Possible involvement of protein kinase C in the induction of adipose differentiation-related protein by Sterol ester in RAW 264.7 macrophages

Regulation of lipid droplets via the PLCβ2-PKCα-ADRP pathway in granulosa cells exposed to cadmium

In steroidogenic cells, steroids are synthesized de novo from cholesterol stored in lipid droplets (LDs). The size of LDs regulated by adipose differentiation-related protein (ADRP) is closely related to cholesterol ester hydrolysis. Many studies reported that cadmium (Cd) had dual effects on steroidogenesis in granulosa cells (GCs). However, the role of LD and its regulation in abnormal steroidogenesis caused by Cd exposure remain unknown. In current study, female rats were exposed to CdCl₂ during gestation and lactation, and influence of such exposure was investigated in ovarian GCs of female offspring. The size of LDs was found much smaller than normal in GCs; ADRP was down-regulated and hormone-sensitive lipase (HSL) phosphorylation was increased, followed by up-regulation of steroidogenic acute regulatory protein (StAR) and cholesterol side-chain cleavage enzyme (CYP11A1); the expression of 1-phosphatidylinositol 4,5-bisphosphate phosphodiesterase beta-2 (PLCβ2) and protein kinase C alpha type (PKCα) were both decreased accompanying the ADRP down-regulation. This series of events resulted in a high level of progesterone in serum. Similar results were demonstrated in GCs treated with 20 μM CdCl₂ for 24 h in vitro. The protein level of ADRP was decreased after gene silencing of PLCβ2/PKCα, and the knockdown of PLCβ2/PKCα/ADRP led to micro-sized LD formation. We found that Cd exposure down-regulated ADRP by inhibiting the PLCβ2-PKCα signaling pathway, reduced the size of LDs, and promoted HSL phosphorylation. StAR and CYP11A1 were both up-regulated following the hydrolysis of cholesterol ester, which led to a high production of progesterone. LD thereby is a target subcellular organelle for Cd to affect steroid hormone synthesis in ovarian GCs. These findings might help to uncover the mechanism of ovarian dysfunction and precocious puberty caused by Cd pollution.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Dihydroaustrasulfone Alcohol (WA-25) Impedes Macrophage Foam Cell Formation by Regulating the Transforming Growth Factor-β1 Pathway

Atherosclerosis is considered an inflammatory disease. However, clinically used anti-atherosclerotic drugs, such as simvastatin, have many side effects. Recently, several unique marine compounds have been isolated that possess a variety of bioactivities. In a previous study, we found a synthetic precursor of the marine compound (austrasulfone), which is dihydroaustrasulfone alcohol (WA-25), has anti-atherosclerotic effects in vivo. However, the detailed mechanisms remain unclear. Therefore, to clarify the mechanisms through which WA-25 exerts anti-atherosclerotic activity, we used RAW 264.7 macrophages as an in vitro model to evaluate the effects of WA-25. In lipopolysaccharide (LPS)-stimulated RAW 264.7 cells, WA-25 significantly inhibited expression of the pro-inflammatory proteins, inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2). In contrast, simvastatin increased the COX-2 expression compared to WA-25. In addition, WA-25 impedes foam cell formation and up-regulated the lysosomal and cyclic adenosine monophosphate (cAMP) signaling pathway. We also observed that transforming growth factor β1 (TGF-β1) was up-regulated by WA-25 and simvastatin in LPS-induced RAW 264.7 cells, and the promising anti-atherosclerosis effects of WA-25 were disrupted by blockade of TGF-β1 signaling. Besides, WA-25 might act through increasing lipolysis than through alteration of lipid export. Taken together, these data demonstrate that WA-25 may have potential as an anti-atherosclerotic drug with anti-inflammatory effects.

