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

Abstract 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 α (GPIb α ) receptor expressed by the human breast carcinoma cell line MCF-7 participates in tumorinduced platelet aggregation, we examined the synthesis and functional characteristics of this MCF-7-immunorelated GPlbα. When 35 S-cysteine-labeled, digitoninlysed MCF-7 cells were immunoprecipitated with platelet-specific monoclonal antibodies (mAbs) to GPlbα, major radioactive bands were observed. Northern blots showed MCF-7 transcripts for GPlbα under both high- and low-stringency hybridization conditions. In the presence of purified human iodine 125-labeled von Willebrand factor ( 125 I-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 × 10 6 ± 445,000 binding sites/cell) in which non-GPlbα vWf binding sites were blocked. 125 I-vWf binding to blocked MCF-7 cells could be selectively and completely inhibited by mAbs specific for the vWf binding domain of GPlbα but not by mAbs against the GPIX subunit, the GPIbβ subunit, or alternate GPlba 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