Regulation of stearoyl coenzyme A desaturase expression in human retinal pigment epithelial cells by retinoic acid

Death and the desaturase: implication of Stearoyl-CoA desaturase-1 in the mechanisms of cell stress, apoptosis, and ferroptosis

Growth and proliferation of normal and cancerous cells necessitate a finely-tuned regulation of lipid metabolic pathways to ensure the timely supply of structural, energetic, and signaling lipid molecules. The synthesis and remodeling of lipids containing fatty acids with an appropriate carbon length and insaturation level are required for supporting each phase of the mechanisms of cell replication and survival. Mammalian Stearoyl-CoA desaturases (SCD), particularly SCD1, play a crucial role in modulating the fatty acid composition of cellular lipids, converting saturated fatty acids (SFA) into monounsaturated fatty acids (MUFA) in the endoplasmic reticulum (ER). Extensive research has elucidated in great detail the participation of SCD1 in the molecular mechanisms that govern cell replication in normal and cancer cells. More recently, investigations have shed new light on the functional and regulatory role of the Δ9-desaturase in the processes of cell stress and cell death. This review will examine the latest findings on the involvement of SCD1 in the molecular pathways of cell survival, particularly on the mechanisms of ER stress and autophagy, as well in apoptotic and non-apoptotic cell death.

All-trans retinoic acid induces lipophagy through the activation of the AMPK-Beclin1 signaling pathway and reduces Rubicon expression in adipocytes

Lipophagy is defined as a lipolysis pathway that degrades lipid droplet (LD) via autophagy. All-trans retinoic acid (atRA), a metabolite of vitamin A, stimulates lipolysis through hormone-sensitive lipase and β-oxidation. However, the regulation of lipolysis by atRA-induced autophagy in adipocytes remains unclear. In this study, we investigated the effect of atRA on autophagy in epididymal fat of mice and the molecular mechanisms of autophagy in 3T3-L1 adipocytes. Western blotting showed that atRA decreased the expression of p62, a cargo receptor for autophagic degradation, and increased the expression of the lipidated LC3B (LC3B-II), an autophagy marker, in epididymal fat. Next, we confirmed that atRA increased autophagic flux in differentiated 3T3-L1 cells using the GFP-LC3-RFP-LC3ΔG probe. Immunofluorescent staining revealed that the colocalization of LC3B with perilipin increased in differentiated 3T3-L1 cells treated with atRA. The knockdown of Atg5, an essential gene in autophagy induction, partly suppressed the atRA-induced release of non-esterified fatty acid (NEFA) from LDs in differentiated 3T3-L1 cells. atRA time-dependently elicited the phosphorylation of AMPK and Beclin1, autophagy-inducing factors, in mature 3T3-L1 adipocytes. Inversely, atRA decreased the protein expression of Rubicon, an autophagy repressor, in differentiated 3T3-L1 cells and epididymal fat. Interestingly, the expression of ALDH1A1, atRA-synthesizing enzymes, increased in epididymal fat with decreased protein expression of Rubicon in aged mice. These results suggest that atRA may partially induce lipolysis through lipophagy by activating the AMPK-Beclin1 signaling pathway in the adipocytes and increased atRA levels may contribute to decreased Rubicon expression in the epididymal fat of aged mice. (248/250 words).

The retinoid X receptor has a critical role in synthetic rexinoid-induced increase in cellular all-trans-retinoic acid

Rexinoids are agonists of nuclear rexinoid X receptors (RXR) that heterodimerize with other nuclear receptors to regulate gene transcription. A number of selective RXR agonists have been developed for clinical use but their application has been hampered by the unwanted side effects associated with the use of rexinoids and a limited understanding of their mechanisms of action across different cell types. Our previous studies showed that treatment of organotypic human epidermis with the low toxicity UAB30 and UAB110 rexinoids resulted in increased steady-state levels of all-trans-retinoic acid (ATRA), the obligatory ligand of the RXR-RAR heterodimers. Here, we investigated the molecular mechanism underlying the increase in ATRA levels using a dominant negative RXRα that lacks the activation function 2 (AF-2) domain. The results demonstrated that overexpression of dnRXRα in human organotypic epidermis markedly reduced signaling by resident ATRA, suggesting the existence of endogenous RXR ligand, diminished the biological effects of UAB30 and UAB110 on epidermis morphology and gene expression, and nearly abolished the rexinoid-induced increase in ATRA levels. Global transcriptome analysis of dnRXRα-rafts in comparison to empty vector-transduced rafts showed that over 95% of the differentially expressed genes in rexinoid-treated rafts constitute direct or indirect ATRA-regulated genes. Thus, the biological effects of UAB30 and UAB110 are mediated through the AF-2 domain of RXRα with minimal side effects in human epidermis. As ATRA levels are known to be reduced in certain epithelial pathologies, treatment with UAB30 and UAB110 may represent a promising therapy for normalizing the endogenous ATRA concentration and signaling in epithelial tissues.

Pretreatment of human retinal pigment epithelial cells with sterculic acid forestalls fenretinide-induced apoptosis

The ratio of saturated to monounsaturated fatty acids, thought to play a critical role in many cellular functions, is regulated by stearoyl-CoA desaturase (SCD), a rate-limiting enzyme in the biosynthesis of monounsaturated fatty acids. Previously, we observed a decrease in both SCD protein and enzymatic activity in apoptosis induced by fenretinide, a synthetic analog of retinoic acid, in the human retinal pigment epithelial (RPE) cell line ARPE-19. Here, we investigated the effect of pretreating ARPE-19 with sterculic acid, a cyclopropenoic fatty acid inhibitor of SCD, on preventing fenretinide-induced apoptosis, given the role of SCD in cell proliferation and apoptosis. We show that sterculic acid pretreatment prevents the effects of fenretinide-induced apoptosis shown by changes in cell morphology, viability, and caspase-3 activation. Analysis of endoplasmic reticulum (ER)-associated proteins shows that sterculic acid pretreatment reduced the fenretinide-induced upregulation of heme oxygenase-1, ATF3 and GADD153 expression that are in response to reactive oxygen species (ROS) generation. Sterculic acid is as effective as allopurinol in inhibition of xanthine oxidase (XDH), and this may play a role in reducing the potential role of XDH in fenretinide-induced ROS generation. Sterculic acid pretreatment also results in a reduction in SOD2 mRNA expression. Dihydroceramide accumulation, compared to ceramide, and ROS generation indicate that a ceramide-independent pathway mediates fenretinide-induced apoptosis, and ROS mediation is borne out by activation of the NF-κBp50 and NF-κBp65 downstream signaling cascade. Its prevention by sterculic acid pretreatment further indicates the latter's antioxidant/anti-inflammatory effect. Taken together, our results suggest that sterculic acid pretreatment can mitigate ROS-mediated fenretinide-induced apoptosis. Thus, sterculic acid may serve as a potential antioxidant and therapeutic agent. These effects may be independent of its effects on SCD activity.

