Thyrotropin prevents apoptosis by promoting cell adhesion and cell cycle progression in FRTL-5 cells

Apoptosis has been shown to be involved in endocrine tissue homeostasis as well as regression due to hormone deprivation. The goal of this study was t o induce apoptosis and to investigate a potential role of TSH as a survival factor in thyroid follicular cells (FRTL-5) in vitro. Our results indicate d that FRTL-5 cells underwent anchorage-dependent apoptosis when plated in the absence of serum and hormones, but when the cells became attached;to th e substrate by addition of TSH in the medium, apoptosis was prevented. The apoptosis was evaluated by positive terminal deoxynucleotidyl transferase-m ediated deoxy-UTP nick end labeling staining, typical apoptotic bodies by e lectron microscopy, DNA ladder by gel electrophoresis, and subdiploidy by p ropidium iodide-stained flow cytometry. TSH was shown to prevent apoptosis and maintain cell viability. cAMP partly mimicked this effect, which was in hibited by a specific inhibitor of protein kinase A, H-89. While investigat ing the mechanisms of apoptosis, we observed that the phosphorylated focal adhesion kinase was strengthened by TSH. Furthermore, FRTL-5 cells were fou nd to undergo growth arrest in the G(1) phase in the absence of TSH, accomp anied by an elevated level of cyclin-dependent kinase inhibitor, p27, and a decreased level of cyclin D. In contrast, TSH promoted transition from G(1 ) to S phase by decreasing P27 protein and increasing cyclin D expression. We concluded that in addition to regulating growth and differentiation, TSH may function as a survival factor in thyroid cells by preventing anchorage -dependent apoptosis in FRTL-5 cells partly via the cAMP pathway.