PTEN enters the nucleus by diffusion

Spatiotemporal distribution of PTEN before directed cell migration in monolayers

The intracellular distribution of phosphatase and tensin homolog (PTEN) is closely related to directed cell migration. In single cells, PTEN accumulates at the rear of the cell before and during directed migration; however, the spatiotemporal distribution of PTEN in confluent cell monolayers, particularly before directed migration, remains unclear. In this study, we wounded a cell in confluent fetal rat skin keratinocytes (FRSKs) and examined the dynamics of PTEN in the cells adjacent to the wounded cell. In contrast to single-cell migration, we found that PTEN translocated to the nucleus before the beginning of directed migration. This nuclear translocation of PTEN did not occur in disconnected cells, and it was also suppressed by importin-β inhibitor and actin inhibitor. When the nuclear localization of PTEN was inhibited by an importin-β inhibitor, cell elongation in the direction of migration was also significantly inhibited. Our results indicate that PTEN translocation is induced by the disruption of cell-cell adhesion and requires the involvement of importin-β and actin cytoskeleton signaling. In addition, phosphatidylinositol 3,4,5-triphosphate (PIP3) may regulate PTEN distribution through its localized accumulation at the cell edge. Our findings suggest that the translocation of PTEN is crucial for directed cell migration and for responding to mechanical environmental changes in confluent cell monolayers.

The E3 ubiquitin ligase CHIP drives monoubiquitylation-mediated nuclear import of the tumor suppressor PTEN

Monoubiquitylation is a principal mechanism driving nuclear translocation of the protein PTEN (phosphatase and tensin homolog deleted on chromosome ten). In this study, we describe a novel mechanism wherein the protein CHIP (C-terminus of Hsc70-interacting protein) mediates PTEN monoubiquitylation, leading to its nuclear import. Western blot analysis revealed a rise in both nuclear and total cellular PTEN levels under monoubiquitylation-promoting conditions, an effect that was abrogated by silencing CHIP expression. We established time-point kinetics of CHIP-mediated nuclear translocation of PTEN using immunocytochemistry and identified a role of karyopherin α1 (KPNA1) in facilitating nuclear transport of monoubiquitylated PTEN. We further established a direct interaction between CHIP and PTEN inside the nucleus, with CHIP participating in either polyubiquitylation or monoubiquitylation of nuclear PTEN. Finally, we showed that oxidative stress enhanced CHIP-mediated nuclear import of PTEN, which resulted in increased apoptosis, and decreased cell viability and proliferation, whereas CHIP knockdown counteracted these effects. To the best of our knowledge, this is the first report elucidating non-canonical roles for CHIP on PTEN, which we establish here as a nuclear interacting partner of CHIP.

Nuclear PTEN's Functions in Suppressing Tumorigenesis: Implications for Rare Cancers

Phosphatase and tensin homolog (PTEN) encodes a tumor-suppressive phosphatase with both lipid and protein phosphatase activity. The tumor-suppressive functions of PTEN are lost through a variety of mechanisms across a wide spectrum of human malignancies, including several rare cancers that affect pediatric and adult populations. Originally discovered and characterized as a negative regulator of the cytoplasmic, pro-oncogenic phosphoinositide-3-kinase (PI3K) pathway, PTEN is also localized to the nucleus where it can exert tumor-suppressive functions in a PI3K pathway-independent manner. Cancers can usurp the tumor-suppressive functions of PTEN to promote oncogenesis by disrupting homeostatic subcellular PTEN localization. The objective of this review is to describe the changes seen in PTEN subcellular localization during tumorigenesis, how PTEN enters the nucleus, and the spectrum of impacts and consequences arising from disrupted PTEN nuclear localization on tumor promotion. This review will highlight the immediate need in understanding not only the cytoplasmic but also the nuclear functions of PTEN to gain more complete insights into how important PTEN is in preventing human cancers.

The non-canonical nuclear functions of key players of the PI3K-AKT-MTOR pathway

The PI3K-AKT-MTOR signal transduction pathway is one of the essential signalling cascades within the cell due to its involvement in many vital functions. The pathway initiates with the recruitment of phosphatidylinositol-3 kinases (PI3Ks) onto the plasma membrane, generating phosphatidylinositol-3,4,5-triphosphate [PtdIns(3,4,5)P₃ ] and subsequently activating AKT. Being the central node of the PI3K network, AKT activates the mechanistic target of rapamycin kinase complex 1 (MTORC1) via Tuberous sclerosis complex 2 inhibition in the cytoplasm. Although the cytoplasmic role of the pathway has been widely explored for decades, we now know that most of the effector molecules of the PI3K axis diverge from the canonical route and translocate to other cell organelles including the nucleus. The presence of phosphoinositides (PtdIns) inside the nucleus itself indicates the existence of a nuclear PI3K signalling. The nuclear localization of these signaling components is evident in regulating many nuclear processes like DNA replication, transcription, DNA repair, maintenance of genomic integrity, chromatin architecture, and cell cycle control. Here, our review intends to present a comprehensive overview of the nuclear functions of the PI3K-AKT-MTOR signaling biomolecules.

Aberrant expression of PI3K/AKT signaling is involved in apoptosis resistance of hepatocellular carcinoma

Phosphatidylinositol 3-kinase (PI3K)/AKT signaling is a crucial pathway for cell survival and proliferation, which are regulated by several growth factors and activated receptors. Upregulated PI3K/AKT signaling molecules were reported in several cancers and they are associated with altered cellular functions, leading to oncogenesis. Here, we have examined the implications of elevated PI3K/AKT expression in the apoptosis resistance of human hepatocellular carcinoma (HCC) Huh7 cells. We showed that PI3K/AKT signaling is significantly upregulated in Huh7 cells by quantitative polymerase chain reaction and protein expression analysis. Also, perversely upregulated PI3K/AKT signaling Huh7 cells are highly resistant to treatment with chemotherapy drugs (docetaxel and sorafenib) and acquired apoptosis resistance through downregulation of tumor suppressor protein PTEN (phosphatase and tensin homolog deleted on chromosome ten). Hence, we have investigated the effect of PTEN overexpression on apoptosis induction in Huh7 cells. We showed that PTEN overexpressed Huh7 cells became more sensitive toward the aforesaid drugs and induced apoptotic cell death due to intracellular reactive oxygen species (ROS) generation. Concurrently, the overexpression of PTEN leads to the activation of mitochondria facilitated intrinsic apoptosis, evidenced by upregulated cytochrome C, caspase 3, and caspase 9. Collectively, our data suggest that the aberrant expression of PI3K/AKT signaling contributes to apoptosis resistance in HCC.

A bacterial tyrosine phosphatase modulates cell proliferation through targeting RGCC

Tyrosine phosphatases are often weaponized by bacteria colonizing mucosal barriers to manipulate host cell signal transduction pathways. Porphyromonas gingivalis is a periodontal pathogen and emerging oncopathogen which interferes with gingival epithelial cell proliferation and migration, and induces a partial epithelial mesenchymal transition. P. gingivalis produces two tyrosine phosphatases, and we show here that the low molecular weight tyrosine phosphatase, Ltp1, is secreted within gingival epithelial cells and translocates to the nucleus. An ltp1 mutant of P. gingivalis showed a diminished ability to induce epithelial cell migration and proliferation. Ltp1 was also required for the transcriptional upregulation of Regulator of Growth and Cell Cycle (RGCC), one of the most differentially expressed genes in epithelial cells resulting from P. gingivalis infection. A phosphoarray and siRNA showed that P. gingivalis controlled RGCC expression through Akt, which was activated by phosphorylation on S473. Akt activation is opposed by PTEN, and P. gingivalis decreased the amount of PTEN in epithelial cells. Ectopically expressed Ltp1 bound to PTEN, and reduced phosphorylation of PTEN at Y336 which controls proteasomal degradation. Ltp-1 induced loss of PTEN stability was prevented by chemical inhibition of the proteasome. Knockdown of RGCC suppressed upregulation of Zeb2 and mesenchymal markers by P. gingivalis. RGCC inhibition was also accompanied by a reduction in production of the proinflammatory cytokine IL-6 in response to P. gingivalis. Elevated IL-6 levels can contribute to periodontal destruction, and the ltp1 mutant of P. gingivalis incited less bone loss compared to the parental strain in a murine model of periodontal disease. These results show that P. gingivalis can deliver Ltp1 within gingival epithelial cells, and establish PTEN as the target for Ltp1 phosphatase activity. Disruption of the Akt1/RGCC signaling axis by Ltp1 facilitates P. gingivalis-induced increases in epithelial cell migration, proliferation, EMT and inflammatory cytokine production.

