Effects of fatty acids on hepatic gene expression

Mediterranean diet affects the metabolic outcome of metabolic dysfunction-associated fatty liver disease

The prevalence of metabolic dysfunction-associated fatty liver disease (MAFLD) is on the rise globally. It is currently one of the most prevalent liver diseases and one of the world's important public health problems. At present, there is no consensus on a pharmacological treatment for MAFLD. By contrast, lifestyle interventions based on exercise and a balanced diet are considered to be the cornerstone of MAFLD management. Mediterranean diet (MD) have a large content of polyphenols, polyunsaturated fatty acids, oleic acid, carotenoids and fiber, which carry out antioxidant, anti-inflammatory and antibacterial benefits. It has been considered to reduce the incidence rate of cardiovascular disease and type 2 diabetes. The purpose of this narrative review is therefore to summarize and analyze the evidence for the effect of MD on metabolic outcomes in MAFLD patients.

Biotechnology Challenges to In Vitro Maturation of Hepatic Stem Cells

The incidence of liver disease is increasing globally. The only curative therapy for severe end-stage liver disease, liver transplantation, is limited by the shortage of organ donors. In vitro models of liver physiology have been developed and new technologies and approaches are progressing rapidly. Stem cells might be used as a source of liver tissue for development of models, therapies, and tissue-engineering applications. However, we have been unable to generate and maintain stable and mature adult liver cells ex vivo. We review factors that promote hepatocyte differentiation and maturation, including growth factors, transcription factors, microRNAs, small molecules, and the microenvironment. We discuss how the hepatic circulation, microbiome, and nutrition affect liver function, and the criteria for considering cells derived from stem cells to be fully mature hepatocytes. We explain the challenges to cell transplantation and consider future technologies for use in hepatic stem cell maturation, including 3-dimensional biofabrication and genome modification.

Effects of perilla extract on productive performance, serum values and hepatic expression of lipid-related genes in Shaoxing ducks

1. The aim of this study was to identify the effect of perilla extract, a source of polyunsaturated fatty acids, on lipid metabolism and expression of lipid-related genes in livers of Shaoxing ducks. 2. Two hundred and forty 28-week-old laying ducks received a commercial diet with perilla extract added at 0 (control) or 200 mg/kg of feed. 3. Ducks fed on a diet with perilla extract had increased laying rates compared with control ducks. 4. Serum concentrations of triglycerides were reduced by perilla extract, while high-density lipoprotein cholesterol and total serum cholesterol increased. 5. The expression of genes involved in hepatic lipogenesis, sterol regulatory element-binding protein-1, acetyl CoA carboxylase, stearoyl CoA desaturase, fatty acid synthase, apolipoprotein B, and apolipoprotein very low density lipoprotein, were decreased in the perilla group. 6. The mRNA expression of peroxisome proliferators-activated receptor alpha and acyl-coenzyme A oxidase was enhanced following treatment with perilla extract, and a similar tendency was observed in the expression of liver fatty acid-binding protein. 7. The results show that a diet with 200 mg/kg perilla extract regulated fat metabolism of Shaoxing ducks by improving egg laying, altering serum lipid profiles, stimulating lipid catabolic gene expression and inhibiting lipogenic gene expression in the liver.

A defect in Δ6 and Δ5 desaturases may be a factor in the initiation and progression of insulin resistance, the metabolic syndrome and ischemic heart disease in South Asians

The high incidence of insulin resistance and the metabolic syndrome in South Asians remains unexplained. I propose that a defect in the activity of Δ⁶ and Δ⁵ desaturases and consequent low plasma and tissue concentrations of polyunsaturated fatty acids such as γ-linolenic acid (GLA), dihomo-γ-linolenic acid (DGLA), arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and formation of their anti-inflammatory products prostaglandin E₁ (PGE₁), prostacyclin (PGI₂), PGI₃, lipoxins, resolvins, protectins, maresins and nitrolipids could be responsible for the high incidence of insulin resistance, the metabolic syndrome and ischemic heart disease (IHD) in South Asians. This proposal is supported by the observation that South Asian Indians have lower plasma and tissue concentrations of GLA, DGLA, AA, EPA and DHA, the precursors of PGE₁, PGI₂, PGI₃, lipoxins, resolvins, protectins, and nitrolipids, the endogenous molecules that prevent platelet aggregation, vasoconstriction, thrombus formation, leukocyte activation and possess anti-inflammatory action and thus, are capable of preventing the development of insulin resistance, atherosclerosis, hypertension, type 2 diabetes mellitus and premature ischemic heart disease. Genetic predisposition, high carbohydrate intake, lack of exercise, tobacco use and low birth weight due to maternal malnutrition suppress the activity of Δ⁶ and Δ⁵ desaturases that leads to low plasma and tissue concentrations of polyunsaturated fatty acids and their products. This implies that adequate provision of polyunsaturated fatty acids and co-factors needed for their metabolism, and efforts to enhance the formation of their beneficial metabolites PGE₁, PGI₂, PGI₃, lipoxins, resolvins, protectins, maresins and nitrolipids could form a novel approach in the prevention and management of these diseases in this high-risk population.

