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• W2054661998 abstract "Stearoyl-coenzyme A desaturase (SCD) catalyzes the desaturation of saturated fatty acids to monounsaturated fatty acids in mammalian cells. Currently, there are four known enzymatic isoforms (SCD1–SCD4) in the mouse genome. The physiological roles for multiple SCD isoforms and their substrate specificities are unknown at present. We report here distinct substrate specificities for the mouse SCD isoforms. Each SCD isoform was able to complement the ole1 mutation in Saccharomyces cerevisiae through heterologous expression of transgenic SCD. Fatty acid analysis showed that mouse SCD1, SCD2, and SCD4 desaturate both C18:0 and C16:0, whereas mouse SCD3 uses C16:0 but not C18:0. We identify SCD3 as a mammalian palmitotyl-CoA Δ9-desaturase, and its existence in mouse helps explain distinct physiological roles for each SCD isoform. Stearoyl-coenzyme A desaturase (SCD) catalyzes the desaturation of saturated fatty acids to monounsaturated fatty acids in mammalian cells. Currently, there are four known enzymatic isoforms (SCD1–SCD4) in the mouse genome. The physiological roles for multiple SCD isoforms and their substrate specificities are unknown at present. We report here distinct substrate specificities for the mouse SCD isoforms. Each SCD isoform was able to complement the ole1 mutation in Saccharomyces cerevisiae through heterologous expression of transgenic SCD. Fatty acid analysis showed that mouse SCD1, SCD2, and SCD4 desaturate both C18:0 and C16:0, whereas mouse SCD3 uses C16:0 but not C18:0. We identify SCD3 as a mammalian palmitotyl-CoA Δ9-desaturase, and its existence in mouse helps explain distinct physiological roles for each SCD isoform. Δ9-desaturase is a fatty acid-modifying enzyme in the biosynthesis of monounsaturated fatty acids. Because this enzyme commonly introduces a cis double bond at the 9,10 position of stearoyl-CoA to form oleoyl-CoA, it has been commonly known as stearoyl-coenzyme A desaturase (SCD) (1Miyazaki M. Ntambi J.M. Role of stearoyl-coenzyme A desaturase in lipid metabolism.Prostaglandins Leukot. Essent. Fatty Acids. 2003; 68: 113-121Abstract Full Text Full Text PDF PubMed Scopus (240) Google Scholar, 2Ntambi J.M. Miyazaki M. Recent insights into stearoyl-CoA desaturase-1.Curr. Opin. Lipidol. 2003; 14: 255-261Crossref PubMed Scopus (207) Google Scholar, 3Shanklin J. Whittle E. Fox B.G. Eight histidine residues are catalytically essential in a membrane-associated iron enzyme, stearoyl-CoA desaturase, and are conserved in alkane hydroxylase and xylene monooxygenase.Biochemistry. 1994; 33: 12787-12794Crossref PubMed Scopus (644) Google Scholar, 4Fox B.G. Shanklin J. Somerville C. Munck E. Stearoyl-acyl carrier protein delta 9 desaturase from Ricinus communis is a diiron-oxo protein.Proc. Natl. Acad. Sci. USA. 1993; 90: 2486-2490Crossref PubMed Scopus (239) Google Scholar, 5Ntambi J.M. Regulation of stearoyl-CoA desaturase by polyunsaturated fatty acids and cholesterol.J. Lipid Res. 1999; 40: 1549-1558Abstract Full Text Full Text PDF PubMed Google Scholar). Four SCD isoforms (SCD1–SCD4) have been characterized in mice (6Kaestner K.H. Ntambi J.M. Kelly Jr., T.J. Lane M.D. Differentiation-induced gene expression in 3T3-L1 preadipocytes. A second differentially expressed gene encoding stearoyl-CoA desaturase.J. Biol. Chem. 1989; 264: 14755-14761Abstract Full Text PDF PubMed Google Scholar, 7Miyazaki M. Jacobson M.J. Man W.C. Cohen P. Asilmaz E. Friedman J.M. Ntambi J.M. Identification and characterization of murine SCD4, a novel heart-specific stearoyl-CoA desaturase isoform regulated by leptin and