Dual regulation of mouse Δ5- and Δ6-desaturase gene expression by SREBP-1 and PPARα
2002; Elsevier BV; Volume: 43; Issue: 1 Linguagem: Inglês
10.1016/s0022-2275(20)30193-0
ISSN1539-7262
AutoresTakashi Matsuzaka, Hitoshi Shimano, Naoya Yahagi, Michiyo Amemiya-Kudo, Tomohiro Yoshikawa, Alyssa H. Hasty, Yoshiaki Tamura, Jun-ichi Osuga, Hiroaki Okazaki, Yoko Iizuka, Akimitsu Takahashi, Hirohito Sone, Takanari Gotoda, Shun Ishibashi, Nobuhiro Yamada,
Tópico(s)Cancer, Lipids, and Metabolism
ResumoIn the process of seeking sterol regulatory element-binding protein 1a (SREBP-1a) target genes, we identified and cloned a cDNA clone encoding mouse Δ5-desaturase (D5D). The hepatic expression of D5D as well as Δ6-desaturase (D6D) was highly activated in transgenic mice overexpressing nuclear SREBP-1a, -1c, and -2. Disruption of the SREBP-1 gene significantly reduced the expression of both desaturases in the livers of SREBP-1-deficient mice refed after fasting. The hepatic expression of both desaturases was downregulated by dietary PUFA, which were reported to suppress SREBP-1c gene expression. Sustained expression of hepatic nuclear SREBP-1c protein in the transgenic mice abolished the PUFA suppression of both desaturases. Although these data suggested that SREBP-1c regulates D5D and D6D expression, there was no difference in either the D5D or D6D mRNA level between fasted and refed normal mouse livers, indicating a mechanism for fasting induction of both desaturases. Administration of fibrate, a pharmacological ligand for peroxisome proliferator activating receptor α (PPARα), caused a significant increase in expression of both desaturases. The data suggested that D5D and D6D expression is dually regulated by SREBP-1c and PPARα, two reciprocal transcription factors for fatty acid metabolism, and could be involved in lipogenic gene regulation by producing PUFA. —Matsuzaka, T., H. Shimano, N. Yahagi, M. Amemiya-Kudo, T. Yoshikawa, A. H. Hasty, Y. Tamura, J-i. Osuga, H. Okazaki, Y. Iizuka, A. Takahashi, H. Sone, T. Gotoda, S. Ishibashi, and N. Yamada. Dual regulation of mouse Δ5- and Δ6-desaturase gene expression by SREBP-1 and PPARα. J. Lipid Res. 2002. 43: 107–114. In the process of seeking sterol regulatory element-binding protein 1a (SREBP-1a) target genes, we identified and cloned a cDNA clone encoding mouse Δ5-desaturase (D5D). The hepatic expression of D5D as well as Δ6-desaturase (D6D) was highly activated in transgenic mice overexpressing nuclear SREBP-1a, -1c, and -2. Disruption of the SREBP-1 gene significantly reduced the expression of both desaturases in the livers of SREBP-1-deficient mice refed after fasting. The hepatic expression of both desaturases was downregulated by dietary PUFA, which were reported to suppress SREBP-1c gene expression. Sustained expression of hepatic nuclear SREBP-1c protein in the transgenic mice abolished the PUFA suppression of both desaturases. Although these data suggested that SREBP-1c regulates D5D and D6D expression, there was no difference in either the D5D or D6D mRNA level between fasted and refed normal mouse livers, indicating a mechanism for fasting induction of both desaturases. Administration of fibrate, a pharmacological ligand for peroxisome proliferator activating receptor α (PPARα), caused a significant increase in expression of both desaturases. The data suggested that D5D and D6D expression is dually regulated by SREBP-1c and PPARα, two reciprocal transcription factors for fatty acid metabolism, and could be involved in lipogenic gene regulation by producing PUFA. —Matsuzaka, T., H. Shimano, N. Yahagi, M. Amemiya-Kudo, T. Yoshikawa, A. H. Hasty, Y. Tamura, J-i. Osuga, H. Okazaki, Y. Iizuka, A. Takahashi, H. Sone, T. Gotoda, S. Ishibashi, and N. Yamada. Dual regulation of mouse Δ5- and Δ6-desaturase gene expression by SREBP-1 and PPARα. J. Lipid Res. 2002. 43: 107–114. PUFA such as arachidonic acid (AA, 20:4n-6), EPA (20:5n-3), and DHA (22:6n-3) play pivotal roles in a number of biological functions including brain development, cognition, reproduction, inflammatory responses, and hemostasis. In addition to being crucial components of membrane phospholipids and functioning in key steps of cell signaling, PUFA have a unique ability to suppress the transcription of hepatic genes encoding lipogenic enzymes (1Blake W.L. Clarke S.D. Suppression of rat hepatic fatty acid synthase and S14 gene transcription by dietary polyunsaturated fat.J. Nutr. 1990; 120: 1727-1729Google Scholar, 2Jump D.B. Clarke S.D. Thelen A. Liimatta M. Coordinate regulation of glycolytic and lipogenic gene expression by polyunsaturated fatty acids.J. Lipid Res. 1994; 35: 1076-1084Google Scholar, 3Jump D.B. Clarke S.D. Thelen A. Liimatta M. Ren B. Badin M. Dietary polyunsaturated fatty acid regulation of gene transcription.Prog. Lipid Res. 1996; 35: 227-241Google Scholar, 4Ntambi J.M. Dietary regulation of stearoyl-CoA desaturase 1 gene expression in mouse