Progesterone blocks intracellular translocation of free cholesterol derived from cholesteryl ester in macrophages

Towards a Unitary Hypothesis of Alzheimer's Disease Pathogenesis

The "amyloid cascade" hypothesis of Alzheimer's disease (AD) pathogenesis invokes the accumulation in the brain of plaques (containing the amyloid-β protein precursor [AβPP] cleavage product amyloid-β [Aβ]) and tangles (containing hyperphosphorylated tau) as drivers of pathogenesis. However, the poor track record of clinical trials based on this hypothesis suggests that the accumulation of these peptides is not the only cause of AD. Here, an alternative hypothesis is proposed in which the AβPP cleavage product C99, not Aβ, is the main culprit, via its role as a regulator of cholesterol metabolism. C99, which is a cholesterol sensor, promotes the formation of mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a cholesterol-rich lipid raft-like subdomain of the ER that communicates, both physically and biochemically, with mitochondria. We propose that in early-onset AD (EOAD), MAM-localized C99 is elevated above normal levels, resulting in increased transport of cholesterol from the plasma membrane to membranes of intracellular organelles, such as ER/endosomes, thereby upregulating MAM function and driving pathology. By the same token, late-onset AD (LOAD) is triggered by any genetic variant that increases the accumulation of intracellular cholesterol that, in turn, boosts the levels of C99 and again upregulates MAM function. Thus, the functional cause of AD is upregulated MAM function that, in turn, causes the hallmark disease phenotypes, including the plaques and tangles. Accordingly, the MAM hypothesis invokes two key interrelated elements, C99 and cholesterol, that converge at the MAM to drive AD pathogenesis. From this perspective, AD is, at bottom, a lipid disorder.

Progesterone Suppresses Cholesterol Esterification in APP/PS1 mice and a cell model of Alzheimer's Disease

AIMS

Cholesteryl ester(CE), generated from the mitochondria associated membrane (MAM), is involved in the pathogenesis of Alzheimer's Disease (AD). In theory, the different neuroprotective effects of progesterone in AD are all linked to MAM, yet the effect on cholesterol esterification has not been reported. Therefore, this study was aimed to investigate the regulation of progesterone on intracerebral CE in AD models and the underlying mechanism.

METHODS

APP/PS1 mice and AD cell model induced by Aβ 25-35 were selected as the research objects. APP/PS1 mice were daily administrated intragastrically with progesterone and The Morris Water Maze test was performed to detect the learning and memory abilities. Intracellular cholesterol was measured by Cholesterol/Cholesteryl Ester Quantitation Assay. The structure of MAMs were observed with transmission electron microscopy. The expression of acyl-CoA: cholesterol acyltransferase 1 (ACAT1), ERK1/2 and p-ERK1/2 were detected with western blotting, immunohistochemistry or immunofluorescence.

RESULTS

Progesterone suppressed the accumulation of intracellular CE, shortened the length of abnormally prolonged MAM in cortex of APP/PS1 mice. Progesterone decreased the expression of ACAT1, which could be blocked by progesterone receptor membrane component 1 (PGRMC1) inhibitor AG205. The ERK1/2 pathway maybe involved in the progesterone mediated regulation of ACAT1 in AD models, rather than the PI3K/Akt and the P38 MEPK pathways.

SIGNIFICANCE

The results supported a line of evidence that progesterone regulates CE level and the structure of MAM in neurons of AD models, providing a promising treatment against AD on the dysfunction of cholesterol metabolism.

Macrophages Shed Excess Cholesterol in Unique Extracellular Structures Containing Cholesterol Microdomains

Supplemental Digital Content is available in the text.

Objective—

Cells use various mechanisms to maintain cellular cholesterol homeostasis including efflux of cholesterol from the cellular plasma membrane to cholesterol acceptors such as HDLs (high-density lipoproteins). Little is known about the transfer of cholesterol from cells into the extracellular matrix. Using a unique monoclonal antibody that detects ordered cholesterol arrays (ie, cholesterol micro[or nano]-domains), we previously identified that particles containing these cholesterol domains accumulate in the extracellular matrix during cholesterol enrichment of human monocyte-derived macrophages and are found in atherosclerotic lesions. In this study, we further investigate these deposited particles containing cholesterol microdomains and discover their unexpected morphology.

Approach and Results—

Although appearing spherical at the resolution of the conventional fluorescence microscope, super-resolution immunofluorescence and atomic force microscopy of in situ cholesterol microdomains, and immunoelectron microscopy of isolated cholesterol microdomains revealed that the microdomains are not vesicles or 3-dimensional crystals but rather appear as branching irregularly shaped deposits of varying size. These cholesterol microdomain-containing deposits are shed from the plasma membrane into the extracellular matrix.

Conclusions—

To date, research on cellular excretion of excess cholesterol has demonstrated cellular cholesterol efflux in the form of membranous vesicles and discoidal HDL particles released into the fluid-phase medium. Shedding of plasma membrane cholesterol microdomains provides an additional mechanism for cells such as macrophages to maintain plasma membrane cholesterol homeostasis. Furthermore, recognition that macrophages shed cholesterol microdomains into the extracellular matrix is important to our understanding of extracellular buildup of cholesterol in atherosclerosis.