dietary factors.J. Biol. Chem. 2003; 278: 33904-33911Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar, 8Ntambi J.M. Buhrow S.A. Kaestner K.H. Christy R.J. Sibley E. Kelly Jr., T.J. Lane M.D. Differentiation-induced gene expression in 3T3-L1 preadipocytes. Characterization of a differentially expressed gene encoding stearoyl-CoA desaturase.J. Biol. Chem. 1988; 263: 17291-17300Abstract Full Text PDF PubMed Google Scholar, 9Zheng Y. Prouty S.M. Harmon A. Sundberg J.P. Stenn K.S. Parimoo S. Scd3—a novel gene of the stearoyl-CoA desaturase family with restricted expression in skin.Genomics. 2001; 71: 182-191Crossref PubMed Scopus (148) Google Scholar). All mouse SCD genes are colocalized to chromosome 19 (9Zheng Y. Prouty S.M. Harmon A. Sundberg J.P. Stenn K.S. Parimoo S. Scd3—a novel gene of the stearoyl-CoA desaturase family with restricted expression in skin.Genomics. 2001; 71: 182-191Crossref PubMed Scopus (148) Google Scholar).Despite the fact that the four SCD isoforms share >75% nucleotide and amino acid sequence identities, they differ in their 5′-flanking promoter, which results in divergent tissue-specific expression (2Ntambi J.M. Miyazaki M. Recent insights into stearoyl-CoA desaturase-1.Curr. Opin. Lipidol. 2003; 14: 255-261Crossref PubMed Scopus (207) Google Scholar). SCD1 is expressed in lipogenic tissues, including liver and adipose tissue (8Ntambi J.M. Buhrow S.A. Kaestner K.H. Christy R.J. Sibley E. Kelly Jr., T.J. Lane M.D. Differentiation-induced gene expression in 3T3-L1 preadipocytes. Characterization of a differentially expressed gene encoding stearoyl-CoA desaturase.J. Biol. Chem. 1988; 263: 17291-17300Abstract Full Text PDF PubMed Google Scholar). SCD2 is expressed primarily in the brain and neuronal tissue, particularly during the neonatal mylenation period (6Kaestner K.H. Ntambi J.M. Kelly Jr., T.J. Lane M.D. Differentiation-induced gene expression in 3T3-L1 preadipocytes. A second differentially expressed gene encoding stearoyl-CoA desaturase.J. Biol. Chem. 1989; 264: 14755-14761Abstract Full Text PDF PubMed Google Scholar). SCD3 expression is restricted to sebocytes in skin, preputial gland, and Harderian gland (9Zheng Y. Prouty S.M. Harmon A. Sundberg J.P. Stenn K.S. Parimoo S. Scd3—a novel gene of the stearoyl-CoA desaturase family with restricted expression in skin.Genomics. 2001; 71: 182-191Crossref PubMed Scopus (148) Google Scholar, 10Miyazaki M. Kim H.J. Man W.C. Ntambi J.M. 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.J. Biol. Chem. 2001; 276: 39455-39461Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar, 11Miyazaki M. Gomez F.E. Ntambi J.M. Lack of stearoyl-CoA desaturase-1 function induces a palmitoyl-CoA Delta6 desaturase and represses the stearoyl-CoA desaturase-3 gene in the preputial glands of the mouse.J. Lipid Res. 2002; 43: 2146-2154Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar). SCD4 is expressed predominantly in heart (7Miyazaki M. Jacobson M.J. Man W.C. Cohen P. Asilmaz E. Friedman J.M. Ntambi J.M. Identification and characterization of murine SCD4, a novel heart-specific stearoyl-CoA desaturase isoform regulated by leptin and dietary factors.J. Biol. Chem. 2003; 278: 33904-33911Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar). Mouse SCD1 and SCD2 are the best characterized isoforms because of the availability of SCD1 and SCD2 knockout mice (SCD1−/− and SCD2−/−) (12Miyazaki M. Man W.C. Ntambi J.M. Targeted disruption of stearoyl-CoA desaturase1 gene in mice causes atrophy of sebaceous and meibomian glands and depletion of wax esters in the eyelid.J. Nutr. 2001; 131: 2260-2268Crossref PubMed Scopus (222) Google