Adenomatous polyposis coli protein contains two nuclear export signals and shuttles between the nucleus and cytoplasm

Chromosome Region Maintenance 1 (XPO1/CRM1) as an Anticancer Target and Discovery of Its Inhibitor

Chromosome region maintenance 1 (CRM1) is a major nuclear export receptor protein and contributes to cell homeostasis by mediating the transport of cargo from the nucleus to the cytoplasm. CRM1 is a therapeutic target comprised of several tumor types, including osteosarcoma, multiple myeloma, gliomas, and pancreatic cancer. In the past decade, dozens of CRM1 inhibitors have been discovered and developed, including KPT-330, which received FDA approval for multiple myeloma (MM) and diffuse large B-cell lymphoma (DLBCL) in 2019 and 2020, respectively. This review summarizes the biological functions of CRM1, the current understanding of the role CRM1 plays in cancer, the discovery of CRM1 small-molecule inhibitors, preclinical and clinical studies on KPT-330, and other recently developed inhibitors. A new CRM1 inhibition mechanism and structural dynamics are discussed. Through this review, we hope to guide the future design and optimization of CRM1 inhibitors.

The 15-Amino Acid Repeat Region of Adenomatous Polyposis Coli Is Intrinsically Disordered and Retains Conformational Flexibility upon Binding β-Catenin

The tumor suppressor Adenomatous polyposis coli (APC) is a large, multidomain protein with many identified cellular functions. The best characterized role of APC is to scaffold a protein complex that negatively regulates Wnt signaling via β-catenin destruction. This destruction is mediated by β-catenin binding to centrally located 15- and 20-amino acid repeat regions of APC. More than 80% of cancers of the colon and rectum present with an APC mutation. Most carcinomas with mutant APC express a truncated APC protein that retains the ∼200-amino acid long' 15-amino acid repeat region'. This study demonstrates that the 15-amino acid repeat region of APC is intrinsically disordered. We investigated the backbone dynamics in the presence of β-catenin and predicted residues that may contribute to transient secondary features. This study reveals that the 15-amino acid region of APC retains flexibility upon binding β-catenin and that APC does not have a single, observable "highest-affinity" binding site for β-catenin. This flexibility potentially allows β-catenin to be more readily captured by APC and then remain accessible to other elements of the destruction complex for subsequent processing.

Oncogenic Serine 45-Deleted β-Catenin Remains Susceptible to Wnt Stimulation and APC Regulation in Human Colonocytes

The Wnt/β-catenin signaling pathway is deregulated in nearly all colorectal cancers (CRCs), predominantly through mutation of the tumor suppressor Adenomatous Polyposis Coli (APC). APC mutation is thought to allow a “just-right” amount of Wnt pathway activation by fine-tuning β-catenin levels. While at a much lower frequency, mutations that result in a β-catenin that is compromised for degradation occur in a subset of human CRCs. Here, we investigate whether one such “stabilized” β-catenin responds to regulatory stimuli, thus allowing β-catenin levels conducive for tumor formation. We utilize cells harboring a single mutant allele encoding Ser45-deleted β-catenin (β-catΔS45) to test the effects of Wnt3a treatment or APC-depletion on β-catΔS45 regulation and activity. We find that APC and β-catΔS45 retain interaction with Wnt receptors. Unexpectedly, β-catΔS45 accumulates and activates TOPflash reporter upon Wnt treatment or APC-depletion, but only accumulates in the nucleus upon APC loss. Finally, we find that β-catenin phosphorylation at GSK-3β sites and proteasomal degradation continue to occur in the absence of Ser45. Our results expand the current understanding of Wnt/β-catenin signaling and provide an example of a β-catenin mutation that maintains some ability to respond to Wnt, a possible key to establishing β-catenin activity that is “just-right” for tumorigenesis.

The Role of Carcinogenesis-Related Biomarkers in the Wnt Pathway and Their Effects on Epithelial-Mesenchymal Transition (EMT) in Oral Squamous Cell Carcinoma

As oral squamous cell carcinoma (OSCC) can develop from potentially malignant disorders (PMDs), it is critical to develop methods for early detection to improve the prognosis of patients. Epithelial-mesenchymal transition (EMT) plays an important role during tumor progression and metastasis. The Wnt signaling pathway is an intercellular pathway in animals that also plays a fundamental role in cell proliferation and regeneration, and in the function of many cell or tissue types. Specific components of master regulators such as epithelial cadherin (E-cadherin), Vimentin, adenomatous polyposis coli (APC), Snail, and neural cadherin (N-cadherin), which are known to control the EMT process, have also been implicated in the Wnt cascade. Here, we review recent findings on the Wnt signaling pathway and the expression mechanism. These regulators are known to play roles in EMT and tumor progression, especially in OSCC. Characterizing the mechanisms through which both EMT and the Wnt pathway play a role in these cellular pathways could increase our understanding of the tumor genesis process and may allow for the development of improved therapeutics for OSCC.

APC controls Wnt-induced β-catenin destruction complex recruitment in human colonocytes

Wnt/β-catenin signaling is essential for intestinal homeostasis and is aberrantly activated in most colorectal cancers (CRC) through mutation of the tumor suppressor Adenomatous Polyposis Coli (APC). APC is an essential component of a cytoplasmic protein complex that targets β-catenin for destruction. Following Wnt ligand presentation, this complex is inhibited. However, a role for APC in this inhibition has not been shown. Here, we utilized Wnt3a-beads to locally activate Wnt co-receptors. In response, the endogenous β-catenin destruction complex reoriented toward the local Wnt cue in CRC cells with full-length APC, but not if APC was truncated or depleted. Non-transformed human colon epithelial cells displayed similar Wnt-induced destruction complex localization which appeared to be dependent on APC and less so on Axin. Our results expand the current model of Wnt/β-catenin signaling such that in response to Wnt, the β-catenin destruction complex: (1) maintains composition and binding to β-catenin, (2) moves toward the plasma membrane, and (3) requires full-length APC for this relocalization.

Nuclear Export Signal Masking Regulates HIV-1 Rev Trafficking and Viral RNA Nuclear Export

HIV-1's Rev protein forms a homo-oligomeric adaptor complex linking viral RNAs to the cellular CRM1/Ran-GTP nuclear export machinery through the activity of Rev's prototypical leucine-rich nuclear export signal (NES). In this study, we used a functional fluorescently tagged Rev fusion protein as a platform to study the effects of modulating Rev NES identity, number, position, or strength on Rev subcellular trafficking, viral RNA nuclear export, and infectious virion production. We found that Rev activity was remarkably tolerant of diverse NES sequences, including supraphysiological NES (SNES) peptides that otherwise arrest CRM1 transport complexes at nuclear pores. Rev's ability to tolerate a SNES was both position and multimerization dependent, an observation consistent with a model wherein Rev self-association acts to transiently mask the NES peptide(s), thereby biasing Rev's trafficking into the nucleus. Combined imaging and functional assays also indicated that NES masking underpins Rev's well-known tendency to accumulate at the nucleolus, as well as Rev's capacity to activate optimal levels of late viral gene expression. We propose that Rev multimerization and NES masking regulates Rev's trafficking to and retention within the nucleus even prior to RNA binding.

IMPORTANCE

HIV-1 infects more than 34 million people worldwide causing >1 million deaths per year. Infectious virion production is activated by the essential viral Rev protein that mediates nuclear export of intron-bearing late-stage viral mRNAs. Rev's shuttling into and out of the nucleus is regulated by the antagonistic activities of both a peptide-encoded N-terminal nuclear localization signal and C-terminal nuclear export signal (NES). How Rev and related viral proteins balance strong import and export activities in order to achieve optimal levels of viral gene expression is incompletely understood. We provide evidence that multimerization provides a mechanism by which Rev transiently masks its NES peptide, thereby biasing its trafficking to and retention within the nucleus. Targeted pharmacological disruption of Rev-Rev interactions should perturb multiple Rev activities, both Rev-RNA binding and Rev's trafficking to the nucleus in the first place.

