Oxidized VLDL induces less triglyceride accumulation in J774 macrophages than native VLDL due to an impaired extracellular lipolysis

The present study examined the relative contributions of the different path ways by which oxidatively modified VLDL (oxVLDL) promotes the uptake and in tracellular accumulation of lipids in J774 macrophages, VLDL was oxidized f or a maximum of 3 hours, resulting in an increase in thiobarbituric acid-re active substances and an increased electrophoretic mobility on agarose gel. The lipid composition of the relatively moderately oxidized VLDL samples d id not differ significantly from that of nonoxidized VLDL samples, The upta ke of I-125-labeled VLDL by the J774 cells increased with oxidation time an d was completely blocked on coincubation with polyinosinic acid (PolyI), in dicating that oxVLDL is taken up by the cells via the scavenger receptor on ly. Despite the 2-fold increased uptake of oxVLDL protein, the cell associa tion 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 approximate to 3-fold less efficient after incubation with oxVLDL than after incubation with native VLDL. The induction of de novo synthesis of T G with oxVLDL was even further decreased on simultaneous incubation with Po lyI, whereas PolyI did not affect the native VLDL-induced TG synthesis. The se results indicate that oxVLDL induces endogenous TG synthesis predominant ly through particle uptake via the scavenger receptor and much less via the extracellular lipoprotein lipase (LPL)-mediated hydrolysis of TG, as is th e 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 effi cient in inducing TG accumulation in J774 cells as a consequence of a defec t in apoC2 as an activator for the LPL-mediated extracellular lipolysis.