Searching for avidity by chemical ligation of combinatorially self-assembled DNA-encoded ligand libraries

DNA encoded ligands are self-assembled into bivalent complexes and chemically ligated to link their identities.

New technologies for DNA analysis--a review of the READNA Project

The REvolutionary Approaches and Devices for Nucleic Acid analysis (READNA) project received funding from the European Commission for 41/2 years. The objectives of the project revolved around technological developments in nucleic acid analysis. The project partners have discovered, created and developed a huge body of insights into nucleic acid analysis, ranging from improvements and implementation of current technologies to the most promising sequencing technologies that constitute a 3(rd) and 4(th) generation of sequencing methods with nanopores and in situ sequencing, respectively.

Human ATP-binding cassette G1 controls macrophage lipoprotein lipase bioavailability and promotes foam cell formation

OBJECTIVE

The physiological function of the ATP-binding cassette G1 (ABCG1) transporter in humans is not yet elucidated, as no genetic disease caused by ABCG1 mutations has been documented. The goal of our study was, therefore, to investigate the potential role(s) of ABCG1 in lipid metabolism in humans.

METHODS AND RESULTS

Here we report that among the 104 polymorphisms present in the ABCG1 gene, the analysis of the frequent functional rs1893590 and rs1378577 single nucleotide polymorphisms located in the regulatory region of ABCG1 in the Regression Growth Evaluation Statin Study population revealed that both ABCG1 single nucleotide polymorphisms were significantly associated with plasma lipoprotein lipase (LPL) activity. Moreover, we observed that plasma LPL activity was modestly reduced in Abcg1(-/-) mice as compared with control mice. Adipose tissue and skeletal muscle are the major tissues accounting for levels and activity of plasma LPL in the body. However, beyond its lipolytic action in the plasma compartment, LPL was also described to act locally at the cellular level. Thus, macrophage LPL was reported to promote foam cell formation and atherosclerosis in vivo. Analysis of the relationship between ABCG1 and LPL in macrophages revealed that the knockdown of ABCG1 expression (ABCG1 knockdown) in primary cultures of human monocyte-derived macrophages using small interfering RNAs led to a marked reduction of both the secretion and activity of LPL. Indeed, LPL was trapped at the cell surface of ABCG1 knockdown human monocyte-derived macrophages, likely in cholesterol-rich domains, thereby reducing the bioavailability and activity of LPL. As a consequence, LPL-mediated lipid accumulation in human macrophage foam cells in the presence of triglyceride-rich lipoproteins was abolished when ABCG1 expression was repressed.

CONCLUSIONS

We presently report that ABCG1 controls LPL activity and promotes lipid accumulation in human macrophages in the presence of triglyceride-rich lipoproteins, thereby suggesting a potential deleterious role of macrophage ABCG1 in metabolic situations associated with high levels of circulating triglyceride-rich lipoproteins together with the presence of macrophages in the arterial wall.

Effects of deletion of macrophage ABCA7 on lipid metabolism and the development of atherosclerosis in the presence and absence of ABCA1

ABCA7, a close relative of ABCA1 which facilitates cholesterol efflux to lipid-poor apoproteins, has been implicated in macrophage lipid efflux and clearance of apoptotic cells in in vitro studies. In the current study, we investigated the in vivo effects of macrophage ABCA7 deficiency on lipid metabolism and atherosclerosis. Chimeras with dysfunctional ABCA7 in macrophages and other blood cells were generated by transplantation of bone marrow from ABCA7 knockout (KO) mice into irradiated low-density lipoprotein receptor (LDLr) KO mice. Unexpectedly, macrophage ABCA7 deficiency did not significantly affect atherosclerosis susceptibility of LDLr KO mice after 10 weeks Western-type diet feeding. However, ABCA7 deficiency was associated with 2-fold (p<0.05) higher macrophage ABCA1 mRNA expression levels. Combined disruption of ABCA1 and ABCA7 in bone-marrow-derived cells increased atherosclerotic lesion development (1.5-fold (p>0.05) as compared to wild type transplanted mice. However, single deletion of ABCA1 had a similar effect (1.8-fold, p<0.05). Macrophage foam cell accumulation in the peritoneal cavity was reduced in ABCA1/ABCA7 dKO transplanted animals as compared to single ABCA1 KO transplanted mice, which was associated with increased ABCG1 expression. Interestingly, spleens of ABCA1/ABCA7 double KO transplanted mice were significantly larger as compared to the other 3 groups and showed massive macrophage lipid accumulation, a reduction in CD3+ T-cells, and increased expression of key regulators of erythropoiesis. In conclusion, deletion of ABCA7 in bone marrow-derived cells does not affect atherogenesis in the arterial wall neither in the absence or presence of ABCA1. Interestingly, combined deletion of bone marrow ABCA1 and ABCA7 causes severe splenomegaly associated with cellular lipid accumulation, a reduction in splenic CD3+ T cells, and induced markers of erythropoeisis. Our data indicate that ABCA7 may play a role in T cell proliferation and erythropoeisis in spleen.

Apolipoprotein isoform E4 does not increase coronary heart disease risk in carriers of low-density lipoprotein receptor mutations

BACKGROUND

In humans, the E4 allele of the apolipoprotein E gene is associated with increased coronary heart disease risk. Surprisingly, in rodents, apolipoprotein E4 only accelerates the atherosclerotic process when transgenic for the human low-density lipoprotein receptor (LDLR) protein. We therefore investigated whether the LDLR locus interacted with the apolipoprotein E gene genotype on coronary heart disease risk in patients clinically diagnosed with familial hypercholesterolemia with and without LDLR mutation. We investigated whether the presence of an LDLR mutation diminishing LDLR function was protective in E4/E4 carriers.

