Pseudo type III dyslipoproteinemia is associated with normal fibroblast lipoprotein receptor activity

Pseudo type III (PT-III) dyslipoproteinemia is characterized by a plasma accumulation of triglyceride-rich lipoproteins (TRL) and their remnants. It mimics type III, but its etiology can not be ascribed to a genetic apo E defect. In order to determine whether PT-III is associated with a genetic lipoprotein receptor abnormality, we have measured (in cultured fibroblasts from affected and nonaffected individuals) the in vitro activity of three lipoprotein receptors which are implicated in the catabolism of TRL, namely the low-density lipoprotein receptor (LDL-R), the lipoprotein receptor-related protein (LRP) and the lipolysis-stimulated receptor (LSR). Specific cell association and degradation of 125I-LDL by LDL-R-upregulated PT-III fibroblasts was not significantly different from that of control cells (103±10% and 98±17% of controls; 20 μg/ml 125I-LDL). Specific cell association and degradation of rabbit 125I-β-VLDL was also not significantly different. LRP activity was assessed by measuring the ability of PT-III and control cells to bind three different LRP ligands: activated α2-macroglobulin (α2M-MA), lactoferrin and apo E-enriched rabbit β-VLDL. No significant differences were observed (24.0±2.1 vs. 23.4±5.7 fmol/mg for 5 nM of 125I-α2M-MA; 4.8±0.3 vs. 5.2±1.3 μg/mg for 20 μg/ml of 125I-lactoferrin; 319.4±51.2 vs. 309.5±23.2 ng/mg for 5 μg/ml of 125I-β-VLDL, PT-III vs. control, respectively). LSR activity, as assessed by the cell association or degradation of 125I-LDL by fibroblasts in the presence of 0.5 mM oleate and human leptin, was also not different. No evidence was obtained for deficient cellular recognition of PT-III TRL (d<1.006 g/ml) by normal human fibroblasts or mouse macrophages. These results suggest that PT-III dyslipoproteinemia is not due to an accumulation in plasma of poorly recognized TRL, nor due to a genetic defect in LDL-R, LRP or LSR.