Studies of fibronectin synthesized by cultured chick hepatocytes

We have adapted a chick embryo liver cell system for studying the synthesis of proteins secreted by hepatocytes. In primary liver cell cultures maintained for several days in arginine-deficient medium containing ornithine (0.7 mM) and carbamyl phosphate (1 mM), only hepatocytes demonstrated normal morphological and biosynthetic characteristics, indicating that they possessed a functional ornithine cycle as a source of arginine production. Non-parenchymal liver cells, such as fibroblasts, which lack the ornithine cycle were excluded. Hepatocytes in arginine-deficient or arginine-containing medium synthesized fibronectin (Fn) over several days at a constant rate of 3 micrograms +/- 1 microgram/mg cell protein per day, with fibronectin representing approximately 3% of the total secreted hepatocyte proteins during any culture period after the first 24 h. Pulse-chase experiments indicated that Fn synthesis and secretion was relatively rapid (t1/2 = 45 min) and represented approximately 95% of the intracellularly labelled Fn. This Fn is secreted predominantly as a 450 kD dimer with a subunit size that is indistinguishable from the plasma form as assessed by one-dimensional electrophoretic analysis. Continuous exposure of hepatocytes to insulin caused a moderate decrease (26%) in Fn synthesis, whereas there was no effect of short-term exposure. In contrast, dexamethasone stimulated Fn production 2-3-fold, consistent with its known ability to stimulate hepatocyte production of acute phase proteins. Under these conditions, electrophoretic analyses showed that an increased quantity of intact hepatocyte Fn was produced having the same molecular size of plasma Fn.