Targeted Destruction of c-Myc by an Engineered Ubiquitin Ligase Suppresses Cell Transformation and Tumor Formation

Targeted protein degradation using chimeric human E2 ubiquitin-conjugating enzymes

Proteins can be targeted for degradation by engineering biomolecules that direct them to the eukaryotic ubiquitination machinery. For instance, the fusion of an E3 ubiquitin ligase to a suitable target binding domain creates a 'biological Proteolysis-Targeting Chimera' (bioPROTAC). Here we employ an analogous approach where the target protein is recruited directly to a human E2 ubiquitin-conjugating enzyme via an attached target binding domain. Through rational design and screening we develop E2 bioPROTACs that induce the degradation of the human intracellular proteins SHP2 and KRAS. Using global proteomics, we characterise the target-specific and wider effects of E2 vs. VHL-based fusions. Taking SHP2 as a model target, we also employ a route to bioPROTAC discovery based on protein display libraries, yielding a degrader with comparatively weak affinity capable of suppressing SHP2-mediated signalling.

Lipid-mediated intracellular delivery of recombinant bioPROTACs for the rapid degradation of undruggable proteins

Recently, targeted degradation has emerged as a powerful therapeutic modality. Relying on "event-driven" pharmacology, proteolysis targeting chimeras (PROTACs) can degrade targets and are superior to conventional inhibitors against undruggable proteins. Unfortunately, PROTAC discovery is limited by warhead scarcity and laborious optimization campaigns. To address these shortcomings, analogous protein-based heterobifunctional degraders, known as bioPROTACs, have been developed. Compared to small-molecule PROTACs, bioPROTACs have higher success rates and are subject to fewer design constraints. However, the membrane impermeability of proteins severely restricts bioPROTAC deployment as a generalized therapeutic modality. Here, we present an engineered bioPROTAC template able to complex with cationic and ionizable lipids via electrostatic interactions for cytosolic delivery. When delivered by biocompatible lipid nanoparticles, these modified bioPROTACs can rapidly degrade intracellular proteins, exhibiting near-complete elimination (up to 95% clearance) of targets within hours of treatment. Our bioPROTAC format can degrade proteins localized to various subcellular compartments including the mitochondria, nucleus, cytosol, and membrane. Moreover, substrate specificity can be easily reprogrammed, allowing modular design and targeting of clinically-relevant proteins such as Ras, Jnk, and Erk. In summary, this work introduces an inexpensive, flexible, and scalable platform for efficient intracellular degradation of proteins that may elude chemical inhibition.

Degraders upgraded: the rise of PROTACs in hematological malignancies

ABSTRACT

Targeted protein degradation (TPD) is a revolutionary approach to targeted therapy in hematological malignancies that potentially circumvents many constraints of existing small-molecule inhibitors. Heterobifunctional proteolysis-targeting chimeras (PROTACs) are the leading TPD drug class, with numerous agents now in clinical trials for a range of blood cancers. PROTACs harness the cell-intrinsic protein recycling infrastructure, the ubiquitin-proteasome system, to completely degrade target proteins. Distinct from targeted small-molecule inhibitor therapies, PROTACs can eliminate critical but conventionally "undruggable" targets, overcome resistance mechanisms to small-molecule therapies, and can improve tissue specificity and off-target toxicity. Orally bioavailable, PROTACs are not dependent on the occupancy-driven pharmacology inherent to inhibitory therapeutics, facilitating substoichiometric dosing that does not require an active or allosteric target binding site. Preliminary clinical data demonstrate promising therapeutic activity in heavily pretreated populations and novel technology platforms are poised to exploit a myriad of permutations of PROTAC molecular design to enhance efficacy and targeting specificity. As the field rapidly progresses and various non-PROTAC TPD drug candidates emerge, this review explores the scientific and preclinical foundations of PROTACs and presents them within common clinical contexts. Additionally, we examine the latest findings from ongoing active PROTAC clinical trials.

A TRIM21-based bioPROTAC highlights the therapeutic benefit of HuR degradation

Human antigen R (HuR) is a ubiquitously expressed RNA-binding protein, which functions as an RNA regulator. Overexpression of HuR correlates with high grade tumours and poor patient prognosis, implicating it as an attractive therapeutic target. However, an effective small molecule antagonist to HuR for clinical use remains elusive. Here, a single domain antibody (VHH) that binds HuR with low nanomolar affinity was identified and shown to inhibit HuR binding to RNA. This VHH was used to engineer a TRIM21-based biological PROTAC (bioPROTAC) that could degrade endogenous HuR. Significantly, HuR degradation reverses the tumour-promoting properties of cancer cells in vivo by altering the HuR-regulated proteome, highlighting the benefit of HuR degradation and paving the way for the development of HuR-degrading therapeutics. These observations have broader implications for degrading intractable therapeutic targets, with bioPROTACs presenting a unique opportunity to explore targeted-protein degradation through a modular approach.

STUB1/CHIP: New insights in cancer and immunity

The STUB1 gene (STIP1 homology and U-box-containing protein 1), located at 16q13.3, encodes the CHIP (carboxyl terminus of Hsc70-interacting protein), an essential E3 ligase involved in protein quality control. CHIP comprises three domains: an N-terminal tetratricopeptide repeat (TPR) domain, a middle coiled-coil domain, and a C-terminal U-box domain. It functions as a co-chaperone for heat shock protein (HSP) via the TPR domain and as an E3 ligase, ubiquitinating substrates through its U-box domain. Numerous studies suggest that STUB1 plays a crucial role in various physiological process, such as aging, autophagy, and bone remodeling. Moreover, emerging evidence has shown that STUB1 can degrade oncoproteins to exert tumor-suppressive functions, and it has recently emerged as a novel player in tumor immunity. This review provides a comprehensive overview of STUB1's role in cancer, including its clinical significance, impact on tumor progression, dual roles, tumor stem cell-like properties, angiogenesis, drug resistance, and DNA repair. In addition, we explore STUB1's functions in immune cell differentiation and maturation, inflammation, autoimmunity, antiviral immune response, and tumor immunity. Collectively, STUB1 represents a promising and valuable therapeutic target in cancer and immunology.

Beyond canonical PROTAC: biological targeted protein degradation (bioTPD)

Targeted protein degradation (TPD) is an emerging therapeutic strategy with the potential to modulate disease-associated proteins that have previously been considered undruggable, by employing the host destruction machinery. The exploration and discovery of cellular degradation pathways, including but not limited to proteasomes and lysosome pathways as well as their degraders, is an area of active research. Since the concept of proteolysis-targeting chimeras (PROTACs) was introduced in 2001, the paradigm of TPD has been greatly expanded and moved from academia to industry for clinical translation, with small-molecule TPD being particularly represented. As an indispensable part of TPD, biological TPD (bioTPD) technologies including peptide-, fusion protein-, antibody-, nucleic acid-based bioTPD and others have also emerged and undergone significant advancement in recent years, demonstrating unique and promising activities beyond those of conventional small-molecule TPD. In this review, we provide an overview of recent advances in bioTPD technologies, summarize their compositional features and potential applications, and briefly discuss their drawbacks. Moreover, we present some strategies to improve the delivery efficacy of bioTPD, addressing their challenges in further clinical development.

