apoE Secretion Research Articles

Abstract 628: Hepatic Insulin Signaling Regulates ApoA-I Gene Expression Through the Type I Deiodinase.

People with low levels of high density lipoprotein cholesterol (HDLC) and apolipoprotein A-I (ApoA-I) have a higher risk of cardiovascular disease. Low levels of HDLC are common in individuals who are insulin resistant (IR), e.g., with metabolic syndrome and type 2 diabetes mellitus (T2DM). Despite the high prevalence of these two disorders, very little work has been reported regarding the molecular pathways linking insulin signaling or action and the levels of either HDLC or ApoA-1. We reported previously that liver specific insulin receptor (InsR) knockout mice (LIRKO) have markedly reduced plasma HDLC levels that increase after restoration of hepatic Akt signaling. In the present study, we created acute LIRKO mice by injecting an albumin-Cre adenovirus (Ad) into InsR floxed mice and observed marked reductions in HDLC, the expression of ApoA-I, and the expression of the gene coding Type1 iodothyronine deiodinase1, a selenoenzyme expressed highly in the liver that converts thyroxine to 3,5,3’-triiodothyronine (T3) or reverse T3. Deiodinase 1 knockout mice also had significantly reduced hepatic ApoA-I mRNA levels. Overexpression of Dio1 in LIRKO restored HDLC and significantly increased the expression of ApoA-I mRNA. In vitro studies showed that the expression of ApoA-I was significantly reduced after knockdown of either InsR or Dio1 expression in HepG2 cells. Moreover, overexpression of Dio1 restored ApoA-I promoter activity that had been decreased by knockdown of InsR. Deletion analysis of ApoAI promoter regions showed that insulin signaling regulated ApoA-I expression by acting on a region which does not contain any thyroid response elements. Pulse-chase experiments in HepG2 cells showed that deficiency of insulin signaling resulted in decreased synthesis and secretion of ApoAI. Our results indicates that defective hepatic insulin signaling results in reduced expression of Dio1 which, in turn, leads to reduced expression of ApoA-I and decreased synthesis and secretion of ApoA-I from hepatocytes. We believe our studies have defined a novel pathway from insulin signaling to ApoA-I synthesis that may lead to new approaches for increasing HDL levels in people with defective insulin signaling.

Abstract 671: Beyond Cholesterol Efflux: Defining HDL Function in Atherosclerosis and the Requirement for Procollagen C-Endopeptidase Enhancer Protein 2

Recent studies suggest that assessment of high density lipoprotein (HDL) function provides a more reliable metric of athero-protection than plasma concentration alone. One important function attributed to apolipoprotein A-I (apoA-I), the major protein constituent of HDL, is its ability to accept newly effluxed cholesterol from ATP-binding cassette A-I (ABCA1) to form nascent HDL (nHDL). New studies suggest that the protein, procollagen c-endopeptidase enhancer protein 2 (PCOLCE2; PCPE2), is involved in mediating nHDL particle formation. PCPE2 is a 52 kDa glycoprotein sharing 43% amino acid identity with PCOLCE (PCPE1). Both are secreted into the extracellular matrix where they enhance the catalytic cleavage of the C-terminal propeptides of procollagen catalyzed by bone morphogenetic protein 1 (BMP1). The connection between PCPE2 and HDL was discovered after significant correlations were found between single-nucleotide polymorphisms and plasma HDL-cholesterol concentrations in population studies. Consistent with these findings a recent genome-wide siRNA screening study indicated that PCPE2 was a significant modulator of HDL apoA-I secretion from cells, while other studies in mice lacking PCPE2 showed defective ABCA1-mediated cholesterol efflux, despite elevated plasma HDL. Thus, we began the current studies to determine if the increased plasma HDL, in light of defective cholesterol efflux, would be atheroprotective or atherogenic. PCPE2 null mice were crossed with LDL receptor null mice to obtain double knockout, LDLr -/- , PCPE2 -/- mice. Both LDLr -/- and LDLr -/- , PCPE2 -/- mice were fed an atherogenic diet for 12 wks after which the lipid deposition in the aortic root was evaluated. As expected, LDLr -/- , PCPE2 -/- mice showed elevated levels of enlarged HDL compared to control mice, while LDL levels were similar between the two genotypes. Despite elevations in plasma HDL, LDLr -/- , PCPE2 -/- mice showed significantly more atherosclerosis compared to LDLr -/- mice, similar in extent to that found in mice that lack HDL due to an absence of plasma apoA-I (LDLr -/- , apoA-I -/- ). This outcome suggests that PCPE2 is atheroprotective and likely participates at various levels of HDL metabolism, including HDL production, remodeling and catabolism.

