Analysis of PTEN complex assembly and identification of heterogeneous nuclear ribonucleoprotein C as a component of the PTEN-associated complex

The Novel Phosphatase Domain Mutations Q171R and Y65S Switch PTEN from Tumor Suppressor to Oncogene

Phosphatase and tensin homolog deleted on chromosome 10, or PTEN, is a well-characterized tumor suppressor with both lipid and protein phosphatase activities. PTEN is often downregulated by epigenetic mechanisms such as hypermethylation, which leads to constitutive activation of the PI3K-Akt pathway. Large datasets from next-generation sequencing, however, revealed that mutations in PTEN may not only hamper protein function but may also affect interactions with downstream effectors, leading to variable oncogenic readouts. Here, two novel PTEN mutations, Q171R and Y65S, identified in Filipino colorectal cancer patients, were phenotypically characterized in NIH3T3 and HCT116 cells, alongside the C124S canonical mutant and wild-type controls. The novel mutants increased cellular proliferation, resistance to apoptosis and migratory capacity. They induced gross morphological changes including cytoplasmic shrinkage, increased cellular protrusions and extensive cytoskeletal reorganization. The mutants also induced a modest increase in Akt phosphorylation. Further mechanistic studies will help determine the differential oncogenic potencies of these mutants, and resolve whether the structural constraints imposed by the mutations may have altered associations with downstream effectors.

Alternative splicing landscape of the neural transcriptome in a cytoplasmic-predominant Pten expression murine model of autism-like Behavior

Alternative splicing (AS) is a posttranscriptional mechanism regulating gene expression that complex organisms utilize to expand proteome diversity from a comparatively limited set of genes. Recent research has increasingly associated AS with increased functional complexity in the central nervous systems in higher order mammals. This work has heavily implicated aberrant AS in several neurocognitive and neurodevelopmental disorders, including autism. Due to the strong genetic association between germline PTEN mutations and autism spectrum disorder (ASD), we hypothesized that germline PTEN mutations would alter AS patterns, contributing to the pathophysiology of ASD. In a murine model of constitutional mislocalization of Pten, recapitulating an autism-like phenotype, we found significant changes in AS patterns across the neural transcriptome by analyzing RNA-sequencing data with the program rMATS. A few hundred significant alternative splicing events (ASEs) that differentiate each m3m4 genotype were identified. These ASEs occur in genes enriched in PTEN signaling, inositol metabolism, and several other pathways relevant to the pathophysiology of ASD. In addition, we identified expression changes in several splicing factors known to be enriched in the nervous system. For instance, the master regulator of microexons, Srrm4, has decreased expression, and consequently, we found decreased inclusion of microexons in the Pten^m3m4/m3m4 cortex (~10% decrease). We also demonstrated that the m3m4 mutation disrupts the interaction between Pten and U2af2, a member of the spliceosome. In sum, our observations point to germline Pten disruption changing the landscape of alternative splicing in the brain, and these changes may be relevant to the pathogenesis and/or maintenance of PTEN-ASD phenotypes.

Alborixin clears amyloid-β by inducing autophagy through PTEN-mediated inhibition of the AKT pathway

Imbalance in production and clearance of amyloid beta (Aβ) is the primary reason for its deposition in Alzheimer disease. Macroautophagy/autophagy is one of the important mechanisms for clearance of both intracellular and extracellular Aβ. Here, through screening, we identified alborixin, an ionophore, as a potent inducer of autophagy. We found that autophagy induced by alborixin substantially cleared Aβ in microglia and primary neuronal cells. Induction of autophagy was accompanied by up regulation of autophagy proteins BECN1/Beclin 1, ATG5, ATG7 and increased lysosomal activities. Autophagy induced by alborixin was associated with inhibition of the phosphoinositide 3-kinase (PI3K)-AKT pathway. A knock down of PTEN and consistent, constitutive activation of AKT inhibited alborixin-induced autophagy and consequent clearance of Aβ. Furthermore, clearance of Aβ by alborixin led to significant reduction of Aβ-mediated cytotoxicity in primary neurons and differentiated N2a cells. Thus, our findings put forward alborixin as a potential anti-Alzheimer therapeutic lead. Abbreviations: Aβ: amyloid beta; ALB: alborixin; ATG: autophagy-related; BECN1: beclin 1; DAPI: 4, 6-diamidino-2-phenylindole; DCFH-DA: 2,7-dichlorodihydrofluorescein diacetate; fAβ: fibrillary form of amyloid beta; GFAP: glial fibrillary acidic protein; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; MAP2: microtubule-associated protein 2; MTOR: mechanistic target of rapamycin kinase; PTEN: phosphatase and tensin homolog; ROS: reactive oxygen species; SQSTM1: sequestosome 1; TMRE: tetramethylrhodamine, ethyl ester.

