Lipoprotein Lipase Reduces Secretion of Apolipoprotein E from Macrophages

Abstract

Macrophages are a significant source of lipoprotein lipase (LPL) and apolipoprotein E (apo E) in the developing arterial wall lesion, and each of these proteins can importantly modulate lipid and lipoprotein metabolism by arterial wall cells. LPL and apo E share a number of cell surface binding sites, including proteoglycans, and we have previously shown that proteoglycans are important for modulating net secretion of apoprotein E from macrophages. We therefore evaluated a potential role for LPL in modulating net secretion of macrophagederived apo E. In pulse-chase experiments, addition of LPL during the chase period produced a decrease in secretion of apoprotein E from human monocyte-derived macrophages, from the human monocytic THP1 cell line, and from J774 cells transfected to constitutively express a human apo E cDNA. LPL similarly reduced apo E secretion when it was prebound to the macrophage cell surface at 4°C. A native LPL particle was required to modulate apo E secretion; addition of monomers and aggregates did not produce the same effect. Depletion of cell surface proteoglycans by a 72-h incubation in 4-methylumbelliferyl-␤-D-xyloside did not attenuate the ability of LPL to reduce apo E secretion. However, addition of receptor-associated protein attenuated the effect of LPL on apo E secretion. Although LPL could mediate removal of exogenously added apo E from the culture medium, detailed pulse-chase analysis suggested that it primarily prevented release of newly synthesized apo E from the cell layer. Cholesterol loading of cells or antibodies to the low density lipoprotein receptor attenuated LPL effects on apo E secretion. We postulate that LPL sequesters endogenously synthesized apo E at the cell surface by a low density lipoprotein receptor-dependent mechanism. Such post-translational regulation of macrophage apo E secretion by LPL could significantly influence apo E accumulation in arterial vessel wall lesions.Macrophages synthesize and secrete both apo E 1 and LPL (1, 2). These cells, therefore, can be a significant source of each of these proteins in the arterial wall. Both proteins have important roles in systemic lipoprotein metabolism (3, 4) as well as in lipoprotein metabolism at the level of the individual cell (5-14). Regional accumulation of each of these proteins has been detected at sites of atherosclerotic lesion development (15); apo E has been found predominantly on the surface of macrophages and in the matrix surrounding macrophages in atherosclerotic lesions, whereas LPL is also associated with arterial smooth muscle cells. apo E expression in the vessel wall has been shown to be important for modulating vessel wall cholesterol homeostasis (16,17). In fact, macrophage-specific expression of apo E in apo E-null mice has been shown to protect against atherosclerotic lesion development even in the presence of high levels of circulating atherogenic lipoproteins (17). As noted above, LPL also accumulates in diseased vessel wall. A number of cell surface binding sites for LPL have been id...

Degradation of Macrophage ApoE in a Nonlysosomal Compartment

“…We therefore utilized a J774 cell macrophage model, which was transfected to constitutively express a human apoE cDNA. Expression of the apoE cDNA in this cell line is mediated by the human metallothionine IIa promoter and has been previously characterized in detail (3,10,19,20). These cells secrete 0.9 -3.5 g/ml/mg of cell protein of apoE over 24 h depending on culture conditions, an amount similar to that reported for human monocyte-derived macrophages.…”

“…The difference between the fraction of apoE remaining at 40°C between control and sterol/oxysterol-treated cells is significant at p Ͻ 0.05. 20). This effect was rapid and could be observed within 1 h (3,19,20).…”

“…20). This effect was rapid and could be observed within 1 h (3,19,20). In the current studies, we demonstrate the existence of an intracellular locus in which the degradation rate of apoE responds to sterol enrichment of cells.…”

“…25-Hydroxycholesterol was obtained from Steraloids. All other materials were from sources described previously (3,10,19,20).…”

“…Cell Culture-J774 cells stably transfected to constitutively express a wild type human apoE cDNA have been characterized previously in detail (3,10,19,20). Cells were selected and maintained in G418 (400 g/dl) until 1 week prior to the initiation of experiments.…”