Abstract 9: Metabolic Pathways for HDL Subtypes in Low-HDL Overweight Phenotype Compared With Normal

Introduction: A prevailing concept of HDL metabolism in plasma is that HDL is secreted into plasma as a small discoidal particle; and then expands and contracts by uptake and removal of cholesterol. This process has not been studied directly in humans. Hypothesis: We hypothesized that the 4 common sizes of HDL will show interconversions suggesting reverse cholesterol transport and that is impaired in those who have low HDL-c levels. Methods: We studied 12 adults, ages 30 to 70; six had low HDL-c (mean 39 mg/dL) and were overweight, and 6 had high HDL-c (mean 79 mg/dL) and normal body weight. They received a intravenous bolus of trideuterated leucine with serial blood sampling for 94 hours. HDL sizes, very small (<7.1 nm), small (7.1-8.2 nm), medium (8.2-9.5 nm) and large (9.5-12.2), were separated, and the isotope enrichments and masses of apoA-I in each HDL size were measured by mass spectrometry. We tested many kinetic models using SAAM-II. Results: In both HDL groups, we found similar peak enrichment heights and times in the four HDL sizes suggesting mostly independent metabolism which was confirmed by the final model that had excellent fits to the data. Most of each plasma HDL size subfraction originated from direct secretion into plasma. Only small amounts of apoA-I flux went from smaller to larger HDL, 19% in the low vs 2% in the high HDL group (p=0.006); and in the opposite direction, 3% in the low vs 9% in the high HDL group (ns). ApoA-I in medium and large HDL had faster fractional catabolic rates (FCRs) in the low than in the high HDL group (0.61 vs 0.31 pools/day [p=0.004] and 0.51 vs 0.24 pools/day [p=0.01]), respectively). In contrast, apoA-I in very small (prebeta) HDL had slower FCR in the low than high HDL group (0.51 vs 0.73/d [p=0.048]). Secretion rates of apoA-I into plasma HDL of each size were not significantly different between the groups. Total apoA-I secretion was 12 vs 14 mg/kg/d in the low vs high HDL group (ns). Conclusions: Our results question the concepts of reverse cholesterol transport dominating plasma HDL metabolism, and of its impairment in those with low plasma HDL-c levels. Differences in apoA-I FCR of HDL subfractions explained the well-known shift in size distribution toward smaller HDL in those who have low HDL-c.

FGF21 inhibits apolipoprotein(a) expression in HepG2 cells via the FGFR1-ERK1/2-Elk-1 pathway

Lipoprotein(a) [Lp(a)] is a highly atherogenic lipoprotein, whose metabolism is poorly understood. Efficient and secure drugs that can lower elevated plasma Lp(a) concentrations are currently lacking. Fibroblast growth factor-21 (FGF-21), a member of the FGFS super family, regulates glucose and lipid metabolism in hepatocytes and adipocytes via FGFR-ERK1/2 signaling. In this study, we investigated the molecular mechanisms that influence apolipoprotein(a) [apo(a)] biosynthesis. We also determined the effects of FGF21 on HepG2 cell apo(a) expression and secretion, as well as the mechanism of FGF21 in these effects. Results showed that FGF21 inhibited apo(a) expression at both mRNA and protein levels in a dose- and time--dependent manner and then suppressed the secretion of apo(a). These effects were attenuated by PD98059 (ERK1/2 inhibitor) and Elk-1 siRNA. PD166866 (FGFR1 inhibitor) also attenuated the FGF21-mediated inhibition of apo(a) expression and inhibited ERK1/2 and Elk-1 activation. These results demonstrate that FGF21 suppresses apo(a) expression via the FGFR1-ERK1/2-Elk-1 pathway.