Dietary fatty acid composition differently influences retinoylation reaction in rat testes mitochondria

All-trans-retinoic acid (atRA) is incorporated covalently into proteins of rat testes mitochondria. In this study, the effect of three diets with different fatty acid composition on the retinoylation of proteins of rat testes mitochondria has been investigated. Different groups of rats were fed on a basal diet supplemented with 15% of either coconut oil (CO), olive oil (OO) or fish oil (FO). We found that, when compared with CO, the binding of retinoic acid was decreased in FO- and OO-fed rats. Mitochondrial phospholipids composition was differently influenced by dietary treatments; minor changes were observed in fatty acid composition of phospholipids. Few differences were observed in the Arrhenius plots among the three groups of rats. Kinetic analysis revealed a decrease in the V (max) value in FO- and OO- as compared with CO-fed rats. No difference among the three groups were observed in the K ( M ) value. The retinoylation reaction was inhibited by 13-cis-RA and 9-cis-RA.

Mechanisms of oleoylethanolamide-induced changes in feeding behavior and motor activity

Oleoylethanolamide (OEA), a lipid synthesized in the intestine, reduces food intake and stimulates lipolysis through peroxisome proliferator-activated receptor-α. OEA also activates transient receptor potential vanilloid type 1 (TRPV1) in vitro. Because the anorexigenic effect of OEA is associated with delayed feeding onset and reduced locomotion, we examined whether intraperitoneal administration of OEA results in nonspecific behavioral effects that contribute to the anorexia in rats. Moreover, we determined whether circulating levels of other gut hormones are modulated by OEA and whether CCK is involved in OEA-induced anorexia. Our results indicate that OEA reduces food intake without causing a conditioned taste aversion or reducing sodium appetite. It also failed to induce a conditioned place aversion. However, OEA induced changes in posture and reduced spontaneous activity in the open field. This likely underlies the reduced heat expenditure and sodium consumption observed after OEA injection, which disappeared within 1 h. The effects of OEA on motor activity were similar to those of the TRPV1 agonist capsaicin and were also observed with the peroxisome proliferator-activated receptor-α agonist Wy-14643. Plasma levels of ghrelin, peptide YY, glucagon-like peptide 1, and apolipoprotein A-IV were not changed by OEA. Finally, antagonism of CCK-1 receptors did not affect OEA-induced anorexia. These results suggest that OEA suppresses feeding without causing visceral illness and that neither ghrelin, peptide YY, glucagon-like peptide 1, apolipoprotein A-IV, nor CCK plays a critical role in this effect. Despite that OEA-induced anorexia is unlikely to be due to impaired motor activity, our data raise a cautionary note in how specific behavioral and metabolic effects of OEA should be interpreted.

Arachidonic acid, an omega-6 fatty acid, induces cytoplasmic phospholipase A2 in prostate carcinoma cells

For the past 60 years, dietary intake of essential fatty acids has increased. Moreover, the omega-6 fatty acids have recently been found to play an important role in regulation of gene expression. Proliferation of human prostate cells was significantly increased 48 h after arachidonic acid (AA) addition. We have analyzed initial uptake using nile red fluorescence and we found that the albumin conjugated AA is endocytosed into the cells followed by the induction of RNA within minutes, protein and PGE2 synthesis within hours. Here we describe that AA induces expression of cytosolic phospholipase A2 (cPLA2) in a dose-dependent manner and that this upregulation is dependent upon downstream synthesis of PGE2. The upregulation of cox-2 and cPLA2 was inhibited by flurbiprofen, a cyclooxygenase (COX) inhibitor, making this a second feed-forward enzyme in the eicosanoid pathway. Cox-2 specific inhibitors are known to inhibit colon and prostate cancer growth in humans; however, recent findings show that some of these have cardiovascular complications. Since cPLA2 is upstream in the eicosanoid pathway, it may be a good alternative for a pharmaceutical target for the treatment of cancer.

Induction of apoptosis by fatty acid ethyl esters in HepG2 cells

Fatty acid ethyl esters (FAEEs) are esterification products of ethanol and fatty acids which have been found particularly in the organ damaged by ethanol abuse. To evaluate any effect of FAEEs on HepG2 cells, we added FAEEs to cell culture medium. Electrophoresis of DNA from HepG2 cells exposed to 18.5 microM ethyl palmitate (EP) and 10.6 microM ethyl stearate (ES) for 24 h revealed a smear which is typical of non-specific degradation by DNA ladder assay. Apoptosis was characterized by electron microscopy, flow cytometry revealed that the cell cycle of HepG2 cells was perturbed by exposure to FAEEs. In the present study we demonstrate that treatment of HepG2 cells with EP and ES induces apoptosis, as well as perturbing the cell cycle as the number of cells in the G(2)/M and S phases decreased.

