The E3 ubiquitin ligase Itch regulates tumor suppressor protein RASSF5/NORE1 stability in an acetylation-dependent manner

UBE4B interacts with the ITCH E3 ubiquitin ligase to induce Ku70 and c-FLIPL polyubiquitination and enhanced neuroblastoma apoptosis

Expression of the UBE4B ubiquitin ligase is strongly associated with neuroblastoma patient outcomes, but the functional roles of UBE4B in neuroblastoma pathogenesis are not known. We evaluated interactions of UBE4B with the E3 ubiquitin ligase ITCH/AIP4 and the effects of UBE4B expression on Ku70 and c-FLIPL ubiquitination and proteasomal degradation by co-immunoprecipitation and Western blots. We also evaluated the role of UBE4B in apoptosis induced by histone deacetylase (HDAC) inhibition using Western blots. UBE4B binding to ITCH was mediated by WW domains in the ITCH protein. ITCH activation led to ITCH-UBE4B complex formation and recruitment of Ku70 and c-FLIPL via ITCH WW domains, followed by Ku70 and c-FLIPL Lys48/Lys63 branched polyubiquitination and proteasomal degradation. HDAC inhibition induced Ku70 acetylation, leading to release of c-FLIPL and Bax from Ku70, increased Ku70 and c-FLIPL Lys48/Lys63 branched polyubiquitination via the ITCH-UBE4B complex, and induction of apoptosis. UBE4B depletion led to reduced polyubiquitination and increased levels of Ku70 and c-FLIPL and to reduced apoptosis induced by HDAC inhibition via stabilization of c-FLIPL and Ku70 and inhibition of caspase 8 activation. Our results have identified novel interactions and novel targets for UBE4B ubiquitin ligase activity and a direct role for the ITCH-UBE4B complex in responses of neuroblastoma cells to HDAC inhibition, suggesting that the ITCH-UBE4B complex plays a critical role in responses of neuroblastoma to therapy and identifying a potential mechanism underlying the association of UBE4B expression with neuroblastoma patient outcomes.

Targeting proteostasis network in osteoporosis: Pathological mechanisms and therapeutic implications

As the most common bone disease, osteoporosis (OP) increases bone fragility and makes patients more vulnerable to the threat of osteoporotic fractures. With the ageing population in today's society, OP has become a huge and growing public health problem. Unfortunately, the clear pathogenesis of OP is still under exploration, and effective interventions are still scarce. Therefore, exploring new targets for pharmacological interventions to develop promising therapeutic drugs for OP is of great clinical value. Previous studies have shown that normal bone remodeling depends on proteostasis, whereas loss of proteostasis during ageing leads to the dysfunctional proteostasis network (PN) that fails to maintain bone homeostasis. Nevertheless, only a few studies have revealed the pathophysiological relationship between bone metabolism and a single component of PN, yet the role of PN as a whole in the pathogenesis of OP is still under investigation. This review comprehensively summarized the role of PN in the pathogenesis of OP and further discussed the potential of PN as innovative drug targets for the therapy of OP.

The role of NEDD4 related HECT-type E3 ubiquitin ligases in defective autophagy in cancer cells: molecular mechanisms and therapeutic perspectives

The homologous to the E6-AP carboxyl terminus (HECT)-type E3 ubiquitin ligases are the selective executers in the protein ubiquitination, playing a vital role in modulation of the protein function and stability. Evidence shows the regulatory role of HECT-type E3 ligases in various steps of the autophagic process. Autophagy is an intracellular digestive and recycling process that controls the cellular hemostasis. Defective autophagy is involved in tumorigenesis and has been detected in various types of cancer cells. A growing body of findings indicates that HECT-type E3 ligases, in particular members of the neural precursor cell expressed developmentally downregulated protein 4 (NEDD4) including NEDD4-1, NEDD4-L, SMURFs, WWPs, and ITCH, play critical roles in dysregulation or dysfunction of autophagy in cancer cells. The present review focuses on NEDD4 E3 ligases involved in defective autophagy in cancer cells and discusses their autophagic function in different cancer cells as well as substrates and the signaling pathways in which they participate, conferring a basis for the cancer treatment through the modulating of these E3 ligases.

Epigenetics and genetics of hepatoblastoma: Linkage and treatment

Hepatoblastoma is a malignant embryonal tumor with multiple differentiation modes and is the clearest liver malignancy in children. However, little is known about genetic and epigenetic events in Hepatoblastoma. Increased research has recently demonstrated, unique genetic and epigenetic events in Hepatoblastoma, providing insights into its origin and precise treatment. Some genetic disorders and congenital factors are associated with the risk of Hepatoblastoma development, such as the Beckwith-Wiedemann syndrome, Familial Adenomatous polyposis, and Hemihypertrophy. Epigenetic modifications such as DNA modifications, histone modifications, and non-coding RNA regulation are also essential in the development of Hepatoblastoma. Herein, we reviewed genetic and epigenetic events in Hepatoblastoma, focusing on the relationship between these events and cancer susceptibility, tumor growth, and prognosis. By deciphering the genetic and epigenetic associations in Hepatoblastoma, tumor pathogenesis can be clarified, and guide the development of new anti-cancer drugs and prevention strategies.

