CLIPR-59 regulates TNF-α-induced apoptosis by controlling ubiquitination of RIP1

ROS-mediated mitophagy and necroptosis regulate osteocytes death caused by TCP particles in MLO-Y4 cells

Our previous data have revealed TCP particles caused cell death of osteocytes, comprising over 95 % of all bone cells, which contribute to periprosthetic osteolysis, joint loosening and implant failure, but its mechanisms are not fully understood. Here, we reported that TCP particles inhibited cell viability of osteocytes MLO-Y4, and caused cell death. TCP particles caused mitochondrial impairment and increased expressions of LC-3 II, Parkin and PINK 1, accompanied by the elevation of autophagy flux and intracellular acidic components, the accumulation of LC-3II, PINK1 and Parkin in damaged mitochondria, and p62 reduction. The increased LC-3II expression and cell death extent were significantly enhanced by the autophagy inhibitor Baf A1, compared with Baf A1 (or TCP particles) alone, indicating that TCP particles increase autophagic flux and lead to cell even death of MLO-Y4 cells, closely associated with mitophagy. Furthermore, TCP particles induced propidium iodide (PI) uptake and the phosphorylation of RIP1, RIP3 and MLKL, thereby increasing necroptosis in MLO-Y4 cells. The pro-necroptotic effect was alleviated by the RIP1 inhibitor Nec-1 or the MLKL inhibitor NSA. Additionally, TCP particles promoted the production of intracellular reactive oxygen species (ROS) and mitochondrial ROS (mtROS), and increased TXNIP expression, but decreased protein levels of TRX1, Nrf2, HO-1 and NQO1, leading to oxidative stress. The ROS scavenger NAC remarkably reversed mitophagy and necroptosis caused by TCP particles, suggesting that ROS is responsible for mitophagy and necroptosis. Collectively, ROS-mediated mitophagy and necroptosis regulate osteocytes death caused by TCP particles in MLO-Y4 cells, which enhances osteoclastogenesis and periprosthetic osteolysis.

Golgi Complex: A Signaling Hub in Cancer

The Golgi Complex is the central hub in the endomembrane system and serves not only as a biosynthetic and processing center but also as a trafficking and sorting station for glycoproteins and lipids. In addition, it is an active signaling hub involved in the regulation of multiple cellular processes, including cell polarity, motility, growth, autophagy, apoptosis, inflammation, DNA repair and stress responses. As such, the dysregulation of the Golgi Complex-centered signaling cascades contributes to the onset of several pathological conditions, including cancer. This review summarizes the current knowledge on the signaling pathways regulated by the Golgi Complex and implicated in promoting cancer hallmarks and tumor progression.

Bcl-3 promotes TNF-induced hepatocyte apoptosis by regulating the deubiquitination of RIP1

Tumor necrosis factor-α (TNF) is described as a main regulator of cell survival and apoptosis in multiple types of cells, including hepatocytes. Dysregulation in TNF-induced apoptosis is associated with many autoimmune diseases and various liver diseases. Here, we demonstrated a crucial role of Bcl-3, an IκB family member, in regulating TNF-induced hepatic cell death. Specifically, we found that the presence of Bcl-3 promoted TNF-induced cell death in the liver, while Bcl-3 deficiency protected mice against TNF/D-GalN induced hepatoxicity and lethality. Consistently, Bcl-3-depleted hepatic cells exhibited decreased sensitivity to TNF-induced apoptosis when stimulated with TNF/CHX. Mechanistically, the in vitro results showed that Bcl-3 interacted with the deubiquitinase CYLD to synergistically switch the ubiquitination status of RIP1 and facilitate the formation of death-inducing Complex II. This complex further resulted in activation of the caspase cascade to induce apoptosis. By revealing this novel role of Bcl-3 in regulating TNF-induced hepatic cell death, this study provides a potential therapeutic target for liver diseases caused by TNF-related apoptosis.

