WWP1-dependent ubiquitination and degradation of the lung Krüppel-like factor, KLF2

WWP1 E3 ligase at the crossroads of health and disease

The E3 ubiquitin ligase WWP1 (WW Domain-containing E3 Ubiquitin Protein Ligase 1) is a member of the HECT (Homologous to the E6-associated protein Carboxyl Terminus) E3 ligase family. It is conserved across several species and plays crucial roles in various physiological processes, including development, cell growth and proliferation, apoptosis, and differentiation. It exerts its functions through ubiquitination or protein-protein interaction with PPXY-containing proteins. WWP1 plays a role in several human diseases, including cardiac conditions, neurodevelopmental, age-associated osteogenic disorders, infectious diseases, and cancers. In solid tumors, WWP1 plays a dual role as both an oncogene and a tumor suppressor, whereas in hematological malignancies such as AML, it is identified as a dedicated oncogene. Importantly, WWP1 inhibition using small molecule inhibitors such as Indole-3-Carbinol (I3C) and Bortezomib or siRNAs leads to significant suppression of cancer growth and healing of bone fractures, suggesting that WWP1 might serve as a potential therapeutic target for several diseases. In this review, we discuss the evolutionary perspective, structure, and functions of WWP1 and its multilevel regulation by various regulators. We also examine its emerging roles in cancer progression and its therapeutic potential. Finally, we highlight WWP1's role in normal physiology, contribution to pathological conditions, and therapeutic potential for cancer and other diseases.

Neglected PTM in Animal Adipogenesis: E3-mediated Ubiquitination

Ubiquitination is a widespread post-transcriptional modification (PTM) that occurs during protein degradation in eukaryotes and participates in almost all physiological and pathological processes, including animal adipogenesis. Ubiquitination is a cascade reaction regulated by the activating enzyme E1, conjugating enzyme E2, and ligase E3. Several recent studies have reported that E3 ligases play important regulatory roles in adipogenesis. However, as a key influencing factor for the recognition and connection between the substrate and ubiquitin during ubiquitination, its regulatory role in adipogenesis has not received adequate attention. In this review, we summarize the E3s' regulation and modification targets in animal adipogenesis, explain the regulatory mechanisms in lipogenic-related pathways, and further analyze the existing positive results to provide research directions of guiding significance for further studies on the regulatory mechanisms of E3s in animal adipogenesis.

WW Domain-Containing E3 Ubiquitin Protein Ligase 1: A Self-Disciplined Oncoprotein

WW domain-containing E3 ubiquitin protein ligase 1 (WWP1) is a member of C2-WW-HECT E3 ligase family. Although it may execute carcinostatic actions in some scenarios, WWP1 functions as an oncoprotein under most circumstances. Here, we comprehensively review reports on regulation of WWP1 and its roles in tumorigenesis. We summarize the WWP1-mediated ubiquitinations of diverse proteins and the signaling pathways they involved, as well as the mechanisms how they affect cancer formation and progression. According to our analysis of database, in combination with previous reports, we come to a conclusion that WWP1 expression is augmented in various cancers. Gene amplification, as well as expression regulation mediated by molecules such as non-coding RNAs, may account for the increased mRNA level of WWP1. Regulation of enzymatic activity is another important facet to upregulate WWP1-mediated ubiquitinations. Based on the published data, we conclude that WWP1 employs interactions between multiple domains to autoinhibit its polyubiquitination activity in a steady state. Association of some substrates can partially release certain autoinhibition-related domains and make WWP1 have a moderate activity of polyubiquitination. Some cancer-related mutations can fully disrupt the inhibitory interactions and make WWP1 hyperactive. High expression level or hyperactivation of WWP1 may abnormally enhance polyubiquitinations of some oncoproteins or tumor suppressors, such as ΔNp63α, PTEN and p27, and ultimately promote cell proliferation, survival, migration and invasion in tumorigenesis. Given the dysregulation and oncogenic functions of WWP1 in some cancer types, it is promising to explore some therapeutic inhibitors to tune down its activity.

An HECT domain ubiquitin ligase CgWWP1 regulates granulocytes proliferation in oyster Crassostrea gigas

Ubiquitination is involved in the regulation of granulocyte proliferation in vertebrate, and E3 ubiquitin ligase WWP1 has been reported to play an essential role in this process. In the present study, an HECT type E3 ubiquitin ligase (CgWWP1) was identified from oyster Crassostrea gigas, which contained a N-terminal C2 domain, four WW domains, and a C-terminal HECT domain. CgWWP1 was able to bind the activated ubiquitin (Ub) and formed CgWWP1-Ub complex in vitro. The mRNA transcripts of CgWWP1 were expressed in granulocytes, semi-granulocytes and agranulocytes, with the highest expression level in granulocytes. The expressions of potential granulocyte markers CgSOX11 (0.18-fold, p < 0.05) and CgAATase (0.2-fold, p < 0.01) in haemocytes were significantly down-regulated at 24 h after the treatment with Indole-3-carbinol (I3C), a WWP1 inhibitor. The proportions of EdU⁺ granulocytes reduced significantly at 12 h (8.1% ± 1.4%) and 24 h (9.7% ± 2.8%) after I3C treatment, which were significantly lower than that in the sterile seawater treatment (SW) group at 12 h (15.8% ± 4.2%) and 24 h (17.6% ± 0.8%), respectively. Meanwhile, the green EdU signals observed by confocal scanning microscopy in granulocytes of oysters treated by I3C became weaker compared to that in the SW group. These results indicated that CgWWP1 was involved in the regulation of granulocyte proliferation as a ubiquitin-protein ligase.