Roles of vitamin A in the regulation of fatty acid synthesis

Dietary macronutrients and micronutrients play important roles in human health. On the other hand, the excessive energy derived from food is stored in the form of triacylglycerol. A variety of dietary and hormonal factors affect this process through the regulation of the activities and expression levels of those key player enzymes involved in fatty acid biosynthesis such as acetyl-CoA carboxylase, fatty acid synthase, fatty acid elongases, and desaturases. As a micronutrient, vitamin A is essential for the health of humans. Recently, vitamin A has been shown to play a role in the regulation of glucose and lipid metabolism. This review summarizes recent research progresses about the roles of vitamin A in fatty acid synthesis. It focuses on the effects of vitamin A on the activities and expression levels of mRNA and proteins of key enzymes for fatty acid synthesis in vitro and in vivo. It appears that vitamin A status and its signaling pathway regulate the expression levels of enzymes involved in fatty acid synthesis. Future research directions are also discussed.

Beyond Proteostasis: Lipid Metabolism as a New Player in ER Homeostasis

Biological membranes are not only essential barriers that separate cellular and subcellular structures, but also perform other critical functions such as the initiation and propagation of intra- and intercellular signals. Each membrane-delineated organelle has a tightly regulated and custom-made membrane lipid composition that is critical for its normal function. The endoplasmic reticulum (ER) consists of a dynamic membrane network that is required for the synthesis and modification of proteins and lipids. The accumulation of unfolded proteins in the ER lumen activates an adaptive stress response known as the unfolded protein response (UPR-ER). Interestingly, recent findings show that lipid perturbation is also a direct activator of the UPR-ER, independent of protein misfolding. Here, we review proteostasis-independent UPR-ER activation in the genetically tractable model organism Caenorhabditis elegans. We review the current knowledge on the membrane lipid composition of the ER, its impact on organelle function and UPR-ER activation, and its potential role in human metabolic diseases. Further, we summarize the bi-directional interplay between lipid metabolism and the UPR-ER. We discuss recent progress identifying the different respective mechanisms by which disturbed proteostasis and lipid bilayer stress activate the UPR-ER. Finally, we consider how genetic and metabolic disturbances may disrupt ER homeostasis and activate the UPR and discuss how using -omics-type analyses will lead to more comprehensive insights into these processes.

Stearoyl-CoA desaturase 5 (SCD5), a Δ-9 fatty acyl desaturase in search of a function

A large body of research has demonstrated that human stearoyl-CoA desaturase 1 (SCD1), a universally expressed fatty acid Δ9-desaturase that converts saturated fatty acids (SFA) into monounsaturated fatty acids (MUFA), is a central regulator of metabolic and signaling pathways involved in cell proliferation, differentiation, and survival. Unlike SCD1, stearoyl-CoA desaturase 5 (SCD5), a second SCD isoform found in a variety of vertebrates, including humans, has received considerably less attention but new information on the catalytic properties, regulation and biological functions of this enzyme has begun to emerge. This review will examine the new evidence that supports key metabolic and biological roles for SCD5, as well as the potential implication of this desaturase in the mechanisms of human diseases.

Genetic analyses of human fetal retinal pigment epithelium gene expression suggest ocular disease mechanisms

The retinal pigment epithelium (RPE) serves vital roles in ocular development and retinal homeostasis but has limited representation in large-scale functional genomics datasets. Understanding how common human genetic variants affect RPE gene expression could elucidate the sources of phenotypic variability in selected monogenic ocular diseases and pinpoint causal genes at genome-wide association study (GWAS) loci. We interrogated the genetics of gene expression of cultured human fetal RPE (fRPE) cells under two metabolic conditions and discovered hundreds of shared or condition-specific expression or splice quantitative trait loci (e/sQTLs). Co-localizations of fRPE e/sQTLs with age-related macular degeneration (AMD) and myopia GWAS data suggest new candidate genes, and mechanisms by which a common RDH5 allele contributes to both increased AMD risk and decreased myopia risk. Our study highlights the unique transcriptomic characteristics of fRPE and provides a resource to connect e/sQTLs in a critical ocular cell type to monogenic and complex eye disorders.

Targeting stearoyl-CoA desaturase 1 to repress endometrial cancer progression

Stearoyl-CoA desaturase 1 (SCD1) is an established molecular target in many primary tumors including breast, lung, pancreatic, colon and hepatocellular carcinomas. However, its potential role in supporting endometrial cancer growth and progression has not yet been determined. In this study, we evaluated the value of SCD1 as a candidate therapeutic target in human endometrial cancer. Compared with secretory and post-menopausal endometrium, SCD1 was highly expressed in normal endometrium of proliferative phase, endometrial hyperplasia and endometrial carcinoma, while was absent or low expression in non-malignant control stromal cells and adjacent normal endometrium. Knockdown of SCD1 significantly repressed endometrial cancer cell growth and induced cell apoptosis. Both short hairpin RNA targeted knockdown and chemical inhibitor of SCD1 suppressed the foci formation of AN3CA, a metastatic endometrial cell line. Xenograft model further demonstrated that reduced SCD1 expression impaired endometrial cancer growth in vivo. Taken together, these findings indicate that SCD1 is a potentially therapeutic target in human endometrial cancer. Inhibiting lipid metabolism in cancer cells would be a promising strategy for anti-cancer therapy.

Vitamin A Improves Hyperglycemia and Glucose-Intolerance through Regulation of Intracellular Signaling Pathways and Glycogen Synthesis in WNIN/GR-Ob Obese Rat Model

Vitamin A and its metabolites modulate insulin resistance and regulate stearoyl-CoA desaturase 1 (SCD1), which are also known to affect insulin resistance. Here, we tested, whether vitamin A-mediated changes in insulin resistance markers are associated with SCD1 regulation or not. For this purpose, 30-week old male lean and glucose-intolerant obese rats of WNIN/GR-Ob strain were given either a stock or vitamin A-enriched diet, i.e. 2.6 mg or 129 mg vitamin A/kg diet, for 14 weeks. Compared to the stock diet, vitamin A-enriched diet feeding improved hyperglycemia and glucose-clearance rate in obese rats and no such changes were seen in lean rats receiving identical diets. These changes were corroborated with concomitant increase in circulatory insulin and glycogen levels of liver and muscle (whose insulin signaling pathway genes were up-regulated) in obese rats. Further, the observed increase in muscle glycogen content in these obese rats could be explained by increased levels of the active form of glycogen synthase, the key regulator of glycogen synthesis pathway, possibly inactivated through increased phosphorylation of its upstream inhibitor, glycogen synthase kinase. However, the unaltered hepatic SCD1 protein expression (despite decreased mRNA level) and increased muscle-SCD1 expression (both at gene and protein levels) suggest that vitamin A-mediated changes on glucose metabolism are not associated with SCD1 regulation. Chronic consumption of vitamin A-enriched diet improved hyperglycemia and glucose-intolerance, possibly, through the regulation of intracellular signaling and glycogen synthesis pathways of muscle and liver, but not associated with SCD1.

