UBE4B regulates p27 expression in A549 NSCLC cells through regulating the interaction of HuR and the p27 5' UTR.

Ubiquitination factor E4B (UBE4B) has a tumor-promoting effect, demonstrated by its aberrant expression in various types of cancers, and in vitro studies have shown that the retardation of cancer cell proliferation can be induced by targeting UBE4B. However, the molecular pathways through which UBE4B exerts its oncogenic activities have not yet been clearly identified and existing knowledge is limited to p53 and its subsequent downstream targets. In this study, we demonstrated that UBE4B regulates p27 expression in A549 cells via the cap-independent translation pathway following treatment with rapamycin and cycloheximide (CHX). Subsequently, we identified that UBE4B regulates p27 translation by regulating the interaction between human antigen R (HuR) and the p27 internal ribosomal entry site (IRES). First, UBE4B interacts with HuR, which inhibits p27 translation through the IRES. Secondly, the interaction between HuR and the p27 IRES was diminished by UBE4B depletion and enhanced by UBE4B overexpression. Finally, HuR depletion-induced growth retardation, accompanied by p27 accumulation, was restored by UBE4B overexpression. Collectively, these results suggest that the oncogenic properties of UBE4B in A549 cells are mediated by HuR, suggesting the potential of targeting the UBE4B-HuR-p27 axis as a therapeutic strategy for lung cancer.

TRIM22 facilitates autophagosome-lysosome fusion by mediating the association of GABARAPs and PLEKHM1.

Tripartite motif (TRIM) proteins are a large family of E3 ubiquitin ligases implicated in antiviral defense systems, tumorigenesis, and protein quality control. TRIM proteins contribute to protein quality control by regulating the ubiquitin-proteasome system, endoplasmic reticulum-associated degradation, and macroautophagy/autophagy. However, the detailed mechanisms through which various TRIM proteins regulate downstream events have not yet been fully elucidated. Herein, we identified a novel function of TRIM22 in the regulation of autophagy. TRIM22 promotes autophagosome-lysosome fusion by mediating the association of GABARAP family proteins with PLEKHM1, thereby inducing the autophagic clearance of protein aggregates, independent of its E3 ubiquitin ligase activity. Furthermore, a TRIM22 variant associated with early-onset familial Alzheimer disease interferes with autophagosome-lysosome fusion and autophagic clearance. These findings suggest TRIM22 as a critical autophagic regulator that orchestrates autophagosome-lysosome fusion by scaffolding autophagy-related proteins, thus representing a potential therapeutic target in neurodegenerative diseases.Abbreviations: AD: Alzheimer disease; ADAOO: AD age of onset; AICD: APP intracellular domain; APP: amyloid beta precursor protein; BSA: bovine serum albumin; cDNAs: complementary DNAs; CQ: chloroquine; CTF: carboxyl-terminal fragment; EBSS: Earle's balanced salt solution; GABARAP: GABA type A receptor-associated protein; GST: glutathione S-transferase; HA: hemagglutinin; HOPS: homotypic fusion and protein sorting; IFN: interferon; IL1A/IL-1α: interleukin 1 alpha; KO: knockout; MTORC1: mechanistic target of rapamycin kinase complex 1; NFKBIA/IκBα: NFKB inhibitor alpha; NFE2L2/NRF2: NFE2 like bZIP transcription factor; PBS: phosphate-buffered saline; PI3K: class I phosphoinositide 3-kinase; PLA: proximity ligation assay; PLEKHM1: pleckstrin homology and RUN domain containing M1; PSEN1: presenilin 1; SEM: standard errors of the means; SNAREs: soluble N-ethylmaleimide-sensitive factor attachment protein receptors; SNCA: synuclein alpha; SNP: single nucleotide polymorphism; TBS: tris-buffered saline; TNF/TNF-α: tumor necrosis factor; TRIM: tripartite motif; ULK1: unc-51 like autophagy activating kinase 1; WT: wild-type.

Skeletal Muscle-Specific Bis Depletion Leads to Muscle Dysfunction and Early Death Accompanied by Impairment in Protein Quality Control.

