Insulin and insulin-like growth factors (IGFs) elicit distinct but overlapping biological effects in vivo. To investigate whether differences in intrinsic signaling capacity of receptors contribute to biological specificity, we constructed chimeric receptors containing the extracellular portion of the neurotrophin receptor TrkC fused to the intracellular portion of the insulin or IGF-I receptors. Chimeras were stably expressed in 3T3-L1 adipocytes at levels comparable to endogenous insulin receptors and were efficiently activated by neurotrophin-3. The wild-type insulin receptor chimera mediated approximately 2-fold greater phosphorylation of insulin receptor substrate 1 (IRS-1), association of IRS-1 with phosphoinositide 3-kinase, stimulation of glucose uptake, and GLUT4 translocation, compared with the IGF-I receptor chimera. In contrast, the IGF-I receptor chimera mediated more effective Shc phosphorylation, association of Shc with Grb2, and activation of mitogenactivated protein kinase compared with the insulin receptor chimera. The two receptors elicited similar activation of protein kinase B, p70S6 kinase, and glycogen synthesis. We conclude that the insulin receptor mediates some aspects of metabolic signaling in adipocytes more effectively than the IGF-I receptor, as a consequence of more efficient phosphorylation of IRS-1 and greater recruitment/activation of phosphoinositide 3-kinase. Insulin and insulin-like growth factors (IGFs)1 are structurally related polypeptides that exert diverse effects on mammalian tissues. The most prominent actions of insulin in vivo are concerned with the acute regulation of carbohydrate and lipid metabolism in liver, muscle, and fat, whereas IGFs act on skeletal and other tissues to promote growth, differentiation, and survival. The receptors for insulin and IGFs, which mediate these effects (IR and IGFR), are also structurally related and highly homologous, consisting of extracellular ␣-subunits responsible for ligand binding and transmembrane ␤-subunits possessing protein-tyrosine kinase activity, in a disulphide linked ␤-␣-␣-␤ configuration (1-3). Within the intracellular portion, the level of sequence identity between the receptors is greatest in the tyrosine kinase domain (84%) and somewhat less in the flanking juxtamembrane and carboxyl-terminal regions (61 and 44%, respectively). Not surprisingly, the signaling mechanisms of the insulin receptor (IR) and IGF-I receptor (IGFR) are broadly similar. Ligand binding activates tyrosine kinase activity, leading to phosphorylation of intracellular substrates, such as IRS and Shc proteins, and the recruitment and/or stimulation of signal transducing molecules, including phosphoinositide 3-kinase (PI 3-kinase) and Grb2⅐Sos (4, 5). These signal transducers in turn promote activation of proteinserine kinase cascades involving phosphoinositide-dependent kinase/PKB and MAPK/Erk kinase/MAPK, respectively, which modulate the activity of glucose transporters, enzymes, and transcription factors (6, 7).Given the similarity in structu...

Insulin and insulin-like growth factor-1 (IGF-1) have similar cell-surface receptors yet subserve different physiological functions. To examine whether these differences relate to intrinsic signaling properties of the intracellular domains of their respective receptors, chimeric receptors were constructed using the extracellular domain of the neurotrophin-3 (NT-3) receptor, TrkC, and the intracellular domain of either the insulin receptor or the IGF-1 receptor. TrkC-IR (TIR) and TrkC-IGF-1R (TIGR) were stably expressed in 3T3-L1 cells. While TIR and TIGR cell lines expressing similar numbers of chimeric receptors showed a similar dose-response relationship in NT-3 stimulated DNA synthesis, NT-3 stimulated glycogen synthesis was greater in TIR than in TIGR cells (maximum TIGR response was 35% of maximum TIR response). Additionally, the concentration of NT-3 at which significant stimulation of glycogen synthesis was seen was 0.1 ng/ml in TIR and 1 ng/ml in TIGR cells. Basal levels of thymidine incorporation but not glycogen synthesis were consistently higher in TIR than in TIGR expressing cells. No detectable basal autophosphorylation of chimeric receptors was seen in any of the cell lines. However, exposure of cell lines to the phosphatase inhibitor bisperoxovanadate resulted in greater basal autophosphorylation of the TIR and endogenous murine IR than the TIGR and endogenous murine IGF-1R. Thus, in this system, the intracellular domain of the IR appears to couple more effectively to glycogen synthesis than that of the IGF-1R, whereas the intracellular domains of both receptors have a similar capacity to stimulate DNA synthesis.Insulin and insulin-like growth factor-1 (IGF-1) 1 are homologous peptides that are essential for normal metabolism and growth regulation. In the intact organism these hormones have significantly different physiological roles (for review, see Refs. 1-3). Insulin primarily functions as a regulator of carbohydrate, fat, and protein metabolism. In contrast, IGF-1 is largely a mediator of cell growth and differentiation primarily stimulating DNA synthesis and mitogenic events (4 -6). Despite these different functions, the cell surface receptors for insulin and IGF-1 (IR and IGF-1R, respectively) are highly homologous. They are both Type II tyrosine kinase receptors with the same disulfide-linked heterotetrameric structure consisting of two extracellular ␣-subunits containing the ligand-binding domain and two transmembrane ␤-subunits with the ligand-sensitive tyrosine kinase activity (7). The greatest homology (over 80%) is found in the tyrosine kinase domain while differences in the ligand-binding domain (overall homology 44 -60%) account for ligand specificity. The greatest sequence divergence between the two receptors is found in their C termini (44% identity) (8).In addition to the similarity of receptor structure the intracellular signaling events which result from ligand-induced receptor activation are remarkably similar (9). Both receptors phosphorylate insulin-receptor substrates IRS-1 ...

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