Splicing Regulators and Their Roles in Cancer Biology and Therapy

NMR analysis, cytotoxic activity and theoretical study of a complex between SRPIN340 and p-sulfonic acid calix[6]arene

Aim: This study aimed to enhance the aqueous dissolution of SRPK inhibitor N-(2-(piperidin-1-yl)-5-(trifluoromethyl)phenyl)isonicotinamide (SRPIN340). Materials & Methods: A complex with p-sulfonic calix[6]arene (Host) and SRPIN340 (Guest) was prepared, studied via 1H nuclear magnetic resonance (NMR) and theoretical calculations and biologically evaluated on cancer cell lines. Results & conclusion: The 1:1 host (H)/guest (G) complex significantly enhanced the aqueous dissolution of SRPIN340, achieving 64.8% water solubility as determined by 1H NMR quantification analysis. The H/G complex reduced cell viability by 75% for HL60, ∼50% for Nalm6 and Jurkat, and ∼30% for B16F10 cells. It exhibited greater cytotoxicity than free SRPIN340 against Jurkat and B16F10 cells. Theoretical studies indicated hydrogen bond stabilization of the complex, suggesting broader applicability of SRPIN340 across diverse biological systems.

N-Benzylated 5-Hydroxybenzothiophene-2-carboxamides as Multi-Targeted Clk/Dyrk Inhibitors and Potential Anticancer Agents

Numerous studies have reported that Dyrk1A, Dyrk1B, and Clk1 are overexpressed in multiple cancers, suggesting a role in malignant disease. Here, we introduce a novel class of group-selective kinase inhibitors targeting Dyrk1A, Dyrk1B, and Clk1. This was achieved by modifying our earlier selective Clk1 inhibitors, which were based on the 5-methoxybenzothiophene-2-carboxamide scaffold. By incorporating a 5-hydroxy group, we increased the potential for additional hydrogen bond interactions that broadened the inhibitory effect to include Dyrk1A and Dyrk1B kinases. Within this series, compounds 12 and 17 emerged as the most potent multi-kinase inhibitors against Dyrk1A, Dyrk1B, and Clk1. Furthermore, when assessed against the most closely related kinases also implicated in cancer, the frontrunner compounds revealed additional inhibitory activity against Haspin and Clk2. Compounds 12 and 17 displayed high potency across various cancer cell lines with minimal effect on non-tumor cells. By examining the effect of these inhibitors on cell cycle distribution, compound 17 retained cells in the G2/M phase and induced apoptosis. Compounds 12 and 17 could also increase levels of cleaved caspase-3 and Bax, while decreasing the expression of the antiapoptotic Bcl-2 protein. These findings support the further study and development of these compounds as novel anticancer therapeutics.

Pharmacological inhibition of CLK2 activates YAP by promoting alternative splicing of AMOTL2

Yes-associated protein (YAP), the downstream effector of the evolutionarily conserved Hippo pathway, promotes cellular proliferation and coordinates certain regenerative responses in mammals. Small molecule activators of YAP may, therefore, display therapeutic utility in treating disease states involving insufficient proliferative repair. From a high-throughput chemical screen of the comprehensive drug repurposing library ReFRAME, here we report the identification of SM04690, a clinical stage inhibitor of CLK2, as a potent activator of YAP-driven transcriptional activity in cells. CLK2 inhibition promotes alternative splicing of the Hippo pathway protein AMOTL2, producing an exon-skipped gene product that can no longer associate with membrane-bound proteins, resulting in decreased phosphorylation and membrane localization of YAP. This study reveals a novel mechanism by which pharmacological perturbation of alternative splicing inactivates the Hippo pathway and promotes YAP-dependent cellular growth.

Phosphorylation mediated regulation of RNA splicing in plants

For the past two decades, the study of alternative splicing (AS) and its involvement in plant development and stress response has grown in popularity. Only recently however, has the focus shifted to the study of how AS regulation (or lack-thereof) affects downstream mRNA and protein landscapes and how these AS regulatory events impact plant development and stress tolerance. In humans, protein phosphorylation represents one of the predominant mechanisms by which AS is regulated and thus the protein kinases governing these phosphorylation events are of interest for further study. Large-scale phosphoproteomic studies in plants have consistently found that RNA splicing-related proteins are extensively phosphorylated, however, the signaling pathways involved in AS regulation have not been resolved. In this mini-review, we summarize our current knowledge of the three major splicing-related protein kinase families in plants that are suggested to mediate AS phospho-regulation and draw comparisons to their metazoan orthologs. We also summarize and contextualize the phosphorylation events identified as occurring on splicing-related protein families to illustrate the high degree to which splicing-related proteins are modified, placing a new focus on elucidating the impacts of AS at the protein and PTM-level.

