J. Lipid Res. Acyl Labeled PIP's available August 1, 2008
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Originally published In Press as doi:10.1194/jlr.M500237-JLR200 on August 16, 2005

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
M500237-JLR200v1
46/11/2432    most recent
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Li, Y.
Right arrow Articles by Nakamura, M. T.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Li, Y.
Right arrow Articles by Nakamura, M. T.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati  
What's this?
Journal of Lipid Research, Vol. 46, 2432-2440, November 2005
Copyright © 2005 by American Society for Biochemistry and Molecular Biology

Peroxisome proliferator-activated receptor {alpha} is required for feedback regulation of highly unsaturated fatty acid synthesis1

Yue Li, Takayuki Y. Nara and Manabu T. Nakamura2

Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, IL 61801

1 Part of the work was presented at the Experimental Biology Conference [Li, Y., T.Y. Nara, and M.T. Nakamura. 2004. Regulation of highly unsaturated fatty acid synthesis: a new physiological role of peroxisome proliferator-activated receptor alpha (abstract). FASEB J. 18: A863] and cited in reference 1. Back

Published, JLR Papers in Press, August 16, 2005. DOI 10.1194/jlr.M500237-JLR200

2 To whom correspondence should be addressed. e-mail: mtnakamu{at}uiuc.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
{Delta}6 desaturase (D6D), the rate-limiting enzyme for highly unsaturated fatty acid (HUFA) synthesis, is induced by essential fatty acid-deficient diets. Sterol regulatory element-binding protein-1c (SREBP-1c) in part mediates this induction. Paradoxically, D6D is also induced by ligands of peroxisome proliferator-activated receptor {alpha} (PPAR{alpha}). Here, we report a novel physiological role of PPAR{alpha} in the induction of genes specific for HUFA synthesis by essential fatty acid-deficient diets. D6D mRNA induction by essential fatty acid-deficient diets in wild-type mice was diminished in PPAR{alpha}-null mice. This impaired D6D induction in PPAR{alpha}-null mice was not attributable to feedback suppression by tissue HUFAs because PPAR{alpha}-null mice had lower HUFAs in liver phospholipids than did wild-type mice. Furthermore, PPAR{alpha}-responsive genes were induced in wild-type mice under essential fatty acid deficiency, suggesting the generation of endogenous PPAR{alpha} ligand(s). Contrary to genes for HUFA synthesis, the induction of other lipogenic genes under essential fatty acid deficiency was higher in PPAR{alpha}-null mice than in wild-type mice even though mature SREBP-1c protein did not differ between the genotypes. The expression of PPAR{gamma} was markedly increased in PPAR{alpha}-null mice and might have contributed to the induction of genes for de novo lipogenesis.

Our study suggests that PPAR{alpha}, together with SREBP-1c, senses HUFA status and confers pathway-specific induction of HUFA synthesis by essential fatty acid-deficient diets.

Abbreviations: ACBP, acyl-coenzyme A binding protein; AOX, acyl-coenzyme A oxidase; CYP, cytochrome P450; D5D, {Delta}5 desaturase; D6D, {Delta}6 desaturase; HUFA, highly unsaturated fatty acid; PK, pyruvate kinase; PPAR, peroxisome proliferator-activated receptor; PPRE, peroxisome proliferator response element; SCD, stearoyl-coenzyme A desaturase; SREBP, sterol regulatory element-binding protein; TRB3, mammalian tribbles homolog

Supplementary key words {Delta}6 desaturase • arachidonic acid • docosahexaenoic acid • essential fat deficiency • liver • peroxisome proliferator-activated receptor {alpha}-null mouse • polyunsaturated fatty acid • sterol regulatory element-binding protein-1c


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In mammals, highly unsaturated fatty acids (HUFAs) such as arachidonic acid (20:4 n-6) and docosahexaenoic acid (22:6 n-3) are present in membrane phospholipids and are required for many physiological functions, including eicosanoid signaling, skin and hair integrity, vision and brain functions, cardioprotection, and the regulation of gene expression (1). Tissue HUFAs are maintained in a narrow concentration range to perform these functions. Although mammals are unable to synthesize HUFAs from acetyl-CoA, they are capable of synthesizing HUFAs from precursor PUFA, linoleic (18:2 n-6) and {alpha}-linolenic (18:3 n-3) acids (1). {Delta}6 desaturase (D6D) catalyzes the first and rate-limiting reaction of HUFA synthesis (2) and thus is subject to regulation by dietary fatty acids. D6D is induced when animals are fed a diet devoid of all n-6 and n-3 fatty acids (an essential fat-deficient diet), whereas D6D is suppressed by diets containing either a substrate or a product, the latter exerting stronger suppression (3, 4).

The underlying mechanism of this D6D regulation began emerging only recently. The enzyme activity of D6D largely parallels the expression of mRNA (4), which is regulated mainly at the transcriptional level (5). Two transcription factors, sterol regulatory element-binding protein-1c (SREBP-1c) and peroxisome proliferator-activated receptor {alpha} (PPAR{alpha}), are involved in the regulation of the D6D gene. SREBP-1c, a transcription factor of the basic-helix-loop-helix-leucine-zipper family, induces a set of genes for fatty acid and glycerolipid synthesis (6), including all three mammalian desaturases: stearoyl-CoA desaturase (SCD) for monounsaturated fatty acid synthesis (7, 8) and D6D and {Delta}5 desaturase (D5D) for HUFA synthesis (9, 10). SREBP-1c is synthesized as a precursor protein that is inserted into the endoplasmic reticulum membrane. Proteolytic cleavage is required to produce mature SREBP-1c protein, which then enters the nuclei and activates target genes (11). Dietary PUFAs suppress the induction of lipogenic genes in liver, whereas essential fat-deficient diets such as fat-free diet and triolein diet induce these genes (12, 13). SREBP-1c was identified as the factor that at least in part mediates this PUFA effect by binding the PUFA response sequence in promoters of FAS (14), S-14 (15), SCD (7, 16), and D6D (10). Dietary PUFAs exert their effects by reducing both mRNA and proteolytic activation of SREBP-1c (14, 17, 18). However, the mechanism of differential regulation among these SREBP-1c target genes has not been well characterized.

