Induction of inducible nitric-oxide synthase by the heterotrimeric G protein G alpha(13)

While the functions of several G protein alpha subunits such as alpha(s) and alpha(q) are relatively well understood, the action of others such as alpha(13) remain largely undefined. Because of recent interest in regulation of nitric-oxide synthase (NOS) by G protein-coupled signaling systems and findings that receptors for two proinflammatory substances, thrombin and thromboxane couple to alpha(13), we studied the effect of alpha(13) on NOS activity in a renal epithelial cell line. We found that stable overexpression of alpha(13) or its GTPase-deficient mutant, alpha(13Q226L), in a continuous renal epithelial cell line (MCT) increased NOS activity. The increased NOS activity was due to increased expression of the macrophage-inducible form of NOS (iNOS). iNOS protein and activity were not increased in similar cells expressing an activated alpha(s) (alpha(sQ227L)) or were minimally increased in cells expressing activated alpha(i1) (alpha(i1Q204L)) and alpha(q) (alpha(qQ209L)), members of the three other G protein alpha chain families. Transient coexpression of alpha(13) or (alpha(13Q226L)) increased the activity of an iNOS promoter-CAT construct demonstrating that alpha(13) increases iNOS expression through transcription. Consequently, alpha(13) induces iNOS through a novel mechanism that is distinct from that of other G protein a chains and that may mediate the actions of G protein-dependent proinflammatory agents.