Regulation of the components of the 150 kDa IGF binding protein complex in cocultures of rat hepatocytes and Kupffer Cells by 3?,5?-cyclic adenosine monophosphate

Abstract

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 dibutyryl-cAMP (db-cAMP) and 8-bromo-cAMP (8-br-cAMP) on IGF-I, ALS, and IGFBP 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 while 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 did not respond to cAMP treatment indicating the importance of a cellular interaction between KC and hepatocytes for the decreased IGFBP-3 synthesis. The inhibition of IGFBP-3 in db-cAMP-treated cocultures was due to a decrease of IGFBP-3 mRNA level accompanied by a reduced cellular degradation of IGFBP-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 of 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 transfer of IGFs from circulation to peripheral tissues.

The IGF axis and hepatocarcinogenesis

“…Although released from diVerent cellular sources within rat liver, biosynthesis of the individual components of the 150 kDa complex was apparently controlled by the same regulatory mechanism. Maintenance of cocultures of hepatocytes with KCs in the presence of cAMP decreased the biosynthesis of IGFBP-3 (released from KCs) and ALS (released from hepatocytes) synergistically, 19 whereas insulin treatment resulted in a synergistic stimulation of the biosynthesis of these components. 30 This synergistic regulation might be facilitated by a cellular interaction between hepatocytes and KCs because a soluble factor secreted by hepatocytes was necessary for the stimulated IGFBP-3 biosynthesis in KCs in response to insulin.…”

“…In fact, when cocultures of hepatocytes with KCs were incubated at pH 7.4 and 37°C in the presence of iodinated IGFBP-3, a time dependent disappearance of intact IGFBP-3 and the generation of several IGFBP-3 proteolytic fragments of diVerent sizes was observed. 19 This indicated that, after endocytosis, a cathepsin mediated proteolysis of IGFBP-3 in intracellular organelles occurs, accompanied by partial recycling and release of IGFBP-3 fragments into the extracellular medium. The process of endocytosis and subsequent proteolytic degradation of IGFBP-3, as demonstrated in the coculture model with hepatocytes and KCs, might reflect the capacity of the liver to clear IGFBP-3 from the circulation.…”

Ptgs2 activation by endotoxin mediates the decrease in Igf1, but not in Igfbp3, gene expression in the liver

“…Although Igf1 and Igfbp3 genes in the liver are positively regulated by GH, other stimuli differentially affect them. For example, in cocultures of rat hepatocytes and Kupffer cells cAMP decreases Igfbp3, whereas it increases Igf1 biosynthesis (Scharf et al 2001). Furthermore, glucocorticoids inhibit Igf1, whereas they increase Igfbp3 mRNA in the liver (Luo & Murphy 1990, Koedam et al 2000, Priego et al 2005b.…”