The Fbx4 tumor suppressor regulates cyclin D1 accumulation and prevents neoplastic transformation

Squamous cell cancers of the aero-upper digestive tract: A unified perspective on biology, genetics, and therapy

Squamous cell cancers (SCCs) of the head and neck, esophagus, and lung, referred to as aero-upper digestive SCCs, are prevalent in the United States and worldwide. Their incidence and mortality are projected to increase at alarming rates, posing diagnostic, prognostic, and therapeutic challenges. These SCCs share certain epigenetic, genomic, and genetic alterations, immunologic properties, environmental exposures, as well as lifestyle and nutritional risk factors, which may underscore common complex gene-environmental interactions across them. This review focuses upon the frequent shared epigenetic, genomic, and genetic alterations, emerging preclinical model systems, and how this collective knowledge can be leveraged into perspectives on standard of care therapies and mechanisms of resistance, nominating new potential directions in translational therapeutics.

SKP1-CUL1-F-box: Key molecular targets affecting disease progression

The correct synthesis and degradation of proteins are vital for numerous biological processes in the human body, with protein degradation primarily facilitated by the ubiquitin-proteasome system. The SKP1-CUL1-F-box (SCF) E3 ubiquitin ligase, a member of the Cullin-RING E3 ubiquitin ligase (CRL) family, plays a crucial role in mediating protein ubiquitination and subsequent 26S proteasome degradation during normal cellular metabolism. Notably, SCF is intricately linked to the pathogenesis of various diseases, including malignant tumors. This paper provides a comprehensive overview of the functional characteristics of SCF complexes, encompassing their assembly, disassembly, and regulatory factors. Furthermore, we discuss the diverse effects of SCF on crucial cellular processes such as cell cycle progression, DNA replication, oxidative stress response, cell proliferation, apoptosis, cell differentiation, maintenance of stem cell characteristics, tissue development, circadian rhythm regulation, and immune response modulation. Additionally, we summarize the associations between SCF and the onset, progression, and prognosis of malignant tumors. By synthesizing current knowledge, this review aims to offer a novel perspective for a holistic and systematic understanding of SCF complexes and their multifaceted functions in cellular physiology and disease pathogenesis.

Polyploidy in Cancer: Causal Mechanisms, Cancer-Specific Consequences, and Emerging Treatments

Drug resistance is the major determinant for metastatic disease and fatalities, across all cancers. Depending on the tissue of origin and the therapeutic course, a variety of biological mechanisms can support and sustain drug resistance. Although genetic mutations and gene silencing through epigenetic mechanisms are major culprits in targeted therapy, drug efflux and polyploidization are more global mechanisms that prevail in a broad range of pathologies, in response to a variety of treatments. There is an unmet need to identify patients at risk for polyploidy, understand the mechanisms underlying polyploidization, and to develop strategies to predict, limit, and reverse polyploidy thus enhancing efficacy of standard-of-care therapy that improve better outcomes. This literature review provides an overview of polyploidy in cancer and offers perspective on patient monitoring and actionable therapy.

Targeting cyclin D1 as a therapeutic approach for papillary thyroid carcinoma

Cyclin D1 functions as a mitogenic sensor that specifically binds to CDK4/6, thereby integrating external mitogenic inputs and cell cycle progression. Cyclin D1 interacts with transcription factors and regulates various important cellular processes, including differentiation, proliferation, apoptosis, and DNA repair. Therefore, its dysregulation contributes to carcinogenesis. Cyclin D1 is highly expressed in papillary thyroid carcinoma (PTC). However, the particular cellular mechanisms through which abnormal cyclin D1 expression causes PTC are poorly understood. Unveiling the regulatory mechanisms of cyclin D1 and its function in PTC may help determine clinically effective strategies, and open up better opportunities for further research, leading to the development of novel PTC regimens that are clinically effective. This review explores the mechanisms underlying cyclin D1 overexpression in PTC. Furthermore, we discuss the role of cyclin D1 in PTC tumorigenesis via its interactions with other regulatory elements. Finally, recent progress in the development of therapeutic options targeting cyclin D1 in PTC is examined and summarized.

Tumor suppressor mediated ubiquitylation of hnRNPK is a barrier to oncogenic translation

Heterogeneous Nuclear Ribonucleoprotein K (hnRNPK) is a multifunctional RNA binding protein (RBP) localized in the nucleus and the cytoplasm. Abnormal cytoplasmic enrichment observed in solid tumors often correlates with poor clinical outcome. The mechanism of cytoplasmic redistribution and ensuing functional role of cytoplasmic hnRNPK remain unclear. Here we demonstrate that the SCFFbxo4 E3 ubiquitin ligase restricts the pro-oncogenic activity of hnRNPK via K63 linked polyubiquitylation, thus limiting its ability to bind target mRNA. We identify SCFFbxo4-hnRNPK responsive mRNAs whose products regulate cellular processes including proliferation, migration, and invasion. Loss of SCFFbxo4 leads to enhanced cell invasion, migration, and tumor metastasis. C-Myc was identified as one target of SCFFbxo4-hnRNPK. Fbxo4 loss triggers hnRNPK-dependent increase in c-Myc translation, thereby contributing to tumorigenesis. Increased c-Myc positions SCFFbxo4-hnRNPK dysregulated cancers for potential therapeutic interventions that target c-Myc-dependence. This work demonstrates an essential role for limiting cytoplasmic hnRNPK function in order to maintain translational and cellular homeostasis.

