High density lipoprotein metabolism in low density lipoprotein receptor-deficient mice

The LDL receptor (LDLR) and scavenger receptor class B type I (SR-BI) play physiological roles in LDL and HDL metabolism in vivo. In this study, we explored HDL metabolism in LDLR-deficient mice in comparison with WT littermates. Murine HDL was radiolabeled in the protein ((125)I) and in the cholesteryl ester (CE) moiety ([(3)H]). The metabolism of (125)I-/[(3)H]HDL was investigated in plasma and in tissues of mice and in murine hepatocytes. In WT mice, liver and adrenals selectively take up HDL-associated CE ([(3)H]). In contrast, in LDLR(-/-) mice, selective HDL CE uptake is significantly reduced in liver and adrenals. In hepatocytes isolated from LDLR(-/-) mice, selective HDL CE uptake is substantially diminished compared with WT liver cells. Hepatic and adrenal protein expression of lipoprotein receptors SR-BI, cluster of differentiation 36 (CD36), and LDL receptor-related protein 1 (LRP1) was analyzed by immunoblots. The respective protein levels were identical both in hepatic and adrenal membranes prepared from WT or from LDLR(-/-) mice. In summary, an LDLR deficiency substantially decreases selective HDL CE uptake by liver and adrenals. This decrease is independent from regulation of receptor proteins like SR-BI, CD36, and LRP1. Thus, LDLR expression has a substantial impact on both HDL and LDL metabolism in mice.

Scavenger receptor CD36 mediates uptake of high density lipoproteins in mice and by cultured cells

The mechanisms of HDL-mediated cholesterol transport from peripheral tissues to the liver are incompletely defined. Here the function of scavenger receptor cluster of differentiation 36 (CD36) for HDL uptake by the liver was investigated. CD36 knockout (KO) mice, which were the model, have a 37% increase (P = 0.008) of plasma HDL cholesterol compared with wild-type (WT) littermates. To explore the mechanism of this increase, HDL metabolism was investigated with HDL radiolabeled in the apolipoprotein (¹²⁵I) and cholesteryl ester (CE, [³H]) moiety. Liver uptake of [³H] and ¹²⁵I from HDL decreased in CD36 KO mice and the difference, i. e. hepatic selective CE uptake ([³H]¹²⁵I), declined (-33%, P = 0.0003) in CD36 KO compared with WT mice. Hepatic HDL holo-particle uptake (¹²⁵I) decreased (-29%, P = 0.0038) in CD36 KO mice. In vitro, uptake of ¹²⁵I-/[³H]HDL by primary liver cells from WT or CD36 KO mice revealed a diminished HDL uptake in CD36-deficient hepatocytes. Adenovirus-mediated expression of CD36 in cells induced an increase in selective CE uptake from HDL and a stimulation of holo-particle internalization. In conclusion, CD36 plays a role in HDL uptake in mice and by cultured cells. A physiologic function of CD36 in HDL metabolism in vivo is suggested.

Selective uptake of HDL cholesteryl esters and cholesterol efflux from mouse peritoneal macrophages independent of SR-BI

Scavenger receptor class B type I (SR-BI) mediates the selective uptake of HDL cholesteryl esters (CEs) and facilitates the efflux of unesterified cholesterol. SR-BI expression in macrophages presumably plays a role in atherosclerosis. The role of SR-BI for selective CE uptake and cholesterol efflux in macrophages was explored. Macrophages and HDL originated from wild-type (WT) or SR-BI knockout (KO; homozygous) mice. For uptake, macrophages were incubated in medium containing 125I-/3H-labeled HDL. For lipid removal, [3H]cholesterol efflux was analyzed using HDL as acceptor. Selective uptake of HDL CE ([3H]cholesteryl oleyl ether - 125I-tyramine cellobiose) was similar in WT and SR-BI KO macrophages. Radiolabeled SR-BI KO-HDL yielded a lower rate of selective uptake compared with WT-HDL in WT and SR-BI KO macrophages. Cholesterol efflux was similar in WT and SR-BI KO cells using HDL as acceptor. SR-BI KO-HDL more efficiently promoted cholesterol removal compared with WT-HDL from both types of macrophages. Macrophages selectively take up HDL CE independently of SR-BI. Additionally, in macrophages, there is substantial cholesterol efflux that is not mediated by SR-BI. Therefore, SR-BI-independent mechanisms mediate selective CE uptake and cholesterol removal. SR-BI KO-HDL is an inferior donor for selective CE uptake compared with WT-HDL, whereas SR-BI KO-HDL more efficiently promotes cholesterol efflux.