Transcriptome Analysis Reveals that Vitamin A Metabolism in the Liver Affects Feed Efficiency in Pigs

Feed efficiency (FE) is essential for pig production. In this study, 300 significantly differentially expressed (DE) transcripts, including 232 annotated genes, 28 cis-natural antisense transcripts (cis-NATs), and 40 long noncoding RNAs (lncRNAs), were identified between the liver of Yorkshire pigs with extremely high and low FE. Among these transcripts, 25 DE lncRNAs were significantly correlated with 125 DE annotated genes at a transcriptional level. These DE genes were enriched primarily in vitamin A (VA), fatty acid, and steroid hormone metabolism. VA metabolism is regulated by energy status, and active derivatives of VA metabolism can regulate fatty acid and steroid hormones metabolism. The key genes of VA metabolism (CYP1A1, ALDH1A2, and RDH16), fatty acid biosynthesis (FASN, SCD, CYP2J2, and ANKRD23), and steroid hormone metabolism (CYP1A1, HSD17B2, and UGT2B4) were significantly upregulated in the liver of high-FE pigs. Previous study with the same samples indicated that the mitochondrial function and energy expenditure were reduced in the muscle tissue of high-FE pigs. In conclusion, VA metabolism in liver tissues plays important roles in the regulation of FE in pigs by affecting energy metabolism, which may mediate fatty acid biosynthesis and steroid hormone metabolism. Furthermore, our results identified novel transcripts, such as cis-NATs and lncRNAs, which are also involved in the regulation of FE in pigs.

Stearoyl CoA desaturase-1: New insights into a central regulator of cancer metabolism

The processes of cell proliferation, cell death and differentiation involve an intricate array of biochemical and morphological changes that require a finely tuned modulation of metabolic pathways, chiefly among them is fatty acid metabolism. The critical participation of stearoyl CoA desaturase-1 (SCD1), the fatty acyl Δ9-desaturing enzyme that converts saturated fatty acids (SFA) into monounsaturated fatty acids (MUFA), in the mechanisms of replication and survival of mammalian cells, as well as their implication in the biological alterations of cancer have been actively investigated in recent years. This review examines the growing body of evidence that argues for a role of SCD1 as a central regulator of the complex synchronization of metabolic and signaling events that control cellular metabolism, cell cycle progression, survival, differentiation and transformation to cancer.

A novel AhR ligand, 2AI, protects the retina from environmental stress

Various retinal degenerative diseases including dry and neovascular age-related macular degeneration (AMD), retinitis pigmentosa, and diabetic retinopathy are associated with the degeneration of the retinal pigmented epithelial (RPE) layer of the retina. This consequently results in the death of rod and cone photoreceptors that they support, structurally and functionally leading to legal or complete blindness. Therefore, developing therapeutic strategies to preserve cellular homeostasis in the RPE would be a favorable asset in the clinic. The aryl hydrocarbon receptor (AhR) is a conserved, environmental ligand-dependent, per ARNT-sim (PAS) domain containing bHLH transcription factor that mediates adaptive response to stress via its downstream transcriptional targets. Using in silico, in vitro and in vivo assays, we identified 2,2′-aminophenyl indole (2AI) as a potent synthetic ligand of AhR that protects RPE cells in vitro from lipid peroxidation cytotoxicity mediated by 4-hydroxynonenal (4HNE) as well as the retina in vivo from light-damage. Additionally, metabolic characterization of this molecule by LC-MS suggests that 2AI alters the lipid metabolism of RPE cells, enhancing the intracellular levels of palmitoleic acid. Finally, we show that, as a downstream effector of 2AI-mediated AhR activation, palmitoleic acid protects RPE cells from 4HNE-mediated stress, and light mediated retinal degeneration in mice.

Fenretinide induces ubiquitin-dependent proteasomal degradation of stearoyl-CoA desaturase in human retinal pigment epithelial cells

Stearoyl-CoA desaturase (SCD, SCD1), an endoplasmic reticulum (ER) resident protein and a rate-limiting enzyme in monounsaturated fatty acid biosynthesis, regulates cellular functions by controlling the ratio of saturated to monounsaturated fatty acids. Increase in SCD expression is strongly implicated in the proliferation and survival of cancer cells, whereas its decrease is known to impair proliferation, induce apoptosis, and restore insulin sensitivity. We examined whether fenretinide, (N-(4-hydroxyphenyl)retinamide, 4HPR), which induces apoptosis in cancer cells and recently shown to improve insulin sensitivity, can modulate the expression of SCD. We observed that fenretinide decreased SCD protein and enzymatic activity in the ARPE-19 human retinal pigment epithelial cell line. Increased expression of BiP/GRP78, ATF4, and GADD153 implicated ER stress. Tunicamycin and thapsigargin, compounds known to induce ER stress, also decreased the SCD protein. This decrease was completely blocked by the proteasome inhibitor MG132. In addition, PYR41, an inhibitor of ubiquitin activating enzyme E1, blocked the fenretinide-mediated decrease in SCD. Immunoprecipitation analysis using anti-ubiquitin and anti-SCD antibodies and the blocking of SCD loss by PYR41 inhibition of ubiquitination further corroborate that fenretinide mediates the degradation of SCD in human RPE cells via the ubiquitin-proteasome dependent pathway. Therefore, the effect of fenretinide on SCD should be considered in its potential therapeutic role against cancer, type-2 diabetes, and retinal diseases.

Effect of high versus low doses of fat and vitamin A dietary supplementation on fatty acid composition of phospholipids in mice

Dietary fat and vitamin A provide important precursors for potent bioactive ligands of nuclear hormone receptors, which regulate various enzymes involved in lipid homeostasis, metabolism and inflammation. We determined the effects of dietary fat and dietary vitamin A on hepatic expression of two fatty acid metabolizing enzymes, elongase 6 (ELOVL6) and stearoyl-coenzyme A desaturase 1 (SCD1) and the concentration of saturated fatty acids (SAFA) and monounsaturated fatty acid (MUFA) of phospholipids in serum and liver. Mice (n = 6) were fed 4 weeks with diets containing 2, 5 and 25 % of fat or vitamin A (0, 2,500 and 326,500 RE/kg as retinyl palmitate). MUFAs and SAFAs were measured using GC and ESI-MS/MS. Hepatic expression of metabolizing enzymes was determined using QRT-PCR. ELOVL6 was significantly down-regulated in response to a high-fat diet (p < 0.001) and significantly up-regulated in response to low-fat diet (p < 0.05). SCD1 expression was significantly lower in high- versus low-fat diet (p < 0.05). The vitamin A content in the diet did not influence the hepatic expression of both enzymes. In plasma, the amounts of MUFAs bound to phospholipids significantly decreased in response to a high-fat diet and increased after a low-fat diet. This tendency was also observed in the liver for various phospholipids sub-classes. In summary, this study shows that fat content in the diet has a stronger impact than the content of vitamin A on hepatic gene expression of SCD1 and ELOVL6 and thereby on MUFA and SAFA concentrations in liver and plasma.