Bacteria colonizing the oral cavity can induce inflammatory destruction of the periodontal tissues, and are increasingly associated with oral squamous cell carcinoma. P. gingivalis, a major periodontal pathogen, can subvert epithelial pathways that control important physiological processes relating to innate immunity and cell fate; however, little is known about the effector molecules. Here we show that P. gingivalis can deliver a tyrosine phosphatase, Ltp1, within epithelial cells, and Ltp1 phosphatase activity destabilizes PTEN, a negative regulator of Akt1 signaling. The production of RGCC is thus increased and this leads to increased epithelial cell migration, proliferation, a partial mesenchymal phenotype and inflammatory cytokine production. Ltp1 phosphatase activity thus provides a mechanistic basis for a number of P. gingivalis properties that contribute to disease. Indeed, an Ltp1-deficient mutant was less pathogenic in a murine model of periodontitis. These results contribute to deciphering the pathophysiological events that underlie oral bacterial diseases that initiate at mucosal barriers.

Neddylation of PTEN regulates its nuclear import and promotes tumor development

PTEN tumor suppressor opposes the PI3K/Akt signaling pathway in the cytoplasm and maintains chromosomal integrity in the nucleus. Nucleus-cytoplasm shuttling of PTEN is regulated by ubiquitylation, SUMOylation and phosphorylation, and nuclear PTEN has been proposed to exhibit tumor-suppressive functions. Here we show that PTEN is conjugated by Nedd8 under high glucose conditions, which induces PTEN nuclear import without effects on PTEN stability. PTEN neddylation is promoted by the XIAP ligase and removed by the NEDP1 deneddylase. We identify Lys197 and Lys402 as major neddylation sites on PTEN. Neddylated PTEN accumulates predominantly in the nucleus and promotes rather than suppresses cell proliferation and metabolism. The nuclear neddylated PTEN dephosphorylates the fatty acid synthase (FASN) protein, inhibits the TRIM21-mediated ubiquitylation and degradation of FASN, and then promotes de novo fatty acid synthesis. In human breast cancer tissues, neddylated PTEN correlates with tumor progression and poor prognosis. Therefore, we demonstrate a previously unidentified pool of nuclear PTEN in the Nedd8-conjugated form and an unexpected tumor-promoting role of neddylated PTEN.

PTEN Nuclear Functions

For years, clinical and basic researchers have been aware of the presence of PTEN in the nucleus in cell culture, animal models, and both healthy and diseased human tissues. Despite the early recognition of nuclear PTEN, the understanding of the mechanisms of its nuclear localization, function in the nucleus, and importance in biology and human disease has been lacking. Over the last decade, emerging concepts for the complex involvement of nuclear PTEN in a variety of processes, including genome maintenance and DNA repair, cell-cycle control, gene expression, and DNA replication, are illuminating what could prove to be the key path toward a full understanding of PTEN function in health and disease. Dysregulation of nuclear PTEN is now considered an important aspect of the etiology of many pathologic conditions, prompting reconsideration of the therapeutic approaches aimed at countering the consequences of PTEN deficiency. This new knowledge is fueling the development of innovative therapeutic modalities for a broad spectrum of human conditions, from cancer and metabolic diseases, to neurological disorders and autism.

Toward Systems Pathology for PTEN Diagnostics

Germline alterations of the tumor suppressor PTEN have been extensively characterized in patients with PTEN hamartoma tumor syndromes, encompassing subsets of Cowden syndrome, Bannayan-Riley-Ruvalcaba syndrome, Proteus and Proteus-like syndromes, as well as autism spectrum disorder. Studies have shown an increase in the risk of developing specific cancer types in the presence of a germline PTEN mutation. Furthermore, outside of the familial setting, somatic variants of PTEN occur in numerous malignancies. Here we introduce and discuss the prospect of moving toward a systems pathology approach for PTEN diagnostics, incorporating clinical and molecular pathology data with the goal of improving the clinical management of patients with a PTEN mutation. Detection of a germline PTEN mutation can inform cancer surveillance and in the case of somatic mutation, have value in predicting disease course. Given that PTEN functions in the PI3K/AKT/mTOR pathway, identification of a PTEN mutation may highlight new therapeutic opportunities and/or inform therapeutic choices.

Autism-associated PTEN missense mutation leads to enhanced nuclear localization and neurite outgrowth in an induced pluripotent stem cell line

Germline mutation in the PTEN gene is the genetic basis of PTEN hamartoma tumor syndrome with the affected individuals harboring features of autism spectrum disorders. Characterizing a panel of 14 autism‐associated PTEN missense mutations revealed reduced protein stability, catalytic activity, and subcellular distribution. Nine out of 14 (64%) PTEN missense mutants had reduced protein expression with most mutations confined to the C2 domain. Selected mutants displayed enhanced polyubiquitination and shortened protein half‐life, but that did not appear to involve the polyubiquitination sites at lysine residues at codon 13 or 289. Analyzing their intrinsic lipid phosphatase activities revealed that 78% (11 out of 14) of these mutants had twofold to 10‐fold reduction in catalytic activity toward phosphatidylinositol phosphate substrates. Analyzing the subcellular localization of the PTEN missense mutants showed that 64% (nine out of 14) had altered nuclear‐to‐cytosol ratios with four mutants (G44D, H123Q, E157G, and D326N) showing greater nuclear localization. The E157G mutant was knocked‐in to an induced pluripotent stem cell line and recapitulated a similar nuclear targeting preference. Furthermore, iPSCs expressing the E157G mutant were more proliferative at the neural progenitor cell stage but exhibited more extensive dendritic outgrowth. In summary, the combination of biological changes in PTEN is expected to contribute to the behavioral and cellular features of this neurodevelopmental disorder.

PTEN in Chromatin Remodeling

The tumor suppressor phosphatase and tension homolog (PTEN) is frequently mutated in human cancers, and it functions in multiple ways to safeguard cells from tumorigenesis. In the cytoplasm, PTEN antagonizes the PI3K/AKT pathway and suppresses cellular proliferation and survival. In the nucleus, PTEN is indispensable for the maintenance of genomic stability. In addition, PTEN loss leads to extensive changes in gene expression at the transcriptional level. The linker histone H1, generally considered as a transcriptional repressor, binds to the nucleosome to form a structure named the chromatosome. The dynamics between H1 and chromatin play an important role in determining gene expression. Here, we summarize the current understanding of roles of PTEN in controlling chromatin dynamics and global gene expression, which is crucial function of nuclear PTEN. We will also introduce the recent discovery of the PTEN family members and their functions.

