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

From Crypts to Cancer: A Holistic Perspective on Colorectal Carcinogenesis and Therapeutic Strategies

Colorectal cancer (CRC) represents a significant global health burden, with high incidence and mortality rates worldwide. Recent progress in research highlights the distinct clinical and molecular characteristics of colon versus rectal cancers, underscoring tumor location's importance in treatment approaches. This article provides a comprehensive review of our current understanding of CRC epidemiology, risk factors, molecular pathogenesis, and management strategies. We also present the intricate cellular architecture of colonic crypts and their roles in intestinal homeostasis. Colorectal carcinogenesis multistep processes are also described, covering the conventional adenoma-carcinoma sequence, alternative serrated pathways, and the influential Vogelstein model, which proposes sequential APC, KRAS, and TP53 alterations as drivers. The consensus molecular CRC subtypes (CMS1-CMS4) are examined, shedding light on disease heterogeneity and personalized therapy implications.

The FABD domain is critical for the oncogenicity of BCR/ABL in chronic myeloid leukaemia

BACKGROUND

Abnormally expressed BCR/ABL protein serves as the basis for the development of chronic myeloid leukaemia (CML). The F-actin binding domain (FABD), which is a crucial region of the BCR/ABL fusion protein, is also located at the carboxyl end of the c-ABL protein and regulates the kinase activity of c-ABL. However, the precise function of this domain in BCR/ABL remains uncertain.

METHODS

The FABD-deficient adenovirus vectors Ad-BCR/ABL△FABD, wild-type Ad-BCR/ABL and the control vector Adtrack were constructed, and 32D cells were infected with these adenoviruses separately. The effects of FABD deletion on the proliferation and apoptosis of 32D cells were evaluated by a CCK-8 assay, colony formation assay, flow cytometry and DAPI staining. The levels of phosphorylated BCR/ABL, p73, and their downstream signalling molecules were detected by western blot. The intracellular localization and interaction of BCR/ABL with the cytoskeleton-related protein F-actin were identified by immunofluorescence and co-IP. The effect of FABD deletion on BCR/ABL carcinogenesis in vivo was explored in CML-like mouse models. The degree of leukaemic cell infiltration was observed by Wright‒Giemsa staining and haematoxylin and eosin (HE) staining.

RESULTS

We report that the loss of FABD weakened the proliferation-promoting ability of BCR/ABL, accompanied by the downregulation of BCR/ABL downstream signals. Moreover, the deletion of FABD resulted in a change in the localization of BCR/ABL from the cytoplasm to the nucleus, accompanied by an increase in cell apoptosis due to the upregulation of p73 and its downstream proapoptotic factors. Furthermore, we discovered that the absence of FABD alleviated leukaemic cell infiltration induced by BCR/ABL in mice.

CONCLUSIONS

These findings reveal that the deletion of FABD diminished the carcinogenic potential of BCR/ABL both in vitro and in vivo. This study provides further insight into the function of the FABD domain in BCR/ABL.

Mouse Models for Application in Colorectal Cancer: Understanding the Pathogenesis and Relevance to the Human Condition

Colorectal cancer (CRC) is a malignant disease that is the second most common cancer worldwide. CRC arises from the complex interactions among a variety of genetic and environmental factors. To understand the mechanism of colon tumorigenesis, preclinical studies have developed various mouse models including carcinogen-induced and transgenic mice to recapitulate CRC in humans. Using these mouse models, scientific breakthroughs have been made on the understanding of the pathogenesis of this complex disease. Moreover, the availability of transgenic knock-in or knock-out mice further increases the potential of CRC mouse models. In this review, the overall features of carcinogen-induced (focusing on azoxymethane and azoxymethane/dextran sulfate sodium) and transgenic (focusing on Apc^Min/+) mouse models, as well as their mechanisms to induce colon tumorigenesis, are explored. We also discuss limitations of these mouse models and their applications in the evaluation and study of drugs and treatment regimens against CRC. Through these mouse models, a better understanding of colon tumorigenesis can be achieved, thereby facilitating the discovery of novel therapeutic strategies against CRC.

