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 species with insulin-like growth factor (IGF-I) immunoreactivity and HPLC mobility and expresses IGF-I mRNA. Conditioned medium also contained IGF-binding activity with a pattern of displacement of [I-125]-IGF-I characteristic of IGF binding proteins. Western ligand analysis and immunobloting with anti-IGFBP4 antiserum identified a 34 kD IGF-I binding 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 system in the transformed state.