Molecular Analysis of Membrane Targeting by the C2 Domain of the E3 Ubiquitin Ligase Smurf1

As a matter of fat: Emerging roles of lipid-sensitive E3 ubiquitin ligases

The dynamic structure and composition of lipid membranes need to be tightly regulated to control the vast array of cellular processes from cell and organelle morphology to protein-protein interactions and signal transduction pathways. To maintain membrane integrity, sense-and-response systems monitor and adjust membrane lipid composition to the ever-changing cellular environment, but only a relatively small number of control systems have been described. Here, we explore the emerging role of the ubiquitin-proteasome system in monitoring and maintaining membrane lipid composition. We focus on the ER-resident RNF145 E3 ubiquitin ligase, its role in regulating adiponectin receptor 2 (ADIPOR2), its lipid hydrolase substrate, and the broader implications for understanding the homeostatic processes that fine-tune cellular membrane composition.

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.

The regulatory roles of the E3 ubiquitin ligase NEDD4 family in DNA damage response

E3 ubiquitin ligases, an important part of ubiquitin proteasome system, catalyze the covalent binding of ubiquitin to target substrates, which plays a role in protein ubiquitination and regulates different biological process. DNA damage response (DDR) is induced in response to DNA damage to maintain genome integrity and stability, and this process has crucial significance to a series of cell activities such as differentiation, apoptosis, cell cycle. The NEDD4 family, belonging to HECT E3 ubiquitin ligases, is reported as regulators that participate in the DDR process by recognizing different substrates. In this review, we summarize recent researches on NEDD4 family members in the DDR and discuss the roles of NEDD4 family members in the cascade reactions induced by DNA damage. This review may contribute to the further study of pathophysiology for certain diseases and pharmacology for targeted drugs.

Specific interactions of peripheral membrane proteins with lipids: what can molecular simulations show us?

Peripheral membrane proteins (PMPs) can reversibly and specifically bind to biological membranes to carry out functions such as cell signalling, enzymatic activity, or membrane remodelling. Structures of these proteins and of their lipid-binding domains are typically solved in a soluble form, sometimes with a lipid or lipid headgroup at the binding site. To provide a detailed molecular view of PMP interactions with the membrane, computational methods such as molecular dynamics (MD) simulations can be applied. Here, we outline recent attempts to characterise these binding interactions, focusing on both intracellular proteins, such as phosphatidylinositol phosphate (PIP)-binding domains, and extracellular proteins such as glycolipid-binding bacterial exotoxins. We compare methods used to identify and analyse lipid-binding sites from simulation data and highlight recent work characterising the energetics of these interactions using free energy calculations. We describe how improvements in methodologies and computing power will help MD simulations to continue to contribute to this field in the future.

Overexpressed Smurf1 is degraded in glioblastoma cells through autophagy in a p62-dependent manner

Homologous to E6AP C-terminus (HECT)-type E3 ubiquitin ligase SMAD-specific E3 ubiquitin protein ligase 1 (Smurf1) was originally identified to ubiquitinate Smad protein in the TGF-β/BMP signaling pathway. Recently, Smurf1 has been reported to promote tumorigenesis by regulating multiple biological processes. High expression of Smurf1 plays a vital role in brain tumor progression by mediating aberrant cell signaling pathways. Previous reports have shown that Smurf1 is degraded mainly through the ubiquitin-proteasome system, but it remains unclear whether Smurf1 is degraded by autophagy in tumor cells. In this study, we show that autophagy activators promote Smurf1 degradation in glioblastoma (GB) cells. The autophagy receptor p62 colocalizes with ubiquitinated substrates to promote sequestration of cytoplasm cargo into the autophagosome. We report that autophagic degradation of Smurf1 is dependent on p62. Moreover, the autophagic degradation of Smurf1 is prevented in the absence of the HECT domain or E3 ubiquitin ligase activity. We further proved that activation of autophagy leads to a decrease of Smurf1 and the inhibition of the phosphoinositide 3-kinase/protein kinase B signaling pathway in GB cells. Our results suggest that enhancement of autophagic degradation of Smurf1 may be a potential approach to treating GB.

