Skp2 deletion unmasks a p27 safeguard that blocks tumorigenesis in the absence of pRb and p53 tumor suppressors

Comprehensive single cell transcriptomics analysis of murine osteosarcoma uncovers Skp2 function in metastasis, genomic instability and immune activation and reveals additional target pathways

Osteosarcoma (OS) is the most common primary pediatric bone malignancy. One promising new therapeutic target is SKP2, encoding a substrate recognition factor of the SCF E3 ubiquitin ligase responsible for ubiquitination and proteasome degradation of substrate p27, thus driving cellular proliferation. We have shown previously that knockout of Skp2 in an immunocompetent transgenic mouse model of OS improved survival, drove apoptosis, and induced tumor inflammation. Here, we applied single-cell RNA-sequencing (scRNA-seq) to study primary OS tumors derived from Osx-Cre driven conditional knockout of Rb1 and Trp53. We showed that murine OS models recapitulate the tumor heterogeneity and microenvironment complexity observed in patient tumors. We further compared this model with OS models with functional disruption of Skp2: one with Skp2 knockout and the other with the Skp2-p27 interaction disrupted (resulting in p27 overexpression). We found reduction of T cell exhaustion and upregulation of interferon activation, along with evidence of replicative and endoplasmic reticulum-related stress in the Skp2 disruption models, and showed that interferon induction was correlated with improved survival in OS patients. Additionally, our scRNA-seq analysis uncovered decreased activities of metastasis-related gene signatures in the Skp2-disrupted OS, which we validated by observation of a strong reduction in lung metastasis in the Skp2 knockout mice. Finally, we report several potential mechanisms of escape from targeting Skp2 in OS, including upregulation of Myc targets, DNA copy number amplification and overexpression of alternative E3 ligase genes, and potential alternative lineage activation. These mechanistic insights into OS tumor biology and Skp2 function suggest novel targets for new, synergistic therapies, while the data and our comprehensive analysis may serve as a public resource for further big data-driven OS research.

Targeting RCC1 to block the human soft-tissue sarcoma by disrupting nucleo-cytoplasmic trafficking of Skp2

Soft-tissue sarcomas (STS) emerges as formidable challenges in clinics due to the complex genetic heterogeneity, high rates of local recurrence and metastasis. Exploring specific targets and biomarkers would benefit the prognosis and treatment of STS. Here, we identified RCC1, a guanine-nucleotide exchange factor for Ran, as an oncogene and a potential intervention target in STS. Bioinformatics analysis indicated that RCC1 is highly expressed and correlated with poor prognosis in STS. Functional studies showed that RCC1 knockdown significantly inhibited the cell cycle transition, proliferation and migration of STS cells in vitro, and the growth of STS xenografts in mice. Mechanistically, we identified Skp2 as a downstream target of RCC1 in STS. Loss of RCC1 substantially diminished Skp2 abundance by compromising its protein stability, resulting in the upregulation of p27Kip1 and G1/S transition arrest. Specifically, RCC1 might facilitate the nucleo-cytoplasmic trafficking of Skp2 via direct interaction. As a result, the cytoplasmic retention of Skp2 would further protect it from ubiquitination and degradation. Notably, recovery of Skp2 expression largely reversed the phenotypes induced by RCC1 knockdown in STS cells. Collectively, this study unveils a novel RCC1-Skp2-p27Kip1 axis in STS oncogenesis, which holds promise for improving prognosis and treatment of this formidable malignancy.

Targeted inhibition of SCFSKP2 confers anti-tumor activities resulting in a survival benefit in osteosarcoma

Osteosarcoma(OS) is a highly aggressive bone cancer for which treatment has remained essentially unchanged for decades. Although OS is characterized by extensive genomic heterogeneity and instability, RB1 and TP53 have been shown to be the most commonly inactivated tumor suppressors in OS. We previously generated a mouse model with a double knockout (DKO) of Rb1 and Trp53 within cells of the osteoblastic lineage, which largely recapitulates human OS with nearly complete penetrance. SKP2 is a repression target of pRb and serves as a substrate recruiting subunit of the SCFSKP2 complex. In addition, SKP2 plays a central role in regulating the cell cycle by ubiquitinating and promoting the degradation of p27. We previously reported the DKOAA transgenic model, which harbored a knock-in mutation in p27 that impaired its binding to SKP2. Here, we generated a novel p53-Rb1-SKP2 triple-knockout model (TKO) to examine SKP2 function and its potential as a therapeutic target in OS. First, we observed that OS tumorigenesis was significantly delayed in TKO mice and their overall survival was markedly improved. In addition, the loss of SKP2 also promoted an apoptotic microenvironment and reduced the stemness of DKO tumors. Furthermore, we found that small-molecule inhibitors of SKP2 exhibited anti-tumor activities in vivo and in OS organoids as well as synergistic effects when combined with a standard chemotherapeutic agent. Taken together, our results suggest that SKP2 inhibitors may reduce the stemness plasticity of OS and should be leveraged as next-generation adjuvants in this cancer.

SKP2 Knockout in Rb1/p53-Deficient Mouse Models of Osteosarcoma Induces Immune Infiltration and Drives a Transcriptional Program with a Favorable Prognosis

Osteosarcoma is an aggressive bone malignancy with a poor prognosis. One putative proto-oncogene in osteosarcoma is SKP2, encoding a substrate recognition factor of the SCF E3 ubiquitin ligase. We previously demonstrated that Skp2 knockout in murine osteosarcoma improved survival and delayed tumorigenesis. Here, we performed RNA sequencing (RNA-seq) on tumors from a transgenic osteosarcoma mouse model with conditional Trp53 and Rb1 knockouts in the osteoblast lineage ("DKO": Osx1-Cre;Rb1lox/lox;p53lox/lox) and a triple-knockout model with additional Skp2 germline knockout ("TKO": Osx1-Cre;Rb1lox/lox;p53lox/lox;Skp2-/-), followed by qPCR and immunohistochemistry validation. To investigate the clinical implications of our results, we analyzed a human osteosarcoma patient cohort ("NCI-TARGET OS") with RNA-seq and clinical data. We found large differences in gene expression after SKP2 knockout. Surprisingly, we observed increased expression of genes related to immune microenvironment infiltration in TKO tumors, especially the signature genes for macrophages and to a lesser extent, T cells, B cells, and vascular cells. We also uncovered a set of relevant transcription factors that may mediate these changes. In osteosarcoma patient cohorts, high expression of genes upregulated in TKO was correlated with favorable overall survival, which was largely explained by the macrophage gene signatures. This relationship was further supported by our finding that SKP2 expression was negatively correlated with macrophage infiltration in the NCI-TARGET osteosarcoma and the TCGA Sarcoma cohorts. Overall, our findings indicate that SKP2 may mediate immune exclusion from the osteosarcoma tumor microenvironment, suggesting that SKP2 modulation in osteosarcoma may induce antitumor immune activation.

Effect of ubiquitin protease system on DNA damage response in prostate cancer (Review)

Genomic instability is an essential hallmark of cancer, and cellular DNA damage response (DDR) defects drive tumorigenesis by disrupting genomic stability. Several studies have identified abnormalities in DDR-associated genes, and a dysfunctional ubiquitin-proteasome system (UPS) is the most common molecular event in metastatic castration-resistant prostate cancer (PCa). For example, mutations in Speckle-type BTB/POZ protein-Ser119 result in DDR downstream target activation deficiency. Skp2 excessive upregulation inhibits homologous recombination repair and promotes cell growth and migration. Abnormally high expression of a deubiquitination enzyme, ubiquitin-specific protease 12, stabilizes E3 ligase MDM2, which further leads to p53 degradation, causing DDR interruption and genomic instability. In the present review, the basic pathways of DDR, UPS dysfunction, and its induced DDR alterations mediated by genomic instability, and especially the potential application of UPS and DDR alterations as biomarkers and therapeutic targets in PCa treatment, were described.