Activation of human stearoyl-coenzyme A desaturase 1 contributes to the lipogenic effect of PXR in HepG2 cells

The pregnane X receptor (PXR) was previously known as a xenobiotic receptor. Several recent studies suggested that PXR also played an important role in lipid homeostasis but the underlying mechanism remains to be clearly defined. In this study, we found that rifampicin, an agonist of human PXR, induced lipid accumulation in HepG2 cells. Lipid analysis showed the total cholesterol level increased. However, the free cholesterol and triglyceride levels were not changed. Treatment of HepG2 cells with rifampicin induced the expression of the free fatty acid transporter CD36 and ABCG1, as well as several lipogenic enzymes, including stearoyl-CoA desaturase-1 (SCD1), long chain free fatty acid elongase (FAE), and lecithin-cholesterol acyltransferase (LCAT), while the expression of acyl:cholesterol acetyltransferase(ACAT1) was not affected. Moreover, in PXR over-expressing HepG2 cells (HepG2-PXR), the SCD1 expression was significantly higher than in HepG2-Vector cells, even in the absence of rifampicin. Down-regulation of PXR by shRNA abolished the rifampicin-induced SCD1 gene expression in HepG2 cells. Promoter analysis showed that the human SCD1 gene promoter is activated by PXR and a novel DR-7 type PXR response element (PXRE) response element was located at -338 bp of the SCD1 gene promoter. Taken together, these results indicated that PXR activation promoted lipid synthesis in HepG2 cells and SCD1 is a novel PXR target gene.

The oil spill in ageing Bruch membrane

Ageing is the largest risk factor for age-related macular degeneration (AMD), and soft drusen and basal linear deposits are lipid-rich extracellular lesions specific to AMD. Oil red O binding neutral lipid represents a major age-related deposition in the Bruch membrane (BrM) and the first identified druse component. Decades after these seminal observations, a natural history of neutral lipid deposition has been articulated and a biochemical model proposed. Results obtained with multiple biochemical, histochemical, and ultrastructural methods, and supported indirectly by epidemiology, suggest that the RPE secretes apolipoprotein B (apoB)-lipoprotein particles of unusual composition into BrM, where they accumulate with age eventually forming a lipid wall, a precursor of basal linear deposit. The authors propose that constituents of these lesions interact with reactive oxygen species to form pro-inflammatory peroxidised lipids that elicit neovascularisation. Here, the authors summarise key evidence supporting both accumulation of BrM lipoproteins leading to lesion formation and lipoprotein production by the RPE. The authors update their model with genetic associations between AMD and genes historically associated with plasma HDL metabolism, and suggest future directions for research and therapeutic strategies based on an oil-spill analogy.

Effect of synthetic ligands of PPAR α, β/δ, γ, RAR, RXR and LXR on the fatty acid composition of phospholipids in mice

Nuclear hormone receptors are transcription factors that can be activated by nutrition-derived ligands and alter the expression of various specific target genes. Stearoyl-Coenzyme A desaturase (SCD1) converts palmitic acid (16:0) to palmitoleic acid (16:1n-7) as well as stearic acid (18:0) to oleic acid (18:1n-9). At the same time, elongase 6 (ELOVL6) elongates 16:1n-7 and 18:1n-9 to vaccenic acid (18:1n-7) and eicosenoic acid (20:1n-9). We examined how synthetic selective ligands of nuclear hormone receptors alter the gene expression of hepatic enzymes in mice. In addition, we examined how the regulation of these two enzymes influences fatty acid composition of phospholipids in liver and plasma. Mice were gavaged daily for 1 week with synthetic ligands of peroxisome proliferator-activated receptor (PPAR) α, β/δ, γ, liver X receptor (LXR), retinoic acid receptor (RAR) and retinoid-X receptor (RXR) for 1 week. Phospholipids from liver and plasma were analysed using ESI-MS/MS and GC after saponification. Hepatic gene expression of SCD1 and ELOVL6 was measured using QRT-PCR. SCD1 and ELOVL6 expression increased after the gavage of LXR and RXR ligands. The analysis of fatty acid composition of total phospholipids in plasma and liver showed increased percentage contributions of the SCD1 and ELOVL6 products 18:1n-9, 18:1n-7 and 20:1n-9 after LXR and RXR ligand application. Analysis of total phospholipids from plasma and liver revealed a significant increase in monounsaturated fatty acids bound in phosphatidylcholine (PtdCho) and lysophosphatidylcholine (PtdEtn) after LXR and RXR ligand administration. Increased hepatic gene expression of SCD1 and ELOVL6 after gavage of selective RXR or LXR ligands to mice resulted in increased concentrations of their metabolic products in phospholipids of liver and plasma.

Decreased expression of insulin-like growth factor binding protein-5 during N-(4-hydroxyphenyl)retinamide-induced neuronal differentiation of ARPE-19 human retinal pigment epithelial cells: regulation by CCAAT/enhancer-binding protein

Insulin-like growth factor (IGF)-binding protein-5 (IGFBP5), an important member of the IGF axis involved in regulating cell growth and differentiation, acts by modulating IGF signaling and also by IGF-independent mechanisms. We identified IGFBP5 by microarray analysis as a gene differentially regulated during N-(4-hydroxyphenyl)retinamide (4HPR)-induced neuronal differentiation of human retinal pigment epithelial (RPE) cells. IGFBP5 is expressed in human RPE cells, and its expression, mRNA as well as protein, is greatly decreased during the 4HPR-induced neuronal differentiation. Exogenous IGFBP5 does not block the neuronal differentiation indicating that IGFBP5 down-regulation may not be a prerequisite for the neuronal differentiation. IGFBP5 down-regulation, similar to neuronal differentiation, is mediated by the MAPK pathway since U0126, an inhibitor of MEK1/2, effectively blocked it. The overexpression of transcription factor CCAAT/enhancer binding protein-beta (C/EBPbeta) inhibited the 4HPR-induced down-regulation of IGFBP5 expression and the neuronal differentiation of RPE cells. Interestingly, the binding of C/EBPbeta to the IGFBP5 promoter was decreased by the 4HPR treatment as indicated by gel shift and chromatin immunoprecipitation analyses. Further, the deletion of C/EBP response element from IGFBP5 promoter markedly decreased the basal promoter activity and abolished its responsiveness to 4HPR treatment in reporter assays, suggesting that the expression of IGFBP5 is regulated by C/EBP. Thus, our results clearly demonstrate that the IGFBP5 expression is down-regulated during 4HPR-induced neuronal differentiation of human RPE cells through a MAPK signal transduction pathway involving C/EBPbeta.

Stearoyl-CoA desaturase-1: a novel key player in the mechanisms of cell proliferation, programmed cell death and transformation to cancer

As part of a shift toward macromolecule production to support continuous cell proliferation, cancer cells coordinate the activation of lipid biosynthesis and the signaling networks that stimulate this process. A ubiquitous metabolic event in cancer is the constitutive activation of the fatty acid biosynthetic pathway, which produces saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs) to sustain the increasing demand of new membrane phospholipids with appropriate acyl composition. In cancer cells, the tandem activation of the fatty acid biosynthetic enzymes adenosine triphosphate citrate lyase, acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) leads to increased synthesis of SFA and their further conversion into MUFA by stearoyl-CoA desaturase (SCD) 1. The roles of adenosine triphosphate citrate lyase, ACC and FAS in the pathogenesis of cancer have been a subject of extensive investigation. However, despite early experimental and epidemiological observations reporting elevated levels of MUFA in cancer cells and tissues, the involvement of SCD1 in the mechanisms of carcinogenesis remains surprisingly understudied. Over the past few years, a more detailed picture of the functional relevance of SCD1 in cell proliferation, survival and transformation to cancer has begun to emerge. The present review addresses the mounting evidence that argues for a key role of SCD1 in the coordination of the intertwined pathways of lipid biosynthesis, energy sensing and the transduction signals that influence mitogenesis and tumorigenesis, as well as the potential value of this enzyme as a target for novel pharmacological approaches in cancer interventions.

