Stem Cells, Cancer, and MUSASHI in Blood and Guts

Musashi-2 (MSI2) regulation of DNA damage response in lung cancer

Lung cancer is one of the most common types of cancer worldwide. Non-small cell lung cancer (NSCLC), typically caused by KRAS and TP53 driver mutations, represents the majority of all new lung cancer diagnoses. Overexpression of the RNA-binding protein (RBP) Musashi-2 (MSI2) has been associated with NSCLC progression. To investigate the role of MSI2 in NSCLC development, we compared the tumorigenesis in mice with lung-specific Kras-activating mutation and Trp53 deletion, with and without Msi2 deletion (KPM2 versus KP mice). KPM2 mice showed decreased lung tumorigenesis in comparison with KP mice. In addition, KPM2 lung tumors showed evidence of decreased proliferation, but increased DNA damage, marked by increased levels of phH2AX (S139) and phCHK1 (S345), but decreased total and activated ATM. Using cell lines from KP and KPM2 tumors, and human NSCLC cell lines, we found that MSI2 directly binds ATM mRNA and regulates its translation. MSI2 depletion impaired DNA damage response (DDR) signaling and sensitized human and murine NSCLC cells to treatment with PARP inhibitors in vitro and in vivo. Taken together, we conclude that MSI2 supports NSCLC tumorigenesis, in part, by supporting repair of DNA damage by controlling expression of DDR proteins. These results suggest that targeting MSI2 may be a promising strategy for lung cancers treated with DNA-damaging agents.

Down-regulation of Musashi-2 exerts antileukemic effects on acute lymphoblastic leukemia cells and increases sensitivity to dexamethasone

Musashi-2 (MSI2), implicated in the oncogenesis and propagation of a broad array of malignancies, inclusive of certain leukemia, remains a nascent field of study within the context of acute lymphoblastic leukemia (ALL). Using lentiviral transfection, ALL cells with stable MSI2 knockdown were engineered. A suite of analytic techniques - a CCK-8 assay, flow cytometry, qRT-PCR, and western blotting - were employed to evaluate cellular proliferation, cell cycle arrest, and apoptosis and to confirm differential gene expression. The suppression of MSI2 expression yielded significant results: inhibition of cell proliferation, G0/G1 cell cycle arrest, and induced apoptosis in ALL cell lines. Furthermore, it was noted that MSI2 inhibition heightened the responsiveness of ALL cells to dexamethasone. Significantly, the depletion of MSI2 prompted the translocation of GR from the cytoplasm to the nucleus upon dexamethasone treatment, consequently leading to enhanced sensitivity. Additionally, the FOXO1/4 signaling pathway contributed to the biological effects of ALL cells evoked by MSI2 silencing. Our study offers novel insight into the inhibitory effects of MSI2 suppression on ALL cells, positing MSI2 as a promising therapeutic target in the treatment of ALL.

Musashi-2 Deficiency Triggers Colorectal Cancer Ferroptosis by Downregulating the MAPK Signaling Cascade to Inhibit HSPB1 Phosphorylation

BACKGROUND

Musashi-2 (MSI2) is a critical RNA-binding protein (RBP) whose ectopic expression drives the pathogenesis of various cancers. Accumulating evidence suggests that inducing ferroptosis of tumor cells can inhibit their malignant biological behavior as a promising therapeutic approach. However, it is unclear whether MSI2 regulates cell death in colorectal cancer (CRC), especially the underlying mechanisms and biological effects in CRC ferroptosis remain elusive.

METHODS

Experimental methods including qRT‒PCR, immunofluorescence, flow cytometry, western blot, co-immunoprecipitation, CCK-8, colony formation assay, in vitro cell transwell migration and invasion assays, in vivo xenograft tumor experiments, liver and lung CRC metastasis models, CAC mice models, transmission electron microscopy, immunohistochemistry, histopathology, 4D label-free proteomics sequencing, bioinformatic and database analysis were used in this study.

RESULTS

Here, we investigated that MSI2 was upregulated in CRC and positively correlated with ferroptosis inhibitor molecules. MSI2 deficiency suppressed CRC malignancy by inhibiting cell proliferation, viability, migration and invasion in vitro and in vivo; and MSI2 deficiency triggered CRC ferroptosis by changing the intracellular redox state (ROS levels and lipid peroxidation), erastin induced cell mortality and viability, iron homeostasis (intracellular total irons and ferrous irons), reduced glutathione (GSH) levels and mitochondrial injury. Mechanistically, through 4D-lable free proteomics analysis on SW620 stable cell lines, we demonstrated that MSI2 directly interacted with p-ERK and MSI2 knockdown downregulated the p-ERK/p38/MAPK axis signaling pathway, which further repressed MAPKAPK2 and HPSB1 phosphorylation, leading to decreased expression of PCNA and Ki67 and increased expression of ACSL4 in cancer cells. Furthermore, HSPB1 could rescue the phenotypes of MSI2 deficiency on CRC ferroptosis in vitro and in vivo.

CONCLUSIONS

This study indicates that MSI2 deficiency suppresses the growth and survival of CRC cells and promotes ferroptosis by inactivating the MAPK signaling pathway to inhibit HSPB1 phosphorylation, which leads to downregulation of PCNA and Ki67 and upregulation of ACSL4 in cancer cells and subsequently induces redox imbalance, iron accumulation and mitochondrial shrinkage, ultimately triggering ferroptosis. Therefore, targeted inhibition of MSI2/MAPK/HSPB1 axis to promote ferroptosis might be a potential treatment strategy for CRC.

