Novel association of APC with intermediate filaments identified using a new versatile APC antibody

(Pro)renin receptor promotes colorectal cancer progression through inhibiting the NEDD4L-mediated Wnt3 ubiquitination and modulating gut microbiota

BACKGROUND

We previously found that (pro)renin receptor ((P)RR) augments Wnt3 protein without affecting Wnt3 gene transcription in colorectal cancer (CRC) cells, thus contributes to CRC initiation. The present study aims to investigate whether (P)RR further promotes CRC progression following oncogenesis and the related mechanisms. Notably, we deeply elaborate how (P)RR affects Wnt3 protein level and the key enzyme that mediates this process.

METHODS

Immunohistochemistry, western blotting and immunofluorescence were performed to detect protein expression status. A kind of gastrointestinal epithelium-specific ATP6AP2 ((P)RR encoding gene) knock-in mice were generated using Crispr/Cas9 system.

RESULTS

We found that increased (P)RR expression in primary CRC lesions is positively associated with higher Wnt3 protein level and disease progression. Progressive CRC presents less colocalization of Wnt3 and an E3 ubiquitin ligase NEDD4L in primary lesions than non-progressive CRC. In colon cancer cells, (P)RR dramatically inhibits the NEDD4L-mediated Wnt3 protein ubiquitination. ATP6AP2 knock-in mice show more diminished Wnt3-NEDD4L colocalization in their gut epithelium in comparison to wildtype mice. They also have abnormal gut bacterial flora distribution. Especially, Lachnospiraceae_NK4A136 and Bacteroides genus, which are generally protective against CRC, are suppressed in guts of ATP6AP2 knock-in mice.

CONCLUSIONS

Collectively, (P)RR promotes CRC progression through inhibiting the NEDD4L-mediated Wnt3 ubiquitination and modulating gut microbiota. Video Abstract.

Branched actin networks are assembled on microtubules by adenomatous polyposis coli for targeted membrane protrusion

Cell migration is driven by pushing and pulling activities of the actin cytoskeleton, but migration directionality is largely controlled by microtubules. This function of microtubules is especially critical for neuron navigation. However, the underlying mechanisms are poorly understood. Here we show that branched actin filament networks, the main pushing machinery in cells, grow directly from microtubule tips toward the leading edge in growth cones of hippocampal neurons. Adenomatous polyposis coli (APC), a protein with both tumor suppressor and cytoskeletal functions, concentrates at the microtubule-branched network interface, whereas APC knockdown nearly eliminates branched actin in growth cones and prevents growth cone recovery after repellent-induced collapse. Conversely, encounters of dynamic APC-positive microtubule tips with the cell edge induce local actin-rich protrusions. Together, we reveal a novel mechanism of cell navigation involving APC-dependent assembly of branched actin networks on microtubule tips.

Elevated adenomatous polyposis coli in goblet cells is associated with inflammation in mouse and human colon

NEW FINDINGS

What is the central question of this study? What is the localization and distribution pattern of adenomatous polyposis coli (APC) in intestinal epithelial cells? Does this distribution change in different regions of the colon or in the condition of inflammation? What is the main finding and its importance? Colonic epithelia from mice and humans contain a subset of goblet cells displaying high APC levels. The number of APC^high goblet cells increases in inflamed tissue, which also displays increased GRP78, indicating potential stress from mucin production. In cultured human colon cells, expression of interleukin 1 pathway components (inducers of MUC2 expression) is reduced upon APC depletion raising the potential for APC participation in an inflammatory response.

ABSTRACT

Adenomatous polyposis coli (APC) serves as a gatekeeper of intestinal homeostasis by promoting cellular differentiation and maintaining crypt architecture. Although appreciated as a critical colon tumour suppressor, roles for APC in disease states such as inflammation have yet to be fully delineated. This study aimed to characterize the localization of APC protein in gastrointestinal tissues from human patients with active inflammatory bowel disease and mice with dextran sodium sulfate (DSS)-induced colitis. Fluorescence immunohistochemistry revealed a subset of goblet cells with elevated Apc staining intensity in the small intestines and proximal/medial colons of mice. Upon induction of colitis with DSS, these 'APC^high ' goblet cells remained in the proximal and medial colon, but now were also observed in the distal colon. This phenotype was recapitulated in humans, with APC^high goblet cells observed only in the descending colons of patients with active ulcerative colitis. In cultured human colon cells derived from normal tissue, APC depletion reduced expression of mRNAs encoding the interleukin 1 (IL1) signalling pathway components IL1β and interleukin-1 receptor (IL1R), known regulators of Muc2 expression. Treating cancer cells lacking wild-type APC with IL1β, or induction of full-length APC in these cells led to increases in IL1R and MUC2 expression. Combining IL1β treatment with APC induction led to an increase of MUC2 expression greater than expected for additive affects, suggesting that APC sensitizes cells to IL1 signalling. These findings suggest that APC has novel roles in maintaining proper goblet cell function, thus providing further evidence for APC as an important factor in intestinal tissue homeostasis and disease.

