Platelet proaggregating activity of human colorectal tumour cell lines does not correlate with the degree of differentiation.

Six human colorectal tumour cell lines with various degrees of differentiation were studied. Each of the cell lines studied showed an in vitro platelet proaggregating activity, represented by the induction of typical aggregation waves. This activity was dose-dependent and probably related to a thrombin-dependent mechanism. However, the degree of cell differentiation did not correlate with the proaggregating activity. In fact, significant differences were observed between the two well differentiated cell lines, while a comparison between well and poorly differentiated cell lines did not reveal any difference. These results were confirmed by the ultrastructural observations, indicating that similar platelet-tumour cell interaction may be found in all the cell lines studied. The present results suggest that platelet proaggregating activity of the human colorectal tumour cell lines under investigation is unrelated to their degree of differentiation.

Differential platelet proaggregating activity of three Burkitt's lymphoma-derived human cell lines.

We demonstrate a differential platelet in vitro proaggregating activity in three Burkitt's lymphoma--derived human B cell lines, i.e. Daudi, Raji and P3H-R1. Functional and ultrastructural findings indicated the ability of Daudi cells to induce a marked secondary irreversible platelet aggregation, while the Raji cells only induced a primary-type reversible platelet response; no evidence of proaggregating activity has been obtained for P3H-R1 cells. Luminometric assays indicated that contact of Daudi and Raji, but not P3H-R1, cells with platelet rich plasma (PRP) or platelet poor plasma (PPP) was followed by ADP release, in the range of 2,2-3,5 microM and 0.4-0.6 microM respectively for Daudi and Raji cells. After preincubation of PRP with apyrase Daudi cells induced a reversible platelet response similar to that obtained with the use of Raji cells: then, the irreversible complete platelet response induced by Daudi cells was to be related to ADP release from degranulating platelets. Experiments in gel-filtered platelet systems showed that the plasma co-factor inducing ADP release from Daudi and Raji cells was not fibrinogen. Specific inhibition of platelet thrombin receptors, as well as of cycloxygenase and lipoxygenase pathways, did not modify the proaggregating activity of Daudi and Raji cells. Work is in progress to characterize the plasma factor interacting with Daudi and Raji, but not P3H-R1 cells, and the differences between the three cell lines which support this differential interaction.