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Transcription Factor Ctip2 Controls Epidermal Lipid Metabolism and Regulates Expression of Genes Involved in Sphingolipid Biosynthesis during Skin Development

  • Zhixing Wang
    Affiliations
    Department of Pharmaceutical Sciences, College of Pharmacy, Oregon State University, Corvallis, Oregon, USA
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  • Jay S. Kirkwood
    Affiliations
    Department of Pharmaceutical Sciences, College of Pharmacy, Oregon State University, Corvallis, Oregon, USA

    Linus Pauling Institute, Corvallis, Oregon, USA
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  • Alan W. Taylor
    Affiliations
    Linus Pauling Institute, Corvallis, Oregon, USA
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  • Jan F. Stevens
    Affiliations
    Department of Pharmaceutical Sciences, College of Pharmacy, Oregon State University, Corvallis, Oregon, USA

    Linus Pauling Institute, Corvallis, Oregon, USA
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  • Mark Leid
    Affiliations
    Department of Pharmaceutical Sciences, College of Pharmacy, Oregon State University, Corvallis, Oregon, USA

    Molecular Cell Biology Program, Corvallis, Oregon, USA

    Environmental Health Science Center, Oregon State University, Corvallis, Oregon, USA
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  • Gitali Ganguli-Indra
    Affiliations
    Department of Pharmaceutical Sciences, College of Pharmacy, Oregon State University, Corvallis, Oregon, USA

    Molecular Cell Biology Program, Corvallis, Oregon, USA
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  • Arup K. Indra
    Correspondence
    Department of Pharmaceutical Sciences, College of Pharmacy, Oregon State University, 1601 SW Jefferson Avenue, Corvallis, Oregon 97331, USA
    Affiliations
    Department of Pharmaceutical Sciences, College of Pharmacy, Oregon State University, Corvallis, Oregon, USA

    Molecular Cell Biology Program, Corvallis, Oregon, USA

    Environmental Health Science Center, Oregon State University, Corvallis, Oregon, USA

    Department of Dermatology, Oregon Health and Science University, Portland, Oregon, USA
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      The stratum corneum is composed of protein-enriched corneocytes embedded in an intercellular matrix of nonpolar lipids organized as lamellar layers and giving rise to epidermal permeability barrier (EPB). EPB defects have an important role in the pathophysiology of skin diseases such as eczema. The transcriptional control of skin lipid metabolism is poorly understood. We have discovered that mice lacking transcription factor COUP-TF-interacting protein 2 (Ctip2) exhibit EPB defects including altered keratinocyte terminal differentiation, delayed skin barrier development, and interrupted neutral lipid distribution in the epidermis. Here we adapted a targeted lipidomic approach using mass spectrometry and have determined that Ctip2−/− mice (germline deletion of the Ctip2 gene) display altered composition of major epidermal lipids, such as ceramides and sphingomyelins, compared with wild-type mice at different stages of skin development. Interestingly, expressions of several genes involved in skin sphingolipid biosynthesis and metabolism were altered in mutant skin. Ctip2 was found to be recruited to the promoter region of a subset of those genes, suggesting their possible direct regulation by Ctip2. Our results confirm an important role of Ctip2 in regulating skin lipid metabolism and indicate that profiling of epidermal sphingolipid could be useful for designing effective strategies to improve barrier dysfunctions.

      Abbreviations

      Acer1–Acer3
      alkaline ceramidase 1–3
      Asah1 and 2
      acylsphingosine amidohydrolases 1 and 2
      CER
      ceramide
      ChIP
      chromatin immunoprecipitation
      Ctip2
      chicken ovalbumin upstream promoter transcription factor (COUP-TF)-interacting protein 2
      Degs1
      dihydroceramide desaturase 1
      EPB
      epidermal permeability barrier
      eLox3
      epidermis-type lipoxygenase 3
      Gba2
      acid beta-glucosidase
      Lass1–Lass6
      ceramide synthases 1–6
      LC/MS/MS
      liquid chromatography–tandem mass spectrometry
      qRT-PCR
      quantitative real-time reverse-transcriptase PCR
      SC
      stratum corneum
      Sgms1 and 2
      sphingomyelin synthase 1 and 2
      Smpd1 and 2
      sphingomyelinase 1 and 2
      Sphk1 and 2
      sphingosine kinases 1 and 2
      Sptlc1-3
      palmitoyltransferase 1-3
      Ugcg
      glucosylceramide synthase