A Lys49 phospholipase A2, isolated from Bothrops asper snake venom, induces lipid droplet formation in macrophages which depends on distinct signaling pathways and the C-terminal region

MT-II, a Lys49PLA₂ homologue devoid of catalytic activity from B. asper venom, stimulates inflammatory events in macrophages. We investigated the ability of MT-II to induce formation of lipid droplets (LDs), key elements of inflammatory responses, in isolated macrophages and participation of protein kinases and intracellular PLA₂s in this effect. Influence of MT-II on PLIN2 recruitment and expression was assessed, and the effects of some synthetic peptides on LD formation were further evaluated. At noncytotoxic concentrations, MT-II directly activated macrophages to form LDs. This effect was reproduced by a synthetic peptide corresponding to the C-terminal sequence 115–129 of MT-II, evidencing the critical role of C-terminus for MT-II-induced effect. Moreover, MT-II induced expression and recruitment of PLIN2. Pharmacological interventions with specific inhibitors showed that PKC, PI3K, ERK1/2, and iPLA₂, but not P38^MAPK or cPLA₂, signaling pathways are involved in LD formation induced by MT-II. This sPLA₂ homologue also induced synthesis of PGE₂ that colocalized to LDs. In conclusion, MT-II is able to induce formation of LDs committed to PGE₂ formation in a process dependent on C-terminal loop engagement and regulated by distinct protein kinases and iPLA₂. LDs may constitute an important inflammatory mechanism triggered by MT-II in macrophages.

Perilipin 2 (PLIN2)-deficiency does not increase cholesterol-induced toxicity in macrophages

Interventions on macrophages/foam cells to redirect intracellular cholesterol towards efflux pathways could become a very valuable addition to our therapeutic arsenal against atherosclerosis. However, certain manipulations of the cholesteryl ester cycle, such as the inhibition of ACAT1, an ER-resident enzyme that re-esterifies cholesterol, are not well tolerated. Previously we showed that targeting perilipin-2 (PLIN2), a major lipid droplet (LD)-associated protein in macrophages, prevents foam cell formation and protects against atherosclerosis. Here we have assessed the tolerance of PLIN2-deficient bone marrow derived macrophages (BMM) to several lipid loading conditions similar to the found during atherosclerosis development, including exposure to modified low-density lipoprotein (mLDL) and 7-ketocholesterol (7-KC), a free cholesterol (FC) metabolite, in media with or without cholesterol acceptors. BMM isolated from mice that do or do not express PLIN2 were tested for apoptosis (TUNEL and cleaved caspase-3), ER stress (CHOP induction and XBP-1 splicing), and inflammation (TNF-α and IL-6 mRNA levels). Like in other cell types, PLIN2 deficiency impairs LD buildup in BMM. However, while most stress parameters were elevated in macrophages under ACAT inhibition and 7-KC loading, PLIN2 inactivation was well tolerated. The data support the safety of targeting PLIN2 to prevent foam cell formation and atherosclerosis.

Lipid droplets in inflammation and cancer

Accumulation of lipid droplets (also known as lipid bodies or adiposomes) within leukocytes, epithelial cells, hepatocytes and other non-adipocytic cells is a frequently observed phenotype in infectious, neoplastic and other inflammatory conditions. Lipid droplet biogenesis is a regulated cellular process that culminates in the compartmentalization of lipids and of an array of enzymes, protein kinases and other proteins, suggesting that lipid droplets are inducible organelles with roles in cell signaling, regulation of lipid metabolism, membrane trafficking and control of the synthesis and secretion of inflammatory mediators. Enzymes involved in eicosanoid synthesis are localized at lipid droplets and lipid droplets are sites for eicosanoid generation in cells during inflammation and cancer. In this review, we discuss the current evidence related to the biogenesis and function of lipid droplets in cell metabolism and signaling in inflammation and cancer. Moreover, the potential of lipid droplets as markers of disease and targets for novel anti-inflammatory and antineoplastic therapies will be discussed.

