Interaction of the tumor suppressor PTEN/MMAC with a PDZ domain of MAGI3, a novel membrane-associated guanylate kinase

Synthetic lethal targeting of PTEN-deficient cancer cells using selective disruption of polynucleotide kinase/phosphatase

A recent screen of 6,961 siRNAs to discover possible synthetic lethal partners of the DNA repair protein polynucleotide kinase/phosphatase (PNKP) led to the identification of the potent tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN). Here, we have confirmed the PNKP/PTEN synthetic lethal partnership in a variety of different cell lines including the PC3 prostate cancer cell line, which is naturally deficient in PTEN. We provide evidence that codepletion of PTEN and PNKP induces apoptosis. In HCT116 colon cancer cells, the loss of PTEN is accompanied by an increased background level of DNA double-strand breaks, which accumulate in the presence of an inhibitor of PNKP DNA 3'-phosphatase activity. Complementation of PC3 cells with several well-characterized mutated PTEN cDNAs indicated that the critical function of PTEN required to prevent toxicity induced by an inhibitor of PNKP is most likely associated with its cytoplasmic lipid phosphatase activity. Finally, we show that modest inhibition of PNKP in a PTEN knockout background enhances cellular radiosensitivity, suggesting that such a "synthetic sickness" approach involving the combination of PNKP inhibition with radiotherapy may be applicable to PTEN-deficient tumors.

MAGI-1 acts as a scaffolding molecule for NGF receptor-mediated signaling pathway

We have recently found that the membrane-associated guanylate kinase with inverted organization-1 (MAGI-1) was enriched in rat nervous tissues such as the glomeruli in olfactory bulb of adult rats and dorsal root entry zone in spinal cord of embryonic rats. In addition, we revealed the localization of MAGI-1 in the growth cone of the primary cultured rat dorsal root ganglion cells. These results point out the possibility that MAGI-1 is involved in the regulation of neurite extension or guidance. In this study, we attempted to reveal the physiological role(s) of MAGI-1 in neurite extension. We found that RNA interference (RNAi)-mediated knockdown of MAGI-1 caused inhibition of nerve growth factor (NGF)-induced neurite outgrowth in PC12 rat pheochromocytoma cells. To clarify the involvement of MAGI-1 in NGF-mediated signal pathway, we tried to identify binding partners for MAGI-1 and identified p75 neurotrophin receptor (p75NTR), a low affinity NGF receptor, and Shc, a phosphotyrosine-binding adaptor. These three proteins formed an immunocomplex in PC12 cells. Knockdown as well as overexpression of MAGI-1 caused suppression of NGF-stimulated activation of the Shc-ERK pathway, which is supposed to play important roles in neurite outgrowth of PC12 cells. These results indicate that MAGI-1 may act as a scaffolding molecule for NGF receptor-mediated signaling pathway.

The N and C termini of ZO-1 are surrounded by distinct proteins and functional protein networks

BACKGROUND

Biotin ligase tagging with ZO-1 was applied to identify a more complete tight junction proteome.

RESULTS

Identical but also different proteins and functional networks were identified near the N and C ends of ZO-1.

CONCLUSION

The ends of ZO-1 are embedded in different functional subcompartments of the tight junction.

SIGNIFICANCE

Biotin tagging with ZO-1 expands the tight junction proteome and defines subcompartments of the junction. The proteins and functional protein networks of the tight junction remain incompletely defined. Among the currently known proteins are barrier-forming proteins like occludin and the claudin family; scaffolding proteins like ZO-1; and some cytoskeletal, signaling, and cell polarity proteins. To define a more complete list of proteins and infer their functional implications, we identified the proteins that are within molecular dimensions of ZO-1 by fusing biotin ligase to either its N or C terminus, expressing these fusion proteins in Madin-Darby canine kidney epithelial cells, and purifying and identifying the resulting biotinylated proteins by mass spectrometry. Of a predicted proteome of ∼9000, we identified more than 400 proteins tagged by biotin ligase fused to ZO-1, with both identical and distinct proteins near the N- and C-terminal ends. Those proximal to the N terminus were enriched in transmembrane tight junction proteins, and those proximal to the C terminus were enriched in cytoskeletal proteins. We also identified many unexpected but easily rationalized proteins and verified partial colocalization of three of these proteins with ZO-1 as examples. In addition, functional networks of interacting proteins were tagged, such as the basolateral but not apical polarity network. These results provide a rich inventory of proteins and potential novel insights into functions and protein networks that should catalyze further understanding of tight junction biology. Unexpectedly, the technique demonstrates high spatial resolution, which could be generally applied to defining other subcellular protein compartmentalization.

Identification of PDZ Domain Containing Proteins Interacting with 1.2 and PMCA4b

PDZ (PSD-95/Disc large/Zonula occludens-1) protein interaction domains bind to cytoplasmic protein C-termini of transmembrane proteins. In order to identify new interaction partners of the voltage-gated L-type Ca2+ channel 1.2 and the plasma membrane Ca2+ ATPase 4b (PMCA4b), we used PDZ domain arrays probing for 124 PDZ domains. We confirmed this by GST pull-downs and immunoprecipitations. In PDZ arrays, strongest interactions with 1.2 and PMCA4b were found for the PDZ domains of SAP-102, MAST-205, MAGI-1, MAGI-2, MAGI-3, and ZO-1. We observed binding of the 1.2 C-terminus to PDZ domains of NHERF1/2, Mint-2, and CASK. PMCA4b was observed to interact with Mint-2 and its known interactions with Chapsyn-110 and CASK were confirmed. Furthermore, we validated interaction of 1.2 and PMCA4b with NHERF1/2, CASK, MAST-205 and MAGI-3 via immunoprecipitation. We also verified the interaction of 1.2 and nNOS and hypothesized that nNOS overexpression might reduce Ca2+ influx through 1.2. To address this, we measured Ca2+ currents in HEK 293 cells co-expressing 1.2 and nNOS and observed reduced voltage-dependent 1.2 activation. Taken together, we conclude that 1.2 and PMCA4b bind promiscuously to various PDZ domains, and that our data provides the basis for further investigation of the physiological consequences of these interactions.

PTEN modulators: a patent review

INTRODUCTION

PTEN (phosphatase and tensin homolog deleted on chromosome 10) plays a pivotal role in controlling intracellular signaling for cell survival and proliferation by inhibiting the PI3K/Akt pathway, and its dysfunction is associated with several neoplastic diseases. PTEN is frequently found mutated in many pathological conditions highlighting its importance in normal physiological function. Unlike several cellular proteins which are activated by phosphorylation, PTEN is inactivated upon phosphorylation by specific kinases which phosphorylate serine and threonine residues in its C-terminal region. Therefore, development of therapeutic agents that specifically target kinases and kinase-domain-containing proteins affecting PTEN would lead to the treatment of PTEN-related diseases.

AREAS COVERED

With increasing evidence on the role of PTEN in many human diseases, the present review focuses on the clinical relevance of PTEN with a comprehensive list of currently identified modulators of PTEN, and proposes potentially novel molecular targets which could aid in the development of drug candidates for the treatment of PTEN-related diseases such as cardiovascular diseases, diabetes, obesity, cancer, autism, Parkinson's and Alzheimer's diseases.