Role of nuclear receptors in the regulation of gene expression by dietary fatty acids (review)

Long chain fatty acids, derived either from endogenous metabolism or by nutritional sources play significant roles in important biological processes of membrane structure, production of biologically active compounds, and participation in cellular signaling processes. Recently, the structure of dietary fatty acids has become an important issue in human health because ingestion of saturated fats (containing triglycerides composed of saturated fatty acids) is considered harmful, while unsaturated fats are viewed as beneficial. It is important to note that the molecular reason for this dichotomy still remains elusive. Since fatty acids are important players in development of pathology of cardiovascular and endocrine system, understanding the key molecular targets of fatty acids, in particular those that discriminate between saturated and unsaturated fats, is much needed. Recently, insights have been gained on several fatty acid-activated nuclear receptors involved in gene expression. In other words, we can now envision long chain fatty acids as regulators of signal transduction processes and gene regulation, which in turn will dictate their roles in health and disease. In this review, we will discuss fatty acid-mediated regulation of nuclear receptors. We will focus on peroxisome proliferators-activated receptors (PPARs), liver X receptors (LXR), retinoid X receptors (RXRs), and Hepatocyte Nuclear Factor alpha (HNF-4alpha), all of which play pivotal roles in dietary fatty acid-mediated effects. Also, the regulation of gene expression by Conjugated Linoleic Acids (CLA), a family of dienoic fatty acids with a variety of beneficial effects, will be discussed.

Nutrient-gene interactions: a single nutrient and hundreds of target genes

Based on the effects of a selective experimental zinc deficiency in a rodent model we explore the use of transcriptome profiling for assessing nutrient-gene interactions in the liver at the molecular and cellular levels. Zinc deficiency caused pleiotropic alterations in mRNA/protein levels of hundreds of genes. In the context of observed metabolic alterations in hepatic metabolism, possible mechanisms are discussed for how a low zinc status may be sensed and transmitted into changes in various metabolic pathways. However, it also becomes obvious that analysis of such complex nutrient-gene interactions beyond the descriptional level is a real challenge for systems biology.

Peroxisome proliferators and fatty acids negatively regulate liver X receptor-mediated activity and sterol biosynthesis

Peroxisome proliferators (PPs) are potent tumor promoters in rodents. The mechanism of hepatocarcinogenesis requires the nuclear receptor peroxisome proliferator activated receptor-alpha (PPARalpha), but might also involve the PPARalpha independent alteration of signaling pathways that regulate cell growth. Here, we studied the effects of PPs on the mevalonate pathway, a critical pathway that controls cell proliferation. Liver X receptors (LXRs) are nuclear receptors that act as sterol sensors in the mevalonate pathway. In gene reporter assays in COS-7 cells, the basal activity of the LXR responsive reporter gene (LXRE-luc) was suppressed by 10 microM lovastatin and zaragozic acid A, suggesting that this activity was attributed to the activation of native LXRs, by endogenously produced mevalonate products. The potent PP and rodent tumor promoter, pirinixic acid (WY-14643) also inhibited LXR-mediated transcription in a dose related manner (approximate IC(50) of 100 microM). As did several other PPs including ciprofibric acid and mono-ethylhexylphthalate. Polyunsaturated and medium to long chain fatty acids at 100 microM were also potent inhibitors; the arachidonic acid analogue eicosatetraynoic acid being the most active (approximate IC(50) of 10 microM). Of the PPs and fatty acids tested, there was a strong correlation between the ability of these agents to suppress de novo sterol synthesis in a rat hepatoma cell line, H4IIEC3, and inhibit LXR-mediated transcription in COS-7 cells, but a discordance between these endpoints and PPARalpha activation and fatty acid acyl-CoA oxidase induction. Taken together, these results suggest that PPs and fatty acids negatively regulate the mevalonate pathway through a mechanism that is not entirely dependent on PPARalpha activation. Because of the importance of the mevalonate pathway in regulating cell proliferation, the modulation of this pathway by PPs and fatty acids might contribute to their actions on cell growth/differentiation.

Peroxisome proliferator-activated receptors: a family of lipid-activated transcription factors

Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear transcription factors that belong to the steroid receptor superfamily. This family of PPARs includes PPARalpha, PPARdelta, PPARgamma1, and PPARgamma2. These PPARs are related to the T3 and vitamin D(3) receptors and bind to a hexameric direct repeat as a heterodimeric complex with retinoid receptor Xalpha. PPARs regulate the expression of a wide array of genes that encode proteins involved in lipid metabolism, energy balance, eicosanoid signaling, cell differentiation, and tumorigenesis. A unique feature of these steroid-like receptors is that the physiologic ligands for PPARs appear to be fatty acids from the n-6 and n-3 families of fatty acids and their respective eicosanoid products. This review describes the characteristics, regulation, and gene targets for PPARs and relates their effects on gene expression to physiologic outcomes that affect lipid and glucose metabolism, thermogenesis, atherosclerosis, and cell differentiation.

