LncRNA MIR155HG Promotes Temozolomide Resistance by Activating the Wnt/ß-Catenin Pathway Via Binding to PTBP1 in Glioma.

Temozolomide (TMZ) is widely used for glioma therapy in the clinic. Currently, the development of TMZ resistance has largely led to poor prognosis. However, very little is understood about the role of MIR155HG, as a long noncoding RNA, in TMZ resistance. In our study, MIR155HG level was markedly higher in glioma patients than in normal controls and that poor survival was positively correlated with MIR155HG expression. It was apparent that TMZ sensitivity was promoted by downregulation of MIR155HG, and this could be reversed by MIR155HG overexpression in vivo and in vitro. Furthermore, polypyrimidine tract binding protein 1 (PTBP1) was proven to bind with MIR155HG and to regulate MIR155HG-related TMZ resistance. Mechanistic investigation showed that the expression levels of both MIR155HG and PTBP1 influenced the expression of relevant proteins in the Wnt/ß-catenin pathway. Collectively, the study demonstrated that the knockdown of MIR155HG increased glioma sensitivity to TMZ by inhibiting Wnt/ß-catenin pathway activation via potently downregulating PTBP1.

Role of polypyrimidine tract-binding protein 1/yin yang 2 signaling in regulating vascular smooth muscle cell proliferation and neointima hyperplasia.

Abnormal proliferation of vascular smooth muscle cells (VSMCs) is a hallmark of vascular restenosis. We investigated whether polypyrimidine tract-binding protein 1 (PTBP1), a novel regulator of cell proliferation and differentiation, is implicated in VSMC proliferation and neointima hyperplasia responding to injury. C57BL/6 J mice of 10-12 weeks old were randomly divided into sham and carotid artery injury group. Primary VSMCs obtained from thoracic aortas of 10- to 12-week-old mice were treated with physiological saline and platelet derived growth factor-BB (PDGF-BB). Adenovirus expressing shCon, shPTBP1 or shYY2 were transfected into the injured common carotid artery or VSMCs. qRT-PCR and immunoblotting were used to determine the mRNA and protein expression levels, respectively. Immunohistochemical staining of H&E and Ki-67 were used to evaluate restenosis of vessels. Cell counting kit-8 assay and Ki-67 immunofluorescent staining were utilized to evaluate the rate of VSMC proliferation. The expression of PTBP1 were upregulated both in injured arteries and in PDGF-BB-treated VSMCs. PTBP1 inhibition significantly attenuated neointima hyperplasia and Ki-67 positive area induced by injury. Knockdown of PTBP1 in vitro also suppressed VSMC proliferation after PDGF-BB treatment. The effects of PTBP1 inhibition mentioned above were all abolished by knockdown of YY2. Finally, we identified four cell cycle regulators (p53, p21, Cdkn1c, Cdkn2b) that were regulated by PTBP1/YY2 axis both in vitro and in vivo. These findings demonstrated that PTBP1 is a critical regulator of VSMC proliferation and neointima hyperplasia via modulating the expression of YY2.

Identification of MicroRNA-124 as a Major Regulator of Enhanced Endothelial Cell Glycolysis in Pulmonary Arterial Hypertension via PTBP1 (Polypyrimidine Tract Binding Protein) and Pyruvate Kinase M2.

