Although tumor cell-induced platelet aggregation is thought to mediate an early step in the metastatic process, little is known about tumor adhesive receptors responsible for the initial platelet-tumor attachments. Because our preliminary work demonstrated that a platelet-immunorelated glycoprotein Ib alpha (GPIb alpha) receptor expressed by the human breast carcinoma cell line MCF-7 participates in tumor-induced platelet aggregation, we examined the synthesis and functional characteristics of this MCF-7-immunorelated GPIb alpha. When S-35-cysteine-labeled, digitonin-lysed MCF-7 cells were immunoprecipitated with platelet-specific monoclonal antibodies (mAbs) to GPIb alpha, major radioactive bands were observed. Northern blots showed MCF-7 transcripts for GPIb alpha under both high- and low-stringency hybridization conditions. In the presence of purified human iodine 125-labeled von Willebrand factor (I-125-labeled vWf) with or without the addition of ristocetin, unlabeled vWf was observed to competitively bind to fixed MCF-7 cells (50% inhibitory concentration = 10 mu g/ml, dissociation constant = similar to 3.8 +/- 1.9 nmol/L, 2.7 x 10(6) +/- 445,000 binding sites/cell) in which non-GPIb alpha vWf binding sites were blocked. I-125-vWf binding to blocked MCF-7 cells could be selectively and completely inhibited by mAbs specific for the vWf binding domain of GPIb alpha but not by mAbs against the GPIX subunit, the GPIb beta subunit, or alternate GPIb alpha epitopes other than the vWf-binding domain. Finally, when whole blood substrate was incubated with a mAb specific for the GPIb binding epitope of vWf, MCF-7-induced platelet aggregation was virtually abolished in comparison with control specimens (N = 8; p < 0.0009). These findings (1) confirm the synthesis and expression of an MCF-7 protein with homology to platelet GPIb alpha, (2) confirm that the functional activity of this MCF-7-immunorelated GPIb alpha differs from that of platelet GPIb alpha, and (3) suggest that MCF-7-immunorelated GPIb alpha in its adhesive interactions with plasma vWf may constitute an initial event in MCF-7-induced platelet aggregation.