Regulation of gene expression by dietary fat

Dietary fat is an important macronutrient for the growth and development of all organisms. In addition to its role as an energy source and its effects on membrane lipid composition, dietary fat has profound effects on gene expression, leading to changes in metabolism, growth, and cell differentiation. The effects of dietary fat on gene expression reflect an adaptive response to changes in the quantity and type of fat ingested. Specific fatty acid-regulated transcription factors have been identified in bacteria, amphibians, and mammals. In mammals, these factors include peroxisome proliferator-activated receptors (PPAR alpha, -beta, and -gamma), HNF4 alpha, NF kappa B, and SREBP1c. These factors are regulated by (a) direct binding of fatty acids, fatty acyl-coenzyme A, or oxidized fatty acids; (b) oxidized fatty acid (eicosanoid) regulation of G-protein-linked cell surface receptors and activation of signaling cascades targeting the nucleus; or (c) oxidized fatty acid regulation of intracellular calcium levels, which affect cell signaling cascades targeting the nucleus. At the cellular level, the physiological response to fatty acids will depend on (a) the quantity, chemistry, and duration of the fat ingested; (b) cell-specific fatty acid metabolism (oxidative pathways, kinetics, and competing reactions); (c) cellular abundance of specific nuclear and membrane receptors; and (d) involvement of specific transcription factors in gene expression. These mechanisms are involved in the control of carbohydrate and lipid metabolism, cell differentiation and growth, and cytokine, adhesion molecule, and eicosanoid production. The effects of fatty acids on the genome provide new insight into how dietary fat might play a role in health and disease.

Dietary polyunsaturated fatty acids and hepatic gene expression

Dietary polyunsaturated fatty acids (PUFA) have profound effects on hepatic gene transcription leading to significant changes in lipid metabolism. PUFA rapidly suppress transcription of genes encoding specific lipogenic and glycolytic enzymes and induce genes encoding specific peroxisomal and cytochrome P450 (CYP) enzymes. Using the peroxisome proliferator-activated receptor alpha (PPAR alpha)-null mouse, we showed that dietary PUFA induction of acyl CoA oxidase (AOX) and CYP4A2 require PPAR alpha. However, PPAR alpha is not required for the PUFA-mediated suppression of fatty acid synthase (FAS), S14, or L-pyruvate kinase (L-PK). Studies in primary rat hepatocytes and cultured 3T3-L1 adipocytes showed that metabolites of 20:4n-6, like prostaglandin E2 (PGE2), suppress mRNA encoding FAS, S14, and L-PK through a Gi/Go-coupled signal transduction cascade. In contrast to adipocytes, 20:4n-6-mediated suppression of lipogenic gene expression in hepatic parenchymal cells does not require cyclooxygenase. Transfection analysis of S14CAT fusion genes in primary hepatocytes shows that peroxisome proliferator-activated PPAR alpha acts on the thyroid hormone response elements (-2.8/-2.5 kb). In contrast, both PGE2 and 20:4n-6 regulate factors that act on the proximal promoter (-150/-80 bp) region, respectively. In conclusion, PUFA affects hepatic gene transcription through at least three distinct mechanisms: (i) a PPAR-dependent pathway, (ii) a prostanoid pathway, and (iii) a PPAR and prostanoid-independent pathway. PUFA regulation of hepatic lipid metabolism involves an integration of these multiple pathways.

Ratio-fluorescence microscopy of lipid oxidation in living cells using C11-BODIPY(581/591)

A ratio-fluorescence assay was developed for on-line localization and quantification of lipid oxidation in living cells. The assay explores the oxidative sensitivity of C11-BODIPY(581/591). Upon oxidation, the fluorescence of this fluorophore shifts from red to green. The probe incorporates readily into cellular membranes and is about twice as sensitive to oxidation as arachidonic acid. Using confocal microscopy, the cumene hydroperoxide-induced oxidation of C11-BODIPY(581/591) was visualized at the sub-cellular level in rat-1 fibroblasts. Preloading of the cells with tocopherol retarded this oxidation. The data demonstrate that C11-BODIPY(581/591) is a valuable tool to quantify lipid oxidation and anti-oxidant efficacy in single cells.

Multiple mechanisms for polyunsaturated fatty acid regulation of hepatic gene transcription

Dietary polyunsaturated fatty acids (PUFA) have profound effects on hepatic gene transcription leading to significant changes in lipid metabolism. Highly unsaturated n-3 PUFA suppress the transcription of genes encoding specific lipogenic enzymes and induce the expression of genes encoding specific enzymes involved in peroxisomal and microsomal fatty acid oxidation. Our studies have shown that fatty acid effects on hepatic gene expression may involve at least three distinct pathways. One pathway involves peroxisome proliferator-activated receptor (PPARalpha), a fatty acid activated nuclear receptor. PPARalpha is required for the PUFA induction of mRNAs encoding enzymes involved in fatty acid oxidation. However, PPARalpha is not required for PUFA suppression of mRNAs encoding proteins involved in lipogenesis. A second pathway involves prostanoids. In cultured 3T3-L1 adipocytes, cyclooxygenase derived 20:4 n-6 metabolites, like PGE2, suppress mRNAs encoding proteins involved in lipogenesis. However, in hepatic parenchymal cells, 20:4 n-6 suppression of lipogenic gene expression does not require a cyclooxygenase. Nevertheless, PGE2 and PGF2alpha suppress hepatic lipogenic gene expression. 20:4 n-6 cyclooxygenase products can arise from non-parenchymal cells and through a paracrine control process act on a G-protein linked receptor signaling cascade to suppress lipogenic gene expression. The fact that n-3 and n-6 PUFA suppression of lipogenic gene expression does not require PPARalpha or cyclooxygenase activity indicates the presence of a third pathway for the control of hepatic gene transcription. These studies indicate that the pleiotropic effects of PUFA on hepatic lipid metabolism cannot be attributed to a single regulatory mechanism.

