Transcriptional homeostatic control of membrane lipid composition

50 years since the concept of homeoviscous adaptation

This mini review focuses on the phenomenon of homeoviscous adaptation (HVA). The concept, which dominated for decades, had a significant impact on membrane and lipid research. It includes the functional characterization of biological membranes and their domains, the role of lipids and fatty acids in cell metabolic control, and the characterization of fatty acid desaturases and their roles in membrane properties modulation. This hypothesis led to the discovery of a feed-back manner of desaturase expression and membrane-associated temperature sensors in bacteria.

Extreme makeover: the incredible cell membrane adaptations of extremophiles to harsh environments

The existence of life beyond Earth has long captivated humanity, and the study of extremophiles-organisms surviving and thriving in extreme environments-provides crucial insights into this possibility. Extremophiles overcome severe challenges such as enzyme inactivity, protein denaturation, and damage of the cell membrane by adopting several strategies. This feature article focuses on the molecular strategies extremophiles use to maintain the cell membrane's structure and fluidity under external stress. Key strategies include homeoviscous adaptation (HVA), involving the regulation of lipid composition, and osmolyte-mediated adaptation (OMA), where small organic molecules protect the lipid membrane under stress. Proteins also have direct and indirect roles in protecting the lipid membrane. Examining the survival strategies of extremophiles provides scientists with crucial insights into how life can adapt and persist in harsh conditions, shedding light on the origins of life. This article examines HVA and OMA and their mechanisms in maintaining membrane stability, emphasizing our contributions to this field. It also provides a brief overview of the roles of proteins and concludes with recommendations for future research directions.

Homologous Delta-12 Fatty Acid Desaturase (FAD2) Genes Affect Gene Expression and Linoleic Acid Levels in Lentinula edodes under Heat Stress

Delta-12 fatty acid desaturases (FAD2s) actively regulate stress responses and cell differentiation in living organisms. In this study, six homologous FAD2 genes were identified based on the genome sequence of Lentinula edodes. Then, the six FAD2 protein sequences were analyzed using bioinformatics tools, including ExPASy ProtParam, SignalP, TMHMM, and TargetP. These analyses were performed to predict the physical and chemical properties, signal peptides, and transmembrane and conserved domains of these proteins. The polypeptide sequences were aligned, and a maximum likelihood phylogenetic tree was constructed using MEGA 7.0 software to elucidate the phylogenetic relationships between homologous FAD2 sequences. The results demonstrated that the FAD2 proteins contained three conserved histidine-rich regions (HXXXH, HXXHH, and HXXHH), which included eight histidine residues. The linoleic acid content and FAD2 enzyme activity were further analyzed, and the levels in the mutagenic heat-tolerant strain 18N44 were lower than those in the wild-type strain 18. Interestingly, the expression levels of the FAD2-2 and FAD2-3 genes under heat stress in strain 18N44 were lower than those in strain 18. These findings indicated that FAD2-2 and FAD2-3 may play major roles in the synthesis of linoleic acid during heat stress.

Polyunsaturated fatty acid elongation and desaturation in activated human T-cells: ELOVL5 is the key elongase

PUFAs are important constituents of membrane glycerophospholipids. However, changes in the capacities to incorporate and metabolize PUFAs when cells enter the cell cycle have not been thoroughly studied. In this study, differences in the incorporation and metabolism of exogenous PUFAs in resting and proliferating primary human T-cells and in the Jurkat cell line were measured. Overall, proliferating T-cells and Jurkat cells had a greater capacity to incorporate and elongate exogenous 18- and 20-carbon PUFAs compared with resting T-cells. Proliferating T-cells and Jurkat cells also showed a greater capacity to desaturate 18-carbon PUFA substrates. Consistent with these observations, a significant increase in the expression of fatty acid desaturase (FADS) 1, FADS2, and elongation of very long chain fatty acids protein (ELOVL) 5 was measured in proliferating T-cells compared with resting T-cells. No quantifiable ELOVL2 was measured. Knockdown of ELOVL5 in T-cells and Jurkat cells significantly affected cellular monounsaturated and PUFA profiles and strongly impaired the elongation of 18- and 20-carbon PUFAs. In conclusion, the induction of proliferation in human T-cells is associated with a significant increase in the capacity to take up and metabolize exogenous PUFAs, and ELOVL5 is responsible for the elongation of 18- and 20-carbon PUFAs in these cells.

Mitochondria in peroxisome-deficient hepatocytes exhibit impaired respiration, depleted DNA, and PGC-1α independent proliferation

