An Increase in Mucin2 Expression Is Required for Colon Cancer Progression Mediated by L1.

An induction in the expression of the cell adhesion receptor L1, a Wnt target gene, is a characteristic feature of Wnt/ß-catenin activation in colon cancer cells at later stages of the disease. We investigated the proteins secreted following L1 expression in colon cancer cells and identified Mucin2 among the most abundant secreted proteins. We found that suppressing Mucin2 expression in L1-expressing colon cancer cells inhibits cell proliferation, motility, tumorigenesis, and liver metastasis. We detected several signaling pathways involved in Mucin2 induction in L1-expressing cells. In human colon cancer tissue, Mucin2 expression was significantly reduced or lost in the adenocarcinoma tissue, while in the mucinous subtype of colon cancer tissue, Mucin2 expression was increased. An increased signature of L1/Mucin2 expression reduced the survival rate of human colon cancer patients. Thus, induction of Mucin2 expression by L1 is required during mucinous colon cancer progression and can serve as a marker for diagnosis and a target for therapy.

Downregulation of the Tumor Suppressor TFF1 Is Required during Induction of Colon Cancer Progression by L1.

The immunoglobulin family cell adhesion receptor L1 is induced in CRC cells at the invasive front of the tumor tissue, and confers enhanced proliferation, motility, tumorigenesis, and liver metastasis. To identify putative tumor suppressors whose expression is downregulated in L1-expressing CRC cells, we blocked the L1-ezrin-NF-κB signaling pathway and searched for genes induced under these conditions. We found that TFF1, a protein involved in protecting the mucus epithelial layer of the colon, is downregulated in L1-expressing cells and displays characteristics of a tumor suppressor. Overexpression of TFF1 in L1-transfected human CRC cells blocks the pro-tumorigenic and metastatic properties conferred by L1 by suppressing NF-κB signaling. Immunohistochemical analyses revealed that human CRC tissue samples often lose the expression of TFF1, while the normal mucosa displays TFF1 in goblet cells. Identifying TFF1 as a tumor suppressor in CRC cells could provide a novel marker for L1-mediated CRC development and a potential target for therapy.

The Collagen-Modifying Enzyme PLOD2 Is Induced and Required during L1-Mediated Colon Cancer Progression.

The overactivation of Wnt/ß-catenin signaling is a hallmark of colorectal cancer (CRC) development. We identified the cell adhesion molecule L1CAM (L1) as a target of ß-catenin-TCF transactivation in CRC cells. The overexpression of L1 in CRC cells confers enhanced proliferation, motility, tumorigenesis and liver metastasis, and L1 is exclusively localized in the invasive areas of human CRC tissue. A number of genes are induced after L1 transfection into CRC cells by a mechanism involving the cytoskeletal protein ezrin and the NF-κB pathway. When studying the changes in gene expression in CRC cells overexpressing L1 in which ezrin levels were suppressed by shRNA to ezrin, we discovered the collagen-modifying enzyme lysyl hydroxylase 2 (PLOD2) among these genes. We found that increased PLOD2 expression was required for the cellular processes conferred by L1, including enhanced proliferation, motility, tumorigenesis and liver metastasis, since the suppression of endogenous PLOD2 expression, or its enzymatic activity, blocked the enhanced tumorigenic properties conferred by L1. The mechanism involved in increased PLOD2 expression by L1 involves ezrin signaling and PLOD2 that affect the SMAD2/3 pathway. We found that PLOD2 is localized in the colonic crypts in the stem cell compartment of the normal mucosa and is found at increased levels in invasive areas of the tumor and, in some cases, throughout the tumor tissue. The therapeutic strategies to target PLOD2 expression might provide a useful approach for CRC treatment.

A Necessary Role for Increased Biglycan Expression during L1-Mediated Colon Cancer Progression.