Hepatic ATP-binding cassette transporter A1 is a key molecule in high-density lipoprotein cholesteryl ester metabolism in mice

OBJECTIVE

Mutations in ATP-binding cassette transporter A1 (ABCA1), the cellular lipid transport molecule mutated in Tangier disease, result in the rapid turnover of high-density lipoprotein (HDL)-associated apolipoproteins that presumably are cleared by the kidneys. However, the role of ABCA1 in the liver for HDL apolipoprotein and cholesteryl ester (CE) catabolism in vivo is unknown.

METHODS AND RESULTS

Murine HDL was radiolabeled with 125I in its apolipoprotein and with [3H]cholesteryl oleyl ether in its CE moiety. HDL protein and lipid metabolism in plasma and HDL uptake by tissues were investigated in ABCA1-overexpressing bacterial artificial chromosome (BAC)-transgenic (BAC+) mice and in mice harboring deletions of total (ABCA1-/-) and liver-specific ABCA1 (ABCA1(-L/-L)). In BAC+ mice with elevated ABCA1 expression, fractional catabolic rates (FCRs) of both the protein and the lipid tracer were significantly decreased in plasma and in the liver, yielding a diminished hepatic selective CE uptake from HDL. In contrast, ABCA1-/- or ABCA1(-L/-L) mice had significantly increased plasma and liver FCRs for both HDL tracers. An ABCA1 deficiency was associated with increased selective HDL CE uptake by the liver under all experimental conditions.

CONCLUSIONS

Hepatic ABCA1 has a critical role for HDL catabolism in plasma and for HDL selective CE uptake by the liver.

Angiotensin II down-regulates the SR-BI HDL receptor in proximal tubular cells

BACKGROUND

The kidney plays an important role in the metabolism of lipoproteins, but renal cells are also a target of lipids under pathophysiological conditions contributing to organ damage and progression of disease. The majority of studies has focused on the interaction of renal cells with low-density lipoproteins. Relatively little is known of potential metabolism of high-density lipoproteins (HDL) on renal cells However, diverse pathophysiological situations, such as the nephrotic syndrome and acute renal injury, may be associated with an activated renin-angiotensin system as well as altered renal handling of HDL. Therefore, the present study sought to gain insight into the expression of the HDL receptor scavenger receptor class B type I (SR-BI) in cultured renal cells and a potential regulation by angiotensin II (ANG II).

METHODS

Different renal cells lines and primary cultures (proximal tubular and mesangial cells) were screened by western blot for the expression of SR-BI. MCT cells, a mouse proximal tubular cell line, were selected for further studies. SR-BI protein and mRNA expression were determined after treatment with various doses of ANG II in the presence or absence of AT(1)- or AT(2)-receptor blocker. Uptake of HDL-associated cholesteryl ester into MCT cells was determined. Finally, rats were infused intraperitoneally with ANG II for 3-7 days, proximal tubules were isolated by differential centrifugation and SR-BI protein expression was assessed. Results. SR-BI protein was expressed in various primary cultures and permanent renal cell lines. ANG II (10(-10)-10(-6) M) treatment for 24 h induced a significant down-regulation of SR-BI protein and mRNA expression in MCT cells. This suppression was attenuated by an AT(1)-receptor antagonist whereas an AT(2)-blocker was without effect. MCT cells revealed a high selective uptake of HDL cholesteryl ester that was significantly higher than that in syngeneic mesangial cells. ANG II for 24 h significantly reduced this selective HDL cholesteryl ester uptake into MCT, but not mesangial cells. Finally, ANG II- infusion into rats for 3 and 7 days induced a significant decrease of SR-BI protein expression in isolated tubules.