Enhanced circulating retinol and non-esterified fatty acids in pregnancies complicated with intrauterine growth restriction

IUGR (intrauterine growth restriction) increases the incidence of perinatal complications and, although several placental transport functions have been shown to be altered in pregnancies complicated by IUGR, the mechanism behind it is not well understood. The aim of the present study was to investigate factors in maternal and cord blood plasma from normal and IUGR-complicated pregnancies associated with the body weight of newborns. At the time of Caesarean section, 24 women with IUGR pregnancies were compared with a group of 30 normal controls with AGA (appropriate gestational age) fetuses who were studied at Caesarean section, which took place 5 weeks later than IUGR pregnancies, and also to a group of 25 non-delivered gestational age-matched control pregnant women (AGA-35wk). Maternal plasma retinol, gamma- and alpha-tocopherol, NEFAs (non-esterified fatty acids), and palmitic, palmitoleic, gamma-linolenic and arachidonic acids were higher in women with IUGR pregnancies than in AGA-35wk controls, whereas stearic and alpha-linolenic acids were lower. Smaller differences were found when comparing these variables for IUGR and AGA women. However, umbilical vein plasma gamma-tocopherol, cholesterol, triacylglycerols and NEFAs were higher in the IUGR group than in the AGA group, whereas arachidonic acid was lower. Maternal plasma retinol and NEFAs were the only variables negatively correlated with birthweight when multiple linear regressions were analysed. In conclusion, the increased levels of circulating retinol and NEFAs in maternal plasma are negatively associated with birth and placental weights, which may reflect an impaired placental transfer in IUGR pregnancies. As retinoids are involved in the control of gene transcription, it is proposed that a decrease in placental transfer of retinol could underlie the metabolic dysfunction of IUGR pregnancies.

Biochemical and physiological function of stearoyl-CoA desaturase

A key and highly regulated enzyme that is required for the biosynthesis of monounsaturated fatty acids is stearoyl-CoA desaturase (SCD), which catalyzes the D(9)-cis desaturation of a range of fatty acyl-CoA substrates. The preferred substrates are palmitoyl- and stearoyl-CoA, which are converted into palmitoleoyl- and oleoyl-CoA respectively. Oleate is the most abundant monounsaturated fatty acid in dietary fat and is therefore readily available. Studies of mice that have a naturally occurring mutation in the SCD-1 gene isoform as well as a mouse model with a targeted disruption of the SCD gene (SCD-1(-/-)) have revealed the role of de novo synthesized oleate and thus the physiological importance of SCD-1 expression. SCD-1 deficiency results in reduced body adiposity, increased insulin sensitivity, and resistance to diet-induced obesity. The expression of several genes of lipid oxidation are upregulated, whereas lipid synthesis genes are downregulated. SCD-1 was also found to be a component of the novel metabolic response to the hormone leptin. Therefore, SCD-1 appears to be an important metabolic control point, and inhibition of its expression could be of benefit for the treatment of obesity, diabetes, and other metabolic diseases. In this article, we summarize the recent and timely advances concerning the important role of SCD in the biochemistry and physiology of lipid metabolism.

Mitogen-activated protein kinase pathway mediates N-(4-hydroxyphenyl)retinamide-induced neuronal differentiation in the ARPE-19 human retinal pigment epithelial cell line

We have shown previously that N-(4-hydroxyphenyl)retinamide (4HPR, fenretinide), a retinoic acid derivative, induces neuronal differentiation in cultured human retinal pigment epithelial (RPE) cells [Chen et al., J. Neurochem., 84 (2003), 972]. We asked the question whether the mitogen-activated protein kinase (MAPK) pathway is involved in the regulation of the 4HPR-induced neuronal differentiation of RPE (ARPE-19) cells. When we treated ARPE-19 cells with 4HPR, c-Raf and MEK1/2 kinase were activated resulting in activation of the downstream effector ERK1/2 and of SAPK/JNK. By blocking the upstream kinase MEK1/2 with specific inhibitor U0126 we abrogated the 4HPR-induced phosphorylation of ERK1/2 and SAPK/JNK, indicating that the neuronal differentiation occurs through a positive cross-talk between the ERK and the SAPK/JNK pathways. Both U0126 and the suppression of ERK1/2 expression with small interfering RNA effectively blocked the 4HPR-induced neuronal differentiation of RPE cells and the expression of calretinin. The activated ERK1/2 then induced a sequential activation of p90RSK, and increase in phosphorylation of transcription factors c-fos and c-jun leading to transcriptional activation of AP-1. Taken together, our results clearly demonstrate that c-Raf/MEK1/2 signaling cascade involving ERK1/2 plays a central role in mediating the 4HPR-induced neuronal differentiation and calretinin expression in the human ARPE-19 retinal pigment epithelial cell line.

A2E, a pigment of the lipofuscin of retinal pigment epithelial cells, is an endogenous ligand for retinoic acid receptor

Lipofuscin contains fluorophores, which represent a biomarker for cellular aging. Although it remains unsubstantiated clinically, experimental results support that the accumulation of lipofuscin is related to an increased risk of choroidal neovascularization due to age-related macular degeneration, a leading cause of legal blindness. Here, we report that a major lipofuscin component, A2E, activates the retinoic acid receptor (RAR). In vitro experiments using luciferase reporter assay, competitional binding assay, analysis of target genes, and chromatin immunoprecipitation (ChIP) assay strongly suggest that A2E is a bona fide ligand for RAR and induces sustained activation of RAR target genes. A2E-induced vascular endothelial growth factor (VEGF) expression in a human retinal pigment epithelial cell line (ARPE-19) and RAR antagonist blocked the up-regulation of VEGF. The conditioned medium of A2E-treated ARPE-19 cells induced tube formation in human umbilical vascular endothelial cells, which was blocked by the RAR antagonist and anti-VEGF antibody. These results suggest that A2E accumulation results in the phenotypic alteration of retinal pigment epithelial cells, predisposing the environment to choroidal neovascularization development. This is mediated through the agonistic function of A2E, at least in part. The results of this study provide a novel potential therapeutic target for this incurable condition.