A perspective on medicinal chemistry approaches towards adenomatous polyposis coli and Wnt signal based colorectal cancer inhibitors

Colorectal cancer (CRC) is one of the major causes of carcinogenic mortality in numbers only after lung and breast cancers. The mutations in adenomatous polyposis coli (APC) gene leads to formation of colorectal polyps in the colonic region and which develop as a malignant tumour upon coalition with patient related risk factors. The protein-protein interaction (PPI) of APC with Asef (A Rac specific guanine nucleotide exchange factor) overwhelms the patient's conditions by rapidly spreading in the entire colorectal region. Most mutations in APC gene occur in mutated cluster region (MCR), where it specifically binds with the cytosolic β-catenin to regulate the Wnt signalling pathway required for CRC cell adhesion, invasion, progression, differentiation and stemness in initial cell cycle phages. The current broad spectrum perspective is attempted to elaborate the sources of identification, development of selective APC inhibitors by targeting emopamil-binding protein (EBP) & dehydrocholesterol reductase-7 & 24 (DHCR-7 & 24); APC-Asef, β-catenin/APC, Wnt/β-catenin, β-catenin/TCF4 PPI inhibitors with other vital Wnt signal cellular proteins and APC/Pol-β interface of colorectal cancer. In this context, this perspective would serve as a platform for design of new medicinal agents by targeting cellular essential components which could accelerate anti-colorectal potential candidates.

Functions of the APC tumor suppressor protein dependent and independent of canonical WNT signaling: implications for therapeutic targeting

The acquisition of biallelic mutations in the APC gene is a rate-limiting step in the development of most colorectal cancers and occurs in the earliest lesions. APC encodes a 312-kDa protein that localizes to multiple subcellular compartments and performs diverse functions. APC participates in a cytoplasmic complex that promotes the destruction of the transcriptional licensing factor β-catenin; APC mutations that abolish this function trigger constitutive activation of the canonical WNT signaling pathway, a characteristic found in almost all colorectal cancers. By negatively regulating canonical WNT signaling, APC counteracts proliferation, promotes differentiation, facilitates apoptosis, and suppresses invasion and tumor progression. APC further antagonizes canonical WNT signaling by interacting with and counteracting β-catenin in the nucleus. APC also suppresses tumor initiation and progression in the colorectal epithelium through functions that are independent of canonical WNT signaling. APC regulates the mitotic spindle to facilitate proper chromosome segregation, localizes to the cell periphery and cell protrusions to establish cell polarity and appropriate directional migration, and inhibits DNA replication by interacting directly with DNA. Mutations in APC are often frameshifts, insertions, or deletions that introduce premature stop codons and lead to the production of truncated APC proteins that lack its normal functions and possess tumorigenic properties. Therapeutic approaches in development for the treatment of APC-deficient tumors are focused on the inhibition of canonical WNT signaling, especially through targets downstream of APC in the pathway, or on the restoration of wild-type APC expression.

Colorectal cancer: the APC-lncRNA link

The adenomatous polyposis coli (APC) gene plays, among other things, a crucial role in the regulation of cell proliferation and survival through its ability to regulate canonical Wnt signaling. In this issue of the JCI, Wang et al. provide an intriguing new mechanism for APC function involving the regulation of a novel long noncoding RNA (lncRNA), leading to changes in exosome production. APC signaling via this novel pathway can regulate cell proliferation and invasion as well as angiogenesis. In addition to enhancing our understanding of APC function, this new mechanism is of particular clinical significance, as it may provide additional targets for the treatment of APC-mutated cancers.

Mutational Mechanisms That Activate Wnt Signaling and Predict Outcomes in Colorectal Cancer Patients

APC biallelic loss-of-function mutations are the most prevalent genetic changes in colorectal tumors, but it is unknown whether these mutations phenocopy gain-of-function mutations in the CTNNB1 gene encoding β-catenin that also activate canonical WNT signaling. Here we demonstrate that these two mutational mechanisms are not equivalent. Furthermore, we show how differences in gene expression produced by these different mechanisms can stratify outcomes in more advanced human colorectal cancers. Gene expression profiling in Apc-mutant and Ctnnb1-mutant mouse colon adenomas identified candidate genes for subsequent evaluation of human TCGA (The Cancer Genome Atlas) data for colorectal cancer outcomes. Transcriptional patterns exhibited evidence of activated canonical Wnt signaling in both types of adenomas, with Apc-mutant adenomas also exhibiting unique changes in pathways related to proliferation, cytoskeletal organization, and apoptosis. Apc-mutant adenomas were characterized by increased expression of the glial nexin Serpine2, the human ortholog, which was increased in advanced human colorectal tumors. Our results support the hypothesis that APC-mutant colorectal tumors are transcriptionally distinct from APC-wild-type colorectal tumors with canonical WNT signaling activated by other mechanisms, with possible implications for stratification and prognosis.Significance: These findings suggest that colon adenomas driven by APC mutations are distinct from those driven by WNT gain-of-function mutations, with implications for identifying at-risk patients with advanced disease based on gene expression patterns. Cancer Res; 78(3); 617-30. ©2017 AACR.