liver.J. Biol. Chem. 1992; 267: 10925-10930Google Scholar). The availability of 20- and 22-carbon (n-3 and n-6) PUFA is determined by the synthesis of γ-linolenic acid (18:3n-6) and stearidonic acid (18:4n-3) and by the subsequent elongation and desaturation of these fatty acids to 20:4n-6 and 22:6n-3 (5Sprecher H. Biochemistry of essential fatty acids.Prog. Lipid Res. 1981; 20: 13-22Google Scholar). One of two rate-limiting steps in the production of these polyunsaturated fatty acids is the desaturation of eicosatrienoic acid (20:3n-6) and eicosatetraenoic acid (20:4n-3) to 20:4n-6 and 20:5n-3, determined by the activity of Δ5-desaturase (D5D). The other step is the desaturation of linoleic acid (18:2n-6) and α-linolenic acid (18:3n-3) to 18:3n-6 and 18:4n-3 catalyzed by Δ6-desaturase (D6D). Studies based on enzymatic activity suggest that the liver is the primary site for 20-carbon polyenoic fatty acid synthesis because the liver is an organ with a great amount of D6D and D5D activity (6Scott B.L. Bazan N.G. Membrane docosahexaenoate is supplied to the developing brain and retina by the liver.Proc. Natl. Acad. Sci. USA. 1989; 86: 2903-2907Google Scholar). Both enzymes are coordinately regulated by various dietary conditions and hormonal levels in the liver. For instance, D6D activity is induced by peroxisome proliferators and by the supplementation of insulin to diabetic rats (7Igal R.A. Mandon E.C. de Gomez Dumm I.N. Abnormal metabolism of polyunsaturated fatty acids in adrenal glands of diabetic rats.Mol. Cell. Endocrinol. 1991; 77: 217-227Google Scholar, 8Kawashima Y. Musoh K. Kozuka H. Peroxisome proliferators enhance linoleic acid metabolism in rat liver. Increased biosynthesis of omega 6 polyunsaturated fatty acids.J. Biol. Chem. 1990; 265: 9170-9175Google Scholar). It is also highly suppressed by dietary PUFA, indicating that these enzymes are involved in feedback regulation in the production of AA, EPA, and DHA. Remarkable progress in understanding the molecular basis of the nutritional regulation of these desaturases has been made by cloning mammalian D5D and D6D cDNAs (9Cho H.P. Nakamura M. Clarke S.D. Cloning, expression, and fatty acid regulation of the human Δ-5 desaturase.J. Biol. Chem. 1999; 274: 37335-37339Google Scholar, 10Cho H.P. Nakamura M.T. Clarke S.D. Cloning, expression, and nutritional regulation of the mammalian Δ-6 desaturase.J. Biol. Chem. 1999; 274: 471-477Google Scholar). The predicted amino acid sequences were suggested to contain a cytochrome b5-like domain that might dispense with the presence of a separate cytochrome b5. Suppression of D5D and D6D activities by dietary unsaturated fatty acids was shown to be associated with decreased D5D and D6D mRNA levels (9Cho H.P. Nakamura M. Clarke S.D. Cloning, expression, and fatty acid regulation of the human Δ-5 desaturase.J. Biol. Chem. 1999; 274: 37335-37339Google Scholar, 10Cho H.P. Nakamura M.T. Clarke S.D. Cloning, expression, and nutritional regulation of the mammalian Δ-6 desaturase.J. Biol. Chem. 1999; 274: 471-477Google Scholar). Sterol regulatory element-binding proteins (SREBP) are membrane-bound transcription factors that belong to the basic helix-loop-helix leucine zipper family (11Brown M.S. Goldstein J.L. A proteolytic pathway that controls the cholesterol content of membranes, cells, and blood.Proc. Natl. Acad. Sci. USA. 1999; 96: 11041-11048Google Scholar, 12Brown M.S. Goldstein J.L. The SREBP pathway: regulation of cholesterol metabolism by proteolysis of a membrane-bound transcription factor.Cell. 1997; 89: 331-340Google Scholar, 13Brown M.S. Ye J. Rawson R.B. Goldstein J.L. Regulated intramembrane proteolysis: a control mechanism conserved from bacteria to humans.Cell. 2000; 100: 391-398Google Scholar). A sterol-regulated cleavage is required for SREBP to enter the nucleus and activate the transcription of genes involved in cholesterol and fatty acid synthesis by binding to a sterol regulatory element (SRE) or its related sequences, including E-boxes, within their promoter regions (14Briggs M.R. Yokoyama C. Wang X. Brown M.S. Goldstein J.L. Nuclear protein that binds sterol regulatory element of low density lipoprotein receptor promoter. I. Identification of the protein and delineation of its target nucleotide sequence.J. Biol. Chem. 1993; 268: 14490-14496Google Scholar, 15Kim J.B. Spotts G.D. Halvorsen Y.D. Shih H.M. Ellenberger T. Towle H.C. Spiegelman B.M. Dual DNA binding specificity of ADD1/SREBP1 controlled by a single amino acid in the basic helix-loop-helix domain.Mol. Cell. Biol. 1995; 15: 2582-2588Google Scholar). There are three forms of SREBP that have been characterized: SREBP-1a and -1c (also known as ADD1) and SREBP-2 (16Hua X. Yokoyama C. Wu J. Briggs M.R. Brown M.S. Goldstein J.L. Wang X. SREBP-2, a second basic-helix-loophelix-leucine zipper protein that stimulates transcription by binding to a sterol regulatory element.Proc. Natl. Acad. Sci. USA. 1993; 90: 