Recent Insights on the Role of PPAR-β/δ in Neuroinflammation and Neurodegeneration, and Its Potential Target for Therapy

Peroxisome proliferator-activated receptors (PPAR) belong to the family of hormone and lipid-activated nuclear receptors, which are involved in metabolism of cholesterol, sphingolipids, and fatty acids. The transcriptional activity of PPARs is known to engage in a variety of cellular functions including cell differentiation, proliferation, and development (Hong et al. 2019). These receptors heterodimerize with retinoid X receptor (RXR), and the dimer regulates gene expression in response to dietary-derived fatty acids as well as exogenous agonists. Activation of these receptors by endogenous or exogenous ligands can evoke transduction of signals and induce interaction with lipoproteins, coactivators, or corepressors (Evans and Mangelsdorf 2014; Varga et al. 2011). PPARs not only play a role on regulating lipid metabolism and signaling, but also for maintenance of carbohydrates and glucose homeostasis.

Similar to PPAR-α and PPAR-γ in this family, PPAR-β/δ, which is also known as PPAR-δ, was cloned from the mouse genome and identified as an orphan nuclear receptor in the 90 s (Hong et al. 2019). Subsequently, two existing isoforms of this protein were identified by alternative splicing of gene NR1C2. PPAR-β/δ contains the canonical structure domains common to other nuclear receptor family members, including the amino-terminal AF-1 trans-activation domain, a DNA-binding domain, and a dimerization and ligand-binding domain with a ligand-dependent trans-activation function AF-2 at the carboxy-terminal region (Azhar 2010). The amino-terminal AF-1 trans-activation domain is responsible for transcriptional activation. It provides constitutive activation function independent of ligand binding. The DNA-binding domain (DBD, domain C), which is comprised of two zinc-finger motifs, is involved in DNA recognition and protein–protein interaction. While the hinge domain (domain D) is succeeded by the C-terminal Ligand-binding domain (LBD, domains E/F), which contains not only the ligand-binding pocket, but also regions important for dimerization and the AF-2 domain. Ligand binding is thought to induce structural changes in the AF-2 domain, allowing the recruitment of co-activator proteins important for transcriptional activation, thereby serving as a switch to activate PPARs (Brunmeir and Xu 2018). So far, only one post-translational modification for PPAR-β/δ is known. Koo and colleagues showed that PPAR-β/δ SUMOylation at K104 is removed by SUMO-Specific Protease 2 (SENP2) and this promotes the expression of FAO genes in muscle (Koo et al. 2015).

PPAR-β/δ is comprised of 441 amino acids with a molecular weight of 49.9 kDa. According to Gene Cards, this protein is widely expressed and detected in human tissues, including the brain, pancreas, liver, and heart (Hong et al. 2019). Although PPAR-β/δ is expressed in cells in all brain regions, neurons appear to have the highest expression. Warden et al. (2016) demonstrated localization of PPAR isotypes in the adult mouse and human brain (Fig. 1). Using quantitative PCR and double immunofluorescence microscopy, investigation among brain parts indicated highest level of mRNA and proteins in the prefrontal cortex (Warden et al. 2016). In the brain, although all PPAR isoforms have been detected in neuronal and astrocytes, PPAR-β/δ appeared to have low immunoreactivity in microglia as compared with other PPARs members. Analysis of subcellular localization indicated that PPAR-β/δ in neurons is present both in the cytoplasm and nucleus. Nevertheless, its intracellular localization may change depending on patho-physiological conditions and applied therapy (Gamdzyk et al. 2018).

