High density lipoproteins stimulate mitogen-activated protein kinases in human skin fibroblasts

Up-regulated expression of scavenger receptor class B type 1 (SR-B1) is associated with malignant behaviors and poor prognosis of breast cancer

Scavenger receptor class B type 1 (SR-B1) is an integral membrane protein that is expressed in numerous cells and tissue types. The primary role of SR-B1 is to facilitate uptake of cholesteryl esters from high-density lipoproteins (HDL) in the liver. Altered SR-B1 expression contributes to human diseases. This study assessed association of SR-B1 expression in breast tissue specimens with breast cancer development and prognosis. Tissue specimens from 30 cases of adjacent normal breast tissues, ductal carcinoma in situ (DCIS) and invasive ductal breast cancer (IDCA) were subjected to Western blot analysis, and 135 cases of DCIS and IDCA were used for quantitative immunohistochemical analysis of SR-B1 expression. The data showed that SR-B1 was significantly overexpressed in IDCA tissues compared to normal breast and DCIS tissues. SR-B1 expression was associated with pre-menopausal status, tumor size, and worse overall survival of patients. The data from this ex vivo study suggests that up-regulated SR-B1 protein expression is associated with malignant behaviors of breast cancer and that SR-B1 is an independent predictor for poor survival in breast cancer patients.

Signal transduction by HDL: agonists, receptors, and signaling cascades

Numerous epidemiologic studies revealed that high-density lipoprotein (HDL) is an important risk factor for coronary heart disease. There are several well-documented HDL functions such as reversed cholesterol transport, inhibition of inflammation, or inhibition of platelet activation that may account for the atheroprotective effects of this lipoprotein. Mechanistically, these functions are carried out by a direct interaction of HDL particle or its components with receptors localized on the cell surface followed by generation of intracellular signals. Several HDL-associated receptor ligands such as apolipoprotein A-I (apoA-I) or sphingosine-1-phosphate (S1P) have been identified in addition to HDL holoparticles, which interact with surface receptors such as ATP-binding cassette transporter A1 (ABCA1); S1P receptor types 1, 2, and 3 (S1P1, S1P2, and S1P3); or scavenger receptor type I (SR-BI) and activate intracellular signaling cascades encompassing kinases, phospholipases, trimeric and small G-proteins, and cytoskeletal proteins such as actin or junctional protein such as connexin43. In addition, depletion of plasma cell cholesterol mediated by ABCA1, ATP-binding cassette transporter G1 (ABCG1), or SR-BI was demonstrated to indirectly inhibit signaling over proinflammatory or proliferation-stimulating receptors such as Toll-like or growth factor receptors. The present review summarizes the current knowledge regarding the HDL-induced signal transduction and its relevance to athero- and cardioprotective effects as well as other physiological effects exerted by HDL.

Scavenger receptor class B type I regulates cellular cholesterol metabolism and cell signaling associated with breast cancer development

Introduction

Previous studies have identified cholesterol as an important regulator of breast cancer development. High-density lipoprotein (HDL) and its cellular receptor, the scavenger receptor class B type I (SR-BI) have both been implicated in the regulation of cellular cholesterol homeostasis, but their functions in cancer remain to be established.

Methods

In the present study, we have examined the role of HDL and SR-BI in the regulation of cellular signaling pathways in breast cancer cell lines and in the development of tumor in a mouse xenograft model.

Results

Our data show that HDL is capable of stimulating migration and can activate signal transduction pathways in the two human breast cancer cell lines, MDA-MB-231 and MCF7. Furthermore, we also show that knockdown of the HDL receptor, SR-BI, attenuates HDL-induced activation of the phosphatidylinositol 3-kinase (PI3K)/protein Kinase B (Akt) pathway in both cell lines. Additional investigations show that inhibition of the PI3K pathway, but not that of the mitogen-activated protein kinase (MAPK) pathway, could lead to a reduction in cellular proliferation in the absence of SR-BI. Importantly, whereas the knockdown of SR-BI led to decreased proliferation and migration in vitro, it also led to a significant reduction in tumor growth in vivo. Most important, we also show that pharmacological inhibition of SR-BI can attenuate signaling and lead to decreased cellular proliferation in vitro. Taken together, our data indicate that both cholesteryl ester entry via HDL-SR-BI and Akt signaling play an essential role in the regulation of cellular proliferation and migration, and, eventually, tumor growth.

Conclusions

These results identify SR-BI as a potential target for the treatment of breast cancer.

High-density lipoprotein: a novel target for antirestenosis therapy

Restenosis is an integral pathological process central to the recurrent vessel narrowing after interventional procedures. Although the mechanisms for restenosis are diverse in different pathological conditions, endothelial dysfunction, inflammation, vascular smooth muscle cell (SMC) proliferation, and myofibroblasts transition have been thought to play crucial role in the development of restenosis. Indeed, there is an inverse relationship between high-density lipoprotein (HDL) levels and risk for coronary heart disease (CHD). However, relatively studies on the direct assessment of HDL effect on restenosis are limited. In addition to involvement in the cholesterol reverse transport, many vascular protective effects of HDL, including protection of endothelium, antiinflammation, antithrombus actions, inhibition of SMC proliferation, and regulation by adventitial effects may contribute to the inhibition of restenosis, though the exact relationships between HDL and restenosis remain to be elucidated. This review summarizes the vascular protective effects of HDL, emphasizing the potential role of HDL in intimal hyperplasia and vascular remodeling, which may provide novel prophylactic and therapeutic strategies for antirestenosis.

