Heparan Sulfate Proteoglycans Participate in Hepatic Lipaseand Apolipoprotein E-mediated Binding and Uptake of Plasma Lipoproteins, Including High Density Lipoproteins

Zhong-Sheng Ji,Helén L Dichek,R Dennis Miranda,Robert W Mahley

doi:10.1074/jbc.272.50.31285

Zhong-Sheng Ji, Helén L Dichek + Show 2 more

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https://doi.org/10.1074/jbc.272.50.31285

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Abstract

High density lipoprotein (HDL) particles and HDL cholesteryl esters are taken up by both receptor-mediated and non-receptor-mediated pathways. Here we show that cell surface heparan sulfate proteoglycans (HSPG) participate in hepatic lipase (HL)- and apolipoprotein (apo) E-mediated binding and uptake of mouse and human HDL by cultured hepatocytes. The HL secreted by HL-transfected McA-RH7777 cells enhanced both HDL binding at 4 degrees C (approximately 2-4-fold) and HDL uptake at 37 degrees C (approximately 2-5-fold). The enhanced binding and uptake of HDL were partially inhibited by the 39-kDa protein, an inhibitor of low density lipoprotein receptor-related protein (LRP), but were almost totally blocked by heparinase, which removes the sulfated glycosaminoglycan chains from HSPG. Therefore, HL may mediate the uptake of HDL by two pathways: an HSPG-dependent LRP pathway and an HSPG-dependent but LRP-independent pathway. The HL-mediated binding and uptake of HDL were only minimally reduced when catalytically inactive HL or LRP binding-defective HL was substituted for wild-type HL, indicating that much of the HDL uptake required neither HL binding to the LRP nor lipolytic processing. To study the role of HL in facilitating the selective uptake of cholesteryl esters, we used HDL into which radiolabeled cholesteryl ether had been incorporated. HL increased the selective uptake of HDL cholesteryl ether; this enhanced uptake was reduced by more than 80% by heparinase but was unaffected by the 39-kDa protein. Like HL, apoE enhanced the binding and uptake of HDL (approximately 2-fold) but had little effect on the selective uptake of HDL cholesteryl ether. In the presence of HL, apoE did not further increase the uptake of HDL, and at a high concentration apoE impaired or decreased the HL-mediated uptake of HDL. Therefore, HL and apoE may utilize similar (but not identical) binding sites to mediate HDL uptake. Although the relative importance of cell surface HSPG in the overall metabolism of HDL in vivo remains to be determined, cultured hepatocytes clearly displayed an HSPG-dependent pathway that mediates the binding and uptake of HDL. This study also demonstrates the importance of HL in enhancing the binding and uptake of remnant and low density lipoproteins via an HSPG-dependent pathway.

Highlights

High density lipoproteins (HDL)1 participate in the transport of cholesterol to the liver for reutilization and excretion [1,2,3] and to other tissues such as the adrenal glands, ovaries, and testes for hormone synthesis (4 – 6)
Binding of HDL—To determine if secreted hepatic lipase (HL) mediates the direct binding of HDL to the cell surface, nontransfected and HL-transfected McA-RH7777 cells were incubated at 37 °C for 2 h to allow HL to accumulate in the medium as described previously [52] and placed on ice for 20 min in the cold room (4 °C) to minimize the catalytic activity of the secreted HL
Hepatic lipase acts as a ligand that mediates an increased cell surface binding of HDL particles

Summary

Introduction

High density lipoproteins (HDL) participate in the transport of cholesterol to the liver for reutilization and excretion (as bile acids) [1,2,3] and to other tissues such as the adrenal glands, ovaries, and testes for hormone synthesis (4 – 6). Cholesteryl esters of HDL may be taken up by the liver or extrahepatic tissues (or cultured cells) without a parallel uptake of HDL particles. A 476-amino acid protein (Mr ϳ60,000), plays an important role in HDL metabolism (26 –28) It interacts with HDL [29] and catalyzes the hydrolysis of HDL phospholipids and triglycerides [30, 31], converting HDL2 to HDL3 [32, 33], and it promotes the uptake of HDL by perfused rat liver [34, 35]. Apolipoprotein E can associate with HDL and mediate HDL particle uptake by either the LDL receptor [12, 42, 49] or the LRP [20]. HL-mediated selective uptake of cholesteryl esters has been demonstrated in cultured hepatocytes [36, 37] and in Chinese hamster ovary cells transfected with glycophosphatidylinositol-anchored HL [51]

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