Oleic acid-induced ADRP expression requires both AP-1 and PPAR response elements, and is reduced by Pycnogenol through mRNA degradation in NMuLi liver cells

Fatty acids stimulate lipid accumulation in parallel with increased expression of adipose differentiation-related protein (ADRP) in liver cells. Although it is generally considered that the fatty acid effect on ADRP expression is mediated by peroxisome proliferator-activated receptors (PPARs), we identified here an additional molecular mechanism using the NMuLi mouse liver nonparenchymal cell line, which expresses PPARgamma and delta but not alpha. Oleic acid (OA) and specific ligands for PPARgamma and -delta stimulated ADRP expression as well as the -2,090-bp ADRP promoter activity which encompasses the PPAR response element (PPRE) adjacent to an Ets/activator protein (AP)-1 site. When the AP-1 site was mutated, OA failed to stimulate the activity despite the presence of the PPRE, whereas ligands for PPARgamma and -delta did stimulate it and so did a PPARalpha ligand under the coexpression of PPARalpha. DNA binding of AP-1 was stimulated by OA but not by PPAR ligands. Because we previously demonstrated that Pycnogenol (PYC), a French maritime pine bark extract, suppressed ADRP expression in macrophages partly by suppression of AP-1 activity, we tested the effect of PYC on NMuLi cells. PYC reduced the OA-induced ADRP expression along with suppression of lipid droplet formation. However, PYC neither suppressed the OA-stimulated ADRP promoter activity nor DNA binding of AP-1 but, instead, reduced the ADRP mRNA half-life. All these results indicate that the effect of OA on ADRP expression requires AP-1 as well as PPRE, and PYC suppresses the ADRP expression in part by facilitating mRNA degradation. PYC, a widely used dietary supplement, could be beneficial for the prevention of excessive lipid accumulation such as hepatic steatosis.

PAT proteins, an ancient family of lipid droplet proteins that regulate cellular lipid stores

The PAT family of lipid droplet proteins includes 5 members in mammals: perilipin, adipose differentiation-related protein (ADRP), tail-interacting protein of 47 kDa (TIP47), S3-12, and OXPAT. Members of this family are also present in evolutionarily distant organisms, including insects, slime molds and fungi. All PAT proteins share sequence similarity and the ability to bind intracellular lipid droplets, either constitutively or in response to metabolic stimuli, such as increased lipid flux into or out of lipid droplets. Positioned at the lipid droplet surface, PAT proteins manage access of other proteins (lipases) to the lipid esters within the lipid droplet core and can interact with cellular machinery important for lipid droplet biogenesis. Genetic variations in the gene for the best-characterized of the mammalian PAT proteins, perilipin, have been associated with metabolic phenotypes, including type 2 diabetes mellitus and obesity. In this review, we discuss how the PAT proteins regulate cellular lipid metabolism both in mammals and in model organisms.

The PAT family of lipid droplet proteins in heart and vascular cells

Cytoplasmic lipid droplets (LDs) are organelles in which cells store neutral lipids for use as an energy source in times of need, but they also play important roles in the regulation of key metabolic processes. Although LDs are essential for normal cell function, excess accumulation of intracellular lipid is associated with several metabolic diseases, including obesity, type 2 diabetes, and atherosclerosis. The function of LDs is regulated by their associated proteins, including the members of the PAT family: perilipin, adipophilin/adipose differentiation-related protein, tail-interacting protein 47, S3-12, and OXPAT/myocardial LD protein/lipid-storage droplet protein 5. In this review we discuss the PAT proteins in two cardiovascular contexts: 1) in the atherosclerotic vessel wall, where LDs within macrophage foam cells store cholesteryl esters derived from modified lipoproteins, and 2) in the myocardium, where LDs store fatty acids, the major energy substrate for normal heart function, as triglyceride.

Leukocyte lipid bodies - Biogenesis and functions in inflammation

Lipid body accumulation within leukocytes is a common feature in both clinical and experimental infectious, neoplasic and other inflammatory conditions. Here, we will review the contemporary evidence related to the biogenesis and structure of leukocyte lipid bodies (also known as lipid droplets) as inflammatory organelles. Studies of leukocyte lipid bodies are providing functional, ultrastructural and protein compositional evidences that lipid bodies are not solely storage depots of neutral lipid. Over the past years substantial progresses have been made to demonstrate that lipid body biogenesis is a highly regulated process, that culminate in the compartmentalization of a specific set of proteins and lipids, that place leukocyte lipid bodies as inducible cytoplasmic organelles with roles in cell signaling and activation, regulation of lipid metabolism, membrane trafficking and control of the synthesis and secretion of inflammatory mediators. Pertinent to the roles of lipid bodies in inflammation and cell signaling, enzymes involved in eicosanoid synthesis are localized at lipid bodies and lipid bodies are sites for eicosanoid generation. Collectively, lipid bodies in leukocytes are emerging as critical regulators of different inflammatory diseases, key markers of leukocyte activation and attractive targets for novel anti-inflammatory therapies.

