Nephrotic livers secrete normal VLDL that acquire structural and functional defects following interaction with HDL.

Nephrotic livers secrete normal VLDL that acquire structural and functional defects following interaction with HDL. Background Binding of very low-density lipoprotein (VLDL) isolated from serum of nephrotic rats VLDL to endothelial cells is defective. This defect is conferred on normal VLDL by prior incubation with high-density lipoprotein (HDL) from nephrotic, but not control rats. It is not known whether the defect is present in nascent VLDL ( n VLDL) or is acquired after secretion. We test the hypothesis that VLDL is normal at the time of secretion from the liver and the defect in binding to endothelium is conferred following secretion through interaction with HDL. Methods VLDL was synthesized by and collected from isolated perfused livers from either control or nephrotic rats. n VLDL was labeled with 3 H-oleate to measure binding and 35 S methionine to evaluate apolipoprotein exchange and composition. To test whether HDL conferred a binding defect, n VLDL was incubated with HDL obtained either from control or nephrotic rats prior to measurement of binding. To distinguish the effects of proteinuria from reduced albumin concentration we additionally incubated n VLDL with HDL obtained from rats with hereditary analbuminemia. Both HDL and VLDL were reisolated by centrifugation prior to subsequent binding and lipolysis determination. Exchange of 35 S-labeled apolipoprotein E (apoE) among the subsequent VLDL and HDL fractions was determined. To determine the effect of HDL on lipolysis, HDL-treated VLDL was exposed to lipoprotein lipase–coated 96-well plates and 3 H-oleate release measured. To establish whether differences in apoE content could explain the differences in binding and lipolysis, apoE was restored to nephrotic VLDL and lipolysis and binding were subsequently measured. Results Binding of nephrotic n VLDL was greater than control n VLDL (0.58 ± 0.13 vs. 0.75 ± 0.07ng protein bound/mg cell protein) ( P = 0.04, N = 6). Lipolysis was similarly elevated (0.091 ± 0.010 vs 0.064 ± 0.002nmol NEFA released/well/hour) ( P n VLDL obtained from either nephrotic or control livers ( P = 0.02, N = 6). Treatment with nephrotic (vs. control) HDL reduced both binding (control n VLDL + control HDL, 0.64 ± 0.02; control + nephrotic, 0.43 ± 0.06; nephrotic + control, 0.69 ± 0.05; and nephrotic + nephrotic, 0.62 ± 0.04mg VLDL protein/mg cell protein) and lipolysis (control n VLDL + control HDL, 0.053 ± 0.004; control + nephrotic, 0.038 ± 0.004; nephrotic + control, 0.069 ± 0.004; and nephrotic + nephrotic, 0.062 ± 0.004nmol NEFA/well/hour) ( P n VLDL + control HDL) of n VLDL from either source. The apoE content of n VLDL coincubated with control HDL or analbuminemic HDL was increased compared n VLDL incubated with either no HDL or nephrotic HDL ( P P Conclusion Unlike circulating VLDL, binding of n VLDL from isolated livers from nephrotic rats to endothelial cells is greater and its lipolysis is increased compared to control n VLDL. Decreased binding and lipolysis is conferred following incubation with HDL isolated from control, but not nephrotic rats and binding can be restored by reintroduction of apoE. Thus both defects are conferred on VLDL by exposure to HDL obtained from nephrotic animals, possibly a consequence of a failure of nephrotic HDL to enrich VLDL with apoE during clearance.