Until recently, studies on the role of PPAR-β/δ were largely carried out with peripheral organs/tissue (Phua et al. 2020). Its expression is detected at early stage of embryogenesis, and disruption of this gene is lethal due to severe placental defects. The knockout animals are characterized by alterations of skin and fat mass, and impairment of brain development. PPAR-β/δ seems to play a key role in embryo development, and its deletion can induce a high rate of mortality around embryonic day 10.5 (E10.5) (Hall et al. 2008; Nadra et al. 2006). At this time of development, the expression of PPAR-β/δ could be detected in all brain regions, including the cerebral cortex, thalamus, cerebellum and brainstem, and reaching peak levels between E 13.5 and E 15.5 (Gofflot et al. 2007; Braissant and Wahli 1998). The expression of PPAR-β/δ was found in neurons, astrocytes, oligodendrocytes, and recently, also in microglia cells (Schnegg and Robbins 2011; Carniglia et al. 2013). In addition, this receptor is also expressed in brain capillary endothelial cells, suggesting an involvement in regulation of blood/brain barrier (Akanuma et al. 2008). Studies using genetically modified PPAR-β/δ null mice indicated changes in brain weight, and concomitantly, the body weight was also smaller as compared to the wide-type control (Peters et al. 2000). Histological study showed disturbances in myelination in the corpus callosum, more frequently in females comparing to males (Markham et al. 2009).

During the past decade, most studies on PPAR-β/δ were carried out with muscle and other peripheral tissue/organs, and relatively few studies were carried out with the brain (Grimaldi 2007; Phua et al. 2020; Wang et al. 2020). The study by Rosenberger et al. (2002) showed significant alterations of phospholipids and esterified fatty acids, together with gender differences in the brain of PPAR-β/δ null mice. Results with PPAR-β/δ null mice also showed defects in brain peroxisomal acyl-CoA utilization and thus projected a role in myelination. PPAR-β/δ also can influence genes engaged in enzymes responsible for fatty acid β oxidation pathway in mitochondria and peroxisome (Grimaldi 2007; Lamichane et al. 2018). Information in Fig. 2 demonstrates the potential roles of PPAR-β/δ in lipid metabolism in the brain. In many instances, these roles are comparable to those demonstrated in hepatocytes and in some tumor cells (Beyaz and Yilmaz 2016).

PPAR-β/δ can alter brain membrane phospholipids, through post-translational modification via the acylation process. This process may lead to changes in protein functions, such as the myelin proteolipid proteins (PLP) (Campagnoni and Macklin 1988). On the other hand, activation of PPAR-β/δ by long-chain saturated and unsaturated fatty acids (LCSFA, LCUFA) and their metabolites may lead to regulation/modulation of transcription of genes encoding proteins such as fatty acid binding proteins (FABP) and fatty acid translocase (FAT).

PPAR-β/δ is also engaged in regulation of cholesterol release and metabolism. However, despite that adult CNS contains 23% of the total sterol pool in the entire body, little information is available regarding this receptor and cholesterol in the brain (Dietschy and Turtley 2004). Cholesterol is an important constituent of the plasma membrane and is the major component of myelin in adult human brain where it consists 70–80% of the whole brain cholesterol. In human adult brain, cholesterol level reaches 23 mg/g w.bw, however, at birth only 6 mg/g bw and in adult mouse brain about 18 mg/g bw. With participation of apolipoprotein E and A, PPAR-β/δ may alter cholesterol metabolism in the brain, and exert effects on neural and glial cells differentiation. However, the relationship between cholesterol metabolism, PPARs, and neurodegeneration/neuroinflammation is till now not fully elucidated.

In the peripheral system, PPAR-β/δ agonists have been proposed for treatment of metabolic syndrome (MetSD) which is tightly connected with long-chain fatty acid (LCFA) homeostasis (Varga et al. 2011). Nevertheless, molecular sequence of events leading to imbalance of lipid homeostasis is still not well understood. It is suggested that higher levels of circulating LCFA and their availability can induce fat accumulation in adipose tissue, liver, and other tissues, leading to insulin resistance and DMT2. Activation of PPAR-β/δ in animal model leads to improvement of lipid homeostasis and insulin sensitivity (Tanaka et al. 2003). There is evidence that LCFA signaling is mediated by PPAR- β/δ, which also plays a crucial role in lipid absorption and intestinal physiology. Due to the proangiogenic and pro-/anti-carcinogenic properties of PPAR-β/δ ligands, these compounds may serve as therapeutic agents for treating metabolic syndrome, dyslipidemia, and diabetes (Bishop-Bailey and Swales 2008).

