Investigação da apoptose plaquetária induzida por heme e pela hemozoína na malária vivax

Abstract

Malaria is a disease caused by the Plasmodium parasite that is transmitted by the Anopheles mosquito. Although most cases of malaria in the world are caused by P. falciparum, in Brazil most reports are due to infections by P. vivax. Vivax malaria has long been neglected as it was considered a benign infection, but severe forms and complications of the disease, including a sudden drop in platelet count, have been reported. One of the hypotheses to explain thrombocytopenia is platelet apoptosis, which may be induced by stimuli released during infection. To understand whether factors released during vivax malaria were inducing platelet apoptosis, platelets from healthy donors were incubated with plasma from patients infected with P. vivax. It was observed that patient plasma induced the apoptotic phenotype in platelets, evidenced by phosphatidylserine exposure, inner mitochondrial membrane depolarization, and cell shrinkage. It has been shown that heme and hemozoin, two toxins released after the lysis of infected red blood cells, induce apoptosis in trophoblast cells and astrocytes. However, heme- and hemozoin-induced platelet apoptosis has not yet been described. To investigate this, platelets from healthy volunteers were incubated with heme or synthetic hemozoin (sHz) and it was observed that both heme and sHz induced platelet apoptosis. Since the antagonistic balance between pro and anti-apoptotic proteins regulates apoptosis and the degradation of Bcl-xL in response to heme was recently evidenced by our group, we investigated which protein degradation pathway, proteasome or calpains, would be involved in this process. Platelets from healthy donors were then pretreated with the proteasome inhibitor, Bortezomib, or the calcium chelator, Bapta-AM, prior to stimulation with heme and sHz. We observed that treatment with Bortezomib or Bapta-AM was not able to prevent platelet apoptosis induced by heme and sHz. Finally, the addition of hemopexin, a heme chelator, reversed the apoptotic effect of plasma from vivax malaria patients on platelets, suggesting the involvement of heme in platelet apoptosis. Our results therefore indicate that heme and hemozoin toxins, through the induction of platelet apoptosis, may be contributing to the thrombocytopenia observed in vivax malaria.