Retinoic Acid Isomers Facilitate Apolipoprotein E Production and Lipidation in Astrocytes through the Retinoid X Receptor/Retinoic Acid Receptor Pathway

Apolipoprotein E (apoE) is the major cholesterol transport protein in the brain. Among the three human APOE alleles (APOE2, APOE3, and APOE4), APOE4 is the strongest genetic risk factor for late-onset Alzheimer disease (AD). The accumulation of amyloid-β (Aβ) is a central event in AD pathogenesis. Increasing evidence demonstrates that apoE isoforms differentially regulate AD-related pathways through both Aβ-dependent and -independent mechanisms; therefore, modulating apoE secretion, lipidation, and function might be an attractive approach for AD therapy. We performed a drug screen for compounds that modulate apoE production in immortalized astrocytes derived from apoE3-targeted replacement mice. Here, we report that retinoic acid (RA) isomers, including all-trans-RA, 9-cis-RA, and 13-cis-RA, significantly increase apoE secretion to ~4-fold of control through retinoid X receptor (RXR) and RA receptor. These effects on modulating apoE are comparable with the effects recently reported for the RXR agonist bexarotene. Furthermore, all of these compounds increased the expression of the cholesterol transporter ABCA1 and ABCG1 levels and decreased cellular uptake of Aβ in an apoE-dependent manner. Both bexarotene and 9-cis-RA promote the lipidation status of apoE, in which 9-cis-RA promotes a stronger effect and exhibits less cytotoxicity compared with bexarotene. Importantly, we showed that oral administration of bexarotene and 9-cis-RA significantly increases apoE, ABCA1, and ABCG1 levels in mouse brains. Taken together, our results demonstrate that RXR/RA receptor agonists, including several RA isomers, are effective modulators of apoE secretion and lipidation and may be explored as potential drugs for AD therapy.

Bilberries exert an anti-atherosclerotic effect in lipid-loaded macrophages

Abstract We hypothesized that the mechanism responsible for the anti-atherosclerotic action of bilberry extract (BE) is linked to its antioxidant and anti-inflammatory potential, and investigated its direct effect on the regulation of apolipoprotein E (apoE) and cholesteryl ester transfer protein (CETP) secretion from lipid-loaded macrophages. Human THP-1 macrophages were loaded with lipids by incubation with human copper-oxidized LDL (oxLDL) and then exposed to different concentrations of BE (1–5 µg mL−1) obtained from bilberries (mechanically homogenized and solubilized in ethanol). Cellular and secreted proteins, the phosphorylation level of NF-κB and protein kinase A (PKA) were quantified by Western blot and gene expression was evaluated by Real-time PCR. The results showed that BE induced in lipid-loaded macrophages has: (i) an antioxidant effect by reducing the expression of NADPH oxidase subunits, p22phox, p47phox and NOX4, (ii) an anti-inflammatory effect by diminishing the secretion of CRP, MCP-1 and IL-1β and (iii) cholesterol efflux by increasing the secretion of apoE and CETP and by reducing cellular cholesterol content. BE exerted these effects by inhibition of NF-κB and activation of PKA signaling pathways. Our study supports BE therapeutic administration to decrease oxidative and inflammatory stress by a molecular mechanism regulated by NF-κB and PKA signaling pathways in lipid-loaded macrophages.

Abstract 19434: The Novel PPARalpha-Selective Agonist K-877 Improves Dyslipidemia, Enhances Reverse Cholesterol Transport and Decreases Inflammation and Atherosclerosis