Interaction between APC and Fen1 during breast carcinogenesis

Aberrant DNA base excision repair (BER) contributes to malignant transformation. However, inter-individual variations in DNA repair capacity plays a key role in modifying breast cancer risk. We review here emerging evidence that two proteins involved in BER - adenomatous polyposis coli (APC) and flap endonuclease 1 (Fen1) - promote the development of breast cancer through novel mechanisms. APC and Fen1 expression and interaction is increased in breast tumors versus normal cells, APC interacts with and blocks Fen1 activity in Pol-β-directed LP-BER, and abrogation of LP-BER is linked with cigarette smoke condensate-induced transformation of normal breast epithelial cells. Carcinogens increase expression of APC and Fen1 in spontaneously immortalized human breast epithelial cells, human colon cancer cells, and mouse embryonic fibroblasts. Since APC and Fen1 are tumor suppressors, an increase in their levels could protect against carcinogenesis; however, this does not seem to be the case. Elevated Fen1 levels in breast and lung cancer cells may reflect the enhanced proliferation of cancer cells or increased DNA damage in cancer cells compared to normal cells. Inactivation of the tumor suppressor functions of APC and Fen1 is due to their interaction, which may act as a susceptibility factor for breast cancer. The increased interaction of APC and Fen1 may occur due to polypmorphic and/or mutational variation in these genes. Screening of APC and Fen1 polymorphic and/or mutational variations and APC/Fen1 interaction may permit assessment of individual DNA repair capability and the risk for breast cancer development. Such individuals might lower their breast cancer risk by reducing exposure to carcinogens. Stratifying individuals according to susceptibility would greatly assist epidemiologic studies of the impact of suspected environmental carcinogens. Additionally, a mechanistic understanding of the interaction of APC and Fen1 may provide the basis for developing new and effective targeted chemopreventive and chemotherapeutic agents.

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.

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.

β-Catenin Signaling and Roles in Liver Homeostasis, Injury, and Tumorigenesis

β-catenin (encoded by CTNNB1) is a subunit of the cell surface cadherin protein complex that acts as an intracellular signal transducer in the WNT signaling pathway; alterations in its activity have been associated with the development of hepatocellular carcinoma and other liver diseases. Other than WNT, additional signaling pathways also can converge at β-catenin. β-catenin also interacts with transcription factors such as T-cell factor, forkhead box protein O, and hypoxia inducible factor 1α to regulate the expression of target genes. We discuss the role of β-catenin in metabolic zonation of the adult liver. β-catenin also regulates the expression of genes that control metabolism of glucose, nutrients, and xenobiotics; alterations in its activity may contribute to the pathogenesis of nonalcoholic steatohepatitis. Alterations in β-catenin signaling may lead to activation of hepatic stellate cells, which is required for fibrosis. Many hepatic tumors such as hepatocellular adenomas, hepatocellular cancers, and hepatoblastomas have mutations in CTNNB1 that result in constitutive activation of β-catenin, so this molecule could be a therapeutic target. We discuss how alterations in β-catenin activity contribute to liver disease and how these might be used in diagnosis and prognosis, as well as in the development of therapeutics.

APC is an RNA-binding protein, and its interactome provides a link to neural development and microtubule assembly

Adenomatous polyposis coli (APC) is a microtubule plus-end scaffolding protein important in biology and disease. APC is implicated in RNA localization, although the mechanisms and functional significance remain unclear. We show APC is an RNA-binding protein and identify an RNA interactome by HITS-CLIP. Targets were highly enriched for APC-related functions, including microtubule organization, cell motility, cancer, and neurologic disease. Among the targets is β2B-tubulin, known to be required in human neuron and axon migration. We show β2B-tubulin is synthesized in axons and localizes preferentially to dynamic microtubules in the growth cone periphery. APC binds the β2B-tubulin 3' UTR; experiments interfering with this interaction reduced β2B-tubulin mRNA axonal localization and expression, depleted dynamic microtubules and the growth cone periphery, and impaired neuron migration. These results identify APC as a platform binding functionally related protein and RNA networks, and suggest a self-organizing model for the microtubule to localize synthesis of its own subunits.

A pilot study evaluating genetic alterations that drive tobacco- and betel quid-associated oral cancer in Northeast India

The susceptibility of an individual to oral cancer is mediated by genetic factors and carcinogen-exposure behaviors such as betel quid chewing, tobacco use, and alcohol consumption. This pilot study was aimed to identify the genetic alteration in 100 bp upstream and downstream flanking regions in addition to the exonic regions of 169 cancer-associated genes by using Next Generation sequencing with aim to elucidate the molecular pathogenesis of tobacco- and betel quid-associated oral cancer of Northeast India. To understand the role of chemical compounds present in tobacco and betel quid associated with the progression of oral cancer, single nucleotide polymorphisms (SNPs) and insertion and deletion (Indels) found in this study were analyzed for their association with chemical compounds found in tobacco and betel quid using Comparative Toxogenomic Database. Genes (AR, BRCA1, IL8, and TP53) with novel SNP were found to be associated with arecoline which is the major component of areca nut. Genes (BARD1, BRCA2, CCND2, IGF1R, MSH6, and RASSF1) with novel deletion and genes (APC, BRMS1, CDK2AP1, CDKN2B, GAS1, IGF1R, and RB1) with novel insertion were found to be associated with aflatoxin B1 which is produced by fermented areca nut. Genes (ADH6, APC, AR, BARD1, BRMS1, CDKN1A, E2F1, FGFR4, FLNC, HRAS, IGF1R, IL12B, IL8, NBL1, STAT5B, and TP53) with novel SNP were found to be associated with aflatoxin B1. Genes (ATM, BRCA1, CDKN1A, EGFR, IL8, and TP53) with novel SNP were found to be associated with tobacco specific nitrosamines.