PPAR-β/δ plays a crucial role in diseases associated with alterations of lipid and glucose metabolism, including MetSD, DMT2, and atherosclerosis. MetSD is a complex pathological condition together with dyslipidemia, hyperglycemia, central obesity, and hypertension, many are associated with prothrombotic and proinflammatory state. The study by Serrano-Marco et al. (2011) suggested that PPAR-β/δ activation could impede IL6-induced STAT3 activation by inhibition of ERK1/2 and prevention of STAT3 association with Hsp90. This effect may contribute to the suppression of cytokine-induced insulin resistance in adipocytes and possibly may also occur in the brain. Obviously, more studies are needed to better understand the role of this receptor in neurodegenerative and neuroinflammatory diseases.

PPARs are known to modulate inflammatory processes associated with lipid signaling pathways. Suppressing inflammatory processes in the CNS could lead to reduction of brain damage and improvement of motor and cognitive outcome (Villapol 2018). Resident microglia and infiltrated inflammatory cells were regarded as mediators responsible for this process (Salvi et al. 2017).

PPAR-β/δ is not only a lipid sensor, but also a regulator of mitochondrial function, and may influence oxidative stress and inflammation in brain cells as well as proliferation and angiogenesis in vascular endothelial cells (Bishop-Bailey and Swales 2008). There is evidence that PPAR-β/δ regulates vascular function by enhancing VEGFR expression, phosphorylation of AKT, and subsequently regulating endothelial NO production and reducing ROS and inflammation (Jiang et al. 2019).

Systemic inflammatory responses (SIR) evoked by the endotoxin lipopolysaccharide (LPS) may contribute to neurodegenerative disorders (Brown 2019). PPARs are unique set of fatty acid regulated transcription factors controlling both inflammation and lipid metabolism (Varga et al. 2011; Schnegg and Robbins 2011). It was recently reported that PPAR-β/δ agonists exerted significant anti-inflammatory effects and suppressed the genes encoding iNOS, several chemokines such as CXCL1, CXCL2, CXCL10 interleukins IL1, IL6, and other cytokines TNF-α, IFN-γ, and concomitantly enhanced IL10 (Kuang et al. 2012; Chehaibi et al. 2017; Beyaz and Yilmaz 2016). Agonist of PPAR β/δ such as GWO 742 could decrease neutrophil infiltration into the brain during ischemia and protects against neuroinflammation (Chehaibi et al 2017). However, activation of other members of PPARs evoked also anti-inflammatory effect (Varga et al. 2011; Carniglia et al. 2013; Villapol 2018).

In our previous studies, acute SIR evoked by LPS administered i.p. to mice-induced memory impairment showed alterations of transcription of pro-oxidative, inflammatory genes, and genes engaged in cells death signaling (Czapski et al. 2010; 2016; Jacewicz et al. 2009). During the last decade, there has been increasing interest on the involvement of PPAR-β/δ in inflammatory processes (Bishop-Bailey and Bystrom 2009; Piqueraset al 2009; Schnegg and Robbins 2011).

Neurodegeneration and neuroinflammatory processes play a significant role in brain ischemia/hypoxia pathology. Alteration of lipid metabolism, including polyunsaturated fatty acids and synthesis of several eicosanoids and docosahexanoids were recognized as the early and most important events in ischemia/hypoxia encephalophathy (Bazan 1970; Tang and Sun 1985; Strosznajder and Domanska-Janik 1980; Nalivaeva and Rybnikova 2019). Omega 3 fatty acids supplementation was shown to exert protection via anti-inflammatory action by suppressing microglia response in neonatal hypoxic-ischemic brain injury (Zhang et al. 2010). Study by Saganuma et al. (2013) also demonstrated that docosahexaenoic acid (DHA) supplementation may be beneficial in ischemia hypoxia encephalopathy. In a rodent model of brain ischemia, the data by Song et al. (2019) showed that oleic acid (OA) could mediate neuroprotection through PPAR-γ activation and its anti-inflammatory effect.