Spermidine activates RIP1 deubiquitination to inhibit TNF-α-induced NF-κB/p65 signaling pathway in osteoarthritis

Spermidine has been known to inhibit the production of pro-inflammatory cytokines. However, there are no reports about anti-inflammatory effects of spermidine on osteoarthritis (OA). Herein, we examined whether OA progression could be delayed by intraperitoneal injection (i.p.) of spermidine in the anterior cruciate ligament transection (ACLT) and TNF-α induced arthritis (TIA) mouse models. During the process, human FLS cells (H-FLS) were used to investigate the potential ubiquitination mechanism of spermidine-mediated RIP1 in TNF-α-induced NF-κB/p65 signaling. We found that spermidine attenuated synovitis, cartilage degeneration and osteophyte formation, resulting in substantially lower OARSI scores and TNF-α scores in spermidine-treated ACLT and TIA mice. In terms of the mechanism, 9 μM spermidine did not affect the viability, proliferation, cell cycle and apoptosis of H-FLS, and exerted inhibitory effects by activating CYLD-mediated RIP1 deubiquitination on TNF-α-induced NF-κB/p65 signaling in H-FLS. From these data, we can conclude that spermidine attenuates OA progression by the inhibition of TNF-α-induced NF-κB pathway via the deubiquitination of RIP1 in FLS. Therefore, intake of spermidine could be a potential therapy for preventing OA.

The roles of apoptosis, autophagy and unfolded protein response in arbovirus, influenza virus, and HIV infections

Virus infection induces different cellular responses in infected cells. These include cellular stress responses like autophagy and unfolded protein response (UPR). Both autophagy and UPR are connected to programed cell death I (apoptosis) in chronic stress conditions to regulate cellular homeostasis via Bcl2 family proteins, CHOP and Beclin-1. In this review article we first briefly discuss arboviruses, influenza virus, and HIV and then describe the concepts of apoptosis, autophagy, and UPR. Finally, we focus upon how apoptosis, autophagy, and UPR are involved in the regulation of cellular responses to arboviruses, influenza virus and HIV infections.

Abbreviation: AIDS: Acquired Immunodeficiency Syndrome; ATF6: Activating Transcription Factor 6; ATG6: Autophagy-specific Gene 6; BAG3: BCL Associated Athanogene 3; Bak: BCL-2-Anatagonist/Killer1; Bax; BCL-2: Associated X protein; Bcl-2: B cell Lymphoma 2x; BiP: Chaperon immunoglobulin heavy chain binding Protein; CARD: Caspase Recruitment Domain; cART: combination Antiretroviral Therapy; CCR5: C-C Chemokine Receptor type 5; CD4: Cluster of Differentiation 4; CHOP: C/EBP homologous protein; CXCR4: C-X-C Chemokine Receptor Type 4; Cyto c: Cytochrome C; DCs: Dendritic Cells; EDEM1: ER-degradation enhancing-a-mannosidase-like protein 1; ENV: Envelope; ER: Endoplasmic Reticulum; FasR: Fas Receptor;G2: Gap 2; G2/M: Gap2/Mitosis; GFAP: Glial Fibrillary Acidic Protein; GP120: Glycoprotein120; GP41: Glycoprotein41; HAND: HIV Associated Neurodegenerative Disease; HEK: Human Embryonic Kidney; HeLa: Human Cervical Epithelial Carcinoma; HIV: Human Immunodeficiency Virus; IPS-1: IFN-β promoter stimulator 1; IRE-1: Inositol Requiring Enzyme 1; IRGM: Immunity Related GTPase Family M protein; LAMP2A: Lysosome Associated Membrane Protein 2A; LC3: Microtubule Associated Light Chain 3; MDA5: Melanoma Differentiation Associated gene 5; MEF: Mouse Embryonic Fibroblast; MMP: Mitochondrial Membrane Permeabilization; Nef: Negative Regulatory Factor; OASIS: Old Astrocyte Specifically Induced Substrate; PAMP: Pathogen-Associated Molecular Pattern; PERK: Pancreatic Endoplasmic Reticulum Kinase; PRR: Pattern Recognition Receptor; Puma: P53 Upregulated Modulator of Apoptosis; RIG-I: Retinoic acid-Inducible Gene-I; Tat: Transactivator Protein of HIV; TLR: Toll-like receptor; ULK1: Unc51 Like Autophagy Activating Kinase 1; UPR: Unfolded Protein Response; Vpr: Viral Protein Regulatory; XBP1: X-Box Binding Protein 1

Transcriptomic Analysis of Stem Cells Treated with Moringin or Cannabidiol: Analogies and Differences in Inflammation Pathways

