Lucas M, Iverius P H, Strickland D K, Mazzone T
Department of Medicine, Rush Medical College, Chicago, Illinois 60612, USA.
J Biol Chem. 1997 May 16;272(20):13000-5. doi: 10.1074/jbc.272.20.13000.
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 macrophage-derived 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 degrees 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-beta-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.
巨噬细胞是发育中的动脉壁病变中脂蛋白脂肪酶（LPL）和载脂蛋白E（apo E）的重要来源，并且这些蛋白质中的每一种都能显著调节动脉壁细胞的脂质和脂蛋白代谢。LPL和apo E共享许多细胞表面结合位点，包括蛋白聚糖，并且我们之前已经表明蛋白聚糖对于调节巨噬细胞中载脂蛋白E的净分泌很重要。因此，我们评估了LPL在调节巨噬细胞衍生的apo E净分泌中的潜在作用。在脉冲追踪实验中，在追踪期添加LPL导致人单核细胞衍生的巨噬细胞、人单核细胞THP1细胞系以及转染以组成性表达人apo E cDNA的J774细胞中载脂蛋白E的分泌减少。当LPL在4℃预结合到巨噬细胞表面时，它同样减少了apo E的分泌。需要天然LPL颗粒来调节apo E的分泌；添加单体和聚集体不会产生相同的效果。在4-甲基伞形酮基-β-D-木糖苷中孵育72小时以耗尽细胞表面蛋白聚糖，并没有减弱LPL降低apo E分泌的能力。然而，添加受体相关蛋白减弱了LPL对apo E分泌的影响。虽然LPL可以介导从培养基中去除外源性添加的apo E，但详细的脉冲追踪分析表明它主要阻止了新合成的apo E从细胞层释放。细胞的胆固醇负载或针对低密度脂蛋白受体的抗体减弱了LPL对apo E分泌的影响。我们推测LPL通过低密度脂蛋白受体依赖性机制在细胞表面隔离内源性合成的apo E。LPL对巨噬细胞apo E分泌的这种翻译后调节可能会显著影响apo E在动脉血管壁病变中的积累。