METHODS AND RESULTS

In a cohort of 2400 patients clinically diagnosed with familial hypercholesterolemia, we found an LDLR gene mutation in 1383 patients, whereas in 1013 patients, such mutation was not present. In 92 patients homozygous for the apolipoprotein E4, the presence of an LDLR mutation conferred lower coronary heart disease risk (hazard ratio, 0.16; 95% CI, 0.05-0.58; P=0.005). Mirroring these results, the apolipoprotein E4/E4 genotype was also associated with lower coronary heart disease risk in patients with familial hypercholesterolemia with an LDLR mutation (hazard ratio, 0.26; hazard ratio, 0.08-0.80; P=0.02).

CONCLUSIONS

LDLR function is key to the detrimental effects of apolipoprotein E4 in humans. Kinetic studies in humans are now required to study the consequences of our observation for prevention of both coronary heart disease and Alzheimer disease.

Liver X receptors inhibit macrophage proliferation through downregulation of cyclins D1 and B1 and cyclin-dependent kinases 2 and 4

Macrophages serve essential functions as regulators of immunity and homeostasis, and their proliferation contributes to pathogenesis of certain disorders. In this report, we show that induction of macrophage proliferation by the growth factor M-CSF is negatively modulated by agonists that activate the nuclear receptor liver X receptor (LXR), both in vitro and in vivo. Both isoforms LXR α and β are involved in the antiproliferative actions of LXR ligands in macrophages. In contrast, M-CSF does not exert negative effects on LXR-mediated gene expression. Treatment with LXR agonists results in the accumulation of macrophages in the G(0)/G(1) phase of the cell cycle without affecting ERK-1/2 phosphorylation. The use of small interfering RNA or genetically modified mice revealed that, in contrast to other cellular models, functional expression of either the cyclin-dependent kinase inhibitor p27KIP1 or the cholesterol transporters ATP-binding cassette A1 or ATP-binding cassette G1 was not required for the antiproliferative effects of LXR agonists in macrophages. Western blot analysis revealed that protein expression of key molecules that regulate progression through the cell cycle, such as cyclins D1 and B1 and cyclin-dependent kinases 2 and 4, was downregulated upon LXR activation. These observations suggest a role for LXR agonists in limiting macrophage proliferative responses associated to pathogenic disorders.

Genetic variant of the scavenger receptor BI in humans

BACKGROUND

In mice, the scavenger receptor class B type I (SR-BI) is essential for the delivery of high-density lipoprotein (HDL) cholesterol to the liver and steroidogenic organs. Paradoxically, elevated HDL cholesterol levels are associated with increased atherosclerosis in SR-BI-knockout mice. It is unclear what role SR-BI plays in human metabolism.

METHODS

We sequenced the gene encoding SR-BI in persons with elevated HDL cholesterol levels and identified a family with a new missense mutation (P297S). The functional effects of the P297S mutation on HDL binding, cellular cholesterol uptake and efflux, atherosclerosis, platelet function, and adrenal function were studied.

RESULTS

Cholesterol uptake from HDL by primary murine hepatocytes that expressed mutant SR-BI was reduced to half of that of hepatocytes expressing wild-type SR-BI. Carriers of the P297S mutation had increased HDL cholesterol levels (70.4 mg per deciliter [1.8 mmol per liter], vs. 53.4 mg per deciliter [1.4 mmol per liter] in noncarriers; P<0.001) and a reduced capacity for efflux of cholesterol from macrophages, but the carotid artery intima-media thickness was similar in carriers and in family noncarriers. Platelets from carriers had increased unesterified cholesterol content and impaired function. In carriers, adrenal steroidogenesis was attenuated, as evidenced by decreased urinary excretion of sterol metabolites, a decreased response to corticotropin stimulation, and symptoms of diminished adrenal function.

CONCLUSIONS

We identified a family with a functional mutation in SR-BI. The mutation carriers had increased HDL cholesterol levels and a reduction in cholesterol efflux from macrophages but no significant increase in atherosclerosis. Reduced SR-BI function was associated with altered platelet function and decreased adrenal steroidogenesis. (Funded by the European Community and others.).

Enhanced foam cell formation, atherosclerotic lesion development, and inflammation by combined deletion of ABCA1 and SR-BI in Bone marrow-derived cells in LDL receptor knockout mice on western-type diet

RATIONALE

macrophages cannot limit the uptake of lipids and rely on cholesterol efflux mechanisms for maintaining cellular cholesterol homeostasis. Important mediators of macrophage cholesterol efflux are ATP-binding cassette transporter 1 (ABCA1), which mediates the efflux of cholesterol to lipid-poor apolipoprotein AI, and scavenger receptor class B type I (SR-BI), which promotes efflux to mature high-density lipoprotein.

OBJECTIVE

the aim of the present study was to increase the insight into the putative synergistic roles of ABCA1 and SR-BI in foam cell formation and atherosclerosis.

METHODS AND RESULTS

low-density lipoprotein receptor knockout (LDLr KO) mice were transplanted with bone marrow from ABCA1/SR-BI double knockout mice, the respective single knockouts, or wild-type littermates. Serum cholesterol levels were lower in ABCA1/SR-BI double knockout transplanted animals, as compared to the single knockout and wild-type transplanted animals on Western-type diet. Despite the lower serum cholesterol levels, massive foam cell formation was found in macrophages from spleen and the peritoneal cavity. Interestingly, ABCA1/SR-BI double knockout transplanted animals also showed a major increase in proinflammatory KC (murine interleukin-8) and interleukin-12p40 levels in the circulation. Furthermore, after 10 weeks of Western-type diet feeding, atherosclerotic lesion development in the aortic root was more extensive in the LDLr KO mice reconstituted with ABCA1/SR-BI double knockout bone marrow.

CONCLUSIONS

deletion of ABCA1 and SR-BI in bone marrow-derived cells enhances in vivo macrophage foam cell formation and atherosclerotic lesion development in LDLr KO mice on Western diet, indicating that under high dietary lipid conditions, both macrophage ABCA1 and SR-BI contribute significantly to cholesterol homeostasis in the macrophage in vivo and are essential for reducing the risk for atherosclerosis.