MYOD-SKP2 axis boosts tumorigenesis in fusion negative rhabdomyosarcoma by preventing differentiation through p57Kip2 targeting

Rhabdomyosarcomas (RMS) are pediatric mesenchymal-derived malignancies encompassing PAX3/7-FOXO1 Fusion Positive (FP)-RMS, and Fusion Negative (FN)-RMS with frequent RAS pathway mutations. RMS express the master myogenic transcription factor MYOD that, whilst essential for survival, cannot support differentiation. Here we discover SKP2, an oncogenic E3-ubiquitin ligase, as a critical pro-tumorigenic driver in FN-RMS. We show that SKP2 is overexpressed in RMS through the binding of MYOD to an intronic enhancer. SKP2 in FN-RMS promotes cell cycle progression and prevents differentiation by directly targeting p27Kip1 and p57Kip2, respectively. SKP2 depletion unlocks a partly MYOD-dependent myogenic transcriptional program and strongly affects stemness and tumorigenic features and prevents in vivo tumor growth. These effects are mirrored by the investigational NEDDylation inhibitor MLN4924. Results demonstrate a crucial crosstalk between transcriptional and post-translational mechanisms through the MYOD-SKP2 axis that contributes to tumorigenesis in FN-RMS. Finally, NEDDylation inhibition is identified as a potential therapeutic vulnerability in FN-RMS.

SKP2 knockout in Rb1/p53 deficient mouse models of osteosarcoma induces immune infiltration and drives a transcriptional program with a favorable prognosis

Purpose

Osteosarcoma (OS) is an aggressive bone malignancy with a poor prognosis. One putative proto-oncogene in OS is SKP2, encoding a substrate recognition factor of the SCF E3 ubiquitin ligase. We previously demonstrated that SKP2 knockout in murine OS improved survival and delayed tumorigenesis. Here we aim to define the SKP2 drives transcriptional program and its clinical implication in OS.

Experimental Design

We performed RNA-sequencing (RNA-seq) on tumors from a transgenic OS mouse model with conditional Trp53 and Rb1 knockouts in the osteoblast lineage ("DKO": Osx1-Cre;Rb1lox/lox;p53lox/lox) and a triple-knockout model with additional Skp2 germline knockout ("TKO": Osx1-Cre;Rb1lox/lox;p53lox/lox;SKP2-/-). We validated our RNA-seq findings using qPCR and immunohistochemistry. To investigate the clinical implications of our results, we analyzed a human OS patient cohort ("NCI-TARGET OS") with RNA-seq and clinical data.

Results

We found large differences in gene expression after SKP2 knockout. Strikingly, we observed increased expression of genes related to immune microenvironment infiltration in TKO tumors. We observed significant increases in signature genes for macrophages and to a lesser extent, T cells, B cells and vascular cells. We also uncovered a set of relevant transcription factors that may mediate the changes. In OS patient cohorts, high expression of genes upregulated in TKO was correlated with favorable overall survival, which was largely explained by the macrophage gene signatures. This relationship was further supported by our finding that SKP2 expression was negatively correlated with macrophage infiltration in the NCI-TARGET OS and the TCGA Sarcoma cohort.

Conclusion

Our findings indicate that SKP2 may mediate immune exclusion from the OS tumor microenvironment, suggesting that SKP2 modulation in OS may induce anti-tumor immune activation.

Roles of p53-Mediated Host–Virus Interaction in Coronavirus Infection

The emergence of the SARS-CoV-2 coronavirus has garnered global attention due to its highly pathogenic nature and the resulting health crisis and economic burden. Although drugs such as Remdesivir have been considered a potential cure by targeting the virus on its RNA polymerase, the high mutation rate and unique 3’ to 5’ exonuclease with proofreading function make it challenging to develop effective anti-coronavirus drugs. As a result, there is an increasing focus on host–virus interactions because coronaviruses trigger stress responses, cell cycle changes, apoptosis, autophagy, and the dysregulation of immune function and inflammation in host cells. The p53 tumor suppressor molecule is a critical regulator of cell signaling pathways, cellular stress responses, DNA repair, and apoptosis. However, viruses can activate or inhibit p53 during viral infections to enhance viral replication and spread. Given its pivotal role in cell physiology, p53 represents a potential target for anti-coronavirus drugs. This review aims to summarize the relationship between p53 and coronaviruses from various perspectives, to shed light on potential targets for antiviral drug development and vaccine design.

The roles of E3 ubiquitin ligases in cancer progression and targeted therapy

Ubiquitination is one of the most important post-translational modifications which plays a significant role in conserving the homeostasis of cellular proteins. In the ubiquitination process, ubiquitin is conjugated to target protein substrates for degradation, translocation or activation, dysregulation of which is linked to several diseases including various types of cancers. E3 ubiquitin ligases are regarded as the most influential ubiquitin enzyme owing to their ability to select, bind and recruit target substrates for ubiquitination. In particular, E3 ligases are pivotal in the cancer hallmarks pathways where they serve as tumour promoters or suppressors. The specificity of E3 ligases coupled with their implication in cancer hallmarks engendered the development of compounds that specifically target E3 ligases for cancer therapy. In this review, we highlight the role of E3 ligases in cancer hallmarks such as sustained proliferation via cell cycle progression, immune evasion and tumour promoting inflammation, and in the evasion of apoptosis. In addition, we summarise the application and the role of small compounds that target E3 ligases for cancer treatment along with the significance of targeting E3 ligases as potential cancer therapy.

Interfering with the Ubiquitin-Mediated Regulation of Akt as a Strategy for Cancer Treatment

The serine/threonine kinase Akt modulates the functions of numerous substrates, many of them being involved in cell proliferation and growth, metabolism, angiogenesis, resistance to hypoxia and migration. Akt is frequently deregulated in many types of human cancers, its overexpression or abnormal activation being associated with the increased proliferation and survival of cancer cells. A promising avenue for turning off the functionality of Akt is to either interfere with the K63-linked ubiquitination that is necessary for Akt membrane recruitment and activation or increase the K48-linked polyubiquitination that aims to target Akt to the proteasome for its degradation. Recent evidence indicates that targeting the ubiquitin proteasome system is effective for certain cancer treatments. In this review, the functions and roles of Akt in human cancer will be discussed, with a main focus on molecules and compounds that target various elements of the ubiquitination processes that regulate the activation and inactivation of Akt. Moreover, their possible and attractive implications for cancer therapy will be discussed.

Targeting the untargetable: RB1-deficient tumours are vulnerable to Skp2 ubiquitin ligase inhibition

Proteins that regulate the cell cycle are accumulated and degraded in a coordinated manner during the transition from one cell cycle phase to the next. The rapid loss of a critical protein, for example, to allow the cell to move from G1/G0 to S phase, is often regulated by its ubiquitination and subsequent proteasomal degradation. Protein ubiquitination is mediated by a series of three ligases, of which the E3 ligases provide the specificity for a particular protein substrate. One such E3 ligase is SCF^Skp1/Cks1, which has a substrate recruiting subunit called S-phase kinase-associated protein 2 (Skp2). Skp2 regulates cell proliferation, apoptosis, and differentiation, can act as an oncogene, and is overexpressed in human cancer. A primary target of Skp2 is the cyclin-dependent kinase inhibitor p27 (CDKN1b) that regulates the cell cycle at several points. The RB1 tumour suppressor gene regulates Skp2 activity by two mechanisms: by controlling its mRNA expression, and by an effect on Skp2's enzymatic activity. For the latter, the RB1 protein (pRb) directly binds to the substrate-binding site on Skp2, preventing protein substrates from being ubiquitinated and degraded. Inactivating mutations in RB1 are common in human cancer, becoming more frequent in aggressive, metastatic, and drug-resistant tumours. Hence, RB1 mutation leads to the loss of pRb, an unrestrained increase in Skp2 activity, the unregulated decrease in p27, and the loss of cell cycle control. Because RB1 mutations lead to the loss of a functional protein, its direct targeting is not possible. This perspective will discuss evidence validating Skp2 as a therapeutic target in RB1-deficient cancer.

Akt: a key transducer in cancer

Growth factor signaling plays a pivotal role in diverse biological functions, such as cell growth, apoptosis, senescence, and migration and its deregulation has been linked to various human diseases. Akt kinase is a central player transmitting extracellular clues to various cellular compartments, in turn executing these biological processes. Since the discovery of Akt three decades ago, the tremendous progress towards identifying its upstream regulators and downstream effectors and its roles in cancer has been made, offering novel paradigms and therapeutic strategies for targeting human diseases and cancers with deregulated Akt activation. Unraveling the molecular mechanisms for Akt signaling networks paves the way for developing selective inhibitors targeting Akt and its signaling regulation for the management of human diseases including cancer.

