Activation of platelet heparitinase by tumor cell-derived factors

The nature of the cooperation between platelets and tumor cells during the process of blood-borne metastasis is essentially unknown. In previous in vitro studies we showed that platelets participated in the formation of gaps in the endothelial cell lining, and that concomitantly heparan sulfate glycosaminoglycans were degraded by the platelet heparitinase, released on activation of platelets. In the current study we show that the ability to degrade proteoheparan sulfate derived from endothelial extracellular matrix is gradually eliminated when the number of human platelets is decreased from 5 x 10(7) to 10(6) cells/mL. When aliquots of conditioned media or lysates of either Eb or heat-inactivated ESb mouse lymphoma cells (both of which showed no heparanase activity) were added to freeze-thawed lysates of 10(6) platelets, a reappearance of platelet heparitinase activity was observed. A similar activation was not elicited by lysates of several normal mammalian cells. These data suggest that in its native form, a fraction of the platelet heparitinase is stored in an inactive form that can be activated by a factor secreted by lymphoma, but not by normal cells. Partial characterization of the heparitinase-activating factor showed that it is a heat-stable polyanionic molecule, devoid of proteolytic activity and resistant to both proteolytic and chondroitinase digestions. Activation of platelet heparitinase was also observed on coincubation with chondroitinases ABC and AC, suggesting that the inactive form of platelet heparitinase could result from a complex formation with a chondroitinase-sensitive proteoglycan. The lymphoma-derived heparitinase activating factor itself is, however, not a chondroitinase, because activity of chondroitinase could not be detected in Eb and ESb cells. A possible mechanism by which tumor cells recruit and regulate the activity of platelet heparitinase, and its relevance to the progression of blood borne metastasis, is discussed.