The endocannabinoid 2‐arachidonoylglycerol activates human platelets through non‐CB1/CB2 receptors

Summary. Background: The endocannabinoid 2‐arachidonoylglycerol (2‐AG) is an endogenous lipid that acts through the activation of G‐protein‐coupled cannabinoid receptors and plays essential roles in many physiological contexts. In the cardiovascular system 2‐AG is generated by both activated endothelial cells and platelets, and participates in the regulation of inflammation and thrombosis. Although human platelets actively metabolize endocannabinoids, 2‐AG also binds to platelet surface and leads to cell activation. Objective: To investigate the biological consequence of 2‐AG interactions with human platelets and to clarify the role of cannabinoid receptors. Methods: Gel‐filtered platelets were stimulated with 2‐AG in the presence or absence of various inhibitors. Platelet aggregation and secretion were measured in a lumiaggregometer. Calcium ion movements were measured in FURA‐2 loaded platelets. Thromboxane A₂ (TxA₂) generation was evaluated as Thromboxane B₂ accumulation with a commercial EIA assay. Results: 2‐AG induced platelet shape change, aggregation and secretion with a dose‐dependent mechanism that required engagement of platelet TxA₂ receptors. 2‐AG caused also cytosolic calcium increase; however, it was totally dependent on availability of TxA₂. Indeed 2‐AG was able to induce a robust generation of TxA₂ through the cyclooxygenase pathway. Treatment of platelets with inhibitors of monoacylglycerol lipase and fatty acid amide hydrolase did not affect the activation induced by 2‐AG. Moreover, neither CB₁ and CB₂ proteins nor CB₁/CB₂ mRNAs were detected in platelets. Conclusions: 2‐AG can be considered a new physiologic platelet agonist able to induce full platelet activation and aggregation with a non‐CB₁/CB₂ receptor‐mediated mechanism.