Effects of peroxisome proliferators on rat liver phospholipids: sphingomyelin degradation may be involved in hepatotoxic mechanism of perfluorodecanoic acid

Prenatal exposure to poly-/per-fluoroalkyl substances is associated with alteration of lipid profiles in cord-blood

INTRODUCTION

Poly-/per-fluoroalkyl substances (PFAS) are widespread environmental pollutants that may induce metabolic perturbations in humans, including particularly alterations in lipid profiles. Prenatal exposure to PFAS can cause lasting effects on offspring metabolic health, however, the underlying mechanisms are still unknown.

OBJECTIVES

The goal of the study was to investigate the impact of prenatal PFAS exposure on the lipid profiles in cord blood.

METHODS

Herein, we combined determination of bile acids (BAs) and molecular lipids by liquid chromatography with ultra-high-resolution mass spectrometry, and separately quantified cord blood concentrations of sixteen PFAS in a cohort of Chinese infants (104 subjects) in a cross-sectional study. We then evaluated associations between PFAS concentration and lipidome using partial correlation network analysis, debiased sparse partial correlation, linear regression analysis and correlation analysis.

RESULTS

PFAS levels showed significant associations with the lipid profiles; specifically, PFAS exposure was positively correlated with triacylgycerols (TG) and several bile acids. Importantly, exposure to perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), perfluorononanoic acid (PFNA) and perfluorohexane sulfonic acid (PFHxS) were associated with increased levels of TGs with saturated fatty acids while multiple classes of phospholipids were decreased. In addition, several free fatty acids showed significant positive correlations with PFOS.

CONCLUSIONS

Our results indicated that prenatal exposure to PFAS mediated metabolic changes, which may explain the associations reported between PFAS exposure and metabolic health later in life.

Lipid responses to environmental perfluoroalkyl substance exposure in a Taiwanese Child cohort

Although recent epidemiologic studies have focused on some of the health effects of perfluoroalkyl substance (PFASs) exposure in humans, the associations between PFASs exposure and the lipidome in children are still unclear. The purpose of this study was to assess lipid changes in children to understand possible molecular events of environmental PFASs exposure and suggest potential health effects. A total of 290 Taiwanese children (8-10 years old) were included in this study. Thirteen PFASs were analyzed in their serum by high-performance liquid chromatography-tandem mass spectrometry (LC-MS). MS-based lipidomic approaches were applied to examine lipid patterns in the serum of children exposed to different levels of PFASs. LC coupling with triple quadrupole MS technology was conducted to analyze phosphorylcholine-containing lipids. Multivariate analyses, such as partial least squares analysis along with univariate analyses, including multiple linear regression, were used to analyze associations between s exposure and unique lipid patterns. Our results showed that different lipid patterns were discovered in children exposed to different levels of specific PFASs, such as PFTrDA, PFOS, and PFDA. These changes in lipid levels may be involved in hepatic lipid metabolism, metabolic disorders, and PFASs-membrane interactions. This study showed that lipidomics is a powerful approach to identify critical PFASs that cause metabolite perturbation in the serum of children and suggest possible adverse health effects of these chemicals in children.

Effect of perfluorooctanesulfonic acid (PFOS) on the liver lipid metabolism of the developing chicken embryo

Perfluorooctanesulfonate (PFOS) is a well-known contaminant in the environment and it has shown to disrupt multiple biological pathways, particularly those related with lipid metabolism. In this study, we have studied the impact of in ovo exposure to PFOS on lipid metabolism in livers in developing chicken embryos using lipidomics for detailed characterization of the liver lipidome. We used an avian model (Gallus gallus domesticus) for in ovo treatment at two levels of PFOS. The lipid profile of the liver of the embryo was investigated by ultra-high performance liquid chromatography combined with quadrupole-time-of-flight mass spectrometry and by gas chromatography mass spectrometry. Over 170 lipids were identified, covering phospholipids, ceramides, di- and triacylglycerols, cholesterol esters and fatty acid composition of the lipids. The PFOS exposure caused dose dependent changes in the lipid levels, which included upregulation of specific phospholipids associated with the phosphatidylethanolamine N-methyltransferase (PEMT) pathway, triacylglycerols with low carbon number and double bond count as well as of lipotoxic ceramides and diacylglycerols. Our data suggest that at lower levels of exposure, mitochondrial fatty acid β-oxidation is suppressed while the peroxisomal fatty acid β -oxidation is increased. At higher doses, however, both β -oxidation pathways are upregulated.