Intracellular delivery of the PTEN protein using cationic lipidoids for cancer therapy

The phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is a tumor suppressor, mutated or inactive in a large percentage of human cancers. Restoring PTEN activity in cancer cells through gene therapy has shown to inhibit cell growth and induce apoptosis, particularly in cells with a PTEN deficiency. Gene therapy, however, comes with some inherent risks such as triggering an immune response and permanent off target effects. Nanoparticle assisted protein delivery could mitigate these liabilities while maintaining therapeutic integrity. In this report, we evaluated the use of cationic lipid-like (lipidoid) materials to intracellularly deliver the PTEN protein. We synthesized a small library of cationic lipidoid materials and screened for the delivery of PTEN based on cell viability. The lipidoid material EC16-80 was selected for high efficacy and the subsequent lipidoid-protein complex was characterized using DLS, zeta potential, and TEM. Intracellular delivery of PTEN with EC16-80 to the PTEN deficient prostate cancer cell line PC-3 resulted in a significant decrease in activated AKT and induced apoptosis. Interestingly, delivery of PTEN to PTEN deficient prostate cancer cell lines PC-3 and LNCaP compared to the breast cancer cell line, MCF-7 with endogenous PTEN, resulted in significantly lower IC₅₀ values in PC-3 and LNCaP cells indicating that the treatment is predominantly specific to PTEN-deficient cells. Altogether, these results demonstrate the first intracellular delivery of recombinant PTEN using a synthetic delivery vehicle and highlight the potential of intracellular PTEN protein delivery as a potential targeted cancer therapy.

Connexin43 recruits PTEN and Csk to inhibit c-Src activity in glioma cells and astrocytes

Connexin43 (Cx43), the major protein forming gap junctions in astrocytes, is reduced in high-grade gliomas, where its ectopic expression exerts important effects, including the inhibition of the proto-oncogene tyrosine-protein kinase Src (c-Src). In this work we aimed to investigate the mechanism responsible for this effect. The inhibition of c-Src requires phosphorylation at tyrosine 527 mediated by C-terminal Src kinase (Csk) and dephosphorylation at tyrosine 416 mediated by phosphatases, such as phosphatase and tensin homolog (PTEN). Our results showed that the antiproliferative effect of Cx43 is reduced when Csk and PTEN are silenced in glioma cells, suggesting the involvement of both enzymes. Confocal microscopy and immunoprecipitation assays confirmed that Cx43, in addition to c-Src, binds to PTEN and Csk in glioma cells transfected with Cx43 and in astrocytes. Pull-down assays showed that region 266–283 in Cx43 is sufficient to recruit c-Src, PTEN and Csk and to inhibit the oncogenic activity of c-Src. As a result of c-Src inhibition, PTEN was increased with subsequent inactivation of Akt and reduction of proliferation of human glioblastoma stem cells. We conclude that the recruitment of Csk and PTEN to the region between residues 266 and 283 within the C-terminus of Cx43 leads to c-Src inhibition.

