In vivo interference with Skp1 function leads to genetic instability and neoplastic transformation

Transcriptomic Analysis of Hormone Signal Transduction, Carbohydrate Metabolism, Heat Shock Proteins, and SCF Complexes before and after Fertilization of Korean Pine Ovules

The fertilization process is a critical step in plant reproduction. However, the mechanism of action and mode of regulation of the fertilization process in gymnosperms remain unclear. In this study, we investigated the molecular regulatory networks involved in the fertilization process in Korean pine ovules through anatomical observation, physiological and biochemical assays, and transcriptome sequencing technology. The morphological and physiological results indicated that fertilization proceeds through the demise of the proteinaceous vacuole, egg cell division, and pollen tube elongation. Auxin, cytokinin, soluble sugar, and soluble starch contents begin to decline upon fertilization. Transcriptomic data analysis revealed a large number of differentially expressed genes at different times before and after fertilization. These genes were primarily involved in pathways associated with plant hormone signal transduction, protein processing in the endoplasmic reticulum, fructose metabolism, and mannose metabolism. The expression levels of several key genes were further confirmed by qRT-PCR. These findings represent an important step towards understanding the mechanisms underlying morphological changes in the Korean pine ovule during fertilization, and the physiological and transcriptional analyses lay a foundation for in-depth studies of the molecular regulatory network of the Korean pine fertilization process.

A small-molecule Skp1 inhibitor elicits cell death by p53-dependent mechanism

Skp1 overexpression promotes tumor growth, whereas reduced Skp1 activity is also linked with genomic instability and neoplastic transformation. This highlights the need to gain better understanding of Skp1 biology in cancer settings. To this context, potent and cellularly active small-molecule Skp1 inhibitors may be of great value. Using a hypothesis-driven, structure-guided approach, we herein identify Z0933M as a potent Skp1 inhibitor with K_D ∼0.054 μM. Z0933M occupies a hydrophobic hotspot (P1) – encompassing an aromatic cage of two phenylalanines (F101 and F139) – alongside C-terminal extension of Skp1 and, thus, hampers its ability to interact with F-box proteins, a prerequisite step to constitute intact and active SCF E3 ligase(s) complexes. In cellulo, Z0933M disrupted SCF E3 ligase(s) functioning, recapitulated previously reported effects of Skp1-reduced activity, and elicited cell death by a p53-dependent mechanism. We propose Z0933M as valuable tool for future efforts toward probing Skp1 cancer biology, with implications for cancer therapy.

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Z0933M manifests strong binding with Skp1 and inhibits Skp1-F-box PPIs

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Z0933M interacts with a P1 hotspot alongside C-terminal extension of Skp1

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Z0933M alters SCF E3 ligase functioning, leading to substrate accumulation/modulation

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Z0933M causes cell-cycle arrest, and elicits cell death by p53-dependent mechanism

Aberrant SKP1 Expression: Diverse Mechanisms Impacting Genome and Chromosome Stability

The S-phase Kinase-Associated Protein 1 (SKP1) is a core component of the SKP1, Cullin 1, F-box protein (SCF) complex, an E3 ubiquitin ligase that serves to poly-ubiquitinate a vast array of protein targets as a signal for their proteasomal degradation, thereby playing a critical role in the regulation of downstream biological processes. Many of the proteins regulated by SKP1 and the SCF complex normally function within pathways that are essential for maintaining genome stability, including DNA damage repair, apoptotic signaling, and centrosome dynamics. Accordingly, aberrant SKP1 and SCF complex expression and function is expected to disrupt these essential pathways, which may have pathological implications in diseases like cancer. In this review, we summarize the central role SKP1 plays in regulating essential cellular processes; we describe functional models in which SKP1 expression is altered and the corresponding impacts on genome stability; and we discuss the prevalence of SKP1 somatic copy number alterations, mutations, and altered protein expression across different cancer types, to identify a potential link between SKP1 and SCF complex dysfunction to chromosome/genome instability and cancer pathogenesis. Ultimately, understanding the role of SKP1 in driving chromosome instability will expand upon our rudimentary understanding of the key events required for genome/chromosome stability that may aid in our understanding of cancer pathogenesis, which will be critical for future studies to establish whether SKP1 may be useful as prognostic indicator or as a therapeutic target.

The significance of CUX1 and chromosome 7 in myeloid malignancies

PURPOSE OF REVIEW

Loss of chromosome 7 has long been associated with adverse-risk myeloid malignancy. In the last decade, CUX1 has been identified as a critical tumor suppressor gene (TSG) located within a commonly deleted segment of chromosome arm 7q. Additional genes encoded on 7q have also been identified as bona fide myeloid tumor suppressors, further implicating chromosome 7 deletions in disease pathogenesis. This review will discuss the clinical implications of del(7q) and CUX1 mutations, both in disease and clonal hematopoiesis, and synthesize recent literature on CUX1 and other chromosome 7 TSGs.

RECENT FINDINGS

Two major studies, including a new mouse model, have been published that support a role for CUX1 inactivation in the development of myeloid neoplasms. Additional recent studies describe the cellular and hematopoietic effects from loss of the 7q genes LUC7L2 and KMT2C/MLL3, and the implications of chromosome 7 deletions in clonal hematopoiesis.

SUMMARY

Mounting evidence supports CUX1 as being a key chromosome 7 TSG. As 7q encodes additional myeloid regulators and tumor suppressors, improved models of chromosome loss are needed to interrogate combinatorial loss of these critical 7q genes.

