Novel double-negative feedback loop between adenomatous polyposis coli and Musashi1 in colon epithelia

Advances in Targeted and Chemotherapeutic Strategies for Colorectal Cancer: Current Insights and Future Directions

Colorectal cancer (CRC) remains one of the leading causes of cancer-related mortality worldwide, necessitating the continuous evolution of therapeutic approaches. Despite advancements in early detection and localized treatments, metastatic colorectal cancer (mCRC) poses significant challenges due to low survival rates and resistance to conventional therapies. This review highlights the current landscape of CRC treatment, focusing on chemotherapy and targeted therapies. Chemotherapeutic agents, including 5-fluorouracil, irinotecan, and oxaliplatin, have significantly improved survival but face limitations such as systemic toxicity and resistance. Targeted therapies, leveraging mechanisms like VEGF, EGFR, and Hedgehog pathway inhibition, offer promising alternatives, minimizing damage to healthy tissues while enhancing therapeutic precision. Furthermore, future directions in CRC treatment include exploring innovative targets such as Wnt/β-catenin, Notch, and TGF-β pathways, alongside IGF/IGF1R inhibition. These emerging strategies aim to address drug resistance and improve patient outcomes. This review emphasizes the importance of integrating molecular insights into drug development, advocating for a more personalized approach to combat CRC's complexity and heterogeneity.

Comprehensive transcriptional atlas of human adenomyosis deciphered by the integration of single-cell RNA-sequencing and spatial transcriptomics

Adenomyosis is a poorly understood gynecological disorder lacking effective treatments. Controversy persists regarding "invagination" and "metaplasia" theories. The endometrial-myometrial junction (EMJ) connects the endometrium and myometrium and is important for diagnosing and classifying adenomyosis, but its in-depth study is just beginning. Using single-cell RNA sequencing and spatial profiling, we mapped transcriptional alterations across eutopic endometrium, lesions, and EMJ. Within lesions, we identified unique epithelial (LGR5+) and invasive stromal (PKIB+) subpopulations, along with WFDC1+ progenitor cells, supporting a complex interplay between "invagination" and "metaplasia" theories of pathogenesis. Further, we observed endothelial cell heterogeneity and abnormal angiogenic signaling involving vascular endothelial growth factor and angiopoietin pathways. Cell-cell communication differed markedly between ectopic and eutopic endometrium, with aberrant signaling in lesions involving pleiotrophin, TWEAK, and WNT cascades. This study reveals unique stem cell-like and invasive cell subpopulations within adenomyosis lesions identified, dysfunctional signaling, and EMJ abnormalities critical to developing precise diagnostic and therapeutic strategies.

Sonic Hedgehog and WNT Signaling Regulate a Positive Feedback Loop Between Intestinal Epithelial and Stromal Cells to Promote Epithelial Regeneration

BACKGROUND AND AIMS

Active intestinal stem cells are prone to injury by ionizing radiation. We previously showed that upon radiation-induced injury, normally quiescent reserve intestinal stem cells (rISCs) (marked by BMI1) are activated by Musashi-1 (MSI1) and exit from the quiescent state to regenerate the intestinal epithelium. This study aims to further establish the mechanism that regulates activation of Bmi1-CreER;Rosa26eYFP (Bmi1-CreER) rISCs following γ radiation-induced injury.

METHODS

Bmi1-CreER mice were treated with tamoxifen to initiate lineage tracing of BMI1 (eYFP+) cells and exposed to 12 Gy of total body γ irradiation or sham. Intestinal tissues were collected and analyzed by immunofluorescence, Western blot, reverse-transcription quantitative polymerase chain reaction, enzyme-linked immunosorbent assay, and chromatin immunoprecipitation real-time polymerase chain reaction.

RESULTS

After irradiation, increased expression of Msi1 in eYFP+ cells was accompanied by increased expression of Axin2, a WNT marker. Promoter studies of the Msi1 gene indicated that Msi1 is a WNT target gene. Coculture of stromal cells isolated from irradiated mice stimulated Bmi1-CreER-derived organoid regeneration more effectively than those from sham mice. Expression of WNT ligands, including Wnt2b, Wnt4, Wnt5a, and Rspo3, was increased in irradiated stromal cells compared with sham-treated stromal cells. Moreover, expression of the Sonic hedgehog (SHH) effector Gli1 was increased in stromal cells from irradiated mice. This was correlated with an increased expression of SHH in epithelial cells postirradiation, indicating epithelial-stromal interaction. Finally, preinjury treatment with SHH inhibitor cyclopamine significantly reduced intestinal epithelial regeneration and Msi1 expression postirradiation.

CONCLUSIONS

Upon ionizing radiation-induced injury, intestinal epithelial cells increase SHH secretion, stimulating stromal cells to secrete WNT ligands. WNT activators induce Msi1 expression in the Bmi1-CreER cells. This stromal-epithelial interaction leads to Bmi1-CreER rISCs induction and epithelial regeneration.

Phosphorylation of MSI-1 is implicated in the regulation of associative memory in Caenorhabditis elegans

The Musashi family of RNA-binding proteins controls several biological processes including stem cell maintenance, cell division and neural function. Previously, we demonstrated that the C. elegans Musashi ortholog, msi-1, regulates forgetting via translational repression of the Arp2/3 actin-branching complex. However, the mechanisms controlling MSI-1 activity during the regulation of forgetting are currently unknown. Here we investigated the effects of protein phosphorylation on MSI-1 activity. We showed that MSI-1 function is likely controlled by alterations of its activity rather than its expression levels. Furthermore, we found that MSI-1 is phosphorylated and using mass spectrometry we identified MSI-1 phosphorylation at three residues (T18, S19 and S34). CRISPR-based manipulations of MSI-1 phosphorylation sites revealed that phosphorylation is necessary for MSI-1 function in both short- and long-term aversive olfactory associative memory. Thus, our study provides insight into the mechanisms regulating memory-related MSI-1 activity and may facilitate the development of novel therapeutic approaches.

Understanding neural circuits and molecular mechanisms underlying learning and memory are the major challenges of neuroscience. It is a generally accepted model that a learning event causes modification of synapses; strengthening some within a circuit and weakening others (termed “synaptic plasticity”). A plastic nervous system requires not only the ability to acquire and store but also to forget new inputs. While learning and memory is widely investigated, clear-cut evidence for mechanisms involved in forgetting is still sparse. Previously, we demonstrated the role of the protein Musashi (MSI-1) in the active regulation of forgetting in the nematode C. elegans. Here we investigated the role of protein modification (phosphorylation) as a possible regulatory mechanism of the MSI-1 protein activity. We found that MSI-1 protein is modified at different positions and all of these modifications at the protein level contribute to the correct activity of the protein leading to active forgetting of short and long-term memories.