PPAR{alpha}, a member of the nuclear receptor family, induces genes involved with mitochondrial and peroxisomal fatty acid ß-oxidation as well as ketogenesis in liver (19, 20). Studies using targeted disruption of the PPAR{alpha} gene have demonstrated that PPAR{alpha} is essential for the upregulation of genes for fatty acid oxidation during fasting (21–23). Various long-chain fatty acids and hypolipidemic drugs called peroxisome proliferators bind and activate PPAR{alpha} as agonistic ligands (24, 25). Although SREBP-1c and PPAR{alpha} induce almost mutually exclusive sets of genes, three mammalian desaturases, SCD-1, D6D, and D5D, are induced not only by essential fat deficiency but also paradoxically by peroxisome proliferators in rats (26–28) and to a lesser extent in pigs (29). In rodents, peroxisome proliferators cause peroxisome proliferation and hepatomegaly in addition to the induction of fatty acid oxidation enzymes (20, 30). Thus, increased degradation of HUFAs and demand for membrane phospholipids may at least in part account for the marked induction of D6D by peroxisome proliferators. Consistent with this hypothesis, the induction of desaturases by peroxisome proliferator was slower than the induction of fatty acid oxidation enzymes (27, 28). In addition to the possible indirect induction, peroxisome proliferators can also directly, albeit modestly, induce SCD-1 (27) and D6D (5) in cell culture through peroxisome proliferator response element (PPRE) present in the promoters of these genes. The physiological role of PPRE in desaturases is unclear at present. As Matsuzaka et al. (9) implied, a possible function of PPAR{alpha} could be to maintain HUFA synthesis in fasting conditions, when SREBP-1c is drastically decreased (31). It has yet to be determined whether PPAR{alpha} has a previously unidentified essential function in the feedback regulation of HUFA synthesis.

Thus, the objectives of this study were to elucidate an in vivo regulatory role of PPAR{alpha} in 1) HUFA synthesis under essential fat deficiency and 2) differential regulation between genes for HUFA synthesis and genes involved with other lipid metabolism.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals and diets
Breeding pairs of PPAR{alpha}-null mice (129S4/SvJae-Pparatm1Gonz) (32) and wild-type mice (129S4/SvJae) were purchased from Jackson Laboratory (Bar Harbor, ME). Eight week old male mice were housed in individual cages and maintained at 24°C with a 12 h light/dark cycle and provided free access to food during the dark cycle. Before treatment, mice were fed a standard diet, AIN93G (33), for 1 week. Then, animals were divided into five groups (n = 5–6) and fed the following diets for 1 week: control (AIN93G, 7% soybean oil; fed), fasting (AIN93G; fasted), fat-free (soybean oil was replaced with isocaloric anhydrous D-glucose; fed), triolein [soybean oil was replaced with an equal amount of triolein, 99% purity (Sigma Chemical Co., St. Louis, MO); fed], or Wy [AIN93G containing 0.1% Wy14643 (Chemsyn Science Laboratories, Lenexa, KS); fed]. All animals except the fasting group were euthanized at the end of the 12 h meal period. Mice in the fasting group were euthanized after 24 h of fasting. For SREBP-1c protein analysis, liver was processed immediately. For the analyses of RNA and fatty acids in phospholipids, liver was frozen in liquid nitrogen and stored at –80°C. For the analysis of nonesterified fatty acids in liver, blood was expelled from liver by perfusing liver with phosphate-buffered saline through the portal vein, and then liver was immediately frozen in liquid nitrogen. The animal protocol was reviewed and approved by the Institutional Animal Care and Use Committee.

RNA analysis
A detailed procedure has been described elsewhere (29). Briefly, total RNA was prepared using TRIzol reagent (Invitrogen, Carlsbad, CA) and was reverse-transcribed using random hexamers. The mRNA was analyzed by real-time quantitative PCR using SYBR Green fluorescence dye (Applied Biosystems, Foster City, CA). The abundance of mRNA was expressed relative to 18S rRNA.

SREBP-1c protein analysis
Microsomal and nuclear extracts of mouse liver were prepared with a slight modification of the method of Sheng et al. (34). Fresh liver of individual mice was homogenated, and nuclear protein was extracted. Aliquots of liver homogenate for nuclear protein extraction were also used for microsomal protein preparation. Nuclear and microsomal protein was quantitated using the BCA protein assay kit (Pierce Biotechnology, Inc., Rockford, IL). Thirty micrograms of each protein was loaded for Western blotting. Precursor and mature protein were detected by monoclonal anti-SREBP-1 antibody (IgG-2A4) (ATCC, Manassas, VA).

Fatty acid analysis
This procedure was described previously (35). Briefly, total liver lipids were extracted using chloroform-methanol (2:1, v/v). To prevent hydrolysis of fatty acid esters, a piece of frozen liver was homogenized without thawing in the extraction solvent using Polytron PT1200C (Kinematica). C17:0 phosphatidylcholine and free fatty acids were added as internal standards at the extraction step. The extract was separated by thin-layer chromatography into phospholipids and free fatty acids, which then were methylated using methanolic HCl (Supelco, Bellefonte, PA). Each fatty acid species was quantified by HP5890 gas chromatography (Agilent Technologies, Wilmington, DE) with a 30 m x 0.25 mm PAG capillary column (Supelco).

Statistical analysis
Statistical analysis was performed using a commercial software package (Statview 5.0.1; SAS Institute, Cary, NC). Student's t-test was used for comparisons between two groups. One-way ANOVA followed by Fisher's protected least-squares difference posthoc test were used for comparisons among multiple treatment groups. Two-way ANOVA was used to determine genotype and diet effects. P < 0.05 was considered statistically significant.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
D6D mRNA induction by essential fat-deficient diets was diminished in PPAR{alpha}-null mice
Both fat-free and triolein diets were essential fat-deficient and were devoid of both n-6 and n-3 fatty acids. The amount of dietary fat in the triolein diet was kept the same as in the control diet, although all fat was provided as nonessential fat (18:1 n-9). There was no significant difference in body weight between wild-type and PPAR{alpha}-null mice. Animals fasted for 24 h weighed 10% less than mice fed the control diet in both genotypes (data not shown). The liver weight of fasted animals was significantly lower than that of the control group (Table 1). The liver weight of other diet groups was unchanged with the exception that the wild-type mice fed Wy14643, a PPAR{alpha} ligand, showed a marked increase in liver weight as a consequence of peroxisome proliferation in these animals (Table 1). Morphological changes in liver, including fatty liver, were not observed in any of the groups other than liver enlargement in the wild-type group fed Wy14643.


