Fibronectin and Its Integrin Receptors in Cancer

Erkki Ruoslahti    Cancer Research Center The Burnham Institute La Jolla, California 92037

The adhesive extracellular matrix protein fibronectin and its integrin receptors play important roles at several stages of tumor development. Tumor cells are generally less adhesive than normal cells and deposit less extracellular matrix. The loosened matrix adhesion that results may contribute to the ability of tumor cells to leave their original position in the tissue.

Normal cells, when detached, stop growing and undergo anoikis (apoptosis caused by loss of adhesion). Integrin-activated pathways mediated by focal adhesion kinase (FAK) and the adapter protein Shc seem to be particularly important in anchorage dependence; many oncoproteins are capable of shunting these pathways. Malignant cells circumvent anchorage dependence with the help of oncoproteins.

Once invading tumor cells have gained access to the circulation, adhesion to the endothelia and other tissue components facilitates the establishment of tumor colonies at distant sites. Specific tissue affinities may underlie the tendency of some tumors to metastasize preferentially to certain tissues. Interfering with tumor cell attachment with integrin-binding peptides has been shown to be an effective antimetastatic strategy in animal experiments.

Tumor angiogenesis is yet another aspect of malignancy wherein extracellular matrices and integrins are important. Angiogenic endothelial cells in tumor vessels depend on the αv family of integrins for survival. Inhibiting angiogenesis with compounds that block the activity of αv integrins, and targeting drugs into tumors through these integrins, show promise as new anticancer strategies. © 1999 Academic Press.

I INTRODUCTION

Cancer is a disease of tissue architecture. In forming tissues and organs during development, cells specialize and migrate to their appropriate places in an orderly way. After this process is completed, the body plan is strictly maintained, except in cancer. Cancerous cells acquire the ability to breach tissue barriers, trespassing into adjacent tissues and distant sites in the body. This process, metastasis, is the most devastating aspect of cancer. Metastatic cancer has usually reached so many places that cure by surgery becomes impossible. For that reason, invasion into normal tissue and metastasis are the hallmarks of malignancy. A benign tumor that is not removed can get very large; the cells that make up such a tumor obviously overproliferate, but unlike malignant cancer cells, they do not invade or metastasize.

Adhesive interactions are thought to play a major role in the construction of the body plan during development. These interactions comprise an area code system that guides cells to their appropriate locations in the body and anchors them there. Adhesion is also important in the maintenance of the body plan. Thus, it is not surprising that cell adhesion molecules—integrins and their extracellular matrix ligands in particular—are important in cancer.

Integrins are the main class of cell adhesion receptors for extracellular matrices and can also serve as cell—cell adhesion receptors (Hynes, 1992; Springer, 1994; Ruoslahti, 1996). Integrins are membrane proteins that consist of an α and a β subunit, each with a molecular mass in the 100- to 200-kDa range. Both subunits span the cell membrane, with most of the polypeptide outside the cell. The cytoplasmic domains of the integrin subunits, with one exception, are short, 30- to 50-amino acid peptides. There are 15 known α subunits and 8 β subunits, which combine into some 25 different integrins. The β1, β2, and αv subunits can pair with a particularly large number of possible companion subunits, each defining its subfamily among integrins. The extracellular ligand-binding specificity of an integrin is generated jointly by the α and β subunits of the integrin.

Integrins display specificity on several levels. First, they are expressed in a cell-type- and stage-specific manner. Thus, one group of integrins is associated with migration and proliferation in various types of cells. These “emergency integrins” include α5β1, αvβ3, and αvβ6 (Sheppard, 1996). These integrins may be particularly important in cancer. Many other integrins are selectively expressed in a certain cell type or a few cell types. Examples of cell-type-specific integrins include αIIbβ3 in platelets and α6β4 in epithelial cells. Another level of integrin specificity is manifested in their ligand binding. Many of the integrins bind the RGD cell attachment sequence, but they recognize that sequence differentially in the context of various extracellular matrix proteins, such that some bind primarily to fibronectin and others to vitronectin (Ruoslahti, 1996). At yet another level of specificity, individual integrins mediate distinct signals into the interior of the cell (Schwartz et al., 1995; Clark and Brugge, 1995; Juliano and Haskill, 1993).

Integrins have been shown to be signaling molecules capable of generating both common signals and signals that are specific for individual integrins. It has also been found that information can flow in the reverse direction through integrins; their ligand-binding ability is regulated from inside the cell (Chan and Hemler, 1993; Zhang et al., 1996; Kolanus et al., 1996; Hughes et al., 1997).

In this article, I review the current understanding of the role of integrins and their ligands in the development and dissemination of cancer. Also discussed are a number of emerging integrin-based cancer treatments.

II REDUCED ADHESIVENESS IS NEEDED FOR DETACHMENT AND MIGRATION

To be able to emigrate to another tissue, cancer cells have to detach from their original location, invade a blood or lymphatic vessel, travel in the circulation to a distant site, and establish a new cellular colony (Fig. 1). At every one of these steps, they must escape a number of controls that keep normal cells in place.

The first step in cancer invasion is likely to be loosening of the adhesive restraint between cells. Both cell–extracellular matrix adhesion and cell—cell adhesion contribute to keeping cells in place. Indeed, adhesion molecules that mediate these interactions are frequently missing or compromised in cancer cells.

The first adhesion anomaly discovered was the loss of fibronectin matrix in malignantly transformed cells in the early 1970s (Ruoslahti, 1988). Fibronectin is the prototype cell attachment protein found in the matrix surrounding normal cells. The fibronectin matrix mediates cell adhesion and anchorage through a number of fibronectin-binding integrins, including α5β1. The fibronectin matrix appears to be an important constraint on cells, because its restoration by various means suppresses cell migration and tumorigenicity.

Fibronectin does not form matrix spontaneously—the cell uses its α5β1 integrin to capture secreted fibronectin and convert it into the fibrils that are then deposited into the matrix. Forced expression of the α5β1 integrin in tumor cells reduces their motility and tumorigenicity (Giancotti and Ruoslahti, 1990). Conversely, a decrease in α5β1 expression increases the tumorigenicity of CHO cells (Schreiner et al., 1991). Some of the α5β1 integrin effect may be related to a signaling role of the integrin (Varner et al., 1995; Varner and Cheresh, 1996). However, the increase in fibronectin matrix assembly that accompanies an increase in α5β1 expression or activity (Giancotti and Ruoslahti, 1990; Wu et al., 1996) appears to be responsible for most of the effect. Thus, expression of fibronectin from a transfected cDNA can convert tumorigenic cells into nontumorigenic ones (Wu et al., 1996), and the fibrillar form of fibronectin, superfibronectin, has a tumor-suppressive effect in vitro and in vivo. I discuss superfibronectin later on in this review.

Malignant transformation by oncogenes generally reduces cellular adhesiveness, and this may be one critical element in tumorigenesis. At least two oncogenes, Src and Ras, can impair integrin activity (Akamatsu et al., 1996; Hughes et al., 1997). The Src oncogene can directly phosphorylate the β1 integrin subunit, causing loss of β1 integrin ligand-binding affinity (Akamatsu et al., 1996). Src also phosphorylates a number of signaling molecules associated with integrins; the effect appears to be to shunt the integrin signaling pathways that control anchorage dependence (below). The influence of Ras on integrins was discovered in experiments designed to reveal cDNAs capable of reducing integrin affinity for their ligands (Hughes et al., 1997). Importantly, by impairing the α5β1 integrin, both Src and Ras reduce fibronectin matrix deposition. In addition, oncogenic Src reduces fibronectin synthesis by the cells...