Control of SOX2 protein stability and tumorigenic activity by E3 ligase CHIP in esophageal cancer cells

SOX2 is highly expressed and controls tumor initiation and cancer stem cell function in various squamous cell carcinomas including esophageal squamous cancer. However, the molecular mechanism leading to SOX2 overexpression in cancer is incompletely understood. Here, we identified CHIP, a chaperone-associated ubiquitin E3 ligase, as a novel negative regulator of SOX2 protein stability and tumorigenic activity in esophageal squamous carcinoma cells. We showed that CHIP interacted with SOX2 primarily via chaperone HSP70, together they catalyzed SOX2 ubiquitination and degradation via proteasome. In contrast, HSP90 promoted SOX2 stability and inhibition of HSP90 activity induced SOX2 ubiquitination and degradation. Notably, unlike the case in normal esophageal tissues where CHIP was detected in both the cytoplasm and nucleus, CHIP in clinical esophageal tumor specimens was predominantly localized in the cytoplasm. Consistent with this observation, we observed increased expression of exportin-1/CRM-1 in clinical esophageal tumor specimens. We further demonstrated that CHIP catalyzed SOX2 ubiquitination and degradation primarily in the nuclear compartment. Taken together, our study has identified CHIP as a key suppressor of SOX2 protein stability and tumorigenic activity and revealed CHIP nuclear exclusion as a potential mechanism for aberrant SOX2 overexpression in esophageal cancer. Our study also suggests HSP90 inhibitors as potential therapeutic agents for SOX2-positive cancers.

Nucleus-specific RNAi nanoplatform for targeted regulation of nuclear lncRNA function and effective cancer therapy

In the context of cancer therapy, a recently identified therapeutic target is represented by the essential subtype of RNA transcripts - the long noncoding RNAs (lncRNA). While this is the case, it is especially difficult to successfully regulate the expression of this subtype in vivo, particularly due to the protection granted by the nuclear envelope of nuclear lncRNAs. This study documents the development of a nucleus-specific RNA interference (RNAi) nanoparticle (NP) platform for the targeted regulation of the nuclear lncRNA function, in order to effectuate successful cancer therapy. An NTPA (nucleus-targeting peptide amphiphile) and an endosomal pH-responsive polymer make up the novel RNAi nanoplatform in development, which is capable of complexing siRNA. The nanoplatform is capable of accumulating greatly in the tumor tissues and being internalized by tumor cells, following intravenous administration. The exposed complexes of the NTPA/siRNA may conveniently escape from the endosome with the pH-triggered NP disassociation, following which it can target the nucleus by specifically interacting with the importin α/β heterodimer. In orthotopic and subcutaneous xenograft tumor models, this would result in a notable suppression of the expression of nuclear lncNEAT2 as well as greatly impede the growth of tumors in liver cancer.

PROTAC targeted protein degraders: the past is prologue

Targeted protein degradation (TPD) is an emerging therapeutic modality with the potential to tackle disease-causing proteins that have historically been highly challenging to target with conventional small molecules. In the 20 years since the concept of a proteolysis-targeting chimera (PROTAC) molecule harnessing the ubiquitin-proteasome system to degrade a target protein was reported, TPD has moved from academia to industry, where numerous companies have disclosed programmes in preclinical and early clinical development. With clinical proof-of-concept for PROTAC molecules against two well-established cancer targets provided in 2020, the field is poised to pursue targets that were previously considered 'undruggable'. In this Review, we summarize the first two decades of PROTAC discovery and assess the current landscape, with a focus on industry activity. We then discuss key areas for the future of TPD, including establishing the target classes for which TPD is most suitable, expanding the use of ubiquitin ligases to enable precision medicine and extending the modality beyond oncology.

Low NCOR2 levels in multiple myeloma patients drive multidrug resistance via MYC upregulation

MYC upregulation is associated with multidrug refractory disease in patients with multiple myeloma (MM). We, isolated patient-derived MM cells with high MYC expression and discovered that NCOR2 was down-regulated in these cells. NCOR2 is a transcriptional coregulatory protein and its role in MM remains unknown. To define the role of NCOR2 in MM, we created NCOR2 knockout human myeloma cell lines and demonstrated that NCOR2 knockout led to high MYC expression. Furthermore, NCOR2 knockout conferred resistance to pomalidomide, BET and HDAC inhibitors, independent of Cereblon (CRBN), indicating high MYC expression as a cause of multidrug resistance. Moreover, NCOR2 interacted with the nucleosome remodeling and deacetylase (NuRD) complex and repressed the expression of CD180 by directly binding to its promoter and inducing MYC expression. Next, we generated lenalidomide-resistant and pomalidomide-resistant human myeloma cell lines. Whole-exome sequencing revealed that these cell lines acquired the same exonic mutations of NCOR2. These cell lines showed NCOR2 downregulation and MYC upregulation independent of CRBN and demonstrated resistance to BET and HDAC inhibitors. Our findings reveal a novel CRBN independent molecular mechanism associated with drug resistance. Low NCOR2 expression can serve as a potential biomarker for drug resistance and needs further validation in larger prospective studies.

Engineering Single Pan-Specific Ubiquibodies for Targeted Degradation of All Forms of Endogenous ERK Protein Kinase

Ubiquibodies (uAbs) are a customizable proteome editing technology that utilizes E3 ubiquitin ligases genetically fused to synthetic binding proteins to steer otherwise stable proteins of interest (POIs) to the 26S proteasome for degradation. The ability of engineered uAbs to accelerate the turnover of exogenous or endogenous POIs in a post-translational manner offers a simple yet robust tool for dissecting diverse functional properties of cellular proteins as well as for expanding the druggable proteome to include tumorigenic protein families that have yet-to-be successfully drugged by conventional inhibitors. Here, we describe the engineering of uAbs composed of human carboxyl-terminus of Hsc70-interacting protein (CHIP), a highly modular human E3 ubiquitin ligase, tethered to differently designed ankyrin repeat proteins (DARPins) that bind to nonphosphorylated (inactive) and/or doubly phosphorylated (active) forms of extracellular signal-regulated kinase 1 and 2 (ERK1/2). Two of the resulting uAbs were found to be global ERK degraders, pan-specifically capturing all endogenous ERK1/2 protein forms and redirecting them to the proteasome for degradation in different cell lines, including MCF7 breast cancer cells. Taken together, these results demonstrate how the substrate specificity of an E3 ubiquitin ligase can be reprogrammed to generate designer uAbs against difficult-to-drug targets, enabling a modular platform for remodeling the mammalian proteome.