Design, synthesis and evaluation of 4,7-disubstituted 8-methoxyquinazoline derivatives as potential cytotoxic agents targeting β-catenin/TCF4 signaling pathway

Overactivation of Wnt/β-catenin signaling by accumulated β-catenin in the nucleus has been shown to play a crucial role in the etiology of cancer. Interaction of β-catenin with Transcription factor 4 (TCF4) is a key step for the activation of Wnt genes in response to upstream signals of the Wnt/β-catenin pathway. Hence, down regulation of Wnt/β-catenin signaling or targeting downstream events by selective β-catenin/TCF4 protein-protein interaction inhibitors could be a potential therapeutic strategy against such cancers. In this study structure-based drug design approach was followed to design novel 4,7-disubstituted 8-methoxyquinazoline-based derivatives which could act as potential cytotoxic agents inhibiting the β-catenin/TCF4 protein-protein interactions. Fifteen compounds possessing 4,7-disubstituted 8-methoxyquinazoline scaffold were synthesized. Cytotoxic potential of the synthesised derivatives were determined against constitutively activated β-catenin/TCF4 signaling pathway cancer cells (HCT116 and HepG2) using the sulforhodamine B assay. The most potent compound (18B) was selected for detailed biological evaluation. Cell morphology, Hoechst 33342 and Annexin V/PI staining were used to detect apoptosis, while inhibition of cell migration was assessed by in vitro wound healing assay against HCT116 and HepG2 cells. Effect on β-catenin/TCF mediated transcriptional activity was assessed by TOPFlash/FOPFlash assay, TCF4 and β-catenin protein expression by immunocytofluorescence, and Wnt target genes (like c-MYC and Cyclin D1) mRNA levels by RT-PCR against HCT116 cells. Cytotoxic potency of the most potential compound (18B) against primary human gallbladder cancer cells was also evaluated. The derivatives showed interactions with active site residues of β-catenin and were capable of hindering the TCF4 binding, thereby disrupting β-catenin/TCF4 interactions. Cytotoxic potencies (IC50) of these derivatives ranged from 5.64 ± 0.68 to 23.18 ± 0.45 μM against HCT116 and HepG2 cells respectively. Compound (18B), the most potent compound among the series, induced apoptosis and inhibited cell migration against HCT116 and HepG2 cells. Mechanistic studies indicated that compound (18B) downregulated β-catenin/TCF4 signaling pathway, β-catenin and TCF4 protein expression, and mRNA levels of c-MYC andCyclin D1 in HCT116 cells and showed cytotoxicity against primary human gallbladder cancer cells with IC50 value of 8.50 ± 1.44 μM. Thus, novel 4,7-disubstituted 8-methoxyquinazoline derivatives were identified as potential cytotoxic agents with potencies comparable to that of imatinib mesylate. Compound (18B) represents a promising lead molecule as anticancer agent against colon, hepatocellular and gallbladder cancers targeting β-catenin/TCF4 signaling pathway.

Therapeutic Effects of Polyphenols on the Treatment of Colorectal Cancer by Regulating Wnt β-Catenin Signaling Pathway

Colorectal cancer (CRC) is the third most common cause of cancer-related death worldwide in terms of both its rates of incidence and mortality. Due to serious side effects associated with conventional chemotherapeutic treatments, many natural products with fewer adverse side effects have been considered as potential treatment options. In fact, many natural products have widely been used in various phases of clinical trials for CRC, as well as in in vitro and in vivo preclinical studies. Curcumin (CUR) and resveratrol (RES) are classified as natural polyphenolic compounds that have been demonstrated to have anticancer activity against CRC and are associated with minimal side effects. By regulating select target genes involved in several key signaling pathways in CRC, in particular, the Wnt β-catenin signaling cascade, the course of CRC may be positively altered. In the current review, we focused on the therapeutic effects of CUR and RES in CRC as they pertain to modulation of the Wnt β-catenin signaling pathway.

NOTUM promotes thermogenic capacity and protects against diet-induced obesity in male mice

We recently showed that NOTUM, a liver-secreted Wnt inhibitor, can acutely promote browning of white adipose. We now report studies of chronic overexpression of NOTUM in liver indicating that it protects against diet-induced obesity and improves glucose homeostasis in mice. Adeno-associated virus (AAV) vectors were used to overexpress GFP or mouse Notum in the livers of male C57BL/6J mice and the mice were fed an obesifying diet. After 14 weeks of high fat, high sucrose diet feeding, the AAV-Notum mice exhibited decreased obesity and improved glucose tolerance compared to the AAV-GFP mice. Gene expression and immunoblotting analysis of the inguinal fat and brown fat revealed increased expression of beige/brown adipocyte markers in the AAV-Notum group, suggesting enhanced thermogenic capacity by NOTUM. A β3 adrenergic receptor agonist-stimulated lipolysis test suggested increased lipolysis capacity by NOTUM. The levels of collagen and C–C motif chemokine ligand 2 (CCL2) in the epididymal white adipose tissue of the AAV-Notum mice were significantly reduced, suggesting decreased fibrosis and inflammation, respectively. RNA sequencing analysis of inguinal white adipose of 4-week chow diet-fed mice revealed a highly significant enrichment of extracellular matrix (ECM) functional cluster among the down-regulated genes in the AAV-Notum group, suggesting a potential mechanism contributing to improved glucose homeostasis. Our in vitro studies demonstrated that recombinant human NOTUM protein blocked the inhibitory effects of WNT3A on brown adipocyte differentiation. Furthermore, NOTUM attenuated WNT3A’s effects on upregulation of TGF-β signaling and its downstream targets. Overall, our data suggest that NOTUM modulates adipose tissue function by promoting thermogenic capacity and inhibiting fibrosis through inhibition of Wnt signaling.