The E3 Ubiquitin Ligase Fbxo4 Functions as a Tumor Suppressor: Its Biological Importance and Therapeutic Perspectives

Simple Summary

Fbxo4 is an E3 ubiquitin ligase that requires the formation of a complex with S-phase kinase-associated protein 1 and Cullin1 to catalyze the ubiquitylation of its substrates. Moreover, Fbxo4 depends on the existence of posttranslational modifications and/or co-factor to be activated to perform its biological functions. The well-known Fbxo4 substrates have oncogenic or oncogene-like activities, for example, cyclin D1, Trf1/Pin2, p53, Fxr1, Mcl-1, ICAM-1, and PPARγ; therefore, Fbxo4 is defined as a tumor suppressor. Biologically, Fbxo4 regulates cell cycle progression, DNA damage response, tumor metabolism, cellular senescence, metastasis and tumor cells’ response to chemotherapeutic compounds. Clinicopathologically, the expression of Fbxo4 is associated with patients’ prognosis depending on different tumor types. Regarding to its complicated regulation, more in-depth studies are encouraged to dissect the detailed molecular mechanisms to facilitate developing new treatment through targeting Fbxo4.

Abstract

Fbxo4, also known as Fbx4, belongs to the F-box protein family with a conserved F-box domain. Fbxo4 can form a complex with S-phase kinase-associated protein 1 and Cullin1 to perform its biological functions. Several proteins are identified as Fbxo4 substrates, including cyclin D1, Trf1/Pin2, p53, Fxr1, Mcl-1, ICAM-1, and PPARγ. Those factors can regulate cell cycle progression, cell proliferation, survival/apoptosis, and migration/invasion, highlighting their oncogenic or oncogene-like activities. Therefore, Fbxo4 is defined as a tumor suppressor. The biological functions of Fbxo4 make it a potential candidate for developing new targeted therapies. This review summarizes the gene and protein structure of Fbxo4, the mechanisms of how its expression and activity are regulated, and its substrates, biological functions, and clinicopathological importance in human cancers.

F-box only protein 9 and its role in cancer

The F-box proteins (FBP), substrate recognition subunit of the SCF (Skp1-Cullin1-F-box protein complex) E3 ligase, play important roles in the ubiquitylation and subsequent degradation of the target proteins from several cellular processes. Disorders of F-box protein-mediated proteolysis lead to human malignancies. FBP plays an important role in many cellular processes, including cell proliferation, cell cycle, apoptosis, migration, invasion, and metastasis, suggesting that it can be associated with tumorigenesis, cancer development and progression. However, the expression and function of FBXO9 (F-box only protein 9) differ in various types of human cancer. Due to the ability to regulate the stability and activity of oncogenes and tumor-suppressor genes, and the physiological functions of many of the F-box proteins remain subtle, further genetic and mechanistic studies will elaborate and help define FBXO9's role. Targeting F-box protein or F-box protein signaling pathways could be an effective strategy for preventing or treating human cancer. This review is presented to summarize the part of FBXO9 in different types of human cancer and its regulation mechanism, and to pave the way to design FBXO9-targeting anticancer therapies.

The SCF Complex Is Essential to Maintain Genome and Chromosome Stability

The SKP1, CUL1, F-box protein (SCF) complex encompasses a group of 69 SCF E3 ubiquitin ligase complexes that primarily modify protein substrates with poly-ubiquitin chains to target them for proteasomal degradation. These SCF complexes are distinguishable by variable F-box proteins, which determine substrate specificity. Although the function(s) of each individual SCF complex remain largely unknown, those that have been characterized regulate a wide array of cellular processes, including gene transcription and the cell cycle. In this regard, the SCF complex regulates transcription factors that modulate cell signaling and ensures timely degradation of primary cell cycle regulators for accurate replication and segregation of genetic material. SCF complex members are aberrantly expressed in a myriad of cancer types, with altered expression or function of the invariable core SCF components expected to have a greater impact on cancer pathogenesis than that of the F-box proteins. Accordingly, this review describes the normal roles that various SCF complexes have in maintaining genome stability before discussing the impact that aberrant SCF complex expression and/or function have on cancer pathogenesis. Further characterization of the SCF complex functions is essential to identify and develop therapeutic approaches to exploit aberrant SCF complex expression and function.

F-Box protein 4 inhibits progression of papillary thyroid cancer

OBJECTIVE

We aimed to investigate the role of F-Box protein 4 (FBXO4) in the progression of papillary thyroid cancer (PTC) and to reveal the underlying signaling pathways responsible for FBXO4 action in PTC.

METHODS

FBXO4 expression was evaluated in tissues from PTC patients as well as in cell lines. Overexpression of FBXO4 was re-introduced into PTC cell line B-CPAP, followed by analysis of cell migration, invasion, apoptosis and epithelial-mesenchymal transition (EMT) marker profile. An in vivo xenograft tumor mouse model was employed to address the role of FBXO4 in tumorigenesis as well.

RESULTS

Endogenous FBXO4 was downregulated in PTC patient tissues and cell lines. Upon re-introducing its expression, FBXO4 suppressed migration and invasion and induced apoptosis of PTC cells, as well as inhibited EMT. Using a xenograft tumor mouse model, the pro-apoptotic and anti-EMT functions of FBXO4 are also validated in vivo, resulting in considerably slowed tumor growth rate of inoculated FBXO4-expressing PTC cells.

CONCLUSION

Our results therefore propose the potential therapeutic value of FBXO4 in targeted treatments against PTC.

Regulation of Normal and Malignant Hematopoiesis by FBOX Ubiquitin E3 Ligases

Hematopoiesis is responsible for numerous functions, ranging from oxygen transportation to host defense, to injury repair. This process of hematopoiesis is maintained throughout life by hematopoietic stem cells and requires a controlled balance between self-renewal, differentiation, and quiescence. Disrupting this balance can result in hematopoietic malignancies, including anemia, immune deficiency, leukemia, and lymphoma. Recent work has shown that FBOX E3 ligases, a substrate recognition component of the ubiquitin proteasome system (UPS), have an integral role in maintaining this balance. In this review, we detail how FBOX proteins target specific proteins for degradation to regulate hematopoiesis through cell processes, such as cell cycle, development, and apoptosis.