The tight interrelationship between peroxisomes and mitochondria is illustrated by their cooperation in lipid metabolism, antiviral innate immunity and shared use of proteins executing organellar fission. In addition, we previously reported that disruption of peroxisome biogenesis in hepatocytes severely impacts on mitochondrial integrity, primarily damaging the inner membrane. Here we investigated the molecular impairments of the dysfunctional mitochondria in hepatocyte selective Pex5 knockout mice. First, by using blue native electrophoresis and in-gel activity stainings we showed that the respiratory complexes were differentially affected with reduction of complexes I and III and incomplete assembly of complex V, whereas complexes II and IV were normally active. This resulted in impaired oxygen consumption in cultured Pex5(-/-) hepatocytes. Second, mitochondrial DNA was depleted causing an imbalance in the expression of mitochondrial- and nuclear-encoded subunits of the respiratory chain complexes. Third, mitochondrial membranes showed increased permeability and fluidity despite reduced content of the polyunsaturated fatty acid docosahexaenoic acid. Fourth, the affected mitochondria in peroxisome deficient hepatocytes displayed increased oxidative stress. Acute deletion of PEX5 in vivo using adeno-Cre virus phenocopied these effects, indicating that mitochondrial perturbations closely follow the loss of functional peroxisomes in time. Likely to compensate for the functional impairments, the volume of the mitochondrial compartment was increased several folds. This was not driven by PGC-1α but mediated by activation of PPARα, possibly through c-myc overexpression. In conclusion, loss of peroxisomal metabolism in hepatocytes perturbs the mitochondrial inner membrane, depletes mitochondrial DNA and causes mitochondrial biogenesis independent of PGC-1α.

Regulatory role of membrane fluidity in gene expression and physiological functions

Plants, algae, and photosynthetic bacteria experience frequent changes in environment. The ability to survive depends on their capacity to acclimate to such changes. In particular, fluctuations in temperature affect the fluidity of cytoplasmic and thylakoid membranes. The molecular mechanisms responsible for the perception of changes in membrane fluidity have not been fully characterized. However, the understanding of the functions of the individual genes for fatty acid desaturases in cyanobacteria and plants led to the directed mutagenesis of such genes that altered the membrane fluidity of cytoplasmic and thylakoid membranes. Characterization of the photosynthetic properties of the transformed cyanobacteria and higher plants revealed that lipid unsaturation is essential for protection of the photosynthetic machinery against environmental stresses, such as strong light, salt stress, and high and low temperatures. The unsaturation of fatty acids enhances the repair of the damaged photosystem II complex under stress conditions. In this review, we summarize the knowledge on the mechanisms that regulate membrane fluidity, on putative sensors that perceive changes in membrane fluidity, on genes that are involved in acclimation to new sets of environmental conditions, and on the influence of membrane properties on photosynthetic functions.

Improvement of Polyunsaturated Fatty Acid Production in Echium acanthocarpum Transformed Hairy Root Cultures by Application of Different Abiotic Stress Conditions

Fatty acids are of great nutritional, therapeutic, and physiological importance, especially the polyunsaturated n-3 fatty acids, possessing larger carbon chains and abundant double bonds or their immediate precursors. A few higher plant species are able to accumulate these compounds, like those belonging to the Echium genus. Here, the novel E. acanthocarpum hairy root system, which is able to accumulate many fatty acids, including stearidonic and α-linolenic acids, was optimized for a better production. The application of abiotic stress resulted in larger yields of stearidonic and α-linolenic acids, 60 and 35%, respectively, with a decrease in linoleic acid, when grown in a nutrient medium consisting of B5 basal salts, sucrose or glucose, and, more importantly, at a temperature of 15°C. The application of osmotic stress employing sorbitol showed no positive influence on the fatty acid yields; furthermore, the combination of a lower culture temperature and glucose did not show a cumulative boosting effect on the yield, although this carbon source was similarly attractive. The abiotic stress also influenced the lipid profile of the cultures, significantly increasing the phosphatidylglycerol fraction but not the total lipid neither their biomass, proving the appropriateness of applying various abiotic stress in this culture to achieve larger yields.

Analysis of gene expression changes to elucidate the mechanism of chilling injury in precision-cut liver slices

The exact mechanism of chilling injury (by a decrease of temperature to sub-physiological values), especially in the intact organ, is yet unknown. Precision-cut liver slices (PCLS), which closely resemble the organ from which they are derived, are an ideal in vitro model to study the mechanism of chilling injury in the intact organ. In the present study we were able to separate chilling injury from other damaging events such as cryoprotectant toxicity and ice-crystal injury and performed micro-array analysis of regulated genes. Pathway analysis revealed that different stress responses, lipid/fatty acid and cholesterol biosynthesis and metabolism were affected by chilling. This indicates that the cell-membrane might be the primary site and sensor for chilling, which may initiate and amplify downstream intracellular signaling events. Most importantly, we were able to identify gene expression responses from stellate cells and Kupffer cells suggesting the involvement of all liver cell types in the injury. In conclusion, a broad spectrum of previously unknown gene expression changes induced by chilling was identified in the tissue. This is the first report of a systematic investigation on the mechanism of chilling injury in integrated tissue by micro-array analysis under conditions in which other sources of injury are minimal.

SOA genes encode proteins controlling lipase expression in response to triacylglycerol utilization in the yeast Yarrowia lipolytica

The oleaginous yeast Yarrowia lipolytica efficiently metabolizes hydrophobic substrates such as alkanes, fatty acids or triacylglycerol. This yeast has been identified in oil-polluted water and in lipid-rich food. The enzymes involved in lipid breakdown, for use as a carbon source, are known, but the molecular mechanisms controlling the expression of the genes encoding these enzymes are still poorly understood. The study of mRNAs obtained from cells grown on oleic acid identified a new group of genes called SOA genes (specific for oleic acid). SOA1 and SOA2 are two small genes coding for proteins with no known homologs. Single- and double-disrupted strains were constructed. Wild-type and mutant strains were grown on dextrose, oleic acid and triacylglycerols. The double mutant presents a clear phenotype consisting of a growth defect on tributyrin and triolein, but not on dextrose or oleic acid media. Lipase activity was 50-fold lower in this mutant than in the wild-type strain. The impact of SOA deletion on the expression of the main extracellular lipase gene (LIP2) was monitored using a LIP2-beta-galactosidase promoter fusion protein. These data suggest that Soa proteins are components of a molecular mechanism controlling lipase gene expression in response to extracellular triacylglycerol.

