The PDZ protein tax-interacting protein-1 inhibits beta-catenin transcriptional activity and growth of colorectal cancer cells

Blocking the functional domain of TIP1 by antibodies sensitizes cancer to radiation therapy

Non-small-cell lung cancer (NSCLC) and glioblastoma (GB) have poor prognoses. Discovery of new molecular targets is needed to improve therapy. Tax interacting protein 1 (TIP1), which plays a role in cancer progression, is overexpressed and radiation-inducible in NSCLC and GB. We evaluated the effect of an anti-TIP1 antibody alone and in combination with ionizing radiation (XRT) on NSCLC and GB in vitro and in vivo. NSCLC and GB cells were treated with anti-TIP1 antibodies and evaluated for proliferation, colony formation, endocytosis, and cell death. The efficacy of anti-TIP1 antibodies in combination with XRT on tumor growth was measured in mouse models of NSCLC and GB. mRNA sequencing was performed to understand the molecular mechanisms involved in the action of anti-TIP1 antibodies. We found that targeting the functional domain of TIP1 leads to endocytosis of the anti-TIP1 antibody followed by reduced proliferation and increased apoptosis-mediated cell death. Anti-TIP1 antibodies bound specifically (with high affinity) to cancer cells and synergized with XRT to significantly increase cytotoxicity in vitro and reduce tumor growth in mouse models of NSCLC and GB. Importantly, downregulation of cancer survival signaling pathways was found in vitro and in vivo following treatment with anti-TIP1 antibodies. TIP1 is a new therapeutic target for cancer treatment. Antibodies targeting the functional domain of TIP1 exhibited antitumor activity and enhanced the efficacy of radiation both in vitro and in vivo. Anti-TIP1 antibodies interrupt TIP1 function and are effective cancer therapy alone or in combination with XRT in mouse models of human cancer.

Cancer invasion and metastasis: Insights from murine pubertal mammary gland morphogenesis

Cancer invasion and metastasis accounts for the majority of cancer related mortality. A better understanding of the players that drive the aberrant invasion and migration of tumors cells will provide critical targets to inhibit metastasis. Postnatal pubertal mammary gland morphogenesis is characterized by highly proliferative, invasive, and migratory normal epithelial cells. Identifying the molecular regulators of pubertal gland development is a promising strategy since tumorigenesis and metastasis is postulated to be a consequence of aberrant reactivation of developmental stages. In this review, we summarize the pubertal morphogenesis regulators that are involved in cancer metastasis and revisit pubertal mammary gland transcriptome profiling to uncover both known and unknown metastasis genes. Our updated list of pubertal morphogenesis regulators shows that most are implicated in invasion and metastasis. This review highlights molecular linkages between development and metastasis and provides a guide for exploring novel metastatic drivers.

Tax1 binding protein 3 regulates osteogenic and adipogenic differentiation through inactivating Wnt/β-catenin signalling

Tax1 binding protein 3 (Tax1bp3) is a PDZ domain-containing protein that is overexpressed in cancer. Previous studies recognized Tax1bp3 as an inhibitor of β-catenin. Till now it is not known whether Tax1bp3 regulates osteogenic and adipogenic differentiation of mesenchymal progenitor cells. In the current study, the data showed that Tax1bp3 was expressed in bone and was increased in the progenitor cells when induced toward osteoblast and adipocyte differentiation. The overexpression of Tax1bp3 in the progenitor cells inhibited osteogenic differentiation and conversely stimulated adipogenic differentiation, and the knockdown of Tax1bp3 affected the differentiation of the progenitor cells oppositely. Ex vivo experiments using the primary calvarial osteoblasts from osteoblast-specific Tax1bp3 knock-in mice also demonstrated the anti-osteogenic and pro-adipogenic function of Tax1bp3. Mechanistic investigations revealed that Tax1bp3 inhibited the activation of canonical Wnt/β-catenin and bone morphogenetic proteins (BMPs)/Smads signalling pathways. Taken together, the current study has provided evidences demonstrating that Tax1bp3 inactivates Wnt/β-catenin and BMPs/Smads signalling pathways and reciprocally regulates osteogenic and adipogenic differentiation from mesenchymal progenitor cells. The inactivation of Wnt/β-catenin signalling may be involved in the reciprocal role of Tax1bp3.

WNT/β-catenin pathway is a key regulator of cardiac function and energetic metabolism

The WNT/β-catenin pathway is a master regulator of cardiac development and growth, and its activity is low in healthy adult hearts. However, even this low activity is essential for maintaining normal heart function. Acute activation of the WNT/β-catenin signaling cascade is considered to be cardioprotective after infarction through the upregulation of prosurvival genes and reprogramming of metabolism. Chronically high WNT/β-catenin pathway activity causes profibrotic and hypertrophic effects in the adult heart. New data suggest more complex functions of β-catenin in metabolic maturation of the perinatal heart, establishing an adult pattern of glucose and fatty acid utilization. Additionally, low basal activity of the WNT/β-catenin cascade maintains oxidative metabolism in the adult heart, and this pathway is reactivated by physiological or pathological stimuli to meet the higher energy needs of the heart. This review summarizes the current state of knowledge of the organization of canonical WNT signaling and its function in cardiogenesis, heart maturation, adult heart function, and remodeling. We also discuss the role of the WNT/β-catenin pathway in cardiac glucose, lipid metabolism, and mitochondrial physiology.

The Usher syndrome 1C protein harmonin regulates canonical Wnt signaling

Human Usher syndrome (USH) is the most common form of hereditary combined deaf-blindness. USH is a complex genetic disorder, and the pathomechanisms underlying the disease are far from being understood, especially in the eye and retina. The USH1C gene encodes the scaffold protein harmonin which organizes protein networks due to binary interactions with other proteins, such as all USH proteins. Interestingly, only the retina and inner ear show a disease-related phenotype, although USH1C/harmonin is almost ubiquitously expressed in the human body and upregulated in colorectal cancer. We show that harmonin binds to β-catenin, the key effector of the canonical Wnt (cWnt) signaling pathway. We also demonstrate the interaction of the scaffold protein USH1C/harmonin with the stabilized acetylated β-catenin, especially in nuclei. In HEK293T cells, overexpression of USH1C/harmonin significantly reduced cWnt signaling, but a USH1C-R31* mutated form did not. Concordantly, we observed an increase in cWnt signaling in dermal fibroblasts derived from an USH1C ^{R31*/R80Pfs*69} patient compared with healthy donor cells. RNAseq analysis reveals that both the expression of genes related to the cWnt signaling pathway and cWnt target genes were significantly altered in USH1C patient-derived fibroblasts compared to healthy donor cells. Finally, we show that the altered cWnt signaling was reverted in USH1C patient fibroblast cells by the application of Ataluren, a small molecule suitable to induce translational read-through of nonsense mutations, hereby restoring some USH1C expression. Our results demonstrate a cWnt signaling phenotype in USH establishing USH1C/harmonin as a suppressor of the cWnt/β-catenin pathway.

Characterization of the interaction between β-catenin and sorting nexin 27: contribution of the type I PDZ-binding motif to Wnt signaling

Background: Sorting Nexin 27 (SNX27) is a 62-kDa protein localized to early endosomes and known to regulate the intracellular trafficking of ion channels and receptors. In addition to a PX domain common among all of the sorting nexin family, SNX27 is the only sorting family member that contains a PDZ domain. To identify novel SNX27–PDZ binding partners, we performed a proteomic screen in mouse principal kidney cortical collecting duct cells (mpkCCD) using a GST-SNX27 fusion construct as bait. We found that the C-terminal type I PDZ binding motif (DTDL) of β-catenin, an adherens junction scaffolding protein and transcriptional co-activator, interacts directly with SNX27. Using biochemical and immunofluorescent techniques, β-catenin was identified in endosomal compartments where co-localization with SNX27 was observed. Furthermore, E-cadherin, but not Axin, GSK3 or Lef-1 was located in SNX27 protein complexes. While overexpression of wild-type β-catenin protein increased TCF-LEF dependent transcriptional activity, an enhanced transcriptional activity was not observed in cells expressing β-Catenin ΔFDTDL or diminished SNX27 expression.