METTL3-mediated m6A methylation of SPHK2 promotes gastric cancer progression by targeting KLF2

N6-methyladenosine (m⁶A) RNA methylation is profoundly involved in epigenetic regulation, especially for carcinogenesis and tumor progression. Mounting evidence suggests that methyltransferase METTL3 regulates malignant behaviors of gastric cancer (GC). However, the clinical significance and biological implication of SPHK2 and its related m⁶A modification in GC remain unclear. In this study, quantitative real-time PCR (qRT-PCR), western blot and immunohistochemistry were utilized to detect the expression profiles and prognostic significance of SPHK2 in GC. Here, we showed that increased SPHK2 was signified a poor prognosis of GC patients. Phosphorylation and ubiquitination assays were used to investigate the possible mechanisms of SPHK2-mediated KLF2 expression. SPHK2 can promote the phosphorylation of KLF2, which triggers the ubiquitination and degradation of KLF2 protein in GC. Methylated RNA immunoprecipitation (MeRIP) was performed to uncover the m⁶A modification of SPHK2 mRNA. METTL3 promotes translation of SPHK2 mRNA via an m⁶A-YTHDF1-dependent manner. Functionally, SPHK2 facilitates GC cell proliferation, migration and invasion by inhibiting KLF2 expression. SPHK2/KLF2 regulates the cell proliferation, migration, and invasion induced by METTL3 in GC. Overall, our findings reveal that METTL3-mediated m⁶A modification of SPHK2 contributes to GC progression, which extends the understanding of the importance m⁶A methylation in GC and represents a potential target for GC therapy.

MicroRNA-15b shuttled by bone marrow mesenchymal stem cell-derived extracellular vesicles binds to WWP1 and promotes osteogenic differentiation

Background

Osteogenic differentiation is an essential process for bone regeneration involving bone marrow mesenchymal stem cells (BMSCs). BMSC-secreted extracellular vesicles (EVs) enriched with microRNAs (miRs) have vital roles to play in mediating osteogenic differentiation. Therefore, this study aimed to explore the effect of BMSC-derived EVs loaded with miR-15b on osteogenic differentiation.

Methods

Human BMSCs (hBMSCs) were cultured and treated with plasmids overexpressing or knocking down KLF2, WWP1, and miR-15b to define the role of derived EVs in osteogenic differentiation in vitro. The expression of osteogenic differentiation-related marker was measured by Western blot analysis. The interaction among miR-15b, WWP1, and ubiquitination of KLF2 was investigated by dual-luciferase reporter, immunoprecipitation, and GST pull-down assays. Moreover, EVs from hBMSCs transfected with miR-15b inhibitor (EV-miR-15b inhibitor) were injected into ovariectomized rats to verify the effect of miR-15b on bone loss in vivo.

Results

WWP1 was downregulated, and KLF2 was upregulated during osteogenic differentiation. After co-culture with EVs, miR-15b expression was elevated and WWP1 expression was reduced in hBMSCs. Upregulation of miR-15b or KLF2 or downregulation of WWP1 or NF-κB increased ALP activity and cell mineralization, as well as osteogenic differentiation-related marker expression in hBMSCs. Mechanistically, miR-15b targeted and inhibited WWP1, thus attenuating KLF2 degradation and inhibiting NF-κB activity. Co-culture of EVs increased the bone volume and trabecular number, but decreased bone loss in ovariectomized rats, which could be reversed after treatment with EV-miR-15b inhibitor.

Conclusion

Collectively, BMSC-derived EVs loaded with miR-15b promoted osteogenic differentiation by impairing WWP1-mediated KLF2 ubiquitination and inactivating the NF-κB signaling pathway.

Graphical abstract

Transcription Factor KLF2 and Its Role in the Regulation of Inflammatory Processes

KLF2 is a member of the Krüppel-like transcription factor family of proteins containing highly conserved DNA-binding zinc finger domains. KLF2 participates in the differentiation and regulation of the functional activity of monocytes, T lymphocytes, adipocytes, and vascular endothelial cells. The activity of KLF2 is controlled by several regulatory systems, including the MEKK2,3/MEK5/ERK5/MEF2 MAP kinase cascade, Rho family G-proteins, histone acetyltransferases CBP and p300, and histone deacetylases HDAC4 and HDAC5. Activation of KLF2 in endothelial cells induces eNOS expression and provides vasodilatory effect. Many KLF2-dependent genes participate in the suppression of blood coagulation and aggregation of T cells and macrophages with the vascular endothelium, thereby preventing atherosclerosis progression. KLF2 can have a dual effect on the gene transcription. Thus, it induces expression of multiple genes, but suppresses transcription of NF-κB-dependent genes. Transcription factors KLF2 and NF-κB are reciprocal antagonists. KLF2 inhibits induction of NF-κB-dependent genes, whereas NF-κB downregulates KLF2 expression. KLF2-mediated inhibition of NF-κB signaling leads to the suppression of cell response to the pro-inflammatory cytokines IL-1β and TNFα and results in the attenuation of inflammatory processes.

WWP1 knockout in mice exacerbates obesity-related phenotypes in white adipose tissue but improves whole-body glucose metabolism

White adipose tissue (WAT) is important for maintenance of homeostasis, because it stores energy and secretes adipokines. The WAT of obese people demonstrates mitochondrial dysfunction, accompanied by oxidative stress, which leads to insulin resistance. WW domain-containing E3 ubiquitin protein ligase 1 (WWP1) is a member of the HECT-type E3 family of ubiquitin ligases and is associated with several diseases. Recently, we demonstrated that WWP1 is induced specifically in the WAT of obese mice, where it protects against oxidative stress. Here, we investigated the function of WWP1 in WAT of obese mice by analyzing the phenotype of Wwp1 knockout (KO) mice fed a high-fat diet. The levels of oxidative stress markers were higher in obese WAT from Wwp1 KO mice. Moreover, Wwp1 KO mice had lower activity of citrate synthase, a mitochondrial enzyme. We also measured AKT phosphorylation in obese WAT and found lower levels in Wwp1 KO mice. However, plasma insulin level was low and glucose level was unchanged in obese Wwp1 KO mice. Moreover, both glucose tolerance test and insulin tolerance test were improved in obese Wwp1 KO mice. These findings indicate that WWP1 participates in the antioxidative response and mitochondrial function in WAT, but knockdown of WWP1 improves whole-body glucose metabolism.

miR-129-5p and -3p co-target WWP1 to suppress gastric cancer proliferation and migration

Gastric cancer (GC) is a worldwide health problem. Uncovering the underlining molecular mechanisms of GC is of vital significance. Here, we identified a novel oncogene WW domain-containing E3 ubiquitin protein ligase 1 (WWP1) in GC. WWP1 could promote GC cell proliferation and migration in vitro and expedite GC growth in vivo. We also found out two microRNAs (miRNAs): miR-129-5p and -3p could both target WWP1. Interestingly, miR-129-5p bound to the CDS region of WWP1 mRNA. The miR-129 pairs (miR-129-5p and -3p) play pivotal roles in GC to suppress its proliferation and migration in vitro and slow down GC growth in vivo by repressing WWP1. In summary, we identified two tumor suppressive miRNAs which share the same precursor that could regulate the same oncogene WWP1 in GC. Our finding would add new route for GC research and treatment.

