Cdh1 regulates osteoblast function through an APC/C-independent modulation of Smurf1

UPS-dependent strategies of protein quality control degradation

The degradation of damaged proteins is critical for tissue integrity and organismal health because damaged proteins have a high propensity to form aggregates. E3 ubiquitin ligases are key regulators of protein quality control (PQC) and mediate the selective degradation of damaged proteins, a process termed 'PQC degradation' (PQCD). The degradation signals (degrons) that trigger PQCD are based on hydrophobic sites that are normally buried within the native protein structure. However, an open question is how PQCD-specialized E3 ligases distinguish between transiently misfolded proteins, which can be efficiently refolded, and permanently damaged proteins, which must be degraded. While significant progress has been made in characterizing degradation determinants, understanding the key regulatory signals of cellular and organismal PQCD pathways remains a challenge.

Deletion of a core APC/C component reveals APC/C function in regulating neuronal USP1 levels and morphology

Introduction

The Anaphase Promoting Complex (APC/C), an E3 ubiquitin ligase, plays a key role in cell cycle control, but it is also thought to operate in postmitotic neurons. Most studies linking APC/C function to neuron biology employed perturbations of the APC/C activators, cell division cycle protein 20 (Cdc20) and Cdc20 homologue 1 (Cdh1). However, multiple lines of evidence indicate that Cdh1 and Cdc20 can function in APC/C-independent contexts, so that the effects of their perturbation cannot strictly be linked to APC/C function.

Methods

We therefore deleted the gene encoding Anaphase Promoting Complex 4 (APC4), a core APC/C component, in neurons cultured from conditional knockout (cKO) mice.

Results

Our data indicate that several previously published substrates are actually not APC/C substrates, whereas ubiquitin specific peptidase 1 (USP1) protein levels are altered in APC4 knockout (KO) neurons. We propose a model where the APC/C ubiquitylates USP1 early in development, but later ubiquitylates a substrate that directly or indirectly stabilizes USP1. We further discovered a novel role of the APC/C in regulating the number of neurites exiting somata, but we were unable to confirm prior data indicating that the APC/C regulates neurite length, neurite complexity, and synaptogenesis. Finally, we show that APC4 SUMOylation does not impact the ability of the APC/C to control the number of primary neurites or USP1 protein levels.

Discussion

Our data indicate that perturbation studies aimed at dissecting APC/C biology must focus on core APC/C components rather than the APC/C activators, Cdh20 and Cdh1.

Fufang Zhenshu Tiaozhi capsule enhances bone formation and safeguards against glucocorticoid-induced osteoporosis through innovative Mekk2-mediated β-catenin deubiquitination

INTRODUCTION

Bone homeostasis depends on the regulation of β-catenin in osteoblasts. Glucocorticoids (GCs) are known to diminish β-catenin activity via Wnt pathway signaling, leading to osteoporosis. Conversely, activating β-catenin in osteoblasts through mitogen-activated protein kinase kinase kinase 2 (Mekk2) offers an innovative approach to combat GC-induced osteoporosis (GIOP). Fufang Zhenshu Tiaozhi (FTZ) capsules have shown effectiveness in treating GIOP, but the mechanisms behind this are still unclear.

MATERIALS AND METHODS

In this study, Mekk2 knockout mice (Mekk2-/-) was generated by CRISPR/Cas9. These mice were then subjected to Alcian Blue-Alizarin Red staining and immunofluorescence to assess their bone and cartilage development. To establish models of GIOP, both Mekk2-/- and wild-type (WT) mice were treated with dexamethasone (DXMS) and subsequently given FTZ capsules. We analyzed the resulting phenotypic changes in these mice using Micro-CT scans and histomorphological studies. Primary osteoblasts, isolated from both Mekk2-/- and WT mice, underwent qRT-PCR to measure key osteogenesis markers, including Runx2, Sp7, Bgalp, Col1a1 and Alp. Cells were then exposed to treatments with either FTZ or Wnt3a and the phosphorylation levels of β-catenin and Mekk2, along with the protein expression of Runx2, were evaluated using Western blotting and immunoprecipitation. Additionally, C3H10T1/2 cells transfected with TOPflash-luciferase and Renilla luciferase reporters were treated with FTZ and Wnt3a to measure β-catenin activity.

RESULTS

In our study, administering FTZ in vivo effectively prevented bone loss typically induced by GCs. However, it's important to note that this protective effect was substantially reduced in mice lacking Mekk2. Additionally, FTZ showed a significant ability to enhance osteogenic differentiation in primary osteoblasts, doing so by altering the expression of Mekk2. Intriguingly, the impact of FTZ on Mekk2 appears to function through a pathway separate from the traditional Wnt signaling route. Furthermore, our findings indicate that FTZ also promotes the deubiquitination of β-catenin, contributing further to its positive effects on bone health.

CONCLUSIONS

This study suggests that FTZ plays a significant role in protecting bone mass in cases of GIOP. The mechanism through which FTZ confers this benefit involves the activation of Mekk2/β-catenin signaling pathways, which represents a promising alternative strategy to counteract the deleterious effects of GIOP by augmenting osteoblastogenesis.

Coordination of growth and drought responses by GA-ABA signaling in rice

The drought caused by global warming seriously affects the crop growth and agricultural production. Plants have evolved distinct strategies to cope with the drought environment. Under drought stress, energy and resources should be diverted from growth toward stress management. However, the molecular mechanism underlying coordination of growth and drought response remains largely elusive. Here, we discovered that most of the gibberellin (GA) metabolic genes were regulated by water scarcity in rice, leading to the lower GA contents and hence inhibited plant growth. Low GA contents resulted in the accumulation of more GA signaling negative regulator SLENDER RICE 1, which inhibited the degradation of abscisic acid (ABA) receptor PYL10 by competitively binding to the co-activator of anaphase-promoting complex TAD1, resulting in the enhanced ABA response and drought tolerance. These results elucidate the synergistic regulation of crop growth inhibition and promotion of drought tolerance and survival, and provide useful genetic resource in breeding improvement of crop drought resistance.

Research Progress in Function and Regulation of E3 Ubiquitin Ligase SMURF1

Smad ubiquitylation regulatory factor 1 (Smurf1) is an important homologous member of E6-AP C-terminus type E3 ubiquitin ligase. Initially, Smurf1 was reportedly involved in the negative regulation of the bone morphogenesis protein (BMP) pathway. After further research, several studies have confirmed that Smurf1 is widely involved in various biological processes, such as bone homeostasis regulation, cell migration, apoptosis, and planar cell polarity. At the same time, recent studies have provided a deeper understanding of the regulatory mechanisms of Smurf1's expression, activity, and substrate selectivity. In our review, a brief summary of recent important biological functions and regulatory mechanisms of E3 ubiquitin ligase Smurf1 is proposed.

