APC and beta-catenin degradation

Mechanism of APC truncation involved in colorectal cancer tumorigenesis (Review)

Adenomatous polyposis coli (APC) is widely recognized as a heavily mutated gene that suppresses tumor growth in colorectal cancer (CRC). Its mutation is considered to be the primary and early event that occurs in the development of CRC. In addition, APC has a crucial role in inhibiting the canonical Wnt signaling pathway. APC mutations in CRC result in the production of shortened gene products. This impairment of β-catenin destruction complexes causes an accumulation of active β-catenin in the cytoplasm and nucleus. In these compartments, β-catenin can bind with DNA-binding proteins of the transcription factor/lymphoid enhancer-binding factor family, thereby activating the Wnt signaling pathway. Consequently, the balance of numerous cellular processes is disrupted, ultimately driving the formation of tumors. There is a growing body of evidence indicating the prevalent occurrence of APC truncation in the majority of CRC cases. Furthermore, it has been observed that these truncated proteins have a crucial role in the activation of the Wnt signaling pathway and the subsequent loss of tumor inhibitory function. This review aimed to provide an overview of the recent advancements in understanding the mechanism behind APC truncation and its association with the onset and progression of CRC.

The Metabolic Fates of Pyruvate in Normal and Neoplastic Cells

Pyruvate occupies a central metabolic node by virtue of its position at the crossroads of glycolysis and the tricarboxylic acid (TCA) cycle and its production and fate being governed by numerous cell-intrinsic and extrinsic factors. The former includes the cell’s type, redox state, ATP content, metabolic requirements and the activities of other metabolic pathways. The latter include the extracellular oxygen concentration, pH and nutrient levels, which are in turn governed by the vascular supply. Within this context, we discuss the six pathways that influence pyruvate content and utilization: 1. The lactate dehydrogenase pathway that either converts excess pyruvate to lactate or that regenerates pyruvate from lactate for use as a fuel or biosynthetic substrate; 2. The alanine pathway that generates alanine and other amino acids; 3. The pyruvate dehydrogenase complex pathway that provides acetyl-CoA, the TCA cycle’s initial substrate; 4. The pyruvate carboxylase reaction that anaplerotically supplies oxaloacetate; 5. The malic enzyme pathway that also links glycolysis and the TCA cycle and generates NADPH to support lipid bio-synthesis; and 6. The acetate bio-synthetic pathway that converts pyruvate directly to acetate. The review discusses the mechanisms controlling these pathways, how they cross-talk and how they cooperate and are regulated to maximize growth and achieve metabolic and energetic harmony.

Low penetrance genetic polymorphisms as potential biomarkers for colorectal cancer predisposition

Colorectal cancer is a leading form of cancer in both males and females. Early detection of individuals at risk of colorectal cancer allows proper treatment and management of the disease to be implemented, which can potentially reduce the burden of colorectal cancer incidence, morbidity and mortality. In recent years, the role of genetic susceptibility factors in mediating predisposition to colorectal cancer has become more and more apparent. Identification of high-frequency, low-penetrance genetic polymorphisms associated with the cancer has therefore emerged as an important approach which can potentially aid prediction of colorectal cancer risk. However, the overwhelming amount of genetic epidemiology data generated over the past decades has made it difficult for one to assimilate the information and determine the exact genetic polymorphisms that can potentially be used as biomarkers for colorectal cancer. This review comprehensively consolidates, based primarily on results from meta-analyses, the recent progresses in the search of colorectal cancer-associated genetic polymorphisms, and discusses the possible mechanisms involved.

Paracrine and autocrine regulation of gene expression by Wnt-inhibitor Dickkopf in wild-type and mutant hepatocytes

Background

Cells are able to communicate and coordinate their function within tissues via secreted factors. Aberrant secretion by cancer cells can modulate this intercellular communication, in particular in highly organised tissues such as the liver. Hepatocytes, the major cell type of the liver, secrete Dickkopf (Dkk), which inhibits Wnt/ β-catenin signalling in an autocrine and paracrine manner. Consequently, Dkk modulates the expression of Wnt/ β-catenin target genes. We present a mathematical model that describes the autocrine and paracrine regulation of hepatic gene expression by Dkk under wild-type conditions as well as in the presence of mutant cells.

