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Cell Metabolism
Review
Peroxisomes: A Nexus for Lipid
Metabolism and Cellular Signaling
Irfan J. Lodhi
1,
*and Clay F. Semenkovich
1,2,
*
1
Division of Endocrinology, Metabolism and Lipid Research; Department of Medicine
2
Department of Cell Biology and Physiology
Washington University, St. Louis, MO 63110, USA
*Correspondence: ilodhi@dom.wustl.edu (I.J.L.), csemenko@wustl.edu (C.F.S.)
http://dx.doi.org/10.1016/j.cmet.2014.01.002
Peroxisomes are often dismissed as the cellular hoi polloi, relegated to cleaning up reactive oxygen chemical
debris discarded by other organelles. However, their functions extend far beyond hydrogen peroxide meta-
bolism. Peroxisomes are intimately associated with lipid droplets and mitochondria, and their ability to carry
out fatty acid oxidation and lipid synthesis, especially the production of ether lipids, may be critical for gener-
ating cellular signals required for normal physiology. Here, we review the biology of peroxisomes and their
potential relevance to human disorders including cancer, obesity-related diabetes, and degenerative neuro-
logic disease.
Introduction
Peroxisomes are multifunctional organelles present in virtually all
eukaryotic cells. In addition to being ubiquitous, they are also
highly plastic, responding rapidly to cellular or environmental
cues by modifying their size, number, morphology, and function
(Schrader et al., 2013). Early ultrastructural studies of kidney
and liver cells revealed cytoplasmic particles enclosed by a single
membrane containing granular matrix and a crystallinecore (Rho-
din, 1958). These particles were linked with the term ‘‘peroxi-
some’’ by Christian de Duve, who first identified the organelle in
mammalian cells when enzymes such as oxidases and catalases
involved in hydrogen peroxide metabolism cosedimented in equi-
librium density gradients (De Duve and Baudhuin, 1966). Based
on these studies, it was originally thought that the primary function
of these organelles was the metabolism of hydrogen peroxide.
Characterization of peroxisomes (also called ‘‘microbodies’’ in
the early literature) was greatly facilitated by the development of
a cytochemical staining procedure using 3,30-diaminobenzadine
(DAB), which permits visualization of these organelles based on
the peroxidative activity of catalase at alkaline pH (Fahimi, 1969;
Novikoff and Goldfischer, 1969). Using this staining method,
Novikoff and colleagues observed a large number of peroxisomes
in tissues active in lipid metabolism, such as liver, brain, intestinal
mucosa, and adipose tissue (Novikoff and Novikoff, 1982; Novik-
off et al., 1980). Peroxisomes in different tissues vary greatly in
shape and size, ranging from 0.1–0.5 mM in diameter. In adipo-
cytes, peroxisomes tend to be small in size and localized in the
vicinity of lipid droplets. Notably, a striking increase in the number
of peroxisomes was observed duringdifferentiation of adipogenic
cells in culture (Novikoff and Novikoff, 1982). These findings sug-
gest that peroxisomes may be involved in lipid metabolism.
Beevers and colleagues implicated peroxisomes in lipid meta-
bolism by demonstrating that enzymes involved in fatty acid
oxidation are colocalized in plant peroxisome-like organelles
called glyoxysomes, which are capable of converting fatty acids
to metabolic intermediates for carbohydrate synthesis (Cooper
and Beevers, 1969). Based on the finding that the fibrate class
of hypolipidemic drugs promotes peroxisome proliferation, Laz-
arow and De Duve (1976) hypothesized that peroxisomes in an-
imal cells were capable of carrying out fatty acid oxidation. This
was confirmed when they showed that purified rat liver peroxi-
somes contained fatty acid oxidation activity that was robustly
increased by treatment of animals with clofibrate. In a series of
experiments, Hajra and colleagues discovered that peroxisomes
were also capable of lipid synthesis (Hajra and Das, 1996). Over
the past three decades, multiple lines of evidence have solidified
the concept that peroxisomes play fundamentally important
roles in lipid metabolism. In addition to removal of reactive oxy-
gen species, metabolic functions of peroxisomes in mammalian
cells include b-oxidation of very long chain fatty acids, a-oxida-
tion of branched chain fatty acids, and synthesis of ether-linked
phospholipids as well as bile acids (Figure 1). b-oxidation also
occurs in mitochondria, but peroxisomal b-oxidation involves
distinctive substrates and complements mitochondrial function;
the processes of a-oxidation and ether lipid synthesis are unique
to peroxisomes and important for metabolic homeostasis.
Here, we highlight the established role of peroxisomes in lipid
metabolism and their emerging role in cellular signaling relevant
to metabolism. We describe the origin of peroxisomes and
factors involved in their assembly, division, and function. We
address the interaction of peroxisomes with lipid droplets and
implications of this interaction for lipid metabolism. We consider
fatty acid oxidation and lipid synthesis in peroxisomes and their
importance in brown and white adipose tissue (WAT) (sites rele-
vant to lipid oxidation and synthesis) and disease pathogenesis.
Finally, we review what is known about the mechanisms under-
lying human peroxisomal disorders.
Peroxisomal Biogenesis
Despite two decades of research on the process, the origin of
peroxisomes remains controversial (Dimitrov et al., 2013). In
principle, organelles can be derived from (1) growth and fission
of preexisting organelles, (2) templated assembly using an exist-
ing copy of the organelle, or (3) de novo generation (Lowe and
Barr, 2007). For peroxisomes, two alternative theories of biogen-
esis have been proposed (Figure 2A). According to the classical
380 Cell Metabolism 19, March 4, 2014 ª2014 Elsevier Inc.
model of peroxisome biogenesis, peroxisomes are autonomous
organelles that arise from preexisting peroxisomes through
growth and division (Lazarow and Fujiki, 1985). This model
(Figure 2A, top) was supported by the observation that some
peroxisomes in histologic sections of liver are visualized as
dumbbell-shaped interconnected structures. Such structures
were more abundant following induction of peroxisome prolifer-
ation by pharmacological means or by partial hepatectomy
(Legg and Wood, 1970; Reddy and Svoboda, 1971; Rigatuso
et al., 1970). This model was generally accepted for over 20
years. However, subsequent studies suggested that peroxi-
somes could arise de novo. In yeast cells lacking detectable
peroxisomes due to a single gene mutation, reintroduction of
the wild-type version of the gene complements the defect and
restores peroxisome formation (Baerends et al., 1996; Ho
¨hfeld
et al., 1991; Wiemer et al., 1996), consistent with the idea that
peroxisomes may not be exclusively autonomous.