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.

MicroRNA-135b promotes cancer progression by acting as a downstream effector of oncogenic pathways in colon cancer

MicroRNA deregulation is frequent in human colorectal cancers (CRCs), but little is known as to whether it represents a bystander event or actually drives tumor progression in vivo. We show that miR-135b overexpression is triggered in mice and humans by APC loss, PTEN/PI3K pathway deregulation, and SRC overexpression and promotes tumor transformation and progression. We show that miR-135b upregulation is common in sporadic and inflammatory bowel disease-associated human CRCs and correlates with tumor stage and poor clinical outcome. Inhibition of miR-135b in CRC mouse models reduces tumor growth by controlling genes involved in proliferation, invasion, and apoptosis. We identify miR-135b as a key downsteam effector of oncogenic pathways and a potential target for CRC treatment.

The transcription factor c-Jun protects against liver damage following activated β-Catenin signaling

Background

The Wnt/β-Catenin signaling pathway is central for liver functions and frequently deregulated in hepatocellular carcinoma (HCC). Analysis of the early phenotypes and molecular events following β-Catenin activation is therefore essential for better understanding HCC pathogenesis. The AP-1 transcription factor c-Jun is a putative β-Catenin target gene and promotes hepatocyte survival, proliferation, and liver tumorigenesis, suggesting that c-Jun may be a key target of β-Catenin signaling in the liver.

Methodology/Principal Findings

To address this issue, the immediate hepatic phenotypes following deletion of the tumor suppressor Apc and subsequent β-Catenin activation were analyzed in mice. The contribution of c-Jun to these phenotypes was dissected in double mutant animals lacking both, Apc and c-Jun. β-Catenin was rapidly activated in virtually all Apc mutant hepatocytes while c-Jun was induced only after several days, suggesting that its expression was rather a secondary event following Apc deletion in the liver. Loss of Apc resulted in increased hepatocyte proliferation, hepatomegaly, deregulated protein metabolism, and premature death. Interestingly, additional deletion of c-Jun did not affect hepatocyte proliferation but resulted in increased liver damage and mortality. This phenotype correlated with impaired expression of hepatoprotective genes such as Birc5, Egfr Igf1 and subsequently deregulated Akt signaling.

Conclusions/Significance

These data indicate that c-Jun is not a primary target of β-Catenin signaling in the liver, but rather protects against liver damage, which in turn may promote liver tumorigenesis.

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.

A knock-in mouse model reveals roles for nuclear Apc in cell proliferation, Wnt signal inhibition and tumor suppression

Mutation of the tumor suppressor adenomatous polyposis coli (APC) is considered an initiating step in the genesis of the vast majority of colorectal cancers. APC inhibits the Wnt-signaling pathway by targeting the proto-oncogene β-catenin for destruction by cytoplasmic proteasomes. In the presence of a Wnt signal, or in the absence of functional APC, β-catenin can serve as a transcription cofactor for genes required for cell proliferation such as cyclin-D1 and c-Myc. In cultured cells, APC shuttles between the nucleus and the cytoplasm, with nuclear APC implicated in the inhibition of Wnt target gene expression. Adopting a genetic approach to evaluate the functions of nuclear APC in the context of a whole organism, we generated a mouse model with mutations that inactivate the nuclear localization signals (NLSs) of Apc (Apc(mNLS)). Apc(mNLS/mNLS) mice are viable and fractionation of mouse embryonic fibroblasts (MEFs) isolated from these mice revealed a significant reduction in nuclear Apc as compared with Apc(+/+) MEFs. The levels of Apc and β-catenin protein were not significantly altered in small intestinal epithelia from Apc(mNLS/mNLS) mice. Compared with Apc(+/+) mice, Apc(mNLS/mNLS) mice showed increased proliferation in epithelial cells from the jejunum, ileum and colon. These same tissues from Apc(mNLS/mNLS) mice showed more mRNA from three genes upregulated in response to canonical Wnt signal, c-Myc, axin-2 and cyclin-D1, and less mRNA from Hath-1, which is downregulated in response to Wnt. These observations suggest a role for nuclear Apc in the inhibition of canonical Wnt signaling and the control of epithelial proliferation in intestinal tissue. Furthermore, we found Apc(Min/+) mice, which harbor a mutation that truncates Apc, to have an increased polyp size and multiplicity if they also carry the Apc(mNLS) allele. Taken together, this analysis of the novel Apc(mNLS) mouse model supports a role for nuclear Apc in the control of Wnt target genes, intestinal epithelial cell proliferation and polyp formation.

