Ubiquitylation of RUNX3 by RNA-binding ubiquitin ligase MEX3C promotes tumorigenesis in lung adenocarcinoma.

Lung adenocarcinoma (LUAD) is the most common pathological type of lung cancer, but the early diagnosis rate is low. The RNA-binding ubiquitin ligase MEX3C promotes tumorigenesis in several cancers but its mechanism of action in LUAD is unclear. In this study, the biological activity of MEX3C was assessed in LUAD. MEX3C and RUNX3 mRNA levels in the tissues of LUAD patients were determined using reverse transcription­quantitative PCR. The involvement of MEX3C in the growth and metastasis of LUAD cells was measured by EdU assay, CCK-8, colony formation, Transwell assay, TUNEL, and flow cytometry. Expression of apoptosis and epithelial-mesenchymal transition related proteins were determined using western blotting analysis. LUAD cells transfected with si-MEX3C were administered to mice subcutaneously to monitor tumor progression and metastasis. We found that MEX3C is strongly upregulated in LUAD tissue sections, and involved in proliferation and migration. A549 and H1299 cells had significantly higher levels of MEX3C expression compared to control HBE cells. Knockdown of MEX3C dramatically decreased cell proliferation, migration, and invasion, and accelerated apoptosis. Mechanistically, we demonstrate MEX3C induces ubiquitylation and degradation of tumor suppressor RUNX3. Moreover, RUNX3 transcriptionally represses Suv39H1, as revealed by RNA pull-down and chromatin immunoprecipitation assays. The in vivo mice model demonstrated that knockdown of MEX3C reduced LUAD growth and metastasis significantly. Collectively, we reveal a novel MEX3C-RUNX3-Suv39H1 signaling axis driving LUAD pathogenesis. Targeting MEX3C may represent a promising therapeutic strategy against LUAD.

Hsa_circ_0008133 contributes to lung cancer progression by promoting glycolysis metabolism through the miR-760/MEX3A axis.

BACKGROUND: Lung cancer is a very common cancer with poor prognosis and high mortality. Circular RNAs (circRNAs) have been confirmed to be related to the occurrence of lung cancer, and circ_0008133 has been found to be possibly related to lung cancer. METHODS: Expression of circ_0008133, miR-760, and mex-3 RNA binding family member A (MEX3A) messenger RNA (mRNA) was detected using quantitative real-time polymerase chain reaction (qRT-PCR). Cell viability, colony number, migration, and invasion were assessed using cell counting kit-8 (CCK8), colony formation, wound healing, and transwell assays. Glucose consumption and lactate production were detected using commercial kits. Protein expression was measured using western blot. Dual-luciferase reporter assay and RNA pull-down assay were used to analyze the relationships between miR-760 and circ_0008133 or MEX3A. The effects of circ_0008133 knockdown on tumor growth in vivo were examined by the nude mice expriment. Immunohistochemistry (IHC) assay analyzed Ki-67 expression. RESULTS: Circ_0008133 and MEX3A were markedly boosted in lung cancer tissues and cells. Circ_0008133 knockdown decreased lung cancer cell viability, glucose consumption, lactate production, colony formation, migration, and invasion. In mechanism, circ_0008133 might positively regulate MEX3A expression by sponging miR-760. Additionally, knockdown of circ_0008133 inhibited tumor growth in vivo. CONCLUSION: Circ_0008133 accelerated the progression of lung cancer by promoting glycolysis metabolism through the miR-760/MEX3A axis.

circGRAMD1B contributes to migration, invasion and epithelial-mesenchymal transition of lung adenocarcinoma cells via modulating the expression of SOX4.