Structural Basis for the Enzymatic Activity of the HACE1 HECT-Type E3 Ligase Through N-Terminal Helix Dimerization

HACE1 is an ankyrin repeat (AKR) containing HECT-type E3 ubiquitin ligase that interacts with and ubiquitinates multiple substrates. While HACE1 is a well-known tumor suppressor, its structure and mode of ubiquitination are not understood. The authors present the cryo-EM structures of human HACE1 along with in vitro functional studies that provide insights into how the enzymatic activity of HACE1 is regulated. HACE1 comprises of an N-terminal AKR domain, a middle (MID) domain, and a C-terminal HECT domain. Its unique G-shaped architecture interacts as a homodimer, with monomers arranged in an antiparallel manner. In this dimeric arrangement, HACE1 ubiquitination activity is hampered, as the N-terminal helix of one monomer restricts access to the C-terminal domain of the other. The in vitro ubiquitination assays, hydrogen-deuterium exchange mass spectrometry (HDX-MS) analysis, mutagenesis, and in silico modeling suggest that the HACE1 MID domain plays a crucial role along with the AKRs in RAC1 substrate recognition.

Anaphase-Promoting Complex Subunit 1 Associates with Bone Mineral Density in Human Osteoporotic Bone

Genome-wide association studies (GWAS) are one of the most common approaches to identify genetic loci that are associated with bone mineral density (BMD). Such novel genetic loci represent new potential targets for the prevention and treatment of fragility fractures. GWAS have identified hundreds of associations with BMD; however, only a few have been functionally evaluated. A locus significantly associated with femoral neck BMD at the genome-wide level is intronic SNP rs17040773 located in the intronic region of the anaphase-promoting complex subunit 1 (ANAPC1) gene (p = 1.5 × 10-9). Here, we functionally evaluate the role of ANAPC1 in bone remodelling by examining the expression of ANAPC1 in human bone and muscle tissues and during the osteogenic differentiation of human primary mesenchymal stem cells (MSCs). The expression of ANAPC1 was significantly decreased 2.3-fold in bone tissues and 6.2-fold in muscle tissue from osteoporotic patients as compared to the osteoarthritic and control tissues. Next, we show that the expression of ANAPC1 changes during the osteogenic differentiation process of human MSCs. Moreover, the silencing of ANAPC1 in human osteosarcoma (HOS) cells reduced RUNX2 expression, suggesting that ANAPC1 affects osteogenic differentiation through RUNX2. Altogether, our results indicate that ANAPC1 plays a role in bone physiology and in the development of osteoporosis.

Urinary collagen peptides: Source of markers for bone metabolic processes in kidney transplant recipients

INTRODUCTION

Kidney transplant recipients (KTRs) are at an increased risk of fractures. Total urinary hydroxyproline excretion served as marker for bone resorption (BR) but was replaced by β-CrossLaps (CTX), a C-terminal collagen α-1(I) chain (COL1A1) telopeptide. We investigated the low-molecular-weight urinary proteome for peptides associated with changes in bone metabolism after kidney transplantation.

METHODS

Clinical and laboratory data including serum levels of CTX in 96 KTR from two nephrology centers were correlated with signal intensities of urinary peptides identified by capillary electrophoresis mass spectrometry.

RESULTS

Eighty-two urinary peptides were significantly correlated with serum CTX levels. COL1A1 was the predominant peptide source. Oral bisphosphonates were administered for decreased bone density in an independent group of 11 KTR and their effect was evaluated on the aforementioned peptides. Study of the peptides cleavage sites revealed a signature of Cathepsin K and MMP9. Seventeen of these peptides were significantly associated with bisphosphonate treatment, all showing a marked reduction in their excretion levels compared to baseline.

DISCUSSION

This study provides strong evidence for the presence of collagen peptides in the urine of KTR that are associated with BR and that are sensitive to bisphosphonate treatment. Their assessment might become a valuable tool to monitor bone status in KTR.

A Golgi-resident GPR108 cooperates with E3 ubiquitin ligase Smurf1 to suppress antiviral innate immunity

The regulation of antiviral immunity is crucial in maintaining host immune homeostasis, a process that involves dynamic modulations of host organelles. The Golgi apparatus is increasingly perceived as a host organelle functioning as a critical platform for innate immunity, but the detailed mechanism by which it regulates antiviral immunity remains elusive. Here, we identify the Golgi-localized G protein-coupled receptor 108 (GPR108) as a regulator of type Ι interferon responses by targeting interferon regulatory factor 3 (IRF3). Mechanistically, GPR108 enhances the ubiquitin ligase Smad ubiquitylation regulatory factor 1 (Smurf1)-mediated K63-linked polyubiquitination of phosphorylated IRF3 for nuclear dot 10 protein 52 (NDP52)-dependent autophagic degradation, leading to suppression of antiviral immune responses against DNA or RNA viruses. Taken together, our study provides insight into the crosstalk between the Golgi apparatus and antiviral immunity via a dynamic and spatiotemporal regulation of GPR108-Smurf1 axis, thereby indicating a potential target for treating viral infection.

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.

The HECT family of E3 ubiquitin ligases and PTEN

Members of the HECT family of E3 ubiquitin ligases have emerged as prominent regulators of PTEN function, subcellular localization and levels. In turn this unfolding regulatory network is allowing for the identification of genes directly involved in both tumorigenesis at large and cancer susceptibility syndromes. While the complexity of this regulatory network is still being unraveled, these new findings are paving the way for novel therapeutic modalities for cancer prevention and therapy as well as for other diseases. Here we will review the signal transduction and therapeutic implications of the cross-talk between HECT family members and PTEN.

E3 Ubiquitin Ligases: Potential Therapeutic Targets for Skeletal Pathology and Degeneration

The ubiquitination-proteasome system (UPS) is crucial in regulating a variety of cellular processes including proliferation, differentiation, and survival. Ubiquitin protein ligase E3 is the most critical molecule in the UPS system. Dysregulation of the UPS system is associated with many conditions. Over the past few decades, there have been an increasing number of studies focusing on the UPS system and how it affects bone metabolism. Multiple E3 ubiquitin ligases have been found to mediate osteogenesis or osteolysis through a variety of pathways. In this review, we describe the mechanisms of UPS, especially E3 ubiquitin ligases on bone metabolism. To date, many E3 ubiquitin ligases have been found to regulate osteogenesis or osteoclast differentiation. We review the classification of these E3 enzymes and the mechanisms that influence upstream and downstream molecules and transduction pathways. Finally, this paper reviews the discovery of the relevant UPS inhibitors, drug molecules, and noncoding RNAs so far and prospects the future research and treatment.

