INSULIN-LIKE GROWTH-FACTOR-I MESSENGER-RIBONUCLEIC-ACID EXPRESSION IN PORCINE THYROID-FOLLICLES IS REGULATED BY THYROTROPIN AND IODINE

Insulin-like growth factor I (IGF-I) has been shown to be released from thyrocytes in vitro. We investigated IGF-I mRNA expression during treatment with thyrotropin (TSH), forskolin and potassium iodide (KI) in intact porcine thyroid follicles ex vivo. Porcine thyroid follicles were prepared by collagenase digestion and cultured in the presence of TSK, forskolin or EI. After different incubation times, mRNA was isolated and examined by Not-them hybridization with a porcine IGF-I cDNA probe of 405 bp in length. In untreated follicles no IGF-I mRNA was found, whereas in follicles stimulated with TSH an IGF-I mRNA of 7.0 kb was detected after 24h, which persisted for another 24 h. Forskolin treatment mimicked the TSK effect, indicating that IGF-I mRNA expression may be stimulated by the adenylate cyclase pathway. Preincubation of the porcine follicles with KI decreased dose dependently the TSH-induced IGF-I mRNA expression, with complete inhibition at 10 mu mol/l KI. These results suggest that TSK acts via the cAMP pathway to enhance EGF-I mRNA expression, which then may lead to an autocrine IGF-I stimulation. The IGF-ImRNA expression is under negative control of iodide.