Lung adenocarcinoma (LUAD) represents the subtype of non-small-cell lung cancer (NSCLC), with the high morbidity over the world. Mounting studies have highlighted the important roles of circular RNAs (circRNA) in cancers, including LUAD. This study mainly focused on revealing the role of circGRAMD1B and its relevant regulatory mechanism in LUAD cells. RT-qPCR and Western blot were conducted to detect the expression of target genes. Function assays were performed to determine the effect of related genes on migration, invasion, and epithelial-mesenchymal transition (EMT) of LUAD cells. Mechanism analyses were conducted to figure out the specific mechanism with regard to circGRAMD1B and its downstream molecules as well. Based on the experimental results, circGRAMD1B was upregulated in LUAD cells and promoted the migration, invasion, and EMT of LUAD cells. Mechanically, circGRAMD1B sponged miR-4428 to upregulate the expression of SOX4. In addition, SOX4 activated the expression of MEX3A at the transcriptional level, thereby modulating PI3K/AKT pathway to facilitate LUAD cell malignant behaviors. In conclusion, circGRAMD1B is discovered to modulate miR-4428/SOX4/MEX3A axis to further activate PI3K/AKT pathway, finally boosting migration, invasion, and EMT of LUAD cells.

The RNA binding protein MEX3A promotes tumor progression of breast cancer by post-transcriptional regulation of IGFBP4.

PURPOSE: Breast cancer (BC) is the most frequent malignant tumor in women worldwide with exceptionally high morbidity. The RNA-binding protein MEX3A plays a crucial role in genesis and progression of multiple cancers. We attempted to explore its clinicopathological and functional significance in BC in which MEX3A is expressed. METHODS: The expression of MEX3A detected by RT-qPCR and correlated the results with clinicopathological variables in 53 BC patients. MEX3A and IGFBP4 profile data of BC patients were downloaded from TCGA and GEO database. Kaplan-Meier (KM) analysis was used to estimate the survival rate of BC patients. Western Blot, CCK-8, EdU, colony formation and flow cytometry were performed to investigate the role of MEX3A and IGFBP4 in BC cell proliferation, invasion and cell cycle in vitro. A subcutaneous tumor mouse model was constructed to analyze in vivo growth of BC cells after MEX3A knockdown. The interactions among MEX3A and IGFBP4 were measured by RNA pull-down and RNA immunoprecipitation. RESULTS: The expression of MEX3A was upregulated in BC tissues compared to adjacent tissues and high expression of MEX3A was associated with poor prognosis. Subsequent in vitro studies demonstrated that MEX3A knockdown inhibited BC cells proliferation and migration, as well as xenograft tumor growth in vivo. The expression of IGFBP4 was significantly negatively correlated with MEX3A in BC tissues. Mechanistic investigation showed that MEX3A binds to IGFBP4 mRNA in BC cells, decreasing IGFBP4 mRNA levels, which further activated the PI3K/AKT and other downstream signaling pathways implicated cell cycle progression and cell migration. CONCLUSION: Our results indicate that MEX3A plays a prominent oncogenic role in BC tumorigenesis and progression by targeting IGFBP4 mRNA and activating PI3K/AKT signaling, which can be used as a novel therapeutic target for BC.

MEX3C as a potential target for hepatocellular carcinoma drug and immunity: combined therapy with Lenvatinib.

BACKGROUND: The immune microenvironment within hepatocellular carcinoma (HCC) is remarkably intricate. Although the combination of an immune checkpoint inhibitor and Lenvatinib can extend the overall survival of HCC patients, the outcome remains suboptimal. METHODS: We assessed alterations in MEX3C expression during hepatocarcinogenesis by validating multiple databases and subsequently developed a predictive model. Subsequently, we enriched the associated genes of MEX3C to investigate its functional role. We examined the correlation between MEX3C expression levels and immune infiltrating cells. The effects of MEX3C knockdown and Lenvatinib on hepatoma cells were observed by cell function experiments. RESULTS: MEX3C expression is elevated in HCC compared to normal tissues, and its high expression correlates with poor prognosis. Immune checkpoint expression was elevated in the high MEX3C expression group, concomitant with heightened myeloid-derived suppressor cell (MDSC) expression. The combination of MEX3C knockdown and Lenvatinib demonstrated a stronger inhibitory effect on HCC cells compared to Lenvatinib alone. CONCLUSION: MEX3C shows promise as a potential therapeutic target for treating HCC. Furthermore, the combination of MEX3C knockdown and Lenvatinib could offer a novel therapeutic avenue for HCC treatment.

MEX3A induces the development of thyroid cancer via targeting CREB1.