Inhibition of mitogen-activated protein kinase Erk1/2 promotes protein degradation of ATP binding cassette transporters A1 and G1 in CHO and HuH7 cells

Signal transduction modulates expression and activity of cholesterol transporters. We recently demonstrated that the Ras/mitogen-activated protein kinase (MAPK) signaling cascade regulates protein stability of Scavenger Receptor BI (SR-BI) through Proliferator Activator Receptor (PPARα) -dependent degradation pathways. In addition, MAPK (Mek/Erk 1/2) inhibition has been shown to influence liver X receptor (LXR) -inducible ATP Binding Cassette (ABC) transporter ABCA1 expression in macrophages. Here we investigated if Ras/MAPK signaling could alter expression and activity of ABCA1 and ABCG1 in steroidogenic and hepatic cell lines. We demonstrate that in Chinese Hamster Ovary (CHO) cells and human hepatic HuH7 cells, extracellular signal-regulated kinase 1/2 (Erk1/2) inhibition reduces PPARα-inducible ABCA1 protein levels, while ectopic expression of constitutively active H-Ras, K-Ras and MAPK/Erk kinase 1 (Mek1) increases ABCA1 protein expression, respectively. Furthermore, Mek1/2 inhibitors reduce ABCG1 protein levels in ABCG1 overexpressing CHO cells (CHO-ABCG1) and human embryonic kidney 293 (HEK293) cells treated with LXR agonist. This correlates with Mek1/2 inhibition reducing ABCG1 cell surface expression and decreasing cholesterol efflux onto High Density Lipoproteins (HDL). Real Time reverse transcriptase polymerase chain reaction (RT-PCR) and protein turnover studies reveal that Mek1/2 inhibitors do not target transcriptional regulation of ABCA1 and ABCG1, but promote ABCA1 and ABCG1 protein degradation in HuH7 and CHO cells, respectively. In line with published data from mouse macrophages, blocking Mek1/2 activity upregulates ABCA1 and ABCG1 protein levels in human THP1 macrophages, indicating opposite roles for the Ras/MAPK pathway in the regulation of ABC transporter activity in macrophages compared to steroidogenic and hepatic cell types. In summary, this study suggests that Ras/MAPK signaling modulates PPARα- and LXR-dependent protein degradation pathways in a cell-specific manner to regulate the expression levels of ABCA1 and ABCG1 transporters.

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.

Ras/mitogen-activated protein kinase (MAPK) signaling modulates protein stability and cell surface expression of scavenger receptor SR-BI

The mitogen-activated protein kinase (MAPK) Erk1/2 has been implicated to modulate the activity of nuclear receptors, including peroxisome proliferator activator receptors (PPARs) and liver X receptor, to alter the ability of cells to export cholesterol. Here, we investigated if the Ras-Raf-Mek-Erk1/2 signaling cascade could affect reverse cholesterol transport via modulation of scavenger receptor class BI (SR-BI) levels. We demonstrate that in Chinese hamster ovary (CHO) and human embryonic kidney (HEK293) cells, Mek1/2 inhibition reduces PPARα-inducible SR-BI protein expression and activity, as judged by reduced efflux onto high density lipoprotein (HDL). Ectopic expression of constitutively active H-Ras and Mek1 increases SR-BI protein levels, which correlates with elevated PPARα Ser-21 phosphorylation and increased cholesterol efflux. In contrast, SR-BI levels are insensitive to Mek1/2 inhibitors in PPARα-depleted cells. Most strikingly, Mek1/2 inhibition promotes SR-BI degradation in SR-BI-overexpressing CHO cells and human HuH7 hepatocytes, which is associated with reduced uptake of radiolabeled and 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyane-labeled HDL. Loss of Mek1/2 kinase activity reduces SR-BI expression in the presence of bafilomycin, an inhibitor of lysosomal degradation, indicating down-regulation of SR-BI via proteasomal pathways. In conclusion, Mek1/2 inhibition enhances the PPARα-dependent degradation of SR-BI in hepatocytes.

Role of cholesterol in the development and progression of breast cancer

Diet and obesity are important risk factors for cancer development. Many studies have suggested an important role for several dietary nutrients in the progression and development of breast cancer. However, few studies have specifically addressed the role of components of a Western diet as important factors involved in breast cancer initiation and progression. The present study examined the role of cholesterol in the regulation of tumor progression in a mouse model of mammary tumor formation. The results suggest that cholesterol accelerates and enhances tumor formation. In addition, tumors were more aggressive, and tumor angiogenesis was enhanced. Metabolism of cholesterol was also examined in this mouse model. It was observed that plasma cholesterol levels were reduced during tumor development but not prior to its initiation. These data provide new evidence for an increased utilization of cholesterol by tumors and for its role in tumor formation. Taken together, these results imply that an increase in plasma cholesterol levels accelerates the development of tumors and exacerbates their aggressiveness.