Deficiency of adipose differentiation-related protein impairs foam cell formation and protects against atherosclerosis

Foam cells are a hallmark of atherosclerosis. However, it is unclear whether foam cell formation per se protects against atherosclerosis or fuels it. In this study, we investigated the role of adipose differentiation-related protein (ADFP), a major lipid droplet protein (LDP), in the regulation of foam cell formation and atherosclerosis. We show that ADFP expression facilitates foam cell formation induced by modified lipoproteins in mouse macrophages in vitro. We show further that Adfp gene inactivation in apolipoprotein E-deficient (ApoE(-/-)) mice reduces the number of lipid droplets in foam cells in atherosclerotic lesions and protects the mice against atherosclerosis. Moreover, transplantation of ADFP-null bone marrow-derived cells effectively attenuated atherosclerosis in ApoE(-/-) mice. Deficiency of ADFP did not cause a detectable compensatory increase in the other PAT domain proteins in macrophages in vitro or in vivo. Mechanistically, ADFP enables the macrophage to maintain its lipid content by hindering lipid efflux. We detected no significant difference in lesion composition or in multiple parameters of inflammation in macrophages or in their phagocytic activity between mice with and without ADFP. In conclusion, Adfp inactivation in ApoE(-/-) background protects against atherosclerosis and appears to be a relatively pure model of impaired foam cell formation.

FXR deficiency causes reduced atherosclerosis in Ldlr-/- mice

OBJECTIVE

Based on the observation that Fxr-/- mice exhibit a proatherogenic lipoprotein profile, we investigated the role of FXR in the development of atherosclerosis.

METHODS AND RESULTS

Administration of a western diet to Fxr-/- mice or wild-type mice does not result in the development of significant atherosclerotic lesions. Consequently we generated Fxr-/- Ldlr-/- (DKO) mice and compared lesion development with Ldlr-/- mice. After 16 weeks on a Western diet, en face analysis of the aorta indicated that the male DKO mice had reduced atherosclerotic lesions as compared with Ldlr-/- mice. Plasma low-density lipoprotein cholesterol and high-density lipoprotein cholesterol levels were reduced by 40% to 50%, whereas triglyceride levels increased 4-fold in the male DKO mice. Finally, peritoneal macrophages freshly isolated from male DKO mice had reduced expression of CD36 mRNA and decreased neutral lipid accumulation, as compared with Ldlr-/- mice.

CONCLUSIONS

FXR deficiency in male, but not female, Ldlr-/- mice results in a reduction in the size of atherosclerotic lesions in the aorta. The reduction in atherosclerosis may result from a decrease in plasma low-density lipoprotein cholesterol, coupled with reduced expression of CD36 in macrophages of DKO mice.

Hypoxia converts human macrophages into triglyceride-loaded foam cells

OBJECTIVE

Atherosclerotic lesions have regions that are hypoxic. Because the lesion contains macrophages that are loaded with lipid, we investigated whether hypoxia can influence the accumulation of lipids in these cells.

METHODS AND RESULTS

Exposure of human macrophages to hypoxia for 24 hours resulted in an increased formation of cytosolic lipid droplets and an increased accumulation of triglycerides. Exposure of the macrophages to oxidized low-density lipoprotein (oxLDL) increased the accumulation of cytosolic lipid droplets because of an increase in cellular cholesterol esters. The accumulation of lipid droplets in oxLDL-treated cells was further increased after hypoxia, caused by an increased level of triglycerides. Expression analyses combined with immunoblot or RT-PCR demonstrated that hypoxia increased the expression of several genes that could promote the accumulation of lipid droplets. Hypoxia increased the mRNA and protein levels of adipocyte differentiation-related protein (ADRP). It is well known that an increased expression of ADRP increases the formation of lipid droplets. Hypoxia decreased the expression of enzymes involved in beta-oxidation (acyl-coenzyme A synthetase and acyl-coenzyme A dehydrogenase) and increased the expression of stearoyl-coenzyme A desaturase, an important enzyme in the fatty acid biosynthesis. Moreover, exposure to hypoxia decreased the rate of beta-oxidation, whereas the accumulation of triglycerides increased.