Aberrant activation of Wnt/ß-catenin signaling and downstream ß-catenin-TCF target genes is a hallmark of colorectal cancer (CRC) development. We identified the immunoglobulin-like cell adhesion receptor L1CAM (L1) as a target of ß-catenin-TCF transactivation in CRC cells. Overexpression of L1 in CRC cells confers enhanced proliferation, motility, tumorigenesis, and liver metastasis, and L1 is exclusively localized at invasive areas of human CRC tissue. Several genes are induced after L1 transfection into CRC cells by a mechanism involving the L1-ezrin-NF-κB pathway. We conducted a secretomic analysis of the proteins in the culture medium of L1-overexpressing CRC cells. We detected a highly increased level of biglycan, a small leucine-rich ECM component, and a signaling molecule. We found that induction of biglycan is required for the cellular processes conferred by L1, including enhanced proliferation, motility, tumorigenesis, and liver metastasis. The suppression of endogenous biglycan levels or a point mutation in the L1 ectodomain that regulates cell-cell adhesion mediated by L1 blocked the enhanced tumorigenic properties conferred by L1. The mechanism of biglycan induction by L1 involves the L1-NF-κB pathway. Blocking NF-κB signaling in L1 expressing cells suppressed the induction of biglycan and the tumorigenic properties conferred by L1. Biglycan expression was undetectable in the normal colonic mucosa, but expressed at highly increased levels in the tumor tissue, especially in the stroma. The therapeutic strategies to target biglycan expression might provide a useful approach for CRC treatment in L1-overexpressing tumors.

Wnt/ß-Catenin Target Genes in Colon Cancer Metastasis: The Special Case of L1CAM.

Cell adhesion to neighboring cells is a fundamental biological process in multicellular organisms that is required for tissue morphogenesis. A tight coordination between cell-cell adhesion, signaling, and gene expression is a characteristic feature of normal tissues. Changes, and often disruption of this coordination, are common during invasive and metastatic cancer development. The Wnt/ß-catenin signaling pathway is an excellent model for studying the role of adhesion-mediated signaling in colorectal cancer (CRC) invasion and metastasis, because ß-catenin has a dual role in the cell; it is a major adhesion linker of cadherin transmembrane receptors to the cytoskeleton and, in addition, it is also a key transducer of Wnt signaling to the nucleus, where it acts as a co-transcriptional activator of Wnt target genes. Hyperactivation of Wnt/ß-catenin signaling is a common feature in the majority of CRC patients. We found that the neural cell adhesion receptor L1CAM (L1) is a target gene of ß-catenin signaling and is induced in carcinoma cells of CRC patients, where it plays an important role in CRC metastasis. In this review, we will discuss studies on ß-catenin target genes activated during CRC development (in particular, L1), the signaling pathways affected by L1, and the role of downstream target genes activated by L1 overexpression, especially those that are also part of the intestinal stem cell gene signature. As intestinal stem cells are highly regulated by Wnt signaling and are believed to also play major roles in CRC progression, unravelling the mechanisms underlying the regulation of these genes will shed light on both normal intestinal homeostasis and the development of invasive and metastatic CRC.

Recent insights into the role of L1CAM in cancer initiation and progression.

First described as a neuronal cell adhesion molecule, L1CAM was later identified to be present at increased levels in primary tumors and metastases of various types of cancer. Here, we describe the multifaceted roles of L1CAM that are involved in diverse fundamental steps during tumor initiation and progression, as well as in chemoresistance. Recently, Ganesh et al reported that L1CAM identifies metastasis-initiating cells in colorectal carcinoma exhibiting stem-like cell features, increased tumorigenic potential and enhanced chemoresistance. In this review, we highlight recent advances in L1CAM research with particular emphasis on its role in de-differentiation processes and cancer cell stemness supporting the view that L1CAM is a powerful prognostic factor and a suitable target for improved therapy of metastatic and drug-resistant tumors.

Increased expression of cathepsin D is required for L1-mediated colon cancer progression.