EXPERT OPINION

This review describes several target sites that could help in the development of novel drug candidates to regulate or restore the normal physiological functions of PTEN and are essential in the treatment of human diseases where PTEN plays a pivotal role.

A functional network of the tumor suppressors APC, hDlg, and PTEN, that relies on recognition of specific PDZ-domains

APC and PTEN are tumor suppressor proteins that bind through their C-termini to the PDZ domain containing-hDlg scaffolding protein. We have found that co-expression of PTEN and hDlg enhanced the negative regulation of the PI3K/Akt pathway by PTEN, indicating the physiologic importance of these interactions. APC and PTEN share other PDZ domain containing-interacting partners, including the MAGI scaffolding proteins and the MAST family of protein kinases. Mutational analysis revealed that the C-terminal PDZ-binding motifs from APC and PTEN were differentially recognized by distinct PDZ domains. APC bound to the three PDZ domains from hDlg, whereas PTEN mainly bound to PDZ-2/hDlg. This indicates the existence of overlapping, but distinct PDZ-domain recognition patterns by APC and PTEN. Furthermore, a ternary complex formed by APC, PTEN, and hDlg was detected, suggesting that hDlg may serve as a platform to bring in proximity APC and PTEN tumor suppressor activities. In line with this, tumor-related mutations targeting the PDZ-2/hDlg domain diminished its interaction with APC and PTEN. Our results expand the PDZ-domain counterparts for the tumor suppressor APC, show that APC and PTEN share PDZ-domain partners but have individual molecular determinants for specific recognition of PDZ domains, and suggest the participation of the tumor suppressors APC, PTEN, and hDlg in PDZ-domain interaction networks which may be relevant in oncogenesis.

Is the adult Sertoli cell terminally differentiated?

New data have challenged the convention that the adult Sertoli cell population is fixed and unmodifiable. The Sertoli cell has two distinct functions: 1) formation of the seminiferous cords and 2) provision of nutritional and structural support to developing germ cells. For these to occur successfully, Sertoli cells must undergo many maturational changes between fetal and adult life, the main switches occurring around puberty, including the loss of proliferative activity and the formation of the blood-testis barrier. Follicle-stimulating hormone plays a key role in promoting Sertoli cell proliferation, while thyroid hormone inhibits proliferative activity in early postnatal life. Together these regulate the Sertoli-germ cell complement and sperm output in adulthood. By puberty, the Sertoli cell population is considered to be stable and unmodifiable by hormones. But there is mounting evidence that the size of the adult Sertoli cell population and its maturational status is modifiable by hormones and that Sertoli cells can gain proliferative ability in the spermatogenically disrupted hamster and human model. This new information demonstrates that the adult Sertoli cell population, at least in the settings of testicular regression in the hamster and impaired fertility in humans in vivo and from mice and men in vitro, is not a terminally differentiated population. Data from the hamster now show that the adult Sertoli cell population size is regulated by hormones. This creates exciting prospects for basic and clinical research in testis biology. The potential to replenish an adult Sertoli-germ cell complement to normal in a setting of infertility may now be realized.

Sequence analysis of mutations and translocations across breast cancer subtypes

Breast carcinoma is the leading cause of cancer-related mortality in women worldwide, with an estimated 1.38 million new cases and 458,000 deaths in 2008 alone. This malignancy represents a heterogeneous group of tumours with characteristic molecular features, prognosis and responses to available therapy. Recurrent somatic alterations in breast cancer have been described, including mutations and copy number alterations, notably ERBB2 amplifications, the first successful therapy target defined by a genomic aberration. Previous DNA sequencing studies of breast cancer genomes have revealed additional candidate mutations and gene rearrangements. Here we report the whole-exome sequences of DNA from 103 human breast cancers of diverse subtypes from patients in Mexico and Vietnam compared to matched-normal DNA, together with whole-genome sequences of 22 breast cancer/normal pairs. Beyond confirming recurrent somatic mutations in PIK3CA, TP53, AKT1, GATA3 and MAP3K1, we discovered recurrent mutations in the CBFB transcription factor gene and deletions of its partner RUNX1. Furthermore, we have identified a recurrent MAGI3-AKT3 fusion enriched in triple-negative breast cancer lacking oestrogen and progesterone receptors and ERBB2 expression. The MAGI3-AKT3 fusion leads to constitutive activation of AKT kinase, which is abolished by treatment with an ATP-competitive AKT small-molecule inhibitor.

Biochemical and morphological characterization of MAGI-1 in neuronal tissue

The membrane-associated guanylate kinase with inverted organization (MAGI) proteins consist of three members, MAGI-1, MAGI-2 (also known as S-SCAM), and MAGI-3. Although MAGI-2 has been analyzed and shown to interact with a variety of postsynaptic proteins, functional analyses and characterization of MAGI-1 in neuronal tissues have been rare. In this study, we prepared a specific antibody against MAGI-1, anti-MAGI-1, and carried out biochemical and morphological analyses of MAGI-1 in rat neuronal tissues. By Western blotting, a high level of MAGI-1 was detected in nervous tissues, especially in olfactory bulb. Biochemical fractionation clarified that MAGI-1 was relatively enriched in the synaptosomal vesicle and synaptic plasma membrane fractions, whereas MAGI-2 and MAGI-3 appeared to be in the synaptic plasma membrane and postsynaptic density fractions. Immunofluorescent analyses revealed diffuse distribution of MAGI-1 in the cell body and processes of primary cultured rat hippocampal neurons, whereas MAGI-2 and MAGI-3 were likely to be enriched at synapses. Immunohistochemical analyses demonstrated that MAGI-1 was expressed in Purkinje cells, in hypocampal neurons in CA1 region, in the glomerulus region of olfactory bulb, and at the dorsal root entry zone in embryonic rat spinal cord. These results suggest neuronal roles of MAGI-1 different from those of MAGI-2/3.

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.

Na(+)/H(+) exchanger regulatory factor 1 (NHERF1) is required for the estradiol-dependent increase of phosphatase and tensin homolog (PTEN) protein expression

Expression of phosphatase and tensin homologue deleted on chromosome 10 (PTEN) can be induced by estrogens at the posttranscriptional level. However, the molecular mechanism of the process is unclear. In this study, we found that the C terminus (CT) of PTEN is indispensable for 17-β-estradiol (E2)-increased PTEN expression. Therefore, we screened for PTEN-CT-associated proteins using a glutathione-S-transferase pull-down approach in combination with mass spectrometry-based proteomic analyses. Our experiments led to the identification of Na(+)/H(+) exchanger regulatory factor 1 (NHERF1) as a major PTEN-CT binding partner. The first postsynaptic density protein-95/Discslarge/zonula occludens-1 homology domain of NHERF1 and the last four amino acids of PTEN were found to be key determinants of this interaction. By associating with PTEN, NHERF1 could enhance PTEN protein expression by retention of PTEN turnover, as demonstrated by NHERF1 overexpression and small interfering RNA-mediated knockdown experiments, respectively. Furthermore, NHERF1 inhibited ubiquitination of the PTEN protein upon competition with binding of PTEN to neural precursor cell expressed, developmentally down-regulated 4, an ubiquitin E3 ligase. E2 strongly induced the expression of NHERF1 and PTEN only in estrogen receptor (ER)-positive cells but not in ER-negative cells. ICI182780, an ER-specific inhibitor, decreased the expression of both NHERF1 and PTEN, and ICI182780 pretreatment also retarded E2-increased PTEN expression in ER-MDA-MB-231 cells. In both ER-MDA-MB-231 and MCF-7 cells, E2 failed to increase PTEN expression when NHERF1 was knocked down. Taken together, these are the first results that present a possible mechanism for E2-increased PTEN expression. In this process, E2 first induces NHERF1 expression by activating the ER. Upon competition with neural precursor cell expressed, developmentally down-regulated 4, NHERF1 then interacts with PTEN to inhibit PTEN degradation, through an ubiquitination-dependent pathway. This in turn leads to the increase of PTEN expression at the protein level.

