The ubiquitin ligase STUB1 regulates stability and activity of RUNX1 and RUNX1-RUNX1T1

The E3 ligase DTX2 inhibits RUNX1 function by binding its C terminus and prevents the growth of RUNX1-dependent leukemia cells

Transcription factor RUNX1 plays important roles in hematopoiesis and leukemogenesis. RUNX1 function is tightly controlled through posttranslational modifications, including ubiquitination and acetylation. However, its regulation via ubiquitination, especially proteasome-independent ubiquitination, is poorly understood. We previously identified DTX2 as a RUNX1-interacting E3 ligase using a cell-free AlphaScreen assay. In this study, we examined whether DTX2 is involved in the regulation of RUNX1 using in vitro and ex vivo analyses. DTX2 bound to RUNX1 and other RUNX family members RUNX2 and RUNX3 through their C-terminal region. DTX2-induced RUNX1 ubiquitination did not result in RUNX1 protein degradation. Instead, we found that the acetylation of RUNX1, which is known to enhance the transcriptional activity of RUNX1, was inhibited in the presence of DTX2. Concomitantly, DTX2 reduced the RUNX1-induced activation of an MCSFR luciferase reporter. We also found that DTX2 induced RUNX1 cytoplasmic mislocalization. Moreover, DTX2 overexpression showed a substantial growth-inhibitory effect in RUNX1-dependent leukemia cell lines. Thus, our findings indicate a novel aspect of the ubiquitination and acetylation of RUNX1 that is modulated by DTX2 in a proteosome-independent manner.

STUB1/CHIP: New insights in cancer and immunity

The STUB1 gene (STIP1 homology and U-box-containing protein 1), located at 16q13.3, encodes the CHIP (carboxyl terminus of Hsc70-interacting protein), an essential E3 ligase involved in protein quality control. CHIP comprises three domains: an N-terminal tetratricopeptide repeat (TPR) domain, a middle coiled-coil domain, and a C-terminal U-box domain. It functions as a co-chaperone for heat shock protein (HSP) via the TPR domain and as an E3 ligase, ubiquitinating substrates through its U-box domain. Numerous studies suggest that STUB1 plays a crucial role in various physiological process, such as aging, autophagy, and bone remodeling. Moreover, emerging evidence has shown that STUB1 can degrade oncoproteins to exert tumor-suppressive functions, and it has recently emerged as a novel player in tumor immunity. This review provides a comprehensive overview of STUB1's role in cancer, including its clinical significance, impact on tumor progression, dual roles, tumor stem cell-like properties, angiogenesis, drug resistance, and DNA repair. In addition, we explore STUB1's functions in immune cell differentiation and maturation, inflammation, autoimmunity, antiviral immune response, and tumor immunity. Collectively, STUB1 represents a promising and valuable therapeutic target in cancer and immunology.

Driving the degradation of oncofusion proteins for targeted cancer therapy

Oncofusion proteins drive the development of about 16.5% of human cancers {AuQ: Edit OK?}, functioning as the unique pathogenic factor in some cancers. The targeting of oncofusion proteins is an attractive strategy to treat malignant tumors. Recently, triggering the degradation of oncofusion proteins has been shown to hold great promise as a therapeutic strategy. Here, we review the recent findings on the mechanisms that maintain the high stability of oncofusion proteins. Then, we summarize strategies to target the degradation of oncofusion proteins through the ubiquitin-proteasome pathway, the autophagy-lysosomal pathway, and the caspase-dependent pathway. By examining oncofusion protein degradation in cancer, we not only gain better insight into the carcinogenic mechanisms that involve oncofusion proteins, but also raise the possibility of treating oncofusion-driven cancer.