BACKGROUND: Pulmonary arterial hypertension (PAH) is characterized by abnormal growth and enhanced glycolysis of pulmonary artery endothelial cells. However, the mechanisms underlying alterations in energy production have not been identified. METHODS: Here, we examined the miRNA and proteomic profiles of blood outgrowth endothelial cells (BOECs) from patients with heritable PAH caused by mutations in the bone morphogenetic protein receptor type 2 (BMPR2) gene and patients with idiopathic PAH to determine mechanisms underlying abnormal endothelial glycolysis. We hypothesized that in BOECs from patients with PAH, the downregulation of microRNA-124 (miR-124), determined with a tiered systems biology approach, is responsible for increased expression of the splicing factor PTBP1 (polypyrimidine tract binding protein), resulting in alternative splicing of pyruvate kinase muscle isoforms 1 and 2 (PKM1 and 2) and consequently increased PKM2 expression. We questioned whether this alternative regulation plays a critical role in the hyperglycolytic phenotype of PAH endothelial cells. RESULTS: Heritable PAH and idiopathic PAH BOECs recapitulated the metabolic abnormalities observed in pulmonary artery endothelial cells from patients with idiopathic PAH, confirming a switch from oxidative phosphorylation to aerobic glycolysis. Overexpression of miR-124 or siRNA silencing of PTPB1 restored normal proliferation and glycolysis in heritable PAH BOECs, corrected the dysregulation of glycolytic genes and lactate production, and partially restored mitochondrial respiration. BMPR2 knockdown in control BOECs reduced the expression of miR-124, increased PTPB1, and enhanced glycolysis. Moreover, we observed reduced miR-124, increased PTPB1 and PKM2 expression, and significant dysregulation of glycolytic genes in the rat SUGEN-hypoxia model of severe PAH, characterized by reduced BMPR2 expression and endothelial hyperproliferation, supporting the relevance of this mechanism in vivo. CONCLUSIONS: Pulmonary vascular and circulating progenitor endothelial cells isolated from patients with PAH demonstrate downregulation of miR-124, leading to the metabolic and proliferative abnormalities in PAH ECs via PTPB1 and PKM1/PKM2. Therefore, the manipulation of this miRNA or its targets could represent a novel therapeutic approach for the treatment of PAH.

Metabolic and Proliferative State of Vascular Adventitial Fibroblasts in Pulmonary Hypertension Is Regulated Through a MicroRNA-124/PTBP1 (Polypyrimidine Tract Binding Protein 1)/Pyruvate Kinase Muscle Axis.

BACKGROUND: An emerging metabolic theory of pulmonary hypertension (PH) suggests that cellular and mitochondrial metabolic dysfunction underlies the pathology of this disease. We and others have previously demonstrated the existence of hyperproliferative, apoptosis-resistant, proinflammatory adventitial fibroblasts from human and bovine hypertensive pulmonary arterial walls (PH-Fibs) that exhibit constitutive reprogramming of glycolytic and mitochondrial metabolism, accompanied by an increased ratio of glucose catabolism through glycolysis versus the tricarboxylic acid cycle. However, the mechanisms responsible for these metabolic alterations in PH-Fibs remain unknown. We hypothesized that in PH-Fibs microRNA-124 (miR-124) regulates PTBP1 (polypyrimidine tract binding protein 1) expression to control alternative splicing of pyruvate kinase muscle (PKM) isoforms 1 and 2, resulting in an increased PKM2/PKM1 ratio, which promotes glycolysis and proliferation even in aerobic environments. METHODS: Pulmonary adventitial fibroblasts were isolated from calves and humans with severe PH (PH-Fibs) and from normal subjects. PTBP1 gene knockdown was achieved via PTBP1-siRNA; restoration of miR-124 was performed with miR-124 mimic. TEPP-46 and shikonin were used to manipulate PKM2 glycolytic function. Histone deacetylase inhibitors were used to treat cells. Metabolic products were determined by mass spectrometry-based metabolomics analyses, and mitochondrial function was analyzed by confocal microscopy and spectrofluorometry. RESULTS: We detected an increased PKM2/PKM1 ratio in PH-Fibs compared with normal subjects. PKM2 inhibition reversed the glycolytic status of PH-Fibs, decreased their cell proliferation, and attenuated macrophage interleukin-1ß expression. Furthermore, normalizing the PKM2/PKM1 ratio in PH-Fibs by miR-124 overexpression or PTBP1 knockdown reversed the glycolytic phenotype (decreased the production of glycolytic intermediates and byproducts, ie, lactate), rescued mitochondrial reprogramming, and decreased cell proliferation. Pharmacological manipulation of PKM2 activity with TEPP-46 and shikonin or treatment with histone deacetylase inhibitors produced similar results. CONCLUSIONS: In PH, miR-124, through the alternative splicing factor PTBP1, regulates the PKM2/PKM1 ratio, the overall metabolic, proliferative, and inflammatory state of cells. This PH phenotype can be rescued with interventions at various levels of the metabolic cascade. These findings suggest a more integrated view of vascular cell metabolism, which may open unique therapeutic prospects in targeting the dynamic glycolytic and mitochondrial interactions and between mesenchymal inflammatory cells in PH.