Deletion of Wild-type p53 Facilitates Bone Metastatic Function by Blocking the AIP4 Mediated Ligand-Induced Degradation of CXCR4

Background: Management of patients with prostate cancer and bone metastatic disease remains a major clinical challenge. Loss or mutation of p53 has been identified to be involved in the tumor progression and metastasis. Nevertheless, direct evidence of a specific role for wild-type p53 (wt-p53) in bone metastasis and the mechanism by which this function is mediated in prostate cancer remain obscure.

Methods: The expression and protein levels of wt-53, AIP4, and CXCR4 in prostate cancer cells and clinical specimens were assessed by real-time PCR, immunohistochemistry and western blot analysis. The role of wt-p53 in suppressing aggressive and metastatic tumor phenotypes was assessed using in vitro transwell chemotaxis, wound healing, and competitive colocalization assays. Furthermore, whether p53 deletion facilitates prostate cancer bone-metastatic capacity was explored using an in vivo bone-metastatic model. The mechanistic model of wt-p53 in regulating gene expression was further explored by a luciferase reporter assay and chromatin immunoprecipitation (ChIP) assay.

Results: Our findings revealed that wt-p53 suppressed the prostate cancer cell migration rate, chemotaxis and attachment toward the osteoblasts in vitro. The bone-metastatic model showed that deletion of wt-p53 remarkably increased prostate cancer bone-metastatic capacity in vivo. Mechanistically, wt-p53 could induce the ligand-induced degradation of the chemokine receptor CXCR4 by transcriptionally upregulating the expression of ubiquitin ligase AIP4. Treatment with the CXCR4 inhibitor AMD3100 or transduction of the AIP4 plasmid abrogated the pro-bone metastasis effects of TP53 deletion.

Conclusion: Wt-p53 suppresses the metastasis of prostate cancer cells to bones by regulating the CXCR4/CXCL12 activity in the tumor cells/bone marrow microenvironment interactions. Our findings suggest that targeting the wt-p53/AIP4/CXCR4 axis might be a promising therapeutic strategy to manage prostate cancer bone metastasis.

Exosomes Immunity Strategy: A Novel Approach for Ameliorating Intervertebral Disc Degeneration

Low back pain (LBP), which is one of the most severe medical and social problems globally, has affected nearly 80% of the population worldwide, and intervertebral disc degeneration (IDD) is a common musculoskeletal disorder that happens to be the primary trigger of LBP. The pathology of IDD is based on the impaired homeostasis of catabolism and anabolism in the extracellular matrix (ECM), uncontrolled activation of immunologic cascades, dysfunction, and loss of nucleus pulposus (NP) cells in addition to dynamic cellular and biochemical alterations in the microenvironment of intervertebral disc (IVD). Currently, the main therapeutic approach regarding IDD is surgical intervention, but it could not considerably cure IDD. Exosomes, extracellular vesicles with a diameter of 30–150 nm, are secreted by various kinds of cell types like stem cells, tumor cells, immune cells, and endothelial cells; the lipid bilayer of the exosomes protects them from ribonuclease degradation and helps improve their biological efficiency in recipient cells. Increasing lines of evidence have reported the promising applications of exosomes in immunological diseases, and regarded exosomes as a potential therapeutic source for IDD. This review focuses on clarifying novel therapies based on exosomes derived from different cell sources and the essential roles of exosomes in regulating IDD, especially the immunologic strategy.

JNK/Itch Axis Mediates the Lipopolysaccharide-Induced Ubiquitin-Proteasome-Dependent Degradation of Ferritin Light Chain in Murine Macrophage Cells

Ferritin, which is composed of a heavy chain and a light chain, plays a critical role in maintaining iron homeostasis by sequestering iron. The ferritin light chain (FTL) is responsible for the stability of the ferritin complex. We have previously shown that overexpression of FTL decreases the levels of the labile iron pool (LIP) and reactive oxygen species (ROS) in lipopolysaccharide (LPS)-treated murine macrophage cells. The protein level of FTL was downregulated by LPS within a short treatment period. However, the mechanism underlying the LPS-induced changes in the FTL levels is not known. In the present study, we report that LPS induces the ubiquitin-proteasome-dependent degradation of FTL and that the mechanism of LPS-induced FTL degradation involves the JNK/Itch axis. We found that LPS downregulates the protein and mRNA levels of FTL in a time-dependent manner. The proteasome inhibitor MG-132 significantly reverses the LPS-induced decrease in FTL. Furthermore, we observed that LPS treatment cannot cause ubiquitination of the lysine site (K105 and K144) mutant of FTL. Interestingly, LPS-mediated ubiquitin-dependent degradation of FTL is significantly inhibited by the JNK-specific inhibitor SP600125. Moreover, LPS could upregulate the protein level of E3 ubiquitin ligase Itch, a substrate of JNK kinases. Immunoprecipitation analyses revealed an increase in the association of FTL with Itch, a substrate of JNK kinases, in response to LPS stimulation. SP600125 decreased LPS-induced Itch upregulation. Taken together, these results suggest that LPS stimulation leads to the degradation of FTL through the ubiquitin-proteasome proteolytic pathway, and this FTL degradation is mediated by the JNK/Itch axis in murine macrophage cells.

