NONO ubiquitination is mediated by FBW7 and GSK3 β via a degron lost upon chromosomal rearrangement in cancer

FBXW7 in breast cancer: mechanism of action and therapeutic potential

Breast cancer is one of the frequent tumors that seriously endanger the physical and mental well-being in women. F-box and WD repeat domain-containing 7 (FBXW7) is a neoplastic repressor. Serving as a substrate recognition element for ubiquitin ligase, FBXW7 participates in the ubiquitin-proteasome system and is typically in charge of the ubiquitination and destruction of crucial oncogenic proteins, further performing a paramount role in cell differentiation, apoptosis and metabolic processes. Low levels of FBXW7 cause abnormal stability of pertinent substrates, mutations and/or deletions in the FBXW7 gene have been reported to correlate with breast cancer malignant progression and chemoresistance. Given the lack of an effective solution to breast cancer's clinical drug resistance dilemma, elucidating FBXW7's mechanism of action could provide a theoretical basis for targeted drug exploration. Therefore, in this review, we focused on FBXW7's role in a range of breast cancer malignant behaviors and summarized the pertinent cellular targets, signaling pathways, as well as the mechanisms regulating FBXW7 expression. We also proposed novel perspectives for the exploitation of alternative therapies and specific tumor markers for breast cancer by therapeutic strategies aiming at FBXW7.

The Role of FBXW7 in Gynecologic Malignancies

The F-Box and WD Repeat Domain Containing 7 (FBXW7) protein has been shown to regulate cellular growth and act as a tumor suppressor. This protein, also known as FBW7, hCDC4, SEL10 or hAGO, is encoded by the gene FBXW7. It is a crucial component of the Skp1-Cullin1-F-box (SCF) complex, which is a ubiquitin ligase. This complex aids in the degradation of many oncoproteins, such as cyclin E, c-JUN, c-MYC, NOTCH, and MCL1, via the ubiquitin-proteasome system (UPS). The FBXW7 gene is commonly mutated or deleted in numerous types of cancer, including gynecologic cancers (GCs). Such FBXW7 mutations are linked to a poor prognosis due to increased treatment resistance. Hence, detection of the FBXW7 mutation may possibly be an appropriate diagnostic and prognostic biomarker that plays a central role in determining suitable individualized management. Recent studies also suggest that, under specific circumstances, FBXW7 may act as an oncogene. There is mounting evidence indicating that the aberrant expression of FBXW7 is involved in the development of GCs. The aim of this review is to give an update on the role of FBXW7 as a potential biomarker and also as a therapeutic target for novel treatments, particularly in the management of GCs.

RNA-binding proteins as drivers of AML and novel therapeutic targets

Acute myeloid leukemia (AML) is a group of genetically complex and heterogeneous invasive hematological malignancies with a low 5-year overall survival rate of 30%, which highlights the urgent need for improved treatment measures. RNA-binding proteins (RBPs) regulate the abundance of isoforms of related proteins by regulating RNA splicing, translation, stability, and localization, thereby affecting cell differentiation and self-renewal. It is increasingly believed that RBPs are essential for normal hematopoiesis, and RBPs play a key role in hematological tumors, especially AML, by acting as oncogenes or tumor suppressors. In addition, targeting an RBP that is significantly related to AML can trigger the apoptosis of leukemic stem cells or promote the proliferation of stem and progenitor cells by modulating the expression of important pathway regulatory factors such as HOXA9, MYC, and CDKN1A. Accordingly, RBPs involved in normal myeloid differentiation and the occurrence of AML may represent promising therapeutic targets.

Targeting the p300/NONO axis sensitizes melanoma cells to BRAF inhibitors

BRAF inhibitors (BRAFi) that target BRAF V600E kinase, a driver mutation found in 50% of melanomas, show a significant antitumor response, but the common emergence of acquired resistance remains a challenge. Abnormal expression of RAF isoforms CRAF and ARAF reactivates pERK1/2, which plays crucial roles in the acquisition of resistance of melanoma cells. However, the mechanisms of dysregulation of RAF isoforms in resistant melanoma cells remain unknown. Here, we identified NONO interacted with and stabilized both CRAF and ARAF in melanoma cells, and that NONO was acetylated at 198K by p300 acetyltransferase, which stabilized NONO via antagonizing its ubiquitination/degradation mediated by RNF8. The upregulation of both p300 and NONO promoted the rebound of pERK1/2 and the subsequent resistance of melanoma cells to BRAFi, and the activation of ERK1/2 in turn induced p300 to form a positive feedback loop in resistant melanoma cells. There was a positive correlation between p300 and NONO in resistant melanoma cells and clinical samples, and p300 inhibitor C646 overcame the resistance of resistant melanoma cells to BRAF inhibitors in vitro and in vivo. Our findings reveal that targeting the positive feedback loop of p300-NONO-CRAF/ARAF-pERK1/2 may be excellent strategies to overcome the resistance of BRAF inhibitors for melanoma patients.