Effect of dietary omega-3 fatty acid source on plasma and red blood cell membrane composition and immune function in yearling horses

To determine the effect of different sources of dietary n-3 fatty acids (FA) on plasma and red blood cell (RBC) FA composition and immune response, 18 Quarter Horse yearlings were randomly and equally assigned to 1 of 3 treatments: encapsulated fish oil (n = 6), milled flaxseed (n = 6), or no supplementation (control, n = 6). Fish oil contained 15 g of eicosapentaenoic acid (C20:5n-3) and 12.5 g of docosahexaenoic acid (C22:6n-3), and flaxseed contained 61 g of alpha-linolenic acid (C18:3n-3) per 100 g of FA. Horses had free access to bahiagrass pasture during the active growing season and were individually fed a grain mix concentrate at 1.5% BW/d. Fish oil and flaxseed were mixed into the concentrate in amounts to provide 6 g of total n-3/100 kg of BW. Horses were fed their respective treatments for 70 d. Blood samples were obtained to determine plasma and RBC FA composition and for isolation of peripheral blood mononuclear cells. Peripheral blood mononuclear cells were stimulated with concanavalin A and phytohemagglutinin (PHA) to determine lymphocyte proliferation and were challenged with lipopolysaccharide to determine PGE(2) production. In vivo inflammatory response was assessed on d 70 by measuring skin thickness and area of swelling in response to intradermal injection of PHA. Treatment did not affect BW gain, which averaged 0.6 +/- 0.03 kg/d. Horses fed fish oil had greater (P < 0.05) proportions of eicosapentaenoic acid, docosahexaenoic acid, and sum of n-3 in plasma and RBC compared with those in the flaxseed and control treatments. In addition, plasma arachidonic acid was greater (P < 0.05) and plasma linoleic and alpha-linolenic acids were less (P < 0.05) in the fish oil treatment compared with the flaxseed and control treatments. Dietary treatment did not affect lymphocyte proliferation or PGE(2) production. Across treatments, the peak increase in skin thickness was observed 4 to 8 h after PHA injection. At 4 h postinjection, horses fed fish oil and those fed flaxseed had a greater increase in skin thickness than those in the control treatment (P < 0.05) and horses fed fish oil had a larger area of swelling than those in the control treatment (P < 0.05). Skin thickness remained greater (P < 0.05) in horses fed flaxseed than in control horses 6 h after injection. Although fed to supply a similar amount of n-3 FA, fish oil had a greater impact on plasma and RBC n-3 FA content than did flaxseed. However, supplementing horses with both fish oil and flaxseed resulted in a more pronounced early inflammatory response to PHA injection as compared with nonsupplemented horses.

Epigallocatechin-3-gallate affects the growth of LNCaP cells via membrane fluidity and distribution of cellular zinc

OBJECTIVE

To evaluate effects of epigallocatechin-3-gallate (EGCG) on the viability, membrane properties, and zinc distribution, with and without the presence of Zn(2+), in human prostate carcinoma LNCaP cells.

METHODS

We examined changes in cellular morphology and membrane fluidity of LNCaP cells, distribution of cellular zinc, and the incorporated portion of EGCG after treatments with EGCG, Zn(2+), and EGCG+Zn(2+).

RESULTS

We observed an alteration in cellular morphology and a decrease in membrane fluidity of LNCaP cells after treatment with EGCG or Zn(2+). The proportion of EGCG incorporated into liposomes treated with the mixture of EGCG and Zn(2+) at the ratio of 1:1 was 90.57%, which was significantly higher than that treated with EGCG alone (30.33%). Electron spin resonance (ESR) studies and determination of fatty acids showed that the effects of EGCG on the membrane fluidity of LNCaP were decreased by Zn(2+). EGCG accelerated the accumulation of zinc in the mitochondria and cytosol as observed by atomic absorption spectrometer.

CONCLUSION

These results show that EGCG interacted with cell membrane, decreased the membrane fluidity of LNCaP cells, and accelerated zinc accumulation in the mitochondria and cytosol, which could be the mechanism by which EGCG inhibits proliferation of LNCaP cells. In addition, high concentrations of Zn(2+) could attenuate the actions elicited by EGCG.

Unsaturated fatty acids promote hepatoma proliferation and progression through downregulation of the tumor suppressor PTEN

BACKGROUND/AIMS

The impact of dietary fatty acids on the development of cancers is highly controversial. We recently demonstrated that unsaturated fatty acids trigger the downregulation of the tumor suppressor PTEN through an mTOR/NF-kappaB-dependent mechanism in hepatocytes. In this study, we investigated whether unsaturated fatty acids promote hepatoma progression by downregulating PTEN expression.

METHODS

The effects of fatty acids and PTEN-specific siRNAs on proliferation, invasiveness and gene expression were assessed using HepG2 hepatoma cells. The tumor promoting activity of unsaturated fatty acids was evaluated in vivo using HepG2 xenografts in nude mice.