An alternative model (Figure 2A, bottom) holds that peroxi-
somes are semiautonomous organelles deriving their matrix pro-
teins from the cytosol and their membraneproteins and lipids from
the endoplasmic reticulum (ER) (Tabak et al., 2013). A potentialER
origin for peroxisomes is supported by classic morphology
studies showing peroxisomes as clusters surrounded by smooth
ER in absorptive cells of guinea pig intestine (Novikoff and Novik-
off, 1972). Numerous contacts between peroxisomesand smooth
ER were observed, suggesting that peroxisomes arise from
dilated regions of ER. These regions were thought to contain
high concentrations of peroxisomal proteins, such as catalase
(Novikoff and Novikoff, 1972; Reddy and Svoboda, 1971).
Although the existence of direct contact and luminalcontinuity be-
tween peroxisomes and ER has been challenged (Lazarow and
Figure 1. Structure and Functions of
Peroxisomes
The peroxisome is a single membrane-enclosed
organelle that plays an important role in meta-
bolism. The main metabolic functions of peroxi-
somes in mammalian cells include b-oxidation
of very long chain fatty acids, a-oxidation of
branched chain fatty acids, synthesis of bile acids
and ether-linked phospholipids, and removal of
reactive oxygen species. Peroxisomes in many,
but not all, cell types contain a dense crystalline
core of oxidative enzymes.
Fujiki, 1985), recent studies suggest that
preperoxisomal vesicles, precursors of
peroxisomes, bud off of the ER, thus obvi-
ating the need for luminal continuity to
allow transport of peroxisomal contents
(van der Zand et al., 2012). Using a bimo-
lecular fluorescence complementation
assay, these authors monitored peroxi-
somal assembly in real time. This assay
is based on reconstitution of a functional
fluorescent protein mediated by interac-
tions between two peptides fused to two
different nonfluorescent halves of the fluo-
rescent protein (Kerppola, 2008). van der
Zand and colleagues used thistechnology
to show that two distinct preperoxisomal vesicles, each carrying
half of a peroxisomal translocon complex, undergo heterotypic
fusion, forming a functional translocon that permits uptake of
matrix proteins from the cytosol (van der Zand et al., 2012).
Factors Involved in Peroxisomal Biogenesis
Regardless of their origin, peroxisomes require a group of pro-
teins called peroxins for their assembly, division, and inheri-
tance. The budding yeast Saccharomyces cerevisae is a
particularly apt model for studying peroxisomal biogenesis due
to its ease of genetic manipulation and its robust peroxisomal
proliferation induced by oleate-containing medium (Subramani,
1998). Over 30 peroxins, encoded by Pex genes, have been
identified in yeast (Dimitrov et al., 2013). At least a dozen of these
proteins are conserved in mammals, where they regulate various
aspects of peroxisomal biogenesis, including factors that control
assembly of the peroxisomal membrane, factors that interact
with peroxisomal targeting sequences (PTSs) allowing proteins
to be shuttled to peroxisomes, and factors that act as docking
receptors for peroxisomal proteins.
At least three peroxins (Pex3, Pex16, and Pex19) appear to be
critical for assembly of the peroxisomal membrane and import of
peroxisomal membrane proteins (PMPs) (Figure 2B). Pex19 is a
soluble chaperone and import receptor for newly synthesized
PMPs (Jones et al., 2004). Pex3 buds from the ER in a preperox-
isomal vesicle and functions as a docking receptor for Pex19
(Fang et al., 2004). Pex16 acts as a docking site on the peroxi-
somal membrane for recruitment of Pex3 (Matsuzaki and Fujiki,
2008). Peroxisomal matrix proteins are translated on free ribo-
somes in the cytoplasm prior to their import. These proteins
have specific PTSs located either at the carboxyl (PTS1) or amino
(PTS2) terminus (Gould et al., 1987; Swinkels et al., 1991).
Cell Metabolism 19, March 4, 2014 ª2014 Elsevier Inc. 381
Cell Metabolism
Review
Recently, cryptic PTS1 motifs were identified in several glycolytic
enzymes, such as glyceraldehyde-3-phosphate dehydrogenase
(GAPDH) and 3-phosphoglycerate kinase (PGK), in various fungal
species. These motifs are activated by posttranslational pro-
cesses and may direct bona fide cytoplasmic proteins to
peroxisomes under certain metabolic conditions (Freitag et al.,
2012). The import receptor for PTS1-containing peroxisomal
matrix proteins is Pex5 (Dodt and Gould, 1996), whereas Pex7
functions as the import receptor for PTS2-containing proteins
(Braverman et al., 1997). These receptors bind cargo in the cyto-
plasm and transport it to docking sites at the peroxisomal mem-
brane. Once their cargo is delivered to the peroxisomal lumen, the
receptors recycle back to the cytoplasm (Liu et al., 2012b). The
recycling of Pex5 requires ubiquitination (Platta et al., 2007).
Peroxisome Proliferator Activated Receptors in
Peroxisome Biogenesis and Function
Peroxisome proliferator activated receptors (PPARs) are relevant
to peroxisomal lipid oxidation and synthesis and how these pro-
cesses impact disease pathogenesis. Fibrates, used to lower
lipids in humans, and certain xenobiotics promote proliferation
of peroxisomes in cells presumably due to de novo biogenesis.
This property, as well as the capacity of these compounds to
modulate gene expression of the peroxisomal fatty acid oxida-
tion machinery (Reddy et al., 1986), led to the hypothesis that
these chemicals activate a nuclear receptor by a mechanism
analogous to that used by steroid hormones (Issemann and
Green, 1990). Issemann and Green (1990) used reporter assays
to demonstrate that peroxisome proliferating chemicals acti-
vated a PPAR (now referred to as PPARa). Subsequently, two
related nuclear receptors, PPARgand PPARdwere cloned
(Dreyer et al., 1992; Kliewer et al., 1994).
PPARs are recognized as fundamentally important regulators
of lipid metabolism (Wang, 2010). They act as ligand-activated
transcription factors that form obligate heterodimers with reti-
noid X receptors (RXR) and bind to specific DNA sequences
known as PPAR response elements (PPREs) located in promoter
regions of target genes. In the absence of ligand, PPARs are
associated with corepressors. Ligand binding promotes a
conformation change in PPARs, resulting in replacement of re-
pressors with coactivators and subsequent activation of target
gene expression. The three members of the PPAR family mani-
fest distinct tissue distribution patterns and target gene expres-
sion profiles. PPARais the target of fibrate drugs used to treat
lipid disorders in humans. It is highly enriched in liver and brown
adipose tissue (BAT), where it is a key regulator of fatty acid
oxidation. PPARdhas significant functional overlap with PPARa
but is more widely expressed. PPARgis the target of glitazone
drugs used to treat diabetes in humans. It is enriched in adipose
tissue and is absolutely required for adipogenesis (Ahmadian
et al., 2013; Tontonoz and Spiegelman, 2008).
PPARais known to be important for peroxisome proliferation.
PPARaactivation increases not only the expression of fatty acid
BA
Figure 2. Potential Pathways to Peroxisomal Biogenesis
(A) Peroxisomes are generated autonomously through division of preexisting organelles (top) or through a de novo process involving budding from the ER
followed by import of matrix proteins (bottom).