Inflammation is a common feature of many neurodegenerative diseases. The treatment of stem cells as a therapeutic approach to repair damage in the central nervous system represents a valid alternative. In this study, using Next-Generation Sequencing (NGS) technology, we analyzed the transcriptomic profile of human Gingival Mesenchymal Stem Cells (hGMSCs) treated with Moringin [4-(α-l-ramanosyloxy)-benzyl isothiocyanate] (hGMSCs-MOR) or with Cannabidiol (hGMSCs-CBD) at dose of 0.5 or 5 µM, respectively. Moreover, we compared their transcriptomic profiles in order to evaluate analogies and differences in pro- and anti-inflammatory pathways. The hGMSCs-MOR selectively downregulate TNF-α signaling from the beginning, reducing the expression of TNF-α receptor while hGMSCs-CBD limit its activity after the process started. The treatment with CBD downregulates the pro-inflammatory pathway mediated by the IL-1 family, including its receptor while MOR is less efficient. Furthermore, both the treatments are efficient in the IL-6 signaling. In particular, CBD reduces the effect of the pro-inflammatory JAK/STAT pathway while MOR enhances the pro-survival PI3K/AKT/mTOR. In addition, both hGMSCs-MOR and hGMSCs-CBD improve the anti-inflammatory activity enhancing the TGF-β pathway.

Selected Golgi-Localized Proteins and Carcinogenesis: What Do We Know?

The role of the Golgi apparatus in carcinogenesis still remains unclear. A number of structural and functional cis-, medial-, and trans-Golgi proteins as well as a complexity of metabolic pathways which they mediate may indicate a central role of the Golgi apparatus in the development and progression of cancer. Pleiotropy of cellular function of the Golgi apparatus makes it a "metabolic heart" or a relay station of a cell, which combines multiple signaling pathways involved in carcinogenesis. Therefore, any damage to or structural abnormality of the Golgi apparatus, causing its fragmentation and/or biochemical dysregulation, results in an up- or downregulation of signaling pathways and may in turn promote tumor progression, as well as local nodal and distant metastases. Three alternative or parallel models of spatial and functional Golgi organization within tumor cells were proposed: (1) compacted Golgi structure, (2) normal Golgi structure with its increased activity, and (3) the Golgi fragmentation with ministacks formation. Regardless of the assumed model, the increased activity of oncogenesis initiators and promoters with inhibition of suppressor proteins results in an increased cell motility and migration, increased angiogenesis, significantly activated trafficking kinetics, proliferation, EMT induction, decreased susceptibility to apoptosis-inducing factors, and modulating immune response to tumor cell antigens. Eventually, this will lead to the increased metastatic potential of cancer cells and an increased risk of lymph node and distant metastases. This chapter provided an overview of the current state of knowledge of selected Golgi proteins, their role in cytophysiology as well as potential involvement in tumorigenesis.

Death receptor 6 contributes to autoimmunity in lupus-prone mice

Expansion of autoreactive follicular helper T (Tfh) cells is tightly restricted to prevent induction of autoantibody-dependent immunological diseases, such as systemic lupus erythematosus (SLE). Here we show expression of an orphan immune regulator, death receptor 6 (DR6/TNFRSF21), on a population of Tfh cells that are highly expanded in lupus-like disease progression in mice. Genome-wide screening reveals an interaction between syndecan-1 and DR6 resulting in immunosuppressive functions. Importantly, syndecan-1 is expressed specifically on autoreactive germinal centre (GC) B cells that are critical for maintenance of Tfh cells. Syndecan-1 expression level on GC B cells is associated with Tfh cell expansion and disease progression in lupus-prone mouse strains. In addition, Tfh cell suppression by DR6-specific monoclonal antibody delays disease progression in lupus-prone mice. These findings suggest that the DR6/syndecan-1 axis regulates aberrant GC reactions and could be a therapeutic target for autoimmune diseases such as SLE.

Germinal centre (GC) reactions are driven by T follicular helper (Tfh) cells and their dysregulation can cause autoimmune disease. Here the authors show that the orphan receptor DR6 is a Tfh cell marker that binds syndecan-1 on GC B cells driving autoimmunity in lupus-prone mice.

