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Ca2(+)-sensitive binding of thrombospondin to U937 cells is due to the formation of calcium precipitate in the binding medium.
X Sun, D F Mosher
X Sun, D F Mosher
Published January 1, 1991
Citation Information: J Clin Invest. 1991;87(1):171-176. https://doi.org/10.1172/JCI114967.
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Research Article

Ca2(+)-sensitive binding of thrombospondin to U937 cells is due to the formation of calcium precipitate in the binding medium.

Abstract

Thrombospondin (TSP) binds to U937 monocytic cells in a Ca2(+)-enhancible and EDTA-inhibitable manner (Silverstein, R. L., and R. L. Nachman. 1987. J. Clin. Invest. 79:867-874; Silverstein, R. L., A. S. Asch, and R. L. Nachman. 1989. J. Clin. Invest. 84:546-552). We reproduced the results when RPMI cell culture medium, but not when HBSS was used as binding medium. Addition of 1 mM Ca2+ to RPMI medium increased the binding of TSP to suspended U937 cells more than eightfold; the increase was blocked by EDTA but not by heparin. Further studies showed that addition of 1 mM Ca2+ to RPMI medium resulted in an insoluble precipitate, which did not form when EDTA was present or when 1 mM extra Ca2+ was added to HBSS. TSP bound to the precipitate in a saturable and specific manner. The precipitate enhanced binding of TSP to MG63 osteosarcoma cells in a monolayer binding assay. Enhancement of binding in the monolayer assay was observed for fibronectin and vitronectin as well. Our data indicate that there is not a specific Ca2(+)-dependent TSP receptor on U937 cell surface. Instead, the extra binding enhanced by Ca2+ is due to the formation of insoluble salts in the medium.

Authors

X Sun, D F Mosher

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