G protein coupling to the thrombin receptor in Chinese hamster lung fibroblasts

The specific involvement of G proteins in thrombin receptor-mediated Ca2+ mobilization and DNA synthesis has been studied in single Chinese hamster lung fibroblasts (CCL39 cells) activated by the hexapeptide SFLLRN. Immunoblots performed with antibodies directed against the COOH terminus of G protein alpha subunits revealed that alpha q, alpha i, and alpha o were each present in CCL39 cells. The Ca2+ response to SFLLRN was measured after microinjection of anti-alpha q or anti-alpha o antibodies, which produced a total blockade in 71 and 46% of cells, respectively. A partial inhibition of the SFLLRN-induced Ca2+ response was observed in the remaining cells. The lag time between exposure of the cells to SFLLRN and the onset of Ca2+ mobilization was significantly longer (20-24 s) in cells microinjected with anti-alpha q- or anti-alpha o-antibodies than in control cells microinjected with preimmune serum (9 +/- 1 s). Moreover, the peak height of the Ca2+ response to SFLLRN was decreased by 36 and 73%, respectively in cells microinjected with anti-alpha q or anti-alpha o antibodies. SFLLRN-induced DNA synthesis in growth-arrested CCL39 cells was also inhibited (44-78%) by prior microinjection of anti-alpha q or anti-alpha o antibodies. Anti-alpha 1 antibodies had no effect on the SFLLRN-induced Ca2+ response or on DNA synthesis. These results provide direct evidence that the thrombin receptor in CCL39 cells is coupled to two different types of G proteins, Gq and Go, both causing Ca2+ mobilization and mitogenesis.