A novel role for CRTC2 in hepatic cholesterol synthesis through SREBP-2

Cholesterol synthesis is regulated by the transcription factor sterol regulatory element binding protein 2 (SREBP-2) and its target gene 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR), which is the rate-limiting enzyme in cholesterol synthesis. Cyclic adenosine monophosphate-responsive element (CRE) binding protein-regulated transcription coactivator (CRTC) 2 is the master regulator of glucose metabolism. However, the effect of CRTC2 on cholesterol and its potential molecular mechanism remain unclear. Here, we demonstrated that CRTC2 expression and liver cholesterol content were increased in patients with high serum cholesterol levels who underwent resection of liver hemangiomas, as well as in mice fed a 4% cholesterol diet. Mice with adenovirus-mediated CRTC2 overexpression also showed elevated lipid levels in both serum and liver tissues. Intriguingly, hepatic de novo cholesterol synthesis was markedly increased under these conditions. In contrast, CRTC2 ablation in mice fed a 4% cholesterol diet (18 weeks) showed decreased lipid levels in serum and liver tissues compared with those in littermate wild-type mice. The expression of lipogenic genes (SREBP-2 and HMGCR) was consistent with hepatic CRTC2 levels. In vivo imaging showed enhanced adenovirus-mediated HMGCR-luciferase activity in adenovirus-mediated CRTC2 mouse livers; however, the activity was attenuated after mutation of CRE or sterol regulatory element sequences in the HMGCR reporter construct. The effect of CRTC2 on HMGCR in mouse livers was alleviated upon SREBP-2 knockdown. CRTC2 modulated SREBP-2 transcription by CRE binding protein, which recognizes the half-site CRE sequence in the SREBP-2 promoter. CRTC2 reduced the nuclear protein expression of forkhead box O1 and subsequently increased SREBP-2 transcription by binding insulin response element 1, rather than insulin response element 2, in the SREBP-2 promoter.

CONCLUSION

CRTC2 regulates the transcription of SREBP-2 by interfering with the recognition of insulin response element 1 in the SREBP-2 promoter by forkhead box O1, thus inducing SREBP-2/HMGCR signaling and subsequently facilitating hepatic cholesterol synthesis. (Hepatology 2017;66:481-497).

Lactobacillus gasseri [corrected] CHO-220 and inulin reduced plasma total cholesterol and low-density lipoprotein cholesterol via alteration of lipid transporters

This randomized, double-blind, placebo-controlled, and parallel-designed study was conducted to investigate the effect of a synbiotic product containing Lactobacillus gasseri [corrected] CHO-220 and inulin on lipid profiles of hypercholesterolemic men and women. Thirty-two hypercholesterolemic men and women with initial mean plasma cholesterol levels of 5.7±0.32 mmol/L were recruited for the 12-wk study. The subjects were randomly allocated to 2 groups; namely the treatment group (synbiotic product) and the control group (placebo), and each received 4 capsules of synbiotic or placebo daily. Our results showed that the mean body weight, energy, and nutrient intake of the subjects did not differ between the 2 groups over the study period. The supplementation of synbiotic reduced plasma total cholesterol and low-density lipoprotein (LDL)-cholesterol by 7.84 and 9.27%, respectively, compared with the control over 12 wk. Lipoproteins were subsequently subfractionated and characterized. The synbiotic supplementation resulted in a lower concentration of triglycerides in the very low, intermediate, low, and high-density lipoprotein particles compared with the control over 12 wk. The concentration of triglycerides in lipoproteins is positively correlated with an increased risk of atherosclerosis. Our results showed that the synbiotic might exhibit an atheropreventive characteristic. Cholesteryl ester (CE) in the high-density lipoprotein particles of the synbiotic group was also higher compared with the control, indicating greater transport of cholesterol in the form of CE to the liver for hydrolysis. This may have led to the reduced plasma total cholesterol level of the synbiotic group. The supplementation of synbiotic also reduced the concentration of CE in the LDL particles compared with the control, leading to the formation of smaller and denser particles that are more easily removed from blood. This supported the reduced LDL-cholesterol level of the synbiotic group compared with the control. Our present study showed that the synbiotic product improved plasma total- and LDL-cholesterol levels by modifying the interconnected pathways of lipid transporters. In addition, although Lactobacillus gasseri [corrected] CHO-220 could deconjugate bile, our results showed a statistically insignificant difference in the levels of conjugated, deconjugated, primary, and secondary bile acids between the synbiotic and control groups over 12 wk, indicating safety from bile-related toxicity.

Estrogen prevents cholesteryl ester accumulation in macrophages induced by the HIV protease inhibitor ritonavir

Individuals with HIV can now live long lives with drug therapy that often includes protease inhibitors such as ritonavir. Many patients, however, develop negative long-term side effects such as premature atherosclerosis. We have previously demonstrated that ritonavir treatment increases atherosclerotic lesion formation in male mice to a greater extent than in female mice. Furthermore, peripheral blood monocytes isolated from ritonavir-treated females had less cholesteryl ester accumulation. In the present study, we have investigated the molecular mechanisms by which female hormones influence cholesterol metabolism in macrophages in response to the HIV protease inhibitor ritonavir. We have utilized the human monocyte cell line, THP-1 as a model to address this question. Briefly, cells were differentiated for 72 h with 100 nM PMA to obtain a macrophage-like phenotype in the presence or absence of 1 nM 17beta-estradiol (E2), 100 nM progesterone or vehicle (0.01% ethanol). Cells were then treated with 30 ng/ml ritonavir or vehicle in the presence of aggregated LDL for 24 h. Cell extracts were harvested, and lipid or total RNA was isolated. E2 decreased the accumulation of cholesteryl esters in macrophages following ritonavir treatment. Ritonavir increased the expression of the scavenger receptor, CD36 mRNA, responsible for the uptake of LDL. Additionally, ritonavir treatment selectively increased the relative levels of PPARgamma mRNA, a transcription factor responsible for the regulation of CD36 mRNA expression. Treatment with E2, however, failed to prevent these increases at the mRNA level. E2 did, however, significantly suppress CD36 protein levels as measured by fluorescent immunocytochemistry. This data suggests that E2 modifies the expression of CD36 at the level of protein expression in monocyte-derived macrophages resulting in reduced cholesteryl ester accumulation following ritonavir treatment.