Dickkopfs and Wnt/β-catenin signalling in liver cancer

Liver cancer is the fifth and seventh most common cause of cancer in men and women, respectively. Wnt/β-catenin signalling has emerged as a critical player in both the development of normal liver as well as an oncogenic driver in hepatocellular carcinoma (HCC). Based on the current understanding, this article summarizes the possible mechanisms for the aberrant activation of this pathway with specific focus on HCC. Furthermore, we will discuss the role of dickkopfs (DKKs) in regulating Wnt/β-catenin signalling, which is poorly understood and understudied. DKKs are a family of secreted proteins that comprise at least four members, namely DKK1-DKK4, which act as inhibitors of Wnt/β-catenin signalling. Nevertheless, not all members antagonize Wnt/β-catenin signalling. Their functional significance in hepatocarcinogenesis remains to be further characterized for which these studies should provide new insights into the regulatory role of DKKs in Wnt/β-catenin signalling in hepatic carcinogenesis. Because of the important oncogenic roles, there are an increasing number of therapeutic molecules targeting β-catenin and the Wnt/β-catenin pathway for potential therapy of HCC.

β-catenin as a potential key target for tumor suppression

β-catenin is a multifunctional protein identified to be pivotal in embryonic patterning, organogenesis and adult homeostasis. It plays a critical structural role in mediating cadherin junctions and is also an essential transcriptional co-activator in the canonical Wnt pathway. Evidence has been documented that both the canonical Wnt pathway and cadherin junctions are deregulated or impaired in a plethora of human malignancies. In the light of this, there has been a recent surge in elucidating the mechanisms underlying the etiology of cancer development from the perspective of β-catenin. Here, we focus on the emerging roles of β-catenin in the process of tumorigenesis by discussing novel functions of old players and new proteins, mechanisms identified to mediate or interact with β-catenin and the most recently unraveled clinical implications of β-catenin regulatory pathways toward tumor suppression.

APC mutations in colorectal tumours from FAP patients are selected for CtBP-mediated oligomerization of truncated APC

The germline transmission of a mutation in the adenomatous polyposis coli (APC) gene leads to cancer of the gastro-intestinal tract upon somatic inactivation of the remaining allele in familial adenomatous polyposis (FAP) patients. APC mutations result in truncated products that have primarily lost the ability to properly regulate the level of the transcription factor β-catenin. However, colorectal cancer cells from FAP patients always retain a truncated APC product and the reasons for this strong selective pressure are not understood. We describe here the surprising property for the transcriptional repressor C-terminal binding protein (CtBP) to promote the oligomerization of truncated APC through binding to the 15 amino acid repeats of truncated APC. CtBP can bind to either first, third or fourth 15 amino acid repeats, but not to the second. CtBP-mediated oligomerization requires both dimerization domains of truncated APC as well as CtBP dimerization. The analysis of the position of the mutations along the APC sequence in adenomas from FAP patients reveals that the presence of the first 15 amino acid repeat is almost always selected in the resulting truncated APC product. This suggests that the sensitivity of truncated APC to oligomerization by CtBP constitutes an essential facet of tumour development.

Adenomatous polyposis coli protein regulates the cellular response to DNA replication stress

The adenomatous polyposis coli (APC) tumor suppressor traffics between nucleus and cytoplasm to perform distinct functions. Here we identify a specific role for APC in the DNA replication stress response. The silencing of APC caused an accumulation of asynchronous cells in early S phase and delayed S phase progression in cells released from hydroxyurea-mediated replication arrest. Immunoprecipitation assays revealed a selective binding of APC to replication protein A 32kDa subunit (RPA32), and the APC-RPA32 complex increased at chromatin after hydroxyurea treatment. Interestingly, APC knock-down prevented accumulation at chromatin of the stress-induced S33- and S29-phosphorylated forms of RPA32, and reduced the expression of ATR-phosphorylated forms of S317-phospho-Chk1 and γ-H2AX. Using RPA32-inducible cells we showed that reconstitution of RPA32 diminished the S-phase delay caused by loss of APC. In contrast to full-length APC, the truncated APC mutant protein expressed in SW480 colon cancer cells was impaired in its binding and regulation of RPA32, and failed to regulate cell cycle after replication stress. We propose that APC associates with RPA at stalled DNA replication forks and promotes the ATR-dependent phosphorylation of RPA32, Chk1 and γ-H2AX in response to DNA replication stress, thereby influencing the rate of re-entry into the cell cycle.