The ubiquitin ligation machinery in the defense against bacterial pathogens

The ubiquitin system is an important part of the host cellular defense program during bacterial infection. This is in particular evident for a number of bacteria including Salmonella Typhimurium and Mycobacterium tuberculosis which—inventively as part of their invasion strategy or accidentally upon rupture of seized host endomembranes—become exposed to the host cytosol. Ubiquitylation is involved in the detection and clearance of these bacteria as well as in the activation of innate immune and inflammatory signaling. Remarkably, all these defense responses seem to emanate from a dense layer of ubiquitin which coats the invading pathogens. In this review, we focus on the diverse group of host cell E3 ubiquitin ligases that help to tailor this ubiquitin coat. In particular, we address how the divergent ubiquitin conjugation mechanisms of these ligases contribute to the complexity of the anti‐bacterial coating and the recruitment of different ubiquitin‐binding effectors. We also discuss the activation and coordination of the different E3 ligases and which strategies bacteria evolved to evade the activities of the host ubiquitin system.

Binding of Ca2+-independent C2 domains to lipid membranes: A multi-scale molecular dynamics study

C2 domains facilitate protein interactions with lipid bilayers in either a Ca²⁺-dependent or -independent manner. We used molecular dynamics (MD) simulations to explore six Ca²⁺-independent C2 domains, from KIBRA, PI3KC2α, RIM2, PTEN, SHIP2, and Smurf2. In coarse-grained MD simulations these C2 domains formed transient interactions with zwitterionic bilayers, compared with longer-lived interactions with anionic bilayers containing phosphatidylinositol bisphosphate (PIP₂). Type I C2 domains bound non-canonically via the front, back, or side of the β sandwich, whereas type II C2 domains bound canonically, via the top loops. C2 domains interacted strongly with membranes containing PIP₂, causing bound anionic lipids to cluster around the protein. Binding modes were refined via atomistic simulations. For PTEN and SHIP2, CG simulations of their phosphatase plus C2 domains with PIP₂-containing bilayers were also performed, and the roles of the two domains in membrane localization compared. These studies establish a simulation protocol for membrane-recognition proteins.

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Binding of Ca²⁺-independent C2 domains to membranes was explored by MD simulation

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C2 domains from KIBRA, PI3KC2α, RIM2, PTEN, SHIP2, and Smurf2 were compared

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C2 domains formed longer-lived interactions with lipid bilayers containing PIP₂

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For PTEN and SHIP2, simulations of their phosphatase plus C2 domains were performed

SMURF1-mediated ubiquitylation of SHP-1 promotes cell proliferation and invasion of endometrial stromal cells in endometriosis

Background

Endometriosis is a widespread benign gynecological disorder. The signal transducer and activator of transcription 3 (STAT3) signaling pathway plays an important role in the pathogenesis of endometriosis through regulating proliferation and invasion of endometrial stromal cells. Furthermore, the protein tyrosine phosphatase (PTP), SH2 domain-containing phosphatase 1 (SHP-1), negatively regulates STAT3 activation. However, regulation of the SHP-1-STAT3 pathway in the pathogenesis of endometriosis remains unclear.

Methods

Cell proliferation and invasion were assessed by Cell Counting Kit-8 (CCK-8) assay and Transwell analysis, respectively, to investigate the role and regulation of the SHP-1-STAT3 pathway in the proliferation and invasion of endometrial stromal cells. Expression of Smad ubiquitin regulatory factor 1 (SMURF1), SHP-1, matrix metalloproteinase 2 (MMP2), MMP9, STAT3, and phospho-STAT3 (p-STAT3) level in patients with endometriosis were measured by Western blotting and/or immunohistochemical staining. The interaction between SMURF1 and SHP-1 was investigated by co-immunoprecipitation and ubiquitylation analysis.