Critical role of scavenger receptor-BI–expressing bone marrow–derived endothelial progenitor cells in the attenuation of allograft vasculopathy after human apo A-I transfer

Allograft vasculopathy is the leading cause of death in patients with heart transplantation. Accelerated endothelial regeneration mediated by enhanced endothelial progenitor cell (EPC) incorporation may attenuate the development of allograft vasculopathy. We investigated the hypothesis that modulation of EPC biology and attenuation of allograft vasculopathy by increased high-density lipoprotein cholesterol after human apo A-I (AdA-I) transfer requires scavenger receptor (SR)–BI expression in bone marrow–derived EPCs. After AdA-I transfer, the number of circulating EPCs increased 2.0-fold (P < .001) at different time points in C57BL/6 mice transplanted with SR-BI+/+ bone marrow but remained unaltered in mice with SR-BI−/− bone marrow. The effect of high-density lipoprotein on EPC migration in vitro requires signaling via SR-BI and extracellular signal-regulated kinases and is dependent on increased nitric oxide (NO) production in EPCs. Human apo A-I transfer 2 weeks before paratopic artery transplantation reduced intimal area at day 21 3.7-fold (P < .001) in mice with SR-BI+/+ bone marrow but had no effect in mice with SR-BI−/− bone marrow. AdA-I transfer potently stimulated EPC incorporation and accelerated endothelial regeneration in chimeric SR-BI+/+ mice but not in chimeric SR-BI−/− mice. In conclusion, human apo A-I transfer accelerates endothelial regeneration mediated via SR-BI expressing bone marrow–derived EPCs, thereby preventing allograft vasculopathy.

Metabolism of high density lipoproteins in liver cancer

Liver plays a vital role in the production and catabolism of plasma lipoproteins. It depends on the integrity of cellular function of liver, which ensures homeostasis of lipid and lipoprotein metabolism. When liver cancer occurs these processes are impaired and high-density lipoproteins are changed.

Lipoproteins and mitogen-activated protein kinase signaling: a role in atherogenesis?

PURPOSE OF REVIEW

Lipoproteins play a critical role in the development of atherosclerosis, which might result partly from their capacity to induce specific intracellular signaling pathways. The goal of this review is to summarize the signaling properties of lipoproteins, in particular, their capacity to induce activation of mitogen-activated protein kinase pathways and the resulting modulation of cellular responses in blood vessel cells.

RECENT FINDINGS

Lipoproteins activate the extracellular signal-regulated kinase and p38 mitogen-activated protein kinase pathways in all blood vessel cell types. This may require lipoprotein docking to scavenger receptor B1, allowing transfer of cholesterol and sphingosine-1-phosphate to plasma membranes. Subsequent propagation of the signals probably requires the stimulation of G protein-coupled receptors, followed by the transactivation of receptor tyrosine kinases. Lipoprotein-induced extracellular signal-regulated kinase activity favors cell proliferation, whereas lipoprotein-induced p38 mitogen-activated protein kinase activity leads to cell hyperplasia and promotes cell migration. Some signaling pathways and cellular effects induced by lipoproteins have been observed in atherosclerotic plaques and therefore represent potential targets for the development of anti-atherosclerotic drugs.

SUMMARY

The main blood vessel cell types have the capacity to activate protein kinase pathways in the presence of lipoproteins. This induces cell proliferation, hyperplasia and migration, known to be dysregulated in atherosclerotic lesions.

Follicular fluid high density lipoprotein-associated sphingosine 1-phosphate is a novel mediator of ovarian angiogenesis

Angiogenesis plays an important role in the development of the ovarian follicle and its subsequent transition into the corpus luteum. Accordingly, follicular fluid is a rich source of mitogenic and angiogenic factors such as basic fibroblast growth factor and vascular endothelial growth factor secreted by granulosa cells. In the present study, we show that follicular fluid deprived of basic fibroblast growth factor or vascular endothelial growth factor by means of thermal denaturation or antibody neutralization retains its capacity to stimulate endothelial proliferation and angiogenesis. Mass spectrometric analysis of chromatographic fractions stimulating endothelial growth obtained from follicular fluid revealed that the heat-stable mitogenic activity is identical with the subfraction alpha of high density lipoproteins purified from follicular fluid (FF-HDL). Further investigations demonstrated that sphingosine 1-phosphate (S1P), one of the lysophospholipids associated with HDL, accounts for the capacity of this lipoprotein to stimulate endothelial growth and the formation of new vessels. Activation of mitogen-activated protein kinase (p42/44(ERK1/2)), protein kinase C, and protein kinase Akt represent signaling pathways utilized by FF-HDL and S1P to induce endothelial proliferation and angiogenesis. We conclude that FF-HDL represents a novel mitogenic and angiogenic factor present in follicular fluid and that S1P is one of the FF-HDL lipid components accounting for this activity.

Annexin A6 stimulates the membrane recruitment of p120GAP to modulate Ras and Raf-1 activity

Annexin A6 is a calcium-dependent membrane-binding protein that interacts with signalling proteins, including the GTPase-activating protein p120GAP, one of the most important inactivators of Ras. Since we have demonstrated that annexin A6 inhibits EGF- and TPA-induced Ras signalling, we investigated whether modulation of Ras activity by annexin A6 was mediated via altered subcellular localization of p120GAP. First, we exploited our observation that high-density lipoproteins (HDL) can activate the Ras/MAP kinase pathway. Expression of annexin A6 caused a significant reduction in HDL-induced activation of Ras and Raf-1. Annexin A6 promoted membrane binding of p120GAP in vitro, and plasma membrane targeting of p120GAP in living cells, both in a Ca(2+)-dependent manner, which is consistent with annexin A6 promoting the Ca(2+)-dependent assembly of p120GAP-Ras at the plasma membrane. We then extended these studies to other cell types and stimuli. Expression of annexin A6 in A431 cells reduced, while RNAi-mediated suppression of annexin A6 in HeLa cells enhanced EGF-induced Ras and Erk activation. Importantly, the enhancement of Ras activation following RNAi-mediated reduction in p120GAP levels was more marked in annexin A6-expressing A431 cells than controls, indicating that the effect of annexin A6 on Ras was mediated via p120GAP. Finally, we demonstrated that annexin A6 promotes plasma membrane targeting of p120GAP in A431 cells in response to a variety of stimuli, resulting in colocalization with H-Ras. These findings demonstrate an important role for annexin A6 in regulating plasma membrane localization of p120GAP and hence Ras activity.