(B) Peroxisomal membrane protein import. Peroxisomal membrane proteins (PMPs) are imported posttranslationally to the peroxisomal membrane. Pex19 is a
soluble chaperone that binds to PMPs and transports them to the peroxisomal membr ane, where it docks with a complex containing Pex16 and Pex3. Following
insertion of the PMP, Pex19 is recycled back to the cytosol.
382 Cell Metabolism 19, March 4, 2014 ª2014 Elsevier Inc.
Cell Metabolism
Review
oxidation genes but also the abundance of peroxisomes in liver
(Schrader et al., 2012). Its activation induces expression of the
peroxin Pex11 (Cornwell et al., 2004), which may be involved in
peroxisomal biogenesis by promoting peroxisome division (Li
and Gould, 2002), as depicted in Figure 2A. Less is known about
how PPARdand PPARgaffect proliferation. Recently, high-fat
feeding of mice was shown to increase PPARgactivation and
peroxisome proliferation in the hypothalamus. Increased perox-
isome abundance was thought to decrease levels of neuronal
free radicals, which modulate feeding behavior through effects
on the melanocortin system (Diano et al., 2011). PPARgactiva-
tion may be involved in cold-induced proliferation of peroxi-
somes in BAT (Guardiola-Diaz et al., 1999). The transcriptional
coactivator PGC1a, critical for thermogenesis (Puigserver
et al., 1998), promotes cold-induced peroxisome biogenesis
in BAT in a PPARa-independent manner, implicating PPARg
or PPARdin this effect (Bagattin et al., 2010). Our preliminary
studies indicate that treatment of 3T3-L1 preadipocytes with
rosiglitazone substantially increases the expression of several
Pex genes, suggesting that PPARgpromotes peroxisomal
biogenesis in adipocytes (I.J.L and C.F.S, unpublished). Collec-
tively, these findings suggest that PPARs coordinate both perox-
isomal biogenesis and integrative lipid metabolism.
Crosstalk between Peroxisomes and Lipid Droplets
Intracellular lipid storage provides protection against toxic
effects of excessive lipid concentrations, a source of energy for
cellular growth and metabolic processes, and a survival advan-
tage during starvation. Lipid droplets are omnipresent subcellular
organelles that store lipids. Their structure resembles that of
circulating lipoproteins in mammals, consisting of a phospholipid
monolayer surrounding a core of neutral lipids such as triglycer-
ides and cholesteryl esters. Although most prominent in adipo-
cytes, which store excess energy as triglyceride, lipid droplets
are present in most cell types. Lipid droplets vary greatly in
size, ranging from less than 1 mmto100 mm in diameter. Mature
white adipocytes have large unilocular lipid droplets that occupy
the majority of the cytosol. Brown adipocytes have small multi-
locular lipid droplets. In most other cell types, lipid droplets are
less than 1 mm in diameter (Reue, 2011). Like peroxisomes, lipid
droplets are thought to be derived from the ER. According to
the prevailing model of lipid droplet biogenesis, neutral lipids
are incorporated into the interspace of the bilayer leaflets of the
ER membrane, where several enzymes involved in triglyceride
synthesis are localized. Once a certain threshold of triglyceride
accumulation is reached, the outer leaflet of the ER membrane
probably bulges into the cytosol through a budding process,
forming a new lipid droplet that is released from the ER (Farese
and Walther, 2009). Cytosolic lipid droplets increase their volume
through localized de novo lipogenesis, transport of extracellular
lipids into lipid droplets, or fusion with other lipid droplets.
Several lines of evidence suggest that peroxisomes are closely
associated with lipid droplets. In yeast grown in oleate, peroxi-
somes stablyadhere to lipid droplets (Binns et al., 2006). Thisinter-
action appears to be more than a simple juxtaposition; peroxi-
somes extend processes, referred to as pexopodia, into the lipid
droplet core. Proteomic analysis of these cells shows that lipid
droplets are enriched in peroxisomal b-oxidation enzymes (Binns
et al., 2006). Whetherperoxisomes and lipiddroplets intermingle in
animal cells is unknown. However, several studies have reported
close associations between peroxisomes and lipid droplets in
various mammalian cells. Live cell imaging in Cos-7 and HepG2
cells expressing GFP-PTS1 revealed tubulo-reticular clusters of
peroxisomes in close association with lipid droplets (Schrader,
2001). In 3T3-L1 adipocytes and mouse epididymal WAT, immu-
nogold staining with an antibody against catalase showed
numerous small dumbbell-shaped peroxisomes at the periphery
of lipid droplets (Blanchette-Mackie et al., 1995), confirming the
observationof Novikoffand colleagues using DAB staining (Novik-
off and Novikoff, 1982; Novikoff et al., 1980). Recently, several
protein-protein interactions involving lipid droplet resident pro-
teins and peroxisomal markers wereidentified using a bimolecular
fluorescence complementation assay (Pu et al., 2011).
Because peroxisomes and lipid droplets each control lipid
metabolic flux, the close interaction between the two organelles
suggests a coordinated regulation of metabolism and lipid
trafficking across their boundaries. When an organism requires
energy, very long chain fatty acids stored as triglycerides in lipid
droplets can be mobilized and rapidly transported to peroxi-
somes for initiation of fatty acid oxidation. However, it is unlikely
that lipid trafficking between droplets and peroxisomes is unidi-
rectional. As discussed below, peroxisomes are important in the
synthesis of alkyl ether equivalents of acylglycerols. These glyc-
erides may be differentially compartmentalized as compared to
those generated by acylation of glycerol-3-phosphate in the
ER. Mass-spectrometry-based lipidomic analyses show that
up to 20% of neutral lipids in lipid droplets are present as an ether
lipid equivalent of triglyceride, monoalk(en)yl diacylglycerol
(MADAG). MADAG is undetectable in lipid droplets from cells
lacking peroxisomes (Bartz et al., 2007). These studies support
the existence of bidirectional lipid trafficking between peroxi-
somes and lipid droplets, constituting crosstalk between the
two subcellular compartments to balance energy synthesis
and utilization depending on metabolic context.