PROTACs, molecular glues and bifunctionals from bench to bedside: Unlocking the clinical potential of catalytic drugs

The vast majority of currently marketed drugs rely on small molecules with an 'occupancy-driven' mechanism of action (MOA). Therefore, the efficacy of these therapeutics depends on a high degree of target engagement, which often requires high dosages and enhanced drug exposure at the target site, thus increasing the risk of off-target toxicities (Churcher, 2018 [1]). Although small molecule drugs have been successfully used as treatments for decades, tackling a variety of disease-relevant targets with a defined binding site, many relevant therapeutic targets remain challenging to drug due, for example, to lack of well-defined binding pockets or large protein-protein interaction (PPI) interfaces which resist interference (Dang et al., 2017 [2]). In the quest for alternative therapeutic approaches to address different pathologies and achieve enhanced efficacy with reduced side effects, ligand-induced targeted protein degradation (TPD) has gained the attention of many research groups both in academia and in industry in the last two decades. This therapeutic modality represents a novel paradigm compared to conventional small-molecule inhibitors. To pursue this strategy, heterobifunctional small molecule degraders, termed PROteolysis TArgeting Chimeras (PROTACs) have been devised to artificially redirect a protein of interest (POI) to the cellular protein homeostasis machinery for proteasomal degradation (Chamberlain et al., 2019 [3]). In this chapter, the development of PROTACs will first be discussed providing a historical perspective in parallel to the experimental progress made to understand this novel therapeutic modality. Furthermore, common strategies for PROTAC design, including assays and troubleshooting tips will be provided for the reader, before presenting a compendium of all PROTAC targets reported in the literature to date. Due to the recent advancement of these molecules into clinical trials, consideration of pharmacokinetics and pharmacodynamic properties will be introduced, together with the biotech landscape that has developed from the success of PROTACs. Finally, an overview of subsequent strategies for targeted protein degradation will be presented, concluding with further scientific quests triggered by the invention of PROTACs.

CHIP-mediated CIB1 ubiquitination regulated epithelial-mesenchymal transition and tumor metastasis in lung adenocarcinoma

CIB1 is a homolog of calmodulin that regulates cell adhesion, migration, and differentiation. It has been considered as an oncogene in many tumor cells; however, its role in lung adenocarcinoma (LAC) has not been studied. In this study, the expression levels of CIB1 in LAC tissues and adjacent normal tissues were examined by immunohistochemistry, and the relationship between CIB1 expression and patient clinicopathological characteristics was analyzed. The effects of CIB1 on epithelial–mesenchymal transition (EMT), migration, and metastasis of LAC cells were determined in vitro and vivo. Proteins interacting with CIB1 were identified using electrospray mass spectrometry (LS-MS), and CHIP was selected in the following assays. Carboxyl-terminus of Hsp70-interacting protein (CHIP) is a ubiquitin E3 ligase. We show that CHIP can degrade CIB1 via promoting polyubiquitination of CIB1 and its subsequent proteasomal degradation. Besides, lysine residue 10 and 65 of CIB1 is the ubiquitinated site of CIB1. Furthermore, CHIP-mediated CIB1 downregulation is critical for the suppression of metastasis and migration of LAC. These results indicated that CHIP-mediated CIB1 ubiquitination could regulate epithelial–mesenchymal and tumor metastasis in LAC.

Cell cycle-dependent localization of the proteasome to chromatin

An integrative understanding of nuclear events including transcription in normal and cancer cells requires comprehensive and quantitative measurement of protein dynamics that underlie such events. However, the low abundance of most nuclear proteins hampers their detailed functional characterization. We have now comprehensively quantified the abundance of nuclear proteins with the use of proteomics approaches in both normal and transformed human diploid fibroblasts. We found that subunits of the 26S proteasome complex were markedly down-regulated in the nuclear fraction of the transformed cells compared with that of the wild-type cells. The intranuclear proteasome abundance appeared to be inversely related to the rate of cell cycle progression, with restraint of the cell cycle being associated with an increase in the amount of proteasome subunits in the nucleus, suggesting that the nuclear proteasome content is dependent on the cell cycle. Furthermore, chromatin enrichment for proteomics (ChEP) analysis revealed enrichment of the proteasome in the chromatin fraction of quiescent cells and its apparent dissociation from chromatin in transformed cells. Our results thus suggest that translocation of the nuclear proteasome to chromatin may play an important role in control of the cell cycle and oncogenesis through regulation of chromatin-associated transcription factors.

bioPROTACs as versatile modulators of intracellular therapeutic targets including proliferating cell nuclear antigen (PCNA)

Targeted degradation approaches such as proteolysis targeting chimeras (PROTACs) offer new ways to address disease through tackling challenging targets and with greater potency, efficacy, and specificity over traditional approaches. However, identification of high-affinity ligands to serve as PROTAC starting points remains challenging. As a complementary approach, we describe a class of molecules termed biological PROTACs (bioPROTACs)-engineered intracellular proteins consisting of a target-binding domain directly fused to an E3 ubiquitin ligase. Using GFP-tagged proteins as model substrates, we show that there is considerable flexibility in both the choice of substrate binders (binding positions, scaffold-class) and the E3 ligases. We then identified a highly effective bioPROTAC against an oncology target, proliferating cell nuclear antigen (PCNA) to elicit rapid and robust PCNA degradation and associated effects on DNA synthesis and cell cycle progression. Overall, bioPROTACs are powerful tools for interrogating degradation approaches, target biology, and potentially for making therapeutic impacts.

Systematic Nucleotide Exchange Analysis of ESTs From the Human Cancer Genome Project Report: Origins of 347 Unknown ESTs Indicate Putative Transcription of Non-Coding Genomic Regions

Expressed sequence tags (ESTs) provide an imprint of cellular RNA diversity irrespectively of sequence homology with template genomes. NCBI databases include many unknown RNAs from various normal and cancer cells. These are usually ignored assuming sequencing artefacts or contamination due to their lack of sequence homology with template DNA. Here, we report genomic origins of 347 ESTs previously assumed artefacts/unknown, from the FAPESP/LICR Human Cancer Genome Project. EST template detection uses systematic nucleotide exchange analyses called swinger transformations. Systematic nucleotide exchanges replace systematically particular nucleotides with different nucleotides. Among 347 unknown ESTs, 51 ESTs match mitogenome transcription, 17 and 2 ESTs are from nuclear chromosome non-coding regions, and uncharacterized nuclear genes. Identified ESTs mapped on 205 protein-coding genes, 10 genes had swinger RNAs in several biosamples. Whole cell transcriptome searches for 17 ESTs mapping on non-coding regions confirmed their transcription. The 10 swinger-transcribed genes identified more than once associate with cancer induction and progression, suggesting swinger transformation occurs mainly in highly transcribed genes. Swinger transformation is a unique method to identify noncanonical RNAs obtained from NGS, which identifies putative ncRNA transcribed regions. Results suggest that swinger transcription occurs in highly active genes in normal and genetically unstable cancer cells.