Background: Atherosclerosis is characterized by lipid accumulation and chronic inflammation in the arterial wall. Elevated levels of apolipoprotein (apo) B-containing lipoproteins are a risk factor for cardiovascular disease (CVD). By contrast, plasma levels of high-density lipoprotein (HDL) and apoA-I are protective against CVD by enhancing HDL functionality and reverse cholesterol transport (RCT). Activation of peroxisome proliferator-activated receptor-alpha (PPARalpha), a ligand-activated transcription factor, controls plasma lipid metabolism, macrophage cholesterol handling as well as the inflammatory response. Objective: To study whether pharmacological activation of PPARalpha with the highly potent PPARalpha ligand, K-877, improves lipid metabolism, macrophage cholesterol efflux, inflammation and consequently atherosclerosis development in human apoA-I transgenic mice and in the human apolipoprotein E2 knock-in mouse (apo E2KI) model of mixed dyslipidemia and atherosclerosis. Methods and results: In polarized Caco-2/TC7 human intestinal epithelium cells, K-877 treatment decreased apoB secretion and increased the expression of PPARalpha target genes involved in fatty acid oxidation and intestinal HDL production. Moreover, K-877 induced secretion of apoA-I in HDL particles. In primary human macrophages, K-877 promoted macrophage cholesterol efflux to HDL. K-877 treatment of human apoA-I transgenic mice induced plasma HDL cholesterol and apoA-I levels. Moreover, K-877 stimulated in vivo RCT in these mice. In addition, K-877 treatment exerted anti-inflammatory properties in murine and human macrophages. K-877 treatment not only improved the lipid profile, but also reduced inflammatory and macrophage marker gene expression in the aortic crosses as well as aortic atherosclerotic lesion burden in western diet-fed apo E2KI mice. Conclusion: These results demonstrate that the novel PPARalpha agonist K-877 exerts beneficial effects on lipid metabolism, RCT and inflammation resulting in anti-atherogenic properties.

ApoE Production in Human Monocytes and Its Regulation by Inflammatory Cytokines

The apoE production by tissue macrophages is crucial for the prevention of atherosclerosis and the aim of this study was to further elucidate how this apolipoprotein is regulated by cytokines present during inflammation. Here we studied apoE production in peripheral blood mononuclear cells (PBMC) and analysis was made with a newly developed apoE ELISpot assay. In PBMC, apoE secretion was restricted to monocytes with classical (CD14++CD16−) and intermediate (CD14+CD16+) monocytes being the main producers. As earlier described for macrophages, production was strongly upregulated by TGF-β and downregulated by bacterial lipopolysaccharide (LPS) and the inflammatory cytokines IFN-γ, TNF-α and IL-1β. We could here show that a similar down-regulatory effect was also observed with the type I interferon, IFN-α, while IL-6, often regarded as one of the more prominent inflammatory cytokines, did not affect TGF-β-induced apoE production. The TNF-α inhibitor Enbrel could partly block the down-regulatory effect of IFN-γ, IFN-α and IL-1β, indicating that inhibition of apoE by these cytokines may be dependent on or synergize with TNF-α. Other cytokines tested, IL-2, IL-4, IL-12, IL-13, IL-17A and IL-23, had no inhibitory effect on apoE production. In contrast to the effect on monocytes, apoE production by primary hepatocytes and the hepatoma cell line HepG2 was more or less unaffected by treatment with cytokines or LPS.

Hormonal modulators of glial ABCA1 and apoE levels

Apolipoprotein E (apoE) is the major lipid carrier in the central nervous system. As apoE plays a major role in the pathogenesis of Alzheimer disease (AD) and also mediates repair pathways after several forms of acute brain injury, modulating the expression, secretion, or function of apoE may provide potential therapeutic approaches for several neurological disorders. Here we show that progesterone and a synthetic progestin, lynestrenol, significantly induce apoE secretion from human CCF-STTG1 astrocytoma cells, whereas estrogens and the progesterone metabolite allopregnanolone have negligible effects. Intriguingly, lynestrenol also increases expression of the cholesterol transporter ABCA1 in CCF-STTG1 astrocytoma cells, primary murine glia, and immortalized murine astrocytes that express human apoE3. The progesterone receptor inhibitor RU486 attenuates the effect of progestins on apoE expression in CCF-STTG1 astrocytoma cells but has no effect on ABCA1 expression in all glial cell models tested, suggesting that the progesterone receptor (PR) may participate in apoE but does not affect ABCA1 regulation. These results suggest that selective reproductive steroid hormones have the potential to influence glial lipid homeostasis through liver X receptor-dependent and progesterone receptor-dependent pathways.