Scholar, 13Miyazaki M. Dobrzyn A. Elias P.M. Ntambi J.M. Stearoyl-CoA desaturase-2 gene expression is required for lipid synthesis during early skin and liver development.Proc. Natl. Acad. Sci. USA. 2005; 102: 12501-12506Crossref PubMed Scopus (108) Google Scholar). Adult SCD1−/− mice exhibit a decrease in the synthesis of esterified lipids, including triglycerides, cholesteryl esters, and wax esters. As a consequence, SCD1−/− mice are protected against adiposity and liver steatosis, but they develop alopecia and close eye fissure (12Miyazaki M. Man W.C. Ntambi J.M. Targeted disruption of stearoyl-CoA desaturase1 gene in mice causes atrophy of sebaceous and meibomian glands and depletion of wax esters in the eyelid.J. Nutr. 2001; 131: 2260-2268Crossref PubMed Scopus (222) Google Scholar, 14Cohen P. Miyazaki M. Socci N.D. Hagge-Greenberg A. Liedtke W. Soukas A.A. Sharma R. Hudgins L.C. Ntambi J.M. Friedman J.M. Role for stearoyl-CoA desaturase-1 in leptin-mediated weight loss.Science. 2002; 297: 240-243Crossref PubMed Scopus (659) Google Scholar, 15Ntambi J.M. Miyazaki M. Stoehr J.P. Lan H. Kendziorski C.M. Yandell B.S. Song Y. Cohen P. Friedman J.M. Attie A.D. Loss of stearoyl-CoA desaturase-1 function protects mice against adiposity.Proc. Natl. Acad. Sci. USA. 2002; 99: 11482-11486Crossref PubMed Scopus (871) Google Scholar). In contrast, SCD2 mice have a defect in skin permeability barrier formation and show a decrease in the synthesis of lipids during early development but not at late stages of development (13Miyazaki M. Dobrzyn A. Elias P.M. Ntambi J.M. Stearoyl-CoA desaturase-2 gene expression is required for lipid synthesis during early skin and liver development.Proc. Natl. Acad. Sci. USA. 2005; 102: 12501-12506Crossref PubMed Scopus (108) Google Scholar). These results have suggested that although SCD1 is crucial in adult mice, SCD2 controls lipid metabolism in early life.Currently, it is unknown why multiple SCD isoforms exist in mammalian genomes. Both the tissue-specific expression and the resulting phenotypes of SCD1 and SCD2 mouse isoform knockouts suggest that each isoform has a distinct physiological role; however, it is not clear how the enzymes accomplish the different roles biochemically. Liver microsomes from SCD1−/− and SCD2−/− mice displayed a decrease in the conversions of both 18:0-CoA and 16:0-CoA to 18:1-CoA and 16:1-CoA, respectively, suggesting that SCD1 and SCD2 synthesize oleate (18:1Δ9) and palmitoleate (16:1Δ9) (12Miyazaki M. Man W.C. Ntambi J.M. Targeted disruption of stearoyl-CoA desaturase1 gene in mice causes atrophy of sebaceous and meibomian glands and depletion of wax esters in the eyelid.J. Nutr. 2001; 131: 2260-2268Crossref PubMed Scopus (222) Google Scholar, 13Miyazaki M. Dobrzyn A. Elias P.M. Ntambi J.M. Stearoyl-CoA desaturase-2 gene expression is required for lipid synthesis during early skin and liver development.Proc. Natl. Acad. Sci. USA. 2005; 102: 12501-12506Crossref PubMed Scopus (108) Google Scholar). However, microsomes from the Harderian gland of SCD1−/− mice still displayed very high SCD activity toward 16:0-CoA, despite the undetectable level of activity toward 18:0-CoA (10Miyazaki M. Kim H.J. Man W.C. Ntambi J.M. 