Musashi1 regulates breast tumor cell proliferation and is a prognostic indicator of poor survival

The RNA-binding protein Musashi1 affects medulloblastoma growth via a network of cancer-related genes and is an indicator of poor prognosis

Musashi1 (Msi1) is a highly conserved RNA-binding protein that is required during the development of the nervous system. Msi1 has been characterized as a stem cell marker, controlling the balance between self-renewal and differentiation, and has also been implicated in tumorigenesis, being highly expressed in multiple tumor types. We analyzed Msi1 expression in a large cohort of medulloblastoma samples and found that Msi1 is highly expressed in tumor tissue compared with normal cerebellum. Notably, high Msi1 expression levels proved to be a sign of poor prognosis. Msi1 expression was determined to be particularly high in molecular subgroups 3 and 4 of medulloblastoma. We determined that Msi1 is required for tumorigenesis because inhibition of Msi1 expression by small-interfering RNAs reduced the growth of Daoy medulloblastoma cells in xenografts. To characterize the participation of Msi1 in medulloblastoma, we conducted different high-throughput analyses. Ribonucleoprotein immunoprecipitation followed by microarray analysis (RIP-chip) was used to identify mRNA species preferentially associated with Msi1 protein in Daoy cells. We also used cluster analysis to identify genes with similar or opposite expression patterns to Msi1 in our medulloblastoma cohort. A network study identified RAC1, CTGF, SDCBP, SRC, PRL, and SHC1 as major nodes of an Msi1-associated network. Our results suggest that Msi1 functions as a regulator of multiple processes in medulloblastoma formation and could become an important therapeutic target.

Autoregulation of Musashi1 mRNA translation during Xenopus oocyte maturation

The mRNA translational control protein, Musashi, plays a critical role in cell fate determination through sequence-specific interactions with select target mRNAs. In proliferating stem cells, Musashi exerts repression of target mRNAs to promote cell cycle progression. During stem cell differentiation, Musashi target mRNAs are de-repressed and translated. Recently, we have reported an obligatory requirement for Musashi to direct translational activation of target mRNAs during Xenopus oocyte meiotic cell cycle progression. Despite the importance of Musashi in cell cycle regulation, only a few target mRNAs have been fully characterized. In this study, we report the identification and characterization of a new Musashi target mRNA in Xenopus oocytes. We demonstrate that progesterone-stimulated translational activation of the Xenopus Musashi1 mRNA is regulated through a functional Musashi binding element (MBE) in the Musashi1 mRNA 3' untranslated region (3' UTR). Mutational disruption of the MBE prevented translational activation of Musashi1 mRNA and its interaction with Musashi protein. Further, elimination of Musashi function through microinjection of inhibitory antisense oligonucleotides prevented progesterone-induced polyadenylation and translation of the endogenous Musashi1 mRNA. Thus, Xenopus Musashi proteins regulate translation of the Musashi1 mRNA during oocyte maturation. Our results indicate that the hierarchy of sequential and dependent mRNA translational control programs involved in directing progression through meiosis are reinforced by an intricate series of nested, positive feedback loops, including Musashi mRNA translational autoregulation. These autoregulatory positive feedback loops serve to amplify a weak initiating signal into a robust commitment for the oocyte to progress through the cell cycle and become competent for fertilization.

Achaete-scute homologue-1 (ASH1) stimulates migration of lung cancer cells through Cdk5/p35 pathway

Cyclin-dependent kinase 5 (Cdk5) activity is important for the migration and invasion of cancer cells. Our results indicate that in the lung one of the mechanisms that hASH1 regulates—migration—takes place through induction of Cdk5 activity. Our data suggest that Cdk5 and its activator p35 promote lung cancer cell migration through hASH1-mediated signaling.