Musashi-2 causes cardiac hypertrophy and heart failure by inducing mitochondrial dysfunction through destabilizing Cluh and Smyd1 mRNA

Regulation of RNA stability and translation by RNA-binding proteins (RBPs) is a crucial process altering gene expression. Musashi family of RBPs comprising Msi1 and Msi2 is known to control RNA stability and translation. However, despite the presence of MSI2 in the heart, its function remains largely unknown. Here, we aim to explore the cardiac functions of MSI2. We confirmed the presence of MSI2 in the adult mouse, rat heart, and neonatal rat cardiomyocytes. Furthermore, Msi2 was significantly enriched in the heart cardiomyocyte fraction. Next, using RNA-seq data and isoform-specific PCR primers, we identified Msi2 isoforms 1, 4, and 5, and two novel putative isoforms labeled as Msi2 6 and 7 to be expressed in the heart. Overexpression of Msi2 isoforms led to cardiac hypertrophy in cultured cardiomyocytes. Additionally, Msi2 exhibited a significant increase in a pressure-overload model of cardiac hypertrophy. We selected isoforms 4 and 7 to validate the hypertrophic effects due to their unique alternative splicing patterns. AAV9-mediated overexpression of Msi2 isoforms 4 and 7 in murine hearts led to cardiac hypertrophy, dilation, heart failure, and eventually early death, confirming a pathological function for Msi2. Using global proteomics, gene ontology, transmission electron microscopy, seahorse, and transmembrane potential measurement assays, increased MSI2 was found to cause mitochondrial dysfunction in the heart. Mechanistically, we identified Cluh and Smyd1 as direct downstream targets of Msi2. Overexpression of Cluh and Smyd1 inhibited Msi2-induced cardiac malfunction and mitochondrial dysfunction. Collectively, we show that Msi2 induces hypertrophy, mitochondrial dysfunction, and heart failure.

Musashi-2 binds with Fbxo6 to induce Rnaset2 ubiquitination and chemokine signaling pathway during vascular smooth muscle cell phenotypic switch in atherosclerosis

OBJECTIVE

The objective of this study is to determine how Musashi-2 (MSI2) affects vascular smooth muscle cell (VSMC) phenotypic switch and contributes to atherosclerosis (AS).

METHODS

Primary mouse VSMCs were transfected with MSI2 specific siRNA and treated with platelet-derived growth factor-BB (PDGF-BB). The proliferation, cell-cycle, and migration of VSMCs were determined by CCK-8, flow cytometry, wound healing, and transwell assays. Western blot and qRT-PCR were conducted to analyze the protein and mRNA expression. Moreover, the correlation between MSI2, Fbxo6, Rnaset2, and chemokine signaling was predicted and verified using RNAct database, KEGG, wiki, RNA-binding protein immunoprecipitation and co-immunoprecipitation. Moreover, H&E and Oil Red O staining were employed for assessing necrotic core and lipid accumulation in AS mouse aorta tissues. The numbers of B lymphocytes and monocytes, and the levels of triglyceride (TG), total cholesterol (TC), high-density lipoprotein cholesterol (HDLC), and low-density lipoprotein cholesterol (LDL-C) in AS mice blood were investigated using flow cytometry and corresponding commercial kits, respectively.

RESULTS

MSI2 was up-regulated in the PDGF-BB-treated VSMCs. Knockdown of MSI2 inhibited VSMC proliferation, cell-cycle, and migration. Moreover, MSI2 regulated VSMC phenotypic switch through binding with Fbxo6 to induce Rnaset2 ubiquitination. MSI2 knockdown inhibited chemokine signaling via regulating Fbxo6/Rnaset2 axis. In AS mice, knockdown of MSI2 inhibited the formation of necrotic core and atherosclerotic plaque, and inhibited chemokine signaling via regulating Fbxo6/Rnaset2 axis.

CONCLUSION

Our findings demonstrated that MSI2 could bind with Fbxo6 to induce Rnaset2 ubiquitination and the activation of chemokine signaling pathway during VSMC phenotypic switch in AS.

Regulation and signaling pathways in cancer stem cells: implications for targeted therapy for cancer

Cancer stem cells (CSCs), initially identified in leukemia in 1994, constitute a distinct subset of tumor cells characterized by surface markers such as CD133, CD44, and ALDH. Their behavior is regulated through a complex interplay of networks, including transcriptional, post-transcriptional, epigenetic, tumor microenvironment (TME), and epithelial-mesenchymal transition (EMT) factors. Numerous signaling pathways were found to be involved in the regulatory network of CSCs. The maintenance of CSC characteristics plays a pivotal role in driving CSC-associated tumor metastasis and conferring resistance to therapy. Consequently, CSCs have emerged as promising targets in cancer treatment. To date, researchers have developed several anticancer agents tailored to specifically target CSCs, with some of these treatment strategies currently undergoing preclinical or clinical trials. In this review, we outline the origin and biological characteristics of CSCs, explore the regulatory networks governing CSCs, discuss the signaling pathways implicated in these networks, and investigate the influential factors contributing to therapy resistance in CSCs. Finally, we offer insights into preclinical and clinical agents designed to eliminate CSCs.

Musashi-2 (MSI2) promotes neuroblastoma tumorigenesis through targeting MYC-mediated glucose-6-phosphate dehydrogenase (G6PD) transcriptional activation

Neuroblastoma (NB) is the deadliest pediatric solid tumor due to its rapid proliferation. Aberrant expression of MYCN is deemed as the most remarkable feature for the predictive hallmark of NB progression and recurrence. However, the phenomenon that only detection of MYCN in the nearly 20% of NB patients hints that there should be other vital oncogenes in the progression of NB. Here, we firstly show that MSI2 mRNA is augmented by analyzing public GEO datasets in the malignant stage according to International Neuroblastoma Staging System (INSS) stages. Although accumulating evidences uncover the emerging roles of MSI2 in several cancers, the regulatory functions and underlying mechanisms of MSI2 in NB remain under-investigated. Herein, we identified that high-expressed MSI2 and low-expressed n-Myc group account for 43.1% of total NB clinical samples (n = 65). Meanwhile, MSI2 expression is profoundly upregulated along with NB malignancy and negatively associated with the survival outcome of NB patients in the NB tissue microarray (NB: n = 65; Ganglioneuroblastoma: n = 31; Ganglioneuroma: n = 27). In vitro, our results revealed that MSI2 promoted migration, invasion, and proliferation of NB cells via enhancing pentose phosphate pathway. Mechanistically, MSI2 upregulated the key enzyme glucose-6-phosphate dehydrogenase (G6PD) via directly binding to 3'-untranslated regions of c-Myc mRNA to facilitate its stability, resulting in enhancing pentose phosphate pathway. Our findings reveal that MSI2 promotes pentose phosphate pathway via activating c-Myc-G6PD signaling, suggesting that MSI2 exhibits a novel and powerful target for the diagnosis and treatment of NB.