Peroxisomal Fatty Acid Oxidation
Peroxisomes in all eukaryotic organisms are thought to have the
ability to oxidize fatty acids. In plants and most fungi, peroxi-
somes are the only site of fatty acid oxidation (Poirier et al.,
2006). In animal cells, fatty acid oxidation takes place in both
mitochondria and peroxisomes. It is becoming increasingly clear
that peroxisomes have a role in fatty acid oxidation that cannot
be replaced by mitochondria. Peroxisomes in animal cells were
originally thought to play only an ancillary role in fatty acid oxida-
tion, participating in oxidation only when the mitochondrial
oxidative capacity was exceeded. This notion was challenged
by the observation that patients with certain peroxisomal
disorders (discussed below) have increased serum levels of
very long chain (>22 carbon atoms) and branched chain fatty
acids (Brown et al., 1982; Poulos et al., 1986). Subcellular frac-
tionation studies with liver tissue confirmed that b-oxidation of
very long chain fatty acids preferentially occurs in peroxisomes
as compared to mitochondria (Singh et al., 1984). The accumu-
lation of branched chain fatty acids in patients with peroxisomal
disorders suggested a requirement for peroxisomes in a-oxida-
tion (Wanders et al., 2001). Branched chain fatty acids, such as
phytanic acid, have a methyl group on the third carbon atom
(gposition), which prevents b-oxidation. These fatty acids first
Cell Metabolism 19, March 4, 2014 ª2014 Elsevier Inc. 383
Cell Metabolism
Review
undergo oxidative decarboxylation (a-oxidation) in peroxisomes
to remove the terminal carboxyl group as CO
2
. This shortened
fatty acid now has a methyl group on the second carbon and
can be further oxidized in peroxisomes or mitochondria. Unlike
3-methyl branched fatty acids, 2-methyl fatty acids can undergo
b-oxidation with release of a propionyl CoA group, instead of
acetyl CoA, after the first round of b-oxidation (Verhoeven
et al., 1998).
Most fatty acids are metabolized by b-oxidation, which in-
volves the removal of two carbons at the carboxyl terminus of
the molecule and is carried out in peroxisomes and mitochon-
dria. As noted above, branched chain fatty acids are subject to
a-oxidation, which involves the removal of one carbon at the
carboxyl terminus. This process is thought to occur in peroxi-
somes but not in mitochondria; the adaptive rationale for lacking
a-oxidative capacity in mitochondria is not known. Long chain
and very long (> 22 carbons) chain fatty acids can be metabo-
lized by u-oxidation, which involves the removal of one carbon
at the end of the molecule farthest from the carboxyl terminus.
u-oxidation, thought to be a minor pathway under most physio-
logical circumstances, occurs in the smooth ER in a process
dependent on the cytochrome P450 CYP4A subfamily. Dicar-
boxylic acids, the product of u-oxidation, can then be metabo-
lized by b-oxidation in peroxisomes (Reddy and Hashimoto,
2001), suggesting a link between peroxisomal and microsomal
fatty acid metabolism in addition to the well-characterized link
between peroxisomal and mitochondrial fatty acid metabolism.
Similarities and Differences between Peroxisomal and
Mitochondrial Fatty Acid Oxidation
Although there are some fundamental differences in the
biochemistry of mitochondrial and peroxisomal b-oxidation
(Wanders and Waterham, 2006), the overall mechanism is
essentially the same and involves four consecutive enzymatic
steps: (1) dehydrogenation that removes two hydrogen mole-
cules and introduces a trans double bond between the aand b
carbons of the fatty acyl CoA molecule; (2) hydration across
the double bond, forming a 3-L-hydroxyacyl-CoA; (3) dehydro-
genation of the 3-L-hydroxyacyl-CoA, forming 3-ketoacyl CoA;
and (4) thiolytic cleavage of the terminal acetyl CoA group, form-
ing a new acyl CoA molecule that is shorter by two carbons.
Despite these similarities, there are several differences be-
tween mitochondrial and peroxisomal fatty acid oxidation (Wan-
ders and Tager, 1998). First, the enzymes involved in the two
pathways are entirely different. For example, the first step of mito-
chondrial b-oxidation is catalyzed by the flavoenzyme acyl CoA
dehydrogenase to produce FADH2, whose electrons are utilized
to produce two molecules of ATP. In contrast, the first step of
peroxisomal b-oxidation reaction is catalyzed by a different fla-
voenzyme, acyl CoA oxidase. Because peroxisomes lack a respi-
ratory chain, electrons from FADH
2
are transferred directly to O
2
(creating H
2
O
2
), and energy is released as heat, instead of being
used to generate ATP. Thus, peroxisomal fatty acid oxidation is
less favorable in terms of energy storage than mitochondrial fatty
acid oxidation. Because fatty acid oxidation in peroxisomes is not
carried to completion (Osumi and Hashimoto, 1980; Vanhove
et al., 1993), this process is thought to shorten very long chain
fatty acids, which can then be further oxidized in mitochondria.
Consistent with this concept, the substrate specificities of perox-
isomal and mitochondrial b-oxidation systems are different. Fatty
acids of 26 carbon chain length or greater are thought to be
oxidized exclusively in peroxisomes, although the number of
peroxisomal oxidation cycles utilized for these molecules is un-
known. Shorter fatty acids can be oxidized in either organelle
(Wanders et al., 2010), but the quantitative contributions of
each are not well defined, since metabolism varies depending
on substrates and physiological context.
Mitochondrial and peroxisomal b-oxidation systems also differ
in terms of fatty acyl CoA transport from the cytosol. Uptake of
fatty acyl CoAs across the mitochondrial membrane requires a
carnitine exchange system, consisting of two carnitine palmitoyl-
transferases (CPT1 and CPT2) and the transporter protein carni-
tine-acylcarnitine translocase (CACT). CPT1 is associated with
the outer mitochondrial membrane. It exchanges carnitine for
CoA, resulting in a fatty acid carnitine, which is transportedacross
the inner mitochondrial membrane by CACT. Once inside the
matrix, fatty acid carnitine is converted back to fatty acyl CoA
by CPT2. Carnitine does not appear to be required for fatty acyl
CoA uptake into peroxisomes. However, peroxisomes do have
carnitine acyltransferase activity, which may be required for con-
verting acyl CoAs to acylcarnitines so that they can be transferred
to mitochondria for further oxidation (Wanders, 2013). Fatty acyl
CoA uptake into peroxisomes requiresthree ATP binding cassette
transporter D subfamily proteins that are localized to the peroxi-
somal membrane: ABCD1, ABCD2, and ABCD3 (Wanders et al.,
2007). Each can form a functional homodimeric or heterodimeric
transporter by partnering with another ABCD protein. Each sub-
unit contains a transmembrane domain as well as an ATP binding
cassette. ABCD1 (also called ALDP) is mutated in the human
disease adrenoleukodystrophy. As a homodimer, ABCD1 is
involved in transporting very long chain fatty acids from the
cytosol to the peroxisomal lumen for b-oxidation (van Roermund
et al., 2008). ABCD2 is particularly abundant in adipose tissue,
where it may import dietary erucic acid (C22:1) into peroxisomes
(Liu et al., 2012a). ABCD3 (also called PMP70) may import long
chain fatty acids and branched chain acyl CoAs into peroxisomes
for fatty acid oxidation (Imanaka et al., 1999).