Reconstituting regulation of the canonical Wnt pathway by engineering a minimal β-catenin destruction machine

APC and Axin are key negative regulators of Wnt signaling in development and oncogenesis. They form a multiprotein complex targeting the key Wnt effector β-catenin for destruction. Essential components of APC and Axin required for their cooperative function are identified, and the data are used to design a minimal β-catenin–destruction machine.

Negatively regulating key signaling pathways is critical to development and altered in cancer. Wnt signaling is kept off by the destruction complex, which is assembled around the tumor suppressors APC and Axin and targets β-catenin for destruction. Axin and APC are large proteins with many domains and motifs that bind other partners. We hypothesized that if we identified the essential regions required for APC:Axin cooperative function and used these data to design a minimal β-catenin-destruction machine, we would gain new insights into the core mechanisms of destruction complex function. We identified five key domains/motifs in APC or Axin that are essential for their function in reconstituting Wnt regulation. Strikingly, however, certain APC and Axin mutants that are nonfunctional on their own can complement one another in reducing β-catenin, revealing that the APC:Axin complex is a highly robust machine. We used these insights to design a minimal β-catenin-destruction machine, revealing that a minimized chimeric protein covalently linking the five essential regions of APC and Axin reconstitutes destruction complex internal structure, size, and dynamics, restoring efficient β-catenin destruction in colorectal tumor cells. On the basis of our data, we propose a new model of the mechanistic function of the destruction complex as an integrated machine.

Preclinical activity of selinexor, an inhibitor of XPO1, in sarcoma

Selinexor is an orally bioavailable selective inhibitor of nuclear export that has been demonstrated to have preclinical activity in various cancer types and that is currently in Phase I and II clinical trials for advanced cancers. In this study, we evaluated the effects of selinexor in several preclinical models of various sarcoma subtypes. The efficacy of selinexor was investigated in vitro and in vivo using 17 cell lines and 9 sarcoma xenograft models including gastrointestinal stromal tumor (GIST), liposarcoma (LPS), leiomyosarcoma, rhabdomyosarcoma, undifferentiated sarcomas, and alveolar soft part sarcoma (ASPS). Most sarcoma cell lines were sensitive to selinexor with IC50s ranging from 28.8 nM to 218.2 nM (median: 66.1 nM). Selinexor suppressed sarcoma tumor xenograft growth, including models of ASPS that were resistant in vitro. In GIST cells with KIT mutations, selinexor induced G1- arrest without attenuation of phosphorylation of KIT, AKT, or MAPK, in contrast to imatinib. In LPS cell lines with MDM2 and CDK4 amplification, selinexor induced G1-arrest and apoptosis irrespective of p53 expression or mutation and irrespective of RB expression. Selinexor increased p53 and p21 expression at the protein but not RNA level, indicating a post-transcriptional effect. These results indicate that selinexor has potent in vitro and in vivo activity against a wide variety of sarcoma models by inducing G1-arrest independent of known molecular mechanisms in GIST and LPS. These studies further justify the exploration of selinexor in clinical trials targeting various sarcoma subtypes.

Evidence for the Nucleo-Apical Shuttling of a Beta-Catenin Like Plasmodium falciparum Armadillo Repeat Containing Protein

Eukaryotic Armadillo (ARM) repeat proteins are multifaceted with prominent roles in cell-cell adhesion, cytoskeletal regulation and intracellular signaling among many others. One such ARM repeat containing protein, ARM Repeats Only (ARO), has recently been demonstrated in both Toxoplasma (TgARO) and Plasmodium (PfARO) parasites to be targeted to the rhoptries during the late asexual stages. TgARO has been implicated to play an important role in rhoptry positioning i.e. directing the rhoptry towards the apical end of the parasite. Here, we report for the first time that PfARO exhibits a DNA binding property and a dynamic sub-cellular localization between the nucleus (early schizont) and rhoptry (late schizont) during the different stages of the asexual blood-stage life cycle. PfARO possesses a putative nuclear export signal (NES) and the nucleo-apical shuttling was sensitive to Leptomycin B (LMB) suggesting that the nuclear export was mediated by CRM1. Importantly, PfARO specifically bound an A-T rich DNA sequence of the P. falciparum Gyrase A (PfgyrA) gene, suggesting that the DNA binding specificity of PfARO is likely due to the AT-richness of the probe. This is a novel functional characteristic that has not been reported previously for any P. falciparum ARM containing protein and suggests a putative role for PfARO in gene regulation. This study describes for the first time a conserved P. falciparum ARM repeat protein with a high degree of functional versatility.

Structural Basis of Targeting the Exportin CRM1 in Cancer

Recent studies have demonstrated the interference of nucleocytoplasmic trafficking with the establishment and maintenance of various cancers. Nucleocytoplasmic transport is highly regulated and coordinated, involving different nuclear transport factors or receptors, importins and exportins, that mediate cargo transport from the cytoplasm into the nucleus or the other way round, respectively. The exportin CRM1 (Chromosome region maintenance 1) exports a plethora of different protein cargoes and ribonucleoprotein complexes. Structural and biochemical analyses have enabled the deduction of individual steps of the CRM1 transport cycle. In addition, CRM1 turned out to be a valid target for anticancer drugs as it exports numerous proto-oncoproteins and tumor suppressors. Clearly, detailed understanding of the flexibility, regulatory features and cooperative binding properties of CRM1 for Ran and cargo is a prerequisite for the design of highly effective drugs. The first compound found to inhibit CRM1-dependent nuclear export was the natural drug Leptomycin B (LMB), which blocks export by competitively interacting with a highly conserved cleft on CRM1 required for nuclear export signal recognition. Clinical studies revealed serious side effects of LMB, leading to a search for alternative natural and synthetic drugs and hence a multitude of novel therapeutics. The present review examines recent progress in understanding the binding mode of natural and synthetic compounds and their inhibitory effects.

Molecular mechanism of adenomatous polyposis coli-induced blockade of base excision repair pathway in colorectal carcinogenesis

Colorectal cancer (CRC) is the third leading cause of death in both men and women in North America. Despite chemotherapeutic efforts, CRC is associated with a high degree of morbidity and mortality. Thus, to develop effective treatment strategies for CRC, one needs knowledge of the pathogenesis of cancer development and cancer resistance. It is suggested that colonic tumors or cell lines harbor truncated adenomatous polyposis coli (APC) without DNA repair inhibitory (DRI)-domain. It is also thought that the product of the APC gene can modulate base excision repair (BER) pathway through an interaction with DNA polymerase β (Pol-β) and flap endonuclease 1 (Fen-1) to mediate CRC cell apoptosis. The proposed therapy with temozolomide (TMZ) exploits this particular pathway; however, a high percentage of colorectal tumors continue to develop resistance to chemotherapy due to mismatch repair (MMR)-deficiency. In the present communication, we have comprehensively reviewed a critical issue that has not been addressed previously: a novel mechanism by which APC-induced blockage of single nucleotide (SN)- and long-patch (LP)-BER play role in DNA-alkylation damage-induced colorectal carcinogenesis.