Interleukin-like EMT inducer regulates partial phenotype switching in MITF-low melanoma cell lines

ILEI (FAM3C) is a secreted factor that contributes to the epithelial-to-mesenchymal transition (EMT), a cell biological process that confers metastatic properties to a tumor cell. Initially, we found that ILEI mRNA is highly expressed in melanoma metastases but not in primary tumors, suggesting that ILEI contributes to the malignant properties of melanoma. While melanoma is not an epithelial cell-derived tumor and does not undergo a traditional EMT, melanoma undergoes a similar process known as phenotype switching in which high (micropthalmia-related transcription factor) MITF expressing (MITF-high) proliferative cells switch to a low expressing (MITF-low) invasive state. We observed that MITF-high proliferative cells express low levels of ILEI (ILEI-low) and MITF-low invasive cells express high levels of ILEI (ILEI-high). We found that inducing phenotype switching towards the MITF-low invasive state increases ILEI mRNA expression, whereas phenotype switching towards the MITF-high proliferative state decreases ILEI mRNA expression. Next, we used in vitro assays to show that knockdown of ILEI attenuates invasive potential but not MITF expression or chemoresistance. Finally, we used gene expression analysis to show that ILEI regulates several genes involved in the MITF-low invasive phenotype including JARID1B, HIF-2α, and BDNF. Gene set enrichment analysis suggested that ILEI-regulated genes are enriched for JUN signaling, a known regulator of the MITF-low invasive phenotype. In conclusion, we demonstrate that phenotype switching regulates ILEI expression, and that ILEI regulates partial phenotype switching in MITF-low melanoma cell lines.

The virus-induced protein APOBEC3G inhibits anoikis by activation of Akt kinase in pancreatic cancer cells

Pancreatic cancer is one of the more common cancers with a poor prognosis. Some varieties of cancer are related to virus infection. As a virus-induced protein, APOBEC3G (A3G) presents extensive anti-virus ability, but the role of A3G in pancreatic cancer was previously unknown. The expression of A3G in pancreatic cancer was examined using TaqMan real-time qPCR, immunohistochemical and immunofluorescent staining. Subsequently, the role of A3G in pancreatic cancer was evaluated in vivo using the tumor xenograft model. Anoikis was detected by colony formation assay and flow cytometry in vitro. The Akt kinase activity and target protein PTEN were examined by co-immunoprecipitation and immunoblot. The virus-induced protein A3G was significantly up-regulated in pancreatic cancer, and the up-regulation of A3G promoted xenograft tumor formation. A3G inactivated PTEN by binding to the C2 tensin-type and PDZ domains, thereby inducing anoikis resistance through Akt activation. Our results demonstrate that the up-regulation of A3G in pancreatic cancer cells induces anoikis resistance, and they provide novel insight into the mechanism by which A3G affects the malignant behavior of pancreatic cancer cells.

The E3 ubiquitin ligase TRIM32 regulates myoblast proliferation by controlling turnover of NDRG2

Limb girdle muscular dystrophy 2H is caused by mutations in the gene encoding the E3 ubiquitin ligase, TRIM32. Previously, we generated and characterized a Trim32 knockout mouse (T32KO) that displays both neurogenic and myopathic features. The myopathy in these mice is attributable to impaired muscle growth, associated with satellite cell senescence and premature sarcopenia. This satellite cell senescence is due to accumulation of the SUMO ligase PIASy, a substrate of TRIM32. The goal of this investigation was to identify additional substrates of TRIM32 using 2D fluorescence difference gel electrophoresis (2D-DIGE) in order to further explore its role in skeletal muscle. Because TRIM32 is an E3 ubiquitin ligase, we reasoned that TRIM32's substrates would accumulate in its absence. 2D-DIGE identified 19 proteins that accumulate in muscles from the T32KO mouse. We focused on two of these proteins, NDRG2 and TRIM72, due to their putative roles in myoblast proliferation and myogenesis. Follow-up analysis confirmed that both proteins were ubiquitinated by TRIM32 in vitro; however, only NDRG2 accumulated in skeletal muscle and myoblasts in the absence of TRIM32. NDRG2 overexpression in myoblasts led to reduced cell proliferation and delayed cell cycle withdrawal during differentiation. Thus, we identified NDRG2 as a novel target for TRIM32; these findings further corroborate the hypothesis that TRIM32 is involved in control of myogenic cells proliferation and differentiation.

PTEN antagonises Tcl1/hnRNPK-mediated G6PD pre-mRNA splicing which contributes to hepatocarcinogenesis

BACKGROUND

Mounting epidemiological evidence supports a role for phosphatase and tensin homologue (PTEN)-T cell leukaemia 1 (Tcl1) signalling deregulation in hepatocarcinogenesis.

