Adipogenesis stimulates the nuclear localization of EWS with an increase in its O-GlcNAc glycosylation in 3T3-L1 cells

EWSR1::ATF1 Translocation: A Common Tumor Driver of Distinct Human Neoplasms

Cancer is among the leading causes of mortality in developed countries due to limited available therapeutic modalities and high rate of morbidity. Although malignancies might show individual genetic landscapes, recurring aberrations in the neoplastic genome have been identified in the wide range of transformed cells. These include translocations of frequently affected loci of the human genetic material like the Ewing sarcoma breakpoint region 1 (EWSR1) of chromosome 22 that results in malignancies with mesodermal origin. These cytogenetic defects frequently result in the genesis of fusion genes involving EWSR1 and a number of genes from partner loci. One of these chromosomal rearrangements is the reciprocal translocation between the q13 and q12 loci of chromosome 12 and 22, respectively, that is believed to initiate cancer formation by the genesis of a novel, chimeric transcription factor provoking dysregulated gene expression. Since soft-tissue neoplasms carrying t(12;22)(q13;q12) have very poor prognosis and clinical modalities specifically targeting t(12;22)(q13;q12)-harboring cells are not available to date, understanding this DNA aberration is not only timely but urgent. Here, we review our current knowledge of human malignancies carrying the specific subset of EWSR1 rearrangements that leads to the expression of the EWSR1::ATF1 tumor-driver chimeric protein.

The ubiquitin E3 ligase APC/CCdc20 mediates mitotic degradation of OGT

O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) is the sole enzyme that catalyzes all O-GlcNAcylation reactions intracellularly. Previous investigations have found that OGT levels oscillate during the cell division process. Specifically, OGT abundance is downregulated during mitosis, but the underlying mechanism is lacking. Here we demonstrate that OGT is ubiquitinated by the ubiquitin E3 ligase, anaphase promoting complex/cyclosome (APC/C)-cell division cycle 20 (Cdc20). We show that APC/CCdc20 interacts with OGT through a conserved destruction box (D-box): Arg-351/Leu-354, the abrogation of which stabilizes OGT. As APC/CCdc20-substrate binding is often preceded by a priming ubiquitination event, we also used mass spectrometry and mapped OGT Lys-352 to be a ubiquitination site, which is a prerequisite for OGT association with APC/C subunits. Interestingly, in The Cancer Genome Atlas, R351C is a uterine carcinoma mutant, suggesting that mutations of the D-box are linked with tumorigenesis. Paradoxically, we found that both R351C and the D-box mutants (R351A/L354A) inhibit uterine carcinoma in mouse xenograft models, probably due to impaired cell division and proliferation. In sum, we propose a model where OGT Lys-352 ubiquitination primes its binding with APC/C, and then APC/CCdc20 partners with OGT through the D-box for its mitotic destruction. Our work not only highlights the key mechanism that regulates OGT during the cell cycle, but also reveals the mutual coordination between glycosylation and the cell division machinery.

A spatiotemporal proteomic map of human adipogenesis

White adipocytes function as major energy reservoirs in humans by storing substantial amounts of triglycerides, and their dysfunction is associated with metabolic disorders; however, the mechanisms underlying cellular specialization during adipogenesis remain unknown. Here, we generate a spatiotemporal proteomic atlas of human adipogenesis, which elucidates cellular remodelling as well as the spatial reorganization of metabolic pathways to optimize cells for lipid accumulation and highlights the coordinated regulation of protein localization and abundance during adipocyte formation. We identify compartment-specific regulation of protein levels and localization changes of metabolic enzymes to reprogramme branched-chain amino acids and one-carbon metabolism to provide building blocks and reduction equivalents. Additionally, we identify C19orf12 as a differentiation-induced adipocyte lipid droplet protein that interacts with the translocase of the outer membrane complex of lipid droplet-associated mitochondria and regulates adipocyte lipid storage by determining the capacity of mitochondria to metabolize fatty acids. Overall, our study provides a comprehensive resource for understanding human adipogenesis and for future discoveries in the field.