Nuclear p120-catenin regulates the anoikis resistance of mouse lobular breast cancer cells through Kaiso-dependent Wnt11 expression

E-cadherin inactivation underpins the progression of invasive lobular breast carcinoma (ILC). In ILC, p120-catenin (p120) translocates to the cytosol where it controls anchorage independence through the Rho-Rock signaling pathway, a key mechanism driving tumor growth and metastasis. We now demonstrate that anchorage-independent ILC cells show an increase in nuclear p120, which results in relief of transcriptional repression by Kaiso. To identify the Kaiso target genes that control anchorage independence we performed genome-wide mRNA profiling on anoikis-resistant mouse ILC cells, and identified 29 candidate target genes, including the established Kaiso target Wnt11. Our data indicate that anchorage-independent upregulation of Wnt11 in ILC cells is controlled by nuclear p120 through inhibition of Kaiso-mediated transcriptional repression. Finally, we show that Wnt11 promotes activation of RhoA, which causes ILC anoikis resistance. Our findings thereby establish a mechanistic link between E-cadherin loss and subsequent control of Rho-driven anoikis resistance through p120- and Kaiso-dependent expression of Wnt11.

Inhibition of CRM1-dependent nuclear export sensitizes malignant cells to cytotoxic and targeted agents

Nuclear-cytoplasmic trafficking of proteins is a significant factor in the development of cancer and drug resistance. Subcellular localization of exported proteins linked to cancer development include those involved in cell growth and proliferation, apoptosis, cell cycle regulation, transformation, angiogenesis, cell adhesion, invasion, and metastasis. Here, we examined the basic mechanisms involved in the export of proteins from the nucleus to the cytoplasm. All proteins over 40kDa use the nuclear pore complex to gain entry or exit from the nucleus, with the primary nuclear export molecule involved in these processes being chromosome region maintenance 1 (CRM1, exportin 1 or XPO1). Proteins exported from the nucleus must possess a hydrophobic nuclear export signal (NES) peptide that binds to a hydrophobic groove containing an active-site Cys528 in the CRM1 protein. CRM1 inhibitors function largely by covalent modification of the active site Cys528 and prevent binding to the cargo protein NES. In the absence of a CRM1 inhibitor, CRM1 binds cooperatively to the NES of the cargo protein and RanGTP, forming a trimer that is actively transported out of the nucleus by facilitated diffusion. Nuclear export can be blocked by CRM1 inhibitors, NES peptide inhibitors or by preventing post-translational modification of cargo proteins. Clinical trials using the classic CRM1 inhibitor leptomycin B proved too toxic for patients; however, a new generation of less toxic small molecule inhibitors is being used in clinical trials in patients with both hematological malignancies and solid tumors. Additional trials are being initiated using small-molecule CRM1 inhibitors in combination with chemotherapeutics such as pegylated liposomal doxorubicin. In this review, we present evidence that combining the new CRM1 inhibitors with other classes of therapeutics may prove effective in the treatment of cancer. Potential combinatorial therapies discussed include the use of CRM1 inhibitors and the addition of alkylating agents (melphalan), anthracyclines (doxorubicin and daunomycin), BRAF inhibitors, platinum drugs (cisplatin and oxaliplatin), proteosome inhibitors (bortezomib and carfilzomib), or tyrosine-kinase inhibitors (imatinib). Also, the sequence of treatment may be important for combination therapy. We found that the most effective treatment regimen involved first priming the cancer cells with the CRM1 inhibitor followed by doxorubicin, bortezomib, carfilzomib, or melphalan. This order sensitized both de novo and acquired drug-resistant cancer cell lines.

DACT2 is a functional tumor suppressor through inhibiting Wnt/β-catenin pathway and associated with poor survival in colon cancer

Dapper homolog (DACT) 2 is one of the Dact gene family members, which are important modulators of Wnt signaling pathway. We aim to clarify its epigenetic inactivation, biological function and clinical implication in colon cancer. DACT2 was silenced in five out of eight colon cancer cell lines, but robustly expressed in normal colon tissues. The loss of DACT2 expression was regulated by promoter hypermethylation. Restoring DACT2 expression in colon cancer cell lines suppressed tumor cell growth by inducing cell apoptosis and inhibiting cell proliferation both in vitro and in vivo. Moreover, DACT2 overexpression effectively reduced lung metastasis of colon cancer cells in nude mice. These effects by DACT2 were attributed to inhibition of Wnt/β-catenin signaling. Reexpression of DACT2 significantly suppressed the transcriptional activity of both wild-type β-catenin and degradation-resistant form mutant β-catenin (S33Y). DACT2 could actively shuttle into and out of nuclei, with its predominant steady-state localization in the cytoplasm dependent on its nuclear export signal. Co-immunoprecipitation results indicated that DACT2 strongly associated β-catenin as well as lymphoid enhancer-binding factor 1 (LEF1) and directly disrupted the formation of the β-catenin-LEF1 complex in the nucleus. Whereas in the cytoplasm, DACT2 restored junctional localization of E-cadherin-β-catenin complexes and prevented β-catenin nuclear translocation through direct interaction with β-catenin. DACT2 methylation was detected in 43.3% (29/67) of colon cancer tissues, but none in normal controls. Multivariate analysis revealed that patients with DACT2 methylation had a significant decrease in overall survival (P=0.006). Kaplan-Meier survival curves showed that DACT2 methylation was significantly associated with shortened survival in stage I-III colon cancer patients. In conclusion, DACT2 acts as a functional tumor suppressor in colon cancer through inhibiting Wnt/β-catenin signaling. Its methylation at early stages of colon carcinogenesis is an independent prognostic factor.

Nuclear adenomatous polyposis coli suppresses colitis-associated tumorigenesis in mice

Mutation of tumor suppressor adenomatous polyposis coli (APC) initiates most colorectal cancers and chronic colitis increases risk. APC is a nucleo-cytoplasmic shuttling protein, best known for antagonizing Wnt signaling by forming a cytoplasmic complex that marks β-catenin for degradation. Using our unique mouse model with compromised nuclear Apc import (Apc(mNLS)), we show that Apc(mNLS/mNLS) mice have increased susceptibility to tumorigenesis induced with azoxymethane (AOM) and dextran sodium sulfate (DSS). The AOM-DSS-induced colon adenoma histopathology, proliferation, apoptosis, stem cell number and β-catenin and Kras mutation spectra were similar in Apc(mNLS/mNLS) and Apc(+/+) mice. However, AOM-DSS-treated Apc(mNLS/mNLS) mice showed more weight loss, more lymphoid follicles and edema, and increased colon shortening than treated Apc(+/+) mice, indicating a colitis predisposition. To test this directly, we induced acute colitis with a 7 day DSS treatment followed by 5 days of recovery. Compared with Apc(+/+) mice, DSS-treated Apc(mNLS/mNLS) mice developed more severe colitis based on clinical grade and histopathology. Apc(mNLS/mNLS) mice also had higher lymphocytic infiltration and reduced expression of stem cell markers, suggesting an increased propensity for chronic inflammation. Moreover, colons from DSS-treated Apc(mNLS/mNLS) mice showed fewer goblet cells and reduced Muc2 expression. Even in untreated Apc(mNLS/mNLS) mice, there were significantly fewer goblet cells in jejuna, and a modest decrease in colonocyte Muc2 expression compared with Apc(+/+) mice. Colonocytes from untreated Apc(mNLS/mNLS) mice also showed increased expression of inflammatory mediators cyclooxygenase-2 (Cox-2) and macrophage inflammatory protein-2 (MIP-2). These findings reveal novel functions for nuclear Apc in goblet cell differentiation and protection against inflammation-induced colon tumorigenesis.