Bcl-2-interacting cell death suppressor (BIS), also called BAG3, plays a role in physiological functions such as anti-apoptosis, cell proliferation, autophagy, and senescence. Whole-body Bis-knockout (KO) mice exhibit early lethality accompanied by abnormalities in cardiac and skeletal muscles, suggesting the critical role of BIS in these muscles. In this study, we generated skeletal muscle-specific Bis-knockout (Bis-SMKO) mice for the first time. Bis-SMKO mice exhibit growth retardation, kyphosis, a lack of peripheral fat, and respiratory failure, ultimately leading to early death. Regenerating fibers and increased intensity in cleaved PARP1 immunostaining were observed in the diaphragm of Bis-SMKO mice, indicating considerable muscle degeneration. Through electron microscopy analysis, we observed myofibrillar disruption, degenerated mitochondria, and autophagic vacuoles in the Bis-SMKO diaphragm. Specifically, autophagy was impaired, and heat shock proteins (HSPs), such as HSPB5 and HSP70, and z-disk proteins, including filamin C and desmin, accumulated in Bis-SMKO skeletal muscles. We also found metabolic impairments, including decreased ATP levels and lactate dehydrogenase (LDH) and creatine kinase (CK) activities in the diaphragm of Bis-SMKO mice. Our findings highlight that BIS is critical for protein homeostasis and energy metabolism in skeletal muscles, suggesting that Bis-SMKO mice could be used as a therapeutic strategy for myopathies and to elucidate the molecular function of BIS in skeletal muscle physiology.

SF3B4 Depletion Retards the Growth of A549 Non-Small Cell Lung Cancer Cells via UBE4B-Mediated Regulation of p53/p21 and p27 Expression.

Splicing factor B subunit 4 (SF3B4), a component of the U2-pre-mRNA spliceosomal complex, contributes to tumorigenesis in several types of tumors. However, the oncogenic potential of SF3B4 in lung cancer has not yet been determined. The in vivo expression profiles of SF3B4 in non-small cell lung cancer (NSCLC) from publicly available data revealed a significant increase in SF3B4 expression in tumor tissues compared to that in normal tissues. The impact of SF3B4 deletion on the growth of NSCLC cells was determined using a siRNA strategy in A549 lung adenocarcinoma cells. SF3B4 silencing resulted in marked retardation of the A549 cell proliferation, accompanied by the accumulation of cells at the G0/G1 phase and increased expression of p27, p21, and p53. Double knockdown of SF3B4 and p53 resulted in the restoration of p21 expression and partial recovery of cell proliferation, indicating that the p53/p21 axis is involved, at least in part, in the SF3B4-mediated regulation of A549 cell proliferation. We also provided ubiquitination factor E4B (UBE4B) is essential for p53 accumulation after SF3B4 depletion based on followings. First, co-immunoprecipitation showed that SF3B4 interacts with UBE4B. Furthermore, UBE4B levels were decreased by SF3B4 depletion. UBE4B depletion, in turn, reproduced the outcome of SF3B4 depletion, including reduction of polyubiquitinated p53 levels, subsequent induction of p53/p21 and p27, and proliferation retardation. Collectively, our findings indicate the important role of SF3B4 in the regulation of A549 cell proliferation through the UBE4B/p53/p21 axis and p27, implicating the therapeutic strategies for NSCLC targeting SF3B4 and UBE4B.

Hepatocyte-specific deletion of Bis causes senescence in the liver without deteriorating hepatic function.

Bcl-2-interacting cell death suppressor (BIS), also called as BAG3, regulates numerous physiological processes, such as apoptosis, protein quality control, and senescence. Whole-body Bis-knockout (KO) mice exhibit early lethality following cardiac and skeletal muscle dysfunction. The first attempt to generate organ-specific knockout mice resulted in constitutive or inducible heart-specific Bis-knockout mice, which exhibited cardiac dilation and underwent premature death. Here, we generated hepatocyte-specific Bis-knockout (Bis-HKO) mice and found no abnormalities in metabolic function and survival. However, depletion of HSPB8 and accumulation of p62 indicated impaired autophagy in Bis-HKO livers. Interestingly, the number of peroxisomes wrapped by phagophore membranes increased as evidenced by transmission electron microscopy analysis, indicating defects in the progression of pexophagy. In addition, increased dihydroethidine intensities and histone H3 K9me3-positive nuclei indicated increased oxidative stress and senescence induction in Bis-HKO livers. Mechanistically, p27 was upregulated in Bis-HKO livers. In SNU368 hepatocellular carcinoma cells, BIS depletion led to p27 upregulation, and increase in histone H3 K9me3 levels and senescence-associated ß-galactosidase staining; therefore, reproducing the in vivo senescence phenotype. Despite the observation of no metabolic abnormalities, BIS depletion led to defective autophagy, increased oxidative stress, and senescence in Bis-HKO livers. Collectively, our results suggest a role for BIS in maintaining liver regeneration potential under pathological conditions.

