From the Institut für Arterioskleroseforschung,
the ¶ Institut für Biochemie, and the
Institut
für Klinische Chemie und Laboratoriumsmedizin
(Zentrallaboratorium) der Westfälischen
Wilhelms-Universität Münster,
D-48129 Münster, Germany
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() |
---|
We showed recently that a targeted null mutation
in the murine sterol carrier protein 2-/sterol carrier protein x-gene
(Scp2) leads to defective peroxisomal catabolism of
3,7,11,15-tetramethylhexadecanoic acid (phytanic acid), peroxisome
proliferation, hypolipidemia, and enhanced hepatic expression of
several genes that have been demonstrated to be transcriptionally
regulated by the peroxisome proliferator-activated receptor Apart from serving as fuels in energy metabolism, fatty acids have
been proposed to act as regulators in gene expression (reviewed in Ref.
1). Important roles in this process have been assigned to heterodimers
consisting of peroxisome proliferator-activated receptor Sterol carrier protein 2 (SCP2) and sterol carrier protein x (SCPx) are
two peroxisomal proteins that are generated from the same gene via
alternative transcription initiation (11). Based on in vitro
data, it was assumed that SCP2 may play a role in intracellular
cholesterol trafficking (reviewed in Ref. 12), whereas SCPx was
identified as peroxisomal 3-ketoacyl-CoA thiolase with intrinsic lipid
transfer activity (13). Recently, the phenotype of the SCP2/SCPx
knockout mouse, Scp2 ( Our results reveal a strong correlation between phytanic acid serum
concentrations and expression of genes encoding peroxisomal Preparation of cDNA Probes and Northern Blot
Analyses--
Total RNA was isolated from mouse tissues or MH1C1 cells
according to Chomczynski and Sacchi (22) followed by selection of
poly(A)+ RNA on oligo(dT) cellulose. Northern blots were
hybridized with digoxigenin-labeled probes prepared by random priming
using a commercially available kit (Boehringer Mannheim). All probes
were obtained from a mouse liver cDNA library (Stratagene,
Heidelberg, Germany) by polymerase chain reaction amplification with
appropriate primers. Quantification was carried out relative to
expression of glyceraldehyde-3-phosphate dehydrogenase mRNA. The
membranes were rinsed twice in 0.1% SDS, 2× SSC (0.15 M
NaCl and 0.015 M sodium citrate) at room temperature and
then twice in 0.1% SDS, 0.5× SSC at 68 °C for 15 min. Bands were
visualized using the chemiluminescence substrate CDP-Star
(Tropix-Serva, Heidelberg, Germany) and quantified using a Bio-Imager
BAS-KR 1500 (Fuji, Düsseldorf, Germany). DNA sequencing was
performed on an automated laser fluorescence DNA sequencer (Amersham
Pharmacia Biotech) to verify the identity of the polymerase chain
reaction amplification products.
Cell Culture and Transfection--
The rat hepatoma cell line
MH1C1 was obtained from the DSMZ (Braunschweig, Germany) and cultured
in Dulbecco's modified Eagle's medium supplemented with 10% fetal
calf serum and antibiotics. After washing with phosphate-buffered
saline, cells were incubated for 72 h with 250 µM Wy
14,643, bezafibrate, or phytanic acid dissolved in Me2SO
(0.5% v/v). Wy 14,643 was obtained from Biomol (Hamburg, Germany), and
bezafibrate and phytanic acid were obtained from Sigma. HepG2 cells
were cultured in 6-well dishes with Dulbecco's modified Eagle's
medium supplemented with 10% basal medium supplement artificial serum
(Biochrom, Berlin, Germany) and grown to 70% confluency.