The dysregulation of lncRNAs by epigenetic factors in human pathologies

Dysregulation of long noncoding RNAs (lncRNAs) contributes to numerous human diseases, including cancers and autoimmune diseases (ADs). Given the importance of lncRNAs in disease initiation and progression, a deeper understanding of their complex regulatory network is required to facilitate their use as therapeutic targets for ADs. In this review, we summarize how lncRNAs are dysregulated in pathological states by epigenetic factors, including RNA-binding proteins, chemical modifications (N6-methyladenosine, 5-methylcytosine, 7-methylguanosine, adenosine-to-inosine editing, microRNA, alternative splicing, DNA methylation, and histone modification). Moreover, the roles of lncRNA epigenetic regulators in immune response and ADs are discussed, providing new insights into the complicated epigenetic factor-lncRNA network, thus, laying a theoretical foundation for future research and clinical application of lncRNAs.

Combination CDC-like kinase inhibition (CLK)/Dual-specificity tyrosine-regulated kinase (DYRK) and taxane therapy in CTNNB1-mutated endometrial cancer

SM08502 (cirtuvivint) is a novel pan CDC-like kinase (CLK) and Dual specificity tyrosine kinase (DYRK) inhibitor that targets mRNA splicing and is optimized for Wnt pathway inhibition. Previous evaluation of single agent CLK/DYRK inhibition (SM04690) demonstrated inhibition of tumor progression and β-catenin/TCF transcriptional activity in CTNNB1-mutant endometrial cancer (EC). In-vitro analysis of SM08502 similarly decreases Wnt transcriptional activity and cellular proliferation while increasing cellular apoptosis. SM08502 is an active single-agent therapy with IC50's in the nanomolar range for all EC cell lines evaluated. Combination of SM08502 with paclitaxel has synergistic effect in vitro, as demonstrated by Combination Index <1, and inhibits tumor progression in four endometrial cancer models (HEC265, Ishikawa, Ishikawa-S33Y, and SNGM). In our in vivo mouse models, Ishikawa demonstrated significantly lower tumor volumes of combination vs SM08502 alone (Repeated Measures one-way ANOVA, p = 0.04), but not vs paclitaxel alone. HEC265, SNGM, and Ishikawa-S33Y tumors all had significantly lower tumor volumes with combination SM08502 and paclitaxel compared to single-agent paclitaxel (Repeated Measures one-way ANOVA, p = 0.01, 0.004, and 0.0008, respectively) or single-agent SM08502 (Repeated Measures one-way ANOVA, p = 0.002, 0.005, and 0.01, respectively) alone. Mechanistically, treatment with SM08502 increases alternative splicing (AS) events compared to treatment with paclitaxel. AS regulation is an important post-transcriptional mechanism associated with the oncogenic process in many cancers, including EC. Results from these studies have led to a Phase I evaluation of this combination in recurrent EC.

Oncogenic KRAS alters splicing factor phosphorylation and alternative splicing in lung cancer

BACKGROUND

Alternative RNA splicing is widely dysregulated in cancers including lung adenocarcinoma, where aberrant splicing events are frequently caused by somatic splice site mutations or somatic mutations of splicing factor genes. However, the majority of mis-splicing in cancers is unexplained by these known mechanisms. We hypothesize that the aberrant Ras signaling characteristic of lung cancers plays a role in promoting the alternative splicing observed in tumors.

METHODS

We recently performed transcriptome and proteome profiling of human lung epithelial cells ectopically expressing oncogenic KRAS and another cancer-associated Ras GTPase, RIT1. Unbiased analysis of phosphoproteome data identified altered splicing factor phosphorylation in KRAS-mutant cells, so we performed differential alternative splicing analysis using rMATS to identify significantly altered isoforms in lung epithelial cells. To determine whether these isoforms were uniquely regulated by KRAS, we performed a large-scale splicing screen in which we generated over 300 unique RNA sequencing profiles of isogenic A549 lung adenocarcinoma cells ectopically expressing 75 different wild-type or variant alleles across 28 genes implicated in lung cancer.

RESULTS

Mass spectrometry data showed widespread downregulation of splicing factor phosphorylation in lung epithelial cells expressing mutant KRAS compared to cells expressing wild-type KRAS. We observed alternative splicing in the same cells, with 2196 and 2416 skipped exon events in KRASG12V and KRASQ61H cells, respectively, 997 of which were shared (p < 0.001 by hypergeometric test). In the high-throughput splicing screen, mutant KRAS induced the greatest number of differential alternative splicing events, second only to the RNA binding protein RBM45 and its variant RBM45M126I. We identified ten high confidence cassette exon events across multiple KRAS variants and cell lines. These included differential splicing of the Myc Associated Zinc Finger (MAZ). As MAZ regulates expression of KRAS, this splice variant may be a mechanism for the cell to modulate wild-type KRAS levels in the presence of oncogenic KRAS.