View this table:
[in this window]
[in a new window]
  		TABLE 1. Liver weight of wild-type and PPAR{alpha}-null mice fed various diets for 1 week

 
We measured the expression of D6D in the liver of wild-type and PPAR{alpha}-null mice fed essential fat-deficient diets to determine whether PPAR{alpha} is required for D6D induction under essential fat deficiency. Compared with the control diet, D6D expression in wild-type mice was significantly increased by fat-free and triolein diets (Fig. 1A). In contrast, the induction of D6D by fat-free and triolein diets was diminished in PPAR{alpha}-null mice, indicating that PPAR{alpha} is required for full induction of D6D in essential fat deficiency. Wy14643 induced D6D in wild-type mice, whereas D6D induction as well as hepatomegaly were abolished in PPAR{alpha}-null mice (Table 1, Fig. 1A), an observation consistent with a study by Guillou et al. (36), who reported failed induction of D6D by fenofibrate in PPAR{alpha}-null mice. A similar pattern was observed in the expression of D5D, another desaturase in the HUFA synthesis pathway. The D5D mRNA was increased in the fat-free- and triolein-fed wild-type mice (Fig. 1B), whereas no induction of D5D by essential fat deficiency occurred in PPAR{alpha}-null mice. In contrast to rats (26, 28), D5D in mice was not significantly induced by peroxisome proliferators in the present study (Fig. 1B), consistent with other results (36), suggesting possible species differences. This low response could be attributable to PPAR{alpha}-independent high basal expression of D5D mRNA in mice.



View larger version (20K):
[in this window]
[in a new window]
  		Fig. 1. Peroxisome proliferator-activated receptor {alpha} (PPAR{alpha})-null mice were unable to fully induce desaturases for highly unsaturated fatty acid (HUFA) synthesis in liver under essential fat-deficient conditions. Wild-type and PPAR{alpha}-null mice were fed control, essential fat-deficient (fat-free and triolein), or Wy (control + 0.1% Wy14643) diets for 1 week. The fasting group was fed the control diet for 1 week and then fasted for 24 h before euthanasia. The abundance of mRNA was measured by quantitative PCR and expressed as fold increase over the mean value of the control group. Error bars show SD from four to five animals. Groups bearing different letters differ significantly (P < 0.05) by Fisher's protected least-squares difference (PLSD) test. Open bars, wild-type mice; closed bars, PPAR{alpha}-null mice. A: {Delta}6 desaturase (D6D). B: {Delta}5 desaturase (D5D). C: Stearoyl-CoA desaturase-1 (SCD-1). D: FAS. E: Liver-type pyruvate kinase (PK). F: Acyl-CoA binding protein (ACBP).

 
We also quantified the expression of other mRNAs encoding lipogenic enzymes that are involved in nonessential unsaturated fatty acid synthesis but not in HUFA synthesis. SCD-1, which catalyzes monounsaturated fatty acid synthesis, was induced by essential fat-deficient diets as well as by Wy14643 in wild-type mice (Fig. 1C). In PPAR{alpha}-null mice, SCD-1 induction by Wy14643 was abolished, whereas in contrast to D6D and D5D (Fig. 1A, B), SCD-1 was induced even more in PPAR{alpha}-null mice than in wild-type mice by essential fat-deficient diets (Fig. 1C). Induction of FAS showed a similar pattern to SCD-1 except for the lack of induction by Wy14643 in wild-type mice (Fig. 1D). Liver-type pyruvate kinase (PK) shared the same induction trend with FAS, but PK was highly induced in PPAR{alpha}-null mice on all diets, suggesting increased glucose utilization as a fuel source in PPAR{alpha}-null mice (Fig. 1E). The mRNA analysis has demonstrated that PPAR{alpha} is required specifically for the induction of genes for HUFA synthesis, and not for other lipogenic genes under essential fat deficiency.

Recently, the acyl-CoA binding protein (ACBP) gene was reported to be induced by both PPAR{alpha} and SREBP-1c (37). Consistent with that study, ACBP expression by Wy14643 was lower in PPAR{alpha}-null mice than in wild-type mice (Fig. 1F). However, in contrast to the D6D and D5D genes, essential fat-deficient diets had no effect on ACBP in either wild-type or PPAR{alpha}-null mice (Fig. 1F).

PPAR{alpha}-null mice had less HUFA in liver phospholipids than did wild-type mice under essential fat deficiency
Because HUFAs are primarily degraded in peroxisomes (38), targeted disruption of the PPAR{alpha} gene may impair HUFA oxidation, resulting in HUFA accumulation. If this were the case, the lack of D6D induction by essential fat-deficient diets could be attributable to the presence of HUFAs above the normal level in PPAR{alpha}-null mice. Thus, we next examined the amount of HUFAs in liver phospholipids, in which the majority of HUFAs are incorporated. As shown in Table 2, the major products of the D6D pathway, 20:4 n-6 and 22:6 n-3, were significantly lower in PPAR{alpha}-null mice than in wild-type mice, indicating that HUFA composition in liver does not account for the impaired induction of D6D in PPAR{alpha}-null mice. Moreover, low HUFA levels in PPAR{alpha}-null mice further support an essential role of PPAR{alpha} in the induction of synthesis to maintain tissue HUFAs. One week of feeding essential fat-deficient diets led to a significant reduction of 22:6 n-3, whereas 20:4 n-6 was not yet significantly reduced.


View this table:
[in this window]
[in a new window]
  		TABLE 2. Fatty acids in liver phospholipids

 
SREBP-1c nuclear protein was not different between genotypes under essential fat deficiency
Both mRNA and mature protein of SREBP-1c are increased by essential fat-deficient diets and are suppressed by dietary HUFAs (14); thus, SREBP-1c contributes to the feedback regulation of the D6D gene by HUFAs through the sterol regulatory element in the D6D promoter (10). We determined by Western blot analysis whether disruption of the PPAR{alpha} gene impairs the activation of SREBP-1c under essential fat deficiency. The results revealed that there was no difference of SREBP-1c mature protein between the genotypes (Fig. 2A, B), demonstrating that the lack of D6D induction in PPAR{alpha}-null mice is not mediated by SREBP-1c. Furthermore, although SREBP-1c mature protein was increased equally by fat-free diets in both wild-type and PPAR{alpha}-null mice (Fig. 2), D6D was not induced in PPAR{alpha}-null mice (Fig. 1A), indicating that SREBP-1c protein alone is not sufficient, and PPAR{alpha} must be present, to induce D6D.