The positive regulation loop between NRF1 and NONO-TFE3 fusion promotes phase separation and aggregation of NONO-TFE3 in NONO-TFE3 tRCC

TFE3 gene fusions often place TFE3 under the control of a more active promoter and cause overexpression of the TFE3 proteins in renal cell carcinoma associated with Xp11.2 translocations (Xp11.2 tRCC). The purpose of this study was to investigate the transcriptional regulation and aggregation mechanism of NONO-TFE3 in NONO-TFE3 tRCC. In this study, we found that the nuclear aggregation of NONO-TFE3 fusion was significantly more than that of intact TFE3 or PRCC-TFE3 fusion. We observed that NONO fragment mediated-phase separation promoted stabilization and aggregation of NONO-TFE3 fusion. Meantime, we revealed that the positive regulation loop between NONO-TFE3 and NRF1 increased mitochondrial biosynthesis and metabolism in NONO-TFE3 tRCC. Therefore, the present study raises the possibility that mitochondrial metabolism is potentially a fruitful arena for NONO-TFE3 tRCC therapy.

NONO Is a Negative Regulator of SOX2 Promoter

BACKGROUND/AIM

Sex determining region Y (SRY)-box 2 (SOX2) is a transcription factor essential for the maintenance of proliferation and self-renewal of cancer stem cells and is associated with breast cancer initiation. Regulation of cancer stem cell plasticity by SOX2 requires both positive and negative SOX2 transcription factors, but the negative regulator is still largely unknown.

MATERIALS AND METHODS

SOX2 promoter-binding proteins were identified by liquid chromatography-mass spectrometry/mass spectrometry, luciferase assay, and chromatin immunoprecipitation. The effects of one such transcription factor on SOX2 expression was investigated by knockdown and overexpression experiments.

RESULTS

Non-POU domain-containing octamer-binding protein (NONO) (also known as 54-kDa nuclear RNA-binding protein, P54NRB) was identified as a SOX2 promoter-binding protein and a negative regulator of SOX2 expression. Its activity was controlled by its coiled-coil domain and the C-terminal domain.

CONCLUSION

These results suggest that NONO acts as a key regulator of SOX2 transcription through the repression of SOX2 promoter activity in breast cancer cells.

PRMT1 enhances oncogenic arginine methylation of NONO in colorectal cancer

Arginine methylation is an important posttranslational modification catalyzed by protein arginine methyltransferases (PRMTs). However, the role of PRMTs in colorectal cancer (CRC) progression is not well understood. Here we report that non-POU domain-containing octamer-binding protein (NONO) is overexpressed in CRC tissue and is a potential marker for poor prognosis in CRC patients. NONO silencing resulted in decreased proliferation, migration, and invasion of CRC cells, whereas overexpression had the opposite effect. In a xenograft model, tumors derived from NONO-deficient CRC cells were smaller than those derived from wild-type (WT) cells, and PRMT1 inhibition blocked CRC xenograft progression. A mass spectrometry analysis indicated that NONO is a substrate of PRMT1. R251 of NONO was asymmetrically dimethylated by PRMT1 in vitro and in vivo. Compared to NONO WT cells, NONO R251K mutant-expressing CRC cells showed reduced proliferation, migration, and invasion, and PRMT1 knockdown or pharmacological inhibition abrogated the malignant phenotype associated with NONO asymmetric dimethylation in both KRAS WT and mutant CRC cells. Compared to adjacent normal tissue, PRMT1 was highly expressed in the CRC zone in clinical specimens, which was correlated with poor overall survival in patients with locally advanced CRC. These results demonstrate that PRMT1-mediated methylation of NONO at R251 promotes CRC growth and metastasis, and suggest that PRMT1 inhibition may be an effective therapeutic strategy for CRC treatment regardless of KRAS mutation status.

