Differential signaling to glycogen synthesis by the intracellular domain of the insulin versus the insulin-like growth factor-1 receptor

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

Two naturally occurring insulin receptor tyrosine kinase domain mutants provide evidence that phosphoinositide 3-kinase activation alone is not sufficient for the mediation of insulin's metabolic and mitogenic effects

We have recently reported (1) that two naturally occurring mutants of the insulin receptor tyrosine kinase domain, Arg-1174 --> Gln and Pro-1178 --> Leu (Gln-1174 and Leu1178, respectively), both found in patients with inherited severe insulin resistance, markedly impaired receptor tyrosine autophosphorylation, with both mutant receptors; being unable to mediate the stimulation of glycogen synthesis or mitogenesis by insulin when expressed hh Chinese hamster ovary cells, However, these mutations did not fully prevent IRS-1 phosphorylation in response to insulin in these cells, suggesting that IRS-1 alone may not be sufficient to mediate insulin's metabolic and mitogenic effects, In the present study, we have demonstrated that these mutations also impair the ability of the insulin receptor to activate the transcription factor Elk-1 and promote GLUT4 translocation to the plasma membrane, Although at law concentrations of insulin, the mutant receptors were impaired in their ability to stimulate the tyrosine phosphorylation of IRS-1, at higher insulin concentrations we confirmed that the cells expressing the mutant receptors showed significantly increased tyrosine phosphorylation of IRS-1 compared with parental nontransfected cells, In addition, at comparable insulin concentrations, the association of the p85 alpha subunit of phosphoinositide 3-kinase (PI3-kinase) with IRS-1 and the enzymatic activity of IRS-1-associated PI3-kinase were significantly enhanced in cells expressing the mutant receptors, in contrast, no significant stimulation of the tyrosine phosphorylation of Shc, GTP loading of Ras, or mitogen-activated protein kinase phosphorylation was seen in cell lines expressing these mutant receptors. Thus, no activation of any measurable mitogenic or metabolic response was detectable, despite significant insulin-induced phosphorylation of IRS-1 and its association with PI3-kinase in cells stably expressing the mutant insulin receptors, These findings suggest that PI3-kinase activation alone may be insufficient to mediate a wide range of the metabolic and mitogenic effects of insulin, Additionally, the data provide support for the notion that insulin activation of Ras is more closely linked with Shc, and not IRS-1, phosphorylation

Regulation of adipose cell number in man

1. Adipose tissue mass is dependent on both the average volume and the number of its constituent adipocytes, Significant alteration in body mass involves alteration in both adipocyte volume and number