OBJECTIVE

To determine the molecular and biochemical mechanisms by which the PTEN/Tcl1 axis regulates the pentose phosphate pathway (PPP) in hepatocellular carcinoma (HCC).

METHODS

We compared levels of PTEN and glucose-6-phosphate dehydrogenase (G6PD) mRNA in human HCC and healthy liver tissue. We measured PPP flux, glucose consumption, lactate production, nicotinamide adenine dinucleotide phosphate (NADPH) levels and lipid accumulation. We investigated the PTEN/Tcl1 axis using molecular biology, biochemistry and mass spectrometry analysis. We assessed proliferation, apoptosis and senescence in cultured cells, and tumour formation in mice.

RESULTS

We showed that PTEN inhibited the PPP pathway in human liver tumours. Through the PPP, PTEN suppressed glucose consumption and biosynthesis. Mechanistically, the PTEN protein bound to G6PD, the first and rate-limiting enzyme of the PPP and prevented the formation of the active G6PD dimer. Tcl1, a coactivator for Akt, reversed the effects of PTEN on biosynthesis. Tcl1 promoted G6PD activity and also increased G6PD pre-mRNA splicing and protein expression in a heterogeneous nuclear ribonucleoprotein (hnRNPK)-dependent manner. PTEN also formed a complex with hnRNPK, which inhibited G6PD pre-mRNA splicing. Moreover, PTEN inactivated Tcl1 via glycogen synthase kinase-3β (GSK3β)-mediated phosphorylation. Importantly, Tcl1 knockdown enhanced the sensitivity of HCC to sorafenib, whereas G6PD knockdown inhibited hepatocarcinogenesis.

CONCLUSIONS

These results establish the counteraction between PTEN and Tcl1 as a key mechanism that regulates the PPP and suggest that targeting the PTEN/Tcl1/hnRNPK/G6PD axis could open up possibilities for therapeutic intervention and improve the prognosis of patients with HCC.

C-terminally truncated form of αB-crystallin is associated with IDH1 R132H mutation in anaplastic astrocytoma

Malignant gliomas are the most common human primary brain tumors. Point mutation of amino acid arginine 132 to histidine (R132H) in the IDH1 protein leads to an enzymatic gain-of-function and is thought to promote gliomagenesis. Little is known about the downstream effects of the IDH1 mutation on protein expression and how and whether changes in protein expression are involved in tumor formation or propagation. In the current study, we used 2D DIGE (difference gel electrophoresis) and mass spectrometry to analyze differences in protein expression between IDH1(R132H) mutant and wild type anaplastic (grade III) astrocytoma from human brain cancer tissues. We show that expression levels of many proteins are altered in IDH1(R132H) mutant anaplastic astrocytoma. Some of the most over-expressed proteins in the mutants include several forms of αB-crystallin, a small heat-shock and anti-apoptotic protein. αB-crystallin proteins are elevated up to 22-fold in IDH1(R132H) mutant tumors, and αB-crystallin expression appears to be controlled at the post-translational level. We identified the most abundant form of αB-crystallin as a low molecular weight species that is C-terminally truncated. We also found that overexpression of αB-crystallin can be induced by transfecting U251 human glioblastoma cell lines with the IDH1(R132H) mutation. In conclusion, the association of a C-terminally truncated form of αB-crystallin protein with the IDH1(R132H) mutation is a novel finding that could impact apoptosis and stress response in IDH1 mutant glioma.

Proteomic identification of mitochondrial targets of arginase in human breast cancer

We have previously reported arginase expression in human breast cancer cells and demonstrated that the inhibition of arginase by N^ω hydroxy L-arginine (NOHA) in MDA-MB-468 cells induces apoptosis. However, arginase expression and its possible molecular targets in human breast tumor samples and potential clinical implications have not been fully elucidated. Here, we demonstrate arginase expression in human breast tumor samples, and several established breast cancer cell lines, in which NOHA treatment selectively inhibits cell proliferation. The over-expression of Bcl2 in MDA-MB-468 cells abolished NOHA-induced apoptosis, suggesting that the mitochondria may be the main site of NOHA’s action. We, therefore, undertook a proteomics approach to identify key mitochondrial targets of arginase in MDA-MB-468 cells. We identified 54 non-mitochondrial and 13 mitochondrial proteins that were differentially expressed in control and NOHA treated groups. Mitochondrial serine hydroxymethyltransferase (mSHMT) was identified as one of the most promising targets of arginase. Both arginase II (Arg II) and mSHMT expressions were higher in human breast tumor tissues compared to the matched normal and there was a strong correlation between Arg II and mSHMT protein expression. MDA-MB-468 xenografts had significant upregulation of Arg II expression that preceded the induction of mSHMT expression. Small inhibitory RNA (siRNA)-mediated inhibition of Arg II in MDA-MB-468 and HCC-1806 cells led to significant inhibition of both the mSHMT gene and protein expression. As mSHMT is a key player in folate metabolism, our data provides a novel link between arginine and folate metabolism in human breast cancer, both of which are critical for tumor cell proliferation.