Hyperglycemia induces skeletal muscle atrophy via a WWP1/KLF15 axis

Diabetes mellitus is associated with various disorders of the locomotor system including the decline in mass and function of skeletal muscle. The mechanism underlying this association has remained ambiguous, however. We now show that the abundance of the transcription factor KLF15 as well as the expression of genes related to muscle atrophy are increased in skeletal muscle of diabetic model mice, and that mice with muscle-specific KLF15 deficiency are protected from the diabetes-induced decline of skeletal muscle mass. Hyperglycemia was found to upregulate the KLF15 protein in skeletal muscle of diabetic animals, which is achieved via downregulation of the E3 ubiquitin ligase WWP1 and consequent suppression of the ubiquitin-dependent degradation of KLF15. Our results revealed that hyperglycemia, a central disorder in diabetes, promotes muscle atrophy via a WWP1/KLF15 pathway. This pathway may serve as a therapeutic target for decline in skeletal muscle mass accompanied by diabetes mellitus.

β-dystroglycan is regulated by a balance between WWP1-mediated degradation and protection from WWP1 by dystrophin and utrophin

Dystroglycan is a ubiquitous membrane protein that functions as a mechanical connection between the extracellular matrix and cytoskeleton. In skeletal muscle, dystroglycan plays an indispensable role in regulating muscle regeneration; a malfunction in dystroglycan is associated with muscular dystrophy. The regulation of dystroglycan stability is poorly understood. Here, we report that WWP1, a member of NEDD4 E3 ubiquitin ligase family, promotes ubiquitination and subsequent degradation of β-dystroglycan. Our results indicate that dystrophin and utrophin protect β-dystroglycan from WWP1-mediated degradation by competing with WWP1 for the shared binding site at the cytosolic tail of β-dystroglycan. In addition, we show that a missense mutation (arginine 440 to glutamine) in WWP1-which is known to cause muscular dystrophy in chickens-increases the ubiquitin ligase-mediated ubiquitination of both β-dystroglycan and WWP1. The R440Q missense mutation in WWP1 decreases HECT domain-mediated intramolecular interactions to relieve autoinhibition of the enzyme. Our results provide new insight into the regulation of β-dystroglycan degradation by WWP1 and other Nedd4 family members and improves our understanding of dystroglycan-related disorders.

Krüppel-like factors: Crippling and un-crippling metabolic pathways

Krüppel-like factors (KLFs) are DNA-binding transcriptional factors that regulate various pathways that control metabolism and other cellular mechanisms. Various KLF isoforms have been associated with cellular, organ or systemic metabolism. Altered expression or activation of KLFs has been linked to metabolic abnormalities, such as obesity and diabetes, as well as with heart failure. In this review article we summarize the metabolic functions of KLFs, as well as the networks of different KLF isoforms that jointly regulate metabolism in health and disease.

KLF2 in Regulation of NF-κB-Mediated Immune Cell Function and Inflammation

KLF2 (Kruppel-like factor 2) is a member of the zinc finger transcription factor family, which critically regulates embryonic lung development, function of endothelial cells and maintenance of quiescence in T-cells and monocytes. It is expressed in naïve T-cells and monocytes, however its level of expression decreases during activation and differentiation. KLF2 also plays critical regulatory role in various inflammatory diseases and their pathogenesis. Nuclear factor-kappaB (NF-κB) is an important inducer of inflammation and the inflammation is mediated through the transcription of several proinflammatory cytokines, chemokines and adhesion molecules. So, both transcriptional factors KLF2 and NF-κB are being associated with the similar cellular functions and their maintenance. It was shown that KLF2 regulates most of the NF-κB-mediated activities. In this review, we focused on emphasizing the involvement of KLF2 in health and disease states and how they interact with transcriptional master regulator NF-κB.

Krüppel-like factors in mammalian stem cells and development

Krüppel-like factors (KLFs) are a family of zinc-finger transcription factors that are found in many species. Recent studies have shown that KLFs play a fundamental role in regulating diverse biological processes such as cell proliferation, differentiation, development and regeneration. Of note, several KLFs are also crucial for maintaining pluripotency and, hence, have been linked to reprogramming and regenerative medicine approaches. Here, we review the crucial functions of KLFs in mammalian embryogenesis, stem cell biology and regeneration, as revealed by studies of animal models. We also highlight how KLFs have been implicated in human diseases and outline potential avenues for future research.

The 11S Proteasome Subunit PSME3 Is a Positive Feedforward Regulator of NF-κB and Important for Host Defense against Bacterial Pathogens

The NF-κB pathway plays important roles in immune responses. Although its regulation has been extensively studied, here, we report an unknown feedforward mechanism for the regulation of this pathway by Toll-like receptor (TLR) ligands in macrophages. During bacterial infections, TLR ligands upregulate the expression of the 11S proteasome subunit PSME3 via NF-κB-mediated transcription in macrophages. PSME3, in turn, enhances the transcriptional activity of NF-κB by directly binding to and destabilizing KLF2, a negative regulator of NF-κB transcriptional activity. Consistent with this positive role of PSME3 in NF-κB regulation and importance of the NF-κB pathway in host defense against bacterial infections, the lack of PSME3 in hematopoietic cells renders the hosts more susceptible to bacterial infections, accompanied by increased bacterial burdens in host tissues. Thus, this study identifies a substrate for PSME3 and elucidates a proteolysis-dependent, but ubiquitin-independent, mechanism for NF-κB regulation that is important for host defense and innate immunity.

Krüppel-like factor 2 represses transcription of the telomerase catalytic subunit human telomerase reverse transcriptase (hTERT) in human T cells

In normal human T cells, telomerase activity is strictly regulated. T cells are thought to express telomerase to avoid replicative senescence, unlike most normal somatic cells with definite replicative lifespan. T cells in blood and tissues are usually in a state of quiescence without expression of the limiting catalytic subunit of telomerase, human telomerase reverse transcriptase (hTERT). In contrast to activation, repression of hTERT transcription has not been studied well. Our previous studies have found an hTERT promoter element with repressive function. Here we identified KLF2, which represses hTERT transcription by binding to the putative promoter element. KLF2 and hTERT exhibited reciprocal mRNA expression patterns in primary human T cells. In activated T cells, KLF2 binding to the hTERT promoter was eliminated, relieving the repression of hTERT transcription found in resting T cells. Our results suggest that KLF2 is involved in strict repression of hTERT expression through binding to the promoter in primary human T cells.

