# Peroxisome Proliferator-Activated Receptors: “Key” Regulators of Neuroinflammation after  Traumatic Brain Injury

Authors: Philip F. Stahel; Wade R. Smith; Jay Bruchis; Craig H. Rabb
Journal: PPAR Research
 (2008-02-24)

---

## Abstract

Traumatic brain injury is characterized by neuroinflammatory pathological sequelae which contribute to brain edema and delayed neuronal cell death. Until present, no specific pharmacological compound has been found, which attenuates these pathophysiological events and improves the outcome after head injury. Recent experimental studies suggest that targeting peroxisome proliferator-activated receptors (PPARs) may represent a new anti-inflammatory therapeutic concept for traumatic brain injury. PPARs are “key” transcription factors which inhibit NFκB activity and downstream transcription products, such as proinflammatory and proapoptotic cytokines. The present review outlines our current understanding of PPAR-mediated neuroprotective mechanisms in the injured brain and discusses potential future anti-inflammatory strategies for head-injured patients, with an emphasis on the putative beneficial combination therapy of synthetic cannabinoids (e.g., dexanabinol) with PPARα agonists (e.g., fenofibrate).


---

## Body

## 1. Introduction

Research
efforts in recent years have provided increasing evidence that the
intracerebral inflammatory response is in large part responsible for the
devastating neuropathological sequelae and poor outcome of traumatic brain
injury [1–3]. The extent of brain damage
is determined by primary and secondary injury patterns. While the primary
injury results from mechanical forces applied to the skull and brain at the
time of impact, secondary brain injury occurs as a delayed consequence of
trauma [4–7].  Secondary brain injuries are mediated by
endogenous pathophysiological processes which lead to an overwhelming
neuroinflammation in the injured brain
[6, 8–10]. The main risk factors for
developing secondary brain injuries are hypoxemia and systemic hypotension
which occur frequently in the trauma patient [11, 12]. These conditions contribute
to the ischemic brain damage and perpetuate the intracerebral inflammatory
reaction through ischemia/reperfusion-mediated mechanisms [13]. Peroxisome
proliferator-activated receptors (PPARs) are ligand-activated transcription
factors of the nuclear receptor superfamily which have recently been shown to
exert anti-inflammatory properties in acute neurological disorders. These
include cerebrovascular stroke, intracerebral hemorrhage, spinal cord injury,
and traumatic brain injury [14–21]. The present paper provides
an overview on the so far known anti-inflammatory properties of PPARs in brain
injury and discusses potential pharmacological properties of PPAR agonists as
future neuroprotective agents.


## 2. Biological Functions of PPARs

PPARs are nuclear membrane-associated
transcription factors belonging to the nuclear receptor family [22]. Three isotypes with a
differential tissue distribution have been described: PPARα (NR1C1), PPARβ/δ   (NR1C2),
and PPARγ (NR1C3) [23, 24]. While PPARβ/δ  has an
ubiquitous expression, PPARα and PPARγ  are mainly expressed in tissues with high
fatty acid catabolism, such as adipose tissue, liver, kidney, and skeletal
muscle [25]. Mechanistically, PPARs are activated by
heterodimerization with the retinoid-X receptor (RXR) into biologically active
transcription factors. PPAR-RXR heterodimers induce the transcription of
candidate genes by binding to so-called peroxisome proliferator-response
elements (PPRE's) consisting of DNA-specific sequences (see Figure 1).Figure 1
Mechanism of gene transcription
through ligand binding on peroxisome proliferator-activated receptors (PPARs). In presence of coactivating stimuli, PPARs heterodimerize with retinoid
X receptors (RXR) to form active transcription factors. The DNA binding domain
on PPAR-RXR heterodimers induces the transcription of target genes by binding
to peroxisome proliferator-response elements (PPRE's) which consist of
DNA-specific sequences.PPARs exert a wide variety of physiological functions [24, 26]. They
play a central role in the regulation of lipid and lipoprotein metabolism and
glucose homeostasis, and have been shown to mediate cellular proliferation and
programmed cell death (apoptosis) [27–31]. PPARs
have furthermore been involved in bone metabolism and in pathologies of the
cardiovascular system and the lung [32–35]. PPARα has been attributed important immunological
functions due to its expression on monocytes/macrophages, T cells, and vascular
endothelial cells. PPARγ appears to
play a crucial role in the regulation of proliferation and differentiation of
various cell types. While the biological role of PPARβ/δ has not
been defined in detail, recent data imply an antiapoptotic and
anti-inflammatory effect after tissue injury, both in vitro and in vivo [29].From an immunological viewpoint, PPARs have been identified
as important regulators of inflammatory gene expression [36–40]. PPARs
have also been shown to attenuate adaptive immune responses by inhibiting
helper T cell functions and by mediating apoptosis of B cells [41, 42]. PPARs are
activated by naturally ocurring fatty acid derivatives, eicosanoides, and by
synthetic pharmacological agents, such as fibrates (PPARα) and glitazones
(PPARγ) [18, 22, 43]. PPAR ligands
have been shown to exert anti-inflammatory activities in various cell types by
inhibiting the gene expression for proinflammatory cytokines, metalloproteinases,
and hepatic acute-phase proteins.


