Therapeutic Targeting of the GSK3β-CUGBP1 Pathway in Myotonic Dystrophy

Myotonic Dystrophy type 1 (DM1) is a neuromuscular disease associated with toxic RNA containing expanded CUG repeats. The developing therapeutic approaches to DM1 target mutant RNA or correct early toxic events downstream of the mutant RNA. We have previously described the benefits of the correction of the GSK3β-CUGBP1 pathway in DM1 mice (HSALR model) expressing 250 CUG repeats using the GSK3 inhibitor tideglusib (TG). Here, we show that TG treatments corrected the expression of ~17% of genes misregulated in DM1 mice, including genes involved in cell transport, development and differentiation. The expression of chloride channel 1 (Clcn1), the key trigger of myotonia in DM1, was also corrected by TG. We found that correction of the GSK3β-CUGBP1 pathway in mice expressing long CUG repeats (DMSXL model) is beneficial not only at the prenatal and postnatal stages, but also during adulthood. Using a mouse model with dysregulated CUGBP1, which mimics alterations in DM1, we showed that the dysregulated CUGBP1 contributes to the toxicity of expanded CUG repeats by changing gene expression and causing CNS abnormalities. These data show the critical role of the GSK3β-CUGBP1 pathway in DM1 muscle and in CNS pathologies, suggesting the benefits of GSK3 inhibitors in patients with different forms of DM1.

Glycyrrhizic acid alleviates liver fibrosis in vitro and in vivo via activating CUGBP1-mediated IFN-γ/STAT1/Smad7 pathway

BACKGROUND

Hepatic fibrosis, a common pathological feature of chronic liver injuries, is a serious public health problem and lacks effective therapy. Glycyrrhizic acid (GA) is a bioactive ingredient in the root of traditional Chinese medicine licorice, and exhibits remarkable anti-viral, anti-inflammatory and hepatoprotective actions.

PURPOSE

Here we aimed to investigated whether GA provided a therapeutic efficacy in hepatic fibrosis and uncover its molecular mechanisms.

STUDY DESIGN AND METHODS

We investigated the anti-fibrosis effects of GA using CCl₄-induced mouse mode of liver fibrosis as well as TGF-β1-activated human LX-2 cells and primary hepatic stellate cells (HSCs). CUGBP1-mediated IFN-γ/STAT1/Smad7 signaling was examined with immunofluorescence staining and western blot analysis. We designed and studied the binding of GA to CUGBP1 using in silico docking, and validated by microscale thermophoresis (MST) assay.

RESULTS

GA obviously attenuated CCl₄-induced liver histological damage, and reduced serum ALT and AST levels. Meanwhile, GA decreased liver fibrogenesis markers such as α-SMA, collagen α1, HA, COL-III, and LN in the hepatic tissues. Mechanistically, GA remarkably elevated the levels of IFN-γ, p-STAT1, Smad7, and decreased CUGBP1 in vivo and in vitro. Over-expression of CUGBP1 completely abolished the anti-fibrotic effect of GA and regulation on IFN-γ/STAT1/Smad7 pathway in LX-2 cells and primary HSCs, confirming CUGBP1 played a pivotal role in the protection by GA from liver fibrosis. Further molecular docking and MST assay indicated that GA had a good binding affinity with the CUGBP1 protein. The dissociation constant (Kd) of GA and CUGBP1 was 0.293 μM.

CONCLUSION

Our study demonstrated for the first time that GA attenuated liver fibrosis and hepatic stellate cell activation by promoting CUGBP1-mediated IFN-γ/STAT1/Smad7 signalling pathways. GA may be a potential candidate compound for preventing or reliving liver fibrosis.

Fraxinellone alleviates kidney fibrosis by inhibiting CUG-binding protein 1-mediated fibroblast activation

Chronic Kidney Disease (CKD) is a serious threat to human health. In addition, kidney fibrosis is a key pathogenic intermediate for the progression of CDK. Moreover, excessive activation of fibroblasts is key to the development of kidney fibrosis and this process is difficult to control. Notably, fraxinellone is a natural compound isolated from Dictamnus dasycarpus and has a variety of pharmacological activities, including hepatoprotective, anti-inflammatory and anti-cancer effects. However, the effect of fraxinellone on kidney fibrosis is largely unknown. The present study showed that fraxinellone could alleviate folic acid-induced kidney fibrosis in mice in a dose dependent manner. Additionally, the results revealed that fraxinellone could effectively down-regulate the expression of CUGBP1, which was highly up-regulated in human and murine fibrotic renal tissues. Furthermore, expression of CUGBP1 was selectively induced by the Transforming Growth Factor-beta (TGF-β) through p38 and JNK signaling in kidney fibroblasts. On the other hand, downregulating the expression of CUGBP1 significantly inhibited the activation of kidney fibroblasts. In conclusion, these findings demonstrated that fraxinellone might be a new drug candidate and CUGBP1 could be a promising target for the treatment of kidney fibrosis.

