Platelet glycoprotein Ibα (GPIbα) extracellular domain, which is part of the receptor complex GPIb-IX-V, plays an important role in tumor metastasis. However, the mechanism through which GPIbα participates in the metastatic process remains unclear. In addition, potential bleeding complication remains an obstacle for the clinical use of anti-platelet agents in cancer therapy.
Methods and Results:
To generate antibodies that bind to mouse platelet GPIbα, washed mouse platelet lysate was used as the antigen for rat immunization. Obtained hybridoma clones were screened in ELISA for binding affinity to the GPIb-IX complex. Positive clones were further screened for their abilities to inhibit platelet-cancer cell adhesion. Finally, at static condition, two antibodies, 2B4 and 1D12, had virtually no effect on the activation of integrin αIIbβ3, which is used to indicate platelet activation. Then, we characterized the binding sites of 2B4 and 1D12 by 20 purified recombinant GPIbα fragments binding. Results showed that 2B4 and 1D12 shared the same binding sites with vWF.
To determine whether 2B4 and 1D12 affect vWF binding, we tested the binding by flow cytometry using recombined mouse vWF, and then, we investigated platelet aggregation induced by several agonists, including vWF binding agonist ristocetin. Our data demonstrated clearly that 2B4 and 1D12 could inhibit vWF binding.
To investigate whether the inhibition of vWF-GPIbα interaction was associated with tumor metastasis, we examined the effect of 2B4 and 1D12 in each of the following interactions in vitro: between activated platelets and tumor cells, platelets and endothelial cells. Meanwhile, We further investigated the inhibitory effect of these antibodies in vivo using the experimental metastasis model and the spontaneous metastasis model. Results showed that 2B4 and 1D12 could potently inhibit the adhesion of cancer cells in vitro, and metastasisin vivo.
We next investigated whether 2B4 and 1D12 could affect platelet activation and/or induce thrombocytopenia in vivo. Results showed that the addition of 2B4 or 1D12 to PRP did not induce platelet aggregation and injection of 2B4 or 1D12 Fab at appropriate dose did not affect tail-bleeding time and platelet count.
Based on the above findings, we obtained anti-human platelet GPIbα monoclonal antibody YQ3 using the same approach to explore the role of human GPIbα in cancer metastasis. As expected, YQ3 inhibited lung cancer adhesion and demonstrated similar value in metastasis. More importantly, for all three mAbs in our study, none of their Fabs induced thrombocytopenia.
Our results therefore supported the hypothesis that GPIbα contributes to tumor metastasis, and suggested potential value of using anti-GPIbα mAb to suppress cancer metastasis.
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Asterisk with author names denotes non-ASH members.