Although PPARais required for transcriptional regulation
of both pathways (Aoyama et al., 1998), acute regulation of
mitochondrial fatty acid oxidation is different from that of perox-
isomes. Unlike peroxisomal fatty acid oxidation, the mitochon-
drial pathway is exquisitely regulated by malonyl-CoA at the level
of CPT1 (McGarry et al., 1978; McGarry et al., 1977). Whether
there are analogous mechanisms for rapid control of peroxi-
somal fatty acid oxidation is unknown.
Physiological Significance of Peroxisomal Fatty Acid
Oxidation
It is likely that peroxisomal fatty acid oxidation is more complex
than simply shortening chains of very long chain fatty acids that
cannot be directly oxidized in mitochondria. The biochemical
basis underlying the metabolism of very long chain fatty acids
in peroxisomes, but not in mitochondria, is known. Peroxisomal
acyl CoA oxidases and mitochondrial acyl CoA dehydrogenases
manifest different chain length specificities. However, the phys-
iological basis for this organelle-specific functional difference is
not known. Perhaps mitochondrial toxicities associated with
the presence of very long chain fatty acids favored the evolu-
tionary selection of peroxisomes for very long chain metabolism
in mammals. Alternatively, peroxisome-dependent generation of
384 Cell Metabolism 19, March 4, 2014 ª2014 Elsevier Inc.
Cell Metabolism
Review
heat (see below) may provide tissue-specific benefits such as
maintenance of membrane structure, preservation of regional
blood flow to ensure oxygenation, or control of optimum temper-
ature to ensure enzymatic functions required for tissue regener-
ation or repair.
Ether lipids (which encompass alkyl-acylphospholipids and
alkenyl-acylphospholipids, also known as plasmalogens) (Fig-
ure 3) have unique physical properties that impact many aspects
of cell biology (discussed below). Several studies suggest that the
synthesis of ether lipids is linked to peroxisomal b-oxidation in a
manner that is physiologically relevant. Injection of [1-
14
C]ligno-
ceric acid (C24:0) into clofibrate-treated rats results in incorpora-
tion of the radiolabel into ethanolamine plasmalogens, suggesting
that peroxisomal fatty acid oxidation may preferentially channel
acetyl CoAs toward ether lipid synthesis (Hayashi and Oohashi,
1995). Consistent with this possibility, humans and mice with
mutations in ABCD1 have decreased levels of ethanolamine plas-
malogens, coupled with accumulation of very long chain fatty
acids in brain white matter (Khan et al., 2008). ABCD1 was
recently shown to interact with several lipogenic enzymes,
including ATP citrate lyase (ACLY), acetyl CoA carboxylase
(ACC), and fatty acid synthase (FAS) (Hillebrand et al., 2012).
FAS is a large multifunctional enzyme that primarily synthesizes
palmitate (C16:0) using malonyl CoA as a two carbon source
(Lodhi et al., 2011). We recently demonstrated increased fatty
acid oxidation in subcutaneous white fat in mice with adipose-
specific knockout of FAS (Lodhi et al., 2012). It possible that this
increased fatty acid oxidation was primarily peroxisomal since
FAS inactivation results in accumulation of malonyl-CoA, which
inhibits mitochondrial fatty acid oxidation (McGarry et al., 1977).
Lipid Biosynthesis in Peroxisomes
In 1977, Amiya Hajra and colleagues discovered that dihy-
droxyacetone phosphate acyltransferase (DHAPAT), an enzyme
required for the synthesis of acyl DHAP, a precursor for glycerol
ether lipids, was localized to peroxisomes (Jones and Hajra,
1977). This was a surprising discovery because it suggested
that peroxisomes were not only involved in catabolic reactions,
but also the synthesis of phospholipids. In the more common di-
acyl phospholipids, fatty acyl side chains are linked to the sn-1
and sn-2 positions of the glycerol backbone by ester bonds. In
ether lipids (such as alkyl ether phospholids and plasmalogens),
the sn-1 substituent is attached to the glycerol backbone by an
ether bond (Figure 3). In the subsequent 20 years, the Hajra labo-
ratory identified several other components of a peroxisomal ether
lipid synthetic pathway (Hajra and Das, 1996), while others found
that enzymes involved in bile acid synthesis were also localized to
peroxisomes (Pedersen et al., 1997). These studies firmly estab-
lished peroxisomes as a participant in lipid biosynthesis.
Synthesis of Ether Phospholipids
Ether phospholipid synthesis requires a series of enzymes asso-
ciated with the peroxisomal membrane (Figure 4). This pathway is
an alternative route for lysophosphatidic acid (LPA) synthesis (as
opposed to direct acylation of glycerol 3-phosphate) and is oblig-
atory for synthesis of precursors of all ether lipids in mammals,
including platelet activating factors (PAFs) and plasmalogens
(Hajra and Das, 1996). The pathway uses dihydroxacetone phos-
phate (DHAP), produced by dehydrogenation of glycerol 3-pho-
phate (G3P), as a building block for the synthesis of phospho-
lipids. Fatty acids derived from FAS-mediated de novo
lipogenesis or other sources are activated to fatty acyl CoA by
a peroxisome-membrane-associated acyl CoA synthetase
(ACS). Acyl CoA is used by DHAPAT to acylate DHAP or is
reduced to a fatty alcohol by fatty acyl CoA reductase 1 (FAR1)
in an NADPH-dependent reaction. A fatty alcohol is used by alkyl
DHAP synthase (ADHAPS) to convert acyl DHAP to alkyl DHAP.
Acyl DHAP or alkyl DHAP can be reduced to 1-Acyl G3P (LPA)
or 1-O-Alkyl G3P (AGP), respectively, by Acyl/Alkyl DHAP reduc-
tase (ADHAPR). ADHAPR was purified from guinea pig liver and
biochemically characterized (LaBelle and Hajra, 1974), but the
gene encoding this protein was not known in mammals (McIntyre
et al., 2003). We recently identified this gene and renamed the
protein it encodes PexRAP (for Peroxisomal Reductase Acti-
vating PPARg)(Lodhi et al., 2012). Subsequent steps of phospho-
lipid synthesis, such as acylation of the sn-2 position and addition
of the head group (e.g., CDP-choline or CDP-ethanolamine) take
place in the ER (Hajra and Das, 1996).