Thyroid cancer is a prevalent form of endocrine cancer, and its global incidence has been steadily increasing. MEX3A is a protein that is known to be highly expressed in various human malignant tumors, including thyroid cancer, and it has been linked to patient prognosis. However, the molecular mechanisms underlying MEX3A's tumorigenic capabilities in thyroid cancer are not fully understood. In this study, we aimed to investigate the role of MEX3A in thyroid cancer. We confirmed that MEX3A was overexpressed in both thyroid cancer tissues and cell lines. Additionally, we found a positive correlation between high levels of MEX3A and the AJCC stage. To further understand the functional significance of MEX3A in thyroid cancer, we depleted MEX3A expression in B-CPAP and TPC-1 cells. Interestingly, we observed a significant reduction in thyroid cancer cell proliferation and migration, as well as ameliorated cell apoptosis and arrested tumor growth upon MEX3A depletion. These findings strongly suggested that MEX3A played a critical role in the development of thyroid cancer. Furthermore, our study uncovered an important interaction between MEX3A and CREB1 (cAMP response element-binding protein 1). The interaction between MEX3A and CREB1 appeared to contribute to the tumor-promoting effects of MEX3A in thyroid cancer by directly targeting CREB1. Silencing CREB1 was observed to alleviate the malignant phenotypes promoted by MEX3A in thyroid cancer cells. Together, this study highlighted the importance of the MEX3A-CREB1 interaction in thyroid cancer development and suggested the therapeutic potential of targeting MEX3A for the treatment of this disease.

Grass carp (Ctenopharyngodon idella) Mex3B positively regulates innate immunity by promoting the K63-linked ubiquitination of TLR3.

As a member of Mex3 (muscle excess protein-3) family, Mex3B (Mex-3 RNA binding family member B) is crucial in cell proliferation and migration in mammals. In this study, an ortholog of mammalian Mex3B (denominated CiMex3B, MT276802.1) was cloned and identified in grass carp (Ctenopharyngodon idella). CiMex3B is 1578 bp in length and encodes a polypeptide of 525 amino acids. Consistent with its mammalian counterpart, CiMex3B also contains one C-terminal RING domain and two N-terminal conserved tandem KH domains. CiMex3B up-regulates the expressions of IFN1, ISG15, MX2, as well as the expressions of inflammatory cytokines such as IL6, IL8 and TNFα in response to poly(I:C). A screening test for identifying potential targets indicated that CiMex3B is associated with TLR3 and TRIF. CiMex3B co-localizes with TLR3 in the late endosome, mitochondria and endoplasmic reticulum after poly(I:C) stimulation, whereas they are rarely discovered in the lysosomes. CiMex3B serves as a positive regulator in the phosphorylation of IRF3 and induces IFN1 expression. In addition, two truncation mutants of CiMex3B (1-220 and 221-525) were constructed to better understand the molecular mechanism of CiMex3B-mediated ubiquitination of TLR3. In line with wild-type protein, CiMex3B mutant (1-220) was found mainly in the cytoplasm; however, CiMex3B mutant (221-525) resided in the cytoplasm and the nucleus as well, and it was further confirmed that CiMex3B mutant (221-525) still interacts with TLR3. We also observed that CiMex3B promotes the K63-linked ubiquitination of TLR3, while neither of the truncation mutants (1-220 or 221-525) retains this activity. To sum up, this study revealed that CiMex3B potentiates the K63-linked ubiquitination of TLR3, and then elicits the IRF3-mediated antiviral innate immune responses.

MEX3A regulates Lgr5+ stem cell maintenance in the developing intestinal epithelium.

Intestinal stem cells (ISCs) fuel the lifelong self-renewal of the intestinal tract and are paramount for epithelial repair. In this context, the Wnt pathway component LGR5 is the most consensual ISC marker to date. Still, the effort to better understand ISC identity and regulation remains a challenge. We have generated a Mex3a knockout mouse model and show that this RNA-binding protein is crucial for the maintenance of the Lgr5+ ISC pool, as its absence disrupts epithelial turnover during postnatal development and stereotypical organoid maturation ex vivo. Transcriptomic profiling of intestinal crypts reveals that Mex3a deletion induces the peroxisome proliferator-activated receptor (PPAR) pathway, along with a decrease in Wnt signalling and loss of the Lgr5+ stem cell signature. Furthermore, we identify PPARγ activity as a molecular intermediate of MEX3A-mediated regulation. We also show that high PPARγ signalling impairs Lgr5+ ISC function, thus uncovering a new layer of post-transcriptional regulation that critically contributes to intestinal homeostasis.