Large-scale phosphotyrosine proteomic profiling of rat renal collecting duct epithelium reveals predominance of proteins involved in cell polarity determination

Although extensive phosphoproteomic information is available for renal epithelial cells, previous emphasis has been on phosphorylation of serines and threonines with little focus on tyrosine phosphorylation. Here we have carried out large-scale identification of phosphotyrosine sites in pervanadate-treated native inner medullary collecting ducts of rat, with a view towards identification of physiological processes in epithelial cells that are potentially regulated by tyrosine phosphorylation. The method combined antibody-based affinity purification of tyrosine phosphorylated peptides coupled with immobilized metal ion chromatography to enrich tyrosine phosphopeptides, which were identified by LC-MS/MS. A total of 418 unique tyrosine phosphorylation sites in 273 proteins were identified. A large fraction of these sites have not been previously reported on standard phosphoproteomic databases. All results are accessible via an online database: http://helixweb.nih.gov/ESBL/Database/iPY/. Analysis of surrounding sequences revealed four overrepresented motifs: [D/E]xxY*, Y*xxP, DY*, and Y*E, where the asterisk symbol indicates the site of phosphorylation. These motifs plus contextual information, integrated using the NetworKIN tool, suggest that the protein tyrosine kinases involved include members of the insulin- and ephrin-receptor kinase families. Analysis of the gene ontology (GO) terms and KEGG pathways whose protein elements are overrepresented in our data set point to structures involved in epithelial cell-cell and cell-matrix interactions ("adherens junction," "tight junction," and "focal adhesion") and to components of the actin cytoskeleton as major sites of tyrosine phosphorylation in these cells. In general, these findings mesh well with evidence that tyrosine phosphorylation plays a key role in epithelial polarity determination.

Apoptosis regulator through modulating IAP expression (ARIA) controls the PI3K/Akt pathway in endothelial and endothelial progenitor cells

Endothelial and endothelial progenitor cells (ECs and EPCs) play a fundamental role in angiogenesis that is essential for numerous physiological and pathological processes. The phosphatase and tensin homolog (PTEN)/ phosphoinositide 3-kinase (PI3K) pathway has been implicated in angiogenesis, but the mechanism in the regulation of this pathway in ECs and EPCs is poorly understood. Here we show that ARIA (apoptosis regulator through modulating IAP expression), a transmembrane protein that we recently identified, regulates the PTEN/PI3K pathway in ECs and EPCs and controls developmental and postnatal angiogenesis in vivo. We found that ARIA is abundantly expressed in EPCs and regulates their angiogenic functions by modulating PI3K/Akt/endothelial nitric oxide synthase (eNOS) signaling. Genetic deletion of ARIA caused nonfatal bleeding during embryogenesis, in association with increased small vessel density and altered expression of various vascular growth factors including angiopoietins and VEGF receptors. Postnatal neovascularization induced by critical limb ischemia was substantially enhanced in ARIA-null mice, in conjunction with more bone marrow (BM)-derived ECs detected in ischemic muscles. Administration of PI3K or NO synthase inhibitor completely abolished the enhanced neovascularization in ARIA(-/-) mice. Mechanistically, we identified that ARIA interacts with PTEN at the intracellular domain independently of the PTEN phosphorylation in its C-terminal tail. Overexpressed ARIA increased PTEN in the membrane fraction, whereas ARIA-silencing reduced the membrane-associated PTEN, resulting in modified PI3K/Akt signaling. Taken together, our findings establish a previously undescribed mode of regulation of the PTEN/PI3K/Akt pathway by ARIA, and reveal a unique mechanism in the control of angiogenesis. These functions of ARIA might offer a unique therapeutic potential.

Epithelial cell polarity and tumorigenesis: new perspectives for cancer detection and treatment

Loss of cell-cell adhesion and cell polarity is commonly observed in tumors of epithelial origin and correlates with their invasion into adjacent tissues and formation of metastases. Growing evidence indicates that loss of cell polarity and cell-cell adhesion may also be important in early stage of cancer. In first part of this review, we delineate the current understanding of the mechanisms that establish and maintain the polarity of epithelial tissues and discuss the involvement of cell polarity and apical junctional complex components in tumor pathogenesis. In the second part we address the clinical significance of cell polarity and junctional complex components in cancer diagnosis and prognosis. Finally, we explore their potential use as therapeutic targets in the treatment of cancer.

Endosomal targeting of the phosphoinositide 3-phosphatase MTMR2 is regulated by an N-terminal phosphorylation site

MTMR2 is a member of the myotubularin family of inositol lipid phosphatases, a large protein-tyrosine phosphatase subgroup that is conserved from yeast to humans. Furthermore, the peripheral neuromuscular disease Charcot-Marie Tooth disease type 4B has been attributed to mutations in the mtmr2 gene. Because the molecular mechanisms regulating MTMR2 have been poorly defined, we investigated whether reversible phosphorylation might regulate MTMR2 function. We used mass spectrometry-based methods to identify a high stoichiometry phosphorylation site on serine 58 of MTMR2. Phosphorylation at Ser(58), or a phosphomimetic S58E mutation, markedly decreased MTMR2 localization to endocytic vesicular structures. In contrast, a phosphorylation-deficient MTMR2 mutant (S58A) displayed constitutive localization to early endocytic structures. This localization pattern was accompanied by displacement of a PI(3)P-specific sensor protein and an increase in signal transduction pathways. Thus, MTMR2 phosphorylation is likely to be a critical mechanism by which MTMR2 access to its lipid substrate(s) is temporally and spatially regulated, thereby contributing to the control of downstream endosome maturation events.

MAGI-3 competes with NHERF-2 to negatively regulate LPA2 receptor signaling in colon cancer cells

BACKGROUND & AIMS

Lysophosphatidic acid (LPA) is a potent inducer of colon cancer and LPA receptor type 2 (LPA(2)) is overexpressed in colon tumors. LPA(2) interacts with membrane-associated guanylate kinase with inverted orientation-3 (MAGI-3) and the Na+/H+ exchanger regulatory factor 2 (NHERF-2), but the biological effects of these interactions are unknown. We investigated the roles of MAGI-3 and NHERF-2 in LPA(2)-mediated signaling in human colon cancer cells.