KLF2 is a rate-limiting transcription factor that can be targeted to enhance regulatory T-cell production

Regulatory T cells (Tregs) are a specialized subset of CD4(+) T cells that maintain self-tolerance by functionally suppressing autoreactive lymphocytes. The Treg compartment is composed of thymus-derived Tregs (tTregs) and peripheral Tregs (pTregs) that are generated in secondary lymphoid organs after exposure to antigen and specific cytokines, such as TGF-β. With regard to this latter lineage, pTregs [and their ex vivo generated counterparts, induced Tregs (iTregs)] offer particular therapeutic potential because these cells can be raised against specific antigens to limit autoimmunity. We now report that transcription factor Krüppel-like factor 2 (KLF2) is necessary for the generation of iTregs but not tTregs. Moreover, drugs that limit KLF2 proteolysis during T-cell activation enhance iTreg development. To the authors' knowledge, this study identifies the first transcription factor to distinguish between i/pTreg and tTreg ontogeny and demonstrates that KLF2 is a therapeutic target for the production of regulatory T cells.

A Krüppel-like factor downstream of the E3 ligase WWP-1 mediates dietary-restriction-induced longevity in Caenorhabditis elegans

The HECT ubiquitin E3 ligase WWP-1 is a positive regulator of lifespan in response to dietary restriction (DR) in Caenorhabditis elegans. However, substrates of WWP-1 for ubiquitylation in the DR pathway have not yet been identified. Here we identify the C. elegans Krüppel-like factor, KLF-1, as an essential and specific regulator of DR-induced longevity and a substrate for ubiquitylation by WWP-1. Knockdown of klf-1 suppresses the extended lifespan of both DR animals and wwp-1-overexpressing animals, indicating that KLF-1 functions within the same pathway as WWP-1. In addition, overexpression of klf-1 in the intestine is sufficient to extend the lifespan of WT animals on an ad libitum diet, and requires wwp-1 or pha-4/FoxA. We demonstrate that WWP-1 directly interacts with KLF-1 and mediates multiple monoubiquitylation of KLF-1 in vitro and in cellulo. Our data support a model in which modulation of KLF-1 by WWP-1 regulates diet-restriction-induced longevity.

TIPping the balance in adipogenesis: USP7-mediated stabilization of Tip60

Adipogenesis is regulated by a complex interplay between transcription factors, in concert with-among others-transcriptional cofactors, signaling cascades and miRNAs. Several studies have implicated the transcriptional cofactor and acetyltransferase Tip60 in PPARγ signaling and adipocyte differentiation. Since Tip60 protein levels, but not mRNA levels, are upregulated during adipogenesis, and since Tip60 can be degraded by the proteasome, we hypothesized that Tip60 protein may be stabilized through deubiquitination during adipogenesis. Indeed, Tip60 is protected from proteasomal degeradation by the deubiquitinase USP7, which is particularly important for mitotic clonal expansion (MCE), an early step in adipogenesis. Besides this novel role in early differentiation, earlier studies indicated that Tip60 is also important during the later stages of differentiation, indicating a dual role for this protein in adipogenesis. Our recent study sheds new light on the role of Tip60 in cellular differentiation and provide new insights into the importance of a regulatory process that has not been studied intensively in adipogenesis: protein (de)ubiquitination.

Ubiquitin E3 ligase Wwp1 negatively regulates osteoblast function by inhibiting osteoblast differentiation and migration

Ubiquitin E3 ligase-mediated protein degradation promotes proteasomal degradation of key positive regulators of osteoblast functions. For example, the E3 ligases--SMAD-specific E3 ubiquitin protein ligase 1 (Smurf1), Itch, and WW domain-containing E3 ubiquitin protein ligase 1 (Wwp1)--promote degradation of Runt-related transcription factor 2 (Runx2), transcription factor jun-B (JunB), and chemokine (C-X-C) receptor type 4 (CXCR-4) proteins to inhibit their functions. However, the role of E3 ligases in age-associated bone loss is unknown. We found that the expression level of Wwp1, but not Smurf1 or Itch, was significantly increased in CD45-negative (CD45(-)) bone marrow-derived mesenchymal stem cells from 6-month-old and 12-month-old wild-type (WT) mice. Wwp1 knockout (Wwp1(-/-)) mice developed increased bone mass as they aged, associated with increased bone formation rates and normal bone resorption parameters. Bone marrow stromal cells (BMSCs) from Wwp1(-/-) mice formed increased numbers and areas of alkaline phosphatase(+) and Alizarin red(+) nodules and had increased migration potential toward chemokine (C-X-C motif) ligand 12 (CXCL12) gradients. Runx2, JunB, and CXCR-4 protein levels were significantly increased in Wwp1(-/-) BMSCs. Wwp1(-/-) BMSCs had increased amount of ubiquitinated JunB protein, but Runx2 ubiquitination was no change. Knocking down JunB in Wwp1(-/-) BMSCs returned Runx2 protein levels to that in WT cells. Thus, Wwp1 negatively regulates osteoblast functions by affecting both their migration and differentiation. Mechanisms designed to decrease Wwp1 levels in BMSCs may represent a new approach to prevent the decrease in osteoblastic bone formation associated with aging.

WWP1 gene is a potential molecular target of human oral cancer

OBJECTIVE

This study investigates the oncogenic role of WWP1, an ubiquitin ligase linked to tumor promotion, in oral cancer.

STUDY DESIGN

An array-based comparative genomic hybridization was used to detect chromosomal changes in 10 oral cancer specimens. An additional 59 specimens and 6 cultured oral cancer cells were further examined to evaluate changes in the DNA copy number and messenger RNA (mRNA) and protein expression of WWP1.

RESULTS

The copy number of the WWP1 gene and its mRNA levels were significantly increased in the oral cancer specimens. An elevated WWP1 gene expression was observed in 6 cultured oral cancer cell lines. Knockdown of the endogenous WWP1 using small hairpin RNA further showed that deficiency of WWP1 suppressed cell growth and caused apoptosis in oral cancer cells.