11603-11607Google Scholar, 17Tontonoz P. Kim J.B. Graves R.A. Spiegelman B.M. ADD1: a novel helix-loop-helix transcription factor associated with adipocyte determination and differentiation.Mol. Cell. Biol. 1993; 13: 4753-4759Google Scholar, 18Yokoyama C. Wang X. Briggs M.R. Admon A. Wu J. Hua X. Goldstein J.L. Brown M.S. SREBP-1, a basic helix-loophelix leucine zipper protein that controls transcription of the low density lipoprotein receptor gene.Cell. 1993; 75: 187-197Google Scholar). Most organs, including the liver and adipose tissue, express predominantly SREBP-2 and the c isoform of SREBP-1 (19Shimomura I. Shimano H. Horton J.D. Goldstein J.L. Brown M.S. Differential expression of exons 1a and 1c in mRNAs for sterol regulatory element binding protein-1 in human and mouse organs and cultured cells.J. Clin. Invest. 1997; 99: 838-845Google Scholar). Lipogenic enzymes, which are involved in energy storage through the synthesis of fatty acids and triglycerides, are coordinately regulated at the transcriptional level during different metabolic states. In vivo studies demonstrated that SREBP-1c plays a crucial role in the dietary regulation of most hepatic lipogenic genes, whereas SREBP-2 is actively involved in the transcription of cholesterogenic enzymes. These include studies of the effects of the absence or overexpression of SREBP-1 on hepatic lipogenic gene expression (20Horton J.D. Shimomura I. Brown M.S. Hammer R.E. Goldstein J.L. Shimano H. Activation of cholesterol synthesis in preference to fatty acid synthesis in liver and adipose tissue of transgenic mice overproducing sterol regulatory element-binding protein-2.J. Clin. Invest. 1998; 101: 2331-2339Google Scholar, 21Shimano H. Horton J.D. Hammer R.E. Shimomura I. Brown M. Goldstein J.L. Overproduction of cholesterol and fatty acids causes massive liver enlargement in transgenic mice expressing truncated SREBP-1a.J. Clin. Invest. 1996; 98: 1575-1584Google Scholar, 22Shimano H. Yahagi N. Amemiya-Kudo M. Hasty A. Osuga J. Tamura Y. Shionoiri F. Iizuka Y. Ohashi K. Harada K. Gotoda T. Ishibashi S. Yamada N. Sterol regulatory element-binding protein-1 as a key transcription factor for nutritional induction of lipogenic enzyme genes.J. Biol. Chem. 1999; 274: 35832-35839Google Scholar), as well as physiological changes of SREBP-1c protein in normal mice after dietary manipulation, such as placement on high carbohydrate diets, polyunsaturated fatty acid-enriched diets, and fasting-refeeding regimens (23Horton J.D. Bashmakov Y. Shimomura I. Shimano H. Regulation of sterol regulatory element binding proteins in livers of fasted and refed mice.Proc. Natl. Acad. Sci. USA. 1998; 95: 5987-5992Google Scholar, 24Kim H.J. Takahashi M. Ezaki O. Fish oil feeding decreases mature sterol regulatory element-binding protein 1 (SREBP-1) by down-regulation of SREBP-1c mRNA in mouse liver. A possible mechanism for down-regulation of lipogenic enzymes.J. Biol. Chem. 1999; 274: 25892-25898Google Scholar, 25Kim J.B. Sarraf P. Wright M. Yao K.M. Mueller E. Solanes G. Lowell B.B. Spiegelman B.M. Nutritional and insulin regulation of fatty acid synthetase and leptin gene expression through ADD1/SREBP1.J. Clin. Invest. 1998; 101: 1-9Google Scholar, 26Xu J. Nakamura M.T. Cho H.P. Clarke S.D. Sterol regulatory element binding protein-1 expression is suppressed by dietary polyunsaturated fatty acids. A mechanism for the coordinate suppression of lipogenic genes by polyunsaturated fats.J. Biol. Chem. 1999; 274: 23577-23583Google Scholar, 27Yahagi N. Shimano H. Hasty A. Amemiya-Kudo M. Okazaki H. Tamura Y. Iizuka Y. Shionoiri F. Ohashi K. Osuga J. Harada K. Gotoda T. Nagai R. Ishibashi S. Yamada N. A crucial role of sterol regulatory element-binding protein-1 in the regulation of lipogenic gene expression by polyunsaturated fatty acids.J. Biol. Chem. 1999; 274: 35840-35844Google Scholar). Previous reports on the regulation of SREBP-1c have all demonstrated the induction to be at the mRNA level. Promoter analysis revealed that the expression of the SREBP-1c gene is regulated by two factors: SREBP itself, forming an autoloop, and the oxysterol liver X receptor-retinoid X receptor (28Amemiya-Kudo M. Shimano H. Yoshikawa T. Yahagi N. Hasty A.H. Okazaki H. Tamura Y. Shionoiri F. Iizuka Y. Ohashi K. Osuga J-i. Harada K. Gotoda T. Sato R. Kimura S. Ishibashi S. Yamada N. Promoter analysis of the mouse sterol regulatory element-binding protein (SREBP)-1c gene.J. Biol. Chem. 2000; 275: 31078-31085Google Scholar, 29Yoshikawa T. Shimano H. Amemiya-Kudo M. Yahagi N. Hasty A.H. Matsuzaka T. Okazaki H. Tamura Y. Iizuka Y. Ohashi K. Osuga J-I. Harada K. Gotoda T. Kimura S. Ishibashi S. Yamada N. Identification of liver X receptor-retinoid X receptor as an activator of the sterol regulatory element-binding protein 1c gene promoter.Mol. Cell. Biol. 2001; 21: 2991-3000Google Scholar). The mechanism for PUFA suppression of the SREBP-1c promoter is under