The Scavenger Receptor Class B, Type I Is a Primary Determinant of Paraoxonase-1 Association With High-Density Lipoproteins

Objective— To examine the contribution of the scavenger receptor (SR) BI to the mechanism by which high-density lipoprotein (HDL) acquires paraoxonase-1 (PON1).

Methods and Results— Serum PON1 activity contributes to the antioxidant capacity of HDLs and is suggested to be an independent risk factor for atherosclerosis. The association of PON1 with HDL is a major determinant of its serum activity levels. PON1 secretion was studied in stably transfected Chinese hamster ovary and HepG2 models. Complementary analyses were performed in transgenic models. Modulation of SR-BI expression, by SR-BI small and interfering RNA knockdown and pharmacologically, correlated with significant changes ( P <0.01) in PON1 secretion to HDLs and very-low-density lipoproteins. Block lipid transport-1 (BLT1), which increases the affinity of HDL for SR-BI without modulating its expression, was associated with significant increases in secretion. Downregulating postsynaptic density 95/disc-large/zona occludens kinase in HepG2 reduced cell SR-BI protein and lowered enzyme secretion. Serum PON1 activity was significantly reduced in postsynaptic density 95/disc-large/zona occludens kinase knockout mice.

Conclusion— The present study identifies SR-BI as a major determinant of the capacity of HDL to acquire PON1. It reinforces the concept of the receptor as a docking molecule, allowing communication between HDL and the cell, and extends the importance of SR-BI to HDL metabolism and function.

Macrophage adipose triglyceride lipase deficiency attenuates atherosclerotic lesion development in low-density lipoprotein receptor knockout mice

OBJECTIVE

The consequences of macrophage triglyceride (TG) accumulation on atherosclerosis have not been studied in detail so far. Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme for the initial step in TG hydrolysis. Because ATGL knockout (KO) mice exhibit massive TG accumulation in macrophages, we used ATGL KO mice to study the effects of macrophage TG accumulation on atherogenesis.

METHODS AND RESULTS

Low-density lipoprotein receptor (LDLr) KO mice were transplanted with bone marrow from ATGL KO (ATGL KO→LDLr KO) or wild-type (WT→LDLr KO) mice and challenged with a Western-type diet for 9 weeks. Despite TG accumulation in ATGL KO macrophages, atherosclerosis in ATGL KO→LDLr KO mice was 43% reduced associated with decreased plasma monocyte chemoattractant protein-1 (MCP-1) and macrophage interleukin-6 concentrations. This coincided with a reduced amount of macrophages, possibly because of a 39% increase in intraplaque apoptosis and a decreased migratory capacity of ATGL KO macrophages. The reduced number of white blood cells might be due to a 36% decreased Lin(-)Sca-1(+)cKit(+) hematopoietic stem cell population.

CONCLUSIONS

We conclude that the attenuation of atherogenesis in ATGL KO→LDLr KO mice is due to decreased infiltration of less inflammatory macrophages into the arterial wall and increased macrophage apoptosis.

Hepatic glycosphingolipid deficiency and liver function in mice

Recent studies have reported that glycosphingolipids (GSLs) might be involved in obesity-induced insulin resistance. Those reports suggested that inhibition of GSL biosynthesis in animals ameliorated insulin resistance accompanied by improved glycemic control and decreased liver steatosis in obese mice. In addition, pharmacologic GSL depletion altered hepatic secretory function. In those studies, ubiquitously acting inhibitors for GSL biosynthesis have been used to inhibit the enzyme Ugcg (UDP-glucose:ceramide glucosyltransferase), catalyzing the first step of the glucosylceramide-based GSL-synthesis pathway. In the present study a genetic approach for selective GSL deletion in hepatocytes was chosen to achieve complete inhibition of GSL synthesis and to avoid possible adverse effects caused by Ugcg inhibitors. Using the Cre/loxP system under control of the albumin promoter, GSL biosynthesis in hepatocytes and their release into the plasma could be effectively blocked. Deletion of GSL in hepatocytes did not change the quantity of bile excretion through the biliary duct. Total bile salt content in bile, feces, and plasma from mutant mice showed no difference as compared to control animals. Cholesterol concentration in liver, bile, feces, and plasma samples remained unaffected. Lipoprotein concentrations in plasma samples in mutant animals reached similar levels as in their control littermates. No alteration in glucose tolerance after intraperitoneal application of glucose and insulin appeared in mutant animals. A preventive effect of GSL deficiency on development of liver steatosis after a high-fat diet was not observed.

CONCLUSION

The data suggest that GSL in hepatocytes are not essential for sterol, glucose, or lipoprotein metabolism and do not prevent high-fat diet-induced liver steatosis, indicating that Ugcg inhibitors exert their effect on hepatocytes either independently of GSL or mediated by other (liver) cell types.

The E3 ubiquitin ligase IDOL induces the degradation of the low density lipoprotein receptor family members VLDLR and ApoER2

We have previously identified the E3 ubiquitin ligase-inducible degrader of the low density lipoprotein receptor (LDLR) (Idol) as a post-translational modulator of LDLR levels. Idol is a direct target for regulation by liver X receptors (LXRs), and its expression is responsive to cellular sterol status independent of the sterol-response element-binding proteins. Here we demonstrate that Idol also targets two closely related LDLR family members, VLDLR and ApoE receptor 2 (ApoER2), proteins implicated in both neuronal development and lipid metabolism. Idol triggers ubiquitination of the VLDLR and ApoER2 on their cytoplasmic tails, leading to their degradation. We further show that the level of endogenous VLDLR is sensitive to cellular sterol content, Idol expression, and activation of the LXR pathway. Pharmacological activation of the LXR pathway in mice leads to increased Idol expression and to decreased Vldlr levels in vivo. Finally, we establish an unexpected functional link between LXR and Reelin signaling. We demonstrate that LXR activation results in decreased Reelin binding to VLDLR and reduced Dab1 phosphorylation. The identification of VLDLR and ApoER2 as Idol targets suggests potential roles for this LXR-inducible E3 ligase in the central nervous system in addition to lipid metabolism.