Fbxl8 suppresses lymphoma growth and hematopoietic transformation through degradation of cyclin D3

Overexpression of D-type cyclins in human cancer frequently occurs as a result of protein stabilization, emphasizing the importance of identification of the machinery that regulates their ubiqutin-dependent degradation. Cyclin D3 is overexpressed in ~50% of Burkitt’s lymphoma correlating with a mutation of Thr-283. However, the E3 ligase that regulates phosphorylated cyclin D3 and whether a stabilized, phosphorylation deficient mutant of cyclin D3, has oncogenic activity are undefined. We describe the identification of SCF-Fbxl8 as the E3 ligase for Thr-283 phosphorylated cyclin D3. SCF-Fbxl8 poly-ubiquitylates p-Thr-283 cyclin D3 targeting it to the proteasome. Functional investigation demonstrates that Fbxl8 antagonizes cell cycle progression, hematopoietic cell proliferation, and oncogene-induced transformation through degradation of cyclin D3, which is abolished by expression of cyclin D3T283A, a non-phosphorylatable mutant. Clinically, the expression of cyclin D3 is inversely correlated with the expression of Fbxl8 in lymphomas from human patients implicating Fbxl8 functions as a tumor suppressor.

Fbxl8 suppresses cell division, cell proliferation, and tumorigenesis through phosphorylation-dependent degradation of cyclin D3.

Fbxl8 suppresses oncogene-induced transformation of hematopoietic cells and lymphoma cell proliferation through cyclin D3 degradation.

F-Box Proteins and Cancer

Controlled protein degradation is essential for the operation of a variety of cellular processes including cell division, growth, and differentiation. Identification of the relations between ubiquitin ligases and their substrates is key to understanding the molecular basis of cancer development and to the discovery of novel targets for cancer therapeutics. F-box proteins function as the substrate recognition subunits of S-phase kinase-associated protein 1 (SKP1)−Cullin1 (CUL1)−F-box protein (SCF) ubiquitin ligase complexes. Here, we summarize the roles of specific F-box proteins that have been shown to function as tumor promoters or suppressors. We also highlight proto-oncoproteins that are targeted for ubiquitylation by multiple F-box proteins, and discuss how these F-box proteins are deployed to regulate their cognate substrates in various situations.

Cyclin D degradation by E3 ligases in cancer progression and treatment

D cyclins include three isoforms: D1, D2, and D3. D cyclins heterodimerize with cyclin-dependent kinase 4/6 (CDK4/6) to form kinase complexes that can phosphorylate and inactivate Rb. Inactivation of Rb triggers the activation of E2F transcription factors, which in turn regulate the expression of genes whose products drive cell cycle progression. Because D-type cyclins function as mitogenic sensors that link growth factor signaling directly with G₁ phase progression, it is not surprising that D cyclin accumulation is dysregulated in a variety of human tumors. Elevated expression of D cyclins results from gene amplification, increased gene transcription and protein translation, decreased microRNA levels, and inefficiency or loss of ubiquitylation-mediated protein degradation. This review focuses on the clinicopathological importance of D cyclins, how dysregulation of Ubiquitin-Proteasome System (UPS) contributes to the overexpression of D cyclins, and the therapeutic potential through targeting D cyclin-related machinery in human tumors.

Knockout Mouse Models Provide Insight into the Biological Functions of CRL1 Components

The CRL1 complex, also known as the SCF complex, is a ubiquitin ligase that in mammals consists of an adaptor protein (SKP1), a scaffold protein (CUL1), a RING finger protein (RBX1, also known as ROC1), and one of about 70 F-box proteins. Given that the F-box proteins determine the substrate specificity of the CRL1 complex, the variety of these proteins allows the generation of a large number of ubiquitin ligases that promote the degradation or regulate the function of many substrate proteins and thereby control numerous key cellular processes. The physiological and pathological functions of these many CRL1 ubiquitin ligases have been studied by the generation and characterization of knockout mouse models that lack specific CRL1 components. In this chapter, we provide a comprehensive overview of these mouse models and discuss the role of each CRL1 component in mouse physiology and pathology.

The double dealing of cyclin D1

The cell cycle is tightly regulated by cyclins and their catalytic moieties, the cyclin-dependent kinases (CDKs). Cyclin D1, in association with CDK4/6, acts as a mitogenic sensor and integrates extracellular mitogenic signals and cell cycle progression. When deregulated (overexpressed, accumulated, inappropriately located), cyclin D1 becomes an oncogene and is recognized as a driver of solid tumors and hemopathies. Recent studies on the oncogenic roles of cyclin D1 reported non-canonical functions dependent on the partners of cyclin D1 and its location within tumor cells or tissues. Support for these new functions was provided by various mouse models of oncogenesis. Finally, proteomic and transcriptomic data identified complex cyclin D1 networks. This review focuses on these aspects of cyclin D1 pathophysiology, which may be crucial for targeted therapy.Abbreviations: aa, amino acid; AR, androgen receptor; ATM, ataxia telangectasia mutant; ATR, ATM and Rad3-related; CDK, cyclin-dependent kinase; ChREBP, carbohydrate response element binding protein; CIP, CDK-interacting protein; CHK1/2, checkpoint kinase 1/2; CKI, CDK inhibitor; DDR, DNA damage response; DMP1, cyclin D-binding myb-like protein; DSB, double-strand DNA break; DNA-PK, DNA-dependent protein kinase; ER, estrogen receptor; FASN, fatty acid synthase; GSK3β, glycogen synthase-3β; HAT, histone acetyltransferase; HDAC, histone deacetylase; HK2, hexokinase 2; HNF4α, and hepatocyte nuclear factor 4α; HR, homologous recombination; IR, ionizing radiation; KIP, kinase inhibitory protein; MCL, mantle cell lymphoma; NHEJ, non-homologous end-joining; PCAF, p300/CREB binding-associated protein; PGC1α, PPARγ co-activator 1α; PEST, proline-glutamic acid-serine-threonine, PK, pyruvate kinase; PPAR, peroxisome proliferator-activated receptor; RB1, retinoblastoma protein; ROS, reactive oxygen species; SRC, steroid receptor coactivator; STAT, signal transducer and activator of transcription; TGFβ, transforming growth factor β; UPS, ubiquitin-proteasome system; USP22, ubiquitin-specific peptidase 22; XPO1 (or CRM1) exportin 1.