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.

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.

Multi-scale modeling of APC and [Formula: see text]-catenin regulation in the human colonic crypt

Stem cell renewal and differentiation in the human colonic crypt are linked to the [Formula: see text]-catenin pathway. The spatial balance of Wnt factors in proliferative cells within the crypt maintain an appropriate level of cellular reproduction needed for normal crypt homeostasis. Mutational events at the gene level are responsible for deregulating the balance of Wnt factors along the crypt, causing an overpopulation of proliferative cells, a loss of structure of the crypt domain, and the initiation of colorectal carcinomas. We formulate a PDE model describing cell movement and reproduction in a static crypt domain. We consider a single cell population whose proliferative capabilities are determined by stemness, a quantity defined by intracellular levels of adenomatous polyposis coli (APC) scaffold protein and [Formula: see text]-catenin. We fit APC regulation parameters to biological data that describe normal protein gradients in the crypt. We also fit cell movement and protein flux parameters to normal crypt characteristics such as renewal time, total cell count, and proportion of proliferating cells. The model is used to investigate abnormal crypt dynamics when subjected to a diminished APC gradient, a scenario synonymous to mutations in the APC gene. We find that a 25% decrease in APC synthesis leads to a fraction of 0.88 proliferative, which is reflective of normal-appearing FAP crypts. A 50% drop in APC activity yields a fully proliferative crypt showing a doubling of the level of stemness, which characterizes the initial stages of colorectal cancer development. A sensitivity analysis of APC regulation parameters shows the perturbation of factors that is required to restore crypt dynamics to normal in the case of APC mutations.

Suppression of intestinal tumorigenesis in Apc mutant mice upon Musashi-1 deletion

Therapeutic strategies based on a specific oncogenic target are better justified when elimination of that particular oncogene reduces tumorigenesis in a model organism. One such oncogene, Musashi-1 (Msi-1), regulates translation of target mRNAs and is implicated in promoting tumorigenesis in the colon and other tissues. Msi-1 targets include the tumor suppressor adenomatous polyposis coli (Apc), a Wnt pathway antagonist lost in ∼80% of all colorectal cancers. Cell culture experiments have established that Msi-1 is a Wnt target, thus positioning Msi-1 and Apc as mutual antagonists in a mutually repressive feedback loop. Here, we report that intestines from mice lacking Msi-1 display aberrant Apc and Msi-1 mutually repressive feedback, reduced Wnt and Notch signaling, decreased proliferation, and changes in stem cell populations, features predicted to suppress tumorigenesis. Indeed, mice with germline Apc mutations (Apc^Min ) or with the Apc^1322T truncation mutation have a dramatic reduction in intestinal polyp number when Msi-1 is deleted. Taken together, these results provide genetic evidence that Msi-1 contributes to intestinal tumorigenesis driven by Apc loss, and validate the pursuit of Msi-1 inhibitors as chemo-prevention agents to reduce tumor burden.

Insulin signaling regulates a functional interaction between adenomatous polyposis coli and cytoplasmic dynein

Diabetes is linked to an increased risk for colorectal cancer, but the mechanistic underpinnings of this clinically important effect are unclear. Here we describe an interaction between the microtubule motor cytoplasmic dynein, the adenomatous polyposis coli tumor suppressor protein (APC), and glycogen synthase kinase-3β (GSK-3β), which could shed light on this issue. GSK-3β is perhaps best known for glycogen regulation, being inhibited downstream in an insulin-signaling pathway. However, the kinase is also important in many other processes. Mutations in APC that disrupt the regulation of β-catenin by GSK-3β cause colorectal cancer in humans. Of interest, both APC and GSK-3β interact with microtubules and cellular membranes. We recently demonstrated that dynein is a GSK-3β substrate and that inhibition of GSK-3β promotes dynein-dependent transport. We now report that dynein stimulation in intestinal cells in response to acute insulin exposure (or GSK-3β inhibition) is blocked by tumor-promoting isoforms of APC that reduce an interaction between wild-type APC and dynein. We propose that under normal conditions, insulin decreases dynein binding to APC to stimulate minus end-directed transport, which could modulate endocytic and secretory systems in intestinal cells. Mutations in APC likely impair the ability to respond appropriately to insulin signaling. This is exciting because it has the potential to be a contributing factor in the development of colorectal cancer in patients with diabetes.