Clair RW, Rudel LL, Ghosh S. Macrophage-specific transgenic expression of cholesteryl ester hydrolase significantly reduces atherosclerosis and lesion necrosis in Ldlr mice

Accumulation of cholesteryl esters (CEs) in macrophage foam cells, central to atherosclerotic plaque formation, occurs as a result of imbalance between the cholesterol influx and efflux pathways. While the uptake, or influx, of modified lipoproteins is largely unregulated, extracellular acceptor-mediated free cholesterol (FC) efflux is rate limited by the intracellular hydrolysis of CE. We previously identified and cloned a neutral CE hydrolase (CEH) from human macrophages and demonstrated its role in cellular CE mobilization. In the present study, we examined the hypothesis that macrophage-specific overexpression of CEH in atherosclerosis-susceptible Ldlr(-/-) mice will result in reduction of diet-induced atherosclerosis. Transgenic mice overexpressing this CEH specifically in the macrophages (driven by scavenger receptor promoter/enhancer) were developed and crossed into the Ldlr(-/-) background (Ldlr(-/-)CEHTg mice). Macrophage-specific overexpression of CEH led to a significant reduction in the lesion area and cholesterol content of high-fat, high-cholesterol diet-induced atherosclerotic lesions. The lesions from Ldlr(-/-)CEHTg mice did not have increased FC, were less necrotic, and contained significantly higher numbers of viable macrophage foam cells. Higher CEH-mediated FC efflux resulted in enhanced flux of FC from macrophages to gall bladder bile and feces in vivo. These studies demonstrate that by enhancing cholesterol efflux and reverse cholesterol transport, macrophage-specific overexpression of CEH is antiatherogenic.

Differential Regulation of ATP Binding Cassette Protein A1 Expression and ApoA-I Lipidation by Niemann-Pick Type C1 in Murine Hepatocytes and Macrophages

Niemann-Pick type C1 (Npc1) protein inactivation results in lipid accumulation in late endosomes and lysosomes, leading to a defect of ATP binding cassette protein A1 (Abca1)-mediated lipid efflux to apolipoprotein A-I (apoA-I) in macrophages and fibroblasts. However, the role of Npc1 in Abca1-mediated lipid efflux to apoA-I in hepatocytes, the major cells contributing to HDL formation, is still unknown. Here we show that, whereas lipid efflux to apoA-I in Npc1-null macrophages is impaired, the lipidation of endogenously synthesized apoA-I by low density lipoprotein-derived cholesterol or de novo synthesized cholesterol or phospholipids in Npc1-null hepatocytes is significantly increased by about 1-, 3-, and 8-fold, respectively. The increased cholesterol efflux reflects a major increase of Abca1 protein in Npc1-null hepatocytes, which contrasts with the decrease observed in Npc1-null macrophages. The increased Abca1 expression is largely post-transcriptional, because Abca1 mRNA is only slightly increased and Lxr alpha mRNA is not changed, and Lxr alpha target genes are reduced. This differs from the regulation of Abcg1 expression, which is up-regulated at both mRNA and protein levels in Npc1-null cells. Abca1 protein translation rate is higher in Npc1-null hepatocytes, compared with wild type hepatocytes as measured by [(35)S]methionine incorporation, whereas there is no difference for the degradation of newly synthesized Abca1 in these two types of hepatocytes. Cathepsin D, which we recently identified as a positive modulator of Abca1, is markedly increased at both mRNA and protein levels by Npc1 inactivation in hepatocytes but not in macrophages. Consistent with this, inhibition of cathepsin D with pepstatin A reduced the Abca1 protein level in both Npc1-inactivated and WT hepatocytes. Therefore, Abca1 expression is specifically regulated in hepatocytes, where Npc1 activity modulates cathepsin D expression and Abca1 protein translation rate.

Effects of cellular cholesterol loading on macrophage foam cell lysosome acidification

Macrophages incubated with mildly oxidized low density lipoprotein (OxLDL), aggregated low density lipoprotein (AggLDL), or cholesteryl ester-rich lipid dispersions (DISPs) accumulate cholesterol in lysosomes followed by an inhibition of lysosomal cholesteryl ester (CE) hydrolysis. The variety of cholesterol-containing particles producing inhibition of hydrolysis suggests that inhibition may relate to general changes in lysosomes. Lysosome pH is a key mediator of activity and thus is a potential mechanism for lipid-induced inhibition. We investigated the effects of cholesterol accumulation on THP-1 macrophage lysosome pH. Treatment with OxLDL, AggLDL, and DISPs resulted in inhibition of the lysosome's ability to maintain an active pH and concomitant decreases in CE hydrolysis. Consistent with an overall disruption of lysosome function, exposure to OxLDL or AggLDL reduced lysosomal apolipoprotein B degradation. The lysosomal cholesterol sequestration and inactivation are not observed in cholesterol-equivalent cells loaded using acetylated low density lipoprotein (AcLDL). However, AcLDL-derived cholesterol in the presence of progesterone (to block cholesterol egression from lysosomes) inhibited lysosome acidification. Lysosome inhibition was not attributable to a decrease in the overall levels of vacuolar ATPase. However, augmentation of membrane cholesterol in isolated lysosomes inhibited vacuolar ATPase-dependent pumping of H+ ions into lysosomes. These data indicate that lysosomal cholesterol accumulation alters lysosomes in ways that could exacerbate foam cell formation and influence atherosclerotic lesion development.

Lipid imbalance in the neurological disorder, Niemann-Pick C disease

Niemann-Pick C (NPC) disease is a progressive neurological disorder in which cholesterol, gangliosides and bis-monoacylglycerol phosphate accumulate in late endosomes/lysosomes. This disease is caused by mutations in either the NPC1 or NPC2 gene. NPC1 and NPC2 are involved in egress of lipids, particularly cholesterol, from late endosomes/lysosomes but the precise functions of these proteins are not clear. An important question regarding the function of NPC proteins is: why do mutations in these ubiquitously expressed proteins have such dire consequences in the brain? This review summarizes the roles of NPC proteins in lipid homeostasis particularly in the central nervous system.

