The relevance of glycosaminoglycan sulfates to Apo E induced lipid uptake by hepatocyte monolayers

Syndecan-1 and Its Expanding List of Contacts

Significance: The binding of cytokines and growth factors to heparan sulfate (HS) chains on proteoglycans generates gradients that control development and regulate wound healing. Syndecan-1 (sdc1) is an integral membrane HS proteoglycan. Its structure allows it to bind with cytosolic, transmembrane, and extracellular matrix (ECM) proteins. It plays important roles in mediating key events during wound healing because it regulates a number of important processes, including cell adhesion, cell migration, endocytosis, exosome formation, and fibrosis. Recent Advances: Recent studies reveal that sdc1 regulates wound healing by altering integrin activation. Differences in integrin activation lead to cell-type-specific changes in the rate of cell migration and ECM assembly. Sdc1 also regulates endocytosis and the formation and release of exosomes. Critical Issues: Understanding how sdc1 facilitates wound healing and resolution will improve treatment options for elderly and diabetic patients with delayed wound healing. Studies showing that sdc1 function is altered in cancer are relevant to those interested in controlling fibrosis and scarring. Future Directions: The key to understanding the various functions ascribed to sdc1 is resolving how it interacts with its numerous binding partners. The role played by chondroitin sulfate glycosaminoglycan (GAG) chains on the ability of sdc1 to associate with its ligands needs further investigation. At wound sites heparanase can cleave the HS GAG chains of sdc1, alter its ability to bind cytokines, and induce shedding of the ectodomain. This review will discuss how the unique structure of sdc1 allows it to play key roles in cell signaling, ECM assembly, and wound healing.

Structural determinants in the C-terminal domain of apolipoprotein E mediating binding to the protein core of human aortic biglycan

Apolipoprotein (apo) E-containing high density lipoprotein particles were reported to interact in vitro with the proteoglycan biglycan (Bg), but the direct participation of apoE in this binding was not defined. To this end, we examined the in vitro binding of apoE complexed with dimyristoylphosphatidylcholine (DMPC) to human aortic Bg before and after glycosaminoglycan (GAG) depletion. In a solid-phase assay, apoE.DMPC bound to Bg and GAG-depleted protein core in a similar manner, suggesting a protein-protein mode of interaction. The binding was decreased in the presence of 1 m NaCl and was partially inhibited by either positively (0.2 m lysine, arginine) or negatively charged (0.2 m aspartic, glutamic) amino acids. A recombinant apoE fragment representing the C-terminal 10-kDa domain, complexed with DMPC, bound as efficiently as full-length apoE, whereas the N-terminal 22-kDa domain was inactive. Similar results were obtained with a gel mobility shift assay. Competition studies using a series of recombinant truncated apoEs showed that the charged segment in the C-terminal domain between residues 223 and 230 was involved in the binding. Overall, our results demonstrate that the C-terminal domain contains elements critical for the binding of apoE to the Bg protein core and that this binding is ionic in nature and independent of GAGs.

Kinetics of apolipoprotein E isoforms-binding to the major glycosaminoglycans of the extracellular matrix

Apolipoprotein E (apoE), a key lipid transport protein, displays a heparin-binding property that is critical in several apoE functions. The kinetics of the interaction between apoE isoforms and glycosaminoglycans (GAGs) were studied using surface plasmon resonance. The dissociation constant of equilibrium K(D) for apoE3-heparin interaction was estimated to be 12 nM for apoE3 and three common apoE isoforms revealed similar affinities for heparin. ApoE binds to GAGs in the following order: heparin>heparan sulfate>dermatan sulfate>chondroitin sulfate. The affinity parameter of the binding of low molecular weight heparins to apoE is correlated with the chain length. The effective number Z of electrostatic interactions between plasma apoE3 and heparin was assessed to be three. Metal chelators were able to diminish apoE-binding to heparin, suggesting some stabilizing effect of metal ions while reconstitution with lipids did not affect binding affinities for heparin, suggesting that the N-terminal heparin-binding site is responsible for apoE-containing lipoprotein interactions with heparin.