Loss of FBXO9 Enhances Proteasome Activity and Promotes Aggressiveness in Acute Myeloid Leukemia

The hematopoietic system is maintained throughout life by stem cells that are capable of differentiating into all hematopoietic lineages. An intimate balance between self-renewal, differentiation, and quiescence is required to maintain hematopoiesis and disruption of this balance can result in malignant transformation. FBXO9, the substrate recognition component from the SCF E3 ubiquitin ligase family, is downregulated in patients with acute myeloid leukemia (AML) compared to healthy bone marrow, and this downregulation is particularly evident in patients with inv(16) AML. To study FBXO9 in malignant hematopoiesis, we generated a conditional knockout mouse model using a novel CRISPR/Cas9 strategy. Deletion of Fbxo9 in the murine hematopoietic system showed no adverse effects on stem and progenitor cell function but in AML lead to markedly accelerated and aggressive leukemia development in mice with inv(16). Not only did Fbxo9 play a role in leukemia initiation but it also functioned to maintain AML activity and promote disease progression. Quantitative mass spectrometry from primary tumors reveals tumors lacking Fbxo9 highly express proteins associated with metastasis and invasion as well as components of the ubiquitin proteasome system. We confirmed that the loss of FBXO9 leads to increased proteasome activity and tumors cells were more sensitive to in vitro proteasome inhibition with bortezomib, suggesting that FBXO9 expression may predict patients’ response to bortezomib.

Prognostic values of F-box members in breast cancer: an online database analysis and literature review

Introduction: F-box proteins are the substrate-recognizing subunits of SKP1 (S-phase kinase-associated protein 1)–cullin1–F-box protein (SCF) E3 ligase complexes that play pivotal roles in multiple cellular processes, including cell proliferation, apoptosis, angiogenesis, invasion, and metastasis. Dysregulation of F-box proteins may lead to an unbalanced proteolysis of numerous protein substrates, contributing to progression of human malignancies. However, the prognostic values of F-box members, especially at mRNA levels, in breast cancer (BC) are elusive. Methods: An online database, which is constructed based on the gene expression data and survival information downloaded from GEO (http://www.ncbi.nlm.nih.gov/geo/), was used to investigate the prognostic values of 15 members of F-box mRNA expression in BC. Results: We found that higher mRNA expression levels of FBXO1, FBXO31, SKP2, and FBXO5 were significantly associated with worse prognosis for BC patients. While FBXO4 and β-TrCP1 were found to be correlated to better overall survival (OS). Conclusion: The associated results provide new insights into F-box members in the development and progression of BC. Further researches to explore the F-box protein-targetting reagents for treating BC are needed.

Fbxo4-mediated degradation of Fxr1 suppresses tumorigenesis in head and neck squamous cell carcinoma

The Fbxo4 tumour suppressor is a component of an Skp1-Cul1-F-box E3 ligase for which two substrates are known. Here we show purification of SCFFbxo4 complexes results in the identification of fragile X protein family (FMRP, Fxr1 and Fxr2) as binding partners. Biochemical and functional analyses reveal that Fxr1 is a direct substrate of SCFFbxo4. Consistent with a substrate relationship, Fxr1 is overexpressed in Fbxo4 knockout cells, tissues and in human cancer cells, harbouring inactivating Fbxo4 mutations. Critically, in head and neck squamous cell carcinoma, Fxr1 overexpression correlates with reduced Fbxo4 levels in the absence of mutations or loss of mRNA, suggesting the potential for feedback regulation. Direct analysis reveals that Fbxo4 translation is attenuated by Fxr1, indicating the existence of a feedback loop that contributes to Fxr1 overexpression and the loss of Fbxo4. Ultimately, the consequence of Fxr1 overexpression is the bypass of senescence and neoplastic progression.

FBXO4 inhibits lung cancer cell survival by targeting Mcl-1 for degradation

Mcl-1 (myeloid cell leukemia 1) is a prosurvival member of the Bcl-2 family and plays a critical role in cell survival by suppressing apoptosis through inhibiting the activity of proapoptotic proteins. It has been reported that Mcl-1 is frequently overexpressed in lung cancer. However, the exact molecular mechanism underlying Mcl-1 elevation in lung cancer is largely unknown. Here, we reported that Mcl-1 protein levels inversely correlate with FBXO4 expression, but not other F-box proteins examined, in lung cancer cell lines and lung cancer patient samples. Mechanically, FBXO4 is the E3 ubiquitin ligase to interact with and promote Mcl-1 ubiquitination and degradation. As a result, knockdown of Fbxo4 dramatically elevates Mcl-1 protein levels and increases cell survival and resistance to chemotherapeutic drugs, whereas ectopic expression of FBXO4 displays opposite phenotypes. Therefore, our study suggests that the protein stability of Mcl-1 is governed by FBXO4, which plays an important role in cell survival and chemotherapy for lung cancer.

Cyclin D1, cancer progression, and opportunities in cancer treatment

Mammalian cells encode three D cyclins (D1, D2, and D3) that coordinately function as allosteric regulators of cyclin-dependent kinase 4 (CDK4) and CDK6 to regulate cell cycle transition from G1 to S phase. Cyclin expression, accumulation, and degradation, as well as assembly and activation of CDK4/CDK6 are governed by growth factor stimulation. Cyclin D1 is more frequently dysregulated than cyclin D2 or D3 in human cancers, and as such, it has been more extensively characterized. Overexpression of cyclin D1 results in dysregulated CDK activity, rapid cell growth under conditions of restricted mitogenic signaling, bypass of key cellular checkpoints, and ultimately, neoplastic growth. This review discusses cyclin D1 transcriptional, translational, and post-translational regulations and its biological function with a particular focus on the mechanisms that result in its dysregulation in human cancers.

