Fibronectins: multifunctional modular glycoproteins.

Abstract

Fibronectins are large glycoproteins that have been implicated in a wide variety of cellular properties, particularly those involving the interactions of cells with extracellular materials. These properties include cell adhesion, morphology, cytoskel- etal organization, migration, differentiation, oncogenic trans- formation, phagocytosis, and hemostasis. During the past sev- eral years, investigations in many laboratories have analyzed the expression, functions, and structure of fibronectins. These studies have revealed that fibronectins have a complex molec- ular structure consisting of multiple specific binding sites, and the complex biological phenomena in which fibronectins par- ticipate can now be considered in terms of this structure. In this brief article we will review the current understanding of the structure and properties of fibronectins. Because a number of comprehensive reviews on various aspects of fibro- nectins have been published (1-13), we shall focus on overall concepts, recent developments and promising future research directions in this rapidly expanding field. Sources of Fibronectins In vivo, fibronectins are found in body fluids (300 #g/ml in plasma, lesser amounts in other fluids), soft connective tissue matrices, and most basement membranes. Fibronectins are synthesized by a wide variety of cells in vitro (Table I). Fibroblasts and endothelial cells are major producers, but many other cell types, including some epithelial cells, synthe- size fibronectin at lower levels. There are at least two types of fibronectin, termed plasma and cellular fibronectins, although there may well be multiple forms of cellular fibronectin. Cel- lular and plasma fibronectins, although distinguishable, are very similar in structure and properties (see below). One major source of plasma fibronectin appears to be hepatocytes (14, 15) x although endothelial cells (16-18) and macrophages (19-22) could also contribute, given their close association with the bloodstream.