investigation. Both D5D and D6D play crucial roles in the production of PUFA, which in turn suppresses SREBP-1. In addition, D5D, D6D, and SREBP-1c are nutritionally regulated. These lines of evidence suggest that D5D and D6D expression might be related to SREBP-1 activity. In the screening of SREBP-activated genes, we cloned and identified a mouse D5D gene. Our current studies suggest that nutritional expression of D5D and D6D is regulated by both SREBP-1 and peroxisome proliferator activating receptor α (PPARα). We purchased 20:3n-6 from Sigma (St. Louis, MO), Redivue [α-32P]dCTP (6,000 Ci/mmol) from Amersham Pharmacia Biotech (Piscataway, NJ), and restriction enzymes from New England BioLabs (Beverly, MA). Standard molecular biology techniques were used. DNA sequencing was performed with a CEQTM dye terminator cycle sequencing kit and CEQ2000 DNA analysis system (Beckman Coulter, Fullerton, CA). An expression cDNA library of SREBP-1a transgenic liver (21Shimano H. Horton J.D. Hammer R.E. Shimomura I. Brown M. Goldstein J.L. Overproduction of cholesterol and fatty acids causes massive liver enlargement in transgenic mice expressing truncated SREBP-1a.J. Clin. Invest. 1996; 98: 1575-1584Google Scholar) was prepared as previously described for construction of a cDNA library of SREBP-1-deficient mouse adipose tissue, except that poly(A)+ RNA was prepared from livers of SREBP-1a transgenic mice (29Yoshikawa T. Shimano H. Amemiya-Kudo M. Yahagi N. Hasty A.H. Matsuzaka T. Okazaki H. Tamura Y. Iizuka Y. Ohashi K. Osuga J-I. Harada K. Gotoda T. Kimura S. Ishibashi S. Yamada N. Identification of liver X receptor-retinoid X receptor as an activator of the sterol regulatory element-binding protein 1c gene promoter.Mol. Cell. Biol. 2001; 21: 2991-3000Google Scholar). From a DNA microarray system using hepatic poly(A)+ RNA of SREBP-1a transgenic and nontransgenic littermate mice, we identified an expressed sequence tag (EST) clone (GenBank accession number AA068575) that was activated 8-fold in SREBP-1a transgenic liver as compared with wild-type liver, and resembled the mouse D6D. Using this sequence information, an [α-32P]dCTP-labeled DNA probe was prepared and used in the screening from an SREBP-1a transgenic mouse liver cDNA library by colony hybridization. Positive clones were sequenced and the clones containing an open reading frame were used for expression experiments. HEK-293 cells were grown at 37°C in an atmosphere of 5% CO2 in DMEM containing 25 mM glucose, penicillin (100 U/ml), and streptomycin sulfate (100 μg/ml) supplemented with 10% FCS on 100-mm culture plates. At 80% confluence, the mouse D5D expression plasmid or the basic plasmid CMV7 alone (10 μg) was transfected into cells, using SuperFect transfection reagent (Qiagen, Valencia, CA) according to the manufacturer protocol. After transfection, cells were treated with either 100 μM 20:3n-6 diluted in ethanol or ethanol alone for 24 h and then used for fatty acid analysis. Total fatty acid extracted from HEK-293 cells transfected with the mouse D5D expression plasmid or CMV7, in the presence of 100 μM 20:3n-6, was analyzed by gas chromatography of methyl ester derivatives. Lipids were extracted and transmethylated with methanolic HCl, and the fatty acid methyl esters were analyzed as described previously (10Cho H.P. Nakamura M.T. Clarke S.D. Cloning, expression, and nutritional regulation of the mammalian Δ-6 desaturase.J. Biol. Chem. 1999; 274: 471-477Google Scholar). All mice were housed in a controlled environment with a 12-h light/dark cycle and free access to water and diet. For fatty acid or drug experiments, 7-week-old male C57BL/6J mice (21–23 g) were purchased from CLEA (Tokyo, Japan) and adapted to the environment for 1 week. Before sacrifice, each group of animals was fed a diet containing the indicated fatty acids and drugs for 7 days. Fasting and refeeding regimens for wild-type and SREBP-1 knockout mice were previously described (22Shimano H. Yahagi N. Amemiya-Kudo M. Hasty A. Osuga J. Tamura Y. Shionoiri F. Iizuka Y. Ohashi K. Harada K. Gotoda T. Ishibashi S. Yamada N. Sterol regulatory element-binding protein-1 as a key transcription factor for nutritional induction of lipogenic enzyme genes.J. Biol. Chem. 1999; 274: 35832-35839Google Scholar). Transgenic mice overexpressing the nuclear form of SREBP-1a, -1c, and -2 in the liver under the control of the phosphoenolpyruvate carboxykinase promoter were previously described (21Shimano H. Horton J.D. Hammer R.E. Shimomura I. Brown M. Goldstein J.L. Overproduction of cholesterol and fatty acids causes massive liver enlargement in transgenic mice expressing truncated SREBP-1a.J. Clin. Invest. 1996; 98: 1575-1584Google Scholar, 30Shimano H. Horton J.D. Shimomura I. Hammer R.E. Brown M.S. Goldstein J.L. Isoform 1c of sterol regulatory element binding protein is less active than isoform 1a in livers of transgenic mice and in cultured cells.J. Clin. Invest. 