Effects of perfluorooctane sulfonate on genes controlling hepatic fatty acid metabolism in livers of chicken embryos

Per- and polyfluoroalkyl substances (PFAS) are synthetic surfactants with a wide variety of applications; however, due to their stability, they are particularly resistant to degradation and, as such, are classed as persistent organic pollutants. Perfluorooctane sulfonate (PFOS) is one such PFAS that is still detectable in a range of different environmental settings, despite its use now being regulated in numerous countries. Elevated levels of PFOS have been detected in various avian species, and the impact of this on avian health is of interest when determining acceptable levels of PFOS in the environment. Due to its similarities to naturally occurring fatty acids, PFOS has potential to disrupt a range of biological pathways, particularly those associated with lipid metabolism, and this has been shown in various species. In this study, we have investigated how in ovo exposure to environmentally relevant levels of PFOS affects expression of genes involved in lipid metabolism of developing chicken embryos. We have found a broad suppression of transcription of genes involved in fatty acid oxidation and PPAR-mediated transcription with more significant effects apparent at lower doses of PFOS. These results highlight the need for more research investigating the biological impacts of low levels of PFAS to properly inform environmental policy governing their regulation.

Prenatal exposure to perfluoroalkyl and polyfluoroalkyl substances affects leukocyte telomere length in female newborns

Evidence has shown that leukocyte telomere length (LTL) at birth is related to the susceptibility to various diseases in later life and the setting of newborn LTL is influenced by the intrauterine environment. Perfluoroalkyl and polyfluoroalkyl substances (PFASs), as a kind of persistent organic pollutants, are commonly used in commercial and domestic applications and are capable of crossing the maternal-fetal barrier during pregnancy. We hypothesized that intrauterine exposure to PFASs may affect fetal LTL by increasing oxidative stress. To verify this hypothesis, LTL, concentrations of PFASs and reactive oxygen species (ROS) were measured in umbilical cord blood of 581 newborns from a prospective cohort. Our results showed that there were interactions between PFOS/PFDA and sex on LTL and ROS. The LTL was significantly shorter (0.926 ± 0.053 vs 0.945 ± 0.054, P = .023 for PFOS; 0.919 ± 0.063 vs 0.940 ± 0.059, P = .011 for PFDA) and the ROS levels were extremely higher (252.9 ± 60.5 [M] vs 233.5 ± 53.6 [M], P = .031 for PFOS; 255.2 ± 62.9 [M] vs 232.9 ± 58.3 [M], P = .011 for PFDA) in the female newborns whose PFOS or PFDA concentrations fell in the upmost quartile compared with those in the lowest quartile after adjusting for potential confounders. ROS levels were inversely associated with LTL in female newborns (β = -1.42 × 10^-4, P = .022). 13% of the effect of PFOS on female LTL was mediated through ROS approximately by the mediation analyses. However, in male newborns, no relationships among PFASs, ROS and LTL were observed. Our findings suggest a "programming" role of PFASs on fetal telomere biology system in females in intrauterine stage.

Study on the binding interaction between perfluoroalkyl acids and DNA

Perfluoroalkyl acids (PFAAs) are carcinogens, and elucidating their DNA binding properties is crucial for understanding PFAA genotoxicity. We have investigated the binding mode and affinity of five PFAAs to seven DNA molecules using fluorescence displacement and molecular docking analysis. DNA conformational changes upon PFAA binding were also examined by circular dichroism (CD). The data revealed that DNA intercalation was the dominant interaction mode of the PFAAs; however, these molecules also bound to grooves. The dissociation constants for the PFAAs ranged between 0.11 and 1,217.14 μM, and between 3.46 and 2,141.21 μM for DNA intercalation and groove binding, respectively. PFAAs that contain longer carbon chains had stronger DNA intercalation affinities. Binding to DNA was stronger for perfluoroalkyl sulfonates than for perfluorcarboxyl acids that contain the same number of carbons. This observation is postulated to arise from the presence of more fluorine and oxygen atoms in perfluoroalkyl sulfonates acting as hydrogen bond donors that facilitate stronger DNA intercalation. The binding of the PFAAs to DNA showed some CT-DNA sequence selectivity. Molecular docking analysis confirmed the DNA binding mode and affinities of the PFAAs. CD analysis revealed that the PFAAs weakened DNA base stacking and loosened DNA helicity. The present study has improved our understanding of the formation of PFAA-DNA adducts.

