COCULTURE OF PRIMARY RAT HEPATOCYTES AND NONPARENCHYMAL CELLS PERMITSEXPRESSION OF INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-3 IN-VITRO

In biological fluids, the insulin-like growth factors (IGFs) are assoc iated with binding proteins (IGFBPs), which modify IGF distribution an d action. Circulating IGFs are bound predominantly to IGFBP-3, of appa rent hepatic origin, but regulation of IGFBP-3 has been difficult to d issect because of the lack of systems suitable for examining hepatic p roduction of IGFBP-3 in vitro. In the present studies, IGFBP-3 express ion was identified primarily in hepatic nonparenchymal cells, particul arly Kupffer and sinusoidal endothelial cells. Coculture with hepatocy tes enhanced the stability of nonparenchymal cells to express IGFBP-3 in vitro. IGFBP-3 in conditioned medium had apparent mol wt of 150-300 kilodaltons, suggesting formation of a ternary complex with IGFs and the acid-labile subunit. Expression and secretion of IGFBP-3 were horm onally responsive and strongly correlated (r = 0.79; P < 0.001), with 2- to 3-fold stimulation by added insulin or IGF-I (both P < 0.05), bu t not by added GH alone. Our findings suggest that GH may act indirect ly to promote IGFBP-3 generation in vivo via increasing both the secre tion of insulin and the hepatic production of IGF-I; in patients with diabetes mellitus, reduced circulating levels of IGFBP-3 despite high levels of GH may result from both insulin deficiency and inadequate he patic production of IGF-I. Coculture of hepatic nonparenchymal and par enchymal cells should be useful for further analysis of the mechanism of IGFBP-3 regulation.