Oxidized VLDL Induces Less Triglyceride Accumulation in J774 Macrophages Than Native VLDL Due to an Impaired Extracellular Lipolysis

Abstract

Abstract-The present study examined the relative contributions of the different pathways by which oxidatively modified VLDL (oxVLDL) promotes the uptake and intracellular accumulation of lipids in J774 macrophages. VLDL was oxidized for a maximum of 4 hours, resulting in an increase in thiobarbituric acid-reactive substances and an increased electrophoretic mobility on agarose gel. The lipid composition of the relatively moderately oxidized VLDL samples did not differ significantly from that of nonoxidized VLDL samples. The uptake of 125 I-labeled VLDL by the J774 cells increased with oxidation time and was completely blocked on coincubation with polyinosinic acid (PolyI), indicating that oxVLDL is taken up by the cells via the scavenger receptor only. Despite the 2-fold increased uptake of oxVLDL protein, the cell association of triglyceride (TG)-derived fatty acids by the J774 macrophages after incubation with oxVLDL was only 50% of that with native VLDL. In line with these observations, the induction of de novo synthesis of TG by J774 cells was Ϸ3-fold less efficient after incubation with oxVLDL than after incubation with native VLDL. The induction of de novo synthesis of TG with oxVLDL was even further decreased on simultaneous incubation with PolyI, whereas PolyI did not affect the native VLDL-induced TG synthesis. These results indicate that oxVLDL induces endogenous TG synthesis predominantly through particle uptake via the scavenger receptor and much less via the extracellular lipoprotein lipase (LPL)-mediated hydrolysis of TG, as is the case for native VLDL. In line with these observations, we showed that the suitability of VLDL as a substrate for LPL decreases with oxidation time. Addition of oxVLDL to the LPL assay did not interfere with the lipolysis of native VLDL. However, enrichment of the oxidized lipoprotein particle with native apoC2 was able to fully restore the impaired lipolysis. Thus, from these studies it can be concluded that on oxidation, VLDL becomes less efficient in inducing TG accumulation in J774 cells as a consequence of a defect in apoC2 as an activator for the LPL-mediated extracellular lipolysis. (1) the receptor-mediated uptake of intact VLDL particles and (2) the direct uptake of free fatty acids (FFAs) as generated by the extracellular lipoprotein lipase (LPL)-mediated hydrolysis of VLDL-TG, followed by intracellular reesterification into lipids. The resulting cholesterol-enriched remnant particles are thereafter taken up via a receptor-mediated process.In analogy with LDL, 5,6 it has been shown that in vitro exposure of ␤-VLDL to endothelial cells causes oxidation of this lipoprotein. 7,8 This results in a 2-to 3-fold increased degradation by mouse peritoneal macrophages 7 and rabbit smooth muscle cells 8 compared with unoxidized ␤-VLDL and in a 2-to 3-fold increased intracellular cholesterol esterification rate. Isolated human VLDL, like human LDL, was shown to be effectively oxidized in vitro on incubation with free radicals. 9 The degradation of oxidized human VLDL (oxVLD...

Utilisation of Fatty Acid and Triacylglycerol by Rat Macrophages: the Effect of Endotoxin

“…For example increased expression of UCP2 through a TNFα-dependent mechanism [34] has been described for hepatic tissue treated with LPS; increased hydrogen peroxide production by uncoupling proteins may result in oxidative modification of the resulting VLDL yielding efficient substrates for scavenger receptors. Moreover, oxidised VLDL had decreased binding to LPL [35] and induced less TAG accumulation in macrophages due to impaired LPL lipolysis [36]. Similar apo-E contents of both cVLDL and eVLDL [8] argues against enhanced uptake mediated through apo-E-specific routes.…”

The role of lipolysis in mediating the proinflammatory effects of very low density lipoproteins in mouse peritoneal macrophages

“…Third, FFA generated during the lipolysis of VLDL can also be brought into the cells via receptors or by passive diffusion (34). Macrophages are known to secrete active lipoprotein lipase (35), and lipoprotein lipase activity has been shown to be upregulated by exposure to VLDL (8,30). Our data showing the absence of TG and FFA accumulation in macrophages treated with the lipoprotein lipase inhibitor Orlistat are in agreement with other reports (30,33,36) and implicate VLDL remnant and/or FFA uptake as the primary mechanisms by which VLDL induces macrophage foam cell formation.…”

“…TGs in fasting plasma are carried in VLDLs, and these VLDLs have been found within human and experimental atherosclerotic lesions (3,4), providing a rationale to study their direct effects on macrophage functions such as foam cell formation and inflammation. It is well established that VLDL causes the accumulation of TGs and FFAs in macrophages (5)(6)(7)(8)(9). This can occur through three different mechanisms: 1) uptake of intact VLDL particles; 2) uptake of VLDL remnants resulting from lipoprotein lipase-mediated lipolysis; and 3) uptake of FFAs produced by VLDL lipolysis, which can be taken into cells through both passive diffusion and receptor-mediated mechanisms.…”

Adipophilin Enhances Lipid Accumulation and Prevents Lipid Efflux From THP-1 Macrophages

“…2,3 Several studies indicated that oxLDL has a number of diverse effects on macrophage function including growth stimulation 4 proinflammatory effects such as expression of inflammatory cytokines, 5 increase in cytotoxicity and expression of metalloproteinases, 6 inhibition of expression of inducible nitric oxide synthase, 7 and effects on lipid metabolism and accumulation. 8,9 Using either a DNA array approach 10 or a subtractive approach, 11 numerous genes have been shown to be regulated on exposure of THP-1 cells to modified LDL. Adipophilin, or adipose differentiation-related protein (ADRP), a 50-kDa protein initially described in adipocytes, 12 which is considered a marker of lipid accumulation, is among those genes upregulated by modified LDL.…”