## 3. PPARs: “Key” Regulators of Neuroinflammation

Mechanistically, the activation of PPARα   has been shown to inhibit
proinflammatory gene transcription by repressing the central inflammatory
transcription factor, nuclear factor-κB (NF-κB) [43–45]. Along with suppression of
NF-κB, PPARα  acts by inhibition of signal transduction through activator
protein-1 (AP-1) signaling [43]. It appears that the
inhibitory effect of PPARα on these crucial inflammatory transcription factors
creates a negative feedback loop for controlling acute posttraumatic
inflammation [44–46]. First in
vivo data on the involvement of PPARs in the regulation of inflammation were
reported from studies in PPARα knockout
mice [47]. Cuzzocrea et al. showed that
the targeted deletion of the PPARα gene leads
to a significantly increased inflammatory response in different experimental
models of acute inflammation outside the central nervous system (CNS) [47]. Within the CNS, the
constitutive expression of PPARs has been described for some time [48, 49]. Interestingly, PPAR gene
expression was detected not only on vascular endothelial cells in the brain and
spinal cord, but also on resident cells in the CNS, such as neurons and glial
cells [49].


## 4. Role of PPARs in CNS Injury

In recent years, experimental studies in models of cerebral ischemia/reperfusion
injury, ischemic stroke, intracerebral hemorrhage, and spinal cord injury have
revealed a crucial role of PPARs in attenuating neuroinflammation and neuronal
cell death in the injured CNS (see Table1) [14–16, 19, 20, 50, 51, 52]. PPARα gene-deficient mice (PPARα−/−) were shown to have a significantly
worsened neurological outcome, associated with an increased neuroinflammatory
response to experimental spinal cord injury, as compared to wild-type
littermates [16]. The postulated
neuroprotective effects of natural PPARα ligands include the attenuation of
polymorphonuclear leukocyte (PMNL) recruitment and associated neurotoxicity, as
determined by a significantly reduced expression of myeloperoxidase in the
injured spinal cord of PPARα−/− mice [16]. In addition, tumor
necrosis factor (TNF), a “key” mediator of neuroinflammation and neurotoxicity,
was shown to be upregulated and associated with neuronal apoptosis in the
injured spinal cord of PPARα−/−   mice [16]. In traumatic brain
injury, experimental studies in the past
decade have shown that TNF is upregulated in the intracranial compartment
within a few hours after trauma, and contributes to secondary neuronal injury [53–55]. The deleterious neurotoxic effects were shown to be abrogated
by pharmacological inhibition of TNF [56]. Since PPARs inhibit proinflammatory gene transcription by
attenuating NF-κB signaling [43–45], the potent PPAR-mediated
neuroprotective effects may be dependent on
inhibition of NF-κB-dependent proinflammatory cytokines released in the
injured brain, such as TNF, interleukin (IL)-1, IL-8, IL-12, and IL-18 [57–61]. The central role of NF-κB
signaling in inflammation and oxidative stress explains why PPARs have been
considered possible targets for neuroprotection in inflammatory CNS diseases,
including traumatic brain injury [14, 20, 62, 63].Table 1
Selected
publications on the role of PPARs in CNS injury and inflammation.
Models of CNS injury and
neuroinflammationPPAR isotypeMain findingsReference no.Different models of CNS injuryPPARγReview on the mechanisms
of neuroprotection by PPARγ
agonistsKapadia et al. [20]Different models of CNS injuryPPARα, PPARγReview on
pharmacological neuroprotection by PPARsBordet et
al. [14]Brain inflammationPPARγReview on regulation of
microglial activation by PPARγ
agonistsBernardo and Minghetti [63]Spinal cord injuryAll isotypesReview on the role of
PPAR signal transduction in spinal cord injuryVan Neerven and Mey [15]Spinal cord injuryPPARαExperimental model of spinal cord injury in PPARα gene knockout mice. Lack of PPARα leads to worse outcome and increased
neuroinflammation.Genovese et al. [16]Cerebral ischemia/reperfusion injuryPPARγThe PPARγ agonists rosiglitazone and pioglitazone
exert neuroprotective effects in a rat model of cerebral ischemia/reperfusion
injury by reducing neuroinflammation and oxidative stress.Collino et al. [50]Intracerebral hemorrhagePPARγPPARγ expressed by microglia and macrophages
promotes the resolution of intracerebral hemorrhage and attenuates the
neuroinflammatory response.Zhao et al. [19]Traumatic brain injuryPPARαThe PPARα agonist fenofibrate reduces brain edema
and improves the neurological outcome after experimental fluid
percussion brain injury in male Sprague-Dawley rats.Besson et al. [21]Traumatic brain injuryPPARαThe PPARα agonist fenofibrate promotes neurological
recovery by reducing inflammation and oxidative stress in rat brains after
experimental fluid percussion brain injury.Chen et al. [17]NeuroinflammationAll isotypesReview on the interaction between cannabinoids and
PPARs as inhibitors of neuroinflammationSun and Bennett [83]