Detection of a Novel MSI2-C17orf64 Transcript in a Patient with Aggressive Adenocarcinoma of the Gastroesophageal Junction: A Case Report

Adenocarcinoma of the esophagus (EAC) and gastroesophageal junction (GEJ-AC) is associated with poor prognosis, treatment resistance and limited systemic therapeutic options. To deeply understand the genomic landscape of this cancer type, and potentially identify a therapeutic target in a neoadjuvant chemotherapy non-responder 48-year-old man, we adopted a multi-omic approach. We simultaneously evaluated gene rearrangements, mutations, copy number status, microsatellite instability and tumor mutation burden. The patient displayed pathogenic mutations of the TP53 and ATM genes and variants of uncertain significance of three kinases genes (ERBB3, CSNK1A1 and RPS6KB2), along with FGFR2 and KRAS high copy number amplification. Interestingly, transcriptomic analysis revealed the Musashi-2 (MSI2)-C17orf64 fusion that has never been reported before. Rearrangements of the RNA-binding protein MSI2 with a number of partner genes have been described across solid and hematological tumors. MSI2 regulates several biological processes involved in cancer initiation, development and resistance to treatment, and deserves further investigation as a potential therapeutic target. In conclusion, our extensive genomic characterization of a gastroesophageal tumor refractory to all therapeutic approaches led to the discovery of the MSI2-C17orf64 fusion. The results underlie the importance of deep molecular analyses enabling the identification of novel patient-specific markers to be monitored during therapy or even targeted at disease evolution.

Competitive evolved sub-clonal BCR::ABL1 and novel MSI2::PC fusion genes in myelodysplastic syndrome with isolated del(5q)

Myelodysplastic syndrome (MDS) represents a group of neoplasms with extensive heterogeneity. Recurrent mutations in dozens of driver genes have been identified in over 90% of MDS cases, although fusion genes are rarely seen. We first report the competitive evolved sub-clonal breakpoint cluster region (BCR)::ABL1 and novel MSI2::PC fusion gene in MDS with del(5q) in initial diagnosis that underwent dismal progression. However, the BCR::ABL1 clone vanished while the MSI2::PC clone rose to the major one with disease progression. A novel MSI2::PC fusion transcript was identified in initial diagnosis and disease progression of the patient through transcriptome sequencing (RNA-seq) and Quantitative reverse transcription polymerase Chain Reaction (PCR) showed MSI2::PC/ABL1 expression at initial diagnosis and disease progression. In addition, mutation screening of 300 leukemia driver genes identified ARID2 c.5046del/p.F1682Lfs*19 and ZNF292 c.4565A > G/p.Q1522R mutation in bone marrow sample at initial diagnosis and disease progression. In conclusion, the dynamic process of the two fusion and phenotype manifestations may help to understand further the molecular significance of the anomalies of BCR::ABL1, MSI2, and PC in oncogenesis.

MSI2 deficiency in ILC3s attenuates DSS-induced colitis by affecting the intestinal microbiota

Background

The etiology and pathogenesis of inflammatory bowel disease (IBD), including ulcerative colitis (UC) and Crohn's disease (CD), are generally believed to be related to immune dysfunction and intestinal microbiota disorder. However, the exact mechanism is not yet fully understood. The pathological changes associated with dextran sodium sulfate (DSS)-induced colitis are similar to those in human UC. As a subgroup of the innate immune system, group 3 innate lymphoid cells (ILC3s) are widely distributed in the lamina propria of the intestinal mucosa, and their function can be regulated by a variety of molecules. Musashi2 (MSI2) is a type of evolutionarily conserved RNA-binding protein that maintains the function of various tissue stem cells and is essential for postintestinal epithelial regeneration. The effect of MSI2 deficiency in ILC3s on IBD has not been reported. Thus, mice with conditional MSI2 knockout in ILC3s were used to construct a DSS-induced colitis model and explore its effects on the pathogenesis of IBD and the species, quantity and function of the intestinal microbiota.

Methods

Msi2^flox/flox mice (Msi2^fl/fl ) and Msi2^flox/floxRorc^Cre mice (Msi2^ΔRorc ) were induced by DSS to establish the IBD model. The severity of colitis was evaluated by five measurements: body weight percentage, disease activity index, colon shortening degree, histopathological score and routine blood examination. The species, quantity and function of the intestinal microbiota were characterized by high-throughput 16S rRNA gene sequencing of DNA extracted from fecal samples.

Results

MSI2 was knocked out in the ILC3s of Msi2^ΔRorc mice. The Msi2^ΔRorc mice exhibited reductions in body weight loss, the disease activity index, degree of colon shortening, tissue histopathological score and immune cells in the peripheral blood compared to those of Msi2^fl/fl mice after DSS administration. The 16S rRNA sequencing results showed that the diversity of the intestinal microbiota in DSS-treated Msi2^ΔRorc mice changed, with the abundance of Firmicutes increasing and that of Bacteroidetes decreasing. The linear discriminant analysis effect size (LEfSe) approach revealed that Lactobacillaceae could be the key bacteria in the Msi2^ΔRorc mouse during the improvement of colitis. Using PICRUST2 to predict the function of the intestinal microbiota, it was found that the functions of differential bacteria inferred by modeling were mainly enriched in infectious diseases, immune system and metabolic functions.

Conclusions

MSI2 deficiency in ILC3s attenuated DSS-induced colonic inflammation in mice and affected intestinal microbiota diversity, composition, and function, with Lactobacillaceae belonging to the phylum Firmicutes possibly representing the key bacteria. This finding could contribute to our understanding of the pathogenesis of IBD and provide new insights for its clinical diagnosis and treatment.

A novel dual epigenetic approach targeting BET proteins and HDACs in Group 3 (MYC-driven) Medulloblastoma

Background

Medulloblastoma (MB) patients with MYC oncogene amplification or overexpression exhibit extremely poor clinical outcomes and respond poorly to current therapies. Epigenetic deregulation is very common in MYC-driven MB. The bromodomain extra-terminal (BET) proteins and histone deacetylases (HDACs) are epigenetic regulators of MYC transcription and its associated tumorigenic programs. This study aimed to investigate the therapeutic potential of inhibiting the BET proteins and HDACs together in MB.

