Lipolysis Produces Changes in the Immunoreactivity and Cell Reactivity of Very Low Density Lipoproteins

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Abstract
Smaller very low density lipoprotein (VLDL) remnants interact more readily with tissues than do larger “intact” VLDL. This may be related to changes in the availability of VLDL apoproteins on the surface of the lipoproteins. To test this hypothesis VLDL were incubated at 37°C with bovine milk lipase (LPL), and the abilities of LPL-treated VLDL preparations to compete with 125I-low density lipoproteins (LDL) for interaction with cultured normal human fibroblasts were measured. At the same time, the immunologic activities of these preparations were also tested by double antibody radioimmunoassay. Triglyceride (TG) contents of VLDL fell by 30-90% during incubation with LPL and, on zonal ultracentrifugation, VLDL of faster Svedberg unit of flotation (Sf1.063) rates (>150) were gradually converted to smaller VLDL with lower Sf rates (21-60). LPL-treated VLDL competed two to five times more effectively with 125I-LDL for binding to cellular receptors than did control VLDL. Control VLDL incubated with heat-inactivated LPL at 37°C, or with active LPL at 4°C had unaltered cell reactivities and TG contents compared with VLDL incubated without any enzyme. The direct uptake and degradation of LPL-treated VLDL was also assessed by using VLDL 125I-labeled in apoprotein (Apo)B. LPL-treated VLDL-125I-ApoB were taken up and degraded by fibroblast at greater rates than were control VLDL-125I-ApoB. Thus, hydrolysis of VLDL lipids was accompanied by an increased ability of VLDL to interact with fibroblasts. The immunoreactivity of ApoB in the same VLDL preparations, expressed as the “apparent ApoB contents” of LPL-treated VLDL, increased by 10-50% (P < 0.02) in those assays that contained anti-LDL antisera, but the ApoB of control VLDL remained constant. However, assays that contained antisera directed against ApoB isolated from VLDL did not distinguish between LPL-treated and control VLDL. Thus, VLDL lipid hydrolysis was accompanied by changes in the immunoreactivity of VLDL-ApoB, which probably reflect changes in the disposition of ApoB on the surface of VLDL. The altered disposition of ApoB on VLDL “remnants” may be related to their enhanced interaction with cells.