METHODS

We overexpressed or knocked down MAGI-3 in HCT116 and SW480 cells. The effects of MAGI-3 and NHERF-2 in LPA-induced cell migration, invasion, inositol phosphate generation, and nuclear factor-κB activation were determined. Expression of MAGI-3 and NHERF-2 in human colon tumor tissues was analyzed using tissue microarray analysis.

RESULTS

NHERF-2 promoted migration and invasion of colon cancer cells, whereas MAGI-3 inhibited these processes. MAGI-3 competed with NHERF-2 for binding to LPA(2) and phospholipase C-β3. However, NHERF-2 and MAGI-3 reciprocally regulated LPA(2)-induced phospholipase C activity. MAGI-3 increased the interaction of LPA(2) with Gα(12), whereas NHERF-2 preferentially promoted interaction between LPA(2) and Gα(q). MAGI-3 decreased the tumorigenic capacity of LPA(2) by attenuating the activities of nuclear factor-κB and c-Jun N-terminal kinase. MAGI-3 and NHERF-2 were expressed differentially in colon adenocarcinomas, consistent with their opposing effects.

CONCLUSIONS

LPA(2) is dynamically regulated by 2 distinct PDZ proteins via modulation of G-protein coupling and receptor signaling. MAGI-3 is a negative regulator of LPA(2) signaling.

Human T Lymphotropic Virus Type 1 (HTLV-1): Molecular Biology and Oncogenesis

Human T lymphotropic viruses (HTLVs) are complex deltaretroviruses that do not contain a proto-oncogene in their genome, yet are capable of transforming primary T lymphocytes both in vitro and in vivo. There are four known strains of HTLV including HTLV type 1 (HTLV-1), HTLV-2, HTLV-3 and HTLV-4. HTLV-1 is primarily associated with adult T cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-2 is rarely pathogenic and is sporadically associated with neurological disorders. There have been no diseases associated with HTLV-3 or HTLV-4 to date. Due to the difference in the disease manifestation between HTLV-1 and HTLV-2, a clear understanding of their individual pathobiologies and the role of various viral proteins in transformation should provide insights into better prognosis and prevention strategies. In this review, we aim to summarize the data accumulated so far in the transformation and pathogenesis of HTLV-1, focusing on the viral Tax and HBZ and citing appropriate comparisons to HTLV-2.

A new role for PTEN in regulating transient receptor potential canonical channel 6-mediated Ca2+ entry, endothelial permeability, and angiogenesis

Phosphatase and tensin homologue (PTEN) is a dual lipid-protein phosphatase that catalyzes the conversion of phosphoinositol 3,4,5-triphosphate to phosphoinositol 4,5-bisphosphate and thereby inhibits PI3K-Akt-dependent cell proliferation, migration, and tumor vascularization. We have uncovered a previously unrecognized role for PTEN in regulating Ca(2+) entry through transient receptor potential canonical channel 6 (TRPC6) that does not require PTEN phosphatase activity. We show that PTEN tail-domain residues 394-403 permit PTEN to associate with TRPC6. The inflammatory mediator thrombin promotes this association. Deletion of PTEN residues 394-403 prevents TRPC6 cell surface expression and Ca(2+) entry. However, PTEN mutant, C124S, which lacks phosphatase activity, did not alter TRPC6 activity. Thrombin failed to increase endothelial monolayer permeability in the endothelial cells, transducing the Δ394-403 PTEN mutant. Paradoxically, we also show that thrombin failed to induce endothelial cell migration and tube formation in cells transducing the Δ394-403 PTEN mutant. Our results demonstrate that PTEN, through residues 394-403, serves as a scaffold for TRPC6, enabling cell surface expression of the channel. Ca(2+) entry through TRPC6 induces an increase in endothelial permeability and directly promotes angiogenesis. Thus, PTEN is indicated to play a role beyond suppressing PI3K signaling.

Claudin-4 expression is associated with tumor invasion, MMP-2 and MMP-9 expression in gastric cancer

Claudin-4 is a member of the claudin family, a large family of transmembrane proteins that are essential in the formation and maintenance of tight junctions. Matrix metal-loproteinase (MMP)-2 and -9 degrade type IV collagen of the extracellular matrix and basal membranes. Claudin-4 activates MMP-2, indicating that claudin-mediated increased cancer cell invasion may result from the activation of MMP proteins. In the present study, we used immunohistochemistry to examine the expression levels of claudin-4, MMP-2 and MMP-9 in 189 gastric cancer samples, and analyzed their correlation with tumor invasion, clinicopathologic parameters and clinical outcome. The relationship between claudin-4 expression and MMP-2 and -9 expression was also investigated. The expression of claudin-4 was found to be significantly higher in gastric cancer cases with advanced depth of wall invasion, lymph node metastasis, lymphatic invasion and higher TNM stage. Further analysis revealed claudin-4 expression to be significantly correlated with the expression of MMP-2 and -9. Kaplan-Meier survival analysis indicated that MMP-9 expression was correlated with poor prognosis. These results suggest that claudin-4 expression is associated with tumor invasion and with MMP-2 and -9 expression in gastric cancer. Additionally, MMP-9 expression was demonstrated to serve as a prognostic factor in patients with gastric cancer.

Phosphoinositide phosphatases in cell biology and disease

Phosphoinositides are essential signaling molecules linked to a diverse array of cellular processes in eukaryotic cells. The metabolic interconversions of these phospholipids are subject to exquisite spatial and temporal regulation executed by arrays of phosphatidylinositol (PtdIns) and phosphoinositide-metabolizing enzymes. These include PtdIns- and phosphoinositide-kinases that drive phosphoinositide synthesis, and phospholipases and phosphatases that regulate phosphoinositide degradation. In the past decade, phosphoinositide phosphatases have emerged as topics of particular interest. This interest is driven by the recent appreciation that these enzymes represent primary mechanisms for phosphoinositide degradation, and because of their ever-increasing connections with human diseases. Herein, we review the biochemical properties of six major phosphoinositide phosphatases, the functional involvements of these enzymes in regulating phosphoinositide metabolism, the pathologies that arise from functional derangements of individual phosphatases, and recent ideas concerning the involvements of phosphoinositide phosphatases in membrane traffic control.

Isolation of putative FOXO1 genomic elements using an improved in vitro technique to isolate genomic regulatory sequences

The regulation of gene expression drives many biological processes and alterations in normal regulation are integral in the development of the diseased state. Therefore, the ability to screen genomic DNA for direct targets of DNA binding proteins (DNA-BP) would provide valuable information about the mechanisms underlying these processes. At present chromatin immunoprecipitation (ChIP) and its variants are the primary methods for identifying regulatory elements. However, some DNA-BPs, such as the winged-helix transcription factor FOXO1, are difficult to ChIP thereby detracting from the use of this technique as a nonbiased screen to isolate regulatory sequences. In this report we use an improved in vitro method to Pull Out Regulatory Elements (PORE), which uses purified protein with a stable genomic library to isolate regulatory elements directly bound by a DNA-BP, to identify putative FOXO1 genomic regulatory sequences. We first validate this technique using two known DNA-BP (FOXO1 and Pax3) by demonstrating their ability to bind and amplify identified promoter elements when present in a genomic DNA context or when present in the context of our stable genomic library. Subsequent use of this technique with FOXO1 isolated regulatory elements associated with several genes known to be regulated in a FOXO1-dependent manner and multiple genes whose biological functions are consistent with the known biological functions of FOXO1 proving that the in vitro PORE is a valuable and easy to use alternative to ChIP for the isolation of genomic regulatory elements.