NORE1A directs apoptotic switch of TNF signaling through reciprocal modulation of ITCH-mediated destruction of TNFRI and BAX

NORE1A (RASSF5) is a tumor suppressor of the Ras-association domain family (RASSF) that is commonly inactivated in multiple human cancers. However, the molecular mechanism underlying its growth inhibition function remains largely undefined. Here we report that NORE1A antagonizes tumor necrosis factor receptor I (TNFRI) through the assembly of ITCH-mediated destruction complex to suppress TNF-NF-κB signaling and tumorigenesis. Moreover, NORE1A is identified as a transcription target of NF-κB, which directs an apoptotic switch of TNF effect by blocking ITCH interaction with and ubiquitination of BAX. Mechanistically, NORE1A binds directly to TNFRI and ITCH via the C1 and PPXY domains, respectively to facilitate the formation of ITCH-mediated destruction complex followed by ubiquitination-mediated lysosomal degradation of TNFRI. Through this function, NORE1A suppresses TNF-induced NF-κB-mediated transcription of pro-inflammatory and tumor-promoting genes, epithelial-to-mesenchymal transition, invasion and migration of tumor cells, and also debilitates tumor cell activation of macrophage and fibroblast. While NORE1A suppresses TNF receptor-mediated apoptosis, it activates TNF-induced apoptosis through BAX activation by protecting BAX from ITCH binding and ubiquitination. Cytotoxic response to TNF is substantially attenuated in NORE1A-depleted cells and tumors, and NORE1A-induced tumor regression is highly impeded in BAX-depleted tumors. An inverse correlation is shown between NORE1A and TNFRI expression in both cancer cell lines and primary tumors, and NORE1A effect on survival of cancer patients is strongly associated with expression status of ITCH. Collectively, this study uncovers that NORE1A directs a substrate switch of ITCH favoring TNFRI over BAX to terminate TNF signaling and accelerate apoptosis, illuminating the mechanistic consequence of NORE1A inactivation in tumorigenesis.

Itch attenuates CD4 T-cell proliferation in mice by limiting WBP2 protein stability

To mount an antipathogen response, CD4 T cells must undergo rapid cell proliferation; however, poorly controlled expansion can result in diseases such as autoimmunity. One important regulator of T-cell activity is the E3 ubiquitin ligase Itch. Itch deficient patients suffer from extensive autoinflammation. Similarly, Itch deficient mice exhibit inflammation characterized by high numbers of activated CD4 T cells. While the role of Itch in limiting CD4 T-cell cytokine production has been extensively studied, it is less clear whether and how Itch regulates proliferation of these cells. We determined that Itch deficient CD4 T cells are hyperproliferative in vitro and in vivo, due to increased S phase entry. Whole cell proteomics analysis of Itch deficient primary mouse CD4 T cells revealed increased abundance of the β-catenin coactivator WW domain-binding protein 2 (WBP2). Furthermore, Itch deficient cells demonstrate increased WBP2 protein stability, and Itch and WBP2 interact in CD4 T cells. Knockdown of WBP2 in CD4 T cells caused reduced proliferation. Together, our data support that Itch attenuates CD4 T cell proliferation by promoting WBP2 degradation. This study identifies novel roles for Itch and WBP2 in regulating CD4 T cell proliferation, providing insight into how Itch may prevent inflammation.

Aberrant activation of neuronal cell cycle caused by dysregulation of ubiquitin ligase Itch results in neurodegeneration

It is critical for the neuronal cell cycle to remain suppressed in terminally differentiated neurons as its activation results in aberrant cell cycle re-entry that causes neuronal apoptosis (CRNA), which has been observed in several neurodegenerative disorders like Alzheimer's disease (AD). In the present study, we report that E3 ubiquitin ligase Itch is a major regulator of CRNA and elucidated the mechanism via which it is regulated in this process. Neurotoxic amyloid peptide Aβ₄₂-treated neurons or neurons from an AD transgenic mouse model (TgAD) exhibited aberrant activation of the JNK pathway which resulted in the hyperphosphorylation of Itch. The phosphorylation of Itch primes it for autoubiquitination, which is necessary for its activation. These post-translational modifications of Itch facilitate its interaction with TAp73 resulting in its degradation. These series of events are critical for Itch-mediated CRNA and its phosphorylation and autoubiquitination site mutants reversed this process and were neuroprotective. These studies unravel a novel pathway via which neurodegeneration in AD and possibly other related disorders may be regulated by aberrant regulation of the neuronal cell cycle.

Exosomal miR-532-5p from bone marrow mesenchymal stem cells reduce intervertebral disc degeneration by targeting RASSF5

The transplantation of bone marrow mesenchymal stem cells (BMSCs) has been found to be used as an effective therapy of intervertebral disc degeneration (IDD). However, the underlying mechanisms of BMSCs in the progress of IDD are not fully explained. In this study, we found that exosomes derived from BMSCs (BMSCs-Exos) inhibited the apoptotic rate, extracellular matrix (ECM) degradation, and fibrosis deposition in TNF-α-induced nucleus pulposus cells (NPCs). Importantly, the level of miR-532-5p was observed to be decreased in apoptotic NPCs, but abundant in BMSCs-Exos with TNF-α treatment. The results showed that BMSCs-Exos under TNF-α stimuli exerted better effects on NPCs than BMSCs-Exos, which might be mitigated by the inhibition of miR-532-5p in BMSCs-Exos. The gain-of-function results suggested that the direct overexpression of miR-532-5p in NPCs could inhibit TNF-α-induced increase of apoptotic process, activation of apoptotic proteins, imbalance of anabolism/catabolism levels, and accumulation of collagen I. In addition, RASSF5 was demonstrated to be a target of miR-532-5p. Knockdown of RASSF5 could decrease the apoptotic cells and reduce the activated apoptotic protein levels in TNF-α-induced NPCs. Overall, these data indicate that exosomes from BMSCs may suppress TNF-α-induced apoptosis, ECM degradation, and fibrosis deposition in NPCs through the delivery of miR-532-5p via targeting RASSF5. This work provides a promising therapeutic strategy for the progress of IDD.