CONCLUSION

Proteomic and transcriptomic profiling of lung epithelial cells uncovered splicing factor phosphorylation and mRNA splicing events regulated by oncogenic KRAS. These data suggest that in addition to widespread transcriptional changes, the Ras signaling pathway can promote post-transcriptional splicing changes that may contribute to oncogenic processes.

A comprehensive study of alternative splicing in malignant pleural mesothelioma identifies potential therapeutic targets in a new cluster with poor survival

Background

Malignant pleural mesothelioma (MPM) is one of the most aggressive tumors with few effective treatments worldwide. It has been suggested that alternative splicing at the transcriptome level plays an indispensable role in MPM.

Methods

We analyzed the splicing profile of 84 MPM patients from the TCGA cohort by using seven typical splicing types. We classified MPM patients based on their splicing status and conducted a comprehensive analysis of the correlation between the splicing classification and clinical characteristics, genetic variation, pathway changes, immune heterogeneity, and potential therapeutic targets.

Results

The expression of the alternative splicing regulator SRPK1 is significantly higher in MPM tissues than in normal tissues, and correlates with poor survival. SRPK1 deficiency promotes MPM cell apoptosis and inhibits cell migration in vitro. We divided the MPM patients into four clusters based on their splicing profile and identified two clusters associated with the shortest (cluster 3) and longest (cluster 4) survival time. We present the different gene signatures of each cluster that are related to survival and splicing. Comprehensive analysis of data from the GDSC and TCGA databases revealed that cluster 3 MPM patients could respond well to the small‐molecule inhibitor CHIR‐99021, a small‐molecule inhibitor of GSK‐3.

Conclusion

We performed unsupervised clustering of alternative splicing data from 84 MPM patients from the TCGA database and identified a cluster associated with the worst prognosis that was sensitive to a GSK‐3 inhibitor.

Immunomodulatory activity of trifluoromethyl arylamides derived from the SRPK inhibitor SRPIN340 and their potential use as vaccine adjuvant

The serine/arginine-rich protein kinases (SRPK) specifically phosphorylate their substrates at RS-rich dipeptides, which are abundantly found in SR splicing factors. SRPK are classically known for their ability to affect the splicing and expression of gene isoforms commonly implicated in cancer and diseases associated with infectious processes. Non-splicing functions have also been attributed to SRPK, which highlight their functional plasticity and relevance as therapeutic targets for pharmacological intervention. In this sense, different SRPK inhibitors have been developed, such as the well-known SRPIN340 and its derivatives, with anticancer and antiviral activities. Here we evaluated the potential immunomodulatory activity of SRPIN340 and three trifluoromethyl arylamide derivatives. In in vitro analysis with RAW 264.7 macrophages and primary splenocytes, all the compounds modulated the expression of immune response mediators and antigen-presentation molecules related to a tendency for M2 macrophage polarization. Immunization experiments were carried out in mice to evaluate their potential as vaccine immunostimulants. When administrated alone, the compounds altered the expression of immune factors at the injection site and did not produce macroscopic or microscopic local reactions. In addition, when prepared as an adjuvant with inactivated EHV-1 antigens, all the compounds increased the anti-EHV-1 neutralizing antibody titers, a change that is consistent with an increased Th2 response. These findings demonstrate that SRPIN340 and its derivatives exhibit a noticeable capacity to modulate innate and adaptative immune cells, disclosing their potential to be used as vaccine adjuvants or in immunotherapies.

Exploring the roles of the Cdc2-like kinases in cancers

The Cdc2-like kinases (CLKs 1-4) are involved in regulating the alternative splicing of a variety of genes. Their activity contributes to important cellular processes such as proliferation, differentiation, apoptosis, migration, and cell cycle regulation. Abnormal expression of CLKs can lead to cancers; therefore, pharmacological inhibition of CLKs may be a useful therapeutic strategy. This review summarises what is known about the roles of each of the CLKs in cancerous cells, as well as the effects of relevant small molecule CLK inhibitors.