Role of prostaglandin H synthase isoforms in murine ear edema induced by phorbol ester application on skin

Topical application of TPA to a murine ear induced an edema that was accompanied by eicosanoid biosynthesis and an early enhancement of prostaglandin H synthase 2 (PGHS-2) expression. PGHS-2 induction may be correlated with the time-course of TPA-induced edema formation. Treatment with drugs that inhibit AA mobilization such as dexamethasone or manoalide or inhibitors of leukotriene formation such as zileuton or baicalein, reduced TPA-induced edema development and PGHS-2 levels. On the other hand, arachidonic acid (AA) application on the murine ear induced rapid expression of PGHS-2. This effect was not reproduced by other fatty acids such as oleic, linoleic, eicosatetraynoic or eicosapentaenoic acids. PGHS-2 expression induced by AA application was independent of PGHS and lipoxygenase metabolite synthesis. However, topical application of PGE2 on skin induced PGHS-2 overexpression. This study suggests that AA release and/or subsequent metabolism by PGHS may be involved in the induction of PGHS-2 expression in murine TPA- and AA-induced ear oedema.

Polyunsaturated fatty acid regulation of gene expression

For the past three decades, polyunsaturated fatty acids (PUFA) have been recognized as important energy sources and membrane components. PUFA also play key roles in many cellular events, such as gene regulation. Most recently, research has focused on identifying the mechanisms by which PUFA modulate gene transcription, mRNA stability and cellular differentiation. It is the purpose of this review to examine the effects of PUFA on gene expression in lipogenic as well as other tissues. Because the (n-3) and (n-6) series of PUFA are intimately involved in gene regulation, they will be the focus of review. The effects of other fatty acid families on gene expression are reviewed elsewhere.

Dietary fat, genes, and human health

These studies show that a macronutrient like dietary fat plays an important role in gene expression. In the cases presented here, dietary fat regulates gene expression leading to changes in carbohydrate and lipid metabolism. The interesting outcome of these studies is the finding that the molecular targets for dietary fat action did not converge with the principal targets for hormonal regulation of gene transcription, like hormone receptors. Instead, PUFA-RF targets elements that play key ancillary roles in gene transcription. This is important because it shows how PUFA can interfere with hormone regulation of a specific gene without having generalized effect on overall hormonal control, i.e. PUFA effects are promoter-specific. How PUFA-RF interferes with gene transcription will require the isolation and characterization of PUFA-RF along with the tissue-specific factors targeted by PUFA-RF. A different story emerges when fatty acids activate PPAR. Based on the studies presented here and elsewhere, long chain-highly unsaturated fatty acids (like 20:5,n-3 and 22:6, n-3) or high levels of fat activate PPAR. PPAR directly activates genes like AOX, but also inhibits transcription of genes like S14, FAS, apolipoprotein CIII, transferrin. For S14, the mechanism of inhibition involves sequestration of RXR, a critical factor for T3 receptor binding to DNA. Thus, PPAR can have generalized effects on T3 action or on other nuclear receptors, like vit. D (VDR) and retinoic acid (RAR) receptors, that require RXR for action. For apolipoprotein CIII and transferrin, PPAR/RXR heterodimers compete for HNF-4 binding sites (DR + 1). In addition to HNF-4, COUP-TF, ARP-1 and RXR all bind the DR + 1 type motif. These factors are important for tissue-specific regulation of gene transcription. PPAR can potentially interfere with the transcription of multiple genes through disruption of nuclear receptor signaling leading to changes in phenotype. Clearly, more studies are required to assess the role PPAR plays in the fatty acid regulation of gene transcription and its contribution to chronic disease. Finally, it is clear that dietary fat has the potential to affect gene expression through multiple pathways. Depending on the gene examined, PUFA might augment or abrogate gene transcription which leads to specific phenotypic changes altering metabolism, differentiation or cell growth. These effects can be beneficial to the organism, such as the n-3 PUFA-mediated suppression of serum triglycerides or detrimental, like the saturated and n-6 PUFA-mediated promotion of insulin resistance. How such effects contribute to the onset or progression of specific neoplasia is unclear. However, studies in metabolism might provide important clues for this connection.

