SECRETION AND BINDING OF INSULIN-LIKE GROWTH FACTOR-I BY A HUMAN MEDULLARY-THYROID CARCINOMA CELL-LINE

The TT human medullary thyroid carcinoma cell line secretes a 7.5 kD s pecies with insulin-like growth factor (IGF-I) immunoreactivity and HP LC mobility and expresses IGF-I mRNA. Conditioned medium also containe d IGF-binding activity with a pattern of displacement of [I-125]-IGF-I characteristic of IGF binding proteins. Western ligand analysis and i mmunobloting with anti-IGFBP4 antiserum identified a 34 kD IGF-I bindi ng species. Northern analysis identified a 2.1 kb IGFBP4 mRNA species. Cell surface binding of [I-125]-IGF-I identified Type I IGF receptors . TT cells constitute a useful model to study the IGF-I autocrine syst em in the transformed state.