View larger version (32K):
[in this window]
[in a new window]
  		Fig. 2. Sterol regulatory element-binding protein-1c (SREBP-1c) nuclear protein amount was unaffected by disruption of the PPAR{alpha} gene in mouse liver. The animals were treated as described for Fig. 1. A: A representative Western blot (pooled samples of five mice) of SREBP-1c precursor protein (upper panel) and mature protein in the nucleus (lower panel). B: Quantitation of SREBP-1c mature protein. The abundance of protein was quantified using Western blots of individual mice and expressed as fold increase over the mean value of the control group. Open bars, wild-type mice; closed bars, PPAR{alpha}-null mice. Error bars show SD from five animals per group. Groups bearing different letters differ significantly (P < 0.05) by Fisher's PLSD test.

 
PPAR{alpha}-responsive genes were induced by essential fat deficiency
Because our results revealed the requirement of PPAR{alpha} in D6D induction under essential fat deficiency, we reasoned that essential fat-deficient diets would activate PPAR{alpha} and induce other PPAR{alpha}-responsive genes. As shown in Table 3, mRNAs of acyl-CoA oxidase (AOX), cytochrome P450 4A10, and L-bifunctional protein were indeed small but significantly induced when wild-type mice were fed essential fat-deficient diets. The induction of these PPAR{alpha} target genes suggests that endogenous PPAR{alpha} ligands are generated under essential fat deficiency.


View this table:
[in this window]
[in a new window]
  		TABLE 3. Induction of PPAR{alpha}-responsive genes by essential fat-deficient diets in liver of wild-type mice

 
Nonesterified fatty acids were decreased by the fat-free diet in wild-type mice
Nonesterified fatty acids are considered natural ligands of PPAR{alpha} because they bind and activate PPAR{alpha} in vitro (24, 25). Under prolonged essential fat deficiency, nonessential HUFAs, mead acid (20:3 n-9) in particular, increase in tissues (39). We quantitated nonesterified fatty acids in liver of wild-type mice to determine whether there was an increase in any fatty acid species that could act as a PPAR{alpha} ligand under essential fat deficiency. None of the nonesterified fatty acid species above the detection limit (>1 nmol/g liver) increased in animals fed the fat-free diet (Table 4), and 20:3 n-9 remained below the detection limit in both phospholipid and nonesterified fatty acid fractions after 1 week of feeding the fat-free diet (Tables 2, 4). Instead, total as well as most individual nonesterified fatty acids were decreased by the fat-free diet (Table 4), raising the possibility that the endogenous ligand of PPAR{alpha} under essential fat deficiency may not be a fatty acid.


View this table:
[in this window]
[in a new window]
  		TABLE 4. Nonesterified fatty acids in liver from wild-type mice fed a control or fat-free diet

 
Expression of both PPAR{gamma} and mammalian tribbles homolog was increased in PPAR{alpha}-null mice
We investigated two candidate genes that may be responsible for the increased expression of lipogenic genes in PPAR{alpha}-null mice (Fig. 1C–E). Studies showed that the expression of PPAR{gamma}, another member of the PPAR family, was increased in PPAR{alpha}-null mice (40), and that overexpression of PPAR{gamma} in the liver of PPAR{alpha}-null mice induced lipogenic genes (41). Consistent with these reports, a large increase in PPAR{gamma} mRNA was observed in PPAR{alpha}-null mice in all dietary treatments (Fig. 3A).



View larger version (17K):
[in this window]
[in a new window]
  		Fig. 3. Increased expression of both PPAR{gamma} and mammalian tribbles homolog (TRB3) genes in liver of PPAR{alpha}-null mice. The animals were treated as described for Fig. 1. A: PPAR{gamma} mRNA. B: TRB3 mRNA. Open bars, wild-type mice; closed bars, PPAR{alpha}-null mice. Gene expression was measured by quantitative PCR. The abundance of mRNA was expressed as fold increase over the mean value of the control group. Error bars show SD from four to five animals. Groups bearing different letters differ significantly (P < 0.05) by Fisher's PLSD test.

 
Insulin plays critical roles in the induction of lipogenic genes (42). Another candidate gene, mammalian tribbles homolog (TRB3), counters insulin signaling by inhibiting Akt/protein kinase B (43). Moreover, the expression of TRB3 is reportedly dependent on PPAR{alpha} (44). However, contrary to that report (44), TRB3 mRNA was higher in PPAR{alpha}-null mice than in wild-type mice (Fig. 3B). This result was reproducible with the same primer set used by Koo et al. (44) (data not shown). Our study suggests that the modulation of insulin signaling by TRB3 is not the mechanism that causes the high induction of SCD-1, FAS, and PK in PPAR{alpha}-null mice.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
PPAR{alpha} has been shown to play a key role in the metabolic adaptation to fasting by inducing fatty acid oxidation pathways (21–23). By using PPAR{alpha}-null mice, we demonstrated that PPAR{alpha} is also required for the induction of D6D and D5D genes under essential fat deficiency (Fig. 1A, B). The requirement of PPAR{alpha} for D6D gene induction is consistent with the presence of functional PPRE in the human D6D promoter (5). The critical role of PPAR{alpha} in HUFA synthesis is further supported by lower total HUFA levels in PPAR{alpha}-null mice than in wild-type mice (Table 2).

Sophisticated transcriptional regulation of genes in multicell organisms can be achieved by the involvement of multiple transcription factors that respond to different signals (45). Simultaneous activation of two transcription factors, SREBP-1c and PPAR{alpha}, is the likely mechanism by which liver can both sense the demand of HUFAs and induce synthesis. This proposed mechanism would instill a specific induction of the genes for HUFA synthesis among large groups of genes targeted by SREBP-1c and PPAR{alpha}. Among the SREBP-1c-targeted genes tested, the presence of PPAR{alpha} was required only for the induction of D6D and D5D under essential fat deficiency (Fig. 1), suggesting that this dependence of D6D and D5D genes on PPAR{alpha} confers pathway-specific regulation. Likewise, the differential regulation of desaturases from ß-oxidation enzymes could be achieved by the presence of sterol regulatory elements in desaturases. Although functional PPRE is present in the promoters of D6D (5) and SCD-1 (27), fasting does not induce desaturases in the fed state (Fig. 1A) (28), whereas fatty acid oxidation enzymes are induced in a PPAR{alpha}-dependent manner during fasting (21–23). Both protein and mRNA of SREBP-1c in liver decrease drastically during fasting and increase upon refeeding (18, 31). Thus, low SREBP-1c would prevent the desaturases from full induction during fasting.

