Signal transduction through trimeric G proteins in megakaryoblastic cell lines

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- 1/4 , G(i)α-3, G(q)α, and G12α protein. Maturation was accompanied by (1) downregulation of mRNA for G(s)α and disappearance of iloprost-induced Ca2+ 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 G16α mRNA and G16α protein and appearance of thromboxane A2-induced signaling (Ca2+ mobilization and stimulation of prostaglandin I2-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 G16α couples the thromboxane receptor to phospholipase C(β).