Filopodome Mapping Identifies p130Cas as a Mechanosensitive Regulator of Filopodia Stability

Filopodia are adhesive cellular protrusions specialized in the detection of extracellular matrix (ECM)-derived cues. Although ECM engagement at focal adhesions is known to trigger the recruitment of hundreds of proteins ("adhesome") to fine-tune cellular behavior, the components of the filopodia adhesions remain undefined. Here, we performed a structured-illumination-microscopy-based screen to map the localization of 80 target proteins, linked to cell adhesion and migration, within myosin-X-induced filopodia. We demonstrate preferential enrichment of several adhesion proteins to either filopodia tips, filopodia shafts, or shaft subdomains, suggesting divergent, spatially restricted functions for these proteins. Moreover, proteins with phosphoinositide (PI) binding sites are particularly enriched in filopodia. This, together with the strong localization of PI(3,4)P2 in filopodia tips, predicts critical roles for PIs in regulating filopodia ultra-structure and function. Our mapping further reveals that filopodia adhesions consist of a unique set of proteins, the filopodome, that are distinct from classical nascent adhesions, focal adhesions, and fibrillar adhesions. Using live imaging, we observe that filopodia adhesions can give rise to nascent adhesions, which, in turn, form focal adhesions. We demonstrate that p130Cas (BCAR1) is recruited to filopodia tips via its C-terminal Cas family homology domain (CCHD) and acts as a mechanosensitive regulator of filopodia stability. Finally, we demonstrate that our map based on myosin-X-induced filopodia can be translated to endogenous filopodia and fascin- and IRSp53-mediated filopodia.

Regulation and modulation of PTEN activity

PTEN (Phosphatase and tensin homolog deleted on chromosome ten) is a tumor suppressor that is frequently mutated in most human cancers. PTEN is a lipid and protein phosphatase that antagonizes PI3K/AKT pathway through lipid phosphatase activity at the plasma membrane. More recent studies showed that, in addition to the putative role of PTEN as a PI(3,4,5)P3 3-phosphatase, it is a PI(3,4)P2 3-phosphatase during stimulation of class I PI3K signaling pathway by growth factor. Although PTEN tumor suppressor function via it's lipid phosphatase activity occurs primarily in the plasma membrane, it can also be found in the nucleus, in cytoplasmic organelles and extracellular space. PTEN has also shown phosphatase independent functions in the nucleus. PTEN can exit from the cell through exosomal export or secretion and has a tumor suppressor function in adjacent cells. PTEN has a critical role in growth, the cell cycle, protein synthesis, survival, DNA repair and migration. Understanding the regulation of PTEN function, activity, stability, localization and its dysregulation outcomes and also the intracellular and extracellular role of PTEN and paracrine role of PTEN-L in tumor cells as an exogenous therapeutic agent can help to improve clinical conceptualization and treatment of cancer.

Significance of PI3K/AKT signaling pathway in metastasis of esophageal squamous cell carcinoma and its potential as a target for anti-metastasis therapy

Metastasis is the most lethal hallmark of esophageal squamous cell carcinoma (ESCC). The aim of the study is to identify key signaling pathways that control metastasis in ESCC. Highly invasive ESCC sublines (designated I3 cells) were established through three rounds of selection of cancer cells invading through matrigel-coated chambers. Gene expression profile of one of the I3 sublines was compared with that of its parental cell line using cDNA microarray analysis. Gene ontology and pathway analyses of the differentially expressed genes (both upregulated and downregulated) indicated that genes associated with cellular movement and the AKT pathway were associated with increased cancer cell invasiveness. Western blot analysis confirmed increased phosphorylated AKT (p-AKT), N-cadherin and decreased E-cadherin expression in the I3 cells. Immunohistochemistry was used to evaluate the clinical significance of p-AKT expression in ESCC, and the results showed higher p-AKT nuclear expression in lymph node metastases when compared with primary carcinoma. Inactivation of the PI3K/AKT pathway with specific inhibitors, or with PTEN overexpression, resulted in reversed cadherin switching and inhibited cancer cell motility. Inhibition of the pathway by treatment with wortmannin markedly suppressed experimental metastasis in nude mice. Our data demonstrated the importance of the PI3K/AKT signaling pathway in ESCC metastasis and support PI3K/AKT as a valid therapeutic target in treatment of metastatic ESCC.

IGF2 induces CD133 expression in esophageal cancer cells to promote cancer stemness

Failure to eradicate cancer stem cells (CSC) during primary therapy may lead to cancer recurrence. We recently reported that CD133 is a functional biomarker for CSCs in esophageal squamous cell carcinoma (ESCC) but the molecular pathways critical for maintenance of CD133-positive CSCs are largely unknown. Here, we revealed that knockdown of IGF2 or treatment with PI3K/AKT inhibitors markedly inhibited the abilities of CD133-positive ESCC cells to self-renew, resist chemotherapeutic drugs, and form tumors. Further functional analysis identified miR-377 as a downstream regulator of PI3K/AKT signaling, and a mediator of the effects of IGF2 on CD133 expression and CSC properties. We found that the expression levels of IGF2 and CD133 were positively correlated with each other in primary ESCC, and that concurrent elevation of IGF2 and CD133 expression was significantly associated with poor patient survival. Furthermore, in vivo experiments demonstrated that IGF2-neutralizing antibody enhanced the sensitivity of tumor xenografts in nude mice to 5-fluorouracil treatment. This study underpins the importance of the IGF2-PI3K/AKT-miR-377-CD133 signaling axis in the maintenance of cancer stemness and in the development of novel therapeutic strategy for treatment of esophageal cancer.

PTEN/PTENP1: 'Regulating the regulator of RTK-dependent PI3K/Akt signalling', new targets for cancer therapy

Regulation of the PI-3 kinase (PI3K)/Akt signalling pathway is essential for maintaining the integrity of fundamental cellular processes, cell growth, survival, death and metabolism, and dysregulation of this pathway is implicated in the development and progression of cancers. Receptor tyrosine kinases (RTKs) are major upstream regulators of PI3K/Akt signalling. The phosphatase and tensin homologue (PTEN), a well characterised tumour suppressor, is a prime antagonist of PI3K and therefore a negative regulator of this pathway. Loss or inactivation of PTEN, which occurs in many tumour types, leads to overactivation of RTK/PI3K/Akt signalling driving tumourigenesis. Cellular PTEN levels are tightly regulated by a number of transcriptional, post-transcriptional and post-translational regulatory mechanisms. Of particular interest, transcription of the PTEN pseudogene, PTENP1, produces sense and antisense transcripts that exhibit post-transcriptional and transcriptional modulation of PTEN expression respectively. These additional levels of regulatory complexity governing PTEN expression add to the overall intricacies of the regulation of RTK/PI-3 K/Akt signalling. This review will discuss the regulation of oncogenic PI3K signalling by PTEN (the regulator) with a focus on the modulatory effects of the sense and antisense transcripts of PTENP1 on PTEN expression, and will further explore the potential for new therapeutic opportunities in cancer treatment.

MicroRNA-146b promotes PI3K/AKT pathway hyperactivation and thyroid cancer progression by targeting PTEN

Recent studies have shown that miR-146b is the most upregulated microRNA in thyroid cancer and has a central role in cancer progression through mechanisms that remain largely unidentified. As phosphoinositide 3-kinase/protein kinase-B (PI3K/AKT) signaling is a fundamental oncogenic driver in many thyroid cancers, we explored a potential role for miR-146b and its target genes in PI3K/AKT activation. Among the predicted target genes of miR-146b, we found the tumor-suppressor phosphatase and tensin homolog (PTEN). Constitutive overexpression of miR-146b in thyroid epithelial cell lines significantly decreased PTEN mRNA and protein levels by direct binding to its 3'-UTR. This was accompanied by PI3K/AKT hyperactivation, leading to the exclusion of FOXO1 and p27 from the nucleus and a corresponding increase in cellular proliferation. Moreover, miR-146b overexpression led to protection from apoptosis and an increased migration and invasion potential, regulating genes involved in epithelial-mesenchymal transition. Notably, with the single exception of E-cadherin expression, all of these outcomes could be reversed by PTEN coexpression. Further analysis showed that miR-146b directly inhibits E-cadherin expression through binding to its 3'-UTR. Interestingly, miR-146b inhibition in human thyroid tumor xenografts, using a synthetic and clinically amenable molecule, blocked tumor growth when delivered intratumorally. Importantly, this inhibition increased PTEN protein levels. In conclusion, our data define a novel mechanism of PI3K/AKT hyperactivation and outline a regulatory role for miR-146b in suppressing PTEN expression, a frequent observation in thyroid cancer. Both events are related to a more aggressive tumoral phenotype. Targeting miR-146b therefore represents a promising therapeutic strategy for the treatment of this disease.