These results imply importance of the C-terminal PDZ binding motif for the transcriptional activity of β-catenin and propose that SNX27 might be involved in the assembly of β-catenin complexes in the endosome.

Effect of Toxic Metal Binding on Tax-Interacting Protein1 (TIP1): A Protein Related to Brain Diseases

Human tax-interacting protein1 (TIP1), also known as glutaminase-interacting protein (GIP), is a small globular protein containing a PDZ domain. PDZ domains are the most common protein-protein interaction modules present in eukaryotes. In humans, TIP1 plays a very important role in many cellular pathways including β-catenin-mediated Wnt signaling, Rho-activator rhotekin-mediated Rho signaling pathway, and glutamate signaling pathway for the normal activity of the central nervous system. TIP1 also regulates potassium channel expression in the plasma membrane and is a binding partner to many proteins including viral oncoproteins, HTLV-1 Tax and HPV16 E6. Since TIP1 is at a pivotal point in many cellular processes through its interaction with a growing list of partner proteins, any impact on the proper functioning of this protein can have severe consequences on the well-being of a living system. Although metals are essential for plants and animals in trace amounts, elevated levels of heavy metals such as arsenic, cadmium, zinc, and lead are toxic causing various health problems including cardiovascular disorders, neuronal damage, renal injuries, and cancer. Here, we report the effect of heavy metals, arsenic and cadmium, on TIP1 conformation using circular dichroism and fluorescence spectroscopy techniques. Our study revealed these metals have a significant impact on the structure of TIP1 even at very low levels.

Region-Resolved Quantitative Proteome Profiling Reveals Molecular Dynamics Associated With Chronic Pain in the PNS and Spinal Cord

To obtain a thorough understanding of chronic pain, large-scale molecular mapping of the pain axis at the protein level is necessary, but has not yet been achieved. We applied quantitative proteome profiling to build a comprehensive protein compendium of three regions of the pain neuraxis in mice: the sciatic nerve (SN), the dorsal root ganglia (DRG), and the spinal cord (SC). Furthermore, extensive bioinformatics analysis enabled us to reveal unique protein subsets which are specifically enriched in the peripheral nervous system (PNS) and SC. The immense value of these datasets for the scientific community is highlighted by validation experiments, where we monitored protein network dynamics during neuropathic pain. Here, we resolved profound region-specific differences and distinct changes of PNS-enriched proteins under pathological conditions. Overall, we provide a unique and validated systems biology proteome resource (summarized in our online database painproteome.em.mpg.de), which facilitates mechanistic insights into somatosensory biology and chronic pain—a prerequisite for the identification of novel therapeutic targets.

Anti-tax interacting protein-1 (TIP-1) monoclonal antibody targets human cancers

Radiation-inducible neo-antigens are proteins expressed on cancer cell surface after exposure to ionizing radiation (IR). These neo-antigens provide opportunities to specifically target cancers while sparing normal tissues. Tax interacting protein-1 (TIP-1) is induced by irradiation and is translocated to the surface of cancer cells. We have developed a monoclonal antibody, 2C6F3, against TIP-1.

Epitope mapping revealed that 2C6F3 binds to the QPVTAVVQRV epitope of the TIP-1 protein. 2C6F3 binds to the surface of lung cancer (A549, LLC) and glioma (D54, GL261) cell lines. 2C6F3 binds specifically to TIP-1 and ELISA analysis showed that unconjugated 2C6F3 efficiently blocked binding of radiolabeled 2C6F3 to purified TIP-1 protein. To study in vivo tumor binding, we injected near infrared (NIR) fluorochrome-conjugated 2C6F3 via tail vein in mice bearing subcutaneous LLC and GL261 heterotopic tumors. The NIR images indicated that 2C6F3 bound specifically to irradiated LLC and GL261 tumors, with little or no binding in un-irradiated tumors.

We also determined the specificity of 2C6F3 to bind tumors in vivo using SPECT/CT imaging. 2C6F3 was conjugated with diethylene triamine penta acetic acid (DTPA) chelator and radiolabeled with ¹¹¹Indium (¹¹¹In). SPECT/CT imaging revealed that ¹¹¹In-2C6F3 bound more to the irradiated LLC tumors compared to un-irradiated tumors. Furthermore, injection of DTPA-2C6F3 labeled with the therapeutic radioisotope, ⁹⁰Y, (⁹⁰Y-DTPA-2C6F3) significantly delayed LLC tumor growth. 2C6F3 mediated antibody dependent cell-mediated cytotoxicity (ADCC) and antibody dependent cell-mediated phagocytosis (ADCP) in vitro.

In conclusion, the monoclonal antibody 2C6F3 binds specifically to TIP-1 on cancer and radio-immunoconjugated 2C6F3 improves tumor control.

Mechanisms of RhoA inactivation and CDC42 and Rac1 activation during zebrafish optic nerve regeneration

When axons of the mammalian central nervous system (CNS) are injured, they fail to regenerate, while those of lower vertebrates undergo regeneration after injury. Wingless-type MMTV integration site family (Wnt) proteins play important roles in the CNS, and are reported to be activated after mammalian spinal cord or brain injury. Moreover, for axon growth to proceed, it is thought that small G-proteins, such as CDC42 and Rac1, need to be activated, whereas RhoA must be inactivated. However, the cell and molecular mechanisms involved in optic nerve regeneration remain unclear. In this study, we investigated axonal regeneration after injury using the zebrafish optic nerve as a model system. We sought to clarify the role of Wnt proteins and the mechanisms involved in the activation and inactivation of small G-proteins in nerve regeneration. After optic nerve injury, mRNA levels of Wnt5b, TAX1BP3 and ICAT increased in the retina, while those of Wnt10a decreased. These changes were associated with a reduction in β-catenin in nuclei. We found that Wnt5b activated CDC42 and Rac1, leading to the inactivation of RhoA, which appeared to be dependent on increased TAX1BP3 mRNA levels. Furthermore, we found that mRNA levels of Daam1a and ARHGEF16 decreased. We speculate that the decrease in β-catenin levels, which also further reduces levels of active RhoA, might contribute to regeneration in the zebrafish. Collectively, our novel results suggest that Wnt5b, Wnt10a, ICAT and TAX1BP3 participate in the activation and inactivation of small G-proteins, such as CDC42, Rac1 and RhoA, during the early stage of optic nerve regeneration in the zebrafish.

Canonical WNT Signaling Regulates the Pituitary Organizer and Pituitary Gland Formation

The pituitary organizer is a domain within the ventral diencephalon that expresses Bmp4, Fgf8, and Fgf10, which induce the formation of the pituitary precursor, Rathke's pouch, from the oral ectoderm. The WNT signaling pathway regulates this pituitary organizer such that loss of Wnt5a leads to an expansion of the pituitary organizer and an enlargement of Rathke's pouch. WNT signaling is classified into canonical signaling, which is mediated by β-CATENIN, and noncanonical signaling, which operates independently of β-CATENIN. WNT5A is typically classified as a noncanonical WNT; however, other WNT family members are expressed in the ventral diencephalon and nuclear localized β-CATENIN is observed in the ventral diencephalon. Therefore, we sought to determine whether canonical WNT signaling is necessary for regulation of pituitary organizer function. Using a conditional loss-of-function approach, we deleted β-catenin within the mouse ventral diencephalon. Mutant embryos have a smaller Rathke's pouch, resulting from a reduced pituitary organizer, especially Fgf8. This result suggests that canonical WNT signaling promotes pituitary organizer function, instead of inhibiting it. To test this hypothesis, we stimulated canonical WNT signaling in the ventral diencephalon using an inducible gain-of-function allele of β-catenin and found that stimulating canonical WNT signaling expands the domain of Fgf8 and results in a dysmorphic Rathke's pouch. These results demonstrate that canonical WNT signaling in the ventral diencephalon is necessary for proper expression of pituitary organizer genes and suggests that a balance of both canonical and noncanonical WNT signaling is necessary to ensure proper formation of Rathke's pouch.