1997; 99: 846-854Google Scholar, 31Shimano H. Shimomura I. Hammer R.E. Herz J. Goldstein J.L. Brown M.S. Horton J.D. Elevated levels of SREBP-2 and cholesterol synthesis in livers of mice homozygous for a targeted disruption of the SREBP-1 gene.J. Clin. Invest. 1997; 100: 2115-2124Google Scholar). SREBP-1c and -2 transgenic mice were homozygous for the transgene (32Ren B. Thelen A. Jump D.B. Peroxisome proliferator-activated receptor alpha inhibits hepatic S14 gene transcription. Evidence against the peroxisome proliferator-activated receptor alpha as the mediator of polyunsaturated fatty acid regulation of s14 gene transcription.J. Biol. Chem. 1996; 271: 17167-17173Google Scholar). These three transgenic lines and wild-type controls (nontransgenic littermates of SREBP-1a transgenic mice) were put on a high protein/low carbohydrate diet to induce transgenic mice for 2 weeks, and were fasted for 12 h before sacrifice. For fasting and refeeding treatment, SREBP-1-deficient and wild-type mice were fasted for 24 h and fed a high sucrose/fat-free diet for 12 h. Total RNA was extracted from mouse livers and various tissues with TRIzol reagent (Life Technologies, Rockville, MD). RNA samples were run on a 1% agarose gel containing formaldehyde and transferred a nylon membrane (Hybond-N; Amersham Pharmacia Biotech). The probes used were labeled with [α-32P]dCTP, using the Megaprime DNA labeling system kit (Amersham Pharmacia Biotech). The cDNA probe for mouse D5D was prepared by digesting the cloned cDNA with NotI and EcoRI. The cDNA probes for mouse D6D and ribosomal phosphoprotein P0 (36B4) were prepared as described previously (10Cho H.P. Nakamura M.T. Clarke S.D. Cloning, expression, and nutritional regulation of the mammalian Δ-6 desaturase.J. Biol. Chem. 1999; 274: 471-477Google Scholar, 22Shimano H. Yahagi N. Amemiya-Kudo M. Hasty A. Osuga J. Tamura Y. Shionoiri F. Iizuka Y. Ohashi K. Harada K. Gotoda T. Ishibashi S. Yamada N. Sterol regulatory element-binding protein-1 as a key transcription factor for nutritional induction of lipogenic enzyme genes.J. Biol. Chem. 1999; 274: 35832-35839Google Scholar). The membranes were hybridized with the radiolabeled probe in Rapid-Hyb buffer (Amersham Pharmacia Biotech) at 65°C and washed in 0.1× SSC, 0.1% SDS at 65°C. The resulting bands were quantified by exposure of the filters to BAS2000 with BASstation software (Fuji Photo Film, Tokyo, Japan). DNA microarray analysis identified an EST clone whose expression was increased 7-fold in the livers of SREBP-1a transgenic as compared with wild-type mice. This clone (GenBank accession number AA068575) was originally named an EST clone slightly similar to mouse D6D in the database. Mouse D6D was cloned and reported (10Cho H.P. Nakamura M.T. Clarke S.D. Cloning, expression, and nutritional regulation of the mammalian Δ-6 desaturase.J. Biol. Chem. 1999; 274: 471-477Google Scholar). The sequences suggested that this clone should encode something similar, but not identical, to D6D. Using this EST clone as a probe, we cloned the cDNA clone that contains an open reading frame (ORF) from an SREBP-1a transgenic liver cDNA library. The DNA sequence analysis revealed that the predicted amino acid sequence was highly similar to human and rat D5D (9Cho H.P. Nakamura M. Clarke S.D. Cloning, expression, and fatty acid regulation of the human Δ-5 desaturase.J. Biol. Chem. 1999; 274: 37335-37339Google Scholar). Alignment of the predicted amino acid sequences for mouse and human D5D is shown in Fig. 1. Eighty-eight percent of the amino acid sequence for the mouse and human homologs was identical. The high homology includes the presence of a cytochrome b5 domain, transmembrane domains, and histidine-rich regions, all of which were reported for human D5D and mouse D6D. The high similarity of the predicted amino acid sequence from the ORF of the cloned mouse cDNA to the human D5D amino acid sequence suggested that the cDNA encodes the mouse D5D. To confirm this, the cDNA was expressed in HEK-293 cells supplemented with C20:3n-6. As shown in Table 1, gas chromatography analysis of the cellular fatty acids revealed that overexpression of this putative D5D clone increased the relative amounts of C20:4n-6 and C22:4n-6, accompanying a decrease in C20:3n-6 in the cells as compared with mock-transfected cells. The data suggested that the cells transfected with the cDNA were capable of synthesizing 20:3n-6 from eicosadienoic acid (20:2n-6), indicating that the cDNA product has a D5D activity.TABLE 1.Fatty acid profiles of HEK-293 cells transfected with mouse Δ5-desaturaseFatty Acid+100 μM 20:3n-6CMV7D5Dmol% of fatty acidsC14:02.80 ± 0.162.80 ± 0.14C16:025.63 ± 0.2826.58 ± 0.78C16:1n-73.24 ± 0.033.26 ± 0.08C18:011.23 ± 0.3611.58 ± 0.51C18:1n-915.10 ± 0.2415.39 ± 0.08C18:2n-62.69 ± 0.122.58 ± 0.08C20:3n-626.55 ± 0.8220.65 ± 0.88aP < 0.01 versus CMV7.C20:4n-65.69 ± 0.248.98 ± 0.10bP < 0.0005 versus CMV7.C22:4n-62.20 ± 