Proteomics-Based Characterization of miR-574-5p Decoy to CUGBP1 Suggests Specificity for mPGES-1 Regulation in Human Lung Cancer Cells

MicroRNAs (miRs) are one of the most important post-transcriptional repressors of gene expression. However, miR-574-5p has recently been shown to positively regulate the expression of microsomal prostaglandin E-synthase-1 (mPGES-1), a key enzyme in the prostaglandin E₂ (PGE₂) biosynthesis, by acting as decoy to the RNA-binding protein CUG-RNA binding protein 1 (CUGBP1) in human lung cancer. miR-574-5p exhibits oncogenic properties and promotes lung tumor growth in vivo via induction of mPGES-1-derived PGE₂ synthesis. In a mass spectrometry-based proteomics study, we now attempted to characterize this decoy mechanism in A549 lung cancer cells at a cellular level. Besides the identification of novel CUGBP1 targets, we identified that the interaction between miR-574-5p and CUGBP1 specifically regulates mPGES-1 expression. This is supported by the fact that CUGBP1 and miR-574-5p are located in the nucleus, where CUGBP1 regulates alternative splicing. Further, in a bioinformatical approach we showed that the decoy-dependent mPGES-1 splicing pattern is unique. The specificity of miR-574-5p/CUGBP1 regulation on mPGES-1 expression supports the therapeutic strategy of pharmacological inhibition of PGE₂ formation, which may provide significant therapeutic value for NSCLC patients with high miR-574-5p levels.

Correction of RNA-Binding Protein CUGBP1 and GSK3β Signaling as Therapeutic Approach for Congenital and Adult Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a complex genetic disease affecting many tissues. DM1 is caused by an expansion of CTG repeats in the 3′-UTR of the DMPK gene. The mechanistic studies of DM1 suggested that DMPK mRNA, containing expanded CUG repeats, is a major therapeutic target in DM1. Therefore, the removal of the toxic RNA became a primary focus of the therapeutic development in DM1 during the last decade. However, a cure for this devastating disease has not been found. Whereas the degradation of toxic RNA remains a preferential approach for the reduction of DM1 pathology, other approaches targeting early toxic events downstream of the mutant RNA could be also considered. In this review, we discuss the beneficial role of the restoring of the RNA-binding protein, CUGBP1/CELF1, in the correction of DM1 pathology. It has been recently found that the normalization of CUGBP1 activity with the inhibitors of GSK3 has a positive effect on the reduction of skeletal muscle and CNS pathologies in DM1 mouse models. Surprisingly, the inhibitor of GSK3, tideglusib also reduced the toxic CUG-containing RNA. Thus, the development of the therapeutics, based on the correction of the GSK3β-CUGBP1 pathway, is a promising option for this complex disease.

Mir-206 partially rescues myogenesis deficiency by inhibiting CUGBP1 accumulation in the cell models of myotonic dystrophy

Objectives: In this study, we aim to determine how CUG-expansion and the abundance of Celf1 regulates normal myocyte differentiation and reveal the role ofmiR-206 in myotonic dystrohy and explore a possible gene therapy vector. Methods: we set up CUG-expansion and Celf1 overexpression C2C12 cell models to imitate the myocyte differentiation defects of DM1, then transfected AdvmiR-206 into cell models, tested the level of myogenic markers MyoD, MyoG, Mef2c, Celf1 by RT-PCRand Western Blotting, detected myotube formation by myosin heavy chain immunostaining. Result: 3'-UTR CUG-expansion leads to myotube defects and impaired myoblasts differentiation. Overexpression of Celf1 inhibits myoblast differentiation and impairs differentiation. Knockdown of Celf1 partially rescues differentiation defects of myoblasts harboring CUG-expansion. miR-206 incompletely reverses myoblast differentiation inhibition induced by CUG-expansion and partially recuses myoblast differentiation defects induced by Celf1 overexpression. Conclusions: Ectopic miR-206 mimicking the endogenous temporal patterns specifically drives a myocyte program that boosts myoblast lineages, likely by promoting the expression of MyoD to rectify the myogenic deficiency by stimulating the accumulation of Celf1. Abbreviations: DMPK: (dystrophia myotonica protein kinase); 3'-UTR: (3'-untranslated region); MBNL1: (muscleblind-like [Drosophila]); DM1: (myotonic dystrophy type 1); GFP: (green fluorescent protein); RT-PCR: (quantitative reverse transcriptase-polymerase chain reaction); shRNA: (short hairpin RNA).