The proposed mechanism of D6D regulation by simultaneous activation of SREBP-1c and PPAR{alpha} could further explain the robust induction of D6D by Wy14643 (Fig. 1A). Because the peroxisome proliferator was administered in the fed state, PPAR{alpha} was activated under the presence of SREBP-1c, resulting in a strong induction of D6D compared with the state in which only either SREBP-1c (Fig. 2, control) or PPAR{alpha} (fasted state) is active. An increased demand of HUFAs attributable to peroxisome proliferation and hepatomegaly (Table 1) by Wy14643 administration could also contribute to the marked induction of D6D (Fig. 1A) (28). However, smaller but significant induction of desaturases by a peroxisome proliferator was observed in pigs without hepatomegaly (29), further supporting a direct role of PPAR{alpha} in desaturase induction. Moreover, differential effects of peroxisome proliferators and HUFAs in fish oil on desaturase expression can also be explained by the proposed requirement of the simultaneous activation of PPAR{alpha} and SREBP-1c for the full induction of D6D. Both peroxisome proliferators and dietary fish oil induce peroxisomal ß-oxidation enzymes (46–49), and the effect of fish oil is also dependent on PPAR{alpha} (49). Thus, excess dietary HUFAs in fish oil are likely to act as PPAR{alpha} ligands in vivo, inducing their own degradation pathway. However, in contrast to peroxisome proliferators, dietary fish oil strongly suppresses D6D expression (4). It is conceivable that the differential effects of fish oil and peroxisome proliferators on D6D are the result of strong inhibition of SREBP-1c activity by fish oil (14, 15), whereas peroxisome proliferators had no effect on SREBP-1c (Fig. 2).

Peroxisomal enzymes were induced under essential fat deficiency (Table 3). This induction may contribute to the upregulation of docosahexaenoic acid synthesis, because the last step of its synthesis, one cycle of ß-oxidation, is performed in peroxisomes (50–52). PPAR{alpha}-dependent induction of PPAR{alpha} target genes (Table 3) suggests the production of endogenous ligands under essential fat deficiency. On the other hand, AOX mRNA in the fat-free group was higher than in the control group even in the absence of PPAR{alpha} (Table 3), suggesting that an unidentified, PPAR{alpha}-independent mechanism may also be present and upregulate AOX in response to low docosahexaenoic acid in PPAR{alpha}-null mice fed an essential fat-deficient diet (Table 2).

Postulated PPAR{alpha} ligands generated under essential fat deficiency have yet to be identified. Decreased nonesterified fatty acids in the liver from the animals fed a fat-free diet (Table 4) suggest that, unlike under fasting, a molecule other than a fatty acid may act as a ligand of PPAR{alpha} under essential fat deficiency. Lysophospholipids represent a candidate for the endogenous ligands of PPAR{alpha} in an essential fat-deficient condition. A recent report has shown that lysophosphatidic acids act as a ligand of PPAR{gamma} (53). Membrane phospholipids undergo a deacylation/reacylation cycle through hydrolysis to PUFAs and 1-acyl 2-lyso phospholipids followed by their reesterification (54). When an essential fat-deficient diet was fed, the most notable change in nonesterified fatty acid fractions was a drastic reduction of precursors (18:2 n-6 and 18:3 n-3) and a product, 20:4 n-6 (Table 4). The marked decrease in nonesterified PUFAs may shift the deacylation/reacylation equilibrium toward increased lysophospholipids, which then may act as a PPAR{alpha} ligand.

Desaturases supply unsaturated fatty acids that are required to maintain the physical properties of biological membranes (1). The present study revealed that total unsaturated fatty acids in liver phospholipids were remarkably well maintained regardless of genotype and diet (Table 2). However, this stability of unsaturation in liver phospholipids does not explain the much higher induction of genes for 18:1 synthesis (SCD-1, FAS, and PK) in PPAR{alpha}-null mice than in wild-type mice when essential fat-deficient diets were fed (Fig. 1C–E). First, despite the large difference in gene expression between genotypes (Fig. 1C–E), there was no difference in 18:1 in phospholipids between genotypes (Table 2). Furthermore, although high 18:1 in phospholipids in the triolein-fed groups (Table 2) suggests a sufficient supply of 18:1 from the diet for phospholipid synthesis, SCD-1 mRNA was much higher in triolein-fed PPAR{alpha}-null mice than in wild-type mice (Fig. 1C). Together, our results suggest that the induction of lipogenic genes in PPAR{alpha}-null mice fed essential fat-deficient diets may be attributable to dysregulation of lipid synthesis rather than to compensatory upregulation.

PPAR{gamma} and TRB3 are candidates that could be responsible for the dysregulation of lipogenesis in PPAR{alpha}-null mice. Although the expression of PPAR{gamma} is low in normal liver, PPAR{gamma} is induced in PPAR{alpha}-null mice (Fig. 3A) (40), and ectopic expression of PPAR{gamma} in liver induces lipogenic genes (41). Thus, the compensatory increase in PPAR{gamma} mRNA in the PPAR{alpha}-null mouse (Fig. 3A) may be responsible for the hyperinduction of lipogenic genes (Fig. 1C–E). On the other hand, TRB3, a proposed insulin signaling inhibitor (43), was increased in PPAR{alpha}-null mice in our study (Fig. 3B), whereas another study reported decreased TRB3 in PPAR{alpha}-null mice (44). The reason for the opposite response of TRB3 in PPAR{alpha}-null mice in the two studies is currently unclear. However, a marked difference in feeding protocols should be noted. Koo et al. (44) fed diets ad libitum, whereas we fed animals for 12 h/day during the dark cycle. Although PPAR{alpha}-null mice fed ad libitum grow normally, they are unable to adapt to fasting (21–23) and lose body weight even under 3 h/day meal feeding (unpublished data). Thus, when PPAR{alpha}-null mice are fed ad libitum, their eating pattern is likely to shift from a dark-cycle eater to an all-day nibbler, resulting in a loss of the diurnal rhythm of lipogenic gene induction seen in wild-type animals (40). Our feeding protocol was designed to minimize the difference in eating patterns between genotypes. Thus, it is possible that TRB3 expression could be regulated in the same manner as lipogenic genes, resulting in the opposite outcome in two different feeding protocols.