Inhibition of H-Ras and MAPK is compensated by PKC-dependent pathways in annexin A6 expressing cells

High-density lipoprotein (HDL)-induced activation of the Ras/MAPK pathway can be mediated by protein kinase C (PKC)-dependent and independent pathways. Although both pathways co-exist in cells, we showed that binding of HDL to scavenger receptor BI (SR-BI) in CHO cells activates Ras and MAPK in a PKC-independent manner. We have recently identified that HDL-induced activation of Ras and Raf-1 is reduced in annexin A6 expressing CHO cells (CHOanx6). In the present study we demonstrate that despite the loss of Ras and Raf-1 activity, HDL induces MAPK phosphorylation in CHOanx6 cells. Since annexin A6 is a PKCalpha-binding protein we therefore investigated the possible involvement of PKC in HDL-induced Ras and MAPK activation in CHOanx6 cells. Taken together our findings demonstrate that HDL-induced H-Ras and MAPK activation is PKC-dependent in cells expressing annexin A6 to compensate for the loss of PKC-independent activation of H-Ras and MAPK.

Human scavenger receptor class B type II (SR-BII) and cellular cholesterol efflux

Although studies in recombinant cells indicate that scavenger receptor class B, type I (SR-BI) can promote cholesterol efflux, investigations in transgenic mice overexpressing or deficient in SR-BI endorse its physiological function as selectively sequestering cholesteryl esters from high-density lipoproteins (HDLs). Less clear is the role of SR-BII, a splice variant of the SR-B gene that differs only in the C-terminal cytoplasmic domain. Here, we identify several putative signalling motifs in the C-terminus of human SR-BII, which are absent from SR-BI, and hypothesize that these motifs interact with signalling molecules to mobilize stored cholesteryl esters and/or promote the efflux of intracellular free cholesterol. 'Pull-down' assays using a panel of tagged SH3 (Src homology 3) domains showed that cytoplasmic SR-BII, but not cytoplasmic SR-BI, bound the SH3 domain of phospholipase C-gamma1; this interaction was not, however, detected under more physiological conditions. Specific anti-peptide antisera identified SR-BII in human monocyte/macrophage THP-1 cells and, in recombinant cells, revealed receptor localization to caveolae, a plasma membrane microdomain that concentrates signal-transducer molecules and acts as a conduit for cholesterol flux between cells and lipoproteins. Consistent with its caveolar localization, expression of human SR-BII in recombinant Chinese hamster ovary cells (CHO-SR-BII) was associated with increased HDL-mediated cholesterol efflux. Nevertheless, when CHO-SR-BII cells were pre-loaded with cholesteryl [(3)H]oleate and incubated with HDL, cholesteryl ester stores were not reduced compared with control cells. We conclude that although human SR-BII is expressed by macrophages, contains cytoplasmic signalling motifs and localizes to caveolae, its ability to stimulate cholesterol efflux does not reflect enhanced hydrolysis of stored cholesteryl esters.

Apolipoprotein A-I activates cellular cAMP signaling through the ABCA1 transporter

It has been suggested that the signal transduction pathway initiated by apoA-I activates key proteins involved in cellular lipid efflux. We investigated apoA-I-mediated cAMP signaling in cultured human fibroblasts induced with (22R)-hydroxycholesterol and 9-cis-retinoic acid (stimulated cells). Treatment of stimulated fibroblasts with apoA-I for short periods of time (<or=45 min) increased ATP binding cassette A1 (ABCA1) phosphorylation in a concentration-dependent manner. Concomitantly, apoA-I increased the intracellular level of cAMP in a concentration- and time-dependent manner. The maximal cAMP level was reached within 10 min at 10 microg/ml apoA-I representing a 1-fold increase. The ability of apoA-I to mediate cAMP production was only observed in stimulated fibroblasts. Furthermore, overexpression of ABCA1 in Chinese hamster ovary cells resulted in a 1.5-fold increase in apoA-I-mediated cAMP accumulation as compared with untransfected cells. In contrast, forskolin increased cAMP production significantly in unstimulated fibroblasts as well as in untransfected Chinese hamster ovary cells. Pharmacological inhibition of protein kinase A (H89) completely blocked apoA-I-mediated ABCA1 phosphorylation. Naturally occurring mutations of ABCA1 associated with Tangier disease (C1477R, 2203X, and 2145X) severely reduced apoA-I-mediated cAMP production, ABCA1 phosphorylation, (125)I-apoA-I binding, and lipid efflux, without affecting forskolin-mediated cAMP elevation. In contrast, the protein kinase A catalytic subunit was able to phosphorylate ABCA1 similarly from mutant and normal cell lines in vitro. Together, our results indicate that apoA-I activates ABCA1 phosphorylation through the cAMP/protein kinase A-dependent pathway, apoA-I-mediated cAMP production required high level expression of functional ABCA1, and Tangier disease mutants have defective apoA-I-mediated cAMP signaling. These findings suggest that apoA-I may activate cAMP signaling through G protein-coupled ABCA1 transporter.