Integrated bioinformatics analysis of validated and circulating miRNAs in ovarian cancer

Recent studies have focused on the early detection of ovarian cancer (OC) using tumor materials by liquid biopsy. The mechanisms of microRNAs (miRNAs) to impact OC and signaling pathways are still unknown. This study aims to reliably perform functional analysis of previously validated circulating miRNAs' target genes by using pathfindR. Also, overall survival and pathological stage analyses were evaluated with miRNAs' target genes which are common in the The Cancer Genome Atlas and GTEx datasets. Our previous studies have validated three downregulated miRNAs (hsa-miR-885-5p, hsa-miR-1909-5p, and hsalet7d-3p) having a diagnostic value in OC patients' sera, with high-throughput techniques. The predicted target genes of these miRNAs were retrieved from the miRDB database (v6.0). Active-subnetwork-oriented Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis was conducted by pathfindR using the target genes. Enrichment of KEGG pathways assessed by the analysis of pathfindR indicated that 24 pathways were related to the target genes. Ubiquitin-mediated proteolysis, spliceosome and Notch signaling pathway were the top three pathways with the lowest p-values (p < 0.001). Ninety-three common genes were found to be differentially expressed (p < 0.05) in the datasets. No significant genes were found to be significant in the analysis of overall survival analyses, but 24 genes were found to be significant with pathological stages analysis (p < 0.05). The findings of our study provide in-silico evidence that validated circulating miRNAs' target genes and enriched pathways are related to OC and have potential roles in theranostics applications. Further experimental investigations are required to validate our results which will ultimately provide a new perspective for translational applications in OC management.

Transcriptome analysis of clock disrupted cancer cells reveals differential alternative splicing of cancer hallmarks genes

Emerging evidence points towards a regulatory role of the circadian clock in alternative splicing (AS). Whether alterations in core-clock components may contribute to differential AS events is largely unknown. To address this, we carried out a computational analysis on recently generated time-series RNA-seq datasets from three core-clock knockout (KO) genes (ARNTL, NR1D1, PER2) and WT of a colorectal cancer (CRC) cell line, and time-series RNA-seq datasets for additional CRC and Hodgkin’s lymphoma (HL) cells, murine WT, Arntl KO, and Nr1d1/2 KO, and murine SCN WT tissue. The deletion of individual core-clock genes resulted in the loss of circadian expression in crucial spliceosome components such as SF3A1 (in ARNTL^KO), SNW1 (in NR1D1^KO), and HNRNPC (in PER2^KO), which led to a differential pattern of KO-specific AS events. All HCT116^KO cells showed a rhythmicity loss of a crucial spliceosome gene U2AF1, which was also not rhythmic in higher progression stage CRC and HL cancer cells. AS analysis revealed an increase in alternative first exon events specific to PER2 and NR1D1 KO in HCT116 cells, and a KO-specific change in expression and rhythmicity pattern of AS transcripts related to cancer hallmarks genes including FGFR2 in HCT116_ARNTL^KO, CD44 in HCT116_NR1D1^KO, and MET in HCT116_PER2^KO. KO-specific changes in rhythmic properties of known spliced variants of these genes (e.g. FGFR2 IIIb/FGFR2 IIIc) correlated with epithelial-mesenchymal-transition signalling. Altogether, our bioinformatic analysis highlights a role for the circadian clock in the regulation of AS, and reveals a potential impact of clock disruption in aberrant splicing in cancer hallmark genes.

Long non-coding RNA AGAP2-AS1 is up regulated in colorectal cancer

Accumulating evidence has indicated that, aberrant lncRNA expression plays essential roles in the colorectal cancer (CRC) tumorigenesis. AGAP2-AS1 is upregulated in some cancers, however, its involvement in the CRC tumorigenesis in the population of North-West of Iran has remained unknown. In this study, we evaluated its deregulation in CRC microarray datasets, colon cell lines, CRC tumor, adenomatous colorectal polyps and their paired normal tissues. The results showed that AGAP2-AS1 is upregulated in CRC and might be considered as a potential biomarker for CRC development. Moreover, our results suggest AGAP2-AS1 promoted CRC progression by sponging the hsa-miR-15/16 family and upregulation of their targets.

The pseudogene PRELID1P6 promotes glioma progression via the hnHNPH1-Akt/mTOR axis