RESULTS

Incubation of HepG2 cells with unsaturated fatty acids, or PTEN-specific siRNAs, increased cell proliferation, cell migration and invasiveness, and altered the expression of genes involved in inflammation, epithelial-to-mesenchymal transition and carcinogenesis. These effects were dependent on PTEN expression levels and were prevented by mTOR and NF-kappaB inhibitors. Consistent with these data, the development and size of subcutaneous HepG2-derived tumors in nude mice xenografts were dramatically increased when mice were fed with an oleic acid-enriched diet, even in the absence of weight gain.

CONCLUSIONS

These data demonstrate that dietary unsaturated fatty acids promote hepatoma progression by reducing the expression of the tumor suppressor PTEN.

Growth and development alter susceptibility to acute renal injury

Many of the studies of acute renal injury have been conducted in young mice usually during their rapid growth phase; yet, the impact of age or growth stage on the degree of injury is unknown. To address this issue, we studied three forms of injury (endotoxemic-, glycerol-, and maleate-induced) in mice ranging in age from adolescence (3 weeks) to maturity (16 weeks). The severity of injury within each model significantly correlated with weight and age. We also noticed a progressive age-dependent reduction in renal cholesterol content, a potential injury modifier. As the animals grew and aged they also exhibited stepwise decrements in the mRNAs of HMG CoA reductase and the low density lipoprotein receptor, two key cholesterol homeostatic genes. This was paralleled by decreased amounts of RNA polymerase II and the transcription factor SREBP1/2 at the reductase and lipoprotein receptor gene loci as measured by chromatin immunoprecipitation. Our study shows that the early phase of mouse growth can profoundly alter renal susceptibility to diverse forms of experimental acute renal injury.

CD36-dependent regulation of muscle FoxO1 and PDK4 in the PPAR delta/beta-mediated adaptation to metabolic stress

The transcription factor FoxO1 contributes to the metabolic adaptation to fasting by suppressing muscle oxidation of glucose, sparing it for glucose-dependent tissues. Previously, we reported that FoxO1 activation in C(2)C(12) muscle cells recruits the fatty acid translocase CD36 to the plasma membrane and increases fatty acid uptake and oxidation. This, together with FoxO1 induction of lipoprotein lipase, would promote the reliance on fatty acid utilization characteristic of the fasted muscle. Here, we show that CD36-mediated fatty acid uptake, in turn, up-regulates protein levels and activity of FoxO1 as well as its target PDK4, the negative regulator of glucose oxidation. Increased fatty acid flux or enforced CD36 expression in C(2)C(12) cells is sufficient to induce FoxO1 and PDK4, whereas CD36 knockdown has opposite effects. In vivo, CD36 loss blunts fasting induction of FoxO1 and PDK4 and the associated suppression of glucose oxidation. Importantly, CD36-dependent regulation of FoxO1 is mediated by the nuclear receptor PPARdelta/beta. Loss of PPARdelta/beta phenocopies CD36 deficiency in blunting fasting induction of muscle FoxO1 and PDK4 in vivo. Expression of PPARdelta/beta in C(2)C(12) cells, like that of CD36, robustly induces FoxO1 and suppresses glucose oxidation, whereas co-expression of a dominant negative PPARdelta/beta compromises FoxO1 induction. Finally, several PPRE sites were identified in the FoxO1 promoter, which was responsive to PPARdelta/beta. Agonists of PPARdelta/beta were sufficient to confer responsiveness and transactivate the heterologous FoxO1 promoter but not in the presence of dominant negative PPARdelta/beta. Taken together, our findings suggest that CD36-dependent FA activation of PPARdelta/beta results in the transcriptional regulation of FoxO1 as well as PDK4, recently shown to be a direct PPARdelta/beta target. FoxO1 in turn can regulate CD36, lipoprotein lipase, and PDK4, reinforcing the action of PPARdelta/beta to increase muscle reliance on FA. The findings could have implications in the chronic abnormalities of fatty acid metabolism associated with obesity and diabetes.

Influence of Pancreatic Status and Sex on Polyunsaturated Fatty Acid Profiles in Cystic Fibrosis

Background: Some but not all studies have reported abnormal polyunsaturated fatty acid composition in cystic fibrosis (CF) patients. We investigated the influence of pancreatic status and sex on the fatty acid profile in plasma and erythrocyte membranes in patients with CF.

Methods: After a 1-step transesterification with acetyl chloride on plasma and washed erythrocyte membranes, we quantified fatty acid methyl esters by use of GC-MS in 124 CF patients and 80 age-matched healthy controls. In the CF group, mean (SD) age was 17.5 (11.3) years, and 51.6% were male. Pancreatic insufficiency was diagnosed in 78% of the CF population.

Results: A decrease in docosahexaenoic acid concentrations was observed in CF patients independently of pancreatic status. Pancreatic insufficient CF patients displayed lower concentrations of linoleic acid and arachidonic acid and higher concentrations of dihomo-γ-linolenic acid and eicosatrienoic acid (mead acid) in plasma and erythrocyte membranes compared with healthy controls and pancreatic sufficient CF patients. Male CF patients had significantly lower docosahexaenoic acid and higher eicosatrienoic acid in plasma and erythrocyte membranes compared with female CF patients.