Properties and Functions of Ether Phospholipids
Ether lipids, such as plasmalogens, make up approximately 20%
of the total phospholipid mass in humans. Tissue levels of these
lipids vary greatly. Ether phospholipids are particularly abundant
in the brain, heart, and white blood cells. Up to two thirds of the
ethanolamine phospholipid in the brain and 12% of the total
myelin phospholipid pool is plasmalogen (Nagan and Zoeller,
2001). In neutrophils, up to 46% of the phosphatidylcholine
pool is in the plasmanyl form (Bra
¨utigam et al., 1996; Mueller
et al., 1982, 1984). In contrast, intracellular ether lipids are scant
in the liver (Braverman and Moser, 2012). This is surprising,
because many of the enzymes in the ether lipid synthetic
Figure 3. Chemical Structures of Diacyl-
and Ether-Linked Phospholipids
In conventional diacy l phospholipids, fatty acyl
side chains are linke d to the sn-1 and sn-2
positions of the glycerol backbone by ester
bonds. Ether-linke d phospholipids are a special
class of glyceropho spholipids that have an
alkyl chain attached to the sn-1 position by an
ether bond. The sn-2 substituent of ether lipids
is generally an ester-l inked acyl chain as in
diacylphospholipids. The head group of ether
phospholipids is usually choline or ethanol-
amine. Some of the ether linked phospholipids
have a cis double bond adjacent to the ether
bond and are referred to as alkenyl-acylphospholipids, or more commonly as plasmalogens. According to convention, plasmalog en form of
phospholipids have the prefix ‘‘plasmenyl,’’ as in plasmenylcholine. Alkyl-acylphospholipids have the prefix ‘‘plasmanyl.’’
Cell Metabolism 19, March 4, 2014 ª2014 Elsevier Inc. 385
Cell Metabolism
Review
pathway were originally purified from liver. Studies by Vance and
colleagues suggest that liver synthesizes ether phospholipids
but does not store them. Instead, hepatocytes preferentially
secrete ether phospholipids lipids in the form of lipoproteins.
Up to 30% of phosphatidylethanolamine in lipoproteins is in
the form of plasmalogen (Vance, 1990). Others have confirmed
significant levels of plasmalogens in human serum (Bra
¨utigam
et al., 1996). The precise molecular function of intracellular and
circulating ether lipids is unknown. In brain, several lines of evi-
dence suggest that ether lipids in myelin may be critical for
modulating ion transport, and similar effects may be at play in
heart (Ford and Hale, 1996). In white blood cells such as neutro-
phils, ether lipids may create a sink for polyunsaturated fatty
acids such as arachidonic acid utilized for inflammatory signaling
through the generation of eicosanoids (Nagan and Zoeller, 2001).
Regardless of the precise processes driven by ether lipids, dra-
matic developmental abnormalities in humans and mice with
defective ether lipid synthesis suggest that these lipids are
responsible for fundamental cellular tasks that cannot be carried
out by diacyl phospholipids.
Phospholipids are the predominant component of plasma
membranes and intracellular membranes. The presence of
ether-linked alkyl chains in phospholipids alters their physical
properties and modulates membrane dynamics. This effect is
presumably due to the lack of a carbonyl oxygen at the sn-1 po-
sition, which affects hydrophilic head group interactions (Lohner,
1996; Nagan and Zoeller, 2001). Model semisynthetic mem-
branes enriched in ethanolamine plasmalogens form nonlamellar
structures at around 30C, but diacylphospholipids form these
nonbilayer structures at much higher temperatures (68C).
This property of plasmalogens is thought to increase membrane
permeability and promote membrane-membrane fusion (Lohner,
1996). Plasmalogens are also important for the organization of
detergent resistant microdomains, cholesterol-rich membrane
regions that are thought to be involved in cell signaling. Plasmal-
ogen-deficient mice lacking DHAPAT (encoded by gnpat) show
evidence of disruption of these membrane microdomains;
cholesterol is inappropriately localized to a perinuclear compart-
ment, and flotillin-1 is decreased in fractions of neurological
tissue from these mice (Rodemer et al., 2003).
Ether lipids may be involved in cancer pathogenesis. It has
been known for decades that cancer cells have high levels of alkyl
ether lipids (Albert and Anderson, 1977; Howard et al., 1972;
Roos and Choppin, 1984; Snyder and Wood, 1969). Roos and
Choppin (1984) determined the tumorigenecity of mouse fibro-
blast cell lines differing in ether lipid content. One such line, F
40
,
with 30-fold higher ether lipid content as compared to its parental
line, required 1,000-fold fewer cells to form tumors in irradiated
mice, and these tumors were aggressive. The expression of the
ether lipid synthetic enzyme ADHAPS (also called alkylgycerone-
phosphate synthase [AGPS]) is increased in various cancer cell
lines and primary tumors (Benjamin et al., 2013). AGPS knock-
down impaired experimental cancer pathogenesis, including
cell survival, migration, and invasion. Conversely, AGPS overex-
pression increased ether lipids and promoted tumor growth.
While its precise role in cancer progression is unclear, AGPS
may generate structural and signaling lipids that promote carci-
nogenicity (Benjamin et al., 2013).
Figure 4. Acyl DHAP Pathway of Phospholipid Synthesis
This pathway is obligatory for synthesis of ether-linked phospholipids and is also an alternative route for syn thesis of diacylphospholipids. Phospholipid synthesis
begins in peroxisomes and is completed in the ER. The pathway uses dihydroxacetone phosphate (DHAP), generated by glycerol 3-phosphate dehydrogenase
(G3PDH)-mediated dehydrogenation of G3P, as a building block for the synthesis of phospholipids. Fatty acyl CoA produced by de novo lipogenesis is used by
DHAPAT (DHAP acyltransferase) to acylate DHAP or is reduced to a fatty alcohol by a peroxisomal membrane-associated fatty acyl CoA reductase in an NADPH-
dependent reaction. The fatty alcohol is used by alkyl DHAP synthase (ADHAPS) to convert acyl DHAP to alkyl DHAP. Acyl DHAP or alkyl DHAP can be reduced to
1-acyl G3P (lysophosphatidic acid) or its ether lipid equivalent, 1-O-alkyl G3P (AGP), respectively, by acyl/alkyl DHAP reductase (also called PexRAP). The
subsequent steps of phospholipid synthesis, including acylation at the sn-2 position, occur in the ER.
386 Cell Metabolism 19, March 4, 2014 ª2014 Elsevier Inc.
Cell Metabolism
Review
Emerging evidence suggests that alkyl ether phospholipids
may be important in cellular signaling. Recent studies with
DHAPAT knockout mice demonstrate that ether lipids likely
serve as self ligands to activate invariant natural killer T (iNKT)
cells (Facciotti et al., 2012). As discussed below, results from
our laboratory and others suggest that peroxisome-derived
lipids may also be involved in activating the nuclear receptor
PPARg.
Peroxisomes in Adipose Tissue Development and
Function
Adipose tissue is an important metabolic organ that regulates
whole-body energy balance. Two major types of adipose tissue
are found in mammals, white fat and brown fat. Both types store
energy as triglyceride in intracellular lipid droplets and secrete a
host of hormones, called adipokines, which influence metabolic
homeostasis. Whereas WAT primarily stores fat, which can be
mobilized in times of need, BAT transforms the chemical energy
in food into heat through uncoupled respiration. Because of their
contribution to lipid metabolism, peroxisomes may regulate
adipose tissue development and function (Figure 5).
Peroxisome Proliferation in Brown and White
Adipocytes
Peroxisomes in adipocytes are small in size and belong to a
subclass of the organelle referred to as microperoxisomes.