      Introduction

      The exterior layer of skin, the stratum corneum (SC), has a critical role in skin permeability barrier formation and maintenance (
      • Fuchs E.
      • Raghavan S.
      Getting under the skin of epidermal morphogenesis.
      ;
      • Elias P.M.
      The epidermal permeability barrier: from the early days at Harvard to emerging concepts.
      ). It forms a protective barrier against external stress and environmental insults. Skin barrier defects are linked with altered levels of protein(s) involved in keratinocyte terminal differentiation, cornified envelope crosslinking, as well as formation and maintenance of skin lipid barrier (
      • Elias P.M.
      • Menon G.K.
      Structural and lipid biochemical correlates of the epidermal permeability barrier.
      ;
      • Nemes Z.
      • Steinert P.M.
      Bricks and mortar of the epidermal barrier.
      ;
      • Feingold K.R.
      Thematic review series: skin lipids. The role of epidermal lipids in cutaneous permeability barrier homeostasis.
      ,
      • Feingold K.R.
      The outer frontier: the importance of lipid metabolism in the skin.
      ). Mammalian SC contains a large number of lipids such as ceramides (CERs), sphingomyelins, cholesterols, and fatty acids (
      • Elias P.M.
      • Menon G.K.
      Structural and lipid biochemical correlates of the epidermal permeability barrier.
      ;
      • Feingold K.R.
      Thematic review series: skin lipids. The role of epidermal lipids in cutaneous permeability barrier homeostasis.
      ,
      • Feingold K.R.
      Lipid metabolism in the epidermis.
      ;
      • Ishikawa J.
      • Narita H.
      • Kondo N.
      • et al.
      Changes in the ceramide profile of atopic dermatitis patients.
      ). CERs are the most abundant lipid types in SC (50% by weight) and can be divided into at least 11 species (
      • Oda Y.
      • Uchida Y.
      • Moradian S.
      • et al.
      Vitamin D receptor and coactivators SRC2 and 3 regulate epidermis-specific sphingolipid production and permeability barrier formation.
      ;
      • Ishikawa J.
      • Narita H.
      • Kondo N.
      • et al.
      Changes in the ceramide profile of atopic dermatitis patients.
      ). It has been shown that reduced content of CERs leads to epidermal water loss and skin epidermal dysfunctions and causes diseases such as atopic dermatitis or eczema (
      • Imokawa G.
      A possible mechanism underlying the ceramide deficiency in atopic dermatitis: Expression of a deacylase enzyme that cleaves the N-acyl linkage of sphingomyelin and glucosylceramide.
      ;
      • Ishikawa J.
      • Narita H.
      • Kondo N.
      • et al.
      Changes in the ceramide profile of atopic dermatitis patients.
      ). Spingomyelins are another large group of epidermal sphingolipids, which have been reported as precursors of CERs (
      • Schmuth M.
      • Man M.Q.
      • Weber F.
      • et al.
      Permeability barrier disorder in Niemann-Pick disease: sphingomyelin-ceramide processing required for normal barrier homeostasis.
      ;
      • Uchida Y.
      • Hara M.
      • Nishio H.
      • et al.
      Epidermal sphingomyelins are precursors for selected stratum corneum ceramides.
      ;
      • Holleran W.M.
      • Takagi Y.
      • Uchida Y.
      Epidermal sphingolipids: metabolism, function, and roles in skin disorders.
      ). Cholesterols and fatty acids are also major skin lipids, and cholesterol-3-sulfate is a ubiquitous metabolite of cholesterol, which is involved in cornified envelope formation by interacting with transglutaminase1 (
      • Nemes Z.
      • Demény M.
      • Marekov L.N.
      • et al.
      Cholesterol 3-sulfate interferes with cornified envelope assembly by diverting transglutaminase 1 activity from the formation of cross-links and esters to the hydrolysis of glutamine.
      ).
      CER biosynthesis is regulated at multiple levels. Four pathways have been reported so far: the de novo pathway, the sphingomyelinase pathway, the salvage pathway, and the exogenous CER-recycling pathway (
      • Kitatani K.
      • Idkowiak-Baldys J.
      • Hannun Y.A.
      The sphingolipid salvage pathway in ceramide metabolism and signaling.
      ). In the de novo pathway, serine and fatty acyl-CoA are the initiators of a series of reactions. CER synthases 1–6 (Lass1–Lass6) have an important role as each of them has specificities on synthesizing certain CERs (
      • Mizutani Y.
      • Mitsutake S.
      • Tsuji K.
      • et al.
      Ceramide biosynthesis in keratinocyte and its role in skin function.
      ). Other enzymes such as palmitoyltransferases (Sptlc1-3), dihydroceramide desaturase (Degs1), and 3-ketodihydrosphingosine reductase are also key enzymes in the de novo pathway (
      • Merrill A.H.
      De novo sphingolipid biosynthesis: a necessary, but dangerous, pathway.
      ). The salvage pathway has been reported recently as a complex mechanism, commencing as the catabolism of complex sphingolipids into glucosylceramide by glucosylceramide synthase (Ugcg) and followed by the formation of CER by acid beta-glucosidase (Gba2) (
      • Ogretmen B.
      Biochemical mechanisms of the generation of endogenous long chain ceramide in response to exogenous short chain ceramide in the A549 human lung adenocarcinoma cell line. role for endogenous ceramide in mediating the action of exogenous ceramide.