Conclusions: These results support the concept that multiple abnormalities of polyunsaturated fatty acid composition participate in the CF disease phenotype and that pancreatic status plays a major role in such abnormalities. Moreover, patient sex influences the polyunsaturated fatty acid spectrum in CF, with more marked abnormalities in males.

Enhanced sterol response element-binding protein in postintervention restenotic blood vessels plays an important role in vascular smooth muscle proliferation

Postintervention restenosis (PIRS) after balloon angioplasty or stent implantation is a limitation for these interventional procedures even with the advent of new drug-eluting stents. Sterol regulatory element-binding proteins (SREBP) are transcription factors governing cellular lipid biosynthesis and thus critical in the regulation of the lipid-rich cell membranes. PIRS following injury results partially from newly proliferating cells expressing vascular smooth muscle cell (VSMC) markers. Platelet-derived growth factor (PDGF), lysophosphatidic acid (LPA) and alpha(1)-adrenergic receptor stimulation are well recognized diverse mitogens for VSMC activation in PIRS. We examined whether PDGF, LPA and alpha(1)-adrenergic receptor stimulation with phenylephrine (PE) regulate SREBP expression and subsequently, VSMC proliferation. Our results show that PDGF, LPA and PE upregulate SREBP-1 in a time- and dose-dependent manner. PDGF, LPA and PE-mediated proliferation is dependent on SREBP since inhibition of SREBP expression using targeted knockdown of the SREBP precursor SREBP activating protein (SCAP) by siRNA led to an attenuation of SREBP expression and decreased PDGF, LPA and PE induced proliferation. In two different in vivo PIRS models we found that SREBP-1 was enhanced in the injured blood vessel wall, especially within the neointima and co-localized with alpha-smooth muscle actin positive cells. Thus, SREBP is enhanced in the vessel wall following PIRS and is important in the regulation of pro-hyperplasia molecular signaling. SREBP inhibition may be a powerful tool to limit PIRS.

Membrane lipid physiology and toxin catabolism underlie ethanol and acetic acid tolerance in Drosophila melanogaster

Drosophila melanogaster has evolved the ability to tolerate and utilize high levels of ethanol and acetic acid encountered in its rotting-fruit niche. Investigation of this phenomenon has focused on ethanol catabolism, particularly by the enzyme alcohol dehydrogenase. Here we report that survival under ethanol and acetic acid stress in D. melanogaster from high- and low-latitude populations is an integrated consequence of toxin catabolism and alteration of physical properties of cellular membranes by ethanol. Metabolic detoxification contributed to differences in ethanol tolerance between populations and acclimation temperatures via changes in both alcohol dehydrogenase and acetyl-CoA synthetase mRNA expression and enzyme activity. Independent of changes in ethanol catabolism, rapid thermal shifts that change membrane fluidity had dramatic effects on ethanol tolerance. Cold temperature treatments upregulated phospholipid metabolism genes and enhanced acetic acid tolerance, consistent with the predicted effects of restoring membrane fluidity. Phospholipase D was expressed at high levels in all treatments that conferred enhanced ethanol tolerance, suggesting that this lipid-mediated signaling enzyme may enhance tolerance by sequestering ethanol in membranes as phophatidylethanol. These results reveal new candidate genes underlying toxin tolerance and membrane adaptation to temperature in Drosophila and provide insight into how interactions between these phenotypes may underlie the maintenance of latitudinal clines in ethanol tolerance.

Stearoyl-CoA desaturase-1 mediates the pro-lipogenic effects of dietary saturated fat

Dietary saturated fats have often been implicated in the promotion of obesity and related disorders. It has been shown recently that saturated fats act through the transcription factor SREBP-1c (sterol regulatory element-binding protein-1c) and its requisite coactivator, peroxisome proliferator-activated receptor-gamma coactivator-1beta (PGC-1beta), to exert their pro-lipogenic effects. We show here that a diet high in the saturated fat stearate induces lipogenic genes in wild-type mice, with the induction of the Scd1 (stearoyl-CoA desaturase-1) gene preceding that of other lipogenic genes. However, in Scd1-/- mice, stearate does not induce lipogenesis, and Srebp-1c and Pgc-1beta levels are markedly reduced. Instead, genes of fatty acid oxidation such as Cpt-1 (carnitine palmitoyltransferase-1) as well as Pgc-1alpha are induced. Mitochondrial fatty acid oxidation is increased, and white adipose tissue and hepatic glycogen stores are depleted in stearate-fed Scd1-/- mice. Furthermore, AMP-activated protein kinase is also induced by stearate feeding in Scd1-/- mice. These results indicate that the desaturation of saturated fats such as stearate by SCD is an essential step mediating their induction of lipogenesis. In the absence of SCD1, stearate promotes oxidation, leading to protection from saturated fat-induced obesity. SCD1 thus serves as a molecular switch in the promotion or prevention of lipid-induced disorders brought on by consumption of excess saturated fat.

