Regulation of soluble insulin-like growth factor-II/mannose 6-phosphate receptor in hepatocytes from intact and regenerating rat liver.

A soluble, circulating form of the insulin-like growth factor-II/mannose 6-phosphate receptor has been proposed to result from proteolytic cleavage of intact cellular receptors. This study examines receptor release in hepatocytes from normal and regenerating rat liver, where receptor levels are elevated. After partial hepatectomy, serum receptor increased from 0.64 +/- 0.02 to 1.36 +/- 0.15 microgram/ml at 72 h after surgery, reflected by an increase in receptor secretion from 18.5 +/- 3.6 ng/mg protein per 24 h in cells from sham-operated animals (n = 14) to 100.9 +/- 10.8 ng/mg protein per 24 h in cells from regenerating liver (n = 8). A wide range of protease inhibitors had little or no effect on soluble receptor secretion, indicating that extracellular proteolysis of cell surface receptor is not the major route of production in hepatocytes. Neither insulin-like growth factor-II nor mannose 6-phosphate altered receptor secretion, suggesting that neither ligand has a role in elevating receptor levels in liver regeneration. Inhibitors of endocytosis were examined to determine whether soluble receptor formation occurred during receptor recycling. Chloroquine, NH4Cl and monensin did not inhibit soluble receptor release, whereas the microtubule disrupting agents, colchicine and nocodazole, caused a dose-related increase that was reversible by the microtubule stabilizing agent, taxol. This suggests that alteration of early endosome pH does not alter soluble receptor production, but that subsequent disruption of late endosomes may result in increased formation and release of soluble receptor into the culture medium.