Association of Heterotrimeric Gi with the Insulin-like Growth Factor-I Receptor

Abstract

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 ␤␥ subunits in IGF-I signaling was shown. In the present study, we have investigated the role of heterotrimeric G i in IGF-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 ␣i and G ␤ subunits were associated with IGF-IR immunoprecipitates. Similarly, in IGF-IR-null mouse embryo fibroblasts transfected with the human IGF-IR, G i was complexed with the IGF-IR. G ␣s was not associated with the IGF-IR in any cell type. IGF-I induced the release of the G ␤ subunits from the IGF-IR but had no effect on the association of G ␣i . These results demonstrate an association of heterotrimeric G i with the IGF-IR and identify a discrete pool of G ␤␥ subunits available for downstream signaling following stimulation with IGF-I.Many receptors are coupled to heterotrimeric GTP-binding proteins (G-proteins). Prototypic G-protein coupled receptors (GPCRs) 1 contain a seven-membrane spanning region (1). Activated GPCRs bind to G-proteins and induce the release of G ␤␥ subunits from G ␣ subunits, which allows for the exchange of GDP for GTP on the G ␣ subunit. Activated G i subunits and G ␤␥ heterodimers interact with numerous signaling effectors, including adenylyl cyclase, ion channels, protein kinases, and phospholipases (2-4).In addition to their role in fully differentiated cells, GPCRs have been linked to mitogenesis and development (5-8). A specific role for G i in the induction of mitogenesis has been highlighted by the use of pertussis toxin, which inactivates G i by ADP-ribosylation of the G ␣ subunit. However, G ␣ subunits from several classes of G-proteins are not strongly mitogenic. Rather G ␤␥ heterodimer subunits activate a series of nonreceptor tyrosine kinases, which in turn activates p21 ras and extracellular signal-regulated kinases (or MAPK). Thus, G ␤␥ subunits serve to bridge intracellular signaling of classical GPCRs and mitogenic tyrosine kinase receptors (RTKs).G i also appears to be involved in the mitogenic actions of RTKs. Pertussis toxin variably inhibits the metabolic actions of insulin, both in vitro and in vivo (9 -16), and the insulin receptor may associate with G i (17)(18)(19). Importantly, mice with targeted knockout of G i have defects in insulin signaling (20). EGF-dependent signaling is also impaired by pertussis toxin in rat hepatocytes (21-23) and other cells (24 -27).The insulin-like growth factor-I receptor (IGF-IR), which has strong homology to the insulin receptor, exists as an ␣ 2 -␤ 2 -heterodimer and contains a cytoplasmic tyrosine kinase domain (28, 29). U...

Insulin-like Growth Factors Mediate Heterotrimeric G Protein-dependent ERK1/2 Activation by Transactivating Sphingosine 1-Phosphate Receptors

“…The first proposal is that the receptors possess the ability to directly catalyze heterotrimeric G protein guanine nucleotide exchange (32). Consistent with this, several groups have reported that heterotrimeric G protein subunits co-precipitate with the IGF-1 and insulin receptors (22,23,25) or have demonstrated that small peptides derived from the insulin and IGF-2 receptors promote a mastoparan-like acceleration of GTP exchange on purified heterotrimeric G proteins in vitro (31,(33)(34)(35). The second model advanced to explain heterotrimeric G protein-dependent signaling by non-GPCRs is that the receptors generate some signal that activates an endogenous GPCR, which in turn catalyzes G protein activation.…”

“…In Rat1 fibroblasts, IGF-1-stimulated activation of the ERK1/2 mitogen-activated protein kinase cascade is largely pertussis toxin-sensitive and can be blocked by expressing a G␤␥ subunit sequestrant peptide derived from the C terminus of G proteincoupled receptor kinase 2 (GRK2ct) (19). Similar pertussis toxin-sensitive ERK1/2 activation has been reported in undifferentiated 3T3-L1 cells (20), human intestinal smooth muscle cells (21), primary rat cerebellar granule neurons, and NG-108 neuronal cells (22). In HIRcB cells and 3T3-E1 adipocytes, treatment with pertussis toxin or microinjection of a GRK2ct-glutathione S-transferase fusion protein blocks the mitogenic effect of IGF-1 but not of insulin or EGF (23).…”

Insulin-like growth factor I is a comitogen for hepatocyte growth factor in a rat model of hepatocellular carcinoma

“…39,45 Studies in neuronal and intestinal smooth muscle cells report that activation of the IGF-IR leads to the release of a Gi␤␥ subunit and downstream MAPK activation. 47,48 These data raise the possibility of a similar pathway existing in HCC, activation of which acts to alter HGF signaling to stimulate mitogenesis. Although a promitogenic role for IGF-I/ HGF-SF seems likely in this model of HCC, the pleiotropic nature of IGF-I and HGF-SF raises the possibility of other effects mediated either directly or indirectly.…”

The small-molecule BGP-15 protects against heart failure and atrial fibrillation in mice

“…However, the content of G ai2 with IGF1R was significantly lower in ventricular tissue from the HF þ AF model but not BGP-15-treated mice. G ai has been shown to associate with IGF1R in other tissue/cell types 54,55 , and G ai2 provides protection in the heart 56,57 . Thus, this could represent one mechanism by which BGP-15 is mediating protection via IGF1R in the HF þ AF model.…”