The tumor suppressor APC differentially regulates multiple β-catenins through the function of axin and CKIα during C. elegans asymmetric stem cell divisions

The APC tumor suppressor regulates diverse stem cell processes including gene regulation through Wnt-β-catenin signaling and chromosome stability through microtubule interactions, but how the disparate functions of APC are controlled is not well understood. Acting as part of a Wnt-β-catenin pathway that controls asymmetric cell division, Caenorhabditis elegans APC, APR-1, promotes asymmetric nuclear export of the β-catenin WRM-1 by asymmetrically stabilizing microtubules. Wnt function also depends on a second β-catenin, SYS-1, which binds to the C. elegans TCF POP-1 to activate gene expression. Here, we show that APR-1 regulates SYS-1 levels in asymmetric stem cell division, in addition to its known role in lowering nuclear levels of WRM-1. We demonstrate that SYS-1 is also negatively regulated by the C. elegans homolog of casein kinase 1α (CKIα), KIN-19. We show that KIN-19 restricts APR-1 localization, thereby regulating nuclear WRM-1. Finally, the polarity of APR-1 cortical localization is controlled by PRY-1 (C. elegans Axin), such that PRY-1 controls the polarity of both SYS-1 and WRM-1 asymmetries. We propose a model whereby Wnt signaling, through CKIα, regulates the function of two distinct pools of APC - one APC pool negatively regulates SYS-1, whereas the second pool stabilizes microtubules and promotes WRM-1 nuclear export.

Comprehensive analysis of β-catenin target genes in colorectal carcinoma cell lines with deregulated Wnt/β-catenin signaling

Background

Deregulation of Wnt/β-catenin signaling is a hallmark of the majority of sporadic forms of colorectal cancer and results in increased stability of the protein β-catenin. β-catenin is then shuttled into the nucleus where it activates the transcription of its target genes, including the proto-oncogenes MYC and CCND1 as well as the genes encoding the basic helix-loop-helix (bHLH) proteins ASCL2 and ITF-2B. To identify genes commonly regulated by β-catenin in colorectal cancer cell lines, we analyzed β-catenin target gene expression in two non-isogenic cell lines, DLD1 and SW480, using DNA microarrays and compared these genes to β-catenin target genes published in the PubMed database and DNA microarray data presented in the Gene Expression Omnibus (GEO) database.

Results

Treatment of DLD1 and SW480 cells with β-catenin siRNA resulted in differential expression of 1501 and 2389 genes, respectively. 335 of these genes were regulated in the same direction in both cell lines. Comparison of these data with published β-catenin target genes for the colon carcinoma cell line LS174T revealed 193 genes that are regulated similarly in all three cell lines. The overlapping gene set includes confirmed β-catenin target genes like AXIN2, MYC, and ASCL2. We also identified 11 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways that are regulated similarly in DLD1 and SW480 cells and one pathway – the steroid biosynthesis pathway – was regulated in all three cell lines.

Conclusions

Based on the large number of potential β-catenin target genes found to be similarly regulated in DLD1, SW480 and LS174T cells as well as the large overlap with confirmed β-catenin target genes, we conclude that DLD1 and SW480 colon carcinoma cell lines are suitable model systems to study Wnt/β-catenin signaling and associated colorectal carcinogenesis. Furthermore, the confirmed and the newly identified potential β-catenin target genes are useful starting points for further studies.

Analysing the impact of nucleo-cytoplasmic shuttling of β-catenin and its antagonists APC, Axin and GSK3 on Wnt/β-catenin signalling

The canonical Wnt signalling pathway plays a critical role in development and disease. The key player of the pathway is β-catenin. Its activity is mainly regulated by the destruction complex consisting of APC, Axin and GSK3. In the nucleus, the complex formation of β-catenin and TCF initiates target gene expression. Our study provides a comprehensive analysis of the role of nucleo-cytoplasmic shuttling of APC, Axin, and GSK3 and the inactivation of β-catenin by the destruction complex in Wnt/β-catenin signalling. We address the following questions: Can nucleo-cytoplasmic shuttling of APC, Axin and GSK3 increase the [β-catenin/TCF] concentration? And, how is the [β-catenin/TCF] concentration influenced by phosphorylation and subsequent degradation of nuclear β-catenin? Based on experimental findings, we develop a compartmental model and conduct several simulation experiments. Our analysis reveals the following key findings: 1) nucleo-cytoplasmic shuttling of β-catenin and its antagonists can yield a spatial separation between the said proteins, which results in a breakdown of β-catenin degradation, followed by an accumulation of β-catenin and hence leads to an increase of the [β-catenin/TCF] concentration. Our results strongly suggest that Wnt signalling can benefit from nucleo-cytoplasmic shuttling of APC, Axin and GSK3, although they are in general β-catenin antagonising proteins. 2) The total robustness of the [β-catenin/TCF] output is closely linked to its absolute concentration levels. We demonstrate that the compartmental separation of β-catenin and the destruction complex does not only lead to a maximization, but additionally to an increased robustness of [β-catenin/TCF] signalling against perturbations in the cellular environment. 3) A nuclear accumulation of the destruction complex renders the pathway robust against fluctuations in Wnt signalling and against changes in the compartmental distribution of β-catenin. 4) Elucidating the impact of destruction complex inhibition, we show that the [β-catenin/TCF] concentration is more effectively enhanced by inhibition of the kinase GSK3 rather than the binding of β-catenin to the destruction complex.

A Comprehensive Overview of Skeletal Phenotypes Associated with Alterations in Wnt/β-catenin Signaling in Humans and Mice

The Wnt signaling pathway plays key roles in differentiation and development and alterations in this signaling pathway are causally associated with numerous human diseases. While several laboratories were examining roles for Wnt signaling in skeletal development during the 1990s, interest in the pathway rose exponentially when three key papers were published in 2001-2002. One report found that loss of the Wnt co-receptor, Low-density lipoprotein related protein-5 (LRP5), was the underlying genetic cause of the syndrome Osteoporosis pseudoglioma (OPPG). OPPG is characterized by early-onset osteoporosis causing increased susceptibility to debilitating fractures. Shortly thereafter, two groups reported that individuals carrying a specific point mutation in LRP5 (G171V) develop high-bone mass. Subsequent to this, the causative mechanisms for these observations heightened the need to understand the mechanisms by which Wnt signaling controlled bone development and homeostasis and encouraged significant investment from biotechnology and pharmaceutical companies to develop methods to activate Wnt signaling to increase bone mass to treat osteoporosis and other bone disease. In this review, we will briefly summarize the cellular mechanisms underlying Wnt signaling and discuss the observations related to OPPG and the high-bone mass disorders that heightened the appreciation of the role of Wnt signaling in normal bone development and homeostasis. We will then present a comprehensive overview of the core components of the pathway with an emphasis on the phenotypes associated with mice carrying genetically engineered mutations in these genes and clinical observations that further link alterations in the pathway to changes in human bone.

The WNT signaling pathway from ligand secretion to gene transcription: molecular mechanisms and pharmacological targets

Wingless/integrase-1 (WNT) signaling is a key pathway regulating various aspects of embryonic development; however it also underlies several pathological conditions in man, including various cancers and fibroproliferative diseases in several organs. Investigating the molecular processes involved in (canonical) WNT signaling will open new avenues for generating new therapeutics to specifically target diseases in which WNT signaling is aberrantly regulated. Here we describe the complexity of WNT signal transduction starting from the processes involved in WNT ligand biogenesis and secretion by WNT producing cells followed by a comprehensive overview of the molecular signaling events ultimately resulting in enhanced transcription of specific genes in WNT receiving cells. Finally, the possible targets for therapeutic intervention and the available pharmacological inhibitors for this complex signaling pathway are discussed.