      ;
      • Becker K.P.
      Selective inhibition of juxtanuclear translocation of protein kinase C II by a negative feedback mechanism involving ceramide formed from the salvage pathway.
      ). These CERs can be broken down into sphingosines by ceramidases (Asah1 and Asah2), which are reused to produce CERs (
      • Bernardo K.
      • Hurwitz R.
      • Zenk T.
      • et al.
      Purification, characterization, and biosynthesis of human acid ceramidase.
      ;
      • Koch J.
      • Gärtner S.
      • Li C.M.
      • et al.
      Molecular cloning and characterization of a full-length complementary DNA encoding human acid ceramidase. Identification Of the first molecular lesion causing Farber disease.
      ;
      • Li C.M.
      • Hong S.B.
      • Kopal G.
      • et al.
      Cloning and characterization of the full-length cDNA and genomic sequences encoding murine acid ceramidase.
      ;
      • Tani M.
      • Okino N.
      • Mori K.
      • et al.
      Molecular cloning of the full-length cDNA encoding mouse neutral ceramidase. A novel but highly conserved gene family of neutral/alkaline ceramidases.
      ;
      • Mao C.
      • Obeid L.M.
      Ceramidases: regulators of cellular responses mediated by ceramide, sphingosine, and sphingosine-1-phosphate.
      ). In the sphingomyelinase pathway, CER is formed by hydrolysis of sphingomyelins, and the reverse reaction is controlled by sphingomyelin synthase (
      • Marchesini N.
      • Hannun Y.A.
      Acid and neutral sphingomyelinases: roles and mechanisms of regulation.
      ). Exogenous short-chain CER can also be utilized to generate sphingosine, thus leading to the synthesis of endogenous long-chain CERs (
      • Ogretmen B.
      Biochemical mechanisms of the generation of endogenous long chain ceramide in response to exogenous short chain ceramide in the A549 human lung adenocarcinoma cell line. role for endogenous ceramide in mediating the action of exogenous ceramide.
      ;
      • Sultan I.
      • Senkal C.E.
      • Ponnusamy S.
      • et al.
      Regulation of the sphingosine-recycling pathway for ceramide generation by oxidative stress, and its role in controlling c-Myc/Max function.
      ).
      COUP-TF-interacting protein 2 (Ctip2), also known as Bcl11b, is a C2H2 zinc-finger protein expressed in many organs and tissues, including developing mouse epidermis (
      • Avram D.
      • Fields A.
      • Pretty On Top K.
      • et al.
      Isolation of a novel family of C(2)H(2) zinc finger proteins implicated in transcriptional repression mediated by chicken ovalbumin upstream promoter transcription factor (COUP-TF) orphan nuclear receptors.
      ,
      • Avram D.
      • Fields A.
      • Senawong T.
      • et al.
      COUP-TF (chicken ovalbumin upstream promoter transcription factor)-interacting protein 1 (CTIP1) is a sequence-specific DNA binding protein.
      ;
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      ). It has been reported that Ctip2-null mice exhibit altered epidermal proliferation and late terminal differentiation, as well as barrier defects during embryogenesis (
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      ). The barrier defects were correlated with altered surface lipid distribution in the absence of Ctip2 (
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      ).
      Liquid chromatography–tandem mass spectrometry (LC/MS/MS) has been used to characterize CER species in mouse and human SC (
      • Masukawa Y.
      • Narita H.
      • Shimizu E.
      • et al.
      Characterization of overall ceramide species in human stratum corneum.
      ;
      • Park H.
      • Haynes C.A.
      • Nairn A.V.
      • et al.
      Transcript profiling and lipidomic analysis of ceramide subspecies in mouse embryonic stem cells and embryoid bodies.
      ;
      • Ishikawa J.
      • Narita H.
      • Kondo N.
      • et al.
      Changes in the ceramide profile of atopic dermatitis patients.
      ;
      • Janssens M.
      • van Smeden J.
      • Gooris G.S.
      • et al.
      Lamellar lipid organization and ceramide composition in the stratum corneum of patients with atopic eczema.
      ;
      • van Smeden J.
      • Hoppel L.
      • van der Heijden R.
      • et al.
      LC/MS analysis of stratum corneum lipids: ceramide profiling and discovery.
      ). Here we adopted the LC/MS/MS methodology to elucidate the CER containing nonhydroxy fatty acids (N) and sphingosine (S) [CER (NS)], sphingmyelin, and sphingosine profiles in wild-type and Ctip2-null (Ctip2−/−) embryonic skin epidermis. The expression levels of genes encoding various enzymes involved in sphingolipid metabolism were analyzed by quantitative real-time reverse-transcriptase PCR (qRT-PCR) and immunoblotting in wild-type and Ctip2-null skin. Furthermore, Ctip2 recruitment to the promoter regions of several genes that were altered in Ctip2−/− mice was evaluated by chromatin immunoprecipitation (ChIP) analyses. Results indicated that Ctip2 controls sphingolipid metabolism in the developing skin by directly or indirectly regulating the expression of a subset of genes involved in the sphingolipid biosynthesis pathways.