Substrate selection by the proteasome through initiation regions

Proteins in the cell have to be eliminated once their function is no longer desired or they become damaged. Most regulated protein degradation is achieved by a large enzymatic complex called the proteasome. Many proteasome substrates are targeted for degradation by the covalent attachment of ubiquitin molecules. Ubiquitinated proteins can be bound by the proteasome, but for proteolysis to occur the proteasome needs to find a disordered tail somewhere in the target at which it initiates degradation. The initiation step contributes to the specificity of proteasomal degradation. Here, we review how the proteasome selects initiation sites within its substrates and discuss how the initiation step affects physiological processes.

Broad-Spectrum Proteome Editing with an Engineered Bacterial Ubiquitin Ligase Mimic

Manipulation of the ubiquitin-proteasome pathway to achieve targeted silencing of cellular proteins has emerged as a reliable and customizable strategy for remodeling the mammalian proteome. One such approach involves engineering bifunctional proteins called ubiquibodies that are comprised of a synthetic binding protein fused to an E3 ubiquitin ligase, thus enabling post-translational ubiquitination and degradation of a target protein independent of its function. Here, we have designed a panel of new ubiquibodies based on E3 ubiquitin ligase mimics from bacterial pathogens that are capable of effectively interfacing with the mammalian proteasomal degradation machinery for selective removal of proteins of interest. One of these, the Shigella flexneri effector protein IpaH9.8 fused to a fibronectin type III (FN3) monobody that specifically recognizes green fluorescent protein (GFP), was observed to potently eliminate GFP and its spectral derivatives as well as 15 different FP-tagged mammalian proteins that varied in size (27-179 kDa) and subcellular localization (cytoplasm, nucleus, membrane-associated, and transmembrane). To demonstrate therapeutically relevant delivery of ubiquibodies, we leveraged a bioinspired molecular assembly method whereby synthetic mRNA encoding the GFP-specific ubiquibody was coassembled with poly A binding proteins and packaged into nanosized complexes using biocompatible, structurally defined polypolypeptides bearing cationic amine side groups. The resulting nanoplexes delivered ubiquibody mRNA in a manner that caused efficient target depletion in cultured mammalian cells stably expressing GFP as well as in transgenic mice expressing GFP ubiquitously. Overall, our results suggest that IpaH9.8-based ubiquibodies are a highly modular proteome editing technology with the potential for pharmacologically modulating disease-causing proteins.

Design and Functional Characterization of Synthetic E3 Ubiquitin Ligases for Targeted Protein Depletion

A number of techniques now exist for decreasing the expression of cellular proteins without the need for genomic modification. One such technique involves engineered protein chimeras that combine the ubiquitination activity of E3 ubiquitin ligases with the binding affinity and substrate specificity of designer binding proteins (DBPs). These chimeras, called "ubiquibodies," are capable of selectively and controllably steering virtually any protein to the ubiquitin proteasome pathway (UPP) for degradation, making ubiquibodies a powerful addition to the protein knockout toolbox. A distinguishing feature of ubiquibodies is their modularity-simply swapping DBPs can generate a new ubiquibody with specificity for a different substrate protein. Moreover, by employing DBPs that recognize particular protein states (e.g., active versus inactive conformation, mutant versus wild-type, post-translational modification), it becomes possible to deplete certain protein subpopulations while sparing others. This protocol outlines the steps necessary to design and functionally evaluate ubiquibodies for customizable silencing of cellular proteins. © 2018 by John Wiley & Sons, Inc.

Targeted Intracellular Delivery of Antibodies: The State of the Art

A dominant area of antibody research is the extension of the use of this mighty experimental and therapeutic tool for the specific detection of molecules for diagnostics, visualization, and activity blocking. Despite the ability to raise antibodies against different proteins, numerous applications of antibodies in basic research fields, clinical practice, and biotechnology are restricted to permeabilized cells or extracellular antigens, such as membrane or secreted proteins. With the exception of small groups of autoantibodies, natural antibodies to intracellular targets cannot be used within living cells. This excludes the scope of a major class of intracellular targets, including some infamous cancer-associated molecules. Some of these targets are still not druggable via small molecules because of large flat contact areas and the absence of deep hydrophobic pockets in which small molecules can insert and perturb their activity. Thus, the development of technologies for the targeted intracellular delivery of antibodies, their fragments, or antibody-like molecules is extremely important. Various strategies for intracellular targeting of antibodies via protein-transduction domains or their mimics, liposomes, polymer vesicles, and viral envelopes, are reviewed in this article. The pitfalls, challenges, and perspectives of these technologies are discussed.

Sculpting ion channel functional expression with engineered ubiquitin ligases

The functional repertoire of surface ion channels is sustained by dynamic processes of trafficking, sorting, and degradation. Dysregulation of these processes underlies diverse ion channelopathies including cardiac arrhythmias and cystic fibrosis. Ubiquitination powerfully regulates multiple steps in the channel lifecycle, yet basic mechanistic understanding is confounded by promiscuity among E3 ligase/substrate interactions and ubiquitin code complexity. Here we targeted the catalytic domain of E3 ligase, CHIP, to YFP-tagged KCNQ1 ± KCNE1 subunits with a GFP-nanobody to selectively manipulate this channel complex in heterologous cells and adult rat cardiomyocytes. Engineered CHIP enhanced KCNQ1 ubiquitination, eliminated KCNQ1 surface-density, and abolished reconstituted K⁺ currents without affecting protein expression. A chemo-genetic variation enabling chemical control of ubiquitination revealed KCNQ1 surface-density declined with a ~ 3.5 hr t_1/2 by impaired forward trafficking. The results illustrate utility of engineered E3 ligases to elucidate mechanisms underlying ubiquitin regulation of membrane proteins, and to achieve effective post-translational functional knockdown of ion channels.

Cells are surrounded by a membrane that separates the outside of the cell from its inside. Proteins called ion channels are embedded within this membrane and allow charged ions to move in and out of the cell. The movement of ions generates electrical currents that are essential for many processes that keep us alive, including our heartbeat and the activity within our brain.

Like many other proteins, newly made ion channels undergo several steps before they mature and become active. Cells destroy any proteins that do not mature properly, as well as those that become damaged or are simply no longer needed. A small protein called ubiquitin helps to mark such unwanted proteins for destruction. Enzymes known as E3 ligases attach ubiquitin to target proteins in a process known as ubiquitination. This process regulates both the quality and amount of proteins within cells.

To understand the role of a particular protein, it is often necessary to remove it from the cell and then examine the consequences. In the past, researchers have harnessed the ubiquitin system to remove many kinds of proteins, but this approach had not previously been used to target an ion channel.