Hepatic stem cells and transforming growth factor β in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common and lethal cancers worldwide. It arises from modulation of multiple genes by mutations, epigenetic regulation, noncoding RNAs and translational modifications of encoded proteins. Although >40% of HCCs are clonal and thought to arise from cancer stem cells (CSCs), the precise identification and mechanisms of CSC formation remain poorly understood. A functional role of transforming growth factor (TGF)-β signalling in liver and intestinal stem cell niches has been demonstrated through mouse genetics. These studies demonstrate that loss of TGF-β signalling yields a phenotype similar to a human CSC disorder, Beckwith-Wiedemann syndrome. Insights into this powerful pathway will be vital for developing new therapeutics in cancer. Current clinical approaches are aimed at establishing novel cancer drugs that target activated pathways when the TGF-β tumour suppressor pathway is lost, and TGF-β itself could potentially be targeted in metastases. Studies delineating key functional pathways in HCC and CSC formation could be important in preventing this disease and could lead to simple treatment strategies; for example, use of vitamin D might be effective when the TGF-β pathway is lost or when wnt signalling is activated.

Actin nucleators in the nucleus: an emerging theme

Actin is an integral component of the cytoskeleton, forming a plethora of macromolecular structures that mediate various cellular functions. The formation of such structures relies on the ability of actin monomers to associate into polymers, and this process is regulated by actin nucleation factors. These factors use monomeric actin pools at specific cellular locations, thereby permitting rapid actin filament formation when required. It has now been established that actin is also present in the nucleus, where it is implicated in chromatin remodelling and the regulation of eukaryotic gene transcription. Notably, the presence of typical actin filaments in the nucleus has not been demonstrated directly. However, studies in recent years have provided evidence for the nuclear localisation of actin nucleation factors that promote cytoplasmic actin polymerisation. Their localisation to the nucleus suggests that these proteins mediate collaboration between the cytoskeleton and the nucleus, which might be dependent on their ability to promote actin polymerisation. The nature of this cooperation remains enigmatic and it will be important to elucidate the physiological relevance of the link between cytoskeletal actin networks and nuclear events. This Commentary explores the current evidence for the nuclear roles of actin nucleation factors. Furthermore, the implication of actin-associated proteins in relaying exogenous signals to the nucleus, particularly in response to cellular stress, will be considered.

APC gene deletions in gastric adenocarcinomas in a Chinese population: a correlation with tumour progression

INTRODUCTION

The adenomatous polyposis coli (APC) gene encodes a tumor suppressor protein that acts as an antagonist of the Wnt signaling pathway. It has been shown to be involved in genetic instability and to be down-regulated in several human carcinomas. The chromosome locus of APC, 5q21-22, is frequently deleted in gastric cancers (GCs). The functional impact of such regions needs to be extensively investigated in large amount of clinical samples.

PATIENTS AND MATERIALS

Case-matched tissues of GC and adjacent normal epithelium (n = 141) were included in this study. Quantitative PCR was carried out to examine the copy number as well as mRNA expression of APC gene in gastric malignancies.

RESULTS

Our results showed that copy number deletions of APC were present in a relatively high percentage (25.9%, 34 out of 131) of gastric cancer samples. There was a correlation between APC deletion and tumor progression (p < 0.01) as well as gene expression (p < 0.05) in collected GC samples. On the other hand, mRNA levels of APC were also impaired in GC samples with unaltered copy numbers.

CONCLUSION

Sporadic GCs exhibit different mechanisms of APC regulation.

The CRM1 nuclear export protein in normal development and disease

CRM1 (Chromosomal Maintenance 1, also known as Exportin 1) is the major mammalian export protein that facilitates the transport of large macromolecules including RNA and protein across the nuclear membrane to the cytoplasm. The gene encoding CRM1 was originally identified in yeast as required to maintain higher order chromosome structure. In mammalian cells, CRM1 was found to bind several nuclear pore proteins hence its role in nuclear-cytosolic transport was discovered. In addition to nuclear-cytosolic transport, CRM1 also plays a role in centrosome duplication and spindle assembly, especially in response to DNA damage. The crystal structure of CRM1 suggests a complex protein that binds the Ran protein bound to GTP, allowing for a conformational change that facilitates binding to different cargo proteins through a nuclear export signal (NES). Included in the cadre of cargo are multiple tumor suppressor and oncoproteins as p53, BRCA1, Survivin, NPM, and APC, which function in the nucleus to regulate transcription or aid in chromosomal assembly and movement. An imbalance in the cytosolic level of these proteins has been observed in cancer cells, resulting in either inactivation (tumor suppressor) or an excess of anti-apoptotic activity (oncoprotein). Thus, the concept of inhibiting CRM1 has been explored as a potential therapeutic intervention. Indeed, inhibition of CRM1 by a variety of small molecules that interfere with cargo-NES binding results in cancer cell death. Whether all of these proteins together are responsible for this phenotype or whether specific proteins are required for this effect is unclear at this time.

Nuclear export of proteins and drug resistance in cancer

The intracellular location of a protein is crucial to its normal functioning in a cell. Cancer cells utilize the normal processes of nuclear-cytoplasmic transport through the nuclear pore complex of a cell to effectively evade anti-neoplastic mechanisms. CRM1-mediated export is increased in various cancers. Proteins that are exported in cancer include tumor-suppressive proteins such as retinoblastoma, APC, p53, BRAC1, FOXO proteins, INI1/hSNF5, galectin-3, Bok, nucleophosmin, RASSF2, Merlin, p21(CIP), p27(KIP1), N-WASP/FAK, estradiol receptor and Tob, drug targets topoisomerase I and IIα and BCR-ABL, and the molecular chaperone protein Hsp90. Here, we review in detail the current processes and known structures involved in the export of a protein through the nuclear pore complex. We also discuss the export receptor molecule CRM1 and its binding to the leucine-rich nuclear export signal of the cargo protein and the formation of a nuclear export trimer with RanGTP. The therapeutic potential of various CRM1 inhibitors will be addressed, including leptomycin B, ratjadone, KOS-2464, and specific small molecule inhibitors of CRM1, N-azolylacrylate analogs, FOXO export inhibitors, valtrate, acetoxychavicol acetate, CBS9106, and SINE inhibitors. We will also discuss examples of how drug resistance may be reversed by targeting the exported proteins topoisomerase IIα, BCR-ABL, and galectin-3. As effective and less toxic CRM1 export inhibitors become available, they may be used as both single agents and in combination with current chemotherapeutic drugs. We believe that the future development of low-toxicity, small-molecule CRM1 inhibitors may provide a new approach to treating cancer.

APC as a mobile scaffold: regulation and function at the nucleus, centrosomes, and mitochondria

Genetic mutations of adenomatous polyposis coli (APC) predispose to high risk of human colon cancer. APC is a large tumor suppressor protein and truncating mutations disrupt its normal roles in regulating cell migration, DNA replication/repair, mitosis, apoptosis, and turnover of oncogenic β-catenin. APC is targeted to multiple subcellular sites, and here we discuss recent evidence implicating novel protein interactions and functions of APC in the nucleus and at centrosomes and mitochondria. The ability of APC to shuttle between these and other cell locations is hypothesized to be integral to its cellular function.

Update on Wnt signaling in bone cell biology and bone disease

For more than a decade, Wnt signaling pathways have been the focus of intense research activity in bone biology laboratories because of their importance in skeletal development, bone mass maintenance, and therapeutic potential for regenerative medicine. It is evident that even subtle alterations in the intensity, amplitude, location, and duration of Wnt signaling pathways affects skeletal development, as well as bone remodeling, regeneration, and repair during a lifespan. Here we review recent advances and discrepancies in how Wnt/Lrp5 signaling regulates osteoblasts and osteocytes, introduce new players in Wnt signaling pathways that have important roles in bone development, discuss emerging areas such as the role of Wnt signaling in osteoclastogenesis, and summarize progress made in translating basic studies to clinical therapeutics and diagnostics centered around inhibiting Wnt pathway antagonists, such as sclerostin, Dkk1 and Sfrp1. Emphasis is placed on the plethora of genetic studies in mouse models and genome wide association studies that reveal the requirement for and crucial roles of Wnt pathway components during skeletal development and disease.

