Association of heterotrimeric Gi with the insulin-like growth factor-I receptor -: Release of Gβγ subunits upon receptor activation

The insulin-like growth factor-I receptor (IGF-IR) is a key regulator of cell proliferation and survival. Activation of the IGF-IR induces tyrosine autophosphorylation and the binding of a series of adaptor molecules, thereby leading to the activation of MAPK. It has been demonstrated that pertussis toxin, which inactivates the G(i) class of GTP-binding proteins, inhibits IGF-I-mediated activation of MAPK, and a specific role for G(beta gamma) subunits in IGF-I signaling was shown. In the present study, we have investigated the role of heterotrimeric Gi in IG;F-IR signaling in neuronal cells. Pertussis toxin inhibited IGF-I-induced activation of MAPK in rat cerebellar granule neurons and NG-108 neuronal cells. G(alpha i) and Gp subunits were associated with IGF-IR immunoprecipitates. Similarly, in IGF-IR-null mouse embryo fibroblasts transfected with the human IG;F-IR, Gi was complexed with the IGF-IR. G(alpha s) was not associated with the IGF-IR in any cell type. IGF-I induced the release of the Gp subunits from the IGF-IR but had no effect on the association of G(alpha i). These results demonstrate an association of heterotrimeric Gi with the IGF-IR and identify a discrete pool of G(beta gamma) Subunits available for downstream signaling following stimulation with IGF-I.