SENP1 regulates PTEN stability to dictate prostate cancer development

SUMO protease SENP1 is elevated in multiple carcinomas including prostate cancer (PCa). SENP1 exhibits carcinogenic properties; it promotes androgen receptor-dependent and -independent cell proliferation, stabilizes HIF1a, increases VEGF, and supports angiogenesis. However, mice expressing an androgen-responsive promoter driven SENP1-transgene (SENP1-Tg) develop high-grade prostatic intraepithelial neoplasia, but not carcinoma. We now show that tumor suppressive PTEN signaling is induced in SENP1-Tg to enhance prostate epithelial cell apoptosis. SENP1 blocks SUMO1-dependent ubiquitylation and degradation of PTEN. In the absence of SENP1, SUMO1-modified PTEN is sequestered in the cytosol, where binding to ubiquitin-E3 ligase WWP2 occurs. Concurrently, WWP2 is also SUMOylated, which potentiates its interaction with PTEN. Thus, SENP1 directs ubiquitin-E3-substrate association to control PTEN stability. PTEN serves as a barrier for SENP1-mediated prostate carcinogenesis as SENP1-Tg mice develop invasive carcinomas only after PTEN reduction. Hence, SENP1 modulates multiple facets of carcinogenesis and may serve as a target specifically for aggressive PTEN-deficient PCa.

Exosomes Transfer Among Different Species Cells and Mediating miRNAs Delivery

Exosomes, the natural vehicles of intercellular communication, transfer proteins, mRNAs, and microRNAs (miRNAs) and mediate many physiological and pathological processes. It is not clear that whether exosomal miRNAs could regulate gene expression across species, though some studies suggest interactions of exosomal miRNAs between cells. In this report, we have isolated exosomes from rat PC12 cells and assessed their internalization by human cancer Hela cells. The internalized exosomes were located in Hela lysosomes. Human PTEN expression was significantly deregulated due to miR-21 delivered by rat cell exosomes. Our results prove that exosomes could incorporate between cells of different species and could regulate the protein expressions in the recipient cells by delivering the enclosed miRNAs. Thus our study foreshadows a futuristic treatment approach of utilizing miRNA enclosed exosome vehicles sans species concerns in combating various diseases/ regulating abnormal proteins. J. Cell. Biochem. 118: 4267-4274, 2017. © 2017 Wiley Periodicals, Inc.

Nuclear phosphoinositide regulation of chromatin

Phospholipid signaling has clear connections to a wide array of cellular processes, particularly in gene expression and in controlling the chromatin biology of cells. However, most of the work elucidating how phospholipid signaling pathways contribute to cellular physiology have studied cytoplasmic membranes, while relatively little attention has been paid to the role of phospholipid signaling in the nucleus. Recent work from several labs has shown that nuclear phospholipid signaling can have important roles that are specific to this cellular compartment. This review focuses on the nuclear phospholipid functions and the activities of phospholipid signaling enzymes that regulate metazoan chromatin and gene expression. In particular, we highlight the roles that nuclear phosphoinositides play in several nuclear-driven physiological processes, such as differentiation, proliferation, and gene expression. Taken together, the recent discovery of several specifically nuclear phospholipid functions could have dramatic impact on our understanding of the fundamental mechanisms that enable tight control of cellular physiology.

A study of the dynamics of PTEN proteins in living cells using in vivo fluorescence correlation spectroscopy

PTEN (phosphatase and tensin homolog on chromosome 10) is one of the most important tumor-suppressor proteins, which plays a key role in negative regulation of the PI3K/AKT pathway, and governs many cellular processes including growth, proliferation, survival and migration. The dynamics of PTEN proteins in single living cells is as yet unclear owing to a shortage of suitable in vivo approaches. Here, we report a single-molecule method for in vivo study of the dynamics of PTEN proteins in living cells using fluorescence correlation spectroscopy (FCS). First, we established a monoclonal H1299 stable cell line expressing enhanced green fluorescent protein (EGFP) and PTEN (EGFP-PTEN) fusion proteins; we then developed an in vivo FCS method to study the dynamics of EGFP-PTEN both in the nucleus and the cytoplasm. We investigated the diffusion behaviors of EGFP and EGFP-PTEN in solution, nucleus and cytosol, and observed that the motion of PTEN in living cells was restricted compared with EGFP. Finally, we investigated the protein dynamics in living cells under oxidative stress stimulation and a cellular ATP depletion treatment. Under oxidative stress stimulation, the EGFP-PTEN concentration increased in the nucleus, but slightly decreased in the cytoplasm. The diffusion coefficient and alpha value of EGFP-PTEN reduced significantly both in the nucleus and cytoplasm; the significantly decreased alpha parameter indicates a more restricted Brownian diffusion behavior. Under the cellular ATP depletion treatment, the concentration of EGFP-PTEN remained unchanged in the nucleus and decreased significantly in cytosol. The diffusion coefficient of EGFP-PTEN decreased significantly in cytosol, but showed no significant change in the nucleus; the alpha value decreased significantly in both the nucleus and cytoplasm. These results suggest that the concentration and mobility of PTEN in the nucleus and cytoplasm can be regulated by stimulation methods. Our approach provides a unique method for real-time monitoring of protein dynamics in different subcellular compartments under different stimulation treatments.

Characterization of PTEN mutations in brain cancer reveals that pten mono-ubiquitination promotes protein stability and nuclear localization

PTEN is a PIP3 phosphatase that antagonizes oncogenic PI3-kinase signalling. Due to its critical role in suppressing the potent signalling pathway, it is one of the most mutated tumour suppressors, especially in brain tumours. It is generally thought that PTEN deficiencies predominantly result from either loss of expression or enzymatic activity. By analysing PTEN in malignant glioblastoma primary cells derived from 16 of our patients, we report mutations that block localization of PTEN at the plasma membrane and nucleus without affecting lipid phosphatase activity. Cellular and biochemical analyses as well as structural modelling revealed that two mutations disrupt intramolecular interaction of PTEN and open its conformation, enhancing polyubiquitination of PTEN and decreasing protein stability. Moreover, promoting mono-ubiquitination increases protein stability and nuclear localization of mutant PTEN. Thus, our findings provide a molecular mechanism for cancer-associated PTEN defects and may lead to a brain cancer treatment that targets PTEN mono-ubiquitination.

PTEN and TRAIL genes loaded zein nanoparticles as potential therapy for hepatocellular carcinoma

Gene therapy is one of the recent approaches in treatment of hepatocellular carcinoma (HCC). Development of a vector or vehicle that can selectively and efficiently deliver the gene to target cells with minimal toxicity is an urgent demand. In the present study, phosphatase and tensin homolog (PTEN) and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) genes were loaded to zein nanoparticles (ZNPs). The formulated PTEN and TRAIL-loaded ZNPs were tested for their in vitro and in vivo potential antitumor efficacy using liver tumor cells (HepG2) and HCC-induced rats as animal model. Also, mRNA expression of p53, VGEF and MMP-2 were carried out as markers of apoptosis, angiogenesis and metastasis in animal liver tissues. The results of the study showed that both PTEN and TRAIL-loaded ZNPs proved anti-proliferative activity against HepG2 cell lines with IC₅₀ values of 0.09, 0.25 µg/ml, respectively. In vivo assay confirmed decrease in mRNA expression of both VEGF and MMP-2 with increased in P53 expression level in liver tissues of the treated animals. Therefore, authors introduced new integration between gene therapy and nanotechnology in the form of PTEN and TRAIL-loaded ZNPs that proved potential to be used in gene therapy for the treatment of HCC.