Effects of hepatic expression of the high-density lipoprotein receptor SR-BI on lipoprotein metabolism and female fertility

The etiology of human female infertility is often uncertain. The sterility of high-density lipoprotein (HDL) receptor-negative (SR-BI(-/-)) female mice suggests a link between female infertility and abnormal lipoprotein metabolism. SR-BI(-/-) mice exhibit elevated plasma total cholesterol [with normal-sized and abnormally large HDL and high unesterified to total plasma cholesterol (UC:TC) ratio]. We explored the influence of hepatic SR-BI on female fertility by inducing hepatic SR-BI expression in SR-BI(-/-) animals by adenovirus transduction or stable transgenesis. For transgenes, we used both wild-type SR-BI and a double-point mutant, Q402R/Q418R (SR-BI-RR), which is unable to bind to and mediate lipid transfer from wild-type HDL normally, but retains virtually normal lipid transport activities with low-density lipoprotein. Essentially wild-type levels of hepatic SR-BI expression in SR-BI(-/-) mice restored to nearly normal the HDL size distribution and plasma UC:TC ratio, whereas approximately 7- to 40-fold overexpression dramatically lowered plasma TC and increased biliary cholesterol secretion. In contrast, SR-BI-RR overexpression had little effect on SR-BI(+/+) mice, but in SR-BI(-/-) mice, it substantially reduced levels of abnormally large HDL and normalized the UC:TC ratio. In all cases, hepatic transgenic expression restored female fertility. Overexpression in SR-BI(-/-) mice of lecithin:cholesterol acyl transferase, which esterifies plasma HDL cholesterol, did not normalize the UC:TC ratio, probably because the abnormal HDL was a poor substrate, and did not restore fertility. Thus, hepatic SR-BI-mediated lipoprotein metabolism influences murine female fertility, raising the possibility that dyslipidemia might contribute to human female infertility and that targeting lipoprotein metabolism might complement current assisted reproductive technologies.

ApoA-I Lipidation in Primary Mouse Hepatocytes

The liver is the major site of both apolipoprotein A-I (apoA-I) synthesis and ATP-binding cassette transporter A1 (ABCA1) expression. Here, we compare the lipidation with cholesterol and phospholipid of newly synthesized human apoA-I (hapoA-I) using adenoviral vector-mediated endogenous expression or exogenously added hapoA-I in wild type and ABCA1-null hepatocytes. Hepatocytes were labeled with [3H]cholesterol (delivered with LDL or methyl-beta-cyclodextrin), [3H]mevalonate, or [3H]choline. ABCA1 deficiency decreased apoA-I phospholipidation by 80%, but acquisition of de novo synthesized and exogenous cholesterol only decreased by 40-60%. The transfer of de novo synthesized cholesterol to apoA-I was decreased at all time points, but that of exogenously delivered cholesterol was independent of ABCA1 activity at the early time points. Progesterone does not affect apoA-I synthesis or its lipidation but inhibited the early phase of apoA-I cholesterol lipidation in both wild type and ABCA1-null hepatocytes. Fast protein liquid chromatography analysis of medium lipoproteins confirmed that with ABCA1 deficiency, the proportion of secreted high density lipoprotein-associated apoA-I and cholesterol decreased by about 50%. The very low density lipoprotein (VLDL)/LDL size fraction also contained a significant level of cholesterol in ABCA1 deficiency, consistent with the result of immunoprecipitations showing the presence of lipoproteins with both apoA-I and murine apoB. ApoA-I lipidation with newly synthesized cholesterol in ABCA1-null hepatocytes was significantly decreased by brefeldin A and monensin. In conclusion, we demonstrate that: (i) whereas most hepatic phospholipidation of apoA-I is mediated by ABCA1, acquisition of cholesterol depends on active transfer from intracellular compartments by ABCA1-dependent and -independent pathways, both sensitive to progesterone and (ii) there is separate regulation of phospholipid and cholesterol lipidation of apoA-I in hepatocytes.

Effects of 25-Hydroxycholesterol on Cholesterol Esterification and Sterol Regulatory Element-binding Protein Processing Are Dissociable

The regulatory pool of cholesterol is located in the endoplasmic reticulum (ER) and is key to how mammalian cells sense and respond to changes in cellular cholesterol levels. The extent of cholesterol esterification by the ER-resident protein, acyl-coenzyme A:cholesterol acyl-transferase (ACAT), has become the standard method for monitoring cholesterol transport to the ER and is assumed to reflect the regulatory pool of ER cholesterol. The oxysterol, 25-hydroxycholesterol (25HC), is thought to trigger intracellular cholesterol transport to the ER. In support of this contention, we confirmed previous reports that 25HC activates cholesterol esterification and is a potent suppressor of the sterol regulatory element-binding protein (SREBP) pathway. Processing of the ER membrane-bound SREBP into a soluble transcription factor is controlled by cholesterol levels in the ER. In this study, we addressed whether or not cholesterol esterification necessarily reflects cholesterol movement to the cholesterol homeostatic machinery in the ER as determined by SREBP processing. We found that three agents that inhibited the ability of 25HC to induce cholesterol esterification (progesterone, nigericin, and monensin) did not have a corresponding effect on 25HC suppression of SREBP processing. Moreover, ACAT inhibition did not alter the sensitivity of SREBP processing to 25HC. Therefore, cholesterol esterification by the ER-resident protein ACAT is dissociable from cholesterol transport to the cholesterol homeostatic machinery in the ER. In light of our results, we question the security of previous work that has inferred cholesterol transport to the ER regulatory pool based solely on cholesterol esterification.