Cell polarity in motion: redefining mammary tissue organization through EMT and cell polarity transitions

Epithelial to mesenchymal transition (EMT) and its reversion via mesenchymal to epithelial transition (MET), represent a stepwise cycle of epithelial plasticity that allows for normal tissue remodelling and diversification during development. In particular, epithelial-mesenchymal plasticity is central to many aspects of mammary development and has been proposed to be a key process in breast cancer progression. Such epithelial-mesenchymal plasticity requires complex cellular reprogramming to orchestrate a change in cell shape to an alternate morphology more conducive to migration. During this process, epithelial characteristics, including apical-basal polarity and specialised cell-cell junctions are lost and mesenchymal properties, such as a front-rear polarity associated with weak cell-cell contacts, increased motility, resistance to apoptosis and invasiveness are gained. The ability of epithelial cells to undergo transitions through cell polarity states is a central feature of epithelial-mesenchymal plasticity. These cell polarity states comprise a set of distinct asymmetric distributions of cellular constituents that are fashioned to allow specialized cellular functions, such as the regulated homeostasis of molecules across epithelial barriers, cell migration or cell diversification via asymmetric cell divisions. Each polarity state is engineered using a molecular toolbox that is highly conserved between organisms and cell types which can direct the initiation, establishment and continued maintenance of each asymmetry. Here we discuss how EMT pathways target cell polarity mediators, and how this EMT-dependent change in polarity states impact on the various stages of breast cancer. Emerging evidence places cell polarity at the interface of proliferation and morphology control and as such the changing dynamics within polarity networks play a critical role in normal mammary gland development and breast cancer progression.

Gene expression profile of androgen modulated genes in the murine fetal developing lung

BACKGROUND

Accumulating evidences suggest that sex affects lung development. Indeed, a higher incidence of respiratory distress syndrome is observed in male compared to female preterm neonates at comparable developmental stage and experimental studies demonstrated an androgen-related delay in male lung maturation. However, the precise mechanisms underlying these deleterious effects of androgens in lung maturation are only partially understood.

METHODS

To build up a better understanding of the effect of androgens on lung development, we analyzed by microarrays the expression of genes showing a sexual difference and those modulated by androgens. Lungs of murine fetuses resulting from a timely mating window of 1 hour were studied at gestational day 17 (GD17) and GD18, corresponding to the period of surge of surfactant production. Using injections of the antiandrogen flutamide to pregnant mice, we hunted for genes in fetal lungs which are transcriptionally modulated by androgens.

RESULTS

Results revealed that 1844 genes were expressed with a sexual difference at GD17 and 833 at GD18. Many genes were significantly modulated by flutamide: 1597 at GD17 and 1775 at GD18. Datasets were analyzed by using in silico tools for reconstruction of cellular pathways. Between GD17 and GD18, male lungs showed an intensive transcriptional activity of proliferative pathways along with the onset of lung differentiation. Among the genes showing a sex difference or an antiandrogen modulation of their expression, we specifically identified androgen receptor interacting genes, surfactant related genes in particularly those involved in the pathway leading to phospholipid synthesis, and several genes of lung development regulator pathways. Among these latter, some genes related to Shh, FGF, TGF-beta, BMP, and Wnt signaling are modulated by sex and/or antiandrogen treatment.

CONCLUSION

Our results show clearly that there is a real delay in lung maturation between male and female in this period, the latter pursuing already lung maturation while the proper is not yet fully engaged in the differentiation processes at GD17. In addition, this study provides a list of genes which are under the control of androgens within the lung at the moment of surge of surfactant production in murine fetal lung.

Tight junctions: a barrier to the initiation and progression of breast cancer?

Breast cancer is a complex and heterogeneous disease that arises from epithelial cells lining the breast ducts and lobules. Correct adhesion between adjacent epithelial cells is important in determining the normal structure and function of epithelial tissues, and there is accumulating evidence that dysregulated cell-cell adhesion is associated with many cancers. This review will focus on one cell-cell adhesion complex, the tight junction (TJ), and summarize recent evidence that TJs may participate in breast cancer development or progression. We will first outline the protein composition of TJs and discuss the functions of the TJ complex. Secondly we will examine how alterations in these functions might facilitate breast cancer initiation or progression; by focussing on the regulatory influence of TJs on cell polarity, cell fate and cell migration. Finally we will outline how pharmacological targeting of TJ proteins may be useful in limiting breast cancer progression. Overall we hope to illustrate that the relationship between TJ alterations and breast cancer is a complex one; but that this area offers promise in uncovering fundamental mechanisms linked to breast cancer progression.

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

PTEN (phosphatase and tensin homolog deleted on chromosome 10) is well characterized for its role in antagonizing the phosphoinositide 3-kinase pathway. Previous studies using size-exclusion chromatography demonstrated PTEN recruitment into high molecular mass complexes and hypothesized that PTEN phosphorylation status and PDZ binding domain may be required for such complex formation. In this study, we set out to test the structural requirements for PTEN complex assembly and identify the component(s) of the PTEN complex(es). Our results demonstrated that the PTEN catalytic function and PDZ binding domain are not absolutely required for its complex formation. On the other hand, PTEN phosphorylation status has a significant impact on its complex assembly. Our results further demonstrate enrichment of the PTEN complex in nuclear lysates, suggesting a mechanism through which PTEN phosphorylation may regulate its complex assembly. These results prompted further characterization of other protein components within the PTEN complex(es). Using size-exclusion chromatography and two-dimensional difference gel electrophoresis followed by mass spectrometry analysis, we identified heterogeneous nuclear ribonucleoprotein C (hnRNP C) as a novel protein recruited to higher molecular mass fractions in the presence of PTEN. Further analysis indicates that endogenous hnRNP C and PTEN interact and co-localize within the nucleus, suggesting a potential role for PTEN, alongside hnRNP C, in RNA regulation.

Epithelia and integration in sponges

An epithelium is important for integrity, homeostasis, communication and co-ordination, and its development must have been a fundamental step in the evolution of modern metazoan body plans. Sponges are metazoans that are often said to lack a true epithelium. We assess the properties of epithelia, and review the history of studies on sponge epithelia, focusing on their homology to bilaterian epithelia, their ultrastructure, and on their ability to seal. Electron micrographs show that adherens-type junctions are present in sponges but they can appear much slighter than equivalent junctions in other metazoans. Fine septae are seen in junctions of all sponge groups, but distinct septate junctions are only known from Calcarea. Similarly, all sponges can have collagenous sheets underlying their epithelia, but only homoscleromorphs are established to have a distinct basal lamina. The presence of most, but not all, gene families known to be involved in epithelial development and function also suggests that sponge epithelia function like, and are homologous to, bilaterian epithelia. However, physiological evidence that sponge epithelia regulate their internal environment is so far lacking. Given that up to six differentiated epithelia can be recognized in sponges, distinct physiological roles are expected. Recognition that sponges have epithelia challenges the perception that sponges are only loose associations of cells, and helps to relate the biology and physiology of the body plan of the adult sponge to the biology of other metazoans.