The APC tumor suppressor is required for epithelial cell polarization and three-dimensional morphogenesis

The Adenomatous Polyposis Coli (APC) tumor suppressor has been previously implicated in the control of apical-basal polarity; yet, the consequence of APC loss-of-function in epithelial polarization and morphogenesis has not been characterized. To test the hypothesis that APC is required for the establishment of normal epithelial polarity and morphogenesis programs, we generated APC-knockdown epithelial cell lines. APC depletion resulted in loss of polarity and multi-layering on permeable supports, and enlarged, filled spheroids with disrupted polarity in 3D culture. Importantly, these effects of APC knockdown were independent of Wnt/β-catenin signaling, but were rescued with either full-length or a carboxy (c)-terminal segment of APC. Moreover, we identified a gene expression signature associated with APC knockdown that points to several candidates known to regulate cell-cell and cell-matrix communication. Analysis of epithelial tissues from mice and humans carrying heterozygous APC mutations further supports the importance of APC as a regulator of epithelial behavior and tissue architecture. These data also suggest that the initiation of epithelial-derived tumors as a result of APC mutation or gene silencing may be driven by loss of polarity and dysmorphogenesis.

An APC:WNT Counter-Current-Like Mechanism Regulates Cell Division Along the Human Colonic Crypt Axis: A Mechanism That Explains How APC Mutations Induce Proliferative Abnormalities That Drive Colon Cancer Development

APC normally down-regulates WNT signaling in human colon, and APC mutations cause proliferative abnormalities in premalignant crypts leading to colon cancer, but the mechanisms are unclear at the level of spatial and functional organization of the crypt. Accordingly, we postulated a counter-current-like mechanism based on gradients of factors (APC;WNT) that regulate colonocyte proliferation along the crypt axis. During crypt renewal, stem cells (SCs) at the crypt bottom generate non-SC daughter cells that proliferate and differentiate while migrating upwards. The APC concentration is low at the crypt bottom and high at the top (where differentiated cells reside). WNT signaling, in contrast, is high at the bottom (where SCs reside) and low at the top. Given that WNT and APC gradients are counter to one another, we hypothesized that a counter-current-like mechanism exists. Since both APC and WNT signaling components (e.g., survivin) are required for mitosis, this mechanism establishes a zone in the lower crypt where conditions are optimal for maximal cell division and mitosis orientation (symmetric versus asymmetric). APC haploinsufficiency diminishes the APC gradient, shifts the proliferative zone upwards, and increases symmetric division, which causes SC overpopulation. In homozygote mutant crypts, these changes are exacerbated. Thus, APC-mutation-induced changes in the counter-current-like mechanism cause expansion of proliferative populations (SCs, rapidly proliferating cells) during tumorigenesis. We propose this mechanism also drives crypt fission, functions in the crypt cycle, and underlies adenoma development. Novel chemoprevention approaches designed to normalize the two gradients and readjust the proliferative zone downwards, might thwart progression of these premalignant changes.

APC binds intermediate filaments and is required for their reorganization during cell migration

The tumor suppressor APC binds to the intermediate filament vimentin and is required for its microtubule-dependent rearrangements during astrocyte migration.

Intermediate filaments (IFs) are components of the cytoskeleton involved in most cellular functions, including cell migration. Primary astrocytes mainly express glial fibrillary acidic protein, vimentin, and nestin, which are essential for migration. In a wound-induced migration assay, IFs reorganized to form a polarized network that was coextensive with microtubules in cell protrusions. We found that the tumor suppressor adenomatous polyposis coli (APC) was required for microtubule interaction with IFs and for microtubule-dependent rearrangements of IFs during astrocyte migration. We also show that loss or truncation of APC correlated with the disorganization of the IF network in glioma and carcinoma cells. In migrating astrocytes, vimentin-associated APC colocalized with microtubules. APC directly bound polymerized vimentin via its armadillo repeats. This binding domain promoted vimentin polymerization in vitro and contributed to the elongation of IFs along microtubules. These results point to APC as a crucial regulator of IF organization and confirm its fundamental role in the coordinated regulation of cytoskeletons.

APC/β-catenin-rich complexes at membrane protrusions regulate mammary tumor cell migration and mesenchymal morphology

Background

The APC tumor suppressor is mutated or downregulated in many tumor types, and is prominently localized to punctate clusters at protrusion tips in migratory cells, such as in astrocytes where it has been implicated in directed cell motility. Although APC loss is considered an initiating event in colorectal cancer, for example, it is less clear what role APC plays in tumor cell motility and whether loss of APC might be an important promoter of tumor progression in addition to initiation.