Co-transfection of HepG2 cells with 1.5 µg/well pcDNA3-mPPAR Cloning, Expression, and Purification of GST/LBD-mPPAR Dietary Intervention Studies--
Mice were fed a standard chow
diet (Altrumin, Hannover, Germany) containing 0.8 mg/g (w/w) of various
sterols, mainly cholesterol and Ligand Binding Assay--
Ligand binding to recombinant
GST/LBD-mPPAR We demonstrated recently that the loss of the Scp2 gene function
led to drastically elevated phytanic acid serum concentrations accompanied by peroxisome proliferation, hypolipidemia, impaired body
weight control, neuropathy, and markedly altered hepatic gene
expression (14). To characterize in more detail the impact of the gene
disruption on modulation of hepatic gene expression, we exposed C75Bl/6
and Scp2 ( (PPAR
). As a broad range of fatty acids activates PPAR
in
vitro, we examined whether the latter effects could be because of
phytanic acid-induced activation of this transcription factor. Dietary
phytol supplementation was used to modulate the concentration of
phytanic acid in C57Bl/6 and Scp2 (
/
) mice. We found
that the serum concentrations of phytanic acid correlated well with the
expression of genes encoding peroxisomal
-oxidation enzymes and
liver fatty acid-binding protein, which have all been demonstrated to
contain functionally active peroxisome proliferator response elements
in their promoter regions. In accordance with these findings, a
stimulating effect on acyl-CoA oxidase gene expression was also
observed after incubation of the rat hepatoma cell line MH1C1 with
phytanic acid. Moreover, reporter gene studies revealed that phytanic
acid induces the expression of a peroxisome proliferator response
element-driven chloramphenicol transferase reporter gene comparable
with strong peroxisome proliferators. In addition, the ability of
phytanic acid to act as an inductor of PPAR
-dependent
gene expression corresponded with high affinity binding of this dietary
branched chain fatty acid to recombinant PPAR
. We conclude that
phytanic acid can be considered as a bona fide physiological ligand of murine PPAR
.
INTRODUCTION
Top
Abstract
Introduction
References
(PPAR
)1 and retinoid X
receptor
(RXR
), both of which are members of the superfamily of
nuclear hormone receptors that function as ligand-dependent
transcription factors (2-5). RXR
/PPAR
heterodimers alter the
transcription of target genes after binding to PPREs, which consist of
a degenerate direct repeat of the recognition motif TGACCT spaced by 1 nucleotide (also called DR1 element) (2-4). Functionally active PPREs
have been identified within the control regions of various genes
implicated in lipid metabolism (overview in Ref. 6). The finding that
several endogenous unsaturated fatty acids such as oleic acid,
arachidonic acid, or linoleic acid activate PPAR
in vitro
supports the assumption that fatty acids could represent biological
ligands for this nuclear hormone receptor (3, 7-10). It has been
suggested that fatty acids regulate the transcription of genes involved
in their own degradation by activating PPAR
(7). On the other hand,
a great number of chemically diverse peroxisome proliferators activate
PPAR
to a similar or even higher extent than all natural fatty
acids, implying that the specificity of the fatty acid-mediated effect on PPAR
may be low (9, 10). Therefore, it cannot be excluded that
these agonists exert their effects indirectly by either being metabolized in the cell to an active form or by inducing the release or
synthesis of a common endogenous ligand (9, 10).
/
), did not provide evidence for a role of
the gene in intracellular cholesterol trafficking but revealed instead
defective peroxisomal degradation of certain natural methyl-branched
fatty acyl-CoAs such as phytanic and pristanic acid, which are
metabolized in peroxisomes. The metabolic abnormalities were associated
with marked peroxisome proliferation, hypolipidemia, and enhanced
expression of genes encoding peroxisomal
oxidation enzymes (14).
Similar observations were made after feeding mice with fibrates (6,
15). Because the analysis of the PPAR
knockout mice indicated that
the fibrates exert their effects through PPAR
(16), we investigated
whether phytanic acid could act as a fibrate-like natural agonist of
PPAR
. Phytanic acid was also identified as a weak agonist of RXR
,
the obligate heterodimerization partner of PPAR
(17, 18). Therefore,
we further examined if the altered hepatic gene expression in our
transgenic model could be because of phytanic acid-induced activation
of RXR
. A synergistic effect on gene expression in the presence of
ligands for both nuclear receptors has previously been described
to occur in vitro (19, 20).
