Signal Transduction Through Trimeric G Proteins in Megakaryoblastic Cell Lines

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Abstract
Abstract The biogenesis of trimeric G proteins was investigated by measurement of the expression of α-subunits in the megakaryoblastic cell lines MEG-01, DAMI, and CHRF-288-11, representing stages of increasing maturation, and compared with platelets. Megakaryoblasts and platelets contained approximately equal amounts of G i α-1/2, G i α-3, G q α, and G 12 α protein. Maturation was accompanied by (1) downregulation of mRNA for G s α and disappearance of iloprost-induced Ca 2+ mobilization, (2) upregulation of the long form of G s α protein (G s α-L) and an increase in iloprost-induced cAMP formation, and (3) upregulation of G 16 α mRNA and G 16 α protein and appearance of thromboxane A 2 -induced signaling (Ca 2+ mobilization and stimulation of prostaglandin I 2 –induced cAMP formation). G z α protein was absent in the megakaryoblasts despite weak expression of G z α mRNA in DAMI and relatively high levels of G z α mRNA and G z α protein in platelets. These findings reveal major changes in G protein–mediated signal transduction during megakaryocytopoiesis and indicate that G 16 α couples the thromboxane receptor to phospholipase C β .