Apolipoprotein A-I activates protein kinase C alpha signaling to phosphorylate and stabilize ATP binding cassette transporter A1 for the high density lipoprotein assembly

ATP-binding cassette transporter A1 (ABCA1) plays an essential role in the helical apolipoprotein-mediated assembly of high density lipoprotein, and the apolipoporteins stabilize ABCA1 against calpain-mediated degradation during the reaction ((2002) J. Biol. Chem. 277, 22426-22429). Protein kinase C (PKC) inhibitors suppressed both ABCA1 stabilization and cellular lipid release mediated by apolipoprotein A-I (apoA-I) but not ABCA1 increase by calpain inhibitors. The increase of ABCA1 and the cellular lipid release by apoA-I were both suppressed by a phosphatidylcholine phospholipase C (PC-PLC) inhibitor but not by the inhibitors of phosphatidylinositol-PLC and phosphatidylinositol 3-kinase. A protein phosphatase inhibitor further enhanced the ABCA1 increase by apoA-I. Biochemical and microscopic evidence indicated that apoA-I activated PKC alpha, and phosphorylation of ABCA1 was directly demonstrated by apoA-I via PKC. Finally, digestion of sphingomyelin increased ABCA1, and a PC-PLC inhibitor suppressed it. We conclude that apoA-I activates PKC alpha by PC-PLC-mediated generation of diacylglycerol initiated by the removal of cellular sphingomyelin ((2002) J. Biol. Chem. 277, 44709-44714), and subsequently phosphorylates and stabilizes ABCA1.

Involvement of Cdc42 signaling in apoA-I-induced cholesterol efflux

Cholesterol efflux, an important mechanism by which high density lipoproteins (HDL) protect against atherosclerosis, is initiated by docking of apolipoprotein A-I (apoA-I), a major HDL protein, to specific binding sites followed by activation of ATP-binding cassette transporter A1 (ABCA1) and translocation of cholesterol from intracellular compartments to the exofacial monolayer of the plasma membrane where it is accessible to HDL. In this report, we investigated potential signal transduction pathways that may link apoA-I binding to cholesterol translocation to the plasma membrane and cholesterol efflux. By using pull-down assays we found that apoA-I substantially increased the amount of activated Cdc42, Rac1, and Rho in human fibroblasts. Moreover, apoA-I induced actin polymerization, which is known to be controlled by Rho family G proteins. Inhibition of Cdc42 and Rac1 with Clostridium difficile toxin B inhibited apoA-I-induced cholesterol efflux, whereas inhibition of Rho with Clostridium botulinum C3-exoenzyme exerted opposite effects. Adenoviral expression of a Cdc42(T17N) dominant negative mutant substantially reduced apoA-I-induced cholesterol efflux, whereas dominant negative Rac1(T17N) had no effect. We further found that two downstream effectors of Cdc42/Rac1 signaling, c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK), are activated by apoA-I. Pharmacological inhibition of JNK but not p38 MAPK decreased apoA-I-induced cholesterol efflux, whereas anisomycin and hydrogen peroxide, two direct JNK activators, could partially substitute for apoA-I in its ability to induce cholesterol efflux. These results for the first time demonstrate activation of Rho family G proteins and stress kinases by apoA-I and implicate the involvement of Cdc42 and JNK in the apoA-I-induced cholesterol efflux.

Experimental and interventional dietary study in humans on the role of HDL fatty acid composition in PGI2 release and Cox-2 expression by VSMC

BACKGROUND

High-density lipoproteins (HDLs) induce prostacyclin (PGI2) release in vascular smooth muscle cells (VSMCs) by up-regulation of cyclooxygenase-2 (Cox-2). Our goal was to analyze the role of human HDL lipid moiety on Cox-2-dependent PGI2 synthesis in human VSMCs and to assess the impact that the intake of diets with different fatty acid composition exert on HDL-induced PGI2 release.

MATERIALS AND METHODS

Human VSMCs were treated with HDL or fatty acids in the presence or absence of different cell signalling inhibitors and PGI2 (by enzyme immunoassay) and Cox-2 protein levels (by Western blot) were analyzed. High-density lipoproteins were obtained from a plasma pool or from plasma of 12 volunteers subjected to a longitudinal dietary interventional study of three consecutive diets periods enriched in monounsaturated fatty acids (MUFAs), polyunsaturated fatty acids n-6 (PUFA n-6) or n-3 (PUFA n-3).

RESULTS

High-density lipoprotein delipidation attenuated the effect of HDL on both PGI2 synthesis and Cox-2 up-regulation, while arachidonic acid (AA) but not other fatty acids mimicked the effects of HDL. Arachidonic acid induced PGI2 synthesis and Cox-2 expression through similar mechanisms to those activated by HDL [pertussis toxin-sensitive G proteins, p42/44 mitogen-activated protein kinase (MAPK), p38MAPK, and c-Jun N-terminal kinase-1 (JNK-1) pathways]. Finally, we observed that HDL from the PUFA n-3 dietary period induced lower PGI2 release than that from the PUFA n-6 period (64% vs. 100%).

