Inhibitors of specific aminoacyl-tRNA synthetases prevent thyrotropin-induced desensitization in cultured human thyroid cells.

The effect of specific competitive inhibitors of aminoacyl-tRNA synthetase on thyrotropin (TSH) desensitization in human thyroid cell monolayers was examined. Thyroid cells were preincubated with o r without 60 milliunits/ml of TSH for 16 h at 37 “C in histidinefree medium supplemented with either 2 m~ histidinol or 2 mM histidine (control medium). All cells were then exposed to TSH for 30 min in fresh, histidine-containing medium, and cellular CAMP levels were th n measured. As expected, cells pre-exposed to TSH in control medium had a 55% lower CAMP response than cells preincubated in the same medium without TSH, indicating the development of TSH desensitization. In contrast, cells pre-exposed to TSH in histidinol-containing medium did not develop TSH desensitization. Addition of equimolar (2 mM) histidine to the preincubation medium containing histidinol plus TSH allowed the development of TSH desensitization. TSH stimulation performed in histidinol-containing medium revealed a normal acute CAMP response; however, CAMP levels did not subsequently decline as they did in the presence of histidine. Histidinol was effective in preventing TSH desensitization over a wide range of TSH concentrations (0.1 to 100 milliunits/ml). Once TSH desensitization was induced, histidinol in histidine-free medium was ineffective in reversing this desensitization. The efficacy of histidinol in inhibiting protein synthesis in cultured human thyroid cells was confirmed by determining incorporation into acid-insoluble material of a mixture of 3H-amino-acids. The ability of histidine to reverse protein synthesis inhibition was also demonstrated by the same method. 0-Methylthreonine, a competitive inhibitor of isoleucine-tRNA synthetase, similarly prevented TSH desensitization, and this effect was reversed by the concomitant presence of an equimolar concentration of isoleucine. The reversal by histidine of histidinol-induced protein synthesis inhibition was necessary for approximately 90 min before TSH desensitization was produced, These data provide strong evidence for the involvement of protein synthesis in the development of TSH-induced desensitization in cultured human thyroid cells.