Methods

The localization of APC and β-catenin was analyzed in multiple cell lines, including non-transformed epithelial lines treated with a proteasome inhibitor or TGFβ to induce an epithelial-to-mesenchymal transition (EMT), as well as several breast cancer lines, by immunofluorescence. APC expression was knocked down in 4T07 mammary tumor cells using lentiviral-mediated delivery of APC-specific short-hairpin (sh) RNAs, and assessed using quantitative (q) reverse-transcriptase (RT)-PCR and western blotting. Tumor cell motility was analyzed by performing wound-filling assays, and morphology via immunofluorescence (IF) and phase-contrast microscopy. Additionally, proliferation was measured using BrdU incorporation, and TCF reporter assays were performed to determine β-catenin/TCF-mediated transcriptional activity.

Results

APC/β-catenin-rich complexes were observed at protrusion ends of migratory epithelial cells treated with a proteasome inhibitor or when EMT has been induced and in tumor cells with a mesenchymal, spindle-like morphology. 4T07 tumor cells with reduced APC levels were significantly less motile and had a more rounded morphology; yet, they did not differ significantly in proliferation or β-catenin/TCF transcriptional activity. Furthermore, we found that APC/β-catenin-rich complexes at protrusion ends were dependent upon an intact microtubule cytoskeleton.

Conclusions

These findings indicate that membrane protrusions with APC/β-catenin-containing puncta control the migratory potential and mesenchymal morphology of mammary tumor cells and suggest that APC loss during later stages of tumor progression might impact tumor cell dissemination or colonization.

The pan-ErbB negative regulator Lrig1 is an intestinal stem cell marker that functions as a tumor suppressor

Lineage mapping has identified both proliferative and quiescent intestinal stem cells, but the molecular circuitry controlling stem cell quiescence is incompletely understood. By lineage mapping, we show Lrig1, a pan-ErbB inhibitor, marks predominately noncycling, long-lived stem cells that are located at the crypt base and that, upon injury, proliferate and divide to replenish damaged crypts. Transcriptome profiling of Lrig1(+) colonic stem cells differs markedly from the profiling of highly proliferative, Lgr5(+) colonic stem cells; genes upregulated in the Lrig1(+) population include those involved in cell cycle repression and response to oxidative damage. Loss of Apc in Lrig1(+) cells leads to intestinal adenomas, and genetic ablation of Lrig1 results in heightened ErbB1-3 expression and duodenal adenomas. These results shed light on the relationship between proliferative and quiescent intestinal stem cells and support a model in which intestinal stem cell quiescence is maintained by calibrated ErbB signaling with loss of a negative regulator predisposing to neoplasia.

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.

Changes in gene expression and cellular architecture in an ovarian cancer progression model

BACKGROUND

Ovarian cancer is the fifth leading cause of cancer deaths among women. Early stage disease often remains undetected due the lack of symptoms and reliable biomarkers. The identification of early genetic changes could provide insights into novel signaling pathways that may be exploited for early detection and treatment.

METHODOLOGY/PRINCIPAL FINDINGS

Mouse ovarian surface epithelial (MOSE) cells were used to identify stage-dependent changes in gene expression levels and signal transduction pathways by mouse whole genome microarray analyses and gene ontology. These cells have undergone spontaneous transformation in cell culture and transitioned from non-tumorigenic to intermediate and aggressive, malignant phenotypes. Significantly changed genes were overrepresented in a number of pathways, most notably the cytoskeleton functional category. Concurrent with gene expression changes, the cytoskeletal architecture became progressively disorganized, resulting in aberrant expression or subcellular distribution of key cytoskeletal regulatory proteins (focal adhesion kinase, α-actinin, and vinculin). The cytoskeletal disorganization was accompanied by altered patterns of serine and tyrosine phosphorylation as well as changed expression and subcellular localization of integral signaling intermediates APC and PKCβII.

CONCLUSIONS/SIGNIFICANCE

Our studies have identified genes that are aberrantly expressed during MOSE cell neoplastic progression. We show that early stage dysregulation of actin microfilaments is followed by progressive disorganization of microtubules and intermediate filaments at later stages. These stage-specific, step-wise changes provide further insights into the time and spatial sequence of events that lead to the fully transformed state since these changes are also observed in aggressive human ovarian cancer cell lines independent of their histological type. Moreover, our studies support a link between aberrant cytoskeleton organization and regulation of important downstream signaling events that may be involved in cancer progression. Thus, our MOSE-derived cell model represents a unique model for in depth mechanistic studies of ovarian cancer progression.

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.