CONCLUSIONS

Our results suggest that lipid moiety modulates HDL-induced PGI2 release/Cox-2 up-regulation in human VSMCs, and that changes in fatty acids as accomplished with the diet can modulate vascular PGI2 homeostasis.

High density lipoprotein-induced signaling of the MAPK pathway involves scavenger receptor type BI-mediated activation of Ras

High density lipoprotein (HDL) stimulates multiple signaling pathways. HDL-induced activation of the mitogen-activated protein kinase (MAPK) pathway can be mediated by protein kinase C (PKC) and/or pertussis toxin-sensitive G-proteins. Although HDL-induced activation of MAPK involves Raf-1, Mek, and Erk1/2, the upstream contribution of p21(ras) (Ras) on the activation of Raf-1 and MAPK remains elusive. Here we examine the effect of HDL on Ras activity and demonstrate that HDL induces PKC-independent activation of Ras that is completely blocked by pertussis toxin, thus implicating heterotrimeric G-proteins. In addition, the HDL-induced activation of Ras is inhibited by a neutralizing antibody against scavenger receptor type BI. We conclude that the binding of HDL to scavenger receptor type BI activates Ras in a PKC-independent manner with subsequent induction of the MAPK signaling cascade.

High density lipoprotein-induced endothelial nitric-oxide synthase activation is mediated by Akt and MAP kinases

High density lipoprotein (HDL) activates endothelial nitric-oxide synthase (eNOS), leading to increased production of the antiatherogenic molecule NO. A variety of stimuli regulate eNOS activity through signaling pathways involving Akt kinase and/or mitogen-activated protein (MAP) kinase. In the present study, we investigated the role of kinase cascades in HDL-induced eNOS stimulation in cultured endothelial cells and COS M6 cells transfected with eNOS and the HDL receptor, scavenger receptor B-I. HDL (10-50 microg/ml, 20 min) caused eNOS phosphorylation at Ser-1179, and dominant negative Akt inhibited both HDL-mediated phosphorylation and activation of the enzyme. Phosphoinositide 3-kinase (PI3 kinase) inhibition or dominant negative PI3 kinase also blocked the phosphorylation and activation of eNOS by HDL. Studies with genistein and PP2 showed that the nonreceptor tyrosine kinase, Src, is an upstream stimulator of the PI3 kinase-Akt pathway in this paradigm. In addition, HDL activated MAP kinase through PI3 kinase, and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase inhibition fully attenuated eNOS stimulation by HDL without affecting Akt or eNOS Ser-1179 phosphorylation. Conversely, dominant negative Akt did not alter HDL-induced MAP kinase activation. These results indicate that HDL stimulates eNOS through common upstream, Src-mediated signaling, which leads to parallel activation of Akt and MAP kinases and their resultant independent modulation of the enzyme.

Contribution of HDL-apolipoproteins to the inhibition of low density lipoprotein oxidation and lipid accumulation in macrophages

High-density lipoprotein (HDL) is known as a protective factor against atherosclerosis. However, whether HDL-apolipoproteins (apo-HDL) contribute to the protection in arterial cells remains unclear. The localization patterns of human apolipoproteins in atherosclerotic arteries were determined using immunohistochemical examination. The results indicate that several apolipoproteins are retained in component cells of the coronary artery walls. To elucidate the possible roles of apo-HDL in the protection of atherosclerotic lesion formation, we investigated the effects of apo-HDL on the formation of conjugated diene (CD) in a cell-free system and thiobarbituric acid-reactive substances (TBARS) in the medium of a macrophage-mediated LDL oxidation system. The results showed that apo-HDL significantly exerted an inhibitory effect on LDL lipid oxidation in vitro. In addition, apo-HDL decreased cholesterol influx but enhanced cholesterol efflux from J774 macrophages in a dose-dependent manner. These results are consistent with the notion that there is reduced intracellular lipid accumulation in apo-HDL treated macrophages. These data provide a direct evidence for apo-HDL in protecting LDL from oxidative modification and in reducing the accumulation of cholesterol and lipid droplets by J774 macrophages.

Protein kinase A site-specific phosphorylation regulates ATP-binding cassette A1 (ABCA1)-mediated phospholipid efflux

ATP-binding cassette A1 (ABCA1) is a key mediator of cholesterol and phospholipid efflux to apolipoprotein particles. We show that ABCA1 is a constitutively phosphorylated protein in both RAW macrophages and in a human embryonic kidney cell line expressing ABCA1. Furthermore, we demonstrate that phosphorylation of ABCA1 is mediated by protein kinase A (PKA) or a PKA-like kinase in vivo. Through site-directed mutagenesis studies of consensus PKA phosphorylation sites and in vitro PKA kinase assays, we show that Ser-1042 and Ser-2054, located in the nucleotide binding domains of ABCA1, are major phosphorylation sites for PKA. ApoA-I-dependent phospholipid efflux was decreased significantly by mutation of Ser-2054 alone and Ser-1042/Ser-2054 but was not significantly impaired with Ser-1042 alone. The mechanism by which ABCA1 phosphorylation affected ApoA-I-dependent phospholipid efflux did not involve either alterations in ApoA-I binding or changes in ABCA1 protein stability. These studies demonstrate a novel serine (Ser-2054) on the ABCA1 protein crucial for PKA phosphorylation and for regulation of ABCA1 transporter activity.