Lysophosphatidic acid 2 receptor-mediated supramolecular complex formation regulates its antiapoptotic effect

The G protein-coupled lysophosphatidic acid 2 (LPA(2)) receptor elicits prosurvival responses to prevent and rescue cells from apoptosis. However, G protein-coupled signals are not sufficient for the full protective effect of LPA(2). LPA(2) differs from other LPA receptor subtypes in the C-terminal tail, where it contains a zinc finger-binding motif for the interactions with LIM domain-containing TRIP6 and proapoptotic Siva-1, and a PDZ-binding motif through which it complexes with the NHERF2 scaffold protein. In this report, we identify a unique CXXC motif of LPA(2) responsible for the binding to TRIP6 and Siva-1, and demonstrate that disruption of these macromolecular complexes or knockdown of TRIP6 or NHERF2 expression attenuates LPA(2)-mediated protection from chemotherapeutic agent-induced apoptosis. In contrast, knockdown of Siva-1 expression enhances this effect. Furthermore, a PDZ-mediated direct interaction between TRIP6 and NHERF2 facilitates their interaction with LPA(2). Together, these results suggest that in addition to G protein-activated signals, the cooperation embedded in the LPA(2)-TRIP6-NHERF2 ternary complex provides a novel ligand-dependent signal amplification mechanism that is required for LPA(2)-mediated full activation of antiapoptotic signaling.

MAGI-1 modulates AMPA receptor synaptic localization and behavioral plasticity in response to prior experience

It is well established that the efficacy of synaptic connections can be rapidly modified by neural activity, yet how the environment and prior experience modulate such synaptic and behavioral plasticity is only beginning to be understood. Here we show in C. elegans that the broadly conserved scaffolding molecule MAGI-1 is required for the plasticity observed in a glutamatergic circuit. This mechanosensory circuit mediates reversals in locomotion in response to touch stimulation, and the AMPA-type receptor (AMPAR) subunits GLR-1 and GLR-2, which are required for reversal behavior, are localized to ventral cord synapses in this circuit. We find that animals modulate GLR-1 and GLR-2 localization in response to prior mechanosensory stimulation; a specific isoform of MAGI-1 (MAGI-1L) is critical for this modulation. We show that MAGI-1L interacts with AMPARs through the intracellular domain of the GLR-2 subunit, which is required for the modulation of AMPAR synaptic localization by mechanical stimulation. In addition, mutations that prevent the ubiquitination of GLR-1 prevent the decrease in AMPAR localization observed in previously stimulated magi-1 mutants. Finally, we find that previously-stimulated animals later habituate to subsequent mechanostimulation more rapidly compared to animals initially reared without mechanical stimulation; MAGI-1L, GLR-1, and GLR-2 are required for this change in habituation kinetics. Our findings demonstrate that prior experience can cause long-term alterations in both behavioral plasticity and AMPAR localization at synapses in an intact animal, and indicate a new, direct role for MAGI/S-SCAM proteins in modulating AMPAR localization and function in the wake of variable sensory experience.

Phosphatase and tensin homologue deleted on chromosome 10: extending its PTENtacles

Since its discovery in 1997, phosphatase and tensin homologue deleted on chromosome 10 (PTEN) has become one of the most important molecules in tumor biology. Mutations, deletions or dysregulation of PTEN is found in many human tumors. Recent studies have extended the reach of PTEN to include diabetes and neurological diseases such as Parkinson's and autism. In this review, we summarize the traditionally characterized function of PTEN as the lipid phosphatase that dephosphorylates PI-3,4,5-P(3), and several other newly discovered functions. The inhibition of the phosphatidylinositol-3-kinase (PI3K)/AKT signaling pathway may account for most of PTEN's tumor suppressing function. However, other growth inhibiting functions of PTEN may not involve this pathway. PTEN can also inhibit growth through its protein phosphatase activity and in ways not related to its enzymatic activity at all. We survey the many functions and biochemical interactions of PTEN in cytoplasm, the nucleus and throughout the cell in this paper.

PTEN acetylation modulates its interaction with PDZ domain

The PTEN tumor suppressor gene is frequently inactivated in human cancer. As a major tumor suppressor, PTEN function must be tightly regulated. Both phosphorylation and membrane association have been reported to regulate PTEN activity. In addition, the COOH terminus of PTEN has a typical PDZ domain-binding motif that interacts with several PDZ domain-containing proteins. In this report, we show that PTEN is acetylated on Lys(402), which is in the COOH-terminal PDZ domain-binding motif. We show that CBP plays a major role in PTEN acetylation, whereas the SIRT1 deacetylase is mainly responsible for PTEN deacetylation. Interestingly, Lys(402) acetylation modulates PTEN interaction with PDZ domain-containing proteins, indicating a potential role of acetylation in regulating PTEN function.

Proteomic analysis of the balance between survival and cell death responses in cisplatin-mediated ototoxicity

Cisplatin, a widely used anticancer drug, preferentially damages outer hair cells (OHCs) of the inner ear. In this study, an antibody microarray was used to identify early changes in protein expression in the rat cochlea induced by cisplatin. Only small changes in hearing thresholds (4-34 dB elevation) were detected two days after cisplatin treatment (12 mg/kg). OHC function, measured by otoacoustic emissions, was slightly depressed (10 dB), and little or no receptor cell loss was observed. However, cisplatin induced large changes in the expression of 19 proteins involved in apoptosis, cell survival, or progression through the cell cycle. Fifteen of the proteins are novel to the study of the inner ear. Immunoblotting confirmed increases in the levels of the pro-survival activating transcription factor 2 (ATF2), of pro-apoptotic serine-threonine protein kinase, receptor interacting protein, and a 70/75 kDa nitrotyrosine bearing doublet of unknown function. Anti-nitrotyrosine antibodies localized these oxidatively damaged proteins to the stereocilia of OHCs, the Golgi-centrosome region of Hensen's cells, nuclei of outer pillar cells, and tunnel crossing fibers innervating OHCs. The results of this proteomic analysis reflect the commencement of ototoxic and cell survival responses before the observation of a significant functional or anatomical loss.

Par-3-mediated junctional localization of the lipid phosphatase PTEN is required for cell polarity establishment

PDZ domain-containing scaffold protein Par-3 is the central organizer of the evolutionarily conserved cell polarity-regulatory Par-3.Par-6.atypical protein kinase C complex. The PDZ domains of Par-3 have also been implicated as potential phosphoinositide signaling integrators, since its second PDZ domain binds to phosphoinositides, and the third PDZ interacts with phosphoinositide phosphatase PTEN. However, the molecular basis of Par-3/PTEN interaction is still poorly understood. Additionally, it is not known whether the regulatory function of PTEN in cell polarity is specifically mediated by its interaction with Par-3. The structures of Par-3 PDZ3 in both its free and PTEN tail peptide-bound forms determined in this work reveal that Par-3 PDZ3 binds to PTEN with two discrete binding sites: a canonical PDZ-ligand interaction site and a distal, opposite charge-charge interaction site. This distinct target recognition mechanism confers the interaction specificity of the Par-3.PTEN complex. We show that the Par-3 PDZ3-PTEN binding is required for the enrichment of PTEN at the junctional membranes of Madin-Darby canine kidney cells. Finally, we demonstrate that the junctional membrane-localized PTEN is specifically required for the polarization of Madin-Darby canine kidney cells. These results, together with earlier data, firmly establish that Par-3 functions as a scaffold in integrating phosphoinositide signaling events during cellular polarization.

