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

Abstract

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/β-catenin signaling pathway is an excellent model for studying the role of adhesion-mediated signaling in colorectal cancer (CRC) invasion and metastasis, because β-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/β-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 β-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 β-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.

Keywords: EMT; L1; Lgr5; NF-κB; Wnt target genes; cancer stem cells; cell adhesion; colon cancer; invasion and metastasis; β-catenin.

Figure 1

Domain structure and binding partners of L1. Note the numerous types of L1 ligands in the ectodomain as well as in the cytoplasmic tail domain of the molecule.

Figure 2

L1 is exclusively expressed at the invasive front of human colorectal cancer (CRC) tissue in cells expressing β-catenin in their nuclei. (A) Immunohistochemical staining of human CRC tissue for L1. Note the preferential localization of L1 in invasive areas of the tumor (black arrowheads), but not in the inner more differentiated areas of the tumor. (B) In contrast to L1 localization, a serial tissue section stained with anti β-catenin antibody displays a uniform staining of the same CRC tissue area. (C) Enlarged picture of the boxed area in (A) showing the membranal localization of L1. Single CRC cells invading into the stroma could also be seen (red arrowheads). (D) Magnified picture of the boxed area shown in (B) localizing β-catenin staining in both the cytoplasm and nuclei of CRC tissue cells and in the nuclei of single invasive cells (red arrowheads) at the tumor tissue edge [19]. Scale bar: (A,B) 375 μm, (C,D) 75 μm.

Figure 3

An NF-κB-ezrin signaling pathway is involved in L1 signaling in CRC cells. (A) The cytoskeletal protein ezrin is recruited to the cytoplasmic tail of L1 after it is activated by ROCK phosphorylation. The binding of activated ezrin to L1 involves Tyr1151 on the L1 cytoplasmic tail. (B) The L1-activated ezrin complex recruits the cytoplasmic NF-κB–IκB complex and leads to increased phosphorylation of IκB. (C) Elevated IκB phosphorylation results in its increased degradation by the proteasome and the release of NF-κB from the complex followed by NF-κB migration into the nucleus and transcriptional activation of target genes [40].

Figure 4

Genes induced by L1 in CRC cells include intestinal stem cell signature genes. (A) Insulin like growth factor receptor 2 (IGFBP-2) staining of CRC tissue revealed strong staining of the tumor tissue throughout the tissue, while in the adjacent normal mucosa, IGFBP-2 staining was exclusively confined to the bottom of colonic crypts (black arrowheads). (B) The intestinal stem cell signature gene secreted modular calcium-binding matricellular protein-2 (SMOC-2) was detected at the bottom of colonic crypts in normal colonic mucosa (red arrowheads). (C) In CRC tissue, SMOC-2 was localized in more differentiated areas of the tumor with stronger staining of invasive areas of the tumor (blue arrowhead) [44,46]. Scale bar: (A) 250 μm, (B) 50 μm, (C) 100 μm.