HUMAN BREAST-CARCINOMA CELLS SYNTHESIZE A PROTEIN IMMUNORELATED TO PLATELET GLYCOPROTEIN-IB-ALPHA 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 attachme nts. Because our preliminary work demonstrated that a platelet-immunor elated glycoprotein Ib alpha (GPIb alpha) receptor expressed by the hu man breast carcinoma cell line MCF-7 participates in tumor-induced pla telet aggregation, we examined the synthesis and functional characteri stics of this MCF-7-immunorelated GPIb alpha. When S-35-cysteine-label ed, 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. I n the presence of purified human iodine 125-labeled von Willebrand fac tor (I-125-labeled vWf) with or without the addition of ristocetin, un labeled vWf was observed to competitively bind to fixed MCF-7 cells (5 0% inhibitory concentration = 10 mu g/ml, dissociation constant = simi lar 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 bin ding to blocked MCF-7 cells could be selectively and completely inhibi ted 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 w hole blood substrate was incubated with a mAb specific for the GPIb bi nding epitope of vWf, MCF-7-induced platelet aggregation was virtually abolished in comparison with control specimens (N = 8; p < 0.0009). T hese findings (1) confirm the synthesis and expression of an MCF-7 pro tein with homology to platelet GPIb alpha, (2) confirm that the functi onal 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.