Correction of GSK3β at young age prevents muscle pathology in mice with myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a progressive neuromuscular disease caused by expanded CUG repeats, which misregulate RNA metabolism through several RNA-binding proteins, including CUG-binding protein/CUGBP1 elav-like factor 1 (CUGBP1/CELF1) and muscleblind 1 protein. Mutant CUG repeats elevate CUGBP1 and alter CUGBP1 activity via a glycogen synthase kinase 3β (GSK3β)-cyclin D3-cyclin D-dependent kinase 4 (CDK4) signaling pathway. Inhibition of GSK3β corrects abnormal activity of CUGBP1 in DM1 mice [human skeletal actin mRNA, containing long repeats ( HSA^LR) model]. Here, we show that the inhibition of GSK3β in young HSA^LR mice prevents development of DM1 muscle pathology. Skeletal muscle in 1-yr-old HSA^LR mice, treated at 1.5 mo for 6 wk with the inhibitors of GSK3, exhibits high fiber density, corrected atrophy, normal fiber size, with reduced central nuclei and normalized grip strength. Because CUG-GSK3β-cyclin D3-CDK4 converts the active form of CUGBP1 into a form of translational repressor, we examined the contribution of CUGBP1 in myogenesis using Celf1 knockout mice. We found that a loss of CUGBP1 disrupts myogenesis, affecting genes that regulate differentiation and the extracellular matrix. Proteins of those pathways are also misregulated in young HSA^LR mice and in muscle biopsies of patients with congenital DM1. These findings suggest that the correction of GSK3β-CUGBP1 pathway in young HSA^LR mice might have a positive effect on the myogenesis over time.-Wei, C., Stock, L., Valanejad, L., Zalewski, Z. A., Karns, R., Puymirat, J., Nelson, D., Witte, D., Woodgett, J., Timchenko, N. A., Timchenko, L. Correction of GSK3β at young age prevents muscle pathology in mice with myotonic dystrophy type 1.

Reconstitution of HuR-Inhibited CUGBP1 Expression Protects Cardiomyocytes from Acute Myocardial Infarction-Induced Injury

AIM

Myocardial infarction (MI) is one of the leading causes of death in elderly people. Expanding the knowledge of the molecular mechanisms underlying MI is of profound importance to developing a cure for MI. The CUGBP- and ETR-3-like factor (CELF) proteins, a family of RNA-binding proteins, play key roles in RNA metabolism. To determine the functions and molecular mechanisms of CELF proteins in MI, an animal model of acute myocardial infarction (AMI) was used in our study.

RESULTS

We found that the CUG triplet repeat RNA-binding protein 1 (CUGBP1)/CELF1 expression levels were decreased in AMI-injured hearts, and further studies showed that two highly conserved adenylate-uridylate-rich (AU-rich) elements in the 3'UTR of CUGBP1 were responsible for the decreased CUGBP1 expression. Upon AMI, human antigen R (HuR) was relocated to the cytoplasm from the nucleus and interacted with these AU-rich elements to affect the expression of CUGBP1. Reintroduction of CUGBP1 via gene delivery by recombinant adenovirus improved cardiac function in AMI mice. Our studies also indicated that CUGBP1 protected cardiomyocytes from ischemia-induced injury through the promotion of angiogenesis and inhibition of apoptosis by regulating the vascular endothelial growth factor-A gene. Innovation and Conclusion: Our studies indicate a role for CUGBP1 in cardiac disease and reveal a novel MI post-transcriptional gene regulatory mechanism. The reconstitution of CUGBP1 could be developed as a potential therapeutic option for the management of MI. Antioxid. Redox Signal. 27, 1013-1026.