Reduced atherosclerosis in hormone-sensitive lipase transgenic mice overexpressing cholesterol acceptors

Macrophage-specific overexpression of cholesteryl ester hydrolysis in hormone-sensitive lipase transgenic (HSL Tg) female mice paradoxically increases cholesterol esterification and cholesteryl ester accumulation in macrophages, and thus susceptibility to diet-induced atherosclerosis compared to nontransgenic C57BL/6 mice. The current studies suggest that whereas increased cholesterol uptake could contribute to transgenic foam cell formation, there are no differences in cholesterol synthesis and the expression of cholesterol efflux mediators (ABCA1, ABCG1, apoE, PPARgamma, and LXRalpha) compared to wild-type macrophages. HSL Tg macrophages exhibit twofold greater efflux of cholesterol to apoA-I in vitro, suggesting the potential rate-limiting role of cholesteryl ester hydrolysis in efflux. However, macrophage cholesteryl ester levels appear to depend on the relative efficacy of alternate pathways for free cholesterol in either efflux or re-esterification. Thus, increased atherosclerosis in HSL Tg mice appears to be due to the coupling of the efficient re-esterification of excess free cholesterol to its limited removal mediated by the cholesterol acceptors in these mice. The overexpression of cholesterol acceptors in HSL-apoA-IV double-transgenic mice increases plasma HDL levels and decreases diet-induced atherosclerosis compared to HSL Tg mice, with aortic lesions reduced to sizes in nontransgenic littermates. The results in vivo are consistent with the effective efflux from HSL Tg macrophages supplemented with HDL and apoA-I in vitro, and highlight the importance of cholesterol acceptors in inhibiting atherosclerosis caused by imbalances in the cholesteryl ester cycle.

Expression of scavenger receptor BI facilitates sterol movement between the plasma membrane and the endoplasmic reticulum in macrophages

Scavenger receptor BI influences multiple aspects of cellular sterol metabolism. In this series of studies, we evaluated the effect of scavenger receptor BI expression on the distribution and movement of sterol between the plasma membrane and the endoplasmic reticulum in macrophages, by comparing control J774 cells to J774 cells in which SR-BI expression was constitutively increased 3-fold. J774 cells with increased expression of SR-BI (J774-SRBI cells) esterified plasma membrane cholesterol more rapidly as compared to control cells. The esterification of endogenously synthesized cholesterol was also more rapid in cells with increased SR-BI expression; this could be partially suppressed by removing cholesterol from the plasma membrane. The increased plasma membrane sterol esterification in J774-SRBI cells was not due to increased acyl-coA:cholesterol acyltransferase activity and was observed even though J774-SRBI cells manifested a smaller free cholesterol pool in the endoplasmic reticulum. Cholesterol ester hydrolysis was also more rapid in J774-SRBI cells. Increased expression of SR-BI also facilitated the clearance of cellular cholesterol ester to HDL(3). This latter observation, combined with the measurement of the smaller ER free cholesterol pool in J774-SRBI cells, suggests that the free cholesterol derived from the hydrolysis of cholesterol ester was rapidly transported back to the plasma membrane. It is concluded that expression of SR-BI in macrophages increases the rate of free cholesterol transport, and modulates free cholesterol distribution between the plasma membrane and the internal membrane compartments in macrophages.

Sex-related differences in the regulation of macrophage cholesterol metabolism

Men have an earlier onset and higher incidence of coronary heart disease than women, independent of environmental risk factor exposure. As a consequence, there has been considerable interest in the potential role of sex hormones in atherogenesis. An emerging body of evidence suggests that sex-specific tissue and cellular characteristics may mediate sex-specific responses to a variety of stimuli. Recent studies have shown that oestrogen, progesterone and androgens all regulate processes integral to human macrophage foam cell formation, a key event in atherogenesis, in a sex-specific manner; findings that may have important implications for understanding the sex gap in atherosclerosis. Physiological levels of 17beta-estradiol and progesterone are both associated with a female-specific reduction in cholesteryl ester accumulation in human macrophages. By contrast, androgens increase cholesteryl ester formation in male but not in female donor human macrophages. This review summarizes current data concerning the sex-specific effects of sex hormones on processes important to macrophage foam cell formation and the basic mechanisms responsible for the sex specificity of such effects. Future research in this promising field may eventually lead to the novel concept of 'sex-specific' treatments directed at inhibiting atherogenesis.

Cholesterol ester accumulation: an immediate consequence of acute in vivo ischemic renal injury

BACKGROUND

Cholesterol is a major constituent of plasma membranes, and recent evidence indicates that it is up-regulated during the maintenance phase of acute renal failure (ARF). However, cholesterol's fate and that of the cholesterol ester (CE) cycle [shuttling between free cholesterol (FC) and CEs] during the induction phase of ARF have not been well defined. The present studies sought to provide initial insights into these issues.

METHODS

FC and CE were measured in mouse renal cortex after in vivo ischemia (15 and 45 minutes)/reperfusion (0 to 120 minutes) and glycerol-induced myoglobinuria (1 to 2 hours). FC/CE were also measured in (1) cultured human proximal tubule (HK-2) cells three hours after ATP depletion and in (2) isolated mouse proximal tubule segments (PTSs) subjected to plasma membrane damage (with cholesterol oxidase, sphingomyelinase, phospholipase A2, or cytoskeletal disruption with cytochalasin B). The impact of cholesterol synthesis inhibition (with mevastatin) and FC traffic blockade (with progesterone) on injury-evoked FC/CE changes was also assessed.

RESULTS

In vivo ischemia caused approximately threefold to fourfold CE elevations, but not FC elevations, that persisted for at least two hours of reperfusion. Conversely, myoglobinuria had no effect. Isolated CE increments were observed in ATP-depleted HK-2 cells. Neither mevastatin nor progesterone blocked this CE accumulation. Plasma membrane injury induced with sphingomyelinase or cholesterol oxidase, but not with phospholipase A(2) or cytochalasin B, increased tubule CE content. High CE levels, induced with cholesterol oxidase, partially blocked hypoxic PTS attack.

CONCLUSIONS

In vivo ischemia/reperfusion acutely increases renal cortical CE, but not FC, content, indicating perturbed CE/FC cycling. The available data suggest that this could stem from specific types of plasma membrane damage, which then increase FC flux via aberrant pathways to the endoplasmic reticulum, where CE formation occurs. That CE levels are known to inversely correlate with both renal and nonrenal cell injury suggests the potential relevance of these observations to the induction phase of ischemic ARF.