NEDD4 E3 Ligases: Functions and Mechanisms in Bone and Tooth

Protein ubiquitination is a precisely controlled enzymatic cascade reaction belonging to the post-translational modification of proteins. In this process, E3 ligases catalyze the binding of ubiquitin (Ub) to protein substrates and define specificity. The neuronally expressed developmentally down-regulated 4 (NEDD4) subfamily, belonging to the homology to E6APC terminus (HECT) class of E3 ligases, has recently emerged as an essential determinant of multiple cellular processes in different tissues, including bone and tooth. Here, we place special emphasis on the regulatory role of the NEDD4 subfamily in the molecular and cell biology of osteogenesis. We elucidate in detail the specific roles, downstream substrates, and upstream regulatory mechanisms of the NEDD4 subfamily. Further, we provide an overview of the involvement of E3 ligases and deubiquitinases in the development, repair, and regeneration of another mineralized tissue—tooth.

Acetylation-dependent regulation of BRAF oncogenic function

Aberrant BRAF activation, including the BRAF^V600E mutation, is frequently observed in human cancers. However, it remains largely elusive whether other types of post-translational modification(s) in addition to phosphorylation and ubiquitination-dependent regulation also modulate BRAF kinase activity. Here, we report that the acetyltransferase p300 activates the BRAF kinase by promoting BRAF K601 acetylation, a process that is antagonized by the deacetylase SIRT1. Notably, K601 acetylation facilitates BRAF dimerization with RAF proteins and KSR1. Furthermore, K601 acetylation promotes melanoma cell proliferation and contributes to BRAF^V600E inhibitor resistance in BRAF^V600E harboring melanoma cells. As such, melanoma patient-derived K601E oncogenic mutation mimics K601 acetylation to augment BRAF kinase activity. Our findings, therefore, uncover a layer of BRAF regulation and suggest p300 hyperactivation or SIRT1 deficiency as potential biomarkers to determine ERK activation in melanomas.

In tumor cells, hyperactivation of the BRAF protein kinase propels uncontrolled cell proliferation. BRAF hyperactivation is also achieved through several post-translational mechanisms. Dai et al. present an acetylation-dependent regulation of BRAF kinase function in melanoma cells, which serves to enhance BRAF oncogenic function and contributes to BRAF inhibitor resistance.

E3 Ubiquitin Ligase-Mediated Regulation of Osteoblast Differentiation and Bone Formation

The ubiquitin–proteasome system (UPS) is an essential pathway that regulates the homeostasis and function of intracellular proteins and is a crucial protein-degradation system in osteoblast differentiation and bone formation. Abnormal regulation of ubiquitination leads to osteoblast differentiation disorders, interfering with bone formation and ultimately leading to osteoporosis. E3 ubiquitin ligases (E3) promote addition of a ubiquitin moiety to substrate proteins, specifically recognizing the substrate and modulating tyrosine kinase receptors, signaling proteins, and transcription factors involved in the regulation of osteoblast proliferation, differentiation, survival, and bone formation. In this review, we summarize current progress in the understanding of the function and regulatory effects of E3 ligases on the transcription factors and signaling pathways that regulate osteoblast differentiation and bone formation. A deep understanding of E3 ligase-mediated regulation of osteoblast differentiation provides a scientific rationale for the discovery and development of novel E3-targeting therapeutic strategies for osteoporosis.

CDC20 promotes bone formation via APC/C dependent ubiquitination and degradation of p65

The E3 ubiquitin ligase complex CDC20‐activated anaphase‐promoting complex/Cyclosome (APC/C^CDC20) plays a critical role in governing mitotic progression by targeting key cell cycle regulators for degradation. Cell division cycle protein 20 homolog (CDC20), the co‐activator of APC/C, is required for full ubiquitin ligase activity. In addition to its well‐known cell cycle‐related functions, we demonstrate that CDC20 plays an essential role in osteogenic commitment of bone marrow mesenchymal stromal/stem cells (BMSCs). Cdc20 conditional knockout mice exhibit decreased bone formation and impaired bone regeneration after injury. Mechanistically, we discovered a functional interaction between the WD40 domain of CDC20 and the DNA‐binding domain of p65. Moreover, CDC20 promotes the ubiquitination and degradation of p65 in an APC11‐dependent manner. More importantly, knockdown of p65 rescues the bone loss in Cdc20 conditional knockout mice. Our current work reveals a cell cycle‐independent function of CDC20, establishes APC11^CDC20 as a pivotal regulator for bone formation by governing the ubiquitination and degradation of p65, and may pave the way for treatment of bone‐related diseases.

The protective role of MC1R in chromosome stability and centromeric integrity in melanocytes

Variants in the melanocortin-1 receptor (MC1R) gene, encoding a trimeric G-protein-coupled receptor and activated by α-melanocyte-stimulating hormone (α-MSH), are frequently associated with red or blonde hair, fair skin, freckling, and skin sensitivity to ultraviolet (UV) light. Several red hair color variants of MC1R are also associated with increased melanoma risk. MC1R variants affect melanoma risk independent of phenotype. Here, we demonstrated that MC1R is a critical factor in chromosome stability and centromere integrity in melanocytes. α-MSH/MC1R stimulation prevents melanocytes from UV radiation-induced damage of chromosome stability and centromere integrity. Mechanistic studies indicated that α-MSH/MC1R-controlled chromosome stability and centromeric integrity are mediated by microphthalmia-associated transcription factor (Mitf), a transcript factor needed for the α-MSH/MC1R signaling and a regulator in melanocyte development, viability, and pigment production. Mitf directly interacts with centromere proteins A in melanocytes. Given the connection among MC1R variants, red hair/fair skin phenotype, and melanoma development, these studies will help answer a question with clinical relevance “why red-haired individuals are so prone to developing melanoma”, and will lead to the identification of novel preventive and therapeutic strategies for melanomas, especially those with redheads.

Posttranslational Modifications of Smurfs: Emerging Regulation in Cancer

Smad ubiquitination regulatory factors (Smurfs) belong to the Nedd4 subfamily of HECT-type E3 ubiquitin ligases. Under normal situations, Smurfs are exactly managed by upstream regulators, and thereby strictly control tumor biological processes, including cell growth, differentiation, apoptosis, polarization, epithelial mesenchymal transition (EMT), and invasion. Disruption of Smurf activity has been implicated in cancer progression, and Smurf activity is controlled by a series of posttranslational modifications (PTMs), including phosphorylation, ubiquitination, neddylation, sumoylation, and methylation. The effect and function of Smurfs depend on PTMs and regulate biological processes. Specifically, these modifications regulate the functional expression of Smurfs by affecting protein degradation and protein interactions. In this review, we summarize the complexity and diversity of Smurf PTMs from biochemical and biological perspectives and highlight the understanding of their roles in cancer.