In conclusion, this study has demonstrated that 1) PPAR{alpha} is specifically required for the full induction of D6D and D5D genes under essential fat deficiency, but not for other SREBP-1c-targeted genes; 2) PPAR{alpha} is also required to maintain HUFA levels in liver; and 3) essential fat deficiency induces PPAR{alpha}-responsive genes, suggesting the generation of endogenous ligand. In contrast, genes for monounsaturated fatty acid synthesis were increased in PPAR{alpha}-null mice under essential fat deficiency without affecting the abundance of the product in membrane phospholipids. Increased PPAR{gamma} may be involved with the induction of lipogenic genes in PPAR{alpha}-null mice. Simultaneous activation of two transcription factors, SREBP-1c and PPAR{alpha}, is the likely mechanism by which liver can sense the deprivation of dietary essential fatty acids and specifically induce genes for HUFA synthesis.


   ACKNOWLEDGMENTS
 
The authors thank Dr. Marc Montminy of the Salk Institute for Biological Studies (La Jolla, CA) for providing the primer sequence of mTRB3. This study was supported in part by a Scientist Development Grant from the American Heart Association to M.T.N.

Manuscript received June 9, 2005 and in revised form July 29, 2005 and in re-revised form August 4, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
  1. Nakamura, M. T., and T. Y. Nara. 2004. Structure, function, and dietary regulation of delta6, delta5, and delta9 desaturases. Annu. Rev. Nutr. 24: 345–376.[CrossRef][Medline]

  2. Sprecher, H. 1981. Biochemistry of essential fatty acids. Prog. Lipid Res. 20: 13–22.[CrossRef][Medline]

  3. Peluffo, R. O., A. M. Nervi, and R. R. Brenner. 1976. Linoleic acid desaturation activity of liver microsomes of essential fatty acid deficient and sufficient rats. Biochim. Biophys. Acta. 441: 25–31.[Medline]

  4. Cho, H. P., M. T. Nakamura, and S. D. Clarke. 1999. Cloning, expression, and nutritional regulation of the mammalian delta-6 desaturase. J. Biol. Chem. 274: 471–477.[Abstract/Free Full Text]

  5. Tang, C., H. P. Cho, M. T. Nakamura, and S. D. Clarke. 2003. Regulation of human delta-6 desaturase gene transcription: identification of a functional direct repeat-1 element. J. Lipid Res. 44: 686–695.[Abstract/Free Full Text]

  6. Horton, J. D., J. L. Goldstein, and M. S. Brown. 2002. SREBPs: activators of the complete program of cholesterol and fatty acid synthesis in the liver. J. Clin. Invest. 109: 1125–1131.[CrossRef][Medline]

  7. Tabor, D. E., J. B. Kim, B. M. Spiegelman, and P. A. Edwards. 1999. Identification of conserved cis-elements and transcription factors required for sterol-regulated transcription of stearoyl-CoA desaturase 1 and 2. J. Biol. Chem. 274: 20603–20610.[Abstract/Free Full Text]

  8. Liang, G., J. Yang, J. D. Horton, R. E. Hammer, J. L. Goldstein, and M. S. Brown. 2002. Diminished hepatic response to fasting/refeeding and liver X receptor agonists in mice with selective deficiency of sterol regulatory element-binding protein-1c. J. Biol. Chem. 277: 9520–9528.[Abstract/Free Full Text]

  9. Matsuzaka, T., H. Shimano, N. Yahagi, M. Amemiya-Kudo, T. Yoshikawa, A. H. Hasty, Y. Tamura, J. Osuga, H. Okazaki, Y. Iizuka, et al. 2002. Dual regulation of mouse {Delta}5- and {Delta}6-desaturase gene expression by SREBP-1 and PPAR{alpha}. J. Lipid Res. 43: 107–114.[Abstract/Free Full Text]

  10. Nara, T. Y., W. S. He, C. Tang, S. D. Clarke, and M. T. Nakamura. 2002. The E-box like sterol regulatory element mediates the suppression of human delta-6 desaturase gene by highly unsaturated fatty acids. Biochem. Biophys. Res. Commun. 296: 111–117.[CrossRef][Medline]

  11. Brown, M. S., and J. L. Goldstein. 1997. The SREBP pathway: regulation of cholesterol metabolism by proteolysis of a membrane-bound transcription factor. Cell. 89: 331–340.[CrossRef][Medline]

  12. Clarke, S. D., and D. B. Jump. 1994. Dietary polyunsaturated fatty acid regulation of gene transcription. Annu. Rev. Nutr. 14: 83–98.[CrossRef][Medline]

  13. Jump, D. B., and S. D. Clarke. 1999. Regulation of gene expression by dietary fat. Annu. Rev. Nutr. 19: 63–90.[CrossRef][Medline]

  14. Xu, J., M. T. Nakamura, H. P. Cho, and S. D. Clarke. 1999. 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. 274: 23577–23583.[Abstract/Free Full Text]

  15. Mater, M. K., A. P. Thelen, D. A. Pan, and D. B. Jump. 1999. Sterol response element-binding protein 1c (SREBP1c) is involved in the polyunsaturated fatty acid suppression of hepatic S14 gene transcription. J. Biol. Chem. 274: 32725–32732.[Abstract/Free Full Text]

  16. Ntambi, J. M. 1999. Regulation of stearoyl-CoA desaturase by polyunsaturated fatty acids and cholesterol. J. Lipid Res. 40: 1549–1558.[Abstract/Free Full Text]

  17. Xu, J., M. Teran-Garcia, J. H. Park, M. T. Nakamura, and S. D. Clarke. 2001. Polyunsaturated fatty acids suppress hepatic sterol regulatory element-binding protein-1 expression by accelerating transcript decay. J. Biol. Chem. 276: 9800–9807.[Abstract/Free Full Text]

  18. Xu, J., H. Cho, S. O'Malley, J. H. Park, and S. D. Clarke. 2002. Dietary polyunsaturated fats regulate rat liver sterol regulatory element binding proteins-1 and -2 in three distinct stages and by different mechanisms. J. Nutr. 132: 3333–3339.[Abstract/Free Full Text]

  19. Cherkaoui-Malki, M., K. Meyer, W. Q. Cao, N. Latruffe, A. V. Yeldandi, M. S. Rao, C. A. Bradfield, and J. K. Reddy. 2001. Identification of novel peroxisome proliferator-activated receptor alpha (PPARalpha) target genes in mouse liver using cDNA microarray analysis. Gene Expr. 9: 291–304.[Medline]

  20. Kramer, J. A., E. A. Blomme, R. T. Bunch, J. C. Davila, C. J. Jackson, P. F. Jones, K. L. Kolaja, and S. W. Curtiss. 2003. Transcription profiling distinguishes dose-dependent effects in the livers of rats treated with clofibrate. Toxicol. Pathol. 31: 417–431.[CrossRef][Medline]