RNA Binding Protein CUGBP1 Inhibits Liver Cancer in a Phosphorylation-Dependent Manner

Despite intensive investigations, mechanisms of liver cancer are not known. Here, we identified an important step of liver cancer, which is the neutralization of tumor suppressor activities of an RNA binding protein, CUGBP1. The translational activity of CUGBP1 is activated by dephosphorylation at Ser302. We generated CUGBP1-S302A knock-in mice and found that the reduction of translational activity of CUGBP1 causes development of a fatty liver phenotype in young S302A mice. Examination of liver cancer in diethylnitrosamine (DEN)-treated CUGBP1-S302A mice showed these mice develop much more severe liver cancer that is associated with elimination of the mutant CUGBP1. Searching for mechanisms of this elimination, we found that the oncoprotein gankyrin (Gank) preferentially binds to and triggers degradation of dephosphorylated CUGBP1 (de-ph-S302-CUGBP1) or S302A mutant CUGBP1. To test the role of Gank in degradation of CUGBP1, we generated mice with liver-specific deletion of Gank. In these mice, the tumor suppressor isoform of CUGBP1 is protected from Gank-mediated degradation. Consistent with reduction of CUGBP1 in animal models, CUGBP1 is reduced in patients with pediatric liver cancer. Thus, this work presents evidence that de-ph-S302-CUGBP1 is a tumor suppressor protein and that the Gank-UPS-mediated reduction of CUGBP1 is a key event in the development of liver cancer.

RNA-binding protein CUGBP1 regulates insulin secretion via activation of phosphodiesterase 3B in mice

AIMS/HYPOTHESIS

CUG-binding protein 1 (CUGBP1) is a multifunctional RNA-binding protein that regulates RNA processing at several stages including translation, deadenylation and alternative splicing, as well as RNA stability. Recent studies indicate that CUGBP1 may play a role in metabolic disorders. Our objective was to examine its role in endocrine pancreas function through gain- and loss-of-function experiments and to further decipher the underlying molecular mechanisms.

METHODS

A mouse model in which type 2 diabetes was induced by a high-fat diet (HFD; 60% energy from fat) and mice on a standard chow diet (10% energy from fat) were compared. Pancreas-specific CUGBP1 overexpression and knockdown mice were generated. Different lengths of the phosphodiesterase subtype 3B (PDE3B) 3' untranslated region (UTR) were cloned for luciferase reporter analysis. Purified CUGBP1 protein was used for gel shift experiments.

RESULTS

CUGBP1 is present in rodent islets and in beta cell lines; it is overexpressed in the islets of diabetic mice. Compared with control mice, the plasma insulin level after a glucose load was significantly lower and glucose clearance was greatly delayed in mice with pancreas-specific CUGBP1 overexpression; the opposite results were obtained upon pancreas-specific CUGBP1 knockdown. Glucose- and glucagon-like peptide1 (GLP-1)-stimulated insulin secretion was significantly attenuated in mouse islets upon CUGBP1 overexpression. This was associated with a strong decrease in intracellular cAMP levels, pointing to a potential role for cAMP PDEs. CUGBP1 overexpression had no effect on the mRNA levels of PDE1A, 1C, 2A, 3A, 4A, 4B, 4D, 7A and 8B subtypes, but resulted in increased PDE3B expression. CUGBP1 was found to directly bind to a specific ATTTGTT sequence residing in the 3' UTR of PDE3B and stabilised PDE3B mRNA. In the presence of the PDE3 inhibitor cilostamide, glucose- and GLP-1-stimulated insulin secretion was no longer reduced by CUGBP1 overexpression. Similar to CUGBP1, PDE3B was overexpressed in the islets of diabetic mice.

CONCLUSIONS/INTERPRETATION

We conclude that CUGBP1 is a critical regulator of insulin secretion via activating PDE3B. Repressing this protein might provide a potential strategy for treating type 2 diabetes.

Identification of CELF1 RNA targets by CLIP-seq in human HeLa cells

The specific interactions between RNA-binding proteins and their target RNAs are an essential level to control gene expression. By combining ultra-violet cross-linking and immunoprecipitation (CLIP) and massive SoliD sequencing we identified the RNAs bound by the RNA-binding protein CELF1, in human HeLa cells. The CELF1 binding sites deduced from the sequence data allow characterizing specific features of CELF1-RNA association. We present therefore the first map of CELF1 binding sites in human cells.

CUG-binding protein 1 (CUGBP1) expression and prognosis of brain metastases from non-small cell lung cancer

BACKGROUND

The brain is a frequent site of metastases from non-small cell lung cancer (NSCLC). The purpose of this study was to detect the expression of CUG-binding protein 1 (CUGBP1) messenger ribonucleic acid (mRNA) and Ki-67 in metastasized brain tissue from NSCLC and determine the relationship between CUGBP1 and brain metastases.