The neuropilin 2 isoform NRP2b uniquely supports TGFβ-mediated progression in lung cancer

Neuropilins (NRP1 and NRP2) are co-receptors for heparin-binding growth factors and class 3 semaphorins. Different isoforms of NRP1 and NRP2 are produced by alternative splicing. We found that in non-small cell lung cancer (NSCLC) cell lines, transforming growth factor-β (TGFβ) signaling preferentially increased the abundance of NRP2b. NRP2b and NRP2a differ only in their carboxyl-terminal regions. Although the presence of NRP2b inhibited cultured cell proliferation and primary tumor growth, NRP2b enhanced cellular migration, invasion into Matrigel, and tumorsphere formation in cultured cells in response to TGFβ signaling and promoted metastasis in xenograft mouse models. These effects of overexpressed NRP2b contrast with the effects of overexpressed NRP2a. Hepatocyte growth factor (HGF)-induced phosphorylation of the kinase AKT was specifically promoted by NRP2b, whereas inhibiting the HGF receptor MET attenuated NRP2b-dependent cell migration. Unlike NRP2a, NRP2b did not bind the PDZ domain scaffolding protein GAIP carboxyl terminus-interacting protein (GIPC1) and only weakly recruited phosphatase and tensin homolog (PTEN), potentially explaining the difference between NRP2b-mediated and NRP2a-mediated effects. Analysis of NSCLC patient tumors showed that NRP2b abundance correlated with that of the immune cell checkpoint receptor ligand PD-L1 as well as with epithelial-to-mesenchymal transition (EMT) phenotypes in the tumors, acquired resistance to epidermal growth factor receptor (EGFR) inhibitors, disease progression, and poor survival in patients. NRP2b knockdown attenuated the acquisition of resistance to the EGFR inhibitor gefitinib in cultured NSCLC cells. Thus, in NSCLC, NRP2b contributed to the oncogenic response to TGFβ and correlated with tumor progression in patients.

The PTEN tumor suppressor gene and its role in lymphoma pathogenesis

The phosphatase and tensin homolog gene PTEN is one of the most frequently mutated tumor suppressor genes in human cancer. Loss of PTEN function occurs in a variety of human cancers via its mutation, deletion, transcriptional silencing, or protein instability. PTEN deficiency in cancer has been associated with advanced disease, chemotherapy resistance, and poor survival. Impaired PTEN function, which antagonizes phosphoinositide 3-kinase (PI3K) signaling, causes the accumulation of phosphatidylinositol (3,4,5)-triphosphate and thereby the suppression of downstream components of the PI3K pathway, including the protein kinase B and mammalian target of rapamycin kinases. In addition to having lipid phosphorylation activity, PTEN has critical roles in the regulation of genomic instability, DNA repair, stem cell self-renewal, cellular senescence, and cell migration. Although PTEN deficiency in solid tumors has been studied extensively, rare studies have investigated PTEN alteration in lymphoid malignancies. However, genomic or epigenomic aberrations of PTEN and dysregulated signaling are likely critical in lymphoma pathogenesis and progression. This review provides updated summary on the role of PTEN deficiency in human cancers, specifically in lymphoid malignancies; the molecular mechanisms of PTEN regulation; and the distinct functions of nuclear PTEN. Therapeutic strategies for rescuing PTEN deficiency in human cancers are proposed.

ATM-mediated PTEN phosphorylation promotes PTEN nuclear translocation and autophagy in response to DNA-damaging agents in cancer cells

PTEN (phosphatase and tensin homolog), a tumor suppressor frequently mutated in human cancer, has various cytoplasmic and nuclear functions. PTEN translocates to the nucleus from the cytoplasm in response to oxidative stress. However, the mechanism and function of the translocation are not completely understood. In this study, topotecan (TPT), a topoisomerase I inhibitor, and cisplatin (CDDP) were employed to induce DNA damage. The results indicate that TPT or CDDP activates ATM (ATM serine/threonine kinase), which phosphorylates PTEN at serine 113 and further regulates PTEN nuclear translocation in A549 and HeLa cells. After nuclear translocation, PTEN induces autophagy, in association with the activation of the p-JUN-SESN2/AMPK pathway, in response to TPT. These results identify PTEN phosphorylation by ATM as essential for PTEN nuclear translocation and the subsequent induction of autophagy in response to DNA damage.

Assessing PTEN Subcellular Localization

PTEN subcellular localization is fundamental in the execution of the distinct PTEN biological activities, including not only its PI(3,4,5)P3 phosphatase activity when associated to membranes but also its subcellular compartment-specific interactions with regulatory and effector proteins, including those exerted in the nucleus. As a consequence, PTEN subcellular localization is tightly regulated in vivo by both intrinsic and extrinsic mechanisms. The plasma membrane/nucleus/cytoplasm partitioning of PTEN has been the focus of several studies, both from a mechanistic and from a disease-association point of view. Here, we summarize the current knowledge on PTEN plasma membrane/nucleus/cytoplasm distribution, and present subcellular fractionation, immunofluorescence, and immunohistochemical methods to study the distribution and shuttling of PTEN between these subcellular compartments.

A caveolin-dependent and PI3K/AKT-independent role of PTEN in β-catenin transcriptional activity

Loss of the tumour suppressor PTEN is frequent in human melanoma, results in MAPK activation, suppresses senescence and mediates metastatic behaviour. How PTEN loss mediates these effects is unknown. Here we show that loss of PTEN in epithelial and melanocytic cell lines induces the nuclear localization and transcriptional activation of β-catenin independent of the PI3K-AKT-GSK3β axis. The absence of PTEN leads to caveolin-1 (CAV1)-dependent β-catenin transcriptional modulation in vitro, cooperates with NRAS(Q61K) to initiate melanomagenesis in vivo and induces efficient metastasis formation associated with E-cadherin internalization. The CAV1-β-catenin axis is mediated by a feedback loop in which β-catenin represses transcription of miR-199a-5p and miR-203, which suppress the levels of CAV1 mRNA in melanoma cells. These data reveal a mechanism by which loss of PTEN increases CAV1-mediated dissociation of β-catenin from membranous E-cadherin, which may promote senescence bypass and metastasis.

Opening the conformation is a master switch for the dual localization and phosphatase activity of PTEN

Tumor suppressor PTEN mainly functions at two subcellular locations, the plasma membrane and the nucleus. At the plasma membrane, PTEN dephosphorylates the tumorigenic second messenger PIP3, which drives cell proliferation and migration. In the nucleus, PTEN controls DNA repair and genome stability independently of PIP3. Whereas the concept that a conformational change regulates protein function through post-translational modifications has been well established in biology, it is unknown whether a conformational change simultaneously controls dual subcellular localizations of proteins. Here, we discovered that opening the conformation of PTEN is the crucial upstream event that determines its key dual localizations of this crucial tumor suppressor. We identify a critical conformational switch that regulates PTEN's localization. Most PTEN molecules are held in the cytosol in a closed conformation by intramolecular interactions between the C-terminal tail and core region. Dephosphorylation of the tail opens the conformation and exposes the membrane-binding regulatory interface in the core region, recruiting PTEN to the membrane. Moreover, a lysine at residue 13 is also exposed and when ubiquitinated, transports PTEN to the nucleus. Thus, opening the conformation of PTEN is a key mechanism that enhances its dual localization and enzymatic activity, providing a potential therapeutic strategy in cancer treatments.