Our previous data suggested that the human basic helix–loop–helix transcription factor achaete-scute homologue-1 (hASH1) may stimulate both proliferation and migration in the lung. In the CNS, cyclin-dependent kinase 5 (Cdk5) and its activator p35 are important for neuronal migration that is regulated by basic helix–loop–helix transcription factors. Cdk5/p35 may also play a role in carcinogenesis. In this study, we found that the neuronal activator p35 was commonly expressed in primary human lung cancers. Cdk5 and p35 were also expressed by several human lung cancer cell lines and coupled with migration and invasion. When the kinase activity was inhibited by the Cdk5 inhibitor roscovitine or dominant-negative (dn) Cdk5, the migration of lung cancer cells was reduced. In neuroendocrine cells expressing hASH1, such as a pulmonary carcinoid cell line, knocking down the gene expression by short hairpin RNA reduced the levels of Cdk5/p35, nuclear p35 protein, and migration. Furthermore, expression of hASH1 in lung adenocarcinoma cells normally lacking hASH1 increased p35/Cdk5 activity and enhanced cellular migration. We were also able to show that p35 was a direct target for hASH1. In conclusion, induction of Cdk5 activity is a novel mechanism through which hASH1 may regulate migration in lung carcinogenesis.

Characterization of the RNA-binding protein Musashi1 in zebrafish

Musashi (Msi) is an evolutionarily conserved gene family of RNA-binding proteins (RBPs) that is preferentially expressed in the nervous system. The first member of the Msi family was identified in Drosophila. Drosophila Msi plays an important role in regulating asymmetric cell division of the sensory organ precursor cells. The mammalian orthologs, including human and mouse Musashi1 (Msi1), are neural RBPs that are strongly expressed in fetal and adult neural stem/progenitor cells (NS/PCs). Mammalian Msi1 contributes to self renewal of NS/PCs through translational regulation of several target mRNAs. In this study, the zebrafish Msi ortholog zMsi1 was identified and characterized. The normal spatial and temporal expression profiles for both protein and mRNA were determined. A series of splice variants were detected. Overall, zMsi1 was strongly expressed in neural tissue in early stages of development and exhibited similarity to mammalian Msi1 expression patterns. To reveal the in vivo function of zMsi1, morpholinos against Msi1 were introduced into one-cell stage zebrafish embryos. Knock down of zmsi1 frequently resulted in aberrant formation of the Central Nervous System (CNS). These results suggest that Msi1 plays roles in CNS development in vertebrates. This article is part of a Special Issue entitled "RNA-Binding Proteins".

Ringo/cyclin-dependent kinase and mitogen-activated protein kinase signaling pathways regulate the activity of the cell fate determinant Musashi to promote cell cycle re-entry in Xenopus oocytes

Cell cycle re-entry during vertebrate oocyte maturation is mediated through translational activation of select target mRNAs, culminating in the activation of mitogen-activated protein kinase and cyclin B/cyclin-dependent kinase (CDK) signaling. The temporal order of targeted mRNA translation is crucial for cell cycle progression and is determined by the timing of activation of distinct mRNA-binding proteins. We have previously shown in oocytes from Xenopus laevis that the mRNA-binding protein Musashi targets translational activation of early class mRNAs including the mRNA encoding the Mos proto-oncogene. However, the molecular mechanism by which Musashi function is activated is unknown. We report here that activation of Musashi1 is mediated by Ringo/CDK signaling, revealing a novel role for early Ringo/CDK function. Interestingly, Musashi1 activation is subsequently sustained through mitogen-activated protein kinase signaling, the downstream effector of Mos mRNA translation, thus establishing a positive feedback loop to amplify Musashi function. The identified regulatory sites are present in mammalian Musashi proteins, and our data suggest that phosphorylation may represent an evolutionarily conserved mechanism to control Musashi-dependent target mRNA translation.

Stem cell antigen-1 deficiency enhances the chemopreventive effect of peroxisome proliferator-activated receptorγ activation

Stem cell antigen-1 (Sca-1, Ly6A) is a glycerophosphatidylinositol (GPI)-anchored protein that was identified as a murine marker of bone marrow stem cells. Although Sca-1 is widely used to enrich for stem and progenitor cells in various tissues, little is known about its function and associated signaling pathways in normal and malignant cells. Here, we report that the absence of Sca-1 in the mammary gland resulted in higher levels of PPARγ and PTEN, and a reduction of pSer84PPARγ, pERK1/2, and PPARδ. This phenotype correlated with markedly increased sensitivity of Sca-1 null mice to PPARγ agonist GW7845 and insensitivity to PPARδ agonist GW501516. Reduction of Sca-1 expression in mammary tumor cells by RNA interference resulted in a phenotype similar to the Sca-1 deficient mammary gland, as evidenced by increased PPARγ expression and transcriptional activity, resulting in part from a lesser susceptibility to proteasomal degradation. These data implicate Sca-1 as a negative regulator of the tumor suppressor effects of PPARγ.