Inhibition of polypyrimidine tract-binding protein 3 induces apoptosis and cell cycle arrest, and enhances the cytotoxicity of 5- fluorouracil in gastric cancer cells.

BACKGROUND: Human polypyrimidine tract binding protein 3 (PTBP3) was first discovered in 1999 and has been well characterised as a differentiation regulator. However, its role in human cancer has rarely been reported. Our previous study revealed increased PTBP3 protein level in gastric cancer tissues. Downregulation of PTBP3 suppressed the proliferation and differentiation of gastric cancer cells in vivo. METHODS: PTBP3 mRNA levels in human gastric cancer and adjuvant non-tumour tissues were detected. Apoptosis and 5-FU effect were determined in PTBP3-silenced gastric cancer cells. Underlying molecular mechanisms were investigated. RESULTS: MRNA expression of PTBP3 was upregulated in gastric cancer tissues, especially in those at an advanced stage. PTBP3 silencing led to apoptosis, under which modulation of PTB and thereby switch of Bcl-x pre-mRNA splicing pattern might be an important mechanism. Further research found that inhibition of PTBP3 expression enhanced the chemosensitivity of gastric cancer cells towards 5-FU treatment. This was mediated by reduced expression of histone deacetylase 6 (HDAC6), which further inhibited the phosphorylation of Akt and the expression of thymidylate synthase (TYMS), the critical determinant of 5-FU cytotoxicity. CONCLUSIONS: PTBP3 might serve as a biomarker of gastric cancer or potential target for anti-cancer therapy.

CELF and PTB proteins modulate the inclusion of the ß-tropomyosin exon 6B during myogenic differentiation.

Exon 6B from the chicken ß-tropomyosin pre-mRNA is alternatively spliced during myogenic differentiation. Exon 6B is excluded in mRNA from myoblasts and included in mRNA from myotubes. We investigated the regulation of exon 6B inclusion ex vivo in a quail myogenic cell line, which behaves as myoblasts in undifferentiated state and as myotubes after differentiation. We show that the ß-tropomyosin exon 6B is a novel target of CUG-BP and ETR-3-like factor (CELF). Overexpression of CELF proteins in myoblasts activates splicing of exon 6B. Using a dominant-negative form of CELF4, we demonstrate that CELF proteins are involved in switching splicing from exon 6A towards exon 6B inclusion during myogenic differentiation. We also found that polypyrimidine tract binding protein (PTB) is required for splicing repression of exon 6B in myoblasts. CELF and PTB proteins exhibit antagonistic properties toward inclusion of exon 6B during myogenic differentiation. Our results suggest that a change in the protein level of CUGBP1 and PTB proteins, associated with a distinct pattern of PTB during the transition from myoblasts to myotubes is one of the parameters involved in regulating splicing of exon 6B during myogenesis.

hnRNP Q and PTB modulate the circadian oscillation of mouse Rev-erb alpha via IRES-mediated translation.

The physiological and behavioral circadian rhythms of most creatures are controlled by a harmony of functional relationships between clock genes. In mammals, several core clock genes show rhythmic profiles of their mRNA and protein expression. Among them, Rev-erb α functions as a transcriptional repressor, affecting expression patterns of other clock genes. For the continuous and robust oscillation of the molecular clock system, the levels of Rev-erb α protein are expected to be tightly regulated with the correct timing. Here, we demonstrate that Rev-erb α has an internal ribosomal entry site (IRES) in its 5' untranslated region. Furthermore, we demonstrate that heterogeneous nuclear ribonucleoprotein Q and polypyrimidine tract-binding protein (PTB) modulate the IRES-mediated translation of Rev-erb α. We suggest that the rhythmic binding affinity of hnRNP Q to the Rev-erb α IRES and the change in PTB cytosolic levels lead to maintenance of the oscillation profile of the Rev-erb α protein.