Constitutive Musashi1 expression impairs mouse postnatal development and intestinal homeostasis

Evolutionarily conserved RNA-binding protein Musashi1 (Msi1) can regulate developmentally relevant genes. Here we report the generation and characterization of a mouse model that allows inducible Msi1 overexpression in a temporal and tissue-specific manner. We show that ubiquitous Msi1 induction in ∼5-wk-old mice delays overall growth, alters organ-to-body proportions, and causes premature death. Msi1-overexpressing mice had shortened intestines, diminished intestinal epithelial cell (IEC) proliferation, and decreased growth of small intestine villi and colon crypts. Although Lgr5-positive intestinal stem cell numbers remained constant in Msi1-overexpressing tissue, an observed reduction in Cdc20 expression provided a potential mechanism underlying the intestinal growth defects. We further demonstrated that Msi1 overexpression affects IEC differentiation in a region-specific manner, with ileum tissue being influenced the most. Ilea of mutant mice displayed increased expression of enterocyte markers, but reduced expression of the goblet cell marker Mucin2 and fewer Paneth cells. A higher hairy and enhancer of split 1:mouse atonal homolog 1 ratio in ilea from Msi1-overexpressing mice implicated Notch signaling in inducing enterocyte differentiation. Together, this work implicates Msi1 in mouse postnatal development of multiple organs, with Notch signaling alterations contributing to intestinal defects. This new mouse model will be a useful tool to further elucidate the role of Msi1 in other tissue settings.

Interplay among p21Waf1/Cip1, MUSASHI-1 and Krüppel-like factor 4 in activation of Bmi1-CreER reserve intestinal stem cells after gamma radiation-induced injury

Gamma radiation is a commonly used adjuvant treatment for abdominally localized cancer. Since its therapeutic potential is limited due to gastrointestinal (GI) syndrome, elucidation of the regenerative response following radiation-induced gut injury is needed to develop a preventive treatment. Previously, we showed that Krüppel-like factor 4 (KLF4) activates certain quiescent intestinal stem cells (ISCs) marked by Bmi1-Cre^ER to give rise to regenerating crypts following γ irradiation. In the current study, we showed that γ radiation-induced expression of p21^Waf1/Cip1 in Bmi1-Cre^ER cells is likely mitigated by MUSASHI-1 (MSI1) acting as a negative regulator of p21^Waf1/Cip1 mRNA translation, which promotes exit of the Bmi1-Cre^ER cells from a quiescent state. Additionally, Bmi1-specific Klf4 deletion resulted in decreased numbers of MSI1⁺ cells in regenerating crypts compared to those of control mice. We showed that KLF4 binds to the Msi1 promoter and activates its expression in vitro. Since MSI1 has been shown to be crucial for crypt regeneration, this finding elucidates a pro-proliferative role of KLF4 during the postirradiation regenerative response. Taken together, our data suggest that the interplay among p21^Waf1/Cip1, MSI1 and KLF4 regulates Bmi1-Cre^ER cell survival, exit from quiescence and regenerative potential upon γ radiation-induced injury.

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.

Inhibitory effects of Lactobacilli cocktail on HT-29 colon carcinoma cells growth and modulation of the Notch and Wnt/β-catenin signaling pathways

Probiotics could be considered as attractive candidates for preventing tumor growth through maintaining homeostasis. The aim of this study was to evaluate the inhibitory effect of a cocktail of five Lactobacillus species on human colorectal carcinoma cell line HT-29. The anti-proliferative and apoptotic effects of Lactobacilli cocktail were evaluated using MTT and flow cytometry tests, respectively. Quantitative real-time polymerase chain reaction (qPCR) was used to analyze the expression of several genes in the Notch (notch, hes1, msi1, and numb) and Wnt/β-catenin (CTNNB1 and CCND1) pathways, following the treatment of HT-29 cells with Lactobacilli cocktail. The treatment by Lactobacilli cocktail induced a significant anti-proliferative effect and late stage apoptosis among the cancer cells (p < 0.05). Compared to the untreated cells, Lactobacilli cocktail induced the down-regulation of notch, hes1, and msi1 genes and up-regulation of numb gene in the Notch pathway as well as the down-regulation of CTNNB1 and CCND1 genes in the Wnt/β-catenin pathway in a time-dependent manner (p < 0.05). CONCLUSION: Lactobacilli cocktail was shown to have beneficial anti-tumor effects on HT-29 cells by modulating the Notch and Wnt/β-catenin pathways; therefore, the use of Lactobacilli probiotics as nutritional supplements may prevent the incidence of colon cancer.

KH-Type Splicing Regulatory Protein Controls Colorectal Cancer Cell Growth and Modulates the Tumor Microenvironment

KH-type splicing regulatory protein (KHSRP) is a multifunctional nucleic acid binding protein implicated in key aspects of cancer cell biology: inflammation and cell-fate determination. However, the role KHSRP plays in colorectal cancer (CRC) tumorigenesis remains largely unknown. Using a combination of in silico analysis of large data sets, ex vivo analysis of protein expression in patients, and mechanistic studies using in vitro models of CRC, we investigated the oncogenic role of KHSRP. We demonstrated KHSRP expression in the epithelial and stromal compartments of both primary and metastatic tumors. Elevated expression was found in tumor versus matched normal tissue, and these findings were validated in larger independent cohorts in silico. KHSRP expression was a prognostic indicator of worse overall survival (hazard ratio, 3.74; 95% CI, 1.43-22.97; P = 0.0138). Mechanistic data in CRC cell line models supported a role of KHSRP in driving epithelial cell proliferation in both a primary and metastatic setting, through control of the G1/S transition. In addition, KHSRP promoted a proangiogenic extracellular environment by regulating the secretion of oncogenic proteins involved in diverse cellular processes, such as migration and response to cellular stress. Our study provides novel mechanistic insight into the tumor-promoting effects of KHSRP in CRC.

Inhibition of RNA-Binding Protein Musashi-1 Suppresses Malignant Properties and Reverses Paclitaxel Resistance in Ovarian Carcinoma

Background and Aims: Ovarian carcinoma (OC) is one of the most lethal malignant tumors with a high reoccurrence and chemoresistance. The key mechanism relationship with chemoresistance in ovarian carcinoma is still unclear. The existence of cancer stem cells involves in chemoresistance and reoccurrence in OC. The objective of this study was to investigate the expression, suppression of malignant properties and reversal of paclitaxel resistance inhibiting RNA-binding protein Musashi-1 with siRNA in ovarian cancer cells.

Methods: The expression of MSI-1 was analyzed in 39 normal ovarian epithelia tissues, 92 serous cystadenomas, 68 borderline serous cystadenomas, and 97 serous cystadenocarcinomas by immunohistochemistry. pLKO.1-MSI-1-siRNA expression vector was transfected into ovarian carcinoma cell line A2780 and its paclitaxel-resistant cell subline A2780/Taxol. The roles of MSI-1 in proliferation, apoptosis, migration and invasion were explored by cell proliferation analysis, Caspase 3 activity assay, wound healing assay, migration and matrigel invasion assay, respectively. Western Blotting and Real-time quantitative PCR were conducted to detect the expression of MSI-1 and the ERK signaling pathway. Reversal of paclitaxel resistance assay was used to evaluate the role of MSI-1 in paclitaxel resistance of OC cells. Finally, therapeutic effects of MSI-1 inhibition were investigated the xenogratfs of SCID mice in vivo of the paclitacel-resistant.