SCF Ligases and Their Functions in Oogenesis and Embryogenesis-Summary of the Most Important Findings throughout the Animal Kingdom

SCF-dependent proteolysis was first discovered via genetic screening of budding yeast almost 25 years ago. In recent years, more and more functions of SCF (Skp1-Cullin 1-F-box) ligases have been described, and we can expect the number of studies on this topic to increase. SCF ligases, which are E3 ubiquitin multi-protein enzymes, catalyse protein ubiquitination and thus allow protein degradation mediated by the 26S proteasome. They play a crucial role in the degradation of cell cycle regulators, regulation of the DNA repair and centrosome cycle and play an important role in several diseases. SCF ligases seem to be needed during all phases of development, from oocyte formation through fertilization, activation of the embryonic genome to embryo implantation. In this review, we summarize known data on SCF ligase-mediated degradation during oogenesis and embryogenesis. In particular, SCFβTrCP and SCFSEL-10/FBXW7 are among the most important and best researched ligases during early development. SCFβTrCP is crucial for the oogenesis of Xenopus and mouse and also in Xenopus and Drosophila embryogenesis. SCFSEL-10/FBXW7 participates in the degradation of several RNA-binding proteins and thereby affects the regulation of gene expression during the meiosis of C. elegans. Nevertheless, a large number of SCF ligases that are primarily involved in embryogenesis remain to be elucidated.

Ubiquitin signaling in the control of centriole duplication

The centrosome plays an essential role in maintaining genetic stability, ciliogenesis and cell polarisation. The core of the centrosome is made up of two centrioles that duplicate precisely once during every cell cycle to generate two centrosomes that are required for bipolar spindle assembly and chromosome segregation. Abundance of centriole proteins at optimal levels and their recruitment to the centrosome are tightly regulated in time and space in order to restrict aberrant duplication of centrioles, a phenomenon that is observed in many cancers. Recent advances have conclusively shown that dedicated ubiquitin ligase-dependent protein degradation machineries are involved in governing centriole duplication. These studies revealed intricate mechanistic insights into how the ubiquitin ligases target different centriole proteins. In certain cases, a specific ubiquitin ligase targets a number of substrate proteins that co-regulate centriole assembly, prompting the possibility that substrate-targeting occurs during formation of the sub-centriolar structures. There are also instances where a specific centriole duplication protein is targeted by several ubiquitin ligases at different stages of the cell cycle, suggesting synchronised actions. Recent evidence also indicated a direct association of E3 ubiquitin ligase with the centrioles, supporting the notion that substrate-targeting occurs in the organelle itself. In this review, we highlight these advances by underlining the mechanisms of how different ubiquitin ligase machineries control centriole duplication and discuss our views on their coordination.

Differential turnover of Nup188 controls its levels at centrosomes and role in centriole duplication

NUP188 encodes a scaffold component of the nuclear pore complex (NPC) and has been implicated as a congenital heart disease gene through an ill-defined function at centrioles. Here, we explore the mechanisms that physically and functionally segregate Nup188 between the pericentriolar material (PCM) and NPCs. Pulse-chase fluorescent labeling indicates that Nup188 populates centrosomes with newly synthesized protein that does not exchange with NPCs even after mitotic NPC breakdown. In addition, the steady-state levels of Nup188 are controlled by the sensitivity of the PCM pool, but not the NPC pool, to proteasomal degradation. Proximity-labeling and super-resolution microscopy show that Nup188 is vicinal to the inner core of the interphase centrosome. Consistent with this, we demonstrate direct binding between Nup188 and Cep152. We further show that Nup188 functions in centriole duplication at or upstream of Sas6 loading. Together, our data establish Nup188 as a component of PCM needed to duplicate the centriole with implications for congenital heart disease mechanisms.

Identification of Important Modules and Biomarkers in Breast Cancer Based on WGCNA

Introduction

Breast cancer (BRCA) has the highest incidence among female malignancies, and the prognosis for these patients remains poor.

Materials and Methods

In this study, core modules and central genes related to BRCA were identified through a weighted gene co-expression network analysis (WGCNA). Gene expression profiles and clinical data of GSE25066 were obtained from the Gene Expression Omnibus (GEO) database. The result was validated with RNA-seq data from The Cancer Genome Atlas (TCGA) and Oncomine database. The top 30 key module genes with the highest intramodule connectivity were selected as the core genes (R² = 0.40).

Results

According to TCGA and Oncomine datasets, seven genes were selected as candidate hub genes. Following further experimental verification, four hub genes (FAM171A1, NDFIP1, SKP1, and REEP5) were retained.

Conclusion

We identified four hub genes as candidate biomarkers for BRCA. These hub genes may provide a theoretical basis for targeted therapy against BRCA.

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.

Finding of bands of higher molecular weight than expected in three proteins in bovine preimplantation embryos

SummaryWe report here the existence of bands of higher molecular weight after western blot analysis in three proteins - Skp1, p27 and IκBα in bovine preimplantation embryos. This finding is specific to preimplantation embryos (from the 2-cell stage to the blastocyst stage) and not differentiated fibroblast cells in which these bands were of expected molecular weight. We suggest that these bands of higher molecular weight represent a complex of proteins that are characteristic of preimplantation embryos.

Ubiquitin, the centrosome, and chromosome segregation

For over a century, the abnormal movement or number of centrosomes has been linked with errors of chromosomes distribution in mitosis. While not essential for the formation of the mitotic spindle, the presence and location of centrosomes has a major influence on the manner in which microtubules interact with the kinetochores of replicated sister chromatids and the accuracy with which they migrate to resulting daughter cells. A complex network has evolved to ensure that cells contain the proper number of centrosomes and that their location is optimal for effective attachment of emanating spindle fibers with the kinetochores. The components of this network are regulated through a series of post-translational modifications, including ubiquitin and ubiquitin-like modifiers, which coordinate the timing and strength of signaling events key to the centrosome cycle. In this review, we examine the role of the ubiquitin system in the events relating to centriole duplication and centrosome separation, and discuss how the disruption of these functions impacts chromosome segregation.

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.