Depletion of ST6GALNACIII retards A549 non-small cell lung cancer cell proliferation by downregulating transferrin receptor protein 1 expression.

Alterations in sialylation of terminal residues of glycoproteins have been implicated in forming tumor-associated glycans. ST6GALNAC transfers sialyl moiety to N-acetylgalactosamine residue via α2,6 linkage. Although the oncogenic characteristics of ST6GALNACI or II have been demonstrated in various cancer cells, the impact of ST6GALNACIII on tumor progression remains undefined. In this study, we evaluated the effect of ST6GALNACIII knockdown on the growth of A549 non-small cell lung cancer cells. ST6GALNACIII depletion resulted in significant retardation in growth of A549 cells under various culture conditions, including collagen-supported 3D culture and anchorage-independent soft agar culture conditions. Liquid chromatography with tandem mass spectrometry revealed that two glycopeptides of transferrin receptor protein 1 (TFR1) containing N-acetylhexosamine-sialic acid were not detected in ST6GALNACIII-depleted A549 cells compared with control cells. Subsequent lectin binding assay, western blotting, and real-time RT-PCR indicated that TFR1 sialylation was not significantly changed, but TFR1 protein and mRNA expressions were decreased after ST6GALNACIII knockdown. However, cell growth retardation by ST6GALNACIII knockdown was partially rescued by TFR1 overexpression. Additionally, TFR1 mRNA degradation was accelerated following ST6GALNACIII knockdown with concomitant reduction in mRNA levels of iron regulatory protein 1 and 2, the upstream regulators of TFR1 mRNA stability. Therefore, our results indicated an important role of ST6GALNACIII in promoting A549 cell growth through quantitative regulation of TFR1 expression and provided therapeutic implications for ST6GALNACIII targeting in tumor growth suppression in vivo.

KRIBB11 Induces Apoptosis in A172 Glioblastoma Cells via MULE-Dependent Degradation of MCL-1.

KRIBB11, an HSF1 inhibitor, was shown to sensitize various types of cancer cells to treatment with several anticancer drugs. However, the exclusive effects of KRIBB11 in preventing the growth of glioblastoma cells and the related mechanisms have not been elucidated yet. Herein, we aimed to examine the potential of KRIBB11 as an anticancer agent for glioblastoma. Using MTT and colony formation assays and Western blotting for c-PARP, we demonstrated that KRIBB11 substantially inhibits the growth of A172 glioma cells by inducing apoptosis. At the molecular level, KRIBB11 decreased anti-apoptotic protein MCL-1 levels, which was attributable to the increase in MULE ubiquitin ligase levels. However, the constitutive activity of HSF1 in A172 cells was not influenced by the exclusive treatment with KRIBB11. Additionally, based on cycloheximide chase assay, we found that KRIBB11 markedly retarded the degradation of MULE. In conclusion, stabilization of MULE upon KRIBB11 treatment is apparently an essential step for degradation of MCL-1 and the subsequent induction of apoptosis in A172 cells. Our results have expanded the knowledge on molecular pathways controlled by KRIBB11 and could be potentially effective for developing an inhibitory therapeutic strategy for glioblastoma.

An Adult Mouse Model of Dilated Cardiomyopathy Caused by Inducible Cardiac-Specific Bis Deletion.

BCL-2 interacting cell death suppressor (BIS) is a multifunctional protein that has been implicated in cancer and myopathy. Various mutations of the BIS gene have been identified as causative of cardiac dysfunction in some dilated cardiomyopathy (DCM) patients. This was recently verified in cardiac-specific knock-out (KO) mice. In this study, we developed tamoxifen-inducible cardiomyocyte-specific BIS-KO (Bis-iCKO) mice to assess the role of BIS in the adult heart using the Cre-loxP strategy. The disruption of the Bis gene led to impaired ventricular function and subsequent heart failure due to DCM, characterized by reduced left ventricular contractility and dilatation that were observed using serial echocardiography and histology. The development of DCM was confirmed by alterations in Z-disk integrity and increased expression of several mRNAs associated with heart failure and remodeling. Furthermore, aggregation of desmin was correlated with loss of small heat shock protein in the Bis-iCKO mice, indicating that BIS plays an essential role in the quality control of cardiac proteins, as has been suggested in constitutive cardiac-specific KO mice. Our cardiac-specific BIS-KO mice may be a useful model for developing therapeutic interventions for DCM, especially late-onset DCM, based on the distinct phenotypes and rapid progressions.