Results: MSI-1 is overexpressed and associated with an unfavorable prognosis in OC patients. Knockdown of MSI-1 by small interfering RNA (siRNA) inhibits proliferation, promotes apoptosis, and reduces migration and invasion of cancer cells. Moreover, MSI-1 expression inhibition reverses paclitaxel-resistance in OC cells. We further display that MSI-1 effectively protects OC cells from paclitaxel-induced apoptosis by increasing the expression of p-Bcl-2 through ERK signaling pathway activation. In vivo, MSI-1 siRNA clearly showed a strong effect on tumor growth inhibition and paclitaxel-resistance reversal.

Conclusions: These findings suggest that MSI-1 overexpression is associated with the prognosis of OC patients, and knockdown of MSI-1 can suppress malignant properties and reverse paclitaxel-resistance in OC cells. MSI-1 maybe act as a potential prognostic indicator and a therapeutic target in OC.

Therapeutic potential of targeting the Wnt/β-catenin signaling pathway in colorectal cancer

Aberrant Wnt/β-catenin signaling has often been reported in different cancers, particularly colorectal cancer (CRC), and this signaling cascade is central to carcinogenesis. Approximately 80% of CRC cases harbor mutations in the adenomatous polyposis coli gene, and half of the remaining cases feature mutations in the β-catenin gene that affect the Wnt/β-catenin signaling pathway. Unsurprisingly, the Wnt/β-catenin signaling pathway has potential value as a therapeutic target in the treatment of CRC. Several inhibitors of the Wnt/β-catenin signaling pathway have been developed for CRC treatment, but so far no molecular therapeutic targeting this pathway has been incorporated into oncological practice. In this review, we discuss the role of Wnt/β-catenin signaling in CRC and its potential as a target of innovative therapeutic approaches for CRC.

Natural product derivative Gossypolone inhibits Musashi family of RNA-binding proteins

BACKGROUND

The Musashi (MSI) family of RNA-binding proteins is best known for the role in post-transcriptional regulation of target mRNAs. Elevated MSI1 levels in a variety of human cancer are associated with up-regulation of Notch/Wnt signaling. MSI1 binds to and negatively regulates translation of Numb and APC (adenomatous polyposis coli), negative regulators of Notch and Wnt signaling respectively.

METHODS

Previously, we have shown that the natural product (-)-gossypol as the first known small molecule inhibitor of MSI1 that down-regulates Notch/Wnt signaling and inhibits tumor xenograft growth in vivo. Using a fluorescence polarization (FP) competition assay, we identified gossypolone (Gn) with a > 20-fold increase in Ki value compared to (-)-gossypol. We validated Gn binding to MSI1 using surface plasmon resonance, nuclear magnetic resonance, and cellular thermal shift assay, and tested the effects of Gn on colon cancer cells and colon cancer DLD-1 xenografts in nude mice.

RESULTS

In colon cancer cells, Gn reduced Notch/Wnt signaling and induced apoptosis. Compared to (-)-gossypol, the same concentration of Gn is less active in all the cell assays tested. To increase Gn bioavailability, we used PEGylated liposomes in our in vivo studies. Gn-lip via tail vein injection inhibited the growth of human colon cancer DLD-1 xenografts in nude mice, as compared to the untreated control (P < 0.01, n = 10).

CONCLUSION

Our data suggest that PEGylation improved the bioavailability of Gn as well as achieved tumor-targeted delivery and controlled release of Gn, which enhanced its overall biocompatibility and drug efficacy in vivo. This provides proof of concept for the development of Gn-lip as a molecular therapy for colon cancer with MSI1/MSI2 overexpression.

NANOGP8 is the key regulator of stemness, EMT, Wnt pathway, chemoresistance, and other malignant phenotypes in gastric cancer cells

BACKGROUND

Accumulating evidence demonstrated that NANOG1, the key transcription factor for embryonic stem cells, is associated with human cancers. NANOGP8, one of the pseudogenes in NANOG gene family, contains an intact open reading frame and also said to be expressed in cancer tissues. Therefore, a systematic study is greatly needed to address the following questions: among NANOG1 and NANOGP8, which gene is the main contributor for NANOG expression in cancer cells and which one is the key regulator responsible for stemness, epithelial-mesenchymal transition (EMT), metastasis, chemoresistance and other malignant phenotypes. Here we try to explore these issues with gastric adenocarcinoma cell lines in vitro using variety of molecular and cellular techniques.

METHODS

Special primers were designed to distinguish PCR products from NANOG1 and NANOGP8. Sphere-forming cells were cultured with serum-free and selective medium. A stable cell line was established with infection of lentivirus containing NANOGP8. qPCR was performed to measure NANOGP8 expression and its association with stemness, EMT and CSC markers in adherent cells and sphere-forming cells. Western blot analysis was deployed to confirm results of the transcript analysis. Experiments of cell proliferation, migration, invasion, clonogenic assay, sphere cell growth assays, cell cycle analysis, β-catenin accumulation and translocation in nucleus, and drug resistance were conducted to measure the impact of NANOGP8 on malignant statuses of gastric cancer cells. Immunofluorescence staining was used to analyze cell subpopulations with different markers.

RESULTS

NANOGP8 is mainly responsible for NANOG expression in sphere-forming (stem cell-like) cells derived from gastric cancer cell lines regardless their differentiation status. Ectopic expression of NANOGP8 significantly up-regulates stemness transcription factors, EMT inducers, and cancer stem cell markers (CSC) including Lgr5. NANOGP8 also promotes expression of the signature genes vimentin and N-caderin for mesenchymal cells and down-regulates the signature gene E-caderin for epithelial cells whereby confer the cells with mesenchymal cell phenotype. In NANOGP8 over-expressed adherent and sphere-forming cells, Lgr5+ cells are significantly increased. Ectopic expression of NANOGP8 endows gastric cells with enhanced proliferation, migration, invasion, sphere-forming and clonogenic capacity, and chemoresistance. NANOGP8 expression also enhances β-catenin accumulation in nucleus and strengthens Wnt signal transduction.

CONCLUSION

NANOGP8 is the main regulator of gastric cancer stem cells. It is closely associated with EMT, stemness, and CSC marker as well as Wnt signal pathway. NANOGP8 is correlated with cell proliferation, migration, invasion, clonogenic capacity, β-catenin accumulation in nucleus, and chemoresistance in gastric cancer. NANOGP8 is a promising molecular target for clinical intervention of gastric cancer.