  21. Kersten, S., J. Seydoux, J. M. Peters, F. J. Gonzalez, B. Desvergne, and W. Wahli. 1999. Peroxisome proliferator-activated receptor alpha mediates the adaptive response to fasting. J. Clin. Invest. 103: 1489–1498.[Medline]

  22. Leone, T. C., C. J. Weinheimer, and D. P. Kelly. 1999. A critical role for the peroxisome proliferator-activated receptor alpha (PPARalpha) in the cellular fasting response: the PPARalpha-null mouse as a model of fatty acid oxidation disorders. Proc. Natl. Acad. Sci. USA. 96: 7473–7478.[Abstract/Free Full Text]

  23. Hashimoto, T., W. S. Cook, C. Qi, A. V. Yeldandi, J. K. Reddy, and M. S. Rao. 2000. Defect in peroxisome proliferator-activated receptor alpha-inducible fatty acid oxidation determines the severity of hepatic steatosis in response to fasting. J. Biol. Chem. 275: 28918–28928.[Abstract/Free Full Text]

  24. Forman, B. M., J. Chen, and R. M. Evans. 1997. Hypolipidemic drugs, polyunsaturated fatty acids, and eicosanoids are ligands for peroxisome proliferator-activated receptors alpha and delta. Proc. Natl. Acad. Sci. USA. 94: 4312–4317.[Abstract/Free Full Text]

  25. Kliewer, S. A., S. S. Sundseth, S. A. Jones, P. J. Brown, G. B. Wisely, C. S. Koble, P. Devchand, W. Wahli, T. M. Wilson, J. M. Lenhard, et al. 1997. Fatty acids and eicosanoids regulate gene expression through direct interaction with peroxisome proliferator-activated receptors alpha and gamma. Proc. Natl. Acad. Sci. USA. 94: 4318–4323.[Abstract/Free Full Text]

  26. Kawashima, Y., K. Musoh, and H. Kozuka. 1990. Peroxisome proliferators enhance linoleic acid metabolism in rat liver: increased biosynthesis of {omega}6 polyunsaturated fatty acids. J. Biol. Chem. 265: 9170–9175.[Abstract/Free Full Text]

  27. Miller, C. W., and J. M. Ntambi. 1996. Peroxisome proliferators induce mouse liver stearoyl-CoA desaturase 1 gene expression. Proc. Natl. Acad. Sci. USA. 93: 9443–9448.[Abstract/Free Full Text]

  28. He, W. S., T. Y. Nara, and M. T. Nakamura. 2002. Delayed induction of delta-6 and delta-5 desaturases by a peroxisome proliferator. Biochem. Biophys. Res. Commun. 299: 832–838.[CrossRef][Medline]

  29. Cheon, Y., T. Y. Nara, M. R. Band, J. E. Beever, M. A. Wallig, and M. T. Nakamura. 2005. Induction of overlapping genes by fasting and a peroxisome proliferator in pigs: evidence of functional PPARalpha in nonproliferating species. Am. J. Physiol. Regul. Integr. Comp. Physiol. 288: R1525–R1535.[Abstract/Free Full Text]

  30. Gonzalez, F. J. 2002. The peroxisome proliferator-activated receptor alpha (PPAR alpha): role in hepatocarcinogenesis. Mol. Cell. Endocrinol. 193: 71–79.[CrossRef][Medline]

  31. Horton, J. D., Y. Bashmakov, I. Shimomura, and H. Shimano. 1998. Regulation of sterol regulatory element binding proteins in livers of fasted and refed mice. Proc. Natl. Acad. Sci. USA. 95: 5987–5992.[Abstract/Free Full Text]

  32. Lee, S. S., T. Pineau, J. Drago, E. J. Lee, J. W. Owens, D. L. Kroetz, P. M. Fernandez-Salguero, H. Westphal, and F. J. Gonzalez. 1995. Targeted disruption of the alpha isoform of the peroxisome proliferator-activated receptor gene in mice results in abolishment of the pleiotropic effects of peroxisome proliferators. Mol. Cell. Biol. 15: 3012–3022.[Abstract]

  33. Reeves, P. G., F. H. Nielsen, and G. C. Fahey. 1993. AIN-93 purified diets for laboratory rodents: final report of the American Institute of Nutrition Ad Hoc Writing Committee on the reformulation of the AIN-76A rodent diet. J. Nutr. 123: 1939–1951.

  34. Sheng, Z., H. Otani, M. S. Brown, and J. L. Goldstein. 1995. Independent regulation of sterol regulatory element-binding proteins 1 and 2 in hamster liver. Proc. Natl. Acad. Sci. USA. 92: 935–938.[Abstract/Free Full Text]

  35. Nakamura, M. T., A. B. Tang, J. Villanueva, C. H. Halsted, and S. D. Phinney. 1992. Reduced tissue arachidonic acid concentration with chronic ethanol feeding in miniature pigs. Am. J. Clin. Nutr. 56: 467–474.[Abstract/Free Full Text]

  36. Guillou, H., P. Martin, S. Jan, S. D'Andrea, A. Roulet, D. Catheline, V. Rioux, T. Pineau, and P. Legrand. 2002. Comparative effect of fenofibrate on hepatic desaturases in wild-type and peroxisome proliferator-activated receptor alpha-deficient mice. Lipids. 37: 981–989.[Medline]

  37. Sandberg, M. B., M. Bloksgaard, D. Duran-Sandoval, C. Duval, B. Staels, and S. Mandrup. 2005. The gene encoding acyl-CoA-binding protein is subject to metabolic regulation by both sterol regulatory element-binding protein and peroxisome proliferator-activated receptor {alpha} in hepatocytes. J. Biol. Chem. 280: 5258–5266.[Abstract/Free Full Text]

  38. Reddy, J. K., and T. Hashimoto. 2001. Peroxisomal beta-oxidation and peroxisome proliferator-activated receptor alpha: an adaptive metabolic system. Annu. Rev. Nutr. 21: 193–230.[CrossRef][Medline]

  39. Holman, R. T. 1971. Essential fatty acid deficiency. Prog. Chem. Fats Other Lipids. 9: 275–348.

  40. Patel, D. D., B. L. Knight, D. Wiggins, S. M. Humphreys, and G. F. Gibbons. 2001. Disturbances in the normal regulation of SREBP-sensitive genes in PPAR alpha-deficient mice. J. Lipid Res. 42: 328–337.[Abstract/Free Full Text]