The ubiquitin-specific protease USP34 regulates axin stability and Wnt/β-catenin signaling

Wnt proteins control multiple cell behaviors during development and tissue homeostasis. However, pathological activation of Wnt signaling is the underlying cause of various human diseases. The ubiquitin-proteasome system plays important regulatory functions within the Wnt pathway by regulating the activity of several of its core components. Hence, multiple E3 ubiquitin ligases have been implicated in its regulation. Less is known, however, about the role of ubiquitin-specific proteases in Wnt signaling. Analysis of purified axin-containing protein complexes by liquid chromatography-tandem mass spectrometry revealed the presence of the ubiquitin protease USP34. Our results indicate that USP34 functions downstream of the β-catenin destruction complex to control the stability of axin and opposes its tankyrase-dependent ubiquitination. Reflecting on the requirement for tight control of axin homeostasis during Wnt signaling, interfering with USP34 function by RNA interference leads to the degradation of axin and to the inhibition of β-catenin-mediated transcription. Given the numerous human diseases exhibiting spurious Wnt pathway activation, the development of USP34 inhibitors may offer a novel therapeutic opportunity.

The ubiquitin-specific protease USP34 regulates axin stability and Wnt/β-catenin signaling

Wnt proteins control multiple cell behaviors during development and tissue homeostasis. However, pathological activation of Wnt signaling is the underlying cause of various human diseases. The ubiquitin-proteasome system plays important regulatory functions within the Wnt pathway by regulating the activity of several of its core components. Hence, multiple E3 ubiquitin ligases have been implicated in its regulation. Less is known, however, about the role of ubiquitin-specific proteases in Wnt signaling. Analysis of purified axin-containing protein complexes by liquid chromatography-tandem mass spectrometry revealed the presence of the ubiquitin protease USP34. Our results indicate that USP34 functions downstream of the β-catenin destruction complex to control the stability of axin and opposes its tankyrase-dependent ubiquitination. Reflecting on the requirement for tight control of axin homeostasis during Wnt signaling, interfering with USP34 function by RNA interference leads to the degradation of axin and to the inhibition of β-catenin-mediated transcription. Given the numerous human diseases exhibiting spurious Wnt pathway activation, the development of USP34 inhibitors may offer a novel therapeutic opportunity.

Detection of APC gene deletions in colorectal malignancies using quantitative PCR in a Chinese population

The adenomatous polyposis coli (APC) gene has been shown to be involved in genetic instability and to be downregluated in several human carcinomas. The chromosome locus of APC, 5q21-22, is frequently deleted in colorectal cancers (CRCs). The functional impact of such regions needs to be extensively investigated in large amount of clinical samples. Case-matched tissues of CRC and adjacent normal epithelium (n = 134) were included in this study. Quantitative PCR was carried out to examine the copy number as well as mRNA expression of APC gene in colorectal malignancies. Our results showed that copy number deletions of APC were present in a relatively high percentage of colorectal cancer samples (26.1%, 35 out of 134). There was a positive correlation between copy number decrease of APC and tumor progression in CRCs. Furthermore, copy number loss of APC was correlated with decreased mRNA expression. However, mRNA levels of APC were also impaired in CRC samples with unaltered copy numbers, indicating that sporadic CRCs exhibit different mechanisms of APC regulation.

Novel double-negative feedback loop between adenomatous polyposis coli and Musashi1 in colon epithelia

Loss of tumor suppressor adenomatous polyposis coli (APC) is thought to initiate the majority of all colorectal cancers. The predominant theory of colorectal carcinogenesis implicates stem cells as the initiating cells. However, relatively little is known about the function of APC in governing the homeostasis of normal intestinal stem cells. Here, we identify a novel double-negative feedback loop between APC and a translation inhibitor protein, Musashi1 (MSI1), in cultured human colonocytes. We show APC as a key factor in MSI1 regulation through Wnt signaling and identify APC mRNA as a novel target of translational inhibition by MSI1. We propose that APC/MSI1 interactions maintain homeostatic balance in the intestinal epithelium.

Involvement of the APC/β-catenin/TCF signaling pathway in the regulatory effects of octreotide on SW480 colon cancer cells

AIM: To investigate whether the APC/β-catenin/TCF signaling pathway is involved in the regulatory effects of octreotide (OCT) on SW480 colon cancer cells.

METHODS: (1) mRNA testing: SW480 cells were cultured in vitro and divided into control group and OCT group (treated with 10^-10 mol/L OCT). Total RNA was prepared from the two groups of cells and used to detect the mRNA expression of APC2, AXIN, CK1α, cyclin D1 and FZD7 by RT-PCR. (2) Protein testing: SW480 cells were cultured in vitro and divided into control group and OCT group (treated with /L OCT at a concentration of 10^-14, 10^-12 or 10^-10 mol/L). Total protein was prepared from the two groups of cells and used to detect the protein expression of APC2, CK1α and CyclinD1 by Western blot.

RESULTS: (1) mRNA testing: Treatment with OCT increased the mRNA levels of APC2 and CK1α (both P < 0.05), decreased that of cyclin D1 (P < 0.05), but had no significant impact on those of FZD7 and axin (both P > 0.05) in SW480 cells; (2) Protein testing: Treatment with different concentrations of OCT increased the protein levels of APC2 and CK1α and decreased that of cyclin D1 in SW480 cells in a dose-dependent manner (all P < 0.05).

CONCLUSION: OCT can negatively regulate APC/β-catenin/TCF signaling by up-regulating APC2 and CK1α expression and down-regulating cyclin D1 expression.

Epigenetic silencing of beta-spectrin, a TGF-beta signaling/scaffolding protein in a human cancer stem cell disorder: Beckwith-Wiedemann syndrome

Hereditary cancer syndromes provide powerful insights into dysfunctional signaling pathways that lead to sporadic cancers. Beckwith-Wiedemann syndrome (BWS) is a hereditary human cancer stem cell syndrome currently linked to deregulated imprinting at chromosome 11p15 and uniparental disomy. However, causal molecular defects and genetic models have remained elusive to date in the majority of cases. The non-pleckstrin homology domain β-spectrin (β2SP) (the official name for human is Spectrin, beta, nonerythrocytic 1 (SPTBN1), isoform 2; the official name for mouse is Spectrin beta 2 (Spnb2), isoform 2), a scaffolding protein, functions as a potent TGF-β signaling member adaptor in tumor suppression and development. Yet, the role of the β2SP in human tumor syndromes remains unclear. Here, we report that β2SP^+/− mice are born with many phenotypic characteristics observed in BWS patients, suggesting that β2SP mutant mice phenocopy BWS, and β2SP loss could be one of the mechanisms associated with BWS. Our results also suggest that epigenetic silencing of β2SP is a new potential causal factor in human BWS patients. Furthermore, β2SP^+/− mice provide an important animal model for BWS, as well as sporadic cancers associated with it, including lethal gastrointestinal and pancreatic cancer. Thus, these studies could lead to further insight into defects generated by dysfunctional stem cells and identification of new treatment strategies and functional markers for the early detection of these lethal cancers that otherwise cannot be detected at an early stage.