Hyperactivation of Wnt/ß-catenin target genes is considered a key step in human colorectal cancer (CRC) development. We previously identified the immunoglobulin-like cell adhesion receptor L1 as a target gene of ß-catenin/TCF transactivation that is localized at the invasive edge of CRC tissue. Using gene arrays, we discovered a number of downstream target genes and signaling pathways conferred by L1 overexpression during colon cancer progression. Here, we have used a proteomic approach to identify proteins in the secretome of L1-overexpressing CRC cells and studied the role of the increase in the aspartate protease cathepsin D (CTSD) in L1-mediated colon cancer development. We found that in addition to the increase in CTSD in the secretome, the RNA and protein levels of CTSD were also induced by L1 in CRC cells. CTSD overexpression resulted in elevated proliferation under stress and increased motility, tumorigenesis and liver metastasis, although to a lesser extent than after L1-transfection. The suppression of endogenous CTSD in L1-expressing cells blocked the increase in the proliferative, motile, tumorigenic and metastatic ability of CRC cells. Enhancing Wnt/ß-catenin signaling by the inhibition of GSK3ß resulted in increased endogenous CTSD levels, suggesting the involvement of the Wnt/ß-catenin pathway in CTSD expression. In human CRC tissue, CTSD was detected in epithelial cells and in the stromal compartment at the more invasive areas of the tumor, but not in the normal mucosa, indicating that CTSD plays an essential role in CRC progression.

ISG15 induction is required during L1-mediated colon cancer progression and metastasis.

Hyperactivation of Wnt/ß-catenin target gene expression is a hallmark of colorectal cancer (CRC) development. We identified L1-CAM (L1) and Nr-CAM, members of the immunoglobulin family of nerve cell adhesion receptors, as target genes of the Wnt/ß-catenin pathway in CRC cells. L1 overexpression in CRC cells enhances their motile and tumorigenic capacity and promotes liver metastasis. L1 is often localized at the invasive edge of CRC tissue. Using gene arrays and proteomic analyses we identified downstream signaling pathways and targets of L1-mediated signaling. Here, we found that the expression of interferon-stimulated gene 15 (ISG15) that operates much like ubiquitin (is conjugated to proteins by ISGylation), is elevated in the conditioned medium and in CRC cells overexpressing L1. Suppression of endogenous ISG15 levels in L1-expressing cells blocked the increased proliferative, motile, tumorigenic and liver metastatic capacities of CRC cells. ISG15 overexpression, on its own, could enhance these properties in CRC cells, but only to a much lower extent compared to L1. We show that NF-κB signaling is involved in the L1-mediated increase in ISG15, since blocking the NF-κB pathway abolished the induction of ISG15 by L1. Point mutations in the L1 ectodomain that interfere with its binding to L1 ligands, also inhibited the increase in ISG15. We detected high levels of ISG15 in human CRC tissue cells and in the adjacent stroma, but not in the normal mucosa. The results suggest that ISG15 is involved in L1-mediated CRC development and is a potential target for CRC therapy.

Cell-cell adhesion: linking Wnt/ß-catenin signaling with partial EMT and stemness traits in tumorigenesis.

Changes in cell adhesion and motility are considered key elements in determining the development of invasive and metastatic tumors. Co-opting the epithelial-to-mesenchymal transition (EMT) process, which is known to occur during embryonic development, and the associated changes in cell adhesion properties in cancer cells are considered major routes for tumor progression. More recent in vivo studies in tumor tissues and circulating tumor cell clusters suggest a stepwise EMT process rather than an "all-or-none" transition during tumor progression. In this commentary, we addressed the molecular mechanisms underlying the changes in cell adhesion and motility and adhesion-mediated signaling and their relationships to the partial EMT states and the acquisition of stemness traits by cancer cells.

The intestinal stem cell regulating gene ASCL2 is required for L1-mediated colon cancer progression.