Effect of Di-(2-ethylhexyl)-phthalate on Sphingolipid Metabolic Enzymes in Rat Liver

Di-(2-ethylhexyl)-phthalate (DEHP), the most widely utilized industrial plastizer and a ubiquitous environmental contaminant, can act on peroxisome proliferators-activated nuclear hormone receptor family (PPAR) isoforms. To understand the contribution of sphingolipid metabolism to DEHP-induced hepatotoxicity, effect of DEHP exposure on activities of sphingolipid metabolic enzymes in rat liver was investigated. DEHP (250, 500 or 750 mg/kg) was administered to the rats through oral gavage daily for 28 days. The activities of acidic and alkaline ceramidases were slightly increased in 250 mg/kg DEHP-administered rat livers and significantly elevated in 500 mg/kg DEHP-administered ones, although the level of 750 mg/kg DEHP-administered ones was not increased. Neutral ceramidase, acidic and neutral sphingomyelinases, sphingomyeline synthase and ceramide syhthase were not changed at all by DEHP exposure. Therefore, acidic and alkaline ceramidases might play important roles in DEHP-induced hepatotoxicity.

Nrf2- and PPAR alpha-mediated regulation of hepatic Mrp transporters after exposure to perfluorooctanoic acid and perfluorodecanoic acid

Perfluorooctanoic acid and perfluorodecanoic acid (PFDA) are commonly used as emulsifiers and surfactants in fluoropolymer manufacturing and are known peroxisome proliferator-activated receptor alpha (PPAR alpha) agonists. PPAR alpha activation induces beta- and omega-oxidation enzymes such as Cyp4a14 and acyl-CoA oxidase, which are a likely cause of subsequent oxidative stress and peroxisome proliferation. Conversely, NF-E2-related factor-2 (Nrf2) is a transcription factor that protects against oxidative stress and inflammation by regulating several detoxification and xenobiotic transporter genes. Because PFDA markedly induces hepatic metabolism and oxidative stress, we hypothesized that PFDA exposure would increase expression of hepatic efflux multidrug resistance-associated protein (Mrp) transporters. A single PFDA dose (80 mg/kg) administered to mice increased hepatic Mrp3 (fourfold) and Mrp4 (31-fold) mRNA expression. Upregulation of Mrp3 and Mrp4 correlated with elevated serum-conjugated bilirubin and bile acids, respectively. To determine the mechanism of Mrp3 and Mrp4 induction, PFDA was administered to Nrf2-null mice, PPAR alpha-null mice, and mice pretreated with gadolinium chloride, a Kupffer cell-depleting chemical capable of inhibiting the inflammatory cytokine response. In both PPAR alpha- and Nrf2-null mice, maximal induction of Mrp3 and Mrp4 mRNA after PFDA administration was attenuated. Gadolinium chloride pretreatment reduced serum and hepatic tumor necrosis factor-alpha levels after PFDA treatment, as well as Mrp4 mRNA expression by 30%, suggesting that Kupffer cell-derived mediators may contribute to Mrp induction. Thus, after PFDA administration, the liver mounts a compensatory hepatoprotective response via PPAR alpha and Nrf2, markedly increasing Mrp3 and Mrp4 expression, with corresponding increases in serum of known Mrp3 and Mrp4 substrates.

Critical role of PPAR-alpha in perfluorooctanoic acid- and perfluorodecanoic acid-induced downregulation of Oatp uptake transporters in mouse livers

Perfluorooctanoic acid (PFOA) and perfluorodecanoic acid (PFDA) have been detected globally in wildlife and humans. Data from a gene array indicate that PFOA decreases organic anion transporting polypeptides (Oatps) in liver. Na(+)-taurocholate cotransporting polypeptide (Ntcp) and Oatp1a1, 1a4, and 1b2 are major transporters responsible for uptake of bile acids (BAs) and other organic compounds into liver. The purpose of the present study was to determine the effects of two perfluorinated fatty acids, PFOA and PFDA, on mRNA and protein expression of hepatic uptake transporters Oatps and Ntcp, and to determine the underlying regulatory mechanisms by using peroxisome proliferator-activated receptor alpha (PPAR-alpha), constitutive androstane receptor, pregnane-X receptor, NF-E2-related factor 2, and farnesoid X receptor-null mouse models. After 2 days following a single i.p. administration, PFOA did not alter serum BA concentrations, but PFDA increased serum BA concentrations 300%. Furthermore, PFOA decreased mRNA and protein expression of Oatp1a1, 1a4, and 1b2, but not Ntcp in mouse liver. In contrast, PFDA decreased mRNA and protein expression of all four transporters, and decreased the mRNA expression in a dose-dependent manner, with the decrease of Oatp1a4 occurring at lower doses than the other three transporters. Multiple mechanisms are likely involved in the down-regulation of mouse Oatps and Ntcp by PFDA. By using the various transcription factor-null mice, PPAR-alpha was shown to play a central role in the down-regulation of Oatp1a1, 1a4, 1b2, and Ntcp by PFDA. The current studies provide important insight into understanding the mechanisms by which PFDA regulate the expression of hepatic uptake transporters. In conclusion, PFOA and PFDA decrease mouse liver uptake transporters primarily via activation of PPAR-alpha.