MEX3A promotes development and progression of breast cancer through regulation of PIK3CA.

Breast cancer has been identified as the most common malignant tumors among women and the morbidity of breast cancer is still increasing rapidly. MEX3A possesses important functions in the regulation of mRNAs and may be involved in a variety of human diseases including cancer, whose relationship with breast cancer is still not clear. In this study, MEX3A was identified as a potential promotor in breast cancer, whose expression was strongly higher in breast cancer tissues than normal tissues. The in vitro experiments showed that MEX3A is capable of promoting the development of breast cancer through stimulating cell proliferation, inhibiting cell apoptosis, arresting cell cycle and promoting cell migration. The functions of MEX3A were also verified in vivo. Furthermore, a combination of genechip analysis and Ingenuity pathway analysis (IPA) identified PIK3CA as a potential downstream target of MEX3A, knockdown of which executes similar inhibitory effects on breast cancer and could alleviate MEX3A-induced progression of breast cancer. In conclusion, our study unveiled, as the first time, MEX3A as a tumor promotor for breast cancer, whose function was carried out probably through the regulation of PIK3CA.

SMYD2 epigenetically activates MEX3A and suppresses CDX2 in colorectal cancer cells to augment cancer growth.

Dysfunction of the protein methyltransferase SET and MYND domain-containing protein 2 (SMYD2) is frequently linked to multiple diseases including cancer. The study focused on the role of SMYD2 in colorectal cancer (CRC) development. SMYD2 was expressed at high levels in CRC tissues and cells. Knockdown of SMYD2 in LOVO cells reduced cell proliferation, migration and invasiveness in vitro and it suppressed xenograft tumorigenesis in vivo. Overexpression of SMYD2 in HCT116 cells led to inverse trends. Mex-3 RNA binding family member A (MEX3A) was predicted as a target of SMYD2. Chromatin immunoprecipitation (ChIP)-reverse transcription quantitative polymerase chain reaction (qPCR) and cellular assays were performed and validated that SMYD2 activated MEX3A expression by promoting H3K36me2 modification on its promoter. Data in the STRING bioinformatics system indicated caudal type homeobox 2 (CDX2) as an important MEX3A-related gene. Silencing of MEX3A alone blocked proliferation and growth of CRC cells in vitro and in vivo, whereas MEX3A overexpression promoted cell growth by suppressing CDX2. In rescue experiments, MEX3A silencing suppressed the cell growth augmented by SMYD2, and CDX2 downregulation restored the malignance of cancer cells inhibited by MEX3A silencing. Taken together, this study reports that SMYD2-mediated activation of MEX3A augments progression of CRC by suppressing CDX2.

RNA-Binding Protein MEX3A Interacting with DVL3 Stabilizes Wnt/ß-Catenin Signaling in Endometrial Carcinoma.

Disease recurrence and metastasis lead to poor prognosis in patients with advanced endometrial carcinoma (EC). RNA-binding proteins (RBPs) are closely associated with tumor initiation and metastasis, but the function and molecular mechanisms of RBPs in EC are unclear. RBPs were screened and identified using the TCGA, GEO, and RBPTD databases. The effect of MEX3A on EC was verified by in vitro and in vivo experiments. Gene set enrichment analysis (GSEA), immunofluorescence (IF), and co-immunoprecipitation (Co-IP) were used to identify potential molecular mechanisms of action. We identified 148 differentially expressed RBPs in EC. MEX3A was upregulated and related to poor prognosis in patients with EC. In vitro and vivo experiments demonstrated that MEX3A promoted the growth, migration, and invasion capacities of EC cells. Mechanistically, DVL3, a positive regulator of the Wnt/ß-catenin pathway, also increased the proliferation and metastasis of EC cells. MEX3A enhanced EMT and played a pro-carcinogenic role by interacting with DVL3 to stabilize ß-catenin and upregulated the expression of its downstream target genes. MEX3A is upregulated in EC and promotes tumor progression by activating EMT and regulating the Wnt/ß-catenin pathway via DVL3. MEX3A may therefore be a novel therapeutic target for EC.