Aberrant Wnt/ß-catenin signaling is a common event during human colorectal cancer (CRC) development. Previously, we characterized members of the L1 family of cell adhesion receptors as targets of ß-catenin-LEF1/TCF transactivation that are expressed at the invasive CRC tissue edge. Overexpression of L1 in CRC cells confers enhanced motility, tumorigenesis and liver metastasis. We identified several downstream targets of L1-mediated signaling that are considered key intestinal stem cell signature genes. Here, we investigated the involvement of ASCL2, a Wnt target gene and key determinant of intestinal stem cell state, in L1-mediated CRC progression. In L1 overexpressing CRC cells we found an increase in ASCL2, a decrease in E-cadherin and accumulation of nuclear ß-catenin, ß-catenin-LEF1/TCF transactivation and target gene expression. The increase in ASCL2 by L1 overexpression enhanced ASCL2 target gene expression, conferred increased motility, tumorigenesis and metastasis, similar to L1 overexpression. Suppression of ASCL2 in cells expressing L1 blocked these tumorigenic properties. In human CRC tissue, ASCL2 was detected in the nuclei of cells at invasive areas of the tumor that also expressed L1. The results suggest that increased ASCL2 expression is a critical step in L1-mediated CRC progression.

Wnt signaling in cancer stem cells and colon cancer metastasis.

Overactivation of Wnt signaling is a hallmark of colorectal cancer (CRC). The Wnt pathway is a key regulator of both the early and the later, more invasive, stages of CRC development. In the normal intestine and colon, Wnt signaling controls the homeostasis of intestinal stem cells (ISCs) that fuel, via proliferation, upward movement of progeny cells from the crypt bottom toward the villus and differentiation into all cell types that constitute the intestine. Studies in recent years suggested that cancer stem cells (CSCs), similar to ISCs of the crypts, consist of a small subpopulation of the tumor and are responsible for the initiation and progression of the disease. Although various ISC signature genes were also identified as CRC markers and some of these genes were even demonstrated to have a direct functional role in CRC development, the origin of CSCs and their contribution to cancer progression is still debated. Here, we describe studies supporting a relationship between Wnt-regulated CSCs and the progression of CRC.

The Wnt Target Gene L1 in Colon Cancer Invasion and Metastasis.

The Wnt-ß-catenin signaling pathway is highly conserved during evolution and determines normal tissue homeostasis. Hyperactivation of Wnt-ß-catenin signaling is a characteristic feature of colorectal cancer (CRC) development. ß-catenin is a major transducer of the Wnt signal from the cytoplasm into the nucleus where it acts as a co-transcriptional activator of ß-catenin-TCF target genes. ß-catenin is also required for linking cadherin type cell-cell adhesion receptors to the cytoskeleton, and consequently Wnt-ß-catenin signaling is an attractive system for investigating the role of adhesion-mediated signaling in both normal intestinal tissue homeostasis and CRC development. In this review, we summarize our studies on one Wnt-ß-catenin target gene, L1, a member of the immunoglobulin-like cell adhesion transmembrane receptor family. We describe the mechanisms of L1-mediated signaling in CRC cells, its exclusive localization in invasive areas of CRC tissue, and its ability to increase cell motility and confer metastasis to the liver. We discuss the activation (by L1) of genes via an ezrin-NF-κB pathway and the induction of genes also found in the intestinal stem cell signature. By studying L1 (adhesion)-mediated signaling, we expect to learn about mechanisms regulating both normal intestinal homeostasis and CRC development.

Clusterin, a gene enriched in intestinal stem cells, is required for L1-mediated colon cancer metastasis.

Hyperactive Wnt signaling is a common feature in human colorectal cancer (CRC) cells. A central question is the identification and role of Wnt/ß-catenin target genes in CRC and their relationship to genes enriched in colonic stem cells, since Lgr5+ intestinal stem cells were suggested to be the cell of CRC origin. Previously, we identified the neural immunoglobulin-like adhesion receptor L1 as a Wnt/ß-catenin target gene localized in cells at the invasive front of CRC tissue and showed that L1 expression in CRC cells confers enhanced motility and liver metastasis. Here, we identified the clusterin (CLU) gene that is also enriched in Lgr5+ intestinal stem cells, as a gene induced during L1-mediated CRC metastasis. The increase in CLU levels by L1 in CRC cells resulted from transactivation of CLU by STAT-1. CLU overexpression in CRC cells enhanced their motility and the reduction in CLU levels in L1 overexpressing cells suppressed the ability of L1 to confer increased tumorigenesis and liver metastasis. Genes induced during L1-mediated CRC cell metastasis and enriched in intestinal stem cells might be important for both CRC progression and colonic epithelium homeostasis.

c-Kit is suppressed in human colon cancer tissue and contributes to L1-mediated metastasis.