      Results

      Altered profile of sphingolipids in the developing epidermis of Ctip2-null mice

      We hypothesized that impaired epidermal permeability barrier (EPB) in Ctip2-null (Ctip2−/−) mice could be partly due to altered skin lipid composition. To determine the lipid composition of embryonic skin, wild-type and Ctip2-null epidermis were isolated at different developmental stages, embryonic day (E) 16.5, E17.5, E18.5, and neonatal (postnatal day 0, P0). Lipidomic studies were then conducted using the LC/MS/MS technique, and absolute quantities of the different skin sphingolipids were analyzed (for details see Materials and Methods). Both CERs and sphingomyelins were abundant during mouse skin development, especially during the earlier stages of skin development (E16.5, E17.5) (Figure 1).
      Figure thumbnail gr1
      Figure 1Liquid chromatography–tandem mass spectrometry (LC/MS/MS) analyses of mouse epidermal sphingolipids during skin development. The epidermal sphingolipids were extracted from the skin of E16.5, E17.5, and E18.5 embryos and from that of P0 wild-type (WT) and Ctip2−/− mice. The amount of saturated and unsaturated ceramides and sphingomyelins was determined according to their N-acyl chain length using LC/MS/MS analyses. The absolute content of (ad) saturated and (eh) unsaturated ceramides at (a, e) E16.5, (b, f) E17.5, (c, g) E18.5, and (d, h) P0 was quantified using saturated and unsaturated ceramide standards and expressed as pmol per mg of skin (for details see Materials and Methods). The amount of (il) saturated and (mp) unsaturated sphingomyelin was also determined at different developmental stages, namely, (i, m) E16.5, (j, n) E17.5, (k, o) E18.5, and (l, p) P0, using saturated and unsaturated sphingomyelin standards and expressed as pmol per mg of skin (see Materials and Methods). Statistical analyses were performed by Student’s unpaired t-test using Graphpad Prism software. *P<0.05; **P<0.005, ***P<0.001. Data were reported as mean±SEM (n=9). The results represent three separate experiments conducted in triplicate. E, embryonic day; P, postnatal day.
      CER C14:0 level remained very low from E16.5–P0. Long-chain CER C16:0 was the most abundant saturated CER at E16.5 and was 2-fold higher in the mutant epidermis compared with wild type (Figure 1a). Significant induction of CER C18:0 was observed in the E16.5 mutant skin (Figure 1a). The amount of long-chain CERs (C16:0, C18:0, C20:0, and C22:0) was very low at E17.5–E18.5, with no discernable difference observed between wild-type and mutant skin; however, their quantities were larger at P0, each having an approximately 2-fold reduction in Ctip2−/− skin (Figure 1b–d). At E17.5 and E18.5, very long-chain CER C26:0 became the predominant subtype in mouse skin, modestly reduced at E17.5 and 2-fold decreased at E18.5 in Ctip2−/− skin (Figure 1b and c). Modest decrease of CER C24:0 was also observed in Ctip2−/− epidermis at P0 (Figure 1d). Unsaturated CERs were less abundant compared with saturated CERs in the wild-type epidermis. Long-chain unsaturated CER C16:1 was significantly reduced at E16.5 in the mutant epidermis, unaffected at E17.5 and E18.5, with a 2-fold decrease in P0 mutant epidermis (Figure 1e–h).
      Among saturated sphingomyelins, long-chain sphingomyelins C16:0, C20:0, and C22:0 were the most abundant subtypes in embryonic epidermis at most of the stages, which were highly induced in mutant skin, especially at E17.5 and E18.5 (Figure 1j and k). Very long-chain sphingomyelins (C24:1, C26:1, and C28:1) were also increased in the mutant skin at E16.5–E18.5. Interestingly, unsaturated sphingomyelins were less abundant at all developmental stages; however, they showed the same trend as saturated ones, with a higher amount in embryonic mutant epidermis. As early as E16.5, long-chain unsaturated sphingomyelin C24:1 was significantly elevated in Ctip2−/− skin (Figure 1m). Higher quantities of long-chain sphingomyelins C20:1, C22:1, and C24:1 were noted in the mutant epidermis at E17.5 and E18.5 (Figure 1n and o).
      The profiles of sphingosine and sphinganine were also determined by the LC/MS/MS method. Compared with CERs and sphingomyelins, the quantity of free sphingoid bases was smaller in embryonic skin, with no significant changes observed between wild-type and mutant epidermis at any developmental stage (Supplementary Figure S1 online).
      We also performed ultrahigh-pressure liquid chromatography/MS/MS analyses to determine the relative amount of cholesterol and fatty acids. Levels of cholesterol and cholesterol-3-sulfate were significantly reduced in the epidermis of Ctip2−/− mice at E16.5 but were unaltered at all later time points (Supplementary Figure S2a and b online). Although the amount of palmitic acid and stearic acid was similar between wild-type and null skin at E16.5–E18.5, their levels doubled in the P0 Ctip2−/− epidermis compared with the wild-type epidermis (Supplementary Figure S2c and d online). All together, our results indicate that epidermal lipid composition is altered in the absence of transcriptional regulator Ctip2.

