Induction of fibronectin matrix assembly in human fibrosarcoma cells by dexamethasone

Abstract

Previous studies have suggested that the assembly of fibronectin into the extracellular matrix of cultured fibroblasts is mediated by specific matrix assembly receptors that recognize a binding site in the amino terminus of the fibronectin molecule. In the presence of dexamethasone, human fibrosarcoma cells (HT-1080) acquired the ability to specifically bind exogenous plasma fibronectin and incorporate it into a detergent-insoluble extracellular matrix. Dexamethasone-induced fibronectin binding to HT-1080 cells was time dependent, dose dependent, and inhibited by cycloheximide. Saturation binding curves indicated that dexamethasone induced the appearance of 7.7 x 104 matrix assembly receptors per cell. The induced receptors exhibited a dissociation constant (K(D)) for soluble fibronectin of 5.0 x 10-8 M. In parallel experiments, normal fibroblasts exhibited 4.1 x 105 receptors (K(D) = 5.3 x 10-8 M) per cell. In the presence of cycloheximide, the induced fibronectin-binding activity on HT-1080 cells returned to uninduced levels within 12 h. In contrast, fibronectin-binding activity on normal fibroblasts was stable in the presence of cycloheximide for up to 54 h. The first-order rate constant (K(t) = 2.07 x 10-4 min-1) for the transfer of receptor-bound fibronectin to extracellular matrix was four- to fivefold less than that for normal fibroblasts (K(t) = 1.32 x 10-3 min-1). Lactoperoxidase-catalyzed iodination of HT-1080 monolayers indicated that a 48,000-mol-wt cell surface protein was enhanced with dexamethasone. The results from these experiments suggest that dexamethasone induces functional matrix assembly receptors on the surface of HT-1080 cells; however, the rate of incorporation of fibronectin into the matrix is much slower than that of normal fibroblasts.