Functional role of WW domain-containing proteins in tumor biology and diseases: Insight into the role in ubiquitin-proteasome system

The ubiquitin-proteasome system (UPS) governs the protein degradation process and balances proteostasis and cellular homeostasis. It is a well-controlled mechanism, in which removal of the damaged or excessive proteins is essential in driving signal pathways for cell survival or death. Accumulation of damaged proteins and failure in removal may contribute to disease initiation such as in cancers and neurodegenerative diseases. In this notion, specific protein-protein interaction is essential for the recognition of targeted proteins in UPS. WW domain plays an indispensable role in the protein-protein interactions during signaling. Among the 51 WW domain-containing proteins in the human proteomics, near one-quarter of them are involved in the UPS, suggesting that WW domains are crucial modules for driving the protein-protein binding and subsequent ubiquitination and degradation. In this review, we detail a broad spectrum of WW domains in protein-protein recognition, signal transduction, and relevance to diseases. New perspectives in dissecting the molecular interactions are provided.

ITCH as a potential therapeutic target in human cancers

The ITCH/AIP4 ubiquitin E3 ligase was discovered independently by two groups searching for atrophin-1 interacting proteins and studying the genetics of mouse coat color alteration, respectively. ITCH is classified as a NEDD4 family E3 ligase featured with the C-terminal HECT domain for E3 ligase function and WW domains for substrate recruiting. ITCH deficiency in the mouse causes severe multi-organ autoimmune disease. Its roles in maintaining a balanced immune response have been extensively characterized over the past two and a half decades. A wealth of reports demonstrate a multifaceted role of ITCH in human cancers. Given the versatility of ITCH in catalyzing both proteolytic and non-proteolytic ubiquitination of its over fifty substrates, ITCH's role in malignancies is believed to be context-dependent. In this review, we summarize the downstream substrates of ITCH, the functions of ITCH in both tumor cells and the immune system, as well as the implications of such functions in human cancers. Moreover, we describe the upstream regulatory mechanisms of ITCH and the efforts have been made to target ITCH using small molecule inhibitors.

Silencing E3 Ubiqutin ligase ITCH as a potential therapy to enhance chemotherapy efficacy in p53 mutant neuroblastoma cells

P53 mutations are responsible for drug-resistance of tumour cells which impacts on the efficacy of treatment. Alternative tumour suppressor pathways need to be explored to treat p53- deficient tumours. The E3 ubiquitin ligase, ITCH, negatively regulates the tumour suppressor protein TP73, providing a therapeutic target to enhance the sensitivity of the tumour cells to the treatment. In the present study, two p53-mutant neuroblastoma cell lines were used as in vitro models. Using immunostaining, western blot and qPCR methods, we firstly identified that ITCH was expressed on p53-mutant neuroblastoma cell lines. Transfection of these cell lines with ITCH siRNA could effectively silence the ITCH expression, and result in the stabilization of TP73 protein, which mediated the apoptosis of the neuroblastoma cells upon irradiation treatment. Finally, in vivo delivery of the ITCH siRNA using nanoparticles to the neuroblastoma xenograft mouse model showed around 15–20% ITCH silencing 48 hours after transfection. Our data suggest that ITCH could be silenced both in vitro and in vivo using nanoparticles, and silencing of ITCH sensitizes the tumour cells to irradiation treatment. This strategy could be further explored to combine the chemotherapy/radiotherapy treatment to enhance the therapeutic effects on p53-deficient neuroblastoma.

Repurposing old drugs as new inhibitors of the ubiquitin-proteasome pathway for cancer treatment

The ubiquitin-proteasome system (UPS) plays a central role in the degradation of cellular proteins. Targeting protein degradation has been validated as an effective strategy for cancer therapy since 2003. Several components of the UPS have been validated as potential anticancer targets, including 20S proteasomes, 19S proteasome-associated deubiquitinases (DUBs) and ubiquitin ligases (E3s). 20S proteasome inhibitors (such as bortezomib/BTZ and carfilzomib/CFZ) have been approved by the U.S. Food and Drug Administration (FDA) for the treatment of multiple myeloma (MM) and some other liquid tumors. Although survival of MM patients has been improved by the introduction of BTZ-based therapies, these clinical 20S proteasome inhibitors have several limitations, including emergence of resistance in MM patients, neuro-toxicities, and little efficacy in solid tumors. One of strategies to improve the current status of cancer treatment is to repurpose old drugs with UPS-inhibitory properties as new anticancer agents. Old drug reposition represents an attractive drug discovery approach compared to the traditional de novo drug discovery process which is time-consuming and costly. In this review, we summarize status of repurposed inhibitors of various UPS components, including 20S proteasomes, 19S-associated DUBs, and ubiquitin ligase E3s. The original and new mechanisms of action, molecular targets, and potential anticancer activities of these repurposed UPS inhibitors are reviewed, and their new uses including combinational therapies for cancer treatment are discussed.