Skp1: Implications in cancer and SCF-oriented anti-cancer drug discovery

In the last decade, the ubiquitin proteasome system (UPS), in general, and E3 ubiquitin ligases, in particular, have emerged as valid drug targets for the development of novel anti-cancer therapeutics. Cullin RING Ligases (CRLs), which can be classified into eight groups (CRL1-8) and comprise approximately 200 members, represent the largest family of E3 ubiquitin ligases which facilitate the ubiquitination-derived proteasomal degradation of a myriad of functionally and structurally diverse substrates. S phase kinase-associated protein 1 (Skp1)-Cullin1-F-Box protein (SCF) complexes are the best characterized among CRLs, which play crucial roles in numerous cellular processes and physiological dysfunctions, such as in cancer biology. Currently, there is growing interest in developing SCF-targeting anti-cancer therapies for clinical application. Indeed, the research in this field has seen some progress in the form of cullin neddylation- and Skp2-inhibitors. However, it still remains an underdeveloped area and needs to design new strategies for developing improved form of therapy. In this review, we venture a novel strategy that rational pharmacological targeting of Skp1, a central regulator of SCF complexes, may provide a novel avenue for SCF-oriented anti-cancer therapy, expected: (i) to simultaneously address the critical roles that multiple SCF oncogenic complexes play in cancer biology, (ii) to selectively target cancer cells with minimal normal cell toxicity, and (iii) to offer multiple chemical series, via therapeutic interventions at the Skp1 binding interfaces in SCF complex, thereby maximizing chances of success for drug discovery. In addition, we also discuss the challenges that might be posed regarding rational pharmacological interventions against Skp1.

FBXO4 loss facilitates carcinogen induced papilloma development in mice

Cyclin D1 is frequently overexpressed in esophageal squamous cell carcinoma (ESCC) and is considered a key driver of this disease. Mutations in FBXO4, F-box specificity factor that directs SCF-mediated ubiquitylation of cyclin D1, occur in ESCC with concurrent overexpression of cyclin D1 suggesting a potential tumor suppressor role for FBXO4. To evaluate the contribution of FBXO4-dependent regulation cyclin D1 in esophageal squamous cell homeostasis, we exposed FBXO4 knockout mice to N-nitrosomethylbenzylamine (NMBA), an esophageal carcinogen. Our results revealed that loss of FBXO4 function facilitates NMBA induced papillomas in FBXO4 het (+/-) and null (-/-) mice both by numbers and sizes 11 months after single dose NMBA treatment at 2mg/kg by gavage when compared to that in wt (+/+) mice (P < 0.01). No significant difference was noted between heterozygous or nullizygous mice consistent with previous work. To assess cyclin D1/CDK4 dependence, mice were treated with the CDK4/6 specific inhibitor, PD0332991, for 4 weeks. PD0332991 treatment (150mg/kg daily), reduced tumor size and tumor number. Collectively, our data support a role for FBXO4 as a suppressor of esophageal tumorigenesis.

Involvement of F-BOX proteins in progression and development of human malignancies

The Ubiquitin Proteasome System (UPS) is a core regulator with various protein components (ubiquitin-activating E1 enzymes, ubiquitin-conjugating E2 enzymes, ubiquitin-protein E3 ligases, and the 26S proteasome) which work together in a coordinated fashion to ensure the appropriate and efficient proteolysis of target substrates. E3 ubiquitin ligases are essential components of the UPS machinery, working with E1 and E2 enzymes to bind substrates and assist the transport of ubiquitin molecules onto the target protein. As the UPS controls the degradation of several oncogenes and tumor suppressors, dysregulation of this pathway leads to several human malignancies. A major category of E3 Ub ligases, the SCF (Skp-Cullin-F-box) complex, is composed of four principal components: Skp1, Cul1/Cdc53, Roc1/Rbx1/Hrt1, and an F-box protein (FBP). FBPs are the substrate recognition components of SCF complexes and function as adaptors that bring substrates into physical proximity with the rest of the SCF. Besides acting as a component of SCF complexes, FBPs are involved in DNA replication, transcription, cell differentiation and cell death. This review will highlight the recent literature on three well characterized FBPs SKP2, Fbw7, and beta-TRCP. In particular, we will focus on the involvement of these deregulated FBPs in the progression and development of various human cancers. We will also highlight some novel substrates recently identified for these FBPs.

MicroRNA regulation of F-box proteins and its role in cancer

MicroRNAs (miRNAs) are small endogenous non-coding RNAs, which play critical roles in cancer development by suppressing gene expression at the post-transcriptional level. In general, oncogenic miRNAs are upregulated in cancer, while miRNAs that act as tumor suppressors are downregulated, leading to decreased expression of tumor suppressors and upregulated oncogene expression, respectively. F-box proteins function as the substrate-recognition components of the SKP1-CUL1-F-box (SCF)-ubiquitin ligase complex for the degradation of their protein targets by the ubiquitin-proteasome system. Therefore F-box proteins and miRNAs both negatively regulate target gene expression post-transcriptionally. Since each miRNA is capable of fine-tuning the expression of multiple target genes, multiple F-box proteins may be suppressed by the same miRNA. Meanwhile, one F-box proteins could be regulated by several miRNAs in different cancer types. In this review, we will focus on miRNA-mediated downregulation of various F-box proteins, the resulting stabilization of F-box protein substrates and the impact of these processes on human malignancies. We provide insight into how the miRNA: F-box protein axis may regulate cancer progression and metastasis. We also consider the broader role of F-box proteins in the regulation of pathways that are independent of the ubiquitin ligase complex and how that impacts on oncogenesis. The area of miRNAs and the F-box proteins that they regulate in cancer is an emerging field and will inform new strategies in cancer treatment.

Deregulation of F-box proteins and its consequence on cancer development, progression and metastasis

F-box proteins are substrate receptors of the SCF (SKP1-Cullin 1-F-box protein) E3 ubiquitin ligase that play important roles in a number of physiological processes and activities. Through their ability to assemble distinct E3 ubiquitin ligases and target key regulators of cellular activities for ubiquitylation and degradation, this versatile group of proteins is able to regulate the abundance of cellular proteins whose deregulated expression or activity contributes to disease. In this review, we describe the important roles of select F-box proteins in regulating cellular activities, the perturbation of which contributes to the initiation and progression of a number of human malignancies.