USP10 deubiquitinates RUNX1 and promotes proneural-to-mesenchymal transition in glioblastoma

The mesenchymal (MES) subtype of glioblastoma (GBM) is a highly aggressive, malignant and proliferative cancer that is resistant to chemotherapy. Runt-related transcription factor 1 (RUNX1) was shown to support MES GBM, however, its underlying mechanisms are unclear. Here, we identified USP10 as a deubiquitinating enzyme that regulates RUNX1 stabilization and is mainly expressed in MES GBM. Overexpression of USP10 upregulated RUNX1 and induced proneural-to-mesenchymal transition (PMT), thus maintaining MES properties in GBM. Conversely, USP10 knockdown inhibited RUNX1 and resulted in the loss of MES properties. USP10 was shown to interact with RUNX1, with RUNX1 being stabilized upon deubiquitylation. Moreover, we found that USP10 inhibitor Spautin-1 induced RUNX1 degradation and inhibited MES properties in vitro and in vivo. Furthermore, USP10 was strongly correlated with RUNX1 expression in samples of different subtypes of human GBM and had prognostic value for GBM patients. We identified USP10 as a key deubiquitinase for RUNX1 protein stabilization. USP10 maintains MES properties of GBM, and promotes PMT of GBM cells. Our study indicates that the USP10/RUNX1 axis may be a potential target for novel GBM treatments.

VEZF1, destabilized by STUB1, affects cellular growth and metastasis of hepatocellular carcinoma by transcriptionally regulating PAQR4

Hepatocellular carcinoma (HCC) is an aggressive solid malignancy, and recurrence and metastasis are major incentives contributing to its poor outcome. Vascular endothelial zinc finger 1 (VEZF1) has been recognized as an oncoprotein in certain types of cancer, but the expression pattern and regulatory mechanism in HCC remains unclear. This study focused on the functional effect and regulatory basis of VEZF1 in HCC. Microarray analysis identified the differentially expressed VEZF1 in HCC, and we validated its raised expression in HCC clinical samples. Artificial modulation of VEZF1 (knockdown and overexpression) was conducted to explore its role in HCC progression both in vitro and in vivo. It was shown that silencing of VEZF1 suppressed, whereas its overexpression promoted HCC cellular proliferation and metastasis abilities. Mechanistically, VEZF1 transcriptionally activated progestin and adipoQ receptor 4 (PAQR4) to accelerate HCC progression. Furthermore, VEZF1 is confirmed as a substrate of stress-inducible phosphoprotein 1 homology and U-box containing protein 1 (STUB1), and its stability is impacted by STUB1-mediated ubiquitination degradation. Conjointly, our work suggested that VEZF1, destabilized by STUB1, participates in HCC progression by regulating PAQR4. The STUB1/VEZF1/PAQR4 mechanism might provide novel insights on guiding early diagnosis and therapy in HCC patients.

STUB1 regulates antiviral RNAi through inducing ubiquitination and degradation of Dicer and AGO2 in mammals

RNA interference (RNAi) is an intrinsic antiviral immune mechanism conserved in diverse eukaryotic organisms. However, the mechanism by which antiviral RNAi in mammals is regulated is poorly understood. In this study, we uncovered that the E3 ubiquitin ligase STIP1 homology and U-box-containing protein 1 (STUB1) was a new regulator of the RNAi machinery in mammals. We found that STUB1 interacted with and ubiquitinated AGO2, and targeted it for degradation in a chaperon-dependent manner. STUB1 promoted the formation of Lys48 (K48)-linked polyubiquitin chains on AGO2, and facilitated AGO2 degradation through ubiquitin-proteasome system. In addition to AGO2, STUB1 also induced the protein degradation of AGO1, AGO3 and AGO4. Further investigation revealed that STUB1 also regulated Dicer's ubiquitination via K48-linked polyubiquitin and induced the degradation of Dicer as well as its specialized form, termed antiviral Dicer (aviDicer) that expresses in mammalian stem cells. Moreover, we found that STUB1 deficiency up-regulated Dicer and AGO2, thereby enhancing the RNAi response and efficiently inhibiting viral replication in mammalian cells. Using the newborn mouse model of Enterovirus A71 (EV-A71), we confirmed that STUB1 deficiency enhanced the virus-derived siRNAs production and antiviral RNAi, which elicited a potent antiviral effect against EV-A71 infection in vivo. In summary, our findings uncovered that the E3 ubiquitin ligase STUB1 was a general regulator of the RNAi machinery by targeting Dicer, aviDicer and AGO1–4. Moreover, STUB1 regulated the RNAi response through mediating the abundance of Dicer and AGO2 during viral infection, thereby providing novel insights into the regulation of antiviral RNAi in mammals.