Polyunsaturated fatty acid suppression of hepatic fatty acid synthase and S14 gene expression does not require peroxisome proliferator-activated receptor alpha

Dietary polyunsaturated fatty acids (PUFA) induce hepatic peroxisomal and microsomal fatty acid oxidation and suppress lipogenic gene expression. The peroxisome proliferator-activated receptor alpha (PPARalpha) has been implicated as a mediator of fatty acid effects on gene transcription. This report uses the PPARalpha-deficient mouse to examine the role of PPARalpha in the PUFA regulation of mRNAs encoding hepatic lipogenic (fatty acid synthase (FAS) and the S14 protein (S14)), microsomal (cytochrome P450 4A2 (CYP4A2)), and peroxisomal (acyl-CoA oxidase (AOX)) enzymes. PUFA ingestion induced mRNAAOX (2.3-fold) and mRNACYP4A2 (8-fold) and suppressed mRNAFAS and mRNAS14 by >/=80% in wild type mice. In PPARalpha-deficient mice, PUFA did not induce mRNAAOX or mRNACYP4A2, indicating a requirement for PPARalpha in the PUFA-mediated induction of these enzymes. However, PUFA still suppressed mRNAFAS and mRNAS14 in the PPARalpha-deficient mice. Studies in rats provided additional support for the differential regulation of lipogenic and peroxisomal enzymes by PUFA. These studies provide evidence for two distinct pathways for PUFA control of hepatic lipid metabolism. One requires PPARalpha and is involved in regulating peroxisomal and microsomal enzymes. The other pathway does not require PPARalpha and is involved in the PUFA-mediated suppression of lipogenic gene expression.

N-3 polyunsaturated fatty acids and immune cell function

The amount and type of eicosanoids made can be affected by the type of fat consumed in the diet. It is now apparent that both eicosanoids and n-3 PUFAs are potent modulators of lymphocyte and M phi functions in vitro. Inclusion in the diet of high levels of certain lipids containing n-3 PUFAs markedly affects the functions of cells of the immune system subsequently tested in vitro. Cellular components of both natural and acquired immunity are affected. In vivo tests are perhaps the most appropriate approach for determining the effect of different dietary n-3 PUFAs upon immune function. Several studies indicate that diets rich in n-3 PUFAs are anti-inflammatory and immunosuppressive in vivo, although there have been relatively few studies in man. Although some of the effects of n-3 PUFAs may be brought about by modulation of the amount and types of eicosanoids made, it is clear that these fatty acids can also elicit their effects by eicosanoid-independent mechanisms (Fig. 12). Such n-3 PUFA-induced effects may be of use as a therapy for acute and chronic inflammation, for disorders which involve an inappropriately-activated immune response and for the enhancement of graft survival.

Fatty acids involved in signal cross-talk between cell membrane and nucleus

The results presented underline the fact that the nature and the concentration of the non-esterified fatty acids (NEFAs) liberated from membrane lipids, particularly the essential ones issued from lipid nutrition, clearly belong to a large group of factors (hormones, retinoids, growth factors, cytokines...) which control the shift between cell multiplication and differentiation. NEFAs act on this shift, per se or after being metabolized, by influencing, as second messengers or modulators, the intertwined mechanisms of action of growth factors and steroid hormones. These results may explain the molecular links which exist between endocrinology, oncology and nutrition.

Fatty acid regulation of gene expression. Its role in fuel partitioning and insulin resistance

Dietary polyenoic (n-6) and (n-3) fatty acids uniquely regulate fatty acid biosynthesis and fatty acid oxidation. They exercise this effect by modulating the expression of genes coding for key metabolic enzymes and, in doing this, PUFA govern the intracellular as well as the interorgan metabolism of glucose and fatty acids. During the past 20 years, we have gradually elucidated the cellular and molecular mechanism by which dietary PUFA regulate lipid metabolism. Central to this mechanism has been our ability to determine that dietary PUFA regulate the transcription of genes. We have only begun to elucidate the nuclear mechanisms by which PUFA govern gene expression, but one point is clear and that is that it is unlikely that one mechanism will explain the variety of genes governed by PUFA. The difficulty in providing a unifying hypothesis at this time stems from (a) the many metabolic routes taken by PUFA upon entering a cell and (b) the lack of identity of a specific PUFA-regulated trans-acting factor. Nevertheless, our studies have revealed that PUFA are not only utilized as fuel and structural components of cells, but also serve as important mediators of gene expression, and that in this way they influence the metabolic directions of fuels and they modulate the development of nutritionally related pathophysiologies such as diabetes.

