Proteolytic degradation of extracellular matrix in tumor invasion.

The migration of tumor cells through basement membranes and extracellular matrices is an integral component of tumor invasion and metastasis. Laminin and fibronectin are two basement membrane- and extracellular matrix-associated noncollagenous glycoproteins that have been shown to promote both cell adhesion and motility. Purified preparations of laminin and fibronectin stimulated the directed migration of B16 murine metastatic melanoma cells in vitro as assessed in modified Boyden chambers. The stimulation of migration occurred over a concentration range of 1-100 micrograms/ml of laminin or fibronectin, with a peak response occurring between 12.5 and 25 micrograms/ml. The maximal response of these cells was 80-120-fold higher than control migration. Affinity-purified antibody preparations specifically abrogated the migration of these cells in response to the respective proteins. Tumor cells in suspension were preincubated in physiologic levels of plasma fibronectin prior to assay to partially mimic what occurs when a metastasizing cell is in the blood stream. This preincubation with plasma fibronectin had no effect on the subsequent migration of cells in response to either laminin or fibronectin. Furthermore, experiments using filters precoated with fibronectin or laminin indicated that these cells could migrate by haptotaxis to these two proteins. We conclude that tumor cell migration in response to such noncollagenous adhesive glycoproteins could be an important aspect in the invasion and metastasis of certain malignant cell types.

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Laminin and fibronectin promote the haptotactic migration of B16 mouse melanoma cells in vitro.

Mechanisms of metastasis.

Metastasizing tumor cells must traverse diverse extracellular matrices during dissemination. Extracellular matrices consist of two basic types, interstitial stroma and basement membranes. Extracellular matrices are chemically complex structures that interact with cell surfaces by a number of mechanisms. There has been a great deal of effort in recent years to understand the molecular nature of extracellular matrices, especially as it relates to the adhesion of normal and malignant cell types. Adhesive noncollagenous glycoproteins, such as laminin and fibronectin, serve pivotal roles in basement membrane and stromal matrices, respectively. These proteins participate in establishing the architecture of extracellular matrices as well as in attaching to the surface of cells and affecting cellular phenotype. This phenotypic effect ranges from adhesion and motility to growth and differentiation. Changes in adhesive characteristics and motility of cells have long been suspected to play a role in mediating the spread of malignant neoplasms. This article is designed to review extracellular matrix constituents that are currently known that can mediate the adhesion and motility of malignant neoplasms. The adhesion of normal and malignant cells to matrices is a complex process mediated by several distinct mechanisms which are initially manifested by changes in cytoskeletal architecture. The topic of normal and malignant cell adhesion to matrices will also be discussed in this regard, since any explanation of tumor cell migration must account for the complex dynamic interactions of the cell surface with the substratum as well as with the cytoskeleton. Finally, current efforts designed to understand the molecular nature of tumor cell:matrix interactions that contribute to metastatic behavior will also be discussed. The rationale behind these studies is that selective inhibition of specific tumor:extracellular matrix interactions can provide an avenue for therapeutic intervention of metastatic cancer.

The role of cell adhesion proteins--laminin and fibronectin--in the movement of malignant and metastatic cells.

In this review some of the major mechanistic pathways by which tumor cells are thought to invade host tissues are discussed. Tumor invasion has been conceived to be the result of pathological, close-range interactions between malignant cells and host stroma. The sequence of events that characterize invasion can be summarized as follows: (a) Tumor cell clusters break from the confinement of the primary tumor. Loss of intercellular junctions (desmosomes), alterations in the chemical composition and physical properties of the cell surface coat (loss of fibronectin and heparan sulfate; excessive amounts of hyaluronate), and loosening of cell-substrate interactions (loss of hemidesmosomes, fibronectin, and heparan sulfate), are among the most frequently listed causes of tumor cell shedding. (b) Increased proteolytic activities at the invasion front cause focal alterations in the surrounding extracellular matrix, thereby changing its physical properties. Collagenases and cathepsins, as well as elastase and other neutral proteinases are the enzymes most frequently associated with matrix destruction and invasion. In some tissues this process is effectively regulated by inhibitors of matrix-degrading, proteolytic enzymes. (c) Tumor cells migrate into the altered matrix, possibly moving as aggregates along guidance tracks provided by host structures (blood vessels, lymphatics, nerves) or matrix macromolecules (collagen and fibronectin tracks). Migration seems to be preceded by increased swelling of glycosaminoglycan (i.e., hyaluronate) in the matrix, ahead of the migrating cell population. Various host cell types (mast cells, fibroblasts, endothelial cells, macrophages, etc.) may participate in these events.

Tumor invasion and host extracellular matrix.

Two different factors chemotactic for cancer cells were extracted in pseudoglobulin fraction of rat ascites hepatoma transplanted tissue. After chromatography on Sephadex G-50 and CM-Sephadex, these factors were separated by gel filtration on Sephadex G-100. The factora was further fractionated by immunoadsorbent chromatography with goat antirat γ-globulin antibody and then with rabbit antirat hemoglobin antibody; it was a protein with a molecular weight of about 78,000, resembling a chemotactic factor previously reported, and its activity was thermolabile. The previously undescribed factor 6 was also a protein with a molecular weight of about 14,000 and its activity was thermostable. Intradermal injection of these factors at low concentrations induced an extravascular migration of circulating tumor cells and formation of metastatic secondary tumors; and little difference in the in vivo effect between these factors was observed. It was thus assumed that the combined action of these two factors may be involved in malignant invasion.

K. Ushijima

Papers

Characterization of two different factors chemotactic for cancer cells from tumor tissue.