WW-Domain Containing Protein Roles in Breast Tumorigenesis

Protein-protein interactions are key factors in executing protein function. These interactions are mediated through different protein domains or modules. An important domain found in many different types of proteins is WW domain. WW domain-containing proteins were shown to be involved in many human diseases including cancer. Some of these proteins function as either tumor suppressor genes or oncogenes, while others show dual identity. Some of these proteins act on their own and alter the function(s) of specific or multiple proteins implicated in cancer, others interact with their partners to compose WW domain modular pathway. In this review, we discuss the role of WW domain-containing proteins in breast tumorigenesis. We give examples of specific WW domain containing proteins that play roles in breast tumorigenesis and explain the mechanisms through which these proteins lead to breast cancer initiation and progression. We discuss also the possibility of using these proteins as biomarkers or therapeutic targets.

The non-enzymatic RAS effector RASSF7 inhibits oncogenic c-Myc function

c-Myc is a proto-oncogene controlling expression of multiple genes involved in cell growth and differentiation. Although the functional role of c-Myc as a transcriptional regulator has been intensively studied, targeting this protein in cancer remains a challenge. Here, we report a trimodal regulation of c-Myc function by the Ras effector, Ras-association domain family member 7 (RASSF7), a nonenzymatic protein modulating protein-protein interactions to regulate cell proliferation. Using HEK293T and HeLa cell lines, we provide evidence that RASSF7 destabilizes the c-Myc protein by promoting Cullin4B-mediated polyubiquitination and degradation. Furthermore, RASSF7 competed with MYC-associated factor X (MAX) in the formation of a heterodimeric complex with c-Myc and attenuated its occupancy on target gene promoters to regulate transcription. Consequently, RASSF7 inhibited c-Myc-mediated oncogenic transformation, and an inverse correlation between the expression levels of the RASSF7 and c-Myc genes was evident in human cancers. Furthermore, we found that RASSF7 interacts with c-Myc via its RA and leucine zipper (LZ) domains and LZ domain peptide is sufficient to inhibit c-Myc function, suggesting that this peptide might be used to target oncogenic c-Myc. These results unveil that RASSF7 and c-Myc are functionally linked in the control of tumorigenesis and open up potential therapeutic avenues for targeting the "undruggable" c-Myc protein in a subset of human cancers.

Tumor suppressor C-RASSF proteins

Human genome has ten genes that are collectedly called Ras association domain family (RASSF). RASSF is composed of two subclasses, C-RASSF and N-RASSF. Both N-RASSF and C-RASSF encode Ras association domain-containing proteins and are frequently suppressed by DNA hypermethylation in human cancers. However, C-RASSF and N-RASSF are quite different. Six C-RASSF proteins (RASSF1-6) are characterized by a C-terminal coiled-coil motif named Salvador/RASSF/Hippo domain, while four N-RASSF proteins (RASSF7-10) lack it. C-RASSF proteins interact with mammalian Ste20-like kinases-the core kinases of the tumor suppressor Hippo pathway-and cross-talk with this pathway. Some of them share the same interacting molecules such as MDM2 and exert the tumor suppressor role in similar manners. Nevertheless, each C-RASSF protein has distinct characters. In this review, we summarize our current knowledge of how C-RASSF proteins play tumor suppressor roles and discuss the similarities and differences among C-RASSF proteins.

Ubiquitin-Dependent Regulation of the Mammalian Hippo Pathway: Therapeutic Implications for Cancer

The Hippo pathway serves as a key barrier for oncogenic transformation. It acts by limiting the activity of the proto-oncogenes YAP and TAZ. Reduced Hippo signaling and elevated YAP/TAZ activities are frequently observed in various types of tumors. Emerging evidence suggests that the ubiquitin system plays an important role in regulating Hippo pathway activity. Deregulation of ubiquitin ligases and of deubiquitinating enzymes has been implicated in increased YAP/TAZ activity in cancer. In this article, we review recent insights into the ubiquitin-mediated regulation of the mammalian Hippo pathway, its deregulation in cancer, and possibilities for targeting the Hippo pathway through the ubiquitin system.

RAS-mediated oncogenic signaling pathways in human malignancies

Abnormally activated RAS proteins are the main oncogenic driver that governs the functioning of major signaling pathways involved in the initiation and development of human malignancies. Mutations in RAS genes and or its regulators, most frequent in human cancers, are the main force for incessant RAS activation and associated pathological conditions including cancer. In general, RAS is the main upstream regulator of the highly conserved signaling mechanisms associated with a plethora of important cellular activities vital for normal homeostasis. Mutated or the oncogenic RAS aberrantly activates a web of interconnected signaling pathways including RAF-MEK (mitogen-activated protein kinase kinase)-ERK (extracellular signal-regulated kinase), phosphoinositide-3 kinase (PI3K)/AKT (protein kinase B), protein kinase C (PKC) and ral guanine nucleotide dissociation stimulator (RALGDS), etc., leading to uncontrolled transcriptional expression and reprogramming in the functioning of a range of nuclear and cytosolic effectors critically associated with the hallmarks of carcinogenesis. This review highlights the recent literature on how oncogenic RAS negatively use its signaling web in deregulating the expression and functioning of various effector molecules in the pathogenesis of human malignancies.