Independent protective roles for macrophage Abcg1 and Apoe in the atherosclerotic lesion development

OBJECTIVE

ATP-binding cassette transporter G1 (Abcg1) and apolipoprotein E (Apoe) play a role in macrophage cholesterol efflux and consequently the development of atherosclerosis. A possible interaction between Abcg1 and Apoe in cholesterol efflux was postulated, but the potential combined action of these proteins on atherosclerotic lesion formation is unclear.

METHODS

LDL receptor knockout (KO) mice were transplanted with bone marrow from Abcg1/Apoe double KO (dKO) mice, their respective single knockouts, and wild-type (WT) controls and challenged with a high-fat/high-cholesterol diet for 6 weeks to induce atherosclerosis.

RESULTS

No differences were found in serum lipid levels. The mean atherosclerotic lesion area in dKO transplanted animals (187+/-18x10(3)microm(2)) was 1.4-fold (p<0.01) increased compared to single knockouts (Abcg1 KO: 138+/-5x10(3)microm(2); Apoe KO: 131+/-7x10(3)microm(2)) and 1.9-fold (p<0.001) as compared to WT controls (97+/-15x10(3)microm(2)). In vitro cholesterol efflux experiments established that combined deletion of Abcg1 and Apoe leads to a larger attenuation of macrophage cholesterol efflux to HDL as compared to single knockouts.

CONCLUSIONS

Single deletion of macrophage Abcg1 or Apoe does lead to a moderate non-significant increase in atherosclerotic lesion development as tested by ANOVA, while combined deletion of Abcg1 and Apoe induces a more dramatic and significant increase in atherosclerosis. Our results indicate an additive, independent effect for both macrophage Abcg1 and Apoe in the prevention of atherosclerosis.

Apolipoprotein CI inhibits scavenger receptor BI and increases plasma HDL levels in vivo

Apolipoprotein CI (apoCI) has been suggested to influence HDL metabolism by activation of LCAT and inhibition of HL and CETP. However, the effect of apoCI on scavenger receptor BI (SR-BI)-mediated uptake of HDL-cholesteryl esters (CE), as well as the net effect of apoCI on HDL metabolism in vivo is unknown. Therefore, we evaluated the effect of apoCI on the SR-BI-mediated uptake of HDL-CE in vitro and determined the net effect of apoCI on HDL metabolism in mice. Enrichment of HDL with apoCI dose-dependently decreased the SR-BI-dependent association of [(3)H]CE-labeled HDL with primary murine hepatocytes, similar to the established SR-BI-inhibitors apoCIII and oxLDL. ApoCI deficiency in mice gene dose-dependently decreased HDL-cholesterol levels. Adenovirus-mediated expression of human apoCI in mice increased HDL levels at a low dose and increased the HDL particle size at higher doses. We conclude that apoCI is a novel inhibitor of SR-BI in vitro and increases HDL levels in vivo.

The hepatic uptake of VLDL in lrp-ldlr-/-vldlr-/- mice is regulated by LPL activity and involves proteoglycans and SR-BI

LPL activity plays an important role in preceding the VLDL remnant clearance via the three major apolipoprotein E (apoE)-recognizing receptors: the LDL receptor (LDLr), LDL receptor-related protein (LRP), and VLDL receptor (VLDLr). The aim of this study was to determine whether LPL activity is also important for VLDL remnant clearance irrespective of these receptors and to determine the mechanisms involved in the hepatic remnant uptake. Administration of an adenovirus expressing LPL (AdLPL) into lrp(-)ldlr(-/-)vldlr(-/-) mice reduced both VLDL-triglyceride (TG) and VLDL-total cholesterol (TC) levels. Conversely, inhibition of LPL by AdAPOC1 increased plasma VLDL-TG and VLDL-TC levels. Metabolic studies with radiolabeled VLDL-like emulsion particles showed that the clearance and hepatic association of their remnants positively correlated with LPL activity. This hepatic association was independent of the bridging function of LPL and HL, since heparin did not reduce the liver association. In vitro studies demonstrated that VLDL-like emulsion particles avidly bound to the cell surface of primary hepatocytes from lrp(-)ldlr(-/-)vldlr(-/-) mice, followed by slow internalization, and involved heparin-releaseable cell surface proteins as well as scavenger receptor class B type I (SR-BI). Collectively, we conclude that hepatic VLDL remnant uptake in the absence of the three classical apoE-recognizing receptors is regulated by LPL activity and involves heparan sulfate proteoglycans and SR-BI.

Hepatic cell-specific ATP-binding cassette (ABC) transporter profiling identifies putative novel candidates for lipid homeostasis in mice

BACKGROUND

ABC-transporters play an important role in lipid trafficking. Therefore, hepatic expression patterns of ABC-transporters involved in the regulation of cholesterol metabolism were evaluated.

METHODS AND RESULTS

RT-PCR analysis showed that the mRNA expression of 38 ABC-transporters detected in livers of C57Bl/6 mice varied greatly. Although most ABC-transporters were ubiquitously expressed, some members displayed very restricted expression patterns, e.g. ABCA6, A8, B1, B8, B10, B11, C3, D2, and G5/G8 were exclusively (>99%) expressed in parenchymal cells. Interestingly, another 13 ABC-transporters, including ABCA4, A5, A9, A13, B2, B9, C1, C5, D3, D4, F2, G1, and G4 were primarily expressed in Kupffer cells. Although Kupffer cells only contribute to 2.5% of the total liver protein, these 13 genes did contain 9-27% of the total liver expression. Western-type diet feeding (0.25% cholesterol, 15% fat) induced the expression of several primarily Kupffer cell expressed genes, including ABCA5, B9, D3, and D4 (2 to 3-fold higher), whereas the other ABC-transporters were not significantly changed.