Long intergenic non-protein-coding RNA 1567 (LINC01567) acts as a "sponge" against microRNA-93 in regulating the proliferation and tumorigenesis of human colon cancer stem cells

Background

Cancer stem cells (CSCs) are considered to be the major factor in tumor initiation, progression, metastasis, recurrence and chemoresistance. Maintaining the stemness and promoting differentiation of these cells involve various factors. Recently, long non-coding RNAs (lncRNAs) have been identified as new regulatory factors in human cancer cells. However, the function of lncRNAs in colon CSCs is still unknown.

Methods

Primary colon cancer cells were maintained in serum-free medium to form spheres and CD133⁺/CD166⁺/CD44⁺ spheroid cells were selected using FACS technique. Then we detected growth curve, colony formation, invasion and migration ability, and tumorigenicity of CD133⁺/CD166⁺/CD44⁺ cells. LOCCS-siRNA and pcDNA-LOCCS plasmid vectors were constructed and transfected to evaluate impact of the lncRNA. We also performed dual luciferase reporter assay to verify the interaction of LOCCS and miR-93.

Results

The research explored lncRNA expression and the regulatory role of novel lncRNAs in colon CSCs. Using the stem cell markers CD133, CD166 and CD44, we found a subpopulation of highly tumorigenic human colon cancer cells. They displayed some characteristics of stem cells, including the ability to proliferate and form colonies, to resist chemotherapeutic drugs, and to produce xenografts in nude mice. We also found an lncRNA, LOCCS, with obviously upregulated expression in colon CSCs. Knockdown of LOCCS reduced cell proliferation, invasion, migration, and generation of tumor xenografts. Furthermore, microRNA-93 (miR-93) and Musashi-1 mediated the tumor suppression of LOCCS knockdown.

Conclusions

There was reciprocal repression between LOCCS and miR-93. Research on mechanisms suggested direct binding, as a predicted miR-93 binding site was identified in LOCCS. This comprehensive analysis of LOCCS in colon CSCs provides insight for elucidating important roles of the lncRNA–microRNA functional network in human colon cancer.

Electronic supplementary material

The online version of this article (10.1186/s12885-017-3731-5) contains supplementary material, which is available to authorized users.

Roles of microRNAs and RNA-Binding Proteins in the Regulation of Colorectal Cancer Stem Cells

Colorectal cancer stem cells (CSCs) are responsible for the initiation, progression and metastasis of human colorectal cancers, and have been characterized by the expression of cell surface markers, such as CD44, CD133, CD166 and LGR5. MicroRNAs (miRNAs) are differentially expressed between CSCs and non-tumorigenic cancer cells, and play important roles in the maintenance and regulation of stem cell properties of CSCs. RNA binding proteins (RBPs) are emerging epigenetic regulators of various RNA processing events, such as splicing, localization, stabilization and translation, and can regulate various types of stem cells. In this review, we summarize current evidences on the roles of miRNA and RBPs in the regulation of colorectal CSCs. Understanding the epigenetic regulation of human colorectal CSCs will help to develop biomarkers for colorectal cancers and to identify targets for CSC-targeting therapies.

Evasion of regulatory phosphorylation by an alternatively spliced isoform of Musashi2

The Musashi family of RNA binding proteins act to promote stem cell self-renewal and oppose cell differentiation predominantly through translational repression of mRNAs encoding pro-differentiation factors and inhibitors of cell cycle progression. During tissue development and repair however, Musashi repressor function must be dynamically regulated to allow cell cycle exit and differentiation. The mechanism by which Musashi repressor function is attenuated has not been fully established. Our prior work indicated that the Musashi1 isoform undergoes site-specific regulatory phosphorylation. Here, we demonstrate that the canonical Musashi2 isoform is subject to similar regulated site-specific phosphorylation, converting Musashi2 from a repressor to an activator of target mRNA translation. We have also characterized a novel alternatively spliced, truncated isoform of human Musashi2 (variant 2) that lacks the sites of regulatory phosphorylation and fails to promote translation of target mRNAs. Consistent with a role in opposing cell cycle exit and differentiation, upregulation of Musashi2 variant 2 was observed in a number of cancers and overexpression of the Musashi2 variant 2 isoform promoted cell transformation. These findings indicate that alternately spliced isoforms of the Musashi protein family possess distinct functional and regulatory properties and suggest that differential expression of Musashi isoforms may influence cell fate decisions.

Structural Insight into the Recognition of r(UAG) by Musashi-1 RBD2, and Construction of a Model of Musashi-1 RBD1-2 Bound to the Minimum Target RNA

Musashi-1 (Msi1) controls the maintenance of stem cells and tumorigenesis through binding to its target mRNAs and subsequent translational regulation. Msi1 has two RNA-binding domains (RBDs), RBD1 and RBD2, which recognize r(GUAG) and r(UAG), respectively. These minimal recognition sequences are connected by variable linkers in the Msi1 target mRNAs, however, the molecular mechanism by which Msi1 recognizes its targets is not yet understood. We previously determined the solution structure of the Msi1 RBD1:r(GUAGU) complex. Here, we determined the first structure of the RBD2:r(GUAGU) complex. The structure revealed that the central trinucleotide, r(UAG), is specifically recognized by the intermolecular hydrogen-bonding and aromatic stacking interactions. Importantly, the C-terminal region, which is disordered in the free form, took a certain conformation, resembling a helix. The observation of chemical shift perturbation and intermolecular NOEs, together with increases in the heteronuclear steady-state {¹H}-¹⁵N NOE values on complex formation, indicated the involvement of the C-terminal region in RNA binding. On the basis of the two complex structures, we built a structural model of consecutive RBDs with r(UAGGUAG) containing both minimal recognition sequences, which resulted in no steric hindrance. The model suggests recognition of variable lengths (n) of the linker up to n = 50 may be possible.

Stem Cells, Cancer, and MUSASHI in Blood and Guts

The mammalian MSI family of RNA-binding proteins (RBPs) have important roles as oncoproteins in an array of tumor types, including leukemias, glioblastomas, and pancreatic, breast, lung, and colorectal cancers. The mammalian Msi genes, Msi1 and Msi2, have been most thoroughly investigated in two highly proliferative tissues prone to oncogenic transformation: the hematopoietic lineage and the intestinal epithelium. Despite their vast phenotypic differences, MSI proteins appear to have an analogous role in governing the stem cell compartment in both of these tissues, potentially providing a paradigm for a broader understanding of MSI function and oncogenic activities. In this review, we focus on the function of MSI in the blood and the intestine, and discuss therapeutic strategies for targeting this pathway.

