Plasma Membrane Vesicles from Mouse 15091A Tumor Cells: Agent of Platelet Aggregating Activity

We reported that cells from most tumors display platelet aggregating activity (PAA) in heparinized plasma and that this activity contributes to metastasis.Recently, we demonstrated that PAA can be used as a marker of cell transformation in virally infected rat cells. The material responsible for PAA is shed into culture medium. Characterization revealed a material which is particulate and sedimentable at 50,000x g for 60 min.; it contains proteins and lipids with a free cholesterol to phospholipid ratio of 0.556.Delipida-tion as well as complete solubilization abolished PAA.SDS-ME PAGE, 7.5% slab gels, revealed 20 bands.

EM studies of 50,000x g pellets shed by 15091A cells indicated they contained numerous vesicles, some solid bodies, numerous free or vesicle associated small particles, and some amorphous material. Discontinuous sucrose density gradient centrifugation of the 50,000x g pellet yielded at the 1.07-1.17 g/cm³ interface a predominantly vesicular fraction which was the most active interfacial material. The vesicles, visible with phase contrast microscopy, resemble those produced by artificial plasma membrane vesiculation in various cell systems, including normal cells. Since PAA is only shown by transformed cells, vesicles from these must be different or much more numerous. Spontaneous vesiculation by tumor cells may be potentially important in understanding cell transformation and tumor metastases.