Identification of Cathepsin D as a Plasma Biomarker for Alzheimer's Disease.

Although Alzheimer's disease (AD) is the most common neurodegenerative disease, there are still no drugs available to treat or prevent AD effectively. Here, we examined changes in levels of selected proteins implicated in the pathogenesis of AD using plasma samples of control subjects and patients with cognition impairment. To precisely categorize the disease, fifty-six participants were examined with clinical cognitive tests, amyloid positron emission tomography (PET) scan, and white matter hyperintensities scored by magnetic resonance imaging. Plasma cathepsin D levels of the subjects were examined by immunoblotting and enzyme-linked immunosorbent assay (ELISA). Correlation of plasma cathepsin D levels with AD-related factors and clinical characteristics were examined by statistical analysis. By analyzing quantitative immunoblot and ELISA, we found that the plasma level of cathepsin D, a major lysosomal protease, was decreased in the group with amyloid plaque deposition at the brain compared to the control group. The level of plasma cathepsin D was negatively correlated with clinical dementia rating scale sum of boxes (CDR-SB) scores. In addition, our integrated multivariable logistic regression model suggests the high performance of plasma cathepsin D level for discriminating AD from non-AD. These results suggest that the plasma cathepsin D level could be developed as a diagnostic biomarker candidate for AD.

SRSF3 Is a Critical Requirement for Inclusion of Exon 3 of BIS Pre-mRNA.

BCL-2 interacting cell death suppressor (BIS), also known as BAG3, is a multifunctional protein. Aberrant expression and mutation of BIS have been implicated in cancers and myopathy. However, there have only been a few studies on the splicing of BIS pre-mRNA. In the present study, through RT-PCR and sequencing in various cell lines and mouse tissues, we identified for the first time the presence of BIS mRNA isomers in which exon 3 or exons 2-3 are skipped. We also demonstrated that the depletion of SRSF3 promoted the skipping of exon 3 of BIS pre-mRNA in endogenous BIS and the GFP-BIS minigene. SRSF3 specifically interacts with the putative binding sites in exon 3, in which deletion promoted the skipping of exon 3 in the GFP-BIS minigene, which was comparable to the effect of SRSF knockdown. Even though acceleration of exon 3 skipping was not observed in response to various stimuli, SRSF3 depletion, accompanied by the production of a truncated BIS protein, inhibited the nuclear translocation of HSF1, which was restored by the wild-type BIS, not by exon 3-depleted BIS. Therefore, our results suggested that the maintenance of SRSF3 levels and subsequent preservation of the intact BIS protein is an important factor in modulating HSF1 localization upon cellular stress.

Hereditary spastic paraplegia SPG8 mutations impair CAV1-dependent, integrin-mediated cell adhesion.

Mutations in WASHC5 (also known as KIAA0196) cause autosomal dominant hereditary spastic paraplegia (HSP) type SPG8. WASHC5, commonly called strumpellin, is a core component of the Wiskott-Aldrich syndrome protein and SCAR homolog (WASH) complex that activates actin nucleation at endosomes. Although various other cellular roles for strumpellin have also been described, none account for how SPG8-associated mutations lead to HSP. Here, we identified protein interactors of the WASH complex by immunoprecipitation and mass spectrometry and assessed the functions of strumpellin in cultured cells using both overexpression and RNA interference along with cell-spreading assays to investigate cell adhesion. We uncovered a decrease in CAV1 protein abundance as well as endosomal fission defects resulting from pathogenic SPG8 mutations. CAV1, a key component of caveolae, interacted with strumpellin in cells, and strumpellin inhibited the lysosomal degradation of CAV1. SPG8-associated missense mutations in strumpellin did not rescue endosomal tubulation defects, reduction in CAV1 protein abundance, or integrin-mediated cell adhesion in strumpellin-deficient cells. Mechanistically, we demonstrated that the WASH complex maintained CAV1 and integrin protein amounts by inhibiting their lysosomal degradation through its endosomal actin nucleation activity. In addition, the interaction of strumpellin with CAV1 stimulated integrin recycling, thereby promoting cell adhesion. These findings provide a molecular link between WASHC5 mutations and impairment of CAV1- and integrin-mediated cell adhesion, providing insights into the cellular pathogenesis of SPG8.

Functional Characterization of Resting and Adenovirus-Induced Reactive Astrocytes in Three-Dimensional Culture.