Comparison of p53 and the PDZ domain containing protein MAGI-3 regulation by the E6 protein from high-risk human papillomaviruses

Central to cellular transformation caused by human papillomaviruses (HPVs) is the ability of E6 proteins to target cellular p53 and proteins containing PDZ domains, including MAGI-3, for degradation. The aim of this study was to compare E6-mediated degradation of p53 and MAGI-3 under parallel experimental conditions and further with respect to the involvement of proteasomes and ubiquitination. We also compared the degradation of p53 and MAGI-3 by E6 from several HPV types including different variants from HPV-33. All of the E6 genes from different HPV types displayed similar abilities to mediate the degradation of both p53 and MAGI-3 although there may be subtle differences observed with the different 33E6 variants. There were however differences in E6 mediated degradation of p53 and MAGI-3. Proteasome inhibition assays partially protected p53 from E6 mediated degradation, but did not protect MAGI-3. In addition, under conditions where p53 was ubiquitinated by E6 and MDM2 in vivo, ubiquitination of MAGI-3 was not detected. These results imply that although both p53 and MAGI-3 represent effective targets for oncogenic E6, the mechanisms by which E6 mediates p53 and MAGI-3 degradation are distinct with respect to the involvement of ubiquitination prior to proteasomal degradation.

Regulation of the LPA2 receptor signaling through the carboxyl-terminal tail-mediated protein-protein interactions

While it is well known that lysophosphatidic acid (LPA) mediates diverse physiological and pathophysiological responses through the activation of G protein-coupled LPA receptors, the specificity and molecular mechanisms by which different LPA receptors mediate these biological responses remain largely unknown. Recent identification of several PDZ proteins and zinc finger proteins that interact with the carboxyl-terminal tail of the LPA2 receptor provides a considerable progress towards the understanding of the mechanisms how the LPA2 receptor specifically mediates LPA signaling pathways. These findings have led to the proposal that there are at least two distinct protein interaction motifs present in the carboxyl-terminus of the LPA2 receptor. Together, these data provide a new concept that the efficiency and specificity of the LPA2 receptor-mediated signal transduction can be achieved through the cross-regulation between the classical G protein-activated signaling cascades and the interacting partner-mediated signaling pathways.

NHERFs, NEP, MAGUKs, and more: interactions that regulate PTEN

This year marks the 10th anniversary of the discovery of the PTEN/MMAC1/TEP1 tumor suppressor gene (hereafter referred to as PTEN), one of the most commonly mutated genes in cancer. PTEN encodes a lipid phosphatase that dephosphorylates phosphoinositide-3,4,5-triphosphate (PIP(3)), thereby counteracting mitogenic signaling pathways driven by phosphoinositol-3-kinases (PI3K). By opposing PI3K signaling, PTEN inhibits the activation of the critical PI3K effector proteins Akt1-3 (also known as protein kinase B or PKB). Given its central role in antagonizing PI3K signaling, one might expect that like PI3K, the activity of the PTEN protein would be highly regulated by numerous protein/protein interactions. However, surprisingly little is known about such interactions. This fact, combined with the generally accepted notion that phosphatases are less exquisitely regulated than kinases, has led to the idea that PTEN may function in a relatively unregulated fashion. Here we review the identities and proposed functions of known PTEN-interacting proteins, and point out avenues of investigation that we hope may be fruitful in identifying important new mechanisms of PTEN regulation in mammalian cells.

Cellular localization of MAGI-1 caspase cleavage products and their role in apoptosis

MAGI-1 is a membrane-associated guanylate kinase (MAGUK) protein present at adherent and tight junctions, where it acts as a structural and signaling scaffold. During apoptosis, MAGI-1 is cleaved by caspases at Asp761 into N- and C-terminal cleavage products, allowing further dismantling of the cell junctions, one of the key features of apoptosis. Here, we investigated the cellular distribution and possible proapototic role of MAGI-1 caspase cleavage products. Full-length MAGI-1 exhibited submembrane localization, while the N-terminal caspase cleavage product of MAGI-1 is translocated to the cytosol and the C-terminal caspase cleavage product accumulates in the nucleus. When overexpressed in MDCK cells, both N- and C-terminal MAGI-1 caspase cleavage products exhibited minor proapoptotic activity, although their role in apoptosis is probably more passive.

Na+/H+ exchanger regulatory factor 1 inhibits platelet-derived growth factor signaling in breast cancer cells

INTRODUCTION

The gene encoding Na+/H+ exchanger regulatory factor 1 (NHERF1) is a putative tumor suppressor gene that harbors frequent loss of heterozygosity (LOH) and intragenic mutations in breast carcinoma. The exact biologic activity of NHERF1 in mammary glands, however, remains unclear. It was recently proposed that NHERF1 forms a ternary complex with platelet-derived growth factor receptor (PDGFR) and phosphatase and tensin homolog (PTEN), linking NHERF1 suppressor activity to PDGF-initiated phosphoinositide-3 kinase (PI3K)/PTEN signaling.

METHODS

The effect of NHERF1 on the kinetics of PDGF-induced Akt activation was determined in cells with varied NHERF1 background. Levels of active Akt in mammary gland of NHERF1 knockout and wild-type mice were compared. We also examined how NHERF1 expression status affects cell sensitivity to PDGFR inhibitor. A plausible connection between NHERF1 and PTEN pathway was explored at the genetic level.

RESULTS

We showed that NHERF1, through its PDZ-I domain, interacts directly with the carboxyl-terminal tail of PTEN. Knocking down NHERF1 expression in Zr75.1 cells markedly delayed the turnover of PDGF-induced phospho-Akt. Conversely, NHERF1 over-expression in MCF10A cells led to accelerated phospho-Akt degradation. The slowed decay of phospho-Akt that resulted from NHERF1 loss was evident in mouse embryonic fibroblasts isolated from NHERF1 knockout mice. In agreement with this, mammary gland tissues from these mice exhibited markedly elevated phospho-Akt. The responses of breast cancer cells to PDGFR inhibition were also altered by changes in NHERF1 expression level. Zr75.1 cells with NHERF1 knockdown were more resistant to STI-571-induced apoptosis than parental cells. Similarly, over-expression of NHERF1 rendered MCF10A cells more sensitive to STI-571. NHERF1-induced apoptotic response relies on an intact PTEN pathway; over-expression of NHERF1 in MCF10A cells with PTEN knockdown did not affect STI-571 sensitivity. It was found that NHERF1 LOH-positive breast cancer cells had reduced NHERF1 expression. Interestingly, these cells more frequently had wild-type PTEN or PI3KCA gene than the LOH-negative lines.