      Ctip2 directly regulates the expression of a subset of genes involved in lipid metabolism during skin development

      Previous studies have identified four major pathways involved in CER synthesis: the de novo pathway, the salvage pathway, the sphingomyelinase pathway, and the exogenous CER-recycling pathway (
      • Kitatani K.
      • Idkowiak-Baldys J.
      • Hannun Y.A.
      The sphingolipid salvage pathway in ceramide metabolism and signaling.
      ;
      • Mizutani Y.
      • Mitsutake S.
      • Tsuji K.
      • et al.
      Ceramide biosynthesis in keratinocyte and its role in skin function.
      ). We hypothesized that altered sphingolipid profiles in mutant mice could be because of altered expression of genes encoding proteins involved in the sphingolipid metabolism. To test this hypothesis, we evaluated the expression of genes involved in the different CER biosynthesis pathways by qRT-PCR analyses in wild-type control and mutant skin (for details see Materials and Methods). Expressions of CER synthase 3 (Lass3) and palmitoyltransferases (Sptlc1 and Sptlc3) were downregulated at E16.5 and E17.5, whereas CER synthases Lass1 and Lass2 were significantly reduced at E18.5 in Ctip2−/− skin (Figure 2a–c). Lass6 was consistently downregulated from E16.5 to E18.5 in mutant skin (Figure 2a–c). Thus, genes encoding enzymes in the de novo pathway were altered after ablation of Ctip2. Similarly, transcript levels of enzymes involved in the sphingomyelinase pathway, such as sphingomyelinases (Smpd1 and Smpd2), were significantly downregulated at E18.5, whereas sphingomyelin synthase (Sgms2) level was upregulated at E16.5 in null embryos compared with the wild-type ones (Figure 2d–f). Expression of glucosylceramide synthase (Ugcg), which turns complex sphingolipids into glucosylcerarimde, was also reduced at E18.5, whereas acid beta-glucosidase (Gba2), which is involved in the salvage pathway, was downregulated in the skin of null mice from E16.5 to E18.5 (Figure 2d–f). Ceramidases are involved in the breaking down of short-chain CER, facilitating its conversion to sphingosine. In particular, the reduced expression of neutral ceramidase 2 (Asah2) and alkaline ceramidase 1 (Acer1) was observed at all developmental stages in the mutant epidermis (Figure 2g–i). Levels of all other members of acid ceramidases (Asah1 and Asah3) and alkaline ceramidases (Acer2 and Acer3) were unaltered in the mutants at all stages of development (Figure 2g–i). Similarly, sphingosine kinases (Sphk1 and Sphk2) are responsible for the disposal of sphingosine to sphingosine-1-phosphate, the expression level of which was reduced at E16.5 in the mutant epidermis (Figure 2g). No significant changes in the relative mRNA levels of several other enzymes involved in CER synthesis pathways were observed between wild-type and Ctip2−/− embryonic skin (Supplementary Figure S3a–c online). Furthermore, protein levels of Lass2, Gba2, and epidermis-type lipoxygenase 3 (eLox3) were significantly reduced in Ctip2-null skin compared with wild-type skin at E18.5 (Figure 3a and b) (
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      ). Taken together, our results indicate that the expression of a subset of genes encoding enzymes involved in the sphingolipid biosynthesis pathways was altered during skin organogenesis in the absence of Ctip2.
      Figure thumbnail gr2
      Figure 2COUP-TF-interacting protein 2 (Ctip2) regulates the expression of genes encoding lipid-metabolizing enzymes during skin development. Expression of (ac) ceramide synthases (Lass1–Lass3, Lass6), serine palmitoyltransferases (Sptlc1, Sptlc3); (df) sphingomyeinases (Smpd1, Smpd2), sphingomyelin synthase 2 (Sgms2), glucosylceramide synthase (Ugcg), and acid beta-glucosidase (Gba2); (gi) N-acylsphingosine amidohydrolases (acid and neutral ceramidases: Asah1, Asah2), sphingosine kinases (Sphk1, Sphk2), and alkaline ceramidases (Acer1, Acer2) was analyzed by quantitative real-time reverse-transcriptase PCR using specific primers indicated in online. All values represent the relative transcript level after normalization with Gapdh transcripts. Statistical analyses were performed by Student’s unpaired t-test using Graphpad Prism software. *P<0.05; **P<0.005. Data were reported as mean±SEM (n=9). The results represent three separate experiments conducted in triplicate. E, embryonic day.
      Figure thumbnail gr3
      Figure 3Immunoblot analyses of lipid-metabolizing enzymes in the developing murine epidermis. (a) Western blot analyses were performed on protein extracted from embryonic day 18.5 wild-type (WT) and Ctip2−/− mouse epidermis to study the expression levels of eLox3, Gba2, and Lass2 using specific antibodies. β-Actin was used as an internal control. (b) Protein levels of eLox3, Gba2, and Lass2 were quantified in wild-type and mutant epidermis by densitometry analyses of western blots. All data were normalized by the corresponding β-actin levels. Statistical analyses were performed by Student’s unpaired t-test using Graphpad Prism software. *P<0.05. Data were reported as mean±SEM (n=9). The results represent three separate experiments conducted in triplicate. KO, knockout.
      To investigate whether some of the lipid-metabolizing genes, which are dysregulated in mutant skin, are direct targets of transcriptional regulator Ctip2, we conducted in vivo ChIP assay on neonatal (P0) mouse epidermal keratinocyte extracts. The promoter regions of several genes encoding lipid-metabolizing enzymes such as Lass1-3, 6, Sptlc1-3, Smpd1-2, Ugcg, Gba2, and eLox3 (
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      ), the expressions of which were altered in Ctip2−/− skin, were selected and primers were designed to determine possible Ctip2 binding within -6kb upstream of transcription starting sites. Our results confirmed that Ctip2 was recruited within the -1kb promoter region of Lass2, Gba2, and eLox3, as well as between the 5 and 6kb promoter region of eLox3 relative to transcription starting sites (Figure 4a–c). However, we did not observe the direct recruitment of Ctip2 to the promoter regions of several other lipid-metabolizing genes (Supplementary Figure S4a–k online). Our results indicate that Lass2, Gba2, and eLox3 are possible direct targets of Ctip2 in the regulation of skin lipid metabolism. All together, the results suggest that Ctip2 directly or indirectly regulates expression of a subset of lipid-metabolizing genes and controls epidermal sphingolipid biosynthesis during skin organogenesis.
      Figure thumbnail gr4
      Figure 4Chromatin immunoprecipitation (ChIP) analyses on murine keratinocytes for lipid-metabolizing genes. ChIP assay was performed on freshly isolated neonatal mouse epidermal keratinocytes using anti-COUP-TF-interacting protein 2 (anti-Ctip2) antibody and the results were analyzed by quantitative reverse transcriptase in real time using primers indicated in online. Rat Gig was used as the control. Ctip2 was recruited to the promoter (relative to transcription start site) regions of (a) Lass2, (b) Gba2, and (c) eLox3. Statistical analyses were performed by Student’s unpaired t-test using Graphpad Prism software. *P<0.05; **P<0.005. The results represent three separate experiments conducted in triplicate. 3′-UTR, untranslated region.