PDZK1 inhibits the development and progression of renal cell carcinoma by suppression of SHP-1 phosphorylation

Renal cell carcinoma (RCC) is one of the most aggressive urologic cancers, however, the mechanism on supporting RCC carcinogenesis is still not clear. By using gene expression profile analysis and functional clustering, PDZ domain-containing 1 (PDZK1) was revealed to be downregulated in human clear cell renal cell carcinoma (ccRCC) samples, which was also verified in several independent public ccRCC data sets. Using PDZK1 overexpression and knockdown models in ccRCC cell lines, we demonstrated that PDZK1 inhibited cell proliferation, cell cycle G1/S phase transition, cell migration and invasion, indicating a tumor-suppressor role in the development and progression of ccRCC. Our study further demonstrated that PDZK1 inhibited cell proliferation and migration of ccRCC via targeting SHP-1. PDZK1 was further identified to suppress cell proliferation by blocking SHP-1 phosphorylation at Tyr536 via inhibition of the association between SHP-1 and PLCβ3, and then retarding Akt phosphorylation and promoting STAT5 phosphorylation in ccRCC cells. Moreover, the inhibitive effects of PDZK1 on SHP-1 phosphorylation and the tumor growth were verified in vivo by xenograft tumor studies. Accordingly, PDZK1 expression was negatively correlated with SHP-1 activation and phosphorylation, advanced pathologic stage, tumor weight and size, and prognosis of ccRCC patients. These findings have provided first lines of evidences that PDZK1 expression is negatively correlated with SHP-1 activation and poor clinical outcomes in ccRCC. PDZK1 was identified as a novel tumor suppressor in ccRCC by negating SHP-1 activity.

Expression profiling and bioinformatic analyses suggest new target genes and pathways for human hair follicle related microRNAs

BACKGROUND

Human hair follicle (HF) cycling is characterised by the tight orchestration and regulation of signalling cascades. Research shows that micro(mi)RNAs are potent regulators of these pathways. However, knowledge of the expression of miRNAs and their target genes and pathways in the human HF is limited. The objective of this study was to improve understanding of the role of miRNAs and their regulatory interactions in the human HF.

METHODS

Expression levels of ten candidate miRNAs with reported functions in hair biology were assessed in HFs from 25 healthy male donors. MiRNA expression levels were correlated with mRNA-expression levels from the same samples. Identified target genes were tested for enrichment in biological pathways and accumulation in protein-protein interaction (PPI) networks.

RESULTS

Expression in the human HF was confirmed for seven of the ten candidate miRNAs, and numerous target genes for miR-24, miR-31, and miR-106a were identified. While the latter include several genes with known functions in hair biology (e.g., ITGB1, SOX9), the majority have not been previously implicated (e.g., PHF1). Target genes were enriched in pathways of interest to hair biology, such as integrin and GnRH signalling, and the respective gene products showed accumulation in PPIs.

CONCLUSIONS

Further investigation of miRNA expression in the human HF, and the identification of novel miRNA target genes and pathways via the systematic integration of miRNA and mRNA expression data, may facilitate the delineation of tissue-specific regulatory interactions, and improve our understanding of both normal hair growth and the pathobiology of hair loss disorders.

MAGI3 negatively regulates Wnt/β-catenin signaling and suppresses malignant phenotypes of glioma cells

Gliomas are the most common primary brain malignancies and are associated with a poor prognosis. Here, we showed that the PDZ domain-containing protein membrane-associated guanylate kinase inverted 3 (MAGI3) was downregulated at the both mRNA and protein levels in human glioma samples. MAGI3 inhibited proliferation, migration, and cell cycle progression of glioma cells in its overexpression and knockdown studies. By using GST pull-down and co-immunoprecipitation assays, we found that MAGI3 bound to β-catenin through its PDZ domains and the PDZ-binding motif of β-catenin. MAGI3 overexpression inhibited β-catenin transcriptional activity via its interaction with β-catenin. Consistently, MAGI3 overexpression in glioma cells C6 suppressed expression of β-catenin target genes including Cyclin D1 and Axin2, whereas MAGI3 knockdown in glioma cells U373 and LN229 enhanced their expression. MAGI3 overexpression decreased growth of C6 subcutaneous tumors in mice, and inhibited expression of β-catenin target genes in xenograft tumors. Furthermore, analysis based on the Gene Expression Omnibus (GEO) glioma dataset showed association of MAGI3 expression with overall survival and tumor grade. Finally, we demonstrated negative correlation between MAGI3 expression and activity of Wnt/β-catenin signaling through GSEA of three public glioma datasets and immunohistochemical staining of clinical glioma samples. Taken together, these results identify MAGI3 as a novel tumor suppressor and provide insight into the pathogenesis of glioma.

Glutaminases

Mammalian glutaminases catalyze the stoichiometric conversion of L-glutamine to L-glutamate and ammonium ions. In brain, glutaminase is considered the prevailing pathway for synthesis of the neurotransmitter pool of glutamate. Besides neurotransmission, the products of glutaminase reaction also fulfill crucial roles in energy and metabolic homeostasis in mammalian brain. In the last years, new functional roles for brain glutaminases are being uncovered by using functional genomic and proteomic approaches. Glutaminases may act as multifunctional proteins able to perform different tasks: the discovery of multiple transcript variants in neurons and glial cells, novel extramitochondrial localizations, and isoform-specific proteininteracting partners strongly support possible moonlighting functions for these proteins. In this chapter, we present a critical account of essential works on brain glutaminase 80 years after its discovery. We will highlight the impact of recent findings and thoughts in the context of the glutamate/glutamine brain homeostasis.

Genome-Wide MicroRNA and Gene Analysis of Mesenchymal Stem Cell Chondrogenesis Identifies an Essential Role and Multiple Targets for miR-140-5p

microRNAs (miRNAs) are abundantly expressed in development where they are critical determinants of cell differentiation and phenotype. Accordingly miRNAs are essential for normal skeletal development and chondrogenesis in particular. However, the question of which miRNAs are specific to the chondrocyte phenotype has not been fully addressed. Using microarray analysis of miRNA expression during mesenchymal stem cell chondrogenic differentiation and detailed examination of the role of essential differentiation factors, such as SOX9, TGF-β, and the cell condensation phase, we characterize the repertoire of specific miRNAs involved in chondrocyte development, highlighting in particular miR-140 and miR-455. Further with the use of mRNA microarray data we integrate miRNA expression and mRNA expression during chondrogenesis to underline the particular importance of miR-140, especially the -5p strand. We provide a detailed identification and validation of direct targets of miR-140-5p in both chondrogenesis and adult chondrocytes with the use of microarray and 3'UTR analysis. This emphasizes the diverse array of targets and pathways regulated by miR-140-5p. We are also able to confirm previous experimentally identified targets but, additionally, identify a novel positive regulation of the Wnt signaling pathway by miR-140-5p. Wnt signaling has a complex role in chondrogenesis and skeletal development and these findings illustrate a previously unidentified role for miR-140-5p in regulation of Wnt signaling in these processes. Together these developments further highlight the role of miRNAs during chondrogenesis to improve our understanding of chondrocyte development and guide cartilage tissue engineering.

Rho-Kinase Inhibition During Early Cardiac Development Causes Arrhythmogenic Right Ventricular Cardiomyopathy in Mice

OBJECTIVE

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is characterized by fibrofatty changes of the right ventricle, ventricular arrhythmias, and sudden death. Though ARVC is currently regarded as a disease of the desmosome, desmosomal gene mutations have been identified only in half of ARVC patients, suggesting the involvement of other associated mechanisms. Rho-kinase signaling is involved in the regulation of intracellular transport and organizes cytoskeletal filaments, which supports desmosomal protein complex at the myocardial cell-cell junctions. Here, we explored whether inhibition of Rho-kinase signaling is involved in the pathogenesis of ARVC.