Perfluorocarboxylic acids induce cytochrome P450 enzymes in mouse liver through activation of PPAR-alpha and CAR transcription factors

Cytochrome p450 enzymes (Cyps) are major phase-I xenobiotic-metabolizing enzymes. Cyps are regulated by many environmental chemicals and drugs. However, knowledge about regulation of Cyps by perfluorocarboxylic acids (PFCAs), which are persistent in the environment, is limited. Two days after a single i.p. administration (50 mg/kg) of perfluorooctanoic acid (PFOA) and perfluorodecanoic acid (PFDA) increased mRNA expression of Cyp2B10 (20-fold), 3A11 (two-fold), and 4A14 (32-fold), but not Cyp1A1/2 in mouse livers. PFDA and PFOA also markedly increased protein expression of Cyp2B (50-fold) and 4A (10-fold). PFDA increased Cyp4A14 mRNA expression at relatively low doses (0.5 mg/kg), but increased Cyp2B10 mRNA expression only at high doses (> 20 mg/kg). By using constitutive androstane receptor (CAR)-, pregnane-X receptor (PXR)-, peroxisome proliferator-activated receptor alpha (PPAR)-alpha-, and farnesoid X receptor-null mouse models, PPAR-alpha and CAR were shown to play central roles in the induction of Cyps by PFDA. Specifically, PFDA increased Cyp4A14 mRNA expression in wild-type (WT) mice, but much less in PPAR-alpha-null mice. PFDA increased Cyp2B10 mRNA expression in WT mice, but not in CAR-null mice. In addition, PFDA increased mRNA expression and nuclear translocation of the transcription factor CAR. Therefore, the current studies provide important insight into understanding the regulatory mechanisms initiated by PFCAs, and may help to better predict and understand the toxicokinetics and toxicodynamics of various PFCAs. In conclusion, PFCAs increased Cyp2B10 and 4A14 expression by activating PPAR-alpha and CAR nuclear receptors, respectively. PPAR-alpha is activated at much lower doses of PFDA than CAR.

Modes of action and species-specific effects of di-(2-ethylhexyl)phthalate in the liver

The industrial plasticizer di-(2-ethylhexyl)phthalate (DEHP) is used in manufacturing of a wide variety of polyvinyl chloride (PVC)-containing medical and consumer products. DEHP belongs to a class of chemicals known as peroxisome proliferators (PPs). PPs are a structurally diverse group of compounds that share many (but perhaps not all) biological effects and are characterized as non-genotoxic rodent carcinogens. This review focuses on the effect of DEHP in liver, a primary target organ for the pleiotropic effects of DEHP and other PPs. Specifically, liver parenchymal cells, identified herein as hepatocytes, are a major cell type that are responsive to exposure to PPs, including DEHP; however, other cell types in the liver may also play a role. The PP-induced increase in the number and size of peroxisomes in hepatocytes, so called 'peroxisome proliferation' that results in elevation of fatty acid metabolism, is a hallmark response to these compounds in the liver. A link between peroxisome proliferation and tumor formation has been a predominant, albeit questioned, theory to explain the cause of a hepatocarcinogenic effect of PPs. Other molecular events, such as induction of cell proliferation, decreased apoptosis, oxidative DNA damage, and selective clonal expansion of the initiated cells have been also been proposed to be critically involved in PP-induced carcinogenesis in liver. Considerable differences in the metabolism and molecular changes induced by DEHP in the liver, most predominantly the activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)alpha, have been identified between species. Both sexes of rats and mice develop adenomas and carcinomas after prolonged feeding with DEHP; however, limited DEHP-specific human data are available, even though exposure to DEHP and other phthalates is common in the general population. This likely constitutes the largest gap in our knowledge on the potential for DEHP to cause liver cancer in humans. Overall, it is believed that the sequence of key events that are relevant to DEHP-induced liver carcinogenesis in rodents involves the following events whereby the combination of the molecular signals and multiple pathways, rather than a single hallmark event (such as induction of PPARalpha and peroxisomal genes, or cell proliferation) contribute to the formation of tumors: (i) rapid metabolism of the parental compound to primary and secondary bioactive metabolites that are readily absorbed and distributed throughout the body; (ii) receptor-independent activation of hepatic macrophages and production of oxidants; (iii) activation of PPARalpha in hepatocytes and sustained increase in expression of peroxisomal and non-peroxisomal metabolism-related genes; (iv) enlargement of many hepatocellular organelles (peroxisomes, mitochondria, etc.); (v) rapid but transient increase in cell proliferation, and a decrease in apoptosis; (vi) sustained hepatomegaly; (vii) chronic low-level oxidative stress and accumulation of DNA damage; (viii) selective clonal expansion of the initiated cells; (ix) appearance of the pre-neoplastic nodules; (x) development of adenomas and carcinomas.