Now, Kanner et al. set out to selectively eliminate ion channels via targeted ubiquitination. The experiments showed that previous approaches that could destroy proteins within the cell were not effective against ion channels. Kanner et al. then engineered a particular E3 ligase so that it could selectively attach ubiquitin to the desired ion channels. This approach successfully prevented the channels from reaching the cell membrane, thereby silencing the electrical currents that they normally generate. Additionally, a new tool was developed to stop ion channels in their tracks, essentially with a flip of a chemical switch. Kanner et al. then used this approach to manipulate ion channels in a highly controlled manner, within their normal environment of heart muscle cells.

These new approaches form a toolset that scientists can now exploit to study diverse ion channels. In the future, the toolkit could potentially be used to develop treatments for disorders such as epilepsy, chronic pain, and irregular heartbeats, where too many channels are active or present at the cell membrane.

Carboxy-terminus Hsc70 interacting protein exerts a tumor inhibition function in head and neck cancer

Several independent studies have reported the roles of the E3 ubiquitin ligase, carboxy-terminus Hsc70 interacting protein (CHIP) in various types of cancers. However, the biological effects of CHIP vary in regards to different cancers, and the role of CHIP in head and neck cancers (HNCs) remains unknown. In the present study, CHIP overexpression plasmids and CHIP knockdown lentivirus were constructed to affect the expression levels of CHIP protein and biological behaviors in HNC cell lines bilaterally. The biological behaviors regulated by CHIP in HNCs were investigated both in vivo and in vitro with a series of assays and analyses. A tissue microarray was stained and analyzed for the clinical significance of CHIP expression in HNCs. We identified that CHIP suppressed the malignant behaviors of HNCs in a series of in vitro and in vivo experiments, but not its two loss-of-function mutants. However, we observed an altered expression pattern of CHIP from a well, moderate, to poor differentiation pathological status in HNC specimens. In a retrospective cohort of HNCs, lower expression of CHIP indicated a poor differentiation status in tumors and a lower overall survival rate. The present study demonstrated that CHIP functions as a tumor suppressor in HNCs. In conclusion, we demonstrated that suppressed expression of CHIP may result in the progression of HNCs.

Synergistic role of Cul1 and c-Myc: Prognostic and predictive biomarkers in colorectal cancer

Colorectal cancer (CRC) is one of the most common malignant tumors, and its high rates of recurrence and metastasis are the important causes of treatment failure in CRC. Therefore, the development of valuable molecular markers to accurately predict the prognosis of CRC patients is vital. In the present study, we determined the expression of Cullin1 (Cul1) and c-Myc in a CRC tissue microarray containing 470 cancer and corresponding normal tissues by immunohistochemistry. We found that Cul1 and c-Myc expression was significantly upregulated in the CRC cancer tissues compared with that noted in the adjacent non-cancer tissues. High Cul1 expression in cancer tissues was associated with depth of invasion (P=0.005), lymph node metastasis (P=0.001) and TNM stage (P=0.015). High c-Myc expression in cancer tissues was significantly positively association with age (P=0.004), depth of invasion (P<0.001), lymph node metastasis (P<0.001) and TNM stage (P<0.001). Multivariate Cox regression analysis revealed that Cul1 or c-Myc expression was an independent and unfavorable prognostic factor for CRC patients [hazard ratio (HR), 0.749, 95% confidence interval (CI), 0.563-0.996, P<0.05; and HR, 0.384, 95% CI, 0.257-0.472, P<0.001, respectively]. Furthermore, Cul1 and c-Myc exhibited synergistic potential for the prediction of CRC prognosis, and the patients with low expression of both Cul1 and c-Myc had a favorable survival outcome (P<0.001).

The E3 ubiquitin ligase CHIP selectively regulates mutant epidermal growth factor receptor by ubiquitination and degradation

Somatic mutation in the tyrosine kinase domain of epidermal growth factor receptor (EGFR) is a decisive factor for the therapeutic response to EGFR tyrosine kinase inhibitors (EGFR-TKIs) in lung adenocarcinoma. The stability of mutant EGFR is maintained by various regulators, including heat shock protein 90 (Hsp90). The C terminus of Hsc70-interacting protein (CHIP) is a Hsp70/Hsp90 co-chaperone and exhibits E3 ubiquitin ligase activity. The high-affinity Hsp90-CHIP complex recognizes and selectively regulates their client proteins. CHIP also works with its own E3 ligase activity independently of Hsp70/Hsp90. Here, we investigated the role of CHIP in regulating EGFR in lung adenocarcinoma and also evaluated the specificity of CHIP's effects on mutant EGFR. In HEK 293T cells transfected with either WT EGFR or EGFR mutants, the overexpression of CHIP selectively decreased the expression of certain EGFR mutants (G719S, L747_E749del A750P and L858R) but not WT EGFR. In a pull-down assay, CHIP selectively interacted with EGFR mutants and simultaneously induced their ubiquitination and proteasomal degradation. The expressions of mutant EGFR in PC9 and H1975 were diminished by CHIP, while the expression of WT EGFR in A549 was nearly not affected. In addition, CHIP overexpression inhibited cell proliferation and xenograft's tumor growth of EGFR mutant cell lines, but not WT EGFR cell lines. EGFR mutant specific ubiquitination by CHIP may provide a crucial regulating mechanism for EGFR in lung adenocarcinoma. Our results suggest that CHIP can be novel therapeutic target for overcoming the EGFR TKI resistance.

A recombinant chimeric protein specifically induces mutant KRAS degradation and potently inhibits pancreatic tumor growth

Pancreatic cancer is one of the most lethal human diseases, with an all-stage 5-year survival rate below 5%. To date, no effective and specific therapy is available for this disease. Mutations in KRAS are frequently reported in pancreatic and many other cancers; thus, KRAS is an attractive therapeutic target. Our objective was to specifically eliminate mutant KRAS and induce cell death of tumors expressing this mutant protein. We thus constructed several chimeric proteins by connecting the C-terminal domains of several adaptor proteins of E3 ubiquitin ligases such as CBL, CHIP, E6AP, and VHL, as well as VIF encoded by human immunodeficiency virus type 1 (HIV-1), to the Ras binding domain (RBD) of Raf. Although all of these chimeric proteins caused the degradation of mutant KRAS and the death of KRAS-mutant-tumor cell lines, the RBD-VIF with a protein transduction domain (PTD), named PTD-RBD-VIF, had the strongest tumor-killing effect. Intraperitoneally administered recombinant PTD-RBD-VIF potently inhibited the growth of xenografted KRAS-mutant pancreatic cancer cells. Our findings indicate that recombinant PTD-RBD-VIF, a chimeric protein with a combined cellular-viral origin, could be further developed for the treatment of various tumors harboring mutant or over-activated KRAS, especially for cases presenting with pancreatic cancer recurrence after surgery.