Methods

Using clinically relevant BET inhibitors (JQ1 or OTX015) and a pan-HDAC inhibitor (panobinostat), we evaluated the effects of combined inhibition on cell growth/survival in MYC-amplified MB cell lines and xenografts and examined underlying molecular mechanism(s).

Results

Co-treatment of JQ1 or OTX015 with panobinostat synergistically suppressed growth/survival of MYC-amplified MB cells by inducing G2 cell cycle arrest and apoptosis. Mechanistic investigation using RNA-seq revealed that co-treatment of JQ1 with panobinostat synergistically modulated global gene expression including MYC/HDAC targets. SYK and MSI1 oncogenes were among the top 50 genes synergistically downregulated by JQ1 and panobinostat. RT-PCR and western blot analyses confirmed that JQ1 and panobinostat synergistically inhibited the mRNA and protein expression of MSI1/SYK along with MYC expression. Reduced SYK/MSI expression after BET (specifically, BRD4) gene-knockdown further confirmed the epigenetic regulation of SYK and MSI1 genes. In addition, the combination of OTX015 and panobinostat significantly inhibited tumor growth in MYC-amplified MB xenografted mice by downregulating expression of MYC, compared to single-agent therapy.

Conclusions

Together, our findings demonstrated that dual-inhibition of BET and HDAC proteins of the epigenetic pathway can be a novel therapeutic approach against MYC-driven MB.

Increased expression of the RNA-binding protein Musashi-2 is associated with immune infiltration and predicts better outcomes in ccRCC patients

RNA-binding proteins (RBPs) mainly contribute to abnormalities in posttranscriptional gene regulation. The RBP Musashi-2, an evolutionarily conserved protein, has been characterized as an oncoprotein in various tumors. However, the prognostic value and potential roles of Musashi-2 in clear cell renal cell carcinoma (ccRCC) have not yet been elucidated. In this study, we found that Musashi-2 was mainly expressed in the normal distal tubular cells and collecting duct cells of the kidneys, while its expression was significantly decreased in ccRCC. And higher expression levels of Musashi-2 indicated better overall survival (OS) in ccRCC. Furthermore, immunohistochemistry demonstrated that PD-L1 expression was negatively correlated with Musashi-2 expression, and Musashi-2 was found to be remarkably correlated with multiple immune cells and immune inhibitors, including CD8+ T cells, CD4+ T cells, regulatory T (Treg) cells, PDCD1, CTLA4, Foxp3, and LAG3. Functional enrichment analysis revealed that Musashi-2 might be involved in ccRCC metabolic reprogramming and immune infiltration and further predicted the therapeutic sensitivity of ccRCC. Taken together, Musashi-2 is a prognostic biomarker for ccRCC patients that may provide novel insights into individualized treatment strategies and guide effective immunotherapy.

The Role of RNA-Binding Proteins in Hematological Malignancies

Hematological malignancies comprise a plethora of different neoplasms, such as leukemia, lymphoma, and myeloma, plus a myriad of dysplasia, such as myelodysplastic syndromes or anemias. Despite all the advances in patient care and the development of new therapies, some of these malignancies remain incurable, mainly due to resistance and refractoriness to treatment. Therefore, there is an unmet clinical need to identify new biomarkers and potential therapeutic targets that play a role in treatment resistance and contribute to the poor outcomes of these tumors. RNA-binding proteins (RBPs) are a diverse class of proteins that interact with transcripts and noncoding RNAs and are involved in every step of the post-transcriptional processing of transcripts. Dysregulation of RBPs has been associated with the development of hematological malignancies, making them potential valuable biomarkers and potential therapeutic targets. Although a number of dysregulated RBPs have been identified in hematological malignancies, there is a critical need to understand the biology underlying their contribution to pathology, such as the spatiotemporal context and molecular mechanisms involved. In this review, we emphasize the importance of deciphering the regulatory mechanisms of RBPs to pinpoint novel therapeutic targets that could drive or contribute to hematological malignancy biology.

[Advances in research of Musashi2 in solid tumors]

RNA binding protein (RBP) plays a key role in gene regulation and participate in RNA translation, modification, splicing, transport and other important biological processes. Studies have shown that abnormal expression of RBP is associated with a variety of diseases. The Musashi (Msi) family of mammals is an evolutionarily conserved and powerful RBP, whose members Msi1 and Msi2 play important roles in the regulation of stem cell activity and tumor development. The Msi family members regulate a variety of biological processes by binding and regulating mRNA translation, stability and downstream cell signaling pathways, and among them, Msi2 is closely related to embryonic growth and development, maintenance of tumor stem cells and development of hematological tumors. Accumulating evidence has shown that Msi2 also plays a crucial role in the development of solid tumors, mainly by affecting the proliferation, invasion, metastasis and drug resistance of tumors, involving Wnt/β-catenin, TGF-β/SMAD3, Akt/mTOR, JAK/STAT, Numb and their related signaling pathways (Notch, p53, and Hedgehog pathway). Preclinical studies of Msi2 gene as a therapeutic target for tumor have achieved preliminary results. This review summarizes the molecular structure, physiological function, role of Msi2 in the development and progression of various solid tumors and the signaling pathways involved.

Msi2-mediated MiR7a-1 processing repression promotes myogenesis

BACKGROUND

Most of the microRNAs (MiRs) involved in myogenesis are transcriptional regulated. The role of MiR biogenesis in myogenesis has not been characterized yet. RNA-binding protein Musashi 2 (Msi2) is considered to be one of the major drivers for oncogenesis and stem cell proliferation. The functions of Msi2 in myogenesis have not been explored yet. We sought to investigate Msi2-regulated biogenesis of MiRs in myogenesis and muscle stem cell (MuSC) ageing.