Early B-cell-specific inactivation of ATM synergizes with ectopic CyclinD1 expression to promote pre-germinal center B-cell lymphomas in mice

Ataxia telangiectasia-mutated (ATM) kinase is a master regulator of the DNA damage response. ATM is frequently inactivated in human B-cell non-Hodgkin lymphomas, including ~50% of mantle cell lymphomas (MCLs) characterized by ectopic expression of CyclinD1. Here we report that early and robust deletion of ATM in precursor/progenitor B cells causes cell autonomous, clonal mature B-cell lymphomas of both pre- and post-germinal center (GC) origins. Unexpectedly, naive B-cell-specific deletion of ATM is not sufficient to induce lymphomas in mice, highlighting the important tumor suppressor function of ATM in immature B cells. Although EμCyclinD1 is not sufficient to induce lymphomas, EμCyclinD1 accelerates the kinetics and increases the incidence of clonal lymphomas in ATM-deficient B-cells and skews the lymphomas toward pre-GC-derived small lymphocytic neoplasms, sharing morphological features of human MCL. This is in part due to CyclinD1-driven expansion of ATM-deficient naive B cells with genomic instability, which promotes the deletions of additional tumor suppressor genes (i.e. Trp53, Mll2, Rb1 and Cdkn2a). Together these findings define a synergistic function of ATM and CyclinD1 in pre-GC B-cell proliferation and lymphomagenesis and provide a prototypic animal model to study the pathogenesis of human MCL.

Significance of FBX8 in progression of gastric cancer

F-box only protein 8 (FBX8), a novel component of F-box proteins, has recently been observed in several malignancies. However, its clinical implication in the progression of gastric cancer still remains unclear. The aim of this study was to explore the role of FBX8 in gastric cancer (GC) and analyze its correlation with tumor progression and prognosis. The expression of FBX8 in GC cell lines and matched pairs of fresh gastric cancer tissues were detected by real-time RT-PCR and Western blotting. Immunohistochemistry was used to analyze clinicopathological patterns of FBX8 in 136 cases of clinical paraffin-embedded GC tissues. A series of functional assays were conducted to evaluate the effect of FBX8 on proliferation and invasion in vitro and metastasis in vivo. FBX8 was markedly down-regulated in GC tissues compared to adjacent normal tissues. Patients with low FBX8 had shorter overall survival time and poor prognosis. Knocking down FBX8 obviously promoted proliferation and invasion in BGC823 cells, while over-expression of FBX8 in SGC7901 and AGS cells had the opposite effects. Moreover, FBX8 was sufficient to suppress metastasis in nude mice. Down-regulation of FBX8 significantly correlates with invasion, metastasis and poor survival time in GC patients. FBX8 may serve as a promising therapeutic target for inhibition of GC metastasis.

Dual regulation of Fbw7 function and oncogenic transformation by Usp28

Fbw7, the substrate recognition subunit of SCF(Fbw7) ubiquitin ligase, mediates the turnover of multiple proto-oncoproteins and promotes its own degradation. Fbw7-dependent substrate ubiquitination is antagonized by the Usp28 deubiquitinase. Here, we show that Usp28 preferentially antagonizes autocatalytic ubiquitination and stabilizes Fbw7, resulting in dose-dependent effects in Usp28 knockout mice. Monoallelic deletion of Usp28 maintains stable Fbw7 but drives Fbw7 substrate degradation. In contrast, complete knockout triggers Fbw7 degradation and leads to the accumulation of Fbw7 substrates in several tissues and embryonic fibroblasts. On the other hand, overexpression of Usp28 stabilizes both Fbw7 and its substrates. Consequently, both complete loss and ectopic expression of Usp28 promote Ras-driven oncogenic transformation. We propose that dual regulation of Fbw7 activity by Usp28 is a safeguard mechanism for maintaining physiological levels of proto-oncogenic Fbw7 substrates, which is equivalently disrupted by loss or overexpression of Usp28.

Expression and significance of Pin1, β-catenin and cyclin D1 in hepatocellular carcinoma

The aim of the present study was to examine the expression and significance of peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (Pin1), β‑catenin and cyclin D1 in hepatocellular carcinoma (HCC). A total of 24 samples of HCC and adjacent normal tissues were analyzed. The expression of Pin1, β‑catenin and cyclin D1 in HCC were detected using immunohistochemistry, western blot analysis, polymerase chain reaction and immunofluorescence. The expression of Pin1, β‑catenin and cyclin D1 in HCC tissues were significantly higher than that in adjacent tissues. Pin1 was not markedly expressed in the adjacent normal tissues, while expression in the cytoplasm and nucleus of HCC cells was high. However, β‑catenin and cyclin D1 only revealed a weak expression in the cytoplasm and nucleus of HCC cells. Immunoprecipitation analyses demonstrated two clear bands at 19 and 34 kDa, and a brown band at 55 kDa as expected. Immunofluorescence analysis of HCC cells indicated that Pin1 was present in the cytoplasm and nucleus, and β‑catenin and cyclin D1 were present in the nucleus. In conclusion, the present study indicated that Pin1, β‑catenin and cyclin D1 were highly expressed in HCC. Therefore, detection of the expression of Pin1, β‑catenin and cyclin D1 may be useful for the development of novel diagnostic and treatment strategies for HCC.