CONCLUSIONS

The results demonstrate that exposure of human macrophages to hypoxia causes an accumulation of triglyceride-containing cytosolic lipid droplets. This indicates that the hypoxia present in atherosclerotic lesions can contribute to the formation of the lipid-loaded macrophages that characterize the lesion and to the accumulation of triglycerides in such lesions.

Expression of adipose differentiation-related protein (ADRP) is conjointly regulated by PU.1 and AP-1 in macrophages

ADRP is associated with intracellular lipid droplets. We demonstrate the regulatory mechanism for ADRP expression in RAW264.7 macrophages. The ADRP mRNA expression was stimulated by PMA, and synergistically enhanced in association with its protein level in the presence of lipids. A proteasome inhibitor protected the protein from degradation under the lipid-free conditions. One of the possible sites of the PMA action was proved to be an Ets/AP-1 element in the promoter, since mutations of this site reduced the PMA-induced promoter activity, and ligation of this element led to a significant increase in the PMA-responsiveness of homologous or heterologous promoters. Mutations of this site diminished the synergistic effect on the promoter activity induced by PMA and oleic acid, suggesting a possible interaction between this site and the downstream PPARdelta site. EMSA revealed that PU.1 and AP-1 conjointly bound to this site. The juxtaposition of the two sequences was requisite for full activity, since spacer sequences between them decreased the PMA-induced activity. PI3 kinase inhibitor was found to reduce the PMA-induced mRNA expression and promoter activity in parallel with PU.1/AP-1 complex formation on EMSA. From these results, we concluded that the Ets/AP-1 site is an important cis-acting element that regulates the ADRP gene expression in macrophages.

ADRP/adipophilin is degraded through the proteasome-dependent pathway during regression of lipid-storing cells

Adipose differentiation-related protein (ADRP) is a major protein associated with lipid droplets in various types of cells, including macrophage-derived foam cells and liver cells. However, the role of ADRP in the processes of formation and regression of these cells is not understood. When J774 murine macrophages were incubated with either VLDL or oleic acid, their content of both ADRP and triacylglycerol (TG) increased 3- to 4-fold. Induction of ADRP during TG accumulation was also observed in oleic acid-treated HuH-7 human liver cells. Addition of triacsin C, a potent inhibitor of acyl-CoA synthase, for 6 h decreased the amount of TG in VLDL-induced foam cells and oleic acid-treated liver cells; it decreased the amount of ADRP protein in parallel, indicating the amount of ADRP reduced during regression of the lipid-storing cells. Addition of a proteasome inhibitor during triacsin C treatment abolished the ADRP decrease and accumulated polyubiquitinated ADRP. In addition, the proteasome inhibitor reversed not only the degradation of ADRP but also TG reduction by triacsin C. These results suggest that cellular amounts of ADRP and TG regulate each other and that the ubiquitin-proteasome system is involved in degradation of ADRP during regression of lipid-storing cells.

Cytosolic Lipid Droplets Increase in Size by Microtubule-Dependent Complex Formation

Objectives— Adipocyte differentiation-related protein (ADRP)-containing lipid droplets have an essential role in the development of insulin resistance and atherosclerosis. Such droplets form in a cell-free system with a diameter of 0.1 to 0.4 μm, while the droplets present in cells vary in size, from small to very large, suggesting that the droplets can increase in size after being assembled. We have addressed this possibility.

Methods and Results— Experiments in NIH 3T3 cells demonstrated that the lipid droplets could increase in size independently of triglyceride biosynthesis. NIH 3T3 cells were either microinjected with ADRP–GFP (green fluorescent protein) or stained with Nile Red and followed by confocal microscopy and time-lapse recordings. The results showed that lipid droplets formed complexes with each other, with a volume equal to the sum of the merging particles. The formation of complexes could be inhibited by the nocodazole-induced depolymerization of the microtubules; thus, the process is dependent on microtubules. The presence of dynein on ADRP-containing droplets supports a role for this motor protein.