Acid ceramidase promotes nuclear export of PTEN through sphingosine 1-phosphate mediated Akt signaling

The tumor suppressor PTEN is now understood to regulate cellular processes at the cytoplasmic membrane, where it classically regulates PI3K signaling, as well as in the nucleus where multiple roles in controlling cell cycle and genome stability have been elucidated. Mechanisms that dictate nuclear import and, less extensively, nuclear export of PTEN have been described, however the relevance of these processes in disease states, particularly cancer, remain largely unknown. We investigated the impact of acid ceramidase on the nuclear-cytoplasmic trafficking of PTEN. Immunohistochemical analysis of a human prostate tissue microarray revealed that nuclear PTEN was lost in patients whose tumors had elevated acid ceramidase. We found that acid ceramidase promotes a reduction in nuclear PTEN that is dependent upon sphingosine 1-phosphate-mediated activation of Akt. We were further able to show that sphingosine 1-phosphate promotes formation of a complex between Crm1 and PTEN, and that leptomycin B prevents acid ceramidase and sphingosine 1-phosphate mediated loss of nuclear PTEN, suggesting an active exportin-mediated event. To investigate whether the tumor promoting aspects of acid ceramidase in prostate cancer depend upon its ability to export PTEN from the nucleus, we used enforced nuclear expression of PTEN to study docetaxel-induced apoptosis and cell killing, proliferation, and xenoengraftment. Interestingly, while acid ceramidase was able to protect cells expressing wild type PTEN from docetaxel, promote proliferation and xenoengraftment, acid ceramidase had no impact in cells expressing PTEN-NLS. These findings suggest that acid ceramidase, through sphingosine 1-phosphate, promotes nuclear export of PTEN as a means of promoting tumor formation, cell proliferation, and resistance to therapy.

Amplification Target ADRM1: Role as an Oncogene and Therapeutic Target for Ovarian Cancer

Approximately 25,000 ovarian cancers are diagnosed in the U.S. annually, and 75% are in the advanced stage and largely incurable. There is critical need for early detection tools and novel treatments. Proteasomal ubiquitin receptor ADRM1 is a protein that is encoded by the ADRM1 gene. Recently, we showed that among 20q13-amplified genes in ovarian cancer, ADRM1 overexpression was the most highly correlated with amplification and was significantly upregulated with respect to stage, recurrence, and metastasis. Its overexpression correlated significantly with shorter time to recurrence and overall survival. Array-CGH and microarray expression of ovarian cancer cell lines provided evidence consistent with primary tumor data that ADRM1 is a 20q13 amplification target. Herein, we confirm the ADRM1 amplicon in a second ovarian cancer cohort and define a minimally amplified region of 262 KB encompassing seven genes. Additionally, using RNAi knock-down of ADRM1 in naturally amplified cell line OAW42 and overexpression of ADRM1 via transfection in ES2, we show that (1) ADRM1 overexpression increases proliferation, migration, and growth in soft agar, and (2) knock-down of ADRM1 results in apoptosis. Proteomic analysis of cells with ADRM1 knock-down reveals dysregulation of proteins including CDK-activating kinase assembly factor MAT1. Taken together, the results indicate that amplified ADRM1 is involved in cell proliferation, migration and survival in ovarian cancer cells, supporting a role as an oncogene and novel therapeutic target for ovarian cancer.