MEX3D is an oncogenic driver in prostate cancer.

BACKGROUND: Prostate cancer (PCa) is the most common visceral malignancy and the second leading cause of cancer deaths in US men. The two most common genetic alterations in PCa are expression of the TMPRSS2/ERG (TE) fusion gene and loss of the PTEN tumor suppressor. These genetic alterations act cooperatively to transform prostatic epithelium but the exact mechanisms involved are unclear. METHODS: Microarray expression analysis of immortalized prostate epithelial cells transformed by loss of PTEN and expression of the TE fusion revealed MEX3D as one of the most highly upregulated genes. MEX3D expression in prostate cancer was examined in patient samples and in silico. In vitro and in vivo studies to characterize the biological impact of MEX3D were carried out. Analysis of the TCGA PanCancer database revealed TCF3 as a major target of MEX3D. The induction of TCF3 by MEX3D was confirmed and the biological impact of TCF3 examined by in vitro studies. RESULTS: MEX3D is expressed at increased levels in prostate cancer and is increased by decreased PTEN and/or expression of the TE fusion gene and drives soft agar colony formation, invasion and tumor formation in vivo. The known oncogenic transcription factor TCF3 is highly correlated with MEX3D in prostate cancer. MEX3D expression strongly induces TCF3, which promotes soft agar colony formation and invasion in vitro. CONCLUSIONS: Loss of PTEN and expression of the TE fusion gene in prostate cancer strongly upregulates expression of MEX3D and its target TCF3 and promotes transformation associated phenotypes via this pathway.

The effects of MEX3A knockdown on proliferation, apoptosis and migration of osteosarcoma cells.

BACKGROUND: Osteosarcoma is an aggressive malignant tumor which has attracted worldwide attention. MEX3A may be associated with tumors while has not yet seen its coverage on osteosarcoma. Herein, this study was to investigate the correlation between MEX3A and the progression of osteosarcoma. METHODS: Firstly, we determined that expression of MEX3A was significantly higher in osteosarcoma tissues than that in marginal bone by immunohistochemical staining. Additionally, MEX3A expression was downregulated by the RNAi-mediated knockdown. The functions of MEX3A knockdown on proliferation, apoptosis, cell cycle, migration was assessed by MTT assay, flow cytometry, wound-healing assay and Transwell assay, respectively. Knockdown of MEX3A resulted in suppressing cell proliferation, increasing cell apoptosis, inducing the G2 phase cell cycle arrest, and attenuating cellular migration. Furthermore, mouse xenograft model confirmed inhibitory effects of MEX3A knockdown on osteosarcoma formation. RESULTS: The preliminary exploration on the molecular mechanism of MEX3A in osteosarcoma cells showed that the induction of apoptosis needs the participation of a series of apoptosis- associated factors, such as upregulation of Caspase 3, Caspase 8 and HSP60, downregulation of HSP27 and XIAP. CONCLUSIONS: In summary, these findings predicated that therapy directed at decreasing MEX3A expression is a potential osteosarcoma treatment.

MEX3A promotes triple negative breast cancer proliferation and migration via the PI3K/AKT signaling pathway.

Triple-negative breast cancer (TNBC) has the characteristics of fast growth, easy invasion, metastasis, poor prognosis, low tumor-free survival rate and overall survival rate. In this study, the RNA-binding protein MEX3A was selected by using the methods of TCGA database analysis, mRNA microarrays, and tissue chip immunohistochemistry experiments. The high expression of MEX3A is associated with malignancy and poor prognosis of TNBC. In addition, MEX3A knockdown can inhibit the growth and migration of TNBC cells while MEX3A overexpression shows the opposite effect. In vivo experiments, we also demonstrated that downregulating MEX3A can inhibit the tumorigenicity of TNBC cells. The mRNA microarrays and Ingenuity pathway analysis (IPA) were used to explore the downstream of MEX3A, and verified the relationship between PI3K/AKT signaling pathway and MEX3A. Additionally, we have simultaneously up-regulated MEX3A and treated with pathway inhibitors in vitro experiments and found that it can slow down the growth of TNBC cells. In short, we identified MEX3A as a tumor promoter, potential prognostic indicator and therapeutic target for TNBC, may function through the regulation of the PI3K/AKT signaling pathway.