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.J. Biol. Chem. 2001; 276: 39455-39461Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar), suggesting the existence of 16:0-CoA-specific Δ9-desaturase in the Harderian gland of mouse. Although each isoform explicitly desaturates at the Δ9 position of saturated acyl-CoAs, it is clear that both 16:0 and 18:0 fatty acids can be used as substrates.To determine the physiological basis of each mouse SCD isoform, we conducted substrate specificity assays to ascertain isoform preferences for 16:0 and 18:0 acyl-CoAs. We demonstrate in this study that SCD1, SCD2, and SCD4 use both C16 and C18 saturated acyl-CoAs. SCD3 desaturates 16:0-CoA but not 18:0-CoA.MATERIALS AND METHODSCloning of full-length mouse SCD cDNAsThe full coding regions of mouse SCD isoforms (mSCD1, M21280; mSCD2, M26269; mSCD3, AF272037; mSCD4, AY430080) were generated by PCR using mouse tissue cDNAs as templates and 5′ primers, which contain a sequence of N-terminal hemagglutinin epitope tag and a EcoRI restriction enzyme site, and 3′ primers, which contain a stop codon and a XhoI restriction enzyme site. The resulting PCR product was cloned into either a yeast expression vector, p426GPD (16Mumberg D. Muller R. Funk M. Yeast vectors for the controlled expression of heterologous proteins in different genetic backgrounds.Gene. 1995; 156: 119-122Crossref PubMed Scopus (1571) Google Scholar), or a mammalian expression vector, pcDNA3 (Invitrogen).Functional analysisThe p426GPD constructs harboring the mouse SCDs were transformed into Saccharomyces cerevisiae strain L8-14C (17Stukey J.E. McDonough V.M. Martin C.E. The OLE1 gene of Saccharomyces cerevisiae encodes the delta 9 fatty acid desaturase and can be functionally replaced by the rat stearoyl-CoA desaturase gene.J. Biol. Chem. 1990; 265: 20144-20149Abstract Full Text PDF PubMed Google Scholar) (provided by Dr. Charles Martin, Rutgers University), which contains a disruption of the yeast Δ9-desaturase gene OLE1 using the lithium acetate standard method (18Elble R. A simple and efficient procedure for transformation of yeasts.Biotechniques. 1992; 13: 18-20PubMed Google Scholar). This S. cerevisiae strain requires unsaturated fatty acids for growth. The transformed yeast cells were plated onto a synthetic dextrose medium containing 1% Tergitol NP-40, 0.5 mM oleic acid, and 0.5 mM palmitoleic acid but lacking uracil. To test the genetic complementation of the mutant yeast strain, transformed yeast cells were plated onto yeast peptone dextrose (YPD) medium lacking unsaturated fatty acids. Plates were incubated at 30°C for 3 days.HeLa cells were cultured at 37°C in a humidified 5% CO2 atmosphere in Dulbecco’s modified Eagle’s medium supplemented with 10% (v/v) fetal bovine serum and penicillin/streptomycin. The cells were resuspended in cytomix buffer (120 mM KCl, 0.15 mM CaCl2, 25 mM HEPES/KOH, pH 7.6, 2 mM EGTA, and 5 mM MgCl2), and 400 μl of suspension was transferred to a 0.4 cm electroporation cuvette (Invitrogen). Thirty-five micrograms of pcDNA3 DNA harboring one of the mouse SCD constructs was transfected into HeLa cells. Plasmid DNA was added to the cell suspension in the cuvette and mixed. The mixture was then exposed to a single electric pulse of 300 V with a capacitance of 1,000 μF using an Invitrogen pulse system. The cells were allowed to recover in culture medium at 37°C (5% CO2 atmosphere) for 48 h before harvesting and SCD activity assays.Fatty acid analysisYeast cells were grown in liquid YPD medium lacking unsaturated fatty acids for 3 days. Cells were pelleted and washed twice with water, followed by suspension in 0.5 ml of 2 M NaOH in methanol. The mixture was then heated to 80°C for 1 h and acidified with formic acid. Fatty acids were extracted according to Bligh and Dyer’s method (19Bligh E.G. Dyer W.J. A rapid method of total lipid extraction and purification.Can. J. Biochem. Physiol. 