Targeting MDM4 as a Novel Therapeutic Approach in Prostate Cancer Independent of p53 Status

Metastatic prostate cancer is a lethal disease in patients incapable of responding to therapeutic interventions. Invasive prostate cancer spread is caused by failure of the normal anti-cancer defense systems that are controlled by the tumour suppressor protein, p53. Upon mutation, p53 malfunctions. Therapeutic strategies to directly re-empower the growth-restrictive capacities of p53 in cancers have largely been unsuccessful, frequently because of a failure to discriminate responses in diseased and healthy tissues. Our studies sought alternative prostate cancer drivers, intending to uncover new treatment targets. We discovered the oncogenic potency of MDM4 in prostate cancer cells, both in the presence and absence of p53 and also its mutation. We uncovered that sustained depletion of MDM4 is growth inhibitory in prostate cancer cells, involving either apoptosis or senescence, depending on the cell and genetic context. We identified that the potency of MDM4 targeting could be potentiated in prostate cancers with mutant p53 through the addition of a first-in-class small molecule drug that was selected as a p53 reactivator and has the capacity to elevate oxidative stress in cancer cells to drive their death.

Crocodile Oil Modulates Inflammation and Immune Responses in LPS-Stimulated RAW 264.7 Macrophages

Crocodile oil (CO) is generated from the fatty tissues of crocodiles as a by-product of commercial aquaculture. CO is extensively applied in the treatment of illnesses including asthma, emphysema, skin ulcers, and cancer, as well as wound healing. Whether CO has anti-inflammatory properties and encourages an immune response remains uncertain. The impact of CO on inflammatory conditions in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells and the mechanisms behind it were examined in this work. Cells were treated with 0.125–2% CO dissolved in 0.5% propylene glycol with or without LPS. The production and expression of inflammatory cytokines and mediators were also examined in this research. CO reduced the synthesis and gene expression of interleukin-6 (IL-6). Consistently, CO inhibited the expression and synthesis of inflammatory markers including cyclooxygenase-2 (COX-2), prostaglandin E2 (PGE2), nitric oxide (NO), and nuclear factor kappa B (NF-κB). Furthermore, CO reduced the effects of DNA damage. CO also increased the cell-cycle regulators, cyclins D2 and E2, which improved the immunological response. CO might thus be produced as a nutraceutical supplement to help avoid inflammatory diseases.

Synergistic Silencing of Skp2 by siRNA Self-Assembled Nanoparticles as a Therapeutic Strategy for Advanced Prostate Cancer

Advanced prostate cancer, harboring multiple mutations of tumor suppressor genes, is refractory to conventional therapies. Knockout of the Skp2 gene blocks pRb/p53 doubly deficient prostate cancer in mice, which inspired the authors to develop an approach for delivering siRNA that would efficiently silence Skp2 (siSkp2) in vivo. Here, a facile strategy is reported to directly assemble siSkp2 with the natural compound quercetin (Que) into supramolecular nanoparticles (NPs). This carrier-free siSkp2 delivery system could effectively protect siSkp2 from degradation in serum and enhance its cellular internalization. Furthermore, the siSkp2/Que NPs exhibit synergistic effects in Skp2 silencing, because they can degrade the mRNA and protein of Skp2 simultaneously. Indeed, siSkp2/Que NPs remarkably diminish the Skp2 abundance and further inhibit the proliferation and migration of TMU cells (RB1/TP53/KRAS triple mutations) in vitro. The in vivo results further show that i.v. administration of siSkp2/Que NPs efficiently accumulates in tumor sites and strongly inhibits the growth of TMU tumors in nude mice. Importantly, the siSkp2/Que NPs do not induce any abnormality in the treated mice, which suggests satisfactory biocompatibility. Collectively, this study describes a tractable siRNA self-assembled strategy for Skp2 silencing, which might be a promising nanodrug to cure multitherapy-resistant advanced prostate cancer.

Anti-inflammatory effects and enhancing immune response of freshwater hybrid catfish oil in RAW264.7 cells

The present study assessed the effect of freshwater hybrid catfish oil (FFO) on the inflammatory status of lipopolysaccharide (LPS)-stimulated RAW264.7 cells and investigated the underlying mechanisms. RAW264.7 cells were supplemented with various concentrations [0.125-2% in 0.5% propylene glycol (v/v)] of FFO with or without LPS (1 µg/ml) for 24 h. Inflammatory cytokines and mediators were quantified using ELISA and reverse transcription-quantitative PCR. The results revealed that FFO treatment inhibited the secretion and mRNA expression of the pro-inflammatory cytokines IL-6, IL-1β, TNF-α. In line with this, FFO suppressed the expression and secretion of the inflammatory mediators cyclooxygenase-2 and prostaglandin E2. FFO also reduced apoptotic body formation and DNA damage. Correspondingly, FFO enhanced the immune response by modulating the cell cycle regulators p53, cyclin D2 and cyclin E2. Accordingly, FFO may be developed as a nutraceutical product to prevent inflammation.

The Enhancing Immune Response and Anti-Inflammatory Effects of Caulerpa lentillifera Extract in RAW 264.7 Cells

Background: Caulerpa lentillifera (CL) is a green seaweed, and its edible part represents added value as a functional ingredient. CL was dried and extracted for the determination of its active compounds and the evaluation of its biological activities. The major constituents of CL extract (CLE), including tannic acid, catechin, rutin, and isoquercetin, exhibited beneficial effects, such as antioxidant activity, anti-diabetic activity, immunomodulatory effects, and anti-cancer activities in in vitro and in vivo models. Whether CLE has an anti-inflammatory effect and immune response remains unclear. Methods: This study examined the effect of CLE on the inflammatory status and immune response of lipopolysaccharide (LPS)-stimulated RAW 264.7 cells and the mechanisms involved therein. RAW264.7 cells were treated with different concentrations of CLE (0.1–1000 µg/mL) with or without LPS (1 µg/mL) for 24 h. Expression and production of the inflammatory cytokines, enzymes, and mediators were evaluated. Results: CLE suppressed expression and production of the pro-inflammatory cytokines IL-6 and TNF-α. Moreover, CLE inhibited expression and secretion of the inflammatory enzyme COX-2 and the mediators PGE2 and NO. CLE also reduced DNA damage. Furthermore, CLE stimulated the immune response by modulating the cell cycle regulators p27, p53, cyclin D2, and cyclin E2. Conclusions: CLE inhibits inflammatory responses in LPS-activated macrophages by downregulating inflammatory cytokines and mediators. Furthermore, CLE has an immunomodulatory effect by modulating cell cycle regulators.

Targeting RFWD2 as an Effective Strategy to Inhibit Cellular Proliferation and Overcome Drug Resistance to Proteasome Inhibitor in Multiple Myeloma

The potential to overcome resistance to proteasome inhibitors is greatly related with ubiquitin-proteasome system during multiple myeloma (MM) treatment process. The constitutive photomorphogenic 1 (RFWD2), referred to an E3 ubiquitin ligase, has been identified as an oncogene in multiple cancers, yet important questions on the role of RFWD2 in MM biology and treatment remain unclear. Here we demonstrated that MM patients with elevated RFWD2 expression achieved adverse outcome and drug resistance by analyzing gene expression profiling. Moreover, we proved that RFWD2 participated in the process of cell cycle, cell growth and death in MM by mass spectrometry analysis. In vitro study indicated that inducible knockdown of RFWD2 hindered cellular growth and triggered apoptosis in MM cells. Mechanism study revealed that RFWD2 controlled MM cellular proliferation via regulating the degradation of P27 rather than P53. Further exploration unveiled that RFWD2 meditated P27 ubiquitination via interacting with RCHY1, which served as an E3 ubiquitin ligase of P27. Finally, in vivo study illustrated that blocking RFWD2 in BTZ-resistant MM cells overcame the drug resistance in a myeloma xenograft mouse model. Taken together, these findings provide compelling evidence for prompting that targeting RFWD2 may be an effective strategy to inhibit cellular proliferation and overcome drug resistance to proteasome inhibitor in MM.