The IL-37-Mex3B-Toll-like receptor 3 axis in epithelial cells in patients with eosinophilic chronic rhinosinusitis with nasal polyps.

BACKGROUND: The role of IL-37, an immunosuppressive cytokine, in patients with inflammatory diseases is unclear. OBJECTIVE: We sought to explore the expression and pathogenic function of IL-37 in patients with chronic rhinosinusitis (CRS). METHODS: Expression levels of IL-37, IL-18 receptor α, IL-1 receptor 8, Mex3 RNA binding family member B (Mex3B), and thymic stromal lymphopoietin (TSLP) in nasal samples were studied by using quantitative RT-PCR, immunohistochemistry, Western blotting, and ELISA. Human nasal epithelial cells (HNECs) and the BEAS-2B cell line were stimulated with various cytokines and Toll-like receptor (TLR) agonists. In some experiments BEAS-2B cells were transfected with Mex3B small interfering RNA or overexpressing lentiviruses. Genes regulated by IL-37b in HNECs were studied by using RNA sequencing analysis. IL-37b function was confirmed in mice in vivo. RESULTS: Compared with control subjects, although mRNA and protein expression of IL-37 were upregulated in diseased tissues, especially in nasal epithelial cells, in patients with CRS without nasal polyps or in patients with chronic rhinosinusitis with nasal polyps (CRSwNP), IL-37 levels in nasal secretions were reduced in patients with eosinophilic CRSwNP. Type 2 cytokines inhibited IL-37 secretion from HNECs. HNECs expressed IL-37 receptors, IL-18 receptor α, and IL-1 receptor 8. IL-37b downregulated the expression of Mex3B, a TLR3 coreceptor, in HNECs. IL-37b suppressed polyinosinic-polycytidylic acid-induced TSLP production in HNECs in vitro and in murine nasal epithelial cells in vivo. Knocking down or overexpressing Mex3B in BEAS-2B cells abolished the inhibitory effect of IL-37b. Secreted IL-37 levels negatively correlated with Mex3B and TSLP levels and eosinophil numbers in patients with eosinophilic CRSwNP. CONCLUSIONS: The suppressed IL-37 secretion caused by a type 2 milieu can enhance Mex3B-mediated TLR3 activation and subsequent TSLP production in nasal epithelial cells and therefore promotes eosinophilic inflammation in patients with CRSwNP.

MEX3A knockdown inhibits the development of pancreatic ductal adenocarcinoma.

BACKGROUND: Pancreatic ductal adenocarcinoma (PDA) is one of the most serious causes of death in the world due to its high mortality and inefficacy treatments. MEX3A was first identified in nematodes and was associated with tumor formation and may promote cell proliferation and tumor metastasis. So far, nothing is known about the relationship between MEX3A and PDA. METHODS: In this study, the expression level of MEX3A in PDA tissues was measured by immunohistochemistry. The qRT-PCR and western blot were used to identify the constructed MEX3A knockdown cell lines, which was further used to construct mouse xenotransplantation models. Cell proliferation, colony formation, cell apoptosis and migration were detected by MTT, colony formation, flow cytometry and Transwell. RESULTS: This study showed that MEX3A expression is significantly upregulated in PDA and associated with tumor grade. Loss-of-function studies showed that downregulation of MEX3A could inhibit cell growth in vitro and in vivo. Moreover, it was demonstrated that knockdown of MEX3A in PDA cells promotes apoptosis by regulating apoptosis-related factors, and inhibits migration through influencing EMT. At the same time, the regulation of PDA progression by MEX3A involves changes in downstream signaling pathways including Akt, p-Akt, PIK3CA, CDK6 and MAPK9. CONCLUSIONS: We proposed that MEX3A is associated with the prognosis and progression of PDA,which can be used as a potential therapeutic target.

The Role of the RNA-Binding Protein Family MEX-3 in Tumorigenesis.