In Vivo and In Vitro Effects of an Apolipoprotein E Mimetic Peptide on Amyloid-β Pathology

Apolipoprotein E (ApoE) is the major apolipoprotein present in the high-density lipoprotein-like particles in the central nervous system (CNS). ApoE is involved in various protective functions in CNS including cholesterol transport, anti-inflammatory, and antioxidant effects. An ApoE peptide would be expected to exert protective effects on neuroinflammation. To determine the effects of an ApoE mimetic peptide Ac-hE18A-NH2 on amyloid-β pathology. Using human APP/PS1ΔE9 transgenic mice and in vitro studies, we have evaluated the effect of an ApoE mimetic peptide, Ac-hE18A-NH2, on amyloid plaque deposition and inflammation. Administration of Ac-hE18A-NH2 to APP/PS1ΔE9 mice for 6 weeks (50 μg/mouse, 3 times a week) significantly improved cognition with a concomitant decrease in amyloid plaque deposition and reduced activated microglia and astrocytes, and increased brain ApoE levels. Oligomeric Aβ42 (oAβ42) and oxidized PAPC (ox-PAPC) inhibited secretion of ApoE in U251 cells, a human astrocyte cell line, and this effect was ameliorated in the presence of peptide Ac-hE18A-NH2. The peptide also increased Aβ42 uptake in a cell line of human macrophages. Peptide Ac-hE18A-NH2 attenuates the effects of oxidative stress on ApoE secretion, inhibits amyloid plaque deposition, and thus could be beneficial in the treatment of Alzheimer's disease.

Enhancement of FGF-1 release along with cytosolic proteins from rat astrocytes by hydrogen peroxide

We previously observed that the production and release of fibroblast growth factor (FGF-1) are increased in rat astrocytes during in vitro long-term culture, that FGF-1 enhances the generation of apoE-containing high density lipoproteins (apoE/HDL), and that the wound healing of brain cryoinjury delays in apoE-deficient mouse. The detail mechanism underlying these phenomena remains unknown. In this study, we examined effects of oxidative stress on release of FGF-1 from cultured rat astrocytes.The treatment of rat astrocytes with 100µM hydrogen peroxide (H2O2) for 10min enhanced FGF-1 release without inducing apoptosis. The conditioned medium prepared from the cells cultured in a fresh medium after the treatment with H2O2 had the FGF-1-like activities, which enhanced cholesterol synthesis, signalings to phosphorylate Akt and ERK, and apoE secretion. The oxidative stress induced by H2O2 enhanced the release of cytosolic proteins such as HSP70 and HSP90 in addition to FGF-1. Antioxidants such as ascorbic acid and ebselen suppressed the release of cytosolic proteins induced by H2O2 treatment. The addition of lipoproteins such as low density lipoproteins (LDL), furthermore, canceled H2O2-induced release of FGF-1 and cytosolic proteins. Proteolysis of cytosolic proteins in the H2O2-treated rat astrocytes was enhanced in the presence of exogenous trypsin, which was attenuated by the pretreatment with LDL, suggesting that H2O2 increases the permeability of the membrane of cells, which was prevented by the addition of lipoproteins. These findings suggest that oxidative stress is one of the candidates which triggers FGF-1 release from astrocytes in the brain, and that the lipid homeostasis in the cell membrane may regulate H2O2-induced release of FGF-1.

HDL inhibit endoplasmic reticulum stress by stimulating apoE and CETP secretion from lipid-loaded macrophages

The role of HDL in the modulation of endoplasmic reticulum (ER) stress in macrophage-derived foam cells is not completely understood. Therefore, we aimed to investigate whether HDL may inhibit ER stress in correlation with the secretion of apoE and CETP from lipid-loaded macrophages. To this purpose, THP-1 macrophages were loaded with lipids by incubation with human oxidized LDL (oxLDL) and then exposed to human HDL3. ER stress signaling markers, protein kinase/Jun-amino-terminal kinase (SAPK/JNK p54/p46) and eukaryotic initiation factor-2α (eIF2α), as well as the secreted apoE and CETP, were evaluated by immunoblot analysis. Out of the many different bioactive lipids of oxLDL, we tested the effect of 9-hydroxy-octadecadienoic acid (9-HODE) and 4-hydroxynonenal (4-HNE) on ER stress. Tunicamycin was used as positive control for ER stress induction. Results showed that oxLDL, 9-HODE and 4-HNE induce ER stress in human macrophages by activation of eIF-2α and SAPK/JNK (p54/p46) signaling pathways. OxLDL stimulated apoE and CETP secretion, while tunicamycin determined a reduction of the secreted apoE and CETP, both in control and lipid-loaded macrophages. The addition of HDL3 to the culture medium of tunicamycin-treated cells induced: (i) the reduction of ER stress, expressed as decreased levels of eIF-2α and SAPK/JNK, and (ii) a partial recovery of the secreted apoE and CETP levels in lipid-loaded macrophages. These data suggest a new mechanism by which HDL3 diminish ER stress and stimulate cholesterol efflux from lipid-loaded macrophages.