-oxidation enzymes (acyl-CoA oxidase (ACO), peroxisomal bifunctional enzyme, peroxisomal 3-ketoacyl-CoA thiolase), and liver fatty acid-binding protein. In addition, we demonstrate that phytanic acid
does not only bind to recombinant PPAR
but also induces the
expression of a PPRE-driven CAT reporter gene comparable with strong
peroxisome proliferators. The identification of phytanic acid as a bona
fide physiological ligand of PPAR
is of special interest, as an
accumulation of this dietary fatty acid is not only observed in Scp2
(
/
) mice but also in several inherited human diseases,
e.g. Refsum disease and Zellweger syndrome (21).
EXPERIMENTAL PROCEDURES
(5, 23) and 1.5 µg of pCAT-iPPRE was performed with Fugene transfection reagent (Boehringer Mannheim). pcDNA3-mPPAR
was a
friendly gift from Dr. P. Holden (Zeneca), and the reporter gene
construct pCAT-iPPRE was prepared by cloning the previously identified "ideal" PPRE sequence 5'-tgtgacctttgacctagttttg-3' (24)
into plasmid pCAT3 (Promega, Heidelberg, Germany). Transfection with
0.5 µg/well pSV-
-Gal (Promega) was performed as the internal control. After transfection, cells were incubated for 42 h with 200 µM indicated compound, dissolved in 1%
Me2SO (arachidonic acid, 100 µM). CAT and
-galactosidase concentrations were measured with an enzyme-linked
immunosorbent assay detection kit (Boehringer Mannheim). Normalized CAT
expression was determined and plotted as fold induction relative to
untreated cells. Each experiment was performed six times with similar results.
Fusion
Protein--
The ligand binding domain of mPPAR
was amplified by
polymerase chain reaction from a murine liver cDNA library with a
5' primer that introduced an EcoRI site and a 3' primer that
introduced a BamHI site downstream of the natural stop codon
of mPPAR
cDNA. The resulting fragment was appropriately digested
and subcloned into a EcoRI/BamHI-digested GST
fusion vector (pGEX-2T, Amersham Pharmacia Biotech). GST/LBD-mPPAR
expression in Escherichia coli strain XL-1-Blue (Stratagene)
was induced by addition of
isopropyl-1-thio-b-D-galactopyranoside to the growth media
(0.2 mM final concentration). After culturing for 5 h,
bacterial extracts were prepared by sonication (50 W, 2 × 20 s) followed by 5 freeze/thaw cycles. The fusion protein was purified on
a glutathione-Sepharose 4B column as per the manufacturer's recommendations (Amersham Pharmacia Biotech).
-sitosterol, 0.075 mg/g (w/w) of
nonesterified phytol, and 0.2 mg/g (w/w) of phytanic acid.
Phytol-enriched diets were prepared from these diets by adding 5 mg/g
of phytol (Aldrich). Bezafibrate was added to the standard diet at a
concentration of 2.5 mg/g, treatment with 9-cis RA was
performed by daily gavage of 10 µg of 9-cis RA (Sigma)/g
of body weight. Animals were kept individually, and food intake and
body weights were monitored daily.
fusion protein was performed with the fluorescent
fatty acid trans-parinaric acid (25, 26). The concentration
of trans-parinaric acid in absolute ethanol was determined
spectrophotometrically (
310 = 84,000 M
1). Protein solution (0.1 to 0.4 µM in phosphate-buffered saline) was titrated with
trans-parinaric acid at 25 °C using a fluorescence spectrophotometer (LS 50 B, Perkin-Elmer). For excitation and emission,
wavelengths of 320 and 412 nm and a slit width of 2.5 and 20 nm were
used. Ethanol concentration never exceeded 1% (v/v). All binding
experiments were performed at least four times, and the dilution was
subtracted from original data. The binding isotherms were fitted using
a nonlinear Marquardt algorithm. For competition experiments,
GST/LBD-mPPAR
fusion protein (0.1 to 0.4 µM in
phosphate-buffered saline) was saturated with
trans-parinaric acid, which was then displaced from the
protein using various ligands dissolved in ethanol (80 to 100 µM).