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The E3 ubiquitin ligase, STUB1, is a novel regulator of the RNAi machinery in mammals.

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STUB1 induces ubiquitination and degradation of Dicer and AGO proteins in mammals.

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STUB1 deficiency enhances a potent antiviral effect against EV-A71 infection in vivo.

Carboxyl terminus of HSP70-interacting protein attenuates advanced glycation end products-induced cardiac injuries by promoting NFκB proteasomal degradation

Advanced glycation end products (AGEs), which are highly reactive molecules resulting from persistent high-glucose levels, can lead to the generation of oxidative stress and cardiac complications. The carboxyl terminus of HSP70 interacting protein (CHIP) has been demonstrated to have a protective role in several diseases, including cardiac complications; however, the role in preventing AGE-induced cardiac damages remains poorly understood. Here, we found that elevated AGE levels impaired cardiac CHIP expression in streptozotocin-induced diabetes and high-fat diet-administered animals, representing AGE exposure models. We used the TUNEL assay, hematoxylin and eosin, Masson's trichrome staining, and western blotting to prove that cardiac injuries were induced in diabetic animals and AGE-treated cardiac cells. Interestingly, our results collectively indicated that CHIP overexpression significantly rescued the AGE-induced cardiac injuries and promoted cell survival. Moreover, CHIP knockdown-mediated stabilization of nuclear factor κB (NFκB) was attenuated by overexpressing CHIP in the cells. Furthermore, co-immunoprecipitation and immunoblot assay revealed that CHIP promotes the ubiquitination and proteasomal degradation of AGE-induced NFκB. Importantly, fluorescence microscopy, a luciferase reporter assay, electrophoretic mobility shift assay, and subcellular fractionation further demonstrated that CHIP overexpression inhibits AGE-induced NFκB nuclear translocation, reduced its binding ability with the promoter sequences of the receptor of AGE, consequently inhibiting the translocation of the receptor AGE to the cell membrane for its proper function. Overall, our current study findings suggest that CHIP can target NFκB for ubiquitin-mediated proteasomal degradation, and thereby potentially rescue AGE-induced cardiac damages.

A histone modifier, ASXL1, interacts with NONO and is involved in paraspeckle formation in hematopoietic cells

Paraspeckles are membraneless organelles formed through liquid-liquid phase separation and consist of multiple proteins and RNAs, including NONO, SFPQ, and NEAT1. The role of paraspeckles and the component NONO in hematopoiesis remains unknown. In this study, we show histone modifier ASXL1 is involved in paraspeckle formation. ASXL1 forms phase-separated droplets, upregulates NEAT1 expression, and increases NONO-NEAT1 interactions through the C-terminal intrinsically disordered region (IDR). In contrast, a pathogenic ASXL mutant (ASXL1-MT) lacking IDR does not support the interaction of paraspeckle components. Furthermore, paraspeckles are disrupted and Nono localization is abnormal in the cytoplasm of hematopoietic stem and progenitor cells (HSPCs) derived from ASXL1-MT knockin mice. Nono depletion and the forced expression of cytoplasmic NONO impair the repopulating potential of HSPCs, as does ASXL1-MT. Our study indicates a link between ASXL1 and paraspeckle components in the maintenance of normal hematopoiesis.

