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 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 Gi alpha-1/2, Gi alpha-3, Gq alpha, and G12 alpha protein. Maturation was accompanied by (1) downregulation of mRNA for Gs alpha and disappearance of iloprost-induced Ca2+ mobilization, (2) upregulation of the long form of Gs alpha protein (Gs alpha-L) and an increase in iloprost-induced cAMP formation, and (3) upregulation of G16 alpha mRNA and G16 alpha protein and appearance of thromboxane A2-induced signaling (Ca2+ mobilization and stimulation of prostaglandin I2-induced cAMP formation). Gz alpha protein was absent in the megakaryoblasts despite weak expression of Gz alpha mRNA in DAMI and relatively high levels of Gz alpha mRNA and Gz alpha protein in platelets. These findings reveal major changes in G protein-mediated signal transduction during megakaryocytopoiesis and indicate that G16 alpha couples the thromboxane receptor to phospholipase C beta.

Dimerization of the Human Receptors for Prostacyclin and Thromboxane Facilitates Thromboxane Receptor-mediated cAMP Generation

“…8). A previous study in DAMI and CHRF cell lines, representing middle and late stage megakaryocyte maturation, also observed a synergistic induction of cAMP formation following coactivation of IP and TP (39). Thus, the net generation of cAMP in response to TP activation was maximized when IP was present and activated.…”

Gα16 Couples Chemoattractant Receptors to NF-κB Activation

“…In human T cells, alteration of the G␣ 16 expression level by cell-activating agents affects TCR-mediated functions (44). In other hemopoietic cells, G␣ 16 is known to couple the P2U purinergic receptor to calcium mobilization (20), and has been suggested to mediate some of the functions of thromboxane A2 receptor (45). G␣ 16 is expressed at higher levels in early stages of hemopoietic cells, and in poorly differentiated leukemia cells (46).…”

Thromboxane A2 Signalling in Humans: a ‘Tail’ of Two Receptors

“…Whereas many of the latter reported studies have implicated various G protein α subunits in mediating TP activation [45][46][47][48][49][50][51][52][53][54][55] and the latter studies [33,60] mobilization by TPα and TPβ indicating that the TP isoforms may mediate opening of L-type channels in a Gα 12 dependent signalling mechanism [61]. Taken together, these studies investigating the signalling by the TP isoforms indicate that both TPα and TPβ couple to both the G q and G 12 family with no apparent isoform specific differences in their signalling behaviour.…”