CONCLUSIONS

Our findings underscore the importance of cellular localization for studying the regulation of key ABC-transporters in liver cholesterol homeostasis. Furthermore, several novel ABC-transporters, including ABCA5, B9, D3, and D4 were identified as putative novel candidates involved in liver macrophage cholesterol homeostasis.

Combined deletion of macrophage ABCA1 and ABCG1 leads to massive lipid accumulation in tissue macrophages and distinct atherosclerosis at relatively low plasma cholesterol levels

OBJECTIVE

The purpose of this study was to evaluate the effect of the combined deletion of ABCA1 and ABCG1 expression in macrophages on foam cell formation and atherosclerosis.

METHODS AND RESULTS

LDL receptor knockout (KO) mice were transplanted with bone marrow from ABCA1/ABCG1 double KO (dKO) mice. Plasma cholesterol levels after 6 weeks of Western-type diet (WTD) feeding were significantly lower in dKO transplanted mice than ABCA1 KO, ABCG1 KO, and control transplanted animals. Extreme foam cell formation was present in macrophages of various tissues and the peritoneal cavity of dKO transplanted animals. Furthermore, severe hypoplasia of the thymus and a significant decrease in CD4-positive T cells in blood was observed. Despite relatively low plasma cholesterol levels dKO transplanted animals developed lesion sizes of 156+/-19x10(3) microm2 after only 6 weeks of WTD feeding. Lesions, however, were smaller than single ABCA1 KO transplanted animals (226+/-30x10(3) microm2; P<0.05) and not significantly different from single ABCG1 KO (117+/-22x10(3) microm2) and WT transplanted mice (112+/-15x10(3) microm2).

CONCLUSIONS

Macrophage ABCA1 and ABCG1 play a crucial role in the prevention of macrophage foam cell formation, whereas combined deletion only modestly influences atherosclerosis which is associated with an attenuated increase in WTD-induced plasma cholesterol and decreased proinflammatory CD4-positive T cell counts.

Coexistence of foam cells and hypocholesterolemia in mice lacking the ABC transporters A1 and G1

The concept that macrophages can become foam cells as a result of a disturbed balance between the uptake of cholesterol from lipoproteins and cholesterol efflux is generally accepted. ABCA1 and ABCG1 are two cholesterol transporters that may act sequentially to remove cellular cholesterol, but currently their combined role in vivo is unknown. We report here that targeted disruption of both ABCA1 and ABCG1 in mice, despite severe plasma hypocholesterolemia, leads to massive lipid accumulation and foam cell formation of tissue macrophages. A complete ablation of cellular cholesterol efflux in vitro is observed, whereas in vivo macrophage-specific reverse cholesterol transport to the feces is markedly decreased. Despite the massive foam cell formation of tissue macrophages, no lipid accumulation was observed in the vascular wall, even in mice of 1 year old, indicating that the double knockout mice, possibly because of their hypocholesterolemia, lack the trigger to attract macrophages to the vessel wall. In conclusion, even under hypocholesterolemic conditions macrophages can be converted into foam cells, and ABCA1 and ABCG1 play an essential role in the prevention of foam cell formation.

Scavenger receptor BI facilitates the metabolism of VLDL lipoproteins in vivo

Scavenger receptor class B type I (SR-BI) functions as an HDL receptor that promotes the selective uptake of cholesteryl esters (CEs). The physiological role of SR-BI in VLDL metabolism, however, is largely unknown. SR-BI deficiency resulted in elevated VLDL cholesterol levels, both on chow diet and upon challenge with high-cholesterol diets. To specifically elucidate the role of SR-BI in VLDL metabolism, the plasma clearance and hepatic uptake of (125)I-beta-VLDL were studied in SR-BI(+/+) and SR-BI(-/-) mice. At 20 min after injection, 66 +/- 2% of the injected dose was taken up by the liver in SR-BI(+/+) mice, as compared with only 22 +/- 4% (P = 0.0007) in SR-BI(-/-) mice. In vitro studies established that the B(max) of (125)I-beta-VLDL binding was reduced from 469 +/- 30 ng/mg in SR-BI(+/+) hepatocytes to 305 +/- 20 ng/mg (P = 0.01) in SR-BI(-/-) hepatocytes. Both in vivo and in vitro, limited to no selective uptake of CEs from beta-VLDL was found. Interestingly, HDL effectively competed for the association of beta-VLDL in the presence as well as in the absence of SR-BI, indicating a second common recognition site. In conclusion, SR-BI plays an important physiological role in the metabolism of VLDL (remnants).

Platelet activation by oxidized low density lipoprotein is mediated by CD36 and scavenger receptor-A

OBJECTIVE

The interaction of platelets with low density lipoprotein (LDL) contributes to the development of cardiovascular disease. Platelets are activated by native LDL (nLDL) through apoE Receptor 2' (apoER2')-mediated signaling to p38(MAPK) and by oxidized LDL (oxLDL) through lysophosphatidic acid (LPA) signaling to Rho A and Ca2+. Here we report a new mechanism for platelet activation by oxLDL.

METHODS AND RESULTS

Oxidation of nLDL increases p38(MAPK) activation through a mechanism that is (1) independent of LPA, and (2) unlike nLDL-signaling not desensitized by prolonged platelet-LDL contact or inhibited by receptor-associated protein or chondroitinase ABC. Antibodies against scavenger receptors CD36 and SR-A alone fail to block p38(MAPK) activation by oxLDL but combined blockade inhibits p38(MAPK) by >40% and platelet adhesion to fibrinogen under flow by >60%. Mouse platelets deficient in either CD36 or SR-A show normal p38(MAPK) activation by oxLDL but combined deficiency of CD36 and SR-A disrupts oxLDL-induced activation of p38(MAPK) by >70%.