CONCLUSION

Our results reveal that WWP1 might play an oncogenic role in oral cancer cells.

WW domain containing E3 ubiquitin protein ligase 1 (WWP1) negatively regulates TLR4-mediated TNF-α and IL-6 production by proteasomal degradation of TNF receptor associated factor 6 (TRAF6)

BACKGROUND

Toll-like receptors (TLRs) play a pivotal role in the defense against invading pathogens by detecting pathogen-associated molecular patterns (PAMPs). TLR4 recognizes lipopolysaccharides (LPS) in the cell walls of Gram-negative bacteria, resulting in the induction and secretion of proinflammatory cytokines such as TNF-α and IL-6. The WW domain containing E3 ubiquitin protein ligase 1 (WWP1) regulates a variety of cellular biological processes. Here, we investigated whether WWP1 acts as an E3 ubiquitin ligase in TLR-mediated inflammation.

METHODOLOGY/RESULTS

Knocking down WWP1 enhanced the TNF-α and IL-6 production induced by LPS, and over-expression of WWP1 inhibited the TNF-α and IL-6 production induced by LPS, but not by TNF-α. WWP1 also inhibited the IκB-α, NF-κB, and MAPK activation stimulated by LPS. Additionally, WWP1 could degrade TRAF6, but not IRAK1, in the proteasome pathway, and knocking down WWP1 reduced the LPS-induced K48-linked, but not K63-linked, polyubiquitination of endogenous TRAF6.

CONCLUSIONS/SIGNIFICANCE

We identified WWP1 as an important negative regulator of TLR4-mediated TNF-α and IL-6 production. We also showed that WWP1 functions as an E3 ligase when cells are stimulated with LPS by binding to TRAF6 and promoting K48-linked polyubiquitination. This results in the proteasomal degradation of TRAF6.

Metabolic and immunomodulatory effects of n-3 fatty acids are different in mesenteric and epididymal adipose tissue of diet-induced obese mice

In studies emphasizing antiobesogenic and anti-inflammatory effects of long-chain n-3 polyunsaturated fatty acids (LC-n-3 PUFA), diets with very high fat content, not well-defined fat quality, and extreme n-6/n-3 PUFA ratios have been applied frequently. Additionally, comparative analyses of visceral adipose tissues (VAT) were neglected. Considering the link of visceral obesity to insulin resistance or inflammatory bowel diseases, we hypothesized that VAT, especially mesenteric adipose tissue (MAT), may exhibit differential responsiveness to diets through modulation of metabolic and inflammatory processes. Here, we aimed to assess dietary LC-n-3 PUFA effects on MAT and epididymal adipose tissue (EAT) and on MAT-adjacent liver and intestine in diet-induced obese mice fed defined soybean/palm oil-based diets. High-fat (HF) and LC-n-3 PUFA-enriched high-fat diet (HF/n-3) contained moderately high fat with unbalanced and balanced n-6/n-3 PUFA ratios, respectively. Body composition/organ analyses, glucose tolerance test, measurements of insulin, lipids, mRNA and protein expression, and immunohistochemistry were applied. Compared with HF, HF/n-3 mice showed reduced fat mass, smaller adipocytes in MAT than EAT, improved insulin level, and lower hepatic triacylglycerol and plasma NEFA levels, consistent with liver and brown fat gene expression. Gene expression arrays pointed to immune cell activation in MAT and alleviation of intestinal endothelial cell activation. Validations demonstrated simultaneously upregulated pro- (TNFα, MCP-1) and anti-inflammatory (IL-10) cytokines and M1/M2-macrophage markers in VAT and reduced CD4/CD8α expression in MAT and spleen. Our data revealed differential responsiveness to diets for VAT through preferentially metabolic alterations in MAT and inflammatory processes in EAT. LC-n-3 PUFA effects were pro- and anti-inflammatory and disclose T cell-immunosuppressive potential.

FBW7 regulates endothelial functions by targeting KLF2 for ubiquitination and degradation

F-box and WD repeat domain-containing 7 (FBW7), the substrate-binding subunit of E3 ubiquitin ligase SCF(FBW7) (a complex of SKP1, cullin-1 and FBW7), plays important roles in various physiological and pathological processes. Although FBW7 is required for vascular development, its function in the endothelium remains to be investigated. In this study, we show that FBW7 is an important regulator of endothelial functions, including angiogenesis, leukocyte adhesion and the endothelial barrier integrity. Using RNA interference, we found that the depletion of FBW7 markedly impairs angiogenesis in vitro and in vivo. We identified the zinc finger transcription factor Krüppel-like factor 2 (KLF2) as a physiological target of FBW7 in endothelial cells. Knockdown of FBW7 expression resulted in the accumulation of endogenous KLF2 protein in endothelial cells. FBW7-mediated KLF2 destruction was shown to depend on the phosphorylation of KLF2 via glycogen synthase kinase-3 (GSK3) at two conserved phosphodegrons. Mutating these phosphodegron motifs abolished the FBW7-mediated degradation and ubiquitination of KLF2. The siRNA-mediated knockdown of FBW7 showed that KLF2 is an essential target of FBW7 in the regulation of endothelial functions. Moreover, FBW7-mediated KLF2 degradation was shown to be critical for angiogenesis in teratomas and in zebrafish development. Taken together, our study suggests a role for FBW7 in the processes of endothelial cell migration, angiogenesis, inflammation and barrier integrity, and provides novel insights into the regulation of KLF2 stability in vivo.

Fbw7γ-mediated degradation of KLF13 prevents RANTES expression in resting human but not murine T lymphocytes

RANTES (CCL5) is a chemokine implicated in many human diseases. We previously showed that the transcription factor Kruppel-like factor 13 (KLF13) controls the late (3-5 days after activation) expression of RANTES in T lymphocytes and that KLF13 itself is translationally regulated through the 5'-untranslated region of its mRNA. Here, we show that KLF13 levels are further regulated by ubiquitination and degradation. KLF13 protein is undetectable in resting human T lymphocytes, but treatment with either proteosomal or lysosomal inhibitors increases KLF13 protein levels. Glycogen synthase kinase 3β (GSK3β)-mediated phosphorylation of KLF13 triggers the ubiquitination of KLF13 by the E3 ligase Fbw7γ, resulting in KLF13 protein degradation. Knockdown of either Fbw7γ or GSK3β by small interfering RNA increases KLF13 expression in resting human T lymphocytes. In contrast, in murine T lymphocytes, KLF13 protein is abundant because of the absence of Fbw7γ. Treatment of unactivated human lymphocytes with lysosomal inhibitors stabilizes KLF13 protein, resulting in an increase of RANTES mRNA and protein. Taken together, these studies found that tightly regulated control of both synthesis and degradation allows rapid changes in the level of KLF13 in human T lymphocytes.