The transmembrane neural cell adhesion receptor L1 is a Wnt/ß-catenin target gene expressed in many tumor types. In human colorectal cancer, L1 localizes preferentially to the invasive front of tumors and when overexpressed in colorectal cancer cells, it facilitates their metastasis to the liver. In this study, we investigated genes that are regulated in human colorectal cancer and by the L1-NF-κB pathway that has been implicated in liver metastasis. c-Kit was the most highly suppressed gene in both colorectal cancer tissue and the L1-NF-κB pathway. c-Kit suppression that resulted from L1-mediated signaling relied upon NF-κB, which directly inhibited the transcription of SP1, a major activator of the c-Kit gene promoter. Reconstituting c-Kit expression in L1-transfected cells blocked the biological effects conferred by L1 overexpression in driving motility and liver metastasis. We found that c-Kit expression in colorectal cancer cells is associated with a more pronounced epithelial morphology, along with increased expression of E-cadherin and decreased expression of Slug. Although c-Kit overexpression inhibited the motility and metastasis of L1-expressing colorectal cancer cells, it enhanced colorectal cancer cell proliferation and tumorigenesis, arguing that separate pathways mediate tumorigenicity and metastasis by c-Kit. Our findings provide insights into how colorectal cancer metastasizes to the liver, the most common site of dissemination in this cancer.

L1-mediated colon cancer cell metastasis does not require changes in EMT and cancer stem cell markers.

Aberrant activation of Wnt/ß-catenin signaling is common in most sporadic and inherited colorectal cancer (CRC) cells leading to elevated ß-catenin/TCF transactivation. We previously identified the neural cell adhesion molecule L1 as a target gene of ß-catenin/TCF in CRC cells. Forced expression of L1 confers increased cell motility, invasion, and tumorigenesis, and the induction of human CRC cell metastasis to the liver. In human CRC tissue, L1 is exclusively localized at the invasive front of such tumors in a subpopulation of cells displaying nuclear ß-catenin. We determined whether L1 expression confers metastatic capacities by inducing an epithelial to mesenchymal transition (EMT) and whether L1 cosegregates with cancer stem cell (CSC) markers. We found that changes in L1 levels do not affect the organization or expression of E-cadherin in cell lines, or in invading CRC tissue cells, and no changes in other epithelial or mesenchymal markers were detected after L1 transfection. The introduction of major EMT regulators (Slug and Twist) into CRC cell lines reduced the levels of E-cadherin and induced fibronectin and vimentin, but unlike L1, Slug and Twist expression was insufficient for conferring metastasis. In CRC cells L1 did not specifically cosegregate with CSC markers including CD133, CD44, and EpCAM. L1-mediated metastasis required NF-κB signaling in cells harboring either high or low levels of endogenous E-cadherin. The results suggest that L1-mediated metastasis of CRC cells does not require changes in EMT and CSC markers and operates by activating NF-κß signaling.

Nuclear factor-kappaB signaling and ezrin are essential for L1-mediated metastasis of colon cancer cells.