Protein Kinase C Controls Vesicular Transport and Secretion of Apolipoprotein E from Primary Human Macrophages

Macrophage-specific apolipoprotein E (apoE) secretion plays an important protective role in atherosclerosis. However, the precise signaling mechanisms regulating apoE secretion from primary human monocyte-derived macrophages (HMDMs) remain unclear. Here we investigate the role of protein kinase C (PKC) in regulating basal and stimulated apoE secretion from HMDMs. Treatment of HMDMs with structurally distinct pan-PKC inhibitors (calphostin C, Ro-31-8220, Go6976) and a PKC inhibitory peptide all significantly decreased apoE secretion without significantly affecting apoE mRNA or apoE protein levels. The PKC activator phorbol 12-myristate 13-acetate (PMA) stimulated apoE secretion, and both PMA-induced and apoAI-induced apoE secretion were inhibited by PKC inhibitors. PKC regulation of apoE secretion was found to be independent of the ATP binding cassette transporter ABCA1. Live cell imaging demonstrated that PKC inhibitors inhibited vesicular transport of apoE to the plasma membrane. Pharmacological or peptide inhibitor and knockdown studies indicate that classical isoforms PKCα/β and not PKCδ, -ε, -θ, or -ι/ζ isoforms regulate apoE secretion from HMDMs. The activity of myristoylated alanine-rich protein kinase C substrate (MARCKS) correlated with modulation of PKC activity in these cells, and direct peptide inhibition of MARCKS inhibited apoE secretion, implicating MARCKS as a downstream effector of PKC in apoE secretion. Comparison with other secreted proteins indicated that PKC similarly regulated secretion of matrix metalloproteinase 9 and chitinase-3-like-1 protein but differentially affected the secretion of other proteins. In conclusion, PKC regulates the secretion of apoE from primary human macrophages.

Apolipoprotein E protects astrocytes from hypoxia and glutamate-induced apoptosis

Apolipoprotein E (apoE) is predominantly synthesized by astrocytes in the brain. In this study, we investigated the role of apoE in astrocyte apoptosis. We demonstrated that apoE protects astrocytes from hypoxia-induced apoptosis in a dose-dependent manner. Glutamate release from astrocytic cultures is significantly lower from WT mice than from apoE knockout mice. Furthermore, the protective effect of apoE is mimicked by an NMDA receptor antagonist, MK-801. Finally, the apoE activator T0901317 significantly reduced the effect of glutamate-induced apoptosis of astrocytes. These results suggest that apoE protects astrocytes from hypoxia-induced apoptosis associated to NMDA receptor activation. Approaches that elevate apoE secretion in astrocytes might provide a novel strategy in the protection of neuronal ischemic injury.

Two apolipoprotein E mimetic peptides with similar cholesterol reducing properties exhibit differential atheroprotective effects in LDL-R null mice

ObjectiveWe investigated two apoE mimetic peptides with similar long-term plasma cholesterol reducing abilities for their effects on atherosclerotic lesions in Western diet-fed female LDL-receptor (LDL-R) null mice. Methods and resultsSingle doses of peptides Ac-hE18A-NH2 and mR18L were administered retro-orbitally to LDL-R null mice on Western diet and plasma cholesterol was measured at 10 min, 4 h, and 24 h post administration. Peptide mR18L and not Ac-hE18A-NH2 reduced plasma cholesterol levels significantly at 4 h post administration. However, multiple administrations (100 μg/mouse twice weekly for 8 weeks) resulted in a similar reduction in plasma cholesterol. Only the plasma from the Ac-hE18A-NH2 group had significantly reduced reactive oxygen species levels at the end of the treatment protocol. Both mR18L and Ac-hE18A-NH2 showed reduced atherosclerotic lesion areas. However, peptide Ac-hE18A-NH2 was significantly more effective in inhibiting atherosclerosis. Both peptides reduced total plaque macrophage load compared to the saline treated animals, with peptide Ac-hE18A-NH2 having a greater reduction. Incubation of HepG2 cells and THP-1 monocyte-derived macrophages with both peptides in the presence of oxidized phospholipid showed that Ac-hE18A-NH2 promotes the secretion of apoE from the cells whereas mR18L does not. ConclusionsDespite similar reductions in plasma cholesterol levels, Ac-hE18A-NH2 was more effective in inhibiting lesions than mR18L, possibly due to its ability to promote the secretion of apoE from hepatocytes and macrophages.