CONCLUSIONS

Our data show that ANG II mediates down-regulation of SR-BI expression on proximal tubular cells in vivo and in vitro. However, the effects were small and additional experiments are necessary to confirm these first observations. The attenuated SR-BI expression is functionally relevant and associated with a decrease in cholesteryl ester uptake. ANG II-mediated suppression may contribute to various pathophysiological situations, such as acute tubular injury, the nephrotic syndrome and atherosclerosis.

Scavenger Receptor Class B Type I Mediates the Selective Uptake of High-Density Lipoprotein–Associated Cholesteryl Ester by the Liver in Mice

OBJECTIVE

High-density lipoprotein (HDL) cholesteryl esters (CE) are taken up by liver and adrenals selectively, ie, independent from particle internalization. Class B type I scavenger receptor (SR-BI) mediates this uptake in vitro. The role of SR-BI in HDL metabolism was explored in mice.

METHODS AND RESULTS

Mice with a mutation in the SR-BI gene (SR-BI KO) and wild-type (WT) littermates were used. Mutants had increased HDL cholesterol. HDL was labeled with 125I (protein) and [3H] (CE). After HDL injection, blood samples were drawn and finally the mice were euthanized. In WT, the plasma decay of HDL-associated [3H] is faster compared with 125I and this represents whole-body selective CE uptake. In SR-BI KO, the decay of both tracers is similar, yielding no selective CE removal. In WT liver and adrenals, uptake of [3H] is higher than 125I, showing selective uptake. In SR-BI KO, liver uptake of [3H] and 125I are similar, proposing no selective HDL CE uptake. In SR-BI KO adrenals, selective uptake is reduced; however, even in the absence of SR-BI, this uptake is detected using WT-HDL.

CONCLUSIONS

SR-BI mediates selective uptake of HDL CE by the liver. In adrenals, an alternative mechanism or mechanisms can play a role in selective CE uptake.

Hepatic lipase mediates an increase in selective uptake of HDL-associated cholesteryl esters by cells in culture independent from SR-BI

Scavenger receptor class B type I (SR-BI) mediates the selective uptake of HDL cholesteryl esters (CEs) by the liver. Hepatic lipase (HL) promotes this lipid uptake independent from lipolysis. The role of SR-BI in this HL-mediated increase in selective CE uptake was explored. Baby hamster kidney (BHK) cells were transfected with the SR-BI cDNA yielding cells with SR-BI expression, whereas no SR-BI was detected in control cells. These cells were incubated in medium containing 125I [3H]cholesteryl oleyl ether-labeled HDL3 (d = 1.125-1.21 g/ml) and HL was absent or present. Tetrahydrolipstatin (THL) blocked lipolysis. In control BHK cells and in BHK cells with SR-BI, HDL3 selective CE uptake (3H-125I) was detectable and SR-BI promoted this uptake. In both cell types, HL mediated an increase in selective CE uptake from HDL3. Quantitatively, this HL effect was similar in control BHK cells and in BHK cells with SR-BI. These results suggest that HL promotes selective uptake independent from SR-BI. To investigate the role of cell surface proteoglycans on the HL-mediated HDL3 uptake, proteoglycan deficiency was induced by heparinase digestion. Proteoglycan deficiency decreased the HL-mediated promotion of selective CE uptake. In summary, the stimulating HL effect on HDL selective CE uptake is independent from SR-BI and lipolysis. Proteoglycans are a requisite for the HL action on selective uptake. Results suggest that (a) pathway(s) distinct from SR-BI mediate(s) selective CE uptake from HDL.

Scavenger receptor BI (SR-BI) mediates a higher selective cholesteryl ester uptake from LpA-I compared with LpA-I:A-II lipoprotein particles