0.134.43 ± 0.35cP < 0.005 versus CMV7.C22:5n-31.79 ± 0.081.88 ± 0.25C22:6n-31.84 ± 0.111.78 ± 0.08Fatty acid composition of HEK-293 cells transfected with the mouse Δ5-desaturase and CMV7, grown in the presence of 100 μM 20:3n-6. Results are expressed as the mol percentage of total fatty acids and as means ± SEM for three independent cultures. Statistical significance was assessed by unpaired l-test.a P < 0.01 versus CMV7.b P < 0.0005 versus CMV7.c P < 0.005 versus CMV7. Open table in a new tab Fatty acid composition of HEK-293 cells transfected with the mouse Δ5-desaturase and CMV7, grown in the presence of 100 μM 20:3n-6. Results are expressed as the mol percentage of total fatty acids and as means ± SEM for three independent cultures. Statistical significance was assessed by unpaired l-test. Expression of D5D and D6D was examined in various tissues as estimated by Northern blot analysis. Mouse D5D is a single transcript that is approximately 4.5 kb in size (Fig. 2). The expression was high in the adrenal gland, liver, brain, and testis, tissues where lipogenesis and steroidogenesis are active. The tissue distribution pattern of mouse D6D expression was similar, suggesting that D5D and D6D cooperatively produce polyenoic fatty acids. We performed Northern blot analysis of liver RNA from nuclear SREBP-1a, -1c, and -2 transgenic and wild-type mice to estimate the effects of nuclear SREBP on the expression of D5D as well as D6D. As shown in Fig. 3, the hepatic mRNA level of D5D was robustly increased by overexpression of nuclear SREBP-1a, -1c, and -2 at this order of magnitude. Hepatic D6D mRNA levels were also induced by SREBP overexpression in a pattern similar to that of D5D mRNA. A shorter mouse D6D transcript was barely detectablefrom livers of fasted wild-type mice, but became prominent in SREBP transgenic livers. This minor, short transcript was identified in human D6D and was reported to be nutritionally regulated as well as the major transcript. The data indicate that every isoform of SREBP can activate the expression of both desaturase genes. Nutritional regulation of hepatic D5D and D6D mRNA levels was estimated in wild-type and SREBP-1 deficient mice. As shown in Fig. 4, these mice were fasted and then refed with a high sucrose/fat-free diet. Interestingly, the wild-type mice did not show a marked change in D5D or D6D mRNA level between fasting and refeeding. In the fasted state, the SREBP-1-deficient mice showed hepatic expression levels of both D5D and D6D similar to those of wild-type mice. After refeeding, there was a considerable reduction in the expression of both desaturases in SREBP-1 null mice, showing a much lower level than in the refed wild-type mice. Expression of PPARα was induced by fasting and suppressed by refeeding, whereas the SREBP-1c mRNA level showed the opposite response. These data demonstrate that the hepatic expression of D5D and D6Dis not upregulated by refeeding, but that SREBP-1 contributes strongly to their expression in the refed state. The nutritional regulation of these enzymes is distinct from that of regular lipogenic enzymes, which are suppressed by fasting and are markedly induced by refeeding, although both these desaturases and lipogenic enzymes are regulated by SREBP-1. Numerous dietary studies indicate that the hepatic activity and expression of both desaturases are induced by diets low in essential fatty acids and suppressed by diets rich in PUFA such as vegetable or marine oils. We compared mouse hepatic D5D and D6D expression levels in mice fed a fat-free high carbohydrate diet with various kinds of fatty acids. Figure 5 showed that stearate (C18:0) or oleate (C18:1) did not affect the expression of D5D and D6D. However, when a high carbohydrate diet was supplemented with linoleate or EPA, the expression of both desaturases was considerably suppressed, and the reduction was more marked with fish oils rich in EPA and DHA. These data were consistent with previous reports on the expression of human and rat desaturase genes. We also tested PUFA suppression of their expression in SREBP-1c transgenic mice (Fig. 6). This line of transgenic mice expresses amounts of nuclear SREBP-1c protein, derived from the transgene, similar to the physiological level of nuclear SREBP-1c in the livers of normal refed mice (27Yahagi N. Shimano H. Hasty A. Amemiya-Kudo M. Okazaki H. Tamura Y. Iizuka Y. Shionoiri F. Ohashi K. Osuga J. Harada K. Gotoda T. Nagai R. Ishibashi S. Yamada N. A crucial role of sterol regulatory element-binding protein-1 in the regulation of lipogenic gene expression by polyunsaturated fatty acids.J. Biol. Chem. 1999; 274: 35840-35844Google Scholar). The sustained level of nuclear SREBP-1c completely abolished PUFA suppression of D5D and D6D mRNA levels. These data suggest that endogenous SREBP-1c couldbe involved in maintaining D5D and D6D expression in a fed state.Fig. 6.Northern blot of D5D and D6D mRNA from livers