Ezrin ubiquitylation by the E3 ubiquitin ligase, WWP1, and consequent regulation of hepatocyte growth factor receptor activity

The membrane cytoskeleton linker ezrin participates in several functions downstream of the receptor Met in response to Hepatocyte Growth Factor (HGF) stimulation. Here we report a novel interaction of ezrin with a HECT E3 ubiquitin ligase, WWP1/Aip5/Tiul1, a potential oncogene that undergoes genomic amplification and overexpression in human breast and prostate cancers. We show that ezrin binds to the WW domains of WWP1 via the consensus motif PPVY⁴⁷⁷ present in ezrin’s C-terminus. This association results in the ubiquitylation of ezrin, a process that requires an intact PPVY⁴⁷⁷ motif. Interestingly ezrin ubiquitylation does not target the protein for degradation by the proteasome. We find that ezrin ubiquitylation by WWP1 in epithelial cells leads to the upregulation of Met level in absence of HGF stimulation and increases the response of Met to HGF stimulation as measured by the ability of the cells to heal a wound. Interestingly this effect requires ubiquitylated ezrin since it can be rescued, after depletion of endogenous ezrin, by wild type ezrin but not by a mutant of ezrin that cannot be ubiquitylated. Taken together our data reveal a new role for ezrin in Met receptor stability and activity through its association with the E3 ubiquitin ligase WWP1. Given the role of Met in cell proliferation and tumorigenesis, our results may provide a mechanistic basis for understanding the role of ezrin in tumor progression.

WWP1: a versatile ubiquitin E3 ligase in signaling and diseases

WW domain-containing E3 ubiquitin protein ligase 1 (WWP1) is a multifunction protein containing an N-terminal C2 domain, four tandem WW domains for substrate binding, and a C-terminal catalytic HECT domain for ubiquitin transferring. WWP1 has been suggested to function as the E3 ligase for several PY motif-containing proteins, such as Smad2, KLF5, p63, ErbB4/HER4, RUNX2, JunB, RNF11, SPG20, and Gag, as well as several non-PY motif containing proteins, such as TβR1, Smad4, KLF2, and EPS15. WWP1 regulates a variety of cellular biological processes including protein trafficking and degradation, signaling, transcription, and viral budding. WWP1 has been implicated in several diseases, such as cancers, infectious diseases, neurological diseases, and aging. In this review article, we extensively summarize the current knowledge of WWP1 with special emphasis on the roles and action of mechanism of WWP1 in signaling and human diseases.

Krüppel-like factors in lymphocyte biology

The Krüppel-like factor family of transcription factors plays an important role in differentiation, function, and homeostasis of many cell types. While their role in lymphocytes is still being determined, it is clear that these factors influence processes as varied as lymphocyte quiescence, trafficking, differentiation, and function. This review will present an overview of how these factors operate and coordinate with each other in lymphocyte regulation.

The E3 ubiquitin ligase SMAD ubiquitination regulatory factor 2 negatively regulates Krüppel-like factor 5 protein

The zinc finger transcription factor Krüppel-like factor 5 (KLF5) is regulated posttranslationally. We identified SMAD ubiquitination regulatory factor 2 (SMURF2), an E3 ubiquitin ligase, as an interacting protein of KLF5 by yeast two-hybrid screen, coimmunoprecipitation, and indirect immunofluorescence studies. The SMURF2-interacting domains in KLF5 were mapped to its carboxyl terminus, including the PY motif of KLF5 and its zinc finger DNA-binding domain. KLF5 protein levels were reduced significantly upon overexpression of SMURF2 but not catalytically inactive SMURF2-C716A mutant or SMURF1. SMURF2 alone reduced the protein stability of KLF5 as shown by cycloheximide chase assay, indicating that SMURF2 specifically destabilizes KLF5. In contrast, KLF5(1-165), a KLF5 amino-terminal construct that lacks the PY motif and DNA binding domain, was not degraded by SMURF2. The degradation of KLF5 by SMURF2 was blocked by the proteasome inhibitor MG132, and SMURF2 efficiently ubiquitinated both overexpressed and endogenous KLF5. In contrast, knocking down SMURF2 by siRNAs significantly enhanced KLF5 protein levels, reduced ubiquitination of KLF5, and increased the expression of cyclin D1 and PDGF-A, two established KLF5 target genes. In consistence, SMURF2, but not the E3 ligase mutant SMURF2-C716A, significantly inhibited the transcriptional activity of KLF5, as demonstrated by dual luciferase assay using the PDGF-A promoter, and suppressed the ability of KLF5 to stimulate cell proliferation as measured by BrdU incorporation. Hence, SMURF2 is a novel E3 ubiquitin ligase for KLF5 and negatively regulates KLF5 by targeting it for proteasomal degradation.

"Go with the flow": how Krüppel-like factor 2 regulates the vasoprotective effects of shear stress

Laminar shear stress is known to confer potent anti-inflammatory, antithrombotic, and antiadhesive effects by differentially regulating endothelial gene expression. The identification of Krüppel-like factor 2 as a flow-responsive molecule has greatly advanced our understanding of molecular mechanisms governing vascular homeostasis. This review summarizes the current understanding of Krüppel-like factor 2 action in endothelial gene expression and function.

WW domain-containing E3 ubiquitin protein ligase 1 (WWP1) delays cellular senescence by promoting p27(Kip1) degradation in human diploid fibroblasts

WW domain-containing E3 ubiquitin protein ligase 1 (WWP1) plays an important role in the proliferation of tumor cells and the lifespan of Caenorhabditis elegans. However, the role of WWP1 in cellular senescence is still unknown. Here, we show that the expression patterns of p27(Kip1) and WWP1 are inversely correlated during cellular senescence. Moreover, the overexpression of WWP1 delayed senescence, whereas the knockdown of WWP1 led to premature senescence in human fibroblasts. Furthermore, we demonstrate that WWP1 repressed endogenous p27(Kip1) expression through ubiquitin-proteasome-mediated degradation. Additionally, WWP1 had a strong preference for catalyzing the Lys-48-linked polyubiquitination of p27(Kip1) in vitro. Finally, we demonstrate that WWP1 markedly inhibited the replicative senescence induced by p27(Kip1) by promoting p27(Kip1) degradation. Therefore, our study provides a new molecular mechanism for the regulation of cellular senescence.