CONCLUSION

These findings reveal a novel platelet-activating pathway stimulated by oxLDL that is initiated by the combined action of CD36 and SR-A.

Total body ABCG1 expression protects against early atherosclerotic lesion development in mice

OBJECTIVE

ABCG1 has recently been identified as a facilitator of cholesterol and phospholipid efflux from macrophages to HDL. In bone marrow transplantation studies, we and others have now shown that the absence of macrophage ABCG1 may differentially influence atherosclerotic lesions dependent on the experimental setting and/or the stage of atherosclerotic lesion development. To further define the role of ABCG1 in atherogenesis, we investigated in the current study the effect of total body deficiency of ABCG1 on atherosclerotic lesion development.

METHODS AND RESULTS

ABCG1-/- mice and wild-type littermates were fed an atherogenic diet for 12 weeks to induce atherosclerotic lesion formation. Both before and after the start of the atherogenic diet, serum lipid levels and lipoprotein profiles did not differ significantly between the two groups. In addition no significant difference in serum apoE levels was found after diet feeding. In wild-type mice the atherogenic diet induced the formation of macrophage-rich early lesions (size: 24+/-7x10(3) microm2 [n=6]). Feeding ABCG1-/- mice the atherogenic diet led to a significant 1.9-fold stimulation of atherosclerotic lesion size (46+/-6x10(3) microm2 [n=7]; Student t test P=0.034 and Mann-Whitney test P=0.050) compared with controls, suggesting a clear antiatherogenic role for ABCG1. At the same time, excessive lipid accumulation was observed in macrophage-rich areas of the lungs and spleens of ABCG1-/- mice as compared with wild-type mice.

CONCLUSIONS

Total body ABCG1 expression protects against early atherosclerotic lesion development.

The pharmacokinetic behavior of the photosensitizer meso-tetra-hydroxyphenyl-chlorin in mice and men

PURPOSE

Meso-tetra-hydroxyphenyl-chlorin (mTHPC) is a hydrophobic photosensitizer that binds to plasma lipoproteins after intravenous injection. In vitro experiments with human plasma have shown that mTHPC initially binds to an unknown protein and subsequently redistributes to lipoprotein fractions. It has been suggested that this might explain the unusual pharmacokinetic profile of mTHPC humans. In humans, unlike in rodents, reappearance of mTHPC has been reported, resulting in a second plasma peak after intravenous injection. However, previous studies analyzed only limited time points during the first 24 h after injection. Our aim was to determine the pharmacokinetics of mTHPC in detail, and to investigate whether the pharmacokinetic behavior of the drug is affected by binding of mTHPC to lipoproteins in vivo.

METHODS

Plasma of cancer patients and mice, intravenously injected with mTHPC, was analyzed for total drug content and drug distribution over the lipoprotein fractions.

RESULTS

Pharmacokinetic profiles of mTHPC in a group of human subjects showed that apparent steady state drug levels were maintained for at least 10 h. Closer examination of individual profiles showed that the initial (5 min) plasma drug levels were on average 86% of the maximal plasma concentration, which occurred at about 5 h after injection. In mice, however, plasma pharmacokinetics were described by a standard bi-exponential decline of the drug concentration. The majority (>58%) of mTHPC injected into both BALB/c nude mice and patients initially bound to the HDL plasma fraction. We extended our study to ApoE -/- mice, with highly elevated lipoprotein levels, and SR-BI -/- mice, which are lacking the main clearance pathway for HDL associated cholesteryl esters, to take into account the differences between lipoprotein levels and clearance in mice and man. Although mTHPC distribution over the lipoproteins changed in these mice, pharmacokinetic profiles of mTHPC remained the same.

CONCLUSIONS

We conclude that neither lipoprotein levels nor cholesterol metabolism affects the pharmacokinetics of mTHPC in plasma.

Regulation of cholesterol homeostasis in macrophages and consequences for atherosclerotic lesion development

Foam cell formation due to excessive accumulation of cholesterol by macrophages is a pathological hallmark of atherosclerosis. Macrophages cannot limit the uptake of cholesterol and therefore depend on cholesterol efflux pathways for preventing their transformation into foam cells. Several ABC-transporters, including ABCA1 and ABCG1, facilitate the efflux of cholesterol from macrophages. These transporters, however, also affect membrane lipid asymmetry which may have important implications for cellular endocytotic pathways. We propose that in addition to the generally accepted role of these ABC-transporters in the prevention of foam cell formation by induction of cholesterol efflux from macrophages, they also influence the macrophage endocytotic uptake.

Macrophage ABCG1 deletion disrupts lipid homeostasis in alveolar macrophages and moderately influences atherosclerotic lesion development in LDL receptor-deficient mice

OBJECTIVE

ABCG1 has recently been identified as a facilitator of cellular cholesterol and phospholipid efflux to high-density lipoprotein (HDL). Its expression in macrophages is induced during cholesterol uptake in macrophages and by liver X receptor (LXR). The role of macrophage ABCG1 in atherosclerotic lesion development is, however, still unknown.

