COUPLING OF THE THROMBIN RECEPTOR TO G(12) MAY ACCOUNT FOR SELECTIVE EFFECTS OF THROMBIN ON GENE-EXPRESSION AND DNA-SYNTHESIS IN 1321N1 ASTROCYTOMA-CELLS

In 1321N1 astrocytoma cells, thrombin, but not carbachol, induces AP-1 -mediated gene expression and DNA synthesis. To understand the diverge nt effects of these G protein-coupled receptor agonists on cellular re sponses, we examined G(q)-dependent signaling events induced by thromb in receptor and muscarinic acetylcholine receptor stimulation. Thrombi n and carbachol induce comparable changes, in phosphoinositide and pho sphatidylcholine hydrolysis, mobilization of intracellular Ca2+, digly ceride generation, and redistribution of protein kinase C; thus, activ ation of these G(q)-signaling pathways appears to be insufficient for gene expression and mitogenesis. Thrombin increases Ras and mitogen-ac tivated protein kinase activation to a greater extent than carbachol i n 1321N1 cells. The effects of thrombin are not mediated through G(i), since ribosylation of G(i)/G(o) proteins by pertussis toxin does not prevent thrombin-induced gene expression or thrombin-stimulated DNA sy nthesis. We recently reported that the pertussis toxin-insensitive G(i ), protein is required for thrombin-induced DNA synthesis. We demonstr ate here, using transfection of receptors and G proteins in COS-7 cell s, that G alpha(1/2) selectively couples the thrombin receptor to AP-1 -mediated gene expression. This does not appear to result from increas ed mitogen-activated protein kinase activity but may reflect activatio n of a tyrosine kinase pathway. We suggest that preferential coupling of the thrombin receptor to G(12) accounts for the selective ability o f thrombin to stimulate Ras, mitogen-activated protein kinase, gene ex pression, and mitogenesis in 1321N1 cells.