Characterization of SCF-Complex during Bovine Preimplantation Development

The degradation of maternal proteins is one of the most important events during early development, and it is presumed to be essential for embryonic genome activation (EGA), but the precise mechanism is still not known. It is thought that a large proportion of the degradation of maternal proteins is mediated by the ubiquitin-proteolytic system. In this study we focused on the expression of the Skp1-Cullin1-F-box (SCF) complex, a modular RING-type E3 ubiquitin-ligase, during bovine preimplantation development. The complex consists of three invariable components—Cul1, Skp1, Rbx1 and F-box protein, which determines the substrate specificity. The protein level and mRNA expression of all three invariable members were determined. Cul1 and Skp1 mRNA synthesis was activated at early embryonic stages, at the 4c and early 8c stage, respectively, which suggests that these transcripts are necessary for preparing the embryo for EGA. CUL1 protein level increased from MII to the morula stage, with a significant difference between MII and L8c, and between MII and the morula. The CUL1 protein was localized primarily to nuclei and to a lesser extent to the cytoplasm, with a lower signal in the inner cell mass (ICM) compared to the trophectoderm (TE) at the blastocyst stage. The level of SKP1 protein significantly increased from MII oocytes to 4c embryos, but then significantly decreased again. The localization of the SKP1 protein was analysed throughout the cell and similarly to CUL1 at the blastocyst stage, the staining was less intensive in the ICM. There were no statistical differences in RBX1 protein level and localization. The active SCF-complex, which is determined by the interaction of Cul1 and Skp1, was found throughout the whole embryo during preimplantation development, but there was a difference at the blastocyst stage, which exhibits a much stronger signal in the TE than in the ICM. These results suggest that all these genes could play an important role during preimplantation development. This paper reveals comprehensive expression profile, the basic but important knowledge necessary for further studying.

Genome-wide analysis of phylogeny, expression profile and sub-cellular localization of SKP1-Like genes in wild tomato

SKP1 is a core component of SCF complex, a major type of E3 ubiquitin ligase catalyzing the last step in ubiquitin-mediated protein degradation pathway. In present study, SKP1 gene family in Solanum pimpinellifolium (SSK), a wild species of tomato, was investigated. A total of 19 SSK genes were identified through homologous search. Their chromosomal locations, gene structures, phylogeny, expression profiles, sub-cellular localizations and protein-protein interaction patterns with putative F-box proteins were analyzed in detail. The high homology and similar expression patterns among clustered SSK genes in chromosome suggested that they may have evolved from duplication events and are functionally redundant. Sub-cellular localization indicated that most of the SSK proteins are distributed in both cytosol and nucleus, except for SSK8, which is detected in cytosol only. Tissue-specific expression patterns suggested that many SSK genes may be involved in tomato fruit development. Furthermore, several SSK genes were found to be responsive to heat stress and salicylic acid treatment. Based on phylogenetic analysis, expression profiles and protein interaction property, we proposed that tomato SSK1 and SSK2 might have similar function to ASK1 and ASK2 in Arabidopsis.

Role of vacuolar ATPase and Skp1 in Sjögren's syndrome

Immune mechanisms alone cannot directly account for exocrine gland dysfunction and extraglandular features such as renal tubular acidosis, neuropathy, hearing loss and fatigue in Sjögren's syndrome (SS). Absence of Vacuolar ATPase (V-ATPase) has been reported in SS related renal tubular acidosis (RTA). We hypothesise how defect in V-ATPase could account for decreased neurotransmitter release leading onto exocrine dysfunction, neuroendocrine manifestations and hearing loss which are well described manifestations in SS. S-phase-kinase-associated protein-1 (Skp1) is a constituent of RAVE which is involved in V-ATPase assembly. It is also a component of SCF ligase which is crucial in NFκB signalling. SKP1 also interacts with TRIM 21/Ro 52 which is an autoantigen in SS. By virtue of these interactions, we postulate how a defective skp1 could fit into the existing pathogenesis of SS and also account for increased risk of lymphoma in SS as well as congenital heart block in fetus of mothers with SS.

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.

An SCF complex containing Fbxl12 mediates DNA damage-induced Ku80 ubiquitylation

The Ku heterodimer, composed of Ku70 and Ku80, is the initiating factor of the nonhomologous end joining (NHEJ) double-strand break (DSB) repair pathway. Ku is also thought to impede the homologous recombination (HR) repair pathway via inhibition of DNA end resection. Using the cell-free Xenopus laevis egg extract system, we had previously discovered that Ku80 becomes polyubiquitylated upon binding to DSBs, leading to its removal from DNA and subsequent proteasomal degradation. Here we show that the Skp1-Cul1-F box (SCF) E3 ubiquitin ligase complex is required for Ku80 ubiquitylation and removal from DNA. A screen for DSB-binding F box proteins revealed that the F box protein Fbxl12 was recruited to DNA in a DSB- and Ku-sensitive manner. Immunodepletion of Fbxl12 prevented Cul1 and Skp1 binding to DSBs and Ku80 ubiquitylation, indicating that Fbxl12 is the F box protein responsible for Ku80 substrate recognition. Unlike typical F box proteins, the F box of Fbxl12 was essential for binding to both Skp1 and its substrate Ku80. Besides Fbxl12, six other chromatin-binding F box proteins were identified in our screen of a subset of Xenopus F box proteins: β-TrCP, Fbh1, Fbxl19, Fbxo24, Fbxo28 and Kdm2b. Our study unveils a novel function for the SCF ubiquitin ligase in regulating the dynamic interaction between DNA repair machineries and DSBs.