METHODS AND RESULTS

To assess the role of macrophage ABCG1 in atherosclerosis, we generated low-density lipoprotein (LDL) receptor knockout (LDLr-/-) mice that are selectively deficient in macrophage ABCG1 by using bone marrow transfer (ABCG1-/- --> LDLr-/-). Peritoneal macrophages isolated from donor ABCG1-/- mice exhibited a 22% (P=0.0007) decrease in cholesterol efflux to HDL. To induce atherosclerosis, transplanted mice were fed a high-cholesterol diet containing 0.25% cholesterol and 15% fat for 6 and 12 weeks. Serum lipid levels and lipoprotein profiles did not differ significantly between ABCG1-/- --> LDLr-/- mice and controls. In lungs of ABCG1-/- --> LDLr-/- mice a striking accumulation of lipids was observed in macrophages localized to the subpleural region. After 6 weeks of high-cholesterol diet feeding the atherosclerotic lesion size was 49+/-12x10(3) microm2 for ABCG1+/+ --> LDLr-/- mice versus 65+/-15x103 microm2 for ABCG1-/- --> LDLr-/- mice and after 12 weeks of high-cholesterol diet feeding 124+/-17x10(3) microm2 for ABCG1+/+ --> LDLr-/- mice versus 168+/-17x10(3) microm2 for ABCG1-/- --> LDLr-/- mice. Atherosclerotic lesion size depended on both time and the macrophage ABCG1 genotype (P=0.038 by 2-way ANOVA, n > or = 8), indicating a moderately 33% to 36% increase in lesion formation in the absence of macrophage ABCG1.

CONCLUSIONS

Macrophage ABCG1 deficiency does lead to heavy lipid accumulation in macrophages of the lung, and also a moderately significant effect on atherosclerotic lesion development was observed.

Scavenger receptors: friend or foe in atherosclerosis?

PURPOSE OF REVIEW

Scavenger receptors were originally defined by their ability to bind and internalize modified lipoproteins. Nowadays the family of scavenger receptors is composed of structurally different surface receptors which recognize a broad pattern of common ligands which include, besides modified lipoproteins, apoptotic cells and pathogens. This review focuses on the role of scavenger receptors in the development of atherosclerotic lesions.

RECENT FINDINGS

Recent studies indicate that scavenger receptor A activity can be regulated by phosphorylation, glucosidases, 8-isoprostane, high glucose and nobiletin. Modulation of these regulatory components may beneficially influence scavenger receptor A's proatherogenic function. It appears that statins do lead to a reduction in CD36 transcription and could modulate in this way CD36-mediated atherosclerotic foam cell formation. Macrophage scavenger receptor BI appears to facilitate the development of small fatty streak lesions, whereas the formation of advanced atherosclerotic lesions is reduced, indicating a unique dual role for macrophage scavenger receptor BI in the pathogenesis of atherosclerosis.

SUMMARY

It is proposed that the presence of scavenger receptors in macrophages is beneficial because they remove potential deleterious material from the arterial wall. Inadequate handling of the internalized material by the macrophages will lead to foam cell formation. If adequate levels of ATP-binding cassette transporters and accepting HDL are present, however, the macrophage is perfectly able to metabolize and secrete the internalized atherogenic substances whereby HDL facilitates further transport from the arterial wall to the liver, leading to release in bile.

Scavenger receptor BI and ATP-binding cassette transporter A1 in reverse cholesterol transport and atherosclerosis

PURPOSE OF REVIEW

The appearance of scavenger receptor class B type I (SR-BI) and ATP-binding cassette transporter A1 (ABCA1) in macrophages and liver implicates these transporters in different stages of reverse cholesterol transport. This review focuses on the role of SR-BI and ABCA1 in reverse cholesterol transport in the context of atherosclerotic lesion development.

RECENT FINDINGS

Recent studies indicate that hepatic expression of ABCA1 and SR-BI is important for the generation of nascent HDL and the delivery of HDL cholesteryl esters to the liver, respectively. Although macrophage SR-BI and ABCA1 do not contribute significantly to circulating HDL levels, the perpetual cycle of HDL lipidation and delipidation by the liver ensures the availability of acceptors for cholesterol efflux that maintain cholesterol homeostasis in arterial macrophages, thereby reducing atherogenesis. In addition to its established role in the selective uptake of HDL cholesteryl esters, there is now evidence that hepatic SR-BI facilitates postprandial lipid metabolism, and that hepatic secretion of VLDL is dependent on ABCA1-mediated nascent HDL formation. Thus, remnant and HDL metabolism are more intimately intertwined in hepatic lipid metabolism than has previously been appreciated.

SUMMARY

Recent advances in the understanding of the role of ABCA1 and SR-BI in HDL metabolism and their atheroprotective properties indicate the significant potential of modulating ABCA1 and SR-BI expression in both arterial wall macrophages and the liver for the treatment of atherosclerotic coronary artery disease.

Adenovirus-mediated hepatic overexpression of scavenger receptor class B type I accelerates chylomicron metabolism in C57BL/6J mice

The function of scavenger receptor class B type I (SR-BI) in mediating the selective uptake of HDL cholesteryl esters is well established. In SR-BI-deficient mice, we recently observed a delayed postprandial triglyceride (TG) response, suggesting an additional role for SR-BI in facilitating chylomicron (CM) metabolism. Here, we assessed the effect of adenovirus-mediated hepatic overexpression of SR-BI (Ad.SR-BI) in C57BL/6J mice on serum lipids and CM metabolism. Infection of 5 x 10(8) plaque-forming units per mouse of Ad.SR-BI significantly decreases serum cholesterol (>90%), phospholipids (>90%), and TG levels (50%), accompanied by a 41.4% reduction (P < 0.01) in apolipoprotein B-100 levels. The postprandial TG response is 2-fold lower in mice treated with Ad.SR-BI compared with control mice (area under the curve = 31.4 +/- 2.4 versus 17.7 +/- 3.2; P < 0.05). Hepatic mRNA expression levels of genes known to be involved in serum cholesterol and TG clearance are unchanged and thus could not account for the decreased plasma TG levels and the change in postprandial response. We conclude that overexpression of SR-BI accelerates CM metabolism, possibly by mediating the initial capture of CM remnants by the liver, whereby the subsequent internalization can be exerted by additional receptor systems such as the LDL receptor (LDLr) and LDLr-related protein 1.