Chromosomal localization of Ewing sarcoma EWSR1/FLI1 protein promotes the induction of aneuploidy

Ewing sarcoma is a pediatric bone cancer that expresses the chimeric protein EWSR1/FLI1. We previously demonstrated that EWSR1/FLI1 impairs the localization of Aurora B kinase to the midzone (the midline structure located between segregating chromosomes) during anaphase. While localization of Aurora B is essential for faithful cell division, it is unknown whether interference with midzone organization by EWSR1/FLI1 induces aneuploidy. To address this, we generated stable Tet-on inducible cell lines with EWSR1/FLI1, using CRISPR/Cas9 technology to integrate the transgene at the safe-harbor AAVS1 locus in DLD-1 cells. Induced cells expressing EWSR1/FLI1 displayed an increased incidence of aberrant localization of Aurora B, and greater levels of aneuploidy, compared with noninduced cells. Furthermore, the expression of EWSR1/FLI1-T79A, containing a threonine (Thr) to alanine (Ala) substitution at amino acid 79, failed to induce these phenotypes, indicating that Thr 79 is critical for EWSR1/FLI1 interference with mitosis. In contrast, the phosphomimetic mutant EWSR1/FLI1-T79D (Thr to aspartic acid (Asp)) retained the high activity as wild-type EWSR1/FLI1. Together, these findings suggest that phosphorylation of EWSR1/FLI1 at Thr 79 promotes the colocalization of EWSR1/FLI1 and Aurora B on the chromosomes during prophase and metaphase and, in addition, impairs the localization of Aurora B during anaphase, leading to induction of aneuploidy. This is the first demonstration of the mechanism for EWSR1/FLI1-dependent induction of aneuploidy associated with mitotic dysfunction and the identification of the phosphorylation of the Thr 79 of EWSR1/FLI1 as a critical residue required for this induction.

O-GlcNAc: A Sweetheart of the Cell Cycle and DNA Damage Response

The addition and removal of O-linked N-acetylglucosamine (O-GlcNAc) to and from the Ser and Thr residues of proteins is an emerging post-translational modification. Unlike phosphorylation, which requires a legion of kinases and phosphatases, O-GlcNAc is catalyzed by the sole enzyme in mammals, O-GlcNAc transferase (OGT), and reversed by the sole enzyme, O-GlcNAcase (OGA). With the advent of new technologies, identification of O-GlcNAcylated proteins, followed by pinpointing the modified residues and understanding the underlying molecular function of the modification has become the very heart of the O-GlcNAc biology. O-GlcNAc plays a multifaceted role during the unperturbed cell cycle, including regulating DNA replication, mitosis, and cytokinesis. When the cell cycle is challenged by DNA damage stresses, O-GlcNAc also protects genome integrity via modifying an array of histones, kinases as well as scaffold proteins. Here we will focus on both cell cycle progression and the DNA damage response, summarize what we have learned about the role of O-GlcNAc in these processes and envision a sweeter research future.

Reduced O-GlcNAcase expression promotes mitotic errors and spindle defects

Alterations in O-GlcNAc cycling, the addition and removal of O-GlcNAc, lead to mitotic defects and increased aneuploidy. Herein, we generated stable O-GlcNAcase (OGA, the enzyme that removes O-GlcNAc) knockdown HeLa cell lines and characterized the effect of the reduction in OGA activity on cell cycle progression. After release from G1/S, the OGA knockdown cells progressed normally through S phase but demonstrated mitotic exit defects. Cyclin A was increased in the knockdown cells while Cyclin B and D expression was reduced. Retinoblastoma protein (RB) phosphorylation was also increased in the knockdown compared to control. At M phase, the knockdown cells showed more compact spindle chromatids than control cells and had a greater percentage of cells with multipolar spindles. Furthermore, the timing of the inhibitory tyrosine phosphorylation of Cyclin Dependent Kinase 1 (CDK1) was altered in the OGA knockdown cells. Although expression and localization of the chromosomal passenger protein complex (CPC) was unchanged, histone H3 threonine 3 phosphorylation was decreased in one of the OGA knockdown cell lines. The Ewing Sarcoma Breakpoint Region 1 Protein (EWS) participates in organizing the CPC at the spindle and is a known substrate for O-GlcNAc transferase (OGT, the enzyme that adds O-GlcNAc). EWS O-GlcNAcylation was significantly increased in the OGA knockdown cells promoting uneven localization of the mitotic midzone. Our data suggests that O-GlcNAc cycling is an essential mechanism for proper mitotic signaling and spindle formation, and alterations in the rate of O-GlcNAc cycling produces aberrant spindles and promotes aneuploidy.

