BACKGROUND: Platelets are small circulating blood cells mostly known for their crucial role in hemostasis. Their alpha granules contain diverse cargoes, which platelets actively secrete in response to vascular injury. While over 300 proteins have been reported in platelet granules, the mechanisms that govern their secretion are not well understood. It has been suggested that platelets can regulate their secretion by preferentially releasing pro- or anti-angiogenic proteins in response to different stimuli. Differential secretion is still highly controversial, as not all groups were able to reproduce this. Most published literature focuses on a very limited number of secreted proteins and have used few agonists to investigate signaling pathways leading to platelet secretion. We therefore sought to explore the secretion patterns of 19 different proteins with angiogenic or inflammatory properties in response to 6 different platelet activation pathways.
METHODS AND RESULTS: Blood was drawn from consenting healthy adults and platelets were isolated and washed of plasma proteins. Platelets were then activated with agonists targeting different classical activation pathways (arachidonic acid, ADP, collagen, PAR-1 and PAR-4 receptor-activating peptides and epinephrine). Following aggregation, supernatants containing proteins released by platelets were collected. Quantification of 19 different cytokines having either an inflammatory and/or angiogenic profile was assessed by flow cytometry using multiplex kits (AimPlex Biosciences). All platelet agonists induced secretion to different extents. Of all the agonists tested, epinephrine induced the highest release. However, there was no trend towards differential secretion of pro- or anti-angiogenic factors, nor pro- or anti-inflammatory factors in response to specific activation pathways.
CONCLUSION: Unlike previously reported, we were not able to confirm differential secretion by platelets in response to different stimuli. However, all agonists were not equally potent at inducing platelet secretion, epinephrine appearing an important secretagogue.