Pericentrin contains five NESs and an NLS essential for its nucleocytoplasmic trafficking during the cell cycle

Pericentrin, a conserved centrosomal component, provides the structural scaffold to anchor numerous centrosomal proteins, and thus plays an essential role in the organization and function of the centrosome and the mitotic spindle. Although pericentrin was shown to localize in the cytoplasm and reported to be sensitive to leptomycin B (LMB), a specific inhibitor of Crm1, the regions within pericentrin that serve as signals for transporting in and out of the nucleus have not yet been identified. In this study, we identified five novel nuclear export signals (NESs) in pericentrin with diverse export activities. All of the five NESs could bind to Crm1 in a LMB-sensitive way when mediating the nuclear export of pericentrin. We also demonstrated that the region of amino acids 8-42 in pericentrin contains a tripartite nuclear localization signal (NLS) consisting of three clusters of basic amino acids. The NLS of pericentrin binds to importin beta directly or via the adaptor importin alpha to form the import complex, which could be disrupted by RanQ69L, a dominant-negative Ran GTPase possessing high affinity for importin beta. Furthermore, we found that mutation of the NESs in full-length pericentrin results in both nuclear and cytoplasmic localization, and mutation of the NLS abolishes the nuclear import of pericentrin. On the basis of these results, we suggest that the NESs and NLS of pericentrin are essential for its subcellular localization and nucleocytoplasmic trafficking during the cell cycle.

Nuclear export of the yeast hexokinase 2 protein requires the Xpo1 (Crm1)-dependent pathway

Hexokinase 2 (Hxk2) from Saccharomyces cerevisiae was one of the first metabolic enzymes described as a multifunctional protein. Hxk2 has a double subcellular localization; it functions as a glycolytic enzyme in the cytoplasm and as a regulator of gene transcription of several Mig1-regulated genes in the nucleus. However, the mechanism by which Hxk2 enters and leaves the nucleus is still unknown. In low glucose conditions, Hxk2 is phosphorylated at serine 14, but how this phosphorylation may affect glucose signaling is also unknown at the moment. Here we report that the Hxk2 protein is an export substrate of the carrier protein Xpo1 (Crm1). We also show that the Hxk2 nuclear export and the binding of Hxk2 and Xpo1 involve two leucine-rich nuclear export signals (NES) located between leucine 23 and isoleucine 33 (NES1) and between leucine 310 and leucine 318 (NES2). We also show that the Hxk2 phosphorylation at serine 14 promotes Hxk2 export by facilitating the association of Hxk2 with Xpo1. Our study uncovers a new cargo for the Xpo1 yeast exportin and identifies Hxk2 phosphorylation at serine 14 as a regulatory mechanism that controls its nuclear exit in function of the glucose levels.

Nuclear APC

Mutational inactivation of the tumor suppressor gene APC (Adenomatous polyposis coli) is thought to be an initiating step in the progression of the vast majority ofcolorectal cancers. Attempts to understand APC function have revealed more than a dozen binding partners as well as several subcellular localizations including at cell-cell junctions, associated with microtubules at the leading edge of migrating cells, at the apical membrane, in the cytoplasm and in the nucleus. The present chapter focuses on APC localization and functions in the nucleus. APC contains two classical nuclear localization signals, with a third domain that can enhance nuclear import. Along with two sets of nuclear export signals, the nuclear localization signals enable the large APC protein to shuttle between the nucleus and cytoplasm. Nuclear APC can oppose beta-catenin-mediated transcription. This down-regulation of nuclear beta-catenin activity by APC most likely involves nuclear sequestration of beta-catenin from the transcription complex as well as interaction of APC with transcription corepressor CtBP. Additional nuclear binding partners for APC include transcription factor activator protein AP-2alpha, nuclear export factor Crm1, protein tyrosine phosphatase PTP-BL and perhaps DNA itself. Interaction of APC with polymerase beta and PCNA, suggests a role for APC in DNA repair. The observation that increases in the cytoplasmic distribution of APC correlate with colon cancer progression suggests that disruption of these nuclear functions of APC plays an important role in cancer progression. APC prevalence in the cytoplasm of quiescent cells points to a potential function for nuclear APC in control of cell proliferation. Clear definition of APC's nuclear function(s) will expand the possibilities for early colorectal cancer diagnostics and therapeutics targeted to APC.

A novel function of adenomatous polyposis coli (APC) in regulating DNA repair

Prevailing literature suggests diversified cellular functions for the adenomatous polyposis coli (APC) gene. Among them a recently discovered unique role of APC is in DNA repair. The APC gene can modulate the base excision repair (BER) pathway through an interaction with DNA polymerase beta (Pol-beta) and flap endonuclease 1 (Fen-1). Taken together with the transcriptional activation of APC gene by alkylating agents and modulation of BER activity, APC may play an important role in carcinogenesis and chemotherapy by determining whether cells with DNA damage survive or undergo apoptosis. In this review, we summarize the evidence supporting this novel concept and suggest that these results will have implications for the development of more effective strategies for chemoprevention, prognosis and chemotherapy of certain types of tumors.

The APC tumor suppressor inhibits DNA replication by directly binding to DNA via its carboxyl terminus

BACKGROUND & AIMS

The APC tumor suppressor is well known for its ability to regulate Wnt signaling through mediation of beta-catenin levels in the cell. Transient over expression of the tumor suppressor gene APC in colon cancer cells prevents entry into S phase of the cell cycle, a phenotype only partially restored by cotransfection of a transcriptionally active form of beta-catenin. In an attempt to define its transcription-independent tumor suppressor functions, we tested whether APC directly affects DNA replication.

METHODS

A transcriptionally quiescent in vitro DNA replication system, the polymerase chain reaction, DNA binding assays, and transient transfections in colon cancer cell lines were used to determine the effects of APC on DNA replication and the mechanism by which it works.

RESULTS

We report that exogenous full-length APC inhibits replication of template DNA through a function that maps to amino acids 2140-2421, a region of the protein commonly lost by somatic or germline mutation. This segment of APC directly interacts with DNA, while mutation of the DNA-binding S(T)PXX motifs within it abolishes DNA binding and reduces inhibition of DNA replication. Phosphorylation of this segment by cyclin-dependent kinases also reduces inhibition of DNA replication. Furthermore, transient transfection of an APC segment encoding amino acids 2140-2421 into a colon cancer cell line with mutant APC prevents cell cycle progression into or through S phase.

CONCLUSIONS

Our results suggest that APC can negatively regulate cell cycle progression through inhibition of DNA replication by direct interaction with DNA.

Beta-catenin can bind directly to CRM1 independently of adenomatous polyposis coli, which affects its nuclear localization and LEF-1/beta-catenin-dependent gene expression

Nuclear beta-catenin affects the developmental process and progression of tumors. However, the precise mechanism for the nuclear export of beta-catenin is not completely understood. We found that beta-catenin can bind directly to CRM1 through its central armadillo (ARM) repeats region, independently of the adenomatous polyposis coli (APC) protein. CRM1 overexpression transports nuclear beta-catenin into the cytoplasm and decreases LEF-1/beta-catenin-dependent transcriptional activity, which is also affected by the co-overexpression of E-cadherin. CRM1 competed with E-cadherin and LEF-1 for binding to beta-catenin. beta-catenin could interact directly with APC through its essential sequences between amino acids 342 and 350. The site-directed beta-catenin mutant (NES2(-)), which could interact with CRM1, but not with APC, still retained its ability to export from the nucleus and its transactivational activity. This suggests that CRM1 can function as an efficient nuclear exporter for beta-catenin independently of APC. These results strongly suggest that the CRM1-mediated pathway is involved in the efficient transport of nuclear beta-catenin in the nucleus of cells.

Nucleocytoplasmic trafficking and transcription effects of huntingtin in Huntington's disease

There are nine genetic neurodegenerative diseases caused by a similar genetic defect, a CAG DNA triplet-repeat expansion in the disease gene's open reading frame resulting in a polyglutamine expansion in the disease proteins. Despite the commonality of polyglutamine expansion, each of the polyglutamine diseases manifest as unique diseases, with some similarities, but important differences. These differences suggest that the context of the polyglutamine expansion is important to the mechanism of pathology of the disease proteins. Therefore, it is becoming increasingly paramount to understand the normal functions of these polyglutamine disease proteins, which include huntingtin, the polyglutamine-expanded protein in Huntington's disease (HD). Transcriptional dysregulation is seen in HD. Here we discuss the role of normal huntingtin in transcriptional regulation and misregulation in Huntington's disease in relation to potentially analogous model systems, and to other polyglutamine disease proteins. Huntingtin has functional roles in both the cytoplasm and the nucleus. One commonality of activity of polyglutamine disease proteins is at the level of protein dynamics and ability to import and export to and from the nucleus. Knowing the temporal location of huntingtin protein in response to signaling and neuronal communication could lead to valuable insights into an important trigger of HD pathology.