Novel Cell and Tissue Acquisition System (CTAS): microdissection of live and frozen brain tissues

We developed a novel, highly accurate, capillary based vacuum-assisted microdissection device CTAS - Cell and Tissue Acquisition System, for efficient isolation of enriched cell populations from live and freshly frozen tissues, which can be successfully used in a variety of molecular studies, including genomics and proteomics. Specific diameter of the disposable capillary unit (DCU) and precisely regulated short vacuum impulse ensure collection of the desired tissue regions and even individual cells. We demonstrated that CTAS is capable of dissecting specific regions of live and frozen mouse and rat brain tissues at the cellular resolution with high accuracy. CTAS based microdissection avoids potentially harmful physical treatment of tissues such as chemical treatment, laser irradiation, excessive heat or mechanical cell damage, thus preserving primary functions and activities of the dissected cells and tissues. High quality DNA, RNA, and protein can be isolated from CTAS-dissected samples, which are suitable for sequencing, microarray, 2D gel-based proteomic analyses, and Western blotting. We also demonstrated that CTAS can be used to isolate cells from native living tissues for subsequent recultivation of primary cultures without affecting cellular viability, making it a simple and cost-effective alternative for laser-assisted microdissection.

Determining PTEN functional status by network component deduced transcription factor activities

PTEN-controlled PI3K-AKT-mTOR pathway represents one of the most deregulated signaling pathways in human cancers. With many small molecule inhibitors that target PI3K-AKT-mTOR pathway being exploited clinically, sensitive and reliable ways of stratifying patients according to their PTEN functional status and determining treatment outcomes are urgently needed. Heterogeneous loss of PTEN is commonly associated with human cancers and yet PTEN can also be regulated on epigenetic, transcriptional or post-translational levels, which makes the use of simple protein or gene expression-based analyses in determining PTEN status less accurate. In this study, we used network component analysis to identify 20 transcription factors (TFs) whose activities deduced from their target gene expressions were immediately altered upon the re-expression of PTEN in a PTEN-inducible system. Interestingly, PTEN controls the activities (TFA) rather than the expression levels of majority of these TFs and these PTEN-controlled TFAs are substantially altered in prostate cancer mouse models. Importantly, the activities of these TFs can be used to predict PTEN status in human prostate, breast and brain tumor samples with enhanced reliability when compared to straightforward IHC-based or expression-based analysis. Furthermore, our analysis indicates that unique sets of PTEN-controlled TFAs significantly contribute to specific tumor types. Together, our findings reveal that TFAs may be used as “signatures” for predicting PTEN functional status and elucidate the transcriptional architectures underlying human cancers caused by PTEN loss.

PTEN level in tumor suppression: how much is too little?

The importance of PTEN (phosphatase and tensin homolog located on chromosome 10) in cancer has surpassed all predictions and expectations from the time it was discovered and has qualified this gene as one of the most commonly mutated and deleted tumor suppressors in human cancer. PTEN levels are frequently found downregulated in cancer, even in the absence of genetic loss or mutation. PTEN is heavily regulated by transcription factors, microRNAs, competitive endogenous RNAs (such as the PTEN pseudogene), and methylation, whereas the tumor suppressive activity of the PTEN protein can be altered at multiple levels through aberrant phosphorylation, ubiquitination, and acetylation. These regulatory cues are presumed to play a key role in tumorigenesis through the alteration of the appropriate levels, localization, and activity of PTEN. The identification of all these levels of PTEN regulation raises, in turn, a key corollary question: How low should PTEN level(s) or activity drop in order to confer cancer susceptibility at the organismal level? Our laboratory and others have approached this question through the genetic manipulation of Pten in the mouse. This work has highlighted the exquisite and tissue-specific sensitivity to subtle reductions in Pten levels toward tumor initiation and progression with important implications for cancer prevention and therapy.

PTEN-Associated Complexes: An Overview

PTEN is a tumor suppressor best characterized for its role as a lipid phosphatase in antagonizing the PI3-kinase pathway. Several recent studies have identified proteins that form high molecular weight complexes with PTEN in different subcellular compartments. PTEN is critical for early embryonic development, cell proliferation, cell survival and stem cell function. The discovery of PTEN complex components may help our understanding of its biological functions. In this review, PTEN complex components, functions and their regulation will be discussed.