Human breast carcinoma cells synthesize a protein immunorelated to platelet glycoprotein-Ibα with different functional properties

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 Ibα (GPIbα) 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 GPIbα. When ³⁵S-cysteine-labeled, digitoninlysed MCF-7 cells were immunoprecipitated with platelet-specific monoclonal antibodies (mAbs) to GPIbα, major radioactive bands were observed. Northern blots showed MCF-7 transcripts for GPIbα under both high- and low-stringency hybridization conditions. In the presence of purified human iodine 125-labeled von hybridization conditions. In the presence of purified human iodine 125-labeled von Willebrand factor (¹²⁵I-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 μg/ml, dissociation constant = ~ 3.8 ± 1.9 nmol/L, 2.7, x 10⁶ ± 445,000 binding sites/cell) in which non-GPIα vWf binding sites were blocked. ¹²⁵I-vWf binding to blocked MCF-7 cells could be selectively and completely inhibited by mAbs specific for the vWf binding domain of GPIbα but not by mAbs against the GPIX subunit, the GPIbβ subunit, or alternate GPIbα 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 GPIbα, (2) confirm that the functional activity of this MCF-7-immunorelated GPIbα differs from that of platelet GPIbα, and (3) suggest that MCF-7-immunorelated GPIbα in its adhesive interactions with plasma vWf may constitute an initial event in MCF-7-induced platelet aggregation.