APPROACH AND RESULTS

Using 2 novel mouse models with SM22α- or αMHC-restricted overexpression of dominant-negative Rho-kinase, we show that mice with Rho-kinase inhibition in the developing heart (SM22α-restricted) spontaneously develop cardiac dilatation and dysfunction, myocardial fibrofatty changes, and ventricular arrhythmias, resulting in premature sudden death, phenotypes fulfilling the criteria of ARVC in humans. Rho-kinase inhibition in the developing heart results in the development of ARVC phenotypes in dominant-negative Rho-kinase mice through 3 mechanisms: (1) reduction of cardiac cell proliferation and ventricular wall thickness, (2) stimulation of the expression of the proadipogenic noncanonical Wnt ligand, Wnt5b, and the major adipogenic transcription factor, PPARγ (peroxisome proliferator activated receptor-γ), and inhibition of Wnt/β-catenin signaling, and (3) development of desmosomal abnormalities. These mechanisms lead to the development of cardiac dilatation and dysfunction, myocardial fibrofatty changes, and ventricular arrhythmias, ultimately resulting in sudden premature death in this ARVC mouse model.

CONCLUSIONS

This study demonstrates a novel crucial role of Rho-kinase inhibition during cardiac development in the pathogenesis of ARVC in mice.

Adaptable hydrogel networks with reversible linkages for tissue engineering

Adaptable hydrogels have recently emerged as a promising platform for three-dimensional (3D) cell encapsulation and culture. In conventional, covalently crosslinked hydrogels, degradation is typically required to allow complex cellular functions to occur, leading to bulk material degradation. In contrast, adaptable hydrogels are formed by reversible crosslinks. Through breaking and re-formation of the reversible linkages, adaptable hydrogels can be locally modified to permit complex cellular functions while maintaining their long-term integrity. In addition, these adaptable materials can have biomimetic viscoelastic properties that make them well suited for several biotechnology and medical applications. In this review, an overview of adaptable-hydrogel design considerations and linkage selections is presented, with a focus on various cell-compatible crosslinking mechanisms that can be exploited to form adaptable hydrogels for tissue engineering.

PDZ Domain Recognition: Insight from Human Tax-Interacting Protein 1 (TIP-1) Interaction with Target Proteins

Cellular signaling is primarily directed via protein-protein interactions. PDZ (PSD-95/Discs large/ZO-1 homologous) domains are well known protein-protein interaction modules involved in various key signaling pathways. Human Tax-interacting protein 1 (TIP-1), also known as glutaminase interaction protein (GIP), is a Class I PDZ domain protein that recognizes the consensus binding motif X-S/T-X-V/I/L-COOH of the C-terminus of its target proteins. We recently reported that TIP-1 not only interacts via the C-terminus of its target partner proteins but also recognizes an internal motif defined by the consensus sequence S/T-X-V/L-D in the target protein. Identification of new target partners containing either a C-terminal or internal recognition motif has rapidly expanded the TIP-1 protein interaction network. TIP-1 being composed solely of a single PDZ domain is unique among PDZ containing proteins. Since it is involved in many important signaling pathways, it is a possible target for drug design. In this mini review, we have discussed human TIP-1, its structure, mechanism of function, its interactions with target proteins containing different recognition motifs, and its involvement in human diseases. Understanding the molecular mechanisms of TIP-1 interactions with distinct target partners and their role in human diseases will be useful for designing novel therapeutics.

Targeting the β-catenin nuclear transport pathway in cancer

The nuclear localization of specific proteins is critical for cellular processes such as cell division, and in recent years perturbation of the nuclear transport cycle of key proteins has been linked to cancer. In particular, specific gene mutations can alter nuclear transport of tumor suppressing and oncogenic proteins, leading to cell transformation or cancer progression. This review will focus on one such factor, β-catenin, a key mediator of the canonical wnt signaling pathway. In response to a wnt stimulus or specific gene mutations, β-catenin is stabilized and translocates to the nucleus where it binds TCF/LEF-1 transcription factors to transactivate genes that drive tumor formation. Moreover, the nuclear import and accumulation of β-catenin correlates with clinical tumor grade. Recent evidence suggests that the primary nuclear transport route of β-catenin is independent of the classical Ran/importin import machinery, and that β-catenin directly contacts the nuclear pore complex to self-regulate its own entry into the nucleus. Here we propose that the β-catenin nuclear import pathway may provide an opportunity for identification of specific drug targets and inhibition of β-catenin nuclear function, much like the current screening of drugs that block binding of β-catenin to LEF-1/TCFs. Here we will discuss the diverse mechanisms regulating nuclear localization of β-catenin and their potential as targets for anticancer agent development.

Family-wide investigation of PDZ domain-mediated protein-protein interactions implicates β-catenin in maintaining the integrity of tight junctions

β-catenin is a multifunctional protein that plays a critical role in cell-cell contacts and signal transduction. β-catenin has previously been shown to interact with PDZ-domain-containing proteins through its C terminus. Using protein microarrays comprising 206 mouse PDZ domains, we identified 26 PDZ-domain-mediated interactions with β-catenin and confirmed them biochemically and in cellular lysates. Many of the previously unreported interactions involved proteins with annotated roles in tight junctions. We found that four tight-junction-associated PDZ proteins-Scrib, Magi-1, Pard3, and ZO-3-colocalize with β-catenin at the plasma membrane. Disrupting these interactions by RNA interference, overexpression of PDZ domains, or overexpression of the β-catenin C terminus altered localization of the full-length proteins, weakened tight junctions, and decreased cellular adhesion. These results suggest that β-catenin serves as a scaffold to establish the location and function of tight-junction-associated proteins.

High-throughput identification of putative receptors for cancer-binding peptides using biopanning and microarray analysis

Phage-display peptide biopanning has been successfully used to identify cancer-targeting peptides in multiple models. For cancer-binding peptides, identification of the peptide receptor is necessary to demonstrate the mechanism of action and to further optimize specificity and target binding. The process of receptor identification can be slow and some peptides may turn out to bind ubiquitous proteins not suitable for further drug development. In this report, we describe a high-throughput method for screening a large number of peptides in parallel to identify peptide receptors, which we have termed "reverse biopanning." Peptides can then be selected for further development based on their receptor. To demonstrate this method, we screened a library of 39 peptides previously identified in our laboratory to bind specifically to cancers after irradiation. The reverse biopanning process identified 2 peptides, RKFLMTTRYSRV and KTAKKNVFFCSV, as candidate ligands for the protein tax interacting protein 1 (TIP-1), a protein previously identified in our laboratory to be expressed in tumors and upregulated after exposure to ionizing radiation. We used computational modeling as the initial method for rapid validation of peptide-TIP-1 binding. Pseudo-binding energies were calculated to be -360.645 kcal mol(-1), -487.239 kcal mol(-1), and -595.328 kcal mol(-1) for HVGGSSV, TTRYSRV, and NVFFCSV respectively, suggesting that the peptides would have at least similar, if not stronger, binding to TIP-1 compared to the known TIP-1 binding peptide HVGGSSV. We validated peptide binding in vitro using electrophoretic mobility shift assay, which showed strong binding of RKFLMTTRYSRV and the truncated form TTRYSRV. This method allows for the identification of many peptide receptors and subsequent selection of peptides for further drug development based on the peptide receptor.

Tax-interacting protein 1 coordinates the spatiotemporal activation of Rho GTPases and regulates the infiltrative growth of human glioblastoma

PDZ domains represent one group of the major structural units that mediate protein interactions in intercellular contact, signal transduction and assembly of biological machineries. TIP-1 protein is composed of a single PDZ domain that distinguishes TIP-1 from other PDZ domain proteins that more often contain multiple protein domains and function as scaffolds for protein complex assembly. However, the biological functions of TIP-1, especially in cell transformation and tumor progression, are still controversial as observed in a variety of cell types. In this study, we have identified ARHGEF7, a guanine nucleotide exchange factor (GEF) for Rho GTPases, as one novel TIP-1 interacting protein in human glioblastoma cells. We found that the presence of TIP-1 protein is essential to the intracellular redistribution of ARHGEF7 and rhotekin, one Rho effector, and the spatiotemporally coordinated activation of Rho GTPases (RhoA, Cdc42 and Rac1) in migrating glioblastoma cells. TIP-1 knockdown resulted in both aberrant localization of ARHGEF7 and rhotekin, as well as abnormal activation of Rho GTPases that was accompanied with impaired motility of glioblastoma cells. Furthermore, TIP-1 knockdown suppressed tumor cell dispersal in orthotopic glioblastoma murine models. We also observed high levels of TIP-1 expression in human glioblastoma specimens, and the elevated TIP-1 levels are associated with advanced staging and poor prognosis in glioma patients. Although more studies are needed to further dissect the mechanism(s) by which TIP-1 modulates the intracellular redistribution and activation of Rho GTPases, this study suggests that TIP-1 holds potential as both a prognostic biomarker and a therapeutic target of malignant gliomas.

