Thrombospondin binding by keratinocytes: modulation under conditions which alter thrombospondin biosynthesis

Our recent studies have shown that thrombospondin (TSP) is a potent adhesion factor for normal human keratinocytes. Stimulation of adhesion is presumed to result from the binding of TSP to high-affinity receptors on the surface of responsive cells. The present study indicates that keratinocytes bind TSP in a receptor-like manner. Binding is time- and concentration-dependent, saturable, reversible and specific. Approximately 180 ng of TSP can be bound per 1 x 10(5) cells at saturation and half-maximal binding occurs at 22 nM. A series of monoclonal antibodies to various regions of the TSP molecule were examined for effects on TSP binding and TSP-induced adhesion. An antibody directed against the heparian-binding domain of the TSP molecule significantly inhibited TSP binding but had no effect on adhesion. In contrast, three antibodies which recognize epitopes in the 140-kDa fragment of the molecule inhibited both binding and adhesion. In a previous study we showed that treatment of keratinocytes with interferon-gamma inhibited TSP production and inhibited adhesion under unstimulated conditions as well as in response to TSP. The present study shows that interferon-gamma also inhibits TSP binding by keratinocytes. When the data from the present study are taken together with our past data, they suggest that normal human keratinocytes have the capacity to bind TSP and use this capacity to bind endogenously synthesized TSP. This provides a mechanism for utilizing the endogenously produced TSP to stimulate adhesion.