CONCLUSION

Our data indicate that the interaction of NHERF1 with PTEN counterbalances PI3K/Akt oncogenic signaling and may affect how cells respond to PDGFR inhibition in breast cancer. The dependence of NHERF1 responses on PTEN and genetic segregation of NHERF1 and PTEN (or PI3KCA) alterations suggest that NHERF1 is an active component of the PTEN pathway. Collectively, our study indicates that the biologic activity of NHERF1 in mammary gland is related to PTEN signaling.

New insights into PTEN

The functions ascribed to PTEN have become more diverse since its discovery as a putative phosphatase mutated in many human tumors. Although it can dephosphorylate lipids and proteins, it also has functions independent of phosphatase activity in normal and pathological states. In addition, control of PTEN function is very complex. It is positively and negatively regulated at the transcriptional level, as well as post-translationally by phosphorylation, ubiquitylation, oxidation and acetylation. Although most of its tumor suppressor activity is likely to be caused by lipid dephosphorylation at the plasma membrane, PTEN also resides in the cytoplasm and nucleus, and its subcellular distribution is under strict control. Deregulation of PTEN function is implicated in other human diseases in addition to cancer, including diabetes and autism.

MAGI-2 Inhibits cell migration and proliferation via PTEN in human hepatocarcinoma cells

MAGI-2, a multidomain scaffolding protein, contains nine potential protein-protein interaction modules, including a GuK domain, two WW domains and six PDZ domains. In this study, we examined eight human hepatocarcinoma cell lines (HHCCs) and found that MAGI-2 was expressed only in 7721 cells. After 7721, 7404 and 97H cells were transfected with myc-MAGI-2 plasmid, their migration and proliferation was significantly inhibited, which was associated with downregulation of p-FAK and p-Akt. It is known that p-FAK is a substrate of PTEN and p-Akt can be regulated by PTEN via PIP(3). We demonstrated that PTEN was upregulated after myc-MAGI-2 transfection, which was due to the enhancement of PTEN protein stability rather than mRNA levels. Furthermore, MAGI-2-induced inhibition of cell migration and proliferation was attenuated in 7721 cells with PTEN silence or in PTEN-null cell line U87MG, and PTEN transfection could restore the effect of MAGI-2 in U87MG cells. Finally, the molecular association between PTEN and MAGI-2 was confirmed. Our results suggested that PTEN played a critical role in MAGI-2-induced inhibition of cell migration and proliferation in HHCCs.

PTEN: its deregulation and tumorigenesis

The tumor suppressor phosphatase and tensin homolog (PTEN) functions as a phosphoinositide 3-phosphatase, that antagonizes phosphatidylinositol 3-kinase action, and negatively regulates cell proliferation and survival signals. Inactivation of PTEN by loss-of-function mutations gives rise to deregulated hyperproliferation of cells, leading to oncogenic transformation. Recent studies have identified a number of upstream regulatory factors for PTEN and unveiled that the impairment in the PTEN regulatory system potentially becomes a causal factor for oncogenic transformation of cells. This article will review the PTEN inactivation mechanism which is linked to human tumorigenesis, particularly focusing on recent research progress in PTEN regulators.

PDZ protein interactions underlying NMDA receptor-mediated excitotoxicity and neuroprotection by PSD-95 inhibitors

In neuronal synapses, PDZ domains [postsynaptic density-95 (PSD-95)/Discs large/zona occludens-1] of PSD-95 proteins interact with C termini of NMDA receptor [NMDAR (NR)] subunits, linking them to downstream neurotoxic signaling molecules. Perturbing NMDAR/PSD-95 interactions with a Tat peptide comprising the nine C-terminal residues of the NR2B subunit (Tat-NR2B9c) reduces neurons' vulnerability to excitotoxicity and ischemia. However, NR subunit C termini may bind many of >240 cellular PDZs, any of which could mediate neurotoxic signaling independently of PSD-95. Here, we performed a proteomic and biochemical analysis of the interactions of all known human PDZs with synaptic signaling proteins including NR1, NR2A-NR2D, and neuronal nitric oxide synthase (nNOS). Tat-NR2B9c, whose interactions define PDZs involved in neurotoxic signaling, was also used. NR2A-NR2D subunits and Tat-NR2B9c had similar, highly specific, PDZ protein interactions, of which the strongest were with the PSD-95 family members (PSD-95, PSD-93, SAP97, and SAP102) and Tax interaction protein 1 (TIP1). The PSD-95 PDZ2 domain bound NR2A-NR2C subunits most strongly (EC50, approximately 1 microM), and fusing the NR2B C terminus to Tat enhanced its affinity for PSD-95 PDZ2 by >100-fold (EC50, approximately 7 nM). IC50 values for Tat-NR2B9c inhibiting NR2A-NR2C/PSD-95 interactions (approximately 1-10 microM) and nNOS/PSD-95 interactions (200 nM) confirmed the feasibility of such inhibition. To determine which of the PDZ interactions of Tat-NR2B9c mediate neuroprotection, one of PSD-95, PSD-93, SAP97, SAP102, TIP1, or nNOS expression was inhibited in cortical neurons exposed to NMDA toxicity. Only neurons lacking PSD-95 or nNOS but not PSD-93, SAP97, SAP102, or TIP1 exhibited reduced excitotoxic vulnerability. Thus, despite the ubiquitousness of PDZ domain-containing proteins, PSD-95 and nNOS above any other PDZ proteins are keys in effecting NMDAR-dependent excitotoxicity. Consequently, PSD-95 inhibition may constitute a highly specific strategy for treating excitotoxic disorders.

The cytoplasmic plaque of tight junctions: a scaffolding and signalling center

The region of cytoplasm underlying the tight junction (TJ) contains several multimolecular protein complexes, which are involved in scaffolding of membrane proteins, regulation of cytoskeletal organization, establishment of polarity, and signalling to and from the nucleus. In this review, we summarize some of the most recent advances in understanding the identity of these proteins, their domain organization, their protein interactions, and their functions in vertebrate organisms. Analysis of knockdown and knockout model systems shows that several TJ proteins are essential for the formation of epithelial tissues and early embryonic development, whereas others appear to have redundant functions.

Cleavage of MAGI-1, a tight junction PDZ protein, by caspases is an important step for cell-cell detachment in apoptosis

MAGI-1, a member of the MAGUK family of proteins, is shown to be rapidly cleaved during Fas-induced apoptosis in mouse 3T3 A31 cells, and in UV irradiation- and staurosporine-induced apoptosis in HaCaT cells. This generates a 97 kDa N-terminal fragment that dissociates from the cell membrane; a process that is largely prevented in the presence of the caspase inhibitor Z-VAD-fmk. In addition, we show that in vitro translated radiolabelled MAGI-1 is efficiently cleaved into 97 kDa and 68 kDa fragments by caspases-3 and -7 at physiological concentrations and mutating the MAGI-1 Asp(761) to Ala completely abolished the caspase-induced cleavage. Moreover, in HaCaT cells overexpressing the MAGI-1 Asp(761)Ala mutant the disruption of cell-cell contacts was delayed during apoptosis, whereas other caspase-dependent processes such as nuclear condensation were not affected, suggesting that cell detachment is parallel to them. Thus, MAGI-1 cleavage appears to be an important step in the disassembly of cell-cell contacts during apoptosis.