Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 7473–7478, June 1999
A critical role for the peroxisome proliferator-activated receptor ?
(PPAR?) in the cellular fasting response: The PPAR?-null
mouse as a model of fatty acid oxidation disorders
(nuclear hormone receptors?lipid metabolism?transcriptional control?dietary regulation)
TERESA C. LEONE*†, CARLA J. WEINHEIMER*†, AND DANIEL P. KELLY*‡§¶
*Center for Cardiovascular Research, Departments of‡Medicine and§Molecular Biology and Pharmacology, Washington University School of Medicine,
St. Louis, MO 63110
Communicated by William H. Daughaday, University of California at Irvine, Balboa Island, CA, April 29, 1999 (received for review April 5, 1999)
transcription factor, the peroxisome proliferator-activated
receptor ? (PPAR?), plays a pivotal role in the cellular
metabolic response to fasting. Short-term starvation caused
hepatic steatosis, myocardial lipid accumulation, and hypo-
glycemia, with an inadequate ketogenic response in adult mice
lacking PPAR? (PPAR????), a phenotype that bears remark-
able similarity to that of humans with genetic defects in
mitochondrial fatty acid oxidation enzymes. In PPAR????
mice, fasting induced the hepatic and cardiac expression of
PPAR? target genes encoding key mitochondrial (medium-
chain acyl-CoA dehydrogenase, carnitine palmitoyltrans-
ferase I) and extramitochondrial (acyl-CoA oxidase, cyto-
chrome P450 4A3) enzymes. In striking contrast, the hepatic
and cardiac expression of most PPAR? target genes was not
induced by fasting in PPAR????mice. These results define a
critical role for PPAR? in a transcriptional regulatory re-
sponse to fasting and identify the PPAR????mouse as a
potentially useful murine model of inborn and acquired
abnormalities of human fatty acid utilization.
We hypothesized that the lipid-activated
Starvation triggers a complex array of adaptive metabolic
responses. A prominent feature of the energy-metabolic re-
ketones for energy production (1–4) and an augmentation in
the capacity for mitochondrial fatty acid oxidation (FAO) in
tissues with high oxidative energy demands such as heart and
liver (5). The importance of the fasting-inducible capacity for
cellular lipid utilization is underscored by the dramatic phe-
notype of human inborn errors in mitochondrial FAO enzymes
(6). Children afflicted with genetically determined enzymatic
defects in the FAO pathway typically are asymptomatic under
normal feeding conditions. However, short-term fasting, such
as that associated with an infectious illness, precipitates a
dramatic and often fatal clinical picture characterized by
hypoketotic hypoglycemia, liver dysfunction, and cardiomyop-
athy (6–8). Postmortem studies of FAO enzyme-deficient
patients have demonstrated marked intracellular accumula-
tion of neutral lipid in liver and heart. The capacity to oxidize
fats is also diminished in several common acquired cardiac
diseases including cardiac hypertrophy and myocardial isch-
emia (9–17). The molecular pathogenesis of target organ
dysfunction from inherited and acquired alterations in cellular
FAO has not been elucidated.
A previous study in rodents demonstrated that the hepatic
expression of genes encoding mitochondrial FAO enzymes is
induced, at the transcriptional level, in response to fasting (5).
This transcriptional regulatory response likely plays a key role
in the fasting-induced augmentation of FAO capacity in liver
and other oxidative tissues. The mechanisms involved in the
fasting-induced transcriptional activation of FAO enzyme
genes are unknown. However, recent studies have identified a
role for a nuclear receptor, the peroxisome proliferator-
genes encoding mitochondrial FAO enzymes in heart and liver
(16, 18–23). PPAR? first was identified based on its control of
genes encoding peroxisomal FAO enzymes in response to
peroxisome proliferators such as fibric acid derivatives (24).
PPAR? now is known to regulate the transcription of genes
encoding peroxisomal, mitochondrial, and certain cytochrome
P450 enzymes (18, 24–26) involved in long-chain FAO. Fatty
acids and peroxisome proliferators are among the compounds
known to activate PPAR? (27–29), which binds its cognate
elements as a heterodimeric partner with the retinoid X
receptor (30). The results of recent studies have indicated that
endogenous PPAR? ligands are metabolized by peroxisomal
(31) and mitochondrial (18) FAO pathways, providing further
evidence that fatty acid intermediates activate PPAR? in vivo.
Accordingly, PPAR? as a lipid-activated nuclear receptor is an
excellent candidate for mediating the physiologic and dietary
control of genes encoding cellular fatty acid utilization en-
zymes. We sought to test the hypothesis that PPAR? is
involved in the metabolic response to fasting. We predicted
be unable to increase the capacity for cellular fatty acid
utilization in the context of short-term starvation.
Animal Studies. The PPAR????and PPAR????mice were
a generous gift of Frank J. Gonzalez (National Cancer Insti-
tute, Bethesda, MD) and have been described (32, 33). The
PPAR????and PPAR????lines were pure-bred on an sv129
background (32). All experiments were performed with mice
ranging in age from 12 to 16 weeks (20–30 g). Male and female
littermate or age-matched mice were separated into individual
cages at the beginning of each fasting experiment. Fasting was
initiated at 9:00 a.m. For the gene expression studies, fasting
was initiated at 5:00 p.m. The mice were fasted for 24 h or 48 h
as indicated in the text. Control mice (fed ad libitum) and
fasted mice were kept in identical light?dark cycles. The
control mice were allowed free access to standard lab chow
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PNAS is available online at www.pnas.org.
Abbreviations: PPAR, peroxisome proliferator-activated receptor;
FAO, fatty acid oxidation; NEFA, nonesterified free fatty acids;
MCAD, medium-chain acyl-CoA dehydrogenase; CPT I, carnitine
palmitoyltransferase I; ACO, acyl-CoA oxidase; CYP, cytochrome
P450; GAPDH, glyceraldehyde-3-phosphate dehydrogenase.
†T.C.L. and C.J.W. contributed equally to this work.
¶To whom reprint requests should be addressed at: Center for
Cardiovascular Research, Washington University School of Medi-
cine, 660 South Euclid Avenue, Campus Box 8086, St. Louis, MO
63110. e-mail: firstname.lastname@example.org.
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