Polyunsaturated fatty acids regulate lipogenic and peroxisomal gene expression by independent mechanisms

Polyunsaturated fatty acids of the (n-6) and (n-3) families uniquely coordinate hepatic lipid synthesis and oxidation by suppressing the transcription of hepatic genes encoding lipogenic and glycolytic enzymes while concomitantly inducing the activity of enzymes in mitochondrial and peroxisomal fatty acid oxidation. Recently a group of fatty acid activated nuclear transcription factors termed peroxisome proliferator activated receptors (PPARs) were cloned. The discovery of PPARs led us to hypothesize that polyunsaturated fatty acids coordinately modulated the transcription of lipogenic and oxidative genes via a PPAR mediated process. Rats and mice were fed a potent PPAR activator, 5,8,11,14-eicosatetraynoic acid (ETYA), to ascertain if the expression of hepatic fatty acid synthase and peroxisomal acyl-CoA oxidase were coordinately suppressed and induced in response to PPAR activation. Expectedly, ETYA increased peroxisomal acyl-CoA oxidase mRNA abundance, but PPAR activation neither suppressed fatty acid synthase transcription nor reduced the level of fatty acid synthase mRNA. In fact, ETYA prevented the suppression of hepatic fatty acid synthase expression that characteristically results from feeding corn oil. Fatty acid composition analyses indicated that ETYA interfered with 18:2 (n-6) conversion to 20:4 (n-6). Thus, it appears that PPAR is not the sole factor responsible for the coordinate regulation of lipid synthesis and oxidation by polyunsaturated fatty acids. In addition, our data indicate that the active polyenoic fatty acid responsible for the regulation of gene transcription must undergo delta-6 desaturation.

Regulation of hepatic stearoyl-CoA desaturase gene 1 by vitamin A

The effect of vitamin A supplementation on stearoyl-CoA desaturase gene 1 expression in mouse liver was characterized. Normal BALB/c mice were fed 0.01% and 0.1% retinol palmitate as components of nonpurified diets. This treatment resulted in a 3-fold and a 7-fold induction of SCD1 mRNA levels, respectively, as determined by RNase protection analysis. Vitamin A-deficient animals were also fed diets containing 0.01% and 0.1% retinol palmitate, resulting in a similar pattern of SCD1 mRNA induction. Fatty acid synthase and beta-actin mRNA levels did not respond consistently or significantly to retinoic acid treatment. Dietary and hormonal studies were carried out to investigate the role of the retinoid X receptor in the regulation of SCD1 by type II steroid hormones. A receptor-saturating dose of thyroid hormone, triiodothyronine, repressed vitamin A-elevated SCD1 mRNA levels in vivo. Peroxisome proliferator-elevated SCD1 mRNA levels were unaffected by administration of thyroid hormone. This suggests that the retinoic acid receptor transcriptionally regulates SCD1 through a traditional mechanism of heterodimerization with the retinoid X receptor.

Arachidonic acid up-regulates and prostaglandin E2 down-regulates the expression of pancreatic-type phospholipase A2 and prostaglandin-endoperoxide synthase 2 in uterine stromal cells

It is well known that arachidonic acid, as a substrate of prostaglandin G/H synthase (PGHS), is converted into prostaglandins of the two-series. In this work, we attempted to determine whether arachidonic acid and prostaglandin E2 might regulate the expression of PGHS and the pancreatic-type phospholipase A2 (PLA2I), which may be involved in the liberation of arachidonic acid from membrane phospholipids. For this purpose, we used the uterine stromal cell line UIII, which produces prostaglandin E2 and expresses both the constitutive and inducible PGHS enzymes (PGHS1 and PGHS2) and PLA2 I. The results show that PGHS1, which is expressed at a high level in UIII cells, was not modified by arachidonic acid. The expression of PGHS2 and PLA2 I was up-regulated by increasing arachidonate concentrations (1-10 microM). The maximal response was obtained at 24 h, reaching a 2.3-fold and 2.6-fold increase for PGHS2 and PLA2 I expression, respectively, compared to the control level. To discriminate between the effect of arachidonic acid and that of prostaglandins, which are highly increased in the presence of exogenous arachidonic acid, we treated the cells with two inhibitors of PGHS activity, aspirin and meclofenamic acid. Both inhibitors failed to suppress the arachidonate-induced increase of PLA2 I and PGHS2 expression and even enhanced it either in the presence or absence of arachidonic acid. In contrast, the addition of prostaglandin E2 to the culture medium decreased the expression of both enzymes in a dose-dependent manner, the maximal response being reached at 1 microM. We conclude that arachidonic acid up-regulates the expression of PLA2 I and PGHS2 in the uterine stromal cells, independently of prostanoids, and that prostaglandin E2 is capable of down-regulating enzyme expression.