Studies of myo-inositol and plasmalogen metabolism in rat brain

Plasmalogens are ether-linked phospholipids that are abundant in nervous tissues. Their biological role is unclear, but may involve membrane structure/function and antioxidant activities. This study further investigates a recent report that chronic administration of myo-inositol in rats increased brain phosphatidylethanolamine plasmalogen (PlsEtn). We examined the effects of myo-inositol administration on the incorporation of [2-(13)C]ethanolamine ([2-(13)C]Etn) into rat brain phospholipids using NMR spectroscopy. Rats received either acute myo-inositol (single dose) +/- [2-(13)C]Etn, or chronic myo-inositol (10-day treatment) + [2-(13)C]Etn. Controls received saline rather than myo-inositol. Acute myo-inositol produced a 68% increase in brain [myo-inositol] and an increase in the incorporation of [2-(13)C]Etn into phospholipids (P < .05). The PlsEtn/phosphatidylethanolamine ratio and the [PlsEtn] were increased by 27% and 30%, respectively. The PlsEtn content as a mole percentage of total phospholipids was elevated (P < or = .05). Acute administration of myo-inositol + ethanolamine illustrates a positive correlation between the brain [myo-inositol] and the biosynthesis of ethanolamine phospholipids, with preferential synthesis of PlsEtn.

Predicting the biodegradation products of perfluorinated chemicals using CATABOL

Perfluorinated chemicals (PFCs) form a special category of organofluorine compounds with particularly useful and unique properties. Their large use over the past decades increased the interest in the study of their environmental fate. Fluorocarbons may have direct or indirect environmental impact through the products of their decomposition in the environment. It is a common knowledge that biodegradation is restricted within non-perfluorinated part of molecules: however, a number of studies showed that defluorination can readily occur during biotransformation. To evaluate the fate of PFCs in the environment a set of principal transformations was developed and implemented in the simulator of microbial degradation using the catabolite software engine (CATABOL). The simulator was used to generate metabolic pathways for 171 perfluorinated substances on Canada's domestic substances list. It was found that although the extent of biodegradation of parent compounds could reach 60%, persistent metabolites could be formed in significant quantities. During the microbial degradation a trend was observed where PFCs are transformed to more bioaccumulative and more toxic products. Perfluorooctanoic acid and perfluorooctanesulfonate were predicted to be the persistent biodegradation products of 17 and 27% of the perfluorinated sulphonic acid and carboxylic acid containing compounds, respectively.

Chromatin sphingomyelin changes in cell proliferation and/or apoptosis induced by ciprofibrate

It has been shown that neutral-sphingomyelinase and sphingomyelin-synthase activities are present in chromatin and they modify the sphingomyelin (SM) content. The activity of the first enzyme is stimulated and the second inhibited, when the hepatocytes enter into the S-phase after partial hepatectomy, thus suggesting that ceramide may have a pivotal role in cell proliferation. An opposite function was attributed to ceramide in hepatocytes which undergo apoptosis after lobular ligature. In order to clarify this point, a model was developed in which the same liver cells undergo proliferation followed by induced apoptosis. To this purpose, the rats were treated for 7 days with ciprofibrate and then left without treatment for 4 days. During the treatment, the peroxisome enzyme markers increase their activity and the number of proliferating cells increases, reaching a maximum after 3 days of treatment, as shown by the number of cells positive for the proliferating cell nuclear antigen. At the same time, the chromatin sphingomyelinase activity reaches the maximum, while a similar increase is not found in the cytoplasm or in the isolated nuclei. On the contrary, SM-synthase activity is depressed in chromatin, but not in the nuclei in which a peak is shown after 3 days of ciprofibrate treatment. After drug withdrawal, the hepatocytes undergo apoptosis as confirmed by the increase of Bax and tissue transglutaminase (tTGase) expression; the chromatin SM increases as a consequence of an increase of SM-synthase activity. It can be hypothesised that chromatin SM may have a role in cell duplication by influencing the chromatin structure stability.

