We present results from studies of human cell culture models to support the premise that the extracellular transport of lysosomal acid lipase has a function in lipoprotein cholesteryl ester metabolism in vascular tissue. Vascular endothelial cells secreted a higher fraction of cellular acid lipase than did smooth muscle cells and fibroblasts. Acid lipase and lysosomal jl-hexosaminidase were secreted at approximately the same rate from the apical and basolateral surface of an endothelial cell monolayer. Stimulation of secretion with NH4CI did not affect the polarity. We tested for the ability of secreted endothelial lipase to interact with connective tissue cells and influence lipoprotein cholesterol metabolism in a coculture system in which endothelial cells on a micropore filter were suspended above a monolayer of acid lipase-deficient (Wolman disease) fibroblasts. After 5-7 d, acid lipase activity in the fibroblasts reached 10 %-20% of the level in normal cells; cholesteryl esters that had accumulated from growth in serum were cleared. Addition of mannose 6-phosphate to the coculture medium blocked acid lipase uptake and cholesterol clearance, indicating that lipase released from endothelial cells was packaged into fibroblast lysosomes by a phosphomannosyl receptor-mediated pathway. Supplementation of the coculture medium with serum was not required for lipase uptake and cholesteryl ester hydrolysis by the fibroblasts, but was necessary for cholesterol clearance.Results from our coculture model suggest that acid lipase may be transported from intact endothelium to cells in the lumen or the wall of a blood vessel. We postulate that delivery of acid hydrolases and lipoproteins to a common endocytic compartment may occur and have an impact on cellular lipoprotein processing. IntroductionWe are studying the extracellular transport of lysosomal acid lipase in human cell culture models of vascular tissue to determine its role in lipoprotein cholesteryl ester metabolism and to learn how it relates to the abnormal lysosomal hydrolase activity and lipid accumulation that are characteristic of atherogenesis. Lysosomal acid lipase, also termed acid cholesteryl ester hydrolase, or cholesterol esterase (3.1.1.13), is critical for the hydrolysis of triglycerides and cholesteryl esters that cells acquire by receptormediated endocytosis of lipoproteins (Goldstein and Brown, 1977). A hereditary deficiency of acid lipase activity, in patients with Wolman disease-or the clinically less severe condition, cholesteryl ester storage disease-is characterized by accumulation of esterified cholesterol

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