Conclusions— Lipid droplets can grow after they have been assembled. This increase in size is independent of triglyceride biosynthesis and involves formation of complexes, which requires intact microtubules.

Redistribution of macrophage cholesteryl ester hydrolase from cytoplasm to lipid droplets upon lipid loading

Hydrolysis of intracellular cholesteryl esters (CEs) represents the first step in the removal of cholesterol from lipid-laden foam cells associated with atherosclerotic lesions. Neutral cholesteryl ester hydrolase (CEH) catalyzes this reaction, and we recently cloned the cDNA for the human macrophage CEH and demonstrated increased mobilization of intracellular CE droplets by CEH overexpression. The present study was undertaken to test the hypothesis that for CE hydrolysis, CEH must become associated with the surface of the cytoplasmic lipid droplets. Our data show the redistribution of CEH from cytosol to lipid droplets upon lipid loading of human THP-1 macrophages. Depletion of triacylglycerol (TG) by incubation with the acyl-CoA synthetase inhibitor Triacsin D had no effect on CEH association with the lipid droplets, suggesting that CEH associates with mixed (CE + TG) as well as TG-depleted CE droplets. However, CEH had 2.5-fold higher activity when mixed droplets were used as substrate in an in vitro assay, consistent with the reported higher cholesterol efflux from cells containing mixed isotropic droplets. Perilipin as well as adipophilin, two lipid droplet-associated proteins, were also present on the lipid droplets in THP-1 macrophages. In conclusion, CEH associates with its intracellular substrate (lipid droplets) and hydrolyzes CE more efficiently from mixed droplets.

Magnolol stimulates lipolysis in lipid-laden RAW 264.7 macrophages

This study investigated the effect of magnolol, a compound isolated from Magnolia officinalis, on lipolysis in lipid-laden RAW 264.7 macrophages. Treatment of macrophages with magnolol led to dissolution of lipid droplets. This phenomenon was accompanied by a dose-dependent release of glycerol and cholesterol and a concomitant reduction in intracellular levels of glycerol and cholesterol. Furthermore, adipose differentiation-related protein (ADRP), a lipid droplet-associated protein, was down-regulated by magnolol in a dose- and time-dependent manner by Western blot analysis. Immunofluorescence studies also showed that ADRP became detached from the surface of lipid droplets after magnolol treatment. The lipolytic effect of magnolol was not mediated through the cAMP-protein kinase A (PKA) system, an authentic lipolytic pathway for macrophages, since magnolol did not induce an increase of intracellular cAMP levels, and pretreatment with either of PKA inhibitors, PKI and KT5720, did not abrogate the lipolytic response to magnolol. We conclude that magnolol induce-lipolysis of lipid-laden macrophages by down-regulation of ADRP expression and detachment of ADRP from the lipid droplet surface by a cAMP-independent mechanism. Lipolysis of lipid-laden macrophages may occur when the amount of ADRP on the surface of lipid droplets is not enough to stabilize the lipid droplets.

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.

Coordinated upregulation of oxidative pathways and downregulation of lipid biosynthesis underlie obesity resistance in perilipin knockout mice: a microarray gene expression profile

Obesity is a major risk factor for diabetes and heart disease. We previously reported that the inactivation of the gene for perilipin (plin), an adipocyte lipid droplet surface protein, produced lean and obesity-resistant mice. To dissect the underlying mechanisms involved, we used oligonucleotide microarrays to analyze the gene-expression profile of white adipose tissue (WAT), liver, heart, skeletal muscle, and kidney of plin(-/-) and plin(+/+) mice. As compared with wild-type littermates, the WAT of plin(-/-) mice had 270 and 543 transcripts that were significantly up- or downregulated. There was a coordinated upregulation of genes involved in beta-oxidation, the Krebs cycle, and the electron transport chain concomitant with a downregulation of genes involved in lipid biosynthesis. There was also a significant downregulation of the stearoyl CoA desaturase-1 gene, which has been associated with obesity resistance. Thus, in response to the constitutive activation of lipolysis associated with absence of perilipin, WAT activated pathways to rid itself of the products of lipolysis and activated pathways of energy expenditure that contribute to the observed obesity resistance. The biochemical pathways involved in obesity resistance in plin(-/-) mice identified in this study may represent potential targets for the treatment of obesity.