Characterization of the adipose tissue atrophy induced by peroxisome proliferators in mice

In the present study, we characterized the effects of peroxisome proliferators (PP) on adipose tissue in mice. Treatment with potent PP, such as perfluorooctanoic acid (PFOA), 2-methyl-2-(p(1,2,3,4-tetrahydroxy-naphthyl)-phenoxy)propionic acid, (4-chloro-6-(2,3-xylidino)2-pyrimidinylthio) acetic acid, and di(2-ethylhexyl)phthalate, caused dramatic decreases in adipose tissue weight, whereas the moderately potent PP, acetylsalicylic acid, had a relatively weak effect. This decrease in weight reflects a loss of fat from adipocytes rather than a loss of cells, as demonstrated by constant DNA content. The dose-dependency and time-course experiments indicate that peroxisome proliferation occurs simultaneously with or prior to adipose tissue atrophy. Thus, hepatic peroxisome proliferation might result in the increased mobilization of lipids and lipid utilization in liver. The enhanced adipose tissue hormone-sensitive lipase (HSL) activity and down-regulated lipoprotein lipase (LPL) activity observed upon PP treatment might, at least in part, explain the loss of fat via increased FA release from adipocytes and/or decreased FA uptake from the circulation, respectively. In addition, the possible involvement of the increased tumor necrosis factor alpha expression found upon PFOA treatment in reducing the insulin sensitivity of adipose tissue and thereby altering LPL and HSL activities is discussed.

Major clofibrate effects on liver and plasma lipids are independent of changes in polyunsaturated fatty acid composition induced by dietary fat

The effects of clofibrate on the content and composition of liver and plasma lipids was studied in mice fed for 4 wk on diets enriched in n-6 or n-3 polyunsaturated fatty acids (PUFA) from sunflower oil (SO) or fish oil (FO), respectively; both oils were fed at 9% of the diet (dry weight basis). Only FO was hypolipidemic. Both oil regimes led to slightly increased concentrations of phospholipids (PL) and triacylglycerols (TG) in liver as compared with a standard chow diet containing 2% fat. Clofibrate promoted hypolipidemia only in animals fed SO. Its main effect was to enlarge the liver, such growth increasing the amounts of major glycerophospholipids while depleting the TG. SO and FO consumption changed the proportion of n-6 or n-3 PUFA in liver and plasma lipids in opposite ways. After clofibrate action, the PUFA of liver PL were preserved better than in the absence of oil supplementation. However, most of the drug-induced changes (e.g., increased 18:1n-9 and 20:3n-6, decreased 22:6/20:5 ratios) occurred irrespective of lipids being rich in n-6 or n-3 PUFA. The concentration of sphingomyelin (SM), a minor liver lipid that virtually lacks PUFA, increased with the dietary oils, decreased with clofibrate, and changed its fatty acid composition in both situations. Thus, oil-increased SM had more 22:0 and 24:0 than clofibrate-decreased SM, which was significantly richer in 22:1 and 24:1.

Conjugated linoleic acids exhibit a strong fat-to-lean partitioning effect, reduce serum VLDL lipids and redistribute tissue lipids in food-restricted rats

Effects of conjugated linoleic acids (CLA) on a series of metabolic events are expected to depend on the feeding regimen and levels of energy ingested. This study was the first examining the mode of action of CLA on body composition, tissue lipids, lipoproteins and hepatic enzymes in situations of enhanced fat store mobilization. Two groups of male growing Sprague-Dawley rats were fed for 3 wk a diet containing 0 (control group) or 3 g/100 g of a CLA mixture at the expense of sunflower oil, and were then subjected to a weight-loss feeding regimen for another 18 d. Rats fed the CLA-fortified diet gained 11% less weight than the control rats (P<0.05). Rats fed the high CLA diet had less body fat (1.47+/-0.16 vs. 1.07+/-0.09 g/100g, P<0.05) and a higher lean deposition (25.6+/-0.2 vs. 28.4+/-0.3 g/100 g, P<0.05) than control rats. CLA-fed rats had a 41% lower cholesterol concentration in liver than the control rats (P<0.05). Some differences in glycerophospholipid subclass profile of liver and erythrocyte membrane were observed; the hepatic concentrations of phosphatidylethanolamine (4.76+/-0.46 vs. 6.86+/-0.99 micromol/g, P = 0.07) and phosphatidylcholine (12.9+/-0.5 vs. 15.3+/-1.2 micromol/g, P = 0.09) tended to be greater and the level of phosphatidylcholine in erythrocyte membranes was significantly greater (1.40+/-0.12 vs. 1.83 +/-0.16 micromol/g, P<0.05) in the CLA-treated group than in the control group. The activities of catalase and ornithine decarboxylase in liver did not differ between the groups. Further, CLA-treated rats had significantly lower serum concentrations of VLDL lipids than control rats, whereas concentrations of LDL and HDL lipids were unaffected. The results indicate that a high dose of a CLA mixture is a strong repartitioning agent and a modulator of lipid metabolism under conditions of enhanced fat store mobilization in rats.