Results

Our spatial model describes the competition of Dkk and Wnt at receptor level, intra-cellular Wnt/ β-catenin signalling, and the regulation of target gene expression for 21 individual hepatocytes. Autocrine and paracrine regulation is mediated through a feedback mechanism via Dkk and Dkk diffusion along the porto-central axis. Along this axis an APC concentration gradient is modelled as experimentally detected in liver. Simulations of mutant cells demonstrate that already a single mutant cell increases overall Dkk concentration. The influence of the mutant cell on gene expression of surrounding wild-type hepatocytes is limited in magnitude and restricted to hepatocytes in close proximity. To explore the underlying molecular mechanisms, we perform a comprehensive analysis of the model parameters such as diffusion coefficient, mutation strength and feedback strength.

Conclusions

Our simulations show that Dkk concentration is elevated in the presence of a mutant cell. However, the impact of these elevated Dkk levels on wild-type hepatocytes is confined in space and magnitude. The combination of inter- and intracellular processes, such as Dkk feedback, diffusion and Wnt/ β-catenin signal transduction, allow wild-type hepatocytes to largely maintain their gene expression.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-017-0470-9) contains supplementary material, which is available to authorized users.

Genetic differences stratified by PCR-based microsatellite analysis in gastric intramucosal neoplasia

BACKGROUND

Although genetic alterations in patients with advanced gastric cancer have been extensively studied, those in patients with intramucosal neoplasia (IMN) are still poorly understood.

METHODS

We evaluated genetic differences in 158 IMNs, including 51 low-grade dysplasias, 58 high-grade dysplasias (HGDs), 30 intramucosal cancers (IMCs), and 19 mixed tumors (composed of IMC and HGD within the same tumor), using PCR-based microsatellite analysis [allelic imbalance (AI) and microsatellite instability (MSI)]. We classified the DNA methylation status as a hypermethylated epigenome, a moderately methylated epigenome, or a hypomethylated epigenome. In addition, p53 overexpression, β-catenin nuclear localization, and mucin expression were also examined.

RESULTS

From cluster analysis, the IMNs examined were categorized into four subgroups as follows. Tumors in subgroup 1 were characterized by MSI-high status, a hypermethylated epigenome, and loss or reduction of expression of MLH-1. Tumors in subgroup 2 showed a mixed pattern consisting of AI and MSI. In contrast, tumors in subgroup 3, which showed accumulation of multiple AIs, were closely associated with HGD, IMC, or mixed tumor and exhibited nuclear expression of β-catenin. Tumors in subgroup 4, which were generally low-grade dysplasias, exhibited a low frequency of AIs and no MSI. Although the mucin phenotype was not correlated with any subgroup, expression of mucin was associated with some subgroups. Overexpression of p53 was common in all subgroups.

CONCLUSION

The approach described herein was useful for studying genetic differences in IMNs. In addition, we suggest that stratification of genetic differences may help to identify genetic molecular profiles in IMNs.

Modulation of Inflammatory Responses by Wnt/β-Catenin Signaling in Dendritic Cells: A Novel Immunotherapy Target for Autoimmunity and Cancer

The Wnt/β-catenin pathway is an evolutionarily conserved signaling pathway critical for several biological processes. An aberrant Wnt/β-catenin signaling is linked to several human diseases. Emerging studies have highlighted the regulatory role of the Wnt/β-catenin signaling pathway in normal physiological processes of parenchymal and hematopoietic cells. Recent studies have shown that the activation of Wnt/β-catenin pathway in dendritic cells (DCs) play a critical role in mucosal tolerance and suppression of chronic autoimmune pathologies. Alternatively, tumors activate Wnt/β-catenin pathway in DCs to induce immune tolerance and thereby evade antitumor immunity through suppression of effector T cell responses and promotion of regulatory T cell responses. Here, we review our work and current understanding of how Wnt/β-catenin signaling in DCs shapes the immune response in cancer and autoimmunity and discuss how Wnt/β-catenin pathway can be targeted for successful therapeutic interventions in various human diseases.

