Isolation of hepatocellular lipid droplets: the separation of distinct subpopulations

Thioredoxin domain containing 5 is involved in the hepatic storage of squalene into lipid droplets in a sex-specific way

Hepatic thioredoxin domain-containing 5 (TXNDC5) is a member of the protein disulfide isomerase family found associated with anti-steatotic properties of squalene and located in the endoplasmic reticulum and in lipid droplets. Considering that the latter are involved in hepatic squalene accumulation, the present research was aimed to investigate the role of TXNDC5 on hepatic squalene management in mice and in the AML12 hepatic cell line. Wild-type and TXNDC5-deficient (KO) mice were fed Western diets with or without 1% squalene supplementation for 6 weeks. In males, but not in females, absence of TXNDC5 blocked hepatic, but not duodenal, squalene accumulation. Hepatic lipid droplets were isolated and characterized using label-free LC-MS/MS analysis. TXNDC5 accumulated in this subcellular compartment of mice receiving squalene and was absent in TXNDC5-KO male mice. The latter mice were unable to store squalene in lipid droplets. CALR and APMAP were some of the proteins that responded to the squalene administration in all studied conditions. CALR and APMAP were positively associated with lipid droplets in the presence of squalene and they were decreased by the absence of TXNDC5. The increased squalene content was reproduced in vitro using AML12 cells incubated with squalene-loaded nanoparticles and this effect was not observed in an engineered cell line lacking TXNDC5. The phenomenon was also present when incubated in the presence of a squalene epoxidase inhibitor, suggesting a mechanism of squalene exocytosis involving CALR and APMAP. In conclusion, squalene accumulation in hepatic lipid droplets is sex-dependent on TXNDC5 that blocks its secretion.

Pgc1a is responsible for the sex differences in hepatic Cidec/Fsp27β mRNA expression in hepatic steatosis of mice fed a Western diet

Hepatic fat-specific protein 27 [cell death-inducing DNA fragmentation effector protein C (Cidec)/Fsp27] mRNA levels have been associated with hepatic lipid droplet extent under certain circumstances. To address its hepatic expression under different dietary conditions and in both sexes, apolipoprotein E (Apoe)-deficient mice were subjected to different experimental conditions for 11 wk to test the influence of cholesterol, Western diet, squalene, oleanolic acid, sex, and surgical castration on Cidec/Fsp27 mRNA expression. Dietary cholesterol increased hepatic Cidec/Fsp27β expression, an effect that was suppressed when cholesterol was combined with saturated fat as represented by Western diet feeding. Using the latter diet, neither oleanolic acid nor squalene modified its expression. Females showed lower levels of hepatic Cidec/Fsp27β expression than males when they were fed Western diets, a result that was translated into a lesser amount of CIDEC/FSP27 protein in lipid droplets and microsomes. This was also confirmed in low-density lipoprotein receptor (Ldlr)-deficient mice. Incubation with estradiol resulted in decreased Cidec/Fsp27β expression in AML12 cells. Whereas male surgical castration did not modify the expression, ovariectomized females did show increased levels compared with control females. Females also showed increased expression of peroxisome proliferator-activated receptor-γ coactivator 1-α (Pgc1a), suppressed by ovariectomy, and the values were significantly and inversely associated with those of Cidec/Fsp27β. When Pgc1a-deficient mice were used, the sex differences in Cidec/Fsp27β expression disappeared. Therefore, hepatic Cidec/Fsp27β expression has a complex regulation influenced by diet and sex hormonal milieu. The mRNA sex differences are controlled by Pgc1a.

Ethanol-induced steatosis involves impairment of lipophagy, associated with reduced Dynamin2 activity

BACKGROUND

Lipid droplets (LDs), the organelles central to alcoholic steatosis, are broken down by lipophagy, a specialized form of autophagy. Here, we hypothesize that ethanol administration retards lipophagy by down-regulating Dynamin 2 (Dyn2), a protein that facilitates lysosome re-formation, contributing to hepatocellular steatosis.