METHODS

We detected the expression of Msi2 in MuSCs and differentiated myotubes by quantitative reverse transcription PCR (RT-qPCR) and western blot. Msi2-binding partner human antigen R (HuR) was identified by immunoprecipitation followed by mass spectrometry analysis. The cooperative binding of Msi2 and HuR on MiR7a-1 was analysed by RNA immunoprecipitation and electrophoresis mobility shift assays. The inhibition of the processing of pri-MiR7a-1 mediated by Msi2 and HuR was shown by Msi2 and HuR knockdown. Immunofluorescent staining, RT-qPCR and immunoblotting were used to characterize the function of MiR7a-1 in myogenesis. Msi2 and HuR up-regulate cryptochrome circadian regulator 2 (Cry2) via MiR7a-1 was confirmed by the luciferase assay and western blot. The post-transcriptional regulatory cascade was further confirmed by RNAi and overexpressing of Msi2 and HuR in MuSCs, and the in vivo function was characterized by histopathological and molecular biological methods in Msi2 knockout mice.

RESULTS

We identified a post-transcription regulatory cascade governed by a pair of RNA-binding proteins Msi2 and HuR. Msi2 is enriched in differentiated muscle cells and promotes MuSC differentiation despite its pro-proliferation functions in other cell types. Msi2 works synergistically with another RNA-binding protein HuR to repress the biogenesis of MiR7a-1 in an Msi2 dose-dependent manner to regulate the translation of the key component of the circadian core oscillator complex Cry2. Down-regulation of Cry2 (0.6-fold, vs. control, P < 0.05) mediated by MiR7a-1 represses MuSC differentiation. The disruption of this cascade leads to differentiation defects of MuSCs. In aged muscles, Msi2 (0.3-fold, vs. control, P < 0.01) expression declined, and the Cry2 protein level also decreases (0.5-fold, vs. control, P < 0.05), suggesting that the disruption of the Msi2-mediated post-transcriptional regulatory cascade could attribute to the declined ability of muscle regeneration in aged skeletal muscle.

CONCLUSIONS

Our findings have identified a new post-transcriptional cascade regulating myogenesis. The cascade is disrupted in skeletal muscle ageing, which leads to declined muscle regeneration ability.

The RNA-Binding Protein Musashi1 Regulates a Network of Cell Cycle Genes in Group 4 Medulloblastoma

Medulloblastoma is the most common malignant brain tumor in children. Treatment with surgery, irradiation, and chemotherapy has improved survival in recent years, but patients are frequently left with devastating neurocognitive and other sequelae. Patients in molecular subgroups 3 and 4 still experience a high mortality rate. To identify new pathways contributing to medulloblastoma development and create new routes for therapy, we have been studying oncogenic RNA-binding proteins. We defined Musashi1 (Msi1) as one of the main drivers of medulloblastoma development. The high expression of Msi1 is prevalent in Group 4 and correlates with poor prognosis while its knockdown disrupted cancer-relevant phenotypes. Genomic analyses (RNA-seq and RIP-seq) indicated that cell cycle and division are the main biological categories regulated by Msi1 in Group 4 medulloblastoma. The most prominent Msi1 targets include CDK2, CDK6, CCND1, CDKN2A, and CCNA1. The inhibition of Msi1 with luteolin affected the growth of CHLA-01 and CHLA-01R Group 4 medulloblastoma cells and a synergistic effect was observed when luteolin and the mitosis inhibitor, vincristine, were combined. These findings indicate that a combined therapeutic strategy (Msi1 + cell cycle/division inhibitors) could work as an alternative to treat Group 4 medulloblastoma.

Overview of the molecular mechanisms of migration and invasion in glioblastoma multiforme

Glioblastoma (GBM) is one of the most devastating cancers, with an approximate median survival of only 16 months. Although some new insights into the fantastic heterogeneity of this kind of brain tumor have been revealed in recent studies, all subclasses of GBM still demonstrate highly aggressive invasion properties to the surrounding parenchyma. This behavior has become the main obstruction to current curative therapies as invasive GBM cells migrate away from these foci after surgical therapies. Therefore, this review aimed to provide a relatively comprehensive study of GBM invasion mechanisms, which contains an intricate network of interactions and signaling pathways with the extracellular matrix (ECM). Among these related molecules, TGF-β, the ECM, Akt, and microRNAs are most significant in terms of cellular procedures related to GBM motility and invasion. Moreover, we also review data indicating that Musashi-1 (MSI1), a neural RNA-binding protein (RBP), regulates GBM motility and invasion, maintains stem cell populations in GBM, and promotes drug-resistant GBM phenotypes by stimulating necessary oncogenic signaling pathways through binding and regulating mRNA stability. Importantly, these necessary oncogenic signaling pathways have a close connection with TGF-β, ECM, and Akt. Thus, it appears promising to find MSI-specific inhibitors or RNA interference-based treatments to prevent the actions of these molecules despite using RBPs, which are known as hard therapeutic targets. In summary, this review aims to provide a better understanding of these signaling pathways to help in developing novel therapeutic approaches with better outcomes in preclinical studies.

Musashi-2 (MSI2) regulates epidermal growth factor receptor (EGFR) expression and response to EGFR inhibitors in EGFR-mutated non-small cell lung cancer (NSCLC)

Non-small cell lung cancer (NSCLC) has limited treatment options. Expression of the RNA-binding protein (RBP) Musashi-2 (MSI2) is elevated in a subset of non-small cell lung cancer (NSCLC) tumors upon progression, and drives NSCLC metastasis. We evaluated the mechanism of MSI2 action in NSCLC to gain therapeutically useful insights. Reverse phase protein array (RPPA) analysis of MSI2-depleted versus control Kras^LA1/+; Trp53^R172HΔG/+ NSCLC cell lines identified EGFR as a MSI2-regulated protein. MSI2 control of EGFR expression and activity in an NSCLC cell line panel was studied using RT-PCR, Western blots, and RNA immunoprecipitation. Functional consequences of MSI2 depletion were explored for cell growth and response to EGFR-targeting drugs, in vitro and in vivo. Expression relationships were validated using human tissue microarrays. MSI2 depletion significantly reduced EGFR protein expression, phosphorylation, or both. Comparison of protein and mRNA expression indicated a post-transcriptional activity of MSI2 in control of steady state levels of EGFR. RNA immunoprecipitation analysis demonstrated that MSI2 directly binds to EGFR mRNA, and sequence analysis predicted MSI2 binding sites in the murine and human EGFR mRNAs. MSI2 depletion selectively impaired cell proliferation in NSCLC cell lines with activating mutations of EGFR (EGFR^mut). Further, depletion of MSI2 in combination with EGFR inhibitors such as erlotinib, afatinib, and osimertinib selectively reduced the growth of EGFR^mut NSCLC cells and xenografts. EGFR and MSI2 were significantly co-expressed in EGFR^mut human NSCLCs. These results define MSI2 as a direct regulator of EGFR protein expression, and suggest inhibition of MSI2 could be of clinical value in EGFR^mut NSCLC.