Microperoxisomes are difficult to detect by electron microscopy,
because they lack a crystalline nucleoid found in larger peroxi-
somes, such as those in hepatocytes (Novikoff and Novikoff,
1982). Nevertheless, these small peroxisomes are abundantly
present in adipocytes and are closely associated with lipid drop-
lets. Using histochemical or immunogold staining for catalase,
a large number of microperoxisomes were observed in 3T3-L1
adipocytes and mice epididymal WAT (Blanchette-Mackie
et al., 1995; Novikoff et al., 1980).
Figure 5. Relationship between Peroxisomes and PPARg
PPARgis a key regulator of adipocyte differentiation as well as function and is activated by multiple endogenous ligands, including alkyl ether phospholipids,
which are synthesized in peroxisomes. PPARgexists as a heterodimer with RXR and regulates expres sion of a large number of genes harboring PPAR response
elements (PPRE), including genes involved in adipogenesis, lipid metabolism, and glucose homeostasis. Emerging studies indicate that PP ARgalsoregul ates the
expression of genes involved in peroxisomal biogenesis, suggesting a feed-forward mechanism of PPARgactivation.
Cell Metabolism 19, March 4, 2014 ª2014 Elsevier Inc. 387
Cell Metabolism
Review
A large number of catalase-positive particles were also ob-
served in rat BAT and increased dramatically with cold exposure,
suggesting the presence of physiologically relevant microperox-
isomes in brown adipocytes (Ahlabo and Barnard, 1971). Subse-
quent studies showed that a hydrogen peroxide-producing
oxidase was present in these particles together with catalase,
consistent with their identification as bona fide peroxisomes
(Pavelka et al., 1976). Pex genes involved in peroxisomal biogen-
esis are significantly increased during differentiation of brown
adipocytes in culture and in BAT of mice exposed to cold temper-
atures in a manner dependent on PGC1a(Bagattin et al., 2010).
Peroxisomal metabolism may be related to heat production in
BAT. Acetyl CoAs generated by peroxisomal fatty acid oxidation
may enter the tricarboxylic acid cycle in brown fat mitochondria
to fuel thermogenesis mediated by UCP1, a mitochondrial un-
coupling protein that allows protons to return to the mitochon-
drial matrix without synthesizing ATP. Alternatively, because
peroxisomal b-oxidation itself generates heat instead of ATP
due to the lack of a peroxisomal electron transport system, per-
oxisomes may be involved in adaptive thermogenesis indepen-
dent of UCP1. Together, these studies indicate that peroxisomes
affect white and brown adipocyte differentiation and function.
Peroxisome-Derived Phospholipids in PPARgSignaling
and Adipocyte Differentiation
The nuclear receptor PPARgis required for adipocyte differenti-
ation and function. Studies in mice and cultured cells have estab-
lished PPARgas both necessary and sufficient for adipocyte
differentiation. Moreover, PPARgregulates whole-body lipid
metabolism and insulin sensitivity through its transcriptional con-
trol of various adipocyte activities, including fat storage, lipogen-
esis, adipokine production, and thermogenesis (Tontonoz and
Spiegelman, 2008). Genome-wide mapping of PPARgregulatory
elements using chromatin immunoprecipitation (ChIP) assays
combined with microarrays revealed several thousand distinct
PPARgbinding sites in adipocytes (Lefterova et al., 2008). How
does PPARgregulate such a large number of genes in adipo-
cytes? One possibility is that PPARgis activated by multiple
endogenous ligands that in turn recruit different transcriptional
coactivators controlling unique subsets of genes. PPARghas a
large ligand binding pocket capable of engaging a variety of
agonists (Xu and Li, 2008). Originally identified as the target of
the diabetes drugs thiazolidinediones, PPARgbinds several
different lipids, with variable effects on transcriptional activity
(Schupp and Lazar, 2010). Identification of specific endogenous
ligands of PPARghas been difficult. A study using a PPARg-
dependent reporter assay suggested that endogenous ligands
for PPARgcould be produced during early phase adipogenesis,
but these ligands remain unidentified (Tzameli et al., 2004).
Our previous studies suggested that de novo lipogenesis
mediated by FAS is involved in generating an endogenous ligand
for PPARain liver (Chakravarthy et al., 2009). To determine if
FAS is also involved in endogenous activation of PPARgin adi-
pocytes, we generated mice with adipose-specific deletion of
FAS. Studies with these mice and embryonic fibroblasts derived
from these animals suggested that FAS is part of a lipogenic
pathway that promotes PPARgactivation in adipose tissue. Us-
ing a mass-spectrometry-based approach, we identified several
FAS-dependent alkyl ether-linked phosphatidylcholine species
that were associated with PPARgin a rosiglitazone-displaceable
manner. Treatment of cells with one of these lipid species pro-
moted PPARg-dependent luciferase reporter activity, suggest-
ing that it could activate PPARg(Lodhi et al., 2012). As noted
above, the peroxisomal acyl-DHAP pathway of lipid synthesis
produces ether phospholipids. Activities of various enzymes in
this pathway increase during adipogenesis (Hajra et al., 2000).
Knockdown of PexRAP, the terminal enzyme in this pathway
(Figure 4), impairs PPARgactivation and adipogenesis in
cultured cells. Antisense oligonucleotide-mediated knockdown
of PexRAP in mice decreases expression of PPARgdependent
genes, reduces fat mass, increases leanness, and improves
insulin sensitivity (Lodhi et al., 2012).
We identified alkyl ether phospholipids as potential endoge-
nous ligands of PPARgin an unbiased screen, and there is
precedent for PPARgactivation by this class of lipids. Azelaoyl
phosphatidylcholine (1-O-hexadecyl-2-O-(9-carboxyoctanoyl)-
sn-glyceryl-3-phosphocholine), an oxidatively shortened
phospholipid in LDL, was reported to be a high affinity ligand
(equipotent to rosiglitazone) for PPARg(Davies et al., 2001). An
alkyl ether analog of LPA, 1-O-alkyl glycerol 3-phosphate
(AGP), a direct product of the PexRAP-catalyzed reaction (see
Figure 4), has been shown to be a high affinity partial agonist
for PPARg(McIntyre et al., 2003; Tsukahara et al., 2006; Zhang
et al., 2004). Since PPARgcan be activated by a variety of lipids,
it is possible that endogenous ligands of this nuclear receptor in
general exhibit partial agonism, thus permitting selective recruit-
ment of coactivators to regulate different subsets of genes.