METHODS

Primary hepatocytes were isolated from male Wistar rats fed Lieber-DeCarli control or EtOH liquid diets for 6-8 wk. Hepatocytes were incubated in complete medium (fed) or nutrient-free medium (fasting) with or without the Dyn2 inhibitor Dynasore or the Src inhibitor SU6656. Phosphorylated (active) forms of Src and Dyn2, and markers of autophagy were quantified by Western Blot. Co-localization of LDs-with autophagic machinery was determined by confocal microscopy.

RESULTS

In hepatocytes from pair-fed rats, LD breakdown was accelerated during fasting, as judged by smaller LDs and lower TG content when compared to hepatocytes in complete media. Fasting-induced TG loss in control hepatocytes was significantly blocked by either SU6656 or Dynasore. Compared to controls, hepatocytes from EtOH-fed rats had 66% and 40% lower content of pSrc and pDyn2, respectively, coupled with lower rate of fasting-induced TG loss. This slower rate of fasting-induced TG loss was blocked in cells co-incubated with Dynasore. Microscopic examination of EtOH-fed rat hepatocytes revealed increased co-localization of the autophagosome marker LC3 on LDs with a concomitant decrease in lysosome marker LAMP1. Whole livers and LD fractions of EtOH-fed rats exhibited simultaneous increase in LC3II and p62 over that of controls, indicating a block in lipophagy.

CONCLUSION

Chronic ethanol administration slowed the rate of hepatocyte lipophagy, owing in part to lower levels of phosphorylated Src kinase available to activate its substrate, Dyn2, thereby causing depletion of lysosomes for LD breakdown.

A method for lipid droplet isolation from human placenta for further analyses in clinical trials

We describe a method to isolate lipids droplets from human placental tissue for future lipid analyses. We collected placentas at term from healthy women (n=5) and tested three methods published for lipids droplets isolation in other tissues. Only one of the methods, when modified, isolated lipids droplets from placental tissue, whereas all three methods allowed lipids droplets isolation from rat liver (control tissue) and separation of lipids droplets from blood contamination of the tissue. The placental lipids droplets layer was characterized by the presence of adipophilin while no N+ /K+-ATPase as plasma membrane contamination was detected. Intraplacental triglyceride content showed a high coefficient of variation (~42%), whereas for cholesterol and phospholipids this was lower. One method was effective for isolation of placental lipids droplets. It is necessary to collect a pool of placental tissue pieces for placental lipids droplets analyses. Freezing in liquid nitrogen is recommended.

Analysis of lipid droplets in hepatocytes

The liver plays an important role in triacylglycerol (TG) metabolism. It can store large amounts of TG in cytosolic lipid droplets (CLDs), or it can package TG into very-low density lipoproteins (VLDL) that are secreted from the cell. TG packaged into VLDL is derived from TG stored within the endoplasmic reticulum in lumenal lipid droplets (LLDs). Therefore, the liver contains at least three kinds of LDs that differ in their protein composition, subcellular localization, and function. Hepatic LDs undergo tremendous changes in their size and protein composition depending on the energetic (fasting/feeding) and pathological (viral infection, nonalcoholic fatty liver disease, etc.) states. It is crucial to develop methodologies that allow the isolation and analyses of the various hepatic LDs in order to gain insight into the differential metabolism of these important lipid storage/transport particles in health and disease. Here, we present detailed protocols for the isolation and analysis of CLDs and LLDs and for monitoring CLD dynamics.

Rab GTPases associate with isolated lipid droplets (LDs) and show altered content after ethanol administration: potential role in alcohol-impaired LD metabolism

BACKGROUND

Alcoholic liver disease is manifested by the presence of fatty liver, primarily due to accumulation of hepatocellular lipid droplets (LDs). The presence of membrane-trafficking proteins (e.g., Rab GTPases) with LDs indicates that LDs may be involved in trafficking pathways known to be altered in ethanol (EtOH) damaged hepatocytes. As these Rab GTPases are crucial regulators of protein trafficking, we examined the effect EtOH administration has on hepatic Rab protein content and association with LDs.