The Integrative Analysis Identifies Three Cancer Subtypes and Stemness Features in Cutaneous Melanoma

Background: With the growing uncovering of drug resistance in melanoma treatment, personalized cancer therapy and cancer stem cells are potential therapeutic targets for this aggressive skin cancer. Methods: Multi-omics data of cutaneous melanoma were obtained from The Cancer Genome Atlas (TCGA) database. Then, these melanoma patients were classified into different subgroups by performing "CancerSubtypes" method. The differences of stemness indices (mRNAsi and mDNAsi) and tumor microenvironment indices (immune score, stromal score, and tumor purity) among subtypes were investigated. Moreover, the Least Absolute Shrinkage and Selection Operator (LASSO) and Support Vector Machine-Recursive Feature Elimination (SVM-RFE) algorithms were performed to identify a cancer cell stemness feature, and the likelihood of immuno/chemotherapeutic response was further explored. Results: Totally, 3 specific subtypes of melanoma with different survival outcomes were identified from TCGA. We found subtype 2 of melanoma with the higher immune score and stromal score and lower mRNAsi and tumor purity score, which has the best survival time than the other subtypes. By performing Kaplan-Meier survival analysis, we found that mRNAsi was significantly associated with the overall survival time of melanomas in subtype 2. Correlation analysis indicated surprising associations between stemness indices and subsets of tumor-infiltrating immune cells. Besides, we developed and validated a prognostic stemness-related genes feature that can divide melanoma patients into high- and low-risk subgroups by applying risk score system. The high-risk group has a significantly shorter survival time than the low-risk subgroup, which is more sensitive to CTLA-4 immune therapy. Finally, 16 compounds were screened out in the Connectivity Map database which may be potential therapeutic drugs for melanomas. Conclusion: Thus, our finding provides a new framework for classification and finds some potential targets for the treatment of melanoma.

RNA-Binding Protein MSI2 Binds to miR-301a-3p and Facilitates Its Distribution in Mitochondria of Endothelial Cells

Numerous miRNAs have been detected in mitochondria, which play important roles in many physiological and pathophysiological processes. However, the dynamic changes of miRNA distribution in mitochondria and their mechanisms in reactive oxygen species (ROS)-induced endothelial injury remain unclear. Therefore, miRNA levels in whole cells and mitochondria of H₂O₂-treated endothelial cells were analyzed by small RNA sequencing in the present study. The results showed that H₂O₂ significantly reduced the relative mitochondrial distribution of dozens of miRNAs in human umbilical vein endothelial cells (HUVECs). Among the high-abundance miRNAs, miR-301a-3p has the most significant changes in the redistribution between cytosol and mitochondria confirmed by absolute quantitative polymerase chain reaction (qPCR). To unravel the mechanism of miR-301a-3p distribution in mitochondria, RNA pull-down followed by label-free quantitative proteomic analysis was performed, and RNA-binding protein Musashi RNA binding protein 2 (MSI2) was found to specifically bind to miR-301a-3p. Western blotting and immunofluorescence colocalization assay showed that MSI2 was located in mitochondria of various cell types. H₂O₂ significantly downregulated MSI2 expression in whole endothelial cells, promoted the distribution of MSI2 in cytosol and decreased its distribution in the mitochondria. Moreover, overexpression of MSI2 increased the mitochondrial distribution of miR-301a-3p, whereas inhibition of MSI2 decreased its distribution in mitochondria. Thus, MSI2 might be responsible for the distribution of miR-301a-3p between cytosol and mitochondria in endothelial cells. Our findings revealed for the first time that MSI2 was involved in the regulation of miRNA distribution in mitochondria and provided valuable insight into the mechanism of mitochondrial distribution of miRNAs.

Identification and Validation of an Aspergillus nidulans Secondary Metabolite Derivative as an Inhibitor of the Musashi-RNA Interaction

RNA-binding protein Musashi-1 (MSI1) is a key regulator of several stem cell populations. MSI1 is involved in tumor proliferation and maintenance, and it regulates target mRNAs at the translational level. The known mRNA targets of MSI1 include Numb, APC, and P21^WAF-1, key regulators of Notch/Wnt signaling and cell cycle progression, respectively. In this study, we aim to identify small molecule inhibitors of MSI1-mRNA interactions, which could block the growth of cancer cells with high levels of MSI1. Using a fluorescence polarization (FP) assay, we screened small molecules from several chemical libraries for those that disrupt the binding of MSI1 to its consensus RNA. One cluster of hit compounds is the derivatives of secondary metabolites from Aspergillus nidulans. One of the top hits, Aza-9, from this cluster was further validated by surface plasmon resonance and nuclear magnetic resonance spectroscopy, which demonstrated that Aza-9 binds directly to MSI1, and the binding is at the RNA binding pocket. We also show that Aza-9 binds to Musashi-2 (MSI2) as well. To test whether Aza-9 has anti-cancer potential, we used liposomes to facilitate Aza-9 cellular uptake. Aza-9-liposome inhibits proliferation, induces apoptosis and autophagy, and down-regulates Notch and Wnt signaling in colon cancer cell lines. In conclusion, we identified a series of potential lead compounds for inhibiting MSI1/2 function, while establishing a framework for identifying small molecule inhibitors of RNA binding proteins using FP-based screening methodology.

RNA-binding protein MSI2 isoforms expression and regulation in progression of triple-negative breast cancer

BACKGROUND

The RNA-binding protein Musashi-2 (MSI2) has been implicated in the tumorigenesis and tumor progression of some human cancers. MSI2 has also been reported to suppress tumor epithelial-to-mesenchymal transition (EMT) progression in breast cancer, and low MSI2 expression is associated with poor outcomes for breast cancer patients; however, the underlying mechanisms have not been fully investigated. This study investigated the expression and phenotypic functions of two major alternatively spliced MSI2 isoforms (MSI2a and MSI2b) and the potential molecular mechanisms involved in triple-negative breast cancer (TNBC) progression.

