Identification and characterisation of a platelet GPIb/V/IX-like complex on human breast cancers: Implications for the metastatic process

The glycoprotein (GP) Ib/V/IX receptor complex is an important adhesion mol ecule, originally thought to be unique to the megakaryocytic lineage. Recen t evidence now indicates that GPIb/V/IX may be more widely expressed. In th is study we report the presence of all subunits of the complex on four brea st cancer cell lines, and 51/80 primary breast tumours. The surface express ion of GPIb/V/IX was confirmed by flow cytometry, and by immunoprecipitatio n of biotin surface-labelled tumour cells. Western blotting of cell lysates under reducing conditions revealed that tumour cell-GPIb alpha had a relat ive molecular weight of 95 kDa as compared to 135 kDa on platelets. Despite the discrepant protein size, molecular analyses on the tumour cell-GPIba s ubunit using RT-PCR and DNA sequencing revealed 100% sequence homology to p latelet GPIb alpha. Tumour cell-GPIb/V/IX was capable of binding human von Willebrand factor (vWf), and this binding caused aggregation of tumour cell s in suspension. Tumour cells bound to immobilised vWf in the presence of E DTA and demonstrated prominent filapodial extensions indicative of cytoskel etal reorganisation. Furthermore, in a modified Boyden chamber assay, prior exposure to vWf or a GPIba monoclonal antibody, AK2, enhanced cell migrati on. The presence of a functional GPIb/V/IX-like complex in tumour cells sug gests that this complex may participate in the process of haematogenous bre ast cancer metastasis.