Coupled activation and degradation of eEF2K regulates protein synthesis in response to genotoxic stress

The kinase eEF2K [eukaryotic elongation factor 2 (eEF2) kinase] controls the rate of peptide chain elongation by phosphorylating eEF2, the protein that mediates the movement of the ribosome along the mRNA by promoting translocation of the transfer RNA from the A to the P site in the ribosome. eEF2K-mediated phosphorylation of eEF2 on threonine 56 (Thr⁵⁶) decreases its affinity for the ribosome, thereby inhibiting elongation. Here, we show that in response to genotoxic stress, eEF2K was activated by AMPK (adenosine monophosphate-activated protein kinase)-mediated phosphorylation on serine 398. Activated eEF2K phosphorylated eEF2 and induced a temporary ribosomal slowdown at the stage of elongation. Subsequently, during DNA damage checkpoint silencing, a process required to allow cell cycle reentry, eEF2K was degraded by the ubiquitin-proteasome system through the ubiquitin ligase SCF(βTrCP) (Skp1-Cul1-F-box protein, β-transducin repeat-containing protein) to enable rapid resumption of translation elongation. This event required autophosphorylation of eEF2K on a canonical βTrCP-binding domain. The inability to degrade eEF2K during checkpoint silencing caused sustained phosphorylation of eEF2 on Thr⁵⁶ and delayed the resumption of translation elongation. Our study therefore establishes a link between DNA damage signaling and translation elongation.

FBXO11 targets BCL6 for degradation and is inactivated in diffuse large B-cell lymphomas

BCL6 is the product of a proto-oncogene implicated in the pathogenesis of human B-cell lymphomas. By binding specific DNA sequences, BCL6 controls the transcription of a variety of genes involved in B-cell development, differentiation and activation. BCL6 is overexpressed in the majority of patients with aggressive diffuse large B-cell lymphoma (DLBCL), the most common lymphoma in adulthood, and transgenic mice constitutively expressing BCL6 in B cells develop DLBCLs similar to the human disease. In many DLBCL patients, BCL6 overexpression is achieved through translocation (~40%) or hypermutation of its promoter (~15%). However, many other DLBCLs overexpress BCL6 through an unknown mechanism. Here we show that BCL6 is targeted for ubiquitylation and proteasomal degradation by a SKP1–CUL1–F-box protein (SCF) ubiquitin ligase complex that contains the orphan F-box protein FBXO11 (refs 5, 6). The gene encoding FBXO11 was found to be deleted or mutated in multiple DLBCL cell lines, and this inactivation of FBXO11 correlated with increased levels and stability of BCL6. Similarly, FBXO11 was either deleted or mutated in primary DLBCLs. Notably, tumour-derived FBXO11 mutants displayed an impaired ability to induce BCL6 degradation. Reconstitution of FBXO11 expression in FBXO11-deleted DLBCL cells promoted BCL6 ubiquitylation and degradation, inhibited cell proliferation, and induced cell death. FBXO11-deleted DLBCL cells generated tumours in immunodeficient mice, and the tumorigenicity was suppressed by FBXO11 reconstitution. We reveal a molecular mechanism controlling BCL6 stability and propose that mutations and deletions in FBXO11 contribute to lymphomagenesis through BCL6 stabilization. The deletions/mutations found in DLBCLs are largely monoallelic, indicating that FBXO11 is a haplo-insufficient tumour suppressor gene.

mTOR generates an auto-amplification loop by triggering the βTrCP- and CK1α-dependent degradation of DEPTOR

DEPTOR is a recently identified inhibitor of the mTOR kinase that is highly regulated at the posttranslational level. In response to mitogens, we found that DEPTOR was rapidly phosphorylated on three serines in a conserved degron, facilitating binding and ubiquitylation by the F box protein βTrCP, with consequent proteasomal degradation of DEPTOR. Phosphorylation of the βTrCP degron in DEPTOR is executed by CK1α after a priming phosphorylation event mediated by either the mTORC1 or mTORC2 complexes. Blocking the βTrCP-dependent degradation of DEPTOR via βTrCP knockdown or expression of a stable DEPTOR mutant that is unable to bind βTrCP results in mTOR inhibition. Our findings reveal that mTOR cooperates with CK1α and βTrCP to generate an auto-amplification loop to promote its own full activation. Moreover, our results suggest that pharmacologic inhibition of CK1 may be a viable therapeutic option for the treatment of cancers characterized by activation of mTOR-signaling pathways.

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.

FBXW5 controls centrosome number

Regulatory mechanisms to prevent centriole overduplication during the cell cycle are not completely understood. In this issue, FBXW5 is shown to control the degradation of the centriole assembly factor HsSAS-6. Moreover, the study proposes that FBXW5 is a substrate of both PLK4 and APC/C, two established regulators of centriole duplication.

Involvement of Grb2 adaptor protein in nucleophosmin-anaplastic lymphoma kinase (NPM-ALK)-mediated signaling and anaplastic large cell lymphoma growth

Most anaplastic large cell lymphomas (ALCL) express oncogenic fusion proteins derived from chromosomal translocations or inversions of the anaplastic lymphoma kinase (ALK) gene. Frequently ALCL carry the t(2;5) translocation, which fuses the ALK gene to the nucleophosmin (NPM1) gene. The transforming activity mediated by NPM-ALK fusion induces different pathways that control proliferation and survival of lymphoma cells. Grb2 is an adaptor protein thought to play an important role in ALK-mediated transformation, but its interaction with NPM-ALK, as well as its function in regulating ALCL signaling pathways and cell growth, has never been elucidated. Here we show that active NPM-ALK, but not a kinase-dead mutant, bound and induced Grb2 phosphorylation in tyrosine 160. An intact SH3 domain at the C terminus of Grb2 was required for Tyr(160) phosphorylation. Furthermore, Grb2 did not bind to a single region but rather to different regions of NPM-ALK, mainly Tyr(152-156), Tyr(567), and a proline-rich region, Pro(415-417). Finally, shRNA knockdown experiments showed that Grb2 regulates primarily the NPM-ALK-mediated phosphorylation of SHP2 and plays a key role in ALCL cell growth.