Circular RNA-ITCH Suppresses Lung Cancer Proliferation via Inhibiting the Wnt/β-Catenin Pathway

As a special form of noncoding RNAs, circular RNAs (circRNAs) played important roles in regulating cancer progression mainly by functioning as miRNA sponge. While the function of circular RNA-ITCH (cir-ITCH) in lung cancer is still less reported, in this study, we firstly detected the expression of cir-ITCH in tumor tissues and paired adjacent noncancer tissues of 78 patients with lung cancer using a TaqMan-based quantitative real-time PCR (qRT-PCR). The results showed that the expression of cir-ITCH was significantly decreased in lung cancer tissues. In cellular studies, cir-ITCH was also enhanced in different lung cancer cell lines, A549 and NIC-H460. Ectopic expression of cir-ITCH markedly elevated its parental cancer-suppressive gene, ITCH, expression and inhibited proliferation of lung cancer cells. Molecular analysis further revealed that cir-ITCH acted as sponge of oncogenic miR-7 and miR-214 to enhance ITCH expression and thus suppressed the activation of Wnt/β-catenin signaling. Altogether, our results suggested that cir-ITCH may play an inhibitory role in lung cancer progression by enhancing its parental gene, ITCH, expression.

TGF-β Targets the Hippo Pathway Scaffold RASSF1A to Facilitate YAP/SMAD2 Nuclear Translocation

Epigenetic inactivation of the Hippo pathway scaffold RASSF1A is associated with poor prognosis in a wide range of sporadic human cancers. Loss of expression reduces tumor suppressor activity and promotes genomic instability, but how this pleiotropic biomarker is regulated at the protein level is unknown. Here we show that TGF-β is the physiological signal that stimulates RASSF1A degradation by the ubiquitin-proteasome pathway. In response to TGF-β, RASSF1A is recruited to TGF-β receptor I and targeted for degradation by the co-recruited E3 ubiquitin ligase ITCH. RASSF1A degradation is necessary to permit Hippo pathway effector YAP1 association with SMADs and subsequent nuclear translocation of receptor-activated SMAD2. We find that RASSF1A expression regulates TGF-β-induced YAP1/SMAD2 interaction and leads to SMAD2 cytoplasmic retention and inefficient transcription of TGF-β targets genes. Moreover, RASSF1A limits TGF-β induced invasion, offering a new framework on how RASSF1A affects YAP1 transcriptional output and elicits its tumor-suppressive function.

The E3 ligase Itch in immune regulation and beyond

Itch or itchy E3 ubiquitin ligase was initially discovered by genetic studies on the mouse coat color changes, and its deletion results in an itchy phenotype with constant skin scratching and multi-organ inflammation. It is a member of the homologous to E6-associated protein C-terminus (HECT)-type family of E3 ligases, with the protein-interacting WW-domains for the recruitment of substrate and the HECT domain for the transfer of ubiquitin to the substrate. Since its discovery, numerous studies have demonstrated that Itch is involved in the control of many aspects of immune responses including T-cell activation and tolerance and T-helper cell differentiation. Itch is also implicated in other biological contexts such as tumorigenesis, development, and stress responses. Many signaling pathways are regulated by Itch-promoted ubiquitylation of diverse target proteins. Itch is also involved in human diseases. Here, we discuss the major progress in understanding the biological significance of Itch-promoted protein ubiquitylation in the immune and other systems and in Itch-mediated regulation of signal transduction.

Ras Regulates Rb via NORE1A

Mutations in the Ras oncogene are one of the most frequent events in human cancer. Although Ras regulates numerous growth-promoting pathways to drive transformation, it can paradoxically promote an irreversible cell cycle arrest known as oncogene-induced senescence. Although senescence has clearly been implicated as a major defense mechanism against tumorigenesis, the mechanisms by which Ras can promote such a senescent phenotype remain poorly defined. We have shown recently that the Ras death effector NORE1A plays a critical role in promoting Ras-induced senescence and connects Ras to the regulation of the p53 tumor suppressor. We now show that NORE1A also connects Ras to the regulation of a second major prosenescent tumor suppressor, the retinoblastoma (Rb) protein. We show that Ras induces the formation of a complex between NORE1A and the phosphatase PP1A, promoting the activation of the Rb tumor suppressor by dephosphorylation. Furthermore, suppression of Rb reduces NORE1A senescence activity. These results, together with our previous findings, suggest that NORE1A acts as a critical tumor suppressor node, linking Ras to both the p53 and the Rb pathways to drive senescence.

LPS impairs oxygen utilization in epithelia by triggering degradation of the mitochondrial enzyme Alcat1

Cardiolipin (also known as PDL6) is an indispensable lipid required for mitochondrial respiration that is generated through de novo synthesis and remodeling. Here, the cardiolipin remodeling enzyme, acyl-CoA:lysocardiolipin-acyltransferase-1 (Alcat1; SwissProt ID, Q6UWP7) is destabilized in epithelia by lipopolysaccharide (LPS) impairing mitochondrial function. Exposure to LPS selectively decreased levels of carbon 20 (C20)-containing cardiolipin molecular species, whereas the content of C18 or C16 species was not significantly altered, consistent with decreased levels of Alcat1. Alcat1 is a labile protein that is lysosomally degraded by the ubiquitin E3 ligase Skp-Cullin-F-box containing the Fbxo28 subunit (SCF-Fbxo28) that targets Alcat1 for monoubiquitylation at residue K183. Interestingly, K183 is also an acetylation-acceptor site, and acetylation conferred stability to the enzyme. Histone deacetylase 2 (HDAC2) interacted with Alcat1, and expression of a plasmid encoding HDAC2 or treatment of cells with LPS deacetylated and destabilized Alcat1, whereas treatment of cells with a pan-HDAC inhibitor increased Alcat1 levels. Alcat1 degradation was partially abrogated in LPS-treated cells that had been silenced for HDAC2 or treated with MLN4924, an inhibitor of Cullin-RING E3 ubiquitin ligases. Thus, LPS increases HDAC2-mediated Alcat1 deacetylation and facilitates SCF-Fbxo28-mediated disposal of Alcat1, thus impairing mitochondrial integrity.