1959; 37: 911-917Crossref PubMed Scopus (42174) Google Scholar) and transmethylated with 1 ml of 14% BF3 in methanol (Sigma). The resulting fatty acid methyl esters were extracted with hexane and analyzed by gas-liquid chromatography (12Miyazaki M. Man W.C. Ntambi J.M. Targeted disruption of stearoyl-CoA desaturase1 gene in mice causes atrophy of sebaceous and meibomian glands and depletion of wax esters in the eyelid.J. Nutr. 2001; 131: 2260-2268Crossref PubMed Scopus (222) Google Scholar, 20Watts J.L. Browse J. A palmitoyl-CoA-specific Δ9 fatty acid desaturase from Caenorhabditis elegans.Biochem. Biophys. Res. Commun. 2000; 272: 263-269Crossref PubMed Scopus (107) Google Scholar, 21Sakai H. Kajiwara S. A stearoyl-CoA-specific Δ 9 fatty acid desaturase from the basidiomycete Lentinula edodes.Biosci. Biotechnol. Biochem. 2003; 67: 2431-2437Crossref PubMed Scopus (15) Google Scholar). Exogenous saturated fatty acids (0.2 mM) were added into YPD in the presence of 1% Tergitol NP-40 (Sigma). The double bond positions of the monounsaturated fatty acid methyl ester were determined by GC-MS analysis of the dimethyl disulfide derivatives (11Miyazaki M. Gomez F.E. Ntambi J.M. Lack of stearoyl-CoA desaturase-1 function induces a palmitoyl-CoA Delta6 desaturase and represses the stearoyl-CoA desaturase-3 gene in the preputial glands of the mouse.J. Lipid Res. 2002; 43: 2146-2154Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar, 22Buser H.R. Arn H. Guerin P. Rausher S. Determination of double bond position in mono-unsaturated acetates by mass spectrometry of dimethyl disulfide adducts.Anal. Chem. 1983; 55: 818-822Crossref Scopus (392) Google Scholar).Δ9-desaturase activityMicrosomes were purified from HeLa cells by differential centrifugation and resuspended in a 0.1 M potassium phosphate buffer (pH 6.8). Δ9-desaturase activity was assayed at 25°C for 7 min with either [14C]stearoyl-CoA or [14C]palmitoyl-CoA, 2 mM NADH, and 100 μg of microsomal protein (12Miyazaki M. Man W.C. Ntambi J.M. Targeted disruption of stearoyl-CoA desaturase1 gene in mice causes atrophy of sebaceous and meibomian glands and depletion of wax esters in the eyelid.J. Nutr. 2001; 131: 2260-2268Crossref PubMed Scopus (222) Google Scholar).Immunoblot analysisYeast protein extract was electrophoresed by 8% SDS-PAGE and transferred to a nitrocellulose membrane (Millipore). The membrane was blocked at room temperature for 1 h in TBS containing 0.1% Tween 20 containing 1% BSA and then incubated at room temperature with 100 ng/ml anti-hemagglutinin monoclonal antibody (clone 3F10; Roche) in TBS containing 1% BSA for 1 h. After washing with TBS containing 0.1% Tween 20, the membrane was incubated with a 1:20,000 dilution of horseradish peroxidase-conjugated anti-rat IgG (Sigma) for 30 min at room temperature. The signal was visualized with the ECL Western blot detection kit (Pierce).Statistical analysisAll data are expressed as means ± SEM. An unpaired Student’s t-test was used to determine significance.RESULTSTo study the function of mouse SCD genes (SCD1–SCD4), the open reading frames of the genes were subcloned in the episomal yeast expression vector p426GPD, which encodes uracil prototrophy under the constitutive glyceraldehyde-3-phosphate dehydrogenase promoter. The resulting plasmid was used to transform L8-14C, a Δ9-desaturase (OLE1)-deficient yeast strain. As shown in Fig. 1, yeast transformed with plasmids containing each SCD were able to grow on YPD plates lacking unsaturated fatty acids, indicating that the mouse Δ9-desaturases were functional in yeast by their ability to complement the ole1 mutation. The fatty acid compositions of transformants as determined by gas-liquid chromatography are shown in Table 1. Yeast expressing mSCD1 and mSCD2 had similar fatty acid compositions and