Signal transduction through trimeric G proteins in megakaryoblastic cell lines

The biogenesis of trimeric G proteins was investigated by measurement of the expression of alpha-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) alpha-1/2, G(i) alpha-3, G(q) alpha, and G(12)alpha protein. Maturation was accompanied by (1) downregulation of mRNA for G(s) alpha and disappearance of iloprost-induced Ca2+ mobilization, (2) upregulation of the long form of G(s) alpha protein (G(s) alpha-L) and an increase in iloprost-induced cAMP formation, and (3) upregulation of G(16)alpha mRNA and G(16)alpha protein and appearance of thromboxane A(2)-induced signaling (Ca2+ mobilization and stimulation of prostaglandin I-2-induced cAMP formation). G(z) alpha protein was absent in the megakaryoblasts despite weak expression of G(z) alpha mRNA in DAMI and relatively high levels of G(z) alpha mRNA and G(z) alpha protein in platelets. These findings reveal major changes in G protein-mediated signal transduction during megakaryocytopoiesis and indicate that G(16)alpha couples the thromboxane receptor to phospholipase C beta.