ANTXR-1 and -2 independent modulation of a cytotoxicity mediated by anthrax toxin in human cells

Several animal models have shown that anthrax toxin (ATX) elicits a cytotoxic effect on host cells through anthrax toxin receptor (ANTXR) function. In this study, compared with mouse cells, cells obtained from humans exhibited low sensitivity to ATX-mediated cytotoxicity, and the sensitivity was not correlated with expression levels of ANTXRs. ATX treatment also induced a cytotoxic effect in other cultured human cells, human embryonic kidney (HEK) 293 cells, that express ANTXRs at undetectable levels. Furthermore, ectopic expression of ANTXRs in HEK293 cells did not affect the sensitivity to ATX treatment. These findings suggest that there is an ANTXR-independent cytotoxic mechanism in human cells.

Apoptosis, autophagy and unfolded protein response pathways in Arbovirus replication and pathogenesis

Arboviruses are pathogens that widely affect the health of people in different communities around the world. Recently, a few successful approaches toward production of effective vaccines against some of these pathogens have been developed, but treatment and prevention of the resulting diseases remain a major health and research concern. The arbovirus infection and replication processes are complex, and many factors are involved in their regulation. Apoptosis, autophagy and the unfolded protein response (UPR) are three mechanisms that are involved in pathogenesis of many viruses. In this review, we focus on the importance of these pathways in the arbovirus replication and infection processes. We provide a brief introduction on how apoptosis, autophagy and the UPR are initiated and regulated, and then discuss the involvement of these pathways in regulation of arbovirus pathogenesis.

Double-Stranded RNA Is Detected by Immunofluorescence Analysis in RNA and DNA Virus Infections, Including Those by Negative-Stranded RNA Viruses

Early biochemical studies of viral replication suggested that most viruses produce double-stranded RNA (dsRNA), which is essential for the induction of the host immune response. However, it was reported in 2006 that dsRNA could be detected by immunofluorescence antibody staining in double-stranded DNA and positive-strand RNA virus infections but not in negative-strand RNA virus infections. Other reports in the literature seemed to support these observations. This suggested that negative-strand RNA viruses produce little, if any, dsRNA or that more efficient viral countermeasures to mask dsRNA are mounted. Because of our interest in the use of dsRNA antibodies for virus discovery, particularly in pathological specimens, we wanted to determine how universal immunostaining for dsRNA might be in animal virus infections. We have detected the in situ formation of dsRNA in cells infected with vesicular stomatitis virus, measles virus, influenza A virus, and Nyamanini virus, which represent viruses from different negative-strand RNA virus families. dsRNA was also detected in cells infected with lymphocytic choriomeningitis virus, an ambisense RNA virus, and minute virus of mice (MVM), a single-stranded DNA (ssDNA) parvovirus, but not hepatitis B virus. Although dsRNA staining was primarily observed in the cytoplasm, it was also seen in the nucleus of cells infected with influenza A virus, Nyamanini virus, and MVM. Thus, it is likely that most animal virus infections produce dsRNA species that can be detected by immunofluorescence staining. The apoptosis induced in several uninfected cell lines failed to upregulate dsRNA formation.

IMPORTANCE

An effective antiviral host immune response depends on recognition of viral invasion and an intact innate immune system as a first line of defense. Double-stranded RNA (dsRNA) is a viral product essential for the induction of innate immunity, leading to the production of type I interferons (IFNs) and the activation of hundreds of IFN-stimulated genes. The present study demonstrates that infections, including those by ssDNA viruses and positive- and negative-strand RNA viruses, produce dsRNAs detectable by standard immunofluorescence staining. While dsRNA staining was primarily observed in the cytoplasm, nuclear staining was also present in some RNA and DNA virus infections. The nucleus is unlikely to have pathogen-associated molecular pattern (PAMP) receptors for dsRNA because of the presence of host dsRNA molecules. Thus, it is likely that most animal virus infections produce dsRNA species detectable by immunofluorescence staining, which may prove useful in viral discovery as well.