Physiological role of hepatic NPC1L1 in human cholesterol and lipoprotein metabolism: New perspectives and open questions

The importance of the cholesterol transporter Niemann-Pick C1 like protein 1 (NPC1L1)in the small intestine for efficient absorption of cholesterol is well documented and its physiological importance is clear (1). Similarly, the drug ezetimibe (Zetia™), which blocks function of this transporter, reduces plasma cholesterol, primarily LDL, by dramatically reducing intestinal absorption of dietary and biliary cholesterol (2). There are controversies about the exact mechanism by which NPC1L1 functions: whether the protein is on the apical cell membrane or intracellular vesicles or both (3–5), and whether ezetimibe directly blocks NPC1L1 or interrupts its association with other proteins in the sterol absorption and transport pathway (6–8). Nevertheless, it is now accepted that this protein is the key player in sterol absorption, although other transporters may play ancillary roles (9, 10).

Modulation of lipid metabolism with the overexpression of NPC1L1 in mouse liver

Niemann-Pick C1-like 1 protein (NPC1L1), a transporter crucial in intestinal cholesterol absorption, is expressed in human liver but not in murine liver. To elucidate the role of hepatic NPC1L1 on lipid metabolism, we overexpressed NPC1L1 in murine liver utilizing adenovirus-mediated gene transfer. C57BL/6 mice, fed on normal chow with or without ezetimibe, were injected with NPC1L1 adenovirus (L1-mice) or control virus (Null-mice), and lipid analyses were performed five days after the injection. The plasma cholesterol levels increased in L1-mice, and FPLC analyses revealed increased cholesterol contents in large HDL lipoprotein fractions. These fractions, which showed α-mobility on agarose electrophoresis, were rich in apoE and free cholesterol. These lipoprotein changes were partially inhibited by ezetimibe treatment and were not observed in apoE-deficient mice. In addition, plasma and VLDL triglyceride (TG) levels decreased in L1-mice. The expression of microsomal triglyceride transfer protein (MTP) was markedly decreased in L1-mice, accompanied by the reduced protein levels of forkhead box protein O1 (FoxO1). These changes were not observed in mice with increased hepatic de novo cholesterol synthesis. These data demonstrate that cholesterol absorbed through NPC1L1 plays a distinct role in cellular and plasma lipid metabolism, such as the appearance of apoE-rich lipoproteins and the diminished VLDL-TG secretion.

Cholesterol accumulation inhibits ER to Golgi transport and protein secretion: studies of apolipoprotein E and VSVGt

Cholesterol excess is typical of various diseases including atherosclerosis. We have investigated whether cholesterol accumulation in the ER (endoplasmic reticulum) can inhibit exit of vesicular cargo and secretion of proteins by studying apoE (apolipoprotein E), a significant glycoprotein in human health and disease. CHO (Chinese hamster ovary) cells expressing human apoE under a cholesterol-independent promoter incubated with cholesterol-cyclodextrin complexes showed increased levels of cellular free and esterified cholesterol, inhibition of SREBP-2 (sterol-regulatory-element-binding protein 2) processing, and a mild induction of ER stress, indicating significant accumulation of cholesterol in the ER. Secretion of apoE was markedly inhibited by cholesterol accumulation, and similar effects were observed in cells enriched with lipoprotein-derived cholesterol and in primary human macrophages. Removal of excess cholesterol by a cyclodextrin vehicle restored apoE secretion, indicating that the transport defect was reversible. That cholesterol impaired protein trafficking was supported by the cellular accumulation of less sialylated apoE glycoforms, and by direct visualization of altered ER to Golgi transport of thermo-reversible VSVG (vesicular stomatitis virus glycoprotein) linked to GFP (green fluorescent protein). We conclude that intracellular accumulation of cholesterol in the ER reversibly inhibits protein transport and secretion. Strategies to correct ER cholesterol may restore homoeostatic processes and intracellular protein transport in conditions characterized by cholesterol excess.

