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 pro tein at 4 h, similar to 50% of which was bound to cell-surface heparan sulfate proteoglycans (HSPG). The newly synthesized HL possessed enzy matic activity. When rabbit beta-very low density lipoproteins (beta-V LDL) and canine chylomicrons or chylomicron remnants were incubated wi th HL-secreting cells, remnant binding and uptake were enhanced 3-fold compared with nontransfected cells. Furthermore, fluorescence microsc opy showed enhanced uptake of ecyl-3,3,3',3'-tetramethylindocarbocyani ne-labeled beta-VLDL by the HL-transfected cells. When I-125-beta-VLDL were added to conditioned medium from HL-secreting cells, the HL in t he media enhanced the binding and uptake of the remnant lipoproteins b y nontransfected cells about 3-fold. Likewise, surface-bound HL (witho ut 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 cel l-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 nontransfected cells p revented the enhanced binding and uptake of beta-VLDL incubated with c onditioned medium from HL-secreting cells. As binding was not enhanced in the absence of HSPG, an HL-HSPG initial interaction appears essent ial. Addition of apolipoprotein (apo) E to the beta-VLDL did not facil itate HL-mediated binding and uptake; in fact, beta-VLDL from apoE-nul l mice demonstrated a similar degree of enhanced binding as did rabbit beta-VLDL with or without added apoE. On the other hand, beta-VLDL fr om transgenic mice overexpressing binding-defective apoE(Arg(142) --> Cys) did not display any enhanced binding and uptake by the HL-secreti ng cells, and it appears that the apoE(Arg(142) -->( Cys) actually inh ibited the HL-mediated interaction. This mutant form of apoE is associ ated with a dominant mode of expression of type III hyperlipoproteinem ia in contrast to the more commonly occurring recessive disorder. Impa ired HL interaction with the apoE(Arg(142) --> Cys) beta-VLDL may cont ribute to remnant lipoprotein accumulation in the plasma of patients w ith this mutant form of apoE. Thus, HL contributes to the enhanced cel l association of specific types of remnant lipoproteins by initiating their binding to cell-surface HSPG.