The oncogenic RNA-binding protein Musashi1 is regulated by HuR via mRNA translation and stability in glioblastoma cells

Musashi1 (Msi1) is an evolutionarily conserved RNA-binding protein (RBP) that has profound implications in cellular processes such as stem cell maintenance, nervous system development, and tumorigenesis. Msi1 is highly expressed in many cancers, including glioblastoma, whereas in normal tissues, its expression is restricted to stem cells. Unfortunately, the factors that modulate Msi1 expression and trigger high levels in tumors are largely unknown. The Msi1 mRNA has a long 3' untranslated region (UTR) containing several AU- and U-rich sequences. This type of sequence motif is often targeted by HuR, another important RBP known to be highly expressed in tumor tissue such as glioblastoma and to regulate a variety of cancer-related genes. In this report, we show an interaction between HuR and the Msi1 3'-UTR, resulting in a positive regulation of Msi1 expression. We show that HuR increased MSI1 mRNA stability and promoted its translation. We also present evidence that expression of HuR and Msi1 correlate positively in clinical glioblastoma samples. Finally, we show that inhibition of cell proliferation, increased apoptosis, and changes in cell-cycle profile as a result of silencing HuR are partially rescued when Msi1 is ectopically expressed. In summary, our results suggest that HuR is an important regulator of Msi1 in glioblastoma and that this regulation has important biological consequences during gliomagenesis.

Activation of the aryl hydrocarbon receptor represses mammosphere formation in MCF-7 cells

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor. Recent studies have reported the anti-tumor effects of the AhR in breast cancer. In this study, we investigated the anti-tumor effect of AhR activation based on the cancer stem cell hypothesis. We show that AhR activation suppressed mammosphere formation of MCF-7 cells and decreased the proportion of cells with high ALDH-1 (aldehyde dehydrogenase 1) activity. In addition, we also demonstrate that AhR activation regulates self-renewal signaling by down-regulating Wnt/β-catenin and Notch.

Aberrant expression of CD133 and musashi-1 in preneoplastic and neoplastic human oral squamous epithelium and their correlation with clinicopathological factors

BACKGROUND

The present study focuses on the expression pattern of the stem cell markers CD133 and Musashi-1 in precancerous and cancerous tissues of oral epithelium. The study also aims to investigate the correlation of CD133 and Musashi-1 expression with clinicopathological factors.

METHODS

Immunohistochemical analysis was done to investigate the expression pattern of CD133 and Musashi-1, whereas, the coexpression of CD133 and Musashi-1 was studied using immunofluorescence analysis.

RESULTS

A gradual increase in the expression of CD133 and Musashi-1 was observed from normal to dysplasia to carcinoma. In addition, the expression of CD133 and Musashi-1 shows significant difference between the stages and histological types of oral carcinoma. Interestingly, coexpression of CD133 and Musashi-1 was observed in oral carcinoma and CAL27 cells.

CONCLUSIONS

A gradual increase in the expression of CD133 and Musashi-1 from normal to dysplasia to carcinoma suggests the possible involvement of these 2 proteins in oral carcinogenesis. The overexpression of CD133 and Musashi-1 in advanced stages and also in poorly differentiated tumors reveals their relationship with invasion and differentiation status of oral carcinoma cells. Moreover, the significant positive correlation between CD133 and Musashi-1 expression suggests that they might have a functional relationship in oral carcinoma cells, which needs further investigation.