Using GW0742, a specific agonist for PPAR-β/δ receptor, the study by Gamdzyk et al. (2018) demonstrated that stimulation of PPAR-β/δ could exert neuroprotective effects in a rat model of neonatal hypoxic–ischemia (HI). In this study, administration of GW0742 reduced brain infarct area, brain atrophy, apoptosis, and improved neurological function at 72 h and 4 weeks post HI. Additionally, GW0742 administration induced several molecular processes, e.g., enhancing the transcription of gene coding PPAR-β/δ, increase in miR-17-5p level, and downregulation of the Thioredoxin Interacting Protein (TXNIP) in the ipsilateral hemisphere. These events also led to inhibition of the Apoptosis Signal-regulating Kinase 1 (ASK1/p38) signaling pathway and reduced apoptotic cell death. In contrary, GSK3787, an antagonist of this receptor, was shown to reverse the protective effects evoked by intranasal administration of GW0742. The study of Hack et al. (2012) and Zaveri et al (2009) demonstrated the effect of PPAR-β/δ antagonists GSK3787, GSK0660, and SR13904, respectively.

In the recent review article, Gamdzyk et al. (2020) compared neuroprotective efficacy of PPAR-β/ δ agonists to PPAR-α and PPAR-γ and conclude that despite of being the most highly expressed in CNS, the available data on the effect of this receptor agonists in stroke as well as other neurological disorders are relatively poor and thus needing further investigations. However, the last results of Chehaibi et al. (2017) demonstrated several ameliorating effects of PPAR-β/δ agonist GW0742 in mice brain ischemia evoked by occlusion of middle cerebral artery (MCA). The significant anti-inflammatory effect was exerted by this agonist, which decreased the neutrophil infiltration, the level of several chemokines and interleukins such as IL-1β, IL6, and other cytokines. Using the same model of brain ischemia. Pialat et al. (2007) previously demonstrated in magnetic resonance imaging (MRI) scan that PPAR-β/δ-null mice comparing to control wild mice indicated significant differences in lesion volume. The effect of GW0742 was also investigated by Tang et al. (2020) in a collagenase-induced intracerebral hemorrhage (ICH) mouse model. In this study, PPAR-β/δ agonist was administered (intraperitoneally in a dose of 3 mg/kg body weight) 30 min before ICH, and its neuroprotective effects included mitigation of behavioral dysfunction and molecular pathways associated with activation of inflammation and apoptosis. A previous study by Paterniti et al. (2010) evaluated the involvement of PPAR-β/δ in spinal cord injury (SCI) in mice evoked by application of vascular clips (force of 24 g) to the dura via a four-level T5 to T8 laminectomy. GW0742 (administered i.p. in a dose of 3 mg/kg body weight) exerted significant neuroprotective effect, and ameliorated the recovery of limb function. The protective effects of this agonist include inhibition of neutrophil infiltration, expression of proinflammatory cytokines and altered molecular processes leading to cell death through changes of transcription of pro- and anti-apoptotic proteins (Fasl, Bax, Bcl2). The protective processes evoked by GW0742 could be eliminated by specific receptor antagonist (GSK0660), which was administered (1 mg/kg bw) at 30 min before GW0742. This high-affinity PPAR-β/δ agonist GW0742 was able to evoke significant neuroprotective effects in secondary damage, during experimental spinal cord injury (SCI) in mice (Paterniti et al. 2010). In this study, GW0742 treatment (0.3 mg/kg⁻¹ i.p) at 1 and 6 h after SCI, significantly reduced inflammation, nitric oxide synthesis, nitrotyrosine formation and activation of apoptotic signaling. Moreover, this agonist protected against edema and showed positive effect on motor recovery score. The study of Esposito et al. (2012) indicated that GW0742, through targeting divergent downstream pathways regulating PPAR-β/δ receptors, could decrease changes on both molecular and cellular levels that take place in spinal cord damage. CNS hypoxia–ischemia hemorrhage and traumatic injury are closely connected with vascular alterations, oxidative stress and hypertension. In a preclinical study, Toral et al. (2016) showed antihypertensive effects of PPAR-β/δ in spontaneously hypertensive rats (SHR) as well as in other animal models. Pharmacological activation of PPAR-β/δ exerted several protective effects, improved the endothelial dysfunction, decreased vascular inflammation and vasoconstriction responses. There is evidence that other isoforms of PPARs can also show protective effects on cerebral ischemia damage. In a study by Wu et al. (2016), cultured neurons were subjected to in vitro oxygen–glucose deprivation (OGD), and treatment with GW9662, an antagonist for PPAR-γ, could ameliorate neuronal apoptosis and inhibit p22phox subunit of NADPH oxidase (Wu et al. 2016). It is possible to suggest that agonist acting simultaneously on PPAR-γ and PPAR-β/δ could be more effective in OGD model and in brain ischemia pathology comparing to PPAR-γ alone. Despite of evidence indicating ability for PPAR-β/δ agonists to exert neuroprotective effects on cerebral ischemia injury, there were also negative results, probably depending on the type of agonists used and method of administration. For example, in a study by Knauss et al. (2018) oral administration with SAR 145, a known lipophilic agonist for PPAR-β/δ, could not improve short or long outcomes after focal cerebral ischemia induced to mice through middle cerebral artery occlusion (Knauss et al. 2018).