Research over the past decade has suggested important roles for pseudogenes in glioma. This study aimed to show that pseudogene PRELI domain-containing 1 pseudogene 6 (PRELID1P6) promotes glioma progression. Aberrant expression of genes was screened using The Cancer Genome Atlas database. We found that mRNA level of PRELID1P6 was highly upregulated in glioma and was associated with a shorter survival time. Functional studies showed that the knockdown of PRELID1P6 decreased cell proliferation, sphere formation, and clone formation ability and blocked the cell cycle transition at G0/G1, while overexpression of PRELID1P6 had the opposite effects. Mechanistically, knockdown of PRELID1P6 changed the cellular localization of heterogeneous nuclear ribonucleoprotein H1 (hnRNPH1) from nucleus to cytoplasm, which promoted ubiquitin-mediated degradation of hnRNPH1. RNA-sequence and gene set enrichment analysis suggested that knockdown of PRELID1P6 regulates the apoptosis signaling pathway. Western blotting showed that PRELID1P6 increased TRF2 expression by hnRNPH1-mediated alternative splicing effect and activated the Akt/mTOR pathway. Furthermore, Akt inhibitor MK2206 treatment reversed the oncogenic function of PRELID1P6. PRELID1P6 was also found to be negatively regulated by miR-1825. Our result showed that PRELID1P6 promotes glioma progression through the hnHNPH1-Akt/mTOR pathway. These findings shed new light on the important role of PRELID1P6 as a novel oncogene for glioma.

Alternative splicing and cancer: a systematic review

The abnormal regulation of alternative splicing is usually accompanied by the occurrence and development of tumors, which would produce multiple different isoforms and diversify protein expression. The aim of the present study was to conduct a systematic review in order to describe the regulatory mechanisms of alternative splicing, as well as its functions in tumor cells, from proliferation and apoptosis to invasion and metastasis, and from angiogenesis to metabolism. The abnormal splicing events contributed to tumor progression as oncogenic drivers and/or bystander factors. The alterations in splicing factors detected in tumors and other mis-splicing events (i.e., long non-coding and circular RNAs) in tumorigenesis were also included. The findings of recent therapeutic approaches targeting splicing catalysis and splicing regulatory proteins to modulate pathogenically spliced events (including tumor-specific neo-antigens for cancer immunotherapy) were introduced. The emerging RNA-based strategies for the treatment of cancer with abnormally alternative splicing isoforms were also discussed. However, further studies are still required to address the association between alternative splicing and cancer in more detail.

Emerging Roles of SRSF3 as a Therapeutic Target for Cancer

Ser/Arg-rich (SR) proteins are RNA-binding proteins known as constitutive and alternative splicing (AS) regulators that regulate multiple aspects of the gene expression program. Ser/Arg-rich splicing factor 3 (SRSF3) is the smallest member of the SR protein family, and its level is controlled by multiple factors and involves complex mechanisms in eukaryote cells, whereas the aberrant expression of SRSF3 is associated with many human diseases, including cancer. Here, we review state-of-the-art research on SRSF3 in terms of its function, expression, and misregulation in human cancers. We emphasize the negative consequences of the overexpression of the SRSF3 oncogene in cancers, the pathways underlying SRSF3-mediated transformation, and implications of potential anticancer drugs by downregulation of SRSF3 expression for cancer therapy. Cumulative research on SRSF3 provides critical insight into its essential part in maintaining cellular processes, offering potential new targets for anti-cancer therapy.

The biological function and clinical significance of SF3B1 mutations in cancer

Spliceosome mutations have become the most interesting mutations detected in human cancer in recent years. The spliceosome, a large, dynamic multimegadalton small nuclear ribonucleoprotein composed of small nuclear RNAs associated with proteins, is responsible for removing introns from precursor mRNA (premRNA) and generating mature, spliced mRNAs. SF3B1 is the largest subunit of the spliceosome factor 3b (SF3B) complex, which is a core component of spliceosomes. Recurrent somatic mutations in SF3B1 have been detected in human cancers, including hematological malignancies and solid tumors, and indicated to be related to patient prognosis. This review summarizes the research progress of SF3B1 mutations in cancer, including SF3B1 mutations in the HEAT domain, the multiple roles and aberrant splicing events of SF3B1 mutations in the pathogenesis of tumors, and changes in mutated cancer cells regarding sensitivity to SF3B small-molecule inhibitors. In addition, the potential of SF3B1 or its mutations to serve as biomarkers or therapeutic targets in cancer is discussed. The accumulated knowledge about SF3B1 mutations in cancer provides critical insight into the integral role the SF3B1 protein plays in mRNA splicing and suggests new targets for anticancer therapy.

The PI3K-AKT network at the interface of oncogenic signalling and cancer metabolism

The altered metabolic programme of cancer cells facilitates their cell-autonomous proliferation and survival. In normal cells, signal transduction pathways control core cellular functions, including metabolism, to couple the signals from exogenous growth factors, cytokines or hormones to adaptive changes in cell physiology. The ubiquitous, growth factor-regulated phosphoinositide 3-kinase (PI3K)-AKT signalling network has diverse downstream effects on cellular metabolism, through either direct regulation of nutrient transporters and metabolic enzymes or the control of transcription factors that regulate the expression of key components of metabolic pathways. Aberrant activation of this signalling network is one of the most frequent events in human cancer and serves to disconnect the control of cell growth, survival and metabolism from exogenous growth stimuli. Here we discuss our current understanding of the molecular events controlling cellular metabolism downstream of PI3K and AKT and of how these events couple two major hallmarks of cancer: growth factor independence through oncogenic signalling and metabolic reprogramming to support cell survival and proliferation.

