Processing and transport of lysosomal enzymes in human monocyte line U937.

Precursors of cathepsin D and beta-hexosaminidase synthesized in the U937 monocyte line are processed to mature forms with similar kinetics as in fibroblasts. In U937 cells the processing of the precursor of the beta-chain of beta-hexosaminidase, however, results in a larger fragment that resembles a processing intermediate in fibroblasts. This difference is explained by differences in the equipment of the cells with proteinases, since cross-feeding of the precursors to the cells results in a processing characteristic for the recipient cell type. In sucrose gradients the precursors are found partly in a low- and partly in a high-density region. Mature polypeptides and activity of lysosomal enzymes fractionate mainly in the higher density region. In U937 cells the transport and maturation of endogenous lysosomal enzymes are less sensitive to bases (NH4Cl, chloroquine, tilorone) and to antibody against the mannose 6-phosphate specific receptors than in fibroblasts. A small portion of enzymes released from U937 cells contains the markers recognized by the mannose-6-phosphate specific receptors. U937 cells express these receptors and utilize them for transport of endogenous and exogenous lysosomal enzymes. It appears, however, that a fraction of lysosomal enzymes is transported in U937 cells independent of the mannose-6-phosphate-specific receptors.