THYROID-HORMONE RECEPTORS AND 3,5,3'-TRIIODOTHYRONINE BIOLOGICAL EFFECTS IN FRTL5 THYROID FOLLICULAR CELLS

Specific thyroid hormone (T3) receptors are present in thyroid follicular cells, including the rat FRTL5 clonal line, but little is known about the effects of T3 on the growth and differentiated function of the thyroid. Unlike primary cultures of animal or human thyroid cells, FRTL5 do not secrete appreciable amounts of thyroid hormones. We now have studied the effects of T3 by itself and in combination with TSH and insulin-like growth factor-I (IGF-I) on [H-3]thymidine incorporation into DNA, iodide uptake, and cAMP production in FRTL5. We also have investigated the expression of different c-erbA mRNAs in these cells. Specific binding of T3 to FRTL5 cell nuclei in intact cells occurred with a binding capacity of 0.1-0.15 ng T3/mg DNA and an apparent K(d) of 0.4 nm. Using an RNase protection assay on total cellular FRTL5 RNA and specific cRNA probes, we demonstrated the presence of c-erbA-alpha and -beta mRNAs, both encoding T3 receptors. Biological effects were assessed in serum-free medium or buffer containing 0.1% BSA after maintaining quiescent culture of cells for at least 5 days in hormone-free medium containing 5% calf serum. T3 alone stimulated a dose-dependent increase in [H-3]thymidine incorporation that reached a plateau at 188% of the control value at 10 nM T3. At 10(-11) M TSH, T3 potentiated TSH-stimulated [H-3]thymidine incorporation (2.2-fold), but at TSH concentrations greater than 5 x 10(-11) m, T3 had no effect or reduced the response to TSH. T3 potentiated the [H-3]thymidine response to 2 and 10 ng/ml IGF-I by 1.5- to 1.7-fold. T3 alone had no effect on iodide uptake, but attenuated iodide uptake stimulated by TSH. T3 was more potent in inhibiting TSH-stimulated iodide uptake than in enhancing TSH-stimulated DNA synthesis. T3 did not affect either basal or TSH-stimulated cAMP accumulation. Thus, in FRTL5 thyroid follicular cells 1) T3 receptors are expressed, as measured by direct binding assays and by the expression of c-erbA mRNAs; and 2) T3 acts as a growth factor and weak antidifferentiation factor. We suggest that T3 may modulate the actions of TSH and growth factors in thyroid epithelium.