Cullin 1 functions as a centrosomal suppressor of centriole multiplication by regulating polo-like kinase 4 protein levels

Abnormal centrosome and centriole numbers are frequently detected in tumor cells where they can contribute to mitotic aberrations that cause chromosome missegregation and aneuploidy. The molecular mechanisms of centriole overduplication in malignant cells, however, are poorly characterized. Here, we show that the core SKP1-cullin-F-box component cullin 1 (CUL1) localizes to maternal centrioles and that CUL1 is critical for suppressing centriole overduplication through multiplication, a recently discovered mechanism whereby multiple daughter centrioles form concurrently at single maternal centrioles. We found that this activity of CUL1 involves the degradation of Polo-like kinase 4 (PLK4) at maternal centrioles. PLK4 is required for centriole duplication and strongly stimulates centriole multiplication when aberrantly expressed. We found that CUL1 is critical for the degradation of active PLK4 following deregulation of cyclin E/cyclin-dependent kinase 2 activity, as is frequently observed in human cancer cells, as well as for baseline PLK4 protein stability. Collectively, our results suggest that CUL1 may function as a tumor suppressor by regulating PLK4 protein levels and thereby restraining excessive daughter centriole formation at maternal centrioles.

Regulation of postsynaptic RapGAP SPAR by Polo-like kinase 2 and the SCFbeta-TRCP ubiquitin ligase in hippocampal neurons

The ubiquitin-proteasome pathway (UPP) regulates synaptic function, but little is known about specific UPP targets and mechanisms in mammalian synapses. We report here that the SCF(beta-TRCP) complex, a multisubunit E3 ubiquitin ligase, targets the postsynaptic spine-associated Rap GTPase activating protein (SPAR) for degradation in neurons. SPAR degradation by SCF(beta-TRCP) depended on the activity-inducible protein kinase Polo-like kinase 2 (Plk2). In the presence of Plk2, SPAR physically associated with the SCF(beta-TRCP) complex through a canonical phosphodegron. In hippocampal neurons, disruption of the SCF(beta-TRCP) complex by overexpression of dominant interfering beta-TRCP or Cul1 constructs prevented Plk2-dependent degradation of SPAR. Our results identify a specific E3 ubiquitin ligase that mediates degradation of a key postsynaptic regulator of synaptic morphology and function.

Altered pattern of Cul-1 protein expression and neddylation in human lung tumours: relationships with CAND1 and cyclin E protein levels

The Cul-1 protein is the scaffold element of SCF complexes that are involved in the proteasomal degradation of numerous proteins regulating cell cycle progression. Owing to this central role in cell growth control, aberrant expression of the components of SCF is thought to play a role during tumourigenesis. Nothing is known about Cul-1 expression in human tumours. In this study, we have analysed its status in a series of 128 human lung carcinomas, comprising 50 non-small cell lung cancers (NSCLCs; 29 squamous cell carcinomas and 21 adenocarcinomas) and 78 neuroendocrine (NE) lung tumours (24 typical and atypical carcinoids, 19 large cell NE carcinomas and 35 small cell lung carcinomas), using immunohistochemistry. We report for the first time an altered pattern of Cul-1 expression in human tumours; indeed, we show that Cul-1 expression is up-regulated in 40% (51/128) of all lung tumours as compared to normal lung tissues, including 34% (17/50), 75% (18/24) and 30% (16/54) of NSCLCs, carcinoids and high grade neuroendocrine lung carcinomas, respectively. Furthermore, we demonstrate that high levels of Cul-1 protein are associated with a low KI67 proliferative index (p = 0.005) and with a decrease in the cyclin E oncoprotein (p = 0.0003), one of the major targets of SCF complexes. These data suggest that up-regulation of Cul-1 could protect cells from hyperproliferative signals through cyclin E down-regulation. Cul-1 is modified by neddylation, a post-translational modification that grafts ubiquitin-like Nedd8/Rub1 residues and controls Cul-1 activity. We also provide evidence that neddylated forms of Cul-1 are specifically expressed in high-grade NE lung tumours and are associated with down-regulation of the Cul-1 inhibitor CAND1 (p = 0.03) and a high level of cyclin E (p = 0.0002). These data support the notion that alterations in the Cul-1 neddylation/deneddylation pathway could contribute to the development of these highly aggressive lung tumours.

Human delta-lactoferrin is a transcription factor that enhances Skp1 (S-phase kinase-associated protein) gene expression

Delta-lactoferrin is a cytoplasmic lactoferrin isoform that can locate to the nucleus, provoking antiproliferative effects and cell cycle arrest in S phase. Using macroarrays, the expression of genes involved in the G(1)/S transition was examined. Among these, Skp1 showed 2-3-fold increased expression at both the mRNA and protein levels. Skp1 (S-phase kinase-associated protein) belongs to the Skp1/Cullin-1/F-box ubiquitin ligase complex responsible for the ubiquitination of cellular regulators leading to their proteolysis. Skp1 overexpression was also found after delta-lactoferrin transient transfection in other cell lines (HeLa, MDA-MB-231, HEK 293) at comparable levels. Analysis of the Skp1 promoter detected two sequences that were 90% identical to those previously known to interact with lactoferrin, the secretory isoform of delta-lactoferrin (GGCACTGTAC-S1(Skp1), located at - 1067 bp, and TAGAAGTCAA-S2(Skp1), at - 646 bp). Both gel shift and chromatin immunoprecipitation assays demonstrated that delta-lactoferrin interacts in vitro and in vivo specifically with these sequences. Reporter gene analysis confirmed that delta-lactoferrin recognizes both sequences within the Skp1 promoter, with a higher activity on S1(Skp1). Deletion of both sequences totally abolished delta-lactoferrin transcriptional activity, identifying them as delta-lactoferrin-responsive elements. Delta-lactoferrin enters the nucleus via a short bipartite RRSDTSLTWNSVKGKK(417-432) nuclear localization signal sequence, which was demonstrated to be functional using mutants. Our results show that delta-lactoferrin binds to the Skp1 promoter at two different sites, and that these interactions lead to its transcriptional activation. By increasing Skp1 gene expression, delta-lactoferrin may regulate cell cycle progression via control of the proteasomal degradation of S-phase actors.