Adenomatous polyposis coli (APC) plays multiple roles in the intestinal and colorectal epithelia

The adenomatous polyposis coli (APC) gene is mutated in familial adenomatous polyposis and in most sporadic colorectal tumors. During both embryonic and postnatal periods, APC is widely expressed in a variety of tissues, including the brain and gastrointestinal tract. The APC gene product (APC) is a large multidomain protein consisting of 2843 amino acids. APC downregulates the Wnt signaling pathway through its binding to beta-catenin and Axin. Most mutated APC proteins in colorectal tumors lack the beta-catenin-binding regions and fail to inhibit Wnt signaling, leading to the overproliferation of tumor cells. Several mouse models (APC580D, APCDelta716, APC1309, APCMin, APC1638T) have been established to investigate carcinogenesis caused by APC mutations. APC also binds to APC-stimulated guanine nucleotide exchange factor, the kinesin superfamily-associated protein 3, IQGAP1, microtubules, EB1, and discs large (DLG). APC has both nuclear localization signals and nuclear export signals in its molecule, suggesting its occasional nuclear localization and export of beta-catenin from the nucleus. APC is highly expressed in the intestinal and colorectal epithelia and may be involved in homeostasis of the enterocyte renewal phenomena, in which proliferation, migration, differentiation, and apoptosis are highly regulated both temporally and spatially. Through the many binding proteins mentioned, APC can exert multiple functions involved in epithelial homeostasis.

G2E3 is a nucleo-cytoplasmic shuttling protein with DNA damage responsive localization

G2E3 was originally described as a G2/M-specific gene with DNA damage responsive expression. The presence of a conserved HECT domain within the carboxy-terminus of the protein indicated that it likely functions as a ubiquitin ligase or E3. Although HECT domains are known to function in this capacity for many proteins, we demonstrate that a portion of the HECT domain from G2E3 plays an important role in the dynamic subcellular localization of the protein. We have shown that G2E3 is a nucleo-cytoplasmic shuttling protein with nuclear export mediated by a novel nuclear export domain that functions independently of CRM1. In full-length G2E3, a separate region of the HECT domain suppresses the function of the NES. Additionally, G2E3 contains a nucleolar localization signal (NoLS) in its amino terminus. Localization of G2E3 to the nucleolus is a dynamic process, and the protein delocalizes from the nucleolus rapidly after DNA damage. Cell cycle phase-specific expression and highly regulated subcellular localization of G2E3 suggest a possible role in cell cycle regulation and the cellular response to DNA damage.

Dancing in and out of the nucleus: p120ctn and the transcription factor Kaiso

The catenin p120 (hereafter p120(ctn)) was first identified as a Src kinase substrate and subsequently characterized as an Armadillo catenin member of the cell-cell adhesion cadherin-catenin complex. In the past decade, many studies have revealed roles for p120(ctn) in regulating Rho family GTPase activity and E-cadherin stability and turnover, events that occur predominantly at the plasma membrane or in the cytoplasm. However, the recent discovery of the nuclear BTB/POZ-ZF transcription factor Kaiso as a p120(ctn) binding partner, coupled with the detection of p120(ctn) in the nucleus of some cell lines and tumor tissues, suggested that like the classical beta-catenin, p120(ctn) undergoes nucleocytoplasmic trafficking and regulates gene expression. Indeed, p120(ctn) has a classic nuclear localization signal and does traffic to the nucleus. Moreover, nuclear p120(ctn) regulates Kaiso DNA-binding and transcriptional activity, similar to beta-catenin's modulation of TCF/LEF transcription activity. However unlike beta-catenin, p120(ctn) does not appear to be a transcriptional activator. Hence it remains to be determined whether the sole role of nuclear p120(ctn) is regulation of Kaiso or whether p120(ctn) binds and regulates other transcription factors or nuclear proteins.

Truncated APC regulates the transcriptional activity of beta-catenin in a cell cycle dependent manner

Most colon cancer cells express truncated versions of the tumour suppressor Adenomatous Polyposis Coli (APC). These molecules are selected during tumourigenesis for impaired beta-catenin degrading activity. In this study, we describe that truncated APC can still control the activity of beta-catenin in colon cancer cell lines via its first 20 amino acid repeat. First, we show that both endogenous and ectopically expressed truncated APC molecules can bind to beta-catenin. Second, reduction of the levels of truncated APC by RNA interference increases the activity of a beta-catenin-dependent reporter gene and stimulates the expression of the beta-catenin target gene AXIN2/conductin. This occurs without alterations of the amounts of cytosolic beta-catenin. Conversely, ectopic expression of truncated APC decreases beta-catenin-dependent transcription without affecting the intensity of immunofluorescence staining of beta-catenin in transfected cells. Third, we reveal that the APC level increases when cells reach the G1-S boundary during cell cycle progression. Simultaneously, the amount of beta-catenin bound to APC increases and the transcriptional activity of beta-catenin drops in an APC-dependent manner. Again, this occurs independently of the amounts of either total or phosphorylated cytosolic beta-catenin. Together, these results indicate that truncated APC controls the ability of beta-catenin to activate transcription. As we also show that the inhibition involves the first 20 amino acid repeat of APC, our data suggest that colon cancer cells retain a truncated APC molecule containing at least the first 20 amino acid repeat to modulate the transcriptional activity of beta-catenin in a cell cycle-dependent manner.

Role of nucleotide excision repair deficiency in intestinal tumorigenesis in multiple intestinal neoplasia (Min) mice

Mice deficient in the Xeroderma pigmentosum group A (Xpa) gene are defective in nucleotide excision repair (NER) and highly susceptible to skin carcinogenesis after dermal exposure to UV light or chemicals. Min (multiple intestinal neoplasia) mice, heterozygous for a germline nonsense mutation in the tumor suppressor gene adenomatous polyposis coli (Apc), develop intestinal tumors spontaneously and show additional intestinal tumors after exposure to the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). In this study, we investigated the impact of loss of XPA function on PhIP-induced intestinal tumorigenesis in F1 offspring of Min/+ (Apc(+/-)) mice crossed with Xpa gene-deficient mice. Apc(+/-) mice lacking both alleles of Xpa had higher susceptibility towards toxicity of PhIP, higher levels of PhIP-DNA adducts in the middle and distal small intestines, as well as in liver, and a higher number of small intestinal tumors at 11 weeks, compared with Apc(+/-) mice with one or two intact Xpa alleles. Localization of tumors was not affected, being highest in middle and distal small intestines in all genotypes. At 11 weeks of age, the number of spontaneous intestinal tumors was not significantly increased by homozygous loss of Xpa, but untreated Apc(+/-)/Xpa(-/-) mice had significantly shorter life-spans than their XPA-proficient littermates. Heterozygous loss of Xpa did not affect any of the measured end points. In conclusion, the Xpa gene and the NER pathway are involved in repair of bulky PhIP-DNA adducts in the intestines and the liver, and most probably of DNA lesions leading to spontaneous intestinal tumors. These results confirm a role of the NER pathway also in protection against cancer in internal organs, additional to its well-known importance in protection against skin cancer. An effect of Apc(+/-) on adduct levels, additional to that of Xpa(-/-), indicates that the truncated APC protein may affect a repair pathway other than NER.