Brain is a rich environment where neurons and glia interact with neighboring cells as well as extracellular matrix in three-dimensional (3D) space. Astrocytes, which are the most abundant cells in the mammalian brain, reside in 3D space and extend highly branched processes that form microdomains and contact synapses. It has been suggested that astrocytes cultured in 3D might be maintained in a less reactive state as compared to those growing in a traditional, two-dimensional (2D) monolayer culture. However, the functional characterization of the astrocytes in 3D culture has been lacking. Here we cocultured neurons and astrocytes in 3D and examined the morphological, molecular biological, and electrophysiological properties of the 3D-cultured hippocampal astrocytes. In our 3D neuron-astrocyte coculture, astrocytes showed a typical morphology of a small soma with many branches and exhibited a unique membrane property of passive conductance, more closely resembling their native in vivo counterparts. Moreover, we also induced reactive astrocytosis in culture by infecting with high-titer adenovirus to mimic pathophysiological conditions in vivo. Adenoviral infection induced morphological changes in astrocytes, increased passive conductance, and increased GABA content as well as tonic GABA release, which are characteristics of reactive gliosis. Together, our study presents a powerful in vitro model resembling both physiological and pathophysiological conditions in vivo, and thereby provides a versatile experimental tool for studying various neurological diseases that accompany reactive astrocytes.

The Ca2+-activated chloride channel anoctamin-2 mediates spike-frequency adaptation and regulates sensory transmission in thalamocortical neurons.

Neuronal firing patterns, which are crucial for determining the nature of encoded information, have been widely studied; however, the molecular identity and cellular mechanisms of spike-frequency adaptation are still not fully understood. Here we show that spike-frequency adaptation in thalamocortical (TC) neurons is mediated by the Ca2+-activated Cl- channel (CACC) anoctamin-2 (ANO2). Knockdown of ANO2 in TC neurons results in significantly reduced spike-frequency adaptation along with increased tonic spiking. Moreover, thalamus-specific knockdown of ANO2 increases visceral pain responses. These results indicate that ANO2 contributes to reductions in spike generation in highly activated TC neurons and thereby restricts persistent information transmission.

Comparison of the Efficacy of Fluorometholone With and Without Benzalkonium Chloride in Ocular Surface Disease.

PURPOSE: The purpose of this study was to compare the cytotoxicity and antiinflammatory effect of preserved and unpreserved 0.1% fluorometholone (FML). METHODS: Drug-induced morphological changes and cytotoxicity were examined in human corneal epithelial cells. Dry eye was induced in mice by treatment with 0.2% benzalkonium chloride (BAC) for the first 2 weeks, and then, the eyes (4 groups; Normal saline, BAC, preserved FML, and unpreserved FML) were treated thrice daily with each formulation for the next 2 weeks. Corneal tissues were embedded in paraffin and stained with hematoxylin and eosin for histopathological examination. Immunofluorescence staining was performed for tumor necrosis factor-α, interleukin-6, and human leukocyte antigen-DR. Terminal deoxynucleotidyl transferase dUTP nick end labeling assay was performed to evaluate drug-induced cytotoxicity. RESULTS: BAC and preserved FML caused cell shrinkage and detachment from the plate in a dose-dependent manner, and cell viability decreased significantly. However, cytotoxicity was reduced on treatment with unpreserved FML. Hematoxylin-eosin staining revealed surface desquamation, irregular surface, loss of cell borders, and stromal shrinkage in the group treated with BAC. On BAC exposure, tumor necrosis factor-α, interleukin-6, and human leukocyte antigen-DR were strongly detected, and cytotoxicity was markedly increased, as evidenced by a positive result in the terminal deoxynucleotidyl transferase dUTP nick end labeling assay. Ocular surface damage and inflammation were slightly reduced on treatment with preserved FML. In comparison, unpreserved FML did not induce morphological changes; moreover, decreased cell cytotoxicity and ocular surface inflammation were observed. CONCLUSIONS: The cytotoxicity of antiinflammatory eye drops evaluated in this study was induced by the preservative BAC. Accordingly, unpreserved FML is more effective than preserved eye drops in decreasing ocular inflammation.

Cyclosporine A Downregulates MMP-3 and MMP-13 Expression in Cultured Pterygium Fibroblasts.

