CELL-SURFACE PROTEOGLYCANS MODULATE NET SYNTHESIS AND SECRETION OF MACROPHAGE APOLIPOPROTEIN-E

Using a macrophage cell line that constitutively expresses a human apo liprotein E (apoE) cDNA, we have investigated the post-translational m etabolism of endogenously produced apoE. Inhibition of lysosomal or cy steine proteases led to significant inhibition of apoE degradation but did not increase apoE secretion, indicating that cellular degradation is not limiting for apoE secretion in macrophages. Treatment of macro phages with inhibitors of proteoglycan synthesis (4-methylum-bellifery l-beta-D-xyloside) or sulfation (sodium chlorate) enhanced the release of apoE from cells and significantly attenuated the increase in secre tion produced by incubation with phosphatidylcholine vesicles (PV). Th ese observations suggested that a significant fraction of the apoE ret ained by cells (and released by incubation with PV) was associated wit h proteoglycans. Treatment of cells with exogenous heparinase led to a greater than 4-fold increase in apoE secretion and similarly attenuat ed the response to PV, suggesting that apoE was trapped in an extracel lular proteoglycan matrix. This conclusion was confirmed in studies sh owing that PV could enhance the release of apoE from cells during an i ncubation at 4 degrees C, but this enhanced release was abolished in p roteoglycan-depleted cells. Incubation with lactoferrin at 4 or 37 deg rees C produced the existence of a cell surface pool of apoE. Pulse-ch ase studies showed that the apoE trapped in the proteoglycan matrix wa s susceptible to rapid cellular degradation such that net synthesis of apoE (secreted plus cell-associated) was increased significantly in p roteoglycan-depleted cells compared with control cells as early as 45 min during a chase period.