The regulation of NONO by USP11 via deubiquitination is linked to the proliferation of melanoma cells

Ubiquitin‐specific protease 11 (USP11) has been implicated in the regulation of DNA repair, apoptosis, signal transduction and cell cycle. It belongs to a USP subfamily of deubiquitinases. Although previous research has shown that USP11 overexpression is frequently found in melanoma and is correlated with a poor prognosis, the potential molecular mechanism of USP11 in melanoma remains indefinitive. Here, we report that USP11 and NONO colocalize and interact with each other in the nucleus of melanoma cells. As a result, the knockdown of USP11 decreases NONO levels. Whereas, overexpression of USP11 increases NONO levels in a dose‐dependent manner. Furthermore, we reveal that USP11 protects NONO protein from proteasome‐mediated degradation by removing poly‐ubiquitin chains conjugated onto NONO. Functionally, USP11 mediated melanoma cell proliferation via the regulation of NONO levels because ablation of USP11 inhibits the proliferation which could be rescued by ectopic expression of NONO protein. Moreover, a significant positive correlation between USP11 and NONO concentrations was found in clinical melanoma samples. Collectively, these results demonstrate that USP11 is a new deubiquitinase of NONO and that the signalling axis of USP11‐NONO is significantly involved in melanoma proliferation.

NONO and tumorigenesis: More than splicing

The non-POU domain-containing octamer-binding protein NONO/p54^nrb , which belongs to the Drosophila behaviour/human splicing (DBHS) family, is a multifunctional nuclear protein rarely functioning alone. Emerging solid evidences showed that NONO engages in almost every step of gene regulation, including but not limited to mRNA splicing, DNA unwinding, transcriptional regulation, nuclear retention of defective RNA and DNA repair. NONO is involved in many biological processes including cell proliferation, apoptosis, migration and DNA damage repair. Dysregulation of NONO has been found in many types of cancer. In this review, we summarize the current and fast-growing knowledge about the regulation of NONO, its biological function and implications in tumorigenesis and cancer progression. Overall, significant findings about the roles of NONO have been made, which might make NONO to be a new biomarker or/and a possible therapeutic target for cancers.

Recent insight into the role of FBXW7 as a tumor suppressor

FBXW7 (also known as Fbw7, Sel10, hCDC4, or hAgo) is a tumor suppressor and the most frequently mutated member of the F-box protein family in human cancers. FBXW7 functions as the substrate recognition component of an SCF-type E3 ubiquitin ligase. It specifically controls the proteasome-mediated degradation of many oncoproteins such as c-MYC, NOTCH, KLF5, cyclin E, c-JUN, and MCL1. In this review, we summarize the molecular and biological features of FBXW7 and its substrates as well as the impact of mutations of FBXW7 on cancer development. We also address the clinical potential of anticancer therapy targeting FBXW7.

Long non-coding RNA SSTR5-AS1 facilitates gemcitabine resistance via stabilizing NONO in gallbladder carcinoma

Gallbladder carcinoma (GBC) is the most aggressive carcinoma of the biliary tract, effective chemotherapy was critical for the patients with unresectable GBC. However, chemotherapy resistance is still problematic for clinicians. Here, we identified a specific long non-coding RNA, SSTR5-AS1, in GBC patient that facilitates gemcitabine resistance. SSTR5-AS1 is significantly increased in GBC samples and cell lines, especially in gemcitabine-resistant cell lines, and higher SSTR5-AS1 expression was correlated with poorer overall survival rate in GBC patients. Our data revealed that upregulated SSTR5-AS1 facilitates gemcitabine resistance via inhibiting apoptosis. Knockdown of SSTR5-AS1 sensitized drug resistant GBC cells to gemcitabine in vitro and strongly inhibited xenografts formed by drug resistant GBC cells in vivo. Moreover, we found via streptavidin pull down assay that NONO specifically binds to sense sequence of SSTR5-AS1 and prevented proteasome mediated NONO degradation, which resulted in increased NONO protein level without affecting the transcription of NONO. NONO functions as the downstream effector of SSTR5-AS1 and is required for SSTR5-AS1 mediated gemcitabine resistance. Collectively, our data provided novel insights into lncRNA-mediated chemotherapy resistance and suggested a novel therapeutic target to improve chemotherapy strategies for unresectable GBC patients.