      Discussion

      The establishment of an effective physical barrier protecting the living organism from its surrounding environment is the key function of mammalian skin (
      • Hoffjan S.
      • Stemmler S.
      On the role of the epidermal differentiation complex in ichthyosis vulgaris, atopic dermatitis and psoriasis.
      ). The transport of most substances across the SC takes place through the lipid bilayer, suggesting its essential role in the epidermal barrier function (
      • Jungersted J.M.
      • Hellgren L.I.
      • Jemec G.B.E.
      • et al.
      Lipids and skin barrier function – a clinical perspective.
      ). The major SC lipids, CERs, cholesterol, and fatty acids, have distinct roles in maintaining normal barrier functions (
      • Elias P.M.
      • Menon G.K.
      Structural and lipid biochemical correlates of the epidermal permeability barrier.
      ;
      • Ishikawa J.
      • Narita H.
      • Kondo N.
      • et al.
      Changes in the ceramide profile of atopic dermatitis patients.
      ). CERs and cholesterol were shown to have an influence on (and are influenced by) the integrity of the SC, whereas free fatty acids have a major role in lipid bilayer formation and pH maintenance (
      • Baroni A.
      • Buommino E.
      • De Gregorio V.
      • et al.
      Structure and function of the epidermis related to barrier properties.
      ). We have identified Ctip2 as a key regulator of skin lipid metabolism, which has an important role in the establishment of EPB during development. Ablation of Ctip2 leads to altered lipid composition in the developing mouse epidermis, such as reduced CER at E17.5–P0 as well as increased sphingomyelins at E16.5–E18.5. Ctip2 regulates sphingolipid homeostasis in mouse embryonic skin by modulating the expression of several key enzymes involved in lipid metabolism. Here we demonstrate that the transcription factor Ctip2 is recruited to the promoter regions of a subset of genes (elox3, Lass2, and Gba2) involved in the sphingolipid biosynthesis pathways and could directly regulate their expression (Supplementary Figure S5 online).
      Skin barrier defects are caused not only by altered levels of protein(s) involved in keratinocyte terminal differentiation, cross-linking of cornified envelopes, and formation of intercellular junctions but also by impaired formation and maintenance of skin lipid barrier (
      • Furuse M.
      • Hata M.
      • Furuse K.
      • et al.
      Claudin-based tight junctions are crucial for the mammalian epidermal barrier: a lesson from claudin-1-deficient mice.
      ;
      • Kirschner N.
      • Brandner J.M.
      Barriers and more: functions of tight junction proteins in the skin.
      ). Previous studies have shown that Ctip2 regulates keratinocyte proliferation and late terminal differentiation in a cell-autonomous manner; the ultrastructural analyses of the epidermis have revealed defects in lipid disc formation, loosely packed corneocytes, and disorganized intercellular lamellar body membranes in Ctip2-null mice (
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      and unpublished data). In this study, we have focused on the role of Ctip2 in controlling the epidermal lipid barrier by modulating lipid metabolism. Herein we have applied a mass spectrometry method to systemically detect and profile CERs, sphingomyelins, sphingosines, and sphinganines simultaneously in murine epidermis. As a crucial member of SC lipids, there are few reports on the profiling of skin sphingomyelins. Our present data confirm deregulated levels of both long-chain and very long-chain saturated and unsaturated CERs in the mutant epidermis at different stages of skin development. Along the same line, the overall levels of long-chain and very long-chain saturated and unsaturated sphingomyelins were significantly enhanced in the mutants at all stages analyzed. In the present study, we have specifically looked at CER [NS] containing nonhydroxy fatty acids (N) and sphingosines (S) but were unable to conduct mass spectrometry studies on other subclasses containing (N) and phytosphingosines (P)-[NP]; (N) and 6-hydroxy sphingosines (H)-[NH]; hydroxy fatty acids (A) and sphingosines (S)-[AS]; [AP]; [AH]; ester-linked fatty acids (E), hydroxy fatty acids (O), and (S)-[EOS]; and [EOP] and [EOH] because of the lack of available deuterated CER internal standards for each subclass. Additional analyses for the different subclasses will be performed in future using a specific internal standard for each subclass.
      Our results indicate that the altered expressions of CER synthases 1–6 (Lass 1–6), sphingomyelinases (Smpd1 and Smpd2), glucosylceramide synthase (Ugcg), and that of acid beta-glucosidase (Gba2) at different developmental stages of Ctip2-null skin contribute to an overall decrease in the epidermal CER levels in mutant embryonic skin. It was also shown that several ceramidases involved in CER metabolism (Asah2 and Acer1) are reduced in Ctip2−/− skin. As CER biosynthesis is a complex process regulated by multiple pathways, it is still unclear which pathway contributes most to the reduction of epidermal CER content in the ablation of Ctip2. Lass 1–6 have been reported to be involved in UVB-induced apoptosis and their expression was increased after UVB irradiation in normal human keratinocytes but was reduced in aged skin (
      • Uchida Y.
      • Nardo A.D.
      • Collins V.
      • et al.
      De novo ceramide synthesis participates in the ultraviolet B irradiation-induced apoptosis in undifferentiated cultured human keratinocytes.
      ;
      • Jensen J.-M.
      • Förl M.
      • Winoto-Morbach S.
      • et al.
      Acid and neutral sphingomyelinase, ceramide synthase, and acid ceramidase activities in cutaneous aging.
      ). A recent study has reported that Lass3-mutant mice die after birth because of increased skin barrier defects, characterized by increased transepidermal water loss, loss of ultra-long-chain CERs, and nonfunctional cornified envelopes (
      • Jennemann R.
      • Rabionet M.
      • Gorgas K.
      • et al.
      Loss of ceramide synthase 3 causes lethal skin barrier disruption.
      ). The upregulation of Sgms2 and the reduced expression of Smpd1 and Smpd2 in Ctip2−/− mice may partially account for the increased amount of sphingomyelins observed at later stages in mutant skin (Figure 1j and k). Furthermore, loss of Gba2, the expression of which is altered in the mutant epidermis, has been reported to lead to barrier defects in patients with Gaucher’s disease (
      • Holleran W.M.
      • Ginns E.I.
      • Menon G.K.
      • et al.
      Consequences of beta-glucocerebrosidase deficiency in epidermis. Ultrastructure and permeability barrier alterations in Gaucher disease.
      ). Similarly, eLox3 and its substrate 12R-lipoxygenase (12R-LOX) are involved in the formation of oxidized CERs in mouse epidermis; 12R-LOX-deficient mice die shortly after birth because of severe barrier abnormalities with the absence of protein-bound EOS CER (
      • Zheng Y.
      • Yin H.
      • Boeglin W.E.
      • et al.
      Lipoxygenases mediate the effect of essential fatty acid in skin barrier formation: a proposed role in releasing omega-hydroxyceramide for construction of the corneocyte lipid envelope.
      ). Interestingly, we have reported earlier that eLox3 expression is downregulated in Ctip2−/− embryonic skin (
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      ). Here we have furthered our studies and have shown that Ctip2 could directly regulate the expressions of eLox3, Gba2, and Lass2 in mouse embryonic skin by recruitment to their promoter regions. In addition, the catabolism and metabolism of sphingosines were both downregulated in Ctip2−/− epidermis, which may explain the consistency of its amount between wild-type and null mouse epidermis. All together, we have identified Ctip2 as a key regulator of several lipid-metabolizing genes and hence of epidermal sphingolipid biosynthesis during skin development.
      Sphingolipids are key components in skin barrier homeostasis, as well as in other cellular processes such as cell cycle arrest, apoptosis, and stress responses (
      • Hannun Y.A.
      Functions of Ceramide in Coordinating Cellular Responses to Stress.
      ;
      • Ogretmen B.
      • Hannun Y.A.
      Biologically active sphingolipids in cancer pathogenesis and treatment.
      ). CER is a central metabolite, whose biosynthesis is regulated at multiple levels with spatial separation of enzymes involved in CER formation (
      • Futerman A.H.
      • Riezman H.
      The ins and outs of sphingolipid synthesis.
      ;
      • Kitatani K.
      • Idkowiak-Baldys J.
      • Hannun Y.A.
      The sphingolipid salvage pathway in ceramide metabolism and signaling.
      ). It remains unclear whether alteration of the enzymes in one of the pathways could have an effect on other pathways and finally modulate the biological functions. It would be interesting to systemically compare the lipid composition in healthy subjects with that of patients with skin disorders, such as atopic dermatitis and psoriasis. It is possible that, besides Ctip2, other regulatory pathways are also involved in controlling CER biosynthesis. B-cell lymphoma/leukemia 11A (Bcl11a/Ctip1) protein is a homolog of Ctip2, which was found to be upregulated in Ctip2-null mice at E18.5 (
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      ). The role of Ctip1 in epidermal barrier development and maintenance is still unclear. Further investigation on the expression pattern and distribution of Ctip1, as well as its role in lipid barrier homeostasis in mouse epidermis, will be performed using Ctip1−/− mice.

      Materials and Methods

      Mice

      The floxed Citp2 allele was generated by flanking exon 4 of the Ctip2 locus, which encodes for three-fourths of the open reading frame (
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      ). Ctip2-null (Ctip2−/−) mice were generated by breeding mice containing the floxed Ctip2 allele with cytomegalovirus promoter–driven Cre recombinase transgenic mice (
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      ). OSU IACUC approval was obtained for animal experiments.