RESULTS
/
) mice to a standard laboratory chow diet (low phytol
diet) and to a diet supplemented with 5 mg/g of nonesterified phytol
(high phytol diet). Phytol is rapidly converted into phytanic acid in
both strains of mice (14). Effects on hepatic gene expression were
evaluated by Northern blot analyses with liver RNA isolated from the
four groups: low phytol C57Bl/6, low phytol Scp2 (
/
), high phytol
C57Bl/6, and high phytol Scp2 (
/
). We selected to study four genes
that comprise functionally active PPREs: ACO (23), peroxisomal
bifunctional enzyme (27), peroxisomal 3-ketoacyl-CoA thiolase (28), and
liver fatty acid-binding protein (29). As shown in Fig.
1A, expression of all of these genes was induced considerably in the two high phytol groups. Lowest
expression was consistently seen in the low phytol C57Bl/6 group,
followed by the low phytol Scp2 (
/
) group (1.5-to 3-fold higher)
and the high phytol C57Bl/6 group (3- to 7-fold higher). The most
drastic induction was evident in the high phytol Scp2 (
/
) group in
whom expression was between five- (liver fatty acid-binding protein)
and more than 10-fold (peroxisomal bifunctional enzyme) higher than in
the low phytol C57Bl/6 group. Thus, hepatic expression of PPAR
target genes seemed to parallel phytanic acid serum concentrations
(Fig. 1B).
View larger version (14K):
[in a new window]
Fig. 1.
Expression of
PPAR -dependent genes correlates
with phytanic acid serum concentrations. A, Northern
blot analyses with poly(A)+ RNA from liver of Scp2 (
/
)
and wild type mice (+/+) fed a standard chow diet or a diet
supplemented with 5 mg/g of phytol (phyt). The abbreviations
for the cDNA probes are given in the text. Northern blots were
reprobed with rat glyceraldehyde-3-phosphate dehydrogenase
(GAPDH) cDNA to exclude lane-loading differences.
PBE, peroxisomal bifunctional enzyme; pTHIOL,
peroxisomal 3-ketoacyl-CoA thiolase; L-FABP, liver fatty
acid binding protein. B, phytanic acid concentrations in
sera from Scp (
/
) and wild type (+/+) mice. Results are expressed
in µmol/liter. Quantification and identification was performed as
described previously (11).
To exclude hormonal or strain-specific influences on
PPAR-dependent gene expression (30, 31), we next
investigated whether phytanic acid could also induce the expression of
target genes in a cell culture model. Therefore, we incubated the rat
hepatoma cell line MH1C1 with phytanic acid and examined ACO mRNA
expression by Northern blot analyses. MH1C1 cells have previously been
shown to retain the ability of peroxisome proliferation in response to
nafenopin and to express significant amounts of PPAR
(32). In
accordance with our in vivo findings in Scp2 (
/
) mice,
we found a 3- to 4-fold elevated ACO mRNA expression after
incubation of MH1C1 cells with 250 µM phytanic acid for 3 days (Fig. 2). The increase on ACO
mRNA expression was more pronounced than that obtained after
incubation of this cell line with 250 µM bezafibrate (2- to 3-fold) but less prominent than that obtained with 250 µM Wy 14,643 (4- to 5-fold) (Fig. 2).
|
To gain further insights into the mechanism of phytol-induced
modulation of gene expression, we treated Scp2 (/
) and C57Bl/6 mice
with bezafibrate and 9-cis RA and compared ACO gene
expression in their livers with the corresponding effects of dietary
phytol administration. Bezafibrate has been demonstrated to be an
activator of PPAR
(9), whereas 9-cis RA was identified as
a weak activator of RXR
(17, 18). As evident from Fig.