Adaptors as the regulators of HECT ubiquitin ligases

The HECT (homologous to E6AP C-terminus) ubiquitin ligases (E3s) are a small family of highly conserved enzymes involved in diverse cellular functions and pathological conditions. Characterised by a C-terminal HECT domain that accepts ubiquitin from E2 ubiquitin conjugating enzymes, these E3s regulate key signalling pathways. The activity and functional regulation of HECT E3s are controlled by several factors including post-translational modifications, inter- and intramolecular interactions and binding of co-activators and adaptor proteins. In this review, we focus on the regulation of HECT E3s by accessory proteins or adaptors and discuss various ways by which adaptors mediate their regulatory roles to affect physiological outcomes. We discuss common features that are conserved from yeast to mammals, regardless of the type of E3s as well as shed light on recent discoveries explaining some existing enigmas in the field.

SMURF1, a promoter of tumor cell progression?

Overexpression of HECT-type E3 ubiquitin ligase SMURF1 is correlated with poor prognosis in patients with various cancers, such as glioblastoma, colon cancer, and clear cell renal cell carcinoma. SMURF1 acts as a tumor promoter by ubiquitination modification and/or degradation of tumor-suppressing proteins. Combined treatment of Smurf1 knockdown with rapamycin showed collaborative antitumor effects in mice. This review described the role of HECT, WW, and C2 domains in regulating SMURF1 substrate selection. We summarized up to date SMURF1 substrates regulating different type cell signaling, thus, accelerating tumor progression, invasion, and metastasis. Furthermore, the downregulation of SMURF1 expression, inhibition of its E3 activity and regulation of its specificity to substrates prevent tumor progression. The potential application of SMURF1 regulators, specifically, wisely choose certain drugs by blocking SMURF1 selectivity in tumor suppressors, to develop novel anticancer treatments.

Nanoscale Copper(II)-Diethyldithiocarbamate Coordination Polymer as a Drug Self-Delivery System for Highly Robust and Specific Cancer Therapy

Disulfiram (DSF), an old alcohol-aversion drug, has been repurposed for cancer therapy, and mechanistic studies reveal that it needs to be metabolized to diethyldithiocarbamate (DTC) and subsequently coordinates with copper(II) to form the DTC-copper complex (CuET) for anticancer activation. Here, we utilized this mechanism to construct a CuET self-delivery nanosystem based on the metal coordination polymer for highly robust and selective cancer therapy. In our design, the nanoparticles were facilely prepared under mild conditions by virtue of the strong coordination between Cu²⁺ and DTC, yielding 100% CuET loading capacity and allowing for further hyaluronic acid (HA) modification (CuET@HA NPs). The CuET@HA NPs could selectively deliver into cancer cells and release the active component of CuET in response to both endo/lysosome acidic pH and intracellular abundant GSH, which induces strong cytotoxicity toward cancer cells over normal cells taking advantage of the p97 pathway interference mechanism. Upon intravenous injection, the self-assembled system could passively accumulate into a tumor and elicit potent tumor growth inhibition at a dose of 1 mg/kg without any noticeable side effects. Given the cost-effective and easily scaled-up preparation, our designed nanosystem provides a promising strategy to pave the way for clinical translation of DSF-based cancer chemotherapy.

WWP1 Gain-of-Function Inactivation of PTEN in Cancer Predisposition

BACKGROUND

Patients with PTEN hamartoma tumor syndrome (PHTS) have germline mutations in the tumor-suppressor gene encoding phosphatase and tensin homologue (PTEN). Such mutations have been associated with a hereditary predisposition to multiple types of cancer, including the Cowden syndrome. However, a majority of patients who have PHTS-related phenotypes have tested negative for PTEN mutations. In a previous study, we found that the E3 ubiquitin ligase WWP1 negatively regulates the function of PTEN.

METHODS

In a prospective cohort study conducted from 2005 through 2015, we enrolled 431 patients with wild-type PTEN who met at least the relaxed diagnostic criteria of the International Cowden Consortium. Patients were scanned for WWP1 germline variants. We used the Cancer Genome Atlas (TCGA) data set as representative of apparently sporadic cancers and the Exome Aggregation Consortium data set excluding TCGA (non-TCGA ExAC) and the noncancer Genome Aggregation Database (gnomAD) as representative of population controls without a reported cancer diagnosis. We established both in vitro and murine in vivo models to functionally characterize representative WWP1 variants.

RESULTS

The existence of germline WWP1 variants was first established in a family with wild-type PTEN who had oligopolyposis and early-onset colon cancers. A validation series indicated that WWP1 germline variants occurred in 5 of 126 unrelated patients (4%) with oligopolyposis as a predominant phenotype. Germline WWP1 variants, particularly the WWP1 K740N and N745S alleles, were enriched in patients who did not have PHTS but had prevalent sporadic cancers, including PTEN-related cancer types in TCGA (odds ratio, 1.5; 95% confidence interval, 1.1 to 2.1; P = 0.01). The prioritized WWP1 variants resulted in gain-of-function effects, which led to aberrant enzymatic activation with consequent PTEN inactivation, thereby triggering hyperactive growth-promoting PI3K signaling in cellular and murine models.

CONCLUSIONS

In this study involving patients with disorders resulting in a predisposition to the development of multiple malignant neoplasms without PTEN germline mutations, we confirmed the function of WWP1 as a cancer-susceptibility gene through direct aberrant regulation of the PTEN-PI3K signaling axis. (Funded by the National Institutes of Health and others.).

Cdh1-APC Regulates Protein Synthesis and Stress Granules in Neurons through an FMRP-Dependent Mechanism

Maintaining a balance between protein degradation and protein synthesis is necessary for neurodevelopment. Although the E3 ubiquitin ligase anaphase promoting complex and its regulatory subunit Cdh1 (Cdh1-APC) has been shown to regulate learning and memory, the underlying mechanisms are unclear. Here, we have identified a role of Cdh1-APC as a regulator of protein synthesis in neurons. Proteomic profiling revealed that Cdh1-APC interacts with known regulators of translation, including stress granule proteins. Inhibition of Cdh1-APC activity caused an increase in stress granule formation that is dependent on fragile X mental retardation protein (FMRP). We propose a model in which Cdh1-APC targets stress granule proteins, such as FMRP, and inhibits the formation of stress granules, leading to protein synthesis. Elucidation of a role for Cdh1-APC in regulation of stress granules and protein synthesis in neurons has implications for how Cdh1-APC can regulate protein-synthesis-dependent synaptic plasticity underlying learning and memory.