METHODS

Male Wistar rats were pair-fed Lieber-DeCarli diets for 5 to 8 weeks. Whole liver and isolated LD fractions were analyzed. Identification of LDs and associated Rab proteins was performed in frozen liver or paraffin-embedded sections followed by immunohistochemical analysis.

RESULTS

Lipid accumulation was characterized by larger LD vacuoles and increased total triglyceride content in EtOH-fed rats. Rabs 1, 2, 3d, 5, 7, and 18 were analyzed in postnuclear supernatant (PNS) as well as LDs. All of the Rabs were found in the PNS, and Rabs 1, 2, 5, and 7 did not show alcohol-altered content, while Rab 3d content was reduced by over 80%, and Rab 18 also showed EtOH-induced reduction in content. Rab 3d was not found to associate with LDs, while all other Rabs were found in the LD fractions, and several showed an EtOH-related decrease (Rabs 2, 5, 7, 18). Immunohistochemical analysis revealed the enhanced content of a LD-associated protein, perilipin 2 (PLIN2) that was paralleled with an associated decrease of Rab 18 in EtOH-fed rat sections.

CONCLUSIONS

Chronic EtOH feeding was associated with increased PLIN2 and altered Rab GTPase content in enriched LD fractions. Although mechanisms driving these changes are not established, further studies on intracellular protein trafficking and LD biology after alcohol administration will likely contribute to our understanding of fatty liver disease.

Quantitative optical trapping on single organelles in cell extract

We have developed an optical trapping method to precisely measure the force generated by motor proteins on single organelles of unknown size in cell extract. This approach, termed VMatch, permits the functional interrogation of native motor complexes. We apply VMatch to measure the force, number and activity of kinesin-1 on motile lipid droplets isolated from the liver of normally fed and food-deprived rats.

Nuclear lipid droplets: a novel nuclear domain

We investigated nuclear neutral-lipid (NL) composition and organization, as NL may represent an alternative source for providing fatty acids and cholesterol (C) to membranes, signaling paths, and transcription factors in the nucleus. We show here that nuclear NL were organized into nonpolar domains in the form of nuclear-lipid droplets (nLD). By fluorescent confocal microscopy, representative nLD were observed in situ within the nuclei of rat hepatocytes in vivo and HepG2 cells, maintained under standard conditions in culture, and within nuclei isolated from rat liver. nLD were resistant to Triton X-100 and became stained with Sudan Red, OsO4, and BODIPY493/503. nLD and control cytosolic-lipid droplets (cLD) were isolated from rat-liver nuclei and from homogenates, respectively, by sucrose-gradient sedimentation. Lipids were extracted, separated by thin-layer chromatography, and quantified. nLD were composed of 37% lipids and 63% proteins. The nLD lipid composition was as follows: 19% triacylglycerols (TAG), 39% cholesteryl esters, 27% C, and 15% polar lipids; whereas the cLD composition contained different proportions of these same lipid classes, in particular 91% TAG. The TAG fatty acids from both lipid droplets were enriched in oleic, linoleic, and palmitic acids. The TAG from the nLD corresponded to a small pool, whereas the TAG from the cLD constituted the main cellular pool (at about 100% yield from the total homogenate). In conclusion, nLD are a domain within the nucleus where NL are stored and organized and may be involved in nuclear lipid homeostasis.

A high-cholesterol, n-3 polyunsaturated fatty acid diet causes different responses in rats and hamsters