The chimeric ubiquitin ligase SH2-U-box inhibits the growth of imatinib-sensitive and resistant CML by targeting the native and T315I-mutant BCR-ABL

Chronic myeloid leukemia (CML) is characterized by constitutively active fusion protein tyrosine kinase BCR-ABL. Although the tyrosine kinase inhibitor (TKI) against BCR-ABL, imatinib, is the first-line therapy for CML, acquired resistance almost inevitably emerges. The underlying mechanism are point mutations within the BCR-ABL gene, among which T315I is notorious because it resists to almost all currently available inhibitors. Here we took use of a previously generated chimeric ubiquitin ligase, SH2-U-box, in which SH2 from the adaptor protein Grb2 acts as a binding domain for activated BCR-ABL, while U-box from CHIP functions as an E3 ubiquitin ligase domain, so as to target the ubiquitination and degradation of both native and T315I-mutant BCR-ABL. As such, SH2-U-box significantly inhibited proliferation and induced apoptosis in CML cells harboring either the wild-type or T315I-mutant BCR-ABL (K562 or K562R), with BCR-ABL-dependent signaling pathways being repressed. Moreover, SH2-U-box worked in concert with imatinib in K562 cells. Importantly, SH2-U-box-carrying lentivirus could markedly suppress the growth of K562-xenografts in nude mice or K562R-xenografts in SCID mice, as well as that of primary CML cells. Collectively, by degrading the native and T315I-mutant BCR-ABL, the chimeric ubiquitin ligase SH2-U-box may serve as a potential therapy for both imatinib-sensitive and resistant CML.

An Inducible System for Rapid Degradation of Specific Cellular Proteins Using Proteasome Adaptors

A common way to study protein function is to deplete the protein of interest from cells and observe the response. Traditional methods involve disrupting gene expression but these techniques are only effective against newly synthesized proteins and leave previously existing and stable proteins untouched. Here, we introduce a technique that induces the rapid degradation of specific proteins in mammalian cells by shuttling the proteins to the proteasome for degradation in a ubiquitin-independent manner. We present two implementations of the system in human culture cells that can be used individually to control protein concentration. Our study presents a simple, robust, and flexible technology platform for manipulating intracellular protein levels.

Expression of HAX-1 in colorectal cancer and its role in cancer cell growth

Colorectal cancer (CRC) is one of the most common types of cancer worldwide. Hematopoietic cell-specific protein 1-associated protein X-1 (HAX-1) has been found to be involved in several types of cancer. However, the role of HAX-1 in CRC remains to be elucidated. The aim of the present study was to investigate whether the expression of HAX-1 is associated with the progression of CRC, and to determine the effects of HAX-1 on the apoptosis and proliferation of CRC cells. Tumor tissues and adjacent noncancerous tissues were collected from 60 patients with CRC, following the provision of informed consent. The expression levels of HAX-1 and the association with clinical and pathological characteristics were then analyzed. The expression levels of HAX-1 were significantly higher in the cancerous tissues from the patients with CRC, particularly in tissues of an advanced stage of cancer. In addition, HAX-1 expression was associated with malignant progression and poor prognosis. Furthermore, SW480 CRC cells, overexpressing HAX-1, exhibited increased resistance to camptothecin in vitro, and promoted proliferation in vitro and in vivo. By contrast, HAX-1 knockdown significantly decreased the proliferation. In addition, the expression levels of ki-67 and phosphorylatedakt were inhibited following HAX-1 knockdown. In conclusion, the expression levels of HAX-1 were increased in cancerous tissue from patients with CRC, and were associated with progression of the disease. These results suggested that HAX-1 may contribute to chemotherapy resistance and malignant progression in CRC.

Tetramethylpyrazine inhibits the proliferation of acute lymphocytic leukemia cell lines via decrease in GSK-3β

Tetramethylpyrazine (TMP) has been proven to be an anticancer agent in many studies. However, its effectiveness in acute lymphoblastic leukemia (ALL) and its molecular mechanisms are still unclear. The present study aimed to evaluate the effect of TMP against Jurkat and SUP-B15 ALL cell lines and to investigate the possible detailed mechanism of action of TMP. A Cell Counting Kit-8 (CCK-8) assay was employed to examine the proliferation of Jurkat and SUP-B15 cells. Flow cytometric analysis was conducted to detect the cell cycle distribution and apoptotic rate. The expression of total glycogen synthase kinase-3β (GSK-3β), cox-2, survivin, bcl-2 and p27 RNA and protein levels was detected by quantitative real-time PCR and western blot assay, respectively. Additionally, western blot analysis was used to determine the whole-cell and nuclear protein levels of GSK-3β downstream transcription factors, NF-κB (p65) and c-myc. TMP inhibited the proliferation of Jurkat and SUP-B15 cells in a dose- and time-dependent manner, with IC₅₀ values of 120 and 200 µg/ml, respectively at 48 h. TMP induced the apoptosis of Jurkat and SUP-B15 cells and synergistically blocked cell cycle progression at the G0/G1 phase. Cells treated with TMP exhibited significantly attenuated GSK-3β, NF-κB (p65) and c-myc expression, followed by downregulation of bcl-2, cox-2 and survivin and an upregulation of p27. The results showed that TMP induced apoptosis and caused cell cycle arrest in Jurkat and SUP-B15 cells through the downregulation of GSK-3β, which may have further prevented the induced translocation of NF-κB and c-myc from the cytoplasm to the nucleus.

Ubiquitin-specific peptidase 22 overexpression may promote cancer progression and poor prognosis in human gastric carcinoma

Ubiquitin-specific peptidase 22 (USP22) was recently identified as a new tumor cell marker, and previous studies demonstrated its expression in a variety of tumors and its correlation with tumor progression. Because tumor progression plays an important role in cancer, researchers are paying more attention to the correlation between USP22 expression and metastatic potential, resistance to chemotherapy, and patient prognosis. This study showed that USP22 is highly expressed in gastric cancer tissues, and significant differences in USP22 expression (P < 0.01) were identified between different types of gastric cancer (the highest expression was found in poorly differentiated adenocarcinomas). In addition USP22 expression was found to be correlated with the promotion of cancer evolution, tumor invasion, and lymph node metastasis. The C-myc protein was also shown to have synergistic effects with USP22 in gastric cancer tissue. On the basis of the results, USP22 expression may play an important role in gastric carcinoma tissue, particularly in precancerous lesions during the gastric cancer evolution process.