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.

Regulation of E3 ubiquitin ligases by homotypic and heterotypic assembly

Protein ubiquitylation is essential for the maintenance of cellular homeostasis. E3 ubiquitin ligases are key components of the enzymatic machinery catalyzing the attachment of ubiquitin to substrate proteins. Consequently, enzymatic dysfunction has been associated with medical conditions including cancer, diabetes, and cardiovascular and neurodegenerative disorders. To safeguard substrate selection and ubiquitylation, the activity of E3 ligases is tightly regulated by post-translational modifications including phosphorylation, sumoylation, and ubiquitylation, as well as binding of alternative adaptor molecules and cofactors. Recent structural studies identified homotypic and heterotypic interactions between E3 ligases, adding another layer of control for rapid adaptation to changing environmental and physiological conditions. Here, we discuss the regulation of E3 ligase activity by combinatorial oligomerization and summarize examples of associated ubiquitylation pathways and mechanisms.

The functions and regulation of Smurfs in cancers

Smad ubiquitination regulatory factor 1 (Smurf1) and Smurf2 are HECT-type E3 ubiquitin ligases, and both Smurfs were initially identified to regulate Smad protein stability in the TGF-β/BMP signaling pathway. In recent years, Smurfs have exhibited E3 ligase-dependent and -independent activities in various kinds of cells. Smurfs act as either potent tumor promoters or tumor suppressors in different tumors by regulating biological processes, including metastasis, apoptosis, cell cycle, senescence and genomic stability. The regulation of Smurfs activity and expression has therefore emerged as a hot spot in tumor biology research. Further, the Smurf1- or Smurf2-deficient mice provide more in vivo clues for the functional study of Smurfs in tumorigenesis and development. In this review, we summarize these milestone findings and, in turn, reveal new avenues for the prevention and treatment of cancer by regulating Smurfs.

New Aspects of HECT-E3 Ligases in Cell Senescence and Cell Death of Plants

Plant cells undergo massive orderly changes in structure, biochemistry, and gene expression during cell senescence. These changes cannot be distinguished from the hydrolysis/degradation function controlled by the ubiquitination pathway, autophagy, and various hydrolases in cells. In this mini-review, we summarized current research progress that the human HECT (homologous to the E6AP carboxyl terminus)-type ubiquitin E3 ligases have non-redundant functions in regulating specific signaling pathways, involved in a number of human diseases, especially aging-related diseases, through the influence of DNA repair, protein stability, and removal efficiency of damaged proteins or organelles. We further compared HECT E3 ligases’ structure and functions between plant and mammalian cells, and speculated new aspects acting as degrading signals and regulating signals of HECT E3 ligase in cell senescence and the cell death of plants.

The WW1 Domain Enhances Autoinhibition in Smurf Ubiquitin Ligases

Downregulation of ubiquitin (Ub) ligase activity prevents premature ubiquitination and is critical for cellular homeostasis. Nedd4 Ub ligases share a common domain architecture and yet are regulated in distinct ways through interactions of the catalytic HECT domain with the N-terminal C2 domain or the central WW domain region. Smurf1 and Smurf2 are two highly related Nedd4 ligases with ~70% overall sequence identity. Here, we show that the Smurf1 C2 domain interacts with the HECT domain and inhibits ligase activity in trans. However, in contrast to Smurf2, we find that full-length Smurf1 is a highly active Ub ligase, and we can attribute this striking difference in regulation to the lack of one WW domain (WW1) in Smurf1. Using NMR spectroscopy and biochemical assays, we identified the WW1 region as an additional inhibitory element in Smurf2 that cooperates with the C2 domain to enhance HECT domain binding and Smurf2 inhibition. Our work provides important insights into Smurf regulation and highlights that the activities of highly related proteins can be controlled in distinct ways.

Emerging roles of HECT-type E3 ubiquitin ligases in autophagy regulation

Autophagy is a conserved self-eating process that delivers cytoplasmic material to the lysosome to allow degradation of intracellular components, including soluble, unfolded and aggregated proteins, damaged organelles, and invading microorganisms. Autophagy provides a homeostatic control mechanism and is essential for balancing sources of energy in response to nutrient stress. Autophagic dysfunction or dysregulation has been implicated in several human pathologies, including cancer and neurodegeneration, and its modulation has substantial potential as a therapeutic strategy. Given the relevant clinical and therapeutic implications of autophagy, there is emerging intense interest in the identification of the key factors regulating the components of the autophagic machinery. Various post-translational modifications, including ubiquitylation, have been implicated in autophagy control. The list of the E3 ubiquitin protein ligases involved in the regulation of several steps of the autophagic process is continuously growing. In this review, we will focus on recent advances in the understanding of the role of the homologous to the E6AP carboxyl terminus-type E3 ubiquitin ligases in autophagy control.

LMCD1 promotes osteogenic differentiation of human bone marrow stem cells by regulating BMP signaling

Human bone marrow stem cells (BMSCs) are heterogeneous progenitor cells with two defining features, self-renew and multi-lineage differentiation. As one of the differentiation directions, osteogenesis is vital for bone homeostasis. A growing body of evidences show that ubiquitin-dependent protein degradation plays an essential role in the osteogenic differentiation of BMSCs. In this study, we found that LMCD1 was upregulated during osteogenic differentiation process of BMSCs by analyzing GSE80614. In vitro and in vivo functional studies confirmed that LMCD1 was critical to the osteogenic commitment of BMSCs. Compared to those of the controls, downregulation of LMCD1 significantly restrained osteogenic differentiation and enhanced adipogenic differentiation, while upregulation of LMCD1 increased the osteogenic differentiation and suppressed adipogenic differentiation. Mechanically, we found that LMCD1 could protect RUNX2 and Smad1 protein from Smurf1-induced ubiquitination degradation thereby regulating BMP signaling. In conclusion, our findings suggest that LMCD1 is a novel regulator of osteogenic differentiation and may be a potential therapeutic target for bone metabolism related diseases.