Focus on PTEN Regulation

The role of phosphatase and tensin homolog on chromosome 10 (PTEN) as a tumor suppressor has been for a long time attributed to its lipid phosphatase activity against PI(3,4,5)P₃, the phospholipid product of the class I PI3Ks. Besides its traditional role as a lipid phosphatase at the plasma membrane, a wealth of data has shown that PTEN can function independently of its phosphatase activity and that PTEN also exists and plays a role in the nucleus, in cytoplasmic organelles, and extracellularly. Accumulating evidence has shed light on diverse physiological functions of PTEN, which are accompanied by a complex regulation of its expression and activity. PTEN levels and function are regulated transcriptionally, post-transcriptionally, and post-translationally. PTEN is also sensitive to regulation by its interacting proteins and its localization. Herein, we summarize the current knowledge on mechanisms that regulate the expression and enzymatic activity of PTEN and its role in human diseases.

Biophysical methods for the characterization of PTEN/lipid bilayer interactions

PTEN, a tumor suppressor protein that dephosphorylates phosphoinositides at the 3-position of the inositol ring, is a cytosolic protein that needs to associate with the plasma membrane or other subcellular membranes to exert its lipid phosphatase function. Upon membrane association PTEN interacts with at least three different lipid entities: An anionic lipid that is present in sufficiently high concentration to create a negative potential that allows PTEN to interact electrostatically with the membrane, phosphatidylinositol-4,5-bisphosphate, which interacts with PTEN's N-terminal end and the substrate, usually phosphatidylinositol-3,4,5-trisphosphate. Many parameters influence PTEN's interaction with the lipid bilayer, for example, the lateral organization of the lipids or the presence of other chemical species like cholesterol or other lipids. To investigate systematically the different steps of PTEN's complex binding mechanism and to explore its dynamic behavior in the membrane bound state, in vitro methods need to be employed that allow for a systematic variation of the experimental conditions. In this review we survey a variety of methods that can be used to assess PTEN lipid binding affinity, the dynamics of its membrane association as well as its dynamic behavior in the membrane bound state.

Lung neuroendocrine tumors: correlation of ubiquitinylation and sumoylation with nucleo-cytosolic partitioning of PTEN

Background

The tumor suppressor phosphatase and tensin homolog (PTEN) is a pleiotropic enzyme, inhibiting phosphatidyl-inositol-3 kinase (PI3K) signaling in the cytosol and stabilizing the genome in the nucleus. Nucleo-cytosolic partitioning is dependent on the post-translational modifications ubiquitinylation and sumoylation. This cellular compartmentalization of PTEN was investigated in lung neuroendocrine tumors (lung NET).

Methods

Tumor tissues from 192 lung NET patients (surgical specimens = 183, autopsies = 9) were investigated on tissue microarrays. PTEN was H-scored by two investigators in nucleus and cytosol using the monoclonal antibody 6H2.1. Results were correlated with immunoreactivity for USP7 (herpes virus-associated ubiquitin-specific protease 7) and SUMO2/3 (small ubiquitin-related modifier protein 2/3) as well as PTEN and p53 FISH gene status. Clinico-pathologic data including overall survival, proliferation rate and diagnostic markers (synaptophysin, chromogranin A, Mib-1, TTF-1) were recorded.

Results

The multicentre cohort included 58 typical carcinoids (TC), 42 atypical carcinoids (AC), 32 large cell neuroendocrine carcinomas (LCNEC) and 60 small cell lung carcinomas (SCLC). Carcinoids were smaller in size and had higher synaptophysin and chromogranin A, but lower TTF-1 expressions. Patients with carcinoids were predominantly female and 10 years younger than patients with LCNEC/SCLC. In comparison to the carcinoids, LCNEC/SCLC tumors presented a stronger loss of nuclear and cytosolic PTEN associated with a loss of PTEN and p53. Concomitantly, a loss of nuclear USP7 but increase of nuclear and cytosolic SUMO2/3 was found. Loss of nuclear and cytosolic PTEN, loss of nuclear USP7 and increase of cytosolic SUMO2/3 thus correlated with poor survival. Among carcinoids, loss of cytosolic PTEN was predominantly found in TTF1-negative larger tumors of male patients. Among SCLC, loss of both cytosolic and nuclear PTEN but not proliferation rate or tumor size delineated a subgroup with poorer survival (all p-values <0.05).

Conclusions

Cellular ubiquitinylation and sumoylation likely influence the functional PTEN loss in high grade lung NET. Both nuclear and cytosolic PTEN immunoreactivity should be considered for correlation with clinico-pathologic parameters.

Membrane association of the PTEN tumor suppressor: neutron scattering and MD simulations reveal the structure of protein-membrane complexes

Neutron reflection (NR) from planar interfaces is an emerging technology that provides unique and otherwise inaccessible structural information on disordered molecular systems such as membrane proteins associated with fluid bilayers, thus addressing one of the remaining challenges of structural biology. Although intrinsically a low-resolution technique, using structural information from crystallography or NMR allows the construction of NR models that describe the architecture of protein-membrane complexes at high resolution. In addition, a combination of these methods with molecular dynamics (MD) simulations has the potential to reveal the dynamics of protein interactions with the bilayer in atomistic detail. We review recent advances in this area by discussing the application of these techniques to the complex formed by the PTEN phosphatase with the plasma membrane. These studies provide insights in the cellular regulation of PTEN, its interaction with PI(4,5)P2 in the inner plasma membrane and the pathway by which its substrate, PI(3,4,5)P3, accesses the PTEN catalytic site.

Identification of intrinsically disordered regions in PTEN and delineation of its function via a network approach

Intrinsically disordered proteins (IDPs) are proteins that lack stable higher order structures for the entire protein molecule or a significant portion of it. The discovery of IDPs evolved as an antithesis to the conventional structure-function paradigm wherein a higher order structure dictates protein function. Over the last decade, a number of proteins with functionally relevant unstructured regions have been discovered, which includes tumor suppressor PTEN. The protein domains that lack structure provide "hot-spots" for post-translational modifications (PTMs) and protein-protein interactions (PPIs), which facilitate their regulation and participation in multiple cellular processes. Consequently, dysregulation in IDPs contribute to aberrant cellular pathophysiology. Herein, we present PTEN and its translational isoform PTEN-L as a hybrid protein possessing ordered domain and intrinsically disordered C-terminal and an N-terminal tails. We review the role of intrinsic disorder in PTEN function and propose a methodology for the use of intrinsic disorder to study PTEN-regulated higher order protein-networks by associating basic principles of network biology to functional pathway analysis at the systems level.

Study of PTEN subcellular localization

The tumor suppressor PTEN is a key regulator of a plethora of cellular processes that are crucial in cancer development. Through its lipid phosphatase activity PTEN suppresses the PI3K/AKT pathway to govern cell proliferation, growth, migration, energy metabolism and death. The repertoire of roles fulfilled by PTEN has recently been expanded to include crucial functions in the nucleus, where it favors genomic stability and restrains cell cycle progression, as well as protein phosphatase dependent activity at the endoplasmic reticulum (ER) and mitochondria-associated membranes (MAMs), where PTEN interacts with the inositol 1,4,5-trisphosphate receptors (IP3Rs) and regulates Ca²⁺ release from the ER and sensitivity to apoptosis. Indeed, PTEN is present in definite subcellular locations where it performs distinct functions acting on specific effectors. In this review, we summarize recent advantages in methods to study PTEN subcellular localization and the distinct biological functions of PTEN in different cellular compartments. A deeper understanding of PTEN’s compartmentalized-functions will guide the rational design of novel therapies.

