DISSOCIATION OF THYROTROPIN RECEPTOR FUNCTION AND THYROTROPIN DEPENDENCY IN RAT-THYROID TUMOR-CELL LINES DERIVED FROM FRTL-5

Spontaneously transformed somatic thyrocyte mutants, FRTL-5/TA and FRT L-5/TP, are thyrotropin (TSH) independent for growth and show loss of the thyroid-specific phenotype with absent thyroglobulin and thyroid p eroxidase gene expression. To investigate the role of TSH-receptor (TS H-R) activation in rat thyroid growth and function, binding of TSH and TSH-induced cAMP production were measured in intact cells under ident ical assay conditions. TSH binding did not differ in terms of affinity and receptor number and presence of 5.6 kb and 3.3 kb mRNA rat TSH-R transcripts was determined in ail variants. Bg contrast. basal cAMP wa s 11-fold lower in FRTL-5/TA and 6-fold lower in FRTL-5/TP than in wil d-type FRTL-5 (1.1+/-0.4; P<0.01). Maximal cAMP production was similar between wild-type and cell variants and stimulation by bovine, rat an d recombinant human TSH revealed normal activation patterns. Therefore . a dissociation was present between the loss of TSH control on growth and function, and the presence of a normally functioning TSH-R. Subse quent to TSH incubation FRTL-5/TP and FRTL-5/TA cells showed a differe nt expression pattern of TSH-R and the proto oncogenes c-mye and fos t han FRTL-5 wild-type. The data indicated that the cause of the TSH-ind ependency is located down-stream of the cAMP cascade, influencing gene s that control the expression of cell cycle-related proto-oncogenes an d thyroid-specific genes.