Scavenger receptor class B, type I (SR-BI) mediates the selective uptake of high-density lipoprotein- (HDL-) associated cholesteryl esters (CE), i.e. lipid uptake independent from HDL holo-particle internalisation. This pathway contributes to the HDL-mediated CE delivery to the liver. From human plasma HDL, two major lipoprotein subfractions can be isolated: one contains apolipoprotein (apo) A-I and apo A-II (LpA-I:A-II) as dominant protein components, whereas in the other population apo A-II is absent (LpA-I). In this investigation the role of SR-BI in selective CE uptake from HDL, LpA-I and LpA-I:A-II was explored. HDL(3) (d=1.125-1.21 g/ml), LpA-I and LpA-I:A-II were isolated from human plasma and radiolabeled in the protein (125I) as well as in the CE moiety ([3H]). Baby hamster kidney (BHK) cells were stably transfected with the full-length human SR-BI cDNA and these cells demonstrate SR-BI expression in immunoblots. In contrast, no SR-BI protein was detectable in control BHK cells (vector). To investigate lipoprotein uptake, cells incubated (37 degrees C, 4 h) in medium containing radiolabeled HDL(3), LpA-I or LpA-I:A-II and finally cellular tracer content was determined. For both types of BHK cells, the rate of apparent lipoprotein particle uptake according to the lipid tracer ([3H]) was in substantial excess over that due to the protein tracer (125I) demonstrating selective CE uptake ([3H]-(125)I) from HDL(3), LpA-I and LpA-I:A-II. SR-BI expression increased cellular selective CE uptake from labeled HDL(3) up to 24-fold. In BHK cells without SR-BI expression, selective CE uptake was higher from LpA-I compared with LpA-I:A-II. Analogously, in BHK cells with SR-BI expression, the rate of selective CE uptake was higher from LpA-I compared with LpA-I:A-II. In summary, SR-BI significantly increases selective CE uptake from HDL(3), LpA-I and LpA-I:A-II. Concerning HDL subfractions, the rate of SR-BI-mediated selective CE uptake is greater from LpA-I compared with LpA-I:A-II and this result suggests that SR-BI preferentially facilitates the CE transfer from LpA-I lipoprotein particles to cells.

Lipoprotein lipase mediates an increase in selective uptake of HDL-associated cholesteryl esters by cells in culture independent of scavenger receptor BI

Scavenger receptor class B type I (SR-BI) mediates the selective uptake of HDL cholesteryl esters (CEs) by the liver. LPL promotes this selective lipid uptake independent of lipolysis. In this study, the role of SR-BI in the mechanism of this LPL-mediated increase in selective CE uptake was explored. Baby hamster kidney (BHK) cells were transfected with the SR-BI cDNA, and significant SR-BI expression could be detected in immunoblots, whereas no SR-BI was visualized in control cells. Y1-BS1 murine adrenocortical cells were cultured without or with adrenocorticotropic hormone, and cells with no detectable or with SR-BI were obtained. These cells incubated without or with LPL in medium containing 125I/[3H]cholesteryl oleyl ether- labeled HDL3; tetrahydrolipstatin inhibited the catalytic activity of LPL. In BHK and in Y1-BS1 cells without or with SR-BI expression, apparent HDL3 selective CE uptake ([3H]CEt - 125I) was detectable. Cellular SR-BI expression promoted HDL3 selective CE uptake by approximately 250-1,900%. In BHK or Y1-BS1 cells, LPL mediated an increase in apparent selective CE uptake. Quantitatively, this stimulating LPL effect was very similar in control cells and in cells with SR-BI expression. The uptake of radiolabeled HDL3 was also investigated in human embryonal kidney 293 (HEK 293) cells that are an established SR-BI-deficient cell model. LPL stimulated [3H]cholesteryl oleyl ether uptake from labeled HDL3 by HEK 293 cells substantially, showing that LPL can induce selective CE uptake from HDL3 independent of SR-BI. To explore the role of cell surface proteoglycans on lipoprotein uptake, we induced proteoglycan deficiency by heparinase treatment. Proteoglycan deficiency decreased the LPL-mediated promotion of HDL3 selective CE uptake. In summary, evidence is presented that the stimulating effect of LPL on HDL3 selective CE uptake is independent of SR-BI and lipolysis. However, cell surface proteoglycans are required for the LPL action on selective CE uptake. It is suggested that pathways distinct from SR-BI mediate selective CE uptake from HDL.