NMR spectroscopic analyses of liver phosphatidylcholine and phosphatidylethanolamine biosynthesis in rats exposed to peroxisome proliferators-A class of nongenotoxic hepatocarcinogens

Peroxisome proliferators (PPs) are commercial/industrial chemicals that display tumor promoter activity in rodents. The mechanism is not completely understood, and our ability to predict tumorigenicity a priori is even less developed. Wy-14,643, perfluorooctanoic acid (PFOA), and di(2-ethylhexyl)phthalate (DEHP) are strong, moderate, and weak tumor promoters, respectively, while perfluorodecanoic acid (PFDA) lacks promoter activity. This investigation examined the effects of these PPs on the biosyntheses of phosphatidylcholine (PtdC) and phosphatidylethanolamine (PtdE) in rat liver. After exposure to PPs, rats were administered [1-(13)C]choline + [2-(13)C]ethanolamine and liver extracts were analyzed by (31)P and (13)C NMR. The ratio of choline-derived to ethanolamine-derived phospholipids, R(c/e), was significantly affected by all PPs (p < 0. 05). R(c/e) values were in the order Wy-14,643 > PFOA > DEHP > control > PFDA. The amounts of PtdC derived via the CDP-choline pathway versus PtdE-N-methyltransferase (PEMT) activity was 71 vs 29% in controls. This distribution was significantly affected by treatments with Wy-14,643 (95 vs 5%), DEHP (87 vs 13%), and PFDA (39 vs 61%) (p < 0.02). Data suggest that Wy-14,643, PFOA, and DEHP cause a preference for choline and the CDP-choline pathway for biosynthesis of PtdC. Additionally, Wy-14,643 and DEHP inhibited the PEMT pathway. In contrast, PFDA-treated rats showed a preference for ethanolamine, and PtdC was predominately synthesized through the PEMT pathway. These data corroborate studies by Vance and co-workers which suggest that the pathways for PtdC biosynthesis are important for hepatocarcinogenesis. Further studies to evaluate the potential of these measurements as a biomarker for PP-associated tumorigenesis is warranted.

High dietary levels of a conjugated linoleic acid mixture alter hepatic glycerophospholipid class profile and cholesterol-carrying serum lipoproteins of rats

To investigate the body composition, hepatic lipids, and serum lipoproteins in response to graded levels of a conjugated linoleic acid (CLA) mixture added to a high linoleate diet, adult male Sprague-Dawley rats were randomly assigned into four dietary groups of 10 rats each and fed for 5 weeks controlled amounts of diets containing 0%, 1%, 3%, or 5% of a CLA mixture in exchange for sunflower oil. The various dietary lipid treatments did not significantly influence growth and body partitioning, although there was a trend toward decreased contents of extractable lipids in carcass (whole bled body without liver and gut) with increasing CLA. When carcass lipids of CLA-treated rats were extracted, a distinct accumulation of total CLA was observed. A dietary level of 1% CLA mixture exhibited only weak effects on hepatic glycerophospholipid levels. CLA levels of 3% and 5% caused distinct changes in phospholipid subclass distribution. These changes were reduced levels of lysophosphatidylethanolamine (LPE) and ethanolamine plasmalogen (EPL) and increased levels of phosphatidylcholine (PC). Further, a 5% level of CLA increased the hepatic concentration of phosphatidylserine (PS) compared with the other treatments. The incorporation of total CLA into individual phospholipids followed a dose-responsive manner. The extent of incorporation of CLA was not the same among the glycerophospholipid species analyzed, the order being cardiolipin > phosphatidylethanolamine and PC > LPE/EPL > phosphatidylinositol > PS. Further, CLA increased the proportions of n-3 fatty acids in the individual glycerophospholipids. High CLA diets containing 3% and 5% of a CLA mixture were associated with increased activity of catalase in the peroxisome-enriched cell fraction of liver and exhibited marked reductions of cholesterol in the low and high density lipoproteins relative to rats receiving no CLA.

