Carl P. Sparrow

27-Hydroxycholesterol Is an Endogenous Ligand for Liver X Receptor in Cholesterol-loaded Cells

The nuclear receptors liver X receptor α (LXRα) (NR1H3) and LXRβ (NR1H2) are important regulators of genes involved in lipid metabolism, including ABCA1,ABCG1, and sterol regulatory element-binding protein-1c (SREBP-1c). Although it has been demonstrated that oxysterols are LXR ligands, little is known about the identity of the physiological activators of these receptors. Here we confirm earlier studies demonstrating a dose-dependent induction of ABCA1 and ABCG1 in human monocyte-derived macrophages by cholesterol loading. In addition, we show that formation of 27-hydroxycholesterol and cholestenoic acid, products of CYP27 action on cholesterol, is dependent on the dose of cholesterol used to load the cells. Other proposed LXR ligands, including 20(S)-hydroxycholesterol, 22(R)-hydroxycholesterol, and 24(S),25-epoxycholesterol, could not be detected under these conditions. A role for CYP27 in regulation of cholesterol-induced genes was demonstrated by the following findings. 1) Introduction of CYP27 into HEK-293 cells conferred an induction of ABCG1 and SREBP-1c; 2) upon cholesterol loading, CYP27-expressing cells induce these genes to a greater extent than in control cells; 3) in CYP27-deficient human skin fibroblasts, the induction of ABCA1 in response to cholesterol loading was ablated; and 4) in a coactivator association assay, 27-hydroxycholesterol functionally activated LXR. We conclude that 27-hydroxylation of cholesterol is an important pathway for LXR activation in response to cholesterol overload.

Activation of PPARδ alters lipid metabolism in db/db mice

Peroxisome proliferator‐activated receptors (PPARs) are nuclear receptors, which heterodimerize with the retinoid X receptor and bind to peroxisome proliferator response elements in the promoters of regulated genes. Despite the wealth of information available on the function of PPARα and PPARγ, relatively little is known about the most widely expressed PPAR subtype, PPARδ. Here we show that treatment of insulin resistant db/db mice with the PPARδ agonist L‐165 041, at doses that had no effect on either glucose or triglycerides, raised total plasma cholesterol concentrations. The increased cholesterol was primarily associated with high density lipoprotein (HDL) particles, as shown by fast protein liquid chromatography analysis. These data were corroborated by the chemical analysis of the lipoproteins isolated by ultracentrifugation, demonstrating that treatment with L‐165 041 produced an increase in circulating HDL without major changes in very low or low density lipoproteins. White adipose tissue lipoprotein lipase activity was reduced following treatment with the PPARδ ligand, but was increased by a PPARγ agonist. These data suggest both that PPARδ is involved in the regulation of cholesterol metabolism in db/db mice and that PPARδ ligands could potentially have therapeutic value.

Cholesterol Efflux Potential and Antiinflammatory Properties of High-Density Lipoprotein After Treatment With Niacin or Anacetrapib

Objective—To examine the effects of treatments with niacin or anacetrapib (an inhibitor of cholesteryl ester transfer protein) on the ability of high-density lipoprotein (HDL) to promote net cholesterol efflux and reduce toll-like receptor–mediated inflammation in macrophages. Methods and Results—A total of 18 patients received niacin, 2 g/d, for 4 weeks; 20 patients received anacetrapib, 300 mg/d, for 8 weeks; and 2 groups (n=4 and n=5 patients) received placebo. HDL samples were isolated by polyethylene glycol precipitation or ultracentrifugation, tested for the ability to promote cholesterol efflux in cholesterol-loaded THP-I or mouse peritoneal macrophages, or used to pretreat macrophages, followed by lipopolysaccharide exposure. HDL cholesterol levels were increased by 30% in response to niacin and by approximately 100% in response to anacetrapib. Niacin treatment increased HDL-mediated net cholesterol efflux from foam cells, primarily by increasing HDL concentration, whereas anacetrapib treatment increased cholesterol efflux by both increasing HDL concentration and causing increased efflux at matched HDL concentrations. The increased efflux potential of anacetrapib-HDL was more prominent at higher HDL cholesterol concentrations (>12 &mgr;g/mL), which was associated with an increased content of lecithin–cholesterol acyltransferase (LCAT) and apolipoprotein E and completely dependent on the expression of ATP binding cassette transporters (ABCA1 and ABCG1). Potent antiinflammatory effects of HDL were observed at low HDL concentrations (3 to 20 &mgr;g/mL) and were partly dependent on the expression of ABCA1 and ABCG1. All HDL preparations showed similar antiinflammatory effects, proportionate to the HDL cholesterol concentration. Conclusion—Niacin treatment caused a moderate increase in the ability of HDL to promote net cholesterol efflux, whereas inhibition of cholesteryl ester transfer protein via anacetrapib led to a more dramatic increase in association with enhanced particle functionality at higher HDL concentrations. All HDLs exhibited potent ability to suppress macrophage toll-like receptor 4–mediated inflammatory responses, in a process partly dependent on cholesterol efflux via ABCA1 and ABCG1.