Diet induced regulation of genes involved in cholesterol metabolism in rat liver parenchymal and Kupffer cells

BACKGROUND/AIMS

Feeding rodents atherogenic diets enriched in cholesterol or cholic acid changes hepatic cholesterol metabolism. In the present study, the effect of an atherogenic diet enriched in cholesterol and cholic acid on cellular hepatic cholesterol metabolism was studied.

METHODS

Gene and protein expression analysis was performed on parenchymal, endothelial, and Kupffer cells isolated from rats fed a chow or atherogenic diet using quantitative real-time PCR and immunoblotting, respectively.

RESULTS

The atherogenic diet raised the serum cholesterol concentration 11-fold, mostly in the VLDL fraction, and led to heavy lipid loading of rat liver parenchymal and Kupffer cells. Only moderate changes in the expression of genes involved in cholesterol metabolism were observed in parenchymal cells on the diet, while PPAR delta expression was 6.8-fold decreased. Kupffer cells, however, showed a highly adaptive response with a 2- to 9-fold induction of SR-BI, ABCA1, and ABCG5/G8, and an 82-fold induction in CYP7A1 mRNA expression, respectively.

CONCLUSIONS

Heavy lipid loading of parenchymal cells leads to moderate gene expression changes, while Kupffer cells respond in a highly adaptive fashion by stimulating the expression of genes involved in cholesterol metabolism and transport.

Scavenger receptor class B type I is solely responsible for the selective uptake of cholesteryl esters from HDL by the liver and the adrenals in mice

Scavenger receptor class B type I (SR-BI) has been identified as a functional HDL binding protein that can mediate the selective uptake of cholesteryl ester (CE) from HDL. To quantify the in vivo role of SR-BI in the process of selective uptake, HDL was labeled with cholesteryl ether ([(3)H] CEt-HDL) and (125)I-tyramine cellobiose ([(125)I]TC-HDL) and injected into SR-BI knockout (KO) and wild-type (WT) mice. In SR-BI KO mice, the clearance of HDL-CE from the blood circulation was greatly diminished (0.043 +/- 0.004 pools/h for SR-BI KO mice vs. 0.106 +/- 0.004 pools/h for WT mice), while liver and adrenal uptake were greatly reduced. Utilization of double-labeled HDL ([(3)H]CEt and [(125)I]TC) indicated the total absence in vivo of the selective decay and liver uptake of CE from HDL in SR-BI KO mice. Parenchymal cells isolated from SR-BI KO mice showed similar association values for [(3)H]CEt and [(125)I]TC in contrast to WT cells, indicating that in parenchymal liver cells SR-BI is the only molecule exerting selective CE uptake from HDL. Thus, in vivo and in vitro, SR-BI is the sole molecule mediating the selective uptake of CE from HDL by the liver and the adrenals, making it the unique target to modulate reverse cholesterol transport.

Scavenger receptor BI plays a role in facilitating chylomicron metabolism

The function of scavenger receptor class B type I (SR-BI) in mediating the selective uptake of high density lipoprotein (HDL) cholesterol esters is well established. However, the potential role of SR-BI in chylomicron and chylomicron remnant metabolism is largely unknown. In the present investigation, we report that the cell association of 160 nm-sized triglyceride-rich chylomicron-like emulsion particles to freshly isolated hepatocytes from SR-BI-deficient mice is greatly reduced (>70%), as compared with wild-type littermate mice. Competition experiments show that the association of emulsion particles with isolated hepatocytes is efficiently competed for (>70%) by the well established SR-BI ligands, HDL and oxidized low density lipoprotein (LDL), whereas LDL is ineffective. Upon injection into SR-BI-deficient mice the hepatic association of emulsion particles is markedly decreased ( approximately 80%) as compared with wild-type mice. The relevance of these findings for in vivo chylomicron (remnant) metabolism was further evaluated by studying the effect of SR-BI deficiency on the intragastric fat load-induced postprandial triglyceride response. The postprandial triglyceride response is 2-fold higher in SR-BI-deficient mice as compared with wild-type littermates (area-under-the-curve 39.6 +/- 1.2 versus 21.1 +/- 3.6; p < 0.005), with a 4-fold increased accumulation of chylomicron (remnant)-associated triglycerides in plasma at 6 h after intragastric fat load. We conclude that SR-BI is important in facilitating chylomicron (remnant) metabolism and might function as an initial recognition site for chylomicron remnants whereby the subsequent internalization can be exerted by additional receptor systems like the LDL receptor and LDL receptor-related protein.

Selection of antisense oligodeoxynucleotides against glutathione S-transferase Mu

The aim of the present study was to identify functional antisense oligodeoxynucleotides (ODNs) against the rat glutathione S-transferase Mu (GSTM) isoforms, GSTM1 and GSTM2. These antisense ODNs would enable the study of the physiological consequences of GSTM deficiency. Because it has been suggested that the effectiveness of antisense ODNs is dependent on the secondary mRNA structures of their target sites, we made mRNA secondary structure predictions with two software packages, Mfold and STAR. The two programs produced only marginally similar structures, which can probably be attributed to differences in the algorithms used. The effectiveness of a set of 18 antisense ODNs was evaluated with a cell-free transcription/translation assay, and their activity was correlated with the predicted secondary RNA structures. Four phosphodiester ODNs specific for GSTM1, two ODNs specific for GSTM2, and four ODNs targeted at both GSTM isoforms were found to be potent, sequence-specific, and RNase H-dependent inhibitors of protein expression. The IC50 value of the most potent ODN was approximately 100 nM. Antisense ODNs targeted against regions that were predicted by STAR to be predominantly single stranded were more potent than antisense ODNs against double-stranded regions. Such a correlation was not found for the Mfold prediction. Our data suggest that simulation of the local folding of RNA facilitates the discovery of potent antisense sequences. In conclusion, we selected several promising antisense sequences, which, when synthesized as biologically stable oligonucleotides, can be applied for study of the physiological impact of reduced GSTM expression.