converted 85% and 77% of 18:0 to 18:1Δ9 and 51% and 36% of 16:0 to 16:1Δ9, respectively. mSCD4 used 8% of 16:0 and 13% of 18:0, but the conversion rates were lower than those of SCD1 and SCD2. Therefore, the mouse SCD1, SCD2, and SCD4 enzymes desaturate 16:0 and 18:0 to 16:1Δ9 and 18:1Δ9, respectively, although the three enzymes preferably use 18:0 compared with 16:0. SCD3 converted 49% of 16:0 to 16:1Δ9, but the conversion of 18:0 to 18:1Δ9 was <2%, suggesting that this isoform prefers C16:0 as a substrate. Immunoblot analysis with a hemagglutinin antibody showed that all Δ9-desaturase proteins were expressed in yeast at the expected size (Fig. 1B). The Δ9 positions of the double bond in all monounsaturated fatty acids were determined by GC-MS analysis of the dimethyl disulfide derivatization. As expected, the mass spectra of 16:1 and 18:1 in yeast expressing any SCD showed the characteristic Δ9 unsaturated fragment ions at m/z 217 and 185 (data not shown).TABLE 1Fatty acid composition of total lipids in L8-14C transformantsFatty acid compositionSCD Isoform16:016:1n-718:018:1n-918:1n-7Percent conversion of 16:0Percent conversion of 18:0%SCD149.927.93.318.30.636.484.8SCD241.642.93.311.21.051.477.3SCD342.740.016.20.01.149.00.3SCD460.15.927.85.60.69.716.8SCD, stearoyl-coenzyme A desaturase. Data represent the content of each fatty acid in percentage of total fatty acids (n = 5). Standard errors of the mean were all <10% and are omitted for clarity. Open table in a new tab To determine whether the mouse SCD isoforms desaturate other saturated fatty acids, we exogenously provided the saturated fatty acids (0.2 mM) to yeast expressing each SCD isoform (Fig. 2). SCD1 and SCD2 converted 13% and 14% of 13:0 to 13:1Δ9, 11% and 14% of 14:0 to 14:1Δ9, 43% and 31% of 17:0 to 17:1Δ9, and 30% and 45% of 17:0 to 17:1, respectively. mSCD3 converted 14% of 12:0, 32% of 13:0, and >50% of 14:0 to 12:1Δ9, 13:1Δ9, and 14:1Δ9, respectively. The conversion of 17:0 to 17:1Δ9 was undetectable in yeast expressing SCD3. SCD4 used only 2.9% of 14:0 and 5.7% of 17:0, suggesting that mSCD4 may use other acyl-CoAs as major substrates. None of the SCD isoforms used a C10:0 or C20:0 saturated fatty acid.Fig. 2Conversion of saturated fatty acids to Δ9 monounsaturated fatty acids. Fatty acids (0.2 mM) were added in the presence of 1% Tergitol NP-40, and yeast cells were cultured for 2 days. Values represent mean conversion (%) ± SEM (n = 5).View Large Image Figure ViewerDownload Hi-res image Download (PPT)To determine whether the mouse SCDs displayed similar substrate specificities in mammalian cells, we recloned the Δ9-desaturases into a mammalian expression vector and transfected them into HeLa cells (human cervical cancer cells), which have very low Δ9-desaturase activity compared with other human cell lines, including HEK-293, HepG2, CHO, MDA, and MCF-7 cells (data not shown). 16:0-CoA and 18:0-CoA were used as substrates because 16:1Δ9 and 18:1Δ9 are the major monounsaturated fatty acids in mammalian cells. The substrate preferences of all of the Δ9-desaturases in mammalian cells are consistent with those found in yeast experiments. HeLa cells overexpressing mSCD1, mSCD2, and mSCD4 used 16:0-CoA and 18:0-CoA, but with 2.2-, 1.6-, and 2.0-fold higher Δ9-desaturase activity, respectively, toward 18:0-CoA than 16:0-CoA. mSCD3 showed high activity toward 16:0-CoA, but its activity toward 18:0-CoA was not detectable (Fig. 3).Fig. 3Δ9-desaturase activity in HeLa cells overexpressing mouse Δ9-desaturases. Microsomal fractions (100 μg) were incubated with either [14C]stearoyl-CoA or [14C]palmitoyl-CoA in the presence of NADH. Each value represents the mean ± SEM (n = 4) * P < 0.001 versus 16:0-CoA.View Large Image Figure ViewerDownload Hi-res image Download (PPT)DISCUSSIONThere are four known Δ9-desaturase isoforms in mouse that are required for the biosynthesis of monounsaturated fatty acids, mainly oleate and palmitoleate (2Ntambi J.M. Miyazaki M. Recent insights into stearoyl-CoA desaturase-1.Curr. Opin. Lipidol. 