Stearoyl-CoA desaturase mRNA expression during bovine adipocyte differentiation in primary culture derived from Japanese Black and Holstein cattle

Stearoyl-CoA desaturase (SCD) catalyzes the synthesis of monounsaturated fatty acids (MUFAs). In cattle, the MUFAs are related to softness and flavor of meat. In order to investigate gene expression profile during bovine preadipocyte differentiation, we isolated stromal-vascular cells from perirenal adipose tissues of Japanese Black and Holstein steers. Gene expression level of adipocyte type fatty acid binding protein (FABP4), SCD, sterol regulatory element binding protein 1 (SREBP1) and CCAAT/enhancer binding protein alpha (C/EBP-alpha) were elucidated by real-time PCR assay. The levels of SCD mRNA expression were significantly increased to 10.8 and 6.3-fold in Japanese Black and Holstein, respectively, on day 1 of the culture. The difference in SCD expression between the two breeds may reflect differences in the fat development characteristics of the cattle breeds. Although transcription factors SREBP1 and C/EBP-alpha are supposed to regulate SCD expression, expression levels of the two factors were not completely consistent with that of SCD.

Influence of dietary lipids on the fatty acid composition and stearoyl-CoA desaturase expression in hybrid tilapia (Oreochromis niloticusxO. aureus) under cold shock

To improve hybrid tilapia (Oreochromis niloticusxO. aureus) survival under cold shock, the influence of diets containing various dietary lipids was investigated. Four different diets were used which consisted of 12% fish oil, 12% palmitoleic oil 12% coconut oil, and a mixture of fish oil (7%) and corn oil (5%). Our results showed that during cold shock, the proportion of saturated fatty acids in the fish steadily and significantly decreased for all of the diets, but the proportion of monounsaturated fatty acids increased. Proportions of polyenoic fatty acids initially increased then stabilized for the mixed, fish, and coconut oil diets, but did not significantly increase until day 4 for the palmitoleic oil diet. The stearoyl-CoA desaturase (SCD) activity was the lowest on day 0 and then gradually increased for all diets. At any point, the enzymatic activity of SCD was the highest for fish on the mixed and the coconut oil diet, followed by the palmitoleic oil diet, and was lowest for the fish oil diet. The expression of SCD mRNA steadily increased for all diets, but increased more substantially for the mixed diet. On day 6, the expression was the highest for fish on the mixed diet, followed by the coconut oil diet, with the lowest levels for those on the palmitoleic and fish oil diets. These results show that dietary lipids strongly affect the fatty acid composition and SCD expression in tilapia under cold shock, and cold tolerance of this species is also affected.

N-(4-hydroxyphenyl)retinamide induces apoptosis in human retinal pigment epithelial cells: retinoic acid receptors regulate apoptosis, reactive oxygen species generation, and the expression of heme oxygenase-1 and Gadd153

N-(4-hydroxyphenyl)retinamide (4HPR, fenretinide), a retinoic acid (RA) derivative and a potential cancer preventive agent, is known to exert its chemotherapeutic effects in cancer cells through induction of apoptosis. Earlier work from our laboratory has shown that relatively low concentrations of 4HPR induce neuronal differentiation of cultured human retinal pigment epithelial (ARPE-19) cells (Chen et al., 2003, J Neurochem 84:972-981). However, at higher concentrations of 4HPR, these cells showed morphological changes including cell shrinkage and cell death. Here we demonstrate that ARPE-19 cells treated with 4HPR exhibit a dose- and time-dependent induction of apoptosis as evidenced by morphological changes, mono- and oligonucleosome generation, and increased activity of caspases 2 and 3. The 4HPR-induced apoptosis as well as the activation of caspases 2 and 3 were blocked by both retinoic acid receptors (RAR) pan-antagonists, AGN193109 and AGN194310, and by an RARalpha-specific antagonist AGN194301. 4HPR treatment also increased reactive oxygen species (ROS) generation in ARPE-19 cells in a time-dependent manner as determined from the oxidation of 2',7'-dichlorofluorescin. In addition, the increase in the expression of heme oxygenase-1 (HO-1), a stress response protein, and the growth arrest and DNA damage-inducible transcription factor 153 (Gadd153) in response to the ROS generation were also blocked by these receptor antagonists. Pyrrolidine dithiocarbamate (PDTC), a free-radical scavenger, inhibited 4HPR-induced ROS generation, the expression of its downstream mediator, Gadd153, and apoptosis in the pretreated cells. Therefore, our results, clearly demonstrate that 4HPR induces apoptosis in ARPE-19 cells and that RARs mediate this process by regulating ROS generation as well as the expression of Gadd153 and HO-1.

Identification of Biomarkers Modulated by the Rexinoid LGD1069 (Bexarotene) in Human Breast Cells Using Oligonucleotide Arrays

Retinoids have been found to be promising chemopreventive agents that play an important role in regulating cell growth, differentiation, and apoptosis. The action of retinoids is mediated by retinoid receptors (retinoic acid receptors and retinoid X receptors), which are nuclear transcription factors that, when bound to retinoids, regulate gene expression. LGD1069 is a highly selective RXR agonist that has reduced toxicity compared with retinoids. Our previous studies have shown that RXR-selective ligands (or "rexinoids"), including LGD1069, can inhibit the growth of normal and malignant breast cells and can suppress the development of breast cancer in transgenic mice. For the current study, we attempted to identify biomarkers of the chemopreventive effect of the RXR-selective retinoid LGD1069. In these experiments, we used Affymetrix microarrays to identify target genes that were modulated by LGD1069 in normal human breast cells. Affymetrix and dChip analysis identified more than 100 genes that were up-regulated or down-regulated by LGD1069 treatment. We then tested 16 of these genes in validation experiments using quantitative reverse transcription-PCR and Western blotting of independently prepared samples, and found that 15 of 16 genes were modulated in a similar manner in these validation experiments as in the microarray experiments. Genes found to be regulated include known retinoid-regulated genes, growth regulatory genes, transcription factors, and differentiation markers. We then showed that the expression of several of these rexinoid-regulated biomarkers is modulated in vivo in mammary glands from mice treated with LGD1069. These critical growth-regulating proteins will be promising targets of future agents for the prevention and treatment of breast cancer.

Cell density-dependent nuclear/cytoplasmic localization of NORPEG (RAI14) protein

NORPEG (RAI14), a developmentally regulated gene induced by retinoic acid, encodes a 980 amino acid (aa) residue protein containing six ankyrin repeats and a long coiled-coil domain [Kutty et al., J. Biol. Chem. 276 (2001), pp. 2831-2840]. We have expressed aa residues 1-287 of NORPEG and used the recombinant protein to produce an anti-NORPEG polyclonal antibody. Confocal immunofluorescence analysis showed that the subcellular localization of NORPEG in retinal pigment epithelial (ARPE-19) cells varies with cell density, with predominantly nuclear localization in nonconfluent cells, but a cytoplasmic localization, reminiscent of cytoskeleton, in confluent cultures. Interestingly, an evolutionarily conserved putative monopartite nuclear localization signal (P(270)KKRKAP(276)) was identified by analyzing the sequences of NORPEG and its orthologs. GFP-NORPEG (2-287 aa), a fusion protein containing this signal, was indeed localized to nuclei when expressed in ARPE-19 or COS-7 cells. Deletion and mutation analysis indicated that the identified nuclear localization sequence is indispensable for nuclear targeting.