The interaction of SKP2 with p27 enhances the progression and stemness of osteosarcoma

Osteosarcoma is a highly aggressive malignancy for which treatment has remained essentially unchanged for years. Our previous studies found that the F-box protein SKP2 is overexpressed in osteosarcoma, acting as a proto-oncogene; p27Kip1 (p27) is an inhibitor of cyclin-dependent kinases and a downstream substrate of SKP2-mediated ubiquitination. Overexpression of SKP2 and underexpression of p27 are common characteristics of cancer cells. The SCFSKP2 E3 ligase ubiquitinates Thr187-phosphorylated p27 for proteasome degradation, which can be abolished by a Thr187Ala knock-in (p27T187A KI) mutation. RB1 and TP53 are two major tumor suppressors commonly coinactivated in osteosarcoma. We generated a mouse model with a double knockout (DKO) of Rb1 and Trp53 within cells of the osteoblastic lineage, which developed osteosarcoma with full penetrance. When p27T187A KI mice were crossed on to the DKO background, p27T187A protein was found to accumulate in osteosarcoma tumor tissues. Furthermore, p27T187A promoted apoptosis in DKO tumors, slowed disease progression, and significantly prolonged overall survival. RNA sequencing analysis also linked the SCFSKP2 -p27T187A axis to potentially reduced cancer stemness. Given that RB1 and TP53 loss or coinactivation is common in human osteosarcoma, our study suggests that inhibiting the SKP2-p27 axis may represent a desirable therapeutic strategy for this cancer.

Post-Translational Modifications That Drive Prostate Cancer Progression

While a protein primary structure is determined by genetic code, its specific functional form is mostly achieved in a dynamic interplay that includes actions of many enzymes involved in post-translational modifications. This versatile repertoire is widely used by cells to direct their response to external stimuli, regulate transcription and protein localization and to keep proteostasis. Herein, post-translational modifications with evident potency to drive prostate cancer are explored. A comprehensive list of proteome-wide and single protein post-translational modifications and their involvement in phenotypic outcomes is presented. Specifically, the data on phosphorylation, glycosylation, ubiquitination, SUMOylation, acetylation, and lipidation in prostate cancer and the enzymes involved are collected. This type of knowledge is especially valuable in cases when cancer cells do not differ in the expression or mutational status of a protein, but its differential activity is regulated on the level of post-translational modifications. Since their driving roles in prostate cancer, post-translational modifications are widely studied in attempts to advance prostate cancer treatment. Current strategies that exploit the potential of post-translational modifications in prostate cancer therapy are presented.

Cross-species identification of PIP5K1-, splicing- and ubiquitin-related pathways as potential targets for RB1-deficient cells

The RB1 tumor suppressor is recurrently mutated in a variety of cancers including retinoblastomas, small cell lung cancers, triple-negative breast cancers, prostate cancers, and osteosarcomas. Finding new synthetic lethal (SL) interactions with RB1 could lead to new approaches to treating cancers with inactivated RB1. We identified 95 SL partners of RB1 based on a Drosophila screen for genetic modifiers of the eye phenotype caused by defects in the RB1 ortholog, Rbf1. We validated 38 mammalian orthologs of Rbf1 modifiers as RB1 SL partners in human cancer cell lines with defective RB1 alleles. We further show that for many of the RB1 SL genes validated in human cancer cell lines, low activity of the SL gene in human tumors, when concurrent with low levels of RB1 was associated with improved patient survival. We investigated higher order combinatorial gene interactions by creating a novel Drosophila cancer model with co-occurring Rbf1, Pten and Ras mutations, and found that targeting RB1 SL genes in this background suppressed the dramatic tumor growth and rescued fly survival whilst having minimal effects on wild-type cells. Finally, we found that drugs targeting the identified RB1 interacting genes/pathways, such as UNC3230, PYR-41, TAK-243, isoginkgetin, madrasin, and celastrol also elicit SL in human cancer cell lines. In summary, we identified several high confidence, evolutionarily conserved, novel targets for RB1-deficient cells that may be further adapted for the treatment of human cancer.

[Targeting Cullin-RING E3 ligases for anti-cancer therapy: efforts on drug discovery]

Cullin-RING E3 ligases (CRLs) are the major components of ubiquitin-proteasome system, responsible for ubiquitylation and subsequent degradation of thousands of cellular proteins. CRLs play vital roles in the regulation of multiple cellular processes, including cell cycle, cell apoptosis, DNA replication, signalling transduction among the others, and are frequently dysregulated in many human cancers. The discovery of specific neddylation inhibitors, represented by MLN4924, has validated CRLs as promising targets for anti-cancer therapies with a growing market. Recent studies have focused on the discovery of the CRLs inhibitors by a variety of approaches, including high through-put screen, virtual screen or structure-based drug design. The field is, however, still facing the major challenging, since CRLs are a large multi-unit protein family without typical active pockets to facilitate the drug design, and enzymatic activity is mainly dependent on undruggable protein-protein interactions and dynamic conformation changes. Up to now, most reported CRLs inhibitors are aiming at targeting the F-box family proteins (e.g., SKP2, β-TrCP and FBXW7), the substrate recognition subunit of SCF E3 ligases. Other studies reported few small molecule inhibitors targeting the UBE2M-DCN1 interaction, which specifically inhibits CRL3/CRL1 by blocking the cullin neddylation. On the other hand, several CRL activators have been reported, such as plant auxin and immunomodulatory imide drugs, thalidomide. Finally, proteolysis-targeting chimeras (PROTACs) has emerged as a new technology in the field of drug discovery, specifically targeting the undruggable protein-protein interaction. The technique connects the small molecule that selectively binds to a target protein to a CRL E3 via a chemical linker to trigger the degradation of target protein. The PROTAC has become a hotspot in the field of E3-ligase-based anti-cancer drug discovery.

Targeted Inhibition of the E3 Ligase SCFSkp2/Cks1 Has Antitumor Activity in RB1-Deficient Human and Mouse Small-Cell Lung Cancer

The RB1 tumor suppressor gene is mutated in highly aggressive tumors including small-cell lung cancer (SCLC), where its loss, along with TP53, is required and sufficient for tumorigenesis. While RB1-mutant cells fail to arrest at G₁-S in response to cell-cycle restriction point signals, this information has not led to effective strategies to treat RB1-deficient tumors, as it is challenging to develop targeted drugs for tumors that are driven by the loss of gene function. Our group previously identified Skp2, a substrate recruiting subunit of the SCF-Skp2 E3 ubiquitin ligase, as an early repression target of pRb whose knockout blocked tumorigenesis in Rb1-deficient prostate and pituitary tumors. Here we used genetic mouse models to demonstrate that deletion of Skp2 completely blocked the formation of SCLC in Rb1/Trp53-knockout mice (RP mice). Skp2 KO caused an increased accumulation of the Skp2-degradation target p27, a cyclin-dependent kinase inhibitor, which was confirmed as the mechanism of protection by using knock-in of a mutant p27 that was unable to bind to Skp2. Building on the observed synthetic lethality between Rb1 and Skp2, we found that small molecules that bind/inhibit Skp2 have in vivo antitumor activity in mouse tumors and human patient-derived xenograft models of SCLC. Using genetic and pharmacologic approaches, antitumor activity was seen with Skp2 loss or inhibition in established SCLC primary lung tumors, in liver metastases, and in chemotherapy-resistant tumors. Our data highlight a downstream actionable target in RB1-deficient cancers, for which there are currently no targeted therapies available. SIGNIFICANCE: There are no effective therapies for SCLC. The identification of an actionable target downstream of RB1, inactivated in SCLC and other advanced tumors, could have a broad impact on its treatment.