This study was designed to investigate the response to a high-cholesterol, n-3 polyunsaturated fatty acid (PUFA) or n-6 PUFA diet in rats and hamsters. Animals were fed n-3 or n-6 PUFA with a cholesterol-free diet, or with a diet enriched with cholesterol (0.5%, w/w) for 2 weeks. In rats and hamsters fed a cholesterol-free diet, plasma cholesterol, triglycerides and very-low-density lipoprotein (VLDL)-triglyceride levels in n-3 PUFA group were significantly lower than those in n-6 PUFA group. In contrast, when diets were supplemented with 0.5% cholesterol, the plasma cholesterol- and triglyceride-lowering effect of dietary n-3 PUFA disappeared. In hamsters fed with the atherogenic diet (0.5% dietary cholesterol) for 2 weeks, n-3 PUFA induced hypercholesterolemia more than n-6 PUFA, the increase being in the VLDL and low-density lipoprotein (LDL) fractions. Our data thus indicate that elevation of VLDL- and LDL-cholesterol in hamsters by n-3 PUFA, compared with n-6 PUFA, is dependent on 0.5% dietary cholesterol supplementation. In rats, on the other hand, dietary n-3 PUFA did not induce hypercholesterolemia more than n-6 PUFA when 0.5% cholesterol was supplemented. Although the effects of n-3 PUFA on plasma cholesterol, triglycerides and VLDL-triglycerides were similar in hamsters and rats, the interactive effects of n-3 PUFA and cholesterol on plasma and lipoprotein cholesterol levels differed in the two species. It was also found that plasma triglycerides, cholesterol and lipoprotein cholesterol levels in hamsters are higher than in rats in the presence and absence of dietary cholesterol. In addition, cholesterol feeding induces hypertriglyceridemia and hypercholesterolemia only in hamsters. Moreover, liver triglyceride concentrations increased in rats fed a cholesterol-rich diet and hepatic triglyceride levels of the n-3 PUFA-fed rats were significantly lower than those in the n-6 PUFA-fed rats in the presence and absence of dietary cholesterol. However, triglycerides did not accumulate in the liver in hamsters fed a cholesterol-rich diet and hepatic triglyceride levels of the n-3 PUFA-fed hamsters were not significantly different from those in the n-6 PUFA-fed hamsters in the presence and absence of dietary cholesterol. Therefore, these studies confirm marked species differences in response to the interactive effects of dietary n-3 PUFA and cholesterol.

Hepatitis C virus core protein shows a cytoplasmic localization and associates to cellular lipid storage droplets

There is now abundant evidence to substantiate an important role of hepatitis C virus (HCV) core protein in cellular gene expression as well as in the viral cycle. Thus the subcellular localization of this protein has important implications. However, several studies have shown controversial results: the HCV core has been, indeed, described as cytoplasmic or nuclear depending on the size of the protein or on the genotype analyzed. We have studied the localization of the HCV core protein in two different cell lines, one nonhepatic (CHO) and the other hepatic (HepG2). Double immunofluorescence staining using a nuclear membrane marker and confocal analysis showed the core protein pattern to be cytoplasmic and globular. This pattern is not cell cycle-regulated. Electron microscopy analysis revealed the nature of the globular staining observed in immunofluorescence. The HCV core protein accumulated at the surface of lipid droplets that were also the unique morphological feature of nonhepatic core transfected cells. The lipid droplets were isolated by sequential ultracentrifugation on the basis of their density; biochemical analysis revealed a prevalence of triglycerides. In addition the core protein colocalized with apolipoprotein AII at the surface of the lipid droplets as revealed by confocal microscopy. Moreover analysis of liver biopsies from chronically HCV-infected chimpanzees revealed that HCV core is cytoplasmic and localized on the endoplasmic reticulum and on lipid droplets. These results clearly define the subcellular localization of the HCV core protein and suggest a relationship between the expression of the HCV core protein and cellular lipid metabolism.

Reduced hepatic release of apoprotein B after enteral nutrition in rats

To investigate the effects of enteral and parenteral alimentation on VLDL release from the liver, a lipid-free liquid nutriment was continuously administered to free-moving rats via the oral cavity (oral group), stomach (enteral group) or superior caval vein (parenteral group). After 1-week of nutrition, the plasma VLDL concentrations were significantly lowered in the enterally-fed group. By immunoblotting assay using a specific antiserum, plasma contents of both apoprotein B-100 and B-48, the major components of rat apoprotein B, were found to be decreased in the enteral group, whereas only that of apoprotein B-48 was reduced in the parenteral group as compared with the oral group. Sucrose gradient centrifugation of the lipid droplets in the liver from the enteral group showed an increase of the free-triacylglycerol fraction with a concomitant increase of the apoprotein B-48-rich triacylglycerol fraction. These results suggest that enteral nutrition causes triacylglycerol accumulation in the liver, at least in part by impairment of lipoprotein release from the liver.

