Thyrotropin modulates interferon-γ-mediated intercellular adhesion molecule-1 gene expression by inhibiting Janus kinase-1 and signal transducer and activator of transcription-1 activation in thyroid cells

TSH is known as an important hormone that plays the major role not only in the maintenance of normal physiology but also in the regulation of immunomodulatory gene expression in thyrocytes. The adhesion molecule intercellular adhesion molecule-1 (ICAM-1) was identified as one of the proteins that are abnormally expressed in the thyroid gland during autoimmune thyroid diseases. In this study we found that TSH inhibits interferon-gamma (IFN gamma)-mediated expression of the ICAM-1 gene, and we investigated the involved mechanisms in rat FRTL-5 thyroid cells. After exposure to IFN gamma, ICAM-1 expression is positively regulated at the level of transcription. This effect occurs via the IFN gamma-activated site (GAS) element in the ICAM-1 promoter as a consequence of the activation of STAT1 (signal transducer and activator of transcription-1), but not of STAT3. On the other hand, after exposure to TSH plus IFN gamma, ICAM-1 transcription is negatively modulated. Sire found that this inhibitory effect of TSH also occurs via the GAS element. Electrophoretic mobility shift assays confirmed that the IFN gamma-induced DNA-binding activities of STAT1 were reduced by TSH. Furthermore, our results showed that the inhibitory effect of TSH on IFN gamma signaling is caused by inhibition of tyrosine phosphorylation on STAT1, Janus kinase-l (Jak1), and IFN gamma receptor alpha, but not Jak2. In conclusion, we have identified a novel mechanism in which TSH modulates the IFN gamma-mediated Jak/STAT signaling pathway through the inhibition of Jak1 and STAT1.