Engineering ePTEN, an enhanced PTEN with increased tumor suppressor activities

The signaling lipid phosphatidylinositol (3,4,5)-trisphosphate (PIP3) is a key regulator of cell proliferation, survival, and migration and the enzyme that dephosphorylates it, phosphatase and tensin homolog (PTEN), is an important tumor suppressor. As excess PIP3 signaling is a hallmark of many cancers, its suppression through activation of PTEN is a potential cancer intervention. Using a heterologous expression system in which human PTEN-GFP is expressed in Dictyostelium cells, we identified mutations in the membrane-binding regulatory interface that increase the recruitment of PTEN to the plasma membrane due to enhanced association with PI(4,5)P2. We engineered these into an enhanced PTEN (ePTEN) with approximately eightfold increased ability to suppress PIP3 signaling. Upon expression in human cells, ePTEN decreases PIP3 levels in the plasma membrane; phosphorylation of AKT, a major downstream event in PIP3 signaling; and cell proliferation and migration. Thus, the activation of PTEN can readjust PIP3 signaling and may serve as a feasible target for anticancer therapies.

MicroRNA-216a/217-induced epithelial-mesenchymal transition targets PTEN and SMAD7 to promote drug resistance and recurrence of liver cancer

Tumor recurrence and metastases are the major obstacles to improving the prognosis of patients with hepatocellular carcinoma (HCC). To identify novel risk factors associated with HCC recurrence and metastases, we have established a panel of recurrence-associated microRNAs (miRNAs) by comparing miRNA expression in recurrent and nonrecurrent human HCC tissue samples using microarrays (recurrence is defined as recurrent disease occurring within a 2-year time point of the original treatment). Among the panel, expression of the miR-216a/217 cluster was consistently and significantly up-regulated in HCC tissue samples and cell lines associated with early tumor recurrence, poor disease-free survival, and an epithelial-mesenchymal transition (EMT) phenotype. Stable overexpression of miR-216a/217-induced EMT increased the stem-like cell population, migration, and metastatic ability of epithelial HCC cells. Phosphatase and tensin homolog (PTEN) and mothers against decapentaplegic homolog 7 (SMAD7) were subsequently identified as two functional targets of miR-216a/217, and both PTEN and SMAD7 were down-regulated in HCC. Ectopic expression of PTEN or SMAD7 partially rescued miR-216a/217-mediated EMT, cell migration, and stem-like properties of HCC cells. Previously, SMAD7 was shown to be a transforming growth factor beta (TGF-β) type 1 receptor antagonist. Here, we further demonstrated that overexpression of miR-216a/217 acted as a positive feedback regulator for the TGF-β pathway and the canonical pathway involved in the activation of phosphoinositide 3-kinase/protein kinase K (PI3K/Akt) signaling in HCC cells. Additionally, activation of the TGF-β- and PI3K/Akt-signaling pathways in HCC cells resulted in an acquired resistance to sorafenib, whereas blocking activation of the TGF-β pathway overcame miR-216a/217-induced sorafenib resistance and prevented tumor metastases in HCC.

CONCLUSION

Overexpression of miR-216a/217 activates the PI3K/Akt and TGF-β pathways by targeting PTEN and SMAD7, contributing to hepatocarcinogenesis and tumor recurrence in HCC.

Critical role of increased PTEN nuclear translocation in excitotoxic and ischemic neuronal injuries

Stroke is the leading cause of disability in developed countries. However, no treatment is available beyond 3 h post-ictus. Here, we report that nuclear translocation of PTEN (phosphatase and tensin homolog deleted on chromosome TEN) is a delayed step causatively leading to excitotoxic (in vitro) and ischemic (in vivo) neuronal injuries. We found that excitotoxic stimulation of N-methyl-d-aspartate (NMDA) resulted in PTEN nuclear translocation in cultured neurons, a process requiring mono-ubiquitination at the lysine 13 residue (K13), as the translocation was prevented by mutation of K13 or a short interfering peptide (Tat-K13) that flanks the K13 residue. More importantly, using a rat model of focal ischemia, we demonstrated that systemic application of Tat-K13, even 6 h after stroke, not only reduced ischemia-induced PTEN nuclear translocation, but also strongly protected against ischemic brain damage. Our study suggests that inhibition of PTEN nuclear translocation may represent a novel after stroke therapy.

ΔNp63α represses nuclear translocation of PTEN by inhibition of NEDD4-1 in keratinocytes

ΔNp63α maintains the proliferative potential of keratinocytes by inhibiting the transcription and nuclear localization of the tumor suppressor PTEN as shown earlier by our laboratory. The goal of this study was to define the mechanisms by which ΔNp63α mediates the nuclear exclusion of PTEN. We demonstrate here that ΔNp63α reduces the ubiquitination of PTEN, a key signaling event in the nuclear translocation of PTEN. The decrease in ubiquitinated PTEN correlated with the ability of ΔNp63α to bind to neuronal precursor developmentally down regulated 4 (NEDD4) promoter and transcriptionally repress the E3 ubiquitin ligase NEDD4-1. Knockdown of NEDD4-1 in cultured keratinocytes was sufficient to attenuate the increase in nuclear PTEN observed upon silencing of ΔNp63α. In vivo examination of normal skin demonstrated that ΔNp63α and NEDD4-1 were both expressed in the basal layer of the epidermis and this correlated with nuclear exclusion of PTEN. Altogether, these studies suggest that ΔNp63α-mediated suppression of nuclear PTEN in basal layer keratinocytes occurs through repression of NEDD4-1.

Fibroblast growth factor 2 induces E-cadherin down-regulation via PI3K/Akt/mTOR and MAPK/ERK signaling in ovarian cancer cells

Fibroblast growth factor 2 (FGF2) is produced by ovarian cancer cells and it has been suggested to play an important role in tumor progression. In this study, we report that FGF2 treatment down-regulated E-cadherin by up-regulating its transcriptional repressors, Slug and ZEB1, in human ovarian cancer cells. The pharmacological inhibition of phosphatidylinositol-3-kinase (PI3K), mammalian target of rapamycin (mTOR), and MEK suggests that both PI3K/Akt/mTOR and MAPK/ERK signaling are required for FGF2-induced E-cadherin down-regulation. Moreover, FGF2 up-regulated Slug and ZEB1 expression via the PI3K/Akt/mTOR and MAPK/ERK signaling pathways, respectively. Finally, FGF2-induced cell invasion was abolished by the inhibition of the PI3K/Akt/mTOR and MAPK/ERK pathways, and the forced expression of E-cadherin diminished the intrinsic invasiveness of ovarian cancer cells as well as the FGF2-induced cell invasion. This study demonstrates a novel mechanism in which FGF2 down-regulates E-cadherin expression through the activation of PI3K/Akt/mTOR and MAPK/ERK signaling, and the up-regulation of Slug and ZEB1 in human ovarian cancer cells.

PtdIns(4,5)P2-mediated cell signaling: emerging principles and PTEN as a paradigm for regulatory mechanism

PtdIns(4,5)P2 (phosphatidylinositol 4,5-bisphosphate) is a relatively common anionic lipid that regulates cellular functions by multiple mechanisms. Hydrolysis of PtdIns(4,5)P2 by phospholipase C yields inositol trisphosphate and diacylglycerol. Phosphorylation by phosphoinositide 3-kinase yields PtdIns(3,4,5)P3, which is a potent signal for survival and proliferation. Also, PtdIns(4,5)P2 can bind directly to integral and peripheral membrane proteins. As an example of regulation by PtdIns(4,5)P2, we discuss phosphatase and tensin homologue deleted on chromosome 10 (PTEN) in detail. PTEN is an important tumor suppressor and hydrolyzes PtdIns(3,4,5)P3. PtdIns(4,5)P2 enhances PTEN association with the plasma membrane and activates its phosphatase activity. This is a critical regulatory mechanism, but a detailed description of this process from a structural point of view is lacking. The disordered lipid bilayer environment hinders structural determinations of membrane-bound PTEN. A new method to analyze membrane-bound protein measures neutron reflectivity for proteins bound to tethered phospholipid membranes. These methods allow determination of the orientation and shape of membrane-bound proteins. In combination with molecular dynamics simulations, these studies will provide crucial structural information that can serve as a foundation for our understanding of PTEN regulation in normal and pathological processes.

