Recognition and receptor-mediated endocytosis of the lysosomal acid lipase secreted by cultured human fibroblasts.

ABSTRACT

We have studied the recognition and uptake of acid lipase by human fibroblasts in order to determine requirements for localization and function of the enzyme in lysosomes. Our approach was based on evidence that a number of acid hydrolases involved in mucopolysaccharide metabolism are secreted from cultured fibroblasts and endocytosed by a phosphomannosyl-dependent, receptor-mediated process. Acid fatty acid ester hydrolase activity secreted from human fibroblasts was separable into two major forms by hydrophobic chromatography. The dominant form from normal cells was deficient in fibroblasts from patients with Wolman's disease, an inherited disorder of lysosomal cholesteryl ester and triglyceride metabolism. The time- and temperature-dependent, saturable uptake of this enzyme by fibroblasts was competitively inhibited by mannose 6-phosphate and was destroyed by pretreatment of the enzyme with phosphatase. Internalized lipase had a half-life of 1 day. Application of the enzyme to Wolman's disease fibroblasts reduced cholesteryl ester storage; this effect was blocked by ammonium chloride, a general inhibitor of lysosomal hydrolysis. Our results indicate that extracellular acid lipase is transported to fibroblast lysosomes by the same receptor-mediated process that functions in the packaging of several lysosomal glycosidases.