of wild-type or TgSREBP-1c mice fed PUFA diets. Wild-type mice were fed a high carbohydrate fat-free diet (lane 1), and homozygous Tg-SREBP-1c mice were fed a high protein diet with 20% triolein (lane 2), 5% EPA ethyl ester plus 20% triolein (lane 3), or 20% sardine fish oil (lane 4) for 7 days and killed in a nonfasted state. Total RNA (10 μg) pooled equally from livers of each group was subjected to Northern blotting, followed by hybridization with the D5D and D6D cDNA probes. A cDNA probe for 36B4 (acidic ribosomal phosphoprotein P0) was used to confirm equal loading.View Large Image Figure ViewerDownload (PPT) As shown in Fig. 4, fasting treatment induces expression of PPARα, crucial for fatty acid oxidation. To estimate the possible involvement of PPARα in D5D and D6D expression, we challenged mice with fibrate, a pharmacological PPAR ligand. Addition of fibrate to the diet increased both D5D and D6D mRNA levels (Fig. 7) whereas troglitazone, which induces PPARγ, had no effect, indicating that both desaturases can be induced by PPARα and not by PPARγ. These results are consistent with the previous report that peroxisome proliferators enhanced linoleic acid metabolism in rat liver (8Kawashima Y. Musoh K. Kozuka H. Peroxisome proliferators enhance linoleic acid metabolism in rat liver. Increased biosynthesis of omega 6 polyunsaturated fatty acids.J. Biol. Chem. 1990; 265: 9170-9175Google Scholar). The current study clearly demonstrates that SREBP activate the expression of both D5D and D6D in the liver. Both hepatic D5D and D6D mRNA levels were markedly increased by overexpression of SREBP, with preference for SREBP-1 relative to SREBP-2. Furthermore, the absenceof SREBP-1 markedly impaired the hepatic expression of these desaturases in a refed state. All these data suggest that D5D and D6D are highly potential SREBP target genes and that SREBP-1 plays a pivotal role in the hepatic expression of these desaturases, at least under lipogenic conditions. The physiological relevance of SREBP activation of these desaturases is currently unknown. Under lipogenic conditions, when SREBP-1 is activated, newly synthesized fatty acids could be dominated by saturated or monounsaturated fatty acids, and availability of essential fatty acids might be limited. In this situation, activation of D5D and D6D might be necessary to meet cellular requirements for AA, EPA, and DHA. Cumulative evidence suggests that the cellular availability of PUFA is crucial for the regulation of lipogenesis. On the assumption that the PUFA that these desaturases synthesize can be used for inhibition of lipogenesis, SREBP-1 induction of these desaturases would contribute to efficient suppression of lipogenesis whenever essential fatty acids such as C18:2n-6 become available. It is consistent with the previous report that D6D activity is important for PUFA suppression of hepatic lipogenic gene expression. This is the first observation that there could be a feedback system for the hepatic SREBP-1c-lipogenesis system, which was thought to depend essentially on the supply of carbohydrates as discussed in the autoloop activation of the SREBP-1c promoterunderlying the overshooting phenomenon in a refed state (28Amemiya-Kudo M. Shimano H. Yoshikawa T. Yahagi N. Hasty A.H. Okazaki H. Tamura Y. Shionoiri F. Iizuka Y. Ohashi K. Osuga J-i. Harada K. Gotoda T. Sato R. Kimura S. Ishibashi S. Yamada N. Promoter analysis of the mouse sterol regulatory element-binding protein (SREBP)-1c gene.J. Biol. Chem. 2000; 275: 31078-31085Google Scholar). Dietary PUFA have been reported to decrease both the activity and expression of D5D and D6D (9Cho H.P. Nakamura M. Clarke S.D. Cloning, expression, and fatty acid regulation of the human Δ-5 desaturase.J. Biol. Chem. 1999; 274: 37335-37339Google Scholar, 10Cho H.P. Nakamura M.T. Clarke S.D. Cloning, expression, and nutritional regulation of the mammalian Δ-6 desaturase.J. Biol. Chem. 1999; 274: 471-477Google Scholar), a finding supported by our current observations. PUFA were reported to inhibit SREBP-1 activity by multiple mechanisms (24Kim H.J. Takahashi M. Ezaki O. Fish oil feeding decreases mature sterol regulatory element-binding protein 1 (SREBP-1) by down-regulation of SREBP-1c mRNA in mouse liver. A possible mechanism for down-regulation of lipogenic enzymes.J. Biol. Chem. 1999; 274: 25892-25898Google Scholar, 25Kim J.B. Sarraf P. Wright M. Yao K.M. Mueller E. Solanes G. Lowell B.B. Spiegelman B.M. Nutritional and insulin regulation of fatty acid synthetase and leptin gene expression through ADD1/SREBP1.J. Clin. Invest. 