Identification of poly (ADP-ribose) polymerase-1 (PARP-1) as a novel Kruppel-like factor 8-interacting and -regulating protein

Krüppel-like factor 8 (KLF8) regulates critical gene transcription and cellular events associated with cancer. However, KLF8-interacting proteins remain largely unidentified. Using co-immunoprecipitation (co-IP), mass spectrometry, and GST pulldown assays, we identified poly(ADP-ribose) polymerase-1 (PARP-1) as a novel KLF8-interacting protein. Co-IP and Western blotting indicated that KLF8 is also a PARP-1 substrate. Mutation of the cysteines in the zinc finger domain of KLF8 abolished PARP-1 interaction. Surprisingly, immunofluorescent staining revealed a cytoplasmic mislocalization of KLF8 in PARP-1(-/-) cells or when the interaction was disrupted. This mislocalization was prevented by either PARP-1 re-expression or inhibition of CRM1-dependent nuclear export. Interestingly, co-IP indicated competition between PARP-1 and CRM1 for KLF8 binding. Cycloheximide chase assay showed a decrease in the half-life of KLF8 protein when PARP-1 expression was suppressed or KLF8-PARP-1 interaction was disrupted. Ubiquitination assays implicated KLF8 as a target of ubiquitination that was significantly higher in PARP-1(-/-) cells. Promoter reporter assays and chromatin immunoprecipitation assays showed that KLF8 activation on the cyclin D1 promoter was markedly reduced when PARP-1 was deleted or inhibited or when KLF8-PARP-1 interaction was disrupted. Overall, this work has identified PARP-1 as a novel KLF8-binding and -regulating protein and provided new insights into the mechanisms underlying the regulation of KLF8 nuclear localization, stability, and functions.

Smurf1 ubiquitin ligase targets Kruppel-like factor KLF2 for ubiquitination and degradation in human lung cancer H1299 cells

Krüppel-like factor 2 (KLF2) has been demonstrated to be essential for normal lung development, erythroid differentiation, T-cell differentiation, migration and homing. However, the mechanisms underlying the regulation of KLF2, in particular its responsible E3 ligase is still unclear. Here we show that the homologous to E6AP carboxyl terminus (HECT)-type ubiquitin ligase Smad ubiquitination regulatory factor 1 (Smurf1) interacts with and targets KLF2 for poly-ubiquitination and proteasomal degradation specifically in lung cancer H1299 cells. The catalytic ligase activity of Smurf1 is required for it to regulate KLF2. Consequently, Smurf1 represses the transcriptional factor activity of KLF2 and regulates the expression its downstream genes such as CD62L and Wee1. This study provided the first evidence that Smurf1 functions as an E3 ligase to promote the ubiquitination and proteasomal degradation of KLF2.

Mammalian Krüppel-like factors in health and diseases

The Krüppel-like factor (KLF) family of transcription factors regulates diverse biological processes that include proliferation, differentiation, growth, development, survival, and responses to external stress. Seventeen mammalian KLFs have been identified, and numerous studies have been published that describe their basic biology and contribution to human diseases. KLF proteins have received much attention because of their involvement in the development and homeostasis of numerous organ systems. KLFs are critical regulators of physiological systems that include the cardiovascular, digestive, respiratory, hematological, and immune systems and are involved in disorders such as obesity, cardiovascular disease, cancer, and inflammatory conditions. Furthermore, KLFs play an important role in reprogramming somatic cells into induced pluripotent stem (iPS) cells and maintaining the pluripotent state of embryonic stem cells. As research on KLF proteins progresses, additional KLF functions and associations with disease are likely to be discovered. Here, we review the current knowledge of KLF proteins and describe common attributes of their biochemical and physiological functions and their pathophysiological roles.

Krüppel-like factors: three fingers in control

Krüppel-like factors (KLFs), members of the zinc-finger family of transcription factors capable of binding GC-rich sequences, have emerged as critical regulators of important functions all over the body. They are characterised by a highly conserved C-terminal DNA-binding motif containing three C2H2 zinc-finger domains, with variable N-terminal regulatory domains. Currently, there are 17 KLFs annotated in the human genome. In spite of their structural similarity to one another, the genes encoding different KLFs are scattered all over the genome. By virtue of their ability to activate and/or repress the expression of a large number of genes, KLFs regulate a diverse array of developmental events and cellular processes, such as erythropoiesis, cardiac remodelling, adipogenesis, maintenance of stem cells, epithelial barrier formation, control of cell proliferation and neoplasia, flow-mediated endothelial gene expression, skeletal and smooth muscle development, gluconeogenesis, monocyte activation, intestinal and conjunctival goblet cell development, retinal neuronal regeneration and neonatal lung development. Characteristic features, nomenclature, evolution and functional diversities of the human KLFs are reviewed here.

Smurf control in bone cells

The homologous to the E6-associated protein carboxyl terminus (HECT) domain E3 ubiquitin ligase Smurf1 is the first E3 ligase to be implicated in regulating bone cell function. The involvement of Smurf1 in multiple signaling pathways and pathological conditions is presently an area of extensive scientific interest. This review highlights recent works exploring Smurf-regulated biological processes in bone cells and highlights recent discoveries surrounding the regulatory mechanisms modulating its catalytic activity and substrate recognition capability. Moreover, we discuss the relevance of targeting the HECT E3s through the development of small-molecule inhibitors as an anticancer therapeutic strategy.

Proteasome inhibitors enhance endothelial thrombomodulin expression via induction of Krüppel-like transcription factors

OBJECTIVE

Impairment of the thrombomodulin-protein C anticoagulant pathway has been implicated in pathological thrombosis associated with malignancy. Patients who receive proteasome inhibitors as part of their chemotherapeutic regimen appear to be at decreased risk for thromboembolic events. We investigated the effects of proteasome inhibitors on endothelial thrombomodulin expression and function.