Experimentally induced monensin-resistant Eimeria tenella and membrane fluidity of sporozoites

Two resistant lines of Eimeria tenella (H) to monensin were developed after 35 passages in chickens medicated with 100-125 ppm or 125-200 ppm monensin in the diet. Drug sensitivity of the induced lines to different level drugs were estimated with mean lesion scores (LS), mean oocyst productions (OP), percentage optimum anticoccidial activity (POAA), reduction of lesion scores (RLS), relative oocyst production (ROP), anticoccidial index (ACI) and global index (GI), respectively. Membrane fluidity of sporozoites of the sensitive line (i.e. the parent line, coded as MON-S((S))) and two resistant lines (coded as MON-R((S))-1 and MON-R((S))-2) with and without in vitro exposure to monensin were determined. Membrane fluidity of MON-R((S))-1 and MON-R((S))-2 were significantly lower than that of MON-S((S)). In vitro exposure to monensin significantly increased membrane fluidity of MON-S((S)), but had a much less effect on those of MON-R((S))-1 and MON-R((S))-2. Sporozoits of the MON-S((S))and MON-R((S))-2 with or without in vitro exposure to monensin were examined by SEM, and the sensitive sporozoites (MON-S((S))) appeared swollen and bulgy after treatment with monensin, while there was no obvious morphological deformation in the resistant sporozoites (MON-R((S))-2). The results suggest that the altered membrane fluidity in the membranes of E. tenella may be related to the decreased sensitivity to monensin.

Membrane fluidity and its roles in the perception of environmental signals

Poikilothermic organisms are exposed to frequent changes in environmental conditions and their survival depends on their ability to acclimate to such changes. Changes in ambient temperature and osmolarity cause fluctuations in the fluidity of cell membranes. Such fluctuations are considered to be critical to the initiation of the regulatory reactions that ultimately lead to acclimation. The mechanisms responsible for the perception of changes in membrane fluidity have not been fully characterized. However, the analysis of genome-wide gene expression using DNA microarrays has provided a powerful new approach to studies of the contribution of membrane fluidity to gene expression and to the identification of environmental sensors. In this review, we focus on the mechanisms that regulate membrane fluidity, on putative sensors that perceive changes in membrane fluidity, and on the subsequent expression of genes that ensures acclimation to a new set of environmental conditions.

Identification of genes regulated by oleic acid in Jurkat cells by suppressive subtractive hybridization analysis

In this study, the effect of oleic acid (50 microM) on gene expression of Jurkat cells (human T lymphocytes cell line) was examined using the suppressive subtractive hybridization approach. This technique allowed us to identify genes with higher or lower expression after cell treatment with oleic acid as compared to untreated cells. Oleic acid upregulated the expression of the translation elongation factor alpha 1 and ATP synthase 8 and downregulated gp96 (human tumor rejection antigen gp96), heat-shock protein 60 and subtilisin-like protein 4. These results suggest that oleic acid, at plasma physiological concentration, can regulate the expression of important genes to maintain the machinery that ensures cell functioning.

Vascular endothelial growth factor activation of sterol regulatory element binding protein: a potential role in angiogenesis

By stimulating the migration and proliferation of endothelial cells (ECs), vascular endothelial growth factor (VEGF) is a potent angiogenic factor. However, the molecular mechanism involved in the VEGF-induced angiogenesis remains elusive. We hypothesized that sterol regulatory element binding proteins (SREBPs), transcription factors governing cellular lipid homeostasis, play an important role in regulating angiogenesis in response to VEGF. VEGF activated SREBP1 and SREBP2 in ECs, as demonstrated by the increased SREBPs, their cleavage products, and the upregulation of the targeted genes. VEGF-induced SREBP activation depended on SREBP cleavage-activating protein (SCAP), because knocking down SCAP by RNA interference (RNAi) inhibited SREBP activation in response to VEGF. SREBP activation was also blocked by 25-hydroxycholesterol (25-HC). To verify the functional implication of SREBPs in VEGF-induced angiogenesis, we tested the role of SREBPs in EC migration and proliferation. SCAP RNAi or 25-HC inhibited VEGF-induced pseudopodia extension and migration of ECs. Both treatments inhibited VEGF-induced EC proliferation, with cell growth arrested at the G(0)/G(1) phase and a concomitant decrease of the S phase. Blocking the PI3K-Akt pathway inhibited the VEGF-activated SREBPs, demonstrating that PI3K-Akt regulates SREBPs. Consistent with our in vitro data, SREBP1 was detected in newly developed microvasculatures in a rabbit skin partial-thickness wound-healing model. SREBP inhibition also markedly suppressed VEGF-induced angiogenesis in chick embryos. In summary, this study identifies SREBPs as the key molecules in regulating angiogenesis in response to VEGF.

Role of oleic acid as a neurotrophic factor is supported in vivo by the expression of GAP-43 subsequent to the activation of SREBP-1 and the up-regulation of stearoyl-CoA desaturase during postnatal development of the brain

We have recently reported that albumin, a serum protein present in the developing brain, stimulates the synthesis of oleic acid by cultured astrocytes by inducing stearoyl-CoA 9-desaturase, the rate-limiting enzyme in oleic acid synthesis, through activation of the sterol regulatory element-binding protein-1. In this work, we offer evidence supporting the in vivo occurrence of this process during the postnatal development of the rat brain. Our results show that albumin reaches maximal brain level by day 1 after birth, coinciding with activation of the sterol response element binding protein-1, which is responsible for the transcription of the enzymes required for oleic acid synthesis. In addition, the developmental profile of stearoyl-CoA 9-desaturase-1 mRNA expression follows that of sterol regulatory element-binding protein-1 activation, indicating that these phenomena are tightly linked. In a previous work, we showed that oleic acid induces neuronal differentiation, as indicated by the expression of growth associated protein-43. Here, we report that the expression of growth associated protein-43 mRNA peaks at about day 7 after birth, following the maximal expression of stearoyl-CoA 9-desaturase-1 mRNA that occurs between days 3 and 5 postnatally. In conclusion, our results support the hypothesis that the synthesis of oleic acid is linked to neuronal differentiation during rat brain development.