APC/Cdh1 targets PECAM-1 for ubiquitination and degradation in endothelial cells

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is a member of the immunoglobulin superfamily and is expressed by hematopoietic and endothelial cells (ECs). Recent studies have shown that PECAM-1 plays a crucial role in promoting the development of the EC inflammatory response in the context of disturbed flow. However, the mechanistic pathways that control PECAM-1 protein stability remain largely unclear. Here, we identified PECAM-1 as a novel substrate of the APC/Cdh1 E3 ubiquitin ligase. Specifically, lentivirus-mediated Cdh1 depletion stabilized PECAM-1 in ECs. Conversely, overexpression of Cdh1 destabilized PECAM-1. The proteasome inhibitor MG132 blocked Cdh1-mediated PECAM-1 degradation. In addition, Cdh1 promoted K48-linked polyubiquitination of PECAM-1 in a destruction box-dependent manner. Furthermore, we demonstrated that compared with pulsatile shear stress (PS), oscillatory shear stress decreased the expression of Cdh1 and the ubiquitination of PECAM-1, therefore stabilizing PECAM-1 to promote inflammation in ECs. Hence, our study revealed a novel mechanism by which fluid flow patterns regulate EC homeostasis via Cdh1-dependent ubiquitination and subsequent degradation of PECAM-1.

Interplay between c-Src and the APC/C co-activator Cdh1 regulates mammary tumorigenesis

The Anaphase Promoting Complex (APC) coactivator Cdh1 drives proper cell cycle progression and is implicated in the suppression of tumorigenesis. However, it remains elusive how Cdh1 restrains cancer progression and how tumor cells escape the inhibition of Cdh1. Here we report that Cdh1 suppresses the kinase activity of c-Src in an APC-independent manner. Depleting Cdh1 accelerates breast cancer cell proliferation and cooperates with PTEN loss to promote breast tumor progression in mice. Hyperactive c-Src, on the other hand, reciprocally inhibits the ubiquitin E3 ligase activity of APC^Cdh1 through direct phosphorylation of Cdh1 at its N-terminus, which disrupts the interaction between Cdh1 and the APC core complex. Furthermore, pharmacological inhibition of c-Src restores APC^Cdh1 tumor suppressor function to repress a panel of APC^Cdh1 oncogenic substrates. Our findings reveal a reciprocal feedback circuit of Cdh1 and c-Src in the crosstalk between the cell cycle machinery and the c-Src signaling pathway.

The Anaphase Promoting Complex adaptor protein Cdh1 tightly controls cell cycle progression to restrain tumorigenesis but the mechanism is not completely known. Here, the authors show that reciprocal inhibition between Cdh1 and the c-Src signaling pathway regulate breast cancer tumorigenesis.

A multi-lock inhibitory mechanism for fine-tuning enzyme activities of the HECT family E3 ligases

HECT E3 ligases control the degradation and functioning of numerous oncogenic/tumor-suppressive factors and signaling proteins, and their activities must be tightly regulated to prevent cancers and other diseases. Here we show that the Nedd4 family HECT E3 WWP1 adopts an autoinhibited state, in which its multiple WW domains sequester HECT using a multi-lock mechanism. Removing WW2 or WW34 led to a partial activation of WWP1. The structure of fully inhibited WWP1 reveals that many WWP1 mutations identified in cancer patients result in a partially active state with increased E3 ligase activity, and the WWP1 mutants likely promote cell migration by enhancement of ∆Np63α degradation. We further demonstrate that WWP2 and Itch utilize a highly similar multi-lock autoinhibition mechanism as that utilized by WWP1, whereas Nedd4/4 L and Smurf2 utilize a slightly variant version. Overall, these results reveal versatile autoinhibitory mechanisms that fine-tune the ligase activities of the HECT family enzymes.

HECT type E3 ligases are key regulators of cell growth and proliferation. Here the authors present the crystal structures of the Nedd4 family E3 ligase WWP1 in a closed and semi-open state and in combination with mutagenesis experiments identify a multi-lock regulatory mechanism that allows the fine-tuning of activities of Nedd4 family E3 ligases.

K27-linked ubiquitination of BRAF by ITCH engages cytokine response to maintain MEK-ERK signaling

BRAF plays an indispensable role in activating the MEK/ERK pathway to drive tumorigenesis. Receptor tyrosine kinase and RAS-mediated BRAF activation have been extensively characterized, however, it remains undefined how BRAF function is fine-tuned by stimuli other than growth factors. Here, we report that in response to proinflammatory cytokines, BRAF is subjected to lysine 27-linked poly-ubiquitination in melanoma cells by the ITCH ubiquitin E3 ligase. Lysine 27-linked ubiquitination of BRAF recruits PP2A to antagonize the S365 phosphorylation and disrupts the inhibitory interaction with 14-3-3, leading to sustained BRAF activation and subsequent elevation of the MEK/ERK signaling. Physiologically, proinflammatory cytokines activate ITCH to maintain BRAF activity and to promote proliferation and invasion of melanoma cells, whereas the ubiquitination-deficient BRAF mutant displays compromised kinase activity and reduced tumorigenicity. Collectively, our study reveals a pivotal role for ITCH-mediated BRAF ubiquitination in coordinating the signals between cytokines and the MAPK pathway activation in melanoma cells.

Hippo signaling is intrinsically regulated during cell cycle progression by APC/CCdh1

The Hippo-YAP/TAZ signaling pathway plays a pivotal role in growth control during development and regeneration and its dysregulation is widely implicated in various cancers. To further understand the cellular and molecular mechanisms underlying Hippo signaling regulation, we have found that activities of core Hippo signaling components, large tumor suppressor (LATS) kinases and YAP/TAZ transcription factors, oscillate during mitotic cell cycle. We further identified that the anaphase-promoting complex/cyclosome (APC/C)^Cdh1 E3 ubiquitin ligase complex, which plays a key role governing eukaryotic cell cycle progression, intrinsically regulates Hippo signaling activities. CDH1 recognizes LATS kinases to promote their degradation and, hence, YAP/TAZ regulation by LATS phosphorylation is under cell cycle control. As a result, YAP/TAZ activities peak in G1 phase. Furthermore, we show in Drosophila eye and wing development that Cdh1 is required in vivo to regulate the LATS homolog Warts with a conserved mechanism. Cdh1 reduction increased Warts levels, which resulted in reduction of the eye and wing sizes in a Yorkie dependent manner. Therefore, LATS degradation by APC/C^Cdh1 represents a previously unappreciated and evolutionarily conserved layer of Hippo signaling regulation.

APC/CCdh1 regulates the balance between maintenance and differentiation of hematopoietic stem and progenitor cells

Hematopoietic stem and progenitor cells (HSPCs) represent the lifelong source of all blood cells and continuously regenerate the hematopoietic system through differentiation and self-renewal. The process of differentiation is initiated in the G1 phase of the cell cycle, when stem cells leave their quiescent state. During G1, the anaphase-promoting complex or cyclosome associated with the coactivator Cdh1 is highly active and marks proteins for proteasomal degradation to regulate cell proliferation. Following Cdh1 knockdown in HSPCs, we analyzed human and mouse hematopoiesis in vitro and in vivo in competitive transplantation assays. We found that Cdh1 is highly expressed in human CD34+ HSPCs and downregulated in differentiated subsets; whereas, loss of Cdh1 restricts myeloid differentiation, supports B cell development and preserves immature short-term HSPCs without affecting proliferation or viability. Our data highlight a role of Cdh1 as a regulator of balancing the maintenance of HSPCs and differentiation into mature blood cells.

