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

Background/Aims: The liver is the main source of circulating insulin-l ike growth factor binding proteins. In man, the cellular origin of ins ulin-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 expressi on of individual insulin-like growth factor binding proteins and of th e acid-labile subunit of the insulin-like growth factor ternary bindin g protein complex was studied by Northern blotting, secretion of insul in-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-li ke growth factor binding proteins of molecular weights of 24 kD, 30 kD , 34 kD, 43 kD and 46 kD, respectively. Using polyclonal antisera, the se proteins mere identified as insulin-like growth factor binding prot ein-1, -2 and the insulin-like growth factor binding protein-3 doublet . Neutral size chromatography of culture supernatants showed the prese nce of an insulin-like growth factor binding protein complex of approx imately 40 kD, but absence of the high molecular weight ternary comple x of 150 kD. Conclusions: It is concluded that in man parenchymal live r cells have to be regarded as a source of acid-labile subunit and of circulating insulin-like growth factor binding proteins including insu lin-like growth factor binding protein-3.