3, treatment of Scp2 (
/
) and control
mice with 9-cis RA alone stimulated ACO gene expression only
very moderately, leading to a 1.5-fold increase that was not
statistically significant. Most efficient stimulation of ACO gene
expression was observed in Scp2 (
/
) mice that had been treated with
either phytol or bezafibrate (Fig. 3). However the simultaneous
administration of 9-cis RA and bezafibrate to Scp2 (
/
)
and control mice did not lead to a synergistically enhanced ACO gene
expression that was observed in rat hepatocyte cultures (19, 20).
|
These results pointed to similarities that seemed to exist between the
effects of dietary phytol intake and treatment with bezafibrate. The
good correlation between plasma phytanic acid concentrations and
expression of PPAR target genes led to our hypothesis that this
fatty acid may act as a direct agonist of PPAR
, especially as it has
been demonstrated that a broad range of fatty acids binds to and
thereby activates this transcription factor (3, 7-10). To evaluate
this hypothesis, we tested binding of phytanic acid to a recombinant
glutathione-S-transferase/murine PPAR
ligand binding
domain fusion protein (GST/LBD-mPPAR
) and compared its affinity with
a number of well characterized PPAR
activators. We used a
fluorescence binding assay in which increasing concentrations of
trans-parinaric acid were incubated with a constant amount
of GST/LBD-mPPAR
fusion protein. The assay takes advantage of the
known fact that binding of trans-parinaric acid to proteins changes its spectral properties, leading to sensitized fluorescence with a maximum at a wavelength of 412 nm (excitation at 320 nm) (24).
As is evident from Fig. 4A,
saturable binding of trans-parinaric acid to the purified
GST/LBD-mPPAR
fusion protein could be demonstrated. In contrast,
trans-parinaric acid did not bind to purified recombinant GST, thus excluding the possibility that the GST part of the fusion protein contributed significantly to the binding activity (26).
|
To compare the binding affinities of several known PPAR activators
with that of phytanic acid, we performed competition experiments. As
shown in Fig. 4B, Wy 14,643 revealed the best displacement of trans-parinaric acid from GST/LBD-mPPAR
fusion protein
and thus the highest binding affinity. Surprisingly, the natural
branched chain fatty acid phytanic acid bound to recombinant mPPAR
far better than the well known PPAR
activators bezafibrate,
arachidonic acid, and palmitic acid (7, 9). In accordance with previous studies demonstrating that erucic acid does not activate PPAR
(7,
10), we observed no displacement of trans-parinaric acid from GST/LBD-mPPAR
fusion protein after adding this very long chain
fatty acid and, thus, no binding.
The ability of phytanic acid to induce the expression of a CAT reporter
gene linked to a PPRE was examined by co-transfection of HepG2 cells
with a mPPAR-expressing plasmid (5, 23). The addition of PPAR
ligands to the culture medium at a concentration of 200 µM (arachidonic acid, 100 µM) revealed a
strong correlation between the binding affinity of the compounds toward
mPPAR
and their respective trans-activation ability. The
administration of Wy 14,643 led to a 10-fold increase in CAT
expression, followed by phytanic acid (6.5-fold), bezafibrate
(4.0-fold), arachidonic acid (3.1-fold), and palmitic acid (2.2-fold)
(Fig. 5). Therefore, phytanic acid is not
only a high affinity ligand but also a potent activator of murine
PPAR
.
|
![]() |
DISCUSSION |
---|
In a previous study, we demonstrated that Scp2 (/
) mice had a
defect in peroxisomal catabolism of phytanoyl-CoA (14). The data
pointed to a dual role played by the two Scp2-encoded gene
products, SCP2 and SCPx, which are both localized in peroxisomes as
follows. 1) Reduced peroxisomal phytanoyl-CoA import seemed to relate
to the absence of phytanoyl-CoA carrier function that was shown to be
associated with SCP2. 2) Defective thiolytic cleavage of
3-ketopristanoyl-CoA was apparently because of absence of the 3-ketopristanoyl-CoA thiolase activity that was shown to be associated with SCPx (13, 33, 34). In addition to the metabolic defect, we
observed profound peroxisome proliferation, hypolipidemia, and
increased expression of genes encoding proteins that function in
peroxisomal and mitochondrial
-oxidation (14). The purpose of the
present work was to characterize the latter effects of the gene
disruption in more detail.