Spy1 induces de-ubiquitinating of RIP1 arrest and confers glioblastoma's resistance to tumor necrosis factor (TNF-α)-induced apoptosis through suppressing the association of CLIPR-59 and CYLD

Glioblastoma multiforme (GBM), a grade-IV glioma, is resistant to TNF-α induced apoptosis. CLIPR-59 modulates ubiquitination of RIP1, thus promoting Caspase-8 activation to induce apoptosis by TNF-α. Here we reported that CLIPR-59 was down-regulated in GBM cells and high-grade glioma tumor samples, which was associated with decreased cancer-free survival. In GBM cells, CLIPR-59 interacts with Spy1, resulting in its decreased association with CYLD, a de-ubiquitinating enzyme. Moreover, experimental reduction of Spy1 levels decreased GBM cells viability, while increased the lysine-63-dependent de-ubiquitinating activity of RIP1 via enhancing the binding ability of CLIPR-59 and CYLD in GBM, thus promoting Caspase-8 and Caspase-3 activation to induce apoptosis by TNF-α. These findings have identified a novel Spy1-CLIPR-59 interplay in GBM cell's resistance to TNF-α-induced apoptosis revealing a potential target in the intervention of malignant brain tumors.

ClipR-59 interacts with Elmo2 and modulates myoblast fusion

Recent studies using ClipR-59 knock-out mice implicated this protein in the regulation of muscle function. In this report, we have examined the role of ClipR-59 in muscle differentiation and found that ClipR-59 knockdown in C2C12 cells suppressed myoblast fusion. To elucidate the molecular mechanism whereby ClipR-59 regulates myoblast fusion, we carried out a yeast two-hybrid screen using ClipR-59 as the bait and identified Elmo2, a member of the Engulfment and cell motility protein family, as a novel ClipR-59-associated protein. We showed that the interaction between ClipR-59 and Elmo2 was mediated by the atypical PH domain of Elmo2 and the Glu-Pro-rich domain of ClipR-59 and regulated by Rho-GTPase. We have examined the impact of ClipR-59 on Elmo2 downstream signaling and found that interaction of ClipR-59 with Elmo2 enhanced Rac1 activation. Collectively, our studies demonstrate that formation of an Elmo2·ClipR-59 complex plays an important role in myoblast fusion.

The two faces of receptor interacting protein kinase-1

Receptor Interacting Protein Kinase-1 (RIPK1), a key player in inflammation and cell death, assumes opposite functions depending on the cellular context and its posttranslational modifications. Genetic evidence supported by biochemical and cellular biology approaches sheds light on the circumstances in which RIPK1 promotes or inhibits these processes.

Necrosis-dependent and independent signaling of the RIP kinases in inflammation

It is now widely accepted that some forms of necrosis are controlled by a dedicated signaling pathway triggered by various cell surface and intracellular receptors. This regulated form of necrosis is mediated by the kinase activity of receptor-interacting protein kinase 1 (RIP1/RIPK1) and/or RIP3/RIPK3. A number of studies using the RIP1 kinase inhibitor Necrostatin-1 (Nec-1) and its derivatives, or RIP3-deficient mice demonstrated that RIP1 and RIP3 are involved in various infectious and sterile inflammatory diseases. As a consequence, these specific phenotypes were construed to depend on necrosis. However, emerging evidence indicates that the RIP1 kinase activity and RIP3 can also control apoptosis and inflammatory cytokine production independent of necrosis. Therefore, we may need to re-interpret conclusions drawn based on loss of RIP1 or RIP3 functions in in vivo models. We propose that studies of RIP1 and RIP3 in different inflammatory responses need to consider cell death-dependent and independent mechanisms of the RIP kinases.

MicroRNA-23a is involved in tumor necrosis factor-α induced apoptosis in mesenchymal stem cells and myocardial infarction

Cell therapy has emerged as an attractive therapeutic modality to treat myocardial infarction (MI) via repairing damaged myocardium, and mesenchymal stem cells (MSCs) are an appealing therapeutic approach for cardiac regeneration. However, the clinical application of MSC-based therapy is restricted because of the poor survival of implanted cells, and this poor survival remains poorly understood. Using a tumor necrosis factor (TNF)-α-induced bone marrow (BM)-MSC injury model in vitro and a rat MI model in vivo, we showed in the current study that miR-23a was involved in TNF-α-induced BM-MSC apoptosis through regulating caspase-7 and that the injection of BM-MSCs overexpressing miR-23a could improve left ventricular (LV) function and reduce infarct size in the rat MI model. Our findings elucidate the etiology of MI and provide an alternative treatment strategy for patients with heart failure caused by MI who are not optimal candidates for surgical treatment.