Therapeutically viable generation of neurons with antisense oligonucleotide suppression of PTB.

Methods to enhance adult neurogenesis by reprogramming glial cells into neurons enable production of new neurons in the adult nervous system. Development of therapeutically viable approaches to induce new neurons is now required to bring this concept to clinical application. Here, we successfully generate new neurons in the cortex and dentate gyrus of the aged adult mouse brain by transiently suppressing polypyrimidine tract binding protein 1 using an antisense oligonucleotide delivered by a single injection into cerebral spinal fluid. Radial glial-like cells and other GFAP-expressing cells convert into new neurons that, over a 2-month period, acquire mature neuronal character in a process mimicking normal neuronal maturation. The new neurons functionally integrate into endogenous circuits and modify mouse behavior. Thus, generation of new neurons in the dentate gyrus of the aging brain can be achieved with a therapeutically feasible approach, thereby opening prospects for production of neurons to replace those lost to neurodegenerative disease.

The polypyrimidine tract-binding protein is required for efficient dengue virus propagation and associates with the viral replication machinery.

The polypyrimidine tract-binding protein (PTB) functions primarily as an IRES trans-acting factor in the propagation of hepatitis C virus and picornaviruses. PTB interacts with secondary structures within the 3'- and 5'-untranslated regions of these viral genomes to mediate efficient IRES-mediated viral translation. PTB has also been reported to bind to the untranslated region of the single-stranded RNA dengue virus (DENV), suggesting a similar function for PTB in flaviviruses. Indeed small interfering RNA-mediated PTB knockdown inhibited the production of infectious DENV, and this inhibition was specific to PTB knockdown and not due to a nonspecific anti-viral state. In fact, PTB depletion did not inhibit the production infectious yellow fever virus, another flavivirus. Nevertheless, whereas PTB knockdown led to a significant decrease in the accumulation of DENV viral RNAs, it did not impair translation. Moreover, PTB was shown to interact with the DENV nonstructural 4A protein, a known component of the viral replication complex, and with the DENV genome during infection. These data suggest that PTB interacts with the replication complex of DENV and is acting at the level of viral RNA replication.

Lowly expressed lncRNA PVT1 suppresses proliferation and advances apoptosis of glioma cells through up-regulating microRNA-128-1-5p and inhibiting PTBP1.

BACKGROUND: Glioma is a primary intracranial malignancy with poor prognosis, of which the pathogenesis remains to be elucidated. Therein, the aim of this study is to discuss the impacts of lncRNA plasmacytoma variant translocation 1 (PVT1)/microRNA-128-1-5p (miR-128-1-5p)/polypyrimidine tract-binding protein 1 (PTBP1) axis on the biological characteristics of glioma cells. METHODS: Glioma tissue samples (72 cases) and normal brain tissue samples (35 cases) were harvested. The expression of PVT1, miR-128-1-5p and PTBP1 in glioma tissues and cells was detected. Glioma cells were transfected with sh-PVT1, miR-128-1-5p mimics or miR-128-1-5p inhibitors to verify the impacts of PVT1 and miR-128-1-5p on DNA damage, cell colony formation, invasion, proliferation, migration and apoptosis of glioma U87 and U251 cells. The growth of transplanted tumor was tested by tumor xenograft in nude mice. The combination of PVT1 and miR-128-1-5p and the targeting relationship between miR-128-1-5p and PTBP1 were verified. RESULTS: PVT1 and PTBP1 expression was enhanced and miR-128-1-5p expression was degraded in glioma tissues and cells. Overexpressed miR-128-1-5p and lowly-expressed PVT1 promoted DNA damage, suppressed colony formation, invasion, proliferation and migration as well as boosted apoptosis of U251 and U87 cells. Up-regulating miR-128-1-5p and down-regulating PVT1 reduced transplanted tumor volume and weight of glioma in mice. Low expression miR-128-1-5p reversed the effect of low expression PVT1 on the biological characteristics of glioma cells. PVT1 specifically bound to miR-128-1-5p and PTBP1 was the target gene of miR-128-1-5p. CONCLUSION: This study suggests that down-regulated PVT1 or up-regulated miR-128-1-5p boosts apoptosis and attenuates proliferation of glioma cells by inhibiting PTBP1 expression. This study is essential for finding new therapeutic targets for glioma.