METHODS

The expression of CUGBP1 mRNA and Ki-67 in metastasized brain tissue from NSCLC was investigated by semiquantitative polymerase chain reaction and immunohistochemistry, respectively. The expression of CUGBP1 and Ki-67 in metastasized brain tissue from NSCLC was related to clinical characteristics, as assessed using the chi-square test. The prognostic significance was assessed by univariate and multivariate analyses using the Cox hazard model.

RESULTS

The expression of CUGBP1 mRNA and Ki-67 was overexpressed in metastasized brain tissue from NSCLC and was correlated with differentiation. In addition, by both univariate and multivariate survival analyses, CUGBP1 expression, Ki-67 expression, and age were noted to be independent indicators of a shorter postsurgical survival.

CONCLUSION

The expression of CUGBP1 is an important factor in the development of brain metastases from NSCLC.

Celf1 regulates cell cycle and is partially responsible for defective myoblast differentiation in myotonic dystrophy RNA toxicity

Myotonic dystrophy is a neuromuscular disease of RNA toxicity. The disease gene DMPK harbors expanded CTG trinucleotide repeats on its 3'-UTR. The transcripts of this mutant DMPK led to misregulation of RNA-binding proteins including MBNL1 and Celf1. In myoblasts, CUG-expansion impaired terminal differentiation. In this study, we formally tested how the abundance of Celf1 regulates normal myocyte differentiation, and how Celf1 expression level mediates CUG-expansion RNA toxicity-triggered impairment of myocyte differentiation. As the results, overexpression of Celf1 largely recapitulated the defects of myocytes with CUG-expansion, by increasing myocyte cycling. Knockdown of endogenous Celf1 level led to precocious myotube formation, supporting a negative connection between Celf1 abundance and myocyte terminal differentiation. Finally, knockdown of Celf1 in myocyte with CUG-expansion led to partial rescue, by promoting cell cycle exit. Our results suggest that Celf1 plays a distinctive and negative role in terminal myocyte differentiation, which partially contribute to DM1 RNA toxicity. Targeting Celf1 may be a valid strategy in correcting DM1 muscle phenotypes, especially for congenital cases.

Alternative polyadenylation regulates CELF1/CUGBP1 target transcripts following T cell activation

Alternative polyadenylation (APA) is an evolutionarily conserved mechanism for regulating gene expression. Transcript 3' end shortening through changes in polyadenylation site usage occurs following T cell activation, but the consequences of APA on gene expression are poorly understood. We previously showed that GU-rich elements (GREs) found in the 3' untranslated regions of select transcripts mediate rapid mRNA decay by recruiting the protein CELF1/CUGBP1. Using a global RNA sequencing approach, we found that a network of CELF1 target transcripts involved in cell division underwent preferential 3' end shortening via APA following T cell activation, resulting in decreased inclusion of CELF1 binding sites and increased transcript expression. We present a model whereby CELF1 regulates APA site selection following T cell activation through reversible binding to nearby GRE sequences. These findings provide insight into the role of APA in controlling cellular proliferation during biological processes such as development, oncogenesis and T cell activation.

GSK3β is a new therapeutic target for myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1), an incurable, neuromuscular disease, is caused by the expansion of CTG repeats within the 3′ UTR of DMPK on chromosome 19q. In DM1 patients, mutant DMPK transcripts deregulate RNA metabolism by altering CUG RNA-binding proteins. Several approaches have been proposed for DM1 therapy focused on specific degradation of the mutant CUG repeats or on correction of RNA-binding proteins, affected by CUG repeats. One such protein is CUG RNA-binding protein (CUGBP1). The ability of CUGBP1 to increase or inhibit translation depends on phosphorylation at Ser302, which is mediated by cyclin D3-CDK4. The mutant CUG repeats increase the levels of CUGBP1 protein and inhibit Ser302 phosphorylation, leading to the accumulation of CUGBP1 isoforms that repress translation (i.e., CUGBP1^REP). Elevation of CUGBP1^REP in DM1 is caused by increased GSK3β kinase, which reduces the cyclin D3-CDK4 pathway and subsequent phosphorylation of CUGBP1 at Ser302. In this review, we discuss our recent discovery showing that correction of GSK3β activity in the DM1 mouse model (i.e., HSA^LR mice) reduces DM1 muscle pathology. These findings demonstrate that GSK3β is a novel therapeutic target for treating DM1.