Basic Fibroblast Growth Factor (Basic FGF) in Isolated Ovine Thyroid Follicles: Thyrotropin Stimulation and Effects of Basic FGF on DNA Synthesis,Iodine Uptake and Organification,and the Release of Insulin-Like Growth Factors (IGFs) and IGF-Binding Proteins

We examined the effects of thyroid-stimulating hormone (TSH) on basic fibroblast growth factor (basic FGF) expression in isolated ovine thyroid follicles in vitro, and the effects of exogenous basic FGF on thyroid growth and function, to elucidate the significance of increased basic FGF expression during TSH-induced rat thyroid hyperplasia in vivo. Primary cultures of ovine thyroid follicles were maintained in serum-free Ham's modified F-12M medium containing transferrin, somatostatin, and glycyl-histidyl-lysine (designated 3H) with or without basic FGF alone, or in combination with TSH (100 μU/mL) and cortisol (10 nM). Following 48 h incubation, cells were harvested and total RNA prepared for the detection of basic FGF mRNA using Northern blot analysis and ribonuclease protection assay. Basic FGF in the cytoplasm and extracellular matrix fractions was quantified by radioimmunoassay. Basic FGF mRNA transcripts of 3.7,3.0, and 2.2 kb, respectively, were found in thyroid follicles cultured in 3H medium, and the abundance of each increased between 2- and 3-fold following incubation with 10–50 μU/mL TSH, although higher concentrations of TSH were less effective. Similar results were seen using a more sensitive ribonuclease protection assay. Cells cultured in control, 3H medium contained 2.4 ± 0.5 fmol immunoreactive basic FGF/μg cell DNA within the cytoplasm and 21.1 ± 1.5 fmol/μg DNA within the extracellular matrix (mean ± SD, n = 6). A significant increase (p < 0.05) in basic FGF content was seen in both cell compartments following incubation with 50 or 100 μU/mL TSH, while 250 μU/mL was less effective. The addition of TSH and cortisol for 48 h did not alter the total DNA content per culture well compared with incubation in 3H medium alone, but the further addition of basic FGF (0.55 nM) caused a significant (p < 0.05) 50% increase in DNA content. Subsequent incubation with Na¹²⁵I showed that the ability of TSH and cortisol to stimulate optimally iodine uptake and organification was abolished by the presence of 1.1 nM basic FGF. Since TSH-stimulated iodine uptake and organification have been shown to be partially dependent on the availability of endogenous insulin-like growth factors (IGFs), we measured the release of IGF-I and IGF-II into conditioned culture medium using radioimmunoassays, and the presence of IGF binding proteins (IGFBPs) using ligand blot analysis. Thyroid follicles released both IGF-I and IGF-II, and four species of IGFBP of 44–50, 34, 28, and 19 kDa, respectively, when cultured in 3H medium. Neither TSH and cortisol nor basic FGF altered the release of IGF-I or IGF-II. However, the release of all IGFBP forms was reduced following incubation with TSH and cortisol, while the presence of basic FGF (1.1 nM) increased IGFBP release in control incubations, and reversed the inhibitory effects of TSH and cortisol. The results suggest that an increased expression of basic FGF by thyroid follicular cells may contribute to the hyperplastic effects of high physiological concentrations of TSH in vivo. Basic FGF also inhibited TSH-stimulated iodine uptake and organification; this may have been mediated partly by increased IGFBP release, in turn limiting IGF bioavailability and the synergy between IGFs and TSH on thyroid function.