METHODS

The Illumina sequencing platform was used to analyze the mRNA transcriptomes of TNBC and normal tissues, while quantitative reverse transcription-polymerase chain reaction and immunohistochemistry validated MSI2 isoform expression in breast cancer tissues. The effects of MSI2a and MSI2b on TNBC cells were assayed in vitro and in vivo. RNA immunoprecipitation (RIP) and RNA sequencing were performed to identify the potential mRNA targets of MSI2a, and RIP and luciferase analyses were used to confirm the mRNA targets of MSI2.

RESULTS

MSI2 expression in TNBC tissues was significantly downregulated compared to that in normal tissues. In TNBC, MSI2a expression was associated with poor overall survival of patients. MSI2a overexpression in vitro and in vivo inhibited TNBC cell invasion as well as extracellular signal-regulated kinase 1/2 (ERK1/2) activity. However, MSI2b overexpression had no significant effects on TNBC cell migration. Mechanistically, MSI2a expression promoted TP53INP1 mRNA stability by its interaction with the 3'-untranslated region of TP53INP1 mRNA. Furthermore, TP53INP1 knockdown reversed MSI2a-induced suppression of TNBC cell invasion, whereas ectopic expression of TP53INP1 and inhibition of ERK1/2 activity blocked MSI2 knockdown-induced TNBC cell invasion.

CONCLUSIONS

The current study demonstrated that MSI2a is the predominant functional isoform of MSI2 proteins in TNBC, that its downregulation is associated with TNBC progression and poor prognosis and that MSI2a expression inhibited TNBC invasion by stabilizing TP53INP1 mRNA and inhibiting ERK1/2 activity. Overall, our study provides new insights into the isoform-specific roles of MSI2a and MSI2b in the tumor progression of TNBC, allowing for novel therapeutic strategies to be developed for TNBC.

RNA Regulators in Leukemia and Lymphoma

Posttranscriptional regulation of mRNA is a powerful and tightly controlled process in which cells command the integrity, diversity, and abundance of their protein products. RNA-binding proteins (RBPs) are the principal players that control many intermediary steps of posttranscriptional regulation. Recent advances in this field have discovered the importance of RBPs in hematological diseases. Herein we will review a number of RBPs that have been determined to play critical functions in leukemia and lymphoma. Furthermore, we will discuss the potential therapeutic strategies that are currently being studied to specifically target RBPs in these diseases.

RNA-Targeted Therapies and High-Throughput Screening Methods

RNA-binding proteins (RBPs) are involved in regulating all aspects of RNA metabolism, including processing, transport, translation, and degradation. Dysregulation of RNA metabolism is linked to a plethora of diseases, such as cancer, neurodegenerative diseases, and neuromuscular disorders. Recent years have seen a dramatic shift in the knowledge base, with RNA increasingly being recognised as an attractive target for precision medicine therapies. In this article, we are going to review current RNA-targeted therapies. Furthermore, we will scrutinise a range of drug discoveries targeting protein-RNA interactions. In particular, we will focus on the interplay between Lin28 and let-7, splicing regulatory proteins and survival motor neuron (SMN) pre-mRNA, as well as HuR, Musashi, proteins and their RNA targets. We will highlight the mechanisms RBPs utilise to modulate RNA metabolism and discuss current high-throughput screening strategies. This review provides evidence that we are entering a new era of RNA-targeted medicine.

[Musashi-1 positively regulates growth and proliferation of hepatoma cells in vitro]

OBJECTIVE

To investigate the regulatory role of Musashi-1 (MSI1) in the proliferation and growth of hepatocellular carcinoma (HCC) cells.

METHODS

We examined the expression of MSI1 in HCC and paired adjacent tissues from 24 patients using immunohistochemistry and Western blotting. A MSI1-expressing vector was constructed and stably transfected into HepG2 cells, and short hairpin RNAs (shRNAs) that targeted MSI1 mRNA were ligated into the vector and stably transfected in Huh7 cells. The effects of MSI1 overexpression and silencing on the proliferation, viability and cell cycle of HepG2 cells were investigated using flow cytometry or MTT assay. The expressions of PCNA, cyclin D1, APC and β-catenin in the HCC cells were detected with Western blotting.

RESULTS

MSI1 expression was significantly up-regulated in HCC tissues as compared with that in the adjacent tissues. Overexpression of MSI1 in HepG2 cells resulted in significantly enhanced cell growth (P < 0.01) and significantly reduced G0/G1 phase cells from (58.42±3.18)% to (40.67±1.22)% and increased S phase cells from (28.51± 1.93)% to (40.06±1.92)% (P < 0.01), causing also increases in the expressions of PCNA and Cyclin D1. Knockdown of MSI1 in Huh7 cells obviously inhibited the cell growth and caused cell cycle arrest at the G1/S phase (P < 0.01) with reduced protein expressions of PCNA and cyclin D1. Overexpression of MSI1 in HepG2 cells also down-regulated the expression of APC and up-regulated the expression of β-catenin protein, while MSI1 knockdown caused reverse changes in Huh7 cells.

CONCLUSIONS

MSI1 promotes the progression of HCC through positive modulation of cell growth and cell cycle via the Wnt/β-catenin pathway.

Crystal and solution structures of human oncoprotein Musashi-2 N-terminal RNA recognition motif 1

Musashi‐2 (MSI2) belongs to Musashi family of RNA binding proteins (RBP). Like Musashi‐1 (MSI1), it is overexpressed in a variety of cancers and is a promising therapeutic target. Both MSI proteins contain two N‐terminal RNA recognition motifs and play roles in posttranscriptional regulation of target mRNAs. Previously, we have identified several inhibitors of MSI1, all of which bind to MSI2 as well. In order to design MSI2‐specific inhibitors and compare the differences of binding mode of the inhibitors, we set out to solve the structure of MSI2‐RRM1, the key motif that is responsible for the binding. Here, we report the crystal structure and the first NMR solution structure of MSI2‐RRM1, and compare these to the structures of MSI1‐RBD1 and other RBPs. A high degree of structural similarity was observed between the crystal and solution NMR structures. MSI2‐RRM1 shows a highly similar overall folding topology to MSI1‐RBD1 and other RBPs. The structural information of MSI2‐RRM1 will be helpful for understanding MSI2‐RNA interaction and for guiding rational drug design of MSI2‐specific inhibitors.