2003; 14: 255-261Crossref PubMed Scopus (207) Google Scholar). However, except for rat SCD1 (17Stukey J.E. McDonough V.M. Martin C.E. The OLE1 gene of Saccharomyces cerevisiae encodes the delta 9 fatty acid desaturase and can be functionally replaced by the rat stearoyl-CoA desaturase gene.J. Biol. Chem. 1990; 265: 20144-20149Abstract Full Text PDF PubMed Google Scholar), the substrate specificities of mammalian Δ9-desaturases have not been characterized in detail. In addition, the reasons for the existence and tissue-specific expression of the multiple isoforms remain unknown. Using two different models [1) a Δ9-desaturase-deficient yeast strain that requires unsaturated fatty acids for growth, and 2) HeLa cells (human cervical cancer cells), which have very low activity of Δ9-desaturase], we demonstrated that mouse SCD isoforms have different substrate specificities. Based on the results from both experiments, mSCD1 and mSCD2 are capable of using 16:0-CoA and 18:0-CoA, although there was an ∼2-fold higher preference toward 18:0-CoA over 16:0-CoA. SCD4 also used both 16:0 and 18:0, but the activity was lower than those of other isoforms. Therefore, the preferential substrates of SCD4 may be undetermined. SCD3 uses C16:0 but not C18:0. Based on this observation, SCD3 should be named palmitoyl-coenzyme A Δ9-desaturase-1 (PCD1).The substrate recognition of plant acyl-acyl-carrier protein Δ9-desaturase has been studied extensively because of the availability of the crystal structure (23Cahoon E.B. Shah S. Shanklin J. Browse J. A determinant of substrate specificity predicted from the acyl-acyl carrier protein desaturase of developing cat's claw seed.Plant Physiol. 1998; 117: 593-598Crossref PubMed Scopus (86) Google Scholar). Using structural prediction and site-specific mutagenesis, the tryptophan at position 118 was shown to control the chain length specificity of plant 16:0-acyl-carrier protein-specific Δ9-desaturase (23Cahoon E.B. Shah S. Shanklin J. Browse J. A determinant of substrate specificity predicted from the acyl-acyl carrier protein desaturase of developing cat's claw seed.Plant Physiol. 1998; 117: 593-598Crossref PubMed Scopus (86) Google Scholar). However, all of the tryptophans in the predicted catalytic sites located between the first histidine box and the C terminus are conserved in the mouse Δ9-desaturases. Despite the distinct substrate preferences described above, the presumed catalytic sites of mouse SCDs, particularly between SCD1 and PCD1/SCD3, are very similar (>94%) (7Miyazaki M. Jacobson M.J. Man W.C. Cohen P. Asilmaz E. Friedman J.M. Ntambi J.M. Identification and characterization of murine SCD4, a novel heart-specific stearoyl-CoA desaturase isoform regulated by leptin and dietary factors.J. Biol. Chem. 2003; 278: 33904-33911Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar, 9Zheng Y. Prouty S.M. Harmon A. Sundberg J.P. Stenn K.S. Parimoo S. Scd3—a novel gene of the stearoyl-CoA desaturase family with restricted expression in skin.Genomics. 