Tumor suppressor genes and allele-specific expression: mechanisms and significance

Recent findings indicate that allele-specific expression (ASE) at specific cancer driver gene loci may be of importance in onset/progression of the disease. Of particular interest are loss-of-function (LOF) of tumor suppressor gene (TSGs) alleles. While LOF tumor suppressor mutations are typically considered to be recessive, if these mutant alleles can be significantly differentially expressed relative to wild-type alleles in heterozygotes, the clinical consequences could be significant. LOF TSG alleles are shown to be segregating at high frequencies in world-wide populations of normal/healthy individuals. Matched sets of normal and tumor tissues isolated from 233 cancer patients representing four diverse tumor types demonstrate functionally important changes in patterns of ASE in individuals heterozygous for LOF TSG alleles associated with cancer onset/progression. While a variety of molecular mechanisms were identified as potentially contributing to changes in ASE patterns in cancer, changes in DNA copy number and allele-specific alternative splicing possibly mediated by antisense RNA emerged as predominant factors. In conclusion, LOF TSGs are segregating in human populations at significant frequencies indicating that many otherwise healthy individuals are at elevated risk of developing cancer. Changes in ASE between normal and cancer tissues indicates that LOF TSG alleles may contribute to cancer onset/progression even when heterozygous with wild-type functional alleles.

AR mRNA stability is increased with AR-antagonist resistance via 3'UTR variants

Advanced prostate cancer is often treated with AR antagonists which target the androgen receptor (AR) on which the growth of the tumour depends. Prostate cancer often develops AR-antagonist resistance via a plethora of mechanisms, many of which are as yet unknown, but it is thought that AR upregulation or AR ligand-binding site mutations, may be responsible. Here we describe the production of cell lines based on LNCaP and VCaP, with acquired resistance to the clinically relevant AR antagonists, bicalutamide and enzalutamide. In these resistant cells, we observed, via RNA-seq, that new variants in the 3'UTR of the AR mRNA were detectable and that the levels were increased both with AR-antagonist treatment and with hormonal starvation. Around 20% of AR transcripts showed a 3 kb deletion within the 6.7 kb 3'UTR sequence. Actinomycin D and luciferase fusion studies indicated that this shorter mRNA variant was inherently more stable in anti-androgen-resistant cell lines. Of additional interest was that the AR UTR variant could be detected in the sera of prostate cancer patients in a cohort of serum samples collected from patients of Gleason grades 6-10, with an increasing level correlated to increasing grade. We hypothesise that the shorter AR UTR variant is a survival adaptation to low hormone levels and/or AR-antagonist treatment in these cells, where a more stable mRNA may allow higher levels of AR expression under these conditions.

PRMT5 methylome profiling uncovers a direct link to splicing regulation in acute myeloid leukemia

Protein arginine methyltransferase 5 (PRMT5) has emerged as a promising cancer drug target, and three PRMT5 inhibitors are currently in clinical trials for multiple malignancies. In this study, we investigated the role of PRMT5 in human acute myeloid leukemia (AML). Using an enzymatic dead version of PRMT5 and a PRMT5-specific inhibitor, we demonstrated the requirement of the catalytic activity of PRMT5 for the survival of AML cells. We then identified PRMT5 substrates using multiplexed quantitative proteomics and investigated their role in the survival of AML cells. We found that the function of the splicing regulator SRSF1 relies on its methylation by PRMT5 and that loss of PRMT5 leads to changes in alternative splicing of multiple essential genes. This explains the requirement of PRMT5 for leukemia cell survival. We show that PRMT5 regulates binding of SRSF1 to mRNAs and proteins and provide potential biomarkers for the treatment response to PRMT5 inhibitors.