1998; 101: 1-9Google Scholar, 26Xu J. Nakamura M.T. Cho H.P. Clarke S.D. Sterol regulatory element binding protein-1 expression is suppressed by dietary polyunsaturated fatty acids. A mechanism for the coordinate suppression of lipogenic genes by polyunsaturated fats.J. Biol. Chem. 1999; 274: 23577-23583Google Scholar, 27Yahagi N. Shimano H. Hasty A. Amemiya-Kudo M. Okazaki H. Tamura Y. Iizuka Y. Shionoiri F. Ohashi K. Osuga J. Harada K. Gotoda T. Nagai R. Ishibashi S. Yamada N. A crucial role of sterol regulatory element-binding protein-1 in the regulation of lipogenic gene expression by polyunsaturated fatty acids.J. Biol. Chem. 1999; 274: 35840-35844Google Scholar, 33Worgall T.S. Sturley S.L. Seo T. Osborne T.F. Deckelbaum R.J. Polyunsaturated fatty acids decrease expression of promoters with sterol regulatory elements by decreasing levels of mature sterol regulatory element-binding protein.J. Biol. Chem. 1998; 273: 25537-25540Google Scholar, 34Xu J. Teran-Garcia M. Park J.H. Nakamura M.T. Clarke S.D. Polyunsaturated fatty acids suppress hepatic sterol regulatory element-binding protein-1 expression by accelerating transcript decay.J. Biol. Chem. 2001; 276: 9800-9807Google Scholar). Considering our new finding that D5D and D6D are potential SREBP targets, it is likely that down-regulation of D5D and D6D expression by PUFA could be mediated through suppression of SREBP-1. This hypothesis is further supported by the observation that PUFA suppression of D5D and D6D was completely abolished by the sustained amount of nuclear SREBP-1c protein forcibly expressed in SREBP-1c transgenic mice. This unique fashion of nutritional regulation (activation by SREBP-1c, but no change by a fasting-refeeding treatment) places D5D and D6D in a position distinct from regular lipogenic enzymes. In particular, in SREBP-1 knockout mice, D5D and D6D mRNA levels are considerably higher in fasted liver than in refed liver. These observations can be explained by the hypothesis that, in additionto SREBP-1c-mediated induction as observed in the refed state of normal mice, there is another regulatory mechanism that activates the expression of D5D and D6D in a fasted state. Our results from administration of fenofibrate, a PPARα ligand, indicate that PPARα is responsible for the fasting induction of these desaturases. Furthermore, we and others previously reported that starvation increases the expression of PPARα, which should enhance fasting-induced activation of D5D and D6D in addition to the ligand activation of PPARα (27Yahagi N. Shimano H. Hasty A. Amemiya-Kudo M. Okazaki H. Tamura Y. Iizuka Y. Shionoiri F. Ohashi K. Osuga J. Harada K. Gotoda T. Nagai R. Ishibashi S. Yamada N. A crucial role of sterol regulatory element-binding protein-1 in the regulation of lipogenic gene expression by polyunsaturated fatty acids.J. Biol. Chem. 1999; 274: 35840-35844Google Scholar, 35Kersten S. Seydoux J. Peters J.M. Gonzalez F.J. Desvergne B. Wahli W. Peroxisome proliferator-activated receptor alpha mediates the adaptive response to fasting.J. Clin. Invest. 1999; 103: 1489-1498Google Scholar). In a fasted state, lipolysis from adipose tissue is enhanced and circulating free fatty acids enter the liver for β-oxidation that is controlled by PPARα. Many of these free fatty acids are essential fatty acids, and can be both ligands for both PPARα and substrates for D5D and D6D. Activation of D5D and D6D by PPARα in the liver might be important to produce required amounts of PUFA. Taken together, the expression of D5D and D6D could be regulated by two distinct mechanisms, one through SREBP-1c as is shown by its induction during refeeding, and the other through PPARα that is inducible by fasting. Dual regulation of D5D and D6D by the two factors SREBP-1 and PPARα, which possess opposing activation properties depending on nutritional status, caused no remarkable change in the overall expression of these enzymes between fasting and refeeding as schematized in Fig. 8. This dual regulation of both desaturases contributes to a stable production of PUFA that are essential for cellular functions regardless of energy state. Further analysis of the promoter regions of the D5D and D6D genes will be required. Interestingly, these genesare located close to each other in an inverted orientation, sharing an 11-kb upstream region (9Cho H.P. Nakamura M. Clarke S.D. Cloning, expression, and fatty acid regulation of the human Δ-5 desaturase.J. Biol. Chem. 1999; 274: 37335-37339Google Scholar). It would be intriguing if D5D and D6D shared in this region enhancers that are important for nutritional regulation, such as an SRE responsible for SREBP activation and/or a peroxisome proliferator response element, which could explain PPAR ligand activation. The current study suggests that PUFA play an essential role in energy metabolism by regulating or modifying these lipid transcription factors and that D5D and D6D could be involved in this system by producing PUFA. This study was supported by the Organization for Pharmaceutical Safety and Research (OPSR, Promotion of Fundamental Studies in Health Science), and by Health Sciences Research grants (Research on Human Genome and Gene Therapy) from the Ministry of Health and Welfare, Japan.
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