METHODS AND RESULTS

Proteasome inhibitors as a class markedly induced the expression of thrombomodulin and enhanced the protein C activating capacity of endothelial cells. Thrombomodulin upregulation was independent of NF-kappaB signaling, a principal target of proteasome inhibitors, but was instead a direct consequence of increased expression of the Krüppel-like transcription factors, KLF2 and KLF4. These effects were confirmed in vivo, where systemic administration of a proteasome inhibitor enhanced thrombomodulin expression that was paralleled by changes in the expression of KLF2 and KLF4.

CONCLUSIONS

These findings identify a novel mechanism of action of proteasome inhibitors that may help to explain their clinically observed thromboprotective effects.

Regulation of the nuclear localization of the human Nedd4-related WWP1 protein by Notch

Nedd4 family ubiquitin ligases regulate trafficking and degradation of numerous target substrates in different cellular compartments, including at the plasma membrane, in endosomes, in the secretory pathway and in the nucleus. WWP1 is a Nedd4 family protein closely related to mouse Itch and Drosophila Su(dx), both of which have been shown to regulate the Notch receptor. To investigate the possibility that WWP1 is also associated with Notch signalling we coexpressed human Notch1 and WWP1 in mouse myoblast cells. We found that WWP1 could localize to both the nucleus and cytoplasm in a context dependent manner. Coexpression of human Notch1 (hN1) depleted WWP1 from the nucleus to colocalise with hN1 in early endosomes, dependent on the presence of the C-terminal HECT domain. Furthermore we found that full-length expressed WWP1 could interact in vitro with the cytoplasmic domain of human Notch1. The Notch receptor has multiple roles in development, mediating a short-range signal that controls cell fate and pattern formation. The canonical Notch signal involves proteolytic release of the soluble Notch intracellular domain and the activation by the latter of the transcription factor Suppressor of Hairless/CBF-1 in the nucleus. This pathway does not however account for all of the activity of Notch. The ability of Notch to regulate the nuclear localization of WWP1 suggests a possible alternative mechanism by which Notch may communicate a signal to the nucleus. Drosophila Notch similarly regulated the nuclear localization of the Drosophila Nedd4 family protein, Suppressor of deltex, implying conservation of this mechanism during evolution.

Overexpression of WWP1 is associated with the estrogen receptor and insulin-like growth factor receptor 1 in breast carcinoma

WWP1, a HECT type E3 ubiquitin ligase frequently amplified and overexpressed in breast cancer, has the potential to become a useful clinical biomarker and therapeutic target in breast cancer. Here, we performed immunohistochemical staining in formalin-fixed and paraffin-embedded tissue sections from 187 cases of primary invasive mammary carcinoma [137 ductal carcinomas (IDC) and 50 lobular carcinomas (ILC)] by using a monoclonal anti-WWP1 antibody. The normal breast epithelium and adjacent benign epithelium are essentially negative for WWP1. Cytoplasmic WWP1 immunoreactivity was observed in 76/187 (40.6%) tumors and showed a positive correlation with ERalpha (p = 0.05) and IGF-1R proteins (p = 0.001) in this cohort. The positive correlations between WWP1 and ER/IGF-1R were also observed in a panel of 12 breast cancer cell lines by Western blot. Interestingly, the ER levels are decreased when WWP1 is silenced in ER positive MCF7 and T47D breast cancer cell lines. Finally, WWP1 ablation collectively inhibits cell proliferation with tamoxifen in MCF7 and T47D, as measured by (3)H-thymidine incorporation assays. These findings suggest that WWP1 may play an important role in ER positive breast cancer.

Tumour-promoting activity of altered WWP1 expression in breast cancer and its utility as a prognostic indicator

WWP1 is a ubiquitin ligase, associated with the post-translational regulation of several tumour-promoting and tumour suppressor proteins. Here we show that WWP1 expression is up-regulated in a subset of breast tumour cell lines and primary breast tumours. We overexpressed WWP1 in MCF10A breast epithelial cells and demonstrated increased cell growth and anchorage-independent colony formation. RNAi knockdown of WWP1 expression in T47D and MCF7 breast tumour cell lines reduced anchorage-independent colony formation. We used WWP1 protein expression levels, in combination with its sub-cellular localization, to classify breast tumours into four categories. Surprisingly, a category with low/absent WWP1 expression displayed a consistently worse prognosis compared with WWP1-expressing tumours. Importantly, the association with disease-free survival was independent of the status of other commonly used prognostic indicators. Thus, WWP1 is a prognostic marker and may be a potential therapeutic target for a subset of breast tumours.

The WW domain containing E3 ubiquitin protein ligase 1 upregulates ErbB2 and EGFR through RING finger protein 11

The WW domain containing E3 ubiquitin protein ligase 1 (WWP1) is a homologous to the E6-associated protein C terminus-type E3 ligase frequently overexpressed in human prostate and breast cancers due to gene amplification. Previous studies suggest that WWP1 promotes cell proliferation and survival; however, the mechanism of WWP1 action is still poorly understood. Here, we showed that WWP1 upregulates and maintains erythroblastic leukemia viral oncogene homolog 2 (ErbB2) and epithelial growth factor receptor (EGFR) in multiple cell lines. WWP1 depletion dramatically attenuates the EGF-induced ERK phosphorylation. WWP1 forms a protein complex with RING finger protein 11 (RNF11), a negative regulator of ErbB2 and EGFR. The protein-protein interaction is through the first and third WW domains of WWP1 and the PY motif of RNF11. Although WWP1 is able to ubiquitinate RNF11 in vitro and in vivo, WWP1 neither targets RNF11 for degradation nor changes RNF11's cellular localization. Importantly, inhibition of RNF11 can rescue WWP1 siRNA-induced ErbB2 and EGFR downregulation and growth arrest. Finally, we demonstrated that RNF11 is overexpressed in a panel of prostate and breast cancer cell lines with WWP1 expression. These findings suggest that WWP1 may promote cell proliferation and survival partially through suppressing RNF11-mediated ErbB2 and EGFR downregulation.

Role of Krüppel-Like Transcription Factors in Endothelial Biology

Krüppel-like factors are members of the zinc finger family of transcription factors that have been implicated as playing key roles in regulating cellular differentiation and tissue development. Studies over the past several years support an important role for this family of factors in vascular biology. This review summarizes the role of Krüppel-like factors in endothelial cell biology.