Specific inhibition of splicing factor activity by decoy RNA oligonucleotides.

Alternative splicing, a fundamental step in gene expression, is deregulated in many diseases. Splicing factors (SFs), which regulate this process, are up- or down regulated or mutated in several diseases including cancer. To date, there are no inhibitors that directly inhibit the activity of SFs. We designed decoy oligonucleotides, composed of several repeats of a RNA motif, which is recognized by a single SF. Here we show that decoy oligonucleotides targeting splicing factors RBFOX1/2, SRSF1 and PTBP1, can specifically bind to their respective SFs and inhibit their splicing and biological activities both in vitro and in vivo. These decoy oligonucleotides present an approach to specifically downregulate SF activity in conditions where SFs are either up-regulated or hyperactive.

Knockdown of polypyrimidine tract-binding protein suppresses ovarian tumor cell growth and invasiveness in vitro.

Polypyrimidine tract-binding protein (PTB) is an RNA-binding protein with multiple functions in the regulation of RNA processing and IRES-mediated translation. We report here overexpression of PTB in a majority of epithelial ovarian tumors revealed by immunoblotting and tissue microarray (TMA) staining. By western blotting, we found that PTB was overexpressed in 17 out of 19 ovarian tumor specimens compared to their matched-normal tissues. By TMA staining, we found PTB expression in 38 out of 44 ovarian cancer cases but only in two out of nine normal adjacent tissues. PTB is also overexpressed in SV40 large T-antigen immortalized ovarian epithelial cells compared to normal human ovarian epithelial cells. Using doxycycline-inducible small interfering RNA technology, we found that knockdown of PTB expression in the ovarian tumor cell line A2780 substantially impaired tumor cell proliferation, anchorage-independent growth and in vitro invasiveness. These results suggest that overexpression of PTB is an important component of the multistep process of tumorigenesis, and might be required for the development and maintenance of epithelial ovarian tumors. Moreover, because of its novel role in tumor cell growth and invasiveness, shown here for the first time, PTB may be a novel therapeutic target in the treatment of ovarian cancer.

Relocalization of the polypyrimidine tract-binding protein during PKA-induced neurite growth.

Neurite RNA binding proteins are important for neurite growth, a process critical for neuronal development and regeneration after injury. It has been known that many RNA binding proteins undergo nucleocytoplasmic shuttling but how their nucleocytoplasmic distributions are regulated during neurite growth has not been well explored. Here we found that the polypyrimidine tract binding protein (PTB) was exported from the nucleus and accumulated at growing neurite terminals upon activation of the PKA pathway in PC12 cells in a PKA-target Ser16-dependent manner. RNA interference (RNAi) of PTB significantly disrupted the neurite growth. We then examined the role of cytoplasmic PTB in relation to mRNAs involved in neurite growth. We found that PTB was preferentially associated with the beta-actin mRNA transcripts in cytoplasmic fractions. RNAi of PTB reduced neurite accumulation of the endogenous actin proteins. It is thus likely that, during PKA-induced neurite growth, PTB is relocalized through Ser16 phosphorylation to the cytoplasm where it is associated with beta-actin mRNA and is critical for the mRNA localization to neurites.

The RNA-binding protein ROD1/PTBP3 cotranscriptionally defines AID-loading sites to mediate antibody class switch in mammalian genomes.

Activation-induced cytidine deaminase (AID) mediates class switching by binding to a small fraction of single-stranded DNA (ssDNA) to diversify the antibody repertoire. The precise mechanism for highly selective AID targeting in the genome has remained elusive. Here, we report an RNA-binding protein, ROD1 (also known as PTBP3), that is both required and sufficient to define AID-binding sites genome-wide in activated B cells. ROD1 interacts with AID via an ultraconserved loop, which proves to be critical for the recruitment of AID to ssDNA using bi-directionally transcribed nascent RNAs as stepping stones. Strikingly, AID-specific mutations identified in human patients with hyper-IgM syndrome type 2 (HIGM2) completely disrupt the AID interacting surface with ROD1, thereby abolishing the recruitment of AID to immunoglobulin (Ig) loci. Together, our results suggest that bi-directionally transcribed RNA traps the RNA-binding protein ROD1, which serves as a guiding system for AID to load onto specific genomic loci to induce DNA rearrangement during immune responses.