New partner proteins containing novel internal recognition motif for human glutaminase interacting protein (hGIP)

Regulation of gene expression in cells is mediated by protein-protein, DNA-protein and receptor-ligand interactions. PDZ (PSD-95/Discs-large/ZO-1) domains are protein-protein interaction modules. PDZ-containing proteins function in the organization of multi-protein complexes controlling spatial and temporal fidelity of intracellular signaling pathways. In general, PDZ proteins possess multiple domains facilitating distinct interactions. The human glutaminase interacting protein (hGIP) is an unusual PDZ protein comprising entirely of a single PDZ domain and plays pivotal roles in many cellular processes through its interaction with the C-terminus of partner proteins. Here, we report the identification by yeast two-hybrid screening of two new hGIP-interacting partners, DTX1 and STAU1. Both proteins lack the typical C-terminal PDZ recognition motif but contain a novel internal hGIP recognition motif recently identified in a phage display library screen. Fluorescence resonance energy transfer and confocal microscopy analysis confirmed the in vivo association of hGIP with DTX1 and STAU1 in mammalian cells validating the previous discovery of S/T-X-V/L-D as a consensus internal motif for hGIP recognition. Similar to hGIP, DTX1 and STAU1 have been implicated in neuronal function. Identification of these new interacting partners furthers our understanding of GIP-regulated signaling cascades and these interactions may represent potential new drug targets in humans.

Expression of Tax-interacting protein 1 (TIP-1) facilitates angiogenesis and tumor formation of human glioblastoma cells in nude mice

Glioblastoma is the most common and fatal type of primary brain tumors featured with hyperplastic blood vessels. Here, we performed meta-analyses of published data and established a correlation between high TIP-1 expression levels and the poor prognosis of glioblastoma patients. Next, we explored the biological relevance of TIP-1 expression in the pathogenesis of glioblastoma. By using orthotopic and heterotopic mouse models of human glioblastomas, this study has characterized TIP-1 as one contributing factor to the tumor-driven angiogenesis. In vitro and in vivo functional assays, along with biochemical analyses with microarrays and antibody arrays, have demonstrated that TIP-1 utilizes multiple pathways including modulating fibronectin gene expression and uPA protein secretion, to establish or maintain a pro-angiogenic microenvironment within human glioblastoma. In conclusion, this work supports one hypothesis that TIP-1 represents a novel prognostic biomarker and a therapeutic target of human glioblastoma.

Expression of TIP-1 confers radioresistance of malignant glioma cells

BACKGROUND

Malignant gliomas represent one group of tumors that poorly respond to ionizing radiation (IR) alone or combined with chemotherapeutic agents because of the intrinsic or acquired resistance. In this study, TIP-1 was identified as one novel protein that confers resistance of glioma cells to IR.

METHODOLOGY/PRINCIPAL FINDINGS

Meta-analysis indicated that high TIP-1 expression levels correlate with the poor prognosis of human malignant gliomas after radiotherapy. Studies with established human glioma cell lines demonstrated that TIP-1 depletion with specific shRNAs sensitized the cells to IR, whereas an ectopic expression of TIP-1 protected the glioma cells from the IR-induced DNA damage and cell death. Biochemical studies indicated that TIP-1 protein promoted p53 ubiquitination and resulted in a reduced p53 protein level. Furthermore, p53 and its ubiquitination are required for the TIP-1 regulated cellular response to IR. A yeast two-hybrid screening identified that TIP-1, through its single PDZ domain, binds to the carboxyl terminus of LZAP that has been studied as one tumor suppressor functioning through ARF binding and p53 activation. It was revealed that the presence of TIP-1 enhances the protein association between LZAP and ARF and modulates the functionality of ARF/HDM2 toward multi-ubiquitination of p53, while depleting TIP-1 rescued p53 from polyubiquitination and degradation in the irradiated glioma cells. Studies with a mouse xenograft model indicated that depleting TIP-1 within D54 cells improved the tumor growth control with IR.

CONCLUSIONS/SIGNIFICANCE

This study provided the first evidence showing that TIP-1 modulates p53 protein stability and is involved in the radioresistance of malignant gliomas, suggesting that antagonizing TIP-1 might be one novel approach to sensitize malignant gliomas to radiotherapy.

Specificity and promiscuity in human glutaminase interacting protein recognition: insight from the binding of the internal and C-terminal motif

A large number of cellular processes are mediated by protein-protein interactions, often specified by particular protein binding modules. PDZ domains make up an important class of protein-protein interaction modules that typically bind to the C-terminus of target proteins. These domains act as a scaffold where signaling molecules are linked to a multiprotein complex. Human glutaminase interacting protein (GIP), also known as tax interacting protein 1, is unique among PDZ domain-containing proteins because it is composed almost exclusively of a single PDZ domain rather than one of many domains as part of a larger protein. GIP plays pivotal roles in cellular signaling, protein scaffolding, and cancer pathways via its interaction with the C-terminus of a growing list of partner proteins. We have identified novel internal motifs that are recognized by GIP through combinatorial phage library screening. Leu and Asp residues in the consensus sequence were identified to be critical for binding to GIP through site-directed mutagenesis studies. Structure-based models of GIP bound to two different surrogate peptides determined from nuclear magnetic resonance constraints revealed that the binding pocket is flexible enough to accommodate either the smaller carboxylate (COO(-)) group of a C-terminal recognition motif or the bulkier aspartate side chain (CH(2)COO(-)) of an internal motif. The noncanonical ILGF loop in GIP moves in for the C-terminal motif but moves out for the internal recognition motifs, allowing binding to different partner proteins. One of the peptides colocalizes with GIP within human glioma cells, indicating that GIP might be a potential target for anticancer therapeutics.

The PDZ protein TIP-1 facilitates cell migration and pulmonary metastasis of human invasive breast cancer cells in athymic mice

Tax-interacting protein 1 (TIP-1, also known as Tax1bp3) inhibited proliferation of colon cancer cells through antagonizing the transcriptional activity of beta-catenin. However, in this study, elevated TIP-1 expression levels were detected in human invasive breast cancers. Studies with two human invasive breast cancer cell lines indicated that RNAi-mediated TIP-1 knockdown suppressed the cell adhesion, proliferation, migration and invasion in vitro, and inhibited tumor growth in mammary fat pads and pulmonary metastasis in athymic mice. Biochemical studies showed that TIP-1 knockdown had moderate and differential effects on the beta-catenin-regulated gene expression, but remarkably down regulated the genes for cell adhesion and motility in breast cancer cells. The decreased expression of integrins and paxillin was accompanied with reduced cell adhesion and focal adhesion formation on fibronectin-coated surface. In conclusion, this study revealed a novel oncogenic function of TIP-1 suggesting that TIP-1 holds potential as a prognostic biomarker and a therapeutic target in the treatment of human invasive breast cancers.

Aberrant nuclear localization of EBP50 promotes colorectal carcinogenesis in xenotransplanted mice by modulating TCF-1 and β-catenin interactions

Dysregulation of canonical Wnt signaling is thought to play a role in colon carcinogenesis. β-Catenin, a key mediator of the pathway, is stabilized upon Wnt activation and accumulates in the nucleus, where it can interact with the transcription factor T cell factor (TCF) to transactivate gene expression. Normal colonic epithelia express a truncated TCF-1 form, called dnTCF-1, that lacks the critical β-catenin-binding domain and behaves as a transcriptional suppressor. How the cell maintains a balance between the two forms of TCF-1 is unclear. Here, we show that ERM-binding phosphoprotein 50 (EBP50) modulates the interaction between β-catenin and TCF-1. We observed EBP50 localization to the nucleus of human colorectal carcinoma cell lines at low cell culture densities and human primary colorectal tumors that manifested a poor clinical outcome. In contrast, EBP50 was primarily membranous in confluent cell lines. Aberrantly located EBP50 stabilized conventional β-catenin/TCF-1 complexes and connected β-catenin to dnTCF-1 to form a ternary molecular complex that enhanced Wnt/β-catenin signaling events, including the transcription of downstream oncogenes such as c-Myc and cyclin D1. Genome-wide analysis of the EBP50 occupancy pattern revealed consensus binding motifs bearing similarity to Wnt-responsive element. Conventional chromatin immunoprecipitation assays confirmed that EBP50 bound to genomic regions highly enriched with TCF/LEF binding motifs. Knockdown of EBP50 in human colorectal carcinoma cell lines compromised cell cycle progression, anchorage-independent growth, and tumorigenesis in nude mice. We therefore suggest that nuclear EBP50 facilitates colon tumorigenesis by modulating the interaction between β-catenin and TCF-1.