Rb and p53-Deficient Myxofibrosarcoma and Undifferentiated Pleomorphic Sarcoma Require Skp2 for Survival

Myxofibrosarcoma (MFS) and undifferentiated pleomorphic sarcoma (UPS) are highly genetically complex soft tissue sarcomas. Up to 50% of patients develop distant metastases, but current systemic therapies have limited efficacy. MFS and UPS have recently been shown to commonly harbor copy number alterations or mutations in the tumor suppressor genes RB1 and TP53. As these alterations have been shown to engender dependence on the oncogenic protein Skp2 for survival of transformed cells in mouse models, we sought to examine its function and potential as a therapeutic target in MFS/UPS. Comparative genomic hybridization and next-generation sequencing confirmed that a significant fraction of MFS and UPS patient samples (n = 94) harbor chromosomal deletions and/or loss-of-function mutations in RB1 and TP53 (88% carry alterations in at least one gene; 60% carry alterations in both). Tissue microarray analysis identified a correlation between absent Rb and p53 expression and positive expression of Skp2. Downregulation of Skp2 or treatment with the Skp2-specific inhibitor C1 revealed that Skp2 drives proliferation of patient-derived MFS/UPS cell lines deficient in both Rb and p53 by degrading p21 and p27. Inhibition of Skp2 using the neddylation-activating enzyme inhibitor pevonedistat decreased growth of Rb/p53-negative patient-derived cell lines and mouse xenografts. These results demonstrate that loss of both Rb and p53 renders MFS and UPS dependent on Skp2, which can be therapeutically exploited and could provide the basis for promising novel systemic therapies for MFS and UPS. SIGNIFICANCE: Loss of both Rb and p53 renders myxofibrosarcoma and undifferentiated pleomorphic sarcoma dependent on Skp2, which could provide the basis for promising novel systemic therapies.See related commentary by Lambert and Jones, p. 2437.

The Biology of F-box Proteins: The SCF Family of E3 Ubiquitin Ligases

F-box proteins function as substrate adaptors for the S-phase kinase-associated protein 1 (SKP1)-cullin 1 (CUL1)-F-box protein (SCF) ubiquitin ligase complexes, which mediate the proteasomal degradation of a diverse range of regulatory proteins. 20 years since the F-box protein family has been discovered, our understanding of substrate-recognition regulation and the roles F-box proteins play in cellular processes has continued to expand. Here, we provide an introduction to the discovery and classification of F-box proteins, the overall structural assembly of SCF complexes, the varied mechanisms by which F-box proteins recognize their substrates, and the role F-box proteins play in diseases and their potentials in targeted therapies.

Targeting SCF E3 Ligases for Cancer Therapies

SKP1-cullin-1-F-box-protein (SCF) E3 ubiquitin ligase complex is responsible for the degradation of proteins in a strictly regulated manner, through which it exerts pivotal roles in regulating various key cellular processes including cell cycle and division, apoptosis, and differentiation. The substrate specificity of the SCF complex largely depends on the distinct F-box proteins, which function in either tumor promotion or suppression or in a context-dependent manner. Among the 69 F-box proteins identified in human genome, FBW7, SKP2, and β-TRCP have been extensively investigated among various types of cancer in respective of their roles in cancer development, progression, and metastasis. Moreover, several specific inhibitors have been developed to target those E3 ligases, and their efficiency in tumors has been determined. In this review, we provide a summary of the roles of SCF E3 ligases in cancer development, as well as the potential application of miRNA or specific inhibitors for cancer therapy.

Ubiquitin-Regulated Cell Proliferation and Cancer

Ubiquitin ligases (E3) play a crucial role in the regulation of different cellular processes such as proliferation and differentiation via recognition, interaction, and ubiquitination of key cellular proteins in a spatial and temporal regulated manner. The type of ubiquitin chain formed determines the fate of the substrates. The ubiquitinated substrates can be degraded by the proteasome, display altered subcellular localization, or can suffer modifications on their interaction with functional protein complexes. Deregulation of E3 activities is frequently found in various human pathologies, including cancer. The illegitimated or accelerated degradation of oncosuppressive proteins or, inversely, the abnormally high accumulation of oncoproteins, contributes to cell proliferation and transformation. Anomalies in protein abundance may be related to mutations that alter the direct or indirect recognition of proteins by the E3 enzymes or alterations in the level of expression or activity of ubiquitin ligases. Through a few examples, we illustrate here the complexity and diversity of the molecular mechanisms related to protein ubiquitination involved in cell cycle regulation. We will discuss the role of ubiquitin-dependent degradation mediated by the proteasome, the role of non-proteolytic ubiquitination during cell cycle progression, and the consequences of this deregulation on cellular transformation. Finally, we will highlight the novel opportunities that arise from these studies for therapeutic intervention.

KPC1 alleviates hypoxia/reoxygenation-induced apoptosis in rat cardiomyocyte cells though BAX degradation

Bax triggers cell apoptosis by permeabilizing the outer mitochondrial membrane, leading to membrane potential loss and cytochrome c release. However, it is unclear if proteasomal degradation of Bax is involved in the apoptotic process, especially in heart ischemia-reperfusion (I/R)-induced injury. In the present study, KPC1 expression was heightened in left ventricular cardiomyocytes of patients with coronary heart disease (CHD), in I/R-myocardium in vivo and in hypoxia and reoxygenation (H/R)-induced cardiomyocytes in vitro. Overexpression of KPC1 reduced infarction size and cell apoptosis in I/R rat hearts. Similarly, the forced expression of KPC1 restored mitochondrial membrane potential (MMP) and cytochrome c release driven by H/R in H9c2 cells, whereas reducing cell apoptosis, and knockdown of KPC1 by short-hairpin RNA (shRNA) deteriorated cell apoptosis induced by H/R. Mechanistically, forced expression of KPC1 promoted Bax protein degradation, which was abolished by proteasome inhibitor MG132, suggesting that KPC1 promoted proteasomal degradation of Bax. Furthermore, KPC1 prevented basal and apoptotic stress-induced Bax translocation to mitochondria. Bax can be a novel target for the antiapoptotic effects of KPC1 on I/R-induced cardiomyocyte apoptosis and render mechanistic penetration into at least a subset of the mitochondrial effects of KPC1.

Gartanin is a novel NEDDylation inhibitor for induction of Skp2 degradation, FBXW2 expression, and autophagy

Gartanin, a 4-prenylated xanthone, has been identified from the purple mangosteen fruit as a potent growth inhibitor of various cancer cell lines, including prostate cancer. However, much of Gartanin's anticancer mechanism remains unknown. We have discovered that Gartanin docked onto the regulatory subunit of the precursor cell-expressed developmentally downregulated 8 (NEDD8)-activating enzyme (NAE) complex and next to the NEDD8 binding complex, which leads to inhibit NEDD8 conjugations to both Cullin1 and Ubc12 in prostate cancer cell lines and Ubc12 NEDDylation in an in vitro assay. The S phase kinase-associated protein (Skp2) and F-box and WD-repeat domain-containing 2 (FBXW2), the NEDD8 family members of E3 ubiqutin ligases, were also downregulated and upregulated by Gartainin, respectively. Knock-down of NEDD8 expression by short harpin (Sh) RNAs blocked or attenuated these effects of Gartainin. Finally, Gartanin demonstrated its ability to inhibit growth of prostate cancer lines via autophagy initiation. Our data support that Gartanin is a naturally occurring NEDDylation inhibitor and deserves further investigation for prostate cancer prevention and treatment.

Fbxw7 and Skp2 Regulate Stem Cell Switch between Quiescence and Mitotic Division in Lung Adenocarcinoma

BACKGROUND/AIMS

The molecular mechanism of dormancy initiation of cancer stem cells (CSCs) is not clear. This study was to explore the molecular mechanism by which CSCs switch from mitotic division to quiescence.

METHODS

MTT assays, flow cytometry, Western blotting, qRT-PCR, and immunofluorescence staining were used to test cell viability, cell cycle and expression of F-box and WD repeat domain-containing 7 (Fbxw7), c-myc, S phase kinase associated protein-2 (Skp2), cyclin-dependent kinase inhibitor 1B (p27), octamer-binding transcription factor 3/4 (Oct3/4), and β catenin gene in 5-fluorouracil (5-FU)-treated A549 cells. Lung adenocarcinoma xenograft models were employed to detect the effects of Fbxw7 on tumor growth.

