Insulin-like growth factor binding protein-3 interacts with the thyroid hormone receptor α1 and modulates transcription of thyroid hormone responsive gene.

OBJECTIVE: To study the interaction between insulin-like growth factor binding protein-3 (IGFBP-3) and thyroid hormone receptor α1 (TRα1) and the modulatory effect of IGFBP-3 on transcription of the thyroid hormone responsive gene.
METHODS: The interaction between IGFBP-3 and TRα1 was detected with glutathione-S-transferase pull-down method, co-immunoprecipitation, fluorescence resonance energy transfer test. The cellular distribution of these two proteins was observed by confocal laser scanning microscopy. The effect of IGFBP-3 on the growth hormone promoter activity stimulated by triiodothyronine (T3) was determined by dual-luciferase reporter assay.
RESULTS: IGFBP-3 interacted with TRα1 both in vivo and in vitro. IGFBP-3 and TRα1 were shown to co-localize in the nucleus of HEK-293 cells. The overexpressed IGFBP-3 inhibited the growth hormone promoter activity stimulated by T3 (P<0.01).
CONCLUSIONS: IGFBP-3 interacts with TRα1 and inhibits T3 responsive gene transcription. This finding further confirms the insulin-like growth factor-independent role of IGFBP-3 in the nucleus.