Comparison of the GPVI inhibitors losartan and honokiol

Abstract

Losartan and honokiol are small molecules which have been described to inhibit aggregation of platelets by collagen. Losartan has been proposed to block clustering of GPVI but not to affect binding of collagen. Honokiol has been reported to bind directly to GPVI but only at a concentration that is three orders of magnitude higher than that needed for inhibition of aggregation. The mechanism of action of both inhibitors is so far unclear. In the present study, we confirm the inhibitory effects of both agents on platelet aggregation by collagen and show that both also block the aggregation induced by the activation of CLEC-2 or the low affinity immune receptor FcγRIIa at similar concentrations. For GPVI and CLEC-2, this inhibition is associated with a reduction in protein tyrosine phosphorylation of multiple proteins including Syk. In contrast, on a collagen surface, spreading of platelets and clustering of GPVI (measured by single molecule localisation microscopy) was not altered by losartan or honokiol. Furthermore, in flow whole-blood, both inhibitors suppressed the formation of multi-layered platelet thrombi at arteriolar shear rates at concentrations that hardly affect collagen-induced platelet aggregation in platelet rich plasma. Together, these results demonstrate that losartan and honokiol have multiple effects on platelets which should be considered in the use of these compounds as anti-platelet agents.

Keywords:

• CLEC-2
• GPVI
• GPVI
• honokiol
• losartan
• platelets

Acknowledgements

SPW holds a BHF Chair (CH03/003) and ATH holds a BHF Studentship (FS/15/71/31677). JAE is supported by Deutsche Forschungsgemeinschaft (DFG grant: Eb177/13-1).

Authorship Contributions

M-B.O. designed and performed research, analysed and interpreted data, made the Figures and wrote the manuscript; M.N., C.P., and J.A.P. performed experiments, analysed and interpreted data, and made figures; G.P., L.G.C. and N.S.P. performed experiments; J.A.E. purified rhodocytin and edited the manuscript; J.M.W.H. designed research, interpreted data and edited the manuscript; S.P.W. supervised research, analysed and interpreted data, wrote the manuscript. All authors read and approved the final manuscript.

Disclosure Statement

The authors declare no competing financial interests.

Supplementary Material

Supplemental data for this article can be accessed here.

Additional information

Funding

This work was supported by the British Heart Foundation [RG/13/18/30563].