In the βTC3 insulin-secreting β-cell line, glucose rapidly induces the tyrosine phosphorylation of the 97-kDa insulin receptor β-subunit. Phosphorylation is transient, with fourfold stimulation by 2 min and subsequent dephosphorylation to basal levels by 10–15 min. Elevating the extracellular KC1 concentration equipotently initiates receptor phosphorylation. Preventing insulin secretion with 1 μmol/l epinephrine or by removing extracellular Ca2+ blocks the effect. In the absence of glucose-induced secretion, exogenous insulin also stimulated insulin receptor autophosphorylation transiently and with an ED50 of 4 × 10−9 mol/1. In addition, functional insulin-like growth factor I (IGF-I) receptors are also expressed by these β-cells, as indicated by IGF-I-induced receptor tyrosine phosphorylation (ED50 = 5 × 10−9 mol/1) and also by detection of hybrid insulin/IGF-I receptor autophosphorylation at 10−7 mol/1 IGF-I. Both glucose and insulin stimulate the tyrosine phosphorylation of the insulin receptor substrate (IRS) IRS-1 and increase by two- to fivefold the rapid association of IRS-1 with the 85-kDa α-subunit of the phosphatidylinositol-3-kinase, as determined by co-immunoprecipitation assays. These results demonstrate that in these β-cells, glucose-induced insulin secretion activates the β-cell surface insulin receptor tyrosine kinase and its intracellular signal transduction pathway, suggesting a new autocrine mechanism for the regulation of β-cell function.

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