Upregulation of MMP-9 expression in MDA-MB231 tumor cells by platelet granular membrane.

The interaction between tumor cells and platelets facilitates the formation of metastasis in a way depending on the platelet aggregating ability of the tumor cell, but the mechanism remains to be elucidated. We have shown, by zymography and Western blot, that platelets greatly increased the secretion to the culture medium of MMP-9 by human mammary tumor cells MDA-MB231. This increase, which was dependent on protein synthesis, was caused by the platelet aggregates interacting with the tumor cells and not by the soluble factors released during platelet activation. Platelet subcellular fractionation allowed the localization of the inducing factor to the membrane fraction of the platelet granules, thus requiring platelet aggregation in order to become accessible on the platelet surface.