Deficiency in Inducible Nitric Oxide Synthase Results in Reduced Atherosclerosis in Apolipoprotein E-Deficient Mice

Inducible NO synthase (iNOS) present in human atherosclerotic plaques could contribute to the inflammatory process of plaque development. The role of iNOS in atherosclerosis was tested directly by evaluating the development of lesions in atherosclerosis-susceptible apolipoprotein E (apoE)−/− mice that were also deficient in iNOS. ApoE−/− and iNOS−/− mice were cross-bred to produce apoE−/−/iNOS−/− mice and apoE−/−/iNOS+/+ controls. Males and females were placed on a high fat diet at the time of weaning, and atherosclerosis was evaluated at two time points by different methods. The deficiency in iNOS had no effect on plasma cholesterol, triglyceride, or nitrate levels. Morphometric measurement of lesion area in the aortic root at 16 wk showed a 30–50% reduction in apoE−/−/iNOS−/− mice compared with apoE−/−/iNOS+/+ mice. Although the size of the lesions in apoE−/−/iNOS−/− mice was reduced, the lesions maintained a ratio of fibrotic:foam cell-rich:necrotic areas that was similar to controls. Biochemical measurements of aortic cholesterol in additional groups of mice at 22 wk revealed significant 45–70% reductions in both male and female apoE−/−/iNOS−/− mice compared with control mice. The results indicate that iNOS contributes to the size of atherosclerotic lesions in apoE-deficient mice, perhaps through a direct effect at the site of the lesion.

Effects of pH and Low Density Lipoprotein (LDL) on PCSK9-dependent LDL Receptor Regulation

Mutations within PCSK9 (proprotein convertase subtilisin/kexin type 9) are associated with dominant forms of familial hyper- and hypocholesterolemia. Although PCSK9 controls low density lipoprotein (LDL) receptor (LDLR) levels post-transcriptionally, several questions concerning its mode of action remain unanswered. We show that purified PCSK9 protein added to the medium of human endothelial kidney 293, HepG2, and Chinese hamster ovary cell lines decreases cellular LDL uptake in a dose-dependent manner. Using this cell-based assay of PCSK9 activity, we found that the relative potencies of several PCSK9 missense mutants (S127R and D374Y, associated with hypercholesterolemia, and R46L, associated with hypocholesterolemia) correlate with LDL cholesterol levels in humans carrying such mutations. Notably, we found that in vitro wild-type PCSK9 binds LDLR with an ∼150-fold higher affinity at an acidic endosomal pH (KD = 4.19 nm) compared with a neutral pH (KD = 628 nm). We also demonstrate that wild-type PCSK9 and mutants S127R and R46L are internalized by cells to similar levels, whereas D374Y is more efficiently internalized, consistent with their affinities for LDLR at neutral pH. Finally, we show that LDL diminishes PCSK9 binding to LDLR in vitro and partially inhibits the effects of secreted PCSK9 on LDLR degradation in cell culture. Together, the results of our biochemical and cell-based experiments suggest a model in which secreted PCSK9 binds to LDLR and directs the trafficking of LDLR to the lysosomes for degradation.

A PCSK9-binding antibody that structurally mimics the EGF(A) domain of LDL-receptor reduces LDL cholesterol in vivo

Proprotein convertase subtilisin-like/kexin type 9 (PCSK9) regulates LDL cholesterol levels by inhibiting LDL receptor (LDLr)-mediated cellular LDL uptake. We have identified a fragment antigen-binding (Fab) 1D05 which binds PCSK9 with nanomolar affinity. The fully human antibody 1D05-IgG2 completely blocks the inhibitory effects of wild-type PCSK9 and two gain-of-function human PCSK9 mutants, S127R and D374Y. The crystal structure of 1D05-Fab bound to PCSK9 reveals that 1D05-Fab binds to an epitope on the PCSK9 catalytic domain which includes the entire LDLr EGF(A) binding site. Notably, the 1D05-Fab CDR-H3 and CDR-H2 loops structurally mimic the EGF(A) domain of LDLr. In a transgenic mouse model (CETP/LDLr-hemi), in which plasma lipid and PCSK9 profiles are comparable to those of humans, 1D05-IgG2 reduces plasma LDL cholesterol to 40% and raises hepatic LDLr protein levels approximately fivefold. Similarly, in healthy rhesus monkeys, 1D05-IgG2 effectively reduced LDL cholesterol 20%–50% for over 2 weeks, despite its relatively short terminal half-life (t1/2 = 3.2 days). Importantly, the decrease in circulating LDL cholesterol corresponds closely to the reduction in free PCSK9 levels. Together these results clearly demonstrate that the LDL-lowering effect of the neutralizing anti-PCSK9 1D05-IgG2 antibody is mediated by reducing the amount of PCSK9 that can bind to the LDLr.

