Insulin binding and biological activities in the FRTL-5 rat thyroid cell line

A cloned rat thyroid cell line (FRTL-5) was examined for both insulin binding and responsiveness. The characteristics of insulin binding to thyroid cells were similar to those observed in typical insulin target cells. The ¹²⁵I-insulin binding was time and temperature dependent and Scatchard analysis suggested the presence of two major binding sites with high and low affinity constant (Kd = 1.4 × 10^-10 mol/L and 1.5 × 10^-9 mol/L, respectively). ¹²⁵I-insulin was also internalized and degraded in a temperature-dependent manner. IGF₁ was weakly effective in completing ¹²⁵I-insulin binding to FRTL-5 cells (57% inhibition at 333 nmol/L), whereas noninsulin-related peptide hormones were ineffective. Exposure of FRTL-5 cells to insulin stimulated both methyl-aminoisobutyric acid (M-AIB) and 2-deoxy-D-glucose (2DG) transport. These effects were evident at 10^-9 mol/L and maximal at 10^-7 mol/L insulin. Maximal stimulation was three- to four-fold over basal value for both M-AIB and 2DG transport. These data suggest that insulin specifically binds to FRTL-5 cells and regulates different biological functions of this thyroid cell line.