Multidrug resistance P-glycoprotein is not involved in cholesterol esterification

The aim of the present paper is to reinvestigate the role of multidrug resistance P-glycoprotein MDR1 and MDR-associated protein (MRP1) in cholesterol esterification using well-characterized inhibitors. Using specific substrate efflux assay, we show that GF120918 (0.2 microM) and probenecid (5 mM) were specific inhibitors of MDR1 and MRP1, respectively. In HepG2 cells, neither of them affect the esterification of cholesterol derived from the uptake of cholesterol-rich lipoprotein, while both verapamil (100 microM) and progesterone (100 microM) were able to inhibit cholesterol esterification. Similar results were obtained with verapamil, progesterone, and GF120918 in the MDR1-overexpressing cells MCF7/ADR. The capacity of progesterone to reduce cholesterol esterification is not correlated with its ability to inhibit MDR1 but is rather due to direct inhibition of acyl-CoA:cholesterol acyltransferase (ACAT). We conclude that the esterification of cholesterol is not correlated with MDR1 or MRP1 activity, thus excluding their role in the intracellular transport of endocytosis-derived cholesterol.

Sterols and inhibitors of sterol transport modulate the degradation and secretion of macrophage ApoE: requirement for the C-terminal domain

Macrophage-derived apoE, produced in the vessel wall, may have important effects during atherogenesis. The production of apoE by macrophages can be regulated at a transcriptional level by cellular differentiation state, cytokines and sterol loading. In addition, there are post-transcriptional and post-translational loci for regulation. We have recently identified an intermediate density cell membrane fraction in which the degradation of apoE can be modulated by sterols. Suppressing degradation of apoE in this fraction by pre-incubating cells in sterols led to enhanced apoE secretion. In this report we demonstrate that the suppressive effect of sterols on the degradation of newly synthesized apoE in this fraction depends on the presence on its C-terminal domain, by studying a macrophage cell line transfected to express a mutant form of apoE in which amino acids beyond amino acid 202 were deleted. In addition, two modulators of cellular sterol transport, progesterone and U1866A, inhibited the degradation of full-length apoE. In contrast, incubation of cells in the acyl-CoA:cholesterol acyltransferase inhibitor S58035 did not influence apoE degradation. As would be predicted based on the results of degradation assays, U1866A, but not S58035, increased the secretion of apoE from a cell line transfected to constitutively express full-length apoE cDNA. The effect of U1866A on apoE degradation, like the effect of sterol, required the presence of the apoE C-terminal domain. Our results indicate that alteration of intracellular sterol homeostasis by pre-incubation in sterols or by drugs that modify the subcellular transport of sterol, modulates the susceptibility of apoE to degradation and that this modulation requires the presence of C-terminal lipid binding domains.

Estrogen and progesterone reduce lipid accumulation in human monocyte-derived macrophages: a sex-specific effect

BACKGROUND

Males have an earlier onset and greater prevalence of clinical atherosclerosis than age-matched females, which is consistent with an atheroprotective effect of the female sex steroids, estrogen and progesterone. We therefore examined the effects of estrogen and progesterone on human foam cell formation, a key early event in atherogenesis.

METHODS AND RESULTS

Monocytes from healthy female and male donors were obtained from white cell concentrates and allowed to differentiate into macrophages over 10 days. These human monocyte-derived macrophages (MDMs) were exposed to either control (0.1% vol/vol ethanol) or estrogen or progesterone treatment on days 3 through 10. Lipid loading was achieved on days 8 through 10 by incubation with acetylated LDL. Lipid from the MDMs was then extracted for analysis of cholesteryl ester (CE) content. 17beta-Estradiol at both physiological (2 nmol/L) and supraphysiological (20 and 200 nmol/L) concentrations produced a significant reduction in macrophage CE content (88+/-3%, 88+/-2%, and 85+/-4%, respectively; P<0.02 compared with control). Physiological and supraphysiological levels of progesterone (2, 10, and 200 nmol/L) produced an even more dramatic reduction in CE content (74+/-9%, 56+/-10%, and 65+/-8%, respectively; P<0.002 compared with control). This effect could be abrogated by coincubation with the progesterone receptor antagonist RU486. Neither estrogen nor progesterone produced a reduction in lipid loading in male-donor-derived MDMs. Detailed lipid trafficking studies demonstrated that both estrogen and progesterone altered macrophage uptake and/or processing of modified LDL.

CONCLUSIONS

Physiological levels of estrogen and progesterone are associated with a female-sex-specific reduction in human macrophage lipid loading, which is consistent with an atheroprotective effect.

Progesterone metabolism in human fibroblasts is independent of P-glycoprotein levels and Niemann-Pick type C disease

Progesterone inhibits intracellular transport of lysosomal cholesterol in cultured cells, and thus at least in part mimics the biochemical phenotype of Niemann-Pick type C disease (NPC) in human fibroblasts. The goal of this study was to determine whether metabolism of progesterone to other steroids is affected by the NPC mutation or by P-glycoprotein (a known progesterone target). We found that human fibroblasts metabolize progesterone in three steps: rapid conversion to 5alpha-pregnane-3,20-dione, which is then reduced to 5alpha-pregnane-3beta(alpha)-ol-20-one with subsequent 6alpha-hydroxylation. The pattern and rates of progesterone metabolism were not significantly different in a variety of fibroblasts from normal individuals, NPC patients, and obligate heterozygotes. Inhibition of steroid 5alpha-reductase with finasteride completely blocked metabolism of progesterone but had no effect on inhibition of LDL-stimulated cholesterol esterification (IC50 = 10 microM). Progesterone also partially inhibited 25-hydroxycholesterol-induced cholesterol esterification, with similar dose-dependence in normal and NPC fibroblasts. P-glycoprotein levels varied significantly among the various fibroblasts tested, but no correlation with NPC phenotype or rate of progesterone metabolism was noted, and P-glycoprotein inhibitors did not affect conversion of progesterone to products. These results indicate that metabolism of progesterone in human fibroblasts is largely independent of its ability to interfere with cholesterol traffic and P-glycoprotein function.