O-GlcNAc glycosylation stoichiometry of the FET protein family: only EWS is glycosylated with a high stoichiometry

Of the FET (fused in sarcoma [FUS]/Ewing sarcoma protein [EWS]/TATA binding protein-associated factor 15 [TAF15]) family of heterogeneous nuclear ribonucleoprotein particle proteins, FUS and TAF15 are consistently and EWS variably found in inclusion bodies in neurodegenerative diseases such as frontotemporal lobar degeneration associated with FUS. It is speculated that dysregulation of FET proteins at the post-translational level is involved in their cytoplasmic deposition. Here, the O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation stoichiometry of the FET proteins was chemoenzymatically analyzed, and it was found that only EWS is dynamically glycosylated with a high stoichiometry in the neural cell lines tested and in mouse brain. It was also confirmed that EWS, but not FUS and TAF15, is glycosylated with a high stoichiometry not only in the neural cells but also in the non-neural cell lines tested. These results indicate that O-GlcNAc glycosylation imparts a physicochemical property on EWS that is distinct from that of the other FET proteins in most of cell lineages or tissues.

The glycosylation stoichiometry of EWS species in neuronal cells

Although Ewing sarcoma protein (EWS) is known to be glycosylated by O-linked β-N-acetylglucosamine (O-GlcNAc), the dynamics and stoichiometry of its glycosylation remain obscure. Here, we report a dynamic change in the glycosylation stoichiometry of EWS species during neuronal differentiation of embryonic carcinoma P19 cells. Our findings suggest that O-GlcNAc glycosylation participates in the regulation of EWS functions in neuronal cells.

O-GlcNAc modification of Sp3 and Sp4 transcription factors negatively regulates their transcriptional activities

The addition of O-linked N-acetylglucosamine (O-GlcNAc) on serine or threonine modifies a myriad of proteins and regulates their function, stability and localization. O-GlcNAc modification is common among chromosome-associated proteins, such as transcription factors, suggesting its extensive involvement in gene expression regulation. In this study, we demonstrate the O-GlcNAc status of the Sp family members of transcription factors and the functional impact on their transcriptional activities. We highlight the presence of O-GlcNAc residues in Sp3 and Sp4, but not Sp2, as demonstrated by their enrichment in GlcNAc positive protein fractions and by detection of O-GlcNAc residues on Sp3 and Sp4 co-expressed in Escherichia coli together with O-GlcNAc transferase (OGT) using an O-GlcNAc-specific antibody. Deletion mutants of Sp3 and Sp4 indicate that the majority of O-GlcNAc sites reside in their N-terminal transactivation domain. Overall, using reporter gene assays and co-immunoprecipitations, we demonstrate a functional inhibitory role of O-GlcNAc modifications in Sp3 and Sp4 transcription factors. Thereby, our study strengthens the current notion that O-GlcNAc modification is an important regulator of protein interactome.

An essential role for Ewing sarcoma gene (EWS) in early white adipogenesis

OBJECTIVE

White adipose tissue is important for mammalian energy homeostasis and metabolism. It was previously demonstrated that Ewing sarcoma gene (EWS) is essential for early classical brown fat lineage determination, but its role in white adipocyte differentiation is not known.

METHODS

Mouse embryonic fibroblasts (MEFs) lacking Ews and shRNA-mediated silencing of Ews in 3T3L1 preadipocytes were used to investigate the role of EWS in adipogenesis. White fat differentiation was determined by analyzing the expression of key adipogenic genes and by Oil red O staining.

RESULTS

Following adipogenic stimulation, Ews expression arose rapidly in 3T3L1 cells during early induction period. Ews-null MEFs and 3T3L1 cells with reduced Ews expression failed to undergo adipogenesis. This was accompanied by significant reduction in the expression of critical early adipogenic regulators, Bmp2, Bmp4 (bone morphogenic protein 2 and 4), Cebpβ, and Cebpδ (CCAAT/enhancer binding protein β and δ). Complementation of recombinant BMP2 or BMP4 partially rescued adipogenesis in Ews-depleted 3T3L1 cells.

CONCLUSIONS

These results demonstrate that EWS is essential during the early steps of white adipocyte differentiation, at least in part through its regulation of BMP2 and BMP4 expression.