Retinoid x receptor agonists increase bcl2a1 expression and decrease apoptosis of naive T lymphocytes

Vitamin A affects many aspects of T lymphocyte development and function. The vitamin A metabolites all-trans- and 9-cis-retinoic acid regulate gene expression by binding to the retinoic acid receptor (RAR), while 9-cis-retinoic acid also binds to the retinoid X receptor (RXR). Naive DO11.10 T lymphocytes expressed mRNA and protein for RAR-alpha, RXR-alpha, and RXR-beta. DNA microarray analysis was used to identify RXR-responsive genes in naive DO11.10 T lymphocytes treated with the RXR agonist AGN194204. A total of 128 genes was differentially expressed, including 16 (15%) involved in cell growth or apoptosis. Among these was Bcl2a1, an antiapoptotic Bcl2 family member. Quantitative real-time PCR analysis confirmed this finding and demonstrated that Bcl2a1 mRNA expression was significantly greater in nonapoptotic than in apoptotic T lymphocytes. The RXR agonist 9-cis-retinoic acid also increased Bcl2a1 expression, although all-trans-retinoic acid and ligands for other RXR partner receptors did not. Treatment with AGN194204 and 9-cis-retinoic acid significantly decreased apoptosis measured by annexin V staining but did not affect expression of Bcl2 and Bcl-xL. Bcl2a1 promoter activity was examined using a luciferase promoter construct. Both AGN194204 and 9-cis-retinoic acid significantly increased luciferase activity. In summary, these data demonstrate that RXR agonists increase Bcl2a1 promoter activity and increase expression of Bcl2a1 in naive T lymphocytes but do not affect Bcl2 and Bcl-xL expression in naive T lymphocytes. Thus, this effect on Bcl2a1 expression may account for the decreased apoptosis seen in naive T lymphocytes treated with RXR agonists.

Stearoyl-CoA Desaturase Is Involved in the Control of Proliferation,Anchorage-independent Growth, and Survival in Human TransformedCells

Saturated and monounsaturated fatty acids are the most abundant fatty acid species in mammalian organisms, and their distribution is regulated by stearoyl-CoA desaturase, the enzyme that converts saturated into monounsaturated fatty acids. A positive correlation between high monounsaturated fatty acid levels and neoplastic transformation has been reported, but little is still known about the regulation of stearoyl-CoA desaturase in cell proliferation and apoptosis, as well as in cancer development. Here we report that simian virus 40-transformed human lung fibroblasts bearing a knockdown of human stearoyl-CoA desaturase by stable antisense cDNA transfection (hSCDas cells) showed a considerable reduction in monounsaturated fatty acids, cholesterol, and phospholipid synthesis, compared with empty vector transfected-simian virus 40 cell line (control cells). hSCDas cells also exhibited high cellular levels of saturated free fatty acids and triacylglycerol. Interestingly, stearoyl-CoA desaturase-depleted cells exhibited a dramatic decrease in proliferation rate and abolition of anchorage-independent growth. Prolonged exposure to exogenous oleic acid did not reverse either the slower proliferation or loss of anchorage-independent growth of hSCDas cells, suggesting that endogenous synthesis of monounsaturated fatty acids is essential for rapid cell replication and invasiveness, two hallmarks of neoplastic transformation. Moreover, apoptosis was increased in hSCDas cells in a ceramide-independent manner. Finally, stearoyl-CoA desaturase-deficient cells were more sensitive to palmitic acid-induced apoptosis compared with control cells. Our data suggest that, by globally regulating lipid metabolism, stearoyl-CoA desaturase activity modulates cell proliferation and survival and emphasize the important role of endogenously synthesized monounsaturated fatty acids in sustaining the neoplastic phenotype of transformed cells.

Regulation of stearoyl-CoA desaturase expression

Stearoyl-CoA desaturase (SCD) is a regulatory enzyme in lipogenesis, catalyzing the rate-limiting step in the overall de novo synthesis of monounsaturated FA, mainly oleate and palmitoleate from stearoyl- and palmitoyl-CoA, respectively. Oleate and palmitoleate are the major monounsaturated FA of membrane phospholipids, TG, wax esters, cholesterol esters, and alkyldiacylglycerol. Several SCD gene isoforms (SCD1, SCD2, SCD3, and SCD4) exist in mice, and two have been characterized in humans. SCD1 gene expression in liver cells is regulated by numerous stimuli including diet and hormones. We are interested in why SCD is such a highly regulated enzyme even though oleate, the major product of this enzyme, is one of the most abundant FA in the diet and is therefore readily available. Dietary oleate is also well known for its TG-lowering effects and, as a major component of olive oil, is expected to have beneficial effects. However, high SCD activity has been implicated in diabetes, obesity, atherosclerosis, and cancer in several animal models; therefore, the role that de novo oleate plays in these disease states has to be carefully evaluated. By using SCD1-/- mice, which are deficient in tissue oleate, we begin to learn more about the physiological role of SCD gene expression and oleate in normal and disease states.

High stearoyl-CoA desaturase protein and activity levels in simian virus 40 transformed-human lung fibroblasts

The precise role of monounsaturated fatty acid (MUFA) synthesis in cell proliferation and programmed cell death remains unknown. The strong correlation of high levels of MUFA and neoplastic phenotype suggest that the regulation of stearoyl CoA desaturase (SCD) must play a significant role in cancer development. In this study, the levels of SCD protein and activity were investigated in normal (WI38) and SV40-transformed (SV40-WI38) human lung fibroblasts. Thus, the activity of SCD on exogenous [14C]stearic acid and endogenous [14C]acetate-labeled fatty acids was increased by 2.2- and 2.6-fold, respectively, in SV40-WI38 compared to WI38 fibroblasts. Concomitantly, a 3.3-fold increase in SCD protein content was observed in SV40-transformed cells. Cell transformation also led to high levels of MUFA, which was paralleled by a more fluid membrane environment. Furthermore, the levels of PPAR-gamma, a well-known activator of SCD expression, were highly increased in SV40-transformed fibroblasts. SCD activity appeared linked to the events of programmed cell death, since incubations with 40 microM etoposide induced apoptosis in SV40 cells, and led to a decrease in fatty acid synthesis, SCD activity and in MUFA cellular levels. Taken together, these results suggest that SCD protein and activity levels are associated with the events of neoplastic cell transformation and programmed cell death.

Recent insights into stearoyl-CoA desaturase-1

PURPOSE OF REVIEW

Stearoyl-Coenzyme A (CoA) desaturase is a central lipogenic enzyme catalyzing the synthesis of monounsaturated fatty acids - mainly oleate (C(18:1)). Oleate is the most abundant monounsaturated fatty acid in dietary fat and is therefore readily available. Why, then, is stearoyl-CoA desaturase a highly regulated enzyme? This review summarizes the recent and timely advances concerning the important role of stearoyl-CoA desaturase in metabolism.