VCP/p97 increases BMP signaling by accelerating ubiquitin ligase Smurf1 degradation

The bone morphogenetic protein (BMP)-Smad signaling pathway plays a crucial role in the control of bone homeostasis by regulating osteoblast activity. It is known that the ubiquitin ligase Smad ubiquitination regulatory factor (Smurf)1 is a master negative regulator of BMP signaling, but how its stability and activity are regulated remains poorly understood. Our study showed that valosin-containing protein/p97, the mutations of which lead to rare forms of Paget's disease of bone (PDB)-like syndrome-such as inclusion body myopathy (IBM) associated with Paget's disease of bone and frontotemporal dementia (IBM-PFD)-together with its adaptor nuclear protein localization (NPL)4, specifically interact with Smurf1 and deliver the ubiquitinated Smurf1 for degradation. Depletion of either p97 or NPL4 resulted in the elevation of Smurf1 protein level and decreased BMP signaling accordingly. Mechanically, a typical proline, glutamic acid, serine, and threonine motif specifically existing in Smurf1 is necessary for its recognition and degradation by p97, and this process is dependent on p97 ATPase activity. More importantly, compared with p97 WT, PDB-associated mutation of p97 (mainly A232E) harboring the higher ATPase activity of p97 further promoted Smurf1 degradation, thus increasing BMP signaling activity. Our findings first establish a link between p97 and Smurf1, providing an in-depth understanding of how Smurf1 is regulated, as well as the mechanism of p97-related bone diseases.-Li, H., Cui, Y., Wei, J., Liu, C., Chen, Y., Cui, C.-P., Li, L., Zhang, X., Zhang, L. VCP/p97 increases BMP signaling by accelerating ubiquitin ligase Smurf1 degradation.

WW domain-mediated regulation and activation of E3 ubiquitin ligase Suppressor of Deltex

The Nedd4 family E3 ligases Itch and WWP1/2 play crucial roles in the regulation of cell cycle progression and apoptosis and are closely correlated with cancer development and metastasis. It has been recently shown that the ligase activities of Itch and WWP1/2 are tightly regulated, with the HECT domain sequestered intramolecularly by a linker region connecting WW2 and WW3. Here, we show that a similar autoinhibitory mechanism is utilized by the Drosophila ortholog of Itch and WWP1/2, Suppressor of Deltex (Su(dx)). We show that Su(dx) adopts an inactive steady state with the WW domain region interacting with the HECT domain. We demonstrate that both the linker and preceding WW2 are required for the efficient binding and regulation of Su(dx) HECT. Recruiting the multiple-PY motif-containing adaptor dNdfip via WW domains relieves the inhibitory state of Su(dx) and leads to substrate (e.g. Notch) ubiquitination. Our study demonstrates an evolutionarily conservative mechanism governing the regulation and activation of some Nedd4 family E3 ligases. Our results also suggest a dual regulatory mechanism for specific Notch down-regulation via dNdfip-Su(dx)-mediated Notch ubiquitination.

Regulating the human HECT E3 ligases

Ubiquitination, the covalent attachment of ubiquitin to proteins, by E3 ligases of the HECT (homologous to E6AP C terminus) family is critical in controlling diverse physiological pathways. Stringent control of HECT E3 ligase activity and substrate specificity is essential for cellular health, whereas deregulation of HECT E3s plays a prominent role in disease. The cell employs a wide variety of regulatory mechanisms to control HECT E3 activity and substrate specificity. Here, we summarize the current understanding of these regulatory mechanisms that control HECT E3 function. Substrate specificity is generally determined by interactions of adaptor proteins with domains in the N-terminal extensions of HECT E3 ligases. These N-terminal domains have also been found to interact with the HECT domain, resulting in the formation of inhibitory conformations. In addition, catalytic activity of the HECT domain is commonly regulated at the level of E2 recruitment and through HECT E3 oligomerization. The previously mentioned regulatory mechanisms can be controlled through protein-protein interactions, post-translational modifications, the binding of calcium ions, and more. Functional activity is determined not only by substrate recruitment and catalytic activity, but also by the type of ubiquitin polymers catalyzed to the substrate. While this is often determined by the specific HECT member, recent studies demonstrate that HECT E3s can be modulated to alter the type of ubiquitin polymers they catalyze. Insight into these diverse regulatory mechanisms that control HECT E3 activity may open up new avenues for therapeutic strategies aimed at inhibition or enhancement of HECT E3 function in disease-related pathways.

Identification of the APC/C co-factor FZR1 as a novel therapeutic target for multiple myeloma

Multiple Myeloma (MM) is a haematological neoplasm characterised by the clonal proliferation of malignant plasma cells in the bone marrow. The success of proteasome inhibitors in the treatment of MM has highlighted the importance of the ubiquitin proteasome system (UPS) in the pathogenesis of this disease. In this study, we analysed gene expression of UPS components to identify novel therapeutic targets within this pathway in MM. Here we demonstrate how this approach identified previously validated and novel therapeutic targets. In addition we show that FZR1 (Fzr), a cofactor of the multi-subunit E3 ligase complex anaphase-promoting complex/cyclosome (APC/C), represents a novel therapeutic target in myeloma. The APC/C associates independently with two cofactors, Fzr and Cdc20, to control cell cycle progression. We found high levels of FZR1 in MM primary cells and cell lines and demonstrate that expression is further increased on adhesion to bone marrow stromal cells (BMSCs). Specific knockdown of either FZR1 or CDC20 reduced viability and induced growth arrest of MM cell lines, and resulted in accumulation of APC/C^Fzr substrate Topoisomerase IIα (TOPIIα) or APC/C^Cdc20 substrate Cyclin B. Similar effects were observed following treatment with proTAME, an inhibitor of both APC/C^Fzr and APC/C^Cdc20. Combinations of proTAME with topoisomerase inhibitors, etoposide and doxorubicin, significantly increased cell death in MM cell lines and primary cells, particularly if TOPIIα levels were first increased through pre-treatment with proTAME. Similarly, combinations of proTAME with the microtubule inhibitor vincristine resulted in enhanced cell death. This study demonstrates the potential of targeting the APC/C and its cofactors as a therapeutic approach in MM.

