hCG-induced TSH receptor activation and growth acceleration in FRTL-5 thyroid cells.

Our previous studies have indicated specificity cross-over between LH/hCG and the TSH receptor. We have now analyzed the ability of the TSH-dependent Fisher Rat thyroid cell line (FRTL-5) to proliferate in a differentiated state under the influence of highly purified hCG (hCG-CR121). TSH receptor activation and growth induction were observed after 7 days suspension from a TSH-induced growth phase. The effect of 10 ug hCG-CR121 was equivalent to 500 +/- 43 (mean +/- SEM) uIU of human TSH (2nd IRP, 80/558) with reference to growth stimulation, as judged by 72-h [3H]-thymidine uptake and to approximately 15 +/- 35 uIU human TSH with reference to receptor activation as judged by cyclic AMP accumulation. These data demonstrate specificity cross-over by hCG for the TSH-dependent FRTL-5 cell line and suggest that hCG has greater growth-stimulating than thyroid-stimulating potential when compared with human TSH.