Gerhard Christofori

A causal role for E-cadherin in the transition from adenoma to carcinoma

Development of malignant tumours is in part characterized by the ability of a tumour cell to overcome cell–cell adhesion and to invade surrounding tissue. E-cadherin is the main adhesion molecule of epithelia and it has been implicated in carcinogenesis because it is frequently lost in human epithelial cancers. Re-establishing the functional cadherin complex in tumour cell lines results in a reversion from an invasive to a benign epithelial phenotype. However, it remained unresolved whether the loss of E-cadherin-mediated cell adhesion was a cause or a consequence of tumour progression in vivo. Here we report that the loss of E-cadherin expression coincides with the transition from well differentiated adenoma to invasive carcinoma in a transgenic mouse model of pancreatic β-cell carcinogenesis (Rip1Tag2). Intercrossing Rip1Tag2 mice with transgenic mice that maintain E-cadherin expression in β-tumour cells results in arrest of tumour development at the adenoma stage, whereas expression of a dominant-negative form of E-cadherin induces early invasion and metastasis. The results demonstrate that loss of E-cadherin-mediated cell adhesion is one rate-limiting step in the progression from adenoma to carcinoma.

The role of the cell-adhesion molecule E-cadherin as a tumour-suppressor gene

E-cadherin-mediated cell-cell adhesion is lost during the development of most epithelial cancers. Recent evidence indicates that the loss of E-cadherin function, besides causing loss of cell-cell adhesion, might also convey signals that actively induce tumour-cell invasion and metastasis.

N-CAM modulates tumour-cell adhesion to matrix by inducing FGF-receptor signalling

Loss of expression of neural cell-adhesion molecule (N-CAM) is implicated in the progression of tumour metastasis. Here we show that N-CAM modulates neurite outgrowth and matrix adhesion of β-cells from pancreatic tumours by assembling a fibroblast-growth-factor receptor-4 (FGFR-4) signalling complex, which consists of N-cadherin, FGFR-4, phospholipase Cγ (PLC-γ), the adaptor protein FRS2, pp60c-src, cortactin and growth-associated protein-43 (GAP-43). Dominant-negative FGFR-4, inhibitors of FGFR signalling and anti-β1-integrin antibodies repress matrix adhesion induced by N-CAM. FGF ligands can replace N-CAM in promoting matrix adhesion but not neurite outgrowth. The results indicate that N-CAM stimulates β1-integrin-mediated cell–matrix adhesion by activating FGFR signalling. This is a potential mechanism for preventing the dissemination of metastatic tumour cells.

Cell adhesion in tumor invasion and metastasis: loss of the glue is not enough

Tumor cells often show a decrease in cell-cell and/or cell-matrix adhesion. An increasing body of evidence indicates that this reduction in cell adhesion correlates with tumor invasion and metastasis. Two main groups of adhesion molecules, cadherins and CAMs, have been implicated in tumor malignancy. However, the specific role that these proteins play in the context of tumor progression remains to be elucidated. In this review, we discuss recent data pointing to a causal relationship between the loss of cell adhesion molecules and tumor progression. In addition, the direct involvement of these molecules in specific signal transduction pathways will be considered, with particular emphasis on the alterations of such pathways in transformed cells. Finally, we review recent observations on the molecular mechanisms underlying metastatic dissemination. In many cases, spreading of tumor cells from the primary site to distant organs has been characterized as an active process involving the loss of cell-cell adhesion and gain of invasive properties. On the other hand, various examples of metastases exhibiting a relatively benign (i.e. not invasive) phenotype have been reported. Together with our recent results on a mouse tumor model, these findings indicate that passive metastatic dissemination can occur, in particular as a consequence of impaired cell-matrix adhesion and of tumor tissue disaggregation.

Cadherins and the tumour progression: is it all in a switch?

Progression to tumour malignancy involves changes in a tumour cells capabilities to adhere and communicate with neighboring cells and with its extracellular environment. Correlation studies in human cancer specimen and functional experiments with cultured tumour cells and transgenic mouse models have indicated that the loss of the cell adhesion molecule E-cadherin is causally involved in the formation of epithelial cancers (carcinomas). More recently, it has been observed that the function of E-cadherin is replaced or overruled by the expression of mesenchymal cadherins, such as N-cadherin. Although the functional implication of such a cadherin switch remains to be elucidated, recent experimental results demonstrating an interaction of cadherins with tyrosine kinase receptors suggest that changes in cadherin expression may not only modulate tumour cell adhesion but also affect signal transduction and, hence, the malignant phenotype.