Post-transcriptional Regulation of Colorectal Cancer: A Focus on RNA-Binding Proteins

Colorectal cancer (CRC) is a major health problem with an estimated 1. 8 million new cases worldwide. To date, most CRC studies have focused on DNA-related aberrations, leaving post-transcriptional processes under-studied. However, post-transcriptional alterations have been shown to play a significant part in the maintenance of cancer features. RNA binding proteins (RBPs) are uprising as critical regulators of every cancer hallmark, yet little is known regarding the underlying mechanisms and key downstream oncogenic targets. Currently, more than a thousand RBPs have been discovered in humans and only a few have been implicated in the carcinogenic process and even much less in CRC. Identification of cancer-related RBPs is of great interest to better understand CRC biology and potentially unveil new targets for cancer therapy and prognostic biomarkers. In this work, we reviewed all RBPs which have a role in CRC, including their control by microRNAs, xenograft studies and their clinical implications.

Small-molecule targeting of MUSASHI RNA-binding activity in acute myeloid leukemia

The MUSASHI (MSI) family of RNA binding proteins (MSI1 and MSI2) contribute to a wide spectrum of cancers including acute myeloid leukemia. We find that the small molecule Ro 08-2750 (Ro) binds directly and selectively to MSI2 and competes for its RNA binding in biochemical assays. Ro treatment in mouse and human myeloid leukemia cells results in an increase in differentiation and apoptosis, inhibition of known MSI-targets, and a shared global gene expression signature similar to shRNA depletion of MSI2. Ro demonstrates in vivo inhibition of c-MYC and reduces disease burden in a murine AML leukemia model. Thus, we identify a small molecule that targets MSI's oncogenic activity. Our study provides a framework for targeting RNA binding proteins in cancer.

Screening Approaches for Targeting Ribonucleoprotein Complexes: A New Dimension for Drug Discovery

RNA-binding proteins (RBPs) are pleiotropic factors that control the processing and functional compartmentalization of transcripts by binding primarily to mRNA untranslated regions (UTRs). The competitive and/or cooperative interplay between RBPs and an array of coding and noncoding RNAs (ncRNAs) determines the posttranscriptional control of gene expression, influencing protein production. Recently, a variety of well-recognized and noncanonical RBP domains have been revealed by modern system-wide analyses, underlying an evolving classification of ribonucleoproteins (RNPs) and their importance in governing physiological RNA metabolism. The possibility of targeting selected RNA-protein interactions with small molecules is now expanding the concept of protein "druggability," with new implications for medicinal chemistry and for a deeper characterization of the mechanism of action of bioactive compounds. Here, taking SF3B1, HuR, LIN28, and Musashi proteins as paradigmatic case studies, we review the strategies applied for targeting RBPs, with emphasis on the technological advancements to study protein-RNA interactions and on the requirements of appropriate validation strategies to parallel high-throughput screening (HTS) efforts.

Luteolin inhibits Musashi1 binding to RNA and disrupts cancer phenotypes in glioblastoma cells

RNA binding proteins have emerged as critical oncogenic factors and potential targets in cancer therapy. In this study, we evaluated Musashi1 (Msi1) targeting as a strategy to treat glioblastoma (GBM); the most aggressive brain tumor type. Msi1 expression levels are often high in GBMs and other tumor types and correlate with poor clinical outcome. Moreover, Msi1 has been implicated in chemo- and radio-resistance. Msi1 modulates a range of cancer relevant processes and pathways and regulates the expression of stem cell markers and oncogenic factors via mRNA translation/stability. To identify Msi1 inhibitors capable of blocking its RNA binding function, we performed a ~ 25,000 compound fluorescence polarization screen. NMR and LSPR were used to confirm direct interaction between Msi1 and luteolin, the leading compound. Luteolin displayed strong interaction with Msi1 RNA binding domain 1 (RBD1). As a likely consequence of this interaction, we observed via western and luciferase assays that luteolin treatment diminished Msi1 positive impact on the expression of pro-oncogenic target genes. We tested the effect of luteolin treatment on GBM cells and showed that it reduced proliferation, cell viability, colony formation, migration and invasion of U251 and U343 GBM cells. Luteolin also decreased the proliferation of patient-derived glioma initiating cells (GICs) and tumor-organoids but did not affect normal astrocytes. Finally, we demonstrated the value of combined treatments with luteolin and olaparib (PARP inhibitor) or ionizing radiation (IR). Our results show that luteolin functions as an inhibitor of Msi1 and demonstrates its potential use in GBM therapy.

Human oncoprotein Musashi-2 N-terminal RNA recognition motif backbone assignment and identification of RNA-binding pocket

RNA-binding protein Musashi-2 (MSI2) is a key regulator in stem cells, it is over-expressed in a variety of cancers and its higher expression is associated with poor prognosis. Like Musashi-1, it contains two N-terminal RRMs (RNA-recognition Motifs, also called RBDs (RNA-binding Domains)), RRM1 and RRM2, which mediate the binding to their target mRNAs. Previous studies have obtained the three-dimensional structures of the RBDs of Musashi-1 and the RBD1:RNA complex. Here we show the binding of MSI2-RRM1 to a 15nt Numb RNA in Fluorescence Polarization assay and time resolved Fluorescence Resonance Energy Transfer assay. Using nuclear magnetic resonance (NMR) spectroscopy we assigned the backbone resonances of MSI2-RRM1, and characterized the direct interaction of RRM1 to Numb RNA r(GUAGU). Our NMR titration and structure modeling studies showed that MSI2-RRM1 and MSI1-RBD1 have similar RNA binding events and binding pockets. This work adds significant information to MSI2-RRM1 structure and RNA binding pocket, and contributes to the development of MSI2 specific and MSI1/MSI2 dual inhibitors.