CXCL10 expression induced by Mxi1 inactivation induces mesangial cell apoptosis in mouse Habu nephritis

MAX interactor 1 (Mxi1) proteins are c-myc antagonists that primarily exert their biological functions by inhibiting Myc-dependent gene transcription. In this study, Mxi1(-/-) mice were used to generate a model of mesangial proliferative glomerulonephritis for the first time. In the present study, we demonstrated that Mxi1(-/-) mice exhibited a more typical and severe pathological phenotype, which was displayed primarily as a noticeable dissolution phenotype with a higher proportion of apoptotic cells and higher chemokine CXCL10 expression during the early days of modeling, compared with wild-type mice. Additionally, we determined that IRF3-mediated TLR4 signaling was likely involved in regulating CXCL10 expression, which might participate in the mesangial dissolution process. We also found increases in CXCL10 expression, caspase 3 activation, and the proportion of apoptotic cells when Mxi1 expression was inhibited in mouse mesangial cells. Furthermore, the proportion of apoptotic cells decreased after inhibiting CXCL10 expression. Therefore, we concluded that the mesangial cell apoptosis observed in this mesangial proliferative glomerulonephritis model was related to CXCL10 expression induced by Mxi1 inactivation. This finding provides a new theoretical basis for the mechanism of mesangial proliferative glomerulonephritis progression and reveals potential intervention targets for the early treatment of this disease.

Ubiquibodies, synthetic E3 ubiquitin ligases endowed with unnatural substrate specificity for targeted protein silencing

The ubiquitin-proteasome pathway (UPP) is the main route of protein degradation in eukaryotic cells and is a common mechanism through which numerous cellular pathways are regulated. To date, several reverse genetics techniques have been reported that harness the power of the UPP for selectively reducing the levels of otherwise stable proteins. However, each of these approaches has been narrowly developed for a single substrate and cannot be easily extended to other protein substrates of interest. To address this shortcoming, we created a generalizable protein knock-out method by engineering protein chimeras called "ubiquibodies" that combine the activity of E3 ubiquitin ligases with designer binding proteins to steer virtually any protein to the UPP for degradation. Specifically, we reprogrammed the substrate specificity of a modular human E3 ubiquitin ligase called CHIP (carboxyl terminus of Hsc70-interacting protein) by replacing its natural substrate-binding domain with a single-chain Fv (scFv) intrabody or a fibronectin type III domain monobody that target their respective antigens with high specificity and affinity. Engineered ubiquibodies reliably transferred ubiquitin to surface exposed lysines on target proteins and even catalyzed the formation of biologically relevant polyubiquitin chains. Following ectopic expression of ubiquibodies in mammalian cells, specific and systematic depletion of desired target proteins was achieved, whereas the levels of a natural substrate of CHIP were unaffected. Taken together, engineered ubiquibodies offer a simple, reproducible, and customizable means for directly removing specific cellular proteins through accelerated proteolysis.

Targeted degradation of KRAS by an engineered ubiquitin ligase suppresses pancreatic cancer cell growth in vitro and in vivo

KRAS is an attractive pancreatic ductal adenocarcinoma (PDAC) therapeutic target. E3 ligase is thought to be the component of the ubiquitin conjugation system that is directly responsible for substrate recognition. In this study, an engineered E3 ubiquitin ligase (RC-U) was generated to target the KRAS oncoprotein for ubiquitination and degradation. The engineered E3 ubiquitin ligases (RC-U) were constructed (pRC-U and lentivirus-expressing RC-U). After transfecting the pRC-U plasmid into human pancreatic cancer cells, KRAS expression levels were determined. KRAS expression was also evaluated in cells transfected with pRC-U and treated with MG-132 or cycloheximide. Interactions between RC-U and KRAS as well as whether RC-U could ubiquitinate KRAS were investigated. Extracellular signal-regulated kinase 1/2 (ERK1/2) and phosphorylated ERK 1/2 (pERK1/2) levels were examined in pancreatic cancer cells transfected with pRC-U. The effects of RC-U on pancreatic cancer cell growth were assessed. RC-U decreased KRAS protein levels. After pRC-U transfection, KRAS stability was increased in the presence of MG-132. HEK 293T cells were transfected with a mutant KRAS construct together with pRC-U and incubated with cycloheximide to inhibit new protein synthesis. The exogenous mutant KRAS oncoprotein was degraded more quickly. RC-U can bind KRAS and KRAS can be ubiquitinated by RC-U. pERK1/2 protein levels were decreased. RC-U resulted in reduced cell proliferation in vitro and in vivo. KRAS destruction by RC-U occurred through a ubiquitin-dependent, proteasome-mediated degradation pathway. RC-U inhibited pancreatic cancer cell growth in vitro and in vivo.

Proteolytic control of the oncoprotein transcription factor Myc

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

2,4-Dichlorophenoxyacetic acid: effects on Syrian hamster embryo (SHE) cell transformation, c-Myc expression, DNA damage and apoptosis

2,4-Dichlorophenoxyacetic acid (2,4-D) is a selective, systemic auxin-type herbicide extensively used throughout the world. The present research was aimed at studying effects of low and non-cytotoxic concentrations of 2,4-D on SHE cells in relation with carcinogenicity. Effects were studied on Syrian hamster morphological cell transformation, c-Myc expression - both at the gene and protein level - DNA damage and apoptosis. 2,4-D significantly induced cell transformation at 11.5 microM and 23 microM (i.e. 2.5 microg/mL and 5 microg/mL). An increase in the expression of the transcription factor c-Myc, measured by use of RT-PCR with respect to mRNA level and by Western blotting for protein level was registered at these concentrations, as well as genotoxic effects evaluated with the single-cell gel electrophoresis (Comet) assay. Consequences for apoptosis of 2,4-D treatment were also investigated. The fluorochrome acridine orange was used to study DNA fragmentation as a marker of apoptosis. No effect on apoptosis was found at 2,4-D concentrations that induced cell transformation. This was confirmed by the unchanged expression of Bcl-2 and Bax, two regulator genes of the mitochondrial pathway of apoptosis. Our results demonstrate the transforming and genotoxic effects of low concentrations of 2,4-D in mammalian cells. This information contributes to a better understanding of the mechanism of 2,4-D toxicity in mammalian cells and demonstrates that 2,4-D should be considered as potentially hazardous to humans.

Ligand-dependent transcription of estrogen receptor alpha is mediated by the ubiquitin ligase EFP

Estrogen-mediated ubiquitylation and subsequent degradation of the estrogen receptor alpha (ERalpha) appears to be involved in the transcriptional activity of ERalpha. We show that the estrogen-responsive finger protein (EFP) interacts with and ubiquitylates ERalpha. EFP promoted the ubiquitylation of ERalphain vitro and in vivo and consequently promoted the degradation of ERalpha. The interaction between EFP and ERalpha was greatly enhanced in the presence of estrogen. The action of EFP on ERalpha in the presence of estrogen resulted in a robust interaction between ERalpha and Tip60, one of the transcriptional coactivators, leading to activation of ERalpha transcriptional activity. However, a dominant negative mutant of EFP lacking the RING domain prolonged the half-life of ERalpha and inhibited the transcription by ERalpha. Our results indicate that EFP functions as a cofactor for ERalpha-mediated transcription, thus suggesting that ERalpha-mediated transcription is closely linked to the ubiquitylation of ERalpha.