Induction of adipose differentiation related protein and neutral lipid droplet accumulation in keratinocytes by skin irritants

Keratinocytes play an important role in skin irritation. In an attempt to investigate mechanistic bases of human skin irritation response, we recently identified the upregulation by skin irritants of adipose differentiation related protein (ADRP) in reconstituted human epidermis. ADRP is a lipid-storage-droplet-associated protein, governing deposition and release of lipids from droplets. The purpose of this study was to characterize, in a human keratinocyte cell line (NCTC 2544), sodium-dodecyl-sulfate-induced ADRP expression, to identify the biochemical events that lead to ADRP expression, and to understand its function in sodium dodecyl sulfate cytotoxicity. Sodium dodecyl sulfate induced a concentration- and time-related production of ADRP that was associated with lipid droplet accumulation. Lipid accumulation following sodium dodecyl sulfate treatment was due to intracellular redistribution rather than lipid neosynthesis, as indicated by equivalent 14C-oleate and 14C-acetate incorporations. Other skin irritants, namely benzalkonium chloride, tributyltin, and 12-O-tetradecanoylphorbol 13-acetate, also induce lipid droplet accumulation. Sodium-dodecyl-sulfate-induced ADRP expression and lipid droplet accumulation were modulated by the calcium chelator BAPTA, indicating a role of calcium in ADRP induction. Decrease of sodium-dodecyl-sulfate-induced ADRP expression by specific ADRP antisense oligonucleotide resulted in increased cytotoxicity, indicating a protective role of ADRP and lipid accumulation in the process of cell damage induced by skin irritants. ADRP expression was also induced in vivo following treatment with sodium dodecyl sulfate in an experimental model of skin irritation, indicating that the in vitro model represents irritation.

Fixation methods for the study of lipid droplets by immunofluorescence microscopy

The study of proteins associated with lipid droplets in adipocytes and many other cells is a rapidly developing area of inquiry. Although lipid droplets are easily visible by light microscopy, few standardized microscopy methods have been developed. Several methods of chemical fixation have recently been used to preserve cell structure before visualization of lipid droplets by light microscopy. We tested the most commonly used methods to compare the effects of the fixatives on cellular lipid content and lipid droplet structure. Cold methanol fixation has traditionally been used before visualization of cytoskeletal elements. We found this method unacceptable for study of lipid droplets because it extracted the majority of cellular phospholipids and promoted fusion of lipid droplets. Cold acetone fixation is similarly unacceptable because the total cellular lipids are extracted, causing collapse of the shell of lipid droplet-associated proteins. Fixation of cells with paraformaldehyde is the method of choice, because the cells retain their lipid content and lipid droplet structure is unaffected. As more lipid droplet-associated proteins are discovered and studied, it is critical to use appropriate methods to avoid studying artifacts.

Oleic acid-induced PKC isozyme translocation in RAW 264.7 macrophages

Fatty acids are important second messengers that mediate various cellular functions, but their role in the formation of macrophage foam cells is not known. High plasma levels of oleic acid (OA) in obese patients are often associated with a high risk for atherosclerosis. In this study, we investigated the protein kinase C (PKC) isozymes involved in OA-induced lipid accumulation in RAW 264.7 macrophages. The results show that OA induces translocation of PKC alpha, beta1, and delta from the cytosol to the cell membrane 5 min after the treatment. After 16 h incubation with OA, PKC delta was found to be colocalized with adipose differentiation-related protein (ADRP) on the surface of lipid droplets, but immunoprecipitation experiments showed that PKC delta was not biochemically associated with ADRP. After 16 h incubation with OA plus phorbol 12-myristate 13-acetate (PMA), PKC delta staining on the lipid droplet surface was not seen, whereas the accumulation of lipid droplets was unaffected. Furthermore, downregulation of PKC delta was confirmed by immunoblotting. These results demonstrate possible involvement of specific PKC isozymes in the early phase of lipid accumulation, possibly during the uptake of OA.