Liver X Receptor Agonists as Potential Therapeutic Agents for Dyslipidemia and Atherosclerosis

The recent identification of liver X receptors (LXR) as regulators of the cholesterol and phospholipid export pump ABCA1 has raised the possibility that LXR agonists could be developed as HDL-raising agents, possibly also acting on the artery wall to stimulate cholesterol efflux from lipid-laden macrophages. Presently several pharmaceutical companies are working to develop such compounds, which will require finding a path for separating these beneficial effects from the detrimental stimulation of triglyceride synthesis also inherent to LXR agonists. Other challenges to the drug development process include species differences, which makes prediction of in vivo effects of LXR agonists in humans difficult. This review summarizes the present state of knowledge on LXR as a drug target and discusses possible solutions for dissociating the favorable effects of LXR agonists from their unwanted effects.

A potent synthetic LXR agonist is more effective than cholesterol-loading at inducing ABCA1 mRNA and stimulating cholesterol efflux

The LXR nuclear receptors are intracellular sensors of cholesterol excess and are activated by various oxysterols. LXRs have been shown to regulate multiple genes of lipid metabolism, including ABCA1 (formerly known asABC1). ABCA1 is a lipid pump that effluxes cholesterol and phospholipid out of cells. ABCA1 deficiency causes extremely low high density lipoprotein (HDL) levels, demonstrating the importance of ABCA1 in the formation of HDL. The present work shows that the acetyl-podocarpic dimer (APD) is a potent, selective agonist for both LXRα (NR1H3) and LXRβ (NR1H2). In transient transactivation assays, APD was ∼1000-fold more potent, and yielded ∼6-fold greater maximal stimulation, than the widely used LXR agonist 22-(R)-hydroxycholesterol. APD induced ABCA1mRNA levels, and increased efflux of both cholesterol and phospholipid, from multiple cell types. Gas chromatography-mass spectrometry measurements demonstrated that APD stimulated efflux of endogenous cholesterol, eliminating any possible artifacts of cholesterol labeling. For both mRNA induction and stimulation of cholesterol efflux, APD was found to be more effective than was cholesterol loading. Taken together, these data show that APD is a more effective LXR agonist than endogenous oxysterols. LXR agonists may therefore be useful for the prevention and treatment of atherosclerosis, especially in the context of low HDL levels.

Biochemical characterization of cholesteryl ester transfer protein inhibitors

Cholesteryl ester transfer protein (CETP) has been identified as a novel target for increasing HDL cholesterol levels. In this report, we describe the biochemical characterization of anacetrapib, a potent inhibitor of CETP. To better understand the mechanism by which anacetrapib inhibits CETP activity, its biochemical properties were compared with CETP inhibitors from distinct structural classes, including torcetrapib and dalcetrapib. Anacetrapib and torcetrapib inhibited CETP-mediated cholesteryl ester and triglyceride transfer with similar potencies, whereas dalcetrapib was a significantly less potent inhibitor. Inhibition of CETP by both anacetrapib and torcetrapib was not time dependent, whereas the potency of dalcetrapib significantly increased with extended preincubation. Anacetrapib, torcetrapib, and dalcetrapib compete with one another for binding CETP; however anacetrapib binds reversibly and dalcetrapib covalently to CETP. In addition, dalcetrapib was found to covalently label both human and mouse plasma proteins. Each CETP inhibitor induced tight binding of CETP to HDL, indicating that these inhibitors promote the formation of a complex between CETP and HDL, resulting in inhibition of CETP activity.

Structural and Biochemical Characterization of the Wild Type PCSK9-EGF(AB) Complex and Natural Familial Hypercholesterolemia Mutants

PCSK9 regulates low density lipoprotein receptor (LDLR) levels and consequently is a target for the prevention of atherosclerosis and coronary heart disease. Here we studied the interaction, of LDLR EGF(A/AB) repeats with PCSK9. We show that PCSK9 binds the EGF(AB) repeats in a pH-dependent manner. Although the PCSK9 C-terminal domain is not involved in LDLR binding, PCSK9 autocleavage is required. Moreover, we report the x-ray structure of the PCSK9ΔC-EGF(AB) complex at neutral pH. Compared with the low pH PCSK9-EGF(A) structure, the new structure revealed rearrangement of the EGF(A) His-306 side chain and disruption of the salt bridge with PCSK9 Asp-374, thus suggesting the basis for enhanced interaction at low pH. In addition, the structure of PCSK9ΔC bound to EGF(AB)H306Y, a mutant associated with familial hypercholesterolemia (FH), reveals that the Tyr-306 side chain forms a hydrogen bond with PCSK9 Asp-374, thus mimicking His-306 in the low pH conformation. Consistently, Tyr-306 confers increased affinity for PCSK9. Importantly, we found that although the EGF(AB)H306Y-PCSK9 interaction is pH-independent, LDLRH306Y binds PCSK9 50-fold better at low pH, suggesting that factors other than His-306 contribute to the pH dependence of PCSK9-LDLR binding. Further, we determined the structures of EGF(AB) bound to PCSK9ΔC containing the FH-associated D374Y and D374H mutations, revealing additional interactions with EGF(A) mediated by Tyr-374/His-374 and providing a rationale for their disease phenotypes. Finally, we report the inhibitory properties of EGF repeats in a cellular assay measuring LDL uptake.