Regulation of α1-acid glycoprotein externalization and intracellular accumulation in glucocorticoid-induced rat hepatoma cells

Abstract

α 1-Acid glycoprotein ( α 1-AGP) is a glucocorticoid inducible gene product that is synthesized and secreted by certain rat hepatoma tissue culture (HTC) cell lines such as M1.54. Exposure to monensin, a Na +-K + ionophore, causes a significant redistribution of α 1-AGP into two distinct fractions; immunoprecipitation of [ 35S]methionine-labeled proteins revealed that a 27% decrease in secretion accounts for a sixfold increase in accumulation of a stable intracellular species. The new intracellular α 1-AGP is more heterogeneous than normal while the extra-cellular form is 6000 Da smaller than normal. These effects are due to selective alterations in carbohydrate maturation; endo-β- N-acetylglucosaminidase H (endo H) digestion demonstrated that both α 1-AGP species contain variable numbers of endo H-resistant oligosaccharide side chains ranging between zero and five. Ricin affinity chromatography revealed that the attachment of galactose residues is strikingly correlated with α 1-AGP externalization while neuraminidase digestions demonstrated that sialic acid attachment appears unessential for its secretion. Taken together, our results suggest that in the presence of monensin the cellular transport of intracellular destined and externalized α 1-AGP proceeds in common through the early segments of the Golgi and at a point prior to or at the compartment containing galactosyl transferase, α 1-AGP becomes committed for secretion.