Regulation of the components of the 150 kDa IGF binding protein complex incocultures of rat hepatocytes and Kupffer cells by 3 ',5 '-cyclic adenosine monophosphate

In the circulation, most of IGFs are bound to a high molecular mass complex of 150 kDa that consists of IGF-I (or IGF-II), IGFBP-3 and the acid-labile subunit (ALS). Within rat liver, biosynthesis of these components has been localized to different cell populations with hepatocytes as source of ALS and nonparenchymal cells (endothelial and Kupffer cells (KC)) as source of IGFBP-3. In the present study, the regulatory effects of the cAMP analogs d ibutyryl-cAMP (db-cAMP) and 8-bromo-cAMP (8-br-cAMP) on IGF-I, ALS, and IGF BP expression were evaluated in primary cultures of rat hepatocytes, KC as well as in cocultures of hepatocytes and KC. In cocultures, biosynthesis of IGFBP-3 and ALS was inhibited dose-dependently by db-cAMP and 8-br-cAMP wh ile that of IGF-I, IGFBP-1, and -4 was stimulated as demonstrated by ligand and Northern blotting. IGFBP-3 expression in primary cultures of pure KC d id not respond to cAMP treatment indicating the importance of a cellular in teraction between KC and hepatocytes for the decreased IGFBP-3 synthesis. T he inhibition of IGFBP-3 in db-cAMP-treated cocultures was due to a decreas e of IGFBP-3 mRNA level accompanied by a reduced cellular degradation of IG FBP-3. We conclude that cAMP stimulate the biosynthesis of IGF-I, IGFBP-1, and -4 in cocultures of hepatocytes and KC thereby enabling the formation o f binary IGF/IGFBP complexes while the formation of the 150 kDa complex is impaired through downregulation of IGFBP-3 and ALS. This complex regulation may be a prerequisite for the effects of cAMP-dependent hormones on the tr ansfer of IGFs from circulation to peripheral tissues. (C) 2001 Wiley-Liss, Inc.