Effects of SRSF1 on subnuclear localization of topoisomerase I

Subnuclear localization of topoisomerase I (top I) is determined by its DNA relaxation activity and a net of its interactions with in majority unidentified nucleolar and nucleoplasmic elements. Here, we recognized SR protein SRSF1 (Serine/arginine-rich splicing factor 1, previously known as SF2/ASF) as a new element of the net. In HeLa cells, overexpression of SRSF1 recruited top I to the nucleoplasm whereas its silencing concentrated it in the nucleolus. Effect of SRSF1 was independent of top I relaxation activity and was the best pronounced for the mutant inactive in relaxation reaction. In HCT116 cells where top I was not released from the nucleolus upon halting relaxation activity, it was also not relocated by elevated level of SRSF1. Out of remaining SR proteins, SRSF5, SRSF7, and SRSF9 did not influence the localization of top I in HeLa cells whereas overexpression of SRSF2, SRSF3, SRSF6, and partly SRSF4 concentrated top I in the nucleolus, most possibly due to the reduction of the SRSF1 accessibility. Specific effect of SRSF1 was exerted because of its distinct RS domain. Silencing of SRSF1 compensated the deletion of the top I N-terminal region, individually responsible for nucleoplasmic localization of the mutant, and restored the wild-type phenotype of deletion mutant localization. SRSF1 was essential for the camptothecin-induced clearance from the nucleolus. These results suggest a possible role of SRSF1 in establishing partition of top I between the nucleolus and the nucleoplasm in some cell types with distinct combinations of SR proteins levels.

SRPK1 is a poor prognostic indicator and a novel potential therapeutic target for human colorectal cancer

Background

Serine/arginine protein kinase 1 (SRPK1) is a protein kinase that belongs to the serine/arginine-rich domain family of splicing factors which are essential for splice-site selection, especially the modulation for RNA metabolism, localization, and translation. High expression of SRPK1 has been found in numerous human cancers, but its mechanism in colorectal cancer (CRC) is still rarely reported.

Purpose

To investigate the expression of SRPK1 in CRC tissues and cells and determine its functions and mechanism in CRC.

Methods

The expression of SRPK1 was explored in human CRC patients and cells by immunohistochemistry, real-time quantitative PCR, and Western blot; Cell Counting Kit-8, Transwell, flow cytometry, and tube formation assay were used to investigate the CRC cell viability, migration, apoptosis, and angiogenesis, respectively.

Results

SRPK1 was overexpressed in CRC tumor tissues and cells, and correlated with tumor node metastasis stage; inhibition of SRPK1 by siRNA resulted in decreased cell growth and migration, significantly increased apoptosis, and suppressed angiogenesis.

Conclusion

SRPK1 can be a prognostic indicator of CRC and may be a therapeutic target for CRC.

Influence of transcriptional variants on metastasis

Cancer metastasis is defined as the dissemination of malignant cells from the primary tumor site, leading to colonization of distant organs and the establishment of a secondary tumor. Metastasis is frequently associated with chemoresistance and is the major cause of cancer-related mortality. Metastatic cells need to acquire the ability to resist to stresses provided by different environments, such as reactive oxygen species, shear stress, hemodynamic forces, stromal composition, and immune responses, to colonize other tissues. Hence, only a small population of cells has a metastasis-initiating potential. Several studies have revealed the misregulation of transcriptional variants during cancer progression, and many splice events can be used to distinguish between normal and tumoral tissue. These variants, which are abnormally expressed in malignant cells, contribute to an adaptive response of tumor cells and the success of the metastatic cascade, promoting an anomalous cell cycle, cellular adhesion, resistance to death, cell survival, migration and invasion. Understanding the different aspects of splicing regulation and the influence of transcriptional variants that control metastatic cells is critical for the development of therapeutic strategies. In this review, we describe how transcriptional variants contribute to metastatic competence and discuss how targeting specific isoforms may be a promising therapeutic strategy.

Serine-arginine protein kinase 1 (SRPK1) is elevated in gastric cancer and plays oncogenic functions

Serine-arginine protein kinase 1 (SRPK1) phosphorylates proteins involved in the regulation of several mRNA processing pathways including alternative splicing. SRPK1 has been reported to be over-expressed in multiple cancers including prostate, breast, lung and glioma. Several studies further identified that inhibition of SRPK1 showed tumor-suppressive effects, thus raising SRPK1 as a novel candidate chemotherapy target. Interestingly, SRPK1 plays tumor suppressing role in mouse embryonic fibroblasts, on that SRPK1-silencing induces cell transformation. Therefore, the effect of SRPK1 seems heterogeneously in different cell types and tissues. The existence and role of SRPK1 in gastric cancer (GC) hasn't been reported. Here we investigated the expression pattern of SRPK1 in GC by immunohistochemistry and found that it was up-regulated in tumor tissues, where its expression was correlated with tumor grade and prognosis. Further, we explored the signaling mechanism of SRPK1 in promoting GC progression, which revealed that both PP2A and DUSP6 phosphatases impaired the oncogenic effects of SRPK1. However, we didn't find any direct interaction between SRPK1 with PP2A or DUSP6, indicating PP2A and DUSP6 function by regulating the downstream effectors of SRPK1. Our study not only revealed the clinical significance of SRPK1 in GC, but also provided new evidence for its signaling modulation which is invaluable for novel chemotherapy development.