Despite of the recognition of PPAR-β/δ in metabolic and inflammatory diseases, there is increasing interest in developing appropriate ligands/antagonists towards treatment of cancer (Wagner and Wagner 2020; Reil and Lee 2008; Liu et al. 2018). However, the molecular mechanism of PPARs in carcinogenesis is still not fully elucidated, and data from in vitro and in vivo studies are still controversial. Tatenhorst et al. (2008) concluded in their review articles that the agonists of PPARs could be promising for new approaches in human CNS tumor therapy. Subsequently, Youssef and Badr (2011) tried to explain the complexity of these receptor responses and their conformational changes that influence their ability to recruit specific functionally distinct coactivators. For better understanding of the complicated role of PPAR-β/δ in carcinogenesis, these authors recalled the work of Mukherjee et al. (1994), showing that some receptors (such as androgen receptors) exhibited capability to interact with 150 proteins/polypeptides, and thus suggested such a possibility for PPAR in carcinogenesis. It also seems that, in the case of PPAR-β/δ, the complex coactivators and repressors in PPAR-β/δ could be subjected to deeper analysis. Recent studies of Yao et al. (2017) showed that PPAR-β/δ could inhibit human neuroblastoma cell tumorigenesis by inducing protein p-53 and SOX2 mediated cell differentiation. These results suggest that combinatorial activation of retinoic acid receptor, PRAR-α and PPAR-β/δ may be promising therapeutic approach for RA-resistant neuroblastoma patients. Ding et al. (2020) demonstrated the impact of PPAR-β/δ and PPAR-γ polymorphism on glioma risk and prognosis in the Chinese Han population.

Considering their anti-inflammatory, neuroprotective, and anti-tumors properties, PPAR-β/δ agonists are promising treatments of AD and other neurodegenerative disorders. A list of the natural and synthetic agonists for PPAR-β/δ is shown in Table 1. These PPAR-β/δ ligands should be applied in various other pathologies as DMT2, MetSD, atherosclerosis, obesity, hepatosteatosis. The role of PPAR-β/δ in cancer should be better elucidated and understood. Besides PPAR-β/δ, agonists of PPAR-α and PPAR-γ may also be involve in neurodegenerative diseases, in MetSD and dyslipidemia. Therefore, future studies should test PPAR-β/δ ligands in combination with ligands of other PPARs receptor in these neurological disorders and in inflammation.