Wnt signaling from membrane to nucleus: β-catenin caught in a loop

β-catenin is the central nuclear effector of the Wnt signaling pathway, and regulates other cellular processes including cell adhesion. Wnt stimulation of cells culminates in the nuclear translocation of β-catenin and transcriptional activation of target genes that function during both normal and malignant development. Constitutive activation of the Wnt pathway leads to inappropriate nuclear accumulation of β-catenin and gene transactivation, an important step in cancer progression. This has generated interest in the mechanisms regulating β-catenin nuclear accumulation and retention. Here we discuss recent advances in understanding feedback loops that trap β-catenin in the nucleus and provide potential insights into Wnt signaling and the development of anti-cancer drugs.

Characteristic Markers of the WNT Signaling Pathways Are Differentially Expressed in Osteoarthritic Cartilage

OBJECTIVE

It is well known that expression of markers for WNT signaling is dysregulated in osteoarthritic (OA) bone. However, it is still not fully known if the expression of these markers also is affected in OA cartilage. The aim of this study was therefore to examine this issue.

METHODS

Human cartilage biopsies from OA and control donors were subjected to genome-wide oligonucleotide microarrays. Genes involved in WNT signaling were selected using the BioRetis database, KEGG pathway analysis was searched using DAVID software tools, and cluster analysis was performed using Genesis software. Results from the microarray analysis were verified using quantitative real-time PCR and immunohistochemistry. In order to study the impact of cytokines for the dysregulated WNT signaling, OA and control chondrocytes were stimulated with interleukin-1 and analyzed with real-time PCR for their expression of WNT-related genes.

RESULTS

Several WNT markers displayed a significantly altered expression in OA compared to normal cartilage. Interestingly, inhibitors of the canonical and planar cell polarity WNT signaling pathways displayed significantly increased expression in OA cartilage, while the Ca(2+)/WNT signaling pathway was activated. Both real-time PCR and immunohistochemistry verified the microarray results. Real-time PCR analysis demonstrated that interleukin-1 upregulated expression of important WNT markers.

CONCLUSIONS

WNT signaling is significantly affected in OA cartilage. The result suggests that both the canonical and planar cell polarity WNT signaling pathways were partly inhibited while the Ca(2+)/WNT pathway was activated in OA cartilage.

Functional implications of structural predictions for alternative splice proteins expressed in Her2/neu-induced breast cancers

Alternative splicing allows a single gene to generate multiple mRNA transcripts, which can be translated into functionally diverse proteins. However, experimentally determined structures of protein splice isoforms are rare, and homology modeling methods are poor at predicting atomic-level structural differences because of high sequence identity. Here we exploit the state-of-the-art structure prediction method I-TASSER to analyze the structural and functional consequences of alternative splicing of proteins differentially expressed in a breast cancer model. We first successfully benchmarked the I-TASSER pipeline for structure modeling of all seven pairs of protein splice isoforms, which are known to have experimentally solved structures. We then modeled three cancer-related variant pairs reported to have opposite functions. In each pair, we observed structural differences in regions where the presence or absence of a motif can directly influence the distinctive functions of the variants. Finally, we applied the method to five splice variants overexpressed in mouse Her2/neu mammary tumor: anxa6, calu, cdc42, ptbp1, and tax1bp3. Despite >75% sequence identity between the variants, structural differences were observed in biologically important regions of these protein pairs. These results demonstrate the feasibility of integrating proteomic analysis with structure-based conformational predictions of differentially expressed alternative splice variants in cancers and other conditions.

Emerging theme: cellular PDZ proteins as common targets of pathogenic viruses

More than a decade ago, three viral oncoproteins, adenovirus type 9 E4-ORF1, human T-lymphotropic virus type 1 Tax, and high-risk human papillomavirus E6, were found to encode a related carboxyl-terminal PDZ domain-binding motif (PBM) that mediates interactions with a select group of cellular PDZ proteins. Recent studies have shown that many other viruses also encode PBM-containing proteins that bind to cellular PDZ proteins. Interestingly, these recently recognized viruses include not only some with oncogenic potential (hepatitis B virus, rhesus papillomavirus, cottontail rabbit papillomavirus) but also many without this potential (influenza virus, Dengue virus, tick-borne encephalitis virus, rabies virus, severe acute respiratory syndrome coronavirus, human immunodeficiency virus). Examination of the cellular PDZ proteins that are targets of viral PBMs reveals that the viral proteins often interact with the same or similar types of PDZ proteins, most notably Dlg1 and other members of the membrane-associated guanylate kinase protein family, as well as Scribble. In addition, cellular PDZ protein targets of viral PBMs commonly control tight junction formation, cell polarity establishment, and apoptosis. These findings reveal a new theme in virology wherein many different virus families encode proteins that bind and perturb the function of cellular PDZ proteins. The inhibition or perturbation of the function of cellular PDZ proteins appears to be a widely used strategy for viruses to enhance their replication, disseminate in the host, and transmit to new hosts.

Epithelial cell polarity: a major gatekeeper against cancer?

The correct establishment and maintenance of cell polarity are crucial for normal cell physiology and tissue homeostasis. Conversely, loss of cell polarity, tissue disorganisation and excessive cell growth are hallmarks of cancer. In this review, we focus on identifying the stages of tumoural development that are affected by the loss or deregulation of epithelial cell polarity. Asymmetric division has recently emerged as a major regulatory mechanism that controls stem cell numbers and differentiation. Links between cell polarity and asymmetric cell division in the context of cancer will be examined. Apical–basal polarity and cell–cell adhesion are tightly interconnected. Hence, how loss of cell polarity in epithelial cells may promote epithelial mesenchymal transition and metastasis will also be discussed. Altogether, we present the argument that loss of epithelial cell polarity may have an important role in both the initiation of tumourigenesis and in later stages of tumour development, favouring the progression of tumours from benign to malignancy.

Promiscuous binding at the crossroads of numerous cancer pathways: insight from the binding of glutaminase interacting protein with glutaminase L

The glutaminase interacting protein (GIP) is composed of a single PDZ domain that interacts with a growing list of partner proteins, including glutaminase L, that are involved in a number of cell signaling and cancer pathways. Therefore, GIP makes a good target for structure-based drug design. Here, we report the solution structures of both free GIP and GIP bound to the C-terminal peptide analogue of glutaminase L. This is the first reported nuclear magnetic resonance structure of GIP in a complex with one of its binding partners. Our analysis of both free GIP and GIP in a complex with the glutaminase L peptide provides important insights into how a promiscuous binding domain can have affinity for multiple binding partners. Through a detailed chemical shift perturbation analysis and backbone dynamics studies, we demonstrate here that the binding of the glutaminase L peptide to GIP is an allosteric event. Taken together, the insights reported here lay the groundwork for the future development of a specific inhibitor for GIP.