CYLD deubiquitinates RIP1 in the TNFα-induced necrosome to facilitate kinase activation and programmed necrosis

Background

Necroptosis/programmed necrosis is initiated by a macro-molecular protein complex termed the necrosome. Receptor interacting protein kinase 1 (RIPK1/RIP1) and RIP3 are key components of the necrosome. TNFα is a prototypic inducer of necrosome activation, and it is widely believed that deubiquitination of RIP1 at the TNFR-1 signaling complex precedes transition of RIP1 into the cytosol where it forms the RIP1-RIP3 necrosome. Cylindromatosis (CYLD) is believed to promote programmed necrosis by facilitating RIP1 deubiquitination at this membrane receptor complex.

Methodology/Principal Findings

We demonstrate that RIP1 is indeed the primary target of CYLD in TNFα-induced programmed necrosis. We observed that CYLD does not regulate RIP1 ubiquitination at the TNF receptor. TNF and zVAD-induced programmed necrosis was highly attenuated in CYLD^-/- cells. However, in the presence of cycloheximide or SMAC mimetics, programmed necrosis was only moderately reduced in CYLD^-/- cells. Under the latter conditions, RIP1-RIP3 necrosome formation is only delayed, but not abolished in CYLD^-/- cells. We further demonstrate that RIP1 within the NP-40 insoluble necrosome is ubiquitinated and that CYLD regulates RIP1 ubiquitination in this compartment. Hence, RIP1 ubiquitination in this late-forming complex is greatly increased in CYLD^-/- cells. Increased RIP1 ubiquitination impairs RIP1 and RIP3 phosphorylation, a signature of kinase activation.

Conclusions/Significance

Our results show that CYLD regulates RIP1 ubiquitination in the TNFα-induced necrosome, but not in the TNFR-1 signaling complex. In cells sensitized to programmed necrosis with SMAC mimetics, CYLD is not essential for necrosome assembly. Since SMAC mimetics induces the loss of the E3 ligases cIAP1 and cIAP2, reduced RIP1 ubiquitination could lead to reduced requirement for CYLD to remove ubiquitin chains from RIP1 in the TNFR-1 complex. As increased RIP1 ubiquitination in the necrosome correlates with impaired RIP1 and RIP3 phosphorylation and function, these results suggest that CYLD controls RIP1 kinase activity during necrosome assembly.

DHHC17 palmitoylates ClipR-59 and modulates ClipR-59 association with the plasma membrane

ClipR-59 interacts with Akt and regulates Akt compartmentalization and Glut4 membrane trafficking in a plasma membrane association-dependent manner. The association of ClipR-59 with plasma membrane is mediated by ClipR-59 palmitoylation at Cys534 and Cys535. To understand the regulation of ClipR-59 palmitoylation, we have examined all known mammalian DHHC palmitoyltransferases with respect to their ability to promote ClipR-59 palmitoylation. We found that, among 23 mammalian DHHC palmitoyltransferases, DHHC17 is the major ClipR-59 palmitoyltransferase, as evidenced by the fact that DHHC17 interacted with ClipR-59 and palmitoylated ClipR-59 at Cys534 and Cys535. By palmitoylating ClipR-59, DHHC17 directly regulates ClipR-59 plasma membrane association, as ectopic expression of DHHC17 increased whereas silencing of DHHC17 reduced the levels of ClipR-59 associated with plasma membrane. We have also examined the role of DHHC17 in Akt signaling and found that silencing of DHHC17 in 3T3-L1 adipocytes decreased the levels of Akt as well as ClipR-59 on the plasma membrane and impaired insulin-dependent Glut4 membrane translocation. We suggest that DHHC17 is a ClipR-59 palmitoyltransferase that modulates ClipR-59 plasma membrane binding, thereby regulating Akt signaling and Glut4 membrane translocation in adipocytes.

Ubiquitin-specific protease 4 promotes TNF-α-induced apoptosis by deubiquitination of RIP1 in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is a common type of cancer. Better understanding of molecular aberrations associated with HNSCC might identify new diagnostic and therapeutic strategies for this disease. In this study, we found ubiquitin-specific protease 4 (USP4) was significantly upregulated in HNSCC. USP4 negatively regulates RIP1-mediated NF-κB activation and promotes TNF-α-induced apoptosis in FaDu cells. USP4 directly interacts with receptor-interacting protein 1 (RIP1) and deubiquitinates K63-linked ubiquitination from RIP1. Therefore, our results indicate that USP4 has tumor suppressor roles in HNSCC and suggest USP4 as a potential therapeutic target for HNSCC.