Suppression of intestinal tumorigenesis in Apc mutant mice upon Musashi-1 deletion

Therapeutic strategies based on a specific oncogenic target are better justified when elimination of that particular oncogene reduces tumorigenesis in a model organism. One such oncogene, Musashi-1 (Msi-1), regulates translation of target mRNAs and is implicated in promoting tumorigenesis in the colon and other tissues. Msi-1 targets include the tumor suppressor adenomatous polyposis coli (Apc), a Wnt pathway antagonist lost in ∼80% of all colorectal cancers. Cell culture experiments have established that Msi-1 is a Wnt target, thus positioning Msi-1 and Apc as mutual antagonists in a mutually repressive feedback loop. Here, we report that intestines from mice lacking Msi-1 display aberrant Apc and Msi-1 mutually repressive feedback, reduced Wnt and Notch signaling, decreased proliferation, and changes in stem cell populations, features predicted to suppress tumorigenesis. Indeed, mice with germline Apc mutations (Apc^Min ) or with the Apc^1322T truncation mutation have a dramatic reduction in intestinal polyp number when Msi-1 is deleted. Taken together, these results provide genetic evidence that Msi-1 contributes to intestinal tumorigenesis driven by Apc loss, and validate the pursuit of Msi-1 inhibitors as chemo-prevention agents to reduce tumor burden.

Msi1 promotes tumor progression by epithelial-to-mesenchymal transition in cervical cancer

Musashi1 (Msi1) is an RNA-binding protein that has been reported to be a pivotal regulator in tumorigenesis and progression in several cancers. However, its function and mechanism in cervical cancer is still unknown. In this study, Msi1 expression was found elevated in cervical cancers by immunohistochemistry and correlated with poor outcomes. Then, endogenous Msi1 was silenced in cervical cancer cell lines by short hairpin RNA, and its function and mechanism were determined. The results showed that the silencing of Msi1 in SiHa and HeLa cells inhibited the cells' migratory and invasive abilities in vitro and tumor progression in vivo. Epithelial-to-mesenchymal transition (EMT) markers were down-regulated, and Wnt activity was inhibited by the silencing of Msi1. In clinical tissues, positive correlations between Msi1 and EMT markers were found. In conclusion, Msi1, a diagnostic marker and potential therapeutic target, promoted the EMT progression through activation of the Wnt signaling pathway in cervical cancers, thereby contributing to poor prognosis.

Msi RNA-binding proteins control reserve intestinal stem cell quiescence

Regeneration of the intestinal epithelium is driven by multiple intestinal stem cell (ISC) types, including an active, radiosensitive Wnthigh ISC that fuels turnover during homeostasis and a reserve, radioresistant Wntlow/off ISC capable of generating active Wnthigh ISCs. We examined the role of the Msi family of oncoproteins in the ISC compartment. We demonstrated that Msi proteins are dispensable for normal homeostasis and self-renewal of the active ISC, despite their being highly expressed in these cells. In contrast, Msi proteins are required specifically for activation of reserve ISCs, where Msi activity is both necessary and sufficient to drive exit from quiescence and entry into the cell cycle. Ablation of Msi activity in reserve ISCs rendered the epithelium unable to regenerate in response to injury that ablates the active stem cell compartment. These findings delineate a molecular mechanism governing reserve ISC quiescence and demonstrate a necessity for the activity of this rare stem cell population in intestinal regeneration.

Musashi mediates translational repression of the Drosophila hypoxia inducible factor

Adaptation to hypoxia depends on a conserved α/β heterodimeric transcription factor called Hypoxia Inducible Factor (HIF), whose α-subunit is regulated by oxygen through different concurrent mechanisms. In this study, we have identified the RNA binding protein dMusashi, as a negative regulator of the fly HIF homologue Sima. Genetic interaction assays suggested that dMusashi participates of the HIF pathway, and molecular studies carried out in Drosophila cell cultures showed that dMusashi recognizes a Musashi Binding Element in the 3' UTR of the HIFα transcript, thereby mediating its translational repression in normoxia. In hypoxic conditions dMusashi is downregulated, lifting HIFα repression and contributing to trigger HIF-dependent gene expression. Analysis performed in mouse brains revealed that murine Msi1 protein physically interacts with HIF-1α transcript, suggesting that the regulation of HIF by Msi might be conserved in mammalian systems. Thus, Musashi is a novel regulator of HIF that inhibits responses to hypoxia specifically when oxygen is available.

The Msi Family of RNA-Binding Proteins Function Redundantly as Intestinal Oncoproteins

Members of the Msi family of RNA-binding proteins have recently emerged as potent oncoproteins in a range of malignancies. MSI2 is highly expressed in hematopoietic cancers, where it is required for disease maintenance. In contrast to the hematopoietic system, colorectal cancers can express both Msi family members, MSI1 and MSI2. Here, we demonstrate that, in the intestinal epithelium, Msi1 and Msi2 have analogous oncogenic effects. Further, comparison of Msi1/2-induced gene expression programs and transcriptome-wide analyses of Msi1/2-RNA-binding targets reveal significant functional overlap, including induction of the PDK-Akt-mTORC1 axis. Ultimately, we demonstrate that concomitant loss of function of both MSI family members is sufficient to abrogate the growth of human colorectal cancer cells, and Msi gene deletion inhibits tumorigenesis in several mouse models of intestinal cancer. Our findings demonstrate that MSI1 and MSI2 act as functionally redundant oncoproteins required for the ontogeny of intestinal cancers.

A Mouse Model of Targeted Musashi1 Expression in Whole Intestinal Epithelium Suggests Regulatory Roles in Cell Cycle and Stemness

The intestinal epithelium is very peculiar for its continuous cell renewal, fuelled by multipotent stem cells localized within the crypts of Lieberkühn. Several lines of evidence have established the evolutionary conserved RNA-binding protein Musashi1 as a marker of adult stem cells, including those of the intestinal epithelium, and revealed its roles in stem cell self-renewal and cell fate determination. Previous studies from our laboratories have shown that Musashi1 controls stem cell-like features in medulloblastoma, glioblastoma, and breast cancer cells, and has pro-proliferative and pro-tumorigenic properties in intestinal epithelial progenitor cells in vitro. To undertake a detailed study of Musashi1's function in the intestinal epithelium in vivo, we have generated a mouse model, referred to as v-Msi, overexpressing Musashi1 specifically in the entire intestinal epithelium. Compared with wild type litters, v-Msi1 mice exhibited increased intestinal crypt size accompanied by enhanced proliferation. Comparative transcriptomics by RNA-seq revealed Musashi1's association with gut stem cell signature, cell cycle, DNA replication, and drug metabolism. Finally, we identified and validated three novel mRNA targets that are stabilized by Musashi1, Ccnd1 (Cyclin D1), Cdk6, and Sox4. In conclusion, the targeted expression of Musashi1 in the intestinal epithelium in vivo increases the cell proliferation rate and strongly suggests its action on stem cells activity. This is due to the modulation of a complex network of gene functions and pathways including drug metabolism, cell cycle, and DNA synthesis and repair.