Targeting phospholipase D1 attenuates intestinal tumorigenesis by controlling β-catenin signaling in cancer-initiating cells

Kang et al. show that genetic or pharmacological inactivation of the enzyme phospholipase D1 (PLD1) disrupts colitis-associated intestinal tumorigenesis by suppressing the self-renewal capacity of colon cancer stem cells.

Expression of the Wnt target gene phospholipase D1 (PLD1) is up-regulated in various carcinomas, including colorectal cancer (CRC). However, the mechanistic significance of its elevated expression in intestinal tumorigenesis remains unknown. In this study, we show that genetic and pharmacological targeting of PLD1 disrupts spontaneous and colitis-associated intestinal tumorigenesis in Apc^Min/+ and azoxymethane/dextran sodium sulfate mice models. Intestinal epithelial cell–specific PLD1 overexpression in Apc^Min/+ mice accelerated tumorigenesis with increased proliferation and nuclear β-catenin levels compared with Apc^Min/+ mice. Moreover, PLD1 inactivation suppressed the self-renewal capacity of colon cancer–initiating cells (CC-ICs) by decreasing expression of β-catenin via E2F1-induced microRNA (miR)-4496 up-regulation. Ultimately, low expression of PLD1 coupled with a low level of CC-IC markers was predictive of a good prognosis in CRC patients, suggesting in vivo relevance. Collectively, our data reveal that PLD1 has a crucial role in intestinal tumorigenesis via its modulation of the E2F1–miR-4496–β-catenin signaling pathway. Modulation of PLD1 expression and activity represents a promising therapeutic strategy for the treatment of intestinal tumorigenesis.

Shaping of interphase chromosomes by the microtubule network

It is well established that microtubule dynamics play a major role in chromosome condensation and localization during mitosis. During interphase, however, it is assumed that the metazoan nuclear envelope presents a physical barrier, which inhibits interaction between the microtubules located in the cytoplasm and the chromatin fibers located in the nucleus. In recent years, it has become apparent that microtubule dynamics alter chromatin structure and function during interphase as well. Microtubule motor proteins transport several transcription factors and exogenous DNA (such as plasmid DNA) from the cytoplasm to the nucleus. Various soluble microtubule components are able to translocate into the nucleus, where they bind various chromatin elements leading to transcriptional alterations. In addition, microtubules may apply force on the nuclear envelope, which is transmitted into the nucleus, leading to changes in chromatin structure. Thus, microtubule dynamics during interphase may affect chromatin spatial organization, as well as transcription, replication and repair.

The Wnt/β-catenin signaling pathway controls the inflammatory response in infections caused by pathogenic bacteria

Innate immunity against pathogenic bacteria is critical to protect host cells from invasion and infection as well as to develop an appropriate adaptive immune response. During bacterial infection, different signaling transduction pathways control the expression of a wide range of genes that orchestrate a number of molecular and cellular events to eliminate the invading microorganisms and regulate inflammation. The inflammatory response must be tightly regulated because uncontrolled inflammation may lead to tissue injury. Among the many signaling pathways activated, the canonical Wnt/β-catenin has been recently shown to play an important role in the expression of several inflammatory molecules during bacterial infections. Our main goal in this review is to discuss the mechanism used by several pathogenic bacteria to modulate the inflammatory response through the Wnt/β-catenin signaling pathway. We think that a deep insight into the role of Wnt/β-catenin signaling in the inflammation may open new venues for biotechnological approaches designed to control bacterial infectious diseases.

The RNA-binding protein Fus directs translation of localized mRNAs in APC-RNP granules

RNA localization pathways direct numerous mRNAs to distinct subcellular regions and affect many physiological processes. In one such pathway the tumor-suppressor protein adenomatous polyposis coli (APC) targets RNAs to cell protrusions, forming APC-containing ribonucleoprotein complexes (APC-RNPs). Here, we show that APC-RNPs associate with the RNA-binding protein Fus/TLS (fused in sarcoma/translocated in liposarcoma). Fus is not required for APC-RNP localization but is required for efficient translation of associated transcripts. Labeling of newly synthesized proteins revealed that Fus promotes translation preferentially within protrusions. Mutations in Fus cause amyotrophic lateral sclerosis (ALS) and the mutant protein forms inclusions that appear to correspond to stress granules. We show that overexpression or mutation of Fus results in formation of granules, which preferentially recruit APC-RNPs. Remarkably, these granules are not translationally silent. Instead, APC-RNP transcripts are translated within cytoplasmic Fus granules. These results unexpectedly show that translation can occur within stress-like granules. Importantly, they identify a new local function for cytoplasmic Fus with implications for ALS pathology.