Automated fluorescent miscroscopic image analysis of PTBP1 expression in glioma.

Multiplexed immunofluorescent testing has not entered into diagnostic neuropathology due to the presence of several technical barriers, amongst which includes autofluorescence. This study presents the implementation of a methodology capable of overcoming the visual challenges of fluorescent microscopy for diagnostic neuropathology by using automated digital image analysis, with long term goal of providing unbiased quantitative analyses of multiplexed biomarkers for solid tissue neuropathology. In this study, we validated PTBP1, a putative biomarker for glioma, and tested the extent to which immunofluorescent microscopy combined with automated and unbiased image analysis would permit the utility of PTBP1 as a biomarker to distinguish diagnostically challenging surgical biopsies. As a paradigm, we utilized second resections from patients diagnosed either with reactive brain changes (pseudoprogression) and recurrent glioblastoma (true progression). Our image analysis workflow was capable of removing background autofluorescence and permitted quantification of DAPI-PTBP1 positive cells. PTBP1-positive nuclei, and the mean intensity value of PTBP1 signal in cells. Traditional pathological interpretation was unable to distinguish between groups due to unacceptably high discordance rates amongst expert neuropathologists. Our data demonstrated that recurrent glioblastoma showed more DAPI-PTBP1 positive cells and a higher mean intensity value of PTBP1 signal compared to resections from second surgeries that showed only reactive gliosis. Our work demonstrates the potential of utilizing automated image analysis to overcome the challenges of implementing fluorescent microscopy in diagnostic neuropathology.

Oncogenic miR­132 sustains proliferation and self­renewal potential by inhibition of polypyrimidine tract­binding protein 2 in glioblastoma cells.

Glioblastoma multiforme (GBM) is the leading type of brain tumor, exhibiting unlimited proliferation and invasion potential. The present study indicated that a high expression level of miR­132 was detected in the neural subtype of GBM and predicted an unfavorable prognosis for patients from The Cancer Genome Atlas cohort (n=526). Cox hazard regression analysis demonstrated miR­132 as an independent prognostic indicator for GBM patients. Further in vitro experiments indicated that miR­132 promoted the proliferation and sphere formation of U87 cells. Unsupervised hierarchical clustering analysis was performed to compare differently expressed genes between two Gene Expression Omnibus (GEO) datasets and Gene Ontology analysis was applied to evaluate the significant signaling pathways modulated by miR­132 in GBM cells within a genetic bioinformatic lab, the Gene­Cloud of Biotechnology Information. By combining the results based on GEO datasets and the miRNA bioinformatic prediction, polypyrimidine tract­binding protein 2 (PTBP2), a brain tissue­specific post­transcriptional protein, was identified as a potential downstream target of miR­132 in GBM. Thus, miR­132 overexpression in GBM cells predicted an unfavorable outcome for patients, and sustained the proliferation and self­renewal abilities of GBM cells in an miR­132/PTBP2 signaling pathway.

PTBP1 and PTBP2 Repress Nonconserved Cryptic Exons.

The fidelity of RNA splicing is maintained by a network of factors, but the molecular mechanisms that govern this process have yet to be fully elucidated. We previously found that TDP-43, an RNA-binding protein implicated in neurodegenerative disease, utilizes UG microsatellites to repress nonconserved cryptic exons and prevent their incorporation into mRNA. Here, we report that two well-characterized splicing factors, polypyrimidine tract-binding protein 1 (PTBP1) and polypyrimidine tract-binding protein 2 (PTBP2), are also nonconserved cryptic exon repressors. In contrast to TDP-43, PTBP1 and PTBP2 utilize CU microsatellites to repress both conserved tissue-specific exons and nonconserved cryptic exons. Analysis of these conserved splicing events suggests that PTBP1 and PTBP2 repression is titrated to generate the transcriptome diversity required for neuronal differentiation. We establish that PTBP1 and PTBP2 are members of a family of cryptic exon repressors.

