Synthesis of insulin-like growth factor binding proteins and of the acid-labile subunit of the insulin-like growth factor ternary binding protein complex in primary cultures of human hepatocytes

Abstract

Background/Aims: The liver is the main source of circulating insulin-like growth factor binding proteins. In man, the cellular origin of insulin-like growth factor binding proteins has remained obscure. Methods: Human hepatocytes isolated from surgical specimens were purified and cultured using a collagen gel immobilization technique. Gene expression of individual insulin-like growth factor binding proteins and of the acid-labile subunit of the insulin-like growth factor ternary binding protein complex was studied by Northern blotting, secretion of insulin-like growth factor binding proteins by Western ligand blotting and immunoblot analysis. Neutral size chromatography of medium samples was used to detect insulin-like growth factors binding protein complexes. Results: In cultured hepatocytes transcripts for insulin-like growth factor binding protein-1, -2, -3, -4 and for acid-labile subunit could be demonstrated. Ligand blotting revealed the secretion of insulin-like growth factor binding proteins of molecular weights of 24 kD, 30 kD, 34 kD, 43 kD and 46 kD, respectively. Using polyclonal antisera, these proteins were identified as insulin-like growth factor binding protein-1, -2 and the insulin-like growth factor binding protein-3 doublet. Neutral size chromatography of culture supernatants showed the presence of an insulin-like growth factor binding protein complex of approximately 40 kD, but absence of the high molecular weight ternary complex of 150 kD. Conclusions: It is concluded that in man parenchymal liver cells have to be regarded as a source of acid-labile subunit and of circulating insulin-like growth factor binding proteins including insulin-like growth factor binding protein-3.