Identification and validation of novel small molecule disruptors of HuR-mRNA interaction

HuR, an RNA binding protein, binds to adenine- and uridine-rich elements (ARE) in the 3'-untranslated region (UTR) of target mRNAs, regulating their stability and translation. HuR is highly abundant in many types of cancer, and it promotes tumorigenesis by interacting with cancer-associated mRNAs, which encode proteins that are implicated in different tumor processes including cell proliferation, cell survival, angiogenesis, invasion, and metastasis. Drugs that disrupt the stabilizing effect of HuR upon mRNA targets could have dramatic effects on inhibiting cancer growth and persistence. In order to identify small molecules that directly disrupt the HuR-ARE interaction, we established a fluorescence polarization (FP) assay optimized for high throughput screening (HTS) using HuR protein and an ARE oligo from Musashi RNA-binding protein 1 (Msi1) mRNA, a HuR target. Following the performance of an HTS of ∼6000 compounds, we discovered a cluster of potential disruptors, which were then validated by AlphaLISA (Amplified Luminescent Proximity Homogeneous Assay), surface plasmon resonance (SPR), ribonucleoprotein immunoprecipitation (RNP IP) assay, and luciferase reporter functional studies. These compounds disrupted HuR-ARE interactions at the nanomolar level and blocked HuR function by competitive binding to HuR. These results support future studies toward chemical probes for a HuR function study and possibly a novel therapy for HuR-overexpressing cancers.

Tumor suppressive microRNA-137 negatively regulates Musashi-1 and colorectal cancer progression

Stem cell marker, Musashi-1 (MSI1) is over-expressed in many cancer types; however the molecular mechanisms involved in MSI1 over-expression are not well understood. We investigated the microRNA (miRNA) regulation of MSI1 and the implications this regulation plays in colorectal cancer. MicroRNA miR-137 was identified as a MSI1-targeting microRNA by immunoblotting and luciferase reporter assays. MSI1 protein was found to be highly expressed in 79% of primary rectal tumors (n=146), while miR-137 expression was decreased in 84% of the rectal tumor tissues (n=68) compared to paired normal mucosal samples. In addition to reduced MSI1 protein, exogenous expression of miR-137 inhibited cell growth, colony formation, and tumorsphere growth of colon cancer cells. Finally, in vivo studies demonstrated that induction of miR-137 can decrease growth of human colon cancer xenografts. Our results demonstrate that miR-137 acts as a tumor-suppressive miRNA in colorectal cancers and negatively regulates oncogenic MSI1.

Natural product (-)-gossypol inhibits colon cancer cell growth by targeting RNA-binding protein Musashi-1

Musashi-1 (MSI1) is an RNA-binding protein that acts as a translation activator or repressor of target mRNAs. The best-characterized MSI1 target is Numb mRNA, whose encoded protein negatively regulates Notch signaling. Additional MSI1 targets include the mRNAs for the tumor suppressor protein APC that regulates Wnt signaling and the cyclin-dependent kinase inhibitor P21(WAF-1). We hypothesized that increased expression of NUMB, P21 and APC, through inhibition of MSI1 RNA-binding activity might be an effective way to simultaneously downregulate Wnt and Notch signaling, thus blocking the growth of a broad range of cancer cells. We used a fluorescence polarization assay to screen for small molecules that disrupt the binding of MSI1 to its consensus RNA binding site. One of the top hits was (-)-gossypol (Ki = 476 ± 273 nM), a natural product from cottonseed, known to have potent anti-tumor activity and which has recently completed Phase IIb clinical trials for prostate cancer. Surface plasmon resonance and nuclear magnetic resonance studies demonstrate a direct interaction of (-)-gossypol with the RNA binding pocket of MSI1. We further showed that (-)-gossypol reduces Notch/Wnt signaling in several colon cancer cell lines having high levels of MSI1, with reduced SURVIVIN expression and increased apoptosis/autophagy. Finally, we showed that orally administered (-)-gossypol inhibits colon cancer growth in a mouse xenograft model. Our study identifies (-)-gossypol as a potential small molecule inhibitor of MSI1-RNA interaction, and suggests that inhibition of MSI1's RNA binding activity may be an effective anti-cancer strategy.

Neural stem and progenitor cell fate transition requires regulation of Musashi1 function

Background

There is increasing evidence of a pivotal role for regulated mRNA translation in control of developmental cell fate transitions. Physiological and pathological stem and progenitor cell self-renewal is maintained by the mRNA-binding protein, Musashi1 through repression of translation of key mRNAs encoding cell cycle inhibitory proteins. The mechanism by which Musashi1 function is modified to allow translation of these target mRNAs under conditions that require inhibition of cell cycle progression, is unknown.

Results

In this study, we demonstrate that differentiation of primary embryonic rat neural stem/progenitor cells (NSPCs) or human neuroblastoma SH-SY5Y cells results in the rapid phosphorylation of Musashi1 on the evolutionarily conserved site serine 337 (S337). Phosphorylation of this site has been shown to be required for cell cycle control during the maturation of Xenopus oocytes. S337 phosphorylation in mammalian NSPCs and human SH-SY5Y cells correlates with the de-repression and translation of a Musashi reporter mRNA and with accumulation of protein from the endogenous Musashi target mRNA, p21^WAF1/CIP1. Inhibition of Musashi regulatory phosphorylation, through expression of a phospho-inhibitory mutant Musashi1 S337A or over-expression of the wild-type Musashi, blocked differentiation of both NSPCs and SH-SY5Y cells. Musashi1 was similarly phosphorylated in NSPCs and SH-SY5Y cells under conditions of nutrient deprivation-induced cell cycle arrest. Expression of the Musashi1 S337A mutant protein attenuated nutrient deprivation-induced NSPC and SH-SY5Y cell death.

Conclusions

Our data suggest that in response to environmental cues that oppose cell cycle progression, regulation of Musashi function is required to promote target mRNA translation and cell fate transition. Forced modulation of Musashi1 function may present a novel therapeutic strategy to oppose pathological stem cell self-renewal.