A posttranscriptional mechanism that controls Ptbp1 abundance in the Xenopus epidermis.

The output of alternative splicing depends on the cooperative or antagonistic activities of several RNA-binding proteins (RBPs), like Ptbp1 and Esrp1 in Xenopus. Fine-tuning of the RBP abundance is therefore of prime importance to achieve tissue- or cell-specific splicing patterns. Here, we addressed the mechanisms leading to the high expression of the ptbp1 gene, which encodes Ptbp1, in Xenopus epidermis. Two splice isoforms of ptbp1 mRNA differ by the presence of an alternative exon 11, and only the isoform including exon 11 can be translated to a full-length protein. In vivo minigene assays revealed that the nonproductive isoform was predominantly produced. Knockdown experiments demonstrated that Esrp1, which is specific to the epidermis, strongly stimulated the expression of ptbp1 by favoring the productive isoform. Consequently, knocking down esrp1 phenocopied ptbp1 inactivation. Conversely, Ptbp1 repressed the expression of its own gene by favoring the nonproductive isoform. Hence, a complex posttranscriptional mechanism controls Ptbp1 abundance in Xenopus epidermis: skipping of exon 11 is the default splicing pattern, but Esrp1 stimulates ptbp1 expression by favoring the inclusion of exon 11 up to a level that is limited by Ptbp1 itself. These results decipher a posttranscriptional mechanism that achieves various abundances of the ubiquitous RBP Ptbp1 in different tissues.

Characterization of a novel posttranslational modification in polypyrimidine tract-binding proteins by SUMO1.

Polypyrimidine tract-binding protein 1 (PTBP1) and its brainspecific homologue, PTBP2, are associated with pre-mRNAs and influence pre-mRNA processing, as well as mRNA metabolism and transport. They play important roles in neural differentiation and glioma development. In our study, we detected the expression of the two proteins in glioma cells and predicted that they may be sumoylated using SUMOplot analyses. We confirmed that PTBP1 and PTBP2 can be modified by SUMO1 with co-immunoprecipitation experiments using 293ET cells transiently co-expressing SUMO1 and either PTBP1 or PTBP2. We also found that SUMO1 modification of PTBP2 was enhanced by Ubc9 (E2). The mutation of the sumoylation site (Lys137) of PTBP2 markedly inhibited its modification by SUMO1. Interestingly, in T98G glioma cells, the level of sumoylated PTBP2 was reduced compared to that of normal brain cells. Overall, this study shows that PTBP2 is posttranslationally modified by SUMO1.

Lineage-specific splicing of a brain-enriched alternative exon promotes glioblastoma progression.

Tissue-specific alternative splicing is critical for the emergence of tissue identity during development, yet the role of this process in malignant transformation is undefined. Tissue-specific splicing involves evolutionarily conserved, alternative exons that represent only a minority of the total alternative exons identified. Many of these conserved exons have functional features that influence signaling pathways to profound biological effect. Here, we determined that lineage-specific splicing of a brain-enriched cassette exon in the membrane-binding tumor suppressor annexin A7 (ANXA7) diminishes endosomal targeting of the EGFR oncoprotein, consequently enhancing EGFR signaling during brain tumor progression. ANXA7 exon splicing was mediated by the ribonucleoprotein PTBP1, which is normally repressed during neuronal development. PTBP1 was highly expressed in glioblastomas due to loss of a brain-enriched microRNA (miR-124) and to PTBP1 amplification. The alternative ANXA7 splicing trait was present in precursor cells, suggesting that glioblastoma cells inherit the trait from a potential tumor-initiating ancestor and that these cells exploit this trait through accumulation of mutations that enhance EGFR signaling. Our data illustrate that lineage-specific splicing of a tissue-regulated alternative exon in a constituent of an oncogenic pathway eliminates tumor suppressor functions and promotes glioblastoma progression. This paradigm may offer a general model as to how tissue-specific regulatory mechanisms can reprogram normal developmental processes into oncogenic ones.