      Lipid extraction

      Fetus from E16.5 to E18.5 and neonatal mice were collected and genotyped (
      • Golonzhka O.
      • Liang X.
      • Messaddeq N.
      • et al.
      Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation.
      ). A minimum of three wild-type and Ctip2−/− fetuses were taken from the same litter and compared between littermates. A minimum of three litters were analyzed from each developmental stages. Skin was isolated, weighed, and incubated overnight in CnT-07 medium (CELLnTEC, Bern, Switzerland) containing 1mgml−1 dispase (Gibco, Grand Island, NY) at 4°C to dissociate the epidermis.
      Raw skin lipid was extracted using the Bligh and Dyer method (
      • Folch J.
      • Lees M.
      • Sloane Stanley G.H.
      A simple method for the isolation and purification of total lipides from animal tissues.
      ;
      • Bligh E.G.
      • Dyer W.J.
      A rapid method of total lipid extraction and purification.
      ). Briefly, skin epidermis was incubated in extraction solvent (chloroform:methanol:water (1:2:0.8), 10–15mg tissue per 1ml extraction solvent) overnight at room temperature. Fresh extraction solvent was added to epidermis samples on the second day and the old extraction solvent was saved. Samples were sonicated for five cycles, for 30seconds each, and the suspension was shaken for 30minutes at room temperature. After centrifugation at 2,000r.p.m. for 10minutes, the pellets were discarded and the extraction solvents were combined. Chloroform and water were added to the extraction solvent at an extraction solvent:chloroform:water ratio of 7.6:2:2. After mixing and centrifuging, the upper phase was discarded and the lower phase (chloroform) was washed twice with chloroform:methanol:water at a ratio of 1:1:0.9. The lower phase was dried in nitrogen, weighed, and stored at -20°C.

      Liquid chromatography–tandem mass spectrometry

      Raw skin lipid was further purified as in previous reports (
      • Merrilljr A.
      • Sullards M.
      • Allegood J.
      • et al.
      Sphingolipidomics: high-throughput, structure-specific, and quantitative analysis of sphingolipids by liquid chromatography tandem mass spectrometry.
      ;
      • Monette J.S.
      • Gómez L.A.
      • Moreau R.F.
      • et al.
      (R)-α-Lipoic acid treatment restores ceramide balance in aging rat cardiac mitochondria.
      ). A volume of 10μl of sphingolipid internal standard mixture (Avanti, Alabaster, Alabama) and 1mg of raw lipid were added to 1.5ml of chloroform:methanol (1:2) and sonicated in the bath sonicator for 30seconds. After 2hours of incubation at 48°C, the samples were cooled on ice. Samples were then incubated for 1hour at 37°C with 75μl of 1M KOH in methanol to remove the free fatty acids and triacylglycerol and cooled on ice. A volume of 12μl of glacial acetic acid was added to neutralize the pH. Phase separation was achieved by adding 2ml of chloroform and 4ml of water, and samples were gently vortexed. After centrifugation for 15minutes at 2000, r.c.f., the chloroform layer was aspirated and dried under nitrogen. The samples were reconstituted in 200μl of chloroform:methanol (3:1) and diluted 1:4 with acetonitrile:methanol:acetic acid (97:2:1) with 5mM ammonium acetate.
      LC/MS/MS was performed and relative quantification was based on skin weight and sphingolipid standards (
      • Monette J.S.
      • Gómez L.A.
      • Moreau R.F.
      • et al.
      (R)-α-Lipoic acid treatment restores ceramide balance in aging rat cardiac mitochondria.
      ). In brief, lipids were separated by HPLC using a Supelco Discovery column (2mm × 50mm; Sigma-Aldrich, St Louis, MD) at a flow rate of 300μlmin−1. Mobile phase A consisting of methanol:water (60:40) and mobile phase B consisting of methanol:chloroform (60:40) both contained 0.2% (v/v) formic acid and 10mM ammonium acetate. The column was pre-equilibrated at 100% phase A and 5-μl sample was injected; 100% mobile phase A was maintained for 1min, followed by a linear increase to 40% mobile phase B in 7minutes. This was followed by a linear increase to 70% mobile phase B in the next 6minutes, which was maintained for the remainder (6minutes) of the 20minute run.
      Sample detection was performed using a triple-quadrupole mass spectrometer operated in positive mode (Applied Biosystems/MDS Sciex, Carlsbad, CA, API300) with multiple reaction monitoring. The results represent three separate experiments conducted in triplicate.

      Ultrahigh-pressure liquid chromatography/MS/MS

      Ultrahigh-pressure liquid chromatography was performed on a Shimadzu Nexera system (Shimadzu, Columbia, MD) coupled with a hybrid quadrupole time-of-flight mass spectrometer (TripleTOF 5600, Applied Biosystems/MDS Sciex) with raw skin lipids. Chromatographic separations were carried out on a Brownlee Analytical C18 column (50 × 2.1mm, 1.9μm; PerkinElmer, Boston, MA). The flow rate was 0.35mlmin−1 and mobile phases consisted of water (A) and acetonitrile (B), both with 0.1% formic acid. The elution gradient was as follows: 0minutes, 25% B; 2minutes, 55% B; 7minutes, 100% B; and 9minutes, 100% B. The column was equilibrated 3minutes before each injection. Column temperature was held at 40°C and the injection volume was 3μl.
      Mass spectrometry was performed on an AB SCIEX TripleTOF 5600 equipped with an electrospray ionization source. The instrument was operated in information-dependent MS/MS acquisition mode with the collision energy set at 30V and a collision energy spread of 10V. TOF MS acquisition time was 0.15seconds and MS/MS acquisition time was 0.10seconds. Scan range was 70–1000m/z for TOF MS and 40–1000m/z for MS/MS. Ion source gas 1 and 2 and curtain gas (all nitrogen) were set at 50 and 40 and 25, respectively. Source temperature was set at 500°C and IonSpray voltage at 5.5/-5.5kV. A set of standards was run periodically throughout the batch to assess system variance and stability. The instrument was automatically calibrated every five runs.
      Stearic acid, palmitic acid, cholesterol, and cholesterol-3-sulfate were identified using authentic standards with accurate mass measurements, isotope distribution, MS/MS fragmentation, and retention time. The results represent three separate experiments conducted in triplicate.