RECENT FINDINGS

Recent findings using mice that have a naturally occurring mutation in the SCD1 gene isoform as well as a mouse model with a targeted disruption of the stearoyl-CoA desaturase gene-1 (SCD1-/-) have revealed the role of de-novo synthesized oleate and thus the physiological importance of SCD1 expression. In the highlighted references, it is shown that the SCD1-/- mice have reduced body adiposity, increased insulin sensitivity, and are resistant to diet-induced obesity. The expression of several genes of lipid oxidation is upregulated, whereas lipid synthesis genes are downregulated. SCD1 was also found to be a component of the novel metabolic response to the hormone leptin.

SUMMARY

SCD1, therefore, appears to be an important metabolic control point, and inhibition of its expression could be of benefit for the treatment of obesity, diabetes and other metabolic diseases.

Differentiation of human retinal pigment epithelial cells into neuronal phenotype by N-(4-hydroxyphenyl)retinamide

ARPE-19, a human retinal pigment epithelial (RPE) cell line, has been widely used in studies of RPE function as well as gene expression. Here, we report the novel finding that N-(4-hydroxyphenyl)retinamide (fenretinide), a synthetic retinoic acid derivative and a potential chemopreventive agent against cancer, induced the differentiation of ARPE-19 cells into a neuronal phenotype. The treated cells lost their epithelial phenotype and exhibited a typical neuronal shape with long processes (four to five times longer than the cell body). The onset of fenretinide-induced neuronal differentiation was dose and time dependent, started within 1-2 days, and lasted at least 4 weeks. Immunohistochemical studies indicated that the expression of neurofilament proteins (NF160 and NF200), calretinin and neural cell adhesion molecule was increased in these differentiated cells. Western blot analysis indicated that cellular retinaldehyde-binding protein, which is normally expressed in RPE cells, was decreased in treated cells. Protein analysis on a two-dimensional gel followed by matrix-assisted laser desorption ionization-time of flight mass spectrometric analysis demonstrated that heat-shock protein 70 was increased after fenretinide treatment. Thus, fenretinide, a synthetic retinoid, is able to induce neuronal differentiation of human RPE cells in culture.

Role of stearoyl-coenzyme A desaturase in lipid metabolism

Stearoyl-CoA desaturase (SCD) (EC 1.14.99.5) is an endoplasmic reticulum-bound enzyme that catalyzes the delta9-cis desaturation of saturated fatty acyl-CoAs, the preferred substrates being palmitoyl- and stearoyl-CoA, which are converted to palmitoleoyl- and oleoyl-CoA, respectively. These monounsaturated fatty acids are used as substrates for the synthesis of triglycerides, wax esters, cholesteryl esters and membrane phospholipids. The saturated to monounsaturated fatty acid ratio affects membrane phospholipid composition and alteration in this ratio has been implicated in a variety of disease states including cardiovascular disease, obesity, diabetes, neurological disease, skin disorders and cancer. Thus, the expression of SCD is of physiological importance in normal and disease states. Several mammalian SCD genes have been cloned. A single human, three mouse and two rat are the best characterized SCD genes. The physiological role of each SCD isoform and the reason for having three or more SCD gene isoforms in the rodent genome are currently unknown. A clue as to the physiological role of the SCD, at least SCD1 gene and its endogenous products came from recent studies of asebia mouse strains that have a natural mutation in the SCD1 gene and a mouse model with a targeted disruption of the SCD1 gene. In this review we discuss our current understanding of the physiological role of SCD in lipid synthesis and metabolism.

Recent advances in molecular cloning of fatty acid desaturase genes and the regulation of their expression by dietary vitamin A and retinoic acid

Vitamin A, as an essential micronutrient, is involved in higher animals in embryonic development and postnatal growth, reproduction and maintenance of normal skin, immunity and vision. Recently, studies in vivo and in cell lines have shown that vitamin A and its active metabolite, retinoic acid, regulate the expression of fatty acid desaturases including stearoyl-CoA desaturase and delta-5 desaturase. Whereas the former desaturase catalyzes the formation of monounsaturated from saturated fatty acids, the latter enzyme is involved in the desaturation pathway of dietary essential fatty acids for production of polyunsaturated fatty acids. The reaction products of these desaturases serve as critical regulators in a wide range of physiological processes which include fetal growth and development, reproduction, cell differentiation, immune and inflammatory responses.

Transforming growth factor-beta regulates stearoyl coenzyme A desaturase expression through a Smad signaling pathway

The regulation of stearoyl-CoA desaturase (SCD), a rate-limiting enzyme in the synthesis of unsaturated fatty acids, is physiologically important because the ratio of saturated to unsaturated fatty acids is thought to control cellular functions by modulating the structural integrity and fluidity of cell membranes. Transforming growth factor-beta (TGF-beta), a multifunctional cytokine, increased SCD mRNA expression in cultured human retinal pigment epithelial cells. This response was elicited by all three TGF-beta isoforms, beta1, beta2, and beta3. However, SCD mRNA expression was not increased either by other members of the TGF-beta family or by other growth factors or cytokines. TGF-beta also increased SCD mRNA expression in several other cell lines tested. The increase in SCD mRNA expression was preceded by a marked increase in Smad2 phosphorylation in TGF-beta-treated human retinal pigment epithelial cells. TGF-beta did not induce SCD mRNA expression in a Smad4-deficient cell line. However, Smad4 overexpression restored the TGF-beta effect in this cell line. Moreover, TGF-beta-induced SCD mRNA expression was effectively blocked by the overexpression of Smad7, an inhibitory Smad. Thus, a TGF-beta signal transduction pathway involving Smad proteins appears to regulate the cellular expression of the SCD gene, and this regulation may play an important role in lipid metabolism.

Oleoyl-CoA is the major de novo product of stearoyl-CoA desaturase 1 gene isoform and substrate for the biosynthesis of the Harderian gland 1-alkyl-2,3-diacylglycerol

1-Alkyl-2,3-diacylglycerol (ADG) is a unique neutral lipid found in the eyeball-associated Harderian gland (HG) of the mouse and acts as a lubricant to facilitate eyelid movement. We found that the HG of the mice with a disruption in the gene for stearoyl-CoA desaturase 1 (SCD1) (SCD1-/-) is deficient in ADG. The amount of C20:1n-9, which is a major fatty acid of ADG, was reduced by greater than 90% despite normal elongase enzyme activity proposed to elongate it from C18:1n-9. HG from SCD1-/- mice exhibited high desaturase activity toward C16:0-CoA as substrate but had very low desaturase activity toward C18:0-CoA. Feeding diets containing high levels of oleate to the SCD1-/- mice did not increase the levels of C18:1n-9 or C20:1n-9 in the HG and failed to restore the ADG to the levels found in the HG of the wild-type mouse. De novo ADG synthesis as measured by the incorporation of [(3)H]glycerol and [(14)C]glucose was high in the SCD1+/+ mouse but was reduced by greater than 90% in the HG of SCD1-/- mouse. The deficiencies in the levels of ADG and C20:1n-9 were not compensated for by the expression of SCD2 and SCD3 isoforms in the HG of the SCD1-/- mouse. These observations demonstrate that SCD1-synthesized oleoyl-CoA is a major substrate required for the biosynthesis of normal levels of ADG and that the SCD isoforms present in the HG have different substrate specificity.