An update on the molecular pathogenesis and potential therapeutic targeting of AML with t(8;21)(q22;q22.1);RUNX1-RUNX1T1

Acute myeloid leukemia (AML) with t(8;21)(q22;q22.1);RUNX1-RUNX1T1, one of the core-binding factor leukemias, is one of the most common subtypes of AML with recurrent genetic abnormalities and is associated with a favorable outcome. The translocation leads to the formation of a pathological RUNX1-RUNX1T1 fusion that leads to the disruption of the normal function of the core-binding factor, namely, its role in hematopoietic differentiation and maturation. The consequences of this alteration include the recruitment of repressors of transcription, thus blocking the expression of genes involved in hematopoiesis, and impaired apoptosis. A number of concurrent and cooperating mutations clearly play a role in modulating the proliferative potential of cells, including mutations in KIT, FLT3, and possibly JAK2. RUNX1-RUNX1T1 also appears to interact with microRNAs during leukemogenesis. Epigenetic factors also play a role, especially with the recruitment of histone deacetylases. A better understanding of the concurrent mutations, activated pathways, and epigenetic modulation of the cellular processes paves the way for exploring a number of approaches to achieve cure. Potential approaches include the development of small molecules targeting the RUNX1-RUNX1T1 protein, the use of tyrosine kinase inhibitors such as dasatinib and FLT3 inhibitors to target mutations that lead to a proliferative advantage of the leukemic cells, and experimentation with epigenetic therapies. In this review, we unravel some of the recently described molecular pathways and explore potential therapeutic strategies.

Emerging Evidences for an Implication of the Neurodegeneration-Associated Protein TAU in Cancer

Neurodegenerative disorders and cancer may appear unrelated illnesses. Yet, epidemiologic studies indicate an inverse correlation between their respective incidences for specific cancers. Possibly explaining these findings, increasing evidence indicates that common molecular pathways are involved, often in opposite manner, in the pathogenesis of both disease families. Genetic mutations in the MAPT gene encoding for TAU protein cause an inherited form of frontotemporal dementia, a neurodegenerative disorder, but also increase the risk of developing cancer. Assigning TAU at the interface between cancer and neurodegenerative disorders, two major aging-linked disease families, offers a possible clue for the epidemiological observation inversely correlating these human illnesses. In addition, the expression level of TAU is recognized as a prognostic marker for cancer, as well as a modifier of cancer resistance to chemotherapy. Because of its microtubule-binding properties, TAU may interfere with the mechanism of action of taxanes, a class of chemotherapeutic drugs designed to stabilize the microtubule network and impair cell division. Indeed, a low TAU expression is associated to a better response to taxanes. Although TAU main binding partners are microtubules, TAU is able to relocate to subcellular sites devoid of microtubules and is also able to bind to cancer-linked proteins, suggesting a role of TAU in modulating microtubule-independent cellular pathways associated to oncogenesis. This concept is strengthened by experimental evidence linking TAU to P53 signaling, DNA stability and protection, processes that protect against cancer. This review aims at collecting literature data supporting the association between TAU and cancer. We will first summarize the evidence linking neurodegenerative disorders and cancer, then published data supporting a role of TAU as a modifier of the efficacy of chemotherapies and of the oncogenic process. We will finish by addressing from a mechanistic point of view the role of TAU in de-regulating critical cancer pathways, including the interaction of TAU with cancer-associated proteins.

RUNX1 Dosage in Development and Cancer

The transcription factor RUNX1 first came to prominence due to its involvement in the t(8;21) translocation in acute myeloid leukemia (AML). Since this discovery, RUNX1 has been shown to play important roles not only in leukemia but also in the ontogeny of the normal hematopoietic system. Although it is currently still challenging to fully assess the different parameters regulating RUNX1 dosage, it has become clear that the dose of RUNX1 can greatly affect both leukemia and normal hematopoietic development. It is also becoming evident that varying levels of RUNX1 expression can be used as markers of tumor progression not only in the hematopoietic system, but also in non-hematopoietic cancers. Here, we provide an overview of the current knowledge of the effects of RUNX1 dosage in normal development of both hematopoietic and epithelial tissues and their associated cancers.