Peroxisome proliferators induce mouse liver stearoyl-CoA desaturase 1 gene expression

Peroxisome proliferators induce stearoyl-CoA desaturase activity (EC 1.14.99.5) in liver [Kawashima, Y., Hanioka, N., Matsumura, M. & Kozuka, H. (1983) Biochim. Biophys. Acta 752, 259-264]. We analyzed the changes in stearoyl-CoA desaturase 1 (SCD1) mRNA to further define the molecular mechanism for the induction of stearoyl-CoA desaturase by peroxisome proliferators. SCD1 mRNA was analyzed from the livers of BALB/c mice that had been fed diets supplemented with clofibrate or gemfibrozil. Clofibrate was found to induce liver SCD1 mRNA levels 3-fold within 6 hr to a maximum of 22-fold in 30 hr. Gemfibrozil administration resulted in a similar induction pattern. This induction is primarily due to an increase in transcription of the SCD1 gene, as shown by nuclear run-on transcription assays and DNA deletion analysis of transfected SCD1-chloramphenicol acetyltransferase fusion genes. The cis-linked response element for peroxisome proliferator-activated receptor (PPAR) was localized to an AGGTCA consensus sequence between base pairs -664 to -642 of the SCD1 promoter. Clofibrate-mediated induction of SCD1 mRNA was shown to be independent of polyunsaturated fatty acids, with peroxisome proliferators and arachidonic acid having opposite effects on SCD1 mRNA levels. Additionally, the activation of SCD1 mRNA by clofibrate was inhibited 77% by cycloheximide administration. Levels of liver beta-actin and albumin mRNAs were unchanged by these dietary manipulations. Our data show that hepatic SCD1 gene expression is regulated by PPARs and suggest that peroxisome proliferators and poly-unsaturated fatty acids act through distinct mechanisms.

Peroxisome proliferator-activated receptor alpha inhibits hepatic S14 gene transcription. Evidence against the peroxisome proliferator-activated receptor alpha as the mediator of polyunsaturated fatty acid regulation of s14 gene transcription

The peroxisome proliferator-activated receptor (PPARalpha) has been implicated in fatty acid regulation of gene transcription. Lipogenic gene transcription is inhibited by polyunsaturated fatty acids (PUFA). We have used the PUFA-sensitive rat liver S14 gene as a model to examine the role PPARalpha plays in fatty acid regulation of hepatic lipogenic gene transcription. Both PPARalpha and the potent peroxisome proliferator, WY14643, inhibit S14CAT activity in transfected primary hepatocytes. WY14643 and PPARalpha target the S14 T3 regulatory region (TRR, -2.8 to -2.5 kilobases), a region containing 3 T3 response elements (TRE). Transfer of the TRR to the thymidine kinase (TK) promoter conferred negative control to the TKCAT gene following WY14643 and PPARalpha treatment. Gel shift analysis showed that PPARalpha, either alone or with RXRalpha, did not bind the S14TRR. However, PPARalpha interfered with TRbeta/RXRalpha binding to a TRE (DR+4). Functional studies showed that co-transfected RXRalpha, but not T3 receptor beta1 (TRbeta1), abrogated the inhibitory effect of PPARalpha on S14 gene transcription. These results suggest that WY14643 and PPARalpha functionally interfere with T3 regulation of S14 gene transcription by inhibiting TRbeta1/RXR binding to S14 TREs. Previous studies had established that the cis-regulatory targets of PUFA control were located within the proximal promoter region of the S14 gene, i.e. between -220 and -80 bp. Finding that the cis-regulatory elements for WY14643/PPARalpha and PUFA are functionally and spatially distinct argues against PPARalpha as the mediator of PUFA suppression of S14 gene transcription.

Polyunsaturated fatty acid regulation of hepatic gene transcription

Polyunsaturated fatty acids (PUFA) of the n-6 and n-3 families inhibit transcription of a number of hepatic lipogenic and glycolytic genes, e.g. fatty acid synthase. In contrast, saturated and monounsaturated fatty acids exert no suppressive action on lipogenic gene expression. The unique PUFA regulation of gene expression extends beyond the liver to include genes such as adipocyte glucose transporter-4, lymphocyte stearoyl-CoA desaturase 2, and interleukins. Some of the transcriptional effects of PUFA appear to be mediated by eicosanoids, but PUFA suppression of lipogenic and glycolytic genes is independent of eicosanoid synthesis and appears to involve a nuclear mechanism directly modified by PUFA. With the recent cloning of a fatty acid-activated nuclear factor termed peroxisome-proliferator-activated receptor (PPAR) has come the suggestion that PPAR may be the PUFA response factor. This review, however, presents several lines of evidence that indicate that the PPAR and n-6 and n-3 PUFA regulation of lipogenic and glycolytic gene transcription involve separate and independent mechanisms. Thus PPAR appears not to be the PUFA response factor.

Effect of polyunsaturated fatty acids in obese mice

Genetically obese (ob/ob) mice display a variety of metabolic differences from lean litter mates. In the obese state, fatty acid desaturation-elongation in brown adipose tissue mitochondria is apparently altered, resulting in differences in membrane fatty acid composition. This change in membrane lipid environment appears to influence GDP binding and therefore the activity of the proton conductance pathway associated with regulation of energy expenditure in these animals. In liver, binding of insulin to the nuclear membrane is increased by feeding a high polyunsaturated/saturated (P/S) diet fat. Consumption of a high P/S diet decreased mRNA levels for fatty acid synthase, acetyl-CoA carboxylase, malic enzyme, and pyruvate kinase in obese and lean animals. Expression of mRNA for these lipogenic enzymes was higher in obese animals and suggests that obese mice may be resistant to polyunsaturated fatty acid feedback control of gene expression.