In vitro data published earlier (17, 18) showed that
phytanic acid behaves like a weak activating ligand of RXR and thus may act as 9-cis RA-like agonist when present in high
concentrations. Because RXR
is an obligatory partner in
PPRE-dependent gene expression (2, 3), we initially
considered that the effects on gene expression in Scp2 (
/
) mice
were because of enhanced activation of RXR
in this transgenic model.
However, the evidence that we present in the current manuscript does
not support this hypothesis, as follows. 1) Application of
9-cis RA to control mice did not induce ACO gene expression,
although 9-cis RA has been demonstrated to be a more potent
activator of RXR
than phytanic acid. 2) Application of the RXR
agonist 9-cis RA to both strains of mice did not evoke hypotriglyceridemia or peroxisome proliferation,
2 which were observed in Scp2
(
/
) mice, especially after feeding the phytanic acid precursor
phytol. Therefore, it seems unlikely that the effects observed in Scp2
(
/
) mice are because of phytanic acid-induced activation of
RXR
.
Because a broad range of fatty acids has been shown to activate PPAR
in vitro (3, 7-10), we investigated whether the enhanced hepatic gene expression in our mouse model could be because of the
phytanic acid-induced activation of PPAR
. It has been demonstrated that ligand binding to PPAR
induces a conformational change that enables the protein to interfere with basal transcription machinery (35). The DNA binding affinity of PPAR
is also enhanced in the
presence of ligands, at least if the receptor concentration is limiting
(7). Therefore, ligand binding is a necessary prerequisite for the
activation of PPAR
-dependent gene expression. We
measured the ability of phytanic acid and several well known PPAR
activators to bind to a recombinant GST/LBD-mPPAR
fusion protein. So
far, ligands of murine and Xenopus PPAR
have been
primarily identified by indirect binding assays, in which the
ligand-dependent DNA binding activity of PPAR
(7) or the
ligand-induced activation of coactivator proteins (9) were measured.
Kd-values have only been reported for the few cases
in which radiolabeled ligands were available (10, 36). The
trans-parinaric acid competition assay that we used in the
present study allowed us to identify direct binding of ligands to the
soluble GST/LBD-mPPAR
fusion protein. Furthermore, actual
Kd-values for the ligands could be obtained using a
Marquardt algorithm. For Wy 14,643, a Kd value of 4 nM was calculated, followed by phytanic acid (10 nM), bezafibrate (45 nM), arachidonic acid (83 nM), and palmitic acid (100 nM).
The affinities for straight chain fatty acids in binding to PPAR
were found in the range of their respective physiological serum
concentrations (~30 µM) (7, 10). Because phytanic acid bound to the recombinant GST/LBD-mPPAR
fusion protein with at least
one order of magnitude higher affinity than palmitic acid, one might
consider that this dietary fatty acid also binds within its
physiological serum concentration range (1.3-6.5 µM)
(17). The direct binding of phytanic acid to recombinant PPAR
supports the assumption that PPAR
activation is not necessarily
achieved by a common endogenous ligand that mediates the effects of the structural diverse PPAR
activators. The binding affinities of the
compounds toward the recombinant GST/LBD-mPPAR
fusion protein corresponded well with their trans-activation ability,
obtained by co-transfection of a PPRE-driven CAT reporter gene and a
mPPAR
-expressing plasmid into HepG2 cells. In addition, the extent
of induction of acyl-CoA oxidase mRNA expression in MH1C1 cells was
also consistent with the trans-activation ability of the compounds.