Mechanisms Regulating Microtubule Binding, DNA Replication, and Apoptosis are Controlled by the Intestinal Tumor Suppressor APC

Colorectal cancer (CRC) results from the progressive accumulation of both genetic and epigenetic alterations that lead to the transformation of normal colorectal epithelium to benign (adenoma) and invasive (carcinoma) disease. Since its discovery in mutated form as the causative gene for familial adenomatous polyposis coli (FAP), as well as in many sporadic CRCs, the APC tumor suppressor has been shown to possess numerous functions within the cell including regulation of WNT signaling pathways and its transcriptional effects, cell migration, and chromosome separation. In recent years, other novel roles for APC have been investigated and suggest that APC can also repress DNA replication and enhance apoptosis. Further insights into the mechanisms by which APC contributes to tumor suppression will accelerate the diagnosis and treatment of CRC.

Assembly of the base excision repair complex on abasic DNA and role of adenomatous polyposis coli on its functional activity

The assembly and stability of base excision repair (BER) proteins in vivo with abasic DNA and the role of adenomatous polyposis coli (APC) protein in this process are currently unclear. We have studied the assembly of a multiprotein BER complex onto abasic DNA (F-DNA) and characterized the physical and functional activity of the associated proteins. We found that the BER complex contained all the essential components of the long-patch BER system, such as APE1, Pol-β, Fen1, and DNA ligase I. Interestingly, wild-type APC was also present in the BER complex. Kinetics of the assembly of BER proteins onto the F-DNA were rapid and appeared in sequential order depending upon their requirement in the repair process. The presence of wild-type APC in the BER complex caused a decrease in the level of assembly of BER proteins and negatively affected long-patch BER. These results suggest that major BER proteins in the complex are assembled onto F-DNA and are competent in performing DNA repair. Wild-type APC in the BER complex reduces the repair activity, probably because of interaction with multiple components of the system.

DNA polymerase β as a novel target for chemotherapeutic intervention of colorectal cancer

Chemoprevention presents a major strategy for the medical management of colorectal cancer. Most drugs used for colorectal cancer therapy induce DNA-alkylation damage, which is primarily repaired by the base excision repair (BER) pathway. Thus, blockade of BER pathway is an attractive option to inhibit the spread of colorectal cancer. Using an in silico approach, we performed a structure-based screen by docking small-molecules onto DNA polymerase β (Pol-β) and identified a potent anti-Pol-β compound, NSC-124854. Our goal was to examine whether NSC-124854 could enhance the therapeutic efficacy of DNA-alkylating agent, Temozolomide (TMZ), by blocking BER. First, we determined the specificity of NSC-124854 for Pol-β by examining in vitro activities of APE1, Fen1, DNA ligase I, and Pol-β-directed single nucleotide (SN)- and long-patch (LP)-BER. Second, we investigated the effect of NSC-124854 on the efficacy of TMZ to inhibit the growth of mismatch repair (MMR)-deficient and MMR-proficient colon cancer cell lines using in vitro clonogenic assays. Third, we explored the effect of NSC-124854 on TMZ-induced in vivo tumor growth inhibition of MMR-deficient and MMR-proficient colonic xenografts implanted in female homozygous SCID mice. Our data showed that NSC-124854 has high specificity to Pol-β and blocked Pol-β-directed SN- and LP-BER activities in in vitro reconstituted system. Furthermore, NSC-124854 effectively induced the sensitivity of TMZ to MMR-deficient and MMR-proficient colon cancer cells both in vitro cell culture and in vivo xenograft models. Our findings suggest a potential novel strategy for the development of highly specific structure-based inhibitor for the prevention of colonic tumor progression.

Beta-catenin signaling, liver regeneration and hepatocellular cancer: sorting the good from the bad