Dual roles of Oct4 in the maintenance of mouse P19 embryonal carcinoma cells: as negative regulator of Wnt/β-catenin signaling and competence provider for Brachyury induction

Transcription factor Oct4 is expressed in pluripotent cell lineages during mouse development, namely, in inner cell mass (ICM), primitive ectoderm, and primordial germ cells. Functional studies have revealed that Oct4 is essential for the maintenance of pluripotency in inner cell mass and for the survival of primordial germ cells. However, the function of Oct4 in the primitive ectoderm has not been fully explored. In this study, we investigated the role of Oct4 in mouse P19 embryonal carcinoma (EC) cells, which exhibit molecular and developmental properties similar to the primitive ectoderm, as an in vitro model. Knockdown of Oct4 in P19 EC cells upregulated several early mesoderm-specific genes, such as Wnt3, Sp5, and Fgf8, by activating Wnt/β-catenin signaling. Overexpression of Oct4 was sufficient to suppress Wnt/β-catenin signaling through its action as a transcriptional activator. However, Brachyury, a key regulator of early mesoderm development and a known direct target of Wnt/β-catenin signaling, was unable to be upregulated in the absence of Oct4, even with additional activation of Wnt/β-catenin signaling. Microarray analysis revealed that Oct4 positively regulated the expression of Tdgf1, a critical component of Nodal signaling, which was required for the upregulation of Brachyury in response to Wnt/β-catenin signaling in P19 EC cells. We propose a model that Oct4 maintains pluripotency of P19 EC cells through 2 counteracting actions: one is to suppress mesoderm-inducing Wnt/β-catenin signaling, and the other is to provide competence to Brachyury gene to respond to Wnt/β-catenin signaling.

The HPV16 E6 binding protein Tip-1 interacts with ARHGEF16, which activates Cdc42

Background:

Guanidine exchange factor (GEF)-catalysed activation of Rho proteins such as Cdc42 has been shown to have a crucial role in cellular transformation, malignant progression and invasion. We have previously shown that the HPV16 E6 oncoprotein binds to the PDZ domain protein Tax-interacting-protein 1 (Tip-1) and we now report identification and functional analysis of a novel Tip-1 binding GEF.

Methods:

Yeast two-hybrid, in vitro pull-down, site-directed mutagenesis, semiquantitative PCR, co-immunoprecipitation and western blotting were used to identify/confirm novel Tip-1 binding partners and analyse cellular expression levels. In vitro kinetic analyses of recombinant proteins, siRNA gene silencing and in cell assays were used to measure Rho protein activation.

Results:

Tax-interacting-protein 1 was shown to interact with ARHGEF16 by its carboxyl PDZ binding motif. Levels of ARHGEF16 were increased in transformed and immortalised cells expressing ectopic HPV16 E6 and Cdc42 was co-immunoprecipitated by ARHGEF16 in the presence of high-risk HPV E6. In vitro kinetic analysis confirmed that recombinant ARHGEF16 activates Cdc42 and this was increased by the addition of recombinant Tip-1 and E6. Cells expressing HPV16 E6 had higher levels of Cdc42 activation, which was decreased by siRNA silencing of either Tip-1 or ARHGEF16.

Conclusion:

These data suggest that HPV16 E6, Tip-1 and ARHGEF16 may cooperate to activate Cdc42 and support a potential link between the expression of HPV16 E6 and Cdc42 activation.

TIP-1 translocation onto the cell plasma membrane is a molecular biomarker of tumor response to ionizing radiation

Background

Tumor response to treatment has been generally assessed with anatomic and functional imaging. Recent development of in vivo molecular and cellular imaging showed promise in time-efficient assessment of the therapeutic efficacy of a prescribed regimen. Currently, the in vivo molecular imaging is limited with shortage of biomarkers and probes with sound biological relevance. We have previously shown in tumor-bearing mice that a hexapeptide (HVGGSSV) demonstrated potentials as a molecular imaging probe to distinguish the tumors responding to ionizing radiation (IR) and/or tyrosine kinase inhibitor treatment from those of non-responding tumors.

Methodology/Principal Findings

In this study we have studied biological basis of the HVGGSSV peptide binding within the irradiated tumors by use of tumor-bearing mice and cultured cancer cells. The results indicated that Tax interacting protein 1 (TIP-1, also known as Tax1BP3) is a molecular target that enables the selective binding of the HVGGSSV peptide within irradiated xenograft tumors. Optical imaging and immunohistochemical staining indicated that a TIP-1 specific antibody demonstrated similar biodistribution as the peptide in tumor-bearing mice. The TIP-1 antibody blocked the peptide from binding within irradiated tumors. Studies on both of human and mouse lung cancer cells showed that the intracellular TIP-1 relocated to the plasma membrane surface within the first few hours after exposure to IR and before the onset of treatment associated apoptosis and cell death. TIP-1 relocation onto the cell surface is associated with the reduced proliferation and the enhanced susceptibility to the subsequent IR treatment.

Conclusions/Significance

This study by use of tumor-bearing mice and cultured cancer cells suggested that imaging of the radiation-inducible TIP-1 translocation onto the cancer cell surface may predict the tumor responsiveness to radiation in a time-efficient manner and thus tailor radiotherapy of cancer.

Jaagsiekte sheep retrovirus transformation in Madin-Darby canine kidney epithelial cell three-dimensional culture

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of a contagious lung cancer in sheep that shares similarities with human bronchioloalveolar carcinoma (BAC). JSRV is unique because the envelope gene (env) is the oncogene, as it can transform cells in culture and induce tumors in animals. The phosphatidylinositol 3-kinase (PI3K)-Akt-mTOR and H/N-Ras-MEK-mitogen-activated protein kinase (MAPK) pathways have been shown to be critical for Env transformation. However, the question still remains of how disruption of these pathways relates to tumor formation. To address this, JSRV Env transformation was studied in the context of epithelial structure, using the polarized Madin-Darby canine kidney (MDCK) epithelial cell three-dimensional (3-D) culture system. The results indicated that JSRV Env-transformed MDCK cells were larger and had full or multiple lumens, in contrast to the single lumens observed in controls. The altered phenotype was largely mediated by an increase in proliferation, in addition to overcoming the proliferative suppression signal. JSRV Env was not found to disrupt polarity or tight junctions or to inhibit lumen apoptosis. The PI3K-Akt-mTOR pathway was important for Env transformation in MDCK cells, although the mechanisms of action differed in 3-D and monolayer cultures. PI3K-dependent signaling to mTOR occurred in monolayers, while PI3K-independent signaling to mTOR occurred in 3-D culture. In contrast, the H/N-Ras-MEK-MAPK pathway was found to be inhibitory to transformation in both normal and transformed MDCK cells in 3-D culture. However, in monolayer culture, inhibition of MEK reverted the transformed phenotype, suggesting a different mechanism(s) of action in monolayer versus 3-D culture.

Identification and characterization of endonuclein binding proteins: evidence of modulatory effects on signal transduction and chaperone activity

Background

We have previously identified endonuclein as a cell cycle regulated WD-repeat protein that is up-regulated in adenocarcinoma of the pancreas. Now, we aim to investigate its biomedical functions.

Results

Using the cDNA encoding human endonuclein, we have expressed and purified the recombinant protein from Escherichia coli using metal affinity chromatography. The recombinant protein was immobilized to a column and by affinity chromatography several interacting proteins were purified from several litres of placenta tissue extract. After chromatography the eluted proteins were further separated by two-dimensional gel electrophoresis and identified by tandem mass spectrometry. The interacting proteins were identified as; Tax interaction protein 1 (TIP-1), Aα fibrinogen transcription factor (P16/SSBP1), immunoglobulin heavy chain binding protein (BiP), human ER-associated DNAJ (HEDJ/DNAJB11), endonuclein interaction protein 8 (EIP-8), and pregnancy specific β-1 glycoproteins (PSGs). Surface plasmon resonance analysis and confocal fluorescence microscopy were used to further characterize the interactions.

Conclusions

Our results demonstrate that endonuclein interacts with several proteins indicating a broad function including signal transduction and chaperone activity.

Development of a high-throughput method for the systematic identification of human proteins nuclear translocation potential

Background

Important clues to the function of novel and uncharacterized proteins can be obtained by identifying their ability to translocate in the nucleus. In addition, a comprehensive definition of the nuclear proteome undoubtedly represents a key step toward a better understanding of the biology of this organelle. Although several high-throughput experimental methods have been developed to explore the sub-cellular localization of proteins, these methods tend to focus on the predominant localizations of gene products and may fail to provide a complete catalog of proteins that are able to transiently locate into the nucleus.