RESULTS

5-FU inhibited the proliferation of A549 cells, with a median inhibitory concentration (IC50) of 200 μg/ml after 24 h treatment. 5-FU treatment increased the expressions of Oct3/4, Fbxw7, and p27 and increased the number of A549 cells at G0/G1. 5-FU treatment triggered nuclear translocation of β-catenin, decreased the expression levels of c-myc and Skp2, and decreased the number of A549 cells at S phase. Release from 5-FU decreased the expressions of Oct3/4, Fbxw7 and p27; decreased the percentage of cells in the G0/G1 phase; increased the expressions of Skp2 and c-myc; and increased the proportion of cells in S phase. 5-FU treatment led to high expressions of Oct3/4, c-myc, and p27, with low expressions of Fbxw7 and Skp2. Knockdown of Fbxw7 augmented the expression of c-myc and decreased the proportion of A549 cells in Go/G1 phase. Skp2 siRNA increased the expression of p27 and the percentage of G0/G1 phase cells and reduced the proportion of S phase cells. Fbxw7 overexpression inhibited tumor growth in mouse lung adenocarcinoma xenograft models. When Fbxw7 expression was low, Skp2 expression was higher in lung adenocarcinoma tissues and associated with the differentiation of lung adenocarcinoma.

CONCLUSION

5-FU enriches the CSCs in lung adenocarcinoma cells via increasing Fbxw7 and decreasing Skp2 expression, followed by downregulation of c-myc and upregulation of p27, which switches cells to quiescence.

Skp2-mediated ubiquitination and mitochondrial localization of Akt drive tumor growth and chemoresistance to cisplatin

The E3 ligase S-phase kinase-associated protein 2(Skp2) is overexpressed in human cancers and correlated with poor prognosis, but its contributions to tumorigenesis and chemoresistance in nasopharyngeal carcinoma (NPC) are not evident. Herein we show that Skp2 is highly expressed in NPC tumor tissues and cell lines. Knockdown of Skp2 suppresses tumor cell growth, colony formation, glycolysis, and in vivo tumor growth. Skp2 promotes Akt K63-mediated ubiquitination and activation, which is required for EGF-induced Akt mitochondrial localization. Importantly, K63-linked ubiquitination enhances the interaction between Akt and HK2 and eventually increases HK2 phosphorylation on Thr473 and mitochondrial localization. Overexpression of Skp2 impaired the intrinsic apoptotic pathway and confers cisplatin resistance. Moreover, ectopic expression of Myr-Akt1 or phosphomimetic HK2-T473D rescued cisplatin-induced tumor suppression in Skp2 knockdown stable cells. Also, depletion of Akt ubiquitination enhances the antitumor efficacy of cisplatin in vitro and in vivo. Finally, we demonstrated that Skp2 is positively correlated with p-Akt and HK2 in NPC tumor tissues. This study highlights the clinical value of Skp2 targeting in NPC treatment.

[Progress on correlation between cell senescence and idiopathic pulmonary fibrosis]

Cellular senescence is a key factor driving age-related diseases. Recent studies have revealed that senescence-associated secretory phenotype, telomere attrition, epigenetic changes, and mitochondrial autophagy damage may mediate the pathogenesis of senescence-related idiopathic pulmonary fibrosis (IPF). Reducing the level of cellular senescence or clearing senescent cells can down-regulate the expression of fibrosis factors and alleviate the symptoms of IPF. In this review, we outlined the role and mechanism of cellular senescence in IPF.

High Skp2 expression is associated with a mesenchymal phenotype and increased tumorigenic potential of prostate cancer cells

Skp2 is a crucial component of SCFSkp2 E3 ubiquitin ligase and is often overexpressed in various types of cancer, including prostate cancer (PCa). The epithelial-to-mesenchymal transition (EMT) is involved in PCa progression. The acquisition of a mesenchymal phenotype that results in a cancer stem cell (CSC) phenotype in PCa was described. Therefore, we aimed to investigate the expression and localization of Skp2 in clinical samples from patients with PCa, the association of Skp2 with EMT status, and the role of Skp2 in prostate CSC. We found that nuclear expression of Skp2 was increased in patients with PCa compared to those with benign hyperplasia, and correlated with high Gleason score in PCa patients. Increased Skp2 expression was observed in PCa cell lines with mesenchymal and CSC-like phenotype compared to their epithelial counterparts. Conversely, the CSC-like phenotype was diminished in cells in which SKP2 expression was silenced. Furthermore, we observed that Skp2 downregulation led to the decrease in subpopulation of CD44+CD24- cancer stem-like cells. Finally, we showed that high expression levels of both CD24 and CD44 were associated with favorable recurrence-free survival for PCa patients. This study uncovered the Skp2-mediated CSC-like phenotype with oncogenic functions in PCa.

A patent review of the ubiquitin ligase system: 2015-2018

INTRODUCTION

Ubiquitin-proteasome system (UPS) has been validated as a novel anticancer drug target in the past 20 years. The UPS contains two distinct steps: ubiquitination of a substrate protein by ubiquitin activating enzyme (E1), ubiquitin conjugating enzyme (E2), and ubiquitin ligase (E3), and substrate degradation by the 26S proteasome complex. The E3 enzyme is the central player in the ubiquitination step and has a wide range of specific substrates in cancer cells, offering great opportunities for discovery and development of selective drugs. Areas covered: This review summarizes the recent advances in small molecule inhibitors of E1s, E2s, and E3s, with a focus on the latest patents (from 2015 to 2018) of E3 inhibitors and modulators. Expert opinion: One strategy to overcome limitations of current 20S proteasome inhibitors is to discover inhibitors of the upstream key components of the UPS, such as E3 enzymes. E3s play important roles in cancer development and determine the specificity of substrate ubiquitination, offering novel target opportunities. E3 modulators could be developed by rational design, natural compound or library screening, old drug repurposes, and application of other novel technologies. Further understanding of mechanisms of E3-substrate interaction will be essential for discovering and developing next-generation E3 inhibitors as effective anticancer drugs.

Chemical probes of Skp2-mediated p27 ubiquitylation and degradation

Skp2 is a member of the F-box family of proteins that serve as substrate-specific adaptors in Skp1-CUL1-ROC1-F-box (SCF) E3 ubiquitin ligases. Skp2 (Fbxl1) directly binds to the tumor suppressor p27 in the context of the SCF^Skp2 E3 ubiquitin ligase to ubiquitylate and target-phosphorylated p27 for proteasomal degradation. As p27 is a powerful suppressor of growth in a variety of cells, and as Skp2 is also overexpressed in many human cancers, Skp2 is considered an oncogene and an intriguing drug target. However, despite 20 years of investigation, a valid chemical inhibitor of Skp2-mediated degradation of p27 has not been identified. Recently, an increasing number of compounds designed to have this bioactivity have been reported. Here, we conduct a meta-analysis of the evidence regarding bioactivity, structure, and medicinal chemistry in order to evaluate and compare these Skp2 inhibitor compounds. Despite chemically diverse compounds with a wide array of Skp2-mediated p27 ubiquitylation inhibition properties reported by several independent groups, no current chemical probe formally qualifies as a validated pharmaceutical hit compound. This finding suggests that our knowledge of the structural biochemistry of the Skp2-p27 complex remains incomplete and highlights the need for novel modes of inquiry.

Ubiquitin ligases in oncogenic transformation and cancer therapy

The cellular response to external stress signals and DNA damage depends on the activity of ubiquitin ligases (E3s), which regulate numerous cellular processes, including homeostasis, metabolism and cell cycle progression. E3s recognize, interact with and ubiquitylate protein substrates in a temporally and spatially regulated manner. The topology of the ubiquitin chains dictates the fate of the substrates, marking them for recognition and degradation by the proteasome or altering their subcellular localization or assembly into functional complexes. Both genetic and epigenetic alterations account for the deregulation of E3s in cancer. Consequently, the stability and/or activity of E3 substrates are also altered, in some cases leading to downregulation of tumour-suppressor activities and upregulation of oncogenic activities. A better understanding of the mechanisms underlying E3 regulation and function in tumorigenesis is expected to identify novel prognostic markers and to enable the development of the next generation of anticancer therapies. This Review summarizes the oncogenic and tumour-suppressor roles of selected E3s and highlights novel opportunities for therapeutic intervention.

