Polarized secretion of diamine oxidase by intestinal epithelial cells and its stimulation by heparin

The Caco-2 cells have been used as a model system to study the pathways of diamine oxidase secretion by the intestinal epithelium. When grown in Transwell filter chamber devices, the polarized cell monolayers released the enzyme preferentially into the basal chamber. Heparin (1-10 USP U/mL) rapidly induced a marked stimulation of enzyme secretion only when in contact with the basolateral cell membrane, where high affinity binding sites for [3H]heparin were also exclusively located. Among the other glycosaminoglycans tested, only heparan sulfate (150 mg/mL) was able to induce enzyme release; chondroitin sulfate (150 mg/mL) and dermatan sulfate (150 mg/mL) were without effect. Four monoclonal antibodies specific for human diamine oxidase were produced and found to immunoprecipitate a single protein with a molecular weight of 95,000 (under reducing conditions) from the culture medium of Caco-2 cells. Immunofluorescence staining of cryostat sections of human small intestine with these four antibodies localized diamine oxidase at the lateral and basal sides of the villus cells. Staining was markedly reduced in specimens obtained from patients who received doses of heparin in vivo. This study concludes that release of diamine oxidase by intestinal cells occurs specifically at the basolateral aspect of the cells, most likely through the constitutive secretory pathway. Heparin may induce its marked stimulation of enzyme release by complexing with diamine oxidase bound to the cell surface or through interaction with specific binding sites also located in the basolateral membrane. In the intestinal mucosa in vivo, the basal aspect of the villus cells represents the main site of diamine oxidase storage in the presence of normal circulating levels of heparin.