Dorfin-CHIP chimeric proteins potently ubiquitylate and degrade familial ALS-related mutant SOD1 proteins and reduce their cellular toxicity

The ubiquitin-proteasome system (UPS) is involved in the pathogenetic mechanisms of neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). Dorfin is a ubiquitin ligase (E3) that degrades mutant SOD1 proteins, which are responsible for familial ALS. Although Dorfin has potential as an anti-ALS molecule, its life in cells is short. To improve its stability and enhance its E3 activity, we developed chimeric proteins containing the substrate-binding hydrophobic portion of Dorfin and the U-box domain of the carboxyl terminus of Hsc70-interacting protein (CHIP), which has strong E3 activity through the U-box domain. All the Dorfin-CHIP chimeric proteins were more stable in cells than was wild-type Dorfin (Dorfin(WT)). One of the Dorfin-CHIP chimeric proteins, Dorfin-CHIP(L), ubiquitylated mutant SOD1 more effectively than did Dorfin(WT) and CHIP in vivo, and degraded mutant SOD1 protein more rapidly than Dorfin(WT) does. Furthermore, Dorfin-CHIP(L) rescued neuronal cells from mutant SOD1-associated toxicity and reduced the aggresome formation induced by mutant SOD1 more effectively than did Dorfin(WT).

Down-regulation of c-Myc following MEK/ERK inhibition halts the expression of malignant phenotype in rhabdomyosarcoma and in non muscle-derived human tumors

Background

Expression of c-myc proto-oncogene is inappropriate in a wide range of human tumors, and is a downstream target of Ras/Raf/ERK pathway, which promotes c-Myc stability by enhancing c-Myc expression and activity.

The aim of this study was to investigate whether the oncogenic phenotype in the human muscle-derived Rhabdomyosarcoma (RD) cell line and in non muscle-derived human tumor cell lines (SW403, IGR39 and PC3) can be blocked by disrupting the c-Myc pathway either by means of pharmacological MEK/ERK inhibition or by direct inactivation of the c-Myc protein.

Results

We demonstrate that, in all the tumor cell lines used, the MEK/ERK inhibitor U0126 rapidly induces c-Myc de-phosphorylation, which is followed by a marked reduction in its expression level, by inhibition of proliferation and by reversion of anchorage-independent growth. These data suggest that the targeting of pathways controlling c-Myc expression or stability reverses deregulated growth of different tumor-derived cell lines. Indeed, in RD cells, we found a marked down-regulation of cyclins E2, A and B and CDK2, all of which are known to be targets of c-Myc. Moreover, ectopic MadMyc chimera, a c-Myc function antagonist, causes dramatic growth arrest, CDK and cyclin modulation as well as inhibition of anchorage-independent growth in RD cells, as occurs in U0126-treated cells. In particular, we found that the mere inhibition of c-Myc by MadMyc chimera rescues the myogenic program, MHC expression and the acquisition of the myogenic-like phenotype in RD cells.

Conclusion

Our data provide evidence of the key role played by the MEK/ERK pathway in the growth arrest and transformation phenotype of Rhabdomyosarcoma and of non muscle-derived tumor cell lines. In fact, MEK/ERK inhibitor, U0126, induces growth arrest, anchorage-dependent growth of these cell lines. In addition, the results of this study demonstrate that the direct inactivation of c-Myc by Mad/Myc chimera rescues myogenic program and leads to the reversal of the Rhabdomyosarcoma phenotype. In conclusion these data strongly suggest that the targeting of c-Myc by means of the MEK inhibitor can be tested as a promising strategy in anti-cancer therapy.

The Myc oncoprotein as a therapeutic target for human cancer

Myc expression is deregulated in a wide range of human cancers and is often associated with aggressive, poorly differentiated tumors. The Myc protein is a transcription factor that regulates a variety of cellular processes including cell growth and proliferation, cell-cycle progression, transcription, differentiation, apoptosis, and cell motility. Potential strategies that either inhibit the growth promoting effect of Myc and/or activate its pro-apoptotic function are presently being explored. In this review, we give an overview of Myc activation in human tumors and discuss current strategies aimed at targeting Myc for cancer treatment. Such therapies could have potential in combination with mechanistically different cytotoxic drugs to combat and eradicate tumors cells.

Fbxw7 contributes to tumor suppression by targeting multiple proteins for ubiquitin-dependent degradation

Fbxw7 (also known as Sel-10, hCdc4 or hAgo) is the F-box protein component of a Skp1-Cul1-F-box protein (SCF) ubiquitin ligase. Fbxw7 contributes to the ubiquitin-mediated degradation of cyclin E, c-Myc, Aurora-A, Notch and c-Jun, all of which appear to function as cell-cycle promoters and oncogenic proteins. Loss of Fbxw7 results in elevated expression of its substrates, which may lead to oncogenesis. However, it remains largely unclear which accumulating substrate is most related to cancer development in Fbxw7-mutant cancer cells. In the present study, we examined the abundance of cyclin E, c-Myc and Aurora-A in seven cancer cell lines, which harbor wild-type (three lines) or mutant (four lines) Fbxw7. Although these three substrates accumulated in the Fbxw7-mutant cells, the extent of increase in the expression of these proteins varied in each line. Forced expression of Fbxw7 reduced the levels of cyclin E, c-Myc and Aurora-A in the Fbxw7-mutant cells. In contrast, a decrease in the expression of cyclin E, c-Myc or Aurora-A by RNA interference significantly suppressed the rate of proliferation and anchorage-independent growth of the Fbxw7-mutant cells. These findings thus suggest that the loss of Fbxw7 results in accumulation of cyclin E, c-Myc and Aurora-A, all of which appear to be required for growth promotion of cancer cells. Fbxw7 seems to regulate the levels of multiple targets to suppress cancer development.

The ubiquitin-proteasome system at the crossroads of stress-response and ageing pathways: a handle for skin care?

The regulation of gene expression at the transcriptional level has been considered for long as the main mechanism of cellular adaptive responses. Since the turn of the century, however, it is becoming clear that higher organisms developed a complex, sensitive and maybe equally important network of regulatory pathways, relying largely on protein interactions, post-translational modifications and proteolysis. Here we review the involvement of the ubiquitin-proteasome pathway of protein degradation at different levels of cellular life in relation with ageing, and with a special focus on skin. It comes out that the ubiquitin system plays a major role in signal transduction associated with stress and ageing, in skin in particular through the control of retinoid and NF-kappaB pathways. The understanding of specific proteolytic targeting by E3 ubiquitin-ligases paves the way for a new generation of active molecules that may control particular steps of normal and pathological ageing.