Involvement of the IκB Kinase (IKK)-Related Kinases Tank-Binding Kinase 1/IKKi and Cullin-Based Ubiquitin Ligases in IFN Regulatory Factor-3 Degradation

Activation of the innate arm of the immune system following pathogen infection relies on the recruitment of latent transcription factors involved in the induction of a subset of genes responsible for viral clearance. One of these transcription factors, IFN regulatory factor 3 (IRF-3), is targeted for proteosomal degradation following virus infection. However, the molecular mechanisms involved in this process are still unknown. In this study, we show that polyubiquitination of IRF-3 increases in response to Sendai virus infection. Using an E1 temperature-sensitive cell line, we demonstrate that polyubiquitination is required for the observed degradation of IRF-3. Inactivation of NEDD8-activating E1 enzyme also results in stabilization of IRF-3 suggesting the NEDDylation also plays a role in IRF-3 degradation following Sendai virus infection. In agreement with this observation, IRF-3 is recruited to Cullin1 following virus infection and overexpression of a dominant-negative mutant of Cullin1 significantly inhibits the degradation of IRF-3 observed in infected cells. We also asked whether the C-terminal cluster of phosphoacceptor sites of IRF-3 could serve as a destabilization signal and we therefore measured the half-life of C-terminal phosphomimetic IRF-3 mutants. Interestingly, we found them to be short-lived in contrast to wild-type IRF-3. In addition, no degradation of IRF-3 was observed in TBK1(-/-) mouse embryonic fibroblasts. All together, these data demonstrate that virus infection stimulates a host cell signaling pathway that modulates the expression level of IRF-3 through its C-terminal phosphorylation by the IkappaB kinase-related kinases followed by its polyubiquitination, which is mediated in part by a Cullin-based ubiquitin ligase.

Drug discovery in the ubiquitin-proteasome system

Regulated protein turnover via the ubiquitin-proteasome system (UPS) underlies a wide variety of signalling pathways, from cell-cycle control and transcription to development. Recent evidence that pharmacological inhibition of the proteasome can be efficacious in the treatment of human cancers has set the stage for attempts to selectively inhibit the activities of disease-specific components of the UPS. Here, we review recent advances linking UPS components with specific human diseases, most prominently cancer and neurodegenerative disorders, and emphasize potential sites of therapeutic intervention along the regulated protein-degradation pathway.

The X protein of hepatitis B virus binds to the F box protein Skp2 and inhibits the ubiquitination and proteasomal degradation of c-Myc

The HBx protein of hepatitis B virus is involved in deregulation of cell cycle and development of hepatocellular carcinoma. Since c-Myc also plays an important role in cell proliferation and tumor development, we studied its regulation by HBx in a human hepatoma cell line. Co-expression of HBx and c-Myc resulted in increased stability of intracellular c-Myc. HBx blocked the ubiquitination of Myc through a direct interaction with the F box region of Skp2 and destabilization of the SCF(Skp2) complex. We suggest that sustained presence of c-Myc combined with mitogenic activity inherent to HBx may be associated with cell cycle deregulation and transformation.

Control of genomic instability and epithelial tumor development by the p53-Fbxw7/Cdc4 pathway

Mouse models of cancer have provided novel insights into the timing of p53 loss during tumorigenesis. We have recently identified Fbxw7/Cdc4 as a downstream target of p53 loss that controls genomic instability and tumor development in epithelial tumors. Although p53-deficient mice primarily develop lymphomas and sarcomas, the additional loss of one copy of the Fbxw7 gene drives tumor development in a range of epithelial tissues. These data highlight the importance of genetic instability at the chromosome level in the development of common cancer types, and further illustrate the value of mouse models in identifying causal genetic events in epithelial tumor formation.

Ablation of oncogenic ALK is a viable therapeutic approach for anaplastic large-cell lymphomas

Anaplastic large-cell lymphomas (ALCLs) carry chromosome translocations in which the anaplastic lymphoma kinase (ALK) gene is fused to several partners, most frequently, the NPM1 gene. We have demonstrated that the constitutive activation of ALK fusion proteins results in cellular transformation and lymphoid neoplasia. Herein, we specifically down-regulated ALK protein expression by using small hairpin RNA (shRNA) targeting a sequence coding for the catalytic domain of ALK. The ablation of ALK leads to the down-modulation of known ALK downstream effectors, cell growth arrest, and reversion of the transformed phenotype of ALK(+) mouse embryonic fibroblasts in vitro and in vivo. In human ALCL cells lentiviral-mediated ALK knock-down leads to G(1) cell-cycle arrest and apoptosis in vitro and tumor growth inhibition and regression in vivo. Using a specific approach we have demonstrated that the survival and growth of ALK(+) ALCLs are strictly dependent on ALK activation and signaling. Therefore, ALK is a viable target for therapeutic intervention and its inactivation might represent a pivotal approach for the treatment of ALK lymphomas and other ALK-dependent human tumors.

Expression of the cyclin-dependent kinase inhibitor p27 and its deregulation in mouse B cell lymphomas

CDKN1B (p27) regulates cell-cycle progression at the G1-S transition by suppressing the cyclin E/CDK2 kinase complex. In normal lymphocytes and most human B cell non-Hodgkin lymphomas (NHL), there is an inverse correlation between proliferative activity and expression of p27; however, a subset of NHL with high mitotic indices expresses p27, which is inactive due to sequestration in nuclear protein complexes or due to cytoplasmic retention. Our studies of mouse B cell NHL also identified cases with high proliferative activity and high levels of p27 at a surprisingly high frequency. Here, p27 was complexed with D-type cyclins 1 and 3 and with the COPS9 protein, JAB1. In addition, we found cytoplasmic sequestration following phosphorylation by activated AKT.