Free fatty acids modulate intermembrane trafficking of cholesterol by increasing lipid mobilities: novel 13C NMR analyses of free cholesterol partitioning

Cholesterol and free fatty acids in membranes modulate major biological processes, and their cellular metabolism and actions are often coordinately regulated. However, effects of free fatty acid on cholesterol-membrane interactions have proven difficult to monitor in real time in intact systems. We developed a novel (13)C NMR method to assess effects of free fatty acids on molecular interactions of cholesterol within--and transfer between--model membranes. An important advantage of this method is the ability to acquire kinetic data without separation of donor and acceptor membranes. Large unilamellar phospholipid vesicles (LUV) with phosphatidylcholine/cholesterol ratios of 4:1 served as cholesterol donors. Small unilamellar vesicles (SUV) made with phosphatidylcholine were acceptors. The (13)C(4)-cholesterol peak is narrow in SUV, but very broad in LUV, spectra; the increase in intensity of this peak over time monitored transfer. Oleic acid and other long chain free fatty acids [saturated (C12-18) and unsaturated (C18)] dose-dependently increased mobilities of lipids in LUV (phospholipid and cholesterol) and cholesterol transfer rates, whereas short (C8-10) and very long (C24) chain free fatty acids did not. Decreasing pH from 7.4 to 6.5 (+/-oleic acid) had no effect on cholesterol transfer, and 5 mol % fatty acyl-CoAs increased transfer rates, demonstrating greater importance of the fatty-acyl tail over the headgroup. In LUV containing sphingomyelin, transfer rates decreased, but the presence of oleic acid increased transfer 1.3-fold. These results demonstrate free fatty acid-facilitated cholesterol movement within and between membranes, which may contribute to their multiple biological effects.

The mevalonate pathway during acute tubular injury: selected determinants and consequences

Renal injury evokes tubular cholesterol accumulation, mediated in part by increased HMG CoA reductase (HMGCR) levels. The present study was undertaken to define potential molecular determinants of these changes and to ascertain the relative importance of increased cholesterol production versus mevalonate pathway-driven protein prenylation, on the emergence of the so-called postrenal injury "cytoresistant state." Cultured proximal tubule (HK-2) cells were subjected to Fe or ATP depletion injury, followed 1 to 24 hours later by assessments of: 1) sterol transcription factor expression (SREBP)-1 and -2); 2) HMGCR mRNA levels; and 3) Ras/Rho prenylation. HMGCR mRNA and Ras/Rho prenylation were also assessed after in vivo ischemic and Fe-mediated renal damage. Using specific inhibitors, the relative importance of protein prenylation versus terminal cholesterol synthesis on HK-2 cell susceptibility to injury was also assessed. Acute injury induced HK-2 cell SREBP disruption and reductions in HMGCR mRNA. Renal cortical HMGCR mRNA also fell in response to either in vivo ischemic or Fe-mediated oxidant damage. At 24 hours after in vitro/in vivo injury, a time of cholesterol buildup, no increase in Ras/Rho prenylation was observed. Prenylation inhibitors did not sensitize HK-2 cells to injury. Conversely, squalene synthase (terminal cholesterol synthesis) blockade sensitized HK-2 cells to both Fe and ATP depletion attack. We concluded that: 1) acute tubular cell injury can destroy SREBPs and lower HMGCR mRNA. This suggests that posttranscriptional/translational events are responsible for HMGCR enzyme and cholesterol accumulation after renal damage. 2) Injury-induced cholesterol accumulation appears dissociated from increased protein prenylation. 3) Cholesterol accumulation, per se, seems to be the dominant mechanism by which the mevalonate pathway contributes to the postrenal injury cytoresistant state.

Critical role for cataplerosis via citrate in glucose-regulated insulin release

The molecular mechanisms mediating acute regulation of insulin release by glucose are partially known. The process involves at least two pathways that can be discriminated on basis of their (in)dependence of closure of ATP-sensitive potassium (K+(ATP)) channels. The mechanism of the K+(ATP) channel-independent pathway was proposed to involve cataplerosis, the export of mitochondrial intermediates into the cytosol and in the induction of fatty acid-derived signaling molecules. In the present article, we have explored in fluorescence-activated cell sorter (FACS)-purified rat beta-cells the molecular steps involved in chronic glucose regulation of the insulin secretory response. When compared with culture in 10 mmol/l glucose, 24 h culture in 3 mmol/l glucose shifts the phenotype of the cells into a state with low further secretory responsiveness to glucose, lower rates of glucose oxidation, and lower rates of cataplerosis. Microarray mRNA analysis indicates that this shift can be attributed to differences in expression of genes involved in the K+(ATP) channel-dependent pathway, in cataplerosis and in fatty acid/cholesterol biosynthesis. This response was paralleled by glucose upregulation of the transcription factor sterol regulatory element binding protein 1c (SREBP1c) (ADD1) and downregulation of peroxisome proliferator-activated receptor (PPAR)-alpha and PPAR-beta (PPARdelta). The functional importance of cataplerosis via citrate for glucose-induced insulin release was further supported by the observation that two ATP-citrate lyase inhibitors, radicicol and (-)-hydroxycitrate, block part of glucose-stimulated release in beta-cells. In conclusion, chronic glucose regulation of the glucose-responsive secretory phenotype is associated with coordinated changes in gene expression involved in the K+(ATP) channel-dependent pathway, in cataplerosis via citrate and in acyl CoA/cholesterol biosynthesis.