Quantitative Systems Biology to decipher design principles of a dynamic cell cycle network: the "Maximum Allowable mammalian Trade-Off-Weight" (MAmTOW)

Network complexity is required to lend cellular processes flexibility to respond timely to a variety of dynamic signals, while simultaneously warranting robustness to protect cellular integrity against perturbations. The cell cycle serves as a paradigm for such processes; it maintains its frequency and temporal structure (although these may differ among cell types) under the former, but accelerates under the latter. Cell cycle molecules act together in time and in different cellular compartments to execute cell type-specific programs. Strikingly, the timing at which molecular switches occur is controlled by abundance and stoichiometry of multiple proteins within complexes. However, traditional methods that investigate one effector at a time are insufficient to understand how modulation of protein complex dynamics at cell cycle transitions shapes responsiveness, yet preserving robustness. To overcome this shortcoming, we propose a multidisciplinary approach to gain a systems-level understanding of quantitative cell cycle dynamics in mammalian cells from a new perspective. By suggesting advanced experimental technologies and dedicated modeling approaches, we present innovative strategies (i) to measure absolute protein concentration in vivo, and (ii) to determine how protein dosage, e.g., altered protein abundance, and spatial (de)regulation may affect timing and robustness of phase transitions. We describe a method that we name “Maximum Allowable mammalian Trade–Off–Weight” (MAmTOW), which may be realized to determine the upper limit of gene copy numbers in mammalian cells. These aspects, not covered by current systems biology approaches, are essential requirements to generate precise computational models and identify (sub)network-centered nodes underlying a plethora of pathological conditions.

MYC Modulation around the CDK2/p27/SKP2 Axis

MYC is a pleiotropic transcription factor that controls a number of fundamental cellular processes required for the proliferation and survival of normal and malignant cells, including the cell cycle. MYC interacts with several central cell cycle regulators that control the balance between cell cycle progression and temporary or permanent cell cycle arrest (cellular senescence). Among these are the cyclin E/A/cyclin-dependent kinase 2 (CDK2) complexes, the CDK inhibitor p27^KIP1 (p27) and the E3 ubiquitin ligase component S-phase kinase-associated protein 2 (SKP2), which control each other by forming a triangular network. MYC is engaged in bidirectional crosstalk with each of these players; while MYC regulates their expression and/or activity, these factors in turn modulate MYC through protein interactions and post-translational modifications including phosphorylation and ubiquitylation, impacting on MYC's transcriptional output on genes involved in cell cycle progression and senescence. Here we elaborate on these network interactions with MYC and their impact on transcription, cell cycle, replication and stress signaling, and on the role of other players interconnected to this network, such as CDK1, the retinoblastoma protein (pRB), protein phosphatase 2A (PP2A), the F-box proteins FBXW7 and FBXO28, the RAS oncoprotein and the ubiquitin/proteasome system. Finally, we describe how the MYC/CDK2/p27/SKP2 axis impacts on tumor development and discuss possible ways to interfere therapeutically with this system to improve cancer treatment.

Inhibition of Skp2 suppresses the proliferation and invasion of osteosarcoma cells

Osteosarcoma (OS) is a common bone tumor that mainly affects children and young adults. S-phase kinase‑associated protein 2 (Skp2) has been characterized to play a critical oncogenic role in a variety of human malignancies. However, the biological function of Skp2 in OS remains largely obscure. In the present study, we elucidated the role of Skp2 in cell growth, cell cycle, apoptosis and migration in OS cells. We found that depletion of Skp2 inhibited cell growth in both MG-63 and SW 1353 cells. Moreover, we observed that depletion of Skp2 triggered cell apoptosis in two OS cell lines. Furthermore, downregulation of Skp2 induced cell cycle arrest in the G0/G1 phase in OS cells. Notably, our wound healing assay results revealed that inhibition of Skp2 suppressed cell migration in OS cells. Invariably, our western blot results demonstrated that depletion of Skp2 in OS cells inhibited activation of pAkt and increased p27 expression in OS cells, suggesting that Skp2 exerted its oncogenic function partly through the regulation of Akt and p27. Our findings revealed that targeting Skp2 could be a promising therapeutic strategy for the treatment of OS.

RB1: a prototype tumor suppressor and an enigma

In this review, Dyson summarizes some recent developments in pRB research and focuses on progress toward answers for the three fundamental questions that sit at the heart of the pRB literature: What does pRB do? How does the inactivation of RB change the cell? How can our knowledge of RB function be exploited to provide better treatment for cancer patients?

The retinoblastoma susceptibility gene (RB1) was the first tumor suppressor gene to be molecularly defined. RB1 mutations occur in almost all familial and sporadic forms of retinoblastoma, and this gene is mutated at variable frequencies in a variety of other human cancers. Because of its early discovery, the recessive nature of RB1 mutations, and its frequency of inactivation, RB1 is often described as a prototype for the class of tumor suppressor genes. Its gene product (pRB) regulates transcription and is a negative regulator of cell proliferation. Although these general features are well established, a precise description of pRB's mechanism of action has remained elusive. Indeed, in many regards, pRB remains an enigma. This review summarizes some recent developments in pRB research and focuses on progress toward answers for the three fundamental questions that sit at the heart of the pRB literature: What does pRB do? How does the inactivation of RB change the cell? How can our knowledge of RB function be exploited to provide better treatment for cancer patients?

Hippo Signaling Suppresses Cell Ploidy and Tumorigenesis through Skp2

Polyploidy can lead to aneuploidy and tumorigenesis. Here, we report that the Hippo pathway effector Yap promotes the diploid-polyploid conversion and polyploid cell growth through the Akt-Skp2 axis. Yap strongly induces the acetyltransferase p300-mediated acetylation of the E3 ligase Skp2 via Akt signaling. Acetylated Skp2 is exclusively localized to the cytosol, which causes hyper-accumulation of the cyclin-dependent kinase inhibitor p27, leading to mitotic arrest and subsequently cell polyploidy. In addition, the pro-apoptotic factors FoxO1/3 are overly degraded by acetylated Skp2, resulting in polyploid cell division, genomic instability, and oncogenesis. Importantly, the depletion or inactivation of Akt or Skp2 abrogated Hippo signal deficiency-induced liver tumorigenesis, indicating their epistatic interaction. Thus, we conclude that Hippo-Yap signaling suppresses cell polyploidy and oncogenesis through Skp2.

Skp2 deficiency restricts the progression and stem cell features of castration-resistant prostate cancer by destabilizing Twist

Castration-resistant prostate cancer (CRPC) remains a major clinical challenge because of the lack of effective targeted therapy for its treatment. The mechanism underlying how CRPC gains resistance toward hormone depletion and other forms of chemotherapy is poorly understood. Research on understanding the factors that drive these processes is desperately needed to generate new therapies to cure the disease. Here, we discovered a fundamental role of S-phase protein kinase 2 (Skp2) in the formation and progression of CRPC. In transgenic adenocarcinoma mouse prostate model, Skp2 depletion leads to a profound repression of prostate tumor growth and distal metastasis and substantially prolonged overall survival. We revealed that Skp2 regulates CRPC through Twist-mediated oncogenic functions including epithelial-mesenchymal transition (EMT) and cancer stem cell (CSC) acquisitions. Mechanistically, Skp2 interacted with Twist and promoted the non-degradative ubiquitination of Twist. Consequently, Skp2 stabilized Twist protein expression by preventing proteasomal degradation of Twist by β-TrCP. We found that Twist overexpression augments CSC self-renewal and population and that Skp2 inhibition reverts Twist's effects on CSC regulation. Furthermore, genetically depleting or pharmacologically inactivating Skp2 synergistically re-sensitized CRPC cells toward chemotherapies such as paclitaxel or doxorubicin. Together, this study uncovering Skp2-mediated Twist stabilization and oncogenic functions in CRPC offers new knowledge on how CRPC progresses and acquires chemoresistance during tumor progression. It provides proof of principle that Skp2 targeting is a promising approach to combat metastatic CRPC by targeting Twist and CSCs.

Ubiquitin in Cell-Cycle Regulation and Dysregulation in Cancer

Uncontrolled cell proliferation and genomic instability are common features of cancer and can arise from, respectively, the loss of cell-cycle control and defective checkpoints. Ubiquitin-mediated proteolysis, ultimately executed by ubiquitin-ligating enzymes (E3s), plays a key part in cell-cycle regulation and is dominated by two multisubunit E3s, the anaphase-promoting complex (or cyclosome) (APC/C) and SKP1–cullin-1–F-box (SCF) complex. We highlight the role of APC/C and the SCF bound to F-box proteins, FBXW7, SKP2, and β-TrCP, in regulating the abundance of select fundamental proteins, primarily during the cell cycle, that are associated with human cancer. The clinical success of the first proteasome inhibitor, bortezomib, in treating multiple myeloma and mantle-cell lymphoma set the precedent for viewing the ubiquitin–proteasome system as a druggable target for cancer. Given that there are more E3s than kinases, selective, small-molecule E3 inhibitors have the potential of opening up another dimension in the therapeutic armamentarium for the treatment of cancer.

