYB has been shown to promote cell survival by activating antiapoptotic genes such as ApoJ/clusterin and BCL2. Here, we discuss how B-MYB could be implicated in tumourigenesis by regulating gene expression.

Combined c-Myc and caveolin-1 expression in human prostate carcinoma predicts prostate carcinoma progression

BACKGROUND

Over-expression of the oncogene c-Myc has been implicated in the development and progression of human prostate carcinoma. However, previous assessments of c-Myc expression have not revealed its potential for predicting prostate carcinoma progression. Caveolin-1 is associated with prostate carcinoma progression and is a downstream target gene of c-Myc. The observation that caveolin-1 can suppress c-Myc-induced apoptosis suggested the potential for cooperation between c-Myc and caveolin-1 in malignant progression. In this study, the authors evaluated the prognostic potential of combined c-Myc and caveolin-1 expression in human prostate carcinoma progression.

METHODS

Immunostaining with c-Myc and caveolin-1-specific antibodies was performed on paraffin sections from 104 radical prostatectomy specimens from men with lymph node negative prostate carcinoma. Combined c-Myc and caveolin-1 immunostaining scores were related with the clinical and pathologic features and the probability of prostate-specific antigen recurrence after surgery.

RESULTS

The combination of c-Myc and caveolin-1 immunopositivity correlated positively with Gleason score (rho = 0.219; P = 0.0253) and positive surgical margin (rho = 0.333; P = 0.0006). The combination of positive c-Myc and caveolin-1 in patients with clinically confined prostate carcinoma was a significant prognostic marker for the time to disease progression after surgery in both univariate analysis (P = 0.0039; hazard ratio, 3.035) and multivariate analysis (P = 0.0114; hazard ratio, 2.916).

CONCLUSIONS

The coexpression of c-Myc and caveolin-1 showed potential as a useful prognostic marker for human prostate carcinoma. The current results suggest interactions between c-Myc and caveolin-1 in the progression of human prostate carcinoma.

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.

Control of cell cycle-dependent degradation of c-Ski proto-oncoprotein by Cdc34

It is known that excess amounts of Ski, or any member of its proto-oncoprotein family, causes disruption of the transforming growth factor beta signal transduction pathway, thus causing oncogenic transformation of cells. Previous studies indicate that Ski is a relatively unstable protein whose expression levels can be regulated by ubiquitin-mediated proteolysis. Here, we investigate the mechanism by which the stability of Ski is regulated. We show that the steady-state levels of Ski protein are controlled post-translationally by cell cycle-dependent proteolysis, wherein Ski is degraded during the interphase of the cell cycle but is relatively stable during mitosis. Furthermore, we demonstrate that the ubiquitin-conjugating enzyme Cdc34 mediates cell cycle-dependent Ski degradation both in vitro and in vivo. Overexpression of dominant-negative Cdc34 stabilizes Ski and enhances its ability to antagonize TGF-beta signaling. Our data suggest that regulated proteolysis of Ski is one of the key mechanisms that control the threshold levels of this proto-oncoprotein, and thus prevents epithelial cells from becoming TGF-beta resistant.

Transcriptional signature of an adult brain tumor in Drosophila

Background

Mutations and gene expression alterations in brain tumors have been extensively investigated, however the causes of brain tumorigenesis are largely unknown. Animal models are necessary to correlate altered transcriptional activity and tumor phenotype and to better understand how these alterations cause malignant growth. In order to gain insights into the in vivo transcriptional activity associated with a brain tumor, we carried out genome-wide microarray expression analyses of an adult brain tumor in Drosophila caused by homozygous mutation in the tumor suppressor gene brain tumor (brat).

Results

Two independent genome-wide gene expression studies using two different oligonucleotide microarray platforms were used to compare the transcriptome of adult wildtype flies with mutants displaying the adult brat^k06028 mutant brain tumor. Cross-validation and stringent statistical criteria identified a core transcriptional signature of brat^k06028 neoplastic tissue. We find significant expression level changes for 321 annotated genes associated with the adult neoplastic brat^k06028 tissue indicating elevated and aberrant metabolic and cell cycle activity, upregulation of the basal transcriptional machinery, as well as elevated and aberrant activity of ribosome synthesis and translation control. One fifth of these genes show homology to known mammalian genes involved in cancer formation.

Conclusion

Our results identify for the first time the genome-wide transcriptional alterations associated with an adult brain tumor in Drosophila and reveal insights into the possible mechanisms of tumor formation caused by homozygous mutation of the translational repressor brat.

The c-Rel transcription factor and B-cell proliferation: a deal with the devil

Activation of the Rel/NF-kappaB signal transduction pathway has been associated with a variety of animal and human malignancies. However, among the Rel/NF-kappaB family members, only c-Rel has been consistently shown to be able to malignantly transform cells in culture. In addition, c-rel has been activated by a retroviral promoter insertion in an avian B-cell lymphoma, and amplifications of REL (human c-rel) are frequently seen in Hodgkin's lymphomas and diffuse large B-cell lymphomas, and in some follicular and mediastinal B-cell lymphomas. Phenotypic analysis of c-rel knockout mice demonstrates that c-Rel has a normal role in B-cell proliferation and survival; moreover, c-Rel nuclear activity is required for B-cell development. Few mammalian model systems are available to study the role of c-Rel in oncogenesis, and it is still not clear what features of c-Rel endow it with its unique oncogenic activity among the Rel/NF-kappaB family. In any event, REL may provide an appropriate therapeutic target for certain human lymphoid cell malignancies.

When protein destruction runs amok, malignancy is on the loose

Ubiquitin-dependent proteolysis ensures that specific protein functions are turned off at the right time, in the right place, and in a unidirectional fashion. The high substrate specificity of the system is determined by a large family of ubiquitin ligases, which competes with the protein kinases to be the largest family of enzymes in mammals. Given the crucial function of the proteolytic machinery, altered degradation of cellular regulators contributes to the unchecked proliferation typical of cancer cells. Here we review the aberrant activity of a variety of ubiquitin ligases in human cancer, hence the prospect of targeting them in cancer therapy.