Cellular differentiation state modulates the mRNA export activity of SR proteins

SR proteins connect nuclear pre-mRNA processing to mRNA export and translation. Botti et al. develop a quantitative nucleocytoplasmic shuttling assay and show that SR proteins are differentially modified and active in differentiated and pluripotent cells.

SR proteins function in nuclear pre-mRNA processing, mRNA export, and translation. To investigate their cellular dynamics, we developed a quantitative assay, which detects differences in nucleocytoplasmic shuttling among seven canonical SR protein family members. As expected, SRSF2 and SRSF5 shuttle poorly in HeLa cells but surprisingly display considerable shuttling in pluripotent murine P19 cells. Combining individual-resolution cross-linking and immunoprecipitation (iCLIP) and mass spectrometry, we show that elevated arginine methylation of SRSF5 and lower phosphorylation levels of cobound SRSF2 enhance shuttling of SRSF5 in P19 cells by modulating protein–protein and protein–RNA interactions. Moreover, SRSF5 is bound to pluripotency-specific transcripts such as Lin28a and Pou5f1/Oct4 in the cytoplasm. SRSF5 depletion reduces and overexpression increases their cytoplasmic mRNA levels, suggesting that enhanced mRNA export by SRSF5 is required for the expression of pluripotency factors. Remarkably, neural differentiation of P19 cells leads to dramatically reduced SRSF5 shuttling. Our findings indicate that posttranslational modification of SR proteins underlies the regulation of their mRNA export activities and distinguishes pluripotent from differentiated cells.

Trifluoromethyl arylamides with antileukemia effect and intracellular inhibitory activity over serine/arginine-rich protein kinases (SRPKs)

The serine/arginine-rich protein kinases (SRPKs) have frequently been found with altered activity in a number of cancers, suggesting they could serve as potential therapeutic targets in oncology. Here we describe the synthesis of a series of twenty-two trifluoromethyl arylamides based on the known SRPKs inhibitor N-(2-(piperidin-1-yl)-5-(trifluoromethyl)phenyl)isonicotinamide (SRPIN340) and the evaluation of their antileukemia effects. Some derivatives presented superior cytotoxic effects against myeloid and lymphoid leukemia cell lines compared to SRPIN340. In particular, compounds 24, 30, and 36 presented IC₅₀ values ranging between 6.0 and 35.7 μM. In addition, these three compounds were able to trigger apoptosis and autophagy, and to exhibit synergistic effects with the chemotherapeutic agent vincristine. Furthermore, compound 30 was more efficient than SRPIN340 in impairing the intracellular phosphorylation status of SR proteins as well as the expression of MAP2K1, MAP2K2, VEGF, and RON oncogenic isoforms. Therefore, novel compounds with increased intracellular effects against SRPK activity were obtained, contributing to medicinal chemistry efforts towards the development of new anticancer agents.

Splicing factors of SR and hnRNP families as regulators of apoptosis in cancer

SR and hnRNP proteins were initially discovered as regulators of alternative splicing: the process of controlled removal of introns and selective joining of exons through which multiple transcripts and, subsequently, proteins can be expressed from a single gene. Alternative splicing affects genes involved in all crucial cellular processes, including apoptosis. During cancerogenesis impaired apoptotic control facilitates survival of cells bearing molecular aberrations, contributing to their unrestricted proliferation and chemoresistance. Apparently, SR and hnRNP proteins regulate all levels of expression of apoptotic genes, including transcription initiation and elongation, alternative splicing, mRNA stability, translation, and protein degradation. The frequently disturbed expressions of SR/hnRNP proteins in cancers lead to impaired functioning of target apoptotic genes, including regulators of the extrinsic (Fas, caspase-8, caspase-2, c-FLIP) and the intrinsic pathway (Apaf-1, caspase-9, ICAD), genes encoding Bcl-2 proteins, IAPs, and p53 tumor suppressor. Prototypical members of SR/hnRNP families, SRSF1 and hnRNP A1, promote synthesis of anti-apoptotic splice variants of Bcl-x and Mcl-1, which results in attenuation of programmed cell death in breast cancer and chronic myeloid leukemia. SR/hnRNP proteins significantly affect responses to chemotherapy, acting as mediators or modulators of drug-induced apoptosis. Aberrant expression of SRSF1 and hnRNP K can interfere with tumor responses to chemotherapy in pancreatic and liver cancers. Currently, a number of splicing factor inhibitors is being tested in pre-clinical and clinical trials. In this review we discuss recent findings on the role of SR and hnRNP proteins in apoptotic control in cancer cells as well as their significance in anticancer treatments.