Genomics and epigenomics of colorectal cancer

Colorectal cancer is one of the most common cancer types worldwide and accounts for approximately 600,000 deaths annually. Work over the last decades has uncovered a number of tumor-suppressor and oncogenes which are frequently mutated and might thus be responsible for the malignant transformation. However, only with the development of new high-throughput technologies systematic analyses of the genome and epigenomes became feasible. While data generation has increased exponential, we are now faced with new challenges to transform these data into useful models that help predicting the outcome of genomic aberrations and to develop novel diagnostic and therapeutic strategies. As a basis for the modeling it is essential to understand and integrate current knowledge. We review previous and current ideas in colorectal cancer development and focus on a pathway oriented view. We show that colorectal cancer is a multilayer complex disease affecting the genome as well as the epigenome with direct consequences on the gene and microRNA (miRNA) expression signatures. The goal is to illustrate the current principles of colorectal cancer pathogenesis and to illustrate the need for elaborate computer modeling systems.

Destruction complex function in the Wnt signaling pathway of Drosophila requires multiple interactions between Adenomatous polyposis coli 2 and Armadillo

The tumor suppressor Adenomatous polyposis coli (APC) negatively regulates Wnt signaling through its activity in the destruction complex. APC binds directly to the main effector of the pathway, β-catenin (βcat, Drosophila Armadillo), and helps to target it for degradation. In vitro studies demonstrated that a nonphosphorylated 20-amino-acid repeat (20R) of APC binds to βcat through the N-terminal extended region of a 20R. When phosphorylated, the phospho-region of an APC 20R also binds βcat and the affinity is significantly increased. These distinct APC-βcat interactions suggest different models for the sequential steps of destruction complex activity. However, the in vivo role of 20R phosphorylation and extended region interactions has not been rigorously tested. Here we investigated the functional role of these molecular interactions by making targeted mutations in Drosophila melanogaster APC2 that disrupt phosphorylation and extended region interactions and deletion mutants missing the Armadillo binding repeats. We tested the ability of these mutants to regulate Wnt signaling in APC2 null and in APC2 APC1 double-null embryos. Overall, our in vivo data support the role of phosphorylation and extended region interactions in APC2's destruction complex function, but suggest that the extended region plays a more significant functional role. Furthermore, we show that the Drosophila 20Rs with homology to the vertebrate APC repeats that have the highest affinity for βcat are functionally dispensable, contrary to biochemical predictions. Finally, for some mutants, destruction complex function was dependent on APC1, suggesting that APC2 and APC1 may act cooperatively in the destruction complex.

How do they do Wnt they do?: regulation of transcription by the Wnt/β-catenin pathway

Wnt/β-catenin signalling is known to play many roles in metazoan development and tissue homeostasis. Misregulation of the pathway has also been linked to many human diseases. In this review, specific aspects of the pathway's involvement in these processes are discussed, with an emphasis on how Wnt/β-catenin signalling regulates gene expression in a cell and temporally specific manner. The T-cell factor (TCF) family of transcription factors, which mediate a large portion of Wnt/β-catenin signalling, will be discussed in detail. Invertebrates contain a single TCF gene that contains two DNA-binding domains, the high mobility group (HMG) domain and the C-clamp, which increases the specificity of DNA binding. In vertebrates, the situation is more complex, with four TCF genes producing many isoforms that contain the HMG domain, but only some of which possess a C-clamp. Vertebrate TCFs have been reported to act in concert with many other transcription factors, which may explain how they obtain sufficient specificity for specific DNA sequences, as well as how they achieve a wide diversity of transcriptional outputs in different cells.

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.