Smurf1 regulation of DAB2IP controls cell proliferation and migration

Tumor cell proliferation, survival and migration are regulated by the deletion of ovarian carcinoma 2/disabled homolog 2 (DOC-2/DAB2) interacting protein (DAB2IP), a tumor suppressor that serves as a scaffold protein for H-Ras and TRAF2. Importantly, the oncogenic histone methyl-transferase EZH2 epigenetically down-regulates DAB2IP in a variety of tumors. Recently, we demonstrated that DAB2IP is negatively regulated by Akt-dependent phosphorylation and SCF^Fbw7-mediated degradation. Here, we further identify the oncoprotein Smurf1, an E3-ubiquitin ligase, as a novel negative regulator of DAB2IP. Smurf1-mediated cellular proliferation and migration are largely dependent on the presence of DAB2IP, suggesting that DAB2IP is a key effector molecule of Smurf1 oncogenic function. Additionally, we identify that similar to DAB2IP, Smurf1 is also a target of phosphorylation by both Akt1 and Akt2 kinases, which enhances Smurf1 abundance, leading to a reduction in DAB2IP. Given the role of DAB2IP in tumorigenesis and metastasis, our data identify Smurf1 as an upstream oncogenic factor that negatively regulates DAB2IP to govern aberrant cell growth and migration.

CDH1 regulates E2F1 degradation in response to differentiation signals in keratinocytes

The E2F1 transcription factor plays key roles in skin homeostasis. In the epidermis, E2F1 expression is essential for normal proliferation of undifferentiated keratinocytes, regeneration after injury and DNA repair following UV radiation-induced photodamage. Abnormal E2F1 expression promotes nonmelanoma skin carcinoma. In addition, E2F1 must be downregulated for proper keratinocyte differentiation, but the relevant mechanisms involved remain poorly understood. We show that differentiation signals induce a series of post-translational modifications in E2F1 that are jointly required for its downregulation. Analysis of the structural determinants that govern these processes revealed a central role for S403 and T433. In particular, substitution of these two amino acid residues with non-phosphorylatable alanine (E2F1 ST/A) interferes with E2F1 nuclear export, K11- and K48-linked polyubiquitylation and degradation in differentiated keratinocytes. In contrast, replacement of S403 and T433 with phosphomimetic aspartic acid to generate a pseudophosphorylated E2F1 mutant protein (E2F1 ST/D) generates a protein that is regulated in a manner indistinguishable from that of wild type E2F1. Cdh1 is an activating cofactor that interacts with the anaphase-promoting complex/cyclosome (APC/C) ubiquitin E3 ligase, promoting proteasomal degradation of various substrates. We found that Cdh1 associates with E2F1 in keratinocytes. Inhibition or RNAi-mediated silencing of Cdh1 prevents E2F1 degradation in response to differentiation signals. Our results reveal novel regulatory mechanisms that jointly modulate post-translational modifications and downregulation of E2F1, which are necessary for proper epidermal keratinocyte differentiation.

Group-I PAKs-mediated phosphorylation of HACE1 at serine 385 regulates its oligomerization state and Rac1 ubiquitination

The regulation of Rac1 by HACE1-mediated ubiquitination and proteasomal degradation is emerging as an essential element in the maintenance of cell homeostasis. However, how the E3 ubiquitin ligase activity of HACE1 is regulated remains undetermined. Using a proteomic approach, we identified serine 385 as a target of group-I PAK kinases downstream Rac1 activation by CNF1 toxin from pathogenic E. coli. Moreover, cell treatment with VEGF also promotes Ser-385 phosphorylation of HACE1. We have established in vitro that HACE1 is a direct target of PAK1 kinase activity. Mechanistically, we found that the phospho-mimetic mutant HACE1(S385E), as opposed to HACE1(S385A), displays a lower capacity to ubiquitinate Rac1 in cells. Concomitantly, phosphorylation of Ser-385 plays a pivotal role in controlling the oligomerization state of HACE1. Finally, Ser-385 phosphorylated form of HACE1 localizes in the cytosol away from its target Rac1. Together, our data point to a feedback inhibition of HACE1 ubiquitination activity on Rac1 by group-I PAK kinases.

Allosteric auto-inhibition and activation of the Nedd4 family E3 ligase Itch

The Nedd4 family E3 ligases are key regulators of cell growth and proliferation and are often misregulated in human cancers and other diseases. The ligase activities of Nedd4 E3s are tightly controlled via auto-inhibition. However, the molecular mechanism underlying Nedd4 E3 auto-inhibition and activation is poorly understood. Here, we show that the WW domains proceeding the catalytic HECT domain play an inhibitory role by binding directly to HECT in the Nedd4 E3 family member Itch. Our structural and biochemical analyses of Itch reveal that the WW2 domain and a following linker allosterically lock HECT in an inactive state inhibiting E2-E3 transthiolation. Binding of the Ndfip1 adaptor or JNK1-mediated phosphorylation relieves the auto-inhibition of Itch in a WW2-dependent manner. Aberrant activation of Itch leads to migration defects of cortical neurons during development. Our study provides a new mechanism governing the regulation of Itch.

Increased PLEKHO1 within osteoblasts suppresses Smad-dependent BMP signaling to inhibit bone formation during aging

Emerging evidence indicates that the dysregulation of protein ubiquitination plays a crucial role in aging‐associated diseases. Smad‐dependent canonical BMP signaling pathway is indispensable for osteoblastic bone formation, which could be disrupted by the ubiquitination and subsequent proteasomal degradation of Smad1/5, the key molecules for BMP signaling transduction. However, whether the dysregulation of Smad1/5 ubiquitination and disrupted BMP signaling pathway is responsible for the age‐related bone formation reduction is still underexplored. Pleckstrin homology domain‐containing family O member 1 (PLEKHO1) is a previously identified ubiquitination‐related molecule that could specifically target the linker region between the WW domains of Smurf1 to promote the ubiquitination of Smad1/5. Here, we found an age‐related increase in the expression of PLEKHO1 in bone specimens from either fractured patients or aging rodents, which was associated with the age‐related reduction in Smad‐dependent BMP signaling and bone formation. By genetic approach, we demonstrated that loss of Plekho1 in osteoblasts could promote the Smad‐dependent BMP signaling and alleviated the age‐related bone formation reduction. In addition, osteoblast‐specific Smad1 overexpression had beneficial effect on bone formation during aging, which could be counteracted after overexpressing Plekho1 within osteoblasts. By pharmacological approach, we showed that osteoblast‐targeted Plekho1 siRNA treatment could enhance Smad‐dependent BMP signaling and promote bone formation in aging rodents. Taken together, it suggests that the increased PLEKHO1 could suppress Smad‐dependent BMP signaling to inhibit bone formation during aging, indicating the translational potential of targeting PLEKHO1 in osteoblast as a novel bone anabolic strategy for reversing established osteoporosis during aging.