RESUMO
The aim of this study was to evaluate cell maturation and the platelet production capacity of the megakaryoblastic DAMI cell line, to characterize platelet-like particles produced and to investigate the mechanisms involved in their production. DAMI cell maturation was induced by phorbol myristate acetate (PMA) and thrombopoietin (TPO). Expression levels of GATA-1, Fli-1 and NF-E2 were evaluated using real-time PCR and western blot. Platelet-like particles were characterized by the presence of GPIb and GPIIb by flow cytometry, while the soluble fragment of GPIb, glycocalicin, was detected by enzyme immunoassay. Dense and alpha granules were evaluated by mepacrine staining and thrombospondin-1 detection, respectively, and by electron microscopy. Functional capacity of platelet-like particles was studied by measuring P-selectin membrane after thrombin stimulation by flow cytometry and actin polymerization using phalloidin-FITC by immunofluorescence. We found that stimulation of DAMI cells with high concentration of PMA and TPO induced the expression of transcription factors GATA-1 and Fli-1 followed by an increase in the isoform a of NF-E2. Mature DAMI cells give rise to extensions resembling proplatelets and later, produce platelet-like particles expressing GPIIb and GPIb on their surface and containing dense and alpha granules, which were confirmed by electron microscopy. Platelet functionality was demonstrated by the increase in P-selectin membrane expression after thrombin stimulation and by their ability to spread on fibrinogen matrices. DAMI cell line induced to differentiate into mature megakaryocytes is able to produce functional platelets providing a suitable model to study the mechanisms involved in platelet generation.
Assuntos
Plaquetas/citologia , Megacariócitos/citologia , Modelos Biológicos , Actinas/análise , Plaquetas/efeitos dos fármacos , Diferenciação Celular/efeitos dos fármacos , Linhagem Celular , Grânulos Citoplasmáticos/ultraestrutura , Citometria de Fluxo , Fator de Transcrição GATA1/genética , Fator de Transcrição GATA1/metabolismo , Expressão Gênica/efeitos dos fármacos , Humanos , Megacariócitos/metabolismo , Proteínas dos Microfilamentos/genética , Proteínas dos Microfilamentos/metabolismo , Subunidade p45 do Fator de Transcrição NF-E2/genética , Subunidade p45 do Fator de Transcrição NF-E2/metabolismo , Selectina-P/genética , Selectina-P/metabolismo , Contagem de Plaquetas , Glicoproteínas da Membrana de Plaquetas/genética , Glicoproteínas da Membrana de Plaquetas/metabolismo , Polimerização/efeitos dos fármacos , Receptores Citoplasmáticos e Nucleares/genética , Receptores Citoplasmáticos e Nucleares/metabolismo , Acetato de Tetradecanoilforbol/farmacologia , Trombina/farmacologia , Trombopoetina/farmacologia , Trombospondinas/genética , Trombospondinas/metabolismo , TransativadoresRESUMO
TGF-beta1 plays a main role in tissue repair by regulating extracellular matrix production and tissue granulation. Platelets are one of the main sources of this cytokine in the circulation. The aim of this study was to evaluate the presence of the TGF-beta receptors on platelets, the effect of TGF-beta1 on platelet aggregation and the underlying intracellular mechanisms. TGF-beta receptors on platelets were studied by flow cytometry and their mRNA by PCR. Platelet aggregation was assessed by turbidimetric methods and intracellular pathways by Western blot. TGF-beta receptor type II and mRNA codifying for TbetaRI and TbetaRII were found in platelets. We demonstrated that TGF-beta1 did not trigger platelet aggregation by itself but had a modulating effect on ADP-induced platelet aggregation. Either inhibition or increase in platelet aggregation, depending on the exposure time to TGF-beta1 and the ADP concentration used, were shown. We found that platelets possess Smad2 protein and that its phosphorylation state is increased after exposure to TGF-beta1. Besides, TGF-beta1 modified the pattern of ADP-induced tyrosine phosphorylation. Increased phosphorylation levels of 64-, 80- and 125-kDa proteins during short time incubation with TGF-beta1 and increased phosphorylation of 64- and 125-kDa proteins after longer incubation were observed. The modulating effect of TGF-beta1 on platelet aggregation could play a role during pathological states in which circulating TGF-beta1 levels are increased and intravascular platelet activation is present, such as myeloproliferative disorders. In vascular injury, in which platelet activation followed by granule release generates high local ADP concentrations, it could function as a physiological mechanism of platelet activation control.