Hyperactivation of beta-catenin-T-cell-factor (TCF)-regulated gene transcription is a hallmark of colorectal cancer (CRC). The cell-neural adhesion molecule L1CAM (hereafter referred to as L1) is a target of beta-catenin-TCF, exclusively expressed at the CRC invasive front in humans. L1 overexpression in CRC cells increases cell growth and motility, and promotes liver metastasis. Genes induced by L1 are also expressed in human CRC tissue but the mechanisms by which L1 confers metastasis are still unknown. We found that signaling by the nuclear factor kappaB (NF-kappaB) is essential, because inhibition of signaling by the inhibitor of kappaB super repressor (IkappaB-SR) blocked L1-mediated metastasis. Overexpression of the NF-kappaB p65 subunit was sufficient to increase CRC cell proliferation, motility and metastasis. Binding of the L1 cytodomain to ezrin - a cytoskeleton-crosslinking protein - is necessary for metastasis because when binding to L1 was interrupted or ezrin gene expression was suppressed with specific shRNA, metastasis did not occur. L1 and ezrin bound to and mediated the phosphorylation of IkappaB. We also observed a complex containing IkappaB, L1 and ezrin in the juxtamembrane region of CRC cells. Furthermore, we found that L1, ezrin and phosphorylated p65 are co-expressed at the invasive front in human CRC tissue, indicating that L1-mediated activation of NF-kappaB signaling involving ezrin is a major route of CRC progression.

Coordinating changes in cell adhesion and phenotype during EMT-like processes in cancer.

Understanding the progression of a primary cancer to the metastatic stage has been the focus of extensive research for years. Commonly accepted concepts in this process (i.e., that of genetic instability and loss of normal cellular constraints on growth and motility) are well established. Other important paradigms, such as the necessary change from an epithelial cell phenotype displaying cell-cell adhesions to a singular and motile mesenchymal-like cell phenotype (possibly derived from a stem cell-like cell) via a process similar to epithelial to mesenchymal transition (EMT), are less well understood. In this review we will address studies linking EMT and cancer stem cells during cancer development and observations that are challenging these concepts.

The cell adhesion nectin-like molecules (Necl) 1 and 4 suppress the growth and tumorigenic ability of colon cancer cells.

A key step in human colon cancer development includes the hyperactivation of Wnt/beta-catenin signaling and the induction of beta-catenin-TCF target genes that participate in colon cancer progression. Recent studies identified members of the immunoglobulin-like cell adhesion molecules (IgCAM) of the L1CAM family (L1 and Nr-CAM) as targets of beta-catenin-TCF signaling in colon cancer cells. L1 was detected at the invasive front of colon cancer tissue and confers metastasis when overexpressed in cells. In contrast to L1, we did not detect in colon cancer cells significant levels of another IgCAM family of molecules, the nectin-like (Necl) receptors Necl1 and Necl4, while Necl4 was previously found in the normal small intestine and colon tissues. We studied the properties of colon cancer cells in which Necl4 and Necl1 were expressed either alone, or in combination, and found that such cells display a wide range of properties associated with tumor suppression. Expression of both Necl1 and Necl4 was the most efficient in suppressing the tumorigenicity of colon cancer cells. This was associated with enhanced rates of apoptosis and change in several apoptosis-related markers. In contrast to its capacity to suppress tumorigenesis, Necl4 was unable to affect the highly malignant and metastatic capacities of colon cancer cells in which L1 was overexpressed. Our results suggest that various IgCAM receptor families play different roles in affecting the tumorigenic function of the same cells, and that Necl1 and Necl4 can fulfill a tumor suppressive role.

L1 cell adhesion molecule (L1CAM) in invasive tumors.

The L1 cell adhesion molecule (L1CAM) belongs to the immunoglobulin superfamily and was originally identified in the nervous system. Recent studies demonstrated L1CAM expression in various types of cancer, predominantly at the invasive front of tumors and in metastases, suggesting its involvement in advanced stages of tumor progression. Overexpression of L1CAM in normal and cancer cells increased motility, enhanced growth rate and promoted cell transformation and tumorigenicity. Moreover, the expression of L1CAM in tumor cells conferred the capacity to form metastases. These properties of L1CAM, in addition to its cell surface localization, make it a potentially useful diagnostic marker for cancer progression and a candidate for anti-cancer therapy. We review the role of L1CAM in cancer progression with particular emphasis on colon cancer, and the potential of anti-L1CAM antibodies as a therapeutic tool for cancer.