The appearance of neutral lipid signals in the 1H NMR spectra of a myeloma cell line correlates with the induced formation of cytoplasmic lipid droplets

The appearance of high resolution neutral lipid signals in the 1H NMR spectra of myeloma cells grown in the presence of oleate was shown to correlate with the appearance of cytoplasmic lipid droplets observable by electron microscopy. The spin-spin relaxation times of these lipid signals were similar to those measured previously for lipid resonances in other cell types. These data suggest that cytoplasmic lipid droplets could make a significant contribution to the neutral lipid signals observed in the 1H NMR spectra of some cells.

Acyl-CoA: cholesterol acyltransferase and 3-hydroxy-3-methylglutaryl-CoA reductase in carp-liver microsomes: effect of cold acclimation on enzyme activities and on hepatic and plasma lipid composition

Hepatic microsomal activities of acyl-CoA:cholesterol acyltransferase (ACAT) and 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, rate-limiting enzymes in cholesterol esterification and cholesterol synthesis, and the concentration sand compartmentalization of esterified and unesterified cholesterol, were studied in carp acclimated to 10 and 30 degrees C. Irrespective of acclimation temperature, carp-liver ACAT is characterized by an apparent Km-value for oleoyl-CoA of 11-15 microM and displays an optimum activity at pH 7.4. The enzyme activity is reduced approx. 2-fold upon preincubation of microsomes with alkaline phosphatase. Arrhenius plots of ACAT-activity are curvilinear, with curvatures considerably affected by the acclimation temperature of the fish. Carp HMG-CoA reductase has been characterized previously by Teichert and Wodtke ((1987) Biochim. Biophys. Acta 920, 161-170). When measured at 30 degrees C, ACAT activities from 30 degrees C- and 10 degrees C-acclimated carp are identical (approx. 6 pmol/min per mg protein), whilst 'expressed' HMG-CoA reductase activity (18.1 +/- 12.2 pmol/min per mg protein for 30 degrees C-acclimated carp vs. 159.8 +/- 106.6 pmol/min per mg protein for 10 degrees C-acclimated carp) is enhanced 9-fold in the cold environment. This disparity indicates that cold-acclimation results in a massive increase in the capacity for hepatic cholesterol synthesis relative to hepatic cholesterol esterification. At the same time, hepatic compositional analysis reveals identical contents of unesterified cholesterol in either groups of carp but significantly decreased (3-fold) amounts in cholesterol ester (and also in triacylglycerol, 4-fold) in cold-acclimated carp. Moreover, microsomal fractions display lower cholesterol to phospholipid ratios in the cold. In contrast, concentrations of either cholesterol fractions (and of triacylglycerols) in plasma--the mobile compartment for lipoprotein transport--do not differ in cold- and warm-acclimated carp. Based on current concepts of cholesterol metabolism, it is concluded that the cold-enhanced expression of hepatic HMG-CoA reductase activity is a homeostatic response directed against and compensating for a cold-induced but not yet characterized deficiency in hepatic cholesterol availability.

Some characteristics of a high molecular weight lipid-protein aggregate and its possible role in intracellular fatty acid metabolism

Several physical aspects of a high molecular weight lipid-protein aggregate separated by gel chromatography from chick and rat liver cytosol and its possible role in intracellular fatty acid metabolism were investigated. Electron microscopic examination of the high molecular weight lipid-protein aggregate indicated spherical particles with a diameter range of 200-600 A. This structure is consistent with a microemulsion particle of triglyceride encapsulated by phospholipid and protein. Uptake of fatty acids by microsomes occurred from the same lipid-protein aggregate, and the triglycerides synthesized in microsomes also became associated with these particles in the cytosol. The lipid-protein aggregate prepared by different homogenization methods showed identical ratios of components, but these ratios changed following incubation. These findings lend support to the concept that this aggregate plays a physiological role in intracellular lipid metabolism, and may be identifiable with previously reported subcellular fatty acid and triglyceride pools.