Crystallization of free cholesterol in model macrophage foam cells

-The present study examined free cholesterol (FC) crystallization in macrophage foam cells. Model foam cells (J774 or mouse peritoneal macrophages [MPMs]) were incubated with acetylated low density lipoprotein and FC/phospholipid dispersions for 48 hours, resulting in the deposition of large stores of cytoplasmic cholesteryl esters (CEs). The model foam cells were then incubated for up to 5 days with an acyl-coenzyme A:cholesterol acyltransferase (ACAT) inhibitor (CP-113,818) in the absence of an extracellular FC acceptor to allow intracellular accumulation of FC. FC crystals of various shapes and sizes formed in the MPMs but not in the J774 macrophages. Examination of the MPM monolayers by microscopy indicated that the crystals were externalized rapidly after formation and thereafter continued to increase in size. Incubating J774 macrophages with 8-(4-chlorophenylthio)adenosine 3':5'-cyclic monophosphate (CPT-cAMP) in addition to CP-113,818 caused FC crystal formation as a consequence of CPT-cAMP stimulation of CE hydrolysis and inhibition of cell growth. In addition, 2 separate cholesterol phases (liquid-crystalline and cholesterol monohydrate) in the plane of the membrane bilayer were detected after 31 hours of ACAT inhibition by the use of small-angle x-ray diffraction of J774 macrophage foam cells treated with CPT-cAMP. Other compounds reported to inhibit ACAT, namely progesterone (20 microgram/mL) and N-acetyl-D-sphingosine (c(2)-ceramide, 10 microgram/mL), induced cellular toxicity in J774 macrophage foam cells and FC crystallization when coincubated with CPT-cAMP. Addition of the extracellular FC acceptors apolipoproteins (apo) E and A-I (50 microgram/mL) reduced FC crystal formation. In MPMs, lower cell density and frequent changes of medium were conducive to crystal formation. This may be due to "dilution" of apoE secreted by the MPMs and is consistent with our observation that the addition of exogenous apoE or apoA-I inhibits FC crystal formation in J774 macrophage foam cells cotreated with CP-113,818 plus CPT-cAMP. These data demonstrate that FC crystals can form from the hydrolysis of cytoplasmic stores of CEs in model foam cells. FC crystal formation can be modulated by the addition of extracellular FC acceptors or by affecting the cellular rate of CE hydrolysis. This process may contribute to the formation of FC crystals in atherosclerotic plaques.

A mammalian lysosomal membrane protein confers multidrug resistance upon expression in Saccharomyces cerevisiae

Mouse transporter protein (MTP) is a highly conserved polytopic membrane protein present in mammalian lysosomes and endosomes. The role of MTP in regulating the in vivo subcellular distribution of numerous structurally distinct small molecules has been examined in this study by its expression in a drug-sensitive strain of the yeast Saccharomyces cerevisiae. Surprisingly, the expression of MTP in membranes of an intracellular compartment resulted in a cellular resistance or hypersensitivity to a range of drugs that included nucleoside and nucleobase analogs, antibiotics, anthracyclines, ionophores, and steroid hormones. The intracellular bioavailability of steroid hormones was altered by MTP, as determined using an in vivo glucocorticoid receptor-driven reporter assay in yeast, suggesting that the MTP-regulated drug sensitivity arose due to a change in the subcellular compartmentalization of steroid hormones and other drugs. MTP-regulated drug sensitivity in yeast was blocked to varying degrees by compounds that inhibit lysosomal function, interfere with intracellular cholesterol transport, or modulate the multidrug resistance phenotype of mammalian cells. These results indicate that MTP is involved in the subcellular compartmentalization of diverse hydrophobic small molecules and contributes to the inherent drug sensitivity or resistance of the mammalian cell.

Four cholesterol-sensing proteins

What is the connection among the following three medical conditions: Niemann-Pick type C disease (a cause of mental retardation and early death), systemic lipidosis (in which an obscure side effect of numerous drugs transforms lysosomes into lamellar bodies), and holoprosencephaly (a catastrophe in embryonic development)? Recent evidence suggests that the pathogenesis in each use involves impaired sensing of cellular cholesterol.

Oxidative tyrosylation of HDL enhances the depletion of cellular cholesteryl esters by a mechanism independent of passive sterol desorption

It is believed that HDL protects against atherosclerosis by removing excess cholesteryl esters from cells of the artery wall. Previous studies have suggested that HDL depletes cells of cholesteryl esters both by stimulating cholesterol efflux from the plasma membrane and by activating transport processes that divert cholesterol from the cholesteryl ester cycle, but it is unknown if these are independent processes. We previously found that HDL oxidized by tyrosyl radical has a markedly enhanced ability to promote the removal of cholesterol from cultured cells [Francis, G. A., et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 6631-6635]. Here we show that incubation of cholesterol-loaded human fibroblasts with low concentrations of tyrosylated HDL depleted cells of cholesteryl esters and increased cellular free cholesterol without increasing efflux of cholesterol into the medium as compared to incubation with untreated HDL. Cells preincubated with tyrosylated HDL and then exposed to a variety of cholesterol acceptors exhibited significantly higher rates of free cholesterol efflux than did cells preincubated with HDL. This effect was observed in the presence or absence of an inhibitor of acyl CoA:cholesterol acyltransferase (ACAT) and was independent of cholesteryl ester hydrolysis, suggesting that alterations in cholesteryl ester cycle enzymes were not responsible for the loss of cholesteryl esters. In contrast to the reduction of cholesteryl esters, the rates of cholesterol and phospholipid efflux from the plasma membranes of cells exposed to tyrosylated HDL and HDL were identical. These results suggest for the first time that a mechanism exists to deplete cellular cholesteryl esters and the cholesterol substrate pool for esterification by ACAT prior to the removal of cholesterol from the plasma membrane. Identification of products in tyrosylated HDL responsible for this redistribution of cellular cholesterol may provide important insights into mechanisms of intracellular cholesterol trafficking and the ability of modified forms of HDL to protect the artery against wall pathological cholesterol accumulation.