## 5. Pharmacology of Head Injury: Are PPAR-Agonists and Cannabinoids the Long Sought “Golden Bullet”?

A wide variety of natural and synthetic PPARγ  agonists have been described in recent years
as regulators of microglial activation and cerebral inflammation [63]. For
example, the thiazolinedione pioglitazone has been shown to reduce the  extent of
neuroinflammation and the severity of disease in experimental autoimmune
encephalomyelitis (EAE), the animal model for multiple sclerosis (MS) [64, 65]. A recent
case report described the impressive clinical improvement of a patient with
chronic progressive MS, after a 3-year period of treatment with pioglitazone [66]. This
unexpected clinical recovery implies that PPARγ agonists may represent a promising new
strategy for attenuating neuroinflammation in patients with CNS autoimmune
diseases [62, 63, 67].In cerebrovascular stroke, the combination
therapy of a PPARγ agonist
(rosiglitazone) with an antiexcitotoxic glutamate receptor antagonist (MK-801)
led to an improved neurological recovery in rats undergoing middle cerebral
artery occlusion [18]. A study by another group
assessed the therapeutic efficacy of two different PPARγ agonists, rosiglitazone and pioglitazone, in a
rat model of cerebral ischemia/reperfusion injury [50]. The authors showed that
the pretreatment with either compound led to a significant attenuation of
inflammation and oxidative stress in injured rat brains [50].Pharmacological ligands to PPARα, such as
fenofibrate, have also been shown to exert neuroprotective effects in
inflammatory CNS conditions. Deplanque et al. demonstrated a significant
neuroprotective effect of fenofibrate administration in C57BL/6 mice with
cerebrovascular stroke [68]. The authors suggested that
PPARα may represent a new pharmacological target to reduce the neuroinflammatory
and neuropathological sequelae of cerebrovascular stroke [68].In traumatic brain injury, the PPARα agonist
fenofibrate appears to represent a highly promising new anti-inflammatory
compound. Besson et al. assessed the pharmacological role of fenofibrate in a
model of experimental fluid-percussion injury in adult male Sprague-Dawley rats [21]. The authors revealed that
the administration of fenofibrate during a clinically relevant therapeutic
“time window of opportunity” at 1 hour after trauma mediated a significant
posttraumatic neuroprotection. This was demonstrated by improved neurological
scores in the fenofibrate group at 24 hours and 7 days after trauma, compared
to vehicle-treated animals [21]. Morphologically, fenofibrate
treatment resulted in significantly decreased extent of brain edema at 24 hours
after head injury, compared to the placebo group. The authors furthermore
described a marked reduction in intercellular adhesion molecule (ICAM)-1
expression at the protein level by immunohistochemistry in injured rat brain
sections after fenofibrate administration [21]. This finding implies a
reduced extent of intracerebral immunoactivation and neuroinflammation in rats
treated by the PPARα agonist, compared to vehicle controls.A more recent follow-up study by the same
research group assessed the role of PPARα in modulating the oxidative stress in
the injured rat brain [17]. Oxidative stress and
ischemia/reperfusion-mediated injuries contribute significantly to the extent
of posttraumatic intracerebral inflammation and delayed secondary brain damage
after head injury [13, 69, 70]. Pathophysiologically,
contused brain areas are surrounded by a penumbra zone which is hypoperfused
due to traumatic vascular damage, loss of cerebrovascular autoregulation, and
systemic hypotension. After resuscitation, the hypoperfused, ischemic brain
areas in the penumbra zone are reperfused, which leads to activation of the complement
cascade and of reactive oxygen intermediates by activation of the xanthine
oxidase [71, 72]. Oxygen-derived free radicals
such as hydroxyl ions, hydrogen peroxide, and superoxide anion induce lipid
peroxidation, cell membrane disintegration, and delayed neuronal cell death (see
Figure 2). Lipid peroxidation is facilitated in the brain due to its genuine
vulnerability to oxidative stress based on specific morphological
characteristics, such as a high ratio of “membrane to cytoplasm” and high
levels of polyunsaturated fatty acids in the CNS [70]. In addition to reactive
oxygen intermediates, the generation of nitric oxide (NO) by inducible NO
synthase (iNOS) up-regulation also occurs after head injury and adds to the
extent of secondary brain damage [73]. Metabolites emerging from