  41. Yu, S., K. Matsusue, P. Kashireddy, W. Q. Cao, V. Yeldandi, A. V. Yeldandi, M. S. Rao, F. J. Gonzalez, and J. K. Reddy. 2003. Adipocyte-specific gene expression and adipogenic steatosis in the mouse liver due to peroxisome proliferator-activated receptor gamma1 (PPARgamma1) overexpression. J. Biol. Chem. 278: 498–505.[Abstract/Free Full Text]

  42. Griffin, M. J., and H. S. Sul. 2004. Insulin regulation of fatty acid synthase gene transcription: roles of USF and SREBP-1c. IUBMB Life. 56: 595–600.[Medline]

  43. Du, K., S. Herzig, R. N. Kulkarni, and M. Montminy. 2003. TRB3: a tribbles homolog that inhibits Akt/PKB activation by insulin in liver. Science. 300: 1574–1577.[Abstract/Free Full Text]

  44. Koo, S. H., H. Satoh, S. Herzig, C. H. Lee, S. Hedrick, R. Kulkarni, R. M. Evans, J. Olefsky, and M. Montminy. 2004. PGC-1 promotes insulin resistance in liver through PPAR-alpha-dependent induction of TRB-3. Nat. Med. 10: 530–534.[CrossRef][Medline]

  45. Levine, M., and R. Tjian. 2003. Transcription regulation and animal diversity. Nature. 424: 147–151.[CrossRef][Medline]

  46. Halminski, M. A., J. B. Marsh, and E. H. Harrison. 1991. Differential effects of fish oil, safflower oil and palm oil on fatty acid oxidation and glycerolipid synthesis in rat liver. J. Nutr. 121: 1554–1561.

  47. Baillie, R. A., R. Takada, M. Nakamura, and S. D. Clarke. 1999. Coordinate induction of peroxisomal acyl-CoA oxidase and UCP-3 by dietary fish oil: a mechanism for decreased body fat deposition. Prostaglandins Leukot. Essent. Fatty Acids. 60: 351–356.[CrossRef][Medline]

  48. Ide, T., H. Kobayashi, L. Ashakumary, I. A. Rouyer, Y. Takahashi, T. Aoyama, T. Hashimoto, and M. Mizugaki. 2000. Comparative effects of perilla and fish oils on the activity and gene expression of fatty acid oxidation enzymes in rat liver. Biochim. Biophys. Acta. 1485: 23–35.[Medline]

  49. Dallongeville, J., E. Bauge, A. Tailleux, J. M. Peters, F. J. Gonzalez, J. C. Fruchart, and B. Staels. 2001. Peroxisome proliferator-activated receptor alpha is not rate-limiting for the lipoprotein-lowering action of fish oil. J. Biol. Chem. 276: 4634–4639.[Abstract/Free Full Text]

  50. Voss, A., M. Reinhart, S. Sankarappa, and H. Sprecher. 1991. The metabolism of 7,10,13,16,19-docosapentaenoic acid to 4,7,10,13,16, 19-docosahexaenoic acid in rat liver is independent of a 4-desaturase. J. Biol. Chem. 266: 19995–20000.[Abstract/Free Full Text]

  51. Ferdinandusse, S., S. Denis, P. A. Mooijer, Z. Zhang, J. K. Reddy, A. A. Spector, and R. J. Wanders. 2001. Identification of the peroxisomal beta-oxidation enzymes involved in the biosynthesis of docosahexaenoic acid. J. Lipid Res. 42: 1987–1995.[Abstract/Free Full Text]

  52. Su, H. M., A. B. Moser, H. W. Moser, and P. A. Watkins. 2001. Peroxisomal straight-chain acyl-CoA oxidase and D-bifunctional protein are essential for the retroconversion step in docosahexaenoic acid synthesis. J. Biol. Chem. 276: 38115–38120.[Abstract/Free Full Text]

  53. McIntyre, T. M., A. V. Pontsler, A. R. Silva, A. St. Hilaire, Y. Xu, J. C. Hinshaw, G. A. Zimmerman, K. Hama, J. Aoki, H. Arai, et al. 2003. Identification of an intracellular receptor for lysophosphatidic acid (LPA): LPA is a transcellular PPARgamma agonist. Proc. Natl. Acad. Sci. USA. 100: 131–136.[Abstract/Free Full Text]

  54. Yamashita, A., T. Sugiura, and K. Waku. 1997. Acyltransferases and transacylases involved in fatty acid remodeling of phospholipids and metabolism of bioactive lipids in mammalian cells. J. Biochem. (Tokyo). 122: 1–16.[Abstract/Free Full Text]


Add to CiteULike CiteULike   Add to Complore Complore   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us   Add to Digg Digg   Add to Reddit Reddit   Add to Technorati Technorati    What's this?


This article has been cited by other articles:


Home page
 J. Biol. Chem.Home page
A. M. Hebbachi, B. L. Knight, D. Wiggins, D. D. Patel, and G. F. Gibbons
Peroxisome Proliferator-activated Receptor {alpha} Deficiency Abolishes the Response of Lipogenic Gene Expression to Re-feeding: RESTORATION OF THE NORMAL RESPONSE BY ACTIVATION OF LIVER X RECEPTOR {alpha}
J. Biol. Chem., February 22, 2008; 283(8): 4866 - 4876.
[Abstract] [Full Text] [PDF]

Home page
 ReproductionHome page
T. Saether, T. N Tran, H. Rootwelt, H. J Grav, B. O Christophersen, and T. B Haugen
Essential fatty acid deficiency induces fatty acid desaturase expression in rat epididymis, but not in testis
Reproduction, February 1, 2007; 133(2): 467 - 477.
[Abstract] [Full Text] [PDF]

Home page
 J. Nutr.Home page
R. Ringseis, A. Muschick, and K. Eder
Dietary Oxidized Fat Prevents Ethanol-Induced Triacylglycerol Accumulation and Increases Expression of PPAR{alpha} Target Genes in Rat Liver
J. Nutr., January 1, 2007; 137(1): 77 - 83.
[Abstract] [Full Text] [PDF]

Home page
 Physiol. GenomicsHome page
H. J. Atherton, N. J. Bailey, W. Zhang, J. Taylor, H. Major, J. Shockcor, K. Clarke, and J. L. Griffin
A combined 1H-NMR spectroscopy- and mass spectrometry-based metabolomic study of the PPAR-{alpha} null mutant mouse defines profound systemic changes in metabolism linked to the metabolic syndrome
Physiol Genomics, October 11, 2006; 27(2): 178 - 186.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
M500237-JLR200v1
46/11/2432    most recent
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download