Inhibition of chemokine (C-C motif) receptor 7 sialylation suppresses CCL19-stimulated proliferation, invasion and anti-anoikis

Chemokine (C-C motif) receptor 7 (CCR7) is involved in lymph-node homing of naive and regulatory T cells and lymphatic metastasis of cancer cells. Sialic acids comprise a group of monosaccharide units that are added to the terminal position of the oligosaccharide chain of glycoproteins by sialyation. Recent studies suggest that aberrant sialylation of receptor proteins contributes to proliferation, motility, and drug resistance of cancer cells. In this study, we addressed whether CCR7 is a sialylated receptor protein and tried to elucidate the effect of sialylation in the regulation of signal transduction and biological function of CCR7. Our results demonstrated that α-2, 3-sialyltransferase which catalyze sialylation reaction in vivo was overexpressed in breast tumor tissues and cell lines. Lectin blot analysis clearly demonstrated that CCR7 receptor was sialyated in breast cancer cells. Chemokine (C-C motif) ligand 19 (CCL19), the cognate ligand for CCR7, induced the activation of extracellular signal-regulated kinase (ERK) and AKT signaling and increased the expression of cell cycle regulatory proteins and proliferation of breast cancer cells. When cells were pre-treated with a sialyltransferase inhibitor AL10 or sialidase, CCL19-induced cell growth was significantly suppressed. CCL19 also increased invasion and prevented anoikis by up-regulating pro-survival proteins Bcl-2 and Bcl-xL. Inhibition of sialylation by AL10 totally abolished these effects. Finally, we showed that AL10 inhibited tumorigenicity of breast cancer in experimental animals. Taken together, we demonstrate for the first time that CCR7 receptor is a sialylated protein and sialylation is important for the paracrine stimulation by its endogenous ligand CCL19. In addition, inhibition of aberrant sialylation of CCR7 suppresses proliferation and invasion and triggers anoikis in breast cancer cells. Targeting of sialylation enzymes may be a novel strategy for breast cancer treatment.

Pan-cancer genetic analysis identifies PARK2 as a master regulator of G1/S cyclins

Coordinate control of different classes of cyclins is fundamentally important for cell cycle regulation and tumor suppression, yet the underlying mechanisms are incompletely understood. Here we show that the PARK2 tumor suppressor mediates this coordination. The PARK2 E3 ubiquitin ligase coordinately controls the stability of both cyclin D and cyclin E. Analysis of approximately 5,000 tumor genomes shows that PARK2 is a very frequently deleted gene in human cancer and uncovers a striking pattern of mutual exclusivity between PARK2 deletion and amplification of CCND1, CCNE1 or CDK4-implicating these genes in a common pathway. Inactivation of PARK2 results in the accumulation of cyclin D and acceleration of cell cycle progression. Furthermore, PARK2 is a component of a new class of cullin-RING-containing ubiquitin ligases targeting both cyclin D and cyclin E for degradation. Thus, PARK2 regulates cyclin-CDK complexes, as does the CDK inhibitor p16, but acts as a master regulator of the stability of G1/S cyclins.

Roles of F-box proteins in cancer

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

The FBXO4 tumor suppressor functions as a barrier to BRAFV600E-dependent metastatic melanoma

Cyclin D1-cyclin-dependent kinase 4/6 (CDK4/6) dysregulation is a major contributor to melanomagenesis. Clinical evidence has revealed that p16(INK4A), an allosteric inhibitor of CDK4/6, is inactivated in over half of human melanomas, and numerous animal models have demonstrated that p16(INK4A) deletion promotes melanoma. FBXO4, a specificity factor for the E3 ligase that directs timely cyclin D1 proteolysis, has not been studied in melanoma. We demonstrate that Fbxo4 deficiency induces Braf-driven melanoma and that this phenotype depends on cyclin D1 accumulation in mice, underscoring the importance of this ubiquitin ligase in tumor suppression. Furthermore, we have identified a substrate-binding mutation, FBXO4 I377M, that selectively disrupts cyclin D1 degradation while preserving proteolysis of the other known FBXO4 substrate, TRF1. The I377M mutation and Fbxo4 deficiency result in nuclear accumulation of cyclin D1, a key transforming neoplastic event. Collectively, these data provide evidence that FBXO4 dysfunction, as a mechanism for cyclin D1 overexpression, is a contributor to human malignancy.

FBX8 Acts as an Invasion and Metastasis Suppressor and Correlates with Poor Survival in Hepatocellular Carcinoma

Background

F-box only protein 8 (FBX8), a novel component of F-box proteins, is lost in several cancers and has been associated with invasiveness of cancer cells. However, its expression pattern and role in the progression of hepatocellular carcinoma remain unclear. This study investigated the prognostic significance of FBX8 in hepatocellular carcinoma samples and analyzed FBX8 function in hepatocellular carcinoma cells by gene manipulation.

Methodology

The expression of FBX8 was detected in 120 cases of clinical paraffin-embedded hepatocellular carcinoma tissues, 20 matched pairs of fresh tissues and five hepatocellular carcinoma cell lines by immunohistochemistry with clinicopathological analyses, real-time RT-PCR or Western blot. The correlation of FBX8 expression with cell proliferation and invasion in five HCC cell lines was analyzed. Moreover, loss of function and gain of function assays were performed to evaluate the effect of FBX8 on cell proliferation, motility, invasion in vitro and metastasis in vivo.

Conclusions

We found that FBX8 was obviously down-regulated in HCC tissues and cell lines (P<0.05). The FBX8 down-regulation correlated significantly with poor prognosis, and FBX8 status was identified as an independent significant prognostic factor. Over-expression of FBX8 decreased proliferation, migration and invasion in HepG2 and 97H cells, while knock-down of FBX8 in 7721 cells showed the opposite effect. FBX8 negatively correlated with cell proliferation and invasion in 7701, M3, HepG2 and 97H cell lines. In vivo functional assays showed FBX8 suppressed tumor growth and pulmonary metastatic potential in mice. Our results indicate that down-regulation of FBX8 significantly correlates with invasion, metastasis and poor survival in hepatocellular carcinoma patients. It may be a useful biomarker for therapeutic strategy and control in hepatocellular carcinoma treatment.