Selective multifaceted E3 ubiquitin ligases barricade extreme defense: Potential therapeutic targets for neurodegeneration and ageing

Efficient and regular performance of Ubiquitin Proteasome System and Autophagy continuously eliminate deleterious accumulation of nonnative protiens. In cellular quality control system, E3 ubiquitin ligases are significant employees for defense mechanism against abnormal toxic proteins. Few findings indicate that lack of functions of E3 ubiquitin ligases can be a causative factor of neurodevelopmental disorders, neurodegeneration, cancer and ageing. However, the detailed molecular pathomechanism implying E3 ubiquitin ligases in cellular functions in multifactorial disease conditions are not well understood. This article systematically represents the unique characteristics, molecular nature, and recent developments in the knowledge of neurobiological functions of few crucial E3 ubiquitin ligases. Here, we review recent literature on the roles of E6-AP, HRD1 and ITCH E3 ubiquitin ligases in the neuro-pathobiological mechanisms, with precise focus on the processes of neurodegeneration, and thereby propose new lines of potential targets for therapeutic interventions.

Dysregulation of ubiquitin ligases in cancer

Ubiquitin ligases (UBLs) are critical components of the ubiquitin proteasome system (UPS), which governs fundamental processes regulating normal cellular homeostasis, metabolism, and cell cycle in response to external stress signals and DNA damage. Among multiple steps of the UPS system required to regulate protein ubiquitination and stability, UBLs define specificity, as they recognize and interact with substrates in a temporally- and spatially-regulated manner. Such interactions are required for substrate modification by ubiquitin chains, which marks proteins for recognition and degradation by the proteasome or alters their subcellular localization or assembly into functional complexes. UBLs are often deregulated in cancer, altering substrate availability or activity in a manner that can promote cellular transformation. Such deregulation can occur at the epigenetic, genomic, or post-translational levels. Alterations in UBL can be used to predict their contributions, affecting tumor suppressors or oncogenes in select tumors. Better understanding of mechanisms underlying UBL expression and activities is expected to drive the development of next generation modulators that can serve as novel therapeutic modalities. This review summarizes our current understanding of UBL deregulation in cancer and highlights novel opportunities for therapeutic interventions.

p63 threonine phosphorylation signals the interaction with the WW domain of the E3 ligase Itch

Both in epithelial development as well as in epithelial cancers, the p53 family member p63 plays a crucial role acting as a master transcriptional regulator. P63 steady state protein levels are regulated by the E3 ubiquitin ligase Itch, via a physical interaction between the PPxY consensus sequence (PY motif) of p63 and one of the 4 WW domains of Itch; this substrate recognition process leads to protein-ubiquitylation and p63 proteasomal degradation. The interaction of the WW domains, a highly compact protein-protein binding module, with the short proline-rich sequences is therefore a crucial regulatory event that may offer innovative potential therapeutic opportunity. Previous molecular studies on the Itch-p63 recognition have been performed in vitro using the Itch-WW2 domain and the peptide interacting fragment of p63 (pep63), which includes the PY motif. Itch-WW2-pep63 interaction is also stabilized in vitro by the conformational constriction of the S-S cyclization in the p63 peptide. The PY motif of p63, as also for other proteins, is characterized by the nearby presence of a (T/S)P motif, which is a potential recognition site of the WW domain of the IV group present in the prolyl-isomerase Pin1. In this study, we demonstrate, by in silico and spectroscopical studies using both the linear pep63 and its cyclic form, that the threonine phosphorylation of the (T/S)PPPxY motif may represent a crucial regulatory event of the Itch-mediated p63 ubiquitylation, increasing the Itch-WW domains-p63 recognition event and stabilizing in vivo the Itch-WW-p63 complex. Moreover, our studies confirm that the subsequently trans/cis proline isomerization of (T/S)P motif by the Pin1 prolyl-isomerase, could modulate the E3-ligase interaction, and that the (T/S)pPtransPPxY motif represent the best conformer for the ItchWW-(T/S)PPPxY motif recognition.

Hippo Stabilises Its Adaptor Salvador by Antagonising the HECT Ubiquitin Ligase Herc4

Signalling through the Hippo (Hpo) pathway involves a kinase cascade, which leads to the phosphorylation and inactivation of the pro-growth transcriptional co-activator Yorkie (Yki). Despite the identification of a large number of pathway members and modulators, our understanding of the molecular events that lead to activation of Hpo and the downstream kinase Warts (Wts) remain incomplete. Recently, targeted degradation of several Hpo pathway components has been demonstrated as a means of regulating pathway activity. In particular, the stability of scaffold protein Salvador (Sav), which is believed to promote Hpo/Wts association, is crucially dependent on its binding partner Hpo. In a cell-based RNAi screen for ubiquitin regulators involved in Sav stability, we identify the HECT domain protein Herc4 (HECT and RLD domain containing E3 ligase) as a Sav E3 ligase. Herc4 expression promotes Sav ubiquitylation and degradation, while Herc4 depletion stabilises Sav. Interestingly, Hpo reduces Sav/Herc4 interaction in a kinase-dependent manner. This suggests the existence of a positive feedback loop, where Hpo stabilises its own positive regulator by antagonising Herc4-mediated degradation of Sav.