Dehydroepiandrosterone markedly inhibits the accumulation of cholesteryl ester in mouse macrophage J774-1 cells

To clarify the antiatherogenic mechanism of action of dehydroepiandrosterone (DHEA), we investigated the effects of DHEA on the accumulation of cholesteryl ester (CE) in cultured mouse macrophage J774-1 cells. The accumulation of CE in J774-1 cells in the presence of acetyl low density lipoprotein (AcLDL) and 10(-5) mol/l DHEA was significantly reduced to 30% of the control values for 24 h. The marked effect of DHEA was observed as early as 6 h and continued at least for 48 h. This reduction by DHEA was dose-dependent and occurred starting at a DHEA dose of 5 x 10(-7) mol/1 for 24 h. DHEA treatment did not induced any changes in the cell surface binding, cell-association, or degradation of AcLDL. In comparison, the DHEA analogues, 8354 and 8356, which are known to be much stronger inhibitors of glucose 6-phosphate dehydrogenase than DHEA, did not show as marked an effect as DHEA on the accumulation of CE during the first 6 h. However, after 24-48 h of incubation, both 8354 and 8356 caused a marked reduction in the accumulation of CE similar to that observed with DHEA. A quantitative analysis of the cellular cholesterol content revealed that DHEA caused a marked reduction in CE with a concomitant continuous increase in free cholesterol (FC), while the DHEA analogues caused a marked reduction in CE with no change in FC. DHEA demonstrated little inhibitory effect on 25-hydroxycholesterol-driven esterification. Moreover, 10(-5) mol/1 DHEA induced a CE reduction in the foam cells induced by AcLDL. The CE-reducing capacity was also observed in the DHEA analogues. This CE-reducing capacity disappeared, however, when acyl CoA:cholesterol acyltransferase inhibitor, 58-035, was also present. Based on these findings, it can be concluded that the inhibitory effect of DHEA on the CE storage in response to AcLDL can be explained, at least in part, by two mechanisms. First, a recently published mechanism, namely, the inhibitory action of DHEA on lysosomal cholesterol transport, correlates well with the inhibition against foam cell transformation by DHEA in the early phase (at 6 h) observed in our study. With regard to the second mechanism, the CE-reducing capacity of DHEA from CE-laden foam cells, which appears to be related to a decreased cholesteryl ester cycle, may contribute to the inhibitory effect on the CE storage in the late phase (at 24 h and 48 h). These phase-specific inhibitory mechanisms of DHEA on the CE-storage may therefore partly explain the antiatherogenic action of DHEA.

Compartmental isolation of cholesterol participating in the cytoplasmic cholesteryl ester cycle in Chinese hamster ovary 25-RA cells

Using the Chinese hamster ovary cell line, 25-RA, we have demonstrated that lipoprotein-derived cholesterol and endogenously synthesized cholesterol are selectively differentiated with respect to their cellular locations. These cells lack sterol-mediated regulation, spontaneously storing large amounts of esterified cholesterol, which turns over with a half-time of 7.5 h. When [3H]cholesterol was provided to the cells in serum to trace cellular cholesterol, the specific activities of cellular free and esterified cholesterol (6238 +/- 273 and 5128 +/- 277 cpm/ microg, respectively) failed to equilibrate, indicating that bulk cellular free cholesterol is isolated from that participating in the cholesteryl ester cycle. Using [3H]acetate to trace the fate of endogenously synthesized cholesterol, a failure of equilibration was also observed (specific activities of free and esterified cholesterol = 280 +/- 37 and 458 +/- 8 cpm/ microg, respectively). The lower specific activity of the precursor indicates that endogenously synthesized cholesterol is preferentially esterified. When cells radiolabeled with [3H]acetate were post-incubated in the absence of radiolabel, the specific activity of the esterified cholesterol pool remained significantly higher than that of the free cholesterol, suggesting that cholesterol derived from hydrolysis of esterified cholesterol is preferentially re-esterified.

Cellular cholesterol efflux mediated by cyclodextrins

In this study, we compared cholesterol efflux mediated by either high density lipoproteins (HDL3) or beta-cyclodextrins, cyclic oligosaccharides that are able to dissolve lipids in their hydrophobic core. beta-Cyclodextrin, 2-hydroxypropyl-beta-cyclodextrin, and methyl-beta-cyclodextrin at 10 mM induced the release of 50-90% of L-cell [3H]cholesterol after 8 h of incubation, with a major portion of this cholesterol being released in the first 1-2 h of incubation. The cholesterol efflux kinetics are different if cells are incubated with HDL3, which induces a relatively constant rate of release of cholesterol throughout an 8-h incubation. Cholesterol efflux to cyclodextrins was much greater than phospholipid release. To test the hypothesis that maximal efflux rate constants for a particular cell are independent of the type of acceptor, we estimated the maximal rate constants for efflux (Vmax) of cellular cholesterol from L-cells, Fu5AH cells, and GM3468A fibroblasts. The rate constant for HDL3-mediated efflux varied among cell lines in the order Fu5AH > L-cells > fibroblasts. However, these differences were not evident when cyclodextrins were used as cholesterol acceptors. The estimated Vmax values for cyclodextrin-mediated efflux were 3.5-70-fold greater than for HDL3 for the three cell lines. The very high efficiency of cyclodextrins in stimulating cell cholesterol efflux suggests that these compounds can be used in two general ways for studies of atherosclerosis: 1) as research tools to probe mechanisms of cholesterol transport and aspects of membrane structure or 2) as potential pharmacological agents that could modify in vivo cholesterol metabolism and influence the development of the atherosclerotic plaque.