Mediation of growth factor induced DNA synthesis and calcium mobilization by G_q and G_i2

A newly identified subclass of the heterotrimeric GTP binding regulatory protein family, G_q, has been found to be expressed in a diverse range of cell types. We investigated the potential role of this protein in growth factor signal transduction pathways and its potential relationship to the function of other Ga subclasses. Recent biochemical studies have suggested that G_q regulates the β 1 isozyme of phospholipase C (PLCβ₁), an effector for some growth factors. By microinjection of inhibitory antibodies specific to distinct G α subunits into living cells, we have determined that Gα_q transduces bradykinin- and thrombin-stimulated intracellular calcium transients which are likely to be mediated by PLCβ₁. Moreover, we found that Gα_q function is required for the mitogenic action of both of these growth factors. These results indicate that both thrombin and bradykinin utilize G_q to couple to increases in intracellular calcium, and that G_q is a necessary component of the mitogenic action of these factors. While microinjection of antibodies against Gα_i2 did not abolish calcium transients stimulated by either of these factors, such microinjection prevented DNA synthesis in response to thrombin but not to bradykinin. These data suggest that thrombin-induced mitogenesis requires both G_q and G_i2, whereas bradykinin needs only the former. Thus, different growth factors operating upon the same cell type use overlapping yet distinct sets of Ga subtypes in mitogenic signal transduction pathways. The direct identification of the coupling of both a pertussis toxin sensitive and insensitive G protein subtype in the mitogenic pathways utilized by thrombin offers an in vivo biochemical clarification of previous results obtained by pharmacologic studies.