REDUCED EXPRESSION OF NEURAL CELL ADHESION MOLECULE INDUCES METASTATIC DISSEMINATION OF PANCREATIC BETA TUMOR CELLS

As in the development of many human cancers, in a transgenic mouse model of β-cell carcinogenesis (Rip1Tag2), expression of neural cell adhesion molecule (NCAM) changes from the 120-kDa isoform in normal tissue to the 140/180-kDa isoforms in tumors. NCAM-deficient Rip1Tag2 mice, generated by crossing Rip1Tag2 mice with NCAM knockout mice, develop metastases, a tumor stage that is not seen in normal Rip1Tag2 mice. In contrast, overexpression of NCAM 120 in NCAM-deficient Rip1Tag2 mice prevents tumor metastasis. The results indicate that the loss of NCAM-mediated cell adhesion is one rate-limiting step in the actual metastatic dissemination of β tumor cells.

The Tumor-Suppressor Function of E-Cadherin

The transition from benign tumors to invasive, metastatic cancer cells involves changes in the extracellular matrix environment, cell motility, and cell-cell adhesion. Because cell-cell–adhesion molecules are dynamically regulated during human carcinogenesis, they have been implicated in tumorigenesis, especially during the later stages of tumor progression—that is, tumor-cell invasion and metastasis (Thiery 1996). In this review, we focus on the evidence from genetics and cell-culture studies that support a functional role of E-cadherin/catenin adhesion complexes in carcinogenesis.

Molecular mechanisms of tumor angiogenesis and tumor progression

The formation of new blood vessels (angiogenesis) is crucial for the growth and persistence of primary solid tumors and their metastases. Furthermore, angiogenesis is also required for metastatic dissemination, since an increase in vascular density will allow easier access of tumor cells to the circulation. Induction of angiogenesis precedes the formation of malignant tumors, and increased vascularization seems to correlate with the invasive properties of tumors and thus with the malignant tumor phenotype. In the last few years, the discovery and characterization of tumor-derived angiogenesis modulators greatly contributed to our understanding of how tumors regulate angiogenesis. However, although angiogenesis appears to be a rate-limiting event in tumor growth and metastatic dissemination, a direct connection between the induction of angiogenesis and the progression to tumor malignancy is less well understood. In this review, we discuss the most recent observations concerning the modulation of angiogenesis and their implications in tumor progression, as well as their potential impact on cancer therapy.

Recent advances in cancer research: mouse models of tumorigenesis

Over the past 20 years, cancer research has gained major insights into the complexity of tumor development, in particular into the molecular mechanisms that underlie the progressive transformation of normal cells into highly malignant derivatives. It is estimated that the transformation of a normal cell to a malignant tumor cell is dependent upon a small number of genetic alterations, estimated to be within the range of four to seven rate-limiting events. Critical events in the evolution of neoplastic disease include the loss of proliferative control, the failure to undergo programmed cell death (apoptosis), the onset of neoangiogenesis, tissue remodeling, invasion of tumor cells into surrounding tissue and, finally, metastatic dissemination of tumor cells to distant organs. In patients, the molecular analysis of these multiple steps is hampered by the unavailability of tumor biopsies from all tumor stages. In contrast, mouse models of tumorigenesis allow the reproducible isolation of all tumor stages, including normal tissue, which are then amenable to pathological, genetic and biochemical analyses and, hence, have been instrumental in investigating cancer-related genes and their role in carcinogenesis. In this review, we discuss mouse tumor models that have contributed substantially to the identification and characterization of novel tumor pathways. In particular, we focus on transgenic and knockout mouse models that closely mimic human cancer and thus can be used as model systems for cancer research.

Novel forms of acidic fibroblast growth factor-1 are constitutively exported by β tumor cell lines independent from conventional secretion and apoptosis

Acidic and basic fibroblast growth factors (FGF-1 and FGF-2) are strong mitogens for many tumor cell types and potent inducers of angiogenesis in vitro and in vivo. It is notable that these proteins lack classical signal sequences for secretion; the mechanism by which they are released from cells remains obscure. We demonstrate here that FGF-1 is constitutively exported by tumor cell lines derived from highly angiogenic β cell tumors of transgenic mice. Remarkably, FGF-1 is sequestered as a latent form in the conditioned medium, as assessed by a lack of mitogenic activity and heparin affinity. High salt treatment of conditioned medium unveils the sequestered FGF-1 as novel high molecular weight forms with reduced heparin affinity and a molecular mass of approximately 30–40kDa; we refer to these as exported forms of FGF-1 (XP-FGF-1). Reducing and denaturing agents convert XP-FGF-1 into the 18kDa monomeric form of FGF-1, indicating it represents tight aggregates of FGF-1. XP-FGF-1 is found in cell lysate as well as conditioned medium, suggesting that they represent export intermediates. Brefeldin A, an inhibitor of conventional secretion, does not interfere with FGF-1 export. Moreover, cell lysis or apoptosis are not involved in this export pathway. Therefore, these data demonstrate that FGF-1 is constitutively exported by β tumor cell lines via a novel secretory pathway so as to facilitate tumor growth and angiogenesis.