Loss of DLG5 promotes breast cancer malignancy by inhibiting the Hippo signaling pathway

Discs Large Homolog 5 (DLG5) plays an important role in the maintenance of epithelial cell polarity. Recent research showed that DLG5 is decreased in Yes-associated protein (YAP)-overexpressing cells. However, the exact relationship between DLG5 and YAP is not clear. In this study, we showed that loss of DLG5 promoted breast cancer cell proliferation by inhibiting the Hippo signaling pathway and increasing nuclear YAP expression. Furthermore, depletion of DLG5 induced epithelial-mesenchymal transition (EMT) and disrupted epithelial cell polarity, which was associated with altered expression of Scribble, ZO1, E-cadherin and N-cadherin and their mislocalization. Interestingly, we first reported that loss of DLG5 inhibited the interaction of Mst1 and Lats1 with Scribble, which was crucial for YAP activation and the transcription of TEA domain (TEAD) family members. In summary, loss of DLG5 expression promoted breast cancer malignancy by inactivating the Hippo signaling pathway and increasing nuclear YAP.

p27T187A knockin identifies Skp2/Cks1 pocket inhibitors for advanced prostate cancer

SCFSkp2/Cks1 ubiquitinates Thr187-phosphorylated p27 for degradation. Overexpression of Skp2 coupled with underexpression of p27 are frequent characteristics of cancer cells. When the role of SCFSkp2/Cks1-mediated p27 ubiquitination in cancer was specifically tested by p27 Thr187-to-Ala knockin (p27T187A KI), it was found dispensable for KrasG12D-induced lung tumorigenesis but essential for Rb1-deficient pituitary tumorigenesis. Here we identify pRb and p53 doubly deficient (DKO) prostate tumorigenesis as a context in which p27 ubiquitination by SCFSkp2/Cks1 is required for p27 downregulation. p27 protein accumulated in prostate when p27T187A KI mice underwent DKO prostate tumorigenesis. p27T187A KI or Skp2 knockdown (KD) induced similar degrees of p27 protein accumulation in DKO prostate cells, and Skp2 KD did not further increase p27 protein in DKO prostate cells that contained p27T187A KI (AADKO prostate cells). p27T187A KI activated an E2F1-p73-apoptosis axis in DKO prostate tumorigenesis, slowed disease progression and significantly extended survival. Querying co-occurrence relationships among RB1, TP53, PTEN, NKX3-1 and MYC in TCGA of prostate cancer identified co-inactivation of RB1 and TP53 as the only statistically significant co-occurrences in metastatic castration-resistant prostate cancer (mCRPC). Together, our study identifies Skp2/Cks1 pocket inhibitors as potential therapeutics for mCRPC. Procedures for establishing mCRPC organoid cultures from contemporary patients were recently established. An Skp2/Cks1 pocket inhibitor preferentially collapsed DKO prostate tumor organoids over AADKO organoids, which spontaneously disintegrated over time when DKO prostate tumor organoids grew larger, setting the stage to translate mouse model findings to precision medicine in the clinic on the organoid platform.

Interchangeable Roles for E2F Transcriptional Repression by the Retinoblastoma Protein and p27KIP1-Cyclin-Dependent Kinase Regulation in Cell Cycle Control and Tumor Suppression

The mammalian G₁-S phase transition is controlled by the opposing forces of cyclin-dependent kinases (CDK) and the retinoblastoma protein (pRB). Here, we present evidence for systems-level control of cell cycle arrest by pRB-E2F and p27-CDK regulation. By introducing a point mutant allele of pRB that is defective for E2F repression (Rb1^G) into a p27^KIP1 null background (Cdkn1b^-/-), both E2F transcriptional repression and CDK regulation are compromised. These double-mutant Rb1^G/G; Cdkn1b^-/- mice are viable and phenocopy Rb1^+/- mice in developing pituitary adenocarcinomas, even though neither single mutant strain is cancer prone. Combined loss of pRB-E2F transcriptional regulation and p27^KIP1 leads to defective proliferative control in response to various types of DNA damage. In addition, Rb1^G/G; Cdkn1b^-/- fibroblasts immortalize faster in culture and more frequently than either single mutant genotype. Importantly, the synthetic DNA damage arrest defect caused by Rb1^G/G; Cdkn1b^-/- mutations is evident in the developing intermediate pituitary lobe where tumors ultimately arise. Our work identifies a unique relationship between pRB-E2F and p27-CDK control and offers in vivo evidence that pRB is capable of cell cycle control through E2F-independent effects.

Flavokawain A induces deNEDDylation and Skp2 degradation leading to inhibition of tumorigenesis and cancer progression in the TRAMP transgenic mouse model

S phase kinase-associated protein 2 (Skp2) has been shown to be required for spontaneous tumor development that occurs in the retinoblastoma protein (pRb) deficient mice. Here we have demonstrated that flavokawain A (FKA), a novel chalcone from the kava plant, selectively inhibited the growth of pRb deficient cell lines and resulted in a proteasome-dependent and ubiquitination-mediated Skp2 degradation. Degradation of Skp2 by FKA was found to be involved in a functional Cullin1, but independent of Cdh1 expression. Further studies have demonstrated that FKA docked into the ATP binding pocket of the precursor cell-expressed developmentally down-regulated 8 (NEDD8)-activating enzyme (NAE) complex, inhibited NEDD8 conjugations to both Cullin1 and Ubc12 in PC3 cells and Ubc12 NEDDylation in an in vitro assay. Finally, dietary feeding of the autochthonous transgenic adenocarcinoma of the mouse prostate (TRAMP) mice with FKA inhibited the formation of high-grade prostatic intra-epithelial neoplasia lesions (HG-PIN) and prostate adenocarcinomas, reduced the tumor burden and completely abolished distant organ metastasis. Immunohistochemistry studies revealed that dietary FKA feeding resulted in marked anti-proliferative and apoptotic effects via down-regulation of Skp2 and NEDD8 and up-regulation of p27/Kip1 in the prostate of TRAMP mice. Our findings therefore provide evidence that FKA is a promising NEDDylation inhibitor for targeting Skp2 degradation in prostate cancer prevention and treatment.

Treating cancer when pRb and p53 cannot be reactivated

Activation of oncoproteins and inactivation of tumor suppressors induces tumorigenesis. When these events happen upstream of pRb and p53, cancer therapies may initially succeed and then fail when pRb and p53 are activated and then re-inactivated. Therapies might succeed if they remain effective when pRb and p53 are genetically inactivated.

Skp2 is associated with paclitaxel resistance in prostate cancer cells

Prostate cancer is the most commonly diagnosed tumor in men in the United States. Patients with hormone-refractory prostate cancer are often treated with paclitaxel, but most of them eventually develop drug resistance. S-phase kinase associated protein 2 (Skp2) is a component of the SCF (Skp1-Cullin1-F-box) type of E3 ubiquitin ligase complexes. In the present study, we investigated the role of Skp2 in paclitaxel-resistant DU145-TxR or PC-3-TxR cells by Skp2 silencing or using Skp2 inhibitors. We first confirmed that Skp2 expression is up-regulated in DU145-TxR or PC-3-TxR cells compared with their parental cells DU145 or PC-3, respectively. Knockdown of Skp2 or Skp2 inhibitor treatment in DU145-TxR or PC-3-TxR cells restored paclitaxel sensitivity. E-cadherin was decreased while Vimentin was increased in PC-3-TxR or DU145-TxR cells. In addition, p27 expression was inversely correlated with Skp2 expression in DU145-TxR or PC-3-TxR cells. Moreover, p27 was found to increase in both Skp2 silencing PC-3-TxR and DU145-TxR cells. These results suggest that Skp2 is associated with prostate cancer cell resistance to paclitaxel. Skp2 may be a potential therapeutic target for drug-resistant prostate cancer.