HDL and arteriosclerosis: beyond reverse cholesterol transport

The inverse correlation between serum levels of high density lipoprotein (HDL) cholesterol and the risk of coronary heart disease, the protection of susceptible animals from atherosclerosis by transgenic manipulation of HDL metabolism, and several potentially anti-atherogenic in vitro-properties have made HDL metabolism an interesting target for pharmacological intervention in atheroslcerosis. We have previously reviewed the concept of reverse cholesterol transport, which describes both the metabolism and the classic anti-atherogenic function of HDL (Arterioscler. Thromb. Vasc. Biol. 20 2001 13). We here summarize the current understanding of additional biological, potentially anti-atherogenic properties of HDL. HDL inhibits the chemotaxis of monocytes, the adhesion of leukocytes to the endothelium, endothelial dysfunction and apoptosis, LDL oxidation, complement activation, platelet activation and factor X activation but also stimulates the proliferation of endothelial cells and smooth muscle cells, the synthesis of prostacyclin and natriuretic peptide C in endothelial cells, and the activation of proteins C and S. These anti-inflammatory, anti-oxidative, anti-aggregatory, anti-coagulant, and pro-fibrinolytic activities are exerted by different components of HDL, namley apolipoproteins, enzymes, and even specific phospholipids. This complexity further emphasizes that changes in the functionality of HDL rather than changes of plasma HDL-cholesterol levels determine the anti-atherogenicity of therapeutic alterations of HDL metabolism.

Suppression of endothelial cell apoptosis by high density lipoproteins (HDL) and HDL-associated lysosphingolipids

Apoptotic cell death following injury of vascular endothelium is assumed to play an important role in the pathogenesis of atherosclerosis. In this report, we demonstrate that high density lipoproteins (HDL), a major anti-atherogenic lipoprotein fraction, protect endothelial cells against growth factor deprivation-induced apoptosis. HDL blocked the mitochondrial pathway of apoptosis by inhibiting dissipation of mitochondrial potential (Deltapsi(m)), generation of reactive oxygen species, and release of cytochrome c into the cytoplasm. As a consequence, HDL prevented activation of caspases 9 and 3 and apoptotic alterations of the plasma membrane such as increase of permeability and translocation of phosphatidylserine. Treatment of endothelial cells with HDL induced activation of the protein kinase Akt, an ubiquitous transducer of anti-apoptotic signals, and led to phosphorylation of BAD, a major Akt substrate. Suppression of Akt activity both by wortmannin and LY-294002 or by a dominant negative Akt mutant abolished the anti-apoptotic effect of HDL. Two bioactive lysosphingolipids present in HDL particles, sphingosylphosphorylcholine and lysosulfatide, fully mimicked the survival effect of HDL by blocking the mitochondrial pathway of apoptosis and potently activating Akt. In conclusion, the present study identifies HDL as a carrier of endogenous endothelial survival factors and suggests that inhibition of endothelial apoptosis by HDL-associated lysosphingolipids may represent an important and novel aspect of the anti-atherogenic activity of HDL.

Intracellular cholesterol and phospholipid trafficking: comparable mechanisms in macrophages and neuronal cells

During the past ten years considerable evidences have accumulated that in addition to monocytes/macrophages, that are implicated in innate immunity and atherogenesis, neuronal cells also exhibit an extensive cellular metabolism. The present study focuses on the major protein players that establish cellular distribution of cholesterol and phospholipids. Evidences are provided that neuronal cells and monocytes/macrophages are equipped with comparable intracellular lipid trafficking mechanisms. Selected examples are presented that trafficking dysfunctions lead to disease development, such as Tangier disease and Niemann-Pick disease type C, or contribute to the pathogenesis of diseases such as Alzheimer disease and atherosclerosis.

Apolipoprotein A-I stimulates the transport of intracellular cholesterol to cell-surface cholesterol-rich domains (caveolae)

We have studied the effect of lipid-free human plasma apolipoprotein A-I (apoA-I) on the transport of newly synthesized cholesterol to cell-surface cholesterol-rich domains, which in human skin fibroblasts are mainly represented by caveolae. Changes in transport of newly synthesized cholesterol were assessed after labelling cells with [(14)C]acetate at 15 degrees C and warming cells to permit the transfer of cholesterol, followed by the selective oxidation of cholesterol in cholesterol-rich domains (caveolae) in the plasma membrane before their partial purification. ApoA-I, but not BSA added in an equimolar concentration, enhanced the transport of cholesterol to the caveolae up to 5-fold in a dose- and time-dependent manner. The effect of apoA-I on cholesterol transport exceeded its effect on cholesterol efflux, resulting in an accumulation of intracellular cholesterol in caveolae. Methyl-beta-cyclodextrin, added at a concentration promoting cholesterol efflux to the same extent as apoA-I, also stimulated cholesterol trafficking, but was 3-fold less effective than apoA-I. Progesterone inhibited the transport of newly synthesized cholesterol to the caveolae. Treatment of cells with apoA-I stimulated the expression of caveolin, increasing the amount of caveolin protein and mRNA by approx. 2-fold. We conclude that apoA-I induces the transport of intracellular cholesterol to cell-surface caveolae, possibly in part through the stimulation of caveolin expression.