Transformation of the intestinal epithelium by the MSI2 RNA-binding protein

The MSI2 RNA binding protein is a potent oncogene playing key roles in hematopoietic stem cell homeostasis and malignant hematopoiesis. Here we demonstrate that MSI2 is expressed in the intestinal stem cell compartment, that its expression is elevated in colorectal adenocarcinomas, and that MSI2 loss of function abrogates colorectal cancer cell growth. MSI2 gain of function in the intestinal epithelium in a drug inducible mouse model is sufficient to phenocopy many of the morphological and molecular consequences of acute loss of the APC tumor suppressor in the intestinal epithelium in a Wnt-independent manner. Transcriptome-wide RNA-binding analysis indicates that MSI2 acts as a pleiotropic inhibitor of known intestinal tumor suppressors including Lrig1, Bmpr1a, Cdkn1a, and Pten. Finally, we demonstrate that inhibition of the PDK-AKT-mTORC1 axis rescues oncogenic consequences of MSI2 induction. Taken together, our findings identify MSI2 as a central component in an unappreciated oncogenic pathway promoting intestinal transformation.

Musashi1 as a potential therapeutic target and diagnostic marker for lung cancer

Lung cancer remains one of the leading causes of cancer-related deaths worldwide with a 5-year survival rate of less than 20%. One approach to improving survival is the identification of biomarkers to detect early stage disease. In this study, we investigated the potential of the stem cell and progenitor cell marker, Musashi1 (Msi1), as a diagnostic marker and potential therapeutic target for lung cancer. Functional studies in A549 bronchioalveolar carcinoma and NCI-H520 squamous cell carcinoma cells revealed that Msi1 was enriched in spheroid cultures of tumor cells and in the CD133+ cell population. Downregulation of Msi1 by lentivirus-mediated expression of an Msi1 shRNA reduced spheroid colony proliferation. Growth inhibition was associated with reduced nuclear localization of β-catenin and inhibition of the processing of intracellular Notch. In primary lung cancer, Msi1 protein expression was elevated in 86% of 202 tissue microarray specimens, and Msi1 mRNA was increased in 80% of 118 bronchoscopic biopsies, including metastatic disease, but was rarely detected in adjacent normal lung tissue and in non-malignant diseased tissue. Msi1 was expressed in a diffuse pattern in most tumor subtypes, except in squamous cell carcinomas, where it appeared in a focal pattern in 50% of specimens. Thus, Msi1 is a sensitive and specific diagnostic marker for all lung cancer subtypes.

Pharmacological modulation of beta-catenin and its applications in cancer therapy

Beta-catenin (β-catenin) is a multifunction protein with a central role in physiological homeostasis. Its abnormal expression leads to various diseases including cancer. In normal physiology, β-catenin either maintains integrity of epithelial tissues or controls transcription of various genes on extracellular instigations. In epithelial tissues, β-catenin functions as a component of the cadherin protein complex and regulates epithelial cell growth and intracellular adhesion. In Wnt signalling, β-catenin is a major transcriptional modulator and plays a crucial role in embryogenesis, stem cell renewal and organ regeneration. Aberrant expression of β-catenin can induce malignant pathways in normal cells and its abnormal activity is also exploited by existing malignant programmes. It acts as an oncogene and modulates transcription of genes to drive cancer initiation, progression, survival and relapse. Abnormal expression and function of β-catenin in cancer makes it a putative drug target. In the past decade, various attempts have been made to identify and characterize various pharmacological inhibitors of β-catenin. Many of these inhibitors are currently being investigated for their anticancer activities in a variety of cancers. The first half of this review will focus on the role of β-catenin in cancer initiation, maintenance, progression and relapse whereas the second half will briefly summarize the recent progress in development of agents for the pharmacological modulation of β-catenin activity in cancer therapeutics.

The RNA-Binding Protein Musashi1: A Major Player in Intestinal Epithelium Renewal and Colon Cancer Development

Aberrant gene expression is the cause and the consequence of tumorigenesis. A major component of gene expression is translation regulation; a process whose main players are RNA-binding-proteins (RBPs). More than 800 RBPs have been identified in the human genome and several of them have been shown to control gene networks associated with relevant cancer processes. A more systematic characterization of RBPs starts to reveal that similar to transcription factors, they can function as tumor suppressors or oncogenes. A relevant example is Musashi1 (Msi1), which is emerging as a critical regulator of tumorigenesis in multiple cancer types, including colon cancer. Msi1 is a stem marker in several tissues and is critical in maintaining the balance between self-renewal and differentiation. However, a boost in Msi1 expression can most likely lead cells towards an oncogenic pathway. In this article, we discuss the parallels between Msi1 function in normal renewal of intestinal epithelium and in colon cancer.

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.

Structure of Musashi1 in a complex with target RNA: the role of aromatic stacking interactions

Mammalian Musashi1 (Msi1) is an RNA-binding protein that regulates the translation of target mRNAs, and participates in the maintenance of cell ‘stemness’ and tumorigenesis. Msi1 reportedly binds to the 3′-untranslated region of mRNA of Numb, which encodes Notch inhibitor, and impedes initiation of its translation by competing with eIF4G for PABP binding, resulting in triggering of Notch signaling. Here, the mechanism by which Msi1 recognizes the target RNA sequence using its Ribonucleoprotein (RNP)-type RNA-binding domains (RBDs), RBD1 and RBD2 has been revealed on identification of the minimal binding RNA for each RBD and determination of the three-dimensional structure of the RBD1:RNA complex. Unique interactions were found for the recognition of the target sequence by Msi1 RBD1: adenine is sandwiched by two phenylalanines and guanine is stacked on the tryptophan in the loop between β1 and α1. The minimal recognition sequences that we have defined for Msi1 RBD1 and RBD2 have actually been found in many Msi1 target mRNAs reported to date. The present study provides molecular clues for understanding the biology involving Musashi family proteins.

Translational control in germ cell development: A role for the RNA-binding proteins Musashi-1 and Musashi-2

Mammalian gametogenesis is a complex process involving specialised cell cycle progression and differentiation. As part of their differentiation, germ cells experience periods of transcriptional inactivation and chromatin inaccessibility whilst continuing to coordinate the correct temporal and spatial expression of genes required for continued development. To overcome these obstacles, mammalian germ cells express a wide variety of sequence-specific RNA-binding proteins, which assist in the translational control of many mRNA transcripts which are produced and stored during periods of high mRNA synthesis. In this review we focus on the Musashi family of RNA-binding proteins, a highly conserved family of translational regulatory proteins whose recent identification in germ cells of Drosophila and Xenopus, as well as their well described role in processes such as cell cycle progression and stem cell identity, has led us to investigate the role of these proteins in mammalian germ cell development.