Adenomatous polyposis coli regulates axon arborization and cytoskeleton organization via its N-terminus

Conditional deletion of APC leads to marked disruption of cortical development and to excessive axonal branching of cortical neurons. However, little is known about the cell biological basis of this neuronal morphological regulation. Here we show that APC deficient cortical neuronal growth cones exhibit marked disruption of both microtubule and actin cytoskeleton. Functional analysis of the different APC domains revealed that axonal branches do not result from stabilized β-catenin, and that the C-terminus of APC containing microtubule regulatory domains only partially rescues the branching phenotype. Surprisingly, the N-terminus of APC containing the oligomerization domain and the armadillo repeats completely rescues the branching and cytoskeletal abnormalities. Our data indicate that APC is required for appropriate axon morphological development and that the N-terminus of APC is important for regulation of the neuronal cytoskeleton.

The oligomeric state of CtBP determines its role as a transcriptional co-activator and co-repressor of Wingless targets

C-terminal-binding protein (CtBP) is a well-characterized transcriptional co-repressor that requires homo-dimerization for its activity. CtBP can both repress and activate Wingless nuclear targets in Drosophila. Here, we examine the role of CtBP dimerization in these opposing processes. CtBP mutants that cannot dimerize are able to promote Wingless signalling, but are defective in repressing Wingless targets. To further test the role of dimerization in repression, the positions of basic and acidic residues that form inter-molecular salt bridges in the CtBP dimerization interface were swapped. These mutants cannot homo-dimerize and are compromised for repression. However, their co-expression leads to hetero-dimerization and consequent repression of Wingless targets. Our results support a model where CtBP is a gene-specific regulator of Wingless signalling, with some targets requiring CtBP dimers for inhibition while other targets utilize CtBP monomers for activation of their expression. Functional interactions between CtBP and Pygopus, a nuclear protein required for Wingless signalling, support a model where monomeric CtBP acts downstream of Pygopus in activating some Wingless targets.

FBXW7 influences murine intestinal homeostasis and cancer, targeting Notch, Jun, and DEK for degradation

The E3 ubiquitin ligase component FBXW7 modulates homeostasis and inhibits tumorigenesis in the murine intestine.

The Fbxw7 (F-box/WD repeat–containing protein 7; also called CDC4, Sel10, Ago, and Fbw7) component of the SCF (Skp1/Cullin/F-box protein) E3 ubiquitin ligase complex acts as a tumor suppressor in several tissues and targets multiple transcriptional activators and protooncogenes for ubiquitin-mediated degradation. To understand Fbxw7 function in the murine intestine, in this study, we specifically deleted Fbxw7 in the murine gut using Villin-Cre (Fbxw7^ΔG). In wild-type mice, loss of Fbxw7 in the gut altered homeostasis of the intestinal epithelium, resulted in elevated Notch and c-Jun expression, and induced development of adenomas at 9–10 mo of age. In the context of APC (adenomatous polyposis coli) deficiency (Apc^Min/+ mice), loss of Fbxw7 accelerated intestinal tumorigenesis and death and promoted accumulation of β-catenin in adenomas at late but not early time points. At early time points, Fbxw7 mutant tumors showed accumulation of the DEK protooncogene. DEK expression promoted cell division and altered splicing of tropomyosin (TPM) RNA, which may also influence cell proliferation. DEK accumulation and altered TPM RNA splicing were also detected in FBXW7 mutant human colorectal tumor tissues. Given their reduced lifespan and increased incidence of intestinal tumors, Apc^Min/+Fbxw7^ΔG mice may be used for testing carcinogenicity and drug screening.

Cell regulation by the Apc protein Apc as master regulator of epithelia

The adenomatous polyposis coli (Apc) protein participates in many of the fundamental cellular processes that govern epithelial tissues: Apc is directly involved in regulating the availability of beta-catenin for transcriptional de-repression of Tcf/LEF transcription factors, it contributes to the stability of microtubules in interphase and mitosis, and has an impact on the dynamics of F-actin. Thus Apc contributes directly and/or indirectly to proliferation, differentiation, migration, and apoptosis. This particular multifunctionality can explain why disruption of Apc is especially detrimental for the epithelium of the gut, where Apc mutations are common in most cancers. We summarise recent data that shed light on the molecular mechanisms involved in the different functions of Apc.