Somatomedin synthesis by a subclone of Buffalo rat liver cells: characterization and evidence for immediate secretion of de novo synthesized hormone.

Previous studies have reported that cultured Buffalo rat liver (BRL-3A) cells synthesize and release a somatomedin called multiplication-stimulating activity, a major component of which is rat insulin-like growth factor-II (rIGF-II). We have isolated a subclone of hepatocytes (BRL-3SC) from these cells, which release rIGF-II into serum-free medium at a similar but considerably more uniform rate than BRL-3A cells, and have described their morphology and defined some of the conditions regulating the rate of synthesis and secretion of rIGF-II. Time-course experiments indicate that after a 2-h lag, rIGF-II is released at a nearly constant rate of 70 microU insulin-like activity/ml . 24 h for 5 days. These cells demonstrate considerable resistance to variations in environmental pH, with extremes of pH 6.8-8.0 in the initial culture medium causing a negligible effect on rIGF-II release. We found no evidence for the existence of an appreciable amount of intracellular rIGF-II, consistent with the lack of secretion granules in these cells. Moreover, at concentrations not causing irreversible cell damage, inhibitors of protein synthesis, cycloheximide and actinomycin D, produced 87% and 76% reductions, respectively, in rIGF-II release in 24 h, and inhibitors of oxidative phosphorylation, dinitrophenol and sodium azide, reduced rIGF-II release by 72% and 78%, respectively. We conclude from these studies that BRL-3A cells, and particularly those of the subclone BRL-3SC herein described, may provide an excellent model for investigations of somatomedin synthesis and release and that such studies are facilitated by the fact that rIGF-II released into serum-free medium appears to be the immediate product of de novo peptide synthesis.