For several reasons, the identification of phytanic acid as a PPAR
agonist is of special interest. First, phytanic acid does not only
accumulate in Scp2 (
/
) mice but also in several inherited human
diseases like Refsum disease and Zellweger syndrome (21). Although
remarkable differences were observed in the ligand binding affinities
between rodent and human PPAR
(37), we found that phytanic acid
binds to recombinant human PPAR
with a comparable affinity as to
murine PPAR
.3 Second,
phytanic acid is the first identified natural PPAR
ligand that is
primarily degraded in peroxisomes (21, 38). So far, a variety of
endogenous fatty acids have been described as PPAR
activators
without being substrates for peroxisomal degradation. On the other
hand, very long chain fatty acids that are primarily degraded by
peroxisomal
-oxidation neither bind to nor activate PPAR
(7).
Therefore, one might consider that phytanic acid induces its own
degradation via activation of the PPAR
-dependent peroxisomal oxidation pathways. However, because of its
-methyl group, phytanic acid cannot be degraded by
-oxidation. Instead, a
one carbon moiety is split from the molecule by
-oxidation, yielding
pristanic, which is then subjected to six cycles of peroxisomal
-oxidation (21, 39). We demonstrated previously that the expression
of the key step enzyme in phytanic acid
-oxidation, phytanoyl-CoA
hydroxylase, is not increased in Scp2 (
/
) mice, although the serum
phytanic acid concentrations increases up to 1000-fold after phytol
feeding (14). This leads to the conclusion that the initial step in
phytanic acid degradation is not regulated by the substrate
concentration. Instead, we consider that phytanic acid serves as a
dietary signal molecule that induces the catabolism of fatty acids by
activating PPAR
. This assumption is supported by the recent finding
that PPAR
also modulates constitutive expression of genes encoding
several mitochondrial fatty acid-catabolizing enzymes (40). The dietary
uptake of physiological concentrations of phytanic acid together with a
bulk of other fatty acids would lead to an enhanced mitochondrial and
peroxisomal
-oxidation because of the activation of PPAR
. This is
in accordance with our previous findings, which in addition to the
peroxisomal
-oxidation enzymes, the expression of mitochondrial
3-ketoacyl-CoA thiolase mRNA as well as enzymatic activity of
mitochondrial butyryl-CoA dehydrogenase is drastically enhanced in the
liver of Scp2 (
/
) mice (14). The phytanic acid-induced expression
of genes encoding mitochondrial and peroxisomal
-oxidation enzymes
might also explain the observed hypolipidemia in Scp2 (
/
) mice
(14). Therefore, phytanic acid could serve as a dietary signal leading
to the induction of fatty acid catabolism.
![]() |
ACKNOWLEDGEMENT |
---|
We thank P. R. Holden (Zeneca,
Macclesfield, Cheshire) for providing plasmid pcDNA3-mPPAR and
B. Glass and S. Lütke-Enking for expert technical assistance.
![]() |
FOOTNOTES |
---|
* This work was supported by Deutsche Forschungsgemeinschaft Grants S.E. 459/2-2 (to U. S.) and SFB 310/A4 (to F. S.) and the Interdisziplinäres Klinisches Forschungszentrum of the Medical Faculty, University of Münster (Project A4).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
§ These authors contributed equally to this work.
** To whom correspondence should be addressed: Institut für Arterioskleroseforschung, Domagkstr. 3, 48149 Münster, Germany. Tel.: ++49-251-8356197; Fax: ++49-251-8356208; E-mail: seedorfu{at}uni-muenster.de.
The abbreviations used are:
PPAR, peroxisome
proliferator-activated receptor
; mPPAR
, murine PPAR
; RXR
, retinoid X receptor
; PPRE, peroxisome proliferator response
element; SCP2, sterol carrier protein 2; SCPx, sterol carrier protein
x; ACO, acyl-CoA oxidase; CAT, chloramphenicol transferase; GST, glutathione S-transferase; LBD, ligand binding domain; 9-cis RA, 9-cis retinoic acid.
2 U. Seedorf, unpublished observation.
3 C. Wolfrum, unpublished observation.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() |
---|