The muscle excess 3 (MEX-3) protein was first identified in Caenorhabditis elegans (C. elegans), and its respective homologues were also observed in vertebrates, including humans. It is a RNA-binding protein (RBP) with an additional ubiquitin E3 ligase function, which further acts as a post-transcriptional repressor through unknown mechanisms. In humans, MEX-3 proteins post-transcriptionally regulate a number of biological processes, including tumor immunological relevant ones. These have been shown to be involved in various diseases, including tumor diseases of distinct origins. This review provides information on the expression and function of the human MEX-3 family in healthy tissues, as well after malignant transformation. Indeed, the MEX-3 expression was shown to be deregulated in several cancers and to affect tumor biological functions, including apoptosis regulation, antigen processing, and presentation, thereby, contributing to the immune evasion of tumor cells. Furthermore, current research suggests MEX-3 proteins as putative markers for prognosis and as novel targets for the anti-cancer treatment.

The RNA-binding protein Mex-3B plays critical roles in the development of steroid-resistant neutrophilic airway inflammation.

While most asthma can be treated with steroids, about 10%, called severe asthma, is refractory to steroids. It has recently been shown that in a subgroup of severe asthma cases, neutrophils that infiltrate into the airways play an important role in inflammation. However, the mechanisms underlying this increased neutrophil infiltration are not well understood. Here, using a mouse model of steroid-resistant neutrophilic inflammation, we show that mice deficient for the RNA-binding protein Mex-3B have significantly less neutrophil infiltration in the airways than wild-type mice. We further demonstrate that Mex-3B post-transcriptionally upregulates CXCL2, a chemokine that induces neutrophil chemotaxis and migration. Moreover, we show that treatment with either anti-CXCL2 antibody or anti-Mex-3B antisense oligonucleotide suppresses neutrophilic allergic airway inflammation. These results suggest that Mex-3B-mediated induction of CXCL2 is crucial for steroid-resistant neutrophilic allergic airway inflammation. Our findings suggest new strategies for therapeutic intervention in steroid-resistant severe asthma.

The human RNA-binding protein and E3 ligase MEX-3C binds the MEX-3-recognition element (MRE) motif with high affinity.

MEX-3 is a K-homology (KH) domain-containing RNA-binding protein first identified as a translational repressor in Caenorhabditis elegans, and its four orthologs (MEX-3A-D) in human and mouse were subsequently found to have E3 ubiquitin ligase activity mediated by a RING domain and critical for RNA degradation. Current evidence implicates human MEX-3C in many essential biological processes and suggests a strong connection with immune diseases and carcinogenesis. The highly conserved dual KH domains in MEX-3 proteins enable RNA binding and are essential for the recognition of the 3'-UTR and post-transcriptional regulation of MEX-3 target transcripts. However, the molecular mechanisms of translational repression and the consensus RNA sequence recognized by the MEX-3C KH domain are unknown. Here, using X-ray crystallography and isothermal titration calorimetry, we investigated the RNA-binding activity and selectivity of human MEX-3C dual KH domains. Our high-resolution crystal structures of individual KH domains complexed with a noncanonical U-rich and a GA-rich RNA sequence revealed that the KH1/2 domains of human MEX-3C bound MRE10, a 10-mer RNA (5'-CAGAGUUUAG-3') consisting of an eight-nucleotide MEX-3-recognition element (MRE) motif, with high affinity. Of note, we also identified a consensus RNA motif recognized by human MEX-3C. The potential RNA-binding sites in the 3'-UTR of the human leukocyte antigen serotype (HLA-A2) mRNA were mapped with this RNA-binding motif and further confirmed by fluorescence polarization. The binding motif identified here will provide valuable information for future investigations of the functional pathways controlled by human MEX-3C and for predicting potential mRNAs regulated by this enzyme.

MEX-3 proteins: recent insights on novel post-transcriptional regulators.

RNA-binding proteins of the evolutionarily-conserved MEX-3 family are mediators of post-transcriptional regulation in different organisms. Recent studies highlight their involvement in diverse physiological settings, including the maintenance of a balance between stem cell self-renewal and differentiation. Here, we draw attention to their putative role in tissue homeostasis and disease, particularly cancer.