A direct requirement for peroxisome-derived lipids in adipose
tissue development was suggested by studies in Pex7 knockout
mice that lack plasmalogens (Brites et al., 2009, 2011). These
mice have severely reduced body fat. This decreased adiposity
was not due to a significant difference in food intake and could
be rescued by feeding a diet enriched in alkyl-glycerol, which
restored plasmalogen levels (Brites et al., 2011). Exogenously
added alkyl-glycerol enters the ether lipid synthetic pathway
downstream of the peroxisomal steps. Because endogenous
ether phospholipids are present at low abundance in adipose
tissue (our unpublished observation), it is possible that ether
phospholipids have a signaling function. Whether adipose tissue
deficiency in Pex7 knockout mice is due to defective PPARg
signaling remains to be determined. Recently, an unbiased
mass-spectrometry-based metabolomics screen showed that
endogenous synthesis of alkyl-glycerol ether lipids increases
early during 3T3-L1 adipogenesis and that treatment with these
lipids promotes adipocyte differentiation (Homan et al., 2011).
Peroxisomal Disorders
Some degree of peroxisome function is required for health in
humans since there are several devastating genetic disorders
caused by impaired peroxisomal activity or lack of peroxisomes
due to defective peroxisomal biogenesis (Aubourg and Wan-
ders, 2013). Peroxisomal disorders are clinically heterogeneous.
However, they are consistently associated with impaired perox-
isomal lipid metabolism, resulting in the accumulation of VLCFAs
and phytanic acid, and defective synthesis of ether lipids and bile
acids.
Peroxisomal Biogenesis Disorders
Diseases caused by defects in peroxisomal biogenesis include
Zellweger spectrum disorders and rhizomelic chondrodysplasia
388 Cell Metabolism 19, March 4, 2014 ª2014 Elsevier Inc.
Cell Metabolism
Review
punctata type 1. Zellweger spectrum disorders affect 1 in 30,000
infants and include cerebrohepatorenal Zellweger syndrome,
neonatal adrenoleukodystrophy, and infantile Refsum disease.
These diseases result from mutations in one of the dozen Pex
genes involved in peroxisomal biogenesis. Mutations in Pex3,
Pex16, and Pex19, which result in the complete absence of per-
oxisomes, cause the most severe phenotypes. Mutations in
other Pex genes result in ghost peroxisomes. Features of Zell-
weger syndrome, which is the most severe end of the clinical
spectrum, include craniofacial dysmorphism, hepatomegaly,
and neurological abnormalities such as disruption of normal
development, hypotonia, seizures, glaucoma, retinal degenera-
tion, and deafness. Most Zellweger infants do not survive past
1 year of age due to respiratory compromise, gastrointestinal
bleeding, and liver failure. They seldom reach appropriate devel-
opmental milestones. The features of neonatal adrenoleukodys-
trophy and infantile Refsum disease are similar to those of
Zellweger syndrome, but these disorders progress more slowly.
Children with neonatal adrenoleukodystrophy usually die be-
tween the age of 2 and 3 years. Patients with infantile Refsum
disease can live into early adulthood (Aubourg and Wanders,
2013).
Rhizomelic chondrodysplasia punctata type 1 (RCDP1) is
clinically and genetically distinct from Zellweger spectrum disor-
ders. It is characterized by distinctive facial features, including a
prominent forehead, hypertelorism (widely set eyes), and ante-
verted nares (nostrils that face anteriorly due to an upturned
nose). These patients also suffer from growth failure, develop-
mental delay, seizures, and congenital cataracts. Most die
in early childhood. RCDP1 is caused by mutations in Pex7, a
chaperone for PTS2-containing peroxisomal matrix proteins,
including ADHAPS. A mouse model of RCDP1 has been gener-
ated by knocking out the Pex7 gene. Feeding these mice a diet
enriched in alkyl-glycerol partially rescues their phenotype,
implicating defective ether lipid synthesis in the pathophysiology
(Brites et al., 2011).
Disorders Caused by an Impaired Peroxisomal Function
Defects in a single peroxisomal protein in the setting of intact
peroxisomal structure result in adult Refsum disease (ARD),
X-linked adrenoleukodystrophy (X-ALD), RCDP2, and RCDP3.
ARD is characterized biochemically by accumulation of phy-
tanic acid, which is found in dairy products. Phytanic acid,
a branched chain fatty acid, must first undergo a-oxidation
in peroxisomes prior to b-oxidation in mitochondria. The pri-
mary defect in ARD is a mutation in the peroxisomal enzyme
phytanoyl-CoA hydroxylase that catalyzes the first step in
a-oxidation. Features of ARD include night blindness, loss of
peripheral vision, and ataxia. Symptoms evolve slowly and
usually appear in early adulthood. X-ALD is a disorder of
peroxisomal b-oxidation caused by mutations in ABCD1. It is
characterized by loss of myelin in the central nervous system
and disruption of the adrenal cortex. Major features include
learning disabilities, seizures, visual disturbances, and hearing
loss. RCDP2 and RCDP3 are related to RCDP1 (described
above). They are caused by defects in ether lipid synthesis
due to mutations in DHAPAT (GNPAT) and ADHAPS (AGPS),
respectively. A peroxisome database website (http://www.
peroxisomedb.org/) summarizes the genes involved in peroxi-
somal disorders.
Future Directions
As an organelle, peroxisomes get little respect. This may change
given increasing appreciation for the complexity of their interac-
tions with lipid droplets, the manner in which they complement
metabolic functions of mitochondria, and the critical part they
play in the generation of ether lipids. Several aspects of peroxi-
somal biology are ripe for novel translational pathways to human
disease.
Ether lipids have been known for decades to be increased in
a variety of cancers, although the precise implications of this
finding are poorly understood. FAS is linked to ether lipid pro-
duction, FAS expression is known to be increased in certain can-
cers, and ether lipids appear to be increased in particularly
aggressive cancers (Benjamin et al., 2013). It is possible that
targeted inhibition of a peroxisomal lipogenic pathway leading
to ether lipid production specifically in cancer cells could treat
malignancies without inducing developmental abnormalities
seen with congenital loss of ether lipid synthesis. Mitochondrial
function is altered in insulin-resistant states such as obesity
and diabetes. Based on the relationships between peroxisomes
and mitochondria, modulating ether lipid synthesis in the heart (a
site commonly affected in diabetes), white blood cells (which
may propagate diabetes complications through effects on
inflammation), nerves (neuropathy is a common and inade-
quately treated complication of diabetes), and other tissues
could affect mitochondrial endpoints potentially beneficial in
obesity-related diabetes. Defects in ether lipid synthesis cause
protean neurologic disorders, and elevations of certain ether
lipids are found in neurodegenerative disorders such as Alz-
heimer’s disease (Pettegrew et al., 2001). Dietary interventions
to alter peroxisomal activities could impact the clinical course
of neurologic diseases.
Peroxisomes are still mysterious. Uncovering their signaling
mechanisms and how they leverage relationships with lipid drop-
lets and mitochondria might lead to new approaches to disease
and prompt us to view peroxisomes as intracellular patricians
instead of plebians.
ACKNOWLEDGMENTS
This work was supported by NIH grants DK076729, DK088083, T32
DK007120, DK20579, DK56341, and K99 DK094874.
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