Selective uptake of low-density lipoprotein-associated cholesteryl esters by human fibroblasts, human HepG2 hepatoma cells and J774 macrophages in culture

High-density lipoprotein-(HDL) associated cholesteryl esters (CE) are taken up by hepatic and extrahepatic cells at a higher rate than HDL apolipoproteins. This selective uptake of HDL CE is independent from HDL particle uptake. For low-density lipoprotein (LDL), receptor-mediated endocytosis by cells is well established. In this study, the question was addressed if LDL-associated CE are also taken up by cells independently from LDL particles, i.e., selectively. Human LDL (d = 1.02-1.05 g/ml) was doubly radiolabeled with intracellularly trapped tracers: [125I]Tyramine-Cellobiose ([125I]TC) traced apolipoprotein B, [3H]cholesteryl oleyl ether ([3H]CEt) traced CE. The uptake of doubly radiolabeled LDL by normal and LDL receptor-negative human skin fibroblasts, human HepG2 hepatoma cells and murine J774 macrophages was investigated. Each cell type took up LDL particles as indicated by [125I]TC. However, in fibroblasts, HepG2 cells and J774 macrophages the rate of uptake for LDL-associated [3H]CEt was greater than that according to [125I]TC. These results indicate that extrahepatic and hepatic cells selectively take up LDL CE and this uptake is independent from LDL receptor-mediated endocytosis. Loading cells with cholesterol down-regulated selective uptake of LDL CE. In summary, human skin fibroblasts, human HepG2 cells and murine J774 macrophages selectively take up LDL CE, i.e., CE are taken up independently from LDL particles.

Selective uptake of high-density lipoprotein-associated cholesteryl esters by human hepatocytes in primary culture

High-density lipoprotein cholesteryl esters are taken up by many cells without simultaneous uptake of high-density lipoprotein apolipoproteins. This selective uptake was investigated in human hepatocytes in primary culture. Human high-density lipoprotein-3 (density, 1.125 to 1.21 gm/ml) was radiolabeled in both its apolipoprotein and in its cholesteryl ester moiety; uptake of these high-density lipoprotein3 tracers by hepatocytes was investigated. Apparent high-density lipoprotein3 particle uptake as measured with the cholesteryl ester tracer was in excess of that from the apolipoprotein tracer, indicating selective uptake of high-density lipoprotein3 cholesteryl esters by hepatocytes. This selective uptake is a regulated pathway in hepatocytes, as demonstrated by an inverse relationship between cell cholesterol and the rate of selective uptake. Studies on the mechanism of selective uptake have used inhibitors such as monensin, chloroquine, heparin, and a monoclonal antibody directed against low-density lipoprotein receptors. These experiments provide no evidence for a role of cell-secreted apolipoprotein E, endocytosis or retroendocytosis in selective uptake. The intracellular fate of high-density lipoprotein3-associated cholesteryl esters was investigated with [3H]cholesteryl oleate-labeled high-density lipoprotein3. Hepatocytes hydrolyzed [3H]cholesteryl oleate internalized from labeled high-density lipoprotein3; this catabolism was not inhibited by the presence of chloroquine. In parallel hepatocytes were incubated with [3H]cholesteryl oleate-labeled low-density lipoprotein. Cells hydrolyzed [3H]cholesteryl oleate taken up with low-density lipoprotein; however, this hydrolysis was inhibited by chloroquine, indicating lysosomal low-density lipoprotein cholesteryl ester catabolism. These experiments show that high-density lipoprotein3 cholesteryl esters selectively taken up by hepatocytes are hydrolyzed independently from the classical lysosomal catabolic pathway. The question was addressed if selective uptake mediates a net mass uptake of cholesterol rather than an isotope exchange phenomenon. Incubation of hepatocytes with high-density lipoprotein-3 suppressed endogenous sterol synthesis from sodium [14C]acetate. Hepatocytes were incubated in the presence of high-density lipoprotein3; medium cholesteryl esters decreased as a result of incubation with hepatocytes. These results show a net mass delivery of high-density lipoprotein cholesteryl esters to hepatocytes. In conclusion, the pathway for selective uptake of high-density lipoprotein cholesteryl esters could be demonstrated in human hepatocytes in primary culture. A role for selective uptake in high-density lipoprotein-mediated cholesterol delivery to the liver in human beings in vivo is proposed.