Flipping the switch from g1 to s phase with e3 ubiquitin ligases

The cell cycle ensures genome maintenance by coordinating the processes of DNA replication and chromosome segregation. Of particular importance is the irreversible transition from the G1 phase of the cell cycle to S phase. This transition marks the switch from preparing chromosomes for replication ("origin licensing") to active DNA synthesis ("origin firing"). Ubiquitin-mediated proteolysis is essential for restricting DNA replication to only once per cell cycle and is the major mechanism regulating the G1 to S phase transition. Although some changes in protein levels are attributable to regulated mRNA abundance, protein degradation elicits very rapid changes in protein abundance and is critical for the sharp and irreversible transition from one cell cycle stage to the next. Not surprisingly, regulation of the G1-to-S phase transition is perturbed in most cancer cells, and deregulation of key molecular events in G1 and S phase drives not only cell proliferation but also genome instability. In this review we focus on the mechanisms by which E3 ubiquitin ligases control the irreversible transition from G1 to S phase in mammalian cells.

SCFs in the new millennium

Substrate-specific degradation is a key feature of the ubiquitin proteasome system. Substrate specificity is typically directed by the E3 or ubiquitin ligase; such specificity can be conferred either by ligase modification or expression or conversely via modification of substrates that permit their recognition by a specific E3 ligase. The most well-known example of such complexes are the Cullin-RING ligases (CRLs). CRLs are composed of one of seven cullin-family scaffold proteins; the CRL serves as a scaffold that interacts directly with a RING-domain enzyme (Rbx1/2) through an extensive protein-protein interface within the globular C-terminal domain. At the N terminus, the cullin associates with an adaptor protein through cullin-repeat motifs. This adaptor, in turn, facilitates recruitment of a substrate-specifying factor that recruits the target to be ubiquitylated. The prototypical CRL is the cul1-containing complex, commonly referred to as the Skp1-Cul1-Fbox (SCF) ligase. SCF ligases contribute to the timely destruction of numerous substrates thereby ensuring normal cell growth. The importance of SCF function is highlighted by cancer-specific alterations in either the expression or the function of select F-box substrate-specific adaptors that results in neoplastic conversion. Herein, we discuss the current understanding of SCF function and contribution to cell biology.

Genetically engineered mouse models for functional studies of SKP1-CUL1-F-box-protein (SCF) E3 ubiquitin ligases

The SCF (SKP1 (S-phase-kinase-associated protein 1), Cullin-1, F-box protein) E3 ubiquitin ligases, the founding member of Cullin-RING ligases (CRLs), are the largest family of E3 ubiquitin ligases in mammals. Each individual SCF E3 ligase consists of one adaptor protein SKP1, one scaffold protein cullin-1 (the first family member of the eight cullins), one F-box protein out of 69 family members, and one out of two RING (Really Interesting New Gene) family proteins RBX1/ROC1 or RBX2/ROC2/SAG/RNF7. Various combinations of these four components construct a large number of SCF E3s that promote the degradation of many key regulatory proteins in cell-context, temporally, and spatially dependent manners, thus controlling precisely numerous important cellular processes, including cell cycle progression, apoptosis, gene transcription, signal transduction, DNA replication, maintenance of genome integrity, and tumorigenesis. To understand how the SCF E3 ligases regulate these cellular processes and embryonic development under in vivo physiological conditions, a number of mouse models with transgenic (Tg) expression or targeted deletion of components of SCF have been established and characterized. In this review, we will provide a brief introduction to the ubiquitin-proteasome system (UPS) and the SCF E3 ubiquitin ligases, followed by a comprehensive overview on the existing Tg and knockout (KO) mouse models of the SCF E3s, and discuss the role of each component in mouse embryogenesis, cell proliferation, apoptosis, carcinogenesis, as well as other pathogenic processes associated with human diseases. We will end with a brief discussion on the future directions of this research area and the potential applications of the knowledge gained to more effective therapeutic interventions of human diseases.

miR-211 is a prosurvival microRNA that regulates chop expression in a PERK-dependent manner

MicroRNAs typically function at the level of posttranscriptional gene silencing within the cytoplasm; however, increasing evidence suggests that they may also function in nuclear, Argonaut-containing complexes, to directly repress target gene transcription. We have investigated the role of microRNAs in mediating endoplasmic reticulum (ER) stress responses. ER stress triggers the activation of three signaling molecules: Ire-1α/β, PERK, and ATF6, whose function is to facilitate adaption to the ensuing stress. We demonstrate that PERK induces miR-211, which in turn attenuates stress-dependent expression of the proapoptotic transcription factor chop/gadd153. MiR-211 directly targets the proximal chop/gadd153 promoter, where it increases histone methylation and represses chop expression. Maximal chop accumulation ultimately correlates with miR-211 downregulation. Our data suggest a model in which PERK-dependent miR-211 induction prevents premature chop accumulation and thereby provides a window of opportunity for the cell to re-establish homeostasis prior to apoptotic commitment.

CRL Ubiquitin Ligases and DNA Damage Response

Cullin/RING ubiquitin ligases (CRL) comprise the largest subfamily of ubiquitin ligases. CRLs are involved in cell cycle regulation, DNA replication, DNA damage response (DDR), development, immune response, transcriptional regulation, circadian rhythm, viral infection, and protein quality control. One of the main functions of CRLs is to regulate the DDR, a fundamental signaling cascade that maintains genome integrity. In this review, we will discuss the regulation of CRL ubiquitin ligases and their roles in control of the DDR.

SCF ubiquitin ligases in the maintenance of genome stability

In response to genotoxic stress, eukaryotic cells activate the DNA damage response (DDR), a series of pathways that coordinate cell cycle arrest and DNA repair to prevent deleterious mutations. In addition, cells possess checkpoint mechanisms that prevent aneuploidy by regulating the number of centrosomes and spindle assembly. Among these mechanisms, ubiquitin-mediated degradation of key proteins has an important role in the regulation of the DDR, centrosome duplication and chromosome segregation. This review discusses the functions of a group of ubiquitin ligases, the SCF (SKP1-CUL1-F-box protein) family, in the maintenance of genome stability. Given that general proteasome inhibitors are currently used as anticancer agents, a better understanding of the ubiquitylation of specific targets by specific ubiquitin ligases may result in improved cancer therapeutics.