The determinants of head and neck cancer: Unmasking the PI3K pathway mutations

Studies attempting to identify and understand the function of mutated genes and deregulated molecular pathways in cancer have been ongoing for many years. The PI3K-PTEN-mTOR signaling pathway is one of the most frequently deregulated pathways in cancer. PIK3CA mutations are found 11%-33% of head and neck cancer (HNC). The hotspot mutation sites for PIK3CA are E542K, E545K and H1047R/L. The PTEN somatic mutations are in 9-23% of HNC, and they frequently cluster in the phosphatase domain of PTEN protein. PTEN loss of heterozygosity (LOH) ranges from 41%-71% and loss of PTEN protein expression occurs in 31.2% of the HNC samples. PIK3CA and PTEN are key molecules in the PI3K-PTEN-mTOR signaling pathway. In this review, we provided a comprehensive overview of mutations in the PI3K-PTEN-mTOR molecular circuitry in HNC, including PI3K family members, TSC1/TSC2, PTEN, AKT, and mTORC1 and mTORC2 complexes. We discussed how these genetic alterations may affect protein structure and function. We also highlight the latest discoveries in protein kinase and tumor suppressor families, emphasizing how mutations in these families interfere with PI3K signaling. A better understanding of the mechanisms underlying cancer formation, progression and resistance to therapy will inform selection of novel genomic-based personalized therapies for head and neck cancer patients.

Membrane association of the PTEN tumor suppressor: electrostatic interaction with phosphatidylserine-containing bilayers and regulatory role of the C-terminal tail

The phosphatidylinositolphosphate phosphatase PTEN is the second most frequently mutated protein in human tumors. Its membrane association, allosteric activation and membrane dissociation are poorly understood. We recently reported PTEN binding affinities to membranes of different compositions (Shenoy et al., 2012, PLoS ONE 7, e32591) and a preliminary investigation of the protein-membrane complex with neutron reflectometry (NR). Here we use NR to validate molecular dynamics (MD) simulations of the protein and study conformational differences of the protein in solution and on anionic membranes. NR shows that full-length PTEN binds to such membranes roughly in the conformation and orientation suggested by the crystal structure of a truncated PTEN protein, in contrast with a recently presented model which suggested that membrane binding depends critically on the SUMOylation of the CBR3 loop of PTEN's C2 domain. Our MD simulations confirm that PTEN is peripherally bound to the bilayer surface and show slight differences of the protein structure in solution and in the membrane-bound state, where the protein body flattens against the bilayer surface. PTEN's C2 domain binds phosphatidylserine (PS) tightly through its CBR3 loop, and its phosphatase domain also forms electrostatic interactions with PS. NR and MD results show consistently that PTEN's unstructured, anionic C-terminal tail is repelled from the bilayer surface. In contrast, this tail is tightly tugged against the C2 domain in solution, partially obstructing the membrane-binding interface of the protein. Arresting the C-terminal tail in this conformation by phosphorylation may provide a control mechanism for PTEN's membrane binding and activity.

Regulation of axonal growth and neuromuscular junction formation by neuronal phosphatase and tensin homologue signaling

Axonal growth and synaptogenesis are sequential events of neuronal development. Phosphatase and tensin homologue (PTEN) is expressed in motor neurons, and its disruption leads to continued axonal extension, even upon muscle contact, leading to synaptogenic suppression. Thus PTEN is involved in target-mediated cessation of axonal growth and subsequent synaptic differentiation.

During the development of the vertebrate neuromuscular junction (NMJ), motor axon tips stop growing after contacting muscle and transform into presynaptic terminals that secrete the neurotransmitter acetylcholine and activate postsynaptic ACh receptors (AChRs) to trigger muscle contraction. The neuron-intrinsic signaling that retards axonal growth to facilitate stable nerve–muscle interaction and synaptogenesis is poorly understood. In this paper, we report a novel function of presynaptic signaling by phosphatase and tensin homologue (PTEN) in mediating a growth-to-synaptogenesis transition in neurons. In Xenopus nerve–muscle cocultures, axonal growth speed was halved after contact with muscle, when compared with before contact, but when cultures were exposed to the PTEN blocker bisperoxo (1,10-phenanthroline) oxovanadate, axons touching muscle grew ∼50% faster than their counterparts in control cultures. Suppression of neuronal PTEN expression using morpholinos or the forced expression of catalytically inactive PTEN in neurons also resulted in faster than normal axonal advance after contact with muscle cells. Significantly, interference with PTEN by each of these methods also led to reduced AChR clustering at innervation sites in muscle, indicating that disruption of neuronal PTEN signaling inhibited NMJ assembly. We thus propose that PTEN-dependent slowing of axonal growth enables the establishment of stable nerve–muscle contacts that develop into NMJs.

A functional study of nucleocytoplasmic transport signals of the EhNCABP166 protein from Entamoeba histolytica

EhNCABP166 is an Entamoeba histolytica actin-binding protein that localizes to the nucleus and cytoplasm. Bioinformatic analysis of the EhNCABP166 amino acid sequence shows the presence of 3 bipartite nuclear localization signals (NLS) and a nuclear export signal (NES). The present study aimed to investigate the functionality of these signals in 3 ways. First, we fused each potential NLS to a cytoplasmic domain of ehFLN to determine whether the localization of this domain could be altered by the presence of the NLSs. Furthermore, the localization of each domain of EhNCABP166 was determined. Similarly, we generated mutations in the first block of bipartite signals from the domains that contained these signals. Additionally, we added an NES to 2 constructs that were then evaluated. We confirmed the intranuclear localization of EhNCABP166 using transmission electron microscopy. Fusion of each NLS resulted in shuttling of the cytoplasmic domain to the nucleus. With the exception of 2 domains, all of the evaluated domains localized within the nucleus. A mutation in the first block of bipartite signals affected the localization of the domains containing an NLS. The addition of an NES shifted the localization of these domains to the cytoplasm. The results presented here establish EhNCABP166 as a protein containing functional nuclear localization signals and a nuclear export signal.

Nongenomic Mechanisms of PTEN Regulation

A large amount of data supports the view that PTEN is a bona fide tumor suppressor gene. However, recent evidence suggests that derailment of cellular localization and expression levels of functional nonmutated PTEN is a determining force in inducing abnormal cellular and tissue outcomes. As the cellular mechanisms that regulate normal PTEN enzymatic activity resolve, it is evident that deregulation of these mechanisms can alter cellular processes and tissue architecture and ultimately lead to oncogenic transformation. Here we discuss PTEN ubiquitination, PTEN complex formation with components of the adherens junction, PTEN nuclear localization, and microRNA regulation of PTEN as essential regulatory mechanisms that determine PTEN function independent of gene mutations and epigenetic events.

The PTEN and Myotubularin phosphoinositide 3-phosphatases: linking lipid signalling to human disease

Two classes of lipid phosphatases selectively dephosphorylate the 3 position of the inositol ring of phosphoinositide signaling molecules: the PTEN and the Myotubularin families. PTEN dephosphorylates PtdIns(3,4,5)P(3), acting in direct opposition to the Class I PI3K enzymes in the regulation of cell growth, proliferation and polarity and is an important tumor suppressor. Although there are several PTEN-related proteins encoded by the human genome, none of these appear to fulfill the same functions. In contrast, the Myotubularins dephosphorylate both PtdIns(3)P and PtdIns(3,5)P(2), making them antagonists of the Class II and Class III PI 3-kinases and regulators of membrane traffic. Both phosphatase groups were originally identified through their causal mutation in human disease. Mutations in specific myotubularins result in myotubular myopathy and Charcot-Marie-Tooth peripheral neuropathy; and loss of PTEN function through mutation and other mechanisms is evident in as many as a third of all human tumors. This chapter will discuss these two classes of phosphatases, covering what is known about their biochemistry, their functions at the cellular and whole body level and their influence on human health.