Among the adult organs, liver is unique for its ability to regenerate. A concerted signaling cascade enables optimum initiation of the regeneration process following insults brought about by surgery or a toxicant. Additionally, there exists a cellular redundancy, whereby a transiently amplifying progenitor population appears and expands to ensure regeneration, when differentiated cells of the liver are unable to proliferate in both experimental and clinical scenarios. One such pathway of relevance in these phenomena is Wnt/β-catenin signaling, which is activated relatively early during regeneration mostly through post-translational modifications. Once activated, β-catenin signaling drives the expression of target genes that are critical for cell cycle progression and contribute to initiation of the regeneration process. The role and regulation of Wnt/β-catenin signaling is now documented in rats, mice, zebrafish and patients. More recently, a regenerative advantage of the livers in β-catenin overexpressing mice was reported, as was also the case after exogenous Wnt-1 delivery to the liver paving the way for assessing means to stimulate the pathway for therapeutics in liver failure. β-Catenin is also pertinent in hepatic oval cell activation and differentiation. However, aberrant activation of the Wnt/β-catenin signaling is reported in a significant subset of hepatocellular cancers (HCC). While many mechanisms of such activation have been reported, the most functional means of aberrant and sustained activation is through mutations in the β-catenin gene or in AXIN1/2, which encodes for a scaffolding protein critical for β-catenin degradation. Intriguingly, in experimental models hepatic overexpression of normal or mutant β-catenin is insufficient for tumorigenesis. In fact β-catenin loss promoted chemical carcinogenesis in the liver due to alternate mechanisms. Since most HCC occur in the backdrop of chronic hepatic injury, where hepatic regeneration is necessary for maintenance of liver function, but at the same time serves as the basis of dysplastic changes, this Promethean attribute exhibits a Jekyll and Hyde behavior that makes distinguishing good regeneration from bad regeneration essential for targeting selective molecular pathways as personalized medicine becomes a norm in clinical practice. Could β-catenin signaling be one such pathway that may be redundant in regeneration and indispensible in HCC in a subset of cases?

Accelerated liver regeneration and hepatocarcinogenesis in mice overexpressing serine-45 mutant beta-catenin

The Wnt/beta-catenin pathway is implicated in the pathogenesis of hepatocellular cancer (HCC). We developed a transgenic mouse (TG) in the FVB strain that overexpresses Ser45-mutated-beta-catenin in hepatocytes to study the effects on liver regeneration and cancer. In the two independent TG lines adult mice show elevated beta-catenin at hepatocyte membrane with no increase in the Wnt pathway targets cyclin-D1 or glutamine synthetase. However, TG hepatocytes upon culture exhibit a 2-fold increase in thymidine incorporation at day 5 (D5) when compared to hepatocytes from wildtype FVB mice (WT). When subjected to partial hepatectomy (PH), dramatic increases in the number of hepatocytes in S-phase are evident in TG at 40 and WT at 72 hours. Coincident with the earlier onset of proliferation, we observed nuclear translocation of beta-catenin along with an increase in total and nuclear cyclin-D1 protein at 40 hours in TG livers. To test if stimulation of beta-catenin induces regeneration, we used hydrodynamic delivery of Wnt-1 naked DNA to control mice, which prompted an increase in Wnt-1, beta-catenin, and known targets, glutamine synthetase (GS) and cyclin-D1, along with a concomitant increase in cell proliferation. beta-Catenin-overexpressing TG mice, when followed up to 12 months, showed no signs of spontaneous tumorigenesis. However, intraperitoneal delivery of diethylnitrosamine (DEN), a known carcinogen, induced HCC at 6 months in TG mice only. Tumors in TG livers showed up-regulation of beta-catenin, cyclin-D1, and unique genetic aberrations, whereas other canonical targets were unremarkable.

CONCLUSION

beta-Catenin overexpression offers growth advantage during liver regeneration. Also, whereas no spontaneous HCC is evident, beta-catenin overexpression makes TG mice susceptible to DEN-induced HCC.

Role of Wnt/β-catenin signaling in liver metabolism and cancer

Wnt/β-catenin signaling is known for its role in embryogenesis as well as carcinogenesis. In the liver, it plays many critical roles during hepatic development and regeneration, and its dysregulation is evident in aberrant hepatic growth during various liver tumors. Its chief cellular roles in the liver include regulation of processes of cell proliferation, apoptosis, oxidative stress and differentiation, which in turn contributes to hepatic growth, zonation, xenobiotic metabolism and other metabolic processes inherent to the liver. Most of these functions of the Wnt/β-catenin signaling are dictated through the highly temporal and tissue-specific or non-specific transcriptional targets of the pathway. In addition, some of the critical functions such as cell-cell adhesion and perhaps maintenance of various junctions that are critical from an epithelial cell biology perspective are also a function of β-catenin, which is the central component of the canonical Wnt pathway. Various animal models and clinical studies have demonstrated the spectra of Wnt/β-catenin signaling in liver health and disease. Thus therapeutic modulation of this pathway for improved hepatic health is inevitable in the future. The current review discusses the advances in our understanding of the Wnt/β-catenin signaling in liver physiology and pathology especially in hepatic metabolism and various tumors in adult liver and goes on to extrapolate the pre-clinical significance and possible translational implications of such findings.