Binding of beta-VLDL to heparan sulfate proteoglycans requires lipoprotein lipase, whereas ApoE only modulates binding affinity

The binding of beta-VLDL to heparan sulfate proteoglycans (HSPG) has been reported to be stimulated by both apoE and lipoprotein lipase (LPL). In the present study we investigated the effect of the isoform and the amount of apoE per particle, as well as the role of LPL on the binding of beta-VLDL to HSPG. Therefore, we isolated beta-VLDL from transgenic mice, expressing either APOE*2(Arg158-->Cys) or APOE*3-Leiden (E2-VLDL and E3Leiden-VLDL, respectively), as well as from apoE-deficient mice containing no apoE at all (Enull-VLDL). In the absence of LPL, the binding affinity and maximal binding capacity of all beta-VLDL samples for HSPG-coated microtiter plates was very low. Addition of LPL to this cell-free system resulted in a 12- to 55-fold increase in the binding affinity and a 7- to 15-fold increase in the maximal binding capacity (Bmax). In the presence of LPL, the association constant (Ka) tended to increase in the order Enull-VLDL<E2-VLDL<E3Leiden-VLDL, whereas Bmax increased in the reverse order: E3Leiden-VLDL approximately E2-VLDL<Enull-VLDL. Addition of LPL resulted in a marked stimulation of both Ka and Bmax for binding of beta-VLDL samples to J774 cells similar to that found for the binding to HSPG-LPL complexes. Our results indicate that both Ka and Bmax for binding of beta-VLDL to HSPG are increased more than 1 order of magnitude on addition of LPL. In addition, for the binding of beta-VLDL to HSPG-LPL complexes, the presence of apoE is not a prerequisite, but results in an increased binding affinity, depending on the apoE isoform used.

Heparan sulfate proteoglycans participate in hepatic lipaseand apolipoprotein E-mediated binding and uptake of plasma lipoproteins, including high density lipoproteins

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.

The syndecan family of proteoglycans. Novel receptors mediating internalization of atherogenic lipoproteins in vitro

Cell-surface heparan sulfate proteoglycans have been shown to participate in lipoprotein catabolism, but the roles of specific proteoglycan classes have not been examined previously. Here, we studied the involvement of the syndecan proteoglycan family. First, transfection of CHO cells with expression vectors for several syndecan core proteins produced parallel increases in the cell association and degradation of lipoproteins enriched in lipoprotein lipase, a heparan-binding protein. Second, a chimeric construct, FcR-Synd1, that consists of the ectodomain of the IgG Fc receptor Ia linked to the highly conserved transmembrane and cytoplasmic domains of syndecan-1 directly mediated efficient internalization, in a process triggered by ligand clustering. Third, internalization of lipase-enriched lipoproteins via syndecan-1 and of clustered IgGs via the chimera showed identical kinetics (t1/2 = 1 h) and identical dose-response sensitivities to cytochalasin B, which disrupts microfilaments, and to genistein, which inhibits tyrosine kinases. In contrast, internalization of the receptor-associated protein, which proceeds via coated pits, showed a t1/2 < 15 min, limited sensitivity to cytochalasin B, and complete insensitivity to genistein. Thus, syndecan proteoglycans can directly mediate ligand catabolism through a pathway with characteristics distinct from coated pits, and might act as receptors for atherogenic lipoproteins and other ligands in vivo.

Inhibition of lipoprotein lipase induced cholesterol ester accumulation in human hepatoma HepG2 cells

It has been suggested previously that lipoprotein lipase may act as a ligand to enhance binding and uptake of lipoprotein particles. In the present study we have examined the capacity of bovine milk lipoprotein lipase to induce intracellular accumulation of triglyceride and cholesterol ester by VLDL (Sr 60-400) isolated from Type IV hypertriglyceridemic subject (HTg-VLDL) in HepG2 cells, independent of its lipolytic activity. We have also attempted to elucidate the cellular receptor mechanisms responsible for these effects. HTg-VLDL-mediated increases in intracellular triglyceride and cholesterol ester were dependent on the presence of an active lipase. Bovine milk lipoprotein lipase (LPL) increases triglyceride mass by 301% +/- 28% (P < 0.0005) and cholesterol ester mass by 176% +/- 12% (P < 0.0005). These HTg-VLDL-mediated increases in intracellular triglyceride and cholesterol ester did not occur when heat-inactivated lipase was used. Rhizopus lipase could replace LPL and cause equivalent increases in intracellular triglyceride and cholesterol ester (472% +/- 61%(P < 0.005) and 202% +/- 25% (P < 0.025) respectively vs. control). HTg-VLDL treated with LPL and reisolated also caused equivalent increases (274% +/- 18%(P < 0.01) and 177% +/- 12% (P < 0.005) for triglyceride and cholesterol ester). LDL also caused increases in intracellular cholesterol ester (189% +/- 20%(P < 0.005)), although three times more LDL cholesterol had to be added to achieve the same effect. These LDL-induced increases were effectively blocked by monoclonal antibodies directed against the B,E receptor binding domains of apo B (-97% +/- 13% (P < 0.0005) with anti-apo B 5E11 and -68% +/- 13% (P < 0.05) for anti-apo B B1B3) or by anti-B,E receptor antibodies (-77% +/- 7% (P < 0.01) antibody C7). These same antibodies had little effect on the HTg-VLDL+LPL-induced increases in cholesterol ester (+21%, +15% and -22% for 5E11, B1B3 and C7, respectively). Monoclonal anti-apo E antibodies also had no effect on LDL-mediated increases in intracellular cholesterol ester, but had a small and significant effect on VLDL-mediated increases in cholesterol ester. However, heparin, which interferes with cell surface proteoglycan interaction, was very effective at blocking HTg-VLDL-mediated increases in cholesterol ester in the presence of LPL (-86% +/- 8% P < 0.0005). Heparin was also effective in the presence of Rhizopus lipase (-79%) or lipolyzed re-isolated HTg-VLDL (-95%). These results suggest that lipoprotein lipase may enhance the uptake process beyond its role in lipolytic remodelling but does not appear to be an absolute requirement. In contrast, heparin had no effect on LDL-mediated cholesterol ester accumulation. Lactoferrin, which inhibits interaction with the low density lipoprotein receptor-related protein (LRP), was also very effective at inhibiting HTg-VLDL increases in intracellular cholesterol ester (-95% +/- 6%, P < 0.01). However, there was no effect of either heparin or lactoferrin on HTg-VLDL-mediated triglyceride accumulation. Thus cell surface heparin sulphate may facilitate intracellular lipid acquisition by providing a stabilizing bridge with the lipoproteins and enhance uptake through receptor-mediated processes such as LRP.

