Thyrotropin Modulation of Epidermal Growth Factor (EGF) Binding to Receptors on Cultured Thyroid Cells

Previous studies had shown that epidermal growth factor (EGF) will stimulate growth of cultured thyroid cells in vitro, and TSH will stimulate total assayable EGF receptor in cultured porcine thyroid cells. In this study, we report the effect of TSH on EGF binding to human thyroid cells. Addition of bTSH (1 mU/mL) in binding buffer during receptor assay stimulated specific EGF binding to cells, with an increase of 44% observed over the control after 1 h incubation at 37°C. Affinity crosslinking of the [¹²⁵I]EGF–receptor complex showed a single labeled band with molecular size of 170 kD. No additional band was detected in the presence of TSH. Preincubation of cells with chloroquine, which inhibits lysosomal degradative enzyme activity, caused a continuous accumulation of bound EGF over a 4 h study period at 37°C, and TSH stimulated an increase in internalized EGF. In the presence of chloroquine, total specific bound EGF was linearly correlated to incubation time up to 4 h and can be expressed as Bound = slope*time + intercept (time₀) Addition of TSH during the binding assay significantly increased the value of the slope when compared to control (p < 0.002). The rate at which prebound [¹²⁵I]EGF was released into medium was not affected by the presence of TSH, indicating that TSH-enhanced binding may not be attributed to a reduction in EGF degradation. Coincubation of thyroid cells with EGF at 0 and 1 ng/mL and increasing concentrations of TSH (0–10 mU/mL) indicated that EGF stimulated thymidine incorporation, although TSH failed to synergistically enhance EGF-stimulated cell growth. We conclude that (a) EGF receptor binding in human thyrocytes is enhanced by TSH, (b) the increase results from an enhanced rate of binding, (c) this increased binding rate is related to stimulation by TSH of receptor mediated internalization, (d) TSH-enhanced EGF binding to receptor was not translated into stimulation of cell growth in this cell culture system.