Growth arrest of thyrotropic tumors by thyroid hormone is correlated with novel changes in Wnt-10A

The molecular mechanism underlying thyroid hormone inhibition of thyrotrope cell growth is poorly understood. A comprehensive screen for T3-regulated genes involved in thyrotrope cell regulation was performed by Affymetrix MGU74A Genechip microarray analyses, which compared total RNA from hypothyroid versus 24 h T3-treated TtT-97 tumors. Of the 13,000 genes screened, a number of novel, T3-responsive candidate genes were identified. Within the Wnt family of growth factors, only Wnt-10A transcripts were abundantly expressed in hypothyroid TtT-97 tumors, and were down-regulated with T3 by 6 h of treatment. In addition, nuclear β-catenin, which is a downstream mediator of canonical Wnt signaling, was decreased at the protein and functional levels. TtT-97 growth suppression was associated with decreased cyclin A transcript levels. We conclude that treatment of thyrotropic TtT-97 tumors with T3 resulted in the decreased expression of Wnt-10A, and that thyroid hormone may inhibit growth via cyclin A regulation.