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 matu ration, and compared with platelets. Megakaryoblasts and platelets con tained 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) do wnregulation of mRNA for G(s) alpha and disappearance of iloprost-indu ced Ca2+ mobilization, (2) upregulation of the long form of G(s) alpha protein (G(s) alpha-L) and an increase in iloprost-induced cAMP forma tion, and (3) upregulation of G(16)alpha mRNA and G(16)alpha protein a nd appearance of thromboxane A(2)-induced signaling (Ca2+ mobilization and stimulation of prostaglandin I-2-induced cAMP formation). G(z) al pha protein was absent in the megakaryoblasts despite weak expression of G(z) alpha mRNA in DAMI and relatively high levels of G(z) alpha mR NA and G(z) alpha protein in platelets. These findings reveal major ch anges in G protein-mediated signal transduction during megakaryocytopo iesis and indicate that G(16)alpha couples the thromboxane receptor to phospholipase C beta.