Influence of caveolin-1 on cellular cholesterol efflux mediated by high-density lipoproteins

Caveolin-1 is a principal structural component of caveolae membranes. These membrane microdomains participate in the regulation of signaling, transcytosis, and cholesterol homeostasis at the plasma membrane. In the present study, we determined the effect of caveolin-1 expression on cellular cholesterol efflux mediated by high-density lipoprotein (HDL). We evaluated this effect in parental NIH/3T3 cells as well as in two transformed NIH/3T3 cell lines in which caveolin-1 protein levels are dramatically downregulated. Compared with parental NIH/3T3 cells, these two transformed cell lines effluxed cholesterol more rapidly to HDL. In addition, NIH/3T3 cells harboring caveolin-1 antisense also effluxed cholesterol more rapidly to HDL. However, this effect was not due to changes in total cellular cholesterol content. We further showed that chronic HDL exposure reduced caveolin-1 protein expression in NIH/3T3 cells. HDL exposure also inhibited caveolin-1 promoter activity, suggesting a direct negative effect of HDL on caveolin-1 gene transcription. Moreover, we showed that HDL-induced downregulation of caveolin-1 prevents the uptake of oxidized low-density lipoprotein in human endothelial cells. These data suggest a novel proatherogenic role for caveolin-1, i.e., regarding the uptake and/or transcytosis of modified lipoproteins.

Decreased cellular cholesterol efflux is a common cause of familial hypoalphalipoproteinemia: role of the ABCA1 gene mutations

BACKGROUND

High density lipoproteins (HDL) are complex lipoprotein particles involved in reverse cholesterol (C) transport and are negatively associated with the risk for coronary artery disease (CAD). We have described a disorder of familial HDL deficiency (FHD) due to abnormal cellular cholesterol efflux. In the present study, we investigated cellular cholesterol efflux on skin fibroblast from 15 probands with moderate to severe hypoalphalipoproteinemia, including one subject with Tangier disease (TD). We performed family studies on eight of these probands (269 individuals) with familial hypoalphalipoproteinemia (defined as a HDL-C <5th%, and with no known cause of HDL deficiency). We have previously shown that four of our FHD patients and patients with TD have mutations at the ABC1 gene, demonstrating that FHD is a heterozygous form of TD.

METHODS

On each subject, we carried out detailed biochemical analysis and determined apoA-I-mediated cellular cholesterol efflux using 3H-cholesterol labeled skin fibroblasts from study subjects compared with controls. TD has also been associated with abnormal cellular cholesterol efflux. Cell fusion experiments with polyethylene glycol (PEG) were carried out with fibroblasts from a subject with TD and one with FHD in order to determine whether the Tangier cells can complement the FHD defect. In all subjects with a reduced cellular cholesterol efflux, exons of the ABCA1 gene were sequenced.

RESULTS

Familial forms of HDL deficiency, defined as HDL-C levels <5th percentile, are a heterogeneous group of lipoprotein disorders. A reduced cellular cholesterol efflux has been identified in eight subjects from seven kindred (7/14 or 50% of probands tested), being reduced by a mean 59% of controls (range 49-63%). In four of these subjects, a mutation at the ABCA1 gene locus was identified. In three other subjects an efflux defect was idenfified but no critical mutation at the ABCA1 gene locus has been identified. In the remaining subjects, (7/14), no efflux defect was identified. Complementation studies reveal that the FHD defect is not corrected by Tangier cells, confirming that FHD and TD represent a spectrum of the same genetic defect.

CONCLUSION

Familial hypoalphalipoproteinemia syndromes are phenotypically heterogeneous; one form is associated with abnormal cellular cholesterol efflux caused by heterozygous mutations at the ABCA1 gene, that defines familial HDL Deficiency while homozygous mutations or compound heterozygocity causes TD. Other forms are primary hypoalphalipoproteinemia of unknown cause, while the remaining cases are associated with hypertriglyceridemia with or without elevated apoB levels. We conclude that a cellular cholesterol defect is a relatively frequent cause of familial HDL deficiency and that a mutation at the ABCA1 gene can be identified in half of these patients.

Identification of a sequence of apolipoprotein A-I associated with the activation of Lecithin:Cholesterol acyltransferase

We aimed to distinguish between the effects of mutations in apoA-I on the requirements for the secondary structure and a specific amino acid sequence for lecithin:cholesterol acyltransferase (LCAT) activation. Several mutants were constructed targeting region 140-150: (i) two mutations affecting alpha-helical structure, deletion of amino acids 140-150 and substitution of Ala(143) for proline; (ii) two mutations not affecting alpha-helical structure, substitution of Val(149) for arginine and substitution of amino acids 63-73 for sequence 140-150; and (iii) a mutation in a similar region away from the target area, deletion of amino acids 63-73. All mutations affecting region 140-150 resulted in a 4-42-fold reduction in LCAT activation. Three mutations, apoA-I(Delta140-150), apoA-I(P143A), and apoA-I(140-150 --> 63-73), affected both the apparent V(max) and K(m), whereas the mutation apoA-I(R149V) affected only the V(max). The mutation apoA-I(Delta63-73) caused only a 5-fold increase in the K(m). All mutants, except apoA-I(P143A) and apoA-I(Delta63-73), were active in phospholipid binding assay. All mutants, except apoA-I(P143A), formed normal discoidal complexes with phospholipid. The mutation apoA-I(Delta63-73) caused a significant reduction in the stability of apoA-I.phospholipid complexes in denaturation experiments. Combined, our results strongly suggest that although the correct conformation and orientation of apoA-I in the complex with lipids are crucial for activation of LCAT, when these conditions are fulfilled, activation also strongly depends on the sequence that includes amino acids 140-150.