Dominant expression of type III hyperlipoproteinemia. Pathophysiological insights derived from the structural and kinetic characteristics of ApoE-1 (Lys146-->Glu)

Type III hyperlipoproteinemia is characterized by delayed chylomicron and VLDL remnant catabolism and is associated with homozygosity for the apoE-2 allele. We have identified a kindred in which heterozygosity for an apoE mutant, apoE-1 (Lys146-->Glu), is dominantly associated with the expression of type III hyperlipoproteinemia. DNA sequence analysis of the mutant apoE gene revealed a single-point mutation that resulted in the substitution of glutamic acid (GAG) for lysine (AAG) at residue 146 in the proposed receptor-binding domain of apoE. The pathophysiological effect of this mutation was investigated in vivo by kinetic studies in the patient and six normal subjects, and in vitro by binding studies of apoE-1 (Lys146-->Glu) to LDL receptors on human fibroblasts and to heparin. The kinetic studies revealed that apoE-1 (Lys146-->Glu) was catabolized significantly slower than apoE-3 in normals (P < 0.005). In the proband, the plasma residence times of both apoEs were substantially longer and the production rate of total apoE was about two times higher than in the control subjects. ApoE-1 (Lys146-->Glu) was defective in interacting with LDL receptors, and its ability to displace LDL in an in vitro assay was reduced to 7.7% compared with apoE-3. The affinity of apoE-1 (Lys146-->Glu) to heparin was also markedly reduced compared with both apoE-2 (Arg158-->Cys) and apoE-3. These abnormal in vitro binding characteristics and the altered in vivo metabolism of apoE-1 (Lys146-->Glu) are proposed to result in the functional dominance of this mutation in the affected kindred.

Chylomicron-remnant uptake by freshly isolated hepatocytes. Effect of heparin and of hepatic triacylglycerol lipase

Chylomicron remnants labelled biologically with [3H]cholesterol were efficiently taken up by freshly isolated hepatocytes during a 3 h incubation in Krebs bicarbonate medium. Their [3H]cholesteryl ester was hydrolysed (74% net hydrolysis), and 0.1 mM-chloroquine could partially inhibit this hydrolysis, provided that hepatocytes were first preincubated for 2 h 30 min at 37 degrees C. This hydrolysis was also measured in preincubated cells with remnants double-labelled (3H and 14C) on their free cholesterol moiety; [3H]cholesterol arising from [3H]cholesteryl ester hydrolysis was recovered in the free [3H]cholesterol pool. A dose-response study showed saturation of remnant uptake at 180 micrograms of remnant protein/10(7) cells. Heparin (10 units/ml) increased remnant uptake by 63% (P less than 0.01), [3H]cholesteryl ester accumulation in the cell pellet by 110% (P less than 0.025) and hepatic lipase activity secreted in the medium by 2.4-fold (P less than 0.01) and by 3.3-fold (P less than 0.01) at the end of the preincubation and incubation periods respectively. Addition of 100 munits of semi-purified hepatic lipase preparation/flask stimulated remnant uptake by 44-69%, and [3H]cholesteryl ester accumulation in the presence of chloroquine by 2.1-fold (P less than 0.025). When hepatic lipase was incubated solely with the remnants, it decreased their triacylglycerol and phospholipid contents by 24% and 26% respectively. Thus freshly isolated hepatocytes may be used to study chylomicron-remnant uptake. Hepatic lipase, which seems to underly the stimulating effect of heparin, facilitates remnant uptake in vitro, and this could be mediated by at least one (or both) of its hydrolytic properties.