Screening for E3-ubiquitin ligase inhibitors: challenges and opportunities

The ubiquitin proteasome system (UPS) plays a role in the regulation of most cellular pathways, and its deregulation has been implicated in a wide range of human pathologies that include cancer, neurodegenerative and immunological disorders and viral infections. Targeting the UPS by small molecular regulators thus provides an opportunity for the development of therapeutics for the treatment of several diseases. The proteasome inhibitor Bortezomib was approved for treatment of hematologic malignancies by the FDA in 2003, becoming the first drug targeting the ubiquitin proteasome system in the clinic. Development of drugs targeting specific components of the ubiquitin proteasome system, however, has lagged behind, mainly due to the complexity of the ubiquitination reaction and its outcomes. However, significant advances have been made in recent years in understanding the molecular nature of the ubiquitination system and the vast variety of cellular signals that it produces. Additionally, improvement of screening methods, both in vitro and in silico, have led to the discovery of a number of compounds targeting components of the ubiquitin proteasome system, and some of these have now entered clinical trials. Here, we discuss the current state of drug discovery targeting E3 ligases and the opportunities and challenges that it provides.

The ubiquitin E3 ligase SCF-FBXO24 recognizes deacetylated nucleoside diphosphate kinase A to enhance its degradation

The Skp-Cul-F box (SCF) ubiquitin E3 ligase machinery recognizes predominantly phosphodegrons or, less commonly, an (I/L)Q molecular signature within substrates to facilitate their recruitment in mediating protein ubiquitination and degradation. Here, we examined the molecular signals that determine the turnover of the multifunctional enzyme nucleoside diphosphate kinase A (NDPK-A) that controls cell proliferation. NDPK-A protein exhibits a half-life of ∼6 h in HeLa cells and is targeted for ubiquitylation through actions of the F-box protein FBXO24. SCF-FBXO24 polyubiquitinates NDPK-A at K85, and two NH(2)-terminal residues, L55 and K56, were identified as important molecular sites for FBXO24 interaction. Importantly, K56 acetylation impairs its interaction with FBXO24, and replacing K56 with Q56, an acetylation mimic, reduces NDPK-A FBXO24 binding capacity. The acetyltransferase GCN5 catalyzes K56 acetylation within NDPK-A, thereby stabilizing NDPK-A, whereas GCN5 depletion in cells accelerates NDPK-A degradation. Cellular expression of an NDPK-A acetylation mimic or FBXO24 silencing increases NDPK-A life span which, in turn, impairs cell migration and wound healing. We propose that lysine acetylation when presented in the appropriate context may be recognized by some F-box proteins as a unique inhibitory molecular signal for their recruitment to restrict substrate degradation.

High throughput screening for inhibitors of the HECT ubiquitin E3 ligase ITCH identifies antidepressant drugs as regulators of autophagy

Inhibition of distinct ubiquitin E3 ligases might represent a powerful therapeutic tool. ITCH is a HECT domain-containing E3 ligase that promotes the ubiquitylation and degradation of several proteins, including p73, p63, c-Jun, JunB, Notch and c-FLIP, thus affecting cell fate. Accordingly, ITCH depletion potentiates the effect of chemotherapeutic drugs, revealing ITCH as a potential pharmacological target in cancer therapy. Using high throughput screening of ITCH auto-ubiquitylation, we identified several putative ITCH inhibitors, one of which is clomipramine—a clinically useful antidepressant drug. Previously, we have shown that clomipramine inhibits autophagy by blocking autophagolysosomal fluxes and thus could potentiate chemotherapy in vitro. Here, we found that clomipramine specifically blocks ITCH auto-ubiquitylation, as well as p73 ubiquitylation. By screening structural homologs of clomipramine, we identified several ITCH inhibitors and putative molecular moieties that are essential for ITCH inhibition. Treating a panel of breast, prostate and bladder cancer cell lines with clomipramine, or its homologs, we found that they reduce cancer cell growth, and synergize with gemcitabine or mitomycin in killing cancer cells by blocking autophagy. We also discuss a potential mechanism of inhibition. Together, our study (i) demonstrates the feasibility of using high throughput screening to identify E3 ligase inhibitors and (ii) provides insight into how clomipramine and its structural homologs might interfere with ITCH and other HECT E3 ligase catalytic activity in (iii) potentiating chemotherapy by regulating autophagic fluxes. These results may have direct clinical applications.

PLCɛ and the RASSF family in tumour suppression and other functions

Not all proteins implicated in direct binding to Ras appear to have a positive role in the generation and progression of tumours; examples include Phospholipase C epsilon (PLCɛ) and some members of the Ras-association domain family (RASSF). The RASSF family comprises of ten members, known as RASSF1 to RASSF10. PLCɛ and RASSF members carry a common Ras-association domain (RA) that can potentially bind Ras oncoproteins and mediate Ras-regulated functions. RASSF1 to RASSF6 also share a common SARAH domain that facilitates protein-protein interactions with other SARAH domain proteins. The majority of the family are frequently downregulated by epigenetic silencing in cancers. They are implicated in various important biological processes including apoptosis, microtubule stabilisation and cell cycle regulation. Recent studies have reinforced the tumour suppressive properties of the RASSF family, with new evidence of emerging pathways and novel functions that suggest a wider role for these proteins. This review will first describe an emerging role of PLCɛ in tumour suppression and then focus on and summarise the new findings on the RASSF family in the last five years to consolidate their well-established functions, and highlight the new regulatory roles of specific RASSF members.