Visualizing ubiquitination in mammalian cells

Covalent modification of proteins with ubiquitin is essential for the majority of biological processes in mammalian cells. Numerous proteins are conjugated with single or multiple ubiquitin molecules or chains in a dynamic fashion, often determining protein half-lives, localization or function. Experimental approaches to study ubiquitination have been dominated by genetic and biochemical analysis of enzyme structure-function relationships, reaction mechanisms and physiological relevance. Here, we provide an overview of recent developments in microscopy-based imaging of ubiquitination, available reagents and technologies. We discuss the progress in direct and indirect imaging of differentially linked ubiquitin chains in fixed and living cells using confocal fluorescence microscopy and super-resolution microscopy, illustrated by the role of ubiquitin in antibacterial autophagy and pro-inflammatory signalling. Finally, we speculate on future developments and forecast a transition from qualitative to quantitative super-resolution approaches to understand fundamental aspects of ubiquitination and the formation and distribution of functional E3 ligase protein complexes in their native environment.

The ubiquitin ligase RNF38 promotes RUNX1 ubiquitination and enhances RUNX1-mediated suppression of erythroid transcription program

RUNX1 is a member of RUNX transcription factors and plays important roles in hematopoiesis. RUNX1 function is under the tight control through posttranslational modifications, including phosphorylation and ubiquitination. We previously developed a luminescence-based binding assay (AlphaScreen) to systematically detect RUNX1-interacting E3 ubiquitin ligases. In this study, we showed that a nuclear ubiquitin ligase RNF38 induced ubiquitination of RUNX1. RNF38-induced RUNX1 ubiquitination did not promote RUNX1 degradation, but rather stabilized RUNX1 protein. We also found that RNF38 enhanced RUNX1-mediated transcriptional repression of the erythroid master regulator KLF1 in K562 cells. Consequently, RNF38 cooperated with RUNX1 to inhibit erythroid differentiation of K562 cells. Thus, our study identified RNF38 as a novel E3 ligase that modifies RUNX1 function without inducing its degradation.

The nonreceptor tyrosine kinase c-Abl phosphorylates Runx1 and regulates Runx1-mediated megakaryocyte maturation

The transcription factor Runx1 is an essential regulator of definitive hematopoiesis, megakaryocyte (MK) maturation, and lymphocyte differentiation. Runx1 mutations that interfere with its transcriptional activity are often present in leukemia patients. Recent work demonstrated that the transcriptional activity of Runx1 is regulated by kinase-mediated phosphorylation. In this study, we showed that c-Abl, but not Arg tyrosine kinase, associated with Runx1 both in cultured cells and in vitro. c-Abl-mediated tyrosine phosphorylation in the Runx1 transcription inhibition domain negatively regulated the transcriptional activity of Runx1 and inhibited Runx1-mediated MK maturation. Consistent with these findings, increased numbers of MKs were detected in the spleens and bone marrow of abl gene conditional knockout mice. Our findings demonstrate an important role of c-Abl kinase in Runx1-mediated MK maturation and platelet formation and provide a potential mechanism of Abl kinase-regulated hematopoiesis.

Ubiquitin ligase CHIP functions as an oncogene and activates the AKT signaling pathway in prostate cancer

Carboxyl terminus of Hsc-70-interacting protein (CHIP) is an E3 ubiquitin ligase that induces the ubiquitination and degradation of numerous tumor-associated proteins and serves as a suppressor or promoter in tumor progression. To date, the molecular mechanism of CHIP in prostate cancer remains unknown. Therefore, the present study investigated the biological function of CHIP in prostate cancer cells and obtained evidence that CHIP expression is upregulated in prostate cancer tissues. The CHIP vector was introduced into DU145 cancer cells and the cell biological behaviour was examined through a series of experiments, including cell growth, cell apoptosis and migration and invasion assays. The results indicated that the overexpression of CHIP in DU145 prostatic cancer cells promoted cell proliferation through activation of the protein kinase B (AKT) signaling pathway, which subsequently increased cyclin D1 protein levels and decreased p21 and p27 protein levels. The overexpression of CHIP significantly increased the migration and invasion of the DU145 cells, which is possible due to activation of the AKT signaling pathway and upregulation of vimentin. The expression level of CHIP was observed to be increased in human prostate cancer tissues compared with the adjacent normal tissue. Furthermore, the CHIP expression level exhibited a positively association with the Gleason score of the patents. These findings indicate that CHIP functions as an oncogene in prostate cancer.