Astrocyte-synthesized oleic acid behaves as a neurotrophic factor for neurons

Unlike in the adult brain, the newborn brain specifically takes up serum albumin during the postnatal period, coinciding with the stage of maximal brain development. Here we shall summarize our knowledge about the role played by albumin in brain development. The role of this protein in brain development is intimately related to its ability to carry fatty acids. Thus, albumin stimulates oleic acid synthesis by astrocytes from the main metabolic substrates available during brain development. Astrocytes internalize albumin in vesicle-like structures by receptor-mediated endocytosis, which is followed by transcytosis, including passage through the endoplasmic reticulum (ER). The presence of albumin in the ER activates the sterol regulatory element-binding protein-1 (SREBP-1) and increases stearoyl-CoA 9-desaturase (SCD) mRNA, the key enzyme in oleic acid synthesis. Oleic acid released by astrocytes is used by neurons for the synthesis of phospholipids and is specifically incorporated into growth cones. In addition, oleic acid promotes axonal growth, neuronal clustering, and the expression of the axonal growth associated protein, GAP-43. All of these observations indicate neuronal differentiation. The effect of oleic acid on GAP-43 synthesis is brought about by the activation of protein kinase C. The expression of GAP-43 is significantly increased by the presence of albumin in neurons co-cultured with astrocytes, indicating that neuronal differentiation takes place by the presence of oleic acid synthesized and released by astrocytes in situ. In conclusion, during brain development the presence of albumin could play an important role by triggering the synthesis and release of oleic acid by astrocytes, thereby inducing neuronal differentiation.

Transcytosis of albumin in astrocytes activates the sterol regulatory element-binding protein-1, which promotes the synthesis of the neurotrophic factor oleic acid

We have recently reported that albumin, a serum protein present in the developing brain, stimulates the synthesis of oleic acid by astrocytes, which promotes neuronal differentiation. In this work, we gain insight into the mechanism by which albumin induces the synthesis of this neurotrophic factor. Our results show that astrocytes internalize albumin in vesicle-like structures by receptor-mediated endocytosis. Albumin uptake was followed by transcytosis, including passage through the endoplasmic reticulum, which was required to induce the synthesis of oleic acid. Oleic acid synthesis is feedback-regulated by the sterol regulatory element-binding protein-1, which induces the transcription of stearoyl-CoA 9-desaturase, the key rate-limiting enzyme for oleic acid synthesis. In our research, the presence of albumin activated the sterol regulatory element-binding protein-1 and increased stearoyl-CoA 9-desaturase mRNA. Moreover, when the activity of sterol regulatory element-binding protein-1 was inhibited by overexpression of a truncated form of this protein, albumin did not affect stearoyl-CoA 9-desaturase mRNA, indicating that the effect of albumin is mediated by this transcription factor. The effect of albumin was abolished when traffic to the endoplasmic reticulum was prevented or when albumin was accompanied with oleic acid. In conclusion, our results suggest that the transcytosis of albumin includes passage through the endoplasmic reticulum, where oleic acid is sequestrated, initiating the signal cascade leading to an increase in its own synthesis.

Isolation and characterization of the human stearoyl-CoA desaturase gene promoter: requirement of a conserved CCAAT cis-element

Stearoyl-CoA desaturase is the rate-limiting enzyme in the production of mono-unsaturated fatty acids. We have recently cloned and characterized the human Scd cDNA and SCD (the stearoyl-CoA desaturase structural gene) on chromosome 10, as well as the non-transcribed pseudogene on chromosome 17. In order to further define SCD regulation and function, we have isolated and characterized the promoter of the structural gene. Screening of chromosome-10-specific libraries resulted in the isolation of 4.1 kb of SCD sequence upstream of the translation start site. Binding sites for transcription factors critical for mouse Scd1 and Scd2 promoter activity, such as sterol-regulated-element-binding protein and nuclear factor Y, were present in the human SCD promoter (Scd is the mouse stearoyl-CoA desaturase gene). Deletion analysis in HaCaT keratinocytes identified a critical region for promoter activity between nts 496-609 upstream of the translation start site. Site-directed mutagenesis of binding sites in this region identified the CCAAT box as the critical cis-element for SCD promoter activity. An electrophoretic mobility-shift assay confirmed that this element binds nuclear proteins from HaCaT keratinocytes. The polyunsaturated-fatty-acid (PUFA) response element, previously identified in the promoters of mouse Scd1 and Scd2, was found to be conserved in the human SCD promoter, and contained the critical CCAAT cis-element. A minimal promoter construct including this region was responsive to fatty acids, with oleate and linoleate decreasing transcription and stearate increasing it. These studies indicate that CCAAT-box-binding proteins activate SCD transcription in cultured keratinocytes and that fatty acids modulate transcription, most likely through the conserved PUFA response element.