Breast cancer cells produce tenascin C as a metastatic niche component to colonize the lungs

We report that breast cancer cells that infiltrate the lungs support their own metastasis-initiating ability by expressing tenascin C (TNC). We find that the expression of TNC, an extracellular matrix protein of stem cell niches, is associated with the aggressiveness of pulmonary metastasis. Cancer cell-derived TNC promotes the survival and outgrowth of pulmonary micrometastases. TNC enhances the expression of stem cell signaling components, musashi homolog 1 (MSI1) and leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5). MSI1 is a positive regulator of NOTCH signaling, whereas LGR5 is a target gene of the WNT pathway. TNC modulation of stem cell signaling occurs without affecting the expression of transcriptional enforcers of the stem cell phenotype and pluripotency, namely nanog homeobox (NANOG), POU class 5 homeobox 1 (POU5F1), also known as OCT4, and SRY-box 2 (SOX2). TNC protects MSI1-dependent NOTCH signaling from inhibition by signal transducer and activator of transcription 5 (STAT5), and selectively enhances the expression of LGR5 as a WNT target gene. Cancer cell-derived TNC remains essential for metastasis outgrowth until the tumor stroma takes over as a source of TNC. These findings link TNC to pathways that support the fitness of metastasis-initiating breast cancer cells and highlight the relevance of TNC as an extracellular matrix component of the metastatic niche.

Identification of a novel intronic enhancer responsible for the transcriptional regulation of musashi1 in neural stem/progenitor cells

Background

The specific genetic regulation of neural primordial cell determination is of great interest in stem cell biology. The Musashi1 (Msi1) protein, which belongs to an evolutionarily conserved family of RNA-binding proteins, is a marker for neural stem/progenitor cells (NS/PCs) in the embryonic and post-natal central nervous system (CNS). Msi1 regulates the translation of its downstream targets, including m-Numb and p21 mRNAs. In vitro experiments using knockout mice have shown that Msi1 and its isoform Musashi2 (Msi2) keep NS/PCs in an undifferentiated and proliferative state. Msi1 is expressed not only in NS/PCs, but also in other somatic stem cells and in tumours. Based on previous findings, Msi1 is likely to be a key regulator for maintaining the characteristics of self-renewing stem cells. However, the mechanisms regulating Msi1 expression are not yet clear.

Results

To identify the DNA region affecting Msi1 transcription, we inserted the fusion gene ffLuc, comprised of the fluorescent Venus protein and firefly Luciferase, at the translation initiation site of the mouse Msi1 gene locus contained in a 184-kb bacterial artificial chromosome (BAC). Fluorescence and Luciferase activity, reflecting the Msi1 transcriptional activity, were observed in a stable BAC-carrying embryonic stem cell line when it was induced toward neural lineage differentiation by retinoic acid treatment. When neuronal differentiation was induced in embryoid body (EB)-derived neurosphere cells, reporter signals were detected in Msi1-positive NSCs and GFAP-positive astrocytes, but not in MAP2-positive neurons. By introducing deletions into the BAC reporter gene and conducting further reporter experiments using a minimized enhancer region, we identified a region, "D5E2," that is responsible for Msi1 transcription in NS/PCs.

Conclusions

A regulatory element for Msi1 transcription in NS/PCs is located in the sixth intron of the Msi1 gene. The 595-bp D5E2 intronic enhancer can transactivate Msi1 gene expression with cell-type specificity markedly similar to the endogenous Msi1 expression patterns.

Musashi1 cooperates in abnormal cell lineage protein 28 (Lin28)-mediated let-7 family microRNA biogenesis in early neural differentiation

Musashi1 (Msi1) is an RNA-binding protein that is highly expressed in neural stem/progenitor cells (NS/PCs) as well as in other tissue stem cells. Msi1 binds to the 3'-UTR of its target mRNAs in NS/PCs, prevents their translation, and interferes with NS/PC differentiation. We previously showed that Msi1 competes with eIF4G to bind poly(A)-binding protein and inhibits assembly of the 80 S ribosome. Here we show that Msi1 works in concert with Lin28 to regulate post-transcriptional microRNA (miRNA) biogenesis in the cropping step, which occurs in the nucleus. Lin28 and its binding partner terminal uridylyltransferase 4 (TUT4) are known to maintain embryonic stem cell pluripotency by blocking let-7 miRNA biogenesis at the dicing step. Interestingly, we found that during early neural differentiation of embryonic stem cells, Msi1 enhanced the localization of Lin28 to the nucleus and also inhibited the nuclear cropping step of another let-7 family miRNA, miR98. These results suggest that Msi1 can influence stem cell maintenance and differentiation by controlling the subcellular localization of proteins involved in miRNA biogenesis, as well as by regulating the translation of its target mRNA.