Sage weed (Salvia plebeia) extract antagonizes foam cell formation and promotes cholesterol efflux in murine macrophages

Lipid-laden peripheral tissue cells release cholesterol to an extracellular acceptor such as high-density lipoprotein (HDL). Foam cells are formed at the first stage of atherosclerosis development. This study investigated whether sage weed (Salvia plebeia) extract (SWE) influences cholesterol handling of J774A1 murine macrophages. A murine macrophage cell line, J774A1, was used in this study. Oxidized low-density lipoproteins (LDL) treatment was used for foam cell formation, which was confirmed using Oil red O staining. The oxidized LDL uptake and cholesterol efflux from lipid-laden foam cell-associated proteins were detected by western blot analysis. Also, transcriptional levels of these associated genes were examined using reverse transcription-PCR. Also, cholesterol efflux was measured using NBD-cholesterol efflux assay. Non-toxic SWE at ≥10 µg/ml attenuated scavenger receptor (SR)-B1 expression of macrophages induced by oxidized LDL for 6 h, which was achieved at its transcriptional levels. Consistently, SWE suppressed oxidized LDL-stimulated cellular lipid accumulation and foam cell formation due to downregulated SR-B1. SWE upregulated the protein expression and mRNA levels of ATP-binding cassette transporter A1 (ABCA1) and ATP-binding cassette transporter G1 (ABCG1) in lipid-laden foam cells, both responsible for cholesterol efflux. In addition, SWE promoted apolipoprotein E (apoE) secretion from oxidized LDL-induced foam cells. Cholesterol efflux was enhanced by ≥10 µg/ml SWE most likely through the induction of ABCA1 and ABCG1 and the secretion of apoE. Although 10 µM homoplantaginin, a compound mainly present in sage weeds, did not influence cellular expression of ABCA1 and ABCG1, it suppressed oxidized LDL-enhanced SR-B1 induction and foam cell formation. These results demonstrate that SWE antagonized oxidized LDL uptake and promoted cholesterol efflux in lipid-laden macrophages. Therefore, SWE may serve as a protective therapeutic agent against the development of atherosclerosis.

Extracellular Nucleotides Inhibit Insulin Receptor Signaling, Stimulate Autophagy and Control Lipoprotein Secretion

Hyperglycemia is associated with abnormal plasma lipoprotein metabolism and with an elevation in circulating nucleotide levels. We evaluated how extracellular nucleotides may act to perturb hepatic lipoprotein secretion. Adenosine diphosphate (ADP) (>10 µM) acts like a proteasomal inhibitor to stimulate apoB100 secretion and inhibit apoA-I secretion from human liver cells at 4 h and 24 h. ADP blocks apoA-I secretion by stimulating autophagy. The nucleotide increases cellular levels of the autophagosome marker, LC3-II, and increases co-localization of LC3 with apoA-I in punctate autophagosomes. ADP affects autophagy and apoA-I secretion through P2Y13. Overexpression of P2Y13 increases cellular LC3-II levels by ∼50% and blocks induction of apoA-I secretion. Conversely, a siRNA-induced reduction in P2Y13 protein expression of 50% causes a similar reduction in cellular LC3-II levels and a 3-fold stimulation in apoA-I secretion. P2Y13 gene silencing blocks the effects of ADP on autophagy and apoA-I secretion. A reduction in P2Y13 expression suppresses ERK1/2 phosphorylation, increases the phosphorylation of IR-β and protein kinase B (Akt) >3-fold, and blocks the inhibition of Akt phosphorylation by TNFα and ADP. Conversely, increasing P2Y13 expression significantly inhibits insulin-induced phosphorylation of insulin receptor (IR-β) and Akt, similar to that observed after treatment with ADP. Nucleotides therefore act through P2Y13, ERK1/2 and insulin receptor signaling to stimulate autophagy and affect hepatic lipoprotein secretion.