RXR Ligands Negatively Regulate Thrombosis and Hemostasis.

Gagan D. Flora,Marilena Crescente,Alexander P. Bye,Neline Kriek,Tanya Sage,Amanda J. Unsworth,Jonathan M. Gibbins,Parvathy Sasikumar

doi:10.1161/atvbaha.117.309207

Gagan D. Flora, Marilena Crescente + Show 6 more

Open Access

https://doi.org/10.1161/atvbaha.117.309207

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Abstract

Platelets have been found to express intracellular nuclear receptors including the retinoid X receptors (RXRα and RXRβ). Treatment of platelets with ligands of RXR has been shown to inhibit platelet responses to ADP and thromboxane A2; however, the effects on responses to other platelet agonists and the underlying mechanism have not been fully characterized. The effect of 9-cis-retinoic acid, docosahexaenoic acid and methoprene acid on collagen receptor (glycoprotein VI [GPVI]) agonists and thrombin-stimulated platelet function; including aggregation, granule secretion, integrin activation, calcium mobilization, integrin αIIbβ3 outside-in signaling and thrombus formation in vitro and in vivo were determined. Treatment of platelets with RXR ligands resulted in attenuation of platelet functional responses after stimulation by GPVI agonists or thrombin and inhibition of integrin αIIbβ3 outside-in signaling. Treatment with 9-cis-retinoic acid caused inhibition of thrombus formation in vitro and an impairment of thrombosis and hemostasis in vivo. Both RXR ligands stimulated protein kinase A activation, measured by VASP S157 phosphorylation, that was found to be dependent on both cAMP and nuclear factor κ-light-chain-enhancer of activated B cell activity. This study identifies a widespread, negative regulatory role for RXR in the regulation of platelet functional responses and thrombus formation and describes novel events that lead to the upregulation of protein kinase A, a known negative regulator of many aspects of platelet function. This mechanism may offer a possible explanation for the cardioprotective effects described in vivo after treatment with RXR ligands.

Highlights

This study identifies a widespread, negative regulatory role for Retinoid X receptors (RXR) in the regulation of platelet functional responses and thrombus formation and describes novel events that lead to the upregulation of protein kinase A, a known negative regulator of many aspects of platelet function
Many intracellular nuclear receptors are expressed in human platelets that negatively regulate platelet function when stimulated by their endogenous ligands.[1,2,3,4,5,6,7,8,9,10,11,12]
We report that RXR ligands increase protein kinase A (PKA) activity via both cAMP and nuclear factor κ-light-chain-enhancer of activated B cells (NFκB)–dependent mechanisms that have not been described for the endogenous agonist of any other nuclear receptor.[24]

Summary

Introduction

9-cis-RA Increases PKA Activity in Resting and Stimulated Platelets The data presented here demonstrates a role for RXR in the negative regulation of platelet function, this is not associated with prominent alterations in the early platelet signaling events associated with GPVI agonists or thrombin. Similar results were observed after treatment with either docosahexaenoic acid or methoprene acid because both alternative RXR ligands caused a significant increase in VASP S157 phosphorylation that was reversed after treatment with the PKA inhibitor H89 (Figure V in the online-only Data Supplement).

Results

Conclusion