Role of fibronectin as a growth factor for fibroblasts
Fibroblast replication is regulated by exogenous signals provided by growth factors, mediators that interact with the target cell surface and signal the cell to proliferate. A useful model of growth regulation, the "dual control model," suggests that growth factors can be grouped either as competence factors or as progression factors, and that optimal replication of fibroblasts requires the presence of both types of growth factors. Although most growth factors are soluble mediators, recent studies have demonstrated that, for some cell types, the extracellular matrix can replace the requirement for a competence factor. Since fibronectin is an important constituent of the extracellular matrix that interacts with specific domains on the fibroblast surface, we examined the ability of fibronectin to act as a competence factor to promote the growth of human diploid fibroblasts. To accomplish this, fibronectins purified from two sources, human plasma and human alveolar macrophages, were tested for their ability to (a) stimulate fibroblast replication in serum-free medium containing characterized progression factors (insulin or alveolar macrophage-derived growth factor); (b) provide a growth-promoting signal early in G1. Fibronectin stimulated fibroblast replication in a dose-dependent manner in the presence of a fixed dose of a progression factor. Conversely, fibronectin conferred on previously unresponsive fibroblasts the ability to replicate in a dose-dependent manner when cultured with increasing amounts of a progression factor. Moreover, fibronectin signaled growth-arrested fibroblasts to traverse G1 approximately 4 h closer to S phase. No differences were observed in the ability of plasma or macrophage fibronectins to provide a competence signal for fibroblast replication. Since fibronectin is a major component of the extracellular matrix, these observations suggest that it may provide at least one of the signals by which the matrix conveys the "competence" that permits fibroblasts to replicate in the presence of an appropriate progression signal.