2001; 71: 182-191Crossref PubMed Scopus (148) Google Scholar). The amino acid alterations between these two proteins are concentrated in 20 amino acid residues before the third histidine box, suggesting that an amino acid in this portion of the protein might distinguish substrates as a result of their chain lengths. More studies are required to resolve this issue.Although the reason for the existence of PCD1/SCD3 and its ability to produce shorter chain monounsaturated fatty acids is unknown, it could be attributable to the unique tissue-specific distribution of the SCD genes and the difference in melting point of their monounsaturated fatty acid products. PCD1/SCD3 is highly expressed in skin sebaceous glands, which produce lipid secretions (mainly wax ester) referred to as sebum (9Zheng Y. Prouty S.M. Harmon A. Sundberg J.P. Stenn K.S. Parimoo S. Scd3—a novel gene of the stearoyl-CoA desaturase family with restricted expression in skin.Genomics. 2001; 71: 182-191Crossref PubMed Scopus (148) Google Scholar). The most abundant monounsaturated fatty acid in sebum is 16:1 (24Green S.C. Stewart M.E. Downing D.T. Variation in sebum fatty acid composition among adult humans.J. Invest. Dermatol. 1984; 83: 114-117Abstract Full Text PDF PubMed Scopus (51) Google Scholar). Because skin is poikilothermal, sebum is easily affected by the environmental temperature. The melting point of 16:1Δ9 (0.5°C) is lower than that of 18:1Δ9 (16.2°C). Thus, 16:1Δ9 appears to be more resistant to cold temperature and could be preferentially used by acyl-CoA wax alcohol acyltransferases (AWAT1 and AWAT2) (25Turkish A.R. Henneberry A.L. Cromley D. Padamsee M. Oelkers P. Bazzi H. Christiano A.M. Billheimer J.T. Sturley S.L. Identification of two novel human acyl-CoA wax alcohol acyltransferases: members of the diacylglycerol acyltransferase 2 (DGAT2) gene superfamily.J. Biol. Chem. 2005; 280: 14755-14764Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar) in the synthesis of skin waxes. The skin of SCD1−/− mice exhibits alopecia, atrophy of the sebaceous gland, and decreased sebum production (12Miyazaki M. Man W.C. Ntambi J.M. Targeted disruption of stearoyl-CoA desaturase1 gene in mice causes atrophy of sebaceous and meibomian glands and depletion of wax esters in the eyelid.J. Nutr. 2001; 131: 2260-2268Crossref PubMed Scopus (222) Google Scholar, 26Zheng Y. Eilertsen K.J. Ge L. Zhang L. Sundberg J.P. Prouty S.M. Stenn K.S. Parimoo S. Scd1 is expressed in sebaceous glands and is disrupted in the asebia mouse.Nat. Genet. 1999; 23: 268-270Crossref PubMed Scopus (193) Google Scholar). Interestingly PCD1/SCD3 expression was lost in skin sebocytes of SCD1−/− mice (9Zheng Y. Prouty S.M. Harmon A. Sundberg J.P. Stenn K.S. Parimoo S. Scd3—a novel gene of the stearoyl-CoA desaturase family with restricted expression in skin.Genomics. 2001; 71: 182-191Crossref PubMed Scopus (148) Google Scholar, 11Miyazaki M. Gomez F.E. Ntambi J.M. Lack of stearoyl-CoA desaturase-1 function induces a palmitoyl-CoA Delta6 desaturase and represses the stearoyl-CoA desaturase-3 gene in the preputial glands of the mouse.J. Lipid Res. 2002; 43: 2146-2154Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar). These data suggest that 16:1Δ9 synthesized from PCD1/SCD3 is an important fatty acid in the skin functions of wax production and hair growth. In addition, we previously found that Harderian gland sebocytes of SCD1−/− mice have greater SCD activity toward 16:0-CoA than 18:0-CoA, and the reduction in the levels of 16:1Δ9 and its metabolites in the Harderian gland of SCD1−/− mice is less than that of 18:1Δ9 and its metabolites (10Miyazaki M. Kim H.J. Man W.C. Ntambi J.M. 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.J. Biol. Chem. 2001; 276: 39455-39461Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar). Therefore" @default.
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