Results

We have developed a method for examining the nuclear localization potential of human gene products at the proteome scale by adapting a mammalian two-hybrid system we have previously developed. Our system is composed of three constructs co-transfected into a mammalian cell line. First, it contains a PCR construct encoding a fusion protein composed of a tested protein, the PDZ-protein TIP-1, and the transactivation domain of TNNC2 (referred to as ACT construct). Second, our system contains a PCR construct encoding a fusion protein composed of the DNA binding domain of GAL4 and the PDZ binding domain of rhotekin (referred to as the BIND construct). Third, a GAL4-responsive luciferase reporter is used to detect the reconstitution of a transcriptionally active BIND-ACT complex through the interaction of TIP-1 and rhotekin, which indicates the ability of the tested protein to translocate into the nucleus. We validated our method in a small-scale feasibility study by comparing it to green fluorescent protein (GFP) fusion-based sub-cellular localization assays, sequence-based computational prediction of protein sub-cellular localization, and current sub-cellular localization data available from the literature for 22 gene products.

Conclusion

Our reporter-based system can rapidly screen gene products for their ability to be translocated to the nucleus. Large-scale applications of the system presented herein should provide invaluable information for a more complete biological atlas.

The terminal region of beta-catenin promotes stability by shielding the Armadillo repeats from the axin-scaffold destruction complex

Post-translational stabilization of beta-catenin is a key step in Wnt signaling, but the features of beta-catenin required for stabilization are incompletely understood. We show that forms of beta-catenin lacking the unstructured C-terminal domain (CTD) show faster turnover than full-length or minimally truncated beta-catenins. Mutants that exhibit faster turnover show enhanced association with axin in co-transfected cells, and excess CTD polypeptide can compete binding of the beta-catenin armadillo (arm) repeat domain to axin in vitro, indicating that the CTD may restrict beta-catenin binding to the axin-scaffold complex. Fluorescent resonance energy transmission (FRET) analysis of cyan fluorescent protein (CFP)-arm-CTD-yellow fluorescent protein beta-catenin reveals that the CTD of beta-catenin can become spatially close to the N-terminal arm repeat region of beta-catenin. FRET activity is strongly diminished by the coexpression of beta-catenin binding partners, indicating that an unliganded groove is absolutely required for an orientation that allows FRET. Amino acids 733-759 are critical for beta-catenin FRET activity and stability. These data indicate that an N-terminal orientation of the CTD is required for beta-catenin stabilization and suggest a model where the CTD extends toward the N-terminal arm repeats, shielding these repeats from the beta-catenin destruction complex.

Nanoscale elongating control of the self-assembled protein filament with the cysteine-introduced building blocks

Self-assembly of artificially designed proteins is extremely desirable for nanomaterials. Here we show a novel strategy for the creation of self-assembling proteins, named "Nanolego." Nanolego consists of "structural elements" of a structurally stable symmetrical homo-oligomeric protein and "binding elements," which are multiple heterointeraction proteins with relatively weak affinity. We have established two key technologies for Nanolego, a stabilization method and a method for terminating the self-assembly process. The stabilization method is mediated by disulfide bonds between Cysteine-residues incorporated into the binding elements, and the termination method uses "capping Nanolegos," in which some of the binding elements in the Nanolego are absent for the self-assembled ends. With these technologies, we successfully constructed timing-controlled and size-regulated filament-shape complexes via Nanolego self-assembly. The Nanolego concept and these technologies should pave the way for regulated nanoarchitecture using designed proteins.

Molecular mechanism of inward rectifier potassium channel 2.3 regulation by tax-interacting protein-1

Inwardly rectifying potassium channel 2.3 (Kir2.3) is specifically targeted on the basolateral membranes of epithelial and neuronal cells, and it thus plays an important role in maintaining potassium homeostasis. Tax-interacting protein-1 (TIP-1), an atypical PDZ-domain-containing protein, binds to Kir2.3 with a high affinity, causing the intracellular accumulation of Kir2.3 in cultured epithelial cells. However, the molecular basis of the TIP-1/Kir2.3 interaction is still poorly understood. Here, we present the crystal structure of TIP-1 in complex with the C-terminal Kir2.3-peptide (residues 436-445) to reveal the molecular details of the interaction between them. Moreover, isothermal titration calorimetry experiments show that the C-terminal Kir2.3-peptide binds much more strongly to TIP-1 than to mammalian Lin-7, indicating that TIP-1 can compete with mammalian Lin-7 to uncouple Kir2.3 from its basolateral membrane anchoring complex. We further show that the phosphorylation/dephosphorylation of Ser443 within the C-terminal Kir2.3 PDZ-binding motif RRESAI dynamically regulates the Kir2.3/TIP-1 association in heterologous HEK293T cells. These data suggest that TIP-1 may act as an important regulator for the endocytic pathway of Kir2.3.

Structural basis of beta-catenin recognition by Tax-interacting protein-1

Tax-interacting protein-1 (TIP-1) is an unusual signaling protein, containing a single PDZ domain. TIP-1 is able to bind beta-catenin with high affinity and thus inhibit its transcriptional activity. The high-resolution crystal structure of TIP-1 in complex with the C-terminal peptide of beta-catenin provides molecular details for the recognition of beta-catenin by TIP-1. Moreover, structural comparison of peptide-free and peptide-bound TIP-1 reveals that significant conformational changes are required in the betaB-betaC loop region of TIP-1 to avoid clashes with the incoming C-terminal beta-catenin peptide. Such conformational changes have not been observed in other structures of PDZ domains. In addition to the canonical peptide-binding pocket of the PDZ domain, TIP-1 can form a binding cavity to anchor more amino acids through a conserved hydrophobic residue pair (Trp776 of beta-catenin and Pro45 of TIP-1). Structural and biochemical data indicate that the canonical binding pocket together with the hydrophobic residue pair are presumably the major cause of the significantly higher affinity of the beta-catenin C-terminal to TIP-1 than to other PDZ domains, providing a unique binding specificity. Our results reveal the molecular mechanism of TIP-1 as an antagonist in PDZ domain signaling.

Cell polarity and cancer--cell and tissue polarity as a non-canonical tumor suppressor

Correct establishment and maintenance of cell polarity is required for the development and homeostasis of all metazoans. Cell-polarity mechanisms are responsible not only for the diversification of cell shapes but also for regulation of the asymmetric cell divisions of stem cells that are crucial for their correct self-renewal and differentiation. Disruption of cell polarity is a hallmark of cancer. Furthermore, recent evidence indicates that loss of cell polarity is intimately involved in cancer: several crucial cell-polarity proteins are known proto-oncogenes or tumor suppressors, basic mechanisms of cell polarity are often targeted by oncogenic signaling pathways, and deregulation of asymmetric cell divisions of stem or progenitor cells may be responsible for abnormal self-renewal and differentiation of cancer stem cells. Data from in vivo and three-dimensional (3D) cell-culture models demonstrate that tissue organization attenuates the phenotypic outcome of oncogenic signaling. We suggest that polarized 3D tissue organization uses cell-cell and cell-substratum adhesion structures to reinforce and maintain the cell polarity of pre-cancerous cells. In this model, polarized 3D tissue organization functions as a non-canonical tumor suppressor that prevents the manifestation of neoplastic features in mutant cells and, ultimately, suppresses tumor development and progression.

Cadherins and catenins at synapses: roles in synaptogenesis and synaptic plasticity

Synapse formation involves reciprocal interactions between cells resulting in formation of a structure optimized for efficient information transfer. Recent work has implicated constituents of the cadherin-catenin cell-adhesion complex in both synapse formation and plasticity. In this review, we describe recent interesting discoveries on mechanisms of cadherin complex function, in addition to regulating adhesion, that are relevant for understanding the role of this complex in synaptogenesis and plasticity. We describe how this complex acts via (i) recruitment/stabilization of intracellular partners; (ii) regulation of intracellular signaling pathways; (iii) regulation of cadherin surface levels, stability and turnover; (iv) stabilization of receptors; and (v) regulation of gene expression. These exciting discoveries provide insights into novel functional roles of the complex beyond regulating cell adhesion.