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

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

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

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

CUL1, a component of E3 ubiquitin ligase, alters lymphocyte signal transduction with possible effect on rheumatoid arthritis

Ubiquitination affects various immune processes and E3 ubiquitin ligases (E3) play an important role in determining substrate specificity. We identified 11 human E3 ligase genes of potential importance in pathogenesis of autoimmune diseases by search of public databases and screened them for candidacy of biological investigation with case-control linkage disequilibrium tests on multiple SNPs in the genes using rheumatoid arthritis (RA) as a model of autoimmune diseases. Significant association with RA was observed in an SNP in intron 3 of Cullin 1 (CUL1) that affected transcriptional efficiency of the promoter activity in lymphocytic cell lines. Quantitative expression analysis revealed that CUL1 mRNA was highly detected in lymphoid tissues including spleen and tonsil, and was specifically expressed in T and B lymphocytes in fractionated peripheral leukocytes. Histological evaluation of tonsils indicated that CUL1 protein expression was relatively specific for maturing germinal centers. Suppression of CUL1 expression had influence on the phenotype of T-cell line, that is, it inhibited IL-8 induction, which is known to play an important role in the migration of inflammatory cells into the affected area seen in RA. Our data suggest that the regulation of CUL1 expression in immunological tissues may affect the susceptibility of RA via altering lymphocyte signal transduction.

Folate transport gene inactivation in mice increases sensitivity to colon carcinogenesis

Low dietary folate intake is associated with an increased risk for colon cancer; however, relevant genetic animal models are lacking. We therefore investigated the effect of targeted ablation of two folate transport genes, folate binding protein 1 (Folbp1) and reduced folate carrier 1 (RFC1), on folate homeostasis to elucidate the molecular mechanisms of folate action on colonocyte cell proliferation, gene expression, and colon carcinogenesis. Targeted deletion of Folbp1 (Folbp1(+/-) and Folbp1(-/-)) significantly reduced (P < 0.05) colonic Folbp1 mRNA, colonic mucosa, and plasma folate concentration. In contrast, subtle changes in folate homeostasis resulted from targeted deletion of RFC1 (RFC1(+/-)). These animals had reduced (P < 0.05) colonic RFC1 mRNA and exhibited a 2-fold reduction in the plasma S-adenosylmethionine/S-adenosylhomocysteine. Folbp1(+/-) and Folbp1(-/-) mice had larger crypts expressed as greater (P < 0.05) numbers of cells per crypt column relative to Folbp1(+/+) mice. Colonic cell proliferation was increased in RFC1(+/-) mice relative to RFC1(+/+) mice. Microarray analysis of colonic mucosa showed distinct changes in gene expression specific to Folbp1 or RFC1 ablation. The effect of folate transporter gene ablation on colon carcinogenesis was evaluated 8 and 38 weeks post-azoxymethane injection in wild-type and heterozygous mice. Relative to RFC1(+/+) mice, RFC1(+/-) mice developed increased (P < 0.05) numbers of aberrant crypt foci at 8 weeks. At 38 weeks, RFC1(+/-) mice developed local inflammatory lesions with or without epithelial dysplasia as well as adenocarcinomas, which were larger relative to RFC1(+/+) mice. In contrast, Folbp1(+/-) mice developed 4-fold (P < 0.05) more lesions relative to Folbp1(+/+) mice. In conclusion, Folbp1 and RFC1 genetically modified mice exhibit distinct changes in colonocyte phenotype and therefore have utility as models to examine the role of folate homeostasis in colon cancer development.

Folate Transport Gene Inactivation in Mice Increases Sensitivity to Colon Carcinogenesis

Low dietary folate intake is associated with an increased risk for colon cancer; however, relevant genetic animal models are lacking. We therefore investigated the effect of targeted ablation of two folate transport genes, folate binding protein 1 (Folbp1) and reduced folate carrier 1 (RFC1), on folate homeostasis to elucidate the molecular mechanisms of folate action on colonocyte cell proliferation, gene expression, and colon carcinogenesis. Targeted deletion of Folbp1 (Folbp1+/− and Folbp1−/−) significantly reduced (P &lt; 0.05) colonic Folbp1 mRNA, colonic mucosa, and plasma folate concentration. In contrast, subtle changes in folate homeostasis resulted from targeted deletion of RFC1 (RFC1+/−). These animals had reduced (P &lt; 0.05) colonic RFC1 mRNA and exhibited a 2-fold reduction in the plasma S-adenosylmethionine/S-adenosylhomocysteine. Folbp1+/− and Folbp1−/− mice had larger crypts expressed as greater (P &lt; 0.05) numbers of cells per crypt column relative to Folbp1+/+ mice. Colonic cell proliferation was increased in RFC1+/− mice relative to RFC1+/+ mice. Microarray analysis of colonic mucosa showed distinct changes in gene expression specific to Folbp1 or RFC1 ablation. The effect of folate transporter gene ablation on colon carcinogenesis was evaluated 8 and 38 weeks post-azoxymethane injection in wild-type and heterozygous mice. Relative to RFC1+/+ mice, RFC1+/− mice developed increased (P &lt; 0.05) numbers of aberrant crypt foci at 8 weeks. At 38 weeks, RFC1+/− mice developed local inflammatory lesions with or without epithelial dysplasia as well as adenocarcinomas, which were larger relative to RFC1+/+ mice. In contrast, Folbp1+/− mice developed 4-fold (P &lt; 0.05) more lesions relative to Folbp1+/+ mice. In conclusion, Folbp1 and RFC1 genetically modified mice exhibit distinct changes in colonocyte phenotype and therefore have utility as models to examine the role of folate homeostasis in colon cancer development.

