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Heme stimulates platelet mitochondrial oxidant production via the activation of toll-like receptor 4 signaling to mediate targeted granule secretion

Hemolysis is a pathological component of many diseases and is associated with thrombosis and vascular dysfunction. Hemolytic products, including cell-free hemoglobin and free heme directly activate platelets. However, the effect of hemolysis on platelet degranulation, a central process in not only thrombosis, but also inflammatory and mitogenic signaling, remains less clear. Our group showed that hemoglobin-induced platelet activation involved the production of mitochondrial reactive oxygen species (mtROS). However, the molecular mechanism by which extracellular hemolysis induces platelet mtROS production, and whether the mtROS regulate platelet degranulation remains unknown. Here, we demonstrate using isolated human platelets that cell free heme is a more potent agonist for platelet activation than hemoglobin, and stimulates the release of a specific set of molecules from the α-granule of platelets, including the glycoprotein thrombospondin-1 (TSP-1). We uncover the mechanism of heme-mediated platelet mtROS production which is dependent on the activation of platelet TLR4 signaling and leads to the downstream phosphorylation of complex-V by the serine kinase Akt. Notably, inhibition of platelet TLR4 or Akt, or scavenging mtROS prevents heme-induced granule release in vitro. Further, heme-dependent granule release is significantly attenuated in vivo in mice lacking TLR4 or those treated with the mtROS scavenger MitoTEMPO. These data elucidate a novel mechanism of TLR4-mediated mitochondrial regulation, establish the mechanistic link between hemolysis and platelet degranulation, and begin to define the heme and mtROS-dependent platelet secretome. These data have implications for hemolysis-induced thrombo-inflammatory signaling and for the consideration of platelet mitochondria as a therapeutic target in hemolytic disorders. Key points Heme induces platelet mtROS production by inhibiting complex-V activity via TLR4 signaling. Heme stimulated platelet granule secretion is regulated by mtROS.