Role of Wnt/β-catenin signaling in liver metabolism and cancer

Wnt/β-catenin signaling is known for its role in embryogenesis as well as carcinogenesis. In the liver, it plays many critical roles during hepatic development and regeneration, and its dysregulation is evident in aberrant hepatic growth during various liver tumors. Its chief cellular roles in the liver include regulation of processes of cell proliferation, apoptosis, oxidative stress and differentiation, which in turn contributes to hepatic growth, zonation, xenobiotic metabolism and other metabolic processes inherent to the liver. Most of these functions of the Wnt/β-catenin signaling are dictated through the highly temporal and tissue-specific or non-specific transcriptional targets of the pathway. In addition, some of the critical functions such as cell-cell adhesion and perhaps maintenance of various junctions that are critical from an epithelial cell biology perspective are also a function of β-catenin, which is the central component of the canonical Wnt pathway. Various animal models and clinical studies have demonstrated the spectra of Wnt/β-catenin signaling in liver health and disease. Thus therapeutic modulation of this pathway for improved hepatic health is inevitable in the future. The current review discusses the advances in our understanding of the Wnt/β-catenin signaling in liver physiology and pathology especially in hepatic metabolism and various tumors in adult liver and goes on to extrapolate the pre-clinical significance and possible translational implications of such findings.

Pathologic features and biologic importance of colorectal serrated polyps

Serrated polyps of the large intestine comprise a heterogeneous group of mucosal lesions that includes nondysplastic polyps, such as hyperplastic polyps and sessile serrated polyps, and polyps that show overt cytologic dysplasia, namely serrated adenomas and mixed hyperplastic/adenomatous polyps. These polyps have received increased recognition over the past 2 decades, as emerging evidence suggests that a subset may be precursors to colorectal carcinomas that lack chromosomal instability. Several investigators have proposed the concept of the "serrated neoplastic pathway" according to which nondysplastic serrated lesions develop progressively severe dysplasia culminating in the development of microsatellite unstable carcinomas that show DNA hypermethylation and BRAF mutations. A subset of hyperplastic polyps and sessile serrated polyps show mutations in the BRAF gene and abnormal DNA methylation, which can, ultimately, affect the promoter regions of key DNA-repair and tumor suppressor genes, such as MLH1 and MGMT, leading to their decreased transcription and microsatellite instability. On the basis of this hypothesis, many authors have proposed that sessile serrated polyps should be treated and surveilled similar to conventional adenomas, although prospective data are lacking. This review describes the clinicopathologic and molecular features of serrated polyps and discusses the current data regarding their biologic significance.

Calcium, calcium-sensing receptor and colon cancer

There is much evidence that dietary Ca(2+) loading reduces colon cell proliferation and carcinogenesis in humans and rodents, but during carcinogenesis it becomes ineffective or even tumor-promoting. We are beginning to see how Ca(2+) balances the continuous massive cell production in colon crypts by driving the terminal differentiation and eventually the apoptosis of the cells mainly on the mucosal surface, and how this Ca(2+) control is lost during colon carcinogenesis. The rapid proliferation of the transit-amplifying (TA) progeny of the colon stem cells is driven by the so-called "Wnt" signaling mechanism, which involves the stimulation of proliferogenic genes such as those for c-Myc and cyclin D1 and the silencing of the gene for the cell cycle-stopping p21(Cip1/WAF1) protein by nuclear beta-catenin*Tcf-4 complexes. TA cells avoid mitotic damage and premature apoptosis by expressing the protein survivin. It appears that TA cell cycling stops and terminal differentiation starts when the cells reach a higher level in the crypt where there is enough lumenal Ca(2+) to stimulate the expression and activation of CaSRs (Ca(2+)-sensing receptors), the signals from which stimulate the expression of E-cadherin. Along with this, the APC (adenomatous polyposis coli) protein appears and some of it enters the nucleus. There it makes the TA cells susceptible to the eventual apoptotic balancing by stopping survivin expression and the beta-catenin*Tcf-4 complex from driving further cell cycling by releasing beta-catenin from the nucleus, and delivering it to cytoplasmic APC*axin*GSK-3beta complexes for ultimate proteasomal destruction. Cytoplasmic beta-catenin is then prevented from returning to the nucleus by either being intercepted and destroyed by APC*axin*GSK-3beta complexes or locked by the emerging E-cadherin into membrane adherens junctions which tie the cell into the sheet of proliferatively shut-down cells with APC-dependent cytoskeletons moving to the mouth of the crypt and onto the flat mucosal surface. A common first step in sporadic colon carcinogenesis is the loss of functional APC which disorients upwardly directed migration and causes the retention of nuclear beta-catenin and proliferogenic beta-catenin*Tcf-4 complexes as well as genomic instability. Eventually the balance between cell proliferation and terminal differentiation and death is radically tipped in favour of proliferation by the appearance of apoptosis-resistant, survivin-expressing clones of Ca(2+)-insensitive cells which are locked into the proliferative, mutation-prone mode because of CaSR-disabling gene mutations which prevent the stimulation of E-cadherin expression and terminal differentiation.