Novel role of oxidants in the molecular mechanism of action of peroxisome proliferators

Peroxisome proliferators are nongenotoxic rodent carcinogens that act as tumor promoters by increasing cell proliferation; however, their precise mechanism of action is not well understood. Oxidative DNA damage caused by leakage of hydrogen peroxide (H2O2) from peroxisomes was hypothesized initially as the mechanism by which these compounds cause liver tumors. It seems unlikely that oxidants of peroxisomal origin explain the mechanism of action of peroxisome proliferators because treatment with these compounds in vivo does not lead to increased H2O2 production. On the other hand, Kupffer cell-derived oxidants, such as superoxide, may play a role in initiating tumor nerosis factor-alpha (TNF-alpha) production that leads to hepatocyte proliferation. Peroxisome proliferators have been shown to activate Kupffer cells both in vitro and in vivo, and the use of Kupffer cell inhibitors such as methyl palmitate and dietary glycine have demonstrated that Kupffer cells are responsible for hepatocyte proliferation by mechanisms involve TNF-alpha. Moreover, peroxisome proliferators activate the transcription factor NF-kappaB, one of the major regulators of TNF-alpha expression, in Kupffer cells. Importantly, activation of NF-kappaB by peroxisome proliferators was shown to be oxidant-dependent, leading to the hypothesis that oxidants of Kupffer cell origin are involved in the mechanism of action. Many of the effects of peroxisome proliferators, including peroxisome induction and hepatomegaly, involve the peroxisome proliferator-activated receptor-alpha (PPARalpha). Recently, it was shown that peroxisome proliferator-induced cell proliferation and tumors require the PPARalpha. However, PPARalpha is not involved in TNF-alpha production by Kupffer cells because it is not expressed in this cell type. How it is involved in liver tumor remains unclear and one possible explanation is that both Kupffer cell TNF-alpha and parenchymal cell PPARalpha are required. Collectively, recent data are consistent with the hypothesis that oxidants play a role in signaling hepatocellular proliferation due to peroxisome proliferators via activation of NF-kappaB and incrase in mitogenic cytokines such as TNF-alpha.

Dose-response hapatotoxicity of the peroxisome proliferator, perfluorodecanoic acid and the relationship to phospholipid metabolism in rats

Perfluorodecanoic acid (PFDA) is a potent peroxisome proliferator that causes hepatotoxicity but lacks tumor-promoting activity in rats. We previously showed that a single dose of PFDA at 50 mg/kg (approximately LD50) causes an elevation in liver phosphocholine (PCho) and other effects related to phospholipid metabolism. In this study, we examined metabolic effects in the dose range 2-50 mg/kg in rats. At doses < or =20 mg/kg, PFDA is significantly less hepatotoxic than the LD50 as manifested by electron microscopy and measurements of daily food consumption and body weight. At 50 mg/kg rat serum tumor necrosis factor (TNF)-alpha concentration was increased 8-fold, while at 15 mg/kg there was no apparent increase in this cytokine. This lower dose, however, induces metabolic effects similar to those seen at the LD50. Liver fatty acyl-CoA oxidase activity showed a dose-dependent increase from 5-25 mg/kg PFDA. Treatments at 15 and 50 mg/kg caused a significant increase in liver phosphatidylcholine (28 and 66%) and phosphatidylethanolamine (31 and 74%). Both doses caused a significant increase in liver PCho but did not affect liver ATP levels, as manifested in 31P nuclear magnetic resonance (NMR) spectra from rat livers in vivo. These data suggest that the increase in liver [PCho] observed following PFDA exposure in rats represents a specific metabolic response, rather than a broad-range hepatotoxic effect.

Alterations by perfluorooctanoic acid of glycerolipid metabolism in rat liver

The effects of perfluorooctanoic acid (PFOA) feeding on hepatic levels of glycerolipids and the underlying mechanism were investigated. Feeding of rats with 0.01% of PFOA in the diet for 1 week caused an increase in the contents of phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylinositol (PtdIns), phosphatidylserine (PtdSer) and triglyceride (TG), which were 2.2, 2.4, 2.4, 1.6 and 5.2 times over control, respectively, on the basis of whole liver. The activities of glycerol-3-phosphate acyltransferase, diacylglycerol kinase and PtdSer decarboxylase were significantly increased upon PFOA feeding, whereas the activities of CTP:phosphoethanolamine cytidylyltransferase and PtdEtn N-methyltransferase were decreased. On the other hand, the activity of CTP:phosphocholine cytidylyltransferase was not increased by PFOA. Upon PFOA feeding, hepatic level of 16:0-18:1 PtdCho was markedly increased and, by contrast, the levels of molecular species of PtdCho which contain 18:2 were decreased, resulting in the reduced concentration of molecular species of serum PtdCho containing 18:2. The increase in the level of hepatic 16:0-18:1 PtdCho seemed to be due to 3-fold increase in the activities of both delta9 desaturase and 1-acylglycerophosphocholine (1-acyl-GPC) acyltransferase. The mechanism by which PFOA causes the accumulation of glycerolipids in liver was discussed.