Enhanced binding and uptake of remnant lipoproteins by hepatic lipase-secreting hepatoma cells in culture

Rat hepatoma McA-RH7777 cells transfected with a human hepatic lipase (HL) cDNA synthesized and secreted 50-80 ng of human HL/mg of cell protein at 4 h, ∼50% of which was bound to cell-surface heparan sulfate proteoglycans (HSPG). The newly synthesized HL possessed enzymatic activity. When rabbit β-very low density lipoproteins (β-VLDL) and canine chylomicrons or chylomicron remnants were incubated with HL-secreting cells, remnant binding and uptake were enhanced 3-fold compared with nontransfected cells. Furthermore, fluorescence microscopy showed enhanced uptake of 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine-labeled β-VLDL by the HL-transfected cells. When ¹²⁵I-β-VLDL were added to conditioned medium from HL-secreting cells, the HL in the media enhanced the binding and uptake of the remnant lipoproteins by nontransfected cells about 3-fold. Likewise, surface-bound HL (without HL in the medium) also was able to mediate the enhanced binding of the remnants. This HL-enhanced binding was shown to be mediated by an interaction with cell-surface HSPG. Heparinase treatment to remove cell-surface HSPG or chlorate treatment to prevent HSPG sulfation of the HL-secreting cells abolished all the HL-mediated enhanced binding and uptake. Furthermore, heparinase pretreatment of non-transfected cells prevented the enhanced binding and uptake of β-VLDL incubated with conditioned medium from HL-secreting cells. As binding was not enhanced in the absence of HSPG, an HL-HSPG initial interaction appears essential. Addition of apolipoprotein (apo) E to the β-VLDL did not facilitate HL-mediated binding and uptake; in fact, β-VLDL from apoE-null mice demonstrated a similar degree of enhanced binding as did rabbit β-VLDL with or without added apoE. On the other hand, β-VLDL from transgenic mice overexpressing binding-defective apoE(Arg¹⁴² → Cys) did not display any enhanced binding and uptake by the HL-secreting cells, and it appears that the apoE(Arg¹⁴² → Cys) actually inhibited the HL-mediated interaction. This mutant form of apoE is associated with a dominant mode of expression of type III hyperlipoproteinemia in contrast to the more commonly occurring recessive disorder. Impaired HL interaction with the apoE(Arg¹⁴² → Cys) β-VLDL may contribute to remnant lipoprotein accumulation in the plasma of patients with this mutant form of apoE. Thus, HL contributes to the enhanced cell association of specific types of remnant lipoproteins by initiating their binding to cell-surface HSPG.