the interaction between superoxide anion and NO, such as the highly reactive
oxidant peroxynitrite, have been shown to mediate neurotoxicity and delayed
neuronal cell death after traumatic brain injury [74].Figure 2
Working hypothesis of
PPAR-mediated mechanisms of neuroprotection after traumatic brain injury. The
neuropathological sequelae of head injury include the posttraumatic activation
of NFκB-dependent
inflammatory genes. The transcription of neuroinflammatory mediators in the
injured brain induces and perpetuates the intracranial inflammatory response
and leads to formation of brain edema and adverse outcome. Activation of PPARs
by binding of synthetic ligands, such as the PPARα agonist fenofibrate, leads to inhibition of NFκB and of downstream transcribed proinflammatory
and proapoptotic mediators. In addition, cannabinoids have a dual
neuroprotective function, (1) by acting as ligands to PPARs and (2) by
inhibiting “key” mediators of neuroinflammation and apoptosis, such as tumor
necrosis factor (TNF). The combination therapy of synthetic PPAR agonists and
cannabinoids may represent the long sought pharmacological “golden bullet” for
the treatment of traumatic brain injury in the future.The pharmacological administration of the
PPARα agonist fenofibrate after experimental fluid-percussion injury resulted
in a significant decrease of intracerebral iNOS expression [17]. This was associated with a
decreased neuroinflammation in the injured brain and an improved neurological
recovery after trauma [17]. These important findings
imply that the attenuation of oxidative stress may represent a “key”
mechanistic aspect of PPAR-mediated neuroprotection after head injury. The
pleiotropic beneficial effects of PPARs in the injured brain, however, are far
from being elucidated in detail until present. For example, in contrast to PPARα,
no studies have yet been performed to analyze the effect of PPARγ in experimental models of traumatic brain
injury (see Table 1).Despite increasing insights
into the pathophysiological mechanisms of posttraumatic neuroinflammation and
neurodegeneration, clinical neuroprotection trials have failed to provide a
benefit of anti-inflammatory pharmacological strategies with regard to the
outcome after head injury [75, 76]. Cannabinoids have recently
evolved as a promising new therapeutic avenue for neuroprotection after head
injury [77–79]. This group of compounds
consists of natural (endocannabinoids) and synthetic ligands, such as
dexanabinol (HU-211). The endocannabinoid 2-arachidonoyl glycerol (2-AG) has
received increased attention in recent years due to its strong neuroprotective
effect after head injury, by inhibition of proinflammatory cytokines, reactive
oxygen intermediates, and excitotoxic aminoacids, such as glutamate [80, 81]. The pharmacological agent
dexanabinol was shown to mediate neuroprotection by inhibition of TNF
production in injured rodent brains [77, 82] and was recently proposed as
an effective neuroprotective strategy to reduce the extent of secondary brain
injury in humans (see Figure 2) [78, 79]. Dexanabinol (HU-211) is a
nonpsychotropic, synthetic cannabinoid which exerts beneficial effects by
cytokine inhibition and radical scavenging associated with reduction of brain
edema [77–79, 82]. Cannabinoids were attributed
a new role as neuroprotective agents by agonistic action to PPARs [83]. The functional interaction
between cannabinoids and PPARs was first described based on the finding of
oleylethanolamide (OEA), a lipid derivate structurally related to anandamide,
as a regulator of feeding behavior in rats through activation of PPARα [68, 84]. Aside from OEA, which is a
low-affinity agonist to cannabinoid receptors, other cannabinoids were recently
described as PPAR ligands [83]. As such, Δ9-tetrahydrocannabinol (THC) was
found to activate PPARγ in human
cell lines [85]. Of particular
interest for neuroprotection in traumatic brain injury is the finding that the
potent endocannabinoid 2-AG [80, 81] has been found to suppress
the proinflammatory cytokine IL-2 through PPARγ signaling, independent of 2-AG binding
to cannabinoid receptors [86]. Future studies will have to
determine whether cannabinoids represent the long sought “golden bullet” for
reduction of secondary brain damage after head injury. It seems reasonable to
suggest that a combination of neuroprotective cannabinoids, such as
dexanabinol, with other potent anti-inflammatory therapeutic agents, such as
synthetic PPAR ligands, may represent a promising new therapeutic avenue for
improving the outcome of traumatic brain injury.



---

*Source: 538141-2008-03-27.xml*