Results

The present study demonstrated that downregulation of SHP-1 expression in patients with endometriosis was negatively correlated with SMURF1 expression. SMURF1, an E3 ubiquitin ligase, activated the STAT3 pathway via ubiquitylation and degradation of SHP-1. Furthermore, SMURF1 promoted cell proliferation and invasion of endometrial stromal cells by activating STAT3 signaling and expression of its downstream targets, MMP2 and MMP9, whereas SHP-1 demonstrated an inverse effect. Additionally, SHP-1 inhibited SMURF1-mediated cell invasion and proliferation of endometrial stromal cells.

Conclusions

Our findings indicate that SMURF1-mediated ubiquitylation of SHP-1 regulates endometrial stromal cell proliferation and invasion during endometriosis.

The Ubiquitin Proteasome System and Skin Fibrosis

The ubiquitin proteasome system (UPS) is a highly conserved way to regulate protein turnover in cells. The UPS hydrolyzes and destroys variant or misfolded proteins and finely regulates proteins involved in differentiation, apoptosis, and other biological processes. This system is a key regulatory factor in the proliferation, differentiation, and collagen secretion of skin fibroblasts. E3 ubiquitin protein ligases Parkin and NEDD4 regulate multiple signaling pathways in keloid. Tumor necrosis factor (TNF) receptor-associated factor 4 (TRAF4) binding with deubiquitinase USP10 can induce p53 destabilization and promote keloid-derived fibroblast proliferation. The UPS participates in the occurrence and development of hypertrophic scars by regulating the transforming growth factor (TGF)-β/Smad signaling pathway. An initial study suggests that TNFα-induced protein 3 (TNFAIP3) polymorphisms may be significantly associated with scleroderma susceptibility in individuals of Caucasian descent. Sumoylation and multiple ubiquitin ligases, including Smurfs, UFD2, and KLHL42, play vital roles in scleroderma by targeting the TGF-β/Smad signaling pathway. In the future, drugs targeting E3 ligases and deubiquitinating enzymes have great potential for the treatment of skin fibrosis.

Overexpression of TC2N is associated with poor prognosis in gastric cancer

Background: Tac2-N (TC2N) is a tandem C2 domain-containing protein, acting as a novel oncogene or suppressor in different kinds of cancers. However, the status of TC2N expression and its significance in gastric cancer (GC) is still unclear. The present study is aimed to elucidate the clinicopathological significance and prognostic value of TC2N level in GC. Methods: We used sequencing data from the Cancer Genome Atlas (TCGA) database to analyze TC2N expression in GC by UALCAN database and Gene Expression Profiling Interactive Analysis tools (GEPIA). TC2N expression level in 12 pairs of fresh GC tissues and adjacent nontumorous tissues was detected by quantitative real-time reverse-transcription polymerase chain reaction (RT-PCR) and Western blot (WB) assays. Immunohistochemical (IHC) staining was used to detect TC2N protein expression in Paraffin-embedded tissues in our center. In vitro proliferation, migration and invasion assays were used to evaluate the effect of TC2N on functional capability of gastric cancer cells. LinkedOmics was used to identify gene expressions associated with TC2N. Results: The mRNA and protein expression of TC2N in gastric cancer were both significantly higher than normal gastric mucosa. It was also elevated in gastric cancer cells compared with normal gastric epithelium cell. In vitro assays suggested that TC2N facilitated proliferation, migration and invasion of gastric cancer cells. Bioinformatic analysis showed a widespread impact of TC2N on the transcriptome and a strong interaction with tumor associated genes. We also found that TC2N was an independent prognostic factor for long-term survival in GC patients and its high expression was evidently associated with poor overall survival and recurrence-free survival. Conclusions: Our results show that high level of TC2N correlates with poor prognosis in patients with gastric cancer and promotes the development of gastric cancer. Thus, TC2N expression can serve as a prognostic biomarker for patients with gastric cancer.

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.