Multifaceted Effects of Thymoquinone on Platelet Calcium Homeostasis

Thymoquinone (TQ), the main bioactive ingredient of Nigella sativa, exhibits numerous pharmacological activities and is used for the prevention of many diseases including hypertension and cancer. However, information concerning the effects of TQ on platelets is limited. In this study, we used the upgraded laser microparticle analyzer LaSca-TMF for simultaneous analysis of platelet shape change, aggregation, and changes in [Ca²⁺]_i. We showed that TQ acutely inhibited platelet aggregation induced by ADP, Trap-6, and CRP; however, the rise of [Ca²⁺]_i was inhibited only in CRP-stimulated platelets, but not in ADP- or Trap-6-stimulated ones. DTT, a thiol-reducing agent, prevented TQ-induced effects in platelets, indicating that protein disulfide isomerases could be involved in the regulation of TQ effects on platelets. Our results, for the first time, demonstrated acute inhibitory effects of TQ on platelet activation induced by GPCRs and ITAM-containing receptors, which were independent of PKA and caspase-3 activation. To the best of our knowledge, this is the first example in which complete inhibition of ADP- and Trap-6-, but not CRP-induced, aggregation is accompanied by high [Ca²⁺]_i levels. Additional experimental approaches are required to explain some effects of TQ on calcium homeostasis and TQ could be a valuable molecule for the analysis of calcium homeostasis in platelets and other cells.