Oncogenic aberrations of cullin-dependent ubiquitin ligases

Accumulating evidence points to a key role of the ubiquitin-proteasome pathway in oncogenesis. Aberrant proteolysis of substrates involved in cellular processes such as the cell division cycle, gene transcription, the DNA damage response and apoptosis has been reported to contribute significantly to neoplastic transformation. Cullin-dependent ubiquitin ligases (CDLs) form a class of structurally related multisubunit enzymes central to the ubiquitin-mediated proteolysis of many important biological substrates. In this review, we describe the role of CDLs in the ubiquitinylation of cancer-related substrates and discuss how altered ubiquitinylation by CDLs may contribute to tumor development.

Regulation of the ubiquitin proteasome pathway in human lens epithelial cells during the cell cycle

Most proliferating cells follow a series of orderly transitions from one phase to another. These transitions are usually controlled by timed degradation of cell cycle regulators by the ubiquitin-proteasome pathway (UPP). There are no published reports regarding the timing of phases of the human lens cell cycle or regarding cell cycle-related changes in UPP components. Objectives of this study were to characterize the timing of the phases of the human lens epithelial cell cycle and to explore potential functions of critical components of the UPP in controlling lens cell cycle. Human lens epithelial cells were synchronized at G0/G1 phase by contact inhibition. Cell cycle progression upon subculturing was monitored by FACS analysis. It took approximately 40 hr for HLEC to complete one cell cycle, approximately 20 hr for G1 phase, approximately 8-10 hr for S phase and approximately 10 hr for the combination of G2 and M phases. Proteasome-dependent degradation of p21WAF and p27Kip, the dominant Cdk inhibitors, was associated with the G1/S phase transition in these cells. Proteasome inhibition experiments indicate that proteolysis is the predominant process which is responsible for the variations in these regulators during the cell cycle. Levels of specific ubiquitin conjugating enzymes, Ubc7 and Ubc10, increased 6 and 2-fold at the G2/M phase and S/G2/M phases, respectively. Levels of these E2s decreased precipitously upon completion of the M phase. In contrast, levels of ubiquitin activating enzyme (E1) and Ubc3 remained constant during the cell cycle. Cul1, a component of the SCF (an E3), remained relatively constant during cell cycle. The up-regulation of Ubc7 and Ubc10 during the G2/M and S/G2/M phases suggests that these enzymes may be involved in controlling the cell cycle progression at this phase. Taken together, the data indicate that expression of key components of the UPP in the human lens epithelial cells is regulated in a cell cycle-dependent manner. Some of the variations in levels of ubiquitin conjugating enzymes are suggestive of previously undescribed functions.

Transgenic Mice as an In Vivo Model of Lymphomagenesis

This review covers multiple data obtained on genetically modified mice that help to elucidate various intricate molecular mechanisms of lymphomagenesis in humans. We are in a "golden age" of mouse genetics. The mouse is by far the most accessible mammalian system physiologically similar to humans. Transgenic mouse models have illuminated how different genes contribute to human lymphomagenesis. Multiple experiments with transgenic mice have not only confirmed the data obtained for human lymphomas but also gave additional evidence for the role of some genes and cooperative participation of their products in the development of human lymphomas. Genes and gene networks detected on transgenic mice can successfully serve as molecular targets for tumor therapy. This review demonstrates the extraordinary possibilities of transgenic technology, which is presently one of the readily available, efficient, and accurate tools to solve the problem of cancer.

Control of meiotic and mitotic progression by the F box protein beta-Trcp1 in vivo

SCF ubiquitin ligases, composed of three major subunits, Skp1, Cul1, and one of many F box proteins (Fbps), control the proteolysis of important cellular regulators. We have inactivated the gene encoding the Fbp beta-Trcp1 in mice. beta-Trcp1(-/-) males show reduced fertility correlating with an accumulation of methaphase I spermatocytes. beta-Trcp1(-/-) MEFs display a lengthened mitosis, centrosome overduplication, multipolar metaphase spindles, and misaligned chromosomes. Furthermore, cyclin A, cyclin B, and Emi1, an inhibitor of the anaphase promoting complex, are stabilized in mitotic beta-Trcp1(-/-) MEFs. Indeed, we demonstrate that Emi1 is a bona fide substrate of beta-Trcp1. In contrast, stabilization of beta-catenin and IkappaBalpha, two previously reported beta-Trcp1 substrates, does not occur in the absence of beta-Trcp1 and instead requires the additional silencing of beta-Trcp2 by siRNA. Thus, beta-Trcp1 regulates the timely order of meiotic and mitotic events.

NPM-ALK transgenic mice spontaneously develop T-cell lymphomas and plasma cell tumors

Anaplastic Large Cell Lymphomas (ALCLs) carry translocations in which the anaplastic lymphoma kinase (ALK) gene is juxtaposed to various genes, the most common of which is the NPM/B23 gene. ALK fusion proteins result in the constitutive activation of ALK tyrosine kinase, thereby enhancing proliferation and increasing cell survival. A direct role for NPM-ALK in cellular transformation has been shown in vitro with immortalized cell lines and in vivo using retroviral transfer experiments. Nonetheless, there is no direct evidence of its oncogenic potential in T lymphocytes, which represent the most common target of ALK chimeras. Here, we describe a new mouse model of lymphomagenesis in which human NPM-ALK transcription was targeted to T cells. NPM-ALK transgenic (Tg) mice were born with the expected mendelian distribution, normal lymphoid organs, and a normal number and proportion of helper and suppressor T cells. However, after a short period of latency, all NPM-ALK Tg mice developed malignant lymphoproliferative disorders (mean survival, 18 weeks). NPM-ALK Tg thymic lymphomas displayed a T-cell phenotype characteristic of immature thymocytes and frequently coexpressed surface CD30. A subset of the NPM-ALK Tg mice also developed clonal B-cell plasma cell neoplasms. These tumors arose in peripheral lymphoid organs (plasmacytomas) or within the bone marrow and often led to peripheral neuropathies and limb paralysis. Our NPM-ALK Tg mice are a suitable model to dissect the molecular mechanisms of ALK-mediated transformation and to investigate the efficacy of new therapeutic approaches for the treatment of human ALCL in vivo.