PURPOSE: To investigate the regulation of matrix metalloproteinase (MMP)-3 and MMP-13 expression over time and in the presence of cyclosporine A (CsA) in primary cultured human pterygium fibroblasts. We also examined the effects of CsA on cultured human pterygium fibroblasts. METHODS: Primary cultured human pterygium fibroblasts subjected to scratch assays were exposed to 1 and 100 µg/mL of CsA for 3 or 10 minutes. Cells were washed with Dulbecco phosphate-buffered saline, and then incubated with serum-depleted Dulbecco modified Eagle medium/F-12 medium for 48 hours. Expression levels of MMP-3 and MMP-13 proteins and the corresponding mRNA transcripts were determined by western blotting and reverse transcription polymerase chain reaction assays, respectively. RESULTS: Migration of cultured pterygium fibroblast cells was suppressed by pretreatment with CsA compared with controls in a time-dependent and dose-dependent manner (3 minutes, 50.6% ± 1.1 in 1 µg/mL, 60.0% ± 1.2 in 100 µg/mL; 10 minutes, 59.8% ± 5.7 in 1 µg/mL, 60.5 ± 2.4 in 100 µg/mL, respectively, P < 0.01). Pretreatment with CsA also reduced the mRNA (P < 0.05) and protein expression levels (P < 0.05). CONCLUSIONS: CsA was actively involved in the migration of pterygium fibroblasts. Cell migration is inhibited in response to CsA through the inhibition of MMP-3 and MMP-13 expression. These findings reveal the therapeutic potential of CsA on pterygium progression. Further studies will be necessary to elucidate the precise intracellular signal mechanism responsible for CsA-induced downregulation of MMPs in pterygium fibroblasts.

Lipid raft proteome reveals that oxidative phosphorylation system is associated with the plasma membrane.

Although accumulating proteomic analyses have supported the fact that mitochondrial oxidative phosphorylation (OXPHOS) complexes are localized in lipid rafts, which mediate cell signaling, immune response and host-pathogen interactions, there has been no in-depth study of the physiological functions of lipid-raft OXPHOS complexes. Here, we show that many subunits of OXPHOS complexes were identified from the lipid rafts of human adipocytes, C2C12 myotubes, Jurkat cells and surface biotin-labeled Jurkat cells via shotgun proteomic analysis. We discuss the findings of OXPHOS complexes in lipid rafts, the role of the surface ATP synthase complex as a receptor for various ligands and extracellular superoxide generation by plasma membrane oxidative phosphorylation complexes.

TRIM72, a novel negative feedback regulator of myogenesis, is transcriptionally activated by the synergism of MyoD (or myogenin) and MEF2.

TRIM72 is known to be involved in the negative feedback regulation of myogenesis by targeting insulin receptor substrate-1. Here, we found that TRIM72 was more highly expressed in oxidative muscle with the higher activity of MEF2, compared to glycolytic muscle. Indeed, TRIM72 promoter contained an evolutionarily conserved MEF2 site juxtaposed to E-box. TRIM72 promoter activity was decreased by the site-directed mutagenesis of either E-boxes or a MEF2 site and synergistically enhanced by MyoD (or myogenin) and MEF2, which were associated with proximal E-box, and MEF2 site of the TRIM72 promoter, respectively. Taken together all these data, we concluded that the synergism of MyoD (or myogenin) and MEF2 is necessary for TRIM72 expression during C2C12 differentiation.

Mitochondrial oxidative phosphorylation system is recruited to detergent-resistant lipid rafts during myogenesis.

Since detergent-resistant lipid rafts play important roles in the signal transduction for myogenesis, their comprehensive proteomic analysis could provide new insights to understand their function in myotubes. Here, the detergent-resistant lipid rafts were isolated from C2C12 myotubes and analyzed by capillary RPLC/MS/MS. Among the 327 proteins (or protein groups) identified, 28% were categorized to the plasma membrane or raft proteins, 29% to mitochondria, 20% to microsomal proteins, 10% to other proteins, and 13% to unknown proteins. The localization of oxidative phosphorylation (OXPHOS) complexes in the sarcolemma lipid rafts was further confirmed from C2C12 myotubes by cellular fractionation, surface-biotin labeling, immunofluorescence, and lipid raft fractionation. After adding exogenous cytochrome c, the sarcolemma isolated from myotubes had an ability to consume oxygen in the presence of NADH or succinate. The generation of NADH-dependent extracellular superoxide was increased by inhibiting or downregulating OXPHOS I, III, and IV in myotubes, indicating that OXPHOS proteins are major sources for extracellular ROS in skeletal muscle. With all these data, we can conclude that OXPHOS proteins are associated with the sarcolemma lipid rafts during C2C12 myogenesis to generate extracellular ROS.