      Absolute quantification

      Absolute quantification of epidermal CERs, sphingomyelin, sphingosine, and sphinganine was on the basis of comparison with a synthetic sphingolipid standard mixture as previously described (
      • Monette J.S.
      • Gómez L.A.
      • Moreau R.F.
      • et al.
      Characteristics of the rat cardiac sphingolipid pool in two mitochondrial subpopulations.
      ). In brief, 10 synthetic compounds were obtained (Avanti): sphinganine C18 (860498P), sphingosine C18 (860490P), CER C16:0 (860516P), CER C22:0 (860510P), CER C18:1 (860519C), CER C24:1 (860525C), sphingomyelin C18:0 (860586P), sphingomyelin C18:1 (860587C), sphingomyelin C24:0 (860592P), and sphingomyelin C24:1 (860593P). Standard solutions were prepared, and the amount of analyte was determined on the basis of comparison of the peak area of the analyte with that of the sphingolipid. The results represent three separate experiments conducted in triplicate.

      Quantitative real-time reverse-transcriptase PCR

      RNA was extracted and cDNA was synthesized from the skin of mouse fetuses and neonates (
      • Indra A.K.
      • Mohan W.S.
      • Frontini M.
      • et al.
      TAF10 is required for the establishment of skin barrier function in foetal, but not in adult mouse epidermis.
      ;
      • Wang Z.
      • Coleman D.J.
      • Bajaj G.
      • et al.
      RXRα ablation in epidermal keratinocytes enhances UVR-induced DNA damage, apoptosis, and proliferation of keratinocytes and melanocytes.
      ). Real-time PCR was performed on an ABI 7500 Real-Time PCR system using SYBR green methodology (Applied Biosystems) (
      • Indra A.K.
      • Dupe V.
      • Bornert J.M.
      • et al.
      Temporally controlled targeted somatic mutagenesis in embryonic surface ectoderm and fetal epidermal keratinocytes unveils two distinct developmental functions of BRG1 in limb morphogenesis and skin barrier formation.
      ,
      • Indra A.K.
      • Mohan W.S.
      • Frontini M.
      • et al.
      TAF10 is required for the establishment of skin barrier function in foetal, but not in adult mouse epidermis.
      ). Relative gene expression analysis of the qRT-PCR data was performed using Gapdh as an internal control, and the delta-delta CT method was used for quantification as previously described (
      • Wang Z.
      • Coleman D.J.
      • Bajaj G.
      • et al.
      RXRα ablation in epidermal keratinocytes enhances UVR-induced DNA damage, apoptosis, and proliferation of keratinocytes and melanocytes.
      ). All qRT-PCR primers are indicated in Supplementary Table S1 online. The results represent three separate experiments conducted in triplicate.

      Western blotting

      Protein was extracted from mouse epidermis, and western blotting was performed according to previous reports using specific antibodies against Lass2 (1:500), Gba2 (1:1, 000), and eLox3 (1:2, 000) (
      • Wang Z.
      • Coleman D.J.
      • Bajaj G.
      • et al.
      RXRα ablation in epidermal keratinocytes enhances UVR-induced DNA damage, apoptosis, and proliferation of keratinocytes and melanocytes.
      ; Santa Cruz Biotechnology, Santa Cruz, CA). Western blotting represents the results from three separate experiments conducted in triplicate.

      Chromatin immunoprecipitation

      ChIP assay was performed according to
      • Hyter S.
      • Bajaj G.
      • Liang X.
      • et al.
      Loss of nuclear receptor RXRα in epidermal keratinocytes promotes the formation of Cdk4-activated invasive melanomas.
      . ChIP DNA was amplified using an ABI-7500 Real-time PCR machine with specific primers indicated in Supplementary Table S2 online. The results represent three separate experiments conducted in triplicate.

      Statistics

      Statistical significance of differences between groups was analyzed using GraphPad Prism software (Graphpad Software, La Jolla, CA) using Student’s unpaired t-test. All statistical analyses were performed in a double-blinded manner.
      The following CER code is used according to previous reports (
      • Motta S.
      • Monti M.
      • Sesana S.
      • et al.
      Ceramide composition of the psoriatic scale.
      ;
      • Robson K.J.
      • Stewart M.E.
      • Michelsen S.
      • et al.
      6-Hydroxy-4-sphingenine in human epidermal ceramides.
      ): Ceramide EOS (or ceramide 1), ceramide containing ester-linked fatty acids (E), -hydroxy fatty acids (O), and sphingosines (S); Ceramide NS (or ceramide 2), ceramide containing nonhydroxy fatty acids (N) and sphingosines (S); Ceramide NP (or ceramide 3), ceramide containing nonhydroxy fatty acids (N) and phytosphingosines (P); Ceramide EOH (or ceramide 4), ceramide containing ester-linked fatty acids (E), -hydroxy fatty acids (O), and 6-hydroxy sphingosines (H); Ceramide AS (or ceramide 5), ceramide containing -hydroxy fatty acids (A) and sphingosines (S); Ceramide AP (or ceramide 6), ceramide containing hydroxy fatty acids (A) and phytosphingosines (P); Ceramide AH (or ceramide 7), ceramide containing hydroxy fatty acids (A) and 6-hydroxy sphingosines (H); Ceramide NH, ceramide containing nonhydroxy fatty acids (N) and 6-hydroxy sphingosines (H); Ceramide EOP (or ceramide 9), CER containing ester-linked fatty acids (E), hydroxy fatty acids (O), and phytosphingosines (P).

      ACKNOWLEDGMENTS

      We thank Taifo Mahmud, Phil Proteau, and Dong Li for their help in sample preparation, and Xiaobo Liang for mouse colony maintenance. We also thank Walter Vogel from the College of Pharmacy, the OSU Mass Spectrometry Facility, and the Oxidative/Nitrative Stress Core Laboratory of Linus Pauling Institute for technical support in mass spectrometry analyses. We acknowledge Tory Hagen and Liam Finlay for providing two of the ceramide standards and their advice in standard solution preparation. These studies were supported by grant AR056008-03 (AI) from the National Institutes of Health, OHSU Medical Research Foundation grant to AI, and NIEHS center grant (ES00210) to Environmental Health Sciences Center, Oregon State University. We also thank Mark Zabriskie and Gary DeLander of College of Pharmacy, Oregon State University, for their continuous support and encouragement.

      SUPPLEMENTARY MATERIAL

      Supplementary material is linked to the online version of the paper at http://www.nature.com/jid

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