Locomotion of tumor cells as an element of invasion and metastasis.

Malignant tumor cells are endowed with the ability to invade host tissues and to produce metastases. In this review, tumor cell locomotion as an important pathogenic mechanism in the invasive process is discussed. The traffic of neoplastic cells along preformed or newly created tissue pathways will not only depend on cellular cues and/or extracellular stimulatory or inhibitory soluble factors, but also on the interactions of tumor cells with the invaded tissue. Normal tissue exerts its influence on tumor cell migration mainly by providing contact surfaces, as both the growth and locomotor phenomena of tumor cells depend on the cells ability to adhere to structures present along the invasion front. Migrating tumor cells are morphologically characterized by a cell shape change typical for locomoting cells, i.e. polarization. This locomotor phenotype is associated with altered functions of the cytoskeleton induced by agents acting on cell surface receptors, on the signal cascade, or on the cytoskeletal apparatus itself, or evoked by mutations of cytoskeletal proteins. The search for stop signals of tumor cell locomotion is of particular interest, as this may represent an approach to exert an influence on the invasive process.